1. Introduction
Anisotropy of magnetic susceptibility (AMS) is a widely applied and routinely used method for deciphering the complex interplay between magmatic and tectonic processes in batholiths (e.g. Stevenson et al. Reference Stevenson, Owens and Hutton2007; Liu et al. Reference Liu, Liu, Zhang, Huang and Zhang2023; Neves et al. Reference Neves, Ferreira, Sial, Lima, Ardila and Neves2023; Pereira et al. Reference Pereira, Savian, Florisbal, Tomé, do Carmo and da Trindade2024; Tomek et al. Reference Tomek, Žák, Verner, Ježek and Paterson2024; Mattsson et al. Reference Mattsson, McCarthy and Schmiedel2024). The AMS method has proven to be a key tool for characterizing structural fabrics across diverse geological settings, from igneous intrusions to deformed rocks in orogenic belts. Its application enables the interpretation of magma flow dynamics (Bouchez, Reference Bouchez1997; Kratinová et al. Reference Kratinová, Schulmann, Edel, Ježek and Schaltegger2007), post-emplacement deformation (Borradaile & Jackson, Reference Borradaile and Jackson2004; Mamtani & Greiling, Reference Mamtani and Greiling2005), and folded structures lacking clear foliation (Mamtani & Sengupta, Reference Mamtani and Sengupta2010). These studies underscore the versatility of AMS in detecting tectonic imprints in compositionally homogeneous rocks and enhancing the reconstruction of deformation events at multiple scales.
In particular, AMS has been successfully employed to interpret regional strain fields in various plutons and deformation zones, demonstrating its value as a kinematic marker in orogenic contexts (e.g., Knight et al. Reference Knight, Stevenson, Maffione, McCarthy, Burton-Johnston and Lawrence2024; Qayyum et al. Reference Qayyum, Poessé, Kaymakci, Langereis, Gülyüz and Ahsan2022). AMS analyses quantify the preferred orientation of minerals, which can reveal the tectonic strain experienced by a rock body during emplacement and help determine its pre-, syn-, or post-tectonic character (Ferré, Reference Ferré2002; Stevenson et al. Reference Stevenson, Owens and Hutton2007; Benn, Reference Benn2009; Petronis et al. Reference Petronis, O’Driscoll, Troll, Emeleus and Geissman2009, Reference Petronis, O’Driscoll, Stevenson and Reavy2012; Mamtani & Sengupta, Reference Mamtani and Sengupta2010; McCarthy et al. Reference McCarthy, Petronis, Reavy and Stevenson2015; Mattsson et al. Reference Mattsson, Petri, Almqvist, McCarthy, Burchardt, Palma and Galland2021; Cruz et al. Reference Cruz, Sant’Ovaia, McCarthy and Noronha2022).
When applied to the emplacement and deformation history of batholiths, AMS studies must consider several factors, including magma flow patterns, intrusion mechanisms, mineral shape and orientation, tectonic strain indicators, deformation phases, and regional tectonic context (Paterson et al. Reference Paterson, Fowler, Schmidt, Yoshinobu, Yuan and Miller1998; Mamtani et al. Reference Mamtani, Pal and Greiling2013; Liu et al. Reference Liu, Liu, Zhang, Huang and Zhang2023; Nahas et al. Reference Nahas, Gonçalves, Gonçalves and Raposo2023; Tomek et al. Reference Tomek, Žák, Verner, Ježek and Paterson2024). To resolve these complexities, AMS data are most effective when integrated with structural mapping, microstructural analysis, crystallographic preferred orientation, and U–Pb zircon geochronology. This multidisciplinary approach provides a comprehensive understanding of batholith evolution and offers insights into the broader magmatic and tectonic processes active during pluton formation (e.g. Cruden et al. Reference Cruden, Tobisch and Launeau1999; Kratinová et al. Reference Kratinová, Schulmann, Edel and Tabaud2012; Ávila et al. Reference Avila, Archanjo, Hollanda, de Macedo Filho and Lemos-Santos2020; Nke et al. Reference Nke, Njanko, Mamtani, Rochette and Njonfang2022; Mohammad & El Kazzaz, Reference Mohammad and El Kazzaz2022; Viegas et al. Reference Viegas, Montefalco, Yokoyama, Archanjo, Raposo, Seoane and de Miranda Leite2022; Wei et al. Reference Wei, Lin, Chen, Faure, Ji, Hou, Yan and Wang2023).
The internal structure of a pluton may reflect magma dynamics, regional tectonics, or a combination of both (Paterson et al. Reference Paterson, Fowler, Schmidt, Yoshinobu, Yuan and Miller1998, Reference Paterson, Ardill, Vernon and Žák2019; Burton-Johnson et al. Reference Burton-Johnson, Macpherson, Muraszko, Harrison and Jordan2019). Accordingly, AMS fabrics in plutonic rocks can be purely magmatic or overprinted by solid-state deformation (Paterson et al. Reference Paterson, Vernon and Tobisch1989; Zak et al. Reference Žák, Verner and Týcová2008; Paterson et al. Reference Paterson, Ardill, Vernon and Žák2019; Neves et al. Reference Neves, Ferreira, Sial, Lima, Ardila and Neves2023; Siachoque et al. Reference Siachoque, Morales, Cardona, Marulanda and Zapata2024). Distinguishing between these fabric types is essential for reconstructing emplacement histories and understanding the relative contributions of magmatic and tectonic processes (e.g., Vigneresse & Bouchez, Reference Vigneresse and Bouchez1997; Gleizes et al. Reference Gleizes, Leblanc, Santana, Olivier and Bouchez1998; de Saint Blanquat et al. Reference de Saint Blanquat, Tikoff, Teyssier and Vigneresse1998; D’Eramo et al. Reference D’Eramo, Pinotti, Tubía, Vegas, Aranguren, Tejero and Gómez2006; Benn, Reference Benn2009; Zaffarana et al. Reference Zaffarana, Somoza, Orts, Mercader, Boltshauser, González and Puigdomenech2017; Wei et al. Reference Wei, Lin, Chen, Faure, Ji, Hou, Yan and Wang2023).
Moreover, AMS measurements, when combined with field and petrographic data, help differentiate between diapiric ascent and laccolithic inflation in granitoid intrusions, refining models of pluton emplacement (McCarthy et al. Reference McCarthy, Petronis, Reavy and Stevenson2015).
In this context, the present study focuses on the AMS fabric pattern of the Upper Triassic-Lower Jurassic Curaco Batholith, located in the extra-Andean sector of the North Patagonian Massif (Fig. 1, 39°43’00”S-67°40’00”W). This research presents new rock magnetic and AMS data from the eastern sector of the batholith, describing its fabric development and evolution during emplacement. These geophysical results are integrated with previous geological and microstructural studies, contributing to a better understanding of the tectono-magmatic evolution of the extra-Andean North Patagonian Massif during the Upper Triassic-Lower Jurassic, a period marked by significant crustal reorganization associated with the early breakup of Gondwana.
Figure 1. Regional map showing the main magmatic provinces of late paleozoic and early mesozoic rocks of northern Patagonia: Choiyoi Magmatic Province (Carboniferous-Permian) and Chon Aike Silicic Large Igneous Province (Jurassic). Note the E-W Huincul fault zone and the WNW-trending Río Negro fault, two structures that comprise the northeastern limit of the North Patagonian Massif. ScPB: Subcordilleran Plutonic Belt, MCG: Mamil Choique Granitoids, SMG: Sierra del Medio Granitoids, LTG: Laguna del Toro Granitoids, CPC: Chachil Plutonic Complex, HPC: Huechulafquen Plutonic Complex, CPB: Central Patagonian Batholith, LEPVC: La Esperanza Plutonic Volcanic Complex, LM: Los Menucos Group. VR: volcanic rocks, PR: plutonic rocks.
Although prior AMS studies have provided valuable insights into pluton fabrics and deformation histories, the Curaco Batholith offers a unique opportunity to examine the dynamic interaction between magmatic processes and regional tectonics. By integrating new AMS measurements with detailed structural and microstructural data, this study presents novel evidence of how structurally controlled magma emplacement influences the development of magnetic fabric patterns in plutonic rocks. The results contribute to ongoing discussions on emplacement mechanisms and deformation timing in batholiths, advancing our understanding of tectono-magmatic coupling in the North Patagonian Massif and informing broader models of magmatic and tectonic evolution in extensional orogenic settings.
2. Regional geological setting
Following the Gondwanide Eruptive Cycle in the Late Paleozoic, which played a key role in the formation of the Gondwana supercontinent and the generation of the magmatism associated with the Late Paleozoic Choiyoi Magmatic Province (Fig. 1; Kay et al. Reference Kay, Ramos, Mpodozis and Sruoga1989; Sato et al. Reference Sato, Llambías, Basei and Castro2015), the relatively stable Gondwana supercontinent underwent changes in the tectonic regime from a compressional to an extensional regime, accompanied by changes in magmatic activity triggered by the initiation of Gondwana break-up during the Upper Triassic to Lower Jurassic period (Vizán et al. Reference Vizán, Prezzi, Geuna, Japas, Renda, Franzese and Van Zele2017). The collapse of the Gondwanan orogen in Late Triassic times (González et al. Reference González, Giacosa, Lagorio, Ballivian Justiniano, Sato, Cábana, Basei, Busteros and Silva Nieto2021) led to widespread extension of the continental lithosphere, resulting in the formation of half-grabens and associated magmatism, linked to the initial stages of Gondwana’s breakup (Uliana et al. Reference Uliana, Biddle, Cerdan, Tankard and Balkwill1989; Franzese & Spalletti, Reference Franzese and Spalletti2001; Giacosa, Reference Giacosa2020). During the Gondwana break-up, the Karoo mantle plume was invoked to explain the origin of the Chon-Aike Silicic Large Igneous Province (SLIP), extending along its southwest margin throughout Patagonia, the Antarctic Peninsula, and Ellsworth-Whitmore Terrane (Fig. 1; Pankhurst et al. Reference Pankhurst, Leat, Sruoga, Rapela, Márquez, Storey and Riley1998, Reference Pankhurst, Riley, Fanning and Kelley2000). The Chon-Aike intra-plate volcanism likely had a transitional pass to the subduction-related arc volcanism of the proto-Pacific Phoenix and Farallon plates (Pankhurst & Rapela, Reference Pankhurst and Rapela1995; Riley et al. Reference Riley, Leat, Pankhurst and Harris2001; Navarrete et al. Reference Navarrete, Gianni, Encinas, Márquez, Kamerbeek, Valle and Folguera2019, Reference Navarrete, Gianni, Tassara, Zaffarana, Likerman, Márquez, Wostbrock, Planavsky, Tardani and Frasette2024; Zaffarana et al. Reference Zaffarana, Lagorio, Gallastegui, Wörner, Orts, Gregori, Poma, Busteros, Giacosa, Silva Nieto, Ruiz González, Bolsthauser, Puigdomenech and Haller2020, Reference Zaffarana, Lagorio, Gallastegui, Orts, Busteros, Poma, Gregori, Giacosa and Silva Nieto2022; Bastías et al. Reference Bastías, Spikings, Riley, Ulianov, Grunow, Chiaradia and Hervé2021).
The silicic volcanic rocks of the Chon Aike SLIP erupted in three main phases through the Jurassic (Pankhurst et al. Reference Pankhurst, Leat, Sruoga, Rapela, Márquez, Storey and Riley1998, Reference Pankhurst, Riley, Fanning and Kelley2000; Riley et al. Reference Riley, Leat, Pankhurst and Harris2001). The first volcanic phase is a peak eruptive age V1 (188-178 Ma) followed by the V2 (172-167 Ma) and V3 (157-153 Ma) phases. Although volumetrically of minor importance and with a more restricted geographical distribution, the Chon Aike SLIP also records a V0 volcanic phase ranging from 193 to 188 Ma (Pavón Pivetta et al. Reference Pavón Pivetta, Gregori, Benedini, Garrido, Strazzere, Geraldes, Costa Dos Santos and Marcos2019). This precursor phase records a plutonic counterpart ranging from 224-186 Ma, overlapping since ∼193 Ma, and represented by the Central Patagonia Batholith (222-206 Ma, Rapela et al. Reference Rapela, Pankhurst and Harrison1992, Reference Rapela, Pankhurst, Fanning and Hervé2005; Zaffarana et al. Reference Zaffarana, Somoza and de Luchi2014; Lagorio et al. Reference Lagorio, Busteros, Nieto Silva, Zaffarana, Giacosa and González2022), El Sótano Granodiorite (186 ± 9 Ma, Sato et al. Reference Sato, Basei, Tickyj, Llambías and Varela2004), Flores Granite (188 ± 3 Ma, Pankhurst et al. Reference Pankhurst, Rapela and Caminos1993), and Curaco Batholith (224-189 Ma, Saini-Eidukat et al. Reference Saini-Eidukat, Bjerg, Gregori, Beard and Johnson1999, Reference Saini-Eidukat, Migueles, Gregori, Bjerg, Beard, Gehrels and Johnson2002, Reference Saini-Eidukat, Beard, Bjerg, Gehrels, Gregori, Johnson, Migueles and Vervoort2004; Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016; see Fig. 1). A tectonic-magmatic feature that the Central Patagonia and Curaco batholiths share is the tectonic control of the magmatic fabric during the emplacement by the NW-SE to E-W-trending basement fabrics (Zaffarana et al. Reference Zaffarana, Somoza, Orts, Mercader, Boltshauser, González and Puigdomenech2017; Ruiz González et al. Reference Ruiz González, Puigdomenech, Zaffarana, Vizán and Somoza2020; Aramendia et al. Reference Aramendia, Zaffarana, González and Pernich2023; González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025). This structural and magmatic framework is associated with the continental transtensional geodynamic scenario of the early phases of the western Gondwana break-up by the latest Late Triassic, characterized by NE-SW oblique extension and dextral strike-slip faulting along E-W-trending brittle-ductile shear zones (e.g., Zaffarana et al. Reference Zaffarana, Somoza and de Luchi2014; Ruiz González et al. Reference Ruiz González, Puigdomenech, Zaffarana, Vizán and Somoza2020; González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025).
3. Geology of the Curaco study area
The Curaco Batholith (González et al. Reference González, Mungai, Cábana, Zaffarana, Aramendia and Herazo2023), formerly Curaco Plutonic Volcanic Complex (Hugo & Leanza, Reference Hugo and Leanza2001) or Alessandrini Complex (Saini-Eidukat et al. Reference Saini-Eidukat, Migueles, Gregori, Bjerg, Beard, Gehrels and Johnson2002) is located in the northern sector of the North Patagonian Massif, ∼100 km south of General Roca town (39°43’S-67°40’W, Figs. 1, 2a). The primary field relationship (i.e., the observed geological contact between the batholith and the volcanic succession), indicates that the batholith intruded a succession of alternating rhyolitic–dacitic ignimbrites, which records a U–Pb zircon age of 255.3 ± 3.0 Ma and has been related to the Choiyoi Magmatic Province (González et al. Reference González, Mungai, Cábana, Zaffarana, Aramendia and Herazo2023). These ignimbrites, which crop out along the western margin of the batholith, can be regionally correlated with the lower Permian volcanic units of the Los Menucos Group in northern Patagonia, as well as with the Collinao and Las Pampas ignimbrites of the Dos Lomas Complex (Bjerg et al. Reference Bjerg, Gregori and Labudía1997; Falco et al. Reference Falco, Bodnar and Del Río2020; González et al. Reference González, Mungai, Cábana, Zaffarana, Aramendia and Herazo2023). The batholith also contains basement xenoliths of amphibolite and mica-schist of unknown age and lithostratigraphic unit (Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016) but which could be ascribed to the Cushamen Formation (Marcos et al. Reference Marcos, Gregori, Benedini, Barros, Strazzere and Pivetta2018).
Figure 2. Area of study (a) location of the study area (39º 43’ S - 67º 40’ W), 100 km south of General Roca, Río Negro Province. (b) RGB: 741 band combination in Landsat images to show the different responses of the satellite to the different facies. The solid white lines delimit the mylonites, the solid yellow lines delimit the porphyritic monzogranite, the solid light blue lines delimit the granodiorites and diorites, the solid pink lines delimit the muscovite-bearing leucogranites, and the solid green lines delimit the granite porphyry. The dashed white lines indicate the traces of magmatic structures (circular pluton shapes). The ages correspond to (1) Saini-Eidukat et al. (Reference Saini-Eidukat, Bjerg, Gregori, Beard and Johnson1999); (2) Saini-Eidukat et al. (Reference Saini-Eidukat, Beard, Bjerg, Gehrels, Gregori, Johnson, Migueles and Vervoort2004); (3) Gregori et al. (Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016); (4) González et al. (Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025). c) Geological map showing the distribution of the main granitic facies in the eastern part of the Curaco Batholith.
The Curaco Batholith comprises coarse to fine-grained monzogranites, sienites, granodiorites, and diorites (Saini-Eidukat et al. Reference Saini-Eidukat, Migueles, Gregori, Bjerg, Beard, Gehrels and Johnson2002; Báez et al. Reference Báez, Paz, Pino, González, Cábana, Giacosa, García and Bechis2016; Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016; Aramendia et al. Reference Aramendia, Zaffarana, González and Pernich2023). Mylonites originating from these granitoids are also recorded (Saini-Eidukat et al. Reference Saini-Eidukat, Beard, Bjerg, Gehrels, Gregori, Johnson, Migueles and Vervoort2004; Báez et al. Reference Báez, Paz, Pino, González, Cábana, Giacosa, García and Bechis2016; Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016). Both the granitoids and the mylonites are all intruded by swarms of aplo-pegmatitic, andesitic, and rhyolitic dikes (Fig. 2c, Hugo & Leanza, Reference Hugo and Leanza2001; Báez et al. Reference Báez, Paz, Pino, González, Cábana, Giacosa, García and Bechis2016; Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016; Aramendia et al. Reference Aramendia, Zaffarana, González and Pernich2023). Initially, the foliated granites within the Curaco Batholith were classified as Caita Có Granite by Gregori et al. (Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016).
Published Rb-Sr and U-Pb geochronological data from the Curaco Batholith range from 224 to 189 Ma, indicating that the tectonic-magmatic evolution of the Curaco Batholith occurs between the Norian and early Pliensbachian (Saini-Eidukat et al. Reference Saini-Eidukat, Beard, Bjerg, Gehrels, Gregori, Johnson, Migueles and Vervoort2004; Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016). Its emplacement is related to the widespread extension resulting from the collapse of the Gondwanide orogen in the Triassic (Sato et al. Reference Sato, Llambías, Basei and Castro2015) and overlaps with the V0 volcanic phase of the Chon Aike SLIP (193-188 Ma, Pavón Pivetta et al. Reference Pavón Pivetta, Gregori, Benedini, Garrido, Strazzere, Geraldes, Costa Dos Santos and Marcos2019). Using Rb-Sr on whole-rock, the equigarnular monzogranites were dated at 192 ± 0.21 Ma (Saini-Eidukat et al. Reference Saini-Eidukat, Bjerg, Gregori, Beard and Johnson1999), while the U-Pb method on zircons yielded ages of 223 ± 6 and 195 ± 3.1 Ma (Fig. 2b, Saini-Eidukat et al. Reference Saini-Eidukat, Beard, Bjerg, Gehrels, Gregori, Johnson, Migueles and Vervoort2004). In a subsequent study, a new Rb-Sr isochron analysis incorporating granodiorites, granites, and aplitic dikes yielded an age of 195 ± 11 Ma (Saini Eidukat et al. Reference Saini-Eidukat, Migueles, Gregori, Bjerg, Beard, Gehrels and Johnson2002). The rhyolitic and dacitic dikes were dated at 193 ± 13 Ma using the Rb-Sr whole rock isochron method (Saini-Eidukat et al. Reference Saini-Eidukat, Migueles, Gregori, Bjerg, Beard, Gehrels and Johnson2002). The analyses of whole rock granite samples, feldspar, and biotite yielded Rb-Sr isochron ages of 195 ± 6.7 Ma (Fig. 2b, Saini-Eidukat et al. Reference Saini-Eidukat, Bjerg, Gregori, Beard and Johnson1999). Gregori et al. (Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016) reported U-Pb zircon ages for the foliated granites of 224 ± 5, 216 ± 8.4, 206.8 ± 6.7, and 189.1 ± 6.5 Ma (Fig. 2b). González et al. (Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025) documented four U-Pb zircon ages for the granodiorite of 205.3 ± 0.9, 198.7 ± 0.6, 191.6 ± 1.1 and 183.4 ± 1.4 Ma (Fig. 2b). Broadly speaking, the Curaco Batholith ranges in age from 223 ± 6 Ma to 183 ± 1.4 Ma (Saini-Eidukat et al. Reference Saini-Eidukat, Bjerg, Gregori, Beard and Johnson1999, Reference Saini-Eidukat, Migueles, Gregori, Bjerg, Beard, Gehrels and Johnson2002, Reference Saini-Eidukat, Beard, Bjerg, Gehrels, Gregori, Johnson, Migueles and Vervoort2004; Grégori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016; González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025). The 224-189 Ma magmatic event also agrees with the observation that the continental red beds of the Cenomanian-Campanian (Upper Cretaceous) Neuquén Group (Bjerg et al. Reference Bjerg, Gregori and Labudía1997; Hugo & Leanza, Reference Hugo and Leanza2001) unconformably overlie the batholith but are unaffected by the Patu-Co and El Loro lineaments (Fig. 2, Hugo & Leanza, Reference Hugo and Leanza2001). Then, according to the stratigraphic-structural relationships, the batholith emplacement and the associated deformation are pre-Cenomanian in a broader sense.
The Curaco Batholith is bounded by two main lineaments to the north and south (Figs. 2a, b Hugo & Leanza, Reference Hugo and Leanza2001). The ∼ENE–WSW-trending Patu-Co and El Loro lineaments (Hugo & Leanza, Reference Hugo and Leanza2001), together with the NW-trending and NE-dipping Pangaré and La Seña mylonitic belts, are major pre-existing structures that controlled the emplacement of the Curaco Batholith and the opening of the Los Menucos pull-apart basin during the Gondwana Orogeny (Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016). This transtensional setting provided accommodation space for subsequent magma emplacement. Within this tectonic framework, the Caita Có granitic stock was emplaced syn-tectonically as a sheeted and foliated intrusion, structurally aligned with and spatially confined by the active NW-SE trending mylonitic belts (Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016). Later tectonic reactivation introduced E–W-striking dextral strike-slip faults, which further dissected the earlier fabrics and deformational domains (e.g., Mizerit et al. Reference Mizerit, Suárez, Voglino, Aranda, Giacosa and González2014; Báez et al. Reference Báez, Paz, Pino, González, Cábana, Giacosa, García and Bechis2016).
In addition, the batholith is traversed by two ductile shear zones known as Pangaré and la Seña (Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016). The Curaco Batholith intrudes volcanic rocks of the Choiyoi Magmatic Province and constitutes part of the plutonic counterpart of the Chon Aike SLIP (Saini-Eidukat et al. Reference Saini-Eidukat, Migueles, Gregori, Bjerg, Beard, Gehrels and Johnson2002, Reference Saini-Eidukat, Beard, Bjerg, Gehrels, Gregori, Johnson, Migueles and Vervoort2004; González et al. Reference González, Mungai, Cábana, Zaffarana, Aramendia and Herazo2023, Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025). Structural and geophysical mapping carried out in this study reveals that its magmatic fabric–previously considered as non-foliated–is parallel both to the fabric of other SLIP rocks and to the tectonic fabric observed in mylonites within the Pangaré and La Seña shear zones.
4. Materials and methods
As a first step for fieldwork, the mapping of the eastern part of the Curaco Batholith was aided by Landsat 8 satellite images using RGB-741, 754, 852, and 871 band combinations (Fig. 2b). Then we went to the field and drilled; we drilled forty-nine sites across the main lithological units of the Curaco Batholith using a portable drill (Table 1). The sampling included equigranular and porphyritic monzogranites, mafic enclaves, aplitic dikes, granodiorites, diorites, muscovite-bearing leucogranites, granite porphyry, mylonites, and both rhyolitic and andesitic dikes. The sampling sites were strategically selected to encompass representative samples of both plutonic and volcanic facies within the batholith, located away from, near to, and within the Pangaré and La Seña shear zones. This approach ensures a comprehensive assessment of the AMS across different lithological and structural variations present in the study area. Since the study area is arid and there are no permanent streams, we also transported water containers to the outcrops to lubricate and cool the drill. We collected at least five cores of 2.5 cm in diameter and ∼10 cm long from each site. A total of 386 samples (2.2 cm in length and 2.54 cm in diameter) were collected for the AMS core analysis, resulting in an average of 8 specimens per site (Table 1). We also performed forty-five representative thin sections out of forty-nine sites for mineralogical, textural, and microstructural studies under the petrographic microscope. We took photomicrographs with a generic digital camera mounted on the microscope, both hosted at the Instituto de Investigación en Paleobiología y Geología (IIPG, UNRN-CONICET), Argentina.
Table 1. Anisotropy of magnetic susceptibility data of the Curaco Batholith
N: number of cores per sample.
K: magnetic susceptibility.
St. Dev: standard deviation.
L: magnetic lineation.
F: magnetic foliation.
Pj: anisotropy degree (Jelinek, Reference Jelinek1981).
T: shape parameter (Jelinek, Reference Jelinek1981) where 0 < T < 1 indicates oblateness and -1 < T < 0 indicates prolateness.
K1 (maximum) and K3 (minimum) are the principal axes of the AMS ellipsoid.
d and I are declination and inclination of downward direction, respectively.
mean and av describe mean and average values, respectively.
a95max. and a95min are major and minor axes of 95% confidence ellipse about principal axis of magnetic susceptibility, in degrees.
Microfabrics.
Mag: magmatic, SM: sub-magmatic, HT Ss: high-temperature solid-state, MT ss: medium-temperature solid-state, LT ss: low-temperature solid-state, Cat: cataclastic, *: site without thin section.
The AMS data were acquired using an MFK1-FA Kappabridge (Agico) instrument, with a field of 200 A/m and a frequency of 976 Hz, in the Daniel A. Valencio Paleomagnetism Laboratory of the Instituto de Geociencias Básicas, Aplicadas y Ambientales (IGEBA, UBA-CONICET), Buenos Aires University, Argentina. The magnetic susceptibility ellipsoids were calculated for a minimum of five specimens per site with the Anisoft 4.2 program (Jelinek, Reference Jelínek1978, Reference Jelinek1981). Additionally, we examined how certain minerals contribute to the bulk rock magnetic susceptibility to enhance our understanding of the AMS data. Rock magnetic studies of selected powdered samples were performed in the Laboratory of Paleomagnetism at the Universidad Complutense de Madrid (Spain). Thermomagnetic curves were carried out with a CS furnace apparatus in a KLY-4S kappabridge by AGICO. Also, hysteresis loops, isothermal remanent magnetization (IRM) acquisition and backfield demagnetization of saturation IRM (SIRM) curves were performed with a Kazan University (RussianFederation) J-Meter coercivity spectrometer (Jasonov et al. 1998). We carried out magnetic analyses to investigate the magnetic properties of rock minerals. For the acquisition of these thermomagnetic analyses, finely ground whole-rock powders were used to ensure homogeneity and minimize anisotropy effects during magnetic measurements. Susceptibility versus temperature curves were performed by heating the samples from room temperature up to 700ºC at a rate of 12 °C/min under an argon atmosphere and then cooled back to room temperature (Dunlop & Özdemir, Reference Dunlop and Özdemir1997).
Online Supplementary Material at http://journals.cambridge.org/geo’ summarizes stereo plots with structural data and other complementary AMS results in figures (Figs. S1-8).
5. Results
5. a. General description of the Curaco Batholith
The Curaco Batholith is a rhombic-shaped body with an axial ratio of approximately 3.0 and an ENE-WSW-trending long axis, covering an area of about 850 km2 (Figs. 1, 2a). Its eastern sector consists mainly of equigranular and porphyritic monzogranites, accompanied by minor occurrences of granodiorites, diorites, muscovite-bearing leucogranites, and a granite porphyry (Figs. 2b, c). Aplite dikes intrude the equigranular and porphyritic monzogranites. The NW–SE-trending Pangaré and La Seña ductile shear zones, 1-6 km wide and approximately 13 km long, cut across all major granitoid facies and host associated mylonitic shear bands (Fig. 2c). They are composed of heterogeneous intercalations of protomylonites, mylonites, and ultramylonites (Fig. 2c). A ∼1 km thick ultramylonite zone occurs in the central part of the Pangaré shear zone (Figs. 2b, c). Andesitic and rhyolitic dikes intrude all main facies of the Curaco Batholith, including the mylonites (Fig. 2c).
The equigranular monzogranite facies represent the main rock type of the Curaco Batholith, occupying the largest exposed area of the batholith (Figs. 2c, 3). The porphyritic monzogranites occur in transitional contact with the equigranular facies (Figs. 2c, 3). Satellite imagery reveals several nearly circular plutons within the monzogranite facies (Figs. 2b, c). A small, irregular, dark grey pluton of intercalated granodiorite and diorite crops out in sharp contact with both monzogranite facies and is restricted to the north of the El Salado lineament (Figs. 2b, c). Granodiorites and diorites plot in the granodiorite and quartz monzodiorite fields, respectively, on the QAP diagram (Fig. 3). Muscovite-bearing leucogranites crop out as a small pluton within the La Seña mylonite belt, showing sharp contacts with the equigranular monzogranites and brittle fracturing (Figs. 2c, 3). The granite porphyry (monzogranite field in the QAP, Fig. 3) crops out as an elongated NE–SW-trending body located north of the El Loro lineament (Fig. 2c).
Figure 3. Modal QAP classification (Streckeisen, Reference Streckeisen1976) of all facies of the Curaco Batholith as defined in this work. Additionally, we include the modal QAP classification of the Curaco Batholith from Báez et al. (Reference Báez, Paz, Pino, González, Cábana, Giacosa, García and Bechis2016) and the CIPW normative compositions of the Curaco Batholith from the analyses of Bjerg et al. Reference Bjerg, Gregori and Labudía1997. Q: quartz, A: alkali feldspar, and P: plagioclase.
In a general sense, diorites and monzogranites share the same evolutionary trend, the same trend depicted by the granitoids of the Curaco Batholith described by Báez et al. (Reference Báez, Paz, Pino, González, Cábana, Giacosa, García and Bechis2016) and Bjerg et al. (Reference Bjerg, Gregori and Labudía1997). (Fig. 3). From the petrographic point of view, the andesite and rhyolite dikes plot along the same evolutionary trend in relation to the plutonic facies (Fig. 3).
5. b. Field and petrographic observations of the different granitic facies of the Curaco Batholith
The equigranular monzogranites are pink-coloured rocks and exhibit a coarse- to fine-grained texture, commonly displaying tube-like schlieren structures and miarolitic cavities (Figs. 4a, b). On average, this facies shows a well-developed magmatic foliation, defined in the field by the orientation of minerals such as biotite and feldspar, oriented NW-SE (N262/14°NW), with dips ranging from subvertical to shallow (85° to 10°), either to the NE or SW (see sites 4, 23B, 29, 15 in Fig. S4a, b, e, h). Petrographically, these monzogranites have a medium-grained hypidiomorphic texture and are composed of euhedral orthoclase (45%), euhedral plagioclase (30%), subhedral to euhedral quartz (20%), and a remaining 5% distributed among biotite, muscovite, apatite, monazite, epidote, titanite, zircon, fluorite, and opaque minerals (Fig. 4c). Plagioclase commonly shows polysynthetic twinning and zoning. The greenish-brown biotite contains inclusions of apatite and opaque minerals and is strongly altered to chlorite. Magnetite occurs as subhedral to euhedral octahedral crystals forming small mafic glomerules, often associated with biotite and apatite.
Figure 4. Field exposures and photomicrographs of equigranular and porphyritic monzogranites of the Curaco Batholith. (a) Tube-like schlieren structure (in section, probably a migrating tube) in the equigranular monzogranites (see AMS ellipsoid from site 29 in Fig. S4e). (b) Miarolitic cavity filled with quartz and plagioclase in equigranular monzogranites (see AMS ellipsoid in Fig. S4e). (c) Photomicrograph of the equigranular monzogranite showing a hypidiomorphic, inequigranular texture composed of plagioclase, K-feldspar, and quartz, with locally euhedral quartz grains. (d) Porphyritic monzogranite displaying magmatic layering, characterized by alternating biotite-rich layers and layers with accumulations of K-feldspar megacrysts aligned parallel to the layering, indicating a NW–SE magmatic flow direction. (e) Granitic blob hosted by the porphyritic monzogranites, where magmatic foliation defined by the biotite schlieren dips radially at shallow angles (<22°) from the subhorizontal inner part (see AMS ellipsoids from site 12B in Fig. S5b). (f) Photomicrograph of the porphyritic monzogranite showing large K-feldspar phenocrysts embedded in a finer-grained groundmass composed of interstitial quartz and plagioclase. (g) Aplitic dike (26B) crosscutting the granodioritic enclave (26C), both hosted by the porphyritic monzogranite (26A). (h) Photomicrograph of a granodioritic enclave showing oriented euhedral to subhedral biotite crystals, plagioclase, quartz, opaque minerals, and apatite inclusions within biotite. (i) Photomicrograph of an aplitic dike showing graphic texture with intergrowths of quartz, orthoclase, and plagioclase.
The porphyritic monzogranites are characterized by large K-feldspar megacrysts, up to 5 cm long, embedded in a coarse-grained equigranular groundmass (Figs. 4d-f). The porphyritic monzogranites exhibit a compositional layering defined by alternating ∼10 cm thick leucocratic and melanocratic bands (Fig. 4d). The leucocratic layers are enriched in K-feldspar and quartz, while the melanocratic bands are dominated by biotite and plagioclase. These bands strike NW and dip steeply to the NE or SW (commonly around N312/70°NE; Fig. 4d).
These monzogranites host granitic tubes with a distinctly elongated NW–SE shape, delineated by schlieren composed of fine-grained biotite and amphiboles (Fig. 4e). Inside the tubes, the magmatic foliation–defined by the alignment of biotite schlieren and K-feldspar megacrysts–dips radially at shallow angles (<22°), while the magmatic lineation, marked by the alignment of K-feldspar phenocrysts, trends NE and dips gently (<15°) (see site 12B in Figs. 4e, S5a). In addition to these tubes, pipe-like structures defined by subvertical concentrations of K-feldspar phenocrysts are also present.
Mineralogically, the porphyritic monzogranites are composed of euhedral alkali feldspar (orthoclase) phenocrysts (20%) immersed in a fine-grained hypidiomorphic matrix. The groundmass is composed of subhedral quartz (35%), subhedral orthoclase (15%), subhedral plagioclase (25%), and 5% of accessory minerals including biotite, apatite, titanite, rutile, zircon, monazite, and opaque minerals (Fig. 4f). These monzogranites also exhibit a micrographic texture. The greenish-brown biotite is strongly altered to chlorite and contains inclusions of apatite and monazite. Magnetite occurs as subhedral to euhedral octahedral crystals disseminated in the groundmass, often associated with biotite and apatite.
The porphyritic monzogranites host biotite–amphibole granodioritic enclaves (Fig. 4g). These mafic enclaves are elongated in a NNW–SSE direction, and they exhibit partial disaggregation along the margins of the aplitic dikes (Fig. 4g). The magmatic foliation within the enclaves is also oriented towards the NW-SE direction (N270/6°NW), consistent with the fabric of the host porphyritic monzogranites (Fig. 4g). Some enclaves incorporate K-feldspar phenocrysts from the host monzogranite. These associated granodioritic enclaves exhibit a medium-grained hypidiomorphic texture and are composed of euhedral plagioclase (40%), anhedral quartz (25%), anhedral orthoclase (15%), subhedral biotite (10%), subhedral clinopyroxene (5%), and a remaining 5% comprising amphibole, titanite, apatite, and allanite (Fig. 4h). Magnetite occurs as subhedral to euhedral octahedral crystals forming small mafic glomerules, associated with biotite.
Aplitic dikes, up to 1 m thick, intrude the equigranular and porphyritic monzogranite facies. These dikes trend from NW–SE to E–W and dip steeply towards the S–SW (Fig. 4g). They exhibit fine-grained aplitic and graphic textures, composed of subhedral to anhedral quartz (60%), anhedral orthoclase (10%), subhedral plagioclase (15%), and euhedral biotite (10%), with 5% of accessory minerals including apatite, zircon, and opaque minerals (Fig. 4i). Brown biotite commonly contains inclusions of apatite and is locally altered to chlorite. Magnetite occurs as subhedral to euhedral octahedral crystals, mostly as isolated grains with some forming small mafic glomerules.
Granodiorites and diorites crop out as irregular dark grey bodies with sharp contacts against the equigranular monzogranites (Figs. 2b, c; 5a). The magmatic foliation trends NW–SE (N160/78°SW) and is subvertical, marked by the alignment of biotite–amphibole in the diorites and biotite–K-feldspar in the granodiorites (Fig. S6g-j). Magmatic lineation, defined by the alignment of K-feldspar phenocrysts, dips moderately (∼30°) to the SE (Fig. S6g-j). Granodiorites bear a fine-grained hypidiomorphic texture and are composed of plagioclase (40%), quartz (25%), orthoclase (15%), biotite (10%), amphibole (5%), and clinopyroxene (5%), along with accessory minerals such as titanite and apatite. Diorites display a medium-grained equigranular hypidiomorphic texture and are composed of euhedral green amphibole (35%), euhedral plagioclase (30%), subhedral alkali feldspar (orthoclase and microcline, 15%), anhedral quartz (10%), biotite (5%), and 5% accessory minerals such as apatite, epidote, titanite, and opaque minerals (Fig. 5b).
Figure 5. Field exposures and photomicrographs of the diorites, granite porphyry and muscovite-bearing leucogranites. (a) Sharp contact between dioritic stock and the equigranular monzogranite. (b) Photomicrograph of the diorite showing large euhedral amphibole phenocrysts set in a medium- to fine-grained groundmass composed of plagioclase, orthoclase, and quartz. (c) Outcrop of granite porphyry at site 40. (d) Photomicrograph of a granite porphyry displaying well-developed micrographic texture, with intergrowths of quartz and K-feldspar formed during late-stage crystallization. A mineralized vein crosscuts the porphyritic groundmass, suggesting post-magmatic hydrothermal activity. (e) Outcrop of muscovite-bearing leucogranites. (f) Photomicrograph of an equigranular muscovite-bearing leucogranite composed of plagioclase, microcline and quartz. A muscovite-rich microxenolith is enclosed within the felsic matrix.
The granite porphyry crops out as an elongate, E–W-trending body within the equigranular monzogranites, subparallel to the eastern porphyritic monzogranite (Figs. 2c). The magmatic and sub-magmatic foliation marked by igneous flow folds trends N146/76°W, and the magmatic lineation plunges 10° towards 153° (see site 40 in Fig. S6f). Mineralized veinlets are present. The granite porphyry is white and displays a well-developed porphyritic texture, characterized by six-sided euhedral quartz phenocrysts (40%), up to 1 cm in diameter (Fig. 5c), immersed in a coarse-grained micrographic matrix composed of anhedral quartz (10%), anhedral orthoclase (25%), euhedral plagioclase (15%), and a remaining 10% consisting of opaque minerals, apatite, and topaz (Fig. 5d).
Muscovite-bearing leucogranites are white to light pink-coloured rocks with a fine-grained texture (Fig. 5e). Magmatic foliation, defined by biotite, trends NW-SE and dips moderately to the SW (mean foliation tensor: N136/61W), while magmatic lineation, marked by the preferred orientation of muscovite, plunges moderately 34° towards 168° (see site 6 in Fig. S6e). The muscovite-bearing leucogranites exhibit a coarse-grained, inequigranular texture, ranging from seriate to graphic, and they are composed of anhedral quartz (50%), anhedral K-feldspar (20%), subhedral to anhedral plagioclase (15%), subhedral muscovite (10%), subhedral biotite, and 5% opaque minerals. They contain quartz phenocrysts and abundant microxenoliths of schists and gneisses, reaching up to 5 mm in length (Fig. 5f).
5. c. Deformation in the Curaco Batholith
In the Curaco Batholith, microfabrics envolve a progressive transition from magmatic and sub-magmatic conditions to high- and low-temperature solid-state deformation. The mylonitic rocks described here do not represent classical metamorphic mylonites formed from fully crystallized protoliths. Instead, they developed syn-tectonically during magma cooling within an active transtensional regime. This continuous evolution allowed the local preservation of relict magmatic features, such as subgrains in plagioclase and elongated quartz grains with magmatic cores, within the mylonitic fabric (González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025). The mylonites from the Pangaré and La Seña shear zones exhibit a NW-SE-trending (mean foliation tensor: N310/60°NE), sub-vertical mylonitic foliation that is parallel to the margins of the shear zones (Figs. 2c, 6a). Mylonitic lineation, defined by the stretching of quartz and feldspar grains, dips either shallowly or steeply toward the SE or NW (Fig. S7). Their protoliths are the porphyritic and equigranular monzogranites. Mylonites are composed of K-feldspar and plagioclase porphyroclasts embedded in a quartz and K-feldspar foliated recrystallized matrix (Fig. 6b). Asymmetric feldspar porphyroclasts indicate dextral strike-slip displacements within the Pangaré shear zone and dextral and sinistral shear sense indicators within the La Seña shear zone (González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025).
Figure 6. Field characteristics of the mylonites and the main volcanic facies of the Curaco Batholith. (a) Mylonites of the La Seña shear zones showing a vertical NW-SE mylonitic foliation (Smy). (b) Photomicrograph of a mylonite showing porphyroclasts of relictic plagioclase and K-feldspar mantled by quartz-rich bands composed of dynamically recrystallized subgrains. (c) Andesitic dike intruding the porphyritic monzogranite. (d) Andesitic dike with a xenolith of granitic composition. (e) Photomicrograph of an andesitic dike with prominent phenocrysts of amphibole set in a fine-grained groundmass. The amphibole crystals are subhedral to euhedral and locally aligned, indicating flow during emplacement. (f) Rhyolitic dike with a strike of N55°, a dip of 85°, and a thickness of 5 m. (g) Phenocrysts of biotite, orthoclase, and plagioclase set in a fine-grained microgranular groundmass within a rhyolitic dike.
Based on the predominant microstructures, we distinguish and describe six main microfabrics within all facies of the Curaco Batholith (Fig. 7, Table 1). The main microfabrics are magmatic, sub-magmatic, high-temperature solid-state (HT), medium-temperature solid-state (MT), low-temperature solid-state (including mylonites, LT), and cataclastic (C), (Vernon Reference Vernon2000, Reference Vernon2004; Passchier & Trouw, Reference Passchier and Trouw2005). During batholith emplacement and cooling, microstructural overprinting typically proceeds from high- to low-temperature conditions, with lower-temperature fabrics overprinting those formed earlier at higher temperatures.
Figure 7. (a) Map showing microstructure distribution across AMS sites. (b-g) Photomicrographs of microstructures suggesting progressive solid-state deformation occurring with the cooling of the Curaco Batholith, from the magmatic to the low-temperature solid state, up to the cataclastic state. All photomicrographs were taken under cross-polarized light (XPL). (b) Magmatic flow foliation in equigranular monzogranite defined by the alignment of euhedral biotite and plagioclase crystals, with no evidence of internal plastic deformation. (c) Porphyritic monzogranite showing a submagmatic fracture filled with quartz in an alkali feldspar (orthoclase) crystal. (d) Aplitic dike showing a chessboard extinction pattern in quartz and lobate grain boundaries, indicative of high-temperature solid-state deformation. (e) Bent twins in plagioclase in granodiorite (f) Recrystallized quartz aggregates forming bands around a deformed residual orthoclase porphyroclast in a mylonite. (g) Quartz xenocryst in rhyolitic dike showing brittle deformation.
Figure 7a shows the distribution of microstructural fabrics observed in thin sections from the Curaco Batholith. Their spatial distribution varies with distance from the La Seña and Pangaré mylonite belts. Magmatic, sub-magmatic and high-temperature solid-state deformation fabrics are located to the west of the La Seña mylonite belt (Fig. 7a). Magmatic microstructures are more clearly preserved in equigranular and porphyritic monzogranites and andesitic dikes, although in the monzogranites, they are partially overprinted by high- to low-temperature solid-state deformation (Table 1). Both equigranular and porphyritic monzogranites and aplite dikes that exhibit magmatic textures and high-temperature solid-state deformation are located to the west of the La Seña mylonite belt (Fig. 7a). Medium-temperature solid-state microfabrics are found in equigranular and porphyritic granite facies, as well as in muscovite-bearing leucogranites, granodiorites, and diorites near the mylonite belts (Fig. 7a, Table 1). Low-temperature solid-state microfabrics are preserved in mylonites within the La Seña and Pangaré mylonite belts (Fig. 7a; Table 1).
Magmatic microstructures are characterized by either shape-preferred orientation of plagioclase and biotite phases or non-oriented but lacking intracrystalline deformation (Figs. 6b; Vernon Reference Vernon2000, Reference Vernon2004; Passchier & Trouw, Reference Passchier and Trouw2005). These features are locally preserved across all petrofacies of the Curaco Batholith; however, they are commonly overprinted by solid-state deformation microstructures, reflecting the progressive transition from sub-magmatic to high- and low-temperature ductile deformation.
Sub-magmatic microfractures in equigranular and porphyritic monzogranites cut across the rigid crystals of quartz, plagioclase, or K-feldspar, overprinting the magmatic fabric (Paterson et al. Reference Paterson, Vernon and Tobisch1989; Bouchez et al. Reference Bouchez, Delas, Gleizes, Nédélec and Cuney1992; Vernon, Reference Vernon2000). The microfractures formed while the rock was still partially molten, are filled with quartz in crystallographic continuity inside and outside the fractured crystal and with uniform extinction (Fig. 7c).
High-temperature solid-state microfabrics develop after complete solidification under ductile conditions. In some equigranular and porphyritic monzogranites and aplite dikes include a chessboard pattern and undulose extinctions in quartz, interlobate and amoeboid-shaped crystal boundaries in quartz, alkali feldspar, and plagioclase, consistent with grain boundary migration activated at high T (GBM; Paterson et al. Reference Paterson, Vernon and Tobisch1989; Passchier & Trouw, Reference Passchier and Trouw2005; Fig. 7d). A slight chessboard pattern of quartz subgrains is observed at specific sites.
Medium-temperature solid-state microstructures in equigranular and porphyritic monzogranites, aplite dikes, diorites and granodiorites include quartz subgrains formed by subgrain rotation (SGR, e.g., Pryer, Reference Pryer1993; Passchier & Trouw, Reference Passchier and Trouw2005), myrmekites, bent twins in plagioclase, flame perthite along the margins of K-feldspar, and microcline (Fig. 7e).
Low-temperature solid-state microstructures in equigranular and porphyritic monzogranites, diorites, granodiorites and mylonites include oriented bands of quartz subgrains that were formed by dynamic recrystallization, bulging and micro-kink bands in biotite, among others (Fig. 7f). In mylonites, these microstructures are more pervasive and occur alongside stretched quartz ribbons, feldspar porphyroclasts with core-and-mantle structures, and mica fish, all indicative of deformation under low-temperature plasticity conditions (Fig. 7f).
Cataclastic microstructures, including microfractures and minor displacements along cleavage planes (Vernon, Reference Vernon2000), occur at the outcrop scale and are restricted to discrete zones and are observed locally in all rocks of the Curaco Batholith (Fig. 7g).
5. d. Andesitic and rhyolitic dikes of the Curaco Batholith
The ENE-WSW-trending porphyritic andesitic dikes, 0.10–2 m thick, dip steeply to the SE (Figs. 2c, 6c). In the field, their magmatic foliation is parallel to the dike walls, and they commonly contain partially assimilated, elongated granitic xenoliths (Fig. 6d). Petrographically, these dikes exhibit a porphyritic texture defined by euhedral plagioclase and amphibole phenocrysts (30%) immersed in a foliated trachytic-textured groundmass (70%) composed of plagioclase, pyroxene, amphibole, and opaque minerals (Fig. 6e). Quartz xenocrysts with oval shapes and outer reaction rims are also present, typically surrounded by amphibole and pyroxene forming coronitic textures.
The NE-SW-trending rhyolitic dikes, coarser and 10–20 m thick, dip steeply to the SE and cut through the andesitic dikes (Figs. 2c, 6f). This cross-cutting relationship, along with the presence of andesitic fragments within the rhyolitic dikes, suggests mingling between both magmatic pulses. Petrographically, they exhibit a porphyritic texture with euhedral phenocrysts (40%) of quartz, plagioclase, biotite, and orthoclase immersed in an aphanitic, microgranular groundmass (60%) composed of K-feldspar (mainly orthoclase), quartz, glass, apatite, and opaque minerals (Fig. 6g). The quartz phenocrysts show engulfments, while biotite contains apatite inclusions. The groundmass is commonly devitrified into spherulitic aggregates (Fig. 6g).
The magmatic fabric of the granitoids and dikes, as well as the mylonitic foliation within the Pangaré and La Seña shear zones, is cut by a NNW-SSE trending set of steeply dipping brittle fractures that dip variably toward the SW or NE. These fracture sets are associated with conjugate NW-SE and NE-SW trending, sub-vertical dextral strike-slip faults (Figs. 2b, c).
5. e. Rock magnetism
The rock magnetism of the different facies of the Curaco Batholith are presented here, where the visual inspection of the thermomagnetic curves allows us to evaluate the main magnetic carriers of the AMS fabric in each of them. As a general observation, almost all the high-temperature thermomagnetic curves are irreversible on the cooling curve and show the presence of Ti-poor magnetite as the main carrier of the AMS, due to its Curie temperature around 580°C (Fig. 8; Dunlop & Özdemir, Reference Dunlop and Özdemir1997; Tauxe, Reference Tauxe1998). This Curie point of 590°C was inferred due to the Hopkinson Peak or inflection point observed in the curves (Fig. 8; Moskowitz, Reference Moskowitz1981; Tauxe, Reference Tauxe1998). Thermomagnetic curves from representative samples of the Curaco Batholith are shown in Figure 8b. The shape of the hysteresis loops shows the presence of a low coercivity mineral in most of the samples, probably magnetite, as the main carrier of the AMS, and, in some cases, it is remarkable the contribution of the paramagnetic minerals. (Figs. S1-S3).
Figure 8. (a) Day plot of hysteresis and backfield parameters for the representative samples of the eastern sector of the Curaco Batholith. (b) Thermomagnetic curves from nine representative samples of equigranular monzogranite, porphyritic monzogranite, granodiorite, diorite, muscovite-bearing leucogranite, aplitic, andesitic and rhyolitic dikes, to illustrate the rock magnetic mineralogy analyses. Mrs: the saturation intensity of magnetic remanence, Ms: the saturation intensity of induced magnetization, Hcr: the coercivity of magnetic remanence, Hr: coercivity of the measured sample. SD: single-domain, PSD: pseudo-single domain, MD: multidomain.
Here we also show the diagram of Day et al. (Reference Day, Fuller and Schmidt1977) to analyze the domain state of the magnetite (Rochette et al. Reference Rochette, Jackson and Aubourg1992; Chadima et al. Reference Chadima, Cajz and Týcová2009). The Day diagram is generated using data for coercive force (Hc), saturation magnetization (Ms), remanent magnetization (Mr), and remanent coercive force (Hcr) obtained from hysteresis loops, IRM and backfield curves (Fig. 8a; Day et al. Reference Day, Fuller and Schmidt1977; Figs. S1-S3). The main conclusion of the analysis of the diagram in Fig. 8a is that most of the samples analyzed do not exhibit single-domain magnetite (Fig. 8a), which facilitates the interpretation of the magnetic fabric as non-inverse (e.g., Rochette et al. Reference Rochette, Jackson and Aubourg1992).
The equigranular monzogranites are represented by sample 23B, which displays cooling curves with higher susceptibility than the heating curves, suggesting the formation of magnetite during the heating process (Fig. 8b). Additionally, the presence of a Hopkinson peak at 580ºC indicates magnetite. In the Day diagram, the sample plots within the pseudo-single domain (PSD) magnetite field (Fig. 8a).
The porphyritic monzogranites are represented by sample 26A, which shows a susceptibility drop around 590 °C, and sample 5A, which exhibits a Hopkinson peak (Fig. 8b). These features suggest the presence of Ti-poor magnetite in this facies. The Day diagram indicates that the porphyritic monzogranites contain both multidomain (MD) and pseudo-single domain (PSD) magnetite (Fig. 8a).
Sample 20B, representative of the granodiorite facies, shows a marked increase in magnetic susceptibility around 500 °C during both heating and cooling (Fig. 8b). This behavior suggests the formation or reordering of Ti-poor magnetite during heating, as commonly described in oxidized Fe-Ti oxide systems (Dunlop & Özdemir, Reference Dunlop and Özdemir1997; Hrouda, Reference Hrouda2003). The Day plot indicates MD magnetite as the main magnetic carrier (Fig. 8a).
The diorites represented by sample 21A show a decrease in the thermomagnetic curves, indicating that their magnetic susceptibility is dominated by paramagnetic minerals, mainly biotite, with amphibole contributing a minor paramagnetic/weakly ferromagnetic signal (Fig. 8b; Hrouda, Reference Hrouda1982; Dunlop & Özdemir, Reference Dunlop and Özdemir1997).
Thermomagnetic curves for the muscovite-bearing leucogranites (sample 6, Fig. 8b) exhibit considerable noise due to their very low bulk magnetic susceptibility (Km) because the main contribution to anisotropy comes from paramagnetic minerals. Despite this, a clear loss of magnetization at approximately 580°C indicates the presence of trace amounts of magnetite as a magnetic carrier (Fig. 8b).
Thermomagnetic analysis of the aplitic dikes (sample 24B, Fig. 8b) indicates a sharp increase in magnetization during heating from 400°C, followed by a complete loss of signal at 580°C, characteristic of magnetite. The corresponding cooling curve displays a distinct bulge between 400°C and 500°C, consistent with the thermal properties of magnetite. In the Day diagram, the sample plots within the pseudo-single domain (PSD) magnetite field (Fig. 8a).
The thermomagnetic curve for the andesitic dike sample 9A (Fig. 8i) exhibits a clear increase in magnetic susceptibility starting around 300°C, culminating in a prominent Hopkinson peak (Fig. 8b). This peak is immediately followed by a sharp drop in magnetization at approximately 590°C, which is characteristic of magnetite as the dominant magnetic carrier in this sample (Fig. 8b). In the Day diagram, the sample plots within the pseudo-single domain (PSD) magnetite field (Fig. 8a).
The thermomagnetic curve for the rhyolitic dike (sample 22, Fig. 8b) is characterized by a prominent Hopkinson peak immediately preceding a sharp and significant drop in magnetization. This behavior is unequivocally diagnostic of magnetite as the primary magnetic carrier, with its Curie temperature confirmed at approximately 580°C. In the Day diagram, the sample plots within the pseudo-single domain (PSD) magnetite field (Fig. 8a).
All of these samples from the Curaco Batholith show the presence of hematite in a small and variable quantity, which can be recognized in some of the IRM curves (not saturating) and in the susceptibility relict between 580°C and 680ºC in the thermomagnetic curves.
5. f. Anisotropy of the magnetic susceptibility (AMS)
First, we describe the scalar parameters of the AMS, which can be summarized as follows. Mylonitic rocks from the Pangaré and La Seña shear zones exhibit high magnetic anisotropies with low magnetic susceptibilities, suggesting that deformation controls their magnetic fabric. In contrast, all the plutonic rocks of the Curaco Batholith, show significantly lower Pj values (almost, less intense than in the mylonite rocks).
The scalar parameters from the 49 AMS sites are presented along with different maps showing the distribution of scalar AMS parameters by rock type within the Curaco Batholith (Fig. 9), as well as site-level plots of anisotropy degree (Pj) versus bulk susceptibility (Km), and shape parameter (T) versus Pj (Fig. 10). The rhyolitic and andesitic dikes were not included in the maps of Figure 9b-d because they were intruded after the development of the Pangaré and La Seña shear zones. The directional AMS parameters (magnetic foliation and lineation) are then shown and analyzed according to their proximity to the major structures and shear zones (Figs. 11, 12, 13). Structural data of magmatic and mylonitic fabrics observed in the field consistently agree with the magnetic foliations and lineations identified across all deformed facies of the Curaco Batholith, indicating a high level of reliability in the AMS fabrics (Figs. S4-S8). Online Supplementary Materials Figures S1-S8 summarize the complete data set of rock magnetics, the AMS stereograms, and the structural field data of sampled sites.
Figure 9. (a) Map showing all the 49 AMS sites in the eastern part of the Curaco Batholith. (b) Map of the mean magnetic susceptibility (Km). All the Km values presented are from the deformed facies, excluding the undeformed facies, which are the rhyolitic and andesitic dikes. (c) Map of the degree of anisotropy (Pj). All the Pj values presented are from the deformed facies. (d) Map showing the shape parameter (T). All the T values presented are from the deformed facies.
Figure 10. AMS scalar data in the sites of the Curaco Batholith, with sites classified with magmatic and solid-state deformation fabrics (Table 1). (a) Pj vs. Km plot. (b) T vs. Pj plot. Pj: corrected anisotropy degree (Jelinek, Reference Jelinek1981); Km: mean susceptibility; T: shape parameter of the magnetic fabrics (Jelinek, Reference Jelinek1981).
Figure 11. AMS directional data (magnetic foliations) of the plutonic rocks of the Curaco Batholith and of the mylonitic rocks of the Pangaré and La Seña shear zones. Three projections illustrate how foliations become more vertical closer to the shear zones. (a) K3 (pole to the magnetic foliation) statistical distribution within the Pangaré and La Seña shear zones. (b) K3 (pole to the magnetic foliation) statistical distribution near the La Seña shear zone. (c) K3 (pole to the magnetic foliation) statistical distribution away from the La Seña shear zone. The stereonets represent Kamb-contoured equal-area lower-hemisphere stereographic projections made with the software Stereonet 11 ( Allmendinger et al., Reference Allmendinger, Cardozo and Fisher2011; Cardozo & Allmendinger, Reference Cardozo and Allmendinger2013). The scale represents Kamb contours in standard deviation. (d) Map showing the distribution of the magnetic foliation planes (plane perpendicular to K3 axis). Due to the coaxiality observed at some sampling sites, the host granite, associated aplitic dike, and enclaves were grouped and represented as a single data point, as in the cases of CUR26A-B-C, 2-3-8, 12A-B, 20A-B, 21A-B, and 18-19.
Figure 12. AMS directional data (magnetic lineations) of the plutonic rocks of the Curaco Batholith and of the mylonitic rocks of the Pangaré and La Seña shear zones. (a) K1 (magnetic lineation) distribution, within the Pangaré and La Seña shear zones. (b) K1 (magnetic lineation) distribution, near the La Seña shear zone. (c) K1 (magnetic lineation) distribution, away from the La Seña shear zone. The stereonets represent Kamb-contoured equal-area lower-hemisphere stereographic projections made with the software Stereonet 11 ( Allmendinger et al., Reference Allmendinger, Cardozo and Fisher2011; Cardozo & Allmendinger, Reference Cardozo and Allmendinger2013). The scale represents Kamb contours in standard deviation. (d) Map showing the distribution of magnetic lineations (K1 direction). Due to the coaxiality observed at some sampling sites, the host granite, associated aplitic dike, and enclaves were grouped and represented as a single data point, as in the cases of CUR26A-B-C, 2-3-8, 12A-B, 20A-B, 21A-B, and 18-19.
Figure 13. AMS directional data (foliations and lineations) in the andesitic and rhyolitic dikes of the Curaco Batholith. (a) K3 and K1 distributions showing the orientation of the magnetic foliations (K3 is the pole of the magnetic foliation) and lineations in the andesitic and rhyolitic dikes. The stereonets represent Kamb-contoured equal-area lower-hemisphere stereographic projections made with the software Stereonet 11 ( Allmendinger et al., Reference Allmendinger, Cardozo and Fisher2011; Cardozo & Allmendinger, Reference Cardozo and Allmendinger2013). The scale represents Kamb contours in standard deviation. (b) Map showing the orientation and distribution of the dikes and a rose diagram (box) showing the distribution of the strikes of the andesitic and rhyolitic dike, which is mostly ENE-WSW. Note the parallelism of the magnetic foliations to the dike margins, except in Site CUR 11 that shows an inverse magnetic fabric.
5. f.1. Scalar parameters of the AMS fabrics
Site distribution in the different lithologies of the Curaco Batholith is presented in Figure 9a and is listed in Table 1, where 5 sites belong to the equigranular monzogranites, 9 belong to the porphyritic monzogranites, only one site belongs to the muscovite-bearing leucogranites, 8 were drilled in the aplitic dikes, 2 sites belong to the granodioritic enclaves, one site was drilled in the granite porphyry, 2 sites belong to the diorites, and 2 were drilled in the granodiorites. Then, 10 sites belong to the mylonites, and the remaining sites (7 and 2) belong to the andesitic and rhyolitic dikes.
The equigranular monzogranites are located near and away from the La Seña shear zone (Fig. 9a). Their Km values are low, falling within the paramagnetic to weakly ferromagnetic range and generally below 5 × 10⁻3 SI, except for site 4, which shows 5 < Km < 20 × 10−3 SI, indicating that the AMS is controlled by ferromagnetic minerals (Fig. 9b, Hrouda, Reference Hrouda2010). Some sites exhibit even lower Km values–below 0.5 × 10−3 SI–which lie well within the paramagnetic range (Figs. 9b, S4c-j, Table 1). This suggests that the magnetic fabric is primarily controlled by paramagnetic silicates, such as biotite or feldspar (Table 1, Fig. 10b; Tarling & Hrouda, Reference Tarling and Hrouda1993; Bouchez, Reference Bouchez2000; Biedermann et al. Reference Biedermann, Pettke, Angel and Hirt2016; Koopmans et al. Reference Koopmans, McCarthy and Magee2022). Site 29, the only site located near the La Seña shear zone, also shows a low Km value within the paramagnetic range (below 0.5 × 10−3 SI; Fig. 9b).
The magnetic susceptibility of the porphyritic monzogranites and their associated granodioritic enclaves is a bit higher, with a wide range of magnetic susceptibilities, spanning the strongly ferromagnetic range, with Km > 20 × 10−3 SI (sites 12A and 12B) to the paramagnetic range, with Km < 0.5 × 10−3 SI (site 5; Figs. 9a, b, Table 1). In particular, sites 12A and 12B (Fig. 2b), bear the highest magnetic susceptibility of the whole batholith, with strongly magnetic monzogranites having Km values of 78.30 × 10−3 and 57.60 × 10−3 SI (Fig. S5a, b, Table 1). These high magnetic susceptibilities suggest that this parameter is likely controlled by ferromagnetic minerals, primarily magnetite, as supported by rock magnetic and petrographic observations (Figs. 8, 10b, S1, S2, Table 1). The high range of magnetic susceptibilities shown by the porphyritic monzogranites and their granodioritic enclaves is present both near and away from the La Seña shear zone, with the only exception being that the lowest susceptibilities in this facies were found at site 5, which is located away from the shear zone (Km = 0.31 × 10−3 SI, Fig. 9b, Table 1).
The muscovite-bearing leucogranites in site 6 display Km of 0.12 × 10−3 SI within the Km < 0.5 × 10−3 SI range, which is consistent with a magnetic fabric controlled by paramagnetic minerals (Figs. 8b, 9b, S1; Table 1; Biedermann et al., Reference Biedermann, Pettke, Angel and Hirt2016; Koopmans & McCarthy, Reference Koopmans, McCarthy and Magee2022). In turn, the granodiorites, diorites, aplite dikes, and granite porphyry also show low Km values, lower than 0.5 × 10−3 SI, suggesting that magnetic susceptibility is primarily controlled by paramagnetic minerals, which is consistent with magnetic studies (Figs. 8b, 9b, S2, S3; Table 1; Biedermann et al., Reference Biedermann, Pettke, Angel and Hirt2016; Koopmans & McCarthy, Reference Koopmans, McCarthy and Magee2022).
The mylonites of the Pangaré and La Seña shear zones exhibit a wide range of Km values, from Km = 13.70 × 10−3 SI in site 30A (falling into the 5<Km<20 × 10−3 SI category in Fig. 9b) to Km = 0.07 × 10−3 SI in site 27 (falling into the K<0.5 × 10−3 SI range in Fig. 9b), thus indicating that the AMS is influenced by both paramagnetic and ferromagnetic minerals (Fig. 9b; Table 1).
In the rhyolitic dikes, the two sites show Km values ranging from 4.45 × 10−3 to 7.27 × 10−3 SI, suggesting that the AMS is controlled by magnetite (Figs. 8, S3, S8a, b; Table 1). In turn, from the andesitic dikes, samples were taken from seven sites with Km values ranging from 0.49 × 10−3 to 27.40 × 10−3 SI, indicating that the AMS is controlled by magnetite (Figs. 8, S3, S8c-i; Table 1).
The Pj values in the mylonitic rocks of the Pangaré and La Seña shear zones are strongly influenced by deformation showing the hihgest Pj values (> 1.1) within the Curaco Batholith, compared with the granitoids (up to 1.359 in site 30A) than the less intensely deformed rocks (Fig. 10a). Pj values greater than 1.17 are only present within the shear zones; near the shear zones, Pj values are lower than 1.06 (with some values between 1.06 and 1.17), and away from the shear zones, low magnetic anisotropies predominate (Pj < 1.06). with some values between 1.06 and 1.17 (Fig. 9c).
The less intensely deformed rocks (located mostly away from the shear zones, Fig. 7a), show lower Pj values than the mylonitic rocks, and they show a weak but systematic increase of Pj with Km (Fig. 10a). For example, in the porphyritic monzogranites, which showed the highest Km values, the Pj values reach 1.114 in site 12A, but lower Pj values are present in the other facies of the batholith (the equigranular monzogranites, the muscovite-bearing leucogranites, granodiorites, and diorites and the granitic porphyry show a low degree of anisotropy, Pj < 1.06, except for site 4 in equigranular, and sites 39A, 9C, and 13B in porphyritic monzogranites, which have a Pj value between 1.06 and 1.17; Fig. 9c, table 1). In the aplitic dikes, Pj values are mostly below 1.06, except for site 41, which has a Pj value between 1.06 and 1.17 (Fig. 9c; table 1). Finally, the rhyolitic and andesitic dikes show Pj values below 1.05, except for site 30B, which has a Pj value close to 1.10 (Figs. 10a, S8, table 1).
With respect to the shape of the ellipsoids described by parameter T, the mylonitic rocks show mostly triaxial AMS ellipsoids (Figs. 9d, 10b, Fig. S7a-g, i, j), except for site PAN3, which shows a prolate ellipsoid (Fig. S7h). The other rocks, either near or away from the La Seña shear zones, display a broader range of T values, varying from triaxial (-0.5 < T < 0.5) to strongly oblate (T = almost 1; Figs. 9b, 10b). For example, in the equigranular monzogranites, ellipsoids are mainly triaxial, except for sites 23B and 19, which display oblate shapes (Figs. 9d, S4b, f). In turn, the porphyritic monzogranites display both oblate and triaxial ellipsoids (Figs. 9d, S5, S6a-d). The muscovite-bearing leucogranites and the granodiorites and diorites show mostly triaxial ellipsoids (Figs. 10d, S6e, g-i), except for site 21A of the diorites, which is prolate (Figs. 9d, 10b, S6j). Most aplite dikes display oblate ellipsoids (Fig. S4c, d, g, j), except for site CUR 17, which exhibits a triaxial ellipsoid (Figs. 9d, S4i). The granitic porphyry and the andesitic and rhyolitic dikes show oblate AMS ellipsoids (Figs. 9d, 10b, S6f, S8a, b, e-i), except sites 30B and 9B, which are triaxial (Fig. S8c, d).
5. f.2. Vectorial parameters of the AMS in the variably deformed plutonic rocks
Magnetic foliations in the mylonites from La Seña and Pangaré shear zones are predominantly of NW-SE strike (N140/83°SW) and dip steeply (>70°) either to the NE or the SW (Figs. 11a, d, S7). However, moderately (37° to 56°) dipping foliations are also observed, such as at the southern end of the Pangaré shear zone (site PAN5, Fig. 11d). The magnetic lineations in the La Seña and Pangaré shear zones strike predominantly towards the SE, and they are either shallowly or moderately plunging (Figs. 12a, d, S7). In the Pangaré shear zone, magnetic foliations dip steeply to the NE in the north (see site 33 in Fig. S7a) and moderately to the SW in the south (site PAN5; Fig. 11d, S7b). In the La Seña shear zone, magnetic lineations plunge either shallowly towards the north (sites 30A, 28, 32, 31, 27; Fig. S7c, e, g, i, j) or steeply towards the south (sites PAN1, 14, PAN3; Figs. 12d, S7d, f, h).
Near the La Seña shear zone, the porphyritic and equigranular monzogranites, diorites, granodiorites, and the muscovite-bearing leucogranites generally display magnetic foliations with the same general NW-SE trend (N327/38°NW) shown by the shear zones, but their dip is not so high, it is either intermediate to shallow (31° to 70°) towards the NE, although some steeply dipping foliations are also found in this area (site 29 of the equigranular monzogranites, site 13B of the porphyritic monzogranites and sites 21A and 21B of the diorites and granodiorites; Fig. 11d). Magnetic lineations are, in turn, mainly NW-SE striking and subhorizontal in all the granitoids of this area near the La Seña shear zone (Figs. 11b, 12b, S4, S5, S6). Contrastingly, the muscovite-bearing leucogranite at site 6 shows a W-E-trending and moderately dipping towards the south magnetic foliation (Figs. 11d, S6e). It is important to mention that in all cases, the magnetic fabrics of the granodioritic enclaves are parallel to those of their porphyritic monzogranite hosts (see sites 26C and 2 in Fig. S5d, h).
Away from the La Seña Shear Zone, the equigranular and porphyritic monzogranites, along with their aplitic dikes and granite porphyry, exhibit magnetic foliations that still display a predominant NW–SE strike (N312/70°NE), although some E-W strikes are also observed in this area (Figs. 11d, S4). However, these foliation planes dip moderately to shallowly towards the S-SW and N-NE (Fig. 11d). In particular, NW-SE steep foliation planes were observed at site 15 of the equigranular monzogranites, which, despite exhibiting high-temperature magmatic deformation (Fig. 7a), also show cataclastic overprinting with a NW-SE subvertical orientation (Figs. 11d, S4h). Another notable case is the granite porphyry and its associated aplitic dykes (sites 40 and 41 towards the SW of the study area), which also exhibit steeply dipping NW–SE (78°-86°) magnetic foliations (Figs. 11d, S6f).
The magnetic lineations are also similar to those observed in the La Seña Shear Zone and its surrounding areas, although they exhibit greater dispersion (Figs. 11c, d; 12c, d). The aplitic dikes exhibit the same W–E to NW–SE-trending, subvertical magnetic fabric as their monzogranitic host rocks. This fabric is either parallel or transverse to the dike margins but is always consistently coaxial with that of the surrounding equigranular or porphyritic monzogranites. For example, this relationship is evident when comparing sites 24B and 25 (aplitic dikes) with their equigranular monzogranitic host at site 23B, as well as in other dike–host rock pairs such as sites 18–19, 26A–26B (Figs. S4f, g, S5c-e), and 8–3 (Figs. 11d, S5i-j).
5. f.3. Magnetic fabric in the rhyolitic and andesitic dikes
In the rhyolite and andesite dikes, the ∼E-W-trending and subvertical magnetic foliations (N106/87°NE) are generally parallel to the dike margins, and the magnetic lineations are predominantly subhorizontal (K1<30°; Figs. 13, S8). However, there are some exceptions (Fig. 13b). For rhyolite dikes of sites 22 and 11, the NW-SE-trending (N138/34°SW) magnetic foliation, which is either sub-vertical (site 22) or sub-horizontal (site 11), is not parallel to the dike walls, and the magnetic lineation plunges shallowly (∼20°) to the NW (Fig. S8a, b). Furthermore, the magnetic fabric at site 22 differs from the previous ones due to its magnetic foliation deflecting more than 15° with respect to the rhyolite dike margins. In contrast, the andesite dikes exhibit a vertical E-W magnetic foliation parallel to the dike margins and a magnetic lineation with a moderate plunge to the west (Figs. 13b, S8c-i).
6. Discussion
6. a. Origin and significance of the fabrics of the Curaco Batholith
Syn-tectonic plutons form during periods of active regional tectonic deformation (e.g., Hutton & Reavy, Reference Hutton and Reavy1992; Gleizes et al. Reference Gleizes, Leblanc, Santana, Olivier and Bouchez1998; de Saint Blanquat et al. Reference de Saint Blanquat, Tikoff, Teyssier and Vigneresse1998; Benn, Reference Benn2009; Zak et al. Reference Žák, Verner and Týcová2008; Paterson et al. Reference Paterson, Ardill, Vernon and Žák2019; Neves et al. Reference Neves, Ferreira, Sial, Lima, Ardila and Neves2023; Wei et al. Reference Wei, Lin, Chen, Faure, Ji, Hou, Yan and Wang2023). When magmatic fabrics, such as schlieren or the alignment of magmatic minerals, are parallel to secondary solid-state deformation fabrics formed during pluton cooling, and even to mylonitic fabrics developed after final crystallization, this structural concordance provides compelling evidence for a continuous, tectonically controlled emplacement process (e.g., Hutton & Reavy, Reference Hutton and Reavy1992; Jacques & Reavy, Reference Jacques and Reavy1994; Burt et al. 1996; Gleizes et al. Reference Gleizes, Leblanc, Santana, Olivier and Bouchez1998; Roman-Berdiel et al. Reference Román-Berdiel, Aranguren, Cuevas, Tubía, Gapais and Brun2000; Zak et al. Reference Žák, Verner and Týcová2008; Paterson et al. Reference Paterson, Ardill, Vernon and Žák2019; Neves et al. Reference Neves, Ferreira, Sial, Lima, Ardila and Neves2023; Wei et al. Reference Wei, Lin, Chen, Faure, Ji, Hou, Yan and Wang2023; Siachoque et al. Reference Siachoque, Morales, Cardona, Marulanda and Zapata2024; Li et al. Reference Li, Liang, Zhang, Ran, Shen, Wang and Jin2018; Tomek et al. Reference Tomek, Žák and Chadima2014).
Field and microstructural observations indicate that the Curaco Batholith records progressive deformation, characterized by a continuum of magmatic to solid-state microstructures formed under decreasing temperature conditions during ongoing magma evolution and associated rheological changes (e.g., Arzi, Reference Arzi1978; Van der Molen & Paterson, Reference Van Der Molen and Paterson1979; Vigneresse et al. Reference Vigneresse, Barbey and Cuney1996; Scaillet et al. Reference Scaillet, Whittington, Martel, Pichavant and Holtz2000; Vernon Reference Vernon2004; Bergantz et al. Reference Bergantz, Schleicher and Burgisser2017; Paterson et al. Reference Paterson, Vernon and Tobisch1989, Reference Paterson, Ardill, Vernon and Žák2019). The development of these microstructures is not strictly tied to specific petrofacies but is strongly influenced by the spatial proximity of the sampled rocks to the La Seña and Pangaré shear zones (Fig. 7a).
Magmatic structures within the granitoids, such as tubes (Fig. 4e) and pipes, were identified in the field. These features indicate the paleovertical orientation of the pluton, suggesting it was not tilted by post-Jurassic deformation. Additional field evidence points to magma mingling between intermediate and felsic intrusive rocks. For instance, schlieren outlining tube boundaries (Fig. 4e) and granodioritic enclaves within monzogranites (Fig. 4g) reflect the interaction between hotter basaltic/andesitic magma and cooler, more viscous felsic magma. Rapid quenching and crystallization of the intermediate magma inhibited complete mixing of the two melts (Barbarin & Didier, Reference Barbarin and Didier1992; Barbarin, Reference Barbarin2005).
Secondly, the Curaco Batholith displays a strong correlation between macroscopic magmatic fabrics and AMS magnetic fabrics (see Figs. S4-S8). Magnetic susceptibility (Km) values are closely linked to lithology, with each lithological unit exhibiting a distinct Km range that underscores the petrological relevance of this parameter. The highest Km values are found in porphyritic granites, where magnetite is the primary contributor. In equigranular monzogranites, Km reflects the combined influence of magnetite and paramagnetic minerals, although values are roughly an order of magnitude lower than those of the porphyritic facies. In muscovite-bearing leucogranites, granodiorites, diorites, and aplites, Km is mainly governed by paramagnetic minerals. Mylonitic rocks show contributions from both ferromagnetic and paramagnetic minerals, while in rhyolitic and andesitic dikes, Km is predominantly controlled by ferromagnetic phases.
Magnetic mineralogy analyses reveal that when magnetite dominates the magnetic anisotropy signal, it typically occurs in multidomain (MD) or pseudosingle-domain (PSD) grain states. These domain sizes are considered appropriate for reliable AMS interpretation, as they do not significantly distort the magnetic fabric signal (Rochette et al. Reference Rochette, Jackson and Aubourg1992; Borradaile & Jackson, Reference Borradaile and Jackson2004).
AMS analysis of the granodioritic enclaves and their porphyritic monzogranitic host (see sites 26A, 26C, 2 and 3 in Figs. S5c, d, h, j) reveals that both lithologies share coaxial magnetic fabrics, indicating that they were simultaneously in a plastic state during fabric acquisition at the magmatic to sub-magmatic stage (Hrouda et al. Reference Hrouda, Táborská, Schulmann, Ježek and Dolejš1999). A comparable pattern is observed in the aplite dikes, where the magnetic fabric consistently aligns with that of the host rock, whether porphyritic or equigranular monzogranite (Fig. 11d; see sites 23B, 24B, 25, 19, 18, 15, 16, 17 in Fig. S4b-d, f-j; sites 26A, 26B, 8, 3 in Fig. S5c, e, i, j). This parallelism suggests that fabric acquisition in the aplite dikes, granodioritic enclaves, and host rocks occurred while the system was in a crystal mush state, near the melt connectivity transition (MCT). At this stage, the melt fraction remained sufficiently high to permit deformation and the development of a common magnetic fabric prior to solidification, at temperatures around 800°C (Bachmann & Bergantz, Reference Bachmann and Bergantz2004; Rosenberg & Handy, Reference Rosenberg and Handy2005).
Under these conditions, granodioritic enclaves acted as passive strain markers, as the crystallinity of the surrounding monzogranite exceeded 30%, preventing enclave sinking. The consistent orientation of magnetic fabrics in both enclaves and host rocks supports their classification as passive strain markers, differing only in color and chemical composition, and serving as direct indicators of intrusive strain (Hrouda et al. Reference Hrouda, Táborská, Schulmann, Ježek and Dolejš1999; Latimer et al. Reference Latimer, McCarthy, Mattsson and Reavy2024). These enclaves also provide evidence of magma mingling, reflecting interactions between compositionally distinct magmas rather than the intrusion of unrelated bodies. Consequently, the deformation recorded in both enclaves and host monzogranites represents synchronous strain during emplacement and solidification within a crystal mush environment.
This progressive deformation from magma mingling to strain accommodation in the crystal mush stage is consistent with recent studies showing that enclaves and coeval magmatic bodies can record successive strain events during pluton emplacement (e.g., Ferré, Reference Ferré2002; Ferré et al. Reference Ferré, Martín-Hernández, Teyssier and Jackson2004, Reference Ferré, Gébelin, Till, Sassier and Burmeister2014; Sant’Ovaia et al., Reference Sant’Ovaia, Olivier, Ferreira, Noronha and Leblanc2010; McCarthy et al. Reference McCarthy, Petronis, Reavy and Stevenson2015; Cruz et al. Reference Cruz, Sant’Ovaia, McCarthy and Noronha2022; Köpping et al. Reference Köpping, Cruden, Magee, McCarthy, Geissman and Holm2023).
All rocks within the Curaco Batholith exhibit microstructures interpreted to have originated under magmatic and sub-magmatic conditions. As the pluton cooled and solidified, these primary structures were partially or completely overprinted by solid-state deformation (Figs. 7a-g; see section 5c). Accordingly, we classify the microstructural patterns into three broad categories: (1) magmatic, (2) sub-magmatic and high-temperature solid-state deformation, and (3) medium- to low-temperature solid-state deformation (Fig. 7a-g).
Magmatic deformation is characterized by melt displacement and rigid-body rotation of crystals, without sufficient crystal interaction to induce ductile flow (Fig. 7b; Paterson et al. Reference Paterson, Vernon and Tobisch1989). Sub-magmatic deformation occurs at temperatures above 650°C, with a low melt fraction (Fig. 7b, c; Paterson et al. Reference Paterson, Vernon and Tobisch1989; Bouchez et al. Reference Bouchez, Delas, Gleizes, Nédélec and Cuney1992; Passchier & Trouw, Reference Passchier and Trouw2005). High-temperature solid-state deformation takes place at temperatures above 530°C (Fig. 7d; Paterson et al. Reference Paterson, Vernon and Tobisch1989; Passchier & Trouw, Reference Passchier and Trouw2005)), while medium- to low-temperature solid-state deformation occurs between 300°C and 530°C, as indicated by bulging recrystallization in quartz, subgrain rotation, and kink bands in biotite (Figs. 7e, f; Pryer, Reference Pryer1993; Passchier & Trouw, Reference Passchier and Trouw2005). Finally, cataclastic microstructures formed during brittle deformation below 270°C are observed in discrete zones throughout the batholith (Fig. 7g).
Most rhyolitic and andesitic dikes exhibit normal magnetic fabrics that are subparallel to the dike margins and display subhorizontal lineations (section 5.f.3). However, two rhyolite dikes (sites 22 and 11) show anomalous magnetic fabrics, with orientations that deviate from the dike walls and lineations plunging shallowly (∼20°) toward the northwest. At these sites, the NW-SE magnetic foliation is subvertical at site 22 and subhorizontal at site 11, both oblique to the respective dike margins (Fig. S8a, b). Site 11, in particular, presents a magnetic fabric that may correspond to an inverse magnetic fabric, where the foliation is perpendicular to the dike margins (Figs. 13b, S8b), a phenomenon attributed to the anisotropic distribution of multidomain magnetite, commonly observed in volcanic rocks (Chadima et al. Reference Chadima, Cajz and Týcová2009).
Minor deviations in foliation orientation relative to dike contacts and variability in lineation plunge fall within the expected range for imbrication fabrics typically associated with dike emplacement. In general, magnetic foliation planes remain subparallel to dike margins, with deviations typically ≤10°, indicative of fabrics formed during magma flow. Magnetic lineations range from subhorizontal to moderately plunging, consistent with flow-related imbrication fabrics (e.g., Hastie et al. Reference Hastie, Watkeys and Aubourg2011; Petronis et al. Reference Petronis, O’Driscoll, Stevenson and Reavy2012; Mattsson et al. Reference Mattsson, Petri, Almqvist, McCarthy, Burchardt, Palma and Galland2021, Reference Mattsson, McCarthy and Schmiedel2024).
6. b. Relationship between the La Seña and Pangaré shear zones and the granitic rocks
A defining characteristic of the Curaco Batholith is the consistent orientation of both magmatic planar and linear structures, along with superimposed foliations and lineations formed under a range of temperature conditions. This structural coherence is especially evident when comparing the magnetic fabrics of mylonites within the La Seña and Pangaré shear zones to those of the surrounding granitoids (Figs. 11, 12). The observed parallelism between magmatic and tectonic foliations, both within the shear zones and their adjacent areas, strongly supports a syn-tectonic emplacement of the pluton (Archanjo et al., Reference Archanjo, Bouchez, Corsini and Vauchez1994; Bouchez, Reference Bouchez1997; de Saint Blanquat & Tikoff, Reference de Saint Blanquat and Tikoff1997; Gleizes et al. Reference Gleizes, Leblanc, Santana, Olivier and Bouchez1998; D’Eramo et al. Reference D’Eramo, Pinotti, Tubía, Vegas, Aranguren, Tejero and Gómez2006; Benn, Reference Benn2009; Zaffarana et al. Reference Zaffarana, Somoza, Orts, Mercader, Boltshauser, González and Puigdomenech2017; Wei et al. Reference Wei, Lin, Chen, Faure, Ji, Hou, Yan and Wang2023).
Regarding ellipsoid geometry, deformation intensity, expressed by the Pj parameter, is highest within the shear zones, where the most intensely deformed rocks occur. In these zones, Km values are primarily influenced by deformation rather than magnetite content (Fig. 10a). Magnetic ellipsoids in the shear zones are predominantly triaxial, reflecting the combined significance of foliation and lineation. An exception is site PAN3, the only prolate site, where magnetic lineation dominates (Fig. 10b).
In and around the La Seña shear zone, a local horizontal compressive regime is indicated by the shallow to intermediate plunge of the K3 axes (Figs. 11a, b; Knight et al. Reference Knight, Stevenson, Maffione, McCarthy, Burton-Johnston and Lawrence2024). In the same area, NW-SE-oriented K1 axes, representing the magmatic to tectonic stretching direction (e.g., McCarthy, Reference McCarthy2013; Knight et al. Reference Knight, Stevenson, Maffione, McCarthy, Burton-Johnston and Lawrence2024), form a tight cluster and are predominantly horizontal to subhorizontal (Figs. 12a, b), further supporting the presence of local compressive stress (Knight et al. Reference Knight, Stevenson, Maffione, McCarthy, Burton-Johnston and Lawrence2024).
Outside the La Seña shear zone, magnetic fabrics are more variable (Figs. 11c, 12c), yet they generally follow the pattern of near-circular magmatic structures observed in satellite imagery (Figs. 2b, c, 11d), which may correspond to outcrop-scale magmatic foliations of a diapiric, laccolithic pluton (Figs. 4a, e). In these areas, K1 orientations remain subhorizontal but exhibit greater dispersion (Fig. 12c), consistent with localized fabric variations driven by magma flow (Knight et al. Reference Knight, Stevenson, Maffione, McCarthy, Burton-Johnston and Lawrence2024).
Away from the shear zones, magnetic ellipsoids tend to be oblate, Pj values are lower, and magnetite content exerts a stronger influence on magnetic susceptibility (Figs. 9c, d, 10a, b). This oblate magnetic fabric likely reflects vertical flattening or magma inflation under transtensional conditions. The contrast in magnetic anisotropy, higher in mylonitic rocks and lower in the surrounding plutonic units, suggests that anisotropy is primarily governed by the deformational regime rather than by the concentration of magnetic minerals.
6. c. Emplacement of the Curaco Batholith within the regional deformation structures
In the previous section, the structural data of the Curaco Batholith were analyzed at a local scale. Here, we present a comprehensive emplacement model that integrates all observed fabrics within the batholith, including the orientation of andesitic and rhyolitic dikes and minor oblique shear structures, within the broader regional tectonic framework.
The Curaco Batholith is bounded by two prominent lineaments, the Patu-Có and El Loro, as well as by the more regional E-W-trending Huincul and NW-N-trending Río Negro transcurrent fault zones (Figs. 1, 2, 14a). Many of the principal faults in the North Patagonian Massif align with Paleozoic basement structures, exhibiting dominant E-W to ENE-WSW orientations (Giacosa et al. Reference Giacosa, Lema, Busteros, Zubia, Cucchi and Di Tommaso2007; Mizerit et al. Reference Mizerit, Suárez, Voglino, Aranda, Giacosa and González2014; Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016). Several of these structures, inherited from the Gondwanide Orogeny (von Gosen Reference Von Gosen2002, Reference Von Gosen2003, Reference Von Gosen2009; Álvarez et al. Álvarez et al., Reference Álvarez, Peroni, Giacosa, Silva Nieto, Busteros and Lagorio2014; Giacosa et al. Reference Giacosa, Márquez, Nillni, Fernández, Fracchia, Parisi, Alfonso, Paredes and Sciutto2004, Reference Giacosa, González, Silva Nieto, Busteros, Lagorio and Rossi2014, Reference Giacosa, Silva Nieto, Busteros, Lagorio and Hernando2017), were reactivated during the early Mesozoic breakup of Gondwana as a result of the initial collapse of the orogen in Late Triassic times and the widespread lithospheric extension that prevailed during the Jurassic (Sato et al. Reference Sato, Llambías, Basei and Castro2015). These reactivations influenced the emplacement of Triassic–Jurassic batholiths in Patagonia (Zaffarana & Somoza Reference Zaffarana and Somoza2012; Zaffarana et al. Reference Zaffarana, Somoza, Orts, Mercader, Boltshauser, González and Puigdomenech2017; Renda et al. Reference Renda, Alvarez, Prezzi, Oriolo and Vizan2019; Giacosa Reference Giacosa2020; Ruiz González et al. Reference Ruiz González, Puigdomenech, Zaffarana, Vizán and Somoza2020). During this extensional phase, the Patu-Có and El Loro lineaments created accommodation space for the Curaco Batholith, with magma ascending along pre-existing NW–SE basement structures under a NE-SW extensional regime (Fig. 14a). Emplacement likely initiated in the western sector and progressed eastward (González et al., Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025; Figs. 14a, b).
Figure 14. Emplacement model of the Curaco Batholith. (a) Initial stage: Reactivation of pre-existing NW-SE trending faults as a consequence of the collapse of the Gondwanan orogen and the widespread extension of the lithosphere, and emplacement of the main plutonic units (equigranular and porphyritic monzogranites, granodiorites, diorites, granite porphyry, and muscovite-bearing leucogranites) within a transtensional right-stepping releasing stepover associated with NW-SE compression and NE-SW extension. (b) Second stage: Emplacement of the plutonic rocks of the Curaco Batholith controlled by the main displacement zones. The NW-SE trending subvertical foliations are associated with a pre-existing structural control related to basement anisotropies. (c) Final stage: emplacement of andesitic and rhyolitic dikes along inherited anisotropies and brittle structures, including the El Salado lineament. This lineament is interpreted as a synthetic mega-P shear zone within a Riedel-type strike-slip fault system, likely acting as a magma ascent pathway for the emplacement of the andesitic and rhyolitic dikes.
Although the batholith’s long axis is aligned with the main displacement zones, the magmatic and tectonic foliations of its plutons were primarily controlled by reactivated NW-SE basement structures. This accounts for their obliquity relative to the Patu-Có and El Loro lineaments, particularly near and within the La Seña and Pangaré shear zones (Figs. 11d, 14a, 13b). Magma intrusions are known to alter the surrounding stress field and interact with pre-existing anisotropies such as shear zones, lithological boundaries, or hydrated zones (McCarthy, Reference McCarthy2013). These factors influence magma propagation and the transition from ascent to emplacement, often involving reorientation of the local σ3; axis, modulated by hydrostatic conditions, regional stress, and host-rock heterogeneities. The NNW-SSE magnetic lineations observed near the La Seña and Pangaré shear zones reflect tectonic stretching associated with shearing along basement structures during and after pluton emplacement (Fig. 12d).
The Curaco Batholith is interpreted as a composite of syn-kinematic intrusions associated with the Patu-Có and El Loro lineaments and the La Seña and Pangaré shear zones, characterized by a dominant NW-SE subvertical foliation pattern (Figs. 14a, b; González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025). Within a Riedel shear framework, these lineaments acted as dextral mega-shears, generating a right-stepping releasing stepover that created the necessary accommodation space for batholith emplacement. The La Seña and Pangaré shear zones likely represent R′ shears within this extensional system. The Pangaré zone exhibits dextral kinematics, while La Seña shows mixed dextral-sinistral indicators (Gregori et al. Reference Gregori, Saini-Eidukat, Benedini, Strazzere, Barros and Kostadinoff2016; González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025).
This model also explains the emplacement of subvertical NE-SW- and NW-SE-striking andesitic and rhyolitic dikes that crosscut mylonites and exhibit composite microstructures, transitioning from magmatic to submagmatic and ultimately to solid-state deformation (Figs. 13b, 14c; González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025). Their orientations are consistent with the stress field that governed granitoid emplacement, and their dominant strikes (Fig. 13b) correspond to synthetic R and P Riedel shears (Fig. 14c). Rhyolitic dikes, which intrude the andesitic ones, likely represent the final magmatic pulse of the batholith. These dikes are affected by NW-SE brittle fractures, locally parallel to strike-slip faults such as the subvertical NE–SW El Salado lineament, which served as a magmatic ascent pathway for both dike types (Figs. 2b, c, 14c; González et al. Reference González, Zaffarana, Oriolo, Aramendia, Tommasi, González, Cábana, Giacosa and Herazo2025).
Magmatic and tectonic fabrics in the Curaco Batholith are weak away from the La Seña and Pangaré shear zones, often with subhorizontal dips (Fig. 11d), but become more pronounced and even mylonitic in proximity to these structures. Field relationships show that the long axis of certain plutons, such as the porphyritic monzogranite within the La Seña shear zone, is aligned with the NW-SE shear zone, with enclaves elongated in the same direction (Figs. 2b, 4e), indicating that movement along the main displacement zones reactivated these structures.
In summary, the emplacement of the Curaco Batholith was influenced by pre-existing basement structures reactivated under a regional dextral transtensional regime, offering a coherent model for understanding the interplay between magmatism and tectonics in the North Patagonian Massif. Overall findings provide compelling evidence that the Curaco Batholith was emplaced syn-tectonically during an active extensional regime. They offer new insights into the interplay between pluton construction and tectonic processes in transtensional settings, contributing to a broader understanding of crustal evolution in the North Patagonian Massif.
7. Conclusions
The integrated analysis of magmatic and solid-state fabrics in the Curaco Batholith, located in the northern sector of the North Patagonian Massif (Patagonia), along with its associated andesitic and rhyolitic dikes, yields several key conclusions:
(i) In-situ emplacement and progressive deformation: the presence of magmatic tubes and pipes within the batholith indicates that it represents an in-situ, non-tilted magma body. Microstructural evidence reveals a continuous transition from magmatic to high-temperature solid-state deformation, suggesting that the pluton underwent progressive strain during cooling. This links magmatic emplacement directly to subsequent solid-state deformation processes.
(ii) Magnetic fabric coherence and passive strain markers: the coaxial magnetic fabric observed in granodioritic enclaves, host monzogranites, and syn-plutonic aplite dikes implies that the enclaves acted as passive strain markers, acquiring their magnetic fabric during the magmatic stage prior to full crystallization. The consistent parallelism between magmatic and solid-state mineral and magnetic fabrics throughout the batholith supports a syn-tectonic emplacement model, with deformation active throughout magma crystallization.
(iii) Structural alignment and persistent stress regime: the batholith is structurally defined by a dominant NW-SE subvertical foliation and shallow to moderately plunging magnetic lineations, both formed under magmatic and solid-state conditions. Although the andesitic and rhyolitic dikes crosscut earlier mylonitic foliations, their orientations remain compatible with the extensional direction of the same tectonic regime. This suggests a persistent stress field responsible for the formation of a right-stepping releasing stepover during and after dike emplacement.
(iv) Brittle deformation and magmatic ascent pathways: brittle deformation structures are widespread across the batholith, affecting both host granitoids and dikes. These features may be coeval with the intrusion of syn-magmatic aplite dikes. NW-SE-trending brittle fractures often align with strike-slip faults, notably the NE-SW-trending dextral El Salado lineament. This structure is interpreted as a synthetic mega-P shear zone that likely served as a magmatic ascent pathway for both andesitic and rhyolitic dikes.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756825100447
Acknowledgements
This work was financed by grants PIP CONICET 112-200901-00766, PI-40A-631, PI-40A-916 and PI-40A 1072 (Universidad Nacional de Río Negro) and PICT 019–01809. We thank the anonymous reviewers and the reviewer Dr. Astrid Siachoque for their constructive comments. We also thank Professor Roberto Weinberg for his assistance in the field.
Competing interests
The author(s) declare none.
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