12 results
Petrogenesis of a rare Ediacaran tonalite–trondhjemite–granodiorite suite, Egypt, and implications for Neoproterozoic Gondwana assembly
- Abdel-Fattah M Abdel-Rahman
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- Journal:
- Geological Magazine / Volume 158 / Issue 4 / April 2021
- Published online by Cambridge University Press:
- 13 August 2020, pp. 701-722
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Most tonalite–trondhjemite–granodiorite (TTG) suites are Archean–Palaeoproterozoic in age, but those of Neoproterozoic–Phanerozoic age are scarce. A rare Ediacaran high-Al TTG suite has been identified at the Fannani Igneous Complex (FIC) in the northern Arabian–Nubian Shield, which is essentially composed of amalgamated Neoproterozoic island-arc Pan-African composite terranes that contain several ophiolitic sutures. The FIC exhibits a wide range of SiO2, Al2O3, Sr and Zr, shows moderate rare earth element (REE) enrichment, and K, Ti, Nb, Y and heavy REE depletion. It is a subsolvus suite with clear orogenic affinities and strong arc-geochemical signatures. The precise U–Pb zircon thermal ionization mass spectrometry age obtained (607.4 ± 1.95 Ma) indicates oceanic subduction extended to late stages of the East African Orogeny. The FIC exhibits 87Sr/86Sr compositions of 0.70346–0.71091 (Sr(i) ratio, 0.70284), and 143Nd/144Nd of 0.51254–0.51270 (ϵNd(t) = +5.12 to +7.16), typical of modern oceanic-arc rocks (as Japan-arc basalts), and suggestive of mantle sources and island-arc settings. The FIC possesses low values of Yb (1.55 ppm), Nb (14 ppm) and Y (24 ppm), and high ratios of Sr/Y (27), Zr/Sm (46) and Nb/Ta (11.8), typical of magmas produced by anatexis of a basaltic slab. Partial melting models show that the FIC magma was generated by melting (F = 0.25–0.50) of a subducted oceanic crust transformed into eclogite, leaving 10–25% garnet in the residue. The FIC and similar complexes produced via slab melting during the closure of the Mozambique Ocean formed large juvenile belts along the East African Orogen that sutured East and West Gondwana together into a united supercontinent.
Chlorites in a spectrum of igneous rocks: mineral chemistry and paragenesis
- Abdel-Fattah M. Abdel-Rahman
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- Journal:
- Mineralogical Magazine / Volume 59 / Issue 394 / March 1995
- Published online by Cambridge University Press:
- 05 July 2018, pp. 129-141
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The chlorite data presented are from four igneous complexes covering the compositional spectrum of igneous rocks (gabbro to granite) of orogenic and anorogenic settings. The four igneous complexes are; early orogenic gabbro-diorite-tonalite (D-T) suite, late orogenic granodiorite-adamellite (G-A) suite (both are calc-alkaline suites), high-alumina trondhjemite (TR), and anorogenic peralkaline granite (PGR).
Chlorites in these igneous rocks show characteristic compositional fields. The Mg vs Fe plot provides the best discriminant, as data points define three compositionally different groups. Phases in the PGR are Fe-rich, siliceous, interlayered chlorite-smectite (Fe/Mg = 8.6), and differ significantly from those in the calc-alkaline D-T and G-A rocks which are Mg-rich chlorites (Fe/Mg = 0.6–0.8). The X-ray diffraction data for the peralkaline granite samples show superlattice reflections at approximately 31 Å (air-dried) and 34 Å (ethylene glycollated), thus suggesting the presence of an expandable (smectite-like) component in this interlayered (chlorite-smectite) phyllosilicate phase. Chlorites in the peraluminous TR rocks contain Fe/Mg values intermediate between the other two types (Fe/Mg = 1.3). Tetrahedral Al (AlZ) values are remarkably low (0–0.5) in phyllosilicates in the PGR, but vary from 1.9–2.5 in chlorites from the other suites. Yet, these chlorite groups with their generally low AlZ values are distinct from the more stable (type IIb) metamorphic chlorites. Sedimentary chlorites are somewhat similar, in their low AlZ values and metastable structural type, to chlorites in igneous rocks.
In the calc-alkaline rocks, chlorite may have been formed at the expense of both biotite [biotite + 3M + 3H2O = chlorite + A], and calcic amphibole [2 Ca-amphibole + 6H2O + 5O2 + 1.8Al = 1 chlorite + 8SiO2 + A], where M = Fe, Mg, Al, and A = K, Na, Ca. The alteration of alkali amphibole in the peralkaline rocks may have produced interlayered chlorite-smectite via this reaction; [1 Na-amphibole + 7H2O + 2.5O2 + M = 1 chlorite-smectite + A]. The presence of such interlayered chlorite-smectite which typically form at low T (150–200°C) suggests that the region was not affected by any major reheating events, which is consistent with the nature of the feldspars.
Geochemistry of mantle-related intermediate rocks from the Tibbit Hill volcanic suite, Quebec Appalachians
- Abdel-Fattah M. Abdel-Rahman, P. Stephen Kumarapeli
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- Journal:
- Mineralogical Magazine / Volume 62 / Issue 4 / August 1998
- Published online by Cambridge University Press:
- 05 July 2018, pp. 487-500
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We present a study on major and trace element geochemistry of some intermediate lithologies from the predominantly basaltic Tibbit Hill volcanic suite in the Humber Zone of the Quebec Appalachians. The intermediate rocks probably formed as lava flows in the volcanic sequence. Their presence shows that this rift-related, c. 554 Ma volcanic sequence is not bimodal (basaltic-comenditic) as previously thought, but consists of a spectrum of compositions ranging from mafic through intermediate to felsic lithologies. The entire volcanic sequence is poly-deformed and generally metamorphosed to greenschist facies conditions.
The intermediate rocks of the Tibbit Hill Formation are trachyandesitic, trachytic and comenditic in composition, and exhibit a wide range of SiO2 content (52 to 68 wt.%). Mg is highly depleted in most samples. Variations of silica versus the alkalis show that most of the samples are alkaline in nature. The rocks display a tholeiitic trend on a standard AFM diagram.
In general, the examined rocks also exhibit a wide range of Sr (15 to 174 ppm), Rb (0 to 156 ppm), Zr (155 to 899 ppm), Nb (18 to 123 ppm), and Y (18 to 94 ppm). The concentration of Hf and Ta are generally low (6.6–14.8 ppm, and 3.3–6.6 ppm, respectively), compared to those of Zr and Nb. Nevertheless, these rocks contain relatively high concentrations of the HFS elements, thus reflecting an enriched source. The suite is also relatively enriched in the rare earth elements (REE), and exhibits fractionated, subparallel REE pattems; the latter are generally uniform and conformable.
Chemical features of these volcanic rocks are typical of those of anorogenic A1 type suites, related to hotspots, mantle plumes, or continental rift zones. This is consistent with earlier interpretation of volcanism associated with an Iapetan RRR triple junction, occurring shortly before the onset of seafloor spreading. At that stage of crustal evolution, alkaline to transitional basaltic magma pierced into the crust, and experienced fractionation to produce the liquids of intermediate composition. Rare earth element geochemical modelling supports the hypothesis that the most evolved composition for which REE data are available (comendite; 67.9 wt.% SiO2) was produced by 20% fractional crystallization of the least evolved trachyandesite (56.7 wt.% SiO2) of this intermediate volcanic assemblage.
Mineral chemistry and paragenesis of astrophyllite from Egypt
- Abdel-Fattah M. Abdel-Rahman
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- Journal:
- Mineralogical Magazine / Volume 56 / Issue 382 / March 1992
- Published online by Cambridge University Press:
- 05 July 2018, pp. 17-26
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Astrophyllite is a rock-forming mineral (3.4 vol.%) in the metasomatized wallrocks within 2 to 3 km of the contact with the Mount Gharib A-type complex in northeastern Egypt. In the peralkaline A-type granites of the main complex, astrophyllite occurs in accessory amounts (0.1 to 0.5 vol.%), mostly in association with arfvedsonite. Eighteen chemical analyses of astrophyllite from both rock units have been used to discuss mineral chemistry, compositional variations and isomorphism.
Substitutions in the astrophyllite unit cell [(Na,K)3Fe7Ti2Si8(O,OH)31] involve F, Ca, Mn, Zn, Mg, Nb, Zr, Sn and A1. The contents of the unit cell depart from ideal stiochiometry. Unusually high Nb contents (up to 5 wt.% Nb2O5) characterize the Egyptian astrophyllitcs. The replacement of Ti by Nb is balanced by cation vacancies (Nb5+ → Ti4+ + □). The niobophyllite-astrophyllite relationship and the structure of astrophyllite in relation to mica and biopyriboles are discussed.
Astrophyllite may have been formed at the expense of alkali amphiboles by metasomatic reactions involving Ti-Nb-rich alkaline fluids and arfvedsonite;
Most astrophyllite reported in the literature occurs in association with alkali amphiboles, particularly arfvedsonite, thus suggesting that similar metasomatic reactions were responsible for the formation of astrophyllite in many other localities in the world.
Geochemistry, age and origin of the Mons Claudianus TTG batholith (Egypt): insight into the role of Pan-African magmatism in uniting plates of Gondwana
- ABDEL-FATTAH M. ABDEL-RAHMAN
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- Journal:
- Geological Magazine / Volume 156 / Issue 6 / June 2019
- Published online by Cambridge University Press:
- 07 June 2018, pp. 969-988
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The tonalite–trondhjemite–granodiorite (TTG) Mons Claudianus Batholith (MCB) of Egypt is subsolvus, metaluminous to mildly peraluminous, exhibits wide ranges of SiO2, Al2O3, Sr, Rb, Zr, shows large Ba enrichment, is moderately enriched in rare earth elements (REE) and is depleted in K, Ti, Nb, Y, Hf and heavy REE (HREE), reflecting strong arc geochemical signatures. Moderate fractionation of REE and lack of Eu anomaly characterize the MCB. It is typical of high-Al TTGs of volcanic-arc affinities. U–Pb–zircon dating produced a Pan-African age of 664.12 ± 0.38 Ma. The MCB exhibits 87Sr/86Sr isotopic compositions of 0.70352–0.70626 (initial Sr ratio of 0.70259) and 143Nd/144Nd ratios of 0.51261–0.51276 (εNd; –0.5 to +2.4), suggestive of a mantle source. Anatexis of a basaltic slab under eclogitic conditions leaving garnet in the residue produces high-Al TTG rocks characterized by low Yb values (<1.8 ppm). Values (in ppm) of Yb (0.65–1.8), Y (2.2–19), Nb (1.2–6.4), and ratios of Nb/Ta (7–17), (La/Yb)N of 11.7, Sr/Y and Zr/Sm (58 and 45, respectively) are all consistent with anatexis of a basaltic slab under eclogitic conditions, leaving garnet in the residue to produce this high-Al TTG suite. The data conform to magma generation via partial melting (F = 0.25–0.50) leaving 15–25 % garnet in the residue. Voluminous synorogenic magmatic pulses, resulting from slab melting during the closure of the Mozambique Ocean via convergence of east and west Gondwana, produced the MCB and similar large batholiths forming the core of Pan-African belts. These belts welded together vestiges of fragmented Rodinia, assembling them into a united Gondwana.
Mineralogy of the Neoproterozoic epidote-bearing TTG suite, Mons Claudianus batholith (Egypt) and implications for synorogenic magmatism
- Abdel-Fattah M. Abdel-Rahman
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- Journal:
- Mineralogical Magazine / Volume 80 / Issue 7 / December 2016
- Published online by Cambridge University Press:
- 02 January 2018, pp. 1291-1314
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The Neoproterozoic, epidote-bearing Mons Claudianus Batholith (MCB), Egypt, consists of tonalite-trondhjemite-granodiorite (TTG) lithologies, containing variable contents of quartz, feldspars, amphiboles, biotite, and magmatic epidote, with accessory titanite, zircon, allanite, apatite, opaque magnetite and ilmenite. Plagioclase varies from An49 to An19, and K-feldspars possess near end-member compositions (Or97 to Or91). Amphiboles are calcic (Ca = 1.88–1.92 atoms per formula unit (apfu)), Al-rich (average AlT = 1.84 apfu), having an average Fe/(Fe + Mg) ratio of 0.50, and are edenite, ferro-edenite and ferropargasite. The Al-in-Hb barometer produced an average crystallization pressure of 5.5 kbar, consistent with the presence of magmatic epidote; the association epidote – Al-rich-Hb suggests mesozonal crustal levels, and thus a possible average rate of regional uplift for the Nubian Shield would have been in the order of 0.03 mm/yr. Calculated temperatures (using the Hb-Plag geothermometer) range from 729 to 754°C (average 747°C). The calculated P / T values of epidote-bearing MCB rocks fall within the experimentally-determined P-T range of stability of magmatic epidote with fO2 buffered from NNO to HM. Biotites in the MCB are moderately Mg-rich (Fe/(Fe + Mg) = 0.42 to 0.50), and are type 'C'-biotite, typical of calcalkaline orogenic suites, which are distinct from types 'A' and 'P' biotites occurring in anorogenic alkaline, and peraluminous lithologies, respectively. The minor secondary chlorite phases, with their Fe/(Fe + Mg) ratios of 0.37–0.52, are pycnochlorite and ripidolites, and belong to group 'c' chlorites. Minerals of the MCB reflect a petrogenetic history involving a wet, subsolvus, typically orogenic magmatic system. Results of this study could have wide implications for mineralogical characterization, level of emplacement and evolution of magmatic systems of TTG suites occurring in other orogenic belts.
Petrogenesis of Cenozoic mafic–ultramafic alkaline lavas from the Tigris volcanic field, NE Syria
- ABDEL-FATTAH M. ABDEL-RAHMAN, NANCY A. LEASE
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- Journal:
- Geological Magazine / Volume 149 / Issue 1 / January 2012
- Published online by Cambridge University Press:
- 04 March 2011, pp. 1-18
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Mafic–ultramafic Quaternary lava flows form the Tigris volcanic field (covering 1750 km2) at the northeastern tip of Syria and extend into Turkey. This volcanic field occurs between the Euphrates graben and the Bitlis–Zagros collision suture that forms the boundary between the Arabian and Eurasian plates. The rocks are made up of labradorite, clinopyroxene, olivine and opaque phases. The Tigris lavas are compositionally restricted to basanites and alkali basalts, having a narrow range of major element compositions (SiO2, 42.2–48.2 wt%; MgO, 5.7–9.0 wt%, with Mg numbers ranging from 0.51 to 0.62; TiO2, 1.7–3.2 wt%), and are alkaline in nature. The rocks are enriched in HFS elements such as Zr (119–231 ppm), Nb (14–43 ppm) and Y (17–22 ppm). The REE patterns are strongly fractionated ((La/Yb)N = 10.6), indicative of a garnet-bearing source. The 143Nd/144Nd isotopic compositions range from 0.512803 to 0.512908, and 87Sr/86Sr from 0.70327 to 0.70403 (εNd = 3.2–5.3) suggesting strong affinities to ocean island basalts. Modelling using a variety of mantle source materials and different degrees of partial melting indicates that the magma was produced by a small degree of batch partial melting (F = 1.5%) of a primitive, garnet-lherzolite fertile mantle source. The overall petrological/chemical nature supports this interpretation. Shear heating at the base of the lithospheric mantle of the northern boundary of the Arabian plate, caused by a change in plate motion as the Arabian plate moved in a more easterly direction during the Plio-Quaternary, could represent a possible source of the heat necessary for partial fusion and magma generation. Adiabatic decompression and melting represents a more likely process for the generation of the Tigris magma. Elemental ratios such as K/P (4.6), La/Ta (12), La/Nb (0.90), Nb/Y (1.22) and Th/Nb (0.09) indicate that the magma was subjected to minimal crustal contamination.
Pan-African volcanism: petrology and geochemistry of the Dokhan Volcanic Suite in the northern Nubian shield
- Abdel-Fattah M. Abdel-Rahman
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- Journal:
- Geological Magazine / Volume 133 / Issue 1 / January 1996
- Published online by Cambridge University Press:
- 01 May 2009, pp. 17-31
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The Late Proterozoic Dokhan volcanic suite (620 Ma) of the northern Nubian shield is the product of Late Pan-African volcanism. The suite covers the entire spectrum from basalt to high-silica rhyolite and occurs as two units: a dark-coloured unit containing basalt-andesite-dacite, and a light-coloured unit encompassing dacite-rhyodacite-rhyolite. The latter unit is made up largely of ash flow tuffs and ign-imbrites that are locally interstratified with basalt and andesite lava flows. The suite forms a continuum in composition with a wide range of Si02 (48–77 wt%), CaO (0.1–8.9 wt%), Sr (81–906 ppm), Zr (85–340 ppm) as well as most other elements, and is moderately enriched in incompatible elements, including rare earth elements (REE). The suite exhibits fractionated, subparallel REE patterns that are similar overall to Andean andesites and ignimbrites. Well-defined major and trace element trends and fractionated REE profiles are consistent with a fractionated basalt to rhyolite calc-alkaline magma series. It is a typical calc-alka-line orogenic complex and exhibits mineralogical-geochemical traits of arc-related volcanism. The suite neither resembles products of extensional nor transitional tectonic regimes as previously thought, but was produced in a subduction-related tectonic environment. The mafic nature of the least-evolved rocks of the suite, along with its relatively low initial 87Sr/86Sr ratio (0.7039) are considered to indicate a mantle source. A mantle-derived basaltic magma fractionated, with amphibole and plagioclase dominating the fractionating assemblage, to produce the more felsic varieties, as suggested by major and trace element fractionation modelling.
The Euphrates volcanic field, northeastern Syria: petrogenesis of Cenozoic basanites and alkali basalts
- NANCY A. LEASE, ABDEL-FATTAH M. ABDEL-RAHMAN
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- Journal:
- Geological Magazine / Volume 145 / Issue 5 / September 2008
- Published online by Cambridge University Press:
- 29 April 2008, pp. 685-701
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The Plio-Quaternary Euphrates volcanic field of NE Syria includes large discontinuous exposures of basanitic and basaltic lava flows (1200 km2 in area). It represents the northern segment of the Cenozoic volcanic province of the Middle East and is located near the Bitlis collision suture. The rocks consist of olivine (15–20%), clinopyroxene (30–35%), plagioclase (45–55%) and opaque phases. Chemically, the rocks are largely ultrabasic (SiO2 38.2–45.5 wt%, MgO 8.7–13.0 wt% and average Mg number of 0.65). They are enriched in incompatible trace elements such as Zr (133–276 ppm), Nb (25–71 ppm) and Y (17–28 ppm). The REE patterns are strongly fractionated ((La/Yb)N = 19.6), indicative of a garnet-bearing source. The 143Nd/144Nd isotopic compositions range from 0.512868 to 0.512940 (εNd = 4.5 to 5.9), and 87Sr/86Sr from 0.70309 to 0.70352. These chemical and isotopic compositions reflect strong affinities to OIB. Elemental ratios such as K/P (3.4), La/Ta (13) and La/Nb (0.77), and the low SiO2 values, suggest that the Euphrates magma was subjected to minimal crustal contamination. Petrogenetic modelling has been carried out using a variety of mantle source materials, different degrees of partial melting (0.1 to 10%), and a number of scenarios including metasomatized sources. Modelling suggests that the magma could have been produced as a result of a small degree of partial melting of either (1) a garnet-bearing depleted source enriched with a small addition of metasomatizing fluids, or (2) a garnet-bearing fertile source. The overall chemical and petrological characteristics are more consistent with the generation of the Euphrates magma by a small degree of partial melting (F = 1%) of a primitive, garnet-lherzolite mantle source, possibly containing a minor spinel component. The Neogene collision of the Arabian plate with Eurasia along the Bitlis suture resulted in reactivation (beneath the Euphrates basin) of deep-seated fractures, along which lavas may have penetrated the crust.
Cenozoic volcanism in the Middle East: petrogenesis of alkali basalts from northern Lebanon
- ABDEL-FATTAH M. ABDEL-RAHMAN, PHILIP E. NASSAR
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- Journal:
- Geological Magazine / Volume 141 / Issue 5 / September 2004
- Published online by Cambridge University Press:
- 06 October 2004, pp. 545-563
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The Cenozoic volcanic field of the Akkar region in northern Lebanon consists of a thick succession (200 m) of basaltic lava flows, erupted at the junction between a restraining bend (the Yammouneh transform fault) and its northern extension (the Ghab transform) in Syria. Both faults are part of the Dead Sea transform fault system, which represents the boundary between the Arabian and African plates and the Levantine subplate. The lavas are made up of about 15–25 vol. % olivine (Fo79–84), 30–40 % clinopyroxene (salite), 40–50 % plagioclase (An58–67), and opaque Fe–Ti oxides (∼ 5 %). Geochemically, they exhibit a narrow range of SiO2 (44.6 to 47.0 wt %), and MgO (2.9 to 7.5 wt %), are relatively enriched in TiO2 (2.0 to 2.9 wt %), and are classified as alkali basalts. Mg-numbers range from 0.32 to 0.59, with an average of 0.47. The rocks are enriched in incompatible trace elements such as Zr (98–184 ppm), Nb (16–39 ppm) and Y (25–34 ppm). The REE patterns are fractionated ((La/Yb)N=8.2), and are generally parallel to subparallel. Such compositions are typical of those of HIMU-OIB and plume-related magmas. Elemental ratios such as K/P (2.9), La/Ta (21.8), La/Nb (0.80), Nb/Y (0.92) and Th/Nb (0.35), and the low average SiO2 content (46.1 wt %) suggest that the magma was subjected to minimal crustal contamination. This may be related to a rapid ascent of the parental magma, in agreement with the nature (mafic, oceanic crust-like) and the thickness (only about 12 km) of the crust of the Eastern Mediterranean region. Cenozoic volcanism in this region is interpreted to have occurred in association with an episode of localized extension, particularly at the junction between the Yammouneh restraining bend and the Dead Sea–Ghab Transform (that is, in a transtensional tectonic regime). The 143Nd/144Nd isotopic composition of the basaltic rocks of northern Lebanon ranges from 0.512842 to 0.512934 (εNd=4.0 to 5.8), and 87Sr/86Sr from 0.703317 to 0.703579, suggesting a HIMU-like mantle source. Modelling indicates that the magma was produced by a small degree of partial melting (F=2 %) of a primitive, garnet lherzolitic mantle source, possibly containing a minor spinel component.
Mesozoic volcanism in the Middle East: geochemical, isotopic and petrogenetic evolution of extension-related alkali basalts from central Lebanon
- ABDEL-FATTAH M. ABDEL-RAHMAN
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- Journal:
- Geological Magazine / Volume 139 / Issue 6 / November 2002
- Published online by Cambridge University Press:
- 27 February 2003, pp. 621-640
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Mesozoic picritic and alkali basalts from central Lebanon represent a significant part of an extension-related Upper Jurassic to Upper Cretaceous discontinuous volcanic belt which occurs throughout the Middle East. Volcanism was associated with an episode of intraplate extension that followed a period of continental break-up, where Mesozoic micro-continental blocks separated from Gondwana as the Neotethys ocean opened in Jurassic times. This volcanic episode produced mafic lava flows ranging in thickness from 5 to 20 m, along with some minor pyroclastic flows. These flows are stratigraphically intercalated with thick carbonate platform deposits. The basalts are made up of about 15–20% olivine (Fo78–91), 30–35% clinopyroxene (salite), 40–50% plagioclase (An56–71) and opaque Fe–Ti oxides (∼5%). Geochemically, the rocks exhibit a relatively wide range of SiO2 (40.4 to 50.5 wt%) and MgO (5.1 to 15.5 wt%) contents, are relatively enriched in TiO2 (1.7 to 3.7 wt%) and vary in composition from alkali picrite and basanite to alkali basalt. The Mg numbers range from 0.56 to 0.70, with an average of 0.63. The rocks are enriched in incompatible trace elements such as Zr (86–247 ppm), Nb (16–66 ppm) and Y (19–30 ppm). Such compositions are typical of those of HIMU-OIB and plume-related magmas. The REE patterns are fractionated ((La/Yb)N = 11), LREE enriched, and are generally parallel to subparallel. Elemental ratios such as K/P (1.1–4.7), La/Ta (11–13), La/Nb (0.57–0.70), Nb/Y (0.68–1.55) and Th/Nb (0.20–0.36) suggest that crustal contamination was minor or absent. This may be related to a rapid ascent of the magma, in agreement with the nature (mafic, oceanic-like) and the small thickness (about 12 km) of the Mesozoic crust of the Eastern Mediterranean region. The 143Nd/144Nd isotopic compositions of the lavas range from 0.512826 to 0.512886, and 87Sr/86Sr from 0.702971 to 0.703669, suggesting a HIMU-like mantle source. Trace element compositions indicate a melt segregation depth of 100–110 km, well within the garnet lherzolite stability field. The geochemical characteristics of the rocks are typical of within-plate alkali basalts, and suggest that the magmas were derived from a fertile, possibly plume-related, enriched mantle source. Petrogenetic modelling indicates that the magmas were produced by very small degrees of batch partial melting (F = 1.5%) of a primitive garnet-bearing mantle source (garnet lherzolite).
Anorogenic magmatism: chemical evolution of the Mount El-Sibai A-type complex (Egypt), and implications for the origin of within-plate felsic magmas
- ABDEL-FATTAH M. ABDEL-RAHMAN, MAYA M. EL-KIBBI
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- Journal:
- Geological Magazine / Volume 138 / Issue 1 / January 2001
- Published online by Cambridge University Press:
- 20 March 2001, pp. 67-85
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The Mount El-Sibai alkaline granitic complex (eastern Egypt) forms an elongate body, which was emplaced at the extension of a NW-trending shear zone, within voluminous calc-alkaline Pan-African host rocks. The complex is hypersolvus in nature and is composed of perthite, quartz, alkali amphibole, Fe-rich biotite, and accessory zircon, apatite, fluorite, aenigmatite and ilmenite. Data on mineral chemistry show that the amphibole ranges in composition from hastingsite to pure end-member arfvedsonite, and the biotite is largely titaniferous annite. Geochemically, the complex is highly evolved in composition (with 72–78 wt% SiO2, and DI values of 85–98), is enriched in Rb (48–291 ppm), Nb (28–237 ppm), Y (47–269 ppm), Zr (58–618 ppm), Ga (17–41 ppm) and the REE (176–437 ppm), and depleted in Al, Mg, Ca, Sr and Eu. The complex exhibits a wide trace-element compositional range. The REE patterns are uniform, parallel to sub-parallel, fractionated ((La/Yb)n = 4.7), LREE enriched over HREE, and show prominent negative Eu-anomalies. The albitized facies of this complex shows the highest concentrations of large ion lithophile (LIL) and high field strength (HFS) elements. The complex exhibits mineralogical and chemical traits typical of within-plate A-type granites. Mount El-Sibai is interpreted to have been developed during a phase of cooling, relaxation, crustal attenuation, and fracturing of the newly formed Pan-African crust. Results of geochemical modelling indicate that the magma may have formed by a large degree of batch partial melting (F=0.65) of Pan-African calc-alkaline rocks, which had been metasomatized. Metasomatism of source rocks may have been caused by a Na–F-rich fluid phase compositionally similar to that which produced the albitized facies. The volatile flux may have caused fenitization-type reactions along fissures and re-activated Pan-African fractures prior to anatexis, and is considered to have played a role as an important agent of heat transfer. Temperature necessary for crustal anatexis is likely to have been produced as a result of shear heating, caused by a rapid change in the direction of plate motions beneath eastern Egypt during Early Palaeozoic times. The confining pressure must have been released by fissuring of the crust. Magma ascent may have been facilitated by reactivation of pre-existing faults and shear zones. This model may have wider implications for the generation of within-plate felsic magmas in other regions.