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Kaersutite-Bearing Xenoliths and Megacrysts in Volcanic Rocks from the Funk Seamount in the Southwest Indian Ocean
- Arch M. Reid, Anton P. le Roex
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- Journal:
- Mineralogical Magazine / Volume 52 / Issue 366 / June 1988
- Published online by Cambridge University Press:
- 05 July 2018, pp. 359-370
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Eight samples (seven volcanic rocks and one quartz sandstone) have been dredged from the Funk Seamount, 60 km NW of Marion Island in the southwest Indian Ocean (lat. 46° 15′S, long. 37° 20′ E). The volcanic rocks are fine-grained vesicular basanitoids and glass-rich volcanic breccias geochemically similar to the Marion Island lavas. Lavas and breccias contain a suite of megacryst minerals and of small polymineralic xenoliths, in both of which kaersutite is a prominent constituent.
The megacryst suite comprises large unzoned single grains of kaersutite, plagioclase, pyroxene, magnetite and ilmenite, all showing textural evidence of resorption/reaction with the basanitoid host. The megacrysts have a limited range of compositions except for the plagioclase which ranges from oligoclase to labradorite.
The small (2 mm to ∼ 3 cm) xenoliths are mostly two-pyroxene amphibole assemblages with or without olivine, magnetite, ilmenite, plagioclase and apatite. The xenoliths show some evidence of reaction with the basanitoid host and most have undergone recrystallization and/or localised decompression melting.
Xenolith and megacryst assemblages are interpreted as being associated with the formation and partial crystallization of a hydrous basanitoid melt at depth.
Chromian illite-ankerite-quartz parageneses from the Kintail district of southern Ross-shire, Scotland
- T. N. Clifford, D. C. Rex, R. Green, A. P. le Roex, H. S. Pienaar, D. Bühmann
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- Journal:
- Mineralogical Magazine / Volume 63 / Issue 1 / February 1999
- Published online by Cambridge University Press:
- 05 July 2018, pp. 37-52
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The Coire Dhuinnid fault zone contains emerald green chromian illite-ankerite-quartz rocks that are similar in appearance to the fuchsite(or mariposite)-carbonate-quartz parageneses that are commonplace in Archaean greenstone belts but which are rather rare in Phanerozoic rocks. The chromian illite contains 2.3 wt.% Cr2O3, low K2O (7.1–7.6 wt.%) and high H2O+ (5.7 wt.%), and it is a 1M polytype with ≤10% of an illite/smectite interstratification indicative of a formation temperature of c. 175–200°C. The host rocks contain high concentrations of Ni and Cr, and show low concentrations of Ti, Nb, Y and Zr, suggesting a former primitive mafic protolith (boninitic magma?); they are considered to be retrograde remnants of Lewisian rocks. The latter, and the associated rocks of the Moine Series, have been affected by CO2 metasomatism that was accompanied by the addition of Ca(+Sr), Fe and Mg, and by the removal of Na from, and the addition of H2O to the Moine metasediments. Radiogenic isotope studies of mineral separates and whole rock from sample no. 43 yielded ages of 483±2 Ma (Ar-Ar dating on Cr illite), 413±12 Ma (K-Ar dating on Cr illite), and 322±9 Ma (Rb-Sr dating on minerals and whole rock); the significance of this discrepant pattern is discussed.
Geochemical evolution of the Okenyenya sub-volcanic ring complex, northwestern Namibia
- Anton P. Le Roex, Ronald T. Watkins, Arch M. Reid
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- Journal:
- Geological Magazine / Volume 133 / Issue 6 / November 1996
- Published online by Cambridge University Press:
- 07 April 2017, pp. 645-670
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The Okenyenya gabbro-syenite complex, one of a number of intrusive igneous complexes of late-Mesozoic age in northwestern Namibia, was emplaced at the time of opening of the South Atlantic Ocean. The 5-km-diameter complex comprises a wide variety of rock types that can be subdivided into two contrasting magmatic suites, one tholeiitic and the other alkaline, which were emplaced in close proximity over a time-span of ˜5 Ma. The tholeiitic suite of rocks includes picritic gabbro, olivine gabbro through quartz monzodiorite and syenite, whereas the alkaline suite includes alkaline gabbro, essexite, nepheline syenite and a range of lamprophyric rock types. Detailed petrographic, mineralogical and bulk rock geochemical data show that the earliest, saucer-shaped, intrusion of olivine gabbro-quartz monzodiorite rocks can be subdivided into an Inner Zone and an Outer Zone (each comprising three distinct intrusive units). The individual units can be readily distinguished on the basis of bulk rock geochemical variations, together with cryptic and modal mineralogical variations. An unusual feature of the intrusive body is that bulk rock and mineral compositions become more evolved with apparent depth, within the body as a whole and within each unit. Compositional variation within the individual intrusive units requires a complex interplay between in situ crystallization, variable expulsion of interstitial melt, magma recharge, and re-equilibration of primocrysts with trapped interstitial melt. Cross-cutting dykes of picritic gabbro (MgO= 13–21 %) have compositions consistent with olivine control. Incompatible trace element ratios (e.g. Zr/Nb= 12.5 ± 1.3) suggest that the picritic gabbro magmas were derived from a distinct source region compared to that giving rise to the tholeiitic olivine gabbros (Zr/Nb = 6.8 ± 1.1).
Alkaline gabbro occupies the central region of the complex and, on the basis of major, trace and rare earth element variations, can be subdivided into four distinct intrusive bodies, interpreted as remnant magma chambers, each having experienced variable degrees of crystal accumulation. In places, magma chamber processes have given rise to centimetre-scale rhythmic layering. Incompatible trace element ratios (e.g. Zr/Nb = 4.4 ± 1.2) serve to distinguish the source region of the alkaline gabbro magmas from those giving rise to the tholeiitic suite of magmas. Younger rocks of both the tholeiitic and alkaline suites show strong evidence of the effects of extensive crystal fractionation. The quartz syenite is characterized by a strong negative Eu anomaly indicative of substantial feldspar fractionation and also shows evidence for direct contamination by earlier gabbro, whereas the syenite shows evidence for feldspar accumulation. Both syenites have geochemical characteristics suggesting consanguinity with the Outer Zone rocks of the olivine gabbro-quartz monzodiorite intrusion. In contrast, the essexite and nepheline syenite compositions are qualitatively consistent with derivation from one of the alkaline gabbro magmas by extensive fractionation of plagioclase, clinopyroxene, olivine and amphibole. The final stage of magmatism is represented by a suite of alkaline and ultramafic lamprophyres emplaced as dykes and diatremes, the latter carrying a variety of megacrystic and xenolithic material, including mantle nodules. The alternation between tholeiitic and alkaline magmatism evident within the Okenyenya complex is similar to that characteristic of the evolution of many ocean island volcanoes.
Volcanism on Gough Island: a revised stratigraphy
- J. G. Maund, D. C. Rex, A. P. Le Roex, D. L. Reid
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- Journal:
- Geological Magazine / Volume 125 / Issue 2 / March 1988
- Published online by Cambridge University Press:
- 01 May 2009, pp. 175-181
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Recent field work on Gough Island combined with K–Ar dating of the lavas requires revision of the age and volcanic stratigraphy. Four main periods of volcanic activity on the island are recognized. These comprise the eruption of the Older Basalt Group which ranges in age from 2.5 to 0.52 Ma, the intrusion of aegerine-augite trachyte plugs (0.8−0.47 Ma), voluminous trachyte extrusion (0.30–0.12 Ma) and finally to eruption of the Edinburgh Basalt (0.20–0.13 Ma).
Within the Older Basalt Group three phases of activity can be recognized; the earliest involving the eruption of pillow basalts and hyaloclastites when the island emerged from below sea level. This was followed by subaerial as-type lava flows and also dyke intrusion (phase two) which probably contributed to forming a large shield-type volcanic island, which in turn supported the eruption and deposition of flat-lying flows on an angular unconformity (phase three). Intrusion of aegirine-augite trachyte plugs occurred concurrently with the latter stages of Older Basalt eruption. After a period of considerable erosion the voluminous trachyte lavas and pyroclastics were erupted. The Edinburgh Basalt, erupted in the vicinity of Edinburgh Peak, represents the youngest volcanic activity on the island.
Rb-Sr age determinations of rocks from the Okenyenya igneous complex, northwestern Namibia
- Simon C. Milner, Anton P. Le Roex, Ronald T. Watkins
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- Journal:
- Geological Magazine / Volume 130 / Issue 3 / May 1993
- Published online by Cambridge University Press:
- 01 May 2009, pp. 335-343
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The Okenyenya igneous complex is one of a suite of intrusions which define a prominent northeast-trending linear feature in Damaraland, northwestern Namibia. Precise Rb–Sr internal isochron ages range from 128.6 ± 1 to 123.4 ± 1.4 Ma for the major phases of intrusion identified within the complex. The tholeiitic gabbros forming the outer rings of the complex, and the later alkali gabbros which form the central hills, cannot be distinguished in terms of Rb–Sr ages, although field relations clearly indicate the younger age of the latter. The intrusionsof nepheline-syenite and essexite comprising the mountain of Okenyenya Bergon the northern edge of the complex give ages of 123.4 ± 1.4 and 126.3 ± 1 Ma, respectively, and form the final major phase of intrusion. The ages obtained for early and late intrusive phases define a minimum magmatic ‘life-span’ of approximately 5 Ma for the complex. The determined age of the Okenyenya igneous complex (129–123 Ma), when taken together with the few reliable published ages for other Damaraland complexes (130–134 Ma), suggests that these sub-volcanic complexes were emplaced contemporaneously with the widespread Etendeka volcanics (˜ 130 Ma), and relate to magmatism associated with the breakup of southern Africa and South America with the opening of the South Atlantic Ocean. The linear distributionof intrusions in Damaraland is interpreted to be due to magmatism resultingfrom the upwelling Tristan plume being focused along a structural discontinuity between the Pan-African, Damaran terrain to the south, and Proterozoiccratonic basement to the north.
Geochemical evolution of the Okenyenya sub-volcanic ring complex, northwestern Namibia
- Anton P. Le Roex, Ronald T. Watkins, Arch M. Reid
-
- Journal:
- Geological Magazine / Volume 133 / Issue 6 / November 1996
- Published online by Cambridge University Press:
- 01 May 2009, pp. 645-670
-
- Article
- Export citation
-
The Okenyenya gabbro–syenite complex, one of a number of intrusive igneous complexes of late-Mesozoic age in northwestern Namibia, was emplaced at the time of opening of the South Atlantic Ocean. The 5-km-diameter complex comprises a wide variety of rock types that can be subdivided into two contrasting magmatic suites, one tholeiitic and the other alkaline, which were emplaced in close proximity over a time-span of ~5 Ma. The tholeiitic suite of rocks includes picritic gabbro, olivine gabbro through quartz monzodiorite and syenite, whereas the alkaline suite includes alkaline gabbro, essexite, nepheline syenite and a range of lamprophyric rock types. Detailed petrographic, mineralogical and bulk rock geochemical data show that the earliest, saucer-shaped, intrusion of olivine gabbro–quartz monzodiorite rocks can be subdivided into an Inner Zone and an Outer Zone (each comprising three distinct intrusive units). The individual units can be readily distinguished on the basis of bulk rock geochemical variations, together with cryptic and modal mineralogical variations. An unusual feature of the intrusive body is that bulk rock and mineral compositions become more evolved with apparent depth, within the body as a whole and within each unit. Compositional variation within the individual intrusive units requires a complex interplay between in situ crystallization, variable expulsion of interstitial melt, magma recharge, and re-equilibration of primocrysts with trapped interstitial melt. Cross-cutting dykes of picritic gabbro (MgO = 13–21 %) have compositions consistent with olivine control. Incompatible trace element ratios (e.g. Zr/Nb= 12.5 ± 1.3) suggest that the picritic gabbro magmas were derived from a distinct source region compared to that giving rise to the tholeiitic olivine gabbros (Zr/Nb = 6.8 ± 1.1).
Alkaline gabbro occupies the central region of the complex and, on the basis of major, trace and rare earth element variations, can be subdivided into four distinct intrusive bodies, interpreted as remnant magma chambers, each having experienced variable degrees of crystal accumulation. In places, magma chamber processes have given rise to centimetre-scale rhythmic layering. Incompatible trace element ratios (e.g. Zr/Nb = 4.4 ± 1.2) serve to distinguish the source region of the alkaline gabbro magmas from those giving rise to the tholeiitic suite of magmas. Younger rocks of both the tholeiitic and alkaline suites show strong evidence of the effects of extensive crystal fractionation. The quartz syenite is characterized by a strong negative Eu anomaly indicative of substantial feldspar fractionation and also shows evidence for direct contamination by earlier gabbro, whereas the syenite shows evidence for feldspar accumulation. Both syenites have geochemical characteristics suggesting consanguinity with the Outer Zone rocks of the olivine gabbro–quartz monzodiorite intrusion. In contrast, the essexite and nepheline syenite compositions are qualitatively consistent with derivation from one of the alkaline gabbro magmas by extensive fractionation of plagioclase, clinopyroxene, olivine and amphibole. The final stage of magmatism is represented by a suite of alkaline and ultramafic lamprophyres emplaced as dykes and diatremes, the latter carrying a variety of megacrystic and xenolithic material, including mantle nodules. The alternation between tholeiitic and alkaline magmatism evident within the Okenyenya complex is similar to that characteristic of the evolution of many ocean island volcanoes.