科研之友
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摘要
Along the eastern margin of the Mesoproterozoic Namaqua metamorphic province(NMP) of southern Africa are a bimodal volcano-sedimentary succession, the ～1.13–1.10 Ga Koras Group, composed of rhyolitic porphyries and basaltic andesites, and the ～1.11–1.07 Ga late-to post-tectonic granitoids of the Keimoes Suite. This review examines existing whole-rock major-and trace-element data, along with isotope chemistry(with some new isotopic data), to investigate the role these two magmatic successions played in terms of post-collisional magmatism of the eastern NMP near the boundary with the Archean Kaapvaal Craton. The Keimoes Suite comprises variably porphyritic biotite monzogranites and granodiorites, with a charnockitic member. They are metaluminous to weakly peraluminous, ferroan, and calc-alkalic. They exhibit large ion lithophile(LIL) element enrichment relative to the high field strength elements(HFSE) with depletions in Ba, Sr, Nb, P, Eu and Ti, and enrichments in Th, U and Pb. Isotopic values(εNd(t): 2.78 to-2.95,but down to-8.58 for one granite, depleted mantle Nd model ages(TDM): 1.62–1.99 Ga, but up to 2.55 Ga;initial 87Sr/86Sr: 0.652 82–0.771 30) suggest derivation from weakly to mildly enriched(and radiogenic)sources of Meso-to Paleoproterozoic age, the former of more juvenile character. The Koras Group is characterized by a bimodal succession of calcic to calc-alkalic, magnesian and tholeiitic basaltic andesites and mostly metaluminous to peralkaline rhyolitic porphyries. Two successions are recognised, an older, lower succession that extruded at ～1.13 Ga, and a younger, upper succession at ～1.10 Ga. The rhyolitic porphyries of both successions show similar LILE/HFSE enrichment and the same element enrichments and depletions as the Keimoes Suite granitoids. The upper succession is consistently more fractionated in terms of both whole-rock major and trace element chemistry, and, isotopically, has a greater enriched source component(ε(Nd(t):-0.69 to-4.26; TDM: 1.64–2.44 Ga), relative to the lower succession(ε(Nd(t): 0.74–5.62; TDM: 1.28–2.12 Ga). Crystal fractionation of plagioclase and K-feldspar appears to have played a role in bringing about compositional variation in many of the granites. These were derived from partial melting of mainly igneous with subordinate sedimentary sources from mostly lower crustal depths, although some granitoids have indications of a possible mantle source component. The lower succession of the Koras Group was derived by partial melting of subduction-influenced enriched mantle giving rise to mafic magmas that fractionated to give rise to the rhyolitic porphyries. The upper succession rhyolites were derived by crustal melting due to the input of mafic magmatism. Crystal fractionation was the main compositional driver for both successions. The Keimoes Suite granitoids and the Koras Group are associated with extensional regimes subsequent to the main deformational episode in the eastern NMP.
