科研之友
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摘要
The 556 +/- 4 Ma-old Bloedkoppie granite (Central Damara orogen, Namibia) is a metaluminous to slightly peraluminous, alkali-calcic to calc-alkalic and ferroan granite. Its composition implies high-temperature, reduced, and anhydrous conditions during granite formation. The granite is fractionated, but heterogeneous radiogenicisotope data (Sr-82/Sr-88(init.): 0.712 to 0.727; epsilon Nd(init.): -7.2 to -13.1; Pb-206/Pb-204; 17.30-17.72; Pb-207/Pb-204: 15.54-15.67; Pb-208/Pb-204: 37.80-38.23) indicate also that combined assimilation-fractional crystallization processes played an important role in the generation of the granite. Major and trace element compositions and isotope data of the least evolved samples and U-Pb data from zircon cores demonstrate that the source rocks are dominated by ca. 1.95 Ga old felsic orthogneisses from the underlying basement. Zircon saturation temperatures and normative Qz-Ab-Or compositions indicate minimum melting P-T conditions of ca. 860 degrees C at >5 kbar and <5 wt.% H2O. The most likely petrogenetic model involves high temperature partial melting of a Paleoproterozoic felsic source in the lower crust ca. 10-20 Ma before the first peak of regional high-temperature metamorphism. Underplating of the lower crust by magmas derived from the lithospheric mantle may have provided the heat for melting of the undepleted basement to produce reduced and anhydrous melts.
- 出版日期2015-5
