Petrogenesis of synorogenic high-temperature leucogranites (Damara orogen, Namibia): Constraints from U-Pb monazite ages and Nd, Sr and Pb isotopes

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• 作者：A. Paul^a ; ^{andre-paul@hotmail.de" class="auth_mail} ; S. Jung^a ; R.L. Romer^b ; A. Stracke^c ; F. Hauff^d
• 关键词：Leucogranites ; Crustal melts ; Biotite dehydration melting ; U&ndash ; Pb monazite ; Sr Nd Pb isotopes
• 刊名：Gondwana Research
• 出版年：May, 2014
• 年：2014
• 卷：25
• 期：4
• 页码：1614-1626
• 全文大小：2742 K

文摘

Two suites of leucogranites were emplaced at 508 卤 5.9 Ma in the Okombahe District of the Damara belt (Namibia) synchronous with the peak of regional high-temperature metamorphism. The Sr (⁸⁷Sr/⁸⁶Sr_init: 0.707 to 0.711), Nd (蔚Nd_init: 鈭?#xA0;4.5 to 鈭?#xA0;6.6), and Pb isotopic (²⁰⁶Pb/²⁰⁴Pb: 18.51-19.13; ²⁰⁷Pb/²⁰⁴Pb: 15.63-15.69; ²⁰⁸Pb/²⁰⁴Pb: 38.08-38.66) compositions indicate that these peraluminous S-type granites were derived from mid- to lower-crustal rocks, which are slightly different to the metapelitic rocks into which they intruded. Since the leucogranites are unfractionated and show no evidence for assimilation or contamination, they constrain the temperature and pressure conditions of their formation. Calculated Zr and LREE saturation temperatures of ca. 850 掳C indicate high-temperature crustal melts. High Rb/Sr and low Sr/Ba ratios are consistent with biotite dehydration melting of pelitic source rocks. Qz-Ab-Or systematics reveal that melting and segregation for the least fractionated samples occurred at ca. 7 kbar corresponding to a mid-crustal level of ca. 26 km. However, there is no evidence for a mantle component that could have served as a local heat source for crustal melting. Therefore, the hot felsic magmas that formed close to the time of peak metamorphism are the result of long-lasting high temperature regional metamorphic conditions and intra-crustal collision.