Geochemical investigation of Early Cretaceous igneous rocks along an east–west traverse throughout the central Lhasa Terrane, Tibet

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• 作者：Di-Cheng Zhu ; Xuan-Xue Mo ; Yaoling Niu ; Zhi-Dan Zhao ; Li-Quan Wang ; Yong-Sheng Liu ; Fu-Yuan Wu
• 关键词：Bulk-rock geochemistry ; Zircon U– ; Pb dating and Hf isotope ; Petrogenesis ; Early Cretaceous igneous rocks ; Slab break-off ; Bangong– ; Nujiang Tethyan Ocean ; Tectonomagmatic evolution ; Lhasa micro-continental block ; Tibet
• 刊名：Chemical Geology
• 出版年：2009
• 出版时间：30 November 2009
• 年：2009
• 卷：268
• 期：3-4
• 页码：298-312
• 全文大小：2295 K

文摘

Bulk-rock major and trace element, Sr–Nd isotope, zircon U–Pb and Hf-isotope data are reported for Early Cretaceous igneous rocks sampled along an east–west traverse through the central Lhasa subterrane (E80°00′–E89°) in southern Tibet. These results offer new insights into the pre-Cenozoic geology and tectonomagmatic evolution of the Lhasa Terrane, and provide boundary conditions for the origin and evolution of the Tibetan Plateau. Our new data, together with data from the literature indicate that the Early Cretaceous magmatism in the central Lhasa subterrane lasted for a long period (~ 143–102 Ma) with a magmatic flare-up at ~ 110 Ma. The latter is accompanied by mafic magmatism in this terrane. The silicic rocks are metaluminous to peraluminous, and enriched in Rb, Th, and U, and depleted in Ba, Nb, Ta, Sr, P, and Ti, with varying initial ⁸⁷Sr/⁸⁶Sr (0.7073–0.7209), negative ε_Nd(t) (− 13.7 to − 4.6), and negative to positive zircon ε_Hf(t). The andesitic and dioritic enclaves are characterized by initial ⁸⁷Sr/⁸⁶Sr of 0.7088–0.7148, ε_Nd(t) of − 9.9 to −7.8, and zircon ε_Hf(t) of − 9.5 to − 0.2, similar to those of the 110 ± 3 Ma silicic rocks. The bulk-rock Sr and Nd isotopic data and zircon ε_Hf(t) values indicate an increased contribution of a mantle component in the generation of the Early Cretaceous igneous rocks in the central Lhasa subterrane at ~ 110 Ma. Available bulk-rock Nd- and zircon Hf-isotope data indicate that the crust of the central Lhasa subterrane, at least its oldest elements, was emplaced during the Archean. This further indicates that the central Lhasa subterrane with ancient basement must have been a micro-continental block (i.e., the Lhasa micro-continental block) extending in an east–west direction for > 700 km (E82°–E89°) with a width of ~ 100 km. The contemporaneous presence of S-type (more crustal source) and I-type (significant mantle input) melts of the Early Cretaceous igneous rocks in this subterrane are interpreted as the consequences of varying extents of interactions between the existing continental crust and mantle-derived melts (including crustal anatexis). We suggest that the mantle-derived melts resulted from southward subduction of the Bangong–Nujiang Ocean seafloor in a syncollisional setting related to the Lhasa–Qiangtang collision, and that the magmatic flare-up with strong mantle input at ~ 110 Ma was genetically associated with the slab break-off of this subducting seafloor.