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Crustal thickening prior to 43 Ma in the Himalaya: Evidence from lower crust‐derived adakitic magmatism in Dala, eastern Tethyan Himalaya, Tibet
Author(s) -
Dai Zuowen,
Dong Lei,
Li Guangming,
Huizenga Jan Marten,
Ding Jun,
Zhang Linkui,
Huang Yong,
Cao Huawen,
Lu Liu,
Yan Guoqiang
Publication year - 2020
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3639
Subject(s) - zircon , geology , geochemistry , crust , magmatism , continental crust , partial melting , magma , tectonics , paleontology , volcano
The Himalayan–Tibetan orogen is the world's largest active orogen with the thickest continental crust on Earth. However, the timing of crustal thickening of this orogen remains controversial. In recent years, magmatic rocks with adakitic affinities have been widely used to constrain the crustal thickness. Here, we present zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic data for the Dala two‐mica granites (eastern Tethyan Himalaya) that may shed light on this issue. Zircon U–Pb dating shows that the Dala two‐mica granites were emplaced at ca . 43 Ma. The granites display adakitic signatures, including high SiO 2 , Al 2 O 3 and Sr contents, low Y and Yb contents with high Sr/Y (27−67) and La/Yb (35−99) ratios. High K 2 O and Th contents, low Nb/U, Ce/Pb, Ti/Eu and Nd/Sm ratios and low MgO, Mg#, Cr and Ni contents suggest that the Dala two‐mica granites were derived from partial melting of a thickened lower crust. Low negative zircon ε Hf ( t ) values (−10.4 to −0.5) with old two‐stage Hf model ages (1.1–1.8 Ga), high ( 87 Sr/ 86 Sr) i (0.715861–0.717665) with low ε Nd ( t ) (−12.5 to −11.3) and high Pb isotopes [( 206 Pb/ 204 Pb) i =18.783–18.861, ( 207 Pb/ 204 Pb) i =18.783–18.861 and ( 208 Pb/ 204 Pb) i =39.052–39.285], together with previous literature data, indicate that the magma source for the Eocene granites in the Yardoi area was composed of ancient lower crust consisting mainly of garnet‐amphibolite and probably subordinate metapelites of the High Himalayan Crystalline Sequence. The residual mineral assemblage of garnet + rutile + plagioclase ± amphibole in the source region demonstrates that the crust beneath the Himalaya could have been thickened to 50 km in the Eocene. Combining with previous studies, we propose that the Eocene magmatic rocks in the Tethyan Himalaya (e.g. ca . 43 Ma old Dala two‐mica granites) can be best interpreted as the consequence of breakoff of the Neo‐Tethyan oceanic slab.