Crustal thickening prior to 43 Ma in the Himalaya: evidence from lower crust‐derived adakitic magmatism in Dala, eastern Tethyan Himalaya, Tibet

Dai, Zuowen, Dong, Lei, Li, Guangming, Huizenga, Jan Marten, Ding, Jun, Zhang, Linkui, Huang, Yong, Cao, Huawen, Lu, Liu, and Yan, Guoqiang (2019) Crustal thickening prior to 43 Ma in the Himalaya: evidence from lower crust‐derived adakitic magmatism in Dala, eastern Tethyan Himalaya, Tibet. Geological Journal, 55 (5). pp. 4021-4046.

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Abstract

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 SiO2, Al2O3 and Sr contents, low Y and Yb contents with high Sr/Y (27−67) and La/Yb (35−99) ratios. High K2O 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 εHf(t) values (−10.4 to −0.5) with old two‐stage Hf model ages (1.1–1.8 Ga), high (87Sr/86Sr)i (0.715861–0.717665) with low εNd(t) (−12.5 to −11.3) and high Pb isotopes [(206Pb/204Pb)i=18.783–18.861, (207Pb/204Pb)i=18.783–18.861 and (208Pb/204Pb)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.

Item ID: 60369
Item Type: Article (Research - C1)
ISSN: 1099-1034
Keywords: adakitic rocks, crustal thickening, Dala, Himalaya, lower crust, slab breakoff, Tibet
Copyright Information: © 2019 John Wiley & Sons, Ltd.
Funders: National Key R&D Program of China, National Natural Science Foundation of China (NNSFC), China Geological Survey (CGS)
Projects and Grants: National Key R&D Program of China 2018YFC0604103, National Key R&D Program of China 2016YFC0600308, NNSFC grant number: 41802095, CGS grant number: DD20190147
Date Deposited: 18 Sep 2019 00:20
FoR Codes: 04 EARTH SCIENCES > 0402 Geochemistry > 040203 Isotope Geochemistry @ 70%
04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 30%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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