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Cenozoic high Sr/Y volcanic rocks in the Qiangtang terrane, northern Tibet: geochemical and isotopic evidence for the origin of delaminated lower continental melts

SHEN LIU^*,, RUI-ZHONG HU^*, CAI-XIA FENG^*, HAI-BO ZOU, CAI LI, XIAO-GUO CHI, JIAN-TANG PENG^*, HONG ZHONG^*, LIANG QI^*, YOU-QIANG QI^* and TAO WANG^*

^* State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China
Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA
Department of Earth and Space Sciences, Jilin University, Changchun 130026, China

Author for correspondence: liushen{at}vip.gyig.ac.cn

Geochemical and Sr–Nd–Pb isotopic data are presented for volcanic rocks from Zougouyouchaco (30.5 Ma) and Dogai Coring (39.7 Ma) of the southern and middle Qiangtang block in northern Tibet. The volcanic rocks are high-K calc-alkaline trachyandesites and dacites, with SiO₂ contents ranging from 58.5 to 67.1 wt % The rocks are enriched in light REE (LREE) and contain high Sr (649 to 986 ppm) and relatively low Yb (0.8 to 1.2 ppm) and Y (9.5 to 16.6 ppm) contents, resulting in high La/Yb (29–58) and Sr/Y (43–92) ratios, as well as relatively high MgO contents and Mg no., similar to the compositions of adakites formed by slab melting in subduction zones. However, the adakitic rocks in the Qiangtang block are characterized by relatively low _Nd(t) values (–3.8 to –5.0) and highly radiogenic Sr ((⁸⁷Sr/⁸⁶Sr)_i = 0.706–0.708), which are inconsistent with an origin by slab melting. The geochemistry and tectonics indicate that the adakitic volcanic rocks were most likely derived from partial melting of delaminated lower continental crust. As the pristine adakitic melts rose, they interacted with the surrounding mantle peridotite, elevating their MgO values and Mg numbers.

Key Words: adakitic rocks • lower crust • delamination • Cenozoic • Tibet