• journal_title：Geological Magazine
• Contributor：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 ; TAO WANG
• Publisher：Cambridge University Press
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.1017/S0016756808004548
• journal_abbrev：Geological Magazine
• issn：0016-7568
• volume：145
• issue：4
• firstpage：463
• section：ORIGINAL ARTICLES

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 ε_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.