| Abstract: | High-K calc-alkaline magmas from the Cenozoic Qiangtang volcanic field, northern Tibetan Plateau, contain lower crustal two-pyroxene and clinopyroxene granulite xenoliths. The petrology and geochemistry of six mafic and three felsic xenoliths from the Hol Xil area south of Ulan Ul Lake are discussed. Mafic granulites (Pl, Opx, Cpx, Ksp, and Bt) contain 48.76–58.61% SiO2, 18.34–24.50% Al2O3, 3.16–5.41% Na2O, 1.58–3.01% K2O, low Mg# (30–67), LREE and LILE enrichment, high Rb/Sr (0.09–0.21), (La/Yb)N (17.32–49.35), low Nb/Ta (9.76–14.92), and variable Eu anomalies (Eu?=?0.19?0.89). They also have more evolved Sr-Nd-Pb isotopic compositions in comparison with the host dacites 87Sr/86Sr (0.710812 vs. 0.713241), ?Sr (+169.13 vs.?+203.88), 143Nd/144Nd (0.512113 vs. 0.512397), ?Nd (?4.70 to??10.05), 206Pb/204Pb (18.7000 vs. 18.9565), 207Pb/204Pb (15.7135 vs. 15.7662), and 208Pb/204Pb (39.1090 vs. 39.4733). Felsic granulites (Qtz, Pl, Ksp, Bt, and Cpx) show enrichment of LREE and LILE and have evolved Sr-Nd-Pb isotopic compositions with (La/Yb)N (2.04–10.82), 87Sr/86Sr (0.712041–0.729088), ?Sr (+180.71–+430.59), 143Nd/144Nd (0.512230–0.512388); ?Nd (?4.74 to??7.96), 206Pb/204Pb (18.9250–19.1717), 207Pb/204Pb (15.7662–15.7720), and 208Pb/204Pb (39.2109–39.6467). These geochemical data suggest that the protolith of the mafic granulites could have been a hybrid mafic magma (e.g. enriched mantle type II) or metasomatized restite derived from the partial melting of metamafic-intermediate rocks rather than basaltic cumulates, whereas the felsic granulite protolith was a quartzofeldspathic S-type granitic rock. We argue that the lower crust of the northern Tibetan Plateau is hot and heterogeneous rather than wholly gabbroic. Interaction between the mantle-derived magma and the metasedimentary/granitic lower crust of the Tibetan Plateau may have played an important role in the generation of shoshonitic and high-K calc-alkaline andesite-dacite rocks. |
