Petrogenesis of granitoids in the eastern section of the Central Qilian Block: Evidence from geochemistry and zircon U-Pb geochronology

The Caledonian-age Qilian Orogenic Belt at the northern margin of the Greater Tibetan Plateau comprises abundant granitoids that record the histories of the orogenesis. We report here our study of these granitoids from two localities. The Qingchengshan (QCS) pluton, which is situated in the eastern section of the Central Qilian Block, is dated at similar to 430-420 Ma. It has high-K calc-alkaline composition with high SiO2 (> 70 wt%), enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), and varying degrees of negative Sr and Eu anomalies. The granitoids in the Tongwei (TW) area, 150 km east of the QCS, are complex, the majority of which are dated at similar to 440 Ma, but there also exist younger, similar to 230 Ma intrusions genetically associated with the Qinling Orogeny. The Paleozoic TW intrusions also have high SiO2, fractionated REE (rare earth element) patterns, but a negligible Eu anomaly. The whole rock Sr-Nd-Hf isotopic compositions suggest that all these Paleozoic granitoids are consistent with melting-induced mixing of a two-component source, which is best interpreted as the combination of last fragments of subducted/subducting ocean crust with terrigenous sediments. The mantle isotopic signature of these granitoids (Sr-87/Sr-86(i): 0.7038 to 0.7100, epsilon(Nd)(t): -4.8 to -1.3, epsilon(Hf)(t): -0.7 to +4.0) reflects significant (similar to 70 %) contribution of the ocean crust derived in no distant past from the mantle at ocean ridges with an inherited mantle isotopic signature. Partial melting of such ocean crust plus terrigenous sediments in response to the ocean closing and continental collision (between the Qilian and Alashan Blocks) under amphibolite facies conditions is responsible for the magmatism. Varying extents of fractional crystallization (+/- plagioclase, +/- amphibole, +/- garnet, +/- zircon) of the parental magmas produced the observed QCS and TW granitoids. We note that sample HTC12-01 in the TW area shows an A-type or highly fractionated granite signature characterized by elevated abundances and a flat pattern of REEs, weak Nb-Ta anomaly, conspicuous negative Sr and Eu anomalies (Sr/Sr* = 0.09, Eu/Eu* = 0.22), and thus the high Sr-87/Sr-86 ratio (0.7851), and moderate epsilon(Nd)(t) (-4.9) and epsilon(Hf)(t) (-2.0), pointing to the significant mantle contribution. Compared with the Paleozoic granitoids, the similar to 230 Ma granitoids in the TW area represented by sample JPC12-02 have higher initial Sr-87/Sr-86 (0.7073) and lower epsilon(Nd)(t) (-6.2) and epsilon(Hf)(t) (-4.5) values, offering an ideal opportunity for future studies on tectonic effects of juxtaposition of younger orogenesis on an older orogen.