Zircon U-Pb geochronology and geochemistry of syenogranites in western Niubiziliang,northwestern Qaidam,Qinghai

摘要

The study presents the results of U-Pb dating of zircons and whole-rock geochemical analyses of a syenogranite located in the western Niubiziliang area, China with the aim of determining its formation time, petrogenesis and the regional tectonic setting.Zircons within the syenogranite are euhedral-subhedral and display rhythmic growth zoning, indicating a magmatic origin.Zircon U-Pb data obtained by LA-ICP-MS indicate the syenogranite formed in the Late Permian (260.7±1.5 Ma).The w(SiO2) of syenogranites is 70.82%——73.59%, w(Al2O3) is 13.49%——14.82%, and w(Na2O+K2O) is 7.85%——8.52%, and yield K2O/Na2O ratios of 1.06-1.26.Therefore, the syenogranites belong to the high-K calc-alkaline and metaluminous (A/CNK<1, A/NK>1) series which display I-type granites similarly.The syenogranites also show the geochemical characteristics of volcanic arc rocks, being enriched in large-ion lithophile elements (LILEs;K, Rb) and light rare-earth elements (LREEs;La, Ce, Sm, Nd), but depleted in high field strength elements (HFSEs;Ta, Nb, P, Ti).On the whole, trace element ratios are close to the mean of the Earth's crust, indicating a lower-crust magma source.The low Sr (161-214)×10-6 and Yb (1.08-1.80)×10-6 concentrations indicate that plagioclase and hornblende are residual mineral phases in the source.The regional geology and whole-rock geochemistry suggest that the formation of the syenogranites was related to subduction of the Zongwulong Ocean crust, and the north margin of Qaidam Block during the Late Permian was in an active continental margin tectonic setting.