U-Pb zircon SHRIMP ages, geochemical and Sr-Nd-Pb isotopic compositions of intrusive rocks from the Longshan-Tianshui area in the southeast corner of the Qilian orogenic belt, China: Constraints on petrogenesis and tectonic affinity

摘要

The Longshan-Tianshui area is located in the southeast comer of the Qilian orogenic belt. Three intrusions (the Yanjiadian diorite, the Caochuanpu and the Guanshan granites) in this area provide insights into regional tectono-magmatic events, petrogenesis and tectonic evolution. Based on U-Pb zircon SHRIMP dating, the magma emplacement ages of the Yanjiadian diorite, the Caochuanpu and the Guanshan granites are 441 ± 10, 434 ± 10 and 229 ± 7 Ma, respectively. The Yanjiadian diorites have geochemical characteristics similar to island-arc-type rocks, but their formation resulted from northward subduction of the Huluhe (Erlanping) back-arc basin. The Caochuanpu granites with K_2O/Na_2O < 0.9, Y of < 6.9 ppm, Yb of < 0.39 ppm, Sr/Y of 36-97 and (La/Yb)_N of 34.8-80.0 exhibit adakitic geochemical signatures. Combined with Pb-Sr-Nd isotope data, the magma for the Caochuanpu granites was suggested to be derived from partial melting of previously underplated Neoproterozoic mafic rocks in the Qinling (or Longshan-Tianshui) island-arc lower crust. The partial melting resulted from island-arc lower crustal thickening due to northward subduction of the Qinling ocean and closure of the Huluhe back-arc. The Guanshan granites with (La/Yb)_N of 17.5-42.4, Sr/Y of 15.8-50.5, Y of 7.8-15.7 ppm and Yb of 0.64-1.31 ppm also display some adakitic geochemical signatures. However, the geochemical and Pb-Sr-Nd isotopic compositions of the Guanshan granites are quite distinct from those of the Caochuanpu granites. It is proposed that magma for the Guanshan granites was derived from partial melting of subducted South Qinling-West Qinling continental crust during Triassic continental collision between the North China and the South China plates. The result provides an example that the adakitic rocks can be generated from partial melting of subducted continental crust. The tectono-magmatic events and petrogenesis of intrusive rocks in the Longshan-Tianshui area are in agreement with those of the North Qinling tectonic unit in the Qinling orogenic belt. Therefore, the Qilian orogenic belt corresponds to the North Qinling tectonic unit in a regional tectonic framework. Both the Qilian and the Qinling belts had a similar tectonic evolution.