LA-ICP-MS zircon U-Pb dating and Hf isotope geochemical characteristics of Qaidamshan intrusive body

Qaidamshan granite is one of the biggest intrusions in the northern margin of Qaidam Basin. To determine the genesis of Qaidamshan granite and its relationship with the Early Paleozoic orogenic movement in northern Qaidam, the authors applied LA-ICP-MS technique to conducting zircon U-Pb dating and Hf isotope compositions measuring. Augen granite and granodiorite of Qaidamshan intrusion have weighted average 206Pb/238U age of 436.9Ma±2.8Ma and 440.8Ma±7.3Ma respectively, with their Th/U ratios both lower than 0.2, which represent the magmatic formation age. Augen granite and granodiorite have similar zircon Hf isotope geochemical characteristics: of the 35 zircon Hf isotope measurements, 32 zircons have the data 176Lu/177Hf = 0.000735~0.002786, 176Hf/177Hf=0.282264~0.282439, εHf(t)=-8.5~-2.5, and TDM2=1.44~1.77Ga. It is shown that the Qaidamshan granite was formed in 437~446Ma, had multi-phase emplacement characteristics，and its protolith came from Qaidam plate basement. In combination with the Paleozoic tectonic evolution characteristics of northern Qaidam margin, the authors hold that the Qaidamshan intrusion was produced by continent-continent collision.