Abstract:

 During the Indosinian period, small granitic bodies or stocks were wildly developed in the Songpan-Garze orogenic belt. This study focuses on the granitic body in Qiuzhi area of Qinghai Province, located along the southern margin of Songpan-Garze orogenic belt. The petrogenesis and tectonic setting of the granitic body are discussed through petrography, zircon U-Pb chronology, zircon Lu-Hf isotope, Sr-Nd isotope of whole rock and whole rock geochemistry. The results show that the granites in Qiuzhi area are mainly granodiorite, characterized by weak sodium enrichment (Na2O/K2O =0.97-1.34) and a low aluminum saturation index (A/CNK=0.99-1.08), thus classified as weak peraluminous calc-alkaline series granites. Rare earth element patterns show enrichment of light rare earth elements and depletion of heavy rare earth elements, along with a right-leaning distribution. The rocks are enriched in Rb, K,U, Th, La, Ce, Nd, Zr, Hf and Y, while being depleted in Ba, Sr ,Ta, Nb, P and Ti. The weighted average age of granodiorite zircon 206Pb/238U is (215.4±1.4) Ma (MSWD=1.05, n=23), indicating a Late Triassic diagenetic age. Zircon Hf isotopic values (176Hf/177Hf)i range from 0.282 377 to 0.282 437, with εHf(t) between -9.44 and -7.19, and two-stage model ages (TDM2) from 1 840 to 1 702 Ma. The initial Sr isotope values (87Sr/86Sr) i of whole rock range from 0.709 783 to 0.710 530, εNd(t) ranges from -8.2 to -7.8, and the two-stage model ages (TDM2) range from 1 659 to 1 630 Ma. These findings suggest that the magma source is the partial melting of the ancient crust material, exhibiting characteristics of calc-alkaline I type granite. By correlating regional geological data with the geochemical characteristics of the petrosphere, it is concluded that Qiuzhi granitic body is a product of lithospheric delamination, asthenosphere material upwelling, and lower crustal partial melting within the background of Late Triassic post-collision environment.

Key words: Songpan-Garze orogenic belt, post-collision, zircon U-Pb age, zircon Lu-Hf isotope, Sr-Nd-Hf isotope, Late Triassic, I-type granite, geochemistry