Abstract:

The La-ICP-MS zircon U-Pb dating of the Shazaoquan quartz diorites in the southwestern margin of Alxa Block yields a
weighted mean age of 302±9.2 Ma. The rocks show SiO2 contents ranging from 60.66% to 61.37%, MgO from 2.13% to 2.51%, and high Na and low K contents (Na2O=4.20~4.46%，K2O=0.67~0.92%, K2O/Na2O=0.16~0.22), and high Al（Al2O3=17.98~18.44%）， Ca (CaO= 6.08%~6.59%), and Sr （582×10-6~620×10-6） contents, but low Y （11.4×10 -6~13.7×10-6） and Yb （1.02×10-6~1.51×10-6) contents.They also have high Sr/Y （35.15~53.25） , Y/Yb （11.00~11.23） and La/Yb (7.28~15.10) ratios. The quartz diorites also show enrichment in incompatible elements （Rb, Th, K, U, Pb etc.） and LREE, and strong depletion in high field strength elements, such as Nb, Ta and Ti, and HREE, with slight positive Eu anomalies (δEu=1.1). The geochemical characteristics of the studied dioritic rocks resemble those of the Cenozoic adakites from the Pacific margin. Together with the regional geological settings, we suggest that the Shazaoquan quartz diorites originated from the partial melting of the subducted basaltic oceanic crusts under the Alxa Block and underwent metasomatization with the surrounding mantle. It also suggests that the formation of Yabulai-Bayinnuoergong arc began in Late Carboniferous-Early Permian and the closure of the southernmost of the Paleo-Asian Ocean occurred between 275~250 Ma.

Key words: Adakite, petrology and geochemistry, zircon U-Pb age, Paleo-Asian ocean, Alax Block