Geochemical and 40Ar-39Ar isotope chronological characteristics of Wugongshan Indo-Chinese granites in central Qiangtang, Tibetan Plateau

Bensongcuo batholith occupies an area of over 1800km2 and is the greatest granite multiple batholith in central Qiangtang. It consists of granites of three different ages (Carboniferous, Triassic and Jurassic) and possesses material records of magmatism of different epochs, so it can serve as a key showcase for studying the tectonic setting of Qiangtang basin. Wugongshan granites, located in the north of Bensongcuo batholith, were previously thought to have been formed in Jurassic; nevertheless, a few Indo-Chinese granites composed mainly of monzogranite and granitic gneiss were recently found in Wugongshan area, which may be inclusions of Jurassic granites. Geochemical studies indicate that monzogranites are high-potassium peraluminous calc-alkaline granites formed in collisional tectonic setting, similar to other Indosinian intermediated-acid rocks in this area. These intermediated-acid rocks compose the collisional or post-collisional magmatic arc of the Lungmu Co-Shuanghu-Lancang River suture zone. Moreover, 40Ar-39Ar dating was carried out for biotites from the granite gneiss and the dating results show that the granitic gneiss resulted from the reformation of Indo-Chinese granites by Jurassic magmatism at 175.8±1.1Ma, and hence the Bensongcuo batholith should serve as the material record of multi-stage intrusion of intermediate-acid magmatic rocks.