Silurian-Devonian granites and associated intermediate-mafic rocks along the eastern Kunlun Orogen, western China: Evidence for a prolonged post-collisional lithospheric extension

Abstract

The eastern Kunlun Orogen hosts numerous granitoid intrusions and associated intermediate-mafic bodies, that bear significant information on the tectonic evolution of this orogenic belt and the genesis of many ore deposits (e.g., Xiarihamu NiCo sulfide deposit), but the timing and petrogenesis of these rocks remain not well understood. In this contribution, the petrogenesis of representative Silurian-Devonian granites and associated intermediate-mafic rocks at Wulonggou, Xiarihamu and Kayakedengtage has been investigated. Zircon UPb dating results reveal that these intrusions were emplaced over a period of ~50 million years (m.y.) from 427 Ma to 382 Ma. Syenogranites at Wulonggou and Xiarihamu as well as quartz monzonites and porphyritic monzogranites at Kayakedengtage have characters of metaluminous A2-type granites. Whole-rock SrNd isotope data indicate that Syenogranites at Wulonggou (427–414 Ma) and Xiarihamu (396–391 Ma) were probably derived from earlier calc-alkaline I-type diorite-granodiorite, whereas quartz monzonites (ca. 424 Ma) and porphyritic monzogranites (ca. 421 Ma) at Kayakedengtage were most likely produced by partial melting of Early Paleozoic granulites. Monzogranites (394 Ma) and granodiorites (397 Ma) at Kayakedengtage and dolerites (426 Ma) from Wulonggou are characterized by positive εNd(t) values, indicative of a depleted mantle origin. The formation of voluminous A2-type granites with mafic rocks from the asthenosphere since 427 Ma occurred immediately after the subduction, supporting a post-collisional setting and a slab break-off model. The extensive magmatism coupled with asthenospheric upwelling and rapid orogenic uplift suggests two large-scale extension events at 427–414 Ma and 403–380 Ma in the orogen. The 411–406 Ma Xiarihamu NiCo sulfide deposit formed during relatively weak lithospheric extension and asthenospheric upwelling between these two extensional events. These results together with 373–357 Ma metaluminous A2-type syenogranites and I-type granodiorites at Qimantagh suggest a prolonged (~70 m.y.) and episodic post-collisional extension in the orogen, which was controlled by cyclic asthenospheric upwelling.