Late Cretaceous transition from subduction to collision along the Bangong-Nujiang Tethys: New volcanic constraints from central Tibet

Abstract

This study deals with arc-type and subsequent bimodal volcanic rocks interbedded with (late) Cretaceous sedimentary formations near Gaize, central Tibet that shed new light on the Tethyan evolution along the Bangong-Nujiang suture. Unit I consists of trachyandesites interbedded with fine-grained sandstone, slate, and limestone. Zircon dating on a trachyandesite sample yields a 206Pb/238U age of 99±1Ma. The trachyandesites are characterized by strong enrichment in LILE but depletion in HFSE, low zircon saturation temperatures (TZr=642–727°C), and high oxygen fugacity (∆FMQ=−0.67–0.73), indicating their arc affinities. Unit II comprises a bimodal basalt-rhyolite suite interbedded with coarse-grained sandstone and conglomerate. Zircon dating on two rhyolitic samples yield 206Pb/238U ages of 97.1–87.0Ma. In contrast with Unit I trachyandesites, Unit II basalts and rhyolites exhibit OIB-like trace element patterns, high temperatures (T=1298–1379°C for basalts, TZr=855–930°C for rhyolites), and low oxygen fugacity (∆FMQ=−3.37~−0.43), suggesting that Unit II bimodal volcanic rocks probably formed in an extensional setting. The Sr-Nd isotopes of both Unit I (87Sr/86Sri=0.7052–0.7074, εNd(t)=−2.21–1.02) and Unit II (87Sr/86Sri=0.7057–0.7098, εNd(t)=−3.22–0.88) rocks are similar to mantle-wedge-derived volcanic rocks within the southern Qiangtang block. The parental magma of Unit I trachyandesites was formed by fluid induced melting of the mantle wedge above the subducted Bangong-Nujiang Tethyan slab, and contaminated by crustal materials during MASH process within a deep crustal hot zone; and Unit II bimodal volcanic rocks were derived by melting of upwelling asthenosphere and a mildly metasomatized mantle wedge during the Lhasa-Qiangtang collision. We propose that the transition from the Bangong-Nujiang Tethyan subduction to the Lhasa-Qiangtang collision occurred during the Late Cretaceous in central Tibet.