Provenance and tectonic setting of the Sumdo Formation in the Lhasa Terrane, Tibet: Implications for early subduction evolution of the Sumdo Paleo–Tethys Ocean

Abstract

The tectonic setting of the Tangjia–Sumdo metamorphic belt, which separates the North Lhasa Block (NLB) and South Lhasa Block (SLB), remains controversial. Moreover, the sedimentary evolution of the Tangjia–Sumdo metamorphic belt has been little studied, particularly during the early subduction evolution of the Sumdo Paleo-Tethys Ocean (SPTO). Here we present petrological, geochemical, and zircon U-Pb geochronological data on the clastic rocks and metabasaltic interlayers collected from the Sumdo Formation in the Sumdo area. The Sumdo Formation consists of metamorphosed sandstone, graywacke, phyllite, and interlayered metabasalts. Detrital zircon U-Pb dating of the metasandstone and quartz schist identified a youngest zircon age group of 389–316 Ma, which have negative εHf(t) values (−20.5 to −1.5) and old Hf crustal model ages (TCDM = 1450–2654 Ma). The zircon trace element features and Hf isotopic compositions show that the sedimentary detritus was sourced from the NLB, which indicates that this terrane experienced Late Devonian–early Carboniferous magmatism. The ages of the youngest detrital zircons (YC2σ age of 324 ± 13 Ma) indicate that the Sumdo Formation formed mainly during the late Carboniferous–early Permian. Combined with geochemical data of the metabasalts and other regional geological evidence, we conclude that the Sumdo Formation formed in an initial forearc basin. The Sumdo Formation was likely deposited along an active continental margin according to the in–situ suture model, where northward subduction of the Sumdo Paleo–Tethys oceanic lithosphere occurred beneath the NLB. As such, the Sumdo Formation provides a sedimentary record of the early subduction evolution of the SPTO.