Sedimentary Evolution and Provenance of the late Permian-middle Triassic Raggyorcaka Deposits in North Qiangtang (Tibet, Western China): Evidence for a Forearc Basin of the Longmu Co-Shuanghu Tethys Ocean

The tectonic origin of the >500-km-long E-W trending Central Qiangtang metamorphic belt (CQMB), which separates the North Qiangtang block (NQB) and South Qiangtang block (SQB), remains controversial. Moreover, the coeval geological evolution of the southern NQB has been poorly investigated, particularly its tectonic relationship with the CQMB. Here we present stratigraphic, sedimentary, and provenance analyses of the late Permian-middle Triassic depositional succession at Raggyorcaka in the southern NQB and test two radically different hypotheses for origin of the CQMB. A complete marine transgression-regression sequence with two-sided provenance characterizes the late Permian-Triassic sedimentary rocks in the southern NQB. Sandstone petrological analyses reveal a prominent provenance transition to an active volcanic source beginning in the late Changhsingian. Detrital zircon U-Pb geochronological results of the transgression subsequence show a concentrated youngest zircon group of 236–288 Ma (peak at ~248.1 Ma), with negative εHf(t) values (−25.3 to −0.2) and large Hf crustal model ages (T^C _DM; 1,311–2,887 Ma). These new findings show that the Raggyorcaka sequence was most likely deposited in an active continental margin. Combined with other evidence, we further infer that the Carboniferous-Triassic successions of the southern NQB were most likely deposited in a forearc basin under the in situ suture model, that is, the northward subduction of the Longmu Co-Shuanghu Tethys Ocean beneath the NQB. Moreover, the detrital zircon age distribution of the southern NQB suggests that the NQB probably drifted from the Gondwana supercontinent in the early Paleozoic and became adjacent to peri-Cathaysian blocks no later than the Carboniferous.