Radiolarian Biochronology, Detrital Zircon Geochronological and Geochemical Constraints on Provenance and Depositional Environment of Cherts in the Southern Belt of the Western Yarlung Zangbo Suture Zone, Tibet

Ophiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the western part of Yarlung Zangbo suture zone (YZSZ) in Tibet, which in turn is tectonically thrust over a volcanic-sedimentary sequence. Geochemical data and radiolarian fauna of cherts and detrital zircon ages of litho-quartz sandstones in the sequence provide robust constraints to elucidate the stratigraphic and paleo-depositional environments in which these rocks formed. Eight cherts from Purang, Dongbo, Daba Qu, East Daba, and Labuzha massifs in the SB reveal Late Jurassic–Early Cretaceous radiolarians; they are coeval with minimum detrital zircon U-Pb ages of 132 and 149 Ma, respectively, from two litho-quartz sandstones in the northwestern part of Purang massif. Thirty chert samples from five massifs geochemically show that they have high SiO2 contents of 86.51–95.93 wt%, and high mean ratios of Al/(Al+Fe+Mn) ranging from 0.59 to 0.78, indicating a nonhydrothermal, biogenic, and terrigenous origin. Ce/Ce* ratios of cherts range from 0.93 to 1.52, combined with claystone interlayered with radiolarian chert sporadically overlying litho-quartz sandstone and quartzose sandstone, suggesting a continental slope setting. Given the structural and stratigraphic evidences of ophiolites associated with sedimentary strata and no arc-related magmatism in the SB, we propose that SB ophiolites and ophiolitic mélanges represent southward-thrust nappes from the NB.