RECYCLED OROGEN-DERIVED SANDSTONES IN THE MUGAGANGRI MÉLANGE, BANGONG-NUJIANG SUTURE ZONE, CENTRAL TIBET

In a mélange in convergent margins, sandstones dominated by volcanic arc-derived detritus are popular. Sandstones derived from recycled orogen seldom occur but their existence can provide important information about mélange formation. The Mugagangri siliciclastic mélange crops out along the E-W-trending Bangong-Nujiang suture zone (BNSZ) in central Tibet. In Selinco area, central part of the BNSZ, the matrix of the mélange consisted of fine-grained sandstone, siltstone and mudstone. The blocks include massive sandstones and dismembered oceanic strata of northward subducting Bangong-Nujiang Ocean (BNO). The sandstones in both matrix and blocks of the mélange can be divided into two groups based on the point-counting of thin sections and U-Pb ages of detrital zircons. The Group 1 is characterized by andesitic to felsic volcanic detritus and feldspar, with youngest U-Pb ages of detrital zircons (YDZ) peaking at 183-168 Ma. Based on regional geology, we interpret the Group 1 sandstones were derived from a continental arc in the Qiangtang and deposited during the Early to Middle Jurassic. The Group 2 is characterized by abundant quartz and appearance of sedimentary and metamorphic lithic fragments, with few feldspar and few YDZ clustering at the Permian to Late Triassic. We interpret the Group 2 sandstones were derived from a recycled orogen mainly in the Qiangtang.

Two alternative models are proposed to explain the formation processes of the Group 2 sandstones found in the Mugagangri mélange. In the first model, the Group 2 sandstones were deposited along the southern passive continental margin of Qiangtang during the Permian to Late Triassic. Later on they were trapped in accretionary wedges after the BNO initiated subducting northwards during the Early Jurassic. In the second model, the Group 2 sandstones were deposited during the late Middle to Late Jurassic when recycled orogen in Qiangtang was expanding due to the ongoing Lhasa-Qiangtang collision. In both models, the Group 2 sandstones can be correlated with those deposited contemporaneously in shallow marine environments on the southern Qiangtang. However, the second model is in conflict with the stereotyped view that the mélange is unconformably overlain by the Late Jurassic shallow marine Shamuluo Formation and should not be deposited and formed during the Late Jurassic.