Sedimentology of trench-arc sediments and its relation to ophiolite obduction

Abstract

The eastern Himalaya in Xigaze District of southern Tibet has well developed Cretaceous-early Paleogene trench-arc sediments deposited coevally with Gandise magmatic/volcanic arc phase, generated by Tethyan subduction. The Congdu Formation, composed of ophiolitic greywacke turbidite interbeded with volcanic tuff and pelagic radiolarian chert, represents typical trench-floor sediments deposited on the top of a younger oceanic crust composed of pillow lava with thin-bedded chert cover of late Albian to early Cenomanian age. The Cretaceous olistostromes (Zongzhuo Formation) probably of Albian age might be earlier trench-slope deposits. The trench turbidite together with slope olistostromes may constitute the residual accretionary prism of a trench-subduction complex formed before collision. The well known Xigaze Flysch, partly submarine and partly subaerial, rests on the continental crust and was accumulated in the forearc basin as Arc-trench gap sediments during late Aptian to Maestrichtian. It is unlikely that a presumed oceanic crust would be the base of Xigaze Basin. The Yarlung Zangbo ophiolite is suggested to be an older oceanic crust and mantle of upper Jurassic to pre-Albian age because of the radiolarian microfossils associated with the thick-bedded red chert (Lhaze Group). Ophiolites occurring as large thrusted ultramafics, dismembered altered basalts and radiolarite slices, obducted along the suture and formed the tectonic ophiolite melange. The Zangbo Suture Zone involves mainly the Congdu trench turbidite and Zongzhuo olistostromes in addition to obducted ophiolite melange. In other words, the Zongbo Suture is mainly a trench-subduction complex with obducted ophiolite melange system. No evidence exist to suggest that the Xigaze forearc basin sediments were involved either in the ophiolite belt or in the suture zone. Qiuwu Molasse of Eocene age, which unconformably overlies the Gandise granitoids, indicates that the collision occurred during the end of the Cretaceous or at the beginning of the Paleocene. The red subaerial Liuqu Conglomerate of Oligocene to Miocene age that uncomformably overlying the ophiolite, further suggests the ophiolite obduction to be pre-Oligocene. It is nearly contemporary with the continental collision of the early stage. Gandise granitoids in the Trans- Himalaya might be the root of the magmatic arc that followed the present Suture Zone to the north. These granitoids range in age from 120 to 40 Ma and are coeval with intermediate to acidic volcanics of Lingzizong Formation occurring to the north of the Gandise plutons and may correspond to Dras island arc volcanics in Ladakh.