Recognition Of The Bangong Lake MOR-and SSZ-type Ophiolites In The Northwestern Tibet Plateau And Its Tectonic Significance

The Bangong-Nujiang ophiolite zone, located on the Northern Tibet, extends eastward from the Bangong Lake in the Ngari, Tibet via Gerze, Dongqiao to Dengqen, and then southward entering Burma along the Nujiang River. It is over 2000 km in total length and serves as an important boundary between two different terranes in tectonics in Tibet plateau. The Bangong Lake ophiolite is located in the westmost part of the zone, and acts as a key"linker"between west and east Tethys ophiolitic belts.The Bangong Lake ophiolitic massif is represented by two tectonically distinct ophiolitic units: (1) the mantle peridotite unit, mainly including the brecciform Cpx-bearing and the massive Cpx-free harzburgite; and (2) the lava unit, including basaltic lavas (mainly composed of the brecciform basalts, pillow lavas, and massive lavas) and basaltic dykes and isotropic gabbroic dykes. MOR- and SSZ- type ophiolites are recognized in the Bangong Lake by the field work and the detailed study of petrology, mineralogy and geochemistry. MOR-type ophiolite is mainly composed of the brecciform Cpx-bearing harzburgite and brecciform basalts, while SSZ-type ophiolite is chiefly composed of the massive Cpx-free harzburgite and the pillow lavas, basaltic dykes and boninite series volcanic rocks.The brecciform Cpx-bearing harzburgite of MOR-type ophiolite is light serpentinized. Spinel is low Cr# value ranging from 20 to 25 (23 in average, < 60) and HREE of whole rock is consistent with abyssal peridotite, indicating that the brecciform harzburgite formed in the mid-ocean ridge and thought as a residue after 10~15% partial melting of a primordial mantle source. While, the massive Cpx-free harzburgite of SSZ-type ophiolite is heavily serpentinized. Spinel is high Cr# value ranging from 69 to 74 (>60) consisting with that in the Izu-Bonin-Mariana (IBM) peridotite, indicating that the massive harzburgite formed in the Supra-Subduction Zone, and thought as a residue after 35~40% partial melting of a primordial mantle source. The lava unit can geochemically be subdivided into four groups of rocks: (1) the brecciform basalts with high-Ti showing clear P-MORB affinity; (2) the pillow basaltic lavas and andesitic basalts with high-Ti showing Back Arc Basin (BAB) basalts affinity similar to N-MORB; (3) the intrusive basaltic dykes and isotropic gabbroic dykes with high-Ti affinity, but showing clear Island Arc Tholeiite (IAT) affinity; (4) the massive basaltic andesite, andesitic lava breccia, and andesite porphyrite with boninite affinity showing very low abundance of TiO2 (0.23~0.39 wt.%) with respect to above three groups.These different magmatic groups are believed form in Mid-Ocean Ridge (MOR) and Supra-Subduction Zone (SSZ). The first group with P-MORB affinity was formed in the background like the Mid-Atlantic Ocean Ridge. It constitutes MOR-type ophiolite together with the brecciform Cpx-bearing harzburgite. The latter three groups were formed in the setting from forearc to back arc basin within the Supra-Subduction Zone. And these different lavas originated from fractional crystallization from different primitive magmas, which were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extraction. Group 1 have derived from partial melting (ca. 10~15%) of a primordial mantle source, while group 2 and 3 basalts, basaltic dykes, and isotropic gabbro may have derived from partial melting (ca. 25~30%) of a primordial mantle source. Finally, the group 4 boninite series volcanic rocks may have derived from partial melting (ca. 35~40%) of a mantle peridotite previously depleted by primary melt extraction of the group 1 primary melts. They constitute SSZ-type ophiolite together with the massive Cpx-free harzburgite. And the radiolarian in the ribbon-cherts associated with the pillow lavas formed in the back-arc basin has been designed to the Late Jurassic. This suggests that the Bangong Lake SSZ-type ophiolite formed at least in the Late Jurassic.This paper also preliminarily discussed the process of lavas within the SSZ-type ophiolite according as the mechanism of the subduction slab rollback during the ocean-continent subduction. The Bangong Lake boninite oceanic crust was formed in the forearc extension basin caused by the induced corner flow. The subduction slab rollback due to mantle extrusion associated with mantle wedge flow caused the arc-trench retreat and back-arc extension during the continuous subduction where the pillow lavas and basaltic dykes with back-arc affinity were formed by the partial melting of the different depleted mantle wedge peridotites.By contrast, the Bangong Lake ophiolite is similar with the west Tethys typical ophiolite in age and genetic setting. This gives strong evidence support that the Bangong Lake ophiolite is a part of the Neo-Tethys ophiolite. Thus, the Bangong Lake ophiolite zone serves rather as a suture in intra-Gondwana continent than between Gondwana and Laurasia continents. In other word, the Bangong-Nujiang ophiolite zone is not the northern edge of Gondwana continent.