Tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the Sanjiang region, SW China

Abstract

The Sanjiang region in SE Tibet Plateau and NW Yunnan is known to have formed by amalgamation of Gondwana-derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from Paleozoic to Mesozoic. In this paper we provide a synthesis of tectonic evolution, magmatism and metallogeny in the region based on data from literatures. Early Paleozoic ophiolites (473–439Ma) in the Changning–Menglian belt indicate the existence of a Proto-Tethys ocean in this region. Two episodes of subduction-related magmatism in the early-Paleozoic, one occurred in the Baoshan and Tengchong blocks at 502–455Ma and the other occurred in the Simao block at 421–401Ma, are regarded as evidence for two different events of subduction of the Proto-Tethys ocean at different locations. The Proto-Tethys was succeeded in early-Devonian by the Paleo-Tethys which comprised the main ocean and three branches: Ailaoshan, Jinshajiang and Garzê–Litang. The Changning–Menglian main ocean existed from middle-Devonian to middle-Triassic. The remnants of the oceanic crust are preserved in a few places in the Longmu Tso–Shuanghu suture as well as in the Changning–Menglian ophiolite belt. The eastward subduction of the main oceanic plate from early-Permian to early-Triassic formed a prominent arc terrane stretching >1500km from Yunnan to eastern Tibet. From the waning stage of subduction to post-subduction, numerous S-type granite plutons with ages varying between 230 and 219Ma, such as the Lincang batholith in Yunnan were emplaced at or close to the suture. This event produced several hydrothermal W–Sn deposits in the region. The tectonic evolution and associated magmatism of the Jinshajiang and Ailaoshan branch oceans are generally comparable to those of the main ocean. However, the branch oceans were subducted westward instead. The Garzê–Litang branch ocean also underwent westward subduction from middle-Devonian to late-Triassic. Arc-related high Sr/Y porphyry intrusions and associated porphyry-skarn Cu–Mo–Au deposits are common in the Jinshajiang–Ailaoshan region, especially in the Yidun arc which formed prior to Jurassic. The VMS deposits in the Sanjiang region formed in diverse tectonic settings including middle-Silurian back-arc basins, Carboniferous oceanic islands, Paleozoic subduction zones and Triassic post-subduction rifting environments. The Mesozoic and early-Cenozoic evolution of the Baoshan and Tengchong blocks was largely influenced by eastward oceanic subduction of the Meso- and Neo-Tethys from late-Permian to middle-Cretaceous and from late-Cretaceous to ~50Ma, respectively. Abundant early-Cretaceous granitoids and associated skarn-type Pb–Zn and Sn–Fe deposits in the Baoshan and Tengchong blocks were produced in the background of the Shan boundary oceanic slab subduction to the west and the break-off of the Nujiang-Bitu oceanic slab to the north. The subduction of the Neo-Tethys oceanic plate beneath the Tengchong block from Late Cretaceous to Paleogene formed abundant S-type granitoids and many skarn-type and greisen-type Sn–W deposits. Granitoids formed at 105 to 81Ma and contemporaneous hydrothermal W, Mo, Ag and Au deposits, which temporally coincided with the subduction of the Neo-Tethys, are common in the Yidun arc terrane.