Cenozoic tectono-magmatic and metallogenic processes in the Sanjiang region, southwestern China

Abstract

The Sanjiang region in SE Tibet Plateau, and the western Yunnan region in southwestern China constitute a collage of Gondwana-derived micro-continental blocks and arc terranes that were accreted together after the closure of the Paleotethys Oceans in Permo-Triassic. The lithospheric structure in Sanjiang prior to the Cenozoic was dominantly characterized by sub-parallel sutures, subduction-modified mantle and crust, Mesozoic basins between the sutures, and primary polymetallic accumulations. During the Cenozoic, intense deformation, episodic magmatism, and diverse mineralization occurred, jointly controlled by the underthrust of South China lithosphere and the subduction of Pacific plate to the east, the India–Eurasia continental collision and the subduction of Indian oceanic plate to the west. In this paper, we identify the following four main phases for the Cenozoic evolution in the Sanjiang region. (i) Subduction and rollback of Neotethyan oceanic plate before ca. 45–40Ma caused lithosphere shortening, indicated by folding-thrusting in the shallow crust and horizontal shearing in middle crust, and multiple magmatic activities, with associated formation of Sn ore deposits in the Tengchong block, Cu polymetallic ore deposits within Mesozoic basins, and Mo and Pb–Zn ore deposits in the Cangyuan area nearby the Changning–Menglian suture. (ii) Breakoff of Neotethyan slab in 45–40Ma in combination with the India–Eurasia continental hard collision caused the diachronous removal of the lower lithospheric mantle during 42–32Ma, with the resultant potassic–ultrapotassic magmatism and formation of the related porphyry–skarn ore deposits along the Jinshajiang–Ailaoshan suture. (iii) Underthrusting of the South China plate resulting in the kinking of Sanjiang, expressed by block rotation, extrusion, and shearing in the southern Sanjiang during 32–10Ma, with contemporary formation of the orogenic gold deposit along shear zones and the MVT Pb–Zn ore deposits within Mesozoic basins. (iv) Subduction of Indian oceanic plate possibly together with the Ninety East Ridge caused the local extension and volcanism in western Sanjiang, and the interplay between India–Eurasia collision and the Pacific plate subduction induced tensile stress and mantle perturbation in eastern Sanjiang from ca. 10Ma to present. The Cenozoic tectonic process traces a continuum of lithosphere shortening, sub-lithosphere mantle removal, and lithosphere underthrusting. During the lithospheric mantle removal, the simultaneous melting of the metasomatized lithospheric mantle and juvenile lower crust with possible metal enrichment contributed to the formation of potassic–ultrapotassic intrusive rocks and related porphyry–skarn mineralization. It is proposed that the kinking in the Sanjiang region was controlled by the non-coaxial compressions of the South China block and India continent, which are much larger in size than the blocks in Sanjiang. The underthrust continental lithosphere of the South China block caused the formation of orogenic gold deposits due to the release of metamorphic fluids from the front of the underthrust zone and the development of MVT Pb–Zn deposits via fluid circulation in the farther metal-enriched Mesozoic basins. Our study reveals that the pre-Cenozoic lithospheric structure in Sanjiang played an important role in the styles of tectonic movement, the nature and spatial distribution of magmatism, and the large-scale metallogeny during the Cenozoic.