PALEOZOIC COLLISIONAL TECTONICS AND MAGMATISM OF THE CHINESE TIEN-SHAN, CENTRAL-ASIA

The Chinese Tien Shan range is a Palaeozoic orogenic belt which contai ns two collision zones. The older, southern collision accreted a north -facing passive continental margin on the north side of the Tarim Bloc k to an active continental margin on the south side of an elongate con tinental tract, the Central Tien Shan. Collision occurred along the Qi nbulak-Qawabulak Fault (Southern Tien Shan suture). The time of the co llision is poorly constrained, but was probably in in the Late Devonia n-Early Carboniferous. We propose this age because of a major disconfo rmity at this time along the north side of the Tarim Block, and becaus e the Youshugou ophiolite is imbricated with Middle Devonian sediments . A younger, probably Late Carboniferous-Early Permian collision along the North Tien Shan Fault (Northern Tien Shan suture) accreted the no rthern side of the Central Tien Shan to an island arc which lay to its north, the North Tien Shan arc. This collision is bracketed by the Mi ddle Carboniferous termination of arc magmatism and the appearance of Late Carboniferous or Early Permian clastics in a foreland basin devel oped over the extinct arc. Thrust sheets generated by the collision ar e proposed as the tectonic load responsible for the subsidence of this basin. Post-collisional, but Palaeozoic, dextral shear occurred along the northern suture zone, this was accompanied by the intrusion of ba sic and acidic magmas in the Central Tien Shan. Late Palaeozoic basic igneous rocks from all three lithospheric blocks represented in the Ti en Shan possess chemical characteristics associated with generation in supra-subduction zone environments, even though many post-date one or both collisions. Rocks from each block also possess distinctive trace element chemistries, which supports the three-fold structural divisio n of the orogenic belt. It is unclear whether the chemical differences represent different source characteristics, or are due to different e pisodes of magmatism being juxtaposed by later dextral strike-slip fau lt motions. Because the southern collision zone in the Tien Shan is th e older of the two, the Tarim Block sensu stricto collided not with th e Eurasian landmass, but with a continental block which was itself sep arated from Eurasia by at least one ocean. The destruction of this oce an in Late Carboniferous-Early Permian times represented the final eli mination of all oceanic basins from this part of central Asia.