IMPLICATIONS OF PALEOZOIC OPHIOLITES FROM WESTERN JUNGGAR, NW CHINA, FOR THE TECTONICS OF CENTRAL-ASIA

Western Junggar, NW China, contains fragments of several Palaeozoic island arcs, accretionary prisms and ophiolites which were accreted to each other, and possibly to the southern margin of Eurasia, by the end of the Palaeozoic. The ophiolites of Western Junggar are important in reconstructions of these events. Four ophiolite belts are recognized, from south to north these are: Tangbale, Mayila, Dalabute and Barleik/Honggulueleng/Hebukesair. They record the production and destruction of oceanic tracts in this region from the Cambrian to the Carboniferous. The following characteristics are representative of them all. (1) The ophiolitic successions are strongly deformed. (2) Sheeted-dyke swarms are not preserved. (3) Basic igneous rocks show a range of chemical characteristics, from depleted, island arc tholeiite to ocean-island basalt. Altered N-type mid-ocean basalts maybe represented. (4) Large-scale thrusts, typically directed towards the southeast, lie at the base of many individual ophiolitic bodies. (5) The ophiolites rarely mark the position of possible sutures between arcs; they are normally highly deformed and imbricated with other lithologies. Accretion of Palaeozoic arcs and obduction of ophiolites was complete by the end of the Carboniferous, but the timing and sequence of events remain obscure. No continental collisions occurred during this period of assembly; crustal growth occurred by the amalgamation of non-continental blocks.