الملخص الإنجليزي
Many ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra-subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra-oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet-Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault-bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The * Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep-water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back-arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back-arc basin. The ophiolite and accretionary complex constitute a Cambrian-early Ordovician trench-arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block.
