SILURIAN-DEVONIAN INTERACTION OF SUB-PARALLEL SPREADING RIDGE AND TRENCH IN THE WESTERN JUNGGAR AND NORTH TIANSHAN, NW CHINA: IMPLICATION FOR THE MIDDLE PALEOZOIC OROGENIC ARCHITECTURE OF THE WESTERN ALTAIDS

The Western Junggar and North Tianshan in NW China located at the western Altaids or Central Asian Orogenic Belt (CAOB), which was generated by closure of the Paleo-Asian Ocean. They constitute a subduction system since Early Paleozoic and Silurian trench turbidites in the Western Junggar accretionary complex suggest that the subduction lasts until Middle Paleozoic and the coeval arc-related magmas occur in the North Tianshan. At Late Silurian-Early Devonian, the younger episode of backarc basin basalt-like ophiolites occur along the Durbut fault and Baijiantan-Baikouquan areas, and magmatic arc-like rocks, some of which have adakitic properties, intrude and extrude in the Early Paleozoic accretionary complex in the Western Junggar. Meanwhile, the oldest ophiolite in the North Tianshan generates at 386 Ma, younger than the continental margin arc-related magmatism in the North Tianshan and the Western Junggar ophiolites. The arc magmatism and metamorphism paused in the Western Junggar at 350-379 Ma; while continental margin arc-related magmatism and deformation in the North Tianshan were continuous.

Above facts indicate that at Late Silurian-Early Devonian, there develop double magmatic belts, which were separated by back-arc basin basalt-type oceanic crust in the Western Junggar and North Tianshan. Here we interpret this orogenic architecture as a tectonic model of ridge-trench interaction. The spreading ridge sub-parallel subducted beneath former accretionary complex, generating a magmatic belt in the forearc in the Western Junggar and another continental margin arc in the North Tianshan. The Silurian-Devonian back-arc basin basalt-type ophiolites in the Western Junggar and North Tianshan, which have similar ages of the double magmatic belts, generated when the buoyant subducted spreading ridge float back to split the overlaying Western Junggar forearc as a new passive continental margin, where have a magma stagnant period at 350-379 Ma. Meanwhile, the trench has migrated to south of the young ocean and continuously generate continental margin arc-related rocks in the North Tianshan. This orogenic architecture can compare with that of the Miocene-Present ridge-trench interaction in the Western California and Baja California Peninsula in southwest of North American plate.