PALEOZOIC DOCKING OF TACHENG OCEANIC PLATEAU INTO WESTERN JUNGGAR, NW CHINA AND CONTRIBUTION TO ARCHITECTURAL HETEROGENEITY OF WESTERN ALTAIDS

Western Junggar, NW China, locates along-strike between the Boshchekul-Chingiz (BC) Range and Eastern Junggar and Altai belts. We present detailed mapping, structural, and geochemical analyses of the Barleik-Mayile-Tangbale Complex (BMTC) to south of Tacheng block in Western Junggar. The Complex is divisible into Zones I, II and III, which are mainly composed of Cambrian-Silurian rocks. Their studies show that there has a radial outward transition from coral-bearing shallow marine (shelf) to deep ocean (pelagic) environments, and from OIB/E-MORB to N-MORB lava geochemistry away from the Tacheng block. Positive εNd(t) values of volcanic rocks from the Tacan-1 drill-core, and positive εHf(t) values and post-Cambrian ages of detrital zircons from Silurian and Devonian strata indicate that the basement of Tacheng block is a juvenile lithosphere. The BMTC thrust onto Silurian sediments forming imbricate thrust stacks. The tectonic vergence of overturned folds in cherts, drag-related curved cleavages and σ-type structures on the thrust surface suggests top-to-the-NW transport. Comparisons with published data suggest that these positive isotopic values, stratigraphic, structural and geochemical relationships can analogy with the relationships between the Ontong Java oceanic plateau and the Pacific oceanic crust. Therefore we propose that the Tacheng block was an Early Paleozoic oceanic plateau. The BMTC is unconformably overlain by Devonian limestone and conglomerate, suggesting that the Tacheng oceanic plateau initially collides with the Western Junggar during Silurian-Devonian. Western Junggar, BC Range and Eastern Junggar and Altai have parallel strikes. Regional studies suggest that, to north of Tacheng, subduction continues until Late Paleozoic. Paleomagnetic studies on the Carboniferous and Permian dikes and sediments show that the southwest and southeast sides of Tacheng oceanic plateau experience clockwise and anticlockwise rotation, respectively. This changes the strike of Western Junggar to NE-SW. The direction is different from its along-strike counterparts in NW-SE extension and the Tacheng oceanic plateau is distinct with accretionary complexes in BC Range and Eastern Junggar and Altai. They typically display orogen-parallel architectural heterogeneity of western Altaids.