SANUKITOID DYKE SWARMS OF WESTERN JUNGGAR RECORD LATE CARBONIFEROUS RIDGE-TRANSFORM SUBDUCTION IN THE ALTAIDS (CENTRAL ASIAN OROGENIC BELT)

The Western Junggar segment of the southern Altaids is composed of Paleozoic arc volcanic sequences, accretionary complexes, ophiolites, felsic plutons and intermediate-mafic dykes. Four stages of deformation within the Western Junggar accretionary complex are identified. D1 is characteristic of regional symmetric-asymmetric folds in NE-trending (axial surfaces), and imbricated thrusts with NW vergence, reflecting a NW-SE shortening during subduction of the Darbut oceanic plate. D2 is recorded by km to 10-km scale EW-trending folds, partly superposed on the D1 folds. This implies the maximum shortening direction changed from NW-SE to N-S due to clockwise rotation of the convergence vector, inducing a sinistral shear along the Darbut fault. D3 is preserved locally in Permian conglomerates as EW-trending folds due to a weaker N-S shortening. D4 is restricted to the Darbut fault zone, which was active as a dextral strike-slip fault and dragged the core of D3 folds. D1 and D2 were developed between about 308 and 301 Ma; bracketed by the youngest detrital zircons within tuffaceous strata and the zircon crystallization age of undeformed granitic plutons. Two sets of dioritic dyke swarms with orthogonal strikes were emplaced into the accretionary complexes just after the D2 deformation. Hornblende diorite dykes are relatively high SiO₂, and show elevated MgO, Na₂O, K₂O, Mg^#, Ni, Si and Cr contents, La/Yb ratios, very low Y contents. These dykes are also enriched in large ion lithophile and light rare earth elements, which are comparable with those of sanukitoids or adakites. Low (⁸⁷Sr^/86Sr) ratios (0.70366 and 0.70381) and high positive ε_Nd (t) (+6.6 to +8.4) values indicate they were originated from a depleted mantle. These Late Carboniferous dioritic dykes were probably produced by mixing of depleted mantle components that derived from upwelling of asthenosphere through a slab window, and melts that originated by dehydration of subducted oceanic lithosphere. Cross-cutting relations of the dyke swarms reveal that NW/SE-trending dykes are older than NE/SW-trending dykes. The NE/SW-extension was probably associated with a NW/SE-trending slab window, followed by a slightly later NW/SE-extension, which was associated with a NE/SW-trending slab window, a model consistent with a subducted ridge-transform system.