Subduction initiation of the western Paleo-Asian Ocean linked to global tectonic reorganization: Insights from Cambrian island-arc magmatism within the West Junggar, NW China

Abstract

<p>The subduction initiation associated with the beginning of accretionary orogens has been thought to be related to global plate reorganization. To characterize the initial subduction within the western Central Asian Orogenic Belt, this integrated study focuses on Cambrian tholeiitic to calc-alkaline plutons in the Barleik-Mayile-Saleinuohai area of West Junggar, NW China. Zircon U-Pb results of felsic plutons reveal a wide range (511–488 Ma) of ages with older ages up to 514–511 Ma. The felsic rocks exhibit variable SiO₂ (53.0–77.4 wt%) and K₂O (0.05– 2.24 wt%) contents and can be classified as diorite, granodiorite, trondhjemite, and tonalite. On the basis of their low TiO₂ (0.12– 0.71 wt%) contents and characteristic trace element trends as well as high zircon εHf(t) (+10.5 to +14.5) and mantle-like zircon δ¹⁸O (5.0 ± 0.48‰ to 5.4 ± 0.43‰, two standard deviations) values, we interpret that the Cambrian felsic rocks have diverse origins, involving differentiation of arc basalts and partial melting of subducted oceanic crust, arc mafic crust, and metasomatized mantle wedge. The Saleinuohai gabbroic pluton shows zircon δ18O ratios from 4.2 to 4.7‰, which are lower than those of igneous zircons in equilibrium with mantle and thus reflect modification of their mantle source by hydrothermal fluids with seawater-like oxygen isotopes at high temperature. Combined with regional data, we propose that the West Junggar arc represents the extending of the Boshchekul-Chingiz arc in the Early Cambrian, defining a long (>1000 km) E-W–trending subduction zone. The earliest island-arc tholeiitic felsic plutons in the West Junggar took place at ca. 514–511 Ma, which, coupled with other early subduction records (e.g., 530 Ma SSZ-type Kopu-relisay ophiolites) in the western Paleo-Asian Ocean, indicates that initial stages of subduction of the western Paleo-Asian Ocean probably occurred in the Early Cambrian. The simultaneity between the initial subduction of the western Paleo-Asian Ocean, Gondwana assembly, and Laurasia breakup suggests a causal link between the three, collectively correlated to a global plate adjustment event.<br></p>