Ediacaran‐Cambrian intra‐oceanic arc volcanic rocks in southern West Junggar, NW China: New constraints on the initial subduction of the Junggar‐Balkhash Ocean and migration of arc magmatism

Abstract

As numerous blocks of the Ediacaran‐Cambrian supra‐subduction zone ophiolites, seamounts, subduction‐related metamorphic rocks, and arc granitoids have been identified in the ophiolitic mélanges in the contiguous West Junggar (China) and Balkhash (Kazakhstan) area, the initial subduction of the Junggar‐Balkhash Ocean, a major branch of the southern Palaeo‐Asian Ocean is emerging as an interesting topic. In this study, we present new data from diabase and basalt xenoliths in an Ediacaran‐Early Cambrian quartz‐monzonite block within the Tangbale ophiolitic mélange and an isolated dacitic tufflava block within the Saleinuohai ophiolitic mélange in southern West Junggar, NW China, as another line of evidence for the Ediacaran initial subduction of the Junggar‐Balkhash Ocean and the Cambrian migration of arc magmatism in West Junggar. The diabase and dacitic tufflava yield concordant zircon U–Pb ages of 568 and 530 Ma, respectively. The diabase and basalt exhibit high Al2O3 (17.38–17.75 wt%) and low TiO2 (0.69–0.90 wt%), with Mg# = 47–55, whereas the dacitic tufflava is characterized by high Na2O (7.87–8.43 wt%), with Mg# = 46–49. All the samples display similar LREE and large‐ion lithophile element (LILE) enrichment and Nb, Ta, Ti, and varying Zr and Hf depletion, indicating that they were formed in an intra‐oceanic arc setting. An integration of the new and published data shows that the subduction of the Junggar‐Balkhash Ocean was initiated in the Late Ediacaran at least, followed by slab rollback and retreat of subduction zone, which would be responsible for the northward migration of arc magmatism.