Early Cambrian oceanic crust in the Chinese North Tianshan: Evidence of the earliest subduction system within the southwestern Central Asian Orogenic Belt

Abstract

The long-lived accretion of the southwestern Central Asian Orogenic Belt is directly related to the subduction and demise of multiple Paleozoic oceanic basins. However, the evolution of those different oceans, and their relationships, remain poorly constrained. In this paper, we report on an Early Cambrian ophiolite discovered in the North Tianshan suture, China, which represents the oldest oceanic crustal fragments documented in the North Tianshan belt. Two gabbroic blocks collected from the Gangou ophiolitic mélange yielded consistent zircon U–Pb ages of ca. 521 Ma. These gabbros have similar whole-rock major and trace element and Sr–Nd isotopic compositions to those of gabbroic oceanic crust from mid-ocean ridges. In addition, high-An (∼92) plagioclase and high-Mg# (∼91) clinopyroxene indicate that the gabbros crystallized under hydrous conditions, likely in a subduction-related setting. Our data and those for Early Cambrian subduction-related ophiolites in the Junggar, Western Tianshan, and Beishan orogenic belts record the development of the earliest, large-scale, subduction system within the southwestern Central Asian Orogenic Belt, which was probably associated with assembly of Gondwana. This system may have also connected the Early Cambrian subduction systems between the Western Tianshan and Beishan belts.