Early Paleozoic magmatic record from the northern margin of the Tarim Craton: Further insights on the evolution of the Central Asian Orogenic Belt

Abstract

The Southern Tianshan Orogenic Belt (STOB) is an important region to understand the prolonged evolution of the Central Asian Orogenic Belt (CAOB). A comprehensive study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock elements and Sr–Nd isotopes was carried out for two intermediate-mafic intrusions in the Baicheng and Heiyingshan areas of Southwest Tianshan. LA–ICP-MS zircon U–Pb dating yielded crystallization ages of 431±3Ma (MSWD=0.073, n=31) and 430±3Ma (MSWD=0.104, n=30) for the Baicheng diorites, and 423.9±2.1Ma (MSWD=0.5, n=12) for the Heiyingshan gabbros. These data, in combination with other Late Silurian ages previously reported for the intrusive suites from the STOB, indicate an Early Paleozoic magmatic event in the region. In-situ zircon Hf isotope data on two samples from the Baicheng diorites show εHf(t) values of −2.4 to 4.9 and 0.2 to 6, and the Heiyingshan gabbros exhibit similar εHf(t) values from −1 to 1.8. The dioritic pluton exhibits Na-rich, arc-type geochemical and calc-alkaline affinity, and shows trace element patterns characterized by enrichment in large ion lithophile elements (Rb, Ba, K and Sr) and depletion in high strength field elements (Nb, Ta, Ti, Zr and Hf). These features resemble those of arc-type igneous rocks. Combined with initial 87Sr/86Sr ratios of 0.7047 to 0.7064 and εNd(t) of −1.6 to −5.06, we infer that the magma from which the Baicheng dioritic pluton formed was sourced from a mantle wedge which was subjected to metasomatism by fluids and melts related to subducted oceanic slab and the overlying sediments. The Heiyingshan gabbro shows a Na-rich nature and tholeiitic affinity. They not only display supra-subduction zone affinity, but also are geochemically similar to E-MORB, possible implying a back-arc basin setting. The initial 87Sr/86Sr ratios of the Heiyingshan samples range from 0.7046 to 0.7051 and εNd(t) values vary from 1.76 to 5.19. We speculate that the upwelling of the asthenospheric mantle to the supra-subduction wedge, facilitated by subduction roll-back, provided the source for the Heiyingshan gabbroic rocks. Together with other geologic evidence, we correlate the generation of the Baicheng diorite with the southward subduction of the South-Tianshan Ocean during the Early Paleozoic. The Heiyingshan gabbroic plutons were probably formed in a back-arc basin in response to the southward subduction of the Paleo-Tianshan Ocean. In combination with coeval arc-type magmatic rocks widely exposed in the southern margin of the Central Tianshan Block, a double-sided subduction model is proposed to explain the evolution of the South-Tianshan Ocean in the Silurian.