Petrogenesis of Late Carboniferous-Early Permian mafic-ultramafic-felsic complexes in the eastern Central Tianshan, NW China: The result of subduction-related transtension?

Abstract

Knowledge of the Late Paleozoic accretionary processes responsible for the Altaids (or southern Central Asian Orogenic Belt) is critical for a better understanding of continental growth in Central Asia. Here, we systematic study the Late Carboniferous mafic-ultramafic-felsic complexes in eastern Central Tianshan to understand the tectonic evolution of the southern Altaids. Two gabbros yield zircon crystallization ages of 301.5 ± 2.3 Ma and 300.7 ± 2.6 Ma and one granite is dated as 301.6 ± 1.5 Ma. The gabbros are tholeiitic and characterized by typical subduction-related geochemical signatures with low TiO2 contents, enrichments in Rb, Ba, U, K, Pb and Sr, negative Nb and Ta anomalies, and negative to positive Eu and Ti anomalies. The granites are the A2-type granites with high contents of SiO2, K2O + Na2O, Ga, Zr, Nb, Y, HFSE, REE, and high ratios of Fe/Mg and Y/Nb (>1.2). The gabbros and granites have high εNd(t) values of +1.64 to +5.03, zircon εHf(t) values of −2.8 to +9.1, and low (87Sr/86Sr)i (0.704007–0.705437) values. These geochemical characters demonstrate that the rocks in the complexes are fractionation genesis and that the A-type granitic rocks are the highest fractionation facies of tholeiitic mafic rocks. These results suggest that they were originated from the asthenospheric garnet-free mantle in a supra-subduction zone. Combined with regional data, we suggest that the oblique subduction with slab break-off of the Southern Tianshan oceanic plate plausibly induced asthenospheric upwelling to form the tholeiitic magma, and which intrude along the strike-faults the Central Tianshan-Yamansu arc. Accordingly, these results reveal the Paleo-Asian Ocean continued to subduct at the southern Altaids in the Late Carboniferous.