Triassic magmatic reactivation in Eastern Tianshan, NW China: Evidence from geochemistry and zircon U-Pb-Hf isotopes of granites

摘要

Triassic magmatic and tectonic reactivation has played a crucial role in remolding the tectonic framework of Eastern Tianshan, NW China, but is poorly understood. Triassic granites in Eastern Tianshan contain magmatic zircons with euhedral to subhedral shapes and concentric oscillatory zoning structures. These zircons have relatively high Th/U ratios (mostly 0.26-0.93) with LA-ICP-MS U-Pb ages of ca. 250-223 Ma, interpreted as the emplacement ages. The granites have high SiO2 (71.1-77.1 wt.%), Na2O (3.1-3.7 wt.%) and K2O (4.0-4.7) wt.%, but low Al2O3 (11.8-14.7 wt.%), CaO (1.4-2.3 wt.%), FeOt (1.2-2.2 wt.%) and MgO (0.5-1.0 wt.%) contents, and belong to high-K calc-alkaline I-type granite series. They show low Ni (5.2-32.4 ppm) and Cr (6.4-59.4 ppm) contents and low Ti/Zr (8.5-16.3), Ti/Y (58.3-214.6) and Nb/Ta (8.6-15.2) but high Th/Nb (1.0-1.9) and Th/Ta (9.4-26.2) ratios, suggesting that they were most probably derived from partial melt from late Paleozoic crustal rocks, consistent with Hf isotopes of Triassic zircons falling onto the continental evolution lines. Compositional variations of the granites, such as distinct Eu anomalies (8Eu = 0.46-0.52 or 0.66-0.84) and flat or depleted heavy rare earth element (HREE) patterns, indicate variable amounts of residual phases, plagioclase or garnet/amphibole, during partial melting. The apparent subduction-related characteristics, e.g., fractionated FtEE patterns, strong enrichments in large-iron lithophile elements (e.g., Rb, Th and U) and depletions in high field strength elements (e.g., Nb, Ta and Ti), were probably inherited from the crustal sources. Combining our results with those of previous studies, we suggest that Triassic magmatic and tectonic reactivation in Eastern Tianshan and coeval tectonothermal events in the adjacent regions were possibly dynamically linked to the subduction and/or closure of the Paleo-Tethys Ocean.