Tectonic evolution of circum-Rodinia subduction: Evidence from Neoproterozoic A-type granitic magmatism in the Central Tianshan Block, northwest China

Abstract

Neoproterozoic igneous rocks associated with the assembly and configuration of the Rodinia supercontinent are widely distributed in the Central Tianshan Block, southwestern Central Asian Orogenic Belt (CAOB), northwest China. The Central Tianshan Block is an important geological unit in tectonic reconstructions of the CAOB and Rodinia. However, the early Neoproterozoic tectonic framework and evolution of the Central Tianshan Block are poorly understood. Here we present a geochronological, mineral and whole-rock geochemical, and Nd–Hf isotopic study of early Neoproterozoic augen granites from the Bingdaban area. In situ zircon U–Pb dating shows that the augen granites have crystallization ages of 1002–992 Ma. The augen granites have high SiO2 (70.46–76.24 wt%) and K2O (3.36–4.96 wt%) contents, and FeOT/(FeOT + MgO) ratios (0.76–0.84), low Mg# values (29–39), slightly enriched light rare earth element patterns, enriched large-ion lithophile elements, depleted high-field-strength elements (e.g., Nb, Ta, and Ti), and high 10,000 × Ga/Al ratios (2.3–8.3) and Zr + Nb + Ce + Y contents (256–408 ppm). The augen granites have geochemical features typical of A2-type granites, variable εNd(t) values of –1.64 to +6.87 and Nd model ages of 1.78–1.08 Ga, and zircon εHf(t) values of −1.96 to +6.42 with Hf model ages of 1.98–1.48 Ga. These data indicate the granites had a juvenile crustal-dominated source, with subordinate input of ancient crustal components, and formed in a subduction-related extensional setting. The extension might have been related to slab break-off beneath Rodinia. The early Neoproterozoic igneous rocks in the Central Tianshan and adjacent microcontinental blocks in the southwestern CAOB formed at or near an active continental margin, possibly part of the circum-Rodinian subduction zone during or after the assembly of Rodinia. Similar Neoproterozoic magmatism in the Central Tianshan and Yili blocks suggests that they had a comparable crustal evolution and a close tectonic and paleogeographic relationship during the Neoproterozoic. However, significant age and isotopic differences characterize the Central Tianshan–Yili blocks and Tarim Craton, suggesting that the Central Tianshan–Yili blocks were not separated from the Tarim Craton during the early Neoproterozoic.