Geochronology and geochemistry of Late Ordovician–Early Devonian gneissic granites in the Kumishi area, northern margin of the South Tianshan Belt: Constraints on subduction process of the South Tianshan Ocean

Abstract

The South Tianshan Belt has recorded the evolution of the south branches of the Paleo-Asian Ocean and is pivotal for understanding the formation of the southernmost part of the Central Asian Orogenic Belt. In order to place constraints on the tectonic evolution of the South Tianshan Ocean, three highly deformed granitic plutons (monzonitic granites, biotite K-feldspar granites and two-mica granites) exposed in the Kumishi area were collected for geochronological and geochemical studies. Zircon LA-ICP-MS U–Pb dating of the gneissic granites from the three plutons yielded crystallization ages at 446.2±2.2Ma, 431.0±2.8Ma and 407.3±3.1Ma. All of these gneissic granites have geochemical characteristics of calc-alkaline igneous rocks. The biotite monzonitic granites (446Ma) are slightly peraluminous, LREE-enriched and show relatively flat HREE patterns mostly with negative Eu anomalies. The rocks have positive Pb and negative Nb, Ta, Ti anomalies. In combination with their negative εHf(t) values (−7.9 to −11.6) and the corresponding old TDMc ages (1.93–2.16Ga), we suggest that the monzonitic granites were mainly derived from partial melting of the middle-lower crust dominated by Precambrian basement rocks. The biotite K-feldspar granites (431Ma) are characterized by high K2O, low Fe2O3 and MgO contents. They show REE patterns similar to the monzonitic granites, as well as the Nb, Ta, Ti depletions, but have more enrichment in U, Pb, Zr, and Hf. Their depleted εHf(t) values (+0.7 to +5.4) and young TDMc ages (1.07–1.37Ga) imply significant input of mantle-derived juvenile materials, and thus suggest that the K-feldspar granites were possibly derived from a K-enriched crustal source mixed significantly with mantle-derived juvenile materials. The two-mica granites (407Ma) show a typical S-type granite affinity with high Al2O3/TiO2, low CaO/Na2O and K2O/Al2O3 ratios, suggesting a pelitic source. These characteristics, together with their positive εHf(t) values (−0.2 to +3.1) and relatively young TDMc ages (1.20–1.41Ga), indicate that the two-mica granites were most likely generated by partial melting of a pelitic source with less input of juvenile materials. All of the samples from the three plutons show same geochemical characteristics of felsic magmatic rocks formed in an arc-related tectonic setting. Together with the northward thrusting tectonics and the arc magmatic rocks exposed along the South Nalati–Kawablak Fault, a northward subduction of the South Tianshan Ocean is suggested to interpret the formation of the three granitic plutons. Taking into account the distinct tectonic polarity and distribution of magmatic rocks in the western and eastern South Tianshan Belt, we suggest that the South Tianshan Ocean probably underwent different subduction processes in its western and eastern parts. A single northward subduction and a divergent bidirectional subduction are further proposed to interpret the formation of the western and eastern South Tianshan Belt, respectively.