Geochronological and geochemical constraints for a middle Paleozoic continental arc on the northern margin of the Tarim block: Implications for the Paleozoic tectonic evolution of the South Chinese Tianshan

Abstract

In order to reveal the tectonic evolution of the South Chinese Tianshan orogenic belt, we conducted structural, geochemical, and geochronological studies and identified granitic and volcanic rocks along the northern margin of the Tarim block. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of two samples from granitic plutons yielded crystallization ages from 404.8 ± 2.0 Ma to 388.1 ± 2.2 Ma, and two samples of dacite and rhyolite yielded ages of volcanism at ca. 404 Ma, highlighting a middle Paleozoic igneous event. Geochemical data suggest that these Devonian igneous rocks are metaluminous, high-K calc-alkaline felsic volcanic and plutonic rocks. All the samples display relatively enriched light rare earth element (LREE) contents, weak or no negative Eu anomalies, and relative depletion in Nb, Ta, Ti, and P, but they show enrichment in K, Rb, and Ba. In situ zircon Hf isotopic results show a positive range of e Hf (t). These geochemical and isotopic features suggest that this magmatic event probably occurred in an Andean-type arc. Combined with zircon U-Pb and Hf isotopic studies on the basement rocks, the results indicate multiphase Precambrian (ca. 2600–2300 Ma, 1900–1800 Ma, 1140–830 Ma) continental growth and minor involvement of basement in Devonian arc magmatism. Finally, we integrated the structural, geochemical, and geochronological data into a geodynamic model of the South Chinese Tianshan that emphasizes south-directed subduction of the South Tianshan Ocean along the northern margin of the Tarim block during the early to middle Paleozoic.