Journal of Natural Gas Geoscience (Apr 2020)
Petrological, He–Ne–Ar and Sr–Nd–Pb geochemical of volcanic rocks constraint on tectonic settings and geodynamic process of the Carboniferous, East Junggar
Abstract
The Karamaili ophiolite belt is an important tectonic unit in the East Junggar terrane, Xinjiang, China, and provides a material basis for the study of subduction, accretion and collision processes, regional tectonic evolution, and the exploration of oil (gas) reservoirs in the western part of the Central Asian Orogenic Belt (CAOB). In the late Paleozoic, an important tectonic transition from subduction to post–collisional extension occurred in the East Junggar terrane; however, the precise timing of the tectonic transition is still a matter of debate. The volcanic rocks in the Karamaili area, predominately rhyolite and basalt–andesite, are typical post–collisional volcanic rocks and thus provide important constraints on the tectonic setting of the East Junggar. Here, zircon U–Pb geochronology and He–Ne–Ar and Sr–Nd–Pb isotopic analyses were carried out on volcanic rocks from three localities in the Karamaili area. Four zircon U–Pb ages indicate that magmatism occurred in the Carboniferous at 335–310 Ma, which is interpreted as the age of post–collisional extension events. The 4He–3He/4He isotopic ratios of these samples are similar to those of the continental crust and show a weak contribution of mantle He, indicating the presence of mantle He in the orogenic belt. The 20Ne/22Ne, 21Ne/22Ne, and R/Ra–40Ar/36Ar ratios show that the air-like Ne and Ar are derived from dissolved atmospheric and continental crust sources in the subducted supracrustal rocks. Meanwhile, relatively low 87Sr/86Sr ratios of andesite and basalt (0.704–0.707) and high εNd(t) values (4.73–7.22) reveal the presence of depleted mantle components in their source regions. The heterogeneous isotopic signatures of the volcanic rocks in the East Junggar indicate that they formed via different processes or in multiphase settings, suggesting a significant tectonic transition from an island arc setting to a post-collisional extension regime during the Carboniferous period.
