Bimodal magmatism produced by delamination: geochemical evidence from late Palaeozoic volcanic rocks from the Yili Block, Western Tianshan, Northwestern China

Abstract

The Western Tianshan orogenic belt is essential for understanding the evolution of the Central Asian orogenic belt. However, no agreement exists among geologists about its tectonic environment during the Late Palaeozoic. The volcanic rocks of the Yishijilike and Wulang Formation in the Yili Block, Western Tianshan, formed in the Late Carboniferous to Early Permian, mainly consist of a bimodal suite of basalts – basaltic andesites and rhyolites, with only some intermediate rocks. Mafic rocks are slightly enriched in light rare earth elements (LREE) and depleted in Nb, Ta, Zr and Hf, suggesting a subduction-modified depleted mantle source. Some mafic samples in the Early Permian bimodal volcanic rocks have high Ti contents with relatively high concentrations of Nb and high field strength elements (HFSE) and low contents of heavy rare earth elements (HREE). These rocks are similar to the continental flood basalts, which suggests that they formed from an asthenospheric mantle. This paper indicates that mafic members were created by the partial melting of the asthenospheric mantle material and subduction-modified lithospheric mantle mixture. Some rhyolites and dacites in the Wulang formation were enriched in Ga, Nb, Zr, Ce and Y and depleted in Sr and Eu. Additionally, they showed fractionation of rare earth elements (REE) with negative Eu anomalies, which is indicative of an A-type affinity of felsic rocks. The genesis of mafic members and an A-type affinity of felsic members indicate that the Late Carboniferous – Early Permian magmatism in the Western Tianshan area formed as a result of an extensional setting. This study also reveals bimodal magmatism produced by delamination in an extensional tectonic setting.

Carboniferous back-arc extension in the southern Yili-Central Tianshan Block and its significance to the formation of the Kazakhstan Orocline: insights from the Wusun Mountain volcanic belt

“…In this context, the Carboniferous magmatism has been extensively studied with various hypotheses. It was considered to develop in an intracontinental rift setting (Xia et al 2012;Xia and Li 2020), a continental arc setting (An et al 2013;Tang et al 2013;Wang et al 2007b;Yu et al 2016;Zhong et al 2017;Zhu et al 2009), a postcollisional setting (Chen et al 2020;Feng and Zhu 2019;Ge et al 2015;Long et al 2012;Sun et al 2008) or a back-arc extensional setting (Cao et al 2017;Qian et al 2006;Su et al 2018;Wang et al 2018aWang et al , b,2019Yan et al 2015) on the basis of geochemical features of different studied targets. These complex Carboniferous magmatic rocks were subdivided into four main belts, from north to south, as the Keguqinshan-Tulasu, Awulale, Wusun Mountain and Haerk-Nalati domains, according to their geographical distribution characteristics (e.g.…”

“…Complex subduction and accretion processes took place during the aforementioned period, producing massive Late Paleozoic magmatism in the western Chinese Tianshan, which is well recorded in a series of widespread Carboniferous volcanic rocks (XBGMR 1993). Previous studies suggested that they were formed in an intracontinental rift setting (Xia et al 2012;Xia and Li 2020), a continental arc setting (An et al 2013;Tang et al 2013;Wang et al 2007b;Yu et al 2016;Zhong et al 2017;Zhu et al 2009), a post-collisional setting (Chen et al 2020;Feng and Zhu 2019;Ge et al 2015;Long et al 2012;Sun et al 2008;Xiao et al 2010) or a back-arc extensional setting (Cao et al 2017;Qian et al 2006;Su et al 2018;Wang et al 2018aWang et al , b, 2019Yan et al 2015). Due to these controversial interpretations, their tectonic setting, and especially the implication that they have for the formation of the Kazakhstan Orocline, still warrant further elaboration.…”