Geochronology and petrogenesis of Neoproterozoic S‐type granites from the Qilian Block, north‐west China, and their geological implications

Abstract

Early Neoproterozoic granites rarely outcrop in the Qilian Orogenic Belt (QOB) at the north‐eastern margin of the Qinghai–Tibet Plateau; however, they are vital to understanding the Precambrian evolution of the QOB. Thus, we present an integrated study, comprising petrology, geochronology, and whole‐rock geochemistry of two Early Neoproterozoic granitic plutons (Baihuagou and Yamacha plutons) in the eastern Qilian Block (QB). The results provide three key findings. First, their magma emplacement age is ca. 900 Ma (i.e., 918 Ma for Baihuagou and 910 Ma for Yamacha), and the two plutons have similar high‐K calc‐alkaline, strongly peraluminous characteristics, with slight enrichment in light rare earth elements; weak negative Eu anomalies; enrichment in large‐ion lithophile elements (e.g., K, Th, Ba, and Rb); strong depletion in Ti; and weak depletion in Nb, Ta, and, Sr. These characteristics indicate that the granites from these plutons are S‐type granites. Second, the two‐mica granite and gneissic granite samples may have been formed by partial melting of ancient crustal material (mainly psammitic rocks), with contributions from a meta‐igneous component, under high‐temperature and low‐pressure conditions. Third, new data from this study, combined with previously reported dates, indicate a continental margin arc setting for the QB during the Early Neoproterozoic (ca. 900 Ma).

Age and tectonic setting of Neoproterozoic gneissic granites in the southern Yili Block (NW China) and implications for the origins of the continental blocks in SW Central Asian Orogenic Belt

“…On the basis of new data presented in this study and the previous results (Li et al, 2015;Xiong et al, 2019;Zhu et al, 2019), the Neoproterozoic magmatism in the southern Yili Block mainly occurred during 934 and 968 Ma, and these intrusive bodies represent the reworking of mature continental crust. In addition, recent studies also documented Neoproterozoic granites ranging from 873 to 892 Ma in the study area, and these younger granites suggest a more differentiated signature of their source rocks (Xiong et al, 2019).…”

“…They consist of limestones, dolomites, quartzites, phyllites, quartz schists, conglomerates, mudstones, and quartz sandstones (XBGMR, 1993;Zhang et al, 2006). Granitoids ranging from 873 to 962 Ma intruded in these ductilely deformed and weakly metamorphosed sedimentary rocks (Li et al, 2015;Xiong et al, 2019;Zhu et al, 2019Zhu et al, , 2020. The Precambrian basement rocks in the Yili Block were overlain by upper Palaeozoic volcanic and sedimentary sequences (Gao et al, 1998;Liu et al, 2014).…”

“…Available stratigraphic records and increasing geochronological data indicate that the Yili Block experienced similar geological evolution during the Precambrian with the Chinese Central Tianshan and the other continental blocks in the SW CAOB Hu et al, 2000;Huang et al, 2016Huang et al, , 2017Liu et al, 2004Liu et al, , 2014Ma, Shu, Jahn, et al, 2012;Ma, Shu, Santosh, et al, 2012;Zhu et al, 2019). Firstly, the Yili Block and Chinese Central Tianshan develop comparable Precambrian stratigraphy and lithotectonic units mainly consisting of a crystalline and high-grade metamorphic basement, low-grade metamorphic sequences and nonmetamorphic sedimentary covers (XBGMR, 1993;Gao et al, 1998Gao et al, , 2009Shu et al, 2004Shu et al, , 2013 (Hu et al, 2010;Li et al, 2015;Xiong et al, 2019;Zhu et al, 2019) and 969-882 Ma orthogneiss in the Central Tianshan (Chen et al, 2009;Gao et al, 2015;Huang et al, 2014Huang et al, , 2017Huang, He, et al, 2015;Huang, Long, et al, 2015;Meng et al, 2018;Wang, Han, et al, 2014). Thirdly, detrital zircons from both tectonic units show similar age spectra (He, Wang, et al, 2018;Huang et al, 2016Huang et al, , 2017Huang et al, , 2019Ma, Shu, Jahn, et al, 2012;Ma, Shu, Santosh, et al, 2012;Wang, Zhai, et al, 2018), which consist of generally similar major Proterozoic age peaks at 0.8-0.95 Ga, 1.45-1.6 Ga, and 1.7-1.8 Ga (Figure 10a-c).…”