Geochemistry and petrogenesis of the early Palaeozoic appinite-granite complex in the Western Kunlun Orogenic Belt, NW China: implications for Palaeozoic tectonic evolution

Jie Zhu,Junlu Chen,Zongqi Wang,Bing Xiao,Yanjing Chen,Qiugen Li,Haoshu Tang,Xu Chen,Shuwen Liu

doi:10.1017/s0016756817000450

Abstract

AbstractThe Datong pluton, the largest early Palaeozoic granitoid in the Western Kunlun Orogenic Belt (WKOB) in NW China, is a typical appinite-granite complex. It consists of diorites, quartz diorites, monzodiorites, quartz monzodiorites, monzonites, quartz monzonites, syenites, granodiorites and monzogranites. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded crystallization ages of 459 ± 3 Ma for the quartz monzonites and 452 ± 5 Ma for the monzogranites (Late Ordovician). The rocks possess a wide range of SiO2 (56.0–73.4 wt %), MgO (0.17–4.55 wt %) and Mg no. (25–60), with high K2O (2.83–5.29 wt %) contents, exhibiting high-K calc-alkaline to shoshonitic traits. They are characterized by enrichments in large-ion lithophile elements (LILEs) and light rare Earth elements (LREEs), as well as depletions in high-field-strength elements (HFSEs). The rocks have initial 87Sr/86Sr ratios of 0.7086–0.7185, negative εNd(t) values of –3.72 to –1.79 and εHf(t) values vary from –1.6 to +4.7. These features are modelled to show that they were most likely derived from an enriched lithospheric mantle source and that fractional crystallization with minor crustal contamination was involved in their petrogenetic process. Considering the distribution and chronology of the Palaeozoic intrusions – such as Kegang, Bulong, Qiukesu, Yierba, North Kudi, Dongbake, Buya, Ayilixi and Warengzilafu granitoid plutons with ages of c. 420–530 Ma – in conjunction with the Palaeozoic metamorphic overprinting in the WKOB, we propose a divergent double-subduction model to explain the destruction of the Proto-Tethys Ocean and suggest that the Datong pluton was likely emplaced in a post-collisional setting following the termination of subduction in response to slab break-off.

Highlights

The Western Kunlun Orogenic Belt (WKOB), located at the NW border of the Tibetan Plateau, forms the western segment of the Qinling–Qilian–Kunlun Orogenic Belt which transects eastern Asia and, via the bending of the tectonic fabric, terminates eastwards into the approximately N–S-trending Sulu Orogenic Belt (Fig. 1a)
Some models assumed the southwards subduction of the Proto-Tethys Ocean (Mattern & Schneider, 2000; Liao et al 2010; Jia et al 2013; Liu et al 2014), with collision between the NW and SW Kunlun terranes occurring during Late Ordovician – early Silurian time (Fig. 1b; Matte et al 1996; Pan, 1996; Mattern & Schneider, 2000; Ye et al 2008; Liao et al 2010; Jia et al 2013; Liu et al 2014)
As the focus of our systematic investigation, including previously obtained geochemical data (Jiang et al 2002; Liao et al 2010; Yu et al 2011; Gao et al 2013), we found that the Datong pluton had magmatic rock types ranging continuously from subordinate mafic types to the main body of monzonite, quartz monzonite, syenite and granite with highK calc-alkaline to shoshonitic affinities

Summary

Introduction

The Western Kunlun Orogenic Belt (WKOB), located at the NW border of the Tibetan Plateau, forms the western segment of the Qinling–Qilian–Kunlun Orogenic Belt ( referred to as the Central Orogenic Belt; Jiang, Wang & Li, 2000) which transects eastern Asia and, via the bending of the tectonic fabric, terminates eastwards into the approximately N–S-trending Sulu Orogenic Belt (Fig. 1a). As the focus of our systematic investigation, including previously obtained geochemical data (Jiang et al 2002; Liao et al 2010; Yu et al 2011; Gao et al 2013), we found that the Datong pluton had magmatic rock types ranging continuously from subordinate mafic types to the main body of monzonite, quartz monzonite, syenite and granite with highK calc-alkaline to shoshonitic affinities These rocks contain mineralogical and textural features that are indicative of water-rich magmas and form a typical appinite-granite complex. The objective of this research is to determine the tectonic evolution of the Palaeozoic magmatism by using new and reported petrological, geochemical and isotopic data from the WKOB (Fig. 1) In this contribution, we present a comprehensive study of the Datong pluton, including field observations, petrography, zircon U–Pb dating, zircon Hf isotopic compositions, whole-rock geochemistry and Sr–Nd isotopic compositions.

Geological setting and geology of the Datong pluton

Analytical methods

Results

Discussion

Findings

Conclusions