Elsevier

Gondwana Research

Volume 105, May 2022, Pages 201-216
Gondwana Research

Multi-stage metamorphism and deformation of the North Qinling Orogenic Belt: Constraints from petrology, geochronology, and structural analysis of the Qinling Complex

https://doi.org/10.1016/j.gr.2021.09.007Get rights and content

Highlights

  • Four generations of deformation have been recognized in the Qinling Complex.

  • The amphibolite is the product of the retrograde metamorphism of eclogite.

  • The Qinling Complex has similar age spectra of detrital zircon in different parts.

Abstract

The Qinling Complex is the only continental nucleus that outcrops in the North Qinling Orogenic Belt (NQOB) and preserves valuable evolutionary information of NQOB during the early Paleozoic. There are many high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic rocks distributed in the northern and southern parts of the Qinling Complex. Whether the central part (namely the Qihe tectonic slab) underwent a similar HP–UHP metamorphism and what the relationship among different tectonic slabs of Qinling Complex is, are the key to revealing the formation and exhumation process of these HP–UHP rocks. Four episodes of deformation (D1–4) have been recognized in the Qinling Complex. The metamorphic pressure‐temperature conditions of syn-D3 amphibolite in the Qihe tectonic slab are limited to 3.3–6.4 Kbar and 479–538 ℃ based on equilibrium modeling, corresponding to an extensional tectonic setting. The U-Pb dating of the amphibolite yielded a protolith age of 714 ± 46 Ma, an eclogite-facies metamorphic age of 514 ± 4 Ma, and two stages of retrograde metamorphism at 462 ± 3 Ma and 418 ± 5 Ma. The parametamorphic rocks of different tectonic slabs have similar detrital zircon age spectra, which is a very different pattern from adjacent areas of the Qinling Complex. The consistency in the protolith ages, peak metamorphic ages, two stages of retrograde metamorphism, and detrital zircon age from different tectonic slabs in the Qinling Complex indicate that HP–UHP metamorphic rocks from Qinling Complex formed during the same tectonic event. Therefore, the Qinling Complex evolved in deep subduction and underwent eclogite facies metamorphism, followed by superimposed deformation structures and retrograde metamorphism during the two stages of exhumation.

Introduction

The subduction of continental crust generates distinctive high-pressure forms of silica (Chopin, 1984, Smith, 1984). Exhumation of ultrahigh-pressure (UHP) terranes is often attributed to the positive buoyancy of these dominantly quartzofeldspathic terranes relative to the mantle (Hacker and Gerya, 2013, Hacker et al., 2013). Thus, these rocks provide insight into the long-term dynamics and physical conditions in subduction and continental collision zones.

There have been extensive studies about the Qinling Complex, especially on the Early Paleozoic high pressure–ultrahigh-pressure (HP–UHP) metamorphic rocks recently (Fig. 1b), leading to the discovery of diamond inclusions from gneiss (Yang et al., 2003), eclogites, and amphibolites (Wang et al., 2014, Yang et al., 2005), and coesite inclusion from retrograde eclogite (Gong et al., 2016). These HP–UHP metamorphic rocks are distributed in the northern and southern parts of the Qinling Complex (Fig. 2 and Supplementary Table S1). However, HP–UHP metamorphic rocks have not been discovered in the central part of the Qinling Complex, named the Qihe tectonic slab by Wang et al. (1997). Whether the Qihe tectonic slab underwent a similar UHP–HP metamorphism remains not known. Besides, the original tectonic setting of HP–UHP metamorphism has not been fully understood. It could result from the subduction of the Qinling Complex northern margin underneath the Erlangping arc (Wang et al., 2013a, Wu and Zheng, 2013, Yu et al., 2015) or the subduction of the South Qinling Orogenic Belt (SQQB) underneath the Qinling Complex (Liu et al., 2016a). The multi-stage metamorphic and deformation evolution of the Qinling Complex is vital to reveal the evolution and exhumation history of these UHP–HP rocks.

This paper presents new structural data based on detailed structural analyses in the field of the Qinling Complex and age dating of detrital zircon dating. We also provide insights on the metamorphism and geochronology of metamorphic rocks distributed in the Qihe tectonic slab, which can better constrain the tectonic evolution of the Qinling Complex.

Section snippets

Geological background

The Qinling Orogenic Belt (QOB) is bounded by the Luonan-Luanchuan Fault in the north and the Mianlue Suture Zone in the south (Fig. 1a) (Li et al., 2007, Dong et al., 2013, Liu et al., 2013a, Wang et al., 2013a). The QOB was formed through the collision between the North China Block (NCB) and the South China Block (SCB) (Zhang et al., 1996, Zhang et al., 2001) and was subdivided into the North Qinling Orogenic Belt (NQOB) and SQOB by the Shangdan Suture Zone (Zhang et al., 1996, Zhang et al.,

Analytical techniques

Whole-rock geochemical analyses, electron probe, and zircon LA-ICP-MS U–Pb dating were conducted in this study.

Geology of Qinling Complex

The Qinling Complex consists of different tectonic slabs (Fig. 2). We selected nine representative samples in this study, and the sample sites are shown in Fig. 2. The eclogites and amphibolites outcrop as lenses or layers within schist country rocks in the northern part of the Qinling Complex (Liu et al., 2016a, Wang et al., 2013a, Yang et al., 2003). The layers and long axes of the lenses are parallel to the regional foliation of the country rocks. The country rock is muscovite-quartz schist.

Deformation history

Detailed structural analyses have previously been conducted on several major shear zones separating each lithotectonic unit of the NQOB (Mattauer et al., 1985, Ratschbacher et al., 2003) and deformation history within each lithotectonic unit of the NQOB (Wang et al., 2005, Zhao et al., 2015). These studies revealed visible lateral movements along major shear zones and oblique uplift existing in the Qinling Complex. We focused on the detailed deformation history within the Qinling Complex,

Pressure‐temperature condition of D3

The syn-D3 amphibolite that occurs as intercalations within the two-mica quartz schist in the west part of the Qihe slab was selected to determine the pressure‐temperature condition during the D3 deformation.

Zircon U-Pb geochronology of amphibolite

Zircon grains from the sample DQH20-28-2 are spherical to ellipsoidal, 0.05–0.1 mm long, light yellow to colorless, and transparent. CL images mainly show the homogeneous medium-luminescent structure or complex core-rim structure (Supplementary Fig. S6). Therefore, zircon grains have been divided into two types. Type 1 grains have a very low-luminescent core, which is weakly zoned. This indicates that the zircon grains are inherited magmatic zircon and have experienced different degrees of

Interpretation of zircon U-Pb ages of amphibolite

In the zircon grains of sample DQH20-28-2, the inherited cores show very low-luminescent, weakly zoned, steep HREE patterns, high Th/U ratios, and relatively negative Eu anomalies of magmatic origin. Together with some metamorphic zircon, these zircon grains give an upper intercept age of 714 ± 46 Ma, which is interpreted as the time of intrusion of the protoliths of the amphibolite. Similar ages of Neoproterozoic magmatic protolith have been reported from HP–UHP metamorphic rocks in the UHP

Conclusions

1) Four generations of deformation were identified in the Qinling Complex. The D1 is defined by a regionally penetrative schistosity and gneissosity (S1), with some intrafolial tight isoclinal folds (F1). The D2 was mainly formed in the north–south compression deformation, including the ductile shear zone and folds (F2) superposed on D1. The D3 was recognized by refolding (F3) the S1 and older folds (F1 and F2) in the local area of the Qihe tectonic slab. The D4 in the Qinling Complex is a

CRediT authorship contribution statement

Wenbin Kang: Conceptualization, Writing – original draft, Investigation. Wei Li: Investigation, Conceptualization, Writing – review & editing. Yunpeng Dong: Investigation. Le Zhang: Investigation. Jinxiang Zhao: Investigation. Falak Sheir: Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We gratefully acknowledge the anonymous journal reviewers and would like to thank them for their constructive reviews and comments, which substantially improved this work. Financial support for this study was jointly provided by the National Natural Science Foundation of China (grants: 41872218, 41572179, and 41372204) and MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University.

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