Journal of Earth Science  2018, Vol. 29 Issue (5): 1102-1115   PDF    
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Petrology of Garnet Amphibolites from the Hualong Group: Implications for Metamorphic Evolution of the Qilian Orogen, NW China
Yilong Li1, Limin Zhao1, Zhuoyang Li1, Biji Luo1, Jianping Zheng1, Fraukje M. Brouwer2    
1. School of Earth Sciences, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;
2. Geology & Geochemistry Group, Department of Earth Sciences, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
ABSTRACT: The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a systematic study of petrography, mineral chemistry and phase equilibria of garnet amphibolites from the Hualong Group, which represents the Precambrian basement in the southern accretionary belt of the Qilian Orogen. The garnet amphibolites mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. A peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples. Garnet has a homogeneous composition of Alm66-71Grs14-17Prp9-12Sps3-5, amphibole is ferro-hornblende, biotite belongs to the ferro-biotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples witness peak metamorphism at conditions of~4.9-6.3 kbar and~755-820℃ in the upper amphibolite facies to low-temperature granulite facies, and retrograde cooling and decompression at conditions of~2.5-3.1 kbar and~525-545℃. It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.
KEY WORDS: Qilian Orogen    Hualong Group    garnet amphibolite    petrology    metamorphic P-T path    Perple_X    

0 INTRODUCTION

As part of the Central China orogenic system (Li et al., 1978), the Qilian Orogen is located in the northeastern part of the Qinghai-Tibet Plateau, at the junction of the North China Craton in the northeast, the South China Craton in the southeast, and the Tarim Craton in the northwest (Fig. 1a; Song et al., 2013). It is divided into the North Qilian accretionary belt, the central Qilian Block, and the South Qilian accretionary belt (Fig. 1b; Song et al., 2017, 2014, 2013; Feng and He, 1996), and has long drawn international attention for the study of orogeny and continental dynamics (Fu et al., 2018a, b; Song et al., 2017, 2014, 2013, 2006; Zhang Y Q et al., 2017; Xia et al., 2016; Gehrels et al., 2011, 2003; Wu et al., 2010, 2006; Xiao et al., 2009; Yang et al., 2002; Yin and Harrison, 2000; Xu et al., 1994).

Early Paleozoic ophiolites, high-pressure metamorphic rocks (e.g., eclogite and blueschist), numerous pre-, syn- and post- collisional igneous rocks and Precambrian metamorphic rocks crop out in the Qilian Orogen (Xia et al., 2016; Song et al., 2014, 2013). Abundant studies have been carried out on the tectonic affinity of the central Qilian Block (Yan et al., 2015; Tung et al., 2007; Xu et al., 2007; Wan et al., 2006, 2003, 2000; Feng and He, 1996; Xia et al., 1991), and the oceanic subduction to continental collision along the North Qilian and the South Qilian accretionary belts (Fu et al., 2018a, b, c; Song et al., 2017, 2013, 2009, 2007, 2006, 2004; Xia et al., 2016; Huang et al., 2015; Yang et al., 2015; Xiao et al., 2009; Zhang et al., 2009, 2007).

The Precambrian metamorphic basement in the Qilian Orogen consists of the Tuolai Group, the Huangyuan Group, the Maxianshan Group and the Hualong Group. The former three groups are regarded as the basement of the central Qilian Block (He et al., 2010; Wan et al., 2003; Guo et al., 2000). However, two different interpretations persist on the tectonic location of the Hualong Group. One suggests it to be the southeastern part of the central Qilian Block (Yang et al., 2015; Xu et al., 2007), the other ascribes to the South Qilian accretionary belt (Fu et al., 2018a, b; Song et al., 2017; Zhang Y Q et al., 2017; Wang et al., 2016; Yan et al., 2015). The Precambrian metamorphic rocks consist of strongly-deformed gneisses, migmatites, schists, marbles, quartzites and amphibolites (Tung et al., 2012, 2007). Their protolith formation ages and the metamorphic ages were suggested to be Early Neoproterozoic (Yan et al., 2015; Yang et al., 2015; Yu et al., 2012; Tung et al., 2007; Wan et al., 2006, 2003; Guo et al., 2000) and Ordovician–Silurian, respectively (Wang et al., 2016; Yan et al., 2015; Fan and Lei, 2007; Tung et al., 2007). However, there is little information on the metamorphic P-T conditions and the tectonothermal processes that affected the Precambrian metamorphic basement in the Qilian Orogen.

Reconstruction of P-T paths by phase equilibrium modeling in metamorphic rocks is an effective method to constrain the tectonic evolution of orogens. On the basis of detailed fieldwork, the garnet amphibolites from the Hualong Group were selected for study on the petrography, mineral chemistry and phase equilibrium modeling. The data provide constraints on the metamorphism of Precambrian metamorphic basement in the Qilian Orogen and the tectonic mechanism responsible for the accretionary processes of the orogen.

1 GEOLOGICAL SETTING

The Qilian Orogen and its adjacent areas, from north to south, are subdivided into the Alxa Block, the North Qilian accretionary belt, the central Qilian Block, the South Qilian accretionary belt, the Quanji Block, the North Qaidam ultra-high pressure metamorphic (UHPM) belt, and the Qaidam Block (Song et al., 2017, 2013; Zhang Y Q et al., 2017; Chen et al., 2013) (Fig. 1b).

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Figure 1. Tectonic framework of China and location of the study area (modified from Song et al., 2017, 2013; Chen et al., 2013; Yan et al., 2012). (b) Simplified geological map of the Qilian Orogen (modified from Song et al., 2017; Zhang Y Q et al., 2017; Xia et al., 2016; Xu et al., 2016). (c) Geological map of the South Qilian accretionary belt and adjacent areas (modified from Fu et al., 2018a, b; Yan et al., 2015). 1. Precambrian basement; 2. granite; 3. Mid–Late Neoproterozic (848–604 Ma) rift-related volcanics; 4. obducted slice of Late Neoproterozoic–Cambrian (550–497 Ma) oceanic crust; 5. Middle Cambrian–Ordovician rift-related volcanics; 6. Middle Cambrian–Ordovician (514–446 Ma) arc volcanics; 7. Middle Cambrian–Ordovician (517–449 Ma) back-arc volcanics; 8. high-pressure (HP)-ultrahigh pressure (UHP) metamorphic complex; 9. HP blueschist with eclogite; 10. Late Ordovician–Early Silurian (445–428 Ma) post-collisional rift volcanics; 11. Silurian incipient molasse formation; 12. Devonian molasse; 13. fault/inferred fault; 14. Carboniferous–Cenozoic covers; 15. sample locations.

The North Qilian accretionary belt, separating the Alxa Block and the Qilian Block, is mainly made up of Early Paleozoic subduction complexes and overlying Silurian fine-grained siltstone and slate, Devonian poorly-rounded and unsorted terrestrial conglomerate and sandstone, and Carboniferous to Triassic limestone and clastic rocks with coal layers (Song et al., 2013, 2006; Feng and He, 1995). It is regarded as a typical oceanic suture zone (Song et al., 2013, 2006). The Qilian Block is composed of Precambrian basement and an unconformable Phanerozoic overlying sedimentary sequence (Song et al., 2013, 2006; Tung et al., 2012). The latter contains Early Paleozoic volcano-sedimentary series, Late Paleozoic to Triassic shallow marine to continental sediments, and Jurassic to Quaternary continental sediments, with voluminous Early Paleozoic granitoid bodies (Tung et al., 2012, 2007; BGMR-QP, 1991; BGMR-GP, 1989). Intermediate to basic intrusions are also ubiquitous in the block (Tung et al., 2016; Yang et al., 2015; Zhang et al., 2014; Yu et al., 2012; He et al., 2008).

The South Qilian accretionary belt, separating the Qilian Block and the Quanji Block, consists of Cambrian–Ordovician lava flows, pyroclastic rocks and abyssal and bathyal deposits, Silurian flysch, Early Devonian granitoids and Late Devonian molasse (Xu et al., 2006). The Lajishan ophiolite sequence (~530–470 Ma; Fu et al., 2018a, b, c, 2014; Fu and Yan, 2017; Song et al., 2017; Zhang Y Q et al., 2017; Hou et al., 2005; Qiu et al., 1995) and an arc-volcanic sequence (460–440 Ma; Yang H et al., 2015; Yang J S et al., 2002) without high-pressure metamorphic rocks were identified in the belt. The Hualong Group is distributed discontinuously as variably sized tectonic blocks and crops out with a Cambrian–Silurian subduction- accretion complex to the north (Fu et al., 2018a, b, c, 2014; Yan et al., 2015, 2012; Xiao et al., 2009). Therefore, we consider the Hualong Group a tectonic block in the South Qilian accretionary belt (Fig. 1c).

The Hualong Group is well exposed in the Hualong, Jianzha, Ashigong, Qianhu and Duomoji areas and is composed of widespread quartzo-feldspathic gneisses, schists, migmatites and intercalated lenses or layers of marble, quartzite, amphibolite and magmatic veins (BGMR-QP, 1991). Our initial research indicates that the protoliths of paragneisses from the Hualong Group are wackes with a maximum deposition age of ca. 900 Ma formed at an active continental margin, and were affected by amphibolite facies metamorphism at 439.6±4.9 Ma (unpublished data). Quasi-lamellar or lenticular garnet amphibolites are dispersed throughout the gneisses and are oriented parallel to the main tectono-metamorphic fabric in the gneiss. Detailed study on zircon U-Pb geochronology and Hf isotopes of granitic orthogneiss, paragneiss, garnet-bearing amphibolite and quartzite from the Hualong Group were carried out (Yan et al., 2015). Two garnet amphibolite samples (16HL02 and 16HL03) were collected in the Ashigong area (Fig. 1b).

2 ANALYTICAL METHODS 2.1 Scanning Electron Microscope (SEM) and Energy Dispersion Spectrum (EDS) Analysis of Minerals

A HITACHI-SU8010 SEM with an iXRF EDS was used for semi-quantitative analysis of minerals at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (CUG), Wuhan, China. A beam acceleration voltage of 20 kV was employed during the acquisition of X-rays. Detection limits for semi-quantitative analysis are ~0.5 wt.% for most elements.

2.2 Electron Probe Micro-Analysis (EPMA) of Minerals

Microanalyses of minerals were carried out on polished thin sections at the Center for Global Tectonics, School of Earth Sciences, China University of Geosciences (CUG), Wuhan, China. A JEOL JCXA-8230 electron probe micro-analyzer was used with an accelerating voltage of 15 kV, cup current of 20 nA and spot diameter of 2 μm for garnet, pyroxene and plagioclase and 5 μm for amphibole and biotite. The results are listed in Table S1.

2.3 Whole-Rock Major Elements

Powdered samples were analyzed for major elements by X-ray fluorescence (XRF) using a Rigaku RIX 2000 spectrometer at the Department of Geosciences, National Taiwan University. The analytical uncertainties are generally better than 5% for all elements. Analytical details for the major element measurements may be found in Chung et al. (2003) and Yang et al. (2005).

2.4 Background of the Perple_X Models

To decipher the evolution of mineral assemblages with changing metamorphic P-T conditions, P-T pseudosections were calculated for the garnet amphibolites in the NCKFMASHTiMn system using the Perple_X v.6.8.1 software (Connolly, 2005) and the updated version of the internally consistent thermodynamic database hp622ver.dat of Holland and Powell(1998, 1991). The solution models of Holland and Powell (1998) for garnet, chlorite and epidote, Holland and Powell (1996) for clinopyroxene and orthopyroxene, Tajčmanová et al. (2009) for biotite, Holland and Powell (2003) for feldspar and Dale et al.(2005, 2000) for amphibole were used. Quartz is present in both samples, so SiO2 is considered to be available in excess. Water was used as a saturated pure fluid phase since the samples contain abundant hydrous phases. The whole rock composition used in the calculation system comes from XRF measured data. The calculated pressure and temperature range is 1.0–10 kbar and 400–900 ℃ for both garnet amphibolite samples.

3 PETROLOGY AND MINERAL CHEMISTRY

Psammitic paragneiss and granitic orthogneiss are the main components of the Hualong Group. Quartzite, garnet-quartz-mica schist and biotite-quart schist are locally intercalated with the paragneiss. Irregular garnet amphibolite enclaves and layers are common in the outcrops (Fig. 2a). Some of the garnet amphibolites are tens of meters thick and were intruded by still-undeformed granitic and gabbroic dykes (Fig. 2b). The protolith of the garnet amphibolites is interpreted to be a mafic dyke intruded into the paragneisses in the Hualong Group, and was subsequently deformed and transposed.

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Figure 2. Field aspect of garnet amphibolites in the Hualong Group. (a) Garnet amphibolites are intercalated with paragneiss; (b) some of the garnet-amphibolites are tens of meters thick and were intruded by granitic dykes that do not show any evidence of deformation.
3.1 Petrography 3.1.1 Garnet amphibolite sample 16HL02

Photomicrographs of garnet amphibolite 16HL02 show a porphyroblastic texture with a matrix of fine-grained granoblastic texture and a shape preferred orientation of amphibole, plagioclase and quartz crystals (Fig. 3a). It is mainly composed of amphibole (45%–55%), plagioclase (20%–30%), garnet (5%–10%), biotite (5%), quartz (2%–5%), orthopyroxene (4%), clinopyroxene (1%), ilmenite (2%–5%) and chlorite (1%–2%). From the mineral assemblage we infer that the rock was subjected to low temperature granulite facies metamorphism.

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Figure 3. Photomicrographs showing typical textures of garnet grains from sample 16HL02. (a), (b) Inclusions of quartz+ilmenite+amphibole+plagioclase in a garnet porphyroblast; (c), (d) small amounts of biotite and chlorite distributed along the garnet fractures as products of retrograde metamorphism.

Garnet grains are euhedral porphyroblasts with regular edges and a grain size of 0.5–1 mm. Extensive fractures are developed. Garnet usually contains inclusions of quartz+ilmenite+ amphibole+plagioclase with the same orientation as the matrix (Figs. 3a, 3b). The inclusion assemblage may represent the mineral assemblage of the protolith or that of prograde metamorphism. Small amounts of biotite and chlorite grow along the garnet fractures and presumably are products of retrograde metamorphism (Figs. 3c, 3d).

Amphibole grains are green and euhedral to subhedral with grain size of 0.2–1.5 mm. There are three generations of amphibole in the sample. The first generation (Amp1) occurs as fine grained inclusions in garnet and represents products of the prograde metamorphism (Fig. 3). The second generation (Amp2) is porphyroblastic and euhedral with sieve texture and contains unoriented inclusions of plagioclase+ilmenite+quartz (Figs. 4a, 4b). This generation may have been produced during the peak metamorphism. The third generation (Amp3) is the subhedral to anhedral and fine grained main component of the matrix, and has inclusions of residual pyroxene grains (Figs. 4c, 4d), suggesting amphibolite facies retrograde metamorphism. Pyroxene grains are colorless and transparent with a grain size of 0.3–0.5 mm, including mostly orthopyroxene and minor clinopyroxene. The two groups of near-orthogonal pyroxene-type cleavage lines are visible (Figs. 4c, 4d, 4e, 4f).

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Figure 4. Photomicrographs showing typical textures of amphibole grains from sample 16HL02. (a), (b) Inclusions of plagioclase+ilmenite+quartz in amphibole porphyroblast; (c)–(f) residual pyroxene occurs as inclusions in amphibole.

Besides the inclusions in garnet and amphibole, plagioclase occurs as the main component of the matrix. It is anhedral with a size of 0.1–0.2 mm, and some crystals show weak sericitization (Figs. 3, 4). Quartz grains are colorless and anhedral. The quartz in the matrix generally shows wavy extinction (Figs. 4e, 4f) and also occurs as inclusions in garnet and amphibole (Figs. 3, 4).

Based on the mineral crystal morphology and the distribution, three generations of biotite can be identified. The first generation (Bt1) is present as inclusions in garnet, amphibole and quartz with orientations at high angles to that of matrix foliation and cutting the garnet fractures (Fig. 3c). This generation is regarded as product of prograde metamorphism. The second generation (Bt2) is brown and euhedral to subhedral with grain size of 0.5–1.5 mm. It is commonly associated with garnet and may be formed during the peak metamorphism. The third generation (Bt3) exists on the edge or inside of garnet and amphibole and is partly altered to chlorite. It is distributed along garnet fractures and amphibole cleavages and may be the product of retrograde metamorphism (Fig. 3c).

3.1.2 Garnet amphibolite sample 16HL03

Garnet amphibolite sample 16HL03 also shows a porphyroblastic texture with a fine-grained granoblastic matrix showing a shape preferred orientation of amphibole, plagioclase and quartz crystals (Fig. 5a), with similar mineral assemblages and characteristics to sample 16HL02. It is mainly composed of amphibole (35%–45%), plagioclase (20%–30%), garnet (5%–10%), biotite (5%–10%), chlorite (5%–10%), quartz (2%–5%) and ilmenite (2%–5%), with grain size smaller than that in sample 16HL02. The sample has obvious retrogressive characteristics and no residual pyroxene grains are found (Figs. 5a, 5b). Garnet porphyroblasts are euhedral and contain inclusions of amphibole (Amp1)+ plagioclase+quartz+ilmenite (Figs. 5c, 5d). Amphibole porphyroblasts (Amp2) are euhedral with sieve texture and contain oriented inclusions of plagioclase+ilmenite+quartz (Fig. 5e). The inclusion orientation is at high angles to that of matrix, indicating this generation of amphibole may be produced during the prograde or peak metamorphism (Fig. 5e). Fine-grained amphibole (Amp3) occurs as anhedral to subhedral grains in the matrix and underwent weak chloritization on the grain edges (Fig. 5f). In places, fine-grained garnet grains are intergrown with this latest generation of amphibole (Fig. 5f).

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Figure 5. Photomicrographs showing typical textures of garnet amphibolite sample 16HL03. (a), (b) Porphyroblastic texture with a fine-grained granoblastic matrix showing a shape preferred orientation of amphibole+plagioclase+quartz; (c), (d) inclusions of amphibole+plagioclase+quartz+ilmenite in garnet porphyroblast; (e) oriented inclusions of plagioclase+ilmenite+quartz in amphibole porphyroblast; (f) intergrown fine-grained garnet grains with the latest generation of amphibole.
3.1.3 Results of petrographic analysis

Petrographic analysis indicates that three stages of metamorphism were recorded by garnet amphibolite samples from the Hualong Group. The prograde stage (M1) is characterized by mineral assemblage of plagioclase+amphibole+quartz+biotite+ ilmenite inclusions in garnet and amphibole porphyroblasts (Amp2), implying amphibolite facies metamorphism. The peak metamorphism stage (M2) is defined by mineral assemblage of orthopyroxene+clinopyroxene+garnet+amphibole+plagioclase+ biotite+quartz+ilmenite, suggesting a low-temperature granulite facies metamorphism. It is inferred that the peak stage mineral assemblage (M2) is produced by following reactions

$ \begin{array}{l} {\rm{P}}{{\rm{l}}_{\rm{1}}}{\rm{ + Am}}{{\rm{p}}_{\rm{1}}}{\rm{ + Q}}{{\rm{z}}_{\rm{1}}} \to {\rm{Gr}}{{\rm{t}}_{\rm{1}}}{\rm{ + P}}{{\rm{l}}_{\rm{2}}}{\rm{ + Am}}{{\rm{p}}_{\rm{2}}}{\rm{ + Cpx}}\\ {\rm{Am}}{{\rm{p}}_{\rm{1}}}{\rm{ + Q}}{{\rm{z}}_{\rm{1}}} \to {\rm{Cpx + Opx + P}}{{\rm{l}}_{\rm{2}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}}\\ {\rm{Am}}{{\rm{p}}_{\rm{1}}}{\rm{ + Q}}{{\rm{z}}_{\rm{1}}} \to {\rm{Opx + P}}{{\rm{l}}_{\rm{2}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}} \end{array} $

The retrograde stage of metamorphism (M3) has a mineral assemblage of garnet+amphibole+plagioclase+biotite+quartz+ chlorite+ilmenite, indicating an amphibolite facies metamorphic overprint. With declining temperature and pressure after the peak stage, pyroxene broke down to amphibole with some relicts in the core of amphibole (Amp3). Some garnet grains are also formed during this stage and occur intergrown with amphibole. The following reactions are inferred

$ \begin{array}{l} {\rm{Opx + Cpx + P}}{{\rm{l}}_{\rm{2}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{Am}}{{\rm{p}}_{\rm{3}}}{\rm{ + Qz}}\\ {\rm{Opx + Cpx + Am}}{{\rm{p}}_{\rm{2}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{Am}}{{\rm{p}}_{\rm{3}}}{\rm{ + P}}{{\rm{l}}_{\rm{3}}}\\ {\rm{Opx + P}}{{\rm{l}}_{\rm{2}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{Am}}{{\rm{p}}_{\rm{3}}}{\rm{ + Gr}}{{\rm{t}}_{\rm{2}}}{\rm{ + Qz}} \end{array} $
3.2 Mineral Chemistry 3.2.1 Garnet

Two chemical profiles of garnet in samples 16HL02 (Fig. 3a) and 16HL03 (Fig. 5c) were analyzed. Garnet in both samples has a composition of Alm66–71Grs14–17Prp9–12Sps3–5, without obvious compositional zoning from core to rim (Figs. 6a, 6b).

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Figure 6. (a), (b) Compositional profiles of garnet from samples 16HL02 and 16HL03, Xalm=Fe2+/(Fe2++Mn+Mg+Ca), Xsps=Mn/(Fe2++Mn+Mg+Ca), Xprp=Mg/(Fe2++Mn+Mg+Ca), Xgrs=Ca/(Fe2++Mn+Mg+Ca); (c) amphibole compositions plotted in diagram after Leake (1978); (d) profiles of Mg# values of amphibole from sample 16HL02; (e) feldspar classification diagram of Smith (1974), An=Ca/(Ca+Na+K), Ab=Na/(Ca+Na+K), Or=K/(Ca+Na+K); (f) mica classification diagram of Foster (1960).
3.2.2 Amphibole

All amphiboles from the two samples are calcic, in which (Ca+Na)B≥1.34 and NaB < 0.67 (Leake, 1978). Most of them belong to the ferro-hornblende species with Mg# (Mg2+/(Mg2++Fe2+)) values of 0.38–0.51 (Fig. 6c). The amphibole inclusions (Amp1) in garnet have a relatively broad range of Mg# values of 0.51–0.41. The amphibole porphyroblasts (Amp2) show decreasing Mg# values (0.45–0.38) from core to rim, while the amphibole matrix (Amp3) have a homogeneous composition (Fig. 6d).

3.2.3 Pyroxene

Pyroxene in sample 16HL02 is analyzed and classified as ferro-hypersthene with a composition of En40–43Fs57–60. It has a homogeneous Mg# of 0.40–0.43.

3.2.4 Plagioclase

Both plagioclase inclusions in garnet and amphibole and matrix plagioclase from the two samples are analyzed. In sample 16HL02, two matrix-type plagioclase grains are labradorite but reveal distinct compositions. One has homogeneous An values of 60–62 from core to rim, the other has lower and decreasing An values of 56–51 from core to rim. Plagioclase inclusions have An values of 54–49. In sample 16HL03, matrix plagioclase grains have a wide range of composition with An values decreasing from An77 to An41 from core to rim. Plagioclase inclusions have higher An values of 84–85 and are bytownite (Fig. 6e).

3.2.5 Biotite

Matrix-type biotite (Bt2) in sample 16HL02 is analyzed and belongs to the ferrobiotite species (Fig. 6f). It has high values of Mg# (0.42–0.44) and Ti (0.17–0.19), and low AlVI (0.21–0.26). The Mg# values slightly decrease from core to rim (Fig. 6f).

4 P-T PSEUDOSECTION CALCULATIONS

Major element compositions of garnet amphibolite samples 16HL02 and 16HL03 are listed in Table 1.

Table 1 Major element compositions (wt.%) of garnet amphibolite samples from the Hualong Group
4.1 P-T Pseudosection for Sample 16HL02

For sample 16HL02, garnet is present in all observed fields at the calculated P-T range (Fig. 7a). Orthopyroxene coexists with amphibole and biotite in low pressure fields, while clinopyroxene coexists with amphibole and biotite in high pressure fields. There is a narrow belt area with coexisting orthopyronene, clinopyroxene and amphibole, at the P-T conditions of low- temperature granulite facies metamorphism. With increasing temperature, amphibole disappears and orthopyronene and clinopyroxene coexist with biotite. Chlorite is stable in low temperature fields that below 530 ℃ (Fig. 7a).

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Figure 7. (a) P-T pseudosection for sample 16HL02 showing assemblage stability fields for the determined effective bulk rock composition; (b) contour intersections for compositions of biotite (XMg) and amphibole (XMg) for sample 16HL02, XMg=Mg/(Mg+Fe2+); (c) contour intersections for compositions of garnet (Xpyp and Xgrs) and amphibole (XMg) for sample 16HL02, Xpyp=Mg/(Fe2++Mn+Mg+Ca), Xgrs=Ca/(Fe2++Mn+Mg+Ca); (d) interpreted P-T path of sample 16HL02; (e) P-T pseudosection for sample 16HL03 showing assemblage stability fields for the determined effective rock composition; (f) contour intersections for compositions of garnet (Xpyp and Xgrs) and amphibole (XMg) and the P-T path inferred for sample 16HL03.

According to petrographic observations, the mineral assemblage of peak metamorphism consists of orthopyroxene, biotite, amphibole, garnet, plagioclase, clinopyroxene, ilmenite and quartz, corresponding to the Opx+Bt+Amph+Grt+Pl+Cpx+Ilm+Qz stable region in the P-T pseudosection (Fig. 7a). Isopleths of XMg (Mg/(Mg+Fe2+)) contents in biotite and in amphibole are shown in Fig. 7b. The measured XMg (0.45) in the core of Amp2 and XMg (0.44) in the core of Bt2 intercross at the Opx+Bt+Amph+ Grt+Pl+Cpx+Ilm+Qz stable region with P-T conditions of 6.3 kbar and ~820 ℃, which likely represents peak metamorphism (M1, Fig. 7b). Isopleths of Xpyp (Mg/(Fe2++Mn+Mg+Ca)) and Xgrs (Ca/(Fe2++Mn+Mg+Ca)) contents in garnet and XMg contents in amphibole are shown in Fig. 7c. The measured Xpyp (0.12) and Xgrs (0.16) in the core of Grt1 intersect in the peak metamorphic field with slightly lower P-T conditions of 5 kbar and ~800 ℃. Combining the relatively homogeneous composition of garnet with Xpyp (0.12–0.10) slightly decreasing from core to rim, garnet records a stage of growth during exhumation (M2, Fig. 7c). The observed retrograde metamorphic mineral assemblage is composed of plagioclase, biotite, chlorite, amphibole, garnet, ilmenite and quartz, corresponding to the Pl+Bt+Chl+Amph+Grt+Ilm+Qz stable region at 1–5 kbar and 440–530 ℃ (Fig. 7a). The Xgrs isopleths have relatively flat slopes and decrease as pressure declines. The measured XMg in the rim of Amp2 (0.41–0.40) and in Amp3 (0.42–0.41) and Xgrs (0.15) in the rim of Grt1 constrain P-T conditions of 2.5 kbar and ~525 ℃ (M3, Fig. 7c), implying an amphibolite facies retrograde overprint. Therefore, a clockwise P-T path is proposed to garnet amphibolite sample 16HL02 (Fig. 7d).

4.2 P-T Pseudosection for Sample 16HL03

There was no pyroxene in sample 16HL03. The peak metamorphic mineral assemblage consists of plagioclase, amphibole, garnet, biotite, ilmenite and quartz, corresponding to the Pl+Amph+Gt+Bt+Ilm+Qz stable region in the P-T pseudosection (Fig. 7e), defining a P-T range of 4.2–6 kbar and 700–775 ℃ and indicating upper amphibolite facies metamorphism. Isopleths of the Xpyp and Xgrs contents in garnet intersect in the above region at P-T conditions of ~755 ℃ and 4.9 kbar, which probably represents the peak P-T condition recorded by this sample (M1, Fig. 7f). The measured XMg in the rim of Amp2 (0.38) and in Amp3 (0.40–0.38) together with Xgrs (0.16–0.15) in the rim of Grt1 constrain P-T conditions of 3.1 kbar and ~545 ℃ (M2, Fig. 7f), reflecting an amphibolite facies retrograde metamorphism. A clockwise P-T path is also proposed to garnet amphibolite sample 16HL03 (Fig. 7f).

5 GEOLOGICAL IMPLICATION

The Hualong Group, composed of widespread quartzofeldspathic gneisses, schists, migmatites and intercalated lenses or layers of marble, quartzite, amphibolite and magmatic veins (BGMR-QP, 1991), is the dominant component of the South Qilian accretionary belt and represents the Precambrian basement of the Hualong Block (Fu et al., 2018a, b; Yan et al., 2015; He et al., 2011; Xu et al., 2007). Amphibolites in the Hualong Group were subdivided into four types based on their bulk and mineral compositions, ages and tectonic affinities (Fu et al., 2018a). Type Ⅰ is characterized by garnet presence and Proterozoic detrital zircons older than 925 Ma with a sedimentary protolith and was intruded by granitic gneiss at 850 Ma (Yan et al., 2015). The other types have an igneous origin with protolith ages of 1 126–895 Ma (type Ⅱ), 882–580 Ma (type Ⅲ) and 522–440 Ma (type Ⅳ), and are thought to be related to the processes of assembly and breakup of the Rodinia supercontinent and reassembly of the Paleozoic orogenic belt, respectively (Fu et al., 2018a). The garnet amphibolites in this study were collected in the Ashigong area (Fig. 1b) from the same sampling point as the type Ⅳ amphibolites that were reported by Yan et al. (2015) and Fu et al. (2018a), and were intruded by a 442 Ma gabbro (Yan et al., 2015).

Metamorphic ages of ~450 Ma for type Ⅰ garnet amphibolite (Yan et al., 2015) and ~440 Ma for type Ⅳ amphibolite (Wang et al., 2016) determined by LA-ICPMS zircon U-Pb dating on metamorphic overgrowth rims were reported. Fan and Lei (2007) reported a muscovite 40Ar/39Ar age of 418.3±2.8 Ma from a quartzo-feldspathic mylonite in the Hualong Group. We determined a biotite 40Ar/39Ar age of 439.6±4.9 Ma from the paragneisses (unpublished). Therefore, the Hualong Group was affected by an Early Paleozoic metamorphic event.

Xia et al. (2016) synthesized the distribution, age and geochemical data of the Mid–Late Neoproterozoic to Early Paleozoic volcanic rocks from the Qilian Orogen, and suggested that the North Qilian accretionary belt and the South Qilian accretionary belt are two independent and almost simultaneous subduction zones at ca. 550–446 Ma accompanying the closure of the North Qilian Ocean and the South Qilian Ocean, and the northward subduction of the South Qilian Ocean started at ca. 540–520 Ma. The SSZ-type ophiolites (ca. 530–480 Ma) (Fu and Yan, 2017; Song et al., 2017; Zhang Y Q et al., 2017; Fu et al., 2014; Hou et al., 2005; Qiu et al., 1995) at the northern margin of Hualong Group in the Lajishan area represent an Early Paleozoic oceanic basin. A ca. 470–410 Ma continental arc was proposed based on the study of mafic (470–440 Ma) and granitoid plutons (460–410 Ma) that intruded into the Hualong Group (Yan et al., 2015). Therefore, we propose a stage of intra-oceanic subduction at ca. 540–470 Ma (Fig. 8a), followed by oceanic crust-continent subduction since ca. 470 Ma (Fig. 8b) at the southern margin of the central Qilian Block.

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Figure 8. Cartoon showing the tectonic evolution of the South Qilian accretionary belt. (a) Northward intra-oceanic subduction in the South Qilian Ocean during ca. 540–470 Ma; (b) northward subduction of the South Qilian oceanic lithosphere beneath the central Qilian Block during ca. 470–450 Ma resulted in widespread arc magmatic rocks at the southern margin of the central Qilian Block and upper amphibolite facies to low-temperature granulite facies metamorphism of the garnet amphibolites in Hualong Group at ca. 450 Ma; (c) together with Hualong and Quanji blocks, the Qaidam Block accreted to the central Qilian Block during ca. 450–440 Ma; (d) during continental subuduction of the Quanji-Qaidam margin underneath the central Qilian Block and subsequent collision between the central Qilian Block and the Qaidam Block at ca. 440–420 Ma, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.

The P-T paths derived for the garnet amphibolites from the Hualong Group suggest a clockwise P-T evolution. The peak P-T conditions of samples 16HL02 (6.3 kbar, ~820 ℃) and 16HL03 (4.9 kbar, ~755 ℃) record a low pressure and high temperature environment, corresponding to an arc terrane setting and implying a high temperature and low pressure metamorphic belt. The Hualong Group experienced metamorphism and deformation at ca. 450–418 Ma (Wang et al., 2016; Yan et al., 2015; Fan and Lei, 2007). The ca. 450 Ma age of zircon metamorphic overgrowth rims from the garnet amphibolite (Yan et al., 2015) probably represents the time when low-temperature granulite facies peak metamorphism occurred during subduction of oceanic crust underneath the continental margin, which is inferred to be an anomalously hot regime because of the arc terrane setting. Therefore, the oceanic crust-continent subduction may last to ca. 450 Ma (Fig. 8b).

Voluminous 450–420 Ma syn-collisional and post-collisional granitoids that intruded into the Hualong Group and the central Qilian Block were reported (Wang et al., 2017; Cui et al., 2016; Huang et al., 2015; Yan et al., 2015; Yang et al., 2015). Peak ultrahigh pressure metamorphism occurred at 440–420 Ma based on ages obtained from coesite-bearing metapelites and eclogites and diamond-bearing garnet peridotites in the North Qaidam accretionary complex (Zhang J X et al., 2017, and references therein; Xia et al., 2016; Song et al., 2014), which suggested a result from deep subduction of the Qaidam Block beneath the central Qilian Block (Fu et al., 2018b). Therefore, together with the Hualong and Quanji blocks, the Qaidam Block is inferred to have been accreted to the central Qilian Block at ca. 450–440 Ma (Fig. 8c).

During the continent-continent subuduction and collision, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism (Fig. 8d), recorded by a zircon metamorphic overgrowth rim age ca. 440 Ma of type Ⅳ amphibolite (Wang et al., 2016) and a biotite 40Ar/39Ar age 439.6±4.9 Ma of paragneisses (unpublished data) from the Hualong Group. The 418.3±2.8 Ma muscovite 40Ar/39Ar age of the quartzo-feldspathic mylonite in Hualong Group (Fan and Lei, 2007) indicates that the continued continent-continent collision that lasted to ca. 440–420 Ma (Zhang J X et al., 2017; Xia et al., 2016; Song et al., 2014) also affected the Hualong Group until ca. 420 Ma.

6 CONCLUSIONS

(1) Garnet amphibolites from the Hualong Group mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. Garnet has a homogeneous composition of Alm66-71Grs14-17Prp9-12Sps3-5, amphibole is ferro- hornblende, biotite belongs to the ferrobiotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. The mineral assemblages suggest that a peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples.

(2) Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples recorded peak metamorphism at conditions of ~4.9–6.3 kbar and ~755–820 ℃ in the upper amphibolite facies to low-temperature granulite facies, and retrograde cooling and decompression at conditions of ~2.5–3.1 kbar and ~525–545 ℃.

(3) It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block in an arc setting. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.

ACKNOWLEDGMENTS

This paper is dedicated to Prof. Zhendong You for his 90th birthday. The authors are indebted to Dr. Chi-Yu Lee of Department of Geosciences, National Taiwan University for his help with whole-rock major element analysis. This study was funded by the National Natural Science Foundation of China (No. 41520104003), the National Key R & D Program of China (No. 2016YFC0600403), the China Geological Survey (No. DD20160201), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (Nos. CUGL170404, CUG160232). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0850-0.

Electronic Supplementary Materials: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-018-0850-0.


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