Abstract

The Songshugou ophiolite, located in the northern Qinling belt, consists mainly of metamorphosed mafic and ultramafic rocks recording details of deformation and metamorphism that occurred during subduction, accretion, and collision along the Shangdan suture in the Qinling orogenic belt. Electron backscatter diffraction measurements revealed that the harzburgites are dominated by olivine C-type crystal preferred orientations (CPOs), which were possibly induced by high pressure during slab subduction. Olivine A-type CPOs were also observed in some harzburgites, representing the remnants of the original fabric in oceanic mantle rocks formed in the spreading center of the Shangdan ocean. Coarse-grained dunites are characterized by B-type CPOs, which may have been caused by melt-rock reactions and/or high water contents in a suprasubduction-zone setting during exhumation. Fine-grained dunites are also dominated by B-type CPOs, suggesting that grain-size reduction related to mylonitization did not result in fabric variations. Combined with the mineral assemblages, application of geothermometry suggested that the Songshugou ophiolite has experienced metamorphism and deformation under amphibolite-facies conditions. Zircons from garnet-bearing amphibole schist are characterized by flat heavy rare earth element (HREE) patterns and low Th/U ratios and yielded a mass spectrometry U-Pb age of 500.5 ± 8.8 Ma, representing the peak metamorphic age of the metamafic rocks. Other zircons displayed relative HREE enrichment and a clearly negative Eu anomaly and gave an age of 492.5 ± 3.0 Ma, constraining the time of the exhumation of the ophiolite. Integrated with all the available regional geology, our new fabric, geochemical, and geochronological data suggest that the tectonic evolution of the Songshugou ophiolite can be proximately constrained as subduction at ca. 500 Ma and exhumation at ca. 492 Ma.

INTRODUCTION

The Qinling orogenic belt, situated in the middle part of the Central China orogenic system, connects the Dabie orogen to the east and the Qilian and Kunlun orogens to the west (Fig. 1). It has been well documented that the Qinling orogenic belt was formed by polyphase collision between the North China block and the South China block (e.g., Mattauer et al., 1985; Zhang et al., 1995a, 1995b, 2001; Ames et al., 1996; Dong et al., 1998, 2003, 2011a, 2011b; Hacker et al., 1998; Zhai et al., 1998; Ratschbacher et al., 2003, 2006; Bader et al., 2013; Dong and Santosh, 2016; Liu et al., 2016; Tang et al., 2016). Two Paleozoic ophiolitic mélange zones, from north to south, the Erlangping and Shangdan sutures, have been revealed in the Qinling orogenic belt, constraining Paleozoic subduction and collision between the North China block and South China block (Zhang et al., 1995a, 1995b, 2001; Dong et al., 2011a, 2011b; Dong and Santosh, 2016). Although some authors have suggested that the Qinling orogenic belt was formed by southward subduction and collision along the Erlangping suture (Xue et al., 1996; Faure et al., 2001; Wang et al., 2011), most researchers have argued that the Qinling orogenic belt represents the northward subduction of Shangdan oceanic crust between the North China and South China blocks and subsequent collision along the Shangdan suture zone, while the Erlangping suture represents the related back-arc basin (Zhang, 1988; Zhang et al., 1995a; Li et al., 1996; Zhu, 2001; Xu et al., 2002; Dong et al., 2011a, 2011b; Dong and Santosh, 2016). Previous investigations of the geochemistry and geochronology of the ophiolites (Zhang et al., 1995, 2004; Dong et al., 2011b) and magmatic and metamorphic rocks (Liu et al., 1995; Wang et al., 2011), as well as sedimentary assemblages (Meng et al., 1994; Yan et al., 2006; Dong et al., 2013), have constrained the tectonic framework and evolutionary history between the North China and South China blocks; however, the detailed tectonic evolution and processes from subduction to collision along the Shangdan suture zone are still under debate. Ophiolites are the remnants of ancient oceanic crust and upper mantle formed at mid-ocean ridges or suprasubduction zones that were subsequently tectonically emplaced onto continental margins (Pearce, 2008, 2014; Dilek and Furnes, 2011, 2014). Therefore, the structure and fabric of the ophiolite from the Shangdan suture zone may preserve detailed records of deformation responding to tectonic processes ranging from rifting to subduction to collision between the North China and South China blocks.

In this study, we conducted new investigations on the microstructure, crystal preferred orientations (CPOs), mineral geochemistry, and zircon U-Pb ages of the mafic and ultramafic rocks in the Songshugou ophiolite. Our new results provide an opportunity to elucidate the detailed deformation that occurred in response to the subduction of the Shangdan ocean and collision along the Shangdan suture zone, as well as the exhumation of the Songshugou ophiolite on the northern side of the Shangdan suture zone, which may help to better understand the tectonic history of the Qinling orogenic belt.

Geological Setting

The Qinling orogenic belt is currently bounded by the Lingbao-Lushan-Wuyang fault to the north and the Mianlue-Bashan-Xiangguang fault to the south (Zhang et al., 2000; Dong and Santosh, 2016). Separated by the Luonan-Luanchuan fault, the Shangdan suture, and the Mianlue suture/Mianlue-Bashan-Xiangguang fault, the Qinling orogenic belt can be divided into four tectonic belts, from north to south, including the southern sector of the North China block, the North Qinling belt, the South Qinling belt, and the northern sector of the South China block (Fig. 1; Dong et al., 2011a, 2013; Dong and Santosh, 2016).

The southern sector of the North China block was overthrust toward the north onto the Paleozoic–Jurassic successions in the North China block along the Lingbao-Lushan-Wuyang fault (Fig. 1C). The basement is constituted by the Neoarchean–Paleoproterozoic Taihua and Tietonggou Complexes, which are composed dominantly of amphibolite and gneiss. The basement is unconformably overlain by the Mesoproterozoic Xiong’er Group, which consists of low-grade metamorphosed volcanic rocks and minor clastic rocks (Zhang et al., 2001; Dong and Santosh, 2016). The continuous Mesoproterozoic Gaoshanhe and Neoproterozoic Luonan Groups, which, respectively, consist of clastic and carbonate rocks, unconformably overlie the Xiong’er Group. The Luonan Group is occasionally overlain by late Neoproterozoic tillites and Cambrian limestones. In addition, scattered Ordovician, Permian, and Mesozoic strata discontinuously outcrop in the southern sector of the North China block.

The North Qinling belt, located between the Luonan-Luanchuan fault and the Shangdan suture, includes the Kuanping Complex, the Erlangping Group, and the Qinling Complex, from north to south, which are separated from each other by thrust faults or ductile shear zones (Figs. 1B and 1C). The Kuanping Complex includes a Mesoproterozoic to Neoproterozoic ophiolite unit and Neoproterozoic terrestrial clastic sediments (Zhang and Zhang, 1995; Dong et al., 2011b, 2014). The Erlangping Group mainly represents back-arc basin ophiolite related to early Paleozoic northward subduction along the Shangdan suture zone (Sun et al., 1996; Dong et al., 2011a, 2011b). The Qinling Complex chiefly consists of highly deformed and metamorphosed Paleoproterozoic to Mesoproterozoic gneisses, amphibolites, and marbles, which were intruded by voluminous Neoproterozoic and early Paleozoic granitoids, and the complex underwent two-phase amphibolite-facies metamorphism in the Neoproterozoic and early Paleozoic (Dong et al., 2011a). The high-grade basement of the North Qinling belt represents a discrete Precambrian terrane separated from the North China block by the Kuanping Ocean (Dong et al., 2014).

The South Qinling belt was overthrust toward the south onto the South China block along the Mianlue-Bashan-Xiangguang fault during the Middle–Late Triassic and Late Jurassic–Early Cretaceous (Fig. 1C; Zhang et al., 1995b; Dong et al., 2011a, 2015). It is characterized by thin-skinned structures indicating a south-vergent imbricated fold-and-thrust system, and it is composed of pre-Sinian basement overlain by Sinian to Triassic sedimentary successions (Zhang et al., 2001; Dong et al., 2011a, 2015, 2016). In the eastern South Qinling belt, the basement consists of the Mesoproterozoic to Neoproterozoic Wudang and Yaolinghe Groups with volcanic-sedimentary assemblages (Ling et al., 2008) metamorphosed under greenschist-facies conditions (Zhang et al., 2000), while the basement is predominantly composed of Archean–Paleoproterozoic plutonic-metamorphic rocks in the Tongbai-Dabie area (Zhang et al., 2000). The sedimentary cover chiefly includes Sinian clastic and carbonate rocks, Cambrian–Ordovician limestones, Silurian shales, Devonian to Carboniferous clastic rocks with interlayered limestones, and Permian–Early Triassic limestones interlayered with minor sandstones (Dong et al., 2011a, 2013).

The northern sector of the South China block, the foreland fold-and-thrust belt, is separated from the South Qinling belt by the Mianlue-Bashan-Xiangguang fault (Figs. 1B and 1C), and it progressively grades into the undeformed Jurassic to Cretaceous sequences within the Sichuan and Jianghan Basins in the main domain of the South China block. It consists of highly metamorphosed Neoarchean–Paleoproterozoic basement, greenschist-facies metamorphosed Mesoproterozoic/Neoproterozoic transitional basement, and a nonmetamorphosed cover sequence (Zhang et al., 1995a, 2001). Both basement units are exposed in the Huangling and Hannan-Micangshan massifs and are intruded by voluminous Neoproterozoic gabbroic and granitic intrusions (Dong et al., 2011c, 2012). The cover sequence mainly consists of uppermost Neoproterozoic clastic and carbonate rocks, Cambrian–Ordovician limestones, Silurian shales, and Permian–Middle Triassic limestones, which are unconformably covered by Upper Triassic molasses to Cretaceous terrestrial conglomerates and sandstones (Dong et al., 2011a, 2013, 2016).

GEOLOGY OF SONGSHUGOU OPHIOLITE

The Songshugou ophiolite was tectonically emplaced to the northern side of the Shangdan suture zone and thrust into the Qinling Complex of the North Qinling belt (Fig. 2). Together with a series of early Paleozoic ophiolites and subduction-related volcanic and sedimentary rocks along the Shangdan suture zone, the Songshugou ophiolite marks the main tectonic boundary between the North Qinling belt and South Qinling belt (Zhang et al., 1995a, 1995b, 2001; Sun et al., 1996, 2002; Dong et al., 2011a, 2011b). It occurs as a NW-SE lenticular block ∼27 km long and ∼3 km wide (Fig. 2). This tectonic block is bounded by the Jieling ductile shear zone to the north, and it is separated from the Qinling Complex and the Fushui gabbroic intrusion by the Xigou shear zone to the south. The ophiolite consists chiefly of metamorphosed ultramafic and mafic rocks.

The ultramafic rocks occur as tens of rootless blocks tectonically emplaced within the metamorphosed mafic rocks. The largest lenticular outcrop is ∼18 km long and ∼2 km wide, and it is mainly composed of fine- to coarse-grained dunites and harzburgites with minor diopside veins (Figs. 2 and 3). The ultramafic rocks display continuous foliations (Fig. 3A) dipping SW (∼280°–300°) at high angles (∼70°–80°), aligning subparallel to the strike of the massif. Layered and podiform chromites predominantly occur in the coarse-grained dunites (Fig. 3B). Mylonitic deformation is superimposed on the dunites along the margin of the outcrop. Moreover, the dunites have been intruded by numerous diopside veins (Fig. 3C), which have been sinistrally sheared (Fig. 3D) or highly folded (Fig. 3E). Our previous geochemistry and in situ oxygen isotope investigations suggested that the Songshugou ophiolite was probably formed in a mid-ocean-ridge setting (Dong et al., 2008; Sun et al., 2019).

The mafic rocks are mainly of amphibolite, garnet amphibolite, and amphibole schist (Figs. 2 and 3). A few garnet amphibolites occur as discontinuous lenses within the amphibolites. The amphibole schists predominantly crop out as the outer part of the mafic rocks. Strong deformation led to the formation of an intensive foliation and lineation defined by strongly oriented amphiboles and plagioclases. These foliations and lineations are generally parallel to the tectonic contacts between the ultramafic rocks, the mafic rocks, and the Qinling Complex. Some tight folds of foliations (Fig. 3F) were also observed in the amphibole schists near the boundary faults. Several quartz veins occur parallel to the foliation (Fig. 3G). Geochemical investigations suggested that the mafic rocks were derived from a mantle source with a Dupal isotope anomaly (Dong et al., 2008). The garnet amphibolites (Fig. 3H), which occur as discontinuous lenses around the biggest ultramafic slice, were documented as retrograded eclogites (Figs. 2 and 3; Liu et al., 1995; Chen et al., 2015). Some high-pressure (HP) mafic granulites (retrograde metamorphosed eclogites) also crop out within the amphibolites, while a few of the felsic granulites are exposed in the Xigou shear zone (Liu et al., 1996). The formation of the garnet-amphibolites and HP metamorphic rocks was considered to have been associated with the oceanic subduction and emplacement of the Songshugou ophiolite (Liu et al., 1995, 2016; Dong et al., 2008; Chen et al., 2015; Tang et al., 2016).

The formation age of the Songshugou ophiolite has been constrained by maximum Re depletion model ages (TRD) of 1.0–0.8 Ga from the ultramafic rocks (Nie et al., 2017; Yu et al., 2017; Sun et al., 2019), zircon U-Pb ages of 973–729 Ma from the mafic rocks (Liu et al., 2004; Qian et al., 2013; Li et al., 2014; Chen et al., 2015; Yu et al., 2016), and a 40Ar-39Ar age of 848 ± 4 Ma from the diopside vein (Chen et al., 2002).

SAMPLE DESCRIPTIONS

Representative samples of the ultramafic and mafic rocks were collected from the two sections in the Songshugou valley and the Tu’ao area (Fig. 2), and a subset was selected for electron microprobe analyses (EMPA), electron backscatter diffraction (EBSD) measurements, and zircon U-Pb dating.

The ultramafic rocks mainly included harzburgites and coarse-grained and fine-grained dunites (Table 1). The harzburgites were predominantly composed of olivine (∼80%) and orthopyroxene (∼10%), with minor clinopyroxene (0%–3%), chromite (0%–4%), and serpentine (0%–5%). They were mainly characterized by porphyroclastic texture with coarse-grained (up to 2–3 mm) olivine and pyroxene porphyroclasts. The olivine porphyroclasts generally displayed elongated shapes with aspect rations >2 and exhibited high-temperature plastic deformation features, such as undulose extinction, subgrain boundaries, and 120° triple junctions. Some orthopyroxene gains were elongated and displayed kink bands (Fig. 4A); subgrain boundaries were also observed (Fig. 4B). The foliations and lineations were defined by shape preferred orientations (SPOs) of olivines and/or orthopyroxenes (Figs. 4A and 4B).

The medium- to coarse-grained dunites were mainly composed of olivine (>90%), with minor pyroxene (0%–5%), chromite (0%–3%), and serpentine (0%–2%). They were of equigranular (∼0.2–0.5 mm) or porphyroclastic texture, and frequently serpentinized. Dislocation indicators such as undulose extinction and subgrain boundaries of olivines were commonly observed (Figs. 4C and 4D). The foliations and lineations were defined by SPOs of olivines as observed in the field (Figs. 3A–3D). Some dunites were composed of chromite-rich layers, which were parallel to the foliations (Fig. 3B). In addition, the diopside veins were commonly concordant with the foliation of dunites (Figs. 3C–3E). Moreover, orthopyroxene grains were generally in corroded shape and surrounded by fine-grained olivine neoblasts and sparse spinels, suggesting the rocks may have resulted from reaction of orthopyroxene = olivine + SiO2-rich melt between peridotite and boninitic melt, which led to dissolution of pyroxene and precipitation of olivine (Kelemen et al., 1995; Malpas et al., 2003; Tommasi et al., 2004; Sun et al., 2019).

The fined-grained dunites or mylonitic dunites were characterized by intensive ductile deformation showing elongated or fine-grained fabric, with grain size of 0.1–0.2 mm, suggesting that the ophiolite experienced high-temperature plastic deformation. Olivine porphyroclasts occasionally were observed, displaying undulose extinction and/or subgrain boundaries. The foliation and lineations were distinct in these samples, defined by SPOs of olivine and chromite (Figs. 4E–4F). The lineation was cut by anthophyllite and tremolite crystals (Fig. 5). Anthophyllite and tremolite are amphibolite minerals, which are mostly formed by fluid-rock reactions of orthopyroxene + H2O = anthophyllite + olivine, and orthopyroxene + clinopyroxene + H2O = tremolite + olivine, respectively (Cao et al., 2016).

The mafic samples are represented by garnet-bearing amphibole schist, which chiefly consisted of amphibole (45%–55%), plagioclase (20%–40%), and garnet (0%–10%), with minor quartz, apatite, ilmenite, and epidote (Figs. 6A–6C). The garnet grains were partly or totally replaced by neocrystals of plagioclase. A typical retrograde fabric was characterized by fine-grained new crystals of plagioclase surrounding garnet (Fig. 6C), which probably resulted from reaction of garnet + quartz + H2O = Ca-amphibole + plagioclase. The amphibole grains were brownish and bluish in color and exhibited a strong SPO parallel to the foliation and regional structural orientation. Plastic deformation features, which are rarely reported in experimentally or naturally deformed amphiboles (Nyman et al., 1992; Shelley, 1994; Berger and Stünitz, 1996; Imon et al., 2004; Aspiroz et al., 2007), were also not observed in the amphiboles of our samples. Quartz exhibited undulose extinction and bulging dynamic recrystallization behaviors, indicating low-temperature plastic deformation (Fig. 6D; Passchier and Trouw, 2005; Law, 2014).

ANALYTICAL METHODS

Mineral Geochemistry

The major-element compositions of minerals were analyzed using an electron probe microanalyzer (EPMA; JEOL JXA-8230) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. Operating conditions were: accelerating voltage of 15 kV, beam current of 10 nA, and beam diameter of 1 mm. Microprobe standards of natural and synthetic phases were supplied by SPI Company (West Chester, Pennsylvania, USA): diopside for Si, olivine for Mg, hematite for Fe, almandine for Al, Cr2O3 for Cr, diopside for Ca, jadeite for Na, K-feldspar for K, rhodonite for Mn, rutile for Ti, NiSi for Ni, and SrSO4 for Sr.

EBSD Measurements

The mineral CPOs were measured using a scanning electron microscope (JEOL JSM-6490) equipped with an electron backscatter diffraction system (Oxford Nordlys S with HKL Channel 5) at the State Key Laboratory for Mineral Deposition Research, Nanjing University, Nanjing, China. Operating conditions were as follows: 20 kV accelerating voltage, 17–23 mm working distance, and 70° specimen tilt. About 200–300 grains were measured for olivine, plagioclase, and amphibole per thin section, and the computerized indexation of the diffraction pattern was manually checked for each orientation. All index data were determined with a mean angular deviation of <1°.

The CPO strength is described by the J-index, corresponding to a volume-averaged integral of the squared orientation densities. The J-index has a value of 1 for random fabric and infinity for a single crystal (Michibayashi and Mainprice, 2004), and most natural deformed peridotites yield values between 2 and 20 (Ben Ismaïl and Mainprice, 1998). The pfJ index, describing distribution density, equals 1 for random distribution and has a maximum value of ∼60 and 280 for single crystals of olivine and amphibole, respectively (Michibayashi et al., 2006; Barberini et al., 2007). Both the J-index and pfJ-index were calculated for each sample to constrain the CPO data (Mainprice et al., 2000; Michibayashi and Mainprice, 2004; Skemer et al., 2006).

Zircon U-Pb Dating

The U-Th-Pb measurements were obtained using the laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, following the methodology described in Liu et al. (2007). Zircons were extracted from a garnet-bearing amphibole schist sample (SSG04) using standard heavy-liquid and magnetic separation methods and then purified by handpicking under a binocular microscope. A random population of zircons was mounted on adhesive tape, enclosed in epoxy resin, and polished to expose the interior. The internal texture of zircons was observed using cathodoluminescence (CL) images prior to the analysis. The laser-ablation system was a GeoLas 200 M equipped with a 193 nm ArF-excimer laser, and a homogenizing and imaging optical system (MicroLas, Göttingen, Germany). Analyses were performed on the ELAN 6100 ICP-MS from Perkin Elmer/SCIEX (Canada) with a dynamic reaction cell (DRC). The laser-ablation spot size was ∼30 μm, with a rate of 10 Hz and energy of up to 90 mJ. The 207Pb/206Pb, 206Pb/238U, 207Pb/235U, and 208Pb/232Th ratios were calculated using GLITTER 4.0 (Macquarie University) and were corrected for both instrumental mass bias and depth-dependent elemental and isotopic fractionation using Harvard zircon 91500 as the external standard. The ages were calculated using Isoplot 3 (Ludwig, 2003). Common Pb corrections were made following the method of Andersen (2002). Age uncertainties are quoted at the 95% confidence level.

RESULTS

Mineral Chemistry and Geothermometry

The geochemical compositions of individual minerals in harzburgite (samples SSG09–2, SSG11, SSG13–3, SSG17–2, SSG17–3, and SSG19–1), dunite (samples SSG10, SSG12, SSG18–3, and SSG19–4), and amphibolite (samples SSG22–1, SSG22–4, SSG22–5, and SSG06) samples were analyzed by EMPA, and the results are listed in Table DR1 in the GSA Data Repository.1

The olivines were characterized by high forsterite (Fo) contents, ranging from 0.90 to 0.91 (Fig. 7A). The spinels are chromites, as illustrated in the Fe3+-Cr-Al ternary diagram (Fig. 7B). These chromite grains showed a large variation in both Mg# [= Mg/(Mg + Fe2+) atomic ratio] and Cr# [ = Cr/(Cr + Al) atomic ratio], ranging from 0.11 to 0.34 and from 0.65 to 0.97, respectively (Fig. 7C). The TiO2 content of spinel varied in the range of 0–0.288 wt% (Fig. 7D). Together with the microstructure (Fig. 4), the high Cr# and low TiO2 in the spinel, and the high Fo in olivine (Fig. 7) suggest that these samples were associated with boninitic melt in a suprasubduction zone (Dick and Bullen, 1984; Arai, 1994a, 194b; Tamura and Arai, 2006), which was also found in our previous study (Sun et al., 2019). Reaction between mantle peridotites and boninitic melt resulted in the dissolution of pyroxene and precipitation of olivine, forming lenticular dunites (Berger and Vannier, 1984; Kelemen et al., 1995; Malpas et al., 2003; Tommasi et al., 2004). This is also supported by the texture and chemical characteristics of multiphase mineral inclusions in the Songshugou ultramafic rocks (Cao et al., 2016). In addition, EPMA analyses revealed that orthopyroxenes have compositions of enstatite (En) 0.90–0.91, and clinopyroxenes are diopsides. In the amphibolite samples, plagioclase compositions were in the range An 0.36–0.48, while amphiboles were typical hornblende.

The equilibrium temperatures of the dunite samples were estimated using the olivine-spinel thermometer (Ol-SplBal91) based on Fe2+-Mg exchange (Ballhaus et al., 1991), the orthopyroxene thermometer (OpxWS91) based on Al and Cr contents (Witt-Eickschen and Seck, 1991), the orthopyroxene thermometer (OpxBK90) based on Ca contents (Brey and Köhler, 1990), and the clinopyroxene-orthopyroxene thermometer (Cpx-OpxBK90) based on Fe-Mg exchange (Brey and Köhler, 1990). For amphibolite, the hornblende-plagioclase thermometer (Hb-PlHB94) based on nonideal interactions (Holland and Blundy, 1994) was applied to calculate the equilibrium temperatures. Assuming a pressure of 1.5 GPa, which corresponds to equilibration in the spinel stability field and the stability of the peak assemblage in the garnet amphibolite (Tang et al., 2016), the results of the calculation of the equilibrium temperatures are listed in Table 2.

For the dunites, the Ol-SplBal91 thermometer yielded temperatures between 604 ± 6 °C and 640 ± 15 °C, which obviously underestimate the equilibrium temperatures, because the Ol-SplBal91 thermometer gives reasonable results for temperatures higher than 800 °C in spinel peridotite (Ballhaus et al., 1991). The OpxWS91 and OpxBK90 thermometers gave temperatures of 751 ± 17 °C and 813 ± 49 °C for the dunite sample SSG10, respectively. For the harzburgite sample SSG13–3, the application of OpxWS91, OpxBK90, and Cpx-OpxBK90 thermometers yielded equilibrium temperatures of 812 ± 30 °C, 807 ± 30 °C, and 772 ± 15 °C, respectively. For the samples of amphibolite, assuming a pressure of 1.0 GPa, the temperatures estimated using the Hb-PlHB94 thermometer varied from 631 ± 28 °C to 695 ± 18 °C.

Crystal Preferred Orientations

The CPO data from the olivines in harzburgite (Fig. 8) and the coarse-grained (Fig. 9) and fine-grained (Fig. 10) dunite, as well as those of amphiboles and plagioclases in amphibolite (Fig. 11), are presented on equal-area, lower-hemisphere projections in the structural (x-z) reference frame, in which the foliation (x-y plane) is vertical and the lineation (x) is horizontal.

In general, the CPOs of olivine can be mainly classified into five types according to their different slip systems: (010)[100] for A-type, (010)[001] for B-type, (100)[001] for C-type, {0kl}[100] for D-type, and (001)[100] for E-type olivines, respectively (Jung and Karato, 2001; Katayama et al., 2004; Jung et al., 2006; Raterron et al., 2009, 2012; Michibayashi and Oohara, 2013; Hansen et al., 2014; Sun et al., 2016).

The harzburgites dominantly display a C-type olivine CPO (samples SSG08, SSG09–2, SSG17–3, SSG19–1, and SSG19–2), which is characterized by [001] axis concentrations close to the lineation (x direction), [100] axis concentrations normal to the foliation (z direction), and [010] axis concentrations perpendicular to the lineation on the foliation plane (y direction; Fig. 8). The J-index values for these samples were high, varying from 5.38 (SSG08) to 19.64 (SSG19–1), while the pfJ-index values for the [100], [010], and [001] axes were similar for each sample, varying from 1.41 to 4.74. However, some coarse-grained (samples SSG11, SSG13–3, and SSG17–2) and fine-grained (SSG-L5 and SSG-X6) harzburgite samples displayed A-type fabric, defined by the point maxima of [100] axes subparallel to the lineation and [010] axes normal to the foliation (Fig. 8).

The olivines from coarse-grained dunites (samples SSG10, SSG17–6, SSG18–3, SSG19–4, and SSG19–7) showed a preferential alignment of [001] axes parallel to the lineation and [010] axes normal to the foliation, suggesting B-type CPO. Their J-index and pfJ-index values varied from 5.96 to 19.50, and from 1.44 to 4.40, respectively. The fine-grained dunites (samples SSG-L1, SSG-L3, SSG-L8, and SSG-X4) displayed B-type fabric as well (Fig. 10), although some of the CPOs were not strong (e.g., sample SSG-L1).

In the metamafic rocks (samples SSG02–1, SSG06, SSG22–4, and SSG22–5), the amphibole crystals in the amphibole schists displayed strong and consistent CPO patterns (Fig. 11), which were characterized by [001] axis concentrations close to the lineation (x direction), and (100) pole concentrations perpendicular to the foliation (z direction). The distribution of (010) poles was more complicated, with a single concentration at y (SSG02–1), a partial girdle parallel to the foliation with the maximum concentration at y (SSG06), in the middle between y and z (SSG22–4), or a partial girdle perpendicular to x with the maximum concentration at y (SSG22–5). The J-index varied from 10.64 to 12.95. The pfJ-index, varying respectively from 1.71 to 3.43 for (100) and from 1.59 to 3.02 for [001], was higher than that for (010), which varied from 1.36 to1.54. These features suggest that the CPO strengths of amphibole (100) and [001] axes were stronger than (010). However, the plagioclases in the amphibolite samples displayed almost random CPOs, as shown in Figure 11, and their pfJ-index values for each principal crystallographic axis were consistently lower than 1.4.

Zircon U-Pb Geochronology

The zircon grains from the garnet-bearing amphibole schist (sample SSG04) can be classified into two groups according to their CL morphology and Th/U ratios. Group I zircons, including seven representative grains (Fig. 12), were anhedral to subhedral and 50–200 μm long with aspect-ratios of 1–2. These zircon gains were inhomogeneous and gray in color, and some of them showed distinct cores and thin outer rims in the CL images (Fig. 12A). Their Th and U contents varied from 0.028 to 0.544 ppm and 3.68–28.8 ppm, respectively, with very low Th/U ratios of 0.01–0.03 (Table DR2), suggesting their metamorphic origin. This conclusion is also supported by their very low rare earth element (REE) contents (Table DR3). Except one zircon (SSG04–09N) that showed enrichment of heavy (H) REEs, the other six zircons exhibited flat HREE patterns (Fig. 12B). All seven analyses plotted on or near the concordia tight cluster (Fig. 12C), yielding a weighted mean 206Pb/238U age of 500.5 ± 8.8 Ma (mean square of weighted deviates [MSWD] = 0.60; Fig. 12D). Taking together all the CL images, flat HREE distribution patterns, and low Th/U ratios (0.01–0.03), this age can be interpreted as the metamorphic age of the garnet-bearing amphibole schist in the Songshugou ophiolite.

Group II zircons showed distinctive morphology in CL images and Th/U ratios from those of group I zircons. This group included 20 grains, dark in color and mostly homogeneous in CL images (Fig. 13A). Twenty spots were analyzed on different grains, which displayed Th/U ratios ranging from 0.11 to 1.34, and Th and U contents varying from 102 to 1213 ppm and from 240 to 1282 ppm, respectively (Table DR2). In addition, group II zircons exhibited enrichment of HREEs with a negative Eu anomaly (Fig. 13B; Table DR3), which are different from the features of group I zircons. Twenty analyses displayed concordia 206Pb/238U ages varying from 505 Ma to 480 Ma (Table DR2), giving a weighted mean 206Pb/238U age of 492.5 ± 3.0 Ma (MSWD = 1.4; Fig. 13D).

DISCUSSION

Deformation Fabrics

Based on investigations of experimentally and natural deformed samples, A, B, C, D, and E types of olivine CPOs have been distinguished and documented during the past two decades (Jung and Karato, 2001; Katayama et al., 2004; Raterron et al., 2009, 2012; Michibayashi and Oohara, 2013; Sun et al., 2016). The development of these five fabric types depends mainly on the roles of temperature, pressure, stress, water content, strain rate, etc. (Jung and Karato, 2001; Jung et al., 2006; Katayama and Karato, 2006; Michibayashi and Oohara, 2013). The CPOs may be continuously modified by changing physical conditions until frozen at low temperatures (Carter and Avé Lallemant, 1970; Jung and Karato, 2001; Katayama et al., 2004).

The medium- to coarse-grained harzburgite samples are dominated by olivine C-type fabric (samples SSG08, SSG09–2, SSG17–3, SSG19–1, and SSG19–2). C-type CPO is related to dislocation of the (100)[001] slip system, resulting in the maximum concentrations of [100], [010], and [001] axes aligned parallel to the z, y, and x direction, respectively. This olivine CPO is relatively rare in natural samples and poorly understood (Ben Ismaïl and Mainprice, 1998; Mainprice, 2007; Wang et al., 2013). It has been suggested that C-type fabric could be induced by plastic flow under low temperatures and a high strain rate (Carter and Avé Lallemant, 1970), or high water content (Jung and Karato, 2001; Frese et al., 2003), or flow under high pressure (Couvy et al., 2004; Raterron et al., 2007). C-type fabrics have been reported in some orogenic garnet peridotite from ultrahigh-pressure (UHP) metamorphic terranes, such as the Sulu terrane (Xu et al., 2006) and the North Qaidam terrane (Jung et al., 2013) in China and the Western Gneiss Region, Norway (Katayama et al., 2005; Wang et al., 2013), considered as an indicator of UHP and low geothermal gradient (Xu et al., 2006; Wang et al., 2013). In the Songshugou ophiolite, the C-type CPOs observed in the coarse-grained harzburgites were inferred to be induced from the high pressure during subduction of Shangdan oceanic lithosphere, which is consistent with the occurrence of HP and UHP rocks in the surrounding mafic massif (Liu et al., 1995, 2016; Chen et al., 2015; Tang et al., 2016).

Three medium- to coarse-grained (samples SSG11, SSG13–3, and SSG17–2) and two fine-grained (SSG-L5 and SSG-X6) harzburgites are characterized by A-type fabric, which is characterized by [100], [010], and [001] axis concentrations close to the x, z, and y direction, respectively (Fig. 8). The A-type CPO is most common in the normal upper mantle (Mainprice and Silver, 1993; Ji et al., 1994; Jung et al., 2006), which has been widely reported in mantle xenoliths from kimberlites and basalts (Nicolas and Christensen, 1987; Mainprice and Silver, 1993; Ji et al., 1996; Saruwatari et al., 2001; Ohuchi and Irifune, 2014), and it is broadly used to explain seismic data observed in the upper mantle (e.g., Silver, 1996; Savage, 1999). The observation of A-type CPO is also consistent with the geochemistry and O isotope investigations, which suggested that the Songshugou peridotites represent a segment of mid-ocean-ridge mantle (Zhang, 1995; Dong et al., 2008; Lee et al., 2010; Sun et al., 2019). Therefore, the A-type CPOs in some harzburgites probably represent the remnant of original CPOs that were formed in the spreading center of an oceanic upper mantle. The original fabric can be progressively modified at different physical conditions, and the C-type CPOs in the harzburgite probably represent the more intense fabric replacing the A-type CPOs.

The coarse-gained dunites (samples SSG10, SSG17–6, SSG18–3, SSG19–4, SSG19–7) are dominated by B-type CPOs (Fig. 9), which are characterized by the maximum concentrations of olivine [100], [010], and [001] axes aligned parallel to the y, z, and x direction, respectively (Fig. 9). The B-type fabric was previously considered to exist in upper-mantle regions with extremely high water content and high differential stress (Jung and Karato, 2001; Jung et al., 2006; Karato et al., 2008). Recent experimental studies revealed that B-type CPOs may also be induced by high pressure in low water content (Palasse et al., 2012; Lee and Jung, 2015) or completely dry environments (Raterron et al., 2007; Jung et al., 2009; Sundberg and Cooper, 2008). Therefore, B-type CPOs in natural rocks may have two origins: water-induced and pressure-induced origins. The former, induced by high H2O fugacity at low pressure, is likely to be present in the forearc mantle wedge (Kneller et al., 2005, 2007, 2008; Katayama and Karato, 2006), where large volumes of water are released from dehydration reactions in the subducting slab. It agrees with the seismic anisotropy pattern in some mantle wedges, such as Kamchatka, NE Japan, Tonga, and Ryukyu (Mizukami et al., 2004; Nakajima and Hasegawa, 2004; Katayama and Karato, 2006; Long and van der Hilst, 2006; Tasaka et al., 2008; Huang et al., 2011). The latter, induced by high pressure, possibly exists beneath the downgoing plate in subduction zones (Jung et al., 2009). In this case, the observed trench-parallel shear-wave splitting below the subducting slab will be easier to interpret based on trench-perpendicular rather than trench-parallel mantle flow (Russo and Silver, 1994; Peyton et al., 2001; Long and Silver, 2008). In the Songshugou ophiolite, the B-type CPOs were restricted in the dunites, which resulted from the reaction between boninitic melt and harzburgites in a suprasubduction-zone setting (Cao et al., 2016; Sun et al., 2019). Therefore, we suggest that the olivine B-type CPOs were most likely to be formed due to the high water content in the suprasubduction-zone setting. This is also supported by the melt-induced B-type CPO reported in experimental samples (Holtzman et al., 2003).

The fine-grained dunites are dominated by B-type CPOs (samples SSG-L1, SSG-L3, SSG-L8, SSG-X4), which were probably induced by the melt-rock reactions occurring in the same suprasubduction-zone setting as those fabrics in coarse-grained dunites. However, this grain-size reduction was inferred to occur at relatively low temperatures and pressures, which did not result in olivine fabric weakening or fabric variation. This is also consistent with the A-type fabric observed in both fine-grained and coarse-grained harzburgites.

The CPOs of amphibole in our samples are characterized by the [001] axis and (100) pole concentrations close to the x and z directions, respectively, which are consistent with those measured previously in middle- to high-grade metamorphic rocks (Christensen, 1965; Siegesmund et al., 1989; Imon et al., 2004; Barberini et al., 2007; Tatham et al., 2008; Ji et al., 2013). The formation of amphibole CPO is interpreted by either [001] slip along the (100) or (110) plane (Dollinger and Blacic, 1975; Biermann and Van Roermund, 1983; Reynard et al., 1989; Barruol and Kern, 1996) or anisotropic growth (grain boundary migration) and passive rigid-body rotation under various different stresses (Ji et al., 1993; Shelley, 1994; Díaz Aspiroz et al., 2007; Tatham et al., 2008). At present, it is generally accepted that amphibole is one of the strongest and least plastic common rock-forming minerals during deformation (e.g., Shelley, 1994). Because few plastic features such as subgrains, dynamically recrystallized grains, or undulose extinction were observed in the samples, the CPO of amphiboles may have resulted from anisotropic growth or rigid-body rotation, rather than intracrystalline plasticity, which occurs at temperatures higher than amphibolite facies or 700 °C (Nyman et al., 1992; Lafrance and Vernon, 1993; Berger and Stünitz, 1996; Imon et al., 2004). It is also possible that the CPO of amphiboles in retrograde eclogites occurred through mimetic growth after a preexisting CPO of omphacites (McNamara et al., 2012; Heidelbach and Terry, 2013). Similarly, plagioclase grains that were formed during retrograde processes show random crystal orientations. Although the mafic rocks of the Songshugou ophiolite display strong foliation and lineation, it seems that they have not developed dislocation slip–induced fabrics since retrogression from eclogite to amphibolite.

Deformation Conditions

According to the mineral compositions, the deformation temperature was estimated by geothermometry. Assuming pressure (P) = 1.5 GPa, which characterizes the spinel stability field, the application of the OpxWS91, OpxBK90, and Cpx-OpxBK90 thermometers for ultramafic samples yielded temperatures ranging from 751 °C to 813 °C, indicating the equilibrium temperature during the melt-rock reaction that dissolved orthopyroxenes and formed olivine neoblasts. This temperature range is also supported by the metamorphic mineral assemblages. For instance, anthophyllite and tremolite crystals, which are formed by fluid-rock reactions, were observed in fine-grained dunite (Fig. 5A). As illustrated in Figure 5B, anthophyllite is stable at pressures lower than 1 GPa and temperatures of 650–800 °C, while tremolite could be stable at temperatures <900 °C. The coexistence of olivine, tremolite, anthophyllite, and talc in the ultramafic samples may indicate pressure-temperature (P-T) conditions of amphibolite-facies metamorphism (Winter, 2001; Bucher and Grapes, 2011). This is consistent with those in the metamafic rocks.

For mafic samples, the temperatures estimated using the Hb-PlHB94 thermometer varied in the range of 631 °C to 695 °C, i.e., slightly lower than those of the ultramafic samples. This temperature may reflect the retrogression of equilibrium temperatures, as the garnet grains were partly or totally replaced by plagioclase (Fig. 4C). According to the clockwise P-T path and characteristic retrograde textures reported by Tang et al. (2016), the mafic rocks experienced a rapid decompression and cooling history after the peak metamorphism, and the garnets were replaced by plagioclase at ∼<730 °C, agreeing with our data.

Timing of Deformation

The timing of metamorphism and deformation could be revealed by the U-Pb ages of the two groups of zircons separated from the garnet-bearing amphibole schist. Group I zircons are mainly characterized by flat HREE patterns (Fig. 12B), which probably resulted from the coexistence with garnet, because the growth of garnet could progressively deplete the limited HREE reservoir in a closed system (Rubatto, 2002; Whitehouse and Platt, 2003; Wu and Zheng, 2004). Together with the very low Th/U ratios (0.01–0.03), group I zircons are considered to be metamorphic zircons. Therefore, the weighted mean 206Pb/238U age of 500.5 ± 8.8 Ma most likely constrains the peak metamorphic age of the mafic rocks in the Songshugou ophiolite. This age is synchronous with the HP and UHP peak metamorphism in the North Qinling belt (Dong and Santosh, 2016; Liu et al., 2016), which has been well documented by the previously reported U-Pb zircon ages, such as 515 ± 12 Ma (Tang et al., 2016), 501 ± 9 Ma (Liao et al., 2016), 500 ± 8 Ma (Chen et al., 2015), 504 ± 7 Ma (Zhang et al., 2011), 510 ± 8 Ma (Liu et al., 2009), and 511 ± 35 Ma (Yang et al., 2003).

Group II zircons were most likely concentrated in the thin quartz veins paralleling the foliations of the amphibolite (Fig. 3G). Unlike the trace-element characteristics of group I, group II zircons display relative HREE enrichment and a clearly negative Eu anomaly (Fig. 13B). As the pressure decreases during the exhumation processes, garnets host significant HREEs in the mafic rocks and are unstable and progressively replaced by plagioclase, which is strongly enriched in Eu (Fig. 6C). Therefore, the metamorphic overgrowth group II zircons show HREE enrichment and a negative Eu anomaly. Furthermore, the high Th/U ratios of 0.11–1.34 suggest that group II zircons may have been crystallized from melt with a high Th/U ratio. This is coincident with the occurrence of quartz veins, which are parallel to the foliations and relate to metamorphic differentiation in the garnet amphibolites (Fig. 3G). According to their dark color, morphology in the CL images, enrichment of HREE, and high Th/U ratio (0.11–1.34) of the group II zircons, we interpret that 492.5 ± 3.0 Ma represents the crystallization age of the new generation of zircons that were related to fluids during the rapid exhumation of the Songshugou ophiolite. This age is close to the reported retrograde metamorphic ages in the North Qinling belt (Dong and Santosh, 2016; Liu et al., 2016), such as 470 Ma (Bader et al., 2013), 473 ± 4 Ma (Cheng et al., 2011), and 471 ± 8 Ma (Liao et al., 2016).

Tectonic Implications

As a fragment of oceanic lithosphere, the Songshugou ophiolite preserves a detailed record of the tectonic evolution of the Shangdan ocean between the North China block and South China block. Together with previous petrological and geochemical studies of the mafic rocks, the present investigations on the fabrics and geochronology of the deformed mafic and ultramafic rocks provided insights into the tectonic processes along the Shangdan suture zone between the North Qinling belt and South Qinling belt.

According to the geochemical and geochronological data from ophiolites in the North Qinling belt (Dong et al., 2011a; Dong and Santosh, 2016, and the references therein), the spreading of the Shangdan ocean between the North China and South China blocks has been well documented before ca. 534 Ma (Fig. 14A). Due to the extensional stress, the mantle flow in the oceanic lithosphere induced the olivine A-type fabric in the ultramafic rocks of the Songshugou ophiolite, which is the most prevalent fabric formed in normal mantle (Mainprice and Silver, 1993; Ji et al., 1994; Jung et al., 2006).

Up to ca. 500 Ma, deep subduction of the Shangdan oceanic lithosphere underneath the North Qinling belt is indicated by the peak metamorphic P-T conditions of 750–850 °C and 1.5–1.9 GPa (Tang et al., 2016), and a number of zircon U-Pb geochronological data points for the UHP rocks. Our group I metamorphic zircons from garnet amphibolite yielded a LA-ICP-MS U-Pb age of 500.5 ± 8.8 Ma, which is consistent with the U-Pb zircon ages around 500 Ma for the eclogite-facies metamorphism in the literature (Chen et al., 2004; Qian et al., 2013; Liu et al., 2016; Tang et al., 2016). Accordingly, the Songshugou ophiolite is proposed to have subducted underneath the North Qinling belt at ca. 500 Ma. During the subduction process, the increasing high pressure caused the olivine C-type CPO in the ultramafic rocks (Fig. 8). Due to the HP-UHP metamorphism and increasing density, a slab break-off of the subducted oceanic crust occurred, which generated the diapir of the Songshugou mafic-ultramafic complex (Fig. 14B).

At around 492 Ma, the ongoing subduction led to the Songshugou mafic-ultramafic complex thrusting and wedging into the North Qinling belt (Fig. 14C). Due to the pressure and temperature decrease, the eclogite evolved into retrograde metamorphism to form the garnet amphibolite (Fig. 6). The related equilibrium temperatures of retrogression is constrained within 631–695 °C by the application of Hb-PlHB94 thermometer. Our group II zircons from the quartz veins in the amphibole schist yielded a 206Pb/238U age of 492.5 ± 3.0 Ma, constraining the timing of this event. Several lines of geochemical (Fig. 7) and petrological evidence (Figs. 46) indicate that the ultramafic rocks of the Songshugou ophiolite had reacted with melt and fluid as they were exhumed to forearc mantle depth. The boninitic melt-rock reactions dissolved orthopyroxenes and formed olivine neoblasts (Fig. 4) at temperature of 751–813 °C, leading to dunitization. The subsequent fluid-rock reactions occurred at P < 1 GPa and T = 650–800 °C and resulted in the growth of tremolite, anthophyllite, and talc in the ultramafic rocks (Fig. 5). Because large volumes of water in the forearc mantle region were released from dehydration reactions in the subducting slab, the olivine B-type CPO in the dunites may have been induced by the high water content.

Finally, the continuing subduction of the Shangdan oceanic crust during ca. 486–430 Ma, which was constrained by the adjacent subduction-related I-type Huichizi granitoid (Dong and Santosh, 2016), led to the Songshugou mafic-ultramafic complex being thrust into the Qinling Complex and exhumed along the Xigou and Jieling ductile shear zones (Fig. 14D). Due to the intensive regional compression, the Qinling complex and a slice of the Songshugou ophiolite were highly sheared, leading to the mylonitization and foliations along the tectonic boundaries between the different units.

CONCLUSIONS

Together with regional geology, the fabric, geochemical, and geochronological data for the Songshugou ophiolite presented in this study lead to the following major conclusions.

  • (1) Mineral geochemistry and geothermometry investigations suggest that the ultramafic rocks of the Songshugou ophiolite have experienced boninitic melt-rock reactions at temperatures of 751–813 °C and that retrograde metamorphism of the mafic rocks occurred at 631–695 °C.

  • (2) The harzburgites are dominated by C-type CPOs, which may have resulted from high pressure during slab subduction. A-type CPOs are also observed in some harzburgites, representing a remnant of the original fabric formed in the spreading center of the Shangdan ocean. The B-type CPOs were predominantly developed in the dunites, probably induced by the high water content in the mantle wedge.

  • (3) Zircons from garnet-bearing amphibole schist are divided into two groups: group I zircons with flat HREE patterns and low Th/U ratios, which yielded LA-ICP-MS U-Pb ages of 500.5 ± 8.8 Ma, representing the peak metamorphic age, and group II zircons showing relative HREE enrichment and a clearly negative Eu anomaly, which gave an age of 492.5 ± 3.0 Ma, constraining the timing of exhumation.

  • (4) The Songshugou ophiolite had evolved into high-temperature ductile deformation in spreading center of the Shangdan ocean, subduction-related metamorphism, and ductile deformation with peak metamorphic conditions of 750–850 °C at ca. 500 Ma, and exhumation below 631–695 °C at ca. 492 Ma.

ACKNOWLEDGMENTS

Financial support for this study was provided by the National Natural Science Foundation of China (grant nos. 41421002 and 41772208), the National Key Research and Development Program of China (no. 2016YFC0601003 and no. 2016YFC0600202), and the MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University. The crystal preferred orientation figures and calculations of the J-index were made using the programs of D. Mainprice. The critical and constructive reviews by R. Keppler and an anonymous reviewer are highly appreciated.

1GSA Data Repository Item 2019338, Table DR1: Geochemistry of Representative Minerals from Dunite and Amphibolite; Table DR2: LA-ICPMS U-Pb Data for Zircons from Garnet Bearing Amphibole Schist (SSG04); Table DR3: Rare Earth Element Compositions (PPM) of Zircons from Sample SSG04, is available at http://www.geosociety.org/datarepository/2019, or on request from editing@geosociety.org.
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.