Journal of Earth Science  2019, Vol. 30 Issue (3): 460-475   PDF    
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Is the Songshugou Complex, Qinling Belt, China, an Eclogite Facies Neoproterozoic Ophiolite?
Thomas Bader 1,2, Lifei Zhang 1, Xiaowei Li 2     
1. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China;
2. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
ABSTRACT: Ophiolites play a key role in understanding subduction-accretion-collision processes. Herein, we discuss origin and metamorphic evolution of an enigmatic, Neoproterozoic ophiolite candidate-the mafic-ultramafic Songshugou Complex, Qinling belt, China-summarizing published thermobarometry, U/Pb geochronology, and geochemistry and presenting new phase equilibrium modeling. Garnet, rarely preserved in amphibolites of the Songshugou Complex, has prograde zoning and low-pyrope cores[Alm54-71 (Grs+And)25-30Prp1-6Sps5-12]. It formed at quartz eclogite facies conditions of 1.93-2.54 GPa, 462-542¦. During exhumation, garnet first was mantled by plagioclase-rich coronas at about 0.7-1.2 GPa, 660-710¦. During an isothermal uplift to 0.5-0.8 GPa, these coronas evolved widely into σ-shaped aggregates and eventually into whitish ribbons oriented with a steeply southwest dipping mineral stretching lineation. The exhumation into middle-upper crustal levels proceeded till the Late Devonian. The oceanic protoliths of the eclogites were emplaced into continental crust in the Neoproterozoic and dragged into a subduction zone in North Qinling in the Cambrian. The ultramafic rocks of the Songshugou Complex were not subducted with the mafic rocks in a coherent block given the absence of garnet but ubiquitous occurrence of spinel implies a P maximum of~1.7 GPa. Rather, mafic and ultramafic rocks belonged to downgoing and overriding plate, respectively. They were juxtaposed at 0.8-1.7 GPa at Early Ordovician time.
KEY WORDS: Qinling belt    Songshugou Complex    garnet isopleth thermobarometry    eclogite    Early Paleozoic    
0 INTRODUCTION

Ophiolites are relics of oceanic sediments, crust, and upper mantle that were thrust onto continental margins (Dilek and Furnes, 2014, 2011), and they have formed at least since Mesoproterozoic time. Ophiolites provide crucial information on the existence of ancient oceans, on processes at mid-ocean ridges such as partial melting, ascent of magma, and fluid percolation, on the collision between arcs and continents, on the location of sutures, on the descent of matter in subduction zones and its exhumation to the surface, on the development of plate tectonics on Earth, and on super continent cycles (Furnes et al., 2014). Archetypes are the Mesozoic-Cenozoic ophiolites on the island of Cyprus, Mediterranean Sea (Pearce and Robinson, 2010), in the Al Hajar Mountains, Oman and UAE (Goodenough et al., 2014), in the California Coast Range, USA (Giaramita et al., 1998), and on Macquarie Island, Tasmania, Australia (Varne et al., 2000). Yet, the certain recognition of ophiolites in Paleozoicor even Proterozoic orogens is hampered by the ever scarcer geological record, and, hence many of Earth's oldest ophiolites are controversial.

Several Proterozoic-Paleozoic ophiolites occur in the orogens, which welded the three cratons—Yangtze, Noth China, and Tarim—in China (see Zhang et al., 2008 for review). Recent research, for instance, discusses the nature and plate tectonic significance of the ophiolites of Miaowan (1 100-985 Ma; Peng et al., 2012), Huashan (~947 Ma; Shi et al., 2007), and Shimian (~800 Ma; Zhao et al., 2017) at the northwestern Yangtze margin (Fig. 1a). Thereby, it addresses the evolution of a more than 1 000 km long magmatic arc and attempts to clarify, whether Yangtze had a central (Peng et al., 2012) or marginal (Zhao et al., 2017) position in the Rodinia collage.

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Figure 1. (a) Map showing the possible ophiolites at the northern and western margin of Yangtze and the location of the Qinling belt in China. (b) Detailed geologic map of the working area (redrawn from Wang et al., 2005) with metamorphic ages (U/Pb zircon unless otherwise noted) and peak metamorphic conditions of high-pressure granulites and migmatites. Sample locations are: 25291, 33 35.950'N, 110 57.600'E; 25294, 33 35.7167'N; 110 57.9667'E; 25314, 33 36.883'N, 110 56.367'E; 25316, 33 36.550'N; 110 55.167'E; 48175, 110 59.735N, 33 34.551'E; 75282, 33 35.191'N, 110 58.113'E; 75293, 33 35.719'N, 110 57.940'E. 1. Bader et al. (2019a); 2. Tang et al. (2016); 3. Chen et al. (2015); 4. Yu et al. (2015); 5. Li et al. (2014); 6. Bader et al. (2013b); 7. Liu et al. (2013b); 8. Qian et al. (2013); 9. Li et al. (2012); 10. Liu J F et al. (2009); 11. Liu L et al. (1996); 12. Liu L et al. (1995).

The comparably old (~800 Ma; Chen at al., 2015; Li et al., 2014), enigmatic Songshugou Complex in the Qinling Orogen, which welds the Yangtze and North China cratons, might be another ophiolite associated with this arc. As the ophiolites mentioned above, the Songshugou Complex comprises a variety of metamafic and ultramafic rocks (e.g., Dong et al., 2008); in contrast to them, it experienced medium- to high-grade metamorphism and was fused with an association of migmatic and non-migmatic gneisses, which host rare bodies of granulites (e.g., Bader et al., 2019a) and diamond- and coesite-bearing eclogites (Chen et al., 2015; Cheng et al., 2011; Yang et al., 2003). The grade of metamorphism, the relation to abutting metamorphic rocks, and the formation of the Songshugou Complex are controversial or unknown, which is why it is of limited relevance in models of the evolution of the Qinling Orogen and for plate tectonic reconstructions. Yet, given the telling nature of ophiolites (e.g., Dilek and Furnes, 2014, and references therein), and the records of oceanic lithosphere formation and orogenic metamorphism in the Songshugou Complex (Zhang and Yu, 2019), any model of the Qinling Orogen will be incomplete or flawed without its complete integration.

To put the Songshugou Complex into the framework of the Qinling Orogen, we firstly review and critically assess published data. Secondly, based on a regional survey, which includes profile documentation, mapping, and the collection of garnet (-bearing) amphibolites, we derive new pressure-temperature (P-T) paths by phase equilibria modeling and conventional thermobarometry. Thereby, focus is garnet, which is a reliable key phase in the reconstruction of P-T paths (e.g., Spear, 1993) given its ability to retain chemical zoning pattern due to rather low cation diffusion rates (e.g., Chakraborty and Ganguly, 1992). The combination of all the data challenge the view that the Songshugou Complex is an ophiolite but implies a juxtaposition of ultramafic and mafic rocks during exhumation.

1 REGIONAL GEOLOGY

The Songshugou Complex is in fault contact with the Qinling Complex of the Qinling-Tongbai-Dabie-Sulu-Imjingang Orogen (Fig. 1a). Welding the North China and Yangtze cratons, this orogen incorporates several roughly east-west stretching, fault-bounded tectonometamorphic units. This section introduces those relevant in the Early Paleozoic era, from north to south. For details, see Dong and Santosh (2016), Liu et al. (2016), Yu et al. (2015), Bader et al.(2013a, b), and Wu and Zheng (2013). In Section 2, we summarize and critically assess the literature about the Songshugou Complex.

1.1 Kuanping and the Erlangping Complexes

The Kuanping and Erlangping complexes are situated between the North China Craton and the Qinling Complex. The Kuanping Complex comprises a metasedimentary and a metamafic unit. The metasedimentary unit constitutes a clastic wedge, with possible Qinling Complex affinity—both share dearth of Archean-Paleoproterozoic and wealth of ~0.95 Ga zircons—but the metasedimentary unit lacks the ~0.9-0.7 Ga signature of the Qinling Complex (Bader et al., 2013a; Liu X C et al., 2013). The metamafic unit is a relic of a Proterozoic ocean, given MORB and OIB signatures, that might have been closed ~900 Ma ago (Dong et al., 2014). Built up of greenschist to amphibolite facies gabbros, pillow lavas, andesites, rhyolites, ultramafic rocks, cherts, turbidites, and marbles (Dong et al., 2011a; Liu et al., 2011), the Erlangping Complex represents a relatively short-lived (~490-450 Ma) intra- oceanic arc-back-arc system (Bader et al., 2013b, and references therein; Liu X C et al., 2013).

1.2 Qinling Complex

The Qinling Complex is a relic of a ribbon-shaped continental sliver, the Qinling microcontinent. It was derived from a Meso-Neoproterozoic clastic continental wedge and contains metamorphosed siliciclastic and calcsilicate rocks with minor marbles and mafic rocks. Cornerstones of its Neoproterozoic- Early Paleozoic evolution are (1) the intrusion of three suites of granitoids at ~983-873, ~863-814, and ~783-718 Ma (Bader et al., 2013a), (2) (ultra)high-pressure ((U)HP) metamorphism at 518-484 Ma (Fig. S1; Wang et al., 2014, 2011; Cheng et al., 2012, 2011; Yang et al., 2003), (3) upper amphibolite to (HP) granulite facies (ultra-) metamorphism (e.g., Bader et al., 2013b; Liu L et al., 2013) at 520-402 Ma, (4) two climaxes at ~485 and ~426 Ma in continuous magmatism spanning at least 515-400 Ma (Bader et al., 2013b), (5) reheating at ~350 Ma (Bader et al., 2013b), and (6) low to medium-grade retrograde metamorphism and reactivation by ~315 Ma (Zhang et al., 2015; Bader et al., 2013b).

1.3 Danfeng Complex

Cambrian-Silurian cherts, ultramafic and mafic rocks, pillow lavas, and intermediate-acidic plutonites crop out discontinuously along the Shangdan fault zone. Mafic rocks have N-MORB, E-MORB, island-arc, and continental arc signatures (e.g., Dong et al., 2011b). Uranium/lead zircon ages span ~534- 402 Ma; most mafic and intermediate rocks are ~534-470 and 455-402 Ma, respectively (Bader et al., 2013b, and references therein). A 442±7 Ma old subduction-related diorite intruding pillow lavas (Yan et al., 2008) may give a minimum age for accretion to the Qinling Complex, and 40Ar/39Ar ages of 426- 395 Ma (Dong et al., 2011b, and references therein) reflect Devonian metamorphism. A 524 Ma boninite-like dolerite may reflect subduction initiation magmatism (Li et al., 2015).

2 SONGSHUGOU COMPLEX: ASSESSMENT OF KNOWLEDGE 2.1 Structural Relations and Principle Lithologies

The 27 km by 3 km sized Songshugou Complex (Fig. 1c) comprises a variety of metamorphic mafic rocks and dismembered ultramafic bodies. It is fused with the Qinling Complex as the core of a kilometer-scale syncline or sheath fold (Wang et al., 2005, and references therein; Zhang, 1999), the margins of which were overprinted by ductile, steeply dipping shear zones. The fold is outlined by marble, and it belongs to the regionally dominating greenschist to amphibolite facies deformation D3, which reflects the oblique west-ward uplift of the Qinling Complex (Wang et al., 2005). The characteristics of D3 are a steeply dipping foliation, A-type folds with shallowly to intermediately east-southeast-dipping fold axes, and the domination of vertical and horizontal displacement at low- and mid-crustal depths, respectively. Titanite from cleavage domains of two samples yields~420 Ma, ~685 ℃ (Li et al., 2014) and may give estimates for the earlier, near-vertical uplift. However, the eastward migration of batholith emplacement centers (≥420-409 Ma; Wang et al., 2005) is considered as reflecting lateral movement in the Qinling Complex at the same time. The youngest monazite ages in the Qinling Complex (~355 Ma; Bader et al., 2013b) and a U/Pb titanite age of ~352 Ma (~700 ℃) of a HP granulite from the Xigou shear zone (Li et al., 2014) either give a younger limit for the completion of D3, or dating of regional reheating.

2.2 The Suite of Metamafic Rocks

The principle metamafic rock is mafic mylonite; rare constituents are discontinuous layers and lenses of garnet amphibolite, augen amphibolite, and garnet clinopyroxenite (Dong et al., 2008; Zhang, 1999). Garnet amphibolites and garnet clinopyroxenites once were thought to form a contact aureole around the central ultramafic body (Liu et al., 2009; Liu and Sun, 2005). Conversely, newer field observations suggest that they are discontinuous lenses and layers (Zhang et al., 2008).

The P-T evolution of the mafic mylonite and augen amphibolite is unknown. One school of researchers (Yu et al., 2016; Chen et al., 2015; Zhang, 1999) considers garnet amphibolites and clinopyroxenites as overprinted eclogites. Yang et al. (1994) determine exhumation from ≥1.2 GPa, 600 ℃ to 0.6-0.8 GPa, 550-600 ℃ for garnet amphibolites using experimentally determined mineral equilibria. Garnet clinopyroxenites lack matrix plagioclase but contain extensive diopside-plagioclase symplectites, which rarely mantle relic omphacite prisms (Chen et al., 2015). The reintegration of these symplectites points to omphacite precursors with 17 mol%-35 mol% of jadeite, convincing eclogite facies relics (Chen et al., 2015; Zhang, 1999) that formed at ≥1.3 GPa, 690-750 ℃ (Chen et al., 2015). According to Bader et al. (2019b), these symplectites evolved into strongly zoned amphibole porphyroblasts: coronas of dark green to brownish pargasite and hornblende first developed around the symplectites and diffusion of H2O and cations through these coronas permitted further clinopyroxene decomposition, creating pale green actinolite cores. An in-situ SIMS U/Pb age, ~470 Ma, of titanite inclusions in actinolite may date this process, and the corresponding Ti-in-titanite T implies it proceeded at ~675 ℃ (Li et al., 2014).

Zhang (1999) estimated 1.0-1.6 GPa, 830-920 ℃ for the temperature peak of garnet clinopyroxenites on the basis of thermobarometry for the outermost garnet rim and plagioclase and amphibole from the corona mantling this garnet. Given that coronas are disequilibrium textures, formed by decomposition of the enclosed phase, there is no equilibrium between the utilized minerals, the method is not applicable, and these P-T values are coincidental.

Using pseudosections (TNCFMASHO, H2O saturation), Tang et al. (2016) quantify the stability limits of two consecutive mineral assemblages (the prograde of epidote, plagioclase, amphibole, garnet, clinopyroxene, quartz, rutile, and ilmenite, and the peak metamorphic lacking the former three) of a garnet amphibolite at 1.0-1.1 GPa, 720-740 ℃ and 1.5-1.9 GPa, 750-850 ℃, respectively. The following facts challenge these data. (1) At T in excess of 750 ℃, partial melting and melt-loss cause H2O-deficient conditions (White and Powell, 2002). Given that the H2O activity significantly influences the stability limits of phases (e.g., Bader et al., 2014), modeling presuming H2O saturation is unsuitable. (2) The refractory nature and the capability of preserving the growth zoning over a large P-T range make garnet the most important petrogenetic indicator (e.g., Spear, 1993), and its composition should be consistent with the metamorphic P-T path. For verification, we calculated the isopleth intersection for the bulk-rock and garnet rim compositions given in Tang et al. (2016) with the method outlined in Section 3.1 and obtained 1.6 GPa, 610 ℃, a large discrepancy to the published values. (3) The preservation of prograde epidote inclusions in garnet during metamorphism at 750-850 ℃ is unlikely—Perchuk et al.(2008, 2005) demonstrate experimentally that epidote inclusions in garnet melt at 800 ℃.

To summarize, the metamafic suite of the Songshugou Complex contains evidence for eclogite facies metamorphism the peak P of which is unknown; evidence of HP granulite facies metamorphism in the Songshugou Complex itself is not solid.

Geochemically, the metamafic rocks have E-MORB and N-MORB signatures (Tang et al., 2016; Liu et al., 2010; Dong et al., 2008), consistent with formation at a mid-ocean ridge above an anomalous, DUPAL-like mantle (Dong et al., 2008). Interpreted to date protolith formation, five out of eight Sm/Nd whole-rock analyses yielded an isochron age of 1 030±46 Ma (Dong et al., 2008). Yet, the conclusiveness of this selection has been queried by Bader et al. (2013a), and we regard two U/Pb zircon ages of ~796 and ~787 Ma (Chen at al., 2015; Li et al., 2014) as reliable protolith ages. Uranium/lead zircon ages of 518-482 Ma date metamorphism of garnet clinopyroxenite, garnet amphibolite, and mafic mylonite (Table S1 gives details and references).

2.3 The Ultramafic Rocks

The metamafic suite of the Songshugou Complex hosts more than one hundred dismembered ultramafic bodies. The largest one, 16 km by 1-2 km in size (Fig. 1b), is widely pristine—serpentinization, which overprinted most other bodies penetratively, only affected its margin. What follows concerns the pristine sections of this largest body.

About 85% of the ultramafic rocks are pervasively foliated, mylonitic or porphyroclastic spinel dunite. Further constituents are massive, coarse-grained spinel dunite to spinel harzburgite, which form lenses and ribbons several hundred meters long, veins of coarse-grained olivine websterite and olivine clinopyroxenite, which are 10 cm to 5 m wide and in part isoclinally folded, and banded and podiform chromite cummulates (Cao et al., 2017, 2016; Nie et al., 2017; Yu et al., 2017; Bader et al., 2013b). The most widespread mineral assemblage is olivine- spinel-orthopyroxene. The veins are dominated by centimeter- sized orthopyroxene and clinopyroxene; both show exsolution lamellae ubiquitously. Most ultramafics contain actinolite prisms and cummingtonite; the latter mantles actinolite and forms needles.

The foliation and mineral stretching lineation of the mylonitic spinel dunite dip steeply to the southwest; asymmetric S-C fabrics, pressure shadows, and en-echelon veins imply top-to-the-southwest shear sense. These structures formed during exhumation (Cao et al., 2017; Song et al., 1998), and reflect the early, near-vertical uplift during D3.

The origin of the ultramafic rocks is controversial. According to Cao et al. (2016), the spinel dunite and harzburgite are highly refractory residues. In their preferred scenario, boninite-like melts were extracted at anomalously high mantle temperatures of more than 1 100℃ in the fore-arc above a young subduction zone. The olivine-spinel Mg-Fe2+ equilibrium reveals subsequent cooling from ~ 955 to 660 ℃ and reflects grain-size reduction, from coarse-grained into porphyroclastic and mylonitic dunite, during the evolution from a mature subduction zone into orogeny and uplift. An amphibole- forming fluid then percolated at≤0.8 GPa, ≤700 ℃. The latter is in accord with cooling in the Qinling Complex after the second, ~426 Ma magmatic climax (Bader et al., 2013b). The maximal Re depletion model age (TRD) of the ultramafics is 1.2-1.1 Ga, consistent with ages from mantle xenoliths in the Yangtze Craton, but not the North China Craton (Nie et al., 2017). Lead isotopes, which imply a common history of Yangtze Craton, the Qinling Complex, and the mafic and ultramafic rocks of the Songshugou Complex, support this notion (Liu et al., 2016, and references therein; Lee et al., 2010).

Conversely, Yu et al. (2016) propose that the ultramafic rocks are relics of Neoproterozoic oceanic lithosphere, based on similarities of trace element abundances with the ophiolites of Oman and Cyprus. Accordingly, the ultramafic rocks underwent Cambrian-Silurian metamorphism as part of a subducting plate. They interpret their oldest TRD (875, 860 Ma) as reflecting oceanic formation and second this notion with a 40Ar/39Ar isochron age of 848±4 Ma of clinopyroxene megacrysts (Chen et al., 2002).

2.4 Abutting Lithologies

At the Xigou shear zone (Fig. 1b), Liang Liu and his coworkers (Liu et al., 1996, 1995; Liu and Zhou, 1995) discovered HP granulites, which structurally do not belong to the Songshugou Complex itself (Bader et al., 2019a). Petrographically, these are garnet clinopyroxenites, in which orthopyroxene grew during exhumation, and gneisses, which contain antiperthite, perthite, garnet, kyanite, and rutile. They evolved along clockwise P-T paths with peak metamorphism at 1.40-1.58 GPa, 826-887 ℃ and 1.3-1.6 GPa, 800-900 ℃, respectively. Acicular precipitates of rutile, apatite, and quartz in garnet from a felsic HP granulite (Liu et al., 2003) are in accord with these conditions (Axler and Ague, 2015). Concordant U/Pb zircon ages of 506-448 Ma date peak metamorphism, high-temperature uplift, and a first retrograde overprint (Table S1). According to geochemistry, the protoliths of the felsic HP granulites are greywacke, arkose, and, subordinately, acidic subalkaline magmatic rocks that formed at an active continental margin (Wang et al., 1997); inherited ages of ~700 Ma give a maximal age for sedimentation (Liu L et al., 2013).

South of the Songshugou Complex and the HP granulite occurrence, a mafic, multi-phase, syn-subduction batholith— the Fushui Complex—formed 514-477 Ma ago, coevally with the metamorphism in the Songshugou Complex and with paired metamorphism in the Qinling Complex (Table S1). The mafic magma stems from enriched lithosheric mantle (Zhang et al., 2015). It is the largest instance of a suite of ~500 Ma mafic intrusions in the Qinling Complex (Bader et al., 2013a).

Marble and non-migmatic muscovite-biotite gneiss crop out north of the Songshugou Complex. Both contain lenses and dykes of amphibolite. Yu et al. (2016) inferred these amphibolites are overprinted eclogites. While the marble is undated, the gneiss has a spectrum of inherited metamorphic, magmatic, and fluid- modified zircons of 885-558 Ma, the majority of which (810- 690 Ma) is consistent with the Neoproterozoic magmatism in the Qinling Complex and Yangtze Craton (Bader et al., 2013a).

3 NEW PETROLOGY 3.1 Methods

New petrology aims to further elucidate the evolution of the metamorphic mafic rocks of the Songshugou Complex and to check for regional variations. We surveyed the Songshugou Complex on paths perpendicular to strike and along creeks and selected, due to their negligible degree of weathering, two garnet amphibolites (25316A, 48175A) and three garnet relics- bearing augen amphibolites (25314A, 75282C, 75293E) from five outcrops (Fig. 1b) for the determination of P-T paths. For this purpose, we cut the samples perpendicular to the main foliation and parallel to the mineral stretching lineation and prepared polished thin sections from areas with the widest garnet preservation. Then, we recognized and characterized distinct mineral assemblages on these thin sections by optical microscopy, laser Raman spectroscopy, backscattered electron (BSE) imaging, and electron microprobe analysis (EMPA).

For Raman spectroscopy, we used a Renishaw probe with a 514 nm-laser at 50 mW. The measurements employed an objective lens with a fifty-fold magnification and mineral identification utilized the RRUFF database (Downs, 2006). The bulk-rock major and minor element contents were determined at the Institute of Geology and Mineral Resources, Regional Survey of Hebei Province, Langfang, using X-ray fluorescence (XRF) and titration (Fe3+ only). We analyzed samples 75282C and 75293E with the four-spectrometer CAMECA SX-100 EMP at Geoforschungszentrum Potsdam, Germany. The operation conditions were: 15 kV accelerating voltage, 20 nA beam current, 1-25 µm beam diameter, and 10-20 s major and 20-40 s minor element peak-counting time. Standardization used natural and synthetic minerals of the Smithsonian Institute, MAC™, and CAMECA. Raw intensity data were corrected with the PAP program (Pouchou and Pichoir, 1985). We used the four- spectrometer JEOL JXA-8100 EMP at the Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, to analyze the other three samples. Operation conditions were 15 kV acceleration voltage, 10 nA beam current, and 1-10 μm beam size. Matrix correction utilized the PRZ program supplied by the manufacturer. The electronic supplementary material contains representative data of garnet (Table S2), amphibole (Table S3), clinopyroxene (Table S4), and plagioclase (Table S5).

Based on EPMA and whole-rock analyses, we estimated the metamorphic P and T during garnet growth and for a thorough retrogression by isopleth thermobarometry and conventional methods, respectively. To perform isopleth thermobarometry, we calculated equilibrium assemblage and isopleth diagrams, which show the compositional evolution of garnet, with the Domino program and interpreted the diagrams by comparison with the observation. The Theriak-Domino program package (de Capitani and Petrakakis, 2010; de Capitani, 1994; de Capitani and Brown, 1987; available at http://titan.minpet. unibas.ch/minpet/theriak/theruser.html) uses Gibbs-free-energy minimization to calculate equilibrium assemblages together with the modal proportions and compositions of stable phases at given P and T. The fundamental presumption of this modeling is the equilibrium of the considered mineral or mineral assemblage with the bulk-rock composition. For minerals and fluid, the thermodynamic database of Holland and Powell (1998), version tcds55, was used together with the solution models of amphibole of Diener et al. (2007), biotite and melt of White et al. (2007), clinopyroxene of Green et al. (2007), cordierite and epidote of Holland and Powell (1998), feldspar of Holland and Powell (2003), garnet of White et al. (2007; updated version from the tc330-download), osumilite of Kelsey et al. (2004), spinel-magnetite and orthopyroxene of White et al. (2002), and ilmenite of Zeh and Holness (2003). All other minerals present in the thermodynamic database of Holland and Powell (1998) were considered as non-solution phases. To obtain the bulk composition, we transformed the geochemical whole-rock analyses into the model system (TiO2-Na2O-CaO-FeO-Fe2O3-MgO-Al2O3-SiO2-H2O) by omission of Mn and projection from apatite. The model utilized H2O saturation given that, until dry eclogite is formed, prolonged dehydration characterizes subduction zone metamorphism; it omitted CO2, because the fluid in cold subduction zones incorporates little CO2 (e.g., Molina and Poli, 2000) and the small CO2 fraction in it does not have a significant impact on P-T estimates (Wei and Clarke, 2011).

To estimate P and T of retrogression, we calculated the formation T of amphibole from its Ti contents according to Colombi (1988), and amphibole-plagioclase-quartz thermometry followed Bhadra and Bhattacharya (2007).

3.2 Field Relations

Evident from rare outcrops, which are mostly smaller than 20 m2, cubic meter-sized boulders, pebbles, and cobbles, the Songshugou Complex contains a variety of metamafic rocks; mafic mylonite prevails. Most valuable scientifically, yet rarest, clinopyroxene-rich rocks occur at outcrop 25291—the sampling site of several previous studies (Fig. 1b)—and occasionally as cobbles in river beds south of the central ultramafic body. Outcrop 25291 shows a complex succession of garnet- free augen amphibolite (most abundant), mafic mylonite, garnet amphibolite with and without clinopyroxene, garnet clinopyroxenite, and garnetite. Individual rock types extend at the decimeter to meter scale; their transition is in general gradual and parallel to the main foliation.

Garnet amphibolites and augen amphibolites occur as layers (25314A, 25316A) and lenticular bodies across the Songshugou Complex. The layers are centimeters or a few decimeters wide and their contact with the mylonite is foliation- parallel and transitional. Lenticular bodies measure a few to tens of meters in diameter, in part exceeding the size of outcrops (48175A, 75293E). Around the body from which 75282C was taken, the mylonite foliation is oblique to the internal and truncates it. Garnet amphibolite is more abundant in the proximity of the largest, central ultramafic body; it occurs less often near the Jieling shear zone, and occasionally all across the metamafic suite of the Songshugou Complex. Our field work confirmed that garnet (clinopyroxene) amphibolites do not form a mantle around the largest, central ultramafic body, but they crop out discontinuously.

3.3 Microtextures

The cleavage domains of garnet amphibolites and augen amphibolites are 0.1-1.0 mm wide and wrap around garnet and amphibole porphyroclasts and whitish to gray, fine-grained aggregates (augen hereafter, following the terminology of Zhang, 1999). Aside from the dominant amphibole, the cleavage domains contain interstitial plagioclase, ilmenite with titanite coronas, and quartz.

The diameters of augen and garnet vary between ~0.5 and 5 mm across the Songshugou Complex, but it is rather constant in individual layers and bodies. Plagioclase is the principal constituent of the augen. Furthermore, they contain amphibole, clinozoisite, ilmenite, and quartz; garnet relics, titanite, and biotite are occasional constituents.

Inclusions in garnet are amphibole, occurring isolated as well as on microcracks (Fig. 2a), ilmenite (Fig. 2b), titanite mantling ilmenite, quartz, and plagioclase. Garnet very rarely abuts the cleavage domains—rather, there is a continuous transition from less than one hundred micrometers wide, roughly spherical coronas around garnet (Fig. 2b), which consist of the same phases as the augen, to garnet relics in the centers of ovoid to σ-shaped augen (Fig. 2c). This transition is in part visible in a single thin section (e.g., 25314A), but some larger blocks (25316A, 48175A, 75293E) show a wide garnet preservation. Garnet-free augen are elongated (Fig. 2d) to highly stretched (Fig. 2e); the ratios exceed in part 10 : 1 and their long axes follow the mineral stretching lineation defined by oriented amphibole and titanite. Many augen bear asymmetric strain shadows of nearly pure plagioclase or amphibole.

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Figure 2. Photomicrographs, plain polarized light. (a)-(e) A sequence shows the progressive garnet decomposition, from an almost completely preserved porphyroclast (a, 75293E), to a plagioclase-rich corona (b, 25316A), a garnet relic in an augen (c, 25314A), a σ-shaped auge without garnet (d, 25314A), and a stretched auge (e, 25294A). (f) Armored clinopyroxene relic in amphibole (25316A). Mineral abbreviations follow Kretz (1983). Red arrows mark EMP profiles.

Amphiboles of the cleavage domains are oriented, prismatic to anhedral, and less than ~0.2 mm long; they either lack optical zoning or have thin greenish rims on brownish-green cores. Beside, the samples 75293E, 75282C, and 25316A also contain millimeter-sized amphibole porphyroclasts. In these porphyroclasts, a pale-green core is followed by a zone of abundant, oriented, rod-like plagioclase and quartz inclusions, across which the color darkens, and with a greenish-brown to brown rim. Titanite and ilmenite are ubiquitous inclusions in all amphiboles. Remarkably, amphiboles of sample 25316A include armored clinopyroxene relics within their pale green sections (Fig. 2f).

3.4 Mineral Chemistry

The composition of garnet varies little within samples, but somewhat across the Songshugou Complex (Fig. 3; Table S2). The overall rim-ward decrease of spessartine and increase of pyrope characterize the zoning of all garnets, and core and rim compositions are Alm54-71(Grs+And)25-30Prp1-6Sps5-12 and Alm58-78(Grs+And)20-32Prp2-12Sps1-4, respectively. Internal grossular+andradite kick-ups (75282C, 25314A, 25316A) at the expense of almandine and pyrope occur at micro-cracks sealed with amphibole.

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Figure 3. Compositional profiles of garnet. The spots highlighted by orange color are plotted as isopleth intersections in Fig. 5.

Amphiboles belong to the calcic series (Fig. 4; the terminology follows Leake et al., 1997). From the core to the rim of a porphyroclast, Al (0.268-2.382 p.f.u.), (Na+K)A (0.022- 0.625 p.f.u.), and Ti (0.002-0.149 p.f.u.) increase strongly at the expense of Si (7.879-6.363 p.f.u.); the XMg (=Mg/Mg+Fe2+) decreases rim-wards, from 0.828 to 0.372 (Fig. 4; Table S3). Amphiboles in coronas and cleavage domains are compositionally similar to the porphyroclast rims (Fig. 4; Table S3).

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Figure 4. Compositional profiles of amphibole. The terminology follows Leake et al. (1997).

Relic clinopyropxene of 25316A is diopside with an XMg of 0.66 and low Na-contents of~0.020 p.f.u. (Table S4). Plagioclase has broader ranging anorthite in the coronas and augen (19.9-88.5), compared to the cleavage domains (37.4-53.7; Table S5).

3.5 Results of Thermobarometry

The application of garnet isopleth thermobarometry to two garnet amphibolites and three augen amphibolites with garnet relics revealed quartz-eclogite facies P-T conditions: In 25314A, garnet growth started at 2.03 GPa, 506 ℃, and the outermost preserved garnet section pointed to 2.13 GPa, 516 ℃ (Figs. 5a and S2). Garnets of 25316A grew from 1.93 GPa, 511 ℃ to 2.02 GPa, 542 ℃ (Figs. 5b and S3), those of 48175A from 2.01 GPa, 462 ℃ to 2.04 GPa, 473 ℃ (Figs. 5c and S4), those of 75282C from 2.13 GPa, 494 ℃ to 2.41 GPa, 530 ℃ (Figs. 5d and S5), and those of 75293E from 2.25 GPa, 498 ℃ to 2.54 GPa, 530 ℃ (Figs. 5d and S6). Remarkably, the garnet core isopleths intersect at or close to the respective garnet-in of four out of the five samples.

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Figure 5. Equilibrium-assemblage phase diagrams and results of garnet isopleth thermobarometry. Bold numbers refer to EMP analyses shown in Table S2 and marked in Fig. 3. The black arrows show the trends implied by garnet compositions. Assemblages predicted during garnet growth are highlighted. The phases given at the lower right are stable across the entire diagram. The entire isopleth diagrams are shown in Figs. S2–S5. Bulk compositions are: (a) 18.414 Si, 0.513 Ti, 5.965 Al, 3.508 Fe, 3.456 Mg, 4.362 Ca, 1.929 Na, 0.177 K, 60.532 O; (b) 19.041 Si, 0.560 Ti, 5.935 Al, 4.386 Fe, 3.393 Mg 3.911 Ca, 1.593 Na, 0.100 K, 60.933 O; (c) 20.436 Si, 0.552 Ti, 5.534 Al, 6.475 Fe, 0.810 Mg, 2.763 Ca, 1.444 Na, 0.144 K, 61.691 O; (d) 18.414 Si, 0.833 Ti, 5.372 Al, 5.177 Fe, 2.827 Mg, 3.705 Ca, 1.744 Na, 0.166 K, 59.216 O; (e) 18.154 Si, 0.326 Ti, 6.168 Al, 3.345 Fe, 4.339 Mg, 4.887 Ca, 1.378 Na, 0.130 K, 59.537 O.

For sample 25316A—given its well-preserved, bell-shaped spessartine zoning—we evaluated the impacts of chemical fractionation during garnet growth and of X(Fe3+) on the obtained isopleth intersections. Chemical fractionation was implemented by splitting the garnet core-rim path in Fig. 5b into 50 steps and subtracting all the garnet predicted by the modeling at a given step from the bulk composition of this step; the obtained, effective bulk composition was used to calculate the garnet isopleths of the next step. The discrepancy between the non-fractionation and fractionation model was below 0.01 GPa, 1℃.

Plotting garnet isopleths and equilibrium assemblages into T-X(Fe3+)- and P-X(Fe3+)-diagrams revealed that garnet was only stable at 1.93 GPa, 511 ℃, if X(Fe3+)≲0.30. At these conditions, the garnet isopleths depended little on X(Fe3+), and this dependency caused uncertainties of ±0.06 GPa, ±5 ℃ (Fig. S6). This is below the overall error of isopleth thermobarometry (±0.1 GPa; ±35 ℃).

Amphibole-plagioclase-quartz equilibria yielded 0.7-0.9 GPa, 685-700 ℃ for augen of 25314A, 0.8-1.1 GPa, ~700 ℃ for coronas around garnet of 25216A, 0.5-0.6 GPa, 680-710 ℃ for cleavage domains of 48175A, 0.8 GPa, ~690 ℃ for cleavage domains of 75282C, 1.0-1.2 GPa, 660-700 ℃ for coronas around garnet of 75293E, and 0.8 GPa, ~685 ℃ for rims of amphibole porphyroclasts and interstitial plagioclase of 75293E.

4 DISCUSSION 4.1 New P-T Data for the Songshugou Complex, Caveats, and Comparison with Previous Work

The literature discusses a range of P-T conditions, from the eclogite to the (HP) granulite facies, for the diverse metamorphic mafic rocks of the Songshugou Complex (Fig. 6; Section 2.2). Herein, we detail five samples with progradely zoned garnet, which grew during burial and heating (1.93-2.54 GPa, 494-542 ℃; Figs. 5 and S2-S6) according to isopleth thermobarometry. For three reasons, we consider especially garnet core-based P-T data as robust. (1) Given garnet diameters of 1.5-5.5 mm, the maximal T during exhumation (650-710 ℃), and the duration of lower crustal stalling (~50 Ma), the impact of intra-grain diffusion on garnet cores was negligible (Caddick et al., 2010). (2) The intersection of garnet core isopleths at or close to the garnet-in was consistent with negligible post- growth modification and the preservation of the garnet core- bulk-rock equilibrium. The fundamental assumption of phase equilibria modeling was therefore fulfilled. (3) The evaluation showed that the location of garnet isopleths depends only insignificantly on X(Fe3+), and the effect of chemical fractionation during garnet growth was negligible.

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Figure 6. Published and new P-T data from the Songshugou Complex and the Xigou shear zone. Error bars are P- and T-ranges given by the authors. The arrows mark P-T evolutions. Metamafic rocks of the Songshugou Complex are blue. Mafic and felsic granulites from the Xigou shear zone are green and red, respectively. References: 1. Bader et al. (2018); 2. Chen et al. (2015); 3. Bader et al. (2013b); 4. Liu et al. (1996); 5. Liu et al. (1995); 6. Yang et al. (1994).

There are two caveats. (1) Due to the destruction of the outermost garnet sections, this study could not determine the P and T at the end of garnet growth and, hence, the actual peak P may be at least a bit higher than 1.93-2.54 GPa. (2) During exhumation, intra-grain diffusion and interaction with the reactive rock matrix modify the garnet growth zoning (e.g., Caddick et al., 2010), especially at the rims. The compositional profiles (Fig. 3) show little evidence of this: garnets of 25314A, 25316A, and 75293E have slight spessartine kick-ups in the outer 0.05-0.1 mm—hence, we did not use the corresponding EMP spots for isopleth thermobarometry. Except for these rims, especially 25316A, 48175A, and 75293E garnets have a typical prograde zoning with rim-ward decreasing spessartine and increasing pyrope. Garnets of 25316A and 75293E have a flat grossular+andradite zoning (on average); the one of 48175A shows a minor rim-ward grossular+andradite decrease (Fig. 3). Re-equilibration during corona formation and subsequent deformation (0.5-1.2 GPa, 660-710 ℃) should have enriched the garnet rims with grossular+andradite (Figs. S2-S6), and in case of substantial modification wide rims with distinct composition and gradual transition to the garnet interior should have formed (compare Smit et al., 2013). Yet, only one spot of 25314A appears to be clearly effected by Ca-ingress. We therefore concluded that garnet rims widely retained the original composition, and post-growth modification affected the P-T data little (garnet rims) and negligibly (garnet cores; see above).

Amphibole-plagioclase-quartz thermobarometry applied to coronas, augen, and cleavage domains of the five samples yielded consistent T of 660-710 ℃, but scattering P of 0.5-1.2 GPa. This may at least in part be attributed to the low intersection angle of the utilized thermobarometric equilibria. However, P derived from coronas and augen (0.7-1.2 GPa) are on average higher than P based on cleavage domains and the margins of amphibole porphyroclasts (0.5-0.8 GPa; Fig. 6 and Section 3.5). This is consistent with the retrogression sequence— coronas evolved into augen before shearing stretched the augen into ribbons.

To summarize, out of the whole-scale P-T evolution of five samples from the Songshugou Complex, this study quantified two sections: much of the prograde garnet growth at quartz eclogite facies conditions during subduction (1.93-2.54 GPa, 494-542 ℃) and the amphibolitization during exhumation (0.5-1.2 GPa, 660-710 ℃).

Previous research presented in part much higher T for the metamafic suite of the Songshugou Complex (Section 2.2), but assured HP granulites (1.15-1.45 GPa, 810 ℃ to ≥915 ℃) crop out only at the Xigou shear zone—they do not belong structurally to the Songshugou Complex but abut it southerly (Bader et al., 2019a). For the samples studied here, we rule out heating to T distinctly higher than 710 ℃: (1) The modification of garnet by diffusion and interaction with melt or fluid is a significant process at high T (Smit et al., 2013; Caddick et al., 2010; Perchuk et al., 2008, 2005; Chakraborty and Ganguly, 1992), but the prograde garnet growth zoning is well preserved in our samples (Fig. 3). In this manner, garnets of (HP) granulites are commonly retrogradely zoned and pyrope-rich. In two mafic HP granulites from the Xigou shear zone, pyrope decreases rim-wards from 32.0 to 19.1 and 27.3 to 20.4, respectively (Bader et al., 2019a), but our samples have a pyrope maximum of just 11.5 (Fig. 3; Table S2). (2) Granulite-facies amphiboles have higher Ti-contents (Ernst and Liu, 1998)—amphiboles in coronas around garnets from the HP granulites contain 0.197-0.295 p.f.u. Ti (Bader et al., 2019a), but amphiboles of our samples have a maximum of 0.149 p.f.u. Ti (Table S3). (4) There is no compelling thermobarometric evidence for such high T in the Songshugou Complex itself (see the discussion of published data in Section 2.2). (5) Relic diopside included in amphibole (Fig. 2f) is not indicative of granulite facies conditions—the amphibole porphyroclasts emerged from former coronas around omphacite relics and diopside-plagioclase- symplectites (Bader et al., 2019b).

We conclude that the exhumation from the quartz eclogite facies to the base of the crust included heating by about 200 ℃. When the decreasing pressures permitted plagioclase stability, garnet decomposition started with the formation of coronas, which evolved into augen and eventually ribbons. Isothermal uplift reflected by steeply dipping stretching lineations and consistent T in the range of 0.5-1.2 GPa brought the metamorphic mafic rocks into mid-crustal levels. Further exhumation proceeded by westward oblique extrusion, according to the regional shallowly dipping stretching lineations (Wang et al., 2005).

4.2 The Thorough Overprint of the Metamafic Rocks

The Songshugou Complex contains a diversity of metamorphic mafic rocks, and mafic mylonite dominates the lithological spectrum strongly. Only rare blocks and layers show coronas around garnet (Fig. 2b), augen with garnet relics (Fig. 2c), extensive diopside-plagioclase symplectites with omphacite relics (Chen et al., 2015; Zhang, 1999), and diopside relics in amphibole (Fig. 2f). All these fabrics reflect a nearly completed amphibolitization of eclogites, which included the decomposition of garnet by corona formation (0.7-1.2 GPa, 660-710 ℃) and the stretching of the coronas into ribbons (1.93-2.54 GPa, 660-710 ℃). The ages of titanite included in porphyroclastic amphibole—the decomposition product of omphacite and diopside (Bader et al., 2019b)—and titanite oriented with the mineral stretching lineation in the cleavage domains, ~470 and ~420 Ma (Li et al., 2014), respectively, imply the overprint lasted ~50 Ma. Given our thermobarometry— and in accord with T derived from titanites (Li et al., 2014)—this happened at lower crustal levels.

The fabrics of the metamafic rocks imply a pre-overprint mineral assemblage contained garnet and omphacite. According to isopleth thermobarometry, garnet coexisted with amphibole, chlorite, phengite, and lawsonite during growth. The latter three do not occur in our samples, in accord with the long-lasting, lower crustal overprint and the knowledge about reaction kinetics. Water-rich phases such as lawsonite are only preserved, if the exhumation is sufficiently fast or if the uplift trajectory stays in their stability field (e.g., Du et al., 2014, and references therein)—both do not apply to the Songshugou Complex. Dehydration reactions such as the decomposition of chlorite, phengite, and lawsonite have a significant entropy and volume change, and they are therefore expected to be overstepped little (Pattison et al., 2011). Given the overprint happened at~200 ℃ higher T and ~1.0 GPa lower P than eclogite facies metamorphism, it was highly unlikely that chlorite, phengite, and lawsonite survived this overprint metastably.

4.3 Preservation of Garnet

In the mafic rocks of the Songshugou Complex, garnet has been decomposed to varying degrees, including near-preservation of the entire porphyroblast, formation of coronas of increasing thickness around garnet, and complete replacement of garnet by ovoid to σ-shaped aggregates. In case of high strain, these aggregates were stretched ribbon-like. Garnet is more widely preserved in the proximity of the large, central ultramafic body and near the Jieling shear zone. The penetratively serpentinized margin of the central ultramafic body accommodated strain—the serpentinites are penetratively foliated and share the foliation and stretching lineation with the mafic mylonite (Cao et al., 2017; unpublished data acquired during the field survey)—and the rheologically stronger metamafic rocks abutting it experienced less strain. Analogously, yet less pronounced, this happened at the Jieling shear zone, where the metamafic rocks are in contact with rheologically weaker marble. Consequently, deformation triggered garnet decomposition during uplift and the amount of strain governed preservation-decomposition of garnet porphyroblasts. Given that the corona formation requires H2O access, fluid percolation is a second factor: H2O was consumed during the serpentinization of the margin of the central ultramafic body and might have been of limited availability for the formation of hydrous clinozoisite and amphibole in the coronas. Marble might have liberated carbonic fluid, which would have reduced the water activity in the metamafic rocks, favoring preservation of anhydrous minerals such as garnet.

To conclude, the preservation and the decomposition of garnet in the metamafic suite of the Songshugou Complex depended on strain and fluid access: garnet is widely preserved, where abutting rheologically weaker serpentinite and marble accommodated strain and the H2O activity in the percolating fluid was low. We infer that mafic mylonite is the highly strained, completely transformed analogue of augen amphibolite and garnet amphibolite, and all share the P-T evolution.

4.4 Is the Songshugou Complex a Neoproterozoic Ophiolite?

An association of mafic and ultramafic rocks is considered as an ophiolite, if both are spatially and temporarily related (Dilek and Furnes, 2011). The scattered occurrence of more than one hundred smaller ultramafic bodies in the metamafic suite of the Songshugou Complex creates the appearance of a close spacial relation. However, during subduction, the mafic rocks were metamorphosed to eclogites. If the mafic and ultramafic rocks were part of a coherent slice during subduction, both would have evolved along the same P-T path, and garnet should have grown in the ultramafic rocks after the P exceeded ~1.7 GPa (Bader et al., 2013b; Klemme, 2004)—there is no indication for that, and, consequently, the close spatial relation results from tectonic mixing but not from joint genesis.

A temporal relation is harder to decipher given the difficulty to date ultramafic rocks. Published TRD ages are as old as 1.24 Ga (Nie et al., 2017), and they testify that melt was extracted from the ultramafic rocks by the Late Mesoproterozoic, perhaps multiple times. Two protolith ages of the metamafic rocks are ~0.79 Ga (Chen et al., 2015; Li et al., 2014). The age gap of ~0.35 Ga opts against a temporal relation.

Given both reasons, the Songshugou Complex is not an ophiolite in the strict sense.

4.5 Formation of the Songshugou Complex

All the data permit to outline a feasible scenario for the presence of mafic and ultramafic rocks in the Songshugou Complex. The ultramafic rocks were formerly in Rodinia's upper mantle, perhaps in a part that at pre-Rodinia time belonged to Yangtze Craton given Pb isotopic evidence (Liu et al., 2016). After the breakup of Rodinia created a realm of oceans, continents, and continental slivers, they were in the mantle of the continental sliver the relics of which are exposed today in the Qinling Complex; eventually, they were in the base of a fore-arc. The mafic rocks represent ~0.79 Ga old oceanic crust. Considering that oceanic crust has limited persistence—despite a restricted basin in the Mediterranean Sea (~340 Ma; Granot, 2016) the oldest oceanic crust on today's Earth is just 170 Ma old (Müller et al., 2008)—they probably were emplaced into continental crust before they were dragged into a subduction zone about 300 Ma after formation.

During Cambrian plate convergence, the mafic rocks entered the subduction zone and experienced eclogite facies metamorphism peaking at 2.13-2.54 GPa, 516-542 ℃. After exhumation through pressures of ~1.7 GPa, the mafic rocks were juxtaposed with a fraction of the fore-arc mantle and exhumed together with it. The P-T window of the retrogression in the mafic rocks (0.5-1.2 GPa, 660-710 ℃) matches the limited stability field of cummingtonite in the ultramafic rocks (≤0.8 GPa, 650-705 ℃; Bader et al., 2013a), and the juxtaposition was therefore completed when these conditions were reached. An U/Pb age of ~470 Ma of titanite inclusions in the pale green section of amphibole porphyroblasts (Li et al., 2014) gives a minimal age for the juxtaposition. The Songshugou Complex may have resided in the lower crust for ~50 Ma, before consecutive vertical (~420 Ma; Li et al., 2014) and oblique shearing exhumed it into the upper crust.

4.6 The Relation with Contemporaneous HP Granulites and (U)HP Eclogites in the Qinling Complex

The Qinling Complex, with which the Songshugou Complex is fused, contains dismembered, contemporaneous (518- 484 Ma; Fig. S1) diamond- and coesite-bearing eclogites, (HP) granulites, kyanite-bearing migmatites, and arc-related mafic and felsic plutonites. Despite their similar ages and Pb isotope geochemistry (Liu et al., 2016), their genetic relationship among each other and with the Songshugou Complex is controversial: the recent literature attempts to put their formation into a variety of geodynamic frameworks, which include the coexistence of up to three subduction-accretion systems (Bader et al., 2013a), the accretion of a hypothetic South Qinling terrane, which underlays sediments between the Shangdan fault and the Mianlüe suture west of Wudang, to the North China Craton (e.g., and Santosh, 2016; Wu and Zheng, 2013), and the collision of the North China Craton with the Yangtze Craton (Liu et al., 2016) or Australia (Han et al., 2016). Major issues are the penetrative overprint in the lower crust, the incompletely mapped extension of several eclogite and granulite occurrences, and the lack of knowledge about the kinematic regime during peak metamorphism.

Considering all data, the eclogites of the Qinling Complex (including the study area) show numerous characteristics— dispersed occurrence, diverse P-T paths (Fig. S1b), strong heating during exhumation to lower crustal levels, the probable lack of (U)HP nappes, stalling in the lower crust, and juxtaposition with non-serpentinized mantle. Furthermore, they constitute a mixture of matter from an active arc and the subduction channel this arc overrode. Here, we suggest a feasible scenario for the formation of this mixture, which is in accord with the current state of knowledge.

Out of the published, well-detailed eclogite exhumation models, only a diapir-like rise through the mantle wedge explains all these features (Hacker and Gerya, 2013; Yin et al., 2007). In this scenario, the capture of a hanging-wall mantle sliver (the ultramafic rocks of the Songshugou Complex) during exhumation is consequential, and the HP granulites of the Xigou shear zone and from Zhaigen may represent the base of thickened arc crust or testify lower crustal foundering. The P-T path of the former implies juxtaposition with the Songshugou Complex at ~0.8 GPa, 600-700 ℃ (i.e., during D3), but their exhumation from peak P to ~0.8-1.1 GPa happened at ~800-900 ℃; consequently, their exhumation to lower crustal levels proceeded independently to that of the eclogites. Thermomechanical modeling of diapir-like eclogite exhumation (Vogt et al., 2013) revealed both the coexistence and successive formation of individual diapirs of a few tens of kilometers diameter in one and the same subduction zone—the Songshugou Complex matches this dimension, and the UHP eclogite of Qingyouhe, west of the Songshugou Complex (Fig. S1), may have exhumed as part of a different diapir. The suggested model is independent from the plate tectonic framework: the process can operate during oceanic subduction, terrane accretion, or even continental collision, and it gives a possibility to form the complex mixture of rock types in the Qinling Complex above a single subduction zone.

5 CONCLUSIONS

The Songshugou Complex includes suites of ultramafic rocks and metamorphic mafic rocks. In a feasible scenario, the latter are relics of a Neoproterozoic ocean, emplaced into a continental sliver. In a subduction channel, the metamafic rocks were metamorphosed at quartz eclogite facies conditions of 2.13- 2.54 GPa, 516-542 ℃. Exhumation to lower crustal levels involved heating by about 200 ℃ and the subsequent exhumation to the middle to upper crust proceeded by near-vertical and oblique uplift, between ~500 and ~350 Ma. Thereby, all eclogite facies minerals but a small portion of garnet were decomposed. Garnet was preserved at low-strain strain sites with limited fluid percolation. Hence, the suite of metamafic rocks of the Songshugou Complex reflects different degrees of the transformation from eclogite into mafic mylonite, in which the decomposition product of garnet, plagioclase-amphibole-clinozoisite-ilmenite- quartz±titanite, biotite aggregates, were transformed into highly stretched ribbons. Mafic and ultramafic rocks of the Songshugou Complex were juxtaposed at P of 0.8-1.7 GPa, after they were in the subduction channel and belonged to the mantle wedge of the overriding plate, respectively. Given the distinct pre-exhumation evolution of the mafic and the ultramafic constituents, the Songshugou Complex is not an ophiolite in the strict sense.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (Nos. 41350110224, 41750110483, 41730426, 41872066), and the Open Research Project of the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (No. GPMR20 1820). We are grateful to Prof. Xiaochun Liu for his excellent field guidance. We thank Oona Appelt, Guiming Shu, and Xiaoli Li for operating the microprobe. Numerous diagrams were plotted with GMT (Wessel et al., 2013; http://www.soest.hawaii.edu/gmt/). Pointed criticism by two anonymous reviewers and the editors helped to improve the conclusiveness and the clarity of the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1221-1.

Electronic Supplementary Materials: Supplementary materials (Figs. S1–S7, Tables S1–S5) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1221-1.


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