Journal of Earth Science  2019, Vol. 30 Issue (3): 549-562   PDF    
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Metamorphic Petrology of Clinopyroxene Amphibolite from the Xigaze Ophiolite, Southern Tibet: P-T Constraints and Phase Equilibrium Modeling
Yancheng Zhang , Xu-Ping Li , Guangming Sun , Zeli Wang , Wenyong Duan     
Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, College of Earth Science & Engineering, Shandong University of Science and Technology, Qingdao 266590, China
ABSTRACT: The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone (YZSZ), southern Tibet. The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite mélange, four overprinted metamorphic stages are established. They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole, an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite, which is formed during burial process, an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite, and a decomposing retrograde stage M4, which is represented by the amphibolite+plagioclase symplectite+titanite+ rutile+quartz. By applying the THERMOCALC (versions 6.2 and 6.3) technique in the NCFMASHTO system, the P-T conditions estimated from M1 to M4 stages are ca. 8.6 kbar/880℃, 10.8-13.4 kbar/800-840℃, 12.7-13.2 kbar/650-660℃ and < 11.2 kbar/640℃, respectively. The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite, suggesting that the rocks underwent a cooling process during burial from magmatic protolith, and a decompressing stage after the pressure peak metamorphic conditions, which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes. The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies, granulite facies and eclogite facies with a burial depth of 30-40 km. After exhumation of the ophiolitic unit to the shallow crustal levels, the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams. A late subgreenschist facies overprinting subsequently occurs, the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.
KEY WORDS: clinopyroxene amphibolite    thermodynamic modeling    P-T conditions    counterclock P-T path    Bailang terrane    Xigaze ophiolite    Tibet    
0 INTRODUCTION

Convergence between India and Eurasia during the Jurassic and Cretaceous caused destruction of Tethyan Ocean along at least two subduction zones. From the Late Jurassic to the Late Cretaceous, at least one intraoceanic subduction zone was active within the Tethys; this subduction is interpreted to have dipped to the north and to have induced the arc and back-arc ridge accretion (Dupuis et al., 2006, 2005a, b; Dubois-Côté et al., 2005; Hébert et al., 2003, 2001, 2000; Huot et al., 2002; Aitchison et al., 2000; Zhou et al., 1996). The genesis of the Yarlung Zangbo ophiolites, geochronology and tectonic setting of their emplacement are critical regarding the initial collision between the India and Eurasia continents (Hébert et al., 2012; Aitchison et al., 2007, 2003; Ding et al., 2005; Aitchison and Davis, 2004; Malpas et al., 2003; Xia et al., 2003; Mahoney et al., 1998; Zhou et al., 1996).

Amphibolites of the Xigaze ophiolite reached high-pressure and high-temperature (HP-HT) conditions and have been recognized as an important component of supra-subduction zone (SSZ) (Zhang et al., 2016; Guilmette et al., 2012, 2009). The average P-T conditions of garnet-clinopyroxene-amphibolites (Grt-Cpx amphibolite) formed at about 12 kbar/800 ℃ (Guilmette et al., 2012); they experienced peak pressure conditions of ~18 kbar/ ~600 ℃ and reach peak temperatures at about 13–20 kbar/750–875 ℃, which reach to an eclogite facies metamorphic depth (Zhang et al., 2016; Guilmette et al., 2008). Abundant high- precision data has recently been obtained from mafic intrusions to constrain the formation time of the Yarlung Zangbo ophiolites. Most ages indicated that these ophiolites formed at a short duration between 119 to 132 Ma (e.g., Liu et al., 2016; Dai et al., 2013; Hébert et al., 2012; Wang et al., 2006; Malpas et al., 2003). The 40Ar/39Ar step-heating dating of hornblende from Cpx-Grt amphibolite obtained a metamorphic age of 123.6–127.7 Ma (Guilmette et al., 2009). The existence of the Bailang Cpx-Grt amphibolite, therefore, shows that the Xigaze ophiolite has been emplaced soon after it formed, and they are thought to form at the inception of oceanic subduction beneath the hot sub-ophiolitic mantle of the hanging wall (Wu et al., 2014; Guilmette et al., 2012, 2009; Malpas et al., 2003; Nicolas et al., 1981).

If noting the small age difference between the formation of magmatic ophiolite and the metamorphic amphibolites in the Xigaze ophiolite belt, the amphibolites probably resulted from a subduction of young, hot crust beneath a very hot mantle (Hébert et al., 2012; Guilmette et al., 2009, 2008). The onset of this subduction within a pre-existing supra-subduction zone caused the formation of a high-grade metamorphic sole around 130 Ma and provides a mechanism for the trapping of back- or inter-arc lithosphere in a fore-arc setting (Zhang et al., 2016; Guilmette et al., 2012).

The results from Guilmette et al. (2008) were characterized with more detailed studies on phase equilibrium modeling related to the Grt amphibolite. The P-T conditions of 8–10 kbar/~700–750 ℃ for Grt amphibolite were indicated by using traditional geothermobarometrical methods, whereas detailed metamorphic P-T path was not well constrained for Cpx amphibolites. The question is still open which we like to address: Do the Cpx amphibolites share the same metamorphic evolution with Grt amphibolite? In this study, we present results from petrography, mineral chemistry and P-T pseudosection modeling of Cpx amphibolite from the Bailang terrane of the Xigaze ophiolite. The information will then be used to better constrain the evolution of metamorphic portions of the Xigaze ophiolite and its surrounding terranes in South Tibet.

1 GEOLOGICAL SETTING

The Tibetan Plateau consists of numerous terranes which are from north to south, the Kunlun-Qilian, Songpan-Ganze, Qiangtang and Lhasa (Li et al., 2017, 2015a, b; Yin and Harrison, 2000). The Yarlung Zangbo suture zone (YLZB) with an east- west direction is the youngest suture zone in South Tibet (Fig. 1a). Along the YZSZ Jurassic and Cretaceous relicts of ophiolites occur. They refer to the Neo-Tethyan Ocean that was closed prior to the India-Asia collision. During this process portions of the ophiolite were obducted towards the south onto the Great-Indian basement and afterwards thrusted northwards (Guilmette et al., 2012, 2009; Hébert et al., 2012; Bédard et al., 2009; Mo et al., 2007; Dupuis et al., 2006, 2005a, b; Dubois-Côté et al., 2005; Allègre et al., 1984; Nicolas et al., 1981). At least two subduction events could be distinguished as a result of the closure of the Tethyian Ocean at least two subduction events could be distinguished (Dupuis et al., 2006, 2005a, b; Dubois-Côté et al., 2005; Hébert et al., 2003, 2001; Huot et al., 2002; Aitchison et al., 2000; Zhou et al., 1996).

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Figure 1. (a) The Xigaze ophiolite located in Yarlung Zangbo suture zone. YZSZ. Yarlung Zangbo suture zone; BNSZ. Bangong Nujiang suture zone; (b) simplified geological map and sampling locality (yellow star) in the Xigaze ophiolite, southern Tibet.

The Xigaze ophiolite in South Tibet is the largest and best studied ophiolite complexes of the entire YZSZ. It extends over 250 km from east to west and the strike follows the direction Dazhuqu-Angren (Figs. 1a, 1b). It records the northward subduction of the Indian Plate beneath the Lhasa terrane and marks the closure site of the Neo-Tethian Ocean (Zhu et al., 2013; Pan et al., 2012; Yin and Harrison, 2000; Nicolas et al., 1981). In the Xigaze ophiolite the amount of mantle peridotites is more than the amount of mafic rocks; portions of sheeted dyke complex are also exposed (Li et al., 2017; Wu et al., 2014; Girardeau et al., 1985; Nicolas et al., 1981). Strongly foliated amphibolite blocks occur in the ophiolitic mélange that underlies on the bottom of the Xigaze ophiolite, which is exposed from Buma eastwards to Bailang and Luobusha. They may represent remnants of a metamorphic sole formed during the emplacement of the ophiolite as the hangingwall of a newborn subduction zone (Zhang et al., 2016; Guilmette et al., 2012, 2009, 2008).

The Bailang amphibolites occur as xenoliths in a serpentinite mélange, which is located on the south side of the Xigaze ophiolite belt, about 20–30 km northeast of the Bailang County (Fig. 1b). The serpentinite mélange extends in NE-SW direction and is oriented parallel to the regional structural deformation of the major lithostratigraphic units (Sun et al., 2018; Li et al., 2017; Zhang et al., 2016). The Bailang serpentinite mélange also includes deformed Cpx-Grt amphibolite and rodingite blocks (Li et al., 2017; Zhang et al., 2016). The clinopyroxene amphibolite (Cpx amphibolite) profile occurs as ~50 m×50 m blocks within the serpentinite mélange. The margin of the Cpx amphibolite is rodingitized at a width of about 6 m at the contact zone with serperntinite country rock (Fig. 2a). Early coarse-grained clinopyroxene of ~1–5 mm can be observed in hand specimen and thin section under microscope (Figs. 2b, 3a), while late stage clinopyroxene belongs to the metamorphic stage. Late fluid veins were developed in the Cpx amphibolite, which are mostly filled by prehnite and epidote (Figs. 2b, 3a, 3c and 3k). Sometimes, late calcite veins were observed.

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Figure 2. The field outcrop of clinopyroxene amphibolite from the Bailang terrane of the Xigaze ophiolite.
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Figure 3. Photographs of mineral assemblages and textures in the clinopyroxene amphibolite. (a) Pseudomorphism of relict protolithic Cpx1, which is replaced by later Cpx and Amp; (b) Cpx2 is overprinted by Amp3; (c) Amp1 and its evolved Amp2 which includes pyrite+magnetite as inclusions; (d) amphibolites from different stages and Ttn4 and Rt4 in the stage M4; (e) Cpx2 is overprinted by Amp3; (f) Rt2 is included within Ttn3 which in turn is overgrown by Rt4; (g) mineral assemblage of Amp3+Pl3; (h) mineral assemblage of Amp3+Cpx3 and Ab+Prh; (i) Ttn3 and overprinted Rt4; (j) a large plagioclase porphyroblast, which preserved protolithic pseudomorphism (Pl1); (k) modified Pl in different stages, and final subgreenschist assemblage Ab+Ep; (l) relict and metamorphic Pl compositions with variable An values. Photos (b)–(c) were taken under plane-polarized light, (d) under crossed polars; all the others are BSE images.
2 PETROGRAPHY

Clinopyroxene amphibolites contain about 50%–65% amphibole, 15%–25% plagioclase, 5%–15% clinopyroxene and 10%–20% other minerals consist of epidote group, quartz, prehnite and chlorite. Accessory minerals are abundant such as rutile, titanite, apatite, zircon and rare pyrite, magnetite (Fig. 3). Mineral abbreviations follow Whitney and Evans (2010).

On the basis of textural properties, three types of clinopyroxene could be distinguished. The first type shows a relict enhedral crystal (M1, Cpx1), which is a pseudomorph of amphibole (> 3 mm) (Fig. 3a). Cpx1 is commonly kinked and surrounded by a rim of orange or green amphibole. The second stage Cpx2 (M2) is anhedral with a grain size of usually < 1 mm (Figs. 3a, 3e). Fine grained Cpx3 (M3) occurs in the matrix or as vein-like, and directly contacts with amphibole (Figs. 3e, 3h).

Four types of amphiboles have been distingushed. Large brown-reddish enhedral amphibole ~1–5 mm (Fig. 3c) in size shows a relict igneous characteristics and is defined as Amp1 (M1). Green or light brown amphibole of ~0.5–2 mm in size belongs to the second stage (Amp2, M2). It is frequently deformed and contains plagioclase and other early stage minerals such as pyrite, which became oxidized to form magnetite in the late stage (Figs. 3c, 3d). The third type of amphibole (Amp3) is fine-grained and scatters in the matrix or overprints in the rim of Cpx2 (Figs. 3b, 3e, 3j). The forth type of amphibole (Amp4) forms fairly fine-grained grains, which are newly crystallized in the matrix (Fig. 3d).

Plagioclase is a very important major mineral in the Cpx amphibolite and records important information of different metamorphic processes. Almost all of them are replaced by an albite-prehnite subgreenschist assemblage. We found a large pseudomorphic magmatic plagioclase, which is partly replaced by later different metamorphic plagioclases as Pl1 to Pl2, and also subgreenschist facies albite (Figs. 3j3l). Some hypautomorphic grains, intergrown with the Amp3, were taken as Pl3 (Fig. 3g).

Rutile and titanite are ubiquitous minor phases (Figs. 3e3f, 3h3i). Early stage rutile (Rt2) forms as inclusions within titanite (Ttn3), and a later stage rutile (Rt4) is in turn formed at the expense of Ttn3 (Figs. 3e3f, 3l), or associated with titanite of the same stage in the matrix (Fig. 3e).

Other minerals such as epidote, prehnite, albite, and sometimes calcite vein are representive of the late sub-greenschist facies assemblage. Prehnite veins and albite-prehnite intergrowth are typical textures for this stage (Figs. 3a, 3c, 3e and 3h). It is noteworthy that similar prehnitization overprinted on amphibolites had also been reported from other Cpx-Grt amphibolite locations in the Xigaze ophiolite along the YZSZ (Zhang et al., 2016; Guilmette et al., 2012, 2009, 2008).

In summary, irrespective of last subgreen schistfacies overpriting, we can distinguish four different generations of mineral assemblages. The first record of M1 refers to the metamorphic evolved stage after igneous assemblage Cpx1+ Pl1+Amp1, ilmenite is not detected at this stage, probably due to the late metamorphic overprints. The stage M2 represents for pressure progressive assemblage Cpx2+Amp2+Pl2+Rt2. The stable mineral assemblage at the peak pressure stage (M3) is Cpx3+Amp3+Pl3+Ttn3 and the stage M4 is characterized by the decomposing assemblage Amp4+Ab4+Rt4+Ttn4. The final metamorphic stage is subgreenschist facies overprinting under brittle deformation conditions produced the assemblage Prh+Ab±Chl±Ep±Cal.

3 MINERAL COMPOSITION 3.1 Analytical Method

The compositions of minerals were analyzed at the Tongji University, Shanghai, using a JXA-8230 electron microprobe with conditions of 15 kV accelerating voltage, 10 nA probe current and focussed beam. The natural minerals jadeite (Si), forsterite (Mg), hematite (Fe), albite (Na, Al), diopside (Ca), rutile (Ti), rhodonite (Mn) and sanidine (K) served as analytical standards. The representative mineral analyses of Cpx amphobolites are presented in Tables 14.

Table 1 Representative electron microprobe analyses data (wt.%) of clinopyroxene from Cpx amphibolites in the Bailang terrane of the Xigaze ophiolite
Table 2 Representative electron microprobe analyses data (wt.%) of amphibole from clinopyroxene amphibolites in the Bailang terrane of the Xigaze ophiolite
Table 3 Representative electron microprobe analyses of plagioclase for the sample 15BG46 from clinopyroxene amphibolites from the Bailang terrane of the Xigaze ophiolite
Table 4 Representative electron microprobe analyses data (wt.%) of other minerals from clinopyroxene amphibolites in the Bailang terrane of the Xigaze ophiolite
3.2 Clinopyroxene

Clinopyroxenes were distinguished as three types as discussed in the above sections, and the mineral compositions are listed in Table 1. All of the clinopyroxenes are metamorphic diopsides with very low Cr content (mostly < 0.2). The relict Cpx1 at M1 stage is a pseudomorph of igneous clinopyroxene, and was compositionally modified by Cpx2 (Fig. 3a) so that the composition of Cpx1 is no longer available. The early colorless diopside (Cpx2) contains Al2O3 3.00 wt.%–4.62 wt.% and TiO2 ~0.19 wt.%–0.47 wt.%, but the later light green diopside (Cpx3) contains lower Al2O3 and TiO2 0.32 wt.%–3.13 wt.% and ~0–0.35 wt.%, respectively. The early diopside Cpx2, however, presents lower Mg# (~0.57–0.71) than that of late Cpx3 (~ 0.72–0.84).

3.3 Amphibole

On the basis of classification of Leake et al. (2004), amphiboles in the Cpx ampibolite are all Ca-amphibole group in composition (Ti < 0.5 and CaB > 1.5 a.p.f.u.). They are edenite, pargasite, tschermakite, magnesio-hornblende and additional actinolite that occurs in the final subgreeschist facies assemblage together with prehnite, albite and chlorite etc. (Table 2, Fig. 4). The Amp1 contains the highest AlIV ~1.27–1.72, Ti ~0.02–0.15 atom per formula unit (a.p.f.u.) and total alkali content between 0.61 and 1.75 a.p.f.u. (Table 2, Figs. 5a and 5b). Most amphiboles from M1 stage are edinites with some of them being pargasites. In comparison with Amp1, amphibolites (Amp2) at M2 stage are also mainly edinites besides a few pargasites and magnesio- hornblende, which show slightly less AlIV between 1.11 and 1.64 a.p.f.u., slightly lower Ti contents, ranging from 0.04 to 0.13, while amphiboles (Amp3) at M3 stage contain both low AlIV ~0.72–1.34 and Ti ~0.02–0.09 a.p.f.u., which are magnesio- hornblende besides few edinite. Amphiboles from M4 stage (Amp4) are magnesio-hornblende with the lowest AlIV ~0.03– 0.63 a.p.f.u.) and Ti ~0.00–0.01 a.p.f.u. except for actinolites. It shows a negative correlation from Amp1 to Amp4 in the diagrams of AlIV vs. Ti and AlIV vs. (Na+K) (Figs. 5a5b).

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Figure 4. Ca-amphibole nomenclature for Cpx amphibolite after Leake et al. (2004).
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Figure 5. (a) Ti vs. AlIV (a.p.f.u.) diagram for amphiboles. Fields for 'ophiolitic mélange' and 'Beimarang mélange' taken from Dupuis et al. (2005a) and Huot et al. (2002). Amphibole nomenclature from Leake et al. (2004); (b) (Na+K) vs. AlIV diagram for amphiboles. End-member compositions are indicated. EK. Ekermanite; ED. edenite; PG. pargasite; TR. tremolite; TS. tschermakite.
3.4 Plagioclase

The analyzed compositions for a large plagioclase, preserved pseudomorph of magmatic protolith, present a variable An components from 85.56 to 1.74 (Table 3, Figs. 3j3l). That indicates that the large plagioclase records plagioclase compositions of different stages during metamorphic evolutional processes.

This large plagioclase provides relict igneous components (An=85.56–77.61), the first record of metamorphic plagioclase of M1 stage Pl1 (An=40.13–29.22); metamorphic M2 stage Pl2 (An=25.05–23.02); and an analyzed single grain plagioclase, which is associated with Amp3, provides a M3 stage Pl3 composition (An=11.39) (Fig. 3g).

In addition, plagioclases in subgreenschist facies are all albites with compositions of An=5.41–1.74. Albite grains at this stage occur in the matrix, and closely associated with prehnite, chlorite and epidote veins under the last subgreenschist facies conditions (Fig. 3).

3.5 Other Minerals

On the basis of texture characters, two stages of rutile and titanite are identified, which can be used to further constrain the P-T conditions (Table 4). These two minerals have similar mineral compositions in different metamorphic stages respectively. Epidotes contain iron content ~10.11 wt.% in average, which were formed in late veins under the last subgreenschist facies conditions (Fig. 3j). Chlorite is also iron rich with FeO ~19.94 wt.%.

4 PHASE EQUILIBRIA MODELING AND P-T CONDITIONS

Thermodynamic modeling caculations are carried out in the NCFMASHTO (Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2) system for the representative Cpx amphibolite sample (15BG46), which are shown in Figs. 67. Quartz is stable and shown in all pseudosection fields. Pseudosection calculations were performed using THERMOCALC 3.45, using versions 6.2 and 6.3 of the Holland and Powell (2011) dataset (ds6). Activity-composition relationships are taken from data used for clinopyroxene (Green et al., 2016), amphibole (Diener et al., 2007), plagioclase (Holland and Powell, 2003), K-feldspar (Holland and Powell, 2003) and epidote (Holland and Powell, 2011).

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Figure 6. T-X(O) diagram at (a) 12 kbar and (b) 9 kbar for sample 15BG46. Detailed descriptions are shown in this paper.
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Figure 7. P-T pseudosections calculated from effective bulk composition of sample 15BG46. Mineral abbreviations after Whitney and Evans (2010).
4.1 Analytical Method

The whole-rock geochemical analyses were made using a Philips PW1400 X-ray fluorescence spectrometer (XRF) at Ruhr-University Bochum, Germany. Total iron was determined as Fe2O3(t) (wt.%). FeO was analyzed by potentiometry; Fe2O3 was then calculated by difference (Fe2O3(t)−FeO×1.111 3). Water content was determined using Coulomb Karl-Fischer titration method (Johannes and Schreyer, 1981). The major elements oxide compositions of whole-rock for sample 15BG46 are listed in Table 5-(1).

Table 5 Major element concentrations (wt.%) and calculated effective compositions of sample 15GB46

Since Cpx amphibolites in this study were overprinted by rodingitization and sometimes aslo calcite veins in the final subgreenschist facies metamorphism, the phase equilibria modeling diagrams were calculated for sample 15BG46 using an effective bulk-rock compositions by removing the influence of Ca addition and irrespective of the CO2 for simplitity. In addition, there is no biotite, muscovite and K-feldspar in the sample, so that K2O influence was neglected in the calculatations of T-M(O) and P-T pseudosection diagrams. By normalized to the NCFMASHTO system, the using effective compositions are listed in Table 5-(2).

4.2 T-M(O) Diagram

The mineral and petrological evidences indicate the involvement of epidote in the Cpx amphibolite, which occurs in a relatively high fO2 level. In order to define the M(O) contents in T-M(O) diagrams in stable constrains to different metamorphic stages and their P-T conditions, the T-M(O) diagrams were constructed at 12 and 9 kbar, respectively. The choice of M(O) contents is based on the variations of O mole percentage (from 0 to 4) in the bulk rock (Figs. 6a, 6b). The corresponding mole ratios of oxide are normalized to 100% in the NCFMASHTO system as listed in Table 5-(3) and (5), which are calculated on the basis of effective composition (Table 5-(2)). Correspondingly Fe2O3 is variable from Fe3+=0 to Fe3+=91.8% in total iron. Oxygen fugacity shows influence on the stability of Fe3+-bearing minerals (e.g., epidote and magnetite) during metamorphic processes. Epidote is not stable at lower M(O) values before crossing over Ep entrance line as shown in Figs. 6a and 6b. Passing the red solid line, the melt is produced, and the pink dashed line indicates disappearance of H2O when temperature increases. The observed mineral assemblage (M2) is stable for M(O) contents between 0 and 2.21 (long dashed green line based on the predicted phase mineral assemblage of M2 stage of Cpx+Amp+Pl+Qz+Rt+Liq. According to the mineral assemblage of M3 stage, Cpx+Amp+Qz+Ttn+Liq is stable in a M(O) field between 0 and 1.60, while the mineral assemblage of M4 stage Amp+Ab+Rt+Ttn+Liq does not occur in Fig. 6a under relative higher pressure at 12 kbar, probably because of too high pressure value chosen here. A T-M(O) diagram at 9 kbar then was computed, and the mineral assemblage of M4 stage was presented in the lowest-left corner (Fig. 6b). We, therefore, chose a M(O) value of 1.05 (~24.1% Fe3+ in total iron), which was calculated on the basis of effective composition (Table 5-(2)) and then normalized to 100% in the NCFMASHTO system, by consideration of stable fields of mineral assemblages for M2, M3 and M4 three stages, which is marked by red dash line and it is further used to construct the diagram of the P-T pseudosection (Fig. 7). It means that M4 mineral assemblage Amp+Ab+Rt+Ttn only occurs below 625 ℃ (Fig. 6b). In addition, epidote does not occur in these four satges, and it only occur in subgreenschist facies conditions based on the petrological study, and is outside of the modeling T-M(O) diagrams in this study.

4.3 P-T Pseudosection and Metamorphic Stages

The observed mineral assemblage involves clinopyroxene, amphibole, plagioclase, quartz, titanite and rutile that are stable in a wide P-T range. A specific P-T pseudosection with the P-T range of 5–15 kbar and 600–900 ℃ is constructed in the model NCFMASHTO system. The isopleths of An in plagioclase and Ti (Amp) in amphibole are contoured for the relevant mineral assemblages, and to constrain P-T conditions of different metamorphic stages.

4.3.1 Metamorphic M1 stage

As figured out in the previous section, the relict large protolithic plagioclase pseudomorph (Figs. 3j3l) presents high content of An ~77.61–85.56 that suggests the sample reserves a mineral assemblage formed before metamorphism as shown in dashed line after M1 stage in Fig. 7. The maximum Ti preserved in Amp1 (Ti=0.123) (Table 2) and corresponding to Pl1 (An=40.13) further constrain the P-T conditions of M1 stage at P=8.6 kbar, and T=880 ℃.

Although ilmenite and orthopyroxene are not found in this study, as the An contours increase from 40.13 to 85.56, the temperature also increases till outside of the Fig. 7 (> 900 ℃), which implies that ilmenite and orthopyroxene may have existed and a typical igneous mineral assemblage Opx-Cpx- Amp-Pl-Qz-Ilm-Liq might has been reached.

4.3.2 Metamorphic M2 stage

The metamorphic (M2) stage is characterized by the mineral assemblage of clinopyroxene from relict large porphyroblast (Cpx2) (Fig. 3b), amphibolite (Amp2) (Figs. 3c3d), plagioclase (Pl2) from relict large plagioclase porphyroblast, rutile (Rt2) (Fig. 3e) and quartz that falls in the pseudosection field of Cpx+Amp+Pl+Rt+Liq (Fig. 7). This stable mineral field presents the An isopleth of plagioclase dropping from 26 to 16 with increasing pressure. The An value of Pl2 (23.02–25.05) and Ti (Amp) (0.081–0.105) in sample 15BG46 constrain the P-T conditions of the M2 stage at 10.8–13.4 kbar and 800–840 ℃ (Fig. 7).

From the first record of metamorphic M1 stage to M2 stage, the mineral assemblage passes from the possible stable assemblage of Cpx-Amp-Pl-Qz-Ilm-Liq pseudosection field to Cpx+Amp+Pl+Rt+Liq assemblage field, cutting across the early metamorphic M1 stage of assemblage Cpx+Amp+Pl+ Ilm+Liq, where An still preserves high values from 30.06 to 38.08 in the large plagioclase porphyroblast (Table 3, Figs. 3j3l, 7). Since absence of ilmenite and more other mineral compositions, exact P-T conditions of the early metamorphic stage are not able to be constrained.

4.3.3 Metamorphic M3 stage

As for the metamorphic stage M3, the mineral assemblage of clinopyroxene (Cpx3)+amphiblite (Amp3)+plagioclase (Pl3)+ titanite (Ttn3)+quartz occupies a rutile free portion of the Cpx amphibolite field (Fig. 7). It suits the pseudosection field of Cpx+ Amp+Pl+Ttn+Liq, which exhibits more temperature-dependent An (pl) isopleth range of 12–15 (Fig. 7). Rutile is no longer stable at the M3 stage, the metamorphic path from the stage M2 to stage M3 enters a rutile free assemblage of Cpx+Amp+Pl+Ttn+Liq pseudosection field where An content decreases with temperature dropping. Only An (Pl3) value (11.39), which is associated with Amp3, is able to confine the P-T conditions of the stage M3 at ~12.7–13.2 kbar and 650–660 ℃ (Fig. 7).

4.3.4 Metamorphic M4 stage

As discussed above, the retrograde M4 stage represents the development of amphibole (Amp4)+plagioclase (Ab)+rutile (Rt4)+titanite (Ttn4)+quartz (q). This assemblage fits the pseudosection field of Amp+Ab+Rt+Ttn+H2O with existence of albite composition (An < 6) rather than high An value plagioclase. The formation of symplektitic Ab+Amp4 after Amp3 was observed in this stage (Figs. 3e, 3g). The metamorphic path from the stage M3 to stage M4 cuts across the rutile free fields, and stable in a pseudosection field with coexistence of both titanite and rutile as pressure remarkably decreases. The P-T conditions of M4 stage are defined by the mineral assemblage at < 11.2 kbar and < 640 ℃ (Fig. 7).

4.3.5 Final subgreenschist facies metamorphism

This final stage metamorphism, developed during brittle tectonic setting after metamorphic terrane reture back to shallow crust, is a metasomatic process. A mineral assemblage at the subgreenschist facies contains prehnite (vein), epidote, albite, chlorite and calcite, which is an overprint of rodingitization on the amphibolites of the Bailang terrane, Xigaze ophiolite (Li et al., 2017). The P-T conditions is figured out 3.2 kbar, < 285–300 ℃ (Li et al., 2017), which is outside of the modeled pseudosection range (Figs. 67).

5 DISCUSSION AND CONCLUSIONS 5.1 Metamorphic Processes and P-T Path

The P-T path derived for the Cpx amphibolites on the basis of thermodynamic calculations is shown in Fig. 7, which is counterclockwise. The first recorded stage (M1) occurs at ~900 ℃, at pressures ~8.6 kbar, as indicated by high An values in the large plagioclase porphyroblast and with paragenetic highest Ti in amphibole (Amp1). During further burial along with a contemporaneous temperature decrease, P-T conditions of the M2 stage at 10.8–13.4 kbar and ~800–840 ℃ were reached, followed by a M3 stage of a nearly isobabric cooling. This stage of the P-T conditions was documented by coexisting Amp3 and Pl3 and reach a P-T conditions up to 12.7–13.2 kbar, ~650–660 ℃, which indicates the M3 stage could form in a deep crustal level. A subsequent metamorphic process of M4 stage is represented by decompression symplektites of Ab+Amp4 (Figs. 3e, 3g) at < 11.2 kbar and < 640 ℃ (Fig. 7). The last metamorphic/ metasomatic event occurred under subgreenschist facies conditions, documented by the mineral assemblage of Prh+Ab+ Chl+Ep+Qz, which is outside of the chosen P-T conditions in the pseudosection diagram Fig. 7. The final overprint at subgreenschist facies metamorphism occurred the same time with surrounding serpentinization and rodingitization processes, which took place after the Bailang terrane was exhumed to shallow crustal level under brittle tectonic conditions, and it has also been observed in most of the metamorphic blocks of the Xigaze ophiolite (Li et al., 2017; Guilmette et al., 2008; Dupuis et al., 2005a; Huot et al., 2002). Oxidation conditions are documented on the basis of the stable epidote-bearing assemblage of the T-M(O) diagram (Figs. 6a, 6b). Epidote only occurs in subgreenschist facies conditions in this study, and is outside of the modeling T-M(O) diagrams. It can be concluded, however, at low P-T subgreenschist facies conditions, when epidote occurs, the Cpx amphibolite exposes to much higher fO2 condition.

A similar counterclockwise P-T path was determined by Guilmette et al. (2008) and Zhang et al. (2016) for Grt-Cpx amphibolites from other localities of the Xigaze ophiolite, and was also reported for Grt-Cpx amphibolites from Indus- Tsangpo suture zone (Bhowmik and Ao, 2016). These Cpx-Grt amphibolites experienced the high-pressure overprint, which reached up to the transitional area of amphibolite-eclogite- granulite facies conditions (Zhang et al., 2016; Guilmette et al., 2012, 2008), or to an eclogite facies conditions (Bhowmik and Ao, 2016). Geodynamic significance of superposed HP metamorphism (M2–M3) from the Bailang amphibolite unit in this study provides a good evidence of a single cycle of subduction and exhumation along an entire counterclockwise P-T path as indicated by Bhowmik and Ao (2016).

5.2 Geological Implications

The Cpx-amphibolite studied received its metamorphic overprints in a serpentinite shear zone of the Xigaze ophiolite in the central Yarlung Zangbo suture zone (Fig. 1b). Deformational features are common in amphibolites of the Xigaze ophiolite, such as in the Cpx-Grt amphiboliltes from Sangsang, Saga, Buma and Bailang as shown by Guilmette et al.(2012, 2009, 2008) and Zhang et al. (2016). The counterclockwise P-T path for the studied Cpx amphibolite represents the department block, which is involved in serpentinite mélange of the bottom of the Xigaze ophiolite. It is also involved in the metamorphic sole together with Grt-Cpx amphibolites of the Bailang terrane as the same as those Grt-Cpx amphibolites from Sangsang, Saga of the Xigaze ophiolite (Zhang et al., 2016; Guilmette et al., 2012, 2008; Dilek and Whitney, 1997). Some researchers have explicitly defined the inception of subduction and consequent development of a metamorphic sole beneath an ophiolite as emplacement, or at least the first stage of emplacement (e.g., Bhowmik and Ao, 2016; Zhang et al., 2016; Dai et al., 2013; Guilmette et al., 2012, 2008; Bézard et al., 2011; Bédard et al., 2009; Dupuis et al., 2005b; Wakabayashi and Dilek, 2003; Huot et al., 2002; Dilek and Whitney, 1997). Metamorphic soles are thought to form beneath the hot sub-ophiolitic mantle of the hanging wall, the high-grade metamorphism of the sole, therefore, can only occur at the inception of subduction because the hanging wall would be too cold to cause highgrade metamorphism thereafter (e.g., Bhowmik and Ao, 2016; Dupuis et al., 2005b; Wakabayashi and Dilek, 2003; Nicolas et al., 1981).

One important observation on the Bailang Cpx-amphibolite of the Xigaze ophiolite is that the peak metamorphic temperature (M1, 880 ℃) reaches a granulite facies metamorphism. Some researchers have suggested that some ophiolites have formed at mid-ocean ridges (Robertson, 2002; Nicolas, 1989; Coleman, 1981). The Bailang Cpx-Grt amphibolites from the nearby Cpx amphibolites locality, about 2 km south-east of this studied Cpx amphibolite, is reported to exhibit a depleted mantle N-MORB affinity and to have been originated from a mid-oceanic ridge environment; but they also show significant enrichment of large ion lithophile elements and partly depleted in high field strength elements, which proves subduction related genetic information, and probably links to an arc/back-arc basin spreading centers (Zhang et al., 2016). The Cpx amphibolite of this study shows a similar rare earth element pattern and trace element geochemical characteristics (unpublished data). Such ophiolite probably forms in a spreading environment during the initial stages of intra- oceanic subduction, prior to the emergence of any major related oceanic arc (Bhowmik and Ao, 2016; Hébert et al., 2012; Guilmette et al., 2009, 2008; Aitchison et al., 2000). In another word, it is probably produced in an arc/back-arc basin spreading center (Zhang et al., 2016; Dai et al., 2013; Hébert et al., 2012, 2003, 2001; Bézard et al., 2011; Bédard et al., 2009; Guilmette et al., 2008; Dupuis et al., 2006, 2005b; Ding et al., 2005; Dubois-Côté et al., 2005).

ACKNOWLEDGMENTS

We thank Dr. Lingmin Zhang from Tongji University for the helps in electron microprobe analysis. We acknowledge Dr. Cong Zhang and an anonymous reviewer for their comments and suggestions that greatly improved the manuscript. This study was financially supported by the National Natural Science Foundation of China (No. 41572044) and the SDUST Research Fund (No. 2015TDJH101). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1222-0.


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