Contents lists available atScienceDirect
Journal of Asian Earth Sciences
journal homepage:www.elsevier.com/locate/jseaesTectono-metamorphic evolution of the Karakoram Terrane: Constrained
from P–T–t–fluid history of garnet-bearing amphibolites from trans
Himalaya, Ladakh, India
Praveen Chandra Singh
a,d, Himanshu K. Sachan
a,⁎, Aditya Kharya
a, Franco Rolfo
b,c,
Chiara Groppo
b,c, Saurabh Singhal
a, Sameer K. Tiwari
a, Shashi Ranjan Rai
a,e aWadia Institute of Himalayan Geology, 33 GMS Road, Dehra Dun 248 001, IndiabDepartment of Earth Sciences, University of Torino, Via Valperga Caluso 35, Torino 10125, Italy cIGG-CNR, Via Valperga Caluso 35, Torino 10125, Italy
dDepartment of Earth Sciences, Indian Institute of Technology, Bombay, Mumbai, India eDepartment of Earth Sciences, Indian Institute of Technology, Roorkee, India
A R T I C L E I N F O Keywords: Garnet-bearing amphibolite P-T-t evolution Thermodynamic modeling Fluid inclusions U-Pb zircon geochronology Karakoram Metamorphic Complex
A B S T R A C T
This study presents new data sets on the petrological, fluid inclusion and U-Pb zircon geochronological aspects of the garnet-bearing amphibolites from the eastern Karakorum (Pangong-Tso), Ladakh. to delineate the P-T-t-fluid evolution and its tectonic implications.
The calculated phase equilibria in the model system NCKFMASHTO reveal that the peak metamorphism occurred at 8.5–9.5 kbar, 630–655 °C, and retrograde metamorphism crossed a stage at 5.5–5.7 kbar, 620–630 °C. Monophase primary and secondary carbonic and carbonic-aqueous fluid inclusions occur in garnet and quartz. Trapping of the primary inclusion occurred at pressures lower than the estimated peak P-T condi-tions (5.8–6.1 kbar, 629–638 °C) indicative of partially re-equilibrated fluid inclusions during exhumation. The primary inclusions were original as carbonic-aqueous fluid derived from prograde decarbonation of the am-phibolite’s protolith or adjacent carbonate-rich rocks and H2O leached out during deformation, leaving a pure CO2fluid.
U–Pb dating of zircons yielded two age groups: (i) at 128 to 131 Ma interpreted as the age of detrital zircons of sedimentary derivation, and constrain the maximum depositional age of the sedimentary protolith at 131 Ma and (ii) at 114 ± 0.38 Ma is interpreted as the age of peak metamorphism remarkably similar to the meta-morphic age of the surrounding metapelites. This metameta-morphic stage corresponds to the well-known Cretaceous phase of crustal thickening and heating along the active continental margin of South Asia.
1. Introduction
The Karakoram terrane is the western margin of the Qiangtang terrane of central south Tibet (Searle et al., 1989,Searle, 1991, Kapp
et al., 2003). The Karakoram terrane offers a possibility for exploring
the evolution of the South Asian margin prior to India-Asia collision. Geodynamic evolution of the western portion of the Karakoram terrene has been studied in quite a detail byFoster et al. (2004), Fraser et al. (2001) and Palin et al. (2012), but the tectono-metamorphic history of central and eastern Karakoram is significantly less known, and still debated. The metamorphism in the Karakoram terrane was initially interpreted as a consequence of shear heating under fault-guided magmatic upwelling along the Karakoram fault (Rolland and Pêcher,
2001; Rolland et al., 2009; Valli et al., 2008; Thanh et al., 2009, 2011). An alternative view explains the metamorphism in the Karakoram ter-rane as the product of ongoing crustal thickening (Searle et al., 1998, 2011; Streule et al., 2009; Wallis et al., 2013, 2014). This interpretation is based on the evidence that peak metamorphism predates strike-slip deformation on the Karakoram fault (Streule et al., 2009), thus sug-gesting that prograde metamorphism may be the result of regionally widespread crustal thickening and magmatism, rather than the product of localized deformation (Wallis et al., 2014).
The Pangong-Tso area lies in the eastern Karakoram and belongs to the eastern part of the Karakoram Metamorphic Complex (KMC) and comprises garnet-bearing amphibolite, medium- to high-grade meta-pelites and anatectic granites, extending across northern Pakistan and
https://doi.org/10.1016/j.jseaes.2020.104293
Received 9 November 2018; Received in revised form 17 February 2020; Accepted 18 February 2020 ⁎Corresponding author.
E-mail address:hksachan@gmail.com(H.K. Sachan).
Journal of Asian Earth Sciences 196 (2020) 104293
Available online 19 February 2020
1367-9120/ © 2020 Elsevier Ltd. All rights reserved.
Ladakh into western Tibet (Phillips, 2008).Lemennicier et al. (1996) described the structural–metamorphic evolution of the central part of KMC in Baltistan.Streule et al. (2009), Thanh et al. (2011) andSachan et al. (2016)have investigated the evolution of the eastern part of KMC and reported two metamorphic events within the metapelitic complex of the KMC (Pangong-Tso region): peak metamorphism is Cretaceous in age, predates the strike-slip deformation on the Karakoram fault (Streule et al., 2009), and can be linked with similar Cretaceous me-tamorphic events in the Hunza area of the central-western Karakoram (Fraser et al., 2001; Palin et al., 2012).Sachan et al. (2016)described the role of fluids (with special emphasis on CO2) in the metamorphic
P-T history of metapelites and advocated an isothermal path for the ex-humation of high-grade metapelites.
The garnet-bearing amphibolites widely occur as lenticular blocks and bodies within the metapelitic complex of the KMC. The occurrence of these amphibolites was preliminarily mentioned by Lemennicier et al. (1996) and Searle et al. (2010)on the Pakistani side of the KMC complex. More recently,Wallis et al. (2014)reported the amphibolites from the eastern side of the KMC and proposed an empirical geother-mometric estimate for peak metamorphic stage i.e. ~10 kbar and 735 °C. However, no age data are available for the garnet-bearing amphibolites of the Karakoram region.
Fig. 1. Geological map of SE Ladakh (A) with details of the study area (B). Inset shows location in the framework of the Himalayas (afterChaudhury, 1983). Studied samples locations are shown in the map (orange dots). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
This contribution is the comprehensive petrological, fluid inclusion and geochronological study on the garnet-bearing amphibolites from the KMC of the Pangong-Tso region. These garnet-bearing amphibolites thus represent an excellent opportunity for: (i) testing the P–T–t me-tamorphic evolution so far inferred from the associated metapelites only; (ii) discussing the evolution of the fluid phase in relation to the P–T path derived from the thermodynamic modeling; (iii) providing new data on the tectono-metamorphic evolution of the Karakoram terrane before India-Asia collision.
2. Geological setting
The Ladakh terrane comprises five geotectonic units from north to south: the Karakoram block, the Shyok Suture Zone (SSZ), the Ladakh arc, the Indus suture zone, and the Tethys Himalaya (Thakur and Rawat, 1992). In this area, the Indus suture marks the boundary be-tween Indian and Asian continental crust (Molnar and Tapponnier, 1975) and separates the Ladakh magmatic arc from the Indian crust (i.e. Tso–Morari crystalline complex). The northern limit of the Ladakh magmatic arc is represented by the SSZ, and the Karakoram block is located to the north of this suture. The Karakoram block comprises the Karakoram batholith, the KMC and a multiplicity of Permian–Cretac-eous strata (Searle, 1991). The study area (Pangong-Tso region) is lo-cated along the southern margin of the Karakoram block, between the Karakoram batholith and the SSZ (Fig. 1a).
The tectonic unit of the Pangong-Tso region is represented by a ca. 7-km wide pop–up structure (Dunlap et al., 1998; Weinberg et al., 2000). It is an integrated portion of the ca. 1000-km long dextral Kar-akoram Shear Zone, which is one of the major strike–slip faults that regulates the extrusion of Tibet in response to the northward movement of India (Molnar and Tapponnier, 1975, 1978; Searle et al., 1998; Valli et al., 2007; Weinberg et al., 2000). KMC is locally known as Pangong Metamorphic Complex. It is aligned E-W and oblique to the NW-SE regional orientation of the Karakoram belt (Fig. 1a). Migmatites and mylonites are observed along the Karakoram fault adjoining with the Ladakh batholith.
The Karakoram block in the Pangong-Tso region is mainly char-acterized by metapelites, metabasites, calc-silicates, migmatitic or-thogneisses, granodiorites, and leucogranites. The grade of meta-morphism increases biotite to sillimanite zone (Fig. 1b) towards NE to SW (Chaudhry, 1983; Searle et al., 1998) (Fig. 1b).
In the study area, the right–lateral Karakoram fault opens into two branches, a south-western strand (Tangtse fault) and a north-eastern strand (Pangong fault). Here, orthogneisses and amphibolites seen
intruded by leucogranite sheets (Tangtse and Muglib granites) with extensive dike–sill networks (Weinberg and Searle, 1998; Searle et al., 1998; Phillips et al. 2004; Weinberg and Mark, 2008). Staur-olite–bearing schists forming a part of the Pangong Metamorphic Complex, occur immediately adjacent to Pangong fault and stretched to NNE direction (Fig. 1b). The Pangong Metamorphic Complex (PMC) is a 10 km wide, steeply dipping sequence of metapelites, metapsammites, metacarbonates, and amphibolites. It runs alongside the Pangong strand of the Karakoram fault between Muglib and Pangong lake and then extends NE across the lake into an unmapped area in Xinjiang province, China. Dextral C–S fabrics, mylonite zones and horizontal or shallow (20°–40°) plunging stretching lineations (Phillips and Searle, 2007) are ubiquitous.
In this study, among the various lithologies of PMC, garnet-bearing amphibolites have been petrologically investigated in detail. This li-thology crops out as prominent black bands within marbles and meta-pelites. Its mineral assemblage is variable, spanning from the relatively simple Hbl + Pl + Qtz ± Grt ± Bt assemblage to the more complex Cpx- and/or Scp-bearing assemblages. Field evidence (i.e. association of amphibolites, marbles, and metapelites), as well as the observed mi-neral assemblages (i.e. especially the occurrence of scapolite and bio-tite), strongly suggest that these amphibolites are a part of the meta-sedimentary sequence. Derived from a meta-sedimentary protolith (e.g. marl), locally enriched in carbonate minerals (scapolite-bearing am-phibolites). The studied samples are representative of the simpler variety of garnet-bearing amphibolite (Hbl + Pl + Qtz + Grt + Bt) and were collected along the road between Tangtse and Muglib (N34°02′12.9′'; E78°12′42.1′' (4010 m). The outcrop consists of tens of meters –a thick level of amphibolite, pervasively intruded by a network of leucogranite dikes (Fig. 3A & B). The selected samples (HK13A and B) are petrographically similar; therefore, petrography, mineral che-mical data, and P-T pseudosection modeling refer ONLY to sample HK13A.
3. Methods
3.1. Micro-X-ray fluorescence (μ-XRF) maps
The μ-XRF Eagle III-XPL spectrometer equipped with an EDS Si(Li) detector and with an Edax Vision32 micro-analytical system at Department of Earth Sciences, University of Torino (Italy) has been utilized to acquire the micro-XRF maps of the whole thin sections (Fig. 2) and to estimate the mineral modes. Instrument working con-ditions were as follows: 100 ms counting time, 40 kV accelerating voltage, and a probe current of 900 μA. Nearly 65 μm spatial resolution in both x and y directions was used. The software program “Petromod” (Cossio et al., 2002) has been utilized to obtain the quantitative modal percentages of each mineral from the μ-XRF maps.
3.2. Mineral chemistry
Minerals were analyzed using a WDS electron microprobe (Cameca SX100 electron probe microanalyzer) at Wadia Institute of Himalayan Geology, Dehra Dun (India). The operative conditions were as follows; -acceleration voltage of 15 kV, specimen current 20 nA. and beam dia-meter 2 μm. The integration time for all the elements was 20 s except for Fe (30 s). Natural and synthetic silicates and oxides were used for reference and matrix corrections were carried out by the PAP program. The electron microprobe analyses of different minerals are de-scribed inTables 3–6. Structural formulae calculated on the basis of 12 oxygens for garnet, 8 oxygens for plagioclase, 22 oxygens for biotite, and 23 oxygens for amphibole. Fe3+content in garnet calculated by stoichiometry charge balance and for amphibole following Droop (1987).
Fig. 2. Processed major elements μ-XRF map of the whole thin section of the studied sample HK13A.
3.3. Phase diagram computation
To calculate the P-T pseudosections for both the plagioclase- and amphibole-rich layers the Perple_X program package (version 6.7.5) that relies on the Gibbs free energy minimization (Connolly, 1990, 2005, 2009;Connolly and Petrini, 2002) was utilized. The internally consistent thermodynamic dataset and the equation of state for H2O of Holland and Powell (2011)were used for pseudosections modeling.
The bulk rock compositions of the plagioclase- and amphibole-rich
layers were estimated by combining the mineral proportions acquired from the modal calculation of the micro-XRF map (Fig. 2,Table 1) with mineral chemistry obtained at EPMA. The effective bulk rock compo-sitions used to model the plagioclase- and amphibole-rich domains are reported inTable 2.
Both pseudosections are calculated within the P-T range of 4–11 kbar and 510–700 °C. According to mineral assemblages and compo-sitions, the model system NCKFMASHTO (Na2O–CaO–K2O–FeO– MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) considered for the pseudosections Grt Grt Grt Grt Ilm Amp Amp Amp Amp Qtz Qtz Qtz Pl Pl Pl Pl Bt Bt 500 μm Grt Ilm Amp Amp Amp Amp Amp Qtz Pl Bt Bt Bt 500 μm d e Amp Amp Amp Pl Pl 500 μm c Grt Grt Amp Amp Amp Amp Amp Qtz Qtz Qtz Pl Pl Pl Ilm Pl Bt Bt 500 μm f Ilm Bt a b
Fig. 3. Representative field and photomicrographs of the studied garnet-bearing amphibolite. (a,b) The outcrop comprises of a tens of meters –thick level of amphibolite, extensively intervened by a network of leucogranite dykes. (c,d) Details of the amphibole-rich layer (PPL): amphibole defines the primary foliation. Garnet porphyroblast in (b) contains inclusions of amphibole and ilmenite and is surrounded by a matrix of amphibole and plagioclase (detail in a). (e,f) Details of the plagioclase-rich layer (PPL). Garnet is crowded of inclusions of amphibole, quartz, ilmenite and minor biotite.
calculation. SiO2 considered a saturated component with the con-strained amount. MnO was not considered because of its low content and lack of zoning in garnet. 5% of the total FeO content (mole %) was assumed to be Fe2O3, according to the average Fe+3content in the analyzed minerals·H2O considered in excess without fixing its amount; this implies that the modeled solidus is a wet-solidus and represents the minimum P-T conditions at which melt is predicted to be produced if H2O in excess and/or introduced in the system from outside.
Although a small amount of carbonate could have been possibly present in the sedimentary protolith of the studied amphibolites. The CO2was not considered in the calculation of the P-T pseudosections, in the reasonable hypothesis that carbonates were completely consumed during the early prograde evolution.
The significance of this assumption is that the modeled P-T pseu-dosections cannot be used to constrain the prograde P-T evolution of the studied amphibolite, but their validity is limited to the peak and retrograde history. Nevertheless, the influence of CO2-bearing fluids on the stability of the observed mineral assemblage has been tested by calculating a simplified (i.e. model system NCKFMASH-CO2) P/T-X (CO2) pseudosection for the plagioclase-rich layers along a reliable prograde P/T gradient.
The following activity–composition models were used for the P-T pseudosection calculations: amphibole (Green et al., 2016), clinopyr-oxene (Green et al., 2016), chlorite (White et al., 2014), epidote (Holland and Powell, 2011), plagioclase (Newton et al., 1980), garnet (White et al., 2014), biotite (White et al., 2014), ilmenite (White et al., 2014), mica (White et al., 2014) and orthopyroxene (White et al., 2014). Scapolite (Kuhn et al., 2005) and dolomite (Holland and Powell, 1998) have been additionally used for the P/T-X(CO2) pseudosection. Quartz, titanite, rutile, water (and calcite) were considered as end-members. Mineral abbreviations in the pseudosections are as follows: amp – amphibole; mu – muscovite; chl – chlorite; ep – epidote; sph – titanite; opx – orthopyroxene; g – garnet; bi – biotite; pl – plagioclase; ilm – ilmenite; rt – rutile; q – quartz. The P-T pseudosections for both domains have been contoured with the compositional isopleths of garnet (XMg, XCa), plagioclase (XAn) and biotite (XMg, XTi).
3.4. Fluid inclusions study
Double polished thin sections of selected samples with a thickness of 120–150 µm were prepared. Petrography of the fluid inclusions per-formed following the criteria outlined byTouret (2001) and Van den Kerkhof and Hein (2001).
Microthermometric measurements performed with a Linkam heating/freezing system at the Wadia Institute of Himalayan Geology, Dehra Dun (India). The calibrations accomplished at −56.6 °C (triple point of CO2) with pure CO2inclusions in natural samples. Repeated microthermometric measurements account for the precision of micro-thermometric results at ± 0.1 °C and ± 0.2 °C for Tm and Th, respec-tively. The accuracies for heating runs in the range of ± 3 °C; as con-cerning the cooling temperatures, the precision is within ± 0.5 °C.
3.5. Geochronology
U-Pb geochronology of zircon carried out on two garnet-bearing amphibolite samples (HK13A and B). Zircons were separated from 3 to 4 kg of rock samples processed through a jaw crusher, disk mill, Holman-Wilfley water table, isodynamic magnetic separator, and heavy liquids. Thereafter, hand-picked the zircons under a stereomicroscope, and subsequently mounted the selected zircon grains in perfluoroalkoxy alkane (PFA®) Teflon. The mounted zircon grains were polished using 8, 5, 3, 1, and 0.25 μm diamond paste. Cathodoluminescence (CL) images of mounted zircons acquired using a Gatan Chroma CL imaging system with ultraviolet (UV) range attached to a Zeiss EVO 40 extended pressure (EP) scanning electron microscope, with a varying probe current of 10–20 nA, at the Wadia Institute of Himalayan Geology, Dehradun (India).
Zircons of the garnet-bearing amphibolites were analyzed for U-Pb ages via laser-ablation multicollector (LA-MC) ICP-MS at the Wadia Institute of Himalayan Geology, Dehradun (India). The instrument comprises an MC-ICP-MS (Neptune Plus, Thermo Fisher Scientific, Inc.) and a 193 nm excimer laser (UV Laser, Model Analyte G2, Cetec-Photon machine, Inc.), equipped with a high-performance HelEx–II sample chamber. The methodology used is similar to that ofMukherjee et al. (2017). For the in-situ U-Pb analysis of zircons, the spot sizes were 20 μm, and these spots were positioned relative to the CL images. The analyses were carried out with energy density 4 J cm–2, a repetition rate of 5 Hz, 75% laser intensity, and 175 total shots per analysis, i.e., 35 s analysis time for each spot with 10 s background measurement.
4. Results
4.1. Petrography and mineral chemistry
The studied garnet-bearing amphibolite (sample HK13A) shows a granoblastic, medium-grained, with a banded fabric marked by cm-thick alternating amphibole-rich and plagioclase-rich bands (Fig. 2). Amphibole- and plagioclase-rich layers show a similar assemblage but different modal amounts of minerals (Table 1). The main foliation is defined by the preferred orientation of amphibole nematoblasts and biotite lepidoblasts. Amphibole-rich layers mostly consist of amphibole and plagioclase with minor amounts of quartz and biotite, and plagio-clase-rich layers mostly consist of plagioclase and quartz, with minor amphibole and garnet (Table 1, 2). Accessory minerals are ilmenite, apatite, and zircon in both the layers. Microstructural relationships between quartz, plagioclase, amphibole, garnet, biotite, and ilmenite suggest that these minerals all belong to the equilibrium assemblage.
Green amphibole occurs as medium- to coarse-grained idioblasts associated to biotite and plagioclase in the matrix or as inclusions within garnet (Fig. 3c–f). It isstrongly pleochroic from straw yellow to olive green and dark green (Fig. 3c). The amphiboles (Fig. 4a) of both domains belong to the tschermakite group (Leake et al., 1997) and are characterized by marginally different Si (on the basis of 23 oxygens) Table 1
Bulk compositions of garnet amphibolite (sample No HK13A). Oxides AMPHIBOLE RICH LAYER (Wt. %) AMPHIBOLE RICH LAYER (Mole %) PLAGIOCLASE RICH LAYER (Wt. %) PLAGIOCLASE RICH LAYER (Mole %) SiO2 43.66 47.97 64.31 69.24 Al2O3 22.64 14.66 17.40 11.04 MgO 5.94 9.73 2.10 3.37 FeO 11.72 10.23 4.74 4.05 MnO 0.14 – 0.09 – TiO2 2.57 2.12 1.21 0.98 CaO 7.96 9.37 5.26 6.06 Na2O 4.43 4.72 4.68 4.89 K2O 0.94 0.66 0.23 0.16 Fe2O3 0.00 0.54 0.00 0.21 Total 100.00 100.00 100.00 100.00 Table 2
Modal (vol%) of garnet amphibolite (sample NoHK13A). Minerals Whole rock AMPHIBOLE RICH
LAYER PLAGIOCLASE RICHLAYER
QUARTZ 13.98 3.46 28.94 PLAGIOCLASE 38.35 30.41 49.89 AMPHIBOLE 44.67 62.51 18.93 GARNET 0.42 0.00 1.09 BIOTITE 0.74 1.13 0.02 ILMANITE 1.84 2.48 1.12 Total 100.00 100.00 100.00
(Table 6), XNa(XNa= Na/(Na + Ca)), XMgand XFe+3(XFe+3= Fe+3/ Fetot) contents (plagioclase-rich layers: Si = 6.02 – 6.64 a.p.f.u., XNa= 0.21–0.27, XMg= 0.56 – 0.72, XFe+3= 0.22–0.41; amphibole-rich layers: Si = 5.99–6.39 a.p.f.u., XNa= 0.23–0.27, XMg= 0.56 – 0.73, XFe+3= 0.21–0.42). Amphiboles included in garnet from both the layers contain higher XMgand XFe+3as compared to those present in the matrix.
Garnet includes amphibole, quartz, and ilmenite (Fig. 3d,e,f). Medium- to coarse-grained, weakly zoned garnet occurs as a poikilo-blast (up to 12 mm in diameter) (Fig. 3e,f) and present in both layers but rare in the amphibole-rich layers. Garnet is relatively more sieved and fractured in plagioclase-rich layers (Fig. 3e,f), and it is partially replaced by biotite along fractures. It is comparatively rich in alman-dine (up to 0.62) with minor amounts of pyrope (0.16–0.18) and grossular (0.13–0.20) (Fig. 4b). The spessartine and andradite contents are very low, ranging between 0.03 and 0.05 and 0.01– 0.03 respec-tively. XMgincreases and XCadecreases from core to rim (Table 3).
Plagioclase in both domains shows different grain sizes and com-positions. In plagioclase-rich layers, it occurs as a medium- to coarse-grained anhedral grains, associated with quartz and minor amphibole in the matrix (Fig. 3e,f). Its XAncontent varies between 0.46 and 0.54 from the core to the rim (Table 4,Fig. 4c). In the amphibole-rich layers, plagioclase grains are up to 0.3 mm and commonly occur as inter-growths with amphibole in the matrix (Fig. 3c,d). Plagioclase has
labradorite composition (Fig. 4c) with XAnranging between 0.51 and 0.64, from the core to the rim (Table 4).
Biotite occurs as fine- to medium-grained flakes (size 0.2–1.3 mm) in the matrix (Fig. 3c-f) or rarely along with the fractures of garnet (Fig. 3f). It is relatively abundant in the amphibole-rich layers It is associated with amphibole and plagioclase in both the domains. The calculated XMgvalues in amphibole- and plagioclase-rich domains vary in the range of 0.57–0.62 and 0.55–0.61, respectively (Table 5). The TiO2content is slightly higher in biotite of the plagioclase-rich layers (up to 1.68 wt%) in comparison to the amphibole-rich layers (up to 1.57 wt%) (Table 5). In the XMgvs AlVIdiagram, biotite compositions plot between the phlogopite and eastonite fields. In both layers, biotite shows a very close affinity with eastonite composition (Fig. 4d).
Fine- to medium-grained and xenoblastic textured quartz occurs in the plagioclase-rich layers. The accessory minerals, i.e., ilmenite and apatite, are fine-grained and idioblastic and are abundant in the amphibole-rich layers (Fig. 2). Ilmenite and apatite are fine-grained and idioblastic.
4.2. Phase equilibria modeling and the estimate of the P–T condition
To constrain the peak and retrograde metamorphic history of the garnet-bearing amphibolite from the Pangong region, isochemical phase diagrams are formulated for both the plagioclase- and amphibole-rich layers.
Fig. 4. (a) Mg/(Mg + Fe2+) vs. Si per formula diagram showing compositions of amphibole. (b) Garnet compositions plotted in the (spessartine + grossular) -almandine - pyrope diagram. (c) Ternary Ab – Or –An diagram showing composition of plagioclase. (d) XMgvs AlVIdiagram displaying the composition of biotite.
4.2.1. Plagioclase-rich layers
The calculated pseudosection (Fig. 5) shows a relatively simple to-pography consisting of three- to seven-variant fields (Fig. 5). Garnet is predicted to be stable at T > 560 °C and P > 6.5 kbar. The calculated garnet compositional isopleths (Fig. SM1a, b) suggest that XMgincreases as temperature rises in all the field assemblages, whereas XCaisopleths show a more complex trend. Specifically, garnet XCa decreases as temperature rises in the muscovite/chlorite-bearing field assemblages up to 620 °C, but at T > 620 °C it increases as temperature rises in
most of the field assemblages except for the amp + g + bi + pl + q ± ilm ± rt field, where it is mostly P-dependent. Plagioclase is stable in the whole P-T range of interest. The modeled XAnisopleths are de-pendent on both P and T at T < 620 °C (i.e. in the muscovite/chlorite-bearing field assemblages), but at T > 620 °C these isopleths become more P-dependent and XAnincreases as pressure decreases (Fig. SM1c). Biotite is predicted to be stable over the whole P-T range of interest. The calculated XTiisopleths of biotite are mostly T-dependent and in-crease with the rise of temperature (Fig. SM1d). Conversely, the XMg Table 3
Microprobe analyses and structural formulae (on the basis of 12 Oxygens) of Garnet.
PLAGIOCLASE RICH LAYER AMPHIBOLE RICH LAYER
C C R R R R R R C C R R R R Oxides Wt% SiO2 38.42 38.62 38.44 38.64 38.71 38.50 38.43 38.32 38.34 38.35 38.67 38.12 38.36 38.33 TiO2 0.00 0.00 0.05 0.00 0.11 0.06 0.08 0.11 0.08 0.09 0.01 0.13 0.02 0.04 Al2O3 21.22 21.36 21.23 21.24 21.06 21.04 20.99 21.29 20.92 20.96 20.97 20.59 20.72 20.55 Cr2O3 0.01 0.00 0.00 0.02 0.02 0.00 0.00 0.04 0.03 0.03 0.04 0.00 0.06 0.00 FeO* 26.97 26.74 27.71 27.39 27.57 27.80 27.69 27.66 28.32 28.17 28.07 28.98 28.63 28.91 MnO 2.14 1.79 1.56 1.87 1.77 2.01 1.96 1.81 2.37 2.25 2.33 2.20 2.22 2.18 MgO 4.06 4.09 4.68 4.60 4.61 4.36 4.33 4.31 4.03 4.04 4.23 4.30 4.49 4.53 CaO 7.42 7.32 6.16 6.12 6.15 6.40 6.60 6.28 6.05 6.15 5.67 5.88 5.47 5.43 Na2O 0.01 0.01 0.02 0.03 0.02 0.04 0.02 0.03 0.03 0.03 0.04 0.03 0.09 0.02 K2O 0.01 0.02 0.04 0.01 0.07 0.00 0.00 0.03 0.01 0.02 0.01 0.02 0.00 0.02 Total 100.26 99.95 99.89 99.92 100.09 100.21 100.10 99.88 100.18 100.08 100.04 100.25 100.06 100.01 Si 3.018 3.032 3.023 3.035 3.038 3.027 3.025 3.018 3.026 3.027 3.046 3.016 3.030 3.032 Al 1.965 1.977 1.968 1.966 1.948 1.950 1.947 1.976 1.946 1.950 1.947 1.920 1.929 1.916 Ti 0.000 0.000 0.003 0.000 0.006 0.004 0.005 0.007 0.005 0.005 0.001 0.008 0.001 0.002 Cr 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.002 0.002 0.002 0.002 0.000 0.004 0.000 Fe3+ 0.016 0.000 0.007 0.000 0.007 0.020 0.023 0.000 0.021 0.016 0.004 0.057 0.036 0.049 Fe2+ 1.756 1.756 1.816 1.799 1.802 1.808 1.800 1.822 1.848 1.844 1.845 1.861 1.855 1.864 Mn 0.142 0.119 0.104 0.124 0.118 0.134 0.131 0.121 0.159 0.150 0.155 0.147 0.149 0.146 Mg 0.475 0.479 0.548 0.538 0.539 0.511 0.508 0.506 0.474 0.475 0.497 0.507 0.529 0.534 Ca 0.625 0.616 0.519 0.515 0.517 0.539 0.557 0.530 0.512 0.520 0.479 0.498 0.463 0.460 Na 0.002 0.002 0.003 0.005 0.003 0.006 0.003 0.005 0.005 0.004 0.006 0.005 0.014 0.003 K 0.001 0.002 0.004 0.001 0.007 0.000 0.000 0.003 0.001 0.002 0.001 0.002 0.000 0.002 XMg 0.21 0.21 0.23 0.23 0.23 0.22 0.22 0.22 0.20 0.20 0.21 0.21 0.22 0.22 XCa 0.22 0.22 0.18 0.18 0.18 0.19 0.19 0.19 0.18 0.18 0.17 0.17 0.16 0.16
* Total iron as FeO; Grt:XMg= Mg/(Mg + Fe2+); XCa= Ca/(Ca + Fe2++ Mg); C – core; R – rim. Table 4
Microprobe analyses and structural formulae (on the basis of 8 Oxygens) of Plagioclase.
PLAGIOCLASE RICH LAYER AMPHIBOLE RICH LAYER
C C C C C R R R C C C C R R R R Oxides Wt% SiO2 59.03 58.25 56.93 58.81 58.54 55.40 55.56 55.57 52.83 52.51 54.65 54.36 52.13 52.00 52.18 51.48 TiO2 0.00 0.00 0.00 0.01 0.00 0.02 0.01 0.01 0.05 0.00 0.00 0.02 0.03 0.01 0.00 0.00 Al2O3 26.63 26.64 27.57 26.34 26.64 28.69 28.65 28.71 28.27 29.11 27.60 28.83 30.47 29.68 30.54 30.15 Cr2O3 0.09 0.04 0.00 0.01 0.00 0.01 0.03 0.03 0.03 0.00 0.09 0.00 0.00 0.01 0.00 0.00 FeO* 0.11 0.19 0.20 0.03 0.14 0.13 0.12 0.11 0.14 0.17 0.14 0.14 0.21 0.03 0.07 0.00 MnO 0.01 0.10 0.08 0.00 0.05 0.00 0.01 0.01 0.05 0.00 0.00 0.01 0.05 0.01 0.02 0.06 MgO 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 CaO 8.64 8.82 9.21 9.10 8.92 11.09 10.73 10.58 11.89 11.92 11.48 10.82 13.72 13.45 13.25 13.82 Na2O 5.64 5.78 5.82 5.68 5.61 5.20 4.97 4.93 6.25 6.31 5.90 5.46 4.48 4.37 4.11 4.27 K2O 0.05 0.05 0.00 0.01 0.02 0.00 0.03 0.07 0.13 0.10 0.09 0.08 0.05 0.12 0.10 0.07 Total 100.20 99.87 99.81 99.99 99.92 100.55 100.11 100.03 99.64 100.13 99.95 99.72 101.14 99.68 100.27 99.85 Si 2.622 2.604 2.554 2.621 2.611 2.481 2.493 2.494 2.418 2.392 2.478 2.460 2.350 2.374 2.363 2.349 Ti 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.002 0.000 0.000 0.001 0.001 0.000 0.000 0.000 Al 1.394 1.404 1.458 1.384 1.401 1.515 1.515 1.519 1.525 1.563 1.475 1.538 1.619 1.597 1.630 1.622 Cr 0.003 0.001 0.000 0.000 0.000 0.000 0.001 0.001 0.001 0.000 0.003 0.000 0.000 0.000 0.000 0.000 Fe2+ 0.004 0.007 0.008 0.001 0.005 0.005 0.005 0.004 0.005 0.006 0.005 0.005 0.008 0.001 0.003 0.000 Mn 0.000 0.004 0.003 0.000 0.002 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.002 0.000 0.001 0.002 Mg 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 Ca 0.411 0.422 0.443 0.435 0.426 0.532 0.516 0.509 0.583 0.582 0.558 0.525 0.663 0.658 0.643 0.676 Na 0.486 0.501 0.506 0.491 0.485 0.452 0.433 0.429 0.555 0.557 0.519 0.479 0.392 0.387 0.361 0.378 K 0.003 0.003 0.000 0.001 0.001 0.000 0.002 0.004 0.008 0.006 0.005 0.005 0.003 0.007 0.006 0.004 XAn 0.46 0.46 0.47 0.47 0.47 0.54 0.54 0.54 0.51 0.51 0.52 0.52 0.63 0.63 0.64 0.64
Table 5
Microprobe analyses and structural formulae (on the basis of 22 Oxygens) of Biotite.
PLAGIOCLASE RICH LAYER AMPHIBOLE RICH LAYER
M M M M M M M M M M M M Oxides Wt% SiO2 37.06 37.16 36.88 36.69 36.91 36.32 36.57 36.89 36.31 35.91 36.32 36.33 TiO2 1.59 1.56 1.68 1.39 1.38 1.47 1.57 1.45 1.55 1.31 1.36 1.34 Al2O3 18.01 17.96 17.94 16.32 15.84 16.16 17.48 17.08 17.5 17.38 17.36 17.41 FeO* 15.35 15.42 15.51 17.82 17.61 17.66 15.56 15.56 15.58 17.19 17.10 17.15 MnO 0.04 0.09 0.07 0.12 0.02 0.08 0.08 0 0 0.11 0.09 0.19 MgO 13.51 13.25 13.42 12.20 12.04 12.26 13.63 13.44 13.97 12.69 12.92 12.61 CaO 0.07 0.18 0.09 0.19 0.04 0.15 0.05 0.04 0.15 0.21 0.08 0.11 Na2O 0.38 0.18 0.19 0.32 0.23 0.12 0.24 0.22 0.25 0.13 0.37 0.32 K2O 7.55 7.21 7.26 7.94 7.61 8.09 8.23 8.43 8.25 7.49 7.65 7.72 Cr2O3 0.00 0.01 0.06 0.00 0.06 0.00 0.00 0.01 0.00 0.00 0.00 0.00 Total 93.56 93.02 93.10 92.99 91.74 92.31 93.41 93.12 93.56 92.42 93.25 93.18 Si 5.561 5.595 5.557 5.641 5.727 5.628 5.536 5.603 5.494 5.524 5.536 5.546 Al(IV) 2.439 2.405 2.443 2.359 2.273 2.372 2.464 2.397 2.506 2.476 2.464 2.454 ∑ Z 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 Al(VI) 0.746 0.782 0.742 0.598 0.624 0.580 0.654 0.660 0.614 0.674 0.654 0.679 Ti 0.179 0.177 0.190 0.161 0.161 0.171 0.179 0.166 0.176 0.152 0.156 0.154 Cr 0.000 0.001 0.007 0.000 0.007 0.000 0.000 0.001 0.000 0.000 0.000 0.000 Fe2+ 1.926 1.941 1.954 2.291 2.285 2.288 1.970 1.976 1.971 2.211 2.179 2.189 Mn 0.005 0.011 0.009 0.016 0.003 0.010 0.010 0.000 0.000 0.014 0.012 0.025 Mg 3.022 2.974 3.014 2.796 2.785 2.832 3.076 3.043 3.151 2.910 2.936 2.870 ∑ Y 5.133 5.105 5.175 5.862 5.865 5.882 5.234 5.186 5.299 5.961 5.937 5.917 Ca 0.011 0.029 0.015 0.031 0.007 0.025 0.008 0.007 0.024 0.035 0.013 0.018 Na 0.111 0.053 0.055 0.095 0.069 0.036 0.070 0.065 0.073 0.039 0.109 0.095 K 1.445 1.385 1.395 1.557 1.506 1.599 1.589 1.633 1.592 1.470 1.487 1.503 ∑ X 1.567 1.466 1.465 1.684 1.582 1.660 1.668 1.705 1.690 1.543 1.610 1.616 XMg 0.61 0.61 0.61 0.55 0.55 0.55 0.61 0.61 0.62 0.57 0.57 0.57 XTi 0.035 0.035 0.037 0.031 0.031 0.032 0.034 0.032 0.033 0.029 0.030 0.030
* Total iron as FeO, XMg= Mg/(Mg + Fe), XTi= Ti/ (Ti + Fe2++ Mg); M – Matrix.
Table 6
Microprobe analyses and structural formulae (on the basis of 23 Oxygens) of Amphibole.
PLAGIOCLASE RICH LAYER AMPHIBOLE RICH LAYER
IN IN M M M M IN IN M M M M M Oxides Wt% SiO2 41.62 41.57 42.17 43.69 40.98 44.53 41.44 41.77 40.99 42.98 42.88 42.42 42.88 TiO2 0.92 0.93 1.26 1.13 0.91 0.94 1.08 0.79 1.06 1.18 1.14 1.09 1.18 Al2O3 17.99 18.02 14.16 12.86 16.90 10.62 18.31 17.95 17.18 13.17 13.49 14.24 13.37 Cr2O3 0.00 0.00 0.00 0.01 0.00 0.02 0.00 0.00 0.00 0.06 0.00 0.03 0.06 FeO 12.04 12.26 15.63 16.45 14.10 17.65 12.20 11.96 14.09 16.68 16.71 16.14 16.68 MnO 0.08 0.12 0.25 0.27 0.22 0.28 0.08 0.19 0.19 0.13 0.38 0.19 0.13 MgO 10.82 10.71 9.28 9.61 9.84 9.67 10.66 10.87 9.70 9.39 9.46 9.53 9.39 CaO 11.04 10.98 11.14 11.03 11.26 11.24 11.15 10.88 11.24 11.12 11.07 11.08 11.12 Na2O 2.27 2.13 1.96 1.88 2.17 1.62 1.85 2.26 2.11 1.99 1.91 1.87 1.99 K2O 0.46 0.45 0.61 0.52 0.70 0.45 0.45 0.45 0.68 0.56 0.52 0.54 0.56 TOTAL 97.24 97.17 96.46 97.45 97.08 97.02 97.22 97.12 97.24 97.26 97.56 97.13 97.36 Si 6.027 6.016 6.307 6.453 6.038 6.640 5.986 6.041 6.025 6.386 6.332 6.273 6.361 AlIV 1.973 1.984 1.693 1.547 1.962 1.360 2.014 1.959 1.975 1.614 1.668 1.727 1.639 ∑Z 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 AlVI 1.096 1.090 0.803 0.692 0.973 0.506 1.103 1.101 1.002 0.692 0.680 0.755 0.699 Ti 0.100 0.101 0.142 0.126 0.101 0.105 0.117 0.086 0.117 0.132 0.127 0.121 0.132 Cr 0.000 0.000 0.000 0.001 0.000 0.002 0.000 0.000 0.000 0.007 0.000 0.004 0.007 Fe3+ 0.529 0.606 0.352 0.476 0.481 0.495 0.623 0.598 0.470 0.431 0.587 0.578 0.456 Fe2+ 0.929 0.877 1.602 1.556 1.257 1.706 0.850 0.848 1.262 1.641 1.476 1.418 1.613 Mn 0.010 0.014 0.032 0.034 0.027 0.035 0.010 0.023 0.024 0.016 0.048 0.024 0.016 Mg 2.336 2.311 2.069 2.116 2.162 2.150 2.296 2.344 2.126 2.080 2.083 2.101 2.077 ∑Y 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 Ca 1.713 1.702 1.785 1.745 1.777 1.796 1.726 1.686 1.770 1.770 1.751 1.755 1.767 Na 0.637 0.597 0.568 0.538 0.620 0.468 0.518 0.634 0.601 0.573 0.547 0.536 0.572 K 0.085 0.084 0.116 0.098 0.132 0.086 0.083 0.083 0.128 0.106 0.098 0.102 0.106 ∑X 2.435 2.383 2.470 2.382 2.529 2.350 2.327 2.403 2.499 2.449 2.396 2.393 2.446 XMg 0.72 0.72 0.56 0.58 0.63 0.56 0.73 0.73 0.63 0.56 0.59 0.60 0.56
IN– Inclusion, M – Matrix.
* Total iron as FeO, Fe2+and Fe3+calculated by the scheme of Droop (1987), X
isopleths of biotite are mostly T-dependent at T < 620 °C (i.e. in the muscovite/chlorite-bearing fields), whereas at T > 620 °C they be-come P-dependent, increasing as pressure rises (Fig. SM1e).
The assemblage observed in plagioclase-rich layers (amp + g + bi + ilm + pl + qtz) is modelled by a relatively large six-variant field at 620to > 700 °C, 6.5–10.5 kbar (Fig. 5). The modelled compositional isopleths of minerals, i.e. garnet rim: XMg= 0.22–0.23, XCa = 0.18–0.19, plagioclase: XAn = 0.46–0.47, biotite: XTi= 0.035–0.037, XMg= 0.61, constrain peak P–T conditions around 8.5–9.5 kbar and 630–655 °C. The simplified P/T-X(CO2) pseudosection calculated for the plagioclase-rich layers (Fig. 6) shows that the stability field of the observed mineral assemblage is poorly influenced by the fluid composition, i.e. for the estimated peak P-T conditions, the ob-served peak assemblage is predicted to be stable in a wide range of fluid compositions, from a pure aqueous fluid to a fluid with X(CO2) = 0.6. The observed growth of biotite at the expense of garnet suggests that the retrograde P–T path exits from the garnet stability field. Further constraints on the retrograde evolution are given by the inter-section of the compositional isopleths of biotite and plagioclase (biotite: XMg= 0.55, XTi= 0.031–0.032; plagioclase: maximum XAn= 0.54), which suggests nearly isothermal decompression to P-T conditions around 5.5–5.7 kbar and 620–630 °C.
4.2.2. Amphibole-rich layers
The topology of the modeled isochemical phase diagram is rela-tively simple and dominated by four- to eight -variant fields (Fig. 7). It is similar to that calculated for the plagioclase-rich layers, except for the fact that quartz is not predicted to be stable, in agreement with the observed mineral assemblage (seeFig. 2). The modeled compositional isopleths of garnet (XMg, XCa), plagioclase (XAn) and biotite (XMg, XTi) follow the same trend of variation with changing pressure and tem-perature as in the pseudosection calculated for the plagioclase-rich layers (Fig. SM2a–e).
The observed mineral assemblage (amp + g + bi + pl + ilm) is predicted to be stable in a relatively large P-T field at 620 to > 700 °C, 6.2–10.2 kbar. The intersection of mineral compositional isopleths, i.e. garnet rim XMg = 0.21–0.22, XCa = 0.16–0.17; plagioclase: XAn= 0.51–0.52; biotite XTi= 0.032–0.034, XMg= 0.61–0.62, con-strain peak P–T conditions at around 8.2–8.6 kbar and 615–630 °C (Fig. 7).
As already discussed for the plagioclase-rich layers, the retrograde evolution is marked by the (limited) replacement of garnet by biotite, which suggests the entrance in the garnet-absent amp + bi + ilm + pl field. The intersection of the compositional isopleths of biotite (XTi= 0.029–0.030, XMg= 0.57), and plagioclase (maximum modeled XAn = 0.63–0.64) suggests decompression at P-T condition around
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T (
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P (Kbar)
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NCKFMASHTO
Amp bi ilm Amp g bi ilm Amp g bi ilm rt Amp g bi rt Amp chl bi ilm Amp chl g bi ilm Amp chl g mu bi ilm Amp chl g bi ilm rt Amp chl g mu bi ilm rtAmp g mu bi ilm rt
Amp ep g bi rt Amp ep g mu rt Amp ep g mu bi rt Amp ep g mu bi rt Amp ep g mu bi ilm rt Amp ep g mu bi ilm Amp ep mu bi ilm rt Amp ep mu bi ilm Amp chl ep mu bi rt ilm Amp chl ep mu bi rt Amp chl ep mu bi ilm Amp chl ep g mu bi ilm p e l h c p m A h p s i b u m Amp ep mu bi sph Amp ep mu sph Amp ep mu bi rt Amp ep mu bi sph rt Amp chl ep bi ilm Amp chl ep g bi ilm Amp chl bi ilm sph Amp chl ep bi ilm sph Chl ep bi sph Amph chl ep bi sph Chl ep bi sph ilm Chl ep bi sph rt Chl ep bi sph rt ilm Chl ep mu bi sph Amp chl ep bi rt ilm Amp g mu bi ilm XTi bi 0.035 XTi bi 0.037 XTi bi 0.031XTi bi 0.032 XMg bi 0.61 XMg bi 0.55 X Mg g 0.22 X Mg g 0.23 XCa g 0.18 XCa g 0.19 XAn pl 0.46 XAn pl 0.47 XAn pl 0.54 +pl +qFig. 5. P–T pseudosection calculated for the plagioclase-rich layers in the NKCFMASHTO model system. Plagioclase and quartz are in excess. The red square and triangular areas constrain peak and retrograde P-T conditions, respectively, as inferred from the intersection of the following isopleths: XMg(g), XCa(g), XAn(pl), XMg (bi) and XTi(bi). The thick arrow shows the inferred P–T path. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
5.6–6.1 kbar and 610– 620 °C (Fig. 7).
4.3. Fluid inclusions study
4.3.1. Fluid inclusions petrography and chronology
Both the primary and secondary fluid inclusions in garnet and quartz were identified based on their mode of occurrence (Touret, 2001; Van den Kerkhof and Hein, 2001; Roedder, 1984). The inclusions in garnet give information about the peak metamorphic history because garnet in the studied sample is part of the peak assemblage.
Primary fluid inclusions occur as clusters and isolated, observed both in garnet and quartz (Fig. 8). These inclusions display negative crystal shape as well as rectangular and ovoid shapes (Fig. 8) and monophasic at room temperature (ca. 25 °C).
Secondary inclusions were found predominantly in quartz and occur as trails cross-cutting grain boundaries and in form of arrays along fractures collectively point towards fluid trapped during retrogression. These inclusions are either monophasic or biphasic and ovoidal in shape (Fig. 9a & b). Re-equilibrated inclusions in quartz grains are abundant and display several deformation features such as stretching, necking, hook and annular-shaped morphology (Fig. 9b,c,d).
4.3.2. Microthermometric measurements
Fluid densities and isochores were estimated by using the “Flincor” computer program ofBrown (1989)based on the equation ofBrown and Lamb (1989)and ofBowers and Hegelson (1983). The results of microthermometric measurements are shown inFigs. 10 and 11.
The analyzed monophasic primary fluid inclusions show initial melting temperature (Tim) between –56.8 and –56.6 °C, suggestive of pure CO2. In garnet, these inclusions homogenized between −16 and −24 °C (Fig. 10a), and in quartz they homogenized between −6 and −14 °C (Fig. 10b). This implies that primary fluid inclusions trapped in peak metamorphic minerals (i.e. garnet) display lower homogenization temperatures than the quartz hosted primary inclusions. The secondary carbonic fluid inclusions hosted in quartz also show initial melting temperature (Tim) between –56.9 and –56.6 °C, indicating pure CO2 composition. The homogenization temperature of CO2 in these
inclusions are in the range of +14 to +21 °C (Fig. 10c). The secondary carbonic-aqueous fluid inclusions present in the quartz grains also display initial melting temperature (Tim) between –56.9 and –56.6 °C specifying pure CO2composition. The homogenization temperature of CO2in these inclusion lies between +22 to +25 °C (Fig. 11b). The clathrate melting temperature was also observed in some biphase in-clusion which occurred between 7 and 8.1 °C. The total homogenization of such inclusions took place in between 285 and 320 °C (Fig. 11a).
Compositions and densities of the fluid phases observed in the in-clusions are used to calculate the isochores in the P-T space; the minimum and maximum Th values of each category of inclusions considered, the implications are discussed in a later section.
4.4. Geochronology
Zircons in the studied samples are subhedral and stubby to prismatic and having two types of structures in CL images (Fig. 12). The grains display either irregular and blurred zoning or no zoning, indicating either recrystallization of magmatic zircons or metamorphic growth (Corfu et al., 2003).
The results of U-Pb zircon geochronology are reported inFig. 13. The analyzed zircons have a Th/U ratio of 0.36–5.88 (Table 7). These Th/U ratios have a very wide range and hence do not provide any in-dication of the zircons origin. There are two clusters of ages: one is between 128 and 131 Ma (Fig. 13 c,d) and the other one is 114 ± 0.38 Ma (Fig. 13a,b).
5. Discussion
Data obtained by petrologic modeling, geochronology, and fluid inclusions study provide new insights on the P-T-t evolution, compo-sition, and behavior of metamorphic fluids as well as on the peak and exhumation history. Our results reveal the pervasive presence of car-bonic rich fluids during metamorphic and fluid evolution of garnet-amphibolite. This helps in understanding the deep crustal process in Tibetan crust before India-Asia collision.
Fig. 6. P/T-X(CO2) pseudosection calculated for the plagioclase-rich layers in the NKCFMASH-CO2model system along a hypothetical prograde path. Plagioclase and quartz are in excess. The observed mineral assemblage (amp + g + bi + pl + q) is modelled by the blue field. The dashed field corresponds to the inferred peak P-T conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
5.1. Metamorphic P-T evolution
Petrographic observations and phase equilibria modeling suggest a decompression clockwise P–T path for the observed mineral assem-blages in the garnet-bearing amphibolite. The results obtained from plagioclase- and amphibole-rich layers are mutually consistent and constrain peak P-T conditions at 8.5–9.5 kbar, 630–655 °C. A decom-pression stage has been further constrained at 5.5–5.7 kbar, 620–630 °C, thus suggesting that the early retrograde evolution was dominated by nearly isothermal decompression. The late metamorphic evolution after the isothermal decompression is inferred to be domi-nated by cooling with a slight decompression, as inhibited in the co-existing sillimanite-staurolite schists (Sachan et al., 2016).
These results are consistent with the medium- to high-grade meta-morphism in the Karakoram involving an early phase of crustal thick-ening, resulted in metamorphism at peak conditions of up to 10 kbar and 750 °C (Streule et al.,2009), followed by near-isothermal decom-pression. The isothermal decompression paths necessitate that the ex-humation of deep-seated metamorphic rocks was rapid relative to the rate of thermal relaxation and cooling (England and Thompson, 1984; Thompson and England, 1984). The isothermal decompression
P–T path for garnet-amphibolite (Fig. 14) is consistent with crustal thickening phenomena (e.g.Brown, 1993). The P-T path incorporating
2–4 kbar decompression from 7 to 10 kbar. The steep trajectory of the
P–T path is considered to reveal a phase of rapid decompression
con-sequential from deformation controlled, rather than erosion controlled exhumation (Brown,1993). The ultimate near‐isobaric stage in the
P–-T evolution possibly imitates much slower exhumation following
ter-mination of rapid uplift. The P–T path inferred in our study is consistent with previous results given byStreule et al. (2009)(peak conditions at ca. 625 °C, 5–6 kbar), Thanh et al. (2011) (peak conditions at ca. 680 °C, 8.5 kbar) and Sachan et al. (2016) (peak conditions at ca. 650 °C, 8 kbar) for lithologies hosting the garnet-bearing amphibolites.
5.2. Evolution of the fluid phase
Our study points to the pervasive presence of carbonic fluids trapped within quartz and garnet in the studied amphibolite. The primary fluids are trapped in both garnet and quartz, and secondary fluid inclusions only in quartz. The homogenization of primary carbonic fluid inclusions oc-curred in the liquid phase in the temperature range of −24 to −6 °C, which translates into a density range of 1.01–1.05 g/cm3. The secondary carbonic inclusions have low-density carbonic fluids (0.76–0.83 g/cm3) trapped at a later stage, whereas the secondary aqueous-carbonic inclu-sions have a density within the range of 0.711 to 0.751 g/cm3and the bulk density ranged in between 0.911 and 0.923 g/cm3.
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10
XTi bi 0.034 XTi bi 0.032 XT i bi 0.030 XT i bi 0.029 XMg bi 0.61 XMg bi 0.62 XMg bi 0.57 X Mg g 0.21X Mg g 0.22 XCa g 0.17 XCa g 0.16 XAn pl 0.52 XAn pl 0.63 XAn pl 0.64 XAn pl 0.51NCKFMASHTO
Amp bi ilm Amp g bi ilm Amp g bi ilm rt Amp g bi rt Amp chl ep bi ilm sph Amp chl bi ilm Amp chl ep bi ilm Amp chl g bi ilm Amp g mu bi ilm rt Amp chl ep bi sph Chl ep bt sph pe lh c p m A hp si b s m Amp chl ep mu bt rt Amp chl ep mu bt ilm rtAmp chl ep mu bi ilm Amp chl ep g mu bi ilm
Amp chl ep bi ilm rt Amp chl ep g bi ilm Amp chl ep mu bi sph (-pl) Amp ep mu bi sph rt Amp ep mu bi sph rt (-pl) Amp ep mu bi rt Amp ep mu bi ilm rt
Amp ep mu
bi ilm pl
Amp ep g ms
bi ilm
Amp ep g mu bi ilm rtAmp chl g mu bi ilm
Amp chl g mu bi ilm rt Amp g mu bi ilm Amp chl g bi ilm rt
Amp chl ep bi sph rt
Amp chl ep bi pl rt
+pl
Fig. 7. P–T pseudosection calculated for the amphibole-rich layer in the NKCFMASHTO model system. The red square and triangular areas constrain peak and retrograde P-T conditions as inferred from the intersection of the following isopleths: XMg(g), XCa(g), XAn(pl), XMg(bi) and XTi(bi). The thick arrow shows the inferred P–T path. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Representative photomicrographs of primary fluid inclusions in garnet (a, b) and quartz (c, d). All inclusions are monophasic and contain CO2gaseous phase.
Fig. 9. Representative photomicrographs of secondary fluid inclusions in quartz. (a) Trail of secondary CO2and CO2-H2O fluid inclusions. (b) Annular and hook shaped equilibrated fluid inclusions in matrix quartz grain. (c) Detail of stretching textures of fluid inclusions. (d) Detail of stretched and necking phenomena in re-equilibrated fluid inclusions.
As expected, the calculated isochores for primary inclusions in garnet and quartz plot at higher P then the isochores of secondary in-clusions in quartz (Fig. 14). The occurrence of primary fluid inclusions in garnet suggests that the first event of fluid trapping occurred at peak P-T conditions; in spite of the textural evidences of fluid inclusion, however, the calculated isochores for primary inclusions intersect at lower P-T conditions than the peak metamorphic estimates, thus sug-gesting that these inclusions experienced post-peak re-equilibration during retrogression, at P-T conditions consistent with those estimated by petrologic modeling (ca. 5.7 kbar, 600 °C).
The calculated isochores for secondary carbonic and aqueous-car-bonic fluid inclusions intersect the calculated P-T trajectory at ca. 2.2 kbars, 470 °C (Fig. 14). Deformation features observed in the secondary inclusions (e.g. necked and stretched morphologies, hook shape, an-nular textures) suggest that they experienced re-equilibration during isothermal decompression (Sterner and Bodnar, 1989; Boullier et al., 1991;Vityk and Bodnar, 1995; Boullier, 1999; Bodnar, 2003;Kharya, Fig. 10. Histograms showing homogenization temperature (Th) for CO2 in
primary carbonic inclusions in garnet (a), primary carbonic inclusions in quartz (b) and secondary carbonic inclusions in quartz (c).
Fig. 11. (a) Histogram showing total homogenization temperature (Th) of secondary CO2-H2O inclusions in quartz. (b) Histogram showing the homo-genization temperature (Th) for CO2in CO2-H2O inclusions in quartz.
2015; Sachan et al., 2017; Kharya et al., 2020), consistently with the P-T path inferred from petrological modeling.
Overall, the observed fluid evolution from an early, high-density, carbonic fluid (primary inclusions in garnet and quartz) to a late, low density, carbonic and carbonic-aqueous fluid (secondary inclusions in quartz) is perfectly consistent with the inferred isothermal decom-pression from ca. 9 kbar to 2 kbar at ca. 600 °C (Fig. 14).
5.3. Nature and source of fluids
The results indicate that the CO2was the dominant fluid at peak amphibolite-facies conditions. The astonishing absence of significant H2O contents in the primary fluid inclusions contrasts with the presence of abundant hydrous minerals (hornblende, biotite) occurring in close association with the garnet and quartz grains, suggestive of the water-saturated condition. The results of thermodynamic modeling, that predicts an H2O or H2O-CO2fluid in equilibrium with the observed assemblage (Fig. 6). The sedimentary nature (i.e. a marl) inferred for the protolith of the studied amphibolite (seeSection 2), as well as its close association with marbles and calc-silicate rocks, would be con-sistent with the presence of a H2O-CO2(rather than pure CO2) fluid during prograde metamorphism, that could have been potentially trapped as primary fluid inclusions during the growth of garnet and quartz. However, this does not justify the occurrence of pure CO2 pri-mary fluid inclusions. A similar issue was reported by Klein and Fuzikawa (2010) and Faleoris et al. (2010)for amphibolite-facies rocks from Brazil. The study proposes that the water originally present in the primary fluid inclusions was preferentially lost and dispersed by crystal-plastic deformation and recrystallization. Their interpretation is based on the studies byKerrich (1976), Wilkins and Barkas (1978), Hollister (1990) and Johnson and Hollister (1995), who proposed that ductile strain-induced leakage of H2O from mixed CO2+ H2O fluid inclusions in quartz is a reliable mechanism for producing occurrences of pure CO2 fluid inclusions in metamorphic rocks. A similar explanation could be extended to the primary fluid inclusions observed in the studied
amphibolite. Following this hypothesis, the fluid trapped in the primary fluid inclusions would have been an H2O-CO2fluid derived from pro-grade decarbonation of the amphibolite’s protolith or of the adjacent carbonate-rich rocks; its original H2O content would have been lost during deformation, leaving a pure CO2fluid. As concerning the sec-ondary carbonic fluid inclusions observed in quartz, a similar genetic process could be advocated.
5.4. Interpretation of the age data and tectonic implications
Zircons from the studied garnet-bearing amphibolite yield two age groups: a highly scattered group at 128–131 Ma and a well-constrained age of 114 ± 0.38 Ma (Fig. 13). The older ages of 128–131 Ma are interpreted as the (partially re-equilibrated?) age of detrital zircons derived from a sedimentary source and constrain the maximum de-positional age of the sedimentary protolith at 131 Ma. The younger age of 114 ± 0.38 Ma is interpreted as the age of prograde to peak me-tamorphism and is remarkably similar to the monazite U-Pb 108 ± 0.6 Ma age obtained byStreule et al. (2009)for the sillimanite-grade metapelites surrounding the garnet-amphibolite.
Lacassin et al. (2004),Valli et al., (2007, 2008) andRolland et al (2009)advocated that the metamorphic activity in the Karakoram is a result of shear heating due to strike-slip faulting at around 15–16 Ma, along the Karakoram Fault. However,Streule et al. (2009)suggested that the metamorphic rocks were formed at 100–108 Ma (monazite U-Pb geochronology) and hence have no relation with shear heating along the Karakoram fault. Our geochronological study as well asStreule et al. (2009)dataset advocates that metamorphism is pre-faulting and has no relation with Karakoram's fault activity as the metamorphism took place during the Cretaceous time. Moreover, the younger age of zircon is similar to the age of the protolith of the K2 orthogneiss (115–120 Ma:Searle et al., 1990) in northern Pakistan confirming that, in the KMC, medium to high-grade metamorphism took place before collision and accretion of the Kohistan arc and the Indian plate to Asia. We, therefore, propose that the sedimentary protolith of the studied rocks was deposited at < 131 Ma, and was subsequently subjected to a medium- to the high-grade metamorphic event (together with the as-sociated staurolite and kyanite/sillimanite -bearing metapelites) related to the Cretaceous phase of crustal thickening along the south Asian margin. These results, which are in line with the interpretations of Searle and Tirrul (1991),Streule et al. (2009) and Wallis et al. (2014), further enhance our understanding of the tectonic evolution of the Karakoram terrane prior to the Himalayan orogeny, confirming that the South Asian active continental margin experienced an Andean-type tectonism during Cretaceous.
6. Conclusions
1. The peak P–T conditions of eastern Karakoram (Pangong Tso) garnet-bearing amphibolites, have been constrained at 8.5–9.5 kbar and 630–655 °C using the P–T pseudosection approach.
2. The nearly isothermal early decompression, with a P-T stage around 5.5–5.7 kbar, 620–630 °C.
3. Pure CO2 primary fluid inclusions are hosted in garnet and quartz, apparently in contrast with the hydrous mineral assemblage of the amphibolite. It is proposed that the fluid trapped in the primary fluid inclusions was originally an H2O-CO2fluid derived from prograde decarbonation of the amphibolite’s protolith or of the adjacent carbo-nate-rich rocks and that its original H2O content was lost during de-formation, leaving a pure CO2fluid.
4. Zircon U–Pb ages for the garnet amphibolites in the Pangong-Tso area suggest that the peak metamorphic stage occurred at Fig. 12. Cathodoluminescence images of a subset of zircons separated from the
114 ± 0.38 Ma, and 131 Ma as the maximum depositional age of the sedimentary protolith.
5. The Cretaceous crustal thickening and heating in the Karakoram terrane were connected to Andean-type tectonism along the South Asian active continental margin.
CRediT authorship contribution statement
Praveen Chandra Singh: Data curation, Investigation, Writing
-review & editing. Himanshu K. Sachan: Investigation, Data curation, Methodology, Writing - original draft. Aditya Kharya: Investigation, Data curation, Methodology, Writing - review & editing. Franco Rolfo:
Data curation, Investigation, Writing - review & editing. Chiara
Groppo: Data curation, Investigation, Writing - review & editing. Saurabh Singhal: Data curation, Writing - review & editing. Sameer K. Tiwari: Data curation, Writing - review & editing. Shashi Ranjan Rai:
Data curation, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influ-ence the work reported in this paper.
Fig. 13. (A) Concordia of metamorphic ages of zircon showing a prominent peak age at 114 ± 0.38 Ma. (B) Weigted mean average age of peak metamorphism for the garnet-bearing amphibolite sample from Pangong-Tso area. MSWD: mean square of weighted. (C) Concordia age of zircons showing the age of sedimentary protolith. (D) Weighted mean average of older cluster showing maximum depositional age.
Acknowledgements
HKS, AK and PCS, SKT and SR are thankful to the Director, Wadia Institute of Himalayan Geology, Dehradun, for providing lab facilities and encouragement to carry out this work. This study is part of a Cooperation Agreement between the Wadia Institute of Himalayan Geology (Dehradun, India) and the University of Torino, Dept. of Earth Sciences (Torino, Italy). The fieldwork of FR and CG was supported by the University of Torino “Ricerca Locale” [ROLF_RIC_LOC_14_01; GROC_RILO_16_01] and by PRIN 2010/2011 [CARRPRIN12] founds.
Appendix A. Supplementary material
Supplementary data to this article can be found online athttps:// doi.org/10.1016/j.jseaes.2020.104293.
References
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Table 7
Geochronological Data of zircon from garnet-amphibolite.
RATIO AGE 207Pb/235U 2σ 206Pb/238U 2σ 207Pb/235U 2σ 206Pb/238U 2σ Th/U 1 HK17_18 0.1003 0.002 0.01405 0.00025 97.1 1.9 89.9 1.6 0.42 2 HK1317_11 0.248 0.021 0.02091 0.00037 224 16 133.4 2.3 0.43 3 HK1317_13 0.525 0.017 0.0208 0.00033 430 10 132.7 2.1 5.88 4 HK1317_14 0.1598 0.0048 0.01754 0.00016 150.5 4.2 112.1 1 1.72 5 HK1317_15 0.256 0.014 0.01784 0.0002 231 11 114 1.2 0.77 6 hk13b_1 0.1377 0.0022 0.01916 0.00032 131 2 122.3 2 1.67 7 hk13b_12 0.1637 0.0094 0.01544 0.00023 153.5 8.2 98.8 1.5 0.65 8 hk13b_13 0.1326 0.0017 0.01917 0.00016 126.4 1.5 122.4 1 1.83 9 hk13b_14 0.1692 0.0099 0.01996 0.00046 158.5 8.6 127.4 2.9 0.89 10 hk13b_15 0.1357 0.0033 0.01872 0.00018 129.2 2.9 119.6 1.2 2.22 11 hk13b_16 0.1484 0.0038 0.02133 0.00014 140.5 3.4 136.08 0.91 1.00 12 hk13b_17 0.1375 0.0031 0.02019 0.00024 130.8 2.8 128.9 1.5 1.36 13 hk13b_18 0.132 0.0067 0.01794 0.00072 125.8 6 114.6 4.5 1.00 14 hk13b_19 0.1564 0.0051 0.02234 0.00052 147.5 4.5 142.4 3.3 0.85 15 hk13b_2 0.1344 0.0078 0.01741 0.00039 128 6.9 111.3 2.4 0.48 16 hk13b_20 0.1676 0.008 0.02103 0.00031 157.3 6.9 134.2 2 1.62 17 hk13b_21 0.295 0.03 0.01972 0.00013 262 23 125.9 0.8 1.43 18 hk13b_22 0.1195 0.0016 0.01787 0.00018 114.6 1.4 114.2 1.1 0.30 19 hk13b_25 0.259 0.015 0.01787 0.00034 234 12 114.2 2.2 0.69 20 hk13b_27 0.185 0.016 0.0175 0.00036 169 13 111.8 2.3 0.65 21 hk13b_28 0.1613 0.0068 0.01791 0.00018 151.6 5.9 114.4 1.1 0.71 22 hk13b_29 0.1273 0.0034 0.01535 0.00012 121.6 3.1 98.21 0.74 0.46 23 hk13b_3 0.0995 0.003 0.01446 0.00069 96.3 2.8 92.6 4.4 1.20 24 hk13b_30 0.159 0.023 0.01782 0.00034 148 19 113.9 2.1 0.49 25 hk13b_31 0.1225 0.0019 0.0177 0.00022 117.4 1.7 113.1 1.4 0.83 26 hk13b_32 0.192 0.013 0.01753 0.00015 178 11 112.04 0.98 0.69 27 hk13b_35 0.1202 0.0015 0.01799 0.00022 115.2 1.3 114.9 1.4 0.36 28 hk13b_36 0.12 0.001 0.01795 0.00016 115.03 0.91 114.7 1 0.38 29 hk13b_4 0.1431 0.005 0.02023 0.00038 135.8 4.4 129.1 2.4 1.04 30 hk13b_5 0.1396 0.0034 0.02102 0.00019 132.6 3.1 134.1 1.2 1.23 31 hk13b_7 0.147 0.01 0.01809 0.00039 138.7 9.1 115.6 2.4 0.61 32 hk13b_8 0.1463 0.0058 0.01688 0.00021 138.6 5.1 107.9 1.3 0.87
Fig. 14. P–T path of the garnet-bearing amphibolite from the Pangong-Tso area constrained from thermodynamic modelling and fluid inclusions study. P-T conditions inferred from phase equilibria modelling (peak and retrograde stages) are shown by blue line and blue boxes, respectively. Isochores obtained from fluid inclusions microthermometry are shown (green lines: primary CO2 inclusions in garnet; black lines: primary CO2inclusions in quartz; blue lines: secondary CO2inclusions in quartz; red lines: secondary CO2-H2O in quartz). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
