Metamorphic Evolution of Garnet-bearing Epidote-Barroisite Schist From the Meratus Complex in South Kalimantan, Indonesia

DOI:10.17014/ijog.2.3.139-156This paper presents metamorphic evolution of metamorphic rocks from the Meratus Complex in South Kalimantan, Indonesia. Eight varieties of metamorphic rocks samples from this location, which are garnet-bearing epidote-barroisite schist, epidote-barroisite schist, glaucophane-quartz schist, garnet-muscovite schist, actinolite-talc schist, epidote schist, muscovite schist, and serpentinite, were investigated in detail its petrological and mineralogical characteristics by using polarization microscope and electron probe micro analyzer (EPMA). Furthermore, the pressure-temperature path of garnet-bearing epidote-barroisite schist was estimated by using mineral parageneses, reaction textures, and mineral chemistries to assess the metamorphic history. The primary stage of this rock might be represented by the assemblage of glaucophane + epidote + titanite ± paragonite. The assemblage yields 1.7 - 1.0 GPa in assumed temperature of 300 - 550 °C, which is interpreted as maximum pressure limit of prograde stage. The peak P-T condition estimated on the basis of the equilibrium of garnet rim, barroisite, phengite, epidote, and quartz, yields 547 - 690 °C and 1.1 - 1.5 GPa on the albite epidote amphibolite-facies that correspond to the depth of 38 - 50 km. The retrograde stage was presented by changing mineral compositions of amphiboles from the Si-rich barroisite to the actinolite, which lies near 0.5 GPa at 350 °C. It could be concluded that metamorphic rocks from the Meratus Complex experienced low-temperature and high-pressure conditions (blueschist-facies) prior to the peak metamorphism of the epidote amphibolite-facies. The subduction environments in Meratus Complex during Cretaceous should be responsible for this metamorphic condition.


Background
The Meratus Complex lies in the South Kalimantan extending in trend of NE-SW ( Figure 1). The metamorphic rocks cropping out in this location have been considered as part of Cretaceous subduction fossils in central Indonesia, which Metamorphic Evolution of Garnet-bearing Epidote-Barroisite Schist from the Meratus Complex in South Kalimantan, Indonesia widely spread throughout Central Java, South Sulawesi, and South Kalimantan 1998;Parkinson et al., 1998;Kadarusman et al., 2007). The studies of the high-pressure metamorphic rocks, especially their prograde and retrograde pressure-temperature paths, provide important constraint on the tectonic processes of ancient subduction zone in the central Indonesia.

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G Zulkarnain (2003) reported the occurences of quartz-chloritoid rocks from this location and concluded it was derived from pelitic schist in an accretionary complex environment. Moreover, Sikumbang and Heryanto (2009) reported occurences of several metamorphic rocks from this location. One of them is barroisite-epidote schist. Parkinson et al. (1998) described the occurrence of glaucophane-and kyanite-bearing quartz schist in this location and also suggested that the presence of Mg-rich chloritoid implied recrystallization at pressure of ~1.8 GPa or higher, which recommended as a part of high-pressure metamorphic terranes in central Indonesia (Luk-Ulo Complex in Central Java, Bantimala Complex in South Sulawesi). However, detailed observations of P-T evolution, particularly on the reaction texture of prograde, peak, and retrograde  Williams et al. (1988), and Setiawan et al. (2013). The Luk Ulo Complex from Wakita et al. (1994b), Miyazaki et al. (1998), Asikin et al. (2007), and Kadarusman et al. (2007). The Meratus Complex from Parkinson et al. (1998), Sikumbang andHeryanto (2009), andSetiawan (2013). Barru Complex from Setiawan (2013). The Bantimala Complex from Sukamto (1982), Wakita et al. (1994a, Miyazaki et al. (1996), Parkinson et al. (1998), and Setiawan (2013). The Palu Complex from Kadarusman and Parkinson (2000), and Kadarusman et al. (2004). The Malino Complex from Leeuwen et al. (2006). The Pompangeo Complex from Parkinson (1998). Grt = garnet; Qz = quartz; Brs = Barroisite; Ms = muscovite; Gln = glaucophane. stages have not been reported. In this paper, we present a petrographic description and minerals chemistry analysis of representative samples of newly found garnet-bearing epidote-barroisite schist from the Meratus Complex, South Kalimantan (Figures 1 and 2), and discuss its P-T evolution. Mineral chemistries of representative samples were analyzed using JEOL JXA-8530F electron probe micro analyzer (EPMA) at Kyushu University, Japan. The analytical condition of EPMA was set an accelerating voltage of 15 kV, a probe current of 12 nA and a beam diameter of 2 μm. Natural mineral samples (ASTIMEX-MINM-53) and synthesized oxide samples (P and H Block No. SP00076) were used as standards for the quantitative chemical analyses. Mineral abbreviation in this paper follows Whitney and Evans (2010). Constraining the prograde peak and retrograde P-T path are very important for geodynamic interpretations in the region. Particularly in highpressure metamorphic terranes, prograde to retrograde histories are relevant to the processes of deep subduction and subsequent exhumation. The

Geological Outline
The Cretaceous subduction complex, which is represented by the occurrence of accretionary unit including mélanges, pillow basalts, dismembered ophiolites, cherts, serpentinites, and garnet lherzolite are sporadically exposed in the central Indonesia region through the Java, Kalimantan, and Sulawesi Islands (Sukamto, 1982;Wakita et al., 1994aWakita et al., , 1994bWakita et al., , 1996Wakita et al., , 1998Parkinson et al., 1998;Wilson and Moss, 1999;Kadarusman and Parkinson, 2000). The distribution of the accretionary units and metamorphic rocks are shown in Figure  1. Most of the metamorphic rocks exposing in the complexes occur in a limited areas and are bounded by the thrust fault with other units such as dismembered ophiolites, cherts, mélanges, and serpentinites (Sukamto, 1982;Asikin et al., 2007;Sikumbang and Heryanto, 2009).
In the Meratus and Bobaris Mountains of South Kalimantan, the metamorphic rocks crop out in the most southern part of the Meratus Mountains namely as the Meratus Complex. The Meratus Complex extends in the trend of NE-SW (Sikumbang and Heryanto, 2009; Figure 2). The metamorphic rocks occur as wedge-shaped tectonic blocks in fault contact with ultramafic rocks and Cretaceous sedimentary rocks (Parkinson et al., 1998). The dominant lithologies in this complex are serpentinized peridotite and pyroxenite, gabbro, plagiogranite intrusions, shale-matrix mélange with clasts of limestone, chert and basalt (Laut Island), pelagic sediments with a Middle Jurassic-late Early Cretaceous radiolarian biostratigraphy, clastic and carbonate sediments, and various low-grade schists Parkinson et al., 1998;Sikumbang and Heryanto, 2009). These formations are unconformably overlain by Late Cretaceous turbidites and volcaniclastics ( Figure 2). Cretaceous magmatic rocks of island-arc with calc-alkaline affinity intruded the Meratus Complex (Yuwono et al., 1988). The magmatic rocks are rhyolite, dacite, andesite, basalt, granite, diorite, and gabbro with the age ranges of 92 -72 Ma (Yuwono et al., 1988). Sikumbang and Heryanto (2009) reported metamorphic rocks of quartz-muscovite schist, quartzite, barroisite-epidote schist, and metagabbro. Parkinson et al. (1998) described the occurrence of glaucophane-and kyanite-bearing quartz schist in this location. They also suggested that the presence of Mg-rich chloritoid implied recrystallization at a pressure of ~1.8 GPa or higher. The K-Ar dating of various mica schists yielded ages ranging between 110 -180 Ma, which are in the similar age with the metamorphic rocks in South Sulawesi and Central Java Sikumbang and Heryanto, 2009). Furthermore, Wakita et al. (1998) and Parkinson et al. (1998) suggested the occurrence of high-pressure metamorphic rocks in this location were products of a Cretaceous subduction beneath Sundaland.

Modes of Occurrences and Sample Descriptions
The blueschist-to amphibolite-facies rocks (e.g. epidote-barroisite schist discussed in this study) occur in the Aranio River (Figures 2 and  3a -c). The schist consists of garnet-and quartzrich layers which has 80ºW trending foliation from north with dipping 66º to north (Figures 3b and c). In the southern part of the complex, only serpentinized peridotites were identified as metamorphic rocks and the others are ultramafic rocks and mafic rocks such as peridotite, olivinegabbro, and hornblende-gabbro. The exposure of serpentinite could be found throughout in the complex (Figures 2 and 3d). Other types of metamorphic rocks are mainly tremolite-talc schist, muscovite schist, and epidote schist. However, I J O G the geological relationships between the schists and the serpentinites have not yet been clarified because of poor exposures due to deep weathering and heavily vegetated areas.
The mineral assemblages of collected samples are listed in Table 1. General petrographical characteristics of representative metamorphic rocks collected from the Meratus Complex are also described below. Fe 3+ contents of garnet were calculated using algorithm suggested by Droop (1987). Phengite formulae have been calculated on the basis of eleven oxygen atoms with assuming all iron to be Fe 2+ . Nomenclatures and calculated composition of the amphiboles follow Leake et al. (1997).

Garnet-muscovite schists
These rocks have porphyroblastic texture with numerous garnet porphyroblasts ( Figure  4d). These samples mainly consist of garnet, muscovite, quartz, epidote, rutile, albite, and apatite. A lot of cracks appear in the coarsegrained garnet (0.5 -1 mm in diameter). Those cracks are filled with chlorite and albite. Muscovite (~0.25 mm in diameter) and epidote (~0.3 mm in diameter) are abundant in the matrix. Those minerals develop the schistose fabric of these rocks. Secondary chlorite subsequently replaces garnet and other minerals by pseudomorph after them.

Tremolite-talc schist
This rock is rarely found in study area. It consists of tremolite, talc, and quartz (Figure 4e). Sheaf texture of tremolite (~0.5 mm in diameter) is embedded in the abundant talc grains. Interstitial albite and quartz occur filling in the cracks.

Serpentinite
Serpentinite preserves relict mineral phases. Clinopyroxene and olivine are well recognized under polarized microscope despite suffering from crosscut by mesh texture of serpentine ( Figure 4f). Spinel (0.2 -0.5 mm in diameter) occurs in the matrix which consider as a relict mineral.

Petrography and Mineral Chemistries of Garnet-bearing epidote-barroisite Schist
The estimation of P-T paths of the high-pressure metamorphic rock (garnet-bearing epidotebarroisite schist; Sample no. 031601) from the Meratus Complex is described in detail in this section. The estimated metamorphic evolution in this chapter could reflect the evolution of the Meratus Complex of South Kalimantan. General petrography of the rock sample discussed here has already been described in the previous section. Hence, in this section, the description of petrography and mineral chemistry will focus on the selected garnet-bearing epidote-barroisite schist sample with mineral coexistence on that is mainly described in detail.

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G fine-grained garnet (0.1 -0.5 mm) has inclusions of quartz, titanite, apatite, chlorite, and epidote ( Figure 5b). Blue-greenish barroisites (0.5 -1 mm) show compositional heterogeneity suggested by patchy texture of Si-rich and Si-poor barroisites (Figure 5c). In several grains, thin layer of actinolite rims the barroisites (Figure 5d). Some of the barroisites are replaced by chlorite and albite, which indicate secondary phases in this rock.

Mineral chemistry
Mineral chemistries of the garnet-bearing epidote-barroisite schist are described in detail. The analyses were performed using the same system as described in the previous section. Representative mineral chemistry analyses of the garnet-bearing epidote-barroisite schist are presented in Table 2.

Garnet
Euhedral garnet obviously has two domains defined by a different composition, which show core and rim portions ( Figure 6). The garnet has a barrier reef type zoning pattern identified by an irregular/anhedral shape of core portion ( Figure 6). Enami et al. (2011) studied the barrier reef garnet grains from Myanmar eclogite and they suggested that relics of an older core occurring as fragments were dissolved by the new garnet. The chemical zoning on the garnet is clearly identified on the Ca, Fe, Mg, and slightly on the Mn elements ( Figure 6). Epidote, titanite, and apatite are inclusions in the core portion of garnet which are clearly identified particularly by Ca and Ti elements ( Figure 6). Based on the chemical zonation (rim to core),    the garnet core portion has increased in grossular and decreased in almandine and pyrope (Prp10-15Alm53-58Sps8-11Grs18-24; Figure 7). The rim portion of the garnet shows increasing pyrope and almandine but decreasing grossular with spessartine that are relatively flat (Prp16-18Alm59-61Sps7-10Grs13-16; Figure 7). The increasing spessartine components along the garnet cracks (Sps18-20) might be an effect of retrograde metamorphism.

Phengite
Two types of phengite were recognized in the matrix (phengites 1 and 2). The differences between both phengites are obviously identified by the Other minerals Other minerals are epidote, titanite, chlorite, and albite. Those are describing in here. There are no significant differences between epidotes that occur as the inclusion and in the matrix. Both of them have similar ranges of pistacite contents (XFe 3+ = 0.18 -0.30). Titanite as inclusion in garnet core have higher-XAl (0.084 -0.275) than titanite in the matrix (XAl = 0.084 -0.275). Chlorite occurring along cracks of garnet and replacing other minerals has a compositional range of XFe = 0.44 -0.46. Albite occur as interstitial phases along the cracks of barroisite and in the matrix has composition of XAb = 0.98 -1.00.

Discussion of P-T Estimation
Based on the textural and mineral chemical results, the metamorphic evolution of the garnetbearing epidote-barroisite schist is divided into three stages which represents the different metamorphic facies as follows: prograde (blueschistfacies), peak (amphibolite-facies), and retrograde stages (greenschist-facies). Summary of mineral assemblages and their chemical characters are presented in Table 3.

Prograde Stage
The prograde stage of garnet-bearing epidotebarroisite schist might be preserved as mineral inclusions, composed of glaucophane that is included in the barroisite in the matrix and the epidote in the garnet core. As described previously, two kinds of phengite were identified in the matrix ( Figure 8) which are Na-rich phengite (phengite 1) and normal phengite (phengite 2). The Na-rich phengite (phengite 1) might be pseudomorph after paragonite. Hence, the primary stage of this rock might be represented by the assemblage of glaucophane + epidote ± paragonite (Table 3). X Grs X Alm X Prp Figure 7. Chemical composition of zoning profile from core to rim in garnet. The garnet core portion has higher grossular and lower almandine and pyrope. Rim portion shows increasing pyrope and almandine but decreasing grossular with spessartine which are relatively flat.
Titanite and epidote grains are included in the garnet core (Figure 5b). Rutile, which is not observed in the garnet inclusion, appears in the matrix. Manning and Bohlen (1991) suggested geobarometry involving titanite, rutile, epidote The equilibrium reaction of Lws + Jd = Pg + Ep/Zo + Qz + H 2 O + Vapor from Heinrich and Althaus (1988) might give a minimum temperature for primary stage as lawsonite and Naclinopyroxene that could not be found in this rock ( Figure 11). The presence of glaucophane grains included in the barroisite can be used to constrain the maximum temperature of prograde stage. The maximum temperature of glaucophane is based on experimental studies of natural glaucophane  (Table 3). The activity of grossular component in garnet core was calculated from the mixing model of Berman (1990). The titanite, Table 3. Mineral Parageneses in Metamorphic Evolution of Garnet-bearing Epidote-barroisite Schist Figure 11. P-T diagram of garnet-bearing epidote-barroisite schist. The petrogenetic grids are from Evans (1990), the abbreviations as follows; LBS: lawsonite blueschist-facies, E: eclogite-facies, AEA: albite epidote amphibolite-facies, A: amphibolite-facies, GS: greenschist-facies. Experimental determined reactions: [1] Lws + Jd = Pg + Ep/Zo + Qz + Vapor (Heinrich and Althaus, 1988) [2] maximum stability field of glaucophane (Maresch, 1997), [3] Ep + Ttn = Grt + Rt + Qz + H2O (Manning and Bohlen, 1991), and [4] amphiboles solid-solution (Otsuki and Banno, 1990). Garnet-phengite geothermometry: open square from Krogh and Raheim (1978) and open circle from Green and Hellman (1982). Geobarometry of phengite from Massonne and Schreyer (1987). and grossular in eclogite was based on dehydration reaction of Ep + Ttn = Grs + Rt + Qz + H 2 O. Garnet in this sample does not show an increase of grossular between core and rim. However, if other net-transfer reaction producing pyrope and almandine, sometimes the grossular weight *Abbreviations see Table 1.  Manning and Bohlen (1991). Activity of titanite (Table 2) was calculated as X ca X Ti X Si [X o ] 5 with Xo = (5 -X Al -X 3+ ) by assuming that F and OH substitutions were balanced by Al and Fe 3+ , which correspond to a Tin = 0.756. The activity of clinozoisite/ epidote was calculated as X 2 X 2 (1 -Fe 3+ )X 3 with the maximum pistacite selected (Table 2) to give the maximum pressure corresponding to a Czo = 0.367. Other activities of rutile, quartz, and H 2 O are assumed to be 1 (Manning and Bohlen, 1991). The activities of the assemblage yield 1.7 -1.0 GPa for an assumed temperature of 300 -550 ºC interpreted as the maximum pressure limit of the prograde stage ( Figure 11).

Peak Stage
Mineral coexistences at the peak P-T condition are garnet rim, barroisite, phengite 2, epidote, and quartz (Table 3). The temperature condition is estimated using the garnet-phengite geothermometer formulated by Krogh and Raheim (1978) and Green and Hellman (1982). The results give temperature ranges of 547 -636 ºC assuming 1.0 GPa (Figure 11). The peak pressures are estimated using phengite geobarometer formulated from Massone and Schreyer (1980). Maximum and minimum Si contents on phengite 2 are used for this geobarometer to obtain maximum and minimum pressures, respectively. The result gives a range of pressures at 1.1 -1.5 GPa. Petrogenetic grid from Evans (1990) suggests that this peak P-T conditions are plotted on the epidote amphibolite-facies ( Figure 11).

Retrograde Stage
The retrograde decompression stages in this rock are represented by textural relations of amphiboles in the matrix (Figures 5c -d). Following solid-solution diagram of amphiboles from Otsuki and Banno (1990), the retrograde P-T path is explained by changing chemical composition of amphiboles from Si-Na rich barroisite to actinolite rim through Si-Na poor amphiboles (barroisite II; Figures 5c-d, 11). Therefore, there are two stages in the retrograde metamorphism. The second or last retrograde-decompression P-T path should be on the stability field of actinolite (Table 3) which lies near 0.5 GPa at 350 ºC ( Figure 11).

Metamorphic Evolution
The pressure-temperature path of garnetbearing epidote-barroisite schist was estimated by using mineral parageneses, reaction textures, and mineral chemistries. The obtained pressuretemperature path of the garnet-bearing epidotebarroisite schist has a clockwise trajectory. The rock experienced primary stage on the stability field of paragonite + glaucophane + epidote and subsequent increasing pressure and temperature to the stability field of barroisite, which peak P-T condition of this rock was at 547 -690 ºC and 1.1 -1.5 GPa on the albite epidote amphibolitefacies that correspond to the depth of 50 -60 km. The retrograde stage is presented by changing mineral compositions of amphiboles from the Si-rich barroisite-to the actinolite-stability field through Si-poor barroisite/magnesiohornblende/taramite/pargasite, which lies near 0.5 GPa at 350 ºC.

Conclusions
It might be concluded that metamorphic rocks from the Meratus Complex experienced high-pressure condition of the epidote blueschist-facies before the peak metamorphism of the epidote-amphibolite facies. The worldwide blueschist-facies metamorphic rock is consided as markers of fossil subduction zones. As already mentioned before, the K-Ar dating of various mica schists in this location yielded ages ranging 110 -180 Ma, which are in the similar age with the metamorphic rocks in South Sulawesi and Central Java. Therefore, the occurrences of prograde blueschist-facies in Meratus Complex might be concluded that this area was originally in subduction zone during Cretaceous age. Compared to the other high-pressure metamorphic terranes in central Indonesia (e.g. eclogite from Bantimala Complex: 580 -650 ºC at 1.8 -2.4 GPa and 630 -700 ºC at 2.9 -3.1 GPa and eclogite from Luk Ulo Complex: 2.0-2.3 GPa at 365 -410 ºC and 2.15 -2.25 GPa at 550 -625 ºC, the estimated P-T metamorphic condition of garnet-bearing epidote-barroisite schist from the Meratus Complex has a lower peak pressure but giving higher temperature (547 -690 ºC) at pressure 1.1 -1.5 GPa. The high-pressure metamorphic rocks from Bantimala and Luk Ulo Complexes are characterized by low-to very low geothermal gradients. Furthermore, this study shows that garnet-bearing epidote-barroisite schist from the Meratus Complex has a higher geothermal gradient compared to the other metamorphic terranes. Possibly, the Meratus Complex was proximal and the others were distal with respect to the original Cretaceous subduction site. The results reported here and further precise petrological and geochronological studies will contribute toward a better understanding of the Mesozoic tectono-metamorphic development of the eastern margin of the Sundaland.