Peak Metamorphic Conditions of Garnet Amphibolite from Luk Ulo Complex, Central Java, Indonesia: Implications for Medium-Pressure/High-Temperature Metamorphism in the Central Indonesian Accretionary Collision Complex

Garnet amphibolites, which suggest medium-pressure/high-temperature (MP/HT), are widely recognized in Luk Ulo Complex, Central Java. Their occurrences associated with high-pressure/low-temperature (HP/LT; eclogite, blueschist) metamorphic rocks in the Luk Ulo Complex will provide important constraint on the geodynamic model of Central Indonesian Accretionary Collision Complex (CIACC). This study aimed to estimates P-T metamorphic condition of garnet amphibolite from Luk Ulo Complex by using mineral parageneses, thermodynamic data, and NCKFMASHO pseudosection. Prograde stage assemblages represented by inclusions in the garnet, which are garnet core, zoisite, titanite, apatite, and quartz. Mineral coexistences at the peak P-T condition are garnet rim, magnesiohornblende, zoisite, titanite, quartz, albite, and phengite. The retrograde stage represented by secondary minerals fill the crack in the garnet and other minerals, which are chlorite and quartz. P-T metamorphic condition of garnet amphibolite can only be interpreted from the peak metamorphic stage. The temperature of the garnet amphibolite is estimated using the garnet-amphibole and garnet-phengite geothermometers. Meanwhile, the pressure condition is estimated from phengite geobarometer. The results were compared to the stability and compositions of the phases in NCKFMASHO pseudosection in order to constrain the peak P-T metamorphic conditions. It is concluded that the peak P-T metamorphic condition for garnet amphibolite is 0.9 1.4 GPa and 558 606 oC. The estimated peak P-T metamorphic temperature is higher compared to the previously published gradient geothermal of eclogite and tourmaline-eclogite in the Luk Ulo Complex. The MP/HT (amphibolite) and HP/LT (blueschist and eclogite) metamorphic rocks could have similar metamorphic ages if both footwall and hanging wall had initially very high thermal gradients and the rate of subduction was very slow (10 km/Ma or less).


Background
High-pressure/low-temperature (HP/LT) metamorphic rocks (blueschist-and eclogitefacies) record subduction processes and provide geodynamic evolution of convergent plate boundaries. Those rocks are exposed in Central Indonesian Accretionary Collision Complex (CIACC; Kadarusman et al., 2007;Figure 1a) region, particularly in Luk-Ulo (Central Java), Bantimala (South Sulawesi), and Meratus (South Kalimantan) Complexes Parkinson et al., 1998;Kadarusman et al., 2007;Setiawan et al., 2015;Setiawan et al., 2016). Recent Rb-Sr geochronological studies of various metamorphic rocks from these areas yielded Early Cretaceous ages (130 -120 Ma in South Sulawesi and 119 -117 Ma in Central Java; Hoffman et al, 2019; Böhnke et al., 2019). Compared to Bantimala Complex, amphibolitefacies rocks (garnet amphibolite, epidote-albite amphibolite) are widely recognized in Luk Ulo Complex, Central Java (e.g. Setiawan et al., 2013;Hoffman et al., 2019). They are reported to have protolith of mafic rocks (Setiawan et al., 2013). Furthermore, based on the previous and recent studies, this type of rock was not reported yet in the Bantimala Complex, South Sulawesi. The amphibolite-facies rocks suggest mediumpressure/high-temperature (MP/HT) metamorphic processes that are stable over a wide P-T range at the transition from amphibolite through granulite to the eclogite facies. They are reported to have formed at pressure as low as 0.5 GPa and as high as 2 GPa, where plagioclase decomposes (Graham and Powell, 1984). Their occurrences associated with HP/LT metamorphic rocks in the Luk Ulo Complex will provide important constraint on the geodynamic model of CIACC. The P-T estimation of MP/HT metamorphic rocks in Luk Ulo Complex has not been reported. In this study, new observational results of garnet amphibolite from Luk Ulo Complex, Central Java, are presented that preserve textural information of metamorphic processes and estimate their peak P-T condition. Such data are important to suggest the understanding of subduction processes in Luk Ulo Complex and to contrast with other metamorphic terranes in CIACC.

Geological Outline
Cretaceous accretionary complexes including subduction-related metamorphic rocks are sporadically exposed in the central Indonesian region through Java, Kalimantan, and Sulawesi Islands (e.g. Parkinson et al., 1998;Kadarusman et al., 2007;Figure 1a). Most of the metamorphic rocks are exposed in the limited areas as blocks and boulders along the rivers with other units including mélanges, dismembered ophiolites, cherts, and serpentinites (Parkinson et al., 1998;Setiawan et al., 2016). The metamorphic rocks exposed in Java were described in Karangsambung and Jiwo Hills, I J O G Peak Metamorphic Conditions of Garnet Amphibolite from Luk Ulo Complex, Central Java, Indonesia: Implications for Medium-Pressure/High-Temperature Metamorphism in the Central Indonesian Accretionary Collision Complex (N.I. Setiawan et al.) 227 Central Java, and Ciletuh, West Java ( Figure 1a). In Central Java, the most significant metamorphic rocks are exposed on the Luk Ulo Complex in Karangsambung area (Figure 1b). Detailed geology of this area was described by Wakita et al. (1994), Miyazaki et al. (1998), Asikin et al. (2007), and Kadarusman et al. (2007). The Luk Ulo Complex consists of shale, sandstone, chert, basic to ultrabasic rocks, limestone, conglomerate, and metamorphic rocks. Dismembered ophiolites are well recognized in the central part of the complex (Suparka, 1988). Tectonic slices of the complex have a trend of ENE-WSW (Asikin et al., 2007).
Most of the metamorphic rocks in the Luk Ulo Complex are muscovite schist in which albite, quartz, and muscovite are abundant . The epidote amphibolite in which barroisite, garnet, epidote, albite, biotite, and phengite are present, intercalated with garnet-bearing pelitic schists . Small amounts of garnet amphibolite, eclogite, glaucophane rock, and jadeite-quartz-glaucophane rock occur as tectonic blocks embedded in sheared serpentinite .

Field Occurrences and Metamorphic Rock Samples
Most of the metamorphic rocks are found as river boulders in Muncar, Loning, and Luk Ulo Rivers (Figures 1b and 2a) as most of the outcrops have already experienced weathering due to the tropical condition. The HP/LT (eclogite and garnet-glaucophane rock) and MP/HT (garnet amphibolite, amphibolite) metamorphic rocks predominantly occur as boulders on the rivers in the western part of the complex (Muncar and Luk Ulo Rivers; Figure 1b and Figure 2a respectively). While in the eastern part of the complex (Loning River; Figure 1b) mostly medium-to low-grade metamorphic rocks could be found. The boundary of these rock types is between Muncar and Loning Rivers. The metamorphic rocks occurring in this complex includes HP metabasites (eclogite, garnet-glaucophane schist, and glaucophane schist), MP metabasites (amphibolite, garnet amphibolite), and pelitic schist (garnet-muscovite schist, muscovite schist). Other variations of lowgrade schists found in this area are garnet-albiteactinolite schist, garnet-biotite-muscovite schist, apatite-quartz-muscovite schist, and chloriteactinolite schist.
Totally fourteen samples of garnet amphibolite (twelve samples) and amphibolites (two samples) were collected from Muncar River during the fieldwork (Table 1; Figures 2a and 2b). All samples were prepared for thin section, then for microscopic studies using polarization microscope. Furthermore, most representative sample with least hydration and alteration (18T02G) was selected for mineral chemistry analyses using EPMA and the whole rock chemical composition for pseudosection analysis using XRF.

Analytical Methods
Mineral chemistries of representative samples were analyzed with JEOL JXA-8530F hyperprobe EPMA and JEOL JED2140-JSM5301S scan-ning electron microscope with energy dispersive spectrometry system (SEM-EDS) in Kyushu University, Japan. The analytical conditions of EPMA were set on accelerating voltage of 15 kV, probe current of 0.4 nA for EDS and 12 nA, and 2 μm beam diameter for EPMA. 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. The results were processed with ZAF correction method. The analytical conditions of X-ray mapping included 15 kV accelerating voltage and 50 nA probe current. Major, trace, and rare-earth element composition of representative samples (18T02G) were analyzed by X-ray fluorescence spectrometry (XRF) using Rigaku ZSX Primus II on a fused glass disk (sample: flux ratio, 1:2) to generate pseudosection. Mineral chemistry analyses and formulae of garnet, phengite, amphibole, and epidote were calculated using algorithms suggested by Droop (1987), Rieder et al. (1998), Leake et al. (1997), and Armbruster et al. (2006), respectively. Furthermore, chemical analyses and formulae of titanite and chlorite were calculated following Deer et al. (1992). P-T condition of metamorphic rock was estimated by using mineral parageneses, thermodynamic data, and pseudosection. Mineral parageneses were provided by combining textural observation of metamorphic rocks using polarization microscope and mineral analyses obtained by EPMA. The geothermobarometry is based on data from laboratory experiments and thermodynamic calculations, which was facilitated by several reliable publications (e.g. phengite barometer by Massone and Schreyer, 1987; garnet-amphibole geothermometer by Graham and Powell, 1984;etc.). In order to constrain the peak P-T conditions, the mineral parageneses were compared to the stability and phases in pseudosection using Perple_X software (Connolly, 2005). All of mineral abbreviation in this paper follows Whitney and Evans (2010).

Mineral Chemistries Garnet
Representative mineral chemistry analyses of garnet, phengite, epidote/zoisite, and other minerals are presented in Table 2. Fe 3+ contents in garnet were calculated using algorithms suggested by Droop (1987). Garnet in the garnet amphibolite was analyzed for chemical mapping on the Si, Mn, Al, Mg, Fe, Na, and Ti elements ( Figure 4). Subhedral garnet obviously has a strong chemical zoning of the Ca and Mn with slight zoning of the Fe and Mg elements ( Figure  5a). The inclusions of zoisite, titanite, and apatite in the garnet are clearly identified particularly by Ca and Ti elements ( Figure 5a). Based on the chemical zonation, core portion of garnet has higher spessartine and almandine, with lower grossular contents (Prp 11-12 , Alm 48-51 , Sps 14-18 , Grs 31-33 ; Figure 5a) than mantle portion (Prp 11-12 , Alm 51-53 , Sps 2-5 , Grs 31-36 ; Figure 5a). The rim portion of the garnet shows increasing of grossular but decreasing of almandine and spessartine contents (Prp 10-11 , Alm 49-51 , Sps 1-3 , Grs 36-39 ; Figure  5a) compared to the mantle portion. Spessartine component is relatively constant from core to rim (Figure 5a).

Phengite
Phengite formulae have been calculated to eleven oxygen atoms with assuming all iron to be Fe 2+ (Rieder et al., 1998). Phengite in the garnet amphibolite only occurs in the matrix. It shows direct contact with garnet ( Figure 3f). The phengite has X Si values ranging from 0.578 to 0.585 (Figure 5b). Whereas X Mg values range from 0.664-0.692 (Figure 5b).

Amphibole
Nomenclatures and calculated compositions of the amphiboles follow Leake et al. (1997)   6.53 -6.64; Figure 5c). There is no chemical zoning identified on the amphiboles.

Epidote/Zoisite
Cation formulae of epidote have been calculated assuming all iron to be Fe 3+ (Armbruster et al., 2006). Zoisite in the garnet amphibolite shows low pistacite values in the matrix (X Fe 3+ = 0.03 -0.07) compared to the inclusion in the garnet (X Fe 3+ = 0.11 -0.13).

Other Minerals
Titanite in this rock also occurs as inclusions in the garnet rim and in the matrix, which have similar ranges of X Al = 0.04 -0.07. Chlorite occurs along cracks of garnet and replacing other miner- ‡ Data used for calculation peak P-T condition

Pressure-Temperature Conditions for the Garnet Amphibolite
Based on the textural and mineral chemistry results, the metamorphic evolution of the garnet amphibolite is divided into three stages: prograde, peak, and retrograde ones (Table 3). Prograde stage assemblages represented by the inclusions in the garnet are garnet (core), zoisite, titanite, apatite, and quartz. Mineral coexistences at the peak P-T condition are garnet rim, magnesio-hornblende, zoisite, titanite, quartz, albite, and phengite. While retrograde stage represented by the secondary minerals fill the crack in the garnet and other minerals, which are chlorite and quartz.
Due to the limiting of mineral parageneses in each stage (Table 3), P-T condition can only be interpreted from the peak metamorphic stage. The temperature condition of the garnet amphibolite is estimated using the garnet-amphibole geothermometer formulated by Graham and Powell (1984) and garnet-phengite geothermometer formulated by Green and Hellman (1982). The results give temperature ranging from 558 -606 ºC assuming pressure of 0.9 GPa (Figure 6a). The  pressure is estimated using experimental phengite geobarometer formulated by Massone and Schreyer (1987). Since this phengite geobarometer originally proposed on the limiting assemblage with K-feldspar and phlogophite, which are not observed in this rock, the estimated pressure is considered as a minimum pressure of this rock. The result gives range of minimum pressure at 0.9 GPa (Figure 6a). Petrogenetic grid of Oh and Liou (1998) suggests this P-T conditions are plotted on the epidote amphibolite-and amphibolite-facies. Chlorite presents as secondary minerals filling in the cracks of garnet and other minerals. Therefore, this rock should experience retrogression in the greenschist-facies. In order to constrain the peak P-T conditions, the observed mineral assemblages, mineral composition, and the textural relationship were compared with the stability and compositions of the phases in pseudosection. The pseudosection modeling was computed using free-energy minimization of Perple_X software (Connolly, 2005) and end-member thermodynamic data from       Table 4. The whole rock chemical composition of garnet amphibolite with sample number 18T02G (Table 5) is used in this calculation. The Fe 2 O 3 concentration is assumed and ferric-ferrous ratio [Fe 2+ /(Fe 2+ + Fe 3+ )] of 0.86 is used as proposed by Presnall et al. (1979) for basaltic MORB. Therefore, analyses using whole rock chemistry with assumed ferricferrous should be semiquantitative. The peak assemblage of the garnet amphibolite is garnet, magnesio-hornblende, phengite, zoisite, albite, titanite, and quartz. The titanite is not shown in the NCKFMASHO pseudosection, therefore the assemblage yields P-T conditions from 1.3 -1.5 GPa and 600 -640 ºC ( Figure  6b). The garnet-amphibole thermometer (Green and Powell, 1984) yields 606 ºC (P = constant). Based on the NCKFMASHO pseudosection, the garnet + zoisite + phengite + amphibole + albite + phengite + quartz appear around 1.3 -1.4 GPa at 606 ºC ( Figure 6b). The P-T conditions are speculative because the pressure for this stage cannot be estimated. The estimated P-T conditions by pseudosection (1.3 -1.4 GPa and 606 ºC) are higher-pressure condition than estimated by normal P-T diagram (~0.9 GPa and 558 -606 ºC). Ph geobarometry MS (87) Reference P = 1.4 GPa (pseudosection; Fig. 6b Graham and Powell (1984) and garnet-phengite geothermometry open circle from Green and Hellman (1982). Geobarometry of phengite from Massone and Schreyer (1987).
Unless otherwise noted, the compositional variables of x, y, and z may varybetween zero and unity and are determined as a function of the computational variables by free-energy minimization.

I J O G
Peak Metamorphic Conditions of Garnet Amphibolite from Luk Ulo Complex, Central Java, Indonesia: Implications for Medium-Pressure/High-Temperature Metamorphism in the Central Indonesian Accretionary Collision Complex (N.I. Setiawan et al.) However, both of the results are still in agreement since the estimated pressure of 0.9 GPa by phengite barometer from Massone and Shreyer (1987) is considered as a minimum pressure condition in this rock. Therefore, it might be concluded that the estimated peak P-T condition for garnet amphibolite is 0.9 -1.4 GPa and 558 -606 ºC (Figure 6a).

Discussion
Estimated peak P-T metamorphic condition for garnet amphibolite in Luk Ulo is at 0.9 -1.4 GPa and 558 -606 ºC. The estimated peak P-T temperature is higher compared to the estimated gradient geothermal of eclogite and tourmalineeclogite (Figure 7; Kadarusman et al., 2007). It could be in agreement with peak P-T condition of Jd-Qz-Gln rock (Figure 7; Miyazaki et al., 1998) assuming that the exhumation is clockwise P-T path. However in this study, any relict of HP metamorphic mineral coexist could not be found in the inclusion or early stage on garnet amphibolite. As the result, only peak P-T condition, both trajectory of clockwise and counterclockwise P-T paths are still possible in Luk Ulo Complex.
Compared to other metamorphic terranes in CIACC (Figure 1a), the MP/HT metamorphic rocks were not reported yet in the Bantimala Complex, South Sulawesi. However, these types of rock are recognized in other metamorphic complexes in South Sulawesi namely as Barru (garnet-biotite-muscovite schist; Setiawan et al., 2014) and Biru Complexes (epidote-garnet amphibolite; Jaya et al., 2017). In the Meratus Complex, this type of rock is represented by garnet-bearing epidote-barroisite schist (Setiawan et al., 2015).
The estimated peak P-T temperature of garnet amphibolite is higher compared to the estimated retrograde trajectory path of eclogites from the Bantimala Complex (Figure 7; Setiawan et al., 2016). Rubidium-Sr white mica ages of HP/LT rocks in Bantimala Complex yielded 130 -120 Ma (Böhnke et al., 2019) which is older than Luk Ulo Complex (119 -118 Ma; Hoffman et al., 2019). The P-T path of HP/LT metamorphic rocks from Bantimala and Luk Ulo Complexes also differ from the others (Kadarusman et al., 2017;Setiawan et al., 2016). The result of garnet amphibolite peak metamorphic condition emphasizes the differences between these two complexes. Consequently, strike variations of metamorphic terranes with the specific P-T stages and ages record differences in subduction zone geometry and subduction rates of a single converging system. The variations along strike subduction depend on many factors including subduction rates, obliquity along trenches, fluid flow, and subducting slab ages (Plunder et al., 2018). However, these aspects need further attention in future studies.

Conclusion
This study aimed to estimate P-T condition of garnet amphibolite in the Luk Ulo Complex. Such data give significant information constraint to metamorphic evolution in the Luk Ulo Complex. Furthermore, the results are compared to other metamorphic terranes in CIACC. The main results are summarized as follows: The P-T metamorphic condition of garnet amphibolite from Luk Ulo Complex is estimated by using mineral parageneses, thermodynamic data, and pseudosection. The estimated peak P-T condition is at 0.9 -1.4 GPa and 558 -606 ºC.
The estimated peak P-T metamorphic temperature of garnet amphibolite is higher compared to the estimated gradient geothermal of eclogite and tourmaline-eclogite in the Luk Ulo Complex.
The MP/HT (amphibolite) and HP/LT (eclogite and blueschist) metamorphic rocks in the Luk Ulo Complex would have similar Rb-Sr ages (119 -118 Ma for HP/LT metamorphic rocks and 117 Ma for epidote amphibolite) if both footwall and hanging wall had initially very high thermal gradients and the rate of subduction was very slow (10 km/Ma or less).
The estimated peak P-T temperature of garnet amphibolite in the Luk Ulo Complex is higher compared to the estimated retrograde trajectory path of eclogites from the Bantimala Complex. Hence, strike variations of metamorphic terranes, with the specific P-T stages and ages, record differences in subduction zone geometry and subduction rates of a single converging system of CIACC.