Petrochemistry of Ultramafic Rock in Baula - Pomalaa Ophiolite Complex, Southeast Sulawesi, Indonesia

Baula and Pomalaa Ophiolitic Complexes are part of East Sulawesi Ophiolite (ESO). The ultramafic rocks in the Baula and Pomalaa Ophiolite Complex mainly is peridotite and consist of harzburgite, lherzolite and olivine websterite, mostly serpentinized. Chemical and petrological research has focused on minerals, such as olivine, pyroxene, and spinel. This study examines the tectonic setting and temperature of ultramafic rock formation. Twelve ultramafic rock samples were examined using geothermometers made of pyroxene, petrographic examination, and coexisting olivine and spinel analyses. SEM and petrographic analysis of pyroxene lamellae and mylonite-ultramylonite structures allowed for the measurement of the geothermometer of ultramafic rocks. Using SEM-EDS, the coexistence of olivine and spinel was analyzed to determine the type of ultramafic tectonic setting. In the coexistence of olivine and spinel, olivine and spinel oxide compounds as tectonic setting markers in the form of Fo and Cr# values. Ultramafic rocks have different temperature levels, based on pyroxene thermometer, and the first one starts at a high temperature of 1000-1200ºC. It is characterized by thin, elongated augite lamellae. Instead, large lamellae characterize augite at medium temperatures (800–1000ºC). Irregular, anhedral, and broader forms of enstatite lamellae are typical of low temperatures (500–800ºC). Different generations of exsolution lamellae indicate that magma cooling was gradual. The distribution of #Fo ranged from 0.87 to 0.92, and Cr# values ranged from 0.13-0.19. According to coexisting olivine and spinel analysis. On the Olivine-Spinel Mantle Array (OSMA), the Fo and Cr# plot indicates that the peridotites tectonic setting was from the ocean floor and the magmatism was from MORB (Mid Oceanic Ridge Basalt). The Al 2 O 3 vs. TiO 2 pattern in spinel lherzolite also similar with Ampana and Kabaena peridotites magmatism.


Introduction
In Indonesia, Ophiolite complexes are located in Meratus Mountains, eastern and southeastern Sulawesi, Halmahera, and Papua.Some ophiolite complexes are associated with melange, such as in Ciletuh (West Java), Karangsambung (Central Java), and Bantimala (Sulawesi) (Surono and Hartono, 2013).The ophiolite complex on Sulawesi Island is known as the East Sulawesi Ophiolite Belt (ESOB) or the East Sulawesi Ophiolite Belt (LOST).The LOST extends for 500 km along the eastern arm to the southeastern arm (Parkinson, 1998).The LOST can be traced from the East Arm of Sulawesi, starting from Poh, Bunta, to Ampana and Morowali.In the central part of Sulawesi, LOST is distributed in Kolonodale, Bungku, and Kendari.The west coast of the Southeast Arm of Sulawesi, such as Lasusua, Kolaka, and Kabaena (Kadarusman et al., 2004, Hamilton, 1979).
The displacement of ultramafic rocks is important for the study of petrology and petrogenesis because mineralogical parameters related to changes in temperature and pressure during the rock formation process (geothermometer) and displacement can tell us about the early genesis of peridotite rock formation and its tectonic evolution (Lindsley and Andersen, 2012).The formation temperature of ultramafic rocks can be identified based on pyroxene minerals that exhibit lamellar exsolution textures between clinopyroxene and orthopyroxene (Yellappa et al., 2021, Koizumi et al., 2014, Lindsley, 1983).The exsolution process usually occurs due to lamellae growth in the crystal of origin during the cooling process.The determination of the tectonic environment of ultramafic rocks is based on the presence of coexisting olivine and spinel minerals (Olfindo et al., 2020, Payot et al., 2018, Arai, 1994).
The research location for the study of petrology and mineral chemistry of ultramafic rocks is at Pomalaa and Baula, Kolaka Regency, Southeast Sulawesi Province.In this area, there are widely exposed ultramafic rocks, which have generally been serpentinized (Jaya, 2017, Simandjuntak et al., 1993).The research proposal is predicated on the dearth of pertinent studies about the petrology and mineral chemistry of ultramafic rocks in this region.
In this study, the formation temperature of ultramafic rocks was analyzed by making petrographic observations and analyzing pyroxene lamellae (geothermometer) using SEM.This data was then combined with the results of chemical analysis of coexisting olivine-spinel to obtain an overview of the tectonic environment of the study area.

Methodology
A descriptive approach to field geology methods, supported by laboratory analysis, is used to conduct this research.The percentage of mineral composition in petrographic observations has been the subject of quantitative research.Megascopically and microscopically detailed descriptions of ultramafic rock samples serve as primary data.
Fresh ultramafic rocks in the study area are the focus of research.The object was then concentrated on the orthopyroxene-clinopyroxene minerals, which form lamellar exsolution textures, as well as the coexisting olivine and spinel minerals.Thin sections of ultramafic rocks show the presence of these textures.The microscopic description of ultramafic rocks is based on the work of Streckeisen (1976).
The petrographic analysis uses Nikon Eclipse E-100 polarizing microscope at the Geological Engineering Laboratory of Halu Oleo University, with thin sections prepared at the Petrology Laboratory of Hasanuddin University, Makassar.Observed optical properties include crystal shape and size, cleavage, relief, index of refraction, absorbs and pleochroic color, interference color, optical orientation, twinning, and extinguishing angle.This description seeks to identify various characteristics of the olivine and pyroxene minerals present in the thin section, such as specific mineral types, distinctive textures, and other distinguishing characteristics.
If two pyroxene minerals are observed coexisting and forming lamellar exsolution, the formation temperature (geothermometer) of ultramafic rocks can be determined.The two pyroxene minerals analyzed contain clinopyroxene lamellae within an orthopyroxene mineral host.SEM and polarizing microscopy were used to examine these two pyroxene minerals.The formation temperature of ultramafic rocks requires a descriptive analysis based on the texture and chemical composition of lamellar pyroxene.
A Phenom ProX SEM equipped with an EDS detector was used to determine the mineral's chemical composition.Utilizing the EDS detector, chemical data on minerals was gathered.With 15 kV of accelerating voltage and 30 mA of probe current, mineral chemistry tests were conducted in a high vacuum environment.There are sixteen firing points (beams), each of which is composed of a lamellar-forming orthopyroxeneclinopyroxene mineral and a coexisting olivine-spinel mineral.The elements measured include Oxygen (O), Magnesium (Mg), Silicon (Si), Iron (Fe), Nickel (Ni), Calcium (Ca), Aluminum (Al), Manganese (Mn), Chromium (Cr), Titanium (Ti), Sodium (Na), Potassium (K), Zinc (Zn), and Vanadium (V) (V).The chemical data is presented as oxide compounds with weight percent (wt percent) units, including SiO2, MgO, FeO, NiO, CaO, MnO, Al2O3, Cr2O3, TiO2, K2O, Na2O, ZnO, and V2O3.The weight percent (wt percent) data were analyzed and processed in order to determine the mole percent (percent mol) of Mg (percent En), Fe (percent Fs), Ca (percent Wo), Cr#, Mg#, and others.
Together-growing olivine and spinel minerals (coexisting) were used to determine the tectonic environment of ultramafic rocks in the study area.Using SEM-EDS analysis, chemical data were obtained based on the work of Arai (1994).

Geomorphology
The geomorphology of the Baula-Pomalaa area, Kolaka Regency, consists of hills, valleys, and lowlands at 0-800 meters above sea level elevation.Based on van Zuidam (1986) classification, the study area is divided into three geomorphological units, i.e. structural morphology (55%), which is spread in the central and northeast-southeast part of the study area, denudational morphology (40%) which is distributed in the west-southwest part of the study area, and fluvial system (5%) which is spread in the northwest part of the study area.River systems consist of rectangular and dendritic patterns.Rectangular type is found almost throughout the study area, with igneous and metamorphic rocks.The dendritic flow pattern is located southwest to northwest of the study area, consisting of conglomerate rocks and alluvial deposits.

Stratigraphy
The stratigraphy of the Baula-Pomalaa area comprises ultramafic rocks, metamorphic rocks, monomic conglomerates, polymictic conglomerates, and alluvial deposits.Ultramafic rocks consist of peridotite.The primary minerals of ultramafic rocks are olivine, pyroxene, and spinel.The secondary minerals consist of serpentine, chlorite, and talc.Both ultramafic and metamorphic rocks are separated by structural contact of fault.In general, ultramafic rocks have undergone partial or complete serpentinization and have been weathered into laterite deposits.Ultramafic rocks occupy ~40% of the study area (Fig 1).Conglomerates in study area are consist of two type, monomic and polymictic conglomerates.Monomic conglomerates are found in isolated intra-mountain depressions and rivers with >10m fault scarp.Isolated intra-mountain basin formed by tectonic uplift of ophiolite onto metamorphic basement.During that time, conglomerate was formed by ultramafic fragments which deposited in the basin (Surono, 2013).The conglomerates grain fragments are dominated by ultramafic rocks and serpentinite (>90%).The grain size is sand-boulder (>30 cm), dark brown-brown in color, non-carbonate cement, and poor sorted and generally distributed in the southern part and slightly in the north of the study area.
The polymictic conglomerate is found in low-elevation areas in the south-southwest and slightly north of the study area.This area is located outside of isolated basin.Its grain consists of ultramafic, schist, serpentinite, and limestone lithics with boulder-sand size (<30 cm), light grey-brown, non-carbonate cement, and poorly sorted.In some locations, these rocks have not consolidated well.
Alluvial deposits are found in the north and along the coastline of the study area.This area is generally for residential use.The alluvium unit consists of brownish-grey to reddish clay-boulder loose material.

Geological Structure
Geological structures that develop in the study area are strike-slip and thrust faults (Fig 1).The regional tectonics influences the formation of the geological structure of the study area in Sulawesi, which has the direction of the regional structure northwest-southeast.Thrust faults are generally northwest-south-southeast and northeast-southwest, and strikeslip faults are northwest-southeast and northeast-southwest.The formation of structures in the study area is closely related to the collision direction between Southeast Sulawesi and the Buton Microcontinent Fragment which occurred during the Early Miocene and continued until the Late Miocene.

Petrography of Ultramafic Rocks
The ultramafic rocks in the Baula Pomalaa Ophiolite Complex mainly is peridotite and consist of harzburgite, lherzolite and olivine websterite, mostly serpentinized.Ultramafic rocks are structurally adjacent to metamorphic rocks; in some locations, the ultramafic rocks are fully serpentinized.Conglomerates are divided into polymictic and monomic conglomerates, with a predominance of ultramafic grains or fragments.Monomic conglomerates are commonly found in isolated intra-mountain depressions and rivers with >10m inclines.The grain fragments are dominated by ultramafic and serpentinite rocks (>90%).Polymictic conglomerates are found in low elevation areas, outside of isolated basin, with ultramafic rocks, schist, serpentinite and limestone grains.
At some contacts between olivine and orthopyroxene, olivine is also present as milonite and ultramilonite (Fig a).Ultraminolites indicate an intensive deformation process in the shear zone (Matysiak and Trepmann, 2015).The magnetite present is a byproduct of the serpentinization process (Maulana et al., 2015).

5.
Mineral Chemistry of Ultramafic Rocks

Orthopyroxene
The chemical data of orthopyroxene minerals in ultramafic rocks of the Baula and Pomalaa Regional Ofiolite Complex can be seen in Table 3.The orthopyroxene mineral chemistry data is normalized (6 oxygen) with a total cation of 4 apfu.Orthopyroxene is present in all ultramafic rock incisions and is of the Mg-rich enstatite type with compositions En87,1-88,5, Fs9,1-10,8, Wo2,05-2,9.XMg range ~0.92.

Chromite-spinel
The chemical data of spinel mineral of ultramafic rock of the Ophiolite Complex of Baula and Pomalaa Area can be seen in Table 2.The spinel mineral chemistry data (4 oxygen) is normalized with a total cation of 3 apfu.All spinel minerals analyzed at the study sites are chromite-spinel type ([Mg,Fe][Cr,Al]2O4) with Cr# <0.2.

Geothermometer
The geothermometer or formation temperature of ultramafic rocks in this study was determined using two pyroxene thermometers, lamellar exsolution between orthopyroxene and clinopyroxene (Lindsley, 1983).The formation temperature of ultramafic rocks can be obtained through lamellar exsolution of orthopyroxene and clinopyroxene, provided that the content of wolastony + enstatite + ferrophyllite amounts to ≥90%.The diagram used in determining the formation temperature is the enstatite-diopsidhedenbergite-ferosilite (En-Di-Hd-Fs) quadrilateral diagram with a pressure of 5 kbar (Lindsley and Andersen, 2012).The quadrilateral diagram of En-Di-Hd-Fs at a pressure of 5 kbar is used based on Kadarusman et al. (2004) on the metamorphism process of amphibolite facies in the ophiolite complex in Sulawesi that occurred at a pressure of about 4 kbar.
The low-temperature rock formation data, which has a temperature interval of enstatite crystallization between 500-800ºC.The lamellar texture shown in the low-temperature data (Fig ) is interpreted to have been disturbed by deformation and formed after reheating (Pittarello et al., 2019) due to deformation.Deformation is characterized by the irregular shape of the lamellae and wider dimensions compared to other lamellae.The reduction of calcium (Ca) changes the shape and dimension of lamellae (Rajesh, 2006).The geothermometer estimated pyroxene equilibration temperatures from 1000°C to 1200°C.These temperatures reflect magma-induced pyroxene crystallization.Several lamellar generations of pyroxenes formed at lower temperatures, from 500°C to 1000°C, as the magma cooled.Pyroxenes crystallized over a long cooling period, with different lamellar generations reflecting undercooling and progressive or continuous exsolution (Rajesh, 2006).The thin primary pyroxenes suggest low fractionation at high temperatures, while the broader lines between host and lamellae phases indicate slow cooling that allowed various exsolution textures.Due to slow cooling, different generations of exsolution lamellae developed with different temperatures.

Petrogenesis of Ultramafic Rocks
According to petrographic analysis, peridotite dominates ultramafic rocks in the study area.The rocks found are peridotite (harzburgite and lherzolite) and pyroxenite (olivine websterite).The absence of garnet in the ultramafic rocks of the Baula and Pomalaa Ophiolite Complexes indicates that the ultramafic rocks have yet to undergo a high degree of metamorphism.The dominant presence of serpentines in all samples indicates intensive metamorphism at low temperatures.Chlorite and serpentine characterize the continuation of retrograde metamorphism at low temperatures (Frost et al., 2013, Arai, 1994).Serpentine+tremolite+magnetite+talc, especially at the pyroxene rim, indicates a reaction between pyroxene minerals and seawater (Zeng et al., 2012).Another source could also be the decomposition of water from previous serpentine formations.These processes likely occurred during the uplift of the ophiolite from the upper mantle to the surface (Arai et al., 2008).The absence of plagioclase in ultramafic rocks indicates that the uplift process of these ultramafic rocks occurred rapidly (Maulana et al., 2015).
The tectonic setting of ultramafic rocks (Fig ) was determine by putting the Cr# spinel and Fo olivine data from the mineral chemistry of coexisting olivine and spinel (Table 2) onto the Olivine Spinel Mantle Array plot (Arai, 1994).Based on these data, it can be seen that the pattern of olivine spinel chemical data in the study area is similar to the four types of theoretical tectonic environments: ocean floor, oceanic hotspot, Japan arcs, and continent.Based on the chemical data pattern of these tectonic environments, in the oceanic hotspot type, the weight percent of TiO2 is ~1 wt%.The Japan arcs type has several Cr# values >0.5 and Fo <0.87.In the continent type, there are Cr# values >0.5 even up to 0.75; in the ocean floor type, the Fo values range from 0.88-0.92and Cr# 0.1-0.6.The chemical data distribution of the study area has TiO2 ~0.03 wt%, Fo 0.87-0.92,Cr# 0.13-0.19,and none of Fo <0.87.Based on the distribution pattern of mineral chemical data, it can be interpreted that the ultramafic rocks in the study area are suitable for the ocean floor type.The Al2O3 vs TiO2 chemical data of the peridotite spinel minerals of the study area were plotted using plot from Kamenetsky et al. (2001) to determine the tectonic setting of the peridotite rocks.The data was then compared with spinel mineral data on Kabaena and Ampana lherzolites (Kadarusman et al., 2004), which have similar distribution patterns (Error!Not a valid bookmark self-reference.).Al2O3 spinel data distribution in the study area is 46.16-53.64wt% and TiO2 ~0.03 wt%.Based on these data, ultramafic rocks are interpreted as the MORB peridotite type.
The speed of the ocean floor spreading results in different characteristics of the ophiolites (Poli andSchmidt, 2002, Pearce et al., 1984).The ophiolite complexes formed through fastspreading centers are characterized by harzburgite, poor clinopyroxene, and dunite (Maulana et al., 2015).The Baula Pomalaa Ophiolite Complex generally consists of harzburgite and lherzolite with abundant clinopyroxene minerals.The dismembered ophiolitic sequence and the abundance of clinopyroxene (augite or diopside) in the ultramafic rocks indicate that the ophiolite sequence formed through slow spreading center mechanism (Gong et al., 2016, Maulana et al., 2015).The pyroxenite (olivine websterite) also indicates that the Baula Pomalaa Ofiolite Complex formed at a slow spreading center.
The formation process of ultramafic rocks can be interpreted based on the formation temperature of lamellar exsolution between orthopyroxene and clinopyroxene minerals.Ultramafic rocks in the study area formed at a temperature of ~1200°C.This temperature is the crystallization temperature of pyroxene minerals.Due to slow cooling, different generations of exsolution lamellae developed with different temperatures.
The chemical data of the ultramafic rocks of the Baula Pomalaa Ofiolite Complex show some similarities with several other ophiolite complexes on Sulawesi Island.The ultramafic rocks of the Southeast Arm of Sulawesi are part of the Cretaceous-aged East Sulawesi Ophiolite (ESO) (Surono, 2013).The data distribution plot of Al2O3 vs TiO2 spinel peridotite of the study area has similarities with the Kabaena and Ampana patterns, which have previously been studied by Kadarusman et al. (2004) and belongs to the East Sulawesi Ofiolite, i.e. the peridotite at the site is sourced from the MORB peridotite.Based on the data plot of Cr# vs Fo and Al2O3 vs TiO2 (Fig 6), it can be concluded that the tectonic setting of ultramafic rocks in the study area is ocean floor with magmatism sourced from MORB (Mid Oceanic Ridge Basalt).

Conclusion
Ultramafic rocks in the Baula-Pomalaa Ofiolite Complex are dominated by peridotite (harzburgite, lerzolite) and pyroxenite (olivine websterite).Ultramafic rocks have undergone serpentinization and low-degree metamorphism as evidenced by serpentine, talc, and amphibole minerals (tremolite and hornblende).Based on the geothermometer analysis, the formation temperature of ultramafic rocks is divided into three groups, i.e., high temperature (1000-1200°C) as the initial rock formation temperature characterized by thin elongated lamellae.Intermediate temperatures (800-1000°C) are characterized by a relatively wide type of lamellae in orthopyroxene.Anhedral and wider irregular lamella types characterize low temperatures (500-800°C) which formed after pyroxenes reheating (Pittarello et al., 2019) due to tectonic movement.Coexisting olivine and spinel analysis shows a distribution of Fo values ranging from 0.87-0.92and Cr# values ranging from 0.13-0.19.The results of the tectonic setting plot on the Olivine-Spinel Mantle Array (OSMA) obtained that the tectonic setting of ultramafic rocks in the study area belongs to the ocean floor with magmatism sourced from MORB (Mid Oceanic Ridge Basalt).

Fig 1 .
Fig 1. Geologic map and distribution of ultramafic rock sample in the study area Metamorphic rocks in the study area consist of schist and alteration rock serpentinite.Serpentinites are composed of serpentine minerals such as lizardite and chrysotile.Muscovitequartz schists dominate the schists in the study area and belong to the greenschist facies.The schists are scattered in the eastern part of the study area.Conglomerates in study area are consist of two type, monomic and polymictic conglomerates.Monomic conglomerates are found in isolated intra-mountain depressions and rivers with >10m fault scarp.Isolated intra-mountain basin formed by tectonic uplift of ophiolite onto metamorphic basement.During that time, conglomerate was formed by ultramafic fragments which deposited in the basin(Surono, 2013).The conglomerates grain fragments are dominated by ultramafic rocks and serpentinite (>90%).The grain size is sand-boulder (>30 cm), dark brown-brown in color, non-
f). Orthopyroxene is present as subhedral-anhedral crystals of 0.5-5 mm in size, mostly are enstatite.Tremolite-talc and olivine ultramilonite are present at the rim of enstatite in contact with olivine (Fig b).One of the enstatite crystals was also found to have a kinkband texture, and some had undulate extinction (Fig b).These textures are characteristic of rigid/brittle deformation (Kadarusman et al., 2004).Augite is present as subhedralanhedral crystals of 1-5 mm size and as lamellar exsolution within the host mineral enstatite (Fig b and 2e).Hornblende was found to start partially replacing augite (Fig c).Chromite-spinel is found generally coexisting with olivine (Fig c).Olivine typically has euhedral-anhedral crystals measuring 0.1-3 mm, altered to serpentine and chlorite, forming vein and mesh textures (Fig c and 2f).

Fig 4 .
Fig 4. SEM EDS observation of point PMS 01 showing some lamellae textures

Fig 5 .
Fig 5. Plot of Cr# spinel vs Fo olivine of ultramafic rocks of Baula-Pomalaa Ofiolite Complex using OSMA diagram.The red shading color is the result of this study and the yellow shading color is the data from Arai (1994).

Fig 6 .
Fig 6.Plot Al2O3 vs TiO2 in spinel peridotite of the study area using graphs fromKamenetsky et al. (2001) compared with data of Al2O3 vs TiO2 in spinel peridotite of Kabaena and Ampana fromKadarusman et al. (2004)