1
Adi Maulana 1 , David J Ellis 2 , Andrew C Christy 2
1 Geology Department, Hasanuddin University, Makassar, Indonesia
2 Research School of Earth Science, ANU, Canberra, Australia
Contact Author : adi-maulana@unhas.ac.id
ABSTRACT
This study reports the pre-Tertiary tectonic evolution of the South Sulawesi basement rocks complexes constraints from petrological and geochemical data, particularly from the metamorphic rocks. The pre-Tertiary basement rocks complexes in South Sulawesi consist of two separated blocks, Bantimala and Barru block. The metamorphic rocks assemblages from these two blocks show quite different characteristic and different metamorphic history. Whole rock geochemistry indicates that both the Bantimala and Barru blocks were accreted slices from a wide range of tectonic environments. Five different tectonic settings for protolith have been recognised in the Bantimala block, mid oceanic ridge basalt, oceanic island basalt, island arc volcanics, cumulates and continental granodiorities or sediments, and is dominated by the oceanic basalt types. Conversely, the quartzofeldspathic gneisses that make up much of Barru are more felsic and show a consistent arc affinity. The Barru block also shows intrusion of late dacites cutting through the ultramafics, which is not known from Bantimala. Both blocks are heterogeneous and have complex accretion histories. Although roughly the same age
(Cretaceous) and probably both situated at the southeast margin of Sundaland, they may not have been geographically as close as they are now. The Bantimala block records deep subduction of cold ocean floor including MORB, arc-related lavas and seamounts, and exhumation of deeply subducted material, prior to collision with microcontinents to the East and obduction of the ultramafics. Conversely, the Barru block is interpreted to preserve the roots of an old island arc, subduction of some ocean floor with seamounts, and obduction of quite different ocean floor material from the North, and was evidently too warm to preserve blueschist or eclogites.
Therefore, these two blocks must have derived from different sources and tectonic setting.
Keywords: Tectonic evolution, South Sulawesi, Basement rock.
Geological complexity of Sulawesi, the four-armed shape island in the centre of the Indonesian archipelago, has been widely well known (Hamilton, 1979; Sukamto, 1982; Hall & Wilson,
2000). The distinctive multi-armed shape of this island suggests that it is a complex assemblage of tectonic terranes, the details of which are still not fully elucidated (van Leeuwen & Muhardjo,
2005). This region shows evidence of plate convergence involving subduction of oceanic plate
(Hamilton, 1979; Katili, 1978; Wakita et al., 1996), continent–continent collision (Bergman et al., 1996), arc–continent collision (Elburg et al., 2002), sediment accretion and emplacement of dismembered ophiolites (Kadarusman et al., 2004; Villeneuve et al., 2002) and exhumation of high pressure metamorphic rocks (Parkinson, 1996; Wakita, 2000). Based on the overall
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2 geological framework that has emerged from these studies, lithotectonic Sulawesi can be divided into four (4) tectonic provinces, namely (1) the Western and North Sulawesi Pluto-Volcanic Arc,
(2) the Central Sulawesi Metamorphic Belt, (3) the East Sulawesi Ophiolite Belt and (4) the
Banggai-Sula and Tukang Besi continental fragments (Fig.1).
The Tertiary collision(s) have disrupted or buried much of the Mesozoic continental margin, but fragments of pre-Tertiary basement are preserved in the Western Sulawesi volcanic arc province, two examples being those in the Bantimala and Barru areas (Hamilton, 1979; Parkinson, 1998;
Parkinson et al., 1998; Sukamto, 1982; Wakita, 2000; Maulana, 2009). These two isolated blocks, about 30 km apart, formed the South Sulawesi basement rocks which have historically been referred to collectively as the Bantimala Basement Complex.
The basement complexes in these two areas have received increasing attention since the first geological mapping in Sulawesi. A wide variety of lithologies of various ages (Hamilton, 1979;
Miyazaki et al., 1996; Parkinson & Katayama, 1999; Parkinson et al., 1998; Sukamto, 1975; van
Leeuwen, 1981; Wakita et al., 1996; Wilson & Bosence, 1996), paleogeographic anomalies
(Haile, 1978; Sasajima et al., 1980) as well as the regional structure (Berry & Grady, 1987) has been recorded. Details of their history remain uncertain (van Leeuween, 1981), and previous studies have regarded Bantimala and Barru blocks as being exposed parts of a single larger unit, partially exhumed after small-scale eastward subduction in the Makassar Strait (Sukamto,1975;
Katili, 1978; Guntoro, 1999).
Figure 1 Tectonic Setting of Geology of Sulawesi (modified after Hamilton, 1979; Hall &
Wilson, 2000).
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These basement rocks are of particular interest since they provide an insight into the geological processes that were operating at the eastern margin of Sundaland in the Late Mesozoic.
However, no detailed petrological, mineralogical or geochemical data from these two blocks have been reported to date, despite the fact that they can provide important insights into the
Mesozoic evolution of the Indonesian region.
This study is primarily concerned with the metamorphic evolution of the South Sulawesi basement rock complexes. This study will present an overview of the petrographic and geochemical analyses to constrain the pre-Tertiary tectonic evolution of the South Sulawesi basement rocks. The study will be mainly based on a comprehensive on the basement rocks from
South Sulawesi study by Maulana (2009) and Maulana et al. (2014).
RESULTS AND DISCUSSION a. Petrography
Bantimala Block
Based on the petrography, the rocks assemblages in the Bantimala block can be classified into (i) very weakly metamorphosed sedimentary rocks, (ii) metasedimentary rocks, (iii) metabasites (iv) meta-ultramafic rocks and (v) ultramafics rocks (Table 1).
Table 1. Lithologies and localities of the metabasic rocks in the Bantimala blocks
Block Lithology
Rock types
Very weakly metamorphosed sedimentary rocks
Radiolarian chert
Sandstone
Mudstone
Metasedimentary rock
Metabreccia
Metabasic rock
Analyses a a a a
Locality
1,2
1
1
1
High-pressure
Eclogite
Blueschist
Low Pressure
Greenschist
Meta-ultramafic rocks
High-pressure
–
Low Pressure
Tremolite schist
Serpentinite
Ultramafic rock
Olivine clinopyroxenite a,b,c a,b,c a,b,c a,b,c a,b,c
1,2,3
1,2
1,2,4
4
4
Dunite a,b,c 5
Harzburgite
Podiform chromitite
Notes: a: Petrography, b: SEM, c: Bulk chemistry and trace element. 1: Cempaga and Pateteyang Rivers, 2 : Bantimala River, 3:
Batupute River, 4. Batugarencing Hill, 5: Moreno Hill.
The sedimentary lithologies in this block include radiolarian chert, sandstone and mudstone which were weakly metamorphosed. The sandstone includes sample BM09 consists of poorly sorted, subrounded to angular quartz and feldspar grains (up to 80%), with flakes of muscovite and chlorite. Some accessory iron oxides are also present. Examples of mudstone are samples CP02A and CP02C from the Cempaga River, which consist of silicified clay minerals crosscut by quartz and calcite veins. The intense silicification and presence of chlorite in the matrix, as well as calcite veining suggest that these samples underwent low grade metamorphism to the lower greenschist facies.
The metamorphosed sedimentary rocks include metabreccia (CP01) and was found as an isolated block underneath the interbedded mudstone and chert in the Cempaga River section, Bantimala block. It is a highly deformed and altered rock with brecciated texture, composed of angular to subangular lithic clasts (quartzofeldspathic schist, quartzite, mica schist and granitoids) within a matrix rich in quartz grains. Minerals in lithic fragments include quartz, somewhat sericitised
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4 plagioclase laths (up to 0.5 cm in size) and sericitised chlorite which suggests greenschist facies metamorphism.
The metabasic rocks of the Bantimala block show assemblages characteristic of three different metamorphic facies: eclogite, blueschist and greenschist. In most samples, an overprint of metamorphism in a second facies can be identified, particularly for the high-pressure rocks.
Maulana (2009) has reported detail mineral assemblages for each facies, along with the metaultramafic and ultramafic rocks and mineral paragenesis.
The eclogites occur as tectonic blocks within blueschist, typically several metres across.
In addition to garnet + omphacite + quartz ± glaucophane + rutile as primary phases, many of these eclogites contain combinations of chlorite, actinolite, epidote-group minerals and other secondary phases as a result of retrogression. On the basis of mineralogy, the eclogites can be divided into two types: glaucophane eclogite and glaucophane-free eclogite (Fig.2.a and b).
Aluminosilicate minerals (kyanite, sillimanite and andalusite) have not been found in any samples.
Blueschist-facies rocks in the Bantimala block are distinguished from eclogites by the absence of omphacite. They are composed mainly of glaucophane with garnet or albite and other minerals, but without omphacite except in BM04, where omphacite occurs as inclusions in garnet, suggesting overprinting on eclogite assemblage. They are often characterised by pseudomorph formation, such as albite and epidote pseudomorphs after garnet, and also zoisite/epidote after probably lawsonite. Based on mineralogical assemblages, two major types of blueschist have been identified: albite-epidote-glaucophane schist and quartz-glaucophane schist (Fig.2.c). The greenschist-facies rocks contain actinolite + albite + epidote + chlorite assemblages with some samples showing relict igneous minerals in metabasic rocks.
Greenschist-facies meta-ultramafic rocks, discussed separately below, are characterised by tremolite and serpentine. Lithologies include actinolite schist, actinolite-epidote schist, augitebearing actinolite schist, quartz-epidote chlorite schist, quartz-mica schist and metabasalt for metabasic and tremolite schist and serpentinite for meta-ultramafic rocks.
The ultramafic rocks have been metamorphosed to greenschist facies, evidenced by the occurrence of tremolite and chloritised serpentinite. Tremolite schist (BGR02) and serpentinite were found on the Batugarencing Hill. The low to moderately metamorphosed ultramafic rocks occur on the Moreno Hill as olivine clinopyroxenite and serpentinised peridotite (harzburgite, lherzolite and dunite). Podiform chromitite occurs as lens shaped blocks or nodules within the clinopyroxenite and enveloped by dunite (Fig.2.d). a. b
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Figure 2. Photomicrograph of the metabasic rocks from the Bantimala block a.
Porphyroblastic texture with garnet porphyroblast in glaucophane and omphacite matrix (CP03E) from glaucophane-bearing eclogite. b.
Pseudomorph of epidote and chlorite after garnet in glaucophane-free eclogite
(BM15C). c. Photomicrograph of albite-epidote-glaucophane schist (sample
BML01) with crossed polars. Albite (Ab) orange in colour, and epidote (Ep) pseudomorphic after possible lawsonite in the matrix of glaucophane (Gln) and chlorite (Chl). d. Photomicrograph of olivine and clinopyroxene with Cr-rich spinel (Cr-spl) in pyroxenite (MOR02A) with crossed polars. Olivine has been partially replaced by serpentine (Srp).
Barru Block
Unlike the Bantimala block, the metabasic rocks in the Barru block does not have blueschist- or eclogite-facies rocks, but is made up of very weakly metamorphosed sediments, greenschist- and amphibolite facies metabasic and quartzofeldspathic gneiss rocks, and peridotite. The peridotite is intruded by dacitic igneous rock. Lithologies, localities and analyses of the rock assemblages from the Barru blocks are summarised in Table 2.
The very weakly metamorphosed sedimentary rocks in the Barru block include radiolarian chert and breccia. The breccia (samples AM04 and AM12) is composed of rock fragments of schist, gneiss, quartzite and marble of 2.5 – 5 mm size, in a matrix of smaller (0.5 – 2.5 mm) clasts of sandstone, chert and quartzofeldspathic rock, which in turn are set in a groundmass of serpentine.
Table 2. Lithologies and localities of the metabasic rocks in the Barru blocks
Block Lithology
Rock types
Very weakly metamorphosed sedimentary rocks
Radiolarian chert
Breccia
Metabasic rock
High-pressure
–
Low-pressure
Greenschist
Amphibolite
Quartzofeldspathic gneiss
Low-pressure
Gneiss
Igneous rocks
Dacite
Analyses a a a,b,c a,b,c a,b,c
Locality
6
6
6
9
6 a,b,c 6,8
Ultramafic rock
Spinel lherzolite 10,7
Podiform chromitite a,b,c
7
Harzburgite
Notes: a: Petrography, b: SEM, c: Bulk chemistry and trace element. 6: Dengenge River, 7 : Sabangnairi Hill, 8: Camming
Village, 9. Lasitae Mountain, 10. Kamara Village.
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6
Metabasic rocks in the Barru block show greenschist- and amphibolite-facies metamorphism only, with no high-pressure blueschist or eclogite assemblages (Fig.3.a and b.).
Figure.3. Photomicrograph of the metabasic rocks from the Barru block a. Photomicrograph of quartz mica schist (DNG03) with crossed polars light. Garnet (Grt) occurs as grains in the matrix of phengite (Phn) and chlorite (Chl) layers which define the foliation. b. Photomicrograph of amphibolite (AM14) with crossed polars light. It consists of hornblende (Hbl) with calcite (Cal) and plagioclase (Pl) grains.
They were found in the Dengenge River section, and show a north to north-westward increase in metamorphic grade. The greenschist-facies rocks mainly occupy the southeast region of the block, whereas amphibolite was found to the northwest, adjacent to the ultramafics. The greenschist-facies rocks include chloritic phyllite, quartzite, chlorite schist, garnet-mica schist and quartz-mica schist. The amphibolite facies in metabasic rocks is represented by amphibolite
(AM 14) outcropping in the Dengenge River. The quartzofeldspathic gneiss samples are AM06 and AM08 and AM09SC. They consist of 30 – 35% plagioclase (oligoclase), 35 – 40% quartz, 8
– 10% biotite, 5 – 10% garnet, 5 – 10% epidote, 2 – 5% chlorite and < 5% muscovite. Zircon,
Mn-rich ilmenite and rutile occur as accessory minerals. Alternation of plagioclase and quartz bands with biotite and sometimes muscovite layers defines the foliation.
Generally, the ultramafic rocks of the Barru Block have been strongly serpentinised (70 –
90%) and metamorphosed to low temperature (greenschist) or moderately high temperature
(amphibolite) facies. The ultramafic rocks are of spinel lherzolite and harzburgite composition.
Podiform chromitite sometimes occurs as lenses or nodules within peridotite at Sabangnairi Hill and Kamara Village.
The igneous rocks in the Barru Block include samples AM15 and BR04 which were found in the Dengenge River and at Camming Village respectively. They occur as a large intrusion cutting the metamorphic and ultramafic units. b. Geochemistry
Bantimala block
The geochemical analyses of this study can be shown in Maulana (2009). It was shown that the metabasites are derived from products of five different tectonic settings: cumulates, mid oceanic ridge, oceanic island basalt, volcanic island arc, and continental rocks (Table 3).The
Bantimala Block shows a strong tectonic fabric striking NNW-SSE, and most major geological boundaries within the block strike in this direction. The ultramafics are at the eastern margin of
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7 the block, and are thrust over the metamorphic rocks lying to the west. Therefore, the overall scenario is one of subduction of other rocks and obduction of ultramafics, in a westward direction, with rapid uplift of parts of subducted slab, metamorphosed to blueschist or eclogite facies. However, the detailed picture is more complex.
Trace element data show that the MORB basalts were of both normal and enriched types, which may imply two distinct episodes of ocean floor subduction. Seamounts were present on the ocean floor, bringing in basalt with an oceanic island signature. Some of these were subducted to eclogite-facies depths (BM15B), while others, perhaps due to their thickened crust, were barely subducted or deformed, and retain pseudomorphs after the original igneous texture
(CP02D). The 'trachyandesitic' greenschist CP03A may have been derived from a continental fragment, which again was not subducted deep. The glaucophane-free eclogites have cumulate signatures, and many of the blueschists and greenschists are derived from island arc volcanics spanning a composition range from picrobasalt to dacite. Hence, there has been back-arc spreading and arc development in the Bantimala complex, followed by later closure of the basin.
Each of the tectonic settings was represented in two or three of the river sections of this study, implying that small tectonic slices have been intimately mixed in this accretionary complex.
In additional to the meta-igneous rocks, there are bedded cherts in Bantimala which represent pelagic sediments. The presence of continental-derived clastic rocks (meta-breccia, sandstone and mudstone) suggests that continental terrigenous sources got close to the trench as the ocean basin closed. Close proximity to a continent is also supported by quartz-epidotechlorite schist BML03A with its continental affinity. It is noteworthy that marble is rare in this block and was not found in this study, which may indicate that the seafloor was deeper than the carbonate compensation depth for most of subduction. Despite this, the large proportion of OIBderived material indicates that topographically high seamounts were frequent.
Table 3 Mafic to felsic meta-igneous rocks of Bantimala Block, classified by reconstructed protolith from trace element data, bulk composition and metamorphic grade.
Deduced protolith
E-MORB
E-MORB
N-MORB
N-MORB
Gabbroic cumulate
Gabbroic cumulate
OIB
OIB
OIB + Na metasomatism
Highest P of metamorphism
Eclogite
(now mainly blueschist)
Eclogite
Eclogite
Blueschist
Eclogite
Eclogite
Blueschist
Greenschist
Greenschist
Greenschist
Lithology
Ab-Ep-Gln schist with eclogite relicts
Gln-Eclogite
Gln-Eclogite
Ab-Ep-Gln schist
Gln-Eclogite
Gln-free Eclogite
Ab-Ep-Gln schist
Act schist with relict augite
Metabasalt with relict igneous texture
Ab-Act schist Ultramafic cumulate + Si metasomatism
Arc picrobasalt
Arc basalt
Arc basalt
Arc andesite
Greenschist
Blueschist
Greenschist
Blueschist
Grt-Act schist
Ab-Ep-Gln schist
Act-Ep schist
Qtz-Gln schist
Arc dacite
Arc dacite
Continental granodiorite/sediment
Blueschist
Greenschist
Greenschist
Qtz-Gln schist
Qtz-Ep schist
Qtz-Ep-Chl schist
Samples
BM04
BM11, BM15B
BP04
BML04A
CP03E
BP01, BM15C
BML01A
BM06
CP02D
CP04B, CP04C
CP03B
BM03
BM07, CP04A
BM05,
BML03B
CP03C
CP03A
BML03A
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8 b. Barru block
Although geographically only about 30 km to the north of the Bantimala Block, the Barru
Block shows significant differences, as has been made apparent through this study. The main structural grain in the Barru Block is ENE-WSW, almost perpendicular to that in the Bantimala
Block. Nevertheless, the Barru rocks represent a range of tectonic environments which is similarly diverse to that of the Bantimala Block (Table 4) and overall, they are more felsic rocks.
Compared to the Bantimala Block, the analysed samples from the Barru Block are more silicic and calk-alkaline rather than tholeiitic, except for the greenschist (DNG04) (Maulana, 2009).
Thus, the Barru Block also shows evidence of subducted MORB with seamounts and arc related sources. However, there has been no exhumation of deeply subducted slabs in the Barru
Block, and the OIB-derived rocks are more felsic than in the Bantimala Block. The dacitic/granodioritic rocks of Table 4 divide into two groups. The amphibolite-facies gneisses structurally underlie the ultramafics which have been thrust over them from the North, and may have been metamorphosed during emplacement of the ultramafics. However, the dacitic AM15 and BR04 intrude the ultamafics, and hence represent a second, later volcanic arc. Note that the
Barru ultramafics are relatively undepleted lherzolite, quite different from the cumulate of
Bantimala ultramafic. Again, pelagic cherts and breccias containing terrigenous clasts are associated with these meta-igneous rocks, suggesting subduction of deep ocean floor at sometimes, and proximity of continental masses at others.
The absence of the high-pressure and low-temperature metamorphic facies implies that the currently exposed rocks in this block were only subducted to shallow depths, probably not more than 30 km. This may imply either that the subduction angle was shallower than in the
Bantimala Block, and/or that the subducted ocean crust in Barru was younger, warmer and more buoyant.
Table 4. Mafic to felsic meta-igneous rocks of Barru Block, classified by reconstructed protolith from trace element data, bulk composition and metamorphic grade.
Deduced protolith
Highest P of metamorphism
Amphibolite
Lithology Samples
Amphibolite AM14 NMORB-like cumulate
OIB andesite
Arc dacite
Arc dacite
Greenschist
Amphibolite Quartzo- feldspathic
Gneiss
Low Greenschist Dacitic
Intrusives
DNG04
AM06, AM08, AM09SC
AM15, BR04
Tectonic Model
Previous models of plate tectonics for the South Sulawesi region have been formulated using geological and seismic as well as geochronological data (Guntoro, 1999; Wakita et al.,
1996). Nevertheless, this study is the first reporting whole-rock geochemical data for rock units within these complexes. By combining the existing geochronological data and our study results, we have endeavoured to better constrain of the pre-Tertiary tectonic evolution of the Bantimala and Barru Blocks from the Early Jurassic to the end of Paleocene .
In the model presented here, the Bantimala block represents collision near the Sundaland margin between at least one island arc above a subduction zone with continental fragments
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9 which may be of Australian origin, moving in from the East. Back-arc and MORB-type oceanic crust and seamounts were subducted, and subsequently, fragments of subducted slab representing all these types were exhumed, in some cases from depths of > 90 km and metamorphosed to the eclogite facies. Uplift must have been rapid, in order to preserve the relatively low-temperature blueschist and eclogite rocks. The rapid exhumation may have been driven by buoyancy after slab break-off, or may be related to the extension associated with the roll-back of subducting slab as suggested for the New Caledonia high-P belt exhumation process (Spandler, 2004).
Other material was caught in the accretionary prism without being subducted to great depth, and underwent only greenschist-facies metamorphism. This includes oceanic igneous rocks, pelagic sediment, and continental-derived material. The sediments are of broadly mid-
Cretaceous age, implying that the metamorphic rocks are slightly older, probably early
Cretaceous.
Finally, compression in the region caused the obduction of the ultramafics from an oceanic basin from the East, in the Paleocene or Eocene. The Makassar Strait would have opened between southwest Sulawesi and Sundaland by oblique extension in the Paleocene to Early
Eocene (Lee & Lawver, 1995), but presumably after the emplacement of the ultramafics. A new west-dipping subduction system then initiated, which probably responsible for the variation sources in magmatic activity during Tertiary within the region (Elburg & Foden, 1998). Coffield et al.(1993) and Bergman et al.(1996) reported some calc-alkaline and high-potassium volcanics in the region.
The main tectonic features distinguishing the Barru block from Bantimala are the absence of high-pressure metamorphic rocks in the Barru block and the occurrence of high-temperature amphibolite-facies rocks.
The strucurally lowest and presumably oldest rocks recorded in Barru are the amphibolite-facies gneisses of dacitic composition, which represent granodioritic magmas at the root of an island arc. Shallow (greenschist-facies) subduction of a seamount is also recorded, as are barely metamorphosed seafloor sediments which are similar to but may be slightly younger than those from the Bantimala block.
Amphibolite-facies gabbroic cumulate with NMORB-like LREE depletion overlies these rocks as a tectonic sheet and forms a sole underneath the obducted ultramafics. It is interpreted to constitute a first slice of obducted ocean floor, and the ultramafics (lherzolite with chromitite) a second slice of deeper oceanic lithosphere.
Clearly, the detailed history of the two blocks is quite different, which suggests that they are not dismembered parts of a single accretionary complex.These proposed models are also consistent with the west Pacific region tectonic history that has undergone numerous episodes of opening and closing of marginal basin during the Mesozoic to Cenozoic time (Hall, 2002; Harris,
2003; Pigram & Panggabean, 1984). Furthermore, the subduction of various sources of the rocks, particularly young island arcs, has been recognised in the Southeast Asia region where there are small plates, young arc and short-lived subduction zone occur (Closs, 1993).
CONCLUSIONS
The "South Sulawesi Basement Complexes" consists of two separate massifs about 30 km apart, the Bantimala and Barru blocks. Based on field mapping, petrography and geochemistry, the blocks can be described as follows.
1. The Bantimala block forms a ENE-dipping stack of tectonostratigraphic units, of which five major types can be distinguished. a.
Weakly metamorphosed deep-sea sediments, composed of radiolarian chert, sandstone and mudstone.
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10 b.
Metabreccia and metabasalt derived from continental margin and seamount respectively, which have been metamorphosed in the greenschist facies. c.
Low-pressure to high-pressure meta-igneous rocks, derived from various tectonic environments: island arc basalt, cumulates, oceanic island basalt, mid-ocean ridge and continental rocks. They are represented by strongly foliated greenschists, blueschists and metre-scale blocks of eclogite enclosed in blueschist. Phase-equilibrium data, mineral chemistry, and thermobarometry suggest that the eclogites reached a maximum pressure of 28±2 kbar at temperature of 650±50°C, near the quartz-coesite phase boundary. All rocks show some retrogressive greenschist-facies overprint. d.
Arc-derived metavolcanics and continental-derived metasediments, structurally overlying the metabasites. e.
Ultramafic rocks, which have been moderately to strongly tectonised and serpentinised.
They are predominantly harzburgite with and pockets of dunite, and small amounts of olivine clinopyroxenite with podiform chromitite.
2. The Barru block consists of four NNW-dipping structural units, which were all later intruded by volcanic rock: a.
Weakly metamorphosed deep-sea sediment (radiolarian chert) interbedded with continental sediments and breccia which contains fragments of metamorphic and ultramafic rock. b.
Greenschist- to amphibolite-facies metamorphic rocks of island arc affinity. These are structurally overlain by mafic amphibolite and ultramafics. c.
Amphibolite of MORB affinity, forming a sheet at the sole of the ultramafic unit, between the arc-derived rocks and the ultramafics. d.
Ultramafic rocks, which are highly serpentinised spinel lherzolite, locally metamorphosed in to amphibolite facies and lenses of podiform chromitite e.
Volcanic rock, dacitic in compositions, which intruded the metamorphic and ultramafic suites, and are interpreted to represent development of a second, later volcanic arc. The rocks have undergone low-temperature alteration, indicated by fine-grained chlorite and probably epidote rims on plagioclase.
ACKNOWLEDGEMENTS
This work is a small part of first authors` master thesis. We acknowledged the financial support from AusAID for this project. We thank to Dr. Ulrike T from ANU for analytical work.
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