A PRELIMINARY VIEW AND IMPORTANCE OF METAMORPHIC … BPS02.pdf · METAMORPHIC GEOLOGY FROM JIWO HILLS IN CENTRAL JAVA Nugroho Imam Setiawan1, Yasuhito Osanai2, Carolus Prasetyadi3

BPS02 Prosiding Seminar Nasional Kebumian Ke-6 Teknik Geologi Universitas Gadjah Mada, 11-12 Desember 2013

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A PRELIMINARY VIEW AND IMPORTANCE OF METAMORPHIC GEOLOGY FROM JIWO HILLS IN

CENTRAL JAVA Nugroho Imam Setiawan1, Yasuhito Osanai2, Carolus Prasetyadi3

1Geological Engineering Department, Faculty of Engineering, Gadjah Mada University, Jl. Grafika 2, Bulaksumur, Yogyakarta 55281, Indonesia. Email: [email protected]

2Division of Earth Sciences, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

3Geological Engineering Department, Faculty of Mineral Technology, National Development University “Veteran”, Jl. SKW 104, Condongcatur, Sleman, Yogyakarta 55281, Indonesia

Received Date: October 30, 2013

Abstract Jiwo Hills is an isolated hill with approximately occupied 30 km2 in Bayat area of Central Java. The Jiwo Hills has been considered to be composed of very low-grade metamorphic rocks. This contribution explains a preliminary view of the metamorphic rocks from collected rocks sample in the Jiwo Hills.

Common metamorphic rocks found in this area are phyllite, mica schist, calc-silicate schist, and marble with the foliation trend of NE-SW. Most of the metamorphic rock exposures are strongly weathered. Rarely epidote-glaucophane schist crop out near the exposure of serpentinite in western part of this complex. Several carbonate sedimentary rocks are converted to garnet-wollastonite skarn under the contact metamorphism probably caused by diabase intrusion.

Garnet-wollastonite skarn mainly consists of garnet and wollastonite embed in the quartz matrix. The garnets are rich of grossular composition, range from Adr13–19Grs81–85Sps0–2. Those show anisotropic, with sectorial twinning zonings. The skarn deposits might give important information for economic resources in this area.

Epidote-glaucophane schist mainly consists of glaucophane, epidote, quartz, phengite, titanite, and hematite. By the present of this blueschist facies of high-pressure metamorphic rock in this area, it confirms that Jiwo Hills is one of the high-pressure metamorphic terranes together with Luk Ulo Complex of Central Java, Meratus Complex of South Kalimantan, and Bantimala Complex of South Sulawesi. The serpentinites might facilitate exhumation of the blueschist in the Jiwo Hills. Detailed study of the distribution of metamorphic rocks in this area is needed in order to understanding tectonic evolution as well as economic geology.

Keywords: Jiwo Hills, Bayat, blueschist, high-pressure metamorphic rocks, skarn

Introduction Significantly regional metamorphic rocks scattered expose throughout Java Island. Those are exposed in Ciletuh Complex of West Java, Luk Ulo Complex of Central Java, and Jiwo Hills area of Central Java (Fig. 1a). Among those complexes in Java island, Luk Ulo Complex is well-known to be worldwide outcrop of high-pressure metamorphic rocks (eclogite, blueschist) [1, 2, 3, 4], whereas the other complexes (Ciletuh and Jiwo Hills) still lack of publication about the metamorphic rocks and the occurrence of high-pressure metamorphic rocks.


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This contribution explains the occurrence of various metamorphic rocks in the Jiwo Hills. Previously, [5] has reported the metamorphic facies study in western part of this complex (West Jiwo). This paper explains detailed petrographical observation and mineral chemistry analyses on garnet-wollastonite skarn and epidote-glaucophane schist. Furthermore, those results are synthesized to emphasize the contribution of Jiwo Hills metamorphic terrane on the tectonic evolution of Indonesia region. Mineral abbreviations in this paper follow [6]. Geological Outline Jiwo Hills located in Bayat area, which is 45 km NE of Yogyakarta (Figs. 1a, b). In this isolated hills, which approximately has 30 km2, regional metamorphic rocks were expose [5, 7, 8] (Fig. 1b). The Jiwo Hills have been previously considered to be composed of the very low-grade metamorphic rocks [2, 8]. Common metamorphic lithologies in the Jiwo Hills are phyllite, mica schist, calc-silicate schist, and marble. [5] reported the occurrence of amphibolite, blueschist, phyllite and serpentinite in this area. Foliations of the metamorphic rocks are reported to have a trend of NE-SW [8]. No reliable P-T conditions of the metamorphic rocks were reported from this area.

Figure 1. Simplified geological map of Jiwo Hills in Central Java. Modified after [7, 8].


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The regional metamorphic rocks are considered to be the oldest rocks crop out in this complex. [8] reported K-Ar from quartz-mica schist yielded 98 Ma. Those are unconformably overlain by Paleogene and Neogene sedimentary rocks [7] (Fig. 1b). A diabase, which yields K-Ar age of 39.8–31.3 Ma [10], intruded older rocks in this area (Fig. 1b). Modes of Occurrence Field investigation in the Jiwo Hills confirms that very low-grade metamorphic rocks are abundant in this area. However, most of the exposures are strongly weathered. The most predominant rock types are phyllites. Rarely blueschist-facies assemblage of epidote-glaucophane schist was found near the exposure of serpentinite in the western part of this complex (West Jiwo) with the foliation trend varies from N62–63°E dipping 55–70° to the south (Figs. 1b, 2a, b). Garnet-wollastonite skarn and clinopyroxene-epidote skarn were also found in the western part (Fig. 1b). Furthermore, the occurrence of marbles were reported by [5] also found in the western part. Whereas in the eastern part of this complex are dominated by calc-silicate schist and phyllite (Figs. 1b, 2b, c). The calc-silicate schist has a trend of N60°E dipping 34° to the south. The other variations of low-grade schists found in this area are albite-muscovite schist, graphite phyllite, and quartz phyllite.

Figure 2. Modes of occurrence of the metamorphic rocks in Jiwo Hills. (a) Outcrop of serpentinite and (b) epidote-glaucophane schist in West Jiwo. (c) Outcrop of phyllite and

(d) calc-silicate schist in East Jiwo.


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Petrography and Mineral Chemistry In this section, petrographical characteristics of collected metamorphic rock samples from Jiwo Hills area are discussed. Detailed description of mineral assemblages under the microscope are given in Table 1. Two samples of garnet-wollastonite skarn and epidote-glaucophane schist were analyzed their mineral chemistries with a JEOL JED2140-JSM5301S scanning electron microscope with energy dispersive spectrometry system (SEM-EDS) in Kyushu University, Japan. The analytical conditions were set at an accelerating voltage of 15 kV, a probe current of ca. 0.35 nA, and a beam diameter of 1 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. Representative chemical compositions of the analyzed minerals are listed Table 2.

Fe3+ contents of garnet were calculated using algorithms as suggested by [11]. The end members of clinopyroxene were calculated as suggested by [12] with Fe2+/Fe3+ ratios were determined from stoichiometry using the method of [13] as Fe3+ = 4 – 2Si – 2Ti – Al – Cr + Na + K, Fe2+ = Fetotal – Fe3+. Nomenclatures and calculated compositions of the amphiboles follow [14]. Phyllites Phyllites represent the lowest grade of metamorphic rocks in the Jiwo Hills. These rocks contain detrital grains of quartz with fine grain matrix of chlorite, muscovite, and graphite (Fig. 3a). Those have thin layers, in which contains two regions of detrital quartz-rich and chlorite-rich regions. Fine-grained apatite is commonly framed by graphite and chlorite. Calc-silicate schists Calc-silicate schists mainly consist of calcite, quartz, muscovite, and iron-oxide grain minerals (Fig. 3b). Weak schistosity is developed by calcite and muscovite grains. Calcite, quartz, iron-oxide grains are having size of 0.1–0.5 mm in length, whereas muscovite is ~0.2 mm in length. Several calcite veins are cross-cutting this rocks, which is regarded as secondary phases. Albite-muscovite schists These rocks show strong schistosity and mainly consist of albite, chlorite, quartz, muscovite, and graphite (Fig. 3c). Porphyroblastic albites are abundant in this rock type. The strong parallel schistocity is defined by chlorite, muscovite, and graphite. Chlorite and albite might be pseudomorph after garnet (Fig. 3c). Although original garnet grains were not identified in the collected-rock samples. Serpentinites Serpentinite are composed mostly of serpentine together with spinel (Fig. 3d). Spinel is having size of 0.5–2 mm in diameter. Relict grains of protolith rocks were not recognized due to the highly serpentinization and highly weathering condition. Garnet-wollastonite skarn This rock mainly consists of garnet and wollastonite, embed in the quartz matrix. The garnets having size of 0.5–1 mm in diameter, which obviously show sectorial twinning zoning (Figs. 3e, f). Those are rich in grossular composition with the range of Adr13–

19Grs81–85Sps0–2 (Fig 4a). Representative chemical composition analysis of garnet is shown in Table 2. Rim to rim analysis of garnet (A to A’; Fig 3f) shows that the garnet has slightly zone in composition with slightly lower grossular content in the rim than core portions (Fig. 4b). The andradite composition shows opposite pattern (Fig. 4b) with


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Figure 3. The scale bar without expression indicates 1 mm. Photomicrographs of metamorphic rocks from Jiwo Hills. (a) Phyllite has main compositions of chlorite, quartz, muscovite, and graphite with minor apatite. (b) Calc-silicate schist with calcite, quartz, and

iron oxide grains. (c) Chlorite-albite schist with chlorite and albite pseudomorph after garnet and surrounded by graphite. (d) Serpentinite with spinel grains embed in the mesh texture of serpentine. (e) Garnet-wollastonite skarn; garnet has sectorial twinning zoning.

(f) Back-scattered image of garnet-wollastonite skarn with A–A’ is microprobe line analysis.


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Figure 3. (continued) The scale bar without expression indicates 1 mm. (g) Epidote-glaucophane schist has main composition of epidote, glaucophane, quartz,

titanite and hematite. (h) Back-scattered image of epidote-glaucophane schist shows that amphibole has glaucophane composition in core-portion and actinolite composition in rim-

portion.

Figure 4. (a) Chemical compositions of garnets in garnet-wollastonite skarn are plotted on the ternary diagram of grossular, spessartine, and andradite. (b) Zoning profile of garnet in

garnet-wollastonite skarn based on the microprobe analysis line of A–A’ (Fig. 3f). (c) Chemical compositions of clinopyroxenes in garnet-wollastonite skarn are plotted on the

ternary diagram of clinoenstatite, wollastonite, and clinoferrosilite from [15]. (d) Chemical compositions of amphiboles in epidote-glaucophane schist are plotted in the nomenclature

diagram of amphibole from [14].


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Table 2. Representative microprobe analyses of garnet, clinopyroxene, and amphibole.

spessartine relatively flat (Fig. 4b). All of the fine-grained clinopyroxenes in this rock are plotted in the wollastonite area of ternary diagram of clinoestatite-clinoferrosilite-wollastonite suggested from [15] (Fig. 4c). Representative chemical composition analysis of clinopyroxene is shown in Table 2. Epidote-glaucophane schist Epidote-glaucophane schist mainly consists of glaucophane, epidote, quartz, phengite, titanite, and hematite (Figs. 3g, h). Excluding quartz, glaucophane and epidote are ubiquitous in matrix, which having size of ~0.2 mm in diameter and show random orientation. Actinolite occupy in the rim of the glaucophane (Figs. 3h, 4d) as secondary phases of this rock. Chlorite and albite replace other minerals, which also regarded as secondary phases. Representative chemical composition analysis of amphibole is shown in Table 2. Discussion Metamorphic rocks crop out in the Jiwo Hills having grade variation of greenschist, blueschist, hornfels (skarn), and serpentinite (Table 1). Based on their occurrence in the geological map, West Jiwo has more variation of metamorphic rocks than East Jiwo. The metamorphic grades found in the West Jiwo are greenschist, blueschist, hornfels (skarn),


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and serpentinite (Fig. 1). Furthermore, [5] reported the occurrence of marble in the West Jiwo. Whereas in the East Jiwo, so far, only greenschist was found in there (Fig. 1). Implication of the occurrence of skarn in the Jiwo Hills Mineral chemistry analyses have confirmed that grossular-rich composition of anisotropic sectorial twinning zoning garnet together with wollastonite embed in the quartz matrix is generally considered as skarn rock [16]. Carbonate sedimentary rocks should be considered to be host rock of this contact metamorphic process with the intrusion of igneous rock, which is probably diabase. As the preliminary study, we strongly recommend to continue the study of skarn in Jiwo Hills by detail mapping on the skarn zonation and prospect of economic skarn deposit. Correlation with other high-pressure metamorphic complex in central Indonesia Epidote-glaucophane schist is assumed to have a peak metamorphism at blueschist-facies on the glaucophane stability field. Later retrogression was happened in the greenschist-facies on the actinolite stability field, which shown by the amphibole texture (Figs. 3h, 4d). The occurrence of blueschist-facies in the Jiwo Hills suggests that Jiwo Hills is one of the high-pressure metamorphic terranes in Indonesia region together with Luk-Ulo Complex of Central Java, Bantimala Complex of South Sulawesi, and Meratus Complex of South Kalimantan. Relatively close synchronology in the available K-Ar ages of metamorphic rocks in the Jiwo Hills (K-Ar: 98 Ma [8]) with the other terranes (Bantimala Complex K-Ar: 113–137 Ma [2, 17, 18]; Luk Ulo Complex K-Ar: 110–124 Ma [1, 2, 19]; Meratus Complex K-Ar: 110–119 Ma [20, 21]) indicates that these rocks were metamorphosed and subducted northwesternly beneath the Sundaland in the similar time (Cretaceous age).

The occurrence of epidote-glaucophane schist in the Jiwo Hills is adjacent to the serpentinite outcrop, which also observed in the Luk Ulo Complex, Bantimala Complex, and Meratus Complex [2, 22]. [23] pointed out that buoyancy is the only effective force to exhume rocks from deeply subducted levels to the base of the crust. The serpentinites are extremely buoyant with respect to the oceanic crust that mainly consists of mafic rocks that have been transformed to high-pressure metamorphic rocks during subduction and increasing their density, which are denser than average mantle rocks [24]. Thus, serpentinites might be counter-balancing the negative buoyancy, decouple, and facilitate exhumation of the high-pressure rocks [24, 25].

Mélange deposit, ophiolite, and oceanic plate stratigraphy (e.g. chert, pillow lava), which found in the Luk Ulo Complex and Bantimala Complex, were not observed in the Jiwo Hills. [5] and W. Warmada (2008; unpublished data) have analyzed bulk chemistry of several metamorphic rocks in the Jiwo Hills (Table 3). Representative samples were plotted on the ACF diagram from [26] show that the protolith of metamorphic rocks from Jiwo Hills were derived from sedimentary rocks without any basic igneous rock signatures (Fig. 5). This is different from those in the Bantimala, Luk Ulo, and Meratus Complexes, which have protolith of MORB, OIB, and arc signatures more than sedimentary rocks [4, 27] (Fig. 5).

Sedimentary rocks are much more developed in the continental crust. Therefore, it might have possibility that Jiwo Hills was a small continental crust that had subducted beneath the Sundaland during Cretaceous. It can also explain the position of Jiwo Hills relatively located in the east of other high-pressure metamorphic terranes in central Indonesia. However, these are still speculative tectonic interpretation from the preliminary study. Detailed studies particularly of the occurrence of high-pressure metamorphic rocks in the Jiwo Hills are needed in order to determine the tectonic evolution in this region.


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Figure 5. Bulk rock compositions of metamorphic rocks from Jiwo Hills compared to the

other high-pressure metamorphic terranes in central Indonesia are plotted on the ACF diagram from [26].

Conclusion 1. Various metamorphic rocks crop out in the Jiwo Hills area include greenschist,

blueschist, serpentinite, and skarn.

2. The occurrence of skarn in the Jiwo Hills might give prospect information of economic mineral deposits.

3. The occurrence of blueschist-facies in the Jiwo Hills might be correlated with other high-pressure metamorphic terranes in central Indonesia region, which considered as Cretaceous subduction complex.

Acknowledgments We would like to thank to staff members of Geological Engineering Gadjah Mada University for their assistance during the field survey in the Jiwo Hills and valuable discussions. This work is a part of the PhD study supported by JICA AUN/SEED-Net scholarship, and was supported by Grants-in-Aid for Scientific Research (No. 21253008 and 22244063 to Y. Osanai) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.


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A PRELIMINARY VIEW AND IMPORTANCE OF METAMORPHIC … BPS02.pdf · METAMORPHIC GEOLOGY FROM JIWO HILLS IN CENTRAL JAVA Nugroho Imam Setiawan1, Yasuhito Osanai2, Carolus Prasetyadi3