New Perspective On High Grade Metamorphic Regional In

Bayah Complex, Banten Province

Aton Patonah1, Ildrem Syafrie1, Hero Ayasa2,Haryadi Permana3

1. Laboratorium of Petrology and Mineralogy, Departmen of Geologi,

Faculty of Geologi, UNPAD.

2. Student of Geological Department, Faculty of Geology, UNPAD.

3. Geoteknologi LIPI. Jl. Cisitu Lama, Bandung

Abstract

Report of metamorphic rocks research which are exposed in Bayah Complex is still

very less. Many researchers focused on their studies ophiolithic and metamorphic

rock in Ciletuh as the oldest rock in West java. Some of them suggest the

metamorphic rocks in Bayah complex as a product of local metamorphism because

the granodiorite intrusion is present around them. Field investigation showed that

the characteristic metamorphic rocks in Bayah have different from product the

intrusion. We suggest that this complex is produced from regional metamorphism,

it is characterized by present high grade mineral assemblage as product of high

grade metamorphism in this area, that is, garnet biotite schist, biotite schist,

actinolite schist and hornblend schist. With new found this rocks, probably it can

change geological history, especially in this area, West Java and surrounding.

Key words: Bayah complex, regional metamorphism, high grade metamorphism,

garnet schist biotite, schist actinolite

Introduction

Tectonic setting has a role in determining potential natural resources in an

area and its distributuin. The trace can be known, by identifying basement rock

characteristics as one of the approaches. In Part of West Java, this basement is

represented by Pre Tertiary of Ciletuh Bay (Van Bemmelen, 1949; Clemen & Hall,

2007). It is consist of Metamorphic basic and ultrabasic rock with chloritic schist

and phyllite, acctered to margin of Sundaland in the Late Crestaceous (picture 1).

According to Schiller et all (1991), Ciletuh Melange Complex is described as

ophiolite assemblages associated with greenschist, mica schist, amphibolite schist,

and quartzites which has been interpreted as a tectonic mélange (e.g. Patonah et al,

2010; Parkinson et al, 1998; Martodjojo et al, 1978; Thayyib et al, 1977). This

mélange complex has Crestaceous age and it is the same age with Luk Ulo mélange

complex (Parkinson et al, 1998). It has unconformity with Ciletuh Formation which

deposited in shallow marine (Van Bemmelen, 1949). Bayah Formation is founded

about 20 kilometres toward southeast of Ciletuh, which present as the oldest rock in

Bayah area.

In Bayah area, the metamorphic rock consist of Schist, Genes, and

amphibolite which formed probably Early Oligocene – Late Oligocene (Sujatmiko

& Santosa, 1992). These rocks still unknown because lack of publication about

them. This paper try to identify based on data collection field work during May –

September 2014.

1st International

Conference

Geoscience for Energy, Mineral

Resources, and Environment

applieds 2014

Methodology

For a while, this research is described by field investigation and petrography

analysis. For comprehensive study, on going. We chose 40 sample represented this

research. The functions are to determine kind of minerals, texture, structure and

name of the rocks based on Barker classification (1994) and Robert (2000), to

arrange paragenesis of the rocks and determine probably the original of the rock,

also metamorphic facies and metamorphism have developed.

Result and Discussion

Metamorphic rock in this area distributed on Cipager and Cibaong rivers

(figure 2 and 3). These dominantly have foliated, faulted and fractured, some of

them weathered. As long as Cipager river, biotite schist is dominantly, grey –

greenish grey in colors, lgood foliated with trend dominantly west to east, some of

them trend in NW-SE, commonly fine – very fine grain, fractured, locally it is

intruded by diorite. Microscophically, kind of mica schist are biotite schist

dominantly, some garnet biotite schist and chlorite schist. Biotite schist have grey –

greenish grey colors, good foliated, have boudinage structure, and fractured is filled

by epidote, chlorite, quartz, opaq and oxide minerals. The rock consist of

plagioclase and K-feldsphare as porphyroblast, others filled as matrix, they are,

biotite, muscovite, chlorite, quartz, opaq minerals, some contain garnet (picture 4).

Move toward east, is founded green schist, have green colors, good foliated with

trend west to east, dominantly of actinolite, some hornblend, plagioclase, quartz,

biotite and chlorite minerals. Other minerals are opaq and oxide minerals.

In Cibaong river, Mica schist, green schist and basalt are presents. Biotite

is kind of mica that dominantly present in is Schist. It has greenish grey in colors,

good foliated trends in West to East, fracture are filled by sulfide minerals, some of

them are weathered. Amphibolite schist consist of actinolite schist and hornblend

schist. Actinolite schist have green in colors, good foliated, some fractured, consist

of actinolite dominantly, some hornblende and plagioclase present as porphyroblast,

others are quartz, biotite and chlorite (figure 5).

Based on mineral assemblage, protolith of biotite schist is dominantly

semipelite and pelite, some of them are psammite (figure 6). Meanwhile, the

protolith of amphibolite schist is volcaniclastic and mafic rock in origin. Some

geothermometer has been used to temperature estimation. Biotite schist formed

from 425oC to 450oC, is characterized by biotite, muscovite, K-feldspare,

plagioclase and quartz (Deer, dkk; 1992; Barker, 1994). In some place, garnet is

present, it is indicated that the rocks formed from 480oC to 500oC, and indicating

the upper greenschist facies. The retrograde metamorphism is situated to 150oC –

200oC, it is characterized by chlorite and muscovite replaced biotite, K-feldspare

altered to sericite and muscovite.

Hornblende schist and actinolite schist formed from 370oC – 420oC, is

signed by actinolite, chlorite, biotite, plagioclase, quartz minerals (Deer, dkk; 1992;

Barker, 1994) and included to transition of green schist – amphibolite schist. The

rock have retrograde traject which is characterized by hornblende altered to

actinolite and actinolite to chlorite.

Based on characterization of the rocks in this area, metamorphic rock in

this area associated with regional metamorphism, foliated trend West to East, and

boudinage structure. Some folliation have changed which is predicted by intrusion

effect in some place. In this process, is predicted by accretion of Eocene –

Oligocene age.

Conclusion

Metamrphic rocks in Bayah area are dominantly by biotite schist, some

chlorite schist, actinolite and hornblende schist. The rocks come from semipelite

and pelite, some psammite, volcaniclastic and mafic rock. Based on mineral

assemblage, we suggest that regional metamorphism happened in this area and they

have retrograde traject which is produced by tectonic association in Eocene –

Oligocene age.

Reference

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Picture 1. Geology of Regional the research area (Sujatmiko and Santoso, 1992)

Picture 2. Biotite schist and amphibolite schist outcrop in Cipager area.

Daerah penelitian

Picture 4. Biotite schist showed foliated and boudinage structure, consist of biotite, muscovite,

K-feldspare, plagioclase, chlorite and quartz minerals.

Picture 5. Biotite schist is dominantly biotite altered to chlorite, some muscovite, K-feldspare,

plagioclase, quartz and garnet.

Picture 6. Hornblend schist consist of hornblend, actinolite, plagioclase, and quartz minerals,

some biotite mineral and chlorite minerals.

Picture 7. Protolith of biotite schist in Bayah area are dominantly semipelite and pelite, some

psammite.