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PERTEMUAN ILMIAH TAHUNAN IAGI KE-37

HOTEL HORISON BANDUNG, AGUSTUS 2008

139

THE P-T PATH OF METAMORPHIC ROCKS FROM KARANGSAMBUNG

AREA, KEBUMEN, CENTRAL JAVA

Lisnanda A. PERDANA1, AMRIZAL

1, And I G.B. Eddy SUCIPTA

2

1Undergraduate Student, Department of Geological Engineering, Institut Teknologi Bandung,

Bandung 40132, INDONESIA

2Geology and Paleontology Research Group, Institut Teknologi Bandung, Bandung 40132,

INDONESIA

ABSTRACT

Karangsambung have various lithology types which cropped out in Lok Ulo Complex,Central Java. This complex consists of dismembered ophiolites, sedimentary rocks,

and crystalline schists and gneisses occurring as tectonic slabs in a black-shale matrixtectonic mlange. Age of the rocks in this area is around Cretaceous which comes fromthe interaction between Eurasian Plate and Indo-Australian Plate. The same lithology isappearing in Kalimantan and Sulawesi.

During the interaction process, occur melting process of the plate which producedmagma chamber. It is caused the high pressure metamorphism rocks metamorphosedinto amphibolites facies. With the occurrence of fast uplifting causing the pressure andtemperature decrease rapidly, the metamorphic rocks again metamorphosed intogreenschist facies with remnant mineral relics of the old metamorphic facies.Possibly high pressure metamorphism in Karangsambung area, Kebumen, CentralJava, happens around 350-5800C and 10-16 kbar. Produced metamorphic rock intransition facies between blueschist to eclogite. The metamorphism process occur in atdepth 35-50 km.

The P-T path of tourmaline-bearing eclogites is characterized by rising pressures atdecreasing temperatures (stage I to stage III: P = 22.5 kbar and T = 365C), whereasthe normal eclogites show rising temperatures at increasing pressure (stage III: P =20.5 kbar and T = 410C). Thus, these eclogites were subducted to ~70 km depth at ageothermal gradient of ~6 C/km. Stage IV is limited to the P-T range of 810 kbar and350400C for both eclogite type.

SARI

Karangsambung memiliki berbagai jenis tipe litologi yang tersingkap di Kompleks LukUlo, Jawa Tengah. Kompleks ini terdiri dari ofiolit, batuansedimen dan sekis kristalindan gneiss yang terbentuk sebagai hasil gaya tektonik tarikan pada daerah melangeyang memiliki matriks berupa serpih hitam. Umur Batuan di daerah ini sekitar Kapuryang merupakan hasil interaksi antara Lempeng Eurasia dan Indo-Australia. Litologiyang sama juga terdapat di Kalimantan dan Sulawesi

Sewaktu proses interaksi, terjadi proses pelelehan dari lempeng yang membentukdapur magma. Itulah yang menyebabkan batuan metamorfisme tekanan tinggi

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terubahkan menjadi fasies amfibolit. Dengan keterdapatan dari pengangkatanmenyebabkan tekanan dan tenperatur menurun drastis dan batuan fasies amfibolitterubahkan kembali menjadi fasies sekis hijau dengan masih menyisakan mineral reliksisa dari fasies terdahulunya.

Kemungkinan metamorfisme tekanan tinggi di Area Karangsambung, Kebumen, JawaTengah, terjadi sekitar 350-5800C and 10-16 kbar, menghasilkan batuanmetamorfisme fasies transisi antara sekis biru sampai eklogit. Proses metamorfismeterjadi pada kedalaman sekitar 35-50 km.

Jalur P-T dari Turmalin eklogit dicirikan dengan naikknya temperatur (tahap I sampaitahap III: P = 22,5 kbar and T = 365C) dimana eklogit normal menunjukkanpeningkatan temperatur dan tekanan (tahap III: P = 20,5 kbar dan T = 410 0C makakemungkinan eklogit ini telah tersubduksi hingga kedalaman 70 km dengan gradiengeotermal 60C/km. Tahap IV terbatas dengan jangkauan P dan T 8-10 kbar dan 350-4000C dari tiap tipe eklogit.

INTRODUCTION

Background

Luk-Ulo area is one of the greatest

place for many kind of rocks, because it

is place formed in accretional area

which is have a special characteristic in

the petrology, Stratigraphy and

structural. Luk-ulo mlange complex

position is on the subduction areawhich had evolution until now.

The rocks in this area has a different

feature with the other places because

of mlange complex is the place where

continent type rock is mix with oceanic

type rocks, so the observation in this

area will develop our petrological ideas.

Aims and Object ive

The intention of writing of this paper is

to fulfill a requirement fot paper contest

in The 37thIAGI Annual Convention and

Exhibition, Bandung 2008. As a

purpose that I want to reached from

writing of this paper is to know the

caused, the characteristic, and process

in Luk Ulo mlange complex in

Karangsambung area, Kebumen,

Central Java.

REGIONAL GEOLOGY

In the Karangsambung area, Kebumen

- Central Java outcropped the oldest

rocks in Java Island called as Luk Ulo

Mlange Complex which is located

around Luk Ulo River. This complex

has a varieties litology arrangement

with difficult geological structure. This

complex consist of rocks fragment

which is surrounded by groundmass of

shale and claystone which age is upper

cretaceous to Paleocene. Rocks

fragment in Luk Ulo Mlange Complex

can be differentiated to native blocks

and exotic blocks. The form which is

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similar with boudine or boudinage form

as ellipsoid and large that commonly

consist of schist, chert, red limestone,

basalt, serpentinite, amphibolites,

gabro, peridotite and high pressure

metamorphic rocks can be found here

too. The native blocks consist of

greywacke.

The geology of the Karangsambung

area is characterized by the Lok Ulo

Complex, which is unconformably

overlain by Volcanic and SedimentaryRocks of the Eocene Karangsambung

Formation (Asikin et al., 1992). The

rocks of this Formation and of Tertiary

sequences, subdivided into the

Totogan, Waturanda, Penosogan, and

Halang Formations in ascending order,

are gently folded with an E-W trending

vertical axial plane (Figure 1).

The Stratigraphy of Karang Sambung

area, the first is Luk Ulo Mlange

Complex which is consist of rocks

fragment such as schist, chert, red

limestone, basalt, serpentinite,

amphibolites, gabro, peridotite, then the

local high pressure metamorphic rocks

such as eclogite, schist blue which is

surrounded by matrixs of shale and

black claystone which age is Upper

Cretaceous to Paleocene, in the top of

it, Karang Sambung Formation

deposited by unconformity contact

which age is Eocene and it`s olisostrom

deposit type that consists of stone

shale, boulder of limestone,

conglomerate,stone clay and basalt.

Upper of Karang Sambung Formation

deposited Totogan Formation which the

sediment are breccias with clay stone

fragmental, sandstone, basalt in clay

stone groundmass which age is

Oligocene to Early Miocene. Upper of it

deposited Waturanda Formation by

conformity and consist of volcanic

breccias and greywacke sandstone withclaystone interbedded, which age is

Early Miocene. Panosogan Formation

which age is Middle Miocene consists

of sandston, claystone, tuff, marl,

limestone. It is deposited by conformity

upper Waturanda Formation. In some

area of Panosogan Formation found

intercalated Kemangguan Breccias

which age is Late Miocene then Halang

Formation consists of sandstone,

claystone, marl, tuff which age is Late

Miocene-Pliocene.

THE P-T PATH OF

METAMORPHIC ROCKS FROM

KARANGSAMBUNG AREA

The samples were taken from 3

different places which are Muncar river

(KM-1, KM-2), Pucangan village (DP-1)

and Luk Ulo (LU-1).

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Petrography Analysis

Samples KM-1, mica schist-

glaucophane has crystal size range

from middle-coarse, porphyroblastic

texture appeared with garnet as a

porphyroblast component (15%) which

size is >2,5 mm, subhedral-anhedral, in

groundmass of mica/paragonite (25%),

glaucophane (25%), pyroxene/omphasit

(20%), sphene (10%), quartz (3%) and

scapolite (2%) which size is 0,5-1,5 mm

and in some area can be found

schistose structure. Samples KM-2 andDP-1, garnet amphibolites showing

porphyroblastic texture with garnet as

porphyroblast mineral (2%) which size

is 2-5 mm, subhedral-anhedral in

groundmass of actinolite (40%),

hornblende (30%), chlorite (15%),

mica/muscovite (10%), sphene (8%).

Samples LU-1, mica schist-quartz-

actinolite showing heteroblastic texture,

size of grain around 0,3-2 mm consist

of mica/muscovite (25%), quartz (25%),

actinolite (20%), hornblende (15%),

sphene (8%), plagioclase (5%), and

epidote (2%).

Texture and Structure

Observing texture and structure is

needed to know tectonic deformation

process that happen in the same time

with the metamorphism process and to

know overprint process on the

metamorphism process next. Structure

and texture because of tectonic

deformation showed with schistose.

From petrography method,

metamorphic structure and texture just

can be showed in KM-2, DP-1, LU-1.

On KM-2 and DP-1 occur poikiloblastic

texture and corona structure (rim) in

porphyroblast garnet. Poikiloblastic

texture showed by the inclusion of

glaucophane, subhedral-anhedral, in

porphyroblast garnet which is have

coarser grain (1,5-2,5 mm), thiscondition is indicating that before

garnet, glaucophane minerals was form

and in later metamorphic process

glaucophane was well preserved in

garnet so it cant be interfered with the

changes of temperature and pressure

in metamorphism process that form the

garnet. The corona structure showed by

the occurrence of chlorite (low grade

metamorphism) that surrounding

porphyroblast garnet (high pressure

metamorphism). From the

metamorphism structure and texture so

on KM-2 and DP-1 showed that the

metamorphism process begin with high

pressure metamorphism that follow by

the increase of temperature then

decreasing temperature and causing

low grade temperature.

Structure and texture which was

caused by the tectonic deformation just

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appeared in LU-1, indicated with two

schistose structure (S1,S2). Schistose

structure,S2 consist of muscovite,

quartz, actinolite and plagioclase then

schistose structure,S1 consist of

hornblende that only occur in few area

and commonly is being crossed by the

S2 structure. By seeing that condition,

so LU-1 had tectonic deformation two

times, the first deformation that

resulting S1 structure happen in high

pressure metamorphism so that is why

the hornblende was found then togetherwith the decreasing temperature

happen the second deformation that

resulting S2 that consist of muscovite,

quartz, actinolite and plagioclase. The

evidence of glaucophane as a relic

mineral showed that metamorphism

process happen in high pressure.

RESULT AND DISCUSSION

High pressure metamorphism (>4 kbar)

in Karangsambung, Kebumen, Central

Java, characterized by the occurrence

of high pressure metamorphism

diagnostic minerals such as

glaucophane and omphasite in KM-1,

but when we observe from the texturalcomposition, the abundance of those

mineral showed different

metamorphism process.

Glaucophane and omphasite on KM-1

was resulted from metamorphism

process in 10-16 kbar and 350-

5800C which resulting mineral in the

transition facies between blueschist and

eclogite, in the others glaucophane on

KM-2, DP-1 and LU-1, was experienced

metamorphism process three times, the

first when glaucophane mineral was

formed in > 4 kbar ( 4-6 kbar) and

3000C then the isobaric condition

happened then resulting amphibolitesfacies (indicate with hornblende

garnet) that performed in 6 kbar and

6000C and finally experienced the

regressive metamorphism process

(decreasing temperature and pressure)

which resulting greenschist facies

(indicate with actinolite, muscovite,

chlorite) in 400-4500C and 3-6 kbar.

During this process, LU-1 consider to

have higher pressure ( 4-6 kbar) than

KM-2 and DP-1. This was indicated

with the occurrence of glaucophane

which has contact with actinolite

During subduction process the material

from Indo-Australia plate experience

high increasing pressure relatively with

lower increasing in temperature. On

KM-1, increasing pressure can perform

to 10-16 kbar with the temperature

around 350-5800C which is resulting

group of mineral in transition facies

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between blueschist and eclogite. In

other way the increasing temperature

on KM-2, DP-1, LU-1, consider around

4-6 kbar with the temperature

3000C. Metamorphism process in

pressure around 4-6 kbar is happen in

depth around 15-20 km then

metamorphism in 10-16 kbar can only

be happened in depth around 35-50

km (Yardley, 1989 op.cit. Sucipta,

2006). From this data, can be said that

subduction process Indo-Australia plate

to Eurasia Plate in Karangsambungarea happen in late Cretaceous-

Paleogene and reach depth 50 km.

The next metamorphism process

indicated with Isobaric condition in

temperature around 6000C

(amphibolites facies on KM-2, DP-1,

LU-1). The cause of increasing

temperature possibly happen when the

subducting plate begin to melt and

resulting magma kitchen. This

condition cannot be detect on KM-1, it

is because the group mineral that

forming KM-1 still in stable condition in

temperature 6000C (the maximum

temperature for KM-1 5800C) or KM-1

doesnt interfered by the condition of

rising temperature because KM-1 is

was form in greater depth than the

other samples.

The last metamorphism process

characterized with decompression

condition (decreasing pressure rapidly)

with a little of decreasing temperature,

so the group of mineral which was form

before this process is well preserved

(can be found hornblende garnet as

primary mineral on KM-2, DP-1, LU-1).

In tectonic concept, the metamorphism

process possibly happen because there

is fast uplift that related to the changes

of subduction rate or the subducting of

micro-continent component whichcausing the docked and then obducted

(Soesilo and Sutanto, 2000). During

this process the metamorphic rocks still

find the equilibrium condition that

indicated with regressive

metamorphism process. On KM-2, DP-

1, LU-1 is going to changes into

greenschist facies (indicated with

abundance of actinolite, muscovite,

chlorite as a primary mineral).

CONCLUSION

In Karangsambung area occur 4 step

of metamorphism process that can be

proved by the appearance of high

pressure metamorphism diagnostic

mineral such as lawsonite,

glaucophane, pyroxene (jadeit,

omphasite) and aragonite. On Km-1,

the metamorphism process proved by

the appearance of glaucophane and

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pyroxene (omphasite) as an indicator

On KM-2 and DP-1 glaucophane

appear as an inclusion of garnet

porfiroblast and on LU-1, glaucophane

appear in few zone as a relic mineral in

the part of actinolite body.

Then from the data that acquired,

commonly the tectonic history of

Karangsambung area is divided into

two part:

1. Subducting Event

From the samples, we can know that

the rising temperature to 350-580

0

Cand pressure 10-16 kbar was caused

by the the subduction of plate Indo-

Australia onto Eurasia plate and have

reach depth 50 km.

2. Uplift

There are evidences on the KM-2, DP-1

and LU-1 that proved uplift event in

Karangsambung area, it is because on

those samples there is actinolite,

muscovite and chlorite as minerals that

indicate decreasing temperature then

can be found direct contact of

glaucophane minerals with actinolite .

REFERENCES

Asikin, S., Harsolumakso, A.H., Busono,

H. And Gafoer, S. 1992. Geologic

Map of Kebumen Quadrangle,

Java. Geological Research and

Development Center, Bandung.

Kadarusman, Ade., Massonne, H. J.,

Roermund, H. V., Permana, H.,

and Munasri. 2006. P-T Evolution

of Eclogite and Blueschists from

the Luk Ulo Complex of CentralJava, Indonesia. International

Geology Review, Vol. 48.

Sucipta, I G.B.E. 2006. Petrologi

Batuan Metamorf Tekanan Tinggi

di Daerah Karangsambung,

Kebumen, Jawa Tengah. Buletin

Geologi, Vol. 38, No. 2.

Soesilo, J. and Sutanto. 2000. Study on

Garnet Bearing Quartz-Muscovite

Schist Blocks of The Luk Ulo

Melange Complex, Kebumen,

Central Java. Proc. of Indonesian

Association of Geologist, 29th

Annual Convention.

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Figure 1. The geological map of Karangsambung area (modified after Asikin et al., 1992 op.cit

Kadarusman et al., 2006). Karangsambung area consists of Karangsambung, Totogan,

Waturanda, Penosogan, and Halang Formation, are gently folded with E-W trending vertical

axial plane.

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Figure 2. The stratigraphy of Karangsambung area (Asikin et al., 1992). It consist of metamorphic

rocks, sedimentary rocks and ophiolite sequences. The stratigraphiy of this area influenced by

sedimentary and tectonic.

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