PETROGENESIS OF VERY LOW-TO LOW-GRADE …werecropped outs,whichare scaly clay, mica schist, zeolitic rocks, and basaltic lava, respectively from down- to up-stream. The zeolitic rock

PROCEEDING, SEMINAR NASIONAL KEBUMIAN KE-10 PERAN PENELITIAN ILMU KEBUMIAN DALAM PEMBANGUNAN INFRASTRUKTUR DI INDONESIA

13 – 14 SEPTEMBER 2017; GRHA SABHA PRAMANA

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PETROGENESIS OF VERY LOW-TO LOW-GRADE METAMORPHIC ROCKS IN

LUK ULO MÉLANGE COMPLEX, KARANGSAMBUNG, CENTRAL JAVA,

INDONESIA

Renaldi Suhendra1*

Nugroho Imam Setiawan1

I Wayan Warmada1

Andika Bayu Aji2

Hanik Humaida2

1Geological Engineering Department, Engineering Faculty, Universitas Gadjah Mada 2Geochemist in Institute of Geological Hazard Technology Investigation and Development, Yogyakarta

corresponding author: [email protected]

ABSTRACT

Very low-to low-grade metamorphic rocks in Karangsambung area are widespread in the Northern

Part of Luk UloMélangeComplex especially in Gebang River, Kaliwiro Area, Central Java.This

research was conducted in order to understand [1] P and Tcondition of metamorphic rock, [2] protolith

variation, [3] relation between these rocks and other rock units, and [4] tectonic environment pre- and

syn-metamorphism. The systematic sampling method was conducted to record mineralogical change

along Gebang River using petrography, XRD, XRF, and SEM-EDS analyses. Several rocks

werecropped outs,whichare scaly clay, mica schist, zeolitic rocks, and basaltic lava, respectively from

down- to up-stream. The zeolitic rock consist of celadonite-natrolite-palagonitemeta-basalt, zeolite

meta-calcareouspolymictic breccia, zeolite meta-pelite in which based on mineral geochemistry, these

rocks are metamorphosed under zeolite-facies (T= 120–220 °C and P= 1–5Kbar). While, mica schist

consists of garnet-muscovite-zoisite schist (meta-calcsilicate),garnet-tourmaline-muscovite schist,

andgarnet-muscovite greenschist (meta-pelite), as well as garnet-muscoviteortho-greenschist (meta-

basite) in which from pseudosection analysis, these areincluded intogreenschist-facies(T= 510–580 °C

and P= 0.74–1.08 Gpa). Furthermore, very low- to low-grade metamorphic rocks or metamorphosed to

un-metamorphosed rocks transitionsaredominantly composed by structural contact. Ortho-rock and

meta-peliteprotolith analysis show respectively MORB and island arc affinities before subducted and

cropped out in the accretionary wedgeby accretionary type exhumation. New discoveries about

Northern part of Luk Ulo Mélange Complex from very low- to low-grade metamorphic

rockperspectives, hopefully can be used to complete the geological history in this area.

Keyword: zeolite-facies, greenschist-facies, very low- to low-grade metamorphism, Gebang

River

1. Introduction

1.1. Background Study

Temperature/grade range of metamorphism has been divided into five parts, namely

very low-, low-, medium-, high-, and very high-grade metamorphism. In this research,

detail investigation will only be focused on very low- to low-grade metamorphism. Very

low-grade metamorphism covers the transition between diagenesis and metamorphism

(zeolite and prehnite-pumpellyite-facies), while low-grade metamorphism belongs to

greenschist-facies or epizone (Arkai et al., 2007). According to Smulikowski (2007),

diagenesis represents outer crust processes (can be shallow or deep diagenesis) causing

change in chemical, mineralogical, physical, and biological features including weathering

process and alteration of unstable mineral (e.g. smectite to illite or kaolinite to dickite due

to increasing in deep). While, metamorphism is described as change in mineralogy and

chemical composition, as well as structure of the rock due to elevated pressure and

mailto:[email protected]



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temperature (Bucher and Frey, 2002). Very low-grade metamorphism is distinguished from

diagenesis by higher P/T condition and the position in the earth crust (Arkai et al., 2007).

Previous researchers (e.g.Setiawan et al., 2012; 2013; Kadarusman et al., 2010;

2012; and Miyazaki et al., 1998) have conducted detail investigation about medium (MP) to

high pressure (HP) metamorphic rock, especially in the middle to southern part of Luk Ulo

Mélange Complex (Muncar, Luk Ulo, and Gua Rivers). They concluded that the Luk Ulo

Complex was resulted from metamorphism developed in accretionary system).The typical

of metamorphic rock formed in this environment will have been high varieties of

metamorphic rock in age and grade due to diachronous-progressive metamorphism

(Miyashiro, 1994). However, detail investigation about very low- to low grade metamorphic

rock itself is still very lack to be done. In this time, the investigation will be established on

very low- to low grade metamorphic rock in which those are relatively abundant in the

Northern Part of Luk Ulo Mélange Complex (e.g. Gebang River, Loning River, and

Lokidang River).Hopefully, from this research, it can provide an actual data to add and to

complete the geological history of Luk Ulo Mélange Complex from very low- to low-grade

metamorphic rock perspective.

1.2. Location

This research is located in Gebang River, Kaliwiro Area, Wonosobo District,

Central Java, Indonesia (Fig. 1).It is included in the Northern part of Luk Ulo Mélange

Complex and it takes about one-half hour trip from Wonosobo City. Gebang River is the

most upstream part of Luk Ulo River. It is relatively subparallel and in line with Lokidang

and Loning Rivers. This location was selected because several aspects that are representing

the Northern Part of Luk Ulo Mélange Complex, dominantly consist of very low- to low-

grade metamorphic rocks, and good accessibility (this river can be traced from the

metamorphic till the un-metamorphosed rocks). Detail investigation was only committed

along the river startedfrom the triple junction of Gebang and Maetan Riversuntilthe un-

metamorphic rock were found.

1.3. Aims and Objectives

This research was conducted tounderstand the very low- to low-grade metamorphic

condition and to complete the whole metamorphic grade identification that was still lack of

data.Hopefully, all of the data can be used to support and to complete the geological

history interpretation of this area. Furthermore, this research will only be focused to

understand[1] P/T condition of metamorphic rock, [2] protolith variation, [3] relation

between these rocks and other rock units, and [4] tectonic environment pre- and syn-

metamorphism.

2. Methodology

Petrology and geochemistry analyses are the powerful approach to understand the

metamorphic rock. Petrology anaysis includes basic hand speciment and petrographycal

observation, while the geochemstry analysis devided into two categries that are mineral

geochemistry and rock geochemisry. To answer the objectives of this study several analyses

are needed to be commited as follows; petrography, XRD, SEM-EDS, and XRF analyses in

which XRD and SEM-EDS are including into mineral geochemistry, in contrast XRF is

belong to bulk rock geochemistry. Detail explanation about each method will be delivered in

the section below.

2.1. Petrography Analysis



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Several rock samples had been selected by hand specimen observation using

magnifying glass before petrography analysis was conducted. It was committed to get the

representative sample from each facies metamorphism. There are three modes of

observation in this analysis that are Plane Polarized Light (PPL), Cross Polarized Light

(XPL), and conoscopic modes.This analysis will provide data such as mineral assemblage

(including facies metamorphism), mineral abundancy, rock structure, rock texture, and

rock name (refer to Scmidh, 2007). These data will be used as basic information and

support the advanced analysis (e.g. mineral assemblage data will be used to determine the

P and T estimation from pseudo section data based on bulk rock geochemistry data). All of

the analysis procedures were conducted in Optical Geology Laboratory UGM using

Olympus CX31 refractive polarization microscope.

2.2. XRD Analysis

Different form Petrography analysis, XRD analysis was used for very fine- to fine

grain mineral such as clay size minerals (including zeolite minerals).This analysis will

provide mineral assemblage and their abundancy data in which it will be used for facies

metamorphic and P/T metamorphism determination. XRD preparation and analysis

wereconducted in Laboratory Centre of Geological Department UGM using Rigaku

Multiflex 2 Kw andtwo kinds of preparation methods (air dried and ethylene glycol

methods) were used for observe the mineral characteristics (e.g. swelling capacity).

Mineral determination was done referring to table of key line in XRD pattern of mineral in

clay by Chen (1977).

2.3. SEM-EDS Analysis

SEM-EDS/EDX, a quantitative method, is a geochemical for mineral analysis. It

was used to determined and to ensure the kind of mineral that can be established by

petrography analysis. This analysis will produce the element composition (it can be oxide

data or atomic composition) of analyzed mineral and then the mineral composition will be

calculated by DHZ stoichiometry method and classified depending on their mineral group.

While, Fe+3 and Fe+2separation in pyroxene mineral was referred to Droop et al., (1987).

According to Rollinson (1994),the problem of X-Ray analysis is its limit detection in light

element (element which has atomic weight less than 4 such as Be, Li, He, and H)cannot be

detected in this research. However, in this research B element from tourmaline mineral

with 5 atomic mass still cannot be detected even with the small energy electron beam.

Therefore, mineral that contain a lot of these minerals need to be normalized with the

sample standard (in this research using mineral data from Deer et al., (1966)). This analysis

used Field Emission Scanning Electron Microscope FEI quanta 650 type, as well asX-Act

OXFORD EDS/EDX detector, ETD. Secondary detector used VCD Back Scanning

Electron Microscope (BSE), and palladium-gold coating method.

2.4. XRF Analyses

This analysis was committed to get the information about Protolith variation and its

tectonic environment as well as P/T estimation using pseudosection based on bulk rock

geochemistry data.Pseudosection data was obtained using Perplex 6.7.7 software. Bulk

rock geochemistry data was obtained with Wave length (WD)-XRF type using Analytical

AXIOS MAX PW4400 and fresh pellet sample preparation method added by Licowax

Powder and pressed by Atlas Power Specac T.25 (25 ton). SEM-EDS and XRF analyses

were conducted in the laboratory of Institute of Geological Hazard Technology

Investigation and Development, Yogyakarta.



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3. Result

3.1. Facies Metamorphism and Protolith Variation

At least there are two dominant facies metamorphisms that are cropped out along

Gebang River. That are greenschist-facies in the southern part and zeolite-facies in the

northern part of the Gebang River (Fig. 2). These facies metamorphism also are formed

from several protoliths such as pelitic rock, ortho-rock, calc-silicate rock, and meta

calcareousbreccia.Facies and protolith determination are concluded based on mineral

assemblage investigation.Detail explanation will be delivered in the section below.

3.1.1. Zeolite-facies

Zeolite-facies represents very low-grade metamorphic rock. According to

Miyashiro (1993),it is characterized by the presences of zeolite minerals (e.g.

phillipsite, analcime, laumontite, natrolite, etc.) + quartz. Commonly, it also causes a

green color to the rock (Fig. 4c).The zeolite-faciesis cropped out on the northern part

of Gebang River which has normal fault contact with the greenschist-facies in the

southand basaltic lava in the north of Gebang River (Fig. 2).

Pelitic rock

Pelitic rock refers to argillaceous sedimentary rock protolith (e.g. shale and

greywacke). Zeolite-facies from this kind of protolith is known by its remain

sedimentary texture which shows well-bedding and lamination.A best sample of this

type is only seen in stop site 26as a floating rock fragment (Fig. 3g). From XRD

analysis this rock consists of smectite, phillipsite, and illite as very low-grade

metamorphic minerals.Ilt-Php-Smec assemblage indicated temperature metamorphism

range around 120 – 200 °C (Lagat, 2009; Reyes, 1989).

Ortho-rock

Ortho-rock terminologyrefers to igneous rock protolith (Scmidh et al., 2007). This

rock was found in STA 20 as a bed rock and there was also found eroded chert above

the igneous rock surface (Fig. 3d). From petrography, XRD, and SEM-EDS analyses,

this kind of rock contains oligoclase and augiteas a primary mineral as well as

palagonite (Fig. 5c), celadonite (Fig. 7a), zeolites (phillipsite, natrolite, and

laumontite), calcite, goethite, and hematite as a very low-grade metamorphic mineral.

Primary mineral is seen by primary texture and appropriate mineral assemblage for

meta-basalt and ensured by geochemical classification (Fig. 8). Palagonite considered

as initial, metastable, replacing product of sideromelane during palagonitization, the

final form of replacement product being smectite (Pauly et al., 2011). Another unique

alteration mineral found meta-basalt rock is celadonite. Celadonite, one of mica

member mineral, is a late-alteration product to very low-grade metamorphism mineral

from volcaniclastic rock, vesicular basalt, or olivine basalt replacing olivine,

hypersthene, and/or groundmass material (Wilson, 2013; Hendricks and Ross, 1941).

From experimental study, it stables up to temperature 250 °C and may survive up to

350 °C and pressure 2-7 Kbar (Wise and Eugster, 1964 and Velde, 1972 in Wilson,

2013).

The occurrences of palagonite and celadonite in this sample indicate that this sample is

related with basaltic igneous rock (Jacobsson, 1978; Stroncik and Schmincke, 2001),

probably basalt.

Meta-calcareous breccia



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Meta-calcareous breccia is given for the kind of rock which has breccia texture (Fig.

4h) and contains zeolite mineral and a lot of crystalline carbonate mineral (calcite).

Fragments of this rock contain meta-ortho rock, quartzite, meta-pelite, and mica-

schistthat also suffer very low-grade metamorphism. While, matrix of the rock

contains of calcite, clay size mineral, and clay size sedimentary materials (Fig.

4a).Coarse calcite crystal is derived from lime mud (micritic) which underwent very

low-grade metamorphism. Even in phreatic zone diagenesis level, micrite has

sporadically changed into medium to coarse calcite mineral/cementsuch as blocky

isophacus, syntaxial, etc.(Longman, 1980; Bogs, 2006). So, in higher Pand T

condition (very low-grade metamorphism), primary texture of micrite will have

vanished and changed into coarse calcite minerals. The clay sized mineral consists of

quartz, smectite, phillipsite, celadonite, and analcime as very low grade metamorphic

mineral. Celadonite in this type of protolith present as vesicular-filling in basalt

fragment (Fig. 5b).

3.1.2. Greenschist-facies

Greenschist-facies include in low-grade metamorphic rock. It is characterized

by the existence of green mineral (e.g. chlorite, actinolite, epidote), muscovite, and

garnet (Yardley, 1983 and Best, 2003). At least there are three types of protoliths that

were found along Gebang River those are; [1] pelitic rock, [2] ortho-rock, and [3] calc-

silicate rock.

Pelitic rock

Pelitic protolith in greenschist-facies is found dominantly in research area. It shows

well-foliation rock texture (Fig. 3b) which also represents stratification before the

metamorphism.This protolith can be divided into two groups, those are [1] Ms-Chl-

Act bearing schist and [2] Tur-Ms bearing schist. The first group dominantly consist of

quartz-muscovite-chlorite-albite mineral (Fig. 6a), while the second groupconsists of

tourmaline (Fig. 6b) as atrace mineral which also relatively low quartz content

compared than the other pelitic rock (Tab. 1).Tourmaline mineral is proved by

geochemical calculation (Tab. 2) with normalization of B2O3from Deer et al., (1966).

Tur-Ms bearing schist can be interpreted to be formed from shale protolith. Because in

metamorphosed argillaceous sediment (shale), generally tourmaline may be produced

by boron metasomatism or recrystallization process of detrital grain from the original

sediment (Deer et al., 1966). While, Ms-Chl-Act bearing schist is interpreted to be

formed from pelitic rock which contain feldspar mineral, probably Meta-feldspartic

wacke.

Ortho-rock

Ortho-rock protolith was only found as a floating material on bed stream. In hand

specimen, this rock showed greenish black color and higher in density than the pelitic

or calc-silicate protolith. So, it is quite easy to be distinguished from others. It

dominantly consists of actinolite mineral and small number of muscovite, garnet,

epidote, chlorite, and rutile (Tab. 1). This rock showed poor-schistocity texture (Fig.

6c), it is caused by only a few platy minerals who present in this type of protolith.

Calc-silicate rock

Calc-silicate rock represented Ca or Ca-Mg rich sediment (Yardley, 1989). In the

research area, the presence of this kind of protolith is characterized by the occurrence

of zoisite-albite-chlorite-muscovite-sphene mineral (Tab. 1) and only presents in

restricted area (STA 11). Zoisite, a calcium rich epidote, is a typical constituent of



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medium grade regionally metamorphosed rock from marly composition (Deer et al.,

1966). In thin section, it is characterized by high relief, bluish gray interference color

and parallel extinction angel (Fig. 6d). The occurrences of zoisite, muscovite, and

quartz in this rock indicated that this rock may be derived from carbonate bearing

sedimentary rock (marl).

3.2. Change in Facies Metamorphism and Relation with Other Rock Units

3.2.1. Zeolite- and greenschist-facies contact

Zeolite in research area was found in the northern part of Gebang River, while

the greenschist-facies was found in the southern part of the river bounded by normal

fault (Fig. 2). The fault was not found during the fieldwork observation because the

transition of green schist and zeolite-facies is covered by giant chert, red limestone,

basalt, andesite rock fragment.However, based on the additional data from Lokidang

River, there was a normal fault with N15°E/50° in direction between the greenschist in

the south and the zeolite-facies in the north in which greenschist-facies relatively

moved downward toward the zeolite-facies (Fig. 3). It is supported by the elevation

contrast between the zeolite elevation and greenschist elevation. Nevertheless,

generally speaking, this condition suggested that the metamorphic grade is relatively

decrease northward.But, intense tectonic activity causing the green schist facies is

exposed between the zeolite-facies and basaltic lava in the northern part (Fig. 3).

In the section AB (Fig. 2), the contrast condition appeared when the zeolite-

facies was relatively move down toward the greenschist-facies. In this section also

showed a complex fault system which is dominantly composed of normal faulting

with N-S and NE-SW direction.

3.2.2. Metamorphic rocks and un-metamorphosed rock contact

Beside the metamorphic rock and metamorphic rock contact, the metamorphic

rock also directly contacts with the sedimentary rock such as scaly clay in the south

part and basaltic lava in the north part (Fig. 3 A-B section).

Greenschist-facies and scaly clay

In the south part, Gebang River is composed by scaly clay unit (Fig. 4a) covered by

fluvial deposit unconformably. Then, it was faulted and caused direct contact between

the scaly clay and the greenschist-facies (Fig. 3). But,itwas very difficult to determine

the fault type because the fault zone had been strongly altered, so there was no fault

plane to be measured. According to Prasetyadi (2007), the green schist and zeolite-

facies are bounded by normal fault. But, it is very difficult to be trusted because

commonly scaly fabric is formed by thrusting mechanism (Fig. 9) produced when the

oceanic plate carrying argillaceous sediment is subducted in the shallow depth shortly

after entering the subduction zone (Frist et al., 2011). So, in this research the fault

between greenschist-facies and scaly clay is interpreted as a thrust/reverse fault (Fig. 3

BE section).

Greenschist-facies and basaltic lava

Greenschist-facies was also found in the north part whichdirectly contact with the

basaltic lava.Normally the higher grade metamorphic rock was trusted above the un-

metamorphic rock/lower grade metamorphic rock. But in this research area, the low-

grade metamorphic rock which relatively moves downward toward the un-

metamorphic rock is the unique creature developing in the research area. Another

evidence is the discovery of uncommon olistolith in which high pressure metamorphic



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rock descend upon the low pressure metamorphic rock (Fig. 4e). So, it can be

concluded that the intense trusting during subduction caused higher grade

metamorphic rocks were exposed and located above the lower grade metamorphic

rocks or un-metamorphic rocks. Afterwards, extension regime after subduction caused

the higher grade metamorphic rocks were faulted and moved down causing contact

between these rocks in the same crust level (Platt, 1986)

3.3. P/T Metamorphism Condition

3.3.1. Zeolite-facies

P andT condition in zeolite-facies is quite difficult to be determined using

pseudosection analysis. Temperature metamorphism is predicted using mineral

geothermometric by Lagat (2009), Reyes (1990), and Miyashiro (1994).However,

pressure condition in zeolite-facies cannot be predicted using mineral assemblage, so

pressure condition of zeolite-facies is referred to Miyashiro (1994).P/T formation of

this facies from mineral assemblage analysis yielded 120–220 °C (Tab. 3) with 1–5

kbar. This pressure and temperature condition belongs to burial metamorphism type

with relatively little or no movement. According to Erns (1975; 1977) the lowest grade

metamorphic rock tends to be the youngest metamorphic rock and represents oceanic

toward growth accretion zone.

3.3.2. Greenschist-facies

P/T metamorphic condition of greenschist-facies is determined by pseudo-

section approach using bulk rock geochemistry data. Two samples were selected to

exhibit the P/T condition. The first sample is taken from the southern greenschist-

facies (S5-01) that contact with the scaly clay. It yielded 555–580°C and 0.94–1.08

GPa (Fig. 10a). While, the second sample is taken from the northern greenschist facies

(S16-01) before structurally contact with the zeolite-facies. It yielded 510–524°C and

0.74–0.88 GPa (Fig. 10b). From these two samples, it concludes that the Pand T

condition is decrease northward the Gebang River. Based on well-schistosic texture,

PandT condition, and great distribution of various metamorphic facies in the research

area, thisfacies is related with the regional metamorphism with subduction zone

metamorphism environment.

3.4. Tectonic Setting

Tectonic setting determination is conducted on two protolith types, those are pelitic

rock and ortho-rock protolith. The fresh sample of greenschist-facies from ortho-rock

protolith is only found 1 sample. Therefore, to make a good sample distribution, medium-

to high-pressure metamorphic rock data is added to conclude the analysis.According to

Pearce (1973; 1982), using Zr, Y3, and Ti/100 tertiary diagram and Y vs Cr trace element

data, ortho-rock protolith showed from MORB tectonic setting (Fig. 11a).MORB protolith

was also mentioned by Setiawan et al.(2012)as one of tectonic setting beside the within

plate basalt tectonic setting.

While, according to Bhatia and Crook (1986) using major element data such as

total Fe2O3+MgO vs K2O/Na2O and Fe2O3+MgO vs Al2O3/SiO2 as well as Roser and

Korsch (1988 in Rolinson, 1993) using SiO2vs K2O/Na2O diagram, the pelitic protolith

was formed in island arc tectonic setting (Fig. 11b) before subsequently subducted with the

oceanic plate. The occurrence of island arc protolith in the Luk Ulo Mélange Complex has

never beenreported, so this data can be used to complete the geological history about

metamorphic rocks which presents in Karangsambung area.



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Various protolith reported such as meta-basite from within plate basalt, N-MORB,

andE-MORB (Setiawan et al., 2012); meta-psamite/meta-granite from continental

supracrustal parentage (Kadarusman et al., 2010); and meta-pelite from island arc

environment (this research)indicates that that Luk Ulo Complex is not a simple result of

subduction zone but a complex accretion zone.

4. Discussion

4.1. Zeolite-facies and Greenschist-Facies Transition

Along the Gebang River, zeolite- and greenschist-facies were bounded by normal

fault.There is a gap in pressure andtemperature condition between these two facies.

Generally, in prograde metamorphic rock, transition zone between zeolite- and greenschist-

facies is occupied by prehnite-pumpellyite-facies. It is characterized by the presences of

prehnite and/or pumpellyite mineral and disappearing of zeolite mineral in quartz bearing

rock (Miyashiro, 1994). Intense trusting and continued by normal faulting and erosion

processes caused no prehnite-pumpellyite appeared in the surface. An important data about

the presence of prehnite-pumpellyite facies that is the discovery of retrograde garnet-

staurolite pargasitite (retrograde amphibole-facies). This rock contains a lot of retrograde

mineral such as calcite and prehnite minerals cutting medium-pressure metamorphic

minerals (Fig. 12). Thus, it could be interpreted that this rock had reached medium-

pressure condition, then suffered the prehnite-pumpellyite-faciespressure and temperature

condition during exhumation process. Based on mineral prehnite-calcite stability by Lagat

(2009), temperature formation of this facies is around 260 – 320 °C and 1 – 7 Kbar (Winter,

2001). Therefore, based on this data, it can be assumed that the prehnite-pumpellyite-facies

is actually exist below the surface but in unknown crustal level (Fig. 3).

4.2. Green schist-facies soft matrix as exhumation agent of MP to HP metamorphic

rocks

Exhumation mechanism of MP to HP metamorphic rocks in specific subduction

zone is slightly different. Various researcher such as Guillot (2009) with serpentinite

channel, accretionary type, and continental type mechanism. Jolivet et al. (2003) with

coaxial extension associated with a decoupling fault, Steck et al. (1998) buoyancy assisted

by erosion and tectonic process, etc.

The characteristics of MP to HP metamorphic rock in the research area are [1]

dominantly consist of MP (amphibolite facies) metamorphic rock and only a few of HP

metamorphic rocks (blue schist facies) with no mantle-derived materials suffering

retrograde to green schist-facies in the outer part, [2] metric to hectometric lenses blocks

enclosed in para-green schist matrix (Fig. 4b and 4e), and [3] neither covered nor

uncovered by quartz-calcite ± chlorite± epidote ± sphene ± tourmaline minerals before

contact with the green schist matrix, and [4] a few of that have structurally contact with the

green schist matrix.In additional, there also presents stacking of oceanic sediment for

instance chert and red limestone with metric to hectometric in scale. These characteristics

are similar to accretionary type exhumation mechanism proposed by Guillot et al. (2009) in

which it dominantly consists of meta-sediment protolith from oceanic sediments and upper

oceanic crust protolith then respectively subducted, metamorphosed, and exhumed in the

forearc accretionary wedges or prism. The MP to HP metamorphic rock are carried up

from a great depth by thrust fault system during subduction (Guillot, 2009)or by

subduction erosion of oceanic crust (Stern, 2011)which then proceed by trapped in the

green schist-facies matrix and ascent to the surface driven by conduit flow due to

continuous subduction (Fig. 13a). As already mentioned before, contrast P and T condition

may be found in this area because underplating mechanism during compression regime.



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Afterward, it is subsequently followed by normal fault system due to change of

compressional regime into extensional regime (Fig. 13b).

This data shows that the exhumation mechanism in the northern part of

Karangsambung area (this research area) is totally different with the southern part (Kali

Muncar and surroundings). Southern part area dominantly consists of HP metamorphic

rocks (eclogite-facies and blue schist-facies) carried by serpentinite matrix or renowned as

a serpentinite type subduction channel type. While, the Northern part is strongly related

with an accretionary type exhumation mechanism.

5. Conclusion

Pressure and temperature metamorphism condition of zeolite-facies (very low-grade

metamorphic rock) is formedaround 120–220 °C and 1–5 Kbar, while greenschist-facies

(low-grade metamorphic rock) is around 510–580°C and 0.74–1.08 GPa. P/T condition is

decrease northward/ toward the upstream. Prehnite-pumpellyite-facies formed in 260 –

320 °C and 1 – 7 Kbar presents as zeolite- to green schist-facies in unknown depth.

The zeolitic-facies consist of celadonite-natrolite-palagoniteformed from meta-basalt

protolith, zeolite meta-calcareous polymictic breccia is formed from calcareous polymictic

breccia protolith, and zeolite meta-pelite is formed from pelitic rock protolith. While,

greenschist facies thatconsist of garnet-muscovite-zoisite schist is derived from calc-silicate

protolith, garnet-tourmaline-muscovite schist and garnet-muscovite greenschist areproduced

from pelitic rock protolith, as well as garnet-muscovite ortho-greenschist isformed from

meta-basite protolith.

Low-grade to very low-grade metamorphic rock andto un-metamorphosed rock transitions

in Gebang River aredominantlycomposed by geological structure. Greenschist-facies and

zeolite-facies is bounded by NE-SW normal fault, zeolite-facies and basaltic lava bounded

by N-E normal fault, scaly clay and greenschist-facies is bounded by thrust fault.

Meta-ortho rock protolith is resulted from MORB tectonic environment, while meta-pelite

protolith shows indication as island arc protolith. Furthermore, these two tectonic settings

are subducted and metamorphosed in subduction zone environment. Afterward, these rocks

are cropped out in the cretaceous accretionary wedge/prism by accretionary type exhumation

mechanism.

Acknowledgements

The authors acknowledge the input from all of my friend from UGM, especially my

laboratory partner M. Fikri. A and Aloysius. A, Anzja C.I who give me an information about

this research area, Afif. A, Arif. Z, and Hagi. R who accompanied me during one-week

fieldwork.Also, thank you very much to Bapak, Ibu for the advice and keep me on track) and

to all of Institute of Geological Hazard Technology Investigation and Development

Yogyakarta staff who always help me during laboratory activities. The research was covered

by Beasiswa 2000 (a scholarship research from UGM alumni batch 2000)

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1102

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Figure. 2. Location of research area. The observation will only be focused along Gebang River

because fresh metamorphic rock is only found on there.



1103

Figure 2. Facies metamorphism distribution along Gebang River



1104

Figure 3. Geological section along Gebang River showing facies metamorphism distribution



1105

Figure 4. Metamorphic rock outcrops along Gebang River. a) Scaly clay with scaly fabric and various

rock fragment such as quartzite, red limestone, basalt, conglomerate, and mica schist indicating

mélange tectonic process because of subduction, b) low-grade metamorphic rock (mica schist) with

enclosed medium pressure metamorphic rock inside, c) Ze meta-calcareous polymictic breccia

showing green color due to presences of zeolite minerals, d) Cel-pal-ntr meta-basalt rock showing

greenish and reddish black color caused by zeolite and palagonite minerals, high pressure

metamorphic rock structurally contact with low grade metamorphic rock appears as olistolith above

basaltic lava bed rock, f) reddish black basaltic lava, red color probably caused by presence of

palagonite mineral, g) ze meta-pelite rock fragment showing lineation texture and green color, and h)

breccia texture observed from very low-grade metamorphic rock.

Blue schist-facies

Low-grade metamorphic rock

a) b)

c) d)

e) f)

g) h)

Basalt-Andesite frg

Conglomerate frg

Scaly fabric

Clay matrix Basalt-Andesite frg

Mica schist

Medium to – high-pressure metamorphic rock

Low-grade metamorphic rock

Quartzite

IRFrg

Zeolites

Chert Cel-pal-ntr meta-basalt

Chert



1106

Figure 5. Photomicrograph of Zeolite facies from different kinds of protoliths. a) Ze meta-calcareous

polymictic breccia (S18-02) showing breccia texture, igneous and sedimentary rock fragment which

also contain very low-grade metamorphic mineral (smectite and zeolite) with large calcite crystal as

roc matrix, b) Ze meta-calcareous polymictic breccia (S17-03) with dominant of igneous rock

fragment in composition, c) Cel-pal-ntr meta-basalt (S20-01) showing mineral filing cavities and

replacement texture by celadonite and palagonite minerals, d) the appearance of natrolite mineral

replacing oligoclase mineral in Cel-pal-ntr meta-basalt (S20-01)

Figure6. Photomicrograph of Green schist facies from different kinds of protoliths. a) Grt-Ms

greenschist (S5-01) showing schistossic texture and Chl-Act-Ab-Qz-Grt assemblage which may be

Grt

Cal

IRFrg

Smec

Opq

SRFrg

Ze

Cel

Pal Olg

Aug

Ntr

Olg

Olg 0.2 mm

1 mm

0.2 mm

1 mm

1 mm 1 mm

Ms

Chl

Qz

Ab

Ab

Act

Act

Ms

Act

Grt

Ms

Opq

Ab

Zo

Ab

Chl

Ms

Zo

Ms

Grt

a)

c)

IRFrg

IRFrg

Cal Cel

Smec

1 mm Smec

b)

d)

a)

c)

b)

d)

1 mm

Grt

Ab

Chl

Tur

Grt

Qz

Ms



1107

produced from feldspartic wacke protolith, b) Grt-tur-ms greenschist (S15-01) shows schistossic

texture and Grt-Ab-Ms-Qz-Tur assemblage indicating shale protolith, c) Grt-Ms ortho-greenschist

showing poor-schistosity texture and Act-Grt-Ab-Ms assemblage representing ortho rock protolith, d)

Grt-Ms-Zo Schist is dominantly composed by zoisite mineral indicating calc-silicate protolith

Figure 7. Back Scattered Electron images from zeolite and green schist facies. a) Cel-pal-ntr meta-

basalt (S20-01) from BSE image shows different in mineral composition in which bright color

indicating electrolyte mineral, while the dark color is less in metal element composition, b) Grt-tur-ms

greenschist (S15-01) from BSE image exhibits tourmaline and albite minerals

Olg

Cel Pal

Aug Tur Ab

Pore a) b)

a)

b)



1108

Figure 8. Geochemistry mineral classification from S20-01 (Cel-Pal-Php Meta-basalt). a) Plotting of

pyroxene group mineral identified as augite mineral (Modified after Droop et al., 1966) and b) plotting

of plagioclase group mineral identified as oligoclase mineral (Modified after Deer et al., 1992)

Figure 9.Schematic model scaly fabric formation corresponds with Luk Ulo Mélange characteristics

(Modified after Frisch et al., 2011)

Figure 10.Pseudosection for P and T estimation in Green schist facies. Pseudo section from bulk rock

geochemistry saturated with H2O from sample S5-01 (SiO2= 66.01, Al2O3= 14.65, CaO= 1.83, MgO=

3.29, FeOtot= 4.62, Na2O=3.36) and S16-01 (SiO2= 63.08, Al2O3=14.33, CaO= 2.87, MgO= 3.14,

FeO= 6.23, Na2O= 3.72). P and T estimation using pseudosection showed that southern part of green

schist facies (S5-01) is higher than the northern part (S16-01)

a) b)



1109

Figure 11.Tectonic setting determination using geochemical data. Analysis from ortho-rock protolith

produced MORB setting as tectonic setting and b) island arc setting is resulted from pelitic protolith

analysis (After Pearch, 1973; 1982; Bathia and Crook, 1986; Roser and Korsch, 1988 in Rollinson,

1993)

a)

b)



1110

Figure 12.Prehnite-calcite assemblage in retrograde amphibolite facies

Figure 13.Accretionary type exhumation mechanism model and its metamorphic-grade distribution.

Pro-wedge, retro-wedge, and conduit exhumation in subduction zone (Modified after Ernst, 2005) and

b) Schematic succession of HP Cycladic blueschist belt in Greece as underplating followed by normal

fault because of extension regime after compression regime (Modified after Forster and Lister, 2005 in

Guillot, 2009)

Cal

0.2 mm

Prh

Prg

Prg

Prh

Cal

0.2 mm

Prh

Prg

Prg

Prh



1111

Table 1. Mineral assemblages and their facies and protolith interpretation based on petrography analysis

Ab Act Ap Bt Cal Cel Chl Ep SRFrg IRFrg Grt Hem Ms Opq Pal Pl Qz Rt Spn Tur Ze Zo

S2-02 ○ ● - - □ - □ - - - □ - ◊ - - - - - - - - ○ Green schist Ortho-rock Grt-Ms-Zo ortho-greenschist

S3-02 ● □ - - ◊ - ○ - - - ◊ - ◊ □ - - ◊ - □ - - □ Green schist Pelitic Grt-Ms greenschist

S4-02 ● □ - □ ◊ - ◊ - - - □ - ◊ □ - - ◊ - - - - □ Green schist Pelitic Grt-Ms greenschist

S5-01 ● □ - □ □ - □ - - - ◊ - ○ - - - ◊ - □ - - - Green schist Pelitic Grt-Ms greenschist

S6-01 ● - - - - - □ - - - ◊ - ○ □ - - ○ - ◊ - - □ Green schist Pelitic Grt-Ms Schist

S7-01 ● - - - □ - □ - - - ◊ - ○ □ - - ◊ - □ - - □ Green schist Pelitic Grt-Ms greenschist

S8-04 ● □ ● - - - □ - - - ● - ◊ - - - □ □ □ - - - Green schist Pelitic Ms-Grt greenschist

S09-01 ○ ● - - - - ◊ - - - ◊ - ○ - - - □ □ - - - □ Green schist Ortho-rock Grt-Ms ortho-greenschist

S11-01 ● - - - - - ◊ - - - ◊ - ◊ - - - □ - ◊ - - ○ Green schist Calc-silicate Grt-Ms-Zo Schist

S15-01 ● □ - - ◊ - ○ - - - ◊ - ◊ □ - - □ □ □ □ - □ Green schist Pelitic Grt-Tur-Ms greenschist

S16-01 ● □ - - □ - ◊ □ - - ◊ - □ - - ◊ - □ - - □ Green schist Pelitic Grt-Ms greenschist

S16-03 ● □ - - □ - ◊ - - - ○ - ○ - - - ◊ - □ - - □ Green schist Pelitic Grt-Ms greenschist

S16-04 ● - - - ● - ◊ - - - □ - ◊ - - - ◊ - - - - ◊ Green schist Pelitic Grt-Zo-Ms Schist

S17-03 ● - - - - □ - - □ ● - - - - - - - - - - ◊ - Zeolite Meta-calcareous breccia Ze meta-calcareous polymict breccia

S18-02 ● □ - - - □ - - ○ ● - - - □ - □ - - - - ◊ - Zeolite Meta-calcareous breccia Ze meta-calcareous polymict breccia

S20-01 □ - - - □ □ - - - - - □ - ○ ◊ ● □ - - - ◊ - Zeolite Meta-basalt Cel-pal-ntr meta-basalt

Sample

listsFacies Rock Name (IUGS, 2007)

Primary MineralsProtolith

Very abundant ●

Abundant ○

Common ◊

Rare □

Ab= Albite, Act= Actinolite, Ap= Apatite, Bt= Biotite, Cal= Calcite, Cel= Celadonite, Chl= Chlorite, Cpx= Clinopyroxene, Ep= Epidote, IRFrg= Igneous rock Fragmen, SRFrg= Sedimentary rock Fragment, Grt= Garnet, Hem=

Hematite, Ms=Muskovit, Ntr= natrolite Opq= Opaque minerals, Pal= Palagonite, Pl= Plagioclase, Qz= quartz, Rt= Rutile, Spn=Sphene/titanite, Tur=Tourmaline, Ze= Zeolite, Zo= Zoisite

Mineral abbevations (Witney dan Evans, 2010) Mineral abundancy



1112

Table 2.Mineral geochemistry result using SEM-EDS analysis. Zeolite facies form ortho-rock

protolith contain celadonite, palagonite, oligoclase, and augite. Palagonite composition is normalized

by H2O content from Stroncik and Schmincke (2002) and green schist facies showed albite and

tourmaline. Tourmaline was normalized by B2O3from Howie et al. (1966)

Zeolite facies (S20-01) (Green schist facies (S15 01)

Oxide

Celadonite

Palagonite

Oligoclase

Augite

Albite

Tourmaline

SiO2 52.950 23.02 56.410 46.270 70.07 35.76

Al2O3 7.440 4.73 17.490 3.630 19.72 31.01

Fe2O3* 21.660 28.18 3.540 18.440 0.01 6.24

TiO2 0.020 0.37 9.360 1.380 0.02 0.88

MnO 0.000 0.19 0.000 0.300 0.00 0.03

MgO 5.850 2.08 0.640 13.690 0.05 8.23

CaO 0.090 0.26 3.950 15.650 0.11 0.48

Na2O 0.050 00.06 7.490 0.350 9.94 2.74

K2O 11.950 3.86 1.040 0.210 0.07 0.06

Cr2O3 0.000 0.05 0.070 0.090 0.00 0.09

Br2O3 - 0.00 10.73

H2O 0.00 37 0.01 0.00 0.01 4.12

Total 100.01 99.70 99.99 100.01 99.99 100.00

Basis

O/OH/F

24 12 32 6

32 31

Si 8.269 3.265 10.376 1.798 12.126 6.289

Al 1.369 0.791 3.791 0.166 4.02 6.427

Fe* 2.828

3.342 0.544

0.599 0.001 0.917

Fe3+ - - - 0.090 - -

Fe2+ - - - 0.509 - -

Ti 0.002 0.039 1.295 0.040 0.003 0.116

Mn 0.000 0.018 0.000 0.008 0.00 0.003

Mg 1.361 0.439 0.176 0.793 0.013 2.157

Ca 0.015 0.039 0.778 0.651 0.020 0.090

Na 0.015 0.016 2.671 0.026 3.335 0.934

K 2.380 0.696 0.244 0.010 0.015 0.013

Cr 0.000 0.005 0.010 0.003 0.00 0.012

Total 16.241 8.654 19.886 4.096 19.536 16.963

Ab - - 72.318 - - -

An - - 21.075 - - -

Or - - 6.607 - - -

Aug - - - 86.895 - -

Jd - - - 7.160 - -

Ae - - - 8.764 - -

Ca (Wo) - - - 31.753 - -

Mg (En) - - - 38.652 - -

ΣFe (Fs) - - - 29.595 - -



1113

Table 3.Geothermometry mineral based on zeolite facies

100 120 140 160 180 200 220 240 260 280 300 320

Anl

Cal

Cel

Php

Smec

Cel

Cal

Lau

Ntr

Pal

Ilt

Php

Smec

Zeolite meta-

pelite (S30-01)

Temperature Stability °C MineralSamples

zeolite meta-

calcareous

polymictic

breccia (S18-02)

Celadonite-

palagonite-

zeolite meta-

basalt (S20-01)

Mineral abbreviation from White and Evans (2007): Anl: analcime, Cal: calcite, Cel: celadonite, Ilt:

illite, Lau: laumontite, Pal: palagonite, Php: phillipsite, Smec: smectite

Documents

PETROGENESIS OF VERY LOW-TO LOW-GRADE …werecropped outs,whichare scaly clay, mica schist, zeolitic rocks, and basaltic lava, respectively from down- to up-stream. The zeolitic rock