PROCEEDING, SEMINAR NASIONAL KEBUMIAN KE-10 PERAN … · 2018. 3. 1. · proceeding, seminar nasional kebumian ke-10 peran ilmu kebumian dalam pembangunan infrastruktur di indonesia

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

13 – 14 SEPTEMBER 2017; GRHA SABHA PRAMANA

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CHARACTERISTICS OF BASALTIC PILLOW LAVA IN JARUM VILLAGE,

BAYAT: MAGMA EVOLUTION AND PETROGENETIC MODEL

Aditya Hirawan1*

Aya Shika V. Bangun1

Riski Budi Pratiwi1

Anastasia Dewi Titisari1

1Departemen Teknik Geologi, Fakultas Teknik, Universitas Gadjah Mada

Jalan Grafika 2, Yogyakarta

*corresponding author: [email protected]

ABSTRACT

Bayat Region have been recognized as fossils of tertiary volcanoes in Southern Mountains, Central

Java; with volcanic bodies such as dikes, sills, pyroclastics, and lava flows. Former studies reveals

there were four times of magmatisms occured in Southern Mountains which aged early Oligocene to

middle Miocene. However, former studies have not proved whether the outcropped volcanic rocks are

originated from the same magmatism through geochemistry study. This research is aimed to complete

the previous study by comparing the bulk rock geochemistry of basaltic pillow lavas in Jarum Village,

approximately 3 km to the south of Pendul Hill, with the Pendul intrusion body itself. Methods applied

are bulk rock geochemistry analysis (ICP-AES) and trace elements using ICP-MS, supported by

petrographic analysis. Bivariate plots constructed to show cogenetic trends between two sets of above

mentioned samples, while spider diagrams are drawn showing magma origin in respect with the

comparison between the abundancae of LILE towards HFSE. By geochemical meaning, Pendul

gabbroic–dioritic intrusion in Jiwo Hills shows early stage of magmatism while basaltic pillow lavas

in Jarum Village, as the younger-aged volcanic body, is expected to originate from the same

magmatism. The expected result of this research is a cogenetic trend on bivariate plots and in AFM

diagram, as well as the decrease of siderophile compatible trace elements in spider diagrams between

the two sets of samples. This experiment geochemically proves the process of magma evolution and

establishes petrogenetic model of Bayat magmatism in a specific scope.

Keywords: bulk rock chemistry, petrography, cogenetic trend, magma evolution

1. Introduction

Bayat Region, Central Java, is one of the most desired study area for petrogenetic geologists.

This area is very well-known by the outcrop of Central Java pre-tertiary metamorphic

basement, intruded by Early-Middle Tertiary magmatism, and lastly underlain by the tertiary

marine-originated sedimentary rocks. However, the fate of the tertiary magmatism in this area

is still debatable; yet tried to be proven by former researchers such as Surono (2006), Bronto

(2011), Akbar & Setiawan (2015), and Akbar (2016). This research tries an attempt to reveal

the magma evolution and the origin of basaltic pillow lava flows outcropped 3 kilometers to

the south of Jiwo Hills, the research object that had been used for any petrogenetic researches

in Bayat Region this far. Therefore, by describing the geochemical and petrographic

characteristics of other magmatic bodies and compare it to the Jiwo Hill itself, a new

breakthrough in understanding the magmatism in Bayat Region could have been proposed.

1.1. Regional Geology

Jiwo Hills are one of the manifestations of Java basement complex outcrop holding several

distinctive rocks from different ages. The oldest is pre-tertary metamorphic rocks, followed



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by the intrusion body of igneous rocks in the Early Tertiary, and younger sedimentary rock

beds from the Middle and Upper Tertiary age. Jiwo Hills are located in Bayat, Klaten District,

Central Java Province. Jiwo Hills consist of two distinctive elongated hills, both are separated

by the Dengkeng River (Figure 1). The one located in the west of the river is named Jiwo

Barat and the other one on the east is named Jiwo Timur. Intrusive igneous body has become

vastly distributed in both Jiwo Barat and Jiwo Timur, making up the rock formation named

Pendul Diorite in the regional geological map (Surono, 1992). The intrusion body comprises

several types of intermediate to mafic igneous rocks such as micro – gabbro, micro – diorite,

and olivine – gabbro (Akbar & Setiawan, 2015). Former researchers has dated the age of the

igneous rock, resulting 33,15 to 24,25 Mya (Early-Oligocene to Early-Miocene) (Soeria-

Atmadja, 1994). This intrusive igneous rock was introduced into the basement in the same

time with the early subduction of Indian – Australian plate beneath the Eurasian plate that

took place in the southern side of Java. The intrusion body formed as the manifestation of

early oligocene volcanism in Java.

1.2. Regional Magmatism

Former researcher, Bronto (2011) concluded that Pendul and its vicinity in Jiwo Hills are

fossils of Early Tertiary magmatism that had undergone intensive denudation through ages.

This result is based on palaeo-geomorphological observation that Pendul is forming isolated

hills of dioritic to gabbroic rocks. (Bronto, 2011 after Akbar, 2016). Another former

researcher, Surono (2006), stated an opinion that Pendul was a multi-stage intrusion

originated from the melting mantle wedge; associated with subduction zone, based on the

abundance of the REE (Rare Earth Elements). Surono (2006) added that there were three

periods of magmatism occured in Bayat Region: the first one was in Late Eocene-Early

Oligocene, the second was in Late Oligocene-Early Miocene, and the last period was in

Middle Miocene. The first and the third periods were produced and differentiated from

different parental/ primitive magma. This conclusion is based on the distinguishable REE

pattern between both samples.

The Early Tertiary magmatism products did not only outcropped in Jiwo Hills, but also to the

southern part of the hills where the pillow lava flows and columnar joint were found.

However, the geochemical connection of these basaltic lava isolated hills; such as Sepikul

Hill, Jarum Village, and Santren VIllage with Jiwo Hills itself and the three periods of

magmatisms hypothesis is still unknown.

1.3. Magma Evolution and Origin

To have a better understanding toward magma evolution and magma origin (tectonic setting

and partial melting), one have to deal with geochemical data such as main oxide wt% and

trace elements such as LILE (Large Ionic Lithopile Elements), HFSE (High Field Strength

Elements), and REE (Rare Earth Elements). Main oxide is used in Harker Diagram (Bivariate

plots) to compare the cogenetic and magma evolution events between samples, and also

knowing the magma affinity. While trace elements data are used in determining magma origin

and tectonic setting by means of normalization using triangular plots or spider mulltielement

diagram.

2. Research Methods

There are three steps or methods in determining the characteristics basaltic pillow lava in

Jarum Village, Bayat Region, as follows:

2.1. Field Observation



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Field observation is conducted to collect lava samples and to observe the stratigraphical

contact between the lava body and surrounding rocks. The samples collected were the freshest

and least-altered (not adjacent to veins and veinlet system).

There were six samples collected from three different localities, such as Sepikul Hill (SP01

and SP02), Jarum Village (JR01), and Santren VIllage (SN01, SN02, and SN03). The amount

of the samples collected in each locality depended on the size of the lava body in order to

collect the most representative samples.

2.2. Petrographical Observation

Textural and compositional informations under the polarizing microscope give understanding

on the petrogenesis such as magma source, differentiation level, fractional crystallization, and

igneous rock typing. Rock typing includes determining rock name based on mineral

composition using IUGS Standard Normalization. Textural imprints such as mineral zoning,

trachytic, and other volcanic rock textures are considered.

2.3. Geochemical Analysis

Geochemical data are obtained from ICP-AES for main oxide wt% data and ICP-MS for trace

element data. All samples are prepared and analyzed under the authority of ALS Mineral,

Canada.

Geochemical analysis of main oxide wt% is used in determining more accurate rock naming

using TAS (Total Alkali-Silica) Diagram and also used in Harker Diagram (bivariate plots) to

check magma evolution events and cogenetic trend with the samples from Jiwo Hills (Akbar,

2016) as a comparison. Main oxide data is also used in determining magma affinity through

AFM Diagram (Irving & Baragar, 1971) and SiO2 vs K2O Diagram (Pecerillo & Taylor, 1976)

Trace element data is used tectonic setting, process of magma generation and its origin using

normalization triangular plots such as Hf-Th-Ta Diagram (Wood, et.al., 1980), Nb-Zr-Y

Diagram (Meschede, 1986), Ti-Zr-Sr Diagram (Pearce & Cann, 1973), and Ti-Zr-Y Diagram

(Pearce & Cann, 1973). Spidergram is also used to determine the cross-check the origin and

setting of the magma using Chondrite normalization (Sun and McDonough, 1969).

As a comparison, geochemical data from eight samples of Jiwo Hills were collected from

former research (Akbar, 2016) named consecutively as follows: JB08, JB10, JB12, JB13

(West Jiwo), JT01, JT04A, JT04D, and JT07 (East Jiwo).

In the following part of the text, bunch of samples from each locality are called “sample

group”. Thus there are two sample groups considered in this paper, the first one is basaltic

pillow lava sample group that is collected by field investigation from pillow lavas outcropped

in Sepikul Hill, Jarum Village, and Santren VIllage. While the second sample group which is

used as a comparison is having a locality in Jiwo Hills (West Jiwo and East Jiwo), obtained

from former research (Akbar, 2016).

3. Data

3.1. Field Data

Both Sepikul Hill and Santren VIllage form an inlier isolated hill among quarternary alluvial

deposit with the height less than 150 meters, incised by small intermittent streams. Sepikul

Hill is located approximately 4 kilometers to the west of Santren VIllage. While the outcrop

in Jarum Village is found on a plain between those two isolated hills, imbedded among

alluvial deposits with the height not more than 15 meters.



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Basaltic pillow lava in Santren VIllage shows conform relationship with the surrounding

southward - dipping marine volcaniclastic sedimentary rocks. While the relationship between

pillow lava outcropped in Sepikul Hill and the sedimentary rocks could not be determined due

to lack of contact outcrop. Both rocks from Sepikul Hill and Santren VIllage underwent

medium intensity of hydrothermal alteration, manifested in the presence of chlorite and calcite

as amygdules and veins. Basaltic lava outcropped in Jarum Village shows least alteration

intensity among three localities of study area. (Figure 2 to 4)

Field data from Jiwo Hills are obtained from Akbar (2016) which stated that the samples

collected are named olivine gabbro and micro-olivine gabbro. Samples from West Jiwo were

collected from the outcrop in Kebo River, Kebo Hill, and Konang Hill. While samples of East

Jiwo were collected from the outcrop in Konang Hill and Pendul Hill. All outcrop underwent

intense spheroidal weathering.

3.2. Petrographical Data

Selected samples of basaltic pillow lava sample group from three localities (JR01, SP01,

SP02, SN01, and SN03) are analyzed under the polarizing microscope to interpret the magma

generation process. Sample SN02 are not selected due to high intensity of alteration. Figure 5

shows representative photomicrographs of the rock collected from each locality of basaltic

pillow lava sample group.

The sample from Jarum Village (JR01) shows porphyroaphanitic texture of long stubby

plagioclase phenocrysts (1 mm to 1,5 mm) embedded in plagioclase and pyroxene matrix. The

plagioclase matrix are having shape of elongated tiny euhedral laths, while the pyroxenes are

anhedral. The size of lath crystals are longer than the pyroxene, giving the appearance of

subophitic texture.

Samples from Santren Village (SN01 and SN03) also comprise similar petrographical

characteristics with the former sample, that is porphyroaphanitic with distinctive subophitic

texture on the matrix. The abundance of plagiocalse laths is greater than the pyroxene. In

these samples, 0,7 to 1 mm-sized pyroxene phenocrysts are exist with the species of aegirine-

augite.

Samples from Sepikul Hill (SP01 and SP02) also shows porphyroaphanitic texture but with

distinctive ophitic matrix texture. This ophitic texture can be distinguished from the other

samples by the size of the plagioclase laths relative to the pyroxene matrix. These sample

show shorter and less-sizeable lath compared to the pyroxene grains.

3.3. Geochemical Data

This comparative study is conducted by comparing geochemical data of six collected samples

with eight secondary data obtained from former research (Akbar, 2016).

3.3.1. Main Oxide Wt% Data

Main oxide wt% data from fourteen samples are tabulated in Table 1. All samples shows early

stage of magma differentiation, indicated by the low SiO2 content (most samples showing

SiO2 < 50 wt%), with relatively high content on MgO. However, the MgO content of six

basaltic pillow lava samples (4,66 – 5,42 wt%) are lower than the other eight samples from

Jiwo Hills (7,82 – 15,89 wt%). While Loss on Ignition (LOI) of basaltic pillow lava samples

(are higher than that of samples from Jiwo Hills, due to higher alteration intensity.



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Total alkali content (Na2O + K2O) and CaO content of both sample groups shows relatively

similar abundance. Similar behaviors also happen in Fe2O3, TiO2, and MnO content between

two sample groups.

Main oxide data is used to determine the rock type based on total alkali versus silica content

(TAS Diagram). Samples from Santren VIllage and Jarum Village are included in Basalt,

while two samples from Sepikul Hill that show relatively higher K2O among other basaltic

lava samples are included in Trachy-Basalt. This anomaly is conjectured to be caused by

higher alteration intensity.(Figure 6)

3.3.2. Trace Element Data

Trace element data for both sample groups are shown in Table 2. Trace elements include LILE

(K, Rb, Sr, Cs, Ba, Eu) and HFSE (Zr, Nb, Ta, Hf, Th, U, LREE, and HREE). LREE/ Light

Rare Earth Elements include Y, La, Ce, Pr, and Nd; while HREE/ Heavy rare Earth Elements

include Sm, Gd, Tb, Tm, Dy, Ho, Er, Yb, and Lu.

Basaltic pillow lava samples show higher content of LILE than that of HFSE and REE in the

same sample group. Similar pattern also exists in Jiwo Hills sample group. Jiwo Hills sample

group shows higher content of LREE than basaltic pillow lava sample group, such as La, Ce,

Pr and Nd.

Some elements such as Sn and W are below the device detection limit so that their abundance

could not be determined.

4. Result And Discussion

4.1. Magma Differentiation

Magma differentiation level is understood by using Harker Diagram with respect of SiO2

basis. (Figure 7) Both group of samples are symbollized by different shape and color in the

diagram: colored triangles for basaltic pillow lava sample group, and grey to white rectangles

for Jiwo Hill sample group.

Note that there are no distinguishable cogenetic trends between two sample groups. Both

groups comprises low silica content with neither significant decrease in ferromagnesian oxide

such as Fe2O3, CaO, MgO, TiO2, and MnO nor increment in alkali oxide such as Na2O and

K2O.

However, MgO versus SiO2 bivariate plot shows distinguishable set of plotted data between

two sample group. This is due to the significant difference of MgO content, where Jiwo Hill

sample group shows higher MgO content in the similar SiO2 content with the basaltic pillow

lava sample group.

Akbar (2016) stated that gabbroic rocks of Jiwo Hills are having characteristics of primitive

magma based on the MgO content and Mg number. According to Tatsumi and Eggins (1995),

any mafic rocks with MgO content higher than 5 wt% should be having primitive magma

characteristics. Basaltic pillow lava sample group shows an average MgO content above 5

wt% and thus having one similarity with primitive magma characteristics. However, to get

better conclusion whether the rock is having great similarity to primitive magma, one should

integrate all geochemical data; such as Mg number > 65 (Gill, 2010), Ni > 400 ppm, and Cr >

1000 ppm (Best, 203).



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Subophitic and ophitic textures shows the intergrowth process of plagioclase and pyroxene

crystal during magma generation. Intergrowth process can be interpreted as a process of

congruent melting with respect of two-phases criteria; in this case plagioclase and pyroxene.

What can be inferred from this data is that the basaltic pillow lavas are coming from a

different period of magmatism with the gabbroic rocks in Jiwo Hills. Therefore these set of

mafic volcanic rocks are not differentiated from the olivine gabbroic rocks of Jiwo Hills. The

difference is manifested in the distinguishable MgO content, where the basaltic pillow lava

came from lesser Mg content magma source than that of Jiwo Hill gabbroic rocks. Very low

silica content of basaltic pillow lava sample group (46,4 wt% - 49,2 wt%) also becomes one

proof that these mafic volcanic rocks undergone a very little or no differentiation process.

Different period of magmatism can be correlated with the result of former research (Surono,

2006) which stated that Jiwo Hill gabbroic rocks represented the first period of Bayat

magmatism; aged Late Eocene to Early Oligocene. This brings understanding that basaltic

pillow lava of Jarum Village and its vicinity which holds some of the primitive magma

characteristics did not come from the first period of magmatism. To determine from which

period of magmatism they are originated, absolute or radiometric dating is required.

4.2. Magma Affinity

Magma affinity is analyzed using AFM Diagram (Figure 8) and SiO2 vs. K2O Diagram (Figure

9). The result shows that basaltic pillow lava sample group are having subalkaline affinity.

Subalkaline affinity are subdivided into tholeiitic and calc-alkaline affinity.

Sample SP01, SP02, JR01, and SN01 shows a transition between calc-alkaline and tholeiitic

affinity, while sample SN02 and SN03 shows tholeiitic affinity. This affinity shows early

stage of mantle-derived magmatism which commonly produced by mid-oceanic ridge setting

(MORB/ Mid-Oceanic Ridge Basalt). While all Jiwo Hill sample group are showing tholeiitic

affinity.

Therefore, basaltic pillow lava of Jarum Village can be interpreted as the first cycle of

magmatism in southern Java at the early tertiary age, originated from the oceanic island arc

setting.

SiO2 vs. K2O Diagram is less useful because it shows wider range of magma affinity from

tholeiitic to high-K calc-alkaline. This is due to the very low content of SiO2 so that the

magma affinity between samples are not distingushable.

4.3. Tectonic Setting

Tectonic setting is understood by normalizing several fundamental LILE and HFSE in

triangular plots such as as Hf-Th-Ta Diagram (Figure 10), Nb-Zr-Y Diagram (Figure 11), and

Ti-Zr-Y Diagram (Figure 12).

Hf-Th-Ta Diagram give that basaltic pillow lava sample group comprise two tectonic settings;

volcanic arc (VAB) and mid-oceanic ridge (N-MORB). While all samples from Jiwo Hill

sample group are having volcanic arc tectonic setting (VAB). This infer that both basaltic

pillow lava in Jarum Village and gabbroic rocks in Jiwo Hills were derived from ascending

mantle during the early stage of subduction right after the spreading process, and yet

underwent very little differentiation process. The abovementioned subduction process refers

to the oceanic island arc setting of the southern edge of Sundaland during early tertiary era.

Ti-Zr-Y Diagram also shows MORB (mid-oceanic arc) and VAB (volcanic arc) to island arc

tectonic setting. While Nb-Zr-Y Diagram also supports the tectonic setting of the basaltic



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pillow lava sample group, which is N-MORB. However, Jiwo Hill samples are randomly

distributed in the diagram.

Spidergrams of basaltic pillow lava sample group is plotted using LILE and HFSE data. Two

spidergrams were made with two different normalization; first one is normalized to chondrite

and the second is normalized to MORB (Sun & McDonough, 1969). (Figure 13 and 14).

Normalization data are tabulated in Table 3 for chondrite normalization and Table 4 for

MORB normalization.

The result of both normalization shows these three distinguishable characteristics:

The graph spikes down on Nb and Ta in both normalization

Both normalization shows enrichment in incompatible LILE such as Rb, Ba, and K (far left) relatively to HFSE.

The graph is irregular but having gently negative slope from LILE to HFSE

Accoring to Best (2003), these patterns characterize subduction zone basalt (oceanic island

arc setting) with hydrous magma generation. Basalts coming from an oceanic island arc

subduction is enriched in incompatible elements such as Ba, Rb, and K but strongly

impoverished in HFSE such as Nb and Ta due to their low solubility in the migrating fluid

during the process of partial melting. Thus, the migrating fluid that drains some of HFSE such

as Nb and Ta is originated from seawater injected by the subducted slab.

5. Conclusion

This study concludes that the basaltic pillow lava of Jarum Village are originated from

oceanic island arc setting. The magma comes from the process of mantle wedge partial

melting in hydrous condition. The affinity of basaltic pillow lava is tholeiitic to calc-alkaline,

comprising the early stage of magmatism.

Basaltic pillow lava of Jarum Village and its vicinity are not the result of magma

differentiation from olivine gabbroic rocks of Jiwo Hill, due to their distinguishable

geochemical characteristics at similar SiO2 content. Thus, the basaltic pillow lava comes from

the different period of magmatism than the Jiwo Hill gabbroic rocks.

Recommendation

This study has not reveal the absolute date of the generation of basaltic pillow lava in Jarum

Village, or in which period of Bayat magmatism does it come along. Further absolute dating

study is needed to solve the precise time-based relationship among tertiary magmatic rock

bodies in Bayat region in a specific scope and its correlation to the evolution of Java island

arc in general.

Acknowledgement

We would like to express our deep gratitude to Anastasia Dewi Titisari, Ph.D for facilitating

this experiment. We would also like to thank our colleagues in assisting our research,

especially in collecting field data.



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References

Akbar, Aditya M. (2016) Studi Petrologi Batuan Beku dan Hubungannya dengan Proses Magmatisme

di Daerah Perbukitan Jiwo, Kecamatan Bayat, Provinsi Jawa Tengah. S.T., Thesis,

Universitas Gadjah Mada.

Akbar Aditya M. and Nugroho Imam Setiawan (2015) Petrogenesis Batuan Beku Intrusi Di

Daerah Perbukitan Jiwo Barat Dan Timur, Kecamatan Bayat, Kabupaten Klaten,

Provinsi Jawa Tengah. Proceeding, Seminar Nasional Kebumian Ke-8, Universitas

Gadjah Mada, Yogyakarta, p. 675-683

Best, Myron G. (2003) Igneous and Metamorphic Petrology. Blackwell Publishing, Oxford, UK. p.

32, 40-44.

Junitia, B.M. (2014) Studi Mineralisasi dan Alterasi di Perbukitan Jiwo dan Sekitarnya, Kecamatan

Bayat, Kabupaten Klaten, Provinsi Jawa Tengah. Yogyakarta: Universitas Gadjah Mada.

Rahardjo, W. (2004). Geologi Daerah Perbukitan Jiwo, Bayat, Klaten. Yogyakarta: Jurusan Teknik

Geologi FT UGM

Rollinson, Hugh (1993) Using Geochemical Data: Evaluation, Presentation, and Interpretation.

Longman Group Ltd., Burnt Mill, Harlow, Essex, UK. p.135-148.

Surono, Hartono, U. and Permanadewi, S. (2006) Posisi Stratigrafi dan Petrogenesis Intrusi Pendul

Perbukitan Jiwo, Bayat, Kabupaten Klaten, Jawa Tengah. J.S.D.Geol Vol. XVI No.5 September

2006



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Figure 1. Geological map of study area



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Figure 2. Basaltic pillow lava outcrop in Sepikul Hill

Figure 3. Basaltic pillow lava outcrop in Jarum Village

Figure 4. Basaltic pillow lava outcrop in Santren Village

N

N

N



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Figure 5. Photomicrographs of selected basaltic pillow lava samples.

JR01

Px

Plg Px

SN01

Plg

Px

Plg

Px

SN03

Px

Plg

Plg

SP01

Px

Plg

Plg

Plg

Plg

SP02

Plg Px

Plg

Px

Px

Plg

= 1 mm



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Figure 6. TAS (Total Alkali-Silica) diagram of basaltic pilow lava sample group.



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Figure 7. Harker Diagram of two sample groups.



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Figure 8. AFM Diagram of two sample groups (Irving and Baragar, 1971)

Figure 9. SiO2 vs K2O Diagram of two sample groups (Pecerrillo & Taylor, 1993).



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Figure 10. Hf-Th-Ta Diagram of two sample groups (Wood, et,al., 1980).

\ Figure 11. Nb-Zr-Y Diagram of two sample groups (Meschede, 1986)



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Figure 12. Ti-Zr-Y Diagram of two sample groups (Pearce & Cann, 1973)

Figure 13. Chondrite normalized multielement diagram/ spidergram of basaltic pillow lava sample group (Sun &

McDonough, 1969)



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Figure 14. MORB normalized multielement diagram/ spidergram of basaltic pillow lava sample group (Sun &

McDonough, 1969)

Table 1. Main oxide data of two sample groups



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Table 2. Trace element data of two sample groups



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Table 3. Multielement normalization table for basaltic pillow lava sample group.

Documents

PROCEEDING, SEMINAR NASIONAL KEBUMIAN KE-10 PERAN … · 2018. 3. 1. · proceeding, seminar nasional kebumian ke-10 peran ilmu kebumian dalam pembangunan infrastruktur di indonesia