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Petrographic Characteristic of Pyroxene Andesite Lava in Kalibawang Area, Kulon Progo, D. I.
Yogyakarta: a preliminary petrogenetic study of Old Andesite Formation in South of Central
Java
Cendi Diar Permata Dana1, Cahyo Sedewo1, Ilham Dharmawan Putra1, Esti Handini1
1Geological Engineering Department, Faculty of Engineering, Universitas Gadjah Mada
Jl. Grafika No.2 Yogyakarta 55281, Indonesia
Abstract
Andesitic lavas in Kalibawang area are part of Old Andesite
Formation (OAF). These lavas represent the first cycle of
volcanism in Sunda arc, which marked the earliest
development of Sunda arc.
The samples were collected from three locations in the east
of Kulon Progo mountains. Detailed petrographic
observation is used to characterize the samples.
The result of petrographic observation shows that two of the
three collected samples have similar phenocryst
assemblages, composed of clinopyroxene and andesine and
texture. Cryptocrystalline plagioclase is present as the main
component in the ground mass, showing trachytic texture.
Hornblende appears only as oxidized xenocryst. The third
sample is collected from a possible dike remnant, exposed
in the south of the two andesitic lavas, and composed of
equigranular plagioclase laths and remnant of olivine
replaced by iddingsite.
The petrographic analysis suggests that the two pyroxene-
andesite lavas might have resulted from the same magmatic
system. However, it has not been fully understood whether
or not they are coming from the same source. The third
sample, the diabasic dyke, is not cut through the andesitic
lavas. It may represent the sub-volcanic part either of the
same system which erupted prior to the two andesitic lavas
or the whole different system which presumably is older or
younger (?) than the andesitic lava group.
Key Words: Andesite lava, Old Andesite Formation, Sunda
arc, Magmatic system.
Introduction
Pre-Quaternary magmatic activity in Java island started
from approximately 40-19 Mya (late Eocene - Early
Miocene) and generates an arc trail from west to east of
Indonesia islands, which its products have andesitic
properties, while, the Second Magmatism activity occurred
11-2 million years ago (Miocene - Pliocene) and also
producing andesitic product (Soeria Atmadja, et al., 1991;
Harjanto, 2015).
One of the important magmatism activity event in The
Indonesia is the occurrence of movement and contact
subduction of tectonic plates that form a magmatic arc from
the northwestern island of Weh in Sumatra to Banda Api in
the eastern part of the Banda Islands (Whitford, 1975). This
Magmatic arc is known by the name Sunda – Banda Arc. In
Sunda - Banda Arc, There are approximately 102 active
volcanic mountain because of the subduction activity of the
tectonic plates (Padang, 1951).
One of the results of the Pre-Quaternary magmatic activity
can be observed in the Kalibawang Region, Kulon Progo
Regency, Yogyakarta. From observations on the photo
image and on the field, there are various rocks as the past
products of volcanism activity in the Kalibawang area, such
as Andesite Breccia, Tuff, lapilli tuff, lava flows, dike
remnants and others. In the studied area, we found various
rocks that characterize proximal volcanic facies, they are
association between the lava, volcanic breccias and
agglomerates (Bronto , 2006). These Rocks are expected as
products of three ancient volcanoes developed in Kulon
Progo area, they are Mount Gajah in the center, Mount Ijo
in the south, and Mount Menoreh in the north (Bronto 2010
in Wijaya, 2015). For now, these three volcanoes have
undergone an intensive erosion process.
One of the lithology that has interest for further
investigation in the focus area is the presence of lava in the
study area , where there are few lava outcrops that show the
differences in petrological appearance. From the three
analyzed samples, those are CCI - L01, CCI-L02 and CCI -
L03, it can be seen that the first two lava samples have the
same texture that is phorphyroaphanitic with a texture that
shows the direction of flow and a specific orientation that
different from sample CCI-L03.
The differences in Petrological may represent the different
magma properties or different process that form the lava in
focus Study area. One of the previous research said that
volcanism of Mount Gajah produced basaltic andesite lavas
that is rich in Pyroxene, and side basalt intrusion
(Ichsyansyah, 2015). Volcanism of Mount Menoreh
generated lava flows which are initially rich in Pyroxene
and changed into hornblende because of differentiation
process (Ichsyansyah, 2015).
From the analysis, the difference in characteristic such as
phenocryst assemblage in these lavas can be used as a key
to understand the characteristics of old magmatic system in
Kulonprogo area. The same data can also be used to
interpret the magmatic evolution in Kalibawang area, which
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represent the first cycle of volcanism and also the earliest
development of Sunda arc.
Geological Setting
Tectonic development of Java changes magmatic arc turned
from south to the north. (Hamilton, 1979) said that tectonic
evolution of Java island during Paleogen shows continuous
subduction of Hindia-Australian plate that slipped under
Java. Sribudiyani, et al, (2003) said that based on the new
seismic and drilling data in East Java to interpret the
presence of fragments continent (called microplate of East
Java) as the cause changing lanes subduction southwest-
northeastern direction (pattern of Meratus) into east-west
(pattern of Java). Another development is the lane tectonic
subduction Karangsambung-Meratus become inactive due
to clogged by the presence of continental material.
Kulon Progo area is formed by the more or less independent
dome of the Kulon Progo mountains between Purworedjo
and the Progo river. Kulon Progo mountains is a dome that
is separate from South Serayu mountains. The dome formed
by major tectonic force that is process of uplift. The
northern and eastern parts of West Progo is bounded by the
valley of Progo river and the southern part of the coastal
plain bounded by ocean of Indonesia, while the
northwestern part of dealing with the mountains of South
Serayu (Bemmelen, 1949).
During paleogen era, Kulon Progo experienced at least
twice period of tectonic phase which first happened in the
Late Oligocene - Early Miocene and the second occurred in
the Middle Miocene - Late Miocene which produces
magmatic arc. Magmatism activities in the area of Kulon
Progo occurred in the Oligocene - Miocene with the spread
of volcanic rocks is west – east (Bemmelen, 1949).
Stratigraphy of West Progo mountains from the oldest to the
young is Nanggulan Formation, Old Andesite Formation,
Jonggrangan Formation, Sentolo Formation, and
Quaternary Deposit (based on Juhri et al., 1977) in
Subiyanto (1989).
Data and Method
The samples were collected from three locations in the east
of Kulon Progo mountains (Fig. 1). The samples were taken
at the upper and lower part of the outcrop, except in the third
location. The dyke remnant in third location have
homogenous texture and composition. The thin section
preparation is conducted at Central Laboratory at
Geological Engineering Department of Gadjah Mada
University. Petrographic analysis of the samples were
carried out at Optical Geology Laboratory of the same
institution. Detailed petrographic observation using point
counting method is used to determine the mineral
assemblages and microtextures of the samples. The
percentage of phenocryst assemblages as determined using
1000 observation points.
Result and Discussion
1. The Geology and Petrology of the Outcrops
The study area consists of several lithologies such as
andesitic breccia, agglomerate, andesitic lava, and basaltic
dyke. The stratigraphic relation show that the lava is
deposited either below the breccia or the basaltic dyke. The
relation between the lava and the basaltic dyke is difficult to
determine because the location is too far, but probably it is
older than the lava. There is no major structure observed in
this area.
The outcrop of sample CCI-L01 and CCI-L02 show a
weathered condition. The upper part is more weathered than
the lower part shown by the presence of soil and clay also
the color of plagioclase which is brighter. The outcrop has
been deformed showing by the sheeting joint along the
outcrop caused by the exhumation process (Fig. 2). The
sample CCI-L03 show grayish-black color and aphanitic
textures and there are some vesicles. This outcrop also has
deformed showed by columnar joint caused by rapid
cooling process (Fig. 3). The morphology of columnar joint
is colonnade type where the dip slope about 5-750 in
variated direction. The diameter is about 15 centimeters to
1 meters while the length is about 4-25 meters.
2. Petrographic Analysis
Petrographic analysis show that sample CCI-L01 and CCI-
L02 have similar minerals assemblages and textures (Fig.
4). The main textures of these samples are
porphyroaphanitic holocrystalline. The phenocryst mineral
phases show a compositionally zoned as the characteristic
of volcanic rocks (Schmincke, 2005). The groundmass is
showing trachytic texture which caused by the
crystallization of lava flow that create flows line that is a
perpendicular orientation of minerals commonly composed
by feldspar microlite. Another texture that can be observed
is intergranular between plagioclase and pyroxene. The
phenocryst phase mainly composed by andesine and
clinopyroxene group that is diopside in detail. Some opaque
minerals which is probably Fe-Oxide also appears in these
samples as the phenocryst or inclusion. Hornblende appears
only as xenocryst that showing compositionally zoned
where the translucent greenish clinopyroxene cores are
overgrown by iron-rich opaque mineral rims. The diopside
assemblage decrease from sample CCI-L01 to sample CCI-
L02, in other hand andesine and oxy-hornblende
assemblage increase. The sample CCI-L01 has fewer matrix
assemblages than sample CCI-L02 as shown in the table
below.
Composition CCI-L01 CCI-L02
Matrix 54.4% 57.6%
Andesine 27.5% 25.1%
Diopside 10.0% 11.7%
Oxy-Hornblende 7.2% 4.5%
Opaque 0.9% 1.1%
Total 100.0% 100.0%
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The sample CCI-L03 generally has different texture, where
it mainly composed by intergrowth of equigranular crystal
(Fig. 5). It has similar texture with diabasic rock that is sub
ophitic texture where the pseudomorph crystal grow in the
interstitial space of plagioclase laths. This sample has the
different composition with those sample above. It mainly
composed by plagioclase, labradorite in detail and olivine
which has pseudomorphed by iddingsite which replaces
most of the olivine phenocrysts and microphenocrysts.
Some opaque minerals also appear as inclusion, there is no
any clinopyroxene observed in this sample. Labradorite as
the main composition with 63.40% assemblage followd by
iddingsite 32.90% and opaque minerals with 3.70%.
3. Petrographic Comparison with First Cycle Volcanism
Sunda Arc Rocks
The evolution of Indonesian island arc has been dominated
since late Paleozoic (Katili, 1975). It was marked by several
linear volcano-magmatic complexes associated with
sedimentary rock (Whitford, 1975). The “first cycle
volcanism” is correspond to the volcanic activity at the
south and south-west of Sunda arc during the Paleogene
period which culminating in lower Miocene and resulting
the Old Andesite Formation (van Bemmelen., 1949).
Petrographic characteristic of some lavas which represent
the first cycle volcanism are very similar with many of
Quaternary lavas. They have porphyritic textures with
abundant assemblages of plagioclase either as phenocryst
phase and/or groundmass (Whitford, 1975). Both
clinopyroxene and othopyroxene can be found associated
with Fe-Ti-Oxide, amphibole, and olivine either as
phenocryst or microphenocryst (Whitford, 1975).
The intrusive part is characterized with coarse plagioclase
dominated and calcic-clinopyroxene with a sub-ophitic
texture and minor Fe-Ti-oxide, where most of the primary
olivine has replaced by green-brown (Whitford, 1975).
All the description above show a similarity with the
petrographic characteristic of all samples from studied area.
The sample CCI-L01 and CCI-L02 has very similar texture
with the “first cycle volcanism” lava. The mineral
assemblages also show a similarity, although there is no any
olivine and orthopyroxene observed. Hornblende only
appears in some parts, while Fe-Ti-oxide is probably
represent by the opaque minerals. The sample CCI-L03 that
is the basaltic dyke also has similar characteristic with
intrusive part of first cycle volcanism where plagioclase
laths still dominated the rock composition. Clinopyroxene
is not well observed in this thin section. While the
description above said that olivine has replaced possibly by
chlorite, in this sample most olivine was replaced by
iddingsite that is the clay minerals members which mostly
alteration result of olivine.
Generally, the rock petrogenesis can be interpreted from its
textural characteristic and mineral assemblages. The
sampled lavas are composed by andesitic magma where the
sample CCI-L02 is more primitive than sample CCI-L01.
The phenocryst phase of sample CCI-L01 and CCI-L02
show a compositionaly zone which is probably caused by a
new mafic magma mixed with the resident magma
(Schmincke, 2005). This oscillatory zoning also can be
caused by volatile release during the magma eruption that
changes the solubility of the liquid (Fenner, 1926 in
Johannsen, 1933). The intergranular texture indicate that the
nucleation ratio of pyroxene is relatively high so the
pyroxene crystal will concentrated in between of
plagioclase crystal (Hibbard, 1995). The zoning in oxy-
hornblende mainly caused by devolatilization process the
greenish cores may represent early crystallization process of
the magma at high pressure (probably boundary crust-
mantle) (Schimncke, 2005). The sample CCI-L01 has lower
matrix assemblages than sample CCI-L02 which indicate
that sample CCI-L01 is crystallized slower than sample
CCI-L02. The abundant assemblages of labradorite and
pseudomorphed olivine in sample CCI-L03 indicate that it
has more mafic magma source which represent the sub-
volcanic part of the magmatic system.
Based on the petrographic comparison above, the
petrogenetic model may have quite similarity with the
model proposed by Whitford (1975). The petrogenetic
model suggests that the magma generate from partial
melting of modified mantle which has added by water and
melt component from the slab (Whitford, 1975), but it still
need advance study, especially in geochemistry to
determine the more accurate petrogenetic model in this area.
Conclusions
The lavas from studied area is the part of Old Andesite
Formation (OAF) which represent the first cycle of
volcanism. The diabasic dyke is the subvolcanic part of this
system which erupted prior to the two andesitic lavas or the
whole different system which presumably is older or
younger. It also need further research especially in
geochemistry to determine the petrogenetic model of this
system.
References
Schmincke, H., 2005, Volcanism, Springer.
Katili, J. A., 1975, Tectonophysics, 26, 195-212
Whitford, D. J, 1975, Geochemistry and Petrology of
Volcanic Rocks from the Sunda Arc Indonesia,
Australian National University.
Van Bemmelen, R. W. The Geology of Indonesia, 1, 732.
Johannsen, A. 1939, A Descriptive Petrography of the
Igneous Rocks, 1, University of Chicago.
Hibbard, M. J., 1995, Petrography to Petrogenesis, Prentice
Hall, Inc., New Jersey.
PROCEEDINGS
GEOSEA XIV CONGRESS AND 45TH IAGI ANNUAL CONVENTION 2016 (GIC 2016)
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Harjanto, Agus, 2011, Jurnal Ilmiah Magister Teknik
Geologi, 4.
Wijaya, Devy Rizky Panji dan Agus Hendratno. 2015.
Petrogenesis Andesit Basaltik di Daerah Kali Wader
dan sekitarnya, Kecamatan Bener, Kabupaten
Purworejo, Provinsi Jawa Tengah. Yogyakarta :
Universitas Gadjah Mada
Ichsyansyah, Ade Budhi. 2015. Studi Fasies Gunung Api
Purba Daerah Banyuasin Separe dan Sekitarnya,
Kecamatan Loano, Kabupaten Purworejo, Provinsi
Jawa Tengah. Yogyakarta : Universitas Gadjah Mada
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Attachment
Fig. 1. The geological map of studied area showing the lava unit distribution and sampling location.
Fig. 2. (A) Lava outcrop in first location (CCI-L01) and (B) second location (CCI-L02, right) showing sheeting joint structure.
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Fig. 3. Dyke remnant outcrop showing columnar joint structure with dip slope about 5-750 in variated direction
Fig. 4. Photomicrograph of sample CCI-L01 (A) and CCI-L02 (B) showing oxidized hornblende with Fe-Oxide at rim and
clinopyroxene at the core. (Ohb: oxyhornblende; Cpx: clinopyroxene; Chl: chlorite; And: andesine; Opq: opaque).
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Fig 5. Photomicrograph of sample of sample CCI-L03 showing intergrowth texture between labradorite and iddingsite (Lab:
labradorite; Opx: orthopyroxene; Idg: iddingsite).