Temporal Variations of Petrological Characteristics of Tangkil and Rajabasa Volcanic Rocks, Indonesia

Tangkil and Rajabasa Volcanoes are neighbouring subduction-zone volcanoes located on the southeast tip of Sumatra Island (Sunda Strait, Indonesia). Stratigraphic correlation of lavas in Tangkil-Rajabasa volcanic area was established from field observations, morphological analysis, and K-Ar dating analysis. Detailed petrography and geochemical data of two and eleven lava units from Tangkil and Rajabasa, respectively, were then integrated with the stratigraphy to show the temporal variations. Early stage (> 4.3 Ma) effusives of Tangkil Volcano are dacitic to rhyolitic (67-71 wt. % SiO2; Tklf), whereas the later (c. 4.3 Ma) rocks are basalt to basaltic andesite (c. 52 wt. % SiO2; Tklm). Tangkil shows bimodal magmatism, of which the felsic endmember is > 71 wt. % SiO2 and < 0.1 wt. % MgO. Lavas of Rajabasa Volcano are comparatively younger (c. 0.3 to 0.1 Ma) with compositions ranging from basalt to andesite (51-62 wt. % SiO2; Rbs). Chemical variations of Rajabasa accounts for the interactions of at least three endmembers: Mg-rich medium-K basalt magma, low-Mg medium-K basalt magma, and high-K andesitic magma. During the long evolution of Rbs magma system, the temporal chemistry shows rising-falling variation in SiO2 and MgO indicating the three magmas were active. The felsic endmember magma of Rajabasa is fixed in composition (at ~62 wt. % SiO2; ~2.2 wt. % MgO). The rocks from the last Tklf and Rbs indicate open system processes by containing plagioclase and pyroxene phenocrysts that show resorption of evolved core and overgrowth of less evolved mantle. The multiple zones of dissolution-overgrowth in plagioclase crystals and the fluctuating trend in temporal whole-rock variation suggest that the changes of magmatic condition in temperature, H2O, or chemical composition were repetitive.


Introduction
Temporal chemical variations of volcanic rocks from subduction-related volcanoes have been documented in the past decades (e.g. Newhall, 1979;Tsukui, 1985;Camus et al., 1987;Prosser and Carr, 1987;Keller, 2000, 2003;Amma-Miyasaka and Nakagawa, 2003;Wibowo, 2017) by integrating the stratigra-phy, isotopic dating, and whole-rock chemistry to understand the magmatic evolution. The temporal variations are detected at various time scales, ranging from inhomogeneous volcanic products by a single eruption in minutes or hours to variations during a series of eruptions in thousands of years; or even the entire lifetimes of a volcano or volcanic field, up to tens of million years. The variations in many calc-alkaline volcanic rocks chronological evolution of the magma system was not clear. To understand the evolution, age determination of the lava units and detailed petrological study are necessary. The temporal variations are important to clarify the evolution of magmas beneath Tangkil and Rajabasa.
This study aims to provide new insights on the volcanic history and magma evolution of Tangkil and Rajabasa Volcanoes. The lava stratigraphy is established by field observations and geomorphological analysis, supplemented by K-Ar ages. New data of the whole-rock major and minor elements are also presented for rocks from both volcanoes. The approach involves the description of petrological variations by a careful observation on mineral textures, zoning patterns, and mineral assemblages, and they are discussed on the basis of temporal geochemical data. The stratigraphy, whole-rock chemistry, and petrography data presented in this study are directed to investigate a temporal petrological variation. In addition, the observations on mineralogy help to identify the magmatic processes in which the minerals have been through during the evolution of the magma system. Modeling of major elements with MELTS calculation (Ghiorso and Sack, 1995;Gualda et al., 2012) was also carried out.

Geotectonic Setting and Geological Background
Currently, the Indo-Australian Oceanic Plate is subducting beneath the Eurasian Continental Plate with a convergence rate of 6 -7 cm a -1 (McCaffrey, 1991;Hall, 2009). This activity has formed a volcanic arc, namely the Sunda Arc, stretching over 5,600 km from the Andaman Islands to Banda Arc. The Sunda Strait is a transitional zone between the Java perpendicular and Sumatra oblique subductions (e.g. Huchon and Le Pichon, 1984;Malod et al., 1995;Barber et al., 2005). A volcanic alignment across the Sunda Strait runs through from Panaitan Island, via Krakatau Island, the Sebesi and Sebuku Islands, Tangkil and Rajabasa Volcanoes, to the Sukadana basalt plateau from SSW to NNE (Figure1). Tangkil and Raja-basa Volcanoes are neighbours, where Rajabasa Volcano is adjacent to Tangkil Volcano to NW.
The Sunda Strait is tectonically complex (e.g. Ninkovich, 1976). In addition to the N−S compressional force related to the subduction in the southern regime, the strait has undergone an extension as a result of the clockwise rotation of Sumatra relative to Java since Late Cenozoic (Nishimura et al., 1986;Harjono et al., 1991). The Sumatra Fault System (Semangko) is a major strike-slip dextral fault zone that extends NW−SE from Andaman Sea to Sunda Strait (e.g. Barber et al., 2005). In addition to the slab subduction in the southern margin, the volcanism of Tangkil and Rajabasa Volcanoes might have been affected by this complex tectonic setting.
There is no detailed geologic study regarding volcanic activities in Tangkil Volcano yet as the original lava geomorphology has been lost by erosion. In the geological map by Andi Mangga et al. (1994), Tangkil volcanic products are described as the Tertiary andesite lava (Tpv). The volcanic products from Tangkil Volcano are distributed from the southeast of Rajabasa Volcano to the Bakauheni seaport in the southeast. The deposits from Tangkil Volcano are overlain by later pyroclastic fall deposits, which are named as Lampung tuffs (Bemmelen, 1949;Andi Mangga et al., 1994). According to Nishimura (1980) and Nishimura et al. (1984Nishimura et al. ( , 1986, Lampung tuffs were spewed at the southern end of Semangko fault at 1 ± 0.2 Ma to 0.09 ± 0.01 Ma.  Lava morphology is well preserved on Rajabasa Volcano so that studies of lava stratigraphy can be established by observing the overlapping lava morphologies. Andi Mangga et al. (1994) described Rajabasa Volcano as Young Volcanic Deposits (Qhv), dominated by andesite-basalt lava. Volcanic rocks of Rajabasa overlie the Lampung Tuffs. Bronto et al. (2012) established the evolution of Rajabasa Volcano as follows: construction of the pre-Rajabasa composite cone, destruction of the pre-Rajabasa cone by a huge eruption leaving the 25 km-diameter pre-Rajabasa caldera (Figure 2), and the construction of the Rajabasa cone inside the pre-Rajabasa caldera. The volcanic rocks of pre-Rajabasa cone are not exposed because of erosion. The ridge of pre-Rajabasa caldera, however, can be recognized on c. 5 km outward from the northern and eastern base of Rajabasa Volcano by an arc-shaped ridge ( Figure 2). All the existing lava inside the ridge belongs to Rajabasa Volcano formed after the collapse of pre-Rajabasa caldera.   Hasibuan et al.) 139

Samples and analytical methods
A morphological analysis of Rajabasa Volcano was carried out on the basis of the digital elevation model and satellite image. This analysis was done to determine the stratigraphic correlation of lava units. Here, Light Detection and Ranging (LiDAR; only available for Rajabasa Volcano) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were used to delineate each eruptive/flow unit, lineaments, and areal morphology by utilizing Geographic Information System (GIS) 10 and Global Mapper ver. 15 software. The LiDAR data was obtained from Hasibuan (2014) and DEM (ASTER) data is an open access provided by the U.S. Department of Interior, USGS. The DEM describes the topography of Tangkil and Rajabasa which appeared as interpolation of contour lines. This analysis can provide the recognition and localization of the main volcanic features (e.g. Norini et al., 2004). As lava lobe geomorphology is well preserved, the sequence of lava was established based on the geomorphological analysis, as well as the relationship observed at exposures. Field observation is an important stage to ascertain order from morphological analysis.
In this study, the total of twenty lava samples were collected from Rajabasa and five lava and one dyke samples from Tangkil. The sampling covers two units from Tangkil Volcano and eleven units from Rajabasa Volcano. The exposure of fresh volcanic rocks is limited by thick vegetation. For this reason, some lava units from Rajabasa Volcano were not able to be sampled (Rbs 3, Rbs 4, Rbs 5, Rbs 12, Rbs 13, Rbs 14, and Rbs 17). Thin sections and rock powder were prepared from each sample for the petrography and wholerock chemical analysis. Polished thin sections were also prepared for SEM-EDS analysis.
Three lava samples were selected for K-Ar isotope dating; one basaltic lava sample from Tangkil Volcano (sample 24.03), and two andesitic lava samples from the youngest and oldest lava flow units of Rajabasa Volcano (samples 21.03 and 23.02). These samples are representative to constraint the lifespan of the volcanoes. The analysis was carried out at Hiruzen Institute for Geology and Chronology Corp., Okayama, Japan. K-Ar determinations were made using crushed groundmass. The argon contents were analyzed with a single collector by an isotopic dilution method using an argon 38 spike. The errors for the obtained gas are at the two-sigma confidence level. In detail, this analysis refers to Yagi (2006) for sample preparation, Nagao et al. (1984) for potassium analysis, and Itaya et al. (1991); Nagao and Itaya (1988);Nier (1950); Steiger and Jäger (1977) for the argon isotope analysis.
The whole-rock chemical compositions were determined with X-ray fluorescence analysis (ZSX Primus II, Rigaku Co. installed at Akita University) using the glass bead method. The powder samples were prepared by grinding in an agate mill. To determine the LOI, the powder samples were furnaced at 900ºC. The dilution ratio of glass bead is 1:5 (rock: flux of a mixture of Li 2 B 4 O 7 and LiBO 2 ). After grinding the mixtures using an agate mortar and pestle, they were fused at 1,150°C to make glass beads. The compositions of ten major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and nine minor elements (Ba, Zr, Rb, Sr, Nb, Cr, Ni, Y, and V) were determined by matrix-corrected calibration curves obtained from the measurements of fifteen samples of GSJ (Geological Survey of Japan) Igneous Rock Series.
Back-scattered electron images (BSEI) were captured with the scanning electron microscope (SEM: JSM-6610LV, JEOL Co.) and line-scanning analysis of individual minerals were carried out using energy-dispersive X-ray spectroscopy (EDS: INCA X-act, Oxford Instruments) at Akita University. The conditions of this analysis were set at: an acceleration voltage of 15 kV, a probe current of 2.2 nA, and working distance of 10 mm.

Geology of Tangkil and Rajabasa Volcanoes
The volcanic centres of Tangkil and Rajabasa Volcanoes are aligned along a SE -NW direction, over a distance of c. 11 km. The boundary between Tangkil volcanic products and Rajabasa volcanic products is located on the flat plain along the northwestern base of Tangkil Volcano (Semanak Village) where the Lampung tuffs are distributed. Lampung tuffs overlie the volcanic products of Tangkil and are overlain by the volcanic products of Rajabasa.
Tangkil and Rajabasa Volcanoes have different degrees of erosion. Tangkil Volcano has lost its original morphology and thus it is regarded as the remnant of an old volcanic edifice ( Figure 2). Based on DEM observation, the geomorphology of the volcano is a hill with slopes of 5° to 35°. Tangkil Volcano is originally a composite volcano that consists of lava and lahar deposits. Rajabasa Volcano is a steep-sided volcano with radial slopes of more than 20°. Rajabasa Volcano is also a composite volcano, composed of dominant lava, tuff, volcanic breccia, and lahar deposits . Rajabasa Volcano consists of two main volcanic cones ( Figure 2): Rajabasa cone and Balerang cone. The younger eruption centre of Rajabasa cone is located in the NW of the older eruption centre of Balerang cone. Bronto et al. (2012) suggested the two volcanic cones were formed by the movement of eruption centres from SE to NW. Two horseshoe-shaped depressions on the northern and western flanks of the Balerang cone indicate the occurrence of sector collapses or landslides.
The lava stratigraphy of Tangkil and Rajabasa Volcanoes is shown in Figure 3. Tkl 1 from Tangkil Volcano is the oldest unit in the area, which   Hasibuan et al.) was later overlain by Tkl 2. The Tkl 1 is occupied by dacite lava. On the other hand, the Tkl 2 is occupied by basaltic rocks. The deposits of Tangkil Volcano were overlain by Lampung tuffs. The earliest lavas of Rajabasa, Rbs 1 and Rbs 2, overlie the Lampung Tuffs. There are some units drawn in a parallel age, because the stratigraphic orders of which another unit overlies the direct contact cannot be determined (Rbs 1 and Rbs 2, Rbs 7 and Rbs 8, Rbs 9 and Rbs 10) and they are regarded as concurrent products. The lava in the southern and eastern flanks of Rajabasa Volcano (Rbs 1 up to Rbs 12) was derived from the central vent of the Balerang cone. The lava units from the Balerang cone are dominated by andesite, accompanying minor basaltic andesite (Rbs 8) and basalt (Rbs 2) lavas. On the other hand, the basaltic andesite and andesite lavas on the northern and western flanks (Rbs 13 up to Rbs 18) were derived from the younger central vent of Rajabasa cone. The youngest andesite lava (Rbs 18) flowed to the eastern flank, into the horseshoe-shaped depressions of the older volcanic cone.
Field occurrences of the volcanic rocks from Tangkil and Rajabasa are lava, scoria deposit, and dyke. The volcanic rocks of Tangkil are widely distributed in Bakauheni and dominated by dacitic to rhyolitic lavas. The lavas are mostly blocky, and cooling joints (platy and columnar) are distinct in some lava units. All the lavas are more than 6 m thick. The felsic rocks are light grey in colour and show porphyritic to aphanitic glassy textures. The basaltic rocks (Tkl 2 in Figure 3) occur as strombolian scoria deposit and dyke that intruded the dacitic lava (Tkl 1 in Figure 3). In contrast to the felsic rocks, the basaltic rocks are distributed locally in Sidoluhur Village. Scoria deposit and dyke have the thickness of more than 15 m and 30 m, respectively. The scoria clasts are brownish and vesicular. On the other hand, the volcanic rocks of Rajabasa are dominated by basaltic andesite to andesite lavas. Most of the lavas are massive and dense, and some are autobrecciated and vesicular on the exterior. The thickness of the lavas ranges from a few meters to more than 50 m. The lavas are all porphyritic and have similar characteristics in groundmass colour and phenocryst size but varying in phenocryst assemblages. The rocks are light to dark grey in colour and contain phenocrysts with up to 4.3 mm in size. Variations of phenocryst assemblages are described further in the petrography section.

K-Ar Analysis
The results of K-Ar dating are shown in

Petrography
Most of the volcanic rocks from Tangkil and Rajabasa contain phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and opaque minerals, with additional phenocrysts of olivine, spinel, hornblende, or biotite. Spinel occurs only in rocks from Rajabasa Volcano. Aggregates of plagioclase + pyroxene + opaque are common in all samples except in Tkl 2. All the samples are porphyritic, and phenocrysts in mafic samples are more abundant. The groundmass textures are homogeneous and mostly consist of plagioclase, fine-grained pyroxene, apatite, opaque minerals, and glass. Lavas from the felsic Tangkil (sample 23.03 from Tkl 1) and Rajabasa Volcanoes contain plagioclase and pyroxene phenocrysts that show complex zoning (such as oscillatory or patchy zoning) and resorbed textures (dissolution surface or cellular zones). Coexistence of normally and reversely zoned pyroxene is common except the lavas from the early stage of Tangkil (three samples from Tkl 1) in which the orthopyroxene crystals are homogeneous and clear.

Tangkil felsic rocks (Tklf)
Tklf are porphyritic dacite containing phenocrysts of plagioclase, orthopyroxene, and Fe-Ti oxides (Figure 4a). These phenocryst minerals are embedded within a groundmass containing plagioclase, orthopyroxene, augite, apatite, and opaque minerals. Plagioclase in three samples from the early stage of Tangkil (26.02, I.6.03, 18.02) is clear, euhedral to subhedral, and smaller than 1.6 mm. Normal zoning and oscillatory zoning are developed. Hornblende phenocrysts occur in these samples. Orthopyroxene phenocrysts are euhedral to subhedral, and smaller than 1.3 mm. In the sample from the later stage (23.03; last Tklf) most plagioclase crystals are subhedral and the maximum crystal size is 2.4 mm. In this sample, plagioclase contains resorbed cores and cellular zones by An-rich clear rims. The cellular textures are developed either in the centres or margins of plagioclase crystals (Figure 4b). The cellular part comprises small inclusions of glass, pyroxene, and oxide minerals. Clinopyroxene phenocrysts occur together with orthopyroxene, and they are subhedral to anhedral with size of <0.8 mm. The clinopyroxene aggregates with plagioclase and orthopyroxene. Some discrete and clinopyroxene/orthopyroxene in the aggregates show both normal and reverse zoning.

Tangkil mafic rocks (Tklm)
Tklm (samples 24.02 and 24.03 from Tkl 2) are porphyritic basalt containing phenocrysts of plagioclase, Fe-Ti oxides, and olivine replaced by iddingsite (figure 4c). The groundmass comprises plagioclase, augite, pigeonite, opaque minerals, and glass. These rocks are highly porphyritic (33 -37 vol. % phenocrysts). Phenocrysts occur mostly as discrete crystals, and glomerocrysts are not common. Plagioclase crystals are 0.1−2.5 mm in size, and show euhedral to subhedral shape. The most common euhedral plagioclase phenocrysts show normal zoning, consisting of clear cores. The reverse-zoned plagioclase is not found. Some subhedral plagioclase phenocrysts show rounding of edges ( Figure 4c) and sieve textures. Plagioclase crystals without sieve textures are commonly smaller (< 1.8 mm) than plagioclase with sieve textures.

Rocks from Rajabasa (Rbs)
The samples from Rbs are composed of andesite, basaltic andesite, and basalt. They show porphyritic texture with the phenocryst abundance ranging from 38 to 51 vol. %. The phenocrysts are 0.2 to 4.3 mm in size. Phenocrysts of plagioclase, orthopyroxene, clinopyroxene, and Fe-Ti oxides are abundant in all samples. Additionally, some of the samples alternatively contain either a combination of biotite ± hornblende or olivine ± spinel as phenocrysts. Phenocryst minerals occur both as discrete crystals and glomeroporphyritic aggregates (Figure 4d). The aggregates comprise a combination of olivine, plagioclase, orthopyroxene, clinopyroxene, biotite, hornblende, and Fe-Ti oxides. The most common aggregates comprise orthopyroxene, clinopyroxene, plagioclase, and opaque minerals. The groundmass comprises plagioclase, orthopyroxene, augite, apatite, opaque, and minor glass.
Orthopyroxene phenocrysts are commonly more abundant and bigger (<1.5 mm) than clinopyroxene (< 1 mm). The occurrence of orthopyroxene phenocrysts that are mantled by clinopyroxene   (hbl), and pyroxene (opx); b) Subhedral plagioclase (pl) containing cellular zones in the margin of crystals; c) Plagioclase with complex zoning (pl) and iddingsite (id) with altered olivine; d) An aggregate of plagioclase phenocrysts (pl) and discrete augite crystals; e) Orthopyroxene (opx) mantled by clinopyroxene rim (cpx). Melt inclusions developed in resorption boundary between the two phases; f) Phenocrysst assemblages in Rbs, containing spongy cellular or sieved plagioclase crystals (type 1) and "clear" plagioclase crystals (type 2). olivine-rich basaltic andesite samples (Rbs 2 in Table 2). Both homogeneous and zoned pyroxene phenocrysts are present. The zoned pyroxene crystals show either normal or reverse chemical zoning in all lava units, except for Rbs 8 (Figures 5d, 5e). As shown in the back-scattered electron (BSE) images and line scan profiles of augite crystals in an andesite of Rbs 18 (Figure 6), some clinopyroxene crystals show reverse zoning with a low Mg/Fe core and a high Mg/Fe rim (spectrum 1), whereas other coexistent clinopyroxene crystals show normal zoning with a high Mg/Fe core and a low Mg/ Fe rim (spectrum 2). It is also noted that spectrum 2 shows a slight increase of Mg/Fe in the mostrim part. The BSE image (Figure 5e) shows a step reverse zoning in augite of which the composition sharply changes from the iron-rich core (low Mg/ Fe) to the magnesium-rich (high Mg/Fe) mantle. The Fe-rich cores are anhedral with rounded and embayed characteristics. By contrast, rounded and embayed cores do not occur in normally zoned clinopyroxene (Figure 5d). Olivine crystals vary in size, from 0.1 mm to 1.9 mm. Most olivine phenocrysts are subhedral to euhedral. The smaller, the more rounded the olivine crystals are. Olivine occurs as discrete crystals, and in some cases as  Hasibuan et al.) 145 aggregates with clinopyroxene and plagioclase. Spinel often appears as inclusions in big olivine and rarely in pyroxene and plagioclase. Fe-Ti oxide minerals occur as abundant titanomagnetite and rare ilmenite.

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Except for lava from Rbs 8, all unit lavas contain two distinct types of plagioclase crystals; type 1 is characterized by the presence of complex zoning and resorption textures, whereas type 2 is characterized by clear crystal (Figure 4f). The type 1 occurs as subhedral crystals bigger than 0.4 mm. Resorption textures are common, shown by unconformable or irregular surfaces between Ca-poor core and Ca-rich mantle (Figure 5a) and by spongy cellular zones (Figure 5c). The anhedral cores are surrounded by a Ca-rich mantle ( Figure  5a). In addition to the resorbed core, the patchyzoned cores and clear cores are also common. Coarsely spongy cellular zones showing resorption channels which typically occupy the entire crystal from core to rim, whereas finely spongy cellular zones (also termed as "sieved") are overgrown by a clear and zoned rim with the thickness of up to 0.1 mm. Melt inclusions in the resorption channels or sieved zone commonly include glass, pyroxene, and oxide minerals. In some crystals, patchy texture is exhibited in sieved region. Complex oscillatory zoning shows multiple growth zones that are separated by high Ca/Na bands (Figure 5c). From core, the Ca/Na decreases outward, abruptly increases, and gradually decreases again toward the next growth band. The anhedral inner part is dissected by a sharp boundary, dissecting the growth bands discordantly. The multiple   growth zones do not develop in other minerals. On the other hand, type 2 is commonly euhedral and occurs in various size. Clear plagioclase phenocrysts show normal, reverse, or oscillatory zoning. Reversely, zoned plagioclase contains a subhedral Ca-poor core that is overgrown by a Ca-rich mantle ( Figure 6). Type 1 and type 2 plagioclase coexist in individual aggregates, in which the latter type is more abundant.
Spinel occurs in Rbs 2, Rb 6, and Rbs 18. Only andesitic samples with SiO 2 above c. 60 wt % contain biotite and hornblende. The occurrence of olivine in Rajabasa is not correlated with the whole rock composition as olivine is lacking in some andesite samples (Rbs 1,7,9,and 11), but present in more evolved samples (Rbs 10 and Rbs 18) with biotite and hornblende. Olivine crystals in samples with SiO 2 <56 wt % and MgO ≥6 wt % (Rbs 2 and Rbs 8) are up to 1.9 mm in size and abundant, whereas the olivine in samples with SiO 2 >56 wt % is < 0.7 mm in size and sporadic. Basaltic andesite from Rbs 2 is higher in MgO than that of basalt from Rbs 8. Compared to the less magnesian samples, the high MgO samples have more clinopyroxene than orthopyroxene.
Volcanic rocks from Tangkil are classified as medium-K basalt (51-52 wt % SiO 2 ; Tklm) and medium-to high-K dacite to rhyolite (67-71 wt % SiO 2 ; Tklf), whereas those from Rajabasa are basalt, basaltic andesite, and andesite (51−62 wt % SiO 2 ; Rbs) (Figures 8 and 9). The high-SiO 2 samples of Rbs are classified as high-K andesite, whereas the low-SiO 2 lavas are either medium-K basalt or basaltic andesite. Tklf and Tklm show discontinuous, separated iron-enrichment trends on the modified Miyashiro diagram (FeO t /   et al., 1989). Rbs and Tklf rocks are plotted in transition from medium-K to high-K, whereas Tklm rocks plotted in medium-K (above). A discriminating diagram between tholeiitic and calc-alkaline magma series. The separating line (heavy dashed line) is from Miyashiro (1974) which was later modified by Gill (2010)

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Temporal Variations of Petrological Characteristics of Tangkil and Rajabasa Volcanic Rocks, Indonesia (R.F. Hasibuan et al.) 149 (FeO t +MgO) vs SiO 2 ) ( Figure 9) (Miyashiro, 1974;Gill, 2010). The extrapolations of the trends do not cross as they are subparallel. The two samples from Tklm show an iron-enrichment which is typical of tholeiitic basalt. Tklf displays an ironenrichment trend parallel to the basalt trend and crosses the boundary between calc-alkaline and tholeiitic fields. Rbs show two different trends; one is near-constant FeO t /(FeO t +MgO), and the other is a steep iron-enrichment trend. Two samples that are the mafic end of the steep ironenrichment trend are olivine-rich basaltic andesite samples (Rbs 2). The high-silica samples of Rbs cluster at an andesite composition, at which the two trends converge.
Variation diagrams of selected major and minor elements vs. SiO 2 are shown in Figure 10 Rocks from Rajabasa show positive correlations in alkali, Rb, Ba, and Zr but negative correlations in TiO 2 , FeO (t) , MgO, CaO, Al 2 O 3 , and Sr against silica. The silica-variation plots of MgO, Al 2 O 3 , and Sr show two different trends which converge at c. 62 wt % SiO 2 . Two olivine-rich basaltic andesites (Rbs 2) plot at the high-MgO end of the trend on MgO vs. SiO 2 diagram. In addition, they plot off lower the other trend of TiO 2 , FeO (t) , Al 2 O 3 , and Sr. Except for these basaltic andesite samples, Ni and Cr are as low as 8 -23 ppm and 6-41 ppm, respectively. These two samples are extraordinarily high in Ni (95 -114 ppm) and Cr (c. 250 ppm), which are higher than those of basalt samples from Rbs and Tklm. The contents of Ni and Cr from the last Tklf sample (67 wt. % SiO 2 ) are the highest in the series. Furthermore, the content of Ni is slightly higher than samples of Tklm and Rbs except for the two high-MgO samples from Rbs 2. On the diagrams of K 2 O vs MgO and Rb vs Cr, two linear trends of Rbs converge at c. 2.7 wt % K 2 O at the lowest MgO of 2.2 wt % and at c. 83 ppm Rb at the lowest Cr of 5 ppm (Figure 11).
The temporal chemical variation of Tangkil and Rajabasa is shown in Figure 12. The equivocal order of two flow units is depicted in temporal chemical variations with the parallel ages (Rbs 1 -Rbs 2; Rbs 7 -Rbs 8; Rbs 9 -Rbs 10). Before c. 4.3 Ma, Tangkil Volcano was initiated by the eruption of dacite and rhyolite lavas (Tklf). In this period, SiO 2 and Na 2 O+K 2 O initially increased, and then decreased. The opposite trends are apparent in CaO and MgO. In c. 4.3 Ma, Tangkil Volcano changed its eruption products from dacite/rhyolite to basalt (Tklm). The two samples from Tklm show a slight increase in CaO and MgO and a decrease in SiO 2 and Na 2 O+K 2 O. The Al 2 O 3 contents from Tklf and Tklm do not change through time, 16 wt. % and 20 wt. %, respectively. Then at c. 0.31 Ma, Rajabasa Volcano started its activity by erupting basaltic andesite and andesite of which SiO 2 fluctuates from 54 wt. % to 62 wt. %. The majority of Rbs shows rising-falling variation in SiO 2 and MgO. The concentrations of Na 2 O+K 2 O comply with SiO 2 , whereas those of CaO, MgO, and Al 2 O 3 are the contrary. Basaltic andesite and basalt are marked with spikes of CaO (9.1 -10 wt %) and MgO (5.9 -7.1 wt %), while differentiated andesite are produced constantly. The lavas with high-Mg basaltic andesite and basalt composition (Rbs 2 and Rbs 8) are olivinerich rocks and originated from the older Balerang cone. These Mg-rich rocks are intercalated with the differentiated andesite.

The Volcanism of Tangkil and Rajabasa
This study elucidated the history of volcanism in Tangkil and Rajabasa Volcanoes. The volcanism in the area is likely to be affected by the tectonic setting. The clockwise rotation of Sumatra relative to Java which pre-ceded the activity of Rajabasa has started since at least 2.0 Ma (Ninkovich, 1976;Nishimura et al., 1986). The changes of the volcanic centre from Tangkil to Rajabasa and the eruption centre from the Balerang cone to the Rajabasa cone is parallel with the major NW−SE fault zone in Sumatra (Sumatra Fault System). This crustal scale strikeslip faulting may have provided different paths for the magma to rise. A similar idea has been suggested by Bronto et al. (2012) who presumed that the change of eruption centre in Rajabasa Volcano was controlled by a deep fracture. A number of studies have also proposed a similar influence of strike-slip faulting on volcanism (e.g. Fytikas and Vougioukalalis, 1993;Piper and Perissoratis, 2003;Pe-Piper et al., 2005).

Open System Magmatic Process
The open system magmatic processes are indicated by petrographic evidence for the rocks from the last Tklf and Rbs. This evidence includes resorbed textures in plagioclase, such as dissected zoning, irregular and patchy core, as well as cellular or sieved texture with melt inclusions (Figures 4b, f, and 5a -c). The resorbed textures in plagioclase reflect changes in magmatic condi-tion experienced during the crystal growth (e.g. Humphreys et al., 2006). Partial dissolution of plagioclase can be caused by heating, hydration of melt, decompression, or a combination of these processes (e.g. Tsuchiyama, 1985;Nakamura and Shimakita, 1998;Nelson and Montana, 1992). Once in a reservoir, the differentiation of magma continued in cooling, convection, decompression, and recharge of new magmas. Partial

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Temporal Variations of Petrological Characteristics of Tangkil and Rajabasa Volcanic Rocks, Indonesia (R.F. Hasibuan et al.) 153 dissolution of crystal exterior is characterized by the overgrowth of Ca-rich plagioclase on the crystal surface (Figures 5b and 6). This texture is consistent with the experimental results of Nakamura and Shimakita (1998), who invoked the dissolution-recrystallization origin. Anorthite zonation represents the history of ambient magmatic conditions as diffusion rate of NaSi-CaAl is low relative to timescales of magmatic event. The irregular or sieved cores/mantles and clear euhedral rims are formed under different conditions and stages. Dissolution textures were formed by one or combination processes of heating, hydration, or decompression. Then, the recrystallization of higher Ca/Na mantle/rim progressed after the dissolution process The resorption-overgrowth texture in pyroxene crystals also represents the open system processes. Anhedral and rounded cores in pyroxene suggest the resorption process ( Figure  5e). The resorbed cores are only developed in reversely zoned clinopyroxene. The Fe-rich cores are characterized by embayed morphology and unconformable interfaces with mantle, indicating resorption of Fe-rich pyroxene. The dissolution surface in pyroxene develops in response to an external factor such as the temperature increase above their liquidus, compositional change (including oxygen fugacity), or decompression. The resorbed core is overgrown by a less evolved (higher Mg/Fe) mantle or rim, implying resorption-overgrowth was induced by a change in ambient magma composition from low Mg/ Fe to high Mg/Fe (mafic) composition. The Mgrich mantle shows normal zoning with polygonal growth bands (Figures 5d and e), indicating progressively evolving composition of melt during cooling. The coexistence of augite-mantled orthopyroxene with clear euhedral orthopyroxene, as well as normal and reverse zoning of clinopyroxene in the same sample reflects the wide variation (Figure 5f). The texture of orthopyroxene mantled by augite can be explained by heating and compositional change of melt (e.g. Gerlach and Grove, 1982), where the orthopyroxene was resorbed first, overgrown then by augite.
Of the two types of plagioclase and pyroxene, resorption of evolved core and overgrowth of less evolved mantle can be accounted by a temperature rise, influx of volatile (to reduce T of liquidus and to change equilibrium composition), or change of chemical composition of ambient magma to higher MgO/FeO and higher CaO/Na 2 O. The multiple zones of dissolutionovergrowth textures in plagioclase crystals (i.e. Figure 5c) indicate that the changes of magmatic condition in T, H 2 O, or chemical composition were repetitive. The lack of multiple dissolutionovergrowth layers in other minerals implies faster chemical diffusion rate than that in plagioclase (Grove et al., 1984;Morse, 1984).
Magma mixing is indicated by the whole-rock chemistry diagrams (e.g. Figures 10 and 11). Fractional crystallization trends are concaveupward in diagrams of MgO vs SiO 2 . Two linear trends on the diagram (Figure 11), except for a transitional endmember of Rbs 8, indicate the mixing of two mafic endmembers with a felsic endmember. MELTS software (Ghiorso and Sack, 1995;Gualda et al., 2012) was used to evaluate and model the fractional crystallization trends. The composition of 50 wt % SiO 2 for both mafic endmembers was assumed and other elements was determined by extrapolating the trend of the elements to the assumed SiO 2 content; the total obtained for both was 100 wt %. The system was assumed did not contain free-water at liquidus, then by trial-and-error calculations the water content of 2 wt % was determined and fugacity constrained to QFM buffer. The calculation of phase relations under these conditions could produce the phenocryst assemblage of ol+sp+cpx+opx+pl. Three individual isobaric calculations (at 1, 2, and 3 kbar) show that no model result of fractional crystallization resembles the data for each mafic endmember trend, although at lower pressure (1 kbar) the simulated curves approximate the observed trends (Figure 13). In Figure 12, the spikes of CaO and MgO indicate that the recharges of high-Mg mafic endmember are sporadic, whereas the recharges of low-Mg mafic endmember are common during the entire lifetime of Rajabasa. The last Tklf also underwent recharge of mafic magma, indicated by the whole-rock compositional variation of low-SiO 2 , high-MgO, -Ni, and -Cr (Figure 12), and by the mineral textures of plagioclase and pyroxenes shown above.
The evidence of repetitive magma recharges is also represented by the fluctuating temporal whole-rock chemical variation which shows that differentiated andesite is intercalated with Mg-rich rocks ( Figure 12). This variation can be produced by multiple replenishment of the evolved magma reservoir with mafic magma. Belkin et al. (1993), Villemant et al. (1993), and Gertisser and Keller (2003) found the similar variation and suggested multiple replenishments into a pre-existing magma reservoir. Based on the petrographical and geochemical evidences, it is concluded repeating magma recharges during the magmatic evolution of Rajabasa. For the magma system at Rajabasa, at least three endmember magmas are identified: (1) Mg-rich medium-K basalt magma; (2) low-Mg medium-K basalt magma; and (3) high-K andesite magma (Figures 9 and 11). The high-Mg, medium-K basalt magma is a primitive magma with high Cr (>250 ppm) and Ni (>114 ppm) contents. Another basalt magma (2) is more differentiated, indicated by lower MgO, Cr, and Ni contents. The composition of the felsic endmember of Rajabasa Volcano is determined by the intersection of the two linear trends at an andesitic composition of ~2.2 wt % MgO and ~62 wt % SiO 2 ( Figure  11). As discussed above, the Mg-rich medium-K basalt magma and the low-Mg medium-K basalt magma repeatedly injected into the high-K andesite magma.
Tangkil involves bimodal magma system of basalt and felsic magma. The felsic endmember of Tklf is more evolved, > 71 wt % SiO 2 and < 0.1 wt % MgO, than the felsic endmember of Rajabasa ( Figure 11). The last Tklf could be a product of mixing between the felsic endmember and a mafic endmember.
The mineral assemblages in Rbs reflect the features of endmember magmas (Figure 7). The abundance of olivine in the silica-poor rocks implies that the olivine is originated from the mafic endmember. The presence of spinel-bearing rocks in MgO-rich trend indicates that spinel is derived from the Mg-rich medium-K basalt magma. Biotite and hornblende are originally crystallized from felsic endmember since they are abundant in the silica-rich rocks (SiO 2 > 60 wt %). The felsic endmember-derived minerals changed temporally and seem to be affected by temperature conditions in the felsic endmember reservoir. The felsic endmember magma stayed fixed in composition (at c. 62 wt % SiO 2 ), but its  Hasibuan et al.) 155 temperature can change the mineral assemblage. Biotite and hornblende crystallize in a relatively low-T andesitic magma. Biotite and hornblende crystallized when the reservoir was cool, while they were not present when the reservoir was heated by mafic injections or the temperature was still high (like in early period). From the above discussion, cooling and heating of felsic magma reservoirs are repeated, resulted from the repeated injection or recharge of high-T mafic magma.

Conclusion
The volcanostratigraphic result combined with age dating analysis reveals that the volcanism in Tangkil commenced in c. 4.3 Ma and was characterized by dacite to rhyolite (Tklf) and basalt to basaltic andesite (Tklm) rocks; the commencement and cessation of Rajabasa volcanism were in c. 0.3 Ma and c. 0.1 Ma, respectively and characterized by basalt to andesite lavas (Rbs).
The plagioclase and pyroxene phenocrysts from the last Tklf and Rbs show resorption-overgrowth texture, a feature that can be caused by temperature rise, volatile influx, or compositional change of ambient magma. The repetitive changes of magmatic condition are indicated by the multiple zones of dissolution-overgrowth textures in plagioclase phenocryst as well as the fluctuating trend in the temporal whole-rock variation.
In the Rbs magma system, at least three different endmember magmas were involved: Mg-rich medium-K basalt magma; low-Mg medium-K basalt magma; and high-K andesite magma. The rising-falling variations in SiO 2 and MgO during the long evolution of magma indicate the three endmember magmas were active. The mixing of magmas is indicated by the whole-rock chemistry diagrams, petrography, and MELTS. The felsic endmember composition of Rajabasa is ~62 wt % SiO 2 and ~2.2 wt % MgO; Tangkil is > 71 wt % SiO 2 and < 0.1 wt % MgO.
The mineral assemblages in Rbs reflect the features of the three endmember magmas. The repeated injections of the Mg-rich medium-K basalt and the low-Mg medium-K basalt into the high-K andesite magma changed the temperature conditions in the felsic endmember reservoir that can affect the mineral assemblage of biotite and hornblende.
This study shows that the evolution of magma system and processes are reflected in the temporal petrological variations. But why the magmas mixed together during the activity of Rajabasa whereas they did not in Tangkil