Petrogenesis of Botorubuh igneous rocks at Gunungkidul: A preliminary study for paleomagmatism phenomenon in the southern beach of Yogyakarta - Indonesia

Botorubuh Beach is located at Southern Beach of Gunungkidul, in the Southern Mountain of Yogyakarta - Indonesia and is a promontory composed of Middle - Late Miocene igneous rock with a columnar jointing structure. This isolated igneous rock area is surrounded by limestone. Therefore, the regional geological map of Surakarta-Giritontro classifies this area as the Punung-Wonosari Formation which is dominated by limestone. Because of the geological phenomenon of the isolated igneous rock area, it is necessary to study the petrogenesis of the igneous rocks at the area. The petrogenesis research is based on a preliminary study of the petrographical and geochemical characteristics of this igneous rock samples. The petrographic identification of andesite samples shows porphyritic, trachytic, oscillatory zoning, and sieve textures. The results of geochemical analysis (major and trace elements) show that the rock samples are are classified to andesite rocks and calc-alkaline suites. These rocks are enriched in LILEs (Rb, Ba, K) and depleted in HFSEs (Nb, Ti, Ce). Additionally, REE shows a slight enrichment of light-REE and a slight negative anomaly of Eu. The patterns of the trace elements including REE show a typical pattern of calc alkaline arc. Petrographical and geochemical characteristics suggest evidence of magma differentiation process, that is by a mechanism of crystallization fractionation. The andesite was formed in relation to a Middle – Late Miocene paleomagmatism and the Late Miocene-Pliocene subduction zone.


Introduction
A cape is called Botorubuh Beach which is located at the Southern Beach of Gunungkidul, in the Southern Mountain of Yogyakarta -Indonesia, exposes andesitic rocks with a columnar jointing structure. However, on the regional geological map of Surakarta -Giritontro, with a scale of 1: 100,000 compiled by [1], the rocks are grouped into the Wonosari Formation whose constituents are dominated by limestone because the isolated area is surrounded by limestone. In contrast, the Mount Batur composed of andesitic igneous rocks at Wediombo Beach which is located around 2 km to the northwest of Botorubuh Beach area, is classified as the Wuni Formation on the regional geological map. The Wuni formation is composed of andesite to basalt breccias, lava breccias, andesite lava and tuffaceous sandstone of the Middle -Late Miocene Epoch [1]. Meanwhile, the Wuni Formation represents one of the formations in the Southern Mountain whose location is far away in the Pacitan area, East Java, around 60 km to the southeast from Botorubuh area. Furthermore, several studies that have been conducted around the area such as [2], [3], [4], [5] and [6] are more focused on studying the Mount Figure 1. Regional geological map of the southern part of Surakarta-Giritontro geological map, simplified from [1] and the location of studied area.

Research method
More detailed geological mapping was carried out to determine the geological condition and distribution of lithologies in the study area. Rock sampling was collected at several points which represents lithology in the study area. Laboratory analysis was conducted for petrographical observations and geochemical analysis. Petrographic observation for 4 rock samples using a polarizing microscope was carried out at the Optical Geology Laboratory, Department of Geological Engineering, Universitas Gadjah Mada. Geochemical analysis to determine the percentage of major oxide compounds and the concentration of trace elements for representative rock samples using the ICP -AES (Inductively Coupled Plasma -Atomic Emission Spectrometry) and the ICP -MS (Inductively Coupled Plasma -Mass Spectrometry) methods was conducted at ALS Minerals Laboratory, Vancouver, Canada. This study uses additional geochemical data of igneous rocks from previous studies [4], [5] and [6]. The sampling locations and geochemical data for this study and previous researches are summarized on Figure 2 and Tables 1-2, respectively. The types of rocks in the previous studies are andesitic lava, andesitic intrusion [4,5] which is identified as dioritic intrusion by [6].

Figure 2.
Location of this study (red box) and previous studies of [4], [5] and [6] indicated by green box, and their distribution of geochemical sampling locations. Note: solid orange squares indicate sampling locations of [4], solid yellow squares indicate sampling locations of [5] and [6] and solid green squares indicate sampling locations of this study. Table 1. The major oxides abundance (in %) in several igneous rock samples at Botorubuh Beach (this study) and some data from the previous studies [4], [5] and [6].

This study
Previous study [4] Previous study [5] in [6] Andesitic  Table 2. Trace element concentration (in ppm) in several igneous rock samples at Botorubuh Beach (this study) and some data from the previous studies [4] and [5].

Geological Study Area
Based on its dominance, the lithology in the study area can be grouped into three rock units, which is andesite lava units, andesite breccia units, and limestone units. The geological map of the research area can be seen on Figure 3. These igneous rocks are expressed in form of an isolated hilly morphology where andesite lava unit is surrounded by andesite breccia unit. Columnar jointing structure and shear joints of andesite lava are well exposed on coastal cliffs of studied area (Figures 4 and 5, respectively). This research only focuses on the andesite lava units. Megascopically, the andesite is characterized by grayish black (least altered) and brownish gray (slightly weathered) with a massive structure, a porphyritic texture, holocrystalline, subhedral crystal form. Phenocrysts are composed of abundant plagioclase (0.2-4 mm in size) and hornblende, while the groundmass with crystal size <0.01mm is composed of plagioclase and mafic minerals. The samples from previous studies [4], [5] and [6] show that andesitic intrusions [4] and dioritic intrusions [6] are dominated by plagioclase (0.2-6 mm in size), and hornblende and opaque minerals which also can be identified as phenocryst, while the groundmass is dominated by plagioclase. Andesite lava is characterized by plagioclase (<0.01-2 mm in size) both as phenocryst and groundmass. Hornblende and opaque minerals are also common in the andesite lava.

Petrography Characteristics
Petrographic observations show that the andesite is dominated by plagioclase, clinopyroxene, and hornblende minerals with a crystal size of 0.01 -4 mm, with porphyritic texture, and typical textures of trachytic texture and oscillatory zoning (Figures 6A-H). The abundance of minerals is dominated by plagioclase either as phenocrysts or as microcrystal in the groundmass. Hornblende and clinopyroxene also present as phenocrysts.  Figure 6G) are identified in almost all plagioclase phenocrysts together with the oscillatory zoning texture as a characteristic texture of the disequilibrium state. Trachytic texture is clearly recognizable on Figure 6H showing an orientation of plagioclase minerals. The texture of trachytic is a product of flow materials, so that the naming of the andesite lava is supported by evidence of petrographic analysis which indicates the presence of this trachytic texture.

Geochemical Characteristics
Igneous rocks on Botorubuh Beach including rock samples from the previous researches ranges from 58.6 to 63.16%, while the total alkali (Na2O + K2O) ranges from 3.55 to 4.86 wt% ( Table 1). Based on geochemical data plotting on the diagram of Total Alkali vs. Silica ( Figure 7A) and diagram of Nb/Y vs. Zr/Ti ( Figure 7B), the rocks fall within andesite fields. A FeO*/MgO vs. SiO2 diagram with assist line from [10] has been applied to observe the calc-alkaline characteristics of the rock samples ( Figure  8A). The magma suites of the AFM ternary diagram (Irvine and Baragar, 1971 in [11]) of the studied area and previous studies samples is shown on Figure 8B. The magma suites can also be determined using the trace element ratio as the discriminant, namely Ta / Yb vs. Th / Yb [12] is shown on Figure  8C. On the diagram ( Figure 8C), magma suites in the study area and previous studies are also classified as the calc-alkaline magma suites.  (Figure 9). The pattern of the HREE elements (Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu) in the diagram shows depletion in relative to the LREE pattern (La, Ce, Pr, Nd, Sm) ( Figure  10). Moreover, the REE diagram show a slight negative anomaly of Eu for the andesitic samples of Botorubuh area and previous research of [5]. In contrast, Eu element displays a positive anomaly on previous research of [4] samples ( Figure 10). It means that the Eu concentration in the mineral is enriched relative to the other rare-earth elements on those samples. Anomalies of the BR03 sample (Figures 9 -10) indicate enrichments in the average of trace element concentration in comparison to other samples in the study area. A contamination of subducted sediment will show a negative anomaly  [16]. This is consistent with the rare earth elements diagram on the study area showing a negative anomaly of Ce element for the BR03 sample ( Figure 10). Therefore, the anomalies of the BR03 sample are considered and assumed due to the condition of the sample which is as the result of incorporation of subducted sediment to the arc magma sources.   Figure 9. The N-MORB normalized patterns of trace element in some geological settings from [10] are compared with rock sample data of the studied area (Botorubuh) and its surroundings (Mount Batur).

Discussion
Petrographic analysis shows that the porphyritic texture is identified in all rock samples of the study area showing a different crystal size of plagioclase groundmass and plagioclase phenocryst. This difference in the size of the plagioclase crystals indicates the occurrence of two-stage cooling. Referring to the statement of [20], it can confirm that the indication of the two-stage cooling suggests a difference in cooling rate of the magma source of andesitic rock in the research area. It can also be an indication of occurrence of two phases of the crystallization process that is the earlier phase crystallization of phenocryst minerals and the later phase crystallization of groundmass minerals. Typical textures including trachytic texture, oscillatory zoning, and sieve texture are also identified in the rock samples. The trachytic texture in Figure 6H shows that the smaller plagioclase minerals show an alignment pattern 10 around the phenocryst. This texture is common in a product of flow materials. Crystallization of flow materials causes in a tendency for the crystals to position themselves in the direction of the flow. Therefore, the presence of a trachytic texture indicates that andesite in the study area might be associated with a flow process. The oscillatory zoning texture (Figures 6C-D) shows that there are variations in plagioclase composition due to crystal responses to fluctuating external conditions, or due to repeated influx of new magma and mixing of magma with more evolved magma [20]. This zoning texture is able to be an indication of occurrence of pressure and temperature changing during magma movement and volatiles release during magmatic eruptions leading to differentiation of the plagioclase composition (Fenner, 1926 in [21]). The sieve texture ( Figure 6G) shows the appearance of holed plagioclase like corrosion (corroded-like) and then the holes are filled with the later phase magma fluid.
Based on the results of plotting on the diagrams of binary FeO*/MgO vs SiO2 (with assist line from [10]), the AFM magma series by [18] and Ta / Yb vs. Th / Yb by [12] which are shown on Figures 8A-C, respectively, andesite in the Botorubuh coast area is categorized as a calc-alkaline suite. The calcalkaline suites are a characteristic magma suite on a convergent boundary of subduction zone or on an active continental margin of subduction zone [11]. The suggestion is supported by plotted result of Total Alkali Silica (TAS) diagram and diagram of Nb/Y vs. Zr/Ti ( Figures 7A-B, respectively) showing the rock samples classified on andesite, indicate to magma type which formed the andesite rocks, is the intermediate magma. This magma is a typical volcanic rock on the subduction zone [11].
Trace elements of LILE (Large Ion Lithophile Elements) such as Rb, Ba, Sr, K which are enriched relatively to HFSE (High-Field-Strength Elements) such as Th, Nb, Zr, Hf, Ti for andesite of studied area (Figures 9 and 10) suggests a typical pattern of calc alkaline arc. By refers to [22] and is supported by reference work of [11], it can be suggested that the trace element patterns of the rock samples in the study area and in Mount Batur (previous studies) have similarities with the trace element patterns of the calc alkaline island arc tectonic setting (Figure 9). The REE pattern ( Figure 10) shows a consistentcy with the REE pattern of E-MORB magma that the pattern of E-MORB was summarized from [19]. Additionally, the spider diagram of REE shows the enrichment LREE and HREE depletion. Typical subduction zone magma shows a similar phenomenon of that, which is an enrichment of LREE relative to MREE and HREE [23]. Therefore, this typical pattern of trace elements and REE indicates that andesite at studied area was performed in subduction zones and generated from same source. However, there is an exception of one sample (BR03 sample from studied area) that shows a negative anomaly of Ce element, which is considered and assumed due to as the result of incorporation of subducted sediment to the arc magma sources. Plagioclase fractionation is commonly assumed responsible for negative Eu anomalies in calc-alkaline volcanic arc suites (e.g., [24], [25]). The abundance of plagioclase phenocrysts in andesite of studied area ( Figures 6A-H) provides supporting petrographical evidence for this interpretation. Consequently, the slight negative Eu anomaly of the Botorubuh andesite samples ( Figure 10) suggests a plagioclase fractionation within the source magma. Plotted REE on spider diagram of Hendrawan's samples [4] which shows positive Eu anomalies ( Figure 10) suggests that magma source accumulated plagioclase crystals before solidification and its consequent was most of the Eu would be incorporated into the plagioclase, causing a higher concentration of Eu in the mineral versus other REE in that mineral. That produced rock composition which would display a relatively positive Eu anomaly. It can be suggested that the Hendrawan's samples [4] at Mount Batur had experienced solidification at earlier magmatic phase in relative to the other samples from the studied area at Botorubuh. This interpretation is supported by [6] which considered that andesite lava of Mount Batur was formed in the initial magmatic phase (position of Hendrawan's samples), then followed by the diorite intrusion on the area which was formed in the next magmatic phase in Calc-Alkaline Arc.
Refers to the regional geological map [2], the andesitic rocks of Botorubuh studied area can be classified as the Wuni Formation of the Middle -Late Miocene Epoch. Additionally, according to [9] who have reconstructed Tertiary volcanic centers in Java Island and identified Mount Batur as one of the Lower Miocene volcanic center, and also refers to [8] who have explained the three phases of magmatic arcs on the Java Island, therefore it can be summaried that the andesitic rocks of Botorubuh in studied area related with a paleomagmatism of the Middle -Late Miocene Epoch, and in relation to the Late Miocene-Pliocene subduction zone.

Conclusion
Andesitic rocks at Botorubuh Beach are characterized by porphyritic, trachytic, oscillatory zoning, and sieve textures. The results of geochemical analysis (major and trace elements) show that the andesite is typified by calc alkaline suites. These rocks are enriched in LILEs (Rb, Ba, K) and depleted for HFSEs (Nb, Ti, Ce). In addition, REE shows a slight enrichment of light-REE and a slight negatively anomaly of Eu. The patterns of the trace elements including REE show a typical pattern of calc alkaline arc. The petrographical and geochemical characteristics of the andesite rocks in the research area suggests that the lithology was produced by magma differentiation process which was by a mechanism of crystallization fractionation. The andesite was formed in relation to a Middle -Late Miocene paleomagmatism and the Late Miocene -Pliocene subduction zone.