Petrogenesis of Adang and Talaya volcanic rocks, in the Mamuju area and its surrounding, West Sulawesi

The Adang Volcanic and Talaya Volcanic complex located in the Mamuju area are ancient volcanic complexes of the Neogene period. These two complexes consist of pyroclastic, volcaniclastic, and basic-intermediate lava containing the main modal minerals of leucite and clinopyroxene, making these rocks unique and rarely found in Indonesia. This study aims to determine the petrological and geochemical characteristics as well as the tectonic setting of rock formation. The rocks are generally porphyritic and vitrophyric in texture with various rock types as a result of magma differentiation. The Adang Volcanic Lava is composed of trachyandesite, tephriphonolite, trachyte, phonolite, and leucitite. Talaya Volcanic Lava is composed of basalt and trachybasalt rocks. Based on geochemical content, rocks have high alkalinity with wide range concentrations of K2O (0.27 – 10.12%), Na2O (0.54 – 11.06%), and total alkali (4.6 – 12.6%), as well as evidenced by the presence of feldspathoids. Rocks are divided into three groups, sodic, potassic, and ultrapotassic. The ultrapotassic rocks have similarities with Group III (Active Orogenic Zone) and Group I (Lamproite). The intensive enrichment of incompatible elements such as Rb and Th as well as the negative Nb and Ti anomalies reflect that magma derived from mantle metasomatism and is associated with subduction events. The genesis of the rock occurred in the tectonic setting of alkaline arc magmatism or continental extensional zone which was formed after the collision of the Banggai-Sula microcontinent with West Sulawesi. This process is also indirectly influenced by pre-collision subduction of the oceanic lithosphere.


Introduction
Sulawesi Island is an area that is at the junction of the Eurasian, Pacific, and Indo-Australian plates.The interaction of those major plates produces regions with various unique and complex geological characteristics.Sulawesi Island is divided into three geological provinces [1,2,3], namely West Sulawesi (West and North Sulawesi Plutonic-Volcanic Arc), Central Sulawesi (Sulawesi Metamorphic Belt), and East Sulawesi (East Sulawesi Ophiolite Belt).Sulawesi is also composed of the Banggai-Sula Microcontinent in the eastern and southeasternmost part of the island.Rocks formed up the West Sulawesi are more continental in nature consisting of magmatic rock, sedimentary rock (Mesozoic -Quaternary), and metamorphic rock (Cretaceous) [4].Adang and Talaya Volcanic is the product of West Sulawesi magmatism in Neogene Period, located in Mamuju area where potassic and ultrapotassic are found.
There are many implications regarding the tectonic settings and the mechanism of formation of the Adang and Talaya volcanic rocks.Therefore, this study aims to determine the petrological and geochemical characteristics as well as the tectonic setting of rock formation.

Regional Geology and Sample Location
In the western part of the regional geological map of Mamuju area [5], there were several rocks units the oldest to the youngest, namely the Latimojong Formation, Talaya Volcanic, Adang Volcanic, Intrusive Rocks, Mamuju Formation, and Member of Tapalang Mamuju Formation.Lithological formations which are associated with the study area are the Talaya Volcanic and Adang Volcanic which is at the Middle-Late Miocene Epoch.The Talaya Volcanic comprises lithologies of andesitic basaltic volcanic breccia, tuff, and lava, with intercalations of sandstone and marl, locally coal.The Adang Volcanic comprises lithologies of leucite-basaltic tuff, lava, and volcanic breccia, mainly micaceous.Based on volcanostratigraphy, Adang Volcanic encompasses several hummocks: Tapalang, Malunda, Adang, Ampalas, Karampuang, Sumare, and Labuang Rano, while Talaya Volcanic comprises of Kaluku Hummock.

Data and methods
The study area is located in some parts of Mamuju and Mamasa District.The methods used for this study are petrographic and geochemical analyses of primary and secondary data.The primary data has been collected directly from field observation and sampling of relatively fresh to slightly weathered rocks at several points in the study area (figure 1).The samples were processed into 39 thin sections for petrographic observation and 11 samples were carried out in Intertek Laboratories for XRF and ICP-MS/OES analyses to get the major oxides, trace elements, and REE data.However, primary data did not cover some areas well, so secondary data was needed for more representative and precise interpretation.There are 28 additional geochemical data acquired from various authors of previous studies [6,7,8,9], some samples do not have complete REE and trace elements data.For this study, all data were grouped into several clusters based on the location and volcanostratigraphy.The Adang Volcanic encompasses Tapalang, Ampalas, Adang, Sumare, and Labuang Rano groups, while Talaya Volcanic is represented by the Kalukku group.These data were plotted into several diagrams to determine geochemical characteristics such as rock types, magma series, magma evolution, and tectonic setting of volcanic rocks in the study area.The sampling locations of petrographic and geochemical data and their correlation with age, lithology, formation, and stratigraphy are summarized in figure 1 and figure 2.

Petrography
The outcrops found in the study area are lava, pyroclastic rocks, and boulders.The igneous rock classification used to determine the name of the rocks is the classification by Williams et al. (1982) [10].Based on the observation of thin sections, rocks from Adang and Talaya Volcanics generally have a porphyritic texture.The mineralogy of rocks is dominated by the presence of leucite, clinopyroxene, and a small amount of biotite as phenocrysts with volcanic glass and opaque minerals as groundmasses.Those rocks are classified as leucitite.Some of the phenocryst minerals also occur as a groundmass.Some of the rock samples show vitrophyric and trachytic textures, composed of volcanic glass and plagioclase or alkali feldspar as groundmass.Phonolite and trachyte also occurred.

Geochemical analysis
Based on the major oxides data, the silica content of the Adang and Talaya Volcanics lava is (47.06 -65.7%) so the magma is categorized as basaltic-intermediate magma.Magma formed high alkalinity rocks with wide range concentrations of K2O (0.27 -10.12%),Na2O (0.54 -11.06%), and total alkali (4.6 -12.6%).The rocks found in the lava, fragments, and blocks of the Adang Volcanic are classified as trachyandesite, basaltic trachyandesite, trachyte, tephriphonolite, phonotephrite, and andesite.Lava from Talaya Volcanic was plotted within the basalt and trachybasalt fields, with fragments from the volcanic breccia plotted within the dacite field.Almost all of the samples are categorized as alkaline rocks based on their total alkali value (figure 4A).Based on the diagram of Na2O vs K2O (figure 4C) [11] there are three groups of alkaline rocks in the study area: sodic, potassic, and ultrapotassic.Magma series also shows a wide variety from tholeiite to shoshonite based on the diagram of K2O against SiO2 by Peccerillo and Taylor [12] (figure 4B).However, samples might fall in the tholeiite series field due to the significant number of LOI content.Samples of the shoshonite series are the most common.
The trace elements in rocks could be used to determine the process of magma evolution such as crystal fractionation and crustal contamination.By using a variation diagram of Y and Nb elements, the level of crustal contamination could be determined.Rocks formed with high levels of crustal contamination condition have low Y and Nb content.Based on Y vs Nb amd Ba/Th vs Th/Nb diagrams (figure 4E and figure 4F), Adang and Talaya volcanic rocks formed as the result of a wide range level of crustal contamination.For example, samples of Adang Volcanics with high levels of crustal contamination are from the Ampalas group, while samples from the Tapalang group have a high Y and Nb ratio that shows the crustal contamination was not significant.Samples from Talaya in Kalukku also have a high level of crustal contamination.Based on the ratio of Ba/Th and Th/Nb the contamination process dominantly occurred in the lower crust (figure 4F).Tectonic setting determination using Al2O3 vs TiO2 diagram (after Muller and Groves, 2003) [13], shows the rocks of the study area are plotted into both arc-related and within-plate fields (figure 5C).The samples that have more arc-related characteristics are from Ampalas (Adang Volcanic) and Kalukku (Talaya Volcanic) groups, while most samples that show within-plate characteristics are from Tapalang.This might be a clue that the tectonic setting of the study area was transitioning from subduction-related to within-plate magmatism.This can also be seen from the La/Yb vs Th/Nb and La/Yb vs Nb/La diagrams (after Hollocher et al., 2009) [14] (figure 5A and figure 5B) which shows the tectonic setting of rock formation is an alkaline arc.However, there is one sample from the Ampalas group plotted in the continental arc field and two other samples (Ampalas and Kalukku) plotted near the continental arc borderline.Based on Ta/Yb vs Th/Yb diagram (figure 5E) the samples were plotted in the shoshonite series with an alkaline arc tectonic setting.However, one sample from the Ampalas group falls in the calc-alkali series with a volcanic arc tectonic setting.The diagram also shows samples from Ampalas and Kalukku categorized as group III potassic rocks.The variation diagram of Zr vs Nb (figure 5D) indicates potassic rocks formed in subduction and post-collision phase and zone with the characteristic of low content of Zr and Nb and then turned into more evolved due to the enrichment of both elements.
Only seven samples have the criteria for ultrapotassic rocks following the definition of ultrapotassic rocks according to Foley et al. (1987) [15] so those seven samples each from Tapalang, Adang, Ampalas, and Sumare areas were used to determine the ultrapotassic rocks classifications.There are three groups of ultrapotassic rock: Group I (lamproites), Group II (kamafugites), and Group III (active orogenic zone products) [15].Based on the diagram (figure 6) the rocks from the study area are categorized as a combination of group I and group III ultrapotassic rocks.The diagram shows the samples were plotted close with the group borderline.This might indicate those rocks were formed in a transition zone of active orogenic into more within-plate tectonic setting.
The primitive mantle normalized [17] spider diagram for rocks in the study area shows strong enrichment of the incompatible elements like Rb, Ba, Th, and K relative to OIB and MORB patterns.The negative anomaly of the trace elements is shown by Nb, Sr, and Ti while the positive anomaly of trace elements is shown by Hf and Zr (figure 7A).LREE pattern (La, Ce, Pr, Nd, Sm) also shows strong enrichment relative to HREE elements (Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu) in the REE diagrams (figure 7B).The samples from the Tapalang group were the most enriched, while the samples from Kalukku group (Talaya Volcanics) show the least enrichment.Some of the samples have a negative anomaly of Eu.Most of the sample show a similar pattern to the group I (Lamproite) and group III ultrapotassic rocks.This shows that Adang and Talaya Volcanics might have the same tectonic setting as both groups I and III.

Discussion
Based on the petrographic analysis, Adang and Talaya volcanic rocks generally show a porphyritic texture, indicating the minerals are formed in two different cooling stages.First, when magma was far from the surface in the magma chamber by crystallizing coarse-medium-sized minerals to form phenocrysts such as leucite and clinopyroxene, then when it was near the surface so that fine-grained minerals and volcanic glass were crystallized as groundmass.Trachytic texture also shows that magma flows as it rises toward the surface.Crystallization of flow materials causes a tendency for the crystals to position themselves in the direction of the flow.Leucite is a feldspathoid mineral that characterizes potassic and ultrapotassic rocks.This rock is classified as an alkaline rock with a very high K2O content.Usually appears when the magma has low silica saturation.If the magma was saturated with silica, the minerals would appear as feldspar.
Based on the major oxides data, it was found that magma is basaltic-intermediate in composition.This indicates that the magma has partially differentiated.Based on the total alkali (K2O + Na2O) vs silica (SiO2) content (figure 4A), the rocks that appear the most are trachyandesite rocks.Rocks are generally categorized as alkaline rocks.The alkalinity of Adang and Talaya volcanic rocks are also grouped into three types, sodic (Na2O is more dominant than K2O), potassic (K2O is slightly dominant), and ultrapotassic (K2O is extremely dominant).The Magma series also vary from tholeiite to shoshonite, with most samples are shoshonite series.The high LOI content might be the cause of the samples plotted as tholeiite series.Crystal fractionation and crustal contamination occurred during the magma evolution process from calc-alkali to the shoshonite series.Crustal contamination plays a role in the enrichment of incompatible elements such as Ba and Th.The high ratio of Ba/Th shows crustal contamination dominantly occurred in the lower crust.
The Ultrapotassic rocks which have high K2O concentration from the study area were classified as group I and group III ultrapotassic rocks (figure 6).Group I (lamproites) ultrapotassic rocks are typically formed as the product of post-orogenetic magmatic phenomena in regions that have experienced a continental collision with underlying fossil of Beniof zones and are commonly associated with lineaments or fault trends.Group III ultrapotassic rocks are rocks of active orogenic zone tectonic setting with the characteristics of high Al2O3 The tectonic setting of the Adang and Talaya Volcanics is interpreted as an alkaline arc based on La/Yb vs Th/Nb, La/Yb vs Nb/La, and Ta/Yb vs Th/Yb diagrams (figure 4A, figure 4B, and figure 4E).The tectonic setting of the study area is an alkaline arc with a combination of subduction-related and within-plate magmatism.The transitional environment from arc-related to within-plate magmatism is shown by the TiO2 vs Al2O3 diagram (figure 5C) where samples from Kaluku (Talaya Volcanic) and Ampalas group have higher Al2O3 content as typical arc-related characteristics and samples with high TiO2 characterizes the within plate tectonic setting.
The petrogenesis of volcanic rocks in the study area started in Miocene.In Early Miocene, the collision of the Banggai-Sula microcontinent with West Sulawesi occurred [19].Collision causes thickening of the crust, subduction stops, and slab detachment [20].Crustal contamination from previous subduction causes enrichment of REE and incompatible elements such as K, Rb, and Ba so that for a long time it will produce high K calc-alkali magma.In Middle Miocene, magma that previously formed in the continental arc tectonic setting began to evolve into a potassic-ultrapotassic alkaline magma forming porphyritic rocks such as leucitite and phonolite with the main mineral of leucite and pyroxene.The initial stage was initiated by the volcanism at Kalukku (Talaya Volcanic) and Ampalas (Adang Volcanic) areas, characterized by relatively lower Th/Yb, Ta/Yb, Nb/La, and La/Yb ratios.This is supported by the lower position of the spider diagram pattern (figure 7A and figure 7B) relative to other samples from another area such as the Tapalang area.The characteristic feature of this stage is the high intensity of crustal contamination and low Ti, Nb, and Zr content.Furthermore, magma develops along with the collision process.Then in the Late Miocene, there was an uplift and decompression of the lithosphere, the mantle became unstable and triggered an asthenosphere rise and delamination of the lower lithosphere [20].The lower litosphere and asthenosphere interaction form a new type of shoshonite/potassic-ultrapotasic magma such as in the Tapalang area.This magma is also found in Ampalas and other locations.This was indicated by increased Ti and Nb contents, higher Th/Yb, Ta/Yb, La/Yb, and Nb/La ratios, more enriched concentrations of REE and incompatible elements, and higher Zr and Hf.This indicates that in this process the within-plate began to develop but the influence of the previous subduction is still visible, based on the pattern of the spider diagram shown with the negative anomaly of Ti and Nb.

Conclusion
The Adang and Talaya Volcanics have petrological characteristics of leucitite, phonolite, and trachyte rocks with porphyritic textures with the main mineral of leucite and clinopyroxene as phenocrysts in volcanic glass groundmass.Vitrophiric and trachytic textures are also present.The geochemical characteristics of the Adang and Talaya Volcanics are basaltic-intermediate rocks with concentration of TiO2 (0.71 -1.68%), K2O (0.27 -10.12%), and Na2O (0.54 -11.06%).Adang Volcanic magma has a wide range of alkalinity from sodic to ultrapotassic with varying magma series of tholeiitic, calc-alkali, high-K calc-alkali, and shoshonite magmas.The type of rocks based on geochemical data are basaltic trachyandesite, trachyandesite, andesite, tephriphonolite, and trachyte.Talaya Volcanic magma is potassic alkaline and has a shoshonite series with rock types of basalt and trachybasalt.Magma is very

Figure 2 .
Figure 2. The correlation of petrographic and geochemical data with age, formation, stratigraphy, and lithology.The color differences of data in data samples columns shows different data sources.

Table 1 .
Major oxides abundances of primary data sample