Petrogenesis of Andesitic Rocks In Sumalata, North Gorontalo

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1 International Journal Engineering Science Applications Petrogenesis Andesitic Rocks In Sumalata, North Gorontalo Muhammad Kasim1, Ahmad Zainuri2, Nurfaika1 1 Geographical Education Study Program, Gorontalo State University (UNG), Gorontalo, Indonesia 2 Physics Education Study Program, Gorontalo State University (UNG), Gorontalo, Indonesia Correspondence: Muhammad Kasim, Geographical Education Study Program, Gorontalo State University (UNG), Jl. Jenderal Sudirman No. 6, Gorontalo. Tel: (+62) ABSTRACT Sumalata area locates in North Gorontalo or northern coast North Arm Sulawesi. This area is rich in natural resources. One those is a gold mine, which is managed by society. The area is covered by Miocene to Pliocene esite tuffs. Andesitic rocks are members Diorite Boliohuto Formation. This study was conducted to determine petrogenesis esitic rocks. The research method used is field observations. The samples were analyzed by petrographic analysis. Field observations shows esitic rocks consisted esite quartz esite. Most the mineral are presence as opaque minerals such as pyrite chalcopyrite. Petrographic analyses 20 samples indicte four types rocks namely esite, trachyte, latite quartz latite. Those rocks are formed by alkaline-intermediate magma. The differences these rocks are their compositions plagioclase, K-feldspar quartz. In addition, the secondary minerals are chlorite, sericite, carbonates opaque minerals. The presence minerals supposedly due to the alteration primary minerals such as pyroxene plagioclase. This can occur if there is a process alteration metal mineralization. Keywords: Sumalata, Petrogenesis, Andesitic rocks Article history: Received 11 October 2014, last received in revised 19 November 2014 lane in the Northern part Sangihe subduction 1. INTRODUCTION Sumalata area located in North Gorontalo district, Gorontalo Province, or geologically lane in the East South part North Arm Sulawesi [2]. located in North Arm Sulawesi. The regional Based on the regional geologi [7] geologic setting study area can be divided Lokodidi into conglomerate, three groups rocks in general; Formation (TQls) composed sstone, conglomeratic sedimentary rock, volcanic rock, intrusive sstone, black shale tuffaceous sstone rock groups [1]. The geological structure - Pleistocene. Dolokapa Formation (Tmd) is an features in this area is fractures, folds faults. old sedimentary rocks (Miocene), composed Horizontal faults northwest-southeast predominantly trending wake trending conglomerate. a few sstone, siltstone, Pinogu claystone, volcanics agglomerates, (TQpv) northeast-southwest. The study area are located composed tuffs, in the Eastern part North Arm Sulawesi, intermediate to alkaline lava - Upper Pliocene to which is a volcanic arc formed by double Pleistocene. subduction, namely North Sulawesi subduction 37

2 International Journal Engineering Science Applications Figure 1. The map study area. Sumalata District, North Gorontalo. Wobudu Breccia (Tpwv) composed specimen. Sampling locations are plotted onto volcanic breccia, agglomerate, tuff, lapilli tuff base map using GPS. Selected rock samples are intermediate to alkaline lava - Pliocene. require for making thin sections. Samples were Diorite Boliohuto (Tmbo) this unit intruding cut polished up to 0,03 mm thickness Miocene Dolokapa Formation, composed observed with a polarizing microscope. diorite granodiorite, in some places basalt 3. RESULTS dikes are found [Figure 5]. The morphology the study area is This study focused on the dioritic rocks, in particular research on Diorite Boliohuto (Tmbo) composed diorite granodiorite, Wobudu Breccia (Tpwv) are also composed intermediate to alkaline lavas. Petrographic observations are necessary to distinguish mineralogy dioritic rocks in Sumalata area. 2. METHOD The method used in this study is a megascopic rocks descripting in the field with the emphasis on petrographic observations. The geological field observation was carried out characterized by hills located at the foot Mohenti Mountain, East Sumalata District. Most the areas are accessible by foot. The rocks covered this area are intermediate to alkaline igneous rocks. In general, the minerals are altered, such as pyrite, chalcopyrite, malachite, hematite. Calcite quartz veins are found in several places. The rock types are esite esite porphyry. These rocks are underlain by Miocene tuff volcanic rocks. Andesite tuff breccia is a member Wobudu formation. by taking a fresh rock samples with h 38

3 International Journal Engineering Science Applications Figure 2. Microscopic appearance mineral compositions esite with plagioclase ground mass with secondary minerals such as klorite cerisite In Kelapa Dua Padengo encountered minerals size in PHP 2.2 are larger, subhedral to weathered esite outcrops spheroidal anhedral, interference color gray-brown to black, weathering. Oxide minerals indicated by reddish hypocrystalline texture, phanero porphyritic, in- dark brown color. Based on petrological equigranular, the primary mineral consist observation, esite is found in gray to plagioclase masses. The secondary minerals greenish color, hypocrystalline - holohyaline, consist hornblende, sericite, clay minerals, porphyro aphanitic - aphanitic, in-equigranular opaque in PHP 06, while at PHP 2.2 consists hornblende pyroxene, chlorite, sericite, opaque as phenocrysts massive, carbonate quartz veins are also contained. Based on petrographic minerals. observations, Trachyte samples were observed in station esitic rocks in this study area can be divided 5.1 are transparent to brownish color, mineral into esite, trachyte, latite quartz latite, size <10μm-520m, mineral forms subhedral to which has form intermediate alkaline anhedral, interference color gray-brown to black, igneous rock. The differences these esitic hypocrystalline texture, phanero porphyritic, in- rocks are in composition plagioclase, equigranular, the primary mineral consist orthoclase quartz. feldspar masses. Secondary minerals consist Andesite has a dominant plagioclase mineral compared to trachyte. Meanwhile, latite quartz latite only differ in composition quartz mineral where quartz latite increased more than 5 percent quartz. The four rocks are derived from the same esitic magma. Andesite samples were observed at PHP 06 PHP 2.2 is transparent to brownish color, carbonate, sericite, clay, opaque minerals. It is found 25,9-30μm sized vein filled by carbonate minerals. At station 5.2 find transparent to brownish colored, mineral size <10μm-1330μm, subhedral porphyritic-trachytic texture, to anhedral, composed phenocryst alkali feldspar plagioclase. Secondary minerals consist sericite 39

4 International Journal Engineering Science Applications Figure 3. Trachyte appearance under the microscope with a porphyritic-trachytic texture composition major minerals alkali feldspar plagioclase that form the structure the flow with clay minerals chlorite as secondary minerals opaque minerals. At Station PHP 04 the mineral size is <10μm-599μm, has subhedral to anhedral form, hypocrystalline porphyritic texture, phanero in-equigranular. Mineral composition consists plagioclase groundmasses, secondary minerals consist clays, sericite, opaque minerals. Latite samples observed in station C.51 has brownish to black color, mineral size ranges <10µm-1200µm, subhedral to anhedral form, subhedral texture. The mineral composes plagioclase pyroxene. The rock alteration is about 30% to form secondary mineral consisting carbonates, epidote, chlorite, sericite, clays, opaque minerals. 4. DISCUSSION Petrogenesis esitic rocks in study area can be divided into two types (i.e phaneritic igneous volcanic rocks). Both rock types are distinguished by its formation. Andesite is the dominant rock cropping out in this study area. Generally it has phanero-porphyritic texture, is formed as a hypobisal body where plagioclase as its phenocryst within pyroxene matrix. These rocks contain secondary minerals such as chlorite, sericite opaque minerals (metals). These minerals are thought to be present due to interaction remaining solution to alter plagioclase pyroxene into chlorite sericite. Meanwhile, opaque minerals Quartz latite samples observed in station observed macroscopically in the form pyrite H1.01 are strongly altered, transparent to brown magnetite. The alteration plagioclase color, mineral size ranges <10μm-646,4μm, pyroxene into chlorite sericite resulting in subhedral to anhedral, porphyritic texture, increasing composition quartz in the rock. dominated by alkali feldspar mineral glass. Furthermore in several places exposes latite Secondary minerals consist quartz, alunite, quartz latite. Meanwhile, trachyte formed from pyrophyllite, clays, opaque minerals. porphyro-aphanitic molten rock is dominantly composed by K-feldspar. Trachyte, 40

5 International Journal Engineering Science Applications Figure 4. Latite appearance under the microscope with the composition the main minerals pyroxene plagioclase that form the structure the flow the mineral epidote, sericite, chlorite clay minerals as secondary Figure 5. Andesite sample (Station 5.1) shows porphyritic texture latite quartz latite are found in several esite outcrops in Padengo, Lasambo Kelapa Dua. These rocks are difficult to distinguish on macroscopic observation. The process alteration mineralization is the main reason variety esitic rocks in the study area. Figure 6. Morphological view Padengo area shows high weathering cropping out esite rocks. 5. CONCLUSIONS Intermediate rocks become common constituent rocks in Sumalata area, dominated by esite, in some places encountered trachyte content. This latite process with increased supposedly silica due to alteration mineralization occurrences in 41

6 International Journal Engineering Science Applications esite. This is proven by alteration products Bung: chlorite, sericite, carbonates, silica content Pengembangan Geologi. opaque which is a kind metallic minerals. Pusat Penelitian dan [2] Simjuntak, T. O., Tektonika. Publikasi ACKNOWLEDGEMENTS Khusus, Bung: Pusat Penelitian dan Pengembangan Geologi. Our thanks go to Directorate Research Community Services (Ditlitabmas), Directorate General Higher Education, which has given us opportunity for this research. To the President State University Gorontalo who has given the same opportunity for lecturers to conduct research. The Research Institute State University Gorontalo facilitate provide guidance to us tirelessly. Regency Government North Gorontalo, East Sumalata District Government Hulawa Buladu Village Government as well as all those who have helped to make this research went well. And to fellow lecturers who have helped encouraged us. REFERENCES [1] Bachri, S., Sukido, & N. Ratman, 1993, Peta Geologi Lembar Tilamuta, Sulawesi, 42 [3] Carlile, J. C., S. Digdowirogo & K. Darius, 1990, Geological setting, characteristics regional exploration for gold in the volcanic arcs North Sulawesi, Indonesia. Journal Geochemical Exploration. [4] Kavalieris, I., T. M. Van Leeuwen, M. Wilson, 1992, Geological setting styles mineralization, north arm Sulawesi, Indonesia. Journal Southeast Asian Earth Sciences, 7, [5] Trail, D. S., T. V. John, M. C. Bird, R. C. Obial, B. A. Petzel, D. B. Abiong, Parwoto & Sabagio, 1974, The general geological survey block 2, Sulawesi Utara, Indonesia, P.T. Tropic Endeavour Indonesia [6] Van Bemmelen, R. W., 1949, The geology Indonesia, economic geology. Vol. 2. Govt Printing Office: The Hague.