#TITLE_ALTERNATIVE#

• Main Author: SENDJAJA (NIM : 32008004) ; (Pembimbing: Prof. Dr. Emmy Suparka ; Dr. Ir. Chalid Idham Abdul, PURNAMA
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/19173
• Institution: Institut Teknologi Bandung

The Togean Islands are located in the Gulf of Tomini, Tojo Una-Una Regency, Central Sulawesi Province, the geographical location is 0 &#8304; - 0 &#8304; 40 ''30 South and 121 &#8304; - 122 &#8304; 30' East. This archipelago consists of several islands, the main islands being Una-Una, Batudaka, Togean, Talakatoh Waleakodi, and Waleabahi Island. These islands are located between the North Arm and East Arm of Sulawesi and have a complex tectonic regime in the middle of two subduction zones. The south dipping subduction of the Celebes Sea Plate and the west dipping subduction of the Molucca Sea Plate or oceanic plate is in the front of the Banggai Sula Microcontinent. <br /> <br /> This complex tectonic conditions produce a variety of volcanic rocks in Togean Islands, and still a matter of controversy for their petrogenesis. Therefore, in order to evaluate geodynamic setting base on volcanic rocks petrogenesis, a comprehensive study, included field surveys, petrographic, geochemical and radiometric age were conducted. Polarization microscope, SEM-EDS, XRF, ICP-MS, PGA and INAA are the method and instrumentation for petrology and geochmestry analysis, whereas for the age dating was conducted using the fission track method in zircon. <br /> <br /> Volcanic rocks from the Togean Islands can be divided into 3 types based on the location and magma affinities: The Una-Una (UN), Togean (TG) and Walea (WL). These rocks formed in situ around the equator, not as the migration component because of tectonic events. Volcanic rocks of the UN and TG have similar characteristics and geochemical patterns. Both of them are adakites (SiO2 &#8805; 56 %, Al2O3 &#8805; 15 %, MgO < 3 %, Y &#8804; 15 ppm, Yb &#8804; 1.9 ppm, Sr > 300 ppm, rasio Sr/Y > 20, dan La/Yb > 20); consist of basaltic-trachyandesite until trachydasite, in which the magma is calk-alkaline and a high-K calk-alkaline and shoshonite affinity. The volcanic rocks from UN and TG are interpreted as the result of partial melting of the Sulawesi Sea slab within the amphiboles-eclogite zone. <br /> <br /> Geochemical characteristics based on rare earth element patterns show similarities between volcanic rocks of the UN and TG, having 40-150 times the amount of light REE (La, Ce, Pr) and 10 times for heavy REE (Dy, Ho, Er, Tm, Yb, dan Lu). Both of them show an abundance of subduction component elements such as Ba (>1000ppm), Sr (>1000ppm), B (>3ppm), Pb (>25ppm) and show negative Nb depletion, further confirm the evidence that the volcanic rocks formed in a convergent-subduction environment. <br /> <br /> The volcanic rocks of the WL, have a tholeitic affinity and consist of basaltic-andesite and tholeit basalt; they have formed in the mid oceanic ridge (MORB), IAT and CAB. This association is interpreted as being an ophiolite related zone with subduction or suprasubduction ophiolite (SSZ). The subduction system associated with the SSZ ophiolite occurred in the southeast of the Togean Islands involving the Molucca Sea Plate or plate in front of the sea Banggai-Sula microcontinent with the East Sulawesi ophiolite (ESO). <br /> <br /> The Celebes Sea Plate, at the beginning of the Miocene, subducted below the North Arm of Sulawesi with a gentle slope (partial melting of slab at 70-85 km depth, with 250 km distance from the trench), this situation producing adakites in the Una-Una. Along with the continuous subduction process and increasingly steep angle of subduction which the magma source under went and with the mixing process in the mantle wedge and differentiation in the magma chamber more acid rocks were produced, but still giving the impression of being derived from the same magma source. <br /> <br /> In contrast to the process that occurs in the northern part of the Togean Islands, which are indicative of an active subduction of the Celebes Sea Plate since the Miocene, the southern part of the Togean Islands shows a subduction-related product with a different process. The set of ophiolite rocks in the Walea region were formed around 6 Ma., and were related to the process of spreading of oceanic crust in front of the subduction due to roll back of oceanic crust of the Banggai Sula microcontinent or Molucca Sea Plate. <br /> <br /> Intensive movement of the Banggai-Sula microcontinent to the west at this time, resulted in obduction of the ophiolite during collision, ESO emerging to the surface in the East Arm of Sulawesi and SSZ ophiolite in Togean Island; whereas in the UN and TG, volcanic activity is still ongoing, Pyroclastic material from the eruption in 1983 on Una-Una Island, has an acid silica composition with mineralogical assemblages of hornblende- phlogophite-biotite, showing that the petrogenesis of these rocks has been derived from a mixing mantle source, indicating a deepening magma source.