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Subsurface S-type Granitoid Identification Based on Gravity and Seismic Tomography Models in Pacitan, East Java
DOI: http://dx.doi.org/10.55981/eksplorium.2018.4954
Abstract
ABSTRACT
Granitoid outcrop has been observed in Montongan, Tulakan Subdistrict, Pacitan District, East Java. Geochemically, granitoid shows peralluminous S-type granitoid which consists of comparable plagioclase and potassium feldspar leading to adamelite and granodiorite variety with andalusite, fine size corundum and cordierite inside. These modal minerals are consistent with its bulk chemical analysis result that shows alumina rich rock. Highly weathered spotted pinkish soil with remaining quartz gravels characterizes its surface. Lateritic pink soil up to more than 25 meters thick covers the granitoid body and this feature is indicative to locate its surface distribution, while its subsurface distribution is remain uncertain. The research aimed to identify granitoid subsurface distribution. To identify the subsurface body, gravity and seismic tomography models were used. According gravity model, the pluton body is 5 km wide which is rootless downward and seems extends eastward. Meanwhile, the north-south seismic tomographic model across Pacitan Region indicates dense solid body override the recent Java subduction zone. The body is assumed to have correlation with surface granitic rock. It supports an idea that there is a micro continent trapped beneath Southern Mountain of East Java.
ABSTRAK
Singkapan granitoid telah teramati di daerah Montongan, Kecamatan Tulakan, Kabupaten Pacitan, Jawa Timur. Secara geokimia, granitoid Pacitan memperlihatkan granitoid peralumina tipe-S yang tersusun berdasarkan perbandingan plagioklas dan kalium felspar menunjuk pada varian adamelit dan granodiorit dengan andalusit, korundum halus dan kordierit di dalamnya. Mineral modal tersebut konsisten dengan hasil analisis kimia total yang menunjukkan batuan kaya alumina. Tanah berwarna merah muda yang sangat lapuk dengan kerikil sisa kuarsa menjadi ciri khas di permukaannya. Tanah laterit merah muda yang tebalnya lebih dari 25 meter menutupi tubuh granitoid tersebut dan menjadi petunjuk penyebaranya di permukaan, namun penyebaran di bawah permukaannya masih belum pasti. Penelitian ini bertujuan untuk mengetahui penyebaran granitoid di bawah permukaan. Untuk mengidentifikasi tubuh bawah permukaannya, digunakan pemodelan gravitasi dan tomografi seismik. Menurut model gravitasi tubuh pluton mempunyai lebar 5 km dan tampak memanjang ke arah timur yang tidak menentu ke bawahnya. Sementara itu, model tomografi seismik utara-selatan yang memotong wilayah Pacitan, menunjukkan suatu tubuh padat keras berada di atas zona subduksi Jawa saat ini. Tubuh tersebut diasumsikan memiliki hubungan dengan batuan granitik di permukaan. Hal tersebut mendukung ide bahwa terdapat mikro-kontinen terperangkap di bawah Pegunungan Selatan Jawa Timur.
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