Peralihan Rezim Tektonik: Implikasinya pada Konsentrasi Torium di Mamasa dan Tana Toraja, Sulawesi-Indonesia

Sugeng Purwo Saputro, Dwi Ratih Purwaningsih, Bambang Priadi

DOI: http://dx.doi.org/10.17146/eksplorium.2020.41.2.6063

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ABSTRAK Mamasa dan Tana Toraja secara geografis merupakan bagian dari lengan barat Pulau Sulawesi. Batuan-batuan mafik di daerah tersebut dan sekitarnya memiliki nilai laju radiasi tinggi dan anomali kandungan torium (Th). Penelitian ini bertujuan untuk mengetahui mekanisme tataan tektonik yang berperan dalam peningkatan konsentrasi Th. Enam sampel batuan dianalisis menggunakan analisis petrografi dan geokimia (AAS, ICP-MS, NA, dan XRF), dilengkapi dengan pentarikhan umur menggunakan metode 40K-40Ar pada sampel batuan terpilih. Pengamatan petrografi memperlihatkan kehadiran mineral plagioklas, olivin, piroksen, hornblenda, nefelin, dan alanit pada batuan yang diidentifikasi sebagai nefelin-basanit, basalt, trakhibasalt, dan gabro. Sejumlah tekstur yang tampak pada batuan tersebut mengindikasikan kontaminasi dan perubahan kondisi tektonik. Analisis geokimia menunjukkan bahwa nefelin-basanit, basalt, trakhibasalt, dan gabro (absarokit) terbentuk pada batas kontinental aktif (ACM) yang sedang mengalami transisi dari subduksi aktif (penunjaman ke arah barat) menjadi post-subduksi. Perubahan tataan tektonik membuat magma membeku pada kondisi yang sangat ekstrim. Proses pembekuan magma diinterpretasikan terjadi pada umur sekitar 13,10-11,02 Ma. Mekanisme tersebut berperan penting terhadap terjadinya peningkatan konsentrasi torium di Mamasa dan Tana Toraja.

ABSTRACT Mamasa and Tana Toraja geographically are part of the western arm of Sulawesi Island. The mafic rocks in these areas and their surroundings have high radiation dose rate and thorium (Th) anomaly content. This research aim is to determine tectonic setting mechanism which play the important role on the increasing of Th concentration. Six rock samples were analysed using petrography and geochemical analyses (AAS, ICP-MS, NA, and XRF) completed with the 40K-40Ar dating on selected rock samples. Petrography observations show plagioclase, olivine, pyroxene, hornblende, nepheline, and allanite minerals presence in the rocks which identified as nepheline-basanite, basalt, trachybasalt, and gabbro. Numbers of texture appearances in the rocks indicate contamination and changes on tectonic setting. Geochemistry analysis shows that nepheline-basanite, basalt, trachybasalt, and gabbro (absarokite) were formed at the active continental margin (ACM), which is undergoing active subduction (westward subduction) to post-subduction transition. The changing of tectonic setting made magma solidify in extreme conditions. The magma solidify process is interpreted to occur at the age of 13.10-11.02 Ma. These mechanisms play an important role for the increase of thorium concentration in Mamasa and Tana Toraja.


Kata Kunci


Torium; Tana Toraja; potasium-argon; allanit; post-subduksi

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Referensi


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