KARAKTERISTIK ANDESIT FORMASI HULUSIMPANG DAERAH TAMANSARI DAN SEKITARNYA, KABUPATEN TANGGAMUS, LAMPUNG

Abstract

ABSTRAK: Penelitian untuk mempelajari karakteristik batuan andesit Formasi Hulusimpang telah dilakukan di daerah Sumanda dan sekitarnya, Kabupaten Tanggamus, Provinsi Lampung. Secara geologi daerah penelitian berada di Busur Magmatik atau Zona Barisan yang merupakan hasil dari tunjaman pada periode Tersier Awal sampai Resen. Formasi Hulusimpang berumur Oligosen Akhir-Miosen Awal, terdiri dari satuan batuan andesit, basalt dan tuf. Penelitian dilakukan dengan cara observasi lapangan dan analisis petrografi terhadap sayatan tipis batuan andesit yang tersingkap di lapangan. Hasil analisis petrografi menunjukkan bahwa batuan andesit memiliki derajat kristalisasi hipokristalin, tekstur porfiritik, keseragaman butir mineral inequigranular, bentuk kristal subhedral-anhedral, terdiri dari mineral plagioklas yang berlimpah sebagai fenokris dan masa dasar, sanidine dan biotit. Pada sayatan tipis batuan juga ditemukan mineral epidot yang mencirikan temperatur pembentukan berkisar antara 180 hingga 200°C yang mengindikasikan batuan andesit Formasi Hulusimpang mulai mengalami ubahan. Kata Kunci: petrografi, andesit, alterasi, propilitik ABSTRACT: The study about characteristic of andesite rocks in Hulusimpang Formation has been carried out in Sumanda district and its surrounding, Tanggamus regency, Lampung Province. Geologically, the research area located in the Magmatic Arc or Barisan Zone which is the product of subduction in Upper Tertiary to Resent. Hulusimpang Formation was formed in Upper Oligocene to Lower Miocene, consist of andesite, basalt and tuff. The research was conducted by field observation and petrography analysis towards the thin section of andesite that exposed in the research area. The result of petrography analysis shows that the andesite is hipocrystalin in degree of crystalinity, has porphyritic texture, inequigranular with subhedral to anhedral in crystal shape, consist of abundance plagioclass as phenocrysts and groundmass, sanidine and biotite mineral. The thin section also consist epidote mineral which formed in temperature around 180 to 200°C, indicates that the rock has slightly altered.

Keywords: petrography, andesite, alteration, propylitic