Electronic Theses and Dissertation

Komplek Indrapuri di Kecamatan Jantho, Kabupaten Aceh Besar merupakan kawasan yang belum banyak diteliti secara geologi, padahal terletak di jalur tektonik aktif Sesar Sumatera dan mengandung batuan ultramafik yang berpotensi mengalami alterasi hidrotermal. Kurangnya data detail mengenai proses serpentinisasi serta indikasi adanya mineral bernilai seperti magnesium dan besi menjadikan wilayah ini penting untuk diteliti.Penelitian ini dengan luas area sekitar ±25 km². Wilayah ini berada dalam jalur tektonik aktif Sesar Sumatera dan secara geologi terdiri atas empat satuan utama: Komplek Indrapuri (Tuic), Formasi Indrapuri (Qpin), Formasi Seulimeum (QTps), dan Anggota Padangtiji (Tuktp). Penelitian ini bertujuan untuk memahami kondisi geologi, karakteristik litologi, struktur, serta proses alterasi batuan ultramafik berupa serpentinisasi yang berkembang di daerah tersebut. Metode yang digunakan meliputi pemetaan geologi skala 1:25.000, observasi lapangan pada 46 titik singkapan, serta analisis laboratorium menggunakan petrografi dan X-Ray Diffraction (XRD). Secara geomorfologi, wilayah ini diklasifikasikan ke dalam bentuklahan denudasional, struktural, dan fluvial, dengan pola aliran sungai dominan dendritik. Hasil analisis litologi menunjukkan terdapat enam satuan batuan, yaitu: (1) Satuan Endapan pasir kerikil, (2) Satuan Tuf, (3) Satuan Batupasir, (4) Satuan Batupasir Karbonatan, dan (5) Satuan Batu Peridotit Terserpentinisasi. Hubungan stratigrafi antara Formasi Qpin dan Tuic bersifat tidak selaras, mengindikasikan peran aktivitas tektonik dan sedimentasi fluvial. Struktur geologi didominasi oleh kekar berorientasi NW–SE dan NE–SW, membentuk conjugate fractures dan extension joints sebagai jalur pergerakan fluida hidrotermal. Analisis petrografi pada batupasir (MDZ21) menunjukkan klasifikasi lithicwacke, sedangkan tuf (MDZ34) menunjukkan klasfikasi vitrictuff. Batuan ultramafik dijumpai pada satuan Komplek Indrapuri, dengan tekstur faneritik, holokristalin, dan warna hijau kehitaman. Sayatan tipis batuan ultramafik (MDZ17) memperlihatkan mineral olivin ±55% lalu yang telah teralterasi menjadi serpentin ±31%, serta plagioklas ±5% dan mineral opak ±8%. Analisis XRD mengonfirmasi kehadiran olivin, serpentin (lizardite, chrysotile), plagioklas (anorthit), dan magnetit. Proses serpentinisasi diinterpretasikan terjadi akibat interaksi antara batuan ultramafik dan fluida hidrotermal, menghasilkan tekstur mesh dan veinlet, yang terbentuk pada kondisi tekanan 4–11 kbar dan suhu 320–360°C. Temuan ini menunjukkan keterkaitan erat antara struktur geologi, litologi, dan proses hidrotermal dalam membentuk karakteristik batuan di wilayah penelitian.

The Indrapuri Complex in Jantho District, Aceh Besar Regency is an area that has not been widely studied geologically, even though it is located on the active tectonic line of the Sumatran Fault and contains ultramafic rocks that have the potential to experience hydrothermal alteration. The lack of detailed data regarding the serpentinization process and indications of valuable minerals such as magnesium and iron make this area important to study. This research covers an area of approximately ±25 km². This area is located on the active tectonic line of the Sumatran Fault and geologically consists of four main units: the Indrapuri Complex (Tuic), the Indrapuri Formation (Qpin), the Seulimeum Formation (QTps), and the Padangtiji Member (Tuktp). This study aims to understand the geological conditions, lithological characteristics, structures, and alteration processes of ultramafic rocks in the form of serpentinization that developed in the area. The methods used include geological mapping at a scale of 1:25,000, field observations at 46 outcrop points, and laboratory analysis using petrography and X-Ray Diffraction (XRD). Geomorphologically, this area is classified into denudational, structural, and fluvial landforms, with a dominant dendritic river flow pattern. The results of lithological analysis indicate that there are six rock units, namely: (1) Sand-gravel Sediment Unit, (2) Tuff Unit, (3) Sandstone Unit, (4) Carbonate Sandstone Unit, and (5) Serpentinized Peridotite Unit. The stratigraphic relationship between the Qpin and Tuic Formations is unconformable, indicating the role of tectonic activity and fluvial sedimentation. The geological structure is dominated by NW–SE and NE–SW oriented joints, forming conjugate fractures and extension joints as paths for hydrothermal fluid movement. Petrographic analysis of sandstone (MDZ21) shows a lithicwacke classification, while tuff (MDZ34) shows a vitrictuff classification. Ultramafic rocks are found in the Indrapuri Complex unit, with phaneritic, holocrystalline textures, and blackish green color. Thin sections of ultramafic rocks (MDZ17) show ±55% olivine minerals that have been altered to ±31% serpentine, as well as ±5% plagioclase and ±8% opaque minerals. XRD analysis confirmed the presence of olivine, serpentine (lizardite, chrysotile), plagioclase (anorthite), and magnetite. The serpentinization process is interpreted to occur due to the interaction between ultramafic rocks and hydrothermal fluids, resulting in mesh and veinlet textures, which formed at pressure conditions of 4–11 kbar and temperatures of 320–360°C. These findings indicate a close relationship between geological structure, lithology, and hydrothermal processes in shaping the characteristics of rocks in the study area.