No abstract

No abstract

Tangkuban Parahu Volcano is one of the most active volcanoes in West Java, Indonesia, although most of the recent eruptions were relatively mild (e.g. 2013 eruption). However, there is still little information from the volcanic products in the proximal area. Here, a new documentation from the proximal volcanic succession is provided, including tephra-stratigraphy, componentry analysis, and petrography of volcanic products. Detailed mapping of the proximal area shows that the volcanic products are predominantly composed of alternating fine-clay and coarse ash, lapilli tuff, and pyroclastic breccia within ten tephra units. Componentry of ash particles revealed the presence of five components, associated with hydrothermally altered lithics, oxidized lithics, coherent crystalline lithics, magmatic juvenile, and free crystal in entire eruptive products. These indicate that the subvolcanic hydrothermal system has been developed since the Holocene and associated with a continual introduction of magmatic intrusion. Petrographic observation shows the presence of hydrothermal minerals of quartz or silica accompanied by alunite and kaolinite, representing acidic alteration within the crater-conduit. The existence of a silicified zone indicates that the subvolcanic hydrothermal system played an essential role as a cap-rock of pressurized gas and steam at depth (200-500 m), whereas magmatic injection caused the vapour plume expansion. The observation concluded that the proximal volcanic succession captured the evidence of coupled phreatic and phreatomagmatic activities during the latest development of Mount Tangkuban Parahu.

No abstract

No abstract

ABSTRAK Zeolit dijumpai pada batuan piroklastik di daerah Cikembar (Sukabumi) dan Bayah (Banten) dari zona fisiografi pegunungan Bogor dan Bayah. Karakteristik, genesa batuan, dan tipe zeolit di kedua lokasi tersebut belum dijelaskan, terutama kaitannya dengan potensi pengkayaan unsur tanah jarang (UTJ) pada batupasir tufan di daerah Cikembar dan tuf teralterasi di daerah Swakan. Metode yang digunakan pada studi ini terdiri dari pengamatan geologi lapangan, petrografi, difraksi sinar-X (XRD), dan kimia batuan (whole rock) menggunakan XRF dan ICP-MS. Tujuannya adalah untuk mengungkap genesa pembentukkan zeolit dan UTJ di Swakan dan Cikembar. Batupasir tufan dari Cikembar dan tuf dari Swakan menunjukkan kehadiran zeolit bertipe mordenit dan klinoptilolit. Zeolit di daerah Cikembar terdapat sejajar perlapisan, sedangkan di daerah Swakan berasosiasi dengan kumpulan mineral alterasi. Pengendapan zeolit Cikembar dipengaruhi oleh air meteorik yang dicirikan oleh anomali negatif Ce pada pola diagram laba-laba yang di normalisasi terhadap kondrit. Total (∑UTJ) pada batupasir tufan dari Cikembar 82-94 ppm dan pada tuf Swakan 71-83 ppm. Perbedaan kandungan ∑UTJ tersebut lebih mencerminkan komposisi UTJ pada batuan asal. Kata kunci: zeolit, Jawa Barat, mordenit dan klinoptilolit, unsur tanah jarang (UTJ) ABSTRACT -Geology and petrochemistry of zeolite deposits of Bayah and Sukabumi areas. Zeolites are found in pyroclastic rocks in Cikembar (Sukabumi) and Bayah (Banten) from the Bogor and Bayah mountainous physiographic zones. The characteristics, rock genesis, and types of zeolites from these two locations have not been explained, especially in relation to the potential of REEs (Rare Earth Elements) enrichment in Cikembar tuffaceous sandstone and Swakan altered tuff. The method used in this study consists of field geological observations, petrography, Xray diffraction (XRD), and whole rock geochemistry using XRF and ICP-MS. The aim is to uncover the formation of zeolites and REEs in Swakan and Cikembar.The Cikembar tuffaceous sandstones and the Swakan tuffs show the presence of mordenite and clinoptilolite types of zeolites minerals. The zeolites in Cikembar area are parallel to the bedding plane, while in Swakan area are associated with alteration mineral assemblages. The deposition of Cikembar zeolite was influenced by meteoric water which is characterized by the negative Ce anomalies in chondrite-normalized spider diagram pattern. The total (∑REEs) concentration in Cikembar tuffaceous sandstones are 82-94 ppm and Swakan tuffs are 71-83 ppm. The difference in the ∑REE content more likely reflects the parent's rock REEs composition.

No abstract

No abstract