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.
Sunda Arc is located in the Southeast Asia as a result of the Indo-Australian Plate subducted beneath Sundaland. Java Island belongs to the eastern part of Sunda Arc, the arc magmatism started since Eocene to Recent on this island. Java Island can be divided into three parts: West Java, Central Java, and East Java based on the tectonic setting. Most of the previous studies focused on the well-known volcanoes, and there were few geochemical or dating data from the others not famous volcanoes. In this study, we focus on the western part of West Java. Fifty-eight volcanic rocks from five volcanoes (i.e., Danau Volcanic Complex (DVC), North Bayah Dome (NBD), Gede Volcanic Complex (GVC), South Bayah Dome (SBD), and Ciemas) were collected. Major and trace elements, and whole rock Sr-Nd isotopes were also analyzed. Combing the zircon U-Pb ages yield by our group before, we discuss the temporal and spatial geochemical variations in this area. Magmatism shows northward from SBD to DVC during Early-Miocene (16.7 Ma) to Pleistocene (2.4 Ma), however, they show widely erupted between the southern (Ciemas) to the central part of this area (NBD) at 0.7 Ma. All volcanic rocks show island arc signatures (depletion in high-field-strength elements and enrichment in large-ion lithophile elements) with slightly enrichment of light rare earth elements. Whole rock Sr-Nd isotopes results show a similar magma source during 16.7 to 2.4 Ma (εNd value = +3.6 to +1.7), and enriched progressively to GVC (εNd value = +0.8 to +0.9, without exactly ages), and to magmatism at 0.7 Ma (εNd value = -2.0 to -0.4). We suggest that a homogeneous magma was formed after source contamination before 16.7 Ma, and then erupted and migrated from south to north during 16.7 to 2.4 Ma by changing the angle of subduction. Much more crustal material contaminated to magma source after 2.4 Ma and magmatism migrated back to southern part of this area because of slab rollback and/or broken.
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