Some Studies of Volcanology, Petrology and Structure of Mt. Kelut, East Java, Indonesia

Abstract

Mt. Kelut is an active andesite volcano in East Java, Indonesia, part of the Sunda arc, lying within a volcanic belt that is about 150 km above the north-dipping Benioff Zone, where the Indian Ocean plate is being subducted beneath Java. Geologic mapping and stratigraphic studies have led to the recognition that Mt. Kelut volcanic rocks consist of lava flows, lava domes, pyroclastic flows, pyroclastic airfall, and eruption lahar deposits, mostly basaltic andesite in composition, erupted from ten craters located at the summit area. The eruption centre has moved clockwise starting from Lirang crater (238 ka) to Kelut crater (the youngest, perhaps 4 ka). A parasitic cone, Kramasan dome, at the east flank (99 ka) was probably formed after or at the same time as Kombang dome on the Gajahmungkur crater. Another parasitic cone, Pisang dome, on the south flank was probably formed before or during the life of Tumpak crater. The ten craters are located within a 2.5 km wide graben (at least 10 km long) striking southwest-northeast on the west flank of Mt. Kelut. At Kelut, the craters and the graben are controlled by extension which is oriented N 165°E. Crater formation and graben development has controlled other small faults. Based on geological structure, stratigraphy and chemical composition, Kelut volcanic rocks are divided into three periods : Kelut 1 older than 100 ka erupted from Lirang and Gajahmungkur craters ; Kelut 2 (100 to 40 ka) erupted from Tumpak, Sumbing I, and Sumbing II craters ; Kelut 3 (< 40 ka) erupted from Dargo, Gupit, Badak I, Badak II and Kelut craters. Eruption products from the youngest crater (Kelut crater), which are well recorded at a stratigraphic section at Margomulyo, are mostly pyroclastic rocks consisting of 23 layers of pumice lapilli, one layer of pyroclastic flow and 18 layers of ashfall deposits within at least 24 eruption sequences. Based on SEM and grain size analyses, three of the 18 ashfall layers are phreatic deposits, and one of the 18 layers is a phreatomagmatic eruption deposit. The phreatic deposits were produced by Surtseyan and Vulcanian ashfall eruptions with typical column heights of 0.3 - 1.5 km, and the phreato-magmatic deposit was by Surtseyan eruption with typical height of 0.5 - 3 km. One of the strongest eruptions from this crater was the 1951 eruption, with a calculated volume of erupted magma of 87 million m3 and total released energy of 9.3 x 10 23 ergs. The seismicity at Kelut in 1985-1986 as recorded by a tripartite array consists of 3182 earthquakes which are classified into 4 types of volcanic (326 events) and one type of tectonic. Hypocentre calculations can have large errors due to uncertainty in the velocity structure beneath Mt. Kelut and the small size of the tripartite array. Volcanic earthquakes cluster around the west area of the summit 1 or 2 km from the crater at depths less than 5km. Based on the b-value for the volcanic earthquakes (2.16±0.18) they occurred in a non-uniform, thermal stress field which would be consistent with their occurence at the top of a magma body beneath Mt. Kelut. Kelut volcanic rocks are calc-alkaline composed of medium-K basalt to medium-K andesite. In order of time of eruption, Kelut 1 rocks are composed of basalt to andesite, Kelut 2 rocks are basaltic andesite, and Kelut 3 rocks are basalt to basaltic andesite. Most of the rocks are porphyritic with 43 to 68% phenocrysts in microcrystalline or glassy groundmass. The principal phenocryst phases are plagioclase, clinopyroxene, orthopyroxene, titanomagnetite, amphibole and olivine. Petrography, mineral chemistry and bulk rock chemistry indicate dominance of fractional crystallization. Early fractionating minerals are orthopyroxene (or olivine), clinopyroxene and plagioclase followed by plagioclase, clinopyroxene, orthopyroxene, titanomagnetite and amphibole. However, the chemical variations seen are related to the effect of a combination of processes that include the periodic influx of new basaltic magma into shallow magma reservoirs causing magma mixing to disturb a normal pattern of crystal fractionation. The basaltic andesite as a mix between basalt and andesite can be modelled as the middle layer in a shallow zoned magma body. The Kelut magmas are not primary as they have low Mg number (44-54), low Ni (3 - 27 ppm) and low Cr (9 to 33 ppm) compared to primitive magmas from other volcanoes. Several of the more primitive samples from Kelut are characterized by enrichment of LILE and depletion in HFS elements which is similar to the patterns of many arc derived volcanics. The enrichment of LILE can be attributed to metasomatism of the mantle source by fluids released from the subduction slab. The depletion of HFS elements is attributed to higher degrees of partial melting or extreme depletion of the arc source due to previous melt extraction.