Volcanic Stratigraphy and Geochemistry of the Idjen Caldera Complex, East-Java, Indonesia.

Abstract

The Idjen caldera lies in the eastern spur of Java island, part of the Sunda arc, and is situated between 7° 55' 36" - 8° 11' 24" S latitude and 114° 04' 18" - 114° 18' 44" E longitude. The Idjen caldera is classed as a Krakatoa type or stratocone caldera and is characterized by a caldera floor, a caldera escarpment, and older volcanic forms. The caldera diameter ranges from 14 by 16 km at the rim to 11 by 14 km at the floor with a maximum depth of 709 m. Based on the episode of cone collapse, the stratigraphic succession of the study area is divided into pre-caldera, caldera and post-caldera phases. The pre-caldera group (c. 0.3 Ma) is a remnant of a composite cone and is recognized in 500 m of section exposed in the caldera wall. It consists of pyroclastic flow deposits, a series of pyroclastic pumice air fall deposits and at least three sequences of lava flows. The caldera group is mainly composed of large volumes (c. 80 km3) of pyroclastic flow deposits with minor layers of lahar deposits and some layers of air fall deposits. These units are closely related to the formation of the Idjen caldera some time before 50,000 years ago. The Blawan lake sediments within the caldera are evidence of a post-caldera lake. The post-caldera group, which is characterized by many types of eruptions from at least twenty-two separate vents. The last major eruption was of phreatic airfall and pyroclastic flows at 2,590 years ago, and the historic eruptions were reported as phreatic eruptions in 1796, 1817, 1917, 1936 and 1952. All of the study area is underlain by Miocene limestone. Whole rock chemistry indicates that the Idjen volcanic rocks range from 45 to 64 % SiO2 and are mostly basalt and andesite with minor dacite. K2O contents indicate that they are medium to high-K types. Nearly all lavas are porphyritic with either a holocrystalline or hypocrystalline groundmass. The principal phenocryst phases are olivine, plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides, but olivine is absent in the pre-caldera lavas. Biotite is a subordinate phase and is only found in the acid andesite sample from Mt. Glaman lava dome. In hypocrystalline rocks, the phenocrysts are mostly set in a fine-grained groundmass of plagioclase microlites, clinopyroxene, Fe-Ti oxides with or without olivine. Accessory apatite is found as inclusions in plagioclase and clinopyroxene phenocrysts. Plagioclase phenocrysts range in composition from andesine to anorthite and each type of rock shows a similar range of composition except for acid andesites from the caldera group which are more sodic (An34 - An40). Ca-rich pyroxene is diopside and augite. Augite is common in most rock samples. Pigeonite is only found in a single basalt sample from the pre-caldera lavas. Orthopyroxene ranges from Wo3.4, En57.7, Fs37.4 to Wo3.7, En68.5, Fs30.4 ) and is antipathetic with olivine (F040.6 to F075.3). The in-coming of orthopyroxene instead of olivine may be due to increasing silica activity. Titanomagnetite is the only Fe-Ti oxide in the Idjen volcanic rocks. The crystallization sequence of the pre-caldera lava is characterized by mineral assemblages of olivine with apatite - plagioclase - orthopyroxene - clinopyroxene and Fe-Ti oxides or plagioclase - orthopyroxene - 1st clinopyroxene - Fe-Ti oxides 2nd clinopyroxene. The post-caldera lavas display phenocryst assemblages of plagioclase - orthopyroxene - clinopyroxene - Fe-Ti oxides - biotite or olivine - orthopyroxene - clinopyroxene - Fe-Ti oxides. A few samples contain normative corundum, probably owing to alteration. Basic andesite, acid andesite and dacite are oversaturated and quartz normative, whereas basalts are olivine-hypersthene normative with maximum Mg-number (50) and Ni content (41 ppm). The low Mg-number and Ni content suggest that these basalts are not primary magmas, but that they have experienced secondary processes such as magma mixing, fractional crystallization and crystal accumulation. Some major and trace elements of the post-caldera volcanic rocks show inconsistent trends with time and this irregular variation may reflect a complexity of secondary processes in the magmas. They may relate to a combination of magma mixing or influx of new basaltic magma into a shallow reservoir, and then crystallization into more evolved compositions. Depletion in high field strength elements (HFS-elements) relative to MORB can be attributed to depletion in the mantle wedge. Enrichment of LIL-elements are related to migration of fluid or metasomatic processes of enrichment in the mantle wedge.