Petrology and Geochemistry of the Lamongan Volcanic Field, East Java, Indonesia: Primitive Sunda Arc Magmas in an Extensional Tectonic Setting?


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Abstract

New geochemical data are presented from prehistoric and historical eruptive products of the Lamongan volcanic field (LVF), East Java; a region of the Sunda arc covering ∼260 km2 and containing ∼90 eruptive vents plus the historically active Lamongan volcano. LVF lavas include medium-K basalts and basaltic andesites from historical eruptions of Lamongan and prehistoric eruptions in the eastern LVF, along with a high-K suite represented by prehistoric deposits in the western LVF. Although lacking some of the characteristics of truly primary basalts, the least evolved lavas identified in the LVF have some of the lowest SiO2 contents (∼43 wt % SiO2) yet reported in Sunda arc volcanic rocks. Mass balance considerations indicate that two chemically distinct LVF magmas may be parental to suites currently being erupted from the neighbouring volcanoes, Semeru and Bromo. Lamongan’s historical lavas can be related to the medium-K andesitic products of Semeru by fractional crystallization, despite the former’s location at the same distance from the trench as Bromo, a high-K volcano. Extensional tectonics, possibly related to arc segmentation in the region of the LVF, creating conditions that promote the rapid ascent of parental magmas, is probably responsible for this and several other features of the complex.

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