Correlation of Indonesian active volcanic geochemistry with Benioff Zone Depth

Abstract

The active volcanic arc of Indonesia extends almost continuously over 6000 km from north Sumatra to the Molucca Sea. Chemical analyses of 196 rocks from 33 volcanoes have been processed to give molecular norms and multiple linear correlation analyses between various chemical parameters and silicon content or differentiation index (D.I.) and vertical depth of the underlying Benioff Zone, which ranges from 140 to 300 km. The great majority of the volcanic products is of augite-hypersthene andesite or basalt, sometimes with olivine and hornblende, and occasionally with biotite, quartz and tridymite. Leucite occurs in volcanoes overlying the deepest seismic contours. Most rocks are quartz-normative, but many are olivine- and even nepheline-normative where the volcanoes overlie the greatest seismic depths. Significant relationships exist between each of potassium %, alkalis % and the ratio of potassium to alkalis and both silicon content (or D.I.) and depth to the underlying Benioff Zone. The relationship with silicon or D.I. is petrologically controlled and the relationship with Benioff Zone depth may suggest that the magmas are produced at and rise from the Benioff Zone and that some of the chemical variation is controlled by the depth of magma production. However the best correlation obtained cannot explain more than 50 to 60% of the chemical variation in terms of depth of magma production. Future refinement of the measurement of the seismic contours and improvement of the chemical analyses may lead to a closer correlation, but on the basis of the present data it is necessary to conclude that, in addition to seismic depth, other unknown factors play an important role in controlling the chemical variation of the volcanic rocks.