Spatial patterns in the mineralogy of island arc magma series: Sangihe arc, Indonesia

Abstract

The 500-km-long Sangihe arc is the western of two colliding arcs in northeastern Indonesia. In its basalts, olivine ± magnetite crystallization is followed either by plagioclase (the plagioclase basalt series: PBS) or augite (the augite basalt series: ABS). PBS occur at or near the southern volcanic front and evolve to two-pyroxene andesites; ABS occur behind the volcanic front and to the north where collision is more complete, evolve to hornblende andesites, and have higher Al iv in calcic clinopyroxene. Both tholeiitic (TH) and calcalkaline (CA) Fe-enrichment trends occur in PBS whereas ABS are only CA. TH suites occur only at the southern volcanic front, have low LILE concentrations (K 55 < 1) and low CaO/Al 2O 3 (<0.6). CA suites occur more widely, have variable LILE (K 55 = 0.6-2.0), more modal magnetite in andesites, higher modal clinopyroxene/plagioclase ratios, and CaO/Al 2O 3 = 0.6–0.9 in basalts. Quench temperatures are 1110-880°C in basalt to rhyolite, temperatures are higher in andesites from the volcanic front than behind it. Oxygen fugacity is from QFM to above NNO. Silica activity decreases and LILE concentrations increase away from the volcanic front. Consequently, PH 2O is lowest, whereas andesite temperatures plus the percentage of mantle fusion are highest at the southern volcanic front where there is the least coupling between plates.