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

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 (