An introduction to the Sangihe arc: Volcanism accompanying arc—Arc collision in the Molucca Sea, Indonesia

Abstract

In the Molucca Sea region of northeastern Indonesia, the Sangihe and Halmahera arcs are presently in the process of colliding, the earth's only example of a collision between facing volcanic arcs. The collision event is more advanced in the northern Molucca Sea where back-arc thrusting occurs along the Cotobato and Philippine trenches and volcanic centers are inactive and dissected. This paper describes the Sangihe arc, the western arc of the collision. The Sangihe are is approximately 500 km in length extending from the NE tip of Sulawesi to Mindanao, Philippines. The arc comprises over 25 Quaternary volcanic centers, the eight presently active volcanoes being confined to the southern half of the arc. The active volcanic front lies 100–110 km above the top of the west-dipping Benioff zone and volcanoes extend up to 70 km behind the front, thereby defining a volcanic arc overlying earthquakes 100 to 180 km deep. Rock types range from basalt to rhyolite, andesites are dominant. Pyroxene-andesites are common on Sulawesi and at or near the volcanic front from Ruang to Buhias-Pahepa and Buang to Mamanuk. Hornblende-andesites are common at the volcanic front between Mahengetang and Sangihe Island and in centers located well behind the volcanic front. Tholeiitic (TH) suites, defined on the basis of relative degrees of iron enrichment, are confined to the southern volcanic front, lack hornblende, are low-K (K55 < 1.05), have low Rb/Zr ratios (0.10–0.17), low Ba/Y ratios (3.4–4.9), low Rb/Cs ratios (8–14) and have the highest 87Sr/86Sr ratios (0.7039–0.7042). Calcalkaline (CA) suites occur throughout the arc, are low- to high-K (K55 ≅ 0.6–2.0), have variable Rb/Zr ratios (0.15–0.65) and Ba/Y ratios (4.2–12.4), high Rb/Cs ratios (14–82) and low 87Sr/86Sr ratios (0.7035–0.7040). Zr and Y distribution coefficients are lower in TH suites (DZr = 0.0, DY = 0.2–0.5) than in CA suites (DZr = 0.0–0.8, DY = 0.3–1.2). Ti initially increases with increasing SiO2 in some TH suites while decreasing with SiO2 in all CA suites. With the exception of Bukide (Ti/Zr = 138), Ti/Zr ratios of basalts exceed 100 only in TH suites (76–130). LIL-element contents and LIL/HFS-element ratios increase 2 to 3 times across the are away from the volcanic front and 2 to 3 times from south to north along the arc. 87Sr/86Sr ratios decrease away from the volcanic front. The south to north increase in LIL-element contents and LIL/HFS-element ratios, without corresponding changes in Sr-isotopic compositions are interpreted as resulting from decreasing degrees of partial melting northward. This reflects the gradual cessation of volcanism in the north due to collision.