The geochemistry of the neogene Halmahera arc, Eastern Indonesia
The Halmahera arc is a north-south linear intraoceanic arc cutting across the islands of Halmahera and Bacan in NE Indonesia. The arc is the result of the eastward subduction of the Molucca Sea Plate, accommodating the westward movement of the Pacific and Philippine Sea Plates (PSP) against the Eurasian
margin. To the south of the Halmahera arc is a major left-lateral strike-slip fault system: the Sorong Fault Zone
(SFZ), which separates the northward movement of Australia from the westward movement of the PSP. This plate boundary has been stable throughout the Neogene to the present day, and has been responsible for the transfer of continental fragments from the Australian margin into the southern Molucca Sea region.
K/Ar dating has revealed the migration of volcanism along the length of the Halmahera arc from south to north. The oldest volcanics (ca. 11 Ma) are from Obi, the southernmost island in the region, where volcanism is now extinct. Moving north into Bacan, ages range from 7 Ma to the Quaternary, whereas in central Halmahera they range from 6-2 Ma. The present-day arc currently lies to the west of central Halmahera and stretches up
the north-west arm of the island. On the basis of spatial, temporal and geochemical variations a possible seven Neogene volcanic centres can be distinguished along the length of the arc. Major element, trace element and Sr-Nd-Pb-O isotopic analyses reveal a wide diversity in geochemical characteristics between the centres. This is due to heterogeneity within
the arc mantle wedge, the type of arc crust through which the volcanics were erupted and variations in contribution to the mantle wedge from a subducted component. Volcanic rocks from Obi, central Halmahera and north Bacan display geochemical characteristics typical of intraoceanic arc lavas. The lack of a continental component within these centres enables a greater
understanding of the variety of processes and source components affecting arc magmatism in this region. Similarities in certain incompatible trace element characteristics between volcanic rocks of the Mariana and Halmahera arc suggest both arcs are products of a variably depleted mantle beneath the Philippine Sea Plate (PSP). Pb isotopic data from the Halmahera arc, combined with data from back-arc basaltic rocks from the PSP,
suggests an I-MORB-type mantle wedge exists beneath this plate and hence that it was once part of the Indo- Australian plate.
Volcanic rocks from west and south Bacan lie outside the isotopic ranges displayed by lavas from Obi, north Bacan and Halmahera, reaching extreme Sr-Nd-Pb isotopic ratios consistent with the assimilation of a continental component. Isotopic analyses of Permo-Triassic granitic material, found exposed in the Sula- Banggai islands, New Guinea margin, and Queensland, NE Australia, indicate that this is the most likely contaminant of volcanic rocks in the south Bacan region. In contrast, volcanic rocks from west Bacan are contaminated with a component similar in isotopic composition to highly metamorphosed rocks found exposed in the Sibela Mountains, south Bacan. The geochemical signature and age of the Halmahera arc lavas has implications for the arrival and movement of continental crust in the region. Combined with stratigraphic and tectonic knowledge of the region this study has been used to construct a possible model for the development of the Halmahera arc. The contaminated signature of the Bacan Neogene volcanic rocks supports the hypothesis of overthrusting of
ophiolitic and continental material, derived from the PSP and Australian plates respectively, due to collision between the Australian continent and a PSP arc during the Early Miocene. This initiated the development of the Sorong Fault Zone, which was responsible for the recent movement of these 'terranes' into the southern Molucca Sea region.