Isotopic dating and island arc development in the Halmahera region, Eastern Indonesia.
The islands of Halmahera, Morotai, Bacan, Obi and Waigeo lie in a zone of
complex tectonics at the junction between the Eurasian margin and the Philippine Sea and
Australian plates. New age data from the region using Sm-Nd, Rb-Sr and K-Ar systems and
geochemical data are presented and are integrated with existing geological, geochemical and
isotopic data to produce a regional geochronological framework.
Continental metamorphic rocks of probable Palaeozoic age, derived from New
Guinea, are found on Bacan and Obi. Sm-Nd model ages indicate that metamorphic
protoliths contained Precambrian cratonic material which was subsequently mixed with
younger mantle-derived material. Rb-Sr and K-Ar systematics give Neogene ages which
relate to exhumation and emplacement of these rocks by strike-slip processes in the Sorong
Ophiolitic rocks from Halmahera, Obi and Gag are of Philippine Sea plate origin
and are interpreted to have formed an intra-oceanic forearc-arc-backarc system of Jurassic
age based on Sm-Nd, K-Ar dating and geochemical and stratigraphic evidence. Subsequent
Cretaceous to Tertiary arc activity has largely disturbed K-Ar ages of ophiolitic rocks.
Cretaceous calc-alkaline arc activity formed the Gowonli and related volcaniclastic
formations on Obi and Waigeo. Unusual amphibole-rich cumulates, hornblende gabbros and
ultramafic rocks occur adjacent to continental metamorphic rocks. These yield Cretaceous
and younger isotopic ages and are interpreted as the roots of a calc-alkaline island arc
subsequently disrupted and uplifted by faulting in the Early Cretaceous and Neogene.
Diorites and trondjhemites intruded into ophiolitic rocks on Halmahera and Obi date
two phases of arc-related plutonic activity in the Middle to Late Cretaceous. One Paleocene
diorite has also been dated. Isotopic ages from amphibolites, derived from basic protoliths,
and biostratigraphic ages from sedimentary fault-related breccia deposits indicate
deformation of ophiolitic rocks in the Late Cretaceous. Ar-Ar plateau ages and reset K-Ar
ages suggest another period of deformation in the Paleocene.
Isotopic dating shows that Tertiary arc activity occurred in three phases: a brief,
Middle Eocene phase of diorite and andesite formation in eastern Halmahera; a dominantly
Oligocene period of arc activity related to subduction beneath the Philippine Sea plate and a
Neogene phase related to subduction of the Molucca Sea plate. Oligocene arc activity was
terminated by Early Miocene collision of the Philippine Sea plate with the northern
Australian margin. Convergence of the Philippine Sea plate with the Eurasian margin led to
Neogene arc activity above an eastward-dipping Molucca Sea plate slab. Isotopic dating
indicates that Neogene arc volcanism migrated northwards over time. Pliocene
compressional deformation in Halmahera and Bacan caused a westward shift of the arc to its
present position and may be related to collision events within the Sorong Fault system