| Summary: | 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 fault system. 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
|
|---|
