Petrology and Tectonics of the Derewo Metamorphic Belt, West New Guinea

Abstract

The Derewo Metamorphic Belt (also known as the Ruffaer Metamorphic Belt) is a >500 km long, narrow (~10-30 km wide), roughly E-W-trending terrane composed mostly of slate and phyllite, on the northern flank of the Central Range of Papua, Indonesia (west New Guinea). This investigation, the first systematic petrologic study of the belt, is based on petrographic and mineral chemical analysis of 133 samples collected across the 1:250,000 scale Hitalipa Geologic Map Sheet. The southern edge of the Derewo Metamorphics (DM) is a structural boundary, the Derewo fault, in the western half of the map and gradational with unmetamorphosed passive margin strata in the eastern half. The northern boundary of the DM is a fault contact with the Irian Ophiolite Belt. The metamorphic protoliths are Jurassic-Cretaceous Australian passive-margin strata (Kembelangan Group). Most of the rock in the DM is of pelitic bulk composition. The common mineral assemblage is quartz + white mica + chlorite + pyrite + graphite, with many samples containing albite, mixed layer muscovite/paragonite, and/or calcite. Spessartine garnet and iron-rich chloritoid are present but rare. Volumetrically minor protoliths were siltstones or sandstones and rare probable mafic ash beds. Metamorphosed silt- and sandstones contain quartz + albite + potassium feldspar + white mica ± biotite ± calcite. Mafic compositions contain actinolite + epidote + chlorite + albite + white mica + calcite + quartz + titanite. Thermobarometry based on mineral assemblages and compositions indicates the DM in the Hitalipa area experienced peak metamorphic conditions of 250-350°C at 5-8 kbar (burial depths of 15 to 25 km). The DM formed as continental rise and slope sediments that entered a north-dipping subduction zone since 30 Ma. Major uplift from collisional orogenesis began at ~8 Ma when positively buoyant Australian continent-capped lithosphere began to bend downward to subduct. The present-day high topography of the Central Range was established by ~4 Ma when delamination of the subducting plate was complete, and collisional movements changed into a left-lateral transform fault system. Tens of km of strike-slip displacement occurred within the core of the Central Range, offsetting parts of the metamorphic belt along the Derewo and related faults.