Regional high-T/low-P metamorphism of the Kemum Basement High, Bird’s Head, West Papua, Indonesia
Journal of Metamorphic Geology
The northernmost part of the Australian continent is exposed in a basement high in the Bird's Head Peninsula at the NW tip of New Guinea. This basement consists of Silurian–Devonian turbidites that were regionally metamorphosed in the lower greenschist facies. Along its eastern margin, the basement high is intruded by numerous granitoid bodies and records a second, higher-grade metamorphic overprint. We present the first detailed study of metamorphic rocks from this eastern part of the basement high, supported by field and petrographic observations, bulk rock and mineral chemistry, and phase equilibria modelling. Lower-grade basement rocks include chlorite-mica schists, slates, silt-, and sandstones, which give way to andalusite- and cordierite-bearing pelites and psammites, interspersed with layers of quartzite. The dominating pelitic phase assemblage is biotite+white mica±andalusite±cordierite+plagioclase+quartz+ilmenite; incipient sillimanite (fibrolite) is present in some rocks and replaces andalusite in baked margins around the numerous dykes and sills. Other rock types include metasedimentary migmatites, metacarbonates and calcsilicate concretions, rare metavolcanics, and kyanite-bearing segregation veins. Metamorphism is of regional extent and often unrelated to granitoid intrusions. Our observations support the previous notion of an increase in peak metamorphic temperature from west to east. Microstructural relations suggest two phases of metamorphism, which are tentatively correlated with two phases of magmatic activity in the Devonian–Carboniferous and the Permian–Triassic. Based on petrographic observations and phase equilibria modelling, we propose that during the Permian–Triassic, metamorphism proceeded under high temperature/low pressure (HT/LP) or Abukuma-type conditions along a narrow, counter-clockwise P–T–t path with peak metamorphic conditions of the amphibolite facies (~520–650°C and ~2–4 kbar), potentially in an active continental margin setting. While much less evidence is available, similar metamorphic conditions are proposed for the Devonian–Carboniferous metamorphic event.
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