Graphite Flakes from the Luk Ulo Complex, Central Java, Indonesia: A Geothermobarometer Modelling and Host Rock Characteristics

The graphite identified in the metapelites of the Luk Ulo Complex, Central Java, occurs as flakes spanning a broad size range, from fine particles of about 10 µm to large crystals approaching 4500 µm. The flakes coexist with quartz, chlorite, and white mica/phengite, forming well-developed schistosity in the graphite-phyllite rocks. These conditions represent that the graphite flakes are formed under metamorphic conditions. The metamorphic conditions of the graphite-bearing metapelites were evaluated using three analytical techniques: mineral-chemistry thermobarometry involving chlorite and phengite, pseudosection modelling, and Raman spectroscopic determination of CM temperatures. The results indicate that the metamorphic conditions of the graphite-bearing rocks from the Luk Ulo Complex range between 4 to 6 kbar in pressure and from 340 to 380°C in temperature that fall within the Greenschist facies. The geochemical analysis reveals the dominance of SiO2, Al2O3, and Fe2O3, with a rare earth element pattern suggesting a carbon-rich shale or greywacke protolith derived from a Continental Island Arc and Oceanic Island Arc setting. The regional metamorphic event responsible for the formation of graphite flakes is considered to have resulted from a subduction and collision event between Sundaland and various continental fragments derived from Gondwana in central Indonesia during the Cretaceous period. Other regions with similar geological settings in the Cretaceous accretionary-collision complexes in central Indonesia may be potential sources for similar graphite flakes and associated host rocks.