Peak Metamorphic Conditions of Garnet Amphibolite from Luk Ulo Complex, Central Java, Indonesia: Implications for Medium-Pressure/High-Temperature Metamorphism in the Central Indonesian Accretionary Collision Complex

Abstract

DOI:10.17014/ijog.7.3.225-239Garnet amphibolites, which suggest medium-pressure/high-temperature (MP/HT), are widely recognized in Luk Ulo Complex, Central Java. Their occurrences associated with high-pressure/low-temperature (HP/LT; eclogite, blueschist) metamorphic rocks in the Luk Ulo Complex will provide important constraint on the geodynamic model of Central Indonesian Accretionary Collision Complex (CIACC). This study aimed to estimates P-T metamorphic condition of garnet amphibolite from Luk Ulo Complex by using mineral parageneses, thermodynamic data, and NCKFMASHO pseudosection. Prograde stage assemblages represented by inclusions in the garnet, which are garnet core, zoisite, titanite, apatite, and quartz. Mineral coexistences at the peak P-T condition are garnet rim, magnesio-hornblende, zoisite, titanite, quartz, albite, and phengite. The retrograde stage represented by secondary minerals fill the crack in the garnet and other minerals, which are chlorite and quartz. P-T metamorphic condition of garnet amphibolite can only be interpreted from the peak metamorphic stage. The temperature of the garnet amphibolite is estimated using the garnet-amphibole and garnet-phengite geothermometers. Meanwhile, the pressure condition is estimated from phengite geobarometer. The results were compared to the stability and compositions of the phases in NCKFMASHO pseudosection in order to constrain the peak P-T metamorphic conditions. It is concluded that the peak P-T metamorphic condition for garnet amphibolite is 0.9 - 1.4 GPa and 558 - 606 ºC. The estimated peak P-T metamorphic temperature is higher compared to the previously published gradient geothermal of eclogite and tourmaline-eclogite in the Luk Ulo Complex. The MP/HT (amphibolite) and HP/LT (blueschist and eclogite) metamorphic rocks could have similar metamorphic ages if both footwall and hanging wall had initially very high thermal gradients and the rate of subduction was very slow (10 km/Ma or less).

U–Pb zircon geochronology and geochemistry of Luk Ulo River boulders belong to Late Cretaceous-Paleocene metapsammite and metagranite of Luk Ulo Karangsambung, Central Java, Indonesia

“…stated that despite the long subduction history, the Indonesia region had grown mainly from additional continental fragments (estimated to account for approximately 90% of the total area). Several publications were carried out with the idea of a microcontinent that stopped the Cretaceous subduction zone in Central Indonesia, which exposed the high-pressure zone (Clements and Hall 2011; Satyana 2014; Setiawan et al 2020).…”

“…In addition, previous research conducted by Miyazaki (Miyazaki et al 1998) demonstrated that the ages of the metamorphic rocks found in the Karangsambung region correspond to the Early Cretaceous when dated using the isotope K-Ar. Setiawan et al (Setiawan et al 2020) have studied the metamorphic conditions of amphibolite as medium-pressure and high-temperature (MP/HT) and blueschists/eclogites (HP/LT) around Luk Ulo Complex, revealing that similar metamorphic ages with very high thermal gradient and very slow subduction rates. In addition, Al ng et al ( 2021) have used K-Ar and Rb-Sr dating methods in the investigation of the geochronology of metamorphic rocks that are located between the Meratus and Luk Ulo complexes.…”

“…A similar study performed by Mawleda et al (Mawaleda et al 2020) observed that the high or ultrahigh (Eclogite and Blue Schist) metamorphic rocks in the Bantimala Mélange Complex demonstrated the end of the Cretaceous tectonic subduction in the south Arm of Sulawesi. In addition, Setiawan et al (Setiawan et al 2015(Setiawan et al , 2020) evaluated the metamorphic evolution of metamorphism from the Meratus Complex in South Kalimantan, Indonesia, and indicated that the Cretaceous subduction environment was responsible for this metamorphic condition. As a result, many researchers have studied that the Cretaceous subduction exposed the metamorphic rocks throughout various regions of Indonesia (Alam 2020; Al ng et al 2021; Şahin et al 2014).…”

“…Along the Java Trench, this fragment was there before the subduction process started in earnest in its present location. According toSmyth et al (2007), it is possible that the fragment crashed against the edge of Sundaland, bringing an end to the Late Cretaceous phase of the subduction process.Several studies have been published dataset of zircon ages from Western Australia to distinguish it from that obtained from East Java(Setiawan et al 2020;Sircombe and Freeman 1999;Soesilo et al 2015;Townships et al 2017). Their ndings concluded that the zircon ages from Western Australia are highly similar that they identi ed in the Southern Mountains Arc of East Java.…”

Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia

“…The NE–SW trending Luk Ulo‐Meratus metamorphic belt was formed by NW‐directed subduction of oceanic crust below the Sundaland continent followed by East Java micro‐continent collision at the last stage of orogenesis (Hall, 2014; Parkinson et al, 1998). The LUMC consists of a variety of rocks, including (1) mafic‐ultramafic rocks, (2) a mélange unit composed of sandstone, chert, andesite, basalt, limestone, minor amounts of mica schist, and dacitic tuff within scaly clay matrix, and (3) low to high pressure/temperature metamorphic rocks (Kadarusman et al, 2007; Setiawan et al, 2020). The low‐grade metapelites such as carbonaceous phyllite and garnet‐muscovite schist are the predominant metamorphic rocks in the LUMC as typical of accretionary complex.…”