P-T Evolution of Eclogites and Blueschists from the Luk Ulo Complex of Central Java, Indonesia

Abstract

Rocks of the Lok Ulo Complex crop out over a small area in the vicinity of Karangsambung in central Java. They are part of a belt of Cretaceous accretionary-collision complexes that appear sporadically in an arc extending from Java to Kalimantan and Sulawesi. The complex consists of dismembered ophiolites, sedimentary rocks, and crystalline schists and gneisses occurring as tectonic slabs in a black-shale matrix tectonic mélange. High-pressure rocks such as eclogite, glaucophane rock and blueschist crop out in a thin zone between the low-grade schists and a serpentinite zone along the Muncar and Gua rivers. Some of the eclogite blocks contain tourmaline, which is restricted to the outer shells of—and veins in—such blocks. The early metamorphic stage (stage I) of the Lok Ulo eclogites comprises garnet (core) and omphacite + Ca-Na amphibole + phengite + rutile + epidote inclusions in the garnet core. Stage II is characterized by garnet (rims of porphyroblasts) + omphacite + rutile + phengite + Ca-Na amphibole. The matrix constituents, which are similar to those of stages I and II, are related to stage III (late or "peak" eclogitic stage). The blueschist overprint of the eclogites occurred during stage IV. The corresponding assemblage is Na amphibole + chlorite + albite + epidote + quartz + titanite + ilmenite. Subsequently, poikiloblastic tourmaline and apatite grew at the expense of chlorite, epidote, and other minerals in some eclogites (stage V). The P-T path of tourmaline-bearing eclogites is characterized by rising pressures at decreasing temperatures (stage I to stage III: P = 22.5 kbar and T = 365°C), whereas the normal eclogites show rising temperatures at increasing pressure (stage III: P = 20.5 kbar and T = 410°C). Thus, these eclogites were subducted to ~70 km depth at a geothermal gradient of ~6 C°/km. Stage IV is limited to the P-T range of 8-10 kbar and 350-400°C for both eclogite types. The different P-T paths (counterclockwise and clockwise) are explained by metamorphism within a subduction channel. The low geothermal gradient is probably due to a high rate of subduction of a cold oceanic plate.