Aya Yoshimoto

Paleozoic Subduction-Accretion-Closure Histories in the West Mongolian Segment of the Paleo-Asian Ocean: Evidence from Pressure-Temperature-Time-Protolith Evolution of High-Mg and -Al Gneisses in the Altai Mountains

High-Mg, high-Al metasedimentary gneisses from the Altai Mountains, Mongolia, belonging to a subduction-accretion complex within the Central Asian Orogenic Belt can be divided into five rock types on the basis of mineral assemblages. Most rock types have high MgO and Al2O3 content and low CaO, Na2O, Rb, and Sr content. All rock types experienced a similar medium-pressure metamorphism characterized by a “hairpin”-shaped counterclockwise pressure-temperature path. U-Pb zircon and U-Th-Pb monazite ages indicated metamorphism at ca. 356 Ma and 277 Ma and inherited ages of 510–379 Ma, suggesting possible provenance to granitoids comparable to those in the Altai Mountains, China. The zircons that newly nucleated at ca. 356 Ma are characterized by high concentrations of light rare earth elements without a Ce anomaly—features common in zircons from hydrothermally altered rocks and a reducing environment. Petrological and geochronological results in this study suggest the following tectonic evolution: (1) continuous subduction and accretion of paleo-Asian oceanic crust during the Early Paleozoic, resulting in periodic granitoid magmatism in the period 510–380 Ma and a continuous supply of granite-derived sediments providing detrital zircon and monazite grains to the accretionary prism; (2) ridge subduction during the Late Devonian–Early Carboniferous (ca. 356 Ma), resulting in hydrothermal metamorphism of the accretionary prism and interaction with seawater that produced rocks with unusual whole-rock chemistry; and (3) closure of the ocean leading to continental collision in the Early Permian (ca. 277 Ma), with part of the accretionary prism squeezed into lower crustal levels to form medium-pressure metamorphic rocks.

An overview of metamorphic geology from central Indonesia : Importance of South Sulawesi, Central Java and South-West Kalimantan metamorphic terranes

Various metamorphic rocks expose in the central part of Indonesia including Java, Kalimantan and Sulawesi islands. This paper explains results of the compilation of all relevant data regarding to the study of petrological, geochemical and geochronological investigations of metamorphic rocks from Bantimala and Barru Complexes in South Sulawesi, Luk Ulo Complex in Central Java, Meratus Complex in South Kalimantan, and Nangapinoh area of Schwaner Mountains in West Kalimantan. High-pressure and very high-pressure metamorphic rocks crop out in the Bantimala Complex and experienced peak metamorphic condition to eclogite-facies. High-pressure metamorphic rocks also expose in the Luk Ulo Complex in Central Java with peak metamorphic condition in the eclogite-facies. From the South Kalimantan, the P-T condition of metamorphic rocks were estimated from Mg-rich chloritoid that recrystallized at pressure of ~1 .8 GPa. The K-Ar dating of metamorphic rocks from South Sulawesi, Central Java and South Kalimantan were on the same range of Early Cretaceous that have a possibility of separated parts derived from a single subduction complex. Metamorphic rocks exposed in Schwaner Mountains from West Kalimantan are metatonalite and local contact metamorphism with the variety of hornfelsic rocks. No reliable P-T condition has been estimated from these metamorphic rocks. The LA-ICP-MS U-Pb zircon dating of metatonalite from Schwaner Mountains yields magmatic ages of Late Triassic that are older than the K-Ar granitoids ages from Schwaner Mountains (Late Jurassic to Cretaceous) but still in range of K-Ar granitoids ages from northwest Kalimantan (Carboniferous to Triassic). This evidence might indicate that magmatism of granitoids of the several parts in Schwaner Mountains were contemporaneous with northwest Kalimantan domain. On the basis of whole rock chemistry, the protolith of basic metamorphic rocks from South Sulawesi and Central Java are basaltic to andesitic igneous rocks derived from tectonic environment generating MORB or within-plate signature. From the overview, it is confirmed that South Sulawesi, South Kalimantan and Central Java have possibility to be formed a part of a single subduction complex. From West Kalimantan, whole-rock chemistry analyses suggested that the metatonalites and granitoids are derived from a volcanic arc tectonic environment.