No abstract

This contribution reports the metamorphic evolution of the high-pressure metamorphic rocks from the Bantimala Complex, South Sulawesi, Indonesia. Barroisite-bearing and barroisite-free eclogites were examined to assess their metamorphic evolutions, which have implications regarding the tectonic conditions in this region. The eclogites mainly consist of garnet, omphacite, phengite, rutile, and epidote, with or without barroisite. The variations in mineral assemblages are interpreted to depend upon local changes in the bulk chemical composition. The barroisite-bearing eclogites contain two types of euhedral garnet: coarse-(1-1.5 mm) and finegrained (<0.5 mm). Mineral inclusions in the coarse-grained garnet core and mantle show epidote + titanite and glaucophane + epidote assemblages, that stabilized at 0.9-1.5 GPa and 350-550°C within epidote blueschist-facies conditions. Mineral chemistry and chemical-mapping analyses indicate that both fine-grained garnet and the rim of coarse-grained garnet formed at peak P-T conditions, which were estimated as 2.3-2.7 GPa at 615-680°C based on the garnet-omphacite-phengite-quartz equilibrium. Peak P-T conditions for barroisitefree eclogite were similar (2.5-2.7 GPa at 650-690°C) to those for barroisite-bearing eclogite. Actinolite rims overgrowing matrix sodic-calcic amphiboles attest to retrogression at P < 0.5 GPa and T < 350°C in a clockwise P-T path. The very low geothermal gradient experienced during the prograde path (~5°C/km) likely suggests the subduction of an old and cold oceanic crust. The low geothermal gradient on the retrograde path suggests decompressional cooling during exhumation, possibly favored by a serpentinite-dominated matrix within a subduction channel environment.

The Kurosegawa Tectonic Zone, which is one of the least geochronologically understood areas of the Japanese Islands, contains several rock types of contrasting ages in a serpentinite mélange. This paper presents the U -Pb ages of detrital zircons in pelitic schists and quartzite from the Kurosegawa Tectonic Zone, where high -pressure/low -temperature metamorphic rock (e.g., blueschist) is one of the main constituents. We analyzed a variety of samples including glaucophane -bearing pelitic schist from the Itsuki area in Kyushu and the Toba area on the Kii Peninsula, and quartzite from the Anan area in eastern Shikoku. The U -Pb age distributions reflect similar characteristics for all three areas, indicating significant peaks at 450 -500 Ma and ~ 600 Ma, with additional scattered ages of 800 -1800 Ma, and ages older than 2000 Ma. These results suggest that the provenance of the protolith of these high -pressure/low -temperature metamorphic rocks from the Kurosegawa Tectonic Zone would be the same between Kyushu and the Kii Peninsula through the Shikoku area. The distributions of detrital zircon ages in the Kurosegawa Tectonic Zone are similar to those from Permian -Triassic collision -related metamorphic rocks of the Ogcheon Metamorphic Belt and Gyeonggi Massif in South Korea, and from Devonian sedimentary rocks in the South China Craton.

Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical-zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca-zoning patterns of the garnet are different in each rock type. U-Th-Pb monazite geochronology revealed that all rock units experienced a c. 360 Ma event, and three of them were also affected by a c. 260 Ma event. The variations in the microstructures and garnet-zoning profiles are caused by the differences in the (i) whole-rock chemistry, (ii) pressure conditions during garnet growth at c. 360 Ma and (iii) equilibrium temperatures at c. 260 Ma. The garnet with sillimanite inclusions records an increase in pressure at low-P (~5.2-7.2 kbar) and moderate temperature conditions (~620-660°C) at c. 360 Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (~7.0-8.9 kbar at~600-640°C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360 Ma. The differences in the thermal gradients that resulted in varying prograde P-T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260 Ma event is characterized by a relatively high-T/P gradient (~25°C km À1 ) and may be due to collision-related granitic activity and re-equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.