Recent progress in lower crustal process:

Abstract

Ultrahigh-temperature (UHT) and ultrahigh-pressure metamorphism indicate extreme metamorphic conditions in regional metamorphic settings. In particular, the metamorphic temperature of UHT metamorphism is the highest limit of crustal metamorphism, exceeding °C and is equivalent to the temperature of basaltic magma. Typical UHT metamorphic rocks have Mg-Al-rich bulk chemical compositions and are easily identified by a characteristic mineral assemblage of sapphirine + quartz garnet, orthopyroxene + sillimanite quartz, or garnet + osumilite + quartz. Precise reconstruction of the metamorphic evolution of UHT metamorphic rocks on the basis of metamorphic reaction textures, mineral inclusion studies, pressure-temperature estimations, and micro-geochronological analyses is a powerful tool for understanding orogenic processes.

Geochronology and REE geochemistry of zircon and garnet in pelitic gneisses from the Higo metamorphic terrane, Kyushu, Japan: Constraints on the timing of high–temperature metamorphism

“…3b, 3d, and 3g). These distinct ages from detrital Zrns might indicate metamorphism timing, which is quite consistent with previous reports (Sakashima et al, 2003;Takagi and Arai, 2003;Dunkley et al, 2008;Maki et al, 2014;Osanai et al, 2017;Suga et al, 2017).…”

“…The subsequent retrograde stage promoted new Zrn crystallization from melts and Grt breakdown at~110 Ma, finally involving granitic intrusion into pelitic gneisses at~105 Ma. Although the timing of the HT metamorphism in the Higo metamorphic terrane (~110-120 Ma) is consistent with that of previous studies (e.g., Sakashima et al, 2003;Takagi and Arai, 2003;Dunkley et al, 2008;Maki et al, 2014;Osanai et al, 2017;Suga et al, 2017), this study reconfirmed the rapid HT metamorphism framework from detailed observation of Zrn internal textures, REE chemistry, and U-Pb ages, providing critical information to understand the tectonic evolution of this terrane.…”

“…It consists of high-grade metamorphic rocks up to UHT granulite-facies that are characterized by a clockwise P-T evolution and peak metamorphic conditions of 7.8-9.0 kbar and 900-960°C (Osanai et al, 1998). Previously, this terrane was proposed to have metamorphosed at~260-230 Ma (Osanai et al, 1998;Hamamoto et al, 1999;Osanai et al, 2006); however, the recently conducted Zrn geochronology has suggested that the timing of HT-UHT metamorphism is approximately 110-120 Ma because of clustered age data (Sakashima et al, 2003;Takagi and Arai, 2003;Dunkley et al, 2008;Maki et al, 2014;Osanai et al, 2017;Suga et al, 2017). Though its metamorphic evolution in terms of petrography has been well understood, of which the prograde and retrograde metamorphic processes for each metamorphic zone were well reported on the basis of reaction textures, chemical zoning of minerals, and mineral inclusions in porphyroblasts (Hamamoto et al, 1999;Maki et al, 2004;Osanai et al, 2006), it is still obscured in term of P-T-t evolution because of the lack of information regarding the timing of the prograde and retrograde stages.…”

Zircon U–Pb geochronology of “Sashu mylonite”, eastern extension of Higo Plutono‐metamorphic Complex, Southwest Japan: Implication for regional tectonic evolution

“…F I G U R E 1 1 CL image of studied metagabbro (SGN-Gb1914) of Sambagawa metamorphic rocks. Each LA-ICP-MS-analyzed spot is labeled with the concordant 206 Pb/ 238 U age with 2σ error and the Th/U ratio Furthermore, Osanai et al (2017) reported that zircon in garnet-cordierite-biotite gneiss and cordierite-biotite tonalite displays the dark CL image and has weak oscillatory zoning. These features are also almost consistent with our result of the internal texture of zircon grains in tonalite samples from "Sashu mylonite" (Figure 8d,e).…”