Recent progress in lower crustal process:

precise analyses on ultrahigh-temperature metamorphism

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 900°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. In the last 25 years, Japanese geoscientists have highly activated for the research works on the UHT metamorphic rocks worldwide, including in the Napier Complex of East Antarctica, southern Indian peninsular, Highland Complex of Sri Lanka, Kontum Massif of Vietnam, Inner Mongolian Suture Zone in north China, Limpopo Belt in Africa, Lewisian Complex in Scotland and other terranes. These geological and petrological (including experimental petrology) investigations have led to the publications more than 150 research papers. In this review paper including small amount of our new data, we outline our research activities on four different types of Archean-Proterozoic to Cretaceous UHT metamorphism, as well as dehydration melting experiments on F-bearing biotite under UHT conditions.