Multiple breakdown and chemical equilibrium of silicic clinopyroxene under extreme metamorphic conditions in the Kontum Massif, central Vietnam

摘要

Clinopyroxene in ultrahigh-temperature mafic granulites from the Kontum Massif in central Vietnam records multiple metamorphic stages, manifested as exsolution textures (quartz rods and orthopyroxene + hornblende + plagioclase needles), and as symplectitic intergrowths (involving clinopyroxene + plagioclase). These textures suggest a metamorphic evolution characterized by decompression and subsequent cooling from eclogite-facies to amphibolite-facies conditions through ultrahigh-temperature conditions. Quartz rods in clinopyroxene and clinopyroxene + plagioclase symplectites were formed under eclogite conditions prior to ultrahigh-temperature metamorphism. The orthopyroxene + hornblende + plagioclase needles in clinopyroxene are regarded as cooling products after ultrahightemperature metamorphism. Recalculated compositions of precursor clinopyroxene show supersilicic composition. During the metamorphic evolution, the chemical composition varies from silicic (Ca- Eskola-rich) via sodic (Jadeite-rich) to aluminous (Ca-Tschermak-rich) compositions. Presence of supersilicic clinopyroxene suggests that the granulite decompressed from possible ultrahigh-pressure conditions (ca. 800–900 °C at 2–3 GPa) preceding the ultrahigh-temperature stage (1050 °C at 1.3 GPa), which provide strong constraints on the tectonic evolution of the Indochina region, and it also provides insights on crustal exhumation at a continental collision zone. Another significant aspect of this study is that the breakdown textures of clinopyroxene and its chemical variations may provide important information in establishing pre- and post-peak evolution, especially for extremely high-temperature or high-pressure metamorphic rocks.