Relict Majoritic Garnet Microstructures from Ultra-Deep Orogenic Peridotites in Western Norway

Abstract

Protogranular, porphyroclastic and equigranular (or equant-polygonal) garnet microstructures from Mg–Cr type orogenic garnet peridotites, Otrøy, Western Gneiss Region, Norway, have been studied using naked eye, light-optical, electron-optical and confocal laser (fluorescence) microscopy techniques. Protogranular and porphyroclastic garnets contain microstructural evidence for the former existence of majoritic (or super-silicic) garnet. The microstructural evidence consists of exsolution textures involving pyroxene. Two types of exsolution microstructures occur—needles parallel to <111>grt and interstitial grains. The maximum volume percentage for intra-crystalline pyroxene exsolution is 2·7, and 3·6 for inter-crystalline pyroxene exsolution. The maximum pyroxene total volume percentage measured in one single protogranular or porphyroclastic garnet is 4·0. This value, at 1200°C, corresponds to minimum pressures of 6·4 GPa (≥200 km). Excluding exsolved pyroxene needles and interstitials, protogranular and porphyroclastic garnets also contain coarser-grained silicate inclusions of orthopyroxene, clinopyroxene and olivine. However, orthopyroxene is by far the most common solid inclusion and often is the only solid silicate inclusion present. Garnet porphyroclasts with only orthopyroxene inclusions may have interstitial-like orthopyroxene microstructures. The maximum volume percentage of the latter adds up to 6·7, whereas the maximum volume percentage of normal coarser-grained orthopyroxene inclusions in garnet is eight. If all pyroxene inclusions in garnet are due to exsolution of a majoritic garnet component, this implies a minimum pressure of 8 GPa at 1200°C (≥246 km). In contrast, the dominant matrix-type around protogranular and porphyroclastic garnet is dunitic to harzburgitic in composition with ol ≫opx>cpx. This matrix composition thus puts severe constraints on the origin of garnet microstructures characterized by pyroxene inclusions alone.