Following the search of jadeitite in the alluvial deposits at the mouths of the main alpine valleys, a systematic field survey around Monviso, italian western Alps, led to the discovery of a primary jadeitite outcrop. The jadeitite occurs as a tectonic block about 1 m3 in volume near Punta Rasciassa, and is embedded in a serpentinised lherzolite of the basal serpentinite unit of the Monviso meta-ophiolite. On hand specimen, the jadeitite is a very pale grass green, fine-grained massive rock surrounded by a darker retrogression margin and crosscut by coarser grained phlogopite-rich pockets and irregular discontinuous veins. A portion of the boudin consists of a peculiar rock with pegmatoid grain-size. Microscopically, the fine grained jadeitite appears more heterogeneous with clinopyroxene composition ranging from jadeite to omphacite. Three main portions may be distinguished, which consist of clear clinopyroxene, clinopyroxene with accessory rutile, and dusty clinopyroxene, respectively. In all portions the matrix clinopyroxene occurs as aggregates of interlocked stumpy prismatic crystals with sharp compositional zoning, which locally contain larger cloudy clinopyroxene relics. Zircon is a typical ubiquitous accessory mineral. The coarser-grained veins and pockets consist of randomly oriented clinopyroxene idioblasts included in xenoblastic albite or aggregates of phlogopite and Mg-chlorite. In both the rock matrix and the veins a poikiloblastic allanitic epidote locally occurs. The pegmatoid portion consists of Cr-bearing omphacite and chlorite + phlogopite domains. Very rare garnet porphyroblasts locally occur, which contain inclusions of ilmenite and minor apatite. The occurrence of rutile and the equilibration temperature educed by the Y content of garnet suggest eclogite-facies conditions. The ubiquitous presence of zircon suggests that the jadeitite derived from a former felsic dyke (plagiogranite?), originally intrusive into upper mantle peridotite, which experienced a significant metasomatism during peridotite serpentinisation.
Jadeitite in the Monviso meta-ophiolite, Piemonte Zone, Italian western Alps
COMPAGNONI, Roberto;ROLFO, Franco;
2007-01-01
Abstract
Following the search of jadeitite in the alluvial deposits at the mouths of the main alpine valleys, a systematic field survey around Monviso, italian western Alps, led to the discovery of a primary jadeitite outcrop. The jadeitite occurs as a tectonic block about 1 m3 in volume near Punta Rasciassa, and is embedded in a serpentinised lherzolite of the basal serpentinite unit of the Monviso meta-ophiolite. On hand specimen, the jadeitite is a very pale grass green, fine-grained massive rock surrounded by a darker retrogression margin and crosscut by coarser grained phlogopite-rich pockets and irregular discontinuous veins. A portion of the boudin consists of a peculiar rock with pegmatoid grain-size. Microscopically, the fine grained jadeitite appears more heterogeneous with clinopyroxene composition ranging from jadeite to omphacite. Three main portions may be distinguished, which consist of clear clinopyroxene, clinopyroxene with accessory rutile, and dusty clinopyroxene, respectively. In all portions the matrix clinopyroxene occurs as aggregates of interlocked stumpy prismatic crystals with sharp compositional zoning, which locally contain larger cloudy clinopyroxene relics. Zircon is a typical ubiquitous accessory mineral. The coarser-grained veins and pockets consist of randomly oriented clinopyroxene idioblasts included in xenoblastic albite or aggregates of phlogopite and Mg-chlorite. In both the rock matrix and the veins a poikiloblastic allanitic epidote locally occurs. The pegmatoid portion consists of Cr-bearing omphacite and chlorite + phlogopite domains. Very rare garnet porphyroblasts locally occur, which contain inclusions of ilmenite and minor apatite. The occurrence of rutile and the equilibration temperature educed by the Y content of garnet suggest eclogite-facies conditions. The ubiquitous presence of zircon suggests that the jadeitite derived from a former felsic dyke (plagiogranite?), originally intrusive into upper mantle peridotite, which experienced a significant metasomatism during peridotite serpentinisation.
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