ASYMMETRIC TEXTURAL AND COMPOSITIONAL FEATURES OF GRANITIC PEGMATITES, THE EXAMPLE OF GEM-BEARING PEGMATITES OF ELBA ISLAND, TYRRHENIAN SEA, ITALY

The development of different rock units during crystallization, resulting in the formation of zoned intrusive bodies, is one of the most distinguished features of granitic pegmatitic magmas. In a pegmatite population, the largest PM (where PM stays for “pegmatitic macrostructure”, defined in Pezzotta, 2009, 4^th PEG Conference, Recife, Brasil) are the most structurally complex and geochemically evolved. The analysis of the rock textures, of the rock-forming minerals distribution, and of the accessory minerals distribution, evidence that asymmetries are more and more developed as much as the PM are evolved. The identification and the study of such asymmetries are the basis for the understanding of the crystallization processes and of the nature of the pegmatitic-forming medium during PM formation.

In the LCT gem-bearing pegmatites of Elba island (Tyrrhenian sea, Italy), tabular bodies of small size (up to 25 meters in length and up to 2 meters in width), dipping in general from 45° to 80°, contain well-structured PM, characterized by a series of asymmetric features: (1) layered aplitic unit at the foot-wall of the PM; (2) quartz-sekaninaite aggregates systematically distributed between the lower border and core zones; (3) about 70 to 90% of tourmaline and quartz-tourmaline aggregates present in the lower core zone as comb-textures; (4) about 90% of the petalite distributed in the upper part of the lower core zone; (5) feldspars which are dominated by albite in the lower core zone and by pertitic K-feldspar in the upper core zone.

Crystal distribution in miarolitic cavities is totally controlled by the textures and mineralogical composition of the lower and upper core zones. Nevertheless, a zoning perpendicular to the gravity vector exist, characterized by the following elements: over 90% of pollucite masses and crystals and most of the beryl crystals are confined to the upper levels of the cavities; most of the late stage elbaite tiny pencils and acicules are distributed, like a “snow on the roof”, in the upper levels of the cavities, much less are present at intermediate levels, quiet nothing is present at the lowest levels.