Garnet-quartz intergrowths in granitic pegmatites from Bergell and Adamello, Italy

Garnet-quartz intergrowths characterized by graphic texture have been found in granitic pegmatites at three localities in Italy (Sivigia and Val Bona, Bergell; Val Caffaro, Adamello). The intergrowths form nodules which are u p to several centimeters across and are embedded in a matrix consisting of coarse-grained muscovite,potassium feldspar, plagioclase and quartz. Electr on probe microanalysis revealed that garnet is predominantly a spessartine- almandine solid solution with spessartine contents as high as 78 mol%. With in the intergrowths, individual garnet lamellae are often zoned, whereby th e spessartine content decreases considerably from the center to the rim. So me lamellae, however, display only weak zoning or are chemically homogeneou s. X-ray texture goniometry data suggest that the examined area within a nodul e at Sivigia consists of a single garnet crystal which is intergrown with a single quartz crystal, whose c-axis is oriented between the poles to (011) and (112) of garnet. Al Adamello, the garnet consists of a single crystal, but the intergrown quartz is a polycrystalline aggregate, Oxygen isotope data for mineral separates from the Sivigia pegmatite reveal ed that the delta O-18(SMOW) values for quartz within the intergrowths are identical to those of quartz occurring in the pegmatite matrix (11.8 parts per thousand). Oxygen isotope thermometry indicates an equilibration temper ature of approximately 720 degreesC for coexisting garnet and quartz, The h igh delta O-18 values of all major minerals suggest that the Sivigia pegmat ite melt may have been derived from metasedimentary rocks, in agreement wit h whole-rock geochemical data. Moreover, the normal sequence of delta O-18 values exhibited by the major minerals indicates that the pegmatite was not subjected to significant postmagmatic hydrothermal alteration. The observa tions are consistent with a magmatic origin of the garnet-quartz intergrowt hs. The intergrowth texture indicates that garnet and quartz crystallized s imultaneously from the granitic melt, probably as a result of cotectic crys tallization.