Grandidierite from aluminous metasedimentary xenoliths within acid volcanics, a first record in Italy

SUMMARY. Grandidierite has been identified in two aluminous metasedimentary xenoliths from quartz-latitic volcanic rocks from Mt. Amiata and Mt. Cimino, central Italy. Physical and electron microprobe data for the grandidierites and petrological data for the grandidieritebearing xenoliths are presented. The grandidierites formed by a reaction involving pre-existing aluminium rich minerals, possibly at temperatures of at least 8oo ~ and at low pressures. The grandidierite from Mt. Amiata replaces sillimanite. Several common characteristics can be demonstrated for magmatic and metamorphic grandidierite-bearing rocks. It is suggested that metamorphic rocks in which grandidierite occurs have often undergone partial melting. THE rare Mg-, Fe-, Al-borosilicate grandidierite has been identified in the course of a detailed study of metasedimentary xenoliths from acid volcanics of southern Tuscany and northern Latium, Italy. The volcanic rocks are part of two PliocenePleistocene volcanic complexes: Mt. Amiata and Mt. Cimino (fig. I). Both belong to the TuscanLatian Magmatic Province, which is characterized by extrusive and intrusive acidic rocks of crustal anatectic origin (Marinelli, [967). The Mt. Cimino complex is surrounded and partly covered by the Quaternary potassic alkaline products of the Roman Magmatic Province. Chemical, petrological, and volcanological data for Mt. Amiata are