Zoisite-clinozoisite bearing pegmatites and their importance for decompressional melting in eclogites
Franz, Gerhard; Smelik, Eugene A.
European Journal of Mineralogy Volume 7 Number 6 (1995), p. 1421 - 1436
published: Dec 27, 1995
manuscript accepted: Jun 16, 1995
manuscript received: Dec 21, 1994
ArtNo. ESP147050706015, Price: 29.00 €
Abstract Field relations, mineral textures, and mineral compositions for pegmatite segregations in eclogites from Weißenstein (Miinchberg Massif, Germany) indicate that partial melting has occurred during isothermal decompression of the eclogite from 25 kbar to about 10 kbar. The pegmatites have a leuco-tonalitic bulk composition. The matrix of the pegmatite is rich in quartz and plagioclase with abundant graphic intergrowths. Zoisite and phengitic white mica coexist with a silicate melt and occur as cm-sized euhedral phenocrysts; other less abundant phases in the melt are clinozoisite and calcic amphibole. The large zoisite crystals show complex zoning relationships, and are intergrown with narrow lamellae of clinozoisite, albite, and sometimes calcic amphibole along (100) of the zoisite. The complex intergrowth textures and compositional relationships record at least four stages of mineral reaction and growth in the pegmatites during rapid uplift of the eclogites. The P-T path for pegmatite formation has been constrained by experimental data (from the literature) on melting in tonalitic systems and zoisite-clinozoisite phase relations. Mass-balance calculations for H2O production during uplift and amphibolitization of the eclogites indicate that partial melting of about 2.5 vol% would generate enough water-saturated melt to produce the volume of pegmatites observed in the Miinchberg Massif. Our results suggest that partial melting during decompression may be a common process in eclogite terranes, leading to high-pressure pegmatites, with low-Fe epidote minerals as magmatic phases