Composition-Volume Changes during Metasomatic Alteration of Pan African Andesitic Volcanic Rocks at Gabal El Dokhan Area, Northeastern Desert, Egypt.

Abstract

Metasomatic alterations of the Late Proterozoic andesites at Gabal El Dokhan area display obvious change in mineralogical and chemical composition, if compared with the least altered andesitic rock types (assumed protoliths). These altered andesitic rocks have been metamorphosed to lower-middle greenschist facies typified by biotite-actinolite-chlorite-albite-epidote-quartz-opaque minerals/sphene assemblages. Five alteration facies have been recognized: 1) Bio- titization-silicification, 2) Sericitization-silicification, 3) Albitization- silicification, 4) Epidotization-silicification, and 5) Chloritization. The biotitized-silicified rock types are enriched in K-Rb-Ba-FTSM and depleted in LREE relative to the least altered rock. The se- ricitized-silicified rock types are significantly enriched in K-Rb-Ba and depleted in FTSM and LREE. The albitized-silicified rock types are strongly enriched in Na and depleted in LILE and LREE relative to the least altered rock types. The epidotized-silicified rock types are enriched in Ca-Cu-Pb and depleted in LILE and LREE. The chlo- ritized rock types exhibit relative enrichment in Mg, FTSM, Zn-As-Pb and depletion in LILE and LREE. The behavior of the Ti, Al, Fet, P, and most HFS elements during biotitization, sericitization, albitization, epidotization and chlorit- ization suggests that they were inert during metasomatic modification of the andesites. Mass balance calculations suggest that volume change was restricted between zero and up to + 20% for the altered upper andesites and -25 and up to +25% for the altered lower an- desites during the different types of metasomatic facies. Elemental mobilities are interpreted in terms of the controlling mineral phases and the possible nature of the various metasomatising fluids.