Margarite in ultramafic alteration zones (Blackwall): A new occurrence in Barramiya Area, Egypt

摘要

Margarite is recorded here for the first time in the alteration zones (blackwall) of ultramafic rocks. The mineral occurs in metasomatized metapyroxenites (chloritites) of the Barramiya area in the central Eastern Desert of Egypt. These metapyroxenites belong to the Neoproterozoic Pan-African melange of northeast Africa. Two sub-types of chloritites are distinguished, namely margarite-free and margarite-bearing chloritites. Margarite is only found in the metapyroxenites when they are indirect contactwith the melange metadiabases. Margarite is considered here as a metasomatic mineral resulting from the addition of both Ca~(2+) and Al~(3+) from the juxtaposing metadiabases. This is also accompanied by Fe-Mg substitution causing the conversion of the coexisting chlorite species from sheridanite to ripidolite. Also, extensive alteration of ilmenite to rutile is observed. The formation of post-kinematic margarite followed the event of chloritization but it is contemporaneous with the appearance of andalusite (chiastolite) in the matrix metapelites due to the intrusion of within-plate granites (GII). The assemblage margarite-ripidolite-rutile-ilmenite is free of quartz and plagioclase and hence its formation characterizes very low silica activity (μ_(SiO_2)) in the system. It is suggested that the stability limits of the mineral in this case are within the temperature range of 355-405 ℃ at 1-3 kbar.