干旱区生态环境知识资源中心

An ophicalcite occurrence is recorded in the uppermost part of the Precambrian ophiolitic serpentinites at Gebel Mohagara (Wadi Ghadir area) in the Egyptian Eastern Desert. In this locality, the serpentinites and their ophicalcites are sometimes directly overlain by pelagic shales and calcareous sediments along thrust planes. Field relations suggest that these ophicalcites are present as serpentine-carbonate breccias that develop along conjugate shear planes and brecciation zones. Typical sedimentary features are common, such as the presence of micritic carbonate, colloform texture, geopetal-like structures and the presence of vugs. The latter are often filled by coarse calcite spars due to diagenesis and neomorphism. Another older type of less brecciated ophicarbonates (ophimagnesites) is also present and shows extensive replacement of serpentine minerals by magnesite. The ophicalcites are considered as sedimentary breccias formed in a weathered serpentinite lithology with fabrics of typical calichified rocks. It is believed that the calichified serpentinites represent a reworked oceanic calcite that have been formed after the obduction of the ophiolite nappe on the continent. The dissolution of the calcareous material in the pelagic cap furnished the needed carbonate influx to fill the brecciated serpentinite below. (C) 1997 Elsevier Science Limited.