Orthogneisses from the Taba Metamorphic Belt, SE Sinai, Egypt: Witnesses for granitoid magmatism at an active continental margin
Abu El-Enen, Mahrous M.; Zalata, Abdelkader A.; El-Metwally, Ahmed A.; Okrusch, Martin
published: Jul 1, 1999
ArtNo. ESP154017501003, Price: 29.00 €
The gneisses of Taba Metamorphic Belt (TMB) are classified in terms of field, structural, mineralogical and geochemical criteria into two suites of different ages. The older suite, concentrated in the northern part of the study area, comprises three relatively highly deformed gneiss types of predominantly quartz-dioritic to tonalitic composition. These orthogneisses are composed of oligoclase-andesine, amphibole, biotite and quartz with occasional almandine-rich garnet in the older two types. Amphiboles occurring in these gneisses range from edenite to paragasite for type-II; actinolite to magnesio-hornblende for type-III. Geochemically, the older suite is calc-alkaline, and strongly to mildly peraluminous. P-T conditions of the older gneiss suite estimated for the garnet bearing samples, conform to the medium-pressure amphibolite facies. Individual samples yielded average temperatures between about 620 and 660 °C and average pressures between 4.6 and 6.2 kbars. The younger suite comprises three less deformed gneiss types ranging in composition from quartz-monzonite to alkali -granite. In contrast to the older suite, these gneisses are concentrated mainly in the southern part of study area, except for the youngest type that intrudes older gneisses of the northern part. The main mineral phases are plagioclase, K-feldspar, quartz and biotite. In addition, the quartz- monzonitic gneisses of type-IV contain amphiboles of edenite to ferro-edenite composition. The plagioclases are oligoclase to albite in type-V and -VI and andesine to oligoclase in type-IV. The abnormal mineralogical and geochemical characteristics of type-IV are attributed to the assimilation of gabbroic rocks, documented in mafic xenoliths. The younger orthogneisses have alkaline to transitional calc-alkaline and mildly peralumious to metaluminous affinities. The Taba gneisses are derived from calc-alkaline, subduction-related arc granitoids which were emplaced along an active continental margin during the pre- to syn-collision stage. Gneisses of similar provenance are known from other occurrences in the Sinai Peninsula and the Eastern Desert in the Arabian-Nubian Shield of Egypt.