Orthogneisses from the Taba Metamorphic Belt, SE Sinai, Egypt: Witnesses for granitoid magmatism at an active continental margin

The gneisses of Taba Metamorphic Belt (TMB) are classified in terms of fiel d, structural, mineralogical and geochemical criteria into two suites of di fferent ages. The older suite, concentrated in the northern part of the stu dy area, comprises three relatively highly deformed gneiss types of predomi nantly quartz-dioritic to tonalitic composition. These orthogneisses are co mposed of oligoclase-andesine, amphibole, biotite and quartz with occasiona l almandine-rich garnet in the older two types. Amphiboles occurring in the se gneisses range from edenite to paragasite for type-II; actinolite to mag nesio-hornblende for type-III. Geochemically, the older suite is calc alkal ine, and strongly to mildly peraluminous. P-T conditions of the older gneis s suite estimated for the garnet bearing samples, conform to the medium-pre ssure amphibolite facies. Individual samples yielded average temperatures b etween about 620 and 660 degrees C and average pressures between 4.6 and 6. 2 kbars. The younger suite comprises three less deformed gneiss types ranging in com position 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 nor thern part. The main mineral phases are plagioclase, K-feldspar, quartz and biotite. In addition, the quartz-monzonitic gneisses of type-IV contain am phiboles of edenite to ferro-edenite composition. The plagioclases are olig oclase to albite in type-V and -VI and andesine to oligoclase in type IV. T he abnormal mineralogical and geochemical characteristics of type-IV are at tributed to the assimilation of gabbroic rocks, documented in mafic xenolit hs. The younger orthogneisses have alkaline to transitional calc-alkaline a nd mildly peralumious to metaluminous affinities. The Taba gneisses are derived from calc-alkaline, subduction-related are gr anitoids which were emplaced along an active continental margin during the pre- to syn-collision stage. Gneisses of similar provenance an known from o ther occurrences in the Sinai Peninsula and the Eastern Desert in the Arabi an-Nubian Shield of Egypt.