THE ORIGIN OF ZONED NEOPROTEROZOIC IGNEOUS SUITES, SOUTHWEST GABAL MEATIQ, CENTRAL EASTERN DESERT, EGYPT.IMPLICATIONS FROM PLAGIOCLASE-AMPHIBOLE CHEMISTRY AND REE GEOCHEMISTRY

Abstract

Neoproterozoic rocks in the selected area are located southwest of GabalMeatiq, central Eastern Desert of Egypt display compositional zoning including two distinctive rock suites, outer mafic (gabbro-diorite suite) and inner felsic (tonalitegranitoid suite).The mafic suite has alkali -calcic affinity while the felsic suite has calc-alkalic character. Both suites have formed under moderate oxygen fugacity (fO2) and relatively low pressure. The chemical composition trends are functions of the mineralogical composition. FeO, CaO, MgO, Ni, Zr and La have well defined trend indicating predominance of fractional crystallization processes. Trace elements abundance displays slight enrichment of the LILE (Rb, Ba) with respect to HFSE (Zr and Y). The pattern of the felsic suite displays gradual decrease from Rb, Nb and to positive Sr anomalies which consistent with accumulation of anorthite in this suite and fractionation of mafic phases such as amphibole. The enrichment of LILE versus HFSE is a typical signature of subduction related magmatic rocks. Chemical patterns display enrichment of Sr and marked depletion of Ba and Nb which characterized magmatic rocks formed during subduction stages with accommodation of crustal materials. Mafic suite has lower ∑REE values against higher ∑REE values of felsic suite. The thermometric calculations indicated that the original magma has high temperature up to 864°C for mafic suite and up to 963°C for felsic suite from apatite saturation temperature. The estimated zircon and monazite temperatures are lower than those obtained by apatite saturation temperature and plagioclasehornblende thermometer indicating that the original melt didn’t achieve zircon and monazite saturations. Petrological and geochemical data postulated the same magmatic origin for the different rocks in the zoned pluton. The geochemical data support the suggestion that the zoned pluton results in differentiation of an original calc-alkaline magma. Field and geochemical data are consistent with a mixed fractional crystallization/assimilation/multiple emplacement mechanisms for producing the diversity of rock types in the study pluton.