DISCRIMINATION OF LESS AND HIGH FRACTIONATED GRANITIC ROCKS, BY WEATHERING SCHEME, BIRIRIG AREA, CENTRAL EASTERN DESERT, EGYPT.

Abstract

The granitic rocks in Egypt cover more than 40% of the total area of the basement rocks, many efforts were exerted aiming to classify the granitic rocks in Egypt. The present work classifies the Biririg granitic rocks, in the central Eastern Desert of Egypt, on the basis of their mineralogical and chemical composition and discusses the effects of different grades of weathering on their mineralogical and geochemical characteristic. Geochemically, the Biririg granitic rocks (BGR) were derived from calc-alkaline peraluminous parental melts as revealed from agpaitic index, modified alkali-lime index and the aluminum saturation index. Based on geochemical characteristic, the BGR are classified into less fractionated pinkish white granite (LFG) mainly hornblende biotite granite, and highly fractionated pink granite (HFG) mainly biotite granite, the later is evolved, through fractional crystallization from more basic and calcic melts. The BGR were evolved at active continental margin regime, as the geochemical characteristics of the BGR exhibit three main sources of enrichments namely subduction zone, convicted mantle wedge and sub-continental lithosphere. This research discusses, further, the relation between grade of weathering and engineering-pathological properties of the granitic rocks exposed in Biririg area. The chemical index of alteration (CIA) values revealed that, both of LFG and HFG are low weathered, with slight increase of weathering towards the less fractionated pinkish white granite. The pinkish white granite is more basic than pink granite, as the former exhibits more mafic constituents represent about 8% of the total rock volume and are mainly biotite with less frequent amounts of hornblende. The BGR were classified into WI and WII according to the grade of weathering, where the mineralogical, chemical and engineering properties changed. Most elements redistribute in their newly formed secondary minerals. The differences observed in the composition of the WII samples relative to the slightly weathered WI ones, could be attributed to the fractional crystallization of melt, original magmatic features and/or crustal contamination.