TRACING MAGMATIC PHYSICOCHEMICAL CONDITIONS IN ABU KHARIEF ALKALI GRANITE, NORTH EASTERN DESERT, EGYPT EVIDENCES FROM APATITE, MAGNETITE & ILMENITE CHEMISTRY

ABSTRACT
The Abu Kharif Alkali Granite (AKAG) complex in the Northern Eastern Desert consists of syenogranite and alkali feldspar granite. Although the petrogenesis of these rocks is well-known in geologic literatures. Their physical-chemical condition has not been conducted based on the chemical composition of their mineral constituents. Whole-rock geochemical data classify AKAG as highly fractionated A-type with alkaline affinity and an A1 subtype. The mineral chemistry of feldspars, biotite, apatite, and iron oxides was used to determine the physical and chemical conditions and the development of the alkali granite. Whole-rock geochemical data revealed that these rocks possess (La/Sm)_n  = 1.71-4.52, mild HREE depletion (Gd/Yb)_n = 1.34-3.11, and pronounced negative Eu anomalies (Eu/Eu* = 0.09-0.37), characteristic of highly fractionated A-type granites. The biotite thermometer yields crystallization temperature ranges from 610 to 700°C at oxygen fugacity (logfO2) -11.91 and -12.63, which indicates crystallization from relatively oxidized magma that is consistent with low-pressure emplacement. The chemical data for magnetite, ilmenite, and apatite showed that the magmatic oxygen fugacities of the AKAG were a bit higher than the ΔNNO buffer line, and that magnetite and ilmenite were stable at temperatures between 571 and 678°C, which suggests that the oxygen levels decreased as the magma evolved. The rare earth element (REE) patterns of apatite from AKAG show a moderate to strong increase in light REEs (LREEs) like La, Ce, and Pr, along with a small to moderate decrease in Eu (Eu/Eu* < 1), which probably indicates that plagioclase was separated out during the development of the magma. The relatively flat HREE patterns and slight depletion in Y in apatite suggest the absence or minimal involvement of garnet in the residual melt. The trace elements in apatite match the usual characteristics of advanced A-type granites and suggest that apatite formed in conditions with plenty of oxygen. The parental melts of the studied granite were likely F-rich and Cl-poor which supports the high F/Cl ratios derived from apatite compositions. High F/Cl ratios observed in apatite from the studied granitic rocks resulted from a combination of an intrinsically high F/Cl ratio in the melt and the effects of fluid exsolution and degassing during late-stage magmatic evolution. The analyzed apatite data suggest that AKYG underwent late-stage melt-fluid interaction associated with volatile exsolution during crystallization. Apatite samples display low Ce/Pb (0.06-0.395) and Th/U (1.87-12.37), which strongly reflect the involvement of fluid and crustal material during crystallization and imply that the genesis of the AKAG was through partial melting of crustal sources.