The Neoproterozoic pluton of Gabal Gharib granite
Eastern Desert of Egypt is intruded in subduction-related calcalkaline
granitic rocks of granodiorite to adamellite composition.
A zone of metasomatized granite was developed along
the contacts at the expense of the calc-alkaline granite. The
granite of Gabal Gharib is hypersolvus, composed mainly of
orthoclase-microperthite, quartz, and interstitial arfvedsonite.
Fluorite, zircon, ilmenite, allanite, and astrophyllite are the
main accessories. Pegmatite pods as well as miarolitic cavities
(mineral-lined cavities) are common and ranging in size from
a few millimeters to 50 cm. Rare-metal minerals such as
columbite, cassiterite, and fluorite have been identified from
the miarolitic cavities. Geochemical studies revealed that
Gabal Gharib granite is a highly fractionated granite,
homogeneous in composition, with high contents of SiO2,
and alkalis, high Ga/Al, and Fe/Mg ratios, and low concentrations
of Al, Mg, and CaO relative to granodiorite–
adamellite country rocks. Gabal Gharib granite is metaluminous
to peralkaline with ASI (0.94–1.07). Trace element
characteristics of Gabal Gharib granite include abundances of
Rb, Nb, Ta, Sn, Th, U, Y, Ga, Zn, rare earth elements (REEs,
except Eu), and F, and depletion in Sr, and Ba relative to
granodiorite–adamellite country rocks. It has the geochemical
characteristic of anorogenic A-type granite. The uniform
trends of differentiation, normal REE distribution patterns,
and low calculated tetrad effects of REE (<0.2) indicate that
the effect of post-magmatic subsolidus processes were
minimal in the studied granite. Fluid inclusions were studied
in quartz crystals from Gabal Gharib granite, quartz pods, and
metasomatized granite. The study revealed the presence of high-temperature (480–550°C), high-salinity (19.45–39.13 wt.
% NaCl eq.) primary inclusions in both metasomatized and
rare-metal granites coexisting with melt inclusions and
medium-temperature (350–450°C), medium-salinity (10–
16 wt.% NaCl esq.) aqueous inclusions coexisting
hydrocarbon-bearing inclusions. Hydrocarbon is represented
by magmatic CH4 in Gabal Gharib granite, while heavier
aliphatic compounds may be present in quartz pods. Melt
inclusions with temperatures of homogenization >600°C were
also reported. Petrographic, geochemical, and fluid inclusion
studies constrain that the peralkaline anorogenic granite of
Gabal Gharib was derived from highly evolved magma
probably originated by fractional crystallization of mantle
source.