|
Petrogenesis of Abu
Khruq Nepheline Syenites,
South Eastern Desert,
Egypt
By: Hussein A. Hegazy
Geology Department –
Assiut University,
Egypt.
E.mail: hegazy
512000@yahoo.com
Mobile: +201227831604
The Petrogenesis of Abu
Khruq Ring complex rocks which are composed of early uralitized alkaline
gabbros and both silica over and, more common, under saturated nepheline
syenites have been studied. The Abu Khruq ring complex (ARC) about 85km
from the Red Sea is one of several alkaline anorogenic magmatism in
Egypt. It belongs to the youngest group of phanerozoic ring complex
having an age of 89±2Ma (Serecsists et.al., 1981; Lutz et.al., 1988) and
is related to structural lineament trending N30W parallel to the Red
SEA.
Field investigation
revealed that the (ARC) present as a steeply conical hill, rising up to
874 m.a.s.l. and emplaced into the Late-Proterozoic gneisses and schists.
The mass is divided into a central core (about 3 km in diameter) of
nepheline syenite dissected by rhyolite and trachyte plugs and an outer
discontinuous periphery of syenite and quartz syenite. The coexistence
of plutonic and volcanic textures reveal a subvolcanic setting of the
ARC. The older intrusive gabbros were emplaced in irregular masses
beneath the volcanics, syenites were then emplaced in several incomplete
rings decreasing in age toward the center. Early syenites are
oversaturated to saturated while the more common latest syenite are
undersaturated.
The field relationships as
well as the petrographic and chemical characteristics of the ARC reflect
the co- magmatic nature of the entire suite. The chemical data support a
model in which a mantel – derived alkaline mafic magma was emplaced at a
lower crustal level by piecemeal stoping through a collapsed central
block and then extensively fractionated to give an evolved syenitic
liquid. The incompatible trace elements (e.g. Nb, Zr, Ce) show
enrichment trends with silica increase, consistent with fractional
crystallization. Moreover, the low Cr-content in gabbros are consistent
with their derivation from an evolved mafic magma.
Overall,
the compatible trace elements show sharp decreases from gabbros to
syenites.
Open – system crustal
contamination provide a mechanism for nepheline syenite to evolve across
the feldspar join and become oversaturated. This relation is consistent
with the formation of co-genetic silica over and undersaturated syenites
from a critically undersaturated magma by both crustal contamination and
fractionation processes. This provide important insight into the
processes that occur in subvolcanic complexes in continental settings. |