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NEOPROTEROZOIC ALKALINE
MAGMATISM OF ATALLA FELSITES,
EASTERN DESERT OF EGYPT
Abu El-Ela, F.F.
Geology Department,
Faculty of Science, Assiut University, Assiut, Egypt
Abuelelaf@yahoo.com
The Atalla felsites form
an elongate mass 28 km long and 1to 7 km wide, trending NW. They are
composed of an alkaline association of mafic and abundant felsic
volcanic rocks. The mafic volcanics are composed of amygdaloidal basalts
and trachyandesites in the form of sills, sheets and dikes that invaded
and alternated with felsic volcanics. The mafic volcanics were emplaced
shortly after the emplacement of the felsic volcanics. The felsic
volcanic rocks occur as lava flows of porphyritic comendite, sparsely
porphyritic comendite, pantellerite and minor rhyolite tuff, as well as
their equivalent mylonitized rocks. Both felsic and mafic volcanics were
subjected to local mylonitization due to NW sinistral strike-slip
fault. The activity age of NW sinistral strike-slip fault (Najd fault
system) was 588-595 Ma (Fritz et al., 1996).
Mineral chemistry data
of clinopyroxene phenocrysts and groundmass from the mafic volcanics of
the Atalla felsites are classified as augite and minor salite, and they
were derived from an alkaline magma. The chemistry of phenocrysts from
the felsic volcanics of Atalla felsites supports that they are composed
mainly of orthoclase (Or = 91.16-98.47) and albite (An = 0.41-1.17).
On the basis of the
major and trace elements chemistry, the mafic volcanic rocks of the
Atalla felsites are characterized by low Y/Nb ratios, enriched LIL (Sr,
K, Rb, Ba) and HFS (Nb, P, Zr, Ti, Y) elements relative to N-type MORB,
and identical to the humped pattern which characterize the alkali
basalts from within-plate setting. Geochemically, the felsic volcanics
of Atalla felsites are classified as comendite and pantellerite on the
basis of their Zr, TiO2, Nb, Y, FeO* and Al2O3.
They have the characteristics of high potash to shoshonitic suites (K2O=
3.07-6.19), A/CNK (Shand, 1947) =0.96-1.41, K2O/Na2O=
~ 1-4.15 and alkaline to peralkaline suite according to the relation
between SiO2 and alkalinity ratio.
The tectonic environment
using different discriminant diagrams involving the elements, Ti, Mn, P,
Nb, Zr, Y, V. suggests that the mafic volcanic rocks of the Atalla
felsites have the characteristic of alkali basalt, ocean island alkali
basalt and within-plate (oceanic) alkali basalt, and may have been
derived from enriched asthenosphere source (OIA-like source). They were
emplaced in a tectonic setting similar to that associated with
extensional tectonics. The felsic volcanics of the Atalla felsites are
not equivalent to the high silica Dokhan volcanics and represent the
extrusive equivalents to alkaline and peralkaline granitcs which were
emplaced during 590 – 550 Ma in the Eastern Desert of Egypt. |