Petrology and Geochemical Characteristic of the Younger Gabbros of Wadi Shianite Area , Southeastern Desert , Egypt

The present work is a petrological study of the gabbroic rocks of wadi Shianite Southeastern Desert of Egypt. Chemical analyses for major and trace elements showed that there are 3 main gabbro types. These are: 1) pyroxene hornblende gabbronorite; 2) hornblende gabbro; and 3) anorthosite. The opaque minerals study of the gabbroic rocks showed that they composed mainly of ilmenite, magnetite and sulphides. The present gabbroic rocks work are related to calc-alkaline magma type, similar to the younger gabbros in other areas in the Eastern Desert.

El Gaby et al. (op.sit), classified the younger gabbros as intrusive, mantle derived rocks composed commonly of fresh peridotites, gabbro diorite and intruded at the late Cordilleran stage (655 -570 Ma).The present work deals with the petrological and mineral chemistry to identify and detect the magma type and tectonic setting of the younger gabbroic rocks of Wadi Shianite.
The exposed late neoproterozoic rocks in the study area are classified according to Takla (2002)  The younger gabbros form arched outcrop within the hornblende monzogranite and separated from them by wadi alluvium.They form low hills with gentle slope (Figure 1).These rocks are homogenous in composition usually massive and have characteristic boulder-shape weathering at the outer parts.These gabbros are slightly fractured and they classified into: 1) pyroxene hornblende gabbronorite; 2) hornblende gabbro and 3) anorthosite.The modal composition of these gabbros are given in Table 1 and graphically represented on plagioclase-pyroxene-hornblende diagram (Streckeisen, 1976) [10].On this diagram (Figure 2) the studied gabbroic rocks plot within the pyroxene hornblende gabbro norite, hornblende gabbro and anorthosite fields.The pyroxene hornblende gabbronorite composed mainly of plagioclase, orthoand clino-pyroxene, hornblende and opaques.The plagioclases (An 50-65 ), are euhedral prismatic crystal (Plate I(a)) generally fresh, twinned according to albite pericline, and Carlsbad laws (Plate I(b)).Sometimes plagioclase crystals are partially sericitized especially at the contact with the hornblende granites.The clino-pyroxene represented by titanaugite and hypersthene.The augite occurs as anhedral basal section and short prisms (Plate I(c)), while hypersthene occurs as subhedral to anhedral crystals (Plat I(d)) strong pleochroic from pale green to pale brown.The hornblende gabbro is coarse-to medium-grained, orthocumulate, composed essentially of plagioclase as cumulate phase enclosed in intercumulus hornblende and biotite (Plate I(e)), opaque, titanite and apatite are accessories and quartz is secondary mineral constituent.
The anorthosite composed mainly plagioclase (An 50-65 ) as the cumulus phase together with small amount of   Subduction-Related Granitoids (Arc Granites), represented by massive granodiorite and deformed granodiorite.The massive granodiorite rocks occupy as few masses in the central part (Figure 1).They are massive and less weathered, grayish green to green color and cut by acidic dykes and quartz veins.
Under the microscope, They are coarse-to medium-grained, composed of plagioclase (An 20 -An 27 ), potash feldspar and hornblende as essential minerals.Accessory minerals represented by quartz, biotite, sphene, rutile, apatite, zircon and opaque minerals.Calcite, chlorite and saussurite are the main secondary minerals.The textures of the massive granodiorites are holocrystalline, The porphyritic textures.Hypidiomorphic and granular textures are also recorded.
The deformed granodiorites occur in the eastern part of the study area along Gebal Shianite (Figure 1).These rocks form high hills cutting by acidic dykes.The deformed granodiorites are fine to medium-grained rocks that are highly weathered to boulders (Plate I(f)) and show exfoliation structure.

Geochemistry of the Younger Gabbros
Five samples were analyzed for major oxides and trace elements analyses and two samples for rear earth element (REEs) analyses of the studied younger gabbros as given in Table 2.
Table 3 shows the average chemical composition of the studied gabbros and similar rocks from different localities in the Eastern Desert of Egypt.From Table 3 it is clear that the pyroxene hornblende gabbronorite is more rich in SiO 2 than the other types, while the hornblende gabbro is more rich in TiO 2 than the other types.All the types of younger gabbros have similar contents of the other major oxides.From Table 3 it is evident that the pyroxene hornblende gabbronorite of the study area is similar to the younger gabbro of El Bakria (El Mansi,  1996 [6]), while the hornblende gabbros similar to the younger gabbros of Gabal Ambat and gabbro Akarem (Takla et al., 1981 [3]).The anorthosites of the studied area are similar to the leucogabbro of Wadi Beitan (Hamimi, 1992) [11].
To display the trace elements distribution in the studied gabbros they are plotted on spider diagram normalized to Primitive mantle according to Wood et al., 1975 [13] (Figure 3(a)) shows that the studied gabbros are depleted in Cr, Cu, Zr and Ni and enriched in the other element.The REE pattern of the studied gabbronorite and anorthosite (Figure 3(b)) are very similar to each other and similar to that of calc-alkaline rocks.

Typology
The studied gabbroic rocks are chemically classified using the following relationships: On the normative Ab-An-Or diagram (Figure 3(c)) according to Streckeisen, (1976) [10], the studied younger gabbros plot in the gabbro diorite field.On the (Na 2 O + K 2 O) vs. SiO 2 diagram (Figure 3(d)) according to Wilson, 1989 [14] all the analyzed rocks plot in the field of gabbro

Magam Type
On the relation between Alk, vs. SiO 2 (Figure 3(d)) the gabbros plot in the sub alkaline (calc-alkaline) field.The Same conclusion is reached on plotting the analyses of the studied gabbro on Zr vs. P 2 O 5 diagram (Figure 3(e)) according to Winchester and Floyd, 1977 [15], where they plot in sub alkaline field.In the relation between FeO vs. (FeO*/MgO), according to Miyashiro, 1975 [16] (Figure 3(f)) the younger gabbros plot in the calc-alkaline field.In conclusion the younger gabbro of Wadi Shianite area originated from a calc-alkaline magma source similar to other younger gabbro in different areas in the Eastern Desert of Egypt (Takla et al., 2002) [9].

Tectonic Setting
The tectonic setting of the studied gabbroic rocks can be predicted by using TiO 2 -K 2 O-P 2 O 5 ternary diagram (Pearce et al. 1975) [17].On this diagram the studied gabbros plot in the continental field (Figure 4).

Pyroxene
The mineral analyses for the clino-pyroxenes are presented in Table 4(

Amphiboles (Hornblende)
The mineral analyses for the hornblende are given in Table 4(b), which shows that some hornblende crystals are rich in CaO and other rich in Na 2 O, except one sample with high TiO 2 content (3.62 wt%).Basta (1988) [21] indicated that the primary hornblende in the younger gabbros of Sinai are rich in TiO 2 (>1.5%) while the secondary hornblende is poor in TiO 2 .On Si vs. (Na + Ka) diagram (Figure 5(d)) according to Leak (1978) [22], the majority of samples plot in the ferroan pargastic hornblende field and one sample plot in tschermakitic field.

Plagioclases
Plagioclase is ubiquitous mineral in all the studied samples.The mineral compositions are given in Table 4
In conclusion the gabbro of the studied area are pertain to the Egyptian younger gabbros (Takla et al., 1981 [3]) because they contains fresh pyroxene, brown hornblende and fresh plagioclase.The younger gabbro of the studied area is produced from calc-alkaline magma in continental setting similar to the younger gabbros of Samut-Atud (El Mansi, 1996) [6].

Conclusions
The study of the younger gabbros in wadi Shianite area is classifieds into: pyroxene hornblende gabbronorite, hornblende gabbro and anorthosite.Pyroxene hornblende gabbronorite is the predominant type; it has hypidiomorphic granular texture and less common porphyritic texture.It consists of plagioclases, pyroxenes (hypersthenes and augite), brown hornblende and biotite.Hornblende gabbro is a coarse to medium grained rock com-posed of plagioclase, brown hornblende and biotite.Opaque and apatite are accessories.Anorthosite is idiomorphic and granular.It is composed mainly of plagioclase and diallage.
From the study of the opaque mineralogy it is cleared that the opaque minerals in the younger gabbros range from 1% to 11%.They are composed mainly of ilmenite and magnetite.The opaque minerals indicate that they are belonging to younger gabbros of Egypt (Takla, 1971 [1]; and Basta and Takla, 1974) [2].
Geochemically, the studied gabbros are similar to the younger gabbros of Egypt (Takla et al., 1981 [3]).They are sub alkaline formed in a continental arc setting.The pyroxene chemistry (augite) indicates that the host rocks are sub alkaline similar to conclusion reached from the whole-rock geochemistry.

Figure 2 .
Figure 2. Modal plagioclase-pyroxene-hornblende diagram for younger gabbros(Streckeisen, 1976).▲ hornblende gabbros, pyroxene hornblende, gabbronorite anorthosite.diallageas the intercumulus phase (Plate I(f)).The opaque minerals in the studied younger gabbros are mainly represented by ilmenite, magnetite and sulphides.The ilmenite is subhedral homogenous prismatic fresh crystals, sometimes replaced by titanite along grain peripheries (Plate I(g)).Magnetite occurs as discrete subhedral to euhedral grains slightly martitized along (111) plane.Sulphides occur in very small amount as pyrite and pyrrhotite inclusions.Pyrite inclusions in ilmenite are extensively to completely replaced by goethite (Plate I(h)).Subduction-Related Granitoids (Arc Granites), represented by massive granodiorite and deformed granodiorite.The massive granodiorite rocks occupy as few masses in the central part (Figure1).They are massive and less weathered, grayish green to green color and cut by acidic dykes and quartz veins.Under the microscope, They are coarse-to medium-grained, composed of plagioclase (An 20 -An 27 ), potash feldspar and hornblende as essential minerals.Accessory minerals represented by quartz, biotite, sphene, rutile, apatite, zircon and opaque minerals.Calcite, chlorite and saussurite are the main secondary minerals.The textures of the massive granodiorites are holocrystalline, The porphyritic textures.Hypidiomorphic and granular textures are also recorded.The deformed granodiorites occur in the eastern part of the study area along Gebal Shianite (Figure1).These rocks form high hills cutting by acidic dykes.The deformed granodiorites are fine to medium-grained rocks that are highly weathered to boulders (Plate I(f)) and show exfoliation structure.Under the microscope, they are composed of plagioclase (An 18-22 ), showing percline twining.Alkali feldspar (orthoclase and microcline), quartz (undulatory extinction) are the essential minerals.Biotite, muscovite, sphene, zircon, apatite and opaque minerals are the accessory minerals.Chlorite, saussurite and clay minerals are found as secondary minerals.The granodiorites are holocrystalline, porphyritic with hypidiomorphic textures.

Table 1 .
Mineral composition for the younger gabbros of Wadi Shianite area.

Table 2 .
Major oxides, trace elements, CIPW norms and some REE for the studied younger gabbros of Wadi Shianite area.

Table 3 .
Major oxides of younger gabbro form different localities of Egypt.

Table 4 .
Chemical composition and structural formula for (a) clino-pyroxene, (b) hornblende, and (c) plagioclase of the younger gabbros of Wadi Shianit area, South Eastern Desert, Egypt.