Geochemical and Geotectonic Setting for Island Arc Related rocks on Um Taghir Area, Central Eastern Desert, Egypt

Um Taghir area located in theCentral Eastern Desert of Egypt a long road of Safaga-Qena. Itis represented by island arc related rocks that are intruded by late to post tectonic magmatism. The island arc belonging to oceanic terrain, divided into metavolcaniclastic and metagabrroic rocks. Metavolcanoclastic sequence is represented as the older rock unit of the study area,it is noted that metavolcanoclastic intruded by metagabbro. Island arc assemblage is classified into schist and metagabbro that give clean different of their mineralconstituents according to petrographical studies. Geochemically , the investigated rock units and most of dikes have sub alkaline nature. Tectonically , metavolcanoclastic sequence is related to back arc basin basalts, while metagabbro related to volcanic arc.


Introduction
The study area is represented in the Central Eastern Desert of Egypt on the road of Qena-Safaga ( Fig.  1). It covers about 900 km 2 , between longitudes 33° 35ˋ 00ˋˋ and 33° 50ˋ00ˋˋ and latitudes 26° 35ˋ00ˋˋ and 26° 49ˋ00ˋˋ. It forms a part of Neoproterozoic evolution of the North Arabian-Nubian Shield (ANS) in the NE Africa which is belonging to the East African Orogen (EAO) as results of accretion plateaus in the course of consolidation of the Gondwana (Gass, 1982;Stern, 1994;Kröner et al., 1994; I. A. Thabet, 2017; Gahlan H. A., Azer M. K., Asimow P., Al-Kahtany K., 2016; Hamimi Z., Zoheir B. A., Younis M. H., 2015and Stern, 1996). Generally, the ANS is considered one largest regions of mantlederived, juvenile Neoproterozoic crust in the world, which is extended over 3500 km as length and more than 1500 km as width, otherwise African Orogen area (EAO) of Arabian Nubian Shield is covered about 2.7*106 km 2 (Johnson 2014). Investigated rocks represent in the north part of ANS are middlelate Cryogenian age (~ 780 -680 Ma), on the other hand the studied area of the northwestern part of ANS related to the late Cryogenian-Ediacaran age (~ 690-600 Ma) (Johnson, 2014

Geologic setting
The exposed rock units in the studied area have been differentiated into:

Metavolcanoclastic sequence
It considers as older rock unit of the mapped area (Fig. 1), is represented by highly weathered and joints. It is intruded by the metagabbro and late to post orogenic rocks.

Metagabbro
It is restricted in the south part of study area (Fig.1). Metagabbro is recognized by low hills to moderate relief. It is characterized by massive rocks, highly weathered and fine to coarse-grained.

Geochemistry of the investigated rock units
Nine fresh or less altered samples were selected for geochemical analysis in order to work out their geochemical affinity and their geotectonic setting. The major and a range of trace and rare earth elements were analyzed, (at laboratories of Russian Geological Institute and Institute of Biology, Southern Federal University). The results are given (Table.1).

Geochemical classification of the investigated rock units
A lot of parameters are used hereunder to classify and follow up the chemical affinity of the investigated rocks. According to Pearce and Cann (1973), the samples of metavolcanoclastic sequence plot within the field of basalt + dolerite sills (Fig. 2a). On the other hand, according to classification of Na2O + K2O versus SiO2 binary diagram of Cox et al., (1979), the plots of analyzed samples of metagabbro are falling in gabbro (Fig. 3a).

Magma type of the investigated rock units
The magma type of the studied rock units are discussed on the base of the following proposed diagrams. According to Irvine and Baragar (1971) that used (Na2O+K2O)-SiO2 binary diagram to distinguish between the alkaline and sub-alkaline rocks, all the investigated rock units of study area have sub alkaline nature as shown in (Figs. 2b & 3b ). However, both of them tend to be tholeiitic according to AFM diagram of Irvine and Baragar (1971), as shown in (Figs. 2c &3c).

Tectonic setting of the investigated rock units
According to Zr versus Ti relationship of Pearce (1982), shows that the investigated metavolcanoclastic sequence plots in the field of MORB series as shown in (Fig. 2d ),while Floyed (1991) Zr versus Ba relationship, quite shows that the samples are related to the back arc basin basalts (Fig. 2e), on the other hand, Rb versus Y+Nb diagram of Pearce et al., (1984), plotting of the analyzed metagabbro samples are falling in volcanic arc field (Fig. 3d). On the MORB normalizing diagram (Fig. 2f), the metavolcanoclastic is similar to MORB but with enrichments in Ba, Rb and Th with clear declination in Nb and Y, which are characteristic for back arc lavas. On ORG normalizing diagram, metagabbro show enrichments in LILE (eg: Rb and K) and depletion in HFSE with clear negative Nb, similar to the I-type granites from subduction zones. (Fig. 3e).

Rare earth elements of the investigated rock units
The chemical concentrations of the rare earth elements (REE) of the analyzed samples are given in (Table 1). The distribution of the rare earth elements in the metavolcanoclastic varies in the spectrum (La to Lu), which shows slightly increased of REE pattern (La to Pr) and slightly decreased of REE (Pr to Lu) with a significant little or no Eu-anomaly, while the metagabbro shows slightly depletion of REE pattern (La to Lu), and slightly increased of REE pattern (Tb to Lu), all of them have a significant little or no Eu-anomaly as shown in chondrite-normalized REE diagrams of Boynton, (1984), (Figs. 2g& 3f).

Conclusion
Um Taghir located in the Central Eastern Desert of Egypt on the Qena-Safaga road. It represents the diversity of the East African Orogeny (EAO) of Neoproterozoic rocks that belong to the Arabian Nubian Shield (ANS). It is classified into island arc related rocks and late to post-tectonic magmatism. The island arc related rocks are represented by metavolcaniclastic and metagabrroic rocks. Metavolcanoclastic sequnse represents the older rock units of the investigated study area and intruded by the metagabbro and the late to post tectonic magmatism. Geochemically, the investigated rock units have sub-alkaline nature, in addition to metagabbro has tholeiitic affinity. Tectonically, metavolcanoclastic sequence is related to back arc basin basalts, while metagabbro related to volcanic arc intrusions. metavolcanoclastic varies in the spectrum (La to Lu), which shows slightly increased of REE pattern (La to Pr) and slightly decreased of REE (Pr to Lu) while the metagabbro shows slightly depletion of REE pattern (La to Lu), and slightly increased of REE pattern (Tb to Lu).