Petrology, Geochemistry and Mineral Chemistry of Neoproterozoic Granitoids, Wadi Zaghra, Southern Sinai, Egypt

ABSTRACT

The aims of the paper are to study the petrological, geochemical, and mineralogycal characteristics, as well as the geothermobarometry of Wadi Zaghra granitoid rocks.
The granitoid rock outcrops in Wadi Zaghra cover about 165 km 2 in the eastern, western, southern, and northeastern parts of the mapped area as scattered elongated outcrops, trending NW-SE, NE-SW, and roughly parallel to the general trends of the basement exposures of the whole area. The late-syn tectonic granitoid rocks cover about 100 km 2 and comprise the largest plutons of quartz diorites -granodiorites in the western and eastern parts of the traverse along Wadi Zaghra. They intrude metaconglomerate but are intruded by Dokhan volcanics and posttectonic granites. The quartz dioritegranodiorites are cut by andesite and basalt dykes (Fig. 2). The quartz diorites occur as oval and elongated masses intruded in NW-SE trending with high relief (e.g., Gabal Umm Rweiss, 1500 m.a.s.l). They are intruded by the monzogranites at Wadi Magafa, as well as by small bodies of syenogranite to the east of Wadi Zaghra (Fig. 2). On the other hand, the quartz diorites are graduated into granodiorites, but they are also intruded by the monzogranites. They are coarse-grained, dark gray in color, and exhibit sharp intrusive contacts against and/or bear xenoliths of the metasediments (Fig. 2). The granodiorites occur as coarse-grained, lightgray low to moderate masses and/or apophyses, which intrude with sharp contact into and/or bear abundant xenoliths of the metavolcanic-sedimentary rocks.
The post-tectonic granitoid rocks cover an area of about 65 km 2 of monzogranite-syenogranites plutons in the mapped southwestern and southeastern parts.
They enclose enclaves of granodiorites that are cut by andesitic and basaltic dykes (Fig. 2). The monzogranites occur as moderate to high elongated semioval masses trending NW-SE. They intrude the quartz monzodiorites but are intruded by the syenogranites as well as some basic, intermediate, and acidic dykes. The syenogranite is present as small moderate to high elongated masses cutting through the quartz diorite-granodiorite and the monzogranites in the southwestern, eastern, and central parts of the mapped area. This granite is cut by a few basic, intermediate, and acidic dykes.

Petrography
Microscopically, the Wadi Zaghra granitoid rocks are represented by tonalites, granodiorites, monzogranites, syenogranites, and alkali feldspar granites. Quartz-diorites are medium to coarsegrained and composed of plagioclase, hornblende, and biotite with subordinate amounts of quartz. Chlorite and epidote are secondary constituents. Accessory minerals include sphene, zircon, apatite, and iron oxides. Tonalites are coarse-grained and consist mainly of quartz and plagioclase, with subordinate amount of biotite and hornblende (Fig. 3a). Chlorite, epidote, and sericite are secondary minerals. Apatite, zircon, iron oxide, and sphene are the main accessories. Granodiorites are coarsegrained and consist principally of quartz and plagioclase, with small amounts of microcline, biotite, and hornblende ( Fig. 3b and c). Chlorite, epidote, and sericite are secondary minerals. Apatite, zircon, iron oxide, and sphene are the main accessories. Monzogranites are medium to coarsegrained and consist principally of quartz, plagioclase, microcline, and biotite; hornblende is uncommon (Fig. 3d). These mineral constituents often contain zircon, sphene, apatite, and iron oxides as accessory minerals, as well as chlorite, epidote, and sericite as secondary minerals. They exhibit a hypidiomorphic texture and sometimes a porphyritic texture. Syenogranites are medium to coarse-grained and have a pinkish-red color. They consist mainly of quartz, microcline, microcline-perthite, plagioclase, and biotite ( Fig. 3e). Zircon, sphene, apatite, and iron oxides are the main accessory minerals, as well as chlorite, epidote, and sericite as secondary minerals. Alkali feldspar granites are characterized by granular-hypidiomorphic textures. They consist essentially of quartz, potash feldspar, and rarely plagioclase and biotite (Fig. 3f). Zircon, sphene, apatite, and iron oxides are the common accessory minerals, whereas epidote, chlorite, and sericite are secondary minerals. Quartz occurs as anhedral crystals of semicircular shape, filling the interstitial spaces between feldspar crystals. Very often, the quartz crystals enclose plagioclase, biotite, and microcline patches.

Analytical methodology 4.1. Whole rock analysis
Whole-rock chemical analyses for major, trace, and REE elements for 20 representative samples of Wadi Zaghra granitoid rocks were determined by using Xray fluorescence spectrometry (XRF) at Granada University, Granada, Spain, on melt pellets in glass form using the method of Tertiani and Claisse (1982). The rock samples were crushed in a jaw crusher and ground in an electrical agate mortar to a fine powder that passed through a 100 mesh size (sieve 0.0149 mm or =15 microns). One and a half grams of the rock powder mixed well with 7.5 grams of spectromelt A19 (Dilithium tetraborate: lithium metaborate, 66:34) to form a pellet. The mixture was then fused in a furnace for 20 minutes at 1150 °C, after which the fused bead was cooled in pellet form. Two pellets were made for each sample and examined by X-ray fluorescence in the presence of international standard samples (Abbey, 1983). As the X-ray fluorescence determines the FeO and Fe2O3 as total iron, the FeO is determined by the volumetric titration method using 0.1N KMnO4 as well as loss on ignition (L.O.I.) (Shapiro, 1975).

Mineral analysis
Mineral analyses were carried out at the Centro de Instrumentación Científica (C.I.C.) at the University of Granada, Granada, Spain. They sorted out about 38 single-point analyses of amphiboles, biotites, and plagioclase. Mineral majorelement composition was determined by wavelength-dispersive spectrometry (WDS) with a CAMECA SX-100 electron microprobe, operated at 15 kV and 15 nA, and by energy-dispersive spectrometry (EDS) with a Zeiss DSM-950 scanning microscope and a LEO 1430-VP scanning microscope, both operated at 20 kV. Natural and synthetic standards were used, and the precision was better than ±1.5% for analyzing concentrations of 10 wt.%. The electron beam was focused to a spot diameter of about 3 µm, which was occasionally raised up to -10 µm to prevent heat effects on turbed alkali feldspars and perthite. The Ziebold and Ogilive (1996) approach with the Albee and Ray (1970) ZAF correction was applied.

Geochemical characteristics
The major element, trace elements, and rare earth elements concentrations of the studied granitoid rocks in Wadi Zaghra are listed in Table 1 (Table 1).
Moreover, they are characterized by relatively low total REE abundances (∑REE ranges between 33.89 and 170.28 ppm, with an average of 119.95 ppm) ( Table 2).

Mineral chemistry 5.2.1. Amphiboles
The main objective of the present study is to estimate the variation in the chemical composition of the amphiboles to determine their nomenclature and the condition of their formation. The electron microprobe analyses for 6 single points of the amphibole minerals from Wadi Zaghra quartz-diorite and their chemical formulae based on 23 oxygen atoms and ignoring H2O using MinPet software by Richard (1995) are listed in (Table 3).
The amphibole analyses plotted on TSi vs. Mg / (Mg + Fe) diagram (Leake, 1978) are calcic-type in composition and range from actinolite, actinolite hornblende, magnesio-hornblende, to edenite. The amphibole data on the TSi vs. CTi diagram (Leake, 1965) suggest that they are mostly of magmatic origin, with the exception of one sample (SA-94-3-2), which is shifted to the metamorphic amphibole field (Fig. 5).

Biotites
Generally, the main mafic mineral in the studied granitoid rocks is biotite. A total of 14 single points microprobe analyses were performed from Wadi Zaghra syenomonzogranite (SA-97) and quartz-diorite (SA-94). The chemical formulae were calculated (based on 24 oxygen atoms and ignoring H2O) using Minpet software after Richard (1995). The chemical analyses and their chemical formulae are listed in (Table  4).

Feldspars
Eighteen single-point analyses of feldspars from syeno-monzogranites (10 points) and quartz-diorite (8 points) have been analyzed and listed in (Table 5). The chemical formula was computed from the obtained analyses on the basis of (8) oxygen atoms using MinPet software after Richard (1995) and is listed in (Table 5).
The present feldspar data are plotted on the Or-Ab-An ternary diagram (Fig. 7) of Deer et al. (1978). The analyzed samples show that the plagioclase is plotted in the albite and oligoclase fields, except two samples from quartz-diorite lie in the andesine field, as shown in Fig. 7. Table 4. Electron microprobe analyses and structural formulae of biotites for the studied Wadi Zaghra granitoid rocks       Table 5. Electron microprobe analyses and structural formulae of feldspars for the studied Wadi Zaghra granitoid rocks Fig. 7. Or-Ab-An ternary diagram of plagioclase composition of the studied gabbro (after Deer et al., 1978).   (Wright, 1969) and (b) Agpaitic index (AI) vs. SiO2 diagram for the study granitoids (Liègeois and Black, 1987).

Magma Type
The magma type of the Wadi Zaghra granitoids can be deduced by plotting the data on a series of diagrams. On the alkalinity ratio variation diagram of Wright (1969), quartz-diorite, granodiorite, and tonalite show calc-alkaline affinity, whereas syeno-monzogranite and alkali feldspar granite display alkaline affinity (Fig. 9a). On the agpaitic Index (AI) versus SiO2 (wt.%) diagram of Liègeois and Black (1987), quartz-diorite, granodiorite, and tonalite belong to the calc-alkaline series, while alkali feldspar granite and sample SA-93A belong to the alkaline series (Fig. 9b).

Normalized multi-element diagrams and REEs patterns
The trace elements and REEs of the investigated granitoid rocks (Table 1) were normalized to chondritic values after Taylor and McLennan (1985). The spider normalized multi-element diagrams for the studied granitoid rocks (Fig. 12a) show relatively high concentrations of LILE (K, Rb, and Cs), HFSE (Zr, U, and Th), and conspicuous negative anomalies for Ni, Ti, P, Sr (samples; SA-76 and Sa-91) and Zn.
Generally, Zaghra granitoids are characterized by relatively low REE abundances (∑REE ranges between 33.89 and 170.28 ppm, with an average of 119.95 ppm). Rocks show an overall moderately fractionated chondrite-normalized REE pattern (Fig. 12b) as expressed by the chondrite-normalized (La/Yb)N ratio, varying between a most fractionated of 3.4 and the least fractionated of 19.83 (Table 2). In addition, they have (La/Sm)N ratio between 2.54 and 5.97, with an average of 4.49, whereas the (Gd/Lu)N is between 1.00 and 2.33 ppm, with an average of 1.62 ppm, indicating slight fractionation of both LREE and HREE (Table 2).
Chondrite patterns exhibit a negatively steep slope from the light to middle REEs (MREEs), and all have a gentle positive slope toward Lu (Fig. 12b). Alkali feldspar granite and syeno-monzogranite samples have negative Eu anomalies, whereas tonalite and quartz-diorite samples have no to slight positive Eu. LREEs are slightly more abundant than HREEs in terms of normalized chondrite (Fig. 12b).  (Taylor and McLennan, 1985), (b) Normalized REE patterns for the studied granitoid rocks (Taylor and McLennan, 1985).

Tectonic setting
The Y + Nb (ppm) vs. Rb (ppm) diagram (Fig. 13) shows that Wadi Zaghra granitoid rocks lie in a volcanic arc granites field (VAG). The alkali feldspar granites and a few samples of syenogranites and monzogranites are plotted in post-orogenic granites (POG).

P-T conditions
The normative values of quartz, albite, and orthoclase for Zaghra granitoid samples are plotted on the Q-Ab-Or ternary diagram to interpret the temperatures and pressures of the granitoid rock formation (Luth et al., 1964;Winkler et al., 1975). The plot reveals that Zaghra granitoid rocks are formed at a temperature around 670 °C and pressure from 0.5 to 10 kbars, except two spots of quartz-diorite (Fig. 15a). Using the Condie (1973) discrimination diagram, the plots suggest Zaghra granitoid rocks are emplaced at a depth exceeding 30 km (Fig. 15b).

Thermobarometry
It aims to determine the pressure and temperature of crystallization by using amphibole and biotite minerals.

Amphibole Thermobarometry
According to the relation between the Al IV contents of the amphibole versus the temperature (Fig. 16a) after Blundy and Holland (1990), Wadi Zaghra granitoids record a temperature range of amphibole formation around 530-700 ℃ ( Table 6).

Summary and Conclusion
The petrological, geochemical, and mineralogical characteristics of the studied Wadi Zaghra granitoid rocks, as well as their geologic setting and field relationships, are listed in Table 7. Generally, the granitoid rocks of Wadi Zaghra are considered to form at a temperature around 670 °C, a pressure of 0.5 to 10 kbar, and a depth of more than 30 km. According to Blundy and Holland (1990), the formation of amphibole occurs at temperatures between 530 and 700 °C, where the average pressure during crystallization is 1.4 Kbar. Additionally, the average crystallization pressure of biotite by Uchinda et al., 2007