Petrology and geochemistry of multiphase post granitic dikes: A case study from the Gabal Serbal area, Southwestern Sinai, Egypt

: Variable single and/or swarms of post - granitic dikes are widespread at Gabal Serbal, Southwestern Sinai, Egypt. The present article aims to identify and discriminate these multiphase dikes through detailed geological, petro -graphical, and geochemical examinations. These dikes are classi ﬁ ed into two subphases: ( 1 ) acidic dikes ( porphyritic dacite, microgranite, granophyre, and alkaline granophyre dikes ) ; and ( 2 ) basic dikes ( basalt and dolerite dikes ) . They range from vertical or steeply inclined bodies, 0.5 – 15 m wide, pink to black color, and NE – SW to N – S directions. Acidic dikes with di ﬀ erent mineralogical constituents have medium to high k - characters, originating from calc - alka line magma and extruded in a volcanic arc environment. In contrast, basic dikes have medium k - characters, originating from tholeiitic magma and developing within a plate envir -onment. Basic dikes are enriched with opaque minerals, where the basaltic dike contains iron oxides ( magnetite and hematite ) , such as apatite in addition to copper minerals. Dolerite dike comprises magnetite, titanomagne tite, and pyrite.


Introduction
The basement rocks of Sinai represent the northwestern extremity of the Arabian-Nubian Shield (ANS) separated from the main block by the Gulf of Suez and the Gulf of Aqaba. These rocks were formed as a result of complex events of subduction, accretion, and extension during Pan African times [1,2]. The Gabal Serbal area is located in the southwestern part of Sinai Peninsula, between latitudes 28°32′ and 28°40′ 43˝N and longitudes 33°34′ and 33°41′ 30″E (Figure 1). The area under investigation covers about 233 km 2 and is considered a part of the Arabian-Nubian Shield. The Pan-African dike swarms in southwestern Sinai were classified into two swarms: the first (oldest) strikes at 35°and exhibits calc-alkaline chemical properties while the second strikes at 135°and displays transitional to mildly alkaline properties [3]. Geology is considered one of the most important and best sciences in ancient and modern times. It helps discover some of the geological secrets hidden in deep burials and different rocks containing some minerals that have economic feasibility [4]. Dikes are economically significant because they are critical in the formation of world deposits such as tungsten and gold [5,6], and they structurally control the flow and thus the exploration of natural resources such as groundwater and geothermal energy [7,8]. On the other hand, radioactive mineralization linked with red, black, and jasperoid silica veins exhibits apparent brilliant yellow secondary uranium-bearing minerals [9][10][11][12].
Dikes may be straight (and parallel to rift zones), curved, or radiating, with the focal point of the swarm understood to designate the core of a magmatic edifice or mantle plume [13], and are often linked with crustal extension [14][15][16]. It has long been assumed that dike emplacement is somewhat regulated by the regional stress field based on field connections [17,18]. The present study aims to detect the petrography and geochemistry analyses in addition to determine some of the economic mineralization associated with the different dikes at the Gabal Serbal area, Southwestern Sinai, Egypt.

Geological setting of dikes
The post-granitic dikes of the study area occur either as single dikes or as swarms. These dikes are the product of the most recent episodes of magmatism. They have vertical or steeply sloped bodies that have been injected into all of the preceding strata. Their widths range from 0.5 to 15 m. The dike swarms are more resistant to weathering and erosion than their host. The contacts between dikes and their host rocks are sharp. These dikes are classified into acidic and basic dikes. Acidic dikes are represented by porphyritic dacite, microgranite, granophyre, and alkaline granophyre. Porphyritic dacite dikes are massive and fine-grained with a grayish color. These dikes strike at NE-SW, and N-S. Porphyritic dacite dikes have irregular shapes and their width varies between 0.8 and 2.5 m. They cut the gneisses and migmatites in Wadi Agela and the younger granites in Wadi Gebeiy. Microgranite dikes are massive, porphyritic, and fine-grained with pale red color. They exhibit porphyritic texture, where quartz, plagioclase, and K-feldspar are the phenocrysts. These dikes are vertical and strike at NNE-SSW and N-S. They range between 0.5 and 2.5 m in width and cut the younger granites in Wadi Gebeiy. Granophyre dikes are massive, porphyritic, and fine-grained with a greenish-gray color. Biotite, quartz, K-feldspar, and plagioclase phenocrysts are embedded in a quartz-feldspathic matrix. These dikes are of limited distribution and strike at NE-SW with their widths varying between 0.5 m and 5 m. They cut the gneisses in Wadi Agela and the younger granites in Wadi Abura. Alkaline granophyre dikes are massive, porphyritic, and fine-grained with red color. Quartz, Kfeldspar, riebeckite, and plagioclase phenocrysts are embedded in the quartz-feldspathic matrix. These dikes are of limited distribution and strike at NE-SW to ENE-WSW and their widths vary between 1 and 15 m (Figure 2a). They cut the gneisses and migmatites and alkaline granite in Wadi Agela and also the alkali feldspar granite in Wadi Abura. Basic dikes are represented by basalt and dolerite dikes. Basalt dikes are hard, finegrained with a dark green to grayish-green color. They occur as single or as swarms (Figure 2b). They are marked by negative topography and suffer exfoliation weathering forming onion structures. Basaltic dikes strike NNE-SSW, N-S, and NNW-SSE varying in width from 0.5 to 5 m. Also, the thicker dikes attain a well-pronounced chilled margin against the host rock. They are observed cutting the gneisses and migmatites at Wadis: Agela, Allyiat, Rim, and Um Takha also cutting metasediments-metavolcanic association at Wadi Wirqa. They cut older granites between Wadi Ramuz and Wadi Wirqa. These dikes cut the younger granite at Wadis Geba, Gebeiy, and Abura. Dolerite dikes are medium-grained with dark-green to reddish-brown color. They are marked by negative topography relative to their country rocks. These dikes strike at NNE-SSW and NE-SW. They vary in their width between 0.5 and 6 m. The thicker dikes attain a very distinctive chilled margin, where the margin of the dike is fine-grained, and the grain size increases inwards the dyke core. The latter is characterized by porphyritic texture with coarse-grained plagioclase phenocrysts. They cut the gneisses at Wadi Rim: the older granites between Wadi Ramuz and Wadi Wirqa and younger granites at Wadi Geba. One of them cuts the syenogranite at the mouth of Wadi Geba. It exhibits minerals alteration product at its periphery and the contact record high radioactive anomaly. This dike acts as a physical and chemical barrier for the accumulation of radioactive minerals. In the study area, there are about 142 major dikes, and totally, about 278.8 km length was traced and mapped ( Figure 1). The directional trend analysis indicates that these dikes exhibit three main trends, namely, N (33-56°) E with tensional regime N (34-57°) W, N (57-76°) E with tensional regime N (14-33°) W, and N (12-32°) E with tensional regime N (58-78°) W regarding their number and length proportions.

Methods and sample collection
A total of 36 samples were collected from the studied dikes in the assessed region. Chemical analysis was performed on 18 samples from various types of dikes. The samples (about 1 kg) were transferred to the laboratory for analysis. The acquired samples were dried and sieved through a fine mesh (1 mm) (0.256) mm sieve for homologation. To completely dry the samples, they were put in an oven and baked for a day at 100°C. After weighing the samples, they were transported to a 212.6 cm 3 polyethylene measuring cylinder with a 9.5 cm diameter and a 3 cm height. Major oxides and trace elements were detected using the X-ray fluorescence technique. X-ray fluorescence analysis (XRF) is a sophisticated spectroscopic method used to determine a substance's elemental makeup. Numerous elements, from beryllium (Be) through uranium, may be investigated using XRF (U). The X-ray powder diffraction technique collects and analyzes the spectrum produced when the test material is exposed to X-ray radiation. The voltage has been adjusted at 10 kV for light elements, 20-30 kV for medium elements, and 40-50 kV for heavy elements. Additionally, since the environment has a considerable effect on the spectrum when studying light elements, the sample chamber is either evacuated or helium-filled [20][21][22]. The spectrometer was used to determine the chemical element content of a variety of substances, including those that are solid, powdered, or liquid, as well as those that are deposited on the surface or on filters. Heavy minerals were separated using a heavy liquid (bromoform) separation technique, followed by magnetic fractionation using a Frantz isodynamic separator. The binocular microscope was used to collect the heavy minerals, which were then identified using the environmental scanning electron microscope (ESEM)-energy dispersive X-ray (EDX) approach. All studies were conducted at the Nuclear Materials Authority's Laboratories (NMA).

Petrography of dikes
More than 30 samples of post-granitic dikes were investigated using a polarizing microscope in order to detect their mineralogical and textural relationships. They can be classified into acidic and basic dikes as follows.

Porphyritic dacite
Porphyritic dacite is composed of plagioclase, quartz, and K-feldspar as essential felsic minerals, whereas biotite and hornblende are mafic minerals. Porphyritic dacite is marked by the porphyritic texture. Plagioclase crystals range in size from euhedral to subhedral (0.8 mm × 2.1 mm) and are composed of oligoclase and andesine (An 17-36 ). It exhibits albite and pericline twinning and has been changed to saussurite ( Figure 3a). Occasionally, normal zoning of plagioclase phenocrysts is seen. Quartz is found as 1.2 mm × 2.4 mm subhedral to anhedral phenocrysts and as a fine-grained groundmass. Phenocrysts exhibit excessive extinction as a result of the cataclastic effect. K-Feldspar appears as 1.1 mm × 2.3 mm subhedral phenocrysts and as anhedral fine-grained groundmass. Orthoclase and orthoclase microperthite are the minerals that comprise them. Kaolinization of K-feldspar occurs in stages ranging from partial to total. Biotite occurs in the form of subhedral phenocrysts with dimensions of 0.5 mm × 2.1 mm and as a fine-grained groundmass. It demonstrates partial transformation to chlorite and iron oxides at the crystal borders. Hornblende occurs as 0.5 mm × 1.9 mm subhedral to anhedral phenocrysts. Opaques appear as fine crystals ranging from subhedral to anhedral in conjunction with mafic minerals.

Microgranite
Microgranite is composed essentially of quartz, K-feldspar, plagioclase, and biotite as essential minerals, while opaques are accessory minerals. Quartz occurs as euhedral to subhedral phenocrysts (1.1 mm × 2.1 mm) and as fine-grained groundmass. The phenocryst shows weak undulose extinction and corroded edges by the component of the groundmass. K-Feldspar exists as euhedral to subhedral phenocrysts (1.1 mm × 1.9 mm) and as finegrained groundmass. K-Feldspar is represented by orthoclase and microcline microperthite (Figure 3b). It is sometimes altered to kaolinite and poikilitically quartz and plagioclase. Plagioclase exists as subhedral to anhedral phenocrysts (0.8 mm × 2.1 mm) embedded in fine-grained groundmass forming porphyritic texture. It is of albitic to oligoclase (An [8][9][10][11][12][13][14][15] ) in composition. It shows albite/Carlsbad and pericline twinning and altered to saussurite. Normal zoning of phenocrysts is observed. Biotite occurs as small euhedral to subhedral flakes with the pleochroic formula: X = brown and Y = Z = dark brown. It is altered to chlorite, especially along its cleavage planes. Opaques are represented by fine-grained aggregates associated with biotite.

Alkaline granophyre
Alkaline granophyre is macroscopically hard, massive, and fine-grained with red color. The alkaline granophyre is composed of essentially quartz, K-feldspar, plagioclase, and riebeckite. Opaques are accessories. Quartz forms subhedral to anhedral crystals and attains 0.1-1.9 mm in length and 0.1-0.5 mm in width. Sometimes, quartz intergrow with K-feldspar forming a graphic texture. K-Feldspar occurs as subhedral to euhedral crystals and attains 2.1 mm × 5.5 mm across in phenocryst and 0.1 mm × 0.7 mm across in the groundmass. It is represented by perthite and orthoclase micro perthite and partly altered to kaolinite. Plagioclase is rare and forms euhedral to subhedral crystals 0.2 mm × 3.5 mm in phenocryst and 0.1 × 0.9 in the groundmass. It ranges in composition from oligoclase to andesine (An 13-38 ) and shows zoning and partly altered to saussurite. Riebeckite occurs as subhedral phenocrysts and attains 0.2-3.1 mm in length and 0.3-1.2 mm in width. The pleochroic formula is X = dark blue, Y = light blue, and Z = greenish (Figure 3d). Opaques are rare and usually associated with riebeckite.

Basaltic dike
Basaltic dike is composed of essentially plagioclase phenocrysts and groundmass, hornblende, and pyroxene. Apatite, epidote, chlorite, and opaques are accessory minerals. The intergrowth texture is well developed. Plagioclase occurs as subhedral crystals (0.3 mm × 3.5 mm), phenocrysts, and as groundmass (0.2 mm × 0.5 mm) of labradorite composition (An 60-68 ). Plagioclase crystals show intergranular texture and are associated with calcite, chlorite, and epidote. Partial alteration to saussurite is occasionally present. Hornblende exists as fine-grained euhedral to subhedral crystals. It is altered to chlorite and iron oxides. Pyroxene occurs as subhedral to anhedral crystals and is sometimes altered to tremolite and actinolite. Pyroxene crystals are intergrown with plagioclase laths forming intergranular and subophitic textures. Apatite occurs as elongate euhedral to subhedral crystals, which occur either as independent crystals or incorporated within hornblende and plagioclase. Epidote occurs as an anhedral crystal associated with chlorite. Chlorite occurs as an anhedral phenocryst of pennate type (Figure 3e). Opaques occur as anhedral crystals and are associated with hornblende and pyroxene.

Dolerite dike
It composes of plagioclase, hornblende, and pyroxene in addition to trace amounts of biotite, opaques, and sphene. Occasionally, an intergranular texture develops. Plagioclase  is composed of fineto medium-grained subhedral to anhedral crystals (1.1 mm × 2.5 mm) of andesine to labradorite (An 48-56 )composition. It contains albite and albite/Carlsbad twinning, as well as saussurite and calcite. Plagioclase laths have an intergranular pyroxene texture. Hornblende crystals range in size from subhedral to anhedral, measuring 1.1 mm × 2.4 mm. It undergoes partial to full transformation into chlorite at the cleavage planes and crystal boundaries and sometimes displays simple merging. Pyroxene is represented by augite and occurs in crystals ranging in size from subhedral to anhedral to 1.1 mm × 2.5 mm.
Occasionally, augite crystals develop between plagioclase laths, forming a Doleritic texture (Figure 3f). It demonstrates a diuretic-induced modification of tremolite and actinolite. Biotite occurs as subhedral crystals and undergoes a transformation into chlorite and iron oxides. Opaques form anhedral crystals and are seen in association with mafic elements. Sphene crystallizes in anhedral clusters with plagioclase and ferromagnesian minerals.

Geochemistry of dikes
Representative samples from the examined dikes were selected and analyzed for major (%) and trace (ppm) elements ( Table 1).

Geochemical classification and characteristics
The studied dikes are classified based on the total alkalisilica diagram (TAS), as suggested by Le Maitre [23] and Cox et al. [24] (Figure 4a and b). The diagrams clarify that the dolerite and basalt samples fall in the basalt field, while porphyritic dacite samples fall in the dacite field. The other acidic dikes (granophyre, alkaline granophyre, and microgranite) samples fall in the rhyolite field.

Magma type and tectonic setting
On the A-F-M ternary diagram ( Figure 5), the studied dolerite and basalt samples fall within the tholeiitic field. On the other hand, porphyritic dacite, microgranite, granophyre, and alkaline granophyre samples fall within the calc-alkaline field [25]. To illustrate the potassic nature of the studied dikes, the analyses are plotted on the K 2 O vs SiO 2 diagram (Figure 5b) as suggested by Peccerillo and Taylor [26]. It shows that the two samples of dolerite fall in the basalt and medium-k calc-alkaline fields, the majority of samples fall in the basalt field, and a few other samples fall in the medium to high calc-alkaline field. The porphyritic dacite, microgranite, and alkaline granophyre samples fall in dacite and rhyolite, medium-k calc-alkaline fields. The granophyre samples fall in dacite and rhyolite, high k-calc alkaline fields.    : TiO 2 vs Zr diagram for different dikes, after Pearce [28]; symbols as in Figure 4.
the likelihood of these elements being mobilized and implies that these oxides represent magmatic characteristics. Thus, these main oxides may be classified. Zr can be utilized to determine the elemental mobility and to better comprehend magma differentiation. The binary diagrams of Zr-Nb and Zr-Rb ( Figure 7) revealed magmatic tendencies with positive relationships between Nb-Zr and Rb-Zr. Thus, trace elements tend to be the least affected by postcrystallization processes and may thus reflect basic properties. Trace elements seem to be more discriminating than large oxides since they are very sensitive to crystal fractionation, rather than to changes in the fundamental composition. The ternary diagram Y + Zr-TiO 2 *100-Cr after Davies et al. [27] is constructed to show the original differentiation trend of a magmatic suite because Y and Zr are enriched progressively during fractionation like Na 2 O and K 2 O (Figure 8a). The TiO 2 -Zr diagram (Figure 8b), after Pearce [28], shows that basalt and dolerite fall within the plate field, while porphyritic dacite, microgranite, and granophyre fall in the volcanic arc field.

Petrogenesis
The      Figure 11). It is partially or frequently oxidized to hematite (martite). Hematite is steel-gray black with metallic luster in reflected light, with a tendency to a marginal red. Hematite was confirmed by the ESEM-EDX technique and contains 83.95% Fe 2 O 3 ( Figure 12). Apatite is colorless in a thin section, usually found as minute six-sided prismatic crystals, moderate relief, and weak birefringence. It occurs as an anhedral crystal with a subrounded shape and was confirmed by the ESEM-EDX technique ( Figure 13). It contains 25.3% P 2 O 5 , 29.68% SiO 2 , and 27.68% CaO. The copper mineral occurs as an anhedral      Figure 15). It contains 87.58% Fe 2 O 3 . Titanomagnetite is also occurring as euhedral to subhedral minerals. It is less abundant than magnetite and confirmed by the ESEM-EDX technique ( Figure 16). It contains 58.40% Fe 2 O 3 and 7.77% TiO 2 . Sulfide minerals are represented by pyrite, which occurs as cubes and crystalline in the cubic system. It is brass-yellow with a metallic luster in reflected light. It was confirmed by the ESEM-EDX technique ( Figure 17)  It is yellow to brown in color, greasy to earthy luster, yellowish-brown streak, nonmagnetic unless heated, perfect cleavage in one direction, and partially soluble in hydrochloric acid. Goethite was confirmed by the ESEM-EDX technique ( Figure 18) and contains 88.13% Fe 2 O 3 and 6.45% SO 3 ( Figure 19).

Conclusion
The examined area is split by prominent clusters of postgranitic dikes, which create separate and lengthy subparallel dike swarms. The directional trend study of post-granitic dikes reveals three principal trends in terms of quantity and length proportions: N (33-56) E with tensional regime N (34-57) W, N (57-76) E with tensional regime N (14-33) W, and N (12-32) E with tensional regime N (58-78) W. These dikes are classified into acidic (porphyritic dacite, microgranite, granophyre, alkaline granophyre) and basic (basalt and dolerite) dikes. The acidic dikes show quite different contents in plagioclase, quartz, alkali felspar, and biotite. On the other hand, basic dikes are represented by pyroxene, hornblende, plagioclase, and iron oxides as well as the opaque minerals relatively more abundant in the basalt dike. The basic dikes have medium k-characters, originating from tholeiitic magma and extruded within a plate environment. The acidic dikes have medium to high k-characters, originating from calc-alkaline magma and extruded in a volcanic arc environment. The opaques in basalt dikes are represented by magnetite, hematite, apatite, and copper minerals, while dolerite dikes include magnetite, titanomagnetite, pyrite, and goethite minerals.