Geochemistry,radioactivity and magmatic evolution of subduction and A-Type granites from wadi khashab,southern nubian shield,Egypt

Abstract

The Wadi Khashab area in the southern Nubian Shield of Egypt records the transition from subduction-related to post-collisional magmatism, representing a major phase in the evolution of the Arabian-Nubian Shield. Understanding the geochemistry of uranium (U) and thorium (Th) is crucial for tracking magmatic and post-magmatic processes, as these radioactive elements provide valuable insights into crustal evolution. This study integrates field observations, petrography, whole-rock geochemistry, and gamma-ray spectrometric analyses of U and Th to clarify the petrogenesis and geodynamic setting of the granitic rocks. Two distinct magmatic pulses were identified: an older calc-alkaline suite (quartz diorite, tonalite, granodiorite) with low high-field-strength elements (HFSE: Nb = 8-20 ppm, Zr ∼200 ppm) and I-type characteristics, formed in a volcanic arc; and a younger suite of A-type biotite granites and altered varieties (albitized and greisenized), characterized by high SiO₂ and enrichment in incompatible elements. Geochemical modeling suggests that the older suite originated from the hybridization of mantle-derived melts with mafic lower crust, while the younger suite was derived from partial melting of the crust. U-Th geochemistry indicates that magmatic fractionation was overprinted by hydrothermal fluids, leading to significant uranium enrichment in the altered granites, with average contents of 16.2 ppm in albitized and 13.6 ppm in greisenized varieties. These altered granites are classified as uraniferous, with U and Th mainly hosted in resistant accessory minerals. This study links U-Th mobility to specific post-emplacement alteration events, providing new insights into post-accretionary fluid-driven processes in the Nubian Shield and highlighting the area's potential for uranium mineralization.

Keywords

Wadi Khashab El Kharite Shear Zone A-type granite radioactivity

Introduction

Granitoid plutons constitute a significant portion of Egypt's basement complex, accounting for approximately 40% of the exposed Precambrian rocks in the Eastern Desert. Egyptian granitoids are generally classified into three main types: older synorogenic, younger late-orogenic, and post-orogenic granitoids, which include alkaline granites (i.e.,^1–5 Furthermore, there are two main phases in the tectono-magmatic history of the Egyptian basement during the Pan-African Orogeny. The older phase includes the so-called “arc granitoids” that span from 600 to 900 Ma. It is characterized by extensive intrusions of basic and intermediate compositions in addition to their extrusives. There is a progressive evolution from tholeiitic magmas in an immature arc to calc-alkaline series in a mature arc. These rocks formed in a compressive tectonic setting. The younger cycle spans the 600-530 Ma time interval. The transition from a compressive to an extensional regime defines this cycle. The so-called “late- to post-collisional granites” belong to this cycle. The genesis of these granites is attributed to either the late differentiation of the calc-alkaline magma series near the active continental margin^6,7 or due to late to post-collision granites formed by large-scale anatectic melting during the climax of the Pan-African orogeny.^8–10 The tectonic setting of the older granites remains debated. Hussein et al.,⁸ Stern et al.,¹¹ and Stern and Hedge¹² proposed a tectonic regime over the old Benioff zones for the older granitoids associated with subduction, whereas Abdel Rahman,^13,14 Ragab and El Alfy,¹⁰ and Takla et al.¹⁵ suggested an island arc tectonic environment. On the other hand, El Gaby et al.⁶ and El Gaby⁷ considered the ancient granites to represent a type of cordillera originating on an active continental margin. Recently, El Sayed¹⁶ classified Egyptian granitoids into two types: A-type granites associated with anorogenic rifts and I-type granites associated with orogenic arcs. The former, which are calc-alkaline are further subdivided into normal and highly fractionated varieties.

Although numerous studies have investigated granitic rock intrusions in the Arabian–Nubian Shield, the geochemical behavior of uranium and thorium, and their role in constraining magmatic and post-magmatic processes, remain insufficiently addressed. This gap limits the understanding of how these radioactive elements record the transition from subduction-related to post-collision granitic magmatism. This study focuses on the field relationships, petrography, geochemistry and radioactivity of granitic rocks specifically arc and late-collision granites around Wadi Khashab, between latitude 24° 00^׀–24° 15 ^׀ N and longitude 34° 00 ^׀ −34° 30 ^׀ E (Figure 1). This work aims to explain the geodynamic evolution of these rocks, with particular emphasis on uranium–thorium variations as indicators of magmatic fractionation and hydrothermal mobility. Through examining of these rocks, we seek to understand their petrogenesis, geodynamic evolution, and relationship to major accretion structures in the Nubian Shield.

Figure 1.

Geologic map of main rock units in the Eastern Desert after Liégeois and Stern.¹⁷ The dashed black lines represent the boundaries between NED, CED, and SED according to Stern and Hedge¹². The location of the Wadi Khashab intrusions is indicated by the white rectangle.

General Geology of Granite at Wadi Khashab

The granite intrusions in the study area are classified as arc granitic rocks and late-collision granites. The arc granitic rocks include tonalites, granodiorites, and quartz diorites surrounding Wadi Khashab. Quartz diorites form the main intrusions of south of Gabal Homr Akarem, surrounding Gabal Homrit Mikpid, and extending eastward around Gabal El Sheikh (Figure 2). They generally form low hills covered by beds of Nubian sandstone (Figure 3(a)). Tonalite forms modest relief with a gneissose appearance along the southern slope of Wadi El Kharit. Granodiorite forms the main batholiths exposed in the southwestern parts of the Homr Akarem. It appears as boulder-like forms and creates low to moderate intrusions that extend eastward, forming the main body of Gabal Khula and Gabal Miteiweit, it also constitutes minor intrusions along the southern portion of the Wadi El Kharit shear zone (KHSZ) (Figure 2). The contacts of granodiorite masses with the country rocks surrounding Wadi Khashab are sharp and intrusive (Figure 3(b)). They are rich in xenoliths, frequently covered by eolian sands, and highly weathered, displaying spheroidal weathering. The granodiorites of Wadi El Kharit are light grey to reddish-grey, whereas the tonalite is dark grey. These rocks are medium- to coarse-grained and occasionally contain pegmatitic patches rich in plagioclase.

Figure 2.

The detailed new geologic map of the area around Wadi Khashab, and Wadi El Kharite shear zone (KHSZ), South Nubian Shield.

Figure 3.

Field photos showing: (a) a nonconformity surface between late tectonic granites (granodiorite) and the Nubian sandstone along the main asphalt road of Aswan- Sheikh Shadli, (b) a general view showing the main outline and relation between the late and A-type granites of the Homret Mikpid intrusion, (c) an intensively fractured and rough surfaces with high reliefes at Gabel Ghorab Alrayan intrusion, (d) a highly resistant ridges cutting across Gabel Arafite.

Field observations indicate that the younger granite comprises three phases: biotite granite, muscovitized granite, alkali granites and albitized granite. These phases typically appear as solitary, roughly oval, and elongated bodies that align parallel to the strike of regional structures. The contacts of some plutons, especially those located south of Wadi El Kharit, are irregular or semi-circular. Due to the elevated topography, these plutons are erosion resistant and generally have rough surfaces (Figure 3(c)). Additionally, they intersect major fault trends and are crossed by broad Wadis. The dominate fractures in late-collision granites intersect perpendicularly, leading to predominance of a rectangular drainage pattern in the area. The composition of each body is typically homogeneous, with medium- to coarse-grained textures predominating and generally they exhibit sharp contacts with evident thermal effects on the country rocks. The biotite granites Homrit Mikpid represent the first phase, along with a few sporadic minor masses north of Wadi El Sheikh, and Ghorab Rhyan intrusion (Figure 2). West of Gabal Homr Akarem, metasediments are intruded by muscovite-bearing granite, representing the second phase. Plagioclase feldspar, quartz, potash feldspar, and biotite constitute the medium-grained biotite granite at the area. The biotite granite also distinguished by its inequigranular texture and occasionally contains orthoclase phenocrysts up to 1 cm in length. The presence of scattered fluorite and beryl crystals, as well as quartz veins containing both minerals, further characterizes this granite zone.

The main mass of Homr Akarem granite displays zones of metasomatic alteration, that is, greisenization, albitization, and argillic alteration. These zones are found along major fault planes and at the contact between the granite and metasediments. Moving away from these contact zones and fault planes, the degree of modification decreases. The greisenized zone is a yellowish-green rock containing yellow-green mica, quartz, and sericitized feldspar. In contrast, the albitized granite is white, fine-grained, and composed of biotite, quartz, and potash feldspar. The presence of scattered fluorite and beryl crystals, along with quartz veins containing both minerals, is another defining feature of this granite zone (Figure 3(d)). The most prominent set of vein runs in the NW–SE direction. Some veins lack mineralization, while others contain beryl, malachite, fluorite, cassiterite, and molybdenite.

The small, isolated bodies of Gabal Homr Akarem located occasionally at the north and north west parts of the main intrusions (Figure 2), differ significantly from the main mass. They consist primarily of alkali and muscovitized granites, which is composed of quartz, potash feldspar, plagioclase feldspar, and muscovite. Four sets of faults, trending NW–SE, NE–SW, N–S, and E–W, fracture the two masses, with the NW–SE set being the most prominent. Particularly along the fracture planes, feldspars are typically kaolinitized, often followed by iron oxidation (ferruginization).

Methodology

Eleven samples of arc granitic rocks (seven granodiorite, two tonalite, and two quartz-diorite) and 14 of late-collision granites were collected and chemically analyzed for their major oxides and trace elements (Table 1). The fresh granite samples were analyzed at the laboratories of the Nuclear Materials Authority. The major elements were determined on fused pellets prepared according to the method of¹⁸ using lithium tetraborate as a flux. Some trace elements were determined on pressed powder pellets. All XRF elements were calibrated against recommended values of international standards using the data given by Govindaraju.¹⁹ The significant trace elements were determined using X-ray fluorescence method (Philips-PW 1480 x-ray spectrometer X-unique II with automatic sample changer PW 1510); Sc, Hf, Ta, Th and U were analyzed using instrumental neutron activation analysis (INAA). Absolute accuracy was assessed by comparing with international reference materials with <2% variance. The data from chemical analyses, along with CIPW normative values, are presented in Table 1. The rocks under consideration were prepared as 15 sections thin sections and examined using a transmitted-light polarized microscope.

Table 1.

Major oxides, CIPW-norm and trace elements of the arc granitoids of Wadi Khashab area, South Eastern Desert, Egypt.

Rock type	Granodiorite							Tonalite		Quartz diorite
Sample No.	21	39	87	93	1	2	03	81	101	22	152
Major oxides (%)
SiO₂	65.60	66.60	65.70	66.70	67.29	63.52	63.10	64.26	63.80	62.51	62.30
TiO₂	0.46	0.42	0.51	0.40	0.35	0.84	0.94	0.76	0.82	0.88	0.92
Al₂O₃	15.40	15.30	15.35	15.20	15.20	15.96	16.10	15.90	15.94	16.00	16.10
Fe₂O₃	2.20	2.10	2.25	2.05	2.05	1.48	1.60	1.42	1.45	1.70	1.65
FeO	1.32	1.30	1.25	1.26	1.20	3.07	3.10	2.98	3.02	3.20	3.15
MnO	0.06	0.07	0.06	0.07	0.08	0.28	0.25	0.25	0.26	0.33	0.40
MgO	3.28	3.14	3.25	3.32	3.15	3.85	3.90	3.76	3.80	4.00	3.95
CaO	3.92	3.81	3.85	3.75	3.47	5.15	5.23	5.03	5.10	5.40	5.35
Na₂O	3.82	3.70	3.90	3.76	3.90	3.35	3.30	3.45	3.40	3.20	3.25
K₂O	2.26	2.24	2.34	2.29	2.30	1.25	1.22	1.35	1.30	1.10	1.15
P₂O₅	0.42	0.40	0.41	0.38	0.38	0.60	0.65	0.59	0.60	0.42	0.40
LOI	0.91	0.80	0.84	0.76	0.75	0.85	1.02	0.89	0.90	1.10	0.95
Total	99.65	99.88	99.71	99.94	100.12	100.2	100.41	100.64	100.39	99.84	99.57
CIPW norms
Q	22.80	24.87	22.26	24.20	22.66	22.51	21.84	21.29	24.11	22.53	22.50
Or	13.54	13.37	14.00	13.66	8.01	7.73	6.59	6.90	13.65	7.44	7.26
Ab	32.70	31.56	33.34	32.04	29.23	28.88	27.39	27.85	33.08	28.50	28.06
An	17.21	16.72	16.90	16.52	21.50	21.91	24.56	24.55	16.39	22.19	22.28
Cor	0.43	0.71	0.27	0.52	0.17	0.94	0.62	0.63	0.30	1.01	1.23
Di	–	–	–	–	–	–	–	–	–	–	–
Hy	8.30	7.01	8.22	8.46	12.93	13.04	13.80	13.70	7.98	13.26	13.11
Ol	–	–	–	–	–	–	–	–	–	–	–
Mt	3.16	3.07	2.78	3.00	2.06	2.11	2.50	2.43	2.98	2.16	2.38
Il	0.89	0.81	0.98	0.77	1.45	1.57	1.69	1.77	0.67	1.61	1.80
Ap	0.93	0.88	0.91	0.89	1.29	1.32	0.93	0.89	0.83	1.32	1.43
Trace element (ppm)
Rb	39	44	42	46	49	34	30	37	36	25	27
Ba	447	421	430	394	379	518	525	469	478	560	540
Sr	193	174	187	162	143	276	284	228	256	305	297
Ga	21	24	23	25	27	16	15	19	18	12	13
Nb	15	17	16	19	20	11	10	14	13	8	9
Zr	114	137	126	146	167	87	74	105	93	47	56
Y	17	19	18	20	22	13	11	16	14	9	10
Cr	20	18	19	16	14	23	24	21	22	26	25
Ni	8	7	7	6	5	9	10	8	9	12	11
Co	9	8	9	7	6	13	14	10	11	17	16
V	47	45	46	42	36	53	55	49	51	62	58
Cu	52	48	50	45	42	62	65	55	59	70	68
Pb	28	32	30	35	38	20	18	25	22	12	15
Zn	85	98	90	92	105	73	70	81	78	62	65

Sample No.	3	4	9	10	49	50	51	52	53	61	63	67	68	69
Major oxides (%)
SiO₂	73.1	72.8	73.80	73.60	74.20	74.60	74.47	73.60	74.60	72.26	74.90	72.97	73.30	74.47
TiO₂	0.15	0.17	0.13	0.13	0.12	0.09	0.15	0.11	0.12	0.14	0.07	0.11	0.10	0.14
Al₂O₃	14.2	14.50	13.60	14.00	13.10	16.60	13.62	14.20	12.95	14.64	13.51	14.72	14.16	13.41
Fe₂O₃*	1.1	0.80	1.26	1.20	0.80	0.85	0.85	1.20	0.80	0.95	0.87	1.05	0.85	0.86
MgO	0.11	0.14	0.12	0.14	0.14	0.15	0.12	0.20	0.04	0.14	0.14	0.20	0.16	0.18
CaO	1.08	0.78	1.14	1.05	0.90	0.84	0.84	0.95	1.06	0.82	0.67	0.96	0.91	0.66
Na₂O	4.80	4.50	4.20	4.40	4.50	4.28	4.38	4.39	4.27	5.28	4.43	4.39	4.39	4.28
K₂O	4.60	4.30	4.10	4.10	4.64	4.64	4.65	4.40	4.67	3.32	4.56	4.60	4.56	4.78
P₂O₅	0.08	0.20	0.80	–	0.09	0.10	0.09	0.12	0.09	0.14	0.08	0.11	0.09	0.07
LOI	0.62	0.50	0.70	0.75	0.80	0.73	0.71	0.71	0.05	0.71	0.63	0.66	0.76	0.63
Total	99.84	98.69	99.85	99.37	99.29	102.88	99.88	99.88	98.65	98.4	99.86	99.77	99.28	99.48
CIPW norms
Q	25.94	29.36	31.79	30.53	29.66	30.47	29.72	29.47	31.23	27.92	30.51	28.03	28.81	30.15
Or	27.42	25.90	24.64	24.57	27.64	27.68	27.73	26.24	28.01	20.10	27.18	27.45	27.38	28.60
Ab	40.89	38.73	36.06	37.67	38.63	36.48	37.33	37.41	36.6	45.68	37.73	37.44	37.66	36.59
An	3.60	2.75	5.27	4.69	1.95	3.61	3.67	4.04	2.37	3.33	2.88	4.16	4.05	2.90
Cor	–	1.47	0.34	0.62	–	0.21	0.03	0.74	–	1.19	0.23	1.00	0.53	0.13
Di	0.60	–	–	–	0.77	–	–	–	0.22	–	–	–	–	–
Hy	–	0.36	0.30	0.35	–	0.38	0.30	0.50	–	0.36	0.35	0.5	0.41	0.46
Ol	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Mt	1.11	0.81	1.28	1.22	0.81	0.86	0.85	1.21	0.81	0.97	0.88	1.05	0.86	0.87
Il	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Ap	0.18	0.44	0.18	0.22	0.20	0.22	0.20	0.26	0.20	0.31	0.18	0.24	0.20	0.15
Trace element (ppm)
Rb	598	470	387	227	366	322	270	370	140	352	175	245	275	201
Ba	192	38	94	43	34	49	112	30	87	28	82	122	82	129
Sr	12	18	15	14	16	22	28	13	39	16	18	49	18	46
Ga	49	42	13	25	43	43	37	44	24	45	36	34	36	33
Nb	57	80	44	55	105	105	57	88	43	84	66	63	66	58
Hf	11	11	94	11	10	10	10	11	10	10	11	11	11	11
Zr	33	16	12	50	113	113	111	118	212	102	96	133	96	123
Y	116	95	87	90	93	93	110	110	58	95	81	99	81	96
Cr	10	12	12	10	11	11	13	15	8	10	7	12	7	9
Ni	5	5	4	4	5	5	5	5	4	3	6	5	6	5
Co	3	3	3	3	3	3	3	3	5	3	3	4	3	4
V	3	2	2	2	2	2	2	2	3	2	2	2	2	3
Cu	12	11	13	12	10	10	10	10	11	10	12	11	12	11
Pb	9	17	18	14	32	32	18	32	15	41	2	19	22	25
Zn	52	47	134	168	181	181	119	182	75	171	154	112	154	131

A total of 38 samples representing granitic masses in the study area, were analyzed for uranium and thorium content using gamma-ray spectrometric techniques at the Egyptian Nuclear Materials Authority (NMA). The samples were crushed and ground to a grain size of <50 µm, then dried at 80°C to remove moisture. Approximately 300 g of dried samples were sealed in plastic containers and stored for 30 days to achieve secular equilibrium between ²³⁸U and ²³²Th. This set included 15 samples from biotite granite, 5 muscovitized granite, 7 greisenized granite, and 11 albitized granite (Table 2).

Table 2.

Distribution of U and Th in late and post collision granite.

Rock type	Sample no.	eU	eTh	Th/U
Biotite granites	3	13	21	1.6
	4	12	22	1.6
	9	13	20	1.5
	10	16	36	2.25
	53	11	23	2.09
	Average	13	24.4	1 . 8
	49	7	32	4.6
	50	7	32	4.6
	51	12	24	2.0
	52	12	30	2.5
	53	11	23	2.09
	61	6	32	5.3
	63	13	25	1.9
	67	8	24	3.0
	68	13	23	1.7
	69	9	25	2.7
	Average	9.8	27	2 . 7
Muscovite granites	13	2	4	2
	25	5	10	2
	26	2	6	3
	27	1	4	4
	28	3	7	2.3
	Average	2.6	6.2	2.3
Greisenized granites	16	8	16	2
	17	22	30	1.3
	34	10	10	1
	35	14	24	1.7
	43	12	20	1.6
	62	9	30	3.3
	73	20	28	1.4
	Average	13.6	22.6	1.7
Albitized granites	31	9	15	1.6
	32	12	21	1.7
	33	22	30	1.4
	54	20	29	1.4
	55	10	19	1.9
	56	18	27	1.5
	64	28	37	1.3
	75	26	34	1.3
	90	11	20	1.8
	91	7	11	1.5
	92	15	25	1.6
	Average	16.2	23.5	1.4
Av. low Ca-granite*		3.0	17	5.7
Av. granitic rocks**		5.0	18—20	∼4
Average***		1—55	—	—

*Average after Turekian and Wedepohl (1961).²⁰

**Average after Clark et al. (1966)²¹; Rogers and Adams (1969).²²

***Average of albitized granite after Dalkamp (1993).²³

Results

Petrography

Quartz-Diorites

Megascopically, these rocks are coarse- to medium-grained, grey-colored, and display hypidiomorphic to panidiomorphic granular textures. They are holocrystalline under the microscope. Plagioclase, hornblende, quartz, and potash feldspars, along with minor biotite crystals, constitute the major constituents of these rocks. Accessory minerals include zircon, titanite, apatite, epidote, and opaque minerals. Plagioclase appears either fresh or altered, few crystals exhibit oscillatory discontinuous zoning, while other show moderate alteration, especially at the plagioclase cores (Figure 4(a)). Large plagioclase crystals contain fine inclusions of epidote, quartz, apatite, and zircon, and are often corroded by quartz. Hornblende occurs as subhedral to anhedral prismatic crystals or irregular plates. Occasionally, it displays simple twinning and poikilitically encloses plagioclase and opaque minerals. Biotite crystals are subhedral interstitial flakes showing perfect cleavage. Biotite exhibits strong pleochroism, ranging from pale yellow to brown, and commonly engulfs quartz, plagioclase, epidote, and opaque minerals, sometimes, biotite may shows kinking due to local deformation.

Figure 4.

Microscopic photos showing: (a) Plagioclase, hornblende, quartz, and potash feldspars, along with tiny biotite crystals, make up the major constituents of Quartz-diorites, (b) Hornblende, biotite and potash feldspars show a gneissic structure due to being affected by shearing in tonalite, (c) Plagioclases phenocrysts show a cloudy appearance due to saussuritization at tonalite, (d) Plagioclases, quartz, potash feldspars and hornblende main constituents in granodiorites, (e) major components in biotite granites, (f) Quartz, plagioclase and alkali-feldspars form graphic and perthitic textures in biotite granites.

Tonalites

Tonalites are mainly composed of plagioclase, hornblende, biotite, and potash feldspars. Accessory minerals include opaques, sphene, and apatite. These rocks generally exhibit a gneissic structure, reflecting deformation by shearing (Figure 4(b)).Plagioclase often appears cloudy appearance due to saussuritization, which is concentrated in the crystal centers rather than at the outer rims (Figure 4(c)). Quartz forms anhedral crystals with irregular boundaries, either occurring as interstitial fillings between other minerals or as minute grains within hornblende, plagioclase, and biotite. Hornblende occurs as subhedral crystals. It displays simple twinning with varying interference colors, while some crystals show mild chloritization. Biotite occurs as subhedral flaky crystals, poikilitically enclosing quartz and plagioclase, and undergoes partial chloritization along the cleavage planes.

Granodiorites

The rock is composed of plagioclase, quartz, potash feldspar, and hornblende, with opaques, apatite, and epidote as accessory minerals. Most plagioclase crystals are slightly to moderately altered to saussurite and sericite. Intense alteration is observed, particularly at the cores of the crystals, while fresh crystals display albite carlsbad and pericline twinning (Figure 4(d)). Quartz occurs as anhedral crystals with irregular boundaries. Secondary quartz fills the interstices as a result of the recrystallization of primary quartz.

Biotite Granite

The granitic rocks of the Homr Akarem, Um Had, and Homrit Mikpid plutons exhibit considerable similarity in mineralogy, texture, and grain size, although minor differences in mineralization are observed. These rocks are leucocratic, commonly displaying equigranular aggregates of medium grain size (average 3-7 mm). The dominant texture is hypidiomorphic, while subordinate textures such as perthitic, poikilitic, and graphic are also common. Accessory minerals include zircon, apatite, fluorite, beryl, cassiterite, and titanite, along with sparse iron oxide grains (Figure 4(e) and (f)). Common alteration products observed include sericite, chlorite, and kaolinite.

The Altered Granitic Varieties

The biotite granites are cut through by pervasively albitized and greisenized zones. The alteration sequence may involves K-metasomatism, greisenization, muscovitization, fluoritization, or argillic alteration (i.e., kaolinization). Amazonitized granite is characterized by an abundantly developed bluish-green microcline, which imparts a pale to deep green color to the rock. The original texture is partially to completely obliterated. The amazonite displays cross-hatching and pericline twinning of the preexisting replaced albite (Figure 5(a)), with zinnwaldite, mica, and spessartine garnet replacing preexisting minerals. Accessory minerals include fluorite, topaz, columbite, and zircon. Albitized granite is greyish and exhibits an aplitic appearance. Albite occurs as clots of laths aggregated near the grain core or periphery, but not in the concentric orientation typical of magmatic “snow-balled” quartz (Figure 5(b)). It is pronounced at the apical portion of the Homr Akarem granite, whereas in Homrit Mikpid granite, it is localized and controlled by the fracture systems.

Figure 5.

Microscopic photos (a–d) show: (a) The development of large microcline (Mc) crystals enclosing finer quartz and albite in amazonite granites, (b) major components of albitized granites, the fine albite (Ab) inclusions forming clots and laths that aggregate near the quartz (Qtz) rims, (c) fine-grained crystals of quartz and muscovite (Grz) around feldspar phenocrysts forming greisen granite, (d) the development of coarse grained muscovite flakes engulfing pre-existing felsic minerals in altered granites. (e) a close up view of a polished slab showing the development of sericite clots at the expense of K-feldspar with skeletal quartz in yellowish green greisenized granite.

Greisenized granite consists mainly of quartz, muscovite, potash feldspar, and plagioclase, mostly exhibiting a hypidiomorphic texture (Figure 5(c)). Quartz is interstitial and often shows strain effects, indicated by undulatory extinction, it typically replaces the original feldspar. Potash feldspar is mainly microcline, rarely orthoclase, and is often turbid due to kaolinization and sericitization. Plagioclase, composed of oligoclase (An10), forms idiomorphic crystals and is commonly kaolinized and saussuritized (Figure 5(e)). Muscovite shows a distinctive feature, with fine grains entirely enclosed within larger grains, indicating development by replacement processes. Accessory minerals include cassiterite and fluorine-bearing minerals such as fluorite and muscovitized mica. Muscovitized granite is medium- to coarse-grained and hydrothermally altered, with muscovite replacing nearly all biotite crystals, except for some grains occurring as inclusions in quartz (Figure 5(d)). It is mainly composed of quartz, potash feldspars, plagioclase, and muscovite.

Geochemistry

Geochemical Classification of Wadi Khashab Granitic Rocks

The binary (Na₂O + K₂O) versus SiO₂ discrimination diagram, proposed by Middlemost,²⁴ shows that the studied arc granitic rocks plot in both the granodiorite and tonalite fields, while the late-collision granite samples plot in the alkali feldspar granite and granite fields (Figure 6(a)). On the diagram of Irvine and Baragar,²⁵ all studied granitic rocks fall within the subalkaline series (Figure 6(b)). The chemical data were recalculated in terms of (K₂O + Na₂O)–FeO*–MgO and plotted on the ternary AFM diagram of Irvine and Baragar,²⁵ indicating a calc-alkaline suite (Figure 6(c)). The alumina saturation diagram of Shand²⁶ indicates the peraluminous nature of the Wadi Khashab granites (Figure 6(d)).

Figure 6.

Geochemical diagram showing: (a) the binary (Na₂O + K₂O) versus SiO₂ diagram after Middlemost,²⁴ (b) total alkali silica plot after Irvine and Baragar,²⁵ (c) AFM diagram after Irvine and Baragar,²⁵ (d) alumina saturation diagram after Shand,²⁶ (e) binary plots of (100 Ga/Al) against Zr, after,²⁷ (f) ferroan–magnesian diagram after Frost et al.,²⁸ (g) plot of Na₂O versus K₂O (wt%) after Chappell and White,²⁹ (h) the Rb-(Y + Nb) diagram after.³⁰

CIPW (Cross, Iddings, Pirsson, and Washington) normative calculations were used determine the normative mineral composition of the granitic rocks studied. This quantitative approach allow for comparison of samples and aids in interpreting their petrogenesis. The CIPW norm is a standardized calculation method in igneous petrology that was applied to the chemical analyses of the granitic rocks from the Wadi Khashab area. These results quantitatively represent the ideal mineralogical composition of the rocks based on the bulk major oxide chemistry, allowing direct comparisons between different rock suites.

Arc-Related Granitic rocks (Granodiorite, Tonalite, and Quartz Diorite): felsic mineralogy: These rocks are characterized by a high quartz (Q) content, ranging from 21.3% to 24.9%. Feldspar is dominated by plagioclase, with albite (Ab) ranging from 27.4% to 33.3% and anorthite (An) from 16.4% to 24.6%, indicating an overall sodium-rich plagioclase composition. The potassium feldspar component (orthoclase, Or) is sub-ordinate, ranging from 6.6% to 14.0%. Mafic Mineralogy: The primary mafic mineral is hypersthene (Hy), ranging from 7.0% to 13.8%, reflecting a significant ferromagnesian component. Accessory minerals include magnetite (Mt, 2.1–3.2%) and ilmenite (Il, 0.7–1.8%). Minor corundum contents (Cor, 0.3–1.2%) in some samples indicates a slightly peraluminous nature.

Late-Collision Biotite Granite: Felsic mineralogy: This suite is exceptionally rich in feldspars. Quartz content is also high comparable to the arc-related suite (26.0%–31.8%). However, the feldspar composition is dominated by alkali feldspar: albite (Ab) ranges from 36.1% to 45.7% and orthoclase (Or) ranges from 20.1% to 28.6%. Calcium-rich anorthite (An) is very low (1.9%–5.3%), confirming the highly evolved, potassic nature of this granite. The mafic content is very low, consistent with a highly fractionated felsic rock. Hypersthene (Hy) is present only in trace amounts (0.3-0.5%), and magnetite (Mt) is 0.8–1.3%. Ilmenite is virtually absent. Minor diopside (Di) and very low corundum (Cor) indicate a metaluminous to weakly peraluminous character. Using the Ga/Al versus Zr diagram²⁷ and the SiO₂ wt.% versus FeO/(FeO + MgO) plot,²⁸ the studied arc granitic rock samples (quartz-diorite, tonalite, and granodiorite) plot within I- and S-type magnesium-rich granite fields, whereas the late-collision granite samples plot in the ferroan A-type granite field (Figure 6(e) and (f)). To differentiate between I- and S-type granites, the K₂O versus Na₂O diagram²⁹ is used, it shows that the arc-related granites were derived from I-type protoliths (Figure 6(g)). According to Pearce et al.,³⁰ Rb versus (Y + Nb) plots can distinguish volcanic arc, syn-collision, within-plate, and oceanic ridge granites. The Rb–(Y + Nb) diagram indicates that arc granitic rocks fall in the volcanic arc granite (VAG) field, while late-collision granite samples plot in the within-plate granite (WPG) field (Figure 6(h)).

Spider Diagrams

The normalized geochemical pattern for the average trace elements of the studied late-collision granites is illustrated in Figure 7. The normalization factor used is the hypothetical Ocean Ridge Granite (ORG) after Pearce et al.³⁰ The average trace element contents of the studied late-collision granites are compared with those of the Skaergaard within-plate granites of northeast Greenland,³⁰ the monzo-syenogranite of Gabal Kilkbob,³¹ the monzogranites of the Kilkbob area,³¹ the Qash Amir alkali-feldspar granite,³² the Abu-Tiyur alkali-feldspar granite,³³ and Um Shilman alkali feldspar granite.³⁴ The geochemical pattern shows that the late-collision granites of the study area are broadly similar to the Skaergaard within-plate granites, but they are more enriched in high-field-strength elements (HFSE), such as Nb and Y, and depleted in large-ion lithophile elements (LILE), such as Ba and Rb. Moreover, when compared with the younger granites from other Egyptian localities, the studied granites exhibit a generally similar pattern, but with some notable differences: they are depleted in Rb, Zr, and Y and enriched in Ba relative to the Kilkbob monzogranite; depleted in Zr, Rb, Nb, and Y and enriched in Ba relative to the Qash Amir alkali-feldspar granite; and depleted in Ba, Zr, and Y but enriched in Rb and Nb relative to the Abu-Tiyur alkali-feldspar granite. They are enriched by several mobile elements such as Rb, Th and U similar to Um Shilman alkali feldspar granites According to Pearce et al.,³⁰ such geochemical characteristics indicate that these granites originated from magma sources dominated by mantle-derived melts that underwent crustal assimilation.

Figure 7.

Spider diagram of Normalized trace elements of (a) late tectonized granites, (b) A type granites at Wadi Khashab.

Discussion

Petrogenesis and Magma source

The K–Ba variation diagram³⁵ that is based on the average crustal ratio (K/Ba = 65), shows that arc granitic rock samples (tonalites and quartz-diorites) plot below this value, reflecting both lower K₂O contents and K/Ba ratios <65 (Figure 8(a)). In contrast, late-collision granite and granodiorite samples plot above the average crustal ratio, displaying K/Ba >65. These samples are characterized by Ba depletion along with K₂O enrichment, suggesting the involvement of secondary processes act beside crystal fractionation. This process may promotes Ba depletion during magmatic differentiation. The K-Rb diagram (Figure 8(b)) incorporates the magmatic trend (K/Rb = 1000-200) proposed by Shaw³⁶ as well as the average crustal K/Rb ratios of 250³⁷ or 219 ± 69.⁴⁰ The arc granitic rock samples cluster near the mantle line (K/Rb ≈ 1000), while late-collision granites plot below the crustal line (K/Rb ≈ 250). This pattern indicates that arc granitic rocks were likely derived from source regions depleted in Rb or produced through partial melting within the lower or upper mantle.^41,42 The comparatively lower K/Rb ratios in the late-collision granites reflect more evolved and differentiated granitic melts. The Rb-Ba variation diagram (Figure 8(c)) shows that arc granitic samples are scattered between Rb/Ba ratios of 0.1 to 0.2, while late-collision granite samples cluster above a Rb/Ba ratio of 1.0. The large scatter in Rb/Ba ratios for the arc granitic rocks may suggest inhomogeneous source regions. The late-collision granites show higher Rb and lower Ba contents compared to most arc granitic rocks, possibly indicating magma production at shallow depths and/or contamination with crustal materials enriched in Rb,³⁵ or crystal fractionation involving plagioclase. Thus, the Rb/Sr ratio increases with magmatic differentiation due to Sr depletion and Rb enrichment in the liquid magma resulting from feldspar crystallization. The Sr/Ba ratio in arc granitic rock samples clusters between 0.2 and 0.5, while in late-collision granite samples, it clusters between 0.5 and <0.2, indicating Ba enrichment relative to Sr with increasing fractionation, as Ba is enriched in K-feldspars. The Rb/Sr ratio of arc granitic rock samples clusters between 0.1 and 0.2, while that of late-collision granite samples scatters above 5.0. The Rb/Sr ratio increases with the presence of K-feldspars and decreases with the presence of biotite,⁴³ suggesting pre-existing felsic material in the source region.

Figure 8.

Geochemical diagrams showing: (a) the K% against Rb diagram after Shaw,³⁶ the average crustal K/Rb ratio after Taylor,³⁷ (b) Ba–Rb binary plot after Mason,³⁵ (c) ternary plot of A₁–A₂ granite after Eby,³⁸ ternary plot of Laurent et al.³⁹

Variation diagrams of SiO₂ versus major oxides and trace elements (Figures 9 and 10) support a crystal-fractionation model. Quartz-diorite and late-collision granite represent the compositional end members of the temporal sequence, with granodiorites showing intermediate characteristics. The high concentrations of LILE (Ba and Sr) in the arc granitic rocks reflect an undepleted juvenile source and rule out derivation by anatexis of a melt-depleted lower crust. In contrast, the trace element abundances in the late-collision granites—characterized by pronounced depletion in Ba and Sr and enrichment in Rb with increasing differentiation, together with high Rb/Sr ratios—are difficult to attribute to partial melting alone. The sub-linear to curvilinear trends exhibited by major oxides and trace elements indicate that crystal–liquid fractionation played an important role in their magmatic evolution (Figures 9 and 10).

Figure 9.

Variation diagrams of SiO₂ (wt.%) versus major oxides in the Wadi Khashab granites.

Figure 10.

Variation diagrams of SiO₂ (wt.%) versus selected trace elements in the Wadi Khashab granites.

The relationships shown on the Rb/Ba diagram further suggest that crystal fractionation was the dominant process during differentiation from quartz-diorite to A-type granite. Conversely, the Rb/K₂O diagram points to the involvement of additional processes, such as hydrothermal pegmatitic ± metasomatic alteration, during crystallization. The arc granitic rocks have extremely low Rb/Sr ratios (<0.2), and the low Ni (5-12 ppm) and Nb (8-20 ppm) contents of the arc granitic rocks which further indicate significant fractionation. Therefore, crystal fractionation of a mantle-derived basic magma is considered the major process responsible for the evolution of the arc granitic rocks. Finally, the depletion of Sr and Ba and the enrichment of Rb in the late-collision granites contrasts sharply with the compositional features of the arc granitic rocks. This may suggests that, the biotite granites were generated from a felsic magma in which garnet was absent and residual plagioclase was relatively abundant. The ternary diagram after Eby³⁸ indicates that, the granites studied fall within the A₂-type field, suggesting their formation through the melting of crustal materials (Figure 8(d)). This interpretation is further supported by the ternary discrimination diagram after Laurent et al.,³⁹ which shows that, the quartz diorite–tonalite–granodiorite suite of the older phase were originated from a high-K mafic source, while the biotite granites were produced from metasedimentary sources (Figure 8(e)).

Radioactivity

The relationship between uranium and thorium provides important insights into enrichment and depletion processes. The U–Th scatter diagram for the studied granitic samples (Figure 11(a)) shows that most data points plot between the Th/U = 1 and Th/U = 3 lines. The overall increasing trend suggests that the distribution of U and Th was primarily controlled by magmatic processes. However, the scattering of data points from biotite granite to albitized granite may reflect U redistribution by post-magmatic processes. Typically, Th is approximately three times more abundant than U in natural rocks.⁴⁴ Deviations from this ratio indicate U depletion or enrichment, which is clearly observed in the analyzed samples, where decreasing Th/U ratios correspond to uranium enrichment—particularly in the greisenized and albitized granites.

Figure 11.

Variation diagram of SiO₂ (wt.%) against U, Th, and U/Th at figures (a), (b), and (c) respectively.

The curved, decreasing relationship between Th/U ratios and U further suggests that the distribution of Th and U was largely influenced by magmatic fluids. The relationship between Th and Th/U (Figure 11(b)) also shows a visually decreasing trend in Th/U, indicating minor U enrichment. It is well established that U, Th, Zr, Y, Rb, and Nb behave incompatibly in granitic melts, thus if magmatic processes controlled U concentrations, these elements would be expected to increase accordingly. Based on the above relationships, it is evident that the greisenized and albitized granites of the study area can be classified as uraniferous. Despite the relatively high average equivalent uranium contents of these varieties (13.6 and 16.2 ppm, respectively), no primary or secondary uranium minerals were detected, suggesting that U and Th are hosted in resistant accessory minerals such as allanite, sphene, fluorite, zircon, and apatite. Consequently, uranium mobilization from these minerals is difficult. However, processes such as metamictization and tectonic crushing during and/or after the magmatic stages may have facilitated uranium leaching by hydrothermal solutions along grain boundaries and crystal defects, as well as from U adsorbed onto Fe-oxides and hydroxides.

Tectonic Modeling

Despite extensive studies on the granitic rocks of the Eastern Desert, their petrogenesis and evolutionary stages remain debated. A single petrogenetic model cannot yet account for all Egyptian granitic rocks. Several authors (e.g.,⁵^45–49 have proposed that these granites originated through long-term fractionation of mantle-derived melts. Alternatively, granitic rocks may form by partial melting of diverse crustal components with hydrous fluid contributions from the mantle (e.g.,^4,5,50,51

The studied granitic pulses occur within the central part of the Wadi El Kharite Shear Zone (WKHSZ), a major NW-trending post-accretion shear belt associated with the Najd Fault System.^52,53 The geochemical data support a compressional/subduction-related volcanic arc setting, consistent with the structural position of the granites along the WKHSZ. Major structural trends identified in the field and illustrated on a rose diagram reveal a dominant NW–SE fracture system cutting along the Wadi Khashab granitic intrusions (Figure 12). This fracture pattern corresponds closely with the regional trend of Wadi El Kharite, confirming their tectonic affinity.

Figure 12.

Manual lineament extraction and resulted rose diagram of the studied granites.

Figure 13 illustrates a proposed geodynamic model for the two episodes of granitic rock magmatism in the Wadi Khashab region. During early subduction and collision, primary basic magma ascended and ponded within the lower crust. Slab dehydration was the initial step in the formation of subduction-related granitic rocks at Wadi Khashab (Figure 13(a)), releasing fluids that triggered hydrous partial melting of the mantle wedge. Heat supplied by the cooling ponded magmas elevated the continental geotherm initiating melting of the lower crust and producing quartz-dioritic melts. The hybridization of mantle-derived differentiation products and crustal melts generated blended magmas that ascended and evolved further eventually produce the tonalite–granodiorite associations. The presence of the Wadi El Kharite post-accretion shear zone, along with associated deep seated faults and fissures may have caused rapid pressure reduction and adiabatic decompression, allowing the generation of volatiles and heat from the mantle which enhances partial melting of the lower crust (Figure 13(b)). These deep seated fissures potentially driven by buoyancy forces related to asthenospheric upwelling or crustal delamination -acted as channels for heat transfer. During this stage, lower-crustal rocks, particularly diorite and tonalite may have undergone selective melting triggered by decompression and asthenospheric underplating. The derivative melts subsequently experienced extensive fractional crystallization, producing the biotite granites and other altered younger granite types in Wadi Khashab area.

Figure 13.

A proposed tectonic model shows the development stages of granitic rocks at Wadi Khashab area.

Conclusions

The Wadi Khashab region comprises two distinct granitic phases. The older phase consists of slightly deformed, subduction-related calc-alkaline quartz diorite, tonalite, and granodiorite. The younger phase is made up of massive granite intrusions mainly biotite granites and altered varieties like amazonite and albite granites. The older granitic rocks display the geochemical characteristics typically of I-type granites formed in a volcanic arc setting, with lower concentrations of Hf, Zr, Ta, Nb, and alkalis compared to the younger suite. In contrast, the biotite granites and associated altered rocks are classified as A2-type granites that were emplaced during post collisional stages. The subduction-related granites of Wadi Khashab were likely generated through the interaction of partial melts from the mafic lower crust with differentiation products of slab-fluxed mantle melts. The younger granites and their altered suites were derived from crustal sources that experienced significant magmatic differentiation, strongly influenced by albite- and amazonite-rich fluids migrating along fault zones associated with the Wadi El Kharite Shear Zone. The U–Th geochemical variations observed among the different granitic rock types reflect the combined influence of magmatic fractionation and subsequent hydrothermal mobility. The albitized and greisenized granites exhibit the highest uranium contents (16.2 and 13.6 ppm eU, respectively), while the biotite granites also contain elevated uranium levels, averaging 9.8–13 ppm eU. These values indicate that the study area hosts granitic rocks capable of serving as significant uranium sources. The spatial distribution of uranium-rich granitic rocks, combined with intense alteration and local tectonic activity, has played a major role in controlling the localization of uranium enrichment. Furthermore, the presence of the large Wadi El Kharit shear zone likely acts as an efficient structural trap for uranium accumulation within the region.

Footnotes

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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