Geochemical Constraints on the Evolution of Late- to Post-Orogenic Granites in the Arabian Shield, with a Specific Focus on Jabal Al Bayda Area in the Central Hijaz Region, Saudi Arabia

Abstract

The Jabal Al Bayda, located in the Hijaz terrain of northwest Saudi Arabia, comprises magmatic rocks that represent the ending phase in the Precambrian development of the Arabian Shield. Two granitic suites have been studied petrologically and geochemically, the monzogranite and alkali granite suites, to gain knowledge about their origin and geotectonic implications. The geochemical characteristics of the monzogranites align with their formation in a subduction-related environment. These rocks have a composition that is rich in strontium and barium, and low in rubidium, and displays a high-K calc-alkaline to shoshonitic nature. In contrast, alkali granites typically have lower concentrations of Sr and Ba, and higher rubidium contents. The differences in geochemical composition between monzogranites and alkali granites found in Jabal Al Bayda indicate differences in their origin and geotectonic environment. The evolution of granitoid magmatism in the Jabal Al Bayda area is linked to the Hijaz orogenic cycle, during which northwest-dipping subduction led to the formation of the Midyan, Hijaz, and Jeddah arc assemblage, followed by the collision and accretion of these arcs along the Yanbu and Bir Umq sutures. Due to crustal thickening during the subduction-related stage, the deeper parts of the overlying metagraywackes and metatonalites contribute melt to the early crustal magma, which eventually solidifies to form monzogranites. Later on, during the post-orogenic stage, anatexis of metapelites can occur, leading to the generation of magmas that give rise to alkali granites.

1. Introduction

The properties and geochemical signatures of A-type granites contribute to our understanding of the geological history of certain areas, and the tectonic events that have shaped the Earth’s crust. The various proposed mechanisms, including fractional crystallization of mafic magma from the mantle (e.g., [1,2,3,4]), upwelling of magma from the mantle (e.g., [5,6]), partial melting of lower crustal rocks [7,8,9], and hybrid origins [10,11] highlight the ongoing debate among researchers about the specific processes involved in the formation of A-type granites. By studying the geochemical behavior of A-type granites, researchers can interpret the sources of magmas, the processes of magma formation and differentiation, and the interactions between mantle and crustal components. This knowledge contributes to a better understanding of the Earth’s geological history, the formation of granitic rocks, and the distribution of mineral resources.

The development of the Arabian–Nubian Shield (ANS), a large geological region extending across the Arabian Peninsula and northeastern Africa, has been postulated to have occurred in four main phases [12,13,14,15,16]. During the first stage (~950–850 Ma), oceanic crust and island arcs were formed. The second stage, which took place from about 850 to 650 Ma, represents the collision and tectonic assembly of different crustal blocks, that led to the formation of the ANS. The post-collision stage 3 of the ANS evolution, occurring approximately between 650 and 580 Ma, is dominated by intermediate to felsic calc-alkaline magmatism. It is also characterized by a transition from compressional to extensional tectonics around 600 Ma. Alkaline to peralkaline granites, andesites, rhyolites, and several episodes of dyke swarms were formed during the fourth stage (∼600–530 Ma), which represents late- to post-orogenic intracratonic within-plate magmatism. The specific details regarding the processes involved in generating and emplacing magmas, as well as the timing of transitions between stages are indeed the subject of ongoing scientific debate and research.

During an orogenic cycle, which refers to the formation and evolution of a mountain belt, the composition of magma can change significantly due to various tectonic processes. A notable feature of this cycle is the marked change in the properties of magmatism on a regional scale. Understanding the geodynamic transition between the end of early subduction-related magmatism and the formation of late- to post-orogenic granitic magmas is crucial. To facilitate this understanding and to examine the similarities and differences between the Jabal Al Bayda granitoids and other granitoids of the ANS in terms of their tectonic settings and unique geochemical signatures, we used data from previous studies. The purpose of re-plotting the data is to visually depict the geochemical features of the Jabal Al Bayda granitoids and compare them to other ANS granitoids. In this way, we can identify unique geochemical signatures and recognize the tectonic conditions under which these granitoids were formed. This approach ensures that the present geochemical analysis is relevant and informative as it contributes to the broader research goals of understanding the geodynamic transition and delineating the similarities and differences between the various granitoid suites in the region.

The Jabal Al Bayda area lies within the Arabian Shield (AS), which represents a smaller part of the ANS (Figure 1). The study of the Jabal Al Bayda area can provide valuable insights into the geologic evolution, tectonic processes, and magmatic activity within the AS and, consequently, contribute to our understanding of the broader ANS. In our contribution, we have presented major, trace, and rare earth element compositions of the granitoids of the Jabal Al Bayda area. In combination with published data from similar granitoids in the ANS (Figure 1), the study aims to (1) provide a detailed petrographical and geochemical description of the Jabal Al Bayda granites; (2) constrain the origin of the granitic magma; (3) compare the pre- and post-orogenic granites of the Jabal Al Bayda with alkaline granites of the ANS; and (4) integrate the geochemical data obtained from the studied granites and provide new information on the evolution of the AS.

2. General Geology

During the Neoproterozoic, the dominant geological process in the AS was the accretion of island arcs accompanied by several depositional and tectonic cycles [21,22,23]. Eight distinct accreted tectonostratigraphic terranes have been identified, namely, the western terrains that include Asir (including Jiddah), Hijaz, and Midyan, and the eastern terrains that include Afif and Hail terranes (Figure 1). These terranes are separated by four different suture zones: (1) Yanbu Suture zone that separates the Hijaz and Midyan terranes; (2) Thurwah-Bir Umq Suture zone that separates the Asir and Hijaz terranes; (3) Nabitah Suture zone that separates the Afif terrane from the combined Hijaz-Asir terranes; (4) Al Amar Suture zone that separates the Ar Rayn and Afif terranes.

The classification of granitoids in the AS into three distinct groups was described by [24], providing insights into the geodynamic processes that influenced their formation. The first group is dominated by diorite and trondhjemite, and is characterized by syn-kinematic emplacement between 700 and 900 Ma. The second group is governed by monzogranite and monzonite, and is characterized by late-kinematic emplacement between 620 and 700 Ma. The third group includes intrusives that were emplaced after the major tectonic activity had ceased (540 to 620 Ma), and are dominantly alkaline to per-alkaline in composition.

3. Geology of the Study Area

The Jabal Al Bayda area is located within Al Madinah quadrangle which is covered by Harrat Rahat basalt for about 75% of its outcrops. The other 25% is made up of Precambrian basement rocks occupying the west edge and the northeast corner of the quadrangle, and are represented by distinct units [25]. Three types of rocks have been identified: Precambrian rocks (Al Ays Group and Furayh Group), plutonic rocks, and Tertiary-Quaternary volcanics (Figure 2). Proterozoic rocks are divided into two groups, namely, the Al Ays Group and the Furayh Group. The Al Ays Group is dominated by old volcanic rocks (700–745 Ma; [26]) that include andesite, dacite, trachyte, rhyolite, and derivative epiclastic and detrital sedimentary rocks. The Al Ays Group includes two main formations: the Farshah Formation comprises several thousand meters thickness of andesite and pyroclastic rock, and the Urayfi Formation consists of felsic volcanic rocks intercalated with volcanic tuff, rhyolite, and transported sedimentary rocks. The Furayh Group (630–660 Ma; [27]) unconformably overlies the Al Ays Group, and its lower part, named Qidirah Formation, consists of mafic volcanic rocks that include andesite, basalt, volcanic breccia, and volcanic tuff (Figure 2). The upper part, named Dawnak Formation, consists of sandstone with conglomerate and thin layers of fine-grained greywacke and siltstone. The granitic rocks outcrop extensively in the Jabal Al Bayda area (Figure 2) and are intersected by a group of felsic and mafic dykes (Figure 3a).

Alkali granites are represented by the granitic outcrops of Jabal Al Bayda. Field observations of these granites imply a relatively homogeneous composition and display pinkish to light gray color and medium-to coarse-grained crystals. Recent petrological and geochemical studies of the Jabal Al Bayda granites indicate that they exhibit post-orogenic A-type geochemical characteristics, and have likely formed in within-plate related tectonic settings [28,29].

4. Petrographic Characteristics

Based on petrographical analysis, the Jabal Al Bayda pluton is subdivided into monzogranites and alkali granites (Figure 3a,b). Monzogranites are represented by the Jabal Ad Dulu in the northeastern part, the Jabal Jimmah in the central part, and the Jabal Asfa in the southwestern part of the study area (Figure 2). These rocks are megascopically medium- to coarse-grained and have a porphyritic texture. Plagioclase is the dominant mineral in the rock (35–40% by volume), forming sub-euhedral to euhedral crystals and shows zoning (Figure 3c,d). In places, plagioclase may have undergone alteration, resulting in the formation of epidote and sericite. Quartz is the second dominant mineral, comprising approximately 25–30% of the rock’s volume. K-feldspar (20–25% by volume), specifically microcline, is medium-to-coarse-grained and commonly displays Carlsbad twinning. Biotite is present in the rock as euhedral flakes and elongated aggregates. Myrmekitic intergrowths are a distinctive texture observed in this monzogranite. Zircon, magnetite, monazite, and rare titanite and apatite are present as accessory minerals.

Petrographically, alkali granites are dominated by K-feldspars, which make up approximately 55–60% of the rock’s composition. Orthoclase appears as subhedral to anhedral tabular crystals. Microcline and microcline-perthite are also present in the alkali granites (Figure 3e), contributing to the overall K-feldspar content. They occur as subhedral crystals showing cross-hatch twinning. Alkali granites contain quartz, comprising about 25–30% of the rock’s composition. Plagioclase (oligoclase) is present as compositionally zoned prismatic crystals, constituting approximately 8–11% of the rock’s composition. Biotite is present in minor amounts as small irregular flakes, representing less than 2% of the rock’s composition. Sub-solidus myrmekite, anti-perthite, and graphic textures are seen (Figure 3f). Accessory minerals are represented by zircon, sphene, and iron oxides, while epidote, sericite, chlorite, and kaolinite are the main secondary minerals.

5. Geochemistry

5.1. Analytical Techniques

Fifty samples were collected from various locations within the Jabal Al Bayda area, representing the spatial variability of the granitic rocks. These samples were prepared for petrographic examination in the “Laboratory for Rock and Thin Section Preparation” at King Abdulaziz University. Once a small piece of a rock sample (2–3 cm in length) has been glued to a slide, it can be further cut, ground, and polished to achieve the desired thickness (typically around 30 microns), which can then be examined under a petrographic microscope. Based on the petrographic observations, 25 samples were selected for subsequent whole-rock geochemical analysis. We focused on the analysis of granitic rocks that had undergone only minimal changes in their chemical composition and mineralogy, i.e., samples that showed little or no replacement of the original minerals by secondary minerals due to alteration processes such as argillization or other hydrothermal changes.

The selected samples were sent to the laboratories of ACME Analytical Laboratories in Vancouver, Canada, for geochemical analysis. The major element analysis was performed using X-ray fluorescence (XRF), with an accuracy of ±0.01 wt%. In addition, the number of volatiles in the form of loss on ignition (LOI) was measured with an accuracy of 0.01%. Trace and rare earth elements (REE) were measured using the induced coupled plasma mass spectrometer (ICP-MS) method, with an accuracy of 0.01 ppm to 1 ppm. The obtained geochemical data are presented in

Table 1. Major (wt%), trace (ppm), and rare earth elements (ppm) of selected alkali granites from the Jabal Al Bayda of Central Hijaz Region, Saudi Arabia.

Sample	ST-56A	ST-56B	ST-61A	ST-61B	ST-62A	ST-62B	ST-63	ST-64	ST-65	ST-66	ST-67	ST-78	ST-79	ST-80
SiO2	75.07	74.78	76.57	75.37	76.67	75.37	74.87	77.17	79.96	74.28	72.88	76.17	76.27	77.57
TiO2	0.16	0.11	0.08	0.11	0.12	0.09	0.15	0.11	0.13	0.15	0.06	0.14	0.11	0.10
Al2O3	11.84	11.24	11.48	11.97	11.34	12.01	12.20	11.30	10.19	12.24	12.24	11.56	12.12	10.99
Fe2O3	2.50	3.35	1.87	1.82	2.18	1.93	2.16	1.64	1.06	2.32	1.62	1.44	0.92	1.93
MnO	0.01	0.05	0.03	0.02	0.02	0.03	0.04	0.02	<0.01	0.04	0.04	0.04	0.02	0.04
MgO	0.05	0.08	0.04	0.10	0.04	0.08	0.25	0.04	0.03	0.20	0.13	0.07	0.10	0.03
CaO	0.18	0.43	0.35	0.53	0.30	0.26	0.85	0.31	0.18	0.69	1.86	0.56	0.33	0.52
Na2O	3.64	4.22	4.03	4.11	3.81	4.19	4.05	3.71	3.27	4.07	3.36	4.00	3.84	3.66
K2O	5.06	4.27	4.61	4.60	4.44	4.65	4.25	4.74	4.26	4.74	5.24	4.28	4.61	4.59
P2O5	<0.01	<0.01	<0.01	0.02	<0.01	0.01	0.03	<0.01	<0.01	0.02	<0.01	<0.01	<0.01	<0.01
Cr2O3	0.03	0.03	0.01	0.01	0.02	0.02	0.02	0.01	0.02	0.02	0.01	0.01	0.01	0.01
LOI	0.47	0.57	0.39	0.65	0.52	0.38	0.42	0.44	0.49	0.46	1.97	0.85	0.81	0.43
Sum	99.02	99.12	99.46	99.31	99.45	99.02	99.28	99.48	99.58	99.22	99.42	99.11	99.13	99.87
La	110.27	23.13	44.37	59.42	41.28	38.88	40.98	58.62	62.61	62.91	16.95	59.62	164.41	58.03
Ce	207.48	48.95	95.81	116.15	98.40	80.86	88.13	121.04	137.29	132.20	35.49	124.13	94.91	127.22
Pr	25.58	6.05	12.16	12.66	12.19	11.22	10.59	14.34	16.50	15.34	4.52	16.32	34.58	14.87
Nd	89.23	20.24	43.27	43.27	43.97	36.89	36.29	47.66	58.62	55.43	16.45	52.64	131.70	48.45
Sm	17.53	5.79	11.10	10.49	11.70	11.09	8.58	10.13	12.47	12.53	4.70	15.57	32.85	11.10
Eu	0.24	0.10	0.09	0.18	0.13	0.12	0.40	0.12	0.11	0.26	0.17	0.15	0.49	0.11
Gd	15.63	6.17	10.90	10.45	11.92	11.03	8.34	9.16	10.38	11.78	5.08	14.78	37.27	9.89
Tb	2.36	1.19	1.84	1.91	2.10	2.00	1.52	1.44	1.67	1.99	0.99	2.68	4.55	1.65
Dy	12.36	7.09	10.69	11.46	12.16	12.20	9.50	8.38	9.76	12.64	5.91	16.48	20.19	10.15
Ho	2.42	1.64	2.25	2.49	2.53	2.44	1.86	1.62	1.98	2.52	1.26	3.47	3.49	1.96
Er	7.03	5.13	6.19	6.99	7.59	6.93	5.53	4.86	5.57	7.11	4.06	9.60	8.38	6.16
Tm	1.10	0.87	1.04	1.13	1.10	1.10	0.92	0.69	0.89	1.18	0.66	1.52	1.20	0.99
Yb	7.18	5.66	5.97	7.30	6.71	5.97	5.64	5.07	6.04	7.30	4.57	9.68	7.13	6.30
Lu	1.21	0.98	0.92	1.13	1.09	0.99	0.96	0.77	1.03	1.14	0.69	1.53	1.18	1.06
Y	75.47	46.66	66.00	67.90	73.58	67.50	58.62	49.15	56.83	80.16	39.38	97.81	97.71	55.33
Hf	15.15	8.47	9.17	14.56	12.66	11.47	9.67	9.37	12.66	12.46	4.49	15.15	9.77	10.97
Nb	44.47	48.25	37.29	59.02	45.96	43.97	42.67	33.10	49.45	57.23	24.03	58.92	49.55	43.37
Ta	3.09	3.79	2.39	4.59	2.79	2.79	3.79	2.19	4.09	4.59	2.69	4.39	3.69	2.59
Th	21.63	11.96	12.36	18.05	15.35	15.35	19.14	14.06	17.95	23.83	23.93	21.83	19.04	15.55
Zn	41.87	92.72	88.73	85.74	69.79	61.81	54.84	43.87	11.96	64.81	28.91	47.86	11.96	59.82
Co	1.30	1.00	0.60	1.30	0.80	0.90	1.99	0.70	0.20	1.69	1.60	0.90	0.80	2.39
Ni	5.18	2.89	1.99	2.19	3.99	2.69	4.59	1.79	2.19	2.69	2.69	1.69	1.89	1.60
Ba	50.85	39.88	9.97	108.67	61.81	84.75	253.2	24.93	9.97	140.5	208.3	62.81	109.6	12.96
V	12.96	10.97	10.97	12.96	9.97	7.98	14.96	<8	<8	9.97	10.97	<8	<8	<8
Cu	3.69	6.68	1.89	2.69	3.49	4.79	1.99	3.89	2.79	3.39	2.09	1.50	2.79	1.30
Sr	12.66	10.97	7.68	40.88	31.60	25.12	87.24	15.15	6.98	46.26	80.26	29.41	44.17	5.98
Zr	528.51	257.43	275.87	359.52	370.58	308.37	261.31	322.83	363.61	357.32	92.12	390.43	253.04	307.67
Rb	168.39	183.85	172.98	189.13	148.85	179.76	190.93	150.95	152.84	226.82	191.82	192.22	183.65	155.43
As	1.00	1.20	2.19	1.00	0.60	1.50	1.30	1.69	2.19	1.60	<0.5	5.88	<0.5	6.48
Be	3.99	11.96	3.99	8.97	3.99	1.99	4.99	3.99	2.99	4.99	9.97	2.99	15.95	7.98
Sb	0.10	0.10	0.20	0.10	0.10	0.20	0.20	0.10	0.10	0.20	<0.1	0.20	<0.1	0.20
Sn	7.98	12.96	6.98	6.98	6.98	5.98	4.99	6.98	9.97	7.98	3.99	13.96	4.99	8.97
U	5.18	5.48	5.18	7.28	6.08	6.38	8.28	4.89	5.98	7.58	11.07	9.67	4.09	5.38
W	2.19	<0.5	0.50	1.60	1.50	1.60	1.69	2.39	1.60	1.00	<0.5	3.29	0.80	1.89
Cd	<0.1	0.10	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Cs	1.10	0.90	1.89	1.69	1.20	1.69	1.99	2.09	2.19	4.19	0.90	3.09	1.10	1.60
Ga	23.33	31.21	24.63	26.22	24.23	24.33	22.13	23.13	21.83	25.62	19.84	25.92	24.93	22.23
Mo	5.58	2.89	2.19	2.79	2.89	2.69	2.49	2.49	1.69	2.89	1.79	2.99	1.00	4.69
Pb	11.86	15.85	8.18	5.28	10.77	7.88	12.16	9.27	6.38	9.07	15.45	8.47	4.09	9.67
Ag	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Au	2.69	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	0.80	0.70	1.40	<0.5	1.50	<0.5	<0.5
Eu/Eu*	0.044	0.051	0.025	0.052	0.034	0.033	0.144	0.038	0.029	0.065	0.106	0.030	0.043	0.032
Y/Nb	1.70	0.97	1.77	1.15	1.60	1.54	1.37	1.48	1.15	1.40	1.64	1.66	1.97	1.28
Rb/Sr	13.30	16.76	22.53	4.63	4.71	7.15	2.19	9.96	21.90	4.90	2.39	6.54	4.16	25.98
Nb/Ta	14.39	12.74	15.58	12.87	16.46	15.75	11.26	15.09	12.10	12.48	8.93	13.43	13.43	16.73

and

Table 2. Major (wt%), trace (ppm), and rare earth elements (ppm) of selected monzogranites from the Jabal Al Bayda of Central Hijaz Region, Saudi Arabia.

Sample	ST-74A	ST-75	ST-76	ST-82	ST-85	ST-86	ST-89	ST-90	ST-91	ST-92	ST-94
SiO2	65.10	65.40	68.79	68.59	71.58	71.29	68.00	69.39	69.49	69.39	66.80
TiO2	0.57	0.51	0.53	0.50	0.26	0.29	0.54	0.39	0.36	0.34	0.39
Al2O3	16.43	16.85	14.66	14.52	14.21	14.66	14.55	14.31	14.60	14.63	14.16
Fe2O3	3.19	2.59	2.85	3.07	1.76	2.05	3.44	2.87	2.78	2.72	2.48
MnO	0.05	0.06	0.04	0.06	0.03	0.03	0.05	0.08	0.06	0.06	0.05
MgO	1.24	0.69	1.14	1.22	0.43	0.52	1.19	0.53	0.48	0.45	0.37
CaO	2.41	1.85	2.22	2.27	1.12	1.70	2.40	1.08	1.35	1.03	2.70
Na2O	5.08	5.53	4.57	4.35	4.28	4.15	4.31	4.64	4.77	4.77	4.44
K2O	3.85	4.83	3.70	4.02	4.51	3.86	3.80	4.75	4.69	4.75	4.40
P2O5	0.19	0.12	0.16	0.18	0.07	0.10	0.12	0.09	0.09	0.07	0.08
Cr2O3	0.02	0.01	0.01	0.01	0.01	0.02	0.02	0.01	0.02	0.02	0.01
LOI	1.48	0.81	0.67	0.40	0.88	0.58	0.89	1.10	0.53	0.93	3.22
Sum	99.61	99.25	99.33	99.18	99.14	99.24	99.29	99.22	99.20	99.14	99.09
La	32.50	41.77	30.11	30.51	31.01	24.13	19.94	42.07	33.10	38.48	36.39
Ce	64.61	97.21	58.62	63.01	59.02	49.75	44.77	88.73	73.78	80.46	78.76
Pr	7.41	11.92	6.60	7.51	6.61	5.37	5.37	10.41	8.42	9.01	9.10
Nd	25.32	36.99	22.23	26.42	22.63	16.65	19.94	37.49	29.51	32.60	36.19
Sm	4.37	6.50	3.90	4.78	3.80	2.69	4.12	6.97	5.89	5.53	6.43
Eu	1.31	1.37	1.04	1.10	0.83	0.78	1.01	0.91	0.80	0.79	0.83
Gd	3.18	4.69	2.70	3.50	2.70	1.78	3.67	6.30	5.28	5.23	6.12
Tb	0.44	0.66	0.35	0.50	0.32	0.19	0.59	1.06	1.02	0.97	1.05
Dy	2.06	3.31	1.81	2.44	1.73	0.82	3.43	6.45	6.37	5.41	6.16
Ho	0.42	0.62	0.32	0.49	0.30	0.17	0.74	1.37	1.37	1.14	1.46
Er	1.16	1.82	0.72	1.52	0.70	0.42	2.09	4.15	4.16	3.40	4.10
Tm	0.15	0.25	0.12	0.24	0.11	0.05	0.32	0.65	0.69	0.57	0.65
Yb	1.05	1.63	0.72	1.39	0.70	0.38	2.21	4.69	4.54	3.69	4.27
Lu	0.15	0.24	0.14	0.24	0.09	0.06	0.38	0.75	0.73	0.58	0.66
Y	10.77	17.65	9.37	15.05	8.97	4.39	20.94	39.98	38.68	33.80	40.48
Hf	5.38	7.38	4.69	5.28	4.39	4.79	5.28	10.17	8.87	8.67	9.07
Nb	8.97	10.37	8.18	9.07	12.26	7.28	5.98	9.27	9.77	9.47	8.57
Ta	0.60	0.80	0.70	0.70	0.80	0.40	0.50	0.80	0.90	0.60	0.80
Th	7.58	6.08	8.77	9.67	11.86	7.58	6.88	12.66	10.87	11.07	9.87
Zn	42.87	39.88	26.92	30.91	36.89	37.89	22.93	55.83	27.92	42.87	27.92
Co	6.68	3.19	7.28	6.48	2.59	3.39	7.18	3.19	3.39	3.09	3.09
Ni	11.57	2.29	9.67	8.57	2.69	3.89	10.17	3.29	3.99	3.59	3.29
Ba	1154.53	1409.76	746.75	968.09	823.52	925.22	532.40	372.88	342.97	370.88	330.01
V	47.86	32.90	43.87	42.87	19.94	20.94	42.87	18.94	23.93	23.93	21.93
Cu	6.58	4.09	2.29	2.19	3.99	5.88	7.38	4.59	25.72	6.98	3.59
Sr	837.68	521.03	650.34	674.17	328.41	369.39	259.52	130.01	125.42	132.00	101.20
Zr	200.80	312.46	172.18	178.16	159.82	159.12	213.56	352.44	317.54	302.89	328.71
Rb	68.29	61.12	88.73	94.81	134.50	105.08	98.60	143.07	129.91	127.62	114.36
As	0.90	1.00	1.20	0.50	0.60	1.20	1.10	7.68	<0.5	<0.5	<0.5
Be	<1	<1	1.99	3.99	1.00	1.99	1.00	1.99	3.99	8.97	7.98
Sb	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	0.20	0.50	0.60	0.70	0.10
Sn	<1	1.00	<1	<1	<1	<1	1.99	2.99	4.99	3.99	2.99
U	2.59	1.99	2.59	4.89	2.69	1.60	2.59	4.69	5.08	3.89	2.89
W	<0.5	<0.5	<0.5	<0.5	1.90	<0.5	<0.5	1.00	<0.5	<0.5	<0.5
Cd	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	0.10
Cs	1.00	1.20	1.69	3.29	1.20	1.69	1.40	1.79	3.99	1.99	1.40
Ga	19.84	18.84	19.74	17.35	19.54	19.64	17.55	19.74	18.84	18.44	18.25
Mo	0.80	0.80	1.10	1.30	0.60	0.90	1.20	2.09	2.39	2.69	1.99
Pb	4.49	3.59	3.49	2.59	5.48	3.59	5.48	16.45	9.67	16.65	11.07
Ag	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Au	<0.5	<0.5	0.60	<0.5	6.78	<0.5	1.50	1.40	14.46	11.66	8.67
Eu/Eu*	1.071	0.756	0.976	0.820	0.789	1.084	0.792	0.418	0.437	0.447	0.403
Y/Nb	1.20	1.70	1.15	1.66	0.73	0.60	3.50	4.31	3.96	3.57	4.72
Rb/Sr	0.08	0.12	0.14	0.14	0.41	0.28	0.38	1.10	1.04	0.97	1.13
Nb/Ta	15.00	13.00	11.71	13.00	15.38	18.25	12.00	11.63	10.89	15.83	10.75

.

5.2. Bulk-Rock Geochemistry

The granites examined can be classified using the R1–R2 cationic classification of [30], with the monzogranite samples being assigned to the monzogranite field, with the exception of two samples that are assigned to the quartz-monzonite and quartz-syenite fields (Figure 4). The majority of the alkali granites from the Jabal Al Bayda are plotted within the alkali-feldspar granite field, with one sample that straddles the boundary between alkali-feldspar granite and monzogranite fields, and another one that falls in the monzogranite field (Figure 4).

The granitic samples from the study area exhibit a range of silica (SiO₂) content, the low-silica monzogranite (65.10–71.58 SiO₂ wt%), and high-silica alkali granite that contains 72.88–79.96 SiO₂ wt%. Based on Figure 5, it is observed that the selected major and trace elements exhibit medium to weak correlations with silica (SiO₂) variance in the granite samples. However, there is a definite clustering for different granitic types into two distinct fields. Na₂O, Al₂O₃, TiO₂, P₂O₅, CaO, MgO, Fe₂O₃, Co, V, Ba, Ni, and Sr contents show negative correlations with SiO₂ contents. The values of these elements are remarkably higher in monzogranites compared to alkali granites. Similarly, Ta contents display a negative correlation with silica (SiO₂), while Nb and Sn contents show a positive correlation with SiO₂. The values of these elements are higher in alkali granites (Figure 5). The differences in magmatic trends observed for K₂O, Rb, Y, Hf, Th, and U contents among the different granitic types in the Jabal Al Bayda area suggest that a simple fractional crystallization model is inadequate to explain the origin of the granitoids in this region.

In the classification based on the ACNK versus ANK diagram [31], the monzogranites from the Jabal Al Bayda are classified as metaluminous to weakly peraluminous (Figure 6a). Conversely, alkali granites straddle the boundary between metaluminous and peralkaline fields. Similarly to the alkaline granites and monzogranites from the ANS, the studied granitoids are characterized by an enrichment in alkalis (Na₂O + K₂O = 7.53–10.36 wt%). Furthermore, the data suggest that all the samples are plotted within the alkaline field (Figure 6b). In the K₂O vs. SiO₂ diagram (Figure 6c), these samples are plotted within the field of the shoshonite series. Monzogranites, on the other hand, typically exhibit K₂O/Na₂O ratios less than 1, which aligns with their high-K calc-alkaline nature. However, it is also important to mention that some samples within the monzogranite group are plotted within the shoshonite series. The studied alkali granites and monzogranites are plotted within the ferroan field in the FeO_t/(FeO_t + MgO) versus SiO₂ diagram (Figure 6d). However, a few monzogranite samples are plotted in the magnesian field. The ferroan nature of the studied granitoids is similar to the most of rare-metal granites in the ANS. It is also important to note that there are exceptions within the ANS, where certain intrusions display a magnesian nature, e.g., alkali feldspar granite from the Um Taghir area, Egypt [32] and monzogranite from the El Fereyid area, Egypt [33].

The chondrite normalized REE patterns of the alkali granites and monzogranites are quite different (Figure 7a,b). The REE contents of alkali granites are higher than those of the monzogranites. The two patterns demonstrate fractionation in LREEs and flat HREEs (Figure 7a,b). The alkali granites display a strong negative Eu anomaly (Eu/Eu* = 0.025–0.144, with an average of 0.052), and show similar chondrite-normalizing REE patterns of the ANS alkali-feldspar granites. Conversely, the monzogranites show a low Eu anomaly (average Eu/Eu* = 0.73).

The normalization of trace element concentrations in the alkali granites to ORG values [4] indicates that the alkali granites have broadly similar patterns to those of ANS alkali-feldspar granites. Both are enriched in K, Rb, Th, Ta, and Nb, and have negative Ba and Zr anomalies (Figure 7c). This indicates that crustal materials contributed significantly to the composition of these granites. Concerning monzogranites, as shown in Figure 7d, two groups of elements are observed in these granites and their comparison with ANS monzogranites. The first group of elements consists of large ion lithophile elements (LILE), including K, Rb, Ba, and Th, which are observed to be enriched relative to the ORG values of [4], and display higher concentrations compared to the corresponding monzogranites of the ANS. The second group of elements includes Zr, Sm, Y, and Yb, which are observed to be less than one in most cases. The observed trace element patterns in the studied monzogranites align with those typically found in ANS monzogranites associated with island/continental arc settings.

6. Discussion

6.1. Tectonic Setting

In the Ta, Hf, and Rb diagram of [47], the Jabel Al Bayda monzogranites are plotted within the volcanic-arc granite (VAG, Figure 8a), similarly to the monzogranites from the Wadi Um Sidra area, Egypt [48]. However, most of the alkali granites samples are plotted in the within-plate granite field (WPG), comparable to the alkaline granites of Saudi Arabia (e.g., Jabal Khur Dukhan, Jabal az Zuhd, Al-Hamra, and Al-Ghurayyah areas), and monzogranite from the Wadi Al-Baroud area, Egypt [43], while only one sample of the alkali granites is plotted within the post-COLG setting, similarly to alkali granites from the Wadi Um Sidra area, Egypt [48].

The Th/Ta ratio vs. Yb diagram helps to outline the tectonic environment of magmatism (Figure 8b). It is suggested that the alkali granites formed in an ensimatic rift setting, comparable to the alkaline granites of Saudi Arabia, e.g., the alkali feldspar granite from the Al-Hamra area [39], Jabal Sayid area [40], Idah area [37], and Jabal az Zuhd area [38]. On the other hand, the monzogranites are regarded as subduction-related arc granitoids at an active continental margin setting, similarly to Egyptian monzogranites from the El Fereyid area [33], Wadi Um Sidra area [48], and Wadi Al-Baroud area [43].

The Zr vs. (Nb/Zr)_N diagram enables a clear discrimination between two magmatic series (Figure 8c). The early magmatic series, represented by the Jabal Al Bayda monzogranites, is plotted in the subduction-related field, similarly to the Egyptian monzogranite from the Sukari area [42], Wadi Um Sidra area [48], Um Taghir area [32], and Feinan area, Jordan [41], whereas the late magmatic series of alkali granites falls within the field of intraplate granites, except three samples that belong to the collision-related granites, similarly to the alkali feldspar granites from Saudi Arabia, e.g., the Jabal Sayid area [40], Al-Hamra area [39], and Jabal az Zuhd area [38]; A2 alkaline granites from Sahara area, Sinai [45]; and alkali feldspar granite from Egypt, e.g., the Qash Amir area [44] and Um Taghir area [32]. There are similarities between the studied granitoids and the 559 ± 6 Ma Sukari monzogranite of [51]), the 556–572 Ma Feinan Ghuweir Magmatic Suite of [41], the 593 ± 2.4 Ma Jabal Sayid alkali granite of [52], the 564 ± 140 Ma Jabal az Zuhd alkali feldspar granite of [38], and the 608–578 Ma Sahara alkaline granites of [45]. The presence of two magmatic series may indicate a change in tectonic processes or conditions during the evolution of the magmatic system in the ANS. The observed transition from a subduction-related signature to a collision or intraplate signature from the early to late magmatic series provides valuable insights into the evolving tectonic processes and geodynamic history of the region.

Based on the Nb-Y-3Ga ternary diagram for discrimination of A1- and A2-type granites [2], samples of Al Bayda alkali granites are plotted into A2-type granites (Figure 8d), which is in agreement with the general understanding of the alkaline granites in the ANS, where numerous alkaline granites have been documented to have a crustal origin (e.g., Jabal Sayid and Al-Hamra granites), except one sample that fall in A1-type granite, similarly to the alkali feldspar granite from the Qash Amir area, Egypt [44] and alkali feldspar granite from Um Taghir area [32]. Samples of Al Bayda monzogranites straddle the boundary between A1- and A2-type magmatic suites, similarly to the monzogranite from the Wadi Al-Baroud area [43], Sukari area [42], Feinan area [41], El Fereyid area [33], and the alkali-feldspar granite from Al-Ghurayyah area [40], Qash Amir area [44], and Um Taghir area [32].

Group A1 and Group A2 represent different types of granitoids formed in different tectonic environments. Group A1 granitoids are associated with global continental and oceanic rifting, resulting in horizontal crustal extension and lithospheric plate fracturing. The magmas originate from the melting of the subcontinental lithospheric mantle and lower crust. Group A2 granitoids, on the other hand, form in local expansion zones in intracontinental and continental margin areas. They are the result of the expansion and thinning of the continental crust, with the gaps being filled by upwelling magmas originating from the mantle [53]. The Jabal Al Bayda A2 monzogranites, typically derived from subduction-related processes (Figure 8c), may interact with the crustal rocks in the collision zone. This interaction can lead to mixing of magmas and crustal material, resulting in the formation of granites with anorogenic properties. The Y/Nb ratio is a geochemical parameter used to evaluate the association of granitic rocks with crustal processes. A Y/Nb ratio above 1.2 is often considered to indicate an influence of the crust on granite formation. In the case of the Jabal Al Bayda granites, as mentioned in Table 1 and Table 2, the Y/Nb ratio indicates an association with crustal processes.

6.2. Petrogenesis of the Jabal Al Bayda Granitic Magma

Several models have been postulated to explain the petrogenesis of the granitic rocks in the ANS. These alternative models include (1) the fractional crystallization model, as proposed by [54,55,56,57], and other researchers, which suggests that the granitic rocks in the ANS formed through the process of fractional crystallization of mantle-derived magmas; (2) the magma mixing model, as suggested by [58], which proposes that the granitic rocks in the ANS formed through mixing of felsic and mafic magmas; and (3) the partial melting model, as proposed by [59,60,61,62,63,64], and others, which suggests that the granitic rocks in the ANS formed through partial melting of either an ancient, enriched lithospheric mantle or a relatively young and less evolved crust. By examining the geochemical characteristics of the Jabal Al Bayda granitic rocks, it would be possible to assess which of these alternative mechanisms best explains their petrogenesis.

The high silica contents (average of SiO₂ is 68 wt% and 76 wt% for monzogranites and alkali granites, respectively) and enriched and fractionated LREE patterns observed in the Jabal Al Bayda monzogranites and alkali granites are inconsistent with a model of fractional crystallization of mantle materials. The Rb/Sr ratio is a geochemical indicator that can be used to distinguish between mantle-derived magmas and those derived from continental crust. Mantle-derived magmas typically have very low Rb/Sr ratios (Rb/Sr < 0.1; [65]), while continental crustal rocks tend to have higher Rb/Sr ratios [66]. In the case of the Jabal Al Bayda granitic rocks, the average Rb/Sr ratios of 0.53 for the monzogranites and 10.51 for the alkali granites, as reported in Table 1 and Table 2, are higher than the Rb/Sr ratios expected for mantle-derived magmas. These elevated Rb/Sr ratios are more consistent with a continental crustal source rather than a mantle source (Figure 9a).

The Nb/Ta ratios and Y/Nb ratios further support the model of the Jabal Al Bayda granitic rocks originating from the partial melting of a relatively juvenile crust. Mantle-derived rocks typically exhibit higher Nb/Ta ratios (>17.5), whereas continental crustal materials have lower Nb/Ta ratios (<17.5) [66,67]. In the case of the Jabal Al Bayda granitic rocks, the average Nb/Ta ratios of 13.40 for monzogranites and 13.66 for alkali granites, as reported in Table 1 and Table 2, are consistent with crystallization from crustal-sourced magmas rather than mantle-derived sources. Additionally, the Y/Nb ratio can also help in distinguishing between magmas derived from the mantle and those derived from the continental crust. Mantle-derived sources typically have low Y/Nb ratios (<1.2), while crustal sources exhibit higher Y/Nb ratios (>1.2) [2]. In the case of the Jabal Al Bayda granitic rocks, the average Y/Nb ratios of 2.46 for monzogranites and 1.48 for alkali granites, as reported in Table 1 and Table 2, are consistent with a crustal source rather than a mantle source.

The absence of significant evidence of magma mixing in the petrography of the Jabal Al Bayda granitic rocks indeed suggests that magma mixing may not have played a prominent role in their formation. Additionally, the consistent chondrite-normalized REE and trace element patterns observed in all the studied samples, as depicted in Figure 7, further support the idea that the granitic rocks at the Jabal Al Bayda were not formed through a magma mixing process.

Figure 9. (a) Rb/Sr vs. SiO₂ binary diagram. (b) I-, S-, M-, and A-type granite discrimination diagram after [68]. (c) Zr + Nb + Ce + Y (ppm) versus SiO₂, with division line of A-type granites from [68]. (d) Zr vs. SiO₂ discrimination diagram for I-type from A-type granitoid rocks [6]. (e) Molar CaO/(MgO + FeO_total) vs. Al₂O₃/(MgO + FeO_total) (After [69]). (f) Sr–Rb–Ba ternary plot for the studied granites (modified after [70]). Symbols are as indicated in Figure 6.

Figure 9. (a) Rb/Sr vs. SiO₂ binary diagram. (b) I-, S-, M-, and A-type granite discrimination diagram after [68]. (c) Zr + Nb + Ce + Y (ppm) versus SiO₂, with division line of A-type granites from [68]. (d) Zr vs. SiO₂ discrimination diagram for I-type from A-type granitoid rocks [6]. (e) Molar CaO/(MgO + FeO_total) vs. Al₂O₃/(MgO + FeO_total) (After [69]). (f) Sr–Rb–Ba ternary plot for the studied granites (modified after [70]). Symbols are as indicated in Figure 6.

As already mentioned, the geochemical compositions of the Jabal Al Bayda granitic plutons show an evolutionary trend from high-K calc-alkaline to alkaline (shoshonite series) types. In the discrimination diagram by [68], the shoshonite series of the alkali granites and monzogranites are plotted as A-type (Figure 9b–d). The high-K calc-alkaline to shoshonite monzogranites are plotted in A-type field and extend into the fractionated felsic I/S/M-granites field. Therefore, we can divide the Jabal al Bayda granitoids into two groups: the high-K calc-alkaline I-type monzogranites, and shoshonite A-type alkali granites and some of monzogranites. The two groups of granites are clearly distinguished in Figure 9b, in which one can also see a trend from older I-types to younger A-types in the studied region.

By plotting the CaO/(MgO + FeO_total) vs. Al₂O₃/(MgO + FeO_total) ratios of the studied samples (Figure 9e), it is possible to determine the likely source of the magmas that formed the Jabal Al Bayda granitic rocks. Most of the Jabal Al Bayda granitoids are plotted in metapelite and metagraywackes source fields, supporting a continental crustal source. Tonalite and metasediments as crustal sources were also suggested for numerous alkaline A-type granites in the ANS [43,71]. According to the information provided, it can be concluded that the ‘high-Ca’ monzogranite and ‘low-Ca’ alkali granite at the Jabal Al Bayda have different sources and likely originated from distinct types of protoliths. The ‘high-Ca’ monzogranite is interpreted to represent partial melts of magma derived from clay-poor, plagioclase-rich greywackes sources, with a contribution of tonalitic source, while the ‘low-Ca’ alkali granite is inferred to have originated from metapelitic sources (Figure 9e).

The distinctive geochemical features observed in the monzogranite suite at the Jabal Al Bayda, including enrichment in Sr and Ba but depletion in Rb with low Rb/Sr ratios (Figure 9f), are consistent with characteristics commonly found in high Ba-Sr granitoids (e.g., [72,73]). It is evidenced that almost all of the high Ba-Sr granitic intrusions in the world are located on continental margins [74]. On the contrary, the alkali granite shows the lowest strontium and barium values and the highest Rb contents (178 ppm, on average). The variation in Sr, Ba, and Rb contents in the different granitic types can indeed be influenced by the relative contents of feldspars in these rocks. As evidenced by a petrographical study, the monzogranite is enriched in plagioclase, while alkali granite is high in K-feldspar content. Such differences observed in the monzogranites and alkali granites are similar to many alkaline intrusions in the ANS (Figure 9f) and can be attributed to a combination of factors, including the composition of the source materials and the conditions under which melting occurs. Ref. [38] studied the isotopic composition of the Jabal az Zuhd granites, in particular, Sr isotopes, and concluded that it was derived from the partial melting of anhydrous metasedimentary lower crust. Ref. [52] investigated the Jabal Sayid granite and interpreted its source to be the result of partial melting of the lower crust of the juvenile AS. Ref. [75] suggested that the source of the Al Ghurayyah granite could be either the mantle or juvenile crust. In Egypt, studies by [33,43,48] focused on different areas such as Wadi Um Sidra, Wadi Al-Baroud, and El Fereyid. They concluded that the monzogranite and alkali feldspar granites in these areas were likely the result of the partial melting of metasedimentary sources or pre-existing granitic crustal rocks.

From the above discussion, the Neoproterozoic granites from the Jabal Al Bayda area could have resulted from the partial melting of diverse metasedimentary and metaigneous sources with variable amounts of metagraywacke, metapelite, and metatonalite source (Figure 9e). The change in tectonic regime from oblique compression to extension can promote the development of steeply inclined shear extension structures within subducted slabs, which can enhance permeability and facilitate the ascent of magma, potentially leading to the formation of high-K calc-alkaline to shoshonite parent magma and A2-type intraplate granites. The sedimentary component of the source material could have originated from two potential sources: (1) During crustal thickening, lower crustal material can be displaced downwards into the mantle. (2) Some sediments can be incorporated and carried with the descending slab during subduction. These sediments can become entrained within the subduction zone and be transported to deeper levels. The depth to which they are carried can vary but can reach depths of 20–30 km [76].

The findings of [53] suggest that A2-type granites can also form in convergent plate boundary settings that are oriented at an angle, rather than orthogonally, to the direction of plate movement. In such cases, slip movements can occur alongside compression strains, creating a transpression regime. In the post-collision and late-collision settings, this transpressive movement can result in feathering extension structures that align with the direction of plate motion. Previous studies by [77,78] have suggested that the resultant magma from A2-type granites is generated through the melting of lower crustal rocks in extensional settings. This is consistent with the formation of post-orogenic alkaline granites in the Arabian Shield. According to [79], the εNd(T) values of the AS range from approximately +4 to +10. These isotopic values suggest that the formation of post-orogenic alkaline granites in the AS involved partial melting of the lower part of the juvenile crust, with or without direct contribution from the upper mantle material. The studies by [45,52,80] further support the idea that the post-orogenic alkaline granites in the AS resulted from the partial melting of the lower crustal rocks. The published geochronologic data indicate that the transition from compressional volcanic arc settings to extensional intraplate settings likely occurred around 650–630 million years ago (Hassan et al., 2016 [78]). This transition coincided with the initiation of the Najd Fault System and the peak metamorphism of the second metamorphic event in the ANS [81].

6.3. Geodynamic Implications

Refs. [82,83,84,85] interpreted the evolution of the AS in terms of three main stages of development: (1) The Hijaz orogenic cycle occurred from about 950 to 680 Ma, and involved plate subduction, leading to the formation of the volcanic and associated plutonic rocks in the southern AS. (2) The Nabitah orogeny, occurring from 680 to 630 Ma, represents a significant stage in the tectonic evolution of the AS. During this period, smaller tectonic plates within the AS underwent convergence and collision, leading to the formation of mountain belts and associated intense deformation, and metamorphism of the rocks involved. (3) The Najd orogeny stage occurred from 630 to 550 Ma and involved a period of tectonic deformation and magmatism within the AS. Based on geochemistry and strontium isotope initial ratios, ref. [84] concluded that the volcanic and plutonic rocks in the southern AS were developed during the Hijaz orogenic cycle. These rocks were formed within intraoceanic island arcs that were associated with northeast-dipping subduction zones. The collision and accretion of the Hijaz and Asir arcs along the Bir Umq suture represent the last stage of the northeast-dipping subduction and collision process.

A different model that interprets the development of the AS during the Pan-African period was proposed by [85]. It was based on three stages as follows: (1) the accretion of oceanic island arcs stage (950–680 Ma), where the development of the Nabitah mobile belt occurred, is considered a suture zone resulting from west-dipping subduction between two allochthonous plates: the western Hijaz-Asir province and the eastern Afif province; (2) a compressional orogeny stage, during the time frame of 680–630 Ma, which occurred as a result of the collision between the Hijaz-Asir and Afif crustal blocks, as well as between the Afif and Ar Rayn crustal blocks; and (3) the final orogenic phase, which occurred from 620 to 550 Ma in the northern Arabian Shield; this phase is characterized by several key geological processes, including extensive molassic sedimentation, granitic plutonism, and the development of the Najd faulting.

In the context of the Jabal Al Bayda area, it is crucial to understand the geodynamic transition that occurred between the termination of subduction-controlled magmatism during the late stages of the East African Orogen and the appearance of post-orogenic granitic magmas. Geochemical similarities between the studied monzogranites and the alkaline stage I (609–602 Ma) of [45] and the 620–600 Ma Idah Suite of [86] suggest a late-orogenic magmatism producing the Jabal Al Bayda monzogranites that occurred at this time. On the other hand, geochemical similarities between the Jabal Al Bayda alkali granites and the 572–556 Ma Feinan A-type granites of [41], the 608–578 Ma alkaline stage II granites of [45], and the 564 ± 140 Ma Jabal az Zuhd granite of [38] suggest a post-orogenic magmatism producing the Jabal Al Bayda Alkali granites that occurred at this time.

Based on the regional geology and magmatic evolutionary history, it is suggested that the Jabal Al Bayda monzogranite was formed through the partial melting of a metagraywacke source within a subduction setting (Figure 10A). This late-orogenic magmatism is believed to have occurred in the range of 620–600 Ma, corresponding to the post-collision stage 3 of the ANS evolution. It follows the accretion of juvenile arc terranes and the formation of volcanic and stratigraphic successions associated with the Al Ays group (745–700 Ma) and the Furayh group (660–630 Ma). The high Ba-Sr signatures of the monzogranite are in agreement with an enriched source related to the partial melting of metagraywackes and metatonalites in a subduction zone. The Jabal Al Bayda A2-type alkali granite shows low Ba-Sr signatures and high Rb content, suggesting a different source and tectonic setting. It is proposed that this alkali granite was formed through partial melting of a metapelitic source in a post-collisional setting (Figure 10B), corresponding to alkaline to peralkaline granite magmatism during the fourth stage of the ANS evolution (∼600–530 Ma).

7. Conclusions

The granitoids of the Jabal Al Bayda area can be classified in terms of petrographic observations and geochemical data into monzogranite and alkali granite. The geochemical characteristics of the monzogranite, including high Ba-Sr signatures, high-K calc-alkaline to shoshonitic composition, and metaluminous to mildly peraluminous nature, suggest its formation in a subduction-related setting. The Jabal Al Bayda alkali granites exhibit different geochemical characteristics compared to the monzogranite. They have lower contents of Ba and Sr and a higher content of Rb. These features, along with their A-type characteristics, suggest a post-orogenic magmatic origin. The composition of the Jabal Al Bayda granitoids, including both the monzogranite and alkali A-type granites, are similar to many other granites found in the ANS. These Neoproterozoic granites are supposed to have formed by the partial melting of a heterogeneous metasedimentary source, consisting of variable proportions of metapelites and metagraywackes, with little contribution of metatonalitic source.

It is concluded that the studied Jabal Al Bayda plutons were emplaced in a single orogenic cycle that encompasses the Pan-African orogeny. This implies that the emplacement of the plutons occurred during the late stages of island-arc accretion and extended into the post-orogenic period. The transition from island-arc accretion to post-orogenic events, followed by orogeny relaxation and the shift to anorogenic tectonic processes, suggests a complex and dynamic geological history. The development of the Najd fault system likely played a significant role in influencing these tectonic changes and the subsequent emplacement of the Jabal Al Bayda plutons.

In connection with the regional geology and the magmatic evolutionary history, we assume that the Jabal Al Bayda monzogranite was formed by the partial melting of metagraywackes and tonalites in a subduction-related environment. The time of formation of these granites is attributed to late-orogenic magmatism (620–600 Ma). On the other hand, the low Ba–Sr and high Rb signatures of the Jabal Al Bayda A2-type alkali granite were generated by the partial melting of a metapelitic source in a post-collisional setting. The timing of the formation of these granites is attributed to the late-orogenic magmatism (608–556 Ma), which marks the beginning of extension during the fourth stage of ANS evolution (∼600–530 Ma).