Introduction and scope

Current interpretations consider the development of the Gondwanaland as Neoproterozoic continental assembly during the Pan African event initiated shortly after the break-up of supercontinent Rodinia (~ 870 − 550 Ma)1,2,3. The crystalline basement of the Arabian-Nubian Shield (ANS; ~2.7 million km2; Fig. 1a) constitutes the northern segment of this Neoproterozoic Pan-African event2,3,4. The Arabian Shield, situated in the western part of the Arabian Peninsula, comprises eight distinct terranes formed through the Late Tonian-Cryogenian accretion (870 − 620 Ma) and the orogenic collision5,6,7, followed by post- to an-orogenic magmatism associated with local basin developed at Cryogenian-Ediacaran period. The easternmost terranes are suggested to be the most recently formed in the Arabian Shield2,8, including Afif (700 − 680 Ma), Ad-Dawadmi (615 Ma), Ha’il (740 − 683 Ma), and Ar Rayn terranes (690 − 615 Ma; Fig. 1b).

The crustal evolution of these terranes originated in the Cryogenian (740 Ma) as an arc intra-oceanic accretion9,10,11,12 within the framework of the East African Orogen and closing the Mozambique Ocean13. Within the context of the East African Orogen, various deformation patterns and orientations are discussed, along with the tectonic evolution14,15. The initial magmatic evolution of the upper crust preserved the tectonic evolution of the East African Orogen during this transformative stage13,14,15,16. The tectonic models elucidating the evolution of the ANS incorporated the geometric characteristics of the suture zones and the arc-related intrusive rocks. In the eastern Arabian Shield, the collision between the Ad-Dawadmi Terrane in the east and the Afif Terrane in the west gave rise to the formation of the Halaban suture zone and the Suwaj arc. According to Stoeser and Camp17 and Johnson et al.2, ophiolite suture zones are typically used to define the tectonostratigraphic terranes. This paleosubduction zone evolved into a significant north-northwest trending thrust belt (Nabitah suture zone) in the eastern ANS (Fig. 1). The initiation of magmatic events involving arc-related rock suites can be traced back to the early phase of plate convergence, leading to the amalgamation of the Afif and Ad-Dawadmi terranes into a new continental landmass. The Afif Terrane witnessed the Cryogenian-Ediacaran magmatic event, aligning chronologically with the compressional accretionary stage. The distinctive tonalite-trondhjemite-granodiorite (TTG) plutonic assemblages observed in island arcs, predominantly exhibiting a composition ranging from tholeiitic to low-K calc-alkaline, are a manifestation of magmatic processes. These compositions signify the incorporation of juvenile material into the crust through subduction zones at active margins18. The Cryogenian (TTG) plutonic assemblages are likely the magmatic products of west- or southwest-directed subduction19.

The Afif Terrane, as one of the major terranes in the Arabian Shield, can be distinguished into four subterranes due to variations in both ages and provenance20,21,22, and recently proposed to be subdivided into Siham-Khida, Suwaj, and Nuqrah terranes23. The Suwaj Terrane constitutes the easternmost segment of the Afif Terrane, within which the Suwaj suite batholith and locally exposed roof pendants supracrustal Ajal group are identified as the oldest rock type24,25,26. The Suwaj suite, recognized as a middle Cryogenian unit26, has been dated using U-Pb by Stacey et al.24, who determined gabbro and tonalite ages of 695 and 677 Ma, respectively. The U-Pb age of 685 ± 5 Ma obtained by Cole and Hedge25 for the same rocks is deemed reliable considering its consistency with the 40Ar/39Ar dating range (681 − 675 Ma) provided by Al-Saleh et al.27, who interpreted these ages as indicative of cooling processes.

Fig. 1
figure 1

(a) Regional map showing the Arabian-Nubian Shield between continental terranes of East and West Gondwana, after Johnson and Woldehaimanot11. (b) Tectonostratigraphic terranes in the Arabian-Shield (modified after Stoeser and Camp17); the protolith ages for the Arabian-Shield are from Johnson et al.2. This figure has been drawn by using CorelDRAW® Graphics Suite X5.

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The classical definition of adakite rocks related their formation to slab partial melting under high pressure28. Adakites might be the product of complex high-pressure crystallization of mafic magma produced from the mantle in island arc environments29,30,31,32. Within the Arabian Nubian Shield, arc intra-oceanic accretion and arc-related magmatism are common9,10,11. However, records of Adakitic magmatism were limited to the Shaitian tonalite-trondhjemite-granodiorite (TTG) associations and the Dokhan volcanics in the Nubian Shield, in addition to Jebel Tays granites within the Ad-Dawadmi Basin in the Eastern Arabian Shield33,34,35.

Understanding the development of the Arabian-Nubian accretionary orogen requires comprehending the petrogenesis and tectonic imprint of the forming Neoproterozoic rocks. This ongoing study complemented a comprehensive field investigation with detailed petrographic and geochemical examinations to improve the petrogenetic origins and geochemical implications of the Suwaj magmatic events. The aim of this study is (i) to describe the geochemical characteristics of the Neoproterozoic rocks of the Suwaj suite intrusives, (ii) to present current interpretations of geochemical data, and (iii) to test if these intrusions represent arc-related magmatism with adakite affinity or not. The study further seeks to discern the broader implications of these events on the subduction-collision processes occurring within the easternmost segment of the Afif Terrane and its important implications in understanding the geodynamics processes in the ANS. The overarching goal is to refine our understanding of the tectono-magmatic evolution of the local terrane, contributing to the broader vision of the development or reconstruction of the Arabian Shield.

Geologic setting

The Suwaj Terrane lies on the eastern periphery of the Afif Terrane and is situated just to the west of the Al-Dawadmi Terrane as an elongated batholith trending NNW to the west of the Halaban suture (Fig. 2). The study area lies between latitudes 23° 15’ and 24° 30’ N and longitudes 43° 35’ and 44° 25’ E (Fig. 2). The Saudi Geological Survey conducted prior mapping research programs that included the study area36. The Suwaj Terrane comprises diverse Neoproterozoic basement rocks (Fig. 2), including oceanic mafic-ultramafic rocks (the Halaban ophiolite), Cryogenian intrusives (the Suwaj suite), post-amalgamation sedimentary rocks (the Murdama group), and late-Cryogenian to Ediacaran magmatism (late- to post-tectonic intrusive and volcanic).

Fig. 2
figure 2

Simplified geological map of the study area at the Afif Terrane at the eastern margin of the basement of the Arabian-Shield (modified after Delfour et al.26; Johnson36). The geologic map was traced using OLI-8 false color composite image (7–5-3 as RGB, WGS 84 UTM zone 36 N coordinate system; http://earthexplorer.usgs.gov/) processed by ENVI®software (version 5.1) and CorelDRAW® Graphics Suite X5.

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Between the Afif and Ad-Dawadmi terranes, the Halaban suture zone is marked by the Halaban ophiolite, a disjunctive fragmented mafic-ultramafic belt (U-Pb age of 694 ± 8 Ma and 695 Ma)3,24, which extends in the NNW-SSE direction (Fig. 1). Constituting the low-lying ophiolitic rocks, the dark-colored metagabbros with subordinate serpentinites exhibit a width ranging from one to less than two kms.

The western boundary of the Suwaj suite is characterized by an unconformable contact beneath the post-amalgamation sedimentary rocks (Murdama group). Occasionally, the Suwaj suite and the Murdama group appear separated, marked by a sporadic thin band of either the Hulayfah group’s volcanic or granophyric-granodioritic rocks (Fig. 3a). As evidenced by the recorded unconformity surface along the western boundary of the study area, the Suwaj suite predates the deposition of the Murdama group (< 650 Ma) in a superimposed back-arc basin37. Both the Suwaj batholith and the Murdama group have experienced intrusion by several late- to post-tectonic intrusive rocks. Tiny isolated and infrequent exposures of metavolcanic and metasedimentary rocks (amphibolite-grade for the Ajal Group) occurred as roof pendants atop the Suwaj intrusive. These occurrences are classified as supracrustal rocks37,38.

The denomination of the Suwaj Terrane derives from the Cryogenian Suwaj intrusives, which constitute a significant elongated batholithic belt trending NNW and primarily composed of diorite, quartz diorite, granodiorite, and tonalite with less abundance of gabbro occurrences as well as subvolcanic equivalent and sparse dykes39, and scarce of the pyroxenite rocks. Attempting to delineate these units independently during field mapping poses considerable challenges due to the gradational contact of the less common gabbro with the diorite and other Suwaj rock varieties. Consequently, the entire Suwaj suite is distinguished into two associations: diorite-quartz diorite and tonalite-granodiorite25. Although grain size, texture, and relief of these associations are almost indistinguishable, the main distinguishing features between them primarily rely on variations in mafic minerals and quartz contents. These associations are prominently exposed in low- to moderate-relief hills to the west of the Halaban ophiolites (Fig. 3a,b). They predominantly exhibit medium- to coarse-grained textures, with occasional instances of fine- to very fine-grained textures. Typically, the fresh broken surface of the Suwaj suite rocks exhibits a mottled light grey to greenish grey color, which may occasionally be obscured by intense alteration and the presence of secondary minerals at a weathered surface (Fig. 3c). Predominantly, the majority of northern exposures are found to have mafic variations up to gabbroic composition. Within the Suwaj suite, some mineralized quartz veins rich in iron oxide and hydroxide cross the major rocks of the Suwaj suite and have been excavated as previous quarries (Fig. 3d). Notably, well-developed gneissic features of the Suwaj suite appear near the western flank of these intrusive rocks, most of which are periodically covered by the Murdama group. The hardly perceptible volcanic equivalents for the more mafic varieties occurred as roof pendants along the northwestern margin of the Suwaj intrusive37,38, making the Suwaj more of a plutono-volcanic suite.

Fig. 3
figure 3

Field photographs of Suwaj suite at the eastern part of the Afif Terrane. (a) Low lying hills of gray massive medium-grained diorite of the Suwaj suite in Jabal As Sawda domain (Lat= 24°13’16.25"N, Long=45°22’41.01"E). (b) Fresh, pale-gray granodiorite, dissected by mafic veins with alteration, may be pyrolusite (Lat=23°40’2.62"N, Long= 45° 8’39.30"E). (c) The effect of mineralization on the quartz diorite rocks (Lat= 24°24’7.71"N, Long= 44°51’4.87"E). (d) One-meter-thick mineralized quartz vein (N60°W/20°NE) intrudes dioritic rocks (old mine Lat= 24°57’16.02"N, Long= 44°45’13.65"E). (e) Sheared diorite unit of Suwaj suite following the Najd Fault system (Lat=23°51’55.21"N, Long= 45°13’4.93"E). (f) Fresh massive greenish andesite porphyry as a volcanic variety of island arc Suwaj suite (Lat= 24°41’58.81"N, Long= 44°50’36.02"E).

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The diorite-quartz diorite association forms low-relief topography, without distinct sharp contacts, and mainly consists of diorite and quartz diorite varieties, displaying a distinctive spotted grey to greenish grey color. The association is heterogeneous and occasionally exhibits crude banding of diffuse hornblende-rich and hornblende-plagioclase-rich bands, showcasing varied grain sizes and textures. Although the diorite-quartz diorite association predominantly exhibits low-strain, some localized deformation indications, including mylonitized and cataclastic, are visible (Fig. 3e). Spatially, the diorite is exposed as a dark brown or brown-green, medium-grained, equigranular texture or as microdiorite varieties presenting a greenish diabasic appearance with fine-grained amphibole laths (Fig. 3f). Locally, this association intruded by tonalite-granodiorite units of the Suwaj suite, according to field observations and differentiation37.

The second association within the Suwaj suite (tonalite-granodiorite) is exposed as large and uniform bodies of biotite-hornblende tonalite and granodiorite. This association predominantly forms moderate hills that are situated within and west of the diorite-quartz diorite association (Fig. 3b). Due to challenges in independently mapping tonalite and granodiorite, they are combined into a single assemblage. The medium-grained microgranular rock delineates the periphery of the pluton, encircling the coarse-grained material. Generally, the tonalite and granodiorites are whitish grey to dark-grey, coarse-grained, variably strained, highly weathered, and exfoliated. These rocks display localized gradational contact with the diorite and quartz diorite rocks and contain rounded to subrounded xenoliths of diorite and the more mafic rocks.

Petrography

Petrographic examination of the plutonic rocks in the Suwaj Terrane reveals their classification into predominantly diorites, quartz diorites, tonalities, and granodiorites, less common gabbro with scarce of pyroxenite. Gabbro comprises plagioclase and hornblende, with less common clinopyroxene as either single crystals or as remnants in the uralitized hornblende, and opaques. Plagioclase laths enclose partially or completely within the clinopyroxene and the uralitized hornblende. The gabbro suffered a low grade of metamorphism, witnessed by the breakdown of clinopyroxene to hornblende, retrograde hornblende to actinolite and chlorite, and sericitization of the plagioclase. The opaques are fine-grained spread between both plagioclase and mafic minerals. In the more mafic varieties (up to clinopyroxenite), which represented the early magmatic differentiated rock variety, amphibole grains may enclose remnants of clinopyroxene (Fig. 4a). The rock is coarse-grained and composed of clinopyroxene, commonly breakdown to hornblende with some relicts within the hornblende, in addition to fine-grained opaque minerals and sericitized plagioclase accessories.

Fig. 4
figure 4

Photomicrographs of Suwaj suite at the most eastern part of the Afif Terrane. (a) Remnant of clinopyroxene (Cpx) in the amphibole grains in the clinopyroxenite. (b) Poikilitic elongated subhedral amphibole crystal (Amph) in the hornblende gabbro contains inclusions of Plagioclase. (c) Amphibole (Amph), opaque, and plagioclase crystals scattered within much larger hornblende crystal in quartz diorite. (d) severe alteration affecting plagioclase (Pl) and amphibole (Amph) in quartz diorite. (e) Subhedral amphibole grain is intimately associated with deformed quartz and highly altered plagioclase in the tonalite. (f) Quartz diorite is made up mostly of amphibole and extensively altered plagioclase (Pl); coarse euhedral cubic of opaque iron oxyhydroxides (Iox) mineral associated with mafic minerals. (g) Severe alteration affecting plagioclase (Pl) and amphibole (Amph) in quartz diorite. (h) Granodiorite photomicrograph shows well-developed perthitic intergrowth and myrmekitic texture in potash feldspar (Ksp) and plagioclase (Pl) respectively. (i) Oligoclase (Pl) altered into saussuritic aggregates with partial to complete replacement of hornblende by biotite (Bt), with minor amounts of orthoclase and quartz (Qz).

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The diorites primarily composed of plagioclase and hornblende (Fig. 4a). Biotite, quartz, and secondary chlorite occur as minor minerals, whereas titanite, apatite, and opaques are accessories. Plagioclase (up to 60 vol%) forms euhedral to subhedral crystals. Most crystals with indistinct twinning show moderate to intense sericitization, involving the replacement of minerals by sericite, epidote, and calcite. The subhedral pleochroic hornblende occasionally aggregates in clusters (up to 40 vol%) and appears as poorly terminated prismatic or xenomorphic elongate crystals. The large amphibole crystals are poikilitic and may contain plagioclase, apatite, and opaques as inclusions (Fig. 4c). Crystals of quartz are infrequent and typically anhedral with wavy extinction. Large secondary chlorite grains are anhedral and may contain magnetite.

The quartz-diorite has medium-grained, equigranular texture, and primarily consists of plagioclase, quartz, hornblende, and biotite with a subordinate amount of orthoclase, as well as opaques, titanite, zircon, and apatite as accessory minerals. Plagioclase composition ranges from oligoclase to andesine. The plagioclase crystals range from xenomorphic to idiomorphic shapes and display moderate to intense alteration (Fig. 4d). Hornblende occasionally aggregate in clusters. Some crystals poikilitically enclose apatite and opaques. Chlorite is the main alteration product. The mafic component of the rock exhibits variability, where hornblende predominates as the primary mafic mineral, while biotite assumes this role in more acidic varieties. Quartz occurs as interstitial anhedral crystals filling the spaces between the early-formed minerals. They range from a few percent up to 10% of the rock’s composition (Fig. 4e). Biotite occurs as subhedral to anhedral flakes and is strongly pleochroic from pale brown to dark brown and shows an alteration to green chlorite and iron oxyhydroxides. Opaque minerals (up to 5 vol%) occur as interlocked aggregates or as coarse euhedral cubic crystals of magnetite associated with mafic minerals (Fig. 4f).

Tonalite is composed essentially of plagioclase, quartz, hornblende, biotite, and minor accessory zircon, opaques, and apatite. Secondary minerals assemblages comprise muscovite, chlorite, epidote, sericite, kaolinite, and saussurite. Saussuritization predominantly affects the calcium-rich core of plagioclase, with varying degrees of alteration detected within grains. The majority of plagioclase crystals are frequently corroded with quartz, biotite, and hornblende. Quartz occurs as either coarse-grained xenomorphic crystals or drop-like inclusions within both plagioclase and hornblende. Quartz crystals enclosing sphene, apatite, zircon, and opaques are also encountered. Hornblende occurs as prismatic greenish brown subhedral poorly terminated crystals forming twined grains, which are locally and slightly altered. Biotite is found as scattered subhedral flakes and plates in association with hornblende. Most of the biotite flakes enclose primary accessory minerals such as zircon, titanite, iron oxyhdroxides, and apatite. Biotite is partly or completely altered to chlorite and epidote with the liberation of iron oxides along the cleavage planes. Epidote, chlorite, and opaque minerals are the main decomposition products of mafic minerals and are usually associated with amphibole minerals or embedded within their crystals (Fig. 4g). Epidote may fill the cracks as secondary mineral veinlets of filling material.

Granodiorite shares similarities with tonalite. However, it can be distinguished by a higher content of alkali feldspars (< 10 vol% of total feldspar in tonalite). They are essentially composed of plagioclase, quartz, potash feldspar, and biotite with subordinate amount of hornblende. Iron oxyhydroxides, apatite, zircon, and titanite are primary accessory minerals, whereas chlorite, epidote, saussurite, muscovite, and calcite are the secondary minerals. Quartz occurs as either interstitial grains between other constituents or poikilitic inclusions within the feldspar and biotite. K-feldspars occur as subhedral and anhedral microcline crystals, in addition to a subordinate amount of orthoclase crystals. Perthitic and myrmekitic textures are the two types of intergrowth textures (Fig. 4h). Saussurite and epidote are the main alteration products of plagioclase (Fig. 4i). Biotite occurs as subhedral to anhedral flakes with partial alteration to chlorite. It commonly possesses inclusions of iron oxyhydroxides, zircon, apatite, and titanite. The investigated biotites are partly altered to chlorite, releasing iron oxides, especially along cleavage planes.

It is worth mentioning that the gneissic texture is conspicuous in certain samples, characterized by alternating bands of felsic minerals, primarily plagioclase (potentially with quartz), and mafic bands predominantly composed of hornblende, biotite, and chlorite. Feldspar crystals display deformation imprints indicated by bending of their lamellar twinning. Quartz is observed as interstitial elongated grains displaying deformational symptoms clarified by augend and protomylonitic character. Mafic and Secondary minerals clarify the foliation and the deformation microstructures such as bending and fracturing.

Methodology

Thirty-eight fresh samples were carefully selected to avoid the alteration effect. The samples were then crushed and prepared as powder for geochemical analyses. The geochemical analyses of the powdered whole rock samples were conducted at the geochemical laboratories of the Saudi Geological Survey (SGS). Major oxides analyses were made using a fully automatized BRUKER-AXS-S8 TIGER X-Ray fluorescence (XRF), where 1gm of the powdered sample was add to Flux (8 gm of Lithium Tetraborate) in platinum crucible at melting temperature 1000 °C for 20 min to produce a glass disk. Trace and rare earth element concentrations were determined using PERKIN ELMER ICP-OES OPTIM8300DV (inductively coupled plasma spectrophotometer), where 0.5 gm of the powdered samples were digested by mixture of four concentrated acids (HF + aqua regia + HClO4 + HCl) at 96 °C for overnight. Calibration was done using international standards. Loss on ignition is determined by heating known weights of the pulverized samples to 1000 °C for one hour. The followed lithostratigraphic classification of rock suits and formations in the Arabian-Shield basement rocks is related to Johnson26.

Results

Bulk rock chemistry

The compositional analyses of thirty-eight representative samples are listed in Table 1 (Supplementary Data), detailing the major oxides and trace elements integral to elucidating the characteristics of the Suwaj suite rocks. The Suwaj suite shows a broad spectrum of SiO2 contents ranging from ~ 48 to ~ 69 wt% and has a low K2O/Na2O ratio (less than 0.42), with a wide range in FeOt (1.9–12.5 wt%), MgO (0.7–9.1 wt%), CaO (2.7–8.8 wt%), K2O (0.3–3.8 wt%), Na2O (3.1–6.5 wt%), and a narrow range of Al2O3 (12.5–17.8 wt%), MnO (0.1–0.2 wt%), and P2O5 (0.1–0.5 wt%, Table 1; Supplementary Data). Of particular interest, the loss on ignition (L.O.I) values of the selected Suwaj samples do not positively correlate with K2O or Na2O (Fig. 5a,b). This observation indicates that the values of K2O or Na2O have been minimally affected, if at all, by potassic and sodic alteration processes. Thus, they likely represent the geochemical signature of the primary magma.

In the P-Q characteristic diagram after Debon and Le Fort44 (Fig. 5c), the selected samples from the Suwaj suite are plotted between gabbro and granodiorite traversing diorite, quartz diorite, tonalite, and quartz monzodiorite fields. The A/NK vs. A/CNK diagram (Fig. 5d) reveals the metaluminous nature, with A/CNK ranging from 1.06 to 1.14. Additionally, a subtle peraluminous character is observed in only two samples, categorized as granodiorite. The A/CNK [molar Al2O3/ (CaO + K2O + Na2O)] values of all the samples range from 1.1 to 2.3, indicative of strongly peraluminous nature40.

Agpaitic Index (Na + K)/Al (atom %) ranges from 0.105 to 0.730 and reveals a typical calc-alkaline nature. The plotted data exhibits a distribution spanning both low-K and high-K calc-alkaline when presented on the SiO2 vs. K2O classification diagram41,42 (Fig. 5e). Furthermore, the alkali-lime index vs. SiO2 diagram of Frost et al.43, emphasizes the markedly calc-alkalic nature of the examined samples (Fig. 5f).

Fig. 5
figure 5

Geochemical discrimination diagrams of the studied rocks. (a) and (b) Binary diagrams of L.O.I vs. K2O and Na2O. (c) P–Q multicationic plot of Debon and Le Fort44. (d) Alumina index diagram A/CNK-A/NK for all granitoid rocks (after Maniar and Piccoli40). Boundary between I-type and S-type granite is after Chappell and White45. (e) K2O vs. SiO2 classification, after Peccerillo and Taylor46. (f) Na2O + K2O–CaO vs. SiO2 classification diagram.

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On the binary variation diagrams (Fig. 6), a clear inverse correlation with SiO2 is observed for most oxides, including FeOt, TiO2, CaO, and MgO (Fig. 6a–d). Conversely, Na2O and K2O display distinct separated trends for gabbros, diorites, as well as quartz diorites, quartz monzodiorites, and granodiorites (Fig. 6e,f). P2O5 shows both positive and negative trends in gabbros, diorites, and quartz monzodiorites, with a different pattern observed in quartz diorites, tonalities, and granodiorites (Fig. 6g). Additionally, Al2O3 and the summation of rare earth elements (∑REE) in relation to SiO2 demonstrate contrasting trends that notably intersect in the presence of quartz monzodiorite samples (Fig. 6e,g).

Fig. 6
figure 6

Bivariate variation diagrams illustrate the major oxides behavior in relation to silica in the studied rocks.

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The REE characteristics of the studied Suwaj rocks (Table 1; Supplementary Data) indicate a low ∑REE content ranging from 38.8 to 148.7 ppm, with a mean of 68.12 ppm. In the chondrite-normalized REE diagram, the studied samples exhibit a pattern of relatively enriched light rare earth elements (LREE), with a pronounced depletion in Nd for tonalities and enrichment in Ce for granodiorite. Furthermore, there is a heterogeneous pattern observed for heavy rare earth elements (HREE), marked by a significant enrichment of Ho. The LREE relative enrichment demonstrates a (La/Yb)N ratio ranging from 0.78 (gabbro) to 23.8 (granodiorite) with a mean of up to 3.78. Eu/Eu* values manifest a positive anomaly signifying an enrichment of Eu concentration in the mineral compared to other REEs (Fig. 7a; Table 1 Supplementary Data).

Fig. 7
figure 7

Normalized multi-element diagrams of the studied rocks. (a) Chondrite-normalized REE. (b) Primitive mantle-normalized multi-element. Compositions are normalized to the chondrite and primitive mantle of Sun and McDonough47.

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On the primitive mantle-normalized trace elements patterns (Fig. 7b), the gabbros prominently display depletion in Ba, while the granodiorites exhibit enrichment in La, a distinctive feature contrasting with the other samples. Moreover, a pronounced enrichment is observed in Th, Pb, Sr (excluding tonalites), and Eu across all the plotted samples. Notably, all plotted samples consistently show depletion in Zr, Ti, and Y (Fig. 7b; Table 1; Supplementary Data).

In term of both typological and paleotectonic considerations using the Nb-Y and SiO2-Nb discrimination diagrams introduced by Pearce et al.48, the examined rock samples exhibit characteristics indicative of a volcanic arc-related granite nature (Fig. 8a,b). The (Zr + Nb + Ce + Y) content of the studied rocks is notably low, marginally below the (FeOt/MgO) ratio of the highly fractionated I-type granite, situating the studied rocks within the category of unfractionated granite field (Fig. 8c). According to the geochemical criteria postulated by Defant and Drummond28, almost the entire plotted data exhibit elevated concentrations of Y and Sr (Fig. 8d), thereby imparting upon them an adakitic character that accords with the derivation by partial melting of basalts in subducting zones31,49.

Fig. 8
figure 8

Geotectonic discrimination plots of the studied rocks. (a) Nb (ppm) vs. SiO2 after Pearce et al.48. (b) Nb (ppm) vs.Y (ppm) after Pearce et al.48. (c) FeOt/MgO vs. Zr+ Nb+ Ce+Y (ppm) diagram after Whalen et al.50. (d) Y vs. Sr/Y classification diagram for adakitic rocks (after Defant and Drummond28).

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Discussion

Petrogenesis of Suwaj suite

Granitic melts can originate in a wide range of tectonic settings and be linked to various geological processes, including the melting of meta-igneous or metasedimentary source rocks, the fractional crystallization of certain basaltic magmas, or the mixing of melts derived from both the mantle and the crust51,52,53,54,55. Together with to field observations and petrological investigations, whole-rock geochemistry of the intrusive rocks of the Suwaj suite within the Afif Terrane serves as a useful indication and guide for identifying the tectonic setting and highlighting the developing geological processes.

Field and petrological investigations have revealed significant compositional variation, which is consistent with the conversion in mineralogy from gabbro-diorite to tonalite-granodiorite. The absence of discernible gaps among the examined rock samples, coupled with the coherent patterns observed in X-Y variation diagrams (Fig. 6), implies their cogenetic nature. Hence, the uniform compositional variation within the Suwaj suite can been ascribed to one magmatic process in more than one pulse. A comparable compositional variation can only be produced under specific circumstances. The La-La/Sm diagram, illustrated in Fig. 9b, reveals a discernible linear trend in the majority of the Suwaj samples, indicating that the magmas were predominantly influenced by partial melting processes56,57. Altherr et al.58 have proposed a binary diagram that integrates the molar ratios of CaO/(MgO + FeOt) and Al2O3/(MgO + FeOt), providing an effective means of distinguishing between diverse sources of granitic rocks. As per this diagram, the Suwaj suite predominantly falls within the partial melt region associated with a metabasaltic source (Fig. 9c). Figure 7a and b show that the examined intrusives are characterized by conspicuous enrichment of Light Rare Earth Elements (LREEs) and large ion lithophile elements (LILEs), which is concomitant with the depletion of High Field Strength Elements (HFSEs). Such trace element pattern is ascribed to a subduction-related setting59. The granitoids of the Suwaj suite are presumed to have originated primarily through the process of partial melting of a metabasaltic source during the subduction of an oceanic plate.

The low SiO2, high Mg# (> 50), and total FeO (> 10 wt%) of the gabbroic samples aren’t totally compatible with liquids formed from the partial melting of any crustal rocks30. Therefore, the mantle as a contributing source for the Suwaj suite is more conventional. Furthermore, the relatively low Co, Cr, and Ni concentrations in these gabbroic samples indicate relatively evolved melts that stemmed from the lithospheric mantle60,61.

The FeO/MgO ratios gradually increase from gabbroic samples to granodiorite, passing through diorite, quartz diorite, tonalite, and quartz monzodiorite (Table 1; Supplementary Data). This progressive elevation of FeO/MgO ratios is interpreted to reflect constrained magmatic differentiation within their respective parental magmas50. The distinct negative trend on the Al2O3/TiO2 vs. TiO2 diagram (Fig. 9a) represents consistent magmatic differentiation of the studied intrusives62.

The studied granitoid samples have distinctive characteristics, including slightly elevated silica content and reduced levels of MgO, TiO2, and P2O5. Their composition is mostly metaluminous, and their calc-alkaline nature ranges from low- to high-K (Table 1; Supplementary Data, Figs. 5 and 6). The I-type nature of the Suwaj intrusives is much indicated according to several mineralogical features such as absence of normative corundum and occurrence of modal hornblende with accessory titanite and allanite (not shown; Table 1 Supplementary Data). These elemental geochemical attributes minimally match those commonly exhibited by nearby I-type Najirah granitoids and are not consistent with those commonly exhibited by nearby S-type Khurs granites at the Ad-Dawadmi Terrane63.

The geochemical criteria of elevated concentrations of Y and Sr (Fig. 8d) defined the Suwaj intrusives as an adakite derived by the partial melting of basalts in subducting zones. The consistent geochemical compositions and the lack of mafic microgranular enclaves within the Suwaj adakitic granitoids declared invalid magma mixing64. The typical outcome of such magma mixing is the formation of intermediate and high-Mg adakitic rocks, such as high-Mg andesites64,65,66. This contrasts with the Suwaj adakitic granitoids, characterized by slightly elevated SiO2 levels (up to 69.62 wt% in granodiorites) and increased concentrations of Y and Sr (Fig. 8d). Adakitic melts, originating from either subducted slabs or delaminated lower crust, exhibit elevated levels of Mg#, MgO, Cr, and Ni content67,68. This enrichment is attributed to interactions with mantle components during the subsequent ascent of magma (Fig. 9d,e,f) and may also involve the influence of the delaminated lower crust. It is crucial to emphasize that there is a mounting body of evidence suggesting that mantle peridotite plays a direct or indirect role in the genesis of the majority of adakites32. In addition to the low MgO variety, certain adakitic rocks derived from the lower crust exhibit high MgO, low FeO/MgO ratios relative to the given MgO content (or high Mg#), and elevated levels of compatible trace elements (e.g., Ni, Cr), indicating a signature associated with mantle peridotite. In the TiO2 vs. Cr/Ni diagram, the examined gabbro-diorite samples exhibit a distinct low-silica adakite nature (Fig. 9g). As a result, we infer that the adakitic rocks scrutinized in this study are a product of the partial melting of a thickened mafic lower crust during the subduction of oceanic crust (Fig. 9c-g), as noted in the findings of Desouky et al.69.

Fig. 9
figure 9

Geotectonic discrimination diagrams of the studied rocks. (a) Al2O3/TiO2 vs. TiO2 binary diagram after Sun and Nesbitt62. (b) La/Sm vs. La partial melting and fractional crystallization discrimination diagram after Zhao et al.70. (c) Granitic source binary diagram after Altherr et al.58. (d, e, and f) SiO2 vs. MgO, SiO2 vs. Ni, and SiO2 vs. Cr after Grimes et al.71. (g) TiO2 vs. Cr/Ni diagram after Castillo32.

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Tectonic evaluation

The Afif Terrane is one of the major terranes in the Arabian Shield, characterized by its expansive and diverse Precambrian basement26. This terrane represents an extended record of the evolution of the eastern Arabian Shield. The early stages of development observed within the eastern part of the Afif Terrane (Suwaj Terrane) are indicative of microplate amalgamation and the accretion of crustal fragments of the northern East African Orogeny. The granitoids of the Suwaj suite are presumed to have originated primarily through the process of partial melting of a metabasaltic source (Fig. 9) during the subduction of an oceanic plate and show a significant adakitic signature (Figs. 5f and 8d, and 9e,f,g).

Petrogenetic models devised for magmatic systems often exhibit considerable complexity, incorporating numerous assumptions and constraints, thereby posing challenges in assessing their applicability to natural systems. Conversely, simpler models, which effectively capture the geochemical characteristics of rocks without necessitating the satisfaction of additional, often conflicting, conditions, may be favored. In our perspective, the generation of I-type adakaite Suwaj intrusives can be interpreted in the following paragraphs.

The genesis of adakite I-type granites during the ocean closure phase encompasses a sequence of interrelated mechanisms, encompassing subduction, partial melting, magma migration, and interaction with surrounding rocks (e.g., protolith; Fig. 10). In a broader sense, as the active overriding plate docks during the closure of the ocean, it leads to the partial subduction or underthrusting of the cold crustal margin beneath the accretionary prism72. This process sets the stage for subsequent geological events18. The crust undergoing underthrusting is often composed of calc-alkaline diorites-granodiorites and gabbros. Continuous collision and high-pressure conditions lead to partial melting within this crust (Fig. 9b). This partial melting produces acidic-type magma, which is the precursor to adakite I-type granites (Figs. 5d,f and 8). The generated granitic magma, being less dense than the surrounding rock, tends to accumulate and migrate upward. This migration occurs along pathways, often guided by fractures and faults in the crust73,74.

Fig. 10
figure 10

A suggested carton showing the geodynamic setting for the studied Suwaj suite during the Arc-collision stage.

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In a broader context, the Suwaj suite (U-Pb age of 685 ± 5 Ma and 674 ± 6)3,24 predates the basin closure and the maximum deposition ages for the Abt formation3. The close age correlation between and the Suwaj suite the Halaban ophiolite (U-Pb age of 694 ± 8 Ma and 695 Ma)3,24 aligns with the hypothesis that adakite formation occurs exclusively in convergent margins where young and, therefore, still hot oceanic slabs are being subducted29,30,31,32. The Halaban ophiolitic zones have been previously interpreted as having a boninite origin75. Their general character refers to the fact the fact that these ophiolitic zones are more related to Supra Subduction Zone ophiolites that originated above a subduction zone as the second melting of a previously depleted mantle after the first melting at an ocean ridge or mantle plume75,76. Moreover, considering that the Ad-Dawadmi is interpreted as a back-arc basin75, this aligns with models suggesting that partial melting of subducted crust should occur closer to the trench28,29,77.

The Suwaj Terrane primarily comprises two rock types that formed relatively concurrently: the fragmented mafic-ultramafic belt of the Halaban ophiolite (695 Ma)3,24 and the Suwaj suite (U-Pb age of 685 ± 5 Ma)24,25. Therefore, it is crucial to elucidate that the genesis of the Suwaj suite stems from the subduction of the Paleo-Ad-Dawadmi oceanic slab or mantle wedge during the amalgamation between the Afif and Ad-Dawadmi terranes. This alignment situates the Suwaj suite within the original genetic framework of Adakite, typified by intermediate magmatism featuring elevated SiO2 and Al2O3 contents67 (Table 1: Supplementary Data). The high Na2O/K2O ratios imply a potential origin from low-K tholeiitic primary magmas, which are commonly formed in island arc or active continental margin environments and are primarily associated with the melting of oceanic crust18,72. The investigated rock samples exhibit high Sr content (averaging 602.03 ppm) coupled with low Y (averaging nine ppm), alongside elevated Sr/Y ratios (98–125, average 112) and La/Yb ratios (average 21.3), indicative of typical geochemical characteristics of adakites28,49,67. Furthermore, the samples exhibit Cr/Ni ratios (averaging 2.6) consistent with typical features of adakite derived from melting of the basaltic portion of oceanic crust subducted beneath volcanic arcs67,78 (Figs. 8d and 9b,d,e,f,g).

The crystallization ages of Cryogenian arc-related rocks of the Suwaj suite span a brief interval of approximately ca. 15 Ma. The identification of a broad (~ 20 km-wide) magmatic arc extending westward from the eastern boundary of the Afif Terrane into and beneath the Murdama group between ca. 695 and 675 Ma indicates a low-angle, non-steep subduction of the Palaeo-Ad-Dawadmi oceanic plate beneath the Afif Terrane, persisting for a duration of around 20 million years (Fig. 10).

During the evolutionary stages spanning approximately 700 to 650 Ma, characteristic of the ANS and the entire East African Orogeny, the genesis of arc-related magma is commonly attributed to the partial melting of the subducted oceanic crust2,35. This magma ascends as diapers through the convicting asthenosphere, giving rise to the TTG rock association characterized by calc-alkaline affinity18,35,79. Substantial volumes of syn-orogenic magma have been intruded into the metavolcanic and metasedimentary rocks (Ajal group), forming the Suwaj suite. The geochemical analysis classifies these rocks as calc-alkaline, metaluminous I-type granitoids (Fig. 5d,e,f), analogous to the TTG rock association observed in similar terranes within island arc or active continental margins. The primary source of melting for the TTG association is the subducted oceanic slab at convergent plate boundaries30,79,80. The partial melting of these extensively underplated components likely interacted variably with mantle batches, resulting in the formation of most of the Suwaj suite magma. The emplacement of subsequent more potassic granitic magma subsequent to the emplacement of TTG-Like association indicative of a significant phase of crustal thickening occurring subsequent to arc collision and amalgamation, thereby contributing to the formation of the proto-crust within the Arabian-Nubian shield33.

The previously identified Neoproterozoic adakites in the Ad Dawadimi Basin, which is a forearc basin in the Eastern Arabian Shield3, are slightly similar in composition but very different from the discussed adakites in the Afif Terrane in terms of age (633.2 Ma34) and tectonics (as post-tectonic34). The gneissose granitoids (tonalite-trondhjemite-granodiorite) of the Shait area in the Southern Egyptian Eastern Desert in the Nubian shield represent adakitic magmatic pulses slightly similar in composition and tectonic (as island arc terranes resulted from subduction of older oceanic plates beneath younger oceanic lithospheres35). These gneissose granitoids are different from the discussed adakites in the Afif Terrane in age (~ 800 Ma to 717 Ma35). The adakitic Dokhan volcanics in the northern Nubian Shield were generated through partial melting of delaminated mafic lower crust interacting with overlying mantle-derived magma33. These adakitic volcanics are faintly similar in composition but very different from the discussed adakites in the Afif Terrane in terms of age (630 − 590 Ma) and tectonics (as post-collisional suites that associated with subduction zones33).

Conclusions

The extended record of the evolution of the eastern Arabian Shield is predominantly developed with the microplate amalgamation and the accretion of crustal fragments of the northern East African Orogeny. The Afif Terrane is one of the significant terranes in the eastern part of the Arabian Shield that contains a large-scale and diverse Precambrian basement. An area of about 1000 km2 in the Afif Terrane is covered by the Suwaj suite plutonism. Our field study compiled with the petrological and geochemical investigation of this suite demonstrated that this large-scale igneous pluton is metaluminous I-type granitoids with calc-alkaline affinity and comprises two associations, including the diorite-quartz diorite and tonalite-granodiorite associations. The microplate amalgamation and the accretion of crustal fragments led to subducting oceanic crust that partially melted, generating arc-setting magma. This magma rises as diapirs up, forming the TTG rock association of the Suwaj suite, which is emplaced within metavolcanic and metasedimentary supracrustal rocks at an island arc or active continental margin.

The age relationship between the Halaban ophiolite and Suwaj suite (695 − 675 Ma) corresponds to the initial belief that adakite occurs only in convergent margins where young oceanic slabs are subducted. The short range of crystallization ages of Suwaj suite rocks and the wide magmatic arc point to a short, episodic subduction of the Palaeo-Ad-Dawadmi oceanic plate beneath the Afif Terrane.