Classification, origin and nature of magma
The R1-R2 diagram (Fig. 15a) of De La Roche et al. (1980) is used to classify the plutonic rocks of El Mansouri Ring Complex, which indicate that they plot inside the fields of quartz syenite and nepheline syenite, compatible with the field and petrographic studies. Whilst, the volcanic rocks of El Mansouri Ring Complex were categorized by using the binary diagram of total alkalis (Na2O + K2O) vs. SiO2 of Wilson (1989), on which they fall inside the trachyte and trachy-andesite fields (Fig. 15b). According to the total alkalis (Na2O + K2O) vs. silica (SiO2) diagram of Le Maitre et al. (1989; Fig. 15c), all the samples of El Mansouri Ring Complex fall inside the field of alkali, except four samples of pyroclastic rocks owing to their relatively depletion in alkalis and excess in silica contents (Supplementary Table 6). Whilst, the associated carbonatites fall in the calciocarbonatite field according to the ternary diagram (MgO- CaO- Fe2O3 + MnO) of Woolley and Kempe (1989; Fig. 15d), the binary diagram (FeO + Fe2O3 + MnO)/MgO vs. Cao/(CaO + Fe2O3 + MnO) of Harmer and Gittins (1997; Fig. 15e) and the ternary diagram (CaO-MgO- (FeO + Fe2O3 + MnO) of Bell (1989, Fig. 15f).
According to Bonin and Giret (1985), silica-oversaturated and silica-undersaturated rock units from different Ring Complexes worldwide are enriched in sodic clinopyroxenes. Peralkaline silica-saturated types are among of these rocks, comprising El Mansouri Ring Complex, are distinguished by the crystallization of sodic (Na)-rich clinopyroxene during the development of magma. Accordingly, the production of Na-rich clinopyroxene (aegirine) in the investigated rocks was controlled by the magma peralkalinity. Mineralogically, the presence of both sodic pyroxene (aegirine) and alkali amphiboles (arfvedsonite) in all rock types, indicating their peralkaline nature. Furthermore, the imperceptible rises in the majority of incompatible elements (like, Zr, Nb and Rb) from syenites to more felsic peralkaline granites coincide with rising peralkalinity (Weaver et al. 1990). In comparison to other granitoids, A-type granite of peralkaline nature and syenite require a much higher temperature during their origination, indicating that mantle derived magmas played an essential goal in their origin. Some trace elements proportions like Th/Ta, Th/ Nb and Rb/Nb can be used to determine the source of magma from which the final rocks formed (Moreno et al. 2014). This is due to the fact that these trace elements stay hugely constant during the magmatic differentiation process (Wang et al. 2017). When plotted on the (Y/Nb)N vs. (Th/Nb)N diagram of Moreno et al. (2014), the El Mansouri alkaline rocks fall inside the field of ocean island magmatism, except the samples pyroclastic rocks (Fig. 16a).
Several diagrams were performed to indicate the type of magma among of these the diagram of agpaitic index (AI = Na + K/Al) vs. SiO2, which indicates high value of agpaitic index (AI (Na + K)/Al) = 0.9–1.2; Supplementary Table 6) of the investigated rocks and reveals their peralkaline nature (Liégeois et al. 1998), except the samples of pyroclastic rocks have calc-alkaline character (Fig. 16b). When plotted on the (Na + K)/Al vs. Zr (ppm) diagram of Shellnutt and Zhou (2007) and Shellnutt et al. (2009a), the El Mansouri Ring Complex plot inside the field of peralkaline, except the pyroclastic rocks fall in the metaluminous field (Fig. 16c). According to the diagram of Frost et al. (2001) of the alkaline affinity, all the samples of El Mansouri rocks are plotted in the field of alkaline, except the pyroclastic rocks fall inside the calc-alkaline field (Fig. 16d). When plotted on the A/NK vs. A/CNK binary diagram of Maniar and Piccoli (1989), the El Mansouri Ring Complex have been categorized as peralkaline rocks, except the pyroclastic rocks fall in the metaluminous field (Fig. 16e). According to the K2O versus Na2O diagram of Raguin (1965), the El Mansouri Ring Complex fall in the potassic-sodic field, except the pyroclastic rocks lie in sodic field (Fig. 16f).
The analyzed samples of El Mansouri alkaline rocks reveal comparatively high rates of Nb/Ta (range 12–36) and Zr/Hf (range 20–230; Supplementary Table 6). These amounts are near to the estimated mantle Nb/Ta (17.5; Green 1995) and Zr/Hf (35–45, Wedepohl 1995) proportions and varies remarkably from that of crust (Nb/Ta = 10–12; Zr/Hf < 30), meaning that the crystal fractionation of mantle-derived magmas is responsible for the formation of El Mansouri alkaline rocks.
Geotectonic significance of El Mansouri alkaline rocks
According to Whalen et al. (1987); Eby (1990) and Bonin (2007), the A-type granitoids are mainly emplaced in anorogenic extensional or post-collisional environments and have been divided by Eby (1992) into two types A1 and A2. The granitoids of A1 type have been produced by fractional crystallization of basaltic magma derived from oceanic island basalt (OIB) sources and are usually emplaced in anorogenic settings, whilst the granitoids of A2 type have been produced in post-orogenic settings from subcontinental lithosphere or lower crust magmas (Eby 1992).
Several discrimination diagrams were used to display the tectonic setting of alkaline rocks of El Mansouri Ring Complex among of these the binary diagram of Y vs. Nb which was adjusted by (Whalen and Hildebrand 2019; Fig. 17a), reveals that the El Mansouri alkaline rocks fall in within plate granite field, except the pyroclastic rocks plot inside the field of slab failure. This emphasizes that the rocks of El Mansouri Ring Complex are within plate anorogenic granites associated with crustal extension and/or rifting except the pyroclastic rocks. Furthermore, the ferroan and alkaline features of the investigated rocks according to the binary diagram (SiO2 vs. FeOt/FeOt + MgO) of (Frost et al., 2001) (Fig. 17b), are typical of A1 types and vary from A2 types that are commonly alkali calcic (Dall’Agnoll et al. 2012). When plotted on the SiO2 vs. FeOt/MgO diagram of Eby (1990), the El Mansouri alkaline rocks plot inside the A-type granite field, except the pyroclastic rocks (Fig. 17c). Based on the classification diagrams (Y versus Nb) and (Nb + Y versus. Nb/Y) of Whalen and Hildebrand (2019) that distinguish between slab-failure, arc and A-type rocks, showing all the El Mansouri alkaline rocks fall inside the A-type field, except the pyroclastic rocks plot in the slab- failure field (Figs. 17d, e). According to the binary diagram (Rb/Nb versus Y/Nb) of Eby (1992), the studied alkaline rocks have been produced from mantle-derived magma within an anorogenic setting (A1), except the pyroclastic rocks fall inside the A2 field (Fig. 17f). On the ternary (MnO*10- TiO2 and P2O5*10) discrimination diagram of Mullen (1983), the El Mansouri alkaline rocks fall inside the oceanic island basalts (OIB) field, except the pyroclastic rocks (Fig. 17g). When plotted on the Y/Nb versus Ce/Nb discrimination diagram of Eby (1990) for A-type granites, the El Mansouri alkaline rocks fall in the OIB field, except the pyroclastic rocks plot inside the average continental crust (C) field (Fig. 17h). It is inferred that a mantle source played an essential role in the formation of the El Mansouri Ring Complex.
Petrogenesis of El Mansouri alkaline rocks
Generally, three prevalent patterns for the genesis of alkaline granites of A-type were known as follow: (1) Partial melting of granulitic residue with high-temperature, which enhanced in F- and/or Cl-bearing phases (Whalen et al. 1987), (2) Fractional crytallization from either magma, which is composed of basalt (Turner et al. 1992) or magma, which is composed of mildly alkaline trachyte by low-pressure crystal-melt fractionation (Nardi and Bonin 1991), and (3) Development of the A-type granitoids including magma mixing, metasomatism, thermogravititional diffusion, etc. (Bowden et al. 1987). The majority of African alkaline complexes have been distinguished by two post-orogenic phases (Bowden 1985). The first phases are associated with the terminal stage of major orogenies (not strictly anorogenic), whilst the second phases are regarding to the progressive uplift, long-term doming and rifting (true anorogenic).
The geochemical characteristics of the investigated rocks dispute against their production from partial melting of crustal rocks where; (a) In general, rocks which generated by partial melting of crustal magma possess a peraluminous composition (Martin 2006), as opposed to the peralkaline character of the El Mansouri rocks, (b) The obvious lack of negative anomalies of Ta and Nb in all El Mansouri alkaline rocks (Fig. 13a), conflicting with their production by melting of crustal materials (Rudnick and Gao 2003) and (c) Magmas of syenite cannot empirically be formed from partial melting of crustal rocks, until at high pressure up to 25 kbar (Litvinovsky et al. 2000). Furthermore, different geochemical parameters exclude the influence of crustal contamination during the production of El Mansouri intrusive rocks. For example, the investigated rocks possess Th/Nb amounts ranging between 0.07 and 0.3 (Supplementary Table 6), which is perfectly near to the amount of primitive mantle (0.117; Green 1995) and notably various from the mean amount of continental crust (Th/Nb = 0.44, Rudnick and Gao 2003). This excludes that, the production of El Mansouri alkaline rocks is controlled by the crustal contamination.
Since the inspected alkaline rocks cannot be produced from partial melting of crustal materials, the generation of these rocks throughout prolonged fractional crystallization of mafic magma is the most credible mechanism. The Fractional crystallization of alkaline basaltic melts with low or no crustal assimilation is one of the most well recognized patterns for the generation of peralkaline granite and syenite magma (Eby 1992, 1990; Frost and Frost 2011). Thus, we dispute that the El Mansouri alkaline rocks have been formed by the fractional crystallization of oceanic island basalts (OIB)-like mafic magmas (Figs. 17g, h), created from the uplift of lithospheric mantle during the Mesozoic extension as shown in Fig. 18.
The studied amphiboles have noticeably high SiO2 with low MgO and CaO contents (Table 5), compatible with magmatic evolution during the fractional crystallization (Pe-Piper 2007). From quartz syenite to alkali feldspar syenite, the compatible trace elements like Ba and Sr progressively reduce, whilst the incompatible trace elements like Zr, Y and Nb, clarify enhancement trends with SiO2, compatible with their origination by fractional crystallization from the same parental magma (Fig. 11).
The existence of negative K, Ba, Sr, Eu, and Ti anomalies in the studied alkaline rocks (Fig. 13a), can be attributed to the high degree of fractional crystallization. Furthermore, the negative anomalies of Ti displayed by these rocks reflect the fractionation of amphiboles and/or Fe-Ti oxide stages. The function of feldspar fractionation has been detected by variance between Sr and Ba versus Rb (Figs. 19a, b), wherever Sr and Ba reduce methodically with a rising Rb from quartz syenite to alkali feldspar syenite. Ba and Sr exhibit significant cosistent behavior, which coincides with the K-feldspar and plagioclase fractionation through the magmatic development of the ring (Figs. 19a, b). In conclusion, the studied alkaline rocks of El Mansouri area have been probably produced by fractional crystallization of ocean island basalt (OIB)-like mantle melt through which the fractionation of feldspars and mafic minerals played an essential function.
Genesis and geodynamic setting of carbonatites
Commonly, three main processes were suggested to show the origin of carbonatites in aggregation with alkaline rocks comprise: (a) Direct melting from carbonate-rich mantle peridotite or wehrlite (Dalton and Wood 1993; Wyllie et al. 1998); (b) Low-pressure liquid immiscibility from magmas of parental nephelinite (Le Bas 1987) and (c) Extensive crystal fractionation from magmas, which enriched in carbonated alkaline silicate such as, carbonated nephelinite or melilitite (Gittins and Jago 1998). The low concentration of REE in some of these samples is thought to be a regional feature that is characteristic of the lithospheric mantle in that area (Brady and Moore 2012) and has been attributed to post-magmatic depletion because of the hydrothermal recrystallization of the carbonate (Andersen 1986; Halama et al. 2005). By using geological, petrographic and mineralogical criteria, as well as trace and rare earth elements geochemistry, particularly in relation to the associated silicate lithologies, it is possible to identify whether a melt originated from carbonated syenitic by liquid immiscibility or crystal fractionation (Fig. 18). The symmetry in the compositions of trace element from the alkaline igneous rocks and carbonatites in the El Mansouri Ring Complex reveal that they possess a common, mantle-derived source. The high field strength elements (HFSE) are extermely informative in this area. Nb, Y and Zr are among the data available for both carbonatites and alkaline rocks from El Mansouri Ring Complex. Nb is an extermely incompatible element, while Zr and Y are high compatible. Thus, it may be anticipated that Nb partitioned into the melt, which enriched in carbonatites, relative to Zr and Y both during crystal fractionation and liquid immiscibility. The predominance of Nb in the carbonatites compared to the alkaline rock supports the inspected petrogenetic processes. Noticeable, the absence of any gap in the Nb-Zr/Nb and Zr-Zr/Y diagrams (Figs. 19c, d) indicate crystal fractionation from a carbonated syenitic melt. Accordingly, the carbonatites have been originated from crystal fractionation of a parental carbonated syenitic magma (Fig. 18).
Possibility of carbonatite-syenite complexes for REE mineralization
Internationally, carbonatite-related rare earth elements (REEs) deposits with high grades of REEs are preferable objectives for exploration (Hou et al. 2015a; Xie et al. 2016; Rasilainen et al. 2023). There are two processes of REE-enrichment, which are prerequisite to produce such as deposits (Xie et al. 2016): (1) the formation of fertile carbonatite and (2) the subsequent upgrade by processes of magmatic-hydrothermal. The fertile carbonatites which host REE deposits mostly possess high contents of REE, Ba, and Sr, but fertility is not related to CaO: MgO or FeO: MgO ratios (Figs. 19e, f). The REE, Ba, and Sr concentrations for the carbonatite associated with El Mansouri Ring Complex are lower than those at giant or large REE deposits elsewhere in the world, with data for the carbonatite associated with El Mansouri Ring Complex plotting inside the field of barren or near to the boundary between the fields of barren and fertile in the diagrams of (REEs vs. CaO/MgO, FeO/MgO, Ba and Sr/Ba) (Figs. 19e, f, g and h). The high REE concentrations in some samples propose that the carbonatite at the El Mansouri Ring Complex possess the possibility to host large or giant deposits of REE. The outcomes of petrographic investigations, geological features, as wel as chemical analyses which were accomplished during this work manifest potential reasons for the increment of mineralization.
In the study area, the mineralization is controlled by two main trends W-NW and E-W (El Nisr and Saleh 2001). Local tectonic activity in the study area form fissures or fractures in the carbonatite-syenite complexes during the early phases of mineralization, assisting liquid cycling and modifying the liquid chemistry through water-rock interaction. In tectonic fractures, circulation of ore liquids causes the alteration in the carbonatites and gives rise to high contents of REE, F, Cl, CO2, and volatiles in the liquids (Fig. 18). This model supposes that the area of alkaline rocks and carbonatites bounded or crosscut by faults and fractures along the margins possess distinctive potential for like deposits of rare earth elements (REEs) and radiaoactive minerals.
Uranium and thorium radioactivity
Because of the existence of Th- mineralization and accessory minerals of radiogenic character like uranothorite, zircon and monazite, the equivalent uranium (eU) and equivalen thorium (eTh) in the investigated rocks are very high. The eTh/eU diagrams of the investigated quartz syenite, alkali feldspar syenite, trachyte, pyroclastic rocks and carbonatites (Figs. 20a and 21a), exhibits an enhancement of uranium concentrations where the greatest values of both eU and eTh are for carbonatites (Fig. 21a). The majority of samples of different rock units are in between 5.0 and 8.0 eTh/eU ratio with a simultaneous increase in eTh and uranium remained relatively immobile in the original rock (Figs. 20a and 21a). According to this diagram, the radioelement rises progressively during magmatic fractionation, but the proportion varies owing to the various processes of alteration (Heikal et al. 2019; Saleh et al. 2022).The eU/eTh ratio vs. eU diagram (Figs. 20b and 21b), displays a strong direct slope which shows hydrothermal increment of U. Therefore, the various processes of alteration play an essential goal in the mobilization of uranium. On the other hand, The eU/eTh vs. eTh diagram (Figs. 20c and 21c), exhibits a reverse relationship, in which eU/eTh ratio reduces with rising the value of eTh for most plotted samples of quartz syenite, alkali feldspar syenite, trachyte, pyroclastic rocks and carbonatites, clarifying the hydrothermal solutions play a distinctive role in the redistribution of these elements. The majority of samples of the investigated quartz syenite, alkali feldspar syenite, trachyte, pyroclastic rocks and carbonatites have lower U concentration than that of the hypothetical distribution of uranium (Figs. 20d and 21d) and thus the mobilization awards mostly negative values (Cambon 1994; Saleh et al. 2021), which in turn exhibit that uranium of these various investigated rock units is leaching out (migration).