Introduction

The Arabian-Nubian Shield (ANS) is one of the largest tracts of juvenile continental crust of Neoproterozoic age on Earth (Patchett and Chase 2002). It evolved as the northern segment of the East African Orogen, which formed in Neoproterozoic time through three main phases referred to as pre-collisional, collisional and post-collisional (Stern 1994, 2002; Patchett and Chase 2002; Stoeser and Frost 2006; Eyal et al. 2010; Be'eri-Shlevin et al. 2009, 2011). The pre-collisional phase (~820–700 Ma) comprises metamorphosed volcano-sedimentary rocks, ophiolite suites, and calc-alkaline metagabbro-diorite-granitoid rocks. The collisional phase (670–630 Ma) is characterized by an abundance of weakly deformed calc-alkaline gabbros and granodiorites. The post-collisional phase (~620–580 Ma) is widely distributed in the northern part of the ANS and characterized by vast intrusions of calc-alkaline and alkaline/peralkaline granitoids and their associated volcanics. The ANS was stabilized before the development of an extensive peneplain in mid-Cambrian time (~530 Ma) and then exhumed in the Neogene due to Red Sea rifting and flank uplift (Garfunkel 1999).

Post-collisional A-type rocks in southern Sinai comprise dominant granites with lesser syenitic rocks. Previous studies on A-type igneous suites in southern Sinai have often focused only on granites. However, many types of granite classified as A-type are very similar chemically to highly fractionated I-type calc-alkaline granites associated with post-orogenic igneous suites (Bonin 2007; Eyal et al. 2010). Indeed, previous studies on Sinai granites reveal a large overlap between calc-alkaline and alkaline granites in many standard classification diagrams (Litvinovsky et al. 2015). Consequently, studies in southern Sinai based on granites may include misclassified samples and are likely to yield ambiguous answers about the distinctive processes associated with A-type magmatism.

The current contribution aims to define the petrogenesis of the Abu Rumeil syenites, distinctively alkaline members of the A-type igneous suite of the northern ANS (Sinai Peninsula, Egypt). New field, petrographical, geochemical, and mineralogical data are integrated to determine the petrological characteristics and history of the Abu Rumeil syenites. By constraining the source contributions to the magma and the processes and conditions of magmatic evolution, we can better understand the prevailing type of magmatism in the northern ANS (southern Sinai) during the Late Ediacaran, propose a tectonic and geochemical model for their origin, and evaluate their possible significance.

Geological background

The basement rocks of Sinai are the northernmost outcrops of the ANS. They are characterized by abundant post-collisional plutonic and volcanic sequences intruding and overlying older metamorphosed complexes. The post-collisional phase includes both calc-alkaline (ca. 635–590 Ma) and alkaline (ca. 610–580 Ma) plutonic suites and associated volcano-sedimentary sequences (Ali et al. 2009; Be’eri-Shlevin et al. 2009; Azer and El-Gharbawy 2011). There is an overlap between the end of calc-alkaline magmatism and the beginning of alkaline activity (Be’eri-Shlevin et al. 2009, 2011; Azer et al. 2010; Eyal et al. 2010). The post-collisional alkaline phase include A-type igneous rocks and is represented by alkaline volcanics, granites, ring complexes, and dyke swarms (Mushkin et al. 2003; Katzir et al. 2007; Samuel et al. 2007; Farahat and Azer 2011).

Ring complexes of Neoproterozoic age in many parts of the ANS (e.g., Harris 1985; Vail 1989; Katzir et al. 2007; Moghazi et al. 2011) were emplaced during the post-collisional phase of the shield (610–590 Ma). In the southern Sinai Peninsula in particular, the ring complexes are of late Ediacaran age (605–580 Ma; Katzir et al. 2007; Moreno et al. 2012, 2014; Eyal et al. 2014, Azer et al. 2014). The Katherina Ring complex (KRC) is a typical example.

The Katherina area (Fig. 1a) is characterized by emplacement of large bodies of weakly to un-deformed calc-alkaline plutonic and volcanic igneous associations from ca. 635 to 590 Ma (Moreno et al. 2012; Be’eri-Shlevin et al. 2009; Be’eri-Shlevin et al. 2011). Field investigations revealed that the KRC cross-cuts late Pan-African, subduction-related calc-alkaline granitoids and their volcanic equivalents (Bentor 1985; Garfunkel 1999). In the study area, the transition from calc-alkaline to alkaline magmatism is marked by the intrusion of spectacular dyke swarms of varying composition ranging from rhyolite to andesite and basalt. These dyke swarms cut across the calc-alkaline rocks but in turn are truncated by the KRC. The country rocks of the KRC in the investigated area are the Rutig volcanics and associated quartz-diorite. The quartz-diorite represents the oldest rock unit in the study area and yielded a single-crystal U-Pb zircon of 844 ± 4 Ma (Bea et al. 2009). The age of calc-alkaline granitoids is 610 ± 5 Ma (Be’eri-Shlevin et al. 2009), while the Rutig volcanics gave ages from 619 ± 4 to 607 ± 3 Ma (Be’eri-Shlevin et al. 2011).

Fig. 1
figure 1

a Geological map of the Katherina ring complex (modified after Katzir et al. 2007). Inset shows simplified geological map of the late Proterozoic rocks in south Sinai, and the location of the KRC is indicated. Also, the location of Fig. 1b is shown. b Geologic map of Abu Rumeil area (modified after Eyal et al. 2014)

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The rock units of KRC in the study area include volcanics, subvolcanic rocks, ring dykes, and a granitic pluton. Based on field observations, the KRC started with explosive eruption of the Katherina volcanics and their pyroclastics, followed by emplacement of subvolcanics, ring dykes, and eventually the Katherina pluton. The available ages for the different rock units of the KRC (583–602 Ma; Be’eri-Shlevin et al. 2009; Moreno et al. 2012) clearly show that it postdates the Rutig volcanics.

The volcanic rocks include rhyolite, ignimbrite, and associated pyroclastics. The subvolcanic rocks include perthite granite and albite granite. The ring dykes are semicircular in shape and consist mainly of quartz syenite, quartz monzonite, and microgranite (Fig. 1a). The granitic pluton forming the inner core of the KRC is made up of alkaline syenogranite and alkali feldspar granite.

The syenitic rocks of Abu Rumeil form the inner ring dyke of the KRC (Fig. 1b). The dyke has a semicircular shape and highly variable width (few meters—1300 m). The dyke intrudes the Rutig and Katherina volcanics along sharp and subvertical contacts. In the northwestern part, it is cut by the Katherina pluton. It consists mainly of syenites with less quartz-monzonite and microgranite. The syenites include alkali feldspar syenite and quartz syenite. They have a red color and medium-grained texture. The alkali feldspar syenite is dominant and represents the early phase, while quartz syenite was emplaced later.

Petrography

Based on petrographic studies, the syenitic rocks of Abu Rumeil are distinguished into alkali feldspar syenite and quartz syenite, the former being more abundant. The alkali feldspar syenite forms strips of dark brown to light red rocks in outcrop; it is fine- to medium-grained and exhibits hypidiomorphic and seriate textures. It is composed of alkali feldspar, plagioclase, amphiboles, pyroxenes, and subordinate biotite; quartz is entirely absent. Fe-Ti oxides, allanite, and apatite are the common accessory minerals. Feldspars are the dominant minerals and constitute about 65 % of the mode. Alkali feldspars are dusty in appearance and include both orthoclase and microcline. Perthitic texture is very common in the orthoclase. Microcline often exhibits microperthitic texture and Carlsbad twinning, sometimes overprinted by cross-hatch twinning (Fig 3a). Plagioclase occurs as isolated, corroded crystals showing simple twinning, usually associated with orthoclase. Pyroxene includes both clinopyroxene and orthopyroxene. Clinopyroxene is represented by augite and occurs as pale green, euhedral to subhedral crystals. Some augite crystals are twinned, zoned, and partially transformed into winchite (Fig. 2b) or other amphiboles (Fig. 2c). The orthopyroxene is enstatite rimmed by margins of secondary amphiboles. Primary amphiboles are represented by hornblende with rare of them overgrown by primary K-feldspars. Biotite is partly transformed into aggregates of muscovite (Fig 2d).

Fig. 2
figure 2

Microphotographs of the syenitic rocks at Abu Rumeil area, all photomicrographs in cross-polarized light: a cross-hatched twinning in microcline, b augite crystal partly transformed into winchite, c augite crystal partly transformed into amphiboles, d biotite crystal partly transformed into aggregates of muscovite, e corroded orthoclase with perthite texture, f corroded plagioclase phenocryst associated with orthoclase, g rounded xenocryst of clinopyroxene, h small clinopyroxene inclusions in the orthoclase phenocrysts. The mineral abbreviations are Mc microcline, Win winchite, Aug augite, Am amphibole, Bt biotite, Ms muscovite, Or orthoclase, Pl plagioclase, and Cpx clinopyroxene

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The quartz syenite of Abu Rumeil is a dark reddish brown porphyritic rock. Phenocrysts make up 50 to 75 % by volume. The groundmass is fine-grained but shows the same mineralogy as the phenocryst assemblage. Phenocrysts include K-feldspar (45–65 %), plagioclase (5–12 %), quartz (5–10 %), and mafic minerals (2–10 %). All of these show extensive corrosion and reaction rims (Fig. 2e–h). Accessory minerals are zircon, allanite, apatite, and monazite. The groundmass consists mainly of alkali feldspar and quartz, with lesser amounts of plagioclase, biotite, amphibole, and Fe-Ti oxides. The K-feldspar phenocrysts are mainly perthite (Fig. 2e), turbid, and stained with pale, reddish-brown color. Plagioclase occurs as both large corroded phenocrysts (Fig. 2f) and small laths in the groundmass. Quartz occurs as small anhedral crystals or interstitially in the groundmass. Mafic minerals are mostly pyroxene, hornblende, and biotite. Pyroxene includes both clinopyroxene and orthopyroxene. The former occurs as prismatic and tabular phenocrysts, partially replaced or rimmed by hornblende. Orthopyroxene is represented by anhedral crystals of enstatite. Sparse rounded xenocrysts of clinopyroxene (Fig. 2g) and small clinopyroxene inclusions in the orthoclase phenocrysts (Fig. 2h) are relicts of the mafic stages of evolution of the parental magma.

Analytical methods

Fifteen representative samples have been chemically analyzed for major and trace elements by X-ray fluorescence (XRF); 12 samples were carried out at the ACME Analytical Laboratories, Canada, and 3 samples were analyzed at the GeoAnalytical Lab, Washington State University, USA. The REEs for seven representative samples were determined by solution-source inductively coupled plasma mass spectrometry (ICP-MS) at the ACME Analytical Laboratories, Vancouver, Canada. The analytical precision for XRF analyses, as calculated from duplicate samples, is better than 1 % for most major elements and better than 5 % for most trace elements. Accuracy in this lab has been established by analysis of established geostandards, whose accepted compositions are routinely recovered within the stated precision.

Mineral analyses and backscattered electron images were collected at the GPS Division Analytical Facility, California Institute of Technology, using a five-spectrometer JEOL JXA-8200 electron microprobe. The analytical conditions were 15 kV accelerating voltage, 25 nA beam current, a focused beam (1 μm), 20 s on-peak counting times, and a mix of natural and synthetic mineral standards. Also, some mafic minerals in the polished thin sections were analyzed using a Cameca SX50 and JEOL 8900 microprobes housed at the University of Nevada, Las Vegas, USA, with a similar protocol except for 20 nA beam current and defocused 5–10 μm beam size.

Mineral chemistry

Representative microprobe analyses of the major mineral phases from the investigated syenitic rocks are given in Tables 1, 2, 3, 4, and 5.

Representative feldspar analyses from alkali feldspar syenite and quartz syenite are listed in Table 1. They include plagioclase (oligoclase) and alkali feldspars (orthoclase and anorthoclase) (Fig. 3a); microcline would also be called orthoclase in this nomenclature. Orthoclase in the alkali feldspar syenite ranges in composition from Or50.1 to Or81.6, while those in the quartz syenite range from Or45.2 to Or86.2. The anorthoclase shows variation in composition ranging from Or35.6 to Or38.4. Plagioclase in the alkali feldspar syenite ranges from An19.1 to An29.8 with an average An25.0. Plagioclase in the quartz syenites is slightly more sodic, ranging between An 9.6 and An21.9 with an average An16.5.

Table 1 Representative microprobe analyses (wt.%) of feldspars in the Abu Rumeil syenites
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Fig. 3
figure 3

a Or-Ab-An ternary diagram of the analyzed feldspars in the syenites (Deer et al. 1992). b, c Classification diagram of the pyroxene minerals (Morimoto et al. 1988); curves of temperature estimation are after Lindsley (1983). d TiO2-FeO(t)-MgO ternary diagram for the biotite of Abu Rumeil (Nachit et al. 2005). e FeO(t) vs. Al2O3 biotite discriminant diagram for the analyzed biotites (Abdel-Rahman 1994). f Classification of amphibole on the basis of Si and Mg/(Mg + Fe2+) (Leake 1997)

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Pyroxene analyses from syenites are listed in Table 2. Two types of pyroxene are found; clinopyroxene is more abundant than orthopyroxene. No compositional zoning could be distinguished within individual pyroxene grains. According to the classification given by Morimoto et al. (1988), the clinopyroxene is classified as augite with minor pigeonite. The orthopyroxene is enstatite (Fig. 3b, c). Clinopyroxene has low Al2O3 (<2.01 wt.%), TiO2 (<0.75 wt.%), and Na2O (<0.67 wt.%), and its Mg# [(Mg/(Mg+Fe2+)] ranges from 0.54 to 0.73 with an average of 0.63. The enstatite contents of orthopyroxene range from 55.1 to 66.9 % (mole fraction), and its Mg# ranges between 0.56 and 0.64 with an average of 0.61.

Table 2 Representative microprobe analyses (wt.%) of pyroxenes in the Abu Rumeil syenites
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Biotites are similar in both alkali feldspar and quartz syenites (Table 3): iron-rich, siliceous biotites (near annite) with FeO*/MgO ratio ranging from 1.35 to 2.52, with an average of 2.12. They show the chemical characteristics of primary igneous biotite as well as somewhat re-equilibrated magmatic instances (Fig. 3d). The composition of biotite can be used to discriminate the association of parental magma (Nachit et al. 1985; Abdel-Rahman 1994), and on the Al2O3 vs. FeO(t) biotite discrimination diagram of Abdel-Rahman (1994), the analyzed biotites plot in the field of alkaline suites (Fig. 3e).

Table 3 Representative microprobe analyses (wt.%) of biotite in the Abu Rumeil syenites
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Representative chemical analyses of the primary amphiboles are listed in Table 4. According to the classification of Leake (1997), they are calcic amphiboles classified as magnesio-hornblende (Fig. 3f).

Table 4 Representative microprobe analyses (wt.%) of amphiboles in the Abu Rumeil syenites
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Fe-Ti oxides include both magnetite and ilmenite (Table 5). Ti-rich magnetite (4.0–6.0 wt.% TiO2) was observed in both rock types. Ilmenite is compositionally uniform with 46.1 to 47.3 wt.% FeO and 46.9 to 49.7 wt.% TiO2. It is enriched in MnO (3.3–3.7 wt.%), perhaps due to substitution of Mn for Fe2+ with increasing oxygen fugacity under magmatic conditions (Deer et al. 1992).

Table 5 Representative microprobe analyses (wt.%) of Fe-Ti oxides in the Abu Rumeil syenites
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Geochemical characteristics

Major and trace element analyses and calculated normative minerals of the Abu Rumeil syenites are given in Table 6. Normative mineral compositions of the syenites were calculated using the Minpet Software (Richard 1995). The norms indicate that the alkali feldspar syenite is, on average, higher in color index (7.91–10.37) than the quartz syenites (6.34–9.48), although there is significant overlap. The syenites have a wide range of silica contents (62.3–67.8 wt.% SiO2), and thus, their differentiation indices (Thornton and Tuttle 1960) range from 78.8 to 90.6 (Table 6). They are all quartz-normative (1.4–3.2 vol.%); none of the analyzed samples contain the normative nepheline characteristic of undersaturated syenitic rocks. The syenites have relatively high abundances of total alkalis (9.96–11.35 wt.%), Al2O3 (15.33–17.31 wt.%), and Ba (982–2628 ppm). The chemical index of alteration (CIA = molecular [Al2O3 / (Al2O3 + CaO + Na2O + K2O)] × 100) in the studied syenites varies between 46.03 and 49.03 (Table 6), within the range of fresh rocks (45–55; Nesbitt and Young 1982).

Table 6 Chemical and normative compositions of the syenites of G. Abu Rumeil, south Sinai, Egypt
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In the classification of Streckeisen and Le Maitre (1979), the analyzed samples plot in the syenitic fields (Fig. 4a). All samples plot in the field of alkaline syenite in the total alkali-silica (TAS) diagram (Fig. 4b) and in the shoshonitic field in the K2O vs. SiO2 diagram (Fig. 4c). The studied samples are geochemically characterized by high total alkali metals (9.96–11.35 K2O + Na2O %), low TiO2 (<0.99 wt.%), high A12O3 (15.33–17.31 wt.%), and enrichment in the LILE. All these values are characteristic of shoshonitic rocks (Morrison 1980). Plotting the syenite data in the shoshonite field of the Th-Co diagram (Fig. 4d) further confirms their shoshonitic affinity. The agpaitic indices of the investigated syenites are more than 0.87 (Table 6), consistent with their alkaline affinity (Liégeois et al. 1998). They are metaluminous with ASI [=molar Al2O3 / (CaO + K2O + Na2O)] ranging between 0.86 and 0.97.

Fig. 4
figure 4

a Q/(F/)-ANOR diagram for normative classification of Abu Rumeil syenites (Streckeisen and Le Maitre 1979). b Total alkalis vs. silica (TAS) diagram (Le Bas et al. 1986) with the discriminating boundary between alkaline and sub-alkaline fields after Miyashiro (1978). c SiO2 vs. K2O plots (Pearce et al. 1990). d Co vs. Th diagram; the field demarcations are after Hastie et al. (2007). Symbols are as in Fig. 3

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Selected major and trace elements for syenites are plotted against SiO2 (Figs. 5 and 6). They show continuous trends with no compositional gap, although some elements show a break in slope at the boundary between alkali-feldspar syenites and quartz syenites at about 65 % SiO2. Major elements show a clear decrease of Al2O3, TiO2, Fe2O3, MgO, CaO, and P2O5, while Na2O and K2O are constant with increasing SiO2. Regarding trace elements, Rb, Nb, Y, Zr, Th, Ga, and Ta increase whereas Sr, Ba, and V decrease with increasing SiO2 content. According to the criteria of Frost et al. (2001), the syenites of Abu Rumeil are ferroan A-type rocks (Fig. 7a). Furthermore, they plot in the field of A-type granites on a plot of Ga/Al vs. FeO(t)/MgO (Fig. 7b).

Fig. 5
figure 5

Silica variation diagrams of major oxides for the syenites of Abu Rumeil. Symbols are as in Fig. 3

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Fig. 6
figure 6

Silica variation diagrams of some trace elements for the syenites of Abu Rumeil. Symbols are as in Fig. 3

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Fig. 7
figure 7

a FeO(t)/(FeO(t) + MgO) vs. SiO2 diagram for the studied syenites (Frost et al. 2001). b Ga/Al vs. FeO(t)/MgO (Whalen et al. 1987). c Chondrite-normalized REE plots. d NMORB-normalized multi-element diagrams for the averages of alkali feldspar syenite and quartz syenite. Symbols are as in Fig. 3

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The REE concentrations of Abu Rumeil syenites are given in Table 7. Their chondrite-normalized REE patterns (using the chondrite values of Pearce 1983) are generally similar, but there are clear systematic trends (Fig. 7c). All samples show enrichment of light over heavy REE [(La/Lu)N = 7.58–25.17], with a surprising decrease in slope from the more primitive alkali feldspar syenites towards the more evolved quartz syenites. They display a wide range of Eu anomalies [(Eu/Eu*)n = 0.45–1.50], with increasingly negative Eu anomalies in the more evolved quartz syenites. Negative Eu anomalies (Eu/Eu = 0.45–1.13) in the quartz syenite suggest little feldspar accumulation in this rock, whereas the positive Eu anomaly (Eu/Eu = 1.48–1.50) in the alkali feldspar syenite could be consistent with a cumulate origin.

Table 7 Rare earth element (ppm) concentrations and significant elemental ratios of the Abu Rumeil syenites
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N-MORB-normalized multi-element diagrams for the averages of alkali feldspar syenite and quartz syenite (Fig. 7d) are again generally similar, but the more fractionated nature of the trace element pattern in the less-evolved alkali feldspar syenites is again evident. All patterns show about one order of magnitude enrichment in the LILE (K, Rb, and Ba) and Pb relative to HFSE (Nb, Ta, and Ti). This signature is generally consider to be due to enrichment by fluid addition and is often attributed to modification of magmatic sources by fluids derived from a subducting slab (Kay and Mahlburg-Kay 1991). The presence of such a signature in intra-plate magmas is puzzling, as discussed below.

Discussion and conclusions

The post-collisional phase (630–580 Ma) of the northernmost ANS included voluminous intrusion and eruption of calc-alkaline and alkaline magmatism (e.g., Be’eri-Shlevin et al. 2009; Eyal et al. 2010; Farahat and Azer 2011). The origin of this magmatism is controversial. A variety of petrogenetic models have been proposed for the post-collisional A-type magmatism of the ANS, including mantle origin with a significant crustal contribution (e.g., Stern and Voegeli 1987; Beyth et al. 1994; Kessel et al. 1998; Mushkin et al. 2003; Azer 2006; Samuel et al. 2007; Jarrar et al. 2008) and anatectic melts of various crustal sources (e.g., Clemens et al. 1986; Creaser et al. 1991; King et al. 1997; Abdel-Rahmen 2006; Ali et al. 2009). The data presented herein from the Abu Rumeil syenites address the nature and source of the parental magma as well as the conditions under which the suites of observed rocks were generated by magmatic differentiation processes.

Geotectonic affinities

Previous studies dealing with the tectonic setting of alkaline magmatism in southern Sinai have reached contradictory conclusions (e.g., Samuel et al. 2007; Azer 2013; Eyal et al. 2014). An important question is whether or not emplacement of alkaline rocks in southern Sinai was related to rifting. Some authors have concluded that the alkaline rocks of the KRC mark a non-orogenic period of tensional stress, block faulting, and differential uplift (Bentor 1985), while others considered them to be rift-related (Abdel Maksoud et al. 1993; Abdel Khalek et al. 1994). The syenitic rocks of southern Sinai are spatially associated with tectonic lineaments that developed either during or following the post-collisional phase. This spatial association, together with mantle-derived isotopic signatures (Moreno et al. 2014), suggests that parental magmas were derived from deep sources and that the lineaments formed conduits to reach the upper crust. Field investigations reveal that the syenites of Abu Rumeil represent the inner ring dyke of the KRC. The intrusion of the KRC was preceded by the intrusion of dykes with various compositions that cut across the late Neoproterozoic calc-alkaline rocks and mark the switch from calc-alkaline to alkaline magmatism, as a result of extension (Friz-Töpfer 1991). These dykes are in turn intruded by the alkaline phase, which represents the youngest igneous activity in the study area.

The geochemical affinity and mineral chemistry of the Abu Rumeil syenites are consistent with post-collisional activity and a within-plate tectonic setting. In the tectonic discrimination diagram of Pearce et al. (1984), they plot in post-collisional and within-plate fields (Fig. 8a). Geochemically, all the syenites are oversaturated despite their alkaline affinity; undersaturated rocks are not observed. This is inconsistent with the character of magmatism developed in intra-continental rifts (Black et al. 1985; Wilson 1994; Menuge et al. 2002). Classifications based on the mafic magmatic minerals in the Abu Rumeil syenites are consistent with this tectonic inference. In particular, the analyzed biotites are mostly iron-rich and similar to biotites of alkaline igneous rocks of anorogenic tectonic settings (Abdel-Rahman 1994).

Fig. 8
figure 8

a Rb vs. Y + Nb diagram for the syenites of Abu Rumeil (Pearce et al. 1984). b–f Some major and trace element relationships in the syenites of Abu Rumeil. VAG volcanic-arc granites, syn-COLG syn-collisional granites, WPG within-pate granites, ORG ocean-ridge granites. Post-collisional granite field is from Pearce (1996). Symbols are as in Fig. 3

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Petrogenetic source

In general, alkaline syenitic rocks associated with A-type granites have been described from various A-type complexes worldwide (Bonin 2007; Whalen et al. 1987; Liégeois et al. 1998; Azer 2006; Jahn et al. 2009; Litvinovsky et al. 2011). Several petrogenetic models have been proposed for the formation of syenites, which mainly include (1) fractionation from mantle-derived basaltic-andesitic magmas (e.g., Currie 1989; Sutcliffe et al. 1990; Litvinovsky et al. 2002; Yang et al. 2005; Bonin 2007; Turner et al. 1992; Miyazaki et al. 2003; Conceição and Green 2004; Wang et al. 2005; Shellnutt et al. 2009), (2) partial melting of crustal material (Whalen et al. 1987; Allen and Chappell 1992; Lubala et al. 1994; Tchameni et al. 2001; Jahn et al. 2009; Litvinovsky et al. 2011), and (3) interaction between mantle-derived magmas and crustal rocks followed by subsequent differentiation of the resulting hybrid melts (e.g., Zhao et al. 1995; Stevenson et al. 1997; Litvinovsky et al. 1999; Mingram et al. 2000; Vernikovsky et al. 2003; Jung et al. 2007; Ying et al. 2007).

In southern Sinai, the syenitic rocks are rare and are closely associated with A-type granitoids. Previous discussions of the genesis of southern Sinai syenitic rock have focused on their close association with granites, rather than the individual petrological character of the syenites themselves. All petrogenetic models agree that A-syenites in southern Sinai were emplaced in post-collisional and within-plate tectonic settings. The studied syenites have a shoshonitic character, but the origin of shoshonitic syenites is also a matter of debate. Echoing the conflicting theories proposed for syenites worldwide, the A-type igneous rock suites in the ANS including southern Sinai have been attributed either to extensive fractional crystallization of mantle-derived magmas (e.g., Stern and Gottfried 1986; Bonin 2007) to partial melting of crustal material (e.g., Clemens et al. 1986; Creaser et al. 1991; King et al. 1997; Abdel-Rahmen 2006; Farahat et al. 2007; Ali et al. 2009) or to a mantle source with extensive crustal contamination (e.g., Azer 2007; Samuel et al. 2007; Be’eri-Shlevin et al. 2009).

In the following discussion, we will argue that the present data can rule out both end-member models of pure mantle source or pure crustal source. Both components are required, but the dominant petrogenetic mechanism is differentiation from mantle-derived mafic magmas, as often advocated for the ANS A-type igneous rocks (e.g., Stern and Voegeli 1987; Beyth et al. 1994; Kessel et al. 1998; Mushkin et al. 2003; Jarrar et al. 2008), with relatively minor crustal contamination. We do have isotope data on the Abu Rumeil suite, but a number of other syenitic rocks in southern Sinai have isotope signatures (Hf, U, and Pb) indicating a predominantly mantle rather than crustal source (Morag et al. 2011).

Syenitic magmas can occur in the silica undersaturated or saturated alkaline igneous series (Lameyre and Bowden 1982). Silica-saturated syenites can be related to the sodic (Na2O/K2O >2; Le Maitre 2002) or shoshonitic series (Na2O/K2O ≈ 1; Morrison 1980). The studied syenites are silica saturated and have Na2O/K2O ratios (0.99–1.11) characteristic of shoshonitic rocks. The shoshonitic series can be produced by decompression melting of a metasomatized mantle containing small amounts of phlogopite and pargasite (Conceição and Green 2004). The syenitic rocks of Abu Rumeil are characterized by high K2O + Na2O (9.96–11.35 wt.%). Although high alkali concentrations are characteristic of evolved crustal rocks, numerous geochemical and experimental studies (Conceição and Green 2004) argue that such high-alkali rocks are commonly derived from K-enriched basaltic and andesitic mantle magmas, with only a minor crustal contribution. The positive Eu anomaly and low Th/Ce ratios (0.06–0.15) in the more mafic members of the Abu Rumeil syenites indicate that crustal contamination, at least in the alkali-feldspar syenites, was limited (Taylor and McLennan 1995; Sun and McDonough 1989). Furthermore, experiments show that syenites and melts plausibly parental to syenites are not produced directly by anatexis of common crustal sources (Montel and Vielzeul 1997).

However, direct derivation of these syenites from a mantle source is difficult to reconcile with the available geochemical data, which indicate some role for crustal contribution during the evolution of syenites. Overlap in the REE patterns of the studied syenites excludes simple fractional crystallization as the sole process of magma evolution; differentiation must have been accompanied by some assimilation, most likely a crustal contribution. Finally, there is textural evidence for a contamination process. There are large mafic xenocrysts overgrown by primary K-feldspars. They show a spongy cellular texture due to the dissolution of crystals by the enclosed magma under disequilibrium conditions. These xenocrysts may represent either a remnant high-temperature phase that crystallized at greater depths or possibly entrapped crystals. In either case, they point to a petrogenesis involving mafic, presumably mantle-derived magma interacting with an evolved component.

The studied syenites have moderate depletion in Ta-Nb relative to the neighboring LILE, which is a geochemical characteristic feature of subduction-related magmas. Moreover, they show Ti depletion as commonly observed in subduction-related magmas. Shoshonitic rocks from continental within-plate settings may exhibit either arc-type geochemical features (Wyborn 1992; Leat et al. 1998) or OIB-type ones (Rogers et al. 1998). Since active subduction is not considered plausible in the within-plate post-collisional tectonic setting of the ANS, the subduction signature for the studied syenites is presumably inherited, due to partial melting of a lithospheric mantle source following a previous subduction event beneath the ANS (Azer et al. 2012; Khalil et al. 2015). This interpretation is not unique at this time; other means of inheriting a subduction signature could be proposed, and further data may be required to distinguish among them.

Crystallization conditions

The chemistry of some minerals can be used to elucidate and constrain both parental magma composition and the magmatic conditions at which they crystallized (e.g., Lindsley 1983; Blundy and Holland 1990; Putirka 2008; Ridolfi et al. 2010). The application of several amphibole geobarometers to the syenites, using the AMPH-IMA97 Excel Sheet (Hammarstrom and Zen 1986; Hollistaer et al. 1987; Schmidt 1992), yielded crystallization pressure estimates between 1.5 and 3.7 kbar with an average of 2.1 kbar (the uncertainty reflects both the heterogeneity in measured Al contents of amphibole and the differences among barometric calibrations). These crystallization pressures indicate a relatively shallow depth of emplacement (<10 km).

Several geothermometers were used to calculate the crystallization temperatures of the Abu Rumeil syenites. A range of two-pyroxene thermometers (Putirka 2008) yield temperatures between 894 and 1107 °C. The clinopyroxene thermometer of Lindsley (1983) is sensitive to the heterogeneity among analyzed grains and gives a wide range of crystallization temperatures, from 800 to 1100 °C (Fig 4b). These temperatures of clinopyroxene crystallization represent temperatures that existed in magma chambers or conduits well before emplacement, since petrographic observations indicate that clinopyroxene is the earliest crystallized mineral phase.

The Fe2+/(Fe2+ + Mg) ratios of the primary amphiboles of the Abu Rumeil syenites are plotted against AlIV in Fig. 9. The low Fe2+/(Fe2+ + Mg) (0.31–0.45) ratios indicate strikingly high oxygen fugacity conditions (Anderson and Smith 1995).

Fig. 9
figure 9

Fe2+/(Fe2+ + Mg) vs. AlIV diagram of the primary amphiboles (Anderson and Smith 1995). Symbols are as in Fig. 3

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Evidence of fractional crystallization and accumulation

The geochemical characteristics of Abu Rumeil syenites mostly support their evolution from a parental magma by fractional crystallization accompanied by minor contamination. On Rb vs. K2O/Rb plots (Fig. 8b), the studied syenites exhibit a single compositional trend suggestive of derivation from a common parental magma. The general trends on variation diagrams of major oxides and some trace elements (Figs. 5 and 6) are broadly consistent with fractional crystallization and help to identify the important controlling phases. The decreases in CaO, MgO, FeO, TiO2, and P2O5 with increasing SiO2 indicate significant roles for pyroxene, amphibole, plagioclase, apatite, and magnetite and/or ilmenite. The role of feldspar fractionation is indicated by covariation on plots of Sr vs. Ba and Rb/Sr vs. Rb/Ba (Figs. 8 c, d), with decreasing Sr and Ba implying plagioclase fractionation during magmatic evolution. Increasing Rb/Ba ratios are especially indicative of alkali feldspar fractionation (Henderson 1982). Decreasing Fe2O3 and MgO with increasing SiO2 (Fig. 5) suggests fractionation of primary mafic minerals as well. MgO vs. Rb/Sr and TiO2 vs. Rb/Sr variation diagrams (Figs. 8 e, f) suggest fractionation of amphibole, pyroxene, and Fe-Ti oxides. The increasingly sodic plagioclase and iron-rich clinopyroxene in the quartz syenites compared to alkali feldspar syenites is consistent with ongoing fractional crystallization to reach the higher SiO2 members of the suite (Shellnutt et al. 2016).

The opposite signs of the Eu anomalies in the alkali feldspar syenites and the quartz syenites, however, point to a problem with a simple fractional crystallization model for the entire suite. In fact, there are subtle changes in slope at 65 % SiO2 on most of the major and trace element variation diagrams that imply that different processes may be responsible for the compositional variations within the two rock types. Unless the parental magma inherited a strongly positive Eu anomaly, the REE patterns require that the alkali feldspar syenites were generated by crystal accumulation whereas the quartz syenites represent a liquid line of descent generated by crystal fractionation. We attempt to develop a quantitative model of these dual processes by starting with a liquid plotting between the two suites, at about 64.8 % SiO2 and lacking a Eu anomaly. Evidently, a liquid with 64.8 % SiO2 is not a primary mantle-derived melt; considerable differentiation and possible assimilation will already have affected it. In the absence of mafic members of the suite, however, modeling of the earlier history of evolution is a poorly constrained exercise. We therefore restrict our attention to the relationships among the collected samples.

We computed the liquidus minerals of our selected starting liquid with the rhyolite MELTS model (Gualda et al. 2012) and then subtracted an assemblage of these minerals to simulate the quartz syenite trend while adding a slightly different assemblage of these minerals to simulate the alkali feldspar syenite trend. Trace element partition coefficients for each mineral are based on McKenzie and O’Nions (1991, 1995). We were guided by the pressure, temperature, and oxygen fugacity inferences discussed above in our search for crystallization conditions that produce an optimal fit. However, finding a set of conditions where the selected primary liquid is multiply saturated with the full assemblage of plagioclase, alkali feldspar, ilmenite, pyroxene, and amphibole proved impossible. Most conditions lead to one mineral or another being first on the liquidus, which gives an interval where the liquid line of descent has the wrong slope in one or more variation diagrams. Some such simulations are shown in Fig. 10; water-saturated conditions have ilmenite or magnetite on the liquidus and TiO2 or Fe2O3 decrease too quickly, whereas most conditions with lower water activity have plagioclase on the liquidus and K2O increases too much before alkali feldspar appears. Hence, the model shown here is a generalization from the rhyolite-MELTS results rather than a single simulation run.

Fig. 10
figure 10

Differentiation model for the Abu Rumeil syenites using major oxides. Symbols are as in Fig. 3

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We selected the compositions at the first appearance of each phase on the liquidus at 3 kbar, oxygen fugacity one log unit above the QFM buffer, and with 2.3 wt.% H2O in the initial liquid. The liquidus temperature of 978 °C is consistent with the thermometry results above. Given these mineral compositions, fractionation of an assemblage of 50 % alkali feldspar, 38.5 % plagioclase, 6 % clinopyroxene, 1 % apatite, 1 % ilmenite, and 0.35 % Ti-magnetite matches reasonably well the slope of the quartz syenite data in all the major and trace element variation diagrams (Figs. 5 and 6). The model predicts that some of the incompatible elements (Zr, Y, Nb, Th, Ta) become too enriched, compared to the evolved quartz syenites, after extensive fractionation, which may be attributed to imperfect fractional crystallization or to the appearance of unmodeled accessory phases such as zircon and monazite. The compatible behavior of V during fractionation is consistent with the oxidizing conditions selected. The selected fractionating assemblage generates a negative Eu anomaly of suitable magnitude and an overall flattening of the REE pattern (decreasing La/Yb) with progressive fractionation.

On the other hand, assimilation of 45 % alkali feldspar, 43 % plagioclase, 8 % clinopyroxene, 0.8 % apatite, 1.8 % ilmenite, and 1.5 % Ti-magnetite reproduces the major and trace element trends of the alkali feldspar syenites, including the increasing positive Eu anomaly and steeper REE patterns with decreasing SiO2. These phase proportions are broadly similar to those that appear in the rhyolite-MELTS simulations, but were tuned within the uncertainties of the calculations in order to best fit the data.

Although amphibole, biotite, quartz, and orthopyroxene appear late in the fractionation trends, they are not necessary to explain the whole-rock chemical variations and it is likely that these phases appeared so late in the crystallization of the quartz syenites that they crystallized in situ without a fractional effect on the bulk compositional evolution of the rock suite. The differentiation model shown in Figs. 10 and 11 is not unique, but it is thermodynamically and geologically plausible and it explains the bulk of the data. We put it forward as a representative example.

Fig. 11
figure 11

Differentiation model for the Abu Rumeil syenites using trace elements. Symbols are as in Fig. 3

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