Research ArticleGeochemistry and mineralogy of the Jebel Aja Igneous Intrusion and the associated exotic pegmatites, Arabian Shield, Saudi Arabia
Graphical abstract
Introduction
The Arabian-Nubian Shield (ANS), an assembly of Neoproterozoic (850–590 Ma) crystalline rocks, represents one of the largest tracts of juvenile crust on Earth (Johnson, 2003; Meert, 2003; Stoeser and Frost, 2006). It constitutes the northern part of the East African Orogen (Stern, 1994) and was a contiguous mass until the opening of the Red Sea, which divided it into the Nubian Shield to the west and the Arabian Shield to the east. The ANS comprises a collage of ophiolite sequences and associated volcanic arcs, which amalgamated during the assembly of West and East Gondwana and were then intruded by a voluminous range of geochemically distinctive granitoids and mafic-ultramafic intrusions (e.g., Abdel-Karim, 1992, Abdel-Karim, 1996; Ali et al., 2010; Azer et al., 2017; Genna et al., 2002; Stern, 1994; Stoeser and Frost, 2006).
The Arabian Shield hosts a number of discrete post-collisional A-type alkaline/peralkaline igneous complexes and pegmatites with significant enrichment of rare metals (e.g., Abdallah et al., 2020; Abuamarah, 2020; Elliott et al., 1999; Moghazi et al., 2011, Moghazi et al., 2015; Qadhi Talal, 2007). Among these, the Jebel Aja igneous intrusion (JAII), outcropping at the very northeastern limit of basement outcrop, is a well-exposed example. The main rock units of the JAII have been studied by several authors (Abdallah et al., 2020; Abuamarah, 2020; Ekren et al., 1987; Hereher and Abdullah, 2017; Qadhi Talal, 2007; Stuckless et al., 1984), but there are no published studies of the pegmatites. Hence the main focus of this work is to present the first report concerning the exotic pegmatites associated with the JAII.
A number of the minerals hosted in these pegmatites (pyroxmangite, thortveitite, thalénite-(Y)) are reported here for the first time in the whole ANS, and two exotic Sc-rich phases found in cleavage-plane fill in the pyroxmangite are entirely new. Pyroxmangite, thortveitite and thalénite-(Y) are rare silicate minerals. Pyroxmangite (MnSiO3) is a Mn-rich end-member of the pyroxenoid group, with some economic importance because of its use in jewelry and ornamental objects. It was first described by Ford and Bradley (1913) and its crystal structure was determined by Liebau (1959). Pyroxmangite is very similar to its dimorph rhodonite; they may occur together as bladed intergrowths (Jefferson et al., 1980; Michailidis and Sofianska, 2010; Millsteed et al., 2005; Ohashi et al., 1975; Pinckney and Burnham, 1988). Both are triclinic but they can be distinguished by Raman spectra or X-ray diffraction, and rhodonite generally contains higher Ca contents than pyroxmangite. The equilibrium phase boundary between pyroxmangite and rhodonite in pure MnSiO3 has a positive Clapeyron slope, with pyroxmangite on the high-pressure, low-temperature side (Maresch and Mottana, 1976). Thortveitite is a scandium yttrium silicate mineral, nominally (Sc,Y)2Si2O7. It is the primary source of scandium and occurs mainly in granitic pegmatites. Thalénite-(Y) has a nominal formula of Y3Si3O10F, usually occurring within fluorite in granitic pegmatites.
The present study aims to provide complete mineralogical characterization of the JAII pegmatite mineralogy using classical methods and advanced techniques such as electron probe microanalyzer (EPMA), Raman spectroscopy, and scanning electron microscopy (SEM). This is presented alongside detailed geological, mineralogical, and geochemical context data on the host syenogranite and alkaline/peralkaline granite, in order to shed light on the magmatic sources and petrogenetic processes responsible for formation of the JAII and its pegmatites. We discuss the evidence for high pressure crystallization of the pegmatite mineralogy and its possible implications for vertical tectonic motions on the edge of the Arabian Shield in the post-collisional extensional period.
Section snippets
Geological setting
The Arabian Shield (Fig. 1) was formed by the accretion of inter-oceanic island arcs upon closure of the Mozambique Ocean (Nehlig et al., 2002; Robinson et al., 2014). It is characterized by presence of distinct terranes that are marked by the presence of ophiolitic ultramafic bodies within orogenic belts (Agar, 1992; Robinson et al., 2014). After the accretion, a period of extensional tectonism, orogenic collapse, and emplacement of post-collisional alkaline magmas resulted in one of the
Analytical methods
Whole rock geochemical analyses were performed at ALS Geochemistry, Vancouver BC, Canada. Whole rock samples were crushed and pulverized in an agate ring mill. Major elements were determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) following lithium metaborate/lithium tetraborate (LiBO2/Li2B4O7) fusion and digestion in 4% HNO3/2% HCl. Trace elements, including rare earth elements, were determined by Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) using the same
Petrography
The granitic rocks of the JAII show wide variations in their textures and mineralogical compositions (Abdallah et al., 2020; Abuamarah, 2020). According to Abuamarah (2020), the early phase of the JAII includes monzogranite, syenogranite, granophyre and alkali feldspar granite, whereas the later phase includes alkaline granite and peralkaline granite. In the present work, the pegmatites associated with JAII are described for the first time. Brief petrographic descriptions of the pegmatites and
Electron microprobe, SEM/EDS, EBSD and Raman spectroscopy analyses
The identification of the minerals under the microscope was refined and supported by EPMA, SEM/EDS, EBSD and Raman methods. EPMA analyses of the essential minerals were performed in samples of syenogranite, peralkaline granite, and pyroxmangite-bearing pegmatite. The analyzed minerals in the syenogranite and peralkaline granite included K-feldspar, albite, katophorite, sodic amphibole, aegirine, Fe-Ti oxides and apatite. The analyzed minerals in the pyroxmangite-rich pegmatite sample are
Geochemical characteristics
Whole-rock geochemical analyses are provided for 15 samples representing the syenogranite, alkaline/peralkaline granites and pegmatites. Major oxides and calculated CIPW norms are given in Table 1, trace element concentrations in Table 2, and rare-earth elements with selected normalized REE rations in Table 3. Although all samples are high-SiO2 granitoids, there is a distinction between the SiO2 contents of the pegmatites (74.1–76.2 wt%) and the other samples (72.5–74.2 wt%). Using the R1-R2
Petrogenesis
Studies of the JAII published to date have drawn a diverse conclusion about its magmatic source and petrogenetic evolution, leaving considerable unresolved controversy (Abdallah et al., 2020; Abuamarah, 2020; Ekren et al., 1987; Hereher and Abdullah, 2017; Qadhi Talal, 2007; Stuckless et al., 1984). On the Rb/Nb vs. Y/Nb and Nb-Y-Ce diagrams of Eby (1990), the studied granitoid and pegmatite samples straddle the boundary between the A1 and A2 types that are thought to be associate with mantle
Summary
We document the first known occurrence of pyroxmangite, the high-pressure dimorph of MnSiO3, in the whole ANS, in a pegmatite associated with the Jebel Aja Igneous Intrusion. It is associated with an assemblage of Mn, Y, Sc, and REE ore minerals including thortveitite, spessartine, pyrochroite, thalénite-(Y), a novel (Sc, Y, vacancy)-rich variety of pyroxmangite, and a vacancy-rich bustamite with excess octahedral Si. The JAII consists of two consecutive but apparently cogenetic pulses of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to extend their appreciation and gratitude to the Deanship of Scientific Research, King Saud University for funding and supporting this work through Research Group No. RG-1436-036. Analytical work at the Caltech GPS Division Analytical Facilities is partially supported by NSF Grants EAR-0318518 and DMR-0080065. PDA acknowledges NSF award 1911902. The authors highly appreciate thoughtful reviews by the two anonymous reviewers. In addition, the authors are also indebted to
References (76)
- et al.
Petrogenesis of the post-collisional rare-metal-bearing Ad-Dayheen granite intrusion, Central Arabian Shield
Lithos
(2021) The tectono-metallogenic evolution of the Arabian Shield
Precambrian Res.
(1992)- et al.
Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the Allaqi-Heiani suture, South Eastern Desert of Egypt
Gondwana Res.
(2010) - et al.
A classification of volcanic and plutonic rocks using R1-R2 diagrams and major-element analyses-its relation with current nomenclature
Chem. Geol.
(1980) - et al.
Nb–Th–Zr mineralization in microgranite – microsyenite at Jabal Tawlah, Midyan region, Kingdom of Saudi Arabia
J. Afr. Earth Sci.
(1986) The A-type granitoids: a review of their occurrence and chemical characteristics and speculations on their petrogenesis
Lithos
(1990)- et al.
Rare earth abundances in chondritic meteorites
Geochim. Cosmochim. Acta
(1978) - et al.
Proterozoic tectonism of the Arabian Shield
Precambrian Res.
(2002) Island arc and continent-building magmatism: a review of petrogenetic models based on experimental petrology and geochemistry
Tectonophysics
(1980)The application of trace elements to the petrogenesis of igneous rocks of granitic composition
Earth Planet. Sci. Lett.
(1978)