Elsevier

Lithos

Volumes 400–401, 1 November 2021, 106395
Lithos

Research Article
Geochemistry and mineralogy of the Jebel Aja Igneous Intrusion and the associated exotic pegmatites, Arabian Shield, Saudi Arabia

https://doi.org/10.1016/j.lithos.2021.106395Get rights and content

Highlights

  • Two consecutive, cogenetic granitoid pulses make up the Jebel Aja Igneous Intrusion.

  • The whole suite is alkaline and A-type in character and post-collisional.

  • First record of pyroxmangite, thortveitite and thalénite-(Y) in the Arabian Shield

  • A novel (Sc, Y, vacancy)-rich pyroxenoid and a vacancy-rich bustamite are observed.

  • Pyroxmangite and exotic pyroxenes suggest an elevated pressure of emplacement.

Abstract

We describe the Jebel Aja igneous intrusion (JAII), which sits at the farthest northeastern edge of the exposed Arabian Shield in Saudi Arabia. The JAII consists of late Neoproterozoic post-collisional A-type granites, and was emplaced during tectonic extension. It is a composite pluton consisting of a granitic core (monzogranite, syenogranite and alkali feldspar granite) surrounded by a later phase of alkaline and peralkaline granites. Pegmatites occur as pockets and dykes at the contact between the alkaline/peralkaline granites and syenogranite. The pegmatites have sharp contacts with their host granites and show variable colors (principally pink and white), grain sizes, textures, and mineral compositions. We report here that the white pegmatites host pockets with a unique mineralogy that includes the first observation of pyroxmangite, thortveitite and thalénite-(Y) in the whole Arabian Shield as well as a novel (Sc, Y, vacancy)-rich pyroxenoid and a vacancy-rich bustamite with excess octahedral Si. The pyroxmangite-dominated lithology also features accessory quartz, albite, spessartine, pyrochroite, muscovite, and fluorite. The presence of pyroxmangite rather than rhodonite, together with vacancy-rich and Si-excess pyroxenes, suggests an elevated pressure of emplacement. The granites of the JAII are highly evolved and have undergone extreme fractional crystallization of feldspars, leading to increases in the concentration of volatile and incompatible elements in the residual melt, which resulted eventually in injection of coeval pegmatites into the mostly solidified host granite. The syenogranite and alkaline/peralkaline granite of the JAII contain sodic amphibole and have geochemical characteristics common among within-plate rare-metal bearing A-type rocks. They have positive Nb-Ta anomalies that increase from syenogranite to alkaline/peralkaline granite. The pink pegmatite is highly mineralized and contains high concentrations of Nb (1540–1769 μg/g), Ta (103–136 μg/g), Y (1116–1616 μg/g), Zr (6362–9707 μg/g), Hf (215–264 μg/g), Th (278–384 μg/g), U (110–147 μg/g), and ∑REE (2334–3251 μg/g).

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

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