Origin and Petrogenetic Implications of Spessartine Garnet in Highly‐Fractionated Granite from the Central Eastern Desert of Egypt

Abstract

A highly‐fractionated garnet‐bearing muscovite granite represents the marginal granitic facies of the Abu‐Diab multiphase pluton in the Central Eastern Desert of Egypt. New electron microprobe analyses (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) data from garnets are reported, in order to constrain their origin and genesis. Garnet in the Abu‐Diab host granite is euhedral to subhedral, generally homogeneous and, in rare cases, it shows weak zonation. The garnet contains appreciable amounts of MnO and FeO, with lesser amounts of MgO and CaO, yielding an end‐member formula of Sps61–72Alm25–35Prp1–4Adr0–1. Moreover, it is depleted in large ion lithophile elements (LILE) with lower values of Ba, Nb and Sr relative to the primitive mantle. Additionally, it contains high concentrations of HREE and Y and their REE pattern shows strong negative Eu anomalies. The garnet was crystallized under relatively low temperature (646°C–591°C) and pressure (< 3 kbar) conditions. The textural and chemical features indicate that the garnet is magmatic in origin and is chemically similar to that from highly‐fractionated A‐type granite. It was probably formed at the expense of biotite in a highly‐evolved MnO‐rich magma and/or by hydroxyl complexing of Mn during the ascending fluid phases.

Unraveling the Genesis of Highly Fractionated Rare-Metal Granites in the Nubian Shield via the Rare-Earth Elements Tetrad Effect, Sr–Nd Isotope Systematics, and Mineral Chemistry

“…The Neoproterozoic ANS juvenile continental crust was formed by various geodynamic and tectonic processes including the breakup of Rodinia, collision between East and West Gondwana, followed by late to post-collision extension (∼650–540 Ma), and stabilization of the ANS crust. ,, There is an agreement in most recent studies (e.g., refs ,) that the lithospheric delamination and slab breakoff is the main geodynamic process responsible for the formation of most highly fractionated calc-alkaline and rare-metal granites in the ANS.…”

Magmatic Evolution and Rare Metal Mineralization in Mount El-Sibai Peralkaline Granites, Central Eastern Desert, Egypt: Insights from Whole-Rock Geochemistry and Mineral Chemistry Data

“…Various petrogenetic models have been proposed to explain the formation of A-type rocks in general [55] and in the ANS in particular [56][57][58]. However, no single model adequately explains the wide range of chemical compositions observed among these granitic rocks.…”

Magmatic and post-magmatic evolution of post-collisional rare-metal bearing granite: The Neoproterozoic Homrit Akarem Granitic Intrusion, south Eastern Desert of Egypt, Arabian-Nubian Shield

“…The texture and chemical composition of garnet can be used to indicate pressure-temperature-fluid activity conditions during its growth (du Bray, 1988;Whitworth, 1992;Villaros et al, 2009;Zhang et al, 2012;Bekele and Sen, 2020). Spessartine-almandine garnets in general have many possible origins: (1) they may crystallize directly form Mn-rich peraluminous magma under suitable temperature and pressure conditions (du Bray, 1988;Dahlquist et al, 2007;Yang et al, 2013;Sami et al, 2020); (2) they may crystallize as phenocrysts in primary magma at deeper levels and be transported to higher crustal levels and emplaced (Harangi et al, 2001); (3) they may be a residual phase during partial melting (René and Stelling, 2007); (4) they may form as a secondary…”