Magmatic unmixing in spinel from late Precambrian concentrically-zoned mafic–ultramafic intrusions, Eastern Desert, Egypt Ahmed Hassan Ahmed a , Hassan Mohamed Helmy b, ⁎ , Shoji Arai c , Masako Yoshikawa b a Geology Department, Faculty of Science, Helwan University, Cairo, Egypt b Institute of Geothermal Sciences, Kyoto University, Japan c Department of Earth Sciences, Kanazawa University, 920-1192, Japan Received 21 March 2007; accepted 27 November 2007 Available online 14 December 2007 Abstract Spinel is widespread in the ultramafic core rocks of zoned late Precambrian mafic–ultramafic complexes from the Eastern Desert of Egypt. These complexes; Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues of Alaskan-type complexes, they are not metamorphosed although weakly altered. Each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin. Silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma. Relatively high Cr 2 O 3 contents are recorded in pyroxenes (up to 1.1 wt.%) and amphiboles (up to 1.4 wt.%) from the three plutons. The chrome spinel crystallized at different stages of melt evolution; as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles and late-magmatic intercumulus phase. The intercumulus chrome spinel is homogenous with narrow-range of chemical composition, mainly Fe 3+ -rich spinel. Spinel inclusions in clinopyroxene and amphibole reveal a wide range of Al (27–44 wt.% Al 2 O 3 ) and Mg (6–13 wt.% MgO) contents and are commonly zoned. The different chemistries of those spinels reflect various stages of melt evolution and re-equilibration with the host minerals. The early cumulus chrome spinel reveals a complex unmixing structures and compositions. Three types of unmixed spinels are recognized; crystallographically oriented, irregular and complete separation. Unmixing products are Al-rich (Type I) and Fe 3+ -rich (Type II) spinels with an extensive solid solution between the two end members. The compositions of the unmixed spinels define a miscibility gap with respect to Cr–Al–Fe 3+ , extending from the Fe 3+ –Al join towards the Cr corner. Spinel unmixing occurs in response to cooling and the increase in oxidation state. The chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products. Causes of spinel unmixing during late-magmatic stage are analogous to those in metamorphosed complexes. The chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations. Spinels from oxidized magmas are likely to re-equilibrate during cooling and are not good tools for genetic considerations. © 2007 Elsevier B.V. All rights reserved. Keywords: Unmixed spinel; Late-magmatic; Re-equilibration; Alaskan-type; Mafic–ultramafic; Egypt 1. Introduction Spinels are important accessory minerals in all mafic–ultra- mafic magmas of various tectonic settings (e.g., Irvine, 1965, 1967; Hamlyn and Keays, 1979; Dick and Bullen, 1984; Barnes and Roeder, 2001, and many others). They are the main repo- sitory of Cr 2 O 3 in mafic–ultramafic rocks, and host other ele- ments as major constituents like Mg, Al, and Fe. Based on geochemical basis, spinels are classified to three series (Deer et al., 1966) according to the dominant trivalent ion; Spinel series (Al +3 -dominant), Magnetite series (Fe +3 -rich) and Chromite series (Cr +3 -rich). Significant solid-solution composi- tional variations occur naturally within and between the three spinel series. The modification of spinel chemistry is commonly accompanied by textural diversity, like exsolutions and alter- ation. The various geological and physicochemical parameters which may influence the texture and chemistry of spinel have Available online at www.sciencedirect.com Lithos 104 (2008) 85 – 98 www.elsevier.com/locate/lithos ⁎ Corresponding author. E-mail address: hmhelmy@yahoo.com (H.M. Helmy). 0024-4937/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2007.11.009