Petrogenesis of the Concordia Granite Gneiss and its relation to W Mo mineralization in western Namaqualand, South Africa

• Published in: Precambrian research Vol. 70; no. 3; pp. 303 - 335
• Main Author: Raith, J.G.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 1995
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/0301-9268(94)00049-W

The 1.1 Ga Concordia Granite Gneiss (CGG) is part of the late to postorogenic Spektakel Suite in the western Namaqualand Metamorphic Complex, South Africa. It intruded synkinematically, with respect to the main (D2) deformation event, into lower to middle crustal rocks and granite emplacement was more or less coeval with the peak of granulite-facies metamorphism ( > 800°C, ∼ 5 kbar). Several genetically related rock types, megacrystic garnet-bearing granite, minor aplitic leucogranites and pegmatites are distinguished. All varieties are SiO 2-rich (69–79 wt.%) peraluminous granites and show subalkaline-monzonitic magma characteristics. Geochemical differences in whole-rock chemistry between megacrystic granite and aplitic leucogranites (e.g., lower Al 2O 3, MgO, CaO, Ba, Zr; higher K 2O, Rb, Nb, W, Rb/Sr, Ga/Al) and the decrease of e.g., CaO, MgO, Fe 2O 3, Ba, Zr, Th/U with increasing SiO 2 in the megacrystic granites as well as the variation in Fe/Mn of magmatic garnets are best explained with crystal fractionation processes. Fractional crystallization of plagioclase produced potassium- and silica-rich residual melts characterised by very high Rb/Sr, Rb/Ba, U/Th, Mn/Fe ratios and higher concentration of W, Cu and Zn. Crystal fractionation processes also resulted in a relative LREE depletion and HREE enrichment (megacrystic granite: La/Lu) cn = 8.87−31.67; aplitic leucogranite and pegmatites: La/Lu) cn = 0.71−1.44) and evolution of pronounced negative Eu-anomalies. The crystallization sequence (near-solidus crystallization of biotite prior to alkali feldspar) suggests that the CGG magmas were H 2O-undersaturated over a long period of their evolution. Water saturation during late-stage crystallization is, however, indicated by coarse late-stage eutectic mineral textures, pegmatites and W Mo-bearing siliceous rocks. Furthermore the economic potential of the CGG is supported by its geochemical signature (e.g., high U, Th contents) which is similar to evolved high heat production (HHP) granites. The granitic magmas are attributed to partial melting of peraluminous crustal source rocks and are tentatively interpreted as fractionated S-type granites. The W Mo deposits represent vein-type and pegmatitic deposits genetically related to a deep-seated granitic system.