ORCID Profile
0000-0001-6349-0324
Current Organisation
Centre National de la Recherche Scientifique
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Publisher: Elsevier BV
Date: 2020
Publisher: Oxford University Press (OUP)
Date: 10-07-2021
DOI: 10.1093/PETROLOGY/EGAB059
Abstract: The Bushmanland Subprovince of the Mesoproterozoic Namaqua-Natal orogenic belt in southern Africa hosts numerous occurrences of monazite-magnetite-(biotite-apatite-sulfide)-bearing veins and granitoid dykes, including the Steenk skraal vein system, which is one of the highest-grade REE-Th ore deposits in the world. Here, we provide whole-rock geochemical data along with zircon and monazite U-Pb dates and monazite Sm-Nd isotope analyses of these REE-Th-rich veins and granitoid dykes. The U-Pb geochronology indicates that the monazite-rich veins and granitoid dykes formed between 1050 and 1010 Ma, contemporaneously with late-Namaqua granulite-facies metamorphism. They are also coeval with the Koperberg Suite mafic magmas at 1060–1020 Ma and the late stage of a major event of A-type granitoid magmatism that occurred from 1100 to 1030 Ma (i.e. the Spektakel Suite). Similar to the mafic intrusive rocks from the Koperberg Suite, monazite-rich veins and granitoid dykes, located in the southern part of the Bushmanland Subprovince, have more radiogenic Nd isotopic compositions (ɛNd(t) ∼ −1 to zero) than equivalent dykes and veins to the north (ɛNd(t) ∼ −12 to −6). Mafic rocks of the Koperberg Suite reach Th and La concentrations of & ppm that significantly exceed those of other rock types from the region, except for the monazite-rich veins and granitoid dykes, which suggests a genetic link between these rocks. Within veins and granitoid dykes, monazite, biotite and magnetite are commonly anhedral and occur interstitially between the felsic minerals they are, thus, late crystallizing phases. The whole-rock REE-Th concentrations of the granitoid dykes increase with Fe-Mg contents. Therefore, their incompatible element enrichment is not linked to assimilation-fractional crystallization processes. The Nd isotopic signature as well as Fe-Mg- and REE-Th-rich character of the Koperberg Suite and monazite-rich granitoid dykes might reflect partial melting of lithospheric mantle domains, metasomatized during previous Namaqua subduction events, and the mixing of mantle-derived melts with REE-Th-rich metamorphic fluids during their ascent through the crust. We propose that the monazite-magnetite vein mineralizations represent Fe-P-rich immiscible liquids that exsolved from mantle-derived magmas with compositions similar to the most mafic and monazite-rich granitoid dykes. Within this petrogenetic model, conjugate silicate-rich immiscible liquids formed the more felsic granitoid dykes characterized by lower modal abundances of biotite, magnetite and monazite. Although they do not reach similarly high REE-Th concentrations, other A-type granitoids from the region, represented by the Spektakel Suite, also share geochemical affinities with mafic igneous rocks from the Koperberg Suite they may have originated by melting of underplated equivalents of these late-orogenic mafic rocks.
Publisher: Elsevier BV
Date: 04-2020
Location: No location found
No related grants have been discovered for Christophe Ballouard.