Age and origin of NYF pegmatites from the Mesoproterozoic Orange River pegmatite belt (Namaqualand, South Africa): insights from monazite and titanite U-Pb geochronology and Nd isotope systematics

The 450 km long Orange River pegmatite belt consists of >30,000 pegmatite dykes that intruded the Namaqua Sector of the Mesoproterozoic Namaqua-Natal Metamorphic Province in Southern Africa at ca. 1 Ga. The extremities of the belt mostly host pegmatites with Li-Cs-Ta (LCT) signatures. In contrast, the Kakamas Domain, located in the central part of the Namaqua Sector, is dominated by the occurrence of Nb-Y-F-enriched (NYF) pegmatites with economic potential for REE and Th. The origin of NYF pegmatites, including processes of REE-Th enrichment, remain poorly understood. Here, we provide in-situ U-Pb ages along with Sm-Nd isotopic compositions of titanite and monazite from those pegmatites in order to constrain their source and tectonomagmatic context of emplacement. The Kakamas NYF pegmatites consist of (i) simple, relatively homogeneous dykes with low concentrations of REE-Y-Th-U-rich minerals, contrasting with (ii) complex, variably zoned dykes, characterised by a higher proportion of monazite, thorite, and xenotime. Similar to other pegmatites from the Namaqua Sector, simple and complex NYF pegmatites from the Kakamas Domain yield U-Pb ages from 1043 ± 5 to 1025 ± 6 Ma (titanite) and from 991 ± 15 to 962 ± 3 Ma (monazite), respectively, suggesting an increase in the ‘rare metal-fertility’ of pegmatite-forming magmatism over time. Pegmatite crystallisation postdates I- and A-type granitic magmatism in the Kakamas Domain from 1120 to 1080 Ma, and complex pegmatite emplacement was coeval with a major transcurrent deformation event in the Namaqua Sector dated by previous studies at ca. 1005–960 Ma. Vertical structures associated with strike-slip tectonics likely facilitated the upward migration of rare metal-rich melts. Simple pegmatites have average ɛNd(t)_titanite values between - 5.8 and - 2.9 and complex pegmatites have average ɛNd(t)_monazite values of - 4.1 to - 3.1, corresponding to similar two-stage depleted mantle model ages (T_2DM) of 2.1 to 1.9 Ga and 1.9 to 1.8 Ga, respectively. Those model ages are younger than those of Namaqua LCT pegmatites, but overlap with those of orthogneissic, granitic and granulite-facies metapelitic country rocks from the Kakamas Domain suggesting a potential local crustal source for NYF pegmatites. Granulite-facies restitic metapelites as well as A-type and I-type granitoids that constitute a voluminous component of the Kakamas crust show appreciable concentrations of elements characteristically enriched in NYF pegmatites. This geochemical background may explain the prevalence of NYF pegmatites in the Kakamas Domain.