The Keimoes Suite redefined: The geochronological and geochemical characteristics of the ferroan granites of the eastern Namaqua Sector, Mesoproterozoic Namaqua-Natal Metamorphic Province, southern Africa

Abstract

Voluminous granite gneisses and granites straddle the boundary between the Kakamas and Areachap Terranes in the eastern Namaqua Sector (NS) of the Mesoproterozoic Namaqua-Natal Metamorphic Province (NNMP). These rocks have been previously poorly defined and loosely grouped into the Keimoes Suite, but a recent U-Pb age study has suggested the suite be subdivided into syn-tectonic and post-tectonic groups relative to the main phase of the Namaqua Orogeny. This study adds new whole rock geochemical, isotopic and age data for these granites that confirms the subdivision is appropriate. The older group of syn-tectonic granite gneisses, dated between 1175 and 1146 Ma, have penetrative foliations and are largely derived from fractionated, leucogranitic metaluminous to peraluminous magmas with low maficity, low Ti, Mn and Ca. They were derived from mildly depleted sources (εNd(t): −1.47 to 1.78), with Meso-to Paleoproterozoic Nd model ages (1.57–1.91 Ga), and high initial Sr ratios (0.71970–0.75567) suggesting mixing between younger depleted and older, arc-like sources imparting an arc-like signature to the magmas. High initial Sr ratios appear to be an intrinsic character of these granites reflecting those of granites in the region and the highly radiogenic nature of the NS.The weakly to unfoliated late-to post-tectonic megacrystic granodiorites and monzogranites, including charnockites, intruded between 1110 and 1078 Ma and constitute the Keimoes Suite proper. They have I-type characteristics, being strongly metaluminous and locally hornblende- and orthopyroxene-bearing with moderate SiO2 and with arc-type affinities (LILE enrichment relative to the HFSE, Ta-Nb, Ti and P anomalies). However, the granitoids also have high Fe/Mg ratios, along with high HFSE, LILE and REE contents more indicative of A-type granites. They show an increasing maficity, metaluminous character, and general decreasing degree of fractionation with decreasing age. They are similar to the syn-tectonic granites in having εNd(t) values close to zero (−2.95 to 2.83) and Meso-to Paleoproterozoic model ages (TDM: 1.38–1.99 Ga) but lower initial Sr ratios (<0.723 in general) suggesting derivation from relatively depleted sources with a variable enriched and/or crustal component.The timing of emplacement of the post-tectonic granites places peak D2 deformation in the eastern NS predominantly at ∼1.13–1.10 Ga, with peak D2 varying from ∼1.18–1.13 Ga. There was more voluminous granitic magmatism in the Areachap Terrane to the east during the late-to post-tectonic magmatic episode, whereas the earlier, ∼1.18–1.14 Ga syn-tectonic magmatic episode is more concentrated to the west in the Kakamas Terrane. The broad, protracted period of magmatism in the eastern NS attests to a long-lived duration of high-heat flow in this portion of the southern African crust at this time. Nd model ages of Meso-to Paleoproterozoic age reflect those in other granites throughout the NS suggesting extensive reworking of Paleoproterozoic crust during the 1.2–1.0 Ga Namaquan Orogeny.