Lithostratigraphy of the Naros Granite (Komsberg Suite), South Africa and Namibia

Abstract

The Naros Granite occurs as a large, northwest-trending ovoid batholith roughly 30 km long and 15 km wide straddling the Orange River border between South Africa and Namibia, 25 km northeast of Onseepkans. It consists mainly of a leucocratic to mesocratic grey, coarse-grained equigranular hornblende-biotite granite-granodiorite that is locally mildly feldspar porphyritic. Small, ovoid mafic autoliths are common and characteristic of the Naros Granite. The composition of the unit varies from granite to granodiorite with a minor leucogranitic phase observed along the southern margin of the batholith. Hornblende and biotite are ubiquitous mafic minerals but small amounts of orthopyroxene occur locally. The Naros Granite has yielded tightly-constrained U-Pb zircon ages between 1 114 Ma and 1 101 Ma. The Naros Granite is generally unfoliated to weakly deformed with only localised shearing along contacts with the surrounding country rocks giving rise to orthogneissic fabrics. It has an intermediate to felsic composition (mean SiO2: 63.9 ± 2.2 wt.%) and is strongly metaluminous. This, together with its biotite-hornblende ± orthopyroxene mineral assemblage and the abundance of mafic autoliths, suggests it is an I-type granitoid, with the source magma produced by partial melting of older igneous rocks that had not undergone any significant chemical weathering. The Naros Granite is the youngest and most evolved member of the ~1.11 Ga Komsberg Suite, a collection of late-to post-tectonic I-type metaluminous, intermediate to felsic, biotite ± hornblende granitoids and their charnockitic equivalents that have intruded the older pre-tectonic gneisses of the Kakamas Domain of the Namaqua Metamorphic Sector