PETROGRAPHY, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE METAVOLCANIC ROCKS OF THE MESOPROTEROZOIC LEERKRANS FORMATION, WILGENHOUTSDRIF GROUP, SOUTH AFRICA - BACK-ARC BASIN TO THE AREACHAP VOLCANIC ARC

Abstract

The Leerkrans Formation of the Wilgenhoutsdrif Group is a succession of highly sheared metasedimentary and metavolcanic rocks separating the western margin of the Archean Kaapvaal Craton from the polydeformed and highly metamorphosed Proterozoic Namaqua Sector of the Namaqua-Natal Province. This study presents the results of a comprehensive petrographic, geochemical and isotopic study of the metavolcanic rocks of the Leerkrans Formation. Rocks of a basaltic member, a quartz porphyry and a second stratigraphically higher basaltic unit were sampled, with those of the lower basaltic member further subdivided based on their petrographic features and geochemical characteristics. Extensive alteration (chloritisation and epidotisation) of the volcanic rocks make identification of the precursor lithology difficult. Highly chloritised and epidotised metabasalts from the Lower Basalt are typically flow-banded, massive, vesicular or amygdaloidal, and have primitive tholeiitic, MORB-like geochemical characteristics and positive epsilon Nd(t) (e Nd(t) ) values (+4.2 to +6.1), with enriched large ion lithophile element (LILE) contents relative to primitive mantle suggesting derivation from a depleted mantle source. Some of the basalts of this unit have alkaline within-plate tholeiite characteristics. The Upper Quartz Porphyry is comprised of less altered felsic volcanic rocks which have pyroclastic or tuffaceous textures. The geochemistry of these rocks, which display a tholeiitic affinity with fractioned REE patterns and LILE enrichment relative to the high field strength elements (HFSE) and primitive mantle, along with negative e Nd(t) values (−4.8) indicate that the rocks are derived from crustal melting. U-Pb single zircon SHRIMP dating of zircons from a felsic pyroclastic rock from this unit yields an age of 1289 ± 9 Ma. The Upper Basalt and Mixed Zone of the Leerkrans Formation are comprised of basaltic lavas showing similar geochemical features to the Lower Basalt. The metavolcanic rocks of the Leerkrans Formation overlap in age with the oldest units of the ~1.3 to ~1.23 Ga Areachap Group, the polydeformed and highly metamorphosed remnants of a volcanic arc that separates the western margin of the Kaapvaal Craton from the Namaqua Sector. The Leerkrans Formation likely represents the remnants of a related back-arc basin to the volcanic arc which was accreted onto the western margin of the craton. The Leerkrans Formation escaped high-grade metamorphism and polyphase deformation that affected the Areachap arc during the Namaquan Orogeny due to being thrust back onto the Kaapvaal Craton during early arc accretion.