Magmatic clasts in the Saldanha ignimbrites, and Trekoskraal beach pebbles: missing pieces from the volcanic puzzle in the Cape Granite Suite

Abstract

Abstract Previous studies have shown that the 542 Ma Saldanha eruption centre, situated on the west coast of South Africa, consists of the basal Saldanha Ignimbrite, which is partly intermingled with and partly overlain by the Jacobs Bay Ignimbrite, both having S-type characteristics. Together, the Saldanha eruption centre and the Postberg eruption centre (to the south, across Saldanha Bay) form part of the volcanic phase of the Cape Granite Suite. The lowermost parts of the Jacob’s Bay Ignimbrite contain magma clasts that are chemically dissimilar to their host ignimbrites. Some clasts are recrystallized ignimbrites that are chemically distinct from any unit that has outcrop expression, and are inferred to form part of a previously unrecognised volcanic event. Other clasts are non-fragmental, hypabyssal rocks that were evidently intruded prior to the explosive intracaldera eruptions that formed the Saldanha ignimbrites. Beach cobbles and pebbles, sampled from the Trekoskraal coastal area, include three texturally and chemically distinct groups – rhyolitic ignimbrites, rhyolitic hypabyssal rocks and dacitic hypabyssal rocks or lavas. Only a minority of these rocks (from the rhyolitic ignimbrite group) show some chemical affinities with the Saldanha Bay ignimbrites. The other pebble types show neither chemical nor textural similarities with the rocks of either the Saldanha or the Postberg eruption centres. The pebbles and cobbles also have no chemical affinities with any of the granitic intrusive rocks of the region. Their chemical and isotopic characteristics suggest that a variety of different magma batches were formed through partial melting of heterogeneous Malmesbury Group metamorphic rocks, at depth. LA-ICP-MS dating of igneous zircon crystals from two of the pebbles (a low-silica rhyolite ignimbrite and a dacite) yielded magmatic ages of 540 ± 4 Ma and 533 ± 4 Ma, respectively. Taking uncertainty brackets into account, these new dates suggest that there may have been a 3 Myr hiatus in eruptive activity, between the eruptions responsible for the exposed Saldanha ignimbrites and the eruptions that produced the volcanic units from which the pebbles were derived. This confirms the inference that there was a previously unidentified, later, volcanic event associated with the Cape Granite Suite in the Saldanha area.