Porphyry Bay and the Trekoskraal Cauldron – the third volcanic eruption centre of the Cape Granite Suite and its regional implications

Abstract

Pan-African volcanic cauldrons of the Saldania Belt, along the South African West Coast have been named the Postberg and Saldanha centres. Outcrops of subvolcanic and volcanic rocks in the Trekoskraal area indicate the presence of a third volcanic centre that is mostly hidden just off shore. In coastal outcrops of the Porphyry Bay area, porphyritic varieties of the Trekoskraal Granite are unconformably overlain by silicic crystal tuffs and ignimbrites that are, in turn, intruded by the Porphyry Bay microgranite. New zircon U-Pb dates for the rocks include the Trekoskraal Granite (535.4 ± 0.9 Ma), the overlying Trekoskraal crystal tuff (536.1 ± 1.3 Ma), the intruding Porphyry Bay microgranite (534.9 ± 1.1 Ma) and a late, leucomicrogranite dyke (523.1 ± 4.7 Ma). Age and field relationships indicate that the Trekoskraal Granite was emplaced at shallow depth and unroofed to form the substrate for the overlying, folded Trekoskraal tuffs and ignimbrites, within an interval of ≤1.5 Myr, during regional deformation. Inherited zircon in the Porphyry Bay rocks has a ca. 839 Ma peak that represents the only inherited age common to all three units. The youngest concordant detrital (inherited) zircons in the basal tuff and the Porphyry Bay microgranite have ages of ca. 570 Ma, suggesting the presence of Ediacaran metasediments in the anatectic source region. The chemical scatter in major-oxide parameters for the S-type rocks in this area, both within and between different units, is consistent with heterogeneity inherited from metasedimentary magma sources. Such differences show that the Cape Granite Suite (CGS) S-type magmas formed from various metasedimentary units with distinct ages and somewhat different sediment sources, perhaps at different crustal depths. The late-stage (523.1 Ma) leucomicrogranite indicates that minor crustal melting continued for at least 10 Myr after the main phase of CGS magmatism. The field and age relationships underline the close spatial and temporal relationships between volcanic and near-surface plutonic activity, related to the CGS, in this part of the Saldania Belt.