- © 2016 Geological Society of South Africa
The Cape Fold Belt (CFB) along the southern coast of South Africa contains several tectonic windows that expose low-grade “basement” rocks as inliers that are inferred to be late Neoproterozoic in age subjected to tectono-metamorphism during the Pan African Saldanian Orogeny (ca. 650 to 550 Ma). Whilst a Saldanian tectonic history along the northwest to southeast trending western branch of the CFB has been well documented, such Neoproterozoic deformation along the east-west trending southern branch of the CFB is not established. This work presents new field observations and SHRIMP U/Pb geochronologic data from the Kleinrivier Sequence within the Gamtoos Inlier exposed along the eastern-most known tectonic window, near Port Elizabeth, and which is here renamed the Gamtoos Complex. The study area includes 3 tectonic sequences from the Gamtoos Complex (“pre-Cape”) and 2 sequences from the overlying Table Mountain Group of the Cape Supergroup. The lower sequences of the Gamtoos Complex comprise north-east verging thrust packages within a regional antiform, all of which have a common oriented planar tectonic fabric (S2) coincident with those in the lower Paleozoic rocks. Phyllites from the Kleinrivier and upper Sardinia Bay sequences (spanning the pre-Cape and Cape boundary) are similarly affected by a slaty cleavage (S1), crenulation cleavage (S2) and multiple generations of quartz veins. U/Pb data are from zircons of metasediments and igneous rocks from the Gamtoos Complex and from the overlying rocks of the Table Mountain Group. Meta-greywackes of the Kleinrivier Sequence (youngest zircon: 523 ± 6 Ma) are probably Cambrian in age. However, some units from the Kleinrivier Sequence are intruded by felsic sills with a Concordia age: 530.2 ± 4.4 Ma and therefore require further U/Pb detrital zircon analyses. Mafic sills and dykes also intrude the Kleinrivier Sequence and were subsequently deformed and contain S2. Boulder conglomerates in the Sardinia Bay Sequence (youngest matrix zircon: 521 ± 6 Ma) are separated from the Kleinrivier Sequence by an angular unconformity. It also contains granite-boulders (Concordia age: 530.2 ± 4.7 Ma), and is overlain by the feldspathic psammites and quartzites of the Sardinia Bay Sequence with zircons as young as 510 ± 7 Ma (e.g. middle Cambrian). In total, 75% of the detrital zircons from all sequences date between early-Mesoproterozoic and early-Neoproterozoic (ca. 1455 to 852 Ma) and are likely sourced from the gneisses of the Namaqua-Natal province that are known to underlie the CFB, whilst the Neoproterozoic to mid-Cambrian zircons (ca. 828 to 431 Ma) have been sourced from Pan African mobile belts, possibly the Mozambican and/or Saldanian Belts to the east and west, respectively. Such “Pan African” detrital zircons are more prevalent in the Sardinia Bay Sequence and form the dominant component in the Lower Table Mountain Group, suggesting a lesser source influence from the Namaqua Natal Mobile Belt. The conglomerates at Sardinia Bay possibly represent mid-Cambrian rift sequences, similar to the Klipheuwel Group that overlie the Malmesbury Sequence and Cape granites along the western branch of the CFB. The Sardinia Bay Sequence has zircons equivalent to those of the Peninsula Formation (youngest zircon: 516 ± 5 Ma), and therefore should be linked to represent the lower Table Mountain Group.