AGE AND TECTONIC SIGNIFICANCE OF THE VOLCANIC BLOUBERGSTRAND MEMBER IN THE PAN-AFRICAN SALDANIA BELT, SOUTH AFRICA

Abstract

A zircon LA-ICP-MS U-Pb concordia age of 554.5 ± 5.4 Ma (2σ) from a felsic tuff of the Bloubergstrand Member (Tygerberg Formation) in the Pan-African Saldania Belt of the Western Cape provides the first direct age constraint on the deposition of the low-grade metamorphic volcanosedimentary sequence, collectively referred to as the Malmesbury Group. This age is identical within uncertainty to previously documented U-Pb ages of detrital zircons in the metaturbiditic Tygerberg Formation and constrains deposition of the Malmesbury Group to the latest Neoproterozoic. The age of the sub-aerial volcanism is, within error, identical to the emplacement of the earliest granites of the Cape Granite Suite that cross-cut regional metamorphic fabrics and structures of the Malmesbury Group. This suggests sedimentation and volcanism along an active convergent margin and is consistent with the findings of regional studies that postulate sedimentation and subsequent deformation of the Malmesbury Group in a fore-arc region. Concordant spot ages obtained from inherited xenocrystic cores of zircons in the Bloubergstrand Member define four main age groups at ca. 1200 to 1020 Ma, ca. 970 to 950 Ma, ca. 750 to 720 Ma and ca. 660 Ma. These ages can be correlated with earlier magmatic and/or metamorphic events in terrains to the north of the Saldania belt and point to a trench-parallel sediment supply from north to south and during the diachronous closure of the Adamastor ocean in the late Neoproterozoic.