Petrogenesis and provenance of distal volcanic tuffs from the Permian–Triassic Karoo Basin, South Africa: A window into a dissected magmatic province

Abstract

We present zircon rare earth element (REE) compositions integrated with U-Pb ages of zircon and whole-rock geochemistry from 29 volcanic tuffs preserved in the Karoo Supergroup, South Africa, to investigate the history of magmatism in southern Gondwana. Whole-rock compositions suggest a subduction-driven magmatic arc source for early (before 270 Ma) to middle Permian (270-260 Ma) Karoo tuffs. After ca. 265 Ma, the magmatic source of the volcanic deposits transitioned toward intraplate shallow-sourced magmatism. Zircon U-Pb ages and REE chemistry suggest that early to middle Permian magmas were oxidizing, U-and heavy (H) REE-enriched, melts; middle Permian to Triassic zircons record HREE-depleted, more reduced magmatism. Middle Permian to Triassic tuffs contain increasingly large volumes of zircon cargo derived from assimilated crustal material; therefore magmas may have been zircon undersaturated, resulting in less zircon growth and increased inheritance in late Permian to Triassic Gondwanan volcanics. Zircon U-Pb ages and zircon REE chemistry suggest a shift from arc magmatism in the early Permian to extensional magmatism by the late Permian, which may be associated with development of a backarc magmatic system adjacent to western Antarctica that predates known extensional volcanism elsewhere in Gondwana. Opening of the Southern Ocean in the Jurassic-Cretaceous paralleled this extensional feature, which may be related to reactivation of this Permian-Triassic backarc. This study demonstrates the potential of zircon U-Pb age and REE compositions from volcanic tuffs preserved in sedimentary strata to provide a more complete record of magmatism, when the magmatic province has been largely lost to active tectonism.

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

“…262-268 Ma) were sourced from the same volcanic center as the Choiyoi Group of South America, that they proposed to have experienced a period of Triassic zircon exhaustion and recycling of older inherited grains (McKay et al, 2015). However, recent Rare Earth Element (REE) data indicate that the Karoo Basin ash beds were more likely sourced from Antarctica, and do not share a common magmatic history with the South American ash beds (McKay et al, 2016).…”

“…This "inverted" basin stratigraphy was ascribed to an episode of zircon exhaustion and magmatic recycling (as opposed to a disturbed U-Pb system). The placement of the PTB within the upper Ecca Group was further reinforced by a presumed "tuff gap" in the Karoo Basin located above this horizon (Waterford Formation), similar to other Permian−Triassic depositional sites along the southern Panthalassan margin of Gondwana (McKay et al, 2016;McKay et al, 2015;Veevers, 2004).…”

“…U-Pb ages published from the Ecca Group yield maximum depositional ages ranging from ca. 274-250 Ma (Fildani et al, 2007;Fildani et al, 2009;McKay et al, 2016;McKay et al, 2015). The younger ages in this range are problematic in that they require the PTB to be placed within the middle Ecca Group, as opposed to the biostratigraphically-supported placement in the overlying terrestrial fluvio-lacustrine strata of the Beaufort Group (Figs.…”

“…2), and we interpret this as mainly resulting from the contrasting interpretations of complicated U-Pb datasets. (Fildani et al, 2007;Fildani et al, 2009;McKay et al, 2016;McKay et al, 2015). The former age calculations were based on the single youngest grain (Fildani et al, 2007;Fildani et al, 2009) or sub-population of youngest grains (McKay et al, 2016;McKay et al, 2015), whereas our analysis was based on the weighted mean 206 Pb/ 238 U age of all isolated Permian grains (< 280 Ma) that met the specified quality criteria (section 3.3).…”

“…274-250 Ma age range for deposition of the underlying Ecca Group, and therefore a Triassic age for the Beaufort Group ( Fig. 2) (Fildani et al, 2007;Fildani et al, 2009;McKay et al, 2016;McKay et al, 2015). This depositional age assignment requires that the PTB be placed within the upper Skoorsteenberg Formation of the Ecca Group, ~2000 m below the biostratigraphically constrained PTB in the upper Beaufort Group (Fig.…”