Influence of the Gondwanide Magmatic Arc and Cape Fold Belt on the Karoo Basin - South Africa.

摘要

The Permian Ecca Group in South Africa preserves a thick succession of clastic basin fill, including mud-rich turbidite sequences and hemipelagic mudstone units. Provenance of mud influx into the basin is an important constraint for understanding the tectonic processes influencing regional basin development and distribution of organic-rich Lower Ecca Group mudstone and subsequent depositional style of the Upper Ecca Group turbidite systems. In this study, we interpret the Ecca Group mudstones were sourced by the Cape Fold Belt and Gondwanide Magmatic Arc based on geochemistry and mineralogy of 38 mudstone samples. Muscovite and high quartz content (average 32.9%) indicate metamorphic and recycled sedimentary sources. Chemical index of alteration (CIA) values are greatest (CIA = 76 -- 82) and Index of Compositional Variability (ICV) is depressed (ICV = 0.5 -- 0.8) at the base of the Ecca Group, and indicate less weathering upsection. Rare earth element abundances display LREE enrichment and a negative Eu anomaly, and various trace element plots are consistent with mixing of sediment derived from Old Upper Continental Crust (OUC) and Young Differentiated Arc (YDA) terranes, such as the Cape Fold Belt and Gondwanide Magmatic Arc, respectively. Epsilon Nd and chondritic model ages are consistent with mixing of OUC and YDA sources. These results indicate that the southern Gondwanide Magmatic Arc and the Cape Fold Belt affected subsidence and sedimentation during early development of the Karoo Basin, and that the basin is a retroarc foreland basin.