INTERACTIONS BETWEEN THICK RHYOLITE SILLS AND WET, UNLITHIFIED LACUSTRINE SEDIMENTS IN A MESOPROTEROZOIC VOLCANIC ARC SETTING, SW NAMIBIA

Our recent mapping of 1.2 Ga volcanic arc rocks in the Barby Formation in SW Namibia indicates that parts of the unit formed in one or more subsiding intra-arc basins. Lithofacies patterns suggest that volcanism took place in settings where numerous lakes existed in poorly drained areas with relatively low topographic relief. Lacustrine strata consist mostly of planar-bedded volcaniclastic sandstone and mudstone and are intercalated with andesitic pyroclastic deposits erupted from small monogenetic volcanoes; andesitic and rhyolitic lavas occur in smaller amounts. Thick lacustrine sequences are intruded by locally abundant andesitic sills, implying that interconnected sill systems played an important role in subsurface transport of magma. Peperite occurs along sill margins where fragments of quenched andesite were intermixed with disrupted sediment, indicating that the host strata were wet and unlithified during sill intrusion.

Hypabyssal felsic intrusions are much less common in the areas we have mapped so far, and here we describe an unusual example where lacustrine strata were intruded by several rhyolite sills that provide insight into how viscous felsic magma interacts with thick sequences of wet sediment in the shallow subsurface. The two best examples are 60 and 140 m thick and are completely exposed in cross section. Originally glassy zones a few meters thick occur in the lowermost parts of the sills and exhibit flow banding extending down to the sub-mm scale. These zones pass up into thick zones of homogeneous felsite with regular columnar jointing developed parallel to sill bases, recording slow cooling in sill interiors. The upper third to half of each sill consists of zones where pervasive thin flow banding initially developed and was deformed by complex flow folds prior to brittle fragmentation and rotation of clasts to form massive flow breccia. In the uppermost parts of the sills, the flow breccia grades into zones of peperite ~10 m thick, in which flow-banded clasts are chaotically arranged within fine-grained host sediment that shows partial destruction of sedimentary structures. There is no evidence for explosive interactions between rhyolite and wet sediment, which may have been inhibited by the pressure exerted by the overlying sediments and lake water.