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