Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
2004 GSA Annual Meeting, Denver, Colorado, November 7–10, 2004
Relict Grain-Cement Textures in Metasandstones Revealed Using SEM-Based Cathodoluminescence Imaging
Robert M. Reed
Unmetamorphosed quartz sandstones show cathodoluminescence (CL) textures consisting of generally more luminescent, but varied, core quartz grains surrounded by generally less luminescent authigenic quartz cement overgrowths. Previous workers recognized that this CL contrast between quartz grains and authigenic quartz cement was homogenized at metamorphic temperatures. The light-amplification and increased contrast provided by SEM-based CL detectors allow improved imaging and increased understanding of this process. A number of quartzites of known clastic origin and varying metamorphic grade and deformational intensity have been examined. The CL “homogenization” process turns out to be not entirely homogeneous. Some textural evidence of quartz grain-cement features is retained to high temperatures, particularly in less deformed rocks. Typically the longer wavelengths of CL emission (UV-blue) retain distinctive grain and cement overgrowth textures, whereas orange-red CL emissions are homogenized. SEM-CL analysis conducting using an UV-blue filter has proven effective for imaging these relict textures, even within some rocks that have undergone upper amphibolite facies metamorphism.
A number of rocks of unknown or uncertain protolith (feldspathic gneisses and fine-grained, quartz-rich rocks) have also been examined. Some of these rocks show relict CL textures indicative of the rocks having a sandstone protolith. SEM-CL imaging has the potential to increase the recognition of metaclastic rocks in metamorphic terranes. Based on preliminary results, metamorphic rocks with sandstone protoliths may be more common than previously recognized.
Recrystallization related to deformation may be as important as heating in the CL homogenization process. Penetratively deformed rocks seem to preserve clastic textures only in some areas within a thin section. These areas may represent less recrystallized, low-strain zones.
In addition to overgrowth cement textures being preserved, low-T microfractures lined with quartz cement are also visible in some cases. SEM-CL imaging shows that quartz-lined microfractures of varying relative ages are common in quartz-rich metamorphic rocks. The degree of homogenization can provide clues to whether microfractures are pre- or post-metamorphic.