PROVENANCE OF MOUNT SOMERS QUARTZ ARENITES: COMBINED SEM-CL AND OPTICAL MICROSCOPY ANALYSIS

Scanning electron microscopy – cathodoluminescence (SEM-CL) combined with optical microscopy on the same individual quartz crystals is a promising tool for provenance analysis. SEM-CL is useful to identify primary internal features in quartz crystals, acquired during crystallization, such as randomly oriented microcracks and healed fractures, zoning, homogenous or patchy appearance, which are not visible with an optical microscope. Deformation related secondary internal features such as preferably oriented microcracks, deformation lamellae, or recrystallization can be seen with both techniques. Additional information on extinction behavior and poly- or microcrystallinity can be gained from optical microscopy. When combining SEM-CL with information from optical microscopy on the same quartz grains it is possible to distinguish between volcanic, deformed volcanic, plutonic, deformed plutonic and metamorphic/recrystallized quartz in sediment. The approach works best for grains in the coarse to medium sand size range, and can be used to identify potential sediment source areas. We present here a study on Eocene shallow marine quartz rich sandstone from the Mount Somers area, South Island, New Zealand to test this approach. This quartz rich sandstone has been described in the past as locally derived sediment from volcanic source rocks and minor input from metamorphic sources. Using the new combined approach we show that only about half of the quartz grains are locally derived, while ~20% are from metamorphic sources. The other ~30% of quartz grains are from plutonic sources and have been imported by longshore drift due to the lack of local plutonic sources. Recycling of originally plutonic quartz grains from low-grade metasedimentary rocks is possible but would only be a minor contribution and could not account for 30% of the grains. Furthermore, the metasedimentary rocks of the nearby Torlesse Supergroup experienced prehnite-pumpellyite facies metamorphic overprint which is in the 300-400ºC temperature range that causes the onset of quartz recrystallization. Therefore, quartz grains form such source rocks would be identified as metamorphic instead of plutonic quartz. The combined SEM-CL/optical microscopy approach has a great potential to improve provenance determination of quartz rich sediments.