OPTICAL CATHODOLUMINESCENCE MICROSCOPY OF ARCHEAN QUARTZOFELDSPATHIC GNEISSES OF THE EASTERN BEARTOOTH MOUNTAINS, MT/WY: INSIGHTS INTO THE DEFORMATIONAL AND FLUID INFLUX HISTORY

Optical cathodoluminescence (CL) microscopy has traditionally been used for examination of carbonate-bearing rocks, but it is becoming increasingly important for investigating features that develop in a wider range of silicate-rich metamorphic rocks. The challenge has been to capture true-color images from the relatively low amount of CL light typical of most silicate minerals using standard CL instrumentation. An expansion of the CL capability has come with the addition of a low-light fluorescence camera attached to a CL microscope and coupled with flexible imaging software. This updated tool has been applied to Archean quartzofeldspathic (metaplutonic) rocks of the eastern Beartooth Mountains, MT/WY. Most of these rocks were intruded as numerous sheet-like bodies of trondhjemite-tonalite-granodiorite (TTG) at mid-crustal levels during a major crust-forming subduction-related event at 2.8 Ga, but they also incorporate other quartzofeldspathic gneisses (as old as 3.5 Ga) as well as other metasedimentary and meta-igneous rocks that are tectonically intercalated. Relations in quartzofeldspathic gneisses can be petrographically enigmatic, but CL observations provide clues to the evolution of these rocks. Feldspars and apatite are particularly useful because they luminesce in an array of distinctive colors, clearly revealing features such as compositional zoning, exsolution, multiple generations of myrmekite formation, radiation damage and late alteration – each associated with the distinctive periods of deformation or interactions with fluids. Different quartzofeldspathic units (2.8 Ga vs. older enclaves) have distinctive textures. For example, the 2.8 TTG gneisses display features typical of slowly cooled plutonic granitic rocks such as bright-blue luminescing K-feldspar with red perthitic albitic exsolution and red plagioclase with blue antiperthic K-feldspar. However, the development of multiple generations of myrmekites and later alteration becomes more clearly defined. The older enclaves of quartzofeldspathic rocks exhibit a range of CL colors in the plagioclase (red-green-yellow-blue) that relates to the distinctive trace element signature of the local lithology. Strongly zoned apatite (purple-brown cores and yellow rims) mark later timing of reactive fluids.