CATHODOLUMINESCENCE (CL) RESPONSE OF FELDSPARS IN MULTIPLE VEINS SETS AS A MEANS TO IDENTIFY SOURCE REGION: IMPLICATIONS FOR GEOLOGIC EVOLUTION OF THE SAWTOOTH METAMORPHIC COMPLEX, ID, USA

Multiple sets of felsic igneous veins crosscut metasupracrustal rocks in the Sawtooth Metamorphic Complex (SMC) of central Idaho. The leucocratic veins, collected from spatially separate regions throughout the Complex, appear superficially similar consisting of plagioclase, quartz, alkali feldspar ± biotite, zircon, and apatite. However, their provenances are uncertain and could be related to high-grade metamorphic event(s) and/or to injection from the surrounding igneous rocks. Potential sources for these veins, each with a distinct age range, are: (1) anatectic melts developed during high-grade metamorphism(s) (ca. >85 Ma); (2) the Atlanta lobe of the Idaho batholith (granodioritic – ca. 80-67 Ma); (3) the Sawtooth batholith (granitic, ca. 45 Ma); (4) another previously undefined melting and/or hydrothermal event; or (5) a combination of these sources.

To provide an easy, first-order criterion for identifying the relationships among of the various veins, optical cathodoluminescence (CL) images were obtained prior to more involved U/Pb geochronologic studies. Samples from the SMC contain alkali feldspars and plagioclases with distinct CL colors and zoning patterns: Group 1 veins contain green-luminescing plagioclase; Group 2 veins contain plagioclase with blue cores and red rims; Group 3 veins have zoned plagioclase with pink cores and green rims; Group 4 veins contain yellowish-green plagioclase; and Group 5 veins contain plagioclase with bright red CL. Alkali feldspars within Group 5 luminesce purple compared to the blue luminescence of alkali feldspars in groups 1-4.

Reference samples of the anatectic rocks from within the SMC, Idaho batholith, and Sawtooth batholith provide comparative CL characteristics. Plagioclase from the anatectic melts luminesces blue (likely trace Eu²⁺, Ti, and/or Al-O^—Al centers) similar to Group 2 veins. Idaho batholith plagioclase has green CL (likely trace Mn²⁺) similar to Group 1 veins. The Sawtooth batholith samples are similar to Group 5 veins with bright-red CL (likely trace Fe³⁺) in plagioclase. This relatively simple technique provides a means for comparing and refining sample selection for geochronologic studies whereas determining the vein source helps to constrain the timing of emplacement and uplift history of the SMC.