PETROGRAPHY AND GEOCHEMISTRY OF MEGACRYSTIC K-FELDSPARS FROM A GRANITOID INTRUSION, NORTHWESTERN NEVADA

Potassium feldspar (K-feldspar) is a common rock-forming mineral found in igneous intrusions world-wide. While most igneous intrusions contain K-feldspar (<1cm), relatively few intrusions have exceptionally large crystals (>4cm –megacrystic). By studying the petrography and geochemistry of these megacrystic K-feldspars, we hope to gain a better understanding of the physio-chemical environment in which these crystals were formed.

This study focuses on a suite of megacrystic K-feldspars from the Granite Peak stock in northwestern Nevada. Petrographic observations indicate that all of the crystals are euhedral in shape, contain complex zonation boundaries, sectoral zoning, and abundant inclusions (quartz, plagioclase, biotite, Fe-Ti oxides, apatite, and zircon). Many of the inclusions are preferentially oriented with their long-axis parallel to zonation boundaries. EPMA data reveals that the host megacryst is primarily orthoclase (~An₈₀ - An₉₀), while the plagioclase inclusions are predominately andesine (~An₃₀ – An₅₀) with oligoclase to albite rims (~An₅ to An₁₇). EPMA data also indicates that the plagioclase inclusions are regularly zoned with Ca-rich cores (4.24 - 7.79 wt%) and Na-rich rims (9.02 - 11.79 wt%). Image analysis of a Granite Peak stock outcrop demonstrates a preferential orientation of the megacrysts (N =107) with an average NW to SE direction, suggesting that megacrysts were free to rotate into alignment during their formation. Geochemical analyses obtained by μ-XRF reveal that zonation boundaries are correlated with Ba content (0 - 2.35 wt%). Other notable trace elements include: Rb (0 - 80 ppm), Sr (0 - 287 ppm), Zr (0 – 339 ppm), and Ce (0 – 3842 ppm). These data can help us better understand the system in which these crystals were forming and why these crystals grew so large. With distinct Ba zonation patterns, regular zoning within plagioclase inclusions, and the presence of igneous microstructures (no observed sub-solidus deformation), the megacrysts appear have a magmatic origin.