Potassium Feldspar Megacrysts as Archives of Magmatic Systems and Processes

The timing of K-feldspar megacryst formation bears on a fundamental issue in granite petrogenesis, namely whether the textural and chemical features preserved within granitoid intrusions reflect primary magmatic processes, or whether these features represent final crystallization and subsolidus overprinting. This study attempts to place constraints on the origins and processes responsible for megacryst formation by examining a K-feldspar megacrystic intrusion in northwestern Nevada (Granite Peak stock - GPS).

GPS megacrysts are commonly euhedral, ranging from 1 to 8 cm in length (avg. length of 4 to 5 cm). Petrographic observations indicate that zoning (oscillatory and sector zoning) is ubiquitous within all of the GPS megacrysts. Microscopy also reveals abundant inclusions of plagioclase (85-90%), quartz (5-10%), biotite (1-5%), and accessory phases ( 1%) that are preferentially oriented parallel to zonation boundaries. Oscillatory zoning boundaries (0.25 - 1 mm) are interpreted to reflect nearly continuous growth, although some zones show evidence of resorption and/or erosion. Electron probe micro-analyses (EPMA) reveals that the host megacrysts are orthoclase (Or₈₀ - Or₉₀) and contain normally-zoned plagioclase inclusions (An₃₀ -An₅₀) that gradually become more sodic toward the megacyrst rims. Some of the plagioclase inclusions have albitic overgrowths (An₅ - An₁₇). The higher anorthite component in the plagioclase inclusions suggests that the inclusions formed earlier than plagioclase grains in the groundmass (An₂₀-An₃₀). GPS megacrysts also contain complex Ba zonation patterns with concentrations ranging from 0.53 - 2.72 wt%.

Textural observations and EPMA data suggest that K-feldspar crystallization initiated earlier than predicted by experimental phase relationships. High Ba concentrations within the GPS system may provide a possible explanation for early K-feldspar nucleation and growth. While a magmatic origin is favored for the formation of the GPS megacrysts, textural observations suggest that the GPS system was dynamic, possibly involving textural coarsening and late subsolidus overprinting. Unraveling the complex textural and chemical history of these megacrysts provides new insights into the dynamic processes and thermal histories of magmatic systems.