Investigating the textural, chemical, and chronological records preserved within crystal populations can provide insight into the processes which operate during magma ascent, emplacement, and crystallization. K-feldspar megacrysts offer an excellent opportunity to explore these records, particularly in chemically-evolved systems. Understanding 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 late-stage crystallization and subsolidus reorganization. To expand our understanding of megacryst formation, we investigated a suite of K-feldspar megacrysts from the Sheep Hole Pluton (SHP) in Southern California.
SHP megacrysts are euhedral, ranging from 1-8cm in length. Petrographic analysis and SEM/EDS mapping reveals abundant plagioclase (~40%), quartz (~35%), biotite (~10%), titanite (~10%), and hornblende (~5%) inclusions. Other accessory phases include Fe-Ti oxides, apatite, allanite, and zircon. Many of these inclusions, especially euhedral plagioclase, biotite, and titanite, are preferentially orientated along diffuse oscillatory zoning boundaries in the host megacryst. EPMA analyses collected along megacryst core-to-rim traverses reveal Or78-93 compositions with dramatic fluctuations in Ba concentrations (0.89 - 2.73 wt%). Core and rim analyses of plagioclase inclusions were also collected via EPMA. These analyses reveal that plagioclase inclusions contain oligoclase to andesine cores (An19 - An34) and albite-rich rims (An3 - An10).
Although SHP megacrysts are much older than megacrysts described in previous studies, the textural and chemical observations are strikingly similar. We favor a magmatic origin for these megacrysts and interpret these similarities to suggest that a common magmatic process is responsible for K-feldspar megacryst formation.
Wolfe, Claire; Loocke, Matthew; and Brown, Kenneth, "Megacrystic Potassium Feldspar Magmatism In The Southern Mojave Desert, California" (2022). Annual Student Research Poster Session. 98.