MEGACRYSTIC POTASSIUM FELDSPAR MAGMATISM IN THE SOUTHERN MOJAVE DESERT, CALIFORNIA

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.