CONSTRAINING K-FELDSPAR MEGACRYST GROWTH THROUGH INVESTIGATIONS OF PLAGIOCLASE INCLUSIONS

While K-feldspar megacryst formation is not yet completely understood, two models have been proposed to explain their origins. One model proposes relatively early growth within a melt-dominated environment, while the other proposes relatively late growth through textural coarsening. To provide additional constraints on megacryst formation, this study examined the characteristics of plagioclase inclusions found within four sector-zoned K-feldspar megacrysts (3-4 cm) from the Granite Peak Stock intrusion of the Santa Rosa Range, northwestern Nevada.

Microstructural observations reveal subhedral to euhedral plagioclase inclusions, ranging from 0.25 mm to 1 mm. These inclusions are crystallographically aligned within the K-feldspar host and commonly exhibit oscillatory zoning and distinct overgrowths. Megacryst hosts contain two distinct sectors: a plagioclase-rich domain and a biotite-rich domain. Plagioclase inclusions appear to be equally distributed throughout both sectors.

Plagioclase inclusions were analyzed via EPMA from their cores to rims. These inclusions were also analyzed along core to rim traverses within the host K-feldspar megacryst. Our analyses reveal that inclusions are predominantly oligoclase (An₁₃-An₂₈), but andesine cores (An₃₀-An₄₁) were also observed. Most of the inclusions exhibit normal zoning, but some exhibit reverse or complex zoning patterns. Overgrowths found on many of the inclusions are rims of albite (An_2-5). Inclusions within the plagioclase-rich domains and biotite-rich domains are comparable, with only slightly higher anorthite contents within the plagioclase-rich domains.

Microstructural observations and geochemical data suggest an early stage of growth for K-feldspar megacrysts. This interpretation is supported by the euhedral shape, normal zoning, and crystallographic alignment of the plagioclase inclusions. These data indicate that the K-feldspar megacryst-hosted plagioclase inclusions were formed in an evolving melt prior to being incorporated.