USING MICROANALYTICAL TECHNIQUES TO UNRAVEL K-FELDSPAR MEGACRYST FORMATION

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 processes responsible for megacryst formation by examining the megacrystic Granite Peak stock (GPS) in northwestern Nevada.

GPS K-feldspar megacrysts are commonly euhedral, ranging from 1 to 8 cm in length (avg. 5 cm). Petrographic observations reveal both ubiquitous oscillatory and sector zoning. Oscillatory zoning boundaries vary from 0.25 mm to 1 mm in width and commonly exhibit sharp as well as resorption and/or erosion microstructures. Microscopy also reveals abundant crystallographically oriented inclusions of plagioclase (85-90%), quartz (5-10%), biotite (1-5%), and accessory phases (~1%). EPMA analyses indicate that the host megacrysts are orthoclase (Or₈₁ - Or₉₇) with highly variable Ba concentrations ranging from 0.1 - 2.9 wt%. Normally zoned plagioclase inclusions are common, but complexly zoned inclusions are also present. Plagioclase inclusions are typically oligoclase (An₁₃ – An₂₈), but occasionally contain andesine cores (An₃₂ – An₄₁) and albitic overgrowths (An₂ – An₅).

We interpret these data to indicate that K-feldspar crystallization was initiated relatively early in a melt-dominated system. High Ba concentrations within the magmatic system may provide a possible explanation for this early growth. While a magmatic origin is favored, textural observations also suggest that the GPS system experienced relatively late-stage subsolidus overprinting and exsolution. Unraveling the complex textural and chemical history of these megacrysts provides new insights into the dynamic processes and thermal histories of silicic systems.