USING MICROANALYTICAL TECHNIQUES TO UNRAVEL K-FELDSPAR MEGACRYST FORMATION
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.