UNDERSTANDING MELT SEGREGATION FROM CUMULATES AT SPIRIT MOUNTAIN BATHOLITH

The Spirit Mountain Batholith is located in southern Nevada, consisting of about 250 km² of exposed granitic rock that accumulated over ~2 Ma (Walker et al., 2007). Westward tilting of the batholith reveals a cross section from base to roof, allowing us to interrogate the records of magmatic processes within. The granite body shows evidence of repeated episodes of injection, crystal accumulation to produce feldspar rich cumulates, and melt segregation to produce a large, crystal poor, leucogranite cap in a patchwork of segregations from pockets below and injection into the roof zone over the lifetime of the batholith (Claiborne et al., 2006). Internally, Walker identified one ~3km thick section of granite that appears to represent a single intrusion, grading from crystal rich cumulate at its base to crystal poor leucogranite at its paleotop. Bridge Canyon transects this discrete unit from east to west allowing us to carefully investigate the crystal and textural records of formation and extraction of high-silica melts through processes of interstitial melt segregation from cumulate granites. A ~3km transect of samples was taken from top to bottom of this intrusion. Feldspars, including plagioclase and potassium feldspar, from each sample, both larger phenocrysts and matrix crystals, were imaged using Backscatter Electron imaging and Electron Dispersive Spectroscopy phase mapping to elucidate major element zoning. Laser ablation ICPMS was used to identify the trace element composition and any zoning of feldspars. Preliminary results reveal chemical zoning, with changes in potassium, sodium, and barium concentrations, in only the largest feldspar phenocrysts (>15mm) with none in smaller, matrix-forming crystals (2-5mm). This, combined with the field evidence, suggests that the history of this section might be a simple one - one intrusion followed by growth and accumulation of larger crystals and segregation of interstitial melt to produce a high-silica melt, a snapshot of the processes that appears to have occurred repeatedly over time to produce the 250 km² batholith and its cap. Larger crystals may have grown before intrusion, experiencing multiple stages of growth. The results of this study support use of this field unit to ground truth future models of melt intrusion, crystallization, and crystal accumulation and melt segregation in granitic systems.