CRYSTAL SIZE DISTRIBUTION (CSD) ANALYSIS OF THE DEVIL'S THRONE INTRUSION, CERRILLOS HILLS LACCOLITH, NEW MEXICO

The ~150 m thick Devil’s Throne intrusion is one of the steeply dipping, sill-like outer limbs of the ~35 Ma Cerrillos Hills laccolith, a Christmas Tree style laccolith exposed over an ~30 km² area in northern New Mexico (Maynard, 2005). The intrusion consists of a gray andesitic porphyry containing phenocrysts of elongated hornblende and plagioclase crystals in a fine grained matrix. Twenty rock samples were collected systematically across the width of the intrusion and scanned at 3200 dpi and individual plagioclase crystal outlines of ~13,000 crystals were traced using Inkscape. Crystal dimensions (major and minor axes) were measured using ImageJ and then imported into Higgins CSD software for final crystal size distribution analysis. Vertical profiles of mean crystal size variations and phenocryst abundance were made using data from CSD analyses. CSD analysis results reveal that mean crystal size varies from ~0.39 to 1.2 mm across the width of the intrusion however, there is no systematic increase in crystal size from the outer margins to the interior of the intrusion. Instead, the mean crystal size varies randomly and abruptly across the vertical profile. The abundance of plagioclase phenocrysts varies from 20-40%, with the upper part generally consisting of <27% phenocrysts while the lower part generally contains >27% phenocrysts. The random and abrupt variations in mean plagioclase crystal size and phenocryst abundance suggests that the intrusion was emplaced by multiple magma injections. SEM analyses of plagioclase phenocrysts also show strongly zoned crystals with core compositions of andesine with albite rims indicating a multi-stage crystallization history. We would like to thank Donald Hodder, Laurel Mutti, Dr. Garapic, and Jeremy Truitt for their assistance with this project.