PETROLOGY AND GEOCHEMISTRY OF THE GRANITE BUTTE PORPHYRY, COAST RANGE, HUMBOLDT COUNTY, CALIFORNIA

The Granite Butte Porphyry (GBP) is a 35 Ma trachyte plug in the Coast Range of Humboldt County, California (Meyer and Naeser, 1970). Results from petrographic analysis indicate that the GBP is a porphyritic sanidine syenite, with a phenocrysts assemblage composed primarily of plagioclase with hornblende, sanidine, biotite, quartz, epidote and Fe-Ti oxides. Plagioclase is the most common mineral. It ranges from 0.5 to 3mm in diameter and displays oscillatory zoning. Calcite also occurs in the GBP, but it is likely an alteration mineral due to its globular appearance and its occurrence in 0.5mm-1mm cavities. In some samples, fine-grained Fe-Ti oxides rim hornblende phenocrysts. In other samples, hornblende and biotite occur as psuedomorphs that have been completely replaced by Fe-Ti oxides. Dark gray, quenched mafic inclusions also occur in the GBP. Results of whole-rock geochemical analyses via X-ray Fluorescence indicate a compositional range from andesite to trachyte, where major element concentrations (in wt%) are 60.93-66.37 SiO₂, 0.14-0.76 TiO₂, 16.78-17.16 Al₂O₃, 3.71-6.84 Fe₂O₃, 0.12-0.36 MnO, 0.04-2.89 MgO, 0.37-6.16 CaO, 3.30-6.19 Na₂O, 1.66-5.46 K₂O, and 0.02-1.66 P₂O₅. Compared to similarly aged alkali intrusive rocks in the Oregon Coast Range, the GBP is higher in SiO₂ and K₂O, and lower in Na₂O. These results are consistent with the GBP forming from the shallow intrusion of an andesite-trachytic magma intruding into fore arc sediments either due to oblique extension or slab-window processes as previously suggested (Meyer and Naeser, 1970).