PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF THE CAMAS LAND SILL, CHELAN COUNTY, WASHINGTON

The Eocene Camas Land sill of Chelan County, Washington was studied to demonstrate whether cooling was accompanied by fractional crystallization. This coarse, hypabyssal, diabasic sill, variously exposed in a northwest trending syncline, intruded the clastic sedimentary rocks of the Swauk formation (Eocene). The sill is up to 500 feet thick in the northwest. Several samples were collected from a vertical quarry face for both petrographic and geochemical (XRF and ICP-MS) comparison. Modal mineralogy was estimated (using JMicroVision v.1.2.7) and shows that the rocks near the base of the sill are finer-grained and more mafic, with higher proportions of clinopyroxene, magnetite-ilmenite and possibly relict olivine (now altered to iddingsite) whereas those from near the top of the sill are lighter in color due to a higher ratio of plagioclase to clinopyroxene. Whole rock geochemical analyses from this study and others, as plotted on major element Harker variation diagrams, show linear trends suggestive of fractional crystallization. The samples are LREE enriched with no visible europium anomaly. The major, trace and REE element compositions of the sill rocks have been compared to Teanaway formation basalts and basalt of the Columbia River Basalt province.