MINERAL CHEMISTRY OF THE CAMAS LAND DIABASE, CENTRAL CASCADES, WASHINGTON: A SILL WITHIN THE EOCENE CHUMSTICK FORMATION

The undated Camas Land Diabase consist of dark colored diabase with lesser gabbro. It occurs as a sill within the Eocene Chumstick Formation. These rocks are finer-grained near the base, and are folded. To determine the petrogenises of the Camas Land Diabase we analyzed plagioclase (n=21) and pyroxene (n=18) from two diabase and a fine-grained gabbro by an EPMA at the Florida Center for Analytical Electron Microscopy. The diabase is primarily ophitic to subophitic, while the gabbro is ophitic to slightly poikilitic. Plagioclase commonly crystalized before pyroxene. None of the analyzed minerals display chemically zoning. Plagioclase feldspars have compositions that range from An₃₉ to An₇₂, and average An₅₈. They are mostly labradorite (n=17), with two andesine and two bytownite. The pyroxene from two samples are all augite, while coexisting augite and three pigeonites were found in a diabase. The augite range from En₄₃ to En₅₀ and Wo₃₁ to Wo₄₀, and pigeonite ranges from En₆₅ to En₆₇ and Wo₆ to Wo₈. Pyroxene from all three samples plot as subalkaline and tholeiitic. Ti and Cr suggest the samples are predominately orogenic. Pyroxenes plot in the volcanic arc and ocean floor overlap fields on tectonic discrimination diagrams, whereas the pigeonites plot primarily in the volcanic arc fields. The An% of plagioclase decreases as the Mg# of the pyroxene decreases. The plagioclase and pyroxene compositions suggest the sill’s magma was low-K tholeiitic, as well as transitional between mafic and intermediate. This agrees with whole-rock data from previous researchers. The pyroxene compositions suggest the Camas Land Diabase has both volcanic arc and non-arc affinities. Based on this data, the possibility of the sill forming through normal subduction can be eliminated. It is likely the samples formed with an arc influence while also recording decompression melting where MORB can occur. The chemical composition of the pyroxene shows that either ridge subduction, slab breakoff, or a combination of the two, may be the process that occurred to form the sill. These arc and non-arc affinities could be due to Siletzia accretion. The Siletzia terrain accretion, and associated ridge subduction and/or slab breakoff, occurred around the same time as the formation of the Chumstick Formation, which this sill intrudes.