TIMING AND ERUPTION MECHANISM OF FELSIC EOCENE DIKES IN THE VALLEY AND RIDGE PROVINCE OF VA AND WV

In this study, we constrain the age and eruption characteristics of felsic magmas that are part of the alkaline igneous suite known as the Shenandoah Igneous Province (SIP) located in Highland County and Rockingham County, VA, and Pendleton County, WV. Paleomagnetic data and K-Ar and Ar-Ar age dates on limited samples indicate the SIP volcanics are Eocene in age (48-35 Ma) and represent the youngest known igneous activity in the Eastern United States (Southworth 1993). Although many of the igneous bodies are basalts or picrobasalts with SiO₂ <50wt%, there are a number of trachytes and trachydacites with SiO₂>63wt% containing phenocrysts of biotite, amphibole, and feldspars. The felsic magmas may have formed from basalt through assimilation and fractional crystallization (AFC) processes within the lithosphere. SIP dikes follow pre-existing structures, which may have slowed magma ascent through some fractures, allowing AFC to occur. We investigated magma ascent time in three felsic dikes from Highland County, VA, by observing the presence or absence of amphibole dehydration reaction rims under the polarizing microscope. Amphibole phenocrysts had no detectable reaction rims in thin section. These results indicate a maximum ascent time of <4 days for the felsic magmas (Rutherford and Hill 1993). Fast ascent means that differentiation must occur on a very small scale within each dike system, or before ascent within a deeper magma chamber. Mineral separates of biotite and amphibole for the three dikes were prepared for Ar-Ar age dating at the USGS in Reston using a Frantz Isodynamic separator and heavy liquids. Samples were irradiated at the USGS in Denver. Ar isotope analyses were completed using the VG1200 and MAP216 mass spectrometers at the USGS in Reston. One of the samples, a dike from the Hightown Quarry, has a previously determined Ar-Ar age date on biotite of 47.7±0.6 Ma (Southworth 1993). The Ar-Ar ages determined in this study will help constrain the duration and timing of SIP magmatism and the timing of felsic magmatism versus mafic magmatism.