BIOTITE COMPOSITION FROM FELSIC IGNEOUS ROCKS: SHENANDOAH IGNEOUS PROVICE: PRELIMINARY RESULTS

Cenozoic-age bentonite in the Atlantic Coastal Plain is rare due the paucity of volcanism within the region. A rare occurrence of bentonite of demonstrable volcanic origin occurs in the 46 Ma Castle Hayne limestone in the Coastal Plain of North Carolina. Nusbaum et al. (1988) considered three possible source areas with Eocene volcanism: the Shenandoah igneous province (SIP) of northern Virginia, the Trans Pecos province of Texas, and the Bermuda Rise. Based on the anomalously high BaO content of unaltered biotite pyroclasts Nusbaum et al. (1988) proposed that the SIP is the source of the bentonite. The purpose of our research is to further explore this conclusion through analysis of biotite phenocrysts in two felsic samples from the SIP.

We examined biotite compositions with an electron microprobe from two nearly identical hypabyssal intrusions collected near Monterey, Virginia. The first sample consisted on biotite crystals separates from a 47 Ma felsic rock (sample 336-B, Fullagar and Bottino, 1969). The second sample analyzed consists of subhedral plagioclase and biotite phenocrysts surrounded by plagioclase microlites which exhibit trachytic texture.

Compared to mean biotite compositions of Castle Hayne bentonite, SIP biotites have considerably higher total FeO (27 wt% vs. 15 wt %) and lower MgO (6.1 wt% vs. 14.0 wt %) indicating a more felsic composition of SIP magma. However, anomalous BaO contents for Castle Hayne biotite pyroclasts (X = 1.06 wt %) compare favorably for both SIP samples (X = 0.85 wt %).

When considered with age constraints, source distance and prevailing wind directions, anomalously BaO content of biotite from this study is consistent with a SIP source area for the Castle Hayne biotite. However, we recognize that biotite samples from intermediate composition hypabyssal samples may provide a compositional match for biotite, and will continue to pursue analyses of more samples.