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