EPIGENETIC TOURMALINE IN SEDIMENTARY RED-BEDS - AN EXAMPLE FROM THE SILURIAN ROSE HILL FORMATION, VIRGINIA

Acicular epigenetic tourmaline is developed hemimorphically on the c p ole of detrital tourmaline in a Silurian red-bed sandstone of the Rose Hill Formation from western Virginia. Rounded quartz with minor detri tal tourmaline and clay-mineral ooids are cemented by berthierine and hematite that formed epigenetically during a late Paleozoic influx of fluid associated with the Alleghenian orogeny. Detrital grains of tour maline are very Fe- and Ti-rich but Al-poor. Compositional signatures of the detrital tourmaline suggest a provenance from a highly oxidized granitic rock or a quartz-tourmaline vein. Relative to the detrital s ubstrates, the composition of the epigenetic overgrowths is distinctly different, consisting of Al-rich schorl-dravite with a Fe/(Mg+Fe) val ue of 0.66 and a significant X-site vacancy. Al-enrichment and the X-s ite vacancy reflect operation of the exchange vector: square AlNa-1Mg- 1, a consequence of growth in an Al-rich but Na- and Ca-poor environme nt. A high proportion of X-site vacancies is commonly observed in epig enetic tourmaline from sandstones, but the epigenetic tourmaline in th is red-bed sequence is the most Fe-rich reported. Epigenetic tourmalin e is a sensitive recorder of the chemical environment during early sta ges of metamorphism of the red-bed sequence.