Open-system, constant-volume development of slaty cleavage, and strain-induced replacement reactions in the Martinsburg Formation, Lehigh Gap, Pennsylvania

No important changes in the volume of mudrocks occur across the mudstone-to-slate transition in the Martinsburg Formation at Lehigh Gap. Mass-balance calculations based on chemical analyses and specific-gravity measurements of 48 mudstones and 26 graywackes from the Martinsburg Formation across this 130-m strain gradient show constant ratios of Al ₂ O ₃ , TiO ₂ , FeO _T , MgO, MnO, Y, V, and Zr. This reflects the relatively low differential mobility of these components during diagenesis and slaty cleavage development, as well as the uniformity of compositions of the mudstones and graywackes at deposition. The inertness of most major components over the 100-m outcrop requires: 1) that the composition of the fluid passing through the rocks was closely buffered by the quartz-albite-muscovite-chlorite assemblage, 2) that the volume of the fluid was small, and 3) that the P-T gradient down which it traveled was gentle. -from Authors