CH₄-rich inclusions from quartz veins in the Valley-and-Ridge province and the anthracite fields of the Pennsylvania Appalachians

The temperatures and pressures of diagenesis and very low-grade metamorphism of the Palaeozoic formations of the Valley-and-Ridge province in the central Appalachians are revealed by analyzing their fluid inclusions. Samples from two areas were studied and compared: (1) the Silurian and Devonian Delaware Water Gap area and (2) the anthracite region of eastern Pennsylvania. Gaseous CH₄-CO₂ and aqueous (±CH₄) inclusions were studied by microthermometry; gas compositions were measured by Raman analysis. The two fluids are thought to have been immiscible, saturated fluids. The isochores calculated for both types of inclusions indicate fluid trapping P-T conditions of about 195–245 °C, 0.8–1.3 kbar for the anthracite region, but higher pressures (1.3–1.9 kbar at 180–230 °C, and possibly as much as 2.2–2.5 kbar) for the Delaware Water Gap area. The pressure difference reflects the relative stratigraphic position of the rock formations, and a higher thermal gradient for the anthracite region. The fluid trapping temperatures and pressures are consistent with conodont alteration indices (CAI), illite crystallinities, the anthracite rank, slaty cleavage development, and the stability of pyrophyllite.