Mineralogic domains in very low grade pelitic rocks

Carbonaceous pelites from northeastern Pennsylvania that have undergone very low grade metamorphism during the Allegheny orogeny contain domains (approximately 20 × 500 µm) in which one of two mineral assemblages, kaolinite + quartz or pyrophyllite + quartz, occur. These phases are related by the reaction kaolinite + quartz = pyrophyllite + H2O. The bulk mineralogy determined by X-ray diffraction incorrectly suggests that the phases kaolinite, pyrophyllite, and quartz coexist. The occurrence of these phases in rocks metamorphosed under different P-T conditions does not result from internal buffering of µH2O or from the metastable persistence of kaolinite or pyrophyllite due to slow reaction rates. Rather, equilibrium among these minerals and accompanying fluid within the domains was apparently approached, while disequilibrium reigned between domains. It is not at present possible to determine the process by which the domains formed or were maintained. Regions of inferred high permeability (consisting dominantly of quartz grains) do not coincide with the mineralogic domains, suggesting that µH2O was not controlled by the passage of fluid through the rock. Immiscibility between CH4 (provided by coaly matter) and H2O (provided from prograde mineral reactions) is likely to have occurred and may have led to the initial segregation of fluids into discrete domains.