Oxygen Isotopic Studies of Calcareous and Pelitic Metamorphic Rocks, New England

Oxygen isotope fractionations between coexisting quartz and calcite (Δqc) have been measured in calcareous quartzites, quartz-calcite veins, metavolcanic rocks and marbles collected throughout Vermont. Excluding dolomite-bearing samples, Δqc tends to decrease with metamorphic grade up to the upper garnet zone, although there is a large scatter in fractionation values in each zone. The maximum temperatures in each zone calculated from Δqc values agree approximately with geologically inferred values. Many quartz-calcite pairs appear to have re-equilibrated retrogressively to temperatures well below the metamorphic maximum. Maximum preserved temperatures for a given isograd in southern Vermont are higher than those in northern Vermont, possibly due to a regional pressure gradient from north to south. Whole rock isotopic composition appears to have been inherited, with little change, from a protolithic sedimentary mixture of marine limestone, sand, and shale, and does not vary with metamorphic grade. Quartz-magnetite fractionations in rocks from Vermont, New Hampshire, and Maine decrease with increasing metamorphic grade, but also show considerable scatter (overlap between grades), suggesting retrogression. Maximum temperatures at each grade, calculated from these fractionations, also agree with geological estimates. Quartz-mica fractionations vary little with metamorphic grade and are widely scattered, apparently due to retrogressive alteration.