PLUTON MIGRATION THROUGH CRUST LEAVES MIGMATITES BEHIND

ALLEN, Tim, and CHAMBERLAIN, C. Page, Department of Earth Sciences, Dartmouth College, 6105 Fairchild Hall, Hanover, NH 03755-3571

At the northern end of the Acadian metamorphic high in New England (Maine), the metamorphism is characterized by a low-pressure, high-temperature regime associated with granite plutonism, while at the southern end (Massachusetts), the metamorphism is characterized by very high grade, deep-level gneisses and granulites. In between these two extremes lies a large area (New Hampshire) characterized by metamorphic "hot spots." One interpretation of these hot spots is that they are the result of channelized fluid flow (of unknown source) through the crust.

We have carried out detailed stable isotope, petrologic and geochemical studies of migmatites forming one of these hot spots, in Pinkham Notch, NH. We measured d18O of non-migmatized schists and adjacent migmatites formed from the same parent rock. The observed lowering of d18O in the migmatites, enriched fluorine content of biotites, and spatial associations with pegmatites indicate that the migmatization occurred as a result of magmatic water infiltration. This result supports an alternate interpretation of the NH hot spots—that they represent the conduits through which plutons passed as they migrated from their source deep in the crust (MA) to be emplaced at higher crustal levels (ME). Regional mapping shows that structures in this migmatite hot spot are consistent with such an interpretation.

Additionally, d18O values of quartz mineral separates from migmatite leucosomes and melanosomes suggest that there is a significant fractionation of oxygen isotopes between melts and residual solids during fluid-driven partial melting of pelitic rocks.

1992 Geological Society of America Abstracts with Program 24(7): A338.