New Hampshire Migmatites: Oxygen Isotope Fractionation During Partial Melting, and Suggestions of Structurally Enhanced Magma Migration.

T Allen (Geology Department-Mailstop 2001, Keene State College, Keene, NH 03435-2001; e-mail:

Detailed stable isotope, petrologic and geochemical studies of migmatites forming a "hot spot" in the Acadian metamorphic high of New England suggest that these migmatites appear to have formed as a result of low-temperature melting driven by fluid infiltration.

Within the migmatites, the delta 18-O values of quartz mineral separates from migmatite leucosomes are up to 1 per mil higher than for quartz from migmatite melanosomes. This difference between the two components of the migmatites cannot be obtained by the isotopic re-equilibration of the leucosome and melanosome mineral assemblages during cooling, nor by the influence of the infiltrating fluids either during migmatization or by selective post-migmatization alteration. Rather, it is suggested that there is a significant fractionation of oxygen isotopes between melts and residual solids during fluid- driven partial melting of pelitic rocks. The observed oxygen isotope fractionation has been modelled using migmatization reactions determined from comparisons between the migmatites and adjacent un- migmatized parent schists, in order to determine appropriate melt- mineral fractionation factors.

The structural setting of this migmatite "hot spot" suggests that it may represent a conduit through which plutonic magma bodies passed as they migrated from their source deep in the crust to be emplaced at higher crustal levels. This model may help explain the anomalous "hot spot" metamorphic pattern in New Hampshire, and variations in PT paths along the strike of the New England Acadian orogen.

1994 EOS, Transactions of the American Geophysical Unions 75(16): 360-361.