PRESSURE-TEMPERATURE-TIME CONDITIONS, ND AND PB ISOTOPIC COMPOSITIONS AND DETRIAL ZIRCON GEOCHRONOLOGY OF THE MASSABESIC GNEISS COMPLEX, NEW HAMPSHIRE

The Massabesic Gneiss Complex of southeastern New Hampshire is a peri-Gondwanan inlier with either Ganderian or Avalonian affinities. To help determine the relationship of the complex to these peri-Gondwanan terranes, we dated amphibole, muscovite, and biotite in Massabesic amphibolites and paragneisses by the 40Ar/39Ar method, determined bulk-rock Nd and Pb isotopes of Massabesic orthogneisses, and determined the ages of detrital zircons in a Massabesic quartzite by SHRIMP.

The 40Ar/39Ar analyses yielded uniform ages of ~255-260, ~240, and ~238 Ma for amphibole, muscovite, and biotite respectively, showing cooling from the Permian at ~9°C/m.y. Metamorphic pressures and temperatures calculated using standard mineral thermobarometers yield peak anatectic metamorphic conditions of ~9 Kb and ~700°C, and conditions in overprinting schistosities of 7 Kbars at the same 700°C temperatures. Compared to Avalonia of southern Connecticut, one-dimensional thermal modeling shows that the slightly younger ages and the slightly slower cooling rate is explained by a 2-3 km deeper level of Permian metamorphism. These results are consistent with P-T-t paths in all exposed Avalonian rocks in New England, and confirm a Permian clockwise P-T path.

The majority of Avalonian rocks have positive bulk-rock εNd compared to more negative values for Ganderian plutons, however there is considerable overlap in εNd between the two terranes. Massabesic orthogneiss εNd and 207Pb/204Pb values are inconclusive for terrane determination because they plot in the overlapping fields of Ganderia and Avalonia. However, detrital zircons from a Massabesic quartzite are similar to inherited zircons from Ganderian plutons and those in the Lyme dome of southern Connecticut. Mismatches in zircon ages and abundances between Avalonian supracrustals and the Massabesic quartzite make an Avalonian correlation doubtful. Thus while the detrital zircon data suggest a Massabesic – Ganderia correlation, the P-T-t data indicate that the Massabesic was coupled with Avalonia of southern New England during the Alleghanian.