P-T-T CONDITIONS OF THE MASSABESIC GNEISS COMPLEX, NEW HAMPSHIRE: ALLEGHANIAN LOADING AND EXHUMATION OF A PORTION OF THE AVALON TERRANE

Several lines of evidence indicate that the Massabesic Gneiss Complex (MGC) of southeastern New Hampshire is an inlier of the Avalon terrane. Common characteristics of the MGC and the Avalon terrane of southeastern New England include 1) similar rock types, 2) similar igneous ages ( ~620 Ma), and 3) similar Nd isotopic signatures. To this list we add indistinguishable Alleghanian metamorphic P-T-t histories. The P-T path of the MGC is defined by several geothermometers and barometers including garnet and/or amphibole-bearing assemblages from which metamorphic pressures and temperatures of 7-10 Kbars and 670-700o C are calculated. From these conditions in the kyanite field, post-peak pressure sillimanite in sheared schistose rocks and chlorite replacements in orthoamphibolites contribute to defining a clockwise P-T path that loops to lower pressures through the sillimanite field. An Alleghanian exhumation history of the MGC is suggested by the Permian ages of titanite and monazite in the literature. To these we add Permian hornblende 40Ar/39Ar cooling ages (~260 Ma) and early Triassic cooling ages of muscovite and biotite (~240 and 235 Ma respectively). Together these P-T-t data show that the MGC was metamorphosed in the Alleghanian at P-T conditions that were high enough to overprint most evidence of Acadian metamorphism. These metamorphic conditions are distinctly higher grade than those of the adjacent Merrimack and Central Maine metasedimentary rocks to the southeast and northwest respectively. This juxtaposition indicates a major Alleghanian metamorphic and structural discontinuity between the Massabesic Gneiss Complex and its enclosing rocks. Km-scale vertical motion, and arguable strike-slip motion during the Alleghanian on bounding faults are implicated.