THE BOUNDARY BETWEEN THE VERMONT AND NEW HAMPSHIRE TERRANES: AN ACADIAN CONUNDRUM

A long recognized conundrum of New England geology is the enigmatic relationship between the Acadian orogeny in Vermont and New Hampshire. The difference is manifest in seemingly synchronous yet different metamorphic styles of different Siluro-Devonian stratigraphies. The Buchan style metamorphism of New Hampshire, with counterclockwise P-T paths and regional high-T, low-P metamorphism contrasts markedly with the Barrovian metamorphism of Vermont, with its clockwise P-T paths culminating in maximum burial depths of ca 30-35 km.

The VT & NH terranes meet in the vicinity of the Chicken Yard line - the traditional stratigraphic boundary - forming the Connecticut Valley Metamorphic Low (CVML). In the CVS of Vermont and along strike in Massachusetts, the array of accumulated ion microprobe ²⁰⁸Pb/²³²Th spot ages from matrix and inclusion monazite grains is largely confined to 350-390 Ma. ⁴⁰Ar/³⁹Ar cooling ages from muscovite in these same rocks range from 330 Ma to 350 Ma. In Massachusetts, Cambro - Ordovician rocks have temperature-time histories that are identical to Siluro-Devonian units. The age signature of the VT metamorphism continues east across the CVML into the garnet zone in the NH stratigraphic sequence.

To the east, the Bronson Hill terrane consists of a sequence of nappes, each with a unique P-T path, that are stacked in an inverted metamorphic sequence and punctuated by late gneiss domes. Each of the domains of the Bronson Hill is separated by a post metamorphic fault that records a significant break in peak P and/or T and each unit has its own characteristic monazite age array. The lowest and western most unit, the big staurolite domain, contains monazite as old as ca 360 Ma, similar to the Vermont monazite ages, and, most significantly, 320 Ma monazite as inclusions in staurolite and 280 Ma matrix monazite grains. Muscovite ⁴⁰Ar/³⁹Ar ages along and east of the BHT range from 335 Ma at Fall Mtn. to around 250-290 Ma over much of central NH. Accordingly, the rocks of the big staurolite domain underwent metamorphism after the VT rocks cooled through the muscovite closure temperature and after the peak of metamorphism in the overlying NH nappes, presumably during juxtaposition of the two terranes during the Alleghanian and certainly not during the Acadian.