ION MICROPROBE DATING OF MONAZITES FROM THE CONNECTICUT VALLEY METAMORPHIC LOW: GASSETTS TO PUTNEY VERMONT

An apparent metamorphic discontinuity between the staurolite grade rocks of the New Hampshire terrane and the chlorite to garnet grade rocks of the Vermont terrane produces the Connecticut Valley Metamorphic Low. Peak P increases westward some 6 kbar (~20 km) (from 4 to 10 kbar) across 10 km in eastern Vermont from the low grade rocks to the higher grade rocks of the Chester Dome. To the east, metamorphism of the New Hampshire rocks in response to intrusion of the New Hampshire magma series was followed by thrust faulting that produced an inverted metamorphic sequence. Peak metamorphic pressures in the lowest nappes require the nappe sequence to be ~ 20km thick. U-Th-Pb spot ages of monazites from 12 pelitic schist samples have been acquired from the UCLA ion microprobe. The standard error for each spot age typically ranges from 2% to 5% or from 5 to 20Ma . The spot ages of the monazites from the six New Hampshire rocks are all younger than the 400-410 Ma age of the New Hampshire Magma Series and thus record the time for the metamorphism of the nappes. Five of the Vermont samples are from the Gassetts Schist, the other sample is from the Waites River Formation. The spot ages from these rocks are consistent with Early Acadian metamorphism in pre-Silurian rocks of the domes at 410 - 430Ma followed by the main Acadian event in pre-Silurian rocks at 380-390 with metamorphism of Siluro-Devonian rocks at ~ 350Ma.