DATING THE ACADIAN OROGENY IN SOUTHWESTERN MASSACHUSETTS

Fifty-three in situ Th-Pb ion microprobe spot ages were obtained on monazite grains from thirteen muscovite schist samples using the UCLA Ion Microprobe. The dated monazites in these Paleozoic meta-sedimentary rocks occur as inclusions in staurolite and garnet and within the muscovite-biotite matrix. These pelite samples have mineral fabrics consistent with a complex clockwise, Barrovian P-T evolution.

The monazite grains are age zoned with individual grains ranging in age by as much as 52Ma (discounting outlying ages).. The samples are from an east west transect along US Route 20 in southwestern Massachusetts that extends from the Mesozoic Connecticut Valley boarder fault on the east to the Berkshire Massif on the west. Four samples are from the pre-Silurian Rowe and Cobble Mountain formations, which may have been effected by Taconian metamorphism, and nine are from the Siluro-Devonian Waits River and Goshen Formations, The majority of the monazite ages range from 364±3to 376±3in the Rowe and Cobble Mountain formations and from 363±3 to 389±3 in the Waits River and Goshen Formations. The samples yielded only Acadian ages. Muscovite from the nine Siluro-Devonian samples were previously dated by ⁴⁰Ar/³⁹Ar and yielded ages that range from 332±3. to 356±5 Ma (Blanchard et al., 1995; Henderson et al.1996). Preliminary geothermobarometry based on garnet-biotite, and plagioclase, garnet aluminosilicate quartz from Henderson (1996) show increasing metamorphic pressures from 0.6 GPA in the east to 1.1 GPa in the west.

These monazite ages from southwestern Massachusetts are consistent with ages reported in similar rocks in southern Vermont. Monazite ages differ from muscovite cooling ages by as little as 12 Ma. The resulting cooling rate of ca. 20 degrees C/ m.y. may be accommodated by erosion but the E-W pressure gradient requires some 15 km of differential exhumation at 350Ma along some as yet unknown structures