MONAZITE AGES FOR METAMORPHISM AND FAULT ZONE ACTIVITY IN THE NASHOBA TERRANE, EASTERN MASSACHUSETTS

Jim Skehan began his studies in what is known today as the Nashoba terrane in the early 1960's with detailed mapping of the Wachusett-Marlborough water tunnel and he identified many of the major fault zones in the area at this time. However, then as now, the ages of the shearing events were poorly constrained, as were the periods of metamorphism and migmatization in this high-grade terrane. Recent mapping and detailed structural studies in the Shrewsbury quadrangle, coupled with the electron microprobe dating of monazites associated with three intra-terrane shear zones, provide new evidence for the tectonic history of the Nashoba terrane.

The oldest monazite core age encountered, ~ 450 Ma, is from a grain within a garnet porphyroblast in a mylonitic schist that may represent a detrital grain. A second core age of 438 Ma in a now mylonitized granite likely represents an original igneous crystallization age. The first regional metamorphism (M1) affecting the terrane occurred in the interval ~ 430-420 Ma. A second metamorphic event (M2), associated with widespread migmatization, occurred between ~ 410 Ma and 390 Ma and may be separable into two phases, the older coincident with the youngest phase of the Andover Granite. Sinistral ductile shearing accompanied both the M1 and M2 events. Following M1 and M2, peak metamorphic assemblages in the shear zones were overprinted during several intervals from ~ 375-340 Ma and the shearing style shifted from ductile to more brittle-ductile conditions.