Northeastern Section - 40th Annual Meeting (March 14–16, 2005)

Paper No. 8
Presentation Time: 10:40 AM


SPEAR, Frank S., EES, RPI, 110 8th St, Troy, NY 12180 and CHENEY, John T., Geology, Amherst College, Department of Geolgy, Amherst College, Amherst, MA 01002,

A stratigraphic unit traditionally mapped as Devonian Littleton Formation crops out along the Bronson Hill anticlinorium from southern Connecticut to northern New Hampshire. The unit has similar parageneses along strike with early, pre-foliation, small (1-5 mm) garnet, and late, post-fabric large (1-5 cm) staurolite. In contrast, rocks immediately overlying this unit have early, pre-fabric staurolite, and late garnet, and rocks immediately below have early garnet and no staurolite. A marked discontinuity in peak metamorphic P and T conditions exist between this unit (560 C, 600 MPa) and units lying immediately below (480 C, 350 MPa). Furthermore, the P-T path of rocks from this unit is different from that of rocks structurally above and below. Based on these petrologic criteria, we believe this to be a discrete, fault bounded structural unit which we have named the “big staurolite” nappe.

Monazite is abundant in rocks of the "big staurolite" nappe and occurs in the matrix and as inclusions in garnet and staurolite. Monazite is typically unzoned in Y, Nd, P, but has higher Th cores than rims, typical of low to medium grade monazite. Th/Pb ages were determined on six samples from around 50 km along strike using the IMS 1270 at UCLA. Twenty-seven separate ages range from 370 to 320 (± 3 my, 1 s.e.). This range of ages is in marked contrast to monazite from structurally higher nappes which have ages that range from ca. 410 Ma to 350 Ma and lower units, which have ages that range from 420 to 340 Ma. Hornblende and biotite Ar-Ar ages of 280 and 260, respectively, indicate that the “big staurolite” nappe was not cooled until the Alleghenian orogeny.

It is proposed that the “big staurolite” nappe represents a fundamental suture in central New England that binds the dominantly low-pressure, high level nappes of central New Hampshire (the Fall Mountain and Chesham Pond nappes) with the dominantly Barrovian metamorphic rocks west of the Bronson Hill anticlinorium and into eastern Vermont. The suture was apparently active at mid-crustal depths (ca. 20 km) from ca. 350 – 320 Ma, after metamorphism had ceased in higher level nappes, and before the Alleghenian orogeny.