POLYMETAMORPHISM AND DISEQUILIBRIUM IN THE BRONSON HILL TERRANE, NORTHERN CONNECTICUT

The Bronson Hill Terrane in northern Connecticut provides an opportunity to study the overprinting effects of the Acadian and Alleghanian Orogenies on rocks originally deposited during the Taconian Orogeny. Though the rocks of New England have been studied for over a century, the relationship between the Taconian (Ordovician), Acadian (Devonian) and Alleghanian (Permian-Pennsylvanian) orogenies is not well understood. An attempt to use the Holland and Blundy (1994) hornblende-plagioclase thermometer to define the extent of the Alleghanian overprint produced results indicative of disequilibrium.

One of the samples studied was located within the Ellington quadrangle of Connecticut and is a amphibole + plagioclase + quartz + epidote + minor allanite schist. The sample is located within the amphibolite of the Bronson Hill, which is believed to have been extruded during the Taconian Orogeny (Tucker and Robinson, 1990). In BSE, the allanite is divided into two parts. One part has sharp oscillatory zoning that outlines euhedral crystal growth that we believe to be igneous in origin. The second portion of the allanite is an anhedral overgrowth which we conclude to be metamorphic. The overgrowth is non-centered around the igneous core and fingers into the surrounding epidote. Preliminary analysis of the allanite has revealed that the metamorphic and igneous portions of the crystal have different chemical compositions. In the igneous core the moles of lanthium, cesium, thorium, and niobium are lower than in the metamorphic overgrowth. The igneous levels of silica, aluminum, and calcium are higher than the metamorphic portion of the allanite.

Catlos et al. (2000) have developed a method of dating allanite with the ion-microprobe using thorium-lead ratios. With the electron microprobe we analyzed and calculated an initial estimate of age for the metamorphic overprint of 404 Ma (Acadian). Boyd et al. (1993) used 40Ar/39Ar to determine the cooling ages of muscovite and amphibole within the Bronson Hill Terrane. At a latitude similar to the allanite-bearing sample, the muscovite cooling age was ~250 Ma and the amphibole ~300 Ma. The cooling ages are indicative of cooling from a Permian (Alleghanian) loading event (Goeke et al., 2000).