DEVONIAN AND CARBONIFEROUS AGES OF MULTIPLE CLEAVAGES IN THE WAITS RIVER FORMATION, EAST ATHENS DOME AREA, VERMONT

Cleavages of several generations have been identified in the Waits River Fm. from the eastern flank of the Athens dome. The mineralogy, cross cutting relationships, and new 40Ar/39Ar cooling ages show that cleavages formed during both prograde Acadian and post-Acadian metamorphism.

Biotite grade rocks contain early shallowly dipping cleavages (S1) crenulated by steep cleavages (S2). Both cleavages are composed of muscovite + chlorite; however, S2 is also overprinted by S2 trending biotite porphyroblasts. These in turn are cut by randomly oriented biotites. Electron microprobe analyses of muscovite from the two cleavages show greater phengite content in S2. This suggests both a dissolution-precipitation mechanism of cleavage formation and increased pressure conditions during the formation of S2. This higher pressure coupled with the higher temperature biotite growth in S2 requires S1 and S2 to be prograde fabrics. Muscovite separates from these samples give 40Ar/39Ar cooling ages of ~355 Ma. While they are not plateaus, the age spectra are relatively flat, and imply a single dominant age population of muscovite. Thus, the temperature of the biotite grade metamorphism (>400 C?) was not high enough to homogenize the phengite compositions of these white micas, but it was high enough to cause the diffusive loss of radiogenic Ar, rendering a single age population of muscovite. These results suggest that the Acadian orogeny produced several prograde muscovite-chlorite cleavages culminating in a biotite-grade fabric followed by a strain-free period (peak Acadian temperature?) during which randomly oriented biotite grew.

The textures and mineralogy of these rocks contrast with those in garnet grade rocks located along strike one km north. Here S1 is crenulated by a steep S2 cleavage. Both are defined by finely intergrown muscovite and chlorite, completely lack biotite, and both locally replace garnet. Thus these cleavages are retrograde to Acadian metamorphism. Regional early Carboniferous cooling ages make these fabrics Carboniferous or younger. It is therefore possible that the Carboniferous fabrics identified by Spear et al. 2003 near the Connecticut River may penetrate as far west as the eastern margin of the Athens dome.