POLYMETAMORPHISM OF MAFIC AND PELITIC ROCKS SURROUNDING ULTRAMAFICS IN STOCKBRIDGE, VERMONT

Multiply deformed mafic and pelitic rocks in Stockbridge, Vermont, east of the Green Mountain anticlinorium surround a 300-meter long and 25 to 100-meter wide serpentinized ultramafic body that was likely emplaced within a Taconian subduction zone. New electron microprobe data from intercalated amp-chl-epi-qtz-alb-sph greenstone and gar-bio-mus-qtz-epi-ilm schist provide evidence for medium-high pressure metamorphism preserved in a fault bounded slice within greenschist facies terrain (?Acadian).

The ultramafic body consists of narrow interlayered chlorite and serpentine zones that contain coarse-grained tremolite, chromite, magnetite, carbonate and talc pockets of variable size and shape. Amphiboles in the greenstone are chemically zoned with barroisite cores and actinolite rims. The enrichment of Na^M4SiCa_-1Al_-1 (PL) and Al^VIAl^IVMg_-1Si_-1 (TK) substitutions in the cores indicates that they were formed under higher pressure and temperature than the rims, which are greenschist facies because they are in equilibrium with epidote, chlorite and albite. PL and TK substitution values of 0.6 and 1.6 in the cores, respectively, are similar to barroisite cores at Tillotson Peak that occur with Taconian blueschist and eclogite assemblages. Mafic rocks in Stockbridge most likely record subduction metamorphism in amphibole cores and a lower temperature and pressure metamorphic overprint in amphibole rims.

The reaction 4ann + 3chl + mus + 3qtz = 5phl + 4alm + 12H₂O was calculated at 390° C and 0-10 kilobars applying TWEEQ (Berman, 1991) to garnet, biotite and muscovite compositions from Stockbridge. Garnet-biotite geothermometry (Bhattacharya et al., 1992) gives temperatures between 460° C and 420° C. TWEEQ and geothermometric results suggest that the garnet-bearing schist is greenschist facies. High Mn contents allowed garnet to grow at lower temperatures than for typical Barrovian garnet zone.

Mafic and pelitic rocks in Stockbridge may have been initially metamorphosed in the Taconian subduction zone and subsequently retrograded to greenschist facies during the Acadian. Electron microprobe data from the mafic rocks require that the southern limit of medium-high pressure metamorphism in Vermont be extended to Stockbridge, which is closer to higher grade Acadian overprint in southeast Vermont.