POLYMETAMORPHIC REACTION SEQUENCE IN ULTRAMAFICS FROM STOCKBRIDGE, VERMONT: A PATHWAY THROUGH P-T-H2O-CO2-T SPACE

Exhumed ultramafic rocks in Stockbridge, VT preserve an early Paleozoic polymetamorphic reaction history that consisted of serpentinization, carbonatization, and chloritization. The net-transfer reaction sequence with respect to pressure, temperature, aH₂O and aCO₂ is characterized using textural data, modal space analyses, and Perple_X geothermobarometry. Original metamorphism of peridotite occurred along three net-transfer reactions: 1. 6en + 3H₂O = tlc + srp 2. 9en + srp = 3fo + 2tlc 3. 12fo + 2tlc + 18H₂O = 10srp. Initially, Reactions 1 and 2 progressed simultaneously with decreasing temperature above ~730°C and ~20 Kbar. This resulted in net modal increases of fo and tlc and net loss of en; srp mode remained 0. The start of Reaction 3 at ~730°C marked the onset of serpentinization, which proceeded along with Reactions 1 and 2 with decreasing temperature and to completion by ~670°C. At that point, CO₂ was added to the system and partial carbonatization of serpentinite was dictated by the reaction 2srp + 3CO₂ = 3mgs + tlc + H₂O (4) with increasing aCO₂ and decreasing pressure down to ~2-3 Kbar.

Subsequent deformation triggered Al and Ca metasomatism and chloritization along 2 net-transfer reactions: 5. 4srp + 5tlc + Al₂Mg_-1­Si_-1 (TK) + 6CaMg_-1 (CM) = 3tr + chl + 6H₂O 6. 7tlc + 6mgs + TK + 6CM = 3tr + chl + 6CO₂. H₂O released by Reaction 5 induced hydration via Reaction 4 and srp pseudomorphed mgs, while Reaction 6 resulted in chl pseudomorphs after tlc. Coarse-grained tr was formed concurrently along Reactions 5 and 6, which were both initiated between ~1-3 Kbar and ~550-610°C. The last reaction in the sequence, qz + cal + rt = ttn + CO₂ (7), took place with decreasing temperature along the contact with Ca- and Si-rich country rock as aCO₂ approached 0. Reaction 7 explains the occurrence of ttn cored by rt.