METAMORPHOSED MAFIC ROCKS IN THE AMPHIBOLE AND MODAL SPACES OF J.B. THOMPSON, JR. WITH EXAMPLES FROM VERMONT, USA

Variations in mineral compositions and modes in metamorphosed mafic volcaniclastic and igneous rocks in VT are used to build upon J.B. Thompson’s compositional and modal spaces and provide insight into the evolution of metamorphic terranes. Condensed on simple substitutions with tr as the end member component, TK (Al₂Mg_-1Si_-1), PL (NaSiCa_-1Al_-1), and ED (NaAl□_-1Si_-1) increase in amp with metamorphic grade; while PL is greater for higher pressure facies series metamorphism. A N-S trending envelope of high PL_amp associated with ultramafic and pelitic rocks is considered to be a result of Ordovician subduction metamorphism. Using TWEEQ (Berman, 1991) blueschist from the north end of this envelope formed at 16-18 Kb and 400-550°C, while 13-16 Kb and 300-400°C are calculated for core compositions at the south end near Stockbridge. Metamorphism of actinolite rims is estimated at 6-9 Kb and 250-300°C.

Modal space illustrates conveniently metamorphic zoning and grade variations. The following net transfer reactions, continuous on the coupled substitutions ED (amp), PL (amp, plg), and TK (amp, chl), are used to delimit modal space for mafic rocks: α) ab→4qz+ED, δ) cl+6cz+10PL+7qz→10ab+tr+6H₂O, and γ) 12cz+7cl+14qz→12tr+25TK+22H₂O. Progress along each reaction is calculated in oxygen units from bulk composition and electron microprobe analyses. Published experimental studies at 2 and 7 Kb show that γ increases with temperature. Progress along δ is favored at 2 Kb, while at 7 Kb negative progress along δ is observed. Reaction δ is negative in VT except where andalusite is stable in intercalated pelitic schist. In the Rowe-Hawley Zone (RHZ), southeastern VT, TWEEQ calculations for mafic rocks are 8-12 Kb and 350-500°C.

Fault-bounded units within the RHZ show different δ - γ paths. The utility of modal space is not limited by bulk composition because the orientations of mineral isopleths are only dependent on the stoichiometry of the net-transfer reactions (Thompson, 2002). Increased modal amp with decreased modal chl and epi are predicted with dehydration and are obvious between greenschist and amphibolite. To explain an increase in modal amp of low TK, we appeal to polymetamorphism as was predicted for pelitic rocks from VT by Thompson and Norton and by Albee in 1968.