CALCULATED PHASE EQUILIBRIA FOR AMPHIBOLITES AND IMPLICATIONS FOR FACIES TRANSITIONS IN MAFIC ROCKS

Metamorphosed basalt is an important constituent in many meta-igneous terranes. Changes in mineral assemblage with varying P-T-x conditions are well documented in natural metabasalts, but the limited experimental data does little to suggest how these changes are accomplished. New thermodynamic data, coupled with chemical data from natural rocks, allows for rigorous calculation of mafic phase equilibria. The Carolina Terrane in Georgia (USA) preserves a Neoproterozoic volcanic arc containing fault bounded lithotectonic blocks. Metabasalt from a low pressure block (650-700°C and 4-5 kbar) contains the assemblages Amp + Pl (An 30-50) + Ilm + Mgt ± Qtz while metabasalt from a high pressure block (650-700°C and 10-13 kbar) contains Amp + Pl (An 20-40) + Chl + Ep + Ilm ± Qtz. Mineralogic evidence from interbedded felsic rocks show high water activities. With temperatures constrained to a narrow range, variations in mineral assemblage must be due to the pressure differential. Phase equilibria were calculated for quartz-bearing assemblages in the system NCMASH, using compositional data for a sample from near the boundary between the low and high-pressure blocks. The resulting P-T grid involves 28 reactions and 7 invariant points. The invariant points fall into two groups; a low-T An-absent group, and a high-T Chl-absent group. These groups are connected by the degenerate reaction Tsc + Tr + 2Ab=8Qtz + 2Prg. This reaction has a very shallow dP/dT slope and may account for P-dependent pargasite enrichment trends observed in amphibole in the Carolina Terrane and in independent data sets. A series of reactions associated with the Chl-absent invariant points separate low-P zoisite free rocks (amphibolite facies) from higher-P zoisite bearing rocks (epidote amphibolite facies). Two reactions are independent of the calculated invariant points. A low-T reaction (400°C at 6 kbar) accounts for the appearance of anorthite, and thus calcic plagioclase, in mafic systems. The second reaction (550°C at 6 kbar) accounts for the disappearance of chlorite. Both of these reactions have relatively steep dP/dT slopes. Significantly, this series of reactions provide a mechanistic background for the temperature dependent greenschist - amphibolite facies transition at low pressures, and greenschist - epidote-amphibolite - amphibolite transitions at higher pressures.