| Summary: | Includes bibliographical references. Includes illustrations and maps. Metamorphosed ultramafic rocks in the contact aureole of the Kiglapait intrusion, Labrador, were found to have the following series of prograde assemblages: I. ol + tr (+ cum + tc), II. ol + en + en ± di ± sp ± chl (+ cum), III. ol + en + an + hb + sp + chl (+ cum), IV. ol + en + an + hb + sp + chl, V. ol + en + an + hb + sp, VI. ol + en + an + hb, VII. ol + en + an + hb + di, VIII. en + an + di ± ol. These assemblages are similar to assemblages found in other ultramafic rocks. ' Several geothermometers were applied to the assemblages. The two-pyroxene thermometers of Wood and Banno (1973) and Wells (1977) were found to be poorly calibrated, at least for metamorphic temperatures, and the two-pyroxene thermometer of Ross and Huebner (1975) yielded an expected trend, but gave temperatures about 50 to 100 degrees lower than the temperature versus distance curve of Berg and Docka (in press). Inherent problems in the distribution of Mg and Fe^+ between coexisting pyroxenes and ilmenite made the thermometers of Bishop (1980) essentially useless. Poor calibration and lack of thermodynamic data cause the coexisting spinel and olivine thermometers of Roeder et al. (1979) and Fabries (1979) to yield temperatures which are incoherent. A new geothermometer, based on the partitioning of Mg and Fe²⁺ between chlorite and olivine coexisting with en + sp has been formulated. Temperatures of several pairs of coexisting chl + ol were derived from the temperature versus distance curve of Berg and Docka (in press). A plot of ln K[sub D] versus 1000/T yielded the equation: T, K = 2759/(ln K[sub D] + 1.9198). A comparison with chl + ol pairs from Frost (1975) suggests that the equation is correct to ± 50°C. Theoretical calculations for the model system SiO₂ - Al₂O₃ - CaO - MgO were performed using Schreinemakerb method. Results from the model system combined with observed assemblages suggest a revision to the facies concept for ultramafic rocks after O'Hara (1967) and Frost (1976). Furthermore, one assemblage exhibiting the retrograde reaction an + en + ol = hb + sp places this reaction at lower temperatures than the breakdown of chlorite (at about 2.2 kb). The observed assemblages seem to agree well with the model system, although possible disequilibrium cannot be ruled out due to the symplified nature of the model system. M.S. (Master of Science)
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