Low-grade metamorphism of proterozoic rocks from the Esplanade Range, British Columbia
The lithology and petrography of the Horsethief Creek Group of the Esplanade Range and adjacent areas is discussed. The mineral assemblages from zones lower in metamorphic grade than the almandine zone are described and certain mineral compositions determined. The most significant mineral assemblages present are: 1) Muscovite - chlorite - quartz 2A Muscovite - chlorite - quartz 3A Muscovite - chlorite 4) Muscovite - chlorite - quartz - paragonite 5) Muscovite - chlorite - quartz - paragonite - chloritoid 6) Muscovite - chlorite - quartz - albite - epidote 7) Doiomite - muscovite - chlorite - quartz 8) Muscovite - chlorite - quartz - siderite These assemblages are indicative of greenschist facies metamorphism and are characteristic of the biotite zone. Biotite is very rare and its occurence is strongly controlled by bulk chemical composition. The feldspar in feldspathic assemblages is almost entirely albite. Potassium feldspar (microcline) was identified in one sample only and it may be that some plagioclase is albitized potassium feldspar. X-ray data indicate that the feldspar has a low structural state. From x-ray data, chemical analysis of a muscovite-rich sample, and some refractive index measurements, it is concluded that the potassium micas are phengitic. X-ray data suggests that the Al content of of chlorites from various mineral assemblages does not vary much, and that the Fe content of the chlorites is consistently high. Physical conditions during metamorphism are determined and the relative importance of bulk chemical composition and physical conditions discussed. A temperature of approximately 400°C during metamorphism is suggested on the basis of the Mg content of calcite which coexists with dolomite. Load pressure during metamorphism is estimated from structural considerations to have been in the range 2.7 kb to 5.5 kb. Estimates of fo2, fS2, and fco2 are made by comparing the mineral assemblages present to experimentally studied systems. Graphite is a common and extremely important phase.
Bibliography: p. 95-100.
Jones, J. W. (1970). Low-grade metamorphism of proterozoic rocks from the Esplanade Range, British Columbia (Unpublished master's thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/17666