The Blosseville Coast basalts of East Greenland: Their occurrence,composition and temporal variations |
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Authors: | C K Brooks T F D Nielsen T S Petersen |
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Institution: | (1) Department of Geology, University of Manchester, UK;(2) Present address: School of Applied Geology, University of New South Wales, 2033 Kensington, N.S.W., Australia |
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Abstract: | In the system CaO-MgO-A12O3-SiO2 the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatitess+sillimanite pyrope-grossularss+quartz (3), anorthite+enstatitess pyrope-grossularss+diopsidess+quartz (2), anorthite+pyrope-grossularss+ quartz diopsidess+kyanite (4) and anorthite+diopsidess grossular-pyropess +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En0.5Fs0.5). Enstatite and plagioclase (An0.5Ab0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5. |
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