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High Pressure Liquidus Studies of the Inferred Magma Composition of the Kiglapait Layered Intrusion, Labrador, Canada

ABSTRACT HIGH PRESSURE LIQUIDUS STUDIES OF THE INFERRED MAGMA COMPOSITION OF THE KIGLAPAIT LAYERED INTRUSION, LABRADOR, CANADA DEBORAH CASE BANKS MCINTOSH, B.A., WHEATON COLLEGE (MA) M.S., UNIVERSITY OF MASSACHUSETTS, AMHERST Directed by: Professor Stearns A. Morse The depth and nature of the source region of the Kiglapait Layered Intrusion, Labrador, Canada, were experimentally explored using rocks representing the troctolitic bulk composition of the intrusion. Melting and crystallization experiments were performed in graphite using a 19mm piston cylinder apparatus at pressures ranging from 7 to 15 kilobars and temperatures ranging from 1240 oC to 1305 oC. Liquidus and sub-liquidus phase relations in this pressure range were mapped. The P-T equation for the combined liquidi of these bulk compositions was determined to be T = 6.7P +1208, R2 = 0.99, where T is in degrees C and P is in kilobars. The olivine-to-plagioclase ratio at the cotectic among plagioclase, olivine, and liquid is found to be indifferent to pressure in the range 5-15 kb. Plagioclase was found to be stable with olivine from 5 to 11 kb, replaced by the tie line between two aluminous pyroxenes at 12 kb, with a reaction of SP+Al-Cpx to GT+Al-Cpx at 13 kb. The field of spinel relative to garnet is restricted to a small pressure interval £ 2kb. The data suggest a harzburgite source composition from the Kiglapait magma that could have begun melting in the garnet field near 1300°C, 13 kb. The magma could have separated from the source in the plagioclase field near 1280°C, 11kb before emplacement at 3-5 kb ~1240°C. It is estimated that 30-40% of olivine fractionation could have occurred on ascent from the primary to the parent magma.
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