ABSTRACT

The Kiglapait Intrusion on the north coast of Labrador is a bowl-shaped body dominated by troctolite about 3500 km3 in initial volume and was created during an intrusive event perhaps lasting thousands of years. It was emplaced into anorthosite and metasedimentary rocks at an estimated roof depth of 9.6 km and a magma depth of 8.4 km; hence with a floor at 18 km depth. The primary magma for the intrusion is thought to have been of harzburgite composition; a large volume of olivine crystallized in transit, to the extent that the magma became saturated with plagioclase by the time it reached the site of emplacement or soon after. To test this hypothesis, piston-cylinder experiments were made at 5–15 kbar in graphite using the Kiglapait Intrusion bulk composition with Fo-rich olivine added. Results at 13 kbar yielded saturation with garnet, olivine, spinel, orthopyroxene, clinopyroxene, plagioclase, and melt. This assemblage is compatible with a lherzolite solidus at 1375 °C. A postulated harzburgite solidus at 15 kbar would be hotter, perhaps 1410 °C. Partial melt from this harzburgite rising into a hot, thinned lithosphere is presumed to have shed large amounts of olivine to produce the plagioclase-saturated troctolitic basal Lower Zone of the intrusion. Conditions of emplacement are schematically developed in ternary Al–Ca–Fe diagrams. Some high-pressure experimental compositions of clino- and orthopyroxene are metastably enriched in Al but do not affect the interpretation of the magmatic history. We show that olivine fractionation will pass through the compositions of these aluminous minerals to reach a relatively evolved saturation with only plagioclase and olivine, resulting in the voluminous Lower Zone of troctolite. The amount of olivine crystallized to reach this result is calculated, using multiphase Rayleigh fractionation and a standard MELTS routine, to between 30% and 50%. Previously published argon-argon mineral dates on hornblende, biotite, and feldspars have captured a cooling history from ∼1258 °C to the ambient ∼100 °C over the time interval 1307 to 1220 Ma, hence the 87 million year history claimed in the title.

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