Journal of Petrology Advance Access originally published online on September 16, 2004
Journal of Petrology 2004 45(11):2225-2259; doi:10.1093/petrology/egh054
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Journal of Petrology 45(11) © Oxford University Press 2004; all rights reserved
Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 kbar in Graphite
1 DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003-9297, USA
2 DEPARTMENT OF GEOLOGY, SMITH COLLEGE, NORTHAMPTON, MA 01063, USA
3 DEPARTMENT OF GEOLOGICAL SCIENCES, BINGHAMTON UNIVERSITY, BINGHAMTON, NY 13902, USA
The inferred crystallization history of the troctolitic Lower Zone of the Kiglapait Intrusion in Labrador is tested by melting mineral mixtures from the intrusion, made to yield the observed crystal compositions on the cotectic trace of liquid, plagioclase, and olivine. Melting experiments were made in a piston-cylinder apparatus, using graphite capsules at 5 kbar. Lower Zone assemblages crystallized from 1245°C, 5% normative augite in the liquid, to 1203°C, 24% normative augite in the liquid at saturation with augite crystals. This transit is consistent with modal data and the large volume of the Lower Zone. The 1245°C cotectic composition matches the average Inner Border Zone composition. Quenched troctolitic liquid from the Upper Border Zone, and others from nearby Newark Island, plot on or near our experimental cotectic, supporting a common fractionation history. Olivine–plagioclase intergrowths from cotectic troctolitic melt show mosaic textures reflecting the differing barriers to nucleation of these two phases. The linear partitioning of XAb in plagioclase–melt yields an intercept constant KD = 0·524 for these mafic melts. Observed subsolidus exchange of Ca between plagioclase and olivine elucidates the loss of Ca from plutonic olivines. The bulk composition of the intrusion is revised downward in Fo and An.
KEY WORDS: experimental; olivine; plagioclase; Kiglapait; partitioning
Abbreviations: AP, MT, IL, OR, AB, AN, DI, HY, OL, FO, NE, Q, FSP, AUG: (Oxygen) Normative components; Ap, Aug, Ilm, Ol, Pl: Phases; Ab, An, Di, Fa, Fo, Or, Wo: Phase components; also ternary endmembers; BSE: Back-scattered electron; CaTs: Calcium Tschermak's component, CaAlAlSiO6; D: Partition coefficient; f: Fugacity; FL: Fraction of the system present as liquid = 1 – (PCS/100); FMQ: Fayalite = magnetite + quartz buffer; IBZ: Inner Border Zone; IW: Iron = wüstite buffer; kbar: kilobar, 108 pascal; KD: Exchange coefficient; KI: Kiglapait Intrusion; L: Liquid phase; LLD: Liquid line of descent; Ma: Mega-annum, age; Myr: Mega-year, time; OLHY: Normative OL + HY; OLRAT: The ratio OLHY/(OLHY + AUG); P: Pressure; P: Phosphorus; PCS: Percent solidified (volume); SMAR: South Margin average composition; T: Temperature, °C; UBZ: Upper Border Zone; WM: Wüstite = magnetite buffer; Wo: Wollastonite component of pyroxene; X: Mole fraction; XMg: Molar ratio Mg/(Mg + Fe2+); 
, XMg(0): Initial XMg before MT is formed in the norm calculation; X: Coordinate, horizontal axis; Y: Coordinate, vertical axis
* Corresponding author. E-mail: tm{at}geo.umass.edu
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