Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 Kbar in Graphite

JOURNAL OF PETROLOGY VOLUME 45 NUMBER 11 PAGES 2225–2259 2004 doi:10.1093/petrology/egh054 Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 kbar in Graphite S. A. MORSE1*, J. B. BRADY2 AND B. A. SPORLEDER3 1DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003-9297, USA 2DEPARTMENT OF GEOLOGY, SMITH COLLEGE, NORTHAMPTON, MA 01063, USA 3DEPARTMENT OF GEOLOGICAL SCIENCES, BINGHAMTON UNIVERSITY, BINGHAMTON, NY 13902, USA RECEIVED OCTOBER 29, 2002; ACCEPTED JUNE 28, 2004 ADVANCE ACCESS PUBLICATION SEPTEMBER 16, 2004 The inferred crystallization history of the troctolitic Lower Zone of the D : Partition coefficient Kiglapait Intrusion in Labrador is tested by melting mineral mixtures f: Fugacity from the intrusion, made to yield the observed crystal compositions on FL: Fraction of the system present as liquid ¼ 1 À (PCS/100) the cotectic trace of liquid, plagioclase, and olivine. Melting experi- FMQ: Fayalite ¼ magnetite þ quartz buffer ments were made in a piston-cylinder apparatus, using graphite IBZ: Inner Border Zone capsules at 5 kbar. Lower Zone assemblages crystallized from IW: Iron ¼ wuustite€ buffer 8 1245C, 5% normative augite in the liquid, to 1203C, 24% kbar: kilobar, 10 pascal normative augite in the liquid at saturation with augite crystals. This KD: Exchange coefficient transit is consistent with modal data and the large volume of the KI: Kiglapait Intrusion Lower Zone. The 1245C cotectic composition matches the average L: Liquid phase LLD: Liquid line of descent Inner Border Zone composition. Quenched troctolitic liquid from the Ma: Mega-annum, age Upper Border Zone, and others from nearby Newark Island, plot on Myr: Mega-year, time or near our experimental cotectic, supporting a common fractionation OLHY: Normative OL þ HY history. Olivine–plagioclase intergrowths from cotectic troctolitic melt OLRAT: The ratio OLHY/(OLHY þ AUG) show mosaic textures reflecting the differing barriers to nucleation of P : Pressure these two phases. The linear partitioning of XAb in plagioclase– P: Phosphorus melt yields an intercept constant KD ¼ 0Á524 for these mafic PCS: Percent solidified (volume) melts. Observed subsolidus exchange of Ca between plagioclase and SMAR: South Margin average composition olivine elucidates the loss of Ca from plutonic olivines. The bulk T: Temperature, C composition of the intrusion is revised downward in Fo and An. UBZ: Upper Border Zone WM: Wuustite€ ¼ magnetite buffer Wo: Wollastonite component of pyroxene KEY WORDS: experimental; olivine; plagioclase; Kiglapait; partitioning X: Mole fraction 2þ XMg: Molar ratio Mg/(Mg þ Fe ) o XMg, XMg(0): Initial XMg before MT is formed in the norm LIST OF ABBREVIATIONS calculation AP, MT, IL, OR, AB, AN, DI, HY, OL, FO, NE, Q, FSP, X: Coordinate, horizontal axis AUG: (Oxygen) Normative components Y: Coordinate, vertical axis Ap, Aug, Ilm, Ol, Pl: Phases Ab, An, Di, Fa, Fo, Or, Wo: Phase components; also ternary endmembers INTRODUCTION BSE: Back-scattered electron The precise identification of liquid compositions CaTs: Calcium Tschermak’s component, CaAlAlSiO6 parental to the rocks of layered intrusions has long been Journal of Petrology 45(11) # Oxford University Press 2004; all rights *Corresponding author. E-mail: [email protected] reserved JOURNAL OF PETROLOGY VOLUME 45 NUMBER 11 NOVEMBER 2004 an elusive goal of igneous petrology. Even in the case of the found by partitioning relations made on longer runs to Skaergaard Intrusion, the elegant box model and careful achieve compositional equilibrium. volume estimates of Nielsen (2004), when combined with Experiments at pressure are, of course, relevant to the the zone averages of McBirney (1989), contain significant natural intrusive setting of a layered intrusion, but they amounts of olivine on the liquidus (as we shall show), have added advantages. Graphite containers tend to sta- inconsistent with the exposed olivine þ plagioclase rocks bilize the oxygen fugacity near that of the input charge, of the Skaergaard Lower Zone. For the much larger and the crystals produced near the liquidus tend to be volume of the Kiglapait Intrusion (about 3500 km3, com- large, euhedral and unzoned. They are, therefore, easy to pared to the more tractable 280 Æ 23 km3 of Nielsen’s characterize by electron microprobe, and to interpret Skaergaard estimate), a finer scale of sampling was pro- texturally. Our experiments were conducted in the vided by a unified stratigraphic representation of rock Experimental Petrology Laboratory at Smith College, compositions and volumes (Morse, 1969, 1979a, 1979b). Northampton, MA. This study is an outgrowth of a These compositions have been summed over their Masters thesis by Sporleder (1998), which contains volumes to obtain estimates of liquid compositions (Morse, further details. We begin by assuming that the summa- 1979b,1981b). One means of testing the results of any tion liquid composition of Morse (1981b) is an appropri- such volumetric summation is to determine experimen- ate starting point in terms of the phase abundances, tally, at pressure, the equilibrium compositions of melts but not in terms of normative plagioclase and olivine that yield cotectic crystals of the observed natural com- compositions. position at any given stratigraphic level. Such a test furnishes one answer to an inverse problem: the success- ful melt compositions could have given rise to the PREVIOUS WORK observed rocks. That test is the goal of the research Field relations reported here. The Kiglapait Intrusion was emplaced into Archean A study of olivine compositions in the Kiglapait Intru- migmatites, Proterozoic supracrustal rocks and anortho- sion (Morse, 1996) was undertaken because the well sites of the Nain Plutonic Suite (NPS), Labrador (Ryan, known partitioning of Fe–Mg between olivine and liquid 1990) at 1307 Æ 2 Ma (age reviewed in Morse, 1996, could give an objective constraint on the liquid composi- p. 1038; Yu & Morse, 1992). It is the youngest dated tions to compare with the summation model calculated member of the NPS, and one of a suite of Proterozoic from the chemical compositions of the rocks (Morse, troctolitic intrusions that typically cut anorthosite, both in 1981b). The summation liquid is much too Mg-rich to Labrador and elsewhere (Scoates & Mitchell, 2000). The account for the observed olivine compositions in the contact rocks are dry granulites (Berg, 1977) previously rocks of the intrusion. This result led to the conviction heated by emplacement of anorthosite (Yu & Morse, that the same problem would also arise for the plagioclase 1992). The country rocks to the north-west and north composition. These problems were clearly identified by are metamorphosed sedimentary rocks of the Aphebian Blundy (1997), who concluded that the liquid parental to Snyder Group (Speer, 1978) lying unconformably on the actual Kiglapait rocks must have been significantly Archean gneisses and overlain by the Falls Brook more evolved (less refractory) in terms of AN and FO Group of mafic and ultramafic igneous rocks, banded than the chilled margin sample. That sentiment is the iron formation, calc-silicate rocks and pyroxene paragran- foundation stone of the present investigation, whose pur- ulite (Schuh, 1981). The southern and western contact pose it is to ask: What plausible cotectic liquids could rocks and roof rocks are anorthosite, and nearby to the yield the observed An and Fo contents of the Lower west is the large, troctolitic Hettasch Intrusion (Berg, Zone rocks, and at what temperatures? Where does the 1980; Berg et al., 1994). The emplacement pressure of cotectic trace lie in the ternary? How far away from the the Kiglapait Intrusion was near 2Á5 kbar (Berg, 1977, starting point is the saturation point with augite? 1979, 1980; Berg & Docka, 1983). The present experi- To answer these questions, we made mixtures of sepa- mental study was conducted at 5 kbar (500 MPa) in view rated Kiglapait minerals that, when melted just below the of the difficulty of maintaining long runs at lower pres- liquidus, would yield liquids on or near the cotectic con- sures in the piston-cylinder apparatus. dition L(Ol, Pl), and crystallize plagioclase and olivine The history of studies on the bowl-shaped Kiglapait having the compositions observed in the intrusion. Intrusion was reviewed, with map, cross-section and stra- Then, by incrementally adding an augite component tigraphic column, by Morse (1996). The initial depth of along the cotectic, we eventually found the saturation the magma body is estimated at nearly 9 km, almost all of point with augite crystals, defining the boundary between which is preserved at the center, but much of which is lost the Lower and Upper Zones of the intrusion. Our in the presumed eroded wings (Morse, 1969). The former emphasis was on producing liquids of the appropriate Outer Border Zone (Morse, 1969) is now equated with composition, for which the crystal compositions were the Falls Brook Group, which may contain sills of 2226 MORSE et al. EXPERIMENTAL PETROLOGY OF THE KIGLAPAIT INTRUSION Kiglapait material (Morse, 1979a). An Inner Border Zone of these excluded minerals is divided by their calculated (IBZ) of medium- to very coarse-grained olivine gabbro abundance in the modal summation liquid, the results surrounds and underlies the entire layered series and give consistent estimates of the residual porosity, which is grades upward into troctolites of the Lower Zone. Some found to decrease systematically with stratigraphic

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