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JOURNAL OF PETROLOGY VOLUME 37 NUMBER 6 PAGES W37-M61 1996 INSUNG LEE AND EDWARD M. RIPLEYf DEPARTMENT OF GEOLOGICAL SCIENCES, INDIANA UNIVERSITY, BLOOMINGTON, IN 47403, USA Mineralogic and Oxygen Isotopic Studies of Open System Magmatic Processes in the South Kawishiwi Intrusion, Spruce Road Area, Duluth Complex, Minnesota The South Kawishiwi intrusion, located along the western variations in mineral chemistry suggest that discontinuities margin of the Duluth Complex, Minnesota, is one of several within the major units developed by in situ boundaryAayer composite intrusions that are found in the Complex. The Duluth equilibrium crystallization ofsolidification zones ~ 20—50 m in Complex is the principal exposed plutonic portion of the 1-1 Ga thickness, followed by recharge of fresh magma. Upward Midcontineni Rift system. In the Spruce Road area the South enrichment of incompatible elements, olivine Fa content, and Kawishiwi intrusion is divided into seven distinct units that are plagioclase Ab content may be effectively explained by this propart of the broader South Kawishiwi Troctolite Series defined by cess. 5 0 values of uncontaminated rock types are strongly corSeverson (Tech. Rep. NRRI/TR-91/13a, Natural Resources relative with modal mineralogy, and can also be modeled by Research Institute, University of Minnesota, Duluth, 1994). boundary-layer fractionation. A parental magma 5 0 value of Units may be characterized as follows: Unit I—basal accu- ~6-3%o is calculated for Unit VII based on olivine and plamulation of heterogeneous gabbro, troctolite, and norite; Unit gioclase values, and is similar to that of several other large, II—norite with abundant inverted pigeonite; Unit III—troc- layered mafic intrusives. tolite and olivine gabbro with local oxide-rich layers; Unit IV—melatroctolite, troctolite, olivine gabbro; Unit V— KEY WORD& Duluth Compltx; South Kawishiwi Intrusion; high-Al increased plagioclase abundance in troctolites and leucocratic divine tholeiite; open system crystallizfltion; oxygen isotopes troctolites; Unit VI—strongly altered troctolite; Unit VII— similar to Unit V, troctolite and leucocratic troctolite. Country rocks in the Spruce Road area are granodiorite to quartz mon- INTRODUCTION zonite of the Archean Giants Range Batholith. Sutfide miner- The Duluth Complex of northeastern Minnesota alization, consisting of 1-5 vol. % of disseminated pyrrhotite, (Fig. 1) constitutes the principal plutonic portion of cubanite, chalcopyrite andpentlandite, occurs in Units I, II, III, the ~ 1 • 1 Ga Midcontinent Rift System (Van and VI. Oxygen isotopic analyses indicate that Unit II has Schmus & Hinze, 1985). The Complex is subdivided experienced extensive crustal contamination. 518O values of into four series based on rock type and intrusive Unit II range from 6m9 to 7-l%o and are 180 enriched com- relationships (Weiblen & Morey, 1980). An early pared with values of 5-1-6-8%o found in other units. Silica phase of gabbroic cumulates is known as Nathan's contamination is indicated based not only on 5' 0 values, but layered series, and is thought to have originated also by the predominance of orthopyroxene in the unit. Possible from repeated injections of variably evolved magmas high-' 0 contaminant rocks include the Giants Range Bath- (Nathan, 1969). A felsic series refers to a group of olith and pelitic rocks of the Lower Proterozoic Virginia granophyric intrusions located along the roof zone of Formation or Biwabik Iron Formation. Mass balance compu- the Complex. Anorthositic series rocks (Miller & tations suggest that units in the Spruce Road area may be rela- Weiblen, 1990) occupy the highest structural levels ted through varying degrees offractionation of a high-Al, olivine of the Complex, and represent multiple intrusions of tholeiite magma. Modeling of trace element concentrations and plagioclase crystal mushes. The layered series is •Present address: Korea Basic Science Center, Isotope Research Group, Yeoeun Dong 224-1, Yusung Ku, Yusung P.O. Box 41, Taejcan 305-333, Korea f Corresponding author. © Oxford University Press 1996 JOURNAL OF PETROLOGY VOLUME 37 NUMBER 6 DECEMBER 1996 48° ~ Minnesota Ar Figure 2 MIDDLE PROTEROZOIC Intrusive Rocks I, I Subvokanlc mafic rocks I* .1 Grnmtoid rocks r77! Troctolltic and I- I gabbrolc cumulates | | Anorthositic cumulates |—| Early gabbrotc cumulates llllllll Logan sills Volcanic Rocks |7T| N North Shore Volcanic Croup LOWER PROTEROZOIC pri] Anlmikle Croup 20 I 40 I 60 km _J IATI ARCHEAN Fig. 1. Generalized geologic map of northeastern Minnesota [modified from Miller & Weiblen (1990)], ihowing location of the Duluth Complex. composed of predominantly sheet-like mafic intrusions, with lithologies that include troctolite, gabbro, norite, anorthosite, and rare peridotite. Recent U— Pb isotopic studies by Paces & Miller (1993) indicate that intrusive activity was episodic, but that most intrusions of the Complex were emplaced between 1107 and 1096 Ma. Miller & Ripley (in press) have recently reviewed stratigraphic variability within individual intrusions of the layered series, and highlighted possible controls on the differences with respect to the open or closed nature of the magma chamber. For example, the Sonju Lake intrusion (Stevenson, 1974; Miller et al., 1993a) is composed of a cumulate sequence that is interpreted as resulting from closed system differentiation of a tholeiitic magma by unidirectional (bottom-up) fractional crystallization. Both the Partridge River (Taib & Ripley, 1993; Chalokwu et al., 1993) and the South Kawishiwi (Severson, 1994) intrusions show varying degrees of open system behavior, whereas the Duluth layered series (Miller et al., 19936) exhibits intervals of strong differentiation frequently interrupted by recharge and eruption events. To date, the most detailed petro- logic studies have focused on the Partridge River intrusion (Fig. 2, Grant & Moiling, 1981; Rao & Ripley, 1983; Tyson & Chang, 1984; Ripley & AlJassar, 1987; Chalokwu & Grant, 1990; Grant & Chalokwu, 1992; Taib & Ripley, 1993; Chalokwu et al., 1993), in part owing to the presence of potentially exploitable Cu-Ni sulfide mineralization. The purpose of this study is to document geochemical and mineralogical variations in the lesser studied South Kawishiwi intrusion (Fig. 2), and to use the data to constrain possible open system magmatic processes. Although macroscopic features, such as the presence of basal melatroctolite units overlain by troctolitic to gabbroic rocks, and the occurrence of sharp grain size and textural contacts, suggest that multiple inputs of liquid were important in petrogenesis, detailed studies of combined mineral and bulk-rock chemistry and isotopic variations have been few (e.g. Foose, 1984; Foose & Weiblen, 1986). Studies of this type are required not only to better understand the processes that produced the complicated stratigraphic variations in the Complex, but also to better predict the locations of potential CuNi-PGE deposits. 1438 LEE AND RIPLEY SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX 91°45' EXPLANATION Main Augite Troctolite Anorth Troctohte & Troctolite (mixed) Basal Heterogeneous Group Inclusion Biwabiktype Iron Formation Or Porphyritic I Hombleode > Adamdlrte and Monzomte Geological Boundary 2a Fault 1 km Fig. 2. (a) Generalized geologic map of the w a t e m margin of the Duluth Complex ihowing the Partridge River and South Kawiihiwi intrusions [modified from Severson (1991)]. Locations of principal Cu-Ni proipecu: 1, Spruce Road; 2, Maturi; 3, Dunka Pit; 4, Serpentine; 5, Babbitt; 6, Dunka Road; 7, Wetlegi; 8, Wyman Creek; 9, Water Hen. (b) Geologic map of the Spruce Road area, modified from Sevenon (1994). Locations of drill corei examined in thii itudy are ihown as filled cirdei. 1439 JOURNAL OF PETROLOGY VOLUME 37 This study is based on detailed analyses of drill core 34870-A, located in the northern portion of the South Kawishiwi intrusion within the area of the Spruce Road Cu-Ni prospect (Figs 2 and 3). International Nickel Company (now INCO Ltd) drilled ~ 175 holes in the area, but only a few cores remain owing to a core shed fire. Much of the geologic information regarding the intrusion and associated mineralization is based on company drill core records, and more recent study of the remaining drill cores (Severson, 1994). Outcrops are few in the Spruce Road area. Country rocks are part of the Giants Range Batholith, an Archean composite granite—granodiorite body. However, inclusions of sedimentary Proterozoic Animikic Group Virginia Formation and Biwabik Iron Formation (Figs 1 and 2), as well as Keweenawan basalt, are found in the South Kawishiwi intrusive rocks. The presence of metasedimentary inclusions is important as it indicates that the magmas must have traversed these rock types, which are potential contaminants and sources of elements such as sulfur for ore genesis (Lee & Ripley, 1995). Severson (1994) has identified a consistent stratigraphic arrangement of rock types in the eastern Babbitt-Spruce Road area that he refers to as the South Kawishiwi Troctolite Series. This sequence differs from that defined by Severson & Hauck (1990) for the Partridge River intrusion, based on NUMBER 6 DECEMBER 1996 observation of drill core from Wetlegs to Babbitt (Fig. 2). Severson (1994) has defined 17 distinct units, although not all are present in all localities of the South Kawishiwi intrusion. A brief description of Severson's (1994) units is given in Table 1. Our division of drill core 34870-A conforms to that of Severson (1994), but we have further subdivided basal units based on petrochemical data. Figure 3 is a cross-section constructed from drill core observation. Although cores 32706, 32709, and 32715 were examined, only reconnaissance chemical analyses were undertaken. SAMPLING AND ANALYTICAL METHODS Samples for this project were taken from drill cores provided by the American Copper and Nickel Company, Denver, Colorado (a subsidiary of INCO), and by the Minnesota Department of Natural Resources, Hibbing, Minnesota (core 34870-A). The entire 740 m length of core 34870-A was sampled, but only the upper 213 m of cores 32706, 32709, and 32715 were available for collection. One hundred polished thin sections were examined using standard transmitted and reflected light microscopy. Modal analyses were obtained for 75 sections by counting a minimum of 2500 points per section. BH' Sulfide-beoring Basal Heterogeneous Rocks ^^— . Sulfide > 1 vol % , eszzzm . Sulfide 1 - 0 5 vol % Giants Range Batholith 500 A 1000 A' 1500 2000 m A" Fig. 3. Cross-section along lines A-A' and A'-A" of Fig. 2. Rock types are those of Severson (1994): BH, basal heterogeneous zone; ATT.anorthositic troctolite and troctolite. Units I-VII have been defined as part of this study. 1440 LEE AND RIPLEY SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX Table 1: Summary of the key characteristics of units in the South Kaivishiwi Troctolite Series (Severson, 1994) Top Unit Description Anorthoirtic Troctolite Homogeneous—textured with High PicriteNo. 1 at its base; >366m Anorthosrtic Troctolite and Troctolite Interlayered rock types, gradational contact with underiying Main Augrte Troctolrte; 366-610 m Updlp Wedge Present only locally; lulfide-bearing troctolite, anorthosrtic troctolite. and augrte troctolrte; heterogeneous textured; two ultra mafic subunits are present within thick sections; 31-274 m Main Augrte Troctolite Homogeneous whh coarse, ophlttc cpx and composite ilmenite-magnetite grains; 83-421 m UltramaficOne Picrtte and peridotHe with interiayered homogeneous troctolita; surfide-bearing; 1 -5-131 m Basal Heterogeneous Zone Heterogeneous- and homogeneous-textured troctolite and lesser anorthosrtic troctolrte; sharp to gradational contacts; sutftde-bearing; 27-518 m Anorthositlc Troctolite to Troctolita UltramaflcTwo Homogeneous, surflde-barren; present above the PegmatKic Unit 21-366 m Picrite, perldotlte, olivine-nch troctolite alternate with troctolite; massive oxide layers are absent sulflde-bearing only when contained in the Basal Heterogeneous Unit 1 6-130 m Pegmatrtlc Unit of Foose (1984) Pegmatitic troctolrte to anorthosrtic troctolite, minor augrte troctolrte and olivine gabbro; consistently above the Ultramafic Three Unit locally sulfide bearing; 3-79 m Ultramaflc Three Picrite, perldotlte, olrvine-rich troctolite alternate with troctolite; massive oxide layers occur up to Augrte Troctolite-Norile Heterogeneous textured; nortte more common at the base and augrte troctolite in the upper portion; ~ 15 m thick; all rocks are sulfide bearing; 1 -125 m Bottom sulfide-bearing, locally massive; maximum Cu values up to 1 -6 wt%; 3-116m Electron microprobe analyses were performed using a CAMECA SX-50 microprobe at Indiana University and a similar instrument at the University of Chicago. Accelerating voltage was maintained at 15 keV, and sample current varied as follows: plagioclase, 15 nA; oxides, 20 nA; and olivine, orthopyroxene, clinopyroxene and other silicates, 25 nA. Beam size for most analyses was 2-3 /zm, but was reduced when analyzing ilmenite and spinel lamellae. Standards included native elements, and both natural and synthetic minerals. Raw counts were converted to oxide concentrations using the PAP correction scheme (Pouchou & Pichoir, 1984). Major and trace element (Zr, Y, Cr, Rb, Sr, Nb, Ba) compositions for 65 whole-rock samples were analyzed by X-ray fluorescence spectromctry (X-RAY Assay Laboratories, Don Mills, Ontario). Analyses were performed on a Philips 1600 unit using glass disks. Detection limits are 0-01% and 10 p.p.m. for major and trace elements, respectively. Analytical precisions, based on replicate analyses, are <l-5% RSD (relative standard deviation) for major elements and <6% RSD for trace elements. Yttrium was determined for the same samples using an acid digestion followed by inductively coupled plasma spectrometry at Indiana University to achieve a 10 p.p.m. detection limit and enhanced precision. Total carbon was measured for 45 whole-rock powders using a LECO C/S 244 instrument at Indiana University. Multiple analyses of standards gave an absolute standard deviation of ±0-02%. A total of 82 oxygen isotope measurements were made on whole rocks and mineral separates. Mineral separates for oxygen isotopic analyses (plagioclase, olivine, orthopyroxene) were prepared by magnetic separation of 60—100 mesh powder followed by hand-picking to >99 % purity. Mineral separates and whole-rock powders were stored in vacuum at 120°C for at least 24 h before analysis. Oxygen was liberated using the bromine pentafluoride method of Clayton & Mayeda (1963). Extracted oxygen was converted to CO2 by reaction with a hot graphite disk wrapped with Pt-Rh wires. The CO2 gas was analyzed on a Finnigan Delta-E stable isotope ratio mass spectrometer. NBS-28 quartz has a value of 9-6±0-2%o in our laboratory. Analytical precision of <5I8O measurements is ~0-05%o. High-resolution analyses of trace elements (Cu, Zn, Y, Zr) in silicate minerals were undertaken at Brookhaven National Laboratory using the X-ray fluorescence microprobe of the National Synchrotron Light Source (Lu et al., 1989). 1441 JOURNAL OF PETROLOGY VOLUME 37 MINERALOGICAL AND CHEMICAL VARIATIONS IN DRILL CORE 34870-A Variations in mineralogy, texture, both bulk-rock and mineral chemical composition, and isotopic (S, O) values have been utilized to divide drill core 34870-A into seven distinct units (Fig. 4). Rock names are based on the modal scheme proposed by Miller & Weiblen (1990), and follow the conventional nomenclature sanctioned by the IUGS. Primocryst minerals are olivine and plagioclase. To avoid any genetic connotations we have elected not to use cumulus terminology. As described below, we attribute most of the units at Spruce Road to in situ crystallization processes. Unit I is a basal accumulation of heterogeneous noritic, gabbroic, and locally troctolitic rock types. Unit II is characterized by decreased olivine content, and increased amounts of orthopyroxene and inverted pigeonite. Both chemical and isotopic data (see below) indicate that Unit II has experienced more crustal contamination than other units. Units I and II are sulfide-bearing, with 1-5 vol. % disseminated sulfide characterized by enriched S34^ values of 6-10%o (Lee & Ripley, 1995). Units I and II in core 34070-A are part of Severson's (1994) Bottom Augite Troctolite-Norite and Basal Heterogeneous units (Table 1). Unit III consists of melatroctolite, troctolite, and olivine gabbro with local oxide-rich layers. Unit III is also locally sulfide bearing, with <534S values between 4 and 6%o. This unit is correlative with Severson's U3 ultramafic. Unit IV is characterized by two distinct cycles. The lower sub-unit consists of melatroctolite overlain by troctolite and olivine gabbro, whereas the upper subunit is composed of troctolite and augite troctolite overlain by leucotroctolite. Thin layers (~2-3m) of less olivine-rich rock types may occur within the basal melatroctolite, but the sequence is much enriched in olivine compared with the overlying rocks. Unit V is a texturally uniform sequence of troctolite to leucotroctolite with evidence of differentiation culminating in the formation of roof-zone pegmatitic layers. Units IV and V are sulfide poor, and <5MS values of 0 ± 2%o are very different from mineralized units. Units IV and V are both included in Severson's Basal Heterogeneous unit. Unit VI is a strongly altered, predominantly troctolitic sequence with up to 5 vol. % disseminated sulfides that have 534S values of 4—6%o. The unit is thought to be correlative with Severson's Ul ultramafic. Based on drill core relationships both the Ul and U3 units of Severson (1994) appear to be crosscutting in detail, and may be intrusive into the Basal Heterogeneous NUMBER 6 DECEMBER 1996 Zone. Unit VII is similar to Unit V, and contains troctolite and leucocratic troctolite. Sulfide content is low (200-300 p.p.m. S) with ^ S values of 0± 2%o. This unit is correlative with Severson's homogeneous Anorthositic Troctolite and Troctolite. Giants Range Batholith Footwall rocks in all drill cores examined are quartz monzonitc to granodiorite of the Giants Range Batholith. The predominant mineral assemblage consists of plagioclase, K-feldspar, and lesser amounts of quartz, hornblende, biotite, magnetite, and apatite. K-feldspar tends to decrease in abundance away from the contact with the South Kawishiwi intrusion. Chlorite and actinolite alteration of plagioclase and mafic minerals is more common within the contact zone. Clots of chlorite and actinolite (0-9—2-9 mm) occur frequently associated with fine-grained sulfides similar in composition to those in the overlying intrusion (e.g. pyrrhotite, cubanite, chalcopyrite, and pentlandite). These sulfide occurrences are restricted to within 40 m of the intrusive contact. Below this depth sulfur content in the Giants Range Batholith is very low (<0-04 wt %) and the primary sulfide assemblage consists of finely disseminated pyrite and minor chalcopyrite. South Kawishiwi Intrusion Mineralogy and mineral chemistry Olivine. Olivine occurs in two principal textural forms in the rock types from core 34870-A. Granular olivine that ranges in size from 0-7 to 2 mm in diameter is most abundant. A second variety is interstitial or poikilitic in form, and is found primarily in Unit I (1-2-3-6 mm). Modal abundance varies from 0 to ~40% in melatroctolites (Fig. 5). Within troctolitic units olivine and plagioclase occur in nearcotectic proportions (~7:3). Melatroctolites represent an accumulation of olivine. Olivine compositions range from F052 to F069 (Fig. 6) with no evidence of zoning. Trace element content shows values of Zn, Y, Cu, and Zr ranging from 213 to 306 p.p.m., from 1 to 9 p.p.m., 0 p.p.m., and from 3 to 9 p.p.m., respectively. Plagioclase. Plagioclase varies from anhedral to subhedral, lath-shaped crystals that range in grain diameter from 1-5 to 14 mm. Plagioclase is the most abundant primocryst phase in most rocks examined. Overgrowths on cores are common, and may be exceptionally rich in included minerals such as orthopyroxene, clinopyroxene, biotite, apatite, ilmenite, and sulfides. Core compositions range from An^ to An67 (Fig. 6) and are generally correlative 1442 LEE AND RIPLEY SOUTH KAWISHTWI INTRUSION, DULUTH COMPLEX 34870-A Melon Feet O-i-O Troctofite I U I M I • | U I Ofivine Anorthostte Troctofite Qabbro UNIT VII 100- Troctofite -soo 200- -Norite UNIT VI TroctoOts -Gabbronorite CHivine Qabbro 300- -1000 "Pegmatite Zone TroctodtB \\ \ UNIT V OHvine Gabbro wtth minor OUvtne Anorthosite TroctoHto 400- UNIT IV OBvine Qabbro to Metatroctolite _tf—Troctottte -OUvine Qabbro -TroctoBe -Oxide and Troctoflte Zone -1500 Wfr soo- \W\ kNorite with minor TroctoBte \—TroctoWe Melagabbronorite OBvineGabbro Mebnorite UNIT m UNIT II UNIT I kNortte wtth minor OUvine Gabbro 600L 2000 Quartz Monzonite to Qranodiorits 0.5-1.0 vol.% Sutfide >1.0vol.% <0.5 vol.% Fig. 4. Summary of itratigraphic variations within drill core 34870-A, South Kawishiwi intrusion, Spruce Road area. with olivinc compositions. Variations in plagioclase core compositions for any particular section fall within a range of 4 mol % An. Zoning is ubiquitous, and may be normal, reversed or oscillatory. Other researchers (e.g. Ripley & Alawi, 1986; Chalokwu & Grant, 1990; Miller & Weiblen, 1990) have noted similar erratic zoning patterns in feldspars of the Duluth Complex. Grain-to-grain variations from normal to reverse zoning are identified on the scale of a thin section. Plagioclase may also occur with clinopyroxene, orthopyroxene, biotite, and ilmenite as a more clearly interstitial mineral, and with orthopyroxene in symplectitic intergrowths along plagioclaseolivine contacts. Orthopyroxene. Most orthopyroxene from the 1443 JOURNAL OF PETROLOGY VOLUME 37 NUMBER 6 DECEMBER 1996 600 Fig. 5. Modal abundance of olivine, plagioclase, orthopyToxene, dinopyroxene, biotite, and oxides from drill core 34870-A. Open squares repreient values in excess of the maximum shown. Spruce Road deposit occurs as an interstitial mineral. Within noritdc rock types orthopyroxene is commonly poikilitic in form, and encloses laths of 50 Fo% 60 Ni ppm 70 0 1000 An% 2000 iO 55 60 65 70 600 Fig. 6. Variation of olivine Fo, olivine NiO, and plagioclase An in drill core 34870-A. plagioclase. In olivine-bearing rocks orthopyroxene commonly occurs as a thin (generally mm) rim on olivine as well as an irregular interstitial mineral. Orthopyroxene may also occur in symplectitic intergrowths with plagioclase along plagioclase or olivine grain boundaries. Orthopyroxene lamellae are present in many poikilitic dinopyroxene grains. Within Unit II orthopyroxene has inverted from pigeonite. Composition varies from Ens7 to Enyo with no zoning detected. Compositions of orthopyroxene rims vary sympathetically with olivine. dinopyroxene. Clinopyroxenc occurs exclusively as ophitic to subophitic grains that enclose primocryst minerals. Grain size is variable, ranging from 0-6 mm to 13-7 mm in pegmatitic horizons. Clinopyroxene normally contains lamellae of exsolved orthopyroxene and platelets of Fe—Ti oxides. Modal abundance reaches a maximum of 20% in gabbros of Units I, V, and VII (Fig. 5). Compositions vary from Wo37) En38, Fs13 to Wo^, E1144, Fs28- Trace clement analyses show values of Zn, Y, Cu, and Zr, ranging from 42 to 83 p.p.m., from 14 to 213 p.p.m., from 0 to 43 p.p.m., and from 17 to 304 p.p.m., respectively. Biotite. Biotite is a common interstitial mineral that normally occurs along the margins of oxide or sulfide grains. Modal abundance varies from < 1 to ~15% locally (Fig. 5). Compositions range from 1444 LEE AND RIPLEY SOUTH KAWISHTWI INTRUSION, DULUTH COMPLEX Ph37 to Pb.79 and although trends are normally con- or chlorite—actinolite—albite-prehnite—calcite assemcomitant with those of olivine and orthopyroxene, blages in Unit VI. local deviations occur. The most Fe-rich biotites occur associated with incompatible-element enriched pegmatitic intervals. F and Cl contents are not high, Whole-rock chemistry and range from 010 to 0-68 wt % and from 008 to Results of whole-rock major and trace element ana0-82 wt%, respectively. F/Cl ratios correlate with lyses are shown in Figs 7 and 8. Major element conFe/Mg ratio and suggest a crystal chemical control centrations strongly reflect modal mineralogy on halogen composition (Munoz, 1984; Volfinger et throughout the drill core. Generally low values of al., 1985). CaO, A12O3, and SiO2 in Units I-IV correlate with Fe-Ti oxides. Ilmenite and magnetite are the most low plagioclase modal abundance. Conversely, Units abundant oxide minerals in core 34870-A. Both I-IV are characterized by relatively high values of minerals may occur interstitially to silicates. Within FeO and MnO that correlate with a high proportion Unit III massive to semi-massive (20-60%) oxide of Fe-rich silicates or oxides. layers are found intercalated with troctolite. TitaUnits I and II are characterized by high modal niferous magnetite with up to 2 - 8% Cr2O3 is the abundances of ophitic and interstitial orthopyroxene, most abundant oxide in these layers, and forms clinopyroxene, and locally biotite and olivine. Units subhedral to euhedral grains from 0-6 to 1-7 mm in I and II are chemically similar, with only subtle diameter. Exsolved rods of Cr—Al spinel and differences observable. Unit II is characterized by ilmenite may occur along (100) planes of titani- lower values of MgO, MnO, Y, and P2O5, and ferous magnetite. Cr—Al-Ti-bearing magnetite is higher values of SiO2 and Na 2 O. Although mgalso the most common oxide mineral in Unit II, numbers of Unit I (38-49) and Unit II (32-41) are where it occurs in amounts up to 10%. Ilmenite is low, neither is enriched in incompatible elements the most common oxide in Units I, IV, V, VI, and compared with highly evolved, low-m^-number, Fe— VII, where it occurs in interstices, with clino- Ti-P-rich basal units in the Partridge River pyroxene, orthopyroxene, and biotite. Eutectdc intrusion (Chalokwu & Grant, 1990; Severson, 1991; intergrowth of magnetite and ilmenite is observed in Taib & Ripley, 1993). Elevated values of Cr in Unit samples from Unit V. II are related to the presence of Cr-bearing magSulfides. Sulfide minerals at Spruce Road are con- netite. centrated within two specific intervals. Disseminated High concentrations of FeO, MgO, and Cr in mineralization occurs within Units I—III and Unit Unit III and the basal portion of Unit IV reflect the VI. Modal abundance in these units varies from 0-5 abundance of olivine and magnetite. The mgto 5 vol. %. Although host rocks vary significantly numbers of Units III (excluding layers of oxide the nature of sulfide mineralization is similar. Fine- accumulation) to VII range from 50 to 60, with to medium-grained sulfides occur interstitially to most values in the more restricted interval of 50-55. primocryst minerals, and are associated with other Alteration in most units is minor (see above), but interstitial minerals. Principal sulfide minerals are within Unit VI alteration is locally intense with up pyrrhotite, cubanite, chalcopyrite, and pentlandite. to 60% chlorite plus actinolite. The presence of the A more thorough account of sulfide mineral dis- assemblage chlorite—actinolite—epidote—prehnite tribution and genesis has been given by Lee & indicates that alteration temperatures were less than Ripley (1995). 400°C (Liou et al., 1985), but does not preclude Other accessory minerals. Other minerals that occur higher temperature alteration. Enrichment of C in as primary intercumulus phases include apatite, Unit VI is a result of hydrothermal alteration and zircon, baddeleyite, and graphite. Apatite is parti- the formation of calcite in low-temperature cularly abundant in areas of incompatible element alteration assemblages. enrichment. Within pegmatitic zones apatite abunIncompatible element variations may reflect either dance may reach 5 vol. %. Fluid inclusions are fluctuations in the amount of interstitial minerals, or plentiful in coarser-grained apatite, and point to the changes in their composition with depth. Elements involvement of a fluid phase in pegmatite genesis. such as Zr, Y, P, Ba, and Ti illustrate strong positive Alteration minerals. Minor amounts (1-5 vol. %) of correlations. Ratios of incompatible elements, such chlorite, actinolite, hornblende, epidote, iddingsite, as Zr/Y, remain relatively constant from unit to unit serpentine, and magnetite are found in most rock (Fig. 7). Because K 2 O abundance is typically near types as alterations of plagioclase, olivine, and pyr- 0-35 wt % in plagioclase, whole-rock K 2 O abunoxene. More intense alteration is locally observed in dance is related to the abundance of plagioclase as the form of abundant serpentinization of olivine well as to interstitial minerals such as biotite. Trends 1445 JOURNAL OF PETROLOGY VOLUME 37 NUMBER 6 DECEMBER 1996 •a *3 to e 0 c 0 1 4 4 I S 0 s O d> c R S S « R5 S » S K 3 H S ( * K>) O'H •g = O '5* o c o •a 1446 LEE AND RIPLEY SOUTH KAWISHTWI INTRUSION, DULUTH COMPLEX granodiorite to quartz monzonite of the Giants Range Batholith have <518O values from 80 to 96%o. Temperatures of plagioclase-olivine and plagioclase—orthopyroxene equilibration were determined for eight samples (Fig. 9b, Table 3) using mineralmelt fractionation factors of Kyser et al. (1982). Olivine is generally believed to be more resistant to exchange during cooling than is plagioclase, and hence temperatures are thought to be minimum estimates. Computed temperatures for plagioclase— olivine pairs from Unit VII range between 1050 and 1150°C. A plagioclase-orthopyroxene pair from Unit I gave a temperature of ~ 1130°C. Chalokwu et al. (1993) have estimated a crystallization temperature for the Partridge River Intrusion of ~1150°C based on numerical simulation of phase equilibria. Y (RP.m) 10 20 30 600 Fig. 8. Variation in the concentration of incompatible elements (Zr, P2O5, Y) in drill core 34870-A. Dajhed lines separate uniu into inferred differentiation or mixing cycles. PARENTAL MAGMAS AND PROCESSES RESPONSIBLE FOR INTER-UNIT VARIATIONS AT SPRUCE ROAD Estimation of the composition of magma parental to individual units at Spruce Road is complicated by many factors, including possible crustal assimilation, magma mixing, recharge, and extrusion, as well as local evidence for fractionation by crystal accumulation (e.g. oxide layers of Unit III and olivinc-rich layers of Unit IV). Still it is of interest to determine whether the rock types present at Spruce Road can be produced by a common parent, or if different parental magmas derived from distinct sources are required. An evaluation of this type is of particular importance if we are to understand the localization of sulfide mineralization within certain units. Although considerable debate continues over the nature of magmas parental to the Duluth Complex (e.g. Weiblen & Morey, 1980; Klewin, 1989; Miller Oxygen isotopic composition & Weiblen, 1990; Chalokwu et al., 1993), most Oxygen isotopic values of Duluth Complex rocks at researchers agree that the abundance of high-Al rock Spruce Road range from 4-3 to 7-2%o (Table 2), and types in the Complex and the overlying North Shore for Units III—VII are correlative with modal miner- Volcanic Group (e.g. Green, 1972; Brannon, 1984) alogy (Figs 5 and 9a). 5i8O values less than ~6-0%o strongly points to the importance of high-Al, olivine are found only in rocks with relatively high olivine tholeiitic liquids. The Al-rich nature of all units at or oxide contents, in line with the expected frac- Spruce Road is apparent from a comparison of unittionation for minerals crystallized from mantle- to-unit average weighted compositions (Table 4). derived magmas (e.g. Kyser et al., 1982). Most troc- Based on variations in textures and mineral assemtolitic to gabbroic rocks in the Spruce Road area are blages, coupled with chemical differences and characterized by <518O values between 60 and 68%o. reversals, we suggest that multiple inputs of magma Four samples of noritic to troctolitic rocks from Unit have been involved in the generation of the stratiII show high values in the range 6-9-7-\%o. Footwall graphic sequence of the South Kawishiwi intrusion and fluctuations in incompatible element concentrations are particularly informative when evaluating possible models of magma crystallization and replenishment. Several units at Spruce Road (IV— VII) contain cycles characterized by upward increases in incompatible elements and olivine Fa content followed by reversals to lower values (Figs 6 and 8). The thickness of individual cycles marked by such trends varies from 20 to 50 m. Elevated incompatible element abundances within pegmatitic intervals at the top of the second cycle in Unit V (Fig. 9), also suggest an enrichment during crystallization and possible compositional convection. 1447 JOURNAL OF PETROLOGY (a) VOLUME 37 (b) 618o 5.0 6.0 NUMBER 6 DECEMBER 1996 6 l 8 o (°/~; 4.0 7.0 5.0 6.0 7.0 100 50 200 VI £ 300 TOO a a 0) d 400 a IV 150 500 600 200 x Fig. 9. (a) Whole-rock oxygen isotopic values vi depth in drill core 34870-A. (b) Oxygen isotopic variations in whole rocks, plagioclase, and olivine from Unit VII, South Kawishiwi intrusion, Spruce Road area. at Spruce Road. Chalokwu et al. (1993) have argued for a closed system in situ (crystals nucleate and grow in equilibrium with liquid) process of magma crystallization in the Partridge River intrusion. We concur that in situ crystallization has also been important in the petrogenesis of the South Kawishiwi intrusion as discussed more fully below. The similarities in composition of several of the units at Spruce Road to known or inferred liquid compositions strengthens the proposition that in situ crystallization has been an important mechanism, and that many units may have been emplaced as crystalfree liquids (e.g. Miller & Weiblen, 1992). For example, the weighted average composition of Unit VII is very similar to that of the chilled margin of the Pigeon Point sill (Mudrey, 1973), which is a candidate for a parental magma composition. Unit VII is only slightly more aluminous than Brannon's (1984) intermediate olivine tholeiites of the overlying North Shore Volcanic Group. The bulk composition of Unit I is similar to Fe-rich tholeiites defined by Brannon (1984). Mass balance computations confirm that low-pressure fractionation of olivine and plagioclase from an intermediate olivine tholeiite (Brannon, 1984) produces a liquid similar in composition to Unit I. Units III and IV may represent the emplacement of more primitive liquids based on elevated MgO and Cr contents. Alternatively, mass balance computations suggest that Unit IV may be a mixture of ~20% olivine, < 1 % chrome-bearing spinel, and a liquid of an intermediate olivine tholeiite composition. Crystallization of only a few percent olivine crystals in a staging chamber, followed by mechanical accumulation, would change the composition of a parental magma only slightly. Hence basal melatroctolites of Unit IV may be a result of olivine concentration by a process such as flow differentiation (Bhattacharji, 1966, 1967). Units I and II show oxygen isotopic and chemical evidence for crustal contamination by other than volatile elements (see discussion of sulfur addition and hydrothermal alteration below). c518O values of ~7%o in Unit II are anomalous relative to most basaltic rocks (e.g. Kyser et al., 1982), as well as other rock types in the Spruce Road area. The presence of large amounts of orthopyroxene in Unit II is also thought to reflect an increase in the activity of 1448 LEE AND RIPLEY SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX Table 2: Oxygen isotopic values of whole rocks from drill core 34870-A, South Kawishiwi intrusion, Spruce Road area Sample Depth •5 1 B O no. (m) (X* SMOW) Unit Rock Sample Depth a"o type no. (m) (K^SMOIW) Unit Rock type SP10A 15-2 6-2 VII T SP275 413-9 6-4 IV SP17 30-6 6-1 VII TR SP278 421-8 5-6 rv mTR SP21 39-6 62 VII TR SP281 429-2 6-1 IV TR SP29 52-1 6-4 VII OPAN SP285 437-3 4-3 IV (mTR) SP36 60-7 6-1 VII TR SP1 443-6 6-1 III TR SP46 77-6 6-3 VII GB SP292 449-3 6-3 III TR SP54 92-8 6-2 VII TR SP296 457-3 5-7 III OLGB SP65 106-7 6-2 VII TR SP301 464-0 6-1 III NO SP72 118-9 6-4 VII TR SP306 472-6 5-1 III mTR SP82 134-1 6-4 VI! TR SP310 480-4 6-6 III TR SP96 152-4 60 VII TR SP314 487-4 4-9 III mOLOXGB SP1O7 1680 6-4 VII TR SP2 495-9 70 II (NO) SP117 184-1 6-0 VII TR SP324 506-1 70 II NO SP127 198-3 6-1 VII TR SP329 512-7 69 II TR SP133 205-1 6-2 VI TR SP332 518-2 7-1 II NO SP139 213-2 6-5 VI TR SP335 526-1 6-3 I NO SP163 229-1 6-9 VI TR SP341 535-3 6-4 I TR SP185 241-6 6-6 VI TR SP345 543-5 6-4 I mGBNO SP212 259-2 6-5 V GBNO SP347 6480 6-4 1 OLGB SP224 274-5 6-2 V OLGB SP3 559-5 6-5 1 mNO OLAN SP229 292-2 6-8 V OLGB SP357 564-5 6-2 1 NO SP234 304-8 6-8 V (TR) SP6 5700 6-3 1 OLGB SP238 316-4 6-1 V TR SP7 578-2 63 1 NO SP242 328-6 6-2 V OLGB SP371 5850 62 1 NO SP244 334-7 6-2 V OLAN SP374 691-9 6-4 1 NO SP246 340-8 6-0 V OLGB SP38S 606-6 8-5 FWrock GDtoQM SP250 3530 6-7 V OLAN SP394 626-7 83 FWrock GDtoQM SP256 368-5 6-3 IV TR SP401 643-1 9-2 FWrock GDtoQM SP261 380-6 6-1 IV TR SP414 670-3 83 FWrock GDtoQM SP265 389-2 6-0 IV OLGB SP425 700-4 80 FWrock GDtoQM SP268 397-5 6-0 IV mTR SP43B 732-7 9-6 FWrock GDtoQM SP271 405-1 6-4 IV GBNO SP268G 397-5 8-1 Fable dike TR, troctolite; OP AN, orthopyroxene anorthosite; GB, gabbro; NO, norite; OLGB, olivine gabbro; OL AN, olivine anorthosite; m, mela-; OL OX GB, olivine oxide gabbro; GB NO, gabbronorite; GD, granodiorite; QM, quartz monzonite; FW, footwall. Parentheses denote the presence of alteration minerals in excess of 5 vol.%. SiC>2 resulting from country rock assimilation. Both units are characterized by high Ba contents which may be a result of contamination. Oxygen isotopic results from Unit I may also be indicative of minor amounts of country rock contamination. Unit I is characterized by lower plagioclase contents than units such as V and VII, and because of the abundance of interstitial orthopyroxene or clinopyroxene was expected to show lower <518O values. Values from Unit I are 6-2-6-4%o and cannot be readily distinguished from those of more plagioclase-rich units above. Ripley & Al-Jassar (1987) and Zanko et ai. (1994) have shown that highly anomalous 5lBO values may be found around metasedimentary xenoliths in the Babbitt and Serpentine deposits (Fig. 2). (518O values of unmetamorphosed Virginia Formation varies from 8-2 to 13-9%o, and values as high as 11 to 12%o have been measured for troctolites and norites within ~ 3 m of Virginia Formation xenoliths (Ripley & Al-Jassar, 1987). Ripley & Al-Jassar 1449 JOURNAL OF PETROLOGY VOLUME 37 Table 3: Oxygen isotopic values ofmineral separates from Unit VII, and computed oxygen isotopic equilibration temperatures based on thefractionationfactor ofKyser et aL (1982) Sample « 18 0(X.VSM0W) Depth A " AS (m) SP17 30-5 SP36 60-7 WR PI 6-1 6-6 6-1 01 6-6 5-5 1-1 1131 50 1-6 1078 NUMBER 6 DECEMBER 1996 gradients were observed around the xenoliths. At Spruce Road, footwall rocks of the Giants Range Batholith have lower 8i8O values than the metasedimentary footwall material to the southwest. For this reason the detection of possible assimilation of footwall material utilizing 5 O values is more difficult. However, 6 S values of mineralized zones (Lee & Ripley, 1995) suggest that liquids were in contact with metasedimentary rocks at depth, and hence a source of elevated 5leO values was available at some point in the evolutionary history of at least the sulfide-rich magmas. Also, the presence of Virginia Formation and Biwabik Iron Formation xenoI ft liths indicates that contamination by high- O 17 6-2 6-4 1068 SP54 92-8 4-7 material is a possibility. Depending on magma/con65 1-6 SP72 118-9 6-4 4-9 1078 taminant ratios and contaminant (518O values the 6-4 1-6 1078 SP95 152-4 6-0 4-8 detection of less than ~5—10% country rock con2-0 SP117 184-1 6-6 1033 6-8 4-8 tamination is difficult. 0-9 1154 Klewin (1989), Miller & Weiblen (1990), and SP127 6-1 198-3 6-6 5-6 Jerde (1991) have reviewed evidence for polybaric A PI _jt18/-l PI — o Oa; WR, whole rock. fractionation in the production of lavas and intrusive rocks of the Midcontinent Rift. In particular, pri(1987) modeled the 8i8O gradients around xenoliths mitive high-Al, olivine tholeiites are thought to be as a result of diffusive exchange, but after the xeno- generated by high-pressure fractionation of mantleliths had experienced peak temperatures and loca- derived melts, followed by various degrees of fraclized partial melting. No major or trace element tionation in low-pressure chambers to produce a Table 4: Weighted averages of the chemical compositions ofunits in drill core 34870-A, South Kawishiuri intrusion, Spruce Road area Elements Unit 1 Unit II Unit III Unit IV UnitV Unit VI Unit VII Footwall Granite CeO 8-54 8-17 6-18 7-94 10-63 8-86 9 65 MgO 7-87 5-97 12-27 11-27 6-30 898 707 1-49 AfeO, 16-86 17-45 13-77 16-51 21-00 18-21 19 82 16-84 SK3 2 45-64 46-95 39-80 46-34 48-63 4309 47-58 65-29 K2O 045 0-38 0-25 0-44 0-58 0-37 0-50 4-21 Na2O 233 2-63 1-76 2-16 2-88 1-80 2-66 6-22 104 1-16 0-54 1-28 0-27 207 TIOj 1-63 1-81 2-34 FeO 15-38 15-66 22-59 14-19 9-39 13-89 10-97 2-83 MnO 0-16 0-18 0-19 0-16 0-11 0-13 0-13 0-07 PjOs 0-21 on 008 0-15 0-21 0-08 0-18 0-14 H20 0-29 0-40 0-57 0-69 0-49 3-43 0-46 Cr (p.p-m.) 137 198 796 284 70 120 57 0-22 29 Y (p.p.m.) 17 8 4 11 23 6 16 20 Zr (p.p.m.) 84 47 47 72 84 37 90 200 19 21 2 22 21 22 31 120 249 293 193 185 356 256 273 644 Nb (p.pm) 31 29 64 37 28 31 34 41 Be (p-pm) 172 156 88 88 118 72 94 1439 99-81 99-90 100-29 99-37 Rb (p.p.m.) Sr (p.p.m.) Total 99-26 99-72 1450 100-31 98-65 LEE AND RIPLEY SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX suite of compositions observed in the lavas and plutonic rocks. Kinzler & Grove (1992a,i) have shown that high-Al tholeiites may be produced by partial melting of spinel and plagioclase lherzolite at pressures from ~10 to 20 kbar. Compositions similar to those that may be parental to the South Kawishiwi intrusion are not thought to represent primary magmas based on high Fc contents, and relatively high concentrations of incompatible elements such as Y, Zr, and P. In summary, it is clear that varying degrees of low-pressure fractionation of olivine and plagioclase from high-Al olivine tholeiites, augmented by mechanical crystal accumulation processes and limited crustal contamination, can produce the range of rock types present in the South Kawishiwi intrusion at Spruce Road. A MODEL OF CHEMICAL AND ISOTOPIC DIFFERENTIATION PRODUCED DURING IN SITU CRYSTALLIZATION AND MAGMA RECHARGE Variations in mineral chemistry and trace element concentrations Petrographic and chemical data illustrate several points regarding petrogenetic evolution of the South Kawishiwi intrusion in the Spruce Road area that are shared with other sequences in the troctolite series of the Duluth Complex. The distinct modal and compositional characteristics of rock types in the Spruce Road area are interpreted to indicate a history of repeated magmatic injections. Internal differentiation of many pulses appears to be characterized by trends of upward increasing incompatible element concentrations, olivine Fa content, and plagioclase Ab. Such trends may be produced by at least three processes. One method involves closed system in situ crystallization of solidification zones from the base upwards, coupled with convective fractionation of interstitial liquid (e.g. Tait & Jaupert, 1992). Langmuir (1989) has modeled such a process by equilibrium crystallization of solidification zones followed by expulsion of interstitial melt and mixing with overlying liquid. Nielsen & DeLong (1992) referred to the process as boundary-layer equilibrium fractionation, and have also modeled the effects of fractional crystallization in the solidification zone (boundary-layer fractional crystallization). Abrupt reversals in chemical trends may occur when a second solidification zone begins to form in the fractional crystallization model, but would not form as a result of equilibrium crystal- lization. Langmuir (1989) has shown that in situ boundary-layer equilibrium crystallization coupled with expulsion of evolved residual liquid and mixing with overlying magma may lead to large increases in incompatible element content comparable with those that occur during fractional crystallization. However, relative to the results of fractional crystallization, only modest increases in olivine Fa and plagioclase Ab result. A second process for the generation of observed chemical trends involves fractional crystallization of an initial magma pulse followed by periodic recharge of unfractionated liquid (e.g. O'Hara, 1977; O'Hara & Matthews, 1981). Chemical discontinuities marked by a return to more primitive compositions define intervals of new magma input. A third possible process centers on the trapping of various proportions of fractionated, interstitial melt. Upward increases in incompatible elements would then be a function of increases in trapped liquid amount, rather than concentration. Chalokwu & Grant (1990) have described a similar process for the Partridge River intrusion, and have explained the presence of ironrich olivines as a result of reequilibration with varying amounts of trapped liquid. Variations in the proportion of trapped liquid may occur in conjunction with other methods of differentiation. The first and third processes may both be augmented by periodic magma recharge. To evaluate these various possibilities, we have modeled several different fractionation processes at Spruce Road using trace element contents and olivine plus plagioclase compositions. In all cases, the starting magma composition is that of an intermediate olivine tholeiite, here taken as the weighted average of Unit VII. Phase-equilibrium based computer programs of Nielsen (1988), Weaver & Langmuir (1990), and Ariskin et al. (1993) were utilized to simulate both equilibrium and fractional crystallization in the various models considered. At Spruce Road olivine Fo contents range from 52 to 69 but vary only ~ 1 0 % in any unit. Incompatible element abundances may vary from < 5 p.p.m. to 35 p.p.m. in the case of Y, < 10 p.p.m. to > 100 p.p.m. for Zr, and 002 wt% to > 0 3 0 wt% for P 2 O 5 . Closed system homogeneous fractional crystallization The first process to be evaluated for comparative purposes was closed system homogeneous fractional crystallization (i.e. Neilsen & DeLong, 1992). This type of fractionation produces smooth trends of upward decreasing olivine Fo content (~ 80-50 at 80% crystallization; Fig. 10a) as well as gradual enrichment of incompatible trace elements to 90% 1451 JOURNAL OF PETROLOGY VOLUME 37 (a) o o o o DECEMBER 1996 (b) 100 100 80 80 •a o = NUMBER 6 s 60 = 60 40 o o S 40 20 20 45.0 55.0 65.0 75.0 50 85.0 100 Y (RRm) Olivine Fo Fig. 10. (a) Computed olivine compositional trend as a result of closed system homogeneous fractional crystallization of an intermediate olivine tholeiite. Derived using TRACE.FOR (Nielsen, 1988) with a crystallization increment of 02%. (b) Variation in the abundance of a perfectly incompatible element (Y as an example) during closed system fractional or equilibrium crystallization. Computed as described in (a). crystallization followed by pronounced increases in the final stages of crystallization (Fig. 10b). These trends are inconsistent with those observed at Spruce Road, where olivine compositions tend to be much more Fa rich. Possible olivine Fe-Mg exchange with residual trapped liquid to produce more Fa-rich olivine (e.g. Barnes, 1986) was computed utilizing the olivine-liquid Fe—Mg K& value of Roeder & Emslie (1970), an amount of trapped liquid equal to 30%, and olivine abundance computed from the COMAGMAT program (Ariskin et al., 1993). The following equation was used based on mass balance considerations: /FeO\f more magnesium rich than any olivine at Spruce Road, except those found in the basal portions of Units III and IV. In situ boundary-layer crystallization A more attractive mechanism to explain the compositional trends found at Spruce Road is in situ boundary-layer crystallization coupled with periodic exchange with the overlying magma (Langmuir, 1989). In the numerical models employed, crystallization was assumed to proceed from the bottom of a chamber upwards, as proposed by Langmuir (1989) and Nielsen & DeLong (1992). The following equations were utilized: R = (1) where (FeO/MgO)oi is the final, reequilibrated value of olivine and R is the mass ratio of trapped liquid to olivine. Results indicate that shifts in Fo content of ~3—6% may be produced in simulated solidification zones produced by gravitational accumulation of crystals with 30% trapped liquid (Fig. 11). However, the resulting olivine compositions remain ^ s z ( o ) = Q. xi M s z / + C,- R M M s z (1 - / ) (2) Mi, sz(f) = Cit xI M s z / - [M s z (1 - / ) ] (Q, RM —C|, OMl) (3) Mu OM2 = C,-, OMI ( M m - M s z ) - [M s z (1 - / ) ] (Ci, OMi— C,-, RM) (4) where M,jSZ (O) is the mass of element t in the solidification zone before removal of residual melt, 1452 LEE AND RIPLEY SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX u.u 100 0.8 •4 X • final gj 0.7 olivine ^ D I 1 0.6 z1 I 0.5 initial oDvine 80 trapped liquid 0.4 20 40 60 80 100 PERCEKT CRYSTALLIZED Fig. 11. Olivine compositional variation owing to exchange with trapped liquid in a system constructed by the accumulation of crystals and 30% trapped liquid. Initial olivine compositions are average valua for solidification intervali that range from 16 to 26% of the total system. The composition of the trapped liquid is that which remains after the accumulation of crystals in a solidification zone. Initial olivine compositions were determined using the algorithm of Ariilrin el al. (1993) for an intermediate olivine tholeiite. Final olivine compositions were computed by mass balance utilizing the olivine-Hquid Fe-Mg KA value of Roeder & Emilie (1970), and olivine abundance. •M>,sz (0 ls the m a s s °f element i in the solidification zone after removal of residual melt and replacement with overlying melt, Af$z is the mass of solidification zone, f is the fraction of solidification zone crystallized, M,- ( OMI,2-" ™ m a s s of element i in the melt overlying a solidification zone, M m is the mass of original magma, Q R M is the concentration of element i in the residual melt of the solidification zone, C,-jjd is the concentration of element i in the crystallized portion of the solidification zone, and Ci, oMi.2,... is the concentration of element i in the melt overlying a solidification zone. The mass of the basal solidification zones (Langmuir, 1989) was set equal to 20% of Mm, and / s e t to 0-8. Both equilibrium and fractional crystallization models were considered. Residual melt is allowed to exchange with an equivalent mass of overlying, unfractionated melt, and after mixing a new solidification zone begins to crystallize. Results of iterative calculations are shown in Fig. 12. Fractional crystallization in the solidification zone results in cyclic trends in olivine compositions that include more Mg-rich olivine than those observed at Spruce Road and the same problem as encountered with the closed system fractional crystallization model. As with homogeneous fractional crystallization the trapped liquid shift is insufficient to account for the more Fe-rich olivine composition. In contrast, equilibrium crystallization in the solidification zones produces more Fe-rich olivines, and a very gradual overall step-function decrease in Fo content upwards. Trends of this type are clearly more com- u o o 40 D 20 50 55 60 65 70 75 80 85 Olivine Fo Fig. 12. Olivine compositional variations during boundary-layer fractional and equilibrium crystallization of an intermediate olivine tholeiite. Heavy line represents the shift in fractionally crystallized olivine owing to exchange with trapped liquid. Triangles, fractional crystallization; squares, equilibrium crystallization. Computed using the programs of Weaver & Langmuir (1990) and Ariskint/o/. (1993). patible with both the absolute values and spatial compositional variations in olivine at Spruce Road, and throughout most of the troctolitic series rocks of the Duluth Complex. However, the predicted trends of incompatible elements (here assumed to be perfectly incompatible in olivine and plagioclase) show virtually no difference from those computed for homogeneous fractional crystallization, boundarylayer fractional crystallization, and boundary-layer equilibrium crystallization (Fig. 10b). To better explain the observed cyclical trends in incompatible elements an additional process of magma chamber recharge must be involved. Magma chamber recharge Recharge of a chamber undergoing homogeneous fractional crystallization cannot reduce the difficulties associated with the crystallization of olivine more Mg rich than that observed at Spruce Road and will not be considered further. In contrast, 1453 JOURNAL OF PETROLOGY VOLUME 37 recharge of a chamber undergoing in situ equilibrium crystallization could have important consequences with respect to observed chemical trends at Spruce Road. To model the effects of this process it was assumed that a mass of magma equal to one-fifth of that described above underwent equilibrium crystallization, with an initial solidification zone (~ 10% of the total magma mass) that progressed to 70% crystallization. At this time, expulsion of the fractionated liquid occurred followed by exchange and mixing with the overlying melt. Crystallization of successive solidification zones continued in an identical fashion until 80% overall crystallization of the initial magma. A new input of unfractionated magma was then added and mixed with the remaining residual liquid to produce a magma equal in mass to the original. Five cycles were modeled to produce a resultant sequence equal in mass to those described above for systems with no recharge. The model results are shown in Fig. 13a,b. Olivine shows modest iron enrichment in each cycle, with an overall upward change from ~Fo65 to Fo56. Reversals mark the input of fresh, unfractionated magma. Incompatible element trends illustrated by Y show upward increases within each cycle and a return to less enriched compositions when new magma is input. The overall upward enrichment trend is more subdued than that produced by closed system in situ fractionation. The overall trends of olivine compositions and incompatible trace element concentrations observed at Spruce Road are similar to those predicted by this final model, suggesting a crystallization mechanism involving boundary-layer equilibrium crystallization augmented by periodic magma recharge. Recharge in a dynamic rift zone environment is to be expected, and depending on parameters such as the composition of the input magma, state of differentiation, and possible assimilation at depth or en route to an overlying chamber, much more complex geochemical relationships than those outlined here may result. Input of new magma and mixing with more fractionated magma may help to explain the accumulations of titanomagnetite in Unit III, following models described by Irvine (1975, 1977) and Campbell & Murck (1993). Also, reactions in addition to mineral reequilibration, such as deuteric alteration, may occur in the cumulate pile after in situ crystallization and upward extraction of liquid. Lee & Ripley (1995) have described a process of upward movement of D-enriched vapor following H2O saturation of trapped interstitial liquid. Pegmatitic horizons enriched in volatiles and incompatible elements are common in the Duluth Complex, and may be the result of accumulation of volatile- NUMBER 6 DECEMBER 1996 rich liquid near the tops of individual magma sheets. Hydrothermal alteration and element redistribution are logical consequences of such a process. Plagioclase zpnation Plagioclase compositions at Spruce Road are difficult to evaluate because of their complex zonation. The zoning in the plagioclase clearly indicates that nonequilibrium, fractional crystallization occurred or overgrowths resulted from trapped liquid crystallization. However, core compositions show a covariance with olivine compositions that may be related to primary crystallization. Figure 14 contrasts the trends in olivine and plagioclase compositions expected for homogeneous fractional crystallization (Ariskin et al., 1993) versus those obtained in the model of in situ equilibrium crystallization described above. Plagioclase core compositions from Unit VII fall slightly below the trend expected for in situ equilibrium crystallization, but are far more calcic than expected for homogeneous fractional crystallization. This is an expected result based on the plagioclase zoning, but core compositions are not far removed from those predicted for equilibrium crystallization. Ripley & Alawi (1986) suggested that interstitial minerals exert a strong control on the type of zoning present in plagioclase. Normal zoning is predicted where Ca-bearing minerals (i.e. clinopyroxene) form from adjacent interstitial melt, and reverse zoning is predominant in domains where clinopyroxene is less abundant. Implicit in their discussion is the competition between crystal growth rate and diffusivity in producing the zonation (Bacon, 1989). A similar relationship between plagioclase zoning and intercumulus mineralogy is less apparent in the Spruce Road area. Miller & Weiblen (1990) suggested that erratic plagioclase zonation is a reflection of variable crystallization conditions (e.g. Smith & Lofgren, 1983). Plagioclase rim compositions may reflect both large- and small-scale controls, and less efficient cation exchange than olivine. However, both olivine and plagioclase compositions can be largely explained by in situ crystallization processes. Variations in trapped liquid abundance In a process such as in situ boundary-layer crystallization an upward increase in the amount of interstitial minerals, or trapped liquid, is not precluded but may result as a consequence of the mixing of highly evolved liquid with unfractionated liquid. At Spruce Road the estimation of trapped liquid abundance is difficult because of the problems inherent in determining the amount of plagioclase, and to a lesser extent, olivine, that may have formed from the 1454 LEE ANDRIPLEY SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX 100 100 T 80 1 §40 20 -fixed(30%) - increasing(10-30%) 50 55 0 5 10 15 20 25 30 35 40 45 OlivirM Fo Fig. 13. (a) Olivine compositional variations as a result of boundary-layer equilibrium crystallization of an intermediate olivine tholeiite combined with magma chamber recharge (R) by a liquid of the same initial composition, (b) Variation in the composition of an incompatible element (e.g. Y) aj a result of boundary-layer equilibrium crystallization of an intermediate olivine tholeiite combined with magma chamber recharge. Trends for both constant and increasing upward trapped liquid abundance are shown. interstitial melt. Methods of estimating primocryst olivine and plagioclase content by petrography or computation of whole-rock norms provide a minimum estimate of interstitial mineral abundance. As the composition of the remaining liquid becomes more differentiated the proportion of crystallizing clinopyroxene, as well as incompatible elementbearing minerals such as apatite, ilmenite, and zircon will increase. Several of the units at Spruce Road show a strong correlation between normative primocryst minerals (or interstitial minerals) and the abundance of incompatible elements (Fig. 15). The concentration of incompatible elements in rocks characterized by higher abundances of interstitial minerals exceeds that which would result if the compositions were due solely to mixing of variable proportions of primocryst minerals and an interstitial liquid of constant composition. The data are more consistent with an upward increase in both the proportion and incompatible element content of the interstitial liquid. Figure 15 (Unit V) illustrates the trends produced when both C^RM/Q.O anc* A^RM/ Mm increase upward. The ratio CJRM/Q.O *S a n a " logous to the fractionation or enrichment factor denned by Langmuir (1989). Figure 13b illustrates the difference in incompatible element concentrations for the case where trapped liquid abundance remains constant for in situ crystallization, versus the case where trapped liquid percentage increases upward from 10 to 30%. Where the trapped liquid percentage increases upward the concentration of incompatible elements near the top of the magma chamber may exceed that for the case of constant trapped liquid abundance. For example, increases in Y content from 5 to 25 p.p.m. in the basal portion of Unit IV, coupled with an upward increase in olivine iron content from Fo6s to Fo55, are readily explained by a process of in situ crystallization, upward increase in trapped liquid abundance, and accumulation of olivine near the intrusive floor. 1455 JOURNAL OF PETROLOGY • Frac U In sira eq VOLUME 37 • Unit VII 80 _ • • 75 • 70 i". D » • • • • • •» • 55 • 50 45 • 50 • • * • • i 55 • • • • i • 60 1 ' ' 1 • ' ' 65 • I • ' 70 • ' 1 ' ' 75 ' ' 1 80 OIFo% Fig. 14. Olivine Fo and plagioclase An crossplot for simulated in situ equilibrium cryitallization and homogeneoui fractional crystallization, compared with values from Unit VII of drill core 34870-A. The physical mechanism whereby residual liquid is expelled upward and mixes with overlying liquid is not clear. Irvine (1980) in his treatment of infiltration metasomatism referred to crystal compaction and movement of residual fluid from areas of high to low pressure. Chalokwu & Grant (1990) suggested that in the Babbitt area trapped liquid was an ironrich ferrodiorite, and too dense to have risen. Interstitial melt in units of the South Kawishiwi intrusion at Spruce Road was also iron rich, and a similar restriction owing to density differences may be envisioned. Two factors, (1) t h e / o , control on magnetite precipitation and (2) initial- H2O content, are of importance with respect to potential density differences between interstitial liquid and overlying, unfractionated liquid. Computer simulations of the crystallization process (e.g. Ariskin et al., 1993) suggest that at fot conditions near quartz-fayalitemagnetite (QFM), magnetite should saturate in the melt at ~ 4 7 % crystallization. Snyder et al. (1993) have highlighted the importance of iron oxide crystallization in reducing the density of residual liquid in tholeiitic systems. Although density of the remaining liquid in our model system is reduced to 2-58 g/cm 3 vs 2-70 g/cm 3 in the overlying melt, cumulus magnetite and/or ilmenite are not observed in the Spruce Road area, except within massive oxide layers of Unit III. Water concentration will increase in the residual liquid with crystallization, and saturation may eventually be reached in the trapped liquid (e.g. Ripley et al., 1993). Because of the increased water content, density of the intercumulus liquid may be depressed depending on the amount of H 2 O present in the initial liquid. Figure 16 illustrates the change in density with crystal- NUMBER 6 DECEMBER 1996 lization of an intermediate olivine tholeiite liquid with a water content of 0-5 w t % . Water contents of 0-5—1 wt % are not unreasonable for continental flood basalts (e.g. Basaltic Volcanism Study Project, 1981), and are in line with the abundant evidence for late-stage deuteric alteration in rocks of the Complex (Tyson, 1979; Chalokwu, 1985; Sassani & Pasteris, 1988; Ripley et al., 1993). In this simulation / o , is set below that of QFM, between iron-wiistite and wustite-magnetite to prevent saturation in oxide. Christie et al. (1986) suggested that similar/o, conditions characterize mafic magmas before eruption. It is clear that H 2 O content alone, perhaps augmented by crystallization of Fe-Ti oxides, will act to decrease the density of a residual liquid relative to that of overlying parental magma. Oxygen isotopic variations Based on <518O values from Unit VII and the estimated parent melt composition described above, evolution of <518O with in situ crystallization, convective fractionation, and periodic recharge may be modeled. A parent magma Sl8O value was computed by summing individual layers of Unit VII. The computed SO value of the parent magma is 6-3%o, and is very similar to computed parent magma <518O values for a number of other mafic intrusions (Table 5). A value of 6-3%o is higher than that of typical oceanic basalt [(518O ~5-7%o, Kyser et al. (1982)], but Unit VII clearly represents a derivative magma that is more evolved than typical oceanic basalt. Higher <518O values may result from fractional crystallization at depth, as well as possible crustal contamination. A model of Si8O evolution consistent with the trends in major and trace elements presented above is shown in Fig. 17. Isotopic fractionations between Table 5: Comparison of oxygen isotope ratios in mafic—plutonic systems (9tv3VSMOW) Intrusion South Kawishlwi-Spruce Road 6-3 Kiglapait 6-0 Stiltwater 5-9 Bushveld 6-8 Muskox 6-6 Sources of data: Kiglapait (Kalamarides, 1984); Stillwater (Dunn, 1986); Bushveld (Schiffries & Rye, 1989); Muskox (Epstein & Taylor, 1967). 1456 SOUTH KAWISHTWI INTRUSION, DULUTH COMPLEX LEE AND RIPLEY 14 25 - Unit VI 12 10 Y • Unit VII 20 Y 8 « 15 « (RRnUio 6 $ • « 4 5 - 2 0 I 88 90 92 94 80 Normative primocryst minerals (wr%) 25 15 95 UnitV 30 • 90 35 Unit IV 20 Y 85 Normative primocryst minerals (wt%) 25 • 10 Y 20 (Rl>mj 1 5 5 10 • c ) • BA v° X v* * 5 80 85 90 95 1 80 Normative primocryst minerals (wt%) 85 90 95 100 Normative primocryst minerals (wt%) B 4* • • Unit III 6 - Unit II Y Y (PPm) (RP™) 4 • 2 • 70 73 80 85 n 80 90 Normative primocryst minerals (wt%) 25 20 Y i 83 90 95 Normative primocryst minerals (wrN>) 40 35 30 Unit 1 15 • All Units » (Rprn) 20 15 10 80 82 84 88 88 90 70 Normative primocryst minerals (wt%) 80 SO 100 Normative primocryst minerals (wt%) Fig. 15. Plots of Y vs normative plagiodase and olivine (plus oxides for Unit III) for individual units, as well as collectively. A negative correlation between normative plagiodase plus olivine and incompatible dement concentration is observed for several units. Curves shown for Unit V illustrate cases where C^RM/Q.O remains constant (6-67, curve A), and increases upward (6-67-8-9 at 85% normative plagiodase plus olivine, curve B; 667-11 0, curve C; 6-67—14-2, curve D). C,, RM» concentration of clement t in the residual mdt; C;, o. concentration of dement i in the initial magma, here taken as 15 p.p.m. 1457 JOURNAL OF PETROLOGY Dry system VOLUME 37 NUMBER 6 0.5% water DECEMBER 1996 1 0.9 • 0.8 N ^ 0.6 W S" 0.5 mol % crystallized Fig. 16. Denjity variation of an intermediate olivine tholeiite with increasing crystallization. Values from Ariilrin it al. (1993) using the method of Bottinga & Weill (1970). c Ia 0.4 &. u_ olivine, pyroxene, plagioclase, and melt are taken from Kyser et al. (1982). A mass balance expression of the form .18, Liquid = 0.2 — 5 O p iX p i Oinidilmclt — l. 0.3 Res. liquid where Xol, Xph and .YRa.Liquid represent atomic fractions of oxygen in olivine, plagioclase, and residual liquid, respectively, was solved for each increment of crystallization, and a new <518O for the mixture of overlying melt and liquid expelled from the crystallizing pile was computed. Results indicate that variations of no more than 0-1 %o are expected from such a crystallization process, in close agreement with the values observed in Unit VII. CONCLUSIONS Results of mineralogical and geochemical studies of the South Kawishiwi intrusion in the Spruce Road area can be summarized as follows: (1) Based on mineralogy, texture, and chemical and isotopic composition, seven distinct units are recognized in drill core 34870-A. Lithologic variations are: Unit I—basal accumulation of heterogeneous gabbro, troctolite, and norite; Unit II— norite with abundant inverted pigeonite; Unit III— troctolite and olivine gabbro with local oxide-rich layers; Unit IV—melatroctolite, troctolite, olivine gabbro; Unit V—increased plagioclase abundance in troctolites and leucrocratic troctolites; Unit VI— strongly altered troctolite; Unit VII—similar to Unit V, troctolite and leucrocratic troctolite. Sulfide mineralization, consisting of 1—5 vol. % of disseminated pyrrhotite, cubanite, chalcopyrite, and pentlandite occurs in Units I, II, III, and VI. (2) Chemical modeling of trace element concentrations and variations in mineral chemistry 0.1 • 0 6.25 6.30 6.35 618O Fig. 17. Trendi in whole-rock i' 8 O values produced during boundary-layer equilibrium crystallization of an intermediate olivine tholeiite with an initial 5IBO value of 6'3Xo. suggest that unconformities within the major units developed by in situ equilibrium crystallization coupled with expulsion of evolved residual liquid that mixed with overlying magma. Differentiation occurred in zones ranging from ~20 to 50 m in thickness, and was followed by recharge of fresh magma. Upward enrichment of incompatible elements, olivine Fa content, and plagioclase Ab content may be effectively explained by the process. Mass balance computations suggest that the magmas responsible for Units I-VII at Spruce Road may be related through varying degrees of fractionation and limited crustal contamination of a parental high-Al, olivine tholeiite magma. (3) Oxygen isotopic analyses indicate that only Units I and II show strong evidence for crustal contamination by other than volatile elements such as sulfur. (5I8O values of Unit II range from 6-9 to 7-l%o, and are O enriched compared with values of 5-l-6-8%o found in other units. Silica contamination is indicated based not only on 518O values, but also on the predominance of orthopyroxene in the unit. (518O values of Units III-VII can be correlated with 1458 LEEANDRIPLEY SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX Bhattacharji, S., 1967. Scale model experiments on flowage differentiation in sills. In: Wyllie, PJ. (ed.) Ultramafic and Related Rocks. New York: John Wiley, pp. 69-70. Bottinga, Y. & Weill, D. F., 1970. Densities of liquid silicate systems calculated from partial molar volumes of oxide components. American Journal ofScitnct 269, 169-182. Brannon, J. C , 1984. Geochemistry of successive lava flows of Keweenawan North Shore Volcanic Group. Ph.D. Dissertation, Washington University, St Louis, MO, 312 pp. Campbell, I. H. & Murck, B. W., 1993. Petrology of the G and H ACKNOWLEDGEMENTS chromitite zone in Mountain View area of the Stillwater Thanks are extended to Steve Mornis of the Complex, Montana. Journal of Petrology 34(2), 291-316. American Copper and Nickel Company, and to Rick Chalokwu, C. I., 1985. Chemical, petrochemical, and composiRuhanen, Henk Dahlberg, and Jacqueline Jiran of tional study of the Partridge River intrusion, Duluth Complex, the Minnesota Department of Natural Resources for Minnesota. Ph.D. Thesis, Miami University, Oxford, OH, provision of drill core, and assistance in sampling. 230 pp. Mark Sevcrson and Steve Hauck of the Minnesota Chalokwu, C. I. & Grant, N. K., 1990. Petrology of the Partridge River Intrusion, Duluth Complex, Minnesota: 1. Relationships Natural Resources Research Institute and Jim Miller between mineral compositions, density and trapped liquid of the Minnesota Geological Survey openly shared abundance. Journal of Petrology 31, 265-293. data and expertise; their spirit of cooperation is Chalokwu, C. I., Grant, N. K., Ariskin, A. A. &. Barmina, G. S., greatly appreciated. Michael Dorais of Indiana 1993. Simulation of primary phase relations and mineral comUniversity and Ian Steele of the University of positions in the Partridge River Intrusion, Duluth Complex, Chicago patiently provided instruction and advice Minnesota: implications for the parent magma composition. on electron microprobe analysis. Special thanks go to Contributions to Mineralogy and Petrology 114, 539-549. Steve Studley for his expert maintenance of the Christie, D. M., Carmichael, I. S. E. & Langmuir, C. H., 1986. stable isotope mass spectrometry laboratory at Oxidation states of mid-ocean ridge basalt glasses. Earth and Indiana University, and to John Hayes for providing Planetary Science Letters 79, 397-411. Zn reagent for hydrogen isotope analysis. Mark Gil- Clayton, R. N. &. Mayeda, T. K., 1963. The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates strap is thanked for his help with major and trace for isotopic analysis. Geochimica et Cosmochimica Acta 27, 43-52. element chemical analyses, as are Lisa Pratt and Karen Chapman for their help with sulfur and Dunn, T., 1986. An investigation of the oxygen isotope geochemistry of the Stillwater Complex. Journal of Petrology 27, 987-997. carbon analyses. Ruth Droppo graciously processed Epstein, S. & Taylor, H. P., Jr, 1967. Variation of 18 O/ 16 O in the final manuscript. minerals and rocks. In: P. H. Abelson (ed.) Researches in Geochemistry, Vol. 2. New York: John Wiley, pp. 29-62. Thoughtful reviews by Alan Boudreau, Jim Brophy, Mike Foose, David Towell, Haydn Murray, Foose, M. P., 1984. Logs and correlation of drill holes within the and an anonymous Journal of Petrology reviewer South Kawishiwi intrusion, Duluth Complex, Northeastern Minnesota. US Geological Survey Open-File Report 84-14. improved the quality of the manuscript. Guidance Foose, M. P. & Weiblen, P. W., 1986. The physical and petrologic from Roger Nielsen and Alexi Ariskin on efficient use setting and textural and compositional characteristics of sulfides of COMAGMAT is appreciated. Portions of this from the South Kawishiwi intrusion, Duluth Complex, work were supported by NSF Grants EAR 89-15472 Minnesota, USA. In: Fricdrich, G. H., Genlrin, A. D., Naldrett, and EAR 93-03665 to E. M. Ripley. A. J., Ridge, J. D., Sillitoe, R. H. & Vokes, F. M. (eds) Geology and MetaUogtny of Copper Deposits. Berlin: Springer-Verlag, pp. 8 24. Grant, N. K. & Chalokwu, C. I., 1992. Petrology of the Partridge REFERENCES River intrusion, Duluth Complex, Minnesota: II. Geochemistry Ariskin, A. A., Frenkel, M. Y., Barmina, G. S. & Nielsen, R. L., and strontium isotope systematics in Drill Core DDH-221. 1993. 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Zanko, L., Severson, M. J. & Ripley, E. M., 1994. Geology and mineralization of the Serpentine copper-nickel deposit. National Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/93-52. 1461 RECEIVED SEPTEMBER 4, 1994 REVISED TYPESCRIPT ACCEPTED JUNE 18, 1996