Download Mineralogic and Oxygen Isotopic Studies of Open

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. COMAGMAT: a FORTRAN program to model magma
Journal of Petrology 33, 1007-1038.
differentiation processes. Computers and Geosciencts 19(8), 1153—
Grant, N. K. & Moiling, P. A., 1981. A strontium isotope and
1170.
trace element profile through the Partridge River troctolite,
Bacon, C. R., 1989. Crystallization of accessory phases in magmas
Duluth Complex, Minnesota. Contributions to Mineralogy and
by local saturation adjacent to phenocrysts. Geochimica it
Petrology Tl, 296-305.
Cosmochimica Acta 53, 1055-1066.
Green, J. C , 1972. North Shore Volcanic Group. In: Sims, P. K.
Barnes, S. J., 1986. The effect of trapped liquid crystallization on
& Morey, G. B. (eds) Geology of Minnesota: a Centennial Volume. St
cumulus mineral compositions in layered intrusions. Contributions
Paul: Minnesota Geological Survey, pp. 294-332.
to Mineralogy and Petrology 93, 524-531.
Basaltic Volcanism Study Project (BVSP), 1981. Basaltic Volcamsm Irvine, T. N., 1975. Crystallization sequences in the Muskox
intrusion and other layered intrusions. II. Origin of chromitite
on the Terrtstrial Plantts. New York: Pergamon Press, 1286 pp.
layers and similar deposits of other magmatic ores. Geochimica et
Bhattacharji, S., 1966. Mechanics of flow differentiation in ultraCosmochimica Acta 39, 991-1020.
mafic and mafic sills. Journal of Geology 75, 101-112.
modal mineralogy, and mineral isotopic variations
are consistent with boundary-layer fractionation. A
parental magma <518O value of ~6-3%o is calculated
for Unit VII based on olivine and plagioclase values,
and is similar to that of several other large, layered
mafic intrusives.
1459
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 6
DECEMBER 1996
Irvine, T. N., 1977. Origin of chromitite layers in Mink ox intruNathan, H. D., 1969. The geology of a portion of the Duluth
sion and other layered intrusions: a new interpretation. Geology
Complex, Cook County, Minnesota. Ph.D. Thesis, University of
Minnesota, Minneapolis, 198 pp.
5, 273-277.
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REVISED TYPESCRIPT ACCEPTED JUNE 18, 1996