JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 3
B\GES 583-607
1996
W. P. MEURER* AND A. E. BOUDREAU
DEPARTMENT OF GEOLOGY, DUKE UNIVERSITY, BOX 90227, DURJ1AM, NC 27708-0227, USA
Petrology and Mineral Compositions of
the Middle Banded Series of the Stillwater
Complex, Montana
The two olivine-beating zones of the Middle Banded series of (1974), McCallum et al. (1980), Segerstrom &
the Stillwater Complex are characterized by an increase in the Carlson (1982) and Raedeke & McCallum (1984).
number of cumulus minerals with height In each, anorthosite The Stillwater Complex is one of the most intenand anorthositic troctolite dominate the lower part whereas oli- sively studied layered intrusions in the world. Much
vine gabbro and gabbronorite form much of the upper portions. of this interest results from the occurrence of a
Electron microprobe analyses of cumulus minerals indicate little variety of magmatic ore deposits in the lower third of
or no variation of average mineral compositions with height. In the complex (including Pt and Pd deposits, Cu—Ni
addition, no significant lateral variations in cumulus mineral sulfides and chrome ore).
compositions occur along 8 km of section. Plagioclase from
The igneous stratigraphy of the Stillwater
throughout these zones shows the complex, reverse and oscillatoryComplex can be divided into three series (Fig. 2): (1)
zpnation patterns also seen in plagioclasefrom the thick ( >500 the Basal series, which includes mafic dikes and
m) anorthosites that sandwich the zones. The data suggest that gabbroic rocks that make up the lowermost 100 m of
the entire Middle Banded series is genetically related and the complex (Zientek et al., 1985); (2) the Ultramafic
therefore requires models for the origin of the thick anorthosites series, composed of olivine, olivine-orthopyroxene
to also explain the olivine-bearing rocks between them. However, and
orthopyroxene cumulates with cumulus
textural features such as discordant troctolites, pegmatoids, chromite present locally (Raedeke & McCallum,
slump structures and variably developed mineral laminations, 1984); (3) the Banded series, which is composed of
and chemical features such as zpnation in clinopyroxene pro- rocks with cumulus plagioclase and is subdivided
duced by intergranular exchange with orthopyroxene and large into the Lower, Middle and Upper Banded series.
within-sample variations in the mg-number of olivine in lowAlthough there is general agreement about the three
olivine troctolites, indicate that significant modification of these
major series divisions, subdivisions within the
rocks by postcumulus processes has taken place, thereby obscurBanded series are not standardized. In this paper we
ing evidence of their genesis.
use the stratigraphic nomenclature of McCallum et
al. (1980). The Lower Banded series has received
KEY WORDS: Stillwater Complex; Middle Banded series; layered inconsiderable attention from geologists because it
trusions; mineral chemistry; postcumulus processes
hosts an economically significant platiniferous
horizon known as the J-M Reef. Olivine is present in
the Lower Banded series associated with the J - M
INTRODUCTION
Reef in Olivine-Bearing zone I and again just below
The Stillwater Complex is a layered ultramafic to the base of the Middle Banded series after being
mafic pluton of Late Archean age located in absent throughout most of the upper part of the
southwest Montana (Fig. 1). The complex is exposed Ultramafic series and the lower part of the Lower
for ~ 4 8 km along strike with an exposed thickness of Banded series (Fig. 2). The Middle Banded series is
>6-5 km (Page & Zientek, 1985). Results of large- the main subject of this paper and is discussed in
scale mapping of the complex have been reported by detail below. The Upper Banded series contains a
Hess (1960), Jackson (1961), Page & Nokleberg relatively thin olivine-bearing zone that is overlain
'Corresponding author.
Telephone: (919) 681-8169. Rix (919) 684-5833.
e-mail: [email protected] edu
© Oxford Univenity Press 1996
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 3
JUNE 1996
| | || Pifcmolc Cover
G^ibroic - (LBS. UBS)
Anocthoritk - (MBS)
Ultnmaflc - (UMS)
USGS topographic base
Contour Interval 250 ft
Fig. 1. General location map with detail map of the Banded series. Locationj of the Picket Pin Mountain (PPM), Eastern Contact
Mountain (ECM), Central Contact Mountain (CCM) and Western Contact Mountain (WCM) sections, in the Middle Banded series,
are indicated on the detailed topographic map. The Camp Lake section ij located on the southwest corner of Camp Lake.
throughout both zones. Despite the relatively constant average An content, individual grains are often
complexly zoned over 10% mole fraction An, and
variations of up to 20% are found within a standard
thin-section.
In comparison with the thick anorthosites, relatively little work has been done on OB-III and OBIV. McCallum et al. (1980) measured a stratigraphic
section through OB-III and OB-IV in the Contact
Mountain area. Foose (1985) mapped a portion of
the Contact Mountain area including OB-III and
OB-IV, at 1:5000. This map provides a sense of the
lateral continuity of the units but does not express
the small-scale complexities present in OB-III and
OB-IV.
The Middle Banded series differs from the Lower
Banded series and Upper Banded series in several
by a thick
plagioclase-orthopyroxene—augite
cumulate.
The Middle Banded series contains two thick
anorthosite zones, AN-I and AN-II, that sandwich
two olivine-bearing zones, OB-III and OB-IV (Fig.
2). The thick anorthosites have been described by
Hess (1960), McCallum et al. (1980), Scheidle
(1983), Salpas et al. (1983, 1984), Salpas (1985),
Czamanske & Bohlen (1990), Boudreau &
McCallum (1992), Haskin & Salpas (1992) and
Loferski & Arculus (1993). In summary, AN-I and
AN-II are very similar both chemically and texturally. They both contain an average of 90% plagioclase by volume, have plagioclase grains up to three
times more coarse grained than plagioclase in most
other rocks in the complex, and have an average An
content of 78 that remains approximately constant
584
MIDDLE BANDED SERIES, STILLWATER COMPLEX
the mg-number [Mg/(Mg + Fe)] of cumulus mafic
minerals with height in the Lower Banded series and
Upper Banded series, but no such trends have been
discerned in the Middle Banded series. To characterize and understand the internal complexities of
OB-III and OB-IV, the relationships between the
two thick anorthosites and OB-III and OB-IV, and
the relationship between the Middle Banded series
and the rest of the Stillwater Complex, we have
undertaken a detailed study of the olivine-bearing
zones of the Middle Banded series.
|
MEURER AND BOUDREAU
Meters
Gabbronorile-lll
6000-
4000-
Banded series
5000-
Middle Banded series
o.
Olivinc-bcaring V V
Anorthositc II
(AN-II)
Olivine-bearing IV
(OB-IV)
Olivine-besring III
(OB-III)
GEOLOGY AND
STRATIGRAPHY OF THE
MIDDLE BANDED SERIES
Anorthosite I
(AN-I)
Lower Banded series
Olivinc-bcanng II /
3000-
Gabbronoritc II
Nome II
Terminology and petrography
N
Olivinc-beanng I \
Gabbrooonle I
Norite I
2000-
1000-
o-
Ultramafic senes
Bronzitite
zone
Peridotitc
zone
Basal scries
Fig. 2. Subdivisions of the Stillwater Complex used in thii paper
and their approximate thicknejies [after McCallum et al. (1980)].
The nomenclature for cumulus rocks has been a
source of confusion and controversy (Irvine, 1982).
A system for naming cumulates on the basis of their
cumulus mineralogy is inherently a genetic classification but provides valuable information about the
rock modes and texture. In this paper we follow the
lead of McCallum et al. (1980) and name cumulates
based on their cumulus mineralogy (Table 1),
ignoring the cumulus mineral proportions in
applying rock names.
Determining whether or not a mineral is a
cumulus phase can be complicated by a number of
factors (McBirney & Noyes, 1979). We have ignored
chemical considerations and characterize minerals as
cumulus or postcumulus on the basis of grain morphology. Cumulus minerals (1) are assumed to be
primocrysts (i.e. thefirstminerals to crystallize), (2)
are equant and usually euhedral-subhedral, (3)
typically have at least one boundary in common
with another grain of the same mineral (not always
true for olivine in plagioclase-rich troctolites), and
(4) are found throughout the rock. Postcumulus
minerals (1) are anhedral and commonly subpoikilitic or poikilitic, (2) occur interstitial to
ways. In both the Lower Banded series and Upper
Banded series clinopyroxene appears after orthopyroxene in the stratigraphic sequence of cumulus
minerals, whereas the sequence is clinopyroxene followed by orthopyroxene in the Middle Banded
series. The mineral proportions in the Lower Banded
series and Upper Banded series are approximately
equal to those predicted for cotectic crystallization; Table 1: Cumulus mineralogy and rock names
in contrast, the Middle Banded series contains on
average 82% plagioclase by volume, well in excess of Cumulus mineral(s)
Rock type
the cotectic abundance of 60% (McCallum et al.,
1980). Much of this plagioclase is concentrated in Plagioclase
AnorthositB
AN-I and AN-II and in anorthosites and olivine- Plagioclase + olivine
Troctoltte
poor troctolites in the lower portions of OB-III and Plagloclase+dinopyroxene
Gabbfo
OB-IV. Gabbros with approximately cotectic Plagioctasa+cllnopyroxene + oSvine
Olivine gabfaro
cumulus mineral proportions occur in the upper Plagioclase+orthopyroxene+clinopyroxene
Gaboronorite
portions OB-III and OB-IV. There is a systematic Plagioclase+orthopyroxene+clinopyroxene + olivine Olrvinegabbronorite
decrease of both the An content of plagioclase and
585
JOURNAL OF PETROLOGY
VOLUME 37
cumulus minerals, and (3) make up < 15-20% of the
mode (in most instances much less). Based on these
criteria, plagioclase and olivine occur as cumulus
grains, quartz, apatite and ilmenite are postcumulus
minerals, and clinopyroxene and orthopyroxene may
be texturally of either type.
Cumulus plagioclase occurs throughout OB-III
and OB-IV and has an average maximum dimension
that ranges from 3 to 15 mm. The habit of the plagioclase varies from blocky, nearly equant crystals to
tabular, book-shaped crystals. In general, the higher
the proportion of plagioclase in a rock, the larger the
grain size and the more equant its crystal habit. The
most notable exceptions to this general rule occur in
thin (< 1—2 m) anorthositic layers located in sections
dominated by more mafic rocks. Plagioclase found in
rocks with 25-40% mafic minerals are typically 5-8
mm in the long dimension and tabular in habit.
Rocks that are highly laminated contain plagioclase
with the highest aspect ratios (Meurer, 1995).
Cumulus plagioclase grains are almost always free of
inclusions, although polymineralic inclusions of
clinopyroxene—ilmenite—apatite are found in a few
grains in ~30% of the samples examined. Loferski
& Arculus (1993) reported that this type of inclusion
is also found throughout the two thick anorthosites
that sandwich OB-III and OB-IV. These inclusions
have not been reported for rocks outside of the
Middle Banded series.
Olivine is the second most abundant cumulus
mineral in OB-III and OB-IV. In layered rocks it
generally occurs as 5—10 mm, subhedral, equant
crystals. In discordant bodies, where the olivine is
often much larger (>20 mm) it is 'ameboidal' in
shape and often contains inclusions of rounded plagioclase. Throughout much of the section, the olivine
abundance is relatively low (<10%). Thin layers
that contain as much as 60-70% olivine occur sporadically throughout OB-III and OB-IV, but these
layers cannot be correlated between sections and
most persist for <50 m along strike. Olivine grains
are generally partially altered along fractures to a
fine-grained mixture of magnetite and a serpentinegroup mineral. This alteration rarely makes up >30
vol % of most grains. In some locations olivine is
completely altered to medium-grained tremolite and
talc mixture surrounded by fined-grained chlorite.
The talc—tremolite—chlorite alteration assemblage
represents a higher-temperature alteration than does
the magnetite—serpentine alteration (Page, 1976).
Samples in which the olivine is converted to talctremolite-chlorite are referred to as pervasively
altered to distinguish them from the much more
common magnetite-serpentine alteration.
Both orthopyroxene and clinopyroxene occur as
NUMBER 3
JUNE 1996
cumulus grains in the Middle Banded series. Clinopyroxene joins the cumulus assemblage before
orthopyroxene, and occurs throughout most of OBIII but only in the upper half of OB-IV. Cumulus
orthopyroxene is restricted to the upper 100 m of
OB-III, and occurs only in the highest stratigraphic
levels of OB-IV but is not present everywhere along
strike in OB-IV. Postcumulus clinopyroxene and
orthopyroxene occur throughout OB-III and OB-IV
and are far more abundant than cumulus pyroxenes.
Although coarse exsolution is present in some pyroxene grains, the majority of both cumulus and
postcumulus pyroxene grains contain only fine-scale
exsolution. Some inverted pigeonite is found as
intercumulus grains in the anorthositic sections of
OB-IV but the majority of the low-Ca pyroxene in
OB-III and OB-IV crystallized as orthopyroxene.
Field petrography and stratigraphy
Methods
Three complete stratigraphic sections through OBIII and OB-IV and a portion of a fourth were measured and sampled in the Contact Mountain and
Picket Pin Mountain areas (Fig. 3). These sections
are named, from east to west, (1) Picket Pin
Mountain section, (2) Camp Lake section, (3)
Eastern Contact Mountain section and (4) Western
Contact Mountain section. The Camp Lake section
is a partial section covering only the upper 75 m of
OB-III and the lower 50 m of OB-IV. In addition to
these sections, the measured section presented by
McCallum et al. (1980) and Raedeke (1982) was
studied and sampled. This section is referred to as
the Central Contact Mountain section. The locations
of the stratigraphic sections are shown in Fig. 1. The
Eastern Contact Mountain section is the most complete overall but the Picket Pin Mountain section
provides the most complete coverage of the upper
portion of OB-IV. The lower portion of the Western
Contact Mountain section is well exposed, but much
of OB-IV in this section is not exposed. Exposure of
the Central Contact Mountain section is generally
good, although portions are covered and other portions contain fault repetition.
OB-III and OB-IV were studied by dividing individual sections into layers that were used to construct stratigraphic columns. The sections were
measured using a tape and compass. Samples were
taken as close to the center-line of the section as
possible, but 100-150 m on either side of each
section were studied. Three criteria were used to
define the layers used in constructing the stratigraphic columns: (1) layers must be laterally persistent for the entire width of the section studied (i.e.
586
MEURER AND BOUDREAU
90
70
50
30
MIDDLE BANDED SERIES, STILLWATER COMPLEX
10
onhopyroxene
900 m -
clinopyroxene
olivine
plagioclase
90
70
50
30
10
700 m -
700 m -
700 m -
500 m -
500 m -
500 m -
300 m -
300 m -
I
300 m -
m
100 m -
100 m -
100 m -
90
70
50
30
10
•A
Picket Pin Mountain Section
90
70
50
30
10
10
Western Contact Mountain Section
Eastern Contact Mountain Section
Fig. 3. Stratigraphic columni for Western Contact Mountain (WCM), Eaitem Contact Mountain (ECM) and Picket Pin Mountain
(PPM) lections. Columni indicate the variation in relative proportion! of cumulus mineraU with height. Tick marks on the left side of
the column indicate sample locations. Letters indicate locatiom of distinctive stratigraphic horizons: A, wispy-layered troctolite; B,
homogeneous troctolite; C, disrupted zone; D, plagioclase clots; E, foliated zone; F, mafic layer, G, pervasively altered troctolite; H,
AN-I.5; I,fine-scalelayering; J, Hesi' eggs; K, discordant troctolites.
200-300 m along strike); (2) layers must be at least
half a meter thick to be depicted in the stratigraphic
section; (3) layers can be either texturally or mineralogically distinct from the adjacent layers. To
establish the extent of lateral continuity between the
sections measured, marker units and layers were
chosen that are easily correlated from section to
section. In every section studied there are portions
where exposure is poor, and these are described as
'covered'. Covered intervals may include portions of
the stratigraphy used as marker units. A marker
layer is described as 'not exposed' when it is anticipated to occur in a portion of the stratigraphy that is
covered. A marker layer is 'absent' when the appro-
priate stratigraphic interval is exposed but the
marker layer is missing.
Olivine-Bearingzpru 111—petrography
Wispy-layered and homogeneous troctolite. T h e base of
OB-III is marked by two texturally distinctive and
continuous troctolite layers. It is sharp but irregular
and is denned by the first appearance of cumulus
olivine above AN-I. In the lower troctolite layer, the
olivine occurs in wispy, discontinuous layers and is
termed the 'wispy-layered troctolite'. These olivine
layers are no more than 2 cm in thickness and generally continue for <2~3 m along strike. They are
587
JOURNAL OF PETROLOGY
VOLUME 37
similar in appearance to cross-bedded sedimentary
rocks [also noted by Foose (1985) and McCallum et
al. (1985)]. The wispy-layered troctolite is overlain
by a 'homogeneous troctolite', which contains
^ 20% homogeneously distributed olivine. The
contact between these two layers is sharp and easily
recognized. The homogeneous troctolite is laminated, but the wispy-layered troctolite is not. The
thickness of these two layers varies considerably
along strike. In the Western Contact Mountain
section the combined thickness of these layers is
~ 100 m, but thins to ~ 2 5 m at the Eastern Contact
Mountain section. The upper contact of the homogeneous troctolite is sharp and regular but locally it
is cut by discordant troctolites.
Disrupted zone. Above the homogeneous troctolite
clinopyroxene joins the cumulus assemblage within a
zone of extensive layer disruption and discordant
troctolites termed the 'disrupted zone'. The disrupted zone is 15 m thick at Western Contact
Mountain, thickens to 30 m at Central Contact
Mountain, but is absent at the Eastern Contact
Mountain section. As noted above, the base of the
disrupted zone is both sharp and regular; however,
folding and stretching of layers becomes pronounced
within 3 m above the top of the homogeneous troctolite [see McCallum et al. (1980), fig. 4b].
Abundant blocks of anorthosite are present in the
disrupted zone. These blocks range in size from ~ 10
cm to nearly 1 m in long dimension and the mineral
lamination in the host rock wraps around them.
Small (1 m), discordant troctolites composed of 2530% ameboidal olivine and plagioclase are common
in the disrupted zone. Discordant troctolites cut
across the lamination of the host cumulates but also
finger out laterally, parallel to the layering. Discordant bodies composed predominantly of anorthosite that have an outer zone of troctolite are also
found in the disrupted zone.
NUMBER 3
JUNE 1996
Opx
Fig. 4. Thin-section sketch of plagioclase clot. [Note the blocky
habit of the plagioclase in the clot (lower left) and how the
tabular plagioclase of the host-rock wraps around the clot.]
foliation defined by the lamination of platy minerals
termed the 'foliated zone'. The plagioclase clots are
present throughout most of the foliated zone but
occur only sporadically below this zone. Cumulus
orthopyroxene occurs locally immediately below the
foliated zone but is prevalent in this zone. The plagioclase clots, cumulus orthopyroxene and strong
foliation make the rocks of the foliated zone unique
Plagioclase clots. Approximately 250 m above the in appearance, and it is readily identified in all strabase of OB-III, polycrystalline aggregates of plagio- tigraphic sections. Other features of the foliated zone
clase, 15—35 cm in long dimension, give the olivine include the presence of fractures lined with horngabbro a distinct appearance. These 'plagioclase blende, plagioclase—quartz dikelets with hornblende
clots' are often flattened in the plane of the mineral selvages, hornblende-bearing pegmatoids and myrlamination and the plagioclase in the clots is blocky, mekitic lenses. The top of the foliated zone is defined
unlike the very tabular habit of the host rock plagi- by a pair of texturally distinct layers. The lower of
oclase which wraps around the clots (Fig. 4). Most of the two is three to four times coarser grained than
the clots examined are monomineralic, although one other cumulates of OB-III and is termed the 'coarsecontains interstitial quartz. The plagioclase clots grained layer'. The upper layer has ^ 8 0 % orthooccur over a stratigraphic interval of 75-100 m in all pyroxene + clinopyroxene and is called the 'mafic
sections studied, are relatively uniform in size and layer'. The coarse-grained layer is a gabbronorite
are homogeneously distributed within a given except at the Western Contact Mountain section
where it is a olivine gabbronorite. An orthopyroutcrop but vary in abundance with height.
Foliated zone and mafic layer. The plagioclase clots oxene-rich layer 2 cm thick is present half a meter
first occur 30-50 m below a zone with a strong from the top of the coarse-grained layer and has a
588
MEURER AND BOUDREAU
MIDDLE BANDED SERIES, STILLWATER COMPLEX
minimum lateral persistence of >2 - 5 km. The mafic
layer marks the last occurrence of (1) cumulus
orthopyroxene in OB-III, (2) plagioclase clots and
(3) the well-developed foliation that defines the
foliated zone.
gabbro host. The lower portions of these bodies are
pipe like but they become concordant with irregular
lower contacts at the base of AN-II. The troctolite
bodies (1) are composed of 10-15% ameboidal
olivine, (2) may be zoned, containing up to 15%
clinopyroxene at their outer margins, compared with
30—40% clinopyroxene in the host, and may have
Olivine-Bearing zone IV—-petrography
cores of anorthosite, and (3) are not continuous
Pervasively altered troctolite and AN-I.5. The base of OB-along strike.
IV is defined by a 100-125 m thick anorthosite
termed 'AN-I.5'. As the name implies, this layer is Stratigraphy ofOlivine-Bearing zone III and Olivinevery similar to AN-I and AN-II both texturally and Bearing zone IV
chemically (see below). Like the thicker anorthosites,
AN-I.5 is three to four times coarser grained than An idealized stratigraphic section through OB-III
most other rocks in OB-III and OB-IV and lacks and OB-IV is shown in Fig. 5. Much of the lower
discernible mineral lamination. It is underlain by a portion of the idealized section is adopted from the
troctolite that contains pervasively altered olivine. Eastern Contact Mountain section; this is reflected
This type of alteration is observed in the discordant in the high abundance of anorthosite. The disrupted
troctolites immediately below AN-I in OB-II (Hess, zone is a local feature and is therefore omitted from
the section. Whereas the lower portions of OB-III in
1960) and AN-II in OB-IV but is otherwise rare.
the Eastern Contact Mountain and Western Contact
Fine-scale layering. The top of AN-I.5 is defined by
the first appearance of cumulus olivine above the Mountain sections differ, the upper portions of OBbase of OB-IV. The next 100-150 m of section are III in all measured sections are similar. The
composed of alternating anorthositic and troctolitic thickness of AN-I.5 differs slightly between the
layers with troctolitic layers becoming more Eastern Contact Mountain and Western Contact
dominant up-section. Three to five meters below the Mountain sections; however, the continuity of the
level where abundant clinopyroxene joins the exposure at the Eastern Contact Mountain section is
cumulus assemblage, there is a texturally distinctive better, so the Eastern Contact Mountain thickness is
zone composed of fine-scale layering of clinopyrox- used in the idealized section. The upper half of OBene i olivine in a plagioclase-rich matrix. This zone IV is not continuously exposed at any of the sections.
has been noted in the Western Contact Mountain, The upper part of OB-IV in the idealized section is
Central Contact Mountain and Picket Pin Mountain based on a judicious mixture of observations from
sections, but is not exposed at the Eastern Contact the measured sections, examination of float in
Mountain section. In some locations a single layer covered areas, and on the section presented for
may be defined by both olivine and clinopyroxene Central Contact Mountain by McCallum et al.
(the two minerals do not occur together—they (1980). Cumulus orthopyroxene is present near the
alternate along strike for sections of the layer). top of OB-IV at Central Contact Mountain but is
Olivine layers are thinner than clinopyroxene layers not present at the top of OB-IV at Picket Pin
Mountain. We have opted to include the cumulus
(0-75 cm vs 2 cm) and contain less plagioclase.
orthopyroxene in the idealized section to emphasize
Mess's Eggs. Most of the uppermost 50 m of OB-IV
is composed of olivine gabbro. Just above the fine- the similar progression of rock types observed in OBscale layering is a zone that contains unusual bimi- III and OB-IV.
neralic aggregates of olivine and plagioclase that
Hess (1960) described as egg shaped and are here
termed 'Hess's Eggs'. Hess's Eggs are ovoid in cross- MINERAL CHEMISTRY
section with olivine in the lower half of the egg and
plagioclase in the upper half. This texture is present Methods
in the Western Contact Mountain and Central Mineral compositions were determined using the
Contact Mountain sections but is not exposed in the JEOL 733 Superprobe at the University of
Eastern Contact Mountain section and is poorly Washington. Operating conditions for clinodefined, if present at all, at the Picket Pin Mountain pyroxene, orthopyroxene and olivine were 15 kV
section.
and 25 nA with a beam diameter of 10 fim. OperDiscordant troctolite. Discordant troctolites found 0—ating conditions for plagioclase were similar except a
5 m below the base of AN-II are irregularly shaped 20 fim beam diameter was used. Natural minerals
bodies that cross-cut the lamination in the olivine were used as standards and the raw data were
589
JOURNAL OF PETROLOGY
Height above
AN-I
Foliation
VOLUME 37
NUMBER 3
JUNE 1996
Textures
Proportions of
cumulus minerals
discordant Iroctolites
Hess's Eggs
fine-scale layering
orthopyroxene
clinopyroxene
olivine
plagioclase
AN-I.5
pervasively altered
Foliated Zone
plagioclase clots
-Homogeneous troclolite
Wispy layered troctolite
Fig. 5. Idealized stratigraphic section through OB-III and OB-IV showing variation in cumulus modes. The locations of distinctive
textural features are included. Rose diagrams of crystal orientations and associated alignment factors (0, completely random; 100,
perfectly aligned) are provided to give a sense of the mineral lamination in the cumulates (Meurer, 1995).
reduced using an on-line Bence-Albee (Bence &
Albee, 1968) correction scheme. All mineral compositions determined in this study were compiled by
Meurer (1995).
Plagioclase
More than 3000 microprobe analyses of plagioclase
were collected from over 100 samples from OB-III
and OB-IV. Representative analyses are presented in
Table 2. Cumulus plagioclase occurs throughout the
Middle Banded series and so potentially provides the
best means of assessing igneous processes. Analyses
were performed to characterize compositional variability from the scale of single grains, single thin-sections, outcrops and the entire thickness of the zones.
The data are discussed in order of increasing scale
(i.e. from zonation of grains to variations with stratigraphic height).
Compositional contour maps of 14 plagioclase
grains that range in maximum dimension from < 1
to 14 mm in length were constructed using spot
analyses checked against optical zonation. The
grains were contoured for weight percent FeO and
KQO and mole percent An. Most grains are concentrically zoned although incomplete zones are
common near the margins. Zonation patterns of FeO
and K2O content are inversely correlated, with FeO
generally increasing toward the margin. Variations
in FeO are only loosely correlated with An content,
but K2O generally varies inversely with An content.
The smallest variation in An content observed in a
mapped grain is four (from 74 to 78) and the largest
variation is 14 (from 64 to 78). Most grains have a
range in An of 10. Figure 6 shows a grain contoured
for An content and wt% K2O that has a reversely
zoned core, oscillatory zonation in the middle, and
patchy and incomplete zonation near the margin. Of
the grains that showed concentric zonation about
40% are normally zoned and 60% are reversely
590
Table 2: Selected electron microprobe analyses qfplagioclase
Sample
Type
Section
Height*
SiO2
AlaOj
FeO
CaO
Na2O
K2O
Sumt
Si
Al
Fe
Ca
Na
K
Sumt
An
92-M143I
PBAc
ECM
303
49-76
31-88
0-30
16-37
2-62
0-16
10008
2-271
1-715
0012
0-752
0-232
0009
4-992
76-4
92-M147
PAOc
ECM
316
4906
31-64
0-47
15-52
2-73
0-10
99-51
2-258
1-717
0-018
0-766
0-244
0006
5008
76-8
92-Ml 57
PAOc
ECM
613
49-57
32-30
0-45
1506
2-80
009
100-26
2-259
1-735
0017
0-736
0-248
0005
5000
74-8
92-M161
POc
ECM
518
49-11
31-80
049
15-14
2-82
0-10
99-45
2-259
1-725
0019
0-747
0-252
0006
5-007
74-8
92-M117
PAOc
ECM
154
48-97
31-57
0-41
15-63
2-71
008
99-35
2258
1-716
0016
0-772
0-242
0-005
5008
76-2
92-M116
Pc
ECM
144
49-89
31-49
0-33
15-37
2-91
009
10008
2-279
1-696
0013
0-753
0-258
0005
5004
74-5
91-M5
Pc
ECM
66
49-19
32-39
0-40
15-56
2-73
003
100-30
2-245
1-742
0015
0-761
0-242
0002
5006
75-9
91-M3
POc
ECM
20
48-16
32-19
0-44
16-20
2-34
008
99-41
2-223
1-752
0017
0-801
0-209
0005
5008
79-3
92-M99A
POc
PPM
556
48-99
31-86
0-41
15-62
2-64
009
99-59
2-252
1-727
0-016
0-769
0-236
0006
5005
76-5
92-M85I
PBAc
PPM
303
49-77
31-51
0-29
14-77
2-99
0-19
99-51
2-284
1-704
0011
0-726
0-266
0011
5003
73-2
92-M77
PAOc
PPM
216
48-76
32-30
0-39
15-89
2-35
0-10
99-78
2238
1-747
0O15
0-781
0-209
0006
4-997
78-9
92-M74
PAOc
PPM
209
48-81
32-25
0-43
1603
2-37
009
99-98
2-237
1-742
0017
0-787
0-211
0005
5000
78-9
92-M70
PAOc
PPM
192
48-78
32-54
0-45
15-76
2-54
008
100-13
2-232
1-755
0017
0-772
0-226
0005
5006
77-4
92-M2535
PBAc
CCM
257
49-29
31-55
0-34
15-45
2-65
0-18
99-44
2-268
1-711
0013
0-761
0-237
0010
5000
76-3
91-M49
PBAOc
CCM
333
48-20
32-30
0-38
15-84
2-33
008
99-13
2-228
1-759
0015
0-784
0-209
0005
5000
790
91-M42
POc
CCM
632
49-43
3203
0-52
15-86
2-65
006
100-56
2-253
1-721
0020
0-775
0-235
0004
5006
76-8
91-M36
PAOc
CCM
805
48-76
32-86
0-94
14-84
2-69
0-11
100-20
2-229
1-771
0036
0-727
0-238
0006
5008
76-3
91-M18
PAc
CCM
363
48-87
31-64
0-38
15-81
2-58
on
99-39
2-253
1-720
0-015
0-781
0-231
0006
5006
77-2
91-M9
PBAc
CCM
298
49-31
31-39
0-37
15-14
2-77
0-15
99-13
2-275
1-707
0014
0-748
0-248
0009
5000
75-1
91-M50
Pc
WCM
403
48-55
3205
0-43
16-47
2-23
007
99-80
2-232
1-737
0017
0-811
0-199
0004
5000
80-3
91-M51
POc
WCM
391
49-51
31-77
0-37
15-71
2-58
007
10001
2-265
1-713
0014
0-770
0-229
0004
4-995
77-1
91-M53
PBAc
WCM
298
48-48
32-11
0-42
15-48
2-51
0-12
99-12
2-240
1-748
0016
0-766
0-225
0007
5002
77-3
91-M58
PAOc
WCM
249
47-53
3304
0-57
17-27
1-77
004
100-22
2-183
1-788
0022
0-850
0-158
0002
5003
84-4
91-M63
POc
WCM
93
49-45
31-85
0-41
15-56
2-74
0-10
100-11
2-261
1-717
0016
0-762
0-243
0006
6006
76-8
'Above base of OB-MI. tSum of oxide weight percents. tSum of cations normalized to 8 oxygens. SMafic layer. WCM, Western Contact Mountain; CCM, Central Contact Mountain; ECM,
Eastern Contact Mountain; PPM, Picket Pin Mountain. PC, anorthosite; POc, troctolite; PAc, gabbro; PAOc, olivine gabbro; PBAc, gabbronorite.
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 3
JUNE 1996
0 nun
An
Hi 7 7 -79
^ B 75 -77
I I 73 -75
^^u 71 -73
1
169 -71
1 1 67-69
| ' ' 1 65-67
-65
• «
Fig. 6. Contour maps of An content and weight percent K3O in a plagioclase grain from sample 92-M150 (an olivine gabbro from OBIII). Locations of microprobe analyses are indicated by the black dots. Contours are drawn to be consistent with both compositional
data and optical zonation. Inset figure shows relationship of the mapped grain to surrounding grains.
zoned. Many grains have high-An, incomplete outer
zones. These high-An rims are observed on both
normally and reversely zoned grains.
Scheidle (1983) and Czamanske & Scheidle
(1985) reported on the compositional variability of
plagioclase grains from both AN-I and AN-II. Many
of the features they reported are observed in the
plagioclase throughout OB-III and OB-IV. These
include (1) normal, reverse, oscillatory and irregular
zonation, (2) high-An rims, (3) incomplete outer
zones, (4) truncated zones (as if by grain breakage),
and (5) variability of zonation types within a single
thin-section (e.g. normally and reversely zoned
grains are found in the same thin-section). Czamanske & Scheidle (1985) noted that where plagioclase is in contact with quartz incomplete zones with
lower An content are present, suggesting continued
growth of the plagioclase from trapped liquid. In one
grain we mapped that is in contact with quartz the
relations are not as straightforward. Figure 7 shows
that the zone in contact with the quartz is nearly
complete, but additional incomplete zones with
higher An are present on the other side of the grain.
The plagioclase that makes up the monomineralic
clots in OB-III and the plagioclase from the rocks
that host these clots were analyzed to determine if
they are compositionally distinct. We found that the
average An and K2O contents of the plagioclase
clots are indistinguishable from those of the host
rocks. However, the plagioclase in the clots does
have an average of 0-1 wt% more FeO than that in
the host rock (which typically contain 0-3-035 wt%
FeO). Mapping of plagioclase grains in the clots
shows complex and variable zonation and high-An
rims to be present.
The variation of the average An content of plagioclase in an outcrop is small compared with withinsample variation. Figure 8 depicts the variation of
An with stratigraphic height for a continuously
exposed portion of the Western Contact Mountain
592
MEURER AND BOUDREAU
MIDDLE BANDED SERIES, STILLWATER COMPLEX
An
SO
s<x<80
76£x<78
74<x<76
72<x<74
<72
5 mm
Fig. 7. Contour maps of the An content and weight percent K 2 O and FeO in a plagiodase grain from sample 92-M159 (an anorthoiite
from AN-I.5 at the base of O B - I V ) . Locations of microprobe analyses are indicated by the blade dots. Contours are drawn to be consistent with both compositional data and optical zonation. Inset figure shows relationship of the mapped grain to surrounding grains.
325-
—©
225
.'•>£• » ^ >
—I—
•«»
—i—
75
An
80
85
90
Fig. 8. Detail of the variation of An content in plagiodase with height from the Western Contact Mountain section. Averages are indicated by large circles and individual analyses are represented by smaller squares. A detail from the stratigraphic section showing the
cumulus mineral proportions is included.
section in OB-III. Individual analyses are included
in the plot to emphasize their large variability
(An = 70—88) compared with the variability of the
sample averages (An = 74-5—80). The average analyses cluster around 77, which is the average value of
An for plagiodase in OB-III, and do not define a
trend.
593
Figure 9 shows An content vs stratigraphic height
for the Picket Pin Mountain, Eastern Contact
Mountain, Central Contact Mountain and Western
Contact Mountain sections. An content may
decrease modestly with height through OB-III but
no large-scale variation of An with height is found in
OB-IV. A decreasing trend in OB-III is suggested by
JOURNAL OF PETROLOGY
8OO-1
PPM
AN-n
AN-n
• ••
•
OB-rv
•
•
OB-ra
o-
CCM
AN-II
81
OB-IV
9-:
OB-ni
100
•
• •
71
75
•
oUvine — J
"
••
400
•
1
°
77
WCM
OB-IV
plagtoclue
o
200-
isd§
c
•
r.
OB-III
•
•
OB-IV
•a
f
nafic layer
-T»-l
800-
±
•
•
77
: .
600
400-
75
. FeO
o KjO
•
•
•
AN-D
•
300-
e.
ECM
1
•
•3
JUNE 1996
NUMBER 3
OB-IV
700
r 300
•a
VOLUME 37
•
400-
0.0
OB-UI
75
77
An
75
77
An
Fig. 9. Plot of An content vs height for Picket Pin Mountain
(PPM), Eastern Contact Mountain (ECM), Central Contact
Mountain (CCM) and Western Contact Mountain (WCM)
section!. Averages of approximately 15 analyses per sample are
plotted.
data from the Eastern Contact Mountain and Picket
Pin Mountain sections. However, the wide scatter of
average values of An at any given height and the
normative whole-rock An contents (Meurer, 1995)
argue against the presence of any clear stratigraphic
trends. The range of average values of An from OBIII (~6 mol% An) is larger than that for OB-IV
( ~ 3 m o l % An).
Lateral variations in plagioclase composition can
be evaluated by comparing the average compositions from distinctive layers in each of the sections.
Analyses of plagioclase from both the mafic layer
and the coarse-grained layer at the Western Contact
Mountain, Central Contact Mountain, Eastern
Contact Mountain, Camp Lake and Picket Pin
Mountain sections show modest variations along
strike. Going from east to west, the An content of
the coarse-grained layer is 75-3-75-0-76-1-P-76-0,
and that of the mafic layer is 74-8-74-9-75-5-74-4/
75-9—76-8. The plagioclase analyses from two
samples from a single outcrop of the mafic layer in
the Central Contact Mountain area differ by 1-5
mol % An. The variations within the layers are 1-1
and 2-0 for the coarse-grained and mafic layers,
respectively, and are of the same order as the variation between these two samples and are not
deemed significant.
The K2O and FeO contents of plagioclase from
0.4
Weight %
0.6
Fig. 10. Weight percent K 2 O a n d F e O in plagioclase from the
Eastern Contact M o u n t a i n section is plotted against stratigraphic
height. T h e analyses of the sample from t h e mafic layer a r e
connected to emphasize its distinctive trace-element composition.
Variation of cumulus mineral proportions with height is also
included.
•
n-
0.2
OB-III and OB-IV show no systematic vertical or
lateral variations. However, plagioclases from the
mafic layer have K 2 O and FeO contents that differ
from those for other samples although the An
content is similar. Figure 10 shows the variations of
K 2 O and FeO in plagioclase from the Eastern
Contact Mountain section. Plagioclase in the mafic
layer has a distinctly higher K 2 O content and
notably lower FeO content. Average plagioclase
from OB-III and OB-IV has =$010 wt % K 2 O and
~0-46 wt % FeO. Plagioclase from the mafic layer
from all sections has average K 2 O of 0-17 wt % and
FeO of 0-35 wt%.
Pyroxenes
Over 850 microprobe analyses of clinopyroxene and
over 300 analyses of orthopyroxene were collected
(Tables 3 and 4 contain representative analyses),
and these data are discussed in order of increasing
scale.
Five clinopyroxene grains with an average size of
15 mm2 were mapped using 60-100 analyses per
grain. Three of these grains show no discernible
zonation and none have significant postcumulus
overgrowths characterized by lower mg-number
values [Mg/(Mg +Fe)]. Two zoned grains show
consistent increases in m^-number, wt % CaO and
wt% TiO 2 from the core to the rim (e.g. Fig. 11).
Projection of the clinopyroxene analyses used to
construct Fig. 11 onto the graphical thermometer of
594
MEURER AND BOUDREAU
MIDDLE BANDED SERIES, STILLWATER COMPLEX
Table 3: Selected electron microprobe analyses ofclinopyroxene
Sample:
91-U46
92-M253* 92-M259* 92-M157 92-M146t 92-M143*
91-M25
92-M204
91-M51
92-M99A
Type:
PAOc
PBAc
PBAc
PAOc
PBAc
PBAc
PAOc
PAOc
POc
POc
PAOc
Section:
CCM
CCM
CL
ECM
ECM
ECM
ECM
WCM
WCM
PPM
PPM
Height*
385
357
627
301
303
138
290
391
557
198
52-74
52-44
51 37
52-71
52 18
52-24
5208
51-43
5292
0-43
0-32
0-80
0-32
0-35
0-51
0-49
0-51
0-35
1-72
1-92
1 -71
208
206
1-73
0-15
0-10
0-08
0-13
0-18
643
SI0 2
TiO 2
51-85
52-58
0-78
0-41
92-M70
AI2O3
2-26
1-63
1-69
1-78
2-05
Cr2O3
008
0-11
0-11
0-13
009
0-13
FeO
723
6-95
704
8-54
7-45
608
6-73
5-92
7-35
6-20
0-21
0-17
0-18
0-18
0-20
16-88
1606
15 87
15-63
16-35
MnO
0-19
0-19
0-19
0-21
0 19
0-18
MgO
15-37
1701
17-19
17-62
15-24
16-18
NiO
004
004
001
003
004
002
0 05
006
003
004
0-06
22 38
21-09
2286
21-69
22-71
21-80
CaO
22-42
2086
20-76
18-58
21-92
Na2O
0-13
007
007
001
009
0-14
0-26
009
0-39
007
001
99-22
99-82
99-82
99-70
100-22
98-94
99-99
SumS
Si
100-35
99-81
100-20
99-62
1 918
1-941
1-939
1-940
1-922
1-946
1-929
1-934
0 012
0009
0-010
0014
1-926
1-923
1-949
Ti
0022
0011
0023
0009
0-014
0014
0-010
Al
0099
0071
0073
0078
0091
0075
0-084
0075
0091
0090
0O75
Cr
0002
0-003
0003
0004
0003
0004
0-004
0003
0002
0004
0-005
0-264
0-233
0-188
0-208
0-183
0-227
0-194
0-198
Fe
0-224
0-215
0-217
Mn
0006
0006
0006
0006
0006
0005
0007
0005
0006
0006
0006
Mg
0-848
0-936
0-942
0-972
0-850
0-891
0-931
0-887
0-876
0-871
0898
Ni
0001
0001
0000
0001
0001
0000
0-002
0002
0001
0001
0-002
0-885
0-836
0-907
0-860
0-910
0860
Ca
0-888
0-825
0-818
0-737
0-879
Na
0003
0003
0000
0002
0003
0001
0004
0004
0002
0003
0-004
Sumll
4001
4003
4001
4003
4001
3-995
4002
4006
3-995
4008
3-996
Wo
45-3
41-8
41-4
37 3
44-8
45-1
42-3
45-9
43-8
46-1
440
En
43-3
47-4
47-7
493
43-3
45-4
47-1
44-9
44-6
44-1
45-9
Fs
11-4
10-9
110
13-4
11-9
96
10-5
9-3
11-6
98
10-1
mg-no.
79-1
81-4
81-3
78-6
78-5
82-6
81-7
82-9
79-4
81-8
81-9
'Mafic layer. tCoarse-grained layer. tAbove base of OB-III. §Sum of oxide weight percents. MSum of cations normalized to 6 oxygens.
WCM, Western Contact Mountain; CCM, Central Contact Mountain; ECM, Eastern Contact Mountain; PPM, Picket Pin Mountain. POc,
troctolite; PAOc, olivine gabbro; PBAc, gabbronorite.
Lindsley & Anderson (1983) indicates that minimum
crystallization temperatures for core compositions
are between 1000 and 1100°C but rim compositions
yield submagmatic temperatures between 600 and
800°C.
No systematic variation of /rig-number in clinopyroxene with height is found in either OB-III or
OB-IV (Fig. 12). Cumulus clinopyroxene is absent
throughout much of OB-IV, and therefore it is not
meaningful to evaluate large-scale compositional
variations of clinopyroxene in OB-IV. The average
values of the mg-number from each stratigraphic
595
section in OB-III range from 80-5 to 81-5, and
average values for OB-IV range from 79-0 to 81-5.
Analysis of intercumulus clinopyroxene in three
samples from Eastern Contact Mountain shows them
to have substantially lower average m^-number
values (average 74) than the cumulus clinopyroxene
(average 81). Meurer & Boudreau (1993) reported
on the lack of variation with height of trace elements
(e.g. TiO2, A12O3, Cr2O3> NiO, Na2O) in cumulus
clinopyroxene. Their data are primarily from the
Central Contact Mountain section; however, additional data from Western Contact Mountain,
JOURNAL OF PETROLOGY
VOLUME 37
JUNE 1996
NUMBER 3
Table 4: Selected electron microprobe analyses qforthopyroxene
Sample:
91-M93
92-M85'
92-M83
92-M26OT 92-M146t 92-M143* 92-M142
91-M36
92-M253*
92-M214
Type:
POc
PBAc
PBAc
PBAc
PBAc
PBAc
PBAOc
PAOc
PBAc
PAOc
PAOc
Section:
PPM
PPM
PPM
CL
ECM
ECM
ECM
CCM
CCM
WCM
WCM
Height*
539
303
294
301
303
284
806
257
316
276
92-M203
SiO2
54-52
54-79
54-20
53-89
53-86
55-12
54-49
53-61
54-64
54-18
54-35
TiO 2
0-24
0-21
0-25
0-28
0-39
0-26
0-21
0-29
0-20
0-24
0-23
AI 2 O 3
0-92
0-90
102
0-99
1-00
0-89
1-12
0-91
0-83
104
0-99
Cr 2 O 3
FeO
005
007
007
005
0-05
0-07
007
004
007
007
0-07
13-99
13-76
13-62
15-80
16-76
14-59
14-45
16-23
14-42
14-20
15-17
MnO
0-29
0-26
0-32
0-36
0-33
0-31
0-31
0-27
0-33
0-31
0-30
MgO
27-99
28-39
28-17
26-55
26-37
28-26
27-55
27-22
2835
26-92
27-70
NiO
006
003
004
007
0-04
0-05
006
003
005
003
004
CaO
1-69
1-58
1-60
1-41
1-52
0-99
1-92
1-13
0-96
2-22
1 -31
Na2O
001
001
001
001
001
001
001
000
001
001
001
SumS
99-72
99-96
99-25
99-38
100-29
100-51
100-14
99-73
99-82
99-18
100-14
Si
1-963
1-965
1-958
1-963
1-954
1-969
1-959
1-950
1-966
1-967
1-958
Tl
0006
0-006
0007
0008
0011
0007
0006
0008
0005
0006
0006
Al
0039
0038
0044
0-043
0043
0-038
0048
0039
0036
0045
0-042
Cr
0001
0002
0002
0001
0-0O1
0-O02
0002
0-001
0002
0-002
0002
Fe
0-421
0-413
0-411
0-482
0-508
0-436
0-434
0-494
0-434
0-431
0-457
Mn
0009
0008
0010
0011
0-010
0010
0010
0008
0010
0010
0009
Mg
1-503
1-518
1-518
1-442
1-426
1-505
1-477
1-477
1-621
1-457
1-488
Ni
0002
0001
0001
0002
0001
0-001
0002
0001
0001
0001
0-001
Ca
0065
0061
0062
0055
0059
0-038
0074
0044
0037
0086
0-051
Na
0004
0-002
0002
0005
0-002
0003
0-0O4
0002
0003
0002
0002
Sumll
4004
4002
4004
3-999
4-005
3-997
4004
4015
4-002
3-995
4006
Wo
3-3
3-1
3-1
2-8
30
1-9
3-7
2-2
1-9
4-4
25
En
75-5
76-2
76-2
72-9
71-5
76-1
74-4
73-3
76-3
73-8
74-6
Fs
21-2
20-7
20-7
243
25-5
220
21-9
24-5
21-8
21-8
22-9
mg-x\o.
78-1
78-6
78-7
750
73-7
77-5
77-3
74-9
77-8
77-2
76-5
'Mafic layer. tCourse-grained layer. tAbove base of OB-III. §Sum of oxide weight percents. llSum of cations normalized to 6 oxygens.
WCM, Western Contact Mountain; CCM, Central Contact Mountain; ECM, Eastern Contact Mountain; PPM, Picket Pin Mountain. POc,
troctolite; PAOc, olivine gabbro; PBAc, gabbronorite.
Eastern Contact Mountain and Picket Pin Mountain
also indicate little or no variation. Along-strike variation of average m£-number values in cumulus
clinopyroxene is assessed using analyses from marker
layers. Values of m^-number in the mafic layer going
from east to west (i.e. Picket Pin Mountain—Camp
Lake—Eastern Contact Mountain-Central Contact
Mountain-Western Contact Mountain) are 82-881-5-828-82-4-810, giving a total range of 1-8,
which is not significantly more than the range
expected based on the average of the standard
596
deviation from individual analyses of ±0-84 for mgnumber values in clinopyroxene. The coarse-grained
layer has the similar limited variability in mgnumber along strike (from east to west: 79-6— 79-5—
78-7-P-77-8).
Cumulus orthopyroxene occurs only over a short
interval in both OB-III (20-25 m) and OB-IV (<10
m), although postcumulus orthopyroxene is ubiquitous in both OB-III and OB-IV. Within-grain
variability of cumulus orthopyroxene is small
(~0'5 m£-number) and mapping of three cumulus
MEURER AND BOUDREAU
MIDDLE BANDED SERIES, STILLWATER COMPLEX
AN-II
PPM
ECM
•
600-
•
OB-IV
400"
OB-IV
200"
oB-m
OB-ni
•
•
0-
76
AN-II
m •
m
D
80
84
72
78
CCM
80
8-
WCM
OB-IV
OB-rv
300"
OB-III
•
• j
.•"
J.
OB-III
•
.
20072
78
80
mj-no. in cpx
84
72
76
80
mg-no. in cpx
84
Fig. 12. Plot of m^-number in clinopyroxene vs height for Picket
Pin Mountain (PPM), Eastern Contact Mountain (ECM),
Central Contact Mountain (CCM) and Western Contact Mountain (WCM) sections. Averages of approximately nine analyses
Fig. 11. Contour maps of the mg-number and weight percent CaO
per sample are plotted.
and TiOj in a dinopyroxene grain from lample 92-M142 (an
olivine gabbronorite from the foliated zone in OB-III). Locations
of microprobe analyses are indicated by the black dots. Contours magmatic and reflect intergranular exchange as disare drawn to be consistent with compositional data; no optical cussed below.
zonation is observed. Inset figure shows relationship of the
mapped grain to surrounding grains.
Olivine
Olivine is found throughout OB-III and OB-IV
grains indicates that they are not zoned. Post- except in AN-I.5. Representative analyses of the
cumulus orthopyroxene m^-number values vary from >200 olivine analyses are presented in Table 5.
72-0 to 78-5 whereas those for cumulus orthopyr- Average mg-number values from OB-III range from
oxene range from 73-5 to 79-5. Values of mg-number 69 to 76 whereas those from OB-IV show conin the mafic layer, going from east to west, are 78-7— siderable variability ranging from 64 to 75 (Fig. 14).
77-8-77-7-78-5-78-4, giving a total range of only Olivine from near the base and top of OB-III have
1-0. The coarse-grained layer has a slightly larger lower average m^-number values and larger withinrange in m^-number along strike, again from east to sample variability (range in m£-number values is
west: 76-5-75-2-74-2-?-74-2. The average standard five) than do other samples from OB-III (range in
deviation of mg-number in orthopyroxene is ± 0-64, mg-number values <2). The lower average mgwhich is more than the variability seen in the mafic number values may reflect postcumulus equilibration
layer and only slightly less than that in the coarse- with trapped liquid. This effect should be more siggrained layer. Thus lateral variability in m^-number nificant for rocks with low mafic mineral contents
in clino- and orthopyroxene is deemed minimal.
(Barnes, 1986), as is the case for these samples.
Projection of pyroxene compositions onto the graOlivine from the Middle Banded series typically
phical thermometer of Lindsley & Anderson (1983) contains <005 wt% CaO and Cr2O3. The only
shows that the bulk of the clinopyroxene composi- trace elements present in significant amounts are
tions are consistent with equilibration at tempera- NiO and MnO, which are negatively correlated in
tures between 700 and 900°C and orthopyroxene most samples. The NiO content remains essentially
compositions give similar or slightly lower tempera- constant in the lower half of OB-III and then
tures (Fig. 13). These temperatures are too low to be declines modestly in the upper half as depicted in
597
JOURNAL OF PETROLOGY
VOLUME 37
Wo
NUMBER
JUNE 1996
Wo
_
, . ^ , . .
,
T
. . .
Fs
Fig. 13. Clinopyroxene and orthopyroxene compositions for all analyses projected onto the Wo—En—F» ternary (wollastonite—enstatiteferrosilite). The compositional fields for clinopyroxene and orthopyroxene from individual sections, defined in the ternary plots, are
shown on the pyroxene quadrilateral contoured for temperature (Iindsley & Anderson, 1983).
Fig. 14 but no stratigraphic trends are found in OBIV.
800-
olivinc —- ^M
^^^J
^^^^1 *
600"
1
°
p
o
o
DISCUSSION
a
D
°
o°
plagiociasc
i
* 400-
200-
o PPM
OB
-'V
1 * WCM
«=!« — •
%
J
o-
o
o
1 a ECM
c^
o
^^^^B
66
8
•
a
A AAA
_ a a
71
76
mg-no.
0.1
0.3
NiO
Fig. 14. Compoiite plot of mj-number and NiO in olivine vs
height for Picket Pin Mountain (PPM), Eaitem Contact Mountain (ECM), Centra] Contact Mountain (CCM) and Western
Contact Mountain (WCM) sectioni. Averages of approximately
seven analyies per sample are plotted.
598
Before the cumulus and postcumulus processes that
resulted in the textures and compositions preserved
in OB-III and OB-IV can be understood, an adequate description of the vertical and lateral variations in these units is required. This study is
primarily directed at characterizing these units and
assessing the vertical and lateral variations.
However, preliminary interpretations of the data arc
useful as a means of directing and stimulating discussion and investigation of some of the proposed
processes. In the following sections we consider the
origin of some of the textural and chemical features
observed in these zones. The discussion is organized
in terms of increasing size of the features (i.e. from
thin-section scale to the scale of the entire Middle
Banded series). The treatment of individual features
is not meant to be exhaustive; rather, we present
preliminary interpretations that are consistent with
our data and the results of previous work.
37-97
37-38
350
385
356
ECM
ECM
ECM
CCM
CCM
CCM
WCM
WCM
POc
PAOc
POc
PAOc
PAOc
PBAOc
PAOc
PBAOc
POc
PAOc
91-M3II
92-M125
92-M162
92-M157
91-M37
91-M16
91-M46
91-M54
91-M62
92-M196
93
297
37-86
38-30
37-82
38-30
37-99
627
808
37-66
38-66
37-50
36-63
38-36
38-71
37-86
32-38
37 02
36-89
34-11
37-69
38-18
38-87
MgO
22-94
21-53
22-95
25-26
23-47
22-39
24-87
3001
24-43
24-57
28-55
23-30
22-89
22-32
FeO
0-26
0-28
0-21
0-27
0-18
0-20
0-19
0-21
0-26
0-36
0-26
0-27
0-27
0-29
NiO
0-30
0-29
0-27
0-33
0-28
0-35
0-33
0-46
030
0-31
0-35
0-30
0-33
0-29
MnO
99 02
9906
98-31
100-46
100-11
99-94
101-25
99-50
99-92
10006
100-30
99-39
99-70
99-78
Sumt
0-999
1003
0-995
0-998
0-990
0-998
0-989
0-993
0-998
0-998
0-992
0-997
0-997
0-993
1-483
1-510
1-488
1-436
1-497
1-504
1-469
1-315
1-453
1-448
1-363
1-481
1-492
1-514
Mg
0-506
0-472
0-511
0-555
0-514
0-488
0-541
0-684
0-538
0-541
0-640
0-513
0-502
0-488
Fe
0006
0006
0005
0006
0004
0004
0004
0005
0006
0008
0006
0006
0006
0006
Nl
0-007
0006
0006
0007
0006
0008
0007
0011
0007
0007
0008
0007
0007
0007
Mn
3001
2-997
3005
3002
3010
3002
3011
3007
3002
3002
3008
3003
3007
3003
Sum*
74-5
76-2
74-4
72-1
74-5
75-5
73-1
65-8
730
72-8
68-1
74-3
75-6
74-8
mg-no.
"Above base of OB-III. tSum of oxide weight percents. tSum of cations normalized to 4 oxygens. § Wispy-layered troctoiite. IlHomogeneous troctoiite. WCM, Western Contact Mountain;
CCM, Central Contact Mountain; ECM, Eastern Contact Mountain; PPM, Picket Pin Mountain. POc, troctoiite; PAOc, olivine gabbro; PBAOc, olivine gabbronorite.
WCM
ECM
36-45
37-91
37-93
3703
493
253
20
5
37-83
ECM
POc
661
PPM
PAOc
3803
91-M2S
3801
325
92-M101
PPM
210
PAOc
PPM
PAOc
SiO2
92-M89
Height*
92-M75
Section
Typo
Sample
Table 5: Selected electron microprobe analyses of olivine
bd
d
d1
tw
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 3
JUNE 1996
thereby lowering the plagioclase liquidus temperature.
Although some of the high-An partial rims may
Plagioclase habit
have formed by partial dissolution, the concentric
The habit of the plagioclase varies from blocky and nature of other high-An partial rims (such as
equant in plagioclase-rich layers to tabular in layers depicted in Fig. 7) suggests they did not form as a
with 20% or more mafic minerals. The upper consequence of selective dissolution (which would be
portion of the Sept lies layered intrusion is char- localized at high stress contact points between
acterized by zones with tabular, well-laminated grains). If the quartz depicted in Fig. 7 crystallized
anorthosites in massive anorthosite with blocky pla- before the growth of the incomplete high-Anrimon
gioclase. Higgins (1991) suggested that the well- the opposite side of the grain, then isolated pockets
laminated anorthosites formed as a consequence of of liquid were crystallizing while interstitial liquid
shearing within the boundary layer in which the was flowing through the cumulates. An alternative
crystals were growing. A shear event that could explanation for the relations depicted in Fig. 7 is
influence the shape of the crystals should also that the quartz was deposited by fluids that resorbed
produce a lineation (e.g. Brothers, 1964; Benn & part of the plagioclase grain before precipitating the
Allard, 1989). However, there is no significant quartz. We favor the former interpretation because
lineation in either the laminated anorthosites from the compositional zone in contact with the quartz is
the Sept lies layered intrusion or the rocks of OB-III also in contact with other plagioclase grains. Noneand OB-IV.
theless, both models require that late-stage liquidBecause no lineation is observed, we favor an fluid flow through the cumulates be spatially
alternative model that involves crystal growth restricted: in the former case to produce the high-An
under uniaxial stress to explain the tabular vs rim opposite the quartz, and in the latter case to
blocky habits of the plagioclase. If AN-I, AN-I.5 resorb part of the high-An rim and deposit the
and AN-II formed by accumulation of plagioclase quartz.
suspended in the magma column and not at the
More dramatic evidence of the influence of latefloor of the chamber as has been proposed (e.g. stage liquids-fluids on rock textures and minerRaedeke, 1982), then the blocky habit of their pla- alogies is provided by the discordant troctolites and
gioclase reflects growth under relatively isostatic pegmatites. The textures of the troctolites indicate
stress conditions. Plagioclase with a tabular habit that they formed by metasomatic replacement of
may have grown in a uniaxial stress field as might host olivine gabbro (McCallum et al., 1980;
be expected in the (compacting) crystal pile. McBirney & Sonnenthal, 1990). The textures and
Unfortunately, experimental studies of plagioclase relations of pegmatites from the foliated zone are like
growth habits (e.g. Corrigan, 1982; Lofgren, 1983; those described by Braun et al. (1994) for pegmatites
Kouchi et al., 1986) have not dealt with habits pro- below the J-M reef, interpreted to have formed by
duced under uniaxial stress fields. In the absence of recrystallization of host cumulates by channelized
definitive experimental results, we suggest that the fluid flow.
habit of the plagioclase may provide insight into the
relative stress field under which plagioclase grain Postcumulus reequilibration
growth occurred.
The subsolidus temperatures recorded by rims of the
zoned clinopyroxene grains and coexisting orthoEvidence for late-stagefluid flow
pyroxene reflect intergranular exchange between
Several models have been proposed for the origin of clinopyroxene and orthopyroxene. During cooling,
the high-An rims on cumulus plagioclase. Cza- exchange between ortho- and clinopyroxene
manske & Scheidle (1985) suggested that high-An enriched the rims of the clinopyroxene in CaO at
rims on plagioclase may be a product of pressure- the expense of MgO and FeO. The higher mginduced dissolution along grain margins that would number at therimin the clinopyroxene is consistent
effectively concentrate the anorthite component. with this exchange because clinopyroxene has a
Morse & Nolan (1984) described strongly reversed higher K]-, for Mg than does orthopyroxene [see
zonation in cumulus plagioclase from the Kiglapait. Raedeke (1982)].
The distribution of these rims indicates that they
formed from the last liquid to crystallize. Wager &
Brown (1967) suggested that the reverse rims on Internal evolution of Olivine-Bearing
plagioclase from the Bushveld formed when the zones HI and IV
partial pressure of water increased in the magma The evolution of OB-III and OB-IV is discussed in
Cumulus and postcumulus crystallization
features
600
MEURER AND BOUDREAU
MIDDLE BANDED SERIES, STILLWATER COMPLEX
order of increasing stratigraphic height from the base
of OB-III to the top of OB-IV. However, this
sequence may not represent the sequence of crystallization. As discussed below, there is evidence that
the thicker anorthosites (AN-I.5 and AN-II) were
crystallizing before some of the underlying cumulates.
The sharp textural contrast between the wispylayered and homogeneous troctolites that mark the
base of OB-III may reflect deposition under two
different dynamic regimes. The segregation of olivine
in the wispy-layered troctolite could have been produced by deposition from crystal-laden currents or
by postcumulus slumping of the olivine—plagioclase
mixture. We favor the former interpretation because
of the regular nature of the texture, its widespread
occurrence and the planar upper contact of this
layer. Calculation of settling velocities shows that if
the olivine crystals were even a tenth the size of the
plagioclase crystals they could readily segregate
during deposition because of their greater density
contrast with the liquid (e.g. Irvine, 1980). In either
case, the discontinuous nature of the olivine-rich
layers in the wispy-layered troctolite is a consequence of deposition (or consolidation) in a relatively dynamic system. The homogeneous troctolite
reflects accumulation and crystallization under more
quiescent conditions.
Field relations suggest that the disrupted zone
was formed by slumping of partially consolidated
layers. Anorthosite blocks of similar size are found
at the same stratigraphic level in the disrupted
zone and are consistent with the overstretching of
an anorthositic layer as suggested by Foose
(1985). However, other blocks are too widely
scattered both laterally and vertically to be interpreted as disrupted anorthositic layers and suggest
that portions of the section were jumbled during
slumping, with fragments of anorthositic layers
displaced both vertically and laterally. This
process requires substantial transport of partially
consolidated material. The change in thickness of
this zone and the increase in layer disruption from
Western Contact Mountain to Central Contact
Mountain are consistent with slumping from the
west toward the east. Discordant troctolites found
in the disrupted zone are interpreted to have
formed by reaction with infiltrating fluids as noted
above [see Schiffries (1982) and Braun et al.
(1994)]. The absence of discordant troctolites at
the stratigraphically equivalent position at Eastern
Contact Mountain suggests that slumping elsewhere may have been facilitated by volatile
enrichment of the partially consolidated cumulates.
There is no evidence for any large-scale physical
601
disruption of layering in OB-III above the disrupted zone or in OB-IV.
The section between the disrupted zone and the
first occurrence of plagioclase clots is predominantly anorthositic or troctolitic with olivine
gabbro becoming increasingly abundant with
height. Foose (1985) considered the origin of the
plagioclase clots, and suggested three possible
models: (1) the clots represent ripups of anorthositic layers formerly interlayered with the host
rocks; (2) the clots are the remnants of fine-scale
anorthositic layers that were disrupted by compaction of the cumulates; (3) the clots were
initially megacrysts that were recrystallized to
finer-grained aggregates during compaction. He
ruled out the first explanation because the clots
show no evidence of size sorting and because they
are widespread whereas ripup events arc likely to
be local. We find the second objection more compelling because plagioclase clots were probably
close to neutrally buoyant so no size segregation
should have occurred. There is no evidence that
fine-scale layering ever existed in these rocks and
so we agree with Foose in discounting the second
model. Foose favored the third explanation,
recrystallization of megacrysts, but our data do not
support this model. The complex zonation observed
in plagioclase from throughout the Middle Banded
series is also observed in the plagioclase of the clots
and there is no reason to expect recrystallization to
generate complex zonation patterns. As an alternative to these models, we suggest that the clots
were glomerocrysts formed during initial plagioclase nucleation and growth at the base of either
AN-I.5 or AN-II and were carried down by
plumes or density currents. Cumulus orthopyroxene
occurs in the section just above the first occurrence
of plagioclase clots (see Fig. 5), and abundant
intercumulus orthopyroxene is associated with the
plagioclase-clot-bearing layers just below the first
cumulus orthopyroxene occurrence. Cotectic crystallization of significant amounts of orthopyroxene
along with clinopyroxene and plagioclase might
have caused the crystallizing layer to become
gravitationally unstable and form density currents
that carried the plagioclase clots to the floor. This
model requires that crystallization of anorthosite be
taking place above the floor of the magma
chamber during the time needed to accumulate the
60—100 m thickness of cumulates that host the plagioclase clots.
The lower boundary of the foliated zone lies 40 m
above the lowest occurrence of plagioclase clots.
The foliated zone is both texturally and mineralogically distinct and contains the only cumulus
JOURNAL OF PETROLOGY
VOLUME 37
orthopyroxene in OB-III. Despite the strong
mineral lamination present in this zone, no lineation
is observed to indicate alignment by flow, suggesting that the rock foliation developed by compaction. If compaction did occur, then the texture
of the coarse-grained layer could owe its origin to
recrystallization caused by liquids trapped below
the mafic layer. The compositions of ortho- and
clinopyroxene from the foliated zone and from the
coarse-grained layer in particular support these
conclusions. Figure 15 shows the variation of orthoand clinopyroxene m^-number values in the foliated
zone. The wi^-number for the coarse-grained layer is
significantly lower than that of the underlying
layers. Lower m^-number values in the ortho- and
clinopyroxene of the coarse-grained layer are consistent with its equilibrating with later intercumulus
liquids, possibly those compacted out of underlying
cumulates. Several features of the foliated zone
provide evidence of liquid—fluid redistribution,
including the following: fractures lined with hornblende, plagioclase-quartz dikelets with hornblende
selvages, hornblende-bearing pegmatoids, and myrmekitic lenses. The mafic layer, which marks the
upper limit of the foliated zone, is considerably
more dense than the underlying cumulates (3-25 g/
cm3 vs 2-90 g/cm3), and could have provided the
driving force for the compaction of the foliated
zone. It also seems likely that fluids moving through
the cumulates promoted recrystallization and
thereby facilitated compaction.
NUMBER 3
JUNE 1996
Evolution of the Middle Banded series
Several features of OB-III and OB-IV suggest that
these units are intimately related to AN-I and ANIL Whereas none of the observations, taken individually, unequivocally demonstrates that all of the
zones in the Middle Banded series are related, collectively they provide a basis for reaching this conclusion. Features that tie the Middle Banded series
together are as follows:
(1) AN-I.5 is essentially the same in grain size,
texture and composition as AN-I and AN-I I. The
only occurrences of laterally extensive, pervasively
altered troctolites are found beneath AN-I, AN-I.5
and AN-II. These features suggest that these three
anorthosite horizons formed in a similar manner.
(2) Complex plagioclase zonation patterns noted
in AN-I and AN-II (Scheidle, 1983; Czamanske &
Scheidle, 1985) are also observed in the plagioclase
throughout OB-III and OB-IV. This includes normally zoned grains, reversely zoned grains, high-An
rims and truncation of zonation patterns as if by
grain breakage.
(3) Polyphase inclusions composed of clinopyroxene, apatite and mangano-ilmenite are found
throughout AN-I and AN-II, and OB-III and OBIV, but have not been identified in the Lower
Banded series or Upper Banded series. Loferski &
Arculus (1993) suggested that these inclusions
indicate that the entire Middle Banded series crystallized from a highly polymerized magma in the
The general sequence of rock types in OB-IV is Stillwater chamber.
(4) Plagioclase compositions are nearly constant
the same as in OB-III, and both may have crystallized in a similar manner. The lower portions of throughout the entire Middle Banded series.
both OB-III and OB-IV are predominantly anor- Although this compositional feature is enigmatic, it
thositic and troctolitic (Fig. 5). In each, olivine suggests that AN-I and AN-II, and OB-III and OBgabbro becomes increasingly abundant at the IV, are related by the process responsible for the
expense of anorthosite with height. Near the top of constant An content.
OB-III and locally at the top of OB-IV, cumulus
(5) The Pb isotopic values for AN-I, OB-III and
orthopyroxene joins the assemblage; and three- and AN-II show little variation (Wooden et al., 1991).
four-phase cumulates make up the uppermost porIf the Middle Banded series is considered as one
tions of these zones. The progression of cumulate
types is consistent with increasing fractionation in unit, any model for the genesis of the thick anorthoOB-III and OB-IV. However, the lack of sites must account for the genesis of OB-III and OBdecreasing trends with height in average mg-number IV. Four models for the origin of AN-I and AN-II
values of mafic minerals and in the average An have been proposed:
content of the plagioclase does not support this
(1) Hess (1960) suggested that during the crystalconclusion. Comparison of the average mf-number lization of the Ultramafic Series, plagioclase crystalvalues of cumulus mafic minerals and the average lized and either rose buoyantly or remained in
An content of coexisting plagioclase shows no cor- suspension and was displaced upward by density
relation between these two primary indices of currents. This plagioclase was ultimately resorbed
magma evolution (Fig. 16). Thus, if significant and, along with less dense interstitial liquid that
fractionation took place during the crystallization of escaped from the crystal pile, formed a layer of
OB-III and OB-IV, much of the chemical evidence magma enriched in plagioclase component. Repehas been obscured.
tition of this process during the solidification of the
602
MEURER AND BOUDREAU
MIDDLE BANDED SERIES, STILLWATER COMPLEX
320-j mafic layer
D
D 00
D
a
D
a
ocO a oao DD D
• [ 0 ] DC
280bO
0
'53
DC
Increasing
Foliation
240-
D
QDCD
0
a ao#a D
o oa
74
78
76
mg-no. in cpx
320-,
mDoo •
0
•
Qix^ft DUD
o 9 a mm
c©
a no ISCDO
a
o
280a
'3
DC
a
240-•
74
©a m
a
*
ID DC
Omo o
a B a %-, a
76
Increasing
Foliation
D
78
mg-no. in opx
Fig. 15. Detailed plot of m^-number in clinopyroxene and orthopyroxene in the foliated zone for the Western Contact Mountain section.
Average m^-numbers are indicated by the larger filled circles and individual analyses are indicated by the open squares. (Note the
substantially lower m^-number for both types of pyroxenes in the coarse-grained layer.)
ultramafic series resulted in a stratified magma
chamber with the middle portion enriched in plagioclase component that crystallized to form the thick
anorthosites.
(2) McCallum et al. (1980) and Raedeke (1982)
proposed that during the crystallization of the
Ultramafic Series, plagioclase was crystallizing in the
intermediate portion of the magma chamber because
of a pressure gradient. This plagioclase remained in
equilibrium with the main body of magma during
the crystallization and formed anorthositic 'rockbergs'. Injection(s) of a magma with a different
composition caused additional crystallization and
the rockbergs sank in two episodes forming AN-I
and AN-II. Support for the rockberg model, in
principle, is found in the work of Haskin & Salpas
(1992). They interpreted compositional characteristics of AN-I and AN-II to indicate that these zones
were formed by the amalgamation of aggregates of
crystalline plagioclase.
(3) Irvine (1975) pointed out that because of the
curvature of the cotectic surfaces, mixing of a primitive basaltic magma with a more evolved basaltic
magma can result in crystallization in the plagioclase-only field. It has been suggested that AN-I and
AN-II may therefore be the result of magma mixing
603
JOURNAL OF PETROLOGY
VOLUME 37
85o
o
o
O
C
o
A
AA
13004 o
A
*
75-
D
O
65
70
75
0 clinopyroxene
* orthopyroxene
0 olivine
80
85
Mole Fraction An
Fig. 16. Plot of average mj-number in clinopyroxene, orthopyroxene and olivine vs average An content in coexisting plagiodaie for
all samples.
upon injection of a more primitive magma into the
more evolved Stillwater magma chamber.
(4) Czamanske & Bohlen (1990) argued that AN-I
and AN-II were injected into the Stillwater stratigraphy as plagioclase-phyric mushes derived from a
fractionating magma chamber at the crust-mantle
boundary.
Two of these models are incompatible with the
conclusions reached above. Formation of the Middle
Banded series by mixing across cotectic surfaces
(Irvine, 1975) would require enormous amounts of
more primitive magma to yield anorthositic layers of
the thickness of AN-I, AN-I.5 and AN-II. Also,
crystallization of the hybrid magma, once mafic
minerals are saturated, should be accompanied by
evolution in m^-number in the remaining magma
throughout OB-III and OB-IV. The model of Czamanske & Bohlen (1990) requires that the Stillwater
Complex was invaded by three pulses of plagioclaseladen magma. That AN-I.5 is relatively thin presents a physical problem for intruding a viscous
crystal mush into partially solidified cumulates to
produce a horizon with such a large aspect ratio but
relatively uniform thickness. This model also calls
upon an exotic source for the thick anorthosites and
does not explain their many similarities to OB-III
and OB-IV.
The models proposed by Hess (1960) and by
McCallum et al. (1980) provide explanations for the
thick anorthosites that are not inconsistent with our
results. However, neither model provides an explanation for the compositional features of OB-III and
604
NUMBER 3
JUNE 1996
OB-IV. Both models can be extended to explain the
abundance of complexly zoned plagioclase in OB-III
and OB-IV. In each, the same processes that allowed
the anorthositic zones to form could have resulted in
accumulation of crystalline plagioclase between these
two zones. Thus the lack of evolution in the plagioclase throughout the Middle Banded series can be
explained by the mechanism that allowed the plagioclase to be concentrated. However, neither model
explains the lack of evolution in the mafic minerals
and the appearance of clinopyroxene before orthopyroxene in OB-III and OB-IV.
Two explanations for the different crystallization
sequence are most consistent with the data presented. Both require the addition of some liquid to
the Middle Banded series whose composition changes
the crystallization relations in these zones by
changing the bulk composition of the liquid.
McCallum et al. (1980) suggested that this additional
liquid was a different parental magma injected into
the complex at the level of OB-III and OB-IV.
Assuming this were so, it seems rather fortuitous that
the composition of the added liquid did not change
the composition of the minerals that crystallized
after its introduction from those in equilibrium
before its addition, although periodic introductions
of this liquid could have buffered the composition of
the crystallizing minerals to produce the observed
lack of compositional variation. Alternatively, the
liquid could have been derived from the Ultramafic
series as rejected solute enriched in plagioclase and
clinopyroxene components that rose to the level of
the Middle Banded series as buoyant plumes (Morse,
1986).
Neither the mineral compositions nor the field
relations require the Middle Banded series to have
crystallized from more than one magma. Isotopic
data (Martin, 1989; Lambert et al., 1989, 1994)
have been used to infer that two or more magma
sources were involved in the formation of the
Banded series. Loferski et al. (1994) examined rareearth-element contents of plagioclase and concluded
that at least two parental liquids were responsible
for the formation of the Middle Banded series. The
remarkable continuity in major- and trace-element
compositions of the cumulus minerals throughout
the Middle Banded series suggests that the isotopic
and rare-earth-element data should not be considered as definitive. There is evidence that interaction with postcumulus fluids has disrupted
isotopic systems locally in the Stillwater chromitites
(Marcantonio et al., 1993). Similar or other postcumulus effects such as recrystallization may have
also modified original rare-earth-element contents of
the cumulus minerals.
MEURER AND BOUDREAU
MIDDLE BANDED SERIES, STILLWATER COMPLEX
ACKNOWLEDGEMENTS
CONCLUSIONS
Petrographic study of OB-III and OB-IV demonstrates that only portions of these zones show significant lateral continuity. However, despite lateral
variations in cumulus mineral proportions, electron
microprobe analyses reveal that the average mineral
compositions show little or no lateral variability.
Likewise, no pronounced stratigraphic trends are
observed in their average major-element and traceelement compositions. Detailed examination has
revealed the importance of smaller-scale processes
involved in producing and modifying both the rock
textures and chemistry. For example, correlation of
rock fabric with plagioclase grain habit suggests that
plagioclase grain morphology may be indicative of
the stress conditions present during the solidification
of cumulates. Channelized liquid and/or fluid flow
through largely consolidated cumulates is evidenced
by (on a small scale) high-An partial rims on some
plagioclase grains and (on a larger scale) discordant
bodies and pegmatites. Zonation preserved in clinopyroxene grains and the compositions of coexisting
clino- and orthopyroxene indicate that submagmatic
reequilibratdon has taken place.
Petrographic and compositional data suggest that
the entire Middle Banded series is petrogenetically
related and that discussions of the origin of the two
thick anorthosites (AN-I and AN-II) cannot be
divorced from the origin of OB-III and OB-IV. We
suggest that the thick anorthosite layer at the base of
OB-IV (AN-I.5) formed in the same manner as AN-I
and AN-II. The general sequence of rock types in
OB-III and OB-IV is the same and implies that these
zones formed through similar processes. The increase
in the number of cumulus minerals with height
implies continuous fractionation of the liquid that
produced OB-III and OB-IV. This interpretation
conflicts with the lack of observed fractionation in
mineral compositions with height. Our results are
consistent with both the models of Hess (1960) and
McCallum et al. (1980) for the origin of the thick
anorthosites. The major- and trace-element compositions of the cumulus minerals do not indicate that
more than one parental liquid is required to form OBIII and OB-IV. This conclusion is in apparent conflict with the results of isotopic and rare-earthelement studies that indicate the presence of at least
two distinct parental liquids. However, postcumulus
processes may have obscured or completely overprinted some of the original compositional and textural features of the zones, and recognition of these
processes and their effects is required before further
speculation about the cumulus origins of the Middle
Banded series is warranted.
605
Mike Zientek of the US Geological Survey and
several geologists of the Johns Manville Co., especially Lynn LeRoy and Sam Corson, provided logistical support for the field-work in the Stillwater
Complex. An earlier version of this work was much
improved by the critical reading of Ellen Meurer.
Thorough reviews by Stu McCallum and Patty
Loferski and editorial comments by J.M. Rhodes
improved both the quality of the material presented
and its presentation. Financial support for this work
was provided by GSA and Sigma Xi student
research grants and by NSF Grants EAR 9217664
and 9417144.
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RECEIVED OCTOBER 12, 1995
REVISED TYPESCRIPT ACCEPTED JANUARY 4, 1996