Download Fractionation and Liquid Immiscibility in an Anorthositic

Fractionation and Liquid Immiscibility in an
Anorthositic Pluton of the Nain Complex, Labrador
by R. A. WIEBE
Department of Geology, Franklin and Marshall College, Lancaster, Pennsylvania 17604
ABSTRACT
Fine-grained anorthositic dikes are associated with a massive leuconorite pluton (CI = 15) which
is exposed over an area of about 200 km2. Internally, the pluton shows little compositional
variation; average plagioclase composition ranges from An32 to An48. The dikes are nearly uniform
in composition and similar to the estimated bulk composition of the pluton (55 per cent SiO2). They
therefore appear to represent the parental magma of the leuconorite pluton.
A small body of granite (10 km2) was emplaced within and prior to the complete solidification of
the leuconorite. The granitic intrusion caused local deformation of the leuconorite and filter-pressing
of its late stage interstitial liquids. These liquids occur in the younger hydrous granite as very finegrained, chilled pillows of nearly anhydrous Fe-rich diorite and granite. Most of the pillows are
diorites with approximately 55 per cent SiO2. On oxide plots these lie approximately on a
plagioclase control line passing through the composition of the leuconorite dikes. The entire group
of chilled pillows ranges in composition from 45 to 71 per cent SiO2 with a gap between 57 and 63
per cent SiO2. On oxide plots they produce a smooth trend which is oblique to and truncates the
plagioclase control line. Variation in the pillows can best be explained by late-stage liquid
immiscibility.
Fractionation in the interstitial magma was controlled early by crystallization of plagioclase and
later by plagioclase plus pyroxene. Very late stage differentiation was controlled mainly by liquid
immiscibility and produced FeO- and SiO2-rich liquids.
INTRODUCTION
IN any plutonic complex, recognition of the compositions of magmas, parental and
differentiated, is a major goal of petrologic study. This goal is of particular interest
in anorthositic complexes because experimental and field studies have yet to
produce a consensus about parental magmas and liquid paths appropriate for these
complexes. In plutonic terranes chilled margins of plutons and fine-grained dikes
have long been recognized as possible samples of magma, although problems
related to contamination and fractionation demand that such samples be treated
with caution. Compositions of liquids are also preserved in plutonic situations
where multiple injections of magmas with highly contrasted temperatures can lead
to chilled pillows of the high temperature magma in the cooler magma (Wiebe,
1974).
The purpose of this report is to describe and discuss chilled samples representing
liquids which have been preserved by all of these processes and which are
associated with a single leuconorite intrusion. Although the volume of chilled
differentiates is very small, it is possible to trace liquid compositions from an
anorthositic parent to end-stage Fe-rich dioritic and alkali-rich granitic liquids. As
Uounud of Petrolojy. Vol. 20. P«rt 2. pp. 239-269. 19791
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
(Received 17 January 1978; in revisedform 28 April 1978)
240
R- A. WIEBE
expected, early variation can be explained by separation of plagioclase, and later by
plagioclase plus pyroxene. Late-stage variation within the diorite and granite
liquids is largely dependent on their mutual immiscible relations. The major granitic
plutons are in part contemporaneous with the anorthositic rocks, but are not
related to them by any process of fractionation (Wiebe, in press).
GENERAL GEOLOGIC SETTING
FIELD RELATIONS
Rock nomenclature
I have mainly followed the rock classification of Streckeisen (1976) in which the
boundary between anorthosite and leuconorite is a color index (C.I.) of 10. Nonetheless I have retained the term, anorthositic, for those rocks with a color index between 0 and 20.1 have applied the term, diorite, to all of the associated fine-grained
mafic rocks.
Major rock units
The intrusion under study consists mainly of leuconorite with little internal
structure or compositional variation. It is typical of several anorthositic plutons in
the southern part of the Nain complex. It occurs as a homogeneous map unit as
well as a matrix to blocks of older anorthosite in an extensive map unit of
megabreccia (Fig. 1). The areal extent of the leuconorite and megabreccia units is
approximately 200 km2. A large irregularly-shaped area of older anorthosite which
provides the blocks of the megabreccia unit occurs within the leuconorite unit (Fig.
1). A few small bodies of layered diorite occur within the leuconorite.
The leuconorite occurs in two main textural varieties: (1) seriate porphyritic with
coarse iridescent plagioclase phenocrysts 2 to 12 cm in length and (2) thin tabular
plagioclase 12-15 mm in length with evenly spaced, 2-5 cm poikilitic clots of
pyroxene producing a spotted appearance in the outcrop. Textures intermediate between these two types are common. Color index is nearly constant at about 15.
Interstitial quartz and K-feldspar are commonly visible in hand-specimen; within
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
The Nain anorthosite complex is located along the coast of Labrador and has an
areal extent of roughly 15,000 km2 (Fig. 1). It is dominated by anorthositic
(including leuconorite, leucotroctolite and leucogabbro) and granitic plutons.
Gabbroic and dioritic rocks are relatively minor components. The intrusions are
probably all Proterozoic in age. Radiometric ages of 1420 and 1270 have been
reported for anorthositic and granitic members respectively (Barton, 1974; Krogh
& Davis, 1973). It is yet uncertain how representative these ages are for the terrain
in general or the particular rock types. Field relations (Wiebe, 1976) require that
granitic and anorthositic rocks have overlapping intrusive ages in the southern part
of the Nain complex. Berg (1977) provides evidence from mineral assemblages in
the contact aureoles that emplacement depths corresponded to pressures of 4 to 6
kb. Primary igneous relations are uniformly well-preserved because the region has
been free of significant later metamorphic or intrusive events.
241
THE NAIN PLUTONIC COMPLEX, LABRADOR
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
242
R. A. WIEBE
Chilled dikes
Along Fox Inlet on Tunungayualuk Island, many fine-grained anorthositic dikes
occur within an enclosed block or possible roof pendent of basic granulite. They
follow one or more sets of joints within the granulite and most have thicknesses in
the range of 1 cm to 1 meter (Fig. 2). The mineralogy, modes, and chemistry of
these dikes are very similar to the leuconorite intrusion. These dikes appear to
represent chilled samples of the parent magma of this leuconorite intrusion (Wiebe,
in press).
Many fine-grained diorite dikes occur within the leuconorite and anorthosite
units. Many of the larger dikes (>10 m) display gently dipping layering and
lamination and are obviously cumulates rather than samples of chilled liquid. A few
dikes less than 1 m thick are finer-grained, massive, and homogeneous. These
FIG. 2. Anorthositic dike in basic granulite. This dike contains patchy areas of coarser- and finer-grained
leuconorite which have a similar mode and color index of about 10-18. Most dikes show less textural variation.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
50 meters of some contacts these minerals occur in evenly spaced pods up to
several cm in diameter.
Granitic plutons occur mainly along the western contact of the anorthositic
rocks. Two small bodies occur within the anorthositic rocks. All of the granitic
plutons are remarkably homogeneous and massive, even along their intrusive
margins. The color index is characteristically between 5 and 15. Quartz is generally
present in amounts between 25 and 35 per cent, and alkali feldspar is equal to or
more abundant than plagioclase. Similar granite is found in heterogeneous hybrid
units of mixed granite and diorite which occur between granitic and anorthositic
rocks.
THE NAIN PLUTONIC COMPLEX, LABRADOR
243
appear to be reliable samples of 'Liquid' which could be genetically related to the
leuconorite. The sampled dikes all cut the leuconorite or anorthosite units. A few
similar-appearing dikes occur in basement within a few hundred meters of the
leuconorite.
Hybrid unit of diorite and granite
Heterogeneous mixtures, mainly of diorite 'pillows' in heterogeneous matrix
ranging in composition from granite to monzodiorite form mappable units along
several contact zones between leuconorite and granite. Inclusions of leuconorite are
FIG. 3. Pillow of chilled diorite and inclusion of gneiss in heterogeneous hybrid granitic matrix. Fluid state of
the diorite pillow is indicated by the impression of the gneiss inclusion into the shape of the diorite pillow.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
Eastern contact zone with basement
Leuconorite forms a steep and sharp contact with basement rocks to the east.
The basement consists mainly of quartzofeldspathic gneiss which has undergone
partial melting within several meters of the contact (Wiebe, in press). In some
locations along the contact zone contamination and mixing of leuconoritic and
anatectic granitic magmas appears to have been important. One segment of the
contact consists of a breccia zone several meters wide with angular blocks of
basement and a matrix of medium grained leuconorite carrying plagioclase
phenocrysts typical of the average grain size further within the leuconorite.
Granitic melts generated from the adjacent gneiss locally mixed with the
leuconorite in this zone. Contamination clearly precludes the use of these chilled
rocks in any attempt to reconstruct a liquid line of descent for the leuconorite
pluton.
244
R. A. WIEBE
FIG. 4. Irregularly-shaped chilled pillow of pyroxene granite in a host of medium-grained hornblende-biotite
granite.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
prominent locally; inclusions of basement are scarce but widely distributed. Finegrained, pillow-like bodies of diorite range in size from less than 10 cm to more
than 100 m. They are characterized by strongly chilled margins with a rounded,
often crenulated, external shape. Some appear to have settled and partially molded
against each other and one has molded against an angular inclusion of basement
gneiss (Fig. 3).
The heterogeneous matrix to the pillows appears to represent a more thorough
mixing of diorite and granite. Textural and compositional changes in the matrix are
mostly gradational.
Along the western contact zone (near Zoar in Fig. 1), leuconorite occurs as
rounded bodies several meters in diameter in the hybrid matrix. This leuconorite
has random to radially-oriented plagioclase of high aspect ratio (1 x 10 to 20 mm).
Similar leuconorite occurs within a hundred meters of the contact. The texture is
strongly suggestive of supercooling, and is analogous to spherulitic textures
produced by supercooling a plagioclase liquid (Lofgren, 1974).
The leuconorite unit is essentially free of granite dikes along these contacts with
hybrid zones. Emplacement of granite appears not to have effected any significant
fracturing of the anorthositic rocks. In other zones where the hybrid and diorite are
absent, granite forms an extensive breccia zone with the leuconorite and the brittle
behavior of the leuconorite is evident.
One particularly informative complex of hybrid rocks, the Goodnews complex,
is associated with a granite stock intruded into the leuconorite on northeastern
Tunungayualuk Island (Fig. 1). Most of the hybrid zone occurs as a gently dipping
THE NAIN PLUTONIC COMPLEX, LABRADOR
245
Layered dioritic rocks
Layered dioritic rocks occur in many small bodies within the leuconorite unit. In
several, it is possible to observe gradations downward to typical leuconorite and in
some bodies layers of coarse leuconorite alternate with finer-grained and more
mafic layers of diorite. The Fox Inlet layered body (Fig. 1) has been studied and
sampled most completely because of superb exposures of the important basal and
marginal contact relations. A section approximately 150 meters in thickness
displays strong upward enrichment of pyroxenes, oxides, and apatite. Typical
coarse-grained seriate porphyritic leuconorite occurs below and along the sides of
the layered diorite. Anastomosing dikes and veins of more mafic leuconorite and
fine-grained diorite are abundant below and alongside the diorite body. These
irregular bodies commonly have gradational contacts with the leuconorite and
many appear to fade inperceptibly into the leuconorite.
DISCUSSION OF FIELD RELATIONS
Field relations appear to indicate the existence of a wide range of samples which
represent chilled magma. These include (1) the fine-grained anorthositic dikes at
Fox Inlet, (2) chilled relatively fine-grained leuconorite from the western contact
zone, (3) chilled pillows ranging in composition from Fe-rich diorite to granite, and
(4) fine-grained, massive dioritic dikes.
The occurrence of chilled diorite along extensive contacts between granite and
leuconorite must be explained. The granitic plutons have compositions far removed
from the diorite and do not grade to diorite toward the contact zone. Similar
diorites are common as small massive pods and somewhat larger layered bodies
within the leuconorite and appear to represent late-stage differentiates. Field
relations therefore suggest that the diorite pillows are genetically related to the
leuconorite intrusion and that they may be samples of late interstitial liquid in the
leuconorite. The localization of chilled Fe-rich dioritic liquid in these hybrid contact
zones implies that the leuconorite intrusions were not completely solidified when
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
sheet which steepens as it approaches the small steep-sided granite pluton. A
hybrid zone with chilled diorite pillows rings the granite inside a narrow zone of
homogeneous diorite which extends away from the granite into the leuconorite in
two directions: to the north the diorite continues into a gently dipping layered dike
and to the south massive diorite extends into the leuconorite in an irregular area
with local graduations to leuconorite through minor noritic rocks. The diorite
disappears to the south in irregular anastamosing veins and dikes with some
graduations to leuconorite. The granite stock locally contains many blocks of
basement gneiss. Widely scattered chilled pillows, ranging in composition from Ferich diorite to granite, are also common in the stock. Many have a much lower
color index than the typical diorite pillows in the hybrid sheet (Fig. 4).
Some features of the diorite in this intrusion resemble those in a smaller one at
Fox Inlet. Parts of the base and sides of that body grade by similar veining to
leuconorite. The intrusion at Fox Inlet is not associated with granite, however, and
a hybrid zone with chilled dioritic pillows is absent there.
246
R. A. WIEBE
PETROGRAPHY
Plagioclase compositions reported here were determined by the dispersion
method (Morse, 1968). Compositions of other minerals are from reconnaissance
microprobe data. Some orthopyroxene compositions were also determined by the
dispersion method. Where orthopyroxenes were determined both by probe and
dispersion, average compositions agreed within 2 mole per cent En.
Leuconorite
Rocks of the leuconorite unit have a color index of about 15. Average
plagioclase composition is nearly constant (AnJ2_48) throughout the intrusion.
Moderate normal and oscillatory zoning is common. Individual samples commonly
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
some granitic intrusions rose up within and alongside them. Granitic intrusions
could have deformed the adjacent leuconorites and effected filter-pressing of any
interstitial liquid within the disturbed leuconorite. Chilling of the diorite pillows
can be explained if the granitic magma was at a relatively low temperature.
The structural relations of the Goodnews complex can be best explained by
emplacement of the granitic stock into an existing chamber of fractionated dioritic
magma. The resulting hybrid material was then injected largely to the north in the
form of a gently-dipping sheet which steepens toward the granitic stock. Small
volumes of similar hybrid material partially ring the stock (Fig. 1).
It is necessary to explain why pillows in the gently dipping hybrid sheet are
dominantly dioritic in composition while the pillows in the associated granite stock
are dominantly granitic. The simplest explanation lies in the shapes of the
intrusions and the densities of the pillows. The hybrid sheet has a gently dipping
floor on to which the dense dioritic pillows have settled. In contrast the granitic
stock appears to have no immediate floor; there the dominant granitic pillows must
have had densities comparable to the granitic host magma and therefore largely
remained in suspension. Any diorite pillows in the stock would have sunk in the
granitic host magma at significant velocities until cooling and crystallization of the
host granite resulted in sufficiently high viscosities to restrict settling. Viscosities
and densities of appropriate compositions were calculated after methods described
by Shaw (1972) and Bottinga & Weill (1970) to provide estimates of rates of settling or floating. At 1000 °C for spherical pillows with a 10 cm radius, the most
iron-rich diorite would settle through granite at a rate of about 0-04 cm/sec (or 2-5
cm/minute).
The field relations also strongly suggest that the dikes and veins below and
alongside some layered diorite bodies have acted as feeders for a fractionating pool
of relatively mafic liquid which produced the layered diorites. The source of that
liquid also appears to be the interstices of the leuconorite intrusion. The largest
bodies of diorite are near the intrusive margins of the leuconorite and it is possible
that continued movement in the interior of the leuconorite intrusion deformed
marginal liquid in those zones. The Fox Inlet body is in a unique setting, lying between the only observed blocks of enclosed basement, but it is not clear how that
setting had aided segregation of residual liquid.
43-2
11-3
13-1
15-5
13-1
1-9
1-6
0-3
30 IB
83-6
2-0
2-1
10-1
0-9
0-2
10
01
ite
dike (7 spec])
39-6
3-0
13-4
27-3
7-9
0-1
7-0
1-7
I52B
46-0
Inverted pigeonite*
Ave. En
75-9
2-5
1-1
13-5
2-1
0-9
3-5
0-5
484A
43-4
51-3
39-8
4-8
21-2
59-6
10-1
3-0
01
11
01
32-2
6-6
191
4-6
32-9
1-7
2-6
0-3
1-3
0-5
9-8
0-1
0-2
0-4
—
87-7
540
64-0
4-7
7-9
19-6
1-2
0-4
2-2
nd
574
43-4
41-0
46-5
36-4
72-9
4-7
0-8
14-9
0-5
1-1
4-1
1-0
484B
55-1
1-6
5-4
32-1
2-2
0-1
2-6
0-9
62-2
7-8
6-0
19-8
2-5
0-2
0-8
0-7
53-7
2-2
8-7
28-7
3-0
0-6
2-6
0-5
310B
48-1
47-2
40-6
43-0
38-1
50-5
01
0-4
35-3
0-0
0-7
9-3
3-7
53-5
0-0
0-3
31-6
0-0
0-6
9-2
4-8
41-0
43-4
38-5
484D
484 C
Fox inlet layered dioriles
132 A
571
Chilled leuconorile
Plagioclase'
Ave. An
Max. An
Min. An
352A
I33C
Chilled pillows
87-5
2-7
1-4
6-8
00
01
1-3
0-2
A u p lpiipf^nr^r
r\ VCi ICLlUvJilLH
nd: no determination.
* Determined by the dispersion method (mole per cent).
t Includes I per cent fayalite.
Plagioclase
Quartz
K-feldspar
Pyroxene
Hornblende
Biotite
Opaques
Apatite
Plagioclase
Quartz
K-feldspar
Pyroxene
Hornblende
Biotite
Opaques
Apatite
unit (14 spec)
A 11/7 iniir*r\iir\n to
r\ £/c> iLi«L(y/iC/f if c
Modes (vol.) per cent
T A BLE 1
52-2
4-5
11-0
23-7
3-0
2-6
2-4
0-6
133F
42-8
39-9
46-2
36-3
53-7
0-0
0-5
310
00
0-3
10-1
4-4
42-0
40-7
45-1
37-1
50-0
0-0
0-6
33-2
0-3
0-4
10-0
5-5
484F
2-9
0-3
3-1
nd
25-3
7-4
8-8
52-2
30IA
Chilled pi/lows
484E
from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
45-8
0-0
4-8
38-0
0-8
1-3
8-4
0-9
49-4
1-2
2-1
36-2
3-3
0-5
5-5
1-8
352B\
17-7
30-6
37-4
0-6
9-9
1-5
0-9
0-3
340B
16-9
31-7
40-0
0-3
7-8
2-7
0-4
01
Granites from the
Goodnews complex
I52A
278
o70
D
03
r
X
2rm
o
z
n
n
-1
O
r-
z
m
X
248
R. A. WIEBE
Granitic plutons
The granitic plutons consist of homogeneous granite averaging about 30 per cent
quartz, 20 per cent plagioclase, 35 per cent K-feldspar and a color index between 5
and 15 (Table 1). Alkali-feldspar is coarsely and complexly exsolved with varying
proportions of the unmixed phases; some alkali feldspar may have originally been
hypersolvus. Dominant mafic phases are hornblende and biotite. Minor amounts of
fayalitic olivine (Fo3), ferroaugite (Di44En13Fs43) and less commonly grunerite
e,,,) occur mainly as inclusions in hornblende.
Fox Inlet layered diorite
The Fox Inlet diorite is mineralogically similar to other layered and massive
diorite bodies associated with the leuconorite unit. It grades from coarse-grained
leuconorite (CI = 20) at the base to a fine-grained ferrodiorite (CI = 49) at the
highest exposed level. Average plagioclase composition ranges from An44 to An40.
Plagioclase at the top appears to be slightly more sodic. Antiperthite is prominent
at the base and decreases upward. Opaques increase from 3-5 to 10-0 per cent and
apatite from 0-3 per cent to 5-5 per cent. Interstitial quartz and K-feldspar are
present in significant amounts at the base, but decrease upward to trace amounts.
Textures suggest that all of these rocks crystallized with large amounts of
interstitial liquid. These modal data are summarized in Table 1. The samples
represent a stratigraphic thickness of about 150 m. Coarsely exsolved subcalcic
augite and inverted pigeonite occur in roughly equal abundance.
Composite inclusions occur very rarely within coarser plagioclase grains. These
were examined closely on the assumption that they could represent trapped liquid
variably modified by crystallization. Pyroxene, opaques, apatite, quartz, and alkali
feldspar occur commonly. In the lower samples the inclusions characteristically
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
have zoning between An3J and An4J. The dominant mafic phase is inverted
pigeonite; cores of primary orthopyroxene are scarce and minor interstitial augite is
present in most rocks. Minor amounts of hornblende and biotite occur in some
rocks. Interstitial quartz, K-feldspar, ilmenite and apatite are ubiquitous, and
interstitial magnetite is common. The fine-grained leuconorite dikes have somewhat
more sodic plagioclase, averaging between An48_42; normal zoning is strong with a
range in some dikes as great as AnJ6-An37. The pyroxenes are coarsely exsolved.
The composition of the orthopyroxene host of the inverted pigeonite varies in the
range En 30 -En 40 . Pyroxene characteristically occurs in evenly spaced poikilitic
areas, 2-5 cm in diameter. These areas are zoned: small areas of orthopyroxene
commonly occur in the center; most of the area is inverted pigeonite, and minor
amounts of augite occur in the outer portions. Reconnaissance probe data indicate
that in general, Fe/Mg increases from core to rim of the poikilitic areas. Ilmenite
and less commonly magnetite also form poikilitic areas. The leucocratic areas and
the mafic oikocrysts contain a similar distribution of tabular plagioclase. In the
leucocratic areas the interstitial phases are mainly quartz and K-feldspar. Modes of
the leuconorite unit and anorthositic dikes (Table 1) indicate their compositional
similarity.
THE NAIN PLUTONIC COMPLEX,
M9
\
FIG. 5. Composite inclusion ot K-teldspar, pyroxene, opaques and apatite in cumulus plagioclase from the
upper levels of the Fox Inlet layered diorite. Width of photo is 0-8 mm.
FIG. 6. Composite inclusion of microgranophyre with a single crystal of apatite and an opaque mineral in
cumulus plagioclase from the upper levels of the Fox Inlet layered diorite. Width of photo is 2-3 mm.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
\
LABRADOR
250
R. A. WIEBE
have several small crystals of the first three phases in a matrix of K-feldspar with or
without quartz. At higher levels most composite inclusions show a greater concentration of minute opaque and apatite crystals in a matrix of K-feldspar (Fig. 5).
Rarely some composite inclusions consist mainly of microgranophyre (Fig. 6).
FIG. 7. Typical fine-grained and even-grained texture of diorite in the chilled pillows. Width of photo is 9 mm.
separately. Both types occur as chilled bodies within a granitic or hybrid matrix.
Average grain size commonly varies from less than 0-1 mm at the margin to about
1 mm in the interiors of larger pillows. Textures are dominated in thin section by
small equant grains and larger poikilitic, nearly skeletal, minerals.
Dioritic pillows are dominated by plagioclase (An36_31) and pyroxene (including
subcalcic ferroaugite, inverted pigeonite and late-stage poikilitic orthopyroxene).
Plagioclase phenocrysts up to 1 per cent in volume occur in a very few dioritic
pillows; other phases never occur as phenocrysts. A typical even- and fine-grained
texture is shown in Fig. 7. Hornblende is a minor phase in most rocks and a major
one in a few pillows. It is poikilitic and varies in abundance at the expense of
pyroxene. Where present, hornblende is commonly concentrated at pillow margins,
implying that the necessary water was provided by the relatively hydrous granitic
host rock. Quartz and finely perthitic K-feldspar are prominent interstitial and
poikilitic phases in most pillows. Fe-rich olivine (Fo 18 _ )6 ) occurs in one diorite dike
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
Chilled pillows and fine-grained dioritic dikes
Chilled pillows range in composition from Fe-rich diorite to granite. Modes of
representative pillows, given in Table 1, reflect the existence of two groups, dioritic
and granitic, with mineralogies which can most conveniently be described
THE NAIN PLUTONIC COMPLEX, LABRADOR
251
Summary of mineral chemistry
Variation in the compositions of plagioclase and pyroxene in the anorthositic
dikes, the leuconorite unit, the layered diorites and the chilled diorite pillows is
broadly consistent with the origin of all of these rocks from a common parent.
Plagioclase compositions are summarized for each unit in Fig. 8. The values given
are averages of 10 determinations for each specimen. Replicate determinations
produced averages within 2 mole per cent anorthite. Since the compositional range
in a single hand-specimen is commonly as great as 10 to 15 per cent anorthite,
considerable overlap in composition exists between units. Average plagioclase
composition becomes more sodic from the leuconorite unit to the layered diorites to
the chilled dioritic pillow. Compositions of representative pyroxenes are given in
Table 2. The pyroxenes become increasingly Fe-rich from the leuconorite to the
layered diorites to the chilled dioritic pillows (Fig. 9).
PETROCHEMISTRY
Chemical analyses of all major elements were performed by X-ray fluorescence
using the fusion method of Rose et al. (1963). Trace elements were determined by
the method outlined in Hower (1959). FeO was determined by titration. The
analyses are listed in Table 3 and their mutual relations can be viewed on plots of
the various oxides against SiO2 (Fig. 10). The analysed rocks belong to five
different groups, based on field relations and composition: (1) coarse-grained
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
lacking quartz. Opaques (magnetite and ilmenite) range in abundance from 1-6 to
8-4 per cent. Apatite occurs as abundant minute needles too small to permit a
reliable modal estimate by point counting, and the values for apatite in Table 1 are
probably too low. Zircon is a very scarce accessory.
Pyroxenes are variably exsolved. Compositions of orthopyroxene lamellae
range from En24 to En21. Reconnaissance microprobe data on one pillow indicate
the presence of some homogeneous hypersolvus pyroxene and suggests that
crystallization initiated above the two-pyroxene solvus. Alternatively, these
compositions could reflect metastable crystallization.
In contrast, the granitic pillows are dominated by perthitic K-feldspar, quartz
and minor plagioclase. Alkali feldspar is irregularly exsolved. Proportions of
unmixed phases appear highly variable and the alkali-feldspar may be hypersolvus.
Discrete subhedral grains of plagioclase are scarce. Ferroaugite is the dominant
mafic phase and commonly displays orthopyroxene lamellae on (001). Poikilitic
fayalitic olivine is present in a few pillows and poikilitic hornblende is commonly
present in minor amounts, varying at the expense of pyroxene. Some pillows
contain very scarce microphenocrysts (up to 1 • 5 mm in diameter) of plagioclase,
quartz and inverted pigeonite. The plagioclase phenocrysts are normally zoned
with cores as calcic as An3J. Average plagioclase composition is An23 to An20. The
orthopyroxene host of inverted pigeonite is En23. Minute, equant opaques are
present as an accessory phase; apatite and biotite are scarce.
R. A. WIEBE
252
Leuconorite
unit
-6
L4
45
45
50
55
50
•6
chilled diorite
pillows
30
35
-4
-2
40
% An
FIG. 8. Histograms of average plagioclase compositions (Mol. per cent) from individual specimens of the (1)
leuconorite unit. (2) layered diorite and (3) chilled dioritic pillows, n = the number of specimens.
leuconorite representative of the leuconorite unit, (2) fine-grained anorthositic dikes
from Fox Inlet, (3) intermediate differentiates consisting of chilled leuconorite and
two dikes, one of leuconorite and one of diorite, (4) chilled pillows from the
Goodnews complex located north of Fox Inlet and fine-grained dioritic dikes, and
(5) diorites from the Fox Inlet layered body. Field relations indicate that groups 2,
Mq
Atomic %
Fe
FIG. 9. Plot of representative Ca-rich and Ca-poor pyroxenes from the leuconorite unit (crosses), layered
diorite (open circles) and chilled dionte pillows (solid circles). Tie lines join coexisting lamellae in inverted
pigeonite.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
40
I
n
*
-2
54
47
2
006
50-67
0-77
0-25
29-45
0-70
12-74
5-81
500B'
44
3-997
1-979
0-021
0-015
0-007
0-963
0023
0-742
0-243
0004
100-45
' Leucononte unit and anorthositic dike.
Layered diorite.
1
Chilled diorite.
• Total Fe as FeO.
Mg + Fe
4015
3-984
101-33
TOTAL
"
100-36
001
TOTAL
28-26
0-53
1405
6-28
0-00
25-63
0-48
16-62
4-70
1-966
0-027
0000
0005
0-921
0018
0-816
0-262
0000
018
015
1-992
0-008
0032
0004
0-808
0-015
0-935
0190
0000
50-47
0-59
52-85
0-89
SiO2
AI 2 O,
TiO 2
FeO'
MnO
MgO
CaO
Na 2 O
Si
Al
Al
Ti
Fe
Mn
Mg
Ca
Na
500B'
216'
Spec.
Pigeonite
33
4014
1-966
0-034
0003
0-003
1-187
0-021
0-587
0-203
0005
100-91
008
4919
0-80
0-10
35-50
0-64
9-85
4-75
29 5A
2
25
4-005
1-984
0-016
0003
0-004
1-340
0023
0-443
0-184
0008
99-48
38-80
0-67
7-20
4-15
0-10
012
48-06
0-38
304
i
51
3-981
2001
0000
0032
0003
0-924
0 020
0-974
0027
0000
10115
52-73
0-71
0-11
29-12
0-62
17-21
0-65
0-00
216'
46
4-009
1-973
0-025
0000
0-005
1-057
0-022
0-892
0035
0000
100-57
000
50-59
0-55
0-19
32-41
0-66
15-34
0-83
500B'
41
3-992
1-984
0016
0-027
0004
1-115
0-023
0-770
0053
0000
98-95
000
33-37
0-68
12-93
1-24
013
49-69
0-91
500B'
10107
101-04
34
4-009
24
4-006
1-982
0016
0000
0-003
1-467
0-028
0-461
0-045
0-004
005
002
1-976
0-023
0000
0002
1-263
0-022
0-656
0062
0-001
42-80
0-82
7-55
1-03
010
48-38
0-34
3043
37-80
0-66
11-01
1-47
010
49-48
0-50
295A2
Orthopyroxene
60
4-005
0-851
0-013
1-964
0-036
0028
0-006
0-439
0005
0-663
98-63
11-53
20-58
0-18
016
50-95
1-41
0-20
13-62
500B'
Representative pyroxene analyses and structuralformulae based on 6 oxygens
TABLE2
from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
52
4-005
1-915
0039
0000
0004
0-525
0-011
0-567
0-991
0-013
98-83
48-45
0-84
0-15
15-87
0-34
9-62
23-39
0-17
216'
44
3-996
1-984
0016
0-022
0004
0-590
0009
0-470
0-884
0-017
99-71
1808
0-28
8-08
21-15
0-23
016
50-89
0-84
•295A2
A ugite
34
4-018
1-964
0036
0004
0-005
0-718
0015
0-377
0-881
0018
99-89
20-71
0-24
49-49
0-86
0-16
21-62
0-44
6-37
3O43
D
O
00
•^
.**
>
r
pi
r
TJ
AN
/~\
\J
z
no
O
cH
r
z
>
z
X
m
H
254
R. A. WIEBE
50
60
70
SiO,
FIG. 10. Major element oxide plots (wt. per cent) for all chilled rocks (open circles) and Fox Inlet layered
diorites (crosses). Compositions plotted have been recalculated to 100 per cent anhydrous. Solid circles
represent estimates of parental magma: unlabeled point is the average composition of the leuconorite unit;
point labeled, d, is the average composition of the fine-grained leuconorite dikes at Fox Inlet. Solid diamond
represents the composition of the average plagioclase (An,,,).
3, and 4 are samples of magmas genetically related to the leuconorite unit. Group
(5) includes late-stage cumulates related to the leuconorites.
The homogeneity of the leuconorite unit suggests that its average composition
approximates the composition of its parent magma. An estimate of the average
composition of that unit was made for comparison with the fine-grained
anorthositic dikes which clearly represent chilled liquid (Table 3). The constancy of
plagioclase compositions throughout the intrusion and the apparent constancy of
color index suggest that seventeen samples of leuconorite provide a reliable
estimate of average composition. If the composition is biased, it could be slightly
low in plagioclase, because the coarsest phenocrysts were avoided in sampling the
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
40
THE NAIN PLUTONIC COMPLEX, LABRADOR
255
TABLE 3
Whole-rock chemical analyses
Spec.
A1A
Fe2O,
FeO
MnO
MgO
CaO
Na2O
K2O
P2O3
LOI
55-79
0-60
23-18
0-53
3-29
0-05
1 -33
9-01
4-93
0-90
0-19
0-40
TOTAL
100-20
ppm
Rb
Sr
Ba
Ni
V
Zr
K/Rb
Ca/Sr
9
578
556
22
51
150
830
111
Intermediate
rocks
Leuconorile unit
(average of 17
analyses
540
571
574
54-04
0-93
21-96
0-26
5-09
0-08
2-02
900
4-25
0-83
013
1-24
53-01
1-42
20-35
0-97
6-35
010
1-75
8-48
3-84
1-13
0-19
1-54
54-81
1-41
1915
0-61
7-71
0-14
2-16
7-85
4-23
1-39
0-29
0-76
53-45
1-90
16-17
000
11-23
015
2-44
7-02
3-67
1-75
0-31
1-28
99-83
99-13
100-51
99-37
203
517
nd*
11
97
203
391
117
11
579
490
10
68
155
626
111
Chilled dioritic dikes
Spec.
276
102
104
10
45-00
3-92
11-96
1 89
19-71
0-25
3-35
903
2-83
0-50
1-22
1-27
51-23
2-68
13-55
0-73
14-18
0-20
2-39
6-95
3-12
1-21
0-84
2-50
52-89
2-60
13-74
2-96
11-63
019
2-25
6-84
308
1-50
0-70
1-39
54 •95
2 •05
14 •06
2 •03
11 •64
0 •19
1•45
6 •14
3 •04
2 •14
0 •50
1•10
TOTAL
100-93
99-58
99-77
99 •29
ppm
Rb
Sr
Ba
Ni
V
Zr
K/Rb
Ca/Sr
7
420
nd
<2
240
103
600
154
SiO2
TiOj
A1 2 O 3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
nd: no determination.
24
399
nd
5
195
351
417
125
36
460
nd
8
135
170
344
106
45
408
nd
3
84
762
396
108
44
502
nd
12
108
246
262
112
41
388
nd
9
nd
357
354
129
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
SiO 2
TiO 2
Chilled leuconorile
dikes {average of
7 analyses)
R. A. WIEBE
256
TABLE
3 (continued)
Spec.
SiOj
TiO,
A1 2 O 3
Fe2O3
Dioritic Pillows
278
356A
132A
1S2B
128
31OB
45-63
3-80
46-57
3-28
12-99
4-78
15-65
0-25
3-29
8-45
48-14
3-32
1216
49-60
318
12-53
106
16-64
0-23
2-60
7-70
3 06
1-64
0-87
1-43
50-52
2-63
13-34
1-51
13-61
0-22
2-07
7-24
3 05
1 -54
0-84
1-54
52-46
2-38
14-43
0-29
1317
0-78
1-17
1 56
1815
0-28
2-26
7-87
2-90
0-92
1-02
0-79
49-56
2-84
13-89
2-27
14-07
0-23
2-49
7-85
3-25
1-08
0-94
1-50
101-77
98-87
99-97
100-54
98-11
99-62
LOI
12-72
1-92
18-45
0-25
2-61
8-60
2-67
0-83
1-12
1-27
TOTAL
99-87
FeO
MnO
MgO
CaO
Na2O
K2O
P 2 O,
ppm
Rb
Sr
Ba
Ni
V
Zr
K/Rb
Ca/Sr
11
424
nd
2
297
158
627
145
300
13
446
nd
4
nd
158
498
135
Spec.
106
dl
425
nd
2
179
134
—
132
25
441
nd
7
207
160
360
127
32
378
nd
2
243
132
425
146
27
445
nd
7
162
144
474
116
Dioritic pillows
133F
133A
133C
301A
133B
301B
SiO 2
TiO 2
A1 2 O 3
Fe 2 O 3
52-79
2-29
14-32
FeO
MnO
MgO
CaO
12-17
53-46
2-23
13-74
1-46
13-26
019
1-88
6-91
100
53-89
2-09
14-47
1-07
11-37
0-19
211
6-83
3-87
2-31
0-66
1-89
54-08
2-32
14-04
1-09
12-69
018
1-97
6-48
3-43
1-49
0-71
0-98
54-24
2-07
13-98
0-72
11-65
0-17
1-99
6-70
2-90
2-57
0-63
1-75
54-52
2-21
13-35
2-24
10-62
016
1-82
609
2-91
205
0-69
1-74
98-96
100-75
99-46
99-37
98-40
Na 2 O
K2O
P2O3
LOI
TOTAL
ppm
Rb
Sr
Ba
Ni
V
Zr
K/Rb
Ca/Sr
108
019
213
6-86
3-45
1-50
0-63
2-05
99-46
24
450
nd
8
145
234
521
109
3-12
106
0-65
22
437
nd
7
133
230
400
113
59
416
nd
7
130
216
325
117
20
403
nd
5
141
417
620
115
70
422
nd
5
147
255
304
114
37
350
nd
5
157
348
459
124
019
2-46
7-11
3-29
1-57
0-78
1-49
28
446
nd
8
158
164
464
114
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
152A
THE NAIN PLUTONIC COMPLEX, LABRADOR
TABLE
Spec.
3 (continued)
Granitic pillows
340C
340E
352C
352D
352A
Fe 2 O 3
FeO
MnO
MgO
CaO
Na 2 O
K2O
P2O5
LOI
62-71
1-35
12-94
000
7-58
Oil
0-80
3-72
2-77
5-55
0-33
0-79
62-75
1-36
13-59
000
7-69
018
110
3-47
2-82
5-70
0-38
0-80
66-51
1-28
13-55
0-15
709
010
1-08
3-18
3-62
2-98
0-33
0-70
68-27
106
12-95
0-00
5-45
0-07
0-62
2-45
3-50
4-34
0-23
0-55
68-48
1-01
13-02
000
4-86
008
109
304
2-93
5-55
0-21
0-52
69-74
0-71
12-34
0-61
3-77
007
0-42
2-92
2-90
5-79
018
0-77
TOTAL
98-65
99-84
100-57
99-49
100-79
100-22
111
207
nd
1
55
388
415
105
175
287
nd
2
50
344
275
73
A1A
ppm
Rb
Sr
Ba
Ni
V
Zr
K/Rb
Ca/Sr
77
255
nd
8
74
431
598
104
65
278
nd
2
79
412
728
89
Spec.
31
324
nd
4
66
395
798
70
Fox inlet cumulates
484F
484D
LOI
39-99
5-15
11-22
2-63
19-22
0-26
5-23
9-76
210
0-39
2-18
1 85
41-85
5-26
12-94
3-46
16-34
0-23
5-77
9-70
2-53
0-32
1-45
1-62
42-44
4-61
12-86
101
18-56
0-23
503
9-66
2-43
0-41
1-93
1-80
TOTAL
99-98
101-47
100-97
SiO 2
TiO 2
A12O3
Fe 2 O 3
FeO
MnO
MgO
CaO
Na 2 O
K2O
PA
ppm
Rb
Sr
Ba
Ni
V
Zr
K/Rb
Ca/Sr
89
181
nd
3
60
460
405
97
5
338
nd
2
508
104
661
206
<2
362
nd
18
614
80
—
192
484E
2
381
nd
4
425
88
1700
181
484C
452Z
450
527
42-66
501
1400
4-62
13-43
019
5-46
9-98
2-91
0-41
1-72
1-31
43-61
4-17
15-56
2-01
12-84
0-18
4-42
1014
2-61
0-30
1-92
219
43-78
407
14-63
2-12
1310
018
4-35
10-14
2-74
0-33
1-67
1-65
47-89
3-37
15-89
0-38
12-43
016
3-56
9-39
3-18
0-62
1-24
1-48
101-70
99-95
98-76
99-59
2
370
nd
15
736
108
1700
193
<2
449
nd
12
nd
68
<2
430
nd
7
526
90
165
169
9
440
nd
10
344
127
567
152
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
340D
SiO 2
TiO 2
257
258
R. A . W I E B E
TABLE 3 {continued)
Spec.
Fox inlet cumulates
452X
45 2 Y
SiO 2
TiO 2
AI 2 O 3
Fe 2 O 3
FeO
MnO
MgO
CaO
Na 2 O
K.2O
P 2 O,
LOI
48-50
4-66
15-29
3-98
9-47
018
4-00
8-68
2-61
0-68
0-34
1-33
49-60
2-64
19-35
1-73
7-32
0-10
2-27
9-02
4-24
0-58
0-21
1-48
51-76
1-70
20-49
0-68
6-32
009
1-55
8-99
4-12
0-65
0-26
219
TOTAL
99-72
98-54
98-80
ppm
Rb
Sr
Ba
Ni
V
Zr
K/Rb
Ca/Sr
11
385
nd
30
484
161
509
161
7
558
nd
10
309
113
686
116
<2
601
nd
4
202
110
—
107
484A
452
51-81
3-04
17-74
0-21
11-11
014
3-07
8-58
3-68
0-76
0-22
106
52-82
2-39
19-87
3-32
5-60
010
2-44
8-83
4-33
0-76
0-21
0-54
53-17
1-81
21-09
209
5-60
009
1-90
9-07
4-52
0-68
0-27
0-89
101-42
101-21
101-18
9
572
nd
15
260
160
700
110
<2
599
nd
4
224
127
—
108
484B
2
490
nd
11
330
182
3200
125
porphyritic types. Much of the variation is related to a specimen size inadequate for
sampling the large poikilitic areas of pyroxene. For example, the Fe/Mg ratios of
the analyzed leuconorites ranges much more widely than the anorthite content of
the plagioclase. The variation in Fe/Mg ratio reflects the variation of that ratio
from core to rim of the pyroxene oikocrysts and the irregular occurrence of
ilmenite and magnetite.
The fine-grained anorthositic dikes of Fox Inlet have major and minor element
compositions which closely resemble the proposed average composition of the
leuconorite. The average of seven analyzed samples from three different dikes are
presented in Table 3. Their similarity to the composition of the leuconorite unit
indicates that the dikes can be treated as a sample of the parent magma of this
intrusion. Similar compositions have previously been suggested as parental to
anorthosite complexes (Buddington, 1939). Wiebe (in press) briefly reviews
experimental evidence for the existence of such a magma.
The third category of rocks contains those representing liquids which have been
moderately fractionated from the parent. All three specimens come from the
western margin of the leuconorite pluton near Zoar. Specimen 571 is a chilled
leuconorite with highly tabular, randomly oriented plagioclase within a rounded
homogeneous body about 10 m in diameter from the hybrid zone along the
contact. Specimen 540 is from a fine-grained homogeneous leuconorite dike
approximately 1 meter in width which cuts the older anorthosite. Specimen 574 is
from an extensive area of homogeneous fine-grained diorite with black plagioclase
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
451
259
THE NAIN PLUTONIC COMPLEX, LABRADOR
o
o
o
ft
o o
o
•
1-70+
o
<v
60J
45
50
55
60
SiOo
65
70
FIG. 11. FeO T (100)/FeO T + MgO vs. SiO, (wt. per cent). Symbols as in Fig. 10.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
phenocrysts up to 1 cm in length. Chilled compositions like 571 and 540 appear to
be restricted to this particular contact zone. Specimen,574, although porphyritic, is
essentially identical in composition to aphyric dioritic pillows from the Goodnews
complex.
The fourth category consists of chilled pillows from the Goodnews complex and
massive fine-grained dikes which represent highly differentiated liquids. Most of
these rocks have SiO2 between 53 and 56 and there is an apparent continuum of
compositions to about 45 per cent SiO2. A separate group of pillows has SiO2 between 63 and 71 per cent. Several pillows were sampled and analyzed to check for
possible variation from core to rim. Two of seven pillows showed internal variation
but in these, the rims had compositions further from the matrix than did the cores.
The matrix therefore does not appear to cause detectable contamination.
It is important to realize that the density of points does not in general reflect the
relative volumes of rocks. About 95 per cent of the rocks are leuconorites with an
average bulk composition very nearly equivalent to the parent magma. Rocks lying
between the leuconorite average and the pillows appear to represent small volumes
along the western contact zone. The dioritic to granitic pillows within the hybrid
unit and granitic stock probably represent less than 1 per cent of the volume.
Plagioclase of An J0 (approximately the average plagioclase composition in the
leuconorite) is shown on the oxide plots in order to define appropriate plagioclase
control lines. The dominant pillow composition (SiO2 = 55) and the intermediate
chilled rocks lie on or near the plagioclase control lines. The compositions of these
liquids, then, can be related grossly by removal of plagioclase or, in this pluton, by
filter-pressing interstitial liquid at different stages of consolidation.
The diorites have a higher Fe/Mg ratio than the leuconorite parent and it is
therefore essential that a small amount of pyroxene crystallize along with
plagioclase (Fig. 11). Textures in the leuconorite indicate that an extensive network
260
R. A. WIEBE
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
of plagioclase was established prior to widely-scattered nucleation of pyroxene in
the interstices. The dominant dioritic liquids (with approximately 55 per cent SiO^
would therefore have separated from the leuconorite intrusion after the beginning
of growth of the poikilitic pyroxenes.
The entire group of chilled pillows clearly defines trends at l&rge angles to the
plagioclase control lines. These pillow trends extend in two directions away from
the plagioclase control line, and a gap in pillow compositions exists between 56-5
and 63 per cent SiO2.
On the oxide plots, the dominant liquid path extends from the plagioclase
control line with an essentially linear trend from diorites with 55 per cent SiO2
toward high iron diorites with SiO2 values as low as 45 per cent. CaO, TiO2, P2O5,
MnO and MgO all increase with FeO, while A12O3, K 2 O, Na2O and SiO2 decrease.
The granitic pillows range in SiO2 from 63 to 71 per cent. With the exception of
Na 2 O and K 2 O, their compositions define tight and essentially linear trends on
plots of the various oxides against SiO2. As SiO2 increases, all but K2O and Na2O
decrease. The sampling of granitic pillows appears to be inadequate to allow a
detailed evaluation of the causes of their variation and the scatter of the alkalis. The
granitic pillows are presumed to be genetically related to the dioritic pillows
because they (1) are contemporaneous with the dioritic pillows; (2) on the basis of
chilled relations appear to have temperatures of crystallization comparable to the
diorites, and (3) appear to have liquidus phases with compositions comparable to
the diorites. In addition, the leuconorite intrusion appears to be the only possible
source for the very small volumes of anhydrous granitic magma.
The chemistry of the Fox Inlet layered rocks is exemplary of layered
differentiates within the leuconorite. As noted above, the base of the layered body is
gradational to the leuconorite and the layered sequence grades upward to rocks
highly enriched in Fe, Ti and P. Six samples (484A—F) were collected in one section
and seven other samples elsewhere. Taken all together, they plot as a coherent
group on SiO2-oxide plots with SiO2 ranging from approximately 54 per cent at the
base to 41 per cent at the highest exposed level. FeO, TiO2, MnO, MgO increase
regularly upward. At about 49 per cent SiO2, an abrupt increase in P2O3
apparently marks the beginning of cumulus apatite; above that stratigraphic level,
P 2 O 3 continues to increase. A moderate increase in CaO continues to the highest
stratigraphic level. A12O3, Na2O and K2O decrease markedly upward. The
chemistry is consistent with modal and mineral data presented above.
The relations between the chilled liquids and the cumulates can be seen in Fig.
10. Because the dioritic liquids are not in close association with the Fox Inlet
layered body they need not be in detail equivalent to the liquids which produced it.
Nonetheless, if the trend of fractionation is similar throughout the leuconorite
intrusion, they ought to resemble those liquids. The positions of the Fox Inlet rocks
indicate that they cannot be responsible for the trends of variation in the dioritic
liquids.
Rb, Sr, V, Zr, and Ni were determined for all samples (Table 3). Barium was
determined in the leuconorite unit and anorthositic dikes but interference from Ce
261
THE NAIN PLUTONIC COMPLEX, LABRADOR
precluded determination of Ba in the more differentiated liquids. Plots of Rb, Sr, V,
and Zr v. SiO2 for all samples representing liquids are given in Fig. 12. Ni is of
uniformly low abundance. Rb and Sr values for plagioclase in Fig. 12 represent the
average of 13 plagioclase separates (averaging An^.J from the leuconorite unit
The liquid paths behave much as they do for major elements: early control is
dominated by separation of plagioclase followed by a later splitting into two paths.
Simmons (personal communication, 1977) determined REE abundances and
patterns for several rocks from the Nain province. Two of his analyzed samples
V (ppm)
•200
•Si
0
800
600
Zr (ppm)
400
1-200
oo° ° C
45
50
55
SiO,
60
65
70
45
50
55
60
65
70
0
SiO,
FIG. 12. Plot of Rb, Sr, V and Zr vs. wt. per cent SiO2. Symbols as in Fig. 10.
were collected in the field with the writer. One belongs to the leuconorite unit and
was taken from the same outcrop as a sample contributing to the average of the
leuconorite unit in Table 3. Of the several Nain anorthositic rocks analyzed by
Simmons, this specimen had the highest abundance of REE and the smallest
positive Eu anomaly. Simmons & Hanson (1977) suggest that this specimen comes
closest of their Nain samples to representing a possible parent magma. That
suggestion is consistent with the data presented here which indicates that the
average leuconorite approximates the composition of its parent magma.
The second sample was taken from the same diorite pillow as specimen 356A
(Table 3). Simmons' specimen shows the highest overall concentration of REE for
Nain dioritic rocks and shows a small negative Eu anomaly (Simmons, personal
communication, 1977). The abundances and pattern of REE shown by this diorite
pillow is qualitatively consistent with the data presented here indicating that these
pillows represent residual magma from the leuconorite.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
400
262
R. A. WIEBE
DISCUSSION
Origin of chemical variation in the pillows
Two processes might potentially account for the compositional variation of the
dioritic pillows: fractional crystallization or liquid immiscibility. Fractional crystallization, involving separation of plagioclase, two pyroxenes, ilmenite, magnetite and
apatite, should be capable of driving an earlier low SiO2 liquid to a later high SiO2
liquid. However, cumulates with appropriate ratios of phases have not been found
(Fig. 10). There is no appropriate mixture of solid phases, separation of which
could cause the highest-SiO2 diorite to fractionate toward the low SiO2 diorites.
Immiscible separation of a high SiO2 liquid (identical to the granitic pillows)
could be the major cause of the dioritic trend from 55 to 45 per cent SiO2 and could
also explain the gap in silica between dioritic and granitic pillows. The oxide plots
suggest that variation in the diorites could be effected approximately by separation
of the average composition of the granitic pillows and plagioclase (An41) in a ratio
of 3 to 1 (Table 4). Because of the density relations, it seems reasonable to expect
some flotation of plagioclase along with the immiscible granitic liquid.
The entire range of dioritic pillows shows no detectable systematic variation in
the normative compositions of orthopyroxene or plagioclase (Fig. 13). Deter-
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
Origin of pillows
Field and petrographic relations suggest that the granitic and dioritic pillows
represent residual liquids from an anorthositic parent and that they were preserved
by intermingling with contemporaneous intrusions of relatively low temperature,
hydrous granitic magma. Diorite with 55 per cent SiO2 can logically be derived
from ttie well documented (Wiebe, in press) anorthositic parental magma by
separation of plagioclase and very minor pyroxene, and pillows with this
composition are much more common than pillows with very high or very low SiO2
(Fig. 10). Iridescent phenocrysts of plagioclase with cores of An53_48 occur in some
chilled diorites with SiO2 near 55 per cent, providing a mineralogical link between
anorthositic and dioritic rocks.
The wide compositional range of chilled rocks along the western contact zone
implies that residual liquids within the leucononte were at different temperatures
and stages of fractionation when they mixed with the cooler granitic magma. This
requires that the mixing occurred either over a range of time or of space. If the
leuconorite intrusion varied in temperature and percentage solidified when the
granitic magma was emplaced, a wide compositional range of filter-pressed chilled
rock might occur within the hybrid zone. Field relations in the southern Nain
complex suggest that the granitic intrusions moved upward preferentially along
previously established boundaries between basement and anorthositic plutons. If
the leuconorite intrusion had only partially crystallized, residual liquids may have
been most fractionated near the contact. Where the granitic magma extended
further into the interior of the leuconorite, it would have come in contact with and
chilled progressively less differentiated liquid. While flow was occurring in the
contact zone, some mixing of these chilled liquids and hybrids would be expected.
263
THE NAIN PLUTONIC COMPLEX. LABRADOR
TABLE 4
Calculations on chilled dioritic liquid
Observed
Calculated
Wt
Variable
fraction
A1 2 O,
FeOT
MnO
MgO
CaO
Na2O
K2O
P2O,
55-04
2-48
14-36
13-50
0-18
1-91
6-84
3-42
Di45
Gr.,
0-5148
0-3688
Plagioclase
0-1227
"L of the squares of
the residuals = 0-1590
2-18
0-73
Least squares approximation of chilled diontic liquid with 55 per
cent SiO2 (Di5J) calculated as a linear combination of the average
composition of the chilled granitic pillows (Gr67), chilled dioritic
liquid with 45 per cent SiO2 (Di<5) and plagioclase with a composition
of An4,. The compositions of the dioritic pillows are from the linear
regression of all analyzed dioritic pillows. The method of calculation
is after Bryan et al, 1969. K2O is the oxide which shows the poorest
fit Since there is a high scatter of K2O in the granitic pillows, the
average value may be inappropriate. With the exception of K2O the
agreement in observed and calculated compositions clearly supports the
premise that variation in the dioritic pillows is a result of separation
of an immiscible granitic liquid and minor plagioclase (see text).
mination of plagioclase by the dispersion method supports this lack of variation.
Major changes in liquid compositions could be effected by liquid immiscibility
without causing significant variations in the An or En of plagioclase and pyroxenes.
50o
norm
o
0
o
An 40-
O Oo
° o
o
30-
1
30-
norm.
°
'
1
'
1
'
1
'
O
O
°° ° °:»
o
10-
1
46
1
'
o
o
o
3
En 20-
/1 °
•
1
48
'
1
50
Wt. %
'
1
52
'
1
54
'
o
o
o
1
56
Fio. 13. Plots of normative An of plagioclase and normative En of orthopyroxene vs. wt. per cent SiO2 in
chilled diorite pillows.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
55-08
2-28
14-34
13-52
019
201
6-75
3-29
1-87
0-67
SiO2
TiOj
264
R. A. W1EBE
iO?
CaOFeOT*MnO+
MgO+TiO2«-P2O5
2
Al?03
Fio. 14. Ternary plot of (SiO 2 )-(Na 2 O) + K2O + AljOjMCaO + MnO + MgO + FeO T + TiO2 + P2O3)
(wt. per cent). Symbols as in Fig. 10. Solid ellipse encloses field of liquid immiscibility for the system
K 2 O-FeO-Al 2 Oj-Si0 2 (Roedder, 1951). Diagram is the same as that used by VVeiblen & Roedder (1973)
except that TiOj, P 2 Oj and MnO have been included with MgO + FeO + CaO. Dashed curve represents
suggested field of immiscibility for the late stage differentiates related to the leuconorite unit.
partitioned into the granitic melt even more strongly than K. This would result in
the most differentiated chilled diorites (lowest SiOj) having the highest K/Rb
ratios.
The chemical variation of the chilled rocks can therefore most simply be
explained by late stage liquid immiscibility resulting in the formation of granitic and
dioritic liquids which trend away from each as temperature drops'. The proposed
fractionation trend of liquids can be seen in a ternary plot of SiO27(Na2O + K2O +
A\20^-(Ca0
+ MnO + MgO + FeO T + TiO2 + P2Oj) (Fig. 14). Initial
fractionation of plagioclase drives the parent liquid to lower total alkalis +
aluminum at nearly constant SiO2. Later crystallization of minor pyroxene
produces little visible effect. The liquid changes composition until it encounters a
two-liquid field analogous to the interior two-liquid field in the system SiO 2 Fe2SiO4-KAlSiO4 (Roedder, 1951). The single liquid splits into two liquids: one
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
The major effect would be to change the proportion of phases. On the other hand,
fractional crystallization ought to cause significant variation in the normative
compositions of plagioclase and pyroxene.
The chilled pillows and dikes with lower SiO2 have the lowest values of K and
Rb and the highest K/Rb ratios. The trend from 55 per cent SiO2 to 45 per cent
SiO2 cannot therefore be attributed to fractional crystallization. If the trend were,
however, due to separation of an immiscible granitic liquid the behavior of K and
Rb would depend upon the appropriate distribution coefficients of K and Rb between the coexisting liquids. It seems reasonable to suggest that Rb would be
THE NAIN PLUTONIC COMPLEX, LABRADOR
265
K2o
TiO2
FIG. 15. Ternary plot of KJO-PJO,—TiO 2 (wt. per cent). Dashed lines and arrows indicate approximate liquid
paths during fractionation.
fractional crystallization of a K-bearing mineral, and the removal of ilmenite and
apatite would cause fractionation in the opposite sense required for the dioritic
pillows. In order to be consistent with the oxide plots, the directional sense of the
granitic and dioritic trends must be away from the parent. Clearly the trend on this
ternary diagram can best be explained by liquid immiscibility, K2O being strongly
fractionated into the granitic liquid and TiO2 and P2O5 into the Fe-rich liquid.
With.the exception of Zr, the distribution of major and trace elements between
the dioritic and granitic pillows is essentially compatible with the experimental data
on immiscible liquids (Watson, 1976). The dioritic liquid is strongly enriched in
P2O5, TiO2, FeOT, MnO, MgO and V, and strongly depleted in K2O, SiO2, and Rb.
Sr shows moderate enrichment in the basic liquid. A12O3 and Na 2 O have
distribution coefficients close to 1. The extreme scatter and anomalous behavior of
Zr are at present not understood.
It is possible to gain some insight into the timing of the immiscibility relative to
the sequence of crystallization in the fractionating liquid. A plot of FeO(Tot)/
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
changes continuously toward high iron and low SiOj, and the other, a granitic
liquid, appears removed from the diorites and trends ultimately toward the granite
minimum.
This split in path can also be seen in the concentrations of P2O5, TiO2 and K2O
in the chilled rocks. Early crystallization of plagioclase in the anorthositic parent
would probably have little effect on the mutual ratios of these three oxides in the
residual liquids. On a ternary plot of these three oxides the pillows plot in a
remarkably well defined and linear trend from the K2O apex to TiO 2(7J) P 2 Ojj (25)
(Fig. 15). The average of the leuconorite dikes also plots on that trend and lies between the granitic and dioritic pillows. The overall linear trend cannot be due to
266
R. A. WIEBE
FAFeOT
2
2
MgO
FIG. 16. AFM diagram (wt. per cent) for all chilled rocks. Dashed lines and arrows indicate approximate
liquid paths during fractionation. Symbols as in Fig. 10.
liquid path toward the AF boundary; the specific direction should reflect the ratio
of plagioclase and pyroxene crystallizing. When the two liquid fields is
encountered, the liquid trend splits. The dominant path is the high Fe-trend which
has a directional sense toward the FM edge. The minor path is granitic, back
toward A. Both liquid paths appear to trend to slightly higher Fe/Fe + Mg. The
most surprising 'anomaly' is that the high Fe-trend has the opposite directional
sense commonly assumed for high Fe-trends in layered tholeiitic intrusions (Wager
& Brown, 1967).
Layered diorite
The layered diorites (e.g. Fox Inlet) have upward compositional changes similar
to the low SiO2 trend of the chilled diorites. Like the chilled diorites, there is little or
no change in the compositions of pyroxene and plagioclase. The percentages of
pyroxene, ilmenite, magnetite, and apatite greatly increase upward. I suggest that
these modal changes reflect similar changes in the composition of the fractionating
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
FeO(Tot) + MgO versus SiO2 (Fig. 11) indicates that pyroxene must have begun
to crystallize prior to immiscibility in the interstitial liquid. The relatively constant
ratio of P2O5 and TiO2 within the pillows and the parent magma suggests that
neither apatite nor ilmenite had begun to crystallize prior to immiscibility. The
constant ratio after immiscibility probably reflects strong partitioning of both
elements into the Fe-rich liquid.
A consideration of these liquid trends on an AFM diagram is both instructive
and a bit surprising (Fig. 16). Fractionation of plagioclase defines a liquid path
from the parent away from A. Beginning crystallization of pyroxene deflects the
THE NAIN PLUTONIC COMPLEX, LABRADOR
267
dioritic liquid which produced the layered diorite—changes which are due largely
to separation of an immiscible granitic liquid rather than separation of the phases
which are 'cumulus' in the intrusion. Rare rounded inclusions of microgranophyre
within plagioclase of the Fox Inlet layered diorite (Fig. 6) may represent small
droplets of this silica-rich liquid which were trapped within growing plagioclase
crystals.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
End-stage crystallization of the leuconorite unit
There are two kinds of end-stage liquids associated with this leuconorite
intrusion: Fe-rich diorite and granite. They have been sampled directly as chilled
pillows in hybrid zones along the contact zones between the anorthosite pluton and
granitic plutons. There is also evidence within the leuconorite for the existence of
two distinct end-stage liquids. Small bodies of dioritic culmulates within the
leuconorite demonstrate that the dioritic liquids have managed to segregate into
relatively large pools of magma within the solidifying leuconorite. These layered
bodies grade upward from the leuconorite to high Fe differentiates with low SiO2
and K2O. No separate granitic bodies of comparable size have formed within the
leuconorite, but the granitic end-stage liquids are implied by the prominent
interstitial quartz and K-feldspar which occur uniformly throughout the leuconorite
intrusion. The total volume of rocks formed from segregated residual liquids is very
small. In general, residual liquids cannot have been separated from most of the
leuconorite because most of the leuconorite has a chemical composition very near
to the parent (as recorded in the chilled anorthositic dikes).
The sequence of crystallization displayed in the layered diorites and the liquid
paths recorded by chilled dioritic and granitic pillows should therefore have
occurred interstitially throughout the leuconorite unit. After extensive crystallization of plagioclase in the anorthositic parent magma, pyroxene began to
nucleate and crystallize on widely spaced centers in the interstices between
plagioclase crystals. After crystallization of a few modal per cent of pyroxene, the
residual liquid should have encountered a field of two immiscible liquids
establishing both liquids in the interstices of the leuconorite. Since pyroxene had
already begun to crystallize, existing crystals, rather than new nucleii, continued to
grow mainly from the Fe-rich dioritic liquid producing the zonal sequence typical
of the pyroxene oikocrysts. Plagioclase continued to grow from both liquids.
Ultimately, ilmenite, apatite, and magnetite began to nucleate in the basic liquid.
These phases typically occur within the outer portions or along the margins of the
pyroxene oikocrysts. The textural relations are consistent with the implied
interstitial liquid immiscibility. The spotted appearance of the leuconorite
has probably been enhanced by the immiscibility but the spotted texture originated
from widely spaced nucleation of pyroxene prior to immiscibility.
The immiscible liquids must have been able locally to separate from the
interstices of the leuconorite and from each other. Densities and viscosities of the
two liquids could allow separation of the liquids from each other in isolated pockets
of unmixing magma. Filter pressing was suggested as the mechanism responsible
for removing some of the interstitial liquid from the leuconorite. The dioritic liquid
268
R.A.WIEBE
ACKNOWLEDGEMENTS
I am grateful to J. H. Berg and S. A. Morse for a critical reading of an early draft
of this paper and to S. A. Morse for introducing me to the Nain complex and
providing encouragement and much discussion during these studies. Reviews by R.
F. Emslie and P. R. Whitney were extremely helpful. Field work was supported by
NSF Research Grant EAR73-OO667 to S. A. Morse. Partial support during 1975
was provided by a grant from the American Philosophical Society. Laboratory
studies have been funded by NSF Research Grant EAR75-14423 AOL
REFERENCES
BARTON, J. M , 1974. Progress of isotopic and geochemical investigations, coastal Labrador, 1973. In S. A.
MORSE (ed.), The Nain Anorthosite Project, Labrador: Field Report 1973, Contr. No. 13, Geol.
Dept., Univ. of Mass. 19-28.
BERG, J. H., 1977. Regional geobarometry in the contact aureoles of the anorthositic Nain complex, Labrador.
J. Petrology, 18, 399-430.
BOTTTNGA, Y., & WEILL, D. F., 1970. Densities of liquid silicate systems calculated from partial molar volumes
of oxide components. Am. J. Scl. 269, 169-82.
BRYAN, W. B., FINGER, L. W., & CHAYES, F., 1969. Estimating proportions in petrographic mixing equations
by least-square approximation. Science, 163, 926-7.
BUDDINGTON, A. F., 1939. Adirondack igneous rocks and their metamorphism. Mem. geol. Soc.Am. 7.
HOWER, J., 1959. Matrix corrections in the X-ray spectrographic trace element analysis of rocks and minerals.
Am. Miner. 44, 19-32.
KROGH, T. E., & DAVIS, G. L., 1973. The significance of inherited zircons on the age and origin of igneous
rocks—an investigation of the ages of the Labrador adamellites. Yb. Carnegie Instn. Wash. 72, 610-18.
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
appears to have separated more readily than the granitic liquid, occurring as the
dominant pillow type and contributing to the only layered differentiates. At
1000 °C, the viscosity of dioritic liquid would have been about 3 orders of
magnitude less than that of the granitic liquid and filter pressing would presumably
have caused much more effective removal of the dioritic liquid.
The liquid line of descent for this leuconorite intrusion provides some direct
evidence of the nature of late-stage liquids related to layered tholeiitic intrusions.
Recent work by McBiraey (1975) strongly suggests that the end-stage liquids of
the Skaergaard intrusion encountered a field of immiscibility and separated into
SiO2-rich and FeO-rich liquids. The chilled dioritic and granitic liquids described
here are analogous to those determined experimentally by McBirney (1975). The
end-stage ferrodiorite reported by McBirney has about 44 per cent SiO2 and 27 per
cent FeO T ; the most fractionated ferrodiorite liquid reported here (anhydrous) has
about 45 per cent SiO2 and 21-5 FeOT. Further sampling of the chilled pillows
related to this pluton may yet produce evidence of more extreme liquids.
The liquid path described here represents only one of many possibilities within
the Nain complex. A compositional difference in the parent magma of other
anorthositic plutons should result in significant compositional differences in its
associated late-stage liquids and cumulates, particularly in concentrations of SiO2,
alkalis, TiO2 and P 2 O 5 . Immiscibility is by no means a'necessary end-stage for
other possible parental magmas. For example,-the degree of SiO2 saturation or
under-saturation could permit or preclude late-stage immiscibility. Other intrusions
in the Nain complex appear to have a variety of chilled margins, dikes and pillows
associated with them which could yield evidence of other liquid paths.
THE NAIN PLUTONIC COMPLEX, LABRADOR
269
Downloaded from http://petrology.oxfordjournals.org/ at Penn State University (Paterno Lib) on May 11, 2016
LOFGREN, G., 1974. An experimental study of plagioclase crystal morphology: isothermal crystallization. Am.
J. Sci. 274, 243-73.
MCBIRNEY, A. R., 1975. Differentiation of the Skaergaard Intrusion. Nature, 253, 691-4.
MORSE, S. A., 1968. Revised dispersion method for low plagioclase. Am. Miner. 53, 105-15.
ROEDDER, E., 1951. Low-temperature liquid immiscibility in the system K 2 O-FeO-AljOj-SiO 2 . Ibid. 36,
282-6.
ROSE, H. J., ADLER, I., & FLANAGAN, F. J., 1963. X-ray fluorescence analysis of the light elements in rocks
and minerals. Appl. Spectrosc. 17, 81-5.
SHAW, H. R., 1972. Viscosities of magmatic silicate liquids: an empirical method of prediction. A m.J. Sci. 272,
870-93.
SIMMONS, E. C , & HANSON, G. N., 1977. Geochemistry and petrogenesis of massif-type anorthosites. Abstr.
geol.Soc. Am. 9, 317-18.
STRECKEISEN, A., 1976. To each plutonic rock its proper name. Earth Sci. Rev. 12, 1-33.
WAGER, L. R., & BROWN, G. M., 1967. Layered igneous rocks. San Francisco: W. H. Freeman & Co.
WATSON, E. B., 1976. Two-liquid partition coefficients: Experimental data and geochemical implications.
Contr. Miner. Petrol. 56, 119-34.
WEIBLEN, P. W., & ROEDDER, E., 1973. Petrology of melt inclusions in Apollo samples 15598 and 62295, and
of clasts in 67915 and several lunar soils. Geochim. cosmochim.Acta. Suppl. 4, 1, 681-703.
WIEBE, R. A., 1974. Coexisting intermediate and basic magmas, Ingonish, Cape Breton Island: / . Geol. 82,
74-87.
1976. Contact zone between adamellite and anorthosite and the occurrence of dioritic rocks near Zoar,
Labrador. In S. A. MORSE (ed.). Contr. No. 26, Geol. Dept., Univ. of Mass. 27-33.
1978. Anorthosite and associated plutons, southern Nain complex, Labrador. Can. J. Earth Sci. 15,
1326-40.
(in press). Anorthositic dikes, southern Nain complex, Labrador. Am. J. Sci.