The ca. 1315 Ma Hosenbein massif-type anorthosite pluton (Nain,
Labrador) shows complete exposure of a vertical section, described here
for the first time, comprising a ~2000 m-thick Main Zone bound by a ~200
m-thick Marginal Zone. The pluton as a whole consists mostly of
monocumulate leucogabbronorite, leuconorite, and anorthosite (~94 vol%
of all rocks) with elevated abundances of gabbronorite-ferrodiorite (~27
vol%) in the Marginal Zone. Parental magma likely comprised a
plagioclase-phyric basaltic melt, containing ~15-45 vol% plagioclase
phenocrysts. Crystallization of this magma produced predominantly
massive adcumulate-mesocumulate with less abundant, but regular,
occurrences of(in order of relative abundance) gabbronorite-ferrodiorite
segregations, mottled texture, foliation, pyroxenite aggregates, and
modal layering. Additional key petrographic and compositional features
include high-T deformation microstructures in plagioclase, vertically
consistent mineral compositions, and enrichment of FeTi- P-HFSE-REE in
ferrodiorite. Parts of the Marginal Zone show gneissic-mylonitic fabrics
that developed parallel to magma conduits. Collectively these features
suggest that the textural and structural evolution of the pluton was
controlled by: (a) emplacement of plagioclase-phyric magma, (b)
establishment of vast cumulus crystal frameworks, (c) porous flow of
intercumulus melt, (d) compaction, and (e) synplutonic deformation.
These interpretations contrast with previous work emphasizing the
importance of magmatic flow and crystal settling/flotation.
Un pan vertical des roches d'anorthosite plutonique de
Hoseinbein de massif dont la formation remonterait a environ 1 315 Ma
(de Nain, au Labrador) semble enfierement expose et il est decrit ici
pour la premiere fois. Il s'agit en l'occurrence d'une
zone principale de plus ou moins 2 000 m d'epaisseur, ceinturee par
une zone marginale de plus ou moins 200 m d'epaisseur. Dans son
ensemble, la roche plutonique observee se compose principalement de
monocumulat de leucogabbronorite, de leuconofite et d'anorthosite
(~94% vol. de toutes les roches), dont notamment des quantites elevees
de gabbronorite-jotunite (~27% vol.) dans la zone marginale. Le magma
parent se composerait vraisemblablement d'un magma de basalte a
phenocristaux de plagioclase, contenant approximativement entre 15 et
45% vol. de phenocristaux de plagioclase. La cristallisation de ce magma
a produit surtout de l'adcumulat et du mesocumulat massifs, ainsi
que des mineralisations regulieres mais moins abondantes issues (dans
leur ordre d'abondance) de segregations de gabbronorite-jotunite,
d'une texture marbree, d'une foliation, d'agregats
pyroxenitiques et de rubanements modaux. Parmi les autres
caracteristiques petrographiques et de composition importantes, il y a
des microstructures de deformation de plagioclase survenue a temperature
elevee, des formations minerales uniformes au plan vertical ainsi
qu'un enrichissement de Fe-Ti-P-HFS-REE dans la jotunite. Des
sections de la zone marginale presentent des traces d'une texture
gneissique-mylonitique qui s'est formee parallelement aux
canalisations magmatiques. Dans l'ensemble, ces caracteristiques
portent a croire que l'evolution de la texture et de la structure
des roches plutoniques a ete determinee par les phenomenes suivants: (a)
l'emplacement du magma de phenocfistaux de plagioclase; (b)
l'etablissement de vastes complexes de cristaux de cumulus; (c)
l'ecoulement poreux de magma intercumulus; (d) le compactage; et
(e) une deformation synplutonique. Ces interpretations different quelque
peu des travaux precedents, ou l'accent avait ete mis sur
l'importance de l'ecoulement magmatique, et la decantation et
la flottation de cristaux.
[Traduit par la redaction]
Proterozoic massif-type anorthosite is one of six anorthosite types
(Ashwal 1993), which also include anorthosite in layered mafic
intrusions (e.g., Bushveld Complex) and lunar anorthosite. Based on its
high proportion (~70-100 vol%) of intermediate plagioclase
([An.sub.40-60]) and lack of complementary mafic-ultramafic cumulates,
it is inferred that magmas parental to massif-type anorthosite
originated from the fractional crystallization and contamination of
basaltic magma in the crust-mantle boundary zone (Duchesne 1984; Ashwal
1993; Emslie et al. 1994). The plagioclase-phyric basaltic magmas that
developed through fractionation and contamination were then remobilized
to form massif-type anorthosite intrusions al raid- to upper-levels in
the continental crust (Duchesne 1984; Ashwa11993; Emslie et al. 1994).
The petrogenetic model for massif-type anorthosite differs from the
more in situ origins postulated for other types of anorthosite (Fig. 1).
Lens-like bodies of Archaean megacrystic anorthosite (e.g.,
Fiskenoesset, Bad Vermillion Lake), for example, likely formed through
flotation of plagioclase phenocrysts during in situ fractional
crystallization of basaltic parental magma (Windley et al. 1973; Ashwal
et al. 1983). Anorthosite layers in layered intrusions and ocean basins
are modeled in similar ways (e.g., Kruger and Marsh 1985; Eales et al.
1986). The vast expanses of lunar anorthosite also formed through the
flotation of plagioclase crystals in the same basaltic magma from which
it crystallized (Longhi 1978; Walker 1983; Ashwa11993).
In decades preceding the idea that plagioclase accumulation and
crystallization could occur al separate places in the lithosphere,
petrogenetic models for massif-type anorthosite favoured an in situ
origin similar to those currently accepted for layered intrusive (and
other) anorthosite. These earlier models were built on the assumption
that "significant mafic to ultramafic material was buried beneath
anorthosite massifs" (Ashwal 1993, p. 205), which has since widely
been disproved (Ashwal 1993; Funck et al. 2000). Regardless, the in situ
models influenced early work on massif-type anorthosite from the Nain
area (Labrador), with field investigations widely reporting igneous
textures and structures consistent with magma undergoing vigorous
magmatic convection and fractional crystallization (Morse 1969a, 1972;
Berg 1972, 1973; Davies 1973; Ranson 1975, 1981). Since then, no attempt
has been made to integrate the field characteristics with the newer
petrogenetic models for massif-type anorthosite, leaving some
uncertainty in regards to how viscous plagioclase-phyric parental magmas
could have crystallized under apparently dynamic conditions.
[FIGURE 1 OMITTED]
This article attempts to resolve the contrast between early field
studies on Nain anorthosite and currently accepted petrogenetic models
through detailed mapping of the Hosenbein pluton. This intrusion was
traversed to collect data on the relative abundances of igneous rock
types, textures, and structures, which were then used to construct the
first complete vertical section through an anorthosite pluton in the
Nain batholith and to discuss the relative significance of magmatic
processes involved in its textural and structural evolution (Voordouw
2006). Those data are presented in this paper.
The Hosenbein pluton lies near the center of the 1364-1289 Ma Nain
anorthosite-granite batholith (Fig. 2a), which is arguably the
world's type locality for massif-type anorthosite. The Nain
batholith was emplaced across the intersection between crustal-scale
structures related to the east-west trending Gardar-Voisey's Bay
Fault Zone and the north-south trending Paleoproterozoic Torngat Orogen
(Myers et al. 2008). Together, these structures likely controlled the
ascent and emplacement of magmas (Myers et al. 2008) and formation of
the world-class Voisey's Bay Ni-Cu deposit (Evans-Lamswood et al.
The Nain batholith was emplaced in two magmatic episodes (Myers et
al. 2008), each of which began with emplacement of massif-type
anorthosite (1364-1340 Ma and 1319-1295 Ma) and ended with granitoids
(1322-1319 Ma and 1292-1289 Ma). Anorthosite is synonymous with a range
of plagioclase cumulates that include anorthosite sensu stricto
([greater than or equal to] 90 vol% plagioclase) as well as leuconorite,
leucotroctolite and leucogabbronorite (each containing 67.5-90 vol%
plagioclase). Granitoids consist mostly of granite, quartz monzonite and
monzonite, which range from biotite- and hornblende-bearing to pyroxene-
and olivine-bearing (Emslie and Stirling 1993). Troctolitic and
gabbronoritic magmas (e.g., Kiglapait in Morse 1969b) were emplaced in
both episodes (Myers et al. 2008), sometimes in close association with
ferrodiorite and monzonite (de Waard et al. 1976; Gaskill 2005).
The Hosenbein pluton (Figs 2b, 2c) lies near the center of the Nain
batholith and was emplaced at ca. 1315 Ma (Voordouw 2006), near the
beginning of episode 2. Parts of this pluton were mapped over the last
few decades (Wheeler 1960; Rubins and de Waard 1971; Rubins 1973; Ryan
2000, 2001) with the current boundaries identified through mapping as
part of a thesis study by the author (Voordouw 2006). The structural
subdivisions, textures, and structures of this pluton are described in
more detail below.
[FIGURE 2 OMITTED]
Geological mapping of the Hosenbein pluton was done at 1:10 000 to
improve on 1:50 000 maps published by the Geological Survey of
Newfoundland and Labrador (Ryan 2000, 2001). Exposures are abundant and
were mapped through ~200 km of foot traverses that included recording
detailed descriptions at 441 point locations. The vertical distribution
of rock types, textures, and structures was determined by subdividing
the lower and upper halves of the Main Zone into five intervals each
(Fig. 3), with each containing between 31-54 point locations.
Quantitative mineralogical and geochemical data were determined at
Memorial University of Newfoundland. Mineral compositions for
plagioclase (N = 93), clinopyroxene (N = 65) and orthopyroxene (N = 48)
cores were obtained with a Cameca SX-50 microprobe, using
wavelength-dispersive spectrometry (WDS) with count times between ~10-20
seconds. Whole-rock major element abundances for
anorthosite-leucogabbronorite (N = 5) and gabbronorite-ferrodiorite (N =
3) were determined by X-ray fluorescence on a Fissions/Applies Research
Laboratories model 8420+ sequential WD X-ray spectrometer, using the
operating conditions described by Longerich (1995). Trace and rare earth
elements were determined with a Finnigan Neptune multicollector ICP-MS
following the procedures of Longerich et al. (1990).
New geological mapping outlines the Hosenbein pluton as a T-shaped
basinal structure subdivided into Main and Marginal zones (Fig. 2b, 2c).
The Main Zone (~50 [km.sup.2]) is ~2000 m thick and is bound by the
Marginal Zone (~2 [km.sup.2]), which is an average of ~200 m thick. The
Marginal Zone is further subdivided into Lower and Upper units that
bound the lower half and upper halves of the Main Zone, respectively
(Fig. 2c). The northern and western contacts of the pluton abut
Paleoproterozoic(?) ultramafic-mafic gneiss or dips gently underneath
the ca. 1340 Ma Unity massif-type anorthosite (Voordouw 2006). The
southern part of the pluton is crosscut by the ca. 1310 Ma Kikkertavak
anorthosite complex, and the eastern contact abuts Paleoproterozoic(?)
anorthosite gneiss or lies underwater.
[FIGURE 3 OMITTED]
The Marginal and Main Zones are described below in stratigraphic
succession from bottom to top. The emphasis is on lithologies, texture,
and structure, although some supplemental petrographic, mineralogical,
and geochemical data are also provided.
Lower Marginal Zone
The Lower Marginal Zone is a composite subunit formed by partial
ring dykes, straight dykes, and lenticular sheets. Rock types consist of
medium- to very coarse-grained leucogabbronorite (~65 vol%),
gabbronorite-ferrodiorite (~30 vol%), and anorthosite (~5 vol%).
Intra-subunit contacts range from gradational to sharp, with the latter
lacking chilled margins, and inclusions of any lithology can occur in
the others. Contacts with the overlying Main Zone are gradational
whereas those with underlying Paleoproterozoic(?) gneiss are sharp.
Textures and structures are diverse and include massive, mottled,
modally layered, and foliated rocks. Massive and mottled rocks may show
adcumulate (~93-100% cumulus phase), mesocumulate (~85-93%), or
orthocumulate (~75-85%) texture, with mottled rocks comprising two or
more textures in close association (cm- to m-scale). Modal layering
occurs at ~25% of localities and is defined by sharp to gradational
changes in the modal proportions of plagioclase and pyroxene, over
widths ranging from just ~1 cm to ~10 m. Most layers are moderately
dipping to subvertical (~40[degrees]-90[degrees]) and strike parallel to
the north-south trending Paleoproterozoic(?) gneiss that underlies it.
Textures are typically recrystallised and granoblastic
("gneissic"), rather than primary igneous (Figs 4a, 4b), and
grade into steeply dipping and finely layered rocks comprising relict
igneous crystals in an even finer grained granoblastic groundmass
("mylonitic"). Foliation occurs at ~15% of localities and is
defined by the shape preferred orientation (SPO) of cumulus crystals or
crystal aggregates of pyroxene and/or plagioclase, with the latter
defining foliation in layered gneissicmylonitic rocks. Foliations strike
parallel to modal layering and Paleoproterozoic(?) structures.
Inclusions were found at ~10% of localities, and include both
autoliths and xenoliths. Autoliths consist of any Marginal Zone
lithology hosted within a younger Marginal Zone lithology whereas
xenoliths consist of Paleoproterozoic(?) gneiss and pervasively
recrystallised, ca. 1340, Unity and Mount Lister anorthosites. Many
autoliths and xenoliths have tablet-like forms that lie parallel to
modal layering, foliation and nearby Paleoproterozoic (?) structures.
Pegmatoids form lens-like bodies that consist of coarse- to very
coarse-grained leucogabbronorite, gabbronorite, or ferrodiorite, some of
which have crescumulate texture. Like the inclusions, pegmatoids strike
parallel to most of the other planar features in their vicinity.
Modal mineralogies are defined by plagioclase (~30-95 vol%),
orthopyroxene (<1-45 vol%) and clinopyroxene (<1-25 vol%), with
accessory (<10 vol%) abundances of Feo-Ti oxide, olivine, hornblende,
biotite, apatite, and secondary minerals (e.g., sericite, uralite,
chlorite, carbonate). Plagioclase, olivine and, in places, orthopyroxene
are the only cumulus minerals. Compositions range from ~[An.sub.50-60]
for plagioclase, ~[En.sub.55-59] for orthopyroxene and ~[En.sub.36-40]
for clinopyroxene, and average ~[Fo.sub.45] for olivine (Voordouw 2006).
The Main Zone consists mostly of medium- to coarse-grained
leucogabbronorite-leuconorite (~60 vol%) and anorthosite (~40%) with
<1 vol% gabbronorite-ferrodiorite. Abundances of
leucogabbronorite-leuconorite are especially high, between ~60-80% of
the vertical extent through the Main Zone, whereas anorthosite is
abundant in the lower half (Fig. 5). The gradational contact with the
Marginal Zone is marked by an increase in pyroxene and in the dip of
modal layering. The northeastern ~20 [km.sup.2] of the Main Zone
consists of block structure developed directly underneath the Unity
anorthosite (Fig. 6), with block structure comprising numerous xenoliths
(up to ~100 m long) of pervasively recrystallised Unity anorthosite
hosted by igneous-textured leucogabbronorite-leuconorite (Fig. 7a).
[FIGURE 4 OMITTED]
Leucogabbronorite-leuconorite and anorthosite are mostly massive
with less abundant mottled rocks (~20% of localities). Massive
anorthosite has adcumulate texture comprising tightly packed cumulus
plagioclase with accessory (<10 vol%) abundances of intercumulus
pyroxene and Fe-Ti oxide. The mesocumulate texture that characterizes
leucogabbronorite-leuconorite is defined by higher proportions of
intercumulus pyroxene and Fe-Ti oxide. Most massive rocks, however, show
a subtle interspersing of adcumulate and mesocumulate textures (Fig. 7b)
that in some cases grades into more coarsely segregated mottled rocks
(Fig. 7c). These mottled rocks consist of subspherical to shapeless
patches (~1-1000 cm in size) of mesocumulate
leucogabbronorite-leuconorite hosted in adcumulate anorthosite.
Intercumulus pyroxene may show optical continuity across a single patch,
although this situation appears to be rare. Other mottles approach
gabbronorite-ferrodiorite composition and may further aggregate into
large patches and dykes, as described in more detail below. In both
massive and mottled rocks, cumulus plagioclase crystals impinge on at
least one, and typically more, neighbouring plagioclase crystals.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Foliated leucogabbronorite-leuconorite and anorthosite occur at
~15% of localities, showing mostly uniform distribution except for an
anomalously high concentration at ~20-30% of the vertical extent through
the Main Zone (Fig. 6). Foliation is defined by disc-shaped intercumulus
crystals of pyroxene and/or Fe-Ti oxide (Fig. 7d), and generally strikes
east-west and dips moderately to steeply north
Modally layered rocks composed of leucogabbronorite-leuconorite and
anorthosite occur at just 5% of localities, and are therefore
significantly less abundant than massive, mottled, and foliated rocks.
At ~60-70% of the vertical extent through the Main Zone, however, modal
layering occurs at an anomalously high proportion (~15%) of localities
(Fig. 6). Typical layers comprise lens-like bodies of mesocumulate
leucogabbronorite-leuconorite hosted in adcumulate anorthosite (Fig.
7e). These lenses are typically less than a meter thick and tens of
meters long, and dip at gentle to moderate angles towards the center of
Intercumulus aggregates of relatively large pyroxene crystals occur
at ~15% of localities in the Main Zone and are especially abundant in
the lower half (Fig. 6). They are of three types: (1) vein-like, (2)
pod-like, and (3) corona-like. Vein-like aggregates comprise steeply
dipping layers that are typically a few centimetres thick and up to tens
of meters long (Fig. 8a). Their orientation with respect to foliation
and modal layering ranges from concordant to highly discordant. Pod-like
aggregates are shorter and rounder, and may consist of just a few,
exceptionally large, pyroxene crystals. Corona-like aggregates are
formed at contacts with Unity anorthosite xenoliths (Fig. 8b) and
synplutonic gabbronorite-ferrodiorite dykes, with sizes ranging from
~1-2 cm centimetres thick and ~10-100 cm long.
Gabbronorite-ferrodiorite forms dykes (up to ~10 m thick) and
shapeless aggregates (up to ~500 [m.sup.2]) throughout the Main Zone,
occurring at a remarkable ~35% of localities. Thicker (>30 cm) dykes
are particularly abundant within the uppermost parts of the Main Zone
whereas the lower half contains more thin (<1 cm) dykes (Fig. 6).
Contacts with host rocks range from curved and gradational to sharp and
planar (Fig. 7f), and typically lack chilled margins. Textures range
from orthocumulate to granular, with some granular-textured dykes
showing pervasive foliation and modal layering striking parallel to the
Modal mineralogies of lithologies in the Main Zone typically
consist of plagioclase (40-100 vol%), orthopyroxene (0-30 vol%), and
clinopyroxene (0-15 vol%), with accessory abundances of Fe-Ti oxide,
sericite, uralite, apatite, hornblende, biotite, chlorite, carbonate,
epidote, and quartz. Cumulus plagioclase crystals universally impinge on
their neighbours (Fig. 9), and many have deformation twins, serrated
margins, undulatory extinction, and/or brittle fractures. Core
compositions range between [An.sub.43-60], but are vertically uniform
(Fig. 10), and compositional zoning consists only of narrow Ca-rich rims
(Voordouw 2006). Orthopyroxene and clinopyroxene are intercumulus and
have compositions ranging from [En.sub.28-62] and [En.sub.25-40],
respectively. Besides the occurrence of the most Fe-rich pyroxene within
the uppermost part of the Main Zone, there appears to be no systematic
vertical change in pyroxene composition.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
Whole-rock major-oxide abundances in leucogabbronorite-leuconorite
include high amounts of [Al.sub.2][O.sub.3], [Na.sub.2]O, Ba, and Sr
(Table 1), consistent with high abundances of plagioclase. FeO shows
strong positive correlations (r [greater than or equal to] 0.9) with
MnO, MgO, Y, Zr, Ta, U, and rare earth elements (REE). Spidergrams show
depletions in high field strength elements (HFSE) (Fig. 11) and REE
patterns show a positive Eu anomaly as well as enrichment of light REE
relative to heavy REE. Gabbronorite shows relatively high wt% MgO,
spidergrams that are similar to enriched leucogabbronorite-leuconorite
(Fig. 11), and fiat REE patterns. Ferrodiorite contains high wt%
Ti[O.sub.2], FeO, [P.sub.2][O.sub.5], HFSE, and REE.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
Upper Marginal Zone
The Upper Marginal Zone is compositionally, structurally and
texturally similar to the Lower Marginal Zone, comprising several small
sheet-like bodies of medium-grained to very coarse-grained
leucogabbronorite (~50 vol%), gabbronorite-ferrodiorite (~25 vol%), and
anorthosite (~25 vol%). Contacts between lithologies are likewise sharp
to gradational, with some sharp contacts exhibiting an intrusive nature.
Structures and textures are mostly massive, modally layered (25% of
localities), mottled (15%), and foliated (10%), with massive and mottled
rocks similar to those in the Lower Marginal Zone. Well developed modal
layering is typically associated with foliation and equigranular
granoblastic (or gneissic) texture (Fig. 12a), just like in the Lower
Marginal Zone. Gradations from gneissic to mylonitic layering occur as
well. Orientations strike east-west, parallel to the Voisey's
Bay-Gardar Fault Zone, and are also moderately dipping to subvertical
(~50[degrees]-90[degrees]). Other igneous structures that strike
parallel to this fault zone include foliation and tablet-shaped
[FIGURE 12 OMITTED]
Inclusions consist of autoliths and xenoliths. Autoliths comprise
any type of Upper Marginal Zone lithologies hosted in another type, and
attest to the composite nature of this subunit. Contacts between
autoliths and host rocks are sharp to gradational, with some gradational
contacts formed by mixed zones of disaggregated crystals and host rock
(Fig. 12b). Xenoliths include tablets of reworked Paleoproterozoic(?)
gneiss that lie parallel to modal layering and foliation.
Modal mineralogies consist of plagioclase (15-95 vol%),
orthopyroxene (<1-60 vol%), and clinopyroxene (<1-40 vol%) with
accessory abundances of Fe-Ti oxide, hornblende, biotite, apatite and
secondary minerals (sericite, uralite, chlorite, carbonate). Cumulus
minerals consist of plagioclase and orthopyroxene, and mineral
compositions range from [~An.sub.41-55] for plagioclase, [En.sub.35-59]
for orthopyroxene and [En.sub.27-39] for clinopyroxene (Voordouw 2006).
The Hosenbein pluton shows many of the key structural, textural,
and compositional characteristics of massif-type anorthosite, including
(1) predominance of rocks containing 67.5-100 vol% plagioclase, (2)
minor amounts of gabbronorite and ferrodiorite, (3) lack of
complementary mafic-ultramafic cumulates, (4) predominantly massive
adcumulate-mesocumulate texture, (5) mottling, foliation, and modal
layering, (6) block structure, (7) plagioclase ([An.sub.41-60]) and
orthopyroxene ([En.sub.28-62]) of intermediate composition, and (8)
positive whole-rock Eu and Sr anomalies. As such, the following
discussion on parental magmas and magmatic processes may be relevant to
massif-type anorthosite in general.
Parental Magmas: Crystal to Melt Ratio and Rheology
Parental magmas for massif-type anorthosite comprise
plagioclase-phyric basaltic magmas that were separated from
complementary mafic-ultramafic cumulates in the crust-mantle boundary
zone (Fig. 1) (Duchesne 1984; Asbwai 1993; Emslie et al. 1994). The
composition and crystal to melt ratio of such parental magmas can be
estimated from the bulk composition of the Hosenbein pluton, assuming
most magma crystailized within the boundaries of the pluton. In this
specific case, the estimate is reliable because the plan view and
complete vertical section of the pluton are so well exposed. The
weighted average for each rock type in the pluton is ~60 vol%
leucogabbronorite-leuconorite, ~34 vol% anorthosite and ~6 vol%
gabbronorite-ferrodiorite. Assuming mean plagioclase abundances of 80,
95 and 50 vol% for each lithology, respectively, the resultant bulk
composition of the pluton is ~83 vol% plagioclase and ~17 vol% pyroxene
plus accessory minerals. Further assuming that cotectic ratios of
plagioclase to pyroxene equal ~7:3 (Morse, pers. comm. 2007), it follows
that the parental magma to the pluton had a crystal to melt ratio around
The high crystal to melt ratio for magmas parental to the Hosenbein
pluton and other massif-type anorthosite (e.g., Longhi et al. 1993)
differs from the cotectic basaltic magmas inferred to be parental to
Archaean megacrystic, layered intrusive, ocean basin, and lunar
anorthosite. Because increasing crystal contents cause significant
increases in magma viscosity (Marsh 1981; Longhi et al. 1993; Philpotts
and Carrol11996), it is likely that the crystallization processes of
massif-type anorthosite were different as well. Yet much of the work
published on Nain anorthosite describes evidence for crystallization in
dynamic magmatic environments (e.g., Morse 1969a, 1972; Berg 1972, 1973;
Davies 1973; Ranson 1975, 1981) that are atypical of viscous
crystal-laden magma. In contrast, processes important in the
crystallization of such magmas, like porous flow and cumulate compaction
for example (Philpotts et al. 1996; Meurer and Boudreau 1998a, 1998b),
are rarely invoked. The remainder of this discussion explores whether
the apparent disregard of high viscosity magmatic process is consistent
with textures and structures observed in the Hosenbein pluton.
Before continuing, however, it is worth briefly discussing the
possible origins of Hosenbein gabbronorite-ferrodiorite, which likely
crystallized from magmas with relatively low crystal to melt ratios.
Possible origins for these crystal-poor magmas include (1) derivation
from the same deep staging chamber that produced plagioclase-phyric
magmas, (2) fractional crystallization of plagioclase-phyric magmas at
the emplacement level, and (3) intrusion of magmas unrelated to the
pluton (e.g., derived from other magma chambers, partial melting of
mafic-ultramafic sources). In the first case, some of the basaltic melt
residing in deep staging chambers may have been removed without being
enriched in plagioclase phenocrysts. Such an origin is consistent with
the broadly similar mineralogy of gabbronorite-ferrodiorite to that of
the plagioclase cumulates, and their broadly comagmatic origin as
suggested by gradational contact morphologies. The second case is most
relevant to ferrodiorite, which has compositional features (sodic
plagioclase, Fe-rich pyroxene, high wt% Fe-Ti-P-HFSE-REE) consistent
with a residual origin (see also Mitchell et al. 1996; Bhattacharya et
al. 1998). The third case is also possible but implies the remarkable
coincidence that melts of appropriately similar and, in some cases,
evolved compositions were selectively emplaced into the pluton during
and shortly after its emplacement.
Processes that may have been significant during crystallization of
plagioclase-phyric magma in the Hosenbein pluton include (in approximate
order of increasing magma viscosity) (1) convective flow, (2) crystal
settling and/or flotation, (3) development of cumulate crystal
frameworks, (4) porous flow of intercumulus melt, (5) compaction, and
(6) synplutonic deformation. Additional processes like liquid
immiscibility and deuteric alteration probably played only a minor role
in crystallization, and are therefore omitted.
Convective flow is characteristic of low-viscosity,
phenocrysts-poor, magmas like those parental to Archaean megacrystic and
layered intrusive anorthosite. Igneous structures sometimes interpreted
as palaeo-convective indicators include cross-beds, trough structures,
foliation, and autoliths (e.g., Wager et al. 1960; Irvine et al. 1998),
of which only the latter two occur in the Hosenbein pluton.
Foliation indicative of palaeo-convective flow is typically defined
by the alignment of phenocrysts, with long axes parallel to the flow
direction (e.g., Paterson et al. 1989). In contrast, foliation within
most of the Hosenbein pluton (i.e., Main Zone) is defined by the
alignment of intercumulus pyroxene. Plagioclase phenocrysts, on the
other hand, are randomly oriented in even the most strongly foliated
rocks, suggesting it is unlikely that foliation originated through
convective flow. Alternative magmatic origins for foliation include
diapiric ascent, ballooning, and compaction (e.g., Barnichon et al.
1999; Meurer and Boudreau 1998b). Foliation developed during diapirism
and ballooning would tend to concentrate along the margins of the
pluton, which contrasts with the relatively uniform occurrence of
foliation throughout the Main Zone. Perhaps the most likely origin is
compaction, or other types of synplutonic deformation, as this is
consistent with both the uniform and intercumulus nature of the
foliation. The slightly more pervasive foliation developed ~1/4 of the
way up the Main Zone may have resulted from the optimum combination
of(l) maximum mass of overlying cumulates (to trigger compaction) with
(2) thermal insulation from country rock (which delays solidification so
as to allow for compaction to occur).
Foliation in the Marginal Zone is relatively scarce and only
locally defined by the alignment of plagioclase and/or pyroxene
phenocrysts. If this foliation originated through convection then the
steep dips of these foliations imply that convective flow was driven by
subhorizontal, rather than vertical, temperature gradients. Possible
alternative origins include a combination of intrusion-related flow and
synplutonic deformation, which could explain foliation of phenocrysts
and granular-textured aggregates, as well as the spatial association
with gneissic-mylonitic layering.
Autoliths of low-density roof cumulates hosted in high-density
floor cumulates may have been transported downwards by convective flow
(e.g., Irvine et al. 1998). In the Hosenbein pluton, autoliths occur
only in the Marginal Zone and field relations suggest derivation through
pulsating emplacement rather than transport in convective currents.
Furthermore, Unity xenoliths that form block structure in the Main Zone
are concentrated directly underneath unbroken exposures of Unity
anorthosite, suggesting limited movement of xenoliths and thereby a lack
of convective flow.
The settling and flotation of cumulus crystals is also restricted
to low viscosity aphyric magmas, which would allow rapid and unimpeded
vertical movements of cumulus crystals. Indicators of crystal
settling/flotation include modally-graded and grain size-graded
layering, of which only modal layers were observed in Hosenbein
outcrops, as well as cryptic layering.
Modally graded layers are widespread within layered
mafic-ultramafic intrusions (e.g., Bushveld Complex) where they may form
through the settling and flotation of one or more cumulus minerals.
However, research indicates that magmatic modal layers may originate
through many other processes (Naslund and McBirney 1996), including, for
example, diapiric ascent and synmagmatic deformation (e.g., Ameglio et
al. 1997; Barnichon et al. 1999; Royse et al. 1999; Royse and Park
2000). In the Hosenbein pluton, modal layering is most abundant in the
Marginal Zone, where it shows mostly gneissic texture that locally
grades into mylonite. Furthermore, these gneissic-mylonitic modal layers
are subvertically oriented and strike parallel to conduits that
controlled the ascent and emplacement of magmas, suggesting that modal
layers originated through synplutonic deformation in magma conduits
rather than crystal settling/flotation.
Modal layering in the Main Zone is sparse and significantly
subordinate to massive, mottled, and foliated rocks. Furthermore, these
modal layers are defined by changes in the proportions of one cumulus
phase (plagioclase) and several intercumulus minerals, and so may have
originated through the redistribution of intercumulus liquid rather than
crystal settling/flotation. Evidence for paleo-crystal
settling/flotation is therefore at best equivocal and localized.
Cryptic layering may record paleo-crystal settling and/or flotation
through gradual changes in the composition of cumulus minerals (e.g.,
Morse 1969b), like an upward enrichment of Na in plagioclase and/or Fe
in orthopyroxene. In the Hosenbein pluton, plagioclase is the only
cumulus phase and shows no systematic vertical change in composition
across the Main Zone. The wider range in cumulus plagioclase
compositions in the Marginal Zone is likely related to the interplay of
(1) pulsating emplacement of gabbronorite-ferrodiorite magmas with
slightly different plagioclase compositions, (2) localized fractional
crystallization of these magma pulses, and/ or (3) migration of
ferrodiorite residual melts from Main to Marginal Zone.
Development of plagioclase crystal frameworks
Cumulus plagioclase crystals suspended in silicate melt tend to
link together and, eventually, coagulate into cumulus crystal
frameworks, possibly starting at crystal to melt ratios of ~35:65
(Philpotts and Carroll 1996; Philpotts et al. 1996). Establishment of
these frameworks results in a rapid increase in magma viscosity and the
related inhibition of low viscosity magmatic processes (Philpotts and
The Main Zone contains ~85 vol% plagioclase, with most cumulus
plagioclase crystals impeding on at least one neighbouring crystal. From
this simple observation alone it seems almost certain that vast
frameworks of cumulus plagioclase spanned across much, if not the
entire, Main Zone. Furthermore, the estimated ~45:55 crystal to melt
ratio of the parental magma lies well above the ~35:65 ratio required to
form rigid crystal frameworks, suggesting that plagioclase crystal
frameworks formed shortly after magma emplacement. Crystallization of
the Main Zone may therefore have occurred exclusively through processes
typical of high viscosity magmas, consistent with the previously
discussed scarcity of palaeo-convective, -crystal settling and -crystal
The bulk composition of the Marginal Zone (~74% plagioclase and
~26% pyroxene plus accessory minerals) suggests that the crystal to melt
ratio of parental magma was only ~15:85, well below the ~35:65 threshold
for developing crystal frameworks. Hence, these magmas could have
produced dynamic textures and structures for some time after
emplacement. However, the significant abundance of plagioclase cumulates
in the Marginal Zone (~73 vol%) suggests that the development of cumulus
frameworks was inevitable and would have eventually connected frameworks
in the Main Zone to the walls of the pluton. Such pluton-wide crystal
frameworks play an important role in the promotion of synmagmatic
deformation (Barros et al. 2001), as discussed further below.
Porous flow of intercumulus melt
Cumulus crystal frameworks contain intercumulus melts that are
significantly more mobile than the cumulus phases (McBirney and Hunter
1995; Philpotts and Carroll 1996; Philpotts et al. 1996). These melts
can crystallize in situ to form intercumulus minerals or they can be
aggregated through porous flow and, in places, segregated (e.g.,
Philpotts et al. 1996; Mitchell et al. 1996; Bhattacharya et al. 1998).
In the Hosenbein pluton, possible textural and structural indicators for
the palaeo-porous flow of intercumulus melts include (1) mottled texture
(2) pyroxenite aggregates, and (3) ferrodiorite dykes and patches. As
discussed previously, the composition and gradational contacts of
ferrodiorite bodies are consistent with an origin through the
aggregation and segregation of residual melts (Mitchell et al. 1996;
Bhattacharya et al. 1998).
Mottled rocks can be interpreted as plagioclase cumulate that
crystallized with heterogeneously distributed intercumulus melt, as a
homogeneous distribution would have produced a massive rock. Perhaps
this heterogeneous distribution was the result of low nucleation rates
at a late stage in the crystallization process, which forced dissolved
pyroxene components to move towards existing intercumulus nuclei rather
than nucleating in situ. In this case, these components would have used
porous flow to move towards nucleation sites. There are few other viable
interpretations for mottled texture. A cotectic cumulate origin, for
example, is inconsistent with the lack of optical continuity of
orthopyroxene in individual patches and the lack of crystal-settling
Pyroxenite aggregates may also have originated from a combination
of porous flow and low nucleation rates, which would have facilitated
concentration of pyroxene components and the growth of large crystals.
Vein-like aggregates may have formed in planar zones of high porosity
developed during late-magmatic fracturing of the cumulus plagioclase
framework, possibly in response to cooling (Petersen 1987), tectonic
stresses (Barros et al. 2001), and/or compaction (Philpotts et al.
1996). Pod-like aggregates may have formed in a similar way but had room
to grow in three, rather than just two, dimensions. Corona-type
aggregates that formed on xenolith margins may be analogous to
pegmatoidal "strain shadows" reported in some granitoids,
which formed through the flow of intercumulus melt into low-pressure
areas generated by the rotation of xenoliths during late-magmatic
deformation (Paterson and Miller 1998).
Compaction causes the densification of cumulates and simultaneous
removal of intercumulus melt through porous flow (Hunter 1996). Textural
and structural indicators of compaction include adcumulate texture
(Hunter 1996) and high-T deformation microstructures (Lafrance et al.
1996). Possible driving forces include the weight of overlying cumulates
(McKenzie 1984) and synplutonic stress (Barros et al. 2001), the latter
of which is especially effective if the pluton is traversed by crystal
frameworks (Barros et al. 2001).
Adcumulate textures may evolve through compaction (Hunter 1996) or
efficient chemical exchange between intercumulus liquid and a
continuously convecting, overlying, melt reservoir (Tait et al. 1984;
Morse 1986). Within Main Zone anorthosite, adcumulate textures are
widely developed and associated with low abundances of intercumulus
minerals. The origin of these adcumulate textures through exchange with
an overlying reservoir would be indicated by preservation of either
significant plagioclase-pyroxene rocks with cotectic proportions and/or
complementary mafic-ultramatic cumulates. Neither is significantly
developed within the Hosenbein pluton. In contrast, the widespread
evidence for palaeo-porous flow and, as discussed below, high-T
deformation structures, suggest that an origin through compaction is
The high-T deformation microstructures that are so widely developed
in the Main Zone likely formed during crystallization, as the Nain
batholith cooled and was uplifted almost immediately after its
emplacement (Hill 1982; Wiebe 1985). In this case, possible origins for
these microstructures include compaction and diapiric ascent. The latter
is unlikely since the pluton lacks the diapiric intrusive form as
characterized in previous works (Wiebe 1992; Barnichon et al. 1999).
Compaction, on the other hand, is consistent with the widespread
development of adcumulate texture and the indicators for paleo-porous
flow of intercumulus melt. More specifically, compaction driven by
synmagmatic tectonics is consistent with conduit-parallel
gneissic-mylonitic layering within the Marginal Zone as well as the
inferred development of trans-pluton crystal frameworks. Compaction
driven by the weight of overlying cumulates, on the other hand, is
unlikely because the cumulus minerals had similar to lower density
relative to the intercumulus melt.
Two other features in the Main Zone may be consistent with
compaction; (1) the lower half contains a higher proportion of
anorthosite than the upper half, and (2) gabbronorite-ferrodiorite dykes
in the lower half tend to be thinner than those in the upper hall:.
Collectively, these features suggest that intercumulus melts moved
upwards from the lower half of the Main Zone into the upper hall and,
possibly, the Upper Marginal Zone (possibly explaining the occurrence of
Fe-rich pyroxene in this part of the pluton).
Synplutonic deformation may be related to diapirism and/or
synplutonic tectonic activity, and is indicated by (1) high-T
deformation microstructures within igneous minerals (Laffance et al.
1996), (2) gneissic fabrics within pluton margins (e.g., Barnichon et
al. 1999), and, in some cases, (3) foliation. Possible origins of high-T
deformation and foliation through synplutonic deformation were discussed
Gneissic-mylonitic fabrics are relatively abundant in the Marginal
Zone of the Hosenbein pluton, where they strike parallel to
crustal-scale magma conduits. Hence, these fabrics may have formed
through synplutonic deformation of the Marginal Zone on magma conduits
(e.g., Ameglio et al. 1997; Royse and Park 2000). Alternative origins
include diapiric ascent and ballooning, the former being unlikely as the
pluton lacks a diapir-like structure. Ballooning-related deformation, on
the other hand, is typically driven by swelling as magma is emplaced
into the central part of the pluton. The Marginal Zone, however, strikes
parallel to conduits that were widely exploited during emplacement of
the Nain batholith, suggesting that magmas were emplaced along the
margins rather than in the central part of the pluton.
SUMMARY AND CONCLUSIONS
The key to determining the magmatic processes that controlled
crystallization of the Hosenbein pluton is in unravelling the relative
abundances of igneous textures/structures, and to find the common
denominator for the most abundant features. This approach suggests that
the following processes were important: (a) development of cumulus
crystal frameworks, (b) porous flow of intercumulus melt, (c) compaction
and (d) tectonically induced synplutonic deformation. Just as important
is that these processes are consistent with the crystallization of a
viscous plagioclase-phyric parental magma.
The contrast between interpretations made in this study and those
in previous work illustrate how theoretical models are both developed by
and guide field observations. The seminal work done on Nain anorthosite
in the 1970s occurred shortly after the development of models explaining
layering in the Skaergaard and Bushveld intrusions, motivating field
geologists keen to test this model to search for indicator structures in
other intrusive rocks. Although the Nain anorthosites do indeed contain
such features, the later (1980s and 1990s) development of models
suggesting alternative origins for igneous structures and petrogenetic
schemes for massif anorthosite worked against interpretations that Nain
anorthosite evolved in dynamic magma chambers. The results of this study
also demonstrate that it is important to consider the relative abundance
of igneous structures rather than the occurrence of a single
cross-bedded or size-graded layer.
The Hosenbein pluton is ideally suited for future investigations of
massif-type anorthosite and magmatic processes, as it is the only
intrusive body directly accessible from the town of Nain. Future
investigations could focus on providing geochemical and theoretical
constraints on magmatic processes through, for example, testing for
isotopic disequilibrium between cumulus and intercumulus phases and
detailed structural analysis.
I thank my PhD supervisor, John Myers, for funding and encouraging
the research. Paul Sylvester and Pam King are thanked for producing
whole-rock geochemical data. Mike Schaeffer and Maggie Piranian helped
with electron microprobe analyses. Fieldwork was done with the
assistance of Warren Brown, Johan Voordouw, and Cory Furlong. Thanks
also to Voisey's Bay Nickel Company (VBNC) for logistical support
in the field. The research was funded by Natural Sciences and
Engineering Research Council (NSERC) and VBNC through a CRD project
grant (CRDPJ 233669-99) to John Myers. Additional funds were provided
through an NSERC postgraduate fellowship, the Estate of Alfred K.
Snelgrove and the School of Graduate Studies. The author thanks the
journal reviewers and editor for their helpful comments that led to
clarifications and improvements in the manuscript.
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Editorial responsibility: Sandra M. Ba
RONALD VOORDOUW (1)
Department of Earth Sciences, Memorial University of Newfoundland,
St. John's, Newfoundland and Labrador, A1B 3X5, Canada
(1). Present address: Council for Geoscience, PO box 900,
Pietermaritzburg, South Africa, 3200
Date received: 05 August 2009 [paragraph] Date accepted: 04
Table 1. Major (wt%), trace and rare earth (ppm) element data
for leucogabbronorite-leuconorite (Lgn-Ln), gabbronorite (Gn)
and ferrodiorite (Fd) from the Main Zone (MZ), Lower Marginal
Zone (LGZ) and Upper Marginal Zone (UGZ).
Sample 01-24-14 01-21-30 00-2-4 00-22-11
Subunit MZ MZ MZ MZ
Lithology Lgn-Ln Lgn-Ln Lgn-Ln Lgn-Ln
Si[O.sub.2] 51.61 54.10 52.83 52.87
Ti[O.sub.2] 0.58 0.07 0.29 0.33
[Al.sub.2][O.sub.3] 23.84 27.54 22.85 21.19
Fe[O.sup.t] 4.82 0.76 4.59 6.24
MnO 0.06 0.01 0.07 0.10
MgO 2.47 0.31 3.52 4.76
CaO 10.12 10.75 10.31 8.72
[Na.sub.2]O 3.79 4.50 3.85 3.39
[K.sub.2]O 0.34 0.38 0.35 0.40
[P.sub.2][O.sub.5] 0.02 0.01 0.01 0.06
Total 97.65 98.43 98.67 98.06
Rb 1.46 1.71 0.90 2.18
Ba 414 422 379 372
Sr 902 897 720 593
Y 3 0 4 4
Zr 7 3 11 16
Hf 0.337 0.285 0.412 0.847
Nb 0 0 0 1
Cr 93 14 18 84
V 97 0 57 60
Li 4.79 3.36 10.87 5.34
Ta 0.08 0.05 0.06 0.10
Th 0.07 0.02 0.09 2.53
U 0.02 0.01 0.02 0.03
Pb 1.58 2.72 1.34 1.53
Bi 0.01 0.01 0.01 0.00
Mo 0.12 0.08 0.10 0.20
La 3.396 4.889 3.394 5.547
Ce 6 8 7 11
Pr 0.746 0.734 0.841 1.303
Nd 3.035 2.399 3.590 5.366
Sm 0.570 0.255 0.756 1.004
Eu 0.885 0.744 0.998 0.978
Gd 0.519 0.126 0.758 0.959
Tb 0.075 0.014 0.115 0.142
Dy 0.453 0.070 0.722 0.867
Ho 0.090 0.011 0.144 0.179
Er 0.265 0.034 0.402 0.501
Tm 0.044 0.011 0.065 0.070
Yb 0.246 0.020 0.375 0.482
Lu 0.035 0.000 0.057 0.074
Sample 01-33-31 00-25-9d 1/28/1956 01-34-1
Subunit LGZ LGZ UGZ UGZ
Lithology Lgn-Ln Gn Gn Fd
Si[O.sub.2] 50.89 48.20 48.81 41.75
Ti[O.sub.2] 1.75 1.11 1.49 4.84
[Al.sub.2][O.sub.3] 19.68 14.69 15.06 12.10
Fe[O.sup.t] 8.11 13.27 12.41 17.65
MnO 0.13 0.20 0.18 0.23
MgO 3.10 7.02 7.65 6.02
CaO 9.19 10.01 9.05 10.49
[Na.sub.2]O 3.46 2.41 2.44 2.12
[K.sub.2]O 0.44 0.09 0.22 0.18
[P.sub.2][O.sub.5] 0.35 0.01 0.25 2.01
Total 97.11 97.02 97.56 97.39
Cs 0.01 0.01 0.00
Rb 1.41 0.24 1.13 0.82
Ba 636 120 281 333
Sr 603 178 564 457
Y 18 17 9 32
Zr 17 31 21 44
Hf 0.720 1.393 0.784 2.121
Nb 11 1 4 7
Cr 67 144 87 0
V 99 311 275 436
Li 7.46 5.98 3.46 4.10
Ta 0.54 0.11 0.23 0.22
Th 0.22 2.85 0.10 0.40
U 0.04 0.05 0.03 0.09
Pb 3.77 1.49 1.76 1.90
Bi 0.01 0.01 0.01 0.00
Mo 0.78 0.24 0.29 0.80
La 18.632 2.664 10.158 32.604
Ce 44 7 23 80
Pr 5.274 1.099 3.052 10.498
Nd 22.915 6.005 13.143 48.298
Sm 4.441 1.950 2.529 9.555
Eu 2.211 0.910 1.113 2.556
Gd 4.170 2.682 2.311 8.949
Tb 0.592 0.465 0.313 1.213
Dy 3.404 3.111 1.898 6.636
Ho 0.681 0.669 0.363 1.270
Er 1.818 1.958 0.964 3.218
Tm 0.249 0.289 0.143 0.406
Yb 1.524 1.884 0.861 2.355
Lu 0.229 0.282 0.013 0.329