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 em-placement of magmas (Myers et al. 2008) and formation of the world-class Voisey’s Bay Ni-Cu deposit (Evans-Lamswood et al. 2000).

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 (≥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.

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 spec-trometry (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).

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°–90°) 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 gneissic– mylonitic 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 Fe-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_50–60 for plagioclase, ~En_55–59 for orthopyroxene and ~En_36–40 for clinopyroxene, and average ~Fo₄₅ 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 km² 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).

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.

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 (~30°–90°).

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 the pluton.

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 m²) 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 walls.

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_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_28–62 and En_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.

Whole-rock major-oxide abundances in leucogabbronorite– leuconorite include high amounts of Al₂O₃, Na₂O, Ba, and Sr (Table 1), consistent with high abundances of plagioclase. FeO shows strong positive correlations (r ≥ 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 flat REE patterns. Ferrodiorite contains high wt% TiO₂, FeO, P₂O₅, HFSE, and REE.

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°–90°). Other igneous structures that strike parallel to this fault zone include foliation and tablet-shaped inclusions.

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_41–55 for plagioclase, En_35–59 for orthopyroxene and En_27–39 for clinopyroxene (Voordouw 2006).

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; Ashwal 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 crystallized 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 ~45:55.

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 Carroll 1996), 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 em-placement 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-HFSEREE) 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.