The shapes of plutons and their emplacement mechanisms, the connection between the volcanic and plutonic realms, and the development of batholiths have been of interest to geologists since they realized that plutons were once low viscosity magma. These issues have proven difficult to resolve because there are few places that have enough relief to expose the critical relations. The Great Bear magmatic zone, a Paleoproterozoic continental arc located in northern Canada's Wopmay orogen, provides an informative field setting to resolve some of these issues because the rocks are generally non-metamorphosed and were broadly folded such that calderas, stratovolcanoes, and a wide variety of plutons are exposed in oblique cross-section on fold limbs in an area of subdued topographic relief.
Early mafic plutons intruded co-magmatic pillow basalt piles as thin sheets with aspect ratios of 10–15. Plutons of intermediate composition, temporally associated with andesitic stratocones, have flat or slightly domical roofs and flat floors, and aspect ratios in the range of 5–10. Granodioritic to monzodioritic plutons that cut thick sequences of ash-flow tuff and related volcaniclastic rocks are generally sheet-like bodies with aspect ratios of 10–20, except where they intrude calderas and form resurgent plutons. Granitic plutons intrude at slightly deeper crustal levels, are generally younger, and typically have miarolytic cavities, pegmatites, and associated dike swarms. The granites have flat roofs and floors but generally have lower aspect ratios than the intermediate composition plutons.
Cycles—where magma bodies were first partially evacuated by eruption, then were re-energized and rose into their own ejecta to form plutons—span the compositional range from basalt to rhyodacite. The cycle of eruption, with the partial evacuation of chambers and subsequent rise of remaining magma to even higher levels in the crust explains why it is generally so difficult to link volcanic eruptions to specific plutons.
The overall development of the Great Bear magmatic zone—from small-scale local eruptions of basalt to voluminous eruptions of intermediate composition ash-flow tuff followed by wide-scale emplacement of granitic plutons—is interpreted to represent input of subduction-related magmas, which led to progressive heating, melting, and wholesale upward differentiation of the crust beneath the arc.
Continental magmatic arcs are typically curvilinear zones, hundreds of km long, where voluminous volcanic and plutonic rocks result from the interaction of subduction-generated basalt and continental crust (Hildreth and Moorbath, 1988; Hildreth, 2007; Hughes and Mahood, 2008). Because matching individual plutons with specific volcanic rocks is difficult, one of the great unknowns in our understanding of continental arcs is the relationship of the exposed plutonic rocks to the volcanic rocks (Bachmann et al., 2007). The problem arises in part because there are few places in the world where both are well exposed. For the most part active arcs display the volcanic suprastructure, whereas older eroded equivalents appear to be dominated by gregarious plutonic rocks (Hamilton and Myers, 1967) with only a few mostly unrelated volcanic rocks preserved mainly as pendants and in the cores of calderas (Busby-Spera, 1984; Fiske and Tobisch, 1994; Saleeby et al., 1990; Pitcher, 1993).
On a smaller scale, some of the longest standing problems in igneous petrology involve the three-dimensional (3D) shape of plutons and how they create space for themselves (Daly, 1905, 1933; Lawson, 1914; Grout, 1927; Chamberlin and Link, 1927; Benn et al., 1998; Petford et al., 2000; Glazner and Bartley, 2006; Yoshinobu and Barnes, 2008). The shapes of plutons that constitute Cordilleran batholiths and continental arc terranes were generally considered to be in the form of “bell-jars” with steep sidewalls based mainly on plutonic exposures with several kilometers of structural relief in both the Sierra Nevada batholith and Coastal batholith of Peru (Bateman, 1992; Cobbing et al., 1981; Myers, 1975; Bussel and Pitcher, 1985; Cruden, 1998). Attempting to explain the geometries of compositional and petrological variations in plutons, other workers have argued convincingly that they see oblique cross-sections through tilted plutons (Flood and Shaw, 1979; Hildebrand, 1984b, 1986; Barnes et al., 1986; John, 1988; Haeussler and Paterson, 1993; Verplanck et al., 1999; Miller and Miller, 2002; Walker et al., 2007), some of which are tadpole-shaped (Hutchison, 1982; Hopson and Dehlinger, 1989). Still others, such as Cruden and McCaffrey (2001), used observations combined with geophysical and theoretical methods to conclude that many plutons form two end members: wedge-shaped and tabular. Similarly, Haederle and Atherton (2002) used gravity models to suggest that the entire Coastal batholith of Peru is a thin sheet similar to that suggested by Hamilton and Myers (1967) for Cordilleran batholiths in general.
The main reason why it is so difficult to evaluate the shapes of plutons is that, even in the highest mountain ranges on Earth, there isn't enough structural relief to see both roofs and floors of the same bodies. What is needed to resolve the shapes of plutons and their connections to volcanic rocks are areas where batholiths are folded, or possibly exposed in tilted fault blocks (Seager, 1981; Miller and Miller, 2002), yet are neither metamorphosed nor highly-strained. Over eighty years ago, Grout (1927) suggested that the Canadian shield was the best place to resolve problems relating to the “form, origin, history, and nature of batholiths” and we believe that there was some truth to his remarks.
The Great Bear magmatic zone—located in the northwestern Canadian shield and now mapped in considerable detail over 2° of latitude (Fig. 1)—is a Paleoproterozoic example of a magmatic arc developed upon continental crust (Hildebrand et. al., 1987a). It is one of the best places in the world to study the shapes of plutons because rocks of the zone are well-exposed in glacially-polished outcrops (Fig. 2A, B, C), are of low metamorphic grade, have little penetrative fabric, and are broadly-folded such that sections up to 10 km through the arc crust are visible despite subdued topographic relief (Fig. 2D). The plutons of the zone intrude related volcanic rocks and are epizonal (∼1–5 km depth) as documented by early syn-volcanic unroofing, razor-sharp contacts, narrow metamorphic aureoles, and preservation of thick sections of subgreenschist grade volcanic and sedimentary rocks. The folding allows us to see and map oblique cross-sections that display both floors and roofs of many plutons, from which we conclude that most of the plutons, except massive biotite granites, are not hippo-shaped, but rather thin and sheet-like in overall form. In this paper we emphasize the map relations that document the shapes of a variety of plutons within the Great Bear magmatic zone, discuss the overall evolution of the composite batholith, visit the relationship of plutonism to volcanism, and touch on their implications for other continental magmatic arcs.
The Great Bear magmatic zone is the youngest magmatic belt within Wopmay orogen, a Paleoproterozoic orogen located in the northwestern Canadian Shield (Fig. 1). The orogen formed when Hottah terrane, a Paleoproterozoic arc-bearing microcontinent, collided with the western margin of the Archean Slave craton at ∼1.884 Ga (Bowring and Grotzinger, 1992). During the collision, known as the Calderian orogeny, rift, shelf-rise and overlying foredeep rocks of Proterozoic age, which formed a west-facing package atop the Slave craton, and crystalline basement/forearc cover and plutons of Hottah terrane, were tectonically shortened and transported eastward along a basal décollement (Hildebrand et al., 2010). Eastward subduction following collision led to continental arc magmatism of the Great Bear magmatic zone during the period 1872–1843 Ma (Fig. 1; Hildebrand et al., 1987a). This model is supported by the Lithoprobe seismic experiment, which imaged mantle reflectors interpreted as relicts of eastwardly directed subduction dipping beneath the Great Bear magmatic zone (Cook et al., 1999; Cook and Erdmer, 2005). Shut-down of arc magmatism at ∼1.84 Ga was followed by oblique folding of the arc rocks and finally, east-west shortening, accommodated by north-south extension, resulting in an orogen-wide swarm of conjugate transcurrent faults (Fig. 1). Younger Proterozoic dikes and sills, entirely unrelated to the events discussed here, occur throughout the region (Buchan et al., 2009).
GREAT BEAR MAGMATIC ZONE
The Great Bear magmatic zone (Fig. 1) is an ∼100 km-wide belt of mostly subgreenschist-facies volcanic and plutonic rocks that crop out over a strike length of 450 km. They unconformably overlie and intrude rocks deformed and metamorphosed during the Calderian orogeny (Hildebrand et al., 2010). The zone can be traced along strike for an additional 500 km southward beneath a thin veneer of Paleozoic cover and to the north bends westward then continues for another 300 km (Coles et al., 1976; Hildebrand and Bowring, 1984; Hoffman, 1987; Hildebrand et al., 2010). Thus, its two-dimensional extent is comparable to modern continental arc terranes. The overall structure of the zone is crudely synclinal (Hoffman and McGlynn, 1977; Hildebrand and Bowring, 1984) such that the oldest supracrustal rocks occur along its eastern and western margins, although sparse exposures of the lower part of the pile and its basement occur elsewhere within the zone.
The volcanic and sedimentary rocks of the Great Bear magmatic zone are collectively included within the MacTavish Supergroup, which is divided into three groups (Hoffman, 1978, 1982; Hoffman and McGlynn, 1977; Hildebrand, 2010). The LaBine group crops out along the western side of the zone; the temporally correlative Dumas group along the eastern side; and overlying both in the central part of the zone are rocks of the Sloan group. All of the plutonic rocks are informally termed the Great Bear batholith. We use the term pluton to refer to intrusive magma emplaced within a single outer contact that was mappable in the field. Individual plutons may have resulted from single or multiple pulses of injection. In our terminology, a batholith is simply an amalgamation of plutons that are more or less spatially and temporally related.
Except for early tholeiitic basalts (Reichenbach, 1991; Figures 2E, F), the volcanic rocks of the MacTavish Supergroup are mostly calc-alkaline and span the entire compositional range from basalt to rhyolite (Hildebrand et al., 1987a). Intermediate composition rocks dominate, and even where more siliceous compositions are abundant, andesitic lavas occur intercalated with the more siliceous units (Hildebrand, 1981, 1983, 1985). Modes and geochemistry of plutonic rocks are virtually identical to batholiths of continental arcs and Cordilleran-type batholiths (Hildebrand et al., 1987a).
In the west, LaBine Group consists of voluminous eruptions of intermediate to siliceous ash-flow tuff that when erupted, led to collapse of calderas, many of which were subsequently filled with porphyritic andesitic lavas, breccias and tuffs erupted from andesitic stratovolcanoes (Fig. 2G, H). In places, preserved deposits of the stratocones are up to 3km thick. The ignimbrites of the LaBine group are highly variable, but generally contain 5%–35% phenocrysts, and range from small volume andesitic tuffs, erupted from andesitic stratocones, to voluminous, monotonously-intermediate and compositionally-zoned dacite and rhyodacitic sheets of outflow facies tuff and caldera fill (Fig. 2J, K, L). Contemporaneous eastern facies are predominantly fluvio-lacustrine with thin outflow-facies tuffs and minor lavas, collectively known as the Dumas Group. In general, stratigraphically-highest rocks of the Sloan Group occur within the central part of the zone and constitute sections many kilometers thick dominated by monotonous crystal-rich, densely-welded ignimbrite, typically dacitic with abundant cognate lithic fragments (Fig. 2M), and interbedded with only minor lava flows and sedimentary rocks. Detailed facies studies are incomplete, but preserved thicknesses of ash-flow tuff within the Sloan Group indicate that it comprises complexly nested calderas formed by cataclysmic eruptions of monotonously-intermediate ignimbrite.
All of the supracrustal rocks of the belt are cut by epizonal plutons (Fig. 2C), several of which are interpreted to be synvolcanic based on field relations and by U-Pb geochronology (Bowring, 1985). Although we cannot match any single pluton with a specific eruptive product, in a general sense—and based on U-Pb geochronology and unconformities that deroof plutons—there is little doubt that the batholith intruded its own ejecta. The plutonic rocks are grouped into four suites, based on composition and temporal relations (Hildebrand et al., 1987a). The oldest are gabbroic and comagmatic with early pillow basalts. Slightly younger is a compositionally diverse suite of relatively small intermediate to siliceous bodies, collectively named the “early intermediate intrusive suite.” They are spatially and temporally related to andesitic volcanism. Younger yet are huge plutons, corporated as the “granodiorite-monzogranite suite,” that are temporally and compositionally related to voluminous eruptions of ash-flow tuff of the Sloan Group. The youngest plutons are large bodies of granite with only minor eruptive equivalents. Overall, plutonism within the Great Bear magmatic zone became more siliceous with time.
All of the volcanic rocks and all of the plutons are folded (Fig. 2D). Throughout most of the zone, the folds trend northwest but as the axial traces approach the eastern margin of the zone, they bend progressively N-S, parallel to the edge of the Slave craton (Hildebrand et al., 1987b, 1990; Hildebrand and Bowring, 1988). The folds plunge gently everywhere. The regional map (Fig. 1) shows that many plutonic contacts parallel the northwest-trending or northerly striking axial traces of the younger folds and are exposed in cross section. Stratigraphic sections several km thick are exposed on single fold limbs and throughout most of the zone there is no megascopically recognizable planar fabric related to folding with the exception of cleavage within mudstones and poorly-welded tuffs, located mostly in the east where the folds are tighter than elsewhere due to the buttressing effect along the edge of the Slave craton (Hoffman, 1984; Hildebrand and Bowring, 1988).
The east-west shortening that created the northeasterly-trending, dextral transcurrent faults distorted the original trends of fold axes and contacts because the individual fault blocks and their bounding faults rotated in a counterclockwise fashion during shortening of the entire orogen (Freund, 1970; Tirrul, 1987). The overall effect is to cause originally northerly-trending contacts to trend more northwesterly today.
The majority of volcanic and plutonic rocks have U-Pb ages between ∼1872 Ma and 1860 Ma, but some granites and a few dated volcanic rocks have ages as young as 1843 Ma (Fig. 1; Bowring, 1985). Oblique folding and transcurrent faulting are younger than 1843 Ma and predate a WNW-trending mafic dike swarm dated at 1740 +5 / −4 Ma (Irving et al., 2004).
Mafic Intrusions in the Early Basaltic Lava Pile
The oldest magmatic rocks of the Great Bear magmatic zone are 2–3 km of basaltic lavas of the LaBine group exposed along the western margin of the zone (Fig. 1) at Hottah Lake and Conjuror Bay (McGlynn, 1975; Hoffman and McGlynn, 1977; Hildebrand, 1982; Hildebrand et al., 1983; Reichenbach, 1991). The lavas are cut by gabbroic sills (Fig. 3; Table 1), which range in thickness from a few tens of meters to one kilometer and are many kilometers wide. In general they are at least 10–15 times wider than they are thick. Texturally, they range from fine-grained diabase to coarse-grained equigranular, and aphyric to plagioclase porphyritic. Texturally, mineralogically, and chemically the gabbros are similar to the basalts they intrude, except that they are coarser-grained. For example, both sills and basalts contain identical glomeroporphyritic clots of plagioclase (Fig. 2 E, F), and as the clots occur in chilled selvages of pillows and border phases of sills, the lavas and sills are interpreted to be comagmatic (Hildebrand et al., 1983; Reichenbach, 1986). The sills wrap around fold noses defined by bedding in sedimentary and basaltic wallrocks (Fig. 3). Nearly everywhere their razor-sharp contacts dip parallel to bedding.
The Early Intermediate Intrusive Suite: Plutons Associated with Andesitic Stratovolcanoes
Along the western margin of the Great Bear magmatic zone are accumulations up to 3 km thick of strongly plagioclase-porphyritic andesitic lavas (Fig. 2 G, H), breccias, and related epiclastic rocks that are collectively interpreted to constitute the deposits of stratocones (Hoffman et al., 1976; Hoffman and McGlynn, 1977; Hildebrand, 1981, 1984a). In several cases the cones developed within pre-existing calderas (Hildebrand, 1984a, b). The andesites and their substructure are intruded and considerably altered by a suite of intermediate composition plutons (Figs. 1, 4, 5; Table 1), collectively termed the “early intermediate intrusive suite” (Hildebrand et al., 1987a).
Several plutons of the suite were studied in considerable detail (Furnival, 1939; Tirrul, 1976; Hildebrand, 1984a, b, 1986; Cherer, 1988; Reardon, 1992) as they and their alteration haloes host rich polymetallic veins of native Ag, Ni-Co arsenides, and pitchblende. The plutons are 5–25 km in diameter and 1–2 km thick, of monzonite, monzodiorite, diorite and granite. For the most part they intruded 2–3 km beneath the surface and their contacts with wall rocks are sharp. Xenoliths are sparse. Nearly all of the members of the suite have well-developed alteration haloes up to 1 km wide, comprising an inner albite zone, a central magnetite-apatite-actinolite zone, and an outer pyritic zone (Hildebrand, 1986; Reardon, 1992). At least three of the intrusions were de-roofed soon after emplacement as younger rocks unconformably overlie the intrusions and/or clasts of the intrusions occur in stratigraphically younger units that are themselves cut by plutons and folded.
Overall, the plutons are compositionally heterogeneous, but uniformly medium-grained with a well-developed seriate texture defined by plagioclase (Fig. 2I). Subhedral-euhedral hornblende and biotite are the dominant ferromagnesian minerals but where preserved, border phases (Fig. 2N) locally contain relict clinopyroxene and serpentinized orthopyroxene. Magnetite is the dominant opaque mineral throughout the intrusions.
The plutons crop out in two main areas: the Conjuror Bay-Camsell river area and in the area around Port Radium (Fig. 1). More detailed maps (Figs. 4, 5) show the geological setting and relations, whereas the characteristics of individual plutons are listed in Table 1. In general, the plutons form sheet-like bodies with flat, concordant roofs and floors. Another early pluton, a diorite, intrudes farther east at Ellington Lake (Pelletier, 1988), and also has both roof and floor that are dominantly concordant with bedded wall rocks (Fig. 6).
The granodiorite-monzogranite suite is one of the major plutonic suites of the zone. Most contacts of the plutons are concordant with bedding in their wall rocks, and trend northwest (Fig. 1; Table 1), parallel to the regional fold axes. There is little doubt that they are folded along with their wall rocks as contacts and narrow thermal aureoles ranging in grade up to hornblende hornfels facies swing around fold noses with wall rock stratigraphy (Figs. 7, 8, 9). U-Pb ages of the plutons are generally indistinguishable from those obtained from the ash-flow tuffs they intrude (Bowring, 1985), which, along with similar compositions and textures, suggests in the broad sense that the plutons and tuffs may be genetically related. Most of the members of this suite are large plutons, ranging up to 100 km across, with flat, concordant floors and roofs, but at least two members, both of which appear to be resurgent plutons in the cores of calderas, have domical roofs (Table 1).
Most commonly, members of the suite are medium-grained bodies of granodiorite and monzogranite as the name of the suite suggests; however, more siliceous and more intermediate compositions occur in virtually every body (Fig. 2 P, Q, R, S). In fact, members of this suite have internal modal (Fig. 10) and chemical (Fig. 11) variations nearly as great as the entire batholith. Most members of this suite are characterized by seriate plagioclase and 5%–25% subhedral to euhedral biotite and hornblende with hornblende predominating (Hildebrand et al., 1987a). Anhedral quartz and potassium feldspar fill interstices between plagioclase and ferromagnesian minerals. A few percent clinopyroxene, typically rimmed by hornblende, are locally common. Slender needles of apatite, euhedral zircon and sub-euhedral titanite are ubiquitous accessory minerals. Titano-magnetite ± ilmenite are the dominant opaque phases. The compositional variations between phases within the plutons are gradational and occur over tens of centimeters to tens of meters. Obvious signs of basalt-granite mingling were only found in one small area. There, mafic pillows with well-developed selvages (Fig. 2Y, a) occur in more siliceous rocks.
There is ample evidence in the form of xenoliths and net vein breccias, at least at the final site of pluton emplacement, for wall-rock stoping. In general, the stoped blocks are mostly seen adjacent to the roofs (Fig. 2f, g, I, k), and as there are no accumulations of blocks on the floors, must have been digested. Locally, where plutons cut quartz arenite, they are much more quartz rich apparently owing to disaggregation of the sandstone and incorporation of individual grains (Fig. 2e). Only along the margin of one pluton did we find discontinuous layers, 1–10 cm thick, of variable mineralogical proportions. The layering is more or less parallel to the margins and truncations of successive layers are common (Fig. 2X).
In a detailed study of several of the plutons, Housh (1989) documented that there is no simple relationship between rock types and suggested that convection was not strong enough, or perhaps did not last long enough, for the body to homogenize the batch(es) of magma that filled the chamber or redistribute them consistent with their densities. As such, the variations are due to either complex melting and/or mixing processes or heterogeneities in the source region.
The younger granites are mainly biotite granites and, in contrast to other plutons of the zone, are rather homogeneous bodies (Fig. 2T, U, V). Many of the plutons are strongly porphyritic, containing subhedral to euhedral potassium feldspar phenocrysts to 5 cm in a fine- to medium- grained matrix of anhedral quartz, feldspar, and tiny flakes of biotite. Prismatic hornblende is locally present but everywhere less abundant than biotite. Titanite is rare, but allanite, zircon, apatite, magnetite, ilmenite, monazite, and flourite are locally common accessory minerals. Most members of the suite are slightly corundum normative.
The plutons of this suite differ from all others within the zone in that they commonly have zones with abundant pegmatite, miarolytic cavities, granophyre and aplite (Fig. 2j). Additionally, regionally extensive dike swarms appear in many cases to emanate from individual plutons of this suite (Hildebrand et al., 1987b).
Contacts with wall rocks are everywhere sharp (Fig. 2Z, d). Locally there are spectacular sill complexes (Fig. 2b, c) or swarms of partially digested wall rocks (Fig. 2 h, j). The intimate nature of the sills and their compositional similarity to the main body suggest that the magma was highly fluid and little crystallized at the time of emplacement. This is different from the older three magmatic suites, which, judging from the porphyritic nature of their quenched border phases, were partly crystalline.
The younger granites have contacts that tend to be more discordant to supracrustal rocks than the older, more intermediate-composition plutons. Numerous plutons of this suite have lengthy intrusive contacts that parallel bedding in their wall rocks but in places the contacts climb up or down stratigraphy on the scale of kilometers (Figs. 1, 5, 12).
In the southern part of the mapped area—and based upon the more extensive outcrops of Hottah basement and lowermost Great Bear stratigraphy relative to the north—slightly deeper levels of the zone are exposed and granites are much more common (Fig. 1). The contacts of the granites tend to parallel bedding in supracrustal rocks and parallel axial traces of folds even where the traces swing around to a northerly orientation along the eastern margin of the zone (Fig. 1). Such a geometry could only occur if the plutons were folded and this is reflected in the common northwesterly trends of their contacts. However, the granites also have long stretches where their contacts trend orthogonal to the axial traces of the folds; that is, they strike northeast where axial traces of folds trend northwest. The only way to satisfy such a geometry is for the granitic plutons to be much thicker than more intermediate-composition plutons and have steep side-walls. Therefore, they may be more analogous to the bell jar plutons of other workers (Cobbing et al., 1981; Bussell and Pitcher, 1985; Pitcher, 1993).
SHAPES OF PLUTONS AND DEPTH OF EMPLACEMENT
Magmas of nearly every composition, except the biotite granites, created sill-like plutons, 500m-4km or so thick, with aspect ratios of five to ten or even greater. Intrusions related to the early basaltic eruptions have aspect ratios in the range 10–15. Those magmas, which form dikes and inclined sheets where they intrude basement and older supracrustal rocks, only form sills at the highest structural levels. Plutons of the early intermediate intrusive suite form sheet-like bodies both within and at the base of possibly comagmatic andesitic stratovolcanoes. Those plutons have aspect ratios of 5–10 at a minimum. The monotonous granodiorite-monzogranite bodies, which intrude the thick dacitic ash-flow tuff sheets, have aspect ratios of 10–30. Only the granites appear to be thicker, although due to the lack of supracrustal rocks in their zone of intrusion, accurate thickness estimates were difficult to obtain. An estimate made from cross sections suggests that they have aspect ratios of ∼2–4. None of the bodies were obviously composite; that is, sharp internal contacts caused by multiple intrusions or sheets, were not recognized.
The older, more intermediate and mafic composition plutons tend to feather out laterally in the paleohorizontal plane, whereas in many cases the younger granites appear to have steep sidewalls. Because the floor typically parallels the roof of these plutons there is no evidence that the plutons are triangular with inclined floors and flat roofs.
From the amount of volcanic cover above plutons, other than the biotite granites, they were emplaced 1–4 km beneath the surface. Rocks within the alteration haloes of the early intermediate intrusive suite yield paleodepths, based on paleofluid compositions preserved in inclusions, of a km or less (A.K. Somarin, 2010, personal commun.). Several of the plutons were deroofed shortly after emplacement, so the exact amount of cover is unknown as it was eroded away; but using stratigraphic reconstructions and unconformities, similar emplacement depths seem reasonable.
In spite of voluminous volcanic eruptions and excellent exposures of the roofs of scores of plutons we do not generally see cupolas above their roofs. The upper contacts of most plutons are remarkably planar. Only in the cases of two resurgent calderas and the Contact Lake pluton, do we see dome-like or laccolithic plutons, but if they are part of a larger body it must have been much deeper as it is not exposed.
The Great Bear magmatic zone and, in fact, all of Wopmay orogen appears to have a slight northward plunge created by much younger events, likely Phanerozoic in age. An interesting observation that came out of our studies is that the biotite granite bodies are generally concentrated at apparent lower structural levels than the more intermediate composition plutons. This can be seen by examining Figure 1, and noting the prevalence of granites in the southern and northeastern portions of the study area. There they intrude the lowest stratigraphic units of the Great Bear magmatic zone and its basement. Based only on density, the granites should have been buoyant enough to rise to any level, but they didn't. It may be that they were too viscous to rise any farther, or that they contained more water than other bodies, and so rapidly exsolved volatiles and froze. Because many of the granitic plutons contain miarolytic cavities lined with pegmatite, and because the magma that formed dozens of rhyolite porphyries, many quite substantial in size, did rise to high levels in the volcanic suprastructure, the second hypothesis is more probable.
We believe a major control on the shape of plutons in the epizone is lateral spreading and elevation of their roof rocks because (1) it is mechanically much easier than depressing their floors (Paterson and Fowler, 1993; Petford et al., 2000); and (2) because we don't see inclined floors of plutons (Cruden, 1998). Because the intermediate composition bodies are able to spread out laterally in thick sequences of densely-welded tuff or accumulations of andesite flows, well-defined bedding planes do not appear critical to this process. However, when one compares plutons of the Great Bear that intrude bedded rocks versus more massive units, the contacts are invariably much more planar in detail where they intrude the former.
Supporting these ideas are the type of sills and enclaves found adjacent to the roofs of the plutons themselves. Where the wall rocks are well-bedded, there are typically meter-to centimeter thick sills of intrusive rock and flat enclaves formed as the plutons spall off pieces of wall rock; but where the wall rocks are more massive, enclaves are irregular, and if equigranular igneous rock, are blocky and equant. Where deformed metasedimentary rocks constitute the wall rock, the enclaves are quite irregular and blob-like in overall form (Fig. 2 f, g, i, k).
We nowhere saw evidence near these plutons for forceful emplacement at the outcrop scale. There simply aren't folds, faults, or ductile features at that scale that one can relate to the emplacement of the plutons, even though in many cases plutons intrude bedded rocks that would show deformation. In cases of resurgent plutons in calderas, such as the Hogarth and the Calder, it is clear the plutons were able to lift their roofs, but in the more typical case a pluton is many kilometers to tens of kilometers across and despite superb outcrop we nowhere saw evidence that they depressed their floors. Overall, the plutons were emplaced at such shallow levels in the crust, that it is difficult to envision that they did anything but lift their roofs.
Most of the studies of pluton emplacement and temporal development were done in mesozonal or deeper rocks (Matzel et al., 2006; Coleman et al., 2004; Glazner et al., 2004; Miller et al., 2007) and may or may not be applicable to epizonal plutons, their processes, or even their timescales. Working downwards, some workers attempted to reconstruct plutonic processes from the volcanic realm (Smith, 1979; Bachmann and Bergantz, 2004, 2008; Bachmann et al., 2002, 2007; Hildreth, 2007; Reid, 2008). The oblique crustal sections of the Great Bear magmatic zone span the two and so offer an uncommon opportunity to evaluate and test models of magmatism in the epizone where volcanism and plutonism are presumably connected.
Several of the granodioritic-monzogranitic plutons are more than 100 km across, and while there are typically internal variations down to the scale of a hand specimen, there are no obvious intrusive contacts or sharp compositional breaks. In a few places near the wall-rock contacts there are compositional layers (Fig. 2X), but in general they are rare. Housh (1989) studied several plutons and using a combination of Pb and Nd isotopes, petrography, and whole rock chemistry, corroborated the field observations by demonstrating that they are multicomponent mixes. However, he was unable to discern where these mixes were assembled or how. The processes that fill a 100 km wide pluton are simply unknown. Are they open systems, filled incrementally, in a series of pulses; or are they inflated once and then remain as closed systems, or does some combination of the two operate? Open system behavior would seem to fit the data so long as the pulses were frequent enough not to allow sufficient cooling for sharp internal contacts to develop; yet it seems as though incremental pulses (Miller, 2008; Miller et al., 2009) couldn't reproduce the seemingly random compositional variations throughout the bodies. The general lack of cumulate or anti-cumulate layers in these large plutons suggests that either the bodies did not have enough liquid to allow crystal fractionation (sinking, floating, filter pressing), or possibly that convection was particularly vigorous. The problem with strong convection is that the bodies are not well mixed and there is typically no layering at the margins. We suggest that the plutons were likely emplaced more or less at one time as crystal mushes with ∼40% plagioclase and hornblende phenocrysts, then rapidly froze, preserving internal heterogeneities and inclusion trains.
Most of the large ash-flow tuff sheets within the LaBine group are compositionally zoned, yet for the most part, we do not see obvious systematic compositional zonation or compositional layering within plutons such as the Hogarth or Calder bodies (Table 1). Small volume crystal-poor rhyolitic lavas and porphyritic intrusions suggest that some separation of liquid and crystals took place, but for the most part, even rhyolitic ignimbrites contain 10%–25% broken phenocrysts. Ignimbrites of the Sloan group are weakly zoned, if at all, and are best classified as monotonous intermediates (Hildreth, 1981), yet generally they do not have the composition variability that exists in many of the monzogranite-granodiorite plutons that intrude them. Both compositionally zoned and monotonous intermediates are common in younger belts as well, yet compositionally-zoned subvolcanic plutons are also apparently rare (Lipman, 2007).
We mapped ∼20,000 km2 of the Great Bear magmatic zone, and obvious field evidence for magma mixing and mingling is sparse, unless one relates the internal compositional variations to nearly complete mixing of different magmas. There are exceptions, however, and in the best-exposed area, only a few hundreds of meters across, obvious mafic pillows with well-developed selvages occur in more siliceous rocks (Fig. 2Y, a). Also common in that area are mafic orbicules (Fig. 2W), coarsely-poikiolitic gabbro (Fig. 2O), and granodiorite-quartz monzonite choked with mafic inclusions. These relations attest to the invasion of a siliceous system by mafic magma and likely streaming of volatiles (Moore and Lockwood, 1973). The mixed zone may be easily recognized because it was generated by direct introduction of mafic magma into a siliceous body relatively close to the solidus. Within the batholith as a whole, however, the limited occurrence of obvious mixing and mingling textures, such as pillows and blobs, suggests that epizonal mixing of different bodies of mafic and siliceous magma was not a very important process in constructing magmas of the Great Bear magmatic zone and possibly other arcs as well. If so, then do the common heterogeneities in the magma bodies come from bulk melting of source rocks or do different melts mix during ascent to create the observed inhomogeneities? One possibility, not often considered, is that the magmas were originally more ordered, say in a zoned chamber, but then rose upwards following the disturbance caused by partial evacuation during eruption, to become more chaotic. This is consistent with the observation that nearly every one of the intermediate composition plutons, including both the early intermediate suite and the granodiorite-monzogranite suite, vary widely in composition and contain ∼40% seriate plagioclase and several percent hornblende prisms, which are readily interpreted as early crystallizing phases. Thus, it may be that there were too many crystals for the bodies to be able to convect sufficiently to mix when finally emplaced.
In addition to the general lack of evidence for magma mixing, and despite many kilometers of structural relief, we also do not see mafic magmas beneath the floors of the plutons, which indicates that any influx of mafic magmas to re-energize or remobilize the magma chambers must have taken place at deeper levels before the magma bodies rose higher in the crust. Likewise, in the Southern Rocky Mountain volcanic field of Colorado no epizonal plutons show evidence for mixing of silicious and mafic magma (Lipman, 2007), so it is hard to understand how magmatic underplating could represent a viable widespread possibility to rejuvenate magma bodies at the highest structural levels, even though it is recognized at some volcanoes (Bacon and Lanphere, 2006; Andrews et al., 2008). This does not mean that we don't believe that underplating is a viable process at depth, for it might readily occur there and provide a heat source to re-energize magma chambers after initial eruptions.
We generally don't see cumulate or obvious mush zones in the lower parts of the plutons. Where there are cumulate facies, such as with plagioclase in the Rainy Lake pluton, they are obvious. The general lack of cumulates suggests that magma chambers were not mostly evacuated, nor tapped of the rhyolitic melt component (Bachmann and Bergantz, 2004), to leave behind crystal accumulations.
In the entire area, we mapped only one concentrically zoned complex, the Handley Page intrusive series (Fig. 1; Table 1), which is similar to the Tuolumne Intrusive Suite (Bateman and Chappell, 1979; Bateman, 1992). The series includes diorites and minor gabbroic rocks that were intruded by younger and more siliceous bodies progressively inward to a granitic core (Hildebrand et al., 1987b; Gibbins, 1988). These types of complexes clearly have sharp contacts, which reveal age relations, and are a different type of plutonic suite than the majority of bodies in the Great Bear zone. The concentrically zoned suites were originally considered to show reasonably clear evidence for the involvement of crystal fractionation to produce some of their compositional variations (Bateman and Chappell, 1979; Kistler et al., 1986), but recent studies (Coleman et al., 2004; Gray et al., 2008) suggested major roles for other processes. Overall, the general lack of crystal-poor rhyolitic ignimbrites and the rarity of cumulate rocks in the crustal column of the Great Bear magmatic zone and elsewhere suggests that crystal fractionation is not the predominant mechanism that creates broad compositional variations within arc terranes, at least at mid- to upper-crustal levels. Varying degrees of partial melting of lowermost crustal rocks, mixing with mantle-derived basalt, and assimilation of wall rocks during ascent and emplacement might best produce the compositional variations found in continental arcs.
RELATIONSHIP OF PLUTONISM TO VOLCANISM
When one looks at the relationships between plutonism and volcanism in the Great Bear magmatic zone, an interesting pattern emerges. After each eruption type, plutons of similar composition rose into the volcanic suprastructure. At their current level, the plutons, which by every measure appear comagmatic with their wallrocks, cannot have fed them because they intrude them and are thus demonstrably younger. This cyclical relationship repeats itself over most of the history of the zone.
The oldest volcanic rocks of the Great Bear magmatic zone are the tholeiitic basalts (McGlynn, 1975), which were intruded by comagmatic sill-like plutons. The next phase of development was a mixed assemblage of caldera-forming ignimbrite eruptions ranging in composition from dacite to rhyolite and andesitic stratocones. The calderas were intruded by resurgent plutons of similar composition, whereas the andesitic piles were intruded by plutons of the early intermediate intrusive suite. In both cases, the plutons are strikingly similar to their wallrocks, but are younger. Following the mixed caldera-stratocone phase were voluminous eruptions of dacitic-rhyodacitic ash-flow tuff preserved in the central part of the zone as the Sloan group. The voluminous granodiorite-monzogranite plutons intruded the dacites-rhyodacites, and are mineralogically similar to them, but appear to have greater compositional variability.
Following the large-volume dacitic eruptions and emplacement of the granodiorite-monzogranite suite, there were only a few minor eruptions of siliceous lava and tuff; but abundant large tabular plutons of biotite granite were emplaced near the base of the volcanic pile. These plutons have few known eruptive equivalents, despite many having dike swarms. It is, of course, possible that the volcanic rocks associated with the granitic plutons were eroded, but as the entire Great Bear magmatic zone was folded soon after cessation of magmatism (Hildebrand et al., 1987a) it seems unlikely that there would be few such rocks preserved.
Thus, there are two trends to explain: (1) the overall development of the zone from small-scale eruptions of basalt to voluminous granodiorite-dacite magmatism followed by emplacement of abundant biotite granites (Fig. 13); and (2) the smaller-scale rise and emplacement of plutons into what for all appearances appears to represent their own volcanic ejecta (Fig. 13). These smaller-scale events are independent of composition as they occur in basalts, andesites, dacites, and rhyodacites.
The smaller scale trends of magmatism involving the emplacement of plutons into their own ejecta reveals why it is so difficult to directly relate particular plutons to a given volcanic eruption or sequence of eruptions; not only in the Great Bear magmatic zone, but also elsewhere (Walker et al., 2008). Whereas the plutons clearly rose into their respective volcanic suprastructures and therefore must postdate the volcanic eruptions, two possibilities are obvious: (1) the magma chambers from which the plutons were derived erupted the magmas and then continued their rise toward the surface with smaller volumes and fewer volatiles; or (2) the magma bodies erupted from depth and it was entirely different bodies of magma that later rose and were emplaced into the volcanic suprastructure. We prefer the first possibility because we don't see similar plutons or deflated chambers at depth, and because the plutons we do see don't seem to have erupted in situ, so were likely partly drained of volatiles during previous eruptions. If correct, it means that we are, in fact, looking at bodies of the same magma that erupted at the surface, but at a slightly later stage in their evolution. This implies that the magmas were moderately mixed and didn't fractionate between their initial eruption and their final emplacement. It also implies that there were discrete magma batches that rose from depth, lifted their roofs, and spread laterally. In many cases the plutons were rapidly deroofed. It is difficult to envision these chambers, lacking in sharp internal contacts, starting out as thin sills that were progressively assembled by crack-seal growth or other relatively small-volume replenishment mechanism (Coleman et al., 2004; Glazner et al., 2004; Bartley et al., 2008; Annen, 2009; Miller et al., 2009). Furthermore, as pointed out by Lipman (2007), progressive growth by crack seal appears to preclude large volume eruptions as there would never be enough melt to feed such eruptions.
If the magmas feeding the plutons erupted at the surface, how could they later rise into their own volcanic ejecta, given the obvious loss of volatiles during eruption? The important point is that eruptions reflect episodes of degassing within the upper parts of magma chambers. That is, the eruptions don't evacuate the entire magma chamber, but merely draw from its uppermost portions (Smith, 1979). For example, a circular pluton 50 km in diameter with an aspect ratio of 10 would contain nearly 10,000 km3 of crystalline mush and magma, so a major ash-flow eruption of 1000 km3 would only lead to a 10% drawdown of the chamber.
With re-accumulation of volatiles within the chamber, the magma may erupt again, or if volatiles are not concentrated, say near the roof, the magma may rise toward the surface, and thus have the possibility to intrude its own ejecta. Due to the increased crystallization, solidification, and loss of volatiles, subsequent eruptions would probably be smaller and less explosive, if they occurred at all. Where these bodies of magma formed calderas, their subsequent rise into the caldera produces central plutons and resurgent doming of the intracauldron-facies tuff.
A possible method to re-energize a magma chamber without physically introducing basalt into the chamber itself comes from detailed study of the Soufriere Hills volcano in Monserrat where Couch et al. (2001) described a process called convective self-mixing. In that process a magma body is heated from below, but the two magmas themselves never mix. Instead, plumes of the lowermost part of the upper magma body are heated from below and become gravitationally unstable and rise into the overlying magma. This leads to intermingling of thermally different parts of the same magmas body.
The main problem in applying any model in which basalt provides a direct heat source to re-energize previously and partially evacuated magma chambers is that we don't see extensive bodies of gabbro, except in the oldest part of the zone. One way around this conundrum is for the energizing culprits to have been rising magmas of intermediate composition, similar to the original melts in bulk composition. In that way, different bodies of intermediate composition magmas can mix within the partially evacuated chamber, and leave sparse evidence, perhaps only disequilibrium mineral zoning and resorption and/or isotopic variations.
An additional contributing factor—which could operate in concert with partial drawdown—is the concept of gas sparging, a process in which gases streaming upwards from deeper magmas rejuvenate magma bodies (Bachmann and Bergantz, 2003, 2006). Recharging bodies of wet, crystal-rich magma with volatiles and heat could add enough energy to assist their rise into the volcanic suprastructure (Huber et al., 2010).
Except for the late granites, we didn't find evidence for eruptive conduits such as vesiculated dikes above plutons that would suggest that the bodies erupted after or during final emplacement. The lack of miarolitic cavities, except in the younger granites, suggests that hornblende and biotite were able to incorporate the bulk of magmatic water contained within the bodies.
Whatever the cause of their remobilization, it appears that many magma bodies in the Great Bear magmatic zone erupted then migrated toward the surface to intrude their own ejecta. This process might explain why elsewhere it is typically so difficult to match a pluton with its eruptive products. Any regular chemical and mineralogical zoning that formed in the body just prior to its original eruption was redistributed, stirred, or mixed during its subsequent rise toward the surface, and when the magma arrived in its final plutonic configuration it was too crystalline to convect sufficiently to mix well. Furthermore, because the plutons are essentially large sills, they would have cooled rapidly, which would also have tended to quell convection or fractionation. The dominance of strongly variable compositions demonstrates that, even though many bodies are of sufficient volume, they could not have fed the monotonously intermediate ignimbrites of the Sloan Group from their present locations.
In addition to our field data presented here, Christiansen (2001) developed a model for the Yellowstone volcanic field in which, following major ash-flow eruptions, magma chambers quickly reestablished positive magma pressure and continued to rise buoyantly. More recently, Lipman (2007) pointed out two lines of evidence that suggested to him that magma bodies must erupt at depth then migrate upwards: (1) geological and geophysical data are consistent with the subvolcanic batholith beneath the central San Juan mountains migrating upwards after ignimbrite eruptions and collapse because it is not exposed in the floors of nested calderas that cumulatively had 10–15 km of subsidence; and (2) Quaternary calderas in the western U.S., the Andes, and elsewhere don't have shallow geophysical signatures that would indicate voluminous shallow chambers of largely liquid magma beneath them.
OVERALL EVOLUTION OF THE ARC
The evolution of magmatism within the Great Bear magmatic zone has features in common with other continental arcs (Hildebrand and Bowring, 1984; Hildebrand et al., 1987a) and although it is obvious that no single scheme can explain all the variation and styles in magmatism observed at convergent margins, some broad generalities have emerged. Magmatism started with outpourings of basalt, but volumetrically minor dacites were erupted toward its end. The basaltic phase was followed by a stage where andesitic eruptions were punctuated and intermingled with ash flow eruptions and caldera collapse at eruptive centers. As magmatism continued, it developed into more substantial eruptions of intermediate composition ash-flow tuffs with only minor andesite and rhyolite. Within the Great Bear magmatic zone the final stage of magmatism was the rise and emplacement of tabular granitic plutons.
Obviously, given the complexities of folding and faulting with the Great Bear magmatic zone, we cannot reasonably estimate magmatic flux in either the crust or mantle over time, but the overall development of the zone, from basalt to granite, suggests progressive heating and melting of the crust beneath the arc such that larger volumes of crustal material were processed with time. In an early review of this process Hildebrand and Bowring (1984) concluded that most continental arcs have their base near sea level, which suggested to them that the crust was not appreciably thickened despite the overall influx of basalt. They proposed a model in which the material lost from the region by airfall roughly equaled the mass of basalt arriving at the base of the crust. To this effect must be added any regional extensional effects such as, for example, those caused by rollback of the subducting slab. A survey of currently active continental arcs, such as occur in the Cascades, Japan, New Zealand, Central America, the Kamchatkan, Alaskan and Antarctic peninsulas, the Philippines, Italy, and Sumatra, shows that most remain at or near sea level, albeit with individually high-standing volcanoes, for tens of millions of years, despite continual subduction of oceanic lithosphere and its abyssal veneer of sediment (Hildebrand and Bowring, 1984; Hildebrand, 2009b). This is different than Cordilleran-type batholiths (Hamilton, 1969a, b), such as the Sierra Nevada and Coast Plutonic batholiths, which are flare-ups (Ducea, 2001; de Silva, 2008) characterized by nearly complete crustal re-organization and extreme crustal thickening. They likely owe their origins to a somewhat different process than that caused purely by subduction of oceanic crust and its abyssal cover (Hildebrand, 2009; DeCelles et al., 2009).
CONCLUSIONS AND IMPLICATIONS FOR OTHER CONTINENTAL ARCS
The folded 1875–1844 Ma Great Bear magmatic zone exposes scores of plutons, ranging in composition from basalt to granite, with varying degrees of exposure of their floors and roofs. Over the full range of compositions the plutons have dominantly flat roofs and floors. The major exceptions are a few laccolithic bodies and resurgent plutons in the cores of calderas, which have domical roofs. In the Great Bear most plutonic bodies have aspect ratios of 5–15 over the compositional range of basalt to rhyodacite. Only the biotite granites tend to be thicker, but they still have aspect ratios greater than unity.
Epizonal plutons in the Great Bear magmatic zone do not appear to have erupted. However, their likely precursors—more or less equivalent bodies located at greater depths—produced monotonously-intermediate and compositionally zoned eruptions in the form of ash-flows and unzoned eruptions as lavas. These eruptions apparently did not severely deplete the magma chambers, so following eruption, the magma bodies rose into their own ejecta. This could happen because the initial eruptions did not draw down the magma chamber sufficiently to cause them to stagnate. Instead they had the ability to regain positive magma pressure, perhaps because the previous eruptions only drew down a small percent of their volume, or because they were re-energized either by new magmas or by gas sparging. While epizonal plutons are close compositionally and petrographically to their wall rocks, they did not feed those eruptions from their level of emplacement, but instead they represent the final phase in the dynamic lives of complex magmatic systems.
We'd like to thank our numerous field assistants for their excellent mapping, and our competent bush pilots, who kept us safe in the air and on the water. This paper was near completion when Todd Housh died unexpectedly. We will miss his rigorous intellect and intense interest in geological processes. Reviews by Olivier Bachmann, three anonymous sources, and Mike Williams, resulted in a more focused and improved paper.