“Normal” and “inverted” metamorphic isograds and their relation to syntectonic proterozoic batholiths in the Wopmay Orogen, Northwest Territories, Canada

Tectonophysics, 76 (1981) 295-316 Elsevier Scientific Publishing Company,

295 Amsterdam

- Printed

in The Netherlands

“NORMAL” AND “INVERTED” METAMORPHIC IS&GRADS AND THEIR RELATION TO SYNTE~ONIC PROTEROZOIC BATHOLITHS IN THE WOPMAY OROGEN, NORT~EST TERRITORIES, CANADA

M.R. ST-ONGE Department (Canada) (Received

of Geological Sciences,

Queen’s Uniuersity, Kingston, Ont. K7L-3N6

June 4, 1980; revised version

accepted

December

12, 1980)

ABSTRACT St-Onge, M.R., 1981. “Normal” and “inverted” metamorphic isograds and their relation to syntectonic Proterozoic batholiths in the Wopmay Orogen, Northwest Territories, Canada. Tectonophysics, 76: 295-316. Three progressive metamorphic suites are developed in pelitic rocks of the northern Wopmay Orogen. Two suites are related to the Hepburn Batholith and one to the Wentzel Batholith. All three suites are cut by post-metamorphic wrench faults, some of which have significant vertical displacement. The structural relief so provided reveals that medium- and high-grade isograds associated with the Hepburn Batholith dip inward towards the batholith and are thus “hot-side-up”. Isograds associated with the Wentzel Batholith dip away from the batholith and are thus “hot-side-down”. It is concluded that Hepburn Batholith has the form of the flattened funnel fed from depth, and that Wentzel Batholith is the arched roof of an intrusive complex of unknown shape at depth.

INTRODUCTION

The fact that grade of metamorphism is in part controlled by depth has led to the consideration that a “normal” metamorphic suite is one where lower grade zones overlie higher grade zones. And any me~o~hic suite that has an opposite geometry (high-DDE zones over low-grade zones) is one that is considered to be “inverted”. As stated by Chapman (1953) the terms “normal” and “inverted” are helpful when used purely descriptively to denote the vertical order of zones but no genetic significance should be attached to the terms. Tilley (1925) in his classic account of the metamorphic zones of the Southern Highlands of Scotland, was probably the first to recognize the existence of an inverted me~morphic zonation. He implied that the inversion was due to post-met~o~hic rotation of the zones and of the stratigraphic succession by large SE-vergent recumbent folds. Field work by Elles 0040-1951/81/0000-0000/$02.50

@ 1981 Elsevier

Scientific

Publishing

Company

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and Tilley (1930) supported this model, but the latter was disputed by Bailey and McGallien (1937). Recent work in southern California has shown that some inverted metamorphic suites are related to thrust faults, possibly because of the effect of shear-heating at the thrust plane (Graham and England, 1976); inverted successions of metamorphic zones in the East Greenland Caledonides can possibly be explained the same way (Haller, 1971, p. 195). In the northern Appennines, Carmignani et al. (1978) have postulated a large amount of frictional heating during movement on the Apuane Shear Zone to explain local inversions in metamorphic zonations. Systematic mapping (this study) by the Geological Survey of Canada in Early Proterozoic rocks of north-central Wopmay Orogen (Fig. 1) has shown the presence of three progressive metamorphic suites. An eastern and a central metamorphic suite are both related to the Hepburn Batholith (Fig. 2) and a western suite is associated with the Wentzel Batholith. In the study of the metamorphic zones, an attempt was made to determine the dips of metamorphic isograds. As discussed by Tilley (1925), the dip of an isograd can be inferred from its trace in an area of marked topographic relief or from its strike-separation across a fault with a substantial and known component of dip-slip displacement. Topographic relief in the Hepburn Lake map area (Fig. 2) is insufficient for the first approach; however the metamorphic isograds are cut by major faults, the vertical displacements across some of which

119~00’ 68

ARCTIC

A-/

SLAVE

OCEAN

_

PROVINCE 5

Fig. 1. Tectonic Hoffman, 1980.)

units of the northwest

corner

of the Canadian

Precambrian

Shield.

(After

297

Fig. 2. Batholiths and associated metamorphic suites of the northern Wopmay Orogen. Isograds are shown with ornament on high T side: two dots for biotite, box for andalusite, and V for sillimanite + orthoclase. Full triangles indicate hanging wall of Marceau Thrust. S refers to Hornby Bay Group sandstone cover and M to the Muskox Intrusion.

provide significant structural relief. Provided that both the strike-slip and the dip-slip components of displacement on a fault can be determined from the strike-separations of structural elements of known attitude, the dip of an isograd can then be determined from its strike-separation. This has been done in each of the three metamorphic suites, showing that the two suites associated with the Hepburn Batholith are “inverted” at medium- and highgrade whereas the suite associated with the Wentzel Batholith is “normal”. In Wopmay Orogen the “inverted” isograds can be interpreted unambiguously in terms of a funnel-shaped batholith. This distinguishes the inverted

298

zonations found from those of the northern in the Hepburn Lake shear heating along a of intrusive origin.

WOPMAY

those of the Southern Highlands of Scotland and from Appennines or southern California, since the inversions map area were produced not by structural rotation nor crustal thrust but rather by the shape of a heat source

OROGEN

Wopmay Orogen (McGlynn, 1970) is an Early Proterozoic (2.1-1.8 Ga) tectonic-metamorphic belt that flanks the western edge of the Archean Slave craton (Stockwell, 1961; Hoffman et al., 1970; Fraser et al., 1972). The orogen is exceptionally well exposed and has been interpreted as a complete record of a Wilson cycle (Hoffman, 1980), analogous to those that operated during the Phanerozoic. Wopmay Orogen has been subdivided into four tectonic zones (Hoffman, 1980). From east to west (Fig. 1): Zone I comprises autochthonous EarIy Proterozic sediments that rest unconformably on the Slave craton. A thin (600 m) sandstone-shale-dolostone sequence is overlain by a thin (800 m) distal flysch. Zone 2 is an allochthonous equivalent of zone 1. The elastic-carbonate shelf-sequence is much thicker (2-4 km), and is overlain sharply by a thin (80 m) hemipelagic shale and a thick-bedded sandy feldspathic flysch. The vergence of folding and thrusting of the shelf-sequence is toward the Slave craton to the east. Zone 3 is a continuation of zone 2 and includes the early rift deposits of the Akaitcho Group (Hoffman et al., 1978; Easton, 1980), the continentalrise-and-slope-facies deposits of the Epworth Group (Fraser, 1960; Hoffman et al., 1970; Hoffman, 1972, 1973; Fraser, 1974) and an overlying flysch that is the distal equivalent of the flysch of zones 2 and 1. Zone 3 is characterized by early thrust faults, large upright chevron bedding folds with prominent axial-plane cleavage, ate-metamo~hic folds of foliation, syn- to post-tectonic plutons of the Hepburn Batholith and Wentzel Batholith and the presence of three progressive metamorphic suites. Zone 4, separated from zone 3 by a major fauit of indeterminate displacement (Wopmay Fault), consists of thick talc-alkaline volcanic rocks and continental sediments that are broadly folded and only weakly metamorphosed (Hoffman and MeGlynn, 1977; Hoffman, 1978). Large, tabular epizonal I-type (Chappell and White, 1974) plutons (mostly granitic) intrude the sedimentary and comagmatic pile. Zone 4 in the current plate tectonic model for Wopmay Orogen (Hoffman, 1980) is thought to be an arc-bearing microcontinent that collided with the Slave-Taltson plate during the late Early Proterozoic.

299 HEPBURN

BATHOLITH

AND WENTZEL

BATHOLITH

(ZONE

3)

Within the Hepburn Lake segment of zone 3 (Fig. 2), syn- to post-tectonic plutons are clustered in two N-S-trending large composite batholiths, the Hepburn Batholith in the middle of the zone and the Wentzel Batholith on the west side (Fig. 2). Individu~ plutons vary from 1 km2 to over 400 km2 and are generally homogeneous in composition. The plutons range from biotite granite to pyroxenite (Hoffman et al., 1980) but granite (Steckheisen, 1967) is by far the most extensive unit areally. A very general trend, deduced from cross-cutting relationships in the field, is for the plutons to become more mafic with time. Biotite is ubiquitous; hornblende is rare even in the granodiorite and tonalite plutons. Sparse garnet is common and sillimanite is present but rare. In Chappell and White’s (19’74) classification this mineralogy is typical of S-type granitic rocks. If the bulk of the two batholiths was indeed derived by anatexis of the sedimentary prism and/or upper continental crust, then the compositional trend outlined in the field should be expected. The first melts would be those of a eutectic minimum granitic composition and would be followed during continuing anatexis by melting of more refractory minerals giving progressively more mafic magmas. REGIONAL

METAMORPHISM

(ZONE

3)

Metamorphic isograds based on the first appearance of index mineral assemblages in pelitic rocks have been mapped on both sides of the Hepburn Batholith (eastern and central metamorphic suites, Fig. 2) and on the west side of the Wentzel Batholith (western metamorphic suite, Fig. 2). The metamorphic suites in Fig. 2 are outlined with only 2 or 3 isograds for the sake of clarity. A more detailed isograd map pattern for a pertinent part of each me~morphic suite is given in Figs. 4,7 and 8. In all figures the ornament on each isograd symbol indicates the high-grade side of that isograd. During three summers of field work the isograds have been mapped for a distance of 100 km in the eastern metamorphic suite, 60 km in the central metamorphic suite and 50 km in the western metamorphic suite. Traverses were generally sub-perpendicular to the strike of the isograds and spaced 2-4 km apart. A full account of the study of the metamorphic isograds, the pertinent reactions and the inferred P-T-X conditions from microprobe and field work in the Hepburn Lake map area will be given elsewhere (St-Onge, in prep.). Only data relevant to the problem of the attitude of the isograds are presented here. The isograds in the nietapelites are concordant in map view with the batholiths at a regional scale for the three suites, but in detail can be quite discordant; numerous cross-cutting relationships are noted with the younger plutons of the Hepburn Batholith. Accordingly the met~o~hic isograds are inferred to be related to the large foliated granites which constitute the older phases of the batholiths and many of the largest plutons.

300

In all three metamorphic suites the grade in the pelitic rocks increases towards the batholiths from muscovite-chlorite-plagioclase-quartz assemblages to locally high-grade garnet-sillimanite-cordierite-biotite-orthoclase-plagioclase-quartz-granitic melt (the latter inferred from granitic pods in the metasedimen~). The change in metamo~hic mineralogy is simply a reflection of the metamo~hic temperature gradient, with the batholith the obvious source of heat. Estimated conditions of metamorphism in the Hepburn Lake map area are as follo.ws: temperature ranges from less than 440°C in the chlorite zone (Winkler, 1979, p. 213) to 725°C for the products of the reaction quartz + plagioclase + biotite + sillimanite =+garnet + cordierite f orthoclase + granitic liquid (the latter inferred from granitic pods in the metasediments) (Lee and Boldaway, 1977). In the model pelitie system Si02-A1,0,-Fe0-Mg0K,O-Na,O-HZ0 this anatectic reaction would be univariant and vapor-phaseabsent. Estimated pressures for the regional metamorphism vary from a minimum of 2.2 - lo8 Pa in the northern part of the map area (minima given by the lower-P boundary of bathozone 2 (Carmichael, 1978) as shown by the stable assemblage staurolite-gamet-andalusite-biotite-plagioclase-quartz) to a maximum of 3.8 * 10’ Pa (the upper-P boundary of bathozone 3 (Carmichael, 1978) as shown by the occurrence of andalusite plus s~lim~ite) in the central part of the Hepburn Lake map area. P-gradients, inferred from field work using the bathograd scheme decrease from the central part of the map area towards the south, mirroring the pressure decrease in the north half of the map area (St-Onge, in prep.). EASTERN

METAMORPHIC

SUITE: “HOT-SIDE-UP”

The eastern metamorphic suite is developed on the east side of the Hepburn Batholith in pelites and flysch of the Epworth Group (Hoffman et al., 1978). Isograd attitudes for the eastern suite can be determined in the area covered by Fig. 4 and located in Fig. 3. Four isograds are shown in Fig. 4 as mapped in the field and checked with thin section’ work. It was very difficult to tell from hand samples when biotite first appears in the musco~t~hlo~t~pla~ocl~~qu~z slates. Thus in order to map the biotite isograd, a petro~phic microscope was used in the field to check for biotite in powders made from the lower grade samples. The same procedure was used initially for the higher grade isograds, but it was found that with experience, the first occurrence of andalusite and sillimanite could be noted directly in the field, the two polymorphs being conspicuous and distinctive in the muscovite schists. Full mineral assemblages were later verified.with thin sections. The first occurrence of granitic pods in the metapelites is outlined with the granitic pods isograd in Fig. 4, These are ovoid-shaped lenticles of quiz-pl~ocl~e-Kfeldsp~ with an average long axis of 15 cm parallel to the dominant foliation. In an identical

301 67’00’ 45’

116’

* Akoitcho

30’

45’

1150

15’

L.

Hepburn-

L.

IO km

FIGURE

s 15’

ll6O

Fig. 3. Location

66” 45’

30’

map for Figures

15’

1150

00’ 45’

4, 7 and 8.

manner to the biotite, andalusite, and sillimanite porphyroblasts, the granitic pods are found cutting across the bedding laminae in the pelites; the same granitic pods partly cut across and are partly wrapped by the dominant S1 tectonic .mineral fabric. Similar to the mineral porphyroblasts, the first occurrence of the granitic pods is abrupt and mappable; granitic material comprises over 30% of the original rock within 1 km west of the isograd, giving the metapelite a migmatitic character. The first occurrence of the granitic pods is inferred to represent the onset of in-situ partial melting in the metapelites because their occurrence as random, discreet, non-aligned domains in a rock with bedding still intact, does not point to an origin by dyking or veining nor deformed dykes or veins.

302

_I_

I

LUPIN

MARCEAU THRUST

Fig. 4. “Hot-side-up” isograds east of Hepburn Batholith. Isograds shown with ornament on high T side, two dots for biotite, box for andalusite, three ticks for sillimanite, cross for granitic melt (inferred from granitic pods in metasediments). See text for isograd geometry, Lines A-A’-B and C-C’-D are lines of down-plunge sections of Fig. 5. E refers to cut-off of metamorphic suite by Sinister Fault, F refers to Fz folding of metamorphic isograds, G to Spider Lake Pluton, H to Hornby Bay Sandstone outlier, I to F1 folding of Marceau Thrust.

The first occurrence of sillimanite plus orthoclase in the pelites has been mapped in the area covered by Fig. 4 at lower grade, coincident grade or higher grade than the granitic pods isograd. This isograd and a higher grade zone of garnet-cordierite-Kfeldspar are not essential to the argument presented here and have been omitted for the sake of clarity. Two sets of folds are found in the eastern metamorphic suite. The first set is a series of N-S-trending upright chevron bedding folds (F,) with a well developed vertical or steeply west-dipping axial planar cleavage (S1). Biotite, andalusite and sillimanite porphyroblasts cut across and are wrapped by the S, fabric. This suggests a syntectonic metamorphic mineral growth with respect to the S1 fabric. South of the Coppermine River in the higher grade metapelites, S1 is statistically almost recumbent, and the early folds (F,) are refolded coaxially by a second set of upright folds (F,). F2 also folds the higher grade isograds (Point F, Fig. 4) and the biotite foliation in the older plutons of the Hepburn Batholith. The eastern metamorphic suite is characterized by a number of late, fine-

303

grained “B’‘-granite (Stec~eisen, 1967) plutons that are separate from the main cluster of the Hepburn Batholith. In general these intrusions are less than 3 km2 in plan although the Spider Lake Pluton (Point G, Fig. 4) is a notable exception. The plutons are all found near or above the sillimanite isograd. These granites are massive and clearly cross-cut the metamorphic isograds (Fig. 4). Finally, the map-pattern of the eastern metamorphic suite shown in Fig. 4 is dominated by a NW-trending set of post-metamorphic transcurrent faults. AI1 the faults are characterized by a sinistral strike-slip component of displacement, the greatest being that of the Sinister Fault, whose left-lateral displacement of the near vertical Lupin Fault is 12 km (Fig. 4). Preliminary consideration of the strike-separation of the andalusite isograd relative to that of Marceau Thrust across the Sinister Fault suggested that the isograd dipped westward and was thus “hot-side-up” (St-Onge and Hoffman, 1980). In order to make a more quantitative evaluation of the attitude of all the mapped isograds in the eastern metamorphic suite, two don-plunge st~ct~al cross-sections (Sto~kwe~, 1950) have been drawn, one in the fault block south of Sinister Fault (line A-A’-B, Fig. 4 and Fig. 5a) and one in the fault block north of Sinister Fault (line C-C’-0, Figs. 4 and 5b). Both structural sections were constructed as rigorously as possible, taking full account of measured bedding and cleavage attitudes and contact positions along the lines of section. Formation contacts and marker beds up and down strike were projected onto the inclined plane of the section, making use of the northerly plunge (12”--15” ) of the major fold structures. Marceau Thrust is a major fault in the eastern metamorphic suite (Fig. 4) which places the older pelite-metaquartzite sequence (Odjick Formation) on top of the younger black phyllite-greywacke sequence (Recluse Formation). The attitude of the fault could not be directly measured in the field, the trace of the fault invariably corresponding to a narrow valley. However, constraints on the geometry are given (in the fault block south of Sinister Fault) by matching bedding dips of 50”~80” to the west in both footwall and hanging wall of Marceau Fault. Since Marceau Fault places older Odjick Formation on its west side against younger Recluse Formation on its east side, it seems clear that Marceau is a westerly derived thrust fault that now dips 50”--80’ to the west. The west dipping nature of Marceau Fault is corroborated by the effect of the south plunging syncline and anticline pair on the trace of Marceau Fault in location I, Fig. 4. A similar effect by north plunging folds 12 km north of I, Fig. 4 results in a “west bulge” for the trace of Marceau Fault, the occurrence corresponding to a structural culmination and indicating again a west dip for Marceau Fault. Since regional folds (F,) affect the trace of the fault as they do the bedding it is postulated that Marceau Thrust was essentially a “fait accompli” when folding of bedding occurred. Because the trace of Marceau Fault is parallel to the trace of bedding and the inferred dip of the fault that of the folded bedding (except where the thrust cuts up section) a projection of Marceau Thrust as an

304

explicit feature of the cross-sections of Fig. 5 using the plunge of the major fold structures is then permissible. The lower boundary of the north cross-section and the upper boundary of the south cross-section is the trace of Sinister Fault. Obviously the sections cannot be truly “balanced” in the way of DahlStrom (1969) because of the pervasive internal strain found in the metapelites in the form of a slaty cleavage, schistosity or gneissosity. However, because of the amount of control given by the structural measurements along the lines of section combined with the projected contacts along the plunge .of major structures, it is felt that the two cross-sections are a fair depiction of reality. The north cross-section (Fig. 5b) is drawn from Lupin Fault west to the point where the Middle Proterozoic Muskox Intrusive Complex cuts out the folded metamorphic units. The south cross-section (Fig. 5a) is drawn from Lupin Fault west to the point where bedding is fully transposed on the cleavage, in the general vicinity of the granitic pods isograd. E-W realignment of the two sections was done by matching the vertical axial surface trace of a major syncline of Recluse greywacke, found in both fault blocks and here named Zebra Syncline for reference (Figs. 5a, 5b). This realignment results in a matching of Lupin Fault which is the expression of an early syndepositional fault scarp of Fontano Formation age (Hoffman, 1980). The validity of projecting Lupin Fault into the sections relies on the fact that its strike is parallel to the strikes of the fold axial surfaces. It will be noted that all the other folds line up rather well. This realignment accommodates the 12 km strike-slip displacement of the pelite belt by the Sinister Fault. Vertical restoration of the two sections cannot be done as precisely. No stratigraphic marker can be used to give an absolute value of dip-slip motion across the Sinister Fault because of the amount of stratigraphic section lost across the fault. In a general sense, however, the northern section east of Marceau Thrust, being composed of Recluse greywacke and pelite and Fontano pelite (Fig. 5b) must lie above the southern section east of Marceau Thrust which has older Odjick pelite overlain by Fontano pelite with Recluse greywacke and pelite on top (Fig. 5a). The metamorphic mineral assemblages found in both fault blocks provide further evidence for north-side-down displacement on the Sinister Fault. Section line A--A’-B is characterized by the coincidence of the granitic pods isograd and the sillimanite plus orthoclase isograd. In a petrogenetic grid for ideal pelitic rocks, this corresponds to the pressure value of point A. Fig. 6, which is within the P range of bathozone 3 of Carmichael (1978). Section line C---C’-D is characterized by the occurrence of garnet-andalusitestaurolite-muscovite-biotite-quartz assemblages which typifies Carmichael’s (1978) bathozone 2. The orthoclase plus sillimanite isograd occurs down grade from the granitic pods isograd which also suggests that section line C--C’---0 is lower pressure than point A, Fig. 6. From the exposed meta-

_,^,.^_

PODS

-

F’ MARCEA” +fHRUS~ 7 FRRd BIOTITE

FIGURE

BlOiiTE

5b FI .I_L. ,ULT

Fig. 5. Composite inclined down-plunge cross-section. a. Block south of Sinister Fault. b. Block north of Sinister Fault. Epworth Group units are as follows: simple lines for Odjick Formation; lines and dots for Fontano Formation; lines, dots and heavy lines marking graywacke beds for Recluse Formation. Dashed lines are symbol for S1 fabric. Lines of section identified by letters and located in Fig. 4.

ANDiLUSLTE

FSijGf4~f-41~~~

:

306 9r

ABBREVIATIONS andolutlte

P(lO8

500

6oo

T(Y)

700

Fig. 6. P-T petrogenetic grid for part of the ideal pelite system SiOz-AlzOa-FeOMgO-NazO-KzO-H20, showing the bathozone scheme of Carmichael (1978) modified by St-Onge and Carmichael (1979).

morphic assemblages, line A-A’-B (south block) is inferred to have been more deeply buried at the time of metamorphic “quenching” than line C-C-D (north block) to account for the difference in the determined P conditions. Thus from the stratigraphy of the pelite belt and from the exposed metamorphic mineral assemblages, the south block is of a deeper structural level than the north block, and the net dip-slip component of displacement on Sinister Fault must have been north-side-down. This is contrary to the early conclusions of St-Onge and Hoffman (1980). The presence of an outlier of the Homby Bay Sandstone (point H, Fig. 4) is now taken to indicate only a late, relatively minor, south-side-down component of displacement on a fault system with several displacement events, the net cumulative motion being north-side-down. A structural constraint on the minimum dip-slip motion of Sinister Fault is given by Marceau Thrust. Because the thrust fault places the older pelitemetaquartzite sequence on top of the younger black phyllite-greywacke sequence, it must cut up section in both its hanging wall and footwall. Using‘ that criterion, Marceau Thrust has been interpolated between the two

307

horizontally realigned sections and given the shallowest possible attitude by keeping the trace of the fault parallel to that of the beds in the section (Fig. 5). This construction is interesting because it shows that Marceau Thrust is folded by the folds of bedding (F,) in order to respect the up-cutting criterion, as predicted by the effect of the syncline-anticline pair on the trace of Marceau Thrust at location 1, Fig. 4. The amplitudes of the folds of Marceau between the two sections can be used as a minimum separation distance between the top of the south section and the bottom of the north section. The resulting 1.5 km vertical distance is the minimum amount of section lost to the north-side down dip-slip component of motion for Sinister Fault. If Marceau Thrust is in fact a steeper fault in that interval then, the distance between the two sections will be greater, implying a larger component of dip-slip motion for Sinister Fault. Finally isograds in both fault blocks have been projected into the plane of the cross-section. The assumption that the fold axis orientation is relevant to the isograd surfaces must be made at this stage. The point is not to imply a genetic relationship between the isograds and the fold axis but simply to assume that geometrically they are close, thus enabling a down-plunge projection. The validity of this relies on the fact that the strike of the isograds is near parallel to the strike of the fold axial surfaces. To the extent that the vertical component of displacement on Sinister Fault has been overestimated or underestimated the dip of the isograds will be too shallow or too steep, respectively, but there can be little doubt that all except the biotite isograd are “hot-side-up”. Near point F, Fig. 4, the granitic-pods isograd is affected by the latemetamorphic F2 folds (late- and not post- because no cataclastic fabric is associated with the folds). The fact that higher metamorphic grades are found in the F2 synforms than in the F2 antiforms, corroborates the “hotside-up” metamorphism in the western part of the eastern metamorphic suite. The structural sections (Figs. 5a, b) show clearly that the higher grade isograds have not been inverted by a structural rotation, as this would have shown up in both the fold style and stratigraphy of the cross-sections. Rather the inversion of the isograds appears “primary” and thus it has important implications in regard to the shape of the Hepburn Batholith, the apparent heat source in the area. CENTRAL

METAMORPHIC

SUITE:

“HOT-SIDE-UP”

The central suite is developed along the west side of the Hepburn Batholith in pelites and bimodal volcanic rocks of the upper Akaitcho Group (Easton, 1980). Figure 7 shows the isograds and batholith contacts of the central metamorphic suite near the Coppermine River. Fig. 3 shows the location of the area covered by Fig. 7. Five isograds are mappable in the pelitic units of the Akaitcho Group. The

Fig. 7. “Hot-side-up” isograds west of Hepburn Batholith. Isograds are shown with ornament on high T side: two dots for biotite, triangle for staurolite, three ticks for sillimanite, V for sillimanite + orthoclase, and cross for granitic melt (inferred from granitic pods in metasediments). The biotite isograd is not shown north of the fault because the biotite appears in the metapelites over the width of the dyke. See text for isograd geometry.

first occurrence of biotite was determined in the field by the use of a petrographic microscope. Staurolite and sillimanite are conspicuous and distinct in the muscovite schists with staurolite often typically showing cruciform’ twins. The sillimanite plus orthoclase assemblage follows the breakdown of muscovite in the presence of quartz. The isograd is defined by the first occurrence of orthoclase poikiloblasts in contact with sillimanite. Texturally the change is very marked, the pelitic rock being transformed from a muscovite- sillimanite schist into a “denser” quartz-plagioclase-Kfeldsparsillimanite gneiss. The granitic pods isograd marks the first occurrence of granitic pods in the pelitic units. This isograd is identical to the one mapped in the eastern metamorphic suite and the reader is referred to the previous section for a description of mode of occurrence. Where the suite crosses the Coppermine River, it is cut by an unnamed northeast-trending fault (see Fig. 7). Relative metamorphic pressure determinations interpreted from mineral assemblages north and south of the fault can be used to determine the sense of vertical displacement. North and south of the fault a sillimanite f garnet zone is found immediately upgrade from a

309

staurolite zone, a transition that restricts both fault blocks to Carmichael’s (1978) bathozones 3 and/or 4. The absence of kyanite and sporadic occurrences of andalusite in both fault blocks further limit the pressure variation to that of bathozone 3. North of the fault the sillimanite plus orthoclase and the granitic pods isograds coincide, corresponding to the muscovite-melt bathograd of St-Onge and Carmichael (1979) and to the invariant point (labelled A on Fig. 6) that separates bathozones 3a and 3b. South of the fault, however, the sillimanite plus orthoclase isograd is well down-grade from the granitic pods isograd, indicating a position within bathozone 3a and implying a decrease in pressure (see Fig. 6). By this reasoning the fault apparently is south-side-down. The fault is short (10 km) and transcurrent movement cannot have been great. Correlated felsic tuffs that dip 40” to 60” east have less than 1 km of right-lateral strike separation (not shown on Fig. 7 so as not to clutter the isograd pattern). There is little strike separation of the large granitic dyke (see Fig. 7) but its dip is unknown. Assuming that the large granitic dyke is pre-fault and dips vertically (a fair assumption for a dyke), a south-side-down component of displacement would be required to account for the larger strike separation of the east-dipping felsic tuff units corroborating the relative pressure determinations interpreted from the metamorphic data. The isograds have a right-handed strike separation of 2 km. This is probably an effect of the vertical component of movement. Given that the south went down and that the felsic tuffs dip to the east, geometry dictates that the isograds also dip to the east, even more gently than the tuffs. With an easterly dip and grade increasing east, these isograds must also be “hot-side-up”. No post-metamorphic structures (that could imply a structural rotation of the isogradic surfaces are present in the area. As was the case for the eastern metamorphic suite, the “hot-side-up” attitude for the isograds west of Hepbum Batholith would seem to be primary. WESTERN

METAMORPHIC

SUITE:

“HOT-SIDE-DOWN”

The western suite is developed along the east side of Wentzel Batholith in pelite and arkose of the lower Akaitcho Group. It is characterized by the same mineral assemblages as the central metamorphic suite. Figure 8 shows the detail of the southern part of the Western metamorphic suite in the vicinity of Wentzel Lake, where the suite is cut by the southeast-trending Belleau Fault. Figure 3 gives the location of the area covered by Fig. 8. Belleau Fault is a left-lateral transcurrent fault that underwent dip-slip reactivation following deposition of the post-erogenic Homby Bay Group (Middle Proterozoic) cover (Hoffman, 1980). An outlier of Homby Bay Group Sandstone on the north side of the fault indicates a north-side-down dip-slip displacement for the fault (see Fig. 8). The near-vertical Wopmay Fault, which predates the transcurrent Belleau Fault but is younger than the regional metamorphic event (Hoffman, 1980)

Fig. 8. “Hot-side-down” isograds east of Wentzel Batholith. Isograds are shown with ornaments on high T side; triangle for staurotite, three ticks for sillimanite, V for sillimanite plus orthoclase, cross for granitic melt (inferred from granitic pods in met~edimen~), box for andalusite. See text for isograd geometry. 5’ refers to Hornby Bay Sandstone cover.

can be used as a structure marker. Cut by the Belleau Fault, the Wopmay Fault has less than 2 km of stoke-sep~ation (point A, Fig. 8). In contrast both the isograds and the edge of Wentzel Batholith have stoke-sep~ations of about 20 km (points B, C, Fig. 8). Knowing that the north side of Belleau Fault is down, geometry indicates that the isograds and the edge of the Wentzel Batholith both dip to the east. Because metamo~hi~ grade increases to the west, the isograds are therefore “hot-side~o~“. The conclusion is corroborated in the northern part of the western metamorphic suite, where a number of late-metamorphic folds reveal batholithic rocks in the cores of antifo~s and me~morphic rocks in the synforms. The Wentzel Batholith is presumably the heat source for the western sequence and finding it exposed in the anticlines suggests that temperature increased downwards. SHAPE OF WOPMAY OROGEN BATHOLITHS

The three prograde metamo~hi~ suites are regionally concordant with the Hepburn Batholith and Wentzel Batholith. In each suite the grade increases

311

towards the granitic rocks and it is inferred that the latter are the heat sources. The attitudes of the metamorphic isograds must therefore be dependant on the attitudes of the batholithic contacts, that is, the shape of the batholiths themselves. The eastern and central metamorphic suites are both “hot-side-up”, the isograds dipping towards the batholith in the direction of increasing grade. It is postulated that the batholith contacts have the same geometry and are inward dipping. Hepburn Batholith is thus either a broad funnel with a feeder conduit at depth or a rootless synform. The funnel shape is favored because the presence of garnet and rare sillimanite in batholithic units points to an origin by anatectic melting of high-grade gneisses at pressures greater than 6 - lo* Pa to 9 .lOa Pa (Green and Ringwood, 1968; Flood and Shaw, 1975, Green, 1976) substantially more than the upper pressure limit (3.8 . lo8 Pa) of Carmichael’s (1978) bathozone 3 which characterizes the metapelites. Bathozone 3 implies an upper depth limit of approximately 12 km for the metamorphic rocks adjacent to the Hepburn Batholith. These correspond to the floor of the batholith if the model is correct. What batholith was above the metapelites, imposing a “hot-side-up” geometry on the medium- and high-grade isograds, must then have been “thinner” than 12 km although it is difficult to say by how much. The western metamorphic sequence is “hot-side-down”, the metamorphic isograds dipping in the direction of decreasing grade, away from Wentzel Batholith. The present exposure must then be the roof of the batholith. Whether or not Wentzel Batholith and Hepburn Batholith are connected at depth cannot be deduced at present but may be revealed as this type of analysis is extented southward in Wopmay Orogen. Metamorphic isograds attitudes and probably batholith contacts have been modified by the late-metamorphic Fz folding. The scale of the folding, however, does not distort the overall geometry of the isograds as can be seen in Fig. 5a, and should then have no greater effect on the batholith shapes. FUNNEL-SHAPED

GRANITIC

BODIES

Constraints on the thickness at the edges (less than 12 km) and on the attitude of the east and west contacts (both inward dipping) dictate a funnel shape for the Hepburn Batholith. This geometry for a granitic intrusive is not new (Cloos, 1923; Chamberlin and Link, 1927; Lane, 1931) and has been documented in several erogenic belts. A most striking example is that of Bridgwater et al. (1974), in which lateto post-erogenic Ketilidian intrusions of the rapakivi granite suite in south Greenland are shown to be mushroom-shaped, with a sheet-like upper part overlying a vertical stem. Cordierite-andalusite homfelses in the upper contact aureoles suggest maximum depth of crystallization to be 6 km while hypersthene-bearing migmatites from the floor of the intrusions indicate

312

deeper

sections by 3-4 km (Bridgwater et al., 1974). It is interesting that the total depth to the underside of the plutons is 10 km, an estimate close to that for the country rocks at their present level of erosion near the inferred floor of the Hepburn Batholith. Gravity profiles done on the Upper Triassic Guichon Creek Batholith of southern British Columbia, Canada along with detailed geological surveys have delineated an interesting three-dimensional model for the batholith (Ager et al., 1972). Ager et al. postulate the shape of the intrusive complex to be that of a funnel-like structure. The shape is similar to that proposed for the Hepburn Batholith and if the Guichon Creek Batholith had any thermal effect on the country rocks it is expected to be the “hot-side-up” isograd type. Hutchison (1970) proposes a four-fold classification of main plutonic styles as recognized in the Prince Rupert region of the Central Coast Mountains, British Columbia. One category is the para-autochthonous ‘tongueshaped, recumbent plutons and is typified by the Alastair and Ponder Plutons (Hu~hison, 1970). Both these plutons are characterized by westdirected protuberances which concordantly overlie the country gneisses. The extent of the overlap is at least 8 miles (12.9 km). Thus direct field evidence shows that some plutons do have shallow dipping floors and that lateral spreading can be documented. Again with such structures, “hot-side-up” metamorphism would be expected to underlie the intrusions. Hamilton and Myers (1967) in their survey of American batholiths make the point that batholiths in general are formed by magma rising from greater depths, coalescing into shallow and fairly thin complexes. The Sierra Nevada Batholith, a composite mass of plutons of late Jurassic and Cretaceous age is reported by Hamilton and Myers (1967) to be in the order of 8 km thick from gravity and to be “thin” from heat flow data. The New England Appalachian gneiss belt is visualized by Hamilton and Myers (1967) to have formed beneath a batholith analoguous to that of the Sierra Nevada. What predominates in Hamilton and Myers’ work is the notion of batholiths as thin granitic sheets produced by magma formed at depth, rising to coalesce in the upper parts of the crust. Certainly the Hepburn Batholith fits well in this model, The Upper Cretaceous Boulder Batholith of Montana is seen by Hamilton and Myers (1974a, 1974b) as a thin. floored sheet. However, this view is not held by everyone; Klepper et al. (1974) argue instead that the Boulder Batholith widens downwards. The aim of this section of the paper has been to illustrate, with a brief selection from the literature, that the funnel shape proposed for the Hepbum Batholith is not extravagant and that in fact the same model is put forward for several other batholiths. What the previous studies have not documented however, is the imposed attitude of isograds by any batholith that has the shape of a funnel. Hamilton and Myers (1974a, p. 375) remark quite rightly that for their model for

313

“beneath the outer parts of the batholith sedimenthe Boulder Batholith: tary rocks metamorphosed by heat conducted downward from the widening batholith above are presumably present.” Work in the Wopmay Orogen has shown that such metamorphic rocks do exist as documented by the “hot-side-up” isograds and that these are spatially related to the Hepburn Batholith. “HOT-SIDE-UP”

ISOGRADS

AND INVERTED

ZONES

OF METAMORPHISM

Past geological work has shown the common occurrence of inverted zones of metamorphism in erogenic belts. If Tilley’s (1925) interpretations are correct, the Southern Highlands of Scotland are characterized by an inverted metamorphic sequence. Work by Williams and Zwart (1977) in the Seve-Koli Caledonian nappe complex (Scandinavia) shows the existence of a similar sequence of metamorphic zones that was gradually inverted by large recumbent folds. Chapman (1953) attributes the inverted zones of metamorphism in western New Hampshire to an “uneven advance of metamorphism”; at a large scale the whole metamorphic succession is normal (“hot-side-down”) but at a small scale irregular parts are inverted of “hot-side-up” because of tongue-like projections of Bethlehem Gneiss of Kingsman Quartz Monzonite. Soper and Brown (1971) have argued that the metamorphic sequence of the north part of the Moine Nappe in Scotland, is inverted and “hot-side-up” due to another type of heat source, a sheet-like unit of migmatites. Finally in the Himalayas several localities of inverted sequences of metamorphic rocks are reported. They are related: to the Main Central Thrust by Pecher (1975) in the Annapurna Range, and LeFort (1975) in Central Nepal; to a zone of ductile rock deformation above (and related to) the Main Central Thrust by Bordet (1977), in the lower Tibetan Slab and to the thrusting of the infra-structural migmatite complex on the Daling-Darjeeling thrust block (Roy, 1974) in the Kalinpong Hills. The question is left open for the inverted sequence in the Jutogh Series (Simla Klippe) by Naha and Ray (1970) and in the metasediments of the Mishmi Formation by Thakur and Jain (1975). Gansser (1964) explains the occurrence of upward-increasing metamorphism and reversed stratigraphic sections in the Lower Himalayas as a result of large recumbent folds. He recognizes that this explanation is not applicable where reversed metamorphism is coupled with normal stratigraphic sections. The so-called inverted progressive regional metamorphism of the Almora Group (Kuman Himalaya) is considered by Ghose et al. (1974) to be simply the result of a structural juxtaposition of rocks of contrasting grades. Thus it would seem an “inverted” or “hot-side-up” sequence of metamorphic zones could be produced by post-metamorphic recumbent folds, thrusting of hot rocks over colder ones, shear heating on crustal thrusts or emplacement of a funnel-shaped intrusive complex such as the Hepburn Batholith.

314

Careful regional mapping is required in order to determine if a sequence of metamorphic zones is “hot-side-up” or “hot-side-down.” The obvious value to determining the attitude of metamorphic isograds is in the constraints then provided by the geometry on the source of heat of metamorphism. ACKNOWLEDGEMENTS

This paper is part of the author’s Ph.D. thesis. Field work was carried out during the summers of 1977-1979 while employed as senior assistant to Dr. Paul Hoffman of the Geological Survey of Canada. Financial support was granted by NSERC in the form of two postgraduate scholarships covering the time span of 1977-1981. ~though stimulating discu~ions were carried out with several colleagues, two people stand out; Dugald Carmichael, the thesis advisor, for his patience and healthy innovative approach to the problems of metamorphic petrology and for his contributions during the first 7 weeks of the project, in developing mapping methods appropriate to the pelite belts; and Paul Hoffman of the Geological Survey of Canada, who shared the joy of discovery in the field and showed unlimited enthusiasm. Both have kindly read and improved the present manuscript. Capable field assistance was provided by Ian de Bie during the first field season. REFERENCES Ager, C.A., McMillan, W.J and Ulrych, T.S., 1972. Gravity Magnetics and geology of the Guichon Creek Batholith. B. C. Dep. Mines Pet. Resour., Bull., 62: 17 pp. Bailey E.B. and McGallien. W.J., 1937. Perthshire tectonics: Schiehallion to Glen Lyon. Trans. R. Sot. Edinb., 59: 79-118. Bordet, P., 1977. Geologic de la Dalle du Tibet (Himalaya central). In: Livre B la memoire de Albert F. de Lapparent (1905-1975) consacre aux recherches geologiques dans les chafnes alpines de 1’Asie du sud-ouest. M&m, Sot. Geol. Fr., Hors. Ser., 8: 235-250. Bridgwater, D., Sutton J. and Watterson, J., 1974. Crustal downfolding associated with igneous activity. Tectonophysics, 21: 57-77. Carmichael, D.M., 1978. Metamorphic bathozones and bathograds: a measure of the depth of post-metamorphic uplift and erosion on the regional scale. Am. J. Sci., 278: 769-797. Carmignani, L., Giglia, G. and Kligfield, R., 1978. Structural evolution of the Apuane Alps; an example of continental margin deformation in the northern Appenines, Italy. J. Geol., 86: 487-504. Chamberlin R.T. and Link, T.A., 1927. The theory of laterally spreading batholiths. J. Geol., 35(4): 319-352. Chapman, C.A., 1953. Problem of inverted zones of metamorphism in western New Hampshire. Ill. Acad. Sci. Trans., 46: 115-123. Chappell, B.W. and White, A.J.R., 1974. Two contrasting granitic types. Prac. Geol., 8: 173-174. Cloos, H., 1923. Das Batholithenproblem. Fortschr. Geol. Palaeontol., l(1): l-80. Dahlstrom, C.D.A., 1969. Balanced cross-sections. Can. J. Earth Sci., 6: 743-757. Easton, R.M., 1980. Stratigraphy and geochemistry of the Akaiteho Group Hepburn Lake map area, District of Mackenzie. An initial rift succession in Wopmay Orogen (Early Proterozoic). In: Current Research, Part B. Geol. Surv. Can., Pap. 80-l: 47-57.

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