New Developments in Grenville Geology and Remote Sensing Fort-Coulonge Region, Western Quebec

GM 66922 NEW DEVELOPMENTS IN GRENVILLE GEOLOGY AND REMOTE SENSING, GUIDEBOOK FIELD TRIP B3, FORT-COULONGE REGION, WESTERN QUEBEC NEW DEVELOPMENTS IN GRENVILLE GEOLOGY AND REMOTE SENSING FORT-COULONGE REGION, WESTERN QUEBEC

GM 6 692 2 By

Kamal N.M. Sharma', Vernon H. Singhroy2 Louis Madore', Josée Lévesque2 Emmanuelle Giguère', Jules Cimon'

1 - Ministère des Ressources naturelles du Québec 2 - Canada Centre for Remote sensing

GUIDEBOOK FIELD TRIP B3 OTTAWA '97 GAC/MAC Annual Meeting

Ressources naturelles et Faune, Québec

1 3 MARS 2013

DIR. INFORM. GÉOL. 2 INTRODUCTION

Due to its geographic location, the Fort-Coulonge region (Figure 1) stretching from the Ottawa River in the south to the Cabonga Reservoir in the north, occupies a structurally unique and interesting position within the western part of the Grenville Province in Québec. It includes parts of two major lithotectonic domains : the western portion of the Central Metasedimentary Belt (CMB), and west of it, an extensive portion of the Central Gneiss Belt (CGB). The nomenclature used is the one introduced by Wynne-Edwards (1972). The Cayamant Lineament (Sharma et al. 1992) marks the limit between the Grenville Supergroup supracrustal rocks of the CMB to the east and the tonalitic - dioritic - granodioritic - granitic gneiss complexes and amphibolites of the CGB to the west. The Baskatong-Desert Lineament subdivides the CGB into two terranes, the Baskatong-Dozois Terrane and the Pythonga Terrane, respectively to the north and to the south of the lineament (Sharma, 1991 and Sharma et al., 1992. The present level of erosion lies very close to the surface or surfaces along which the supracrustal sequence of the CMB was transported towards NW and overthrust upon the adjacent gneissic terranes of the CGB. The sectors situated west of the Cayamant Lineament are thus dominated by the gneiss complexes of the CGB overlain by the metasedimentary rocks once part of the main body of the CMB. Consequently, the metasedimentary rocks and associated anorthositic rocks are present to the west of the Cayamant Lineament only as parts of the thrust nappes and in numerous small bodies representing the erosional relics of nearly entirely eroded nappes. The larger nappes consisting of supracrustal rocks are exemplified by the Fort-Coulonge and Cabonga nappes. During the northwest overthrusting of the metasedimentary pile over the adjacent gneissic terranes, the gneiss complexes composing these terranes did not remain unaffected, nor did they behave passively. In fact, these gneisses suffered intense deformation and were themselves thrust towards NW along subhorizontal to gently dipping decollements within the gneiss complexes, resulting in numerous thrust nappes consisting of gneiss complex rocks. It is within these deformation zones that numerous tectonic slivers of ultramafic rocks have been observed in the Fort-Coulonge area, as well as further northwestward in the Temiscaming region. 1

3 4 The field excursion is devoted to the observations of the structural and lithological characteris- tics of the Cayamant Lineament, Pythonga Terrane, thrust nappes consisting of the supracrustal rocks and of the gneiss complex rocks, and the occurrence of the ultramafic rocks.

PREVIOUS WORK

The earliest geological observations made in the area were in 1845 by Sir William Logan (1847) who examined the rocks along the Ottawa river during the reconnaissance work between Bytown (Ottawa) and Lake Temiscamang (Témiscamingue). In 1876, Henry G. Vennor and his assistant Lewis R. Ord carried out investigations along the Ottawa river and the southern portions of the Coulonge and Black (Noire) rivers. R.W. Ells (1907) made longer traverses along the Black (Noire) and Coulonge rivers up to a distance of about 70 miles northward from their junction with the Ottawa river.

More recently, the geological mapping done within parts of the area covered by the excursion include : reconnaissance mapping at 1:253 440 scale along Coulonge and Black (Noire) rivers by Retty (1933).

j mapping of Portage-du-Fort and lac Saint-Patrice area at 1:F6 720 scale by Katz (1976).

excellent detail mapping of Fort-Coulonge - Otter lake - Kazabazua area at 1:63 360 scale by Kretz (1977). MAJOR STRUCTURES AND TECTONIC SUBDIVISIONS

We started our field observations in the Fort-Coulonge region in 1989 with the objective to revise and reinterpret the pre-existing geological maps. Between 1989 and 1992, an equivalent of about two summer's work was devoted to a vast reconnaissance program to familiarize ourselves with the area and to outline the problems. At the same time we undertook a detailed structural interpretation of the satellite SEASAT-RADAR images covering 200 km x 100 km, 5 and of the Airborne - RADAR images covering 200 km x 18 km (Figure 2). The high-reso- lution Airborne-RADAR images were flown especially for us in 1989 by the Canada Centre for Remote Sensing to study the nature of the limit between the supracrustal rocks of the CMB and the gneiss complexes of the CGB. These interprations (Figure 3) provided high quality structural information, and have been instrumental to bring out several major structures, of fundamental importance, and which were previously unknown, e.g. Cayamant Lineament, Baskatong-Desert Lineament, NW-trending nappe structures, rare NE-trending nappe structure, brittle fractures related to the Ottawa-Bonnechère graben system etc. All these structures were well understood by 1991 (Lévesque and Sharma, 1991 ; Sharma, 1991).

Following these studies, a systematic mapping program has been in progress since 1993 to present involving K.N.M. Sharma, Louis Madore, Emmanuelle Giguère, Jules Cimon, Josée Lévesque, and Vernon H. Singhroy. Between 1993 and 1996 an area covering approximately 10 000 km2 has been systematically mapped from the Ottawa River in the south to the Baskatong-Desert Lineament towards north (Figure 1).

Cavamant Lineament

The two major lithotectonic domains - the Central Metasedimentary Belt and the Central Gneiss Belt - are juxtaposed along the Cayamant Lineament or the Cayamant Shear Zone (Figures 1 and 3). This lineament is significant for several reasons, the most important being that in addition to marking the western limit of the main body of the CMB, it also represents a major shear zone resulting from the overthrusting, towards W and NW, of the supracrustal pile, transported from the CMB, on top of the adjacent gneissic terranes (Pythonga and Baskatong- Dozois Terranes, subdivisions of the CGB). The Cayamant- Lineament thus constrains the western side of the «Central Metasedimentary Belt Boundary». This term must be reserved exclusively for the limits of the main body of the CMB. The southern side of the CMB Boundary is mostly hidden beneath the Paleozoics in Ontario. Our task now is to constrain the northern and eastern sides of the CMB Boundary which are not well understood at the present time. 6 7 8 Most importantly, as a major shear zone, the Cayamant Shear Zone is the first visible manifes- tation of the Basal Thrust Zone. The erosional remnants of this zone are well preserved in numerous exposures all along the Cayamant Lineament. Although the individual exposures may show minor local variations, the Waltham-type Zone is quite representative of the Basal Thrust Zone. Its upper part is composed of strongly tectonized straight-banded diverse lithologies including calc-silicate rocks, carbonate injections, and pegmatitic injections. The carbonate injections originate as a result of the remobilization and migration of the marbles originally present within the overlying tectonic pile. The lower part of the zone is characterized by straight-banded lithologies with diverse protoliths, such as granite, pegmatite, tonalite, diorite etc. and only little carbonate injections and calc-silicate rocks. Elsewhere, in its simplest form, the Basal Thrust Zone may simply consist of strongly tectonized metasedimentary rocks above and the gneiss complex rocks below, with a sharp contact between the two. In such cases the gneiss complexes usually show considerable migmatization.

Because of erosion it is difficult to estimate as to what could have been the total thickness of the Basal Thrust Zone. The field observations suggest that it must be in the order of several hundred metres.

These observations eloquently point out an important fact that the present level of erosion lies very close to the surface or surfaces along which the supracrustal sequence of the CMB was transported towards W and NW and overthrust upon the adjacent gneissic terranes of the CGB. During this event, and at this level, both groups of rocks underwent upper amphibolite to granulite grade metamorphic conditions. This would mean that the present level of observation corresponds to the depth where once the conditions of upper amphibolite to granulite facies predominated. In other words, we are now able to examine quite a deep section — mid-crustal or deeper — through the Grenvillian crust. Such an inference is nothing new. It was already postulated by Wynne-Edwards more than two decades ago : « ... the Grenville Province is a deep section part of an orogenic belt eroded to a level at which reworked sialic basement is the dominant rock-type exposed. » — Wynne-Edwards, 1972. Essentially the same message has been reiterated by Hanmer (1988), Davidson (1986) and others. 9 Now let us move westward from the Cayamant Lineament into the CGB.

Central Gneiss Belt and « Tectonic outliers » of supracrustal and anorthositic rocks

The presence of the Cayamant Lineament, marking the western side of the CMB Boundary, does not imply that there are no metasedimentary rocks present west of this lineament. In fact, we do find small and large, isolated to somewhat continuous, occurrences of metasediments with or without the associated anorthositic rocks west of the Cayamant Lineament for several tens of kilometres. The larger examples of these are represented by the Fort-Coulonge and Cabonga nappes, both composed of supracrustal and associated anorthositic rocks. In all these cases it can easily be proven that the metasedimentary rocks are tectonically overlying the gneiss complexes of the Pythonga and Baskatong-Dozois terranes of the CGB situated west of the CMB. Even where no visible remnant exposures of metasediments are preserved overlying the gneiss complex, the proof of the proximity of the earlier (before erosion) metasediments on top of the gneiss complexes is demonstrated by the occurrence of pinkish carbonate injections into the underlying gneiss complexes, and in places by the presence of unusually strong tectonic banding, intense deformation and/or migmatization within the gneiss complex.

We believe that during the overthrusting of the metasedimentary sequence on top of the gneiss complexes, under the prevailing metamorphic conditions of amphibolite to granulite grade, the marbles were highly mobile, and found their way as injections into the underlying gneisses in the form of pink carbonates which were also accompanied by a large amount of fluid activity which facilitated the development of unusually abundant skarns in their vicinity. In places such skarns have developed quite extensively. At the same time, in such zones variable portions of the originally tonalitic composition gneisses have become transformed into « scapolite gneisses » by the wholesale alteration of their plagioclase into scapolite, but at the same time perfectly preserving their original textures. Such cases may also be characterized by the development of diopside, sphene, apatite etc. to varying degrees. Furthermore, in areas where the metasedimentary rocks outcrop rather extensively, it is possible to observe the tectonically underlying rocks of the gneiss complexes due to the presence of erosional windows. 10 There is ample evidence that during the overthrusting of the supracrustal pile, transported from the CMB, over the adjacent gneissic terranes, towards W and NW, the gneiss complexes composing these terranes did not remain unaffected nor did they behave passively. In fact, these gneisses suffered as strong deformation as the once overlying metasedimentary rocks, developed a strong foliation and tectonic banding in the process, and were themselves transported towards W and NW along with their overriding load of metasediments. These phenomena are spectacularly evident in the structural interpretations of RADAR data, and confirmed by field studies, where the existence of several NW-trending nappes (rarely NE-trending) and their erosional remnants, hitherto unrecognized, has been demonstrated. Thus, during the thrusting event, packages of gneiss complexes were themselves thrust towards NW along subhorizontal to gently dipping decollements within the gneiss complexes. It is within these deformation zones that numerous tectonic slivers of ultramafic rocks have been observed in the Fort-Coulonge region, as well as further northwestward in the Temiscaming region.

KINEMATICS

There is an overwhelming accumulated evidence from the studies carried out, in Québec and in Ontario within equivalent terranes, by Harmer and Ciesielski (1984), Hanmer (1988), Madore and Sawyer (1992), Davidson (1986), Easton (1986, 1991), Hanmer and McEachern (1992), and from our own work, to suggest that the major tectonic transport took place towards W and NW during the compressive phase of the Grenvillian Orogenic Cycle. As a result, all the major fabrics observed in the rocks are attributed to this important phase of deformation and meta- morphism. However, in addition, we find evidence everywhere of a later phase of movements in extension (attributable to factors such as tectonic readjustment, back-sliding, collapse of a thickly stacked pile, stress release, etc.) in the form of low-temperature mylonites, cataclasites, lateral displacements, SE-verging folds, normal faults, etc. The observable displacements along these structures are of the order of millimetres, centimetres, metres to up to a few metres. We have not observed any single structure developed in extension to which we may attribute enormous displacements of the order reported, from isotope studies, by Van der Pluijm and Carlson (1989). Although the accumulative displacements resulting from the various individual 11 structures developed in extension could possibly add up to a large amount, but up until now we have not observed any single structure which would suggest a large displacement.

CENTRAL METASEDIMENTARY BELT FORELAND ZONE

It is important to understand this terminology intended to clearly express the tectonic relationships existing and observable between the metasedimentary rocks and the gneiss complexes. Because the thrusting event synchronously affects and involves not only the supracrustal sequence transported from the CMB, but also the gneiss complexes of the CGB, we have always preferred, since 1992, to designate the whole thrust zone as the « Central Metasedimen- tary Belt Foreland Zone », and have provided a clear definition of this terminology as follows :

« Central Metasedimentary Belt Foreland Zone »

Applicable to such an extent' within the gneiss complex terranes where the gneisses have suffered strong deformation and transport synchronously with their once tectonically overriding load of metasedimentary rocks during the main thrusting event attributed to the Grenvillian Orogenic Cycle (Sharma et al. 1992).

Obviously, the CMB Foreland Zone must extend for more than 100 km westward from the Cayamant Lineament where the whole thrusting event was initiated. This is due to the fact that our systematic mapping now extends to about 100 km from the Cayamant Lineament and we have not yet observed the termination of the effects of the thrusting event.

We stated in 1992 (Sharma et al. 1992) that we prefer the use of this recommended term to adequately designate the whole thrust zone. An alternative, which to us seemed much less favourable, was to at least modify the existing term as follows :

The amount of space or surface that something occupies or the distance over which it extends - Webster's Ninth New Collegiate Dictionnary, pp. 440. 12 « Central Metasedimentary Belt Boundary Thrust Zone »

During the same period Harmer and McEachern proposed exactly the same name.

BASKATONG-DESERT LINEAMENT

Another major structure identified from the interpretation of RADAR-images and confirmed by field studies is the Baskatong-Desert Lineament (Figure 1 and 3, Sharma, 1991) which is a steeply dipping to subvertical dextral shear zone of kilometric thickness. To date it has been traced from north of Baskatong Reservoir through Lac Désert, and along the Temiscaming Road to west of Lac Brodtkorb - a distance of about 70 km.

Near Baskatong Reservoir its orientation is NNE (parallel to the western limit of the CMB), then continuing westward, it swings to a NE direction near Lac Désert and finally becomes ENE to nearly E-W. It is a steeply dipping to subvertical structure manifested by straight gneisses, porphyroclastic gneisses (definitions of Harmer, 1988), mylonites, banded gneisses and strongly gneissic rocks. The lithologies involved in this deformation zone are tonalitic gneisses, dioritic gneisses, amphibolites, granitic gneisses (earlier granites-pegmatites) which have all been strongly transposed. In general, the structure dips toward E or SE, but where it becomes subvertical northwesterly to northerly dips are also observed, particularly in its western parts. Everywhere it shows excellent subhorizontal mineral lineations and extension lineations. The kinematic indicators suggest a dextral displacement.

The recognition of the Baskatong-Désert Lineament led us to subdivide the CGB of this region into two terranes : Baskatong-Dozois Terrane north of the lineament, and the Pythonga Terrane south of the lineament.

Radiometric work carried out by Guo and Dickin (1992, and personal communications, 1992, 1993 and 1994) of McMaster University shows Archean Sm/Nd model ages for the gneisses of the Baskatong-Dozois Terrane, and Proterozoic Sm/Nd model ages for the Pythonga Terrane. 13 Thus, a difference of about 1000 million years in the model ages of these two terranes confirms the importance of the Baskatong-Désert Lineament as a major structure within the Grenville Province. 14

FORT-COULONGE REGION

FIELD-LOG

15 DAY-1 — JIM LAKE ROAD

0.0 Proceed W on Route 148 starting in front of the Fort-Coulonge covered bridge.

2.00 Turn right on Route du lac Jim (Chemin du Bois-Franc).

0.0 Reset the odometer to zero here at the beginning of the Route du lac Jim.

9.2 STOP-1 Long, fresh road-cut outcrops for about 500 m. Near the southern part very good straight-banded, fine to medium grained amphibolites with tabular habit, oriented NE with gentle SE dip, and SE lineations. Observe the intense deformation in the transposed pinkish leucocratic granitic-pegmatitic material. The degree of deformation is not so

3 evident in the amphibolites, except for the strong tabular nature. The amphibolites possess good granoblastic texture. It contains plagioclase and hornblende in roughly

i x equal proportions with minor amounts of clinopyroxene, orthopyroxene and biotite. The v l etLAPY accessories include iron oxides, sulphides and apatite. The iron oxides and sulphides are .11 distributed in tiny grains near; grain mmargins of other minerals. Sulphides also form coatings on joint planes. ;: ~ p_D n

f Further north, outcrop on E side of the road. Chondrodite - olivine - diopside - sphene - phlogopite bearing marble with good tectonic banding. Bands of garnet-sillimanite gneiss, with pale purple garnet. Bands of deformed pegmatite, some with rusty surface. Bands rich in diopside-scapolite. Large patches of phlogopite and coarsely crystalline diopside, scapolite and pink calcite.

Still further north long cliff on W side of the road. Gently dipping amphibolites, amphibolitized diopside - scapolite - sphene - apatite bearing calc-silicate rocks, gameti- gerous deformed pink pegmatite-granite bands. Coarse sulphides associated with pegmatite and as coatings on joint planes. Locally large allanite crystals in pegmatites.

17.2 STOP-2 Large outcrops west of road. Pegmatite with large diopside crystals and patches rich in diopside. Minor pinkish carbonate zones containing diopside-phlogopite. Patches rich in phlogopite books. Calc-silicate gneisses amphibolitized to varying degrees.

17.6 STOP-3 Large cliff on W side of road. Banded sequence consisting of marble or carbonate injections, calc-silicate gneisses, and strongly deformed pegmatite-granite bands. Gently dipping towards SE, with a SE lineation. The{carbona a,contains diopside, scapolite, 1-' sphene, phlogopite, apatite etc. The deformed pegmatite-granite bands were subjected to considerable grain-size reduction and contain diopside, sphene, apatite and disseminated sulphides. f ~✓,~ (Aa4L AOC+C_R`.,. r.) ,,,`.mow.` " , 20.1 STOP-4 '''`. to Large, long outcrops west of road. Leucocratic, pink pegmatites-granite's with large ,• 20.3 diopside crystals, and patches and masses of varying size, find shape)rich in diopside. '` (diopsidite) or diopside - scapolite - sphene - apatite. There may be theoccasional G presence of patches and pickets of pinkish coarse carbonate. The pegmatite-granite also P \()A_ contain, biotite and allanite crystals. The pegmatite-granite or devoid of quartz in the vicinity of diopside and calc-silicate minerals rich patches. The silica being largely consumed in the formationiopside.

The close ass c of pegmatite and diopside - scapolite - sphene - carbonate bearing patches test `le proximity to marbles which were part of the overriding tectonic pile during the thrusting event as explained earlier. 30.8 STOP-5 Large road-cut outcrop west of road. Leucocratic pegmatite-granite, diopsidite, calc- silicate rocks and banded granitic gneisses with SE dips. The calc-silicate rocks contain diopside, scapolite, phlogopite, sphene, apatite etc. Excellent large prismatic crystals of scapolite. Pinkish carbonate coarse to very coarse grained, also containing diopside, greenish-bluish apatite crystals, phlogopite-biotite etc. )

f" fat^. ....~'' . w 4tx:r..,t;#-ï 36.3 STOP-6 i Large outcrop E of road. Strongly foliated tonalitic gneiss, fine to medium grained, granoblastic with pinkish granitic-pegmatitic layers transposed parallel to foliation. .Theirsgong deformation has disrupted these layers and reduced their grain size, thus leaving only a few relic porphyroclasts. There are different generations of these felsic layers. The foliation in the tonalitic gneiss is defined by nematoblastic hornblende, lepidoblastic biotite and slender quartz ribbons. The accessory minerals include zircon, apatite, magnetite, pyrite, allanite, sphene etc. The gneisses are oriented NE, with SE dip and SE lineations. Look for and examine the kinematic indicators.

48.8 STOP-7 Long outcrop area W, of road. Intensely deformed granoblastic tonalitic gneisses with streaky mafic and pencil gneiss textures, NW orientation and SE lineations. Minor dioritic horizons. Whitish mobilizate as well as pinkish granitic material transposed to varying degrees parallel to foliation. Nematoblastic hornblende is the dominant mafic mineral with very minor biotite, disseminated magnetite and accessory apatite zircon.

50.5 STOP-8 Large outcrop, E of road, of strongly deformed, NW oriented tonalitic gneiss, boudinaged dioritic gneiss, with layers of whitish mobilizate and pinkish granitic-pegmatitic material strongly transposed parallel. to foliatiopn. Reddish or pale purple garnet, fine grained as well as porphyroblastic; (omnipresent) in all the lithologies. Within the deformed granitic-pegmatitic layers there are abundant relic porphyroclasts, some of which are good kinematic indicators. A decimetric band of transposed deformed granitic-pegmatitic material is composed of granoblastic microcline and plagioclase, quartz ribbons, equigranular to porphyroblastic pale purple garnet and sillimanite, and lepidoblastic to random biotite. Some garnets contain tiny inclusions of olive-green spinel. The garnets are usually poikilitic with inclusions of quartz, feldspar, biotite, sillimanite, magnetite etc. Biotite shows some chloritization. Muscovite occurs in a very minor amount. The accessory minerals include apatite, zircon, magnetite, pyrite (etc.

( Note the presence of a decimetric, pinkish-reddish, fine-grained layebof « garnetite . ~~j,,'r. _ ;, Such mocks represent ,..,.,boit,., ,,,,.,.t., veins quartz -rich ,....anit:,. p„_ titib - - s. \ They possess an excellent polygonal texture typical of perfectly annealed rocks. It consists mainly of garnet and quartz with very minor plagioclase which is nearly entire- ' ly sericitized. The accessory minerals include disseminated magnetite, pyrite, apatite, \ !• _— biotite, zircon etc. Elsewhere in the region, similar rocks may also contain diopside, .. scapolite, sphene, plagioclase, microcline etc. in addition to garnet and quartz. In some cases the diopside may even exceed garnet in relative proportions. The development of s the calc-silicate minerals in these rocks iif attributed to the infiltrations of carbonate during deformation and recrystallization. Once formed these bands mae boudinaged. <

Also note the presence of a quartz-rich pegmatite dyke traversing the gneisses and >va containing garnets up to 4 cm in size.

52.9 STOP-9 Large outcrop area E of road. Gently dipping rocks of the gneiss complex consisting of tonalitic and dioritic gneisses, and transposed pinkish granitic-pegmatitic Raterial. The M S^- lextt rocks are well foliated, streaky to banded. Long SE lineations are (bservable) in several places where the tonalitic gneisses are exposed parallel to the foliation plane. Bu#~when -these-are-exposed in sections at an angle or perpendicular to the foliation, ti rocksf give `~ KM 19

an impression that they are less deformed.' This is because these sections show the terminations of the lineations of the pencil gneisses.

53.1 About 100 m north of the(above)outcrop there is an intersection with a side-road on the west side of the main road. Follow this side-road which goes towards lac à l'Ours and lac Didace.

Side-road STOP-10 0. Large outcrop area at the intersection with the roads going towards lac l'Ours and lac Didace. Q~ ~ 4,;.:1 4C.) ~,;Zm.•~-- r,"~ 1„. `' 9 ,- NW oriented, highly lineated excellent streaky tonalitic gneiss and zones of dioritic a5 7 gneiss. Very long and prominent SE lineations. The gneisses are in he granulite facies. The tonalitic gneisses are fine to medium grained, with granoblastic plagioclase, quartz and minor disseminated microcline, ribbon quartz, lepidoplastic biotite, and stubby pyroxene grains. The clinopyroxene largely predominates over orthopyroxene. Hornblende is very minor, developing mostly in small patches within some clinopy- roxéines. The accessory minerals ' elude zircon, apatite, magnetite, pyrite etc,./ The b Nit. dioritic gneiss iLysty_ma - and ominated by the mafic minerals with only minor plagioclase. Hornblende is by far the most common mafic mineral followed by garnet, orthopyroxene and clinopyroxene. The accessory minerals include apatite, magnetite, rte. zircon/ etc.` Among these apatite and magnetite are quite prevalent.

Side-road to STOP-11 lac Didace Excellent example of a « tectonic outlier » of supracrustal rocks. Large outcrop area W

of road consisting of marble and zones of strongly deformed pegmatite-granite. The marble is very coarse, whitish to pale pinkish, with disseminated diopside, phlogopite, graphite etc. There are abundant boudins of varying and compositio~is ;scattered within the marble. Some boudins show good reaction rims around them. Near the road, a strongly deformed zone, with rectilinear banding. These rocks are derived from the

^ j 226 A. !!'t

L ' C~~ n- J deformation of whitish pegmatite-granite, as evidenced by the common presence of whitish feldspar porphyroclasts. Such bands may vary in orientation, but they possess consistent strong SE lineations. They are characterized by the presence of very long monocrystalline quartz ribbons, polygonal plagioclase, strongly lepidoblastic biotite, minor muscovite and garnet porphyroblasts. The garnets have their long dimensions parallel to the foliation and are highly poikilitic with inclusions of any of the principal and accessory minerals present in the rock. The accessory minerals include apatite, magnetite, sphene, zircon, allanite, carbonate These strongly deformed rocks possess excellent « flaky » structure with prominent quartz lineations on every flake visible in the outcrop.

Several fine-grained pinkish « garnetite » lenses and layers containing garnet, diopside, scapolite, quartz, feldspars and magnetite, with good polygonal textures.

Further westward from this outcrop we go back into the gneiss comple,(rockonsisting of tonalitic and dioritic gneisses, and strongly deformed granite-pegmatite layers, all showing very long lineations.

Side-road to STOP-12 lac Didace Large outcrop (E of road) of « flaky » gneiss complex rocks : tonalitic and dioritic gneisses, and strongly deformed and transposed whitish to pinkish granitic-pegmatitic material. The gneisses are an excellent example of pencil gneisses with very long prominent lineations on each flake. The foliation is subhorizontal.

From here we return to the Route du lac Jim.

53.1 Route du lac Jim. Proceed towards the north. 65.4 STOP-13 Large outcrop of straight gneisses E of the road. The rocks are well banded with fine to medium grained pinkish leucocratic bands of granitic composition and mafic bands of dioritic composition. Usually the pinkish bands are thicker and the mafic bands are thinner. The dips vary from gentle towards SE, to subhorizontal, to gentle towards NW. ~a.rt.e.. A few relic rootless isoclinal hinges present in between the bands. The pinkish bands are composed of fine to medium grained equigranular polygonal quartzofeldspathic grains where microcline largely dominates over plagioclase. Quartz also forms ribbons parallel to foliation. These bands are very leucocratic with very minor amounts of mafic minerals such as magnetite, biotite and hornblende. The accessory minerals may include zircon, apatite, allanite. The mafic bands contain leucocratic and mafic minerals in roughly equal proportions. Plagioclase dominates the leucocratic minerals. The mafic minerals include nematoblastic hornblende, lepidoblastic biotite and magnetite. Apatite is more common here than in leucocratic bands.

69.9 STOP-14 Large outcrops on both sides of road - the straight gneisses east of road and the ultramafic rocks west of road.

The straight gneisses, trending NNW, are mostly tonalitic in composition with minor bands that are granitic or mafic in composition. The gneisses possess the typical greenish colour of the rocks in granulite facies. The tonalitic gneisses are fine to medium grained with polygonal plagioclase as the dominant feldspar. Quartz may occur as equigranular polygonal grains or as short ribbons. The me minerals present are orthopyroxene, clinopyroxenegarne ; and minor olive-green hornblende and reddish C lk. v- .~ 6 r ~ . t,~ brown biotite. The pyroxenes form stubby grains. The orthopyroxene is slightly uns- a„ f~ table, altering in parts to an aggregate of chlorite, biotite, carbonate etc. The clinopy- ..V ,.. ~ . i r roxene is more stable. The garnet is equigranular to porphyroblastic, subhedral to anhedral, and uniformly distributed. Magnetite, zircon, apatite, pyrite, allanite are accessory minerals. The ultramafic (peridotitic) rocks form large massive outcrops west of road. The rocks are porphyritic with phenocrysts of pyroxenes (ortho - and clinopyroxenes) and olivine. The pyroxene crystals stand out in relief on weathered surfaces due to their resistance to erosion. In broken fresh samples, the pyroxenes show as large shiny crystals. The large primary pyroxene crystals are characterized by an abundance of fme inclusions (schiller structure), where as the smaller, polygonal pyroxene grains are devoid of such inclusions. Olivine occurs as phenocrysts, smaller grains, as well as inclusions within large pyroxenes. Olivine may be fresh, or serpentinized and replaced by iddingsite to varying degrees. The serpentinization is accompanied by little development of magnetite, indicating the magnesian nature of the olivines. Other minerals present in the rock may include phlogopite, tremolite-actinolite, hornblende, chlorite, magnetite, ilmenite, spinel, carbonates, sulphides etc.

73.3 Intersection with a road going toward E to a network of lumber exploitation roads where several outcrops will be visited. All these outcrops are grouped within STOP 15.

Lumber STOP-15 roads The outcrops in this zone include various straight gneisses, « flaky » gneisses, banded gneisses and ultramafic rocks. The straight gneisses typically consist of felsic, leucocratic, pink or white bands and mafic bands which are of variable thickness in different exposures and are subhorizontal to gently dipping. The felsic bands also possess an excellent millimetric rectilinear foliation showen by minor grain-size variations, mafic contents, quartz ribbons etc. These fine-scale foliations are best observed on weathered surfaces. The felsic bands may be fine to medium to coarse grained, with equigranular, polygonal quartz, microcline and plagioclase. Quartz also occurs as monocrystalline to polycrystalline ribbons. The presence of fme disseminated magnetite is characteristic. The other mafic minerals present in minor amounts may be biotite, hornblende, garnet, chlorite. Orthopyroxene and clinopyroxene usually in association with garnet are present when these bands are in granulite facies. The mafic bands are also fine to medium grained, equigranular, with polygonal plagioclase, nematoblastic hornblende, minor lepidoblastic biotite, garnet, magnetite, sphene, apatite etc. Orthopyroxene and clinopyroxene are present when these rocks are in granulite facies.

In certain sectors, the felsic bands are characterized by the presence of sillimanite and garnet. The presence of sillimanite is considered as an indicator of intense deformation (Sharma et al. 1990). In a few thin-sectionsÇherpresenee-e9spinel was also observed. The sillimanite-bearing zones are identifiable by the typical yellowish-brownish alteration colour seen in outcrops.

The protoliths of the felsic bands are pegmatitic or very coarse granitic rocks. Zones of tonalitic gneisses and ultramafic rocks are also present in places. Locally, centimetric garnetite bands are also observed.

77.0 STOP-16 Large outcrop E of road, about 300 m long and with good tabular habit. The rock is fine to medium grained, NW trending and gently dipping tonalitic gneiss in the granulite facies. Fine red garnetdisseminated in the rock and in places formes high concen- trations on the foliation plane. In fresh surface the rock shows the typical green colour of the rocks in granulite facies. The mobilizate is somewhat paler. The tonalitic gneiss has an excellent granoblastic texture with polygonal quartz and plagioclase. Quartz may also occur as short, narrow, monocrystalline or polycrystalline ribbons. The mafic minerals are garnet, clinopyroxene, orthopyroxene, hornblende and minor biotite. Clinopyroxene largely dominates over orthopyroxene. The orthopyroxene is somewhat — j unstable and being replaced in part by chlorite-carbonate etc. Hornblende is oliv green. Garnet forms equigranular to porphyroblastic euhedral, subhedral or anhedral crystals. The pyroxenes, hornblende and garnet form excellent trains of minerals thus defining excellent fine-scale foliations. Zircon, magnetite, apatite, zoned allanite etc. are the main accessories. The mobilizate layers are somewhat coarser, leucocratic and contain mesoperthitic orthoclase and quartz as the principal minerals, and plagioclase, garnet,

pyroxenes, hornblende, biotite etc. in minor amounts. Quartz forms good short monocrystalline ribbons.

88.0 STOP-17 Large outcrop east of road. Excellent pink porphyroclastic augen and flaser gneiss derived from the intense deformation of very coarse granite or pegmatite. The foliation shows crenulations, minor folds and narrow shear zones. Look for kinematic indicators.

90.3 STOP-18 Long outcrops on east side of road and some smaller ones on west side of road. Excel- lent straight-gneisses and porphyroclastic gneisses showing very good rectilinear banding. The protoliths of these gneisses are tonalites, diorites and granites-pegmatites. The gneisses are fine to medium grained granoblastic except for the feldspar porphyroclasts which may be up to a few cm in length parallel to the banding. The rocks have been somewhat retrograded from the granulite facies. Fine red garnet is omnipresent irrespective of the composition of the bands. Look for the kinematic indicators in the porphyroclastic gneiss bands. Excellent long quartz ribbons are very common. The outcrops west of the road show a few rusty horizons within these gneisses. These zones also show some development of graphite.

95.8 Intersection with the road going westward toward Lac McCann. Follow this road.

0.0 Set the odometer to zero at the intersection.

2.0 STOP-19 Large outcrops south of road, forming a slope towards road. Very good straight gneisses and porphyroclastic gneisses whose protoliths were tonalitic gneisses, dioritic gneisses, granites-pegmatites. Fine red garnet is present in all lithologies. Common quartz ribbons. Certain bands of porphyroclastic gneiss derived from the deformation of pegmatitic material contain potash feldspar porphyroclasts up to 15 cm in size. Look 25

for the kinematic indicators. In parts of the outcrop there are excellent bands of fine- grained granoblastic garnetite containing garnet, diopside, quartz and feldspars. Younger pegmatite dykes contain biotite books and magnetite patches.

3.8 STOP-20 Very good porphyroclastic and straight gneisses south of road resulting from the intense deformation and transposition of dioritic-tonalitic gneisses and granitic-pegmatitic material. Fine red garnet is more abundant in mafic bands than in felsic bands. Excel- lent quartz ribbons in felsic bands. The feldspars are granoblastic, polygonal. Garnet forms equigranular to porphyroblastic subhedral to anhedral crystals. The mafic minerals present in mafic bands are hornblende and biotite; "whey âs'the felsic bands contain mostly reddish biotite. Pyroxenes are also present in miiïdr amounts both in felsic and mafic bands. Look for kinematic indicators.

26 DAY-2 RIVIÈRE COULONGE - WALTHAM - ANORTHOSITES

KM 0.0 Proceed W on Route 148 starting in front of the Fort-Coulonge covered bridge.

2.0 Turn right on Route du lac Jim (Chemin du Bois-Franc). Follow the road signs to Chutes Coulonge.

Chutes- STOP-21 Coulonges At Chutes Coulonge there are excellent large outcrops around the water falls on Coulonge river. These exposures are part of the Basal Thrust Zone. They are characterized by the presence of intensely deformed gneisses, banded gneisses, straight gneisses, carbonate injections, « tectonic mélange with carbonate matrix » (tectonic breccias with carbonate matrix). The gneisses had various protoliths, such as granitic gneisses, mig- matized tonalitic gneisses, mafic gneisses, granites-pegmatites etc. The carbonate injections may be whitish or pinkish, and are usually concordant to the general foliation in the gneisses but may occasionally be discordant to foliation. Some such injections may be accompanied by a reaction rim near their contacts with the surrounding rocks. f The tectonic melange with carbonate matrix is characterized by the presence of fragments and boudins of various sizes, shapes and compositions surrounded by whitish to pinkish, coarse to very coarse carbonate. The fragments may be pieces of various rocks, or single crystals such as diopside, hornblende, sphene, apatite, potash feldspar, olivine, chondrodite, scapolite, quartz, allanite. All these rock fragments and single crystals may develop a thin margin of crushed and recrystallized material forming a kind of mortar texture resulting from the deformation suffered during the transport. Among the rock fragments the calc-silicate rocks are quite prevalent. They contain a combination of the following minerals : diopside, scapolite, phlogopite, sphene, apatite, olivine, chondrodite, epidote, garnet, hornblende, biotite etc. In places relic isoclinal hies preserved in the banded gneisses. Look for kin6matic indicators. KM 27

For the next outcrop, we return toward the main highway, but on our way out we will make a small detour to see another outcrop at the hydroelectrical powerhouse of Hydro- Pontiac.

Outcrop at STOP-22 Powerhouse Excellent outcrop forming a large cliff near the powerhouse. These outcrops are just downstream from the ones visited earlier and present essentially the same features. But, this outcrop is significant because it helps to appreciate the nature and thickness of the Basal Thrust Zone, and observe the effects of infiltration of carbonates resulting from the remobilization of the marbles once part of the overriding tectonic pile.

Return to the main highway. Reset the odometer to zero. Proceed W on Route 148.

9.8 STOP-23 Long outcrops N of the road. Banded gneisses derived from the intense deformation and transposition of pegmatitic-granitic material of different generations, calc-silicate bands, mafic bands etc. There are several very coarse, whitish to pinkish, carbonate bands of varying thickness (cm to m) disposed parallel to the general banding in the outcrop. Such carbonate layers are believed to represent carbonate injections originating from the remobilization of the marbles. Observe the fragments of intensely deformed pink pegmatite found as inclusions within the pink, very coarse carbonate.

The whole sequence is NE trending with gentle SE dips and lineations.

The calc-silicate bands may be composed of diopside - scapolite - sphene in varying proportions and grain sizes. The granoblastic fine to medium grained bands contain excellent polygonal scapolite and disseminated crystals of diopside and sphene. Very minor epidote may partially replace some scapolite grains. The pink pegmatitic material, deformed to different degrees, may be rich in large diopside crystals in the vicinity of carbonate layers. In such cases, the diopside is more common in pegmatitic material nearer to the carbonate layer, and the diopside diminishes in size and abundance farther away from the carbonate-pegmatite contact. The content of diopside in pegmatite-granite has a direct influence upon the content of quartz. The portions of pegmatite-granite containg more diopside may be completely devoid of quartz, whereas farther away from the diopside-bearing portions, the pegmatite-granite resume their normal quartz content.

Certain intensely deformed leucocratic granitic-pegmatitic bands are rich in pale purple garnet, accompanied by minor sillimanite. Biotite is reddish-brown. Quartz forms excellent monocrystalline and polycrystalline ribbons, where as the feldspars (plagioclase and microcline) occur as granoblastic polygonal grains. Garnet porphyroblasts contain abundant inclusions of plagioclase, quartz, sillimanite, magnetite, biotite etc.

12.0 STOP-24 Long and large outcrops forming cliffs on N side of the road. These outcrops continue for about 2.5 km westward. The previous outcrop and these outcrops are all part of the Waltham Deformation Zone observed near the western margin of the Fort-Coulonge 6 Nappe (Madore et al.) .). A few stops will be made in this zone of outcrops characterized by the banded to straight gneisses generally consisting of pinkish leucocratic bands and mafic bands which may vary in relative proportions and thicknesses. The protoliths of pinkish bands are granites and pegmatites. Petrographically they are similar to the straight gneisses described earlier. In western parts of this group of outcrops some bands may be of tonalitic gneisses. The calc- silicate rock bands and injections of carbonates may be absent or present in very minor amounts. Very good SE lineations. In places good examples of nearly undeformed granite-pegmatite dykes emplaced in extension. Continue westward on Route 148. Past Waltham there is a junction, to the right, with the road going towards Chapeau. Reset the odometer to zero at the junction. Follow the road to Chapeau. At about 850 m there is a road going E toward Rivière Noire. Follow this road which parallels Rivière Noire for a few km and then continues towards NW. On this gravel road and on a secondary gravel road going W a few outcrops of anorthositic rocks will be visited.

Rivière STOP-25 Noire Road The anorthositic rocks form an elongated body several km in length and a few km in width near the western margin of the Fort-Coulonge Nappe (Madore et aL tu,.) and at the interface between the supracrustal rocks to the east, and the rocks of the gneiss complex to the west. In addition, a few other smaller discontinuous exposures were also mapped in a similar position around the Fort-Coulonge Nappe. All these anorthositic rocks were recognized for the first time during the course of our field-work in 1992, 1993 and 1994. A major portion of the largest anorthositic body lies within the Waltham Township. Therefore, it is designated as a lithodemic unit called « Waltham Anorthosite » (« Anorthosite de Waltham »).

The lithologies observed within this unit are anorthosite, gabbroic anorthosite and anorthositic gabbro. The degree of deformation suffered by these rocks is quite variable and heterogeneous. It is possible to observe zones of practically undeformed anorthosite with primary igneous textures, and even well preserved magmatic layering in places, through variously deformed varieties to excellent mylonites. The degree of deformation may vary considerably within short distances and even within the same outcrop. For example, there may be an intensely deformed and recrystallized gneissic anorthosite, and nearby we may observe zones and patches of practically undeformed anorthosite with well preserved igneous textures and structures.

The physical appearance of the rock is controlled by the degree of deformation affecting the rock. The least deformed anorthosites may be grey, dark grey, black, purplish in colour, coarse to very coarse grained with dark coloured large plagioclase, pyroxenes, and patches and pods of magnetite-ilmenite. The deformed and recrystallized anorthosites become finer grained and lighter in colour, and may still conserve scattered dark relic plagioclase crystals. There may be new development of hornblende, biotite, garnet etc. The mylonitic varieties are fine to medium grained, light coloured rocks with good mylonitic banding.

During the field-trip severals stops will permit the observation of different varieties of anorthosites and their primary and tectonic textures and structures. Another interesting feature in these outcrops is the presence of young pegmatite dykes with euhedral to subhedral quartz crystals.

Sheenboro STOP-26 Large fresh road-cut outcrops of tonalitic gneisses in the granulite facies. The rocks posses typical greenish colour of the rocks in granulite facies, but still preserve the characteristic streaky texture of the fine to medium grained tonalitic gneisses. Finely disseminated to porphyroblastic garnet is conspicuous in the outcrop. The rock is granoblastic with plagioclase, quartz and minor orthoclase (or microcline). The pyroxenes are largely retrograded and replaced by olive-green to brownish hornblende, and aggregates of chlorite - carbonate - hornblende where occasionally orthopyroxene relics may be seen. Reddish-brown biotite is also present. Garnet is abundant in small subhedral to anhedral crystals, or forms poikiloblasts. These mafic minerals form mineral trains and define excellent foliation. Zircon, apatite, magnetite, allanite are accessory minerals.

Younger pegmatite dykes present in the outcrops are also composed of greenish feldspars which are mostly mesoperthitic orthoclase-microcline in large crystals and only minor plagioclase in smaller crystals. Quartz occurs in large grains interstitial to feldspars. The coarse mafics are largely replaced by an aggregate of chlorite - carbonate - magnetite - poikilitic biotite where good relics of orthopyroxene can be noted. Biotite also occurs in large separate crystals. Hornblende may also be present.

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