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Résumé des présentations

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Images sismiques du nord de l'Abitibi

Président: P. Verpaelst

09:00 Clowes, R.M., Ludden, J., and Hynes, A. Introduction

09:30 Calvert*, A.J., and Ludden, J. The assembly of an Archaean continent: an overview of seismic reflection results from the Abitibi-Grenville trans t.

09:55 Sawyer*, E.W., Benn, K., Ghassemi, M., and Lacroix, S. Opatica-Abitibi-Pontiac traverse across two collision zones: northern leg.

10:20 Panse

10:40 Benn*, K., Sawyer, E.W., Chassent, M., and Lacroix, S. Opatica-Abitibi-Pontiac traverse across two collision zones: southern leg.

11:05 Kerrich*, R., Ludden, J., and Calvert, A. Evolution of fluid regimes in the geodynamic framework of the Abitibi sub-province.

11:30 Moser*, D., Krogh, T.E., and Hubert, C. Archean on top, Paleoproterozoic at the bottom? U-Pb age constraints on seismically reflective structures in buried and exposed deep crust, southern Superior subprovince.

12:00 Lunch

Président: L. Nadeau

13:45 Culshaw*, N., Jamieson, R.A., Ketchum, J.W.F., Wodicka, N., Corrigan, D., Timmermann, H., and Reynolds, P.H. Trangert across the southwestem Grenville orogen, Georgian Bay, Ontario: Polystage convergence and extension in the lower orogenic crust.

14:10 Ketchum*, J.W.F., and Davidson, A. Crustal architecture of the Central Gneiss Belt, Grenville Province, in Ontario and western Quebec.

14:35 Corriveau*, L, van Breemen, O., Boggs, K.J.E., Harris, L, Morin, D., and Rivard, B. The Mont- Laurier area: A record of pre-1.19 Ga Grenvillian convergence and 1.165 Ga magmatic activity with reactivation along major structures.

15:00 Sharma*, K.N.M., Madore, L., Giguère, E., Singhroy, V.H., Lévesque, J. Central Metasedimentary belt foreland zone: relationships between the Central Gneiss Belt and the Central Metasedimentary Belt, western Québec.

15:25 Pause

15:45 Indares*, A., Dunning, G., and Coi, R. Manicouagan imbricate zone: an exposed deep crustal segment in the eastern Grenville Province.

16:10 Hynes*, A., Eaton, D., Indares, A., Rivers, T., and Gobeil, A. Lithoprobe Line 55 (Manicouagan):

Modelling the subsurface geometry.

16:35 Francis*, D., and Scowen, P. Troctolite magmatism across the Manicouagan.

17:00 Dickin, A. Application of crustal formation age dating to the Grenville Province.

Président: J. Ludden

09:00 Martignole*, J., Calvert, A., Friedman, R., Reynolds, P. The Grenville Province in Western Quebec:

tectonic and time framework along the lithoprobe seismic profile.

09:25 Mareschal*, J.-C., Guillou-Frottler, L, Cheng, L.Z., Dhaussy, P., Gariépy, C., and Jaupart, C. Heat flow and gravity characteristics of the southeastern Superior Province and the Grenville Province.

09:50 Mereu*, RF., Roy, B., and Winardhl, S. Deep crustal structure and Moho characteristics of the southeastern Superior and Grenville provinces.

10:15 Pause

10:35 Ji*, S., Rondenay, S., Mareschal, M., ami Senechal, G. Obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones.

11:00 Skulski*, T., Percival, J., Nadeau, L, and Mortensen, J.K. Abitibi-Opatica correlations in the Minto block.

11:25 Rivera*, T., and Corrigan, D. Convergent margin on southeastern Laurentia during the Mesoproterozoic:

Tectonic implications.

Le projet LITHOPROBE est un projet de recherche transca- nadien sur les relations structurales et géométriques des diffé- rentes unités géologiques régionales du territoire canadien. Le projet LITHOPROBE Abitibi-Grenville, entrepris au Québec et en Ontario, avait comme principal objectif la caractérisation de la croûte de l'Abitibi et de la province de Grenville, depuis l'ouest de l'Ontario jusqu'à l'est du Québec. Ces deux seg- ments de croûte reflètent deux processus d'évolution terrestre fondamentalement différents : la région de l'Abitibi représente une croûte juvénile formée et accrétée rapidement vers la fin de l'Archéen ; la province de Grenville est composée en grande partie de croûte remobilisée. Un ensemble de coupes Est-Ouest de cette province permet d'obtenir une perspective tridimensionnelle de l'organisation d'une chaîne de montagnes précambrienne.

Comment la croûte du bouclier précambrien canadien s'est- elle stabilisée ? Quels sont les rôles joués, dans la croissance crustale continentale, par les racines lithosphériques profondes (plus de 100 km) et froides ? Par les injections magmatiques à la base et dans la croûte continentale ? Et par l'épaississe- ment tectonique ? Telles sont les questions fondamentales de géologie du Précambrien, soulevées par l'étude du transect Abitibi-Grenville.

Un des aspects primordiaux du projet LITHOPROBE, parti- culièrement en ce qui concerne l'Abitibi et Sudbury, a été le transfert de connaissances en sismique de la croûte vers l'in- dustrie minière comme outil d'exploration. Dans le cadre du projet, l'industrie minière et le ministère des Ressources natu- relles du Québec ont financé bon nombre d'études à haute résolution. Le but de ces études était la définition de la géo- métrie de sub-surface des zones minéralisées de la région de Matagami et du Groupe de Blake River. Ces études ont été grandement profitables : aujourd'hui, l'industrie minière a adopté la sismique 3-D et la combine à des programmes de forage et de diagraphie en exploration minérale dans les cein- tures de roches vertes, à Sudbury et ailleurs dans le monde, pour l'étude des grandes provinces minières d'Afrique du Sud et d'Australie, par exemple.

Au total, le projet Abitibi-Grenville compte plus de 2000 km de profils régionaux de sismique réflexion. Pour chaque cas, les études sismiques sont accompagnées d'une carto- graphie géologique régionale et de géochronologie U/Pb et Ar/Ar qui viennent s'ajouter aux études géochimiques. Des programmes géophysiques intégrant les données régionales magnétiques et gravimétriques, l'aperçu de sismique réfraction et une tentative de sismique passive avaient pour but de repré- senter le manteau sous la région Abitibi-Grenville. Ces études se sont concentrées sur quatre grands axes géographiques :

- depuis le Nord de la ceinture plutonique d'Opatica au travers de la ceinture d'Abitibi et de la partie centrale de la province de Grenville jusqu'à environ 100 km au Nord de Montréal ;

- les régions de Parry Sound et Bancroft dans la partie Ouest de la province de Grenville ;

- depuis Manicouagan jusqu'à Wabush dans la partie Est du Grenville ;

- la structure de Sudbury.

en 1984 ; dans sa globalité le projet a impliqué plus de 100 chercheurs provenant des universités canadiennes, de la CGC, du MRNQ, de l'OGS et de l'industrie minière. Le fmance- ment provenait du CRSNG du Canada, de la CGC, du MRNQ et de l'industrie minière.

Pour de plus amples informations au sujet du projet LITHO- PROBE et du projet Abitibi-Grenville, vous pouvez consulter le site World Wide Web suivant :

http://www.geop.ubc.ca/Lithoprobe/lithopb.html Coordination du projet :

Responsables de transects :

- J. LUDDEN, CRPG-CNRS, Nancy, France - A. HYNES, McGill Univ. Montréal, Canada Sismique réflexion :

- A. CALVERT, École Polytechnique, Montréal - B. MILKEREIT, Univ. Kiel, Allemagne - D. WHITE, Service géologique du Canada - D. EATON, Service géologique du Canada Sismique réfraction :

- B. MEREU, Univ. Western Ontario Electromagnétique :

- R. KURTZ, Service géologique du Canada Champ potentiel :

- J. C. MARESCHAL, Univ. du Québec à Montréal Région Abitibi :

- J. LUDDEN, CRPG-CNRS et Univ. Montréal Grenville Est :

- A. HYNES, McGill Univ. Montréal Grenville Centre :

- J. MARTIGNOLE, Univ. Montréal Grenville Ouest :

- S. CARR, Carleton Univ.

Liaison MRNQ : - P. VERPAELST Liaison GSC-Québec : - L. NADEAU

SANCE CRUSTALE PRÉCAMBRIENNE

John Ludden (CRGP-CNRS Nancy et Université de Montréal, co-responsable du transect Abitibi-Grenville) Cet exposé comprendra une présentation d'une section crustale profonde, du Nord de la ceinture plutonique d'Opatica à l'Anorthosite de Morin. Les images sismiques montrent la partie inférieure de la ceinture d'Abitibi plongeant dans le manteau, sous la ceinture plutonique d'Opatica : ceci est inter- prété comme une zone de suture associée à l'accrétion des lithologies de l'Abitibi. Les lithologies de croûte moyenne de la sous-province de Pontiac plongent sous la partie Sud de la ceinture d'Abitibi et il y a maintenant certaines évidences de l'existence d'une vieille croûte sous-jacente, ce qui indiquerait peut-être qu'une bonne part de la zone Sud de la province serait autochtone sur un socle volcano-plutonique. Un intérêt particulier à la partie archéenne est la détermination de la nature de la croûte inférieure. La géochronologie ainsi que les résultats des études sismiques permettent d'avancer que l'en- fouissement de la croûte s'est déroulé jusque tard dans l'évolu- tion de la ceinture de l'Abitibi (aux alentours de 2500 Ma), tandis que sous la ceinture plutonique d'Opatica, la croûte inférieure semble avoir été stabilisée avant l'accrétion (2700 Ma). Ces arguments flagrants de formation de croûte et leur relation avec les structures régionales de la croûte supérieure, telles que les zones de cisaillement importantes, peuvent cons- tituer des contraintes intéressantes pour l'histoire des fluides dans cette région et pour l'évolution des dépôts minéralisés.

La section Grenville procure une image claire de la limite croûte-manteau, avec le plus grand épaississement crustal (50 km) situé loin au Sud de la situation supposée pour la collision principale grenvillienne. Presque la moitié du transect est réalisé sur une séquence parautochtone archéenne sur laquelle les assemblages grenvilliens sont charriés en écailles relativement minces. La rampe tectonique de Baskatong est une structure à l'échelle de la croûte qui recoupe les roches autochtones. La base des terrains allochtones semble être située au niveau de cette rampe tectonique, qui s'amortit à 30 km de profondeur environ dans une croûte intermédiaire relati- vement peu réflective. Le chevauchement basal des unités allochtones supérieures démontrent qu'il y a eu une propaga- tion vers le Nord-Ouest lors de l'orogénie grenvillienne.

TIONS DE LA SISMIQUE HAUTE RÉSOLUTION À L'EXPLORATION MINÉRALE

Andrew Calvert (École polytechnique, coordinateur des programmes sismiques) :

L'origine des réflexions sismiques dans les zones de « ro- ches dures » est souvent complexe et difficile à comprendre, requérant de nombreuses données géologiques et de forage.

On peut observer des gisements directement sur les profils, mais il n'est pas toujours facile de distinguer leur signal des autres réflexions fortes comme les horizons de gabbro faillés.

Cependant, les profils sismiques permettent une interprétation structurale détaillée des systèmes de failles dans les camps miniers, permettant d'envisager une perspective d'exploration future.

Dans le cas de la séquence des Mines de Noranda, l'assem- blage bimodal (andésite-rhyolite) volcano-stratigraphique de la séquence n'est pas clairement visualisé. Cependant les réflexions les plus continues, comme les filons-couches et les failles qui recoupent les unités volcaniques sur de larges zones, sont faciles à visualiser et peuvent être utilisées pour repro- duire la géométrie tridimensionnelle du camp minier. Les vides correspondant aux galeries de la mine Ansil profondes de 1400 m sont aussi aisément visibles. Bien que non applicable à ce cas d'étude, l'imagerie sismique 3D des ouvrages miniers est maintenant utilisée comme un guide à l'exploitation minière (comme le montre l'exemple des mines d'or profondes dans les dépôts du Witswatersrand en Afrique du Sud).

Dans le camp minier de Matagami, la stratigraphie des dépôts volcano-sédimentaires semble continue et peut être visualisée bien qu'elle apparaisse plus faiblement que certains filons-couches de gabbro. Néanmoins, une excellente image de l'anticlinal de Galinée peut être obtenue qui permet une représentation détaillée de la fracturation des groupes de Wabasee et du Lac Watson, aussi bien que dans l'intrusion de la Rivière Bell sous-jacente. L'interprétation des données suggère qu'il y a eu une répétition tectonique de l'assemblage volcano-sédimentaire. Une ligne sismique a été enregistrée directement au-dessus du gisement de sulfures massifs volca- nogènes de Bell Allard Sud : le dépôt est visible dans le profil sismique.

PROJET LITHOPROBE ABITIBI-GRENVILLE

The LITHOPROBE project is a pan-Canadian research project aimed at understanding the assembly of the Canadien landmass. The Abitibi-Grenville LITHOPROBE project was carried out in Quebec and Ontario and had as its major objective the characterisation of the crust in the Abitibi region and in the Grenville province, from western Ontario to eastem Quebec. These two segments of crust reflect fundamentally different processes h Earth's evolution: The Abitibi region represents new crust that was formed and accreted rapidly towards the end of the Archaean era; the Grenville province is largely composed of reworked crust, for which a series of east to west sections allow a unique three dimensional perspective to the construction of a Precambrian mountain chain.

How the crust of Canada's Precambrian shield was finally stabilized, and particularly the roles, deep (>100 km) cold-lithospheric root-zones, magmatic intraplating and underpiating and tectonic thickening in continental crustal growth, are among the fundamental questions of Precambrian geology which will be addressed by the Abitibi-Grenville Transect.

The Grenville province contains large areas of reworked crust of pre-Grenvillian age (before about 1300 Ma), some of which were once buried to as much as 30 km, which have been thrust northwestward over and against the Superior province. The Grenville province is exposed from Lake Huron northeastwards to the coast of Labrador, and its boundary with the older provinces to the northwest is a prominent tectonic feature, the Grenville Front, which shows clearly on regional grave/ and magnetic maps. :ased on metamorphic grade, other geological studies, the GLIMPSE seismic survey, and a series of seismic corridors completed by LITHOPROBE in the western Grenville h 1991, a mountain-building process involving stacking of crustal slices, and comparable to that of modem continental mountain chains is suggested. This process appears to have varied from east to west across the Grenville province, as do the lithologies comprising the tectonic stacks. Two additional LITHOPROBE research corridors will be completed across the province.

An important aspect of the LITHOPROBE project and in particular in Abitibi and in Sudbury has been the transfer of expertise in crustal seismics to the mining industry as an exploration tool. The project has successfully completed a number of industry and MERQ funded high-resolution studios in the Abitibi which were amed at defining near surface geometry in mining regions in the Matagami region and in the Blake River group.

These studios have been remarkably successful, with the hard-rock mining community now using 3D seismics combhed with drilling and logging programmes in minerai exploration in greenstone betts, in Sudbury and on an international scale for characterising large mining regions in South Africa and Australia.

In total the Abiliii-Grenville project has completed more than 2000 kdometers of regional seismic reflection profiles. In ail cases the seismic work was complemented by regional geological mapping, U-P and Ar/Ar geochronology, in addition to other geochemical studies. Additional geophysics programmes involved regional gravily and magnetic studios, an extensive seismic refraction survey and a passive seismic experiment aimed at imaging the mantle below the Abitbi-Grenville region. These studios were concentrated along four main corridors:

* From north of the Opatica Plutonic bett across the Abitibi bett and the central Grenville Province, terminating 100 km north of Montreal

The Party Sound and Bancroft regions in the western Grenville

* From Manicouagan to Wabush in the eastem Grenville.

* The Sudbury structure

The AbitilDi-Grenville project is now h ils synthesis phase. The planning and conception of the project was started in 1984, and in aIl the project involved more than 100 scientists from universities across Canada, the GSC, the MERQ and the OGS and the mining industry. Funding was provided by CRSNG-Canada, the GSC,

MERQ and the mining industry. Further information on the LITHOPROBE project and the Abitibi project can be found in the World Wide Web at: http://mistralere.umontrealcaf-kiepurayindex_new.html

PROJECT COORDINATION

Transect leaders: J. Ludden, CRPG-CNRS, Nancy, France A. Hynes, McGiII University

Seismic reflection: Andrew Calvert, Ecole Polytechnique (coordonnateur) B. Milkereit, Univ. Kiel, Germany

D. White, Geological Survey of Canada D. Eaton, Geological Survey of Canada Seismic refraction: B. Mereu, University of Western Ontario Electromagnetics: R. Kurtz, Geological Survey of Canada

Potential Field: J.C. Mareschal, Université du Quebec à Montreal Abitibi Region: J. Ludden, Université de Montreal

Eastem Grenville: A. Hynes, McGiII University

Central. Grenville: J. Martignole, Université de Montreal Western Grenville: S. Carr, Carleton University

Liaison MERQ: P. Verpaelst Liaison GSC-Quebec: L. Nadeau

The two following presentations were given at the conference "Séminaire d'Information sur la recherche géologique et minérale 1996, MRNQ"

John Ludden (co-transect leader) "Towards an understanding of Precambrian crustal growth"

This presentation will involve a presentation of a deep crustal section from north of the Opatica plutonic belt to the Morin anorthosite. The seismic images show the lower-plate of the Abitibi-beit dipping below the Opatica plutonic belt into the mantle; this feature is interpreted as a suture-zone associated with the accretion of the Abitbi lithologies. The mid-crustal Ifthologies of the Pontiac subprovince dip below the southem Abitibi beit, and there is now relatively convincing evidence for an older crustal section underlying the Abitibi, perhaps indicating that much of the southem parts of the region are autochthonous on a volcano-plutonic basement. Of particular interest in the Archaean section is the nature of the lower crust. Geochronology and also seismic results provide relatively convincing evidence that underplating of crust occurred until late in the evolution of the Abitibi beit (circa 2500 Ma). While below the Opatica plutonic beit the lower crust appears to have been stabilised prior to accretion (2700 Ma). These gross features of crustal formation and their relation to regional upper- crustal features, such as major shear zones, may provide important constraints on the fluid history of the region and the evolution of minerai deposits.

The Grenville section provides a clear image of the crust-mantle boundary, with the greatest crustal thickness (50 Km) occurring far south of the inferred location of the main Grenvillian collision. Almost hall of the transect is made up of an Archaean parautochthonous sequence over which Grenvillian assemblages are thrust as relatively thin veneer. The Baskatong ramp is a crustal scale feature which truncates the autochthonous rocks. The base of the aliochthonous terranes is likely to be this ramp, which flattens at around 30 km depth into a relativety transparent intermediate crust. Ramp anticlines and overriding basal thrust of the upper allochthons demonstrate the northwest directe(' propagation during the Grenvillian orogeny.

Andrew Calvert (seismic coordinator) "Technology transfer : applications of high-resolution seismics to minerai exploration"

The origin of seismic reflections in hard rock areas is often complex and difficuit to understand, requiring detailed geological and borehole log data. The direct observation of VMS deposits may be possible, but distinguishing them from other strong reflections, such as faulted gabbro sills, may not be easy. However, seismic profiles can permit detailed structural interpretation of the fauit systems in mining camps, allowing the identification of future exploration prospects.

In the case of the Noranda mine series, the dipphg bimodal (andesite-rhyolite) volcano-stratigraphy of the Mine Series is not easity inaged. However, the more laterally continuous reflections, sills and faults, which cut through pre-existing lithologies over a vide area are easier to image and can be used to provide a three- dirnensional geometry to the mining camp. Interestingly voids such as the galléries of the 1400 m-deep Ansil mine are easily irnaged. Atthough not applicable in this case study, 3D seismic imaging of mine workings are now being used as a guide to mine exploitation (for example deep gold mining in the Witswatersrand deposits in S. Africa)

In the Matagami mining camp, the voicano-stratigraphy does appear to be laterally continuous, and can be inaged, although stil) appears weaker than some gabbros sills. Nevertheless, an excellent image of the Galinee anticline can be obtained, which altows the detailed imaging of faulting in the Wabasee and Watson Lake groups, as well as n the underlying Bell River iltrusion. Interpretation of these data suggest that there has been overthrusting and repetition of the volcano-stratigraphy, and intrusion. One line was recorded directly above the Bell Allard South VMS deposit, and this appears to be imaged in the seismic profile as a short, moderately strong reflection.

THE ASSEMBLY OF AN ARCHAEAN CONTINENT: AN OVERVIEW OF SEISMIC REFLECTION RESULTS FROM THE ABITIB1-GRENVILLE TRANSECT

CALVERT, A.J., département de Génie minéral, École Polytechnique, CP 6079, succursale centre-ville, Montréal, Québec, H3C 3A7, Canada, calvert@geo.polymtl.ca; LUDDEN, J.N., Centre de recherches pétrographiques et géochimiques, 15 Rue N.D. des Pauvres, 54501, Vandoeuvre-les-Nancy, BP 20 Cedex, France, ludden@crpg.cnrs-nancy.fr

Three seismic reflections surveys have been shot over the Superior and Grenville provinces as part of the Lithoprobe Abitibi-Grenville transect, defining a 1000 km long corridor that extends from north of the Opatica plutonic boit, across the entire Abkbi greenstone belt to just north of Montreal. These surveys demonstrate that the southem Superior Province evolved primarily through the northward underthrusting of accreted terranes along one or more north-dipping subduction zones.

Upper amphbolite facies rocks, the highest grade Archaean rocks found along the corridor, are exposed in the central Opatica boit. The seismic reflection profiles show that these rocks were exhumed from mid- crustal depths during an approximately north-south collisional event as proposed previously on the basis of geological mapping. However, the seismic data aiso show that the Opatica belt overties a north-dipping mantle suture zone that extends,down to more than 70 km depth, suggesting that the Opatica orogen was linked to underthrusting of Abitibi iithologies, possible closure of an Archaean ocean and subduction into the mentie.

The Opatica belt formed as a long-lived volcanic arc, and may well represent the central core of the eastem Superior Province to which younger terranes were subsequently accreted. The existence of shallowty north- dipping reflections beneath the Abitibi greenstone boit, and comparison with the mid-crustal section exposed in the Kapuskasing uplift, suggest that underthrusting of both accreted sediments and mafic units, the latter perhaps derived from a downgoing oceanic plate, occurred. Final assembly of the Superior Province in this area le associated with the collision of a larger microcontinent, which now forms the reworked Archaean crust underlying much of the Grenville Province of western Quebec. Although this evolutionary process is consistent with much of the available geological data, a number of puzzles remain, inciuding the origin of the deep seismic reflections, which may possbly be associated with sheet-like mafic intrusions in the upper and mid-crust. The generally low metamorphic grade and the absence of any crustal-scale extensional features in the eastem Superior Province is n marked contrast to more modem orogens and is not easily explained, but may be linked to the rapid formation of a lithospheric root.

METAMORPHIC HISTORY OF THE CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE AND ADJACENT FOOTWALL ROCKS IN THE ALGONQUIN DOMAIN (CENTRAL GNEISS BELT);

BANCROFT - WHITNEY - BARRY'S BAY AREA, ONTARIO GRENVILLE

CARR, S., Ottawa-Carleton Geoscience Centre, Carleton University, Ottawa, Ontario K1S 5B6, scarr@ccs.carleton.ca; BERMAN, R., 601 Booth Street, Geological Survey of Canada, Ottawa, Ontario K1A 0E8, barman gsc.NRCan.gc.ca.

The -10 km thick Central Metasedimentary Bett boundary thrust zone (CMBbtz) comprises generally southeast-dipping gneisses and mytonfte betts, and is interpreted as a northwesterly-directed ductile thrust fauft that was active at -1060-1080 Ma and, in part at >1180 Ma (Hanmer and McEachem 1992, and references therein, McEachem and van Breemen 1992, Burr and Carr 1994). Predominantly amphibolite-facies rocks of the Central Metasedimentary Boit were carried h the hanging wall of the CMBbtz and were thrust over amphbolitized granulite-facies rocks of the Central Gneiss Bett (CGB) in the footwall. A primary objective of our study of rocks from the CMBbtz and adjacent CGB footwall is to deterrnine the conditions and timing of metamorphism, and the magnitude of displacement on the CBBbtz.

Therrnobarometric data have been obtained for ten samples that span an approximately 30 km transect across the CMBbtz and adjacent CGB. Most samples dsplay sharp grain boundaries and few signs of quenched reactions in progress. Exceptions are rare, and hclude partial plagioclase coronas and biotite-quartz syrnplectftes around gamet. Uniform chemical compositions indicate well equilibrated semples. An exception is one paragneiss semple from near Centreview that displays large variations in grossular content of gamet and anorthite content of plagioclase; more work is needed to interpret P-T data for this semple.

Near-peak pressures and temperatures were computed with the TWQ software (version 2.02 which uses recently revised thermodynamic data for end-members and solid solutions, Berman and Aranovich 1996).

Resutts for individuel semples give excellent convergence between independent thermometers (gt-bi, gt-cpx, gt-opx Fe-Mg «change; Al-opx net transfer) and barometers (gt-opx-plg-qz, gt-cpx-plg-qz, gt-hb-plg-qz), further supporthg the attainment of chemical equilbrium. Resutts for the group of samples as a whole are remarkably uniform: 8.9 ± 0.9 kbar and 800 ± 40°C, with no spatial con-elatims between higher or Iower P-T values. Taken at face value, these new data strongly suggest that any significant component of vertical displacement along the CMBbtz must have occurred prior to thermal equilbration at approximately 30 km depth. However, geochronology data suggest that ail P-T data may not have been synchronous.

U-Pb geochronology data from metasedimentary rocks of the CMBbtz reveal multiple episodes of metamorphic zircon and monazite growth at -1100, -1060-1080 and -1040 Ma. ln part, this may represent local magmatic and/or fluid influx events. Minerai growth apparently predated or was coincident with ductile strain n the CMBbtz at 1060-1080 Ma (based on cross-cutting relationships; McEachem and van Breemen 1992, Burr and Carr 1994); however, our preliminary P-T data indicates that the thermal peak outlasted any significant component of vertical motion on the CMBbtz.

Two rocks have been dated from >5 km structurally below the CMBbtz in the Algonquin domain of the Central Gneiss Bert. The age of a 1029 +8/- 4 Ma undeformed pegmatite dates local retrogression to arnphbolfte facies of a granulite-facies orthogneiss located west of Barrys Bay (Stop 1-1 - Easton and Davidson 1994). A granulite-facies paragneiss (-.780°C, 8.3 kbar) located -6 km SE of Whitney yielded 1133 Ma metamorphic zircons, and ca. 1145 Ma and 1100 Ma monazite agas. tt is uncertain, at present, whether these dates are local or are characteristic of a regional metamorphism h the Central Gneiss Bett. These data do not necessarlly date the granulite-facies metamorphism which may be older than 1133 Ma; however, it is likely that the rocks remahed at temperatures below 700°C (monazite closure) alter 1100 Ma. Our initial data suggests that the similarity of P-T conditions at >1100 Ma in the CGB and at <1080 Ma in the CMBbtz is fortuitous.

Additional mapping, geochronology and P-T work will be carried out to further assess the age and conditions of metamorphism in the CGB.

THE MONT-LAURIER AREA A RECORD OF PRE-1.19 GA GRENVILLIAN CONVERGENCE AND 1.165 GA MAGMATIC ACTIVITY WITH REACT1VATION ALONG MAJOR STRUCTURES

CORRIVEAU, L, Geological Survey of Canada, CGC-Québec, P.O. Box 7500, Sainte-Foy, QC, G1V 4C7, corriveau@gsc.nrcan.gc.ca; van BREEMEN, O., Geological Survey of Canada, 610 Booth St., Ottawa, ON, K1A 0E8, ovanbree@gsc.nrcan.gc.ca; BOGGS, K.J.E., Dept. of Geology and Geophysics, University of Calgary, 2500 University Drive N.W., Calgary, AB, T2N 1N4, boggs @ geo.ucalgary.ca;

HARRIS, L, Tectonics Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands WA 6907, Australia, lharris@geoluwa.edu.au; MORIN, D., INRS- Géoressources, P.O. Box 7500, Sainte-Foy, QC, G1V 4C7, dmorin@gsc.nrcan.gc.ca; RIVARD, B.

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Benolt.Rivard @ Ualberta.ca

The marble and quartzite sequences and felsic gneiss complexes of the Mont-Laurier area h the Central Metasedirnentary Bert n Québec (CMB-Q) represent a collage of rheologically contrasting lithotectonic domains where successive Grenvillian orogenic events can be discriminated and characterized using 1.165 Ga, 1.08 Ga and 1.06 Ga intrusive suites as regional tectonic markers. The gneiss complexes record early Grenvillian convergence that culminated at 1.19 Ga (Tmax 950°C, Pmax 10 kbar) and led to westward accretion of the CMB-Q and the adjacent Morin terrane to Laurentia. By 1.165 Ga, the complexes constituted rigid crustal domains and their homogeneous nature compared to that of surrounding paragneisses translates into low reflective domains in the LITHOPROBE seismic profile. Emplacement of monzonite, gabbronorite and composite mafic-felsic dykes at 1.165 Ga mark the onset of renewed intense orogenic activity at a time of AMCG magmatism n the adjacent Morh terrane and stitches both domains to Laurentia. Sheet-like intrusions occur along steepty-dipping, N-S corridors and along the northem boundary of the CMB-Q, while vertically- Iayered mafic intrusions tend to be scattered around gneiss complexes. The magmatic corridors correspond to strong, east-deping reflectors h the LITHOPROBE images that are thought to represent crustal-scale listric ductile shear zones which tapped both felsic and mafic magmas reservoirs below the Morin terrane in an extensional setting. Following the main magmatic activity in the CMB-Q, late compressive deformation was focussed along the N-S corridors and waned clown by 1.155 Ga. West-verging lobs of 1.165 Ga-type dykes in marble along the Cayamant lineament are compatible with a renewed westward thrusting of the CMB-Q.

At 1.08 Ga, potassic alkaline and shoshonitic magmatism occurred with emplacement of composite nested plutons n marble-rkh sequences and dykes with exotic xenoliths in gneiss complexes. The Rivard dyke is the first dyke with lower crustal and mantle xenoliths ever found in the Grenville Province. As each fragment is a sample of the unexposed lithosphere at 1.08 Ga, the xenoliths, h conjunction with surficial geology and LITHOPROBE seismic data, gives a pré- and a post-1080 Ma view of the tectonic setting of the CMB-Q. At 1.06 Ga, white orogenic activity raged in other parts of the Grenville province, we can only fend in the CMB-Q evidence for volumetrically minor mafic and felsic magmas emplacement such as the Guenette granite sheet- like intrusion and associated dykes.

TRANSECT ACROSS THE SOUTHWESTERN GRENVILLE OROGEN, GEORGIAN BAY, ONTARIO:

POLYSTAGE CONVERGENCE AND EXTENSION IN THE LOWER OROGENIC CRUST.

CULSHAW, N.G., JAMIESON, KETCHUM, J.W.F., WODICKA, N., CORRIGAN, D., TIMMERMANN, H., and REYNOLDS, P.H.

The Grenville orogenic cycle, between ca. 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20-30 km, offers some insights on the thermal and mechanical behaviour of the Iower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted Iargely of Mesoproteromk (ca. 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (ca. 1160 Ma) Parry Sound domain over the craton at or shortly before 1080 Ma. This stage of transport was followed by out-of-sequence thrusting and further convergence along successively deeper, foreland-propagating ductile thrust zones. A major episode of extension at ca. 1020 Ma resulted n southeast-directed transport of allochthonous rocks along the mid-crustal Shawanaga shear zone;

this structure re-activated an earlier thrust. The final stage of convergence involved deformation and metamorphism n the Grenville Front Tectonic Zone at ca. 1000-980 Ma. Peak metamorphism along most of the transect at 1065-1045 Ma followed initial transport of allochthonous rocks over the craton by 15-35 m.y.

Regional cooling, which post-dated peak metamorphism by >70 m.y., was probably delayed by the combined affects of late-stage extension and convergence. Ductile extension post-dated high-grade metamorphism and most thrusting at the present level of exposure. Extension was accompanied by the formation of regional transverse folds with axes parallel to the stretching direction. The distribution of migmatite and granulite clearly hfluenced the locus and style of crustal flow at the present level of exposure. Thrusting was facilitated by weak migmataic horizons in the mid-orogenic crust. Extension along a mid-crustal decollement was accompanied by widespread ductile shear at the present Ievel of exposure.

APPLICATION OF CRUSTAL FORMATION AGE DATING TO THE GRENVILLE PROVINCE

DICKIN, A.P., Dept. of Geology, McMaster University, Hamilton, Ontario L8S 1N8, dickin@rncmasterca Crustal formation ages are important in interpreting Lithoprobe data for the Grenville Province because they tell us about the geological history of crustal terranes which were tectonised in the Grenville Orogeny and whose reflection signatures are examined on the seismic profiles.

lt is well known that lithotectonic terranes n the Grenville Province have a variety of crustal formation ages, spanning over one billion years of Barth history. However, the Grenville belt romains one of the least well understood provinces of the Shield, in that we are stil! some distance away from being able to draw a crustal formation age map for the whole province. This is due to the pervasive nature of Grenville uplift and high-grade metamorphism, which rnakes geological mapping and correlation difficult.

lt is in this context that Nd model age dating can provide valuable insights, by estabiishing crustal formation age provinces and mapping the boundaries between them. These boundaries may be Grenville-age shear zones or pre-Grenville crustal sutures. The hterpretation of Nd model ages as crustal formation ages requires constraints from other geological information, principally U-Pb ages. However, given these constraints, Nd isotope data provide a powerful insight into the geological evolution of the Grenville boit from 2.7 to 1 Byr ago.

Results of Nd model age mapping in Ontario and Western Quebec will be compared with evidence from U-Pb and Pb-Pb data. Nd isotope data allow precise mapping of the extent of exposed Archean basement, and suggest that hidden Archean basement does not extend significantly further to the SE than its exposed extent. There is very strong evidence for an Early Proterozoic crust-forming event, which may represent an oceanic arc accreted in the Penokean orogeny. Subsequently, this part of the SE margin of Laurentia was probably an Andean-type boundary for much of the Mid Proterozoic.

Preliminary evidence from the Manicouagan area suggests a simitar history, with Archean basement truncated against an Early Proterozoic arc terrane. However, a large Mid Proterozoic terrane is also found in the Central Quebec Grenville Province. This terrane may have been accreted before the Grenville Orogeny as a large arc fragment.

It is concluded that Nd isotope data make a substantial contribution to understanding the Grenville Parautochthonous Bett of Ontario and Ouebec. These data can also help in understanding the Grenville allochthons and the nature of the hidden basement which underlies them.

3-D SEISMIC REFLECTION MODELLING ALONG THE MANICOUAGAN CORRIDOR

EATON, D., Dept. of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7, deaton@julian.uwo.ca; HYNES, A., Dept. of Earlh and Planetary Sciences, McGiII University, Montreal, Quebec, H3A 2A7, andrew_h egeosci.lan.mcgill.ca

Seismic reflection profiling along the Manicouagan corridor (line 55) has imaged potydeformed metamorphic terranes n the interior of the eastem Grenville province (Eaton et al., 1995). Following route 389, the line runs locally parallel to the structural fabric in some areas, and elsewhere crosses it at a high angle. Identification of out-of-plane reflections is therefore expected to be not onty significant, but critical to a valid interpretation of the data. The presence of abundant out-of-plane reflectivity raises questions about the validity of 2-D migration procédures applied to the seismic data.

We have constructed an upper cristal model along the Manicouagan corridor, to 12 km depth, consisting of 8 hypothetical 2-D surfaces. Each surface represents a regionally extensive lithologie contact and/or structural disc,ontinuityacross which a jump in acoustic properties is expected to occur. 3-D forward modelling of the zero-offset seismic reflection response for this model along the CDP slalom line was carried out using the Bom approximation (Eaton, 1996). Comparison of the modelling resuits with the stacked seismic section provides a way to update the mode! and to constrain the interpretation of the seismic section.

References:

Eaton, D., Hynes, A., Indares, A. and Rivers, T., 1995. Seismic images of ectogites, crustal-scale extension, and Moho relief in the eastem Grenville province, Quebec: Geology, 23, 855-858.

Eaton, D., 1996. BMOD3D: A program for three-dimensional seismic modelling using the Bom approximation:

Geological Survey of Canada, Open File 3357.

TROCTOUTE MAGMAT1SM ACROSS THE MANICOUAGAN CORRIDOR

FRANCIS, D., and SCOWEN, P., Earth and Planetary Sciences, McGiII University, Montréal, Québec H3A 2A7, Don_FOGEOSCI.LAN.MCGILL.CA

A survey of ten troctolite-anorthosite intrusive complexes across the Manicouagan Corridor of the Abitibi- Grenville transect reveals that there are two different magmatic suites. The dominant one is characterized by the crystallization sequence olivine - plagioclase - orthopyroxene - clinopyroxene - oxide and is associated with hypersthene-normative fine-grained dyke rocks. The other is cornprised of the Brien and Lac Raudot intrusions, on either sicle of the Hart-Jaune fault, which are characterized by the crystallization sequence olivine - plagioclase - oxide - clinopyroxene and associated with fine-grained marginal rocks which are nepheline normative. In both suites, the most primitive rocks are dunite cumulate layers whose maximum olivine Fo contents are 75 and 79 respectively. Plagioclase became a cumulate phase at an olivine Fo content of apprœdrnately 73 in the hypersthene-normative suite and 70 in the nepheline-normative suite, as compare to most recent terrestrial magmas which are saturated in plagioclase and plagioclase by olivine Fo contents of 85. This requires that the parental magmas for both these troctolite suites were rich in Fe, with those of the nepheline-normative suite reaching Fe-contents never before reported for primitive magmatic rocks on the Earth. The source nagions for the nepheline-normative troctorrte magmas, in particular, must be unusually Fe- rich, and are not seen in Phanerozoic terrestrial magmatism.

Of particular note are the high Ni contents of olivines in the Brien intrusion, as compared to those of the Lac Raudot complex, all the hypersthene-normative troctolite intrusions of the Manicouagan corridor, and even the Reid Brook troctolite intrusion which hosts the Voisey Bay deposit h Labrador. The Brien nepheline- normative troctolite intrusion differs from the Raudot intrusion, and all the others, in having gamet in the symplectite coronas that have developed between plagioclase and olivine, suggesting a greater depth of emplacement. The presence of gamet in coronas and nepheline-normative character might be provide simple criteria for identifying other such Ni-rich intrusions in the Grenville Province.

NOUVELLES DONNÉES GÉOCHRONOLOGIQUES U-Pb DANS LE COMPLEXE MÉTAMORPHIQUE DE MANICOUAGAN

GOBEIL, André, CLARK, Thomas, et DAVID, Jean, ministère des Ressources naturelles, 5700, 4°

Avenue Ouest, Chariesbourg, Oc, G1H 6R1

Les roches du Complexe métamorphique de Manicouagan font partie de l'allochtone polycyclique de la province géologique de Grenville. L'ensemble est constitué de gabbronorite métamorphisé, de paragneiss variés, de roches calcosilicatées et de gneiss à pyroxènes. Il est recoupé par des intrusions de composition variée allant des roches ultramafiques aux granitoides à pyroxène.

Les roches du Complexe métamorphique de Manicouagan sont métamorphisées au faciès des granulites;

à proximité des zones de déformation qui le bordent, elles sont graduellement rétrogradées au faciès des amphibolites.

Le projet de datation aval pour objectit d'établir la chronologie de la mise en place de certaines lithologies du complexe métamorphique, de définir l'âge du métamorphisme régional (D1) (faciès granulitique) et de préciser la chronologie de la deuxième phase de déformation pour mieux comprendre l'évolution structurale régionale.

La mise en place d'une pegmatite mafique associée à une intrusion litée de gabbronorite syn- à pré-D2 a été datée à partir de zircons à 1510 ± 7 Ma; cette intrusion de gabbronorite contient des enclaves de granulite mafique gneissique ce qui implique un métamorphisme granulitique antérieur à la mise en place du gabbronorite (> 1510 Ma).

Trois analyses de monazite provenant d'une monzonite porphyrique sont pratiquement concordantes à 1487+7/-4 et sont interprétées comme l'âge du métamorphisme granulitique associé à D2. La déformation associée à D2 affecte l'assemblage minéralogique du métamorphisme plus ancien (> 1510 Ma).

Contribution ri° 96-5110-07 du rrinistère des Ressources naturelles

GÉOLOGIE DE LA RÉGION DE L'ASCENSION(31J/10)

HÉBERT, C., LACOSTE, P., NANTEL S. et NADEAU, J., ministère des Ressources naturelles, 5700, 4e avenue Ouest, Charlesbourg, Qc G1H 6R1, sgq mm.gouv.qc.ca

La région de l'Ascension est située dans les Laurentides au nord de Montréal à quelques dizaines de kilomètres à l'est de Mont-Laurier. Les roches de la région appartiennent à la province géologique de Grenville.

Les roches supracrustales sont les plus vieilles et couvrent plus de 60% de la région. Par ordre d'importance elles sont constituées de quartzite, de paragneiss fortement rouillés par endroits, de marbres calcitiques et dolomitiques, de roches c,aloosilicatées et d'amphibolite.

Une suite de roches ignées de composition tonalitique, dioritique et gabbroique semble s'être mise en place après les roches supracrustales. Ces roches correspondraient à la série de Lacoste. Elles sont logées le long et à l'ouest d'une importante zone de déformation connue sous le nom de «cisaillement de Labelle».

Ces roches affleurent surtout de part et d'autre de la rivière Rouge.

Dans la partie ouest de la carte, on observe plusieurs filons-couches de métagabbro qui se sont logés dans les roches supracrustales. Ces métagabbros sont généralement amphibotitisés et parfois grenatifères.

On y observe par endroits des niveaux de gabbro, gabbro-norite et quelques fois d'anorthosite qui sont parfois interstratifiés et qui définissent ainsi un litage magmatique.

Le pluton ultrapotassique de Sainte-Véronique occupe la partie SW de la carte tandis qu'un peu plus au nord, il y a un granite blanc à biotite, muscovite, fluorine et tourmaline qu'on a nommé «Granite de Lanthier».

Selon la carte du gradient vertical ce granite occuperait près de la surface une superficie presque équivalente à la demie ouest de la carte. En effet, même s'il n'affleure que sur quelques dizaines de kilomètres carrés, ailleurs, d se manifeste sous forme de nombreux dykes de granite, de pegmatites à tourmaline ou d'injections intraformationnelles à travers les roches supracrustales sus-jacentes.

La nature des roches supracrustales du côté est et du côté ouest de la rivière Rouge nous semble assez différente surtout en ce qui a trait aux gneiss nodulaires qui sont visibles uniquement du côté ouest. Ceci s'applique aussi aux filons-couches qui sont présents uniquement du côté est.

De plus, deux zones principales de déformation affectent les roches supracrustales suivant des orientations NNE-SSW à l'est de la rivière Rouge et NE-SW du côté ouest.

Le faciès métamorphique est aussi différent de part et d'autre de la rivière. À test, il est de l'ordre des granulites tandis qu'à l'ouest, ë est au faciès amphbolfte. De plus, du côté est, de nombreuses zones de gneiss droits forment un patron anastamosé. Ils sont caractérisés par une linéation généralement SE. Ces linéations sont associées à un chevauchement des différentes unités du SE vers le NW avec une légère composante horizontale en décrochement senestre. Dans les zones moins déformées on peut encore y mesurer une gneissosité ancienne orientée à environ N130 avec un pendage variant du SSW au SSE et ce à la grandeur de la demie est du feuillet 31J/10.

La différence de métamorphisme de part et d'autre de la rivière Rouge semble suggérer que le côté est serait à un niveau d'érosion plus profond que le côté ouest.

THREE GENERATIONS OF ANORTHOSITE-MANGERME-CHARNOCKITE-GRANITE (AMCG) MAGMATISM, CONTACT METAMORPHISM AND TECTONISM IN THE SAGUENAY-LAC-ST-JEAN REGION OF THE GRENVILLE PROVINCE, CANADA.

HIGGINS, M.D., Sciences de la Terre, Universké du Québec à Chicoutimi, Chicoutimi, Canada, G7H 2B1, mhiggins@ uqac.uquebec.ca; Van BREEMEN, O., Geochronology Laboratory, Geological Survey of Canada, 601 Booth St., Ottawa, Canada, K1A 0E8, ovanbree@gsc.NRCan.gc.ca

Four A-type granitoid plutons from the Saguenay-Lac-St-Jean region of the Grenville Province, lacking regionally imposed solid-state deformation, give U-Pb zircon igneous crystallisation ages of 1146 ± 3 Ma, 1082

± 3 Ma, 1067 ± 3 Ma, and 1020+4/-3 Ma. These ages indicate that the region has not been affected by the Ottawan period of the Grenville orogeny. A compilation of these and existing U-Pb age data reveals three periods of Mesoproterozoic magmatism in this region: 1160-1140 Ma, 1082-1050 Ma and 1020-1010 Ma.

Each of these periods appears to have had some or all of the components of the well-known Anorthosite- Mangerite-Chamockite-Granite suite, and the first two periods can be correlated with magmatism elsewhere in the Grenville Province. Sparse U-Pb metamorphic ages also fall within these periods and are interpreted to reflect regional-scale contact metamorphism produced by the large plutons.

In addition, the few ages of strke-slip movements on major shear-zones also fall within these time periods.

It is difficuit to interpret these data as reflecting separate orogenic events, as there is littie définitive evidence of calc-alkaline magmatism, thrusting or regional metamorphism in this region. Perhaps during the interval 1160 to 1010 Ma magmatism was driven by repeated upwelling of hot mentie, either in a plume or above a sinking lithospheric slab, but access to the mid-levels of the crust currently exposed was controlled by stiike-slip movements on major vertical shear-zones that resulted from plate tectonic activity.

LITHOPROBE LINE 55 (MANICOUAGAN): MODELLING THE SUBSURFACE GEOMETRY

HYNES, A., Dept. Earth & Planetary Sciences, McGill University, 3450 University St., Montreal, Quebec H3A 2A7, andrew@stoner.eps.mcgill.ca; EATON, D., Geological Survey of Canada, 1 Observatory Cres., Ottawa, Ontario K1A 0Y3, eaton cg.nrcan.gc.ca; INDARES, A., and RIVERS, T., Dept. Earth Sciences, Memorial University, St. John's, Newfoundland A1B 3X5, afin@sparky2.esd.mun.ca, trivers@sparky2.esd.mun.ca; GOBEIL, A., Min. Energie Ressources, 1620 Boul. de l'Entente, Quebec, Quebec G1S 4N6

Lithoprobe Line 55 crosses the Grenville province of eastem Quebec obliquely, beginning in nappes probably transported onto North America during the Grenville Orogeny and ending in parautochthonous Gagnon Group rocks near the Grenville front. A heterogeneous assemblage of high-pressure metamorphosed (eclogitic) rocks, of uncertain tectonic affinity, the Manicouagan Shear Bett (MSB) is intercalated between the parautochthonous and transported rocks, and is exposed in an arc extending more thon 100 km west of the seismic line.

Major structures within the transported rocks dip predominantly southeast, whereas the seismic line trends north and north-northeast. Most reflections h at least the first 10 s of data are therefore derived from northwest of the line. We have modelled the reflectivity, constraining our modal with the observed stries of tectonic boundaries and assuming constant dips to depth. We can match both the depths and apparent dips of reflectors on the line, and infertile depths of boundaries over a considerable region to the northwest of the line.

The roof fault to the MSB, a normal fault, dips about 50° and has a substantial northward kink on an otherwise north-trending strike near Manicouagan Reservoir and adjacent to the large arc of the MSB. The preserved MSB is lensoid, with minimum vertical thicknesses of more than 20 km on and northwest of the central part of Line 55. The present-day thickness of pre-Grenvillian (Superior basement and parautochthonous Gagnon Group) rocks decreases to less than 7 km near the thickest part of the lens, from more than 30 km to the north and south. Line 55 crosses a major ramp in the pre-collisional continental crust, which has a kink in it beneath that observed in the roof fautt of the MSB. The MSB may owe its conspicuous development to the kink in the ramp, and the roof normal fault may have formed during emplacement of the MSB rather than as an orogenic-collapse feature.

MANICOUAGAN IMBRICATE ZONE: AN EXPOSED DEEP CRUSTAL SEGMENT IN THE EASTERN GRENVILLE PROVINCE

INDARES, A., DUNNING, G., and COX, R., Earth Sciences, MUN, St. John's NFLD, A1B 3X5, afin © sparky2.esd.mun.ca

The Manicouagan lmbricate Zone (MIZ) is a 3000 km2 slice stack recentiy discovered in the Eastem Grenville Province, west of AG-LITHOPROBE line 55. It consists of mainly metaigneous rocks whose protolith ages range from Labradorian to Grenvillian, and was metamorphosed under eclogite fades conditions during the Grenvillian orogeny. MIZ constitutes the largest occurrence of well-preserved high-pressure rocks known in the Proterozoic. MIZ is located between the parautochthonous Gagnon terrane and the allochthonous Hart Jaune terrane. 1t comprises two contrasting lithotectonic packages. The structurally lower Lelukuau Terrane (LT) consists of Labradorian anorthosite gabbroftroctolite and granite intruded by 1299 Ma granite. LT occurs as a S-dipping thrust stack that overiies Gagnon terrane along a thrust contact. The overtying Tshenukutish domain (TD) is mainly composed of 1458 Ma granodiorite, with rafts of metasediments, intruded by 1170 Ma metagabbro and anorthosite. Within TD, Labradorian cumulates and gabbros are exposed in a tectonic window. The SE margin of TD is injected by 1015 Ma granite. TD, as well as the boundary between TD and LT, is characterized by SE-dipping extensional shear zones that truncate earlier thrust related structures ("stage I" extension). These n tum are truncated by a SE-dipping extensional fault that separates MIZ form Hart Jaune terrane.

Evidence of high-pressure metamorphism is widespread throughout MIZ with best preserved exampies LT. These include Grt-Ky-clinopyroxenites (eclogites si), Grt-Cpx-Ky assemblages in anorthosites and Ky- Grt-Kfs restites in granitoids. Phase relationships and thermobarometry indicate peak P-T conditions of 1800 MPa (mhinum) and 800-900°C for LT, implying burial to depths in excess of 50 km, and an externat heat source. These features are compatible with flow of hot asthenosphere following detachment and sinking of mantle lithosphere at the frontal part of a subducted slab. In such a context LT wouid have been imbricated and exhumed from close to the detachment zone, wlth œdiumation occurring in two stages: first by NW-directed thrusting over Gagnon terrane, and extension active at the rear of the stack ("stage I"), and second by extensional collapse of the orogen ("stage

SEISMIC REFLECT1VITY OF A FINELY LAYERED GRANULITE-FACIES DUCTILE SHEAR ZONE IN THE SOUTHERN GRENVILLE PROVINCE (QUEBEC)

JI, Shaocheng, LONG, C., MARTIGNOLE, Jacques, Department of Geology, University of Montreal, C.P.

6128, Succursale Centre-Ville, Montreal, Quebec, H3C 3J7, jish@ ere.umontreal.ca; SALISBURY, M.H., Geological Survey of Canada, Atlantic Centre, Bedford Institute of Oceanography, Box 1006, Dartmouth, Nova Scotia, B2Y 4A2

Lith'oprobe reflection profile 54 across the granulite-facies Morin shear zone (MSZ) in the Grenville Tectonic Province (Quebec, Canada) reveais a west-dipping zone of strong reflections. The origin of these reflections has been kwestigated by detailed field mapping, laboratory measurements of P-wave velocities and densifies of representative rock samples from the main lithological units, and forward synthetic modeling. Petrofabric and mierostructural analyses were performed in order to interpret the measured seismic properties. Synthetic seismograms generated from the velocity and den* data demonstrate that déformation- and metamorphism- induced layerhg of lithologie unis with different acoustic impedanceS and with resutting interference of seismic energy are the primary cause of the strong composite reflections observed in the MSZ. Clusters of fine layers of mafic granulite within granitic or chamockkic mylonite are the ultimate source of this reflectivity. This provides insight into the origin of reflections from other granulite facies shear zones h the world. Although some previous studios have reported that LPO-induced anisotropy might induce significant reflections in greenschist facies mylonite zones (e.g., Fountain et ai, 1984; Rey et al., 1994) or significantly affect the reflectivity in amphbolite facies shear zones (e.g., Christensen and Szymanski, 1988), it is not applicable to the granulite- facies MSZ because LPO-induced anisotropy is tao weak within this shear zone to cause reflections. The low anisotropy is due to the lack of readily-oriented minerais such as mica and amphibole h the rocks and to opposite influence of feldspar and quartz, or feldspar and pyroxene, on the seismic anisotropy of the individual unis. Confining pressure does not affect the reflectivity significantly because the reflection coefficients, at least in this study, are not sensitive to pressure. Synthetic reflection seismograms from the MSZ suggest that high inpedance layers with a bulk volume fraction as small as several per cent can produce strong reffections by constructive interference provided they are lateralty continuous. At a different scale of investigation, this may help to explain the strong reflectivity from some deep crustal sections with low réfraction velocities.

OBLIQUITY BETWEEN SEISMIC AND ELECTRICAL ANISOTROPIES AS A POTENTIAL INDICATOR OF MOVEMENT SENSE FOR DUCTILE SHEAR ZONES

JI, Shaocheng, département de Géologie, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, Québec, H3C 3J7; RONDENAY, Stephane, MARESCHAL, Marianne and SENECHAL, Guy, Génie minéral, École Polytechnique, C.P. 6079, Succursale Centre-Ville, Montréal, Québec, H3C 3A7

Teleseismic shear-wave splitting and magnetotelluric data have been measured along seismic profile 15 for the Canadien LFTHOPROBE Abitibi-Grenville transect (Mareschal et al., 1995; Senechal et al., 1996; Ji et al., in press). This profile extends 250 km across the Grenville front between the Archaean craton and the Proterozoic Grenville Province n the regions of the Pontiac and northwestem Grenville (Canada). The rasas show for the first time a consistent obliquity between the polarization direction of fast split shear wave and the most electrically conductive direction h the upper mentie transcurrent shear zones. At ail well-recorded stations, the fast polarization direction is nearly N100E white the most conductive direction is approximately N80E. Studies of mentie xenoliths from a kimberlite pipe (Rapides des Quinze) in the region suggest that the observed seismic and electrical anisotropies are controlled by lattice preferred orientation and shape preferred orientation of mentie minerais (mainly olivine), respectively. The obliquity between the electrical and seismic anisotropies gives a dextral shear sense of the mentie shear zone beneath the study region. This sense is consistent with that inferred from the surface geology of the crustal shear zones. The southeast-trending and the east-trending transcurrent fautts may be inferred as R and P shears, respectively. Both these shears are synthetic and are oriented symmetrically with respect to the main mentie shear zone. The interpretation impties that déformation of the crust and the subcrustal upper mentie in the lithosphere was principally coherent in the study region. The data irnplies also that the roots of the Canadian Shield have remained fixed to the crust and prevented from significant rotation or tectonic reworking since the late Archean. The survival of both the seismic and electrical anisotropies in the upper mentie since the tate Archean indicates a rapid cooling at the end of the Archean; low temperatures (<800-900°C) and high yield strength (considerably higher than regional differential stress) prevent plastic flow and grain-boundary migration of mentie minerais. The absence of a change il eller the direction or the magnitude of seismic and electric anisotropies across the Grenville Front suggests that the Grenvillian terranes were thrust over a cooled, rigid Archaean upper mentie during the Proterozoic.

References

Ji, S. et al., Geology, in press.

Mareschal, M. et al., Nature, 375, 134-137, 1995.

Sénéchal, G. et al., Geophys. Res. Let., 23, 2255-2258, 1996.

SEQUENTIAL TENSILE FRACTURES OF GARNET CRYSTALS AS AN INDICATOR FOR SUCCESSIVE UPLIFT OF METAMORPHIC TERRANES IN THE GRENVILLE PROVINCE

JI, S., ZHAO, P., and SARUWATARI, K., département de Géologie, Université de Montréal, C.P. 6128, Montréal, Québec, H3C 3J7, jish ere.umontrealca

Granulite-facies mybnites from the Morin shear zone in the Grenville Province (Quebec) contain extensively fractured gamet crystals. The size of gamet crystals varies from 0.01 to 9.0 mm with an average value of 1.51 mm. The aspect-ratios of the gamet crystals range from 1.0 to 7.0 with an average value of 1.93. These crystals are characterized by pervasive, closely spaced, relatively straight, tensile fractures aligned systematically normal to the mykmitic foliation and lineation. The fractures developed preferentially in coarse grains (>0.2 mm) or h grahs with large aspect ratio (>2) because a coarse grain contains generaily more and large microflaws than a fine grain and stress transferred from the incompetent matrix to the competent grain increases wlIh fts aspect ratio. The fractured segments of each gamet crystal have not been separated or fillecl with the matrix minerais such as quartz and feldspars or with retrogressive minerais such as biotite, muscovite and chlorite. This suggests that the gamets were fractured at shallow crustal depths (T <300°C, or <15 km for a thermal gradient 20°C/km) where their mylonitic matrix was no longer capable of plastic flow at geologicalty reasonable strain rates. We proposed that the fractures in the gamet crystals were formed by anisotropic response of anisotropic mylonites to a horizontal extension during uplifting and cooling of the metamorphic terrane. Based on fundamental assumptions in the classical stress transfer theory, we developed a new mechanical model fora particle-matrix system. The particle is assumed to be a rotational body which can be an ellipsoidal, a truncated ellipsoid, a sphere or a body with any given longitudinal section. Such a system is the most appropriate approach for naturel mylonites containing porphyroclasts which have commonly an ellipsoidal shape. Our new model explained well why tensile fracturing took place prof erentially in competent gamet rather than in incompetent felsic material, why two sets of tensile fractures formed in the same gamet grain, and why the fractures are not spaced equally in an ellipsoid grain. We suggest that sequential tensile fractures of gamet grains could serve as an indicator for successive uplift of metamorphic terranes.

CRUSTAL ARCHITECTURE OF THE CENTRAL GNEISS BELT, GRENVILLE PROVINCE, IN ONTARIO AND WESTERN QUEBEC

KETCHUM, J.W.F., Department of Earth Sciences, Memorial University, St. John's, NF, Al B 3X5, ketch @sparicy2.esd.mun.ca; DAVIDSON, A., Geological Survey of Canada, 601 Booth St., Ottawa, ON, K1A 0E8, tdavidson @ gsc.NRCan.gc.ca

Parautochthonous crust and overthrust allochthonous domains charactertze the northwestem and southeastem Central Gneiss Bett, respectively. The mutuel boundary of these first-order divisions, the Allochthon Boundary Thrust (ABT), has not been traced between is known positions near Georgian Bay in central Ontario and Lake Kipawa in western Quebec. Without knowledge of its location, structural character, and age, the regional tectonic history of this part of the southwestem Grenville Province cannot be fully realized. Near Georgian Bay, the ABT coincides with the Shawanaga shear zone, a 100-km-long, northwest-directed thrust zone that was reactivated as an extensional shear zone at ca. 1.02 Ga. In the Lake Kipawa region, the ABT is marked by a shallowly-dipping high-strain zone separating overthrust granulites from amphibolite-facies parautochthonous rocks. In both regions, footwall rocks contain dismembered metabasite dykes (with -1.00 Ga metamorphic zircon), correlated by chemistry and limited age data with the 1.24 Ga Sudbury swarm northwest of the Grenville Front. In contrast, hanging wall units host pods and lenses of metabasite with relict eclogite-facies assemblages, and bodies of -1.16 Ga coronitic metagabbro (with -1.05 Ga metamorphic zircon), both which are chemically distinct from Sudbury diabase. Throughout the Central Gneiss Balt, these footwall and hanging wall metabasites are not known to occur together within structurally coherent crust and may be mutually exclusive. We suggest that their occurrence may serve to distinguish parautochthonous and allochthonous crust. If this criterion is valid, then metabasites in the North Bay region provide first-order constraints on the location of the ABT between Georgian Bay and Lake Kipawa. For example, coronitic metagabbros at North Bay, assigned on petrographic and geochemical grounds to the 1.16 Ga suite, constrain the ABT to pass west of this location. This radically redefines the position of the ABT in Ontario suggested by earlier studios. An important implication of our hypothesis is that rocks of parautochthonous affinity occur at the base of the allochthonous thrust stack and are potentially, although not necessarily, far-travelled. This is not unusual or unique, however, as domains of Labradorian parentage occur on both sides of the ABT in the eastem Grenville Province. Basal accretion within a crustal-scale, forward-propagating thrust stack provides one means of incorporating Nparautochthonous" rocks into the allochthonous assemblage. In this presentation we will review additional data that lend support to our revised position for the ABT.