SKULSKI, T., and PERCIVAL, GSC, Ottawa, Ont. K1A 0E8; NADEAU, L., Centre geoscientifique de Quebec, Ste Foy, QC, GIV 4C7; MORTENSEN, Dept. Geological Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4.
Ancient tonalite gneisses h the Minto block (Vtzien ben) are 3010 +23/-17 Ma, 2941 ± 3 Ma and 2916 ± 11 Ma.
Minto volcanic crust formed between 2.84 and 2.79 Ga has oceanic affinities and is isotopically juvenile. In the Qalluviartuuq boit these comprise 2840 +4/-1 Ma calc-alkaline basait and andesite, 2831 ± 1 Ma granodiorite, and 2831 ± 2 Ma tholeiitic anorthositic gabbro and basait. In the Vizien boit these include 2786 ± 1 Ma peridotite, gabbro and basait. Isotopically juvenile crust was recycled in the Qalluviartuuq and Duquet belts by 2775 Ma quartz diorite and granodiorite. Contemporaneous calc-alkaline volcanism and LREE- enriched tholeiitic basait volcanism occurred in the Duquet boit at 2775 +5/-2 Ma. Post 2775 Ma Minto volcanic betts have continental affinkies and hclude isotopically °volved crust. In the Kogaluc boit these include cals-alkaline volcanic rocks at 2757 +6/-4 Ma and <2759 Ma. Minto granodiorite plutons intruded between 2733 Ma and 2721 Ma have variable Nd isotopic compositions reflecting crustal recycling. Coeval Vizien calc-alkaline volcanic rocks are 2724 ± 1 Ma. Younger Vizien volcanic sequences include 2.72 Ga tholeiitic, LREE-enriched basait and rhyolRe, and 2700 ± 3 Ma mafic crust. The latter lies at the top of a sedimentary sequence deposited on 2941 Ma tonalite gneiss. Vizien ductile deformation and amphibolite metamorphism occurred between 2700 Ma and 2693 Ma. Qalluviartuuq syntectonic granite is 2675 +4/-3 Ma. Upper amphibolite to granulite metamorphism and anatectic granitic magmatism h the southwest Minto occurred between 2700 Ma and 2671 Ma.
Pre 2.82 Ga crust is not reported south of the Minto block il Quebec, although Pontiac Gp. and Caste Fm.
(Lacome block) metasediments include 3029-2925 Ma detrital zircons. Juvenile tonalitic gneisses h the Opatica have ages of 2825-2820 Ma, and tonalite gneiss in the La Grande boit is 2811 ± 2 Ma. The protolith age of isotopically juvenile Opatica Grey gneiss is 2773 Ma. Possible correlatives to 2757 Ma Kogaluc volcanic rocks and associated metasediments and BIF include Lac Guyer rhyolitic tuf (La Grande bett) for which we report an age of 2749 ± 2 Ma. Plutons and volcanic rocks formed between 2730 Ma and 2700 Ma are found in the Quebec segment of the Abitibi boit and are isotopically juvenile with oceanic affinities. Syntectonic plutons in the Abitbi belt recycled this juvenile crust and range h age from 2703 Ma to 2685 Ma. These plutons are contemporaneous with deformation and metamorphism in the southem Minto
GRAVITY TRANSECT ACROSS THE OPATICA, NEMISCAU, AND LA GRANDE SUBPROVINCES TELMAT, H., MARESCHAL, J.C., ANTONUK, C.N., DAVID, J., and GARIÉPY, C., GEOTOP, Université
du Québec à. Montréal, B.P. 8888, Succ."Centre-Ville«, Montréal, Québec, H3C 3P8, hamid @ volcan. geotop. uqam.ca
New gravity data were collected in 1996 at - 2 km interval across the Nemiscau and La Grande subprovinces along the LG2 road. The - 350 km long transect extends northward the gravity data of Lithoprobe Une 48 which were collected at 1 km interval across the Opatica belt and a part of Abitibi subprovince (Antonuk and Mareschal, 1994). In order to do 2.5 D modelling, the two sets of data were projected on a N-S profile perpendicular to the strike of the major structures. The total length of the profile after projection is about 470 km. Because of the difference in sampling distance, the data were interpreted separately. The modelling used generalized linear inversion for iterative improvement of selected modal parameters. The main results are:
a) Abitiai-Opatica transect: The Bouguer anomaly decreases from about -40 mGal in the south over the subprovince to about -75 mGal over the Opatica belt, then increases in the northem end of the transect towards the Nemiscau subprovince (--50 mGal). The preliminary modal was improved due to botter understanding of the geology (Davis et ai, 1995) and the seismic study (Calvert et al., 1995). The final modal suggests underthrusting of the Abitibi under the Opatica. The geometry of the shallower crustal sequences is delineated as the volcano-sedimentary sequence in the Abitibi subprovince, the Ouescapis pluton, the Frotet-Evans greenstone sequence, and the Lac Rodayer pluton in the Opatica.
b) Nemiscau-La Grande transect: The Bouguer anomaly fluctuates around -50 mGal in the Nemiscau, decreases steeply to about 70 mGal in the La Grande subprovince with two peaks ( 45 mGal and -50 mGal) due to the presence of metasedimentary sequences; it increases to about -35 mGal over volcanic bands h the northem end of the transect. Despite the lack of constraints from other geophysical information, preliminary modellrig suggests underthrusting of the Nemiscau under La Grande subprovince with a transport of the supracrustal sequences toward the south.
Antonuk, C.N. & Mareschal, J.C. Abitibi-Grenville workshop, Report 41 (1994).
Calvert, A.J. et al. Nature 375, 670-674 (1995).
Davis, W.J., Gariépy, C., Sawyer, E.W. & Benn, K. Can. J. Earth Sci. 32, 113-127 (1995).
GÉOLOGIE DE LA PARTIE NORD-EST DU GROUPE DE WAKEHAM, PROVINCE DE GRENVILLE VERPAELST, P.', MADORE, L2, BRISEBOIS, DAVID, J.3 , DION, D.J' , CHOINIÈRE, J.1 , LÉPINE,
TRZCIENSKI, W.4, et HOCQ, M.'
'Ministère des Ressources naturelles, 5700, 4 °Avenue Ouest, Charlesbourg, Qc, G1H 6R1, sgq@mm.gouv.qc.ca; 2Centre d'études en Ressources minérales, 5700,
e
Avenue Ouest, Charlesbourg, Oc, G1H 6R1, sgq@mm.gouv.qc.ca; 3Géotop, UQAM, C.P. 8888, succ. Centre-ville, Montréal, Oc, geotop@eruqam.ca;tép. de Géologie, U. de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, Qc H3C 3J7, trzw ere.umontreal.caDe nouvelles données géologiques, géochimiques, géophysiques et géochronologiques, obtenues dans le cadre du Programme d'exploration du Moyen-Nord, permettent de réinterpréter la séquence des événements géologiques dans la partie Nord-Est du Groupe de Wakeham et les unités adjacentes.
Nous proposons l'histoire géologique suivante:
1) mise en place des sédiments et des volcanites du Groupe de Wakeham (>1 500 Ma) sur un socle gneissique (qui pourrait faire partie du Complexe de Boulain*);
2) intrusion du Porphyre de Kataht dans les sédiments (entre 1510±10 Ma et 1495±2 Ma);
3) déformation des roches supracrustales en plis ouverts orientés NE;
4) intrusion de la Suie felsique d'Olomane dans le porphyre et les sédiments du Wakeham (-1240 Ma), et métamorphisme de haute température (-550°C et basse pression (-2,5-3 kb);
5) développement de la Zone de cisaillement de Natashquan (ZCN) entre les domaines sédimentaire (Wakeham) et intrusif (Suite d'Olomane);
6) mise en place de l'essaim de dykes et des plutons de gabbro de la Suite mafique de Lillian ;
7) poursuite du mouvement le long de la ZCN et réorientation des domaines de part et d'autre de cette zone ;
8) intrusion de la Suite felsique de La Galissonnière (993±3 Ma);
9) intrusion du Gabbro de Le Doré (dyke de gabbro à olivine) (<1000 Ma).
*Le Complexe de Boule, qui regroupe des gneiss, des paragneiss, des quartzites et des gneiss tonalitiques, est sok le socle gneissique sur lequel les roches supracrustales du Wakeham reposent de façon «autochtone»
ou «allochtone», soit une partie du Groupe de Wakeham métamorphisé et remobilisé dans la catazone au contact avec les granites de la Suite felsique d'Olomane.
Ces nouvelles données permettent de mieux comprendre les roches du Groupe de Wakeham, leur âge et leurs relations tectoniques avec les unités adjacentes de la Province de Grenville.
Contribution No 96-5110-08 du Ministère des Ressources naturelles du Québec.