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The 1.87 Ga LaBine Group outcrops along the western margin o f Wopmay orogen at Great Bear Lake and rests on a deformed and metamorphosed 1.92 Ga sialic basement complex. It is overlain by rocks of the mainly rhyodacitic Sloan Group. Synto post-volcanic plutons of the Great Bear batholith intrude both groups. From Echo Bay northward to .Hornby Bay the oldest rocks of the LaBine Group are mainly andesitic lavas, breccias, and pyroclastic rocks at least 3000 m thick, interpreted to be the remains o f a number o f large stratovolcanoes. Overlying, and in part interfingering with the stratovolcanoes, are seven major ash-flow t u f f sheets, which are locally intercalated with andesite, dacite, rhyolite flows and domes, and a diverse assemblage of fluvial and lacustrine sedimentary rocks. Sheets of ash-flow tuff include units thicker than 800 m deposited within cauldrons and thin cooling units deposited outside the cauldrons. Intercalated with intracauldron tu f f are wedges of breccia and megabreccia, presumably derived from the walls o f the cauldrons during subsidence. Facies relations and the overall evolution o f the field from early gas-poor andesitic eruptions to gas-rich eruptions o f ash-flow t u f f are closely comparable to Oligocene volcanic fields of the United States believed related to subduction. Rocks of the LaBine field were hydrothermally altered by high-level geothermal processes but on the basis of S i02 , T i 0 2 , REE, and phenocryst mineralogy they can be classified as calc-alkaline. Therefore, it is concluded that the LaBine Group represents an early Proterozoic volcanic arc developed upon continental crust. Preserved stratovolcanoes and other high-level volcanic strata indicate that the LaBine Group was erupted into a basin which was subsiding concomitant with eruptions. The basin was probably generated in a wrench zone related to oblique convergence. Laccoliths in Athapuscow Aulacogen together with recent geochronological and field data suggest that the LaBine Group postdates continent-microcontinent collision in Wopmay Orogen and was probably generated above an eastward-dipping Benioff zone which was either segmented or became shallower with time. Le groupe de Labine, qui date de 1.87 Ga, affleure le long de la marge ouest de llorog&ne de Wopmay, dans la re'gion du Grand lac de ['Ours, et repose sur un complexe rocheux de caractere sialique, d6forrn6 et me'tarnorphise', cfg6 de 1.92 Ga. I1 est recouvert par les roches principalement rhyodacitiques ciu groupe de Sloan. Des plutons synvolcaniques h postvolcaniques du batholite du Grand lac de I1Ours traversent les deux groupes. De la baie dlEcho au nord h la baie Hornby, les plus anciennes roches du groupe de Labine sont principalement des laves and6sitiques. des breches et des roches pyroclastiques d'au moins 3 000 m d16paisseur, qui d1apr8s certaines interpre'tations, seraient les restes de quelques grands stratovolcans. Sept vastes nappes de t u f s re'pandues sous forme de coule'es de cendres, qui recouvrent les stratovolcans et parfois pre'sentent des interdigitations avec ceux-ci, sont localement intercale'es dans des andbites, dacites, coule'es et ddmes rhyolitiques, et divers assemblages de roches se'dimentaires dlorigine fluviatile et lacustre. Les nappes de tufs re'pandues sous forme de coule'es de cendres contiennent des unite's de puissance supe'rieure h 800 m. d6pose'es 6 11int6rieur des caldeiras, et de minces coule'es de lave, de'pose'es h l'exte'rieur de celles-ci. Des formations en biseau, forme'es de braches et me'gabr&ches, probablement d6rive'es des parois des caldeiras pendant la subsidence de celles-ci, sont intercale'es avec les tu fs de Ifint6rieur des caldeiras. Les relations de faci&s et l'e'volution globale ciu terrain, d'abord soumis h des 6ruptions andksitiques pauvres en e'manat ions gazeuses, puis aux e'ruptions gazeuses de tu fs re'pandus en coul6es de cendres rappellent fortement les secteurs volcaniques oligocanes des Etats-Unis, que l'on estime associe's aux phdnornhnes de subduction. Les roches du secteur de Labine ont e'te' alte're'es par des re'actions hydrotherrnales intenses, mais en raison de leur teneur en S i02 , en Ti02 et REE e t de la rningralogie des phe'nocristaux, on peut les classer dans les roches calcoalcalines. On en conclut donc que le groupe de Labine correspond h un arc volcanique d16ge prot6rozo?que in fk ieur , form6 au-dessus de la crodte continentale. Les stratovolcans e t autres niveaux volcaniques cre'6s par une activite'volcanique intense, e t encore conserve's, indiquent que le groupe de Labine s'est e'coule' lors d16ruptions dans un bassin, dont l'affaissernent a accornpagne' les e'ruptions. Le bassin s'est probablernent form6 dans une zone de de'chirement r6sultant d'une convergence oblique. Dans 11aulacog8ne dlAthapuscow, llexistence de laccolites e t les r6centes donne'es ge'ochronologiques e t donne'es obtenues sur le terrain sernblent indiquer que le groupe de Labine est ult6rieur h la collision entre continent e t microcontinent qui a eu lieu lors de 110rog8ne de Wopmay, e t a probablement 6 t6form6 au-dessus d'une zone de Benioff plongeant vers l'est, qui s lest fragrnente'e ou est devenue moins profonde au cours des temps. INTRODUCTION During the next 25 years geological work was mainly In recent years the concept of plate tectonics has confined t o detailed studies of the mineral deposits a t LaBine provided an actualistic framework to interpcet the geology of Point (Campbell, 1955, 1957; Jory, 1964; Robinson, I97 1; continents, which contain the vast majority of recorded earth Robinson and Morton, 1972; Robinson and Badham, 1974) and history. Geologists now examine pre-Mesozoic terranes with in the Conjuror Bay-Camsell River region (Badham, 1972, an eye toward determining an evolutionary scheme for the 1973a, b, 1975; Badham e t al., 1972; Shegelski, 1973; Badharn and Morton, 1976). Mursky (1973) compiled much of the development of the earth based on similarities with, and previously restricted data collected by the Geological Survey dissimilarities to, post-Mesozoic terranes. of Canada durinr! the war. Studies of post-Mesozoic volcanic and plutonic rocks suggest i t is possible to relate magmatic belts to specific tectonic settings. In fact, studies in Cenozoic volcanoplutonic terranes indicate that most can be related to lithospheric plate motions. Thus, igneous rocks may provide valuable insight into tectonic processes in older, cratonic regions where there is no sea floor record. The purpose of this paper is t o present stratigraphic and geochemical data from the LaBine Group, a 1.87 Ga volcanic field located along the eastern shore of Great Bear Lake, and to discuss the tectonic setting and regional implications.

Early Proterozoic Labine Group of Wopmay Orogen: Remnant of a Continental Volcanic Arc Developed During Oblique Convergence