Petrological Features and Magnetic Properties of Pillow Lavas from the Thetford Mines Ophiolite (Quebec)

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  • Petrological Features and Magnetic Properties of Pillow Lavas from the Thetford Mines Ophiolite (Quebec)

    Dipartement de Giologie et Miniralogie, Universiti Laval, Quibec, P.Q. GIK 7P4 Received November 29,1974

    Revision accepted for publication April 23, 1975

    A detailed examination was made to evaluate the variations of the remanent magnetization and magnetic susceptibility through four ophiolitic pillow lavas of Cambrian age. These pillowed metabasalts, which are covered by a thin layer of cherty argillite. derive from an olivine tholeiite of probable oceanic origin. They have been metamorphosed in the greenschist facies under a regime of low pressure, moderate temperature. and in the presence of water but absence of significant stress. They still display their primary structural zoning characterized by a thin outer shell, a much wider globulitic intermediate zone, and a porphyritic core. Morphologies of quenched microphenocrysts of olivine and plagioclase are well preserved.

    The magnetization resides in very fine-grained magnetite and, in spite of avery low remanent magnetization, the primary magnetic imprint appears to be still discernible. Remanent magnetiza- tion and Koenigsberger ratio vary from the pillow margin to its center in a fashion similar to the magnetic signature of some recent and fresh oceanic basalts. The magnetic zoning matches the textural. mineralogical, and chemical zoning characteristic of these pillow lavas. We found also that the N.R.M. vector is consistent and relatively stable within the pillow inner part of the intermediate zone and the outer part of the core and therefore that samples from these zones can be used for a regional paleomagnetic study of the ophiolitic complex.

    Un examen dCtaille a ete realis6 en vue d'kvaluer les variations de la magnetisation rema- nente et de la susceptibilit6 magnetique 1 traven quatre laves en coussins ophiolitiques d'?tge Cambrien. Ces metabasaltes coussines, qui sont couverts d'une mince couche d'argilite sili- ceuse, derivent d'une tholeiite i olivine d'origine probablement ockanique. 11s ont Cte mitamor- phos6s dans le faciis schiste-vert sous faible pression. tempkature modtree, et en prtsence d'eau mais en absence de pression dirig6e. Leur structure interne zonie, qui est encore bien conservee, se caracttrise par une mince crodte ou enveloppe externe, par une zone intermt- diaire beaucoup plus large et 1 texture globulitique, et par une zone interne ou cceur du coussin B texture porphyrique. Les formes originales de refroidissement rapide des microph6nocris- taux d'olivine et de plagioclase sont encore bien prkservies.

    La memoire magnetique se trouve IogCe dans de la magnetite trks fine et en dCpit d'une magnktisation rkmanente t&s faible, la signature magnitique primaire parait encore discernable. Elle se canctQrise par une variation systematique de la magnitisation kmanente et du rapport de Koenigsberger de la bordure au centre du coussin. Cette zonation magnetique est analogue i celle de certaines laves en coussins oceaniques actuelles et se superpose exactement B la structure concentrique des textures, de la minkrologie, er de la composition chimique de ces coussins. De plus, la direction du magnftisme rkmanent nature1 est relativement stable dans les zones internes des coussins rendant possible une etude paldomagnttique regionale du complexe ophiolitique de Thetford-Mines.

    Introduction The paleomagnetism of ophiolitic pillow lavas

    is interesting both for its own properties and its relevance to the hypothesis of an oceanic origin. Oceanic pillow lavas have recently been obtained through the Deep Sea Drilling Project. The detailed magnetic studies carried out on these basaltic samples, which are partially oriented since their vertical' direction is known, provide good references. Now it becomes possible to compare not only the structural, mineralogical, and chemical features of oceanic basalts and

    ophiolitic metabasalts but also their magnetic signature.

    It must be understood, however, that we must be cautious in interpreting the paleomagnetic features because the pattern observed is likely to , result from several causes. Most metabasaltic I volcanic rocks have N.R.M. intensities much lower than those of the fresh basalts (Fox and Opdyke 1973). In metabasalts the primary magnetic features have been partly or completely erased by later overprints. Therefore, selection of the samples is important if we want to obtain

    1 Can. J. Earth Sci., 12, 1406-1420 (1975)

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  • SEGUIN AND LAURENT: PILLOW LAVAS 1407

    some knowledge of the primary magnetic im- print in spite of alteration, tectonic deformation, and metamorphism that the rocks may have undergone. For this study, we have selected the least metamorphosed pillow lavas of the Thet- ford Mines, Quebec, ophiolite. Because the original fine quench textures of the samples chosen are still recognizable, we believe that they are in a very good state of preservation and that they are acceptable and useful material for a paleomagnetic study. The aim of this paper is to describe the structural and petrological charac- teristics, as well as the paleomagnetic properties of the ophiolitic pillow lavas of Thetford Mines, and to compare them with their possible analogs from the sea floor. Finally we shall discuss the origin and evolution of the features observed.

    Geological Setting The ophiolite belt of southeastern Quebec is

    part of a discontinuous string of peridotite bodies and associated rocks scattered along the Appalachian Mountains (Hess 1955). In the Eastern Townships of Quebec, the ophiolite belt reaches a width of several kilometres over a distance of more than 250 km. The big peridotite bodies of Orford, Asbestos, and Thetford Mines, where some of the largest asbestos-mining operations in the world are located, are inter- preted as the lower unit of allochthonous stratified sheets which, prior to their folding with the country rock, have been thrust over the Cambrian formations of the Cambro-Ordovician Inner Zone of the Quebec Appalachians (Laurent 1975; St. Julien and Hubert 1975). The peridotites are overlain by an upper structural unit consisting of a well-layered sequence of chromite-bearing du- nite, pyroxenite, gabbro, diabase, pillowed meta- basaltic lava, and cherty argillite. The main features of these complexes are similar to the Early Paleozoic Appalachian ophiolites of West- ern Newfoundland and to the Mesozoic Alpine ophiolites of the Tethyan Zone.

    Study of the Thetford Mines ophiolite, the largest of the Appalachian belt of Quebec, is of special interest because this complex displays a complete ophiolite suite. Various aspects of its geology have been described in the past. The most significant contributions are those of Dresser (1913), Cooke (1937), and Riordon (1953, 1954). More recently, Kacira (1971) demonstrated the mantle origin of the asbestos

    peridotites (harzburgites of Alpine-type), and St. Julien (1 972) and Lamarche (1 972) recognized the ophiolitic nature of the complex. Then, Laurent (1973) defined the stratigraphic sequence of the ophiolite and suggested that it represents a slice of oceanic crust and mantle formed in Middle Cambrian or earlier times (Laurent and Vallerand 1974) and probably emplaced in its present position during the Early Ordovician Epoch.

    Structural and Petrographic Features of Pillow Lavas

    The pillowed metabasalts, whose cumulative thickness reaches locally a maximum of about 600 m, make up the upper part of the Thetford Mines ophiolite. Their high magnesia content (Laurent and HCbert 1974) and the presence of abundant magnesian chlorite (14 A) pseudo- morphs after olivine phenocrysts indicate that they derive from olivine tholeiites. Through hydrothermal alteration, metasomatic exchange, and low grade regional metamorphism, their primary minerals have been converted into assemblages of the greenschist facies. Olivine is chloritized or silicified; pyroxene is replaced by tremolitic actinolite, and the calcic plagioclases are replaced by albite, actinolite, and a small amount of epidote. The rocks also contain numerous veins of chlorite, epidote, quartz, calcite, and disseminated sulfides such as pyrite and chalcopyrite. Many oceanic metabasalts are similarly altered or metamorphosed (see Melson and van Andel 1966; Aumento and Loncarevic 1969; Miyashiro et al. 1971; Hekinian and Au- mento 1973).

    Primary forms and internal structures of the upper pillow lava flows are well preserved. Individual pillows have the shape of potato bags, about two times or more longer than their largest diameter. The long axis is parallel to the direction of flow, which can be determined on sections parallel to it because the pillow extremity pointing frontwards is bulbous while its back- ward extremity is flattened and pinched (Fig. 1). Sections across the pillows have diameters be- tween 5 and 90 cm (Z = 36 cm); they show variable asymmetrical outlines. The upper part is semi-circular (convex upwards) while the base is more even, generally moulded on the upper surface of the underlying pillows (Fig. 1). Radial jointing is absent or poorly developed, except in

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