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

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<ul><li><p>Petrological Features and Magnetic Properties of Pillow Lavas from the Thetford Mines Ophiolite (Quebec) </p><p>Dipartement de Giologie et Miniralogie, Universiti Laval, Quibec, P.Q. GIK 7P4 Received November 29,1974 </p><p>Revision accepted for publication April 23, 1975 </p><p>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. </p><p>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. </p><p>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. </p><p>La memoire magnetique se trouve IogCe dans de la magnetite trks fine et en dCpit d'une magnktisation rkmanente t&amp;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. </p><p>Introduction The paleomagnetism of ophiolitic pillow lavas </p><p>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 </p><p>ophiolitic metabasalts but also their magnetic signature. </p><p>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 </p><p>1 Can. J. Earth Sci., 12, 1406-1420 (1975) </p><p>Can</p><p>. J. E</p><p>arth</p><p> Sci</p><p>. Dow</p><p>nloa</p><p>ded </p><p>from</p><p> ww</p><p>w.n</p><p>rcre</p><p>sear</p><p>chpr</p><p>ess.</p><p>com</p><p> by </p><p>Uni</p><p>vers</p><p>ity o</p><p>f A</p><p>uckl</p><p>and </p><p>on 1</p><p>2/01</p><p>/14</p><p>For </p><p>pers</p><p>onal</p><p> use</p><p> onl</p><p>y. </p></li><li><p>SEGUIN AND LAURENT: PILLOW LAVAS 1407 </p><p>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. </p><p>Geological Setting The ophiolite belt of southeastern Quebec is </p><p>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. </p><p>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 </p><p>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. </p><p>Structural and Petrographic Features of Pillow Lavas </p><p>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). </p><p>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 </p><p>Can</p><p>. J. E</p><p>arth</p><p> Sci</p><p>. Dow</p><p>nloa</p><p>ded </p><p>from</p><p> ww</p><p>w.n</p><p>rcre</p><p>sear</p><p>chpr</p><p>ess.</p><p>com</p><p> by </p><p>Uni</p><p>vers</p><p>ity o</p><p>f A</p><p>uckl</p><p>and </p><p>on 1</p><p>2/01</p><p>/14</p><p>For </p><p>pers</p><p>onal</p><p> use</p><p> onl</p><p>y. </p></li><li><p>1408 CAN. J. EARTH SCI. VOL. 12, 1975 </p><p>BULBOUS FRONT </p><p>J </p><p>60 crn </p><p>FIG. 1 . Structure and typical sections of metabasaltic pillow lavas from the Thetford Mines ophio- lite. The convex upper surface indicates the top of the pillow and both the axis of the longitudinal section and the bulbous front indicate the direction of flow. </p><p>FIG. 2. 'Hollow' pillow, whose former hollow center is filled with secondary albite, quartz, chlorite, and iron oxides. The floor (flat) of hollow tubes indicates the horizontal level at the time of the pillow formation. </p><p>some rare examples of hollow pillows (Fig. 2), whose former hollow center is now filled with secondary albite, quartz, chlorite, and iron oxides. Hollow tubes in pillows are interesting features because their floor, which is flat, indi- cates the horizontal level at the time of the formation of pillows (Macdonald 1967). The lava flows grade vertically land laterally into pillow breccias and hyaloclastites. An inter- </p><p>stitial clayey matrix is found only in flows at the very top of the volcanic sequence near the con- tact with the cherty argillite of the ophiolite sedimentary cover. Forms and relations between pillows and their associated facies seem analo- gous to what can be seen in sea floor photographs (Aumento 1968; Moore and Fiske 1969; Dangeard et al. 1973). </p><p>The pillows can be divided into three concen- </p><p>Can</p><p>. J. E</p><p>arth</p><p> Sci</p><p>. Dow</p><p>nloa</p><p>ded </p><p>from</p><p> ww</p><p>w.n</p><p>rcre</p><p>sear</p><p>chpr</p><p>ess.</p><p>com</p><p> by </p><p>Uni</p><p>vers</p><p>ity o</p><p>f A</p><p>uckl</p><p>and </p><p>on 1</p><p>2/01</p><p>/14</p><p>For </p><p>pers</p><p>onal</p><p> use</p><p> onl</p><p>y. </p></li><li><p>SEGUIN AND LAURENT. PILLOW LAVAS </p><p>$2&lt; *A</p></li><li><p>1410 CAN. J. EARTH SCI. VOL. 12, 1975 </p><p>inner zone (111) is yellowish green, more homog- eneous, coarser grained than the others, and contains the largest dark patches of magnesium- rich chlorite. It seems probable that before recrystallization the margin was glassy, the intermediate zone partly glassy, partly crystal- line, and the core mainly crystalline. The pillows were also slightly vesicular, as indicated by the presence of small amygdules. These amygdules have a whitish filling of albite and quartz and appear to be scattered mainly in the inner part of the intermediate zone and in the core (Fig. 3). In hollow pillows, the amygdules are much more abundant, of variable sizes and shapes, and are concentrated in the core around the center (Fig. 2). The three zones of these ophiolitic pillows correspond respectively to the glassy skin, variolitic zone, and crystalline core of the oceanic pillows described by Moore (1966), Watkins et al. (1970), Marshall and Cox (1971a), Bryan (1972), and Yeats et al. (1973). </p><p>The glassy skin has recrystallized in a fine- grained (0.01 to 0.10 mm) hypidiomorphic mix- ture of epidote and chlorite in equal amounts (90% of zone I). Actinolite, magnetite, and albite are minor components. The dark patches de- scribed above are olivine phenocrysts and skeletal microphenocrysts pseudomorphosed in magnesian chlorite, as indicated by our X-ray diffraction and microprobe data. </p><p>The intermediate zone is quite complex; a more detailed description of the quench textures and morphologies of crystals will be reported else- where (Laurent, manuscript in preparation). This zone displays a globulitic structure (Fig. 3) characterized by crystal-sheafs of tremolitic actinolite, epidote, and albite (light-green globu- lites) in a porphyritic intersertal matrix (dark green recrystallized glass) of magnesian chlorite, actinolite, albite, and dusts of magnetite. The grain-size ranges between 0.01 and 0.40 mm. Actinolite is the main component (60%), fol- lowed by chlorite (20x1, and albite (15%). Epidote is a minor component (3%). The plagio- clases of the matrix are acicular in the outer part of zone 11, where they locally form star-like aggregates. In the inner part, they present the "belt-buckle" forms described in submarine basalts by Bryan (1972). The frame of the microlite is actinolite and the core is albite. Quench olivine is abundant; the "lantern-like" forms and "doubly swallow-tailed" forms described by Bryan (1972) are recognizable. The </p><p>olivine is replaced either by a magnesian chlorite or by a microcrystalline aggregate of quartz, more rarely by calcite and chlorite. In a chloritic dark patch situated a few mm below the margin, we have found a beautifully preserved relic of microdendritic growth of olivine. It occurs in an area of about 0.5 mm2, and resembles a "micro- spinifex" similar to the skeletal growth of fayalite in smelter slag photographed by Naldrett (1972, p. 148, Fig. 7a). This may represent another quench phase or the skeletal growth of olivine in a glass devitrified after reheating (Lofgren 1971). In the inner part of zone I1 and in the core, the dark patches of chlorite tend to be oriented radially, to be of larger size than in the outer zones, and to have a polygonal outline. They appear to represent cumulophyric clusters of former olivine phenocrysts. The globulites are formed by arborescent groups of tremolitic actinolite, albite, and epidote, which we interpret as pseudomorphs of former featherlike inter- growths of plagioclase and pyroxene. At high magnification in thin-sections, boundaries of globulites are not as sharp as they look in hand- specimen, because the arborescent crystal-sheafs merge gradually into the matrix. </p><p>The core has an intersertal porphyritic texture and is locally amygdaloidal. Actinolite remains the main component (60%) of the core, which is poorer in chlorite (10%) and richer in albite (2073, epidote...</p></li></ul>