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

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 magnetization 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 imprint 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 characteristics, 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 metabasaltic 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) pseudomorphs 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 between 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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1408 CAN. J. EARTH SCI. VOL. 12, 1975

BULBOUS FRONT

J

60 crn

FIG. 1 . Structure and typical sections of metabasaltic pillow lavas from the Thetford Mines ophiolite. 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.

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.

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, indicates 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-

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).

The pillows can be divided into three concen-

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SEGUIN AND LAURENT. PILLOW LAVAS

$2< *A<< . .<p *:. - %.< <. ,

FIG. 3. Detail of the internal structure of the pillow lava A from Mount Adstock. The intermediate zone, between the pillow margin and the core, is globulitic. The globulites, which are set in a dark green chlorite-rich matrix, are light green and formed by crystal-sheafs of tremolitic actinolite, epidote, and albite. They are very small and scattered in the margin and the outer part of the intermediate zone, but increase exponentially in size and number inwards until they merge together and grade into the core. The white amygdules are another distinct feature; they are former vesicles filled with secondary quartz and albite.

tric zones on the basis of composition and fabric scattered in the margin and the outer part of (Fig. lc). First, the margin is a thin rim (I), zone 11, but increase exponentially in size and brownish yellow, that contains small dark patches number inwards (Fig. 3). In the innermost part of chlorite. Second is a thick zone (11) with a of zone 11, the globulites finally merge together in globulitic structure characterized by light green a budding fashion and grade into the inner core. globulites in a (devitrified?) dark green matrix. Dark patches, 2 mm long, of chlorite are spread These globulites are very small and widely throughout. Third is the core of the pillow. This

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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 crystalline, 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).

The glassy skin has recrystallized in a fine- grained (0.01 to 0.10 mm) hypidiomorphic mixture of epidote and chlorite in equal amounts (90% of zone I). Actinolite, magnetite, and albite are minor components. The dark patches described above are olivine phenocrysts and skeletal microphenocrysts pseudomorphosed in magnesian chlorite, as indicated by our X-ray diffraction and microprobe data.

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 globulites) 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%), followed by chlorite (20x1, and albite (15%). Epidote is a minor component (3%). The plagioclases 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

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.

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 (5x1, and quartz (5%) than the intermediate zone. Grains are lath-shaped and larger (0.10 to 1 mm) than the acicular crystals of the outer chilled zones. However, forms of "lantern-shaped" olivine, hollow olivine, and "belt-buckle" plagioclases are still abundant. The olivine is more frequently replaced by quartz than by magnesian chlorite. Some are replaced by both. The amygdules are filled by concentric layers of porphyroblastic albite and quartz. The feldspars adorn the wall of the amygdules (former vesicles) and develop in radiate groups; they can also occupy the center and be surrounded by a rim of quartz, chlorite, and iron oxides.

Morphologies of the microlites and the phenocrysts from these ophiolitic pillow lavas are identical to the quench plagioclases and olivines described in submarine basalts by Muir and Tilley (1966), Bryan (1972), Yeats et al. (1973), and recently found by Liou (1974) in the glassy basalts of an ophiolite from Taiwan.

Table 1 gives the chemical and normative compositions of a metabasaltic pillow from

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SEGUIN AND LAURENT: PILLOW LAVAS 141 1

TABLE 1. Chemical and normative compositions in wt % of the pillow lava A from Mount Adstock

Oxides* #1 #2 #3 #4 #5 X

SiOz 39.66 47.45 52.64 58.49 54.02 51.08 Ti02 0.21 0.12 0.01 0.01 0.10 0.07 A1203 16.98 9.81 12.18 11.67 11.19 11 .60 Fez03 3.80 2.15 1.73 1.43 1.28 1.91 FeO 7.53 7.89 6.17 5.17 6.39 6.70 MgO 14.92 17.40 12.31 11.02 12.84 13.98 MnO 0.17 0.18 0.14 0.10 0.13 0.15 CaO 8.97 8.51 6.04 4.94 6.29 6.91 Na20 0.45 0.76 2.83 3.83 3.24 2.21 K20 0.01 0.34 0.19 0.09 0.09 0.20 pzo5 0.03 0.02 0.02 0.02 0.02 0.02 HzO+ 5.72 3.98 4.35 2.72 3.09 3.91 H20- 0.58 0.36 0.27 0.22 0.20 0.30 c o z 0.09 0.14 0.99 0.10 0.30 0.42 Total 99.12 99.11 99.87 99.81 99.18 99.46

Fe203/Fe0 0.50 0.27 0.28 0.27 0.20 0.28 FeO + Fe203/Mg0 0.75 0.57 0.64 0.59 0.59 0.61 Index SI** 51.9 60.9 52.9 51.1 53.8 55.9

C.Z.P. W. (Total = 100) Quartz - - 2.00 7.05 0.80 0.57 Orthoclase - 2.50 1.00 0.50 0.50 1.15 Plagioclase 50.85 30.80 47.40 49.25 45.75 41.93

(Ang2) (An,?) (An45) (An30) (An361 - (An521 Diopside 15.60 7.60 7.80 12.20 10.48 Olivine 25.98 12.30 - - - -

(FOES) (FOSZ) Hypersthene 18.30 36.40 40.00 33.90 39.40 43.82

(Ens3) (En821 (Enso) (Enso) (En801 (Ens11 Magnetite 4.28 2.40 2.00 1.50 1.35 2.05 Ilmenite 0.30 - - - - - Apatite 0.13 - - - - - Corundum 0.16 - - - - -

Key to numbers: #I = Margin of pillow; 2 and 3 = intermediate zone of pillow; 4 and 5 = wre of pillow (5 = geometrical center of pillow). X, average com-

position of the pillow A. The volumes have been calculated in assuming a cylindrical shape for the pillow. They are respectively 8% for the margin, 62% for the inter-

mediate zone, and 3% for the core: X = H'f iuJ l + ''/o#;) + + 1$1%5)

*Si02 analyzed by X-ray Fluorescence (XF) and Colorimetry (C), Ti02 by XF, AI2O3 by XF and C, Total Fe by XF and FeO by Titrimetry, MgO by Atomlc Absorpt~on (AA), MnO by XF, C a 0 by XF, NatO by AA, K1O by XF and AA, P20s by C, H20+ by Gravimetry, and CO, by Volumetry.

**KUNO'S Index of differentiation: SI = MgO x IOO/FeO + Felon + MgO + Na10 + K1O.

Mount Adstock ("pillow A" discussed later with the magnetic properties). The sampling includes analyses from the rim (#I) to the geometrical center (#5) of the pillow. The average composition (X) given was calculated in assuming a cylindrical shape for the pillow and in using the calculated volumes of its margin (8%), intermediate zone (62%), and core (30%) to balance the results.

Our lava pillow is characterized by high MgO and H,O contents, relatively low AI,O,, CaO, FeO, Fe203, and very low K,O, TiO, and P205 concentrations. The MgO content is similar to some picritic tholeiites (e.g., flow of 1840,

Nanawale Bay, Hawaii (Macdonald 1949; Muir et al. 1957)), while the high amount of water is presumably related to the chloritization of the olivine phenocrysts and to the amphibolitization of the other components of the rock. The pillow has some of the most distinctive chemical features of oceanic tholeiites (Engel et al. 1965), that is, a low K,O content and a high Na20/K20 ratio. However, it is much poorer in TiO, than the present basalts of the oceanic ridges. The same is true for most of the basaltic meta- volcanics from the Thetford Mines ophiolite, whose TiO, content is often lower than 0.5%.

The variation diagram (Fig. 4) illustrates the

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PILLOW A

MARGIN INTERMEDIATE ZONE

CORE

FIG. 4. This diagram shows variations of chemical composition across the pillow A, Mount Adstock (see Table 1 for chemical data).

compositional changes within the pillow. MgO, FeO + Fe203, and CaO decrease, while SiO, and Na,O increase significantly from the rim to the center. Similar variations have been described in pillow lavas from the Alps (Vuagnat 1946), California (Hopgood 1962), and the Mid- Atlantic Ridge (Aumento 1968). The pattern is difficult to explain because it is likely to result from the combined effect of magmatic differentiation, submarine weathering, and metamorphism. Miyashiro et al. (1969) have shown that submarine weathering increases the amount of water and the oxidation of iron and decreases the SiO, content of the weathered zone. The margin of our pillow has the highest water and the lowest SiO, contents, and a Fe,03/Fe0 ratio

about twice as large as those of the inner samples. Therefore, the chemical features of the pillow margin can be partly inherited from submarine weathering, whereas the internal variations observed are perhaps mainly of metamorphic origin. We assume that they must have developed under a regime of low lithostatic pressure and moderate temperature during the slow recrystallization of the rock.

Magnetic Properties of Pillow Lavas A study of the radial variations in magnetic

properties was made on four samples (A, B, C, and D) of pillow lavas. Samples A and B were cut in the same pillow, A, which comes from an outcrop (N46°01'-W71012') located at the southern base of Mount Adstock. Sample C is another pillow from the same outcrop, and sample D was collected in the same volcanic formation but from another outcrop (N46"001- W71°14') 4 km to the southwest of Mount Adstock (consult Map 415A, Thetford Sheet (Cooke 1937)). Part of the core of pillow D (Fig. 2) was originally hollow, but has later been filled with secondary quartz, albite, chlorite, and iron oxides. All samples were cut into cubes of 1 in.3 (16 cm3) along the large diameter of the pillow perpendicular section (Fig. 1C) except sample B, which was cut in the same section but at a right angle to our standard direction of sampling. The different cubes were numbered from margin to core and classified in the three internal zones defined by the petrographic features. The long axis or flow direction of pillows, which is perpendicular to the perpendicular section (see Fig. l), was selected as the direction of reference to define the orientation of the N.R.M. vector. Sizes of samples are given in Table 2.

The remanent magnetization of each cube specimen was measured with a spinner magne- tometer and the susceptibility with a low field A.C. bridge. The results of the measurements are presented in Table 3. The susceptibility is relatively large in the pillow crust (zone I), but decreases rapidly beneath it, within zone 11, and increases again toward the center (Fig. 5). In some pillows, the susceptibility is two to three times smaller in the intermediate zones than in the crust. Most of the susceptibility high in the margin occurs in the outer 1 to 3 cm thick crust. In contrast, the intensity of the remanent magne- I tization is low in the crust but increases sub-

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SEGUIN AND LAURENT: PILLOW LAVAS

TABLE 2. Sizes of pillow samples

Width in em along radius Diameter

Sample (in cm) Margin (I) Internal zone (11) Core (III)

stantially within zone 11, although there are some exceptions, and then undergoes a slow decrease toward the pillow center (Fig. 6). In the pillows studied the direction of the N.R.M. vector is generally more consistent and constant within the inner part of zone I1 and the core than within the outer zones. Figure 7 is an example showing that the N.R.M. direction of the different cubes in sample A, from A, to A,,, i.e., within the inner part of zone I1 and core, can be defined. It tends to cluster around azimuth 175" and inclination 42"N with respect to the direction of reference (long axis or flow direction of pillows). A study of this type provides some basic information relative to the degree of homogeneity or dis- persion of the N.R.M. orientation in pillow lavas before regional paleomagnetic studies are undertaken.

The vsriation of the Koenigsberger ratio R = J/klTI in the diRerent zones of pillow lavas is pictured in Fig. 8. (TI is defined as the intensity of the present earth's field, J as the intensity of remanent magnetization, and k as the magnetic susceptibility. The values of R are low in the crust of pillows (zone I); they increase within zone I1 and the outer part of the core, and then they decrease toward the pillow center. The values of R are of the order of 0.05 to 0.10 in the crust and the center, and of 0.10 to 0.30 in the intermediate region. These values are 1000 to 1500 times smaller than the values obtained by Marshall and Cox (1971a) on recent pillow lavas of the Northern Pacific and Central Indian oceans. Variations of the Koenigsberger ratio with depth, from the margin to the center of pillows, are instructive since this ratio represents an index of stability of the N.R.M. vector. In previous studies of igneous rocks, R has generally been found to be also a sensitive indicator of cooling rate, showing a sharp decrease from the quenched margin to the interior of the igneous body (Hatherton 1954; Cox and Doell 1962). In pillow lavas, however, the relationship of the

wAkln INTERMEDIATE ZONE

CORE

ZONES OF PILLOWS FIG. 5. Variation of magnetic susceptibility K with

distance from the margin to the center of pillows (see Table 3 for magnetic data). Distances are arbitrary. The samples, which vary in size (see Table 2), are compared on a common base. [Symbols of pillows: A (o), B (t), C (x), D (El)].

Koenigsberger ratio to cooling rate is more complex than in other types of igneous bodies because of the occurrence of a relatively thick glassy quenched zone. Marshall and Cox (1971a) have found that in their L-type recent pillows R increases away from the quenched margin of the pillow toward the interior, whereas in their H- type recent pillows R decreases in the same direction. The variation of R in the pillow lavas studied here is similar to the oceanic L-type pillows described by Marshall and Cox in spite of the fact that the values of R decrease rapidly from the outer part of the core to the geometrical center of pillows. The geometric mean

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TABLE 3. Magnetic properties*

Natural rernanence orientation

Width Susceptibility Intensity Koenigsberger Sample Zone of zone (emu/cm3) (emu/cm3) Declination Inclination Ratio number number in cm (K) x ( J ) x ( D ) (1) (R)

A1 I 1.2 4.75 0.86 053" 05" 0.031 A2 ::} 8.7

4.25 1.04 045" - 02" 0.424 A3 3.01 0.66 013" 11" 0.038 A4 I1 3.29 0.78 176" 13" 0.041 A5 IIIA 2.05 4.02 167" 10" 0.341 A6 1.98 0.95 175" 48" 0.083

12.1 3.01 A7 12.20 018" 69" 0.701 A8 I I IA 2.40 0.52 161" 21" 0.038 A9 IIIB 3.42 1.80 271" 68" 0.091 A10 IIM 2.67 0.44 226" 40" 0.028 Al l ::::\ 12.1 2.74 0.71 181" 39" 0.045 A12 3.22 0.58 197" 51" 0.037 A13 T I T A 3.22 1.44 160" 20" 0.077 A14 ii 1 5.8 2.88 1.17 187" 42" 0.071 A15 3.29 0.57 306" 64" 0.030

B1 1.5 4.96 1.10 225" 08" 0.038 B2 " 1 6.6 3.94 0.69 174" - 09" 0.030 B3 I1 2.63 1.62 156" 24" 0.107 B4 :;EIkA) 6.1

2.92 1.08 342" 60" 0.064 B5 2.92 11.90 138" 55" 0.708 B6 IIIB 3.21 1.55 019" 59" 0.083 B7 IIIA 1 6.1 2.78 - - - - B8 I11 A 2.63 1.07 060" 24" 0.070 B9 i: 1 6.4 3.79 2.02 083" - 54" 0.092 B10 3.21 2.04 055" - 12" 0.110 B11 I 1.1 4.38 1.57 187" - 54" 0.062

C1 I 1.7 7.94 2.19 358" - 61" 0.048 C2 I1 3.6 3.65 1.26 260" 46" 0.069 C3 :::IkA) 9.3

3.16 0.48 297" - 28" 0.026 C4 3.71 0.40 090" - 22" 0.019 C5 IIIA 3.16 0.46 107" -31" 0.025

C6 1 3.4 4.10 0.19 102" - 23" 0.008 C7 3.94 0.58 004" - 07" 0.026

D l I 1.8 4.24 2.81 039" 25" 0.115 D2 ;:} 8.1

2.26 1.73 190" 26" 0.133 D3 2.41 3.30 308" 40" 0.237 D4 I1 2.99 2.37 112" 56" 0.137 D5 i:;:} 6.6

3.21 0.97 212" 11" 0.053 D6 3.57 0.35 289" - 67" 0.017 D7 IIIA 2.99 0.36 191" - 48" 0.021 D8 HOLLOW

1 2.92 3.32 065" - 36" 0.198 D9 HOLLOW 0.51 7.05 128" 82" 2.390 Dl0 HOLLOW " 1.10 1.99 328" 08" 0.310 Dl1 HOLLOW 1.61 9.01 292" 78" 0.970 Dl2 i::: \ 5.4 3.65 2.03 286" 08" 0.962 Dl3 3.72 3.31 044" 50" 0.154 Dl4 I1 5.3 2.92 1.99 286" - 05" 0.118 Dl5 I 1.1 6.87 1.99 172" 30" 0.050

*Field intensity at site: 0.557 Oe. Volume of sample: 1 in.3 (16 cm". Orientation of the N.R.M. vector with respect to the long axis or flow direction of pillows which is perpendicular to the perpendicular section

C (Fig. 1).

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SEGUIN AND LAUR ENT: PILLOW LAVAS

m A ~ l N INTERMEDIATE ZONE

CORE

ZONES OF PILLOWS

FIG. 6. Variation of intensity J of remanent magnetization with distance from the margin to the center of pillows (see Table 3 for magnetic data).

of the susceptibilities, N.R.M. intensities, and Koenigsberger ratios for the different zones of the pillow lavas studied are presented in Table 4.

The specimens were demagnetized progressively in alternating magnetic fields with ampli- tude up to 300 Oe. The AF demagnetization results are illustrated by two typical examples (specimens A7 and B5). The results, presented in Table 5, show that the remanent magnetization of specimen A7 is relatively stable, while that of specimen B5 is not. The N.R.M. intensity of this specimen decreases very rapidly when increasing the AF demagnetization field and its N.R.M. orientations become rapidly scattered. A knowledge of the degree of stability of the N.R.M. component is essential before a regional paleomagnetic study is undertaken, and our results indicate that the intermediate Zone I1 has the best stability index in the different pillow lavas studied. The stability index used in this study is described by Symons and Stupovski (1973).

Small fragments from each sample were coarsely crushed in an agate mortar to make

FIG. 7. Variation in pillow A of the direction (D, I ) of remanent magnetization with distance along the large diameter of the perpendicular section. The variations in the direction of the N.R.M. vector are given in respect of the long axis or flow direction of pillows, which is perpendicular to the perpendicular section (see Fig. 1). Table 3 gives the key to specimen numbers.

-* MARGIN INTERMEDIATE

ZONE CORE

ZONES OF PILLOWS

FIG. 8. Variation of Koenigsberger ratio R = ~ / k @ l with distance from the margin to the center of pillows (see Table 3 for magnetic data).

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CAN. J. EARTH SCI. VOL. 12, 1975

TABLE 4. Average variations of magnetic properties*

Susceptibility Intensity of N.R.M. component Koenigsberger Zone number K (emu/cm3) x lo-" J (emu/cm3) x ratio R

I 5.79 I1 3.10

IIIA 2.72 IIIB (Geometrical center) 3.54

*Geometric means for 4 pillow lavas.

TABLE 5. Variations of the intensity and orientation of the N.R.M. vector in A.C. demagnetizing field

Orientation Intensity

Sample Zone AF Demagnetizing (emu/cm3) Declination Inclination number number intensity (4) (J) x ( D ) (1)

A7 I11 A 0 12.2 018" 69" 50 7.39 020" 70"

100 3.96 051" 61" 150 1.97 003" 78"

BS IIIA 0 11.9 138" 55" 50 0.72 076" 60"

100 0.49 040" 39" 1 50 0.37 008" - 05"

10 mg powdered rock samples for thermomagnetic analyses. High-field (800-3000 0e) thermomagnetic curves were obtained with an electromagnetic balance recording the variation of the high-field magnetization J with increasing temperature. Fast heating rates were used, raising the temperature from 20 to 900 "C in approximately 25 min. The thermomagnetic curves indicate the presence of a single magnetic mineral having its Curie temperature in the range 563 to 575 "C, with an average at 570 "C. This is the Curie temperature of magnetite, which sug- gests that the main magnetic memory in the pillow lavas studied is present in magnetite. No hematite was detected. We have also to mention that some native iron was found by the thermomagnetic method in samples from the intermediate zone of pillow A. Contamination of our samples by a small amount of metallic iron during the sample preparation is possible. How- ever, the presence of rock-forming native iron cannot be discarded and is the object of our present studies. Deutsch and Rao (1973) apparently have detected the presence of pure or nearly pure iron in pillow lavas from the Betts Cove ophiolitic complex of Newfoundland.

Discussion The structural, textural, mineralogical, chem-

ical, and magnetic features observed lead us to

believe that an oceanic origin for the pillow lavas of the Thetford Mines ophiolite is highly probable. This origin is corroborated by their association with a thin cover of red and green cherty argillites, which are characteristic of a deep sea sedimentary facies.

The petrographic study has shown progressive changes in textures of the rock and morphology of crystals. From the rim to the core of the pillow, the textures vary from originally vitrophyric (?) to arborescent and intersertal porphyritic, sizes of phenocrysts increase as well as their tendency to form cumulophyric clusters, and habits of microlites change from acicular to lath-shaped. These changes indicate that the rate of cooling decreased continuously from the rim to the core. The globulitic structure that has develo~ed in the intermediate zone illustrates well the relation between rate of cooling and degree of crystallization. Globulites of these pillows have presumably formed through the fast arborescent growth of crystalline germs in a viscous isotropic medium undergoing rapid cooling. The relation is demonstrated by the ex~onential increase in size and number of the arborescent globulites from the pillow margin to the innermost part of the intermediate zone (Fig. 3). The globulitic structure vanishes when globulites merge into the core of the pillow where a less significant amount of liquid solidified as a

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I SEGUIN AND LAURE

glass. The petrographic study indicates also that olivine and plagioclase, which both appear as still recognizable microphenocrysts and phenocrysts in the chilled outer zones, were the first liquidus phases at the time of lava extrusion and pillow formation. They were followed by the simultaneous crystallization of pyroxene (now replaced) and more plagioclase, as indicated by the intergrowth of these two mineral phases in the arborescent globulites of the intermediate zone.

The chemical characteristics of the pillows suggest that there are significant differences between them and present oceanic olivine tholeiites. These differences may result from a combination of changes in the composition of the source and in magmatic processes. Green (1973) believed that elements such as K, Ti, and P, which are in very low concentrations in the pillows studied, are inhomogeneously distributed within the upper mantle. Therefore, it can be expected that their concentrations in the magmas formed through partial melting of the pyrolite will also vary in space and time if the source composition is itself variable in these components. The high MgO and relatively low A1,03 contents of the pillows may depend upon the processes of magma formation and history of extrusion. From the results of the experimental work of Green and Ringwood (1967), one can predict that, at constant partial melting, magmas formed under high pressure (> 10 kb) will have high MgO and low A1,03 contents relative to lower pressure magmas (poor in MgO and rich in A1,03) such as those of the present oceanic ridges (Kay et al. 1970). Furthermore, shallow fractional crystallization of the pillow magma, before extrusion, has presumably not occurred because KUNO's index of differentiation, which ranges from 51 to 61 in the samples analyzed, indicates an early stage of evolution. We can provisionally assume, therefore, that the primary magma was picritic to tholeiitic in composition (olivine-hypersthene to quartz-hypersthene normative), that it was generated at high pressure (great depth), and that it did not undergo large scale fractional crystallization before its extrusion at the surface of the ancient sea-floor.

The metamorphic assemblage 'actinolite- epidote-chlorite-albite-quartz' is typical of the greenschist facies. But the recrystallization must have occurred under low lithostatic pressure (2 kb or less) and moderate temperature (around 250°C or less), because the finest volcanic textures

INT: PILLOW LAVAS 1417

of the rock have been preserved and water was not expelled. The metamorphic overprint did not induce changes in grain size and did not obliterate the structures. The primary minerals seem to have been replaced volume by volume without apparent change in size and form. This seems to be similar to the case of the well preserved quench crystals discovered by GClinas and Brooks (1974) in Archean metabasalts from the Abitibi volcanic belt of Quebec. These conditions are significantly different from those of many greenschists. The stability field of actinolite is wide and ranges from the upper part of the prehnite-pumpellyite facies (Coombs 1960) to most of the greenschist facies. Hashimoto (1972) pointed out that the formation of actinolite in different metamorphic basic rocks results from various reactions occurring generally at low temperature, and that the simple replacement of augite by actinolite is a separate reaction, which does not define an actinolite isograd. Epidote is a hydrous silicate whose formation could depend more upon a high water vapor pressure than a high lithostatic pressure. This appears to be documented by the distribution of the epidotes, which are mainly concentrated in the margin of our pillows. We are led to assume consequently that during the recrystallization of these rocks the partial pressure of H,O was high, as suggested by the chloritization of olivine, the amphibolitization of the other mineral components, and the development of an epidote-rich rim, whereas the partial pressure of CO, was low because actinolite developed extensively, but calcite did not (see Miyashiro 1968). These conditions are probably compatible with a recrystallization in a deep sea environment.

The remanent magnetization of the pillow lavas studied is not carried by titanomagnetite or titanomaghemite, which are the usual oxidation products (secondary components of magnetization) of recent weathered oceanic basalts (Irving et al. 1970; Marshall and Cox 1971b, 1972; Lowrie et al. 1973a, b; Fox and Opdyke 1973), but by magnetite. Probably most if not all of this magnetite is not primary, but results from the replacement of the main ferromagnesian minerals by secondary minerals of metamorphism. For example, some magnetite has formed through the decomposition of olivine into a mixture of chlorite, quartz, and magnetite according to reactions such as:

fayalite + oxygen -+ magnetite + quartz forsterite + water + magnesian chlorite

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1418 CAN. J. EARTH SCI. VOL. 12. 1975

What, then, does the remanent magnetization characterize? Was the magnetic imprint acquired at the time of the solidification of the lava or at the time of the recrystallization of the rock when the actual magnetite was formed? Studies of a great number of sea floor basalts have shown that the titanomagnetite originally contained in these rocks, when they are fresh, is rapidly replaced in the oxidized basalts by titanomaghemite. However, the direction of remanence of the newly formed titanomaghemite is not controlled by the direction of the geomagnetic field during the process of oxidation but by the direction of the original thermoremanent magnetization of the rock (e.g. Fox and Opdyke 1973). This could be true also for the case of a low-grade metamorphic recrystallization, because it is evident that our pillow lavas have retained magnetic features very similar to the magnetic signature of recent and fresh oceanic basalts.

During more than 500 m.y. and through alteration and geomagnetic field reversals, our pillow lavas have progressively acquired com- plicated components of viscous magnetization. Their remanent magnetization, which is a vector sum of the magnetization of many domains, is very weak because these domain moments are now almost random. However, a primary magnetic imprint appears to be still discernible. Remanent magnetization and Koenigsberger ratio vary from the pillow margin to its center in a fashion similar to the oceanic L-type pillows described by Marshall and Cox (1971a). Yet, the magnetic susceptibility of our pillows decreases sharply inwards from the rim, whereas Marshall and Cox's L-type susceptibilities show little variation with depth. The direction of the N.R.M. vector in our pillows is consistent and relatively stable within the inner part of the intermediate zone and the outer part of the core. As recorded by the rock textures and mineralogy, the original olivine tholeiite of these pillow lavas was metamorphosed under a regime of low pressure, moderate temperature, and in the presence of water and absence of significant stress. Such conditions are expected to control the recrystallization of basalts near present oceanic ridges, and it is therefore possible that the primary metamorphism of the pillow lavas studied has taken place in the oceanic environment rather than during or after the tectonic emplacement of the ophiolites into their present setting. If this is

right, then the main metamorphic overprint and lowering of the remanent magnetization has occurred soon after the extrusion and solidification of these lavas.

The pillow crust is characterized by high magnetic susceptibility and low remanent magnetization, the intermediate zone by low magnetic susceptibility and low remanent magnetization. The outer part of the core has low magnetic susceptibility and high remanent magnetization, while the geometrical center of pillows has low magnetic susceptibility and low remanent magnetization. Because of the absence of similar studies on metabasaltic pillow lavas of various ages and from different environments, it is only possible to compare this signature with oceanic basalts, but we have no evidence that pillows in other environments do not present this type of magnetic zoning. In the pillow lavas studied, however, the magnetic zoning is matched by a textural, a mineralogical, and a chemical zoning, which are common features of deep sea basalts but are rare or absent in pillowed basalts of other origin.

The intensity of the remanent magnetization and the Koenigsberger ratio of the pillow lavas from the Thetford Mines ophiolite are respectively at least 1000 times and 100 to 200 times smaller than values obtained for similar lavas of present oceans. Nevertheless, progressive demagnetization of our samples has shown that the N.R.M. orientation is relatively stable in the inner part of the intermediate zone and the outer part of the core and therefore that samples from these zones could be used for a regional paleomagnetic study of the ophiolitic complex.

Acknowledgments -

We are grateful to Professor E. R. Deutsch of Memorial University for helpful discussion. The field work and petrological study were supported by grant A8293 from the National Research Council of Canada to R. Laurent. The paleomagnetic work was financed through grant A7070-110 from the National Research Council of Canada to M. K. Seguin.

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I I SEGUIN AND LAURENT: PILLOW LAVAS 1419

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