The structures of two ophiolite massifs, Bay-of-Islands, Newfoundland: A model for the oceanic crust and upper mantle

Tectonophysics, 77 (1981) 1-34 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THE STRUCTURES NEWFOUNDLAND: MANTLE

J. GIRARDEAU Laboratoire

OF TWO OPHIOLITE MASSIFS, BAY-OF-ISLANDS, A MODEL FOR THE OCEANIC CRUST AND UPPER

* and A. NICOLAS

de Tectonophysique,

Nantes

(France)

(Received May 12, 1980; revised version accepted December 4, 1980)

ABSTRACT Girardeau, J. and Nicolas, A., 1981. The structures of two ophiolite massifs, Bay-ofIslands, Newfoundland: a model for the oceanic crust and upper mantle. Tectonophysics, 77: l-34. For a detailed analysis of their main structures (accumulation planes, foliation planes and lineations), the peridotite-gabbro units of Table Mountain and Blow-Me-Down Mountain, Bay-of-Islands, Newfoundland, have been divided into several structural units, each one being characterized by specific microstructures of deformation or accumulation. A fishbone pattern with the accumulation plane in the mafic cumulates dipping toward the ridge and the foliation plane in the ultramafic tectonites dipping away from it, is postulated for the structure of the lithosphere formed at a spreading center. The tectonic structures in the peridotites are related to asthenospheric flow in the uppermost mantle, and the direction of the ridge is inferred from the lineation trend in the peridotites. The structures in the cumulates are related to subsidence of the magma chamber floor during spreading. At the base of the massifs, the tectonic structures in the mafic metamorphic rocks are parallel to those observed. The latter have been deformed at moderate temperatures under high stresses and display mylonitic textures at their contact with the mafic tectonites. From these superimposed textures we infer a kinematic model for the oceanic thrusting of the lithosphere. The direction of thrusting is given by the lineation trend and its sense by the analysis of shear. All these data are combined into a paleogeographic scheme for the Newfoundland Caledonides.

INTRODUCTION

Geophysical, geochemical and petrological studies conducted on the oceanic crust and/or dredged rocks, directly contribute to the understanding of the formation of the oceanic crust and lithosphere. An indirect but important contribution is provided by the study of ophiolite complexes * Present address: Institut de Physique du Globe, Place Jussieu 4, 75230 Paris Cedex 05, France. 0040-1951/81/0000-0000/$

02.50 @ 1981 Elsevier Scientific Publishing Company

2

whose identity with the oceanic crust is nowadays commonly admitted, though one may question whether they represent the normal oceanic situation. Many petrological studies of ophiolites are now available (references in Coleman, 19’77), but structural studies are far less abundant (Ave Lallemant, 1976; Juteau et al., 1977; Dewey and Kidd, 1977; Karson and Dewey, 1978; Nicolas et al., 1979; Prinzhofer et al., 1980). Table Mountain and Blow-Me-Down Mountain, two massifs of the Bay-ofIslands complex (Western Newfoundland), show a complete ophiolitic sequence which represents a slice of oceanic lithosphere thrusted on to the western continental margin of the Canadian shield (Willies and Smyth, 1973). The petrological and geochemical nature of these massifs is well known (Malpas, 1973, 1976, 1978, 1979) but the structure, mainly that of the ultramafic sequence, has been largely ignored. The situation is surprising considering that the internal structure in the massif seems to be homogeneous (Smith, 1951, 1958; Mercier, 1977) and not perturbed by late low-temperature deformations. This complex thus seemed particularly favourable for a structural study. A structural model for the formation of oceanic crust at a spreading center is proposed on the basis of a systematic analysis of the orientations of internal structures in the peridotite-gabbro unit. Moreover, through the study of microstructures and deformations in the basal peridotites and underlying metamorphic rocks, one may characterize the evolution of this lithosphere in a subduction zone env~onment. Accepting the hypothesis that the massifs have not suffered large body rotations after their detachment, a paleogeographic scheme is finally proposed for the Newfoundland Caledonides. GEOLOGICAL

SETTING

With an approximate area of 4000 km2, the ophiolites cover less than 3% of Newfoundland. They are located in three zones (Fig. 1): (I) the Western Platform where they form the upper terms of allochthonous series, (2) the boundary between the Western Platform and the Central Province where they outcrop as discont~uous bodies along a line running no~h~outh from Baie Verte to Cap Ray, respectively and (3) the eastern edge of the Central Province where they represent large autochtonous complexes. The two ophiolite belts on each side of the Central Province play a key role in the interpretation of the structural evolution of Newfoundland in terms of plate tectonics. According to Strong et al. (1974), the structural evolution of the island began during the Cambrian by the opening of a proto-Atlantic ocean, the Iapetus, with formation of oceanic crust. This ocean began to close during the lower Ordovician with the development of an important island-arc volcanism and thrusting of the Bay-of-Islands ophiolites on to the western continental margin of the Canadian shield (Fig. 2). The Bay-of-Islands complex comprises four separate massifs, all in the

0

Ikkm a

ophiolites

Fig. 1. Location of ophiolites in Newfoundland (from Malpas, 1977). Thick solid lines are the boundaries between the Western Platform, the central province and the Eastern Platform. A = Lomond zone - continental (Grenvillian) basement; B = Hampden zone allochthonous, poly-deformed and metamorphosed rocks; C = Fleur-de-Lys zone -highly deformed and metamorphosed continental margin deposits; D = Notre-Dame zone island arcs. E = Exploits zone; F = Botwood zone (E and F - Ordovician and Silurian sediments); G = Gander zone - continental margin sediments capping gneisses; H = Avalon zone - continental and marine volcanics and sediments on platform.

same structural position; they seem to represent erosional remnants of a once continuous slice (Williams, 1975). Two of the massifs, Blow-Me-Down Mountain and North-Arm Mountain (Fig. 3), display a complete ophiolitic suite comprising, from bottom to top, a metamorphic aureole mainly composed of amphibolites, an ultramafic unit grading from minor lherzolites to harzburgites and dunites, a mafic unit of gabbros, a sheeted-dike complex and a pillow-lava unit. The Table-Mountain and Lewis-Hills massifs are incomplete. This study focused on Blow-Me-Down Mountain and TableMountain because the former massif shows a complete ophiolitic sequence and the latter displays beautiful sections of the contact between the metamorphic rocks and the overlying peridotites which even include some, lherzolites.

I

A LATE WADRYNIAN

I

SILUFtlAN-DEVONIAN

1

CARBWJIFEROUS-JURASS~

Ulhanaflc - mdc

Fig. 2. Geological evolution of Newfoundland (from Strong et al., 1914).

PETROLQGY

The four massifs af the Bay-of-Islands complex present a m&axnorphic belt which is structurally below the ultramafic series along their eastern side.

LEGEND LOWER ORDOVICIAN m

Ophiolite

m

Ophiolits

0

Clastic

0

Carbonate

CAMERIAN m

AND CAMBRIAN Mofiter Peridotites

Allochthon Autochthon

AND/OR

Flew-de.Lys

EARLIER Group

B

Burlington

a

Gronodiorite

Plutons

UIIIB

Pro-Elsonion

Basement

-

Limit

of

Group

Allochthon

Fig. 3. Geological setting for the Bay-of-Islands complex (from Mercier, 1977).

(Williams and Smyth, 1973; Malpas et al., 1973). This belt is less than 300 metres in thickness and is composed of garnet and pyroxene granulites, amphibolites and greenschists. These rocks are derived from ocean-floor volcanics through a contact metamorphism with an extreme thermal gradient (2”C/m), yielding metamorphic facies ranging from granulite facies at the contact with the overlying ultramafic rocks to greenschist facies at the base of the unit. This metamorphism is accompanied by an intense deformation which also decreases from the overlying ultramafic rocks downward (Malpas, 1979). This dynamothermal metamorphic aureole has been ascribed to obduction (Williams and Smyth, 1973).

6

The thickness of this is approximately 5000 metres in Table Mountain and 4500 metres in Blow-Me-Down Mountain. It is composed of three different

/ t3oded equont gabbros

:ew layered gabbros

4’

Iioyered abbros with feldspath s c dunltes,wehrlites, 1tractoliter obbros, olivine ,gobbros on *!I onorthosite bonds

’

1 ILithalaglc contact SI_ Itilghly foliated massive dunites Lo ered depleted harzbur ites wit K numerousdunite pate II es pyroxenlte dikes and dunlte lenses

Foliated hOmageneWS horzburgltes

I’

.

Foliated layered horzburgites with numeraus dunlts lenses and pyroxenlte dlkee

Highly folioted layered

<’ horzburgite . .

Extremely foliated lherzolite with minor websterlte bonds

1

Highly defarmed granulttes and amphlbolltes

Fig. 4. Synthetic cross-section for the peridotite gabbro sequence in Table Mountain. Continuous lines represent the foliation S1 and the accumulation pIane SM. The smaller their spacing, the larger the strain.

types of rocks: (1) Lherzolites. These are located only on the southeastern side of Table Mountain (Fig. 4) with a maximum thickness of 300 m. The lherzolites are poor in clinopyroxene, this mineral being mostly located at the rim of orthopyroxenes which also show exsolutions of light brown spinel. The mantle origin of this association has been inferred by Mercier and Nicolas (1975). However, hornblende, phlogopite and corundum are also present in minor abundances in these lherzolites (Malpas, 1976; Mercier, 1977). Mercier (1977) considers that they are not representative of the suboceanic mantle and have undergone HzO, Si, Al, Na and K metasomatism. According to Suen et al. (1979), they represent the residue left after a low degree of partial melting. (2) Harzburgites. These form the major part of the ultramafic unit in Table Mountain, with an average thickness varying from 2600 m on the southeastern side of the massif to 3500 m to the northeastern side. In BlowMe-Down Mountain, this variation is even greater, from 1500 m on the southeastern side to 4500 m to the northwest. The harzburgites are very homogeneous in their mineralogy and geochemistry (Malpas, 1977). They are often rich in orthopyroxene which appears either as large isolated grains or as recrystallized aggregates. As already observed by Mercier (1977), the orthopyroxene is larger (3-5 mm) in the lower part of the section than in the upper part (l-3 mm) where it displays sinuous and concave contours, probably due to an important partial melting. A few lherzolite bands, less than one metre thick, are observed on the northeastern side of Table Mountain. They are rather rich in orthopyroxene (15-30s) and clinopyroxene (5%); both minerals have sinuous contours. Poikilitic and rounded brown spine1 are associated with orthopyroxene. Such lherzolites, interlayered within the upper harzburgites and dunites could have a cumulate origin (Nicolas and Jackson, 1972, Allegre et al., 1973) or result from impregnation (Sinton, 1977; Dick, 1977; Nicolas et al., 1979). Approximately 200 m below the contact with the massives dunites, the harzburgites become strongly banded and rich in dunite patches similar to those described by Loney et al. (1977) and Cassard et al. (1981). These patches, several metres in dimension, have irregular and digitated contacts with the surrounding harzburgites. Foliation and banding in the harzburgites cut through the patches keeping the same orientation. The dunites have a coarse porphyroclastic texture (Mercier and Nicolas, 1975) with rounded reddish spinels also observed by Cassard et al. (1980). The transition to the massive dunites is marked by very depleted harzburgites interlayered with fine dunite bands. This transition zone is also’ rich in pyroxenite dikes and dunite lenses discordant on the foliation plane. (3) Massive &mites. These represent the upper part of the ultramafic sequence. In Table Mountain the thickness of this unit varies from 10 to 20 m to the northeast to 600 m to the southwest. It is much thicker in Blow-

8

Me-Down Mountain varying from 200 m to the northeast to approximately 3000 m to the southeast. The dunites are mainly composed of olivine with minor black, red and, more rarely, green spinel. In the lower dunites, the spine1 is concentrated in seams of small lateral extension. Higher in the dunites, the plagioclase appears first as isolated grains and, in the upper dunites, as narrow lenses with again a limited lateral extension. Clinopyroxene in the dunites may appear with concave contours and magmatic growth twins. Clinopyroxene and plagioclase in these rocks may thus represent the interstitial phase of a cumulate or may result from magmatic impregnation (Sinton, 1977; Dick, 1977; Nicolas et al., 1979). Although massive dunites have been generally interpreted as cumulates (e.g. Jackson, 1971; Greenbaum, 1972; Juteau, 1974; Malpas, 1973, 1976, 1978; Jackson et al., 1975), some authors now consider that there are such residual dunites (e.g. George, 1978; Sinton, 1979; Dick and Sinton, 1979; Nicolas et al., 1979). Bay-of-Islands dunites do not show large stratiform chromites with magmatic spinels (Menzies and Allen, 1974) which are characteristic of ultramafic cumulates (Thayer, 1960) and no magmatic features have been observed in these dunites. On the contrary, the progressive transition between harzburgites and dunites by size reduction of orthopyroxene and the presence of dunite patches inside the highly depleted harzburgites, rather favour a residual origin. This interpretation is also supported by the thickness (3000 m in Blow-Me-Down Mountain) and homogeneity of the massive dunites. The mafic unit

This unit has a constant thickness of about 3000 m in the Blow-Me-Down massif where the mafic part of the ophiolite suite is complete and can be divided in three parts: (1) Basal gabbros. This unit called the “critical zone” by Smith (1958) and Malpas (1973) is about 200 m thick in Table Mountain and 20 m in Blow-Me-Down Mountain. The basal gabbros are composed of feldspathic dunites, wehrlites, troctolites, olivine gabbros, gabbros, and anorthosites which all define a layering. The contact between the different types of rocks is always sharp, thereby yielding isomodal bands (Jackson, 1967,197l) with thickness varying from one centimetre to several metres and with a lateral extension from 1 m to less than 10 m. The layering can be interpreted as a result of fractional crystallization (Jackson et al., 1975) and/or of successive magma injections (Cambell, 1977). The cumulate origin of the gabbros is confirmed by the presence of rare sedimentary features such as slumps and cross-cutting layers, as well as that of rounded xenoliths of troctolite. Olivinerich gabbros are abundant in the lower part of this unit and disappear progressively upward. However, at the very top of this unit, in the central parts of the massif, dunite layers about 3 m thick, suggest the possibility of magma reinjection into the chamber. Church and Riccio (1977) have suggested

9

that the crystallization sequence and the cryptic variations found within the cumulate series result from the combination of at least three different types of basaltic magmas; the magma chamber would have thus operated as an open system with a continuous or semicontinuous injection of magma. (2) Lower gabbros. The average thickness of this unit is 2000 m in BlowMe-Down Mountain and 1300 m in Table Mountain. Here, the gabbros are more homogeneous with a non-cyclic layering of coarse-grained and finegrained gabbros. Brown magmatic hornblende gabbros are found in the upper part of this unit. (3) Upper gabbros. These outcrop in Blow-Me-Down Mountain where they have an average thickness of 1300 m, and have probably been eroded away in Table Mountain. These gabbros are coarse-grained, mostly equant with a discrete layering. They are commonly rich in brown hornblende and green metamorphic amphiboles derived from the original clinopyroxenes. Whereas the high-grade metamorphism located only in the basal metamorphic aureole is commonly ascribed to abduction (Williams and Smyth, 1973; Malpas et al., 1973; Parrot and Whitechurch, 1978; Nicolas et al., 1979; Nicolas and Le Pichon, 1980), this lower grade metamorphism affecting the cumulate gabbros is probably related to thermal or hydrothermal activity (Bonatti et al., 1975; De Wit and Stern, 1976). STRUCTURAL

DATA

The Table-Mountain and Blow-Me-Down Mountain massifs have been studied by applying the method of structural analysis described by Nicolas and Poirier (1976). Oriented samples have been systematically collected, and the orientation of the main field structures was measured, including the compositional banding plane So in peridotites, the magmatic accumulation plane Slvr and magmatic lineation L, in gabbros, the foliation plane S1 and associated lineation L1 and minor structures such as shear planes, partings and fault planes. Nearly 800 stations of measurement have yielded 3000 data for about 600 samples collected. All laboratory data have been processed by using Bouchez’s (1971) computer programs. A rapid survey of the structural maps shows that Table Mountain (Figs. 5 and 6) and Blow-Me-Down Mountain (Figs. 7 and 8) are structurally quite homogeneous. In the Table-Mountain massif, the foliation plane S, in the ultramafic unit has an average strike of N45” with a moderate dip (40”) to the west (Fig. 9A). The trend of the lineation L1 is nearly north-south (Fig. 9) with a moderate plunge (20”) to the north. The magmatic lineation (LM) trend in the mafic unit is approximately the same but shows a larger dip (45”) to the north (Fig. 9D). The magmatic accumulation plane &,, in the gabbros has an average strike of N50” with a stronger dip (60”) to the northwest (Fig. 9C). In the Blow-Me-Down Mountain massif, both the foliation plane S1 and the magmatic accumulation plane Shl (Fig, 9E and F) have an orientation similar to that observed in Table Mountain. However, the spine1

10

f/

_ -

- _ I’ --

_

_OW ME

Fig. 7. Map of the planar structures in the Blow-Me-Down Mountain massif.

lineation L1 in the peridotites (Fig. 9) is 30” west from that in Table Mountain on average. The magmatic lineations L1 in the Blow-Me-Down gabbros are too scattered for a significant comparison with Table Mountain (Fig. 9H). For the detailed analysis of their structure, the two massifs have been subdivided into the five structurally homogeneous units (Fig. 4) described next. The microstructural characteristics of these units will be described further. Basal peridotites These comprise lherzolites and harzburgites and have a total thickness of about 1000 m. The structures observed in this unit result from a very strong

Fig. 8. Lineation map of the Blow-Me-Down Mountain massif.

defo~~~~o~ ~u~e~po~ed on the foliation S1 and Eneation L1. Pclr this reason, the new foliation plane has been called 23; and the lineatian L;. The foliation 23;has an average orientation of 45”N 45” (Fig. 10A) and the lamellar enstatite and spine1 lineation L; exhibits an average orientation of 175”N 25”, (Fig. IOB); The shear sense, determined in thin sections by considering the obliquity between the extinction under crossed polarizers and the trace of the foliation in the X2 plane (Nicolas and Puirier, 1976), is normal (thirteen measurements normal against one inverse) indicating a northward movement of the overlying block with respect to the underling one.

Fig. 9. Structural elements for Table Mountain (A,B, C, D) and Blow-Me-Dawn mountain (E, F, G, Hf. (Lower -hemisphere, equal-area.) A. Si-p = foliation in peridotites (388 measurements; contours: 0.3,2,4,5,lO%,t. 3. L1-p = lineation in peridotites (280 measurements, contours: 1,3, 5,7%). C. SM-g = accumulation plane in gabbros (125 measurements; contours 1.5, 3, 8, 11%). D. L~eg =: magmatic line&ion in gabbros (175 measurements; contours: 0.7, 2, 8, 5%). E. S,-p = foliation in peridotites (121 measurements; contours: 0.8, 5,9,1X, 16%). F. Z&-g = accumulation plane in gabbros (123 measurements: contours: 0,5,1.5, 3,5%). G. _&l-p = line&ion in peridotite (99 measurements; contours: 1,3,5,8,X3%). H. &-g E magmatic line&ion in gabbros (89 measurements; contours: 1, 2, 8, 6%).

Lower peridotites These harzburgi$es have an average thickness of 2400 m. They have beert deformed at moderate temperatures and show a well-defined foliation plane S1 which has an average orientation of 40”N 31” (Fig. 1OC). The lineation

Fig. 10. Structural elements, for the Table-Mountain massif units. Convertion as in Fig. 9. A. S;-p = foliation plane in basal peridotites (49 measurements; contours: 2, 5, 8, 20%). B. I,;-p = lineation in basal peridotites (43 measurements; contours; 4,6, 12%). C. Sr-p = foliation plane in lower peridotites (173 measurements; contours: 1, 2,4, 7, 11, 15%). D. Li-p = lineation in lower peridotites (143 measurements; contours: 1, 2, 5, 8, 13%). E. SI-p = foliation plane in upper peridotites (146 measurements; contours: 1, 2, 5, 7%). F. Li-p = lineation in upper peridotites (142 measurements; contours: 1, 2, 4, 6%). G. So-p = orthopyroxenite and dunite layers in peridotites (102 measurements; contours: 1, 2, 6, 12, 16%). H. Sr-g = foliation in basal gabbros (76 measurements, contours: 1, 4, 8, 12%). I. Li-g = lineation in basal gabbros (118 measurements, contours: 1, 2,6, 10%). J. SM-g = accumulation plane in lower gabbros (49 measurements; contours 4, 6, 10%). K. LM-g = magmatic lineation in lower gabbros (56 measurements, contours 2, 4, 7%).

16

L1 (spine1 lineation and the direction perpendicular to the tabular enstatite lineation; Darot and Boudier, 1975) has an average orientation of 15’N 25” (Fig. 10D). The shear sense is now inverse (twelve measurements inverse against four normal); hence, the overlying rocks moved southward relative to the underlying ones. Upper peridotites These comprise harzburgites and dunites and have an average thickness of 2200 m. The peridotites have been strongly deformed at high temperatures. The foliation plane S, has an average orientation of 25”N 40” (Fig. 10E) and the lineation L, (with the same definition as above) an average orientation of 160”N 25” (Fig. 10F). The shear sense is normal again (fourteen measurements normal against four inverse). Everywhere in the massif, the compositional banding is parallel to the foliation plane measured in the peridotites (Fig. 1OG). Basal gabbros These have a thickness of less than 200 m. These gabbros have been plastically deformed and have a foliation plane S1 whose orientation is approximately 30”N 40” (Fig. 101) and an aggregate and pyroxene lineation L1 with an orientation of nearly 0”N 45” (Fig. 10H). Lower gabbros These have an average thickness of 1300 m and they are not deformed. The accumulation plane S1, has an average orientation of 60”N 75” (Fig. 10K) and the magmatic pyroxene and plagioclase lineation LM, an average orientation of 55”N 35” (Fig. 10). The main results on Table Mountain and Blow-Me-Down Mountain are summarized in Table I. NW-SE sections through the two massifs (Figs. 11 and 12) illustrate how analogous are the basal metamorphic contacts and the lithologic contacts S, between the peridotites and the cumulate gabbros. Another important feature is the rotation of the magmatic accumulation plane Shl with respect of the lithologic contact planes Sr,, as already mentioned by Casey et al. (1979). Dikes are abundant in the upper harzburgites. Pyroxenite dikes are most commonly without dunite walls. The numerous dunite lenses are thought to represent residual walls after a partial melting relating to pyroxenite and gabbro dike injections (Boudier and Nicolas, 1972,1977). The dunite lenses tend to be parallel to the foliation plane S1 in the surrounding rocks (Fig. 13A). The pyroxenite and gabbro dikes show no preferred orientation (Fig. 13B, C). The chronological sequence of the dikes, based on intersection criteria, is dunites, pyroxenites and gabbros. In the cumulate

m

I

DOWN

IS00

MOU

.

Mom

2200 .

24w .

loaom

200 .

Ihiekm**

XOUNTAIN

of the main structural

BLOW HE

TABLE

Summary

TABLE

and petrological

data of the Table-Mountain

and Blow-Me-Down

Mountain

massifs

254 TO 375 b

NW

NW

O-

700m-

O-

2000m

20m 25OOm

hnrzburgites

horzhrgites

IOOOm

very deformed

SE

WOm

200m

rnetomorphic

SE

Fig. 11. NW-SE cross-sections through the Table-Mountain massif from top to bottom, the sections are for the northern, central and southern parts of the massif respectively. sed. = sediments; dun. = dunites; harz. = harzburgites; met. = metamorphic aureole; Zherz. = lherzolites; f = fault; p. = peridotites; S.L. = lithologic contact plane; dashed line = magmatic accumulation plane; solid line = foliation plane.

C

B

A

NW

19 SE

NW ~00~

Jsheeted

dyke

I

0’ 6%

35OOm

4CQOm

dunites

harzburglta

harzbuqites

J 26cQm

283Om

IWOm

Lll

Fig. 12. NW-SE cross-sections through the Blow-Me-Down Mountain massif. Convertions as in Fig. 11. Sdc = sheeted dike complex.

A

C

D

Fig. 13. Dike and lenses in Table Mountain (structural reference system: Sr : plane of projection and Lr : solid dot. A. Dunite lenses (66 measurements; contours: 2,4, 5%). B. Pyroxenite dikes (118 measurements; contours: 1, 2,3%). C. Gabbro dikes (21 measurements; contours: 5,10,20%). D. Pegmatitic and plagioclase gabbro dikes (50 measurements; including Blow-Me-DownMountain data; contours; 2,4,6%).

20

gabbros, plagioclase dikes and pegmatitic gabbro dikes are common. They are probably related to a late magmatic remobilization along small faults and have no preferred orientation (Fig. 13D). Intrusive dikes of diabase postdate them and seem to be either parallel or at high angle to the magmatic accumulation plane S,. MICROSTRUCTURAL

DATA

The structural units described above are characterized by specific microstructures of deformation or magmatic accumulation. The analysis of petrofabric data and of dislocation microstructures and the measure of the shape and elongation of minerals show an evolution of the deformation from bottom to top of the ophiolitic series.

Basal perido tites The contact between the metamorphic rocks of the thermal aureole and the overlying peridotites is marked by a mylonitic band less than 10 m thick. The analysis of a fine-grained porphyroclastic lherzolite (Fig. 14A) located at approximately 40 metres above the contact, shows that this rock has been deformed in a rotational regime with a dominant [ 1001 (010) slip system. Such an elongation of minerals and the high degree of preferred orientation for olivine, indicate that these, rocks have been submitted to very large strains. Stresses have been estimated from the neoblast size using Mercier’s (1977) and Post’s (1973) calibrations. Though these techniques provide crude absolute values (Poirier and Guillope, 1979), they yield useful relative stress estimates. In the basal unit, the stress would thus vary from more than 1000 bars at the basal contact to approximately 500 bars at the top of the unit.

Fig. 14. Olivine preferred-orientation for the principal microstructural types of peridotites in Table Mountain (normalized projection: solid line: foliation; solid dot: lineation). A. Fine-grained porphyroclastic lherzolite from the basal zone (96 measurements; contours: 1,4,6,10%). B. Finegrained porphyroclastic harzburgite from the lower zone (50 measurements; contours: 1,4,8,10%). C. Equant equigranular dunite from the upper zone (101 measurements; contours: 1,4, 6, 8%). D. Olivine gabbro with ribbon texture from the basal zone (102 measurements; contours: 1, 2,4, 5, 6%). E. Troctolite with ribbon texture from the basal zone (97 measurements; contours: 1, 2, 3,4%).

_A_

-

0..

_C-

_

D-

22

Lower perido tites Fine-grained porphyroclastic textures are common in this unit. Orthopyroxenes are not elongated and spinels have a length/width ratio less than two. However, the strong preferred orientation suggests that the deformation was large. A rotational deformation regime is inferred from the overall symmetry, with the [ 1001 (010) slip system being dominantly activated. Upper perido tites Dominant fine-grained porphyroclastic textures are observed in the harzburgites of this unit. The textures of dunites are equigranularequant or more rarely tabular. Spinels are elongated with a length/width ratio between 2 and 10; aligned grains may be inclusions in large olivine crystals. The preferred orientation for olivine shows that the upper peridotites have been deformed at high temperature in a rotational regime with a dominant [ 1001 (010) slip system (Fig. 14C). The elongation of spinels and the strong olivine preferred orientation indicate that deformation was important in this unit. Stress estimates are in the range of 175-275 bars. Basal gab bros This unit, with a thickness of less than 200 m, is deformed only along its contact with the underlying peridotites. Two major types of microstructures are observed in the gabbros: ribbon and equant textures. The ribbon textures are very common in wehrlites and trotolites and scarcer in olivine gabbros. Plagioclase, clinopyroxene, and olivine crystals are apparently undeformed, exhibiting polygonal contours. They form a regular banding in gneissose rocks with a strong aggregate lineation. Such a facies suggests an origin by ductile deformation. Analysis of preferred orientation of olivine in two specimens (Fig. 14D, E) indicates that these rocks have been plastically deformed at high temperature with a dominant [ 1001 (010) slip system. The unstrained aspect of the minerals is accounted for by annealing at high temperatures. The equant textures are common in olivine and plagioclase gabbros. They are typical undeformed adcumulate textures (Wagner et al., 1960), with cumulus plagioclase and clinopyroxene and interstitial olivine. Therefore, the limit between drastically deformed facies and undeformed ones lies within the basal gabbros at the transition from ribbon to equant textures. Lower and upper gabbros These homogeneous gabbros have adcumulate textures. In Blow-Me-Down Mountain, orthocumulate textures are observed in some brown-hornblende gabbros. The upper gabbros are not deformed and the lower gabbros only

23

Fig. 15. Preferred orientation of the shear zones. Structural reference system: SL (plane of projection) and lineation (solid dot) (39 measurements; contours: 2, 5,9%).

exhibit local shear zones with two preferred orientations. One family is parallel to the layering (Fig. 15) with a normal sense of shear. These shear zones are generally less- than l-m thick and are composed either of mylonitic gabbros or of foliated amphibolites. The shearing occurred after consolidation of the gabbro at moderate temperatures (500-6OO”C), probably during hydrothermal circulation in the vicinity of the ridge axis (Girardeau and Mevel, 1981). The other family of shear zones is more oblique to the layering (Fig. 15). They are narrow (l-15 cm) and also composed of mylonitic gabbros or foliated amphibolites formed at lower temperatures. INTERPRETATION

AND DISCUSSION

It has been assumed that the Bay-of-Islands massifs represented a slice of oceanic lithosphere thrust on to the western continental margin of the Canadian shield. The remarkable homogeneity of the structures in the TableMountain and Blow-Me-Down-Mountain massifs make it possible to construct structural models for this oceanic lithosphere. Accepting the hypothesis that the tectonic emplacement of the Bay-of-Islands ophiolites has not modified its initial oceanic orientation, these models can be used for paleogeographic reconstitutions. The structural information contained in the various units allows a.separate analysis of (1) the creation of oceanic lithosphere and (2) the thrusting in a subduction zone environment. The geometry of the structures of the Bay-ofIslands ophiolites suggests that they have been formed along a linear accretion zone and paleogeographic considerations indicate that they originated in a true ocean rather than in a marginal basin.

24

Structural model for creation of oceanic lithosphere at a spreading center The model is based on the approach of Juteau et al. (1977) and of Prinzhofer et al. (1980), in defining first a paleo-horizontal plane and then the direction of the ridge axis. The trace of the major lithological planes inside the cumulates, which are also parallel to the dunite-gabbro contact plane, is used as a paleo-horizontal plane because it corresponds to the horizontal stratification in the oceanic crust and to the crust-mantle boundary. All structural data have thus to be rotated by approximately 50 degrees. The lineation L,, which is the trace of the plastic flow direction in the tectonite foliation (Nicolas et al., 1972), is presumed to be perpendicular to the ridge direction as was also assumed by Prinzhofer et al. (1980). Though it can be exceptionally different (Vogt and Johnson, 1975; Bird and Phillips, 1975), the plastic flow direction in the mantle generally makes a high angle

NE

PL SD cUG LG -

Fig. 16. Structural

model for the creation of oceanic lithosphere

at a spreading center.

PL = pillow lavas; SDC = sheeted dike complex; UG = upper gabbros; LG = lower gabbros; BG = basal gabbros; UP = upper peridotites; LP = lower peridotites; MC = magma chamber; SM = magmatic accumulation plane; SL = lithologic contact plane; Sr = foliation plane; L 1 = lineation. The crustal part is similar to that proposed by Dewey and Kidd (1977). The rotation of the magmatic accumulation plane SM is related to the subsidence of the magma chamber floor due to the rising asthenosphere and spreading. The foliation Sr in the upper mantle is at first parallel to the main lithologic contact plane SL in the cumulates (horizontal), then dips progressively downwards to about 30 degrees. The sense of shear in the lower peridotites moves away the newly created lithosphere with respect to the underlying asthenosphere. The opposite sense of shear in the upper peridotites relative to the basal gabbros could result from subsidence of the magma chamber floor.

25

with the ridge direction, The compared pattern of seismic anisotropy in ocean and in ophiolites (Peselnick and Nicolas, 1978), also favours this hypothesis. The structural model for the oceanic lithosphere (Fig. 16) is obtained through rotation of the lithological contact plane Sr, to an horizontal position (Fig. 17), using the direction perpendicular to the lineation as the ridge direction. The side of the ridge where the crust has been generated is determined by the inclination of foliation away from the ridge in the mantle (see below). Because the dip in the cumulates is in the opposite direction it is concluded, as was also inferred by Dewey and Kidd (1977), that the accumulation and lamination planes in the crustal cumulates are dipping toward the magma chamber. In our case, this magma chamber would have been permanent and thus related to a fast spreading ridge: had it been transient, chilled margins representing the limits of successive magma chambers should have been observed within the cumulate gabbros. In the peridotites, the foliation S1 is parallel to the contact plane SL between peridotites and gabbros in the vicinity of this contact. Downwards, it progressively dips away from the ridge at about 30 degrees. In order to explain the active plastic flow in the mantle at the ridge, this inclination has

Fig. 17. The main structures in Table Mountain and Blow-Me-Down-Mountain after rotation of SL to the horizontal. The ridge direction is supposed to be perpendicular to the average lineation in the upper and lower peridotites. Sr = foliation plane in the upper peridotites; Sz = foliation plane in the lower peridotites; SM 1 = foliation plane in the basal gabbros; SM~ = accumulation plane in the lower gabbros; SM~ = accumulation plane in the upper gabbros; Lt = lineation in the upper peridotites; 152 = lineation in the lower peridotites; s.d.c. = sheeted dike complex (Salisbury and Christensen’s 1978, data); S,, = contact plane between ultramafics and metamorphic rocks. Two important results appear on the nets: (1) the parallelism between the presumed ridge axis and the intersection between the average accumulation plane in gabbros and foliation in peridotites and (2) the parallelism between the peridotites-gabbros contact SL and the peridotiteslnetamorphic rocks contact SRM.

26

to be parallel to that of the 1000°C isotherm taken as the limit between lithosphere and asthenosphere (Nicolas, pers. comm., 1980). Thus, the ridge axis is normal to the dip of the foliation in the peridotites. Such a disposition is now supported by most of the recent thermal ridge models (e.g. Bottinga and Allegre, 1973; 1976; Sleep, 1975; Tapponnier and Francheteau, 1978). This inclination is probably the best criterion by which to recognize on which flank of the ridge the lithosphere was created. Here the southeast dip of the foliation S, and flow plane in the upper mantle relative to the ridge direction shows that the considered lithosphere was created on the southern flank of a SW-NE trending ridge. In the cumulates, the accumulation plane Sill and the foliation plane S1 are inclined in the opposite sense at about 30 degrees. In cross-section, such a “fishbone” pattern, with the accumulation plane Slvrin the cumulates dipping toward the ridge and the foliation plane S, in the upper mantle dipping away from it, is perhaps a common feature in oceanic lithosphere. In the lower peridotites, the shear sense inferred for the high-temperature plastic flow under moderate stresses shows that the newly created lithosphere moves away from the ridge relative to the underlying asthenosphere. Opposite shear senses are observed in the upper peridotites deformed at high temperature and in the gabbros deformed only in narrow shear zones at moderate temperature. The later shear zones may be related to subsidence of the magma-chamber floor during spreading (Dewey and Kidd, 1977). However, the opposite sense of shear in the tipper peridotites is not explained. In the restored lithosphere, dunite lenses are subhorizontal whereas pyroxenite and gabbro dikes have no preferred orientation. On the basis of Williams and Malpas (1972) and Salisbury and Christensen’s (1978) data the dolerite dikes of the sheeted-dike complex in the Blow-Me-Down-Mountain massifs are subvertical with their azimuth at a high angle to the ridge axis as defined here (Fig. 17). This observation is not yet understood and raises the problem of the doleritedike orientation for the sheeted complex relative to the ridge axis. The observation of open fissures oriented parallel to the rift valley axis in submerged ridges (Arcyana, 1975; Ballard et al., 1975; Fornari et al., 1978) or in emerged segments of ridges such as in Iceland (Daignieres et al., 1975) or the Asal rift in Afar (Stieltjes et al., 1976), suggests that dikes are injected in tension fractures parallel to the ridge axis. This hypothesis is generally accepted (e.g. Moores and Vine, 1971; Cann, 1974; Strong and Malpas, 1975) since all the dikes seem to be parallel in most of the sheeted complexes. For the formation of sheeted dike complexes, Kidd and Cann (1974) have also proposed a discrete off-axis volcanic model and a diffuse injection model in which dikes, injected throughout the whole complex, have no preferred orientation. However, such a pattern has not yet been described, possibly owing to the scarcity of detailed structural studies in sheeted-dike complexes.

27

Kinematic model for oceanic thrusting Parrot and Whitechurch (1978), Cakir et al. (1978) and Malpas (1979) have shown that basal metamorphic rocks associated with ophiolites were derived from ocean floor formations during over-thrusting of the peridotites. Considering the physical condition for basal peridotites in ophiolites and the geophysical conditions in trench environments, Nicolas and Le Pichon (1980) have proposed a model for oceanic thrusting in front of a subduction zone. The high temperatures (800”-.9OO”C) required for metamorphism of the upper part of the down-going slab and for plastic deformation in the overriding peridotites are obtained by shear-strain heating. The temperature of the peridotites of the overriding slab at the depth of initial decollement was around 600” C. In our case, the decollement was initiated at about 15 km beneath the sea floor. Taking into account the thermal structure of the oceanic lithosphere, the lithosphere considered here had to be less than 30 m.y. old. The time of crystallization of the Bay-of-Islands gabbros has been recently determined by Jacobsen and Wasserburg (1979), as being 508 f 6 m.y. and 501 + 13 m.y. by two Sm-Nd mineral isochrons for Blow-Me-Down Mountain gabbros. This result is in good agreement with the 504 f 10 m.y. isotopic age of zircons in trondhjemites proposed by Mattinson (1976). The metamorphic aureole has been dated as 460 + 5 m.y. by 40Ar/39Ar spectra of hornblende (Dallmeyer and Williams, 1975), and as 470 m.y. by the K/Ar method applied to amphiboles (Archibald and Farrar, 1976). Malpas (1979) has shown that the metamorphic history of the aureole has involved several successive stages of deformation and annealing. As the amphiboles are secondary minerals, dating them gives only a minimum age for the high-grade metamorphic event. A period of 10 to 1 m.y. corresponding to the duration of motion of the slabs, could have separated it from the initiation of thrusting if their velocity had been between 1 and 10 cm/yr and the displacement 100 km. Thus, it is concluded that a period of 20-47 m.y. separated the time of crystallization of the Bay-of-Islands cumulates from their oceanic thrusting. The thrusting may have thus originated, as for a young lithosphere, in front of a subduction zone as proposed by Nicolas and Le Pichon (1980). The fact that, in a reconstructed lithosphere section (Figs. 4 and 17), the thrusting plane is subhorizontal as in present subduction zones at the depth here considered, also favours this hypothesis. Such a model is also in good agreement with the scheme presented by Jamieson (1979) for the formation of the metamorphic aureole underlying the White-Hills peridotites at Hare Bay. In our model (Fig. 18), the direction of thrusting is given by the direction of the lineation L; in the basal peridotites and its sense by the analysis of shearing. Here, the direction was N160-N170”‘and the vergence to the northwest.

28 THRUST PLANE

EASTERN CONTINENTAL MAFKW

CRUSTAL FORMA Fig. 18. Schematic model for the oceanic thrusting of the Bay-of-Islands ophiohtes. The thrusting was initiated in front of a SE dipping subduction zone producing the metamorphism of the underlying crustal formations. The lineation ~5; observed in the basal peridotites gives the direction of thrusting and the shearing its sense.

Fig. 19. Schematic diagram of the Bay-of&lands complex showing different possible orientations of the paleo-ridge, Ln Fig. A the ridge is parallel to the orientation of dikes within the sheeted dike complex SD.; (I = Christensen and Salisbury’s (1979) data; 2 and 3 * Williams and Malpas (1972) data; RI, R2, R3 are the corresponding ridge orientations; T.F. is the orientation of the observed transcurrent fault; P.T.F. of the presumed transform fault and R4 of the corresponding ridge according to Karson and Dewey’s (1978) data. In Fig. B the ridge is supposed perpendicular to the lineation in peridotites. Arrows give the trend of the Iineation, and RI, Rz, R3 show the corresponding ridge orientations. (a = data from Girardeau, 1979; b s data from Mercier, 1976; c = data from Christensen and Salisbury, 1979).

29

PALEOGRAPHY

The results presented here give new information for paleogeographic reconstructions provided that the ophiolite massifs have not suffered large bodily rotations after their thrusting: (1) Puleo-orientation of the ridge. It was oriented N45”-N70” on the basis of the flow-lineation trend in the peridotites (Fig. 19B). Different orientations could be obtained when using other criteria to determine its paleo-orientation (Fig. 19A). (2) Primitive position of the thrusted lithosphere relative to the ridge axis: The Bay-of-Islands ophiolites were formed on the southeastern flank of a NE-SW trending ridge according to both our criterion (inclination of the foliation) and Dewey and Kidd’s (inclination of the accumulation planes). (3) Direction and sense of the oceanic thrusting: The analysis of the lineation trends and the sense of shear in the basal peridotites, indicate a NNWthrusting along a SSE-dipping thrust plane. From this it is thus concluded that the oceanic lithosphere originated in front of a ME-dipping subduction zone. As the geochronology suggests that the lithosphere was between 20 and 47 m.y old at the time of thrusting, the thrust had to be initiated close

Western A

Platform

I+,++ ++ 3==

Bay of Islands

Eastern

___ --_

Platform

*

1c-

ophiolites

Fig. 20. Schematic plate tectonic model of-the Newfoundland Caledonides (from Strong et al., model modified). A. In late Hadryan, after a period of continental rifting marked by extrusions of tholeitic lavas from NE-SW fissures in the Western platform (Strong and Williams, 1972), oceanic crust was formed at a NE-SW-trending ridge and consumed along a SSE-dipping subduction zone located on the Eastern margin of the ocean, B. In Cambrian, a slice of oceanic crust was overthrust along a SSE dipping thrust plane seaward of the subduction zone; arc insular volcanism began whereas a marginal basin possibly developed eastward of the subduction zone. C. The Ordovician time was marked by the creation of island arc formations and by the closing of the ocean with possible development of successive oceanic thrusts westward of the subduction zone. The final emplacement of the Bay-of-Islands ophiolites on to the western continental margin has probably resulted from the collision of the island arc with the western margin.

30

to the ridge, perhaps on the ridge itself. These paleogeographic conclusions are remarkably consistent and improve the qualitative evolutionary model (Fig. 20) proposed by Strong et al. (1974) for Newfoundland. ACKNOWLEDGEMENTS

The authors wish to thank D.F. Strong, J.C. Mercier and J.C. Allegre for constructive discussions and P.Y.F. Robin for reviewing the manuscript. Technical support was provided by A. Cossard and C. Mercier-Ronnat for the illustration and by G. Branchu for the thin sections. Financial and logistical support was obtained from Saint-John’s University of Newfoundland through D.F. Strong and from the Centre National de la Recherche Scientifique (ERA 547, ATP Geodynamique). This is Institut de Physique du Globe contribution NS 472. REFERENCES Allegre, C.J., Montigny, R. and Bottinga, Y., 1973. Cortege ophiolitique et cortege oceanique, gdochimie comparde et mode de g&&e. Bull. SOC. GBol. Fr., (7), XV (5-6): 461-477. Archibald, D.A. and Farrar, E., 1976. K-Ar ages of amphiboles from Bay of Islands ophiolite and Little Port Complex, western Newfoundland. Can. J. Earth Sci., 13: 520529. Arcyana, 1975. Transform fault and rift valley from bathyscaph and diving saucer. Science, 190: 108-116. Ave Lallemant, H.G., 1976. Structure of the Canyon Mountain (Oregon) ophiolite and its implication for sea floor spreading. Geol. Sot. Am., Spec. Pap., 1973: 49 pp. Ballard, R.D., Bryan, W.B., Heirtzler, J.R., Keller, G., Moore, J.G. and Van Andel, T.J., 1975. Manned submersible observations in the FAMOUS area: Mid Atlantic ridge. Science, 190: 103-108. Bird, P. and Phillips, J.D., 1975. Oblique spreading near the Oceanographer fracture. J. Geophys. Res., 80129: 4021-4027. Bonatti, E., Honnorez, J., Kirst, P. and Radicatti, F., 1975. Metagabbros from the MidAtlantic ridge at 06’N: contact-hydrothermal-dynamic metamorphism beneath the axial valley. J. Geol., 83: 61-78. Bottinga, Y. and Allegre, C.J., 1973. Thermal aspect of sea-floor spreading and the nature of the oceanic crust. Tectonophysics, 18: l-17. Bottinga, Y. and Allegre, C.J., 1976. Geophysical, petrological and geochemical models of the oceanic lithosphere. Tectonophysics, 32: 9-59. Bouchez, J.L., 1971. Exemples de Traitement automatique des Don&es numeriques en Gkologie structurale et en Petrologic. These 38me cycle, Nantes, 117 pp. Boudier, F. and Nicolas, A., 1972. Fusion partielle gabbroi’que dans la lherzolite de Lanzo (Alpes Piemontaises). Bull. Suisse Min. Petrol., 52: 39-56. Boudier, F. and Nicolas, A., 1977. Structural controls on partial melting in the Lanzo peridotites. In: H.J.B. Dick (Editor), Magma Genesis. Oregon, Dep. Geol. Miner. Ind., Bull., 96: 63-78. Cakir, U., Juteau, T. and Whitechurch, H., 1978. Nouvelles preuves de I’dcaillage intraocdanique precoce des ophiolites tethysiennes: les roches metamorphiques infra-peridotiques du massif de Pozanti-Karsanti (Turquie). Bull. Sot. GBol. Fr., (7), XX: 6170.

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