Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: Formation during the final stages of a spreading centre?

31

Tectonophysics, 206 (1992) 3 l-53

Elsevier Science Publishers B.V., Amsterdam

Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: formation during the final stages of a spreading centre? Cider

Lkpartment of Earth Sciences, Memorial

Suhr * University of hkwfoundland,St. John’s,

Njld. AlB 3X5, Canada

(Received May 27, 1991; revised version accepted October 15, 1991)

ABSTRACT Suhr, G., 1992. Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: formation during the final stages of a spreading centre? Tectonophysics, 206: 31-53. The structure of upper mantle rocks in ophiolites reflects the high-temperature flow beneath an oceanic spreading centre, locally overprinted by detachment-related strain. The structure of the mantle section in the ophiolitic Table Mountain massif, Bay of Islands Complex (BOX), Newfoundland, was investigated in detail. It is correlated with other massifs in the BOIC and the spreading-related mantle flow history is reconstructed. Six structural units can be recognized on the basis of changes in the attitude of the high-tem~rature mineral stretching lineation. The relative age relationships between the structural units can partly be established. They support the plate-thickening model for oceanic lithosphere formation. The structural history of the mantle section suggests that early plate-driven flow was followed by forced flow, underplating of an off-axis diapir to the base of the lithosphere, and detachment. This complex history is explained in a model of a decreasing spreading rate which tracks the waning stages of spreading before detachment of the ophiolite. During this final history, the rate of spreading was decoupled from the rate of mantle upwelling so that a large forced flow field could build up. In the Blow Me Down (BMD) massif of the BOIC, all spreading-related flow structures are parallel to the ridge, suggesting that the structures and the massif were frozen in near the ridge itself. The presence of several kilometres of massive dunites in the BMD massif might indicate that in the final stages of spreading, melt production was higher than could be accommodated by spreading so that melt ponded at the base of the crust. Lherzolites occurring at the base of the BOIC are considered as product of non-adiabatic mantle upwelling prevalent in the final stages of spreading.

Introduction The Bay of Islands Complex (BOIC) is a wellpreserved Ordovician ophiolite in western Newfoundland. It represents vestiges of the Iapetus or related marginal basins (Church and Stevens, 1971; Dewey and Bird, 1971; Williams, 1973). It is exposed in four massifs which are called, from north to south: Table Mountain (TM), North

* Correspondence to: G. Suhr, Institut fir Mineralogie und Petrographie, Universitlt zu K&t, Ziilpicher Str. 49, 5000 K&r 1, Germany. 0040-1951/92/$05.00

Arm Mountain (NAM), Blow Me Down Mountain (BMD), and Lewis Hills (LH). Within the eastern part of each of these massifs, upper mantle peridotites are extensively exposed (Fig. la). Previous work in upper mantle rocks on ophiolites has shown that the peridotites should be considered as tectonites whose structure reflects the high-temperature flow beneath an oceanic spreading centre. The spreading-related structures have been locahy overprinted during detachment of the ophiolite. A knowledge of the orientation of paleogeographical elements of the oceanic spreading centre (e.g., paleo-horizontal, trend of ridge axis> is required in order to interpret the geometry of flow (Juteau et ai., 1977;

0 1992 - Elsevier Science Publishers B.V. Ah rights reserved

G. SUWR

32

_-I 4 a

t-lumber Arm Supergroup Skinner Cove Fm.

m

COASTAL COMPLEX (CC)

m

trondjhemitic intrusives in CC

BAY OF ISLANDS COMPLEX a

parallochthonous

sed. rocks

volcanics sheeted dykes, with mean trend D

gabbros

i

massive dunites and ultramafic cumulates

m

harzburgite and lherzolite

m

metamorphic sole 0

kilometres

-_ BkwMeDown

Fig. 1. (a) Geology of the Bay of Islands work. (b) Paleogeograph~c

Ophiolite,

after Casey et al. (1983), Karson

setting proposed

for the Bay of Islands

Nicolas and Violette, 1982; Ceuleneer et al., 1988; Nicolas, 1989). Several paleogeographic reconstructions have been proposed for the BOIC. The models of Karson and Dewey (1978) and Casey et al. 0983) suggested that the four massifs of the BOIC were aligned in the oceanic environment in an orienta-

(19841, Williams

Ophiol~te

and Cawood

(1989) and this

by Casey et al. (1983).

tion nearly normal to, and to the north of, the spreading axis (Fig. lb). Consequently, the four massifs would represent oceanic lithosphere of different age: the TM massif would represents the oldest, the LH massif the youngest lithosphere. Further relative age relationships may be de-

UPPER

MANTLE

OPHIOLITES

IN BAY

OF ISLANDS

OPHIOLITE

rived from the Parker and Oldenburg (1973) model. In this model for oceanic lithosphere formation, the thickness of the lithosphere grows with age and increasing distance from the ridge axis (plate-thickening model; Forsyth, 1977). If the plate-thickening model applies to the BOIC, then upper mantle rocks located at shallow levels beneath the Moho have been frozen in structurally earlier (i.e. when the lithosphere was closer to the ridge axis) than upper mantle located several kilometres below the Moho (Rabinowicz et al., 1984, 1987; Ceuleneer et al., 1988; Nicolas, 1989). Upper mantle rocks of structurally different ages may thus be preserved within a single massif and between different massifs of the BOIC. By looking at the structural variation of upper mantle rocks in the BOIC, it should be possible to reconstruct a significant period of their high-temperature flow history. Previous work

The BOIC represents the highest structural slice within the Humber Arm Allochthon. The allochthon was tectonically assembled and emplaced onto the North American Craton during the Taconian orogeny (Williams, 1973, 1975; Williams and Cawood, 1989). Reactivation and gravitational sliding of the BOIC took place during the Acadian orogeny (Cawood, 1990). To the west of the three northern massifs of the BOIC is the Little Port Complex (LPC; Williams, 1973). It was assigned to the formation of an island arc (Williams and Payne, 1975; Malpas, 1979b; Searle and Stevens, 1984; Jenner et al., in press) or was interpreted as the high-level expression of a fracture zone (Karson, 1984). According to Karson and Dewey (1978) and Karson (1984), the deeper level part of the same fracture zone is exposed in the western LH, and both the LPC and the western LH together are referred to as the Coastal Complex (CC; Fig. la). In the latter model, a high-temperature contact between the CC and the BOIC is preserved in the LH massif. Mantle peridotites are abundant only in the BOIC. The harzburgites and lherzolites of the BOIC are consistent with an origin as a residuum

33

of intense partial melting (Irvine and Findlay, 1972; Malpas, 1978). The predominance of lherzolites near the base of the mantle section has, however, puzzled previous workers (Church, 1972; Church and Riccio, 1977; Malpas, 1978; Girardeau and Nicolas, 1981; Spray, 1984; Girardeau and Mercier, 19881. Massive dunites are developed in a variable thickness at the top of the mantle sequence. In the two southern massifs of the BOIC, they reach the remarkable thickness of 3 km (Girardeau and Nicolas, 1981; Karson et al., 1984; Karson and Elthon, 1987). In the LH massif, the dunites contain wehrlitic to gabbroic lenses (Karson, 1979). Structural work in the peridotites of the BOIC demonstrated that the flow-related foliations in the mantle section are subparallel to the orientation of major lithological contacts within the BOIC and are thus of paleo-horizontal attitude (Girardeau, 1979; Girardeau and Nicolas, 1981). This feature is characteristic of mantle return flow (Nicolas and Violette, 1982). A feature which remained unexplained in the study of Girardeau and Nicolas (1981) and in the paleogeographic reconstruction of Casey et al. (19831 is the origin of different flow directions preserved between the BMD and TM massif. A shear sense reversal discovered in the upper part of the mantle section of the TM massif (Girardeau and Nicolas, 1981) was later interpreted in a more general model of mantle flow as the transition from forced to plate-driven flow (Rabinowicz et al., 1984, 1987). Detachment structures dominate in the basal part of the mantle section and are located above the high-temperature metamorphic sole (Malpas, 1979a; Girardeau and Nicolas, 1981; McCaig, 1983). In this paper, the detailed structural make-up of the upper mantle section of the TM massif is reported. The structure of the TM section is compared and correlated with that of the BMD massif (Girardeau and Nicolas, 19811 as well as with the less known, spreading-related, high-temperature structures of the NAM massif (Casey, 1980; Dunsworth et al., 1986). A modified palinspastic reconstruction of the BOIC is presented and a history of accretion of the mantle rocks is proposed.

.

sole

structural

m”““,“n-ilmlt

, ,f3structural

+

__---

.

TU = transitional

crustal rocks. (1979)

with plunqe

boundarIes

stretching

dumte

-

Humber

unit

I

1

Arm Allochthon;

L

‘Ikm

\ Mountain

I

massif. heavy black arrows

= movement

h

’

for gabbros

\

lithologies;

Cumulates

after circles = intermediate

and ultramafic of upper block.

sole and related

Data

i\

direction

(1989); MS = metamorphic

in the Table and Cawood

boundaries

from Williams

structural

Cove Formation,

and

low ,“,I, fault, iate very high temperature &a*r?nn= high temperature sheolLullC ultromvlonite

and lhenolite

massive

harzburgite

mainly

_.^:^/., ““,kh ( !,‘U”“J y”““‘”

-L __c + * * *

’ htgh angle faull low anale fault. I‘-“earl”,

.

G

0

I

I I_

lineations

units in ultramafics

unit

very high sheorzones metamorphic

unit; SC = Skinner

Fig. 2. Map showing

Lx

1 ,2,.

,

ljneation traces. temperature hneation traces,

253

x_

traces

traces.

afterGirardeau

lineotlon

lineotion

I

- %_?F 3*a

structural

Girardeau

slices of

(197%

F 2 S

4 pue SolqqeS 10~ e$ea '~!ssewu!e$unom

%u!pUeq lE3!%O[Oql![ pue Suo!legoJ paugap-pxauy

'2 %.J osfe aas pualal lad QjL6l)neap.Ie~y~ la]JesaleInwn3 3!Jetue~lfn a[qeJ_aqj u!ql!m (UO!le!lOJ 01 anb![qo alaw)

30 sapnlgle ueatu %u!MoqS dqq ‘E ‘3rd

~!w!+u!ddw ---------

36

Field- and microstructures The Table Mountain massif preserves the thickest mantle section within the BOIC. Structural data of the mantle section are summarized in Figures 2 and 3. The detailed maps of the structural data base are given in Suhr (1991). Suhr (1991) also presents olivine petrofabric data which demonstrate that mineral-defined foliations and stretching lineations are lying close (generally within 100) to the flow plane and line, respectively. In accordance with previous studies (summarized in Nicolas, 1989) the orientation of mineral-defined shape fabrics in the mantle peridotites is therefore considered as good approximation of the flow fabric. Gabbros and layered ultramafic cumulates have not been mapped in this study. Their relationships, taken from Girardeau and Nicolas (19811, are compiled in Figures 2 and 3. Like the NAM and BMD massifs, the TM massif is folded into a syncline (Williams, 1973). The western limb of the syncline is, however, poorly developed. Only data from the eastern limb are discussed in this paper. In the northern and southern parts of the TM massif, several steep cross-faults are located. Their concentration near the margins of the massif suggests that they are connected with the break-up of the BOIC into single massifs during the final gravitational emplacement. This is supported by an overall sinistral offset pattern of the marginal cross-faults which mimics the offset pattern between the different massifs (Fig. la>. Field mapping of the upper mantle structure in the TM massif has confirmed the overall regular arrangement of the mineral-defined foliations within the TM mantle section (Fig. 3; Girardeau and Nicolas, 1981). However, the orientation of the mineral stretching lineation in the massif appears domainal. Six domains can be defined by changes of the mineral stretching lineation and are considered structural units (Fig. 2% The description of the mantle section follows the division into structural units defined by changes in the mineral stretching lineation. Microstructural descriptions follow a classification described in Nicolas (1986a) and Ceuleneer et al. (1988). In this scheme, asthenospheric and

G. SUHR

lithospheric microstructures are distinguished. The asthenospheric microstructure shows a coarse olivine structure with high-angle grain boundaries and rarely visible dislocation walls. The mineral shape fabric tends to be weak, but the olivine lattice fabric is strong. Melt-impregnation features are common and are evidence for hypersolidus deformation. The lithospheric microstructure is of lower temperature, higher stress nature. Olivine neoblasts are abundant and fine-grained, undulose extinction is common, grain boundaries are not well-defined and of low-angle nature. Depending on the proportion of neoblasts to porphyroclasts, a weakly and a strongly lithospheric microstructure is here distinguished. An origin by either spreading- or detachment-related tectonics is not implied for the lithospheric microstructures, particularly for weakly lithospheric peridotites. Microstructurally determined shear senses follow the method of Nicolas et, al. (1972). It is based on the obliquity between the shape fabric (foliation) and olivine lattice fabric (extinction parallel to tilt walls) as observed in thin sections cut perpendicular to the foliation and parallel to the mineral stretching lineation. Where this obliquity is < 4”, the shear sense was considered neutral. More detailed microstructural descriptions, illustrations, and olivine petrofabric data are given in Suhr (1991) and will be published at a later date. Unit 1

Unit 1 is only developed on the southern side of the eastern limb. Its boundary to unit 2 either coincides or lies just below the dunite-harzburgite boundary (Fig. 2). Lithologically, unit 1 is mainly represented by massive dunites. Isolated, spine&rich, wehrlitic, or plagioclase-wehrlitic bands occur in the dunites and are parallel to the foliation. The bands are locally cut by dyke-like dunites. The mineral-defined shape fabric changes rapidly between more planar (S > L) to more linear (L > S). The lineation plunges moderately to steeply to the west-northwest (Fig. 4a), the foliation appears to be sigmoidally arranged (Figs. 3 and 5a). Peridotites at the base of the

UPPER

MANTLE

OPHIOLITES

Fig. 4. Stereographic

IN BAY

projections

and lower hemisphere

OF ISLANDS

of mineral projection.

OPHIOLITE

stretching Contour

lineations

massive dunites are locally well banded. The bands consists of one to several millimetre thick, clinopyroxenitic or plagioclase-bearing bands parallel to the foliation. Orthopyroxenite dykes display coarse-grained, undeformed fabrics and their orientation pattern is diffuse (Fig. 6a). Field evi-

Fig. 5. Stereographic

projections

lower hemisphere

of mineral-defined

projection.

Contour

in the Table

levels are given in percent

foliations

Mountain

mantle

section,

eastern

limb. Equal

area

of 1% area in lower right of each stereonet.

dence for the dominant sense of shear has not been found and the microstructure-based shear sense method is not conclusive (three out of five determinations neutral). Olivine microstructures in the dunites are mildly lithospheric to asthenospheric. Spinels are

in the Table Mountain

levels are given in percent

mantle

section,

eastern

limb. Equal

of 1% area in lower right of each stereonet.

area and

G. SUHR

Fig. 6. Stereographic

projections

lower hemisphere

of orthopyroxenite projection,

contours

dykes

in the Table

in percent

small and spindle-like and define a good shape fabric. In rare cases, larger, subhedral grains with serpentinized inclusions are present. Rare clinopyroxene is located interstitially to olivine or partially rims plagioclase in plagioclase wehrlite bands. Clinopyroxene appears less deformed than plagioclase. Unit 2 Within unit 2, clinopyroxene-rich harzburgites occur. Minor massive dunites may be developed above the harzburgites. Pyroxenes are finegrained (l-l.5 mm or smaller) and scattered along the foliation plane but the grain size increases and the degree of scattering decreases in the lower part of unit 2. Lithological banding in the host harzburgites is dunitic, locally clinopyroxenitic, or is defined by a very subtle variation in the pyroxene content. Orthopyroxene-rich banding in peridotite is more common in the lower part of unit 2 whereas dunite bands are then rare. Dunites which cross-cut the foliation and the banding are common in the upper part. Adjacent to these dunites, orthopyroxene-bearing banding is replaced by dunite-banding. The mineral-defined fabric is of aggregate nature and shows a well-developed lineation (L = S fabric). The foliation dips moderately to steeply to the westnorthwest (Fig. 5b), the mineral stretching lineation is subhorizontal (Fig. 4b). In the south, folding of the foliation occurs on a scale of several hundred metres (Fig. 3). No mesoscopic folds and no new fabric development have been observed associated with the folds.

Mountain

mantle

section,

eastern

limb. Equal

area

and

of 1% area. The dykes are very rare in units 5 and 6.

The majority of orthopyroxenite dykes clusters in a near vertical, ENE-WSW-trending attitude (Fig. 6b). The dykes locally preserve primary magmatic features like high-angle branching, pegmatitic or comb-textures, en-echelon arrangement, and absence of boudinage. These features are also present in orthopyroxenites of unit 1 and the uppermost part of unit 3. Rarely, orthopyroxenite dykes are buckled. Millimetre-thick clinopyroxenite dykelets are common. The microstructurally determined shear sense is dextral and indicates a NNE movement of the hangingwall (14 dextral, 3 sinistral, 10 neutral; Fig. 2). The dextral domains extends for 500 m into unit 3. However, a sinistral sense of shear within unit 2 is suggested by local occurrences where the foliation is weakly folded in the vicinity of orthopyroxenite dykes (Fig. 7a). Microstructures in the upper part of unit 2 are asthenospheric. Clinopyroxene is common as phase related to melt-infiltration and is enriched in layers parallel to the foliation (or the flow plane). Orthopyroxene grains are recrystallized into equant neoblasts. The neoblasts may also be scattered in the olivine matrix. Spinels are small, scattered, and not associated with another phase. Towards the base of unit 2, microstructures are transitional to those of unit 3 (see below). Unit 3 Within unit 3, harzburgites are clinopyroxenepoor. Lithological banding is defined by orthopyroxene-rich bands in harzburgites, rarely by dunite-bands. The average orthopyroxene grain

UPPER

MANTLE

OPHIOLITES

IN BAY

OF ISLANDS

39

OPHIOLITE

size is somewhat coarser than in unit 2 (1.5-2 mm). Locally, grains up to 5 mm are preserved. The mineral-defined fabric tends to be weak and S > L. It can best be defined as an aggregate fabric, as pull-apart behaviour of orthopyroxene is common. In the pull-apart fabrics, single grains of orthopyroxene are elongated nearly normal to the kinematically significant foliation. The foliation dips moderately to the northwest (Fig. 5~1, the stretching Iineation plunges to the northnorthwest (Fig. 4~). In the lower part of unit 3, the fohation dips to the north-northwest. Folding of the fithological banding is common, particularly next to orthopyroxenite dykes. Fold axes of the banding tend to be at high angle to the stretching lineation suggesting that strain was only moderate after formation of the folds. The asymmetry of the folds indicates a SE movement of the hangingwaIl (Fig. 7bl. The microstructurally determined shear sense is consistent with such a movement (8 times hangin~all to the south-southeast, Cl to the northwest, 1 neutral; Fig. 2). An axial planar fabric is developed with the mesoscopic folds of the banding. In the lower part of unit 3, the folds of the lithological banding contain small, parasitic folds which display a reverse asymmetry to the hosting fold (Fig. 7~1. Orthopyroxenite dykes are commonly boudinaged and subparallel to the foliation. Elongations of boudins tend to be at a high angle to the mineral stretching lineation. The detaited orientation pattern of orthopyroxenite dykes (Fig. 6~) shows a partial girdle which extends from the maximum density to progressiveiy steeper NWdipping dyke attitudes. This pattern is consistent with deformation of steeply dipping, WNWESE-trending dykes (abundant in unit 2) by deformation involving a southeast movement of the hangingwall. Microstructures of unit 3 are mildly Iithospheric. Orthopyroxene grains may be recrystallized into fairly irregular-shaped neobiasts. Pullapart of orthopyroxene is common. Spine1 tends to be spatially associated with orthopyroxene but the spine&orthopyroxene clusters become progressively disintegrated towards the top of unit 3 and within unit 2.

Unit 4 Lithologically, this unit is equivalent to unit 3. Dunite pods and bands are, however, more common in the SE part of the unit. The mineral-defined fabric varies between weak (equivalent to unit 3) to strong in areas of high strain concentration. The foliation is dipping shaIIow to the northwest (Fig. Sd), the stretching lineation trends shahow NNE-SSW to NE-SW (Fig. 46). Along the boundary to unit 3, lower-strain rocks tend to record mineral lineation trends typical for unit 3 whereas high-strain zones record the NNE- to NE-directed lineation. The boundary to unit 4 is drawn at the highest level occurrence of the high-strain zones with the NNE- to NE-trending lineation. In the high-strain zones, orthopyroxenites may be highly attenuated (Fig. 7d). Orthopyroxenite dykes are subparallel to the mean fohation (Fig. 6d). Clinopyroxenite dykes do not occur. The microstructurally determined shear sense indicates a SW movement of the hangin~all (8 to the southwest, 0 to the northeast, 2 neutral; Fig. 2). Microstructurally, the lower strain rocks appear equivalent to those of unit 3. In the higherstrain zones, the microstructure is moderately to highly lithospheric. Olivine shape fabrics are then Iocally oblique to the foliation defined by spine1 and orthopyroxene and alIow a rapid determination of the shear sense (Van der Wal et al., 1990). T~~n~itio~ul unit The region between units 4 and 5 was singled out as a transitional unit (TU). In the TU, the mineral stretching lineation has an overall ENEWSW trend (Fig. 4e). The TU is broad in the north, narrow in the south. However, foliations are very shallow in the north (Fig. 5e) so that the total thickness of the TU (measured perpendicular to the foliation) remains low. The TU shares lithological and structura1 features of units 4 and 5. In the north, an obliquity between the foliation and the lithological banding is characteristic. No mesoscopic folds are, however, observed with the obliquity. In the north, the lithological banding of

Fig. 7. Selected field relationships from the Table Mountain mantle section. (a) Weak f lexure of the foliation and lithological banding next to an orthopyroxenite dyke. Dyke is weathered and runs in groove from upper left to lower right; looking northwest; unit 2. (b) S-fold of the lithological banding symmetrically arranged to boudinaged orthopyroxenite dyke; hammer for scale is circled; looking W, unit 3; sinistral shear sense. (c) S-like fold of the lithological banding as in (b), containing parasitic, isoclinal folds with a reverse asymmetry. Fold is symmetrically arranged and offset along a cross-cutting orthopyroxenite. Drawing from photograph; looking W, unit 3. Cd) Highly attenuated boudin of orthopyroxenire dyke within high strain zone of unit 4. Pen for scale lies on top of boudin; looking W, sinistral shear sense.

icr

UPPER

MANTLE

OPHIOLITES

IN BAY

OF ISLANDS

41

OPHIOLITE

unit 5 (see below) appears deflected to the north, in the south, the deflection is undetermined. The maximum distribution of tilt walls measured during the shear sense determinations often yielded two peaks symmetrically arranged to the normal of the foliation (2 to east-northeast, 2 to westsouthwest, 6 symmetric = neutral) so that the dominant sense of shear remains unresolved. Microstructures are moderately lithospheric.

Unit 5 Within unit 5, harzburgites are coarser grained (orthopyroxene 2-3 mm, up to 5 mm> and contain a higher amount of cpx than in unit 4. The clinopyroxene content increases towards the lower part of the unit. The lithological banding is defined by websteritic or orthopyroxenitic bands in harzburgite and may display small scale buckle folds. Boundaries between the bands and the host peridotites can be very sharply defined and thus contrast markedly with the diffuse pyroxene-rich bands abundant in peridotites of unit 2. The lithological banding appears to define a synforma1 structure (Fig. 3). Particularly in the lower part of the unit, the mineral shape fabric is very poor. It is either defined as an aggregate orthopyroxene fabric or by single grains of spinel. The foliation dips moderately to steeply to the northwest (Fig. 5fI, the mineral stretching lineation is down-dip (Fig. 4f). A high-angle relationship between foliation and lithological banding is very widespread (Fig. 3). In orthopyroxene-rich peridotites, the foliation appears parallel to the banding but it is difficult to determine. Dunites are rare, orthopyroxenite dykes are extremely rare. The number of samples used for the microstructural shear sense determination is too small to be conclusive (2 times hangingwall to the southeast, 3 times to the northwest). Microstructurally, the upper part of the unit resembles unit 3. The lower part, however, displays a characteristic microstructure. It shows an asthenospheric to weakly lithospheric olivine microstructure and pristine clinopyroxene melt impregnation features. Orthopyroxene occurs mainly

as weakly to moderately recrystallized grains with poor shape fabrics. Spine1 is commonly intergrown with opx f cpx as small clusters ( < 1 mm).

Unit 6 Peridotites of unit 6 are coarse-grained lherzolites (orthopyroxene 2-7 mm). Lithological banding is defined as clinopyroxenitic, websteritic and orthopyroxenitic bands in lherzolite. The mineral shape fabric may be weak in the upper part of the unit, but is otherwise moderate to very strong. It is defined as an aggregate fabric and by single, elongated orthopyroxene grains and is invariably parallel to the lithological banding. In the southwestern part of unit 6, an original, high-temperature contact with highly strained amphibolites of the metamorphic sole is present. Farther north, the structural style of the metamorphic sole changes drastically and a complex thrust belt of amphibolites, harzburgites, and greenschists is juxtaposed against higher-level rocks of the TM mantle sequence (Figs. 2, 3). The maximum density of the measured foliations indicates a northwest dip (Fig. 5g1 and is subparallel to the trend of the metamorphic sole. However, even in close vicinity to the metamorphic sole, the foliation and the banding may be at high angle to the trend of the metamorphic sole. The trend of the mineral stretching lineation is poorly defined (Fig. 4g). It appears that an early, moderate to strong mineral shape fabric became refolded during progressive shearing. Deflection patterns of the foliation into higher-strain zones have a mostly sinistral offset component in plan view. Microstructurally determined shear senses yielded no consistent picture (15 determinations). The kinematics of unit 6 must have been complex and are not fully understood. Orthopyroxenite dykes are absent, but some coarse-grained, deformed olivine clinopyroxenites do occur. Microstructures of unit 6 are, apart from the top of the unit, highly lithospheric. Orthopyroxene ribbon grains may reach aspect ratios up to 1OO:l. In addition, coarse spine1 grains ( > 1 mm) occur. They are interpreted as porphyroblasts (Suhr, 1991).

G. SVHR

42

Relative age of structtiral uaits

By using overprinting criteria, the relative structural age between some of the structural units can be determined. On the basis of the deflection of the flow pattern of unit 1 into unit 2 (Fig. 31, unit 2 appears to structurally postdate unit 1. The apparently undeformed nature of orthopyroxenites in unit 1 as opposed to unit 3 supports an early tectonic freezing of unit 1. A relative later age of unit 3 compared to unit 2 is based on the following three arguments: (1) Orthopyroxenite dykes are deformed by sinistral shear (hangin~all to the south-southeast) within unit 3, whereas the majority of the orthopyroxenites are only weakly deformed within the dextrally sheared unit 2. (2) In the lower part of unit 3, early folds related to dextral shear (hangin~al1 to the north-northeast) are overprinted by later folds indicating sinistral shear (Fig. 7~). This suggests that dextral shear, typical for unit 2, once also affected unit 3, but was Iater overprinted by sinistral shear. (3) Subtle, local flexures of the foliation along cross-cutting orthopyroxenites within unit 2 (Fig. 7a) indicate locatly a sinistral shear sense in unit 2. This is interpreted as the highest level expression of the later sinistral shear (dominant within unit 3) overprinting early dextral shear in unit 2. The strain associated with sinistral overprint must have been low so that the microstructural evidence for a dextral shear sense remained preserved. Unit 4 is in many respects similar to unit 3, but shows a different stretching lineation. In highstrain zones of unit 4, moderately to highly lithospheric microstructures are developed and orthopyroxenites may be extremely deformed. Along the boundary between units 3 and 4, the NEtrending stretching lineation is only recorded in high-strain zones. It is inferred that peridotites typical of unit 3 were heterogeneously overprinted by strain associated with unit 4. Structural overprinting between units 4 and 5 is complex and is concentrated within the TU. No consistent style of transposition appears to be present within the TU. The homogeneous sinis-

tral shear sense from unit 4 is lost. Microstructures of unit 5 are locally asthenospheric and evidence for high accumulated strain is absent, contrasting with lithospheric microstructures and higher strain of unit 4. Unit 5 is considered more “primitive” than unit 4. The not fully resolved style of transposition between units 4 and 5 does not permit to safely determine the relative age. Because of the lower-temperature microstructures of unit 4 compared to unit 5, a structurally later age of unit 4 is preferred. Unit 6 shows lower-temperature microstructures and more heterogeneous strain than unit 5. In lower-strain areas of unit 6, microstructures typical for unit 5 can still be recognized. In the southwestern part of the massif, unit 6 is structurally continuous with the metamorphic sole. The latter clearly reflects the later detachment history of the ophiolite. Unit 6 is thus associated with detachment and postdates the partly asthenospheric unit 5. As a result, the deformational sequence began with formation of unit 1 and 2. Unit 2 probably once extended into what is now unit 3. Then strain characteristic for unit 3 developed. It is suggested that deformation related to unit 3 reached at least as far as the base of the current unit 4 and was followed by accretion of the partly asthenospheric unit 5. Finally, the moderately to strongly lithospheric structures formed, affecting the hangin~all of unit 5 (unit 4) and the base of the ophiolite (unit 6). If unit 4 is assigned to detachment (see below), this relative age sequence fully supports the applicability of the plate thickening model to the spreading history of the TM massif. Correlation with other massifs

In the BMD massif, spreading-related stretching lineations plunge to the northwest (Girardeau and Nicolas, 1981). Apart from a narrow section of i&defined movement directions below the gabbros, the shear sense indicates a SE movement of the hangingwall. The bulk of the mantle section in the BMD massif appears thus related to unit 3 in TM. Units 1, 2, 4 and 5 cannot be recognized with the currently available data. Lithology and

UPPER

MANTLE

OPHlOLlTES

IN BAY

Lewis Hills

Karson

43

OPHIOLITE

Casey 1980; ..#UYU.uS” Girardeau 1979;

Dunsworlh et al. 1986

Ginrdaau and ~~edun~e - - c ductitety deformed Nicdas 1981; *

A-

Table Mountain

North Ann Mou~in

Blow Me Down Moun~in

and

Dewey 1978; Suhr and Cawood (submitted)

l

OF ISLANDS

l

metamorphic sole shear sense

Girardeau 1979; this work;

Suhr and Cawood (submitted)

Fig. 8. Proposed correlation of structural units established in the TM mantle section with other massifs of the Bay of Islands Complex. For details see text.

structure of peridotites near the base of the massif (Girardeau and Nicolas, 1981) relate to unit 6 in TM. Data for the mantle section of NAM are very limited. Casey (1980) indicated that a change in the trend of the stretching lineation occurs in the upper part of the mantle section of NAM. It could relate to the change associated with unit 2 to 3. The trend of the mineral stretching lineations in the uppermost harzburgites and plasticaliy deformed transition zone is compatible with those of unit 2 (Casey, 1980; Dunsworth et al., 1986; Bedard, 1991). A foliation pattern mapped by T. Calon in the uppermost harzburgites of NAM (pattern shown in Neyens, 1986) is consistent with dextral shear. Structures and lithologies of the LH massif are difficult to evaluate as they could be influenced by the proximity to a fracture zone &arson and Dewey, 1978; Karson and Elthon, 1987; Suhr et al., 1991) and possible rotations of the entire massif associated with final empIacement (WiIliams and Malpas, 1972; Rarson and Dewey, 1978; Casey et al., 1983). Crustal structures &arson and Dewey, 1978; Karson, 1979) as well as at least the mantle structures in the southern LH (Dunsworth et al., 1986; Suhr et al., 1991) are

very peculiar and unlike those of the three other massifs. The LH massif will therefore not be considered here. The suggested correlation of the structural units across the three northern massifs of the BOIC is shown in Figure 8. Discussion

Palinspastic reconstruction The arguments of Karson and Dewey (1978) and Casey et al. (1983) are followed with some modifications. The paleo-horizontal is taken as the regional attitude of the layered cumulate to harzburgite boundary (petrological Moho). Due to the controversial origin of massive dunites (mantle or crustal origin, see below), the Moho is difficult to locate where thick dunite sequences are developed. In the TM massif, the current attitude of the Moho is dipping steeply to the west-northwest. As a consequence of rotating the Moho to horizontal, the detachment-related base of the mantle sequence (unit 6) becomes subhorizontal. Unit 4 and the TU develop a peculiar paleo-northeast dip (Fig. 9). The trend of the ridge axis was taken as the parallel to the mean trend of the sheeted dykes

G. SUHR

Fig. 9. Simplified reoriented Surface the

with

with dotted

surface

lithological sheets dykes;

view of the Table respect

margin

= mineral banding

with ruled arrows

Mountain

to the paleo-ridge

pattern

= movement

with two heads = movement

frame.

plane;

lines on

lineation:

heavy

lines =

oblique

= mean

section,

foliation

= mean

stretching where

mantle reference

to foliation;

attitude

direction

and

of orthopyroxenite

of upper

undetermined.

lenses block;

Unit numbers

arrow are

also indicated.

(NW-SE; Casey et al., 1983) or the normal to the trend of the fracture zone as seen in the LPC (WNW-ESE; Karson and Dewey, 1978). The angular difference between these models is 25” and thus minor. The orientation of Karson and Dewey (1978) is preferred: during spreading, ridge axes align either normal to the trend of fracture zones or in such a way that the transform segment between offset ridges is shortened (Atwater and Macdonald, 1977; Fujita and Sleep, 1978). By combining the ridge axis trend of Casey et al. (1983) with the fracture zone orientation of the LPC, the transform segment would, however, have been lengthened (cf. Fig. lb). Casey et al. (1983)

avoided this problem by assuming a late, 25” anti-clockwise rotation of the LPC with respect to the BOIC. In the model of Karson and Dewey (1978) and Casey et al. (1983, 19851, the location of the BOIC is to the north of the ridge axis. This conclusion is critical for the model presented below and is thus further evaluated using data of this study. In TM, mineral stretching lineations of unit 2 are directed normal to the inferred ridge trend and foliations are parallel to the Moho. If the Moho is taken as having been regionally paleo-horizontal, then unit 2 of TM constitutes mantle return flow in a direction normal to the ridge axis. By taking account of the dextral shear sense of the unit (hangingwall to the north-northeast), the flow plane within unit 2 was dipping slightly to the north with respect to a rotated, horizontal Moho. This dip would position the ridge axis to the south of TM (e.g., Nicolas, 1989). More importantly, the inferred ridge axis trend implies that mantle flow within the BMD massif was nearly parallel to the ridge axis even at a depth of several kilometres below the Moho. Flow parallel to the ridge is common within ophiolites but is restricted to the vicinity of the ridge (Nicolas and Violette, 1982; Ceuleneer et al., 1988). Preservation of a singular, spreading-related flow structure parallel to the ridge requires that the BMD massif has been frozen in near the ridge. Otherwise, during continued spreading, a component of flow directed normal to the ridge axis would have to be preserved due to the transform damming effect (Vogt and Johnson, 1975; Nicolas and Violette, 1982; Nicolas, 1989). Unit 3 in TM, which was correlated with the BMD spreading structures, shows a component of flow normal to the ridge. This indicates that the BMD massif was closer to the ridge axis than the TM massif and that the ridge axis was located to the south of the TM massif. Both conclusions about the position of the paleo-ridge are consistent with previous arguments &arson and Dewey, 1978; Casey et al., 1983). In addition, the preservation of ridge-parallel flow structures in the BMD massif requires that this massif was not located adjacent to the fracture zone of the Coastal Complex (CC). As the

UPPER

MANTLE

OPHIOLITES

‘/

IN BAY

OF ISLANDS

T - Table Mountain

OPHIOLITE

#

N - North Ann Mountain B - WOW Me Down Mountain L - MS Hills Fig. 10. Suggested configuration of massifs from the Bay of Islands Complex in the oceanic environment. Deformation structures observed in the LH do not permit to conclusively locate the massif either to the north or the south of the spreading centre (Suhr et al., 1991). Thin double line indicates ridge axis, heavy line denotes fracture zone.

CC represents older and thicker lithosphere than that of the BOIC, it would have acted as a barrier for ridge-parallel, asthenospheric flow (Vogt and Johnson, 1975; Nicolas, 1989). As a result, flow would have been deflected into a direction parallel to the CC. This was apparently the case in the LH massif (Suhr et al., 1991). Only the LH massif was thus positioned next to the CC (Karson and Elthon 1987). It is suggested that, with the exception of the LH, the BOIC formed at a considerable distance from the CC (Fig. 10). This interpretation differs from previous ones (Karson and Dewey, 1978; Casey et al., 1983, 1985). The spatial separation of the LH massif from the three other massifs might have lasted until the stage of folding of the massifs: synclinal folding is only observed in the three northern massifs, whereas the LH shows simple tilting of the eastern flank of the massif (Karson, 1979). Also, marginal cross-faults related to the break-up of the three northern massifs during final emplacement (Fig. la) are notably absent in the LH massif (Karson, 1979). The current proximity of the LH and BMD massif might thus be the result of a late juxtaposition of the two massifs during final emplacement. Structural

history

Near a spreading centre, upper mantle oceanic lithosphere is formed by conversion from as-

45

thenosphere. The conversion is marked by the cessation of significant deformation in the upper mantle along the lithosphere-asthenosphere boundary (LAB). In the plate-thickening model, the LAB is represented by an isotherm. For a given spreading rate, the thermal structure of oceanic lithosphere near a spreading centre allows one to calculate the thickness of the lithosphere and the distance from the ridge axis at which the lithosphere was formed. For slow spreading rates, the LAB is steeper than for fast spreading rates (e.g., Parker and Oldenburg, 1973; Sleep, 1975; Boudier et al., 1988) so that a thick lithosphere is present in close proximity to the ridge. The relative age relationships of the different structural units in the TM section suggest that the plate thickening model is applicable at least to the upper part of the mantle section, but probably to the entire mantle section. By choosing some boundary conditions, it is possible to estimate the distances from the ridge axis at which each of the structural units were structurally frozen in, i.e. were converted into lithosphere. This concept was outlined by Ceuleneer et al. (1988) and Nicolas (1989) and was adapted to estimate the distances given in Figure 11. Figure 11 is based on the thermal model presented in Boudier et al. (1988). The following assumptions are made: (1) As the plutonic crustal section within the BOIC is well developed (Casey and Karson, 1981; Karson and Elthon, 19871, a spreading rate of 5 cm/yr is assumed for the BOIC. For low spreading rates, the plutonic crustal section is poorly developed (e.g., Sleep, 1975; Kusznir and Bott, 1976; Sleep and Rosendahl, 1979). (2) In TM, the top of the crust has been eroded. The mean crustal thickness of the BOIC (about 6 km, cf. Karson et al., 1984) is adapted for the TM section. In the northern massifs of the BOIC, the crustal thickness might even have been higher (Karson and Elthon, 1987). (3) A critical parameter to be chosen is the temperature at which deformation ceased in the mantle section. This temperature must be the same as the one which is locked into the microstructure of spreading-related peridotites in

46

G. SUHR

the TM mantle section. I-IypersoIidus microstructures are restricted to the very top of the mantle section and to the base of unit 5. Judging from the solidus of depleted upper mantle peridotite under low pressures (Jaques and Green, 19801, the hypersolidus microstructures formed at 12001250°C. The bulk of the mantle section shows mildly lithospheric microstructures. Their temperatures of deformation are difficult to estimate, but should lie at least at the 1000°C estimated by Nicolas (1986a) for lithospheric microstructures. A value of 1000°C is chosen as LAB. It represents a minimum temperature. Note that the preservation of different temperatures of spreading-related structures within the mantle section implies that the rheological LAB was deviating slightly

from being an isotherm. This indicates the presence of complication not predicted in the platethickening model. The first structures to freeze were those of units 1 and 2. The pahnspastic reconstruction of the TM mantle section suggests that within unit 1, flow occurred parallel to the ridge (Fig. 9). The geometry of flow, particularly the zonal arrangement of the foliations around the lineation, is typical for flow parallel to the ridge (Ceuleneer et al., 1988). Ridge-parallel flow could relate to forced flow to which dunites with interstitial melt and minor harzburgites were subjected. Forced flow is driven by the buoyancy related to a partial melting instability (Rabinowicz et al., 1984, 1987; Scott and Stevenson, 1989; Buck and Su, 1989).

a decreasing spreading rate

motion

Fig. 11. History of accretion of upper mantle peridotites to the base of the oceanic crust within the Table Mountain section. The section is located parallel to the trend of the fracture zone. Unit 2 formed during plate-driven flow, unit 3 solidified during forced flow, and unit 5 represents an off-axis diapir. During formation of unit 3 and 5, the lithosphere was slightly reactivated. Unit 1 is not shown. During detachment, unit 4 overprinted the base of unit 3, and unit 6 overprinted part of unit 5. Subsolidus asthenosphere represents the region where hypersolidus flow structures are overprinted by spreading-related, subsolidus deformation. Thermal structure from Boudier et al. (1988).

UPPER

MANTLE

OPHIOLITES

IN BAY

OF ISLANDS

OPHIOLITE

Alternatively, the peridotites of unit 1 were sucked laterally into the parting plates at the ridge. Shear sense information from unit 2 indicates that during formation of this unit, the plates moved apart passively (plate-driven flow). Consistently, stretching lineations of unit 2 are parallel to the spreading direction, the latter being parallel to the trend of the fracture zone. The preservation of hypersolidus flow microstructures within the upper part of unit 2 was probably favoured by two features: (1) the particle path within the subsolidus asthenosphere (the region between the 1000” and 1200°C isotherms) is shorter near the ridge axis than farther away from it (Fig. 11). The shorter the path, the less intense will be the subsolidus overprint of hypersolidus microstructures; (2) at the boundary between mafic and ultramafic rocks near the Moho, deformation within the uppermost ultramafics will tend to partition into the mafic crust due to rheological constraints (Chen and Morgan, 1990). This would enhance the rapid freezing of the uppermost mantle rocks and the preservation of hypersolidus microstructures. The base of the lithosphere was situated 15-20 km off the ridge axis when accretion of unit 3 commenced. Shear sense information from unit 3 indicates that it was dominated by active spreading (forced flow) and that flow was directed obliquely to the contact to the fracture zone. The reason for the slight spatial mismatch between the unit boundary (based on the stretching lineation) and the shear sense reversal is explained in Suhr (1991). It could lie in a technical problem of the microstructural method of shear sense determination which occurs when during a shear sense reversal the flow plane is kept constant. A possible explanation for the shear sense reversal is that the TM mantle section came under the influence of the forced flow field of an emerging diapir. The sharp lineation change from unit 2 to unit 3 suggests that the lithosphere was slightly reactivated at this stage. Otherwise, a more gradual change of the lineation would be expected during the time when passive and active spreading forces were superimposed. Orthopyroxenites intruding after freezing of

47

unit 2 will be undeformed in that unit but will be deformed within unit 3 (Fig. 11). Further downsection within unit 3, the increasing angle between the foliation and the Moho (Fig. 9) may suggest that the LAB steepened due to a decrease of the spreading rate. A slight change of the stretching lineation of unit 3 to more northerly trends with increasing depth might record the progressive damming of flow by a fracture zone. No entirely reliable criteria have been found for assigning unit 4 to either spreading- or detachment-related tectonics. Based on the presence of moderately to strongly lithospheric microstructures in unit 4, it is assumed that the structure of the unit is the result of an overprint during later detachment. Information about the original mantle structures within this unit would therefore be lost. Probably, lithologies of unit 4 and the TU were subjected to the same deformation as unit 3 and then solidified to form the base of the lithosphere against which unit 5 accreted at about 40-50 km distance from the ridge axis. The formation of unit 5 must explain the preservation of pristine melt impregnation features during extended return flow and the odd arrangement of the lithological banding with respect to the foliation. All this is coupled with a transition into less depleted, lherzolitic chemistries (Suhr and Calon, 1990). Unless a component of vertical (diapiric) motion is invoked, it is very difficult to explain the presence of hypersolidus deformation features: during extended return flow, the particle path intersects progressively colder isotherms and indicates cooling under constant pressure, not fusion. The preferred model for unit 5 suggests underplating of an off-axis diapir to the base of the lithosphere. This would readily explain the preservation of melt-infiltration features. The odd arrangement of the lithological banding could relate to structures formed when the diapir was plated against the base of the lithosphere. In this model, the foliation in unit 5 is a relatively late, high temperature feature associated with underplating. The TU is the zone where remnant structures typical for unit 5 are preserved but overprinted by higher strain in the diapiric roof or, alternatively, associated with detachment. The

48

less depleted chemistry of units 5 and 6 is also explained in this model. Due to a non-adiabatic ascent of the mantle associated with the diapir, lherzolitic instead of harzburgitic residues could result (McKenzie, 1984; Nicolas, 1986b). Essentially, along the boundary of units 4 to 5, two mantle sections are juxtaposed which have experienced a different flow path and partial melting history. The absence of orthopyroxenite dykes within unit 5 is consistent with this model. At the time of the major period of intrusion of the dykes between the formation of units 2 and 3, the mantle section of unit 5 was spatially separated from the region of orthopyroxenite dyke emplacement (Fig. 11). The structure of unit 6 is the result of an overprint during detachment of the ophiolite. The total thickness of the TM lithosphere (6 km crust plus 6 km upper mantle) would have formed within a distance of about 55 km from the ridge axis. The value is subject to change depending on the thermal model chosen, as well as the temperature of the LAB, the spreading rate and any variation of it, and the crustal thickness assumed. The geographical distance between the BMD and the TM massifs is 60 km. It is reduced to 45 km if the current gap of ophiolitic exposure between the BMD and NAM massifs is not erosional but the result of a divergent movement of the (previously connected) massifs during final emplacement. The distances imply that at the time when the currently preserved thickness of the TM mantle lithosphere was structurally frozen in, the BMD massif was positioned near the ridge axis. Such a position of the BMD massif was already inferred from the internal flow structure of the BMD massif. Overall, the estimated distances appear to be in the right order of magnitude. Figure 12 summarizes the information. The early structures in TM (unit 2) are related to the predominance of passive plate movement (Fig. 12a). Plate driven flow carried the TM massif away from the ridge axis before the BMD massif was formed. Similar structures to unit 2 in TM are preserved in the top of the NAM section, but not within the BMD section. Subsequently, a

Fig. 12. Flow fields (thin arrows) and movement paths of massifs (heavy arrows) inferred for the Bay of Islands Complex during (a) passive and (b) active spreading. TM, BMD, and LH as in Fig. 1. For details see text.

forced flow field began to build up. It is related to an emerging diapir and its geometry can be derived from the flow lines of the BMD massif and unit 3 in TM (Fig. 12b). Down-section within unit 3, the slightly changing stretching lineations could be a result of the transform damming effect. Further down-section, the relationships in TM have been obscured by the structures of unit 4 which are thought to have formed during detachment. The BMD massif froze in at the ridge and at considerable distance from any fracture zone. It therefore does not record the transform damming effect. Note also the position of the suspected off-axis diapir in TM. Figure 12b would approximate the massif configuration during cessation of spreading and probably also during initiation of detachment. Model

The structural history of the upper mantle rocks in the BOIC can be summarized in a four stage model which describes the late history of a spreading centre. Stage 1: unrestricted spreading, adiabatic upwelling

Unit 1 is related to minor ridge-parallel flow which occurred near the ridge axis. It was structurally frozen in before overprinting by flow with a component directed normal to the ridge could occur. For unit 2 of TM, plate driven flow is indicated by the sense of shear combined with

UPPER

MANTLE

OPHIOLITES

IN BAY

OF ISLANDS

49

OPHIOLJTE

evidence about the inferred position of the ridge. Unit 2 is not recognized in the younger oceanic lithosphere of the BMD massif. The lithology of unit 2 is harzburgite with clinopyroxene derived by melt-infiltration. Microstructures in the upper part are asthenospheric and of low stress nature which militates against any significant stress concentrations during plate driven flow. Stage 2: w~~~~y restricted s~reading~ adiabatic upwelling

At this stage, the spreading rate, controlled by passive movement of the plates, was decreasing, possibly due to larger-scale plate reorganizations affecting also the plate movement in the ocean basin of the BOIC. The rate of mantle upwelling, normally directly correlated to the spreading rate (e.g., Nicolas, 1986b; Scott and Stevenson, 1989), did not react immediately to changes in the spreading rate. This sluggish behaviour was caused by a column of several tens of kilometres of melt-charged peridotites beneath the ridge (McKenzie, 1984). The buoyant meIt represents an additional force to drive mantIe upwelling (Rabinowicz et al., 1984, 1987; Scott and Stevenson, 1989) and caused an overriding of the (weakened) passive spreading force by active spreading forces. This configuration is considered responsible for formation of the structure of unit 3. A steeper paleo-dip of the flow plane in unit 3 may also indicate a slower spreading rate. Note that the forced flow field was very far-reaching (45 km from the ridge) compared to the Oman ophiolite (20 km; Ceuleneer et al., 1988). Harzburgites (i.e. not Iherzolites) did form synchronously at the ridge axis (peridotites of the BMD massif) and suggest that mantle upwelling was still adiabatic (Boudier and Nicolas, 1985; Nicolas, 1986b). The more lithospheric microstructures of unit 3 compared to unit 2 may indicate that stress concentrations are more likely to occur during forced flow conditions than during plate driven flow. Stage 3: restricted spreading, non-adiabatic upwelling

Unit 5 in TM could relate to the roof of an off-axis diapir which brought up less depleted,

lherzolitic peridotites from depth. This would indicate diffuse mantIe upwelling. There is evidence that channeling of mantle up-flow beneath a ridge is weakened during slow spreading (NicoIas and Rabinowicz, 1984; Chen and Morgan, 19901, a feature which would facilitate the formation of off-axis diapirs. However, an enhanced channeling with a decreasing spreading rate has also been suggested (Girardeau and Mercier, 1988; Scott and Stevenson, 1989). Lherzolites also occur at the base of the three southern massifs of the BOIC. It is difficult to make off-axis diapirism responsible for all these more fherzolitic rocks. More likely, the lherzolites indicate that mantle upwelling became nonadiabatic even below the ridge, so that melting did not proceed to produce a harzburgitic residue. Lherzolites from the base of the BOIC may represent non-adiabatic mantle upweiling prevailing during the dying stages of an ocean ridge system. Massive dunites in ophioiites have been related to intense melt percolation through peridotite (in-situ formation of dunites; Sinton, 1977; Arai, 1980; Nicolas and Prinzhofer, 1983; Benn et al., 1988; Girardeau and Mercier, 1988; Nicolas, 1989). ~ternative~y, they are considered the products of olivine fractionation from melt (magmatic model; e.g., Malpas, 1978; George, 1978; Casey et al., 1981; Lippard et al., 1986; Karson and Elthon 1987). Notwithstanding their unresolved origin, they indicate a high melt flux into the base of the crust. The presence of unusually thick dunite sequences in the BMD and LH massifs therefore suggest intense melt-movement in the Moho-region of the two southern massifs of the BOIC, i.e. in the massifs inferred to have been located near or at the ridge. The formation of the dunites may have been enhanced by the restricted extension: as spreading was insufficient to accommodate melt production, melt ponded at the base of pre-existing crust instead of producing new crust by extension. Stage 4: convergent motion

If the spreading-rate continued to decrease a transition from a divergent to a convergent tectonic regime might have occurred. As a result, intra-oceanic thrusting, i.e. detachment of the

G. SUHR

50

ophiolite, is likely. Clearly, detachment occurred in case of the BOIC. The inferred decreasing spreading rate of the BOIC may thus be coupled to the same tectonic forces which led to detachment. In such a scenario, a short time span between formation of the BOIC and its time of detachment is implied. Currently available radiometric age data suggest an age difference of about 11 m.y. between cessation of spreading and formation of the metamorphic sole (Dallmeyer and Williams, 1975; Dunning and Krogh, 19851. This large age difference appears inconsistent with a progressive evolution from divergence to convergence. However, as shown by Hacker (1991), even when detachment occurred immediately following spreading, an age difference of 6-9 m.y. between formation of the ophiolite and the K-Ar age of amphibolites of the metamorphic sole can be expected due to the cooling characteristics of the K-Ar system. Also, the high temperatures locked into the detachment-related peridotites of unit 6 in TM and the metamorphic sole suggest that residual heat of the ophiolite was still available (Malpas, 1979a). This would indicate very early detachment, closely following spreading. Alternatively, strain heating may possibly explain the high-temperature nature of the detachment structures as temperatures of detachment coincide closely with the buffering temperatures (900-950’0 reached during ductile shearing of olivine-dominated rheologies (Pavlis, 1986). Recently, thermal modelling of Hacker (1991) suggested that strain heating alone would not be sufficient to explain the high peak temperatures of the metamorphic soles in the Oman ophiolite and the BOIC. According to his model, detachment of a very young ophiolite is indicated in both occurrences. Precise determinations of the age of detachment of the BOIC are needed in order to fully resolve whether detachment of the BOIC closely followed its spreading history.

within one massif and between different massifs of the BOIC; the occurrence of lherzolites at the base of all massifs and thick massive dunite sequences at the top of the mantle section in the southern massifs of the BOIC; and the preservation of hypersolidus flow structures within two zones of the TM mantle section. The model is critically dependant on the relative position of the BOIC and the ridge axis. If the ridge axis was to the north of the TM massif, then forced flow would have preceded plate-driven flow. This constitutes the “normal” scenario of a spreadingcentre evolution. In case of the BOIC, the ridge axis is inferred to have been to the south of the ophiolite. Therefore, plate-driven flow gave way with time to forced flow. This scenario was caused by a decoupling of the rate of upwelling and the rate of spreading during the final stages of the spreading history. While numerical modelling of mantle flow is based on constant spreading rates, a model involving a transient regime (decreasing spreading rate) may be more applicable to the BOIC. Acknowledgements

This study was conducted as part of the Ph.D. research of the author at Memorial University of Newfoundland. Funding was provided in part by a Memorial University Graduate Fellowship. The author greatly benefitted from discussions with F. Boudier, T. Calon, P.A. Cawood, S. Edwards, and, in particular, A. Nicolas, as well as with participants and excursion leaders of trips C and D of the Oman Ophiolite Symposium. This paper was carefully reviewed by P. Bird, J. Dewey, and A. Nicolas. References Arai,

Conclusions

The suggested model exlplains several of the major elements in the mantle section of the BOIC: the presence of shear sense reversals and the preservation of different movement directions

S., 1980.

Dunite-harzburgite-chromite

refractory

residue

ern Japan.

J. Petrol.,

Atwater,

as

zone, west-

21: 141-165.

T. and MacDonald,

ters perpendicular

complexes

in the Sangun-Yamaguchi

K.C., 1977. Are spreading

to their transform

faults?

Nature,

cen270:

715-719. Bedard,

J.H.,

1991. Cumulate

in the Bay of Islands

recycling

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and crustal

J. Geol.,

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99: 225-249.

UPPER

MANTLE

OPHIOLITES

IN BAY

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