P−T conditions during emplacement of the Bay of Islands ophiolite complex

Earth and Planetary Science Letters, 63 (1983) 459-473

459

Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands

[51

P - T conditions during emplacement of the Bay of Islands ophiolite

complex A.M. McCaig Department of Earth Sciences, Downing Street, Cambridge, CB2 3EQ (England)

Received March 24, 1982 Revised version accepted February 23, 1983

A high-temperature contact is described between the basal pargasite-bearing spineMherzolites of the Bay of Islands ophiolite complex and underlying garnet-granulite facies metagabbros of its dynamothermal aureole. Three distinct high-temperature hydrous assemblages occur in the basal mylonites of the peridotite, and spinel- and garnet-bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole. On the basis of garnet-clinopyroxene geothermometry and garnet-forming reactions in metabasic rocks, P-T conditions of 7-11 kbar, 750-850°C are estimated for rocks on both sides of the contact. Steep inverted gradients in both temperature and pressure of equilibration occur in the aureole, which most likely represents a thinned, overturned and metamorphosed section through an ophiolite sequence. It is proposed that the aureole formed in a low-angle shear zone cutting the oceanic crust and upper mantle. Age data shows that the Bay of Islands Complex was 30-40 Ma old and therefore relatively cold at the time of formation of the aureole. Prolonged ( > 1 Ma) shear heating must therefore have occurred at high shear stresses and movement rates (>~ 1 kbar, 10 c m / y r ) to produce the high contact temperatures. The displacement surface probably initiated as a discrete fault, evolving into a viscous shear zone with time. Downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite-aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole. The observed pressures at the ophiolite-aureole contact are 3 - 7 kbar in excess of the expected load pressure from the present thickness of the ophiolite. The cause of the pressure excess was removed before formation of lower-grade parts of the aureole. Possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole. A model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures, rapid uplift and erosion during displacement, and the coincidence of K-Ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite.

1. Introduction

Dynamothermal metamorphic aureoles are attached to the basal peridotites of many of the world's ophiolite complexes. Well documented examples occur in the Oman [2,48], Greece and Turkey [2,6,21], Yugoslavia [28,46], Scotland [9,43], Quebec [45] and Western Newfoundland [1,4,5]. Similar aureoles are associated with other ultramafic bodies which may or may not be ophiolitic in origin, as reviewed by Williams and Smyth [1]. In all cases where their relationship to the 0012-82 I X / 8 3 / $ 0 3 . 0 0

@' 1983 Elsevier Science Publishers B.V.

ophiolite can be established, the aureoles are thin (70-700 m) and approximately conformable to lithological boundaries within the overlying ophiolite. They show steeply inverted metamorphic zonations from granulite or amphibolite facies adjacent to ophiolitic peridotites, down to greenschist facies and sometimes unmetamorphosed volcanic or sedimentary rocks further away. The lithology of the aureoles is dominantly metabasic, and many appear to have been derived from oceanic crust. Most authors agree that the aureoles were pro-

460

duced by displacement of ophiolites over oceanic crustal rocks, often well before final emplacement onto a continental margin (e.g. [6,21]). They are therefore the only surviving record of the early stages of obduction, the mechanism and tectonic setting of which has been the subject of so much speculation in the past (e.g. [7,8]). Recently several authors have noted anomalously high-pressure assemblages in the highestgrade parts of aureoles [4,6,9,10,28,45], and apparent gradients in pressure as well as temperature. In most cases either the thickness of the overlying ophiolite is unknown, or the high-pressure assemblages occur in fault slivers of uncertain origin. In this paper similar high-pressure assemblages (7-11 kbar at 750-850°C) are reported from the aureole and basal peridotite of the Bay of Islands Complex of West Newfoundland. Here the thickness of the ophiolite is well known and the contact between the ophiolite and the aureole is unusually well exposed. The severe constraints which these data place on ophiolite emplacement mechanisms are discussed.

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....•........... ~',' ~!ast[L se [{metlt ~

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The Bay of Islands Complex is the highest structural slice in the Humber Arm allochthon [11,12,41], a structural unit which was emplaced onto the Western Newfoundland carbonate platform in Early to Middle Ordovician times. The lower sedimentary slices in the allochthon are thought to represent an imbricated section through the east-facing Cambro-Ordovician continental margin of North America [ 11-13,16]. The complex shows a complete ophiolitic stratigraphic sequence (Fig. 1) 10-12 km in thickness [17,47,58]. This sequence is similar both in seismic velocity structure [47] and geochemistry [60], to modern oceanic crust. The dynamothermal aureole is exposed along the southeast side of the complex. Both the main schistosity in the aureole and its contact with overlying tectonite peridotites dip moderately or steeply to the northwest, and are approximately conformable with the main lithological boundaries within the ophiolite. Rocks interpreted to have formed along a

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Fig. 1. (a) Generalised section through the Bay of Islands Complex [3A4,17,41]. Thicknesses of most units are variable. (b) Measured section through the peridotite-aureole contact. Table Mountain massif, about 2 km north of the east end of Trout River Pond. (c) Section through contact 1 km southwest of (b). (d) Section through contact, south side of Trout River Pond.

northeast-trending oceanic transform fault have been described from the western part of the complex [15], suggesting that the ophiolite may have generated at a northwest-trending spreading centre. This is supported by the trend of dykes within the complex [42]. Stratigraphic evidence suggests that emplacement of the ophiolite began in the early to middle Arenig, and was complete by the Caradoc [11]. During this lengthy process parts of the ophiolite were deeply eroded with unconformable deposition of parallochthonous breccias and mudstones of Llanvirn age [58]. These breccias contain rare amphibolite and metagraywacke clasts possibly derived from the basal aureole. The unconformity

461 postdates some low-angle thrust faults within the ophiolite, and is cut by others. It is clear from the involvement of the aureole in later deformations that it must have formed very early in the emplacement process, probably during the first displacement of the ophiolite over crustal rocks [14,58].

3. Lithological description A complete section of the aureole showing a continuous inverted metamorphic sequence is seen at North Arm, where harzburgite-lherzolite tectonites overlie a few metres of pyroxene amphibolites, about 70 m of garnet amphibolites and amphibolites, and a similar thickness of greenschists and phyllites passing out into unmetamorphosed sediments [ 1,5]. Metasediments are confined to the lower-grade parts of the aureole. Rocks rich in Ti-amphibole, biotite, ilmenite and apatite as well as garnet and orthopyroxene are found locally in the pyroxene amphibolites, and are probably of metasomatic origin [20]. At the eastern end of Trout River Pond in the Table Mountain massif the greenschists have been cut out due to later faulting. The remaining amphibolites appear to be well over 100 m thick, suggesting that the aureole may originally have been considerably thicker in this area than at North Arm. Contact relationships between the peridotite and the aureole are particularly well exposed in this area, and therefore this section was selected for more detailed study. 3.1. Trout River Pond section Uhramafic rocks. The aureole is in contact with spinel-lherzolites (Fig. 1) which form the lowest 500 m of the ultramafic section in the Table Mountain massif [17]. The lherzolites show an early tect0nite fabric, which, close to the basal contact, is overprinted by a fabric sub-parallel to the schistosity in the aureole. In thin section elongate enstatite porphyroclasts show marginal dynamic recrystallisation to small ortho- and clinopyroxenes and tiny spinels. This recrystallisation is accompanied by a decrease in AI203 content of

orthopyroxene from 5-6% to 3-4% [20,64]. A fine-grained olivine (Fog0)-rich matrix flows around the porphyroclasts defining a protomylonitic texture in the terminology of Sibson [18]. Ultramafic mylonites are developed only in the basal 5-10 m of the peridotite. These are associated with the following three high-temperature hydrous assemblages (amphibole terminology according to Leake [ 19]): (1) pargasite lherzolites, (2) Ti-pargasite lherzolites, and (3) amphibolegarnet ariegites. Pargasite-lherzolites are interbanded with metabasic rocks of the aureole at several localities north of Trout River Pond (Fig. lb). They are fine grained (0.2 mm) and show a strong mineral shape fabric with the assemblage pargasite + o r t h o p y r o x e n e + clinopyroxene + olivine + spinel. Compared with nearby lherzolite all minerals are slightly enriched in iron, and pyroxenes have uniformly lower (3-4%) A1203 contents. Spinels are zoned to lower Cr203 contents, which is consistent with release of A1203 from pyroxenes during recrystallisation of original spinel lherzolite (cf. [34]). It is clear that mafic and ultramafic rocks were deformed together at high temperatures, and the interbanding probably resulted from isoclinal folding of an early-formed contact between peridotite and crustal metagabbros [20]. Ti-pargasite lherzolites have been found at one locality on each side of Trout River Pond (Fig. 1c, d) and show a maximum exposed thickness of 2.5 m. Red-brown amphibole-rich bands containing the assemblage titanian pargasite (e.g. Si6.2A12. 2 Ti0.aMg3.aFe0.8Cal.sNa0.8K0.2Oz2(OH)2 ) + titanian phlogopite (Sis.sA12.6Ti0.6Mga.3Fe0.8Na0. 3 K1.5020(OH)4 ) + ilmenite in a partially annealed texture alternate with very fine-grained (0.05 mm) lherzolitic bands showing a mylonitic texture. All minerals have systematically higher F e / M g ratios than lherzolites or pargasite-lherzolites, and steep gradients in F e / M g ratio (e.g. FO87-Fo76 ) occur across the banding over a distance of 1 cm [20]. Compared with normal lherzolite these rocks show enrichment in Ti, K, LREE and many trace elements, and depletion in Mg, Si, Cr and Ni. This metasomatic event is thought to have occurred during mylonitisation [20,27].

462

Amphibole-garnet ariegites [14] are found on the south side of Trout River Pond (Fig. ld). Flaggy bands of ariegite 1-5 cm in width occur in strongly mylonitised and serpentinised lherzolite. The ariegite bands are very fine grained and contain the assemblage clinopyroxene + Ti-pargasite + garnet + orthopyroxene + spinel (+ olivine + plagioclase). All minerals have higher F e / M g ratios than lherzolites or the other hydrous assemblages above [20]. In whole-rock chemistry (Table 2) the amphibole-garnet ariegites correspond to an olivine-rich tholeiitic basalt with a low F e / M g ratio and high Na20. They may be metasomatised tectonic inclusions of mafic aureole rocks similar to the mafic rocks interbanded with the pargasitelherzolites (although of very different chemistry) [20]. The hydrous assemblages and their origin are discussed more fully elsewhere [20,64]. For the purposes of this paper it is sufficient that they

have clearly been highly deformed during emplacement of the ophiolite and hence should reflect the P - T conditions at some stage during this process. Metabasic rocks. The pargasite-lherzolites are in-

terbanded with light green metabasic rocks containing the assemblage plagioclase (Ans0_90)+ clinopyroxene+ garnet with minor pale green hornblende. The rocks are fine grained with a granoblastic texture and correspond to the "hornfels" of Malpas [5]. Larger (2 mm) garnet porphyroblasts often contain inclusions of clinopyroxene and generally have kelyphitic rims containing amphibole and spinel(?). Close to contacts with ultramafic bands garnet, clinopyroxene and plagioclase may co-exist with orthopyroxene and spinel. Alteration of plagioclase to sericite and saussurite is widespread and rodingitic assemblages are locally developed [5]. Although the

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Fig. 2. Sketches of corona textures in coronitic metagabbro TR79-3 (see Fig. lb). Discussion in text.

463 m a f i c / u l t r a m a f i c contacts are strongly altered the low-temperature minerals show no preferred orientation and it seems certain that the interlayering occurred at high temperature and is not a product of low-temperature faulting. Later fault contacts do occur locally, bringing cataclastic brown h o r n b l e n d e amphibolites into contact with serpentinite. Chemically (Table 2), these rocks are very rich in Al203, CaO and Cr203, and have a low F e O / M g O ratio [5]. They are very similar in chemistry to plagioclase-rich mafic cumulates from the "critical zone" at the base of the gabbro in the Bay of Islands Complex [17]. Since the chemistry of metabasic rocks becomes more " n o r m a l " lower down in the aureole [5,20], the distribution of lithologies supports the contention [4] that the aureole represents a thinned and overturned section through an ophiolitic sequence. The "hornfels" grades downwards into brown-hornblende garnet amphibolites 5-10 m from the contact with peridotite. At one locality (Fig. lb) a relatively undeformed metagabbroic pod 1.5 m by 0.3 m in size was found within the aureole 5 m from the interbanded zone. This rock contains well developed corona textures indicating reaction between plagioclase and orthopyroxene or olivine. Fig. 2a shows that the centre of the corona is occupied by chlorite in a polygonal texture. This is surrounded by successive rims of clinopyroxene, garnet, clinopyroxene+ spinel symplectite, and plagioclase (Ans0) full of spinel inclusions. Minor hornblende clearly postdates the corona textures. It is not certain what was originally within the coronas; similar textures in rocks from the Adirondacks [24] have olivine in the centre, sometimes surrounded by orthopyroxene. The chlorite contains 15% AI203 (microprobe), and is unlikely to be a pseudomorph of olivine. Its polygonal texture is very similar to the adjacent clinopyroxene rim, suggesting that it might be replacing a former reaction rim of aluminous orthopyroxene, which may have formed from olivine. Although this hypothesis still requires a considerable amount of chemical transfer during retrogression (orthopyroxenes in the adjacent peridotite contain only 3-4% A1203), it is the only one that can easily

explain the observed sequence of reaction rims. In the coronas garnet is clearly replacing the clinopyroxene-spinel symplectite, which in turn replaces the plagioclase. In certain narrow coronas where no chlorite after orthopyroxene remains, garnet did not form (Fig. 2b) indicating that orthopyroxene was involved in the garnet-forming reaction.

4. Geothermometry and geobarometry 4.1. Analytical details Mineral compositions were determined using a Materials Analysis Company (MAC) 400 electron microprobe fitted with a krisel control automation system using M A G I C correction procedures. All minerals were analysed using 15 kV excitation voltage and a sample current of 300 or 500 /~A. Beam size was 1-2 /~m diameter. A variety of natural mineral standards were used and the standardisation was checked using a kaersutite monitor.

4.2. Geothermometry The geothermometer which can be most widely applied to rocks near the contact is that based on Fe-Mg exchange between garnet and clinopyroxene. Since steep gradients in Fe and Mg exist in these rocks, care was taken to use analyses from adjacent grains and where practicable from spots as close to each other as possible. In some cases the average of several analyses of the same or adjacent grains was used in order to minimise random analytical errors. * In Table 1, temperatures are given using the calibrations of Ellis and Green [30] and Ganguly [31]. Both methods take account of the effects of composition, primarily the Ca content of garnet. Using Ellis and Green's calibration, the temperatures range from 715 to 844°C, whilst Ganguly's calibration yields a range from 825 to 962°C. The amphibole-garnet-ariegite gives the lowest values, but otherwise no pattern is apparent. Fe 2÷ was calculated from microprobe analyses according to * Mineral analyses are available from the author on request.

0.295 0.256 0.166 0.112 0.137 0.502 0.543 0.498

cpx FeT/Mg

1.43 1.65 1.32 1.33 1.30 1.51 1.63 1.65

In K D F e 2+

2 . 9 3 0 + In K o

See Fig. 1 for distances of samples from the aureole-peridotite contact. c Error calculated on basis of 0.6% error in Si (precision of Fleet a n d Barnett [32]). d Error calculated on basis of precisions for Fe and M g [32]. Error calculated on basis of s t a n d a r d deviation in average analysis.

In K n + 1.9034

a T(OK) = 3104xGat + 3 0 3 0 + 10.86 P ( k b a r )

0.148 0.137 0.155 0.150 0.166 0.194 0.253 0.218

XcGt

1.19 1.27 1.01 1.16 1.30 1.29 1.13 1.20

In K D F e "r

b T(OK) = 4801 + 11.07 P ( k b a r ) + 1586X Gt + 1308XMn ~;,

gt rim + cpx gt rim + cpx gt rim + cpx gt + included cpx gt + c p x in c o r o n a gt rim + cpx gt core + cpx core gt rim cpx rim

gt ariegite TR68-13(1) gt ariegite TR68-13(2) " h o r n f e l s " TR79-17 " h o r n f e l s " TR79-2(A) m e t a g a b b r o TR79-3 h b granulite TR68-7 h b granulite TR72-1 hb granulite TR72-1

All temperatures calculated at P = 5 kbar:

Description

Sample

Garnet-clinopyroxene geothermometry

TABLE 1

T(°C) using Fe 2+ 899 825 962 923 933 952 889 877

T ( ° C ) using Fe "r

873 +_36 d 834_+20 d ( + 4 1 ) e 951 + 3 4 d (_+49) ~ 885 + 24 d 84428 d 874 + 22,1 1003+29 d 940 _+ 28 d 790 4- 43 c 715+47 c 833 _+75 c 825 + 105 c 844+62 c 807 4- 29 c 822 + 30 ~ 787 + 29 c

Ganguly [31] b

T(°C) using Fe 2+

Ellis and G r e e n [30] a

.g.

465

the method of Papike et al. [38], which involves very large errors especially in high-Mg pyroxenes. An attempt was made to eliminate these errors by using Fe T instead of Fe 2+ in clinopyroxene for Ellis and Green's method. Table 1 shows that not only are all the temperature estimates higher, as expected, but the scatter is in fact rather greater. This probably indicates that there are real differences in F e 2 + / F e 3+ between the various rocks. If the temperatures in Table 1 are averaged, values of 803 + 38 and 909 + 42°C are obtained by Ellis and Green's and Ganguly's methods respectively. As an absolute temperature estimate Ellis and Green's calibration is preferred since it is based on more experimental data, giving a likely metamorphic temperature in the range 750-850°C. Temperatures were also calculated for the basal peridotites using the two-pyroxene solvus geothermometer of Wells [37]. A range from 800 to 950°C was obtained [20], with a mean of 875 + 49°C. This scatter in results is due partly to the difficulty in assigning cation site occupancies (particularly X ~ ) from microprobe data, as well as a probable lack of equilibrium in some cases. Comparison with feldspar and oxide temperatures in the Adirondacks [33] suggests that this geothermometer tends to overestimate temperatures, so the resuits are probably consistent with the range 750-850°C given above. Another potential geothermometer depends on the Mg-Tschermak's content of orthopyroxenes in spinel-beating assemblages, which has been shown to be almost independent of pressure [25,34]. This again gives temperatures in the range 800-950°C, with the exception of porphyroclast cores in protomylonitic lherzolites (1000-1250°C). This geothermometer is critically dependent on the composition of spinels, which in many of the peridotites are heterogeneous or zoned [20]. However, once again the spread of values is broadly consistent with other geothermometers, beating in mind the higher blocking temperature for A1 mobility compared with Fe and Mg.

4.3. Geobarometry Ultramafic rocks. During deformation the lherzolites and pargasite-lherzolites remained in the

TABLE 2 W h o l e - r o c k analyses a n d n o r m s 1 SiO 2 TiO 2 A1203 Fe203 FeO MnO MgO CaO Na20 K 2° P2Os LOI Total

48.79 20.62 3.11 0.06 7.87 17.15 1.27 1.00 1.00 100.87

2

3

4

46.23 0.07 14.52 0.54 11.80 0.30 12.45 13.00 0.81 0.01 0.03 100.00

43.98 0.85 12.99 I 1.90 a n.d. 0.21 20.38 8.51 1.13 0.00 0.06 3.01 100.66

39.76 1.01 14.43 4.59 14.43 0.32 13.33 11.48 0.56 0.06 0.03 100.00

Normative analyses Mt llm Ab Or An Ne Di 1 H d , i cpx E n "1 Fs ,~ opx F o ~']ol F a .[

0.7 0.2 6.8 35.9 -

5.0 1.8 9.5 30.4 -

6.8 2.1 1.7 0.7 36.8 1.8

25.1 6.2

23.5

_

9.6

8.3 1.3 9.2 1.8

9.2 6.6 -

26.6

20.1

5.7

14.1

4.2 6.1 47.6 3.4

5.6 23.0 1.9

100 M g M g + Fe 100 C a Ca+Na

82

66

83

62

88

90

81

92

a F e 2 + / ( F e Z + + F e 3 + ) ratio a s s u m e d to be the same as for 4 w h e n c a l c u l a t i n g norm. n.d. = not done. 1 = C a - A l - r i c h g a r n e t g r a n u l i t e " h o r n f e l s " from M a l p a s [5]. H i g h K 2 0 content p r o b a b l y p a r t l y due to a l t e r a t i o n of feldspar to sericite; 2 = a l k a l i - p o o r olivine tholeiite of G r e e n and R i n g w o o d [39]; 3 = amphibole-garnet-ariegite, Bay of I s l a n d s [20]; 4 = average of four metapyroxenites, B a l l a n t r a e [10]. All analyses r e c a l c u l a t e d to 100% anhydrous.

spinel-peridotite field without growth of plagioclase or garnet. In the CMAS system this implies [25] pressures of between 5.5 and 12.5 kbar at 850°C and 4.5 and 12 kbar at 750°C. Lherzolites are close to CMAS, but Cr203 in spinels will tend to extend the field of spinel-peridotite to lower

466

110

100

90

•

80

70q

60(

501 3

5

7

9

11

P (kb) Fig. 3. P - T plot showing spinel and garnet-forming reactions in peridotites, gabbros and pyroxenites. Curves A - D in CMAS system. A: f o + a n = o p x + c p x + s p i n e l ; B: c p x + o p x + a n + spinel = gt; from Herzberg [26]. C, D: curves A and B respectively as calculated by Obata [25]. The dashed curves a - d represent extrapolation parallel to curves A - D respectively of Green and Ringwood's [39} experimental data for the spinel-in and garnet-in reactions in alkali-poor olivine tholeiite at 1100°C. Numbered lines are isopleths for X~-aAI2SiO6 cpx in two-pyroxene metagabbroic assemblages [26].

pressure. Production of plagioclase by reaction A (Fig. 3) involves consumption of spinel, and would be expected to produce inverse zoning to higher Cr203 contents in spinel grains (e.g. [34]). In fact the opposite is observed in pargasite-lherzolites, so it is unlikely that plagioclase was present but not observed due to alteration. Mafic rocks. In the uppermost parts of the aureole the assemblage clinopyroxene + plagioclase + garnet ( + hornblende) is well developed. Orthopyroxene has also been reported from these rocks [5]. Compositionally, these rocks are olivine-normative and very rich in A1203 (20%) with very high C a / ( N a + Ca) and M g / ( F e + Mg) ratios (Table

2).

The above assemblage in undersaturated rocks is typical of the intermediate pressure granulite facies [23,39], and can be produced from gabbro by reactions such as A and B in Fig. 3. Evidence for the successive occurrence of reactions of this type is seen in the coronitic metagabbro, where garnet replaces clinopyroxene and spinel which themselves are clearly replacing plagioclase. Green and Ringwood's [39] experiments on natural basaltic compositions indicate that spinel- and garnet-forming reactions occur successively with increasing pressure in Mg-rich undersaturated compositions. In real systems both these reactions are continuous, so reactants and products could coexist over a considerable P-T range. In these circumstances the experimental data on real compositions of Green and Ringwood is likely to be more significant than theoretical extrapolation of results in simple systems such as CMAS. Green and Ringwood's alkali-poor olivine tholeiite is similar in composition to the highest-grade rocks of the aureole (Table 2). The first appearance of garnet in this composition at ll00°C was at 10.1 kbar, and the proportion of olivine decreased considerably in the range 6.8-9 kbar, indicating growth of spinel. In the CMAS system the garnet and spinel-forming reaction curves are strongly curved due to the entropy of mixing of Al in pyroxenes [25,26]. In the absence of reliable data on the mixing properties of other components in the various minerals the data at 1100°C have been extrapolated parallel to the calculated curves in the CMAS system (Fig. 3). This is considered to be the best available means of estimating the pressures. At 850°C pressures of 4.5-5 kbar and 7.5-8 kbar are obtained for the spinel- and garnet-forming reactions respectively (Fig. 3). At 750°C the corresponding pressures are 3.5-4 kbar and 6.5-7 kbar. Some of the garnet-bearing aureole rocks have a higher Mgfl(Mg + Fe) ratio than the experimental composition, and plagioclase in the coronite is Ans0 compared with normative An83 in the alkali-poor olivine tholeiite. Both these changes were shown by Green and Ringwood to shift the garnet-forming reaction to higher pressures and hence 7 kbar is regarded as a reliable minimum pressure at the time of juxtaposition of the peridotite and the aureole.

467 An approximate maximum pressure can be estimated from the presence of plagioclase, which was found to disappear between 12 and 15 kbar for the alkali-poor olivine tholeiite composition [39]. Although tiffs reaction is more difficult to extrapolate to lower temperatures a maximum pressure of about 12 kbar seems reasonable.

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~,

,,/

4.4. P-T-t paths Metagabbros. The chemistry of the granulite-facies metagabbros from the aureole suggest that they first crystallised as cumulates in the oceanic crust (1000°C, 2 kbar). This was followed by cooling for an unknown period (paths (a), (b), Fig. 4) with exsolution of orthopyroxene lamellae in clinopyroxene (Fig. 2), and then by a pressure increase giving successive crossing of the spinel- and garnet-in curves in Fig. 3. These reaction curves are too widely spaced to have been crossed during isobaric cooling as has been suggested in the past [24]. In the coronitic metagabbro, new clinopyroxenes in the coronas and the rims of large clinopyroxenes have Ca-Tschermak's contents of 7-8% and are low ( - 0.1%) in Cr203 and TiO~ [20]. The cores of large clinopyroxenes are lower in CaTschermak's content ( - 5%) and higher in Cr203

/

~

/

\Io)

I

;;"

,

.,

/2

12_ /

Garnet-amphibole-ariegite. Compositionally (Table 2) this rock is far from the CMAS system, and is unlike any of Green and Ringwood's experimental compositions. In view of this, a detailed geobarometric analysis of this rock has not been attempted, although the assemblage is similar to that seen in nearby aureole rocks, and probably formed under similar conditions. This composition is also similar (Table 2) to metapyroxenite in the peridotite of the Ballantrae ophiolite [10], for which minimum pressures of 10-11 kbar have been estimated on the basis of the absence of orthopyroxene and plagioclase in garnet-clinopyroxenea m p h i b o l e - s p i n e l a s s e m b l a g e s . Since b o t h plagioclase and orthopyroxene are present in the ariegites, this can be taken as an approximate maximum pressure in the present case. Thus assemblages on both sides of the peridotite-aureole contact indicate pressures in the range of 7-11 kbar.

It

~e.:

..::: \

/

..'"

~5

'

c ont~ct

t

t

o

(b) 3"~ /'tU

(b)

',o)

t~

t4

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<_.. i " , i / 500

t1

,

,

A 1000

.

.

.

.

J 1500

m (*C) Fig. 4. Possible P-T-t paths within the constraints of thermodynamic data for various levels in the aureole: 1 = lherzolite path assuming initially thick ophiolite slab; 2 = lherzolite path assuming additional nappes emplaced just before formation of the aureole with ophiolite of present-day thickness from time of spreading (Fig. 5). Paths marked (a) correspond to residual heat thermal models; (b) to shear heating models in initially cold crust and mantle, t I to t 4 a r e equivalent times on paths (b). Final cooling not shown. Also shown is garnet-in curve d from Fig. 3.

and TiO 2 ( - 0.5%). The cores must represent original igneous compositions modified only by orthopyroxene exsolution, while the rims evidently formed during corona formation. Examination of the isopleths for Ca-Tschermak's component in clinopyroxene in spinel-bearing two-pyroxene gabbros (Fig. 3) shows that this change could also only have occurred through increase of pressure a n d / o r temperature, and not as a result of cooling. An extension of these paths is seen in the kelyphitic amphibole and spinel(?)-bearing rims around garnets in nearby metagabbros, probably indicating re-entry into the spinel field with decreasing pressure a n d / o r temperature. Finally, replacement of plagioclase by sericite, olivine and orthopyroxene by chlorite, and the development of rodingitic assemblages [5] accompanied serpentinisation of nearby peridotite at low temperature (400°C). Lherzolites. A different P - T path is shown by lherzolites, for which Riccio [17] has estimated initial conditions of 1400°C within the spinellherzolite field (9-20 kbar). This was followed by

468 cooling with pyroxene exsolution and closure of various geothermometers in the 950-1250°C range [17]. The extent of this cooling and the pressure at this time is uncertain, but the lherzolites could have entered the plagioclase peridotite field if equilibrium was incomplete. Later deformation at 800-950°C, 5-12 kbar (in the spinel lherzolite field) resulted in recrystallisation of ortho- and clinopyroxenes to lower A1203 varieties, and growth of new spinel. This deformation phase established the tectonic peridotite-metagabbro contact, and subsequently the P-T-t path of the lherzolite and the uppermost part of the aureole must have been the same. Fig. 4 also shows possible P-T-t paths for the lower-grade parts of the aureole in which there is no evidence for high pressures (cf. [4]).

5. Discussion 5.1. Thermal models

Simple conductive thermal modelling by a number of authors ([1,4,5,9,40], etc.) has shown that residual heat in a hot ophiolite slab can only explain the high "contact" temperatures if initial displacement of the ophiolite over the aureole rocks occurred at or very close to a ridge crest. This is a mechanically attractive site for displacement [20,22], and this model may apply to some ophiolite complexes where spreading and emplacement ages are almost coincident [63]. In the case of the Bay of Islands, however, this model can be ruled out by age data [56,57,59-61]. The spreading age of the ophiolite is constrained by zircon ages from trondjhemites in the Blow-me-Down Massif (504 + 10 Ma) and diorites in the Coastal Complex (508 + 5 Ma) and Sm-Nd mineral isochrons from gabbros (508 + 6 and 501 + 13 Ma). In contrast ages of 460 + 5 Ma (4°mr-39Ar) and 454 5- 9 (K-Ar) have been obtained on amphiboles from the dynamothermal aureole. Since final cooling must have been rapid in order to preserve the steep apparent thermal gradient in the aureole [52], these ages are close to the age of formation of the aureole. Correction of the K-Ar and 4°Ar-39Ar ages using the currently accepted age constants

[44] adds - 8 Ma [62], but this still leaves a gap of about 35 Ma (well outside error) between spreading of the ophiolite and formation of the aureole. Studies of modern heat flow [54] show that oceanic lithosphere of this age would be below 500°C at the level of detachment (12 km) of the ophiolite. Thus shear heating must have been an important source of heat for metamorphism in the aureole, unless the ophiolite was much thicker at the time of displacement than it is now. Unfortunately the possible P-T-t paths do not constrain the amount of cooling before emplacement, as shown in Fig. 4. Another problem is the age of amphiboles in the gabbros and sheeted dykes of the ophiolite. The rapid cooling of oceanic crust means that these ages should be approximately concordant with the spreading ages from zircons and Sm-Nd isochrons. In fact ages of 469 + 7 Ma (4°Ar-39Ar) and 452 + 12 Ma (average K-Ar) have been obtained [61], suggesting that the amphiboles were reset at the time of formation of the aureole. Shear heating could have occurred either on a discrete frictional fault or in a viscous shear zone. At the temperatures involved the latter is more likely, and evidence for ductile deformation is seen at all levels in the Newfoundland aureoles [4,20]. Modelling of viscous shear heating in dry and wet olivine rheologies [40,51,52] suggests that at movement rates of 10 c m / y r maximum temperatures similar to those at the aureole-ophiolite contact could have been achieved in 1-2 Ma (100-200 km of displacement). The thermal anomaly associated with such a zone is 10-20 km wide [51], and amphibole ages in the upper part of the ophiolite could possibly have been affected. Evolution of the aureole in a low-angle shear zone cutting the oceanic crust and upper mantle can explain the observed lithological overturning, with metagabbros overlying " n o r m a l " mafic crustal rocks, and metasediments occurring only in the lower part of the aureole [4,20,22]. Initiation of such a shear zone in cold rocks is very difficult [53] and the movement surface may at first have been a discrete fault, evolving into a shear zone through frictional heating. This explains the lack of overturning in the ultramafic section above the aureole; metagabbros are overlain by lherzolite mylonites rather than dunites or harzburgites as would be

469

expected from complete overturning of the ophiolite section (cf. Fig. 1), suggesting that lherzolites were juxtaposed with gabbros before overturning began. Whatever the mechanism of shear heating, high shear stresses and movement rates (1 kbar, 10 c m / y r ) are required to produce significant temperature increases [40,52]. When different rheologies are sheared together deformation is strongly partitioned into the weaker lithology [51]. Thus as the shear zone brought lherzolites and gabbros over wet diabases and basalts deformation must have been concentrated in the latter, with a consequent decrease in the rate of shear heating [20,52]. This process may have culminated in thrusting of the complete ophioliteaureole sequence over wet sediments at high pore fluid pressures. Preservation of such a steep metamorphic zonation probably could not have occurred without this important cooling mechanism [52].

5.2. Implications of the high-pressure assemblages Assemblages recording pressures of 7-11 kbar are welded to the base of an ophiolite complex only 10-12 km thick; this presents a formidable problem in modelling ophiolite emplacement. High pressures are recorded not only in well-recrystallised ultramafic rocks but also in metagabbros of crustal origin metamorphosed on a P-T-t path involving pressure increase. These cannot simply be relict mantle assemblages brought to high levels •during spreading [49]. Nor are the high-pressure assemblages in exotic slivers somehow incorporated at low temperature between the ophiolite and its aureole; on the contrary they are part of a continuous and coherent sequence formed at high temperature and relatively little disturbed by later movements. Such high-pressure assemblages in ophiolitic aureoles are now sufficiently well constrained and sufficiently widespread [4,6,9, 10,28,45] that they can no longer be ignored or dismissed lightly for want of an adequate model to explain them. Whatever the cause of the high pressure, it must have been removed during assembly of the aureoles since high pressures are not generally recorded in their lower parts [4]. There are three possible mechanisms to explain

the high pressure: (1) Tectonic or hydraulic [28] overpressures. (2) Thinning of the ophiolite during formation of the aureole (Mercier, reported in Nicolas and Le Pichon [3]). (3) Presence of additional nappes or sediments on top of the ophiolite during the early stages of aureole formation.

Overpressures. Assuming that the thermodynamic pressure is equal to the mean stress 6 = ½(a 1 + oz + 03) [29], tectonic "overpressures" equal to 6 - 03 can arise if 03 is vertical. Under "ideal" simple shear conditions, with O I - - 0"2 ~ 0"2 - - 0"3, t h e tectonic overpressure will approximate to the maximum shear stress ½(0.1 - 0 3 ) . Shear stresses of the order 1 kb have been estimated in the basal mylonites of the Bay of Islands lherzolite [55], and similar values are predicted in thermomechanical models [51]. In addition, the coronitic metagabbro shows the high-pressure assemblages but is not deformed; the shear strength of this rock cannot have been exceeded during deformation thus limiting any tectonic overpressure. Hydraulic overpressures could not exceed the tensile strength of the rock without tensile fracture occurring. Since tensile strengths are generally far less than shear strengths, it seems unlikely that the excess pressures can be explained in this way. It appears that tectonic or hydraulic overpressures can at best only explain part of the 3-7 kbar of "excess" pressure seen in the aureole. Thinning. Any tectonic thinning must have been mainly in the ultramafic section of the ophiolite, since the present thickness of the mafic section is close to that of oceanic crust. Since thinning must have occurred during formation of the aureole, ductile deformation of peridotite at high temperatures is the most likely mechanism. Only 10 m of intense basal peridotite mylonite is present, and thinning by 10 km or more across this zone seems unlikely. Thinning in the rest of the peridotite can only have been a low-stress, low-strain rate process at high temperature [55]. Recently J. Casey (personal communication, 1982) has proposed an ingenious model to account for this. He suggests that the inverted metamorphic and lithological se-

470

quence in the aureole evolved over a long period of time (35 Ma) in a subduction zone initiated at a ridge crest. As subduction continued, the subduction zone reduced its angle of dip and thinning occurred in the hot mantle wedge overlying the aureole. Although the mechanism and geometry of this thinning is difficult to visualise, this model does reconcile the P - T data with the thickness of the ophiolite, and minimises the need for shear heating. The main problem seems to lie in timing; if the inverted metamorphic sequence in the aureole resulted from subduction of cold material, then the overlying mantle wedge must have been cooled rapidly both from above and below, leaving little time for ductile thinning at high temperature. The similar ages of amphiboles in the aureole and in the sheeted dyke complex is also not explained, and it seems surprising that the aureole could have remained so coherent during a 35-Ma evolution. Full assessment of this model must await publication. Additional nappes. Any model involving additional nappes or sediments on top of the ophiolite must not only explain their emplacement, but also remove them rapidly during formation of the aureoles. A geometrical model to achieve this based

NW

SE

sl..

km 0

-

n,

6. Conclusions

-- Z"L~\..~ 50

uplilt

beginning

P= 7kbClr 20 km

in
"~

on the ophiolite emplacement model of Nicolas and Le Pichon [3] is illustrated in Fig. 5. An ophiolite slice is detached from below a subduction zone due to compressive beam bending stresses in the downgoing slab. Thus the upper part of the ophiolite could have been partially subducted while a high-pressure aureole was forming at its base. Thrusting in the upper part of the ophiolite [58] and burial may account for resetting of amphibole ages in the diabases and gabbros. A wide thermal aureole generated by shear heating in the aureole (cf. [51]) may also have contributed to this. Rapid uplift of the ophiolite slice would have occurred once it was decoupled from the subducting lithosphere. Density decreases in the ophiolite due to shear heating may have assisted this. One possible problem with this model is again the age of the ophiolite when the aureole was formed; Nicolas and Le Pichon [3] calculate that emplacement of this sort would only occur if lithosphere less than 10 Ma old approached a subduction zone; the Bay of Islands Complex was apparently 30-40 Ma old. This problem might be eased if the Bay of Islands ocean was dominated by transform faults perpendicular to the emplacement direction [15,42], in which case areas of lithosphere of quite different ages might be relatively close to one another, provided the spreading rate was slow. Thus the slice detached in Fig. 5 might have included much younger lithosphere as well as the 30-40 Ma old Bay of Islands Complex.

50

sheGr zone " ~ \ \ ~

Fig. 5. Model for initial emplacement of the Bay of Islands Complex from below a subduction zone. Uplift is due to decoupling of ophiolite from descending lithosphere, and may be assisted by density decrease due to shear heating.

(1) The Bay of Islands Complex was 30-40 Ma old at the time of formation of its metamorphic aureole, and shear heating must therefore have been an important source of heat for metamorphism. (2) The aureole was formed on a low angle thrust fault cutting the oceanic crust and upper mantle, evolving into a viscous shear zone with time. Temperatures of 750-850°C at the peridotite-aureole contact are consistent with a shear zone in mantle rheologies. (3) Preservation of a steep inverted metamorphic zonation in the aureole probably reflects

471

thrusting of the ophiolite-aureole complex over cold sediments in the later stages of emplacement. (4) Pressures recorded in rocks of crustal as well as mantle origin at the peridotite-aureole contact are 3-7 kbar in e x c e s s of the load pressures that could be generated by the present "stratigraphic" thickness of the ophiolite. Anomalously high pressures are not recorded in the lower parts of the aureole. Either the ophiolite was tectonically thinned during formation of the aureole, or more likely it was buried by nappes or sediments early in this process. Both these possibilities must be tested against further data and thermal modelling. (5) A model involving emplacement from below a subduction zone can account for the high pressures by partial subduction of the ophiolite before formation of the aureole. In this model, resetting of amphibole ages in the upper part of the aureole results from burial accompanied by shear heating at the base of the ophiolite, and rapid uplift and erosion (perhaps in part tectonic) results from decoupling of a hot ophiolite from the descending lithosphere.

Acknowledgements This work was undertaken as part of an M.Sc. degree at the University of Western Ontario, and was funded b) an Ontario Graduate Scholarship and N.S.E.R.C. grant No. N-21 to Professor W.R. Church. Professor Church is thanked for his con.stant help and encouragement throughout the project. R.L. Barnett provided invaluable assistance with the microprobe analyses. Drs. J.G. Spray, A.G. Smith, T.J.B. Holland, A. Nicolas and an anonymous reviewer commented on various versions of this paper.

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