Evidence for upper cretaceous transform fault metamorphism in West Cyprus

Earth and Planetar)" Science Letters, 55 (1981) 273-291 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

273

[21

Evidence for Upper Cretaceous transform fault metamorphism in West Cyprus J.G. Spray Department of Earth Sciences. Universi(v of Cambridge, Camhridge, CB2 3E W (U. K.)

and

J.C. Roddick Departmeht of Earth Sciences, Unit~ersit.v of Leeds, Leeds. LS2 9JT (U. K.)

Received May I 1, 1981 Revised version received June 2, 1981

Metamorphic rocks of low-pressure/medium-temperature facies occur in West Cyprus as blocks and slivers with marie and ultramafic screens in high-angle, serpentinite-filled fault zones. A satisfactory explanation for the origin of the metamorphic rocks has previously remained a subject of controversy. The evidence presented here, based on a study of their bulk chemistry, mineralogy and ~Ar/39Ar geochronology, indicates they were produced by greenschist to amphibolite facies dynamothermal metamorphism of alkalic and tholeiitic mafic rocks and associated sediments at between 83 and 90 m.y. Their field relations show similarities with present-day oceanic fracture zones suggesting that metamorphism occurred within strike-slip faults, some of which were probably extensions of the Arakapas transform. We propose that hot crust generated at an oceanic spreading centre provided the heat for metamorphism when juxtaposed against older, cooler rocks during ridge-ridge transform movements. In addition, shear heating may have been facilitated by the strike-slip faulting and contributed to the total heat available. These interpretations are compatible with many aspects of the broader regional geology of Cyprus, provide new constraints on the early evolution of the Troodos Complex and form the basis of a model for transform fault metamorphism.

1. Introduction In W e s t C y p r u s m e t a m o r p h i c rocks are discont i n u o u s l y e x p o s e d in fault zones p e n e t r a t i n g a late Triassic to Cret~iceous basic igneous a n d s e d i m e n t a r y sequence. This sequence, k n o w n as the M a m o n i a C o m p l e x , c o m p r i s e s a. p r e d o m i n a n t l y mafic lower unit a n d an u p p e r m o s t l y allocht h o n o u s s e d i m e n t a r y cover [1]. T h e m e t a m o r p h i c rocks are m a i n l y schistose o r t h o - a m p h i b o l i t e s with s u b o r d i n a t e m e t a s e d i m e n t s in the greenschist to a m p h i b o l i t e facies. T h e y o c c u r as blocks a n d slivers ( < 5 0 0 m thick) a n d as m o r e e l o n g a t e units ( < 2 Cambridge Earth Sciences Series: Contribution No. 126.

k m long) a n d they are always p a r t l y o r wholly in c o n t a c t with serpentinite. The serpentinite, which c o m m o n l y d e l i n e a t e s the a n a s t o m o s i n g fault zones, also c o n t a i n s relatively u n m e t a m o r p h o s e d ultramafic, mafic a n d s e d i m e n t a r y rock fragments. T h e origin of the m e t a m o r p h i c rocks has been the subject of c o n s i d e r a b l e s p e c u l a t i o n since they were first recognized b y G a u d r y [2] a n d subseq u e n t l y briefly d e s c r i b e d by L a p i e r r e [3], Ealey a n d K n o x [4], W o o d c o c k and R o b e r t s o n [5] a n d S w a r b r i c k [6]. T h e y have been variously interpreted as exotic f r a g m e n t s from an external regional m e t a m o r p h i c terrain [4], as s u b - o p h i o l i t e m e t a m o r p h i c rocks [5,7] a n d as the p r o d u c t s of b o t h s h e a r h e a t i n g a n d j u x t a p o s i t i o n of hot crust in

0012-821X/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing Company

274

strike-slip faults [6]. Here we attempt to elucidate the metamorphic and tectonic histories of these rocks and resolve the controversy by detailed study of their field relations, rock and mineral compositions and by dating using the 4°Ar/39Ar method. The results place significant constraints on the origin of the metamorphic rocks and on the geological evolution of Cyprus during the late Mesozoic.

J

z. Geologleal setting The metamorphic rocks were investigated at thre~ localities: Ayia Varvara, Loutra tis Aphroditis and Phasoula (Fig. 1). The largest outcrop of metamorphic rocks occurs north of the village of Ayia Varvara (Fig. 2). Here predominantly amphibolites form a coherent wedge shaped unit within the east-west-trending fault zone. This zone reaches a maximum exposed

J U. Cretaceous-Tertiary sediments

N

Mamonia CompJex

lavas rrTlJ sheeted dykes

STUDIED METAMORPHIC OUTCROPS

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Fig. 1. Simplified geological map of West Cyprus locating the three studied metamorphic outcrops.

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275

width of ca. 1.5 km and is bounded to the north and south by Dhiarizos Group alkali lavas of the Mamonia Complex. North of the amphibolite unit the fault contains serpentinite within which are screens of pillow basalts and subordinate amounts of dolerites, gabbros and harzburgites of supposed Troodos affinities [6]. Southwards the transition from amphibolites to lavas is less abrupt and where exposed occurs as a 3 m wide shear zone. The Loutra tis Aphroditis outcrop is a ca. 500 m wide zone comprising ultramafic and steeply dipping metamorphic rocks. It is bounded to the northwest by Troodos-type mafics and to the southeast by Dhiarizos Group lavas all exposed in a sub-vertical coastal section. Tile depth and lateral extent of the shear zone cannot be determined because of its limited outcrop, but it may continue inland southwest along its strike. The zone comprises grey, quartz-mica phyllites and schists, purple phyllites, marble, massive and schistose amphibolites, serpentinite and serpentinized harzburgite. No systematic changes in metamorphic grade or structural style can be discerned across the shear zone: metamorphosed rocks of both sedimentary and igneous origins and contrasting grade are tectonically intermixed. Three small outcrops of amphibolites were sampled from southwest, west and northwest of the village of Phasoula. The west outcrop is a pearshaped block (ca. 100 × 50 m) entirely surrounded by serpentinite. The southwest and northwest outcrops, like the Ayia Varvara exposure, occur between Dhiarizos Group lavas and serpentinite. The southwest exposure is similar in dimensions to the west Phasoula outcrop. The northwest block has exposed dimensions of ca. 50 x 20 m.

3. Fault dynamics A probable history of movement in the fault zones was established by studying the disposition and tectonic structures of the metamorphic rocks within the fault zones. At Ayia Varvara the amphibolite unit is bounded by a tectonic contact to the south and serpentinite to the north, therefore it must have undergone some displacement since metamor-

phism. The trends of structures within the unit indicate, however, that its displacement was primarily in the vertical a n d / o r horizontal direction(s) within the fault plane and did not involve significant post-metamorphic rotation. Consequently, it may be possible to relate fault rock structure to fault type. For example: the amphibolite unit has a mean SI schistosity of 272/43N, with subhorizontai and approximately east-west-trending FI fold axes (with axial surfaces co-planar to and tightly folding SI) and L1 mineral lineations which are approximately co-axial to F1 hinges. This first deformation coincided with the peak of metamorphism. Some FI folds have been coaxially refolded (F2), followed by later F3 kink folding about steep northeast-plunging axes and subvertical northsouth-trending axial surfaces. Most F3 kinks have " Z " downplunge asymmetry (see stereogram inset of Fig. 2). At Loutra tis Aphroditis the structures are similar to those at Ayia Varvara but are more sheared and disrupted. The metamorphic rocks possess a steep schistosity (066/73S) with F1 fold axes and L 1 mineral lineations trending approximately along strike (FI folds are also co-planar to and tightly fold S1). Here, however, F3 kinks tend to occur in conjugate sets and have both " Z " and "S" asymmetries. At Phasoula the amphibolite outcrops southwest, west and northwest of the village are schistose and dip steeply to the west, northwest and southwest, respectively. Their strikes are subparallel to local trends of the anastomosing fault zone. At all three localities, and especially at Ayia Varvara, it is apparent that the primary internal structural elements of the metamorphic rocks are concordant with the local strikes of the confining fault zones. This is considered strong evidence for deformation of the units having occurred within the fault zones during fault movements. This formation of well-developed schistosities, folds and lineations suggests that the main syn-metamorphic fault movements involved substantial displacement, but it remains difficult to establish whether strike-slip, thrust faulting or both constituted the dominant movement. However, the attitude of the Ayia Varvara F3 kink folds indicates that at least

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Fig. 2. Geological setting and cross-section of the Ayia Varvara metamorphics and related rocks. Mapping and collection of structural data made by J.G. Spray, R.E. Swarbrick and N.H. Woodcock. Stereogram inset shows mean structure trends for only the metamorphic unit: SI schistosities (89 readings); FI and F2 fold axes (16 readings): LI mineral lineations (53 readings) and F3 kink fold axes (18 readings). See text for details.

the later syn-metamorphic fault movements at this locality were strike-slip and dextral.

4. Analytical procedures Mineral compositions of the metamorphic rocks were determined for 11 elements (Na, Mg, A1, Si, P, K, Ca, Ti, Cr, Mn, Fe) using an electronmicroprobe with a Harwell Si (Li) detector and pulse processor [8]• Correction procedures are de-

scribed by Sweatman and Long [9]. Whole rock and trace element analyses were made using a Siemens SRSI X-ray spectrometer. Duplicate fused discs with a lithium tetraborate flux were made for major element determination and calibrated with international standards BCR- l, W-1 and JB-1 [10]. Pressed powder pellets were used for trace element analysis. Mass absorption coefficients for the Phasoula amphibolites were calculated from a 1/M.A.C. versus Nb (background) plot as described by Nisbet et al. [l 1].

277

Otherwise, p r e p a r a t i o n a n d d a t a processing techniques were b a s e d on those used b y N o r r i s h a n d H u t t o n [12]. N a a n d K were d e t e r m i n e d by flame p h o t o m e t r y , F e O b y o x i d i m e t r i c a n d H 2 0 b y Penfield m e t h o d s . 4°Ar/39Ar step heating analysis was m a d e on four a m p h i b o l e separates. T h e a m p h i b o l e s were e x t r a c t e d from host a m p h i b o l i t e s using heavy liquids a n d F r a n z magnet. These mineral separates a n d six aliquots of a m o n i t o r b i o t i t e were then sealed in q u a r t z phials a n d i r r a d i a t e d in the core o f the H e r a l d R e a c t o r A.W.R..E., A l d e r m a s t o n , w h e r e they received a d o s e of ca. 1.58 × 10 Is neut r o n s cm -2. T h e m o n i t o r biotite (LP-6) has a r a d i o g e n i c A r c o n t e n t of 4.316 × 1 0 - 5 c m 3 g - i a K c o n c e n t r a t i o n of 8.343% a n d an age of 124.4 m.y. (Engels, u n p u b l i s h e d data). G a s e x t r a c t i o n a n d d a t a p r o c e s s i n g techniques are given in R o d dick et al. [13] a n d .current c o r r e c t i o n s for Cad e r i v e d A r are n o t e d in S p r a y a n d R o d d i c k [14].

5. C h e m i s t r y and a f f i n i t i e s of the a m p h i b o l i t e s

M a j o r a n d trace e l e m e n t analyses were m a d e of eight a m p h i b o l i t e s : six from the A y i a V a r v a r a o u t c r o p a n d two from L o u t r a tis A p h r o d i t i s . T r a c e e l e m e n t analyses were also m a d e of six a m p h i b o lites from near the village of P h a s o u l a (see Figs. 1 a n d 2 for l o c a t i o n s a n d T a b l e s 1 a n d 2 for data). T h e m a j o r e l e m e n t c h e m i s t r y of the a m p h i b o lites from A y i a V a r v a r a a n d L o u t r a tis A p h r o d i t i s is of a basaltic c h a r a c t e r ( T a b l e 1). However, because m a n y elements are k n o w n to be m o b i l e d u r i n g a l t e r a t i o n a n d m e t a m o r p h i s m , the less mobile trace e l e m e n t s Cr, N b , Ti, Y a n d Z r are used to d e t e r m i n e the likely affinities of the a m p h i b o lites from the three localities. O n the T i / 1 0 0 - Z r - Y × 3 p l o t of Pearce a n d C a n n [15], eight of the f o u r t e e n a n a l y z e d a m p h i b o l i t e s fall within o r j u s t o u t s i d e field B, the field n o r m a l l y o c c u p i e d b y all ocean floor a n d

TABLE 1 Major (wt.%) and trace (ppm) element data for the Ayia Varvara and Loutra tis Aphroditis amphibolites Ayia Varvara

Loutra tis Aphroditis

I

2

3

4

5

6

7

8

49.22 0.50 14.36 3.78 4.83 0.15 9.24 13.83 2.16 0.47 0.04 1.28 0.16 100.02

47.85 I. 10 14.53 4.67 4.73 0.19 6.15 15.84 2.78 0.51 0.13 1.17 0.16 99.81

50.19 0.91 15.41 3.03 7.45 0.16 8.26 9.16 2.78 1.65 0.07 1.28 0.09 100.44

49.73 0.91 15.18 4.91 5.91 0.16 7.84 10.46 2.51 0.94 0.06 1.17 0.19 99.97

41.93 2.81

47.09 0.99 15.10 4.61 5.83 0.19 7.92 13.85 1.78 0.34 0.10

50.06 2.71 15.44 3.15 6.41 0.16 5.77 8.28 2.97

H20 + H20 Total

50.14 1.48 14.16 2.45 6.93 0.27 7.52 I 1.50 2.92 0.27 0.16 1.15 0.70 99.65

1.76

0.13 99.69

1.81 0.07 99.5 I

Cr Nb Ni Rb Sr Ti V Y Zr

307 <5 72 <5 110 8880 399 37 74

507 <5 209 12 88 3000 265 15 24

400 12 135 7 523 6600 269 20 79

394 <5 94 33 95 5460 349 31 52

396 <5 92 29 91 5460 348 30 48

576 68 332 <5 215 16,860 334 28 215

462 <5 279 6 297 5940 281 29 69

275 99 31 47 28O 16,260 209 34 269

SiO2 TiO 2 AI203 Fe203 FeO MnO MgO CaO Na20 K 20

P205

11.71

3.96 8.37 0.02 12.33 12.60

2.18 0.84 0.29 1.93 0.09 99.24

1.65 1.03

278 TABLE 2 Trace element data (ppm) for the amphibolites southwest, west and northwest of Phasoula

Cr Nb Ni Rb Sr Ti V Y Zr

Phasoula southwest

west

9

I1

10

northwest 12

13

14

429 243 249 281 259 246 75 61 133 87 72 186 354 79 59 132 81 34 <5 65 64 41 24 31 130 600 430 435 340 752 12,882 15,008 17,306 14,400 15,613 13,436 280 190 170 266 317 106 39 43 45 36 35 60 166 189 356 268 269 674

some volcanic arc basalts. Five plot in field D, the field of ocean island and continental basalts (Fig. 3). The Y / N b ratio is considered an indicator of the alkalic or tholeiitic nature of basalts altered up to upper greenschist facies [15]. Samples AV-I to 5

Ti//lO0

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Zr

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-~

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*tPw

yx3

Fig. 3. Ti-Y-Zr plots for the 14 amphibolite samples from Ayia Varvara (AV), Loutra tis Aphroditis (LTA) and Phasoula (PH). In addition are shown mean values for upper pillow lavas of the Troodos Complex (UPL), lower pillow lavas and sheeted dykes of the Troodos Complex (LPLD) and lavas from the Dhiarizos Group of the Mamonia Complex (ML). Mean values are from Pearce [181. Island arc hasalts usually plot in fields A and B, ocean floor basalts in field B, calc-alkali basalts in fields B and C and intra-plate basalts in field D.

from Ayia Varvara and LTA-7 from Loutra tis Aphroditis have an average Y / N b ratio of > 4.5. These samples also have low Z r / Y and high Z r / N b ratios typical of slow spreading, mid-ocean ridge or island arc tholeiites [16]. In contrast, samples AV-6 and LTA-8 and all six Phasoula samples have low Y / N b ratios ( < 0.5), high Ti contents and high Z r / Y and low Z r / N b ratios, all indicative of within plate basalts (Table 3). The immobile element data show that the Ayia Varvara amphibolites are predominantly tholeiitic in their affinities, but it is significant that some samples from this exposure have an alkalic character. The same occurs at Loutra tis Aphroditis. In contrast, the Phasoula amphibolites are all alkalic. The occurrence of interlayered amphibolites with contrasting affinities suggests that either the alkalitholeiite combination was originally an igneous feature, or alternatively, tectonic juxtaposition of tholeiitic and alkalic protoliths took place prior to, or during metamorphism (so that both sources were locally metamorphosed and welded together in the fault zones). Any explanation of the origin of the amphibolites must take into account this intermixing, whether it be an igneous or tectonic effect. Both tholeiitic and alkalic lavas and associated sediments occur within and around the fault zones [17] and these may have been the protoliths of the amphibolites and metasediments. To test this hypothesis the trace element contents of the amphibolites are compared with those from the relatively unmetamorphosed Dhiarizos Group and Troodos Complex mafics analyzed by Pearce [18] and Smewing et al. [ 19] in Table 3 and on Ti-Y-Zr and Ti-Cr plots in Figs. 3 and 4. The "tholeiitic" group of amphibolites (samples 1-5, 7) show many similarities with the Troodos Complex mafics, particularly in their immobile element contents, but they are distinct in having a higher concentration of Cr at a given concentration of Ti, Y or Zr. Thus, whereas the Troodos mafics have certain characteristics of island arc tholeiites, the "tholeiitic" amphibolites resemble MORB more closely. ]'he "alkalic" amphibolites (samples 6, 8, 9-14), although comparable with the Dhiarizos Group, show some significant differences, especially in Cr and Nb.

279 TABLE 3 Trace element means (ppm) of the two chemically distinct amphibolite groups compared with means of the alkalic Dhiarizos Group and tholeiitic Troodos Complex mafics * Amphibolite samples 6, 8, 9 - 14 ~

Mamonia Complex

Troodos Complex

Dhiarizos Group

(8)

upper pillow lavas (UPL)

axis sequence (LPLD)

Amphibolite samples 1-5, 7

A(9)

A(19)

(6)

B

B

A(8) Cr Nb Ni Rb Sr Ti V Y Zr

320 98 160 35 398 15,221 234 40 301

Y/Nb Zr/Nb Zr/Y

80 69 . 55 323 15,720 . 33 282

0.41 3.07 7.53

380 1 .

. 29 110 2520

.

. 15 20

0.48 4.09 8.55

405 (31) . 3270 (45) . 15 (44) 31 (44)

15 20 1.40

2.07 (44)

95 1

100 (28) -

. 7 100 6180

5900 (85)

28 66

28 (85) 61 (85)

28 66 2.36

-

2.18 (85)

411 < 6 147 17 201 5890 318 27 58 >4.5 > 9.7 2.14

* Source of data A: Pearce [18], B: Smewing et al. [19]. Bracketed numerals indicate number of samples used to calculate mean.

The comparison, therefore, of amphibolites with the possible protoliths remains inconclusive. Although there are similarities shown in immobile element contents, it is possible that the lavas and amphibolites are unrelated. However, the alkalic and tholeiitic characters shown by the amphibolites most probably reflect the original igneous affinities of their respective source rocks.

olkolic

0

:.o

•

I0000 •

tholeiitic

.o Ti ppm

6. Metamorphic mineralogy

6.1. Amphibolites Most of the amphibolites are schistose and have a foliation defined by tabular amphibole grains often enhanced by a mineral banding comprising layers rich in amphibole, feldspar, epidote or clinopyroxene. Amphibole is the dominant mineral and is mainly associated with plagioclase, epidote, apatite and a Ti-phase (sphene, ilmenite or futile). Some samples also contain pyroxene, garnet or biotite. Particular types of amphibolite are characteristic of each of the three localities and, excluding retrograde phases, their typical mineral assemblages are: Ayia Varoara hornblende-(epidote + albite or andesine)-sphene

1000

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100

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Cr ppm

Fig. 4. Ti-Cr diagram for the 14 amphibolites showing mean values for Troodos and M a m o n i a Complex mafics. Symbols as for Fig. 3.

biotite-hornblende- s p h e n e - rutile-apatite

Loutra tis Aphroditis h o r n b l e n d e - ( e p i d o t e + a l b i t e or oligoclase-andesine)-quartzilmenite-rutile ± garnet ± biotite ± apatite ± pyrite

280 C l i n o p y r o x e n e is p r e s e n t in s e v e r a l a m p h i b o l i t e

Phasoula biotite-hornblende-plagioclase-ilmenite-sphene± apatite ± pyrite

s a m p l e s f r o m P h a s o u l a w h e r e it c o m m o n l y o c c u r s as thin (< 5 mm) layers interbanded with hornb l e n d e w i t h w h i c h it is in e q u i l i b r i u m ( F i g . 5A). All

or: clinopyroxene-hornblende-plagioclase-iimenite-sphene ± apatite--+ pyrite

analyzed

clinopyroxenes were

found

to be

s a l i t e s ( T a b l e 4). P r e - a n d s y n - t e c t o n i c g a r n e t s o c c u r in s e v e r a l

A t all t h r e e l o c a l i t i e s t h e a m p h i b o l e s a r e c a l c i c a n d mostly edenitic or ferroan-pargasitic varieties of

a m p h i b o l i t e s a m p l e s f r o m L o u t r a tis A p h r o d i t i s . L a r g e p r e - t e c t o n i c g a r n e t s (up to 5 m m d i a m e t e r ) are commonly heavily fractured, enclose discor-

h o r n b l e n d e ( T a b l e 4).

d a n t q u a r t z inclusion trails a n d are xenoblastic.

TABLE 4 Microprobe analyses of dated amphiboles and examples of biotite, pyroxenc and garnet. All are from amphibolites Dated amphibole samples a AV-4

AV-5

LTA-7

LTA-8

Biotite

Pyroxene

PH- 14

PH-24

Garnet b LTA- 12 core

SiO2 TiO 2 AI203 Fe203 c FeO MnO MgO CaO Na20 K 20 Total

44.12 0.86 12.63 4.39 I 1.39 0.14 10.82 12.16 0.97 0.66 98.14

Si Ai TM Z

6.46 1.54 8.00

44.99 0.65 11.64 4.48 11.07 0.21 I 1.52 12.11 1.24 0.62 98.53

46.26 0.41 12.17 5.35 8.09 0.22 12.57 I 1.42 1.63 0.21 98.33

44.56 0.49 15.77 8.52 5.26 0.16 I 1.62 10.22 2.07 0.34 99.01

Cations per (23) oxygens 6.56 1.44 8.00

6.64 1.36 8.00

6.31 1.69 8.00

rim

34.19 2.47 16.84 24.65 0.23 12.76 0.34 3.74 95.22

52.37 0.18 1.59 0.60 9.16 0.19 I 1.78 23.33 0.55

38.66 0.20 21.72 28.78 1.09 5.05 5.40

21.84 28.00 0.78 6.03 5.02

99.75

100.90

100.84

(22)

(6)

(24)

(24)

5.2 I 2.79 8.00

1.97 0.03 2.00

6.02

6.06

6.02

6.06

AI vl 0.04 Ti 0.01 Fe 3~ 0.02

AIvt 3.99 Ti 0.02

3.99

Y

4.01

3.90

3.75 0.14 1.17 0.90

3.62 0.10 1.39 0.83

5.96

5.94

AI vl Ti Fe 3+ Fe 2+ Mn Mg Y

0.64 0.09 0.48 1.40 0.02 2.36 4.99

0.56 0.07 0.49 1.35 0.03 2.50 5.00

0.70 0.04 0.58 0.97 0.03 2.69 5.01

0.94 0.05 0.91 0.62 0.02 2.45 4.99

0.23 0.28 3.14 0.03 2.90 6.58

Ca Na K X

1.91 0.28 0.12 2.31

1.89 0.35 0.12 2.36

1.6 0.45 0.04 2.25

1.55 0.57 0.06 2.18

0.06 0.3 0.79

Fe 2+ Mn Mg Ca Na X

0.29 0.01 0.65 0.94 0.04 2.00

39.17

• Amphibole compositions are averages of 20 probe analyses per sample. b Core and rim compositions are from a single garnet porphyroblast. c Ferric iron estimated for amphiboles by summing cations to 13 (excepting C a + N a + K) and for the pyroxene by adjusting cation total to 4. Ferric iron not determined for the biotite or garnet.

281

Many are partially or completely replaced by chlorite and occasionally form "atoll" relics (Fig. 5B). Syn-tectonic garnets are smaller, less fractured, sub-idioblastic and may show rotational inclusion patterns. Compositionally all garnets from the amphibolites are Fe-rich, containing lesser amounts of Mg, Ca and Mn respectively. Mg shows a consistent increase from cores to rims whereas Fe, Ca and Mn decrease (Table 4). Pleochroic brown biotite is present in most thin sections from Phasoula and in the amphibolites with "alkalic" affinities from Ayia Varvara and Loutra tis Aphroditis. It occurs as isolated laths ( < 1 mm long) surrounded by hornblende (Table 4). The mineralogy of the amphibolite outcrops

, . . . .

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~.'-

. 4 ~ ~'~"

~11

.~:~.~.~'~ . ~ ,

• ]

does not reveal the presence of steep metamorphic gradients but rather a uniformity of facies. However, there is some evidence of a marginal increase in grade at Ayia Varvara: over a distance of ca. 500 m there is an apparent increase in metamorphic grade towards the north from epidote amphibolite to low amphibolite facies. Northwards the amphibolites become more abundant in hornblende and less abundant in plagioclase. In addition, the grain size of hornblende becomes larger, contents of AI TM, Ti and Na + K slightly increase and their colour alters from pale to darker green. Plagioclase changes from albite-oligoclase to andesine associated with the breakdown and disappearance of epidote. Retrograde epidote and albite are, however, present throughout the unit but are texturally distinct from their prograde equivalents. The progressive changes in mineral compositions at Ayia Varvara can be related to a northward increasing temperature gradient prevailing during metamorphism, while it is likely that the variable amphibolite mineralogy (i.e. the presence or absence of garnet, clinopyroxene or biotite) has been more closely controlled by bulk chemistry, protolith fabric and the activities of fluid phases during metamorphism.

6.2. Metasediments

Fig. 5. Photomicrographs. A. Layered clinopyroxene amphibolite (sample PH-24). B. Garnet amphibolite (sample LTA-16). Each bar is 2 m m long.

Subordinate amounts of metamorphosed carbonates, cherts, pelites and quartzites occur in association with the amphibolites at Ayia Varvara and Loutra tis Aphroditis. Occasionally the metasediments are found interbanded with the amphibolites but are more common as tectonically separate slivers (normally < 2 m thick). Most of the metasediments are carbonates (marbles) and radiolarian cherts (purple phyllites) with lesser amounts of pelitic and quartzitic phyllites and schists (quartz-garnet-mica-feldspar assemblages). The metamorphosed carbonates and radiolarian cherts probably originated as pelagic sediments, while the compositions of the pelites and quartzites suggest derivation from a terrigenous source. Because some of the metasediments occur interbanded with the amphibolites and have a mineralogy consistent with the same metamorphic grade

282 as the amphibolites, it is inferred that they were associated prior" to metamorphism and were metamorphosed during the same dynamothermal event.

6.3. Metasomatism Localized metasomatism of amphibolite blocks at Phasoula has formed clinopyroxene(ferrosalite)garnet(andradite-grossular)-magnetite layers ( < 5 mm thick), particularly in place of plagioclase-rich bands. Epidote, calcite, sphene, apatite, prehnite and altered feldspar also occur. Hornblende-rich layers have remained largely unaltered. The effects of this metasomatism are mainly restricted to the extremities of blocks causing the formation of incomplete rinds ( < 0.5 m thick). The metasomatic influx of Ca 2÷, and probably Fe 2÷ and Mg 2+, may well have been caused by the release of Ca, Fe and Mg-rich fluids from a clinopyroxenebearing peridotite (i.e. lherzolite) during its hydration to serpentinite at temperatures below 500550°C [20]. The occurrence of amphibolite blocks in direct contact with serpentinite favours this origin and indicates that the main metamorphism (amphibolitization) of mafic protoliths pre-dated serpentinization.

7. 4°Ar/39Argeochronology Four hornblende separates from amphibolites, two from Ayia Varvara (samples AV-4 and 5) and two from Loutra tis Aphroditis (samples LTA-7 and 8), were analyzed by the 4°Ar/39Ar technique. This technique has several advantages over the conventional K-Ar method. Apart from its potential to distinguish samples which have lost radiogenic Ar since crystallization and the presence of extraneous Ar, it can also be used to reveal the distribution of Ca and Cl in amphiboles by studying the release spectra of their artificially derived argon isotopes: 37Ar and 3SAr. Fig. 6 shows the age spectra of the four hornblendes and associated variations in C a / K and C i / K for each heating step (determined from the isotope ratios 37Arca/39ArK and 38Arct/39ArK respectively).

The Ayia Varvara. hornblendes have undisturbed age spectra. Sample AV-5 defines an age plateau at 89-'-2 m.y., while AV-4 has a step pattern which slightly exceeds the error limits with ages from 86 to 89 m.y. However, the integrated age (equivalent to a conventional K-Ar age) of AV-4 (88-+-2 m.y.) compares well with that of sample AV-5. The C a / K and C I / K ratios show similar patterns in both samples, with constant values over the heating steps defining the age plateaux. In contrast, the two hornblende separates from Loutra tis Aphroditis show a greater variation in their age spectra. LTA-8 has a shallow U-shaped pattern ranging from 83 to 90 m.y., while two small gas fractions at the centre of the pattern yield even lower ages of 66 and 78 m.y. The age spectrum of LTA-7 is less clearly defined because 45% of the 39Ar was degassed in the first heating step. However, the subsequent release shows the same step pattern as sample LTA-8, with ages ranging from 82 to 93 m.y. Previously, U-shaped spectra have been interpreted as indicating the presence of excess 4°Ar [21]. However, for these samples it may be attributed to impurities in the mineral separates. Because of the fine grain size of the LTA samples it proved difficult to obtain pure separates and these samples contain significant quantities of feldspar and chlorite. The separate for LTA-7 is ca. 70% hornblende, while LTA-8 is ca. 90% hornblende. Compared with K contents determined from microprobe analyses, K contents based on 39Ar concentrations show that these mineral separates contain about twice as much K as is actually in the hornblende (Tables 4 and 5). The age spectra also support this observation and allow distinction to be made between Ar mainly derived from the amphiboles and Ar located in the impurities. During step heating experiments the AV samples (and in general most hornblendes) do not liberate significant amounts of Ar below 900°C (Fig. 6). However, the LTA samples have already lost ca. 50% of their Ar by this temperature. The ratio plot of C a / K also shows that the Ca sites do not liberate Ar until ca. 900°C and a similar pattern for C! is seen in the C I / K plot. Therefore, the first 50% of gas release is dominated

283

t

't

0

0

2 -

0

,

I

0

I

100I00

90I 80-

80

940 °

LTA- 7

AV-4

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't

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0

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2-I

21_0

0

950 °

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90

I

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i

40

60

80

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60

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~

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~o

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Com. 6. 4°Ar/39Ar spectra plots and normalized ratios of 3SArcl and 37Arc~ to 3'~ArK for the four amphibole samples from Ayia Varvara Loutra tis Aphroditis. Normalization is to the integrated ratio for each complete sample. These ratios reflect variations in C a / K C I / K at the sites releasing Ar. The temperature of the first heating step dominated by Ar release from Ca-sites in hornblende is shown.

284 TABLE 5 Argon isotope data for the four amphibole separates Temperature

36Artr

38Arc1

37Arca

39ARK

( X I 0 - 9 cm 3 STP) a

(°c) A V-4 hornblende (sample weight: 116.1 mg) Blank ¢ 0.0007 700 800 900 920 940 960 990 1020 1040 1060 1095 1150 1400 Total d

0.140 0.013 0.011 0.008 0.012 0.023 0.029 0.03 t 0.018 0.011 0.04 1 0.025 0.029 0.39

Conc./g

3.4

1.049 0.950 2.365 2.626 6.838 25. 100 38.420 46.980 30.190 14.700 59.820 42.520 56.180 327.7 2822.9

1050 1100 1150 1400 Total

0.173 0.046 0.055 0.033 0.034 0.033 0.047 0.42

Conc./g

3.6

I000

7.98

1.276 0.90 I 0.856 0.636 1.229 4.019 5.945 7.193 4.640 2.279 9.194 6.53 I 8.594 53.29 459.0

1400 Total Conc./g

9.440 48.380 85.430 46.610 39.390 39.1 l0 62.030 330.4 2826.0

1.275 0.064 0.070 0.036 0.034 0.03 I 0.054 1.56 13.38

3.062 7.930 14.130 7.715 6.431 6.511 10.240 56.02 479.2

0.054 0.017 0.022 0.016 0.009 0.010 0.026 0.004 0.005 0.16

292.9

0.038 0.021 0.076 0.060 0.037 0.037 0.106 0.02 I 0.022 0.42

1.5

2692.4

3.84

0.016 0.005 0.007

48.77 8.51 7.77 5.59 9.21 27.34 38.70 45.75 29.00 15.07

96.4-~ 4.0 85.3 ± 4.7 87.6 -+ 7.8 87.8 ± 13.0 80.5 ± 3.6 86.9 '. I.I 86.0 -+ 1.1 86.7 -+ 0.7 86.8 '- 0.8 88.4 -~ 5.2 87.5 :-'. 0.7 89.3" 0.7 89.5 + 1.5 87.7 ' 1.8 m.y.

2.4 1.7 1.6 1.2 2.3 7.5 11.2 13.5 8.7 4.3 17.3 12.3 16.1

59.45 41.66 53.97 390.8

93.2 ± 2.7 88.0-+ 1.0 8 8 . 8+- 0.5 87.6 ± 0.7 89.0-'- 0.8 89.0.-- 0.8 89.0*- 0.5 8g.8 =~- 1.8 m.y.

5.5 14.2 25.2 13.8 11.5 11.6 18.3

89.2 ± 82.3 ± 82.5± 86.0:= 86.2± 88.0-+ 91.1 = 91.1 -± 93.0*87.5-+

44.9 12.1 9.9 8.5 5.4 4.4 10.6 2.0 2.2

3366.0

67.81 54.70 90.22 49.39 43.80 43.88 67.63 417.4 3571.0 0.20

7.545 31.290 55.200 47.660 32.320 28.530 66.210 1 1.560

12.620

15.199 4.089 3.350 2.862 1.828 1.503 3.581 0.694 0.760 33.87 311.3

L TA -8 hornblende (86.7 mg) Blank 0.0007 450 500 550

3%r (%)

0.20

L TA -7 hornblende ( 108.8 mg) Blank 0.0007 800 900 950 980 1010 1050 1100 I 150

Apparent age (m.y.) ±2o

0.20 0.722 0.037 0.016 0.009 0.009 0.017 0.017 0.020 0.013 0.006 0.024 0.015 0.021 0.93

A V-5 hornblende (I 16.9 mg) Blank 0.0007 900 950

4OAr

94.10 24.48 22.29 18.99 11.81 10.59 26.50 4.74 55.56 219.1

0.4 1.7 2.1 2.5 3.0 1.5 1.7 2.8 6.8 1.8 m.y.

2013.4 0.20

0.208 0.145 0.392

0.025 0.008 0.004

0.159 0.149 0.334

5.75 2.35 3.68

123.3-"35.0 92.3 -+ 14.0 77.9.+- 10.0

0.5 0.4 1.0

285 TABLE 5 (continued) Temperature (°C)

~6Artr

600 700 750 800 850 900 950 I(X)0 1050 1100 1275 1400 Total

0.006 0.012 0.005 0.005 0.005 0.005 0.024 0.014 0.013 0.013 0.005 0.000 0.14

Conc./g

1.6

37Arca

~XArcl

39ARK

4°Ar

(XIO '~cm3 STP) a 0.799 1.716 1.256 1.874 3.040 5.461 58.240 44.320 34.180 41.450 20.330 0.725 213.4 2461.4

0.005 0.003 0.002 0.003 0.009 0.015 0.212 0.143 0.098 0.123 0.060 0.002 0.71 8.21

1.062 4.182 5.809 4.234 1.150 0.786 5.406 3.726 2.769 3.418 1.734 0.061 34.98 403.4

7.31 25.02 30.41 22.41 6.44 4.46 32.86 22.96 17.52 21.28 10.71 0.36 213.5

Apparent age h (m.y.) ±20 91.1 ± 4.3 89.7 ': 1.3 87.2.'- 0.7 85.8 ' 1.6 77.6± 2.5 66.1 ± 4.4 82.9 ~ 0.8 88.2 ' 2.0 85.6 ± 2.0 88.8 '~ 1.8 91.2 ' 2.7 87.1 ± 46.0 86.4± 1.8 m.y.

3,~Ar (%)

3.0 12.0 16.6 12.1 3.3 2.2 15.5 10.7 7.9 9.8 5.0 0.2

2462.3

All gas quantities corrected for decay, minor products of neutron-induced reactions on major isotopes and for atmospheric "blanks". 37Arc.~=neutron produced from Ca; similarly for Ar isotopes derived from CI and K. tr=trapped; 'U)Ar=trapped plus radiogenic. h Errors for steps are analytical only and do not include error in irradiation parameter J. Ages calculated using decay constants and isotope ratios recommended by Steiger and Jager [47]. Blanks are for all steps except fusion, which may be up to ten times higher. d Includes integrated age. Uncertainty in J (1%) included in error.

by the impurities while the r e m a i n i n g gas release, although having some Ar derived from impurities, will be m a i n l y derived from the h o r n b l e n d e . The C a / K ratios show that the last 40% of Ar release is d o m i n a n t l y from the h o r n b l e n d e . These fractions indicate ages r a n g i n g from 83 to 90 m.y. a n d a n estimated h o r n b l e n d e age of 87-+ 5 m.y. Support for m e t a m o r p h i s m at this time also comes from u n p u b l i s h e d ~ A r / 3 9 A r analyses of a plagioclase separate from LTA-8 which yields a spectrum with most gas fractions having ages from 83 to 88 m.y. Significantly, the ages of the Loutra tis A p h r o d i t i s samples c o n c u r with those from Ayia Varvara. This similarity is not surprising: both localities have geologically c o m p a r a b l e field relations a n d comprise the same rock types, i.e. the evidence is consistent with them having u n d e r g o n e the same p r i m a r y m e t a m o r p h i c event at ca. 8 3 - 9 0 m.y.

8. Summary and interpretation 8.1. Summary of the evidence Investigations of field relations a n d structure, chemistry, mineralogy a n d geochronology of metam o r p h i c rocks in West Cyprus have yielded the following c o n s t r a i n t s on their origin: (1) The m e t a m o r p h i c rocks occur in relatively high-angle, serpentinite-filled fault zones. They were p r o b a b l y deformed within the fault zones d u r i n g fault movements. (2) The m e t a m o r p h i c rocks p r e d o m i n a n t l y consist of amphibolites, some of which have tholeiitic affinities while others have a n alkalic character. M e t a s e d i m e n t s associated with the amphibolites are m e t a m o r p h o s e d pelagic a n d terrigenous sediments. (3) At the three localities there is a general u n i f o r m i t y of m e t a m o r p h i c grade which ranges from the epidote a m p h i b o l i t e to the low a m p h i b o -

286 lite facies. The. P - T conditions of metamorphism cannot be precisely determined, but it is unlikely that temperatures exceeded 600°C. Localized metasomatism of the amphibolites probably occurred during the serpentinization of associated ultramafics after the main metamorphism. (4) Four hornblende separates have 4°Ar/39Ar ages of 83-90 m.y. These ages represent the time when temperatures fell below ca. 550-600°C (i.e. the Ar blocking temperature for the amphiboles). The above results are consistent with the hypothesis that deformation and metamorphism occurred within the host fault zones during fault movements rather than being derived from a regional metamorphic terrain as previously proposed [4]. The metamorphic rocks show certain similarities with dynamothermal "aureoles" associated with the basal thrusts of ophiolites (e.g. [14,22]), and they have been interpreted as such and used as evidence for the allochthoneity of the Troodos Complex [3,5,7]. However, the metamorphic rocks of West Cyprus are not demonstrably located along the base of an ophiolite sequence, nor do they possess steep P - T profiles with grades increasing to upper amphibolite and sometimes granulite facies towards overlying ultramafics (cf. [14,22]). Instead, their occurrence in relatively high-angle, serpentinite-filled faults which penetrate alkalic and tholeiitic basalts and associated sediments indicates an alternative intra-oceanic setting: location in a strike-slip fault complex as suggested by Swarbrick [6]. 8.2. Comparison with oceanic fracture zones

The hypothesis that metamorphism occurred in a strike-slip zone is supported by comparison with present-day oceanic fracture zones. For example: the oceanic fractures which offset the Mid-Atlantic Ridge are part of a series of linear highs and troughs between which are fault scarps with up to 6 km of vertical relief (e.g. [23]). Various rock types have been collected from these scarps including basalts, gabbros, rodingites, amphibolites, peridotites, serpentinites and sediments [24]. Many transverse ridges comprise serpentinite seamounts formed by the vertical protrusion of mantle-derived ultramafics into the fracture zones [25,26].

Amphibolites collected from transverse ridges and axial valleys are usually massive (e.g. [27]), but banded amphibolites derived from gabbros have been sampled and described from the Mid-Atlantic Ridge (e.g. [28]). These comprise alternating amphibole-rich and plagioclase-rich layers interpreted as having formed by contact, dynamic and hydrothermal metamorphism acting on a localized scale less than 1.5 km beneath the axial zone during magma injection and intense faulting. Amphibolites from transverse serpentinite ridges are thought to have been carried to the surface from depths of 1 or 2 km by intrusion of ultramafic diapirs. In a comparable way the blocks and slivers in West Cyprus were probably uplifted from their source areas to finally rest alongside higher level and relatively unmetamorphosed rocks. It has also been shown that modern transt;erse ridges may undergo limited spreading [29] which is often associated with alkalic intrusive-extrusive activity [30]. Similarly, alkalic igneous activity could have provided suitable protolith material for the "alkalic" amphibolites within the fault zones of West Cyprus if they too were once part of an oceanic fracture zone. Evidence of Ca-metasomatism has also been found within transverse ridges of serpentinite from the Mid-Atlantic in the form of rodingitized gabbros. These gabbros are believed to have originally intruded the ultramafics in the lower crust where they were later altered as a result of serpentinization prior to serpentinite protrusion [31]. A comparable process could have been the cause of the serpentinite-related "metasomatism of the amphibolite blocks at Phasoula. It is clear that an oceanic fracture zone was a possible setting for the linear serpentinite bodies and associated metamorphic rocks and mafic and ultramafic screens from West Cyprus. In the next section this analogy is considered relative to the broader geological setting of Cyprus. 9. Palaeoreconstruction 9.1. Regional setting

The interpretation of the serpentinite-filled fault zones of West Cyprus as part of a fossil oceanic

287 fracture zone is strengthened by two additional factors: (1) The occurrence of a fossil transform fault (the Limassol Forest and Arakapas fault belt) striking east-west along the southern flank of the Troodos massif less than 40 km east of the Ayia Varvara and Phasoula fault zones [32,33]. (2) Metamorphism at Ayia Varvara and Loutra tis Aphroditis was approximately synchronous with the igneous crystallization of the Troodos Complex during the Senonian [34-38]. The Arakapas transform fault is well-exposed compared with the fault zones to the west but a number of important similarities can be recognized between the two: All fault zones possess serpentinite cores. Within the serpentinite of the Limassol Forest area are blocks and slivers of layered peridotites, gabbros and dykes [33]; similar lithologies are found in the serpentinite at Ayia Varvara and within many of the serpentinites of West Cyprus. Two of the fault zones show evidence for having undergone dextral strike-slip movement: in the Arakapas fault belt this is indicated by the sense of deviation of dyke trends from north-south to east-west approaching the fault [33]; and at Ayia Varvara, at least as a later movement phase, by the orientation of " Z " kink folds in the amphibolites. In addition, the Ayia Varvara and Phasoula fault zones together have an east-west grain and are along strike from the Arakapas transform and its serpentinite core. These features suggest that the Ayia Varvara and Phasoula localities represent westerly extensions of the Arakapas transform fault. The spatial relationship between these fault zones and the Loutra tis Aphroditis shear zone to the west of the Troodos Complex is not clear. Loutra tis Aphroditis could be part of a distinct fault zone branching from the Arakapas transform, or even a deformed remnant of the spreading centre itself. The second factor supporting the analogy with an oceanic fracture zone concerns the nearby Troodos Complex, which is widely accepted to be a fragment of oceanic lithosphere generated at a spreading centre between ca. 70 and 85 m.y. [3438]. The higher level intrusive rocks of the Complex yield the older ages, e.g. 83 ± 3 m.y. [35],

while the extrusives are apparently younger at ca. 70 m.y. [36,37]. This suggests that fault metamorphism was synchronous with intrusive rather than extrusive igneous activity and that it probably occurred at some depth affecting dykes and gabbros in the strike-slip zone(s). The synchroneity of Troodos Complex igneous crystallization with fault-localized metamorphism is unlikely to be coincidental: depending on the relative position of the Troodos-generating spreading centre at the time of formation of the amphibolites and metasediments, the hot ridge and newly formed crust could have been the heat source for their metamorphism when juxtaposed with older and cooler oceanic crust.

9.2. Palaeoreconstruction Recent research in Cyprus, especially on the sedimentary rocks of the Mamonia Complex southwest of the Troodos massif, is now helping to define the regional geology and history of the Troodos area [3,6,39,40[. The additional constraints provided by this study allow the setting of West Cyprus and the Troodos Complex to be reconstructed for the Upper Cretaceous (see Fig. 7; with palaeoreconstruction illustrated prior to Tertiary anticlockwise rotation of the island through 90 ° [32,41]):

~

CONTINEN|AL ~ I C 4 N LAVAS

Fig. 7. An E-W trending oceanic spreading centre offset by a dextral transform fault approaches a NE-SW oriented continental margin (with alkalic border) as it is consumed by northward subduction during the Senonian. In tl~s setting and during this period the Troodos Complex was generated from one of the ridges and metamorphism was occurring in the transform zone.

288

The north-south orientation of the sheeted dykes of the Troodos Complex a n d their chilling asymmetry show that its spreading centre lay to the present-day west of the massif as a north-south ridge [42]. Secondly, from emplacement vectors and facies analysis of the allochthonous sedimentary rocks of the Mamonia Complex [39], it is likely that the northern limit of the "Troodosbearing" ocean was partly bordered by a NE-SWtrending continental margin (Upper Cretaceous southern "Turkey"). Consequently, as the area of ocean lithosphere to the present-day west (prerotation north) of the Troodos Complex has disappeared, it is likely to have been consumed beneath this margin by northward subduction. In addition, the presence of the Arakapas fossil transform and the Ayia Varvara and Phasoula faults indicates that the spreading ridge was offset. Offsetting could have been caused during initiation of spreading (to accommodate an irregular-shaped rift), by ridge migration ("jumping"), or even by ridge-trench collision if subduction occurred at an angle to the continental margin; processes which all necessitate the inception of a ridge-ridge transform [43]. Fig. 7 shows this reconstruction during the Senonian, with the transform fault initiated prior to ridge-trench collision as the oceanic lithosphere moves northwards and is consumed under the "oblique" continental margin. Although we favour the model shown in Fig. 7, a number of alternative reconstructions are possible, e.g. subduction could have occurred to the south of Troodos in a northward direction (the latter is likely following ridge-trench collision and "locking" in the north).

10. Transform fault metamorphism The ridge-ridge transform system illustrated in Fig. 7 allows localized metamorphism to occur depending on the rate of sea floor spreading and the cooling effect of sea water. Fig. 8 shows a simplified plan of this system in more detail, constructed below the cold surface isotherm and with spreading centres offset by 100 km (a probable minimum displacement for the Troodos situation). Cooling effects and exact isotherm positions are

not considered here, but the limits of isotherrfi perturbation have been determined using the formula:

d- (kL/2v where d = isotherm perturbation distance perpendicular to transform, k = thermal diffusivity, L = distance from ridge parallel to transform and v = half spreading velocity [44]. For k = 10 -2 cm 2 s - t and v = 2 cm yr - 1, L has been varied between 10 and 100 km to show how the isotherms are perturbed relative to the ridges. Because the ridge is offset by the transform, hotter crust is juxtaposed with cooler crust causing metamorphism. Increasing the spreading rate will increase the ratio of hotter crust to cooler crust, while slower spreading will decrease it. Between the ridges metamorphism takes place under shearing conditions. Maximum T gradients are realized

cool r~( le slatiC zone

If

dyr,,amic zono

(~arlwa~)

I

Idextral transf~n~)

a

I

static zone

Iscar/~.~el

b

Fig. 8. Plan of a ridge-ridge dextral transform segment of oceanic crust drawn at ca. 1-2 km below crust surface. Foliated metamorphic rocks are produced between the spreading centres within the transform zone. As new crust moves past each hot offset ridge the shearing ceases, fault plane rocks undergo major thermal overprinting and a "metamorphic wake" is produced. The distribution and shape of isotherms are estimations, but their perturbation limits have been calculated (see text). Insets show two possible sitings of the Troodos Complex (denoted T) during fault metamorphism based on the transform model presented.

289

under static conditions directly opposite each offset ridge, e.g. 2 k m below the ridge axis fresh igneous material at 1000°C juxtaposed with cooler crust at 200°C will yield a contact temperature of ca. 600°C [44]. This position marks the end of shearing as the two segments subsequently move in the same direction outside the transform zone. T gradients decrease away from the ridges but fault plane temperatures may be boosted between the ridges by shear heating if shear stresses and spreading rates are high enough [45]. As spreading continues the metamorphic rocks are carried away from both transform and ridges and temperature gradients diminish as the crust cools. This leaves a transform "'scar" as a narrow (1-2 km.wide) linear zone of ultramafic, mafic and metamorphic rocks (i.e. an inactive fracture zone). The scar may predominantly comprise metamorphic rocks if metamorphism was originally intense within the transform in which case a "metamorphic wake" will be produced. In present-day oceans inactive fracture zones are delineated by topographic troughs and in some examples these can be traced for several thousand kilometres either side of ridge axes (e.g. [46]). Unfortunately their "hard rock" lithologies are usually obscured by sedimentary cover, so it has not yet been possible to study details of their petrology and structure over large areas in order to make rigorous comparisons with supposed ancient fracture zones. However, metamorphic rocks are probably a significant component of oceanic fracture zones. The model presented in Fig. 8 is a gross oversimplification of the thermal complexity of a real ridge-ridge transform system and a more detailed discussion will be presented elsewhere. Undoubtedly the cooling effect of sea water will strongly control the potential of rid.ge-ridge transform systems to cause metamorphism but it remains difficult to quantify. Even a limited amount of water circulation will severely modify the profile shown in Fig. 8. Furthermore, in the case of Cyprus a number of important points should be borne in mind: (1) Evidence indicates that the Troodos Complex was generated near a continental margin and this proximity complicates our simple ridge-ridge

transform model (Fig. 7). Preservation of the Troodos Complex may have originally been due, amongst other later causes, to the oceaniccontinental interface becoming locked soon after its cr3;stallization , perhaps because of ridgecontinent collision. (2) Derivation of amphibolites with both "alkalic" and "tholeiitic" affinities remains problematical, but we prefer a wholly oceanic source for the two without involving the continental margin alkalic rocks, i.e. amphibolites derived from tholeiites produced at a ridge and from alkalic rocks intruded into a "leaky" transform. Terrigenous metasediments probably originated from nearby continental material which had been carried into the "ocean" basin and incorporated into the fault. (3) The similarity in ages between igneous crystallization of the Troodos Complex and faultrelated metamorphism requires metamorphism to have occurred at or near the offset ridges at about the same time as Troodos was being generated. At the time of metamorphism (allowing for blocking temperatures to be reached) this implies that the Troodos Complex must have also been sited at or near a ridge, either (a) within the transform between the spreading centres, or (b) just outside a transform of very limited offset (see Fig. 8). (4) Whether the Troodos Complex is presently rooted or allochthonous has not yet been resolved, but the metamorphic rocks o f West Cyprus do not support the view that this ophiolite has been transported. Contrary to earlier interpretations, these rocks are not the product of obduction but most probably transform metamorphism.

Acknowledgements We thank Drs. R.E. Swarbrick and ,N.H. Woodcock for valuable discussions, criticism of the manuscript and for collaboration in the field. We are also grateful to Drs. D.P. McKenzie, J.A. Pearce and A.G. Smith for their commems on early drafts of the paper. This work was financed by the N.E.R.C.

290

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