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Foliation development in serpentinites, Glenrock, New South Wales
Tec~on[~ph~sjcs, 58 (1979) 81-95 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
FOLIATION DEVELOPMENT SOUTH WALES
IN SERPENTINITES,
GLENROCK,
81
NEW
ADRIAN J. WILLIAMS Geology ~e~art~en~,
Uniuers~~~ of Newcastle,
X.S.W.
230H ~Azis~rulia)
(Received December 5,1978)
ABSTRACT Williams, A.J., 1979. Foliation development in serpentinites, Glenrock, New South Wales. in: T.H. Bell and R.H. Vernon (Editors), Microstructural Processes during Deformation and Metamorphism. Tectonophysics, 58: 81-95. At Glenrock, near the southern end of the Peel Fault System, two fault zones are delineated by melanges in which serpentinite is the main rock type. Protogranular and mylonitic textures are present in relicts of the parent peridotite and in blocks of massive pseudomorphic serpentinite that are surrounded by schistose serpentinite. In schistose serpentinite, the earliest foliation (S,) is defined, microscopically, by the parallel alignment of platy and fibrous serpentine minerals (lizardite and chrysotile) and by trains of magnetite and flattened serpentine pseudomorphs after olivine and pyroxene. It is considered that the schistosity formed perpendicular to the direction of maximum shortening, under conditions in which lizardite and chrysotile were ductile, but antigorite was not, by breakdown of pre-existing serpentine minerals and new growth of lizardite and chrysotile. Post-S, foliations (S2 and S’s) superficially resemble erenulation cleavages in the field but, microscopically, show evidence of shear displacement and are referred to as microshear sets. They probably originated in the ductile-brittle transitional field of serpentine behaviour (Raleigh and Paterson, 1965).
INTRODUCTION
Much of the recent literature in structural geology has been devoted to the discussion of microfabrics, development of foliations (particularly slaty and crenmation cleavages) and multiple deformations in low to medium grade metamorphic rocks. (See Hobbs et al. (1976) for extensive literature sources). These studies have ranged from reconnaissance field surveys to electron microscopic examinations and have mainly been concerned with metasedimentary rocks. Consequently, a tectonically significant group of rocks, the alpine-type serpentinites, has been somewhat neglected. The parent material, peridotite, has been studied in considerable detail by Ave Lallement and Carter (1970), Nicolas et al. {1971), Nicolas and Boudier (1975) and Nicolas and Poirier (1976) because it provides valuable
data on geologic conditions and modes of deformation in the upper mantle. However, apart from studies by Lapham and M&ague (1964), Raleigh and Paterson (1965), Lapham (1967), De Wit et al. (1977), Ross (1977), Saleeby (1978) and MaItman (1978) serpentinites derived from these peridot&es have received little attention, particularly their microscopic characteristics. This may be attributed to the lack of marker horizons in serpentinites and their restricted mineralogy which precludes the development of metamorphic layering_ Also, fabric elements delineated by distinctive metamorphic mineral assemblages, which can be rejated to specific deformational events, do not occur. Detailed studies of serpent&es must improve our understanding of the processes of foliation development. More importantly, by analysing the deformational events in which they have been involved, the number, timing
Fig, 1, Generalized geologic map of northeastern of Glenrock (arrowed).
New South Wales showing the location
83
and nature of movements on faults with which they are intimately associated may be determined. In this context, the Peel Fault System of northeastern New South Wales (Fig, 1) is of primary concern. In this paper, preliminary observations of microscopic characteristics, and a discussion of the possible conditions of formation of the structural elements in serpentinites and associated ultramafic rocks at Glenrock * in northeastern New South Wales (Fig. 1) are presented. A complete geometric analysis of these features is in preparation. GEOLOGIC SETTING
The Peel Fault System (Fig. 1) is a major tectonic element over 300 km in length separating highly deformed lower to middle Palaeozoic rocks of the New England Tablelands Complex in the east from less deformed middle to upper Palaeozoic rocks of the Tamworth Fold Belt to the west (Runnegar, 1974). It is commonly represented as a single fault but is, in fact, a series of parallel faults numbering up to five in some areas (Crook, 1962). It is generally believed that the fault dips steeply towards the east (approximately 65”) and that uplift of the eastern block has occurred (Leitch, 1974). Later lateral movements may also have taken place. However, the number, timing and nature of movement on faults of this system are currently a matter of debate (Schcibner and Glen, 1972; Rod, 1974 and Corbett, 1976). Glenrock is situated on the Peel Fault System approximately 80 km southeast of Tamworth (Fig. l), There, the fault system consists of two main fault zones (Fig. 2) delineated by melange zones comprised dominantly of serpentinite, but also containing blocks of metamorphic rocks including greenschist, blueschist, amphibolite and quartzite. MESOSCOPIC FEATURES OF THE S~RP~~TINITES
Rock
types
Within the melange zones, there are five distinct mesoscopic textural varieties of serpentinite: (1) Massive pseudomorphic - comprised dominantly of serpentine minerals with pseudomorphed pyroxenes (“bastites”) clearly visible to the unaided eye. (2) Massive non-pseudomorphic -- polished and/or slickensided on numerous surfaces and, internally, either mesoscopically featureless or dissected by fine serpentine-fibre veinlets. (3) Schistose - in which, mesoscopically, a11 relict minerals are obliter-
* Refers to the area encompassed 45,000 ha.
by “Glenrock
Station”,
a property of approximately
Upper Carboniferous META - COWGLOMLR*rE @ WNDSTONE b VOLCANIC *c.zxs Devonian
Fig. 2. Simplified geologic map of the Glenrock area compiled from data obtained by the author and staff and students of the Department of Geology, University of Newcastle.
85
ated by a finely penetrative foliation, which is cut by one or more later sets of microshears. (4) Composite - masses composed of phacoids of massive pseudomorphic serpentinite (ranging between approximately 1 cm and 1 m in diameter) set in a matrix of schistose serpentinite. The contact between massive and schistose material is invariably sharp, even in thin section, and may be polished and/or slickensided. The schistose material anastomoses around the blocks, and outcrops are elongate. (5) Brecciated - superficially resembling composite serpentinite, but differing in that the massive fragments are rarely greater than 10 cm in diameter, are angular, and have a random arrangement in a serpentinite matrix which ranges from schistose to non-schistose.
Mesostructures The earliest recognizable secondary foliation in the serpentinites is a schistosity (S,). It is defined by the parallel alignment of platy and fibrous serpentine minerals and is finely penetrative down to the microscale. A fissility is imparted to the rock and weathered masses of this material disintegrate into flaky aggregates. S, is overprinted by two sets of microshears referred to as S2 and S,. The earlier of these (S,) is spaced between 0.5 mm and 10 mm whereas S3 tends to be more widely spaced (approximately 5-30 cm). MICROSCOPIC
STRUCTURES
Characteristics
AND TEXTURES
of the peridotites
Examination of the least altered ultramafic rocks shows that harzburgite is the most common primary ultramafic rock type present. Wehrlite and lherzolite are also represented, but dunite and pyroxenite are absent, except where they occur as monomineralic layers in some peridotite masses. Serpentinization ranges from partial to complete. The primary ultramafic rocks occur either as relatively small (less than 1 m diameter) blocks in composite serpentinite or as elongate slabs 50-100 m long and lo-50 m wide enclosed in schistose or composite serpentinite. The lack of thermal metamorphic effects at contacts between ultramafic and country rocks precludes a magmatic emplacement. They are thought to have been emplaced as cold, tectonic slices (Leitch, 1974). Ages of initial consolidation of the ultramafic rocks, their serpentinization and emplacement are not presently known.
Protogranular
texture
The small blocks in composite serpentinites are generally massive and their grain size, prior to serpentinization, was coarse (approximately 2-5 mm) (Fig. 3). Olivine and orthopyroxene typically exhibit curvilinear and em-
Fig. 3. Protogranular texture in partially serpentinized harzburgite from Glenrock. Crossed polars. Bar scale is 1 mm. 01 = mesh-texture serpentine after olivine: 0p.u = orthopyroxene; M = magnetite.
bayed grain boundaries. Orthopyroxene grains may contain blebs of clinopyroxene and vermicular intergrowths of spinel. Clinopyroxene and spine1 also occur as small grains (less than 1 mm across) adjacent to orthopyroxene grains. Evidence of intracrystalline deformation and of recrystallization is provided, in orthopyroxene grains, by strongly developed undulose extinction and some rare polygonal aggregates at grain boundaries, whereas in olivine grains minor kinking and extensive recrystallization occur. Textures similar to these have been described by Mercier and Nicolas (1975), Nicolas and Poirier (1976) and Basu (1977) and are referred to, by them, as protogranular. According to these authors, such textures arise through. recrystallization of peridot& depleted by partial melting in the upper mantle (Mercier and Nicolas, 1975, p. 478). However, at Glenrock, textural evidence alone is insufficient to discount the possibility that at least some of the protogranular textures are of cumulate origin. Until geochemical data are obtained, it is not possible to discuss the origin of protogranular rocks, in this area, further. Mylonitic texture The larger, elongate slabs of peridot& and some of the small peridotite blocks in composite serpentinite exhibit a segregation banding which is clearly discernible both in the field and in thin section. It is defined by the
87
alternation of pyroxene-rich and olivine-rich (now almost completely serpentinized) layers averaging several millimetres in thickness, but rarely exceeding one centimetre. The texture of these rocks is characterized by highly strained, ragged relict orthopyroxene grains showing undulose extinction and subgrains surrounded by finer, generally strain-free polygonal grains (Fig. 4). Olivine is commonly completely recrystallized. Exsolution lamellae of clinopyroxene and rhombs, or rectangular bars, of spine1 are common in orthopyroxene grains in these rocks. The clinopyroxene lamellae are invariably concentrated in regions of high strain, such as kink band boundaries, suggesting that their formation was strain-related. Similar features have been reported by Etheridge (1975), Basu (1977) and Goode (1978) elsewhere. Kink bands are common in orthopyroxene grains and unrecrystallized olivine grains, and some recrystallized aggregates occur along kink band boundaries in the orthopyroxene grains (cf. Basu, 1977 and Goode, 1978). Spine1 also occurs as irregular-shaped grains scattered throughout the rock. Mercier and Nicolas (1975) used the term “porphyroclastic” to describe textures such as these. However, the term “mylonitic” (Bell and Etheridge, 1973) is more appropriate, genetically, for the rocks at Glenrock. It is suggested that the mylonitic
Fig. 4. Mylonitic texture in partially serpentinized harzburgite from Glenrock. Note undulose extinction in coarser grains and extensive recrystallization. Crossed polars. Bar scale is 1 mm. S = serpentine pseudomorphous after olivine; Opx = orthopyroxene; M = of segregation banding. magnetite ; L = orientation
texture was derived from the protogranular texture by increased deformation. Textures indicative of complete recrystallization have not been observed at Glenrock. However, it is suggested that those portions of the peridotitc. masses which underwent the greatest strains, and consequently the greatest reduction in grain size, became channel-ways for serpentinization and schistose serpentinite development. Watterson (1975), for instance, working in Greenland, suggested that once a region of high strain was established, and a small grain size produced, it tended to persist as a mechanical weakness and to localize subsequent deformations. This is consistent with the hypothesis that schistose serpentinite developed from massive pseudomorphic serpentinite in zones of high strain. Characteristics
of the schistose serpentinite
matrix
Schistosity (S,) Microscopically, the earliest foliation in the serpentinites (S,) is finely penetrative (Fig. 5). It is defined largely by the parallel alignment of platy and fibrous serpentine minerals, which are predominantly a mixture 01 lizardite and orthochrysotile *. Elongate aggregates of magnetite and small, flattened pieces of serpentinized peridotite also parallel the foliation. Where “bastites” comprised essentially of lizardite * are suitably orientated they may be folded, in which case they lie with their axial planes parallel to S,. Such folds are tight to isoclinal and are commonly accompanied by a stretching of the pseudomorph in the plane of the schistosity (Fig. 6). Consequently, this foliation must have been orientated perpendicular to the direction of maximum shortening and was not, therefore, a shear foliation. “Bastites” comprised of antigorite * tend to be less flattened, and pressure-fringes (Spry, 1969) of lizardite and chrysotile have been observed adjacent to these “bastites”, parallel to S, (Fig. 7). The schistosity was thus a ductile feature which formed under conditions in which lizard&e and chrysotile were ductile, but antigorite was not. Limited experimental work by Raleigh and Paterson (1%X), conducted at a strain-rate of ‘7 - 10e4 set-’ , produced such a behaviour at temperatures between 25“ C and 340” C with confining pressures between approximately 3 kb and 4 kb. However, extrapolation of these P-T conditions to geologic strain-rates has not been attempted, so that the figures probably represent an upper limit. In some sections, a transition from massive to schistose serpentinite has been observed (Fig. 8). In these samples, pieces of peridotite have been completely replaced by coarse, bladed antigorite. Near their margins a schistosity defined by a mixture of lizardite and chrysotile anastomoses around lenticular domains of antigorite. The long axes of these lenses parallel the plane _
.__. _.
* All
minerals
X-ray
diffraction
were
identified
camera.
by X-ray
diffraction
using
a Gandolfi
two-rotation
axis
89
Fig. 5. Schistosity
(Sr)
cut
by a later
set of microshears
(S2).
Crossed
polars.
Bar scale is
0.5 mm.
Fig. 6. Portion of a tightly folded and elongate lizardite pseudomorph after pyroxene. Plane-polarized light, Bar scale is 0.5 mm. R = spine1 exsolution rhombs; 01 = serpentine S1, Ss = general orientation pseudomorph after olivine; M = magnetite; Sr = schistosity; of microshear planes.
Fig. 7. Pyroxene pseudomorphed by flaky antigorite with a “pressure fringe” of lizarditc and chrysotile parallel to S,. Crossed polars. Bar scale is 0.1 mm. A = antigorite; C := chrysotile-lizardite mixture;M = magnetite; S, = schistosity.
Fig. 8. Transition from massiqe serpentinite to schistose serpentinite. Bar scale is 0.5 mm. A = antigorite; C = chrysotile-lizardite mixture; orientation.
Crossed polars. S, = sehistosity
91
of the foliation, thus defining a microscopic lineation. Boundaries between schistose and lenticular domains in the transitional zone are sharp, and no microscopic evidence of rotation of earlier antigorite grains into the plane of the foliation has been observed. The schistosity was apparently produced in domains in which serpentinization was well advanced, rather than directly from peridotite, as there is no evidence to suggest that the schistosity was mimetic after a foliation in the peridotite. Metamorphic breakdown of preexisting serpentine minerals (notably antigorite, but also probably lizardite and chrysotile) and orientated, new growth of lizardite and chrysotile in schistose domains is suggested as a mechanism.
Microshear
sets (S, and S,)
Subsequent to the formation of a schistosity in the serpentinites, a set of microshears (S,) was produced which overprinted S1, as seen both microscopically and in hand specimen (Fig. 5). SZ is penetrative throughout both melange zones. Mesoscopically, SZ superficially resembles a crenulation cleavage in that the material between adjacent S2 planes may be crenulated. More commonly, however, S, lies at an angle of approximately 45” or less to S,. The foliation planes are sharply defined and delineated by long-fibre chrysotile, suggesting that they are true planes of shear. In addition, microscopic evidence of S2 as a shear foliation is shown by the rupture and off-set of “bastites” and serpentinized peridotite fragments through which the S, planes pass (Fig. 9), and drag of S, into the foliation planes. The amount of movement that has occurred across a given SZ plane can be estimated from displaced markers (“bastites” and peridotite fragments) and ranges from a few tenths of a millimetre to approximately two millimetres. From the sense of displacement observed at a number of localities throughout the melange zones a movement picture for one of the deformation events that affected the Peel Fault System may be derived. S, microshears are consistently overprinted by a second set of microshears (S,). These are more widely spaced than SZ planes (ranging from a few millimetres to a few tens of centimetres) but, microscopically, exhibit identical features. Mesoscopically, between adjacent S3 planes, the Sz foliation may be folded with the axial planes of the folds orientated at an angle of 45” or less to the S3 planes. Slickensides occur rarely on S3 planes and these plunge sub-horizontally where observed. The conditions of formation of the microshear sets (S, and S,) are of particular interest in that brittle rupture, plastic deformation and new crystal growth apparently all occurred together. Some type of brittle/ ductile transitional behaviour should be invoked to explain these observations. Raleigh and Paterson (1965), in their experimental work, defined a transitional zone which lies at lower confining pressures than the ductile zone discussed earlier. Under experimental strain-rate conditions, temperatures between 25°C and 340°C and confining pressures between 1 kb and
Fig. 9. “Bastite” (B) dragged into S2. Note inferred brittle rupture of “baatite” in places (arrowed). Crossed polars. Bar scale is 0.1 mm. S1 = schistosity; S2 = microshear orientation.
3 kb were observed. Thus, a change from the ductile response, which produced the sehistosity (S,) to transitional behaviour, which produced a set of microshears (S,), suggests a reduction in confining pressure (and probably temperature) from the earlier, inferred, higher P-T conditions present during the formation of schistosity, to these transitional conditions; that is, deformation at shallower crustal levels. Late-stage deformation features
In many areas, post& deformation features, in the form of faults, joints and kinks occur. The faults may be confined to the serpentinite or, in some cases, pass out into the country rocks on either side, in which case clear displacement of the margins of the bodies may be observed. The faults are delineated by serpentinite that is extensively fractured into phacoidaI fragments, and is polished and slickensided on all fracture surfaces. Microscopically, S1, S2 and S3 surfaces appear extremely contorted in these phacoids and numerous small veinlets of cross-fibre chrysotile cut across them. The material readily disintegrates into gravel and cobble sized fragments on weathering and impact. Joints are irregularly dist~buted throughout the area and occur in schistose serpentinite, in which case they overprint the foliations (S,, S2, and
S,), and in massive serpentinite. They are differentiated from S2 and S3 planes because they do not show crenulated material between them, are not penetrative on the scale of an outcrop and are less regularly, and commonly more widely, spaced. Kinks are ubiquitous in the serpentinites but are only visible on the microscale. They occur in long-fibre chrysotile in S2 and S3 planes and are orientated with their axial planes at high angles to the fibres; they are not symmetrical fibre growth curvatures. Time relationships among kinks, joints and late-stage faults have not been determined. CONCLUSIONS
Protogranular and mylonitic textures in partially serpentinized peridotites, or completely pseudomorphed ultramafic rocks, suggest that at least some of the original ultramafic material in the two melange zones at Glenrock may have originated in the upper mantle. Further, this material was emplaced into the earth’s crust as cold, tectonic slices, during which process blocks of metamorphic rocks were caught up with the ultramafic mass. Early serpentinization probably commenced in the stability field of antigorite, but lizardite and chrysotile were probably the dominant serpentine minerals to develop as temperatures decreased. Movement of the masses towards the surface under stress resulted in the metamorphic breakdown of antigorite, and probably also preexisting lizardite and chrysotile, and their replacement by new grains of lizardite and chrysotile. Growth of these two minerals perpendicular to the direction of maximum shortening resulted in the development of a ductile schistosity. This was accompanied by flattening and folding of suitably orientated pseudomorphs. Later movements on the faults produced sets of microshears. These were probably produced within the transitional regime for ductile to brittle behaviour of serpentinite, that is, at shallower crustal levels than for the schistosity. These microshear sets are delineated by long-fibre crysotile and shearing on these is indicated by off-set serpentine pseudomorphs after orthopyroxene and olivine. A consistent sense of displacement is observed, providing a movement picture for one of the deformation events affecting the serpentinites. Late-stage deformation features in the form of joints, faults and microscopic kinks are described but have not been analysed in detail. While these features serve to make structural analysis more difficult, they do not completely obliterate evidence of earlier events. This study has shown that it is possible to interpret the structures,. textures and fabrics in the multiply deformed serpentinites in terms of their tectonic environment; in particular, the timing, number and nature of movements associated with them.
94
ACKNOWLEDGEMENTS
Thanks are extended to Dr. R. Offler for critically reading the manuscript. I am also grateful to B.H.P. (Newcastle) for providing photomicrography facilities, C.S.I.R.O. (North Ryde) for allowing access to X-ray diffraction equipment, and Naroo Pastoral Company for permitting unrestricted access to Glenrock Station. Technical and typing assistance provided by staff at Newcastle TJniversity are also gratefully acknowledged. REFERENCES Ave Lallement, H.G. and Carter, N.L., 1970. Syntectonic recrys~lli~ation and modes of flow in the upper mantle, Geol, Sot. Am. Bull., 81: 2203-2220. Basu, A.R., 1977. Textures, microstructures and deformation of ultramafie xenoliths from San Quentin, Baja, California. Tectonophysics, 43: 213-246. Bell, T.H. and Etheridge, M.A., 1973. Microstructure of mylonites and their descriptive terminology. Lithos, 6: 337-348. Corbett, G.J., 1976. A new fold structure in the Woolomin Beds suggesting a sinistral movement on the Peel Fault. J. Geol. Sot. Aust., 23: 401-406. Crook, K.A.W., 1962. Structural geology of part of the Tamworth Trough. Proc. Linn. Sot. N.S.W., 87: 397-409. De Wit, M.J., Dutch, S., Kligfield, R., Allen, RR. and Stern, C., 1977. Deformation, serpentinization and emplacement of a dunite complex, Gibbs Island, South Shetland Islands: Possible fracture zone tectonics. J. Geol., 85: 745-762. Etheridge, M.A., 1975. Deformation and recrystallization of orthopyroxene from the Gifes Complex, Central Australia. T~onoph~si~s, 25: 87-114. Goode, A.D.T., 1978. High temperature, high strain-rate deformation in the lower crustal Kalka Intrusion, Central Australia. Contrib. Mineral. Petrol., 66: 137-148. Hobbs, BE., Means, W.D. and Williams, P.F., 1976. An Outline of Structural Geology. Wiley, New York, N.Y., 512 pp. Lapham, D.M., 1967, The tectonic history of multiply deformed serpentinite in the Piedmont of Pennsylvania. In: Pd. Wyilie (Editor}, Ultramafic and Related Rocks. Wiley, New York, N.Y., pp. 174-183. Lapham, D.M. and M&ague, H.L., 1964. Structural patterns associated witb the serpentinites of southeastern Pennsylvania. Geol. Sot. Am. Bull., 75: 639-660. Leitch, E.C., 1974. The geological development of the southern part of the New England Fold Belt. J. Geol, Sot. Aust., 21: 133-156. Maltman, A.J., 1978. Serpentine textures in Anglesey, North Wales, United Kingdom. Geol. Sot. Am. Bull., 89: 972-980. Mercier, J.C. and Nicolas, A., 1975. Textures and fabrics of upper mantle peridotites as illustrated by xenoliths from basal&. J. Petrol., 16: 454-487. Nicolas, A. and Boudier, F., 1975. Kinematic interpretation of folds in alpine-type peridotites. Tectonophysics, 25 : 233-260. Nicolas, A. and Poirier, J.P., 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. Wiley, London, 544 pp. Nicolas, A,, Bouchez, J.L., Boudier, F. and Mercier, J.C., 1971. Textures, structures and fabrics due to solid state flow in some European lherzolites. Teetonophysics, 12: 55-86. Raleigh, C.B. and Paterson, MS., 1965. Experimental deformation of serpentinite and its tectonic implications. J. Geophys. Res., 70: 3965-3985. Rod, E., 1974. Structural interpretation of New England region. J. Proc. R. Sot. N.S.W., 107: 90-99.
95 Ross, J.V., 1977. The internal fabric of an alpine peridotite near Pinchi Lake, central British Columbia. Can. J. Earth Sci., 14: 32-44. Runnegar, B., 1974. The geological framework of New England. Geol. Sot. Aust., Qld. Div., Field Guide, pp. 9-19. Saleeby, J., 1978. Kings River ophiolite, southwest Sierra Nevada foothills, California. Geol. Sot. Am. Bull., 89: 617436. Scheibner, E. and Glen, R.A., 1972. The Peel Thrust and its tectonic history. Geol. Surv., N.S.W., Q. Notes, 8: 2-14. Spry, A., 1969. Metamorphic Textures. Pergamon, Oxford, 350 pp. Watterson, J., 1975. Mechanism for the persistence of tectonic lineaments. Nature, 253: 520-522.
FOLIATION DEVELOPMENT SOUTH WALES
IN SERPENTINITES,
GLENROCK,
81
NEW
ADRIAN J. WILLIAMS Geology ~e~art~en~,
Uniuers~~~ of Newcastle,
X.S.W.
230H ~Azis~rulia)
(Received December 5,1978)
ABSTRACT Williams, A.J., 1979. Foliation development in serpentinites, Glenrock, New South Wales. in: T.H. Bell and R.H. Vernon (Editors), Microstructural Processes during Deformation and Metamorphism. Tectonophysics, 58: 81-95. At Glenrock, near the southern end of the Peel Fault System, two fault zones are delineated by melanges in which serpentinite is the main rock type. Protogranular and mylonitic textures are present in relicts of the parent peridotite and in blocks of massive pseudomorphic serpentinite that are surrounded by schistose serpentinite. In schistose serpentinite, the earliest foliation (S,) is defined, microscopically, by the parallel alignment of platy and fibrous serpentine minerals (lizardite and chrysotile) and by trains of magnetite and flattened serpentine pseudomorphs after olivine and pyroxene. It is considered that the schistosity formed perpendicular to the direction of maximum shortening, under conditions in which lizardite and chrysotile were ductile, but antigorite was not, by breakdown of pre-existing serpentine minerals and new growth of lizardite and chrysotile. Post-S, foliations (S2 and S’s) superficially resemble erenulation cleavages in the field but, microscopically, show evidence of shear displacement and are referred to as microshear sets. They probably originated in the ductile-brittle transitional field of serpentine behaviour (Raleigh and Paterson, 1965).
INTRODUCTION
Much of the recent literature in structural geology has been devoted to the discussion of microfabrics, development of foliations (particularly slaty and crenmation cleavages) and multiple deformations in low to medium grade metamorphic rocks. (See Hobbs et al. (1976) for extensive literature sources). These studies have ranged from reconnaissance field surveys to electron microscopic examinations and have mainly been concerned with metasedimentary rocks. Consequently, a tectonically significant group of rocks, the alpine-type serpentinites, has been somewhat neglected. The parent material, peridotite, has been studied in considerable detail by Ave Lallement and Carter (1970), Nicolas et al. {1971), Nicolas and Boudier (1975) and Nicolas and Poirier (1976) because it provides valuable
data on geologic conditions and modes of deformation in the upper mantle. However, apart from studies by Lapham and M&ague (1964), Raleigh and Paterson (1965), Lapham (1967), De Wit et al. (1977), Ross (1977), Saleeby (1978) and MaItman (1978) serpentinites derived from these peridot&es have received little attention, particularly their microscopic characteristics. This may be attributed to the lack of marker horizons in serpentinites and their restricted mineralogy which precludes the development of metamorphic layering_ Also, fabric elements delineated by distinctive metamorphic mineral assemblages, which can be rejated to specific deformational events, do not occur. Detailed studies of serpent&es must improve our understanding of the processes of foliation development. More importantly, by analysing the deformational events in which they have been involved, the number, timing
Fig, 1, Generalized geologic map of northeastern of Glenrock (arrowed).
New South Wales showing the location
83
and nature of movements on faults with which they are intimately associated may be determined. In this context, the Peel Fault System of northeastern New South Wales (Fig, 1) is of primary concern. In this paper, preliminary observations of microscopic characteristics, and a discussion of the possible conditions of formation of the structural elements in serpentinites and associated ultramafic rocks at Glenrock * in northeastern New South Wales (Fig. 1) are presented. A complete geometric analysis of these features is in preparation. GEOLOGIC SETTING
The Peel Fault System (Fig. 1) is a major tectonic element over 300 km in length separating highly deformed lower to middle Palaeozoic rocks of the New England Tablelands Complex in the east from less deformed middle to upper Palaeozoic rocks of the Tamworth Fold Belt to the west (Runnegar, 1974). It is commonly represented as a single fault but is, in fact, a series of parallel faults numbering up to five in some areas (Crook, 1962). It is generally believed that the fault dips steeply towards the east (approximately 65”) and that uplift of the eastern block has occurred (Leitch, 1974). Later lateral movements may also have taken place. However, the number, timing and nature of movement on faults of this system are currently a matter of debate (Schcibner and Glen, 1972; Rod, 1974 and Corbett, 1976). Glenrock is situated on the Peel Fault System approximately 80 km southeast of Tamworth (Fig. l), There, the fault system consists of two main fault zones (Fig. 2) delineated by melange zones comprised dominantly of serpentinite, but also containing blocks of metamorphic rocks including greenschist, blueschist, amphibolite and quartzite. MESOSCOPIC FEATURES OF THE S~RP~~TINITES
Rock
types
Within the melange zones, there are five distinct mesoscopic textural varieties of serpentinite: (1) Massive pseudomorphic - comprised dominantly of serpentine minerals with pseudomorphed pyroxenes (“bastites”) clearly visible to the unaided eye. (2) Massive non-pseudomorphic -- polished and/or slickensided on numerous surfaces and, internally, either mesoscopically featureless or dissected by fine serpentine-fibre veinlets. (3) Schistose - in which, mesoscopically, a11 relict minerals are obliter-
* Refers to the area encompassed 45,000 ha.
by “Glenrock
Station”,
a property of approximately
Upper Carboniferous META - COWGLOMLR*rE @ WNDSTONE b VOLCANIC *c.zxs Devonian
Fig. 2. Simplified geologic map of the Glenrock area compiled from data obtained by the author and staff and students of the Department of Geology, University of Newcastle.
85
ated by a finely penetrative foliation, which is cut by one or more later sets of microshears. (4) Composite - masses composed of phacoids of massive pseudomorphic serpentinite (ranging between approximately 1 cm and 1 m in diameter) set in a matrix of schistose serpentinite. The contact between massive and schistose material is invariably sharp, even in thin section, and may be polished and/or slickensided. The schistose material anastomoses around the blocks, and outcrops are elongate. (5) Brecciated - superficially resembling composite serpentinite, but differing in that the massive fragments are rarely greater than 10 cm in diameter, are angular, and have a random arrangement in a serpentinite matrix which ranges from schistose to non-schistose.
Mesostructures The earliest recognizable secondary foliation in the serpentinites is a schistosity (S,). It is defined by the parallel alignment of platy and fibrous serpentine minerals and is finely penetrative down to the microscale. A fissility is imparted to the rock and weathered masses of this material disintegrate into flaky aggregates. S, is overprinted by two sets of microshears referred to as S2 and S,. The earlier of these (S,) is spaced between 0.5 mm and 10 mm whereas S3 tends to be more widely spaced (approximately 5-30 cm). MICROSCOPIC
STRUCTURES
Characteristics
AND TEXTURES
of the peridotites
Examination of the least altered ultramafic rocks shows that harzburgite is the most common primary ultramafic rock type present. Wehrlite and lherzolite are also represented, but dunite and pyroxenite are absent, except where they occur as monomineralic layers in some peridotite masses. Serpentinization ranges from partial to complete. The primary ultramafic rocks occur either as relatively small (less than 1 m diameter) blocks in composite serpentinite or as elongate slabs 50-100 m long and lo-50 m wide enclosed in schistose or composite serpentinite. The lack of thermal metamorphic effects at contacts between ultramafic and country rocks precludes a magmatic emplacement. They are thought to have been emplaced as cold, tectonic slices (Leitch, 1974). Ages of initial consolidation of the ultramafic rocks, their serpentinization and emplacement are not presently known.
Protogranular
texture
The small blocks in composite serpentinites are generally massive and their grain size, prior to serpentinization, was coarse (approximately 2-5 mm) (Fig. 3). Olivine and orthopyroxene typically exhibit curvilinear and em-
Fig. 3. Protogranular texture in partially serpentinized harzburgite from Glenrock. Crossed polars. Bar scale is 1 mm. 01 = mesh-texture serpentine after olivine: 0p.u = orthopyroxene; M = magnetite.
bayed grain boundaries. Orthopyroxene grains may contain blebs of clinopyroxene and vermicular intergrowths of spinel. Clinopyroxene and spine1 also occur as small grains (less than 1 mm across) adjacent to orthopyroxene grains. Evidence of intracrystalline deformation and of recrystallization is provided, in orthopyroxene grains, by strongly developed undulose extinction and some rare polygonal aggregates at grain boundaries, whereas in olivine grains minor kinking and extensive recrystallization occur. Textures similar to these have been described by Mercier and Nicolas (1975), Nicolas and Poirier (1976) and Basu (1977) and are referred to, by them, as protogranular. According to these authors, such textures arise through. recrystallization of peridot& depleted by partial melting in the upper mantle (Mercier and Nicolas, 1975, p. 478). However, at Glenrock, textural evidence alone is insufficient to discount the possibility that at least some of the protogranular textures are of cumulate origin. Until geochemical data are obtained, it is not possible to discuss the origin of protogranular rocks, in this area, further. Mylonitic texture The larger, elongate slabs of peridot& and some of the small peridotite blocks in composite serpentinite exhibit a segregation banding which is clearly discernible both in the field and in thin section. It is defined by the
87
alternation of pyroxene-rich and olivine-rich (now almost completely serpentinized) layers averaging several millimetres in thickness, but rarely exceeding one centimetre. The texture of these rocks is characterized by highly strained, ragged relict orthopyroxene grains showing undulose extinction and subgrains surrounded by finer, generally strain-free polygonal grains (Fig. 4). Olivine is commonly completely recrystallized. Exsolution lamellae of clinopyroxene and rhombs, or rectangular bars, of spine1 are common in orthopyroxene grains in these rocks. The clinopyroxene lamellae are invariably concentrated in regions of high strain, such as kink band boundaries, suggesting that their formation was strain-related. Similar features have been reported by Etheridge (1975), Basu (1977) and Goode (1978) elsewhere. Kink bands are common in orthopyroxene grains and unrecrystallized olivine grains, and some recrystallized aggregates occur along kink band boundaries in the orthopyroxene grains (cf. Basu, 1977 and Goode, 1978). Spine1 also occurs as irregular-shaped grains scattered throughout the rock. Mercier and Nicolas (1975) used the term “porphyroclastic” to describe textures such as these. However, the term “mylonitic” (Bell and Etheridge, 1973) is more appropriate, genetically, for the rocks at Glenrock. It is suggested that the mylonitic
Fig. 4. Mylonitic texture in partially serpentinized harzburgite from Glenrock. Note undulose extinction in coarser grains and extensive recrystallization. Crossed polars. Bar scale is 1 mm. S = serpentine pseudomorphous after olivine; Opx = orthopyroxene; M = of segregation banding. magnetite ; L = orientation
texture was derived from the protogranular texture by increased deformation. Textures indicative of complete recrystallization have not been observed at Glenrock. However, it is suggested that those portions of the peridotitc. masses which underwent the greatest strains, and consequently the greatest reduction in grain size, became channel-ways for serpentinization and schistose serpentinite development. Watterson (1975), for instance, working in Greenland, suggested that once a region of high strain was established, and a small grain size produced, it tended to persist as a mechanical weakness and to localize subsequent deformations. This is consistent with the hypothesis that schistose serpentinite developed from massive pseudomorphic serpentinite in zones of high strain. Characteristics
of the schistose serpentinite
matrix
Schistosity (S,) Microscopically, the earliest foliation in the serpentinites (S,) is finely penetrative (Fig. 5). It is defined largely by the parallel alignment of platy and fibrous serpentine minerals, which are predominantly a mixture 01 lizardite and orthochrysotile *. Elongate aggregates of magnetite and small, flattened pieces of serpentinized peridotite also parallel the foliation. Where “bastites” comprised essentially of lizardite * are suitably orientated they may be folded, in which case they lie with their axial planes parallel to S,. Such folds are tight to isoclinal and are commonly accompanied by a stretching of the pseudomorph in the plane of the schistosity (Fig. 6). Consequently, this foliation must have been orientated perpendicular to the direction of maximum shortening and was not, therefore, a shear foliation. “Bastites” comprised of antigorite * tend to be less flattened, and pressure-fringes (Spry, 1969) of lizardite and chrysotile have been observed adjacent to these “bastites”, parallel to S, (Fig. 7). The schistosity was thus a ductile feature which formed under conditions in which lizard&e and chrysotile were ductile, but antigorite was not. Limited experimental work by Raleigh and Paterson (1%X), conducted at a strain-rate of ‘7 - 10e4 set-’ , produced such a behaviour at temperatures between 25“ C and 340” C with confining pressures between approximately 3 kb and 4 kb. However, extrapolation of these P-T conditions to geologic strain-rates has not been attempted, so that the figures probably represent an upper limit. In some sections, a transition from massive to schistose serpentinite has been observed (Fig. 8). In these samples, pieces of peridotite have been completely replaced by coarse, bladed antigorite. Near their margins a schistosity defined by a mixture of lizardite and chrysotile anastomoses around lenticular domains of antigorite. The long axes of these lenses parallel the plane _
.__. _.
* All
minerals
X-ray
diffraction
were
identified
camera.
by X-ray
diffraction
using
a Gandolfi
two-rotation
axis
89
Fig. 5. Schistosity
(Sr)
cut
by a later
set of microshears
(S2).
Crossed
polars.
Bar scale is
0.5 mm.
Fig. 6. Portion of a tightly folded and elongate lizardite pseudomorph after pyroxene. Plane-polarized light, Bar scale is 0.5 mm. R = spine1 exsolution rhombs; 01 = serpentine S1, Ss = general orientation pseudomorph after olivine; M = magnetite; Sr = schistosity; of microshear planes.
Fig. 7. Pyroxene pseudomorphed by flaky antigorite with a “pressure fringe” of lizarditc and chrysotile parallel to S,. Crossed polars. Bar scale is 0.1 mm. A = antigorite; C := chrysotile-lizardite mixture;M = magnetite; S, = schistosity.
Fig. 8. Transition from massiqe serpentinite to schistose serpentinite. Bar scale is 0.5 mm. A = antigorite; C = chrysotile-lizardite mixture; orientation.
Crossed polars. S, = sehistosity
91
of the foliation, thus defining a microscopic lineation. Boundaries between schistose and lenticular domains in the transitional zone are sharp, and no microscopic evidence of rotation of earlier antigorite grains into the plane of the foliation has been observed. The schistosity was apparently produced in domains in which serpentinization was well advanced, rather than directly from peridotite, as there is no evidence to suggest that the schistosity was mimetic after a foliation in the peridotite. Metamorphic breakdown of preexisting serpentine minerals (notably antigorite, but also probably lizardite and chrysotile) and orientated, new growth of lizardite and chrysotile in schistose domains is suggested as a mechanism.
Microshear
sets (S, and S,)
Subsequent to the formation of a schistosity in the serpentinites, a set of microshears (S,) was produced which overprinted S1, as seen both microscopically and in hand specimen (Fig. 5). SZ is penetrative throughout both melange zones. Mesoscopically, SZ superficially resembles a crenulation cleavage in that the material between adjacent S2 planes may be crenulated. More commonly, however, S, lies at an angle of approximately 45” or less to S,. The foliation planes are sharply defined and delineated by long-fibre chrysotile, suggesting that they are true planes of shear. In addition, microscopic evidence of S2 as a shear foliation is shown by the rupture and off-set of “bastites” and serpentinized peridotite fragments through which the S, planes pass (Fig. 9), and drag of S, into the foliation planes. The amount of movement that has occurred across a given SZ plane can be estimated from displaced markers (“bastites” and peridotite fragments) and ranges from a few tenths of a millimetre to approximately two millimetres. From the sense of displacement observed at a number of localities throughout the melange zones a movement picture for one of the deformation events that affected the Peel Fault System may be derived. S, microshears are consistently overprinted by a second set of microshears (S,). These are more widely spaced than SZ planes (ranging from a few millimetres to a few tens of centimetres) but, microscopically, exhibit identical features. Mesoscopically, between adjacent S3 planes, the Sz foliation may be folded with the axial planes of the folds orientated at an angle of 45” or less to the S3 planes. Slickensides occur rarely on S3 planes and these plunge sub-horizontally where observed. The conditions of formation of the microshear sets (S, and S,) are of particular interest in that brittle rupture, plastic deformation and new crystal growth apparently all occurred together. Some type of brittle/ ductile transitional behaviour should be invoked to explain these observations. Raleigh and Paterson (1965), in their experimental work, defined a transitional zone which lies at lower confining pressures than the ductile zone discussed earlier. Under experimental strain-rate conditions, temperatures between 25°C and 340°C and confining pressures between 1 kb and
Fig. 9. “Bastite” (B) dragged into S2. Note inferred brittle rupture of “baatite” in places (arrowed). Crossed polars. Bar scale is 0.1 mm. S1 = schistosity; S2 = microshear orientation.
3 kb were observed. Thus, a change from the ductile response, which produced the sehistosity (S,) to transitional behaviour, which produced a set of microshears (S,), suggests a reduction in confining pressure (and probably temperature) from the earlier, inferred, higher P-T conditions present during the formation of schistosity, to these transitional conditions; that is, deformation at shallower crustal levels. Late-stage deformation features
In many areas, post& deformation features, in the form of faults, joints and kinks occur. The faults may be confined to the serpentinite or, in some cases, pass out into the country rocks on either side, in which case clear displacement of the margins of the bodies may be observed. The faults are delineated by serpentinite that is extensively fractured into phacoidaI fragments, and is polished and slickensided on all fracture surfaces. Microscopically, S1, S2 and S3 surfaces appear extremely contorted in these phacoids and numerous small veinlets of cross-fibre chrysotile cut across them. The material readily disintegrates into gravel and cobble sized fragments on weathering and impact. Joints are irregularly dist~buted throughout the area and occur in schistose serpentinite, in which case they overprint the foliations (S,, S2, and
S,), and in massive serpentinite. They are differentiated from S2 and S3 planes because they do not show crenulated material between them, are not penetrative on the scale of an outcrop and are less regularly, and commonly more widely, spaced. Kinks are ubiquitous in the serpentinites but are only visible on the microscale. They occur in long-fibre chrysotile in S2 and S3 planes and are orientated with their axial planes at high angles to the fibres; they are not symmetrical fibre growth curvatures. Time relationships among kinks, joints and late-stage faults have not been determined. CONCLUSIONS
Protogranular and mylonitic textures in partially serpentinized peridotites, or completely pseudomorphed ultramafic rocks, suggest that at least some of the original ultramafic material in the two melange zones at Glenrock may have originated in the upper mantle. Further, this material was emplaced into the earth’s crust as cold, tectonic slices, during which process blocks of metamorphic rocks were caught up with the ultramafic mass. Early serpentinization probably commenced in the stability field of antigorite, but lizardite and chrysotile were probably the dominant serpentine minerals to develop as temperatures decreased. Movement of the masses towards the surface under stress resulted in the metamorphic breakdown of antigorite, and probably also preexisting lizardite and chrysotile, and their replacement by new grains of lizardite and chrysotile. Growth of these two minerals perpendicular to the direction of maximum shortening resulted in the development of a ductile schistosity. This was accompanied by flattening and folding of suitably orientated pseudomorphs. Later movements on the faults produced sets of microshears. These were probably produced within the transitional regime for ductile to brittle behaviour of serpentinite, that is, at shallower crustal levels than for the schistosity. These microshear sets are delineated by long-fibre crysotile and shearing on these is indicated by off-set serpentine pseudomorphs after orthopyroxene and olivine. A consistent sense of displacement is observed, providing a movement picture for one of the deformation events affecting the serpentinites. Late-stage deformation features in the form of joints, faults and microscopic kinks are described but have not been analysed in detail. While these features serve to make structural analysis more difficult, they do not completely obliterate evidence of earlier events. This study has shown that it is possible to interpret the structures,. textures and fabrics in the multiply deformed serpentinites in terms of their tectonic environment; in particular, the timing, number and nature of movements associated with them.
94
ACKNOWLEDGEMENTS
Thanks are extended to Dr. R. Offler for critically reading the manuscript. I am also grateful to B.H.P. (Newcastle) for providing photomicrography facilities, C.S.I.R.O. (North Ryde) for allowing access to X-ray diffraction equipment, and Naroo Pastoral Company for permitting unrestricted access to Glenrock Station. Technical and typing assistance provided by staff at Newcastle TJniversity are also gratefully acknowledged. REFERENCES Ave Lallement, H.G. and Carter, N.L., 1970. Syntectonic recrys~lli~ation and modes of flow in the upper mantle, Geol, Sot. Am. Bull., 81: 2203-2220. Basu, A.R., 1977. Textures, microstructures and deformation of ultramafie xenoliths from San Quentin, Baja, California. Tectonophysics, 43: 213-246. Bell, T.H. and Etheridge, M.A., 1973. Microstructure of mylonites and their descriptive terminology. Lithos, 6: 337-348. Corbett, G.J., 1976. A new fold structure in the Woolomin Beds suggesting a sinistral movement on the Peel Fault. J. Geol. Sot. Aust., 23: 401-406. Crook, K.A.W., 1962. Structural geology of part of the Tamworth Trough. Proc. Linn. Sot. N.S.W., 87: 397-409. De Wit, M.J., Dutch, S., Kligfield, R., Allen, RR. and Stern, C., 1977. Deformation, serpentinization and emplacement of a dunite complex, Gibbs Island, South Shetland Islands: Possible fracture zone tectonics. J. Geol., 85: 745-762. Etheridge, M.A., 1975. Deformation and recrystallization of orthopyroxene from the Gifes Complex, Central Australia. T~onoph~si~s, 25: 87-114. Goode, A.D.T., 1978. High temperature, high strain-rate deformation in the lower crustal Kalka Intrusion, Central Australia. Contrib. Mineral. Petrol., 66: 137-148. Hobbs, BE., Means, W.D. and Williams, P.F., 1976. An Outline of Structural Geology. Wiley, New York, N.Y., 512 pp. Lapham, D.M., 1967, The tectonic history of multiply deformed serpentinite in the Piedmont of Pennsylvania. In: Pd. Wyilie (Editor}, Ultramafic and Related Rocks. Wiley, New York, N.Y., pp. 174-183. Lapham, D.M. and M&ague, H.L., 1964. Structural patterns associated witb the serpentinites of southeastern Pennsylvania. Geol. Sot. Am. Bull., 75: 639-660. Leitch, E.C., 1974. The geological development of the southern part of the New England Fold Belt. J. Geol, Sot. Aust., 21: 133-156. Maltman, A.J., 1978. Serpentine textures in Anglesey, North Wales, United Kingdom. Geol. Sot. Am. Bull., 89: 972-980. Mercier, J.C. and Nicolas, A., 1975. Textures and fabrics of upper mantle peridotites as illustrated by xenoliths from basal&. J. Petrol., 16: 454-487. Nicolas, A. and Boudier, F., 1975. Kinematic interpretation of folds in alpine-type peridotites. Tectonophysics, 25 : 233-260. Nicolas, A. and Poirier, J.P., 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. Wiley, London, 544 pp. Nicolas, A,, Bouchez, J.L., Boudier, F. and Mercier, J.C., 1971. Textures, structures and fabrics due to solid state flow in some European lherzolites. Teetonophysics, 12: 55-86. Raleigh, C.B. and Paterson, MS., 1965. Experimental deformation of serpentinite and its tectonic implications. J. Geophys. Res., 70: 3965-3985. Rod, E., 1974. Structural interpretation of New England region. J. Proc. R. Sot. N.S.W., 107: 90-99.
95 Ross, J.V., 1977. The internal fabric of an alpine peridotite near Pinchi Lake, central British Columbia. Can. J. Earth Sci., 14: 32-44. Runnegar, B., 1974. The geological framework of New England. Geol. Sot. Aust., Qld. Div., Field Guide, pp. 9-19. Saleeby, J., 1978. Kings River ophiolite, southwest Sierra Nevada foothills, California. Geol. Sot. Am. Bull., 89: 617436. Scheibner, E. and Glen, R.A., 1972. The Peel Thrust and its tectonic history. Geol. Surv., N.S.W., Q. Notes, 8: 2-14. Spry, A., 1969. Metamorphic Textures. Pergamon, Oxford, 350 pp. Watterson, J., 1975. Mechanism for the persistence of tectonic lineaments. Nature, 253: 520-522.