NOTES 1483

FIG. A2. Zircon concentrate from RD-72-3-9-3. Note the preponderance of clear, euhedral grains.

essentially euhedral, whereas others are heavily charged dirtier internal domains charged with bubble-like with inclusions, are almost translucent. and are inclusions. subhedral. Between these extremes are cjstals with distinct translucent cores and clear overgrowths. Most grains are generally short (2.4:l lengthlwidth on average; 4.6: 1 maximum).

RD-72-3-9-3 The concentrates are essentially pure euhedral zircon

crystals (Fig. A2) and crystal fragments. Grains vary in habit from stubby to those with a lengthlwidth ratio of about 4.5: 1. Some crystals are twinned. Whereas most grains are transparent and water-clear, many contain

WN-26-74 The concentrates are more than 95% euhedral zircon

crystals with sharp terminations. Most grains are moderately elongate (lengthlwidth up to 4.5:1, but mostly about 3: 1) and clear, with minor proportions of rod, bubble, and opaque inclusions. About 5% of the crystals are heavily charged with inclusions, dust, and debris to the point of being translucent. About 2% of the crystals are twinned in parallel and end-to-end orientation.

Pillow shelves: determination of bedding direction and structural facing direction from shelves in deformed pillow lavas

E. W. SAWYER AND SARAH-JANE BARNES Department of Geology, University of Toronto, Toronto, Ont., Canada M5S IAI

AND

MARCUS J. BUCK Department of Geology, Carleton University, Ottawa, Ont., Canada KIA OE8

Received January 10, 1983 I

Revision accepted April 14, 1983

Pillow shelves form in the upper parts of pillows or lava tubes, and record pauses in their drainage. Thus the recognition of pillow shelves in tectonically deformed pillows allows both the palaeohorizontal (i.e., bedding) direction and approximate way-up direction to be determined. This is demonstrated in an example from the Cape Smith Fold Belt of northern Quebec, where

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1484 CAN. I. EARTH SCI. VOL. 20, 1983

way up and bedding were independently determined. The establishment of way up in strongly deformed pillow sequences enables structural facing direction to be determined and is a crucial step in the elucidation of the structural history in terrains where pillowed basic volcanics predominate, e.g., greenstone belts.

Des structures CtagCes se foment dans les parties supkrieures des coussins de lave ou des tunnels de coulCe de lave, et marquent une pause de leur drainage. Donc, l'identification de ces structures dans des coussins de lave tectoniquement dCform6s pemet de dkterminer la direction pal6ohorizontale (i.e., le litage) et la direction approximative de la polaritC. Nous prksentons un exemple choisi dans la zone de plissement de Cape Smith dans la nord du QuCbec, oh la polarit6 et le litage furent dCterminCs sCparCment. La mesure dans des stquences de coussins de lave fortement dtfom6es permet de d6finir la position de la partie supCrieure des coulCes, ce qui constitue une Ctape cruciale pour reconstituer 1'Cvolution structurale de terrains oh prkdominenet des laves en coussins, par exemple, dans les zones de roches vertes.

[Traduit par le journal] Can. I. Earth Sci. 20, 1483-1487 (1983)

Quebec. The observations are also relevant to strain Introduction determinations, since they may provide the unambig- An unambiguous determination of younging direction uous reference point that Borradaile and Poulsen (1981)

in deformed pillow lavas is essential for at least two suggest is necessary. reasons. Firstly, Borradaile and Poulsen (1981) proposed a method of determining strain from the Pillow structure selvages of pillow lava. Inherent in their study was the ~a l la rd and Moore (1977) and Wells et al. (1979) assumption of constant selvage thickness in the showed that pillows can have a wide variety of forms. undeformed pillows as a means of determining the One type of pillow contains horizontal shelves (Fig. 1). way-up (younging) directions of the pillow lavas, but they stressed the importance of an independent bedding marker to determine the precise way up. Secondly, fold facing directions (that is, younging direction measured in the cleavage plane) and the detection of inverted stratigraphy are a vital part of regional mapping, and a prerequisite to any structural synthesis or reconstruction of the tectonic history of a terrain. In areas such as greenstone belts, where pillow lavas are extensively developed, methods of determining way up have, in the past, been unsatisfactory because results are often ambiguous. In regions of thick and highly strained pillow lava sequences bedding may also be difficult or impossible to determine with any accuracy.

The use of pillow shelves as way-up and bedding indicators is presented below, using an example from the early Proterozoic Cape Smith Fold Belt of northern

Chlor~te schlst

PI I IOW lavas

I

FIG. 1. Sketch of a typical Cape Smith Fold Belt pillow morphology, indicating shelves at the top. Face "a" shows the FIG. 2. Geological sketch map of an area approximately distribution of olivine phenocrysts (dots) around the pillow 50 km east-southeast of Asbestos Hill in northern Quebec, margins and face "b" indicates the distribution of vesicles (open showing the syncline and location of data points used for circles). Fig. 3.

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POLES LINEATIONS SHEAR ZONE

mo Bedding Pillow Elongation - Schistos~ty Poles Q Schistosity Mlneral Orientation + Mineral Lineation

A 4 Plllow Shelves

Open symbols: Narthern llmb of fold Closed: Southern limb

FIG. 3. Stereographic projections of the lower hemisphere onto Lambert equal-area stereographic nets. (a) Orientation of layering in ultramafic flows, schistosity, pillow elongation, and mineral lineation. (b) Orientation of pillow shelves. The .rr axis orientation from bedding and the pillow shelves are shown on the respective diagrams.

Each shelf records the lava surface level in a tube or pillow that was intermittently drained of molten lava. A shelf, therefore, records the temporary level and frozen surface within such a drained pillow (Ballard and Moore 1977, pp. 32-40; Wells et al. 1979, Figs. 5, 12). Shelves tend to develop in the upper part of pillows, and since shelves record the temporary level of lava they are also a record of the horizontal plane.

In their subsequent history the voids between the shelves may become filled with zeolites, and at higher metamorphic grades quartz and calcite may fill the voids. Quartz-filled lenses in the upper parts of pillows from greenstone terrains have been observed by many workers (Viljoen and Viljoen 1969; Hargreaves 1975; Dirnroth et al. 1978; Goodwin 1980, Fig. 7; Wells et al. 1979; Syme et al. 1982, Fig. 19). Although none of these workers have commented upon their potential for structural studies, Viljoen and Viljoen (1969) were the first to recognise that these structures enabled way up to be determined.

An example from the Cape Smith Fold Belt In the tight to isoclinally folded Cape Smith Fold Belt,

pillow lavas form an extensive part of the stratigraphy (Francis et al. 1981) but are not intercalated with sediments to any degree. Way-up directions based on the cuspate bases of pillows can be used where the pillows are not highly strained. However, rocks in much of the fold belt have a very strong schistosity associated with folds and numerous low-angle shear zones that

divide the belt into many thrust blocks (Francis et al. 1981). As a consequence pillow shapes are severely modified either by truncation during shearing or by mutual interference during deformation, causing the development of many cuspate bulges of tectonic origin.

Figure 2 shows a westward plunging, tight syncline where "bedding" orientation was determined from the orientation of mineralogical layering (assumed to have been initially horizontal) in massive pyroxenitic flows. Mineral lineations, minor fold hinges, and the long axes of deformed pillows lie parallel to the major fold hinge as determined from 7~ diagrams (Fig. 3a). Pillow shelves were also measured on the relatively undeformed southern limb (axial ratios approximately 5:2.5:1), the hinge region (approximately 9:3: I), and the northern limb where a major shear zone truncates the fold (approximately 16:4: 1). The orientation of pillow shelves shows a strong correlation with the palaeohori- zontal determined from the orientation of the igneous layering (Fig. 3b; cf. Fig. 3a). Typical pillow shelves are shown in Fig. 4 and profiles of the pillow from across the fold are shown in cartoon form in Fig. 5.

Discussion Pillow shelves appear to be good indicators of the

palaeohorizontal even in strongly deformed rocks. For pillows from the fold hinge (Fig. 5) the usually slit-like shelves have shortened and opened, becoming lensoid and making measurement from a single pillow unreliable. Although the pillow shelves trace the

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1486 CAN. J. EARTH SCI. VOL. 20, 1983

FIG. 4. Pillows from the fold hinge viewed down plunge; arrows indicate pillows with shelves developed. ~

FIG. 5. Cartoon showing distribution of pillow shelves around the fold, based on Fig. 4. Selvages between pillows are stippled. Fold is viewed down plunge.

bedding direction, the normal to the shelves is not analysed (W. M. Schwerdtner, personal communication, necessarily the true way-up direction (Schwerdtner 1982). Consequently, unless the pillow shelves lie in a 1979) in the general case of a deformed pillow (see Fig. principal plane of total strain the way-up direction 6). Precise determination of the way-up direction in any estimated is only an approximation that becomes more pervasively strained rock can be made only where the inaccurate as strain increases. Perhaps the most impor- palaeohorizontal happens to coincide with a principal tant factor for structural mapping and interpretation is plane of total strain, or where the state of strain has been that most of the shelves form in the upper portions of the

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NOTES 1487

FIG. 6. Sketch showing deformation of pillows having various orientations. Note the rotation of the normal to the shelves, i.e., the way-up direction in each pillow.

pillows and so record the pillow tops. If, as is usually the case, the pillows are also flattened in a plane of schistosity then the structural facing direction is readily determined even in extremelv deformed ~illows. The determination of whether folds are upward or downward facing is a means of detecting earlier fold histories (see Shackleton 1957). Some Ungava pillow lavas, particularly those in regions of the lowestmetamorphic grade, show strong slickensides in the selvages between pillows. Such slickensides indicate inhomogeneous deformation in which the pillows have acted as rigid bodies in a ductile matrix and were rotated bodily.

In strongly deformed pillow lavas, the observation of pillow shelves in several adjacent pillows should clearly indicate bedding, and over larger areas folds may thus be detected. This may eliminate what the writers believe to be a common error, that of confusing pillow elongation in a cleavage direction with true bedding, and a consequent failure to recognise folds. This confusion and the failure to recognise folds could lead to a considerable overestimation of the true thickness of pillowed lavas. For strain determination in deformed

I pillow lavas, the shelves provide a valuable reference plane (the palaeohorizontal) .

Conclusion The recognition of pillow shelves within deformed

pillows provides a means of determining bedding orientation and approximate way up. In addition they can be used to determine the structural facing direction, and hence aid in evaluating the structural history of terrains where thick pillow lavas are abundant.

Acknowledgments The writers would like to thank Falconbridge Limited

for permission to publish material obtained while in their employment. We also wish to thank Professor W. M. Schwerdtner for his critical comments, and for pointing out to us that, in the general case, way-up directions are oblique to the deformed bedding plane.

BALLARD, R. D., and MOORE, J. G. 1977. Photographic atlas of the Mid-Atlantic Ridge rift valley. Springer-Verlag, New York, NY, 114 p.

BORRADAILE, G. J., and POULSEN, K. H. 1981. Tectonic deformation of pillow lava. Tectonophysics, 79, pp. T17-T26.

DIMROTH, E., COUSINEAU, P., LEDUC, M., and SANSCHA- GRIN, Y. 1978. Structure and organization of Archean subaqueous basalt flows, Rouyn-Noranda area, Quebec, Canada. Canadian Journal of Earth Sciences, 15, pp. 902-918.

FRANCIS, D. M., HYNES, A. J., LUDDEN, J. N., and BERARD, J. 1981. Crystal fractionation and partial melting in the petrogenesis of a Proterozoic high-MgO volcanic suite, Ungava, Quebec. Contributions to Mineralogy and Petrology, 78, pp. 27-36.

GOODWIN, A. M. 1980. Archean volcanic studies in the Tirnmins - Kirkland Lake - Noranda region of Ontario and Quebec. Geological Survey of Canada, Bulletin 278, 51 p.

HARGREAVES, R. 1975. Volcanic stratigraphy at Utik Lake: summary of geological fieldwork 1975. Mineral Resources Division, Manitoba Department of Mines, Resources and Environmental Management, Geological Paper 2/75, pp. 22-29.

SCHWERDTNER, W. M. 1978. Determination of stratigraphic thickness of strained units. Canadian Journal of Earth Sciences, 15, pp. 1379-1380.

SHACKLETON, R. M. 1957. Downward-facing structures of the Highland border. Quarterly Journal of the Geological Society of London, 113, pp. 361 -392.

SYME, E. C., BAILES, A. H., PRICE, D. P., ~ ~ ~ Z I E H L K E , D. V. 1982. Flin Flon Volcanic Belt: geology and ore deposits at Flin Flon and Snow Lake, Manitoba. Geological Association of Canada, Winnipeg, Man., 1982 Field Trip 6, Guide, 91 p.

VILJOEN, M. J., and VILJOEN, R. P. 1969. The geology and geochemistry of the lower ultramafic unit of the Onverwacht Group and a proposed new class of igneous rocks. Geological Society of South Africa, Special Publication 2, pp. 55-86.

WELLS, G., BRYAN, W. B., and PEARCE, T. H. 1979. Comparative morphology of ancient and modem pillow lavas. Journal of Geology, 87, pp. 422-440.

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