Multiple dikes in Lower Kam Group, Yellowknife ... - GSA Publications

Multiple dikes in Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading? Herwart Helmstaedt Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada W. A. Padgham, John A. Brophy Geology Division, Northern Affairs Program, Yellowknife, Northwest Territories X1A 2R3, Canada

ABSTRACT A mafic intrusive-extrusive complex at the base of the Kam Group in the Archean Yellowknife Supergroup at Yellowknife, Northwest Territories, Canada, grades from gabbros through a zone of multiple and sheeted dikes into massive and pillowed flows with thin beds of interflow sediments. Although rock types and primary structures resemble those in upper parts of Phanerozoic ophiolites, the Kam Group laclcs an ultramafic base and is much thicker than typical ophiolites. Because similar mafic sections occur in certain atypical ophiolites and in plateau areas of modern ocean basins, however, the Kam Group is viewed as a remnant of Archean oceanic crust rather than as a produ ct of intracontinental rift volcanism. The dikes are analogous to sheeted dikes of ophiolites and are suggestive of seafloor spreading in an Archean marginal-basin setting.

INTRODUCTION Interpreted as fragments of ancient oceanic lithosphere, ophiolites have become a "standard part of the plate tectonic paradigm" (Coleman, 1977). Their widespread occurrence in Phanerozoic mountain belts is regarded as evidence that the Wilson cycle, involving the opening and closing of ocean basins (Wilson, 1968), has operated in the geologic past (e.g., Dewey and Bird, 1970; Moores, 1982). Because wellpreserved ophiolites are rare in the Precambrian (e.g., Burke and Kidd, 1980), less agreement exists on the importance of plate tectonics during the Proterozoic, and no consensus on the role of plate tectonics in the evolution of Archean crust has emerged. Whereas Burke et al. (1976) suggested that dismembered ophiolites should exist in Archean greenstone belts, McCall (1981) cited a complete lack of ophiolites in greenstone suites as a major argument against the operation of plate-tectonic processes during the Archean. In this paper we describe a dike and sill complex, near the base of the mafic Kam Group in the Archean Yellowknife greenstone belt, that appears to be analogous to sheeted dikes in Phanerozoic ophiolites. Although not a typical ophiolite, the structure of this complex suggests that the Kam Group evolved in a basin floored by oceanic crust. This interpretation contrasts with a previously proposed intracontinental rift model for the Yellowknife greenstone belt (Henderson, 1981, 1985). 562

GEOLOGIC SETTING The Yellowknife greenstone belt is lccated in the Slave structural province, a late Archean granite-greenstone terrane of the northwestern Canadian Shield (Fig. 1). Supracrustal locks, belonging to the ca. 2.7 Ga Yellowknife Supergroup (Henderson, 1970), are distinguis hed from successions of the Superior province and many other Archean terranes by an abscnce of komatiites and a high proportion of metasedimentary to metavolcanic rocks. Granitoid rocks that intrude the Yellowknife Supergroup consist of granodiorites (2590-2610 Ma) arid crosscutting potassic granites (2575 Ma) (Green and Baadsgaard, 1971). Sporadic occurrences of ca. 3 Ga sialic rocks (Frith, 1978; Nikic et al., 1975; Krogh and Gibbins, 1978) have been interpreted as basement to the Yellowknife Supergroup (Baragar and McGlynn, 1976).

imum U-Pb age of 2.678 Ga (+8.2/-8.0 m.y.), determined from zircons from a quartz-feldspar porphyry dike in the Kam Group (S. Bowring, personal commun.). Although generally obscured by the Western granodiorite, the base of the Kam Group is exposed near the northern end of the belt where pillowed flows that have numerous gabbroic intrusions are in apparently conformable contact with thin beds of banded iron-formation and felsic tuff. It is uncertain whether this contact is depositional or faulted. Deformed granite to the north (Unit 7, Fig. 1) was previously mapped as possible basement (Henderson, 1985) because it is cut by mafic dikes interpreted as feeders to the Kam Group volcanics. We recognize the granite as having intrusive contacts with the Kam Group and the underlying iron-formation; the dikes belong to a postvolcanic swarm (Helmstaedt et al., 1980) (Fig. 3).

STRATIGRAPHY The stratigraphic framework of the Yellowknife greenstone belt has been determine! on the basis of work by Jolliffe (1942,1946), Henderson and Brown (1966), and Henderson (1970, 1985); this framework has been revised recently by Helmstaedt and Padgham (1986) (Fig. 2). The major portion of the belt is underlain by mafic rocks of the Kam Group (Unit 2, Fig. 1), divided into four formations that have a combined thickness of about 11 km (Figs. 2, 3). No repetition of the section by faulting has been recognized. The mafic rocks have a min-

In the northern part of the belt, the Kam Group is truncated by an unconformity beneath conglomerates and sandstones of the Jackson Lake Formation (Fig. 1). Farther south, the tholeiitic Kam Group is overlain by calc-alkalic rocks of the Banting Group (Baragar, 1966; Cunningham, 1984) that, in turn, are overlain by turbidites of the Burwash Formation (Duncan Lake Group). At the southwestern end of the belt, the Yellowknife Bay Formation overlaps a sequence; of older volcanic and sedimentary rocks of the Octopus Formation (Figs. 1, 2). A more detailed discussion of stratigraphic GEOLOGY, v. 14, p. 562-566, July 1986

General Geology of

YELLOWKNIFE BAY

E=z3

PROTEROZOIC FAULTS

rf-IO«

PROTEROZOIC DIABASE DIKES

x 9

LATE ARCHEAN GRANITES

EE!

GRANODIORITE

en

EARLY DEFORMED GRANITOIDS JACKSON LAKE Fm.

ESS

DUNCAN LAKE GROUP

z4 —

BANTING GROUP DUCK Fm. KAM GROUP

EM

YELLOWKNIFE BAY Fm.

T T

TOWNSITE Fm.

nn

CRESTAURUM Fm.

n

CHAN Fm.

n

Figure 1. Geologic map of Yellowknife greenstone belt, generalized from published maps of Geological Survey of Canada and Department of Indian and Northern Affairs, Yellowknife, Northwest Territories. Black star on inset map indicates location of Yellowknife Bay; black areas are other supra crustal belts in Slave province.

OCTOPUS Fm.

150 km

67°-

\ U ' J

'^Slave Province

• 'I 63°-

"

Great'' Slave Lake

relationships in the Yellowknife Supergroup at Yellowknife is given in Helmstaedt and Padgham (1986). Rocks of the Kam Group are metamorphosed and deformed by numerous shear zones. Metamorphic grade increases from greenschist to amphibolite facies toward the granitoid intrusions. Although primary minerals are replaced by metamorphic assemblages, primary structures, grain size variations, and intrusive relationships are well preserved outside the major shear zones. The entire greenstone belt is dissected by numerous transcurrent faults of Proterozoic age (Fig. 1). GEOLOGY, July 1986

fa

'

/

/^Churchill Province 110°

VOLCANIC-INTRUSIVE R E L A T I O N S H I P S IN THE K A M G R O U P The Kam Group is a northeasterly striking, homoclinal sequence of pillowed and massive mafic flows and minor felsic pyroclastic rocks and interflow sediments that dip steeply and face uniformly to the southeast. Among the numerous gabbroic intrusions within the Kam Group, Henderson and Brown (1966) distinguished dikes, sills, and irregular intrusions, many of which were thought to be synvolcanic (see also Baragar, 1966). The mafic intrusive-extrusive complex of the

Chan Formation (Fig. 3), best developed between Chan Lake and Berry Hill (Fig. 1), grades from a lower, gabbro-dominated part through a multiple-dike complex into massive and pillowed flows and thin beds of interflow sediments. At the base of the section is a sheetlike body of massive, medium- to coarsegrained, locally layered gabbro that had intruded a sequence of pillowed flows, remnants of which are preserved at three levels. The upper boundary of this body is marked by a relatively sharp transition into the dike complex, which consists of numerous, fine- to medium-grained metadiabase dikes as well 563

as septa and irregular bodies of medium- to coarse-grained gabbro between which screens of pillowed flows can be recognized. The dikes have symmetric and asymmetric chilled margins and range in width from less than 1 m to over 10 m. They trend between 140° and 170°, and are almost perpendicular to the strike of the overlying flows. Most of the irregular gabbro bodies are multiple intrusions that have abundant chilled margins and extremely complex contact relationships. Igneous layering is generally absent at this level, but a sheetlike body of gabbroic anorthosite, up to 100 m thick, was recognized (Fig. 3). It is surrounded entirely by gabbro that has chilled margins against the anorthosite. Xenoliths of leucogabbro and gabbroic anorthosites, many with diffuse outlines, are common within the gabbro. In the transition zone from dike complex to pillowed flows, a number of dikes "bud" into adjacent pillows; this fact suggests that they were intruded close to the sea floor and acted as a

WEST

feeder system to the growing volcanic pile (de Wit and Stern, 1978). Although pillowed and massive mafic flows predominate above the dike complex, sills and irregular bodies of gabbro, many of them multiple intrusions, E.re still common in the upper parts of the C han Formation. The top half of the Kam Group (Fig. 3) is dominated by pillowed and massive flows, but it contains intercalations of felsic tuffs and tuffaceous sediments, some of which are continuous along strike for more than 10 km and allow stratigraphic correlation across Proterozoic transcurrent faults (Fig. 1). Several pillowed variolitic flows are also laterally continuous and have been used as marker flows by Henderson and Brown (1966) (Fig. 3). Synvolcanic intrusions consist principally of sills, some of which are connected to dike swarms, best developed in parts of the Yellowknife Bay Formation (Fig. 3). The entire section, Including the overlying Banting Group, is intruded by NORTH

of Y e l l o w k n i f e

EAST Bay

at least two swarms of (Archean) postvolcanic dikes, many of which contain megacrystic plagioclase. DISCUSSION The Yellowknife greenstone belt has been interpreted as a margin of an Archean turbiditefilled, intracontinental rift with volcanism restricted to the boundary faults (Henderson, 1981,1985); however, Helmstaedt and Padgham (1986) have pointed out that the submarine character and the thickness of the Kam Group are incompatible with this model. Other workers have proposed that the mafic sequences of Archean greenstone belts must have been deposited in proto-oceanic basins similar in size to the Red Sea (Windley, 1976) or in marginal-basin settings (Tarney et al., 1976). D e Wit and Stern (1980), de Wit et al. (1986), and Harper (1986) have described greenstone belts from the Barberton Mountains and Wyoming that include all rock types of the typical ophiolite assemblage. Burke et al. (1986), suggesting that greenstone belts mark places where oceans have closed, showed that intracontinental rift models emphasize early stages of greenstone belt evolution but ignored the later oceanic and collisional history of many belts. If they are indeed oceanic, certain greenstone sequences should be viewed as Archean analogues of Phanerozoic ophiolites. Although the gradation from high-level gabbros through a zone of multiple dikes into a mafic volcanic complex is typical of upper parts of Phanerozoic ophiolites, without cumulates and a tectonized ultrabasic base the Chan Formation does not represent the complete ophiolite section defined by Anonymous (1972). Together with the mafic flows and sills of the Crestaurum and Yellowknife Bay Formations (Fig. 3), the total section of the Kam Group is also much thicker than that of typical Phanerozoic ophiolites (Coleman, 1977), even if allowance is made for the fact that part of the excess thickness may be due to lateral onlap rather than original stratigraphic superposition (see also Henderson, 1985). Furthermore, the Kam Group differs from most ophiolites in that it has a greater abundance of felsic material.

Figure 2. Stratigraphy of Archean rocks in Yellowknife greenstone belt. After Helmstaedt and Padgham (1986). 564

However, similar deviations from the standard section are known from Phanerozoic ophiolites, which are far too diverse to fit a simple model for either Phanerozoic or modern oceanic crust (Coleman, 1977, 1984). It seems unlikely that Archean oceanic crust could have been less diverse so as to conform to such a model section. The type of setting in which the Kam Group could have originated is constrained by the observation that many of its features are known only from ophiolites or modern oceanic and marginal-basin environments, whereas few are characteristic of mafic GEOLOGY, July 1986 564

sequences in known intracontinental rift settings. In terms of stratigraphy, internal structure, and tectonic history, the Kam Group most closely resembles the late Mesozoic Rocas Verdes ophiolites in southern Chile (Dalziel et al., 1974; de Wit and Stern, 1981) that are preserved more or less in situ in a back-arc-basin setting and have served as prototype for a marginal-basin model of greenstone belts (Tarney et al., 1976). These atypical ophiolites have no ultramafic base. They grade from gabbros with multiple and sheeted dikes into a sequence of pillowed flows with tholeiitic affinities. The dike complex is interpreted as evidence for an origin by sea-floor spreading. The Chilean ophiolites are intruded by calc-alkalic batholiths representing the root of a nearby volcanic arc that shed felsic to intermediate detritus into the basin. In the Yellowknife belt, calc-alkalic

rocks are represented by the Banting Group and the Western granodiorite batholith (Fig. 1), and a nearby arc would account for the felsic intercalations within the Kam Group. The Chilean back-arc basin exhibits considerable alongstrike variation, reflecting the transition from an ensialic rift with diked margins to a typically oceanic basin and the complex history of basin closure (de Wit and Stern, 1981). A similarly complex evolution may explain why in the Yellowknife area apparently contradictory field evidence for an ensialic-rift origin (Henderson, 1981, 1985), an island-arc setting (Folinsbee et al., 1968), and apparently oceanic crust can be found in a single greenstone belt. The important geotectonic question, whether the dikes of the lower Kam Group constitute field evidence for Archean sea-floor spreading, hinges on the analogy with Phanerozoic ophio-

BANTING

lites. Although it only locally consists of 100% dike rock, the Kam dike complex, situated between gabbros and pillowed flows, would undoubtedly be accepted as part of an ophiolite were it encountered in Phanerozoic rocks. In the Chan Formation, gabbros are not continuous with a layered complex and tectonized peridotites but are intrusive into pillowed flows locally underlain by iron-formation. An example of gabbro intrusions in preexisting flows of a typical ophiolite section has been observed in the Mesozoic Point Sal ophiolite, California, and was attributed by Hopson and Frano (1977) to the j u m p of a spreading center and the consequent buildup of composite oceanic crust. The observation, in the same ophiolite, that such renewed intrusion may produce sill complexes, with or without dikes, could have important implications for the interpretation of

GROUP

Km

Kam

Bode Yellorex

Sills

Member

Flows

YELLOWKNIFE BAY

Negus

FORMATION

Flows

10M a s s i v e and Pillowed Flows

Townsite TOWNSITE FORMATION

t

CRESTAURUM

Sills

Post-volcanic Gabbro Dikes 2

Fox Flows

pv - Cemetery

Stock

Tuff

Flows

FORMATION Ranney Tuff Ranney Chert I r r e g u l a r Gabbros G a b b r o Sills T r a n s i t i o n to Pillowed Flows

Post-volcanic Gabbro Dikes 1

Figure 3. Schematic section through Kam Group representing relationships between volcanic and intrusive rocks. Marker beds, flows, and sills in upper part of section are after Henderson and Brown (1966). Deformed granite at base corresponds to Unit 7, and Ouckfish Lake granite to Unit 9 in Figure 1.

M a s s i v e Disbasie Gabbro

Multiple and Sheeted Dikes with screen of Pillowed Flows

Anorthosite

CHAN FORMATION

Coarse Gabbros R e m n a n t s of Pillowed Flows

—

Duckfish L a k e Granite

Iron Formation D e f o r m e d Granite

GEOLOGY, July 1986

565

A r c h e a n greenstones. In the K a m G r o u p (Fig. 3), as in m a n y o t h e r A r c h e a n greenstone sequences, synvolcanic sills a r e very c o m m o n b u t have not b e e n considered as possible evidence for sea-floor spreading. It is well k n o w n f r o m the central Pacific that f r e q u e n t j u m p s of spreading centers a r e correlated with faster t h a n n o r m a l spreading rates a n d lead to the f o r m a tion of a b n o r m a l l y thick crust typically f o a n d u n d e r o c e a n i c plateaus ( W i n t e r e r , 1976). C o m p o s i t e o c e a n i c crust p r o d u c e d b y this m e c h a n i s m m a y b e a realistic a n a l o g u e for certain A r c h e a n g r e e n s t o n e sections. Field evidence at Y e l l o w k n i f e is thus consistent with a n interpretation of the K a m G r o u p as a f r a g m e n t of unusually thick, c o m p o s i t e o c e a n ic crust. A s the intermittent influx of felsic material m a y b e related to n e a r b y a r c volcanism, this crust p r o b a b l y evolved b y spreading in a marginal-basin setting. F o r m a t i o n of the intrusive-extrusive c o m p l e x in t h e C h a n F o r m a t i o n is considered t o be t h e c o n s e q u e n c e of a m a j o r j u m p in locus of spreading within t h e basin, w h e r e a s the sills a n d related dikes in the u p p e r parts of the K a m G r o u p represent a n u m b e r of later intrusive stages d u r i n g further b u i l d u p of the section. D e p o s i t i o n of the overlying Banting G r o u p m a r k e d the t r a n s f o r m a t i o n into a convergent regime a n d the beginning of basin closure.

REFERENCES CITED Anonymous, 1972, Penrose field conference on ophiolites: Geotimes, v. 17, p. 24-25. Baragar, W.R., 1966, Geochemistry of the Yellowknife volcanic rocks: Canadian Journal of Earth Sciences, v. 3, p. 9-30. Baragar, W.R., and McGlynn, J.C., 1976, Early Archean basement in the Canadian Shield: A review of the evidence: Geological Survey of Canada Paper 76-14, 21 p. Burke, K.C., and Kidd, W.S.F., 1980, Volcanism on Earth through time, in Strangway, D., ed., The continental crust of the Earth and its mineral deposits: Geological Association of Canada Special Publication 20, p. 503-522. Burke, K., Dewey, J.F., and Kidd, W.S.F., 1976, Dominance of horizontal movements, arc and microcontinental collisions during the later per-mobile regime, in Windley, B.F., ed., The early history of the Earth: London, Wiley, p. 113-129. Burke, K., Sengor, C., and Fakultesi, M., 1986, Greenstone belts are not intracontinental rifts. What then are they? [abs.]: Workshop on tectonic evolution of greenstone belts: Supplement to Lunar and Planetary Institute Contribution 584, p. 27-28. Coleman, R.G., 1977, Ophiolites: Berlin, Heidelberg, New York, Springer-Verlag, 229 p. 1984, The diversity of ophiolites: Geologie en Mijnbouw, v. 63, p. 141-150. Cunningham, M., 1984, Petrochemistry of the Yellowknife greenstone belt, Yellowknife, N.W.T. [M.Sc. thesis]: Edmonton, University of Alberta, 191 p. Dalziel, I.W.D., de Wit, M.J., and Palmer, K.F., 1974, Fossil marginal basin in the southern Andes: Nature, v. 250, p. 291-294.

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Dewey, J.F., and Bird, J.M., 1970, Mountain belts and the new global tectonics: Journal o:r Geophysical Research, v. 75, p. 2625-2647 de Wit, M.J., and Stern, C.R., 1978, Pillow lalk: Journal of Volcanology and Geothermal Research, v. 4, p. 55-80. 1980, A 3500 My ophiolite complex from the Barberton greenstone belt, S.A., in (Jlover, J.F., and Groves, D.I., eds., Extended abstracts, 2nd International Archean Symposium, Perth: Geological Society of Australia, p. 85-86. 1981, Variations in the degree of crustal extension during formation of a back-arc bas n: Tectonophysics, v. 72, p. 229-260. de Wit, M.J., Hart, R., and Hart, R., 1986, A midArchean ophiolite complex, Barberton Mountain land [abs.]: Workshop on tectonic evolution of greenstone belts: Lunar and Planetary Institute Contribution 584, p. 27-29. Folinsbee, R.E., Baadsgaard, H., Cumming, G.L., and Green, D.C., 1968, A very ancient island arc, in Kropoff, L., Drake, C.L., and Hart, P.J., eds., The crust and upper mantle of the Pacific area: American Geophysical Union Geophysical Monograph 12, p. 441-448. Frith, R.A., 1978, Tectonics and metamorphism along the southern boundary between the Bear and Slave Structural Provinces, in Fraser, J.A., and Heywood, W.W., eds., Metamorphism in the Canadian Shield: Geological Survey of Canada Paper 78-10, p. 103-114. Green, D.C., and Baadsgaard, H., 1971, Temporal evolution and pedogenesis of an Archean crustal segment at Yellowknife, N.W.T.: Journal of Petrology, v. 12, p. 177-217. Harper, G.D., 1986, Dismembered Archean ophiolite, Wind River Mountains, Wyoming (USA): Ofioliti (in press). Helmstaedt, H., and Padgham, W.A., 1986, A new look at the stratigraphy of the Yellowknife Supergroup at Yellowknife, N.W.T.—Implications for the age of gold-bearing shear zones and Archean basin evolution: Canadian Journal of Earth Sciences (in press). Helmstaedt, H., King, J., and Boodle, R., 19S0, Geology of the Banting and Walsh Lakes mapareas, NTS 85 J/9: Yellowknife, Northwest Territories, Department of Indian and Northern Affairs, EGS 1980-5. Henderson, J.B., 1970, Stratigraphy of the Yellowknife Supergroup, Yellowknife Bay-Prosperous Lake area, District of MacKenzie: Geological Survey of Canada Paper 70-26, 12 p. 1981, Archean basin evolution in the Slave Province, Canada, in Kroner, A., ed., Plate tectonics in the Precambrian: Amsterdam, Elsevier, p. 213-235. 1985, Geology of the Yellowknife-Hearne Lake area, District of MacKenzie: A segment across an Archean basin: Geological Survey of Canada Memoir 414, 135 p. Henderson, J.F., and Brown, I.C., 1966, Geology and structure of the Yellowknife greenstone M t , District of MacKenzie: Geological Survey of Canada Bulletin 141, 87 p.

Reviewer's

Hopson, C.A., and Frano, C.J., 1977, Igneous history of the Point Sal ophiolite, southern California, in Coleman, R.G., and Irwin, W.P., eds., North American ophiolites: Oregon Department of Geology and Mineral Industries Bulletin 95, p. 161-183. Jolliffe, A.W., 1942, Prosperous Lake: Geological Survey of Canada Map 868A, scale 1:63,360. 1946, Yellowknife Bay, District of MacKenzie, N.W.T.: Geological Survey of Canada Map 709A, scale 1:63,360. Krogh, T.E., and Gibbins, W., 1978, U-Pb isotopic ages of basement and supracrustal rocks in the Point Lake area of the Slave Structural Province, Canada: Geological Association of Canada Abstracts with Programs, v. 3, p. 438. Moores, E.M., 1982, Origin and emplacement of ophiolites: Reviews of Geophysics and Space Physics, v. 20, p. 735-760. McCall, G.J.H., 1981, Progress in research into the early history of the Earth: A review, 19701980, in Glover, J.E., and Groves, D.I., eds., Archean Geology, Second International Archean Symposium, Perth 1980: Geological Society of Australia Special Publication 7, p. 3-18. Nikic, Z., Baadsgaard, H., Folinsbee, R.E., and Leech, A.P., 1975, Diatreme containing boulders of 3030 m.y. old tonalite gneiss, Con Mine, Yellowknife, Slave Craton: Geological Society of America Abstracts with Programs, v. 7, p. 1213-1214. Tarney, J., Dalziel, I.W.D., and de Wit, M.J., 1976, Marginal basin 'Rocas Verdes' complex from S. Chile: A model for Archean greenstone belt formation, in Windley, B.F., ed., The early history of the Earth: London, Wiley, p. 131-146. Wilson, J.T., 1968, Static or mobile Earth: The current scientific revolution, in Gondwanaland revisited: American Philosophical Society Proceedings, v. 112, p. 309-320. Windley, B.F., 1976, New tectonic models for the evolution of Archean continents and oceans, in Windley, B.F., ed., The early history of the Earth: London, Wiley, p. 105-111. Winterer, E.L., 1976, Anomalies in the tectonic evolution of the Pacific, in Sutton, G.H., Manghnani, M.H., and Moberly, R., eds., The geophysics of the Pacific Ocean basin and its margin: American Geophysical Union Geophysical Monograph 19, p. 269-278.

ACKNOWLEDGMENTS Funded by Northern Affairs Program, Yellowknife, Northwest Territories (field mapping), and Natural Science and Engineering Research Council Grant A8375 (laboratory component). A stimulating field visit by M. J. de Wit provided the impetus for this paper, and discussions with G. Bailey, J. M. Dixon, E. Farrar, D. M. Carmichael, and D. J. Schulze were very helpful. Manuscript received May 7, 1985 Revised manuscript received March 19, 1986 Manuscript accepted March 19, 1986

comment

Provides the first detailed a n d d o c u m e n t s ! evidence for A r c h e a n seafloor spreading. J u h n Liou

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GEOLOGY, July 1986