Geochemistry of Neohelikian Nauyat plateau basalts, Borden rift basin, northwestern Baffin Island, Canada1

J. DOSTAL Department of Geology, Saint Mary's University, Hal*, N.S. , Canada B3H 3C3

AND

G. D. JACKSON AND A. GALLEY Geological Survey of Canada, 601 Booth St., Ottawa, Ont., Canada KIA OE8

Received July 15, 1988

Revision accepted March 29, 1989

Subaerial basalt flows of the Neohelikian Nauyat Formation from northwestern Baffin Island, Northwest Territories, consti- tute an approximately 360 m thick unit that overlies an Archean-Aphebian metamorphic basement. The lavas have under- gone a low-grade regional metamorphism that affected the abundances of Na, K, and related trace elements. The basalts are continental tholeiites possessing some characteristics of mid-ocean-ridge basalts. They underwent fractional crystallization of clinopyroxene, plagioclase, and olivine. Mantle-normalized patterns show an enrichment of the lithophile elements, includ- ing Th and light rare-earth elements, relative to the high-field-strength elements and a distinct depletion of Nb and Sr. The parental magma of the basalts was derived either from oceanic-type mantle and subsequently affected by lower crustal contamination or from the subcontinental lithospheric mantle. The Nauyat basalts are probably related to the initial stage of the opening of the Poseidon (Proto-Arctic) Ocean.

Les coul6es de basalte subakriennes de la Formation de Nauyat, d'dge nCohClikien, du nord-ouest de l'ile Baffin, des Terri- toires du Nord-Ouest, constituent une unit6 lithologique d'6paisseur approximative de 360 m, reposant sur le socle m6tamor- phique archten-aph6bien. Une activitt mttamorphique r6gionale de faible degr6 a affect6 les laves et les teneurs en Na, K et les BlCments traces reli6s furent modifi6es. Les basaltes sont des thol6iites continentales montrant certaines affinit6s avec les basaltes de ride m6dio-ochnique. Ils ont 6volu6 par une cristallisation fractionnk de clinopyrodne, plagioclase et olivine. Les diagramrnes normalis6s au mantau expriment un enrichissement des tl6ments lithophiles, incluant Th et terres rares legkres, relativement aux 616ments h force de champ 6levte et un appauvrissement en Nb et Sr. Le magma parent des basaltes a dCriv6, soit d'un manteau de type ochnique et subs6quemment affect6 par une contamination de la crofite infkrieure, soit du mantau continental lithosph6rique. Les basaltes de Nauyat sont probablement relits au stage initial de l'ouverture de l'octan Poseidon (Proto-Arctique).

[Traduit par la revue]

Can. J. Earth Sci. 26, 2214-2223 (1989)

Introduction The Neohelikian Bylot Supergroup is displayed in imposing

castellated cliffs of northwestern Baffin Island and northern Bylot Island (Fig. 1). As much as 6.1 km thick, the supergroup is composed chiefly of diverse sedimentary rocks deposited in a variety of continental and marine environments within the Borden rift basin (Blackadar 1970; Geldsetzer 1973a, 19736; Jackson and Davidson 1975; Jackson and Iannelli 1981; Jack- son et al. 1975, 1985; Lemon and Blackadar 1963; Olson 1977, 1984). Nauyat, which is the lowermost formation of the Bylot Supergroup contains tholeiitic flows that have been deposited on continental crust (Galley 1978; Galley et al. 1983). An erosion surface marks the nonconformity between the Archean-Aphebian gneiss complex below and the Bylot Supergroup above. Late Proterozoic Borden ( - 850 Ma old) and Franklin ( - 750 Ma old) diabase dykes intrude the base- ment complex and the Bylot Supergroup (Christie and Fahrig 1983; Fahrig and Schwarz 1973; Fahrig et al. 1971). Paleozoic and Cretaceous -Eocene strata unconformably overlie the Bylot Supergroup.

Whole-rock K-Ar ages determined for the Nauyat vol- canic~ range from 762 to 1221 Ma, and a 1129 Ma Rb-Sr errorchron age has also been obtained (Jackson and Iannelli 1981). The best indication to date of the age of the Bylot Supergroup was provided by Fahrig et al. (1981), who inter-

'Geological Survey of Canada Contribution 23288. Printed in Canada 1 Imprim6 au Canada

preted the paleomagnetic pole for the Nauyat and Adams Sound formations as being about 1220 Ma (Neohelikian), and the pole for the Strathcona Sound Formation, near the top of the succession, as being about 16.5 Ma younger. Thus the oldest Nauyat K-Ar age of 1221 Ma agrees with the age of the Nauyat - Adams Sound pole as indicted by its position on the North American polar wandering curve.

The objectives of this paper are to geochemically character- ize the Nauyat basalts, discuss their petrogenesis, and evaluate whether or not their composition is compatible with the earlier suggestion of Jackson and Iannelli (1981) that the Nauyat vol- canism is related to the opening of the Poseidon Ocean.

Nauyat Formation The Nauyat Formation has an average thickness of nearly

240 m near Admiralty Inlet and may attain 430 m east of local- ities A and B (Fig. 1); it comprises a lower sedimentary mem- ber and an upper basalt member (Jackson and Iannelli 1981). The lower (NA1) member is composed chiefly of quartzaren- ite. One or two amygdaloidal plateau basalt flows are com- monly present in the middle part of the NA1 member and are similar to the overlying (NA2) member flows (Fig. 2). The NA1 member has a maximum thickness of more than 200 m east of Admiralty Inlet.

The upper (NA2) member of the Nauyat Formation contains as many as seven basalt flows separated in part by minor baked sedimentary sequences up to several metres thick (Fig. 2).

DOSTAL ET AL.

PHANEROZOIC

LATE PROTEROZOIC rIOHEilKIAN) ........ ......... ........ ......... Undivided ........ .....

Ilia Nauyat basalt

ARCHEAN-EARLY PROTEROZOIC (APHEBIAN)

A, B,C Location of stratigraphic sections

~vvvvvv Fault

4 Downthrown side

FIG. 1. Regional geological setting of the Nauyat basalts.

Near Admiralty Inlet the NA2 member is commonly 100- 130 m thick and may have a maximum thickness of more than 200 m. The total thickness and the number of flows decrease southeastward to 8 m and to one to two flows west of Milne Inlet. Thicknesses on Bylot Island range from 13 to 30 m and may include some sills. All flows in the Nauyat Formation are aphanitic to fine grained, although some have medium-grained interiors. Most flows are increasingly amygdaloidal upward, but the basal and top flows are amygdale free.

Petrography of flows Most of the basalt has a subophitic to intersertal texture. The

principal primary minerals are Ca plagioclase, augite, olivine, titano-magnetite, and accessory apatite. Plagioclase and augite also occur as phenocrysts. Secondary minerals are abundant and include chlorite, albite, calcite, hematite, sphene, alkali feldspar, quartz, and lesser amounts of epidote, actinolite, pumpellyite, prehnite, illite - sericite, leucoxene, sulphide minerals, and iddingsite. Olivine pseudomorphs are common and comprise chlorite, iron oxide, and iddingsite. Arnygdales

are filled chiefly with agate, chalcedony, and chlorite. The secondary mineral assemblages indicate regional metamorphic conditions ranging from the prehnite-pumpellyite to green- schist facies of regional metamorphism (Jackson and Morgan 1978; Galley et al. 1983).

Tectonic setting Nauyat plateau basalts are spatially related to north-

west-trending faults. The latter, which may themselves have acted as feeders, represent rifting that initiated the Borden Basin during marine transgression (from what is now the northwest) of a peneplaned Archean - Aphebian basement complex (Jackson and Iannelli 198 1).

The Nauyat flows are one of several tholeiitic volcanic sequences and (or) sills of similar age and chemistry that are found along what is now the northwest edge of the Canadian Shield. These basalts represent Mackenzie igneous events (Fahrig and Jones 1969; Fahrig et al. 1981; Fahrig 1987; Heaman and LeCheminant 1988) and were emplaced in simi- lar or related environments (continental mantle plume, rift

2216 CAN. J . EARTH SCI. VOL. 26,1989

L E G E N D

El Amygdaloidal basalt

Massive basalt v v v v

Tuffaceous stromatolitic dolostone

1 Gneissic complex

15 Sample number * indicates approximate position from nearby section

R Red

py Pyrite

FIG. 2. Columnar sections of the Nauyat basalts showing major features of petrography. AA, Archean-Aphebian gneiss complex; NA1, lower member; NA2, upper member; AS, Adams Sound Formation.

basin) (Baragar 1977; Fahrig et al. 1981; Jackson and Iannelli 1981; Galley et al. 1983). They, along with the Mackenzie dyke swarm, have been related to the 1250 Ma opening to the northwest of the Poseidon (Proto-Arctic) Ocean (Jackson and Iannelli 1981 ; Galley et al. 1983; Fahrig 1987).

The Borden rift basin formed during this ocean opening, probably as an aulacogen (Olson 1977). Thule Basin to the north, southern Somerset Island area to the west, and Borden Basin may be parts of a single larger basin (Jackson 1986; Jackson and Iannelli 198 1, 1984; Jackson et al. 1985). Some movement of Greenland away from North America probably occurred during this rifting event.

Sampling and analytical notes The Nauyat basalt samples analyzed were collected during

the measurement of three stratigraphic sections in the Milne Inlet Trough (Figs. 1, 2). Section A, studied by Galley (1978), lies about 17 km north of section B near the south side of western Milne Inlet Trough. Section C lies about 100 km to the northeast of A and B, almost at the junction of the Milne Inlet and Eclipse troughs at the northwest tip of Navy Board High (Fig. 1).

Seventeen samples were analyzed by X-ray fluorescence (XRF) for major elements and Rb, Sr, Ba, Y, Zr, Nb, V, Cr, Ni, Cu, and Zn; by instrumental neutron-activation for rare- earth elements (REE), Co, Sc, Hf, and Th; and by atomic absorption for Li. The precision and accuracy of the analytical data have been given by Dostal et al. (1986). Briefly, the pre- cision of the trace-element data is generally better than 10%. These samples were selected as representative from a set of 65

samples (Fig. 3) analyzed for major and some trace elements by XRF (Galley et al. 1983). Rocks from three sections were included to determine whether or not there is any lateral as well as vertical change in composition. Samples containing arnygdules and secondary veins were avoided.

Alteration Most of the analyzed basalt samples were affected by altera-

tion that changed the original chemical composition to various degrees. The effect of secondary chemical alteration of the basalts is shown by high loss on ignition (LOI) values and the I

low content of Ca (Table 1). The abundances of Na and K are highly variable. Some altered basalts in sections A and B are strongly enriched in Na and depleted in K, whereas some rocks from section C have high K abundances.

Several trace elements, including Rb, Sr, and Ba, vary in so wide a range that the abundances cannot be readily explained by magmatic processes. These elements display a negative correlation with Na, implying their depletion during the secon- dary sodium enrichment of the basaltic rocks. Light rare-earth elements (LREE) are also depleted in Na-rich rocks but to a significantly smaller degree (Table 2). This is consistent with some studies that showed that under certain conditions REE, particularly LREE, are affected by low-T alteration and spilitization (Ludden and Thompson 1979). One very strongly altered sample (38) containing K20 > 6%, shows a small but distinct enrichment in heavy rare-earth elements (HREE) , Zr , Hf, and Th. However, overall the REE patterns of the basalts were not significantly modified by the secondary processes, and their La/Yb ratios vary within a small range (Fig. 4). The

DOSTAL ET AL.

FIG. 3. Frequency diagrams for the abundances of TiO, and Zr in 65 samples and Cr in 64 samples of the Nauyat basalts. Reference numbers for the samples are arranged in each column in order of increasing content downward and correlate with the tics at the base of the columns. Samples whose numbers are joined by "&" have the same value (e.g., 21 & 22; 12 & 19) and are shown by only one tic.

h h

. . .. .. .I 5 b a s - a - % & $ ~ e p s ~ % a

0 m Z Z G c a Z Z u z ~ c C x x w

available data on Zr, Y, and Nb (Table 2) indicate that these elements, which are generally considered immobile during alteration (Pearce 1983), have not been redistributed. An exception is sample 1 1 with Na20 = 5.8 % , which seems to

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DOSTAL

be slightly depleted of Zr. The transition elements, including Cu, Zn, Ni, Co, Cr, and V, also display a certain degree of mobility. Cu varies in a wide range and is depleted in Na-rich rocks. The other transition elements exhibit uncharacteristic- ally weak correlations among each other (e.g., Mg-Ni or Cr-Ni) compared with fresh basaltic sequences.

Many elements in the basalts underwent a certain amount of redistribution. However, if samples with anomalously high Na20 (>4.5%) and K20 (>6%) and low CaO (<4%) are excluded, the remaining basalt samples display a relatively small variation in composition and a correlation of REE, Y, Th, and transition elements with less mobile high-field- strength elements (HFSE) such as Zr, Nb, and Ti, suggesting that all these elements retained their primary magmatic distri- bution. The major- and trace-element composition of the representative basalt samples is given in Tables 1 and 2, where the samples were divided into three groups: the least-altered basalts, the Na-rich basalts, and the K-rich or Ca-depleted basalts.

Geochemistry The least-altered samples are tholeiitic basalts similar in

major-element composition to recognized continental tho- leiites (Irvine and Baragar 1971 ; Baragar 1977; Wright et al. 1973; Dupuy and Dostal 1984). Compared with the average continental tholeiite of Manson (1967), they are lower in Al, Ca, Ti, and P. The abundances of Ti and P are similar to those of mid-ocean-ridge basalts (MORB) of equivalent MgO con- tents. On a conventional AFM diagram, the Nauyat Formation basalts display an iron-enrichment trend, characteristic of tholeiitic suites.

The basalts are olivine normative, except one sample (42) that is quartz normative. In comparison with continental flood basalts, which are usually strongly fractionated (Cox 1980), the Nauyat Formation rocks are rather primitive. The [Mg] values (Mg/(Mg + Fe2+) with Fe3+/Fe2+ taken as 0.15) of the basalts vary in a relatively narrow range of 0.67-0.58. These [Mg] values together with the MgO contents resemble those of MORB and also early Mesozoic continental tholeiites, which are related to the initial stages of the opening of the Atlantic Ocean and are scattered along both continental mar- gins of the Atlantic (e.g., Bryan et al. 1977; Dostal and Dupuy 1984; Dupuy et al. 1988).

In the analyzed basalts, Si02 varies from about 50 to 53% and displays, like Al2O3, a crude negative correlation with MgO. These features observed in many tholeiitic suites are consistent with crystal fractionation of a gabbroic assemblage involving clinopyroxene + plagioclase + olivine (Cox 1980). Most major elements, including Ti and P, vary only within narrow limits. An exception is K, which shows a large range of variations, probably as a result of mobility during alteration and metamorphism.

When plotted against [Mg] values, Ti and V display a tholeiitic differentiation trend. Both these elements increase slightly with increasing fractionation, whereas the Ti/V ratio remains almost constant (Ti/V = 19-23) with values typical of MORB (Sun et al. 1979). Ti is also correlated positively with immobile incompatible elements, including Zr, implying that it has an incompatible character. These variations suggest that Fe-Ti oxides played a negligible role during differen- tiation.

Regarding transition trace elements, the abundances of V in the basalts are similar to those of Prinz's (1967) average

ET AL.

1

FIG. 4. Chondrite-normalized REE abundances of the Nauyat For- mation basalts. (a) Least-altered basalts: 0, sample 12; a, sample 9; X , sample 24; a, sample 40; +, sample 42. (b) Strongly altered basalts (dashed lines). Na-rich samples: a, sample 16 (short-dashed line); M, sample 19 (long-dashed line); Ca-poor sample: A , sample 10. Solid lines in b outline the range of the least-altered basalts.

tholeiite, whereas Cr, Ni, and Co contents are higher. The values for all these elements are, however, well within their concentrations in MORB of similar MgO contents. Ni and Cr together with [Mg] display a negative correlation with incom- patible elements such as Zr, Ti, P, and Hf, suggesting that the variations of these transition elements are related mainly to fractionation of mafic minerals, particularly pyroxenes and olivine. The rather uniform concentrations of Zn resemble those of MORB and continental tholeiites. Like other con- tinental tholeiites (Andriambololona and Dupuy 1978), most of the basalt samples exhibit a distinct enrichment in Cu that does not show any obvious correlation with differentiation or stratigraphic position.

The concentrations of Li, Rb, and Ba exhibit a large spread, with Rb varying by an order of magnitude. However, the abundances are within the range reported for continental tholeiites. The lack of any obvious correlations between these elements and degree of differentiation as represented by [Mg] or immobile incompatible elements such as Zr indicates that even in the group of the least-altered basalts, these elements were affected by alteration and (or) low-grade metamorphism.

The chondrite-normalized REE patterns of the least-altered basalts (Fig. 4) are subparallel and characterized by a slight LREE enrichment and fractionated HREE with a La/Yb ratio of 4 - 7 (Table 2). This REE variation can be accounted for by fractional crystallization of the modal minerals - olivine, pyroxene and plagioclase - all of which have REE partition coefficients < 1 in equilibrium with basaltic melts. Most sam- ples display a small negative Eu anomaly indicative of plagio- clase fractionation. The REE abundances resemble those of some ocean-floor basalts (e.g., Frey et al. 1974) and also the early Mesozoic tholeiites from the Atlantic continental mar- gins (e.g., Bryan et al. 1977; Bertrand et al. 1982; Dupuy et al. 1988). The similarities to Mesozoic tholeiites and

CAN. J . EARTH SCI. VOL. 26,1989

FIG. 5. Ti/Y versus Nb/Y diagram for basaltic rocks. Fields after Pearce (1983): 1, volcanic-arc basalts; 2, ocean-floor basalts; 3, within- plate basalts; TH, tholeiitic; TR, transitional; ALK, alkali basalts. The dashed line delineates the field of the early Mesozoic tholeiites related to the opening of the Atlantic (Alibert 1985; Bertrand et al. 1982; Greenough and Papezik 1987; Papezik and Hodych 1980; Papezik and Barr 1981; Papezik et al. 1988), whereas the solid line indicates the field of the Nauyat basalts (the strongly altered lavas with >6% KK,O were excluded).

MORB extend to the distribution of the HFSE, including Zr, Hf, Nb, and Y, which are all generally positively intercorre- lated. Like the Mesozoic tholeiites, these rocks have immobile-incompatible-element ratios such as Ti/Zr, Ti/P, Ti/Y, ZrISm, HfILu, and Zr/Nb very similar to those of some MORB (Le Roex et al. 1983). On various discrimination dia- grams, including Ti/Y versus Nb/Y (Fig. 5), the Nauyat basalts plot within the field of the Mesozoic tholeiites, which is also within the MORB field. On the contrary, most con- tinental tholeiites lie in the field of within-plate basalts.

The geochemical characteristics of the Nauyat Formation basalts are summarized in a mantle-normalized trace-element diagram in Fig. 6a, where the elements are arranged accord- ing to increasing bulk-distribution coefficients for typical mantle mineral assemblages from left to right (after Thompson et al. 1983). In order to decrease the effects of fractional crys- tallization and the variable degree of partial melting, the ele- ment ratios (elementlyb) were used in lieu of the absolute concentrations. These ratios were then normalized to the chon- dritic ratios. If it is assumed that the magmas were derived by a moderate degree of partial melting ( > 5 - 10% ; Frey et al. 1978), the resulting pattern suggests that their upper mantle source was enriched in the more strongly incompatible ele- ments relative to the chondritic mantle used for normalization.

The Nauyat basalts show enrichment of the lithophile ele- ments Rb, K, Th, and LREE relative to the HFSE - Nb, Zr, Hf, and Ti (Tables 1, 2; Fig. 6a). In particular, the patterns of the Nauyat basalts increase from Tb to Th with a distinct depletion of Nb and Sr. The profiles, which are typical of con- tinental tholeiites, differ from those of MORB or oceanic- island tholeiites. The pattern for oceanic basalts is either flat between Th and Tb or gradually increases from Tb to Nb. These patterns for oceanic rocks are accompanied by higher Nb/La and lower Th/La ratios relative to those of continental tholeiites. The distinct depletion of Nb in many continental tholeiites (e.g., Hawkesworth et al. 1986) is accompanied by low Sr abundances relative to REE. Thus it can be argued that the observed Sr depletion in the least-altered Nauyat basalts (Fig. 6) is not related chiefly to secondary alteration but may reflect a primary magmatic feature.

The available data do not show any systematic primary mag-

matic variations in basalt composition with stratigraphic posi- tion or differences between various profiles. Such variations might, however, be partially masked by alteration or meta- morphism, since the secondary processes affected to a certain degree the distribution of large-ion-lithophile elements (LILE), which usually display the largest variations.

Petrogenesis Unlike MORB, including N and P types, and oceanic-island

tholeiites, the Nauyat basalts are depleted of Nb, as shown in Fig. 6. A marked depletion of Nb relative to other trace ele- ments of a comparable degree of "incompatibility", such as LREE and Th, cannot be readily explained by the presence of HFSE-rich accessory phases in the residue of the mantle source or by their separation during fractional crystallization, since the other incompatible elements present in these phases are not correspondingly depleted in the basalts. In addition, these minerals (e.g., Fe-TI oxides) do not crystallize in tholeiitic basalts early enough to produce such a NB impoverishment.

The incompatible-element enrichment patterns for the Nauyat basalts, including their depletion in Nb, are similar to those observed in subduction-related basic lavas (e.g., Pearce 1983) and in continental crust (e.g., Thompson et al. 1983; Dostal et al. 1983). This might indicate either derivation from mantle that was affected by subduction-related processes before the episode of the Nauyat volcanism (i.e., the Nb anomaly and the pattern reflect the compositional characteris- tics of the mantle source) or alternatively crustal contarnina- tion of the magma.

Although the presence of a subduction-zone component in the subcontinental mantle could account for the Nb depletion in continental tholeiites (Cox 1983), it cannot readily explain the Sr impoverishment. The volcanic-arc rocks have signifi- cantly higher SrILREE and Sr/Nb ratios, and their trace- element patterns do not display negative Sr anomalies (Hawkesworth and Powell 1980; Pearce 1983). These fea- tures, together with a Th enrichment relative to LREE, are typical of continental crustal rocks, and it may be suggested that such a trace-element distribution is characteristic of the

DOSTAL ET AL.

FIG. 6. Abundance ratios (elementlyb) in the basalts normalized to those of chondrites (normalizing values are from Thompson et al. 1983). Elements are arranged according to the degree of incompatibility (Thompson et al. 1983). (a) a, sample 17; x , sample 12; A , sample 42. (b) a, North Mountain basalts (average; Dostal and Dupuy 1984); X, P-type MORB (Sun et al. 1979); A, N-type MORB (Sun et al. 1979).

subcontinental lithosphere beneath old cratons that differ from the oceanic mantle (Hawkesworth et al. 1984). Thus, the Nauyat lavas could have been derived from the nonconvecting mantle part of the subcontinental lithosphere. Alternatively, the trace-element characteristics of the Nauyat basalts can be accounted for by an interaction of MORB-type magma with continental crust. The high ThILa ratio ( - 0.27) in the Nauyat basalts, which approaches average crustal content ( - 0.35), suggests that they have undergone crustal contamination. In fact, calculations using the equations of De Paolo (1981) show that concurrent crustal contamination and fractional crystalli- zation of MORB-type magma could produce the trace-element distribution observed in the Nauyat basalts (Dostal et al. 1983, 1986), including relative depletion of Nb and Sr. The distinc- tive and narrow range of composition of the basalts particu- larly for trace elements argues against near-surface crustal contamination, which might be expected to produce a haphaz- ard or continuous and progressive range of compositions. Likewise, the lack of extensive occurrence of silicic magmatic rocks (possible contaminants) and the absence of relic megacrysts and xenocrysts are not in accord with widespread near-surface contamination processes. However, crustal con- tamination of the magma in the lower crust followed by exten- sive magma homogenization in a deep-seated magma chamber can probably explain the observed characteristics of the basalts. Such a process has been suggested for the Mackenzie dykes, which are compositionally very similar (Gibson et al. 1987).

Summary The Nauyat Formation basalts are continental tholeiites,

which, however, differ compositionally from typical continental-flood basalts, including Proterozoic tholeiitic sequences of the Canadian Shield such as those of the Copper- mine River area (Dostal et al. 1983), and resemble MORB in several aspects. Although all the Nauyat samples possess characteristics such as a negative Nb anomaly and Th enrich- ment relative to REE that are typical of continental tholeiites,

they appear to have characteristics transitional between those of continental tholeiites and those of MORB. Geochemically the Nauyat basalts closely resemble Mesozoic tholeiites related to the opening of the Atlantic Ocean (Figs. 5, 6). This may suggest a similar origin and possibly relate the Nauyat basalts to the opening of the Poseidon (Proto-Arctic) Ocean, as suggested by Jackson and Iannelli (1981, 1984) and by Gal- ley et al. (1983). The incompatible-element distribution pat- terns of the Nauyat basalts with a high ratio of LILE including Th and LREE relative to HFSE and particularly Nb indicate either derivation of the parental magma from a subcontinental lithosphere compositionally different from the oceanic mantle or lower crustal contamination of the MORB-type magma. Subsequently, the magma experienced fractional crystalliza- tion of clinopyroxene, plagioclase, and olivine prior to erup- tion at the surface.

Acknowledgments The writers wish to thank Mr. K. Bencik for drafting

assistance, Drs. R. F. Emslie, C. Harris, and H. C. Halls for critically reviewing the manuscript, and Dr. W. R. A. Baragar for helpful comments. We are also grateful to Mr. T. R. Iannelli for his assistance in the field and lab. The study was supported by the Natural Sciences and Engineering Research Council of Canada (operating grant A3782). Polar Continental Shelf Project provided assistance in the field.

ALIBERT, C. 1985. A Sr-Nd isotope and REE study of the Late Tri- assic dolerites from the Pyrenees (France) and the Messejana Dyke (Spain and Portugal). Earth and Planetary Science Letters, 73: 81 -90.

ANDRIAMBOLOLONA, R., and DUPUY, C. 1978. Repartition et com- portement des tltments de transition dans les roches volcaniques. I. Cuivre et zinc. Bulletin du Bureau de rkcherches gtologiques et minikres, 2: 121 - 138.

BARAGAR, W. R. A. 1977. Volcanism of the stable crust. In Volcanic regimes in Canada. Edited by W. R. A. Baragar, L. C. Coleman, and J. M. Hall. Geological Association of Canada, Special Paper 16, pp. 377-405.

2222 CAN. J . EARTH SCI. VOL. 26,1989 I BERTRAND, H., DOSTAL, J., and DUFTJY, C. 1982. Geochemistry of

early Mesozoic tholeiites from Morocco. Earth and Planetary Science Letters, 58: 225 -239.

BLACKADAR, R. G. 1970. Precambrian geology northwestern Baffin Island, District of Franklin. Geological Survey of Canada, Bulletin 191.

BRYAN, W. B., FEY, F. A., and THOMPSON, G. 1977. Oldest Atlantic sea-floor: Mesozoic basalts from western North Atlantic margin and eastern North America. Contributions to Mineralogy and Petrology, 64: 223 -242.

CHRISTIE, K. W., and FAHRIG, W. F. 1983. Paleomagnetism of the Borden dykes of Baffin Island and its bearing on the Grenville Loop. Canadian Journal of Earth Sciences, 20: 275 -289.

Cox, K. G. 1980. A model for flood basalt volcanism. Journal of Petrology, 21: 629 -650.

1983. The Karoo Province of southern Africa: origin and trace element enrichment pattern. In Continental basalts and man- tle xenoliths. Edited by C. J. Hawkesworth and M. J. Norry. Shiva Press, Nantwich, pp. 139- 157.

DE PAOLO, D. J. 1981. Trace element and isotopic effects of combined wall-rock assimilation and fractional crystallization. Earth and Planetary Science Letters, 53: 189 - 202.

DOSTAL, J., and DUPUY, C. 1984. Geochemistry of the North Moun- tain basalts (Nova Scotia, Canada). Chemical Geology, 45: 245 -261.

DOSTAL, J., BARAGAR, W. R. A., and DUPUY, C. 1983. Geochemistry and petrogenesis of basaltic rocks from Coppennine River area, Northwest Temtories. Canadian Journal of Earth Sciences, 20: 684 - 698.

1986. Petrogenesis of the Natkusiak continental basalts, Vic- toria Island, Northwest Territories, Canada. Canadian Journal of Earth Sciences, 23: 622-632.

Duwu, C., and DOSTAL, J. 1984. Trace element geochemistry of some continental tholeiites. Earth and Planetary Science Letters, 67: 61 -69.

DUFTJY, C., MARSH, J., DOSTAL, J., MICHARD, A., and TESTA, S. 1988. Asthenospheric and lithospheric sources for Mesozoic dolerites from Liberia (Africa): trace element and isotopic evi- dence. Earth and Planetary Science Letters, 87: 100 - 110.

FAHRIG, W. F. 1987. The tectonic settings of continental mafic dyke swarms: failed arm and early passive margin. In Mafic dyke swarms. Edited by H. C. Halls and W. F. Fahrig. Geological Association of Canada, Special Paper 34, pp. 331 -348.

FAHRIG, W. F., and JONES, D. L. 1969. Paleomagnetic evidence for the extent of the Mackenzie igneous events. Canadian Journal of Earth Sciences, 6: 679-688.

FAHRIG, W. F., and SCHWARZ, E. J. 1973. Additional paleomagnetic data on the Baffin diabase dikes and a revised Franklin pole. Cana- dian Journal of Earth Sciences, 10: 576-581.

FAHRIG, W. F., IRVING, E., and JACKSON, G. D. 1971. Paleomag- netism of the Franklin diabases. Canadian Journal of Earth Sciences, 8: 455 -467.

FAHRIG, W. F., CHRISTIE, K. W., and JONES, D. L. 1981. Paleomag- netism of the Bylot basins: evidence for Mackenzie continental ten- sional tectonics. In Proterozoic basins of Canada. Edited by F. H. A. Campbell. Geological Survey of Canada, Paper 81-10, pp. 303-312.

FREY, F. A., BRYAN, W., and THOMPSON, G. 1974. Atlantic ocean floor: geochemistry and petrology of basalts from legs 2 and 3 of the Deep-Sea Drilling Project. Journal of Geophysical Research, 79: 5507 -5527.

FREY, F. A., GREEN, D. H., and ROY, S. D. 1978. Integrated models of basalt petrogenesis: a study of quartz tholeiites to olivine melili- tites from south eastern Australia utilizing geochemical and experimental petrological data. Journal of Petrology, 19: 463- 513.

GALLEY, A. G. 1978. The petrology and chemistry of the Nauyat For- mation volcanics, Borden Peninsula, northwestern Baffin Island. B.Sc. thesis, Carleton University, Ottawa, Ont.

GALLEY, A. G., JACKSON, G. D., and IANNELLI, T. R. 1983. Neo- helikian subaerial basalts with ocean-floor type chemistry, north- western Baffin Island [abstract]. Geological Association of Canada, Program with Abstracts, 8: A25.

GELDSETZER, H. 1973a. A syngenetic dolomitization and sulfide mineralization. In Ores in sediments. Edited by G. C. Amstutz and A. J. Bernard. Springer-Verlag , Berlin, pp. 1 15 - 127.

1973b. The tectono-sedimentary development of an algal- dominated Helikian succession on northern Baffin Island, N.W.T. In Symposium of Arctic Geology. Edited by J. D. Aitken and D. J. Glass. Geological Association of Canada, Memoir 19, pp. 99- 126.

GIBSON, I. L., SINHA, M. N., and FAHRIG, W. F. 1987. The geochemistry of the Mackenzie dyke swarm, Canada. In Mafic dyke swarms. Edited by H. C. Halls and W. F. Fahrig. Geological Association of Canada, Special Paper 34, pp. 109- 121.

GREENOUGH, J. D., and PAPEZIK, V. S. 1987. The petrology of North Mountain basalts from the wildcat oil well Mobil Gulf Chinampas N-37, Bay of Fundy, Canada. Canadian Journal of Earth Sciences, 24: 1255 - 1260.

HAWKESWORTH, C. J., and POWELL, M. 1980. Magma genesis in the Lesser Antilles island arc. Earth and Planetary Science Letters, 51: pp. 297-308.

HAWKESWORTH, C. J., ROGERS, N. W., VAN CALSTEREN, P. W. C., and MENZIES, M. A. 1984. Mantle enrichment processes. Nature (London), 311: 331 -335.

HAWKESWORTH, C. J . , MARSH, J. S., DUNCAN, A. R., ERLAND, A. J., and NORRY, M. J. 1986. The role of continental lithosphere in the generation of the Karoo volcanic rocks: evidence from com- bined Nd- and Sr-isotope studies. In Petrogenesis of the volcanic rocks of the Karoo Province. Edited by A. J. Erlank. Geological Society of South Africa, Special Publication 13, pp. 341 -354.

HEAMAN, L. M., and LECHEMINANT, A. N. 1988. U-Pb bad- deleyite ages of the Muskox intrusion and MacKenzie dyke swarm, N.W.T., Canada [abstract]. Geological Association of Canada, Program with Abstracts, 13: A53.

IRVINE, T. N., and BARAGAR, W. R. A. 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 8: 513-548.

JACKSON, G. D. 1986. Notes on the Proterozoic Thule Group, north- ern Baffin Bay. In Current research, part A. Geological Survey of Canada, Paper 86-l A, pp. 541 -552.

JACKSON, G. D., and DAVIDSON, A. 1975. Bylot Island map-area, Dis- trict of Franklin. Geological Survey of Canada, Paper 74-29.

JACKSON, G. D., and IANNELLI, T. R. 1981. Rift-related cyclic sedimentation in the Neohelikian Borden Basin, northern Baffin Island. In Proterozoic basins of Canada. Edited by F. H. A. Camp- bell. Geological Survey of Canada, Paper 81-10, pp. 269 - 302.

1984. Borden Basin, N.W. Baffin Island: mid-Proterozoic rifting and possible ocean opening [abstract]. Geological Associa- tion of Canada, Program with Abstracts, 9: A76.

JACKSON, G. D., and MORGAN, W. C. 1978. Precambrian metamor- phism on Bafin and Bylot islands. In Metamorphism in the Cana- dian Shield. Edited by J. A. Fraser and W. W. Heywood. Geological Survey of Canada, Paper 78-10, pp. 249-267.

JACKSON, G. D., DAVIDSON, A., and MORGAN, W. C. 1975. Geology of the Pond Inlet map-area, Baffin Island, District of Franklin. Geological Survey of Canada, Paper 74-25.

JACKSON, G. D., IANNELLI, T. R., KNIGHT, R. D., and LEBEL, D. 1985. Neohelikian Bylot Supergroup of Borden rift basin, north- western Baffin Island, District of Franklin. In Current research, part A. Geological Survey of Canada, Paper 85-lA, pp. 639-649.

LEMON, R. R. H., and BLACKADAR, R. B. 1963. Admiralty Inlet area, Baffin Island, District of Franklin. Geological Survey of Canada, Memoir 328.

LE ROEX, A. P., DICK, H. J. B., ERLANK, A. J., REID, A. M., FREY, F. A., and HART, S. R. 1983. Geochemistry, mineralogy and petro- genesis of lavas erupted along the southwest Indian Ridge between the Bouvet triple junction and 1 1 degrees east. Journal of Petrol-

DOSTAL ET AL. 2223

LUDDEN, J. N., and THOMPSON, G. 1979. An evaluation of the behaviour of the rare earth elements during the weathering of sea- floor basalts. Earth and Planetary Science Letters, 43: 85-92.

MANSON, V. 1967. Geochemistry of basaltic rocks: major elements. In Basalts. Vol. 1 . Edited by H. H. Hess and A. Poldervaart . Inter- science, New York, pp. 215 -269.

OLSON, R. A. 1977. Geology and genesis of zinc-lead deposits within a late Precambrian dolomite, northern Baffin Island, N.W.T. Ph.D. thesis, The University of British Columbia, Van- couver, B.C.

1984. Genesis of paleokarst and stratabound zinc-lead sul- fide deposits in a Proterozoic dolostone, northern Baffin Island, Canada. Economic Geology, 79: 1056- 1103.

PAPEZIK, V. S., and BARR, S. M. 1981. The Shelburne dike, an early Mesozoic diabase dike in Nova Scotia: mineralogy, chemistry, and regional significance. Canadian Journal of Earth Sciences, 18: pp. 1346-1355.

PAPEZIK, V. S., and HODYCH, J. P. 1980. Early Mesozoic diabase dikes of the Avalon Peninsula, Newfoundland: petrochemistry, mineralogy, and origin. Canadian Journal of Earth Sciences, 17: 1417- 1430.

PAPEZIK, V. S., GREENOUGH, J. D., COLWELL, J. A., and MALLIN- SON, T. J. 1988. North Mountain basalt from Digby, Nova Scotia: models for a fissure eruption from stratigraphy and petrochemistry. Canadian Journal of Earth Sciences, 25: 74- 83.

PEARCE, J. A. 1983. Role of the subcontinental lithosphere in magma genesis at continental margins. In Continental basalts and mantle xenoliths. Edited by C. J. Hawkesworth and M. J. Norry. Shiva Press, Nantwich, pp. 230 -249.

PIUNZ, M. 1967. Geochemistry of basaltic rocks: trace elements. In Basalts. Vol. 1. Edited by H. H. Hess and A. Poldemaart. Inter- science, New York, pp. 271-324.

SUN, S. S., NESBITT, R. W., and SHARASKIN, A. Y. 1979. Geochemi- cal characteristics of mid-ocean ridge basalts. Earth and Planetary Science Letters, 44: 119- 138.

THOMPSON, R. N., MORRISON, M. A., DICKIN, A. P., and HENDRY, G. L. 1983. Continental flood basalts . . . . Arachnids rule OK? In Continental basalts and rnantle xenoliths. Edited by C. J. Hawkesworth and M. J. Norry. Shiva Press, Nantwich, pp. 158-185.

WRIGHT, T. L., GROLIER, M. J., and SWANSON, D. A. 1973. Chemi- cal variation related to the stratigraphy of the Columbia River basalts. Geological Society of America Bulletin, 84: 371 -386.