Eur. J. Mineral. 1998, 10,79-93
Geochemical constraints on the origin and evolution of early Mesozoic dikes in Atlantic Canada JAROSLAV DOSTAL and MICHAEL DURNING
Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada B3H 3C3 e-mail:
[email protected]
Abstract: Two prominent early Mesozoic (~ 201 Ma) diabase dikes in eastern Canada (Shelburne dike of Nova Scotia and Caraquet dike of New Brunswick) are part of the eastern North America (ENA) basalt province between Alabama and Newfoundland. The dikes form lineaments more than 200 km long. They consist of quartz-normative tholeiitic basalts. In contrast to the Shelburne dike, the Caraquet dike contains modal olivine and has a lower content of incompatible trace elements with lower (La/Yb)n (1.6-1.8 vs. 3.1-3.9) and isotopically less enriched Sr and Nd isotopes. The dikes have low Mg#, Ni and Cr, indicating that the magmas underwent extensive fractionation. The magmas were variably contaminated with continental crust, particularly the Shelburne dike as evidenced by high Th/La and its radiogenic initial Sr isotope ratios. The Shelburne dike could have been derived from the Caraquet-type magma by a crustal assimilationfractional crystallization process. The Caraquet magma is inferred to be derived from a sub-continental lithospheric spinel-bearing mantle. The Shelburne and Caraquet dikes and possibly other ENA basalts were probably generated in response to lithospheric extension associated with the opening of the North Atlantic over a region of anomalously hot mantle related to a mantle plume. Key-words: geochemistry, petrogenesis, basalt, dike, Mesozoic.
Introduction Early Mesozoic basaltic dikes, dike swarms, sills and lavas crop out throughout the length of the Appalachian orogen, along the eastern continental margin of North America from Alabama to Newfoundland. This eastern North America (ENA) basaltic province is part of a larger, circum-Atlantic province that embraces the continental margins of North America, western Europe and North Africa and is associated with the onset of continental rifting that preceded the opening of the north Atlantic Ocean {ca. 195 Ma; May, 1971). The suites have been extensively studied in part to shed some insight into mechanism and processes that accompany continental breakup and the initiation of seafloor spreading. Weigand & Ragland (1970) recognized that basalts of the ENA province are tholeiitic (hyper-
sthene-normative) but variable in composition. For example, the southern portion of the ENA province (Carolinas and Virginia) contains mainly olivine-normative tholeiites whereas quartz-normative tholeiites characterize the basalts in the northern (northeastern United States and eastern Canada) part of the province. Weigand & Ragland (1970) attributed these differences to fractional crystallization. Because of the large compositional differences among and between some of these groups, several other processes including crustal contamination (Dostal & Dupuy, 1984; Philpotts & Martello, 1986), variable depths of melting (Keller & Hoover, 1989) and heterogeneous mantle source (Puffer & Philpotts, 1989; Dostal & Greenough, 1992) have also been proposed. However, the petrogenesis of various tholeiitic types as well as the relationship between these types is still a matter of controversy. 0935-1221/98/0010-0079 $ 3.75 © 1998 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart
80
J. Dostal, M. Durning
Geological setting, petrography, mineralogy
Fig. 1. Maps showing the locations of Nova Scotia, New Brunswick and Shelburne and Caraquet dikes and the North Mountain basalts. Sample sites: CH - Cherry Hill; P - Pubnico; C - Caraquet.
Conspicuous examples for ENA basalts in Atlantic Canada are the Shelburne dike of Nova Scotia and the Caraquet dike of New Brunswick (Fig. 1). Previous studies showed that they belong to different ENA basaltic types. The Shelburne dike (Papezik & Barr, 1981) is of the high-TiO2, quartz-normative type of Weigand & Ragland (1970) with modal orthopyroxene, whereas the Caraquet dike is a low-TiO2 variety, usually with modal olivine (Greenough & Papezik, 1986). Detailed comparison of these two dikes can provide insight into the relation between the early Mesozoic basaltic rocks along the ENA margin. This paper uses mineral chemistry and major, trace and isotope whole-rock data to constrain the petrogenesis of the Shelburne and Caraquet dikes. Because of its significantly better exposure, the Shelburne dike is described in greater detail than the Caraquet dike.
The ENA basaltic province is represented in eastern Canada by several dikes and lava flows around the Bay of Fundy. The largest occurrence of basalts is in the Bay of Fundy graben, one of the largest and northernmost of the basins along the continental margin of North America. These subaerial basalts, known as the North Mountain basalt (Fig. 1), form a northeast-trending belt, about 200 km long with thickness decreasing northeastward from about 400 to 275 m (Dostal & Dupuy, 1984; Dostal & Greenough, 1992). They typically are high-TiO2, quartz-normative tholeiites, similar to those described by Weigand & Ragland (1979) from northeastern United States. These rocks and their intrusive counterparts have been isotopically dated. The North Mountain basalt yields a whole-rock K/Ar isochron age of 191 ±2 Ma (Hayatsu, 1979), consistent with the earliest Jurassic age based on fossils (Olsen et al., 1982, 1987). Several Mesozoic dikes in Atlantic Canada have also been dated. Like comparable basaltic dikes and lavas in the northeastern United States, the dikes yielded a K/Ar age of about 190 Ma. Hodych & Hayatsu (1988) reported K/Ar whole-rock isochron ages of 193 ± 2 Ma for the Shelburne dike, 191 ± 1 Ma for the Caraquet dike and 189 ±3 Ma for an intrusion in southeastern Newfoundland, known as the Avalon dike (Fig. 1; Hodych & Hayatsu, 1980; Papezik & Hodych, 1980). However, recent U/Pb ages on zircon and baddeleyite from the ENA sills suggest that the age of the ENA magmatic province is 201 ±2 Ma (Sutter, 1988; Dunning & Hodych, 1990). The early Mesozoic age and the northeastern trending (Fig. 1) imply that all these basaltic bodies were emplaced in the extensional environment associated with the initial stages of the opening of the Atlantic Ocean.
1. Shelburne dike The Shelburne dike (Fig. 1) is a steeply (75-80° SE) dipping basaltic body that cuts Cambro-Ordovician flyshoid metasediments of the Meguma Group and Devono-Carboniferous granitoid plutonic rocks (Papezik & Barr, 1981). The dike sporadically crops out between Pubnico and Cherry Hill, Nova Scotia (Fig. 1). Its magnetic signature can be traced offshore, particularly across the Gulf of Maine. Its total length is in excess of 250 km
Petrogenesis of Mesozoic dikes in Atlantic Canada (Papezik & Barr, 1981; Pe-Piper & Loncarevic, 1989). The width of the dike is variable, ranging from about 60 m at Pubnico to about 100 m at Cherry Hill; it is offset by faults in some places. Samples of the Shelburne dike (Fig. 1) were col lected from one transect at Cherry Hill (20 sam ples) and another at Pubnico (10 samples). Most of the dike is composed of medium- to coarse-grained gabbro/diabase with subophitic or subhedral granular textures. Near the margins, the rocks are fine grained and holocrystalline; there are also about 3 mm thick glassy chilled margins. The gabbro/di abase is made up of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides (ilmenite and titaniferous magnetite) and sometimes apatite. Fine-grained groundmass which is abun dant around the margins of the dike, is composed of equigranular anhedral plagioclase, pyroxenes and Fe-Ti oxides. At Pubnico, the pyroxene/plagioclase ratio in creases toward the centre of the dike probably due to flowage differentiation. At Cherry Hill, plagio clase makes up the bulk of these rocks, with or thopyroxene and clinopyroxene becoming less abundant in the centre. Locally, clinopyroxene rims orthopyroxene. Highly fractionated rocks in the central parts of the dike contain up to 15 vol.% graphic alkali feldspar-quartz intergrowths that enclose acicular apatite crystals. At the Cherry Hill transect, the dike probably crystallized slowly from the margin to the centre. The magma, in moving northeastward (see below), may have slowed enough for flowage differentiation to cease to become a significant factor at Cherry Hill, allowing fractionation mainly in situ. The mineralogy of the dike is similar to many Mesozoic diabases and basalts from the northern parts of the EN A magmatic province (Weigand & Ragland, 1970; Puffer & Philpotts, 1989; Dunn & Stringer, 1990). Typical mineral compositions are given in Table 1. Analyses of plagioclase cores and rims range from An 73 to An60 and An 70 to An46, respectively (see Table 1 for analytical tech nique for determination of mineral composition). In detail, plagioclase shows oscillatory zoning. There is little variation between plagioclase from the margin and centre of the dike. Clinopyroxenes are mostly augites with com position ranging between Wo3oEn58Fs12 and \V035En32Fs33 (Table 1; Fig. 2). Some grains have up to Wo42; cores of these pyroxenes tend to be more calcic than the rims. Groundmass clino pyroxene from the margin is augite to pigeonite
81
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with a trend toward iron enrichment. This trend is also evident among clinopyroxene phenocrysts (Fig. 2) suggesting that the oxygen fugacity was too low to stabilize Fe-Ti oxides during the crys tallization of clinopyroxene phenocrysts. The cli nopyroxene compositions also display a distinct variation along the Cherry Hill section. The con tent of Si, Mg and Cr of clinopyroxene as well as its X Mg decrease from the margin to the centre of the dike while Fe shows an opposite trend. Clino pyroxenes rimming orthopyroxenes are chemi cally similar to augite phenocrysts. The limited amount of data does not allow an evaluation of the variation of clinopyroxene composition across the Pubnico transect. The ranges of mineral com positions at Pubnico are, however, comparable to those at Cherry Hill. Orthopyroxene compositions fall in the bronzite range (Fig. 2) and show chemical variation from core to rim (Wo4En85Fsπ to W05E1175FS20). The iron enrichment trend of orthopyroxene is significantly smaller than that of clinopyroxene (Fig. 2). The limited range of orthopyroxene com positions and the lack of significant zoning sug gest that the crystallization period of ortho pyroxene was relatively short compared to clinopyroxene. Opaque minerals include titanifer-
Table l. Representative microprobe analyses of minerals of the Shelburne and Caraquet dikes.
Orthopyroxene
Clinopyroxene Caraquet
Shelburne 34 Si02 Ti02 A1203 Cr203 FeO* MnO MgO CaO Na20 K20 P2O5 Total ca Mg Fe
c
14
m
m
A8 core 51.99 0.60 1.96
A4 core 51.92 0.43 1.71
12.57 0.34 18.77 13.09 0.29
0.11 100.37 35.52 33.21 21.27
core 51.20 0.65 1.70 0.15 13.58 0.29 15.38 17.17 0.23
rim 51.05 0.78 1.60 0.04 15.81 0.36 13.96 16.87 0.01
core 50.82 0.67 2.51 0.06 11.78 0.32 15.54 17.48 0.42
rim 49.95 0.86 1.58 0.01 18.97 0.39 11.29 16.79 0.42
0.02 100.37
0.10 100.58
0.21 99.80
34.91 43.51 21.58
34.70 39.93 25.37
36.20 44.73 19.02
0.669
0.611
0.702
Shelburne m
0.515
21
c
14
13.32 0.40 18.28 13.39
core 54.99 0.14 1.71 0.36 11.97 0.12 29.07 2.79
rim 54.09 0.05 1.58 0.21 13.54 0.38 27.99 2.44
99.61
99.45
101.60
100.61
101.15
26.69 53.28 20.03
27.21 51.67 21.12
4.52 77.53 17.95
4.61 75.59 19.80
5.29 76.94 17.77
0.812
0.792
0.812
Caraquet
Shelburne
A4
rim 54.47 0.23 1.37 0.19 13.26 0.30 28.38 2.41
0.710
Fe-Ti Oxide
m
core 54.92 0.25 1.88 0.33 12.17 0.19 29.46 2.38
0.727
01ivine
core 38.20
24 m rim 38.05
39.98 0.26
21.47 0.43 39.58 0.32
100.28
99.44
99.85
4.72 74.95 20.33
77.20 22.80
76.70 23.30
0.786
21.00
m
0.772
Ilm
Mt
50.79
15.16 1.42
47.08 1.52
77.77 0.43
0.767
34 - Pubnico section; 14, 21, 24 - Cherry Hill section; m - dike margin; c - dike centre; Ilm - ilmenite; Mt - magnetite. Mineral compositions were determined using a JEOL Superprobe 733 at Dalhousie University (Halifax, Nova Scotia) equipped with four wavelength-dispersive spectrometers and one energy-dispersive spectrometer, and operated with a beam current of 15kV at 5nA. Data were reduced using ZAF corrections.
Petrogenesis of Mesozoic dikes in Atlantic Canada ous magnetite, ilmenite and traces of pyrite and chalcopyrite. Analyses of magnetite and ilmenite give rather uniform compositions across the dike. Magnetite composition averages 47.2% magnetite and 52.8% ulvospinel while ilmenite composi tions average 95.1 % ilmenite and 3.5% hematite. The Fe-Ti oxides appear to be late crystallizing phases. Felsic dikes and/or patches about 10 cm thick occur at the contact with the Shelburne dike at Pubnico. The felsic dikes which are believed to be a result of partial melting of country rocks in duced by the intruding Shelburne dike, are com posed of intergrowth of alkali feldspar and quartz with accessory biotite (sample 43 in Table 2).
2. Caraquet dike The Caraquet dike trends NE-SW (Fig. 1) and has been traced magnetically for about 500 km from Caraquet southwestward into Maine (Burke et al, 1973); with the exception of northern New Bruns wick, its exposure is poor. The dike varies in thickness (9-45 m) and is subvertical (Burke et al, 1973). The gabbro/diabase dike was sampled at sev eral localities in the Caraquet area. Its chilled mar gins are glassy to fine-grained; the dike centre is medium- to coarse-grained. Primary phases in clude zoned plagioclase and clinopyroxene with minor olivine, pigeonite and Fe-Ti oxides. The chilled margins locally are plagioclase- and clinopyroxene-phyric; these phenocrysts can form glomeroporphyric clusters. Petrographically, it differs from the Shelburne dike primarily by the presence of olivine and the absence of orthopyroxene. It is also relatively homogeneous. The petrography and mineralogy of these rocks are comparable to those of other low-Ti, quartz-normative tholeiites from the ENA prov ince (Weigand & Ragland, 1970; Puffer & Philpotts, 1989). Plagioclase compositions range from An74 (core) to An51 (rim). Clinopyroxene is pres ent as both dominant subcalcic augite (Fig. 2) and pigeonite (~Woi0). Augite is typically Wo25_39 En50_48 Fsπ_23 (Table 1). The rims are enriched in Fe up to Wo24En41Fs35. Olivine phenocrysts occur in most samples. Their core-rim composition varies normally from Fo 79 to Fo75, a range similar to that reported for the Caraquet dike by Greenough&Papezik(1986).
83
Geochemistry 1. Analytical techniques and alteration Thirty samples of the Shelburne dike and 23 samples of the Caraquet dike have been analyzed for major and some trace (Rb, Sr, Ba, Zr, Nb, Y, Ga, Cu, Cr and Ni) elements using X-ray fluores cence (fused glass discs and powder pellets) at the Geochemical Centre of Saint Mary's University, Halifax. Fourteen samples were chosen for deter mining a wider range of trace elements including rare-earth elements (REE), Th, U, Nb and Hf using inductively coupled plasma mass spectrometry at Memorial University of Newfoundland. Results are given in Table 2. The method was de scribed by Longerich et al (1990). The precision and accuracy are discussed in Dostal et al (1986, 1994). In general, the precision is better than ± 5 % for major elements and 2 - 1 0 % for trace elements. Nine of these samples were sub sequently selected for Nd and Sr isotope analyses. Sm, Nd, Rb and Sr abundances and Sr and Nd iso tope ratios were determined by isotope dilution mass spectrometry in the AURIF lab of Memorial University of Newfoundland. Measured 143Nd/144Nd values were normalized to a l46Nd/l44Nd ratio of 0.7219. The LaJolla standard, analyzed as part of every run, yielded average 143Nd/144Nd = 0.511849 ±9. The 87Sr/86Sr was corrected using 86 Sr/88Sr = 0.1194. Replicate runs for the NBS 987 Sr standard gave 87Sr/86Sr = 0.710250+11. Initial Nd and Sr isotope ratios and epsilon values (eNd) were calculated using the U/Pb age (201 Ma) of Dunning & Hodych (1990). The rocks of both dikes appear to be fresh al though few samples show the mineralogical char acteristics of limited alteration such as the seriali zation of plagioclase, development of chlorite after pyroxene, and serpentine after olivine and the rare presence of calcite. The altered rocks were not incuded in the analyzed set. 2. Major and trace elements Shelburne dike Most rocks of the Shelburne dike have SiO 2 rang ing from 52 to 54 wt% (LOI-free) and their major element compositions correspond to quartz-nor mative tholeiitic basalts. Compositional variations within the dike are illustrated in Fig. 3 for selected major and trace elements. Compared to a primi-
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