Une longue periode d'extension de la lithosphere a encouragk la fusion d'un plus grand volume de crofite et 1'Crup- tion de picrites et basaltes dans la rCgion ...
Early Proterozoic (1.88-1.87 Ga) tholeiitic magmatism in the New QuCbec orogen THOMAS SKULSKI~ AND ROBERT P. WARES~ Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montrkal, QC H3A 2A7, Canada AND
ALAND. SMITH^
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De'paaement de giologie, Universite' de Montre'al, P.O. Box 6128, Station A, Montre'al, QC H3C 3J7, Canada
Received December 14, 1992 Revision accepted June 30, 1993 The New QuCbec orogen contains two volcano-sedimentary sequences bounded by unconformities. Each sequence records a change from continental sedimentation and alkaline volcanism to marine sedimentation and tholeiitic volcanism. The first sequence records 2.17 Ga rifting and the development, by 2.14 Ga, of a passive margin along the eastern part of the Superior craton. The second sequence developed between 1.88 and 1.87 Ga in pull-apart basins that reflect precollisional dextral transtension along the continental margin. Second-sequence magmatism comprises ( i ) carbonatitic and lamprophyric intrusions and mildly alkaline mafic to felsic volcanic rocks; (ii) widespread intrusion of tholeiitic gabbro sills, and submarine extrusion of plagioclase glomeroporphyritic basalts and younger aphyric basalts and picrites; and (iii) late-stage, mafic to felsic volcanism and intrusion of carbonatites. Crustal thinning allowed primitive tholeiitic magmas to equilibrate at progressively lower pressures before more buoyant derivative liquids could erupt. Early primitive melts were trapped at the base of the crust and crystallized olivine and orthopyroxene with minor crustal contamination. Derivative melts, similar to transitional mid-oceanridge basalts, migrated upward into mid-crustal magma chambers where they became saturated in calcic plagioclase. Subsequent tapping of these magma chambers allowed plagioclase ultraphyric magmas to intrude the sedimentary pile and erupt on the sea floor. Prolonged lithospheric extension resulted in more voluminous mantle melting and eruption of picrites and basalts in the south. Primitive magmas in the north were trapped beneath thicker crust and crystallized wehrlite cumulates. Resulting basaltic melts intruded the volcano-sedimentary pile, or erupted as aphyric basalts. L'orogbne du Nouveau QuCbec inclut deux skquences volcano-~Cdimentaireslimitkes par des discordances. Chaque sCquence tCmoigne du changement d'une skdimentation continentale et d'un volcanisme alcalin a une skdimentation marine et 21 un volcanisme tholkiitique. La premibre sCquence permet de documenter la formation du rift datC de 2,17 Ga et le dCveloppement, il y a 2,14 Ga, d'une marge passive le long de la partie orientale du craton du SupCrieur. La deuxibme sCquence dCveloppCe entre 1 3 8 et 1,87 Ga dans des bassins en > qui reflbtent un regime de transtension dextre avant collision le long de la marge continentale. Le magmatisme de la deuxibme sequence inclut (i) des intrusions carbonatitiques et lamprophyritiques et des volcanites modCrCment alcalines a felsiques; (ii) I'intrusion rCpandue de filons-couches de gabbro tholkiitique, et I'extrusion de basaltes ?i plagioclase glomtroporhyritique ainsi que des basaltes et picrites aphanitiques plus jeunes; et (iii) un stade tardif de volcanisme de composition mafique ?i felsique et d'intrusion de carbonatites. Les magmas tholkiitiques primitifs ont equilibrC a des pressions progressivement moindres suivant un amincissement de la crofite, avant que les liquides rCsiduels moins dense puissent faire eruption. Au dCbut, les magmas primitifs furent piCgCs a la base de la croQteet I'olivine et l'orthopyroxbne ont cristallisC en association avec une contamination crustale mineure. Les magmas secondaires, similaires ?i ceux des basaltes de transition de cr6te mCdio-ockanique, ont migrt par ascension dans les chambres magmatiques ou ils sont devenus saturCs en plagioclase calcique. Les soutirages subskquents dans ces chambres magmatiques ont permis aux magmas ultraphyriques a plagioclase de s'injecter dans les sCdiments empilCs et de s'Cpandre sur le plancher oceanique. Une longue periode d'extension de la lithosphere a encouragk la fusion d'un plus grand volume de crofite et 1'Cruption de picrites et basaltes dans la rCgion sud. Les magmas primitifs dans la rCgion nord furent piCgCs sous une crofite plus Cpaisse avec cristallisation de cumulats de wehrlite. Les magmas basaltiques produits ont pCnCtrC les matCriaux volcanoskdimentaires empilCs, ou ont fait Cruption sous forme de basaltes aphanitiques. [Traduit par la rcklaction] Can. I. Earth Sci. 30, 1505- 1520 (1993)
Introduction The New QuCbec orogen (formerly known as Labrador Trough; Hoffman 1988) is an 800 km long, northwesttrending continental collisional zone extending from the Grenville Front to Ungava Bay. It includes three broad belts comprising eight lithotectonic zones (Fig. 1): (i) a western parautochthonous to allochthonous sedimentary belt (Chioak, MClbzes, and Schefferville zones); (ii) a central allochthonous 'Present address: Continental Geoscience Division, Geological Survey of Canada, 615 Booth Street, Ottawa, ON KIA OE8, Canada. 2Present address: SociCtC de Recherche Ixion Inc., 4450 Fabre Street, Montreal, QC H2J 3V3, Canada. 'Present address: Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, Republic of China. Printed ~n Canada / ImprlrnC au Canada
volcano-sedimentary belt intruded by voluminous gabbro sills (Howse, Baby, and Doublet zones); and (iii) an eastern hinterland of metasedimentary rocks, granitoids, and remobilized Archean basement (Laporte and Rachel zones). The foreland of the orogen represents the vestiges of the Early Proterozoic margin of the Superior craton (Kaniapiskau Supergroup), which was deformed during southwesterly directed collision with the southeastern Rae Province (Harrison et al. 1970; Boone and Hynes 1990; Hoffman 1990a, 1990b; Wares and Goutier 1990a; Wares and Skulski 1992). The foreland consists of two major volcano-sedimentary sequences separated by unconformities (Le Gallais and Lavoie 1982). Each sequence records a transition from continental sedimentation and local alkaline volcanicm to progressively deeper water basinal sedimentation and tholeiitic basaltic volcanism. Second-
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CAN. J. EARTH SCI. VOL. 30, 1993
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Volcano-sedimentary
Undifferentiated Archean De Pas-Kuujjuaq batholith: granite, tonalite, granodiorite
r
X
X
x
x
X
x X
"
X
j X
t
r X
Rae Province "
X
X
*
*
x
x
FIG. 1. Lithotectonic zones of the New Quebec orogen and location map (modified after Wardle et al. 1990b). Map in lower left shows eastern segments of the Trans-Hudson belt. Chioak zone comprises parautochthonous first-, second-, and third-sequence sedimentary rocks. Schefferville zone includes allochthonous Chioak zone sedimentary rocks and second-sequence alkaline igneous rocks. Howse zone is an allochthon of first-sequence volcano-sedimentary rocks intruded by gabbro sills. MClkzes zone comprises allochthonous second-sequence sedimentary rocks. The Baby zone includes second-sequence basalts and turbidites intruded by gabbro sills, whereas the Doublet zone in the south also includes picrites (Willbob Formation). Laporte and Rachel zones comprise amphibolite-grade rocks of the hinterland.
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SKULSKI ET AL.
sequence magmatism (1884 - 1870 Ma; ChevC and Machado 1988; Machado et al.4) postdates 2.0 Ga rifting on the Ungava craton (Chandler and Parrish 1989). In this paper we focus on the origin and tectonic setting of second-sequence tholeiitic magmatism. Our work is mostly based on data collected in the foreland (Baby zone) of the northern New QuCbec orogen (Fig. 2) (Wares and Goutier 1990~). The two volcano-sedimentary sequences within the New QuCbec orogen have been interpreted as a long-lived passive margin succession recording multiple rifting events (Baragar and Scoates 1981; Wardle and Bailey 1981; Le Gallais and Lavoie 1982; Clark and Thorpe 1990). The younger volcanosedimentary pile has also been interpreted as a migrating foredeep sequence formed at the toe of a thrust-fold belt (Hoffman 1987). A more recent model suggests that late-stage basaltic volcanism formed in a dextral oblique extensional setting along the western margin of the southeastern Rae Province and above an east-dipping subduction zone (Hoffman 1990b). The latter model makes specific, testable predictions about the petrological evolution of late-stage volcanic sequences in the New QuCbec orogen. The petrology of the second-sequence basalts in the New Quebec orogen and a comparison with volcanism in other parts of the Trans-Hudson belt suggest a setting in which progressive rifting along an older continental margin resulted in the eruption of relatively evolved tholeiitic basalts similar to transitional mid-ocean-ridge basalt (MORB). We suggest that the volcanic activity was triggered by dextral transtension between the Superior and southeastern Rae provinces, resulting in the development of sediment-rich pull-apart basins along the thinned Superior craton edge, as opposed to the margin of the southeastern Rae Province as suggested by Hoffman (1990b).
Geological setting The oldest foreland sedimentary sequence in the New QuCbec orogen (Le Gallais and Lavoie 1982; Hoffman 1987) is exposed in the central and southern parts of the orogen. It consists of (i) basal fluvial redbeds with local lenses of mildly alkaline mafic lavas (Seward Group); (ii) marine shelf quartzites and carbonate rocks (Pistolet Group); (iii) black shale and turbidites (Swampy Bay Group), which interfinger eastward with tholeiitic basalts and rhyolites (Bacchus Formation) and related gabbro sills; and (iv) a regressive carbonate reef complex (Denault-Abner Formation). A gabbro sill intruding redbeds in the older part of the first sequence (Seward Group) has a U/Pb age of 2169 +_ 4 Ma (Rohon 1989). Rhyolite in the Bacchus Formation is 21421; Ma (B. Dressler and T. Krogh, cited in Clark 1984). Thus the onset of rifting was prior to 2169 Ma, and by 2142 Ma most of the first sequence had been deposited. The second sequence (Le Gallais and Lavoie 1982; Hoffman 1987) onlaps the Superior craton throughout most of the autochthon and consists of a transgressive quartzite (Wishart Formation) overlain by shale (Ruth Formation), banded iron formation (Sokoman Formation) with coeval alkaline volcanic and intrusive rocks, and younger turbidites (Menihek Formation). A carbonatite feeder dyke to alkaline tuffs interstratified
EARLY PROTEROZOIC (AUTOCHTHONOUSPARAUTOCHTHONOUS)
Recent fluvioglacial sediments and unmapped areas
EARLY PROTEROZOIC (ALLOCHTHONOUS)
Chioak Formation (3rd cycle)
Undifferentiated hinterland
conglomerate, sandstone
'
Second cycle
a
turbidites, icon formation,
Thevenet Formation (3rd cycle)
qUarfl Lake Group) (Knob
..
ARCHEAN
granulite, gneiss, granite
L thrust fault
antic'ine
Second cycle Montagnais sills
10 krn
quartzite, argillite
gabbro, peridotite
Hellancourt Formation
syndine
tholeiitic basalt
Baby Formation
-n ,5,.
turbidites, iron formation
First cycle Abner Formation dolomite
4N. Machado, T. Clark, and J. David. New constraints on the ages of magmatism, deformation, and mineralization in the New Qukbec orogen: U-Pb evidence. In preparation.
FIG. 2. Geological map of the northern New Qukbec orogen (adapted from Wares and Gouthier 1990a and references therein). See Fig. 1 for location.
CAN. J. EARTH S(21. VOL. 30, 1993
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with Sokoman iron formation in the lower part of the second sequence has an age of 1880 f 2 Ma (ChevC and Machado 1988). Eastern basinal facies of the second sequence consist of turbidites and banded iron formation (Baby Formation) in the north, turbidites-pyroclastics in the south (Murdoch and Thompson Lake formations), and an overlying pile of submarine tholeiitic basalts (Hellancourt and Willbob formations). The basinal sequence is intruded by voluminous Montagnais gabbroic sills (Fig. 2) ranging in age from 1883.8: 1:: Ma (T. Birkett et al., cited in Wardle et al. 1990a) to 1874 f 3 Ma (reported in Machado 1990). Thus secondsequence deposition started at approximately 1884 Ma, and a time of 258 Ma separates the first and second sequences. The youngest available date on a second-sequence lithology, for a rhyodacite from the Lac Lemoyne area (Fig. 1) in the central orogen, is 1870 k 4 Ma (Machado et a1.4). The rhyodacite is interstratified with rhyolites and potassic felsic volcanic rocks, which are in turn intruded by the Lac Lemoyne carbonatite. The two volcano-sedimentary sequences are unconformably overlain by synorogenic units (Chioak and Tamarack River formations) that consist of a molasse containing clasts of westerly derived basement rocks and second-sequence lithologies (Hoffman 1987). The molasse is presumably no older than 1870 Ma, the youngest age of underlying second-sequence rocks. A late-stage flysch (Thtvenet Formation) that overlies Hellancourt tholeiitic basalts in the north may have formed during collision. Clear evidence of an intersequence (2142 - 1883 Ma) deformational event is lacking. Dimroth (1978) stated that the unconformity at the base of the second sequence cuts into gently folded first-sequence lithologies in the Cambrien Lake area. These open folds, however, are of uncertain origin and could reflect basin subsidence. The age of early Hudsonian deformation in the New QuCbec orogen may be as old as 1880 Ma (Chevt and Machado 1988) based on ages obtained from alkaline dykes that intrude parallel to the fracture cleavage of open, concentric folds in first-sequence lithologies in the parautochthon. However, concordance between fracture cleavage and dyke orientation is not a compelling argument for early deformation. A better constraint on the maximum age of deformation in the foreland is the 1870 Ma age (T. Clark, personal communication, 1992) of deformed rhyodacite in the central part of the orogen. A posttectonic monzonite intrusion at Nachicapau Lake is 1813 f 4 Ma, and thus the main deformational event in the New QuCbec orogen occurred between 1870 and 1813 Ma (Machado et al. 1989; Machado 1990). Uranium-lead dates in the northern hinterland of the orogen (Machado et al. 1989) constrain the Superior - southeastern Rae collision at 1840- 1820 Ma, with amphibolite metamorphism continuing to 1793- 1769 Ma and late-stage pegmatites intruding at 1746- 1740 Ma. Second-sequence volcanism and the intrusion of Montagnais sills The oldest second-sequence volcanic rocks in the northern part of the orogen are relatively rare basalt flows found in the middle Baby Formation. These basalts, found in iron formation near the Koke massive sulphide deposit (Fig. 2), consist actinolite albite schists. The main volcanic of chlorite pile (Hellancourt Formation) consists of a thin basal unit ( < 100 m) of plagioclase glomeroporphyritic (GMP), pillowed and massive basalt flows overlain by a thick pile (up to 1000 m) of aphyric massive and pillowed basalts and minor lapilli tuff. The Baby and Hellancourt formations are intruded
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by Montagnais sills of gabbro, GMP gabbro, and peridotite. The sills reach a maximum thickness of 600 m, and individual sills can be followed along strike for tens of kilometres. Some of the thicker Montagnais sills are differentiated into layered gabbro diorite assemblages. peridotite Massive and pillowed GMP basalt flows at the base of the Hellancourt Formation contain 2- 10% white, subrounded plagioclase glomerocrysts up to 2 cm across. The glomerocrysts are saussuritized and replaced by fine-grained albite, calcite, and zoisite. Younger aphyric basalts include massive nonvesicular flows and closely stacked pillow basalts with infrequent intraflow sediment or pillow breccia. Some aphyric pillow basalts contain lath-shaped carbonate and epidote pseudomorphs of plagioclase microphenocrysts. The metamorphic mineralogy of all basalts (aphyric and GMP) consists of actinolite, epidote, chlorite, plagioclase, quartz, and calcite. Montagnais sills of GMP gabbro are far less abundant than equigranular gabbro sills and occur mostly at the contact between the Baby and Hellancourt formations and are locally host to Cu-Ni sulphide deposits (Wares and Goutier 1990b). Differentiated and mineralized GMP sills consist of (i) olivine pyroxenite (actinolite and hornblende pseudomorphs of augite, serpentine pseudomorphs of olivine), overlain by (ii) GMP gabbro containing 1- 15 cm size clots of saussuritized plagioclase ( < 1- 100 vol. %) and lenses of disseminated sulphides (pyrrhotite, chalcopyrite), and (iii) a roof of granophyre containing saussuritized plagioclase and interstitial quartz. Some Montagnais GMP sills contain sedimentary xenoliths (BCrard 1965; SauvC and Bergeron 1965) or rounded quartz xenocrysts. Most equigranular Montagnais sills in the north consist of homogenous ophitic gabbro. However, in the Lac Soucy area, a 500 m thick differentiated equigranular gabbro sill is exposed (Fig. 2). The sill consists of (i) layered olivine gabbro (serpentinized olivine, saussuritized plagioclase, and actinolite after clinopyroxene), overlain by (ii) ophitic gabbro (saussuritized plagioclase, actinolite pseudomorphs of augite that enclose tremolite and chlorite pseudomorphs of olivine), (iii) equigranular gabbro, (iv) ferrogabbro (5- 10% euhedral oxides replaced by leucoxene, stilpnomelane, and minor quartz), (v) quartz diorite, and (vi) a roof zone of granodiorite. Better preserved Montagnais gabbro sills occur in other parts of the orogen. For instance, SauvC and Bergeron (1965) report fresh gabbro in the centre of thick sills in the north, which contain hypersthene, olivine enclosed in augite, and plagioclase. South of the study area, Montagnais gabbro sills are also layered, and contain (i) olivine gabbro (olivine armoured by augite, orthopyroxene, plagioclase, and oxides), overlain by (ii) wehrlite crescumulates, (iii) ophitic gabbro, (iv) equigranular gabbro (augite and plagioclase with less abundant orthopyroxene, olivine, and pigeonite), and (v) a roof of pegmatitic gabbro (augite, plagioclase, quartz, and apatite) (Baragar 1960; Rohon et al. 1988; Rohon 1989). Some Montagnais sills in the study area are ultramafic, or contain ultramafic lenses of peridotite and troctolite cumulates (serpentinized olivine k saussuritized plagioclase, and intercumulus actinolite after augite). Peridotitic sills in the central part of the orogen are confirmed to a narrow belt in the eastern allcohthon (e.g ., Retty sill; Baragar 1960; Rohon et al. 1988; Clark 1989; Beaudoin and Laurent 1989; Rohon 1989). Baragar and Scoates (1981) showed that peridotite sills are common in the hinterland and suggested that they are the youngest intrusive rocks in the orogen. This view is consistent with field exposures in the north, which show ultramafic sills cutting
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I
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SKULSKI ET AL.
TABLE1. Representative chemical analyses of volcanic rocks Sample No. : 1159A-86 1156B-86 1160B-86 1165-86 SY-10-573 1265-86 1271C-86 4049B-86 387-2-262 387-7-379 387-8-397 DYK APH APH APH GMP GMP GMP GMP BAB BAB BM Legend :
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SiO, (wt. %) TiO, A1zO, MgO FeO MnO CaO Na,O K20 Pz05
LO1 Total Rb ( P P ~ ) Sr Ba Sc Y Zr Hf V Nb Ta Cr Ni La Ce Nd Sm Eu Tb Ho Tm Yb Lu Zr/Y Nb/Zr (Laism), (LaIYb),
47.5 1.12 16.1 1.72 7.89 8.76 0.16 10.8 2.54 0.33 0.12 2.34 99.37
49.7 1.27 14.3 2.07 6.47 10.5 0.18 10.1 2.61 0.17 0.09 2.17 99.67
49.1 1.19 13.8 2.07 7.08 10.5 0.20 11.1 1.88 0.15 0.08 2.09 99.28
49.1 1.28 15.1 2.05 5.77 10.4 0.18 11.5 1.90 0.16 0.09 1.81 99.40
48.5 1.39 14.7 2.02 5.53 10.3 0.17 12.9 1.54 0.05 0.11 2.13 99.32
51.5 1.58 14.0 2.05 5.18 10.4 0.18 10.3 2.15 0.04 0.12 2.40 99.96
48.6 1.47 15.2 2.05 6.54 10.4 0.17 9.76 2.74 0.10 0.13 2.68 99.90
50.7 1.53 13.7 2.18 5.51 11.1 0.19 11.0 1.59 0.06 0.13 2.15 99.88
48.9 1.47 13.8 2.32 6.84 11.8 0.20 7.61 0.69 0.06 0.10 4.48 98.28
45.6 1.11 15.3 2.13 10.3 10.98 0.17 5.08 2.04 0.09 0.07 6.92 99.69
50.7 1.22 14.6 2.05 7.61 10.4 0.19 7.03 2.29 0.05 0.09 3.18 99.47
5 261 128 29 18 67 1.26 177 7 0.58 168 116 9.63 20.0 10.0 3.17 1.03 0.40 0.84 0.24 1.78 0.28 3.72 0.10 1.85 3.57
5 118 13 43 26 68 1.75 366 5 0.45 174 77 4.57 11.2 6.85 3.00 0.93 0.68 1.21 0.37 2.51 0.42 2.62 0.07 0.93 1.20
5 77 101 43 24 60 1.77 339 5 0.16 168 79 3.57 9.1 6.93 2.54 0.89 0.48 1.06 0.42 2.27 0.43 2.50 0.08 0.86 1.04
5 123 22 40 27 71 1.65 344 6 0.17 170 72 4.27 10.8 6.90 2.86 0.94 0.59 1.10 0.45 2.64 0.39 2.63 0.08 0.91 1.07
6 248 14 37 26 79 2.50 37 1 4 0.40 102 56 4.40 12.5 9.00 3.08 1.20 0.90 1.20 0.40 2.80 0.44 3.04 0.05 0.8 1.OO
5 126 9 41.6 32 88 3.20 399 6 0.50 120 69 5.10 12.7 10.0 3.20 1.03 0.80 1.30
5 108 29 38.9 29 79 2.80 396 6 0.40 128 81 5.00 14.3 12.2 3.24 0.92 0.90 1.40
-
-
3.00 0.51 2.75 0.07 0.97 1.12
3.10 0.50 2.72 0.08 0.94 1.07
5 119 10 40.6 30 85 2.39 395 7 0.28 126 67 5.41 12.6 9.35 3.52 1.07 0.65 1.28 0.48 2.88 0.48 2.83 0.08 0.94 1.24
5 240 9 46 28 65 2.16 446 5 0.29 194 64 3.85 9.76 7.29 3.10 1.OO 0.64 1.21 0.38 2.67 0.44 2.32 0.08 0.76 0.95
5 40 14 36 19 46 1.SO 326 5 0.16 257 248 2.89 7.04 5.89 2.18 0.85 0.42 0.73 0.31 1.97 0.31 2.42 0.11 0.81 0.97
5 137 9 44 26 65 2.06 372 6 0.22 199 102 4.30 10.2 6.93 2.74 0.83 0.60 0.93 0.45 3.05 0.38 2.50 0.09 0.96 0.93
NOTES:The samples include Hellancourt aphyric (APH) and glomeroporphyritic (GMP) basalts, diabase dyke (DYK), and Baby basalts (BAB). X-ray fluorescence analyses were done at McGill University with a Philips PW 1400 using fused discs and an u-coefficient technique. Analytical precisions ( l o in wt.%) as calculated from 20 replicate analyses of one disc are 0.05 (Si), 0.003 (Ti), 0.03 (Al), 0.058 (Mg), 0.01 (Fe), 0.001 (Mn), 0.01 (Ca), 0.06 (Na), 0.001 (K), and 0.004 (P).Rb, Sr, Zr, Nb, and Y determined by X-ray fluorescence at McGill University on pressed pellets using a Rh KOCompton scatter matrix correction. The precision for these elements is estimated to be 2 %. Cr, Ni, V, and Ba contents were determined with the major elements and have an estimated precision of 5 % . Sc, Hf, Ta, and the REE contents were determined at the UniversitC de MontrCal using instrumental neutron activation analysis. Samples were irradiated in a flux of 10 n . cm-' . s-' for 4 h in a SLOWPOKE I1 reactor and counted over a 6 week period on two Ge detectors with resolutions of 0.6 and 1.2 keV at 122 keV. The precision for La, Sm, Eu, Yb, and Sc is estimated at less than 5 % ; those for Ce, Nd, Ho, Tb, Lu, and Ta are between 5 and 10%. A complete data set is available from the first author. LOI, loss on ignition.
GMP gabbro and Hellancourt basalt (Wares and Goutier 1990b).
Geochemistry of volcanic and intrusive rocks Greenschist to amphibolite-facies metamorphism of Hellancourt volcanic rocks and Montagnais gabbro sills has resulted in remobilization of alkali and alkaline earth elements. To minimize the effects of alteration, least-altered samples were chosen in the field, and the majority of these have loss on ignition (LOI) values less than 5 wt. %. Only peridotite samples and three samples of Baby Formation basalts have higher LO1 values (Tables 1, 2). The Baby Formation basalts are included
here because they are rare and are the only volcanic rocks in this formation. The high LO1 values in peridotites reflect serpentinization of olivine. We have included these rocks for the sake of completeness. The petrogenetic conclusions arrived at below are based on variations of the less mobile elements: Mg, Fe, Al, Si, Ti, rare earth elements (REE), Zr, Nb, and Y. The Hellancourt aphyric and GMP basalts, Montagnais sills, and coeval rocks in the south-central part of the orogen (Willbob Formation) have been shown to be comagmatic and belong to a low-K tholeiitic magmatic lineage (Baragar 1960; Rohon et al. 1988; Stamatelopoulou-Seymour et al. 1991). Further corroboration for a common parental magma includes 1 (n indicates similar (LaISm), and (LaIYb), values of
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CAN. J. EARTH SCI. VOL. 30, 1993
TABLE 2. Representative chemical analyses of intrusive rocks Sample No.: 3199H-86 0610-87 4062-86 Bulk sill 1OllH-86 1189B-86 SY-33-208 SY-33-261 SY-33-332 38C19-87 0002A-87 DIO PER CHILL GAB CHILL GP CH GP MAT GP GAB PYROX GP CH Legend : SiO, (wt. %) TiO,
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A1203
MgO FeO MnO CaO Na20 KzO p205 LO1 Total Rb ( P P ~ ) Sr Ba Sc Y Zr Hf
v
Nb Ta Cr
Ni La Ce Nd Sm Eu
Tb Ho Tm Yb
Lu ZrlY NbIZr (LaISm), (LaIYb), NOTES: The analyses include gabbro (GAB), diorite (DIO), peridotite (PER), and sill chill margins (CHILL). The bulk sill analysis is a calculated analysis of an equigranular sill (series 3199) northeast of Lac Gerido. Glomeroporphyritic sills include plagioclase glomeroporphyritic gabbro (GP GAB), matrix of glomeroporphyritic gabbro (GP MAT), chill margin of glomeroporphyritic gabbro (GP CH), and olivine pyroxenite (PYROX).
chondrite normalized), NbIZr values of -0.9, and an overlapping range of EN^ values from 2.8 to +5.2 (Tables 1- 3). This chemical evidence is supported by field relationships that show feeder dykes leading from GMP sills into overlying GMP lava flows (Wares and Goutier 1990b). Exceptions to the above include a sample of diabase dyke cutting Hellancourt basalts, two samples of gabbro chill margin, and a granodiorite in equigranular sills, all of which have high (LaIYb), and (LaISm), values (e.g., sample 1159a-86, Table 1; sample 3199h-86, Table 2). The Baby basalts are chemically and isotopically similar to aphyric Hellancourt basalts (Tables 1, 3), and thus their geochemical characteristics are discussed along with the younger aphyric basalts.
+
Glomeroporphyritic sills and lavas GMP basalts have slightly higher Fe and Ti and lower Mg contents than aphyric basalts (Figs. 3A, 4A). In addition, olivine pyroxenites in GMP sills have higher FeIMg values rela-
tive to peridotites associated with equigranular sills (Figs. 3B, 3C). The higher Fe content of GMP lavas and sills may reflect their more evolved nature, and in particular a protracted history of plagioclase fractionation. The corroded textures of plagioclase glomerocrysts in GMP magmas attest to a complex history of plagioclase fractionation. The A1 and Si contents of GMP gabbros (Fig. 5B) project approximately towards An 82, a plagioclase composition more calcic than that predicted to coexist with either the groundmass of the gabbros or the basaltic lavas at 1 atm (1 atm = 101.325 kPa) (An 70; calculated using the algorithm of Nielsen 1988). Thus, it is unlikely that the plagioclase glomerocrysts found in the GMP suite developed in situ, but rather are more likely to have originated from a deeper magma chamber than the sills or flows within which they are found. The GMP gabbro sills and basalts have relatively flat chondrite-normalized trace-element profiles, and the basalts have slightly higher REE and incompatible trace-element
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TABLE3. Neodymium isotopic composition of volcanic and intrusive rocks
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Sample No.
Rock type
Sm ID (PP~)
Nd ID (ppm)
147Sm/l4Nd
'43Nd/144Nd measured
P:E
Ma
Baby Formation basalt Baby Formation basalt Baby Formation basalt Gabbro Gabbro chill Diabase dyke Hellancourt Formation basalt Hellancourt Formation GMP basalt Hellancourt Formation GMP basalt Hellancourt Formation GMP basalt GMP gabbro chill GMP gabbro chill Hellancourt Formation basalt Hellancourt Formation basalt Hellancourt Formation GMP basalt Gabbro chill Baby Formation mudstone Baby Formation mudstone Baby Formation mudstone Chemical procedures follow the method of Smith et al. (1990). Samples were analyzed either at the Universitk de MontrCal using a modified NBS NOTES: mass spectrometer (samples marked with an asterisk; data from Smith and Ludden 1989) or at the University of Alberta using a Micromass MM30 (samples SY-90-118, SY-37-281, and SY-2-58) or VG354 (all other samples) mass spectrometers. Neodymium isotopic ratios are reported normalized to '46Nd/'"Nd = 0.7219. Mean values for the La Jolla standard on these three mass spectrometers were 0.511865 (n = lo), 0.511841 (n = 11), and 0.511856 (n = 6 ) , respectively. Measured ratios have been adjusted relative to '43Nd/'44Nd= 0.511860 for La Jolla to correct for intermachine bias. Nd model ages (T,,) are shown for the mudstone samples. ID, isotopic dilution.
(e.g., Nb) abundances than aphyric basalts (Figs. 6A, 6B). ~ ~ of GMP basalts range from +3.4 to The ~ k Ma$ values +4.8, similar to the range of values found in both GMP gabbro chill margins (+2.8 to +4.0) and aphyric basalts (Table 3; Fig. 7). These data indicate that the mantle source of GMP and aphyric magmas had a time-integrated depletion in Sm relative to Nd. Equigranular sills and aphyric basalts Smooth geochemical sections characterize the differentiated equigranular sill east of Lac Soucy and bear on the nature of the shallow-level fractionation processes (Fig. 8). A decrease in Mg content toward the top of the section is accompanied by little change in Si contents throughout the lower 300 m of the sill (Figs. 8A, 8C). There is an abrupt increase in Ti content that accompanies the change from cumulus olivine + plagioclinopyroxene (olivine gabbro), to plagioclase clase clinopyroxene (gabbro; Fig. 8B). The highest Ti contents reflect accumulation of titanomagnetite in ferrogabbros, and fractionation of this phase is in large part responsible for the increase in Si and decrease in Ti in the upper part of the sill (Figs. 8A, 8B). Whereas there is considerable chemical diversity in some sills, the majority of sills and basalts show limited chemical variations (Table 1). The aphyric basalts have A1 and Si contents that lie at the projected end of an olivine-accumulation trend defined by peridotite sills (Figs. 5A, 5C). The basalts (Fig. 5A) overlap in A1 and Si content with olivine gabbros and ferrogabbros in equigranular sills (Fig. 5C). The basalts have major-element compositions that are similar to Chukotat plagioclase-phyric basalts in the Cape Smith belt (Figs. 3A, 4A, 5A), and both trend to higher Fe contents than low-Mg
+
+
MORB basalts and glasses (Francis et al. 1983). The majority of basalts lie on the high-Fe side of the intersection between an olivine-accumulation trend defined by peridotites and an Fe-enrichment trend defined by olivine gabbros (low Fe, high Mg) and ferrogabbros (high Fe, low Mg; Figs. 3A, 3C). The position of the aphyric basalts relative to the peridotites and olivine gabbros suggests that the lavas were saturated in olivine plagioclase f clinopyroxene. The low Ni and Cr abundances, and high Fe/Mg values (Table 1) of the basalts reflect their evolved nature. Primitive (high-Mg) basalts are absent in the north but are found in the central part of the orogen (Willbob Formation, Doublet zone; Fig. 1) (Beaudoin and Laurent 1989; Clark 1989; Rohon 1989), and these have chemical compositions similar to olivine-phyric Chukotat lavas (Fig. 3A). Collectively, the aphyric basalts and gabbros overlap along a steep negative trend on a Ti - Mg diagram, which is primarily defined by olivine gabbros and Ti-rich ferrogabbros (Figs. 4A, 4C). Some Baby basalts are displaced toward higher Mg and Ti contents. Peridotites lie along an olivine-accumulation trend that projects to higher Ti contents than those of olivine gabbros. This indicates that the olivine gabbros are not intermediate in composition between peridotite and basalt, but are rather olivine + plagioclase clinopyroxene cumulates extracted from basaltic melts that produced the ferrogabbro compositions. Willbob picrites with > 10% Mg show an increase in Sc with decreasing Mg content that is consistent with olivine fractionation (Fig. 9). The highest Sc contents are found in the chill margin compositions of gabbroic sills and in some Baby basalts at 10% Mg. Basalts with < 10% Mg show a slight decrease in Sc with decreasing Mg, which may reflect clino-
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CAN. J. EARTH SCI.
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I
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Chukotat oilvin+phyrlc
$
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Chukotat PIag-phyric
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Chukotat Plag-phyrlc
Chukotat Oliv-phyric
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A
IGMP Gabbro sills1
GMP matrlx GMP gabbro Chill margin x Olivine pyroxenite
A GMP matrix
[GMP Gabbro sills]
GMP gabbro Chill margin x Olivine pyroxenite
% X
"x
.=I
1
dm*
IEquigranular Sills]
I *%
*
*
A
*
A
Diorite-granodiorite Gabbro
I Equigranular sills]
Peridotite
0 Chill margin
*
I4 -
0
Diorite- granodiorite Gabbro Peridotite
0
Chill margin
0
'b
A
an
an
a 0
FIG.3. Fe vs. Mg diagram in cation percent units. Chukotat data are from Francis et al. (1983), and the Willbob data (south-central New Quebec orogen) are from Rohon (1989). Cpx, clinopyroxene; plag, plagioclase.
pyroxene fractionation, which preferentially incorporates Sc relative to olivine and plagioclase (Irving 1978 and references therein). The chondrite-normalized trace-element profiles (Fig. 6A) of Baby and Hellancourt aphyric basalts are flat and subparallel, similar to modern transitional MORB. A diabase dyke (sample 1159a-86; Table 1) has anomalously high LaIYb, ZrIY, and LaISm values that may reflect crustal contamination. The aphyric basalts and GMP suites do not show negative Nb abundance anomalies relative to La (Fig. 6A), which
80
* * ** * * ~ * * - *
FIG.4. Ti vs. Mg diagram in cation percent units. Chukotat data are from Francis et al. (1983), and Willbob data are from Rohon (1989). Oliv, olivine.
would be expected had these magmas assimilated significant amounts of continental crust and (or) were formed from a mantle source that had been affected by subduction-related processes. Exceptions include the sample of diabase dyke, the evolved portions of equigranular sills, and some of their chill margins. Although the high LaINb values in the evolved upper parts of equigranular sills may reflect uptake of light REE in cumulus apatite, the high LaINb values of some chill margins of the sills probably reflect crustal contamination. The Hellancourt and Baby basalts have similar trace element profdes to Chukotat plagioclase-phyric lavas in the Cape Smith belt
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0 Aphyric basalt
+
lpillow and Massive ~ a v a s l
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0
100
1
1
Diabase dyke GMP basalt Baby basalt
1
1
1
1
1
1
1
1
1
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Pillowed and Massive Lavas
I +
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Diabasedyke Aphyric basalt
G M P basalt Baby basalt
1
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L
l
I
-
.
A
GMP matrix GMP gabbro 8 Chill margin x Olivine pyroxenite
X XX
olivine
\ .
,
1 Equigranular Sills I
Olivine pyroxenite
Pyroxene [GMP Gabbro sills1
G M P gabbro Chill margin
\
A Diorite-granodiorite
I
I
I
I
I
I
Gabbro * Peridotite 0 Chili margin
A A
A A A A
A
-
m
FIG. 5. A1 vs. Si diagram in cation percent units. Chukotat data (Cape Smith belt) are from Francis et al. (1983), and the Willbob data are from Rohon (1989). In Fig. SB, a mixing trend between GMP gabbros and plagioclase has An 82 as an end member.
(Francis et al. 1983) and are less light-REE enriched than basalts from the Belcher Islands (Fig. 1) (Arndt et al. 1987; Baragar and Scoates 1987). The equigranular sills show a change from flat trace-element profiles in peridotites and gabbros to more light-REE-enriched profiles in some gabbro chill margin samples, and in particular in a granodiorite from the top of a sill (Fig. 6C). Positive Ma ratios of 2.2-5.2 in Baby basalts, Hellancourt aphyric basalts, and gabbro sills (Table 3; Fig. 7) overlap values found in the GMP suite. High, ,& values near + 5
1
Nb
La Ce Nd P Srn Ti Eu Zr Tb Y Yb Lu
FIG. 6. Chondrite-normalized trace element diagram.
reflect derivation from depleted Early Proterozoic ( - 1.9 Ga) mantle as defined by studies of Trans-Hudson mafic volcanic rocks (Chauvel et al. 1987; Smith and Ludden 1989; Hegner and Bevier 1991) (Fig. 7). Relative to other Trans-Hudson Ma values of basalts in the New Qukbec orobasalts, the gen (Fig. 7) are higher than those found in spinifex basalts of the Ottawa Islands, continental basalts of the Eskimo Formation in the Belcher Islands (Chauvel et al. 1987; Arndt et al. 1987), and most of the Povungnituk suite in the Cape Smith belt (Gaonac'h et al. 1992; Hegner and Bevier 1991). The tholeiitic basalts and Montagnais sills of the New Qutbec oro-
CAN. J. EARTH SCI. VOL. 30, 1993
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400
E
E
300
pzzzq
E l
F
F
f
A
diorite-quartz diorite fnrrogabbro gabbro olivine gabbro
F
Bulk
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SHI
0
PA
Povungnituk alkaline
l ~ e w~
u 0rqlenj h
G g GMP gabbro B g GMP basalt (Hellancourt-Wlllbob)
shalelsiltstone (Baby)
FIG. 7. Histogram of E,, data of Early Proterozoic rocks of the Trans-Hudson orogen. All data were recalculated to 1875 Ma. Data from the New Quebec orogen are from this paper (Table 3), Smith and Ludden (1989), Rohon (1989), and Dia et al. (1990). Data from Cape Smith are from Smith and Ludden (1989), Hegner and Bevier (1991), and Gaonac'h et al. (1992). Data from Ottawa Islands, Belcher Islands, and Fox River Belt are from Chauvel et al. (1987).
gen encompass a range of EN^ values similar to that for Cape Smith lavas of oceanic affinity (e.g., Chukotat basalts and basaltic komatiites and rocks of the Purtuniq ophiolite; Fig. 7). It is possible that the lower range of EN^ values measured in gabbros (e.g., +2.2; Rohon 1989) reflects crustal contamination. Montagnais sills intrude Baby Formation sediments that appear to share an Archean provenance with Nd model ages
FIG. 8. Chemical sections through an equigranular sill east of Lac Soucy. Major elements are in cation percent units. Method of calculating the bulk sill composition is given in the text.
(TDM)ranging from 2.36 to 3.0 Ga and have ~ h Ma 7 values ~ that range from -9.9 to -2.1 (Table 3; Fig. 7). The majority of the tholeiites show a negative correlation between EN^ and Mg (Fig. 10). These data indicate that if crustal contamination occurred, it was restricted to the higher temperature, more primitive magmas in the suite. The Mg-rich diabase dyke composition with EN^ of -3.1 (Fig. 10) may represent an extreme example of such a contamination process, although the origin of the dyke is unclear. In summary, if aphyric and GMP magmas interacted with continental crust, then the nearchondritic La/Nb and La/Yb values of these rocks along with their E N ~ - Mvariations ~ require that only small amounts of
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I basalt and gabbro data the Doublet zone (Rohon 1989)
- from
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0 Aphyric basalt
20 0
0
Gabbro chill margin
0
Baby basalt
5
-k Diabase dyke GMP basalt GMP chill margin 0 Gabbro chill margin D Baby basalt
10
Mg
15
-3-
20
(%I
FIG. 9. Sc (ppm) vs. Mg (cation %) diagram. Data from the Willbob Formation are from Rohon (1989). The Willbob picrite data are scattered about an olivine control line with negative slope. Experimentally derived partition coefficients between olivine and basalt (DF?) range between 0.265 (1240°C) and 0.37 (1 112°C) (Irving 1978 and references therein). The more evolved basalts and gabbros show a poorly defined trend with positive slope that reflects the crystallization of clinopyroxene in addition to olivine and plagioclase. values range from 2.7 at around 1250°C Experimentally derived to values as high as 7 at temperatures close to 1170°C (Irving 1978 and references therein).
%r
contamination occurred, and this was largely restricted to Mg-rich compositions. Origin of glomeroporphyritic magmas The origin of plagioclase megacrysts in mafic volcanic and intrusive rocks has been reviewed by Phinney et al. (1988). These authors recognized many similarities between Archean plagioclase megacryst-bearing basalt flows, sills, and dykes and anorthosite complexes. The common link between anorthosites and plagioclase megacryst-bearing suites is the presence of large anorthositic (An 80-90) megacrysts. The plagioclase megacryst-bearing suites occur in both greenstone belts with "oceanic" affinities (e.g., Utik Lake; Phinney et al. 1988) and in gneiss terranes with continental affinities (e.g., Matachewan dykes; Phinney et al. 1988). Basaltic magmas in the greenstone association commonly have flat, REE profiles. Modern examples of plagioclase megacrystic suites include basalt flows in the Galapagos Islands (Cullen et al. 1989). Anorthosites may have been produced when primitive dense tholeiitic magmas were trapped at depth and were forced to fractionate olivine and (or) orthopyroxene (Phinney et al. 1988; Cullen et al. 1989). Upward migration of the resulting Al- and Ca-rich derivative magmas would result in saturation in calcic plagioclase as a result of the expansion of the plagioclase phase volume with decreasing pressure. Periodic tapping of the upper portions of these magma chambers would expel plagioclase megacryst-bearing mafic magmas to the surface, or allow them to pond in shallower magma chambers. Cullen et al. (1989) conclude that the corroded texture, calcic cores, and albitic rims of plagioclase megacrysts and the lack of a negative europium anomaly in the host matrix all reflect mixing between primitive magmas carrying megacrysts and more evolved dense magmas in the upper crust. The fact that the^^^ and aphyric magmas appear to share,
FIG. 10. €,,(I875 Ma) vs. Mg (in cation %). Uncertainty in E,, is shown at the 20 level. The data from the Doublet zone are from the south-central part of the orogen (Fig. 1).
on the basis of trace element ratios and Nd isotopes, a common parental magma is compelling evidence that the differences between the two are process related. The model of Phinney et al. (1988) provides a viable mechanism for producing anorthositic plagioclase megacrysts. That aphyric lavas overlie GMP lavas can be interpreted to indicate that the crustal density barrier forcing initial olivine f orthopyroxene fractionation at depth eventually failed, or was thinned with time (Fig. 11A). This is different from the model of Cullen et al. (1989), which suggests that plagioclase megacryst suites are preferentially formed over thin crust, such as in the northern Galapagos Islands. The olivine pyroxenites found in the lower parts of some GMP sills may be cumulates derived from primitive GMP magmas that pond at the base of GMP sills. The high Fe and Ti contents and the presence of negative europium anomalies in the olivine pyroxenites reflect earlier extraction of plagioclase. Origin of equigranular sills and aphyric basalts Two questions arise in considering the role played by equigranular sills in the petrogenesis of the basaltic magmas: (i) were the sills fed by magmas more primitive than the basalts? and, if so, (ii) do the basalts and sills share a common fractionation sequence? A bulk composition of a differentiated equigranular sill east of Lac Soucy (Table 2; Fig. 8) was calculated by multiplying the composition of each sample by the vertical distance between samples divided by the total thickness of the sill. This calculation gives a Si content greater than that of any Hellancourt basalt (compare Tables 1 and 2). Since individual chill margin compositions are chemically similar to Hellancourt basalts (Table I), it seems that the higher Si contents of the bulk sill may reflect crustal assimilation as suggested by the high La/Nb values of the more evolved parts of the sill. In contrast, Hellancourt basalts appear to have escaped the effects of upper crustal contamination, since they have flat REE profiles, encompass a limited range of major and trace element compositions, and have a narrow range of EN^ ratios that vary between + 3.4 and + 5.2. A major question in understanding the petrogenesis of the lavas is why their chemical compositions are so restricted. This problem has been addressed in the study of modern mid-
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CAN. 1. EARTH SCI. VOL. 30. 1993 (A) GLOMEROPORPHYRITIC BASALTS AND SILLS
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-1 1+ Opx * 0lower crust
(B)APHYRIC BASALTS AND EQUIGRANULAR SILLS CRUST
+
* sediment
FIG. 11. Model of second-sequence basaltic magma genesis in the New Qu6bec orogen. (A) Continental margin extension at 1883 Ma triggered adiabatic melting of the asthenosphere. Initial mantlederived melts were trapped at the base of the crust and crystallized olivine (01) and orthopyroxene (Opx) with minor crustal contamination. Derivative liquids migrated upward into mid-crustal magma chambers where they became saturated in calcic plagioclase (PI). Periodic tapping of these magma chambers released plagioclase ultraphyric magmas into near-surface GMP sills where mixing and reaction between melts and calcic plagioclase resulted in corroded plagioclase glomerocrysts. Early-formed GMP basalts were derived from these upper crustal sills. Au, augite. (B) Further extension triggered more voluminous mantle melting. Picritic magmas in the north were trapped beneath the crust where they crystallized olivine + clinopyroxene and possibly assimilated small amounts of crust before migrating upwards and either erupting at the surface, or ponding within the volcano-sedimentary pile. Sp, spinel.
-
ocean-ridge basalts. Stolper and Walker (1980) suggest that it reflects the operation of a density filter in which magmas fractionate until they become sufficiently buoyant to rise to the surface. The Hellancourt basalts have chemical compositions that lie close to a 1 atm olivine plagioclase f clinopyroxene cotectic and do not appear to have undergone extensive plagioclase fractionation resulting in pronounced Fe enrichment and a density increase. Extensive Fe and Ti enrichment is confined to the upper differentiated parts of equigranular sills (Fig. 8B). The most primitive Hellancourt aphyric basalt has an MgO content of 8.15% and, based on the activity model of Nielsen (1988) (Fig. 12), contains olivine (Fo 84) on the liquidus followed by olivine plagioclase after 1% crystallization. The cation-normative forsterite content of gabbro-peridotite sills
+
+
FIG.12. Fe-Mg diagram (cation %) with a fractional crystallization model of aphyric basaltic magma. The method of Nielsen (1988) is used to calculate the activity of plagioclase (Pl), olivine (Ol), augite (Au), and spinal (Sp) as a function of temperature and melt composition along the QFM buffer. Minerals are removed from a high-Mg basalt (8.2%MgO) in 0.1 % crystallization increments over a crystallization interval of 80%. The fields of aphyric basalts (Fig. 3A) and equigranular sill compositions are shown (Fig. 3C). Tie lines between the liquid path (bold line) and olivine compositions encompass the range of cation-normative forsterite found in peridotite sills (where XF,3+ = 0.10). Other tie lines connect the liquid path (f = fraction of liquid remaining) to calculated cumulates. These calculated cumulates closely resemble those found in the equigranular sills.
analyzed ranges from Fo 77 to Fo 84 assuming XFe3+= 0.1 (-QFM). A peridotite sill in the field area contains relict cumulus olivine with Fo = 80, and the cation-normative forsterite content of three samples of this sill have similar Fo contents when XFe3+ = 0.1 (XFe3+represents mole fraction of ferric iron). Rohon (1989) reported a range of Fo contents from Fo 77 to Fo 84 in olivine cumulate Montagnais sills. Collectively these results indicate that the peridotitic fractions of gabbro sills represent olivine cumulates from basaltic melts with a maximum MgO content of -8.2% (i.e., at the high MgO end of the basalt spectrum). Thus, unlike the Chukotat suite in the adjacent Cape Smith belt (Francis et al. 1983), or amongst parts of the Willbob Formation (Rohon 1989), primary magmas were not erupted during Hellancourt magmatism in the northern New QuCbec orogen, nor were they emplaced into gabbro sills. Even Baby basalts with MgO contents of 10.3% (Table 1) could only have coexisted with Fo 85 olivine. The Nielsen model predicts that with continued fractional crystallization the removal of olivine and plagioclase in proportions of 30:70 will result in augite saturation when the fraction of remaining liquid is equal to 69 % (Fig. 12). After 80% crystallization spinel starts to crystallize and the residual liquid starts to undergo Fe and Ti depletion and Si enrichment. The calculated gabbro and ferrogabbro cumulates produced by fractional crystallization of the most primitive aphyric basalt are similar in composition to cumulate rocks sampled in differentiated sills (Fig. 12). The amount of crystallization predicted by the Nielsen model, which accounts for the range of MgO contents of
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SKULSKI ET AL.
aphyric basalts, is greater than that implied by their trace element contents (e.g., La). For example, the total amount of trace element fractionation between aphyric basalts encompassing MgO contents of 7.7 1- 5.77 % MgO (Table 1) is 11% , assuming that La is perfectly incompatible. However, the range of MgO contents of these two samples corresponds to 34% crystallization of troctolite and gabbro cumulates using the Nielsen model. Furthermore, this result is contrary to what would be expected if the basaltic magmas evolved in a magma chamber undergoing periodic replenishment and eruption. Stamatelopoulou-Seymour et al. (1991) arrived at a similar conclusion based on their study of Hellancourt basalts. A potential explanation for this decoupling between observed and predicted major and trace element contents may be that the basalts underwent early olivine clinopyroxene fractionation at depth. With increasing pressure the clinopyroxene phase volume expands at the expense of plagioclase. Fractionation of olivine and clinopyroxene will result in a decrease in Mg over a smaller fractionation interval than olivine plagioclase crystallization. Furthermore, clinopyroxene fractionation can explain the low Sc contents of the aphyric basalts (Fig. 9). Thus the Hellancourt basalts did not fractionate in Montagnais sills prior to eruption, and the sills merely represent basaltic magmas that were trapped within the upper crust. The similar Nd isotopic variations between aphyric and GMP magmas indicate that differential crustal contamination is not required to explain the chemical and mineralogical differences between the two suites. The lower Nd isotopic ratios of Mg-rich magmas (Fig. 10) may reflect the assimilation of small amounts of lower crust (Fig. 11B). Our calculations show that a model primary magma (e.g., Willbob picrite, 12.2% MgO; 7 X chondritic REE) while assimilating 5 % of estimated average lower crust (Taylor and McLennan 1985) will produce a 7 % MgO liquid with 50.4% Si02 and an approximately flat REE profile (8.5 x chondrite), crudely similar to some gabbro chill margins (Table 3; e.g., sample 1189B-86). If the parental magma has an EN* of +5.2 and the contaminant has an EN^ of - 15, then the resulting liquid has an EN^ of +4.5. To reproduce the lower EN^ ratios and higher total REE contents characteristic of some of the more primitive lavas (Table 3, e.g., sample 1189B-86 EN^ = f 3 . 2 , REE E 11 X chondrite) and still keep the Si02 contents low, the contaminant would have to have had lower EN* values and higher REE contents than those estimated for lower continental crust. An alternative explanation for the Nd isotopic variability of Hellancourt basalts is that they are the products of multiple mantle sources. One plausible scenario is that asthenospherederived, MORB-like basaltic magmas reacted with enriched subcontinental lithospheric mantle, possibly the source of second-sequence alkaline magmas. Two-component mantle mixing models were also considered by Hegner and Bevier (1991) to explain the isotopic variability of Cape Smith basalts. However, in the present case further testing of this model awaits a more detailed examination of likely enriched mantle sources, potentially through further study of Early Proterozoic alkaline magmatism in the New QuCbec orogen. In summary, individual batches of aphyric magma with MgO contents no greater than 8.2% may have cooled in near-surface sills to produce, via fractional crystallization, the spectrum of cumulate melt compositions preserved in equigranular sills (Fig. 11B). Local crustal assimilation during late-stage crystallization is required to explain the composi-
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tions of evolved granodiorites and diabase dykes. However, simple fractional crystallization in these sills cannot account for the trace element compositions of aphyric basalts. The basalts evolved from more primitive parental melts in deeper werhlite and larger magma chambers dominated by dunite cumulates (Fig. 11B). Small amounts ( < 5 %) of lower crustal assimilation may have occurred here, although the Nd isotopic data could be explained by multiple mantle sources. In conclusion, even though aphyric basalts and equigranular gabbros shared a common parental magma, the aphyric basalts did not differentiate within the voluminous gabbro sills. Tectonic setting of 1.88-1.87 Ga magmatism in the New QuCbec orogen The configuration of the continental margin at 1883 Ma influenced the development of second-sequence depositional and volcanic basins (Wares and Skulski 1992). These basins are believed to have formed on an older east-facing passive margin succession (Wardle and Bailey 1981; Wares and Skulski 1992). The fact that pre-dolomite first-sequence lithologies are absent north of latitude 57"N can be explained by an eastfacing crustal promontory in the north (Wares and Skulski 1992). A crustal promontory developed during 2169 Ma continental rifting would have localized the northern site of firstsequence deposition eastward relative to the margin south of 57"N. The second sequence is envisioned to have formed above first-sequence lithologies south of 57"N, and above platform dolomites resting on an Archean crustal promontory in the north. The presence of thicker continental crust beneath the northern second-sequencebasins may have provided a crustal density barrier preventing the eruption of picritic magmas. Hoffman (1990b) suggested that the Hellancourt basalts may have erupted in a tectonic setting that is analogous to that of the Andaman Sea, that is, in pull-apart basins formed above a subduction zone, along the eastern flanks of the Indian plate Asian plate continental collision zone. Although we are unaware of any trace element data on Andaman Sea basalts, we consider that the lack of a diagnostic geochemical signature such as high La/Nb values in Hellancourt volcanic rocks precludes their derivation above a subduction zone. The geological and geochemical data are, however, consistent with an extemional tectonic regime. The following constraints are most convincing (i) the stratigraphy of the second sequence requires a progressively deepening depositional basin within which shallow water platform sediments were overlain by turbidites; and (ii) volcanic activity suggests progressive extension, characterized by 1880 Ma carbonatites and lamprophyres (Castignon Lake complex near Cambrien aulacogen (Fig. 1); Dressler 1978; ChevC and Machado 1988), 1880 Ma mildly alkaline olivine tholeiites and rhyolites (Nimish Formation; Watanabe and Fowler 1991), 1883- 1874 Ma transitional MORB-like basaltic magmatism (Willbob and Hellancourt formation and Montagnais sills), and picritic magmatism (Willbob Formation). Late-stage magmatism at 1870 Ma comprised rhyolitic, potassic, and carbonatitic magmatism (T. Clark, personal communication, 1992). Collision between the southeastern Rae and Superior provinces along the New QuCbec orogen is consistent with dextral transpression (Hoffman 19906; Wardle et al. 1990a; Moorhead and Hynes 1990). Prior to collision, the oblique convergence may have triggered the formation of pull-apart basins (Fig. 13). Modern pull-apart basins such as in the Gulf of
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Volcanic and
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FIG. 13. Tectonic model of the New QuCbec orogen at - 1.88 Ga. The margins of pull-apart basins were the site of alkaline magmatism that included lamprophyres and carbonatites, followed by olivine tholeiites. Gabbro sill injection in sediments is the predominant mechanism of mafic magma emplacement in the axes of pull-apart basins. Early-formed GMP basaltic volcanism was followed soon after by transitional MORB-like basaltic volcanism characterized by Hellancourt aphyric basalts (Fig. 11). OLM, oceanic lithospheric mantle; CLM, continental lithospheric mantle.
California are sediment rich (Einsele 1986) and give rise to tholeiitic gabbro sill - sediment complexes morphologically similar to. those found in the New QuCbec orogen (Wardle and Bailey 1981; Hoffman 1990b). We interpret Hellancourt volcanism to have occurred in pull-apart basins formed over the 2.1 Ga Superior continental margin. Right-stepping dextral strikeslip faults would have the correct sense of displacement (Fig. 13) to accommodate local basin formation. Sinistral transpression along the eastern margin of the southeastern Rae Province in the Abloviak shear zone has a maximum age of 1910 Ma (Wardle et al. 1990a), with most of the displacement having occurred between 1.86 and 1.82 Ga (Bertrand et al. 1990). This is consistent with southward transport of the southeastern Rae Province prior to collision with the eastern margin of the Superior Province (Hoffman 1990b). However, syndepositional ( - 1.88 Ga) dextral faults have not been described in the hinterland of the New QuCbec orogen. The relationship betwen tholeiitic volcanism and possible basement structures is obscured by the minimum of 65% shortening across the foreland, which, when reconstructed, places the western limit of the Hellancourt Formation at least 15 km east of the Rachel fault (Fig. 2) (Boone and Hynes 1990; Wares and Goutier 1990~).Our model requires that prior to collision, dextral transpression between the southeastern Rae and Superior provinces was accommodated in part along the eastern thinned margin of the Superior Province. Localization of pullapart basins along the continental margin, as opposed to within oceanic crust, may reflect the inherent weakness of thin continental lithosphere relative to the cratonic interior, and the relative strength of 2.1 Ga oceanic lithosphere adjacent to the plate boundary at 1.88 Ga. Formation of these pull-apart basins within the proposed crustal promontory (Wares and Skulski 1992) implies that transform fault propagation was controlled by relative plate motions, rather than plate boundary configuration. Extension at 1.88 Ga initially resulted in the deposition of quartzites, shales, and banded iron formation contemporaneous with eruption and intrusion of alkaline magmas (e.g., Castignon Lake complex near Cambrien aulacogen, Fig. 1). Between 1883 and 1870 Ma, the continental margin would have progressively thinned and oblique extensional basins would have subsided and accumulated turbidites followed by the eruption of tholeiitic basalts and intrusion of gabbroic sills. Locally there may have been some late emergent platforms as in the case of the Lac LeMoyne area in the central part of the orogen, where 1870 Ma potassic rhyolites and intrusive car-
bonatite are associated with dolomite, pelite, and sulphide facies iron formation (Machado et al.4). The MORB-like chemical and Nd isotopic compositions of Hellancourt basalts suggest that they were derived from asthenospheric mantle. With progressive rifting and with increasing degrees of partial melting of the asthenosphere, tholeiitic basaltic magmas would have been formed. Initially these magmas may have been trapped at depth, perhaps at the base of the thinned crust, and forced to fractionate olivine & orthopyroxene (Fig. 11A) (Phinney et al. 1989; StamatelopoulouSeymour et al. 1991). The resulting evolved liquids could then rise to shallower levels and become saturated in anorthositic plagioclase to produce GMP magmas. With continued extension the primitive (picritic?) magmas ponded beneath thinned crust in the north where they crystallized olivine and clinopyroxene (Fig. 11B). Periodic tapping of these magmas resulted in eruption of aphyric basalts, or emplacement of shallow level sills which underwent further gabbroic formation. Ultimately, this crustal density barrier must have failed, allowing picritic magmas to be emplaced as late ultramafic sills. The less radiogenic Nd isotopic compositions of some of the more primitive basalts reflect either small amounts of lower crustal contamination ( < 5 % ) or, alternatively, a heterogenous mantle source. Upper crustal assimilation was largely restricted to the upper parts of thick sills, and for the most part erupted basaltic lavas reflect very little crustal interaction. The crust underlying the New QuCbec orogen was transitional and contained elements of both continental sedimentation and volcanism juxtaposed against a submarine basaltic pile similar in composition to transitional MORB. The presence of turbiditic sediments reflecting a cratonic provenance beneath the tholeiitic volcanic pile indicates that the volcanic basins formed near continental crust. On the other hand, the transitional MORB-like geochemical compositions of Hellancourt basalts is an indication that the continental crust beneath the volcanic pile must have been very thin, or was ultimately breached.
Acknowledgments This paper results from a metallogenic research project administered by the Institut de recherche en exploration minkrale - Mineral Exploration Research Institute (IREM MERI) for the ministbre de L'Energie ,et des Ressources du QuCbec (MERQ). We are grateful to MERQ for permission to publish this work (MERQ contribution 92-51 10-54). The
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authors a r e grateful to I R E M -MERI and the Natural Sciences and Engineering Research Council of Canada for financial support during the course of this work. X-ray fluorescence analysis was performed at McGiil University by Tariq Ahmedali and Glenna Keating. Neutron activation analyses were done in the Dtpartement d e gtologie of the Universitt d e MontrCal, and Gilles Gauthier and Briggite Dionne are thanked for their assistance. W e thank John Ludden, T o m Clark, Mark L a Flbche, and Richard Wardle for thoughtful reviews of the paper, Anne Bertrand for her help in drafting some of the figures, and Anne Charland for her help in editing the French abstract. Arndt, N.T., Briigmann, G.E., Lehnert, K., Chauvel, C., and Chappell, B.W. 1987. Geochemistry, petrogenesis and tectonic environment of Circum-Superior belt basalts, Canada. In Geochemistry and mineralization of Proterozoic volcanic suites. Edited by T.C. Pharaoh, T.D. Beckinsale, and D. Rickard. Geological Society Special Publication (London), No. 33, pp. 133 - 145. Baragar, W.R.A. 1960. Petrology of basaltic rocks in part of the Labrador Trough. Bulletin of the Geological Society of America, 71: 1589- 1643. Baragar, W .R.A., and Scoates, R.F.J. 198 1 . The Circum-Superior belt: a Proterozoic plate margin? In Precambrian plate tectonics. Edited by A. Kroner. Elsevier Scientific Publishing Company, Amsterdam, pp. 297- 330. Baragar, W.R. A., and Scoates, R. F. J. 1987. Volcanic geochemistry of the northern segments of the Circum Superior Belt of the Canadian Shield. In Geochemistry and mineralization of Proterozoic volcanic suites. Edited by T.C. Pharaoh, T.D. Beckinsale, and D. Rickard. Geological Society Special Publication (London), No. 33, pp. 113-131. Beaudoin, G., and Laurent, R. 1989. MCtallogCnie des ClCments du groupe du platine dans la region du lac Retty (?one Center et Pogo Lake)-Fosse du Labrador. Ministkre de 1'Energie et des Ressources du QuCbec, sCrie des manuscrits bruts, MB 89-28. BCrard, J. 1965. BCrard Lake area New QuCbec. QuCbec Department of Natural Resources, Geological Report 11 1. Bertrand, J-M., Van Kranendonk, M., Hanmer, S., and Roddick, J.C. 1990. Structural and metamorphic geochronology of the Torngat orogen in the North River - Nutak transect area, Labrador: preliminary results of U-Pb dating. Geoscience Canada, 17: 297-301. Boone, E., and Hynes, A. 1990. A structural cross-section of the Northern Labrador Trough, New Qutbec. In The Early Proterozoic Trans-Hudson Orogen of North America. Edited by J.F. Lewry and M.R. Stauffer. Geological Association of Canada, Special Paper 37, pp. 387-396. Chandler, F.W., and Parrish, R.R. 1989. Age of the Richmond Gulf Group and implications for rifting in the Trans-Hudson Orogen, Canada. Precambrian Research, 44: 277-288. Chauvel, C., Arndt, N.T., Kielinczuk, S., and Thom, A. 1987. Formation of Canadian 1.9 Ga old continental crust. I: Nd isotopic data. Canadia Journal of Earth Sciences, 24: 396-406. ChevC, S.R., and Machado, N. 1988. Reinvestigation of the Castignon Lake carbonatite complex, Labrador Trough, New QuCbec. Geological Association of Canada - Mineralogical Association of Canada, Program with Abstracts, 13: A20. Clark, T. 1984. GCologie de la rCgion,du lac Cambrien, Territoire du Nouveau QuCbec. Ministkre de 1'Energie et des Ressources du QuCbec, ET 83-02. Clark, T. 1989. ~ t u d du e gite de Cu -Ni -Pd -Pt du lac Bleu no 1 , Fosse du Labrador. Ministkre de 1'Energie et des Ressources du QuCbec, MB 89-35. Clark, T., and Thorpe, R.I. 1990. Model lead ages from the Labrador Trough and their stratigraphic implications. In The Early Proterozoic Trans-Hudson Orogen of North America. Edited by J .F. Lewry and M .R. Stauffer . Geological Association of Canada, Special Paper 37, pp. 413 -431.
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