Anorogenic magmatism and the Grenvillian Orogeny

BRIAN F. WINDLEY Department of Geology, The University, Leicester, LEI 7RH, UK

Received Februa~y 19, 1988

Revision accepted August 29, 1988

The Grenvillian Orogeny was preceded by extensive anorogenic volcanism and plutonism in the period 1500- 1300 Ma in the form of rhyolites, epizonal granites, anorthosites, gabbros, alkaline complexes, and basic dykes. An analogue for the mid- Proterozoic anorogenic complexes is provided by the 2000 km by 200 km belt of anorogenic complexes in the Hoggar, Niger, and Nigeria, which contain anorthosites, gabbros, and peralkaline granites and were generated in a Cambrian to Jurassic rift that farther south led to the formation of the South Atlantic. An analogue for the 1 x lo6 km2 area of 1500- 1350 Ma rhyolites (and associated epizonal granites) that underlie the mid-continental United States is provided by the 1.7 X lo6 km2 area of Jurassic Tobifera rhyolites in Argentina, which were extruded on the stretched continental margin of South America immediately preceding the opening of the South Atlantic. The mid-Proterozoic complexes were intruded close to the continental margin of the Grenvillian ocean and were commonly superimposed by the craton-directed thrusts that characterized the final stages of the Grenvillian Orogeny. The bulk of the Keweenawan rift and associated anorogenic magmatism formed about 1100 Ma at the same time as the Ottawan Orogeny in Ontario, which probably resulted from the collision of the island arc of the Central Metasedimentary Belt attached to the continental block in the east with the continental block to the west. The most appropriate modem equivalent would be the Rhine Graben, which formed at the same time as the main Alpine compression.

L'orogenese grenvillienne a CtC prCcCdte, entre 1500 et 1300 Ma, par un pCriode intense de volcanisme et de plutonisme anorogCniques sous forme de rhyolites, granites Cpizonaux, anorthosites, gabbros, complexes alcalins et dykes basiques. Des complexes anorogCniques analogues d'gge protCrozoique moyen ont CtC reconnus dans la ceinture, de 2000 km par 200 km, qui inclut les complexes anorogkniques du Hoggar, Niger et Nigeria, lesquels sont formCs d'anorthosites, gabbros et granites peralcalins et engendrks dans un rift cambrien 2 jurassique qui plus au sud a conduit 2 la formation de 1'Atlantique Sud. Un complexe de rhyolites (et granites Cpizonaux associts), 5gC de 1500- 1350 Ma, couvrant une superficie d ' l X lo6 km2 et formant le substratum gCologique du centre des Etats-Unis est analogue i celui en Argentine de 1,7 X lo6 krn2 form6 des rhyolites de Tobifera d'lge jurassique mises en place sur la marge continentale tntirCe de 1'AmCrique du Sud juste avant I'ouverture de 1'Atlantique Sud. Les complexes du ProtCrozoi'que moyen furent mis en place pres de la marge continentale de l'octan grenvillien, et en gCnCral ils sont recouverts par les nappes de charriages dirigCes vers le craton caractCristiques des demiers stages de I'orogCnie grenvillienne. La majeure partie du rift de Keweenawan accompagnC d'une activitC magmatique anorogtnique s'est formCe il y a environ 1100 Ma en m&me temps que I'orogenkse de l'ottawan en Ontario, rksultat probable de la collison de l'arc insulaire de la Ceinture mCtasCdimentaire centrale attachte dans I'est au bloc continental avec le bloc continental h l'ouest. L'Cquivalent actuel le plus probant est le graben du Rhin contemporain de la compression alpine principale.

[Traduit par la revue]

Can. I. Earth Sci. 26, 479-489 (1989)

Introduction One of the great problems of North American geology is the

mode of evolution of the major mid-Proterozoic anorogenic magmatic belt that extends from the mid-continent region of the United States to eastern Canada (Bridgwater and Windley 1973; Emslie 1978a, 1978b; Van Schmus et al. 1987).

The belt includes rhyolites, granites, anorthosites, tholeiitic basic dykes, and alkaline rocks. Ages range from 1500 to 1300 Ma. It is widely agreed that these rocks formed during periods of extension in stable continental crust, but it is uncer- tain whether the rocks and the extensional processes were related to (i) anorogenic intracontinental rift zones (Emslie 1978a, 1978b); (ii) earlier crustal accretion, thickening, and convergent plate tectonics (Van Schmus and Bickford 1981; Bickford et al. 1986; Van Schmus et al. 1987); (iii) later conti- nental collision tectonics associated with the Grenville Orog- eny (Van Schmus et al. 1987); or (iv) a mantle diapir (Anderson 1983), plume (Morse et al. 1988), or superswell (Hoffman 1988) under a large craton or supercontinent.

In order to constrain ideas on the evolution of these rocks, it is useful to find comparable Phanerozoic equivalents and to relate them to modem concepts of tectonics. Barker et al. Printed In Canada I Imprime au Canada

(1975) pointed out that the anorthosite-granite suite is similar in mode of emplacement, composition, and probable origin to the Ordovician to Jurassic anorogenic ring complexes of Nigeria (Bennett et al. 1984). One of the aims of this paper is to suggest the similarity of the Tobifera rhyolite terrane of Argentina, which formed in relation to the opening of the Atlantic Ocean, to the mid-Proterozoic rhyolite terrane of the United States and from this relationship suggest a connection between the so-called anorogenic magmatism, the Grenvillian ocean, and the succeeding Grenvillian Orogeny.

In contrast to the predominantly felsic anorogenic mag- matism described above, which preceded the Grenvillian Orogeny, the 1100 Ma Keweenawan magmatism was dorni- nantly tholeiitic to alkaline; it was associated with major rifting of the continental foreland (the Midcontinent Rift System), and this was considered as having been induced by collision tec- tonics during the Grenvillian Orogeny (Gordon and Hempton 1986). Previous suggestions that the Keweenawan developed in a failed rift - aulacogen or is comparable to the East African Rift System (Van Schmus and Hinze 1985) are unlikely because it formed contemporaneously with the Grenvillian Orogeny, not prior to it as required by an aulacogen model.

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

TABLE 1. Time chart for the Grenvillian Wilson cycle illustrated by two regions in the Grenville Province and two in the foreland

Central Interlor U-S A , Labrador North Grenv~lle Prov~nce

& S Canada of Grenv~lle Front Labrador-Quebec- Ma Ad~rondacks S E Ontarlo

900 - ( Leuco-gran~tes

NW Thrust~ng to Foreland 1000 -

Post- coll~stonal ' Grenv~ll~an Keweenawan R ~ t t ~ n g Orogeny i E ' L Z ~ ~ g ~ r o g e n y

l o o - I & Magmat~sm (Terminal Colllslon)

1200 - 1 Elzev~r~an Orogeny

(Arc-Continent Coll~s~onl Volcan~cs & Pluton~cs Central Metasedlmentary

Srbley Group Anorthos~te 1 Belt (Island Arc) 1300 - ,

Alkal~ne Nebraska Complexes Gabbros Anorthos~ta D~abases

1400 - Rhyolites & Gran~tes

' I Anorthos~tes & Rapaklv~ Granttes

Wolf RlverGranite 1500

1600 - Mazatzal Orogeny I IS U S-A 1 I Labradorlan OrogenY

1700 -

NOTES: Anorogenic magmatism from 1500 to 1300 Ma records the opening of the ocean. Subduction of the ocean is indicated by the 1280- 1250 Ma island arc of the Central Metasedimentaly Belt. The Elsevirian and Ottawan orogenies record the collisions of the island arc with the continental blocks to the east and west, respectively. The terminal collision was responsible for the formation of the Keweenawan rifting in the foreland. Post-collisional indentation caused thrusting of the mobile belt over the foreland to the northwest. The thrusting caused crustal thickening and the formation of minor leuco-granites in the thrust belt.

Anorogenic magmatism Pre-Grenvillian

Following the 1900- 1760 Ma Penokean Orogeny (Van Schmus and Bickford 1981; Van Schmus et al. 1987), the 1680 - 1640 Ma Labradorian Orogeny (Nunn et al. 1988), and the 1680- 1625 Ma Mazatzal Orogeny (Van Schmus and Bick- ford 1981; Condie 1982), there was a pause in major tectonic and magmatic activity until the onset of pre-Grenvillian anoro- genic magmatism (Table 1). Widespread anorogenic magmatic rocks, mostly formed between 1500 and 1300 Ma, are present in a zone 5000 km long and up to 1000 km wide extending from Labrador to California within and marginal to what is now the Grenville orogenic belt (Fig. 1). The zone is charac- terized by rhyolites and associated silicic pyroclastic rocks, granitic complexes, anorthosites, gabbros, and alkaline rocks. Granites are locally associated with the rhyolites, anorthosites, or alkaline rocks. A general lack of metamorphism (not higher than the chlorite zone) and deformational features suggests these rocks formed under anorogenic extensional conditions.

The mid-continent region of the United States is underlain by a vast, mostly subsurface zone, 2000 km long and 500 km wide, extending from Ohio to Texas (Fig. 1) that consists of 1500- 1300 Ma old predominantly rhyolitic and dacitic ash- flow tuffs and epizonal granitic plutons (Van Schmus and

Bickford 1981; Thomas et al. 1984; Denison et al. 1984; Bick- ford and Van Schmus 1985; Bickford 1988). These rocks form a thin veneer only a few kilometres thick on older stable conti- nental crust. If only 1 km thick, this amounts to lo6 km3 of sili- cic (SiO, > 70 wt.%) and potassic (K20 > 5-6 wt.%) material. Intermediate and mafic igneous rocks are rare. Twenty-five hundred square kilometres of exposure in the St. Francois Mountains of Missouri shows that the rocks there form the remains of calderas, their pyroclastic fill and outflow sheets, and their subvolcanic plutons (Sides et al. 1981; Bick- ford et al. 1986), which include alkaline A-type granites and high-heat-production granites (Kisvarsanyi 1988). Thomas 1 et al. (1984) suggested these silicic volcanic (and associated plutonic) rocks were emplaced in a continental margin setting, because only younger rocks are known to the south.

The distribution of the anorogenic granite complexes is shown in Fig. 1. The best exposures are in Labrador and Wisconsin, and their main age span is from about 1500 to 1300 Ma. Principal rock types are potassic and iron-enriched biotite and hornblende granites with many two-mica granites. The earliest intrusion is probably the 1485 Ma Wolf River granite in Wisconsin (Van Schmus et al. 1975; Anderson 1980), and the youngest was emplaced at 1296 zk 13 Ma in eastern Labrador (Scharer et al. 1986). Crystallization of many

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WINDLEY 481

Central Metasedimentary Belt (Island Arc1

Anorthosites

Q Granites 1490- 1 4 -f 0 Ma - Granites: 14 10-1 340 Ma

Rhyolites: 1500-1 340 Ma

x Alkaline Complexes

1440 Average age (Ma1 of anorogenic event

FIG. 1. Map showing the distribution of the rhyolite area of the central interior region of the United States and of anorogenic granites, anorthosites, and alkaline complexes in relation to the Grenvillian orogenic belt and its Central Metasedimentary Belt (island arc). Sections A, B, and C refer to Figs. 4a, 4b, and 4c, respectively. Compiled from Anderson (1983), Baragar (1977), Bickford et al. (1986), Davidson (1984), and Van Schmus et al. (1987). CGB, Central Gneiss Belt; CGT, Central Granulite Terrane.

of these epizonal subalkalic to marginally peraluminous granite magmas occurred at 650-790°C and at low total pres- sures of less than 200 MPa. The high K, Fe, Mg, Ba, and rare- earth element (REE) compositions reflect 10 - 30 % of fusion of calc-alkaline crust, which resulted from thermal doming in the mantle (Anderson 1983).

There are extensive remnants of felsic volcanic rocks well within the Grenville Province. The 1271 Ma Wakeham Group contains low-grade (greenschist facies) peralkaline acid volcanics with gabbro dykes and sills that occupy an area of about 10000 km2 in Quebec between 300 and 430 km south- east of the Grenville Front (Bourne 1986). It seems likely that extensive areas of predominantly rhyolitic ash-flow tuffs were metamorphosed to a high grade within the Grenville Province and may be difficult to recognise today. Examples may be (i) alaskite bodies in the northwest Adirondacks that have a relict stratigraphy and include gneissic units of granitic alaskite, granite, trondhjemite, quartz diorite, leucodiorite, and charnockitic alaskite (Carl and Van Diver 1975); and (ii) leuco-gneisses in the Adirondacks, interpreted as having been erupted as ash-flow tuffs at 1263 f 25 Ma and metamor- phosed at 20 km depth at 1120 Ma (Grant et al. 1981).

Many massif-type anorthosite complexes of widely differing age (Emslie 1985) are associated with the Grenville Province and the adjacent Nain Province of Labrador (Fig. 1). An early group includes the 1663 f 45 Ma Harp Lake anorthosite and the ca. 1650 Ma Mealy Mountains anorthosite in Labra- dor, both developed in association with the Labradorian Orog- eny. Pre-Grenvillian and Grenvillian intrusions in Labrador include Kiglapait at 1416 f 50 Ma, Shabogamo at 1379 f 65 Ma, and Flowers River at 141 1 f 48 Ma (Sm-Nd on whole rocks and minerals; Ashwal and Wooden 1985); in Ontario, the Pany Sound anorthosite at ca. 1350 Ma (U-Pb on zircons; A. Davidson, personal communication, 1988); in the Adirondacks, the Marcy anorthosite, which has ages of 1288 36 Ma (Sm-Nd on whole rock; Ashwal and Wooden 1985) and 1100- 1050 Ma (U -Pb on zircon and baddeleysite; 3. Chiarenzelli, personal communication via A. Davidson, 1988); and in Quebec, the St. Urbain body, which has an age of 1079 + 22 Ma (Sm-Nd on whole-rock and mineral iso- chron Ashwal and Wooden, 1985).

In this model the pre-Grenvillian anorthosites were emplaced into the thinned continental margin of the Grenvillian ocean; it has been widely concluded that they were intruded under

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

I I i ' 1 0 O ~ ( a - - - _ _ _ _ I -. I

I 2 5O - - - - - - _ _ _ _ _ _ I :- ' . . I --. 12. .. I Hoggar ( b ) -2500 1 . - I --.. I

I 8 I -

1 I 2 oO- I . . 1 :: I - 2000

I . Ai'r 2'. I

I .. I 2 . I I - .

z I I - - 1 5 0 0 2

15'- Y I -0 U

I I :I

3 - .- a tS Niger a, I + 0

I .. I (11 C A (11

I . . I - 1 000 .G I

I n . ' :.. 1 0°- Nigeria I . : I . - F . . I

I

4 . , ,-- -- -,,,----, ,--------- ------------ . I -500

- 5O - 5' N

0 600 500 400 300 200 100 0

Age (Ma)

FIG. 2. (a) Map of west Africa showing the locations of the main Cambrian to Jurassic anorogenic ring complexes. (b) The sequential age trends of the anorogenic complexes in west Africa from 592 Ma in the north to 141 Ma in the south. Separation to form the South Atlantic began after 135 Ma. (a) and (b) Modified after Cahen et al. (1984).

anorogenic conditions associated with continental rifting (Emslie 1987a, 1978b; Brooks et al. 1981; Morse 1982).

The following basic rocks were intruded in Labrador: (i) the Michael gabbro at 1426 + 6 Ma (U - Pb, Scharer et al. 1986); (ii) the Mealy dykes at 1380 + 54 Ma (Rb-Sr, Emslie et al. 1984); (iii) the Shabogamo gabbro at 1375 f 60 Ma (Sm-Nd, Brooks et al. 1981); (iv) the Seal Lake basalts and diabases at 1323 $ 92 Ma (Rb-Sr, Baragar 1978); and (v) the Harp dykes at 1316 k 94 Ma (K-Ar, Meyers and Emslie 1977). In Ontario the Sudbury diabase dykes have isotopic ages of 1238 f 4 Ma (U -Pb, Krogh et al. 1987) and 1225 Ma (Rb -Sr, Van Schmus 1975; Van Schmus et al. 1982).

Several alkaline to peralkaline complexes were intruded w i t h or adjacent to the Grenville orogenic belt, including the Red Wine and Arc Lake complexes at 1392 + 75 Ma (Curtis and Cunie 1981), the Letitia Lake volcanic -plutonic complex at 1327 + 4 Ma (Hill and Thomas 1983), and the Flowers River peralkaline granites at 1262 f 7 Ma (Collerson 1982). These rocks most likely formed in a series of discontinuous pre-Grenvillian rifts (Baragar 1977; Meyers and Emslie 1977).

The above anorogenic magmatic data suggest that there was considerable crustal extension in a zone at least 1000- 1500 km wide for at least 250 Ma before the Grenvillian Orogeny. This zone includes the region now occupied by the Grenville Province as well well as a region up to 1000 km wide in the continental foreland on its northwestern side.

Phanerozoic equivalents Phanerozoic examples of anorogenic igneous rocks that may

be comparable to the pre-Grenvillian plutonic and volcanic rocks are present in two tectonic settings on either side of the South Atlantic:

(1) In the Hoggar (Algeria), Niger, and Nigeria there are over 90 anorogenic complexes (Fig. 2) in a spectacular north-south-aligned chain 2000 km long and up to 200 km wide representing the eroded roots of volcanoes (Wright 1985). The complexes, which commonly formed by cauldron subsidence, decrease progressively in age southwards from 588 Ma to 141 Ma (Cahen et al. 1984). In the Hoggar the com- plexes have isotopic ages ranging from 592 + 10 to 478 f 25 Ma (Boissonnas 1974; Boissonnas et al. 1969, 1970), and in Niger and Nigeria they range from 487 + 5 to 141 + 2 Ma (Raharnan et al. 1984). The complexes contain volcanic and plutonic rocks with mineral and rock chemistry similar to those in the pre-Grenvillian stretched zone. In the north there are mostly rhyolitic ash-flow tuffs, anorthosites (Husch and Moreau 1982; Moreau 1987), gabbros, and peralkaline granitic rocks; in the south, aluminous biotite granites are dominant (Bennett et al. 1984). The complexes lie on the northward projection of the coastline south of the Gulf of Guinea. This projection is the line of continental separation of the South Atlantic, which began in the Early Cretaceous at 135 - 130 Ma (Rabinowitz and La Brecque 1979). The exten-

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I WINDLEY 483

sional tectonics and the earliest complexes began to form about 460 Ma before the opening of the ocean.

(2) In southern South America the Tobifera felsic volcanic rocks occupy an area of at least 1700 km (north-south) X 1000 km (east-west) (Fig. 3) and have an average thickness of 1 km and minimum volume of 1.7 x lo6 km3; this is the largest felsic volcanic field in the world (Dalziel 1974; Bruhn et al. 1978). Subsurface extent under on-land basins and on the continental shelf is known from geophysical and drill-core data (Ludwig et al. 1968; Zambrano and Urien 1970). These volcanic rocks are predominantly rhyolitic ash-flow tuffs with subordinate lavas and ignimbrites; intermediate and mafic volcanic rocks are absent. The volcanic rocks form a thin, unconformable veneer on older basement. Underlying and overlying fossiliferous sediments indicate that the volcanic rocks formed in about a 50 Ma period from the Early to Late Jurassic. They extend as far as the Atlantic continental margin and are closely associated with deep graben whose fault boundaries began to operate in the Late Jurassic and controlled sedimentation well into the Cenozoic (Dalziel 1974). Thus they are associated with extensional faults active during the early opening of the South Atlantic. Initial separation was in the period 135 - 130 Ma (Rabinowitz and La Brecque 1979). Further north, within the flood basalts of the Parani Plateau of Brazil, there is 150000 km2 of Lower Cretaceous felsic volcanic rocks close to the continental margin; these were extruded in the pre-rift stage of the South America - Africa continental separation (Bellieri et al. 1986).

A series of synchronous Mesozoic - Tertiary sedimentary basins are associated with the Brazilian continental margin. Whereas offshore basins show evidence of rifting and attenua- tion before breakup and thermal subsidence (and therefore lower crustal extension) after it, onshore basins only under- went the early rifting and attenuation stage with no subsequent thermal subsidence and thus no apparent extension of the lower crust beneath them. In order to permit such coupled differential stretching of the lithosphere, Ussami et al. (1986) suggested that the offshore and onshore regions must have been con- nected by an eastward-dipping low-angle crustal detachment surface according to the nonuniform extensional simple-shear model of Wernicke (1985). Following the termination of rift- ing in both basins the detachment allowed the transfer of exten- sion from the upper crust to the deeper part of the lithosphere in the offshore region.

This basin analysis suggests there was a link between the Tobifera lavas and tuffs extruded in or deposited in the com- parable offshore basins to the south and a low-angle detach- ment surface that controlled both their formation and that of the subsequent ocean.

I I

The Grenville orogenic belt There are positive reasons to interpret the Grenvillian Orog-

eny in terms of a plate tectonic model (Anderson and Burke 1983; Windley 1986).

In the western Grenville there are two major, deeply eroded, crustal blocks, namely, the Central Gneiss Belt to the west and the Central Granulite Terrane to the east (Baer 1981). Granu- lite-facies rocks and massif-type anorthosites mainly occur at the present erosion level in the Central Granulite Terrane. Between these crustal blocks there is a remnant island arc in the form of the Central Metasedimentary Belt; this arc contains

FIG. 3. Map of southernmost South America showing the outcrop and subsurface extent of Jurassic silicic volcanic rocks. Data from Bruhn et al. (1978), Dalziel(1974), and Urien and Zambrano (1973).

pillowed tholeiitic basalts overlain by calc-alkaline volcanic rocks, both of which formed about 1280-1250 Ma (Easton 1986) and are comparable in type and geochemistry to those of modem island arcs (Brown et al. 1975; Condie and Moore 1977). Bimodal mafic - silicic metavolcanic rocks of the Tur- riff formation and tholeiitic dykes and sills have immobile minor- and trace-element data that suggest they were emplaced in a back-arc basin (Holm et al. 1985, 1986; Smith and Holm 1988). UIPb isotopic compositions of sulphides from mineral deposits in the Central Metasedimentary Belt suggest that this crustal material was newly differentiated from mantle reser- voirs close to 1260 Ma (Fletcher and Farquhar 1982). In the gneisses close to the western boundary of the arc there are gneissic alkaline complexes with nepheline-bearing meta- syenites and metagabbros containing igneous z i ~ o n s with ages as old as 1219 Ma and metamorphic zircons of 1075 - 1040 Ma (Miller 1984). These rocks may be derived by deformation and metamorphism of nepheline-bearing alkaline complexes asso- ciated with continental rifts (Miller 1984) or of evaporites on a passive continental margin (A. Davidson, personal communi- cation, 1988). Collision of the island arc with the continental block of the Central Granulite Terrane to the east gave rise to deformation and metamorphism that can be correlated with the Elzevirian Orogeny at about 1250- 1220 Ma (Fig. 4a) (Easton 1986; Windley 1986). The arc, now situated on the leading edge of the plate, was intruded by the calc-alkaline Elzevir batholith, which has trace-element patterns similar to those of batholiths in modern Andean-type continental margins (Pride and Moore 1983). The 1120- 1039 Ma Ottawan Orogeny (Easton 1986, Fig. 9) can be correlated with the terminal colli- sion of the eastern arc-continent with the Centml Gneiss Belt to the west (Fig. 4a) (Windiey 1986). It is probably impossible to separate isotopically this collisional event from the immedi-

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484

WEST

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

WEST EAST

EAST

klhallne 1288 Ma Complexes

Island Arc of Csnrral A n O r t ~ O S l t e Metewdimenlarv

Subsurface Grenv~lle Fronl Suture

WEST

t b )

EAST

Ottawan Orogeny

1100-1090 Ma 9 9 v Alkal~ne

Gnetsses

Grenvlrle Front

1 - i k Wenvllle

-t \ \\qvv 9 Hurn

post-colllston

Frrt 7 y

t 9-9 'yy\ "'i"' lndentatlon and Ihrusilnrl

o f Central Dnelss Bell Post-~o11~s~onal lndentat~on and thrusting

( a ) ( c )

FIG. 4. (a) A series of diagrams illustrating the possible tectonic evolution of the Grenvillian Orogeny in Ontario (modified after Windley 1986). Thrusts in Central Gneiss Belt after Davidson (1984). (b) Three diagrams illustrating the possible formation of the Grenvillian belt in southern United States in particular the mode of development of the felsic volcanic rocks on an extentional continental margin and their eventual preservation against the Grenvillian belt. (c) Three sketches illustrating the formation of the Grenvillian belt in eastern Labrador, in particular the formation of anorthosites on the extensional continental margin and their final occurrence within the Grenvillian thrust belt and in the foreland to the west. (a), (b), and (c) refer to sections A, B, and C in Fig. 1, respectively.

ately succeeding southeast-directed postcollisional indentation of the Central Gneiss Belt that led to the northwest overriding of the thrust slabs of gneisses and granulites towards the Gren- ville Front and foreland of the orogenic belt (Davidson et al. 1982; Davidson 1985, 1986); this tectonic relationship is com- parable to that on the northern margin of the Indian Plate in the Himalayas (Windley 1986). The northwest-directed thrusting increased the thickness of the crust to more than 60 -70 km in the Grenville Front tectonic zone, which records lithostatic anatectic kyanite-microcline gneisses of 8 kbar (1 kbar = 100 MPa) and is still expressed by a negative Bouguer gravity anomaly of - 100 mGal (1 mGal = m s-') for over 700 km of its length (Wynne-Edwards 1972; Thomas and Tanner 1975). The increased crustal thickness in the foreland thrust belt of the Canadian Rockies is similarly marked by a negative Bouguer anomaly of more than -200 mGal.

The Grenville Province in both the Adirondack Mountains and Labrador contains similar thrust stacks and has a tectonic history that is readily interpretable as being a result of collision

tectonics (McLelland and Isachsen 1985; Rivers and Nunn 1985).

A consequence of a Himalayan-type collisional model for the Grenville orogenic belt is that individual blocks were uplifted as a result of crustal thickening consequent on thrust- nappe stacking, and therefore the so-called anorogenic plu- tonic rocks intruding the stretched basement zone were uplifted and exposed in a modified but recognisable state. If the western part of the Grenville belt is equivalent to the southern part of the Himalayas (i.e., Indian Plate), then the northwest- directed thrusting stopped at the Grenville Front and left the anorogenic complexes of the Nain Province of Labrador (Fig. 4c) and the rhyolite-granite province of the mid- continental United States (Fig. 4b) in a well-preserved state.

A model for pre-Grenvillian extension tectonics

Any model to account for pre-Grenvillian extension tectonics must explain the very wide zone of extension that controlled

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WINDLEY

LOWER PLATE UPPER PLATE:

Moho

1

siiiciF: \

\ Volcanlcs Epizonal Granite

Anorthosite 1 I

\ Grenvillian

\ Anorthosite / Orogenic Belt 'suture /

FIG. 5. A detachment fault model of a passive continental margin to explain the relationships between anorogenic volcanic and plutonic rocks and the Grenvillian orogenic belt. (a) Basic delamination model for continental extension. (b) Extension has given rise to oceanic crust, and lower- and upper-plate margins have different characteristics. In the more complex lower plate a sag basin, which could be filled with sediments or volcanics, overlies the detachment fault, which has been bowed up to expose mid to lower crustal rocks closer to the continental margin. (c) Section across southernmost South America (Fig. 3), illustrated in terms of a detachment model for the passive continental margin. The sag basin is filled with Tobifera silicic volcanics with some possible epizonal granites. The hypothetical anorthosite complexes are located close to the continental margins, being derived from a possible plume trace similar to the one shown in Fig. 2. (d) Section across the central interior region of the United States. During the Grenvillian Orogeny post-collisional thrusting (Himalayan model) was directed towards the foreland; the silicic volcanics and anorogenic granites are preserved farther west. The suture and the anorthosites indicated are hypothetical. (e) Section across the Grenvillian belt in Labrador. The anorthosites (and associated granites) were originally emplaced in rifts on the stretched passive continental margin. Foreland-directed thrusts in the orogenic belt stop at the Grenville Front. Some anorthosites are situated within the Grenville belt, but to the west of the front they have escaped deformation. The suture is hypothetical; it may be within the thrust pile or be offshore to the east. (a) and (b) are after Lister et al. (1986), based on Wernicke (1985).

the voluminous extrusive and intrusive magmatism for at least 200 Ma. Rather than a single pre-Grenvillian rift zone (Baragar 1977), we should envisage multitudes of discontinuous rifts developed over a wide zone or a wide, stretched extensional zone that may not necessarily have had any rifts. The hetero- geneous asymmetrical simple-shear, lithospheric-stretching model of Wernicke (1985), modified by Lister et al. (1986), provides an ideal mechanism to explain these relationships. It involves extension and displacement on a crustal-scale, shal- low-dipping detachment surface, and thus extension of the upper crust in one region is transferred to the lower crust and asthenosphere in another.

The first stage is represented by the formation of a sag basin on the lower plate caused by early movements on the detach- ment surface. Further lithospheric extension by simple shear eventually leads to continental separation and the formation of an ocean, and this leaves a wide zone of upper crustal exten-

sion on one passive continental margin, which is the zone of our interest (Fig. 5).

Whereas the upper-plate margin has little structure and undergoes uplift, the lower-plate margin is highly structured, with the sedimentaq-volcanic sag basin flanked on the oceanward side by an uplifted outer high where the erosion exposes mid-lower crustal rocks in the form of metamorphic core complexes and, with further uplift, extensive mid-lower crustal basement. An important aspect of the model is that the mechanism of lithospheric attenuation is one of mechanical extension by shear aided by permeating magmas in the lower plate. Thus the model provides a unique explanation of mag- matic belts on the lower plate that are far removed from the area of crustal separation and ocean formation. This lower- plate magmatism can be considered in two parts. The inner sag basin of South America was the site of extrusion of the vast Tobifera rhyolites, which are well preserved today precisely

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486 CAN. J . EARTH SCI. VOL. 26, 1989 I because of their location in such a topographic low. Accord- ingly, the mid-Proterozoic rhyolites and associated epizonal granites of the mid-continent region of the United States prob- ably formed in an inner sag basin on a thinned, lower-plate, passive continental margin. The basin has remained a topo- graphic low ever since, accounting for the remarkable preser- vation of such an extensive field of Precambrian extrusive rocks.

The scales of the model and the natural examples are com- parable. Wemicke (1985, Fig. l b ) pointed out that the upper plate in Saudi Arabia on the east side of the Red Sea is about 750 km wide, and this is close to the width of the lower plate represented by the rhyolite-filled basins of the Tobifera and the mid-continental United States.

On the oceanward side of the sag basin there typically lies a zone of uplifted basement close to the continental margin. Such a basement ridge forms the offshore margin of the conti- nental shelf east of the basins containing the Tobifera rhyolites (Urien and Zambrano 1973). Such a basement ridge would be expected to contain intrusions of anorthosite, gabbro, and gran- ite, which are fortuitously preserved, because of lack of crustal separation, along the uplift ridge in Niger and Nigeria (Wright 1985). Taking such relationships into account, we would expect anorthosites, gabbros, and rapakivi granites to be pre- sent in an uplift zone in what is now largely the Grenville belt.

topic data suggest that the rift system (ca. 1100 Ma) is contem- poraneous with the terminal Grenvillian Ottawan Orogeny (1 120- 1030 Ma). The most likely possibility is that there is a causal relationship between these compressional and exten- sional events (Gordon and Hempton 1986). Similarly, the con- temporaneous Abitibi dykes are everywhere normal to the direction of minimum Grenvillian compression (Ranalli and Emst 1986), and this suggests that the dykes were emplaced in response to a stress field related to the terminal Grenvillian orogenesis (Emst et al. 1987). If the Ottawan Orogeny formed as a result of the collision between the island arc of the Central Metasedimentary Belt attached to the Central Granulite Ter- rane and the continental block of the Central Gneiss Belt to the west, the rift system and associated diabase dykes would be contemporaneous with this collision and thus could have formed as a result of extensional forces in the foreland of the collisional orogenic belt. The most appropriate modem ana- logue is the Rhine Graben (Fig. 6 ) , which began to form in the Eocene at the same time as the main Alpine collision to the south (Sengor 1976; Sengor et al. 1978; Illies and Greiner 1978). The igneous activity of the Rhine Graben shares many similarities with that of the Keweenawan. Thus the terminal collision of the Grenvillian Orogeny was most likely directly responsible for the formation of the Keweenawan and the Mid- continent Rift System and its associated magmatism.

Keweenawan rifting and magmatism The Grenvillian Wilson cycle I In contrast to the anorogenic magmatism described above,

which preceded the Grenvillian Orogeny, the Keweenawan magmatism was contemporaneous with that orogeny (Table 1). The Midcontinent Rift System of North America is in the Lake Superior region and comprises the Keweenawan suite of igneous rocks (Van Schmus and Hinze 1985; Green et al. 1987). The mixed clastic-carbonate red bed sediments of the 1339 f 33 Ma Sibley Group were deposited in a playa lake environment (Cheadle 1986); these are contemporaneous with red beds in the Seal Lake Group in Labrador. The wide time gap between the deposition of the Sibley Group and the onset of Keweenawan igneous activity and the absence of any evi- dence of volcanism in the Sibley Group suggest that it is tec- tonically unrelated to the Keweenawan (Van Schmus et al. 1982). The earliest igneous event was possibly the emplace- ment of the alkaline Coldwell complex at ca. 1190 Ma (Turek et al. 1985, 1986). The Great Abitibi alkaline diabase dyke, which is northwest of and parallel to the Grenville Front, has a U - Pb age of 1 140.6 f 2 Ma (Krogh et al. 1987). Ernst et al. (1987) suggested that this 600 km long dyke and associated Abitibi dykes are related to the Keweenawan volcanics of the Lake Superior basin that have similar age and chemistry. The main Keweenawan magmatic activity occurred at about 1100 Ma (Van Schmus and Hinze 1985). The Logan diabase sills have a U-Pb age of 1109 f 5 Ma (Krogh et al. 1987). Other Keweenawan magmatic activity includes the Baraga dykes and the Thunder Bay dykes (Fahrig 1987), the Duluth gabbroic complex and the Qlser Group basalts, and several other diabase dyke swarms and the Keweenawan basaltic lavas (Green et al. 1987).

The origin of the Midcontinent Rift System and its anoro- genic igneous activity has long been problematic (Van Schmus and Hinze 1985; Hoffman 1988). It cannot be a failed rift arm

The extensional and compressional tectonics of the Grenville orogenic belt must be separated by the formation and destruc- tion of an ocean, particularly if one accepts the evidence of an island arc in the Central Metasedimentary Belt. In all the litera- ture of the Grenville we read little about this possible ocean, but it is necessary to consider it in order to obtain a time scale for the Grenvillian Wilson cycle, which may be taken to extend from the earliest extensional stages associated with the anoro- genic magmatism to the final compressional stages of orogeny. The Grenvillian cycle began about 1500 Ma and ended about 1600 Ma. Table 1 summarizes the main events that character- I ize the development of this Precambrian Wilson cycle.

This is a comparatively long cycle of events. One of the i shortest cycles must be that which gave rise to the Early Pro- (

terozoic Wopmay Orogeny, which lasted less than 90 Ma (Hoffman and Bowring 1984). In contrast, the Himalayan Wilson cycle has continued for nearly 300 Ma and is still not complete. The major part of such a cycle is taken up by the opening of the ocean. The Atlantic is a long-lived 200 Ma ocean and is bordered by the largest felsic volcanic field in the world (the Tobifera), which is comparable in extent to the Midcontinent felsic volcanic field that bordered the possible Grenvillian ocean. Let us consider the time span of a Wilson cycle based on the opening and closure of the Atlantic. The anorogenic complexes of the Hoggar, Niger, and Nigeria, which were probably sited over a hot spot - plume trace, were intruded for at least 460 Ma before the opening of the South Atlantic, and the Tobifera rhyolites were extruded at least 50 Ma beforehand. The North Atlantic has been opening for about 200 Ma. Subduction commonly takes place at twice the rate of sea-floor spreading, so it might take up to 100 Ma to destroy the North Atlantic. It would probably take 50 Ma to grow an island arc and close a back-arc basin. Since the forma-

or aulacogen because it does not date from the breakup of the tion of the Indus-Zangbo suture between the collided Indian pre-Grenvillian continental margin. Currently available iso- and Eurasian plates it has taken 50 Ma to form the Himalayan

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WIN

FIG. 6. Comparison between (a) the Midcontinent Rift System in relation to the Grenvillian belt and (b) the Rhine Graben in relation to the Alpine belt.

orogenic belt. Thus the 500 M a time span for the Grenvillian cycle is not significantly different from a long-lived Phanero- zoic cycle.

Rhyolitic volcanism was clearly very important in the mid- Proterozoic in North America, and this is typically interpreted as being a result of extensional tectonics. The Tobifera rhyo- lites provide a new perspective for comparison because they formed in a wide, stretched continental margin immediately prior to the opening of an ocean. The mid-Proterozoic anortho- sites, granites, and alkaline complexes are commonly con- sidered a s having formed in intracontinental rifts. The complexes of Niger and Nigeria, with which they are com- monly compared, also formed a s precursors to the formation of a n ocean. Therefore it is reasonable to postulate the opening and closure of a n ocean within the Grenvillian cycle of events between the formation of the rhyolites and anorogenic com- plexes and the compressional orogeny. Only by considering the presence of such a n ocean can the full extent of the Gren- villian cycle be appreciated.

Acknowledgments I a m grateful for useful discussions with Mike N o n y , Andy

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WINDLEY 489

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