Anorogenic Magmatism, Plate Motion and Atlantic Evolution

Anorogenic magmatism, plate motion and Atlantic evolution

R. M. MACINTYRE

SUMMARY

Emplacement of carbonatites at o--I5, 25, 40, 6o-70, I3o Ma (and possibly 8o and xoo Ma) may coincide with major changes in plate motion. This suggests a common triggering mechanism which previously may have operated at intervals of about 23 ° Ma.

I. Introduction As nN INTVRCONN~CTED MESH of rigid lithospheric plates embraces the earth, changes in rate or direction of motion of one plate may inevitably affect adjacent plates. Such changes have sometimes been associated with the significant marginal development of basic rocks whose ages provide a means of testing global synchronism. However, these rocks are generally unsuitable for precise age determination studies and their relatively rapid disappearance by subduction and erosion makes another approach desirable. The magmatism associated with plate margins and intraplate rifts is associated with regional crustal swells probably derived from deep forceful disturbances (Le Bas I97I ). On the continents these conditions favoured the development of nephelinitic, carbonatitic and kimberfitic rocks. Occasionally geosutures formed and separation occurred, with new spreading axes and oceanic crust, prolific volcanism, coast-parallel dykes and new continental margins determined by the pre-existing rifts. In Africa alkaline igneous rocks and carbonatites are con- centrated in regions of swelling (Bailey x964, Campbell Smith I956, King & Sutherland I96o ) and a close relationship exists between the swelling and their emplacement (Le Bas I97i ). Their ages may therefore indicate when the process responsible for the swellings operated and allow the synchroneity of its action to be examined. With this approach the precision is improved as these rocks are suited to the application of several radiometric methods. The ages accurately determine the times of emplacement, as the complexes cooled quickly and had an uneventful geological history. They indicate the times of first adjustment to the stress field, irrespective of the development of spreading axes or delays between graben formation and physical separation. Moreover, there is no time restriction as older complexes are preserved in plate interiors. The ages of carbonatites and kimberlites may therefore be extremely significant and relate to the history of plate motions.

3l geol. Soy. Lond. vol. x33, I977, pp. 375-384, 2 figs, x table. Printed in Great Britain

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2. Ages of carbonatites and kimberlites Table xa lists all carbonatite complexes of Mesozoic and Cenozoic age for which age determinations are available excluding those of widespread development in

TABLE I: Age measurements on Mesozoic and Cenozoic carbonatites and related rocks (a) carbonatites with ages greater than ca z 5 Ma. (b) other igneous rocks considered to be temporally associated with the initiation of rifting processes in late oTurassic--early Cretaceous times in North Atlantic region and (c) South Atlantic region.

Location Sample V (No) Method Ref. AgeA(Ma)

(a) Bukusu and Busumbu, Uganda am,b (5) K-Ar (1) 26 Tororo, Uganda b (i) " (i) 26 Sukulu, Uganda b,ph (2) " (I) 27

•~mba Dongar, India px (i) " (2) 38+3 Canary Islands px (i) " (3) 39 • 4 Tamazeght, Morocco b,w (5) Rb-Sr (4) 47 ± 4 b (4) K-At (4) 42 ± 3

• Rocky Boy Stock, Montana b (i) " (5) 52 + 2

Endau, Jombo (Mrima), Kenya v (20) " (i) 6O -7O • Lajes, Brazil f,w (3) " (6) 65 • Itatiaia, Brazil am,b (6) " (6) 66 • Tapira, Brazil (7) 7O

*Salitre, Brazil " (7) 81 *Catalao, Brazil " (7) 83 *Araxa, Brazil " (7) 87 -95

Magnet Cove, Arkansas b (2) " (8) 95 + 5, 97 + 5 " " " ap,sp (2) f-t (9) 99 ±iO " " " b (i) Rb-Sr (8) 105 ± 8 Itapirapua, Brazil b,f (2) K-Ar (~) 103 Songwe Scarp, Musensi, Tanzania b,f (2) K-Ar (i01 lO1 +12, 96 + 9

Panda Hill (Mbeya), Tanzania ph (i) K-Ar (17) 113 + 6 Tchivira Bonga, Angol~ b Rb-Sr (18) 119 + 8 Kangankunde, Malawi ph (i) K-Ar (17) 123 f 6 Tundulu Hill, Malawi b (I) " (13) 133 + 7 Chilwa Island, Malawi b (i) " (13) 136 + 7 Damaraland Complexes, Namibia v (11) " (15) 134 ~ 1 (Messum, Doros, Cape Cross, Paresis) w (5) Rb-Sr (15) 135 ~ 4 Serrote, Brazil b,f,w (3) K-Ar (6) 127 Anitapolis, Brazil b,f,w (3) " (6) 129 Jacupiranga, Brazil b,ph,w (5) " (6) 132 Oka, Quebec w (4) Rb-Sr (ii) 131 + 3 n (3) K-Ar (12) 127 Haast River, New Zealand w (6) " (14) 120-130

Shawa, Rhodesia b,px (4) Rb-Sr (16) 209 +16

(b) Lamprophyres, S.W. Greenland b (3) K-At (19) 116 ~ 4,122 + 5,138 + 5 Lamprophyres, Central Newfoundland b,h (3) (20) 115 +20,129 + 7,144 +12 Budgell Harbour Stock, Newfoundland b (2) ii (21) 135 -+ 8,139 ± 9 Lamprophyre, Grand Isle, Vermont b (i) n, (8) 136 +7, Coast-parallel dyke, S.W. Greenland w (i) . (22) 138 Coast-parallel dyke, Portugal w (i) (23) 134 +2 Wolf Rock phonolite, U.K. n,s (8) (24) 131 + 1

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the last few Ma. Apart from four complexes in Namibia ages of related rocks are not included. Ages of some complexes for which only a single measurement has been reported could be in error and, where carbonatites of different ages occur in close proximity, overprinting of earlier ages may have occurred. A striking feature is the number of carbonatites from widely separated locations on four continents which has been emplaced near the Jurassic-Cretaceous boundary (Macintyre I97I , 1973a ). Most ages are indistinguishable within analytical error and average 13o Ma, suggestive of contemporaneous emplacement. The measurements are shown schematically in the form of a histogram in Fig. I b. Certain times (at around 13o, 65, 4o, 25 and o Ma) appear to represent distinct maxima of carbonatite magmatism. The ages of ldmberlites indicate a broader distribution in time. A number of South African kimberlites coincide with the major peak of carbonatite magmatism (Macintyre & Dawson 1976 and unpublished work).

3. Timing of igneous events in the Atlantic region The North Atlantic region is one of recurrent plate displacement since the early Mesozoic and the absence of major subduction or deformation on the margins allows the assumption of rigid plate configurations. The geophysical evidence indicates that major changes in the spreading geometry took place c. 200, 13 ° and 60 Ma with minor re-orientations at approximately t 60, 80, 40-5 o, 20 and 15 Ma (Nairn & Stehli 1974, Vogt & Avery I974, Williams 1975, Woodland I975). Reliable geochronological information is now available for many igneous rocks associated with specific aspects of this rifting history, providing additional evidence for the timing of these changes.

[For Table z on facing page] V am, amphibole; ap, apatite; b, biotite; f, feldspar; h, hornblende; n, nepheline; ph, phlogopite; px, pyroxene; sa, sanldine; sp, sphene; v, various; w, whole-rock. Approximate number of independent samples analysed is given in paren- theses in column 3. In general the higher this number the greater the degree of reliability of the age measurement. A Rb-Sr ages (re-)calculated with A = x.39 xo"ix yr .x. Errors in age are analytical and are those assigned by original investigators. * Probable carbonatite. (x) R. M. Macintyre, unpublished data; (2) Deans & Powell 1968; (3) Abdel-Monem et al. 1971; (4) Agard i974, Tisserant et al., in press; (5) Faul I96o; (6) Amaral et al. I967; (7) Neill i973; (8) Zartman et al. I967; (9) Naeser & Faul x969; (Io) Miller & Brown x963; (x t) Fairbairn et al. I963; (I2) Shafiqullah et al. I97o; (13) Snelling& Rex I966, Woolley & Garson 197o; (I4) Cooper t97t, Wellman & Cooper 197I ; (15) Manton & Siedner I967, Siedner & Mitchell 1976, Siedner & Miller 1968; (16) Nicolaysen et al. I962; (I7) Snelling 1965; (I8) Lapido-Loureiro I968; (t9) Hansen & Larsen I974; (2o) Wanless et al. I965 and I967; (21) Helwig et al. I974; (22) Gale I969; (23) M. R. P. g. Ferreira, pers. comm. 1974; (24) Mitchell et aI. I975; (25) Umpierre & Halpern 197I; (26) Stocken t973.

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In the British Tertiary Province a major phase of magmatic activity occurred around 59 Ma and was probably followed by a minor phase around 53 Ma (Macintyre x973b , Macintyre et al. I975). The ages correspond with those of volcanic tufts in the North Sea (Jacque & Thouvenin x975, Rhys I974). The earlier dates the incidence of separation between Eurasia and Greenland on the Heirtzler et al. (I968) time scale. Most ages in Baffin Island (Farrar I966 ), West Greenland (Athavale & Sharma I975, Deutsch & Kristjansson i974, Beckinsale et al. i974) and East Greenland (Beckinsale et al. I97O ) also concentrate in the interval 60 Ma to 5 ° Ma. Recent palaeontological evidence (Soper et al. I976 ) indicates an age of 52-55 Ma for the basalts in East Greenland so there is not always an exact correspondence in the timing of the igneous activity with continental separation (cf. Siedner & Mitchell I976 ). Fig. z a illustrates the history of igneous activity in the North Atlantic region derived from the available geochronological evidence. Sharp bursts of volcanism occurred contemporaneously over wide areas by exploiting and utilizing fracture

(a) ICELAND ) O- ~,..J KENYA, TANZAN'A.ET.,OP,A. CA~E VERDE (b)

S.W. IRELAND ,~ • • .UGANDA SW IRE~ ~~CANARY IS, MOROCCO,INDIA - 50-4 E. GREEN~D ~EERR~I~¥Y "-~-S3KENYA

- 100- -'T'-F."~ u.s.A. BRAZIL, TANZANIA

NEWFOUNDLAND,PORTUGAL - S.W.ENGLAND ] [ [ I [ [ 1 1 t ] I CANADA,BRAZIL,NAMIBIA, MALAWIoNEW ZEALAND S.w. GREENLAND ~--~ - 150- NORTH SEA,SWEDEN,NORWAY -'3

I N AMERICAN& AFRICANSEABOARDS~ -Z00- ..-[ 7]R.OOESI* g S.W.NORWAY ~

W :I EAD PERMOCA.ON,,ERO%/ -4

Fzo. z. Comparison of timing of igneous activity in North Atlantic region with (world-wide) incidence of carbonatite magmatism. (a) Schematic representation of distribution in time of igneous activity at or near margins of continents bordering North Atlantic. For explanation see text. (b) Histogram of ages of Mesozoic and Cenozoic carbonatites from Table z (a) with occurrences of doubtful status or un- certain age dashed and range in ages reported for kimberlites from (a) Western U.S.A. : Armstrong I969; Hearn & Boyd I975; Helmstaedt & Doig x975 (b) Southern Africa: Macintyre & Dawson x976 (c) North-eastern U.S.A. and Eastern Canada: Wanless et al. I968; Watson I967; Zartman et al. I967 (d) Western Greenland: Andrews & Emeleus I971 and I975.

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patterns in the crust. There is no evidence for localized mantle plumes or hot-spots (Faerseth et al. 1976, Home & Macintyre 1975, Macintyre et al. 1975). Widespread Carboniferous to mid-Jurassic events correlate with major tectonic pulses affecting the North Sea and surrounding areas (Faerseth et al. 1976). A distinct peak of activity occurred near the Jurassic-Cretaceous boundary (Table I b). The mean ages lie between 125 and 138 Ma, agree within analytical error and average 133 Ma. Sporadic intrusive activity followed in the Cretaceous (Roberts et al. 1974, Jones et al. 1974, Durant et al. 1976). Dykes in southwest Ireland appear later than the main phase of Early Tertiary activity (Home & Macintyre 1975). The North Atlantic region experienced increased volcanism in late Tertiary times (Vogt 1972, Burke et al. 1973). These peaks correspond with the changes in plate motion deduced from the geophysical evidence and also correlate with peaks of carbonatite emplacement (Fig. i). So significant events in the evolution of the North Atlantic appear to have coincided with the widespread emplacement of carbonatites. The events which shaped the evolution of the South Atlantic occurred at similar times. Rifting between South America and Africa took place at 13° Ma (Larson & Ladd 1973) and ten degrees of separation occurred before 121 Ma (Siedner & Mitchell 1976). The rifting was accompanied by the intrusion of carbonatites in South Africa, Namibia, Angola and Brazil (Table I). The emplacement of probable carbonatites in Brazil with ages around 8o Ma may correspond to a subsequent change in the pole of rotation (Le Pichon & Hayes 1971). In late Cretaceousnearly Tertiary times a lapse of sedimentation occurred (Perchnielsen et al. 1975). Ring-complexes dated at 56 4- 8 Ma occur on the Cameroun Line (Lasserre 1967) and probable carbonatites developed in Brazil (Table i). At 4o Ma a further lapse of sedimentation occurred (Perchnielsen et al. 1975) and alkaline intrusions were emplaced in Namibia (Kroner 1973)- At 25 Ma the African plate may have become stationary (Burke & Wilson I97~ ). There is a good correlation between times of carbonatite emplacement and of the initiation of spreading changes during development of the Atlantic. Other significant events in the history of plate motions (e.g. the Hawaiian-Emperor bend at 24.6 Ma (Jackson et al. 1972) and/or 43 Ma (Claque & Jarrard 1973) and the initiation of spreading in the Red Sea at 4o Ma (Girdler & Styles 1974) took place at similar times. So global synchroneity of plate interaction is probable.

4. Conclusions Palaeozoic and Precambrian carbonatites are usually associated with major rifts or aligned in belts suggestive of lines of weakness. Examples are the Jordan, East African and St Lawrence rifts, the Kapuskasing High and 38th parallel lineament and occurrences in Angola, Namibia, Brazil and South Africa. Their ages cluster round certain chronological peaks (Macintyre 197 I) illustrated schematically in Fig. 2. There is some geographical bias but few of the peaks are composed exclu- sively of measurements from a single area. The mean times of emplacement occur at intervals of approximately 230 Ma. This repetitive exploitation of major lineaments and rifts over long periods of time may be an outward manifestation of

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[] Kapuskasing. St. Lawrence. West Greenland [] Scandinavia and Kola [] East Africa and Sinai [] Braz it [] South and West Africa [] Bushveld Area [] Other Areas

ffl3"l ffi 1 171 R I I 500 looo lsoo AGE (Ma} Fxo. 2. Histogram of ages measured for Precambrian and Palaeozoic carbonatites and kimberlites. Peak of activity near Jurassic-Cretaceous boundary also shown. Each box represents an age measurement on a separate intrusion or complex. References to majority of age measurements cited in Macintyre (I97I). Additional data from Cooper i97t; Kroner I973; Macintyre & Dawson 1971 and I976; Shaft- quUah et al. I97o; Siedner & Mitchell x976; Vartiainen & Woolley x974; Verwoerd I967; Wellman & Cooper I97I; Woolley & Garson I97o supplemented by unpublished results of the author.

a process deep in the earth which operated regularly from at least middle Pre- cambrian to Permo-Carboniferous times. The period suggests mantle-wide con- vective motion as overturn yields typical sea-floor spreading rates. The correlation with major Precambrian orogenies (Gittins et al. I967) would not be expected if these were caused by random plate collisions. The carbonatite activity is probably symptomatic of increased heat flow at these times. Latterly disturbances became more frequent, perhaps by a proliferation of con- vective cells or the establishment of a more restricted (asthenospheric ?) circulation system. The last few hundred Ma seem characterized by an abundance of carbonatites and kimberlites (Holmes x965) possibly associated with vigorous mantle degassing. Major influxes of COs may have caused lithospheric attenuation and weakening with the resulting explosive activity facilitating plate fragmentation. Despite these minor perturbations the major disturbance took place 'on schedule' and initiated the general break-up of Gondwanaland. The best estimate which can be derived for this disturbance is 133 Ma. Later changes in the motions of plates appear to have been accompanied by the emplacement of carbonatites in zones of weakness in their interiors.

Acra~owL~.DOF.m~rcrs. I thank N.E.R.C., Strathclyde and other Scottish Universities for financial support, Peter Kent, Mike Le Bas, Peter and Walter Ziegler and Alan Blaxland for critically reading an early version of the manuscript and Tom McMenamin for analytical assistance.

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Received 15 September 1976; revised typescript received 13 November 1976. ROBERT MXTeHELL MACmWVR~, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow and Department of Applied Geology, Univer- sity of Strathelyde, Glasgow.

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