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 synchro- nism. 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 kimber- lites 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/4/375/4885450/gsjgs.133.4.0375.pdf by guest on 26 September 2021 376 R. M. Macintyre 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 (C) Syenite, Mariscala, Uruguay w (3) Rb-Sr (25) 128 + 5 Foyaite, Granitberg, Namibia b K-Ar (26) 130 + 2 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/4/375/4885450/gsjgs.133.4.0375.pdf by guest on 26 September 2021 Atlantic evolution 377 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 bound- ary (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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/4/375/4885450/gsjgs.133.4.0375.pdf by guest on 26 September 2021 378 R. M. Macintyre 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 con- tinental 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.