Age and thermal evolution of the Mesoproterozoic Cape Meredith Complex, West Falkland

JOACHIM JACOBS, ROBERT J. THOMAS, RICHARD A. ARMSTRONG and FRIEDHELM HENJES-KUNST
Journal of the Geological Society, 156, 917-928, 1 September 1999, https://doi.org/10.1144/gsjgs.156.5.0917

Abstract

U-Pb SHRIMP zircon and 40Ar/39Ar mineral ages are reported for rocks of the Cape Meredith Complex, West Falkland. Felsic gneisses of the oldest Big Cape Formation give a zircon date of 1118 ± 8 Ma, interpreted as the time of extrusion of the rhyolitic protoliths. The three phases of granitoid which intrude the Big Cape Formation, namely granodiorite orthogneiss (Gl), syntectonic granite gneiss (G2) and post-tectonic granite (G3), gave U–Pb dates of c. 1090, 1067 ± 9 and 1003 ± 16 Ma respectively. Metamorphic overgrowths on zircons from the felsic gneisses were dated at c. 1000 Ma, coeval with the G3 granite, whereas a date of 1135 ± 11 Ma from inherited cores in G2 zircons were probably derived from a slightly older component of the Big Cape Formation. 40Ar/39Ar hornblende ages from amphibo-lites of the Big Cape Formation (1009 ± 14 and 1015 ± 6 Ma), along with muscovite (989 ± 3 Ma) and biotite (989 ± 7 Ma) from G3 pegmatites show that the complex cooled relatively rapidly to below 350°C, with no evidence of any Pan-African (c. 500 Ma) thermal overprinting. These data show that the older rocks of the Cape Meredith Complex are significantly younger than the equivalent rocks of the Natal Metamorphic Province, SE Africa (c. 1200 Ma), though they are comparable in age to those of Western Dronning Maud Land (East Antarctica), areas which Gondwana reconstructions place west and east of the Falkland Microplate respectively. The older rocks of the Cape Meredith Complex may therefore represent younger outboard arc terranes relative to those exposed in Natal. However, the syn- to post-tectonic granites of the Cape Meredith Complex are comparable in age in all three areas (1070–1000 Ma), suggesting similar collisional and post-collisional histories throughout the entire region. The lower temperature history of the Cape Meredith Complex is comparable to data from Natal, but does not show the Pan-African (c. 500 Ma) overprint which is characteristic of much of western Dronning Maud Land. This situation is consistent with the proposed position of the Falkland Microplate between SE Africa and East Antarctica in Gondwana, where a general eastward increase in the intensity of the Pan-African thermal effects has been recorded.

References

    1. Adie R.J.
    (1952) The position of the Falklands Islands in a reconstruction of Gondwana. Geological Magazine 89:401–410.
    OpenUrlAbstract
    1. Arndt N.T.,
    2. Todt W.,
    3. Chauvel M.,
    4. Tapper M.,
    5. Wpbpr K.
    (1991) U-Pb zircon age and Nd isotopic composition of granitoids, chamockites and supracmstal rocks from Heimefrontfjella, Antarctica. Geologische Rundschau 80:759–777.
    OpenUrl
    1. Cingolani C.A.,
    2. Varpla R.
    (1976) Investigaciones, geologicas y geochronologicas en el extremo sur de la isla Gran Malvina, sector de Cabo Belgrano (Cabo Meredith), Islas Malvinas. Actas del sexto congreso Geologico Argentino, 457–473.
    1. Compston W.,
    2. Williams I.S.,
    3. Ktrschvink J.L.,
    4. Zichao Z.,
    5. Guogan M.A.
    (1992) Zircon U-Pb ages for the Early Cambrian time scale. Journal of the Geological Society, London 149:171–184.
    OpenUrlAbstract/FREE Full Text
    1. Cornpll D.H.,
    2. Thomas R.J.,
    3. Bowring S.A.,
    4. Armstrong R.A.,
    5. Grantham G.H.
    (1996) Protolith interpretation in metamorphic terranes: a back arc environment with Besshi base metal potential for the Quha Formation, Natal Metamorphic Province, South Africa. Precambrian Research 77:243–271.
    OpenUrlCrossRefWeb of Science
    1. Cumming G.L.,
    2. Richards J.R.
    (1975) Ore lead isotope ratios in a continuously changing earth. Earth and Planetary Science Letters 28:155–171.
    OpenUrlCrossRefWeb of Science
    1. Curtis M.L.,
    2. Hyam D.M.
    (1998) Late Palaeozoic to Mesozoic structural evolution of the Falkland Islands: a displaced segment of the Cape Fold Belt. Journal of the Geological Society, London 155:115–129.
    OpenUrlCrossRefWeb of Science
    1. Grunow A.,
    2. Hanson R.,
    3. Wilson T.
    (1996) Were aspects of Pan-African deformation linked to Iapetus opening? Geology 24:1063–1066.
    OpenUrlAbstract/FREE Full Text
    1. Gropnpwald P.B.,
    2. Grantham G.H.,
    3. Watkpys M.K.
    (1991) Geological evidence for a Proterozoic to Mesozoic link between southeastern Africa and Dronning Maud Land, Antarctica. Journal of the Geological Society, London 148:1215–1223.
    OpenUrl
    1. Jacobs J.,
    2. Thomas R.J.
    (1994) Oblique collision at about 1.1 Ga along the southern margin of the Kaapvaal continent, south-east Africa. Geologische Rundschau 83:322–333.
    OpenUrlWeb of Science
    1. Jacobs J.,
    2. Thomas R.J.
    (1996) Pan-African rejuvenation of the 1.1 Ga Natal Metamorphic Province (South Africa): K-Ar muscovite and sphene fission track evidence. Journal of the Geological Society, London 153:971–978.
    OpenUrlAbstract/FREE Full Text
    1. Jacobs J.,
    2. Ahrpndt H.,
    3. Krputzpr H.,
    4. Wpbpr K.
    (1995) K-Ar, 40Ar/39Ar and apatite fission track evidence for Neoproterozoic and Mesozoic basement rejuvenation events in the Heimefrontfjella and annfallknausane (East Antarctica). Precambrian Research 75:251–262.
    OpenUrlCrossRefWeb of Science
    1. Jacobs J.,
    2. Baupr W.,
    3. Spapth G.,
    4. Thomas R.J.,
    5. Wpbpr K.
    (1996) Lithology and structure of the Grenville aged (~1.1 Ga) basement of Heimefrontfjella (East Antarctica). Geologische Rundschau 85:800–822.
    OpenUrl
    1. Jacobs J.,
    2. Faltpr M.,
    3. Thomas R.J.,
    4. Kunz J.,
    5. Jpssbprgpr E.K.
    (1997) 40Ar/39Ar thermochronological constraints on the structural evolution of the Mesoproterozoic Natal Metamorphic Province, SE Africa. Precambrian Research 86:71–92.
    OpenUrlCrossRefWeb of Science
    1. Jacobs J.,
    2. Fanning C.M.,
    3. Hpnjps-Kunst F.,
    4. Olesch O.,
    5. Peach H.-J.
    (1998) Continuation of the Mozambique Belt into East Antarctica: Grenville-age metamorphism and polyphase Pan-African high-grade events in central Dronning Maud Land. Journal of Geology 106:385–406.
    OpenUrlCrossRefWeb of Science
    1. Jacobs J.,
    2. Fanning C.M.,
    3. Wpbpr K.
    (1993) Accretion and indentation tectonics at the southern edge of the Kaapvaal craton during the Kibaran (Grenville) orogeny. Geology 21:203–206.
    OpenUrlAbstract/FREE Full Text
    1. Marshall J.
    (1994) The Falkland Islands: a key element in Gondwana palaeogeography. Tectonics 13:499–514.
    OpenUrlCrossRefWeb of Science
    1. Matthews P.E.
    (1972) Possible Precambrian obduction and plate tectonics in southeastern Africa. Nature 240:37–39.
    OpenUrlCrossRefPubMedWeb of Science
    1. Mitchell C.,
    2. Taylor G.K.,
    3. Cox K.G.,
    4. Shaw J.
    (1986) Are the Falkland Islands a rotated microplate? Nature 319:131–134.
    OpenUrlCrossRef
    1. Paces J.B.,
    2. Miller J.D.
    (1989) Precise U-Pb ages of Duluth Complex and related mafic intrusions, Northeastern Minnesota: Geochronological insights to physical, petrogenic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga midcontinent rift system. Journal of Geophysical Research 98B:13 997–14 013.
    OpenUrl
    1. Rapela C.W.,
    2. Pankhurst R.J.
    (1992) The granites of northern Patagonia and the Gastre Fault System in relation to the break up of Gondwana. in Magmatism and the Causes of Continental Break-up Geological Society, London, Special Publications, eds Storey B.C., Alabaster T., Pankhurst R.J. 68:209–220.
    OpenUrl
    1. Rex D.C.,
    2. Tanner P.W.G.
    (1982) in Antarctic Geoscience, Precambrian ages for gneisses at Cape Meredith in the Falkland Islands, ed Cradock C. (University of Wisconsin Press), pp 107–110.
    1. Sambridge M.S.,
    2. Compston W.
    (1994) Mixture modelling of multi-component data sets with application to ion-probe zircon ages. Earth and Planetary Science Letters 128:373–390.
    OpenUrlCrossRefWeb of Science
    1. Shackleton R.M.
    (1996) The final collision zone between East and West Gondwana: where is it? Journal of African Earth Sciences 23:271–287.
    OpenUrlCrossRef
    1. Steigger R.H.,
    2. Jägpr E.
    (1977) Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36:359–362.
    OpenUrlCrossRefWeb of Science
    1. Thistlewood L.,
    2. Lpat P.T.,
    3. Millar I.L.,
    4. Storey B.C.,
    5. Vaughan A.P.M.
    (1997) Basement geology and Palaeozoic-Mesozoic mafic dykes from the Cape Meredith Complex, Falkland Islands: a record of repeated intracontinental extension. Geological Magazine 134:355–367.
    OpenUrlCrossRefWeb of Science
    1. Thomas R.J.
    (1989) A tale of two tectonic terranes. South African Journal of Geology 92:306–321.
    OpenUrlAbstract
    1. Thomas R.J.,
    2. Eglington B.M.
    (1990) A Rb-Sr, Sm-Nd and U-Pb zircon isotopic study of the Mzumbe Suite, the oldest intrusive granitoid in southern Natal, South Africa. South African Journal of Geology 93:761–765.
    OpenUrlAbstract
    1. Thomas R.J.,
    2. Cornell D.M.,
    3. Armstrong R.A.
    (1999) Age of the Quha Formation, Mzumbe Terrane, Natal Metamorphic Province. South African Journal of Geology 102:1–6.
    OpenUrlAbstract
    1. Thomas R.J.,
    2. Eglington B.M.,
    3. Bowring S.A.
    (1993a) Dating the cessation of Kibaran magmatism in Natal, South Africa. Journal of African Earth Sciences 16:247–252.
    OpenUrlCrossRef
    1. Thomas R.J.,
    2. Eglington B.M.,
    3. Bowring S.A.,
    4. Rettef E.A.,
    5. Walraven F.
    (1993b) New isotope data from a Late Proterozoic porphyritic granite-chamockite association from Natal, South Africa. Precambrian Research 61:83–101.
    OpenUrl
    1. Thomas R.J.,
    2. Henjes-Kunst F.,
    3. Jacobs J.
    (1998) Age of pre-lamprophyre dykes of the Cape Meredith Complex, West Falkland. Geological Magazine 135:495–500.
    OpenUrlAbstract
    1. Thomas R.J.,
    2. Jacobs J.,
    3. Weber K.
    (1997) in The Antarctic Region: Geological evolution and processes, Geology of the Mesoproterozoic Cape Meredith Complex, West Falkland, ed Ricci C.A. (Terra Antarctica Publication, Siena), pp 21–30.
    1. Thomas R.J.,
    2. Marshall C.G.A.,
    3. Watkeys M.K.,
    4. Fitch F.,
    5. Millpr J.A.
    (1992) K-Ar and Ar/Ar dating of the Natal Group: a post Pan-African molasse? Journal of African Earth Sciences 15:453–471.
    OpenUrlCrossRef
  35. Thomas, R.J. Walraven, F. & Eglington, B.M. 1995. Zircon evaporation geochronology of granitoids from southern Natal, South Africa (Extended Abstract). In: Geocongress '95, Geological Society of South Africa. Johannesburg, 418–421.
    1. Wareham C.D.,
    2. Pankhurst R.J.,
    3. Thomas R.J.,
    4. Storpy B.C.,
    5. Grantham G.H.,
    6. Jacobs J.,
    7. Eglington B.M.
    (1998) Pb, Nd and Sr isotope mapping of Grenville-age crustal provinces in Rodinia. Journal of Geology 106:647–659.
    OpenUrlCrossRefWeb of Science
    1. Williams I.S.,
    2. Claesson S.
    (1987) Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides II Ion microprobe zircon U-Th-Pb Contributions to Mineralogy and Petrology 97:205–217.
    OpenUrlCrossRefWeb of Science
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Journal of the Geological Society: 156 (5)
Journal of the Geological Society
Volume 156, Issue 5
September 1999
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