29. Sr-, Nd-, AND Pb-ISOTOPIC COMPOSITION OF VOLCANIC ROCKS FROM THE SOUTHEAST GREENLAND MARGIN AT 63°N: TEMPORAL VARIATION IN CRUSTAL CONTAMINATION DURING CONTINENTAL BREAKUP 1

The southeast Greenland seaward-dipping reflector sequence (SDRS) is composed of Paleocene to Eocene volcanic rocks erupted during continental breakup. Volcanic rocks recovered from a transect across the SDRS at 63°N, during Ocean Drilling Program Leg 152, range in composition from picrite to dacite and represent all the magmatic phases in the development of the continental margin. The earliest magmas, represented by the pre-breakup succession at Site 917 (Lower and Middle Series), were strongly contaminated with continental crust, but the degree of contamination declined rapidly during the late stages of breakup (Site 917 Upper Series). Very low concentrations of incompatible elements in the uncontaminated primitive magmas made them extremely sensitive to the isotopic effects of crustal contamination. Basaltic rocks from the most seaward part of th e transect (Site 918) were erupted after breakup and show no signs of contamination with continental crust. Two distinct crustal contaminants can be recognized: (1) lower crustal basic granulite with unradiogenic Nd, Sr, and Pb; and (2) upper crustal amphibolite-facies gneiss with unradiogenic Nd but highly radiogenic Sr and high 208 Pb/ 204 Pb. The first contaminant affected only the earliest magmas, represented by the lower volcanic units in the Lower Series at Site 917. Later continental magmas were affected by the second contaminant, suggesting storage of magmas at progressively shallower levels in the crust as lithospheric extension proceeded toward continental breakup. The nature and degree of contamination are strikingly similar to those observed in the Hebridean Tertiary igneous province, which would have been adjacent to southeast Greenland during continental breakup.

[1]  M. Fram,et al.  Generation and Polybaric Differentiation of East Greenland Early Tertiary Flood Basalts , 1997 .

[2]  P. Kempton,et al.  The heterogeneous Iceland plume: new insights from the alkaline basalts of the Snaefell volcanic centre , 1995, Journal of the Geological Society.

[3]  A. D. Saunders,et al.  Magma sources and plumbing systems during break-up of the SE Greenland margin: preliminary results from ODP Leg 152 , 1995, Journal of the Geological Society.

[4]  C. Villaseca,et al.  Geochemical and isotopic disequilibrium in crustal melting: An insight from the anatectic granitoids from Toledo, Spain , 1995 .

[5]  J. Blichert‐Toft,et al.  Geochemical Constraints on the Origin of the Late Archean Skjoldungen Alkaline Igneous Province, SE Greenland , 1995 .

[6]  A. Hofmann,et al.  The heterogeneous Iceland plume: Nd‐Sr‐O isotopes and trace element constraints , 1993 .

[7]  H. Austrheim,et al.  Geochronology of Archaean and Proterozoic events in the Ammassalik area, South-East Greenland, and comparisons with the Lewisian of Scotland and the Nagssugtoqidian of West Greenland , 1993 .

[8]  L. Larsen,et al.  A review of the 2500 Ma span of alkaline-ultramafic, potassic and carbonatitic magmatism in West Greenland , 1992 .

[9]  F. Kalsbeek,et al.  Discrepancies between neodymium, lead and strontium model ages from the Precambrian of southern East Greenland: Evidence for a Proterozoic granulite-facies event affecting Archaean gneisses , 1992 .

[10]  K. Grönvold,et al.  Dynamic melting of the Iceland plume , 1991, Nature.

[11]  T. Furman,et al.  Chemical constraints on the petrogenesis of mildly alkaline lavas from Vestmannaeyjar, Iceland: the Eldfell (1973) and Surtsey (1963–1967) eruptions , 1991 .

[12]  D. Nelson.  Isotopic characteristics and petrogenesis of the lamproites and kimberlites of central west Greenland , 1989 .

[13]  P. Holm.  Nd, Sr and Pb isotope geochemistry of the Lower Lavas, E Greenland Tertiary Igneous Province , 1988, Geological Society, London, Special Publications.

[14]  E. Ito,et al.  The O, Sr, Nd and Pb isotope geochemistry of MORB , 1987 .

[15]  R. K. O’nions,et al.  The Lead, Neodymium and Strontium Isotopic Structure of Ocean Ridge Basalts , 1982 .

[16]  I. Gibson,et al.  Elemental fingerprints of isotopic contamination of hebridean Palaeocene mantle-derived magmas by archaean sial , 1982 .

[17]  A. Dickin.  Isotope Geochemistry of Tertiary Igneous Rocks from the Isle of Skye, N.W. Scotland , 1981 .

[18]  D. DePaolo.  Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization , 1981 .

[19]  B. Weaver,et al.  Rare earth geochemistry of Lewisian granulite-facies gneisses, northwest Scotland: Implications for the petrogenesis of the Archaean lower continental crust , 1980 .

[20]  David A. Wood,et al.  The application of a ThHfTa diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province , 1980 .

[21]  P. Hamilton,et al.  Neodymium and Strontium Isotope Evidence for Crustal Contamination of Continental Volcanics , 1978, Science.

[22]  W. Leeman,et al.  207Pb/206Pb whole-rock age of gneisses from the Kangerdlugssuaq area, eastern Greenland , 1976, Nature.

[23]  J. Schilling,et al.  Mantle Plume Mixing Along the Reykjanes Ridge Axis: Lead Isotopic Evidence , 1975, Science.

[24]  R. Pankhurst,et al.  The evolution of early precambrian crustal rocks at Isua, West Greenland — Geochemical and isotopic evidence , 1975 .

[25]  B. Jahn,et al.  Lead and strontium isotopes in post-glacial basalts from Iceland , 1975, Nature.