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 BREAKUP1J.G. Fitton,2 B.S. Hardarson,2 R.M. Ellam,3 and G. Rogers3 |
ABSTRACTThe 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 the 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 208Pb/204Pb. 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. |
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