Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton |
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Authors: | TAPPE, SEBASTIAN FOLEY, STEPHEN F. JENNER, GEORGE A. HEAMAN, LARRY M. KJARSGAARD, BRUCE A. ROMER, ROLF L. STRACKE, ANDREAS JOYCE, NANCY HOEFS, JOCHEN |
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Affiliation: | 1 MAX-PLANCK-INSTITUT FÜR CHEMIE POSTFACH 3060, 55020 MAINZ, GERMANY 2 INSTITUT FÜR GEOWISSENSCHAFTEN, UNIVERSITÄT MAINZ BECHERWEG 21, 55099 MAINZ, GERMANY 3 DEPARTMENT OF EARTH SCIENCES, MEMORIAL UNIVERSITY ST. JOHN'S, NEWFOUNDLAND, CANADA A1B 3X5 4 DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF ALBERTA EDMONTON, ALBERTA, CANADA T6G 2E3 5 GEOLOGICAL SURVEY OF CANADA OTTAWA, ONTARIO, CANADA K1A 0E8 6 GEOFORSCHUNGSZENTRUM POTSDAM TELEGRAFENBERG, 14473 POTSDAM, GERMANY 7 GEOCHEMISCHES INSTITUT, UNIVERSITÄT GÖTTINGEN GOLDSCHMIDTSTRASSE 1, 37077 GÖTTINGEN, GERMANY |
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Abstract: | Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withUPb perovskite emplacement ages of 590555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyrecarbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but CrMg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping 87Sr/86Sri ratios (0·703690·70662)and show a narrow range in initial Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (1520wt %) and Ni (200574 ppm) content; the abundant groundmasscarbonate has 13CPDB between 5·7 and 5,similar to primary mantle-derived carbonates, and 18OSMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 47 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (13CPDB 3·7 to 2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, SrNd isotopes; UPb geochronology |
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Keywords: | : liquid immiscibility mantle-derived magmas metasomatism, Sr Nd isotopes U Pb geochronology |
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