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Depleted basaltic lavas from the proto-Iceland plume, Central East Greenland

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New geochemical and isotopic data are presented for volumetrically minor, depleted low-Ti basalts that occur in the Plateau Basalt succession of central East Greenland (CEG), formed during the initial stages of opening of the North Atlantic at 55 Ma. The basalts have MORB-like geochemistry (e.g. depleted LREE) and are distinct from the high-Ti lavas that dominate the sequence. REE geochemistry implies derivation from a source more depleted than the typical MORB source, and suggests polybaric melting and contributions from both spinel and garnet facies mantle. The low-Ti basalts have Sr-Nd-Pb-Hf isotopic characteristics that are similar to those of depleted magmas from Iceland (e.g. Theistareykir) and adjacent ridges (Kolbeinsey and Reykjanes) and distinct from global MORB (e.g. negative ¿207Pb, and Hf and Nd isotope compositions that plot above the mantle reference line). Isotope and trace element data indicate the involvement of two depleted source components. One component has isotopic compositions similar to other depleted components identified in the North Atlantic and has high Rb/Zr and Ba/Nb. The second is isotopically less depleted with lower Rb/Zr and Ba/Nb. Small degrees of crustal contamination (<1%) by both amphibolitic and granulitic crust result in relatively large changes in isotopic composition (ca. 1% lower for 206Pb/204Pb and 0.1% higher for 87Sr/86Sr depending on the contaminant). Negative ¿Nb suggests a MORB affinity for the low-Ti magmas, however they are distinguished from global N-MORB on the basis of vertical deviations from the Northern Hemisphere Reference Line (negative ¿207Pb and positive ¿208Pb), and relative enrichments in Ba, Sr and Pb. Isotopic compositions of the low-Ti CEG basalts suggest correlation with modern depleted components on Iceland and adjacent ridges considered to be derived from upper mantle sources polluted by the Iceland plume. However, small positive Pb peaks when normalised to MORB, and lower Nb distinguish the CEG low-Ti basalts from depleted Icelandic compositions. The lower ¿Nb (<0) and 87Sr/86Sr, and suggestion of higher 206Pb/204Pb in crustally uncontaminated parental melts implies a closer affinity to compositions from the oceanic ridges surrounding Iceland (especially Reykjanes), yet they are subtly distinct on the basis of available trace element data. We suggest that this depleted component was an integral part of the plume that melted primarily during the rapid lithospheric uplift and extension associated with continental break-up.
OriginalsprogEngelsk
TidsskriftJournal of Petrology
Vol/bind53
Udgave nummer8
Sider (fra-til)1569-1596
Antal sider28
ISSN0022-3530
DOI
StatusUdgivet - 2012

ID: 37990868