TY - JOUR

T1 - Combined bulk-rock Hf- and Nd-isotope compositions of Mesoarchaean metavolcanic rocks from the Ivisaartoq Supracrustal Belt, SW Greenland

T2 - Deviations from the mantle array caused by crustal recycling

AU - Szilas, Kristoffer

AU - Hoffmann, J. Elis

AU - Schulz, Toni

AU - Hansmeier, Christina

AU - Polat, Ali

AU - Viehmann, Sebastian

AU - Kasper, Haino Uwe

AU - Münker, Carsten

N1 - Funding information: We thank R. Hoffbauer for help with the XRF-analysis. Dieter Garbe-Schönberg and his team are acknowledged for quadrupole ICP-MS analyses in Kiel and Cologne. The Geological Survey of Denmark and Greenland (GEUS) is thanked for permission to publish this work. JEH and CM acknowledge the Deutsche Forschungsgemeinschaft for grants MU1406/8 and HO4796/1-2. We greatly appreciate the editorial handling and review comments by Dewashish Upadhyay, and acknowledge the constructive critique by an anonymous reviewer, which significantly improved the manuscript. K. Szilas would like to thank Christopher Kirkland for constructive discussions in the field, which helped clarify the implications of the isotope data for the Ivisaartoq rocks.

PY - 2016/12/1

Y1 - 2016/12/1

N2 - Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The εHft calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding εNdt values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon.

AB - Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The εHft calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding εNdt values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon.

KW - Godthåbsfjord

KW - Greenstone belt

KW - Metavolcanic rocks

KW - Nuuk region

KW - Ujarassuit

UR - http://www.scopus.com/inward/record.url?scp=84994462695&partnerID=8YFLogxK

U2 - 10.1016/j.chemer.2016.09.004

DO - 10.1016/j.chemer.2016.09.004

M3 - Article

AN - SCOPUS:84994462695

VL - 76

SP - 543

EP - 554

JO - Chemie der Erde

JF - Chemie der Erde

SN - 0009-2819

IS - 4

ER -