Petrogenesis of Archaean Malene supracrustal rocks, NW Buksefjorden region, West Greenland: geochemical evidence for highly evolved Archaean crust

Abstract

Abstract We report new data for major, minor and trace elements in forty-nine samples of Malene supracrustal rocks, NW Buksefjorden region, West Greenland. The samples consist principally of quartz±cordierite±biotite±garnet±anthophyllite/gedrite±sillimanite, with abundant accessory zircon, and minor rutile, allanite and monazite; plagioclase is rare to absent, while muscovite occurs in only a few cases. Thus, the Malene paragneisses closely resemble cordierite-orthoamphibole rocks found in other metamorphic complexes. The NW Buksefjorden Malene paragneisses have, on average, high SiO2, moderate Al2O3, Fe2O3(T) and MgO, and low K2O, Na2O and (especially) CaO. Distinctive trace element characteristics include: (1) low Sc, Co, Cr, Ni and Sr; (2) high Y, Zr, Nb, Hf, Ta, Th, U and REE, with prominent negative Eu-anomalies in nearly every case; and (3) high Ga (up to 65 ppm), with Ga Al ratios significantly higher than average crustal material. On the basis of AFM assemblage type, the paragneisses can be divided into distinct groups which preserve the field distinctions noted by previous workers. However, these groupings primarily reflect variations in mg (total range from 0.26–0.89), with no meaningful distinctions appearing among trace elements except possibly Cu. The trace element characteristics of the NW Buksefjorden Malene paragneisses are identical to those of the Malene quartz-cordierite gneisses found throughout the greater Godthabsfjord region. These gneisses are different from the Malene quartzite-psammite-pelite group of clastic metasediments. The trace element data of the quartz-cordierite gneisses are compatible with a protolith dominated by material derived from felsic igneous precursors, an interpretation supported by immobile element discriminant plots and ternary La-Th-Sc and Th-Co-Hf plots. However, negative Eu-anomalies and low Sr and Ca suggest that plagioclase was lost from the protolith or from the precursor lithology (or both). The high concentrations of Ga indicate either that the source rocks for the paragneisses were unusually Ga-rich, or that assembly of the protolith was accompanied by a major fractionation of Al from Ga. We propose that the protolith for most of the NW Buksefjorden Malene paragneisses was a quartz-chlorite-sericite schist derived from felsic volcaniclastic sediment hydrothermally altered by seawater prior to metamorphism. This alteration resulted in gain of Mg (and possibly Fe) and loss of lime and alkalis (and possibly Sr and Eu). This hypothesis contrasts with earlier proposals that the protolith included Mg-rich clay (palygorskite) that precipitated directly from seawater, but it is more consistent with the new data. There is a remarkably close correspondence between the element enrichments (and depletions) shown by the Malene paragneisses and those found in A-type granitic rocks and their volcanic equivalents. Consequently, we suggest a comparable source for the paragneisses. The petrogenesis of A-type granites is associated typically with thickened, stable continental crust. This implies that the protolith for the Malene paragneisses formed after, or in conjunction with, a major event of Archaean crustal thickening.