A Geochemical Overview of Mid-Archaean Metavolcanic Rocks from Southwest Greenland

Abstract

Abstract:The present contribution reviews bulk-rock geochemical data for mid-Archaean (ca. 3075-2840 Ma) metavolcanic rocks from the North Atlantic Craton of southwest Greenland. The data set includes the most recent high quality major and trace element geochemical analyses for ten different supracrustal/greenstone belts in the region. When distilling the data set to only include the least altered metavolcanic rocks, by filtering out obviously altered samples, mafic/ultramafic cumulate rocks, late-stage intrusive sheets (dolerites) and migmatites, the remaining data (N = 427) reveal two fundamentally distinct geochemical suites. The contrasting trends that emerge from the filtered geochemical data set, which best represents the melt compositions for these mid-Archaean metavolcanic rocks are: (1) tholeiitic (mainly basaltic) versus (2) calc-alkaline (mainly andesitic). These two rock suites are effectively separated by their La/Sm ratios (below or above three, respectively). It is demonstrated by geochemical modelling that the two contrasting suites cannot be related by either fractional crystallization or crustal assimilation processes, despite occurring within the same metavolcanic sequences. The tholeiitic basaltic rocks were directly mantle-derived, whereas the petrogenesis of the calc-alkaline andesitic rocks involve a significant (>50%) felsic component. The felsic contribution in the calc-alkaline suite could either represent slab-melt metasomatism of their mantle source, mafic-felsic magma mixing, or very large degrees of partial melting of mafic lower crust. At face value, the occurrence of andesites, and the negative Nb-Ta-Ti-anomalies of both suites, is consistent with a subduction zone setting for the origin of these metavolcanic rocks. However, the latter geochemical feature is inherent to processes involving crustal partial melts, and therefore independent lines of evidence are needed to substantiate the hypothesis that plate tectonic processes were already operating by the mid-Archaean.

Archaean Plate Tectonics in the North Atlantic Craton of West Greenland Revealed by Well-Exposed Horizontal Crustal Tectonics, Island Arcs and Tonalite-Trondhjemite-Granodiorite Complexes

“…A Nb/Yb vs. Th/Yb diagram ( Figure 2) of the metabasaltic rocks from each of the Meso-to Neoarchaean supracrustal belts in the West Greenland NAC ( Table 1) illustrates potential relationships between their basaltic source magmas and Archaean continental crust as explained in the figure caption. See also the generalised Figure 10 in Szilas (2018). The element ratios straddle the present-day MORB-OIB array, forming diagonal trends that extend toward an approximate Archaean continental crustal composition.…”

Secular oxidation of Archean upper mantle caused by increasing crustal recycling

“…Geochemical modelling indicates that the compositional diversity of Archean basalts was most likely related to the introduction of oceanic crust-and sediment-derived melts with high fO2, which are features related to the recycling of crustal materials into the mantle on the modern Earth 12,17,36 (Fig. 3d-f; Supplementary Fig.…”

The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 1. Constraints on theP–Tevolution from igneous, metasomatic and metamorphic amphiboles

“…Nutman et al , 2015). Most of the crust in the NAC consists of TTG continental crust, but supracrustal belts including amphibolite-facies pelite and meta-basaltic to meta-andesitic amphibolites are also an important component of the region (Szilas, 2018).…”

“…Polat et al, 2012). Similarly, the andesitic supracrustal rocks of calc-alkaline affinity could be associated with arc magmatism (Garde, 1997(Garde, , 2007Klausen et al, 2017;Szilas, 2018;Szilas et al, 2012aSzilas et al, , 2012bSzilas et al, , 2016aSzilas et al, , 2017. Conversely, the ultramaficmafic norite-associated complexes of the Fiskefjord region, Akia terrane (Garde, 1997;Szilas et al, 2015), appear to have fractionated from a relatively dry melt formed by high degrees of partial melting pointing to a mantle plume origin (Szilas et al, 2018).…”