Geochemical and thermodynamic constraints on Archean comagmatic volcanic and cumulate rocks from southern West Greenland

The extensive exposure of the Archean continental crust in southern West Greenland makes it an impor-tant window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjorneoen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The vol-canic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dat-ing of a leucoamphibolite yields an age of 3077 +/- 6 Ma, which is older than regional orthogneisses. A ser-ies of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively pro-duce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a con-nection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid-to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crys-tallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geody-namic settings in a hotter early Earth.(c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).