Kristoffer Szilas

Eoarchean within-plate basalts from southwest Greenland: COMMENT

[Jenner et al. (2013)][1] reported the occurrence of, what they interpret as, Earth’s oldest ocean island basalts (OIBs) on the island of Innersuartuut, southwest Greenland. However, this interpretation hinges critically on the incompatible trace element contents of the presented rocks. Compared

Hafnium isotope constraints on the origin of Mesoarchaean andesites in southern West Greenland, North Atlantic Craton

Abstract Numerous supracrustal belts in southern West Greenland host leucoamphibolites, which commonly preserve volcaniclastic textures, and are interpreted as meta-andesites. Such rocks are associated with mesocratic amphibolites of tholeiitic basaltic compositions, which display pillow-lava structures and, thus, support eruption in an oceanic environment. Here we present bulk-rock Lu–Hf isotope data for meta-andesites from the approximately 3071 Ma Qussuk supracrustal belt. Surprisingly, we find evidence for the involvement of a source with near-chondritic Hf-isotope composition in the meta-andesites, whereas the metabasalts display more depleted compositions, with around +4. Trace element modelling indicates that fractional crystallization in combination with crustal assimilation (AFC) is not capable of producing the geochemical compositions of the meta-andesitic rocks from a basaltic melt. Instead, these meta-andesites point to large degrees (c. 50%) of magma mixing, involving mafic and felsic end members. This may either represent: (1) a magma chamber process; (2) mantle-wedge overprinting by a silicic component; or (3) large degrees of melting of primitive mafic crust. Given that there is abundant independent structural and metamorphic evidence for horizontal tectonics in the Archaean crust of southern West Greenland, it is likely that these calc-alkaline meta-andesites and tholeiitic metabasalts were produced by Mesoarchaean subduction zone volcanism.

Sm-Nd and Lu-Hf isotope and trace-element systematics of Mesoarchaean amphibolites, inner Ameralik fjord, southern West Greenland

Abstract Fragmented supracrustal rocks are typical components of Archaean high-grade gneiss terranes, such as those in the North Atlantic Craton. Here we present the first major, trace element and Nd-Hf isotope data for amphibolites collected in the yet poorly studied southern inner Ameralik fjord region of southern West Greenland. In addition, new U-Pb zircon ages were obtained from the surrounding TTG gneisses. Based on their trace-element patterns, two different groups of amphibolites can be distinguished. Following screening for post-magmatic alteration and outlying e values, a reduced sample set defines a 147Sm/143Nd regression age of 3038 Ma ±310 Ma (MSWD = 9.2) and a 176Lu/176Hf regression age of 2867 ± 160 Ma (MSWD = 5.5). Initial εNd2970Ma values of the least-altered amphibolites range from 0.0 to +5.7 and initial εHf2970Ma range from +0.7 to +10.4, indicating significant isotopic heterogeneity of their mantle sources with involvement of depleted domains as well as crustal sources. Surprisingly, the amphibolites which are apparently most evolved and incompatible element-rich have the most depleted Hf-isotope compositions. This apparent paradox may be explained by the sampling of a local mantle source region with ancient previous melt depletion, which was re-enriched by a fluid component during subduction zone volcanism or alternatively by preferential melting of an ancient pyroxenite component in the mantle source of the enriched rocks.