Evidence of Sedimentation From a Continent From NeoArchean Greenland

The Neoarchaean Storø Supracrustal Belt, Nuuk region, southern West Greenland: An arc-related basin with continent-derived sedimentation

Authors:

Szilas et al

Abstract:

We present new major and trace element data, as well as Sm–Nd isotope compositions for Archaean supracrustal rocks and a gabbro-anorthosite complex from the island of Storø in Godthåbsfjord, southern West Greenland. We also provide new U–Pb isotope data for zircon extracted from these rocks. The Storø rocks have experienced amphibolite facies metamorphism, and the entire sequence forms an east-dipping frontal thrust ramp in tectonic contact with Mesoarchaean (‘Nûk’) gneisses to the west and Eoarchaean (‘Itsaq’) gneisses to the east. These orthogneisses record a complex regional accretion history, as established by previous work. The present study aims at explaining the geochemical features of the Storø Supracrustal Belt (SSB), which comprises metavolcanic and metasedimentary rocks that are situated within this collisional zone. The SSB has a maximum age of ca. 2800 Ma, constrained by the youngest detrital zircon population found in a metasedimentary unit. The minimum age of the SSB is 2707 ± 8 Ma, defined by previously published Re–Os isotope data. The metavolcanic rocks have a tholeiitic basaltic composition, with relatively flat primitive mantle-normalised trace element patterns and generally negative Nb-anomalies (Nb/Nb* 0.30–0.90). They plot above the mantle array in Th/Yb-Nb/Yb space, consistent with a subduction zone affinity, as also proposed by previous studies. A thin fault contact separates the SSB from a gabbro-anorthosite complex (formally defined here as the ‘Storø Anorthosite Complex’), which has an age of ca. 3050 Ma. The Sm–Nd isotope data of the SSB suggest, that these metavolcanic rocks experienced contamination by a crustal source that was isotopically similar to the Storø Anorthosite Complex (SAC). This in turn suggests that the SAC could have formed the basement for the younger volcanic sequence of the SSB or alternatively that the mantle source of the SSB was contaminated by melts derived from SAC-aged crust or sediments. The metasedimentary rocks of SSB show a mixed mafic–felsic component with variable degrees of maturity. Highly mature metasediments of the SSB contain several age populations of regionally well-known magmatic events, and thus support a significant local crustal provenance. Furthermore, the youngest documented detrital zircon is found in a thin metasedimentary unit within the metavolcanic rocks, and thereby shows that the SSB formed in close proximity to subaerially exposed continental crust (i.e. a back-arc environment) and not a distal island arc setting as previously proposed. Finally, this suggests, that relatively cool lower continental crust, which was capable of supporting subaerial mountains, existed at least locally during the Meso- Neoarchaean.