Palaeoproterozoic (1740 Ma) rift-related volcanism in the Hekla Sund region, eastern North Greenland: field occurrence, geochemistry and tectonic setting
Abstract Two Palaeoproterozoic volcanic successions, the Hekla Sund (HS) Formation and the Aage Berthelsen Gletscher (AaB) Gletscher Formation, occur within the Caledonian orogen of eastern North Greenland. They consist mainly of mafic pillow lavas, deformed and metamorphosed under greenschist-facies conditions during the Caledonian orogeny. Zircons from a rhyolitic ignimbrite in the HS Formation have yielded an age of 1740±6 Ma. In both formations the volcanic rocks are intercalated with immature sandstones and conglomerates that accumulated in the vicinity of active fault scarps; a shallow marine, rifted basin is implied. Relative concentrations of the more immobile minor and trace elements (Ti, Zr, Nb, Y and rare-earth elements) in both rock suites were unaffected by the metamorphism. Fractional crystallisation of olivine, clinopyroxene and, probably, plagioclase, as well as assimilation of crustal rocks was involved in the petrogenesis of the HS Formation. The AaB basalts have higher Mg, Ni and Cr and lower concentrations of incompatible elements than the rocks of the HS Formation, and they could be regarded as more primitive products of the same magmatic event. However, marked differences in incompatible trace element ratios in the two suites are unlikely to reflect either differences in fractionation histories or variable contamination, and suggest compositional differences in the mantle source rocks. Basalts from the two formations have distinct e Nd values (−4.6 and −4.8 for the HS Formation, −5.9–−5.6 for the AaB Gletscher Formation), which is consistent with this interpretation. The volcanic rocks at HS and AaB Gletscher were erupted shortly after a long period of orogenic activity between 2000 and 1750 Ma ago. Following post-orogenic emplacement of granites at ca. 1750–1740 Ma, uplift and erosion took place, and accumulation of extensive immature sediments occurred simultaneously with formation of the volcanic rocks described in this paper. The magmatism that gave rise to the two formations may have been caused by melting during lithospheric extension.