Single-zircon Pb evaporation geochronology constrains basement-cover relationships in the lower Hecla Hoek Complex of northern Ny Friesland, Svalbard

Abstract

The Hecla Hoek Complex, a ∼ 18-km-thick succession in the Svalbard Caledonian Eastern Terrane, has been interpreted to be a more or less continuous volcano-sedimentary sequence. Amphibolite-facies rocks in the lower parts appear to grade conformably up into almost unmetamorphosed Neoproterozoic and Early Paleozoic strata in the middle and upper parts of the succession. However, recent work in the Ny Friesland transpressional orogen has indicated that the lower part of this Hecla Hoek Complex is composed of at least three thrust sheets. Each thrust sheet is made up of Paleoproterozoic ( ∼ 1750 Ma) granites, locally intruded into older metasedimentary rocks, and overlain by younger sedimentary cover, mainly composed of psammites and quartzites, locally with conglomerates. This isotopic study, focuses on basement-cover relationships in one of the thrust sheets in northern Ny Friesland, where a granitic unit (the Instrumentberget Granitic Gneiss) is overlain by conglomerates and quartzites (Polhem Formation). Ages obtained using Pb evaporation and conventional UPb techniques on both the granitic gneisses and granitic boulders in overlying conglomerates, yield similar ages of ∼ 1740 Ma, suggesting that the latter are derived from the former. Additional Pb evaporation analyses on detrital zircons from two quartzite samples higher up in the Polhem stratigraphy have given information about other source ages. One of these quartzitec samples, from northern Ny Friesland, is dominated by detrital zircon ages of 1870–2040 Ma; however, this sample also confirms the existence of a 1740-Ma source. Another quartzite within the same formation, collected 70 km farther south in Ny Friesland and from a higher stratigraphic level, has provided detrital zircon ages of 1320–2710 Ma; the youngest zircon age in this sample constrains the maximum age for the deposition of the Polhem Formation. These sediments may be as young as Early Paleozoic, but are thought to be Mesoproterozoic, because of the intercalation of abundant amphibolites, which are notable for their absence in the eastern Ny Friesland Neoproterozoic and younger succession. The younger age (< 1740 Ma) of some of the zircons in the Polhem sediments strongly support the recent mapping and structural interpretation that the lower part of the Hecla Hoek Complex is a tectonostratigraphic succession composed of several thrust sheets.