Early-Middle Jurassic Mafic Dykes from the H.U. Sverdrupfjella, Antarctica

Abstract

The Early to Middle Jurassic break-up of Gondwana produced large-volume magmatic events, resulting in large igneous provinces (LIPs) such as the Karoo LIP in southern Africa and Ferrar LIP in East Antarctica. The Early-Middle Jurassic mafic dykes from Sverdrupfjella located in western Dronning Maud Land (WDML), Antarctica, are regarded as part of the Karoo LIP [1]. These dykes intrude both the Grunehogna Province (an Archean basement fragment in WDML thought to have been a pre-breakup constituent of the Kalahari Craton) and the Maud Province (broadly co-eval with the Mesoproterozoic Namaqua-Natal metamorphic province in southern Africa). The dykes intruding the Grunehogna Province are considered on-craton, whereas those intruding the Maud Province are considered off-craton. The geochemistry and geochronology of these dykes and basalts found in the Grunehogna Province and the Maud Province (Vestfjella, Heimefrontfjella and Kirwanveggen) have been studied by previous workers [e.g., 1) and have been categorized into two groups: low-Ti (TiO2 <2.5%) and high-Ti (TiO >2.5%) groups. Based on 40Ar/39Ar age of mafic dykes intruding the Grunehogna Province (on-craton), dyke emplacement occurred at ~178 Ma and ~190 Ma [1]. The Vestfjella basalts (off-craton) have K-Ar ages between 170-230Ma, and plagioclase KlecturerAr ages at ~180 Ma [2]. The Kirwanvergen basalts (off-craton) yielded a K-Ar age of 172 ± 10 Ma [3]. Mafic dykes in Sverdrupfjella that intrude Early Jurassic alkaline intrusive bodies (Straumsvola, Tvora and Jutulrora) show two 40Ar/39Ar age peaks: one at 178-175 Ma (Straumsvola) [4] and another at 206-204 Ma (Jutulrora) [5]. These dykes from the H.U. Sverdrupfjella are characterized by low TiO2 and Zr contents. The dykes from Sverdrupfjella (off-craton) strike dominantly NNE-SSW, with dip angles ranging from 60° to 90°. The strike trends are similar to equivalent dykes from the on-craton region of WDML (Grunehogna Province, Almannryggen area). Samples collected from the Sverdrupfjella are fine to medium grained; the groundmass consists of plagioclase, augite and minor amounts of magnetite and ilmenite. Phenocrysts consist of plagioclase, olivine (with inclusions of Cr-spinel) and augite, and pseudomorphs of euhedral olivine and augite.Unaltered dyke samples will be used for age determination using 40Ar/39Ar data. Rb-Sr and Sm-Nd isotopic systematics will be used to evaluate mantle sources and crustal contamination. Initial 87Sr/86Sr ratios and Nd epsilon values of unaltered samples will be used to test models of derivation and petrogenesis. Isotopic ratios of 87Sr/86Sr ranging from 0.703361 to 0.711183, and 143Nd/144Nd180 epsilon values from 1 to -13 [6] would support derivation from heterogeneous mantle sources with melts affected by crustal contamination. Geochemical and geochronological data for the Early-Middle Jurassic dykes from Sverdrupfjella will be used to determine whether there is any overlap with similar data from the Karoo and Ferrar LIPs.