Plume Magmatism at Franz Josef Land

The morphological types of geological sections, newly acquired 40 Ar/ 39 Ar isotope dates, and geochemical and mineralogical data on Jurassic–Cretaceous basaltic rocks at Franz Josef Land indicate that plume magmatism evolved in this archipelago in two episodes. The Jurassic episode (at 192 ± 3 Ma) was marked by eruptions only of low-K tholeiites. The lava flows of this stage are characterized by combination of textures of two types in their flows: colonnade large-prismatic (in the bottom portions of the flows) and randomly oriented faned small-prismatic parting (in the tops of the flows). During the Late Jurassic–Cretaceous episode (at 157 ± 3 Ma, 132 ± 1 Ma, and 115 ± 1 Ma), low-K tholeiites were erupted together with subalkaline tholeiites, which compose flows, most of the sills, and all the dykes we studied in the archipelago. The lava flows of this episode typically have only columnar and block parting structures. The episodes of magmatic activity well correlate with episodes of continental sedimentation on the archipelago and are separated from one another with an episode of marine transgression in the Middle Jurassic. The volcanics of the low-K series typically bear relatively low concentrations of TiO 2 (≤2 wt %) and MgO (mostly 5–6 wt %), low K 2 O concentrations (≤0.3 wt %), and weakly fractionated REE patterns (La n /Yb n = 2.11–2.30). The rocks of the subalkaline series are richer in K 2 O (0.7–1.2 wt %), TiO 2 , HFSE, and LILE and have fractionated REE patterns (La n /Yb n = 3.85–4.45). The mineralogical composition of the rocks of both series is practically exactly similar. The rocks are dominated by pyroxene and plagioclase and contain subordinate amounts of olivine, titanomagnetite, and ilmenite. The pyroxenes are augite ( X Ca = 0.32–0.42), and the low-K tholeiites contain two pyroxenes: augite and subcalcic augite ( X Ca = 0.12–0.25). The Early Cretaceous low-K tholeiites contain PGE minerals (of the Au–Cu–Pd type): cuproauride Au(Cu, Pd), auricuprite Au(Cu, Pd) 3 , and compounds close to skaergaardite PdCu and nilsenite PdCu 3 . It is interesting that the volcanic rocks of both Early Cretaceous series contain magmatic siderite.