Petrography and Mineral Chemistry of the Sälskär Breccia (Alnö Alkaline Complex, Central Sweden)

Abstract: Primitive magmas can provide important details about the mantle from which they are derived. In this thesis, petrographic observations and new mineral chemistry analyses of the Sälskär breccia from the Alnö alkaline complex, central Sweden, are reported. The rock is a volcanic breccia in which three main constituents were identified: spheroidal melilititic lapilli, angular sövite fragments and a carbonatedominated matrix. The mineralogy of the juvenile lapilli (i.e., with olivine, phlogopite, diopside, titanomagnetite, Cr-spinel, monticellite, perovskite, apatite and calcite) are consistent with what is expected to form by small degrees of partial melting of a volatile-rich, metasomatized, mantle source (indicated by the presence of phlogopite and primary calcite). The melilititic lapilli are frequently cored by a large crystal of olivine or phlogopite with concentrically aligned lath-shaped pseudomorphs after melilitite. During ascent the melilititic magma entrained mantle debris and traveled rapidly through the crust without significant fractionation.  The pyroclast textures indicate an explosive eruption, similar to those that produce pelletal lapilli in kimberlite-type volcanism. The sövite fragments and carbonate-dominated matrix were explosively fragmented during the eruption, which also dispersed the melilite mineralogy throughout the matrix. During the eruption a diatreme formed of within the uppermost 1.5 kilometers of the crust. A mismatch between the model of diatreme formation and a previous erosional model of the area leads to the conclusion that the Sälskär diatreme is either: (i) very deep, or (ii) that the diatreme volcanism took place significantly later than the rest of the complex