The Skien lavas, Oslo Rift: petrological disequilibrium and geochemical evolution

The Skien lavas, which form the earliest phase of basaltic magmatism within the Permo-Carboniferous Oslo Rift, contain multiple generations of clinopy roxene which exhibit strong petrological and geochemical disequilibrium. Th ree principal core compositions have been identified: (1) low-jadeite, high -Mg, Cr-diopside cores (CrMgDi) with strongly depleted trace-element signat ures, which are believed to be xenocrystic in origin; (2) Mg-rich, Cr-poor diopside cores (MgDi) with moderately depleted trace-element signatures whi ch probably represent early cognate growth; and (3) more dominant, low-Mg, phenocrystic diopside cores (Phen-Di). Several samples contain CrMgDi or Mg Di cores which have been subjected to resorption and partial reequilibratio n with their host melts, indicative of extensive disequilibrium and magma m ixing. These three core types are overgrown by trace-element-enriched Ti-au gite, which also forms megacrysts and late-stage lava groundmass. Calculate d Ti-augite/melt partition coefficients show clinopyroxene compatibility of the M-HREE, Zr, Hf and Y. The LILE, Sr, and Nb remain incompatible. epsilo n Sr-300 and epsilon Nd-300 Of Ti-augite overgrowths, phenocrystic diopside , and MgDi diopside cores show that intrasample isotopic disequilibrium exi sted when the host basalts were erupted. All epsilon values lie within the range of data previously published for the Skien lavas. Detailed examinatio n of the chemical, isotopic and textural disequilibrium features seen in th ese lavas has enabled us to place constraints upon the magmatic evolution o f this basalt suite, ranging from xenocryst incorporation to cognate multis tage pyroxene growth, as well as identifying clear evidence of magma mixing and possible crustal contamination.