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Clinopyroxene and amphibole crystal populations in a lamprophyre sill from the Catalonian Coastal Ranges (NE Spain): A record of magma history and a window to mineral-melt partitioning

Abstract:

A Cretaceous sill of alkaline lamprophyre (camptonite) cropping out in the northern Catalonian Coastal Ranges includes macrocrysts and microcrysts of clinopyroxene and amphibole with complex zoning patterns. The in-depth petrographic, compositional and barometric study of the different crystal populations, including statistical evaluation of the compositional dataset by principal component analysis, provides insights into the magmatic plumbing system. Macrocryst cores are inherited antecrysts crystallised in a deep (500-800. MPa) magma chamber or conduit. Their reverse zoning patterns reveal repeated injection and mixing of batches of a more primitive and water-rich magma with the resident magma in the magma chamber. Macrocryst rims and microcrysts, in contrast, crystallised during the ascent and shallow emplacement of the magma below 50. MPa pressure. They define normal zoning patterns that can be related to progressive fractionation of the magma. This study shows that an apparently simple porphyritic rock can actually contain a significant amount of recycled crystals, reflecting an open-system magmatic behaviour. Only the groundmass of the sill represents a closed-system, where microphenocryst cores are in chemical equilibrium with the bulk composition of the groundmass. Using trace element compositions obtained by LA-ICP-MS and geochemical modelling, a complete and consistent partition coefficient dataset is obtained for the first time for clinopyroxene and amphibole in camptonite melts. Both minerals show a similar partitioning behaviour for most trace elements. Clinopyroxene-melt and amphibole-melt partition coefficients are overall consistent with data from basic rocks, although clinopyroxene partition coefficients are generally higher than data from basalt and basanite melts. The results from this research support the use of naturally occurring mineral-melt equilibrium pairs for partitioning studies but emphasise the need for a detailed study of the phases used to define the partition relationships. © 2013 Elsevier B.V.