Anomalous internal zonation of small mafic dykes of the Åland-Åboland dyke swarm, SW Finland: Indication of an unknown process of magma differentiation?

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Sofya, Chistyakova; Latypov, Rais
Holding Date 21 September 2008

Detailed cm-scale sampling across two diabase small dykes (7 and 21 cm thick) of the Åland-Åboland dyke swarm has revealed a well-developed internal zonation, with surprisingly systematic compositional variations. From the margins inwards the dykes exhibit a steady decrease in whole-rock MgO, Mg# (100*Mg/(Mg+Fetotal)), normative Opx and pyroxene ratio Opx# (100*Opx/(Opx+Cpx)) and increase in CaO and normative Cpx (indicating a normal fractionation trend) with simultaneous increase in normative An (100*An/(An+Ab)) and decrease in incompatible Zr, Y, TiO2, and P2O5 (indicating a reverse fractionation trend). In addition marginal rocks of dykes contain normative corundum that is apparently associated with their significant depletion in whole-rock CaO.
The extent of margin-to-centre differentiation of dykes in terms of some petrochemical parameters is exceptionally high (e.g. 15 mol.% of normative An) and is comparable with that observed in large layered intrusions several hundreds of meters to several kilometres thick. The dykes are almost glassy and uncontaminated by host rocks, suggesting that their compositional profiles are primary and most likely reflect temporal changes in composition of magma filling the dykes.
A mechanism responsible for the systematic changes in composition of inflowing magma remains, however, elusive since no known processes are able to force magma to evolve simultaneously along both normal and reverse fractionation trends. The study appears thus to indicate some effective, but not yet specified process of magma differentiation.