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Fine-scale chemical zonation in small mafic dykes, Kestiö Island, SW Finland

Published online by Cambridge University Press:  12 November 2008

SOFYA CHISTYAKOVA  and
RAIS LATYPOV
SOFYA CHISTYAKOVA*
Affiliation:
Department of Geosciences, University of Oulu, Oulu, Finland
RAIS LATYPOV
Affiliation:
Department of Geosciences, University of Oulu, Oulu, Finland Geological Institute, Kola Science Centre, Apatity, Russia
*
†Author for correspondence: sofya.chistyakova@oulu.fi
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Abstract

Detailed centimetre-scale sampling across two small dolerite dykes (7 and 21 cm wide) of Kestiö Island, SW Finland, 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 number (100Mg/(Mg+Fetotal)) and normative Opx (indicating a normal fractionation trend) with simultaneous increase in normative An (100An/(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 most components is slight to modest, although in some petrochemical parameters it is quite high (e.g. 15 mol. % of normative An). 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 elusive, however, since no known processes are able to force magma to evolve simultaneously along both normal and reverse fractionation trends. The study thus appears to indicate some not yet specified process of magma differentiation.

Keywords

mafic dykesinternal zonationreverse and normal differentiation trendsunknown process of magma differentiation
Type
Original Article
Information
Geological Magazine , Volume 146 , Issue 4 , July 2009 , pp. 485 - 496
Copyright
Copyright © Cambridge University Press 2008

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