On the development of internal chemical zonation in small mafic dykes

Three dolerite dykes (Small, 3.2 cm; Middle, 13.5 cm; Thick, 50 cm) from Torsholma Island, SW Finland, reveal distinctly different internal zonation that becomes more complicated with increasing thickness of the dykes. The Small Dyke shows a systematic inward increase in normative Pl (An+Ab+Or) and a decrease in normative An (100*An/(An+Ab)), whole-rock MgO, Mg number (100*Mg/(Mg+Fe-total)), TiO2, K2O and Zr. The Middle Dyke exhibits the same compositional pattern at the margins, while the centre is distinguished by an abrupt increase in normative PI and An, whole-rock Sr and Mg number. From the margins inwards, the Thick Dyke displays first a compositional pattern identical to that observed in the Small Dyke and the margins of the Middle Dyke. This is followed by a region where whole-rock MgO, Mg number and normative An start increasing inwards, while the transition to the centre of the dyke is characterized by a compositional pattern similar to that in the centre of Middle Dyke. The origin of chemical zonation in these dykes is attributed to the operation of three independent physico-chemical processes, namely: the Small Dyke formed exclusively by progressive changes in the composition of inflowing magma; the Middle Dyke by changes in composition of inflowing magma (margins) and concentration of plagioclase and olivine phenocrysts by flow differentiation (centre); the Thick Dyke by changes in composition of inflowing magma (margins), in situ cumulate growth against dyke sidewalls (middle) and flow differentiation (centre). Systematic changes in these processes and, as a result, in internal chemical zonation, likely take place in response to crystallization of magma under less Supercooled conditions with increasing dyke thickness. A comprehensive geochemical study of the internal zonation of small mafic dykes worldwide is required to develop a complete understanding of the processes operating in mafic dykes.