Volume characteristics and element transfer of fenite aureoles: a case study from the Iivaara alkaline complex, Finland

Fenitization in granodioritic rocks has been investigated taking the Iivaar a alkaline complex (Finland) as a case study. Five main fenitization reacti ons responsible for the transition from a granodioritic precursor mineral a ssemblage to a syenitic or even cancrinite-syenitic mineral assemblage of t he fenites are set up. An assessment of element transfer and volume change associated with each of these reactions indicates a general volume loss of about 20% due to fenitization. High activities of Na, Ca, Mg and Fe as well as a low H2O/CO2 ratio in the fenitizing fluid phase are indicated. Si, wh ich is the main component released from the precursor rock, was redistribut ed within the aureole and formed pyroxene dominated mineral assemblages in a reaction with the fenitizing fluid. These vein-type pyroxenites can be fo und cross-cutting all fenite types. Compositional data of aegirine augite i n the fenites reveal disequilibria and feldspar compositions point to minim um temperatures for the production of fenites with syenitic compositions of 660 degrees C and also indicate fast cooling. A Sr isotope characterizatio n of the fenites allows assessment of minimum fluid-rock weight-ratios in t he fenite aureole of 0.023-2.3. The element transfer scheme developed in th is study is applicable to other localities since many aureoles formed in,gr anitic to granodioritic host rocks have petrographic characteristics simila r to the Iivaara fenites. (C) 2000 Elsevier Science B.V. All rights reserve d.