Cordilleran Section - 116th Annual Meeting - 2020

Paper No. 25-9
Presentation Time: 4:50 PM


FRED, Riikka Maria, Department of Geosciences and Geography, University of Helsinki, Gustaff Hällströmin katu 2 (Physicum), Helsinki, 00014, Finland, HEINONEN, Aku, Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, Helsinki, FIN-00014, Finland and HEINONEN, Jussi S., Geology and Geophysics Research Programme (GeoHel), Department of Geosciences and Geography, University of Helsinki, P.O. Box 64 (Gustaf Hällströmin katu 2), Helsinki, FI-00014, Finland

The monzodioritic (ferrodioritic) rocks observed in many massif-type anorthosite suites are presumed to represent residual melt compositions left after the formation of anorthositic cumulates. This has also been suggested for the monzodiorites in the 1.64 Ga Ahvenisto anorthosite complex, SE Finland. We present field, petrographic, and geochemical evidence of the monzodioritic rocks and olivine-bearing cumulates of the Ahvenisto complex complemented with crystallization modeling (rhyolite-MELTS, MAGFRAC) to test this hypothesis.

The Ahvenisto monzodioritic rocks are fine- to medium-grained and composed of plagioclase with olivine and pyroxene or pyroxene and amphibole as major phases. The whole-rock compositions seem to form a liquid line of descent (LLD) against decreasing Mg numbers (52–26) supporting melt composition hypothesis. On the other hand, olivine and pyroxene are not in Fe-Mg equilibrium with the whole-rock compositions, but show compositions (Fool 45–25, Enopx 48–63, Mg#cpx 60–69) indicative of equilibration with more evolved melts.

MELTS modeling and MAGFRAC calculations of the Ahvenisto monzodioritic and complementary cumulate rocks suggest that the observed LLD is produced by fractional crystallization at low pressures. On the contrary, the Mg-deficient olivine and pyroxene compositions can be explained by equilibrium crystallization of individual melt batches, where crystals re-equilibrate with residual melt compositions: the observed mineral compositions are in the range of those produced in MELTS equilibrium crystallization models. Consequently, we conclude that the low pressure evolution of the Ahvenisto monzodioritic rocks results from two controlling processes: 1) fractional crystallization of cumulates in shallow magma chambers producing the observed LLD recorded by the whole-rock monzodioritic compositions followed by 2) equilibrium crystallization of monzodioritic residual melt batches recorded by the Mg-deficient mineral compositions.