Mg-Fe-Mn Distribution in Amphiboles, Pyroxenes, and Garnets and Implications for Conditions of Metamorphism of High-Grade Early Archaean Iron-Formation, Southern West Greenland

R. P. Hall
Department of Geology, Portsmouth Polytechnic, Burnaby Road, Portsmouth PO1 3QL

Abstract: Remnants of early Archaean (Akilia) iron-formation occur as enclaves within the c. 3700 Ma Amitsoq gneisses in the eastern Godthåb-Sukkertoppen region of southern West Greenland. Amphibole- and pyroxene-bearing assemblages are preserved respectively in these two areas. The amphibole-facies rocks comprise one, two, and three-amphibole-bearing assemblages, coexisting with magnetite and quartz (and locally garnet). Experimentally determined phase equilibria suggest that the amphibole-bearing types have not exceeded c. 700 °C, while distribution coefficients in the two-pyroxene ironformation indicate a maximum temperature of around 625 °C.

Mg-Fe distribution relationships between the phases are:

actinolite >(1.6) cummingtonite >(1.8) hornblende >(6) garnet


Ca-pyroxene >(1.6) orthopyroxene ≃ hornblende >(5) garnet

where the numbers are generalized distribution coefficients KD(Mg)a−b=[XMga∕(1−XMga)]∕[XMgb∕(1−XMgb)] and XMga =Mg∕(Mg+Fe) in phase a). Mn distribution is:

garnet >(1.4) cummingtonite >(3.3) actinolite ≃ hornblende


garnet >(3.5) clinopyroxene >(1.2) orthopyroxene > hornblende

(YMn = 0) where KD(Mn)a−b=YMna∕YMnb and YMn = Mn/(Mn + Mg + Fe). The distribution coefficients are variable both from one rock to another and internally from one set of grain pairs to another. The development of the observed variable silicate phase assemblages is dependent upon a complex interaction of not only metamorphic grade (P and T) and element availability, but more importantly aH2o and fo2. In particular, the possibility of reactions between observed assemblages containing cummingtonite + actinolite, magnesian actinotite + magnetite and ferrohypersthene+ferrosalite suggest lesser or greater degrees of dehydration under oxidizing or reducing conditions. For example:



reducing cummingonite+actinolite+magnetite+quartz→ferrohypersthene+ferrosalite+H2O

Keywords: amphiboles • pyroxenes • garnets • metamorphism • iron-formation • SW Greenland

Mineralogical Magazine; March 1985 v. 49; no. 350; p. 117-128; DOI: 10.1180/minmag.1985.049.350.18
© 1985, The Mineralogical Society
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