Mineralogical and chemical variations within layered sills of the Deer Lake Complex, Minnesota

Abstract

Layered mafic sills, intrusive into an Archean volcano-sedimentary sequence of north-central Minnesota, are the major components of an igneous series known as the Deer Lake Complex. Differentiation in the sills is simple: a basal peridotite layer (highly serpentinized) is successively overlain by pyroxenite, a lower orthopyroxene-bearing gabbro, an upper orthopyroxene-free gabbro, and quartz diorite. Chilled margins of the sills are morphologically complex and exhibit textures characteristic of supercooling. The Complex has undergone greenschist regional metamorphism. Olivine in the Complex ranges only from Fo81 to Fo85. Clinopyroxene in pyroxenite layers ranges from Wo45En45Fs10 to Wo36En53Fs11, whereas that in a lower gabbro sample averages Wo36En44Fs20. Ferroaugite, Wo38En27Fs35, occurs in local iron-rich gabbroic units. Layered sills of the Deer Lake Complex were produced from basaltic parental magmas characterized by high MgO content (average ∼11%), low TiO2 (0.68%) and Al2O3 (12.9%), and Ca/Al<1 (0.97). Differentiation in the sills follows a normal tholeiitic trend, with modest enrichment of iron and alkalis.