Oxygen isotopic disequilibrium in plagioclase–corundum–hercynite xenoliths from the Voisey's Bay Intrusion, Labrador, Canada

Abstract

Xenoliths of metamorphic country rocks are locally abundant within breccia sequences in gabbroic to troctolitic rocks of the Voisey's Bay Intrusion, Labrador, Canada. Thermochemical interaction with mafic magma has produced restite assemblages in the xenoliths that are composed of Ca-rich plagioclase, corundum, hercynite and minor magnetite. Hercynite was produced as a result of the breakdown of garnet and pyroxene that were originally present in the metamorphic rocks, as well as by the replacement of corundum, which was itself a product of feldspar degradation during xenolith–magma interaction. Hercynite that replaces pyroxene and garnet is granular to bulbous, whereas that which replaces corundum is acicular to skeletal. Ion microprobe analyses indicate that oxygen isotopic equilibrium was neither established during retrograde cooling of the xenolith assemblage, nor during the replacement processes which led to both types of pseudomorphous hercynite. δ 18O values of hercynite that replaces garnet and pyroxene range from 5 to 11.5‰, and in part reflect the elevated δ 18O values of the protolith minerals. Hercynite that replaces acicular corundum is characterized by δ 18O values between 2.5 and 7.6‰. Oxygen isotopic equilibration with mantle-derived magma having a δ 18O value of ∼ 5.5‰ should have produced values of hercynite near 2.5‰. Values of Δ (plagioclase–corundum) range between 0.2 and 6.1‰, whereas D (hercynite–corundum) values, which are expected to be near 0, may be as large as 9.6‰ at the sub-millimeter scale. Oxygen isotopic disequilibrium may have resulted in the xenoliths due to rapid withdrawal of partial melt, and to the crystallization of plagioclase and biotite bands around the xenoliths which prevented isotopic communication of xenoliths with enclosing mafic magma. Although diffusive transport of oxygen in the xenoliths should have been a viable mechanism for isotopic exchange, the lack of isotopic equilibration during cooling and the preservation of steep isotopic gradients in composite hercynite–corundum grains suggest that reaction and crystallization were necessary to promote oxygen isotopic exchange among minerals in the xenoliths contained within the Voisey's Bay Intrusion.