| Refractory (Ca-rich plagioclase, corundum, hercynite) xenoliths in the Voisey's Bay Intrusion are evidence for rapid thermal equilibration and a direct link between crustal contamination and sulfide ore formation. This study focuses on mineral digestion and melts extraction from pelitic, enderbitic, mafic and quartzofeldspathic xenoliths. Five types of xenoliths: light-cored, dark-cored, laminated, massive and variegated have been observed. Most xenoliths are separated from troctolitic matrix by plagioclase and biotite rims. Hercynite is acicular, bulbous or granular. Acicular hercynite partially replaces corundum. The thicknesses of the hercynite bands are used to calculate xenolith residence time in magma, which range between 3000 and 23000 years. Reactions are proposed to explain the breakdown of garnet, orthopyroxene, plagioclase, K-feldspar, sillimanite and biotite to yield hercynite, Ca-rich plagioclase and corundum. Mass balance calculations indicate that Si-, Na-, K-rich melts were lost from xenoliths into the magma. Chemical potential gradients that existed between xenoliths and magma promoted melting reactions. Oxygen isotopic analyses of country rocks, xenoliths and igneous matrix were utilized to trace chemical exchange between xenoliths and magma. Hercynite and plagioclase separates from the xenoliths have an average delta18O value of 6.8‰, which is lower than the average delta18O of 9.3‰ for mineral separates from the country rocks. The igneous matrix has an average delta 18O value of 6.4‰, which is lower than the value for xenoliths. These results confirm that an 18O-rich partial melt was lost from the xenoliths to the magma. The mantle-like delta18O values of the igneous matrix indicate loss of the partial melt to early flow-through magma. Ion microprobe analyses for oxygen isotopic compositions of hercynite and corundum were used to further evaluate oxygen isotope fractionation during xenolith digestion. Corundum has delta18O values between 1.9 and 6.1‰. Hercynites have delta18O values between 2.5 and 11.5‰. Large differences in delta18O values between hercynite and corundum on a thin section-scale indicate oxygen isotopic disequilibrium, which is interpreted as evidence for rapid melt production and extraction, and partial exchange during the replacement of corundum by hercynite. |
