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Abstract

A petrological study was carried out on Mg-skarn-bearing dunite cumulates that are part of the Neo-Proterozoic Ioko-Dovyren intrusion (North Baikal region, Russia). Skarn xenoliths contain brucite pseudomorphs after periclase, forsterite and Cr-poor spinel. Fine-grained forsterite-spinel skarns occur with the brucite skarns or as isolated schlieren. Field relationships reveal that the Mg-skarns formed from silica-poor dolomitic xenoliths by interaction with the mafic magma of the Ioko-Dovyren intrusion. Rapid heating of dolomitic xenoliths by the mafic magma caused the decomposition of dolomite into calcite + periclase, releasing much CO2. Further heating quantitatively melted the calcite. A periclase-rich restite was left behind after extraction of the low-density, low-viscosity calcite melt. The extracted calcite melt mixed with the surrounding mafic melt. This resulted in crystallization of olivine with CaO contents up to 1.67 wt %. A local decrease in the silica concentration stabilized CaAl2SiO6-rich clinopyroxene. Brucite/periclase-free forsterite-spinel skarns probably originated by crystallization from the mafic melt close to the xenoliths at elevated fO(2). The high fO(2) was caused by CO2-rich fluids released during the decomposition of the xenoliths. The above case study provides the first evidence for partial melting of dolomite xenoliths during incorporation by a mafic magma.