Contact metamorphism and selective metasomatism of the layered Bellerophon Formation in the eastern Monzoni contact aureole, northern Italy

Summary

The Monzoni intrusive complex in the central Western Dolomites comprises a series of small monzogabbro, monzodiorite and pyroxenite intrusions, which were emplaced in previously unmetamorphosed Permo-Triassic sediments during the mid-Triassic. Intrusion depth was less than five kilometres, and solidus temperatures were 900–1000 °C. An about one kilometre wide contact aureole developed in the country rocks for which peak metamorphic temperatures of about 800 and 500 °C are derived for the immediate intrusive contact and for a distance of 380 m from the contact, respectively. The Upper Permian Bellerophon Formation is an alternating sequence of dolomite-poor and dolomite-rich limestone layers. These two lithologies behaved differently during contact metamorphism. Whereas the layers derived from dolomite-poor precursors behaved as closed systems, the layers derived from dolomite-rich lithologies were much more susceptible to the infiltration of fluids or melts. The supply of substantial amounts of silica, alumina and ferric iron from the nearby intrusions lead to high temperature metasomatism of these lithologies. Substantial chemical alteration is discernible up to 65 m away from the intrusive contact and produced clinopyroxene rich skarns, which preferentially replaced the layers derived from dolomite-rich protoliths. The presence of clintonite and melilite in the skarn assemblages indicates formation temperatures in excess of 750 °C. In contrast, the high-grade equivalents of the originally dolomite-poor lithologies largely preserved their original chemical and stable isotope compositions and do not show any evidence of pervasive fluid or melt infiltration at high-grade conditions. The syn-metamorphic permeability structure of the Bellerophon Formation appears to have been largely controlled by the lithological layering and was heterogeneous on a decimetre scale. Together with a complex folding of the country rocks this makes large scale syn-metamorphic thermally driven fluid circulation unlikely in the Bellerophon Formation.