Anders Scherstén

Field relationships and petrology of a mid Proterozoic (1624 ±6 Ma) hornblendite from southwestern Sweden demonstrate that it originated as a mafic-ultramafic hydrous melt. Lobes and veins of hornblendite penetrate the surrounding gneiss and small gneiss xenoliths lie in the main body of the hornblendite, evidence that it intruded with relatively low viscosity. The hornblendite has a composition compatible with its derivation from the mantle at 6 GPa and a geochemical subduction signature indicating a subduction zone origin. It is suggested that the hornblendite represents an intrusive equivalent to primitive lavas seen in Phanerozoic subduction zones that form by melting of metasomatised hydrous mantle.