LINKING SMALL PGE-DEPLETED GABBROIC INTRUSIVE ROCKS TO LARGER MAGMA RESERVOIRS AT DEEPER CRUSTAL LEVELS: AN EXAMPLE FROM THE SCANDINAVIAN CALEDONIDES

The Caledonides of Europe contain numerous small gabbroic intrusives generally less than 10 km in diameter. The Skjækerdalen igneous complex is a lenticular intrusion approximately 4 km long and 1.5 km wide situated in Gula Group meta-pelites of the Trondheim Nappe complex in the central Norwegian Caledonides. The intrusion is a polymagmatic mafic breccia consisting of quartz gabbro with a quartz diorite rim and a central brecciated core consisting of gabbro fragments in a leucocratic dioritic matrix. Interstitial magmatic sulfides are present in some of the gabbro fragments in the brecciated core. PGE concentrations in magmatic sulfides at Skjækerdalen are quite low in comparison with deposits of comparable Ni and Cu contents (e.g. Sudbury, Noril'sk). A possible mechanism to explain these lower concentrations is the formation of an early olivine-chromite-PGM phase during differentiation in a magma chamber at deeper crustal levels in which precipitation of Pt-rich phases scavenged much of the PGE from the silicate magma. This olivine-chromite-PGM phase may have solidified early in the differentiation process allowing subsequent droplets of sulfide liquid segregated at higher levels to settle through a PGE-depleted magma. Remobilization of the differentiated gabbroic and dioritic rocks, containing sulfides with low PGE contents, may have occurred before those phases had completely solidified. Diapiric-like emplacement of the differentiated rocks from this magma chamber, starting with the quartz diorite and ending with remobilization of the gabbro, may resolve the mechanical problem of felsic rocks enclosing progressively more mafic rocks at Skjækerdalen. A similar mechanism can be used to explain low PGE values in other small sulfide-rich gabbroic intrusions in orogenic settings.