MAGMA DYNAMICS AT THE BASE OF AN EVOLVING MAFIC MAGMA CHAMBER - INCOMPATIBLE ELEMENT EVIDENCE FROM THE PARTRIDGE RIVER INTRUSION, DULUTH COMPLEX, MINNESOTA, USA

A characteristic feature of the Partridge River intrusion of the Kewee nawan Duluth Complex is the approximately fivefold to ninefold increas e in the concentrations of incompatible elements in the lower zone com pared with cumulates stratigraphically higher. The concentrations of i ncompatible elements decrease from the lower zone upward to steady sta te values, which is ascribed to variations in the proportions of trapp ed liquid rather than variable degrees of fractional crystallization o f a single parental magma. The calculated average composition of trapp ed liquid using our algorithm is similar to typical Keweenawan low alu mina, high TI-P basalts associated with the Duluth Complex but is diff erent from the leading edge ferrodioritic Liquid quenched in the chill ed margin of the intrusion. This difference suggests that the chilled margin does not represent the original (parental) magma composition fr om which the whole intrusion solidified, and that the enrichment of in compatible elements may be related to the local flotation of magmatic suspensions. To test the latter hypothesis numerically, we have used h eat-mass transfer models, assuming a sheet-like magma chamber, to calc ulate the parameters of the model that best reproduce the observed dis tribution of incompatible elements in a mush zone at the base of the P artridge River intrusion. The results indicate that a mush zone enrich ed in the incompatible elements is produced if the velocity of movemen t of the lower solidification front into the magma body was less than the floating velocity of the bulk crystal mush. The dynamic parameters that best reproduce the observed distribution of incompatible element s include a magma emplacement pressure of 2 kbar, critical crystallini ties of 50-68% in the mush zone from which the liquid is being expelle d, and an emplacement temperature of similar to 1160 degrees C for the initial magma.