IRON-ENRICHMENT IN OXIDIZED SILICEOUS MAGMA? AN EXAMPLE AND POSSIBLE MECHANISM FROM THE CORNUCOPIA STOCK, NE OREGON

Unusual magnetite (+ biotite)-rich rocks (up to 70 vol.% magnetite) occur in the Crater Lake trondhjemite (CLt), the youngest of three cordierite trondhjemites in the Cornucopia stock. The CLt differs from the other two only in its lower Fe content. The mt-rich rocks are often associated with leucotonalite, forming composite dikes and sheets near the margin of the CLt, and temporally associated with late shear zones (indicating late emplacement of the composite dikes). Similar mineral assemblages (incl. cord & musc) and oxygen isotope compositions indicate the mt-rich rocks are genetically related to the CLt. Mt-rich compositions are inconsistent with an origin by hydrothermal deposition, liquid immiscibility, or by magnetite accumulation from the CLt. Instead, they are consistent with magnetite accumulation from an Fe-rich residual liquid derived by extensive (~96%) in-situ crystallization of trondhjemitic magma. Mg-rich cordierite (mg no.=0.73-0.77) and high biotite ferric-ferrous ratios suggest the CLt crystallized between the Ni-NiO and Mt-Hem buffers, and that fO2 conditions were no different in the other cordierite-bearing units. These results suggest that magnetite suppression, and resultant Fe-enrichment, in the CLt was not caused by low fO2. Rather, low Fe in the CLt (<1.50 wt.% Fe as FeO) led to early plag + qtz crystallization, effectively keeping magnetite off the liquidus. Most of the Fe-rich residual liquid remained in equilibrium with the plag + qtz-dominated assemblage, but small amounts were able to separate. Although not observed directly, we suggest that deformation related to shearing along the CLt margins was necessary to mobilize the low volumes of residual liquid. Fractionation within the residual liquid led to mt-rich cumulates and leucotonalitic derivative magma, now observed in the composite dikes. Results of this study provide a mechanism for forming Fe-rich rocks in oxidized siliceous magmas, and may help to explain mt-rich rocks observed in calc-alkaline plutons elsewhere.