CYCLICAL PHASE LAYERING IN THE DULUTH COMPLEX AT DULUTH – EVIDENCE FOR PERIODIC MAGMA VENTING FROM A SHALLOW STAGING CHAMBER

Igneous layering is an almost ubiquitous feature in mafic intrusive bodies and yet remains an enigma. One of the more intriguing forms of layering in the 1.1 Ga Duluth Complex of NE Minnesota is cyclical phase layering exhibited by the Layered Series at Duluth (DLS). In the medial section of the 4 km-thick DLS, known as the cyclic zone, at least 5 macrocycles occur wherein troctolitic (Pl+Ol) cumulates grade upward into olivine gabbroic (Pl+Aug+FeOx+Ol) cumulates. The 4-phase gabbros are then abruptly overlain by troctolitic cumulate marking the base of the next macrocycle. A conventional interpretation of such layering may be that it formed by progressive differentiation followed by magma recharge. However, cryptic layering of olivine and pyroxene across the cumulate regressions marking each macrocycle boundary show no significant increase in mg#, which such a model would predict. Instead, Miller et al. (2002) proposed that decompression due to magma venting at shallow depths might better explain this layering by significantly changing phase equilibrium without changing magma composition.

Within the second macrocycle of the cyclic zone is an interval exhibiting meter-scale mesocyclic layering that mimics the textural and mineralogical changes seen in the larger scale macrocycles. Detailed field, petrographic, and geochemical studies of these mesocycles were performed to further test the plausibility of magma recharge as the mechanism responsible for the cyclic phase layering. Analyses of olivine and augite revealed that mg#'s remained within 2% of the mean throughout individual cycles and throughout the entire mesocyclic system of 13 cycles. The lack of cryptic variation, coupled with constant trace element concentrations, reinforces the broader scale observation that no significant chemical change occurs across cycle boundaries. This reaffirms the conclusion that macro- and meso-scale cyclic phase layering in the DLS likely developed by decompression rather than by differentiation and recharge. This conclusion highlights the often overlooked role that magma venting can play in creating layering in shallow mafic intrusions.