IMPLICATIONS OF GEOCHEMICAL, PETROGRAPHIC AND STRATIGRAPHIC ANALYSES OF THE UPPER PORTAGE LAKE VOLCANICS FOR THE KEWEENAW FLOOD BASALTS

Continental flood basalt provinces (CFBPs) often involve emplacement of millions of cubic kilometers of magmatic material. Despite the large amount of mantle melting required for flood basalt generation, most magmas are not primary, and require some processing within the lithosphere before eruption. Flood basalt lavas represent one of the most effective probes of the evolving magma chamber conditions, and thus a well-preserved volcanic stratigraphic section is critical for investigating these processes. However CFBPs often suffer from poor preservation due to subsequent rifting and erosion. Although this problem is typically resolved by focusing on the youngest CFBPs, failed rifts can preserve significant stratigraphic sections. Here we focus on flood basalt flows from the 1.1 Ga Keweenaw CFBP that have been preserved within the failed Midcontinent Rift. We present geochemical, petrographic and stratigraphic analyses of the upper Portage Lake Volcanics erupted during the Keweenaw CFBP main stage volcanism. Basalts from this region are tholeiitic and unusually enriched in incompatible trace elements – an observation that cannot be accounted for by simple fractionation and assimilation. Furthermore, flow by flow analysis of these lavas reveals a seeming paradox where limited variation in MgO is combined with significant deviations in the concentrations of incompatible trace elements such as Ti, Nb and Zr. To investigate these inconsistencies, a recharge, evacuation, fractional crystallization and assimilation model was applied to the geochemical data and petrographic evidence. The model shows that the inconsistency in incompatible trace element concentrations can be resolved by a series of recharge events, and the complex interplay between fractionation, assimilation and magmatic recharge resulting in an evolving magma chamber. This interplay helps to explain why primary melts are not erupted at the surface even during periods of high magmatic flux. These results highlight the complex evolution of lavas in CFBP and demonstrate the need for the collection of stratigraphically controlled lavas to probe magma evolution.