SILICIC VOLCANISM OF THE PORCUPINE VOLCANICS: IMPLICATIONS FOR MAGMA DIFFERENTIATION DURING THE TERMINAL STAGES OF VOLCANISM WITHIN THE MIDCONTINENT RIFT

The ca. 1.1 Ga Midcontinent Rift and associated Keweenaw large igneous province (LIP) represent the most complete record of magmatism in a failed rift. While the Keweenaw LIP is dominated by basalt, silicic volcanism represents a significant volume of erupted material in the rift. There is debate over the relative importance of fractional crystallization, assimilation, partial melting of crustal components, magma mixing, and liquid immiscibility in the generation of silicic magmas. Rhyolitic volcanism, though often viewed as a lesser product of cyclic bimodal magmatism at rifted margins, may preserve evidence of the processes that ultimately led to the end of magmatism in the Keweenaw LIP. In contrast to geochemical analysis of basalts, which probe mantle conditions, rhyolite geochemistry can elucidate crustal magmatic processes. In this study, we aim to understand what processes were dominant in the generation of the late stage rhyolites (ca. 1094 Ma) in the Porcupine Volcanic group (PV), and use geothermobarometric modeling to constrain the crystallization temperature and pressure of the silicic magma system.

Rhyolites in the PV group fall into two mineralogical and chemical groups. Type A PV rhyolites are characterized by aphanitic petrography, high SiO₂ (75-80 wt%), and relative enrichment of heavy rare earth elements and high field strength elements. Type B PV rhyolites are characterized by phaneritic petrography (quartz, alkali feldspar, Fe-Ti oxides), lower SiO₂ (72-75 wt%) and relative depletions of HREE and HFSE. Geochemical trends suggest that both groups result from partial melting of basaltic crust and fractional crystallization. Major element chemistry from glass analysis of Type B PV samples was used to model crystallization temperature and pressure of the system using rhyolite-MELTS, a phase equilibria based geobarometer. The results imply pressures between 150 to 600 MPa, and temperatures between 700 - 1000°C, with a large majority at 500 MPa and temperatures greater than 850°C. The results of the geochemistry and T/P model support the interpretation that these rhyolites were a product of partial melting of basaltic crust in a thermally mature system at moderate crustal depth before crystallization occurred at temperatures in excess of 850°C and pressures at ~500 MPa.