Posteruptive Thermal History of the Proterozoic Basaltic North Shore Volcanic Group of the Midcontinent Rift: Evidence from K/Ar Data of Celadonite

Abstract This study reports three K/Ar ages on celadonite, a dioctahedral K-Fe mica, in the Proterozoic North Shore Volcanic Group (NSVG) of Midcontinent Rift northeastern Minnesota. Celadonite formed during beginning posteruptive, low-temperature conditions at temperatures<100°C and with input meteoric water. between 1062±16 Ma 955.0±12 document remarkably long posteruptive thermal history >100 myrs thick continental basaltic sequence. In stratigraphically lower part NSVG, celadonite formation occurred an amygdule or vesicle filled while another higher flow was dated 1039.4±14 Ma. Both flows are overprinted by later multistage zeolite-phyllosilicate facies assemblage (laumontite-albite-corrensite±chlorite±smectite±prehnite±pumpellyite). sequence, crystallization rim is followed upper zeolite (stilbite-heulandite-smectite assemblage) 955.0±12.4 The constrained time frame 107 indicates long-lived, probably not continuous locally occurring, alteration process. data suggest that depth-controlled temporally spatially inhomogeneous implies progression sequence from close-to-the-surface mode water to burial metamorphic occurring hydrothermal activity due magmatic activity. Volcanism system supposed have lasted 1109 1083 based U/Pb zircon ages. first recorded NSVG ca. 30 after emplacement Silver Bay aplite intrusion NSVG. Burial rates determined be 0.04 km·Ma-1and 0.10 km·Ma-1. under clearly postdates rift-related alkaline tholeiitic magmatism overlaps depositional window sedimentary rocks overlie volcanics, as well crustal-scale fault systems were active Grenvillian tectonic uplift cessation