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

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