DATING THE HYDROTHERMAL-BURIAL ALTERATION EVENT(S) IN THE PROTEROZOIC BASALTIC NORTH SHORE VOLCANIC GROUP, MINNESOTA

The Proterozoic North Shore Volcanic Group (NSVG, approximate thickness 9735 m) has experienced a hydrothermal-low-temperature burial alteration history after deposition reaching upper zeolite facies conditions (heulandite-stilbite-smectite) in the stratigraphically upper part at Tofte through lower zeolite facies conditions (laumontite-mixed-layered smectite/chorite±prehnite±pumpellyite) and greenschist facies conditions (epidote-chlorite-albite) in the stratigraphically lower part at Duluth.

The Midcontinent Rift magmatism is supposed to be the result of an upwelling mantle plume and occurred at ca. 1109-1094 Ma. In the NSVG, U-Pb ages on zircon in a rhyolite quarry at Carlton Peak give the youngest magmatic age of 1092±2.0 Ma for the NSVG. Magmatic ages get older with stratigraphic thickness from 1096.6±1.7 for the Palisade rhyolite and 1098.4±1.9 for the Lakewood lavas [4]. This study reports 3 K/Ar ages of celadonite in basaltic lava flows having formed during the hydrothermal burial metamorphic history.

XRD data show an almost pure celadonite. In thin section, celadonite occurs at the rim of amygdules as the first mineral to have crystallized. One celadonite bearing lava flow is located in the stratigraphically lower Two harbors suite. The flow shows a typical laumontite-mixed-layered smectite/chlorite-albite±pumpellyite assemblage. The assemblage may vary quite considerably in the outcrop scale, probably reflecting various alteration pulses. Celadonite formation occurred at 1062±16.1 Ma in one amygdule, while another celadonite amygdule of the same flow, ca 0.8 km away, was dated at 1039.4±14.5 Ma. In the stratigraphically higher part of the sequence, celadonite in an amygdule rim of a lava flow has experienced lower zeolite facies conditions (stilbite-heulandite-smectite assemblage) and was dated at 955.0±12.4 Ma.

The K/Ar ages of celadonite formation are signifcantly younger than the U/Pb ages of zircon. The K/Ar ages overlap in age within one flow. The obtained data allow to postulate a first time frame for the alteration history of the NSVG. Surprisingly, low-temperature-hydrothermal alteration occurred well after lava deposition and was a long-lasting process, maybe not always continuous, spanning a period of ca. 100 Ma.