Abstract
Abstract 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°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-1and 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.
Highlights
Hydrothermal alteration of basalts is a widespread phenomenon and has been documented from recent and ancient ocean floor basalts such as the Reykjanes Peninsula in western Iceland [1], the Deep Sea Drilling Project (DSDP) Leg 37 from the western flank of the Atlantic Ridge [2], the ODP Site 1001 Leg 165 from Cretaceous basalts from the Caribbean Sea [3], the eastern flank of the Juan de Fuca Ridge at 48°N [4], or the Oman ophiolite [5]
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
The alteration mode changes in space and time within the sequence and implies the progression of the sequence from a close-to-the-surface alteration mode with input of meteoric water [48, 49], to a burial metamorphic mode and locally to a mode of hydrothermal activity due to continuous magmatism, burial load, and heat transfer from nearby intrusions, such as the Duluth, the Beaver Bay, or the Silver Bay Complexes
Summary
Hydrothermal alteration of basalts is a widespread phenomenon and has been documented from recent and ancient ocean floor basalts such as the Reykjanes Peninsula in western Iceland [1], the Deep Sea Drilling Project (DSDP) Leg 37 from the western flank of the Atlantic Ridge [2], the ODP Site 1001 Leg 165 from Cretaceous basalts from the Caribbean Sea [3], the eastern flank of the Juan de Fuca Ridge at 48°N [4], or the Oman ophiolite [5]. Celadonite occurs in the continental flood basalts of the Miocene Grande Ronde Basalts of the Columbia River Basalt Group in Washington [19] where it crystallized slowly at temperatures between 15 and 35°C, with oxygen fugacity buffered by interaction with basalt and related to the Grande Ronde Aquifer It is reported in the flood basalts from the Deccan Traps Volcanic Province by Ottens al. [20] as first rim-forming mineral in cavities It occurs in the Tertiary basalts of the Lessini Mountains of northern Italy where it is assumed to have crystallized during the deuteric cooling stage from a residual fluid with H2O and fluids [21]. Alteration proceeds with later burial alteration assemblages of increasingly higher temperature and local hydrothermal alteration
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