Abstract

The 8-km-thick, Keweenawan metabasaltic, North Shore Volcanic Group in Minnesota shows a continuous zeolite to greenschist facies progression through five zonal divisions. Mafic phyllosilicates are ubiquitous and a well-developed smectite-corrensite-chlorite transition is observed. This sequence provides an excellent framework in which the influence of regional (thermal) as opposed to local scale (lithology: porosity and permeability) factors on this transition, factors that are actively debated, can be comprehensively documented.

On a regional scale, at lowest grades (zones 1 and 2; thomsonite-scolecite and heulandite-stilbite assemblages, respectively), tri-smectitic phyllosilicates dominate and, with increase in grade to zone 3 (laumontite-albite), there is a marked step and compositional gap to corrensite. With further increase in grade to zones 4 (laumontite-albite ± prehnite ± pumpellyite) and 5 (epidote-albite-actinolite), chlorite becomes dominant. Although a spread in the X-ray diffraction and microprobe data suggests that the corrensite to chlorite transition is continuous rather than stepwise, analytically distinct corrensite and chlorite in one flow suggest a clear compositional gap between these two phases. Overall, the data offer some support for a discontinuous smectite-corrensite-chlorite transition.

Above the lowest grade (zones 1 and 2; approximately 150°C), the phyllosilicates vary on a local, within-flow scale related to morphological contrasts in flow lithology. Smectite is dominant in the massive flow units (low porosity and permeability), whereas corrensite or chlorite is dominant in the amygdaloidal flow tops (high porosity and permeability). These changes provide clear indication that, once above a threshold of approximately 150 °C, fluid/rock ratios have a strong control on the metamorphic process at low grade. Similarities in XMgO between smectite, corrensite, chlorite, and XMgO of the whole rock show that even though smectite is a relict phase remaining in massive zones, it continues to undergo crystal chemical change with increasing grade. However, contrasts between AlIV in the same minerals are provisional indicators that other factors, perhaps related to kinetic features linked to advective and diffusive modes of fluid transport, may be involved.

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