IS THE CHAIN LAKES MASSIF, MAINE-QUEBEC, AN ORDOVICIAN CALDERA REMNANT?

IS THE CHAIN LAKES MASSIF, MAINE-QUEBEC, A CALDERA REMNANT?

MOENCH, Robert H., U.S. Geological Survey, MS 973, Denver, CO 80225, bsmoench@indra.com

On the basis of recent mapping and isotopic age determinations C.C. Gerbi et al (2006, 2006ab) proposed a model whereby emplacement of the Chain Lake massif's protolith, now a diamictite body, occurred at some time between 484 Ma and 685 Ma, certainly before Jim Pond and Boil Mountain magmatism (486-479 Ma and 468 Ma respectively). Small and large clasts were produced during syn-metamorphic disaggregation, at~468 Ma, of a pre-existing stratigraphic pile; this was abruptly followed by thrust-faulting that separated the now high-rank diamictite from the heat source that cooked it.

Alternatively, the matrix-supported clasts and the evidence for very shallow, very high-grade metamorphism throughout the massif might be explained as follows: A large caldera depression formed above a mantle plume during Jim Pond and Boil Mountain magmatism; this basin was quickly filled by a thick sequence of conglomeratic debris-flow beds, forming the massif's protolith. At ~468 Ma, heat from the plume cooked and possibly partially melted all the contents of the depression, much as Gerbi et al described. The environment might have been similar to Yellowstone, albeit in a very different tectonic setting.