CONSTRAINTS ON THE MANTLE PLUME MODEL FOR MESOZOIC ALKALINE INTRUSIONS IN NORTHEASTERN NORTH AMERICA

J. GREGORY McHONE
Graduate Liberal Studies Program, Wesleyan University, Middletown, Connecticut 06459-0519, U.S.A.

Abstract

Mesozoic alkaline intrusions occur with elongate patterns of distribution in southern Quebec and central New England, and similar magmas formed the chain-like New England seamounts in the adjacent North Atlantic Ocean basin. Both igneous groups have been linked by a "hotspot" model in which the North American plate overrode a stationary mantle plume between 125 and 70 million years ago. Evidence for a hotspot plume includes mantle enrichment followed by melting to produce alkaline magmas, a progression of radiometric ages for the New England seamounts, and an apparent orientation of intrusions with plate motion. In keeping with the popularity of a mantle plume origin for all seamount chains, the model of a New England hotspot plume is often described as a demonstrated fact rather than as one of several hypotheses. Closer study reveals that major problems exist with a hotspot link between the seamounts and continental alkaline rocks in northeastern North America. In particular, many alkaline igneous intrusions in northeastern North America do not fit any plume track in age or geographic distribution, despite petrological similarities that imply a common origin. A deep columnar plume cannot be the principal cause of magmatism; rather, the geological evidence calls for widespread, heterogeneous source-areas in the mantle that produce alkaline basalts in concert with the tectonic re- activation of lithospheric structures, allowing isolated alkaline intrusions as well as linear chains to form through space and time.