GEON 14 ENIGMAS AND ADVANCES IN UNDERSTANDING THE CRUSTAL EVOLUTION AND PALEOGEOGRAPHY OF EARLY MESOPROTEROZOIC NORTH AMERICA

A GEON 14 synthesis of the SE Laurentian margin, from eastern Canada to the southwestern US, highlights a bipartite configuration with an inboard margin-scale orogenic belt, and an outboard dominantly igneous domain. A record of deposition, regional metamorphism, ductile deformation, dominantly granitic magmatism, and crustal thickening is the hallmark of the inner accretionary orogen. We propose that the recently recognized Picuris Orogeny of the southwest United States and the Baraboo Orogeny of the northern Midcontinent are an extension of the Pinware Orogeny in eastern Canada and that these linked orogenies are the manifestation of a larger system of Andean-type continental margin convergence.

The igneous domain is manifested by a trans-Laurentian ferroan magmatic belt, much of which is buried in the subsurface of the Midcontinent. This event is perhaps unique in Earth history in terms of its chemistry and volume of magmatism, and it remains enigmatic. Tectonic models have invoked crustal extension and mantle upwelling, post-orogenic melting during collapse of over-thickened crust, and delamination of the lithospheric mantle followed by basaltic underplating and crustal heating. Another model links ca. 1450 Ma pluton emplacement to a transpressional or convergent orogenic setting, although melt products directly related to subduction and continental arc magmatism are sparse.

The Grenville Province in eastern Canada has the largest areal extent of early Mesoproterozoic rocks on the continent and, despite Grenville-age reworking, it preserves a good record of a continental arc system, separated geographically in two parts by a composite arc belt formed during rifting, then building of pericratonic island arcs, and subsequent accretion of the latter to the margin. Granitoid magmatism shows consistently a general evolution from calc-alkaline (during the building of the arc(s)) to ferroan compositions, linked with arc rifting and formation of back-arcs. This evolution may provide hints for the interpretation of the US where exposure is more limited.