Rheological controls on Grenvillian intrusive suites: implications for tectonic analysis

Studies of plutonic suites in western Grenville, along with the recent advances in understanding magma ascent and emplacement, and interactions between magma and their country rock, provide insights on the use of intrusive rocks to infer the accretionary history of deeply-exposed Precambrian orogens. In the Grenvillian Central Metasedimentary Belt of Québec, terrane juxtaposition was inferred to be late based on the distribution of 1.08 Ga potassic alkaline plutons. This view was questioned, however, upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting rheologically softer units, for example Grenvillian marble. In this context, preferential ponding of magmas in marble rather than tectonics can control regional distribution of plutons. The spatial association of plutons with marble and their contact relationships are, at the local scale, obscured by transformation of marble to skarn and by mechanical excision of marble from the paragneiss wall-rock sequence. In contrast, plutons and dykes of an older, 1.17 Ga, less alkaline magma association are shown by field and remote-sensing studies to be evenly distributed across the various lithotectonic domains of the belt. Their sheet-like emplacement along the belt boundary constrains Grenvillian tectonic assembly in the region to be early. Contrasting loci, type and degree of deformation of intrusive bodies illustrate that host-rock rheological differences influence the final characteristics of an intrusive suite. Consequently, the nature of the host is an important variable in inferring timing relationships and tectonic scenarios from intrusive suites in high-grade terranes.