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 interacti ons 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 Quebec, terrane juxtaposition was inferred to be late based on the distribution of 1.08 Ga potassic alkaline plutons. This view was quest ioned, however, upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting t heologically softer units, for example Grenvillian marble. In this con text, preferential pending of magmas in marble rather than tectonics c an control regional distribution of plutons. The spatial association o f plutons with marble and their contact relationships are, at the loca l scale, obscured by transformation of marble to skarn and by mechanic al excision of marble from the paragneiss wall-rock sequence. In contr ast, plutons and dykes of an older, 1.17 Ga, less alkaline magma assoc iation are shown by field and remote-sensing studies to be evenly dist ributed across the various lithotectonic domains of the bell. Their sh eet-like emplacement along the belt boundary constrains Grenvillian te ctonic assembly in the region to be early. Contrasting loci, type and degree of deformation of intrusive bodies illustrate that host-rock rh eological differences influence the final characteristics of an intrus ive suite. Consequently, the nature of the host is an important variab le in inferring timing relationships and tectonic scenarios from intru sive suites in high-grade terranes. (C) 1998 Elsevier Science Ltd. All rights reserved.