Pervasive melt migration from migmatites to leucogranite in the Shuswap metamorphic core complex, Canada: control of regional deformation

The Shuswap metamorphic core complex, exhumed in the hinterland of the Cana dian Cordillera, displays a similar to 15 km structural section from migmat ites to leucogranite. It comprises a lower unit dominated by anatectic migm atites exposed in the core of domes. At higher structural level the transit ion from a middle amphibolite-facies unit to an upper crustal unit is marke d by a low-angle detachment associated with the emplacement of laccoliths o f leucogranite. The amphibolite-facies middle unit is permeated by a networ k of granitic veins feeding the laccoliths and structurally connected to th e migmatites of the lower unit, This paper describes the geometric, textura l and structural characteristics, and the distribution of the granitic frac tion at the outcrop scale in the amphibolite-facies unit separating the mig matites from the leucogranite, This analysis constrains the mechanisms of m elt migration within a plurikilometer-scale section across high-grade rocks of the middle crust, In fertile lithologies (metapelites, felsic amphiboli tes), the granitic fraction, in part generated in situ, forms a diffuse net work of concordant veins feeding discordant veins, and structural sites suc h as the foliation, shear zones, and boudin necks. Discordant granitic vein s are oriented perpendicular, and to a lesser extent parallel, to the miner al and stretching lineation, In refractory lithologies, the granitic fracti on is dominantly intrusive and displays sharp contacts with the host rock. However, distribution of the granitic fraction shows the same characteristi cs as for fertile lithologies. The melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as the foliati on plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations. These geometric and textural characteri stics suggest that granite migration in these rocks is achieved dominantly by viscous flow of the melt (+/- solid) through the solid matrix driven by its buoyancy and controlled by mechanical anisotropy of the rock and local dilation created by heterogeneous deformation. The relationship between the regional fabric and the distribution of the granitic fraction indicates th at regional deformation (incremental and finite) played a major role in pro viding pathways for melt migration. The formation of laccoliths of leucogra nite at higher structural levels suggests that upward melt migration led to accumulation of granitic magmas. These observations are consistent with a model of pervasive melt migration through a network of interconnected conco rdant and discordant granitic veins feeding laccolithic plutons extracted f rom the dominantly solid matrix during vertical thinning and compaction at a regional scale. The presence of both transposed and intact granitic netwo rks indicate an overlap between deformation and granite migration as a cont inuum or in pulses?). The methodology and terminology proposed in this pape r provide some insights on the behavior of the granitic fraction in the mid dle crust. This approach should be applicable to other high-grade terrains exhumed in the cores of orogenic belts. (C) 1999 Elsevier Science B.V. All rights reserved.