Journal of Conference Abstracts

Granite Magma Segregation, Transport and Accumulation, NE Gander Zone, Newfoundland

Richard D'Lemos (rsd'lemos@brookes.ac.uk)

Oxford Brookes University OX3 0BP.

Granite segregation, transport and emplacement mechanisms will differ between tectonic environments (e.g. arc versus continental collision), and within a single setting (e.g. cause and timing of melting and magma transport relative to the overall tectonic events and stress regimes). In an orogenic belt where granite magma is present, it is being emplaced in at least three different scenarios. There are magmas which are separating from their source, there are those in transit through the crust, and those which have arrived at some final intended destination. Large volumes of granite magma (=3D plutons) may develop in all three scenarios. The Silurian through Devonian evolution of the NE Gander zone of Newfoundland provides examples of these processes in a single, transpressional orogen. The region was undergoing progressive exhumation through this period so that the successive emplacement features observed reflect processes acting at different crustal levels.

Regionally developed moderately high T low P migmatites underwent short-lived and incomplete biotite dehydration melting and preserve an array of syn-anatexis structures. Migmatites containing variable proportions of melt underwent variable degrees of syn-anatexis deformation. Magma has clearly migrated at all scales through the migmatite pile from mm to 100's m, and probably km. Melt migration was assisted by embrittlement and deformation, with magma redistributed parallel to and across migmatite layering, into fold noses and shear planes. At high melt contents (through greater melting or magma introduction), original migmatite structures became disrupted and larger bodies of mobile, buoyant magma collected which was capable of intruding surrounding migmatite. At the approximate discontinuity between paragneiss and underlying migmatite, granite has accumulated and intruded as layer parallel sheets into contemporaneous migmatite and constructed a relatively thin (km's) pluton.

Highly elongate plutons (c. 10 x 50km) intruded the deformed, cooling, migmatite pile at low-amphibolite to upper greenschist facies conditions, syntectonically with respect to regional sinistral shear. Internal fabrics and sheeting structures reveal a complex history of changing stress regime during pluton assembly. Despite the narrowness of the plutons, significant high temperature aureoles are present. These plutons are considered to be remnants of magma feeders through the crust.

Extensive posttectonic plutons, which cross-cut regional metamorphic isograds and structural grain, contain stoped blocks of their country rocks and develop overprinting metamorphic aureoles. These plutons were emplaced syntectonically with regional brittle - ductile faulting. Fabrics are weak in pluton interiors, but more strongly developed contact parallel magmatic and local solid-state fabrics at margins record pluton inflation. Internal contacts and disequilibria textures record the progressive influx and mixing of granitic magmas in assembling plutons.