ON THE EMPLACEMENT OF TABULAR GRANITES

Granites in both crystalline terranes and continental magmatic arcs te nd to be circular to elliptical in map view and vary in width from abo ut 3 to 100 km. Available gravity and structural data suggests that ma ny of these plutons are tabular in shape with an average thickness of about 3 km. Ductile structures observed around mesozonal granites indi cate that space is created by a combination of lateral and vertical di splacements of wall rocks, whereas contact relationships of epizonal p lutons imply that only vertical displacements are involved during empl acement. In both settings magma arrives at the emplacement site via on e or more vertical feeder zones and flows laterally. With the exceptio n of very high-level epizonal plutons, structural studies suggest that space for many tabular intrusions must be provided mainly by floor-de pression (lopolith emplacement) rather than roof-lifting (laccolith em placement). An emplacement model for this type of tabular granite is p roposed which involves progressive depression of the floor of an initi ally horizontal chamber as it is filled by one or more vertical condui ts. A crustal-scale balance in the rates of melt extraction, magma asc ent and pluton-filling is required by the model, and transfer of mater ial from the source to the pluton is accommodated by broadly distribut ed deformation of low strain magnitude. The process is evaluated with end-member cantilever and piston sinking mechanisms. The models predic t that large (10-100 km wide), tabular plutons (less than or similar t o 3 km thick) can be emplaced quickly (100 a to 1 Ma) with floor-depre ssion and related wall-rock strain rates similar those expected during tectonic deformation (10(-10) to 10(-15) s(-1)). Bulk strains in the intervening crustal column rarely exceed a strain ratio of 1.5, which is likely to remain undetected in the geological record unless the req uired deformation is accommodated on discrete structures such as norma l faults or shear zones at the base of the pluton.