On the emplacement of tabular granites

Abstract

Granites in both crystalline terranes and continental magmatic arcs tend to be circular to elliptical in map view and vary in width from about 3 to 100 km. Available gravity and structural data suggests that many of these plutons are tabular in shape with an average thickness of about 3 km. Ductile structures observed around mesozonal granites indicate that space is created by a combination of lateral and vertical displacements of wall rocks, whereas contact relationships of epizonal plutons imply that only vertical displacements are involved during emplacement. In both settings magma arrives at the emplacement site via one or more vertical feeder zones and flows laterally. With the exception of very high-level epizonal plutons, structural studies suggest that space for many tabular intrusions must be provided mainly by floor-depression (lopolith emplacement) rather than roof-lifting (laccolith emplacement). An emplacement model for this type of tabular granite is proposed which involves progressive depression of the floor of an initially horizontal chamber as it is filled by one or more vertical conduits. A crustal-scale balance in the rates of melt extraction, magma ascent and pluton-filling is required by the model, and transfer of material from the source to the pluton is accommodated by broadly distributed deformation of low strain magnitude. The process is evaluated with end-member cantilever and piston sinking mechanisms. The models predict that large (10–100 km wide), tabular plutons (≤3 km thick) can be emplaced quickly (100 a to 1 Ma) with floor-depression 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 required deformation is accommodated on discrete structures such as normal faults or shear zones at the base of the pluton.