Metamorphic PTt Paths and Granite Tectonics

R.G.Warren and D.J. Ellis

Geology Department, Australian National University, Canberra ACT 0200

The vertical and horizontal reworking of the crust known as granite tectonics is initiated by the influx of hot mantle-derived mafic magma into the lower crust. Bouyant granite melts form in the lower crust, coalesce through lateral migration, and rise into the upper crust. In regions between rising granite, cooler, denser material sinks to replace the granite, and at the surface, rim synclines develop. These are filled by material eroding off the region above the rising granites and by co-magmatic volcanics. The tectonic regime is best described from the Lachlan Fold Belt (e.g., Chappell et al. 1987) which might well serve as the reference area.

Experimental modelling (Ramberg 1982) of gravity-tectonics (encompassing both salt tectonics and granite tectonics) can be used to show vertical and horizontal movements in the crust as granite magma coalesces and ascends, and from these, PTt paths for regions within the crust can be derived . As granite rises, Zone B under the rim synclines moves down and heats up, producing an anticlockwise PTt metamorphic path. At depth, Zone B merges with, or is dragged into Zone D, where migmatites and granulites form by dehydration melting and contribute to the granites. Migration of melt towards the regions of granite upwelling creates the horizontal fabrics which indicate extreme extension and are characteristic of most granulite terrains. Experimental modelling shows that Zone C, adjacent to the rising granite, may be dragged upwards, and together with refractory material in the granite, experience pressure-decrease at near-peak temperature (clockwise PTt path). At upper crustal levels (Zone A), heat from the granite produces andalusite- to sillimanite-bearing assemblages, and lateral spreading of the granite causes local compression and folding of the rim synclines. Granite tectonics is driven by the need to minimize gravitational potential energy, so involves little uplift and no mountain-building. The crustal upheaval ends in near-isostatic equilibrium, and all zones cool from peak temperatures isobarically. Later, unrelated, tectonic activity must occur if deeper crustal levels are to be exposed.

REFERENCES

Chappell, B.W., White, A.J.R. & Wyborn, D., 1987. The importance of residual source material in granite petrogenesis. Journal of Petrology 28, 1111-1138.

Ramberg, H., 1982. GRAVITY, DEFORMATION AND THE EARTH'S CRUST (2nd edition) Academic Press, London 452 pp.