GRANITE GENERATION AND RAPID UNROOFING RELATED TO STRIKE-SLIP FAULTING, AYSEN, CHILE

A stock of biotite-muscovite-garnet leucogranite crops out in the lower course of Rio Cisnes as an unusual minor lithology within the predominantly dioritic to tonalitic North Patagonian Batholith. Foliated and unfoliated varieties are present-the former are nearer to the main lineament of the Liquifie-Ofqui Fault Zone (LOFZ). Two-feldspar thermometry indicates equilibration temperatures above 600 degrees C, for pressures probably not over 3 kbar, as suggested by the Mn-rich garnet composition. A Rb-Sr whole-rock isochron age of 9.6 +/- 0.4 Ma (1 sigma error) probably indicates the time of magma crystallization. 40Ar- 39Ar ages of 6.6 +/- 0.3 Ma on muscovite and 5.5 +/- 0.4 Ma on biotite are cooling ages from which a moderate average uplift/denudation rate (similar to 1 mm/ yr) may be calculated. Paucity of occurrence, distribution close to the LOFZ and a near minimum-melt composition all suggest that the leucogranite magma was derived by partial melting of the lower crust, perhaps by decompression melting at a time when uplift/denudation rates were high (4 mm/yr or more are required). Regional evidence for rapid Holocene uplift in the immediate vicinity of the LOFZ substantiates the feasibility of the proposed petrogenetic model, which may be valid in other strike-slip orogenic environments.