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

A stock of biotite-muscovite-garnet leucogranite crops out in the lowe r course of Rio Cisnes as an unusual minor lithology within the predom inantly dioritic to tonalitic North Patagonian Batholith. Foliated and unfoliated varieties are present-the former are nearer to the main li neament of the Liquifie-Ofqui Fault Zone (LOFZ). Two-feldspar thermome try indicates equilibration temperatures above 600 degrees C, for pres sures probably not over 3 kbar, as suggested by the Mn-rich garnet com position. A Rb-Sr whole-rock isochron age of 9.6 +/- 0.4 Ma (1 sigma e rror) probably indicates the time of magma crystallization. Ar-40- Ar- 39 ages of 6.6 +/- 0.3 Ma on muscovite and 5.5 +/- 0.4 Ma on biotite a re cooling ages from which a moderate average uplift/denudation rate ( similar to 1 mm/ yr) may be calculated. Paucity of occurrence, distrib ution close to the LOFZ and a near minimum-melt composition all sugges t that the leucogranite magma was derived by partial melting of the lo wer crust, perhaps by decompression melting at a time when uplift/denu dation rates were high (4 mm/yr or more are required). Regional eviden ce for rapid Holocene uplift in the immediate vicinity of the LOFZ sub stantiates the feasibility of the proposed petrogenetic model, which m ay be valid in other strike-slip orogenic environments.