AR-40 AR-39 GEOCHRONOLOGY AND P-T-T PATHS FROM THE CORDILLERA DARWIN METAMORPHIC COMPLEX, TIERRA-DEL-FUEGO, CHILE/

Ar-40/Ar-39 data collected from hornblende, muscovite, biotite and K-f eldspar constrain the P-T-t history of the Cordillera Darwin metamorph ic complex, Tierra del Fuego, Chile. These data show two periods of ra pid cooling, the first between c. 500 and c. 325 degrees C at rates gr eater than or equal to 25 degrees C Ma(-1), and the second between c. 250 and c. 200 degrees C. For high-T cooling, Ar-40/Ar-39 ages are spa tially disparate and depend on metamorphic grade: rocks that record de eper and hotter peak metamorphic conditions have younger Ar-40/Ar-39 a ges. Sillimanite- and kyanite-grade rocks in the south-central part of the complex cooled latest: Ar-40/Ar-39 Hbl = 73-77 Ma, Ms = 67-70 Ma, Bt = 68 Ma, and oldest Kfs = 65 Ma. Thermobarometry and P-T path stud ies of these rocks indicate that maximum burial of 26-30 km at 575-625 degrees C may have been followed by as much as 10 km of exhumation wi th heating of 25-50 degrees C. Staurolite-grade rocks have intermediat e Ar-40/Ar-39 ages: Hbl = 84-86 Ma, Ms = 71 Ma, Bt = 72-75 Ma, and old est Kfs = 80 Ma. Thermobarometry on these rocks indicates maximum buri al of 19-26 km at temperatures of 550-580 degrees C. Garnet-grade rock s have the oldest ages: Ms = 72 Ma and oldest Kfs = 91 Ma; peak P-T co nditions were 525-550 degrees C and 5-7 kbar. Regional metamorphic tem peratures for greenschist facies rocks south of the Beagle Channel did not exceed c.300-325 degrees C from 110 Ma to the present, although t he rocks are only 2 km from kyanite-bearing rocks to the north. One-di mensional thermal models allow limits to be placed on exhumation rates . Assuming a stable geothermal gradient of 20-25 degrees C km(-1), the maximum exhumation rate for the St-grade rocks is c. 2.5 mm yr(-1), w hereas the minimum exhumation rate for the Ky + Sil-grade rocks is c. 1.0 mm yr(-1). Uniform exhumation rates cannot explain the disparity i n cooling histories for rocks at different grades, and so early differ ential exhumation is inferred to have occurred. Petrological and geoch ronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c. 20 km of overbu rden. This behaviour can be explained in terms of a continental deform ation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogen eous thinning at mid- to deep-crustal levels. The P-T-t data from Cord illera Darwin (1) are best explained by a 'wedge extrusion' model, in which extensional exhumation in the southern rear of the complex was c oeval with thrusting in the north along the margin of the complex and into the Magallanes sedimentary basin, (2) suggest that differential e xhumation occurred initially, with St-grade rocks exhuming faster than Ky + Sil-grade rocks, and (3) show variations in cooling rate through time that correlate both with local deformation events and with chang es in plate motions and interactions.