Time Markers for the Evolution and Exhumation History of a Late Palaeozoic Paired Metamorphic Belt in North-Central Chile (34{degrees}-35{degrees}30'S) |
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Authors: | WILLNER, ARNE P. THOMSON, STUART N. KRONER, ALFRED WARTHO, JO-ANNE WIJBRANS, JAN R. HERVE, FRANCISCO |
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Affiliation: | 1 INSTITUT FÜR GEOLOGIE, MINERALOGIE UND GEOPHYSIK, RUHR-UNIVERSITÄT, D-44780 BOCHUM, GERMANY 2 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, YALE UNIVERSITY, P.O. BOX 208109, NEW HAVEN, CT 06520-8109, USA 3 INSTITUT FÜR GEOWISSENSCHAFTEN, UNIVERSITÄT MAINZ, BECHERWEG 21, D-55099 MAINZ, GERMANY 4 WESTERN AUSTRALIAN ARGON ISOTOPE FACILITY, DEPARTMENT OF APPLIED GEOLOGY, CURTIN UNIVERSITY OF TECHNOLOGY, HAYMAN ROAD, BENTLEY, W.A. 6102, AUSTRALIA 5 FACULTEIT DER AARDWETENSCHAPPEN, VRIJE UNIVERSITEIT AMSTERDAM, DE BOELELAAN 1085, 1081 HV AMSTERDAM, THE NETHERLANDS 6 DEPARTAMENTO DE GEOLOGIA, UNIVERSIDAD DE CHILE, CASILLA 13518, CORREO 21, SANTIAGO, CHILE |
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Abstract: | A multi-method geochronological approach is applied to unravelthe dynamics of a paired metamorphic belt in the Coastal Cordilleraof central Chile. This is represented by high-pressurelow-temperaturerocks of an accretionary prism (Western Series), and a low-pressurehigh-temperatureoverprint in the retro-wedge with less deformed metagreywackes(Eastern Series) intruded by magmas of the coeval arc. A pervasivetransposition foliation formed in metagreywackes and interlayeredoceanic crust of the Western Series during basal accretion nearmetamorphic peak conditions (350400°C, 711kbar) at 292319 Ma (40Ar/39Ar phengite plateau ages).40Ar/39Ar UV laser ablation ages of phengite record strain-freegrain growth and recrystallization with a duration of 3141Myr during a pressure release of 34 kbar. During earlyaccretion the main intrusion in the arc occurred at 305 Ma (PbPbevaporation; zircon) and the Eastern Series was overprintedby a short high-temperature metamorphism at 3 kbar, 296301Ma (40Ar/39Ar muscovite plateau ages). Fission-track ages ofzircon (206232 Ma) and of apatite (80113 Ma) aresimilar in both series, indicating synchronous cooling duringdistinct periods of exhumation. Early exhumation (period I)during continuing basal accretion proceeded with mean ratesof 0·190·56 mm/yr, suggesting that erosionin a tectonically active area was an important unroofing mechanism.At the same time mean rates were 0·030·05mm/yr in the Eastern Series, where crustal thickening was minor.A shallow granite intruded into the Western Series at 224 Ma,at the end of basal accretion activity, when exhumation ratesdecreased to 0·040·06 mm/yr in both seriesduring period II (100225 Ma). Major extension, basinformation and local bimodal dyke intrusion at 138 Ma were accompaniedby mean cooling rates of 12°C/Myr. Accelerated coolingof 35°C/Myr at 80113 Ma suggests a mid-Cretaceousconvergence event (period III). After 80 Ma cooling rates decreasedto 12°C/Myr (period IV). The pressuretemperaturedeformationtimeinformation for subduction, basal accretion and exhumation inthe accretionary wedge of central Chile illustrates that theseprocesses reflect a continuous cyclic mass flow that lastednearly 100 Myr, while the retro-wedge remained stable. Afterthe cessation of accretion activity a similarly long periodof retreat of the subducting slab occurred; this ended withrenewed convergence and shortening of the continental margin. KEY WORDS: exhumation rates; Ar/Ar geochronology; fission-track geochronology; Chile; paired metamorphic belt |
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Keywords: | : exhumation rates Ar/Ar geochronology fission-track geochronology Chile paired metamorphic belt |
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