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Time markers for the evolution and exhumation history of a Late Palaeozoic paired metamorphic belt in North-Central Chile (34°-35°30′S)

Abstract:

A multi-method geochronological approach is applied to unravel the dynamics of a paired metamorphic belt in the Coastal Cordillera of central Chile. This is represented by high-pressure - low-temperature rocks of an accretionary prism (Western Series), and a low-pressure- high-temperature overprint in the retro-wedge with less deformed metagreywackes (Eastern Series) intruded by magmas of the coeval arc. A pervasive transposition foliation formed in metagreywackes and interlayered oceanic crust of the Western Series during basal accretion near metamorphic peak conditions (∼350-400°C, 7-11 kbar) at 292-319 Ma (40Ar/39Ar phengite plateau ages). 40Ar/39Ar UV laser ablation ages of phengite record strain-free grain growth and recrystallization with a duration of 31-41 Myr during a pressure release of 3-4 kbar. During early accretion the main intrusion in the arc occurred at 305 Ma (Pb-Pb evaporation; zircon) and the Eastern Series was overprinted by a short high-temperature metamorphism at 3 kbar, 296-301 Ma (40Ar/39Ar muscovite plateau ages). Fission-track ages of zircon (206-232 Ma) and of apatite (80-113 Ma) are similar in both series, indicating synchronous cooling during distinct periods of exhumation. Early exhumation (period I) during continuing basal accretion proceeded with mean rates of ≥0.19-0.56 mm/yr, suggesting that erosion in a tectonically active area was an important unroofing mechanism. At the same time mean rates were 0.03-0.05 mm/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 rates decreased to 0.04-0.06 mm/yr in both series during period II (∼100-225 Ma). Major extension, basin formation and local bimodal dyke intrusion at 138 Ma were accompanied by mean cooling rates of ∼1-2°C/Myr. Accelerated cooling of 3-5°C/Myr at ∼80-113 Ma suggests a mid-Cretaceous convergence event (period III). After 80 Ma cooling rates decreased to 1-2°C/Myr (period IV). The pressure-temperature - deformation-time information for subduction, basal accretion and exhumation in the accretionary wedge of central Chile illustrates that these processes reflect a continuous cyclic mass flow that lasted nearly 100 Myr, while the retro-wedge remained stable. After the cessation of accretion activity a similarly long period of retreat of the subducting slab occurred; this ended with renewed convergence and shortening of the continental margin. © The Author 2005. Published by Oxford University Press. All rights reserved.