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P–T evolution and timing of a late Palaeozoic fore-arc system and its heterogeneous Mesozoic overprint in north-central Chile (latitudes 31–32°S)

Published online by Cambridge University Press:  16 August 2011

ARNE P. WILLNER*
Affiliation:
Institut für Geologie, Mineralogie & Geophysik, Ruhr-Universität, D-44780 Bochum, Germany Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany
HANS-JOACHIM MASSONNE
Affiliation:
Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany
UWE RING
Affiliation:
Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand
MASAFUMI SUDO
Affiliation:
Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknechtstr. 24, D-14476 Potsdam, Germany
STUART N. THOMSON
Affiliation:
Department of Geosciences, University of Arizona, 1040 E. 4th St., Tucson, AZ 85721-0077, USA
*
Author for correspondence: arne.willner@rub.de

Abstract

In the late Palaeozoic fore-arc system of north-central Chile at latitudes 31–32°S (from the west to the east) three lithotectonic units are telescoped within a short distance by a Mesozoic strike-slip event (derived peak P–T conditions in brackets): (1) the basally accreted Choapa Metamorphic Complex (CMC; 350–430°C, 6–9 kbar), (2) the frontally accreted Arrayán Formation (AF; 280–320°C, 4–6 kbar) and (3) the retrowedge basin of the Huentelauquén Formation (HF; 280–320°C, 3–4 kbar). In the CMC, Ar–Ar spot ages locally date white-mica formation at peak P–T conditions and during early exhumation at 279–242 Ma. In a local garnet mica-schist intercalation (570–585°C, 11–13 kbar) Ar–Ar spot ages refer to the ascent from the subduction channel at 307–274 Ma. Portions of the CMC were isobarically heated to 510–580°C at 6.6–8.5 kbar. The age of peak P–T conditions in the AF can only vaguely be approximated at ≥ 310 Ma by relict fission-track ages consistent with the observation that frontal accretion occurred prior to basal accretion. Zircon fission-track dating indicates cooling below ~ 280°C at ~ 248 Ma in the CMC and the AF, when a regional unconformity also formed. Ar–Ar white-mica spot ages in parts of the CMC and within the entire AF and HF point to heterogeneous resetting during Mesozoic extensional and shortening events at ~ 245–240 Ma, ~ 210–200 Ma, ~ 174–159 Ma and ~ 142–127 Ma. The zircon fission-track ages are locally reset at 109–96 Ma. All resetting of Ar–Ar white-mica ages is proposed to have occurred by in situ dissolution/precipitation at low temperature in the presence of locally penetrating hydrous fluids. Hence syn- and postaccretionary events in the fore-arc system can still be distinguished and dated in spite of its complex heterogeneous postaccretional overprint.

Type
Original Articles
Copyright
Copyright © Cambridge University Press 2011

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