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)

In the late Palaeozoic fore-arc system of north-central Chile at latitudes 31-32 degrees S (from the west to the east) three lithotectonic units are telescoped within a short distance by a Mesozoic strikeslip event (derived peak P-T conditions in brackets): (1) the basally accreted Choapa Metamorphic Complex (CMC; 350-430 degrees C, 6-9 kbar), (2) the frontally accreted Arrayan Formation (AF; 280-320 degrees C, 4-6 kbar) and (3) the retrowedge basin of the Huentelauquen Formation (HF; 280-320 degrees 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 degrees 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 degrees 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 similar to 280 degrees C at similar to 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 similar to 245-240 Ma, similar to 210-200 Ma, similar to 174-159 Ma and similar to 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.