Pressure-Temperature Evolution of a Late Palaeozoic Paired Metamorphic Belt in North-Central Chile (34{degrees}-35{degrees}30'S)

In the Chilean Coastal Cordillera, two units, the Western andEastern Series, constitute coeval parts of a Late Palaeozoicpaired metamorphic belt dominated by siliciclastic metasediments.The Western Series also contains rocks from the upper oceaniccrust and represents an accretionary prism. Omnipresent high-pressureconditions are reflected by Na–Ca-amphibole and phengitein greenschists. Peak PT conditions of 7·0–9·3kbar, 380–420°C point to a metamorphic gradient of11–16°C/km. Three unique occurrences of blueschistyield deviating conditions of 9·5–10·7 kbar,350–385°C and are interpreted as relics from the lowermostpart of the basal accretion zone preserving the original gradientof 9–11°C/km along the subducting slab. Pervasiveductile deformation related to basal accretion occurred nearpeak PT conditions. Deformation and PT evolution of the metapsammopeliticrocks is similar to that of the metabasites. However, a raregarnet mica-schist yields peak PT conditions of 9·6–14·7kbar, 390–440°C reflecting a retrograde stage aftercooling from a high-temperature garnet-forming stage. It isconsidered to be an exhumed relic from the earliest siliciclasticrocks subducted below a still hot mantle wedge. A retrogradeoverprint of all rock types occurred at 300–380°C.Continuous reactions caused crystal growth and recrystallizationwith abundant free water mostly under strain-free conditions.They record a pressure release of 3–4 kbar without erasingpeak metamorphic mineral compositions. The Eastern Series lacksmetabasite intercalations and represents a less deformed retro-wedgearea. In the study area it was entirely overprinted at a uniformdepth at 3 ± 0·5 kbar with temperatures progressivelyrising from 400°C to 720°C towards the coeval Late Palaeozoicmagmatic arc batholith. The interrelated pattern of PT datapermits a conceptual reconstruction of the fossil convergentmargin suggesting a flat subduction angle of 25° with continuousbasal accretion at a depth of 25–40 km and a short mainintrusion pulse in the magmatic arc. The latter was accompaniedby the formation of a thermal dome in the retro-wedge area,which remained stable relative to the vertically growing accretionaryprism characterized by cyclic mass flow. KEY WORDS: paired metamorphic belt; greenschist; blueschist; central Chile; thermobarometry     

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)

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-pressure–low-temperaturerocks of an accretionary prism (Western Series), and a low-pressure–high-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 (350–400°C, 7–11kbar) at 292–319 Ma (⁴⁰Ar/³⁹Ar phengite plateau ages).⁴⁰Ar/³⁹Ar UV laser ablation ages of phengite record strain-freegrain growth and recrystallization with a duration of 31–41Myr during a pressure release of 3–4 kbar. During earlyaccretion the main intrusion in the arc occurred at 305 Ma (Pb–Pbevaporation; zircon) and the Eastern Series was overprintedby a short high-temperature metamorphism at 3 kbar, 296–301Ma (⁴⁰Ar/³⁹Ar muscovite plateau ages). Fission-track ages ofzircon (206–232 Ma) and of apatite (80–113 Ma) aresimilar in both series, indicating synchronous cooling duringdistinct periods of exhumation. Early exhumation (period I)during continuing basal accretion proceeded with mean ratesof 0·19–0·56 mm/yr, suggesting that erosionin a tectonically active area was an important unroofing mechanism.At the same time mean rates were 0·03–0·05mm/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 0·04–0·06 mm/yr in both seriesduring period II (100–225 Ma). Major extension, basinformation and local bimodal dyke intrusion at 138 Ma were accompaniedby mean cooling rates of 1–2°C/Myr. Accelerated coolingof 3–5°C/Myr at 80–113 Ma suggests a mid-Cretaceousconvergence event (period III). After 80 Ma cooling rates decreasedto 1–2°C/Myr (period IV). The pressure–temperature–deformation–timeinformation 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     

Mineral Chemistry and Pressure-Temperature Evolution of Two Contrasting High-pressure-Low-temperature Belts in the Chonos Archipelago, Southern Chile

The Chonos Metamorphic Complex forms part of a belt of low-grademetamorphic rocks in the Chilean Coastal Cordillera that areinterpreted as Palaeozoic–Mesozoic accretionary complexes.It comprises metapsammopelitic schists, metabasites and meta-ironstonesoccurring in two contrasting units. Special attention duringmicroprobe study of key samples was given to the chemical zonationof minerals. Subsequently, conventional geothermobarometry andthat using thermodynamic calculations were applied. The Easternbelt comprises rocks that are metamorphosed to pumpellyite–actinolitefacies conditions and show a low degree of deformation withwell-preserved sedimentary and igneous structures. Maximum P–Tconditions were around 5·5 kbar and 250–280°C.The rocks of the Western belt are characterized by a transitionbetween greenschist and albite–epidote–amphibolitefacies metamorphism and show a penetrative tectonic transpositionfoliation S₂ formed close to the pressure maximum. Maximum P–Tconditions vary around 8–10 kbar and 380–500°Coverstepping the stilpnomelane + phengite stability. High pressuresin this belt are confirmed by regionally distributed phengiteswith high Si contents up to 3·5 Si per formula unit.Regional distribution of maximum temperatures is reflected bythe composition of actinolitic hornblendes within the metabasites.In a garnet-bearing metabasite of the Western belt, oscillatorygrowth zoning of garnet was observed. The composition of correspondingmineral inclusions suggests that a prograde P–T path duringgarnet growth evolved from 7·5 kbar and 375°C toabout 9·4 kbar and 500°C. Late garnet grew synkinematicallywith penetrative deformation. The retrograde P–T pathin the rocks of the Western belt is constrained by the compositionof mainly late strain-free minerals and involves slight coolingduring decompression. Both belts are part of a subduction system.The apparent P–T gap between the belts is due to theirjuxtaposition during exhumation. The Eastern belt constitutesthe transition towards the backstop system of the accretionaryprism that is represented by the Western belt, whereas the absenceof very low grade rocks west of the Western belt is attributedto tectonic erosion, which was possibly caused by subductionof a ridge. KEY WORDS: Chonos Metamorphic Complex; accretionary complex; high-pressure–low-temperature metamorphism; oscillatory garnet zonation; phengite; P–T paths