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We describe the geometry and kinematics of a Jurassic to Early Cretaceous transpressive sinistral strike-slip system within a metamorphic basement inlier of the Mesozoic magmatic arc near Bahia Agua Dulce at latitudes 31–32°S in north-central Chile and discuss possible relations with the Atacama Fault System further north. Sinistral transpression overprints structures of an accretionary system that is represented by the metamorphic basement. Sub-vertical semi-ductile NNW-striking strike-slip shear zones are the most conspicuous structures. Chlorite and sericite grew, and white mica and quartz dynamically recrystallized, suggesting low-grade metamorphic conditions during semi-ductile deformation. Folds at the 10–100 metre scale developed before and during strike-slip shearing. The folds are deforming a former sub-horizontal transposition foliation that originated during prior accretion processes. The folds have axes sub-parallel to the strike-slip shear zones and sub-vertical axial surfaces indicating a component of shortening parallel to the shear-zone boundaries, suggesting an overall transpressive deformation regime. Transpressive strike-slip deformation also affects Middle Triassic (Anisian) basal breccias of the El Quereo Formation. 40Ar–39Ar laser ablation ages of synkinematically recrystallized white mica in one of the shear zones provide an age of 174–165 Ma for the waning stages of semi-ductile strike-slip shearing. The semi-ductile shear zones are cut by mafic and rhyolite dykes. Two rhyolite dykes yield 40Ar–39Ar ages of 160.5 ± 1.7 Ma and 131.9 ± 1.7 Ma, respectively. The latter dyke has been affected by brittle faulting. Fault-slip analysis shows that the kinematics of the faulting event is similar to the one of the semi-ductile shearing event, suggesting that sinistral transpression continued after ~130 Ma. Timing, kinematics and geographic position suggest that the shear zones at Bahia Agua Dulce represent a southern continuation of the prominent Atacama Fault System that affected the Jurassic/Early Cretaceous arc over its ~1400 km length. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Madre de Dios Metamorphic Complex (MDMC) in southern Chile is a fossil frontal accretionary prism, which is mainly composed of metapsammopelitic rocks, intercalations of oceanic rocks (greenstone and metachert) and platform carbonate. We concentrated on the metabasite to decipher the metamorphic evolution. This rock type contains assemblages of the pumpellyite–actinolite facies: pumpellyite ± actinolite–chlorite ± grandite ± phengite ± epidote–albite–quartz–titanite ± K-feldspar ± calcite. The metamorphic phases mainly grew by prograde hydration reactions during various episodes of restricted fluid influx. Fundamental phase relations of the pumpellyite–actinolite facies and adjacent facies were reproduced by pseudosections calculated for the system K2O–Na2O–CaO–FeO–O2–MgO–Al2O3–TiO2–SiO2–H2O–CO2 at 200–400°C and 1–9 kbar. The calculated stability fields of the metamorphic assemblages as realized in the MDMC metabasite indicate highest metamorphic conditions restricted to 290–310°C, 4–6 kbar for the MDMC, presumably as a result of the main fluid influx at these conditions. Nevertheless, earlier local equilibria are still preserved as a result of strongly kinetically controlled mineral reactions and a lack of recrystallization and compositional homogenization at thin-section scale. Hence, thermodynamic calculations of local multivariant mineral equilibria using the entire compositional variation of minerals in the MDMC show that the prograde PT path evolved from 4 ± 1 kbar, 200–220°C to 5 ± 1 kbar, 290–330°C. The prograde PT path reflects nearly horizontal particle paths after reaching the maximum depth typical for frontal accretionary prisms. Long residence at maximum depth resulted in thermal re-equilibration. 40Ar/39Ar spot ages were measured by in situ UV laser ablation of local phengite concentrations in a deformed metapelite at 233·2 ± 1·8 Ma and in an undeformed metabasite at 200·8 ± 2·4 Ma. Whereas the first age represents an age of accretion, the latter age can be attributed to mineral growth either during a younger stage of accretion or during a retrograde stage. 40Ar/39Ar isotopic analyses of two further metabasite samples reflect a prominent resetting of ages at 152·0 ± 2·2 Ma and white mica growth during external fluid access triggered by either a local intrusion or a late Jurassic extensional episode. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

In the Chilean Coastal Cordillera, two units, the Western and Eastern Series, constitute coeval parts of a Late Palaeozoic paired metamorphic belt dominated by siliciclastic metasediments. The Western Series also contains rocks from the upper oceanic crust and represents an accretionary prism. Omnipresent high-pressure conditions are reflected by Na–Ca-amphibole and phengite in greenschists. Peak PT conditions of 7·0–9·3 kbar, 380–420°C point to a metamorphic gradient of 11–16°C/km. Three unique occurrences of blueschist yield deviating conditions of 9·5–10·7 kbar, 350–385°C and are interpreted as relics from the lowermost part of the basal accretion zone preserving the original gradient of 9–11°C/km along the subducting slab. Pervasive ductile deformation related to basal accretion occurred near peak PT conditions. Deformation and PT evolution of the metapsammopelitic rocks is similar to that of the metabasites. However, a rare garnet mica-schist yields peak PT conditions of 9·6–14·7 kbar, 390–440°C reflecting a retrograde stage after cooling from a high-temperature garnet-forming stage. It is considered to be an exhumed relic from the earliest siliciclastic rocks subducted below a still hot mantle wedge. A retrograde overprint of all rock types occurred at ∼300–380°C. Continuous reactions caused crystal growth and recrystallization with abundant free water mostly under strain-free conditions. They record a pressure release of 3–4 kbar without erasing peak metamorphic mineral compositions. The Eastern Series lacks metabasite intercalations and represents a less deformed retro-wedge area. In the study area it was entirely overprinted at a uniform depth at 3 ± 0·5 kbar with temperatures progressively rising from 400°C to 720°C towards the coeval Late Palaeozoic magmatic arc batholith. The interrelated pattern of PT data permits a conceptual reconstruction of the fossil convergent margin suggesting a flat subduction angle of ∼25° with continuous basal accretion at a depth of 25–40 km and a short main intrusion pulse in the magmatic arc. The latter was accompanied by the formation of a thermal dome in the retro-wedge area, which remained stable relative to the vertically growing accretionary prism characterized by cyclic mass flow. [ABSTRACT FROM AUTHOR]

Within the metamorphic basement of the Coastal Cordillera of central Chile, the Western Series constitutes the high-pressure (HP)/low-temperature (LT) part (accretionary prism) of a fossil-paired metamorphic belt dominated by metagreywackes. In its eastern part, blocks derived from small lenses of garnet amphibolite with a blueschist facies overprint are locally intercalated and associated with serpentinite and garnet mica-schist. Continuously developed local equilibria were evaluated applying various independent geothermobarometric approaches. An overall anticlockwise PT path results. The prograde path evolved along a geothermal gradient of ∼15 °C/km, passing the high-pressure end of greenschist facies until a transient assemblage developed within albite-epidote amphibolite facies transitional to eclogite facies at peak metamorphic conditions (600–760 °C, 11–16.5 kbar; stage I). This peak assemblage was overprinted during an external fluid infiltration by an epidote blueschist facies assemblage at 350–500 °C, 10–14 kbar (stage II) indicating nearly isobaric cooling. The retrograde equilibration stage was dated with a Rb–Sr mineral isochron at 305.3±3.2 Ma, somewhat younger (296.6±4.7 Ma) in an adjacent garnet mica-schist. Localized retrograde equilibration continued during decompression down to ∼300 °C, 5 kbar. The retrograde evolution is identical in the garnet amphibolite and the garnet mica-schist.The counterclockwise PT path contrasts the usual clockwise PT paths derived from rocks of the Western Series. In addition, their ages related to stage II are the oldest recorded within the fossil wedge at the given latitude. Its “exotic” occurrence is interpreted by the path of the earliest and deepest subducted material that was heated in contact with a still hot mantle. Later accreted and dehydrated material caused hydration and cooling of the earliest accreted material and the neighbouring mantle. After this change also related to rheological conditions, effective exhumation of the early subducted material followed at the base of the hydrated mantle wedge within a cooler environment (geothermal gradient around 10–15 °C/km) than during its burial. The exotic blocks thus provide important time markers for the onset of subduction mass circulation in the Coastal Cordillera accretionary prism during the Late Carboniferous. Continuous subduction mass flow lasted for nearly 100 Ma until the Late Triassic. [Copyright &y& Elsevier]

The Chonos Metamorphic Complex forms part of a belt of low-grade metamorphic rocks in the Chilean Coastal Cordillera that are interpreted as Palaeozoic–Mesozoic accretionary complexes. It comprises metapsammopelitic schists, metabasites and meta-ironstones occurring in two contrasting units. Special attention during microprobe study of key samples was given to the chemical zonation of minerals. Subsequently, conventional geothermobarometry and that using thermodynamic calculations were applied. The Eastern belt comprises rocks that are metamorphosed to pumpellyite–actinolite facies conditions and show a low degree of deformation with well-preserved sedimentary and igneous structures. Maximum P–T conditions were around 5·5 kbar and 250–280°C. The rocks of the Western belt are characterized by a transition between greenschist and albite–epidote–amphibolite facies metamorphism and show a penetrative tectonic transposition foliation S2 formed close to the pressure maximum. Maximum P–T conditions vary around 8–10 kbar and 380–500°C overstepping the stilpnomelane + phengite stability. High pressures in this belt are confirmed by regionally distributed phengites with high Si contents up to 3·5 Si per formula unit. Regional distribution of maximum temperatures is reflected by the composition of actinolitic hornblendes within the metabasites. In a garnet-bearing metabasite of the Western belt, oscillatory growth zoning of garnet was observed. The composition of corresponding mineral inclusions suggests that a prograde P–T path during garnet growth evolved from 7·5 kbar and 375°C to about 9·4 kbar and 500°C. Late garnet grew synkinematically with penetrative deformation. The retrograde P–T path in the rocks of the Western belt is constrained by the composition of mainly late strain-free minerals and involves slight cooling during decompression. Both belts are part of a subduction system. The apparent P–T gap between the belts is due to their juxtaposition during exhumation. The Eastern belt constitutes the transition towards the backstop system of the accretionary prism that is represented by the Western belt, whereas the absence of very low grade rocks west of the Western belt is attributed to tectonic erosion, which was possibly caused by subduction of a ridge. [ABSTRACT FROM PUBLISHER]