Thermobarometry, tectonic-metamorphic evolution and geochemistry of blueschists, eclogitic rocks and associated lithotypes of Diego de Almagro Island, Chilean Patagonia

Na ilha Diego de Almagro, localizada na Patagônia Chilena, ocorrem metabasitos de alta pressão que compõem, juntamente com metatufos, xistos cloríticos, metacherts (coticulitos), xistos micáceos, xistos grafitosos e quartzo micaxistos, uma seqüência metavulcanossedimentar. Relíquias de pillow lavas com tufos finos inter pillow e camadas com pillow breccias indicam, em conjunto com a composição geoquímica, que esta seqüência formou-se em ambiente oceânico. As rochas da seqüência vulcanossedimentar foram transformadas em xistos azuis e rochas eclogíticas, os quais registram ainda a evolução retrometamórfica para as fácies epidoto anfibolito e xisto verde. A gênese das rochas de alta pressão é atribuída a uma zona de subducção cretácica desenvolvida na margem oeste do supercontinente Gondwana. Os cálculos termobarométricos, realizados mediante o software TWQ 1.02, apontam para a presença de xistos azuis transicionais para eclogitos, gerados em condições de 18 kbar e 400 °C e rocha eclogítica com pico metamórfico calculado entre 19,5 e 21 kbar e 580 e 650 °C. O registro do retrometamorfismo que afetou os xistos azuis ocorreu sob condições de pressão entre 8,5 e 9,5 kbar e temperaturas entre 420 e 500 °C. A rocha eclogítica evidencia uma primeira etapa de descompressão a 18 kbar, com temperaturas entre 615 e 670 °C, seguida de 10 kbar e 540 °C. Granada anfibolito que ocorre associado à rocha eclogítica mostra condições semelhantes de pico metamórfico, por volta de 20 kbar e 630 °C. Mapas composicionais de raios X em granada, das rochas de alta pressão, associados a seus perfis composicionais, mostram que as rochas eclogíticas são resultado da progressão do metamorfismo dos xistos azuis. Os porfiroblastos de granada das rochas eclogíticas constituem complexas texturas em atol, e registram as condições de pico metamórfico em suas bordas, formadas num estágio posterior ao desenvolvimento dos atóis. Estas texturas parecem indicar que a formação dos atóis na rocha eclogítica ocorreu pela liberação de fluidos, possivelmente durante uma pausa na subducção, e sua retomada culminou com a geração das bordas grossularíticas na granada. As trajetórias metamórficas das rochas de alta pressão são horárias e indicam descompressão com pequeno aquecimento associado. Os minerais dos xistos azuis registram o início da exumação nas bordas dos minerais do domínio microestrutural da foliação Sn, ao passo que os porfiroblastos de granada da rocha eclogítica indicam, no mesmo domínio microestrutural, as condições de pico metamórfico em suas bordas. Geoquimicamente os protolitos dos metabasitos de alta pressão mostram afinidade geoquímica com basaltos de fundo oceânico, transicional entre MORB e intraplaca, e ambiente tectônico de geração do magma precursor em cadeia meso-oceânica, com possível interação com pluma mantélica. Foram também identificadas rochas com alterações hidrotermais superimpostas que modificaram as composições originais dos litotipos, localizadas na continuidade de uma importante feição estrutural da ilha, a Zona de Cisalhamento Seno Arcabuz, que está possivelmente relacionada à exumação das rochas de alta pressão.At Diego de Almagro Island, localized at Chilean Patagonia, occur high pressure metabasites which constitute, in association with meta tuffs, chloritic schists, metacherts (coticules), micaceous schists, graphite schists and quartz micaschist, a metavolcanosedimentary sequence. Relics of pillow lavas, inter pillow fine tuffs and lenses of pillow breccias, together with the geochemical data, indicate that this sequence was formed in an intra-oceanic environment. These lithotypes were transformed in blueschists and eclogitic rocks, recording a late metamorphic evolution to epidote amphibolite and greenschist facies. Their origin is attributed to a cretaceous subduction zone developed at the western margin of the Gondwana supercontinent. Thermobarometry was carried out using the software TWQ 1.02 and points to the presence of blueschists in transitional metamorphic conditions to eclogites, generated at 18 kbar and 400 °C. The eclogitic rock has a calculated metamorphic peak between 19.5 and 21kbar, and temperatures in the range of 580-650 °C. The retrograde metamorphism recorded in the blueschists occurred with pressures between 8.5 and 9.5 kbar and temperatures in the range of 420500 °C. For the eclogitic rock the first decompression points to P=18 kbar with temperatures between 615 and 670 °C, followed by metamorphic conditions of 10 kbar and 540 °C. Garnet amphibolite associated to the eclogitic rock exhibits similar metamorphic peak conditions around 20 kbar and 630 °C. Compositional X rays maps obtained in garnets of high pressure rocks, used together with their compositional profiles, show that eclogitic rocks are products of the blueschists progressive metamorphism. The garnet porphyroblasts of the eclogitic rock comprise complex atoll textures which were generated before the metamorphic peak recorded at their rims. The textures could indicate that the atolls generation occurred as a consequence of fluids release, probably achieved during a subduction cessation, and the subduction restarting was responsible for the formation of grossular rich rims. The high pressure metamorphic P-T paths are clockwise and point out a decompression in association with a little heat increasing. The blueschists minerals record the beginning of exhumation at their rims in the Sn foliation microstructural domain, whilst garnet porphyroblasts of the eclogitic rock, from the similar microstructural domain, indicate metamorphic peak at their rims. Geochemical studies, realized for the high pressure metabasites, exhibit geochemical affinity with ocean floor basalts, transitional between MORB-type and intraplate, and paleotectonic environment of mid ocean-ridge, with possible mid ocean-ridge/plume interactions during the generation of the pristine magma. Rocks overprinted by hydrothermal alterations, which modified their original compositions, were also identified and are located at the continuity of an important structural feature at the island, the Seno Arcabuz Shear Zone, that is probably related to the high pressure rocks exhumation

Reply to Comment by Heilbron and Valeriano on “Tectono Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West Gondwana”

The exercise of paleogeographic/tectonic reconstruction of past orogenic belts is a complex task that includes the interpretation and integration of multi-technique approaches such as basin tectonics, structural geology, petrology, geochemistry, geochronology, and geodynamics. Proterozoic geologic records are fragmented and incomplete, which make efforts to reconstruct paleogeography and orogenic processes even more challenging. Therefore, for our understanding of the tectonic evolution of past orogenic systems to advance, it is essential that any existing model, including the ones proposed by Heilbron and Valeriano (2020, https://doi.org/10.1029/2019tc005897), are repeatedly exposed to debate and testing. In this reply we address the points raised by Heilbron and Valeriano in order to clarify the scientific foundations of our tectonic interpretation for one of the Neoproterozoic Brasiliano/Pan-African orogens in South America (the Central Ribeira Belt), presented in Meira, Garcia-Casco, Hyppolito, et al. (2019, https://doi.org/10.1029/2018TC004959). We also evaluate the proposed paleogeographic reconstructions for this part of West Gondwana and conclude that an intracontinental model better explains the currently available data and observations, including the space problem pointed out in recent publications, and that existing geochemical and geochronologic data by themselves are not conclusive with respect to an unequivocal tectonic environment.This research was financially supported by São Paulo Research Foundation (FAPESP Grants, #2012/154627 and #2016/061146), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq Gran t, #404767/20168), and CIC (University of Granada). Renato Paes de Almeida, Haakon Fossen, Carolina Cavalcante, Renata Schmitt, Carlos Ganade, and Guiseppe de Toni are fully acknowledge for fruitful discussion

A 100-m.y.-long window onto mass-flow processes in the Patagonian Mesozoic subduction zone (Diego de Almagro Island, Chile)

Diego de Almagro Island was formed by the subduction and accretion of several seafloor-derived tectonic slices with very heterogeneous ages and pressure-temperature-time (P-T-t) paths. The highest element of the pile (the Lazaro unit) evidences subduction in the high-P granulite field (∼1.3 GPa, 750 °C) at ca. 163 Ma. Below it, a thin tectonic sliver (the Garnet Amphibolite unit) preserves eclogite-facies remnants (∼570 °C and ∼1.7 GPa) formed at ca. 131 Ma (in situ U-Pb zircon rim ages). Peak assemblages were nearly fully amphibolitized during decompression down to ∼1.2 GPa and ∼600 °C at 125–120 Ma (Rb-Sr multimineral dating). The underlying Blueschist unit has ∼50 m.y. younger metamorphic ages and exhibits slightly cooler peak burial conditions (∼520 °C, 1.7 GPa; ca. 80 Ma, in situ white mica Ar-Ar ages and multimineral Rb-Sr dating) and is devoid of amphibolitization. The mylonites from the sinistral strike-slip Seno Arcabuz shear zone bounding Diego de Almagro Island to the east also exhibit amphibolite-facies (∼620 °C and ∼0.9 GPa) deformation at ca. 117 Ma (multimineral Rb-Sr ages). In situ white mica Ar-Ar dating and multimineral Rb-Sr dating of low-T mylonites (∼450 °C) along the base of the Lazaro unit reveal partial resetting of high-T assemblages during tectonic displacement between 115 and 72 Ma and exhumation of the slice stack.Detrital zircon U-Th-Pb ages indicate that the material accreted on Diego de Almagro Island has been mostly recycled from a Permian–Triassic accretionary wedge (Madre de Dios accretionary complex) exposed along the subduction buttress. Geological and geochronological constraints suggest that the rocks of the Seno Arcabuz shear zone and the Lazaro unit were tectonically eroded from the buttress, while the underlying Garnet Amphibolite and Blueschist units instead derive from the subducted oceanic basin, with increasingly younger maximum depositional ages. The very long residence time of the rocks (∼90 m.y. for the Lazaro unit) along the hanging wall of the subduction interface recorded long-term cooling along the Patagonian subduction zone during the Mesozoic. Diego de Almagro Island therefore represents a unique window onto long-term tectonic processes such as subduction interface down-stepping, tectonic erosion, and episodic underplating near the base of an accretionary wedge (40–50 km)

Tectono-Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West Gondwana

Recent advances in understanding the plate tectonics, intracontinental deformation, and flow of partially molten crust have significantly improved our knowledge of collisional tectonics and the way in which we understand complex ancient orogens. The Central Ribeira Belt represents a Neoproterozoic fold-and-thrust belt formed in the Brasiliano Orogenic Cycle associated with the assembly of West Gondwana. This fold-and-thrust belt is currently interpreted as a result of recurrent collisions and amalgamation of terranes against large cratons. Based on an integrated structural, petrological, and geochronological study in two metamorphic complexes of the Central Ribeira Belt (Embu and Costeiro complexes), we challenge the current model that involves multiple terrane collisions. Our data show for the first time metamorphic ages older than 600 Ma for samples from Costeiro and Embu complexes and suggest that both geological units experienced an intermediate-P metamorphism (M1) at circa 620 Ma and a low-P metamorphism (M2) at circa 575 Ma. Our proposed tectonic model is consistent with an M1 event related to an intracontinental orogeny, formed in response to the collision between the São Francisco Craton and the Paranapanema Block. On the other hand, the later M2 metamorphism records extensional and wrench tectonics associated with orogenic collapse, constrained by the decompression paths of the metasedimentary sequences and M2-related S3 mylonitic foliation. The M2 metamorphism is associated with wide, right-lateral strike-slip shear zones and voluminous peraluminous magmatism in the Embu Domain and widespread partial melting of the middle crust forming migmatitic rocks and peraluminous leucogranites in the Costeiro Domain.supporting information The first author is thankful to CAPES and CNPq for the PhD scholarships and CAPES for postdoctoral fellowship. T. Hyppolito acknowledges São Paulo Research Foundation for a postdoctoral fellowship (FAPESP Grant 2014/23422‐0). Fernando Bea and Pilar Montero are thanked for their help with SHRIMP analysis and data treatment. Lucas Schiavetti and Glaucia Barreto are thanked for field assistance. Haakon Fossen is fully acknowledged for his comments and suggestions on the first version of this manuscript. Aitor Cambeses is acknowledged for fruitful discussions and help with the illustrations. Thanks are due to the COTEC‐IF and workers of the Parque Estadual da Serra do Mar and Parque Estadual de Ilhabela for allowing access and sampling in the conservation area. This research was financially supported by São Paulo Research Foundation (FAPESP Grants 2012/15462‐7 and 2016/06114‐6), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq Grant 404767/2016‐8), and CIC (University of Granada). The data used are listed in the references, tables, and . This is the IBERSIMS Publication 2

Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

International audienceThe Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed in granulite-facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite-facies remnants of the Tethyan seafloor (Zermatt-Saas zone). Near the base of the Mt. Emilius massif, a shear zone presents eclogite-facies hydrofracture systems associated with deformation-induced re-equilibration of granulites during high pressure metamorphism. We herein focus on a pluri-hectometer-length domain of sheared mafic boudins hosted in the granulitic paragneiss. These mafic boudins exhibit garnetites and garnet veins, clinopyroxenites, clinozoisite and calcite veins. We report multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid-rock interaction, metasomatism and volume change. Our integrated petrological, micro-textural and geochemical investigations illustrate the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite-eclogite-facies field (at ~ 2.15-2.4 GPa, 500-550°C). The calcite veins crosscutting the garnetites have relatively low δ18OVSMOW values (c. +6.5 per mil) near those for marble layers (and nearby calc-silicates) embedded within the metasomatized granulites (+8 to +10 per mil). It is proposed that infiltration of externally-derived H2O-rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short-distance transport and re-precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate, and creating long-lasting mechanical instabilities during subduction zone deformation

Geochemical constraints on blueschist- and amphibolite-facies rocks of the Central Cordillera of Colombia: the Andean Barragan region

Subduction zones are one of the most characteristic features of planet Earth. Convergent plate junctions exert enormous influence on the formation and recycling of continental crust, and they are also responsible for major mineral resources and earthquakes, which are of crucial importance for society. A subduction-related geologic unit containing high-pressure rocks occurs in the Barragan area (Valle del Cauca Department) on the western flank of the Central Cordillera of the Colombian Andes. Blueschists and amphibolites, serpentinized meta-ultramafic rocks, graphite-chlorite-muscovite-quartz schists, protocataclasites, and graphite-chlorite-andalusite-andesine-garnet-muscovite +/- titanite schists are exposed in this region. In spite of the petrotectonic importance of blueschists, the high-pressure metamorphism of the Central Cordillera of Colombia has been rarely studied. New geochemical data indicate that protoliths of the blueschist- and amphibolite-facies rocks possessed normal mid-ocean ridge basalt bulk compositions. Ar-40/Ar-39 geochronology for a metapelite rock associated with the blueschists shows a plateau age of similar to 120 million years. We suggest that high-P/T conditions were present from similar to 150 to 125 Ma, depending on the model of generation and exhumation considered.FAPESP foundation of Brazil [2004-10203-7]FAPESP foundation of BrazilFundacion para la Promocion de la Investigacion y la Tecnologia of Colombia [2178]Fundacion para la Promocion de la Investigacion y la Tecnologia of ColombiaNational Council for Scientific and Technological Development agency (CNPq) of BrazilNational Council for Scientific and Technological Development agency (CNPq) of Brazi

Hot subduction in the middle Jurassic and partial melting of oceanic crust in Chilean Patagonia

Rare remnants of a Mesozoic subduction high pressure (HP) accretionary complex are exposed on Diego de Almagro Island in Chilean Patagonia. We herein focus on the Lazaro unit, a coherent slice of oceanic crust exposed on this island that has been first affected by high temperature (HT) metamorphism followed by a lower temperature deformation event (LT). Its Pressure-Temperature-time (P-T-t) evolution is reconstructed using field and petrographic observations, phase relations, thermobarometry and geochronology. Remnants of a primary amphibolite to HP granulite-facies event in mafic rocks comprising garnet (with ilmenite exsolutions), diopside, trondhjemitic melt, pargasite, plagioclase ± epidote are reported for the first time in neosomes, indicating peak P-T conditions of 1.1–1.3 GPa and c. 750 °C. This peak T paragenesis has been thoroughly overprinted by a phengite-chlorite-actinolite assemblage during isobaric cooling down to c. 450 °C. U-Pb dating of zircon metamorphic rims from a metasedimentary rock yielded a homogeneous age population of 162 ± 2 Ma for the HT event. Sm-Nd dating of two peritectic garnet-bearing samples yield ages of 163 ± 2 Ma and 163 ± 18 Ma for the HT event. Multi-mineral Rb-Sr dating of a metasedimentary rock overprinted by LT deformation suggests retrograde shearing between 120 and 80 Ma. Our results show that the HT event in the Lazaro unit took place at around 160–165 Ma, shortly before the onset of Patagonian Batholith emplacement. Partial melting of subducted oceanic crust reported in the Lazaro unit is related to the early stages of hot subduction along the Gondwana western margin. The Lazaro unit remained at c. 40 km depth along the subduction interface for > 80 Ma, recording the deformation and long-term cooling of the subduction channel environment until the upper Cretaceous.Jesus Muñoz is acknowledged for field work assistance and discussions. Silvio Ferrero and Patrick O′Brien are acknowledged for insightful discussions and Philippe Yamato for sharing numerical modeling results. This project has been funded by a Deutsche Forschungsgemeinschaft (DFG) project to S.A. (AN1113-1), and São Paulo Research Foundation (FAPESP) (#2004/10203-7, #2012/01191-1) and received support for analytical costs at CIC from the University of Granada. T.H. acknowledges the grant #2014/23422-0 (FAPESP) for a post-doctoral fellowship and M.C. the Fondecyt grant #1161818. Arne Willner and Victor Ramos are fully acknowledged for insightful reviews on this manuscript. The journal editor Damian Nance is acknowledged for his careful editorial handling. This is IPGP contribution #3794. This is IBERSIMS publication n.36

S3. Rehydration of eclogites and garnet-replacement processes during exhumation in the amphibolite facies

Additional X-ray data on a garnet from sample #39c

S4. Rehydration of eclogites and garnet-replacement processes during exhumation in the amphibolite facies

EPMA and LA-ICP-MS processed data for sample #217