The fast exhumation pattern of a Neoproterozoic nappe system built during West Gondwana amalgamation: Insights from thermochronology

We use a combination of several in situ techniques to assess the P‐T‐t path of high‐pressure granulites from the Passos Nappe in the Southern Brasília Orogen (SE Brazil). Quantitative element mapping and single‐element thermometers (Zr‐in‐rutile and Ti‐in‐quartz) are coupled with P‐T pseudosections and monazite and rutile dating. Compositional and temperature maps, based on cathodoluminescence mapping and in situ analyses of Ti‐in‐quartz, are presented as a novel approach to evaluate crystallization temperature. The studied rocks have a pelitic protolith and record a peak pressure assemblage of garnet + kyanite + rutile + K‐feldspar + quartz + melt ± plagioclase that formed at ~830°C and 1.2 GPa. Retrograde conditions of ~560°C and 0.6 GPa are determined based on the grossular content of garnet and the crystallization of biotite and ilmenite. Metamorphic peak conditions occurred ca. 635 Ma, according to monazite dating, with a younger date of ca. 615 Ma associated with later kyanite crystallization. Rutile ages of ca. 590 Ma are linked to the late retrograde stage (at ~600°C). Results show that the distribution of Ti‐in‐quartz is heterogeneous, decreasing in abundance towards the rim of crystals, though the higher temperatures constrained with Ti‐in‐quartz thermometry are broadly consistent with peak conditions. The peak pressure conditions are consistent with continental collision setting in the Southern Brasília Orogen and were followed by an early cooling/decompression stage and then by a slow cooling during exhumation and transport to shallower crustal levels.

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“…590 Ma. Similar ages and interpretations are also presented for the Andrelândia Nappe System by Reno et al (2012) and Westin et al (2021).…”

Section: Discussionsupporting

confidence: 84%

“…The cooling rate of this final stage of the retrograde P‐T ‐t path is faster ( ~ 15°C/Ma) than calculated for the earlier phase. Similar exhumation/cooling 40 Ar/ 39 Ar biotite ages around 590–570 Ma are described in high‐pressure granulite from Andrelândia Nappe System (Westin et al, 2021).…”

Section: Discussionsupporting

confidence: 63%

Petrochronology of high‐pressure granulite facies rocks from Southern Brasília Orogen, SE Brazil: Combining quantitative compositional mapping, single‐element thermometry and geochronology

Fumes

Luvizotto

Moraes

et al. 2021

Journal Metamorphic Geology

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“…The Andrelândia and Carmo da Cachoeira Nappes attained peak conditions around 650-670°C and 9-10 kbar in ca. 600 Ma, followed by almost isothermal decompression in the period of 600-575 Ma (Frugis et al, 2018;Motta and Moraes, 2017;Reno et al, 2012;Santos et al, 2004;Westin et al, 2021). In summary, the ANS displays an inverted metamorphic pattern, in which rocks in high-P granulite metamorphic facies overlaps structurally those in amphibolite facies conditions.…”

Section: Andrelândia Nappe System (Ans): P-t-t Paths and Tectonic Evo...mentioning

confidence: 90%

“…Proposed ages, based on monazite geochronology, are 660-650 Ma by Reno et al (2012Reno et al ( , 2009 (Coelho et al, 2017;Motta and Moraes, 2017;Rodrigues et al, 2019;Santos et al, 2004). Zircon and monazite U-Pb dating retrieved ages around 620-615 Ma (Coelho et al, 2017;Motta and Moraes, 2017;Westin et al, 2021). Moreover, the metamafic rocks, interpreted as retroeclogites (Campos Neto and Caby, 1999;Coelho et al, 2017;Reno et al, 2009;Trouw et al, 2013), experienced P-T conditions around 700°-800°C and 12-16 kbar (Coelho et al, 2017;Reno et al, 2009;Tedeschi et al, 2017).…”

Section: Andrelândia Nappe System (Ans): P-t-t Paths and Tectonic Evo...mentioning

confidence: 98%

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Kinematics and P-T-t evolution in hot collisional frameworks a comparison between a Neoproterozoic and a Phanerozoic large Hor orogens

Silva¹

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Large hot orogens (LHO) are characterized by an increase in heat the production and thickness of the lithosphere. The increase in heat production, related to the accretion of highradiogenic continental crust, leads to middle and lower crust partial melt and consequently decreases their strength. The gradient in lithostatic pressure between the thickened crust and No Orógeno Brasília Meridional foi estudada uma exposição do Sistema de Nappes Andrelândia, localizado no município de Pouso Alto, sudeste do Brasil. Na área, os três níveis estruturais do sistema de nappes afloram do topo para base: as nappes Pouso Alto, Liberdade e Andrelândia. Os resultados P-T-t-D obtidos indicam uma propagação "em sequência" da pilha de nappes em direção a base do Sistema de Nappes Andrelândia. A propagação das idades dentro do orógeno representaria diferentes períodos em que cada nappe do sistema atingiu metamorfismo progressivo e retrógrado. Este novo conjunto de dados reforça a ideia de que o Orógeno Brasília Sul foi produto da evolução progressiva da colisão continental entre os crátons Paranapanema e São Francisco.Além disso, os dados fornecidos da Nappe Pouso Alto registram condições metamórficas HP-UHT pela primeira vez dentro do Orógeno Brasília Meridional. Os dados P-T indicam que esta rocha fazia parte do canal de subducção continental e atingiu profundidades crustais de ca. 70 km, correspondente à interface entre a crosta inferior e o manto litosférico em uma crosta duplamente espessada. Sugere-se que a condição metamórfica de UHT foi alcançada pela combinação de acúmulo de elementos de produção de calor e menor fluxo de calor do manto. As rochas enfraquecidas pela fusão parcial iniciaram seu fluxo para fora, provavelmente impulsionadas pela força gravitacional. Por fim, propõe-se que o início da exumação registraria a transição de um cinturão orogênico em cunha para um platô orogênico durante a colisão continental.Os resultados desta tese indicam que os LHO podem apresentar uma geometria em forma de cunha (Orógeno Himalaia) e "fold-nappe" (Orógeno Brasília). O "hinterland" pode ser constituído por rochas da crosta média (Orógeno Himalaia) e/ou inferior (Orógeno Brasília). Os gradientes geotérmicos variam dentro do campo Barroviano, e as isotermas permaneceram perturbadas em um intervalo de tempo de 35 a 50 Ma até começarem a se reequilibrar. Em ambos os casos estudados, eles evoluíram por metamorfismo progressivo e deformação devido a um único evento de colisão continental. As dobras e zonas de cisalhamento desempenharam um papel de primeira ordem na acomodação da deformação dúctil durante a montagem e a exumação da crosta média e inferior. Os exemplos estudados seguem uma propagação "em sequência" do hinterland para o foreland, e sugere-se que sua exumação foi impulsionada principalmente pela força gravitacional. v ACKNOWLEDGMENTSAfter a change of country, a pandemic, a broken leg and so many other things, my PhD journey finally comes to an end. I could not fail to thank in a few lines all those who have accompanied me ...

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The potential for using metagreywacke to study metamorphism of amphibolite facies conditions: a comparison study within the Luminárias Nappe, Southern Brasília Orogen (Southeastern Brazil)

et al. 2021

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The fast exhumation pattern of a Neoproterozoic nappe system built during West Gondwana amalgamation: Insights from thermochronology

Cited by 14 publications

References 89 publications

Petrochronology of high‐pressure granulite facies rocks from Southern Brasília Orogen, SE Brazil: Combining quantitative compositional mapping, single‐element thermometry and geochronology

Petrochronology of high‐pressure granulite facies rocks from Southern Brasília Orogen, SE Brazil: Combining quantitative compositional mapping, single‐element thermometry and geochronology

Kinematics and P-T-t evolution in hot collisional frameworks a comparison between a Neoproterozoic and a Phanerozoic large Hor orogens

The potential for using metagreywacke to study metamorphism of amphibolite facies conditions: a comparison study within the Luminárias Nappe, Southern Brasília Orogen (Southeastern Brazil)

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