Widespread Occurrences of Hogbomite-2N2S in UHT Metapelites from the Betroka Belt, Southern Madagascar: Implications on Melt or Fluid Activity during Regional Metamorphism

Rakotonandrasana, N. O. Thierry; Arima, Makoto; Miyawaki, Ritsuro; Rambeloson, Roger A.

doi:10.1093/petrology/egq004

Cited by 16 publications

(10 citation statements)

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“…, 1996), the differences in estimated peak temperature is not surprising given cation exchange was likely during retrograde cooling. Studies applying phase diagrams give temperatures more similar to those reported here, although some argue for appreciably higher temperatures (Rakotonandrasana et al. , 2010).…”

Section: Discussionsupporting

confidence: 79%

The importance of iron speciation (Fe⁺²/Fe⁺³) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

Boger

White

Schulte

2012

Journal Metamorphic Geology

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Metapelitic granulites from the Anosyen domain of southeastern Madagascar are exposed in three intercalated formations: the Amparihy, Bakika and Ihosy formations. Although mineralogically distinct from each other, the rocks from these formations show very similar bulk-rock compositions when measured on a Fe T basis. The preserved mineral assemblages thus do not reflect differences in the ratios of the main rock-forming oxides (i.e. Al 2 O 3 :Fe T :MgO), but instead reflect variations in the premetamorphic oxidation state of the protolith rocks. These differences in oxidation state are manifested via differences in iron speciation -either Fe +2 or Fe +3 . The relatively reduced rocks of the Amparihy Formation preserve the assemblage bi-sp-sill-g-cd, which contrasts markedly with the mostly garnet and spinel-absent bi-cd-sill-mt assemblages preserved in the strongly oxidized rocks of the Ihosy Formation. Compositionally intermediate rocks of the Bakika Formation are garnet bearing, but sillimanite-absent, and contain the assemblage sp-g-cd-mag. Modelling of these rocks in the Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O system suggests that they evolved along a heating and cooling P-T path with only limited decompression accompanying cooling on the retrograde path. Peak temperatures and pressures of 880-920°C and 6-6.5 kbar are inferred for the majority of the Anosyen domain, with slightly lower peak temperatures (840°C) estimated in the extreme northwest of the area. The high-temperature and relatively low-pressure nature of metamorphism suggests high geothermal gradients existed during orogenesis, which in southern Madagascar is related to the amalgamation of Gondwana (580-520 Ma). Although metamorphic temperatures may have been augmented via thermal advection from the emplacement of the syn-to post-tectonic Ambalavao suite, the high geothermal gradients nevertheless suggest thin and consequently hot lithosphere existed prior to orogenesis.

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Section: Discussionsupporting

confidence: 79%

The importance of iron speciation (Fe⁺²/Fe⁺³) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

Boger

White

Schulte

2012

Journal Metamorphic Geology

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“…Recently recognised localities ( Fig. 2; Table 1) include the Acadian Orogen in northeast USA (Ague and Eckert, 2012;Ague et al, 2013), the Garzón Massif in Colombia (Altenberger et al, 2012), the Schirmacher Hills in Dronning Maud Land, Antarctica (Baba et al, 2006;Baba et al, 2010), the Central Indian Tectonic Zone (Bhandari et al, 2011;Bhowmick et al, 2014), the Anosyen domain of SE Madagascar (Jöns and Schenk, 2008;Rakotonandrasana et al, 2010;Jöns and Schenk, 2011;Boger et al, 2012), the South Altay Belt in NW China (Chen et al, 2006;, the Lapland Granulite Belt…”

Section: What Is Uht Crustal Metamorphism?mentioning

confidence: 99%

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

Kelsey

Hand

2015

Geoscience Frontiers

367

285

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a b s t r a c tUltrahigh temperature (UHT) metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 C. UHT crustal metamorphism is recognised in more than 50 localities globally in the metamorphic rock record and is accepted as 'normal' in the spectrum of regional crustal processes. UHT metamorphism is typically identified on the basis of diagnostic mineral assemblages such as sapphirine þ quartz, orthopyroxene þ sillimanite AE quartz and osumilite in MgeAlrich rock compositions, now usually coupled with pseudosection-based thermobarometry using internally-consistent thermodynamic data sets and/or Al-in-Orthopyroxene and ternary feldspar thermobarometry. Significant progress in the understanding of regional UHT metamorphism in recent years includes: (1) development of a ferric iron activityecomposition thermodynamic model for sapphirine, allowing phase diagram calculations for oxidised rock compositions; (2) quantification of UHT conditions via trace element thermometry, with Zr-in-rutile more commonly recording higher temperatures than Ti-in-zircon. Rutile is likely to be stable at peak UHT conditions whereas zircon may only grow as UHT rocks are cooling. In addition, the extent to which Zr diffuses out of rutile is controlled by chemical communication with zircon; (3) more fully recognising and utilising temperature-dependent thermal properties of the crust, and the possible range of heat sources causing metamorphism in geodynamic modelling studies; (4) recognising that crust partially melted either in a previous event or earlier in a long-duration event has greater capacity than fertile, unmelted crust to achieve UHT conditions due to the heat energy consumed by partial melting reactions; (5) more strongly linking UePb geochronological data from zircon and monazite to PeT points or path segments through using Y þ REE partitioning between accessory and major phases, as well as phase diagrams incorporating Zr and REE; and (6) improved insight into the settings and factors responsible for UHT metamorphism via geodynamic forward models. These models suggest that regional UHT metamorphism is, principally, geodynamically related to subduction, coupled with elevated crustal radiogenic heat generation rates.Ó 2014, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

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“…Rakotonandrasana, Arima, Miyawaki, and Rambeloson () calculated P–T conditions ranging from 880°C at 0.45 GPa to 1,060°C at 0.65 GPa for högbomite–hematite‐bearing pelitic rocks within the Anosyen domain just south of Ihosy using pseudosections and WinTWQ. They interpreted that the rocks experienced a component of decompression near peak temperature based on coronal relationships suggesting the reactions spinel+quartz→cordierite, and garnet+sillimanite→cordierite+spinel, but did not quantify the magnitude of decompression.…”

Section: The Anosyen Domain Of Southern Madagascarmentioning

confidence: 99%

“…Raith et al (2008) calculated metamorphic conditions of 800°C/0.6-0.7 GPa from corundum-spinel-sapphirineanorthite-phlogopite gneisses ("sakenites") 75 km northwest of Ihosy, suggesting that the northern part of the Anosyen domain experienced lower peak temperatures, consistent with the interpretations of Boger et al (2012) and J€ ons and Schenk (2011). Rakotonandrasana, Arima, Miyawaki, and Rambeloson (2010) calculated P-T conditions ranging from 880°C at 0.45 GPa to 1,060°C at 0.65 GPa for h€ ogbomite-hematitebearing pelitic rocks within the Anosyen domain just south of Ihosy using pseudosections and WinTWQ. They interpreted that the rocks experienced a component of decompression near peak temperature based on coronal relationships suggesting the reactions spinel+quartz?cordierite, and garnet+sillimanite?cordierite+spinel, but did not quantify the magnitude of decompression.…”

Section: South Americamentioning

confidence: 99%

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

Holder

Hacker

Horton

et al. 2018

Journal Metamorphic Geology

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We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane.

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Widespread Occurrences of Hogbomite-2N2S in UHT Metapelites from the Betroka Belt, Southern Madagascar: Implications on Melt or Fluid Activity during Regional Metamorphism

Cited by 16 publications

References 47 publications

The importance of iron speciation (Fe⁺²/Fe⁺³) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

The importance of iron speciation (Fe⁺²/Fe⁺³) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

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Widespread Occurrences of Hogbomite-2N2S in UHT Metapelites from the Betroka Belt, Southern Madagascar: Implications on Melt or Fluid Activity during Regional Metamorphism

Cited by 16 publications

References 47 publications

The importance of iron speciation (Fe+2/Fe+3) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

The importance of iron speciation (Fe+2/Fe+3) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

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The importance of iron speciation (Fe⁺²/Fe⁺³) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

The importance of iron speciation (Fe⁺²/Fe⁺³) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar