Petrology of Whiteschists and Associated Rocks at Mautia Hill (Tanzania): Fluid Infiltration during High-Grade Metamorphism?

Jöns, Niels; Schenk, Volker

doi:10.1093/petrology/egh044

Cited by 25 publications

(27 citation statements)

References 54 publications

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“…In contrast, comparable peak conditions of 1.0-1.4 GPa and 700-800°C in the reworked Western Granulites (Sommer et al, 2003;Jöns and Schenk, 2004) reflect the younger event at 0.58-0.55 Ga (Cutten et al, 2006;Tenczer et al, 2013). Another difference is that although both Eastern and Western Granulites have a P-T path with substantial prograde and retrograde P change, the former is anticlockwise and the latter clockwise (Appel et al, 1998;Jöns and Schenk, 2004;Tenczer et al, 2011) consistent with suggestions they are distinct thrust nappes with different histories (Fritz et al, 2009.…”

Section: Metamorphism Along the Mozambique Suturesupporting

confidence: 60%

“…Similar conditions of 0.9-1.2 GPa and 750-880°C in the equivalent Vohibory Domain of Madagascar (Nicollet, 1986(Nicollet, , 1990Jöns and Schenk, 2008) are dated at 0.65-0.61 Ga with a younger overprint at 0.58-0.52 Ga (de Wit et al, 2001;Jöns and Schenk, 2008;Collins et al, 2012;Boger et al, 2015). In contrast, comparable peak conditions of 1.0-1.4 GPa and 700-800°C in the reworked Western Granulites (Sommer et al, 2003;Jöns and Schenk, 2004) reflect the younger event at 0.58-0.55 Ga (Cutten et al, 2006;Tenczer et al, 2013). Another difference is that although both Eastern and Western Granulites have a P-T path with substantial prograde and retrograde P change, the former is anticlockwise and the latter clockwise (Appel et al, 1998;Jöns and Schenk, 2004;Tenczer et al, 2011) consistent with suggestions they are distinct thrust nappes with different histories (Fritz et al, 2009.…”

Section: Metamorphism Along the Mozambique Suturementioning

confidence: 90%

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Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

Fitzsimons

2016

Journal of Mineralogical and Petrological Sciences

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Granulites of the southern East African Orogen formed by continental collision during Gondwana assembly. The highest metamorphic gradients of 25-50°C km −1 were attained at 0.58-0.53 Ga in a microcontinental block that was sandwiched between two collisional sutures and is now exposed in Madagascar and southern India. The 50 Myr duration of extreme pressure-temperature (P-T) conditions and lack of coeval mantle magmatism suggest that metamorphism was driven by radiogenic heat accumulation beneath a long-lived orogenic plateau. Bounding sutures most likely record transfer of this microcontinent across the Neoproterozoic Mozambique Ocean, analogous to Gondwanan terranes that crossed Tethys before final India-Asia collision and, like Tibet, these sutures mark the edges of a plateau that formed following terminal ocean closure and collision. Both sutures record moderate metamorphic gradients of 15-25°C km −1 but with quite different ages. Metamorphism along the western suture at 0.65-0.61 Ga followed the end of magmatism in an adjacent 0.85-0.65 Ga ocean-arc terrane. It has an anti-clockwise P-T path that reflects preferential thickening of the hot arc during early stages of collision, and dates ocean closure at the western suture. Metamorphism along the eastern suture at 0.53-0.51 Ga has a clockwise P-T path and is widely assumed to date terminal collision in the East African Orogen. However, this event was coeval with rapid exhumation of granulites in the adjacent plateau and is more likely to reflect reactivation of a much older eastern suture during plateau collapse. Great care should be taken when using metamorphism to date ocean closure in ancient orogens. Rocks with hot metamorphic gradients give poor age constraints on initial collision because peak T is attained >50 Myr after ocean closure if radioactivity is a major part of the heat budget. Suture zone rocks with moderate metamorphic gradients can provide more reliable estimates for the time of ocean closure but are also prone to later reactivation in orogens with protracted histories.

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Section: Metamorphism Along the Mozambique Suturesupporting

confidence: 60%

Section: Metamorphism Along the Mozambique Suturementioning

confidence: 90%

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

Fitzsimons

2016

Journal of Mineralogical and Petrological Sciences

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“…The presence of hematite and elevated amounts of Mn 3? in silicates of the Mautia Hill whiteschists indicate a very high oxygen fugacity (Jöns and Schenk 2004), which is also true for the yoderite-whiteschists intercalated with quartz-hematite bands from the Zambezi belt (Johnson and Oliver 2002). As an exact determination of the oxygen fugacity during these metasomatic processes is not possible, we calculated phase diagrams both at the magnetite-hematite buffer and with oxygen in abundance to cover a wide range of conditions.…”

Section: Discussionmentioning

confidence: 99%

“…Highly oxidizing conditions were also recorded by the yoderite-bearing Kadunguri whiteschists of the Zambezi belt in northern Zimbabwe (Johnson and Oliver 2002). In both African whiteschist occurrences, the percolation of a syn-metamorphic, high-fO 2 metasomatic fluid was postulated (Johnson and Oliver 2002;Jöns and Schenk 2004). The main effect of high fO 2 is the fixation of ferrous iron in oxide minerals and subordinately in silicates, which also may apply to manganese.…”

Section: Effect Of H 2 Omentioning

confidence: 96%

“…Fockenberg and Schreyer 1994) and hematite as well as piemontite and manganian andalusite in the whiteschists of Mautia Hill, Tanzania (McKie 1959;Jöns and Schenk 2004) indicate that these whiteschists experienced elevated oxygen fugacities. Highly oxidizing conditions were also recorded by the yoderite-bearing Kadunguri whiteschists of the Zambezi belt in northern Zimbabwe (Johnson and Oliver 2002).…”

Section: Effect Of H 2 Omentioning

confidence: 99%

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Protoliths and phase petrology of whiteschists

Franz

Romer

Capitani

2013

Contrib Mineral Petrol

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Whiteschists appear in numerous high-and ultrahigh-pressure rock suites and are characterized by the mineral assemblage kyanite ? talc (?-quartz or coesite). We demonstrate that whiteschist mineral assemblages are well stable up to pressures of more than 4 GPa but may already form at pressures of 0.5 GPa. The formation of whiteschists largely depends on the composition of the protolith, which requires elevated contents of Al and Mg as well as low Fe, Ca, and Na contents, as otherwise chloritoid, amphibole, feldspar, or omphacite are formed instead of kyanite or talc. Furthermore, the stability field of the whiteschist mineral assemblage strongly depends on XCO 2 and fO 2 : already at low values of XCO 2 , CO 2 binds Mg to carbonates strongly reducing the whiteschist stability field, whereas high fO 2 enlarges the stability field and stabilizes yoderite. Thus, the scarcity of whiteschist is not necessarily due to unusual P-T conditions, but to the restricted range of suitable protolith compositions and the spatial distribution of these protoliths: (1) continental sedimentary rocks and (2) hydrothermally and metasomatically altered felsic to mafic rocks. The continental sedimentary rocks that may produce whiteschist mineral assemblages typically have been deposited under arid climatic conditions in closed evaporitic basins and may be restricted to relatively low latitudes. These rocks often contain large amounts of the clay minerals palygorskite and sepiolite. Marine sediments generally do not yield whiteschist mineral assemblages as marine shales commonly have too high iron contents. Sabkha deposits may have too high CO 2 contents. Protoliths of appropriate geochemical composition occur in and on continental crust. Therefore, whiteschist assemblages typically are only found in settings of continental collision or where continental fragments were involved in subduction. Our calculations demonstrate that whiteschists can form by closed-system metamorphism, which implies that the chemical and isotopic composition of these rocks provide constraints on the development of the protoliths.

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