Ultra-High Temperature metamorphism corresponds to extreme conditions of the granulites facies conditions (900-1100°C, 7-13 kbar) in the continental crust. In recent years, ultrahigh-temperature (UHT) granulites have been discovered in numerous high-grade gneiss terrains, like in:
- Napier Complex, Rauer group, Antarctique (Ellis, 1980; Grew, 1980; Sandiford, 1985; Harley, 1985, 1986, 1998; Harley et al., 1991, 2000)
- Wilson Lake, Canada (Arima et al., 1986; Currie et Gittins, 1988)
- Eastern Ghats, Palni Hill Ranges, Inde (Lal et al., 1987; Dasgupta, 1995; Raith et al., 1997, Bose et al., 2000; Rickers et al., 2001)
- In Ouzal, Algérie (Kienast et Ouzegane, 1987; Bertrand et al., 1992)
- Labwor Hills, Uganda (Sandiford, 1987)
- Highland Complex, Sri Lanka (Kriegsman et Schumacher, 1999)
- Barro alto complex, Brasil (De Moraes et Fuck, 2000)
- Limpopo belt, Zimbabwe (Droop, 1989)
- Central Alps, Italie (Droop et Busher-Nurminen, 1984)
- Madagascar (Nicollet, 1990)
- Mollendo-Camana Block, Peruvian Andes (Martignole et Martelat, 2003)
These UHT conditions are usually preserved in aluminous granulites, with characteristic minerals like sapphirine, garnet, Al-rich orthopyroxene, cordierite, sillimanite, corundum etc... Owing to their refractory behavior, these rocks commonly preserve numerous and spectacular coronitic textures providing a powerful means to retrieve the PT evolution close to peak conditions.
In this section, the petrographical PT evolution of a UHT granulite from Madagascar is described. This work was done during my PhD Thesis in collaboration with Christian Nicollet (Universiy of Clermont-Ferrand).
Petrology of a suite of UHT granulites from Madagascar
UHT granulites have been reported in the central part of the Andriamena unit (grey unit) (North-Central Madgascar) near the village of Andriamena and Brieville (Nicollet, 1990) (location a, b and c).
They occur as concordant lenses within a composite gneissic foliation composed of tonalitic and granodioritic gneisses with pelitic migmatites and mafic gneisses.
click in the figure to see photos of the area
A reaction texture sequence has been inferred from partial pseudomorph and coronitic textures, which can be interpreted using a petrogenetic grid in the FMAS(H) system (constructed with Perple_X).
click on the assemblages to delimitate their stability fields under anhydrous conditions:
The P-T petrographical path inferred consist of three distinct stages:
(1) UHT followed by a near isobaric cooling at 'high pressure' (above the FMASH invariant point [spl]);
(2) near isothermal decompression at UHT (between ~1000°C and 900°C);
(3) near isobaric cooling at a pressure of about 6-7 kbar below the invariant point [qz].
||click on a part of the petrographical PT path or on ht ereaction numbers to see a photo of the reaction textures associated with.|
|Stage 1: Isobaric cooling @ UHT|
spr-qz = opx-sil
|Stage 2: Near isothermal decompression @ UHT|
grt exsolution in UHT opx
grt = opx-spr-crd
grt-q = opx-crd
|Stage 3: cooling at low pressure|
spr-crd = opx-sil
crd = opx-sil-q