An example of ultrahigh‐temperature metamorphism: orthopyroxene–sillimanite–garnet, sapphirine–quartz and spinel–quartz parageneses in Al–Mg granulites from In Hihaou, In Ouzzal, Hoggar

Abstract

A B S TR A CT Quartz Al-Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual highgrade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxenesillimanite-garnet-quartz, sapphirine-quartz and spinel-quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz Opx+Sil, (2) Opx+Sil Grt+Spr+Qtz, (3) Grt+Sil+Qtz Crd, (4) Grt+Sil Crd+Spr, (5) Grt+Sil+Spr Crd+Spl, (6) Grt+Sil Crd+Spl, (7) Grt+Crd+Sil Spl+Qtz and (8) Grt+Sil Spl+Qtz. Minerals in quartz Al-Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+) range of the main minerals is as follows: cordierite (0.81-0.97), sapphirine (0.77-0.88), orthopyroxene (0.65-0.81), garnet (0.33-0.64) and spinel (0.23-0.56).The reaction textures and the evolution of the mineral assemblages in the quartz Al-Mg granulites indicate a clockwise P-T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.

Metamorfismo de temperatura ultra-alta de granulitos com safirina de 2,0 Ga do Bloco Itabuna-Salvador-Curaçá, Bahia, Brasil

“…Higher X Mg (0.86) corresponds to lower TiO 2 contents (4.98 wt %), repeating the observations on biotite from other sapphirine-bearing granulites (e.g. Ouzegane and Boumaza, 1996;Guidotti, 1984;Henry and Guidotti, 2002 . Differences in contents for other oxides are minor, Na 2 O and K 2 O contents in both types being insignificant, and therefore not shown (Table 4).…”

Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback

“…18a) that abundant large garnets (20-25 vol %) are surrounded by reaction corona of cordierite + quartz symplectite, sillimanite (likely as pseudomorphs after kyanite), and cordierite + hercynite + corundum symplectite. Coexisting hercynite + quartz are preserved as small inclusions within the garnets and represent the peak metamorphic assemblage -a characteristically high-temperature assemblage that has been reported from several UHT (> 900 • C) localities (Lal et al, 1987;Sandiford et al, 1987;Waters, 1991;Dasgupta et al, 1995;Ouzegane and Boumaza, 1996;Morimoto et al, 2004;Tsunogae and Santosh, 2006). Reaction structures within the garnet corona are indicative of near-isothermal decompression, and so record the exhumation history of the Kobipoto Complex in the Kobipoto Mountains.…”

A matter of time: The importance of the duration of UHT metamorphism

“…(b) P-T diagram showing the modelled GO P-T path of (a) in comparison to selected UHT granulites with clockwise UHT-ITD paths. The selected P-T paths are: Mat -Mather Paragneisses, Rauer Islands, east Antarctica (590-530 Ma) (Harley, 1998b(Harley, , 2004Kelsey et al, 2005;Harley, 2008); Ouz -In Ouzzal, Algeria (Ouzegane and Boumaza, 1996;Adjerid et al, 2013); Pal -Palni Hills, southern India (545 Ma) (Raith et al, 1997;Brandt et al, 2011). (c) P-T diagram showing the clockwise UHT-ITD path of the Mather Paragneisses (Mat) in comparison to the UHT-IBC P-T path of the Archaean Napier Complex of Antarctica Harley, 1985Harley, , 1989Harley, , 1998a.…”

“…4b with a selection of P-T paths synthesised from three UHT areas characterised by 'clockwise' overall P-T evolutions terminated by extensive near-isothermal decompression (ITD) or decompression with cooling. These UHT-ITD granulites (Mather Paragneisses, Antarctica (Mat) : Harley, 1998b: Harley, , 2008; Palni, India (Pal): Raith et al, 1997;Brandt et al, 2011;In Ouzzal, Algeria (Ouz): Ouzegane and Boumaza, 1996;Adjerid et al, 2013) experienced considerable decompression at T > 900°C, involving initial steep ITD (dP/ (2004) and Jamieson et al (2004) and that modelled for the rock package (3) (an orogen core nappe thrust over a rigid indentor) in the Grenville Orogen model GO-ST87 shown in Figure 3 Jamieson and Beaumont, 2011). In the HT1 case the rock package crosses into the sillimanite field and achieves granulite facies P-T conditions, Δt 800 , for 16 Myr, but fail to attain UHT.…”