Melt loss and the preservation of granulite facies mineral assemblages

Abstract

The loss of a metamorphic fluid via the partitioning of H 2 O into silicate melt at higher metamorphic grade implies that, in the absence of open system behaviour of melt, the amount of H 2 O contained within rocks remains constant at temperatures above the solidus. Thus, granulite facies rocks, composed of predominantly anhydrous minerals and a hydrous silicate melt should undergo considerable retrogression to hydrous upper amphibolite facies assemblages on cooling as the melt crystallizes and releases its H 2 O. The common occurrence of weakly retrogressed granulite facies assemblages is consistent with substantial melt loss from the majority of granulite facies rocks. Phase diagram modelling of the effects of melt loss in hypothetical aluminous and subaluminous metapelitic compositions shows that the amount of melt that has to be removed from a rock to preserve a granulite facies assemblage varies markedly with rock composition, the number of partial melt loss events and the P-T conditions at which melt loss occurs. In an aluminous metapelite, the removal of nearly all of the melt at temperatures above the breakdown of biotite is required for the preservation of the peak mineral assemblage. In contrast, the proportion of melt loss required to preserve peak assemblages in a subaluminous metapelite is close to half that required for the aluminous metapelite. Thus, if a given proportion of melt is removed from a sequence of metapelitic granulites of varying composition, the degree of preservation of the peak metamorphic assemblage may vary widely.

Metamorphic zoning and geodynamic evolution of an inverted crustal section (Karakorum margin, N Pakistan), evidence for two metamorphic events

“…In contrast, metapelites within the domes have been partly reequilibrated through back-reaction with the melt during cooling (e.g. White and Powell 2002). Consequently, the temperature of 790-850°C estimated from the ultramafic and calc-silicate parageneses should therefore be the closest estimate of the temperature peak ( Fig.…”

Sapphirine granulites from Panasapattu, Eastern Ghats belt, India: Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

“…In the KFMASH system, sapphirine þ silicate melt can coexist above approximately 900 C (Kelsey et al, 2005) implying that the sapphirine assemblages along with the co-spatial magnesian metapelitic granulites are melt-bearing UHT assemblages. Using isochemical sections in the NCKFMASH model system, White et al (2001) and White and Powell (2002) have demonstrated that in a closed system (i.e., one from which no melt was lost), granulite facies mineral assemblages coexist with melt. To avoid complete retrogression, some melt loss or separation from residual mineral assemblages must occur (e.g., Powell and Downes, 1990;Kriegsman, 2001;Brown, 2002;White and Powell, 2002).…”

“…Using isochemical sections in the NCKFMASH model system, White et al (2001) and White and Powell (2002) have demonstrated that in a closed system (i.e., one from which no melt was lost), granulite facies mineral assemblages coexist with melt. To avoid complete retrogression, some melt loss or separation from residual mineral assemblages must occur (e.g., Powell and Downes, 1990;Kriegsman, 2001;Brown, 2002;White and Powell, 2002). In the case of melt segregation rather than loss, the retrogression depends on the size of the equilibration volume (St€ uwe, 1997), which decreases with falling temperature.…”

“…Most calculations of melting processes in metapelitic compositions can be adequately expressed in the model system Na 2 OeCaOeK 2 OeFeOe MgOeAl 2 O 3 eSiO 2 eH 2 OeTiO 2 eFe 2 O 3 (NCKFMASH) (White et al, 2001;White and Powell, 2002). The NCKFMASH peT isochemical sections for the peT range 600e1000 C and 4e11 kbar were computed using the PERPLEX software (Connolly, 2005; for the bulk compositions of the samples see Table 9).…”

“…The isochemical sections have been calculated in the model system Na 2 OeCaOeK 2 OeFeOeMgOe Al 2 O 3 eSiO 2 eH 2 O (NCKFMASH). Several recent studies have demonstrated the potential of the application of isochemical sections ("pseudosections") to evaluate the stability of mineral assemblages and their evolution in peT space during prograde, peak and retrograde metamorphism (e.g., Tinkham et al, 2001;White et al, 2001;White and Powell, 2002;Zeh and Holness, 2003;Kelsey, 2008;Nasipuri et al, 2009;Dharma Rao and Chmielowski, 2010). The results in this study thus provide new insights on UHT conditions in a deeply buried crustal segment of the Eastern Ghats belt with implications for the thermal and tectonic evolution of this collisional orogen.…”