Groppo_etal_2012b

The cordierite-bearing anatectic rocks of the higher Himalayancrystallines (eastern Nepal): low-pressure anatexis, meltproductivity, melt loss and the preservation of cordierite C . G R O P P O , 1 F . R O L F O 1 , 2 A N D P . M O S C A 21Department of Earth Sciences, University of Torino, via Valperga Caluso 35, I-10125, Torino, Italy ([email protected])2IGG – CNR, Via Valperga Caluso 35, I-10125, Torino, Italy Cordierite-bearing anatectic rocks inform our understanding of low-pressure anatectic processes in thecontinental crust. This article focuses on cordierite-bearing lithologies occurring at the upper structurallevels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite-bearing gneissesfrom different geological transects (from Mt Everest to Kangchenjunga) have been studied, in whichcordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likelyreflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar,plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of themicrostructures related to melt production and ⁄ or melt consumption allows the distinction to be madebetween peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (fromtwo-mica metapelite ⁄ metagreywacke to biotite-metapelite) has been investigated at low-pressureconditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineralassemblages and microstructures. The results of the thermodynamic modelling suggest that the modeand type of the micaceous minerals in the prograde assemblage is a very important parametercontrolling the melt productivity at low-pressure conditions, the two-mica protoliths being significantlymore fertile at any given temperature than biotite gneisses over the same temperature interval.
Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and byexhumation along P-T paths with a peculiar dP ⁄ dT slope of about 15–18 bar °C)1. Overall, our resultsprovide a key for the interpretation of cordierite petrogenesis in migmatites from any low-P regionalanatectic terrane. The cordierite-bearing migmatites may well represent the source rocks for the Mioceneandalusite-bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low-P isobaric heating rather than decompression melting may be the triggering process of this peculiarperaluminous magmatism.
Key words: cordierite-bearing anatectic rocks; higher Himalayan crystallines; Himalaya; low-pressureanatexis; P-T pseudosections.
constraints for the understanding of low-pressure Cordierite-bearing anatectic rocks are characteristic of Experimental studies (e.g. Vielzeuf & Montel, 1994; partial melting at low-pressure conditions (e.g. Spear Carrington & Harley, 1995; Stevens et al., 1995, 1997; et al., 1999). Microstructural and petrological study of Montel & Vielzeuf, 1997; Buick et al., 2004; Grant, cordierite-bearing rocks is thus relevant, besides con- 2004; Spicer et al., 2004) and phase equilibria model- tact metamorphism, for the understanding of regional ling (e.g. Waters, 1988; Harley, 1994; Carrington & metamorphic terranes where partial melting occurred Harley, 1995; Fitzsimons, 1996; Buick et al., 1998; at remarkably shallow-crustal pressures. Furthermore, Johnson et al., 2001) focusing on partial melting of cordierite is potentially a powerful mineral monitor of pelitic and ⁄ or psammitic protoliths at low pressure (i.e.
fluid conditions during high-grade metamorphism (Vry £5 kbar) suggest that cordierite can be formed, at et al., 1990; Visser et al., 1994; Carrington & Harley, supra-solidus conditions, by two significantly different 1996; Harley & Carrington, 2001; Thompson et al., processes. First, cordierite may be a peritectic phase 2001; Harley et al., 2002; Bertoldi et al., 2004). Hence, the study of P-T conditions at which cordierite grows, increasing temperature, with or without significant combined with the information on the activities decompression, for example through the dehydration and compositions of fluids, could provide useful melting reaction Qtz + Sil + Bt fi Kfs + Crd + L.

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