WATER-FLUXED MELTING IN LOW-PRESSURE METAPELITIC ROCKS: INSIGHTS FROM PHASE EQUILIBRIA MODELLING

SOLA, A. M.; HASALOVÁ, PAVLINA; WEINBERG, R.; SUZAÑO, N.; BECCHIO, R. A.; HONGN, F. D.; BOTELHO, N.

Tucumán

Congreso; XX CONGRESO GEOLÓGICO ARGENTINO; 2017

Water-fluxed melting has long been thought to have a minor role on the thermal and chemical structure of crust. We report here on amphibolite facies metasedimentary rocks from the Famatinian Orogen in northwest Argentina, that have undergone water-fluxed incongruent biotite melting at relatively low temperature, which have produced and lost a significant volume of melt. The protoliths consist of the turbiditic Puncoviscana Formation (Neoproterozoic to Early Cambrian). The field area exhibits a condensed metamorphic field gradient, from greenschist to amphibolite facies suprasolidus conditions, recording a low-P almost isobaric path, reaching peak conditions estimated at 700 ºC at 4 kbar. On the base of mineral assemblages recognized in metapelites, the study area is divided into four metamorphic zones that according to the increase in the grade are: (i) chlorite - muscovite (ii) muscovite - biotite (iii) sillimanite - K-feldspar and (iv) cordierite - K-feldspar. The area is particularly well-suited for the study of partial melting as unmelted (protolith) as well as various melt-depleted lithologies are exposed. This allowed the modelled and actual melt productivity in these rocks to be directly compared, which make a strong case that fluid-fluxed melting must have occurred, and also allowed the extent of this melting to be quantified. We apply thermodynamic modelling in the MnNCKFMASHTO system to investigate melting at such low P as a function of water content.Calculations using a typical metapelite composition show how small amounts of added free H2O may increase significantly the melt fraction with little or no change in either the melt or residual phase compositions. They indicate negligible difference in normative An-Ab-Or proportions and ferromagnesian contents between melts derived by dehydration and water-fluxed melts. The same is true for the content of H 2 O dissolved in melts, which remains constant and the melt produced is granitic whether or not aqueous fluids are present. Thus, neither the residue nor the melt composition are indicators of the presence of aqueous fluids during anatexis. Recognizing the impact of small additions of H2O to an anatectic terrane may therefore be difficult. The most significant change related to water-fluxing is the relative proportions of minerals and melt fraction, rather than the actual mineral assemblage. The modal proportion of feldspars decreased while those of cordierite and biotite increased in the residual assemblages, as < 5 mol. % of free H2O was added. The impact of this additions is to more than double the proportion of water-undersaturated melt to 25-30 mol. %.We have developed a simple way based on equilibrium phase diagrams and binary geochemical variation plots, to estimate how much melt a residual rock has lost, if the compositional trends of the protoliths are known. In summary, we find that even though the addition of small amounts of free H 2 O impacts significantly on rock fertility, there is little obvious record in the field. The combined application of careful petrological investigation and thermodynamic modelling is the key to identify the influence of aqueous fluids, and exploit systems that became open not only to fluid influx but also to the extraction of melt.