Low‐P melting of metapelitic rocks and the role of H₂O: Insights from phase equilibria modelling

Abstract

Water‐fluxed melting has long been thought to have a minor influence on the thermal and chemical structure of the crust. We report here on amphibolite facies metasedimentary rocks from the 490–450 Ma 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 pressure almost isobaric path, reaching peak conditions estimated at 700°C at 4 kbar. Thermodynamic modelling in the MnNCKFMASHTO system is applied to investigate melting at such low pressure as a function of water content. Calculations using a typical turbidite 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 H2O 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 feldspar decreased while those of cordierite and biotite increased in the residual assemblages, as <5 mol.% of free H2O was added. The impact of this addition is to more than double the proportion of water‐undersaturated melt to 25–30 mol.%. We have also developed a simple way 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 H2O 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.

Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

“…Similar residual pelitic compositions to the SiO 2 -depleted samples in the MIC contact aureole are reported elsewhere in LP-HT settings where H 2 O-flux melting is interpreted to have occurred, including the El Hoyazo granulite enclaves, Mt. Stafford metapelitic migmatites, Famatian orogeny in Argentina, and Kangaroo Island migmatites (Acosta-Vigil et al 2010, 2012Cesare, Mariani, & Venturelli, 1997;Schwindinger et al 2019;Sola et al 2017). These studies suggest that H 2 O-flux melting may be a relatively common process in some LP-HT settings that can easily be overlooked.…”

“…Adding a relatively small proportion of H 2 O can significantly increase both melt production and the modal abundance of residuum minerals, without altering the melt composition, residuum mineral assemblage, and mineral compositions (e.g. Schwindinger, Weinberg, & Clos, ; Sola et al ). Here, H 2 O‐flux melting is consistent with both petrographical and field observations and modelling that indicate (a) peak temperatures of ~800°C and (b) extensive melt production and loss.…”

“…These studies suggest that H 2 O-flux melting may be a relatively common process in some LP-HT settings that can easily be overlooked. As noted by Sola et al (2017) andSchwindinder et al (2019), melt production in these settings estimates may be severely underestimated if peak P-T conditions and dehydration melting reactions are the only parameters used, and the estimated total volume of melt produced is ignored.…”

Wet or dry? The difficulty of identifying the presence of water during crustal melting

“…The presence of small volumes of aqueous fluids during anatexis will therefore remain cryptic because the main change they cause is an increase in melt fraction, with only minor changes to either melt composition or the modal relationships between peritectic minerals when compared to melting in the absence of such fluids, a point also made by Sola et al. ().…”

“…This type of melting reaction is potent and produces large amounts of granitic magma at upper amphibolite facies, but is frequently said to be a subordinate process in the global production of granite magmas (Brown, ; Clemens & Watkins, ). Recent articles have however sparked renewed interest in water‐fluxed melting (Collins, Huang, & Jiang, ; Sola et al., ; Weinberg & Hasalová, ).…”