Linking granulites, intraplate magmatism, and bi-mineralic eclogites with a thermodynamic-petrological model of melt-solid interaction at the base of anorogenic lower continental crust

Abstract

This study reports results from thermodynamic phase equilibrium and trace element modelling of mafic magmatic underplating and solid-liquid interaction in the lower continental crust (LCC) in intraplate settings. The arrival of underplating basalt sills into thin (∼30 km at 8 kbar) and thicker (∼45 km at 12 kbar) andesitic and basaltic LCC precursors was simulated with heating and batch-melting to yield refractory residues. Continued magmatic invasion of the LCC was then modelled at the same pressures with hybridisation between the residual solids and basaltic and picritic liquids (at 80:20 and 50:50 proportions). The first finding is that hybridisation with basalt increases the stability fields of 2-pyroxene-plagioclase and garnet-clinopyroxene-plagioclase granulites, by far the dominant LCC xenoliths found in anorogenic settings. The second finding pertains to situations when the liquid proportion of the hybrid system is lower than the fraction of the incoming mafic liquid. The resulting liquids share many characteristics with silicic volcanic rocks in continental flood basalt (CFB) provinces, including low Al and high Ti-concentrations. Stoichiometric melting reactions from 80:20 residual andesite:basalt hybridisation reproduce trace element patterns of silicic eruptives with potential for decoupling of Sr-Nd-Pb isotope systematics, as found in CFB rhyolites and LCC xenoliths. Incongruent melting of hornblende and garnet (the A in AFC) from the residual solid and dominant peritectic fractional crystallisation of plagioclase (the FC in AFC) from the liquid occur. A notable feature of LCC-picrite hybridisation is that solid residues after felsic liquid extraction, when subjected to higher pressure, are buffered to olivine and quartz-free compositions. Consequently, if these high-pressure granulite residues delaminated, they would convert to the bi-mineralic high-Mg eclogites found as xenoliths in kimberlites. The modelled residues also have the Sr-enrichment and positive Eu-anomalies of eclogite xenoliths. Solid-liquid interaction in intraplate settings has been and remains a significant process in crustal growth and evolution.