Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma

Abstract

We present the results of numerical simulations to explore the effects of periodicity of basalt intrusion on the degree and timescales of melting of mafic lower (arc) crust. Melt production as a function of temperature was determined from parameterisation of published (0.7–1.6 GPa) fluid-absent partial melting experiments on amphibolites. We focus on a periodicity ranging from 20 to 30 000 years, with a total of 1 km of basic material intruded. Emplacing new basalt intrusions on top of earlier ones maximises the amount of silicic melt generated in the overlying protolith, and reduces greatly the heat loss through the base of the pile. Except for the case of a single intrusion, average partial melt temperatures generally exceed 900°C, and maximum temperatures may exceed 1000°C. The degree of partial melting is governed by the initial intrusion temperature and the periodicity, and yields a maximum predicted average melt fraction of 0.38. There is a time lag between maximum melt production and the maximum height reached by the melt column, with melt fractions in excess of 0.2 generated only where the time interval between each new intrusion is ≤200 years. Predicted maximum melt thicknesses do not exceed ca. 100 m, implying that large granitic magma chambers may not develop in the source region during partial melting of mafic lower crust. The ratio of the period of intrusion ( τ i) and the characteristic timescale for heat loss ( τ d) defines an important variable R that can be used to assess the thermal behaviour of the melting column, with both melt temperature and average (maximum) melt fraction maximised where R=1. The ratio R is used as an indicator of composition (expressed through changes in REE content) of a hypothetical partial melt, with smaller numbers of thicker intrusions ( R→0) resulting in higher modelled (La/Yb) N ratios. Partial melting of continental crust beneath intrusions may provide a means of mixing mantle and crustal components at source prior to magma emplacement. Alternatively, if crustal melt freezes at the base, strong compositional (mafic–felsic) layering may result. During partial melting by periodic, multiple intrusion, the lower crust is a dynamic environment, with the amount of silicic (granitic) melt available for extraction in the source region rising and falling in the crustal column with time.