WHAT CONTROLS THE CHEMICAL STRATIFICATION OF AN ENTIRE ARCS CRUST? INSIGHTS INTO LARGE-SCALE MAGMATIC PROCESSES IN OCEANIC ARCS

Multiple lines of evidences, such as seismic velocity data, indicate that both arc and continental crust is characterised by an internal stratification of a more felsic upper crust and a more mafic lower crust. To explain the origin of this stratification is non-trivial. A popular idea is the density has a significant control on the stagnation and differentiation of arc magma in the deep crust, yet but no field studies exist that investigate the chemical and physical properties (density etc) of the plutonic crust throughout the entire arc crust to evaluate this hypothesis.

To test the role of density for magma stagnation we reconstructed the chemical stratification of the entire plutonic Kohsitan arc crust from ~10km to 52km paleo-depths. We used ~270 whole rocks samples for which the intrusion depth during arc build up could be determined. To better understand the observed chemical differentiation we separate the plutonic rocks in cumulates and liquids based on geochemical date (e.g. Sr/Nd Eu/Eu* etc) and we use MELTS and Perplex calculations to calculate the density of rocks and melts throughout the arc crust. Our results indicate that the plutonic arc rocks are always more dense than any reasonable liquid composition including primitive arc magmas. Accordingly a neutral buoyancy lawyer at which melt stagnate does not exist. Our field studies additionally reveal that associated with chemical stratification in the mafic lower and felsic upper crust a structural stratification exist whereby the lower arc crust is dominated by originally horizontal magmatic contacts between layeres/sills whereas the upper crust is dominated vertical contacts. We use these observation in combination to postulate a new model for magma stagnation and differentiation in arcs that explains both the observed chemical and structural stratification of the arc crust.