Hafnium isotopic record of mantle-crust interaction in an evolving continental magmatic system

Abstract

Tracing the origin and evolution of magmas on their pathway through the lithosphere is key to understanding the magmatic processes that eventually produce eruptions. For ancient magmatic provinces, isotope-geochemical tracers are powerful tools to probe the source regions and magma-crust interaction during ascent and storage. Here we present hafnium isotopic compositions of ID-TIMS dated zircons to trace the evolution of the Middle Triassic magmatic province in the Southern Alps (northern Italy) at high temporal resolution. Systematic changes in hafnium isotopic composition with time record progressively stronger crustal assimilation over 3.5 million years, followed by a rapid increase in εHf towards more juvenile compositions for the major pulse of mafic intrusions and post-intrusive volcanic ash beds within one million years. We interpret these trends to reflect variations in mantle-crust interaction through time. Initial intrusions of basaltic dykes into the relatively cold lower crust cause only limited crustal melting and assimilation but an ensuing magma injection into progressively hotter crust results in more extensive partial melting and assimilation of crustal material. Subsequent intrusions into the magmatic lower-crustal roots cannibalize previous intrusions with progressively less isotopic contrast due to dilution with mantle-derived magmas. This is potentially accompanied by an increase in magma flux due to delamination of dense lower crustal cumulates into the subcontinental lithospheric mantle. The observed trends in hafnium isotopic composition therefore do not necessarily require tectonic re-organizations or changes in mantle sources. Instead these trends may trace variations in mantle-crust interaction during thermally induced chemical maturation of the lower crustal magmatic roots progressively replacing ancient pelitic to mafic lower crustal lithologies by juvenile cumulates.