DECOUPLING OF LU-HF AND SM-ND ISOTOPE SYSTEMS DURING CRUSTAL MELTING: AN EXAMPLE FROM THE HIDAKA METAMORPHIC BELT, JAPAN

Radioisotope systems, especially Lu-Hf and Sm-Nd, have been widely used to deduce the source of crustal melts and to understand the geochronological and tectonophysical evolution of continental crust. The utilization of the Lu-Hf and the Sm-Nd isotope pairs to tracing magma sources relies on the principle that the generated melt/magma inherits its geochemical fingerprint from its source rocks.

However, it is unclear how these isotope tracers respond to melting of heterogeneous source regions. This has direct implications for our understanding of whether isotope variations seen in igneous rocks reflect heterogeneities in source compositions, disequilibrium melting, or open system melt formation involving addition of juvenile mantle-derived components. The potential complexity of the Lu-Hf and Sm-Nd systems has been highlighted in recent studies, where the reliability of radiogenic tracers controlled by accessory minerals has been challenged by claims of isotope disequilibrium between crustal protoliths and melts.

The Hidaka Metamorphic Belt, Japan, exposes an almost intact section through island arc crust. This provides a rare opportunity to study melting processes in the heterogeneous lower crust, melt extraction, and the formation of voluminous intrusions with a range of geochemical compositions. To examine the behavior of the Lu-Hf and Sm-Nd isotope systems during lower crustal anatexis, we study the above isotope pairs on a whole rock and mineral-scale. We found that melt produced from metasedimentary layers infiltrate metabasite units, whereby the felsic, metasedimentary-derived melt contains elevated LREE concentrations but lower HREE contents compared to the mafic host rocks. This interaction causes the Nd isotope ratios of the mafic host unit to become locally contaminated by the less radiogenic Nd isotope signature of the felsic melt. In contrast, the Hf isotope signature of the felsic melt is shifted to relatively radiogenic values, imparting a geochemical fingerprint that is different from that of the original metasedimentary protolith.

Our results have significant implications for understanding isotope equilibration during the melting of the lower crust, but also suggest a new mechanism for how the geochemical fingerprints of so-called ‘I-type’ granites are generated.