High Sr/Y and La/Yb ratios: The meaning of the adakitic signature

The name of adakite is used to describe a far too large group of rocks, whose sole common feature is high Sr/Y and La/Yb ratios. However, such a signature can be achieved via different processes: melting of a high Sr/Y (and La/Yb) source; deep melting, with abundant residual garnet; fractional crystallization or AFC; or interactions of felsic melts with the mantle, causing selective enrichment in LREE and Sr over HREE. A database of the compositions of adakitic rocks-including high silica and low silica adakites [Martin, H., Smithies, R.H., Rapp, R.P., Moyen, J.-F., Champion, D.C., 2005. An overview of adakite, tonalite-trondhjemite-granodiorite (TTG) and sanukitoid: relationships and some implications for crustal evolution. Lithos, 79(12), 1-24.], continental adakites and Archaean adakites-was assembled. Geochemical modeling of the potential processes is used to interpret it, and reveals that (1) the classical model of slab melting provides the best explanation for the genesis of high-silica adakites; (2) low-silica adakites are explained by garnet-present melting of an adakite-metasomatized mantle, i.e., at depths greater than 2.5 GPa; (3) Continental adakites is a term encompassing a huge range of rocks, with a corresponding diversity of petrogenetic processes, and most of them are different from both low- and high-silica aclakites; (4) Archaean aclakites show a bimodal composition range, with some very high Sr/Y examples (similar to part of the TTG suite) reflecting deep melting (> 2.0 GPa) of a basaltic source with a relatively high Sr/Y, while lower Sr/Y rocks formed by shallower (1.0 GPa) melting of similar sources. Comparison with the Archaean TTG suite highlights the heterogeneity of the TTGs, whose composition spreads the whole combined range of HSA and Archaean adakites, pointing to a diversity of sources and processes contributing to the TTG suite. (c) 2009 Elsevier B.V. All rights reserved.