Garnet versus amphibole: Implications for magmatic differentiation and slab melting

The garnet signature in the rare earth element (REE) abundances in adakites has been considered a key genetic indicator of these controversial rocks, whose proposed origins include direct melting of subducted oceanic crust (slab melts). We show that the garnet signature may be quantified using the shape coefficients of chondrite-normalized REE patterns. We applied this method to a global data set of Cenozoic and Quaternary volcanic samples described as adakites. The results indicate that many, but not all, suites of rocks labeled as adakites have undergone fractional crystallization of garnet, starting from parental melts attributable to partial melts of garnet-bearing sources. The extreme garnet signatures seen in many examples require hybrid sources, consisting of subducted sediment as well as igneous oceanic crust; however, extensive deep-crustal differentiation obscures the major and trace-element characteristics of these sources, casting doubt on their identification as primitive slab melts.

Automated summary

The garnet signature is an important indicator for determining the origin of adakites, but not all rocks labeled as adakites exhibit this feature. Many examples with extreme garnet signatures require hybrid sources, including subducted sediment and igneous oceanic crust, but extensive deep-crustal differentiation may obscure the characteristics of these sources.