Adakites-the key to understanding LILE depletion in granulites

For more than 20 years, it has been argued that granulites of the lower continental crust are depleted in fluid-mobile elements such as Cs, Rb, U and Th, either because they were removed by melting of the lower crust or through dehydration. We argue that there is little evidence for a simple relationship between granulite-facies metamorphism and element depletion, and propose that, for felsic orthogneisses, the depletion may be a primary feature of crust generation processes. Modern adakites show a range of fluid-mobile element ratios and adakites from the Austral Volcanic Zone (AVZ), Chile show fluid-mobile element ratios very similar to those found in lower crustal felsic granulites. This suite of adakites has a strong slab-derived component. We propose that modern adakites of the type found in the Austral Volcanic Zone are analogous, in their genesis, to lower crustal felsic granulites and that their mafic precursors were already depleted in fluid-mobile trace elements, prior to melting. An incremental melting model, in which fluid-mobile elements are released from an eclogitic slab prior to partial melting, can explain the fluid-mobile element behaviour of both modern adakites and lower crustal felsic granulites.