(e.g., high-Ba-Sr granitoids, low-silica adakites)
I know that plagioclase in the source, and its pressure dependence play an important role on it, but there is no plagioclase in mantle rocks, is there?
Indeed, the mantle does not contain plagioclase, so that cannot be the reason for rocks that are mantle-derived. But if you are asking about granitoids - they are not mantle-derived; at least not directly. They could be derived by fractional crystallisation of mantle-derived magmas, but only if plagioclase did not fractionate, since this would reduce Sr concentrations.
The best way to generate Sr-rich magmas is to start out with a mantle that is enriched in Sr, for instance by subduction processes. If you look at primitive arc magmas, they will show a positive Sr-anomaly in MORB- or PM-normalised trace element diagrams (the idea is that Sr and other fluid-mobile elements were added by an aqueous fluid coming from the subducted slab). To increase Sr concentrations (irrespective if you start with a subduction-related magma or something else), you will need fractionation of minerals in which Sr is incompatible. This could be garnet (at high pressure), or amphibole (at intermediate pressure). For the latter you of course also need volatiles, so that's most easily done, again, in a subduction environment.
Of course you can also start by partially melting something in the stability field of garnet (whereby Sr goes into the melt), and that is how some people believe adakites are generated (melting of the subducted slab, generating high Sr/Y magmas). However, you don't NEED slab melting to get something with an adakitic signature, fractionation of garnet +/-amphibole can do the same trick.
Hope this helps somewhat to clarify the matter! I attach something I wrote about arc magmatism, which talks about the positive Sr anomaly in arc magmas.
Thanks Marlina! it did help
Fractional crystallisation seems to be a choise, as well as the metasomatized mantle. However, for the latter, where the subdcution-related Sr gets into? what is a high-Sr mantle source? (minerals)
Sr gets in the mantle and fertilizes it via subduction of Sr-rich fluids/sediments or by deep-seated evolved fractionated melts passing throughout it in the way up.
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