Adakite is a petrologic term for volcanic or intrusive igneous rocks that are interpreted to form in subduction zones from the mixing of mantle material with felsic partial melts of descending slabs of oceanic crust basalt. Adakites include a range of resulting rock types: they are recognized by specific chemical and isotopic characteristics, mainly high Sr/Y and La/Yb ratios and low Y and Yb trace element content.[1] Adakites have, in the literature, the following reported composition: SiO2 >56 wt percent, Al2O3 >15 wt percent, MgO normally <3 wt percent, Mg number 0.5, Sr >400 ppm, Y <18 ppm, Yb <1.9 ppm, Ni >20 ppm, Cr >30 ppm, >Sr/Y 20, La/Yb >20, and 87Sr/86Sr <0.7045.[2]
There is some debate as to the process that produces adakites. Low magnesium adakites may be representative of relatively pure partial melting of a subducting basalt, whereas high magnesium adakite or high magnesium andesites may represent melt contamination with the peridotites of the overlying mantle wedge.[3] Adakites have also been reported from the continent-continent collision zone beneath Tibet. [4]