Adakites arevolcanic rocks ofintermediate tofelsic composition that havegeochemical characteristics ofmagma originally thought to have formed bypartial melting of alteredbasalt that issubducted belowvolcanic arcs.^[2] Most magmas derived in subduction zones come from themantle above the subducting plate when hydrous fluids are released from minerals that break down in themetamorphosed basalt, rise into the mantle, and initiate partial melting. However, Defant and Drummond recognized that when youngoceanic crust (less than 25 million years old) is subducted, adakites are typically produced in the arc. They postulated that when young oceanic crust is subducted it is "warmer" (closer to themid-ocean ridge where it formed) than crust that is typically subducted. The warmer crust enables melting of the metamorphosed subducted basalt rather than the mantle above. Experimental work by several researchers has verified the geochemical characteristics of "slab melts" and the contention that melts can form from young and therefore warmer crust in subduction zones.^[3]

Thegeochemical characteristics Defant and Drummond gave for adakites are:

• SiO₂ greater than 56 wt %
• Al₂O₃ greater than or equal to 15 wt %
• MgO normally less than 3 wt %
• Sr greater than 400ppm
• Y less than 18 ppm
• Yb less than 1.9 ppm
• ⁸⁷Sr/⁸⁶Sr usually less than 0.7045

Later Defant and Kepezhinskas reviewed the topic in some detail pointing out that adakites are found associated with manymineral deposits includinggold andcopper.^[4]

Drummond and Defant noted thatArcheantrondhjemites (which make up most of the ancient crust of continents) have similar geochemical characteristics to adakites.^[5] They suggested that the entire Archean crust may have been derived from the partial melting of subducted oceanic crust during the Archean (> 2.5 billion years ago) because during early Earth the temperature of the mantle was much hotter and more oceanic crust was generated and subducted younger. The proposal has been controversial and is still being argued among the scientific community. The alternative interpretation is that thecontinental crust was derived from the partial melting of lower crustal basalts. The same idea has also been postulated for the generation of adakites. However, this hypothesis does not explain the correlation between subducted young crust and adakite eruptions nor the fact that the lower Yb and Y in adakites suggest thatgarnet is stable in the source. Garnet forms only under high pressures within the Earth and would not be stable in lower crust below some island arcs that erupt adakites. See Martin et al. for a more recent summary.^[6]

Lowmagnesium adakites may be representative of relatively pure partial melting of a subducting basalt, whereas high-magnesium adakite or high-magnesiumandesites may represent melt contamination with theperidotites of the overlyingmantle wedge.^[7] Adakites have also been reported from thecontinent-continent collision zone beneathTibet andLesser Caucasus.^[8][9]

• Adak Island, Alaska
• Trans-Mexican Volcanic Belt, Mexico
• Mindanao, the Philippines^[10]

• Felsic rocks
• Intermediate rocks
• Volcanic rocks
• Subduction

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