Adakite

Adakites are volcanic rocks of intermediate to felsic composition that have geochemical characteristics of magma originally thought to have formed by partial melting of altered basalt that is subducted below volcanic arcs. Most magmas derived in subduction zones come from the mantle above the subducting plate when hydrous fluids are released from minerals that break down in the metamorphosed basalt, rise into the mantle, and initiate partial melting. However, Defant and Drummond recognized that when young oceanic 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 the mid-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. The geochemical 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 400 ppm 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 many mineral deposits including gold and copper. Drummond and Defant noted that Archean trondhjemites (which make up most of the ancient crust of continents) have similar geochemical characteristics to adakites. 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.