Eclogite residues

Ducea M. (2001) The California arc thick granitic batholiths, eclogitic residues, lithospheric-scale thrusting, and magmatic flare-ups. GSA Today 11(11), 4-10. 

Figure 8. Plot of Li isotopic composition vs. Na O content of eclogites from Trescolmen, Swiss Alps (Zack et al. 2003). Data from pairs of bulk rock ( ) and omphacite mineral separate (O) samples, indicating that bulk rocks generally preserve isotopic systematics of these rocks. Well-preserved, high pressure metamorphosed basaltic rocks were interpreted to retain much of their elemental character, and as such were good recorders of the residue remaining after subduction dehydration. The data indicated that originally isotopically heavy altered sea floor basalt could be transformed during subduction into some of the isotopically lightest materials in the Earth system.

McDonough W. F. (1991) Partial melting of subducted oceanic crust and isolation of its residual eclogitic lithology. Phil. Trans. Roy. Soc. London A 335, 407—418. 

It is not yet clear whether high thorium, primitive lavas worldwide have high °Th excess, or not. °Th excess has been observed in a few arc lavas, notably primitive andesites from Mt. Shasta in the southern Cascades (Newman et al., 1986), and high Mg andesites from the Austral Andes in southernmost Chile (Sigmarsson et al., 2002). This is consistent with the hypothesis that primitive andesites contain a substantial proportion of partial melt from a source rich in residual garnet, such as subducting, eclogite facies sediment and/or basalt (Section 3.18.3.2.5). 

---