The recognition of adakite-type gneisses in the North Dabie Mountain and its implication to ultrahigh pressure metamorphic geology

Adakite([1]) is an igneous rock generated by the partial melting of subduct ing ocean slab in an island are setting. Meanwhile, residuals of the slab-m elting are transformed to eclogite or garnet-amphibolite during a slab subd uction process. This note reports the adakite-type grey gneisses discovered in the North Dabie Mountain. These adakitic rocks are characterized by hig h Al2O3 (14.97%-17.56%), Sr(545-941 mug/g), Sr/Y (44.71-218.98), La/Yb (16. 70-97.50), and low Y (3.32-12.19 mug/g), Yb (0.31-1.34 mug/g), Sc (0.92-4.2 mug/g), as well as positive Sr anomaly but absent or positive Eu anomaly. They are different from the low aluminum grey gneisses with low Al2O3 (13.7 3%-14.38%), Sr (262-409 mug/g), Sr/Y (6.46-15.97) and La/Yb (1.90-22.60), h igh Y(21.79-36.24 mug/g), Yb (2.00-5.59 mug/g), Sc (8.30%-12.70%), and nega tive Sr, Eu anomaly in the North Dabie Mountain. The adakite-type grey gnei sses in the North Dabie Mountain were most probably formed by the partial m elting of subducting ocean slab before the Indo-Chinese epoch. We suggest t hat the subducting of an ocean slab took place prior to the incorporation b etween the Yangtze and North China plates in the Indo-Chinese epoch, result ing in formation of adakite-type igneous rocks and some ultrahigh pressure eclogites related to the subducted ocean slab. Therefore, besides the ultra high pressure metamorphism of the Indo-Chinese epoch, some ultrahigh pressu re ecoglits were most likely formed prior to the Indo-Chinese epoch in the Dabie Mountain. In addition, the adakite-type grey gneisses with are origin occur in the North Dabie Mountain, inferring that the Yangtze plate subduc ted down below the North China plate and the main suture zone between the t wo plates was likely along the southern side to the North Dabie Mountain.