The characteristics and tectonic-metallogenic significances of the adakites in Yanshan period from eastern China

Adakite is a suite of intermediate-acid igneous rocks characterized by HREE depletion and no obvious negative Eu anomaly, indicating the derivation fr om very deep source with garnet in the residue. A lot of Yanshanian interme diate-acid magmatic recks in eastern China have similar geochemical charact eristics to the adakite, their formation environment however is unrelated t o subduction process. In this paper, adakite is divided into two types: one is O-type adakite, which is characterized by Na enrichment and is related to subduction process; another is C-type adakite, which is enriched in II ( most of them are still enriched in Na, a few K-enriched), is probably a pro duct of partial melting of the lower-crust granulite in the thickened crust (> 50km) resulted from underplating of basaltic magma. The occurrence of C -type adakites in eastern China is indicative to the explanation of the geo logical phenomenon of Yanshanian magmatism. Because C-type adakite can pres erve same imprints of the lower crust, it in turn can trace the composition of the lower crust and discuss metallogenesis related to the lower crust a nd crust-mantle process.