The characteristics and tectonic-metallogenic significances of the adakites in Yanshan period from eastern China.
ZHANG Qi, WANG Yan, QIAN Qing, YANG JinHui, WANG YuanLong, ZHAO TaiPing and GUO GuangJunInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China.
Adakite is a suite of intermediate acid igneous rocks characterized by HREE depletion and no obvious negative Eu anomaly, indicating the derivation from very deep source with garnet in the residue. A lot of Yanshanian intermediate acid magmatic rocks in eastern China have similar geochemical characteristics to the adakite, their formation environment however is unrelated to 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 K (most of them are still enriched in Na, a few K enriched), is probably a product 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 geological phenomenon of Yanshanian magmatism. Because C type adakite can preserve some imprints of the lower crust, it in turn can trace the composition of the lower crust and discuss metallogenesis related to the lower crust and crust mantle process.