Geochemistry and Tectonic Setting of the Eshan Granites in the Southwestern Margin of the Yangtze Plate, Yunnan

Abstract

The extensive Eshan granites of Yunnan are made up of three intrusive units distinguished by their field contact relations; in descending order of age they are the Pojiao Unit, the Luzicun Unit and the Mokela Unit. The Pojiao Unit and Luzicun Unit contain petrographically and geochemically similar rocks but contact relationships show that the latter is younger. The Mokela Unit mainly consists of dykes intruding the other two and has petrographic and geochemical differences. Zircon U/Pb dating and zircon crystallization temperature measurements confirm the sequence of intrusions. Major and trace element analyses suggest that the magmas of the Pojiao Unit granites derived by partial melting of a clay-poor source from the upper crust; the magmas of the Luzicun Unit granites derived by partial melting of upper crust with a small proportion of middle crust accompanied by crystallization of albite which triggered strength reduction. Both magmas mixed and underwent with crustal contamination, assimilation and fractional crystallization. The magmas of the Mokela Unit derived from residual melts and assimilation of argillaceous rocks. A time sequence of melting, intrusion and deformation events is derived from these results and compared with other published tectonic models for the evolution of the SW margin of the Yangtze Plate. Magmatism was initiated by exhumation of upper continental crust during which strongly peraluminous porphyritic biotite monzogranite granites were produced at ca. 854–852 Ma, and the genesis of two-mica granite reflected a later batch of exhumed melts with crustal contamination, assimilation and fractional crystallization at ca. 842 Ma. Finally biotite alkali-feldspar granite and tourmaline granite magmas experienced strong fractional crystallization, emplaced in the cooling stage at ca. 823 Ma, indicating the end of exhumation.