Origin of allanite in gneiss and granite in the Dabie orogenic belt, Central East China

Abstract

Allanite is a common accessory mineral phase, representing an important carrier of rare earth elements, Th, U, Sr and other trace elements in most continental rocks. As Th and U can be incorporated into the allanite lattice, the mineral is a good geochronological tool for constraining geological events. Moreover, the trace element features δEu, Th/U ratio and common lead content of allanite are indicators of the forming conditions. Allanite and coexisting epidote-group minerals are abundant in ultrahigh-pressure (UHP) metamorphic rocks from the Dabie-Sulu orogen in central East China. However, if these minerals formed in the Neoproterozoic as magmatic phases, or in the Triassic as metamorphic phases is a matter of long-standing controversy. We report major and trace element analyses of whole rocks, allanite and coexisting epidote-group minerals, together with U-Th-Pb isotopic compositions of allanite in UHP gneiss from the Dabie-Sulu orogen, and allanite in the adjacent Jingshan granite. The granite is emplaced along the southeastern margin of the North China Craton and considered a product of partial melting of the subducted Dabie-Sulu gneiss. Trace elements (low Th/U and La/Sm, high δEu and high Sr) and high common lead concentrations indicate a metamorphic origin of allanite-epidote in the UHP gneiss. On the other hand, coarse-grained allanite from the Jingshan granite shows a corrosion core and a magmatic rim with common 208Pb up to 70% in the core and less than 30% in the rim. The allanite cores are of peritectic and the rims of magmatic origin with ages of ∼160Ma, consistent with the granite crystallization age. In combination with previous studies, we conclude that the allanite of the Jingshan granite has form from the subducted and remolten Dabie-Sulu gneiss. Allanite records Triassic UHP metamorphic ages as well as Jurassic peritectic-magmatic ages as a part of the evolution of the Dabie-Sulu orogen.