Age, geochemistry and petrogenesis of the cumulate gabbro in Tahe, northern Da Hinggan Mountain.

Abstract

In this paper, geochemical data of the major and trace elements, and Hf isotope are reported for the Tahe cumulate gabbro exposed in the northern Da Hinggan Mountain. Basen on these data, its petrogenesis, protolith and tectonic implications are discussed in detail. The Tahe cumulate gabbro, characterized by low SiO2 (42. 59-47. 4wt. %) , TiO2 (0. 5wt. %) , P2O5 (0. 1wt.%) and high Al2O3(15.9 -32.9 wt.%) , CaO (11.7-17.1 wt. %) , is enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) , showing the similar geochemical characteristics to the gabbros formed in the active continental margin or island arc. Zircon laser ablation (LA) -ICP-MS U-Pb analyses indicated that the Tahe gabbro was emplaced at 333±8 Ma with negative eHf(t) values of - 2 - -5. Therefore, it is concluded that the Tahe gabbroic magma was derived from partial melting of the mantle pyroxenite in the enriched lithospheric mantle or lower crust which was metasomatized by fluid released from the subducted slab. Hence, the occurrence of the Tahe cumulate gabbro indicates that a subduction process was conducted beneath this area at -330Ma, which was considered to be related to the assembly of the Songnen and Xing' an blocks.