Geodynamic Evolution of Flat-Slab Subduction of Paleo- Pacific Plate: Constraints From Jurassic Adakitic Lavas in the Hailar Basin, NE China

摘要

Subduction of the Paleo-Pacific Plate caused widespread Mesozoic magmatism, lithospheric deformation and thinning, and mineralization along the continental margin of East Asia. However, the details of this subduction process remain unclear. To investigate the regional geodynamic evolution and subduction processes of the Paleo-Pacific Plate, we carried out a geochronological, geochemical, and Sr-Nd-Hf isotopic study of Middle-Late Jurassic adakitic lavas in the Hailar Basin, northeast (NE) China. These rocks have relatively high Sr contents (434-877 ppm), low heavy rare earth element and Y (6.81-14.37 ppm) contents, and high Sr/Y ratios (56-83), which are characteristic of adakites. The rocks are classified into Middle Jurassic high-K and Late Jurassic low-K adakitic lavas. The high-K adakitic lavas were derived by dehydration melting of thickened garnet-bearing amphibolite-facies lower crust. In contrast, the low-K adakitic lavas could be considered as pristine slab-melts that were likely generated by hydrous melting of subducted eclogitic oceanic crust, followed by limited interaction between the slab melt and peridotite as a consequence of flat-slab subduction. Combined with previous data, our results demonstrate that the late Mesozoic magmatism in NE China records the entire temporal-spatial evolution of subduction of the Paleo-Pacific slab from initial shallowing of the subduction angle to flat-slab subduction, followed by basalt-eclogite phase transformation and finally slab rollback.