关键词: 埃达克岩, 暗色包体, 地球化学, 岩浆混合, 古太平洋俯冲作用

Abstract:  The paleo-Pacific tectonic system controls the formation and evolution of the East Asian continental margin, but there is still controversy over the onset of the subduction of the paleo-Pacific plate and its detailed evolution process.  Shiren Village in Helong Lity is located in south Jilin Province, in the eastern part of the northern margin of the North China plate, which is a favorable area to study the evolution of the paleo-Pacific Ocean. This paper presents a synthesis study of zircon U-Pb geochronology, geochemistry and Hf isotopes on the granodiorites. The studied rocks yield an average zircon LA-ICP-MS U-Pb age of (173±1) Ma, suggesting a Middle Jurassic magmatic event. The rocks are characterizd by high Mg^#(65.23%-65.92%), abnormally high w (Sr)（(1 657-2 467)×10^-6）, and low w (Y)（(7.50-7.98)×10^-6） and w(HREE), very similar to the low Si adakite defined by Martin et al.(2005), reflecting that the initial magma is derived from the mantle wedge metasomatized by plate melting. The significance lies in the existence of very hot subduction in this area around 173 Ma, which may be caused by an oblique subduction or oceanic ridge subduction, and also means that the subduction direction of the paleo-Pacific plate towards Eurasia deflected or the subduction mode changed in the Early Middle Jurassic. However, the studied rocks are different from the typical low Si adakites, while their w (SiO₂) values (61.13%-62.31%)are significantly greater than 56%.The traces of magmatic mixing were observed in both the field and under the microscope, reflecting that the magma experienced the evolution process of magmatic mixing in an open system during the emplacement process.

Key words: adakite; dark enclaves')">

adakite; dark enclaves, geochemistry, magmatic mixing, paleo-Paleo Pacific subduction