Petrogenesis of Middle Jurassic mafic dikes and granites in the eastern Hebei district, North China Craton, China: Implications for westward subduction of the Paleo-Pacific plate

Abstract

The westward subduction of the Paleo-Pacific plate had a significant effect on the evolution of the eastern margin of the Eurasian continent during the Mesozoic. However, records for the initial subduction of the Paleo-Pacific plate from mafic rocks are still lacking. In this paper, we deal with Middle Jurassic mafic rocks and coeval granites in the eastern Hebei district to constrain the impact of the subduction on the igneous event. Our new U-Pb ages of zircon grains confirm the emplacement of these rocks during the Middle Jurassic. These mafic dikes are composed of appinites and lamprophyres with high MgO (7.8–12.2 wt%), Cr (>500 ppm) abundances relative to high-Ti mafic rocks. They display large ion lithophile element enrichment, high field strength element depletion, and enriched isotopic components (ε_Hf (t) = -9.6 – –22.7), indicating arc-like geochemical signatures. These mafic dikes were likely formed from mafic magmas derived from subduction-related metasomatized subcontinental lithospheric mantle sources under the spinel- garnet transition zone. The coeval Shijiatai (SJT) granitic pluton, which is suggested as the appinite-granite suite, is high-silica granites with low Mg# (<30) values. The SJT granitic magmas were likely sourced from the chemically modified lower crust caused by underplating of mafic magmas derived from hydrated lithospheric mantle. More importantly, the new identification of Jurassic, rather than Triassic water-rich appinitic rocks in eastern China strongly points to the subduction of the Paleo-Pacific plate. Accordingly, our study provides new geochemical evidence for westward subduction of the Paleo-Pacific plate.