Ancient lithospheric mantle and the “inverse evolution” of its components in North China Craton

Abstract：The component evolution of ancient lithospheric mantle was constrained by three deep-seated geological processes, i.e., ① melt extraction, ② lithospheric mantle metasomatism of melt/fluid from different sources, and ③ interaction between lithosphere and asthenosphere /asthenospheric melt. Two stage enstatites (Py1 and Py2) were discovered in a sample of peridotite xenolith from Mengyin kimberlite. The enstatites of the early stage are residual minerals in the early peridotites with coarse-grained texture. However, the enstatites of the late stage were formed under the plastic rheological condition. A comparison of major and trace elements between the two kinds of peridotites from Mingyin, Fuxian and South Africa shows that the coarse-grained peridotites have higher Mg’ (0.902~0.93) and lower Al2O3 (0.22%~2.32%) values, whereas the peridotites with shear texture have lower Mg’ and higher Al2O3(1.69%~2.75%) values, suggesting that the late stage peridotites have lower refractory degree than the early stage peridotites. This phenomenon finds expression in the mantle component evolution process not only controlled by melt extraction but also by the interaction between lithosphere and asthenosphere / asthenospheric melt. The action caused the evolution of mantle composition from refractory back to fertile, and such an evolution trend is called “inverse evolution”. At least two magma extraction events and two “inverse evolution” processes occurred in ancient lithospueric mantle of North China Craton approximately during the 3 Ga geological period. Compared with the action ③ in Phanerozoic, it was characterized by smaller scale, lower strength and more obvious heterogeneity.