Pyroxenite Xenoliths Record Complex Melt Impregnation in the Deep Lithosphere of the Northwestern North China Craton

Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ∼90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880–970 oC), and low-Mg# (70–80) plagioclase-bearing websterites with low equilibration temperatures (550–835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr–Nd (εNd = +11·41, 87Sr/86Sr = ∼0·7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ∼0·703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr–Nd isotopic ratios (εNd = -14·20– -16·74, 87Sr/86Sr = 0·7070–0·7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0·706–0·711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian Ocean and the Late Mesozoic lithospheric extension of eastern Asia.