Interaction of Lithospheric Mantle With Melt Derived From Delaminated Lower Crust: Evidence From Eclogite and Dunite Xenoliths in Mesozoic High-Mg Diorites of the Eastern North China Craton

Eclogite and peridotite xenoliths occur in early Cretaceous high-Mg adakites of the eastern North China craton (NCC). The primary mineral assemblage (garnet + omphacite + quartz + rutile pargasite), and garnet and quartz-rod exsolutions in clinopyroxene in the eclogite xenoliths constrains a minimum pressure of >1.5 GPa, while the estimated peak metamorphic temperatures range from 800 to 1060 °C (Xu et al., 2004, 2006). SHRIMP zircon U-Pb age information and the geochemistry of the eclogite xenoliths indicate that eclogite- facies metamorphism occurred in the Triassic (220-240Ma) and that their protoliths were most likely to have been basalts from the NCC basement (1800 and 2500Ma). This implies that the Triassic thickening, foundering and partial melting of the Archean NCC mafic lower crust took place, an interpretation further supported by the complementary major and trace element compositions between these xenoliths and their host high-Mg adakitic intrusions (Xu et al., 2006). The high-Mg character of host intrusions may have resulted from the interaction between melts derived from delaminated lower crust and mantle peridotite. The peridotite xenoliths hosted in adakitic rocks mainly consist of dunite (95%), harzburgite (3%), and lherzolite (2%). In addition to the veins of orthopyroxene and orthopyroxene+phlogopite in dunites, the increase in TiO2, Al2O3, and FeOt contents and decrease in Cr2O3 content from core to rim for chromites, as well as the increase in TiO2, Cr2O3 contents and decrease of NiO content from core to rim for olivines, from dunite xenoliths imply metasomatism of the Archean lithospheric mantle (Gao et al., 2006) by hydrous silicate melts. Further evidence for this is provided by the trace element compositions of these rocks. The dunite and harzbugite xenoliths are relatively enriched in light rare earth elements and large ion lithophile elements, yet have low abundances of heavy rare earth elements and high field strength elements (HFSE), and lack any Eu anomaly. The initial 87Sr/86Sr ratios and epsilon Nd(t) values for the xenoliths range from 0.7058 to 0.7212 and +0.18 to _10.65, respectively. Taken together, these results, combined with the strong depletion in HFSE in the host rocks, suggests that the hydrous silicate melt had been derived from the partial melting of early Mesozoic delaminated lower continental crust. The observed large Re-Os isotopic variations of peridotite xenoliths can be explained by the reaction of pristine Archaean mantle (TRD=2.6-2.8 Ga) with eclogite-derived melt (Gao et al., 2006).