Recycled Lower Continental Crust: Evidence From Peridotite Xenoliths in Mesozoic High-Mg Diorites of the Eastern North China Craton

Recycling of continental lithosphere, together with recycling of oceanic crust, has been proposed to play a key role in plate tectonics, plume magmatism, crustal evolution and formation of chemical heterogeneities within the mantle. However, direct evidence for recycling of the deep continental lithosphere into the mantle is scarce (Gao et al., 2004). Petrological and geochemical data of the harzburgite-dunite-wehrlite-orthopyroxenite xenoliths hosted in Early Cretaceous high-Mg diorites of the North China Craton (NCC) provide direct evidence for recycling of the deep continental lithosphere into the mantle. Firstly, the veined and zoned orthopyroxenes (i.e., orthopyroxenite) surrounding chromite, together sometimes with phlogopite, can be found in spinel-harzburgite and chromite-bearing dunite, implying that an adakitic melt-peridotite interaction had taken place. Additionally, the residual olivine can be found in poikilitic clinopyroxene (1-2 mm) from wehrlite, also suggesting the introduction of a melt into the peridotite. Secondly, the olivine and orthopyroxene from the harzburgite and dunite xenoliths can be subdivided into two groups based on their Mg#'s (where Mg#\ = molar 100 ´ Mg/(Mg + Fetot)), i.e., high-Mg#\ group (89-95, averaging 91.4) and low-Mg#\ group (82-87, averaging 85). The former should be residues by large degrees of polybaric melting, the latter could be attributed to the reaction of an adakitic melt and peridotite . In addition, veined and zoned orthopyroxenes from dunites have much higher rare earth element (REE) abundances than primary orthopyroxenes from harzburgite, whereas poikilitic clinopyroxenes from wehrlite are characterized by strong enrichment in light rare earth elements (LREEs) and depletion in heavy rare earth elements (HREEs). These results indicate that the introduction of an adakitic melt had occurred in the previous depleted mantle peridotites. Thirdly, the dunite, harzbugite, and wehrlite xenoliths display "U" type of REE patterns on chondrite- normalized diagrams, implying that they could have been subjected to refertilization by late metasomatism. The initial 87Sr/86Sr ratios and ɛ 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 and existence of the Archean inherited zircons in the host rocks, suggests that the previous depleted mantle peridotites had been subjected to modification of an adakitic melt, whereas the delaminated NCC lower continental crust is the most likely candidate for the source material of the adakitic melts that appear to be the primary metasomatic agent observed in these xenoliths (Xu et al., 2006). References Gao S. et al. (2004) Nature 432: 892-897. Xu W. L. (2006) Geology 34: 721-724.