Geochemistry of peridotite xenoliths in Early Cretaceous high-Mg# diorites from the Central Orogenic Block of the North China Craton: The nature of Mesozoic lithospheric mantle and constraints on lithospheric thinning

The North China Craton is the best example of an Archean craton that has lost its thick lithospheric keel. Although removal of the Archean keel is generally considered to have occurred in the Meosozoic-Cenozoic in the Eastern Block and Paleoproterozoic in the Central Orogenic Block, the exact timing and areal extent of the removal is debated, partly due to lack of knowledge about the nature of the Meosozoic lithospheric mantle. Here we report mineralogical and geochemical data on rare peridotite xenoliths from the Early Cretaceous high-Mg# diorites from Fushan in the Trans-North China Orogen (TNCO) of the North China Craton (NCC). These xenoliths provide insights into the nature of the lithospheric mantle underlying the central NCC during the Mesozoic. The peridotite xenoliths are dominated by spinel harzburgite and clinopyroxene (Cpx)-poor spinel Iherzolite, with minor chromite-bearing dunite. The harzburgite and Cpx-poor lherzolite have average forsterite contents of 92.3 and are depleted in CaO (0.59-1.06 wt.%) and Al2O3 (0.15-1.47 wt.%). These features are similar to those of Archean cratonic lithospheric mantle, suggesting the presence of Archean cratonic mantle beneath the Central Orogenic Block of the NCC in the Early Cretaceous. Dunites make up similar to 5% of the xenolith population and are characterized by relatively low Mg# (90.1) and the presence of disseminated chromite (Cr# = 73-85). The olivines in the dunites have high Ca (320-770 ppm) and Ti (1829 ppm) concentrations as well as low Ni abundances (2000-2690 ppm) compared to those from harzburgite and Cpx-poor lherzolite (Ca = 40-80 ppm: Ti = 0.23-8.1 ppm; Ni = 2970-3440 ppm), suggesting that the dunites were produced through an interaction between the Archean lithosphere and siliceous melts. The presence of secondary phlogopite and amphibole in the harzburgite and Cpx-poor Iherzolites as well as veined orthopyroxene in the foliated dunite is coupled with light rare earth element enrichments. Together with low Ca/Al (5-18) and La-N/Yb-N ratios (0.41-2.99) and high Ti/Eu ratios (526-1474) of clinopyroxenes, these metasomatic minerals indicate that the Archean lithospheric mantle had been overprinted by a volatile-bearing silicate melt. The Rb-Sr isochron age (111 +/- 23 Ma) of the harzburgite and Cpx-poor lherzolite xenoliths shows that the metasomatic overprinting occurred in the Early Cretaceous, in agreement near the time of the emplacement of the host magma and coeval voluminous igneous activities in the NCC. This section of Mesozoic lithospheric mantle is markedly different from the contemporaneous lithospheric mantle in the eastern NCC, implying that the destruction of the NCC lithosphere was confined to the eastern NCC and that the Archean mantle was preserved in the central NCC. (C) 2009 Elsevier B.V. All rights reserved.