Ultramafic xenoliths from Damaping (Hannuoba region, NE-China): petrogenetic implications from crystal chemistry of pyroxenes, olivine and Cr-spinel and trace element content of clinopyroxene

Olivine, orthopyroxene, clinopyroxene and spinel from 16 different lherzolite mantle xenoliths from Damaping (Hannuoba region, NE-China) have been analyzed by X-ray single crystal diffraction and electron microprobe to define their structure, chemistry and cation distribution. LA-ICP-MS analysis has been carried out on clinopyroxenes to determine their LILE (Large-Ion Lithophile Elements), HFSE (High Field Strength Elements) and REE (Rare Earth Elements) contents. These latter define Depleted, Enriched, U-shaped and Transitional CInormalized patterns suggesting that the xenolith suite here studied experienced at least two metasomatic events related to the porous flow percolation of melts with tholeiitic and alkaline affinity, respectively. The cation distribution of the mineral phases allowed determination of their intra- and inter-crystalline temperatures of equilibration. The closure temperature of intra-crystalline reactions for the constituent phases is in the range 600–800 °C, suggesting a relatively slow cooling rate, while the inter-crystalline temperature interval found out is in the range 866–1048 °C. The pressure conditions estimated for Hannuoba xenoliths through structural considerations correspond to a depth of about 50–60km, consistent with the Moho depth beneath the Hannuoba area,which is estimated at about 42km. The pressures of equilibration, as estimated from the cell and T site volumes of the clinopyroxenes, have been compared with the trace element and REE signatures; they demonstrate that Enriched, Depleted, U-shaped and Transitional clinopyroxenes formed at different pressures and are not restricted to a specific region of the mantle, testifying that the metasomatism took place at different depths in the mantle.