Petrological features of Subei Basin (Eastern China) lithospheric mantle and their relationships with H2O contents in NAMs

The Subei basin is located east of the Tanlu fault, a major discontinuity which separates the Archean North China Craton from the Proterozoic Yangtze Craton. South-West of the Tanlu fault the two cratonic blocks collided during Triassic time, originating the well-known UHP (Ultra-High Pressure) belt of Dabie Shan. Central-eastern China experienced widespread basaltic volcanism during Cenozoic, probably related to extensive lithospheric thinning and mantle upwelling along weak zones of the Archean-Proterozoic lithospheric roots. This volcanism was particular intense in the Subei basin, where it included minor tholeiitic eruptions in the early Paleogene, and more extensive, xenolith-bearing alkali basalt activity in the Neogene. Three localities, Panshishan (PSS), Lianshan (LS) and Fangshan (FS), about 30 km apart, were sampled in the Subei basin and more than 60 peridotite xenoliths were collected. Volcanism in the last locality has been dated at about 9 Ma. Most of the xenoliths are rounded and small to moderate in size (typically 5–10 cm in diameter), Most of the xenoliths are lherzolites ranging from highly fertile (16-23 vol% of clinopyroxene) to cpx-poor lherzolites (with cpx modal content of 6-9%), although few harzburgites, olivine-websterite and dunites are also found. No hydrous nor metasomatic secondary phases were observed. Textures vary from coarse-grained protogranular (~70% of total samples) through porphyroclastic (~20%) to equigranular (~10%) type. Rarely metasomatic textures, mainly spongy clinopyroxene, were observed. Using two pyroxenes geothermometer (Brey and Kohler, 1990), Panshishan and Lianshan show quite low equilibrium temperature (T=816-1010℃), whereas Fangshan samples show temperature between 1011°C and 1208°C, Pressure estimates on the basis of Ca-exchange between olivine and clinopyrossene, range between 12 to 25, 11 to 23 and 11 to 33 Kbar for Panshishan, Lianshan and Fangshan lherzolites, respectively. fO2 conditions calculated for Panshishan and Fangshan samples range over four orders of magnitude from log fO2~ -4.12 to 0.25 FMQ and from 2.26 to -2.13 FMQ, respectively; Lianshan samples present a more restricted range, with log fO2 from 0.55 to -2.4 FMQ. FTIR analyses of nominally anhydrous minerals (NAMs) have been carried out for these xenoliths. Water content in olivines is very low; frequently it reaches the instrument detective limitation (less then 2ppm). Water content varies from 37 to 183 ppm for cpx and 13 to 74 ppm for opx. Fangshan xenoliths have the highest water content for both opx and cpx compare to other two localities, while Panshisan have the lowest water content in opx, leading to anomalously high DH2Ocpx/opx ratios. Panshishan xenoliths show δ18O values ranging from 5.28 to 5.78 ‰ in olivine, 5.87 to 6.53 ‰ in opx, 5.18 to 6.15 ‰ in cpx, and 4.11 to 5.37 ‰ in sp. The results are similar to those reported by Yu et al. (2005), although these authors refer a broader range of δ18O values for ol, opx and cpx. In Lianshan xenoliths, δ18O values range from 5.42 to 5.96 ‰ in olivine, 6.01 to 6.67 ‰ in opx, 5.77 to 6.34 ‰ in cpx, and 4.52 to 5.58 ‰ in sp. In xenoliths from Fangshan δ18O values range from 5.12 to 6.32 ‰ in ol, 5.79 to 6.57 ‰ in opx, 5.33 to 6.31 ‰ in cpx, and 4.37 to 5.39 ‰ in sp. On the basis of the cpx REE patterns the 47 measured xenoliths are subdivided into five different groups. Group I, with LREE-depleted pattern; Group II, with upward convex pattern, Group III, with flat REE pattern; Group IV, with LREE-enriched pattern; and Group V, with spoon-shape pattern. Group I samples reflect low degree of mantle melting process (F less than 10%) and group IV samples has been strong modified during mantle metasomatic event/s. Comparing water content of peridotite minerals (NAMs) with geochemical parameters, such as major and trace element compositions of minerals, melting index (i.e. Mg# value) for whole rock and minerals, oxygen isotopes, as well as physico-chemical parameters such as Temperature, Pressure and Oxygen Fugacity no correlation are envisaged. In conclusion, at least for the Subei Basin lithospheric mantle represented by xenoliths of Panshinshan, Linshan and Funshan water contents in NAMs are not related to the main depletion/enrichment processes occurring in the upper mantle, but it appears as an intrinsic (pristine?) feature of different mantle domains