科研之友
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摘要
We report the finding of peridotite xenoliths in the Early Cretaceous Longmengou olivine-bearing diabase (138 Ma) in the Northern Taihang Mountains in the central North China Craton. Based on the modal proportions of olivine, clinopyroxene, amphibole and anorthite, these peridotite xenoliths can be divided into three zones: clinopyroxene-bearing olivine zone (COZ), olivine-clinopyroxene zone (OCZ), and amphibole-bearing anorthite-clinopyroxene zone (AACZ). The core of olivine grains in clinopyroxene-bearing olivine zone have higher Mg-# (>95), SiO2 (41.80-42.53 wt%) and lower CaO (<0.07 wt%), FeO (3.91-4.54 wt%) than the rim (Mg-# = 92.5-93.4, SiO2 = 41.27-41.98 wt%, CaO = 0.20-0.34 wt%, and FeO = 7.02-8.87 wt%), suggesting that rim is reaction product. The core of olivine grains with higher Mg-# (>95) and lower NiO content (<0.04 wt%) in the clinopyroxene-bearing olivine zone was derived from ultra-depleted mantle subsequently altered by high Mg-# melts/magma with low Ni. Two generations of olivine grains occur in the OCZ where the first generation shows exsolution of ilmenite and magnetite rods containing up to 035 wt%TiO2, and was likely derived from garnet peridotite hydrated by water. The second generation shows high Mg-# (96.2-97.1) and cataclastic texture, and was possibly formed by decomposition of the COZ. The occurrence of aluminous spinel suggests the role of melts with extremely high Al and Mg. Clinopyroxene in the AACZ shows systematic core-rim compositional variation with CaO and SiO2 contents increasing towards the rim, and MgO and Fe2O3 concentrations decreasing from the core to the rim, indicating that the amphibole-bearing anorthite-clinopyroxene zone is a product of the reaction between mantle xenoliths and mafic magma. Plagioclase with high An value (92.0-99.95, average 97.79) indicates that the metasomatic melts have high Ca/Na and Al/Si ratios, possibly produced by the partial melting of ultra-depleted mantle under "wet" conditions. Combined with the data on other mantle xenoliths discovered in the NCC, our results suggest that the Mesozoic lithospheric mantle beneath the North Taihang Mountains within the central NCC is composed of ultra-depleted Archean and Paleoproterozoic peridotites and dunites modified by complex melts. We also propose that the destruction of eastern part of the NCC mainly occurred during Early Cretaceous, and that the boundary of the lithospheric destruction coincides with the Taihang Mountains.
- 出版日期2017-8
- 单位西北大学; 长安大学; 中国地质大学（北京）
