摘要
This paper reports petrographic, mineral chemical, olivine oxygen isotopic, and whole-rock geochemical data for wehrlite xenoliths from the Early Cretaceous Tietonggou high-Mg diorites in western Shandong Province, in the eastern part of the North China Craton (NCC), and describes the origin of these wehrlites and the processes that affected the deep lithospheric mantle in this area.Wehrlite xenoliths are rounded and vary in size between 3 cm × 4 cm 5 cm and 3 cm 2 cm 1 cm.Olivine within these xenoliths occurs as an isolated residual phase within clinopyroxene, has Fo contents between 89 and 91, and contains between 1414 and 3629 ppm Ni, similar to the values of olivine from peridotite xenoliths in the Cenozoic basalts of eastern China, but lower than the values of olivine from harzburgite xenoliths in the Early Cretaceous high-Mg diorites in western Shandong.In situ oxygen isotope analysis yielded 18 O values of olivine from (6.03±0.33)‰ to (6.82±0.35)‰, averaging (6.5±0.4)‰; this is higher than typical mantle-derived olivine ((5.2±0.3)‰).Compared with clinopyroxenes from peridotite xenoliths in the Late Cretaceous and Cenozoic basalts, clinopyroxenes in the wehrlites contain relatively low concentrations of Na 2 O, TiO 2 , and Al 2 O 3 , high concentrations of CaO, and higher Mg # (91.2-94.1) and Ti/Eu ratios (2082-2845), being similar in composition to clinopyroxenes within harzburgite xenoliths in the Early Cretaceous high-Mg diorites.Clinopyroxenes from wehrlite xenoliths are characterized by low total REE abundance, enrichment in light REEs, and depletion in high field strength elements such as Nb, Ta, Zr, and Hf.Moreover, the 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 187 Os/ 188 Os (125 Ma) ratios of these wehrlites vary from 0.70596 to 0.70737, 0.512181 to 0.512416, and 0.12661 to 0.57650, respectively.These data suggest that these wehrlite xenoliths were formed by modification of the lithospheric mantle by melts derived from recycled continental crust.
This paper reports petrographic, mineral chemical, olivine oxygen isotopic, and whole-rock geochemical data for wehrlite xenoliths from the Early Cretaceous Tietonggou high-Mg diorites in western Shandong Province, in the eastern part of the North China Craton （NCC）, and describes the origin of these wehrlites and the processes that affected the deep lithospheric mantle in this area.Wehrlite xenoliths are rounded and vary in size between 3 cm × 4 cm 5 cm and 3 cm 2 cm 1 cm.Olivine within these xenoliths occurs as an isolated residual phase within clinopyroxene, has Fo contents between 89 and 91, and contains between 1414 and 3629 ppm Ni, similar to the values of olivine from peridotite xenoliths in the Cenozoic basalts of eastern China, but lower than the values of olivine from harzburgite xenoliths in the Early Cretaceous high-Mg diorites in western Shandong.In situ oxygen isotope analysis yielded 18 O values of olivine from （6.03±0.33）‰ to （6.82±0.35）‰, averaging （6.5±0.4）‰; this is higher than typical mantle-derived olivine （（5.2±0.3）‰）.Compared with clinopyroxenes from peridotite xenoliths in the Late Cretaceous and Cenozoic basalts, clinopyroxenes in the wehrlites contain relatively low concentrations of Na 2 O, TiO 2 , and Al 2 O 3 , high concentrations of CaO, and higher Mg # （91.2-94.1） and Ti/Eu ratios （2082-2845）, being similar in composition to clinopyroxenes within harzburgite xenoliths in the Early Cretaceous high-Mg diorites.Clinopyroxenes from wehrlite xenoliths are characterized by low total REE abundance, enrichment in light REEs, and depletion in high field strength elements such as Nb, Ta, Zr, and Hf.Moreover, the 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 187 Os/ 188 Os （125 Ma） ratios of these wehrlites vary from 0.70596 to 0.70737, 0.512181 to 0.512416, and 0.12661 to 0.57650, respectively.These data suggest that these wehrlite xenoliths were formed by modification of the lithospheric mantle by melts derived from recycled continental crust.
基金
supported by Chinese Ministry of Science and Technology(Grant No.2009CB825005)
Natural Science Foundation of China(Grant Nos.90814003 and 90714010)
State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences(Wuhan)
State Key Laboratory of Continental Dynamics,Northwest University
