Elemental and Sr–Nd–Pb isotopic geochemistry of Mesozoic mafic intrusions in southern Fujian Province, SE China: implications for lithospheric mantle evolution

JUN-HONG ZHAO, RUIZHONG HU, MEI-FU ZHOU, SHEN LIU
2007 Geological Magazine  
Cretaceous mafic dykes in Fujian province, SE China provide an opportunity to examine the nature of their mantle source and the secular evolution of the Mesozoic lithospheric mantle beneath SE China. The mafic rocks have SiO 2 ranging from 47.42 to 55.40 wt %, Al 2 O 3 from 14.0 wt % to 20.4 wt %, CaO from 4.09 to 11.7 wt % and total alkaline (K 2 O+Na 2 O) from 2.15 wt % to 6.59 wt %. Two types are recognized based on their REE and primitive mantle-normalized trace element patterns. Type-A is
more » ... atterns. Type-A is the dominant Mesozoic mafic rock type in SE China and is characterized by enrichment of light rare earth elements (LREE) ((La/Yb)n = 2.85-19.0) and arc-like trace element geochemistry. Type-P has relatively flat REE patterns ((La/Yb)n = 1.68-3.43) and primitive mantle-like trace element patterns except for enrichment of Rb, Ba and Pb. Type-A samples show EMII signatures on the Sr-Nd isotopic diagram, whereas type-P rocks have high initial 143 Nd/ 144 Nd ratios (0.5126-0.5128) relative to the type-A rocks ( 143 Nd/ 144 Nd = 0.5124-0.5127). The type-A rocks have 207 Pb/ 204 Pb ranging from 15.47 to 15.67 and 206 Pb/ 204 Pb from 18.26 to 18.52. All the type-A rocks show a negative correlation between 143 Nd/ 144 Nd and 206 Pb/ 204 Pb ratios and a positive relationship between 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios, indicating mixing of a depleted mantle source and an EMII component. Geochemical modelling shows that the parental magmas were formed by 5-15 % partial melting of a spinel lherzolite, and contaminated by less than 1 % melt derived from subducted sediment. The type-P magmas were derived from a mantle source unmodified by subduction components. The wide distribution of type-A dykes in SE China suggests that subduction-modified lithospheric mantle was extensive beneath the Cathaysia Block. Geochemical differences between Mesozoic and Cenozoic mafic rocks indicate that lithospheric thinning beneath SE China occurred in two episodes: firstly by heterogeneous modification by subducted components in early Mesozoic times, and later by chemical-mechanical erosion related to convective upwelling of the asthenosphere during Cenozoic times.
doi:10.1017/s0016756807003834 fatcat:xhnntlgwvbcznoxnl5efaidj3i