Geochronology and geochemistry of Cretaceous Nanshanping alkaline rocks from the Zijinshan district in Fujian Province, South China: Implications for crust-mantle interaction and lithospheric extension

摘要

In situ zircon U-Pb ages and Hf isotopic data, major and trace elements, and Sr-Nd-Pb isotopic compositions are reported for Nanshanping alkaline rocks from the Zijingshan district in southwestern Fujian Province (the Interior or Western Cathaysia Block) of South China. The Nanshanping alkaline rocks, which consist of porphyritic quartz monzonite, porphyritic syenite, and syenite, revealed a Late Cretaceous age of 100-93 Ma. All of the rocks show high SiO_2, K_2O + Na_2O, and LREE but low CaO, Fe_2O_3~T, MgO, and HFSE (Nb, Ta, P, and Ti) concentrations. These rocks also exhibit uniform initial ~(87)Sr/~(86)Sr ratios of 0.7078 to 0.7087 and ε_(Nd)(t) values of -4.1 to -7.2, thus falling within the compositional field of Cretaceous basalts and mafic dikes occurring in the Cathaysia Block. Additionally, these rocks display initial Pb isotopic compositions with a ~(206)Pb/~(204)Pb_i ratio of 18.25 to 18.45, a ~(207)Pb/~(204)Pb_i ratio of 15.63 to 15.67, and a ~(208)Pb/~(204)Pb_i ratio of 38.45 to 38.88. Combined with the zircon Hf isotopic compositions (ε_(Hf)(t) = -11.7 to -3.2), which are different from those of the basement rocks, we suggest that Nanshanping alkaline rocks were primarily derived from a subduction-related enriched mantle source. High Rb/Sr (0.29-0.65) and Zr/Hf (37.5-49.2) but relatively low Ba/Rb (4.4-8.1) ratios suggest that the parental magmas of these rocks were most likely formed via partial melting of a phlogopite-bearing mantle source with carbonate metasomatism. The relatively high SiO_2 (62.35-70.79 wt.%) and low Nb/Ta (10.0-15.3) ratios, positive correlation between SiO_2 and (~(87)Sr/~(86)Sr)_I, and negative correlation between SiO_2 and ε_(Nd)(t) of these rocks suggest that the crustal materials were also involved in formation of the Nanshanping alkaline rocks. Combined with geochemical and isotopic features, we infer magmatic processes similar to AFC (assimilation and fractional crystallization) involving early fractionation of clinopyroxene and olivine and subsequent fractionation of biotite-dominated assemblages coupled with a lesser amount of crustal contamination, thereby forming the Nanshanping alkaline rocks. The Nanshanping alkaline rocks appear to be associated with an extensional environment in the Cathaysia Block. This extensional regime could have resulted in the slab break-off and rollback of the subducting paleo-Pacific plate and the upwelling of the asthenospheric mantle, which induced partial melting of the enriched lithospheric mantle in the Cretaceous.