Geochemistry, Petrogenesis, and Tectonic Significance of Mesozoic Mafic Dikes, Fujian Province, Southeastern China

摘要

Major- and trace-element data for Mesozoic mafic dikes from Fujian Province, Southeast China, provide insights into the nature of their mantle source and evolution of the lithospheric mantle beneath the Cathaysian block. These dikes are alkali to subalkali in composition, and are characterized by high Al_20_3 (14.0-20.4 wt percent ) and moderate CaO (4.09-12.7 wt percent ) compared to Cenozoic basalts in the same region. In primitive mantle-normalized plots, they are enriched in LREE and LILE, and depleted in HOSE (Nb and Ta),with positive Pb and negative Ti anomalies. They are divided into two types, based on their REE contents: type 1 had low total REE (53.8-145.5 ppm) with relatively flat chondrite-normalized patterns [(La/Yb)_n velence l.68-4.65], whereas type 2 has much higher total REE (63-247 ppm) with LREE enrichment [(La/Yb)_n velence 4.63-19]. Zr/Y ratios show a good correlation with Y in the type 1 dikes, but not in the type 2 varieties. Rocks in both groups experienced modest crustal contamination. All have slightly higher Pb isotopic ratios than the associated Cenozoic basalts, suggesting that the Mesozoic dikes were derived from a mantle source that had been modified by subducted materials (fluid + sediment). Type 1 dikes are believed to have been derived from garnet-spinel Iherzolite, whereas type 2 dikes were probably derived from spinel-or spinel-plagioclase Iherzolite. Both mantle sources were compositionally heterogeneous and they experienced different degrees of depletion. Asthenospheric upwelling provides a possible mechanism for lithosphere thinning and extension, leading to emplacement of the dikes.