Genesis and dynamic setting of mafic dikes in southeastern Fujian: evidence from Sr-Nd isotopic and major and trace element geochemistry

In southeastern Fujian, the Haibian spessartite dike and Chihu gabbro dike emplaced at late Cretaceous and Paleocene, respectively, are high-K calcalkali and calcalkali rocks. They are characterized by high Al content and Na2O>K2O, and possibly formed by fractional crystallization of olivine and clinopyroxene. The mafic dikes show enrichment in LILE (such as K, Rb, Ba, Sr, Th) and LREE, depletion in HFSE (such as Nb, Ta, and Ti). In primary-mantle-normalized spidergram, they have the feature of continental marginal arc and negative anomalies of Ta, Nb and Ti. Chondrite-normalized REE patterns for the mafic dikes are LREE enrichment (La/Ybn=5.0 similar to 10.9 and 11.2 similar to 12.0 for Haibian and Chihu, respectively). They are characterized by high initial Sr isotopes (Haibian:0.70577 similar to 0.707574; Chihu: 0.70547 similar to 0.70552) and low epsilon(Nd) (t) (Haibian: -8.1 similar to-1.8; Chihu: -0.2 similar to 0.6). According the Sr-Nd isotopic and trace element characteristics and field observation, the mafic dikes are not affected by crustal material during emplacement. Their mantle source may be related with subduction of the Pacific plate, and mixed by normal mantle wedge and oceanic and terrigenous deposits. This results in enrichment of LILE and LREE. The Haibian and Chihu dikes formed extensional continental margin or rifting environment. From late Cretaceous and Paleocene, the crustal extension may happen at southeastern Fujian, which is consistent with the tectonic setting of southeastern China.