Geochronology and Geochemistry of Mafic Dikes from Hainan Island and Tectonic Implications

Abstract:In the present study, the major and trace element compositions, as well as Sr, Nd isotopic compositions and K‐Ar age data in mafic dikes from Hainan Island, China, have been analyzed. Whole‐rock K‐Ar dating yielded a magmatic duration of 61–98 Ma for mafic dikes. Mafic dikes have a very high concentration of incompatible elements, for example, Ba, Rb, Sr, K, rare earth elements, and especially light rare earth elements (LREE), and negative anomalies of Nb, Ta, and Ti in the normalized trace element patterns. The initial 87Sr/86Sr ratios and εSr(t) of the mafic dikes are 0.70634–0.71193 and +27.7 to +112.2, respectively. In the 87Sr/86Sr versus εNd(t) diagram, the Hainan Island mafic dikes plot between fields for depleted mantle and enriched mantle type 2. All these characteristics show that the mantle (source region) of mafic dikes in this area experienced metasomatism by fluids relatively enriched in LREE and large ion lithophile elements. The genesis of Hainan Island mafic dikes is explained as a result of the mixing of asthenospheric mantle with lithospheric mantle that experienced metasomatism by the subduction of the Pacific Plate. This is different from the Hainan Island Cenozoic basalts mainly derived from depleted asthenospheric mantle, and possibly, minor metasomatised lithospheric mantle. This study suggests that the Mesozoic and Cenozoic lithospheric revolutions in Hainan Island can be divided into three stages: (1) the compression orogenesis stage before 98 Ma. The dominant factor during this stage is the subduction of the ancient Pacific Plate beneath this area. The lithospheric mantle changed into enriched mantle type 2 by metasomatism; (2) the thinning and extension stage during 61–98 Ma. The dominant factor during this stage is that the asthenospheric mantle invaded and corroded the lithospheric mantle; and (3) the large‐scale thinning and extension stage after 61 Ma. The large‐scale asthenospheric upwelling results in the strong erupting of Cenozoic basalts, large‐scale thinning of the lithosphere, the southward translating and counterclockwise rotating of Hainan Island, and the opening of the South China Sea.