Geochemistry Of The Mesozoic And Cenozoic Mafic Dikes And The Lithosphere Revolution From The Coastal Areas Of Guangdong Province And Hainan Island, China

Mafic dikes are a common expression of mantle-derived magma generation, which are formed in extension setting and geochemical studies of these dikes can provide information regarding the chemical characteristics of mantle sources and tectonic history of extensional areas. In this paper, though the field geological investigations, and studies of element-isotope geochemistry, mineralogy and the geo-chronology, combining with the regional geotectonic analysis, the geochemical characteristics and petrogenesis of the mafic dikes from the coastal areas of Guangdong Province and Hainan Island are discussed, mafic magma episodes and mantle characteristics are analyzed, and the relationships between mafic magmatism and lithosphere evolution are studied. The following conclusions are reached from the present study:1. Whole rock K-Ar data yielded ages of 146—54Ma for the mafic dikes from the coastal areas of Guangdong Province and Hainan Island, suggesting that emplacement of the mafic dikes took mainly place in the Cretaceous. According to K-Ar dates and geochemistry of mafic dikes and previous works, the K-Ar data can be divided into five periods (146Ma, 132—138 Ma, 105—112Ma, 82—99Ma, 54-75Ma).2. Two series of the mafic dikes (medium-K and high-K) can be distinguished according to K₂O/Na₂O ratios and K₂O contents and the negative "Ta-Nb" anomalies in the MORB-normalized trace element distribution patterns. The K₂O/Na₂O ratios of one series are <0.5, and K₂O contents are <1.3 wt% (0.37-1.24 wt%) with no apparent negative "Ta-Nb" anomalies. The K₂O/Na₂O ratios of another series are >0.5, and K₂O contents >1.3 wt% (0.37%— 1.24wt%) with strong negative "Ta-Nb" anomalies. Using K₂O vs. SiO₂ diagram, first series belongs to medium-K rocks, and another fall into the field of high-K rocks or shoshonitic rocks.3. The geochemical characteristics of the mafic dikes indicate that the medium-K mafic dikes from this area were formed by magma mixing derived from depleted asthenospheric mantle and EMI -type lithospheric mantle, and the high-K mafic dikes...