Geochemical characteristics of cretaceous mafic dikes from Northern Guangdong, SE China: Age, origin and tectonic significance

Abstract

Systematic radiometric, geochemical and Sr -Nd isotopic analyses are reported here for the Cretaceous mafic dikes from Northern Guangdong, SE China in order to provide a better understanding of the origin of magma and its tectonic significance. The mafic dikes were emplaced in three principal episodes: at similar to 140 Ma, similar to 105 Ma and similar to 90 Ma. With the chemical characteristics of tholeiitic basalts, they were derived from a moderate time-integrated LREE depleted mantle source with epsilon Nd(T) values of 5. The trace element chemistry of the dikes shows general similarities to within-plate basalts. However, some of the 140 Ma dike samples exhibit are signatures with Th-enrichment and Zr- and Hf-depletion. Thus, the mafic dikes are interpreted as having been magmatism produced during a lithospheric extension from a mantle source influenced by earlier subduction processes. This mantle source shows an EM2 signature in incompatible trace element ratios, such as La/Nb approximate to 1.12, Ba/Nb approximate to 9.1, Ba/La approximate to 8.2, Th/Nb approximate to 0.16 and Th/La approximate to 0.146, together with a high Th/U ratio of 4.7 similar to the EM1 component. The combined geochemical and geochronological constraints on the origin of the mafic dikes, together with results from previous studies of late Mesozoic granites, volcanic rocks, rifting systems and extensional or rifted basins indicate that SE China was dominated by an extensional tectonic regime during the Cretaceous, with four major episodes of extensional activities taking place at similar to 140 Ma, similar to 124 Ma, similar to 105 Ma and similar to 90 Ma.