Geochronology, geochemistry and petrogenesis of the Longwo granodioritic pluton in Guangdong province.

The Longwo pluton is a typical example of the Early Yanshanian granites with mantle components in Naming region. Lithologically, this pluton consists mainly of granodiorite, and contains melanocratic dioritic enclaves. Zircon ELA-ICP-MS dating for the granodiorites yields an age of 169. 1 +/- 2.5 Ma, indicating that they were formed in Middle Jurassic. Geochemically, the granodiorites are weakly peraluminous (A/NKC = 1.0 similar to 1.1), and are relatively poor in alkalis and rich in potassium (K2O/Na2O = 1.15 similar to 1.45). The rocks also have enriched LREE and LILE (e. g., Rb, Cs, Th, U) concentrations, and low HFSE (e. g., Nb, Ti) abundances. Dioritic enclaves have similar mineral assemblages to those of the host rocks, but contain more mafic minerals and higher contents of siderophile elements (e. g., V, Cr, Co, Ni). The variation trends of major and trace element abundances between the enclaves and the host rocks suggest a mixing process in their petrogenesis. They also display close Sr, Nd isotopic compositions, with I-sr and epsilon(Nd) (t) values of 0.70843 similar to 0.70995, -6.53 similar to -8.89 for the host granodiorites, and of 0.70797 similar to 0.70882, -4.71 similar to -9.24 for the dioritic enclaves, which are all indicative of mixing of crustal materials with mantle components. A simple binary mixing calculation shows that the ratios of mantle components involved in the genesis ranged from 32.9% to 40.4% in the host granodiorites, and from 31.8% to 46.4% in the dioritic enclaves. Integrated geological and geochemical data suggested that the Longwo granodiorites and their dioritic enclaves were generated by mixing of mantle-derived basic magma and its induced crustal felsic magma under a tensile environment.