Emplacement Ages and Geochemical Characteristics of Grabbroic Intrusions and Prospecting Orientation of Related Deposit in Luodian, Guizhou Province

AbstractEmplacement ages, geochemical characteristics and analysis of continental dynamics on gabbroic intrusions in Luodian County, Guizhou Province, have been discussed based on studies of isotopic chronology (the whole‐rock Sm‐Nd and Rb‐Sr isochron methods), major elements, trace elements and PGE. Intrusive activities of the gabbroic intrusions in the study area took place during the Late Yanshanian Orogenic Movement (the Cretaceous Period), as indicated by the Sm‐Nd isochron ages (t)=(99.6±4.5) (2s̀) Ma and by the Rb‐Sr isochron ages t=(97±1.6) (2s̀) Ma. The gabbroic intrusions are attached to mafic rocks in cal‐alkaline basaltic series. They occurred as dikes and might be formed under an extensional background of the continent. Differentiation of their magmatic crystallization resulted in obvious zonation of petrography. In the gabbroic intrusions of this study, large ion lithophile elements and LREE are enriched, and the chondrite‐normalized REE distribution pattern is leftward inclined without anomalies of δCe or δEu, and there are high concentrations of PGE and ratios of Pd/Ir (averaging 4.21). All of these imply that their source areas may be basaltic magma in the upper mantle with high‐level partial melting, derived from EMI‐type enriched mantle. It is different from Emeishan basalt, which may be related to the upper mantle at low‐grade partial melting. Emplacement mechanism of the gabbroic intrusions in this study may suppose to be asthenosphere upheaving as an isolated hot wave in the presence of mantle fluid, resulting in basaltic magma intruded into the continental crust as a diapiric intrusion. Therefore, uplifting of faulting‐block and extensional deformation could take place in the shallow part of the continental crust while vertical amassing and accretion of magmatic materials in the deep part of the continental crust These special processes could supposed to be a special background of continental dynamics for this large‐scale epithermal metallogenic domain, such as Au, As, Sb and Hg deposits, and vertical uplifting of the Yunnan‐Guizhou Plateau in the Cretaceous Period.