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

Emplacement ages, geochemical characteristics and analysis of continental dynamics ongabbroic intrusions in Luodian County, Guizhou Province, have been discussed based on studies ofisotopic chronology (the whole-rock Sm-Nd and Rb-Sr isochron methods), major elements, traceelements and PGE. Intrusive activities of the gabbroic intrusions in the study area took place during theLate Yanshanian Orogenic Movement (the Cretaceous Period), as indicated by the Sm-Nd isochronages (t)=(99.6±4.5) (2σ) Ma and by the Rb-Sr isochron ages t=(97±1.6) (2o) Ma. The gabbroic intrusionsare attached to mafic rocks in cal-alkaline basaltic series. They occurred as dikes and might be formedunder an extensional background of the continent. Differentiation of their magmatic crystallizationresulted in obvious zonation of petrography. In the gabbroic intrusions of this study, large ion lithophileelements and LREE are enriched, and the chondrite-normalized REE distribution pattern is leftwardinclined 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 mantlewith high-level partial melting, derived from EMl-type enriched mantle. It is different from Emeishanbasalt, which may be related to the upper mantle at low-grade partial melting. Emplacementmechanism of the gabbroic intrusions in this study may suppose to be asthenosphere upheaving as anisolated hot wave in the presence of mantle fluid, resulting in basaltic magma intruded into thecontinental crust as a diapiric intrusion. Therefore, uplifting of faulting-block and extensionaldeformation could take place in the shallow part of the continental crust while vertical amassing andaccretion of magmatic materials in the deep part of the continental crust. These special processes couldsupposed to be a special background of continental dynamics for this large-scale epithermalmetaliogenic domain, such as Au, As, Sb and Hg deposits, and vertical uplifting of the Yunnan-GuizhouPlateau in the Cretaceous Period.