Baddeleyite U–Pb age and geochemical data of the mafic dykes from South Qinling: Constraints on the lithospheric extension. (13th December 2017) - Zhang, Guishan
Abstract : The nature of magma sources, time frame, and geodynamic settings of the late Early Paleozoic mafic magmatic activities in the South Qinling Orogen is not clearly understood. The Ziyang area with concentrated and representative outcrops of the late Early Paleozoic mafic dykes was selected as a study area, and detailed petrology, whole‐rock geochemistry, isotopic geochemistry, and baddeleyite U–Pb chronology were combined to reveal the origin of the mafic dykes. Baddeleyite U–Pb chronological study shows that the formation age of the mafic dykes is 425.4 ± 6.4 Ma, indicating they were formed in the Late Silurian. The mafic dykes in the Ziyang area are alkaline rock series, are generally characterized by enriched Rb, Ba, Sr, La, Ce, Nb, and Ta, and have similar geochemical characteristics to those of OIB‐type basalt. The spider diagram of trace elements shows that the mafic dykes are weakly depleted in K and strongly depleted in Zr, Hf, and Ti. The mafic dykes are composed of depleted Nd and Pb isotopes and moderately enriched Sr isotopes, with εNd ( t ) = +3.4 to +3.6, ( 87 Sr/ 86 Sr) i = 0.70382–0.70412, ( 206 Pb/ 204 Pb) i = 17.542–17.802, ( 207 Pb/ 204 Pb) i = 15.557–15.569, and ( 208 Pb/ 204 Pb) i = 37.531–37.880. The elemental and isotopic geochemistry shows that the mafic dykes in the Ziyang area did not undergo crustal contamination during formation and that their geochemical characteristics could reflect the characteristics of their source. Amphibole whose formation was related to metasomatism by carbonate fluids was likely present in the mantle source. Therefore, the mantle source might have underwent metasomatism by carbonate fluids. The mantle metasomatism occurred recently, so the metasomatism of mantle source was likely caused by a subducted ocean plate in the early Neoproterozoic. The mafic dykes in the Ziyang area were formed in an intracontinental rift environment, and mantle plume activities were the main cause of the intracontinental rifting.