Early Paleozoic lithospheric extension law, dynamic mechanism, origin of mafic dykes in Ziyang, South Qinling Block, China

Early Paleozoic mafic dyke swarms are widely exposed in Ziyang area, South Qinling Block, and its research has important geodynamic significance. Nevertheless, the existing research-related is mainly focused on the Silurian mafic dyke swarms, while the research on the Early Paleozoic (such as Cambrian and Ordovician) mafic dyke swarms is still relatively weak. Thus, it restricts the overall grasp of the problems related to the Early Paleozoic lithospheric extension process in the South Qinling Block, such as the temporal and spatial distribution law, mantle properties, dynamic mechanism and related mineralization. In view of the remaining scientific problems, this study selects the mafic dyke swarms in Kangjiaping, Lishuliang, Dazhuping, Qingmingzhai, Goujiashan, Miaoliangshang, Zengjiashan and Maoba Village as the research objects, and carries out the research on mineralogy, petrology, zircon U-Pb chronology, geochemistry and Sr, Nd and Hf isotope, and the age pattern, origin and genetic dynamic background are systematically discussed. The results show that the Early Paleozoic mafic dykes are mainly dolerite and sillite, which are formed in the Ordovician (478.8 similar to 486Ma) and belong to alkaline series rocks (K2O+Na2O variation range is 4.10% and 5.14%). In addition, the mafic dykes in the study area is characterized by the geochemical characteristics of enrichment of LREE, Rb, Ba, Sr, Nb, Ta, Zr, HF and Eu (Eu/Eu*=1.13 similar to 1.35), typical characteristics of depleted HREE, U, Pb and Ti, (Sr-87/Sr-86)i=0.7044 similar to 0.7050, eNd(t)=3.1 similar to 3.6, epsilon(Hf)(t)=4.4 similar to 12, Hf-1176/Hf-177=0.282634 similar to 0.282846, suggesting that the dykes studied is the product of partial melting of the depleted lithospheric mantle or asthenosphere mantle in the process of mantle plume. In addition, olivine and clinopyroxene were separated and crystallized in the genetic process of the dykes, but the influence of crustal contamination is not obvious.