Confirmation and Geological Significance of Early Ordovician Adakite in Embouchure Area of the Eastern Segment of the Lajishan Mixed Belt

The intrusive rocks in embouchure area of the eastern segment of the Lajishan mixed belt can be further disintegrated into Yingyun diorite and granodiorite. The zircon U-Pb isotopic dating is carried out by LA-ICP-MS. The zircon U-Pb age of Yingyun diorite is (480.7±1.9) Ma (MSWD=0.80), which indicates that the emplacement age of rock mass is early Ordovician.The characteristics of principal elements show that the rocks have high silicon (SiO₂=59.10%-63.45%), high aluminum (Al₂O₃=15.40%-16.19%), rich sodium and little potassium (Na₂O/K₂O=1.41-2.03). The rocks belong to calc-alkaline-high-potassium calc-alkaline series rocks. The standardized partition pattern diagram of rare earth element meteorite shows the right dip curve of light rare earth enrichment (ΣLREE=148.22×10^-6-173.46×10^-6) and heavy rare earth loss (ΣHREE=10.04×10^-6-14.40×10^-6), with the negative anomaly of slight Eu (δEu=0.92-0.96). The rocks are obviously enriched in large ion lithophile elements such as Rb,Ba,Th and deficient in high field strength elements such as Nb,Ta,Ti,P. At the same time, the ‘TNT’ valley with loss of Ta,P,Ti is formed, which shows the characteristics of arc granite. The rock has the characteristics of high Sr (591.9×10^-6-922.9×10^-6), low Y (12.44×10^-6-18.79×10^-6) and low Yb (1.24×10^-6-1.90×10^-6), and a high ratio of Sr/Y (34.85×55.76). The geochemical characteristics are consistent with those of the typical adakite. Combined with the comprehensive study of the regional data, it is considered that the early Ordovician Adakite in embouchure area was originated from the partial melting of basement basaltic rocks which was induced by subduction plate fluid metasomatic mantle wedges and rich in hornblende facies, and was formed under the action of subduction and consumption of the volcanic arc environment of the active continental margin. This indicates that the oceanic basin of Laji Mountain was already in the early stage of plate collision in embouchure area during this period.