Abstract
The western Yangtze Block, South China is characterized by voluminous Neoproterozoic extension-related volcano-sedimentary sequences and intrusions (e.g. Emeishan granitoid intrusion) and spatially associated mafic dykes. The origin and tectonic setting of Emeishan granitoid intrusion and mafic dykes in the Yangtze Block is poorly study. Understanding the origin and geodynamic setting of dykes and granitoid intrusion are significant in deciphering thermal structure and composition of mantle, tectonic evolution as well as crustal growth processes in the Yangtze Block. SIMS Zircon U-Pb dating of Emeishan granitoid intrusion and mafic dykes suggests that the granitoids (ca. 818 Ma) and mafic dykes (ca. 814 Ma) are contemporaneous. Compositionally, the Emeishan granitoids are restricted and characterized by high SiO2 (69.11-70.50 wt%), Na2O + K2O (3.52-4.13%) and Al2O3 (14.16-14.47%), but low CaO (1.78-2.03%), MgO (0.72-0.91%) contents. They feature strong LREEenrichment (La/YbN = 7.2-12.3) in conjunction with negative Eu anomalies. The mafic dykes have low SiO2 (48.09-48.65 wt%), K2O (1.94-2.12 wt%) and Na2O (1.64-1.75 wt%) contents, and high Mg# (61.67-62.26). On primitive mantle-normalized trace element plot, they are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dykes and granitoids record similar zircon epsilon Hf(t) values (+2.2 to +12.1, +1.4 to +6.8), and zircon delta O-18 values (4.47 parts per thousand and 10.41 parts per thousand, 4.49 parts per thousand and 12.05 parts per thousand, respectively). The rifting-related geochemical signature of the Emeishan pluton and contemporaneous margin and intra-plate plume-related magmatism suggest that they were derived from mantle melting in extensional tectonic setting during mid-Neoproterozoic.