Zircon U-Pb age and geochemistry of the dioritic rocks in Chuzhou area in Anhui Province: Petrogenesis and dynamics significance

LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U ages of two diorite porphyrite samples from Chuzhou area of Anhui Province are 126.19±0.44Ma and 126.4±0.7Ma, respectively. Previous study shows that the emplacement age of dioritic rocks should be 120~130Ma, sug-gesting early Cretaceous. Geochemical study shows the values of SiO₂ vary from 56.75% to 60.90%, with the characteristics of high Al₂O₃(14.82%~15.77%), MgO(>4%), Sr(>750×10^-6), Sr/Y(62~110), and La/Yb(20~36) but low Y, Yb, LREE as well as enrichment of LREE and LILE but depletion of HREE, with indistinct Eu anomaly, indicating typical adakitic rocks. Mg^# values range from 39 to 45, K₂O/Na₂O vary from 0.57 to 0.96, 0.75 on average, significantly lower than the values of the adakitic rocks in the thickened crustal of Dabie orogenic belt; Ce/Pb ratios are low, mostly concentrated in the range of 3~5, similar to the values of the continental crust but significantly lower than those of the oceanic crust. The authors hold that the adakitic rocks in Chuzhou area of Anhui was formed by partial melting of delaminated lower crust, and the reaction of upward migrating adakitic magmas and mantle peridotite led to the increase of MgO, Cr and Ni melt content. In Early Cretaceous, eastern China crustal extension thinning resulted in lower crust delamination, and mantle material was involved in bringing Cu, Au and other ore-forming elements; hence adakitic rocks can be used as an important prospecting indicator in this area.