关键词: 形成时差, 锆石SHRIMP U-Pb定年, 镁铁质岩墙群, 寄主花岗岩, 地球化学, 福建湄洲岛

Abstract:

Mafic dikes in Meizhou Island, Fujian Province, intrude along joints of the granite. They are mainly hornblende diabase and composed of plagioclase (An=45-55), hornblende and minority of augite and/or quartz. The SHRIMP U-Pb age of zircons from mafic dikes is （90±2） Ma. The host granite, with zircon SHRIMP U-Pb age of （105±3） Ma, is composed of quartz（25%-30％），perthite（50%-60％），plagioclase (An=22-25，10%-15％) and minority of biotite (<5%). The age difference between the mafic dikes and their host granite is approximately 10 Ma. Geochemically, the mafic dikes are of low CaO, MgO, TiO₂, high Al₂O₃, with LREE and LILE enriched, Ta-Nb depleted, (⁸⁷Sr/⁸⁶Sr)_i=0.705 2-0.705 4,ε_Nd(t) =-0.03-1.57. The overall elemental and Sr-Nd isotopic characteristics of the Meizhou Island mafic dikes are similar to those of mafic dikes from the coastal area of Fujian Province. The host granite is rich in silicon, alkalis, w(SiO₂)>75%，A/CNK=1.05-1.12，w（K₂O+Na₂O）=7.68%-8.12%， (⁸⁷Sr/⁸⁶Sr)_i=0.705 5，ε_Nd(t) = -3.92, which is consistent with that of the late Mesozoic high-K calc-alkaline granitoid in the coastal region of Fujian Province. It’s suggested that the host granites in Meizhou Island be formed by partial melting in middle-lower crustal rocks by underplating and heating of basaltic magma produced from partial melting of enriched lithospheric mantle. The mafic dikes were probably derived from the detained basaltic magma experienced fractional crystallization process. The granite and mafic dikes from Meizhou Island formed in the extensional setting and responded to two extension events. The study on the Meizhou Island granite and mafic dikes provides a new case for Late Mesozoic tectonic-magmatic activities and constraint to the time of lithosphere extensional events in the coastal area of Fujian Province.

Key words: age difference, zircon SHRIMP U-Pb dating, mafic dikes, host granite, Meizhou Island in Fujian