40Ar– 39Ar dating and geochemical characteristics of late Cenozoic basaltic rocks from the Zhejiang–Fujian region, SE China: eruption ages, magma evolution and petrogenesis

Abstract

The Zhejiang–Fujian late Cenozoic volcanism took place sporadically in four volcanic belts and constitutes an important diffuse continental rift basalt province in eastern China. The volcanic rocks consist predominantly of basanite, alkali olivine basalt, olivine tholeiite and quartz tholeiite with subordinate nephelinite and rare alkali picrite basalt. Twenty-four new ages of the basaltic rocks and amphibole megacryst determined by 40Ar– 39Ar incremental heating experiments demonstrate that the basaltic lava erupted from 0.9 to 26.4 Ma. Although some basanite dikes and nephelinite pipes in Inner and Inner Middle belts of the Zhejiang area are early Miocene, almost all the late Cenozoic basaltic volcanism occurred following cessation of South China Sea seafloor spreading (≤16 Ma). Based on these results, most late Cenozoic intraplate magmatism surrounding the South China Sea margins may be related to the migration of the South China Sea mid-ocean ridge system beneath SE China since mid-Miocene. However, in the Zhejiang–Fujian region, volcanic activities terminated gradually and propagated westward and northward due to the collision of the Luzon arc with the eastern edge of the Eurasia continent during late Miocene to Pleistocene. The Zhejiang–Fujian basalts exhibit trace element and isotopic affinities with OIB. Sr and Nd isotope compositions range from 0.703264 to 0.704235 and 0.512725 to 0.512961, respectively, similar to the composition of the Leiqiong basalts in South China. The enrichment of LREE coupled with depleted Sr–Nd isotopic compositions in the basaltic rocks imply that recent mantle metasomatism occurred shortly before the Cenozoic magmatism. The Sr–Nd–Pb isotopic compositions as well as high LILE/HFSE ratios found in some basaltic rocks showed that an EM2-type lead isotope signature existed in the continental lithospheric mantle in the Zhejiang–Fujian region. This character may have resulted from mantle metasomatism due to a paleo-subduction event. The Zhejiang–Fujian basaltic rocks were generated by partial melting and mixing of different proportions of depleted asthenospheric mantle (DMM or MORB) with EM2-type lithospheric mantle and have undergone different degree of fractional crystallization when the magma ascended to the surface.