矿床地球化学国家重点实验室知识库

Cenozoic intraplate basalts are low in volume but widespread in eastern China. They are predominantly alkaline and have oceanic island basalt (OIB)-like trace element compositions. Despite numerous studies, the origin of Cenozoic basalts in eastern China remains elusive. Possible roles of lithosphere thickness, subduction of Pacific plate and lithosphere-asthenosphere interaction in triggering spatial geochemical variations have not yet been clarified. Here, we have carried out mineral chemistry, major and trace element and Sr-Nd-Hf isotope analyses of the late Cenozoic (<20 Ma) basaltic rocks from the Changbaishan-Baoqing Volcanic Belt (CVB), NE China, which revealed clear spatial compositional variations. The North CVB is dominated by basanites and alkali basalts with OIB-like trace element patterns and depleted Sr-Nd-Hf isotopic compositions (Sr-87/Sr-86 = 0.7039-0.7047, epsilon(Nd) = 3-5.6; epsilon(Hf) = 6.7-12), which may have been derived from low degree partial melting of a depleted source from asthenospheric mantle beneath a thick lithosphere. On the other hand, the South CVB consists of both alkali and sub-alkali lavas (alkali basalts, tholeiites and basaltic andesites) that display generally higher SiO2, Sm/Nd, Ba/Nb, Th/U, and lower Nb/Th, La/Sm and more enriched Sr-Nd-Hf isotopic compositions (Sr-87/Sr-86 = 0.7038-0.7056, epsilon(Nd) = -2.4- + 3.2; epsilon(Hf) = 3-8.2). These rocks may have been produced by larger degrees of partial melting of asthenospheric mantle beneath a relatively thin lithosphere. Ancient metasomatized lithospheric mantle might also have contributed to their genesis. In addition, the similar ranges of Mn, Ni and Fe/Mn for olivine phenocrysts from both the North and the South CVB suggest that they may have been derived from hybrid mantle sources containing similar proportions of peridotite and pyroxenite/eclogite components. We propose that decompression melting of upwelling asthenosphere and mechanical-chemical erosion of basal lithosphere related to lithosphere-asthenosphere interaction responsible for the genesis of the CVB magmas were likely associated with upper mantle convection and back-arc extension induced by deep subduction of the Pacific plate and its stagnancy in the transition zone.