Elemental and Sr-Nd-Pb isotope geochemistry of the Cenozoic basalts in Southeast China: Insights into their mantle sources and melting processes
We analyzed whole-rock major and trace elements and Sr-Nd-Pb isotopes of the Cenozoic basalts in Southeast China to investigate their mantle source characteristics and melting process. These basalts are spatially associated with three extensional fault systems parallel to the coast line. After correction for the effect of olivine microlites on bulk-rock compositions and the effect of crystal fractionation, we obtained primitive melt compositions for these samples. These primitive melts show increasing SiO 2 , Al 2 O 3 but decreasing FeO, MgO, TiO 2 , P 2 O 5 , CaO and CaO/Al 2 O 3 from the interior to the coast. Such spatial variations of major element abundances and ratios are consistent with a combined effect of fertile source compositional variation and increasing extent and decreasing pressure of decompression melting from beneath the thick lithosphere in the interior to beneath the thin lithosphere in the coast. These basalts are characterized by incompatible element enrichment but varying extent of isotopic depletion. This element-isotope decoupling is most consistent with recent mantle source enrichment by means of low-degree melt metasomatism that elevated incompatible element abundances without yet having adequate time for isotopic ingrowth in the mantle source regions. Furthermore, Sr and Nd isotope ratios show significant correlations with Nb/Th, Nb/La, Sr/Sr @? and Eu/Eu @? , which substantiates the presence of recycled upper continental crustal material in the mantle sources of these basalts. Pb isotope ratios also exhibit spatial variation, increasing from the interior to the coastal area. The significant correlations of major element abundances with Pb isotope ratios indicate that the Pb isotope variations also result from varied extent and pressure of decompression melting. We conclude that the elevated Pb isotope ratios from the interior to coast are consistent with increasing extent of decompression melting of the incompatible element depleted mantle matrix, which hosts enriched Pb isotope compositions.
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