Mantle sources and petrogenesis of the Cenozoic basalts in Fanshi, Shanxi province : geochemical and Sr-Nd-Pb-Hf isotopic evidence.

摘要

The Fanshi basalts located in the vicinity of the Da Hinggan Ling-Taihangshan gravity gradient zone, represent a major component of Cenozoic basalts of the central North China Craton. Previous studies gave the Fanshi basalts whole rock K-Ar ages of 26.3~24.3Ma. The Fanshi basalts from Sumengzhuang and Yingxian two locations all show OIB-like trace element and isotopic signatures, i.e., they are enriched in incompatible elements with highly fractionated LREEs and HREEs ((La/Yb)N=8.42~21.60) without negative Sr and Eu anomalies. They show relatively low Sr (87Sr/86Sr=0.703848~0.704870) and high Nd (143Nd/144Nd=0.512617~0.513057), Hf (176Hf/177Hf=0.282873~0.283001) isotope ratios, and Pb isotope ratios were 206Pb/204Pb=17.2~17.9, 207Pb/204Pb=15.3~15.4 and 208Pb/204Pb=37.5~37.9. All these geochemical features, combined with petrographic observations and major element data, allow us to suggest that the Fanshi basalts were derived from small degree partial melting of asthenospheric mantle with contributions of metasomatized lithospheric mantle. Olivine and clinopyroxene dominated fractional crystallization is an important process in the petrogenesis of these basalts, which may have taken place in magma chamber of lithospheric mantle condition. The rapid ascent explains the lack of crustal contamination. Sumengzhuang samples are characterized by relatively deeper and lower extent of melting, whereas the Yingxian samples have the signature of slightly shallower and higher extent of melting. Together with studies of Cenozoic basalts in other areas near the gravity gradient zone, we suggest that the Cenozoic basalts near the Gravity Gradient zone may be originated from decompression melting of eastward asthenosphere flow when crossing the gravity gradient zone. This study offers some new perspectives on the petrogenesis of the Cenozoic basaltic volcanism in eastern China in general.