The Thermal Structure of the Upper Mantle in Eastern China—Inferred from the Petrological Model

Cenozoic basalt in eastern China contains abundant ultramafic xenoliths which are specimens of pyrolites released during basaltic magma eruption. A total of 405 P-T data of pyroxene in the ultramafic rocks have been collected, which present a more precise pyroxene geotherm. The average geothermal gradient in the upper mantle represented by the pyroxene geotherm is about 3.3°C / km, which is much less than that derived from the conductive thermal model (≡ 14°C / km), implying the great significance of convective heat transfer. The calculation shows that the contributions of convective and conductive heat transfers are 79% and 21%, respectively. The perturbation in the thermal structure of the upper mantle is an important manifestation of the tectonothermal event of Cenozoic continental rifting and intense basaltic volcanism in eastern China. Based on the pyroxene geotherm and its comparison with the current geothermal field derived from the measurements of the surface heat flows, it is suggested that the Moho may be a secondary thermal boundary. The current geothermal field and the thermal structure of the lithosphere in eastern China may mainly reflect the result of the tectonothermal disturbance in the Neogene-Quaternary, in other words, the lithosphere has just begun to cool.