Research Progress on Cenozoic Volcano Genesis and Fluid Action in Northeast China

The tectonic evolution of northeast China is closely related to the subduction of the Pacific plate. The dehydration of the slab subduction process produces metasomatic agents that have important effects on the physical and chemical properties of the mantle wedge, including the decrease of seismic wave velocity and the increase of Poisson’s ratio and electrical conductivity. In order to investigate the tectonic evolution and fluid action of northeast China, this paper compares the previous seismic and electromagnetic imaging results of northeast China and explores the relationship between the genesis of Cenozoic volcanoes and fluid action in northeast China through rheological analysis. The results show that the western Pacific plate subducted into the mantle transition zone beneath northeast China, and sustained dehydration occurred. The upward migration of these released water caused partial melting at the base of the upper mantle. Some of the upwelling streams pierced the weak tectonic boundary under the buoyancy effect, which finally formed the large-scale Cenozoic volcanic events in northeast China.

Different Tectonic Evolution of Fast Cooling Ophiolite Mantles Recorded by Olivine-Spinel Geothermometry: Case Studies from Iballe (Albania) and Nea Roda (Greece)

Mg-Fe2+ diffusion patterns in olivine and chromite are useful tools for the study of the thermal history of ultramafic massifs. In the present contribution, we applied the exponential modeling of diffusion patterns to geothermometry and geospeedometry of chromitite ores from two different ophiolite contexts. The Iballe ophiolite (Northern Albania) hosts several chromitite pods within dunites. Primary and re-equilibrated Mg#, estimated by using an exponential function, provided re-equilibration and primary temperatures ranging between 677 and 996 °C for chromitites and between 527 and 806 °C for dunites. Cooling rates for chromitites are higher than for dunites, suggesting a different genesis for the two lithologies, confirmed also by spinel mineral chemistry. Chromitites with MORB affinity formed in a SSZ setting at a proto-forearc early stage, explaining the higher cooling rates, while dunites, with boninitic affinity, were formed deeper in the mantle in a more mature subduction setting. At the Nea Roda ophiolite (Northern Greece) olivine in chromitites do not show Mg-Fe variations, and transformation into ferrian chromite produced “fake” diffusion patterns within chromite. The absence of diffusion patterns and the low estimated temperatures (550–656 °C) suggest that Nea Roda chromitites were completely re-equilibrated during an amphibolite-facies metamorphic event that obliterated all primary features.