The mineral thermobarometry proposed in literature is used to calculate the equilibrium temperature and pressure of garnet lherzolite and websterite xenoliths within the Cenozoic kamafugite from west Qinling, Gansu Province, China. The results show that the equilibrium temperature and pressure of garnet lherzolites and websterite and 1127–1266°C, 2.9–3.6 Gpa and 1169–1248°C, 2.8–3.2 Gpa respectively. The equilibrium peressures reach or exceed the equilibrium peressure of spinel lherzolites (2.0–3.0 GPa), and fall into the stability range of garnet peridotite. The equilibrium temperature of the xenoliths reach or exceed the ocean geotherm, identical with the melting temperature of kamafugite magma determined by experiments under the conditions of post-orogenic lithosphere extension. So the thermal state of Cenozoic mantle of the west Qinling may be fit to generate the kamafugite magmatism. The research on petrology-mineralogy and geobarothermometry of the xenoliths shows that both garnet lherzolite and websterite are mantle components of the west Qinling, and may be considered as source rocks of the Cenozoic kamafugite magma.
Quaternary loess-soil sequences in China and Tajikistan are valuable archives of the chemical weathering history for arid and semi-arid regions of Asia. The development of new proxies independent of grain size are vital to reconstruct the long-term chemical weathering history of eolian deposits. In this study, we analyze major elemental and Rb, Sr, and Ba concentrations of decarbonated residue from representative loess and soil units along a north-south transect on the Chinese Loess Plateau and compare these concentrations among different grain-size fractions. Results show that most of the elemental abundances and ratios vary considerably among different grain-size fractions for both loess and paleosols, indicating transport-driven compositional differentiation of minerals during subaerial transport. However, (CaO + Na2O + MgO)/TiO2 ratios show little variation among different size fractions for the loess-soil transect. Loess-soil sequences at Lingtai (northern China) and Chashmanigar (southern Tajikistan) for the past 1.50-1.77 Ma show lower (CaO + Na2O + MgO)/TiO2 ratios in soil units than in adjacent loess horizons, in good agreement with greater weathering intensity in soils than in loess units. The (CaO + Na2O + MgO)/TiO2 ratio of the decarbonated residue is therefore a reliable proxy for chemical weathering of loess deposits that is independent of grain size. The (CaO + Na2O+MgO)/TiO2 ratios of the Lingtai and Chashmanigar sections display a relatively regular oscillation within a narrow range in the early Pleistocene, and a rapid increase in average values and in variance from 0.85-0.6 Ma to the present, indicating lowered chemical weathering intensity in both the dust source regions and the depositional areas. This event may be causally related to the expansion of northern hemisphere ice and/or the regional tectonic uplift of high mountains in Asia since the mid-Pleistocene. 相似文献