Ultramafic rocks at the center of the Vredefort structure: Further evidence for the crust on edge model. Abstract Ultramafic rocks from the center of the Vredefort structure previously have been interpreted either as Early Proterozoic igneous rocks genetically related to the Bushveld complex, or as Archean upper mantle rocks.
Re-Os isotopic data from these rocks display very low 187Os/188Os and 187Re/188Os, suggesting that they are not magma chamber cumulates, but rather the residues of melt extraction, similar in Re-Os systematics to subcontinental lithospheric mantle. Whole-rock Re depletion model ages for two samples are 3.3 and 3.5 Ga, indicating that the postulated melt extraction event must have occurred in the Archean. On the basis of these data we conclude that the ultramafic rocks are not related to the Bushveld event, but rather represent upper mantle that rebounded to crustal levels following an impact event. The Character of the Chemical Index of Alteration and Discussion of Subdivision and Correlation of the Nanhua System in Yichang Area.
The chemical index of alteration(CIA)is first proposed as a marker to distinguish the chemical weathering degree of source rocks, and later it is used to distinguish the depositional environment of sediments.
The formula of the chemical index of alteration is CIA= n(Al_2O_3) /[ n(Al_2O_3) + n(CaO*) + n(Na_2O) + n(K_2O) ]×100, here, CaO* represents CaO in silicates, n(CaO*) = n(CaO) - n(CO_2,calcite) -0.5· n(CO_2,dolomite) -10/3· n(P_2O_5) . It is very important to select samples for analyses of the chemical index of alteration. Following two factors have to be considered when sampling in the field. First, the best rocks are the fine clastic rocks. Outline of GLG354. Radiogenic Isotope Geology.