The primary concern of this paper is to analyse the systematic differences in chemical composition between two suites of trap intrusives that bear a close genetic relationship to titaniferous magnetite and nickel sulf...The primary concern of this paper is to analyse the systematic differences in chemical composition between two suites of trap intrusives that bear a close genetic relationship to titaniferous magnetite and nickel sulfide mineralizations respectively.A discussion is also given to the possible mechanisms of differentiation for such differences. The“alkali-ealeium index”(Calk/m=(Ca+Al+Na+K)×100/Mg+Ca+Al+Na+K number of atoms%)ad “acidity”(αsi=Si×100/Si+Ti+Al+Fe+Mn+Mg+Ca+Na+K number of atoms%)were calculated and plotted for various facies belts of the trap intrusives.It is apparent from these diagrams that alkali-calcium indexes of the suites with strong titaniferous magnetite mineralization are systematically higher than that of the niekelsulfide-rieh ones. The former is therefore referred to as “alkali-calcium trap”and the latter “partial-magnesium trap”. The experimental results of D. H. Green and A. E. Ringwood (1964) for samples of olivine thoteiite compositioN agree in many respects with the trend of compositional variation among the two trap groups in Southwestern China. In conjunction with the intimate tectonic relationship between the two suites, it can be inferred that they represent products of the upper and lower daughter magmas derived from a single parent trap magma by fractional crystallization at 12.5--20 kbar and 1200--1300℃. As is evidenced by their compositional features, the Perraian Omeishan basalt floods are probably the eruptive facies of an alkali-caleium daughter magma, but,owing to the peculiarity of differentiation mechanism in the magmatic sources, they exhibit neither rockforming mineral assemblage characteristie of typical olivine tholeiite nor can be regarded as alkali-olivine basalt. A review of available data on various types of magmatie rock formations throughout the world indicates that the chemistry of basic and ultrabasie intrusives related to titaniferous magnetite mineralization is approximately consistent with the characteristics of alkali-calcium trap series, i.e., being rich in calcium and aluminum, slightly rich in alkalis and relatively poor in magnesium. On the other hand, the general petrochemical character of intrusives related to large nickel sulfide deposits is the low contents of alkalis and titanium. It has also been noted that, among these intrusives,a chemical composition comparable to that of “partial-magnesium trap” and. a TiO3 content of less than 1.6% may be much more favorable for nickel sulfide deposition.Furthermore, as an additional requirement, the ultrabmsic facies belts at the lower parts of these intrusives should be ferro-ultrabasic rocks produced by differentiation from tholeiite magma. In many cases, however, little evidence of appreciable nickel sulfide mineralization has been obsernred in magnesium-ultrabasic rocks.展开更多
Recent eruption of Unzen Volcano in 1991-1995 caught attention of scientists all over the world because of disastrous character of previous one in 1792. Intrusion of andesitic magma to the chamber with rhyolitic magma...Recent eruption of Unzen Volcano in 1991-1995 caught attention of scientists all over the world because of disastrous character of previous one in 1792. Intrusion of andesitic magma to the chamber with rhyolitic magma is proposed to be a trigger for these eruptions. T-P-X parameters of two end-member magmas have been estimated several times, but usually estimations are based on phenocrysts assemblages. New results of this research are based on mafic enclaves and groundmass. These results are significant for magma mixing and mingling theory.展开更多
文摘The primary concern of this paper is to analyse the systematic differences in chemical composition between two suites of trap intrusives that bear a close genetic relationship to titaniferous magnetite and nickel sulfide mineralizations respectively.A discussion is also given to the possible mechanisms of differentiation for such differences. The“alkali-ealeium index”(Calk/m=(Ca+Al+Na+K)×100/Mg+Ca+Al+Na+K number of atoms%)ad “acidity”(αsi=Si×100/Si+Ti+Al+Fe+Mn+Mg+Ca+Na+K number of atoms%)were calculated and plotted for various facies belts of the trap intrusives.It is apparent from these diagrams that alkali-calcium indexes of the suites with strong titaniferous magnetite mineralization are systematically higher than that of the niekelsulfide-rieh ones. The former is therefore referred to as “alkali-calcium trap”and the latter “partial-magnesium trap”. The experimental results of D. H. Green and A. E. Ringwood (1964) for samples of olivine thoteiite compositioN agree in many respects with the trend of compositional variation among the two trap groups in Southwestern China. In conjunction with the intimate tectonic relationship between the two suites, it can be inferred that they represent products of the upper and lower daughter magmas derived from a single parent trap magma by fractional crystallization at 12.5--20 kbar and 1200--1300℃. As is evidenced by their compositional features, the Perraian Omeishan basalt floods are probably the eruptive facies of an alkali-caleium daughter magma, but,owing to the peculiarity of differentiation mechanism in the magmatic sources, they exhibit neither rockforming mineral assemblage characteristie of typical olivine tholeiite nor can be regarded as alkali-olivine basalt. A review of available data on various types of magmatie rock formations throughout the world indicates that the chemistry of basic and ultrabasie intrusives related to titaniferous magnetite mineralization is approximately consistent with the characteristics of alkali-calcium trap series, i.e., being rich in calcium and aluminum, slightly rich in alkalis and relatively poor in magnesium. On the other hand, the general petrochemical character of intrusives related to large nickel sulfide deposits is the low contents of alkalis and titanium. It has also been noted that, among these intrusives,a chemical composition comparable to that of “partial-magnesium trap” and. a TiO3 content of less than 1.6% may be much more favorable for nickel sulfide deposition.Furthermore, as an additional requirement, the ultrabmsic facies belts at the lower parts of these intrusives should be ferro-ultrabasic rocks produced by differentiation from tholeiite magma. In many cases, however, little evidence of appreciable nickel sulfide mineralization has been obsernred in magnesium-ultrabasic rocks.
文摘Recent eruption of Unzen Volcano in 1991-1995 caught attention of scientists all over the world because of disastrous character of previous one in 1792. Intrusion of andesitic magma to the chamber with rhyolitic magma is proposed to be a trigger for these eruptions. T-P-X parameters of two end-member magmas have been estimated several times, but usually estimations are based on phenocrysts assemblages. New results of this research are based on mafic enclaves and groundmass. These results are significant for magma mixing and mingling theory.
