非線形システムの形式的線形化による数値解法とその適用に関する研究

九州工業大学博士学位論文　学位記番号:工博乙第35号　学位授与年月日:平成7年6月30日九州工業大学平成7年

Petrogenesis of neoproterozoic granitoids and related rocks from the Seychelles: The case for an Andean-type Arc origin

SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Platinum-group element distribution in the Main Zone and\ud Upper Zone of the Bushveld Complex, South Africa

The platinum-group element (PGE) contents of the upper portions of the Bushveld Complex were investigated with three questions in mind: (a) In natural systems does magnetite concentrate Os, Ir, Ru and Rh (IPGE), as has been observed in experimental systems? (b) Is there a Au–Pd-enriched layer present such as observed in the upper parts of the Skaergaard intrusion? (c) Can changes in metal ratios be used for prospecting for PGE deposits?\ud In the sulfide-poor Main Magnetite Layer of the eastern Bushveld, Ir and Rh are enriched relative to Pt, Pd and Au and this could be because magnetite preferentially concentrated Ir and Rh over Pt, Pd and Au. However, in most other magnetite layers no enrichment was observed. This could be because most magnetite layers contain approximately 1% sulfides and the PGE budget is dominated by the sulfides. These sulfides obscured the effects of magnetite collecting IPGE, because sulfides collect all the PGE and the partition coefficients for the PGE into a sulfide liquid are much greater than the partition coefficient for IPGE into magnetite.\ud \ud The weighted average of the platinum-group elements (PGE) and Au over the 2000 m of sampled stratigraphy is Au 2.1 ppb, Pd 1.7 ppb, Pt 1.7 ppb, Rh 0.18 ppb, Ru<0.5, Ir 0.16 ppb, Os<0.5 ppb. Compared to the marginal rocks (presumed initial liquids) of the Bushveld Complex the PGE and Au are severely depleted. Only one sample (a leuconorite in the first cyclic unit) contained Pt and Pd at economic grade (Pt 2 ppm, Pd 2 ppm). The overall depletion of the PGE in the Upper Zone (despite the presence of 1% to 3% sulfides) could be the result of the PGE having been stripped from the magma by early sulfide liquid which had already settled out of the magma to form the world famous platinum reefs lower in the magma chamber. In addition to the overall depletion of PGE, PGE/S ratios decrease up section indicating that the sulfide fraction is poorer in PGE up section. This is interpreted to be the result of continued depletion of the silicate liquid as sulfides constantly settle out of the silicate liquid. There appears to be little prospect of a Pd-reef type deposit in the Upper Zone.\ud \ud Comparison of the composition of the marginal rocks of the Bushveld with a weighted average for the complete 6 km of the Bushveld cumulates shows that the cumulate pile is much richer in compatible elements (Ir, Rh, Cr) and poorer in incompatible elements (Sm and Hf) than the marginal rocks. Two possible solutions to this are: (a) The magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or (b) fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres

A clinopyroxenite intrusion from the Pilanesberg Alkaline Province, South Africa

&lt;p&gt;A number of circular negative magnetic anomalies (up to 8 km across) exist within the area encompassed by the western Bushveld Complex (150 km by 100 km) on the Kaapvaal craton in South Africa. They are covered by up to 700 m of sedimentary rocks of the Karoo Supergroup, which could not produce these anomalies. Exploration boreholes into one of these magnetic anomalies revealed a hidden volcanic complex, called the Elandskraal Volcano. One of these boreholes intersected an olivine–magnetite–apatite clinopyroxenite body, which we studied. Ages on apatite–clinopyroxene pairs using Sm–Nd dating techniques yield a poorly constrained age of 1207 ± 200 Ma, because there is little variation in Sm–Nd between all the samples. This age correlates with the Pilanesberg Alkaline Province that spans the time period from 1430 to 1200 Ma, but almost all age determinations from this suite give very large errors.&lt;/p&gt; &lt;p&gt;Three other clinopyroxenite bodies closely related to the Pilanesberg Alkaline Province have been reported, but no geochemical data have been presented. Our mineral and whole-rock geochemical data permit an interpretation of the genesis of these clinopyroxenite bodies. The basaltic lavas of the Elandskraal Volcano are extremely unusual in having very high TiO&lt;sub&gt;2&lt;/sub&gt; (over 7 wt%), high Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (16–21 wt%) and high incompatible element contents. Modelling the crystallization sequence using MELTS of the more magnesian lava compositions yields olivine, clinopyroxene, magnetite and apatite as liquidus phases within a temperature interval of less than 30 °C, which matches that observed in the clinopyroxenite body. In our samples the Mg/(Mg + Fe) value for the mafic minerals and incompatible trace elements abundances in clinopyroxene are consistent with crystallization from these unusual basaltic compositions. Contrasts with other clinopyroxenite bodies in the Phalaborwa Complex and the Bushveld Complex are documented in terms of rock associations and mineral compositions (both major and trace elements).&lt;/p&gt