東南極ナピア岩体,パドー山の始生代片麻岩のSm-NdおよびRb-Sr全岩年代

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Sm-Nd and Rb-Sr whole-rock isochron ages determined for Archaean gneisses from Mount Pardoe show the highest-grade metamorphism (granulite-facies) in the Napier Complex. Felsic gneisses at Mount Pardoe give a defined Sm-Nd isochron age of 2589±192 Ma, with an initial ^<143>Nd/^<144>Nd ratio of 0.50909±0.00015, which corresponds to the initial εNd at that time in ranging from -2.0 to -7.0, Mafic gneisses at Mount Pardoe give an age of 2595±262 Ma, with an initial ^<143>Nd/^<144>Nd ratio of 0.50937±0.00024. The initial εNd value which is extremely low compared with that of CHUR (Chondritic Uniform Reservoir) suggesting that these rocks had a long crustal history before 2589 Ma. These Sm-Nd ages obtained from the whole-rock samples do not represent the primary age, but indicate later re-equilibration of ^<143>Nd/^<144>Nd ratios. This tectonothermal event of D_3-M_3 (ca. 2450 Ma) seems to have been strong enough to have reset the Sm-Nd systems. Initial Nd ratios of felsic gneisses at Mount Pardoe (0.50909) and Tonagh Island (0.50897) are similar probably reflecting original similarities. Both of felsic gneisses from Mount Pardoe and Tonagh Island may be a part of related igneous suite. On the other hand, the Nd initial ratio differences between gneisses of Mount Pardoe (0.50937) and Tonagh Island (0.50794) could reflet original protolith differences. The Rb-Sr whole-rock isochron age of felsic gneiss at Mount Pardoe 2919±233 Ma, with an initial ^<87>Sr/^<86>Sr ratio of 0.70581. Four maflc gneisses from Mount Pardoe give an isochron age of 2908±216 Ma and initial ratio of 0.70517±0.00098. These ags are within error of one another. These Rb-Sr isochron ages are older than those of the Sm-Nd whole-rock isochron age for the same felsic gneisses. These Rb-Sr isotope data for the felsic and mafic gneisses indicate D_2-M_2 period of re-equilibration of the isotopic system on a whole rock. It seems that the retrogressive metamorphism had a great effect on the Sm-Nd whole-rock system compared with the Rb-Sr whole-rock system.
特定非営利活動法人日本火山学会の論文
1997-04-25

東南極ナピア岩体,パドー山の始生代片麻岩のSm-NdおよびRb-Sr全岩年代

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