Continental basalts in the accretionary complexes of the South-west Japan Arc: Constraints from geochemical and Sr and Nd isotopic data of metadiabase

Abstract The Ryoke Belt is one of the important terranes in the South‐west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta‐sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian–early Jurassic). Initial ?_Sr (+ 25– + 59) and ?_Nd (? 2.1–?5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB‐signature, rather than those of either IAB or ACMB. The Sr–Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental‐type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr–Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr–Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental‐type lithosphere is widely represented beneath the western part of the Japanese Islands.     

New polar ring galaxies in rich clusters of galaxies

Four polar ring galaxies discovered in rich clusters of galaxies are presented. Brief comments on their structural properties are given.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Characterization of the Mefjell plutonic complex from the Sør Rondane Mountains, East Antarctica: Implications for the petrogenesis of Pan-African plutonic rocks of East Gondwanaland

Abstract The Mefjell plutonic complex consists of 500–550‐Ma Pan‐African plutonic rocks, which intrude into the Precambrian crystalline basement in the Sør Rondane Mountains, East Antarctica, and forms part of the Sør Rondane Suture Zone. The complex comprises syenitic and granitic (mostly monzogranitic) rocks, and is characterized by the presence of iron‐rich hydrous mafic minerals and primary ilmenite, both of which imply its formation at high temperature and under low oxygen fugacity conditions. The syenitic rocks are metaluminous, and are high in alkalis, K₂O/Na₂O, Al₂O₃, FeO_t/(FeO_t + MgO) (0.88–0.98), K/Rb (800–1000), Ga (18–28 p.p.m.), Zr (up to 2100 p.p.m.) and Ba. They also have a low Mg? (Mg/[Mg + Fe²⁺]), Rb, Sr, Nb, Y and F, low to moderate light rare earth element (LREE)/heavy rare earth element (HREE) ratios and positive Eu anomalies in their rare earth element (REE) patterns. The granitic rocks are metaluminous to peraluminous, and have a high Rb content, high Sr/Ba and LREE/HREE ratios, low K/Rb and negative Eu anomalies. Most of the syenitic and granitic rocks have Y/Nb ratios greater than 1.2, and are depleted in Nb, Ti and Sr on the primitive mantle‐normalized spider diagrams, indicating a crustal origin with subduction zone signatures. We interpret both the syenitic and granitic rocks to be derived from an iron‐rich lower crustal source by dehydration melting induced by the heat of mantle‐derived basaltic intrusion, after which they then underwent limited fractional crystallization. The Mefjell plutonic complex has a high Zr content and tectonic discrimination diagram signatures indicative of normal A‐type granitic rocks. Both rock suites may have been generated under the same postorogenic tectonic setting. The Mefjell syenitic rocks are chemically comparable to charnockites in the Gjelsvikjella and western Mühlig‐Hofmannfjella areas of East Antarctica, whereas the granitic rocks are comparable to aluminous A‐type granitic rocks in South India, which were emplaced during formation and evolution of the Gondwanaland supercontinent.