Two new U-Pb ages of 3448 ± 19 Ma and 3527 ± 17 Ma were obtained on zircons from the Older Metamorphic Tonalite Gneisses (OMTG), Singhbhum North Orissa Craton, Eastern India. These dated TTG-type OMTG gneisses are from the least studied northern area of the Singhbhum Craton. They occur as enclaves in the Singhbhum granite batholith (SBG), emplaced during two distinct periods: from 3440 to 3330 Ma (SBG-A) and from 3200 to 3050 Ma (SBG-B). The U-Pb age of 3448 ± 19 Ma age is close to the older SBG-A phase. The U-Pb age of 3527 ± 17 Ma could corresponds to an older intrusive or inherited from a xenocryst population. Dacitic lava from the southern BIF-bearing Iron Ore Group (IOG) greenstone belt has been recently dated at 3506 ± 2.3 Ma suggesting that it is older than both phases of the SBG. It is thus possible that the metasedimentary rocks of the Older Metamorphic Group (OMG), occurring as enclaves in both OMTG and SBG suites, might be partly derived from the IOG rocks.
We conducted U-Pb dating on zircon crystals separated from two drill core samples to ascertain whether a tonalite body discovered by drilling in Matsukawaura east of the Futaba Fault near the city of Soma, belongs to a 300-Ma Paleozoic granite, or to Cretaceous granitoids with age common to the Abukuma Highlands or the Kitakami Mountains.The zircons yield U-Pb ages of 115-114 Ma, which are slightly older than the age of Abukuma granitoids exposed to the area between the Hatagawa and Futaba faults (108-106 Ma).The Matsukawaura tonalite, having a typical adakitic signature, exhibits similarity to Cretaceous Takase Granite in Wariyama Sheared Granodiorite occurring along the east side of the Futaba Fault.
Daisen volcano is located on the Quaternary volcanic front in southwestern Japan. The volcano is composed mainly of andesite and dacite, which chemically resemble adakites, with high Al2O3 and Sr/Y, steep REE patterns, and no negative Eu anomaly. (238U-230Th) disequilibrium (herein, a ratio in parentheses denotes the activity ratio) and trace element analyses of adakites from two volcanic domes, Karasugasen and Misen, indicate two adakite types. Adakite from Karasugasen is characterized by excess (230Th) over (238U), typical of most adakites, whereas adakite from Misen is characterized by excess (238U) over (230Th). The latter is consistent with enrichment in fluid-mobile elements relative to fluid immobile elements compared to rocks from Karasugasen. The values of (230Th/232Th) of adakites from Karasugasen and Misen are, respectively, around 0.75 and 0.81. These low (230Th/232Th) ratios result from the incorporation of subducted sedimentary material. The ratios, nevertheless, are higher than that for the estimate of lower crustal material suggesting significant incorporation of lower crust is unlikely. As adakites from Misen have (238U) excess over (230Th), adakite magma must have interacted with wedge mantle metasomatized by a slab-derived fluid, confirming the presence of a fluid-metasomatized mantle beneath Daisen volcano.
To ascertain factors controlling melt production along a typical distal, ‘hotspot-interacting’ mid-ocean ridge, we investigated the extent and distribution of both plume-related and plume-unrelated basalt from the central Indian ridge (CIR) between 15°S and 20°S. Comprehensive geochemical data of fresh-quenched volcanic glasses and basalts were used. Variation of Sr, Nd, and Pb isotopic compositions and Nb/Zr, Ba/Nb, and Ba/La content were interpreted by mixing of three melt end members: the Indian depleted MORB mantle derived melt; radiogenic and enriched melt derived from source mantle for Rodrigues Ridge and the intermediate series of Mauritius Island (RE2, radiogenic enriched component 2); and radiogenic but depleted melt derived from source mantle for Gasitao Ridge (RD, radiogenic depleted component). On the basis of quantitative mantle melting and melt mixing model, results show that sources for RE2 and RD are geochemically distinct from those of the Réunion plume (RE1, radiogenic enriched melt component 1). Moreover, the geochemical variation of MORB of 15°S to 20°S is unrelated to contamination of the upper mantle by the Réunion plume. These results suggest strongly that plume-unrelated heterogeneity is widespread throughout the upper mantle. The chemical characteristics of RE2 are remarkably pronounced in basalt from the central portion of ridge segment 16 around 18°S, suggesting substantial magma production. The influence of RE2 decreases along with decreasing magma production to the north, and is only slightly identifiable in basalt from the northern part of segment 18. Although the influence of RE2 decreases somewhat to the south, basalts with extreme RE2 signature were produced in the center of segment 15 around 19°S, where magma production is high. In contrast to RE2, the geochemical signature of RD in basalt is geographically limited to two localities: the south end of segment 18 and the center of segment 15. However, these observations reveal that both RE2 and RD contribute strongly to magma production on segment 15. Results show that melting of ancient recycled plate materials with a low melting point regulates voluminous magma production along the CIR.
Abstract The opening of the Japan Sea separated southwest Japan from the Eurasian continent during the Early to Middle Miocene. Since then, diverse igneous activities have occurred in relation to the subduction of the Philippine Sea Plate beneath southwest Japan. The Okinawa Trough formed in the back‐arc region of the Ryukyu Arc since the Late Miocene. In the Koshikijima Islands, off the west coast of Kyushu and near the northern end of the Okinawa Trough, felsic to intermediate igneous rocks with Middle to Late Miocene radiometric ages occur as granitic intrusions and dikes. We obtained zircon U–Pb ages and whole‐rock major‐ and trace‐element compositions of Koshikijima granitic rocks to elucidate their magmagenesis. The U–Pb ages of granitic rocks in Kamikoshikijima and Shimokoshikijima and a dacite dike are about 10 Ma, suggesting that most magmatism on the Koshikijima Islands was coeval with early rifting in the Okinawa Trough. We infer that magmagenesis occurred via melting of lower crustal mafic rocks related to rifting in the Okinawa Trough based on the arc‐like trace‐element compositions of these I‐type granites. Andesitic dikes preceded felsic igneous activity on the Koshikijima Islands, and their ages and petrochemistry will help elucidate the magmatism and tectonics in this area throughout the Miocene.
Element partitioning between wadsleyite, garnet, and hydrous ultramafic melt has been determined at 16 GPa and 1600°C using a Kawai‐type multi‐anvil high‐pressure apparatus. Element partitioning between olivine and hydrous ultramafic melt was also studied at 5 GPa and 1500°C in order to compare element partitioning behavior between wadsleyite and olivine. Major and trace elements analyses of the quenched melt and coexisting wadsleyite, garnet, and olivine were made with an EPMA and LA‐ICP‐MS, respectively. The experimental results demonstrate that LIL (Rb, Sr, Ba) and HFS (Ti, Zr, Nb, Hf, Ta, Th, U) elements are preferably incorporated into wadsleyite relative to olivine, suggesting that the concentration of these elements in the hydrous transition zone could be greater than that in upper mantle. Our results contribute to understand magmatism and chemical differentiation in the Earth's interior.
