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Your search keyword '"Hirata, Takafumi"' showing total 149 results
Start Over Author "Hirata, Takafumi" Publisher elsevier b.v.
149 results on '"Hirata, Takafumi"'

46. New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges, Weng’an and Chengjiang areas, South China

Author

Okada, Yoshihiro, Komiya, Tsuyoshi, HIRATA, TAKAFUMI, Takahata, Naoto, Sano, Yuji, Han, Jian, and MARUYAMA, SHIGENORI

Subjects
geography, Provenance, geography.geographical_feature_category, Rift, Geology, Unconformity, Paleontology, Craton, chemistry.chemical_compound, Volcano, chemistry, Isotopes of carbon, Carbonate, Zircon
Abstract

The Neoproterozoic and Cambrian were two of the most dramatic periods in the history of the Earth, because large multi-cellular animals first appeared then in the so-called “Cambrian Explosion”. To better understand this event, many paleontological and geochemical studies now focus on rocks in South China, because of the fossiliferous succession and good exposure. Since the recognition of the Yangtze Gorges and Chengjiang area as type localities of the Sinian (Ediacaran) and Meishucunian (Early Cambrian) Systems, both sections have been intensively investigated. In order to decipher the relationships between the evolution of life and surface environmental changes, it is necessary to understand their paleontological, geochemical and geo-chronological constraints. This study presents new chronological constraints for the Cryogenian to Cambrian rocks in the Three Gorges, Weng'an and Chengjiang areas, South China. We discovered two tuff layers, one at the base of the Shuijingtuo Fm at Three Gorges and the other at the bottom of the Dahai Member in Chengjiang. In addition, we collected sandstones from Neoproterozoic strata in the Three Gorges, Chengjiang and Weng'an areas for provenance analysis. Zircons, separated from the tuff layers, provide new Nano-SIMS U–Pb ages of 526.4 ± 5.4 Ma at the base of the Shuijingtuo Fm, and 523.9 ± 6.7 Ma at the bottom of the Dahai Member. The tuffaceous beds occur at an unconformity, and nodules are common in the Three Gorges, Meishucun and Taoying sections, indicating that major and relatively wide-scale volcanic and sedimentological events occurred at ca. 525 Ma. Moreover, carbonate carbon isotope chemostratigraphies at Morocco, Siberia, Three Gorges and Meishucun display different characteristics during the Tommotian. One possibility is that the South China Ocean was separated from an outer ocean at that time. Detrital zircons in sandstones have age populations at ca. 2.7, 1.8, 1.6, 1.0 and from 0.9 to 0.75 Ga. indicating that the paleo-hinterland of the Nanhua and Kangdian rift basins was geologically complex. Despite the lack of ca. 1.6 Ga rocks in the Yangtze and Cathaysia Blocks, these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the Yangtze craton.

Published
2014

50. Depletion and refertilization of the Tethyan oceanic upper mantle as revealed by the early Jurassic Refahiye ophiolite, NE Anatolia—Turkey.

Author

Uysal, Ibrahim, Ersoy, E. Yalçın, Dilek, Yildirim, Escayola, Monica, Sarıfakıoğlu, Ender, Saka, Samet, and Hirata, Takafumi

Abstract

We present new whole-rock major and trace element, Re–Os isotope and mineral chemistry data for the upper mantle peridotites and mafic rocks in the Rehafiye ophiolite in NE Anatolia (Turkey), and discuss their significance for the mantle evolution in Neotethys. The Refahiye peridotites include clinopyroxene (cpx)-rich and cpx-poor harzburgites. Assuming an initial fertile mantle composition, the cpx-rich harzburgites have a limited range of Cr-spl Cr# (33–38), implying 14–18% melt extraction. However, TiO 2 contents of the Cr-spl in these rocks are too high (up to 0.24 wt.%) to explain their Ti concentrations with a simple melt extraction model in a mid-ocean ridge (MOR) setting, and suggest re-crystallization from an impregnating fertile melt. The interstitial phases of cpx in these samples have high TiO 2 (up to 0.40 wt.%) and Na 2 O (up to 0.25 wt.%) contents. They also contain interstitial pargasitic amphibole with TiO 2 contents varying between 0.69 and 0.88 wt.%. These textures and the mineral chemistry data indicate refertilization of the previously depleted, MOR-type peridotites by percolating hydrous melts. The whole-rock REE partial melting modeling is consistent with slightly lower degrees of mantle melting (~ 12–14%) compared to the melt extraction degrees obtained from the Cr-spl compositions (~ 15%). Chromian spinel (Cr-spl) phases in the cpx-poor harzburgites show a wider variation of Cr#, ranging between 57 and 75, and reflect 30–40% of partial melting. In contrast, the whole-rock geochemistry of these rocks represents slightly lower degrees of partial melting, varying between 30 and 35%. The enrichment of TiO 2 contents of the Cr-spl (up to 0.20 wt.%) in some of the cpx-poor harzburgites can be explained by their interaction with Ti-rich hydrous melt formed at supra-subduction (SSZ) tectonic setting. Highly unradiogenic Os isotope composition (0.11956) of a cpx-poor harzburgite sample suggests an ancient melt depletion event experienced by these rocks, whereas its high Re content is a manifestation of subduction enrichment. The mafic units of the Refahiye ophiolite show MORB-like to island arc tholeiite (IAT) geochemical signatures typical of SSZ oceanic crust. A U–Pb zircon age of 183 ± 1 Ma obtained from an isotropic gabbro sample suggests that the crystallization of the SSZ-type mafic units in the Refahiye ophiolite is as old as the early Jurassic. [ABSTRACT FROM AUTHOR]

Published
2015
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