Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin   总被引：23，自引：0，他引：23  

Ryuichi Shinjo  Yuzo Kato   《Lithos》2000,54(3-4):117-137

The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have ⁸⁷Sr/⁸⁶Sr (0.7040–0.7042) and ¹⁴³Nd/¹⁴⁴Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher ⁸⁷Sr/⁸⁶Sr (0.7044–0.7047) but similar ¹⁴³Nd/¹⁴⁴Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization (M_a/M_c) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current.  相似文献   

Rare Earth Elements as an Indicator to Origins of Skarn Deposits: Examples of the Kamioka Zn-Pb and Yoshiwara-Sannotake Cu(–Fe) Deposits in Japan     

Yasuhiro Kato 《Resource Geology》1999,49(4):183-198

Abstract: Systematic data of rare earth elements (REEs) are presented in order to put some constraints on the origin of hydrothermal fluids responsible for two contrastive skarn deposits in Japan; the Kamioka Zn-Pb and Yoshiwara-Sannotake Cu(-Fe) deposits. Carbon and oxygen isotopic studies have demonstrated that the hydrothermal fluids responsible for the Kamioka Zn-Pb deposits are of meteoric water origin whereas those for the Yoshiwara-Sannotake Cu(-Fe) deposits are of magmatic water origin. The REE abundances of epidote skarn derived from aluminous rocks, garnet and clinopyroxene in calcic exoskarn derived from limestone, and interstitial calcite associated with sulfide minerals were determined for these contrastive skarn deposits by inductively-coupled plasma mass spectrometry (ICP-MS). A significant difference in the REE concentrations is not found between epidote skarn and aluminous original rock (plagioclase-clinopyroxene rock, called Inishi rock) from the Kamioka Zn-Pb deposits, indicating that the REEs are generally immobile during the formation of epidote skarn, and that the REE concentrations of the hydrothermal fluid are considerably low relative to the aluminous original rock. In contrast, the epidote skarn exhibits enrichment of Eu with increasing total REE concentrations relative to the aluminous original rock (quartz diorite) in the Yoshiwara-Sannotake Cu(-Fe) deposits, implying a contribution of magmatic fluid derived from granitoids during the skarn formation. Limestone generally has much lower REE concentrations related to surrounding aluminous rocks, and thus the REE concentrations of garnet and clinopyroxene in calcic exoskarn, originated from limestone, are variable due to the interaction with the hydrothermal fluids. The chondrite-normalized REE patterns of garnet, clinopyroxene, and interstitial calcite exactly provide useful information on origins of hydrothermal fluids. The REE patterns of these minerals from the Kamioka Zn-Pb deposits show lower (Pr/Yb)cn ratios, and negative Ce and Eu anomalies inherited from limestone with the decrease of This suggests that the hydrothermal fluids responsible for the Kamioka Zn-Pb deposits were depleted in REEs, and were not magmatic water in origin, but presumably meteoric one. In striking contrast, the REE patterns of exoskarn minerals and calcite from the Yoshiwara-Sannotake Cu(-Fe) deposits exhibit a positive Eu anomaly, and high (Pr/Yb)cn ratios with the considerable increase of σREE and the disappearance of negative Ce anomaly, implying that the fluids were dominantly of magmatic origin. The REE indices are very likely to be an excellent indicator to origins of the skarn deposits.  相似文献   

The high-pressure and temperature equation of state of a majorite solid solution in the system of Mg4Si4O12-Mg3Al2Si3O12     

H. Morishima  E. Ohtani  T. Kato  T. Kubo  A. Suzuki  T. Kikegawa  O. Shimomura 《Physics and Chemistry of Minerals》1999,27(1):3-10

The high-pressure and temperature equation of state of majorite solid solution, Mj_0.8Py_0.2, was determined up to 23 GPa and 773 K with energy-dispersive synchrotron X-ray diffraction at high pressure and high temperature using the single- and double-stage configurations of the multianvil apparatuses, MAX80 and 90. The X-ray diffraction data of the majorite sample were analyzed using the WPPD (whole-powder-pattern decomposition) method to obtain the lattice parameters. A least-squares fitting using the third-order Birch-Murnaghan equation of state yields the isothermal bulk modulus, K _T0  = 156 GPa, its pressure derivative, K′ = 4.4(±0.3), and temperature derivative (∂K _T /∂T) _P = −1.9(±0.3)× 10⁻² GPa/K, assuming that the thermal expansion coefficient is similar to that of pyrope-almandine solid solution. Received: 5 October 1998 / Revised, accepted: 24 June 1999  相似文献   

Quantitative investigation on collapse margin of steel high-rise buildings subjected to extremely severe earthquakes     

Xuchuan Lin  Mikiko Kato  Lingxin Zhang  Masayoshi Nakashima 《地震工程与工程振动(英文版)》2018,17(3):445-457

Reponses of structures subjected to severe earthquakes sometimes significantly surpass what was considered in the design. It is important to investigate the failure mechanism and collapse margin of structures beyond design, especially for high-rise buildings. In this study, steel high-rise buildings using either square concrete-filled-tube (CFT) columns or steel tube columns are designed. A detailed three-dimensional (3D) structural model is developed to analyze the seismic behavior of a steel high-rise towards a complete collapse. The effectiveness is verified by both component tests and a full-scale shaking table test. The collapse margin, which is defined as the ratio of PGA between the collapse level to the design major earthquake level (Level 2), is quantified by a series of numerical simulations using incremental dynamic analyses (IDA). The baseline building using CFT columns collapsed with a weak first story mechanism and presented a collapse margin ranging from 10 to 20. The significant variation in the collapse margin was caused by the different characteristics of the input ground motions. The building using equivalent steel columns collapsed earlier due to the significant shortening of the locally buckled columns, exhibiting only 57% of the collapse margin of the baseline building. The influence of reducing the height of the first story was quite significant. The shortened first story not only enlarged the collapse margin by 20%, but also changed the collapse mode.  相似文献