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Plagioclase compositions vary from An0.1–2.5 to An32 with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists, Franz Josef-Fox Glacier area, Southern Alps, New Zealand. This change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade. Oligoclase (An20-24) and albite (An0.1–0.5) are found in biotite zone schists below the garnet isograd. With increasing grade, the plagioclase compositions outline the peristerite gap, which is asymmetric and narrows to compositions of An12 and An6 near the top of the garnet zone. In any one sample, oligoclase is the stable mineral in mica-rich layers above the garnet isograd, whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz-plagioclase layers. The initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene. Carbonate is restricted to the quartz-plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers. The formation of more Ca-rich albite and more Na-rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap. Garnet appears to have only a localized effect on Ca-enrichment of plagioclase in mica-rich layers within the garnet zone. The Na-content of white mica increases sympathetically with increasing Ca-content of oligoclase and metamorphic grade. Comparison of the peristerite gap in the Franz Josef-Fox Glacier schists and schists of the same bulk composition in the Haast River area, 80 km to the S, indicates that oligoclase appears and epidote disappears at lower temperatures, and that the composition gap between coexisting albite and oligoclase is narrower in the Franz Josef-Fox Glacier area. It is suggested that a higher thermal gradient (38-40°C/km) and variations in Si/Al ordering during growth of the plagioclases between the two areas may account for these differences. In the Alpine schists the peristerite gap exists over a temperature and pressure interval of about 370-515°C and 5.5-7 kbar (550-700 MPa) PH2O. 相似文献
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HIRONOBU SAKUMA RYUJI TADA MASAYUKI IKEDA YUICHIRO KASHIYAMA NAOHIKO OHKOUCHI NANAKO O. OGAWA SATOKO WATANABE EIICHI TAJIKA SHINJI YAMAMOTO 《Island Arc》2012,21(2):79-100
After the severest mass extinction event in the Phanerozoic, biotic recovery from the extinction at the Permian–Triassic boundary required approximately 5 my, which covers the entire Early Triassic. It is important to obtain information on the superocean Panthalassa, which occupied most of the world ocean, to explore paleoenvironmental changes during the Early Triassic at the global scale. In order to establish the continuous lithostratigraphy of pelagic sediments in Panthalassa during the Early Triassic, high‐resolution reconstruction of the Lower Triassic pelagic sequence in Japan was conducted for the first time based on detailed field mapping and lithostratigraphic correlation in the Inuyama area, central Japan. The reconstructed Early Triassic sequence is approximately 9.5 m thick, consists of five rock types, and is divided into eight lithological units. For the reconstructed continuous sequence, measurement of carbon isotopic composition of sedimentary organic matter (δ13Corg) was carried out. Stratigraphic variation of the δ13Corg value shows large‐amplitude fluctuations between ?34.4 and ?21.0‰ throughout the sequence. In order to establish a higher resolution age model for the reconstructed Lower Triassic pelagic sequence, we correlated δ13Corg records in the Inuyama area with high‐resolution isotopic profiles of carbonate carbon (δ13Ccarb) from shallow‐marine carbonate sequences in southern China based on the similarity in general variation patterns with age constraints by radiolarian and conodont biostratigraphy. The result provides a high‐resolution time scale for the pelagic sequence of Panthalassa during the Smithian and Spathian. The age model suggests a drastic increase in sedimentation rate during the late Smithian, which should have been caused by the increase in terrigenous input to this site. 相似文献
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To estimate the amplification characteristics of ground motions in the heavily damaged belt zone in Kobe City during the 1995 Hyogo-ken Nanbu earthquake, 3D wave propagation analyses of a 2D deep irregular underground structure model with a vertical discontinuity were performed at an early stage as a preliminary and qualitative study. The hyperelement method was applied to the analyses for incident plane waves expected from the wavefields due to the source mechanism. The observation records at Kobe University of the rock site were used as control motions. The ground motions on the engineering bedrock (assumed to be on the free surface of the Osaka group layers having a shear velocity of 500 m/s) and at ground surface were calculated. The effects of the deep irregular underground structure and shallow surface layers on the ground motion amplification are discussed. Although there are qualifications due to the uncertain characteristics of the input rock motion and shear wave velocities of the underground structure, the analytical results show that the ground motion in the heavily damaged belt zone were amplified due to the focusing effect of the deep irregular underground structure as well as the shallow surface layers, and that the calculated peak ground acceleration (PGA) distribution coincided closely with the distributions of structural damage. © 1997 by John Wiley & Sons, Ltd. 相似文献