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41.
Qiuli Li Shuguang Li Yong-Fei Zheng Huimin Li Hans Joachim Massonne Qingchen Wang 《Chemical Geology》2003,200(3-4):255-265
This paper first reports a high precision U–Pb age of 218±1.2 Ma for rutile in coesite-bearing eclogite from Jinheqiao in the Dabie Mounteins, east–central China. This work shows that the U–Pb mineral (rutile+omphacite) isochron age of 218±2.5 Ma and conventional rutile U–Pb concordia age of 218±1.2 Ma obtained by common Pb correction based on the Pb isotopic composition of omphacite in the same eclogite sample are consistent, proving that the omphacite with low U/Pb ratio (μ=2.8) can be used for common Pb correction in U–Pb dating of rutile. Oxygen isotope analysis of rutile aliquots gave the consistent δ18O values of −6.1±0.1%, demonstrating oxygen isotope homogenization in the rutile of different grains as inclusion in garnet and grain in matrix. Oxygen isotope thermometry yields temperatures of 695±35 and 460±15 °C for quartz–garnet and quartz–rutile pairs, respectively. These oxygen isotopic observations suggest that the diffusion of oxygen in rutile as inclusion in garnet is not controlled by garnet. According to field-based thermochronological studies of rutile, an estimate of the Tc of about 460 °C for U–Pb system in rutile under rapid cooling conditions (20 °C/Ma) was advised. Based on this U–Pb age as well as the reported chronological data with their corresponding metamorphic and/or closure temperature, an improved T–t path has been constructed. The T–t path confirms that the UHPM rocks in South Dabie experienced a rapid cooling following the peak metamorphism before 220 Ma and a long isothermal stage from 213 to 180 Ma around 425 °C. 相似文献
42.
Given the difficulty of separating the three Picea species—P. glauca, P. mariana, and P. rubens (white, black, and red spruce)—in the pollen record, little is known about their unique histories in eastern North America following deglaciation. Here we report the first use of a classification tree analysis (CART) to distinguish pollen grains of these species. It was successfully applied to fossil pollen from eight sites in Maine and one in Massachusetts. We focused on the late glacial/early Holocene (14,000 to 8000 cal yr B.P.) and the late Holocene (1400 cal yr B.P. to present)—the two key periods since deglaciation when Picea has been abundant in the region. The result shows a shift from a Picea forest of P. glauca and P. mariana in the late glacial to a forest of P. rubens and P. mariana in the late Holocene. The small number of P. rubens grains identified from the late glacial/early Holocene samples (<5%) suggests that that species was either absent or rare at most of the sites. The occurrence and distribution of the three species do not reveal any geographic or temporal trend during late glacial time, but the data suggest that they were distributed in local patches on the landscape. The results of this study indicate that the recent population expansion of Picea (1000 to 500 cal yr B.P.) was likely the first time since deglaciation that P. rubens was abundant in the region. 相似文献
43.
台湾本岛南端的恒春半岛,出露隆起珊瑚礁石灰岩和现代珊瑚礁海岸。隆起珊瑚礁石灰岩在地貌上的表现是多阶的台地。最高的公园面可达海拔300m,依次向下,是海拔约100~160m的笼仔埔面,海拔70~80m的埔顶面以及海拔10~20m的垦丁面。在恒春半岛的西侧,珊瑚礁台地面已经倾斜,因此高度的表现不同。ESR及14C定年表明,抬升的更新世珊瑚礁台地形成于0.5Ma B.P.前后及0.13Ma B.P.两时期;海拔10~20m的全新世珊瑚礁台地约形成于6 000a B.P.的时期。这些证据显示了地质时期里全球变迁的蛛丝马迹。 相似文献
44.
45.
A 2-D time-dependent finite-difference numerical model is used to investigate the thermal character and evolution of a convecting layer which is cooling as it convects. Two basic cooling modes are considered: in the first, both upper and lower boundaries are cooled at the same rate, while maintaining the same temperature difference across the layer; in the second, the lower boundary temperature decreases with time while the upper boundary temperature is fixed at 0°C. The first cooling mode simulates the effects of internal heating while the second simulates planetary cooling as mantle convection extracts heat from, and thereby cools, the Earth's core. The mathematical analogue between the effects of cooling and internal heating is verified for finite-amplitude convection. It is found that after an initial transient period the central core of a steady but vigorous convection cell cools at a constant rate which is governed by the rate of cooling of the boundaries and the viscosity structure of the layer. For upper-mantle models the transient stage lasts for about 30 per cent of the age of the Earth, while for the whole mantle it lasts for longer than the age of the Earth. Consequently, in our models the bulk cooling of the mantle lags behind the cooling of the core-mantle boundary. Models with temperature-dependent viscosity are found to cool in the same manner as models with depth-dependent viscosity; the rate of cooling is controlled primarily by the horizontally averaged variation of viscosity with depth. If the Earth's mantle cools in a similar fashion, secular cooling of the planet may be insensitive to lateral variations of viscosity. 相似文献
46.
《The Professional geographer》1997,49(2):257-268
Committee on Protection and Management of Pacific Northwest Anadromous Salmonids Upstream: Salmon and Society in the Pacific Northwest Cook, Ian G; Doel, Marcus A. and Li, Rex (eds.) Fragmented Asia: Regional Integration and National Disintegration in Pacific Asia Crush, Jonathan Power of Development de Blij, H. J., and Muller, Peter O. Physical Geography of the Global Environment Hayward, David J. International Trade and Regional Economies: The Impacts of European Integration on the United States Kennedy, Michael The Global Positioning System and GIS: An Introduction Lowe, Philip; Marsden, Terry and Whatmore, Sarah (eds) Regulating Agriculture Marsden, Terry; Lowe, Philip and Whatmore, Sarah (eds) Labour and Locality: Uneven Development and the Rural Labour Process Marsden, Terry; Lowe, Philip and Whatmore, Sarah (eds) Rural Restructuring: Global Processes and Their Responses Whatmore, Sarah; Marsden, Terry and Lowe, Philip (eds) Gender and Rurality Whatmore, Sarah; Marsden, Terry and Lowe, Philip (eds) Rural Enterprise: Shifting Perspectives on Small-Scale Production Martopo, Sugeng and Mitchell, Bruce (eds) Bali: Balancing Environment, Economy and Culture Ould-Mey, Mohameden Global Restructuring and Peripheral States: The Carrot and the Stick in Mauritania Redclift, Michael and Benton, Ted (eds) Social Theory and the Global Environment Thomas–Slayter, Barbara and Rocheleau, Dianne Gender, Environment, and Development in Kenya: A Grassroots Perspective Wallach, Bret Losing Asia: Modernization and the Culture of Development Wennberg, John E. Dartmouth Atlas of Health Care Wilson, David and Huff, James O. (eds) Marginalized Places and Populations: A Structurationist Agenda 相似文献
47.
刘福来 《吉林大学学报(地球科学版)》1996,(3)
红赐堡—黄土窑一带早前寒武纪孔兹岩系经历一期麻粒岩相变质作用,并可划分四个变质阶段。各阶段石榴石的分布、形态、矿物组合、化学成分及其成因条件均存在明显差异。通过成因矿物学的系统研究,结合多种地质温压计的估算结果及变质反应实验资料,确定本区孔兹岩系石榴石的结晶生长演化史及变晶变形序列。 相似文献
48.
The Uplift of the Longmenshan Thrust Belt and Subsidence of the West Sichuan Foreland Basin 总被引:8,自引:0,他引:8
Liu Shugen Luo Zhili Dai Sulan Changlu Institute of Technology Chengdu Sichuan ChinaDennis Arne Dalhousie U niversity Canadaand C.J.L. Wilson Melbourne U niversity Australia Jiang M inxi 《《地质学报》英文版》1996,70(1):16-26
Based on fission track dating of apatite, and measurement of vitrinite reflectance of rock samples from the Longmenshan (Longmen Mountain)area and the West Sichuan foreland basin and computer modelling it is concluded that (l)the Songpan-Garze fold belt has uplifted at least by 3-4 km with an uplift rate of no less than 0.3-0.4 mm/a since 10 Ma B.P.; (2) the Longmenshan thrust nappe belt has uplifted at least by 5-6 km with an uplift rate of more than 0.5- 0.6 mm /a since 10 Ma B.P.; (3) the Longmenshan detachment belt has uplifted by 1 - 2 km at a rate of 0.016-0.032 mm/a since 60 Ma B.P.; (4) the West Sichuan foreland basin has uplifted by 1.7-3 km at a rate of 0.028-0.05 mm/a since 60 Ma B.P.; (5) the uplift rate of the area on the west side of the Beichuan-Yingxiu-Xiaoguanzi fault for the last 10 Ma is 40 times as much as that on its east side; (6) the uplifting of the the Songpan - Garze fold belt and the subsidence of the West Sichuan foreland basin 60 Ma ago exhibit a mirro-image correlation, i.e 相似文献
49.
In the Dharwar tectonic province, the Peninsular Gneiss was considered to mark an event separating the deposition of the older
supracrustal Sargur Group and the younger supracrustal Dharwar Supergroup. Compelling evidence for the evolution of the Peninsular
Gneiss, a polyphase migmatite, spanning over almost a billion years from 3500 Ma to 2500 Ma negates a stratigraphic status
for this complex, so that the decisive argument for separating the older and younger supracrustal groups loses its basis.
Correlatable sequence of superposed folding in all the supracrustal rocks, the Peninsular Gneiss and the banded granulites,
indicate that the gneiss ‘basement’ deformed in a ductile manner along with the cover rocks. An angular unconformity between
the Sargur Group and the Dharwar Super-group, suggested from some areas in recent years, has been shown to be untenable on
the basis of detailed studies, A number of small enclaves distributed throughout the gneissic terrane, with an earlier deformational,
metamorphic and migmatitic history, provide the only clue to the oldest component which has now been extensively reworked. 相似文献
50.
J. Schäfer H. Neuroth H. Ahrendt W. Dörr W. Franke 《International Journal of Earth Sciences》1997,86(3):599-611
The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370?Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Münchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380?Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Viséan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380?Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Viséan), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30?mm per year, and rates of exhumation of ca. 3?mm/a, and possibly more. 相似文献