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961.
本文对北大别西部的湖北英山万家老屋、罗田七道河尹家店和安徽金寨丁埠乡李集3个片麻岩样品中锆石进行了BSE显微结构分析,并在此基础上对锆石进行了离子探针U-Pb年龄测定。结果表明北大别存在两种类型的片麻岩,它们的原岩分别形成于晋宁期和燕山期,其中英山万家老屋片麻岩的形成年龄为794Ma,后期地质事件对其锆石U-Pb年龄造成的影响较小;七道河尹家店片麻岩的形成年龄为730Ma,并可能受到燕山期岩浆事件的影响,得到了140Ma左右的变质年龄;丁埠乡李集片麻岩的形成年龄为140Ma。结合已有的年代学结果,北大别地体广泛存在晋宁期的岩浆事件,表明其可能为扬子板块的北缘。 相似文献
962.
研究了鸡西,龙爪沟两群生物组合序列,重新划分了两群(组)的时代,并对两群对比关系提出了新认识。 相似文献
963.
964.
U-Pb isotopic analyses of eight single and multi-grain zircon fractions separated from a syenite of the Diana Complex of the
Adirondack Mountains do not define a single linear array, but a scatter along a chord that intersects the Concordia curve
at 1145 ± 29 and 285 ± 204 Ma. For the most concordant analyses, the207Pb/206Pb ages range between 1115 and 1150 Ma. Detailed petrographic studies revealed that most grains contained at least two phases
of zircon growth, either primary magmatic cores enclosed by variable thickness of metamorphic overgrowths or magmatic portions
enclosing presumably older xenocrystic zircon cores. The magmatic portions are characterized by typical dipyramidal prismatic
zoning and numerous black inclusions that make them quite distinct from adjacent overgrowths or cores when observed in polarizing
light microscopy and in backscattered electron micrographs. Careful handpicking and analysis of the “best” magmatic grains,
devoid of visible overgrowth of core material, produced two nearly concordant points that along with two of the multi-grain
analyses yielded an upper-intercept age of 1118 ± 2.8 Ma and a lowerintercept age of 251 ± 13 Ma. The older age is interpreted
as the crystallization age of the syenite and the younger one is consistent with late stage uplift of the Appalachian region.
The 1118 Ma age for the Diana Complex, some 35 Ma younger than previously believed, is now approximately synchronous with
the main Adirondack anorthosite intrusion, implying a cogenetic relationship among the various meta-igneous rocks of the Adirondacks.
The retention of a high-temperature contact metamorphic aureole around Diana convincingly places the timing of Adirondack
regional metamorphism as early as 1118 Ma. This result also implies that the sources of anomalous hightemperature during granulite
metamorphism are the syn-metamorphic intrusions, such as the Diana Complex. 相似文献
965.
Radiocarbon dating of dissolved inorganic carbon in groundwater from confined parts of the Upper Floridan aquifer, Florida, USA 总被引:3,自引:1,他引:3
Geochemical reaction models were evaluated to improve radiocarbon dating of dissolved inorganic carbon (DIC) in groundwater
from confined parts of the Upper Floridan aquifer in central and northeastern Florida, USA. The predominant geochemical reactions
affecting the 14C activity of DIC include (1) dissolution of dolomite and anhydrite with calcite precipitation (dedolomitization), (2) sulfate
reduction accompanying microbial degradation of organic carbon, (3) recrystallization of calcite (isotopic exchange), and
(4) mixing of fresh water with as much as 7% saline water in some coastal areas. The calculated cumulative net mineral transfers
are negligibly small in upgradient parts of the aquifer and increase significantly in downgradient parts of the aquifer, reflecting,
at least in part, upward leakage from the Lower Floridan aquifer and circulation that contacted middle confining units in
the Floridan aquifer system. The adjusted radiocarbon ages are independent of flow path and represent travel times of water
from the recharge area to the sample point in the aquifer. Downgradient from Polk City (adjusted age 1.7 ka) and Keystone
Heights (adjusted age 0.4 ka), 14 of the 22 waters have adjusted 14C ages of 20–30 ka, indicating that most of the fresh-water resource in the Upper Floridan aquifer today was recharged during
the last glacial period. All of the paleowaters are enriched in 18O and 2H relative to modern infiltration, with maximum enrichment in δ18O of approximately 2.0‰.
Electronic Publication 相似文献
966.
碎屑颗粒磷灰石裂变径迹法在制约地层年龄中的应用 总被引:5,自引:0,他引:5
碎屑颗粒热年代学是 2 0世纪 80年代以来发展起来的一种热年代学方法 ,在地质研究中应用广泛。本文以西秦岭地区三岔—四店一带的白垩纪地层为例 ,说明该方法在制约地层年龄中的应用。结果表明这一带的白垩系地层沉积于晚白垩纪 ,或至少含有晚白垩纪的沉积。 相似文献
967.
Takamoto Okudaira Yasutaka Hayasaka Osamu Himeno Koichiro Watanabe Yasuhiro Sakurai Yukiko Ohtomo 《Island Arc》2001,10(2):98-115
Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole-rock–mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis. 相似文献
968.
969.
970.
华北地台前寒武纪变质基底的Sm-Nd同位素地质信息 总被引:6,自引:0,他引:6
华北前寒武纪变质基底的Sm-Nd同位素资料,主要来自冀东、辽东、内蒙古中部,五台-太行、河南和鲁西等地早前寒武纪以基性岩为主的表壳岩和深成酸性侵入体,也有少量副变质岩的分析结果。已搜集的华北地台前寒武纪变质基底的304个Sm-Nd分析资料表明,除同位素年龄信息外,作为壳-幔演化示踪剂,给出了岩石圈早期演化的重要信息:除已由U-pb法证实的冀东曹庄、辽东鞍山有≥3.8Ga的地壳残留外,Sm-Nd资料还暗示吕粱和登封一带存在>3.5Ga的古老地壳。在华北地台上,除>3.5Ga的早期壳-幔分离事件外,3.2~3.6Ga,2.6~3.0Ga和2.0~2.2Ga分别为三次造壳的高峰时间。来自华北不同地区的Sm-Nd资料还表明,在3.0Ga以前即存在一个规模较大的古老地壳,加之晚太古的造壳规模,太古宙地壳规模之宏大已与现代地壳相近似。除壳-幔分离的造壳事件外,华北地台早前寒武纪地壳活化再造十分强烈。活化再造形成的酸性侵入体的Sm-Nd同位素资料显示T_(DM)=2.6~2.8Ga是活化再造的最高峰时期。 相似文献