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91.
Stromatolites associated with cherty dolomites of the Vanivilaspura Formation of the Archaean Dharwar Supergroup show a morphology
indicative of the deposition of the latter in a intertidal to subtidal environment. The cherts are moderately high in their
Al/Al + Fe ratios but depleted in Fe2C3 and also most trace elements. Unlike most other Archaean cherts, the Vanivilaspur cherts exhibit significant negative Ce
anomaly, which is interpreted to have resulted from contemporary manganese deposition. The Rb/Sr ratios in the cherts show
a sufficient spread to define a linear correlation line in the Rb-Sr evolution diagram corresponding to an age of 2512 ± 159
Ma and initial Sr ratio of 0.7128 ± 0.0012 (2σ). While this age is strikingly close to that of regional metamorphism in the
Dharwar craton, the initial ratio is distinctly higher than that of the associated volcanics. Acid leaching experiments on
the cherts suggest that they may have been isotopically equilibrated on a mm to cm scale about 500 Ma later than the time
of regional metamorphism. 相似文献
92.
Y. J. Bhaskar Rao K. Naha R. Srinivasan K. Gopalan 《Journal of Earth System Science》1991,100(4):399-412
The Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking
gneissosity parallel to the axial planes of a set of isoclinal folds (DhF1). These have been over printed by near-coaxial open folding (DhF12) and non-coaxial upright folding on almost N-S trend (DhF2). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well
as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically
with DhF1 deformation.
The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated
REE patterns (CeN/YbN < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7).
A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30
Ma with Sr
i
of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sr
i
= 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal
residence history of less than a 100 Ma. 相似文献
93.
Calculations of isochrons by different methods are compared for the case when both variables are subject to error. Least-squares operations of the kind given by York (1969) and linear methods are compared. The advantage of the linear methods is demonstrated for the case of data reported by Housner and Brennan (1948), which we have modified for nonreplicated sampling. The sampling distribution of the slope is obtained and confidence intervals for it are given. The confidence intervals for the intercept on the ordinate and its distribution are obtained for fixed values of the slope. Examples of application of the method are given, based on data from the literature compiled in Appendix A. 相似文献
94.
一个花岗岩带或复式岩基中各个岩体的~(87)Sr/~(86)Sr和~(87)Rb/~(86)Sr的算术平均值,在等时线图中可获得一条直线。这直线可认为是一种等时线或混合线。如果为混合线,则可认为花岗岩带或复式岩基是混合成因。通常包括因围岩或其它岩浆的加入而发生的岩浆混染作用和成岩前的源岩混合作用;若为等时线,则可据直线的斜率计算出年龄,标示花岗岩的源岩Sr同位素均一化作用的最终时间或岩浆房中岩浆sr同位素均一化的最终时间。 相似文献
95.
胶东是我国最重要的金矿集中区之一,其成矿背景和成矿作用过程在全球金矿中独具特色(Goldfarb and Santosh,2014)。著名的玲珑金矿田就位于胶东西北部“招莱金矿带”东端,包括玲珑、九曲、大开头、东风和东山等矿段或矿区。本文研究的东风金矿床位于玲珑矿田东南部,该区自20世纪80年代末期发现致矿异常,圈定成矿预测区以来,科研工作者对此进行了有关研究,徐晓璐等(2013)等以东风矿床为例,运用Surpac软件对矿床数据库和矿体品位分布模型的建立进行了研究;冯涛等(2009)等对东风金矿171号脉的矿床地质特征进行了研究;对东风金矿区成矿规律,姜维明等(2002)等进行了初步探讨。但是,对该矿床的成矿时代等研究相对薄弱,本文选取东风矿床与金矿关系密切的黄铁绢英岩,对其进行Rb-Sr同位素分析,以期查明东风金矿床成矿时代等问题,为深入地了解和认识该矿床的成因和成矿作用提供新的依据。 相似文献
96.
97.
辽宁新岭铅锌银矿床位于华北克拉通东北缘,矿体赋存于古元古代辽河群变质岩中,受断裂构造控制。为了确定新岭矿床的成岩成矿时代、成矿流体和物质来源及其与中生代岩浆活动的关系,文章对新岭矿床开展了详细的矿床地质特征、年代学及同位素地球化学研究。结果表明,新岭矿床内与成矿有关花岗斑岩的LA-ICP-MS锆石U-Pb年龄为(227.3±1.5)Ma。5件闪锌矿样品的Rb-Sr等时线年龄为(222.9±2.9)Ma,初始Sr同位素比值为0.7104,确认新岭成岩成矿时代为晚三叠世。成矿流体的δ18OW值介于4.7‰~7.6‰,δDW值介于-97.7‰~-82.0‰,指示成矿流体主要为岩浆热液。黄铁矿3He/4He值介于0.15~0.21 Ra,进一步揭示成矿流体具有壳-幔混源特征。黄铁矿样品的LA-ICP-MS原位δ34S值介于+3.9‰~+7.2‰,均值为+5.5‰。6件闪锌矿样品的铅同位素组成为206Pb/204Pb=17... 相似文献
98.
Dipankar Pathak 《Geostandards and Geoanalytical Research》2023,47(1):185-197
Isotope dilution (ID) is a widely used analytical technique to determine elemental abundance to a high degree of accuracy and precision, using a spiked isotope tracer. This technique also enables efficient correction of the inevitable phenomenon of analytical mass fractionation during mass spectrometric analysis. It is also used to determine stable isotope variations of an element in a sample relative to a reference material, using tracers enriched in two isotopes (popularly known as the double spike method). Isotope dilution data reduction can be performed in Microsoft® Excel™ using different algorithmic approaches. Additional software such as iolite and MATLAB® offer algorithm implementations to perform these calculations. These are however limited to use within particular laboratories/research groups, or either require additional cost-bound software or some degree of knowledge in computer programming languages for use, or all of these. To ease this situation, a graphical user interface-based software is proposed (here named Parmanu-Gunak, meaning Atom-Calculator in Sanskrit text) to invert both single and double spiked isotope data, with the aim of making it a standard tool for ID data reduction. Examples of Nd and Mo isotopes are used to demonstrate the robustness of the program. 相似文献
99.
JANOUSEK VOJTECH; GERDES AXEL; VRANA STANISLAV; FINGER FRITZ; ERBAN VOJTECH; FRIEDL GERTRUDE; BRAITHWAITE COLIN J. R. 《Journal of Petrology》2006,47(4):705-744
The Liov Granulite Massif differs from neighbouring granulitebodies in the Moldanubian Zone of southern Bohemia (Czech Republic)in including a higher proportion of intermediatemaficand orthopyroxene-bearing rocks, associated with spinel peridotitesbut lacking eclogites. In addition to dominantly felsic garnetgranulites, other major rock types include quartz dioritic two-pyroxenegranulites, tonalitic granulites and charnockites. Minor bodiesof high-pressure layered gabbroic garnet granulites and spinelperidotites represent tectonically incorporated foreign elements.The protoliths of the maficintermediate granulites (quartz-dioriticand tonalitic) crystallized 360370 Ma ago, as indicatedby laser ablation inductively coupled plasma mass spectrometryUPb ages of abundant zircons with well-preserved magmaticzoning. Strongly metamorphically recrystallized zircons giveages of 330340 Ma, similar to those of other Moldanubiangranulites. For the overwhelming majority of the Liov granulitespeak metamorphic conditions probably did not exceed 800900°Cat 45 kbar; the equilibration temperature of the pyroxenegranulites was 670770°C. This is in sharp contrastto conditions of adjacent contemporaneous Moldanubian granulites,which are characterized by a distinct HPHT signature.The maficintermediate Liov granulites are thought tohave originated during Viséan metamorphic overprintingof metaluminous, medium-K calc-alkaline plutonic rocks thatformed the mid-crustal root of a Late Devonian magmatic arc.The protolith resembled contemporaneous calc-alkaline intrusionsin the European Variscan Belt. KEY WORDS: low-pressure granulites; geothermobarometry; laser-ablation ICP-MS zircon dating; whole-rock geochemistry; SrNd isotopes; Moldanubian Zone 相似文献
100.
西藏扎布耶湖晚更新世沉积物230 年代学研究 总被引:1,自引:0,他引:1
本文应用全溶样品的等时线模式,较成功地测定了藏北高原西南部扎布耶盐湖SZK01孔岩芯中含不等量碳酸盐粘土的230Th/238U年龄,建立了青藏高原腹地海拔4000 m以上湖泊120 ka以来连续的同位素年龄标尺,从而为扎布耶盐湖及其周缘地区古环境古气候演化研究奠定了时间坐标。根据230Th/238U年龄数据,SZK01孔的平均沉积速率约为68 cm/ka。然而不同层段的沉积速率差异较大,变化在20.8~128.8 cm/ka,其快慢变化反映了该湖从短命深湖、动荡浅湖、滨湖到盐湖的环境变化,与高分辨率SZK02孔的研究结果相一致。讨论了测年模式的适应性﹑样品的U封闭性和应用的优越性,提出开展湖区及周边各类型水体的U、Th同位素化学行为研究和检验沉积物中多源Th对定年的影响将是今后年代学研究的重要内容。 相似文献