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Search Results: 1 - 10 of 2508 matches for " 40Ar-39Ar ages "
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Ages and geochemistry of Mesozoic-Eocene back-arc volcanic rocks in the Aysén region of the Patagonian Andes, Chile
Miguel A. Parada,Alfredo Lahsen,Carlos Palacios
Revista geológica de Chile , 2001,
Abstract: Eighteen new radiometric ages (fourteen 40Ar-39Ar, four K-Ar), combined with previously published ages, confirm the existence of three main extensional back-arc volcanic events, previously defined by stratigraphic relationships, in Chilean Patagonia (Aysén region). These three events developed during the Middle Jurassic -Early Cretaceous (160-130 Ma). Cretaceous (114-75 Ma), and Eocene (55-46 Ma). Based on distinct geochemical data and Sr-Nd isotopic characteristics of the back-arc volcanic rocks collected north and south of 46°30'S, two Mesozoic-Eocene magmatic domains are recognized: Northern Magmatic Domain (NMD) and Southern Magmatic Domain (SMD). Most analyzed basalts and intermediate volcanic rocks of the NMD have alkaline affinities and depleted to slightly depleted Sr-Nd isotopic values similar to those derived from an asthenosphere-dominated source. The SMD mafic volcanic rocks have a subalkaline character and more enriched Sr-Nd isotopic signatures, comparable to those derived from a lithospheric source. The felsic volcanic rocks of the SMD have lower eNd values and slightly higher initial 87Sr/86Sr ratios than the NMD felsic rocks, suggesting a larger crustal contribution in the magma sources. The geochemical and isotopic distinction between NMD and SMD felsic rocks could be influenced by the presence of Paleozoic metamorphic rocks as basement of the volcanic rocks of the SMD. Moreover, the compositional distinction between basalts of both domains may correspond to differences in magnitude of extension, the NMD being the one where the extension would have been greater and, consequently, the lithosphere thinner Edades y geoquímica de las rocas volcánicas del trasarco del Mesozoico-Eoceno en la región de Aysén de los Andes patagónicos, Chile. Diez y ocho nuevas edades radiométricas (catorce 40Ar-39Ar, cuatro K-Ar) junto con las ya publicadas confirman la existencia de tres eventos volcánicos (previamente definidos por relaciones estratigráficas) en la Patagonia chilena (Región de Aysén) durante el intervalo Mesozoico-Eoceno: Jurásico Medio-Cretácico temprano (160-130 Ma), Cretácico (114-75 Ma) y Eoceno (55-46 Ma). Sobre la base de las características geoquímicas e isotópicas de Sr y Nd de las rocas volcánicas estudiadas, se pueden reconocer dos dominios magmáticos Mesozoico-Eoceno: Dominio Magmático Norte (DMN) y Dominio Magmático Sur (DMS). Los basaltos y rocas intermedias del DMN tienen afinidades alcalinas y valores isotópicos de Sr-Nd deprimidos a moderadamente deprimidos similares a aquellos derivados de una fuente dominada por material as
Ages and geochemistry of Mesozoic-Eocene back-arc volcanic rocks in the Aysén region of the Patagonian Andes, Chile
Parada,Miguel A.; Lahsen,Alfredo; Palacios,Carlos;
Revista geológica de Chile , 2001, DOI: 10.4067/S0716-02082001000100002
Abstract: eighteen new radiometric ages (fourteen 40ar-39ar, four k-ar), combined with previously published ages, confirm the existence of three main extensional back-arc volcanic events, previously defined by stratigraphic relationships, in chilean patagonia (aysén region). these three events developed during the middle jurassic -early cretaceous (160-130 ma). cretaceous (114-75 ma), and eocene (55-46 ma). based on distinct geochemical data and sr-nd isotopic characteristics of the back-arc volcanic rocks collected north and south of 46°30's, two mesozoic-eocene magmatic domains are recognized: northern magmatic domain (nmd) and southern magmatic domain (smd). most analyzed basalts and intermediate volcanic rocks of the nmd have alkaline affinities and depleted to slightly depleted sr-nd isotopic values similar to those derived from an asthenosphere-dominated source. the smd mafic volcanic rocks have a subalkaline character and more enriched sr-nd isotopic signatures, comparable to those derived from a lithospheric source. the felsic volcanic rocks of the smd have lower end values and slightly higher initial 87sr/86sr ratios than the nmd felsic rocks, suggesting a larger crustal contribution in the magma sources. the geochemical and isotopic distinction between nmd and smd felsic rocks could be influenced by the presence of paleozoic metamorphic rocks as basement of the volcanic rocks of the smd. moreover, the compositional distinction between basalts of both domains may correspond to differences in magnitude of extension, the nmd being the one where the extension would have been greater and, consequently, the lithosphere thinner
40Ar-39Ar geochronology of Cenozoic Linzizong volcanic rocks from Linzhou Basin, Tibet, China, and their geological implications
Su Zhou,Xuanxue Mo,Guochen Dong,Zhidan Zhao,Ruizhao Qiu,Tieying Guo,Liangliang Wang
Chinese Science Bulletin , 2004, DOI: 10.1007/BF03184291
Abstract: Whole-rock and mineral separate Ar-Ar dating was carried out for the Linzizong volcanic rocks at Linzhou Basin in Tibet to constrain the time span of volcanism and the corresponding stratigraphic sequence. Sampling was based on detailed geologic mapping and stratigraphic sequence of Dianzhong, Nianbo, Pana Formations, systematically from the bottom to near the top. The results indicate that the Linzizong volcanic rocks erupted from Paleocene to middle of Eocene (64.43· 43.93 Ma). Among them, the Pana Formation formed from ca. 48.73 to 43.9 Ma, the Nianbo Formation around 54 Ma and the Dianzhong Formation from 64.4 to 60.6 Ma. In combination with evidence from the geochemical characteristics of the volcanic rocks, and from stratigraphy in southern Tibet, it is postulated that the age of the lowest member in the Dianzhong Formation of the Linzizong volcanic rock, which overlies unconformably the Late Cretaceous Shexing Formation, likely corresponds to the inception of the collision between Indian and Asian continents in southern Tibet.
Northernmost paleo-tethyan oceanic basin in Tibet: Geochronological evidence from40Ar/39Ar age dating of Dur’ngoi ophiolite
Liang Chen,Yong Sun,Xianzhi Pei,Ming Gao,Tao Feng,Zhongqing Zhang,Wen Chen
Chinese Science Bulletin , 2001, DOI: 10.1007/BF02900603
Abstract: Whole rock40Ar/39Ar age dating has been conducted on a basalt sample from Dur’ngoi ophiolite, Qinghai Province, which was reported to be the northernmost paleo-tethyan oceanic basin in Tibet. A high temperature plateau age (345.3±7.9 Ma) with an isochorn age (336.6±7.1 Ma) has been obtained, representing the eruption time of oceanic crust. Considering related geological settings, the new age provides constraints on the northernmost paleo-tethyan suture zone in Tibet and the tectonic evolution of Paleo-tethys in Northeast Tibet and adjacent areas.
40Ar-39Ar thermochronology of two ductile shear zones from Yiwulüshan, West Liaoning Region: Age constraints on the Mesozoic tectonic events
Xiaohui Zhang,Tiesheng Li,Zhiping Pu
Chinese Science Bulletin , 2002, DOI: 10.1360/02tb9250
Abstract: Two ductile shear zones trending EW and NNE respectively not only controlled the tectonic framework of the northern North China, but also constrained the geodynamic background for gold mineralization in this region. Field observations and microstructural analyses reveal that the EW trending ductile shear zones are mainly contributed to dextral compressional deformation resulting from top-to-the-southeast oblique thrust shearing, whereas the NNE trending ones are genetically related to sinistral strike-slip and extensional faulting. One sample from the former yielded an 40Ar–39Ar plateau age of (219± 4) Ma (Bi) and two samples from the latter gave 40Ar–39Ar plateau ages of (116± 2) Ma (Bi) and (127±3) Ma (Bi). These ages provide constraints on the top-to-the-southeast oblique thrusting event occurring in Late Triassic and the sinistral extensional and strike-slip faulting event which occurred in Early Cretaceous.
Mantle xenoliths from Late Cretaceous basalt in eastern Shandong Province: New constraint on the timing of lithospheric thinning in eastern China
Jun Yan,Jiangfeng Chen,Zhi Xie,Taixi Zhou
Chinese Science Bulletin , 2003, DOI: 10.1360/03wd0066
Abstract: The age of the alkali basalt from Daxizhuang in Jiaozhou, eastern Shandong, was determined to be 73.5±0.3 Ma by 40Ar-39Ar technique. The basalt gave high εNd(t) values of +7.5 and +7.6, suggesting that the primitive magma was derived from depleted asthenospheric mantle with a formation depth of about 65–95 km. Spinel-lherzolite xenoliths have been discovered in the basalt. The Fo values of olivine from the xenoliths range from 88 to 89. The estimated equilibrium P-T conditions of spinel-lherzolite xenoliths are about 2.0 GPa and 1010–1140°C, suggesting an equilibrium depth of about 65 km. Geochemical characteristics of the Late Mesozoic (125-115 Ma) mantle-derived rocks in Shandong Province suggest an enriched lithospheric mantle along the southern margin of the North China block. However, geochemical characteristics of the Late Cretaceous basalts in Daxizhuang and the equilibrium P-T condition estimated from the xenoliths suggest that the lithosphere at 73 Ma ago was as thick as that of the Cenozoic in the region. Additionally, the xenoliths are rich in basaltic-component, suggesting a derivation from a newly-formed lithosphere. Therefor, lithospheric thinning took place at a time interval between about 120 and 73 Ma along the southern margin of the North China block.
冀西北后沟金矿床交代成因钾长石激光探针40Ar-39Ar年龄*
徐兴旺,蔡新平,刘玉林,张宝林
中国科学 地球科学 中国科学 地球科学 , 2001,
Abstract: ?冀西北后沟金矿床是我国典型的钾化蚀变岩型金矿床,金矿床的形成与叠置于碱性侵入岩之上的钾交代作用有关.采用激光探针40Ar-39Ar微区定年法,测定交代成因钾长石的形成年龄介于176.7Ma至202.6Ma之间,相应的钾交代作用和伴生的金矿成矿作用发生在燕山早期;钾交代的脉动周期约为4Ma左右.
辽西医巫闾山两条韧性剪切带的40Ar-39Ar年龄:中生代构造热事件的年代学约束
张晓晖,李铁胜,蒲志平
科学通报 , 2002,
Abstract: 贯穿辽西北部和东部、分别呈近东西向和北北东向的韧性剪切带控制着华北北部的构造格局,又制约着该区金矿床形成的主要动力学背景.野外构造解析和显微构造分析表明,前者主要具右行剪切特点,反映由北西向南东的斜冲推覆;后者主要具左行剪切特点,体现伸展兼走滑的运动特征.采自东西向剪切带的1个样品给出了(219±4)Ma(Bi)的坪年龄;采自北北东向剪切带的两个样品分别给出了(116±2)Ma(Bi)和(127±3)Ma(Bi)的坪年龄.这些年龄为发生在晚三叠世的推覆事件和发生在早白垩世的伸展走滑事件提供了时代证据.
东川式层状铜矿40Ar-39Ar成矿年龄研究:华南地区晋宁-澄江期成矿作用新证据
邱华宁,J.,R.,Wijbrans,李献华,朱炳泉,朱崇林,曾保成
矿床地质 , 2002,
Abstract: 本文采用40Ar-39Ar真空击碎技术和阶段加热技术,测定了采自落雪矿的“东川式”层状铜矿之两个石英样品的年龄,获得了810~770Ma的40Ar-39Ar等时线年龄,这是首次获得的“东川式”铜矿的成矿年龄。这些年龄与作者以前获得的脉状铜矿石英的年龄范围一致,远远小于赋矿围岩落雪组的年龄。研究结果表明“东川式”层状铜矿并非沉积成因,与脉状铜矿一样,也是热液成因,并且再次证实晋宁-澄江期是东川铜矿的主要成矿期。
广东高凤金矿形成时代的Rb-Sr、40Ar-39Ar年龄测定
李献华,桂训唐,程景平,阴国渠
矿床地质 , 1992,
Abstract: 用含金石英脉流体包裹体Rb-Sr和40Ar-39Ar方法测定了广东高凤金矿的形成时代为印支期(215×106a)。两种方法的结果在误差范围内完全一致,两种测定方法相互验证,结果准确可靠。研究表明含金石英脉流体包裹体Rb-Sr和40Ar-39Ar年代方法对于解决石英脉型金矿的成矿时代具有很好的应用前景。
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