Late Paleozoic to Early Mesozoic Evolution of Neo-Tethys: Geochemical Evidence from Early Triassic Mafic Intrusive Rocks in the Tethyan Himalaya
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Here we report geochronological, geochemical, and Sr-Nd-Hf isotopic data of a basic intrusion exposed in the Jiacha area within the Tethyan Himalaya, southern Tibet. The Jiacha dikes are composed mainly of mafic rocks (gabbros). In zircon U-Pb dating of Jiacha dikes samples, the weighted mean ages of magma emplacement were determined to be 241.5±4.2 to 245.5±3.3 Ma, showing a Middle Triassic magmatic event in the eastern part of the Tethyan Himalaya. The Jiacha dikes have relatively low MgO and total alkali (K2O+Na2O) contents but high TiO2 contents, exhibit weakly fractionated rare earth element (REE) patterns with slight depletions in light REEs and no obvious Eu anomalies, and show enrichment in high field strength elements and depletion in large-ion lithophile elements. Their initial 87Sr/86Sr ratios vary in the range 0.704250–0.704321, with a positive εNd(t = 243 Ma) of +3.07 to +3.67, and their initial 176Hf/177Hf ratios vary in the range 0.282872–0.283044, with a positive εHf(t = 243 Ma) of +8.53 to +14.45. We propose that the Jiacha dikes were derived from spinel lherzolites in the mantle with no crustal contamination and underwent fractional crystallization of clinopyroxene. Considering previous studies, we propose that Jiacha dikes represent oceanic crustal products of the early evolution of the Neo-Tethys, indicating that the ocean had already begun to open in the Middle Triassic.Keywords:
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Geochronology
Fractional crystallization (geology)
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About 7000 km river dikes and sea dikes have been built in the northem provinces of Vietnam. Annually, Vietnamese govemment and people have spent a lot of money for maintenance and protection of the dike system against flooding. Nevertheless, breaks of the dikes still happen, which have serious consequences. The instability of dikes results from defects, termite nests, seepage effects, or distorted dike foundation. The origin of these effects is usually hidden in the dike's body or foundation. It is not directly visible. Methods of dike monitoring are necessary to assess the stability and the state of the dike.
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Foundation (evidence)
Vietnamese
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预定和北方中国 Craton 的破坏的持续时间，它对理解控制因素的破坏机制和它的 geodynamic 枢轴，仍然保持争论。根据岩浆开始和进化的原则，首先，我们构画出能与 cratonic 破坏有关的 magmatic 表情，然后，抑制时间的使用 magmatic 和盆进化趋势石版印刷可伸缩在诺思中国的球的变瘦。主要结论包括：(1 ) 在北方中国 Craton 下面岩石圈变瘦可能开始了，至少局部地，后来迟了含碳迟了三叠纪，在晚侏罗记早的白垩纪期间达到了它的高潮，并且继续到结束为止近来白垩纪早的新生代。北方中国 Craton 的破坏是一相对慢，而非一个戏剧的过程。(2 ) 沿着 craton 的边缘和内部的球的地区玩了的削弱的石版印刷在 cratonic 破坏的一个重要角色，部分为 cratonic 的异构的模式的财务破坏。(3 ) 控制了北方中国 Craton 的破坏的构造因素可能是多重的。在诺思中国和华南之间的 Paleo 亚洲的板和晚三叠纪的碰撞的晚含碳的向南方的 subduction 可能由影响 craton 的热、不可分的结构重新激活 craton。在东方亚洲大陆下面的和平的 subduction 在 cratonic 破坏起了一个决定因素作用，管理中生代以后的盆和主要构造配置的分发模式， magmatism 的时间的变化和纵贯的严肃貌的形成。
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在北方中国 Craton 和 Yangtze 下面的 subcontinental lithospheric 披风的部件和进化 Craton 是在钻石的同位素的作文是的中国和碳的地质的学习的一个当前的话题进 cratonic lithospheric 披风的最直接的探针之一处理。在这份报纸，在原处 SIMS (第二等的离子集体 Spectrometry ) 技术被用来在 Yangtze Craton 在北方中国 Craton 和湖南从山东和辽宁在钻石的不同内部生长地区分析碳同位素作文。这被发现从北方中国 Craton 的钻石的同位素的范围是的碳从那些相当不同 Yangtze Craton；前者有 6.0/ 的一个范围到 2.0 (相对 VPDB ) 在他们的核心区域 3.0 与一般水准珍视，它与全球 peridotitic 钻石一致；然而，从 Yangtze Craton 的钻石有碳有在他们的核心区域的 7.4 的平均价值的从 8.6 ～ 3.0 的同位素的范围，与全球 eclogitic 钻石更一致。在在单个钻石的不同内部生长地区之间的碳同位素比率的变化在地区学习了的三颗钻石是不同的。在在碳的变化之间有清楚的关联钻石溶解和新生长的同位素的作文和阶段，当没有关联在 13C 和内部包括之间被观察时。变化建议披风液体的同位素的作文在钻石结晶化的过程期间正在改变的碳，并且在披风碳水库的同位素的作文是的碳的异质一个更重要的因素比在控制碳的碳同位素分别在钻石的同位素的作文和他们的变化。另外，初步的结果在原处，氮分析证明碳的变化在核心之间的同位素的作文和外部生长地区不与氮丰富相关，暗示钻石在开的环境结晶或同位素的作文和氮在披风液体满足的碳被其它控制，不是还理解因素。试验性的结果提供碳和它的原来的来源的同位素的作文在在北方中国 Craton 和 Yangtze Craton 下面在披风控制钻石形成的交代矿床液体是不同的提示。
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Composite acidic-basic dikes near Winnsboro, South Carolina, consist of cuspate pillow-like masses of basic rock surrounded by granite with no chilled margins or gradational boundaries. These dikes are similar to the net-veined complexes observed in other parts of the world. Most workers agree that these complexes represent coexisting acidic and basic liquids. Explanations of origin fall into two general classes: liquid immiscibility or commingling of miscible liquids. In the first, a homogeneous magma has unmixed to form two contrasting magmas. In the second, mixing was prevented because of high viscosities and rapid crystallization. The origin of these coexisting magmas in the second case has been variously explained by: coincidental intrusion of magma from different sources; melting of acidic rocks by the basic magma; or successive fractional melting of a common parent. These models are tested in the Winnsboro composite dike on the basis that the light REE's are partitioned in favor of the liquid in liquid-crystal equilibrium. In the Winnsboro dike, the light REE's are enriched in the basic rocks and relatively depleted in the granitic rocks. Thus, fractional crystallization of the basic rock or partial melting of the wall rock must be rejected.
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Fractional crystallization (geology)
Igneous differentiation
Magma chamber
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This article is an individual and necessarily brief overview of the history of geochronology in Queensland with emphasis on the contributions of scientists and technicians who played major roles in the implementation of the geochronology facilities at the University of Queensland. Geochronology and isotope geochemistry research summarised here have contributed among others to our understanding of the chemical and isotopic evolution of the early Earth, mantle dynamics, the thermal history of sedimentary basins and time-scales of weathering and climate change.
Geochronology
Geologic record
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