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The southern Lhasa Terrane is famous for its huge magmatic belt which records the magmatism during Mesozoic and Cenozoic. Although the Mesozoic continental-margin setting in the southern Lhasa Terrane has been identified,details of this tectonic setting and the evolution history during the Late Cretaceous remain unclear. To further constrain these issues,we present zircon LA-ICP-MS U-Pb,Hf isotopic and geochemical data of the Gongbari dacites(of the Sangri Group) which intruded by Paleocene granodiorites from the eastern part of the southern Lhasa Terrane,Tibet. New age data indicate that the dacites were generated at ~95.4 Ma,which suggests the Sangri Group volcanism may last to Late Cretaceous. The Gongbari dacites are characterized by high Sr(428–758 ppm) contents,low concentration of heavy rare earth elements and Y(e.g. Yb=0.78–1.14 ppm; Y=8.85–11.4 ppm) with high Sr/Y(41.91–67.59) and La/Yb(22.64–30.64) ratios,similar to those of adakite. The rocks are calc-alkaline,metaluminous,enriched in LILEs,depleted in HFSEs,and have positive εHf(t) values(+7.7 to +11.6). The Gongbari dacites were probably produced by partial melting of young and hot subducted Neo-Tethyan oceanic crust under amphibolite to garnet amphibolite-facies conditions. Though the Gangdese Mountains may have formed before Indo-Asian collision,the southern margin of Lhasa Terrane might not go through obviously crustal thickening during the northward subduction of NeoTethyan oceanic lithosphere.  相似文献
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Listwanite from the Luobusa ophiolite,Tibet,forms a narrow,discontinuous band along the eastern part of the southern boundary fault. We undertook a detailed petrographic and geochemical study to understand the mineral transformation processes and the behaviour of major and trace elements during listwanite formation. Three alteration zones characterized by distinct mineral components and texture are recognized and,in order of increasing degree of alteration,these are: zoneIII is rich in serpentine minerals; zoneII is rich in talc and carbonates; and zoneI is mainly composed of carbonates and quartz. Geochemical data for the three alteration zones show significant modification of some major and trace elements in the protolith,although some oxides show linear correlations with Mg O. Gold mineralization is recognized in the Luobusa listwanite and may signify an important target for future mineral exploration. Gold enrichment occurs in both zoneI and zoneIIand is up to 0.91 g/t in one sample from zoneI. We show that CO2-rich hydrothermal fluids can modify both the occurrence and composition of chromite grains,indicating some degree of chromite mobility. Low-Cr anhedral grains are more easily altered than high-Cr varieties. The compositions of chromite and olivine grains in the listwanite suggest a dunite protolith.  相似文献
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熊发挥  杨经绥  高健  来盛民  陈艳虹  张岚 《岩石学报》2016,32(12):3635-3648
泽当蛇绿岩位于雅鲁藏布江缝合带东段,岩体由地幔橄榄岩、辉长辉绿岩、辉石岩、火山岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和二辉橄榄岩。在方辉橄榄岩中发现7处豆荚状铬铁矿,矿石类型主要有致密块状和浸染状。出露地表的长度0.5~3m,厚0.2~1m。矿体的延伸方向为北西向,与岩体展布的方向一致,铬铁矿的Cr#=67.9~88.5,属于高铬型铬铁矿。泽当地幔橄榄岩岩相学特征以及矿物组合、矿物化学成份及岩相学特征,显示岩体至少存在两次的部分熔融过程,即为早期的MOR构造背景,以及后期SSZ环境的改造。铬铁矿的铂族元素(PGE)以富集Os、Rh、Pd,亏损Ir、Ru、Pt的负斜率分布模式,表明其形成过程中经受后期熔体/流体的改造。对比罗布莎岩体的矿物组合,矿物化学和地球化学等特征,显示泽当豆荚状铬铁矿矿体与典型高铬型具相似性,存在较大的找矿空间。  相似文献
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