Ore geology,fluid inclusions and O-S stable isotope characteristics of Shurab Sb-polymetallic vein deposit,eastern Iran

The Shurab Sb-polymetallic mineralization is a subvolcanic rock-hosted epithermal deposit and located in north Lut Block, eastern Iran. It is one of the most important deposits of the Iranian East Magmatic Assemblage (IEMA) in which numerous Middle-Cenozoic precious and base metals deposits occur. The main lithological units in the area are Paleogene subvolcanic intrusions and minor Jurassic sedimentary rocks. Mineralization occurs as veins in a series of NW-SE and E-W trending faults and fractures in the Eocene-Oligocene dacite and andesite subvolcanic rocks. Mineralization at the Shurab deposit can be subdivided into four stages: pre-ore stage, Cu-Zn-Pb ore stage, Sb-Ag ± As ore stage and post-ore stage. The total sulfide content of the veins in the area is variable, ranging from 1 to 50%, and is dominated by stibnite, chalcopyrite, galena, Fe-poor sphalerite and pyrite with minor chalcostibite, Ag-tetrahedrite and bournonite; gangue minerals are mainly quartz and calcite. Silicic, argillic, propylitic, and sericitic, are the most obvious wall rock alterations. Microthermometric measurements of primary liquid-rich fluid inclusions in quartz and sphalerite indicate that the veins were formed at temperatures between 115 and 290 °C from fluids with salinities between 0.7 and 16.2 wt% NaCl eq., suggesting an epithermal origin. The δ³⁴S values of pyrite, chalcopyrite and galena vary between -2.5 and 0.8‰, and δ¹⁸O values of quartz range between 12.5 and 14.8‰. It is inferred that the Shurab mineralization is of epithermal origin, related to an Eocene-Oligocene magmatic geothermal system involving fluids of magmatic and meteoric origin.     

Petrogenesis and tectonic implications of Triassic A-type granites in southeastern China: insights from zircon U–Pb–Hf isotopic and whole-rock geochemical compositions of the Luoguyan and Guiyantou granites in northwestern Fujian Province

Triassic A-type granites in eastern South China Block (SCB) are abundant in the Wuyi–Yunkai tectonic domain and provide an important opportunity to explore the early Mesozoic evolution of continental crust of the SE part of the SCB. We carried out U–Pb zircon dating, Lu–Hf isotope analyses of zircon, and whole-rock geochemical analyses for two granitic plutons, the Guiyantou (GYT) and Luoguyan (LGY) granites, from northwestern Fujian Province. LA–ICP–MS U–Pb zircon analyses yielded ages of 232 ± 4 to 231 ± 7 Ma and 221 ± 5 Ma (Middle-Late Triassic) for the GYT and LGY granites. These two granites belong to metaluminous to weakly peraluminous high K calc-alkaline A-type granite that are enriched in K, Al, light rare earth element and Rb, Th, U, and Pb, and depleted in Nb, Ta, P, and Ti. Their rare earth element patterns are highly fractionated with (La/Yb)_N ratios of 2–21 and strong negative Eu anomalies (Eu/Eu* = 0.02–0.31). In situ Hf isotopic analysis of zircon from the GYT and LGY granites yielded ε_Hf(t) values ranging from –11.5 to –1.1, with corresponding two-stage Hf model ages from 1.98 to 1.33 Ga, from which it is inferred that the GYT and LGY magmas formed by partial melting of Proterozoic metasedimentary rock in the Cathaysia block. The two granites were emplaced at 232 and 221 Ma and together with Triassic A-type granites in coastal region of the SCB, which is in agreement with an extensional tectonic setting in the Middle-Late Triassic. We suggest that the Middle-Late Triassic A-type granites in eastern SCB were probably formed in an intracontinental, post-orogenic extensional regime that collision was between the SCB and an ‘unknown block’ or the eastern extension of Indochina block.     

Proterozoic VanDieland in Central Victoria: ages,compositions and source depths for late devonian silicic magmas

The Proterozoic to Cambrian VanDieland microcontinent was accreted to mainland Australia at ca 400?Ma, and its northern tip, the Selwyn Block, forms the basement in central Victoria. Here, mainly Late Devonian, silicic magmas were derived from the Selwyn Block and intruded into the shallow crust. We use the phase petrology of Late Devonian, S-type rhyolitic ignimbrites and a xenolith of pelitic migmatite, together with Nd-model ages for the silicic magmatic rocks to constrain the lithological characteristics of the metasedimentary component of the Selwyn Block, to infer minimum depths and temperature conditions here in the Late Devonian, and the likely ages of the source rocks for the S-type magmas. The most abundant source rocks are inferred to be volcaniclastic metagreywackes, with minor metadacites, meta-andesites and metapelites. The metapelitic xenolith cannot have been the source for any of the silicic magmas but constrains the upper amphibolite-facies part of the Selwyn Block to depths around 17?km, where temperatures reached ～775?°C. The older ignimbrite magma was formed by partial melting at perhaps 770?°C and a depth of at least 33?km, while the younger ignimbrite magma formed at ～23?km and 900?°C. These depths suggest source rocks in the Paleoproterozoic to Mesoproterozoic lower parts of the Selwyn Block. Nd-model ages of the silicic magmatic rocks confirm a dominance of Mesoproterozoic to Paleoproterozoic sources. If the inferred rock types in the Mesoproterozoic formations were as current correlations suggest, the sources for the Late Devonian silicic magmas would have to lie in the undocumented Paleoproterozoic basement of the Selwyn Block. Rock types here must include andesitic to dacitic volcanic components as well as volcaniclastic greywackes and minor pelites, which suggests a continental arc setting. The Late Devonian magmatism in the region may record the progression from amphibolite- to granulite-facies conditions during post-orogenic extension, with heat advected to the crust by mantle-derived mafic magmas. These processes would have resulted in mafitisation of the deep Selwyn Block.     

拉萨地体中部上白垩统达雄组的建立及构造隆升意义

拉萨地体中部措勤盆地内,呈东西向发育一套厚度巨大(~1800m)的晚白垩世陆相磨拉石沉积建造。通过对研究区内达雄村附近出露较好的虾格子剖面开展详细的测制工作,并将该套地层自底到顶分为5段,依次为中-粗粒砂岩层段、大套砾岩层段、中粒砂岩夹粉砂岩-泥岩层段、大套紫红色与绿色粉砂岩/泥页岩层段以及均质中粒砂岩层段。地层底部与下伏郎山组含圆笠虫灰岩呈沉积不整合关系,顶部被新生代沉积物覆盖。通过岩相组合与沉积结构构造分析研究,认为该套地层形成于冲积扇-辫状河沉积体系。通过地层顶部火山灰夹层和下伏郎山灰岩的圆笠虫化石显示其形成年龄约为98~91 Ma,这与获得的砂岩碎屑最年轻锆石年龄较为一致。砾岩砾石统计结果显示火山岩砾石为其主要成分。砂岩碎屑成分显示以大量火山岩岩屑为主。这些碎屑成分特征表明其主要来自岩浆弧的物源区。结合碎屑锆石U-Pb年代学和Hf同位素数据表明,中拉萨地体上广泛分布的则弄群火山岩可能为其主要的物源区。本文综合该套晚白垩世沉积的时空分布、岩性组合以及物源区特征,将其新命名为"达雄组",并将其与中拉萨地体北侧的竟柱山组对比表明,两者可能共同代表了中拉萨地体在晚白垩世早期(约98~91 Ma)所经历构造隆升,为青藏高原腹地在印度-亚洲大陆碰撞之前隆升提供了地质证据。     

扬子板块北缘早-中志留世硅质岩成因及古沉积环境的地球化学研究

本文对四川省若尔盖县北部-甘肃省迭部县中-下志留统[兰多维列统(Llandovery)-温洛克统(Wenlock)]富有机碳硅质岩进行了无机化学研究,目的是探讨其成因及形成环境,并建立研究区硅质岩形成及相关元素富集模式。主要对1个钻孔、2个剖面共28件硅质岩的岩相学、扫描电镜、主量、微量、稀土元素、有机碳及Si、O稳定同位素特征进行了研究。(1)岩相学、扫描电镜分析,Fe/Ti、Al/(Al+Fe+Mn)(0.08~0.79)均值0.31、(Fe+Mn)/Ti,(La/Yb)N(0.12~1.41)均值0.54、δEu(0.81~3.36)均值1.55、∑REE(7.59~253.48)均值82.90,Si同位素(-0.2‰~2.1‰)均值0.66‰、O同位素(13.5‰~21.8‰)均值19.53‰、同位素对成岩温度的反演,显示硅质岩为热水沉积成因,并伴有生物作用(Si、O稳定同位素显示),益哇硅质岩形成时陆源物质供给适量,导致与碳酸盐相关生物繁盛,少量样品受陆源物质影响显著。(2)岩相学、扫描电镜分析,MnO_2/TiO_2、Al2O3/(Al2O3+Fe2O3)、Al/(Al+Fe)(0.08~0.80)均值0.33、Th/U、V(28.6×10~(-6)~2261×10~(-6))均值454.42×10~(-6)、Ti/V,δCe(0.65~1.58)均值1.01、(La/Ce)N(0.60~1.314)均值0.97、有机碳(TOC)-微量元素二元图解,显示研究区硅质岩形成于大陆边缘的热水喷流导致海底局部水动力较强的缺氧环境[特别是δCe、(La/Ce)N、稀土配分模式],部分硅质岩由于富集热液元素导致其具有洋脊、大洋盆地硅质岩的特征。结合前人研究成果,认为研究区硅质岩形成于扬子板块北缘(古商丹洋南缘)热水喷流系统发育(导致海底水动能较强的)、缺氧状态的陆缘拉张裂陷环境,这种环境有利于U、V、Mo、Cu、Ni等变价元素的富集,扬子板块北缘早古生代(寒武纪-志留纪)黑色页岩系产自扩张的大陆边缘缺氧-还原环境。     

The role of mafic microgranular enclaves in the generation of Early Cretaceous granitic rocks of SE China: evidence from zircon U–Pb geochronology,geochemistry, and Hf isotopic data for the Liangnong pluton,eastern Zhejiang Province

In order to constrain the timing and petrogenesis of both the hosting rocks and the inner mafic microgranular enclaves (MMEs) of the Liangnong pluton, SE China, we have performed a series of bulk-rock geochemistry, zircon U–Pb, and Hf isotopic analysis, respectively. Zircon laser ablation–inductively coupled plasma–mass spectrometry U–Pb isotopic analysis yielded ages of 106.3 ± 1.1 Ma for the granodiorite and 103.9 ± 1.6 to 105 ± 1.8 Ma for monzogranite phases within the hosting pluton, as well as an age of 104.7 ± 0.8 Ma for the associated MMEs. The host rocks are metaluminous, have A/CNK values of 0.91–1.09, contain relatively high concentrations of SiO₂ and K₂O, are enriched in Rb, Th, Ba, Zr, and Hf, are depleted of Sr, P, Ti, Nd, and Ta, contain high concentrations of the rare earth elements (REE) and the light REE, and have moderately negative Eu anomalies (Eu*/Eu = 0.6–0.8). In comparison, the MMEs contain high concentrations of Al₂O₃, FeO, MgO, and TiO₂, are relatively enriched in Ba, U, and Sr, and are depleted in Th, Nd, and Zr. They have lower total REE concentrations and higher Eu*/Eu values than the hosting granites. The zircons within the hosting granites have Hf crustal model ages (T_DM^C) that show a peak at 1.29–1.85 Ga. Zircons within the MMEs have different εHf(t) values (–3.7 to +4.9) than the zircons within the hosting granites (–10.8 to –1.9). The results indicate that the MMEs and the hosting granites crystallized from magmas with different sources, thereby showing that the Early Cretaceous magmatism in the coastal areas of SE China was generated by the widespread injection of mantle-derived magmas caused by rollback of the subducting palaeo-Pacific Plate.     

Lower bound limit analysis of wedge stability using block element method

A lower bound limit analysis approach based on the block element method is proposed to analyze wedge stability problem. The search for the maximum value of the factor of safety is set up as a nonlinear programming problem. Sequential quadratic programming (SQP) algorithm from a reasonable initial value is applied to obtain the optimal solution. This approach provides a strict lower bound solution considering the sliding mode and rotation effect simultaneously. The deviations of the factor of safety between the present and traditional limit equilibrium methods are positively correlated with both the friction angle and the dip of the discontinuity surface.     

腾冲地块梁河早始新世花岗岩成因机制及其地质意义

腾冲地块梁河地区芒东和青木寨花岗岩是新特提斯洋演化过程中重要的壳源岩浆活动产物。岩石形成年龄为48~51Ma,属于早始新世,与腾冲地块西缘盈江地区大量的酸性和基性侵入岩的形成年龄相近。梁河地区的早始新世花岗岩具有高硅、钾的特征,属于准铝质-强过铝质高钾钙碱性S型花岗岩。这些花岗岩具有高的初始~(87)Sr/~(86)Sr比值和富集的Nd同位素组成,Nd模式年龄显示源岩应为中元古代的地壳岩石。同时,芒东花岗岩具有高的CaO/Na_2O和相对低的Al_2O_3/TiO_2、Rb/Sr和Rb/Ba比值,说明源区为变质杂砂岩。而青木寨花岗岩具有低的CaO/Na_2O和Al_2O_3/TiO_2、相对高的Rb/Sr和Rb/Ba比值,指示其源岩以变泥质岩为主。结合区域内中-新生代岩浆活动特征,我们认为芒东和青木寨花岗岩是印度-亚洲大陆东向初始碰撞或同碰撞时期挤压背景下,腾冲地块中下地壳成熟度较低的杂砂岩以及成熟度较高的泥岩在高温条件下部分熔融的产物。     

城巴断块明月剖面陡山沱晚期沉积环境的地球化学特征

由于造山带地层系统混乱,部分地区甚至无法恢复、鉴别,给古环境、岩石成因及其成矿性研究带来了困难。以震旦系陡山沱组出露较完整的明月剖面为重点研究对象,通过野外观测和室内综合研究,尝试从稀土元素的配分模式、Eu/Eu*、Ce异常（Ce/Ce*和Ce/Ce**）、微量元素Sr/Ba、Sr-Mn等地球化学特征研究该地区沉积环境以及岩石成因,厘定明月剖面陡山沱晚期沉积环境为大陆边缘沉积环境,指出热水沉积作用对硅质岩成因及成矿性的影响,为下一步找矿提供思路。     

Land use dynamics in Lhasa area, Tibetan Plateau

Land use change is the result of the interplay between socioeconomic, institutional and environmental factors, and has important impacts on the functioning of socioeconomic and environmental systems with important tradeoffs for sustainability, food security, biodiversity and the vulnerability of people and ecosystems to global change impacts. Based on the results of the First Land Use Survey in Tibet Autonomous Region carried out in the late 1980s, land use map of Lhasa area in 1990 was compiled for the main agricultural area in Lhasa valley using aerial photos obtained in April, May and October 1991 and Landsat imagery in the late 1980s and 1991 as remotely sensed data sources. Using these remotely sensed data, the land use status of Lhasa area in 1991, 1992, 1993, 1995, 1999 and 2000 were mapped through updating annual changes of cultivated land, artificial forest, grass planting, grassland restoration, and residential area and so on. Land use map for Lhasa area in 2007 was made using ALOS AVNIR-2 composite images acquired on October 24 and December 26, 2007 through updating changes of main land use types. According to land use status of Lhasa area in 1990, 1995, 2000 and 2007, the spatial and temporal land use dynamics in Lhasa area from 1990 to 2007 are further analyzed using GIS spatial models in this paper.