小兴安岭-张广才岭铁力地区侏罗纪辉绿岩年代学、地球化学、锆石Hf同位素特征及其构造意义

小兴安岭-张广才岭地区中生代岩浆活动的成因及动力学背景对于揭示古太平洋的构造演化具有重要意义。本文选取小兴安岭-张广才岭铁力地区出露的辉绿岩墙为研究对象,进行锆石LA-ICP-MS U-Pb年代学、全岩地球化学和锆石Hf同位素等分析,对该基性岩的形成时代、岩石成因、岩浆源区以及形成的大地构造背景进行讨论。研究表明:辉绿岩锆石具有高Th/U比值( 0.3),CL图像显示微弱的振荡环带结构,具有岩浆锆石特征,~(206)Pb/~(238)U加权平均年龄为187±2Ma,即形成于早侏罗世;该岩体主要经历了橄榄石和单斜辉石的分离结晶作用,未遭受明显的地壳混染作用,并且具有Rb、Ba、U、Pb、K和Sr等流体活动元素相对富集,Th、Nb和Ta等非流体活动性元素相对亏损的地球化学特征,暗示其形成于被俯冲流体富集交代的亏损地幔部分熔融,源区可能为尖晶石-石榴石二辉橄榄岩,部分熔融程度约为6%～20%。结合该地区同时代的岩浆岩、变形构造、矿床特征和黑龙江蓝片岩的相关报道,本文认为小兴安岭-张广才岭地区在中生代期间处于活动大陆边缘环境,其岩浆岩的形成主要是由于存在于佳木斯地块和松嫩-小兴安岭地块间的牡丹江洋西向俯冲造成的。     

西准噶尔成矿带晚古生代花岗岩类岩浆活动及其构造意义

中亚造山带是晚古生代地壳显著生长与大规模成矿的重要地区。本文采集了中亚造山带西部的西准噶尔成矿带哈图-别鲁阿嘎希及其附近地区11个岩体共33件花岗岩类样品,对其开展了岩石地球化学与同位素示踪等研究,厘定了该地区晚古生代岩浆活动的特点与大地构造环境,并与哈萨克斯坦境内的巴尔喀什成矿带晚古生代岩浆活动进行了对比。研究表明,哈图地区晚石炭世花岗岩类主要为后碰撞伸展构造环境的A型花岗岩类,别鲁阿嘎希等地区存在洋内俯冲与岛弧环境的埃达克岩,显示了西准噶尔晚古生代构造环境时空变化的复杂性。该地区花岗岩类εNd(t)值较高(+4.62~+7.53)、εSr(t)值为(-57.61~+18.21),具有中亚造山带花岗岩类的共同特征,为古生代增生的新生陆壳,其源区与亏损地幔组分具有亲缘关系,这与巴尔喀什成矿带东段的花岗岩类具有一致性。花岗岩的~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb和~(208)Pb/~(204)Pb比值范围分别为18.2776~19.1677、15.5260~15.5796和38.2080~39.0821,为造山带花岗岩类。     

青海东昆仑造山型金（锑）矿床成矿流体地球化学研究

青海东昆仑是我国重要的、极富潜力的金属成矿带,其中产出的金矿床不仅具有相似的地质特征,而且与本区晚华力西-印支期造山过程有密切的成因联系,并在产出构造背景、矿体特征、控矿构造、围岩蚀变、矿物组成和元素组合等方面与一般造山型金矿床极为相似,因此应属典型的造山型金矿床.矿床流体包裹体十分发育,包括富CO2、CO2-H2O和H2O-NaCl三种类型,显微测温结果显示,成矿流体为一套中低温(120～380℃)、低盐度(0.35～9.54wt%NaCl)的H2O-CO2-NaCl-CH4±N2流体体系.氢、氧、碳同位素及水-岩交换反应研究表明,成矿流体早期主要为变质水和地层建造水,成矿期以来以大气降水为主.流体不混溶、大气降水的不断加入以及水-岩交换作用是导致本区成矿流体中金、锑淀积成矿的主要因素.     

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks,Roman Magmatic Province,Southern Italy

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.70926 to 0.70999, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for ²⁰⁶Pb/²⁰⁴Pb, 15.685–15.701 for ²⁰⁷Pb/²⁰⁴Pb, and 39.048–39.076 for ²⁰⁸Pb/²⁰⁴Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. ⁸⁷Sr/⁸⁶Sr ranges from 0.70665 to 0.70745, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51234 to 0.51238, ²⁰⁶Pb/²⁰⁴Pb ranges from 18.924 to 19.153, ²⁰⁷Pb/²⁰⁴Pb ranges from 15.661 to 15.694, and ²⁰⁸Pb/²⁰⁴Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

滇西大坪金矿床地质特征及成因初探

大坪金矿床被视为哀牢山造山带南段最典型的造山型金矿,但其诸多地质特征明显不同于国外典型造山型金矿床,而呈现出与浅成低温热液型金矿床的一致性.区域上,脉型金铜铅银矿床的产出与中酸性侵入岩及中基性火山岩密切相关.大坪金矿床形成于主要赋矿的桃家寨闪长岩侵位之后约800Ma的新生代构造-岩浆热事件中,金矿床是区域尺度伸展和转换拉伸应力体制的产物;成矿作用过程中,研究区构造动力体制发生了转换,主成矿期容矿断裂带显示为张剪性正断层性质.矿脉多(55条)、薄(0.2～0.8m)、长(200 ～ 1500m)、陡(56°～ 85°)、延深大(约700m),且近平行成带产出;矿体中Au品位高(超过10×10-6),Au/Ag低(o.1 ～o.5),且伴生Pb、Cu、Ag.矿石的充填型结构构造和矿物共生组合指示矿脉形成温度低、深度浅;矿化-蚀变样式以及蛋白石和氧化矿的出现表明成矿体系处于开放的氧化环境,成矿后保存良好.大坪金矿床这种特殊的地质特征可能与其复杂的区域构造背景及成矿演化过程密切相关,金矿床虽然具有造山型金矿的基本特征,但在成矿作用晚期,矿床浅部叠加了浅成低温热液型金铜铅银成矿作用.即大坪金矿床属于深部造山型+浅部低温热液型矿床套叠组合,成矿体系保存完整,现发现的均为浅部矿体,深部找矿潜力巨大.     

近年来强沙尘暴天气气候特征的分析研究

2000～2002年春季(3～5月)中国北方有12次强沙尘暴天气过程发生,其中11次与蒙古气旋有关.作者从干旱气候背景、环流状况、沙尘源、沙尘路径及天气系统等方面进行了分析,并集中对引发强沙尘暴的蒙古气旋进行了诊断分析.结果表明:在这3年中,春季我国北方强沙尘暴天气主要与蒙古气旋的发展移动有关,气旋冷锋后的大风是强沙尘暴天气发生的主要动力因子;蒙古国南部、巴丹吉林沙漠、腾格里沙漠、乌兰布和沙漠和毛乌素沙地是强沙尘暴过程的主要沙尘源地;影响我国的强沙尘暴的沙尘路径至少可分为3种类型,即偏西路径、西北路径和南疆盆地型,以西北路径居多;我国北方春季的连续干旱、气温偏高及冷空气活跃是强沙尘暴天气形成的重要气候背景.     

http://www.sciencedirect.com/science/article/pii/S1674987112000357

Field and remote sensing studies reveal that Au-bearing quartz±carbonate lodes in Romite deposit,in the extreme South Eastern Desert of Egypt,are controlled by NNE-striking shear zones that splay from the ca.660—550 Ma Hamisana Zone.Quartz in releasing bends with sinistral shear geometry and abundant boudinaged quartz-carbonate lodes with serrate ribboned fabrics suggest vein formation throughout a transpressive wrench system.Ubiquitous hydrothermal quartz,carbonate,and subordinate chlorite and sericite within the shear zones and as slivers in veins,indicate that gold deposition and hydrothermal alteration occurred under greenschist fades conditions.The Al（Ⅳ） in chlorite indicates a formation temperature of～300℃.comparable with temperatures estimated from arsenopyrite composition for grains intimately associated with gold in quartz veins. The new geological and geochemical data indicate that splays off the Hamisana Zone are potential gold exploration targets.Quartz veins along the high order（2nd or 3rd） structures of this crustal-scale shear zone are favorable targets.In the Romite deposit and in surrounding areas,a Au-As-Cu-Sb-Co-Zn geochemical signature characterizes mineralized zones,and particularly rock chips with>1000 ppm As and high contents of Cu,Zn,and Co target the better mineralized areas. The carbonateδ¹³C_pdb andδ¹⁸O_Smow isotope signatures preclude an organic source of the ore fluid,but metamorphic and magmatic sources are still valid candidates.The intense deformation and lack of magmatism in the deposit area argue for metamorphic dewatering of greenstone rocks as the most likely fluid source.The narrow ranges ofδ¹³C（-4.6‰to -3.1‰） andδ¹⁸O（11.9‰-13.7‰） in carbonate minerals in lodes imply a corresponding uniformity to the ambient temperature andδ¹³C_CO2(δ¹³C_∑C) of the ore fluids. The calculatedδ¹⁸Oh₂o values of 6.9‰—7.9‰for ore fluids,based onδ¹⁸O values of vein quartz further suggest a likely metamorphic origin.     

Obituary

Geological sections are still best drawn at natural scale. Sections with vertical exaggerations are rarely drawn correctly, and even when computed carefully give a false notion of the structural features. Many schematic sections which are not drawn to scale and are supposed to explain a proposed new tectonic interpretation give such a distorted and frequently absurd picture of the structural features that they cannot be regarded as valid.