维拉斯托稀有金属-锡多金属矿床铌铁矿族矿物特征及其对岩浆-热液演化的指示

大兴安岭南段维拉斯托大型稀有金属-锡多金属矿床的成矿岩体从深部的中粒花岗岩向上逐渐过渡为斑状细粒碱长花岗岩,记录了岩浆演化的不同阶段。为查明铌铁矿族矿物成分、结构变化及其与岩浆演化过程的耦合关系,文章对采自不同深度的3种类型花岗岩（中粒花岗岩、石英斑晶不具雪球结构的斑状细粒富云母碱长花岗岩和具雪球结构的斑状细粒碱长花岗岩）中的铌铁矿族矿物进行了详细的电子探针成分分析和元素面扫工作。维拉斯托矿床铌铁矿族矿物主要为铌铁矿,发育渐变环带结构,从核部到边部Ta含量增加,且斑状细粒富云母碱长花岗岩和斑状细粒碱长花岗岩铌铁矿族矿物边部的铌铁矿、铌锰矿相较于幔部和核部的铌铁矿,其Ta含量具有明显的组分间隔;铌铁矿族矿物Mn/(Fe+Mn)原子比具有多种演化趋势,与分离结晶的矿物相和流体交代作用有关。中粒花岗岩和斑状细粒富云母碱长花岗岩中的铌铁矿结晶于岩浆阶段,斑状细粒碱长花岗岩中的铌铁矿族矿物结晶于岩浆-热液过渡阶段。其中,斑状细粒富云母碱长花岗岩铌铁矿边部Ta含量增加与铁锂云母的分离结晶作用有关,斑状细粒碱长花岗斑岩铌铁矿族矿物边部Ta含量的突变与岩浆-热液过渡阶段的水硅酸盐液体中Ta溶解度的增加...     

湖北黄陵球状花岗闪长岩的发现及其岩相学特征

在扬子陆块核部黄陵地区发现具球状构造的花岗闪长岩。岩石产于新元古代斑状花岗闪长岩与石英闪长岩之间,出露面积极小。球状花岗闪长岩岩体由主岩、球状体及球间基质三部分组成,球状体呈圆形、椭圆形及不规则状,由球核和同心球壳组成,球体大小多为5~12cm。依据球壳及球核的差异,可大致分为具有多壳层同心结构、具单壳层结构、无明显壳层结构3种类型的球状体。单壳层球壳由角闪石、黑云母等暗色矿物含量高的暗色层组成,具放射状构造;多壳层球壳由含角闪石、黑云母等暗色矿物的暗色层和基本不含或含少量暗色矿物、以斜长石为主的浅色层组成韵律层,具梳状、放射状构造;球核成分可以是同源岩浆组分,也可以是异源捕虏体。单壳层球体,球核、球壳、基质的斜长石牌号依次为36、32、26,酸性程度不断增加,内部呈放射状生长,反映其可能由内向外经岩浆结晶作用形成。初步认为多壳层球体球壳富韵律的变化为富含气水热液的岩浆在动荡的环境下结晶形成,球壳中矿物结晶顺序可能由外向内,核部结晶时间相对更长。     

内蒙古中部苏尼特左旗大石寨组流纹岩SHRIMP锆石U-Pb年龄、地球化学特征及其构造意义

内蒙古中部苏尼特左旗地区大石寨组酸性火山岩SHRIMP锆石U-Pb测年结果显示,2件流纹岩样品的206Pb/238U年龄加权平均值分别为272.1±3.1Ma和271.4±1.7Ma,说明大石寨组流纹岩喷发于早二叠世。锆石Hf同位素分析结果表明,εHf(t)值为+10.8~+13.1;Hf-Nd同位素耦合关系计算显示,其与同期形成的大石寨组玄武岩Nd同位素组成特点非常相似。全岩地球化学分析表明,该流纹岩高Si O_2,贫Ca O、Fe_2O_3*和Mg O,大部分样品属于过铝质、钙碱性流纹岩;富集轻稀土元素,轻、重稀土元素分馏中等,具明显的负Eu异常,富集Cs、Rb、Th、U和Zr,亏损Ba、Sr、Nb、Ta、P和Ti,指示其是富水的钙碱性基性地幔岩浆分离结晶作用的产物。结合区域地质资料,认为苏尼特左旗流纹岩形成于大陆岛弧背景。     

东准噶尔青河地区野马滩一带托让格库都克组高铝玄武岩的发现及其地质意义

下泥盆统托让格库都克组高铝玄武岩产自东准噶尔青河地区野马滩一带。高铝玄武岩富含大量斜长石斑晶,属亚碱性系列。它们具有显著高A12O3(质量分数为17.22%～21.31%),低SiO2(47.12%～49.45%),高TFe2O3(8.50%～11.98%)、MgO(2.64%～5.63%)、Ni((5.75～27.87)×10^-6)、Cr((0.99～25.62)×10^-6),以及高Sm/Yb值(1.13～1.96)、U/Th值(0.42～0.81)的特征,相对富集大离子亲石元素和亏损高场强元素,显示出典型的高铝玄武岩地球化学特征。进一步研究表明,其岩浆来源于受俯冲带流体交代地幔楔的部分熔融。此外,高Sr/Y值(43.16～47.07)和低Y(质量分数为(15.69～19.09)×10^-6)、Yb((1.64～2.13)×10^-6)显示玄武质岩浆源区有俯冲带洋壳物质的加入。高Al特征很可能与高压条件下斜长石的滞后结晶有关。地球化学特征表明,高铝玄武岩形成于岛弧环境。东准噶尔早泥盆世高铝玄武岩的发现丰富了本区火山岩岩类信息资料,并为构造演化研究提供了重要依据。     

西南印度洋中脊玄武岩地球化学特征及其对岩浆过程的指示意义

对采自西南印度洋中脊(SWIR)50°E附近5个站位的玄武岩样品进行了岩石学和元素地球化学研究.样品主量元素、TAS分类图解和AFM图解显示,SWIR研究区样品类型主要为低钾拉斑玄武岩.相对原始地幔SWIR区玄武岩具有Ba、Nb、Sr负异常,K表现为正异常.稀土元素分配模式均为左倾型,具有轻微的Eu、Ce正异常;SWI...     

南天山盲起苏花岗岩体的成因及构造意义：来自年代学、地球化学及Nd- Hf同位素证据

南天山造山带位于中亚造山带与塔里木克拉通的接壤地带,是了解克拉通及造山带构造演化的重要地区。但就南天山洋最终闭合和陆陆碰撞的时代目前还存在不同观点。盲起苏花岗岩体位于南天山造山带东段,是该区大型的复式花岗岩体,主要由东部黑云母花岗闪长岩、西部黑云母花岗岩和中部二云母花岗岩组成。LA ICP MS锆石U Pb定年结果显示,东部、西部和中部花岗岩的形成年龄分别为297±4 Ma、294±3 Ma和292±2 Ma。地球化学特征上,东部和西部花岗岩的SiO 2 含量为65. 34%~73. 12%,A/CNK为1. 00~1. 11（平均1. 05）,Na 2 O+K 2 O的含量为7. 26%~8. 52%,属高钾钙碱性弱过铝质岩石;中部花岗岩的SiO 2 含量为71. 97%~72. 84%,A/CNK为1. 09~1. 14（平均1. 12）,Na 2 O+K 2 O的含量为8. 36%~8. 80%,属钙碱性强过铝质岩石。稀土元素上,三者均富集轻稀土元素\[(La/Yb)N=9. 58~44. 94\],亏损重稀土元素（HREE,LREE/HREE=8. 00~22. 97）,并呈现不同程度的Eu负异常（东部 δ Eu=0. 67~0. 81;西部 δ Eu=0. 48~0. 78;中部 δ Eu=0. 44~0. 48）。微量元素上,三者均富集大离子亲石元素（K、Rb、Ba）,亏损高场强元素（Nb、Ta、Ti）。同位素特征上,锆石 ε Hf ( t )值分别为-11. 31~-1. 64（东部）、-5. 41~-0. 50（西部）和-14. 43~-2. 51（中部）,二阶段Hf模式年龄（ t DM2 ）均在1. 30~2. 20 Ga之间;东部岩体的 ε Nd ( t )的值为-6. 92~-10. 10,二阶段模式年龄 t DM2 (Nd)在1. 62~1. 88 Ga之间。综合区域地质和前人研究资料,表明盲起苏复式花岗岩体可能是后碰撞早期阶段由加厚的变质火成岩地壳发生部分熔融,经不同程度结晶分异所形成的,并暗示了南天山洋的闭合及陆陆碰撞的时间应发生于二叠纪之前。     

Olivine-hosted melt inclusions in Pliocene–Quaternary lavas from the Qorveh–Bijar volcanic belt,western Iran: implications for source lithology and cooling history

ABSTRACT

Silicate melt inclusions (SMIs) are small droplets of magma that become trapped in minerals during crystal growth. SMIs in olivine crystals can provide critical information on the range of melt compositions and processes that occur during melt generation, evolution, transport, and eruption. The Pliocene–Quaternary volcanic rocks in the Qorveh–Bijar volcanic belt of western Iran show porphyritic and microlithic textures, with olivine and clinopyroxene being the dominant minerals. Magnesian olivines in these volcanic rocks contain primary SMIs. The composition and characteristic of olivine-hosted SMI of these rocks are investigated to constrain the source lithology for mafic volcanism. Bulk compositions of the SMIs overlap those of their host rocks and extend to higher CaO/Al₂O₃ values. The estimated entrapment pressures and temperatures of the studied SMIs are 9.1–10.3 kbar and 1220–1355°C. The calculated mafic parental melt contains 42.36 wt.% SiO₂, low total alkalis (3.22 wt.%), and high MgO (16.1 wt.%). Exploratory calculations using pMELTS show that this parental composition underwent variable degrees of fractional crystallization, as reflected by the variable compositions of the SMIs. Several lines of evidence including pyroxene xenocrysts and high FeO/MnO, FC3MS (FeO/CaO – 3*MgO/SiO2), and Zn/Fe ratios (14–21), suggest that a metasomatized pyroxenitic source contributed to the genesis of the parental melt. Amphibole in the SMIs indicates a high volatile content in the parental melt, which we conclude was generated from a metasomatized lithospheric mantle source. The pyroxenite source also contained garnet. Our geochemical results lead us to propose a new petrogenetic model. Specifically, we infer that a dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, i.e. lithospheric drip. This drip underwent volatile-enhanced partial melting during descent through the underlying hot asthenosphere and generated the studied volcanic rocks.     

Salt weathering: influence of evaporation rate,supersaturation and crystallization pattern

Micro- and macroscale experiments which document the dynamics of salt damage to porous stone have yielded data which expose weaknesses in earlier interpretations. Previously unexplained differences are found in crystal morphology, crystallization patterns, kinetics and substrate damage when comparing the growth of mirabilite (Na₂SO₄. 10H₂O) and thenardite (Na₂SO₄) versus halite (NaCl). The crystallization pattern of sodium sulphate was strongly affected by relative humidity (RH), while a lesser RH effect was observed for sodium chloride. Macroscale experiments confirmed that mirabilite (crystallizing at RH > 50 per cent) and thenardite (crystallizing at RH < 50 per cent) tend to form subflorescence in highly localized areas under conditions of constant RH and temperature. This crystallization pattern was more damaging than that of halite, since halite tended to grow as efflorescence or by filling the smallest pores of the stone in a homogeneous fashion, a result which contradicts Wellman and Wilson's theoretical model of salt damage. Low RH promoted rapid evaporation of saline solutions and higher supersaturation levels, resulting in the greatest damage to the stone in the case of both sodium sulphate and sodium chloride crystallization. At any particular crystallization condition, sodium chloride tended to reach lower supersaturation levels (resulting in the crystallization of isometric crystals) and created negligible damage, while sodium sulphate reached higher supersaturation ratios (resulting in non-equilibrium crystal shapes), resulting in significant damage. ESEM showed no damage from sodium sulphate due to hydration. Instead, after water condensation on thenardite crystals, rapid dissolution followed by precipitation of mirabilite took place, resulting in stone damage by means of crystallization pressure generation. It is concluded that salt damage due to crystallization pressure appears to be largely a function of solution supersaturation ratio and location of crystallization. These key factors are related to solution properties and evaporation rates, which are constrained by solution composition, environmental conditions, substrate properties, and salt crystallization growth patterns. When combined with a critical review of salt damage literature, these experiments allow the development of a model which explains variations in damage related to combinations of different salts, substrates and environmental conditions. Copyright © 1999 John Wiley & Sons, Ltd.     

喜马拉雅淡色花岗岩结晶分异机制概述

近年来,喜马拉雅淡色花岗岩的高分异成因得到了学术界的高度关注。另外,有调研工作发现,喜马拉雅淡色花岗岩具有良好的稀有金属成矿潜力,是未来矿产勘探的重点靶区。稀有金属元素的富集过程应与岩浆的结晶分异作用密切相关。但是,目前我们对喜马拉雅淡色花岗岩的岩浆分异演化过程缺乏深入的了解,因而也难以对其成矿效应进行有效评估。针对该问题,本文选择喜马拉雅带中两个颇具代表性的、明显发生稀有金属矿化现象的岩体（特提斯喜马拉雅带中的然巴淡色花岗岩和高喜马拉雅带中的告乌淡色花岗岩）开展系统野外地质、岩相学、矿物学、岩石学和地球化学调查研究,就喜马拉雅淡色花岗岩的结晶分异机制进行初步探讨。综合近年来有关花岗岩结晶分异作用的研究进展,我们认为喜马拉雅淡色花岗岩的结晶分异过程可以划分为原地结晶分异和岩浆侵位运移过程中流动分异两种主要机制,大部分的喜马拉雅淡色花岗岩应是两种机制共同作用的产物。     

Constitutive models and salt migration mechanisms of saline frozen soil and the-state-of-the-practice countermeasures in cold regions

A series of saline soil-related problems, including salt expansion and collapse, frost heave and thaw settlement, threaten the safety of the road traffic and the built infrastructure in cold regions. This article presents a comprehensive review of the physical and mechanical properties, salt migration mechanisms of saline soil in cold environment, and the countermeasures in practice. It is organized as follows:(1) The basic physical characteristics;(2) The strength criteria and constitutive models;(3) Water and salt migration characteristics and mechanisms; and(4) Countermeasures of frost heave and salt expansion. The review provides a holistic perspective for recent progress in the strength characteristics, mechanisms of frost heave and salt expansion, engineering countermeasures of saline soil in cold regions. Future research is proposed on issues such as the effects of salt erosion on concrete and salt corrosion of metal under the joint action of evaporation and freeze-thaw cycles.