The Dabieshan Coesite-bearing Eclogite Terrain-A Late Archaean Ultra - high- Pressure Metamorphic Belt

A U -Pb zircon age of 2774±24 Ma for eclogite from the Bixiling rock body of Anhui Province, central China, indicates that the Dabieshan coesite-bearing eclogite was probably formed in the Late Archaean. A phengite Ar-Ar isochron age of 662±13 Ma for the eclogite confines also an upper limit age of its subsequent retrograde metamorphism in the Precambrian. The results of isotopic dating for such type of eclogite coincide with the geological features of its restricted occurrence within the Archaean metamaorphic terrain composed of the Dabie Group. It is believed that the Dabieshan coesite-bearing eclogite terrain might be a Late Archaean ultra-high-pressure metamorphic belt. The Dabie Mountains area was the eastward extension of the southern Qinling structural belt during the Triassic. Both the Dabie Group and the coesite-bearing eclogite hosted therein underwent a late-stage dynamic metamorphic event. The present authors have obtained a muscovite Ar-Ar isochron age of 192.6±2.8 Ma from plagioclase gn     

Determining the direction of contact metamorphic fluid flow: an assessment of mineralogical and stable isotope criteria

ABSTRACT One-dimensional fluid advection-dispersion models predict differences in the patterns of mineralogical and oxygen isotope resetting during up- and down-temperature metamorphic fluid flow that may, in theory, be used to determine the fluid flow direction with respect to the palaeotemperature gradient. Under equilibrium conditions, down-temperature fluid flow is predicted to produce sharp reaction fronts that separate rocks with isobarically divariant mineral assemblages. In contrast, up-temperature fluid flow may produce extensive zones of isobarically univariant mineral assemblages without sharp reaction fronts. However, during contact metamorphism, mineral reaction rates are probably relatively slow compared with fluid velocities and distended reaction fronts may also form during down-temperature fluid flow. In addition, uncertainties in the timing of fluid flow with respect to the thermal peak of metamorphism and the increase in the variance of mineral assemblages due to solid solutions introduce uncertainties in determining fluid flow directions. Equilibrium down-temperature flow of magmatic fluids in contact aureoles is also predicted to produce sharp δ¹⁸O fronts, whereas up-temperature flow of fluids derived by metamorphic devolatilization may produce gradational δ¹⁸O vs. distance profiles. However, if fluids are channelled, significant kinematic dispersion occurs, or isotopic equilibrium is not maintained, the patterns of isotopic resetting may be difficult to interpret. The one-dimensional models provide a framework in which to study fluid-rock interaction; however, when some of the complexities inherent in fluid flow systems are taken into account, they may not uniquely distinguish between up- and down-temperature fluid flow. It is probably not possible to determine the fluid flow direction using any single criterion and a range of data is required.     

变质分异作用中的质量迁移

变质分异作用中的质量迁移徐士进，陆建军，尹宏伟（南京大学地球科学系．南京２１００９３）关键词武功山，变质分异，质量迁移变质分异作用（ｍｅｔａｍｏｒｐｈｉｃｄｉｆｆｅｒｅｎｔｉａｔｉｏｎ）这一名词是Ｆ．Ｌ．Ｓｔｉｌｌｗｅｌｌ于１９１８年首次提出，意指成...     

和龙地区太古宙变质深成侵入体的地质地球化学特征

本文论述了和龙地区太古宙变质深成侵入体的岩石学，岩石化学，地球化学特征。首次提出了该区是以变质深成侵入体为主体的认识。通过其地球化学特征的研究，对太古宙岩浆的成因和演化提出了新的认识，这些认识将对吉林省太古宙变质岩的研究和成矿作用起到了重要的指导作用。     

赣中变质岩带的矿物化学特征及其P—T轨迹的构造变化

本文根据矿物组合及特征变质矿物，将赣中变质岩带划分出5条变质带，由低到高依次为绢云母－绿泥石（Ser-Chl)带、黑云母（Bi)带、铁铝榴石（Gt)带、十字石（St)带、矽线石（Sil)带。其分布的总体趋势是由南向北呈渐进分布的特点，即变质地层在纵向上由新到老具有由弱而强的变化趋势，同时受构造的控制。区域断裂带，从断裂中心向两侧变质程度递减，并常伴随混合岩化作用。本文从矿物化学特征及P、T条件计算结果来讨论变质岩的演化特征及形成的构造环境。     

南岭东段麻粒岩相变质岩的发现及其地质意义

本文报道了在南岭东段闽西南中元古代桃溪组变质岩中发现的麻粒岩相变质岩。其矿物组合和化学成分显示这些变质岩的原岩是沉积岩。成岩格子和温压计算限定它们的峰期变质作用发生在750-880℃和1．1GPa的高温高压条件下，表明这些表壳岩由于构造作用曾经成为下地壳的物质组成，并经历了麻粒岩相变质作用。等温降压的P-T-t轨迹指示它们很可能是在中生代伸展构造背景下快速抬升到浅部。这些麻粒岩的成分显然与东南沿海地区发现的基性麻粒岩捕虏体的不同，暗示这两地区下地壳的物质组成可能存在差异，也许这就是造成两个地区花岗岩类型不同的最主要原因。     

An oxygen isotope study of quartz veins within eclogites from the Dabie terrane

The oxygen isotope composition of minerals from quartz veins and host eclogites in the Dabie terrane was measured in order to place geochemical constraints on the origin and transport of metamorphic fluid. The results are discussed together with structural and petrological relationships between quartz vein and wallrock. The quartz veins can be temporally classified into three groups: (1) synmetamorphic vein which would be formed prior to eclogite-facies recrystallization when they were exhumated from mantle depths to deep crustal levels; (2) early retrogressive vein which was formed in the early stage of eclogite exhumation subsequent to the recrystallization, the vein-forming fluid is still relevant to the eclogites; (3) late retrogressive vein which was formed in the late stage of eclogite exhumation from deep crustal to upper crustal levels, oxygen isotope fractionation between vein quartz and host eclogite significantly deviates from equilibrium values and the vein-forming fluid was principally derived from granitic gneiss hosting the eclogites. For the synmetamorphic vein, it appears that local advective transport of fluid is the predominant mechanism in the processes of vein precipitation; the scale of oxygen isotope homogenization within the veins is much larger than that within the associated eclogites. The vein-forming fluid would be derived from the exsolution of dissolved hydroxyls within eclogite minerals due to significant pressure decrease. Fluid flow prior to the eclogite-facies recrystallization and the early retrogression may occur mainly along pressure gradients.     

陕西八卦庙金矿钠长石的成因

陕西八卦庙金矿产于凤太矿田北部，含矿地层为上泥盆统星红铺组下段中上部。赋矿围岩中含有较多的钠长石。钠长石呈层状顺层产出，镜下观察为砂屑状、次棱角状，与石英及铁白云石呈镶嵌粒状关系，而无交代结构；钠长石成分计算其形成温度超过800℃和400℃，表明其形成与喷流作用有关，并受到后期变质改造；锯钠长石X射线衍射分析，其有序度为0．38，形成温度为620－667℃，说明它不是一般意义上的热水沉积岩产物。上述研究表明：八卦庙金矿中的钠长石是火山喷发沉积－变质重结晶的产物，火山沉积作用参与该地区成矿是形成巨型金矿的最有利条件。含钠长石硅质岩、千枚岩、灰岩组合是找矿评价的重要标志。     

江西省金溪—南城湿合岩和花岗岩的岩石地球化学及同位素地球化学研究

在研究金溪—南城变质混合岩带中混合岩、花岗岩的矿物岩石学基础上,详细研究了它们的Nd、Sr、Pb、O同位素组成.混合岩、花岗岩初始钕同位素组成(143Nd/144Nd)i=0.511689～0.511853,在εNd-T图中位于该地区结晶基底变粒岩、片岩Nd同位素演化域上方,褶皱基底绢云千枚岩Nd同位素演化线下方;初始锶同位素组成(     

华中高压变质柯石英的拉曼谱学研究

采用光学显微镜、X衍射仪和拉曼分子探针对华中皖西南榴辉岩中的柯石英的晶体结构和分子配位体结构进行了研究,在中国自己的实验室中首次测定了自然柯石英的拉曼散射谱并获得下列结论:①通常高压变质柯石英是不可能结晶成大的颗粒,其大小为几个微米至几十微米。②根据因子群分析理论确定石英的465cm~(-1)峰为对称伸缩振动A_1,柯石英的521cm~(-1)峰为反对称伸缩振动Bg。③利用拉曼分子微探针鉴定SiO_2多形具有独特的效果,研究表明在α-方石英(CR)石英(QZ)和柯石英(CS)中Si-O键长基本不变,但是[SiO_4]配位体密度从CR、QZ至CS是递增的,并导致矿物密度及[SiO_4]架状结构振动频率的增加,这为研究分子结构与地质体温度、压力之关系提供了新的依据。