Petrogenesis of andalusite–kyanite–sillimanite veins and host rocks, Sanandaj-Sirjan metamorphic belt, Hamadan, Iran

Quartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al₂SiO₅ polymorphs, and, more rarely, three coexisting Al₂SiO₅ polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al₂SiO₅ polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe₂O₃ relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al₂SiO₅, kyanite‐quartz veins crosscut rocks that lack Al₂SiO₅ (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al₂SiO₅ triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.     

含硫化物镁铁质-超镁铁质侵入体中橄榄石Mg、Ni含量的制约:原理、模式及Voisey's Bay侵入体样品研究

镁铁质-超镁铁质岩浆结晶分离早期形成镁铁矿物,镁铁矿物中的Ni和Mg是相容元素。随着结晶分离作用的进行,Ni、Mg在硅酸盐岩浆及后形成的硅酸盐物质中的丰度下降。橄榄石中Ni含量及硅酸盐物质MgO/FeO比值都与母岩浆的相关值相关,据此可推断母岩浆的信息,它们之间可由实验测得的系数相联系。当岩浆饱和硫化物时,在结晶分离过程中硫化物珠滴会与镁铁硅酸盐物质一道析出,同时,与硫化物非饱和岩浆相比,过多的Ni会随之析出。这也反映在Ni、Mg含量比无硫化物分离时有更迅速的降低上。Ni、Mg含量变化值可以在VoiseysBay侵入体的模式曲线上反映出,加拿大Labrador的这一侵入体赋存了一个世界级的Ni-Cu-Co硫化物矿床。过去的作法是将侵入体中橄榄石的Ni、Mg含量与Simkin和Smith得出的各种火成岩中橄榄石的Ni、Mg含量相比较以确定Ni亏损,进而假定橄榄石来自硫化物饱和、有经济价值的岩浆。现在的研究显示这种简单的对比会导致错误。将样品数据与模式曲线对比并反映出侵入体矿物结晶堆积特征是重要的方法。使用这一方法,样品数据能很好地被模式曲线拟合。以在VoiseysBay的研究为例,当硫化物液相与硅酸盐矿物被去除后,硫化物非饱和的分离作用期就会显现出来,随后是硅酸盐结晶作用期。     

Fluid inclusion studies of the Abawso gold prospect,near the Ashanti Belt,Ghana

The major Ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the Kumasi Basin with the Ashanti and Sefwi Belts, respectively. The investigated area of the Abawso small-scale concession, covering the workings of the old Ettadom mine, is situated 3 km west of the lithological contact of the Birimian metavolcanic rocks of the Akropong Belt in the east with the Birimian metasedimentary rocks of the Kumasi Basin in the west. The rocks of the Abawso concession represent a steeply NW-dipping limb of a SE-verging anticline with an axis plunging to the SW. Quartz veining occurs predominantly in the form of en échelon dilatational veins along NNE–SSW-striking shear zones of a few metres width and shows evidence of brittle and ductile deformation. Also stockwork-style quartz veining occurs in the vicinity of the main shaft of the old Ettadom mine. Hydrothermal alteration includes sericitisation, sulphidation and locally carbonatisation. The auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins; however, some occur in stockwork. Fluid inclusion studies reveal a large number of H₂O inclusions along intragranular trails in auriferous quartz vein samples, as well as an overall dominance of H₂O and H₂O-CO₂ inclusions over CO₂ inclusions. Textural observations and physico-chemical fluid inclusion properties indicate post-entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation. This process is interpreted to be responsible for the generation of the CO₂ inclusions from a H₂O-CO₂ parent fluid. In comparison with mineralisation at the Ashanti and Prestea deposits, which are characterised by CO₂±N₂ inclusions, the observed inclusion assemblage may be due to a shallower crustal level of mineralisation, or different degrees and styles of recrystallisation, or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations. Furthermore, the microthermometric observations allow the reconstruction of a possible retrograde P-T path, depicting near-isothermal decompression in the P-T range of the brittle/ductile transition.Editorial handling: E. Frimmel     

西藏东巧方辉橄榄岩的显微构造特征及其流变学意义

西藏东巧地区蛇绿岩套中橄榄岩是青藏高原出露的为数不多的地幔岩体之一, 对于揭示该地区的上地幔流变学特征具有重要意义.报道了该地区方辉橄榄岩中橄榄石的位错显微构造特征, 估算了上地幔流变学参数.显微构造研究表明, 东巧方辉橄榄岩发育残斑结构, 橄榄石中位错组态类型比较丰富, 包括自由位错、位错壁(包括宽阔型和紧密型)、位错弓弯和位错网, 表明橄榄石的主导变形机制可能为位错蠕变.该区地幔岩变形大致分为2个阶段: (1) 地幔缓慢塑性流动变形, 形成宽阔型位错壁; (2) 蛇绿岩侵位过程中的变形, 产生紧密型位错壁.根据2种位错壁估算的流动应力平均值分别为3 9.3MPa (宽阔型)和113.9MPa (紧密型), 计算结果获得东巧地区上地幔流动速率为1.13× 10-12 ~ 2.95× 10-11s-1, 有效粘度为4.44×1017~ 1.16×1019Pa°s, 这些参数为东巧地区上地幔物理特征提供了流变学的约束条件      

Albite vein formation during exhumation of high-pressure terranes: a case study from alpine Corsica

The formation of late‐stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high‐pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high‐pressure rocks. Albite veins in retrogressed metabasic rocks from high‐pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P–T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P–T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein‐forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein‐forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during high‐pressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high‐pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems.     

塔里木板块东北部坡北岩体内橄榄辉长苏长岩岩石成因

坡北岩体位于塔里木板块东北部,由5个阶段的岩浆作用形成,其中,第二阶段岩浆作用形成的侵入体岩石类型单一,主要为橄榄辉长苏长岩,局部有零星的辉长苏长岩和斜长岩。岩石的SiO₂—FeO/MgO图解显示其化学组成为钙碱性系列,但单斜辉石的Al₂O₃—SiO₂及n（Si）—n（Al^Ⅳ）图解表明母岩浆为拉斑玄武质岩浆。岩浆演化方向与岩石化学系列的转化是同化混染作用的结果,侵入体遭受了长城系古硐井岩群1.38%～3.15%的混染。原生岩浆为MgO含量约7.31%的玄武质岩浆。岩石的TiO₂、Na₂O、K₂O、稀土元素、大离子亲石元素丰度均很低;除一件样品外,其余岩石的ε_Nd（t）值为+2.54～+2.81,这些特征表明岩浆源区属亏损型大陆岩石圈地幔。岩浆在结晶过程中处于物理化学条件相对稳定的近平衡状态,局部不平衡分离结晶形成堆晶结构和纹层状构造,岩浆结晶温度约为1070℃。由于岩浆源区熔融程度低,导致原生岩浆贫硫,且岩浆在演化过程中没有经历充分的分离结晶作用,这些因素使得橄榄辉长苏长岩侵入体不具备硫达到过饱和进而发生铜镍硫化物熔离成矿的潜力。     

地幔橄榄岩中橄榄石的指示意义

橄榄石是地幔橄榄岩和辉石岩的主要组成矿物,但也经常以斑晶和捕虏晶的形式出现在玄武质岩石中。结合近年来在地幔橄榄岩的主要元素(如Mg和Fe)组成特征以及Li、Mg和Fe稳定同位素地球化学方面的研究成果,重点对橄榄石的地球化学特征与华北克拉通岩石圈地幔演化过程之间的联系进行了讨论,旨在加深对华北克拉通岩石圈地幔演化过程的理解。现有研究表明:地幔橄榄岩中橄榄石的矿物学特征、元素和同位素地球化学组成能够很好地指示岩石圈地幔的特征及其演化过程,因而具有重要的意义。对于克拉通地区的地幔橄榄岩来说,橄榄石的Mg#通常可以指示岩石圈地幔的属性,古老、难熔的地幔橄榄岩中的橄榄石一般具有较高的Mg#(>92),而新生的岩石圈地幔橄榄岩中的橄榄石则具有较低的Mg#(<91)。因此,地幔橄榄岩中橄榄石的Mg#在一定程度上具有年龄意义。橄榄岩中橄榄石的Li、Mg和Fe同位素组成也可以明确指示岩石圈地幔的属性及其所经历的演化过程,正常地幔的δ7Li、δ26Mg和δ57Fe组成相对均一,如果上述同位素组成偏离正常地幔值,则说明岩石圈地幔经历了熔体/流体的交代作用。华北克拉通地区地幔橄榄岩捕虏体中橄榄石的Li、Mg和Fe同位素组成研究表明:该区的岩石圈地幔经历了多个阶段、不同来源的熔体/流体的改造过程。     

扬子地块北缘周庵超镁铁质岩体矿物学特征及其对铜镍矿化的启示

扬子地块北缘~635 Ma周庵超镁铁质岩体是一个新发现的含铜镍硫化物矿化的隐伏岩体,主要由二辉橄榄岩组成。文章通过研究周庵岩体中橄榄石、铬铁矿和辉石的矿物成分变化探讨了岩浆演化过程和含矿岩体成因。根据岩石的矿物组合和蚀变程度,岩体从上到下分为3个部分:上部绿泥石-蛇纹石化二辉橄榄岩相带、中部二辉橄榄岩相带和下部绿泥石-角闪石化二辉橄榄岩相带。根据岩体中部带橄榄石和铬铁矿的成分,计算得到母岩浆的Mg^#值为0.63,MgO/FeO摩尔比值为1.72,w(Al₂O₃)为10.2%~11.7%,w(Ni)为476×10^-6,说明其为高镁玄武质岩浆;岩体中部带原生铬铁矿和粒间相铬铁矿核部的Cr₂O₃和Al₂O₃呈正相关关系,说明铬铁矿与粒间硅酸盐熔体发生了平衡交换,铬铁矿的高w(TiO₂) 和Cr^#值与拉张环境中层状岩体的铬铁矿特征一致;根据辉石温压计得到岩体中部单斜辉石和斜方辉石的共结温度为1 017~1 077℃,压力为(3.6~4.5)×10⁸ Pa,暗示形成岩体的浅部岩浆房深度约为12 km。岩体上部和中部带的橄榄石Fo值大部分集中在80 mol%~85 mol%,w(Ni)介于2 255×10^-6~4 455×10^-6,说明这些橄榄石是从没有经过强烈分离结晶和硫化物熔离的岩浆中结晶出来的。岩体下部带橄榄石的Fo值(67 mol%~68 mol%)和w(Ni) (1 500×10^-6~2 000×10^-6)都低于岩体上部和中部带的橄榄石相应值,说明岩体下部带的橄榄石可能形成于演化程度较高、并经历了硫化物熔离的岩浆。因此,笔者认为周庵岩体是由相对原始的和演化了的高镁玄武质岩浆两期侵位形成的。     

岩脉的侵位机制对成矿作用的约束 ———以内蒙古大井锡多金属矿床为例

[摘 要] 岩脉的侵位机制对理解岩脉和矿脉的侵位过程有着重要的意义。本文详细介绍了岩脉 的侵位机制,并以大井锡多金属矿床为例初步探讨了矿区内岩脉与矿脉的关系。研究表明,区域上主要 受NW-SE 向的挤压应力,在海西期向燕山期转变阶段,区域应力场并没有发生变化,只是林西断块发 生了顺时针转动的结果。矿区范围内,同样受NW-SE 向挤压应力作用,块体也发生了相应的变化,推测 矿区西南向北东方向挤压应力逐渐减小,隆升剥蚀量也随之减小。整体上,岩脉侵位于先存裂隙,矿脉 侵位于岩脉形成的构造空间,局部具有互相穿插的现象。通过岩脉和矿脉的侵位过程,认为矿区西部寻 找斑岩型矿床潜力较大。     

SiO2 precipitation in olivine: ATEM investigation of two dunites annealed at 300 MPa in hydrous conditions

Two natural dunites were annealed at pressure P=300 MPa, temperature T=1373, 1473 and 1573 K, and fO₂ within the stability field of olivine. The starting materials contained small amounts of hydroxyls in the form of minor phases of hydrated minerals, which released an aqueous phase during the experiments. A detailed analytical transmission electron microscopy (ATEM) investigation of these materials revealed that small quantities of two types of silica-rich glass formed during heat treatment. The first type of glass, found at triple junctions as rare partially crystallized glass pockets, results from melting dehydration reactions involving the hydrous phases. The second type of glass is found as pure silica precipitates (0.1–0.5 μm in size, for a total of a few 0.1 vol%) within the olivine grains of specimens heated to ≥1473 K. From considerations of the kinetics of the precipitation at 1473 K, we interpret this silica precipitation as resulting from the condensation of olivine metallic vacancies promoted by increasing fluid fugacities during the runs. Our observations, thus, demonstrate that metastable silica can precipitate in olivine from dunites experiencing rapid changes in their thermodynamical environment.