The Greer Lake leucogranite, Manitoba, and the origin of lepidolite-subtype granitic pegmatites

The Archean Greer Lake leucogranite intruded metabasalts of the Bird River Greenstone Belt in the southwestern part of the Superior Province of southeastern Manitoba. The considerably evolved, multiphase, peraluminous, B-, P-, and S-poor leucogranite (K/Rb 132 to 24) was probably generated by fault-friction-assisted anatexis of dominantly metatonalitic rocks and subsequent differentiation. The leucogranite produced interior, transitional, non-crosscutting pods of barren, beryl-columbite- and lepidolite-subtype pegmatites that solidified from local segregations of highly fractionated residual melt. Steep fractionation gradients characterize the granite-to-pegmatite transition, most conspicuously so in the case of the most evolved, Li, Rb, Cs, Be, Mn, Sn, Nb-Ta, F-rich, lepidolite-subtype pod AC #3 (with K/Rb ≥ 16 and Cs 330 ppmwt in accessory K-feldspar, ≥2.5 and ≤11,200 ppmwt, respectively, in lepidolite, Cs ≤28,000 ppmwt in beryl, and Ta/(Ta+Nb) at. ≤ 0.95 in manganotantalite). The Greer Lake example documents beyond any doubt the igneous derivation of lepidolite-subtype pegmatites from a plutonic parent. Most cases of generally very scarce lepidolite-subtype pegmatites obscure this relationship, as the volatile-rich, highly fluid melts stable to relatively low temperatures commonly migrate to great distances from their plutonic sources.     

From granitoid to kyanite-bearing micaschist during fluid-assisted shearing (Ile d’Yeu, France)

We describe amphibolite-facies shear zones affecting an orthogneiss from the Armorican Hercynian belt (Ile d’Yeu, western France). The deformation pattern is consistent with top-to-the-South thrusting followed by E–W extension, as documented elsewhere in the region. Shearing was accompanied by channelled fluid flow that transformed the orthogneiss into a peraluminous micaschist. Structural and mineralogical data indicate rather early strain localization. Then, temperature increase associated with crustal thickening favoured more distributed deformations marked by shear zone stretching and the development of a HT regional foliation. Chemical analyses made across five shear zones show mass transfers that mainly implied losses in Ca and Na, and gains in H₂O, Mg, and K. Most results indicate constant volume transformation, but some suggest records of either gains or losses of volume (between +20% and −30%). This might reflect variable records of fluid-rock interactions according to the timing of initiation and subsequent evolution of individual shear zones, early thrusting stages being marked by up-temperature flow, and late thrusting stages by down-temperature flow. δ¹⁸O analyses suggest that fluids experienced significant isotopic exchange with orthogneisses.     

青海祁漫塔格地区矽卡岩型矿床花岗质岩石矿物学及地质意义

青海祁漫塔格地区位于东昆仑造山带西段,是我国近些年来的重点找矿地区之一。本文对区内与虎头崖矿床I矿带(Fe)、野马泉矿床(Fe)、虎头崖矿床VI矿带(Zn)和卡而却卡矿床B区(Cu)有关的花岗质岩石开展了矿物学研究。结果显示,虎头崖矿床I矿带的斜长石以奥长石为主(An15.0-24.1);虎头崖矿床VI矿带的斜长石主要为钠长石和奥长石(An8.7-20.8);卡而却卡矿床B区的斜长石主要是中长石(An33.8-42.2);野马泉矿床的斜长石主要为奥长石和中长石(An26.2-48.4)。角闪石为典型的钙角闪石,其中,虎头崖矿床I矿带的角闪石属铁浅闪石;卡而却卡矿床B区的角闪石为镁角闪石和铁角闪石。各矿床(带)的黑云母普遍富Fe、Ti、F、Cl,属典型的铁黑云母。矿物学特征显示,各矿床(带)的黑云母结晶于相似的高氧逸度条件下,但在结晶温度和挥发份组成等方面存在差异。黑云母的结晶温度和氧逸度特征与各矿床类型之间未表现出明显的规律性特征。结合前人的研究成果可知,岩浆结晶的温度和氧逸度可能不是控制本区矽卡岩矿床成矿作用类型的主要因素。黑云母的挥发份组成与各成矿元素间表现出明显的规律性特征,其中,以Fe为主的虎头崖矿床I矿带和野马泉矿床的岩浆流体特征基本一致,表现出相对富Cl、贫H2O、F的特点;以Zn为主的虎头崖矿床VI矿带的岩浆流体相对富F贫H2O、Cl;以Cu为主的卡而却卡矿床B区的岩浆流体相对富Cl、H2O贫F,说明岩浆流体不同的挥发份组成可能与不同的矿床类型之间存在密切的联系。综合地质特征和矿物学特征可知,岩浆流体的挥发份组成可能是控制青海祁漫塔格地区矽卡岩矿床成矿作用类型多样性的重要因素之一。     

华北东部晚中生代区域伸展背景下同构造花岗岩体的起源与就位

花岗质岩浆的起源、迁移及就位是研究大陆岩石圈流变学特性的重要方面。然而,板内伸展背景下同构造花岗岩体的岩浆来源、就位机制和岩浆流动与区域应力场的关系等问题缺乏系统性的总结。晚中生代期间华北板块东部逐渐变为区域伸展体制,同时中浅部地壳形成一系列的韧性剪切带、变质核杂岩和拆离断层,这些伸展构造往往伴有同剪切变形的花岗岩体。因此,华北东部是系统研究板内伸展背景下同构造花岗岩体的最佳区域。本文选取多个典型的同构造花岗岩体,进行综合分析。通过归纳总结这些同构造岩体的岩石地球化学和年代学资料,发现多数同构造岩体具有多个岩浆源区,且较早就位的中性岩席(单元)往往来自壳幔混合岩浆或新生下地壳的部分熔融,而较晚的酸性岩席(单元)则主要来源于古老下地壳的部分熔融。这一特点反映了同伸展岩体岩浆源区由深至浅的演化规律,也揭示了区域伸展背景下源自地幔的流体和热量是触发地壳部分熔融的重要因素。通过分析岩浆就位过程中围岩和岩体中形成的定向及变形组构,发现华北东部同伸展岩体的就位模式可分为三大类:以扁平岩床或岩基形式就位于中部地壳的水平韧性剪切带内;岩浆以近直立运移的方式形成长轴平行拆离断层的岩基,就位于变质核杂岩核部或拆离断层下盘;岩浆就位于再活化的先存断裂,通过膨胀作用、挤压围岩获得就位空间并使围岩变形,形成类似底辟作用的就位方式。剪切应力和浮力是影响岩浆运移方向的重要力学参数。岩浆自源区上升的过程中浮力起着主要控制作用,就位于韧性剪切带时剪切应力起着控制作用,就位于浅部地壳的脆-韧性过渡带时浮力的作用再次凸显。     

风化花岗岩崩岗灾害的成因机理

测定了广东珠海大镜山剖面(DJS)、增城剖面(ZC-1、ZC-2)、阳江飞鹅岭和圆周岭剖面(FEI、YZL)的物性指标,通过建立崩岗数学模型,设定不同的坡面参数条件,计算了坡面和稳定临界深度,当坡度＜43°时,坡体的稳定临界深度＞2m。根据雨水在花岗岩风化剖面的渗透特点,计算了一定强度的降雨在不同时间内的湿润前锋,由临界深度与湿润前锋的对比,说明了降雨与崩岗的关系。此外,还结合文献中日本和香港地区崩岗灾害的统计资料结果,探讨了不同时间的降雨对坡体稳定性的影响。     

南岭中西段若干复式花岗岩体的成因模式研究

岩石学、岩石地球化学、黑云母矿物化学和副矿物特征等研究表明，南岭中西段3个复式花岗岩体的演化规律表现为两种方式：一种是以花山复式花岗岩体为代表的不同母岩浆演化方式，其补体美华花岗岩可能经历了与花山主体岩浆相似的分离结晶作用；另一种是以金鸡岭复式花岗岩体和大东山复式花岗岩体为代表的相同母岩浆演化方式，它们的补体螃蟹木花岗岩和猪蹄石花岗岩分别为金鸡岭主体和大东山主体花岗质母岩浆经过分离结晶作用形成。两种不同产出方式的补体花岗岩可能表明它们与主体岩浆的形成机制不同。     

Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton,southern India

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.     

小秦岭文峪和娘娘山花岗岩体锆石SHRIMP U-Pb年龄及其意义

对小秦岭文峪和娘娘山花岗岩体进行的锆石SHRIMP U-Pb定年和岩石地球化学分析结果表明,文峪和娘娘山黑云母二长花岗岩体属于Ⅰ型花岗岩,是太古宙太华群高级变质岩系经部分熔融的产物。它们的成岩年龄分别为138.4±2.5 Ma和141.7±2.5 Ma,略早于本区拆离伸展活动的时限,暗示了中下地壳部分熔融形成的岩浆热穹窿和花岗质岩浆在上地壳的大面积侵位导致了小秦岭变质核杂岩的发育。花岗岩体的成岩年龄都早于本区金矿床的主期成矿时代,从时间上表明了花岗质岩浆活动与金矿成矿作用无直接关系。     

东昆仑东段巴隆花岗质片麻岩年代学、地球化学特征及地质意义

东昆仑东段巴隆南侧古元古界白沙河岩组中可识别出一套花岗质片麻岩。采用LA-MC-ICP-MS方法进行锆石UPb年代学分析,获得加权平均年龄为870.6±7.7Ma(MSWD=2.6)和875.3±6.3Ma(MSWD=0.92),显示该岩体形成于新元古代早期,并经历了早古生代变质作用。地球化学特征显示,岩石具有高Si O2、Al2O3、K2O,低Ti O2、Na2O的特征,A/CNK平均为1.14,A/NK平均为1.47,矿物组合中出现石榴子石和白云母等富铝矿物,显示S型过铝质花岗岩特征;球粒陨石标准化配分曲线显示轻稀土元素富集((La/Yb)N=6.22~25.37),有明显的铕负异常(δEu=0.16~0.44)。原始地幔标准化蛛网图显示,岩石富集Rb、Th,明显亏损Ba、Ta、Nb、Sr、P和Ti元素。综合分析,岩浆来源于陆壳部分熔融,可能为古元古界白沙河岩组长英质岩石发生脱水熔融的产物,形成于同碰撞环境,是东昆仑地区响应Rodinia超大陆汇聚事件的物质记录。     

Triassic magmatism in the eastern part of the South China Block: Geochronological and petrogenetic constraints from Indosinian granites

The granitic dykes in the Badu Group,Zhejiang Province,South China provide important insights on tectonic setting and crustal evolution of the South China Block(SCB) and the Indochina Block during Triassic.Here we report LA-ICP-MS U-Pb data of granitic rocks from the Hucun and Kengkou which show early Triassic ages of 242 ± 2 and 232 ± 3 Ma,respectively,representing their timing of emplacement.The dyke rocks are enriched in K,Al,LREE,Rb,Th.U,and Pb.and are depleted in Nb,Ta,Sr,and Ti.The rocks are characterized by highly fractionated REE patterns with(La/Yb)N ratios of 28.46-38.07 with strong negative Eu anomalies(Eu/Eu* = 0.65-0.73).In situ Hf isotopic analyses of zircons from the Hucun granite yielded ε_(Hf)(t) values of-13.9 to-6.4 and two-stage depleted mantle Hf model ages of 1.68-2.15 Ga,which indicate that the magma was formed by partial melting of the Paleoproterozoic metasedimentary protoliths in the Cathaysia Block.The zircons from the Kengkou granite have ε_(Hf)(t) values ranging from 40.7 to 31.5 and yield two-stage depleted mantle Hf model ages of 0.99-2.49 Ga,indicating magma origin from a mixed source.The Hucun and Kengkou dykes,together with the Triassic A-type granites in SE China were probably generated during magmatism associated with crust-mantle decoupling along the convergent plate boundary between SCB and the Indochina Block.