Rb-Sr and Sm-Nd Mineral Isochron Ages of the Metamorphic Rocks in the Namaqualand Metamorphic Complex, South Africa

Rb-Sr and Sm-Nd isotopic studies were carried out for metamorphic rocks in the Namaqualand Metamorphic Complex, South Africa. The metamorphic rocks give the Rb-Sr mineral isochron ages (whole-rock - biotite - felsic fractions) of 844±85 Ma and 811.6±6.6 Ma for the lower granulite zone and of 776.5±5.4 Ma for the upper granulite zone. The rocks yield the Sm-Nd mineral isochron ages of 1071±18 Ma (whole-rock - garnet - felsic fractions) and 1067±158 Ma (whole-rock - hornblende - biotite rich fraction - felsic fractions) for the lower granulite zone and of 1052.0±3.6 Ma and 1002.5±1.4 Ma (whole-rock - garnet - felsic fractions) for the upper granulite zone. These age data suggest that the granulite facies metamorphism took place at 1060-1000 Ma, and that the rocks cooled down at 850-780 Ma. The Sr and Nd isotopic compositions of metamorphic rocks are different between the lower and upper granulite zones.     

Rb - Sr Whole Rock-Mineral Isochron Ages of Plutonic Rocks from the Wanni Complex, Sri Lanka

Granitoid plutons in different East Gondwana fragments give evidence for vigorous felsic magmatism during the Pan-African period (800 to 500 Ma). The Sri Lankan basement, which is mainly composed of Proterozoic high-grade metamorphic rocks, was intruded by a few late- to post-tectonic syenitic and granitic plutons. Reliable geochronological data for these plutons are few, and some of the available data are inconsistent with the ages of the surrounding metamorphic country rocks.This report presents five Rb-Sr whole-rock-mineral isochron (WRMI) ages and initial Sr isotope ratios of three granitoid plutons from the Wanni Complex, Sri Lanka, namely the Ambagaspitiya, Tonigala and Kotadeniya Granites. Two samples from the Ambagaspitiya Granite yielded ages of 520±5 and 502±15 Ma, with initial Sr ratios of 0.7103±0.0003 and 0.7125±0.0009 respectively, whereas ages of 467±27 and 497±11 Ma and initial Sr ratios of 0.7070±0.0004 and 0.7085±0.0008 were determined for two Tonigala Granite plutons. A single sample from the Kotadeniya Granite gave an age of 533±19 Ma and an initial Sr ratio of 0.7202±0.0018.These ages are consistent with the geological relations between the granitoids and their country rocks, which underwent granulite facies metamorphism between 650 and 550 Ma, and also with available U-Pb zircon ages (550 Ma). The present study gives improved chronological correlation of the felsic magmatism in East Gondwana fragments in Sri Lanka, India, Madagascar, Antarctica and Western Australia. Each of the Sri Lankan plutons exhibits different initial Sr isotope ratios, indicating that their magmas were derived from distinct source materials.     

Geochemical and petrological characteristics of Eppawala phosphate deposits,Sri Lanka

The apatite-bearing carbonate rocks at Eppawala, Sri Lanka occur as massive, discontinuous bodies in a Precambrian, high-grade metamorphic terrain, which weather to form economically important phosphate deposits. The ore bodies at Eppawala contain =42% P ₂O ₅, and citric acid solubility of different components varies from 4 to 6%. The parent rocks are mainly made up of calcite, dolomite and apatite, with lesser amounts of ilmenite, magnetite, pyrite, forsterite, phlogopite, enstatite, magnesite, diopside, tremolite and spinel. Most of minerals show an euhedral habit, with a wide range of crystal sizes (from a few millimetres to several decimetres). The Eppawala rocks are characterised by low silica (=0.41%), high phosphorous (=10.58%) and high strontium content (2,960–6,819 ppm). Concentrations of light rare-earth elements in these rocks are comparably higher than those of marbles. The REE fractionation of these rocks is pronounced, and La/Yb ratios vary between 14 and 43. Both apatite and calcite show markedly elevated strontium levels (=0.6%). The d ¹³C _PDB and d ¹⁸O _SMOW values of the carbonates are in the range of –3.4 to –2.2 and 7.7 to 16.4‰ respectively. The euhedral habit, as well as the presence of major quantities of apatite and considerable amounts of iron-bearing minerals suggest that the ore host rock has genetic links to an igneous source rather than to an intensely metamorphosed limestone. The higher light REE contents of the rocks, compared to marbles, also argue against a metamorphic or sedimentary origin. The Sr/Mn and Ce/La ratios in the apatite are ~40 and ~2 respectively, suggesting that they were formed in a carbonatite magma. The markedly increased REE concentrations in the bulk chemistry of the rocks have been shown to be mainly controlled by the content of phosphate minerals. Compared to most carbonatites, the Eppawala rocks are generally depleted in selected trace elements, particularly Ba, Nb, Th, V, U and Zr. This depletion may be due to either a primary infertility of the parent magma with regard to such trace elements, or it is a result of fractional crystallisation during the rock formation. The stable isotope ratios do not plot within the defined "mantle carbonatite box", but still lie within the broader range of carbonatitic rocks. With these data at hand, it can be readily argued that the mode of occurrence, petrography and geochemistry of the Eppawala apatite-bearing carbonates provide conclusive evidence of their carbonatitic origin.     

Petrogenesis of the Eppawala carbonatites, Sri Lanka: A cathodoluminescence and electron microprobe study

Field and petrographic investigations, cathodoluminescence (CL) studies as well as microprobe analyses of major rock-forming minerals were conducted to establish the crystallization processes in the Eppawala carbonatites, Sri Lanka. The well preserved magmatic textures and crystal morphologies combined with the chemistry of apatite, calcite and dolomite indicate two major stages of crystal growth, which were accompanied by dynamic crystallization conditions. Initially, nucleation of apatite, ilmenite and possibly olivine was associated with rapid crystal growth during slow cooling of the carbonatite melt at depth. The heat loss through the roof and crystallization processes induced the development of turbulent convective currents, which in turn prevented further nucleation and growth of crystals and led to the dispersion of these earlier formed crystals within the magma chamber. Then, rapid upward movement of magma along structural weaknesses led to (i) the transport of mineral clusters, (ii) deformation of ilmenite, (iii) fracturing of apatite and (iv) the emplacement of the carbonatite melt as dykes. Here, the conditions were favourable for the simultaneous crystallization of magnetite, calcite and dolomite in a non-turbulent environment. Subsequent subsolidus alteration caused the hydrothermal overprint of the documented mineral assemblages, particularly along grain boundaries. The study demonstrates that detailed textural examinations of carbonatites combined with mineral chemical analyses and CL investigations can reveal the crystallization processes within carbonatite melts.     

内蒙古西北部宝音图群Sm—Nd和Rb—Sr地质年代学研究

内蒙古西北部的宝音图群由角闪岩相的变质地层组成。该群中斜长角闪片岩的Sm-Nd全岩等时线年龄为2485±128Ma,可代表宝音图群的形成年龄。等时线的ε_(Nd)(t)值为2.73±2.81,说明岩浆来源于亏损的地幔源区。相同样品的Rb-Sr全岩等时线年龄为623±36Ma,显然反映后期Rb-Sr体系重新均一化的强烈影响,因此该年龄被解释为变质年龄。上述结果表明宝音图群形成于古元古代早期,应属华北板块北缘已知的最古老岩石地层单位,并保留了新元古代末期构造—变质事件的记录。这些认识有助于查明华北板块北缘元古宙构造演化的历史。     

内蒙古北山交叉沟地区变质火山岩Sm-Nd同位素研究

交叉沟变质杂岩套主要由斜长角闪岩、片麻岩、长英片岩、绿片岩和条带状混合岩组成 ,为中国西北北山造山带红石山—黑鹰山地体的重要组成部分。长期以来 ,由于缺少可信的同位素年代学数据 ,该套变质岩的形成时代、成岩环境和物质来源尚不清楚。笔者等首次对交叉沟变质杂岩套代表性变质岩进行了 Sm- Nd同位素研究 ,其中斜长角闪岩 Sm- Nd同位素等时线年龄为 12 6 4± 11Ma、其 1 43Nd/ 1 44 Nd初始值为 0 .5 11375± 13,εNd(t)为 7.2 4± 0 .0 2 (2 σ)。与斜长角闪岩样品相比 ,各类片麻岩的 εNd(12 6 4 Ma)值明显偏低 ,变化范围为 1.18～ - 5 .17。依据上述Sm- Nd同位素数据 ,并且结合野外地质证据和岩相学特征 ,认为交叉沟地区斜长角闪岩的原岩为基性火成岩 ,属于幔源基性岩浆活动的产物 ,成岩时代为中元古代 ,而片麻岩的原岩则多为中酸性火成岩 ,其形成过程与古元古代或太古宙古陆壳岩层的再造作用有关。     

赣中变质岩带的Sm-Nd、Rb-Sr同位素年代研究

赣中变质岩带主要由斜长(云母)变粒岩、十字石榴云母片岩、云母石英片岩夹斜长角闪岩组成。斜长角闪岩Sm-Nd全岩等时线年龄为1113±19Ma,相当于该变质岩带的原岩形成年龄。ε_(Nd)(t)值为2.4±0.1,说明岩浆起源于亏损程度较低的地幔源区:726.6±1.1Ma、403.1±6.4Ma的Rb-Sr等时线年龄表明赣中地区在新元古代末期、加里东期经历了一次强烈的构造变质热作用,其地质、地球化学特征及时代可与浙闽地区陈蔡群、建瓯群进行对比。因此赣中变质岩带并非长期公认的华南加里东褶皱带,应是华夏地块的一部分,这一事实对华南大地构造单元的划分及构造演化具有重要意义。     

赣中变质岩带的Sm—Nd,Rb—Sr同位素年代研究

赣中变质岩带主要由斜长(云母)变粒岩、十字石榴云母片岩、云母石英片岩夹斜长角闪岩组成。斜长角闪岩Sm-Nd全岩等时线年龄为1113±19Ma,相当于该变质岩带的原岩形成年龄。ε_(Nd)(t)值为2.4±0.1,说明岩浆起源于亏损程度较低的地幔源区:726.6±1.1Ma、403.1±6.4Ma的Rb-Sr等时线年龄表明赣中地区在新元古代末期、加里东期经历了一次强烈的构造变质热作用,其地质、地球化学特征及时代可与浙闽地区陈蔡群、建瓯群进行对比。因此赣中变质岩带并非长期公认的华南加里东褶皱带,应是华夏地块的一部分,这一事实对华南大地构造单元的划分及构造演化具有重要意义。     

Application of Rb-Sr ratios to gem exploration in the granulite belt of Sri Lanka

The geochemical distribution of rubidium and strontium in the central granulite belt of Sri Lanka, where many of the gem deposits are found, was studied. The Rb-Sr ratios, particularly in the stream sediments, were found to be useful in delineating gem-bearing areas from the non gem-bearing or low potential areas. Among the main gem minerals that are mined at present are corundum, spinel, zircon and tourmaline. It was observed that higher Rb-Sr ratios correspond to high gem potential and, even within areas of good potential, barren areas could be delineated using these ratios. During pegmatite formation, Rb is enriched, and there is a marked depletion of Sr yielding a high Rb-Sr ratio. Pegmatites, granites and other magmatic bodies are associated with gem formation under granulite facies conditions and, when used in conjunction with geology, structure, and mineralogy, the Rb-Sr ratio could be used effectively to delineate target areas for further exploration.     

Weathering of Monuments at Jethawanaramaya Complex in North-Central,Sri Lanka

Abstract: Sri Lanka has the richest archaeological sites in Asia. Jethawanarama Complex, one of the valuable sites in the country, is suffering from deterioration due to weathering. Monuments were built mainly from stones (granitic gneiss and marble) and clay bricks. The present study aimed to categorize weathering forms and interpret the recently-developing weathering processes. The growing of lichens on surfaces and the development of saline conditions are the major threats on the survival of monuments other than the typical weathering processes of tropical climates Morinite (NaCa2Al2[PO4)]2[F,OH]5·2H2O) is identified as a weathering product of monuments and is generated from lichens.     

大别山高级变质岩云母Rb-Sr等时线年龄测定及其地质意义

应用云母Rb-Sr等时线法对大别山高级变质岩的后期改造历史进行了初步的探讨.采用重液变温技术将云母按比重的区别,分成若干组分,然后用常规的Rb-Sr方法测定.应用这种技术对大别山黄土岭太古代麻粒岩相岩石中的黑云母和产于超高压变质带内的石马含石榴石片麻岩的白云母进行了Rb-Sr等时线年龄测定.黄土岭太古代麻粒岩相片麻岩中的黑云母-全岩Rb-Sr等时线年龄为127±9 Ma,与该区片麻岩中的角闪石的K-Ar和Ar-Ar年龄,及燕山期花岗岩的Rb-Sr年龄一致,说明这一太古代下地壳岩石受到了燕山期深熔-岩浆事件的影响;大别山东南部石马含石榴石片麻岩(榴辉岩相)中的白云母Rb-Sr等时线年龄为191±10 Ma,反映了印支-早燕山期区域性低级变质事件的时代.研究表明,大别山区的不同构造单元中产出的不同类型的高级变质岩自中生代以来有不同的后期演化历史.     

辽宁清原变质镁铁质火山岩Sm－Nd同位素年龄及其地质意义

在辽宁清原浑南群变质镁铁质火山岩中首次获得的Ｓｍ－Ｎｄ全岩等时线年龄为３０１８±１１０Ｍａ，证实了本区中太古代表壳岩的存在。浑南群为高角闪岩相至麻粒岩相。变质镁铁质火山岩的原岩相当于ＴＨ１型拉斑玄武岩，稀土配分为轻稀土亏损型模式。岩石的εＮｄ（ｔ）值为＋０．１９±０．１７，物源来自亏损相对较小的地幔。     

桂东南糯垌—安平地区(变质)中—基性火成岩的年代学、地球化学特征及其构造意义

文章对桂东南糯垌—安平地区出露的阳起石化斜长角闪岩(变质基性岩)和中—基性火山角砾岩进行了锆石U-Pb年代学和全岩地球化学分析研究,结果表明,糯垌地区岩体的形成时代晚于123 Ma,安平地区火山角砾岩记录了两期重要的岩浆活动,一组为燕山期(或之后),年龄上限为～138 Ma,一组为早古生代加里东期(452～450 Ma...     

Geochronology of igneous rocks at and near to the Nezhdaninka gold deposit, Yakutia, Russia: U-Pb, Rb-Sr, and Sm-Nd isotopic data

The intrusive rocks associated with the large Nezhdaninka gold deposit (Au > 470 t) hosted in the Permian carbonaceous terrigenous sequence have been dated on zircon and rock-forming minerals with precision U-Pb (ID-TIMS) and Rb-Sr methods. The lamprophyre of the dike complex that occurs in the ore field and spatially is related to gold mineralization has concordant U-Pb zircon age (121 ± 1 Ma) and the same isochron Rb-Sr age (121.0 ± 2.8 Ma). The concordant U-Pb zircon age of granodiorite that dominates in the Kurum pluton is 94 ± 1 Ma, whereas the Rb-Sr isochron age of various intrusive rocks from this pluton is 1–4 Ma younger. This difference is caused by long-term cooling of the Kurum pluton and later closure of Rb-Sr isotopic system of biotite (300–350°C) and other rock-forming minerals as compared with U-Pb isotopic system of zircon (～ 900°C). The Rb-Sr age of quartz diorite from the Gel’dy group of stocks (92.6 ± 0.8 Ma) coincides within uncertainty limits with the age of the Kurum pluton. Thus, the rocks pertaining to two epochs of magmatic activity, which developed in the South Verkhoyansk Foldbelt and divided by a time span of 25–28 Ma, are documented in the Nezhdaninka ore field. Taking into account that the age of gold mineralization is no less than 120 Ma, the data obtained allow us to specify the previously proposed formation model of the Nezhdaninka deposit. These data give grounds to rule out the Late Cretaceous Kurum pluton and the Gel’dy group of stocks from constituents of the ore-magmatic system, and to suggest that an Early Cretaceous deep-seated magma source existed beneath the deposit. Along with host terrigenous rocks, this magma source participated in the supply of matter to the hydrothermal system. The Nd, Sr, and Pb isotopic systematics of igneous rocks and ore mineralization in the Nezhdaninka ore field show that the Early and Late Cretaceous magma sources were formed in the Precambrian crust dated at ～1.8 Ga.     

粤中地区基底变质岩地球化学特征、锆石U-Pb年代学 及其地质意义

粤中地区经历了多期次的区域变质作用改造,形成一套从绿片岩相到角闪岩相的局部有序总体无序的地质体,其中以浅变沉积岩类为代表的云开岩群构成了该地质体的主体.本文对粤中地区基底变质岩进行了岩石地球化学、锆石U-Pb同位素分析.分析结果表明该变质基底具有比上地壳平均成分更高的Al2O3、TiO2以及更低的SiO2、Fe2O3、Na2O、CaO,岩石成熟度中等.Zr,Hf,Th,U等高场强元素轻度富集,说明沉积物的源区以长英质组分为主或主要由再循环物质组成.综合本文和前人的锆石U-Pb定年数据,认为区内云开岩群的形成时代为新元古代晚期.在锆石年龄频率直方图上,3个样品均出现~1000 Ma的峰值,年龄峰值相对较单一而且年龄谐和性较好,表明原岩主要为格林威尔(Grenville)期碎屑物质组成,综合华夏地块其他地区以往的研究资料,本文认为华夏地块南缘可能存在或极其靠近一个Grenvillian期的造山带,其时代可以与全球性的Grenville造山运动对比.     

湖南东坡矿田金船塘、红旗岭锡多金属矿床Rb-Sr、Sm-Nd同位素年代学研究

与千里山岩体有密切时空联系的东坡矿田是我国主要钨锡多金属矿产资源基地之一。在前人对千里山复式岩体成岩时代和金船塘与千里山岩体第一期似斑状花岗岩侵入活动有关的成矿作用研究的基础上,笔者选择金船塘及红旗岭矿床含矿石英脉中的石英和金船塘矿床矽卡岩矿石中的单矿物,进行石英流体包裹体Rb-Sr和矽卡岩矿物(包括符山石和石榴石)Sm-Nd同位素年代学研究,得出金船塘及红旗岭矿床石英流体包裹体Rb-Sr等时线年龄分别为133.4±5.9 Ma(MSWD=1.3)和143.1±8.7 Ma(MSWD=47),金船塘矿床矽卡岩矿物Sm-Nd等时线年龄为141±11 Ma(MSWD=0.27)。同位素年代学研究表明金船塘矿床至少存在164 Ma±、133~141 Ma两期成矿作用,在测定误差范围内,它们分别与千里山岩体第一期似斑状花岗岩(152 Ma)和第二期等粒黑云母花岗岩(136~137Ma)的侵入活动有关,而红旗岭矿床的成矿作用与千里山岩体第二期岩浆侵入活动有关。