The Composition and Origin of the Connemara Dolomitic Marbles and Ophicalcites, Ireland

The Connemara ophicalcites and associated marbles contain varyingproportions of calcite, dolomite, tremolitic amphibole, serpentinizedolivine, diopside, humite, phlogopite, clinochlore, and quartz.They formed from a chemically precipitated ‘primary’dolomite with a small amount of mica and clay minerals in whichmuch of the trace and minor element content of the rock, e.g.Al, Fe, Ti, Zn, Ni, Cr, Zr, K, Rb, and rare earths was concentrated.The rock was probably silicified after deposition, possiblyduring metamorphism, and was probably not deposited with majoradmixed detrital quartz or feldspar. The formation during metamorphismof complementary segregated layers rich in either olivine (Fo_98?2)or calcite resulted from important chemical changes controlledby the composition of the stable metamorphic minerals, i.e.those for which solution concentrations exceeded the solubilityproduct for the pore fluid. The mineralogy influenced the localconcentrations of both major and trace elements and emphasizesthe importance of solutions and the stable metamorphic mineralogyin manipulating the composition of some metamorphic rocks. Somecriteria for recognizing segregated layers in metamorphic rocksare given. Serpentinization was probably by addition of water and silicaand not by movement of Fe or Mg. Chemical analyses of forty-threesamples each for twenty-six oxides and elements are given andthe first occurrence of humite in Connemara is reported.     

The Composition and Origin of the Connemara Dolomitic Marbles and Ophicalcites, Ireland

The Connemara ophicalcites and associated marbles contain varyingproportions of calcite, dolomite, tremolitic amphibole, serpentinizedolivine, diopside, humite, phlogopite, clinochlore, and quartz.They formed from a chemically precipitated 'primary' dolomitewith a small amount of mica and clay minerals in which muchof the trace and minor element content of the rock, e.g. Al,Fe, Ti, Zn, Ni, Cr, Zr, K, Rb, and rare earths was concentrated.The rock was probably silicified after deposition, possiblyduring metamorphism, and was probably not deposited with majoradmixed detrital quartz or feldspar. The formation during metamorphismof complementary segregated layers rich in either olivine (Fo_98±2)or calcite resulted from important chemical changes controlledby the composition of the stable metamorphic minerals, i.e.those for which solution concentrations exceeded the solubilityproduct for the pore fluid. The mineralogy influenced the localconcentrations of both major and trace elements and emphasizesthe importance of solutions and the stable metamorphic mineralogyin manipulating the composition of some metamorphic rocks. Somecriteria for recognizing segregated layers in metamorphic rocksare given. Serpentinization was probably by addition of water and silicaand not by movement of Fe or Mg. Chemical analyses of forty-threesamples each for twenty-six oxides and elements are given andthe first occurrence of humite in Connemara is reported.     

大别山北部超高压变质大理岩及其地质意义

岩石学研究表明 ,大别山北部镁铁 超镁铁质岩带中白云质大理岩至少经历过三期变质阶段 :(1)榴辉岩相峰期变质阶段 ,矿物组合主要为方解石 +白云石 +金红石 +镁橄榄石 +钛 斜硅镁石 +富镁的钛铁矿±文石±石榴子石 ;(2 )麻粒岩相退变质阶段 ,矿物组合主要为方解石 +白云石 +金云母 +镁橄榄石 +透辉石 +钛铁矿 +尖晶石±斜方辉石等 ;(3)角闪岩相退变质阶段 ,主要矿物组合为方解石 +白云石 +磷灰石 +磁铁矿+榍石等。它的峰期变质矿物组合 ,类似于苏 鲁超高压大理岩 ,形成压力至少大于 2 .5GPa。这进一步证明 ,大别山北部大多数高级变质岩 (包括大理岩等 )都曾经过超高压变质作用 ,应属于印支期扬子俯冲陆壳的一部分。     

Ultra-high-pressure (UHP) marble and eclogite in the Su-Lu UHP terrane, eastern China

A large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the Rongcheng area of the Su-Lu province, eastern China. The marble consists mainly of dolomite, calcite (formerly aragonite), graphite, forsterite, diopside, talc, tremolite and phlogopite. Aggregates of talc and calcite occur at the boundary between dolomite and diopside. Tremolite is a reaction product between talc and calcite. Eclogite blocks are rimmed by dark green amphibolite. The primary mineral assemblage in the core of eclogite is Na-bearing garnet (up to 0.2  wt% Na₂O), omphacitic pyroxene, clintonite and rutile. Secondary minerals are pargasitic/edenitic amphibole, plagioclase, sodic diopside, chlorite, zoisite and titanite. The peak metamorphic conditions, based on stability of the dolomite+forsterite+aragonite (now calcite)+graphite assemblage, under conditions where tremolite is unstable, are estimated at T  =610–660 °C and P =2.5–3.5  GPa (for X _{CO=0.001). A reaction between dolomite and diopside to form talc under tremolite-unstable conditions indicates a temperature decrease under ultra-high-pressure conditions (} P >2.4  GPa, X _{CO<0.0013). The formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post-dating the ultra-high-pressure metamorphism. The temperature decrease under ultra-high-pressure conditions preceding decompression may reflect the underplating of a cold slab, and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root. The} P – T  conditions of the amphibolitization stage are estimated at <0.9  GPa and <460 °C, and are similar to conditions recorded by the surrounding orthogneisses.     

Exsolution of dolomite and application of calcite–dolomite solvus geothermometry in high‐grade marbles: an example from Skallevikshalsen,East Antarctica

Calcite–dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium‐ to high‐grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite–dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite ± apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain ‘negative crystal’ inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850–870 °C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite–graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re‐equilibration of calcite that continued until Mg diffusion ceased at ～460 °C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub‐grain boundaries and formation of fine‐grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high‐grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions.     

Fluid-rock history of granulite facies humite-marbles from Ambasamudram, southern India.

An extensive humite‐bearing marble horizon within a supracrustal sequence at Ambasamudram, southern India, was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics. At peak metamorphic temperatures of 775±73°C, based on calcite‐graphite carbon isotope thermometry, the mineral assemblages suggest layer‐by‐layer control of fluid compositions. Clinohumite + calcite‐bearing assemblages suggest X_CO2 < 0.4 (at 700°C and 5 kbar), calcite + forsterite + K‐feldspar‐bearing assemblages suggest X_CO2>0.9 (at 790°C); and local wollastonite + scapolite + grossular‐bearing zones formed at X_CO2 of c. 0.3. Retrograde reaction textures such as scapolite + quartz symplectites after feldspar and calcite and replacement of dolomite + diopside or tremolite+dolomite after calcite+forsterite or calcite+clinohumite are indicative of retrogression under high X_CO2 conditions. Calcite preserves late Proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration, while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting. Isotopic zonation of calcite and graphite grains was likely produced by localized CO₂ fluid infiltration during retrogression. Contrary to the widespread occurrence of humite‐marbles related to retrograde aqueous fluid infiltration, the Ambasamudram humite‐marbles record a prograde‐to‐peak metamorphic humite formation and retrogression under conditions of low X_H2O.     

Metamorphic Conditions and Fluid Compositions of Scapolite-Bearing Rocks from the Lapis Lazuli Deposit at Sare Sang, Afghanistan

Scapolite and other halogen-rich minerals (phlogopite, amphibole,apatite, titanite and clinohumite) occur in some high-pressureamphibolite facies calc-silicates and orthopyroxene-bearingrocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan.The calc-silicates are subdivided into two groups: garnet-bearingand garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite,the amphibolite facies mineral assemblages in the calc-silicatesinclude clinopyroxene, calcite, quartz and one or more of theminerals scapolite, plagioclase, K-feldspar, titanite, apatiteand rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene,garnet, plagioclase, scapolite, amphibole, quartz, calcite andaccessory dolomite and alumosilicate (kyanite?). Retrogradephases in the rocks are plagioclase, scapolite, calcite, amphibole,sodalite, haüyne, lazurite, biotite, apatite and dolomite.The clinopyroxene is mostly diopside and rarely also hedenbergite.Aegirine and omphacite with a maximum jadeite content of 29mol % were also found. Garnet from the calc-silicates is Grs_45–95Py_0–2and from the orthopyroxene-bearing rocks is Grs_10–15Py_36–43.Peak P–T metamorphic conditions, calculated using availableexchange thermobarometers and the TWQ program, are 750°Cand 1·3–1·4 GPa. Depending on the rock type,the scapolite exhibits a wide range of composition (from Eq_An= 0·07, X_Cl =0·99 to Eq_An = 0·61, X_Cl =0·07).Equilibria calculated for scapolite and coexisting phases atpeak metamorphic conditions yield X_CO2 = 0·03–0·15.X_NaCl (fluid), obtained for scapolite, ranges between 0·04and 0·99. Partitioning of F and Cl between coexistingphases was calculated for apatite–biotite and amphibole–biotite.Fluorapatite is present in calc-silicates, but orthopyroxene-bearingrocks contain chlorapatite. Cl preferentially partitions intoamphibole with respect to biotite. All these rocks have sufferedvarious degrees of retrogression, which resulted in removalof halogens, CO₂ and S. Halogen- and S-bearing minerals formedduring retrogression and metasomatism are fluorapatite, sodalite,amphibole, scapolite, clinohumite, haüyne, pyrite, andlazurite, which either form veins or replace earlier formedphases. KEY WORDS: scapolite; fluid composition; high-pressure; amphibolite facies; Western Hindukush; Afghanistan     

Petrology of chlorite-spinel marbles from NW Spitsbergen (Svalbard)

Small dolomite marble lenses and bands occur in the vast Caledonian migmatite and gneiss area of NW Spitsbergen (Svalbard archipelago). The fine-banded marbles contain numerous assemblages of minerals: calcite, dolomite, olivine, clinohumite, diopside, amphibole, chlorite, spiner and phologopite. The coexistence of calcite + dolomite + olivine + chlorite + spinel over the entire area indicates metamorphic temperatures of 600 to 680° at an estimated pressure of 4 kilobars. A temperature of near 600°C for the peak of metamorphism is suggested by mineral assemblages at the southernmost locality, Jäderinfjellet. Calcite-dolomite geothermometry indicated 595°C at the same locality. The spatial distribution of the marble assemblages suggests that metamorphism occurred under nearly isothermal conditions over an area of at least 25 by 30 kilometres.     

The distribution of certain elements among coexisting calcic pyroxenes, calcic amphiboles and biotites in skarns

Skarns composed predominantly of Ca pyroxene, Ca amphibole, and biotite occur locally in the Grenville metamorphic terrain of the Canadian pre-Cambrian Shield. The enclosing rocks are Grenville gneisses and limestones, gabbroic rocks, veined gneisses, and granitic rocks. The metamorphic grade of the skarns is identical to that of the enclosing rocks, namely the upper region of the amphibolite facies.

Twelve skarn specimens were selected for chemical study. An investigation is made of the concentrations of Al, Fe, Mg, Mn, Ti, Ca, Na, K, Ba, V, Cr, Zr, Y, and Sc in eleven Ca pyroxenes, ten Ca amphiboles, and eleven biotites.

Consideration is given to the distribution of elements among coexisting minerals. Regular relationships appear when

1. (1) the concentration of an element in a mineral is related to the concentration of the element in a coexisting mineral by a linear or non-linear function

2. (2) the linear or non-linear function is itself a function of the concentration of another element in one or both of the coexisting minerals. These relationships indicate that, for the most part, minerals within each skarn specimen closely approached a state of chemical equilibrium.

A preliminary attempt is made to explain the distribution relationships in terms of crystal chemistry.     

Mechanism of Mineral Reactions Inferred from Textures of Impure Dolomitic Marbles from East Greenland

Mineral assemblages and chemical compositions of minerals foundin impure dolomitic marbles embedded in gneisses and migmatitesof the E. Greenland Caledonian fold belt (Scoresby Sund) suggestthat the marbles were metamorphosed near 630 °C at 5 kbpressure. The analysis of complex textural and mineralogicalrelations among minerals such as dolomite, calcite, forsterite,pargasite, chlorite, spinel, diopside and phlogopite led tothe conclusion that the major mineralogical features of therocks were probably caused by sodium metasomatism at constanttemperature and pressure. The effect of the inferred sodiummetasomatism may be summarized by three schematic reactionsall involving modal changes of excess dolomite, calcite, forsterite,chlorite and spinel: (a) nucleation and growth of pargasite,(b) resorption of phlogopite, and (c) growth of pargasite fromphlogopite.     

Geochemical and Mineralogical Characteristics of Pleistocene Lignites and Associated Sediments of Marathousa Coal Field,Central Peloponnese,Greece

The mineralogy and geochemistry data are presented for thirty-seven shales,four concretions,two carbonate sediments and seven lignites from the Marathousa coal field of the Megalopolis Basin in Greece.The argillaceous rocks consist of chlorite,illite,kaolinte,albite,quartz.opal-A,calcite and dolomite;the concretions of aragonite,gypsum and pyrite;and the carbonate rocks of calcite,quartz and illite.The mineral matter in the lignites consists of gypsum,quartz,albite,chlorite,illite,opal-A,dolomite,pyrite,and rarely calcite and kaolinite Athree-factor model explains the total variaition of major and trace elements in the argillaceous sediments.The first factor is an aluminosilicate factor and involves the following elements:Al,Si,Mg,Na,K,Ti,Mn,Nb,Y,Rb,Zn,Cu,Ni,Cr,Nband V,associated with chlorite,albite and illite.The second factor involves the elements Ca,Sr,Ba,Znand Sc and is related to carbonate lithology and mainly the carbonate concretions with gypsum.The third factor involves Fe and Ce with a weak association with Mn.The diagenesis of the Marathousa sediments and lignites was not very advanced as indicated by (a) the total thickness of the sequence (500m),(b) the presence of biogenic silica(opal-A) and (c) the age of the deposit(Pleistocene).FOr these reasons the rpresence of chlorite,illite and kaolinite in the sediments and lignite is due not to diagenetic reactions but to weathering of the flysch and metamorphic rocks at the edges of the Megalopolis Basin and transport of the weathering products(illite,chlorite,kaolinite)into the basin of deposition.The diagenetic minerals of the Marathousa sequence include pyrite,gypsum,dolomite and aragonite.     

Contact Metamorphism of Serpentinite, Chloritic Blackwall and Rodingite at Paddy-Go-Easy Pass, Central Cascades, Washington

The 2 km wide contact aureole produced from serpentinite bythe intrusion of the Mount Stuart Batholith into the IngallsComplex at Paddy-Go-Easy Pass contains the following ultramaficassemblages, in order of increasing grade: serpentine-forsterite-diopside,serpentine-forsterite-tremolite, forsterite-talc, forsterite-anthophyllite,forsterite-enstatite-anthophyllite, forsterite-enstatite-chlorite,forsterite-enstatite-spinel. Associated metarodingites displayfive metamorphic zones, the diagnostic assemblages of whichare, in increasing grade: grossular-idocrase-chlorite, grossular-diopside-chlorite,epidote-diopside-chlorite, epidotediopside-spinel, plagioclase-grossular-diopside.Mafic hornfels in the aureole contains no orthopyroxene, indicatingthat the conditions of pyroxene hornfels facies were not reached. The breakdown of chlorite is best displayed in aluminous blackwallreaction zones around mafic inclusions in the peridotite. Attemperatures above those of the anthophyllite-out isograd, butwithin the field of forsterite+tremolite, these chlorite-richrocks react to form the assemblage: forsterite-enstatite-spinel.Calculations show that cordierite did not form as a result ofchlorite breakdown in the natural system because impurities,such as iron and chromium, displaced the equilibrium: forsterite+cordierite= enstatite+spinel to much lower pressures than the three kilobarsfound in the pure system. The primary chromite of the peridotite has been altered to chrome-magnetitein the serpentinite. This alteration seems to be isochemicalover the whole rock, as true chromite, formed by metamorphism,occurs at grades above that of the forsterite-enstatite-anthophylliteassemblage. Calcic amphibole in high-grade metaperidotite is tremolite,even in the presence of aluminous chromite, whereas that inmetamorphosed blackwall rock grades from tremolite into hornblende.The pattern of substitution appears to be: Mg₂Si₃rlhar2;(Na,K)(Al^VI)₂(Al^IV)₃.     

Sodic amphiboles and pyroxenes from fenites in East Africa

During alkali metasomatism of the country-rock associated with ijolite-carbonatite complexes the development of sodic amphibole and/or pyroxene is characteristic. In this paper, some new chemical analyses of these minerals, together with published analyses from fenites of Kenya, Uganda and Tanzania, include those of co-existing pairs of amphibole and pyroxene. The common amphiboles of the fenites are magnesioarfvedsonites with 100 Mg: Mg+Fe+Mn ranging from 67 to 36. They co-exist with aegirines having 0.75 to 0.89 ions Fe⁺³. Most of these minerals are poor in Ca; co-existing pairs tend to show corresponding increases in Ca and in Fe⁺². In the syenitic fenites of Tororo and Budeda, considered to have formed at higher temperatures, the stable mineral is aegirine-augite. New analyses of richterite, magnesioarfvedsonite and aegirine from carbonate-rich rocks are also presented, and the relation between fenites and carbonatites is discussed.     

Oxygen and Carbon Isotope Studies of Contact Metamorphism of Carbonate Rocks

O¹⁸/O¹⁶ and C¹²/C¹³ ratios have been determined for carbonaterocks and coexisting minerals from two contact metamorphic aureolesat Birch Creek, California and Marble Canyon, Texas. The peliticmetasediments and granitic intrusions of the Birth Creek localitywere also analyzed for their O¹⁸/¹⁶ and D/H ratios. Oxygen andcarbon isotope fractionations in coexisting dolomite and calciteare interrelated but show no obvious correlation with sampledistance from the intrusive contact. Small-scale oxygen isotopicexchange effects between rock units are observed within a fewfeet of the intrusive-country rock contacts and the marble-schistcontacts at Birch Creek. Oxygen isotopic temperatures obtainedfrom quartz—biotite fractionations in the biotite schistsof Birch Creek show a systematic decrease with increasing distanceaway from the intrusive contact; the isotopic contact temperatureis calculated to be 535–45°C. Low O¹⁸/¹⁶ and C¹³/¹²ratiosof the contact metamorphic marbles generally correlate wellwith presence of calc-silicate minerals, indicating that theCO₂ liberated during metamorphic decarbonation reactions isenriched in both O¹⁸and C¹² relative to the carbonates. Materialbalance calculations indicate that the liberated CO₂ is about5 per mille richer in O¹⁸ and about 6 per mille richer in C¹³than coexisting calcite.     

Contact Metamorphism of Serpentinite, Chloritic Blackwall and Rodingite at Paddy-Go-Easy Pass, Central Cascades, Washington

The 2 km wide contact aureole produced from serpentinite bythe intrusion of the Mount Stuart Batholith into the IngallsComplex at Paddy-Go-Easy Pass contains the following ultramaficassemblages, in order of increasing grade: serpentine-forsterite-diopside,serpentine-forsterite-tremolite, forsterite-talc, forsterite-anthophyllite,forsterite-enstatite-anthophyllite, forsterite-enstatite-chlorite,forsterite-enstatite-spinel. Associated metarodingites displayfive metamorphic zones, the diagnostic assemblages of whichare, in increasing grade: grossular-idocrase-chlorite, grossular-diopside-chlorite,epidote-diopside-chlorite, epidote-diopside-spinel, plagioclase-grossular-diopside.Mafic hornfels in the aureole contains no orthopyroxene, indicatingthat the conditions of pyroxene hornfels facies were not reached. The breakdown of chlorite is best displayed in aluminous blackwallreaction zones around mafic inclusions in the peridotite. Attemperatures above those of the anthophyllite-out isograd, butwithin the field of forsterite+tremolite, these chlorite-richrocks react to form the assemblage: forsterite-enstatite-spinel.Calculations show that cordierite did not form as a result ofchlorite breakdown in the natural system because impurities,such as iron and chromium, displaced the equilibrium: forsterite+cordierite=enstatite+spinelto much lower pressures than the three kilobars found in thepure system. The primary chromite of the peridotite has been altered to chrome-magnetitein the serpentinite. This alteration seems to be isochemicalover the whole rock, as true chromite, formed by metamorphism,occurs at grades above that of the forsterite-enstatite-anthophylliteassemblage. Calcic amphibole in high-grade metaperidotite is tremolite,even in the presence of aluminous chromite, whereas that inmetamorphosed blackwall rock grades from tremolite into hornblende.The pattern of substitution appears to be: Mg₂Si₃(Na,K)(Al^VI)₂(Al^IV)₃.     

X射线粉晶衍射仪在大理岩鉴定与分类中的应用

大理岩主要有方解石大理岩、白云石大理岩和菱镁矿大理岩三种。以往大理岩是依据偏光显微镜下观察岩石结构构造及矿物成分进行分类定名,由于方解石、白云石、菱镁矿都属于三方晶系,具有闪突起、高级白干涉色、一轴晶负光性和菱形解理等相同晶体光学特征,偏光显微镜下区分十分困难。为了准确鉴定大理岩中碳酸盐矿物种类及其相对含量,本文利用岩石薄片偏光显微镜和X射线粉晶衍射技术对32件大理岩岩石样品进行分析测试。岩石薄片鉴定结果表明:大理岩造岩矿物主要有方解石、白云石、菱镁矿、石英、斜长石、白云母、黑云母、绿泥石、黏土和金属矿物。根据岩石结构构造及矿物组分特征,可把32件大理岩样品划分为方解石大理岩、长英质方解石大理岩、石英绿泥白云石大理岩、白云石大理岩、云英质白云石大理岩和菱镁矿大理岩等15个类型。X射线粉晶衍射分析表明:大理岩造岩矿物主要有方解石、白云石、菱镁矿、石英、斜长石、钾长石、云母、绿泥石、滑石和蒙脱石。综合分析认为:岩石薄片偏光显微镜鉴定技术很难区分方解石、白云石和菱镁矿等碳酸盐矿物,以及细小的石英、钾长石和斜长石、滑石和白云母等鳞片状硅酸盐矿物;X射线粉晶衍射分析技术不仅能准确检测出大理岩中方解石、白云石和菱镁矿等碳酸盐矿物种类及相对含量(方解石、白云石和菱镁矿的X射线衍射主峰有明显差异,d值分别为0.303 nm、0.288 nm和0.274 nm),而且能够有效鉴别岩石中粉砂级斜长石、钾长石与石英(三种矿物的X射线衍射主峰d值分别为0.319 nm、0.324 nm、0.334 nm);且能区分蒙脱石、绿泥石、云母和滑石等层状硅酸盐矿物(四种硅酸盐矿物的X射线衍射主峰d值分别为1.400 nm、0.705 nm、0.989 nm、0.938 nm)。综合岩石薄片偏光显微镜鉴定和X射线粉晶衍射分析结果,最终确定32件大理岩样品划分为22个岩石类型。研究认为:仅根据岩石薄片偏光显微镜鉴定或X射线粉晶衍射技术其中一种方法不能准确鉴定大理岩岩石,应将大理岩岩石野外观察、岩石薄片鉴定和X射线粉晶衍射技术结合起来,才能准确确定大理岩岩石类型。     

内蒙古大青山地区孔兹岩系中大理岩岩组的地球化学特征及原岩建造

大理岩岩组是内蒙古大青山地区孔兹岩系3个岩组中最上部的一个岩组,该岩组是一套富镁质的大理岩系,以巨厚的、大面积分布的白云质大理岩的发育为特征,底部有少量碎屑岩沉积,矿物组成除白云石外,普遍含蛇纹石化橄榄石、透辉石、金云母等一种或多种富镁硅酸盐矿物。岩石地球化学以富CaO、MgO和LOI为特征,反映了碳酸盐岩的地球化学特征,CaO＋MgO含量达44.96%以上,且CaO/MgO比值较小（1.37～10.94）,其原岩应为一套碎屑含量极低的白云岩,以化学沉积为主。原岩建造的沉积环境：由初期的陆海过渡的三角洲相,过渡到滨海-浅海相,最后到干燥气候条件下有障蔽岛相隔的封闭海盆相。构造环境也从亚稳定状态过渡到稳定构造环境,表明该套变质沉积地层的原岩可能形成于被动大陆边缘,但更类似于中元古界长城系、蓟县系这样的克拉通内裂陷槽或裂谷沉积。     

Mineralogical Evidence for Fluid-Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA

Contact metamorphism of siliceous dolomite in the southern partof the metamorphic aureole of the Alta stock (Utah, USA) producedthe prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite(Fo), and periclase (Per). Calcite (Cc)–dolomite (Do)geothermometry and phase equilibria define a general progradeT–X(CO₂) path of decreasing X(CO₂) with rising temperaturefor the dolomite. High-variance assemblages typify the aureole.Per + Cc and Fo + Cc + Do characterize the inner aureole (Perand Fo zones), and Tr + Do + Cc and Tc + Do + Cc are widespreadin the outer aureole (Tr and Tc zones). Low-variance assemblagesare rare and the thickness of reaction zones (coexisting reactantand product minerals) at the isogradic reaction fronts are narrow(tens of metres or less). The mineral assemblages, calculatedprogress of isograd reactions, and the prograde T–X(CO₂)path all indicate that massive dolomite was infiltrated by significantfluxes of water-rich fluids during prograde metamorphism, andthat the fluid flow was down-temperature and laterally awayfrom the igneous contact. Fluid infiltration continued throughat least the initial retrograde cooling of the periclase zone.Down-T fluid flow is also consistent with the results of Cc–Dogeothermometry and patterns of ¹⁸O depletion in this area. Theclose spatial association of reacted and unreacted chert nodulesin both the tremolite and talc zones plus the formation of tremoliteby two reactions indicate that the outer aureole varied in X(CO₂),and imply that fluid flow in the outer aureole was heterogeneous.The occurrence of dolomite-rich and periclase (brucite)-absent,high-     

A model for the distribution of trace elements between calcite and dolomite

A crystal-growth model is proposed, which allows ions of a trace element to enter the Ca and Mg sites of dolomite in proportion to the size of the ions relative to that of Ca and Mg ions, and which assigns equal portions of the trace element to the Ca site of dolomite and the Ca site of associated calcite. The model produces calcite/dolomite distribution coefficients of 0.79 for Mn and 0.43 for Fe, which may be compared with 0.85 and 0.28 as observed in marble, and a distribution coefficient of 2.0 for Sr and Ba, which may be compared with observed values of 2.3 for Sr and 1.8 for Ba.     

Mineralogy and genesis of the metamorphosed manganese silicate rocks (gondite) of Gowari Wadhona,Madhya Pradesh,India

Manganese silicate rocks, interbanded with manganese oxide orebodies, constitute an important stratigraphic horizon in the Mansar formation of the Sausar Group of Precambrian age in India. The manganese silicate rocks of Gowari Wadhona occupy the westernmost flank of the manganese belt of the Sausar Group. These rocks are constituted of spessartite, calcium-rich rhodonite, quartz, manganoan diopside, blanfordite (manganese bearing member of diopside-acmite series), brown manganese pyroxene (manganese bearing aegirine-augite), winchite (manganese bearing richterite-tremolite), juddite (manganese bearing amphibole with richterite, tremolite, magnesioriebeckite and glaucophane molecules), tirodite (manganese bearing amphibole with richterite, cummingtonite and glaucophane molecules), manganophyllite, alurgite, piedmontite, braunite, hollandite (and other lower oxides of manganese) with minor apatite, plagioclase, calcite, dolomite and microcline. A complete mineralogical account of the manganese-bearing phases has been given in the text. It has been shown that the juxtaposition of manganese silicate rocks with dolomitic marble, regional metamorphism to almandine-amphibolite facies and assimilation of pegmatite veins cutting across the manganese formation, were responsible for the development of these manganese silicate rocks and the unusual chemical composition of some of the constituent minerals. It has been concluded that the manganese silicate rocks of Gowari Wadhona were originally laid down as sediments comprising manganese oxides admixed with clay, silica etc. and were later regionally metamorphosed to almandine-amphibolite facies. All evidences indicate that rhodochrosite was not present in the original sediment and the bulk composition of the sediments was rich in manganese. These rocks agree entirely to the detailed nomenclature of the gondites enunciated by Fermor (1909) and amplified by Roy and Mitra (1964) and Roy (1966).