The role of the grain boundary at chemical and isotopic fronts in marble during contact metamorphism

Carbon and oxygen isotopic profiles around a low pressure metasomatic wollastonite reaction front in a marble of the Hida metamorphic terrain, central Japan, display typical metamorphic fluid-enhanced isotopic zonations. Isotopic profiles obtained from detailed microscale analyses perpendicular to the chemical reaction front in calcite marble show that diffusion-enhanced isotopic exchange may control these profiles. Carbon and oxygen isotopic behaviour in grain boundaries is remarkably different. Oxygen isotopic troughs (¹⁸O depleted rims) around the calcite-grain boundaries are widely observed in this contact aureole, demonstrating that diffusion of oxygen in calcite grain boundary dominates over lattice diffusion in calcite. In contrast, no difference is observed in carbon isotopic profiles obtained from grain cores and rims. There is thus no specific role of the grain boundary for diffusion of carbonic species in the metamorphic fluid during transportation. Carbon chemical species such as CO₂ and CO₃ ions in metamorphic fluid migrate mainly through lattice diffusion. The carbon and oxygen isotope profiles may be modelled by diffusion into a semi-infinite medium. Empirically lattice diffusion of oxygen isotopes is almost six times faster than that of carbon isotopes, and oxygen grain-boundary diffusion is ten times faster than oxygen lattice diffusion. Oxygen isotopic results around the wollastonite vein indicate that migration of the metamorphic fluid into calcite marble was small and was parallel to the aquifer. From the stability of wollastonite and the attainment of oxygen isotopic equilibrium, we suggest that diffusion of oxygen occurred through an aqueous fluid phase. The timescale of formation of the oxygen isotopic profile around the wollastonite vein is calculated to be about 0.76 × 10⁶ years using the experimentally determined diffusion constant. Received: 14 January 1997 / Accepted: 23 April 1998     

板峪口组大理岩中的变质流体

山西五台山区板峪口组大理岩的总体矿物组合为透闪石、金云母、白云石、方解石、微斜长石和石英,岩石变质时受缓冲作用控制。口泉主沟内绿帘石脉体中流体的X_(CO_2)为0.08,而围岩白云岩中X_(CO_2)大于0.4,同一地点脉体和围岩中变质溶液具有不同的X_(CO_2)说明溶液成分受缓冲作用控制。绿帘石脉体内溶液成分保持X_(CO_2)=0.8不变则说明溶液成分受渗滤作用控制。围岩内矿物组合为金云母、透闪石、方解石和白云石也说明溶液成分受渗滤作用控制。总的说来,本地区的变质溶液成分是缓冲作用加渗滤作用的综合结果。 本区变质时所通过的流体数量一般不超过岩石体积的1/4。当岩石内有单矿物脉体时,脉体内所通过的流体数量较高。绿帘石脉内所通过的流体大致相当于岩石体积(99％)。     

Occurrence, Mineral Chemistry, and Metamorphism of Precambrian Carbonate Rocks in a Portion of the Grenville Province

Marble occurs abundantly in a 31,000 km² segment of the southernGrenville Province of the Canadian Precambrian Shield, whereit is associated with quartzite, biotite-garnet gneiss, andamphibolite to form the Grenville Group. An 1800 km² area onthe western margin of this segment, north of the Ottawa river,displays a great variety of carbonate rocks, which may be dividedinto two groups: (I) major marble, with calcite, dolomite, graphite, phlogopite,Ca amphibole, Ca pyroxene, forsterite, humite group minerals, (II) minor marble, with pink calcite, phlogopite, Ca amphibole,Ca pyroxene, K feldspar, scapolite, sphene. Rocks of the first group are associated with plagioclase gneissand amphibolite, and are metamorphosed limestone, little affectedby metasomatism; rocks of the second group, which are less common,are associated with potassium feldspar gneiss and heterogeneousgranitic and syenitic rocks, and are inferred to be metasomaticrocks. Numerous mineral reactions have taken place in the carbonaterocks during metamorphism. The calcite-dolomite reaction, whichgoverns the Mg content of calcite, indicates a metamorphic temperatureof about 650 °C. Forsterite was possibly produced from low-Alamphibole, and forsterite + spinel from high-Al amphibole. Thecrystallization of some silicate minerals in the minor marbleunits, and the enrichment in the contained calcite in Fe andSr are attributed to metasomatic reactions. Metamorphic ion-exchangereactions involving carbonates produced the following distributioncoefficients: Sr in calcite/Sr in dolomite = 2.5 Mn in calcite/Mn in dolomite = 0.89 Fe in calcite/Fe in dolomite = 0.29 from which inferences may be drawn concerning the distributionof these elements between the Ca and Mg sites within dolomiteduring metamorphic crystallization. Ion-exchange reactions involvingsilicates produced the following distribution of Mn: humite group Ca pyroxene.Ca amphibole phlogopite where the numbers are distribution coefficients. An equilibriumdistribution of Fe between silicates and calcite in the minormarble was evidently not attained during metasomatic crystallization.Numerous retrograde reactions have taken place, including thealteration of pyroxene to amphibole, forsterite to serpentine,and the exsolution of dolomite from calcite. Forsterite in marble, and orthopyroxene in the associated gneissesand amphibolites crystallized sporadically in the Laurentianhighlands, but not in the lowlands of the Ottawa rift valley,where peak metamorphic temperatures may have been slightly lower.In the highlands, reactions to produce forsterite and orthopyroxenewere initiated in response to a local increase in temperature,local peculiarities in the chemical composition of amphibole,which produced these minerals, or a local decrease in the activityof CO₂ and H₂O in the grain-boundary phase.     

40Ar/39Ar-dating of phlogopite from mantle xenoliths: Evidence for deep ancient metasomatism of the Siberian craton lithosphere

By applying the ⁴⁰Ar/³⁹Ar-dating method, age estimates for phlogopites of mantle xenoliths with different parageneses from the Udachnaya and Mir kimberlite pipes (Yakutia, Russia) were obtained. The oldest ages determined are 2.6–2.3 Ga, which far transcends the Paleozoic age of kimberlite entrainment. The phlogopite formation of these ages reflects ancient metasomatic events following rearrangement processes in the mantle in the Archean-Early Proterozoic, particularly during and after accretion of the Pangea-0 super-continent. A multistep age spectrum of UV162/09 was obtained from several generations of phlogopite and indicates a later multistage metasomatic process taking place in the mantle under Udachnaya pipe. Several stages of mantle metasomatism of various ages and scales are detected within the Siberian platform.     

Occurrence of Halogen-rich Phlogopite in Late Cenozoic Volcanic Rocks in the Japanese Arcs

Abstract. Halogen-rich phlogopite occurs in the groundmass of andesite and dacite lavas from Late Tertiary to Quaternary volcanoes associated with native sulfur and limonite deposits (Shiretoko-Iwozan, Hachimantai, Adatara, Omeshidake, Masaki) and hydrothermal ore deposits (Harukayama, Muineyama, Hishikari) in Japan. The F contents of the halogen-rich phlogopite range from 3.6 to 5.7 wt%, corresponding to atomic F/(F+C1+OH) ratios ranging from 0.45 to 0.69. On the other hand, the Cl contents of the halogen-rich phlogopite are around 0.2 wt%. The atomic Mg/(Mg+Fe) ratios range from 0.69 to 0.83.
The fluorine intercept value [IV(F)] defined by Munoz (1984) of the phlogopites ranges from 0.79 to 3.17, and the chlorine intercept value [IV(Cl)] ranges from -7.11 to -7.77. The observed IV(F) of the phlogopites broadly overlap the range of the IV(F) for biotites from porphyry copper deposits. On the other hand, the observed IV(Cl) are significantly lower than the IV(Cl) for biotites from porphyry copper deposits. Whereas the F contents of the phlogopite appear more prominent compared to the Cl contents, the calculation of halogen intercept values revealed that the phlogopites are enriched in Cl with respect to the element distribution effect of Mg-Fe substitution. Since the degree of Cl enrichment of the phlogopite is more significant compared to that of biotite in porphyry copper deposits, the phlogopites are considered to have formed under the condition of significantly high activity of halogens. Hydrothermal ore deposits may be formed in magmatic hydrothermal system associated with volcanoes where halogen-rich phlogopite is formed by hypersaline fluid.     

Phase equilibrium and stable isotope constraints on the formation of metasomatic garnet-vesuvianite veins (SW Adamello,N Italy)

 Metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Lower Triassic Prezzo formation in a narrow, 1–5 m wide zone along an intrusive marble-granodiorite contact at the southwestern border of the Tertiary Adamello batholith. The metasomatic mineral assemblage is comprised of garnet, vesuvianite, clinopyroxene, wollastonite, and pyrrhotite, which were precipitated from the vein-forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630° C. The veins are enriched in silicon, aluminum, iron, magnesium, titanium and depleted in calcium with respect to the unaltered contact metamorphic marble. Graphite, which is present in the unaltered Prezzo Marble is absent in the veins. Irregularly shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins. The isotopic compositions of calcite (cc) in the unaltered marble are about δ¹⁸O (SMOW; Standard mean Ocean Water)=21.0‰ and δ¹³C(PDB; Peedee belemnite)=0.0‰. They are reset to significantly lower values within the veins, where δ¹⁸O_cc is 15.0 to 16.0‰ and δ¹³C_cc is −4.5 to −3.5‰. The isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which occurs at the sharp boundary between graphite-bearing and graphite-free material. Within the veins the oxygen isotope fractionation between calcite and garnet (gar) varies systematically with distance from highly silicified zones. The variations in Δ¹⁸O_cc-gar are as large as 2‰, on a millimeter scale, indicating garnet-calcite isotopic disequilibrium. Vein formation was due to the infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures. Removal of graphite from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite. Permeability was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration. Vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid. The isotopic composition of preexisting calcite was initially out of equilibrium with the vein-forming fluid and it was shifted towards equilibrium by surface-reaction controlled calcite-fluid isotopic exchange during calcite recrystallization. Due to the short lifetime of the metasomatic system, calcite-fluid isotopic equilibrium was generally not attained. Within the veins, oxygen and carbon transport was fast relative to mineral-fluid exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral-fluid exchange. Received: 16 June 1994/Accepted: 20 May 1995     

Silica mobility and fluid movement during metamorphism of the Connemara schists, Ireland

Quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of Connemara, and attest to considerable migration of silica. Contrary to common assumptions, there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower P–T down the regional geothermal gradient. Under amphibolite facies conditions, a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated, while cooling over the temperature interval from 650 to 500° C. The absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through-flowing, externally derived fluids. The oxygen isotopic composition of matrix quartz shows considerable differences between different rock types (quartzite, pelite and marble), with a range of δ¹⁸O_SMOW from c.+ 11.5% (quartzite) to + 18.5% (marble). In each rock type, vein quartz compositions closely match those of the matrix quartz. These results demonstrate the importance of local segregation processes in the formation of veins, and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations, if at all. Both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies, with only a relatively minor component of through flow of externally derived fluid. Veins are clearly not the major pathways of metamorphic dewatering. It is proposed that abundant veins in the predominantly pelitic Ballynakill Formation formed during peak metamorphic D3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble. Hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins, while irreversible loss through the rest of the succession was a much less important process. In the central mountains of Connemara, rather pure, unreactive quartzites are cut by widely spaced, laterally extensive quartz veins that are axial planar to D3 folds. These veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile, but here too, oxygen isotope data indicate considerable segregation of locally derived quartz, reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow.     

贺兰山北段牛头沟金矿床成矿流体和成矿物质来源:来自H-O-S-Pb同位素的证据

贺兰山北段牛头沟金矿床为华北克拉通西北缘新发现的金矿床,包括构造破碎带蚀变岩型和石英脉型两种矿化类型,后者可进一步细分为低缓石英脉型和陡窄石英脉型2个亚类。矿区所有矿体均赋存在古元古界贺兰山群变质杂岩和混合花岗岩内,受主干断裂F_1及其上盘次级断裂体系控制。综合本文及前人研究成果表明,破碎带蚀变岩型石英流体包裹体以纯液相水溶液包裹体为主,而低缓石英脉型和陡窄石英脉型石英流体包裹体则以气液两相水溶液包裹体为主,不同矿化类型成矿流体均为中低温(160~210℃)、中低盐度(6%~12%NaCl_(eq))的H_2O-NaCl流体。对矿区内3种矿化类型石英流体包裹体和硫化物分别开展的H-O和S-Pb同位素研究显示:破碎带蚀变岩型和陡窄石英脉型流体包裹体δD_(H2O)组成相近,分别为-75.2‰~-89.3‰和-87.0‰~-93.8‰,而低缓石英脉型流体包裹体则具有较高的δD_(H2O)值(-45.7‰~-67.7‰);流体包裹体δ~(18)O_(H2O)值则由破碎带蚀变岩型(3.7‰~4.4‰)→低缓石英脉型(1.9‰~3.3‰)→陡窄石英脉型(0.5‰~0.9‰)依次降低。破碎带蚀变岩型和陡窄石英脉型δ~(34)S组成均为正值,分别为1.3‰~6.9‰和2.2‰~5.8‰,而低缓石英脉型则具有较低的δ~(34)S值(-5.1‰~-2.6‰)。低缓石英脉型金矿具有明显不同的δD_(H2O)和δ~(34)S组成,可能与含矿断裂性质及其距离导矿构造F_1断裂较远等因素所共同导致的成矿流体氧逸度升高有关。3种矿化类型对应的矿石均表现出明显富集Th放射成因Pb的特点,~(206)Pb/~(204)Pb(16.467~17.994)和~(207)Pb/~(204)Pb(15.382~15.582)组成相对均一,而~(208)Pb/~(204)Pb变化较大(37.413~42.345)。总体上,石英流体包裹体H-O同位素组成表明成矿流体均为岩浆水和大气降水形成的混合流体,其大气降水比例自破碎带蚀变岩型→低缓石英脉型→陡窄石英脉型依次升高;矿石S-Pb同位素指示成矿物质为深部岩浆和具有高Th/U比的基底围岩双重来源。结合区域构造–岩浆演化,笔者将牛头沟金矿床成矿过程概括为晚古生代裂陷盆地形成阶段、中晚侏罗世区域挤压推覆阶段和晚侏罗世至早白垩世岩浆热液活动阶段等3个阶段。     

Hydrothermal mobility of Ti, Zr and REE: examples from the Bergell and Adamello contact aureoles (Italy)

Contact metamorphic marbles, affected by metasomatic fluids at the contact to the Bergell and Adamello Intrusives, contain various accessory minerals (zirconolite, allanite, titanite, rutile, geikielite, hoegbomite), which provide new evidence for the hydrothermal mobility of Ti and Zr. In both examples, Ti and Zr migrated along with U, Th, Y and REE in a metasomatic fluid rich in potassium. The composition of the main minerals (fluorine-rich phlogopite, pargasite and titanian clinohumite) and the abundance of fluor-apatite demonstrate that fluorine and phosphorus were important components of the fluid. The textural relationships indicate that the formation of the accessory phases is linked to the crystallization of the hydrous minerals.     

Syngenetic phlogopite inclusions in kimberlite-hosted diamonds: implications for role of volatiles in diamond formation

We discuss the chemistry of exceptionally rare phlogopite inclusions coexisting with ultramafic (peridotitic) and eclogitic minerals in kimberlite-hosted diamonds of Yakutia, Arkhangelsk, and Venezuela provinces. Phlogopite inclusions in diamonds are octahedral negative crystals following the diamond faceting in all 34 samples (including polymineralic inclusions). On this basis phlogopite inclusions have been interpreted as syngenetic and in equilibrium with the associated minerals. In ultramafic diamonds phlogopites coexist with subcalcic high-Cr₂O₃ pyrope and/or chromite, olivine and enstatite (dunite/harzburgite (H) paragenesis) or with clinopyroxene, enstatite, and/or olivine and pyrope (lherzolite (L) paragenesis). Ultramafic phlogopites have high Mg# [100?Mg/(Mg+Fe)] from 92.4 to 95.2 and Cr₂O₃ higher than TiO₂ in H-phlogopites (1.5–2.5 wt.% versus 0.1–0.4 wt.%, respectively) but lower in L-phlogopites (0.15–0.5 wt.% versus 1.3–3.5 wt.%, respectively). Eclogitic (E) phlogopites show Mg# from 47.4 to 85.3 inclusive, and very broad ranges of TiO₂ up to 12 wt.%. The primary syngenetic origin of phlogopite is indicated, besides other factors, by its compositional consistency with the associated minerals. The analyzed phlogopites are depleted in BaO (0.10–0.79 wt.%), and their F and Cl contents are highly variable reaching 1.29 and 0.49 wt.%, respectively. The latter is in line with high Cl enrichment in some unaltered kimberlites and in nanometric fluid inclusions from diamonds. The presence of syngenetic phlogopite in kimberlite-hosted diamonds provides important evidence that volatiles participated in diamond formation and that at least a part of diamonds may have been related to early stages of kimberlites formation.     

Petrology of amphibolite-facies mafic and ultramafic rocks from the Catalina Schist, southern California: metasomatism and migmatization in a subduction zone metamorphic setting

Abstract The Catalina Schist of southern California is a subduction zone metamorphic terrane. It consists of three tectonic units of amphibolite-, high- P greenschist- and blueschist-facies rocks that are structurally juxtaposed across faults, forming an apparent inverted metamorphic gradient. Migmatitic and non-migmatitic metabasite blocks surrounded by a meta-ultramafic matrix comprise the upper part of the Catalina amphibolite unit. Fluid-rock interaction at high- P , high- T conditions caused partial melting of migmatitic blocks, metasomatic exchange between metabasite blocks and ultramafic rocks, infiltration of silica into ultramafic rocks, and loss of an albitic component from nonmigmatitic, clinopyroxene-bearing metabasite blocks.
Partial melting took place at an estimated P =˜8–11 kbar and T =˜640–750°C at high H₂O activity. The melting reaction probably involved plagioclase + quartz. Trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks.
The metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge. Aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex.     

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.     

Petrographical and geochemical investigation of the Triassic marbles associated with Menderes massif metamorphics,Kavaklıdere,Muğla,SW Turkey

Vast marble deposits occur in a cover sequence of the Menderes Massif, SW Turkey. Four major marble deposits are recognized in Mu?la province based on the stratigraphic levels. These are Permo-Carboniferous aged black marbles (1), Triassic aged marbles (2), Upper Cretaceous aged marbles (3), and Paleocene aged pelagic marbles (4). This study deals with Triassic aged marbles of the southern part of the Menderes Massif. The Triassic marbles from SW Turkey consist of two big marble horizons in the Çayboyu (ÇM) and Kestanecik (KM) regions. The characteristic samples are collected from different stratigraphic levels in marble deposits in the ÇM and KM horizons. Mineralogical and major, trace, and rare earth element (REE) analyses of marble, limestone, and schist were conducted on these samples to reveal their petrographical and geochemical characteristics. The ÇM horizon is represented by calcitic marble layers. Nickel, cobalt, manganese, and iron elements filled in fractures, fissures, and intergranular spaces of calcite crystals and these elements give the pinky colour to the marble from the ÇM horizon. KM marbles were deformed, metamorphosed, and recrystallized under greenschist facies P–T conditions. As a result of the metasomatic reaction of magnesium and manganese rich fluids with marbles, dolomite, and manganese, minerals such as rhodochrosite and pyrolusite have crystallized along vein walls and layers in the KM horizon. Dolomitization was determined in KM marbles, whereas ÇM marbles show the character of limestone. MgO, MnO, Fe₂O₃, Ni, and Zn contents of marbles from the KM horizon are higher than those of ÇM marbles due to metasomatic reactions. The Sr content in white coloured marbles ranges between 11.20 ppm and 112.20 ppm and this concentration reaches up to 272.70 ppm due to metasomatic reactions and fluid intake. The REE content of Triassic marbles is independent of the abundance of carbonate and the REE enrichment observed due to syn-metamorphic fluid flow. The significant negative Eu anomaly in REE patterns indicates that the protoliths of Triassic marbles are carbonate rocks of sedimentary origin.     

Harzburgite to lherzolite and back again: metasomatic processes in ultramafic xenoliths from the Wesselton kimberlite, Kimberley, South Africa

Garnets from phlogopite harzburgite xenoliths from the Wesselton kimberlite show zoning from low-Ca harzburgitic cores to rims with lherzolitic Ca-Cr relations. Garnet cores are depleted in Y and HREE, but have sinuous REE patterns enriched in the MREE. Rimwards increase in Ca and decrease in Cr and Mg is accompanied by increases in Zr, Y, Ti and HREE. Secondary replacement rims on some garnets consist of garnet with low Ca and Cr, but high Mg, Ti and HREE. The zoning, and the secondary replacement rims, are attributed to different stages of a metasomatic process that has converted harzburgite to lherzolite, at temperatures near 1000 °C. Modelling of zoning profiles suggests that the process can be divided into three parts: (a) Inwards diffusion of Ca, Zr and Y over periods of 10,000–30,000 years, from a fluid depleted in Ti, Ga and Y; (b) formation of overgrowths high in Ca, Zr, Y and Ti, followed by annealing over periods of several thousand years; (c) formation of secondary reaction rims of low-Ca garnet, on very short timescales prior to eruption. The sinuous REE patterns of the garnet cores are regarded as “primary” features reflecting an ancient metasomatic event superimposed on a depleted protolith. The high Zr/Y, Zr/Ti and Zr/Ca of the fluids corresponding to stage (a) are ascribed to the presence of phlogopite and garnet in the matrix near the fluid source (presumed to be a melt, possibly a kimberlite precursor), leading to the development of concentration fronts in the percolating fluid. The overgrowths of stage (b) appear to coincide with the precipitation of phlogopite in the rock. The low Ca of the fluid responsible for the secondary replacement rims of stage (c) may reflect the late precipitation of clinopyroxene or Ca-carbonate as part of the metasomatic assemblage. These processes have significantly modified the modal, major- and trace-element composition of the mantle volume sampled by the Wesselton kimberlite, within <1 Ma of eruption. Recognition of such effects and their distribution in time and space is essential to understanding of the evolution of the subcontinental lithospheric mantle. Received: 11 February 1998 / Accepted: 24 June 1998     

Natural sodium phlogopite coexisting with potassium phlogopite and sodian aluminian talc in a metamorphic evaporite sequence from Derrag,Tell Atlas,Algeria

In its only natural occurrence known thus far sodium phlogopite is found in a dolomite containing large porphyroblasts of albite, three other magnesium phyllosilicates, dravite-uvite tourmaline, quartz, rutile, and pyrite. Sodium phlogopites are close to the ideal formula NaMg₃[AlSi₃O₁₀](OH)₂, although they may possibly contain additional Li. They are invariably coated by thin rims of potassium phlogopite with octahedral and tetrahedral occupancies different from those of sodium phlogopite. These rims may have prevented the retrograde hydration of sodium phlogopite which seems to be the main reason for its general absence in natural rocks. For the low-grade metamorphic conditions undergone by the dolomite a solvus relationship is indicated between sodium and potassium phlogopite.Sodium phlogopite also coexists, at least prior to the appearance of K phlogopite, with a talc phase containing Na and Al^[4] substituting for Si. This type of substitution leading from pure talc to sodium phlogopite was found to extend as far as 36 mole percent. However, the nature of this phase as a genuine solid solution or as a disordered mixed-layer between talc and sodium phlogopite could not be identified as yet. The final phyllosilicate appearing in millimeter-size porphyroblasts is an ordered 11 mixed layer between clinochlore and sodian aluminian talc representing a new mineral.Metamorphic temperatures at the supposedly low water and CO₂ fugacities are estimated to have been below 400 °C.     

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。     

Formation and compositional variation of phlogopites in the Horoman peridotite complex,Hokkaido, northern Japan: implications for origin and fractionation of metasomatic fluids in the upper mantle

Harzburgite and lherzolite tectonites from the Horoman peridotite complex, Hokkaido, northern Japan, contain variable amounts of secondary phlogopite and amphibole. Phlogopite-rich veinlets parallel to the foliation planes usually cut olivine-rich parts of the rocks; single-grained interstitial phlogopites are usually associated with orthopyroxene grains. Amphiboles are disseminated in rocks or sometimes occur in the phlogopite-rich veinlets. Within individual veinlets, phlogopites show extensive inter-grain variations in K/(K + Na) ratio (0.96–0.75), generally decreasing from the central (usually the thickest) part towards the marginal parts of veinlets. In contrast, Ti contents are nearly constant in Ti-poor veins or decrease slightly with decreasing K/(K + Na) in T-rich veins. Variation of Ti in phlogopites is very large (0.1–6.8 wt%) and is inversely correlated with Mg/(Mg + Fe^*) (Fe^*, total iron) atomic ratios, which vary from 0.96 to 0.88. Intra-vein variation of phlogopite chemistry (especially K/(K + Na) ratio) could be achieved by in situ fractional crystallization of trapped fluids; variation of Ti, however, cannot be explained by in situ fractionation of the fluids, indicating various Ti contents of the parent fluids. It is suggested that fluids responsible for the formation of the Horoman phlogopites and amphiboles were magmatic volatiles successively released from evolving alkali basaltic magmas. Individual fluids trapped within peridotites were fractionated, precipitating phlogopites successively poorer in K. When the fluids became rich enough in Na, amphiboles co-precipitated with phlogopites. Similar fractional crystallization of phlogopites and amphiboles is expected in the upper mantle on a larger scale if fluids move upwards. This process may control, at least partly, a lateral K/Na distribution in the upper mantle; K and Na may be concentrated in deeper and shallower parts, respectively.     

Chemical Zone of Nephrite in Alamas,Xinjiang, China

The Hetian deposit, located south of the Tarim Basin in Xinjiang, China, is one of the world's largest dolomite-related nephrite deposits. In the Alamas orebody of the deposit, nephrite occurs as veins or lenses along faults or fissures of the adjacent dolomitic marble. Chemical analyses using electronic microscope probe analysis and X-ray fluorescence spectroscope were carried out on nephrite and dolomitic marble samples collected from a cross section in Alamas to investigate zonal structure of the orebody. The nephrite in Alamas is predominately composed of tremolite with minor calcite, titanite and phlogopite, and that dolomitic marble is relatively pure with a FeO content less than 0.20 wt. %. Contents of color-inducing elements, such as Fe, Mn, and Cr, increase gradually as color changes from white through white-green and then to green, resulting in the formation of color-distinctive zones. Tremolite grain size increases as color changes from white through white-green to green. The trend may be consistent with temperature changes from dolomitic marble to granodiorite, which, in turn, suggests that both change in color from white to green and variations of grain size with increasing temperature resulting in formation of the nephrite zonal structure. Both nephrite zonal structure and minor minerals, such as calcite, titanite and phlogopite found in the contact, indicate that this dolomite-related nephrite orebody is of a metasomatic origin under assumed pressure of 100–200 MPa and temperature <550°C.     

Petrogenesis of carbonatitic lamproitic dykes from Sidhi gneissic complex,Central India

Petrographic, mineral chemical and whole-rock geochemical characteristics of two newly discovered lamproitic dykes(Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex(SGC), Central India are presented here. Both these dykes have almost similar sequence of mineral-textural patterns indicative of:(1) an early cumulate forming event in a deeper magma chamber where megacrystic/large size phenocrysts of phlogopites have crystallized along with subordinate amount of olivine and clinopyroxene;(2) crystallization at shallow crustal levels promoted fine-grained phlogopite, K-feldspar, calcite and Fe-Ti oxides in the groundmass;(3) dyke emplacement related quench texture(plumose K-feldspar, acicular phlogopites) and finally(4) post emplacement autometasomatism by hydrothermal fluids which percolated as micro-veins and altered the mafic phases. Phlogopite phenocrysts often display resorption textures together with growth zoning indicating that during their crystallization equilibrium at the crystal-melt interface fluctuated multiple times probably due to incremental addition or chaotic dynamic self mixing of the lamproitic magma. Carbonate aggregates as late stage melt segregation are common in both these dykes, however their micro-xenolithic forms suggest that assimilation with a plutonic carbonatite body also played a key role in enhancing the carbonatitic nature of these dykes. Geochemically both dykes are ultrapotassic(K_2 O/Na_2 O: 3.0-9.4) with low CaO, Al_2 O_3 and Na_2 O content and high SiO_2(53.3-55.6 wt.%)and K_2 O/Al_2 O_3 ratio(0.51-0.89) characterizing them as high-silica lamproites. Inspite of these similarities, many other features indicate that both these dykes have evolved independently from two distinct magmas. In dyke 1, phlogopite composition has evolved towards the minette trend(Al-enrichment) from a differentiated parental magma having low MgO, Ni and Cr content; whereas in dyke 2, phlogopite composition shows an evolutionary affinity towards the lamproite trend(Al-depletion) and crystallized from a more primitive magma having high MgO, Ni and Cr content. Whole-rock trace-elements signatures like enriched LREE, LILE, negative Nb-Ta and positive Pb anomalies; high Rb/Sr, Th/La, Ba/Nb, and low Ba/Rb, Sm/La, Nb/U ratios in both dykes indicate that their pareintal magmas were sourced from a subduction modified garnet facies mantle containing phlogopite. From various evidences it is proposed that the petrogenesis of studied lamproitic dykes stand out to be an example for the lamproite magma which attained a carbonatitic character and undergone diverse chemical evolution in response to parental melt composition, storage at deep crustal level and autometasomatism.     

Origin of Phlogopite in Mantle Xenoliths from Kostal Lake, Wells Gray Park, British Columbia

Phlogopite has been recognized for the first time in ultramaficxenoliths from the Canadian Cordillera. The phlogopite-bearingxenoliths are hosted in post-glacial basanitoid flows and ejectaof the Kostal Lake volcanic center, British Columbia. The xenolithassemblage consists of 60% cumulate-textured wehrlites, and40% coarse-textured lherzolites, harzburgites, dunites, andolivine websterites. The phlogopite occurs: (1) as sub-euhedral grains along grainboundaries in dunite and lherzolite xenoliths; or (2) alongorthopyroxene lamellae exsolved from intercumulus clinopyroxenein the wehrlite xenoliths; or (3) as grains hosted in 10–100pm diameter fluid inclusions in clinopyroxene of all xenoliths.The phlogopites do not show any reaction relationships withother phases in any of the xenoliths studied. Phlogopites ina given xenolith have Mg/Mg + Fe²⁺ similar to that of coexistingolivine, clinopyroxene, and orthopyroxene. The partitioningof Fe and Mg between phlogopite and coexisting olivine and clinopyroxeneis similar to that observed in other phlogopite-bearing mantlexenoliths, and in high-pressure melting experiments on rockswith similar bulk compositions. This indicates that the phlogopitesin xenoliths from Kostal Lake have equilibrated with these coexistingphases. The occurrence of phlogopites in fluid inclusions containingNa, K, Cl, P, and S, suggests that incompatible element-enrichedhydrous fluids/melts fluxed this part of the upper mantle beneatheastern British Columbia. Metasomatism of the upper mantle beneathKostal Lake probably occurred prior to Quaternary alkaline magmatism(7550–400 B.P.) and after the initial volcanism whichformed the wehrlite cumulates (3–5 Ma). Metasomatism causedoverall oxidation of the upper mantle beneath this area butwas not responsible for the anomalously Fe-rich nature of somexenoliths from the Kostal Lake eruptive center.