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1.
Detailed laboratory study has been made on pre-Tertiary coarse-grainedglaucophane schist, garnet-epidote amphibolite, and epidoteamphibolite in the eastern slope of the Central Mountain Range,Taiwan. These petrotectonic assemblages are considered to beexotic tectonic blocks emplaced within the feebly metamorphosedin situ graphite and quartzose schists of the Yuli belt. Thinlenses of Mn-rich metamorphosed tuff are intercalated withinthe metabasaltic rocks. Such high MnO (2 wt. per cent) and lowMgO (3–4 wt. per cent) tuffaceous rocks are similar inbulk composition to some volcanic clays collected in deep oceanbasins. They consist of the characteristic assemblage Mn-bearinggarnet (5–7 wt. per cent MnO and 30 volume per cent inthe rock)+muscovite+epidote+hornblende+quartz+ albite+rutile?pyrite. Successive stages of conversion of garnet-epidote amphiboliteto blueschist assemblages were noticed. The most recrystallizedschists display abundant Mn-bearing garnet, zoned amphibole,phengite, zoned epidote, stilpnomelane, chlorite, quartz, minoralbite, magnetite, and sphene. The recrystallization processis nearly isochemical except the glaucophane schists appearto be more oxidized and contain more Na2O than the relict amphibolites.Intimately associated amphibolites of basaltic composition,in contrast, contain the assemblage hornblende+paragonite+epidote+chlorite+quartz+albite+rutile. Microprobe analyses of the coexisting minerals in glaucophaneschists, garnet-epidote amphibolites and epidote amphibolitesyield the following results: (1) garnets, consisting of almandine,spessartine, and grossular components, are less Mn and Mg-richcompared to those in in situ metabasalts of the Franciscan;(2) rim epidotes of the glaucophane schists are more pistastic(XFe=0?27–0?30) than that of the garnet-epidote amphibolite(0?2–0?22) implying higher fO2 values for the glaucophanization;(3) phengitic micas of the glaucophane schist have less Al2O3content (29 wt. per cent) than those of the garnet-epidote amphibolite(32 wt. per cent) whereas micas of epidote amphibolites areparagonites with K/(K+Na) ratio of 0?04; (4) the zoned amphibolesshow glaucophane occurring marginal to cores of calcic amphibole.Sodic amphiboles with Al2O3 of 6-? to 10?4 wt. per cent arecrossite-glaucophane whereas all calcic amphiboles analyzedare barroisite-pargasite (Al2O3 greater than 10 wt. per cent). The garnet-epidote-rutile bearing glaucophane schist of Taiwanprobably recrystallized at temperatures above 350 ?C (the epidotezone) whereas the lawsonite-sphene glaucophane schists of theFranciscan equilibrated below 350 ?C (the lawsonite zone). TheMn-rich basaltic tuffs and their associated flows appear tohave been metamorphosed at profound depths and at the relativelyhigh temperatures of the epidote amphibolite facies, succeededlater by glaucophane schist facies metamorphism at lower temperatures.  相似文献   

2.
Ninety-seven mineral phases consisting of ten chloritoids, fifteen epidotes, sixteen garnets, four sphenes, seven rutiles, seven pyroxenes, thirteen blue amphiboles, two green amphiboles, eleven phengites, two paragonites, a mariposite, seven chlorites, and two specimens of albite were obtained from the metamorphic rocks of Île de Groix, and their chemical, physical, optical and X-ray properties determined. The chloritoids are all optically positive, monoclinic polymorphs with large 2V, moderate refractive indices and characterized by high densities. Their fluorine contents have been used to propose a new upper limit for OHF substitution in the chloritoid structure, suggesting that partial pressure of fluorine might modify the stability of chloritoids from that determined in pure H2O. The epidotes belong to the Al-Fe epidote series and are epidote sensu stricto. The almandine-rich garnets and the chloromelanites are metastable relics in the glaucophane schists. The grossular contents of the calcareous schist garnets are believed to have become depressed under high CO2 pressure and the low Tschermak's contents of the pyroxenes are to be explained by equilibria involving epidote at high and low temperature when the Tschermak's components will break down to epidote group minerals. The sphenes contain appreciable amounts of combined water, fluorine substituting for oxygen and aluminium substituting for silicon and titanium. The presence of H3O+ is suspected in a specimen of blue amphibole. The barroisite has a composition between glaucophane and hornblende. On account of its high Fe3+ content it is believed to have formed under higher P O 2 than the blue amphiboles. The paragonites which occur in the ohloritoid veins are unstable in the potassium-rich aluminous schists. The phengites show a tendency towards sericitic composition due to post-glaucophanisation readjustments under the lower pressure conditions of the greenschist facies. Some of the Fe3+ contents of the chlorites are interpreted as due to oxidation of ferrous iron, e.g. 2 [Fe(OH)2]2FeOOH + H2. The minerals show strong chemical control of the host rock and their Mn contents are directly related to those of the minerals from which they have evolved through retrogression.Chloritoids and epidotes that are not associated with garnets contain higher amounts of manganese; similarly, the two blue amphiboles with the highest FeMg ratios were obtained from rocks in which garnet has not appeared. It is therefore believed that ottrelite and piemontite would be stable only at the lowest subfacies of the greenschist facies. Also, the ironrich amphiboles must have evolved from low-grade iron-aluminium chlorites, since on the appearance of garnet in a schist iron-aluminium chlorites react with quartz to give almandine and Mg-rich chlorites. The Fe2+Mg ratios of the blue amphiboles therefore reflect the grade of the original schist in which the minerals formed.  相似文献   

3.
Abstract. Pink piemontite-spessartine-bearing and grey-green spessartine-bearing manganiferous quartzose schists derived from siliceous pelagites, and green quartzofeldspathic schists, are described from the greenschist facies of the Haast Schist terrane, near Arrow Junction, western Otago. Electron microprobe data are reported for sphene, spessartine-rich garnet, manganoan epidote, piemontite, tourmaline, phengitic muscovite, chlorite, albite, haematite, rutile, manganoan calcite and chalcopyrite. Metamorphism occurred at about 6.4kbar, 400°C. Xco2 was above the quartz-rutile-calcite-sphene buffer (Xco2± 0.02) throughout the recorded metamorphic history of the piemontite schists. It dropped from above to below this critical buffering value in a spessartine-rich schist and it was close to or below the buffering value in the quartzofeldspathic schists. Production of piemontite required high fO2, believed to be inherited from MnOx in the parent pelagite. Substantial loss of O2 (e.g. minimum of 0.19% by weight in one rock) during diagenesis and/or metamorphism is inferred. In the grey-green schists this inhibited piemontite formation. Slight loss of O2 and Ca2+ accompanied minor late-stage replacement of piemontite by second generation spessartine. Observed zoning and mineral replacements indicate rise of temperature, drop in pressure, or invasion by solutions of lower fO2 and XCO2 equilibrated with surrounding schists. The detailed chemistry of the minerals studied correlates with available Mn and with bulk-rock (Fe3+ x 100)/(Fe2++ Fe3+). The oxidation ratio ranges from 24 in average green quartzofeldspathic schist, through 78 in average grey-green manganiferous quartzose schist, to almost 100 in some piemontite-bearing schists. As Fe2+ gives way to Fe3+, Mg/Fe ratios tend to rise in chlorite, phengite, tourmaline, spessartine, and calcite, Mn increases and Ti decreases in haematite, Mn increases in spessartine and calcite, and Fe increases in rutile. Available divalent cations are depleted relative to Al; chlorite is more aluminous, and phengite more paragonitic than in typical Haast schists.  相似文献   

4.
Summary The formation of cummingtonite in two Ca and Al-poor and Mg-rich amphibolites from the Austroalpine Schneeberg complex occurred at a maximum temperature of about 550°C (5 kb). This is a result of the amphibolite facies Alpine overprint in this part of the Eastern Alps.Textural and chemical relations suggest (Mg–1Si–1Al2)-continuous reactions in the bivariant CMASH-assemblageCam-Cum-Chl * followed by the discontinuous reactionCam+Chl+Qu=Cum+Plg+H2O to be responsible for the formation of cummingtonite in these samples.The Mg–Fe distribution coefficient with values of 0.6–0.7 is similar to cummingtonite-Ca-amphibole pairs from amphibolites with oligoclase+quartz reported in the literature. The Mg/(Mg+Fe) ratio of the calcic amphiboles is lower (0.539–0.555) than the coexisting cummingtonites (0.648–0.662).
Koexistierende Cummingtonite und Hornblenden in Amphiboliten des Schneeberger Zuges, Tirol, Österreich
Zusammenfassung In zwei Ca- und Al-armen Amphiboliten des nördlichen Schneebergerzuges (Rotmoostal) bildete sich Cummingtonit bei Maximaltemperaturen von 550°C (5 kb) bei der Altalpidschen Metamorphose.Texturelle und chemische Beziehungen lassen vermuten, daß sich Cummingtonite sowohl nach kontinuierlichen Reaktionen (in bezug auf den Tschermak-Vektor Mg–1Si–1Al2) gebildet hat, als auch aus Hornblende und Chlorit nach der diskontinuierlichen ACF-ReaktionCam+Chl+Qu=Cum+Plg+H2O hervorgegangen ist.Der Mg–Fe-Verteilungskoeffizient zwischen Hornblende und Cummingtonit entspricht den aus der Literatur bekannten Werten. Er beträgt zwischen 0.6–0.7 für die beginnende Amphibolitfazies. Die Mg/(Mg+Fe)-Verhältnisse sind höher in Cummingtonit (0.648–0.662) als in der koexistierenden Hornblende (0.539–0.555).

With 4 Figures  相似文献   

5.
Hydrothermal alteration of Tertiary gabbros from Skye involved the reaction of igneous olivine, augite, hypersthene, plagioclase, magnetite, and ilmenite with aqueous fluid primarily to combinations of talc, chlorite, montmorillonite, calcic amphibole, biotite, and secondary magnetite. Lesser amounts of calcite, epidote, quartz, sphene, prehnite, and garnet also developed. During mineralogical alteration of gabbro there was a net addition to rock of K, Na, Sr, and H2O and a net loss of Mg. Gabbro was oxidized early in the hydrothermal event and later reduced. Iron and silicon were probably initially lost and later added. There is no evidence for significant change in the Al or Ca content of the gabbros. Hydrothermal alteration of Skye gabbro involved not only large-scale migration of 18O, 16O, D and H but also of K, Na, Sr, Mg, and probably Fe and Si.Mineral thermometry indicates that pyroxenes in the gabbros crystallized at 1000° C–1150° C and were very resistent chemically as well as isotopically to later hydrothermal alteration. Hypothetical equilibrium between primary and secondary mafic silicates suggests that mineralogical alteration of gabbro occurred at 450°–550° C. The lack of correlation between mineralogical and isotopic alteration of gabbro requires that much isotopic alteration occurred at temepratures above those at which the secondary minerals developed, 550°–1000° C. The chemical alteration of gabbro is correlated with its mineralogical alteration and therefore occurred at 450°–550° C.Measured progress of the mineral-fluid reactions was used to estimate the amount of H2O fluid that infiltrated the gabbro as primary olivine was converted to talc+magnetite at 525°–550° C. Calculated fluid-rock ratios are in the range 0.2–6 (volume basis) and are smaller than values estimated from isotopic data (fluid/rock 1–10, volume basis). Both isotopic and petrologic data point to pervasive flow of fluid through crystalline rock at elevated temperatures of 500°–1000° C. Isotopic fluid-rock ratios are larger than petrologic fluid-rock ratios because isotopic alteration of cooling gabbro began earlier and at higher temperatures than did the mineralogical alteration.  相似文献   

6.
Deerite, Fe 12 2+ Fe 6 3+ [Si12O40](OH)10, thus far known from ten localities in glaucophane schist terranes, was synthesized at water pressures of 20–25 kb and temperatures of 550–600 °C under the of the Ni/NiO buffer. The X-ray powder diagram, lattice constants and infrared spectrum of the synthetic phase are closely similar to those of the natural mineral. A solid solution series extends from this ferri-deerite end member to some 20 mole % of a hypothetical alumino-deerite, Fe 12 2+ Al 6 3+ [Si12O40](OH)10. The upper temperature breakdown of ferri-deerite to the assemblage ferrosilite +magnetite+quartz+water occurs at about 490 °C at 15 kb, and 610 °C at 25 kb fluid pressure for the of the Ni/NiO buffer. Extrapolation of these data to lower water pressures indicates that deerite can be a stable mineral only in very low-temperature, high-pressure environments.  相似文献   

7.
This investigation deals with the Nigerian iron-formations and their host rocks and is based on about 560 mineral analyses (electron-microprobe) and 93 whole-rock analyses (64 iron-formations and 29 host rocks). The manganese-rich and Al-bearing iron-formations occurring in various schist belts of the northern and southern part of West-Nigeria consist of the magnetite-free silicate, the magnetite–silicate and the quartz-rich hematite facies.Iron-formations and host rocks originated from submarine-volcanogenic exhalations enriched in Fe, Mn and CO2 and from Al2O3, SiO2 and alkali (K2O and Na2O)-rich continental-derived pelitic to psammitic material. From these sources and their interaction and controlled by the volcanogenic activity, differently composed protoliths were deposited in the marine basin during the Birimian time. Subsequent metamorphism of greenschist to low amphibolite facies conditions during the Eburnian time led to the formation of the metaprotoliths of the magnetite–silicate (consisting of predominantly magnetite and quartz and subordinate of garnet and amphibole), the silicate facies (consisting of garnet, amphibole and rarely Mn-bearing ilmenite and quartz) and the metasediment phyllite. Garnets are predominantly almandine–spessartine solid solutions, whereas amphiboles are Mn and Ca-bearing grunerite–cummingtonite solid solutions. In the course of a second tectono-metamorphic event of Pan-African age, the magnetite–silicate facies iron-formation/phyllite association was transformed into the hematite facies and muscovite/biotite schists, whereas the silicate facies is characterized by extensive silicification features. The hematite facies and the silicified silicate facies are restricted to southern Nigeria where the second and heterogeneous tectono-metamorphic event is more pronounced (amphibolite facies conditions) than in northern Nigeria.The genesis, summarized as the metamorphic model, shows that the carbonate-rich (siderite, rhodochrosite and subordinate magnesite and calcite) protoliths were metamorphically transformed into the silicate and magnetite–silicate facies. The separation of Mn and Fe, leading to manganese-bearing iron-formations and iron-bearing manganese-formations was explained by varying pH-conditions, under which siderite (pH: 6.8–9.4) and rhodochrosite (pH: 9–11) precipitated.Similar to the Gunfit and Biwabik iron-formations of Minnesota, USA, the iron-formation of Bingi (Maru schist belt), now present in the form of the fayalite bearing silicate facies, was overprinted by contact metamorphism caused by a gabbro intrusion.  相似文献   

8.
Soil samples collected from various places in and around Mysore were analyzed for the total trace elements such as Fe, Mn, Cu, Zn, Pb, and Cd. The results of the analysis indicate that the concentration of lead and cadmium in soils is below 2.5 g ml–1 and 0.2 g ml–1, respectively, which are the minimum detection levels, whereas the concentration of iron, manganese, copper, and zinc in most of the samples is within the global average ranges of 3%, 500–1000 g g–1, 15–40 g g–1, and 50–100 g g–1, respectively. The investigated area has the presence of gneisses and schists, in which partly there are igneous intrusions and pegmatitic intrusions. There are amphibolite enclaves in gneisses that account for the higher concentration of trace elements. The lower concentration may be attributed to the presence of silicic type of rock.  相似文献   

9.
The high-pressure schist terranes of New Caledonia and Sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism. The parent rocks were chemically similar, as volcanic-sedimentary trough or trench sequences, and metamorphic temperatures in both belts were 250° to 600° C. From phase equilibria curves, total pressures were higher for New Caledonia (6–15 kb) than for Sanbagawa (5–11 kb) and the estimated thermal gradients were 7–10° C/km and 15° C/km respectively.PT paths identify the higher pressure in New Caledonia (P differences 2 kb at 300° C and 4 kb at 550° C) with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas: lawsonite-epidote-omphacite (New Caledonia) and chlorite-garnet-biotite (Sanbagawa). In New Caledonia the Na-amphibole is dominantly glaucophane and Na-pyroxenes associated with quartz are Jadeite (Jd95–100) and omphacite; in Sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite (Jd50). For both areas, garnet rims show increase in pyrope content with advancing grade, but Sanbagawa garnets are richer in almandine. Progressive assemblages within the two belts can be equated by such reactions as:New Caledonia Sanbagawa glaucophane+paragonite+H2Oalbite+chlorite+quartz glaucophane+epidote+H2Oalbite+chlorite+actinolite and the lower pressure Japanese associations appear as retrogressive phases in the New Caledonia epidote and omphacite zones.The contrasts inPT gradient, regional zonation and mineralogy are believed due to differences in the tectonic control of metamorphic burial: for New Caledonia, rapid obduction of an upper sialic plate over an inert oceanic plate and sedimentary trough; and for Sanbagawa, slower subduction of trench sediments beneath a relatively immobile upper plate.  相似文献   

10.
Iron and manganese redox cycling in the sediment — water interface region in the Kalix River estuary was investigated by using sediment trap data, pore-water and solid-phase sediment data. Nondetrital phases (presumably reactive Fe and Mn oxides) form substantial fractions of the total settling flux of Fe and Mn (51% of Fetotal and 84% of Mntotal). A steady-state box model reveals that nondetrital Fe and Mn differ considerably in reactivity during post-depositional redox cycling in the sediment. The production rate of dissolved Mn (1.6 mmol m–2 d–1) exceeded the depositional flux of nondetrital Mn (0.27 mmol m–2 d–1) by a factor of about 6. In contrast, the production rate of upwardly diffusing pore-water Fe (0.77 mmol m–2 d–1) amounted to only 22% of the depositional flux of nondetrital Fe (3.5 mmol m–2 d–1). Upwardly diffusing pore-water Fe and Mn are effectively oxidized and trapped in the oxic surface layer of the sediment, resulting in negligible benthic effluxes of Fe and Mn. Consequently, the concentrations of nondetrital Fe and Mn in permanently deposited, anoxic sediment are similar to those in the settling material. Reactive Fe oxides appear to form a substantial fraction of this buried, non-detrital Fe. The in-situ oxidation rates of Fe and Mn are tentatively estimated to be 0.51 and 0.16–1.7 mol cm–3 d–1, respectively.  相似文献   

11.
The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP – T – aH2O – XFe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H2O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and XH2O (assuming PH2O +PCO3 = Ptotal) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H2O-rich. A flux of H2O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions.  相似文献   

12.
The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with XFe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with XEpFe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes  相似文献   

13.
The world-class Idrija mercury deposit (western Slovenia) is hosted by highly deformed Permocarboniferous to Middle Triassic sedimentary rocks within a complex tectonic structure at the transition between the External Dinarides and the Southern Alps. Concordant and discordant mineralization formed concomitant with Middle Triassic bimodal volcanism in an aborted rift. A multiple isotopic (C, O, S) investigation of host rocks and ore minerals was performed to put constraints on the source and composition of the fluid, and the hydrothermal alteration. The distributions of the 13C and 18O values of host and gangue carbonates are indicative of a fracture-controlled hydrothermal system, with locally high fluid-rock ratios. Quantitative modeling of the 13C and 18O covariation for host carbonates during temperature dependent fluid-rock interaction, and concomitant precipitation of void-filling dolomites points to a slightly acidic hydrothermal fluid (13C–4 and 18O+10), which most likely evolved during isotopic exchange with carbonates under low fluid/rock ratios. The 34S values of hydrothermal and sedimentary sulfur minerals were used to re-evaluate the previously proposed magmatic and evaporitic sulfur sources for the mineralization, and to assess the importance of other possible sulfur sources such as the contemporaneous seawater sulfate, sedimentary pyrite, and organic sulfur compounds. The 34S values of the sulfides show a large variation at deposit down to hand-specimen scale. They range for cinnabar and pyrite from –19.1 to +22.8, and from –22.4 to +59.6, respectively, suggesting mixing of sulfur from different sources. The peak of 34S values of cinnabar and pyrite close to 0 is compatible with ore sulfur derived dominantly from a magmatic fluid and/or from hydrothermal leaching of basement rocks. The similar stratigraphic trends of the 34S values of both cinnabar and pyrite suggest a minor contribution of sedimentary sulfur (pyrite and organic sulfur) to the ore formation. Some of the positive 34S values are probably derived from thermochemical reduction of evaporitic and contemporaneous seawater sulfates.Editorial handling: P. Lattanzi  相似文献   

14.
Cretaceous melange of the Cordillera de la Costa belt, north–centralVenezuela, there are knockers of eclogite, barroisite-bearingeclogite, and pelitic glaucophane schist. These occur in a metamorphicmelange matrix that locally consists of marble, serpentinite,amphibolite, actinolite schist, feldspathic schist and gneiss,graphitic schist, chloritoid schist, and garnet-bearing micaschist. The protoliths for these various rock types exhibita wide age range (Cambrian to Early Cretaceous?). Recently discoveredknockers of pelitic glaucophane schist contain Mg-glaucophane+ paragonite + kyanite + garnet + talc + graphite + rutile +quartz. The coexistence of kyanite and Mg-glaucophane suggestsminimum P 2000 MPa at T > 600°C. Eclogite knockers fromthe same outcrop contain garnet and clinopyroxene which yield500°C for cores, 700°C for rims, and P 1200 MPa. Theassemblage garnet–biotite–phengite–albitewithin schists of the melange matrix of this locality indicatesmetamorphic conditions of T = 450–520°C at P = 1800MPa. Because all lithologies in this outcrop record high-P conditions,this metamorphic melange formed before or during peak metamorphismin a mid-Cretaceous subduction zone. KEY WORDS: geothermobarometry; high-P pelitic schist; eclogite; Puerto Cabello; Venezuela  相似文献   

15.
Samples of granitic rock from south-central Maine contain primary igneous minerals altered by hydrothermal fluids. The reaction mechanisms (by which the over-all mineralogical change during the alteration was accomplished) involve several different mineral-fluid reactions at different reaction sites in the rock. The reactions involve both molecular and charged species in solution. The different reaction sites correspond to alteration of different primary igneous minerals. Biotite is partially converted to chlorite+sphene; microcline to muscovite; plagioclase to various combinations of muscovite, epidote, and calcite. The different reaction sites are linked by exchange of ions: some reaction sites produce ions consumed at other sites and vice versa. Physical conditions during the hydrothermal event are estimated from mineralogical and thermochemical data: P = 3,500 (±300) bars; T =425 ° (± 25 °)C. The fluid was characterized by X CO 2 = 0–0.13; ln([K+]/[H+ ]) = 10.0; ln([Ca2+]/[H+]2)=9.1; ln([Na+]/[H+]) = 10.5; Fe/(Fe+Mg) = 0.95. Amounts of secondary minerals in altered rock, when compared to the inferred mineral reactions that formed them, indicate that small but significant amounts (0.01–0.3mol/ 1,000cm3 altered rock) of CO2, H2O, H+, and K+ were added to the granites by fluids during the alteration, as well as lesser amounts (< 0.01–0.03 mol/1,000cm3 altered rock) of Mg2+, Fe2+, Fe3+, Mn2+, Na+, and Ti4+. The sole element leached from the granitic rocks during alteration was Ca in amounts 0.1–0.3 mol/1,000 cm3 rock. By estimating the composition of the hydrothermal fluids before and after reaction with the granites and by measuring the amount of material added to or subtracted from the granites during the alteration, the amount and volume of hydrothermal fluid involved can be calculated. Two independent calculations require minimum volumes in the range 100–1,000 cm3 fluid/1,000cm3 altered rock to participate in the hydrothermal event.  相似文献   

16.
Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at temperatures 275±15°C and in the MGS Pfluid is estimated to be 1.5 kbar during alteration. The °D values of alteration phases are:-18 to-29 (fluid inclusions),-47 to-61 (chlorites) and-11 to-31 (epidotes). Chlorite 18O values are +0.2 to +4.3, while 18O values for quartz-K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a - plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200–140°C. The retrograde fluid had D -20 to-30 in all lithologies, but the fluid 18O varied both spatially and temporally within the range-4 to +7. The fO2 of the fluid that deposited the epidotes in the MGS varied with its 18O value, with the most 18O-depleted fluid being the most oxidizing. The D values, together with low (<0) 18O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid 18O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at 400 Ma. However convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later.  相似文献   

17.
This paper describes the distribution of Fe and Ni between the octahedral and tetrahedral sites in pentlandite (Fe,Ni)9S8. The dependence of the distribution on pressure and temperature and the activation energy of the cation exchange reaction were determined through annealing experiments. Synthetic crystals were annealed at 433–723 K and pressures up to 4 GPa, and natural crystals were annealed at 423, 448 and 473 K in evacuated silica capillary tubes for various durations. The cation distributions in the synthetic crystals were determined with an X-ray powder method employing the anomalous dispersion effect of CuK. and FeK radiations, while those of natural crystals were calculated from the cell dimensions. The values of U, S and V for the Fe/Ni exchange reaction are –6818 J mol–1, 20.52 J K–1 mol–1, and 6.99 × 10–6 m3 mol–1, respectively. The dependence of the Fe/Ni distribution on pressure (Pa) and temperature (Kelvin) was determined as lnK = 2.47+8.20 × 102 T –1+8.41 x 10–7 T –1 P, where K = (Fe/Ni)octahedral /(Fe/Ni)tetrahedral. The activation energy of the cation exchange reaction was 185 kJ mol–1.  相似文献   

18.
Five Cu–Au epidote skarns are associated with the Mt. Shea intrusive complex, located in the 2.7–2.6 Ga Eastern Goldfields Province of the Archean Yilgarn craton, in greenstones bounded by the Boulder Lefroy and Golden Mile strike-slip faults, which control the Golden Mile (1,435 t Au) at Kalgoorlie and smaller “orogenic” gold deposits at Kambalda. The Cu–Au deposits studied are oxidized endoskarns replacing faulted and fractured quartz monzodiorite–granodiorite. The orebodies are up to 140 m long and 40 m thick. Typical grades are 0.5% Cu and 0.3 g/t Au although parts are richer in gold (1.5–4.5 g/t). At the Hannan South mine, the skarns consist of epidote, calcite, chlorite, magnetite (5–15%), and minor quartz, muscovite, and microcline. Gangue and magnetite are in equilibrium contact with pyrite and chalcopyrite. The As–Co–Ni-bearing pyrite contains inclusions of hematite, gold, and electrum and is intergrown with cobaltite and Cu–Pb–Bi sulfides. At the Shea prospect, massive, net-textured, and breccia skarns are composed of multistage epidote, actinolite, albite, magnetite (5%), and minor biotite, calcite, and quartz. Gangue and magnetite are in equilibrium with Co–Ni pyrite and chalcopyrite. Mineral-pair thermometry, mass-balance calculations, and stable-isotope data (pyrite δ34SCDT = 2.5‰, calcite δ13CPDB = −5.3‰, and δ18OSMOW = 12.9‰) indicate that the Cu–Au skarns formed at 500 ± 50°C by intense Ca–Fe–CO2–S metasomatism from fluids marked by an igneous isotope signature. The Mt. Shea stock–dike–sill complex postdates the regional D1 folding and metamorphism and the main phase of D2 strike-slip faulting. The suite is calc-akaline and comprises hornblende–plagioclase monzodiorite, quartz monzodiorite, granodiorite, and quartz–plagioclase tonalite porphyry. The intrusions display a wide range in silica content (53–73 wt% SiO2), in ratio (0.37–0.89), and in ratio (0.02–0.31). Chromium (62–345 ppm), Ni (23–158), Sr (311–1361 ppm), and Ba (250–2,581 ppm) contents are high, Sr/Y ratios are high (24–278, mostly >50), and the rare earth element patterns are fractionated . These features and a negative niobium anomaly relative to the normal mid-ocean ridge basalt indicate that the suite formed by hornblende fractionation from a subduction-related monzodiorite magma sourced from metasomatized peridotite in the upper mantle. The magnesian composition of many intrusions was enhanced due to hornblende crystallization under oxidizing hydrous conditions and during the subsequent destruction of igneous magnetite by subsolidus actinolite–albite alteration. At the Shea prospect, main-stage Cu–Au epidote skarn is cut by biotite–albite–dolomite schist and by red biotite–albite replacement bands. Post-skarn alteration includes 20-m-thick zones of sericite–chlorite–ankerite schist confined to two D3 reverse faults. The schists are mineralized with magnetite + pyrite + chalcopyrite (up to 0.62% Cu, 1.6 g/t Au) and are linked to skarn formation by shared Ca–Fe–CO2 metasomatism. Red sericitic alteration, marked by magnetite + hematite + pyrite, occurs in fractured porphyry. The biotite/sericite alteration and oxidized ore assemblages at the Shea prospect are mineralogically identical to magnetite–hematite-bearing gold lodes at Kambalda and in the Golden Mile. Published fluid inclusion data suggest that a “high-pressure”, oxidized magmatic fluid (2–9 wt% NaCl equivalent, , 200–400 MPa) was responsible for gold mineralization in structural sites of the Boulder Lefroy and Golden Mile faults. The sericite–alkerite lodes in the Golden Mile share the assemblages pyrite + tennantite + chalcopyrite and bornite + pyrite, and accessory high-sulfidation enargite with late-stage sericitic alteration zones developed above porphyry copper deposits.  相似文献   

19.
Several mafic rock masses, which have experienced eclogite facies metamorphism, are distributed in flat-lying non-eclogitic schists in an intermediate structural level (thermal core) of the Sanbagawa belt. The largest, Iratsu mass, and an associated peridotite, the Higashi-Akaishi mass, extend E–W for about 8 km, and N–S for about 3 km, and are surrounded by pelitic, basic and quartz schists. The Iratsu mass consists of metabasites of gabbroic and basaltic origin, with intercalations of ultramafic rocks, felsic gneiss, quartz schist and metacarbonate. The Iratsu mass can be divided into two layers along a WNW-trending metacarbonate layer. The Higashi-Akaishi mass consists of peridotite with intercalations of garnet clinopyroxenite. It is situated beneath the western half of the Iratsu mass, and their mutual boundary dips gently or steeply to the N or NE. These masses underwent eclogite, and subsequent epidote-amphibolite facies metamorphism as has been reported elsewhere. The Iratsu–Higashi-Akaishi masses and the surrounding rocks underwent ductile deformation under epidote-amphibolite facies (or lower P–T) metamorphic conditions. Their foliation generally trends WNW and dips moderately to the NNE, and the mineral lineation mostly plunges to the N and NE. In non-eclogitic schists surrounding the Iratsu–Higashi-Akaishi masses, the foliation generally trends WNW and dips gently or steeply to the N or S and the mineral lineation mostly plunges to the NW, N and NE. Kinematic analysis of deformation structures in outcrops and oriented samples has been performed to determine shear senses. Consistent top-to-the-north, normal fault displacements are observed in peridotite layers of the Higashi-Akaishi mass and eclogite-bearing epidote amphibolite layers of the Iratsu mass. Top-to-the-northeast or top-to-the-northwest displacements also occur in non-eclogitic pelitic–quartz schists on the northern side of the Iratsu mass. In the structural bottom of the Iratsu–Higashi-Akaishi masses and to the south, reverse fault (top-to-the-south) movements are recognized in serpentinized peridotite and non-eclogitic schists. These observations provide the following constraints on the kinematics of the rock masses: (1) northward normal displacement of Iratsu relative to Higashi-Akaishi, (2) northward normal displacement of non-eclogitic schists on the north of the Iratsu mass and (3) southward thrusting of the Iratsu–Higashi-Akaishi masses upon non-eclogitic schists in the south. The exhumation process of the Iratsu–Higashi-Akaishi masses can be explained by their southward extrusion.  相似文献   

20.
Phase relations of pumpellyite, epidote, lawsonite, CaCO3, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe3+ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe2+ and Mg are treated as a single component because variation in Fe2+/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:
  1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H2O
  2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H2O
  3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H2O
  4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H2O
  5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H2O + CO2
  6. pumpellyite + chlorite + quartz = epidote + actinolite + H2O
  相似文献   

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