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1.
Carbonaceous material in the Ryoke metamorphic rocks, Kinki district, Japan   总被引：1，自引：0，他引：1  
Ge-Fan Wang   《Lithos》1989,22(4):305-316
Carbonaceous material in the Ryoke pelitic metamorphic rocks in the Wazuka area, Kinki district, Japan, has been studied by X-ray diffraction (XRD) analysis. Detrital graphite in the lower-grade rocks is recognized in both X-ray diffractograms and transmission electron micrographs. Progressive graphitization is considered to have proceeded continuously on the basis of the XRD data of the bulk concentrates of carbonaceous material, and a conspicuous asymmetric shape of XRD peaks in lower-grade samples is ascribed to the mixture of carbonaceous materials with different crystallinities. Fully-ordered graphite does not occur until the highest-grade part of the chlorite-biotite zone. The variation of degree of crystallinity of carbonaceous materials inferred from XRD data is consistent with the prograde mineral zones. The temperatures for the development of fully-ordered graphite are estimated to be between 410° and 440°C in regional metamorphism through comparing the XRD data from low-, medium- and high-pressure types of metamorphic terrains.  相似文献   

2.
A group of high grade metamorphic rocks from the Arendal area in Southern Norway has been analyzed for bulk geochemistry, and carbon and sulfur isotopic composition. A good correspondence between the composition of the Arendal rocks and common unmetamorphosed sedimentary and magmatic rocks suggests that except for some volatile compounds no mass transport took place during metamorphism. The high grade mafic rocks interlayered with the metasediments originate from basaltic tuffs and/or intrusives.Carbon occurs as graphite and also in small amounts in a still unidentified form. The carbon of the silicic metasedimentary rocks is isotopically light with an average 13C of –20.2, but significantly heavier than carbonaceous matter from unmetamorphosed Precambrian sediments. This is probably due to thermal pyrolysis of the original sedimentary organic matter and to the escape of a gas phase enriched in light carbon by reactions with the graphitic substance during metamorphism.Both the metasediments and the high grade mafic rocks have relatively high sulfur contents. The mean 34S values are 3.3 and 1.8, respectively. This can be explained by isotopically heavy sulfur in the original sediment. Some migration of sulfur probably has occurred from the metasediments into the metamafic rocks.No influence of the hypersthene isograd on the chemical and stable isotope composition could be detected.  相似文献   

3.
The ( 18O values of nine Cretaceous granitic rocks from the low P/T type regional metamorphic zone of Japan are +10.0 to +13.2 relative to SMOW, while ten Cretaceous granitic rocks from the non-metamorphic zone are +7.9 to +9.8. The 18O-enrichment in the former rocks is mainly attributed to oxygen isotopic exchange between the granitic magma and the surrounding metamorphic rocks during regional metamorphism. The assimilation of 18O-rich country rocks is also possible in the cases such as gneissose granite and migmatite.The oxygen isotopic ratios of quartz-biotite pairs in the granitic rocks indicate that they are isotopically in near-equilibrium with each other. The quartz-biotite isotopic equilibrium temperatures estimated for these rocks range from 550° to 670° C. Feldspar is occasionally isotopically in disequilibrium with other minerals. This may be caused by exchange of oxygen isotopes between feldspar and hydrothermal or meteoric water after crystallization.  相似文献   

4.
Abstract Optical and X-ray studies of carbonaceous material in the Tono contact metamorphic aureole, Kitakami Mountains, northeast Japan, have revealed that metamorphic graphitization proceeded through two discontinuous changes: first, optically anisotropic domains develop within the coaly phytoclast, forming transitional material, and then, ordered graphite crystallizes by the decomposition of pre-existing carbonaceous materials. Coaly material disappears in the uppermost chlorite zone. Transitional material appears in the middle of the lower chlorite zone. Graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso-grad to the cordierite isograd where all the carbonaceous materials have converted to graphite. The apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials. The temperature of graphitization in the present area is at least 100°C higher than the temperature in the Sanbagawa and New Caledonia high-pressure metamorphic terrains, probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism.  相似文献   

5.
Archean supracrustal sequences of pelitic, quartzitic, calcareous and mafic compositions in the central Laramie Mountains, Wyoming, have been affected by two metamorphic events: a 1.78 Ga amphibolite-grade regional metamorphism, and a 1.43 Ga contact metamorphism resulting from the intrusion of the Laramie Anorthosite Complex (LAC). Rb–Sr whole-rock isotopic data from both outside and within the LAC contact aureole define a linear array that lies along a 1.78 Ga isochron. This date has been independently established as the time of amphibolite facies regional metamorphism associated with collision of the Archean Wyoming province and the Proterozoic Colorado province along the Cheyenne belt. The Rb–Sr isotopic data require that Sr was redistributed during regional metamorphism on a scale of at least tens of metres. Although within the 2 km-wide aureole of LAC the pelitic rocks were thermally metamorphosed at temperatures greater than 800 °C, none of the whole-rock Rb–Sr data from samples within the LAC aureole show evidence of resetting at 1.43 Ga. It is interpreted that the regional metamorphism involved fluid transport which facilitated Sr isotopic resetting, whereas the contact metamorphism occurred in a relatively dry environment in which isotopic mobility was restricted to centimetre-scale or less. Rb–Sr data for biotite, feldspar and whole rock from a regional metamorphosed pelitic schist give an isochron age of 1450±40 Ma, which is interpreted as a cooling age resulting from crustal uplift. Rb–Sr data for biotite, quartz+feldspar and whole rock from a pelitic schist affected by contact metamorphism give an isochron age of 1420±43 Ma, the time of isotopic re-equilibration in response either to crustal uplift or to both contact metamorphism and crustal uplift. This study demonstrates that although the response of isotopic systems to metamorphism is complex, isotopic data provide insight into metamorphic processes that is difficult to obtain by other means.  相似文献   

6.
In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ13C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from -18 to +2‰ and from -7 to -3‰, respectively. The maximum δ13C value of graphite globules in the metasediment rises from -14 to -5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ13C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ13C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively 13C-depleted graphite indicates that the regional metamorphism increased the δ13C values of the Isua graphite. This is consistent with the regional trend of 13C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused 13C-enrichment of Isua graphite. The δ13C values of graphite reported here (δ13C > -18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ13C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13C-enrichment of Isua graphite, previously reported large 13C-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga).  相似文献   

7.
Tetsumaru Itaya 《Lithos》1981,14(3):215-224
Carbonaceous material in pelitic schists of the Sanbagawa metamorphic belt in central Shikoku, Japan, was separated from the host rocks and its X-ray diffraction and chemical composition were studied. Its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well-ordered graphite near the biotite isograd. As graphitization is a rate process, the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism. In an individual metamorphic terraan, however, the degree of graphitization is a useful indicator of relative metamorphic temperature in lower-grade rocks.  相似文献   

8.
Carbon isotope fractionation between coexisting calcite and grpahite ( 13Ccc-gr) has been determined in metamorphosed limestones and calc-silicate rocks from the Ryoke metamorphic belt in the northern Kiso district. In this district, the Ryoke metamorphic rocks, ranging from the lower greenschist facies to the upper amphibolite facies, are widely distributed. The fractionation of 13C/12C between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute 13C values of calcite. This evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification, possibly caused by retrogressive metamorphism, is not distinguished. For T=270–650° C, the fractionation is expressed by the following equation: 13Ccc-gr=8.9×106T–2–7.1 (T in °K).This equation has a slope steeper than the current results on the 13Ccc-gr versus 106T–2 diagram. It can be used as a potential geothermometer for almost the entire temperature range of metamorphism. 13C values of carbonaceous matter in unmetamorphosed limestones in this district are approximately –22, due to its biogenic origin. Graphite from metamorphosed limestones is also considered to be of biogenic origin but shows enrichment of 13C due to isotopic exchange with calcite. 13C values of graphite as well as 13Ccc-gr confirm that zone II represents the lowest grade zone of Ryoke metamorphism. The maximum equilibrium fractionation of 13C between calcite and graphite is considered to be approximately 23%, which corresponds to 270° C. Below this temperature, it seems that carbon isotopic exchange between the minerals does not occur.Calcite in marble from the higher grade zones has relatively lower 13C and 18O values. The depletion of heavy isotopes is considered to be caused by the loss of 13C and 18O enriched carbon dioxide during decarbonation reactions. For oxygen, it is considered that isotopic exchange with metamorphic fluids plays an important role in lowering the 18O value of calcite in some higher grade marbles.  相似文献   

9.
The carbon and oxygen isotopic composition of Fe-carbonate ore and its calcitic to dolomitic Devonian host rocks at the Steirischer Erzberg siderite deposit (Greywacke zone, Upper Austroalpine Unit) were determined in order to constrain the source and nature of the Fe-rich mineralizing fluid. The 18O-values obtained for various Fe-carbonate generations and the carbonate host lie within a similar range between + 14.6 and + 21.6 (V-SMOW). No good correlation exists between the relative ages of the carbonate phases and their O isotopic composition. The variation in 18O-values is due to metamorphic recrystallization with locally variable fluid/rock ratios. The average 13C-value of the carbonate host is +0.5 ± 1.2 (PDB) which corresponds well to worldwide Phanerozoic marine carbonate values. The first Fecarbonate generation has slightly lower 13C-values, on average -1.4 ± 0.8 (PDB). Recrystallization of both the carbonate host minerals and the ankerite/siderite led to significantly lower 13C-values of -4.2 ± 0.6 and-4.7 ± 0.7, respectively. Within the basal breccia of the post-Hercynian transgression series matrix calcite/ dolomite shows an average 13C-value of -2.9 ± 0.7, and matrix siderite/ankerite an average value of-4.1 ± 0.4. These data, together with Sr isotope data published previously, strongly support a late-diagenetic or epigenetic first Fe-mineralization from convecting formation waters. They ascended along extension faults and were driven by an increased heat flow caused by crustal thinning during a Devonian rifting phase that initiated the separation of the Noric terrane from Africa. A potential source of the Fe could have been the underlying Ordovician acid volcanics. Regional metamorphism related to collision tectonics in the Late Carboniferous (Hercynian) and later during the Alpine orogeny, caused intensive recrystallization and partial mobilization of the various carbonate phases.  相似文献   

10.
Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI1, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H2O that took place in psammitic rocks, and the second, BI2, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI1, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.  相似文献   

11.
Micas in 17 pelitic (K-feldspar-free) and 8 psammitic (K-feldspar-bearing) rocks from the Wazuka and the Asemi-gawa areas in the Ryoke and the Sanbagawa metamorphic terrains, respectively, were analyzed on an electron-probe microanalyzer. The deficiency of alkali cations in the low- to middle-grade metamorphic micas is ascribed to the illite substitution, KXII+AlIV=XII (vacancy)+SiIV.At the same metamorphic grade, the deficiency of interlayer cations in micas from the pelitic rocks is greater than that from the psammitic rocks. However, it decreases with rising temperature in both rock-types, irrespective of the pressure of metamorphism.K-feldspar and biotite buffer the illite substitution. Two reactions are proposed to explain the decrease of the alkali-cation deficiency in both muscovite and biotite.  相似文献   

12.
使用Renishaw System-1000型激光拉曼光谱仪, 研究了大兴安岭北部上古生界泥质岩石碳质物的拉曼光谱特征及其对形成温度的表征, 探讨了拉曼光谱参数与镜质体反射率(Ro)的关系。研究表明:研究区上古生界泥质岩石碳质物不具有石墨的拉曼光谱谱带吸收峰, 揭示了地层的变质程度未达到低绿片岩相。利用此次经过完善建立的拉曼光谱地质温度计, 对大兴安岭北部上古生界泥质岩石变质温度的估算结果主要为270~320℃, 表明研究区上古生界遭受了极低级变质作用, 变质程度属近变质带。依据碳质物拉曼光谱参数与镜质体反射率的相互关系, 估算研究区有机质成熟度的Ro值主要分布为3.03%~4.23%, 与实测Ro值吻合, 表明有机质演化处于过成熟阶段, 泥质岩石具有生烃的能力, 部分层位可能具有形成油气资源的潜力。  相似文献   

13.
The degree of recrystallization of carbonaceous material (CM), as monitored by Raman microspectroscopy, was examined as a function of metamorphic grade in two well‐studied contact aureoles containing carbonaceous pelites: the Nelson aureole, British Columbia and the Ballachulish aureole, Scotland. Here, we use (a) the R2 ratio extracted from the Raman spectrum of CM as a proxy for the degree of graphitization (0.0 in perfect graphite then increasing with structural defects) and (b) the second‐order S1 band (~2,700 cm?1) as a marker for the tridimensional ordering of CM. The Nelson aureole (garnet–staurolite–andalusite–sillimanite–K‐feldspar sequence, ~550–650°C, 3.5–4.0 kbar) was developed in rocks that were unmetamorphosed prior to contact metamorphism, whereas the Ballachulish aureole (cordierite–andalusite–K‐feldspar–sillimanite sequence, ~550–700°C, ~3.0 kbar) was developed in rocks that had been metamorphosed to garnet grade conditions (~7 kbar, ~500°C) c. 45 Ma before contact metamorphism. Thirty‐one samples were examined from Nelson and 29 samples from Ballachulish. At Nelson, the R2 ratio steadily decreases from ~0.25 to 0.0 as the igneous contact is approached, whereas at Ballachulish, the R2 ratio remains largely unchanged from regional values (~0.20–0.25) until less than 100 m from the igneous contact. The second‐order S1 band reveals that carbonaceous material (CM) was transformed to highly “ordered” locally tridimensional graphitic carbon at Ballachulish by regional metamorphism prior to contact metamorphism, whereas CM was still a disordered turbostratic (bidimensional) material before contact metamorphism in the case of Nelson. Pretexturation of CM likely induced sluggish recrystallization of CM and delayed graphitization in the Ballachulish aureole. Temperatures of recrystallization of the CM in the two aureoles were estimated using different published calibrations of the thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM), with differences among the calibrations being minor. In the Nelson aureole, temperatures are in reasonable agreement with those indicated by the metapelitic phase equilibria (all within 50°C, most within 25°C). In the Ballachulish aureole, the retarded crystallization noted above results in increasing underestimates of temperatures compared to the metapelitic phase equilibria (up to ~75°C too low within 200 m of the igneous contact). Our study calls for careful attention when using RSCM thermometry in complexly polymetamorphosed rocks to assess properly the meaning of the calculated temperature.  相似文献   

14.
M. Deb 《Mineralium Deposita》1986,21(4):313-321
The sulfur isotope composition of 86 sulfide minerals from the Middle Proterozoic, metamorphosed, stratiform, sediment-hosted Zn-Pb-CU sulfide deposits of Dariba and Sindeswar Kalan located within the Rajpura-Dariba belt in Rajasthan, NW India, have been determined. In addition, 16 carbonaceous and 2 carbonate rock samples from the ore zone have been analyzed for their Ctot and Corg contents and carbon isotope compositions. The sulfur isotope compositions range from 9.1 to –6.7 (mean value of 1.9). Increasing 34S values stratigraphically upward are observed, particularly for pyrite and pyrrhotite suggesting a syngenetic origin for the sulfur. No marked lateral isotopic variations or isotopic variation in minerals from successive laminae in banded ore samples occur. Fractionation of sulfur isotopes between coexisting sulfides suggests that the original isotopic pattern was basically preserved during the amphibolite-facies metamorphism suffered by the deposits. Corg in carbonaceous rocks ranges 0.5–9.3 wt%, with 13C values between –21 and –31 (mean of –25.4) in keeping with the biogenic derivation of the carbon. Recrystallized dolostones have 13C values close to –14.4Geological evidence and isotopic features are consistant with the following genetic scheme: (a) base-metal ores along the belt formed from geothermal emanations carrying H2S, produced by the chemical reduction of seawater sulfates and leaching of mafic volcanics, in a semiclosed (with respect to SO4), shallow-water, rift-related basin with high biological activity; (b) pyrite and pyrrhotite formed diagenetically by bacterial reduction of sulfate in pore seawater in a system open to H2S, thus bringing about the gradual enrichment of 34S in these minerals stratigraphically upward; and (c) northward in the belt, at Sindeswar Kalan, the basin of ore deposition was relatively more open.  相似文献   

15.
The Bleikvassli Zn-Pb deposit is located in the Uppermost Allochthon of the northern Norwegian Caledonides and is enclosed in amphibolite facies, multiply deformed supracrustal rocks. The stratiform orebody occurs stratigraphically above a sequence of gneiss and amphibolite and below a thick carbonate unit. The orebody, spatially associated with a footwall microcline gneiss that contains as much as 12wt K2O, occurs in the lower part of the Mine Sequence which also comprises (kyanite-) mica schist and quartzo-feldspathic to siliceous rocks. The host rock lithology and the metal content of the Bleikvassli orebody are consistent with a SEDEX origin of the deposit. Field relationships and chemistry suggest that the microcline gneiss represents a potassic alteration of pelitic sediments related to the ore-forming process. A 464 ± 22 Ma Rb-Sr isochron for the microcline gneiss is interpreted to be a metamorphic age resulting from resetting of the Rb-Sr isotopic system during the Caledonian orogeny. The U-Pb in the whole rock shows evidence of recent mobilization of uranium and a partial or total resetting of the system during peak metamorphism. As with most SEDEX deposits, the lead isotope composition of the Bleikvassli ore plots close to the orogen growth curve. The geological setting of the ore and the lead — isotope compositions of the galenas indicate a Cambrian age of mineralization. However, the slope of the lead isotope data indicate an age of about 1000 Ma, which is also a maximum age of ore deposition. The lead isotope data for the galena, in conjunction with the compositions of the microcline gneiss during peak metamorphism, support a model whereby the microcline rock was formed as an alteration product by the ore forming fluid and the initial lead isotope composition of the microcline rock was similar to that of the galenas during ore deposition.  相似文献   

16.
Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes were present at an early stage of crystallization of the igneous rocks and thus may represent xenocrystic material assimilated from the wall rocks. The range of 13C values determined for flake graphite in the igneous rocks (–26.5 to –13.8) overlaps the range for flake graphite in the wall rocks (–26.0 to –16.7), and spatial correlation of some 13C values in the plutons and wall rocks supports the assimilation mechanism. The textures of fine-grained irregular aggregates or spherulites of graphite, on the other hand, indicate that they formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a carbon-bearing aqueous fluid phase. Relative to coexisting flake graphite, spherulitic graphite shows isotopic shifts ranging from 1.9 higher to 1.4 lower in both igneous and metasedimentary samples.The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by dehydration of C-O-H fluids initially on or near the graphite saturation boundary. Hydration of silicates causes dehydration of the fluid and drives the fluid composition to the graphite saturation surface. Continued dehydration of the fluid then requires coprecipitation of secondary graphite and hydrous silicates and drives the fluid toward either higher or lower CO2/CH4 depending upon the inital bulk composition. Isotopic shifts in graphite formed at successive reaction stages are explained by fractionation of 13C between secondary graphite and the evolving fluid because 13C is preferentially concentrated into CO2 relative to CH4.Epigenetic graphite in two vein deposits assiciated with the contacts of these igneous rocks is generally enriched in 13C (–15.7 to –11.6) relative to both the igneous and wall-rock 13C values. Values of 13C vary by up to 3.4 within veins, with samples taken only 3 cm apart differing by 2.0 These variations in 13C correlate with textural evidence showing sequential deposition of different generations of graphite in the veins from fluids which differed in proportions of carbon species or isotopic composition (or both).  相似文献   

17.
The O18/O16 and D/H ratios have been determined for rocks and coexisting minerals from several granitic plutons and their contact metamorphic aureoles in the Santa Rosa Range, Nevada, and the Eldora area, Colorado, with emphasis on pelitic rocks. A consistent order of O18/O16 and D/H enrichment in coexisting minerals, and a correlation between isotopic fractionations among coexisting mineral pairs are commonly observed, suggesting that mineral assemblages tend to approach isotopic equilibrium during contact metamorphism. In certain cases, a systematic decrease is observed in the oxygen isotopic fractionations of mineral pairs as one approaches the intrusive contacts. Isotopic temperatures generally show good agreement with heat flow considerations. Based on the experimentally determined quartz-muscovite O18/O16 fractionation calibration curve, temperatures are estimated to be 525 to 625° C at the contacts of the granitic stocks studied.Small-scale oxygen isotope exchange effects between intrusive and country rock are observed over distances of 0.5 to 3 feet on both sides of the contacts; the isotopic gradients are typically 2 to 3 per mil per foot. The degree of oxygen isotopic exchange is essentially identical for different coexisting minerals. This presumably occurred through a diffusion-controlled recrystallization process. The size of the oxygen isotope equilibrium system in the small-scale exchanged zones varies from about 1.5 to 30 cm. A xenolith and a re-entrant of country rock projecting into an intrusive have both undergone much more extensive isotopic exchange (to hundreds of feet); they also show higher isotopic temperatures than the rocks in the aureole. The marginal portions of most plutons have unusually high O18/O16 ratios compared to normal igneous rocks, presumably due to large-scale isotopic exchange with metasedimentary country rocks when the igneous rocks were essentially in a molten state. The isotopic data suggest that outward horizontal movement of H2O into the contact metamorphic aureoles is very minor, but upward movement of H2O is important. Also, direct influx and absorption of H2O from the country rock appears to have occurred in certain intrusive stocks. The D/H ratios of biotites in the contact metamorphic rocks and their associated intrusions show a geographic correlation that is similar to that shown by the D/H ratios of meteoric surface waters, perhaps indicating that meteoric waters were present in the rocks during crystallization of the biotites.Except in the exchanged zones, the O18/O16 ratios of pelitic rocks do not change appreciably during contact metamorphism, even in the cordierite and sillimanite grades; this is in contrast to regional metamorphic rocks which commonly decrease in O18 with increasing grade. Thus, contact metamorphic rocks generally do not exchange with large quantities of igneous H2O, but regional metamorphic rocks appear to have done so.Publications of the Division of Geological Sciences, California Institute of Technology, Contribution No. 1565.  相似文献   

18.
A study of minerals from Proterozoic and Archean series of various regions has been carried out. Sulfate and sulfide sulfur, having doubtlessly passed through the cycle of isotopic fractionation, has been determined in Archean rocks of the Baikal region. In the lazurite deposit of Malaya Bystraya, the country rocks are Precambrian stinking (fetid) dolomites which contain ancient hydrogen sulfide with 34S from +19.0 to +33.4, and sulfate with 34S=+44.9. Carbonate rocks have served as sources of sulfide and sulfate sulfur for a number of minerals (lazurite, scapolites, apatite, pyrrhotite, pyrite, a. o.). A connection between the enrichment of sulfides of metamorphic strata in the 32S isotope or the 34S isotope and the presence in rocks of increased amounts of graphite — the product of metamorphism of the organic matter of ancient sediments — is established. A conclusion has been drawn regarding the isotopic fractionation about 3·109 y. ago.
Zusammenfassung Schwefelisotopenfraktionierung durch den Sulfid-Sulfat-Zyklus findet nur dort statt, wo freier Sauerstoff vorhanden ist. Diese Tatsache wird benutzt, um das Eintreten von freiem Sauerstoff und den Beginn organischen Lebens im Präkambrium festzusetzen. Fraktionierter Sulfid- und Sulfatschwefel in archäischen Gesteinen vom Baikalsee wurde gemessen und ein Zusammenhang zwischen dem Schwefelisotopenverhältnis und einem zunehmenden Gehalt an Graphit festgestellt, der dem Gehalt an organischer Substanz entstammt. Diese Isotopenfraktionierung datiert ungefä hr 3:109 M.J. her.
  相似文献   

19.
In Riphean mudstones of the Yudoma–Maya Trough (southeastern Siberian Platform), which underwent deep catagenetic and metagenetic transformations, 18O values range from 11.2 to 18.9 (18Oaver= 14.4), which are 5–7 lower than those in similar Cenozoic rocks. The isotopic shift is caused by postsedimentary alterations at temperatures up to 200°C under the influence of a moderate influx of metamorphogenic solutions and relative geochemical insulation of rock formations from each other. Within separate formations, we have revealed a distinct positive correlation between 18O and SiO2, which is typical for thermodynamically ordered metamorphic rocks composed of minerals in isotopic equilibrium.  相似文献   

20.
Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif, western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks, which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2 tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning. Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying heavier δ18O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines show intermediate values of 11.3 to 14.6‰ for δ18O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales (≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids, and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism. Received: 18 November 1996 / Accepted: 25 April 1997  相似文献   

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