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1.
Wood-and-Banno temperatures for the coexisting pyroxenes of equilibrated metamorphic rocks in the hornblende granulite subfaoies fall in the range 780–860° C. Minimum temperature estimates for granulites include 760–790 °C, from the dehydration of hornblende to an orthopyroxene assemblage, and about 800 °C, from other evidence. The pyroxene temperatures are generally consistent with these temperature estimates, and are certainly not too low or more than 50 ° too high. Pyroxene temperatures for the three subzones of Broken Hill granulites increase away from the orthopyroxene isograd and are sufficiently precise that the difference between the lowest and intermediate gubzones is statistically significant. Temperatures for pyroxenes in pyroxene-granulite subfacies rocks are greater than 860 °C. The internal consistency, precision and apparent accuracy of the Wood-and-Banno pyroxene geothermometer in the metamorphic temperature range make it an important tool.  相似文献   

2.
The granulites of the Fraser Range are assumed to have formed in a carbon-rich fluid, and are generally devoid of hornblende, and lack obvious hydrous retrograde features. In these granulites, pyroxene, garnet, plagioclase and quartz are the minerals most likely to retain the oxygen isotope ratios fixed at an early stage of initial granulite metamorphism. Temperature estimates using these minerals commonly suggest that oxygen isotopic exchange ceased in the range 600 to 680°C. The peak metamorphic temperature was probably ～ 850°C as based on the stability fields of the coexisting minerals and some cation temperatures from coexisting pyroxenes in these rocks. Ilmenite may be slightly out of isotopic equilibrium with the other minerals. Thus, grains of quartz, feldspar, pyroxene and ilmenite have suffered considerable oxygen isotopic exchange during the retrogressive phase of the metamorphism, in spite of the fact that very little water was present in these granulites. The observed deviation from the peak metamorphic temperatures can be explained by essentially closed system solid-state diffusion (on at least a scale of centimetres) during slow cooling of the rocks from ~850 to 650°C, followed by more rapid cooling down to ～ 300°C. Such an explanation is not at variance with the radiometric data available for rocks from the area, which suggest that the latter phase could have involved uplift rates of ?0.5 mm/yr for a period of about 40 Ma. Wholerock δ18O values on non-quartzose mafic granulites, about 7.2%., fall within the range of basalts affected by seafloor weathering.  相似文献   

3.
Data from analyses of three coexisting pairs of pyroxenes with a wide range of Fe content from each of two localities are used to show the large systematic variation and predictable correlation of Fe (or Mg) of a pyroxene with its content of Al, Mn and Na in mafic granulites. Comparisons of pyroxenes can then be made more meaningful by normalizing Al, or other elements, to an appropriate Mg value. As both P and T may affect the element distribution of the two pyroxenes differently (especially Al and Na) the factor used in normalizing is found to vary from region to region.  相似文献   

4.
Garnetiferous basic granulites occur, as parts of hornblende-pyroxene- and pyroxene granulites, in a Precambrian terrain around Saltora. The chemistry of the garnetiferous basic granulites is broadly similar to that of the hornblende-pyroxene granulites, their immediate precursors, but in detail they have distinctly higher Fe/Mg ratios. The compositions of the major mafic silicates of the garnetiferous varieties do not reflect higher pressures of formation: the Jd/Ts ratios in calcic pyroxenes are similar to those from the non-garnetiferous varieties, and the pyrope contents of garnets are low. Exchange equilibrium in respect of major elements was established among the mafic silicates in spite of garnets being late overprints. The orthopyroxene — calcic pyroxene pairs from the garnetiferous granulites show lower values of K D(Mg-Fe) opx-cpx than those from the non-garnetiferous granulites, pointing to lower temperature of equilibration. The K D(Mg-Fe) opx-hbl K D(Mg-Fe) cpx-hbl relations show that the more magnesian triads equilibrated at lower temperatures; viewed against experimental data regarding the effect of Mg/Fe ratios on the appearance of garnets in basic rocks, formation of garnets by cooling is strongly indicated. Several intergrowth textures, especially garnet-ilmenite and garnet-quartz (±albite) symplectites, and modal relations argue in favour of composite reactions of the type hornblende+ quartz-→calcic pyroxene+garnet+albite+H2O, which couple hornblende breakdown reactions with orthopyroxene+anorthite→garnet reactions. The approximate range of pressure and temperature conditions, estimated from experimental data, are 6–8.5 kb and 750–830° C. Since garnets formed by cooling in iron-rich granulites, the garnetiferous granulites do not represent higher pressure subfacies of the granulite facies.  相似文献   

5.
冀东早太古代迁西群是由火山-沉积岩系变质生成的一套变质杂岩。它的主体是麻粒岩类以及退变质生成的斜长角闪岩类的岩石。该套变质岩系的下部夹有层状和透镜状变质超镁铁质岩(变橄辉岩、变辉橄岩、变辉石岩及变闪辉岩),而上部为变质的含铁岩系。以石渣子山和娄子山为例,它们的含铁岩层分别为辉石磁铁石英岩和英榴易熔岩。 本区的麻粒岩和斜长角闪岩在岩相学上呈渐变关系。绝对不含角闪石和/或富钛黑云母的麻粒岩是很少的。本文将以角闪石和斜长石为主要组成矿物的岩石归为斜长角闪岩类。在斜长角闪岩类岩石中,角闪石含量大大超过辉石,另外常见被角闪石交代的辉石残晶。  相似文献   

6.
Three pairs of coexisting pyroxenes of mafic granulites from each of two locations 100 km apart show large chemical differences, especially in Al, Fe, Mn, Ti and Na. Al content of the pyroxenes at the higher pressure locality is more nearly independent of Al of the host rock than are the pyroxenes from the lower pressure locality. All the data confirm that although no significant difference in temperature has emerged, there was a large difference in pressure between the two localities. Al is found to be a more effective discriminant of metamorphic conditions than . As the three pairs of pyroxenes cover a wide range of Fe at each locality, the close relationship of Al and Na (and of Ca-tschermak and jadeite) to Fe becomes evident. This shows that a normalizing procedure should be adopted before comparing localities with different Al, Mn, Ti, Na and other elements or derived components such as jadeite and Ca-tschermak.  相似文献   

7.
The partition of Si, Al, Ti, Fe3+, Mg, Fe2+, Mn, Ca and Na between coexisting Ca-rich and Ca-poor pyroxenes from a wide variety of igneous and metamorphic rocks have been investigated systematically. Many of the distributions, and especially those for the partition of Ti, Mg, Fe2+, Mn and Na, indicate characteristic trends for pyroxenes from the various petrologic groups identified. The partition of Mg, Fe2+ and Mn correlate with inferred cooling rates, the partition co-efficients of pyroxenes from extruded and other quickly cooled rocks most nearly approaching unity. In contrast, the partition of Si and Ti and the absolute amounts of Al may be related to the physicochemical conditions prevailing during original crystallisation; Ti being particularly relatively enriched in Ca-rich pyroxenes of ultramafic associations. The trends of the compositions of the Ca-rich pyroxenes plotted in the pyroxene quadrilateral also correlate with cooling rates and comparison with the limited data available on the phase relations of coexisting pyroxenes suggests that sub-solidus chemical readjustments have occurred in both phases.  相似文献   

8.
Basic granulites occurring as small enclaves and pods within charnockites contain predominantly orthopyroxene, clinopyroxene, hornblende, plagioclase feldspar and quartz. Chemical composition of coexisting orthopyroxene, clinopyroxene, plagioclase and hornblende has been represented in ACF and AFM diagrams. The mineral assemblages and the textural relationships of the basic granulites have been described. Garnet is notably absent in the basic granulites and this is explained as due to lower (< 8 kbar) pressure and relatively magnesian bulk composition.  相似文献   

9.
Iron-magnesium distribution coefficients for coexisting ortho- and clinopyroxene in 22 amphibolites from the New Jersey Precambrian Highlands range from 1.40 to 1.90. No systematic areal variation of the distribution coefficient is discernable within a 700 mi2 area. The distribution coefficient is, however, systematically related to pyroxene composition. The distribution coefficient tends to increase with increasing pyroxene weight % FeO (Fe as FeO) and decrease with increasing MgO and Al2O3. Data from other workers indicates that the distribution coefficient versus pyroxene composition trends found in the Highlands amphibolites are also present in both igneous and metamorphic rock suites from several other areas. Possible influence of pyroxene CaO on the distribution coefficient is also indicated. The Highlands amphibolite type trends are, however, directly opposite to those previously reported for Australian granulites. Both types of trends are apparently valid since both are present in at least one instance in metamorphic rocks from a relatively small area. The causes for the development of the two types of trends are imperfectly understood. Data presented indicates, however, that the New Jersey amphibolite type trends are apparently more characteristic of Fe-poor pyroxenes, whereas, the Australian granulite type trends are more characteristic of Fe-rich pyroxenes.The distribution coefficient in the Highlands amphibolites is also systematically related to bulk-rock composition due to the sympathetic variation of pyroxene Fe-Mg content with total rock MgO/FeO(Fe as FeO). The observed range of the distribution coefficient in the Highlands amphibolites may, consequently, mostly reflect variation in bulk-rock composition and not variation in crystallization temperature.  相似文献   

10.
Garnet-clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the Sesia zone in the Western Alps. Discrepancies in apparent temperatures of up to 600° C have been found suggesting substantial deviation from ideal behaviour of the garnet-clinopyroxene equilibria in certain assemblages. Assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low K D (2.0) values and correspondingly high apparent temperatures (> 1,000° C). The garnets coexisting with high-Na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes. Departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer. Numerous examples of coexisting garnet-clinopyroxene pairs indicate that there is no obvious relationship of K D with jadeite content in pyroxenes with jadeite content less than 70%, but at higher values, K D varies inversely with the jadeite content. The dependence of K D upon the jadeite content in the high-Na pyroxenes is believed to be due to the preference of M2 sites for Fe2+ in the enstatite-jadeite substitution (Fe2+MgNa–1Al–1). This substitution is usually very limited and has no demonstrable effect upon X Fe Cpx when the M1 (Fe2++Mg) content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes. However, when the M1 (Fe2++Mg) content is low, as in jadeitic pyroxenes, the non-ideality of the enstatite type exchange has a marked effect on X Fe Cpx and consequently on garnet-clinopyroxene calculations.  相似文献   

11.
Distribution coefficients (K D·Fe ++ –Mg) calculated for orthopyroxene-clinopyroxene pairs from 12 basic granulites of the Quairading district, Western Australia, range from 1.87 for magnesian pyroxenes (Opx Mg value=78.1) to 1.70 for iron-rich varieties (Opx Mg value 37.7). Field and petrographic evidence indicates that these pyroxenes have probably reached equilibrium within a narrow temperature range. In order to account for the observed variation of K D values it is suggested that one (or both) of the pyroxene structures is not the ideal Fe++-Mg solution proposed in the thermodynamic model of the pyroxene equilibrium exchange. After consideration of the geometry of the pyroxene cation sites, the relative bond energies of each site (especially crystal field stabilization energy) and structural ordering a model is proposed to explain the non-ideal behaviour of Fe++-Mg in the pyroxene system. The distribution pattern in low-iron pyroxenes will probably show Fe++ favouring the M 2·Opx site; competing unfavourably with Mg++ for the M 1·Cpxsite; and probably excluded by Mg++ from the M 1·Opxsite. As the iron content of the system increases the M 2·Opxsite will begin to become saturated with Fe++ and this ion will enter the M 1·Opx site. Further increases in the iron content of the system will cause the Fe++-Mg distribution to depend increasingly on the relative attraction of the M 1 sites of both pyroxenes. Of these sites Fe++ should show preference to the more distorted M 1·Cpxsite. The distribution coefficient reflects this swing towards a relative enrichment of Fe++ in the clinopyroxene by decreasing regularly with increasing iron content. It is likely that this downward trend will not become evident until the iron content of the M 2·Opx site reaches saturation. This would explain why the K D values for the magnesian pyroxenes remain practically unchanged until the orthopyroxene Mg value is approximately 60; from here on the iron-rich pyroxenes show a rapid decrease in K D value with increasing iron content.The Ca content of the pyroxenes is also significant since the Quairading pyroxenes show a marked increase in mutual solubility with increasing iron content. Calcium taken into the orthopyroxene structure will enter the M 2 site ahead of Fe++ so that this site will reach Fe++ saturation at a lower iron content than when the orthopyroxene is Ca-free.The application of K D values to the regional study of basic granulites, particularly when establishing relative temperature zones on the basis of K D variation, should only be attempted when pyroxenes which extend over a wide range of Fe-Mg content are available.  相似文献   

12.
A thermodynamic model for the Gibbs free energy of igneous pyroxenes with the general formula [Na, Ca, Fe2+, Mg]M2[Fe2+, Mg, Ti, Al, Fe3+]M1[Al, Fe3+, Si]TetSiO6 is calibrated from experimentally determined compositions of coexisting pyroxene and silicate melt. The model is based upon the general formulation, and relies upon the calibration of the “quadrilateral” subsystem, previously published by the present authors. The calibration database of pyroxene-liquid equilibria spans a broad spectrum of temperature, pressure and oxygen fugacity conditions, ranging from 1000°–1600°C, 0.001–30 kbar and iron-wüstite to air. Chemical potentials of endmember pyroxene components as well as exchange potentials between pyroxenes and coexisting liquids are defined utilizing the present authors' thermodynamic melt model. Model parameters are extracted from these relations by regression analysis. The resulting model and derivative endmember properties are internally consistent with an existing standard state thermodynamic database. The success of the model and its applicability to igneous petrogenesis are demonstrated by comparing calculated and experimentally determined liquidus compositions, temperatures and symmetry states for pyroxenes crystallizing from a variety of silicate melts, ranging in composition from tholeiites and angrites through rhyolites to potash ankaratrites.  相似文献   

13.
Distribution of elements in coexisting minerals—biotite, hornblende, augite, hypersthene and plagioclase in charnockitic rocks of West Uusimaa Complex, Finland, is mostly orderly indicating a close approach to chemical equilibrium. The distribution of iron and magnesium in coexisting hornblende and pyroxenes of basic charnockites and other rocks of granulite facies from several different areas is also orderly but the variation in the fugacities of H2O and H2 may cause a disorderly distribution locally in some rocks. The probable oxidation or reduction reactions are discussed on the basis of thermochemical and mineralchemical data.  相似文献   

14.
The Palghat Cauvery Shear Zone (CSZ) is a major shear zone that possibly extends into different fragments of Gondwanaland. In the present study mafic granulites occurring on either side of the CSZ in Namakkal area, southern India are examined. Textural features recorded in the mafic granulites are crucial in elucidating the metamorphic history of the southern granulite terrane (SGT). In the mafic granulites occurring to the south of CSZ, evidence of garnet breaking down during near isothermal decompression (ITD) is indicated by the development of orthopyroxene + plagioclase moats in between quartz and garnet. The presence of comparatively small elongated second generation garnet embedded in pyroxenes from the mafic granulites occurring to the north of CSZ is indicative of the garnet formation via reaction between pyroxenes and plagioclase, which occurred during isobaric cooling (IBC). Rocks occurring to the south of CSZ have recorded comparatively higher temperature and pressure (849‡C and 9.6kbar) than those occurring to the north of the CSZ (731‡C and 8.6kbar) using conventional geothermobarometry. The rocks occurring to the north of CSZ have suffered more complex metamorphic histories in comparison to the southern part. Integrating the results of the present field and metamorphic studies with the earlier investigations and available geochronological data we suggest that the CSZ could represent a suture zone between two different continental blocks that underwent distinct metamorphic evolution.  相似文献   

15.
Analyses of coexisting garnets, clinopyroxenes and plagioclases from eclogites and high pressure granulitic gneisses in the Kristiansund area within the west Norwegian basal gneiss region are used to establish the P-T conditions for the metamorphic peak for these rocks. Based on the distribution of Fe and Mg between coexisting garnet and clinopyroxene in both eclogite and granulites, equilibrium temperatures are estimated to 750 °±50 ° C. Pressures are derived from the absence of orthopyroxenes in the granulites, and from the assemblage clinopyroxene +plagioclase+quartz present in the gneisses. Equilibrium pressures are estimated to 18.5±3.0 kilobars, and these equilibrium conditions are thus compatible with equilibrium conditions derived for both orthopyroxene-free and most orthopyroxene-bearing country-rock eclogites from adjacent areas.  相似文献   

16.
The Tongbai granulites are present mainly as xenoliths in granodioritic gneisses. The xenoliths with a zircon age of 470Ma are older than the host rocks of granodioritic gneisses which yield a zircon age of 435Ma. It is suggested that the granulites were transported from the lower crust to the upper level along with granodioritic magma. Geothermometrical and geobarometrical studies based on the coexisting minerals (Opx-Cpx and Opx-Gar) show that the granulites were crystallized at 818 –840 °C and 9.5−9.8 × 108 Pa corresponding to the lower crust. Tectonically, the Shangdan suture zone constitutes the boundary between the North China and Yangtze plates. The zone is char acterized by the occurrence of ophiolites in the western part and by that of granulites in the eastern part. So the western part marks the upper crustal level of the Qinling belt, while the eastern part represents the exposure of a deeper level. The results of isotopic dating and the geochemical characteristics of the xenoliths are consistent with those of metatholeiites of the ophiolites in the western part. Therefore, it is assumed that both ophiolites found in the west and granulites found in the east all represent the remnants of the ancient Qinling ocean plate. The difference is that the ophiolites are pieces of obducted fragments from the ocean floor during the subduction in the Early Palaeozoic. However, in the Tongbai area, when the ocean floor was subducting towards the lower crust, it underwent a granulite fades metamorphism. Subsequently, granodioritic magma formed by partial melting trapped some fragments of granulite upwards. This project was jointly granted by the National Natural Science Foundation of China and Stiftung Volkswagenwerk of Germany  相似文献   

17.
Garnet occurs widely as a secondary mineral in the granulite-facies rocks of the Lofoten-Vesteraalen islands of North Norway. The garnet-forming reactions may be inferred from the resulting textures and are interpreted as being retrograde reactions. Microprobe analyses show that garnets with high proportions of CaO and MgO occur in coronas around olivine grains in anorthosites, whereas the most iron-rich garnets formed by reaction between plagioclase and the very iron-rich pyroxenes of some monzonitic (mangeritic) rocks. Garnets ranging in composition between these extremes formed by reactions involving biotite, plagioclase and magnetite. Textural features indicate that these reactions have been induced by oxidation of the biotite; the liberated water has converted the pyroxenes to amphibole. The net effect of the reactions is to transform the granulite-facies assemblages to amphibolite-facies assemblages.The secondary garnet is nearly ubiquitous in the granulites of the outer Lofoten islands, which may show no other signs of retrogression. On Langöy in Vesteraalen garnet occurs only within a zone of strongly retrograded gneisses. Differences in bulk composition do not satisfactorily explain the distribution of secondary garnet. This distribution, and the observed oxidation textures, imply widespread penetration of the Lofoten granulites by a transient oxidizing fluid. On Langöy these fluids apparently affected only the narrow retrograded zone. The model suggests that the 1700 to 1800 m.y. Rb/Sr date obtained by Heier and Compston (1969) for the Lofoten granulites represents the time of oxidation, whereas the 2800 m.y. date for the Langöy granulites represents the original granulite-facies metamorphism.Publication number 3 in the Norwegian Geotraverse Project.  相似文献   

18.
青藏高原北缘火山岩中辉石岩包体研究   总被引：7，自引：2，他引：7       下载免费PDF全文
青藏高原北缘可可西里鲸鱼湖、雄鹰台、双泉子火山岩中含有辉石岩包体 ,其主要矿物组成为单斜辉石和斜方辉石。辉石的化学成分与国内外幔源包体 -橄榄岩和辉石岩中的类似 ,与麻粒岩中的明显不同 ;辉石温压计计算结果表明 ,辉石形成的温度为 110 1～ 140 0℃ ,压力为 30× 10 8～ 6 0× 10 8Pa,结合岩浆动力学计算结果 ,推测岩浆的来源深度大于 15 0km ,这与地球物理研究得出本区地下深处存在低速层的结果相吻合。  相似文献   

19.
D.A Carswell 《Lithos》1974,7(2):113-121
The chemistry of the pyroxenes suggests that the garnet lherzolites enclosed in the Norwegian basal gneisses have equilibrated at depths greater than 70 kilometres along an expected sub-continental geotherm. Such depths are somewhat shallower than the apparent depths of origin of most garnet lherzolite xenoliths in kimberlite pipes. Distribution coefficients for Fe2+/Mg2+ and Mn2+/Mg2+ between coexisting clinopyroxenes and garnets support the slightly lower equilibration temperatures deduced for the Norwegian garnet lherzolites compared with the xenolithic garnet lherzolites in kimberlites.The pressure-temperature equilibration conditions deduced for the Norwegian garnet lherzolites (800–1020°C at 22–37 kbs) contrast with previous estimates (625 ± 30° at 14 kbs) for basic eclogite masses in the Norwegian gneisses. This suggests a possible dual paragenesis of the Norwegian eclogites, with the garnet lherzolites being tectonic slices of the sub-continental upper mantle and the basic eclogites deep crustal metamorphic rocks.  相似文献   

20.
Oxygen isotope geothermometry of coexisting minerals from five mafic granulites does not reflect the peak temperature of metamorphism as determined by other methods. Exchange of 18O between phases during slow cooling has (a) lowered the δ 18O of clinopyroxene relative to orthopyroxene in the presence of hornblende, (b) raised the δ 18O of quartz in a manner which is partly dependent on quartz abundance, and (c) lowered the δ 18O of plagioclase in some rocks.  相似文献   

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