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31.
Calculated Phase Relations in the System Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O with Applications to UHP Eclogites and Whiteschists 总被引:2,自引:0,他引:2
Pressuretemperature grids in the system Na2OCaOK2OFeOMgOAl2O3SiO2H2O and its subsystems have been calculatedin the range 1545 kbar and 550900°C, usingan internally consistent thermodynamic dataset and new thermodynamicmodels for amphibole, white mica, and clinopyroxene, with thesoftware THERMOCALC. Minerals considered for the grids includegarnet, omphacite, diopside, jadeite, hornblende, actinolite,glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc,muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibilitydiagrams are used to illustrate the phase relationships in thegrids. Coesite-bearing eclogites and a whiteschist from Chinaare used to demonstrate the ability of pseudosections to modelphase relationships in natural ultrahigh-pressure metamorphicrocks. Under water-saturated conditions, chlorite-bearing assemblagesin Mg- and Al-rich eclogites are stable at lower temperaturesthan in Fe-rich eclogites. The relative temperature stabilityof the three amphiboles is hornblende > actinolite > glaucophane(amphibole names used sensu lato). Talc-bearing assemblagesare stable only at low temperature and high pressure in Mg-and Al-rich eclogites. For most eclogite compositions, talccoexists with lawsonite, but not zoisite, in the stability fieldof coesite. Water content contouring of pressuretemperaturepseudosections, along with appropriate geotherms, provides newconstraints concerning dehydration of such rocks in subductingslabs. Chlorite and lawsonite are two important H2O-carriersin subducting slabs. Depending on bulk composition and pressuretemperaturepath, amphibole may or may not be a major H2O-carrier to depth.In most cases, dehydration to make ultrahigh-pressure eclogitestakes place gradually, with H2O content controlled by divariantor higher variance assemblages. Therefore, fluid fluxes in subductionzones are likely to be continuous, with the rate of dehydrationchanging with changing pressure and temperature. Further, eclogitesof different bulk compositions dehydrate differently. Dehydrationof Fe-rich eclogite is nearly complete at relatively shallowdepth, whereas Mg- and Al-rich eclogites dehydrate continuouslydown to greater depth. KEY WORDS: dehydration; eclogites; phase relations; THERMOCALC; UHP metamorphism; whiteschists 相似文献
32.
The Physico-Chemical Properties of a Subducted Slab from Garnet Zonation Patterns (Sesia Zone, Western Alps) 总被引:3,自引:0,他引:3
KONRAD-SCHMOLKE MATTHIAS; BABIST JOCHEN; HANDY MARK R.; O'BRIEN PATRICK J. 《Journal of Petrology》2006,47(11):2123-2148
Garnets in continentally derived high-pressure (HP) rocks ofthe Sesia Zone (Western Alps) exhibit three different chemicalzonation patterns, depending on sample locality. Comparisonof observed garnet zonation patterns with thermodynamicallymodelled patterns shows that the different patterns are causedby differences in the water content of the subducted protolithsduring prograde metamorphism. Zonation patterns of garnets inwater-saturated host rocks show typical prograde chemical zonationswith steadily increasing pyrope content and increasing XMg,together with bell-shaped spessartine patterns. In contrast,garnets in water-undersaturated rocks have more complex zonationpatterns with a characteristic decrease in pyrope and XMg betweencore and inner rim. In some cases, garnets show an abrupt compositionalchange in core-to-rim profiles, possibly due to water-undersaturationprior to HP metamorphism. Garnets from both water-saturatedand water-undersaturated rocks show signs of intervening growthinterruptions and core resorption. This growth interruptionresults from bulk-rock depletion caused by fractional garnetcrystallization. The water content during burial influences significantly thephysical properties of the subducted rocks. Due to enhancedgarnet crystallization, water-undersaturated rocks, i.e. thoselacking a free fluid phase, become denser than their water-saturatedequivalents, facilitating the subduction of continental material.Although water-bearing phases such as phengite and epidote arestable up to eclogite-facies conditions in these rocks, dehydrationreactions during subduction are lacking in water-undersaturatedrocks up to the transition to the eclogite facies, due to thethermodynamic stability of such hydrous phases at high PTconditions. Our calculations show that garnet zonation patternsstrongly depend on the mineral parageneses stable during garnetgrowth and that certain co-genetic mineral assemblages causedistinct garnet zonation patterns. This observation enablesinterpretation of complex garnet growth zonation patterns interms of garnet-forming reactions and water content during HPmetamorphism, as well determination of detailed PT paths. KEY WORDS: dehydration; high-pressure metamorphism; Sesia Zone; subduction; thermodynamic modelling 相似文献
33.
Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates? 总被引:3,自引:0,他引:3
Layers of Ca-rich garnetclinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the SuLu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards FeTi gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crustmantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped PT pathsfor the garnetclinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnetclinopyroxene rocks; SuLu terrane; UHP metamorphism 相似文献
34.
中国大陆科学钻探工程5000m主孔位于苏鲁超高压变质带南部。该钻孔0~2000m主要由榴辉岩、片麻岩、石榴石橄榄岩和少量片岩和石英岩组成。累积厚度达1000多米的榴辉岩具有不同的矿物组成、不同的矿物含量和不同的全岩化学成分,可划分成富Si的石英榴辉岩、富Ti的金红石榴辉岩、富Al的多硅白云母和蓝晶石榴辉岩、富Mg的双矿物榴辉岩和具有正常玄武岩成分的普通榴辉岩。榴辉岩的原岩包括基性层状侵入岩和变质表壳岩。榴辉岩全岩成分对石榴石和绿辉石中某些化学组分的含量有明显控制,而且直接影响到变质条件估算的准确性。扩散成分环带的广泛发育表明超高压矿物在早期退变质过程中发生了成分再平衡。这一事实以及具有成分生长环带石榴石变斑晶的存在,为榴辉岩形成在更高温度(>940℃)和更高压力(>4.5GPa)条件下提供了有力的证据。 相似文献
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36.
S. Bodorkos M. Sandiford N. H. S. Oliver P. A. Cawood 《Journal of Metamorphic Geology》2002,20(2):217-237
High‐T, low‐P metamorphic rocks of the Palaeoproterozoic central Halls Creek Orogen in northern Australia are characterised by low radiogenic heat production, high upper crustal thermal gradients (locally exceeding 40 °C km?1) sustained for over 30 Myr, and a large number of layered mafic‐ultramafic intrusions with mantle‐related geochemical signatures. In order to account for this combination of geological and thermal characteristics, we model the middle crustal response to a transient mantle‐related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere. This mechanism has the potential to raise mid‐crustal temperatures by 150–400 °C within 10–20 Myr following initiation of the mantle temperature anomaly, via conductive dissipation through the crust. The magnitude and timing of maximum temperatures attained depend strongly on the proximity, duration and lateral extent of the thermal anomaly in the mantle lithosphere, and decrease sharply in response to anomalies that are seated deeper than 50–60 km, maintained for <5 Myr in duration and/or have half‐widths <100 km. Maximum temperatures are also intimately linked to the thermal properties of the model crust, primarily due to their influence on the steady‐state (background) thermal gradient. The amplitudes of temperature increases in the crust are principally a function of depth, and are broadly independent of crustal thermal parameters. Mid‐crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Although pluton‐related thermal signatures significantly dissipate within <10 Myr (even for very large, high‐temperature intrusive bodies), the interaction of pluton‐ and mantle‐related thermal effects has the potential to maintain host rock temperatures in excess of 400–450 °C for up to 30 Myr in some parts of the mid‐crust. The numerical models presented here support the notion that transient mantle‐related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high‐T, low‐P metamorphic belts, especially in those characterised by low surface heat flow, very high peak metamorphic geothermal gradients and abundant mafic intrusions. 相似文献
37.
Constraints on the application of carbon isotope thermometry in high- to ultrahigh-temperature metamorphic terranes 总被引:7,自引:0,他引:7
Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ13C and δ18O values in calcite and heterogeneous single crystal δ13C values (? 1 to ? 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ13C and δ18O values in marbles by infiltration of low δ13C–δ18O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO2‐rich fluid, and that the infiltration had occurred under high temperature and low fO2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ13C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism. 相似文献
38.
Controls on coal metamorphism can be complex. In this paper, we examine four Paleozoic coalfields: the western Kentucky portion of the Illinois Basin, the Pennsylvania anthracite fields, the South Wales Coalfield, and the Bowen Basin. An increase in temperature with depth of burial is certainly a factor in coal metamorphism. In many coalfields, however, including the coalfields reviewed here, it has become apparent that such a simple mechanism does not explain the coal rank patterns observed. The flow of hydrothermal fluids through the coals has been proposed as a cause of coal metamorphism. Evidence includes inverted rank gradients, elevated CFL as an indicator of brine fluids, isotopic evidence for hydrothermal fluids, and vein and cleat mineral assemblages. In any case, multiple hypotheses must often be evaluated in the examination of any coalfield since the simple paradigm of coal rank increases with a simple increase in temperature with increasing depth does not fit the evidence observed in many cases. 相似文献
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