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REE-Depleted Leucogranites, Black Hills, South Dakota: a Consequence of Disequilibrium Melting of Monazite-Bearing Schists 总被引：4，自引：1，他引：3
Two isotopically distinct but otherwise chemically similar leucogranitesuites in the Proterozoic Horney Peak Granite, Black Hills,South Dakota, have contrasting light rare earth element (LREE)concentrations. Most samples of a relatively 18O-depleted suitehave LREE- enriched, chondrite-normalized patterns, typicalof melts derived from metasedimentary protoliths, whereas allsamples of the regionally significant, relatively 18O-enrichedsuite have LREE-depleted patterns. The latter patterns are interpretedto have resulted from disequilibrium melting of schists. Monaziteand perhaps other accessory minerals remained armored by biotiteand garnet which did not partake in the muscovite dehydration-meltingreaction that produced LREE-depleted melts. The REE concentrationsin the LREE-depleted samples are below saturation levels formonazite at reasonable melting temperatures and melt water contents,whereas the REE concentrations in the LREE-enriched samplesyield 700–800C monazite saturation temperatures, reasonablefor biotite dehydration-melting reactions. LREE depletions,analogous to those in the LREE-depleted granites, are also foundin leucosomes of partially molten schists, thought to be theprotolith for the granite. In contrast, the melanosomes holdthe accessory minerals and bulk of the LREEs. KEY WORDS: accessory minerals; leucogranites; Black Hills; monazite; partial melting
*Corresponding author at Department of Geological Sciences, University of Missouri. Telephone: 314-884-6463. Fax: 314-882-5458. e-mail: geolpin{at}showme.missouri.edu. 相似文献
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Conditions of Formation and Crystallization Kinetics of Highly Oxidized Pseudotachylytes from the High Tatras (Slovakia) 总被引：2，自引：0，他引：2
PETRIK IGOR; NABELEK PETER I.; JANAK MARIAN; PLASIENKA DUSAN 《Journal of Petrology》2003,44(5):901-927
Tectonic activity during the Miocene exhumation of the Tatragranitoid basement resulted in frictional melting of granite.The activity marks the early stages of the faulting that isresponsible for uplift of the High Tatras. As indicated by pre-existingcataclasite metamorphic mineral assemblages, the ambient pressurewas about 250–300 MPa, corresponding to depths between10 and 12 km. The pseudotachylytes are Fe rich, highly oxidizedand crystalline. The matrix composition suggests disequilibriumpartial melting of a biotite-dominated assemblage. Oxygen isotopiccompositions of a pseudotachylyte sample and its constituentminerals show equilibration with the host granodiorite and allowfor the introduction of oxidizing external water rather thanoxidation as a result of the dissociation of free water liberatedduring melting. The kinetic information extracted from hematitecrystal-size distributions (CSDs) that are preserved in feldspathicmatrices shows that crystals in most places were accumulatedwhile the system was open. The melt was highly mobile and proneto strong differentiation. The hematite crystals reach a maximumof 26 vol. % (45 wt % Fe2O3). In rare places where the flowceased, the system became closed and produced distinct CSDs.The longest apparent crystallization times (90 s) are recordedmostly in pools in the central parts of the pseudotachylyteswhereas the shortest times (10 s) come from rims and tips offractures. The estimated hematite growth rate was about fiveorders of magnitude higher than that of ilmenite in lava lakes.Such extreme crystallization rates result from high undercoolingsassociated with high cooling rates. Very high cooling ratesare promoted by the extremely high surface/volume ratios ofthe pseudotachylyte sheets. KEY WORDS: pseudotachylyte; hematite; kinetics; High Tatra; Slovakia 相似文献
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The Notch Peak stock, located in the House Range in westernUtah, consists of three crudely concentric sequentially intrudedunits. The earliest unit at the rim is a granite. This is followedby quartz monzonite I and then quartz monzonite II which isin the center. The stock is reversely zoned in major elements,mineralogy, and especially in the concentrations of trace elementsfrom that expected in an inwardly crystallizing pluton. Forexample, strontium and to some extent barium increase towardthe core while rubidium, thorium, uranium, niobium, tantalum,and the heavy REE's decrease toward the core. Assimilation ofcountry rocks, fluid interaction with country rocks, or thermogravitational(Soret) diffusion are not sufficient to explain the observedgcochemical variation. While progressive partial melting ofa Precambrian graywacke source cannot be excluded, such a processwould require complex metamorphic reactions during melting whichare difficult to constrain. Rather, the reverse zoning is easilyexplained by autointrusion of a fractionated diapir. Trace elementand mass-balance calculations suggest that early crystal settlingof plagioclase, quartz, and biotite toward the bottom of themagma chamber resulted in the formation of quartz monzoniteII. The settling moved the composition of the upper parts ofthe diapir toward the three-phase granite minimum where K-feldsparjoined the fractionating assemblage, resulting in the granitecomposition of the top of the diapir. The sequence of intrusions,geometry of the stock, and textures suggest that much of thefractionation occurred prior to emplacement of the diapir intothe magma chamber. Convection within the magma chamber may havebeen inhibited by the downward increase in density of the diapirdue to accumulated crystals. 相似文献
4.
喜马拉雅造山带的地壳上地幔结构——近震反射观测结果 总被引：1，自引：0，他引：1
2002-2005年完成了穿越喜马拉雅山的宽频地震探测,采用了沿剖面紧密排列的台站布置并获得了近震中的反射波资料,这对地壳和岩石圈内部速度界面的研究有重要意义.地震数据及研究结果表明,青藏高原的地震主要发生于上地壳范围内,高原莫霍面反射波的纵波PmP与横波SmS波至清晰可辨且有很强的能量,在跨越雅鲁藏布江时PmP和SmS均发生了错动,表现为北深南浅.在改则至鲁谷沿线所记录到的发生在拉萨地块的Fw21事件的地震数据中,拉萨地块内部的莫霍面反射波,尤其是SmS波至非常清晰连续,地壳纵波平均速度为Vp=6.3km/s,表明拉萨地块内部Moho面平坦、连续,不存在突起与错位.在此Moho面之前出现壳内较强反射界面,埋深45kin可能正是下地壳的顶界面.P208事件中PmP和SmS均清晰可见,然而在距震中450km及其以远地段存在两个清晰的震相PLt及SLt,并且其能量很强,可能是位于160km深处的岩石圈底界面的反映.根据对近震资料的分析研究,本文建立了该地区的地壳上地幔的结构模型. 相似文献
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Carbon isotope fractionations between calcite and graphite in the Panamint Mountains, California, USA, demonstrate the importance of mass balance on carbon isotope values in metamorphosed carbon-bearing minerals while recording the thermal conditions during peak regional metamorphism. Interbedded graphitic marbles and graphitic calcareous schists in the Kingston Peak Formation define distinct populations on a δ13C(gr)–δ13C(cc) diagram. The δ13C values of both graphite and calcite in the marbles are higher than the values of the respective minerals in the schists. δ13C values in both rock types were controlled by the relative proportions of the carbon-bearing minerals: calcite, the dominant carbon reservoir in the marble, largely controlled the δ13C values in this lithology, whereas the δ13C values in the schists were largely controlled by the dominant graphite. This is in contrast to graphite-poor calcsilicate systems where carbon isotope shifts in carbonate minerals are controlled by decarbonation reactions. The marbles record a peak temperature of 531±30 °C of a Jurassic low-pressure regional metamorphic event above the tremolite isograd. In the schists there is a much wider range of recorded temperatures. However, there is a mode of temperatures at c. 435 °C, which approximately corresponds to the temperatures of the principal decarbonation metamorphic reactions in the schists, suggesting that the carbon exchange was set by loss of calcite and armouring of graphite by newly formed silicate minerals. The armouring may explain the relatively large spread of apparent temperatures. Although the modal temperature also corresponds to the approximate temperature of the Cretaceous retrograde event, retrograde exchange is thought less likely due to very slow exchange rates involving well-crystallized graphite, armouring of graphite by silicates during the earlier event, and because of other barriers to retrograde carbon exchange. Thus, only the calcite–graphite carbon isotope fractionations recorded by the marbles demonstrate the high-temperature conditions of the low-pressure Jurassic metamorphic event that was associated with the emplacement of granitic plutons to the west of the Panamint Mountains. 相似文献
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This contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break‐up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ～50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1–4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvianite + garnet + diopside. The assemblages are successfully modelled with T–X(CO2)fluid pseudosections that suggest achievement of CO2‐rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near‐peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high‐grade portions of aureoles is consistent with eventual levelling‐off of temperatures between 350 and 400 °C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion‐wall rock contacts is demonstrated by narrow zones of anomalously low δ13C and δ18O values of carbonate minerals. Although the up to 5‰ decrease of both δ13C and δ18O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in δ13C is attributed to fluid‐mediated exchange with organic low‐13C carbon that is dispersed through the unmetamorphosed rocks and the decrease in δ18O is attributed to fluid‐mediated isotopic exchange with the gabbroic intrusive sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of several decametres; (ii) short high‐temperature regimes prevent achievement of equilibrium metamorphic textures; and (iii) T–X(CO2)fluid paths in calcareous contact aureoles are likely to be complex, reflecting a transition from prograde decarbonation reactions to influx of aqueous fluids during cooling. 相似文献
7.
The effects of infiltration, diffusion, and local structureon the contact metamorphism of the Cambrian Weeks Formation,a calcareous argillite located in western Utah, were examinedusing stable isotope systematics and mineral mass-balance considerations.The emplacement of the Jurassic Notch Peak granitic stock resultedin metamorphism characterized with increasing proximity to theintrusion by phlogopite, diopside, and wollastonite isograds.The isograds are generally concentric around the stock, withdiopside and wollastonite isograds {small tilde}600 and 400m from the contact, respectively, but are deflected near a pre-intrusionfault where the wollastonite isograd is 1. 5 km from the contact.Where the isograds are concentric, the wollastonite isogradmarks an isotopic front with whole-rock 18O values approachingthe 9.5% value of the stock. In contrast, the 18O values inthe unmetamorphosed to diopside-grade rocks range from 16.3to 20.2%. Near the fault the isotopic front extends throughthe diopside zone, suggesting that the fault was a major conduitfor magmatic water. Water—rock ratios for the diopside- and wollastonite-gradesamples determined from mineralogic mass balance are nearlyone order of magnitude larger than ‘one-box’ closed-systemratios determined from shifts in stable isotope ratios. Chromatographicmodels for isotopic exchange and propagation of isograds showthat one-dimensional infiltration of magmatic water throughpore spaces would lead to an isotopic front at 50 m from thestock and the wollastonite isograd would be only 8 km from thestock. These distances are significantly smaller than observed.It is suggested that most of the magmatic water flowed throughfractures or thin permeable layers, with the extent of isotopicexchange between the magmatic water in these conduits and theadjacent rocks being controlled by the extent of reaction progressin the rocks. Considerations of CO2—H2O interdiffusivitiesin fluids indicate that removal of CO2 from the reaction frontstoward the intrusion or the fault was sufficiently rapid tocontrol the geometry of the isograds. 相似文献
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