Comparison of the Garnet--Orthopyroxene Geobarometer with Recent Experimental Studies, and Applications to Natural Assemblages   总被引：1，自引：0，他引：1  

HARLEY  SIMON L. 《Journal of Petrology》1984,25(3):697-712

The garnet-orthopyroxene geobarometer developed by Harley (1984)in the accompanying paper is compared with other recent experimentaldata on alumina solubility in orthopyroxene, obtained both inend-member and more complex systems. The geobarometer, in partbased on recent reversed data in MgO-Al₂O₃-SiO₂ (Perkins etal., 1981), is in reasonable agreement with other recent dataincluding the reversed data of Perkins & Newton (1980) inCaO-MgO-Al₂O₃-SiO₂. Comparison with the experimental data ofWood (1974) in the FeO-MgO-Al₂O₃-SiO₂ system indicates thatless alumina enters orthopyroxene at a chosen P-T conditionthan suggested by that study. Application of the geobarometer to garnet-peridotite xenoliths,and peridotite massifs within gneiss terrains yields broadlyconsistent and reliable pressure estimates provided the temperatureof equilibration is well constrained. Existing data for garnet-peridotitexenoliths in kimberlite yield P-T estimates which lie near steady-statecontinental geotherms but which lie well to the high-temperatureside of such geotherms at high pressures. These data, however,do not necessarily define a simple ‘kinked geotherm’.It is suggested instead that the xenolith data must be viewedin terms of polygenetic models of equilibration and cation exchangeclosure. Garnet-peridotite bodies enclosed in gneisses in the Alps andNorway yield pressure estimates in the range 20–30 kilobars.Such pressure estimates favour a mantle origin and tectonicemplacement model for these bodies. The most recent data relevantto the ‘country-rock’ orthopyroxene-eclogites fromNorway give pressure estimates (18 ? 8 kb) which do not allowdiscrimination between the in situ model of formation and thetectonic emplacement models. Some suites of crustal granulites yield pressure estimates whichare internally consistent, consistent with the presence of characteristicaluminosilicate polymorphs, and similar to pressures calculatedusing other reliable geobarometers where these are available.However, the geobarometer may underestimate pressures in comparisonwith a previous calibration (Harley & Green, 1982), particularlywhere measured values of K^ga-opx _DFe-Mg are high. This discrepancyresults from the sensitivity of the geobarometer equation torelatively small changes in garnet composition, independentof variations in alumina contents of the orthopyroxenes. A diversesuite of samples from a single locality in Enderby Land, Antarctica,illustrates some of these problems and uncertainties.  相似文献   

The Four Plutonic Belts of the Transhimalaya-Himalaya: a Chemical, Mineralogical, Isotopic, and Chronological Synthesis along a Tibet-Nepal Section   总被引：31，自引：0，他引：31  

DEBON  FRANCOIS; FORT  PATRICK LE; SHEPPARD  SIMON M. F.; SONET  JACQUES 《Journal of Petrology》1986,27(1):219-250

The geochemistry (major, trace element, O- and Sr-isotope ratios)and petrology of the Transhimalaya, North Himalaya, High Himalayaand ‘Lesser Himalaya’ plutonic belts are comparedbased on the analyses of up to 492 samples. The composite Transhimalaya batholith is subalkaline or monzoniticin character rather than calc-alkaline. Its genesis was probablyclosely related to subduction processes associated with strike-slipmovement. It was emplaced on both sides of the boundary betweenan earlier metavolcanic arc and a continental margin. Two principalperiods of magmatic activity occurred: Upper Cretaceous and,particularly in this region, Eocene at the time of the India-Eurasiacollision when sediments may have become involved in the subductionprocess. Magmatic differentiation, characterized by two superimposedstages of evolution, and hybridization processes, involvingboth basic and acidic magmas, can account for the genesis ofthe different plutonic units. Although a continental contributionis implied, the isotopic data (6.8 < ¹⁸O < 9?2; 0?704< ⁸⁷Sr/⁸⁶Sr, < 0?707) preclude a significant contributionfrom either old crust or surface derived sediments. The North, High and ‘Lesser’ Himalaya plutonic beltsare fundamentally different and correspond to aluminous associationsof two groups of ages (Lower Palaeozoic for the ‘LesserHimalaya’ and part of the North Himalaya; Upper Cenozoicfor the High Himalaya and part of the North Himalaya). Theyare all high-¹⁸O (9 < ¹⁸O < 14) granites and adamelliteswith high initial ⁸⁷Sr/⁸⁶Sr, ratios (0?709 to < 0?740). TheLower Palaeozoic group was generated within the Gondwana continentalcrust, independent of any true orogenesis, with a probable butlimited contribution from the mantle. High Himalaya and NorthHimalaya Cenozoic plutons are directly linked to the activityof the Main Central Thrust. They were derived by similar anatecticprocesses of the same continental source rocks. The small butdistinct chemical and mineralogical differences among the plutonsare related to the increase in the intensity of anatexis ongoing towards the north and the east.  相似文献   

Formation of an unusual compact Type A refractory inclusion from Allende     

STEVEN B. SIMON  ANDREW M. DAVIS  LAWRENCE GROSSMAN 《Meteoritics & planetary science》1998,33(1):115-126

Abstract— We report the results of a study of TS2, an unusual compact Type A inclusion from Allende. A distinctive, major feature of this inclusion is that many of its melilite crystals have no dominant core-rim zoning but instead consist of 50–200 μm patches of Mg-rich melilite (Åk_32–62, median Åk₅₁) set in or partially enclosed by, and optically continuous with, relatively Al-rich melilite (Åk_25–53, median Åk₃₈). The Al-rich regions have jagged, dendritic shapes but occur within crystals having straight grain boundaries. Another unusual feature of this inclusion is the size and spatial distribution of spinel. In many places, especially in the interior of the inclusion, the aluminous melilite encloses numerous, fine (0.5–5 μm) inclusions of spinel and minor perovskite and fassaite. The latter phases also occur as isolated grains throughout the inclusion. Coarse-grained spinel, ～50–150 μm across, occurs in clumps and chains enclosed in relatively Mg-rich melilite, whereas none of the fine spinel grains are clumped together. The sample also contains a spinel-free palisade body, 1.7 × 0.85 mm, that consists almost entirely of Åk-rich (45–65 mol%) melilite. Within the palisade body are two grains of perovskite with extremely Nb-rich (～4–8 wt% Nb₂O₅) cores and rims of typical composition. All phases in this inclusion have chondrite-normalized REE patterns that are consistent with crystal/melt partitioning superimposed upon a bulk modified Group II pattern. We suggest that TS2 had an anomalous cooling history and favor the following model for the formation of TS2. Precursors having a bulk modified Group II pattern melted. Rapid growth of large, dendritic, nonstoichiometric melilite crystals occurred. The melilite trapped pockets of melt and incorporated excess spinel components and TiO₂. Bubbles formed in the residual melt. As crystallization slowed, coarse spinel grew. Some spinel grains collected against bubbles, forming spherical shells, and others formed clumps and chains. Relatively Åk-rich melilite crystallized from the residual melt between dendritic melilite crystals and from melt trapped in pockets and between arms of dendrites, and incorporated the clumps and chains of coarse spinel. Bubbles broke and filled with late-stage melt, their shapes preserved by their spinel shells. Slow cooling, or perhaps an episode of reheating, allowed the early melilite to become stoichiometric by exsolving fine grains of spinel, perovskite and fassaite, and allowed the melilite to form smooth grain boundaries. Dendritic crystals are indicative of rapid growth and the melilite crystals in TS2 appear to be dendritic. Coarse, dendritic melilite crystals have been grown from Type B inclusion melts cooled at ～50–100 °C/h. If those results are applicable to Type A inclusions, we can make the first estimate of the cooling rate of a Type A inclusion, and it is outside the range (2–50 °C/h) generally inferred for Type B inclusions. The rapid cooling inferred here may be part of an anomalous thermal history for TS2, or it may be representative of part of a normal thermal history common to Types A and B that involved rapid cooling early (at high temperatures) as inferred for TS2, and slower cooling later (at lower temperatures), as inferred for Type B inclusions. We prefer the former explanation; otherwise, the unusual features of TS2 that are reported here would be common in Type A inclusions (which they are not).  相似文献   

STRUCTURAL GLACIOLOGY OF A TEMPERATE MARITIME GLACIER: LOWER FOX GLACIER,NEW ZEALAND     

JOHN R. APPLEBY  MARTIN S. BROOK  SIMON S. VALE  AMANDA M. MACDONALD‐CREEVEY 《Geografiska Annaler: Series A, Physical Geography》2010,92(4):451-467

This paper describes the structural glaciology of the lower Fox Glacier, a 12.7 km‐long valley glacier draining the western side of the Southern Alps, New Zealand. Field data are combined with analysis of aerial photographs to present a structural interpretation of a 5 km‐long segment covering the lower trunk of the glacier, from the upper icefall down‐glacier to the terminus. The glacier typifies the structural patterns observed in many other alpine glaciers, including: primary stratification visible within crevasse walls in the lower icefall; foliation visible in crevasses below the lower icefall; a complex set of intersecting crevasse traces; splaying and chevron crevasses at the glacier margins; transverse crevasses forming due to longitudinal extension; longitudinal crevasses due to lateral extension near the snout; and, arcuate up‐glacier dipping structures between the foot of the lower icefall and the terminus. The latter are interpreted as crevasse traces that have been reactivated as thrust faults, accommodating longitudinal compression at the glacier snout. Weak band‐ogives are visible below the upper icefall, and these could be formed by multiple shearing zones uplifting basal ice to the glacier surface to produce the darker bands, rather than by discrete fault planes. Many structures such as crevasses traces do not show a clear relationship with measured surface strain‐rates, in which case they may be ‘close to crevassing’, or are undergoing passive transport down‐glacier.  相似文献   

Crystal Fractionation and the Evolution of Intra-plate hy-normative Igneous Suites: Insights from their Feldspars   总被引：4，自引：2，他引：2  

NEKVASIL  H.; SIMON  A.; LINDSLEY  D. H. 《Journal of Petrology》2000,41(12):1743-1757

Feldspars and normative feldspar constituents of bulk magmashow trends supportive of fractional crystallization in thethree main types of hy-normative intraplate suites that containqz-oversaturated rocks: ocean island and continental alkalicsuites, ocean island tholeiitic suites and continental tholeiiticsuites. These suites are characterized by the presence of asingle feldspar in each suite member, a shift of this feldsparfrom plagioclase to alkali feldspar, and K enrichment of alkalifeldspar with decreasing temperature in the trachytic members.The modal feldspars provide evidence for a reaction relationshipbetween feldspars and indicate a build-up of magmatic volatilecontent towards saturation with progressive fractionation ofa parental magma having low initial volatile content. The feldsparand normative feldspar evolutionary paths are unique for eachof the three suite types but similar for different suites withinthe same type. This characteristic extends to the felsic members,making it easy to distinguish between rhyolitic or graniticrocks from the different suite types. The feldspars in naturalvolcanic suites commonly show evidence for a polybaric history,particularly in the least-evolved suite members. Late-stagefeldspars of the intermediate members and feldspars of the mostevolved members show paths indicative of significantly lowertemperature and pressure regimes. KEY WORDS: alkalic; intra-plate; feldspars; fractionation; suites  相似文献   

Early Cretaceous Basaltic and Rhyolitic Magmatism in Southern Uruguay Associated with the Opening of the South Atlantic   总被引：8，自引：0，他引：8  

KIRSTEIN  LINDA A.; PEATE  DAVID W.; HAWKESWORTH  CHRIS J.; TURNER  SIMON P.; HARRIS  CHRIS; MANTOVANI  MARTA S. M. 《Journal of Petrology》2000,41(9):1413-1438

The Early Cretaceous volcanic rocks of southern Uruguay comprisemafic and felsic volcanics. The position of these outcrops atthe southern edge of the Paraná–Etendeka continentalflood basalt province provides an opportunity to investigatepossible lateral variations in both mafic and more evolved rocktypes towards the margins of such an area of plume-related magmatism.The mafic lavas are divided into two compositionally distinctmagma types. The more voluminous Treinte Y Trés magmatype is similar to the low-Ti basalts of the Paraná floodbasalt province. The Santa Lucía magma type is a distinctand rare basalt type with ocean-island basalt type asthenosphericaffinities (high Nb/La, low ⁸⁷Sr/⁸⁶Sr_i). The felsic volcanicsare divided into two series, the Lavalleja Series and the AigüaSeries. The Lavalleja Series are chemically and isotopicallysimilar to the Paraná–Etendeka low-Ti rhyolites,and are considered to be related to the Treinte Y Tréslavas by extensive fractionation and crustal assimilation. TheAigüa Series have low ¹⁴³Nd/¹⁴⁴Nd_i and low ⁸⁷Sr/⁸⁶Sr_i andunlike the rhyolites of the Paraná, are interpreted asmelts of pre-existing mafic lower crust that subsequently underwentextreme fractionation. The differences observed in the felsicsuites may be linked to differences in the volumes of the associatedbasalts and the amounts of extension. KEY WORDS: South America; flood basalts; felsic volcanics; crustal melts; plume  相似文献   

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications   总被引：55，自引：1，他引：55  

ZHAO  GUOCHUN; CAWOOD  PETER A.; WILDE  SIMON A.; LU  LIANGZHAO 《Journal of Petrology》2001,42(6):1141-1170

Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M₁) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M₂) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M₃), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M₄), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M₂) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M₃) were estimated at  相似文献   

A Holocene record of coastal landscape dynamics in the eastern Kimberley region,Australia          下载免费PDF全文

ULRIKE PROSKE  DAVID HESLOP  SIMON HABERLE 《第四纪科学杂志》2014,29(2):163-174

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The local geologist 5: The most important fossil in the world     

SIMON KNELL 《Geology Today》1991,7(6):221-224

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The local geologist 2: Making rock records     

SIMON KNELL 《Geology Today》1991,7(2):62-66

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