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The Role of Partial Melting and Fractional Crystallization in Determining Discordant Migmatite Leucosome Compositions 总被引：5，自引：4，他引：5
Anatectic migmatite leucosomes in the Quetico MetasedimentaryBelt (Superior Province) are discordant to the host rock layering.Two morphological varieties within the anatectic leucosome suiteare distinguished. The first type show little compositionalor textural variation either across, or along, the leucosomes.In contrast, the second variety exhibits both compositionaland textural variations in a single leucosome, typically withinternal cross-cutting relationships. Major-oxide contents varycomparatively little in the Quetico anatectic leucosome suite,but there is a considerable range in the incompatible element(REE, Hf, Zr, Y and Th) concentrations. In particular La contentsrange from 1.8 to 78.1 p.p.m. and the La/Yb ratios from 9.1to 101.9. Samples with high REE contents have negative Eu anomalies,whereas those with low total REE abundances have positive Euanomalies, which indicate that feldspar fractionation was importantin their petrogenesis. Three samples which have no Eu anomalies,and which are taken not to have experienced significant feldsparfractionation, are regarded as the closest approximation toa primary melt composition. Petrographic evidence indicates that only the most aluminousbulk compositions in the host rocks have melted, with cordieriteand biotite as the principal residual phases. Batch partialmelting models indicate that the three leucosomes without Euanomalies could have been derived from 40–80 per centpartial melting of the aluminous metasediments, but garnet musthave been a residual phase. Since the residuum from 40 per centpartial melting is more mafic than any of the rocks currentlyexposed in the area, it is concluded that the melting whichgave rise to the leucosomes occurred at greater depth. Crystallization models indicate that the observed range of leucosomecompositions can be derived by crystal fractionation of meltcompositions similar to the three leucosomes lacking Eu anomalies(i.e. the assumed primary melts). Samples with high abundancesof incompatible elements and negative Eu anomalies representfractionated melts, whereas those with low levels of REE andpositive Eu anomalies represent cumulates. Leucosome composition,morphology and texture can be related to crystallization history,notably the timing of crystallization with respect to leucosomeintrusion. In particular, those leucosomes that exhibit compositionaland textural zoning are interpreted to have undergone crystalfractionation during intrusion. Although a suite of migmatite leucosomes may be derived by partialmelting, it is concluded that the trace-element compositionof any particular leucosome depends, to a great extent, uponits segregation and crystallization history. Indeed, the primarymelt composition may not be preserved. 相似文献
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Granitic magmas migrated through Early Proterozoic middle–lower crust at Mt Hay, central Australia, via a diverse network of narrow structurally controlled channelways, during a period of progressive W–SW-directed thrusting (D1a–D1d). They utilized existing folds, boudins and shear zones, or created new channels by magmatic fracture either parallel to layering or, rarely, in irregular arrays. The magmas rose obliquely, parallel to the plunging (50–60°) regional elongation direction, which was defined by coaxial folds, boudin necks and a strong mineral-elongation lineation. Megacrystic charnockitic magmas migrated through metre-scale conduits during D1a–D1b, but leucosomes were generally restricted to smaller (centimetre-scale) structures that existed throughout the entire deformation history. Thus, D1a/D1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma, thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting. The upper–middle crust of the Anmatjira–Reynolds Range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at Mt Hay. They are considered to have formed as syntectonic intrusions emplaced during W–SW-directed thrusting, as at Mt Hay, suggesting that granitic magmas formed near the base of the continental crust passed through the mid-lower crustal level (25–30 km) exposed at Mt Hay and accumulated, in batholithic proportions, at shallower crustal levels (12–20 km) such as the Anmatjira–Reynolds Range area. The observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive, noncoaxial shear deformation. Localization of magmas by sequentially developed, narrow, compressive structures suggests that dilatancy followed successive foliation-forming events, a situation that can occur during steady-state deformation if the effective confining pressures are low, which would be a result of high and possibly variable rates of magma influx. The inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust. 相似文献
3.
Formation of Diatexite Migmatite and Granite Magma during Anatexis of Semi-pelitic Metasedimentary Rocks: an Example from St. Malo, France 总被引：8，自引：2，他引：8
Petrological and geochemical variations are used to investigatethe formation of granite magma from diatexite migmatites derivedfrom metasedimentary rocks of pelitic to greywacke compositionat St. Malo, France. Anatexis occurred at relatively low temperaturesand pressures (<800°C, 4–7 kbar), principally throughmuscovite dehydration melting. Biotite remained stable and servesas a tracer for the solid fraction during melt segregation.The degree of partial melting, calculated from modal mineralogyand reaction stoichiometry, was <40 vol. %. There is a continuousvariation in texture, mineralogy and chemical composition inthe diatexite migmatites. Mesocratic diatexite formed when metasedimentaryrocks melted sufficiently to undergo bulk flow or magma flow,but did not experience significant melt–residuum separation.Mesocratic diatexite that underwent melt segregation duringflow generated (1) melanocratic diatexites at the places wherethe melt fraction was removed, leaving behind a biotite andplagioclase residuum (enriched in TiO2, FeOT, MgO, CaO, Sc,Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocraticdiatexite (enriched in SiO2, K2O and Rb) where the melt fractionaccumulated. Leucocratic diatexite still contained 5–15vol. % residual biotite (mg-number 40–44) and 10–20vol. % residual plagioclase (An22). Anatectic granite magmadeveloped from the leucodiatexite, first by further melt–residuumseparation, then through fractional crystallization. Most biotitein the anatectic granite is magmatic (mg-number 18–22). KEY WORDS: anatexis; diatexite; granite magma; melt segregation; migmatite 相似文献
4.
Disequilibrium Melting and the Rate of Melt-Residuum Separation During Migmatization of Mafic Rocks from the Grenville Front, Quebec 总被引：5，自引：2，他引：5
Migmatites are developed in Archaean metabasites south of theGrenville Front. Relative to equivalent greenschist facies metabasites,those hosting the migmatites have undergone some mobilizationof CaO, Na2O, and Sr, and, in the case of sheared metabasites,the introduction of K2O, Ba, Cs, and Rb, before migmatization.Three types of anatectic migmatite are recognized, based ontheir leucosome-melanosome relationships: (1) non-segregatedmigmatites in which new leucocratic and magic phases are intimatelymixed in patches up to 15 cm across, (2) segregated migmatitesin which the leucosomes are located in boudin necks and shearbands, and are separated from their associated mafic selvedgesby 5–100 cm, and (3) vein-type migmatites where discordantleucosomes lack mafic selvedges. The non-segregated and segregatedmigmatites have a local and essentially isochemical origin,whereas the vein-type represent injected melt. Leucosomes fromthe segregated and vein-type migmatites have similar tonaliticmajor oxide compositions, but they differ greatly in their trace-elementcharacteristics. The vein-type leucosomes are enriched in K2O, Ba, Cs, Rb, LREE,Th, Hf, Zr, and P2O5 relative to their metabasite hosts, andhave greater La/YbN ratios (27 compared with 0?6–17).These veins may have formed by between 5 and 25%equilibriumbatch partial melting of Archaean metabasalt, leaving garnet+ hornblende in the residuum. In contrast, leucosomes from the segregated migmatites are depletedin REE, Sc, V, Cr, Ni, Co, Ti, Th, Hf, Zr, Nb, and P2O5 relativeto their source rocks; the associated mafic selvedges are enrichedin these elements. The leucosomes and mafic selvedges both haveLa/YbN ratios that are similar to those of the source metabasitesirrespective of whether the source is LREE depleted or LREEenriched. The abundances of many trace elements in the leucosomesappear to be controlled by the degree of contamination withresiduum material. Zr concentrations in the leucosomes are between10 and 52% of the estimated equilibrium concentrations in felsicmelts at the temperature (750–775 ?C) of migmatization.A numerical simulation of disequilibrium melting using bothLREE-depleted and LREE-enriched sources yields model melts withtrace element abundances that match those of the natural leucosomes.Mafic selvedge compositions indicate that the segregated migmatitesrepresent a range of between 12 and 36% partial melting of theirhost metamatization. Based upon calculated dissolution times for zircon in wet melts,the melt and residuum were separated in less than 23a, otherwisemelts would have become saturated in Zr. Rapid melt extractionis thought to be driven by pressure gradients developed duringnon-coaxial deformation of the anisotropic palaeosome duringmigmatization. The common occurrence, based on published work, of disequilibriumcompositions in migmatite leucosomes implies that during mid-crustalmelting the melt-segregation rates are greater than the rateof chemical equilibration between melt and the residual solid.In contrast, at the higher temperatures of granite formation,the rate of chemical equilibration exceeds that of melt-segregationand equilibrium melt compositions are reached before segregationcan occur. On the basis of their trace element characteristics,the melt which forms segregated migmatites cannot be the sameas that which forms the vein-like migmatites, or granitoid plutons. 相似文献
5.
Temporal and compositional differences between subsolidus and anatectic migmatite leucosomes from the Quetico metasedimentary belt, Canada 总被引：3，自引：1，他引：3
Abstract Migmatites in the Quetico Metasedimentary Belt contain two types of leucosome: (1) Layer-parallel leucosomes that grew during deformation and prograde metamorphism. These are enriched in SiO2 , Sr, and Eu, but depleted in TiO2 , Fe2 O3 , MgO, Cs, Rb, REE, Sc, Th, Zr, and Hf relative to the Quetico metasediments. (2) Discordant leucosomes that formed after the regional folding events when metamorphic temperatures were at their peak. These are enriched in Rb, Ba, Sr and Eu, but display a wide range of LREE, Th, Zr, and Hf contents relative to the Quetico metasediments.
Layer-parallel leucosomes formed by a subsolidus process termed tectonic segregation. This stress-induced mass transfer process began when the Quetico sediments were deformed during burial, and continued whilst the rocks were both stressed and heterogeneous. Subsolidus leucosome compositions are consistent with the mobilization of quartz and feldspar from the host rocks by pressure solution. The discordant leucosomes formed by partial melting of the Quetico metasediments, possibly during uplift of the belt. The range of composition displayed by the anatectic leucosomes arises from crystal fractionation during leucosome emplacement. Some anatectic leucosomes preserve primary melt compositions and have smooth REE patterns, but those with negative Eu anomalies represent fractionated melts, and others with positive Eu anomalies represent accumulations of feldspar plus trapped melt. 相似文献
Layer-parallel leucosomes formed by a subsolidus process termed tectonic segregation. This stress-induced mass transfer process began when the Quetico sediments were deformed during burial, and continued whilst the rocks were both stressed and heterogeneous. Subsolidus leucosome compositions are consistent with the mobilization of quartz and feldspar from the host rocks by pressure solution. The discordant leucosomes formed by partial melting of the Quetico metasediments, possibly during uplift of the belt. The range of composition displayed by the anatectic leucosomes arises from crystal fractionation during leucosome emplacement. Some anatectic leucosomes preserve primary melt compositions and have smooth REE patterns, but those with negative Eu anomalies represent fractionated melts, and others with positive Eu anomalies represent accumulations of feldspar plus trapped melt. 相似文献
6.
Douglas J. SAWYER Michael D. McGEHEE Julie CANEPA Carleton B. MOORE 《Meteoritics & planetary science》2000,35(4):743-747
Abstract— Water‐soluble ion concentrations from the martian achondrite Nakhla and three asteroidal achon drites are reported. The Nakhla sample contains significant concentrations of chloride, sulfate, Mg, Na, Ca, and K ions. The results are interpreted to indicate that this rock has been in contact with a seawater‐like brine on the martian surface. 相似文献
7.
The subsolidus segregation of layer-parallel quartz-feldspar veins in greenschist to upper amphibolite facies metasediments 总被引：2，自引：0，他引：2
Abstract Layer-parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (D2) compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer-parallel veins show the following trends: (1) an increase in thickness and internal complexity, the latter due to successive boudinage; (2) low-grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz-rich than younger. Mass balance calculations and whole-rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress-induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine-scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing plagioclase/quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagioclase component. 相似文献
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