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1.
CO2–CH4 fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO2-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO2-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO2-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al2SiO5+ water-rich fluid = garnet + melt + CO2–CH4. The presence of CO2-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.  相似文献   

2.
Metasediments of the Rantasalmi-Sulkava area (Finland) showprogressive regional metamorphism with migmatization. The metasedimentsare represented by various types of metapsammites (plagioclase-rich,quartz-rich, and layers of granitic compositions—somerich in microcline and others in plagioclase) and metapelites(dark and light layers). The migmatites of this area are of stromatic type. They consistof leucosomes, mesosomes, and light-coloured plagioclase-richlayers which do not fit the definition of leucosome. Melanosomes,which usually separate leucosomes and mesosomes in stromaticmigmatites, are almost absent. The leucosomes are of three types: (i) quartz-rich; (ii) cordierite-rich;and (iii) granitic. The quartz-rich leucosomes formed firstat subsolidus temperatures through recrystallization. The graniticleucosomes are considered to have developed via partial melting.The cordierite-rich leucosomes are formed—like the graniticones—at supersolidus conditions, but the role of partialmelting is not clear. The mesosomes are the metamorphic portions of the migmatiteswhich are not transformed into leucosomes. They include metapsammiticlayers and light-coloured metapelitic layers, both rich in plagioclase. Besides mineral reactions resulting in new assemblages duringregional metamorphism, the main process changing the protolithsinto migmatites is the conversion of some of the protolith layersinto leucosomes, through (as we believe) an almost isochemicalpartial melting. The migmatites of the Rantasalmi-Sulkava area differ from othermigmatites investigated by the authors in having two differentgenetic types of leucosomes: one formed via partial meltingand the other through subsolidus recrystallization as mentionedabove. The process of migmatization is described and modelledin three steps. Reprint requests to W. Johannes  相似文献   

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, Fe2O3, 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   

4.
Intrusion-related migmatites comprise a substantial part of the high-grade part of the southern Damara orogen, Namibia which is dominated by Al-rich metasedimentary rocks and various granites. Migmatites consist of melanosomes with biotite+sillimanite+garnet+cordierite+hercynite and leucosomes are garnet- and cordierite-bearing. Metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies (5–6 kbar at 730–750 °C). Field evidence, petrographic observations, chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites. The intruding melts were significantly modified by magma mixing with in situ partial melts, accumulation of mainly feldspar and contamination with garnet from the wall rocks. However, it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith. The occurrence of LILE-, HFSE- and LREE-enriched and -depleted residues within the leucosomes implies that both quartzo-feldspathic and pelitic rocks were subjected to partial melting. Isotope ratios of the leucosomes are rather constant (143Nd/144Nd (500 Ma): 0.511718–0.511754, ε Nd (500 Ma): ?3.54 to ?5.11) and Sr (87Sr/86Sr (500 Ma): 0.714119–0.714686), the metasedimentary units have rather constant Nd isotope ratios (143Nd/144Nd (500 Ma): 0.511622–0.511789, ε Nd (500 Ma): ?3.70 to ?6.93) but variable Sr isotope ratios Sr (87Sr/86Sr (500 Ma): 0.713527–0.722268). The most restitic melanosome MEL 4 has a Sr isotopic composition of 87Sr/86Sr (500 Ma): 0.729380. Oxygen isotopes do not mirror the proposed contamination process, due to the equally high δ18O contents of metasediments and crustal melts. However, the most LILE-depleted residue MEL 4 shows the lowest δ18O value (<10). Mass balance calculations suggest high degrees of partial melting (20–40%). It is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites. High degrees of partial melting can be reached as long as the available H2O, derived from the crystallization of the intruding granites, is efficiently recycled within the rock volume. Due to the limited amounts of in situ melting, it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies. The high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas. The Davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures, and illustrates some of the reactions that may modify leucosome compositions. The area provides constraints on melting processes operating in high-grade metasedimentary rocks.  相似文献   

5.
Origin and evolution of a migmatite   总被引：5，自引：0，他引：5  
The development of a stromatic migmatite exposed east and southeast of Arvika (Western Sweden) is described in four stages beginning with the country rock and following evolution through three areas characterized by low, medium and high amounts of leucosomes (areas L, M, and H, respectively).The country rock is a paragneiss composed of thin, alternating fine- and coarse-grained layers. Composition of the layers varies from granitic (fine) to tonalitic (coarse layers).The bulk of the stromatic migmatite is composed of leucocratic layers of magmatic appearance (leucosomes) and darker layers of gneissic aspect (mesosomes). Petrographical and chemical data (given in the form of Niggli values and K2O/SiO2 diagrams) show a close relationship between the fine-grained paragneiss layers and the leucosomes on the one hand and between the coarse-grained layers and the mesosomes on the other.At relatively low temperatures only those gneiss layers with a suitable (granitic) composition are transformed into leucosomes. This process is interpreted to be due to recrystallization of the felsic minerals via partial melting and to the separation of biotite.With increasing metamorphism, leucosomes become broader and more frequent due to partial melting of layers with less suitable composition. Contacts between different generations of leucosome can be recognized in the form of relict melanosomes.These observations favour essentially isochemical melting, followed by later in-situ crystallization. This model of an isochemical layer-by-layer transformation is supported by the preferential formation of hornblende in leucosomes and relict melanosomes, as well as by almost identical compositions of migmatite and country-rock plagioclase.  相似文献   

6.
SAWYER  E. W. 《Journal of Petrology》1987,28(3):445-473
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   

7.
M.I. Holloway  F. Bussy 《Lithos》2008,102(3-4):616-639
Low pressure partial melting of basanitic and ankaramitic dykes gave rise to unusual, zebra-like migmatites, in the contact aureole of a layered pyroxenite–gabbro intrusion, in the root zone of an ocean island (Basal Complex, Fuerteventura, Canary Islands). These migmatites are characterised by a dense network of closely spaced, millimetre-wide leucocratic segregations. Their mineralogy consists of plagioclase (An32–36), diopside, biotite, oxides (magnetite, ilmenite), +/− amphibole, dominated by plagioclase in the leucosome and diopside in the melanosome. The melanosome is almost completely recrystallised, with the preservation of large, relict igneous diopside phenocrysts in dyke centres. Comparison of whole-rock and mineral major- and trace-element data allowed us to assess the redistribution of elements between different mineral phases and generations during contact metamorphism and partial melting.

Dykes within and outside the thermal aureole behaved like closed chemical systems. Nevertheless, Zr, Hf, Y and REEs were internally redistributed, as deduced by comparing the trace element contents of the various diopside generations. Neocrystallised diopside – in the melanosome, leucosome and as epitaxial phenocryst rims – from the migmatite zone, are all enriched in Zr, Hf, Y and REEs compared to relict phenocrysts. This has been assigned to the liberation of trace elements on the breakdown of enriched primary minerals, kaersutite and sphene, on entering the thermal aureole. Major and trace element compositions of minerals in migmatite melanosomes and leucosomes are almost identical, pointing to a syn- or post-solidus reequilibration on the cooling of the migmatite terrain i.e. mineral–melt equilibria were reset to mineral–mineral equilibria.  相似文献   


8.
Sm–Nd (garnet), U–Pb (monazite) and Rb–Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high‐grade regional metamorphism and the duration of regional metamorphic events. Sm–Nd garnet whole‐rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534±5, 528±11 and 539±8 Ma. These results provide substantial evidence for pre‐500 Ma Pan‐African regional metamorphism and melting for this segment of the orogen. Other parts of the migmatite yield younger Sm–Nd ages of 488±9 Ma for melanosome and 496±10, 492±5 and 511±16 Ma for the corresponding leucosomes. Garnet from one xenolith from the leucosomes yields an age of 497±2 Ma. Major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components, but the flat shape of the elemental patterns suggests late‐stage retrograde equilibration. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. Monazite from the leucosomes is reversely discordant and records 207Pb/235U ages between 536 and 529 Ma, indicating that this monazite represents incorporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives 207Pb/235U ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published 207Pb/235U monazite data give ages between 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for monazite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507–505 Ma. These ages are also recorded in 207Pb/235U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm–Nd garnet ages obtained in this study. Taken together, these ages indicate that high‐grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb–Sr whole‐rock data of 520±15 Ma are available. Rb–Sr biotite ages from the different layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late‐stage disturbance of the Rb–Sr isotopic system on the sub‐sample scale. Nevertheless, these ages are close to the youngest Sm–Nd garnet ages, indicating rapid cooling rates between 13 and 20°C Ma?1 and fast uplift of this segment of the crust. Similar Sm–Nd garnet and U–Pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730±30°C. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water‐undersaturated peraluminous melts can be sluggish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high‐grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events.  相似文献   

9.
The migmatites from Punta Sirenella (NE Sardinia) are layered rocks containing 3–5 vol.% of centimeter-sized stromatic leucosomes which are mainly trondhjemitic and only rarely granitic in composition. They underwent three deformation phases, from D1 to D3. The D1 deformation shows a top to the NW shear component followed by a top to the NE/SE component along the XZ plane of the S2 schistosity. Migmatization started early, during the compressional and crustal thickening stage of Variscan orogeny and was still in progress during the following extensional stage of unroofing and exhumation.

The trondhjemitic leucosomes, mainly consisting of quartz, plagioclase, biotite ± garnet ± kyanite ± fibrolite, retrograde muscovite and rare K-feldspar, are locally bordered by millimeter-sized biotite-rich melanosomes. The rare granitic leucosomes differ from trondhjemitic ones only in the increase in modal content of K-feldspar, up to 25%. Partial melting started in the kyanite field at about 700–720 °C and 0.8–0.9 GPa, and was followed by re-equilibration at 650–670 °C and 0.4–0.6 GPa, producing fibrolite–biotite intergrowth and coarse-grained muscovite.

The leucosomes have higher SiO2, CaO, Na2O, Sr and lower Al2O3, Fe2O3, MgO, TiO2, K2O, P2O5, Rb, Ba, Cr, V, Zr, Nb, Zn and REE content with respect to proximal hosts and pelitic metagreywackes. Sporadic anomalous high content of calcium and ferromagnesian elements in some leucosomes is due to entrainment of significant amounts of restitic plagioclase, biotite and accessory phases. The rare granitic leucosomes reveal peritectic K-feldspar produced by muscovite-dehydration melting. Most leucosomes show low REE content, moderately fractionated REE patterns and marked positive Eu anomaly. Proximal hosts and pelitic metagraywackes are characterized by higher REE content, more fractionated REE patterns and slightly negative Eu anomaly.

The trondhjemitic leucosomes were generated by H2O-fluxed melting at 700 °C of a greywacke to pelitic–greywacke metasedimentary source-rock. The disequilibrium melting process is the most reliable melting model for Punta Sirenella leucosomes.  相似文献   


10.
Making a distinction between partial melting and subsolidus segregation in amphibolite facies migmatites is difficult. The only significant melting reactions at lowpressures, either vapour saturated or muscovite dehydration melting, do not produce melanocratic peritectic phases. If protoliths are Si-rich and K-poor, then peritectic sillimanite and K-feldspar will form in scarce amounts, and may be lost by retrograde rehydration. The Roded migmatites of southern Israel (northernmost Arabian Nubian Shield) formed at P = 4.5 ± 1 kbar and T ≤ 700 °C and include Si-rich, K-poor paragneissic paleosome and trondhjemitic leucosomes. The lack of K-feldspar in leucosomes was taken as evidence for the non-anatectic origin of the Roded migmatites (Gutkin and Eyal, Isr J Earth Sci 47:117, 1998). It is shown here that although the Roded migmatites experienced significant post-peak deformation and recrystallization, microstructural evidence for partial melting is retained. Based on these microstructures, coupled with pseudosection modelling, indicators of anatexis in retrograded migmatites are established. Phase diagram modelling of neosomes shows the onset of muscovite dehydration melting at 4.5 kbar and 660 °C, forming peritectic sillimanite and K-feldspar. Adjacent non-melted paleosomes lack muscovite and would thus not melt by this reaction. Vapour saturation was not attained, as it would have formed cordierite that does not exist. Furthermore, vapour saturation would not allow peritectic K-feldspar to form, however K-feldspar is ubiquitous in melanosomes. Direct petrographic evidence for anatexis is rare and includes euhedral plagioclase phenocrysts in leucosomes and quartz-filled embayments in corroded plagioclase at leucosome-melanosome interfaces. In deformed and recrystallized rocks muscovite dehydration melting is inferred by: (1) lenticular K-feldspar enclosed by biotite in melanosomes, (2) abundant myrmekite in leucosomes, (3) muscovite–quartz symplectites after sillimanite in melanosomes and associated with myrmekite in leucosomes. While peritectic K-feldspar formed in melanosomes by muscovite dehydration melting reaction, K-feldspar crystallizing from granitic melt in adjacent leucosome was myrmekitized. Excess potassium was used in rehydration of sillimanite to muscovite.  相似文献   

11.
The processes of ultrametamorphism in the juncture zone between the Aldan shield and Stanovoi folded area are manifested in granitization (volume-for-volume replacement of gneisses by trondhjemite gneisses Lc1) and subsequent migmatization with formation of several leucosome generations Lc2, Lc3, Lc4, and Lc5, which is confirmed by U-Pb zircon dating. It was established that the granitization stage is marked by the input of Si, Na, and Ba and removal of practically all major (including K) and minor elements. Formation of migmatite leucosomes is accompanied by further depletion in transition (Ti, Mg, Fe, V, Cr, Ni) and light rare-earth (La, Ce, Nd, and Eu) elements, and accumulation of HFSE (Pb, U, Th, Nb, Ta, Y) as well as medium and heavy rare-earth elements (Sm, Gd, Yb, Lu). Leucosomes Lc4, in addition, are enriched in K, Rb, and especially HREE due to the appearance of garnet, while Lc5 leucosomes become higher in K, Sr, and Pb. The study of relations of trondhjemite gneisses and migmatite leucosomes with protolith, geochemical features, and opposite trends in variations of Zr/Hf, Zr/Nb, Nb/La, and Eu/Eu*, and LREE/HREE ratios in the series of granitization and migmatization indicate that the trondhjemite gneisses were formed during deep-fluid-assisted infiltration granitization under the amphibolite facies conditions, while migmatite leucosomes were generated during evolving anatexis under conditions of subsequent diatexis and continuing fluid reworking. With time, the composition of the fluid changed changed, the role of K increased, and leucosomes acquired granitic composition. Unlike common K and K-Na types of ultrametamorphism, the considered juncture zone is characterized by specific type of ultrametamorphism-Na type, with formation of granitic leucosomes in subordinate amounts at the final stages.  相似文献   

12.
Pods of granulite facies dioritic gneiss in the Pembroke Valley, Milford Sound, New Zealand, preserve peritectic garnet surrounded by trondhjemitic leucosome and vein networks, that are evidence of high‐P partial melting. Garnet‐bearing trondhjemitic veins extend into host gabbroic gneiss, where they are spatially linked with the recrystallization of comparatively low‐P two‐pyroxene‐hornblende granulite to fine‐grained high‐P garnet granulite assemblages in garnet reaction zones. New data acquired using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA‐ICPMS) for minerals in various textural settings indicate differences in the partitioning of trace elements in the transition of the two rock types to garnet granulite, mostly due to the presence or absence of clinozoisite. Garnet in the garnet reaction zone (gabbroic gneiss) has a distinct trace element pattern, inherited from reactant gabbroic gneiss hornblende. Peritectic garnet in the dioritic gneiss and garnet in trondhjemitic veins from the Pembroke Granulite have trace element patterns inherited from the melt‐producing reaction in the dioritic gneiss. The distinct trace element patterns of garnet link the trondhjemitic veins geochemically to sites of partial melting in the dioritic gneiss.  相似文献   

13.
A correlation between the style of partial melting and synmeta-morphic fluid flow exists in metapelites from the Mount Lofty Ranges, Reynolds Range, and Omeo Zone (Australia). Mount Lofty Ranges migmatites comprise granitic leucosomes in rocks that are still biotite rich, with no indications of other mafic minerals being formed along with the melts. By contrast, in the Reynolds and Omeo migmatites, garnet, cordierite, and/or spinel formed along with the melts. Oxygen isotope data are most consistent with the Mount Lofty Ranges undergoing significant fluid–rock interaction during regional metamorphism, which may have fluxed fluid-present partial melting. By contrast regional metamorphic fluid flow in the Reynolds Range and Omeo Zone was limited, leading to partial melting via fluid-absent reactions. The style of melting reactions may help to constrain the timing of isotopic resetting and fluid flow in metamorphic terrains, which is currently a contentious issue.  相似文献   

14.
Migmatitic rocks developed in metagraywackes during the Variscan orogeny in the Aiguilles-Rouges Massif (western Alps). Partial melting took place 320 Ma ago in a 500 m-wide vertical shear zone. Three leucosome types have been recognised on the basis of size and morphology: (1) large leucosomes > 2 cm wide and > 40 cm long lacking mafic selvage, but containing cm-scale mafic enclaves; (2) same as 1 but with thick mafic selvage (melanosome); (3) small leucosomes < 2 cm and < 40 cm) with thin dark selvages (stromatic migmatites). Types 1 + 2 have mineralogical and chemical compositions in keeping with partial melting experiments. But Type 3 leucosomes have identical plagioclase composition (An19–28) to neighbouring mesosome, both in terms of major- and trace-elements. Moreover, whole-rock REE concentrations in Type 3 leucosomes are only slightly lower than those in the mesosomes, unlike predicted by partial melting experiments. The main chemical differences between all leucosome types can be related to the coupled effect of melt segregation and late chemical reequilibration.

Mineral assemblages and thermodynamic modelling on bulk-rock composition restrict partial melting to  650 °C at 400 MPa. The large volume of leucosome (20 vol.%) thus generated requires addition of 1 wt.% external water. Restriction of extensive migmatization to the shear zone, without melting of neighbouring metapelites, also points to external fluid circulation within the shear zone as the cause of melting.  相似文献   


15.
Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K2O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H2O, K2O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H2O and K2O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K2O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.  相似文献   

16.
A sequence of prograde isograds is recognized within the Dalradian Inzie Head gneisses where pelitic compositions have undergone variable degrees of partial melting via incongruent melting reactions consuming biotite. Three leucosome types are identified. At the lowest grades, granitic leucosomes containing porphyroblasts of cordierite (CRD‐melt) are abundant. At intermediate grades, CRD‐melt mingles with garnetiferous leucosomes (GT‐melt). At the highest grades, CRD‐melt coexists with orthopyroxene‐bearing leucosomes (OPX‐melt), while garnet is conspicuously absent. The prograde metamorphic field gradient is constrained to pressures of 2–3 kbar below the CRD‐melt isograd, and no greater than 4.5 kbar at the highest grade around Inzie Head. A petrogenetic grid, calculated using thermocalc , is presented for the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system for the phases orthopyroxene, garnet, cordierite, biotite, sillimanite, H2O and melt with quartz and K‐feldspar in excess. For the implied field gradient, the reaction sequence predicted by the grid is consistent with the successive prograde development of each leucosome type. Compatibility diagrams suggest that, as anatexis proceeded, bulk compositions may have been displaced towards higher MgO content by the removal of (relatively) ferroan granitic leucosome. An isobaric (P = 4 kbar) T–aH2O diagram shows that premigmatization fluids must have been water‐rich (aH2O > 0.85) and suggests that, following the formation of small volumes of CRD‐melt, the system became fluid‐absent and melting reactions buffered aH2O to lower values as temperatures rose. GT‐ and OPX‐melt formed by fluid‐absent melting reactions, but a maximum of 7–11% CRD‐melt fraction can be generated under fluid‐absent conditions, much less than the large volumes observed in the field. There is strong evidence that the CRD‐melt leucosomes could not have been derived by buoyantly aided upwards migration from levels beneath the migmatites. Their formation therefore required a significant influx of H2O‐rich fluid, but in a quantity insufficient to have exhausted the buffering capacity of the solid assemblage plus melt. Fluid : rock ratios cannot have exceeded 1 : 30. The fluid was channelled through a regionally extensive shear zone network following melt‐induced failure. Such an influx of fluid at such depths has obvious consequences for localized crustal magma production and possibly for cordierite‐bearing granitoids in general.  相似文献   

17.
Integrated, in situ textural, chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western Musgrave Block, central Australia has identified evidence for multiple events of growth and recrystallisation during poly-metamorphism in the Mesoproterozoic. Garnet + sillimanite-bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 Ma, with monazite grains in leucosome recording crystallisation at 1300 Ma. Monazite breakdown during melting is inferred to have occurred in the palaeosome. During a subsequent granulite facies event at 1200 Ma, deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 Ma monazite in leucosome. Growth of new, high-Y (+HREE) monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel, as a result of post-peak isothermal decompression. Diffusive enrichment of resorbed garnet rims in Y + HREE suggests garnet breakdown occurred slower than volume diffusion of REE. Monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150–1130 Ma. The intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or ‘fertile’ local environments provided by the palaeosome/mesosome volumes, which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite. Like reaction textures, it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite REE and U–Th–Pb chemistry and hence ages.  相似文献   

18.
Large garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet‐bearing and garnet‐absent parts of the rock. As a consequence, melt and peritectic K‐feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet‐bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre‐scale physical melt migration and segregation. The near complete preservation of the high‐grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet‐rich leucosomes provide the most likely pathway for melt migration. Decimetre‐scale, coarse‐grained, garnet‐poor leucosomes may represent areas of melt flux through a large‐scale melt transfer network.  相似文献   

19.
在一些俯冲/碰撞造山带中,高压麻粒岩相变质作用通常伴随着广泛的深熔作用。本文以柴北缘超高压变质带都兰地区的基性高压麻粒岩和浅色体为研究对象,在详细的野外观察的基础上,结合岩相学和年代学等研究方法,探讨高压麻粒岩相变质作用与深熔作用的关系及形成机制。从野外关系来看,浅色体主要呈层状、似脉状、补丁状或网络状分布在暗色的基性高压麻粒岩(残留体,residuumormelanosome)中,或与基性高压麻粒岩在露头上互层产出,并显示出混合岩的特征。基性高压麻粒岩主要由石榴子石、单斜辉石、斜长石和石英等矿物组成,在不同样品中还可含有少量蓝晶石、角闪石、金红石、黝帘石/斜黝帘石、黑云母、方柱石、绿泥石;浅色体主要由斜长石、钾长石和石英等矿物组成,一些样品中也含有少量的石榴子石和蓝晶石,与典型的长英质高压麻粒岩的矿物组合特征较为相似。锆石成因年代学结果显示浅色体中既发育深熔锆石,也有变质锆石生长,但两种锆石给出的年龄结果基本一致,其加权平均年龄为434±2Ma(MSWD=1.1),与前人获得的高压麻粒岩相变质作用和深熔作用时代基本一致。因此,综合野外关系、岩相学、地球化学特征及年代学结果,我们推测高压麻粒岩相变质作用及深熔作用可能形成于同一动力学过程,即在俯冲带的上盘环境,(变)基性岩石中的含水矿物(如角闪石、帘石或云母类矿物等)脱水熔融形成高Sr/Y熔体,而基性高压麻粒岩为残留体。  相似文献   

20.
Estimated variations in mineral concentrations across leucosomes suggest that leucosomes are generated during anatexis by a diffusive exchange between the leucosome and the mesosome, and not by the migration of melt from the mesosome. However, the presence of melt is a precondition for the diffusive exchange to take place. Initially a crack is formed due to shear stress. The formation of a crack allows a diffusive exchange to take place through the melt, which causes melting of minerals situated near the crack. The diffusive exchange of material is less efficient in the mesosome where the melt is isolated at grain corners and edges. The microcline enrichment of some granitic leucosomes is thought to be due to the diffusive depletion of the mesosome caused by growth of alkali feldspar during the consolidation of the migmatite. In general, it seems unnecessary to invoke concentrations of water in the leucosome or the intrusion of external fluids or magmas for migmatite formation.  相似文献   

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