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1.
The REE distributions in mesosomes, neosomes, leucosomes and melanosomes of four layered migmatites have been investigated. In one example (Arvika migmatites) the REE patterns in adjacent paragneisses, the presumed parent rock of the migmatites, were also determined. REE patterns of neosomes and mesosomes of Arvika migmatites are similar to the finegrained layers and coarse-grained layers, respectively, observed in the adjacent paragneiss. This is in agreement with the layer-by-layer paragneiss-migmatite transformation model.
The REE patterns of mesosomes and neosomes indicate that these lithologies may have been closed systems (for REE) during the formation of the migmatites. No indication of metasomatic reactions, melt segregation or injection could be detected. Within the neosomes, leucosomes are depleted and melanosomes enriched in REE contents. This is interpreted to be due to separation and concentration of accessory minerals (monazite, epidote, allanite, zircon, sphene, apatite, garnet) into the melanosomes. The behaviour of accessory minerals during migmatite formation is closely allied to that of biotite, which is also concentrated in the melanosomes. 相似文献
2.
The stromatic migmatites of Nelaug (Tvedestrand area, SouthernNorway) are investigated in detail. They show well developedlayers of leucosomes, mesosomes and melanosomes. It is establishedthat the mesosomes and leucosomes of these migmatites are differentfrom each other texturally, mineralogically, and chemically.Also combinations of leucosome plus adjacent melanosome portionsare chemically different from those of the mesosomes. Theseobservations do not agree with the findings of Mehnert (1971)and do not fit into his genetic model. The mesosome layers and the leucosome + melanosome combinationsare taken to represent the chemical compositions of the countryrock, a metagraywacke with relicts of primary rhythmic layering(Touret, 1965). The mineralogical composition of the layersvaries from granitic to tonalitic. Relict textures indicatethat the leucosome portions were initially occupied by layersof granitic composition relatively rich in K-feldspar, whereasthe mesosomes are the representatives of those metagraywackelayers which were relatively rich in plagioclase. An almostisochemical transformation of a paragneiss into the investigatedstromatic migmatite is established. Melting experiments performed at PH2O= 5 Kb yielded solidustemperatures of 640±7 °C for all layers. The Composition of plagioclases present in the different layersis explained by isochemical partial melting and in situ crystallization.The chemical, mineralogical, and textural findings support themodel of almost isochemical transformation already establishedfor the Arvika migmatites (Johannes & Gupta, 1982). 相似文献
3.
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
4.
Werner F. Thöny Peter Tropper Friederike Schennach Erwin Krenn Friedrich Finger Reinhard Kaindl Franz Bernhard Georg Hoinkes 《Swiss Journal of Geoscience》2008,101(1):111-126
Within the Ötztal Complex (ÖC), migmatites are the only geological evidence of the pre-Variscan metamorphic evolution, which led to the occurrence of partial anatexis in different areas of the complex. We investigated migmatites from three localities in the ÖC, the Winnebach migmatite in the central part and the Verpeil- and Nauderer Gaisloch migmatite in the western part. We determined metamorphic stages using textural relations and electron microprobe analyses. Furthermore, chemical microprobe ages of monazites were obtained in order to associate the inferred stages of mineral growth to metamorphic events. All three migmatites show evidence for a polymetamorphic evolution (pre-Variscan, Variscan) and only the Winnebach migmatite shows evidence for a P-accentuated Eo-Alpine metamorphic overprint in the central ÖC. The P-T data range from 670–750 °C and < 2.8 kbar for the pre-Variscan event, 550–650 °C and 4–7 kbar for the Variscan event and 430–490 °C and ca. 8.5 kbar for the P-accentuated Eo-Alpine metamorphic overprint. U-Th-Pb electron microprobe dating of monazites from the leucosomes from all three migmatites provides an average age of 441 ± 18 Ma, thus indicating a pervasive Ordovician-Silurian metamorphic event in the ÖC. 相似文献
5.
6.
S. G. Skublov I. S. Sedova V. A. Glebovitskii I. M. Gembitskaya L. M. Samorukova 《Geochemistry International》2010,48(12):1237-1245
This paper reports the results of chemical and structural study (electron microscopy and ion microprobe) of zircons from different-age
generations of migmatite leucosomes in the basement rocks and Kurumkan Formation within the Nimnyr block, Aldan shield. The
studied zircons show REE distribution pattern with a positive slope from LREE to HREE and positive Ce anomaly, which is typical
of magmatic zircons, but have elevated LREE contents, which implies their crystallization from migmatite melt with subsequent
fluid reworking. The transformations of the zircons were caused by fluid, which was separated during crystallization of the
last LILE-enriched portions of the melt and inherited the geochemical features of the host rock—leucosome. 相似文献
7.
武夷山中段加里东期混合岩的特征及成因讨论 总被引：11，自引：0，他引：11
武夷山中段出露的加里东期混合岩中广泛发育交代结构，可划分出钠－钙交代、钾交代和磋交代等三个阶段．岩石中微斜长石为最大微斜长石．浅色体与基体内外长石的Ａｎ值明显不同，且无环带构造．黑云母成分与围岩中的黑云母接近．岩石化学成分的变化与交代作用的类型有关．微量元素具有与围岩相似的特征，仅重稀土含量高于围岩．质量平衡计算表明，它们形成于开放体系中，矿物颗粒呈聚集分布．这些特征表明它们是由原来的变质岩经减质流体交代形成的。 相似文献
8.
Zircons in ultra-high-temperature (UHT) metamorphosed paragneisses from Mt. Riiser-Larsen in the Napier Complex, East Antarctica, were dated by using ion microprobe (SHRIMP) and electron microprobe (EMP). Both SHRIMP and EMP analyses yield consistent 2520–2460 Ma age populations for garnet–orthopyroxene-bearing paragneiss and leucosomes enclosed within. The peak UHT event was dated at 2480 Ma by SHRIMP analyses on metamorphic zircons from the garnet–orthopyroxene paragneiss and those on magmatic zircons from the leucosomes which are interpreted to be formed at syn-UHT. As obtained by SHRIMP, the UHT metamorphic event was terminated no later than 2460 Ma. Minor 2520-Ma SHRIMP age suggests either the onset of prograde metamorphism or another high-grade metamorphic event unrelated to the UHT. EMP analyses on metamorphic zircons from sapphirine–quartz and osumilite-bearing magnesian paragneisses give c. 2500–2450 Ma ages. Inherited igneous zircon cores of the magnesian paragneisses yield relatively scattered EMP ages ranging over c. 3000–2650 Ma, suggesting that igneous materials of these ages sourced the protoliths of the paragneisses and that they were deposited during the interval c. 2650–2520 Ma. 相似文献
9.
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
10.
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
11.
I. S. Sedova L. M. Samorukova V. A. Glebovitskii S. G. Skublov 《Geochemistry International》2009,47(10):988-1003
This paper reports the results of a geochemical investigation of zircon from a migmatized aluminous gneiss (gn), melanosome (M), and sequential leucosome generations (Lc2, Lc3, Lc4, and Lc5) from an outcrop in the northwestern Ladoga region. The contents of REE, Y, Ti, Hf, Th, U, and P were determined using a
Cameca IMS-4f ion microprobe in 12 zircon grains from the aforementioned rocks, in two-three spots in each grain. All of the
specimens show rather uniform REE distribution patterns. More significant variations were observed in the light and medium
REE (at smaller variations in the heavy REE), as well as in Ti, Y, Th, and U contents between zircons from the host rocks
and from the leucosomes. It was supposed that REE-rich zircons from the gneiss and melanosome (without oscillatory zoning)
are relics, whereas rhythmically zoned zircons with lower REE contents crystallized in the gneiss in the presence of dispersed
anatectic melt. The contents of most REE and Y increase from core to rim in zircons from the gneiss, melanosome, Lc2, Lc4, and Lc5, which is opposite to the compositional trend of zircons from Lc3. It was shown that the decrease of HREE and Y content in zircon in the sequence Lc5 → gn → Lc2, Lc3, Lc4 is related to a decrease in the abundance of these elements in the rocks. The leucosomes do not correspond to a differentiation
series of a single melt (there is no variation trends of Rb/Sr, K/Rb, and Rb/Ba in the rock series). The lower Lu/Hf and Sm/Nd
values in the leucosomes relative to the host rocks allowed us to suppose that their protolith was gneisses (for Lc2) and migmatites (for Lc4 and Lc3). The similarity of the early migmatites and gneisses to Lc3 with respect to major and some trace elements and almost identical Lu/Hf and Sm/Nd values support the possibility of the
formation of this leucosome generation during the beginning of the diatexis of migmatites, which was promoted by a temperature
increase. This resulted in a specific trend in the content of some elements during zircon growth in Lc3, which is different from the trend of zircons from other leucosomes. 相似文献
12.
Mineral and isotope studies were undertaken on migmatites from the Schwarzwald, Moldanubian zone of the Variscan belt. The aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting. Textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre-scale migmatite profiles, respectively, suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation (deformation-enhanced mass transport) rather than by partial melting (anatexis). The results of Rb-Sr thin-slab dating on these profiles indicate that Sr isotopes were not completely reset during migmatization. No true isochron ages, but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating 87 Sr/86 Sr ratios back to various stages in their evolution. The coincidence of these Rb-Sr data with U-Pb ages of monazites from migmatites and non-migmatitic gneisses shows that gneisses and migmatites were formed during the same high-temperature event in the Carboniferous (330-335 Ma). The observation that high-temperature metamorphism failed to equilibrate Sr isotopes on the centimetre-scale imposes limitations on the use of conventional whole-rock isochron techniques in dating migmatites. 相似文献
13.
Kun Zhou Yi‐Xiang Chen Yong‐Fei Zheng Li‐Juan Xu 《Journal of Metamorphic Geology》2019,37(8):1099-1127
Partial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated εHf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages. 相似文献
14.
J. E. GREENFIELD G. L. CLARKE M. BLAND & D. J. CLARK 《Journal of Metamorphic Geology》1996,14(4):413-426
Metasedimentary gneisses show a rapid change in grade within a 10-km-wide low- P /high- T regional aureole at Mt Stafford, Arunta Block, central Australia. Migmatites occur in all but the lowermost of five metamorphic zones, which are characterized by: (1) muscovite–quartz schist; (2) andalusite–cordierite–K-feldspar granofels with small melt segregations; (3) spinel–sillimanite–cordierite–K-feldspar migmatite; (4) garnet–orthopyroxene–cordierite migmatite and minor diatexite; and (5) biotite–cordierite–plagioclase diatexite that shows a transition to granite. A subsolidus unit comprising interbedded sandstone and siltstone is equivalent to bedded migmatite , the main rock type in Zones 2–4. Mesoscopic textures and migmatite classification of this unit vary with grade. In Zone 2, metatexite is developed in siltstone layers that are separated by quartz-rich, unmelted metapsammite layers. Melt segregation was less efficient in Zones 3 and 4, where the dominant migmatite layering is a modified bedding. High proportions of melt were present in Zone 4, in which schlieren migmatite is transitional between bedded migmatite and metapelite-sourced diatexite. The preservation of sedimentary structures and coexistence of melt reactants and products in Zone 4 metapelite imply that melting proceeded in situ without substantial migration of melt. Zone 5 biotite–cordierite–plagioclase diatexite carries rafts of bedded migmatite with strongly resorbed edges, as well as large K-feldspar and quartz augen. This unit of comparatively Ca-rich migmatites is inferred to have been formed by the mixing of locally derived and injected granitic melt. 相似文献
15.
为了深入认识华夏地块早古生代陆内造山作用相应的地壳再造过程,本文选取粤西福湖岭混合岩进行了详细的岩相学以及锆石U-Pb年代学研究。根据混合岩化程度,粤西福湖岭混合岩剖面由上而下可以分为3部分:混合岩化沉积变质岩、条带状混合岩和混合花岗岩。根据岩性与岩相学特征,福湖岭混合岩又可分为古成体、暗色体和浅色体。LA-ICP-MS锆石U-Pb定年结果及与区域上基底变质岩资料的对比研究表明,福湖岭混合岩的原岩(古成体)是形成于新元古代的变质沉积岩。粤西福湖岭混合岩的形成时代为441~435 Ma,是华夏地块早古生代陆内造山事件的重要产物。 相似文献
16.
辽吉东部层状混合岩的成因 总被引：1，自引：0，他引：1
辽吉东部的层状混合岩与上、下围岩始终保持整合接触,反映它的体系基本上是封闭的,没有外来组份的加入。其组构、岩石化学、微量元素和稀土资料等表明层状混合岩不足岩浆型的,而是重熔岩浆型的,其原岩是沉积变质岩。 相似文献
17.
R. A. Jamieson 《Contributions to Mineralogy and Petrology》1984,86(4):309-320
The Kelly's Mountain gneiss complex of Cape Breton Island, Nova Scotia, is a migmatitic paragneiss dominated by biotite- and cordierite-bearing assemblages. Metamorphic grade throughout the complex is in the upper amphibolite facies, with garnet absent and only retrograde muscovite present. In the high grade core of the complex the reaction biotite+andalusite+quartz=cordierite+K-feldspar+sillimanite+ilmenite+H2O is preserved. The pelitic migmatites contain cordierite- and K-feldspar-rich leucosomes and biotite-rich melanosomes. Minor clinopyroxene-bearing amphibolite in the complex does not show migmatitic textures. The migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria. Retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after K-feldspar and muscovite+green biotite+chlorite after cordierite in paragneiss, and sphene after ilmenite in amphibolite. Peak metamorphic conditions of 1–3.5 kb and 580–700° C are estimated. The high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex. The Kelly's Mountain complex represents a rare example of migmatites formed in the low-pressure facies series, and illustrates some of the reactions involving melting in high grade pelitic rocks. 相似文献
18.
Two types of stromatic leucosomes are identified in metasedimentary rocks from the Skagit migmatite complex, North Cascades, Washington state, U.S.A. Both types are trondhjemitic and appear similar in outcrop, but, although both contain low abundances of REE, one type consists of leucosomes that are relatively REE-enriched compared to the other, and contains (1) small (<0.8 mm), Fe-rich garnets that are compositionally and texturally different from mesosome and melanosome garnet; (2) Ti-rich minerals (rutile, titanite) that are not present in the groundmass of the associated mesosomes or melanosomes and (3) CO2-rich fluid inclusions in quartz. Leucosomes of the second type are REE-depleted compared to the first type, lack garnet and Ti-minerals, and contain only H2O-rich fluid inclusions. The first type of leucosome is interpreted to have formed by in situ partial melting accompanied, and perhaps initiated, by an influx of water-rich fluid during upper amphibolite facies metamorphism. These conclusions are based on estimates of metamorphic P-T-Xfluid conditions (9–10 kbar, > 700°C, water-rich fluid present), inferences about the origin of the above-listed mineralogical and fluid inclusion features, and modeling of leucosome trace element abundances. The second type of leucosome is interpreted to have formed entirely by subsolidus processes (e.g., metamorphic differentiation) because these leucosomes lack features consistent with an origin by partial melting.
K-poor (tonalitic/trondhjemitic) leucosomes associated with metasedimentary (biotite-bearing) source rocks may form by water-saturated partial melting or by subsolidus processes. Both general leucosome-forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism. Partial melting may be initiated by syn-metamorphic magmatic activity if crystallizing plutons serve as external sources of the water-rich fluid necessary for ultrametamorphism in the middle crust during orogenesis. Large-scale migmatite complexes such as the Skagit migmatites may form at least in part in response to contact effects of plutonism associated with high-grade metamorphism, so, although migmatite complexes are a volumetrically substantial part of many orogenic belts, they may not themselves represent a significant original source of magma for larger-scale igneous bodies. 相似文献
19.
深俯冲陆壳物质部分熔融产生的熔体,实验岩石学方面已有广泛报道,而天然初始熔体的组分却难以厘定。对此,本文从苏鲁超高压地体荣成混合岩中识别出了深俯冲花岗质陆壳部分熔融产生的天然初始熔体组成。野外露头显示,混合岩中主要矿物组成为钾长石+斜长石+石英的浅色熔体呈不连续的条带状与残余体互层产出,指示了原位或近源区的部分熔融特征。混合岩浅色体锆石CL图像呈明显的核-边结构,继承核部为扬子板块来源的岩浆锆石,形成时代为721±24Ma;新生边部CL图像具震荡环带结构,微量元素上REE呈明显左倾,具有Eu的负异常及Ce的正异常,低的Hf/Y和Th/U比值,具深熔锆石特征,指示形成于花岗质陆壳物质的部分熔融。边部U-Pb谐和年龄为225.9±2Ma,略晚于苏鲁超高压地体超高压峰期变质年龄,表明初始熔融发生在超高压地体折返早期。浅色熔体的全岩地球化学特征表明,主量元素上具有高SiO_2、K_2O及Na_2O含量,低的Fe_2O_3~T、MgO及CaO含量,A/CNK=1.02~1.04,呈弱过铝质亚碱性花岗岩的特征,这与实验岩石学中富硅陆壳物质部分熔融产生的熔体组分极为相近;微量元素上富集大离子亲石元素(如Rb、Ba、Pb等),亏损Nb、Ta、Ti等高场强元素,REE呈较为平坦的配分模式,具弱的Eu负异常并亏损Sr。本文通过上述对天然样品研究,厘定了深俯冲花岗质陆壳部分熔融及其初始熔体的组成,为理解大陆俯冲带壳幔相互作用提供了关键依据。 相似文献
20.
D. L. WHITNEY 《Journal of Metamorphic Geology》1992,10(6):715-725
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