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1.
Rare earth elements (REE) were determined in two suites of Hercynian ‘Younger’ granodiorites and granites, one massive and the other porphyritic. Within each suite, the REE abundances decrease towards the more felsic granite while the REE patterns are almost identical. The patterns of the porphyritic types are only slightly more fractionated than those of the massive rocks. Negative Eu-anomalies are observed in all rocks although those of the granodiorites are smaller than those of the granites. Modeling of the data indicates that the granitic magmas may be derived, by partial melting, from the greywackes and pelites of the orogenic belt, the melts being in equilibrium with a residuum composed of quartz, plagioclase, garnet, orthopyroxene or cordierite, and, possibly, biotite.  相似文献   

2.
The major rock-forming mineral phases (pyroxenes, plagioclase, garnet, hornblende) of a suite of granulite-facies gneisses from the Scourian complex, NW Scotland, have been analyzed for their rare earth element (REE) content. Although host rock compositions range from felsic to ultramafic, REE abundances and patterns for each mineral group show only limited variation. The REEs exhibit regular and consistent distribution patterns for each mineral which suggest, together with major element and textural considerations, that the observed distribution coefficients approach equilibrium. Total REE content follows the sequence hornblende>clinopyroxene>garnet>plagioclase >orthopyroxene and mass balance calculations show that even in the felsic gneisses>60% of the REEs reside in the major rock-forming minerals. Comparisons of both relative REE abundances and distribution coefficients with those in other rock types reveal a striking resemblance with patterns observed in mineral-liquid pairs of dacitic rocks. These similarities may have arisen during a partial melting episode in which granite-granodiorite melts were generated and removed from the Scourian complex; leaving a residuum which is severely depleted in the incompatible elements, including the REEs.  相似文献   

3.
Summary ?Partial melting of the mantle is polybaric which implies that the phase relations change during partial melting. In addition to the pressure the composition of the melt depends on the melting mode. Various melting models have been suggested. Here the basic phase relations of polybaric batch, percolative, and critical melting are considered, using a simple ternary system. The percolative melts are in equilibrium with their residua, but differ somewhat in composition from those of batch melting. Critical melting is a fractional type of melting where the residuum contain interstitial melt. The critical melts differ in composition from batch melts. The linear trends of peridotites from ophiolites show that the extracted melts had nearly constant compositions, and therefore were extracted within a small pressure interval. A comparison between the trends of mantle peridotite and experimental batch melts suggests strongly that the melt extracted from the peridotites are in equilibrium with their residua. This could suggest that either batch or percolative melting are relevant melting modes for the mantle. However, isotopic disequilibria favor instead a critical mode of melting. This inconsistency can be avoided if the ascending melts are accumulated within a source region and equilibrate with the residuum before the melt is extracted from the source region. The evidence for equilibrium suggests that multisaturation of tholeiitic compositions in PT-diagrams is relevant for estimating pressure and temperature of generation of primary tholeiitic magmas. Received September 2, 2001; revised version accepted March 20, 2002  相似文献   

4.
Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The dunite-wehrlite suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of <20–25 kb leaving a residual lherzolite, and that the alkali basalt and basanite melts are formed by small degrees of melting of a garnet-peridotite source at pressures of >20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.  相似文献   

5.
《Precambrian Research》1987,37(4):323-342
A suite of early Proterozoic basic to granitic rocks exposed near St. Cloud, Minnesota are cut by northeast-trending basaltic dikes. Pb isotope data for all of these rocks overlap and plot about an 1800 Ma PbPb correlation line. The basic rocks, which are light REE enriched, have ϵNd values between +0.4 and −4.8. Petrogenetic considerations suggest that the basic rocks were derived from a light REE enriched source which had an Fe/Mg ratio greater than that for pyrolite. The enrichment in Fe/Mg is probably a result of the addition of basic melts to the source. The light REE enrichment may have a mantle origin by the addition of mantle-derived, light REE enriched basic melts or fluids; or a sedimentary origin by the addition of light REE enriched fluids or melts derived from subducted sediments with an early Proterozoic provenance. In either case, the Nd isotopes suggest that the light REE enriched component existed since c. 2300 Ma. The igneous complex may have formed at a convergent margin. An anorthositic gabbro near Mora, has an ϵNd of +5 indicating that it was derived from a mantle source with a history of light REE depletion. This gabbro may have been part of an early Proterozoic ocean crust.  相似文献   

6.
《International Geology Review》2012,54(12):1166-1181
Geological and isotope-geochemical studies of acid volcanics in the Verkhovtsevo greenstone belt and surrounding tonalite-trondhjemite plutons within the central Dnieper gneiss-green- stone terrain were conducted in the search for genetic relationships and increased understanding of the petrogenesis of acid melts. The acid volcanic and plutonic rocks are similar in mineral composition and form a unified calc-alkaline-like trend from dacite/tonalite to rhyolite/ trondhjemite. Dacites and tonalites have the same rare-earth element (REE) patterns with moderately fractionated light and heavy REE as well as small negative Eu anomalies. Rhyolite and trondhjemites have less-fractionated REE patterns with larger negative Eu anomalies. Whole-rock data for the acid volcanic and plutonic rocks yielded a single isochron of 3117 ± 204 Ma, εNd = +1.14 ± 0.80.

The data suggest a temporal and genetic relationship between the acid volcanics of the greenstone sequences and the surrounding plutonic rocks; both appear to belong to a single suite. The positive eNd value tends to suggest that a source of their melts can be traced to mafic materials rather than to older sialic crust. Petrochemical data and REE-model calculations suggest that dacite/tonalite liquids might have formed during partial melting of a mafic source, such as Archaean tholeiite TH-1 in equilibrium with hornblende-pyroxene-plagioclase restite. Subsequent differentiation of these melts in equilibrium with titanoilmenite-pyroxene-plagioclase cumulate may have given rise to the trondhjemites and rhyolites. Such a mineralogy of the restite and cumulate phases suggests that felsic melts containing little water in the Verkhovtsevo greenstone belt were generated at depths up to 30 km, probably in the greenstone belt's mafic basement.  相似文献   

7.
Following previous publication of major–minor elementdata, this paper presents rare earth element (REE) data forheterogeneous (chemically zoned) garnets belonging to the peridotitesuite of mantle xenoliths from the Jagersfontein kimberlitepipe, South Africa. The rim compositions of the garnets in thehighest temperature–pressure (deepest) deformed peridotitesshow a typical megacryst-like pattern, of very low light REE(LREE) increasing through the middle REE (MREE) to a plateauof heavy REE (HREE) at c. 20 times chondrite; these compositionswould be in equilibrium with small-volume melts of the mid-oceanridge basalt (MORB) source (asthenosphere). With decreasingdepth the garnet rims show increasing LREE and decreasing HREE,eventually resulting in humped relative abundance patterns.A set of compositions is calculated for melts that would bein equilibrium with the garnet rims at different depths. Theseshow decreasing relative abundance of each REE from La to Lu,and the La/Lu ratio of the melts increases with decreasing depthof formation. Modelling of the effects of crystal fractionationshows that this process could largely generate the sequenceof garnet rim and melt compositions found with decreasing depth,including the humped REE patterns in high-level garnets. Consideringthe behaviour of major–minor elements as well as REE,a process of percolative fractional crystallization is advocatedin which megacryst source melts percolate upwards through peridotitesand undergo fractionation in conjunction with exchange withthe peridotite minerals. The initial megacryst melt probablyincludes melt of lithospheric origin as well as melt from theMORB source, and it is suggested that the process of percolativefractional crystallization may form a variety of metasomaticand kimberlitic melts from initial megacryst melts. Repeatedmetasomatism of the lower lithosphere by such differentiatingmelts is suggested by consideration of garnet core compositions.Such metasomatism would progressively convert harzburgites tolherzolites by increasing their CaO content, and this may accountfor the fact that the Cr-rich diamond–garnet harzburgiteparagenesis is commonly preserved only where it has been encapsulatedin diamonds. KEY WORDS: cratonic lithosphere; garnet zoning; mantle xenoliths; megacryst magma; metasomatic melt  相似文献   

8.
微量元素模拟限定大别造山带中生代花岗岩类的源岩成分   总被引：1，自引：1，他引：0  
徐启东 《现代地质》1997,11(1):48-57
摘 要　 大陆碰撞造山带中花岗岩浆主要是因下部地壳缺乏流体熔融过程而形成的。对其源 岩成分限制而进行的微量元素模拟不仅要考虑源岩和熔体中有关矿物相比例的变化‚还要考 虑熔融过程中源岩矿物相组合的变化‚并选择合适的元素用于模拟。Rb、Sr、Ba 和 REE 模拟 结果显示‚上溪群杂砂岩作为扬子隆起带中生代花岗岩类的源岩是不合适的：大别隆起带中 的中生代花岗岩类也不可能完全由大别杂岩中的 TTG 质片麻岩熔融形成‚更可能是变基性岩 和 TTG 质片麻岩构成的复合源岩熔融的结果。  相似文献   

9.
Rare earth element (REE) and major element data are presented on 44 Archaean samples which include spinifex textured ultramagnesian lavas (STPK) spinifex textured basalts (STB) and low MgO tholeiites. The samples come from the Yilgarn and Pilbara Blocks (W. Australia), Barberton (South Africa), Belingwe and Que Que (Rhodesia), Abitibi (Canada) and the 3.7 b.y. Isua Belt of Western Greenland. In addition REE data are given on three near primitive mid-ocean ridge basalts (MORB) and a glassy MORB-type basalt from Taiwan. We suggest that REE patterns, particularly the light REE and Eu, can be affected by metamorphism, but argue that the consistency of pattern from samples both within and between areas enables recognition of primary patterns. La/Sm ratios of 2.7 b.y. STPK are characterised by being lower than those of associated basalts. The 3.5 b.y. STPK Barberton material does not show this feature but instead displays significant heavy REE depletion. The separation of garnet from these liquids is suggested as a possible mechanism for the high CaO/Al2O3 ratios, (Al loss) and the heavy REE and Sc depletion. The REE data on Barberton material is equivocal on the derivation of the so-called basaltic komatiites from the peridotitic komatiites. However, REE analyses on STPK and high magnesian lavas from elsewhere suggests that crystal fractionation is not a viable mechanism to produce one from the other. We suggest instead, that varying amounts of partial melting of different sources is responsible for the spectrum of compositions. The STB appear to be an easily recognised rock type within the Archaean. They are characterised by quench (clinopyroxene) textures and a light REE enriched pattern. It is suggested that these are near primary melts and that their REE patterns mirror their mantle source. We propose a two stage model for the 2.7 b.y. mafic complexes, in which, prior to the generation of ultrabasic magmas, the source underwent a small amount of partial melting which resulted in the removal of a melt enriched in incompatible elements. The depletion process could be achieved either during mantle diapirism or by upward migration of interstitial melts into an Archaean low velocity zone. The spread of La/Sm ratios in STPK and STB is used as an argument that the Archaean mantle was chemically heterogeneous and that the degree of heterogeneity was similar to that observed in modern ocean volcanics. As a result, partial melting of the mantle under different P-T conditions produced a spectrum of magma types. The information presently available on Archaean mafic and silicic magmatism and the incompleteness of geochemical data on present day tectonic environments are two major obstacles in formulating Archaean tectonic models. In addition a comparison of present day and Archaean ultramafic and silicic rocks suggests that plate tectonic models as presently understood may not be suitable analogues for all Archaean tectonic environments.  相似文献   

10.
A suite of basanitoids and alkali olivine basalts from Grenada, Lesser Antilles were analyzed for rare earth elements. The REE concentrations of these rocks are characterized by a small variation in the heavy REE (7 to 9 times chondrite) and a large variation in the light REE (17 to 93 times chondrite). Among the possible mechanisms to account for the REE variations, fractional crystallization processes at low and high pressures, and partial melting processes (both batch melting and fractional melting) were examined, using the partition relationships of REE among silicate minerals and melts. It is suggested that the observed REE variations are best explained by variable degrees of batch partial melting, in which garnet is present as one of the solid phases through 2 to 17% melting of a garnet lherzolite parent rock.  相似文献   

11.
The main plutonic complex of the Troodos ophiolite, north of the Arakapas Fault Zone, has been re-examined both from field and geochemical perspectives. Ion microprobe analyses of clinopyroxene crystal cores show that the range of melt compositions added to the lower crust far exceeds that of published lavas in the main Troodos massif. This suggests that the lower crust acted as a filter into which a large range of melt compositions were added and out of which a homogenised (and generally fractionated) derivative was extracted. This crustal-level aggregation homogenised diverse melt fractions from a broad range of degrees of melting. Depleted melts with U-shaped rare earth element (REE) patterns were a significant component of the melts added to the crust, but because of their low incompatible element abundances, mixing with less depleted melts prior to eruption masked their signature in the lavas. The discovery that highly depleted melts constituted a significant component of the melts added to the Troodos crust, but not of the lavas, demonstrates that the spatial distribution of lava-types is not necessarily a good indicator of where different parental melt compositions are generated within the mantle. Compared with normal mid-ocean ridge basalts, the Troodos parental melts were (1) generally depleted in immobile incompatible trace elements, (2) less depleted in light REE (LREE) than would be expected for the concomitant depletion in middle and heavy REE, (3) enriched in Sr with respect to the LREE and (4) more oxidised. Modelling of these characteristics suggests a mantle source that had previously lost a significant melt fraction under relatively reducing conditions. This was followed by remelting under more oxidising conditions in an environment in which Sr and LREE were added to the source consistent with previous models of a supra-subduction zone setting.  相似文献   

12.
We have measured apatite solubility in calcic carbonatitic liquids and determined apatite/melt partition coefficients for a series of trace elements, including the rare earth elements (REE), high field strength elements (HFSE), Rb, Sr, U-Th-Pb. Experiments were performed between 4 and 6 GPa, from 1200 to 1380 °C, using the multianvil apparatus. Trace element concentrations were determined by laser ablation ICP-MS and electron microprobe. In addition, a specific protocol was designed to measure carbon concentration in the apatites, using the electron microprobe. Two starting apatite samples were used in order to test for the effect of apatite chemistry on partitioning behavior.Apatite solubility is lower in calcitic melts by a factor 3-5 compared to dolomitic melts (3-5.5 vs. 10-18 wt.% P2O5 in melt). We interpret this difference in terms of solubility product in the liquid and propose an empirical model for apatite saturation that takes into account melt calcium content. We conclude that calcitic melts that may form by melting of carbonated eclogites could be saturated with residual apatite, contrary to dolomitic melts formed in carbonated peridotites.Compatibility behavior of the REE depends on apatite silica content: REE are compatible in apatites containing 3.5-5 wt.% SiO2, with values between 1.5 and 4, whereas REE are incompatible in apatites containing 0.2 wt.% SiO2. HFSE, U, Th, and Y are compatible in silica-rich apatite, with while . Strontium is always retained in the melt, with of the order of 0.5. Lead appears to be incompatible in apatite, although this finding is weakened by almost complete Pb loss to sample container. High silica concentration favors REE incorporation in apatite by allowing for charged balanced coupled substitution. Sulfur and carbonate may also favor REE incorporation in apatite. Our results allow to reconcile previously published experimental determinations of REE partitioning. We use our experimentally determined partition coefficients to investigate the impact of residual apatite during partial melting of recycled carbonated material (eclogite + sediments) and discuss how the chemical characteristics of the produced liquids can be affected by residual apatite.  相似文献   

13.
Partial fusion hypotheses have been proposed for the origin of lherzolite-harzburgite alpine peridotite associations. Analyzed lherzolites from Othris, Ronda, Lanzo and Beni Bouchera, have light REE depleted to chondritic REE abundances, and clinopyroxenes contain most of the REE relative to depleted olivine and orthopyroxene. Variation in the level of REE enrichment within these lherzolites indicates mantle heterogeneity probably caused by partial melting processes. The Beni Bouchera spinel lherzolite and the Othris plagioclase lherzolite are the best candidates for relatively undepleted mantle based on REE studies. Fractional fusion calculations (15–25%) reveal that partial melts have REE characteristics somewhat similar to oceanic tholeiites. Conversely, computed source peridotites from oceanic tholeiites (Schilling, 1975) are similar to the alpine lherzolites reported here. Alpine lherzolites are, however, depleted in trace elements (K, Rb, Sr and Ba, Menzies and Murthy 1976). Since the lherzolites have an undepleted major, minor and REE chemistry close to that of pyrolite, the lost trace element-rich fraction must represent a small degree of melting. It is proposed that alpine lherzolites are residue left after the loss of a nephelinitic/alkalic fraction, ([Ce/Yb]N=2.0–4.01) representing a small degree of partial fusion. This labile fraction may have existed as an intergranular phase or hydrous mineral prior to melting.  相似文献   

14.
Ultrabasic magmas and high-degree melting of the mantle   总被引：1，自引：0，他引：1  
As the degree of melting of mantle peridotite increases, the liquids that are formed become more basic and less viscous, and the spacing between residual crystals increases. The settling velocities of residual crystals in partial melts consequently will increase by several orders of magnitude, from 9.4 × 10–4 cm/s to 4.3 × 10–1 cm/s for a 1 cm olivine grain, as the proportion of liquid increases from 15 to 60%.To produce an ultrabasic komatiitic magma from a source with commonly assumed mantle composition requires 50 to 80% melting. Before this degree of melting can be reached, a highly fluid picritic magma produced by 30 to 50% melting will segregate from the source. Ultrabasic magmas probably form by a sequential melting process and are derived from a residuum composed of refractory minerals and trapped liquid left by previous episodes of partial melting and magma extraction. Trace element concentrations in ultrabasic komatiite lavas are consistent with this theory.  相似文献   

15.
本文报道了六合-仪征第三纪大陆碱性玄武岩十八个样品的REE、Rb、Ba、Sr、Nb、Zr、Ni、Cr、V、Sc、Y、Ga、Zn、Cu等痕量元素含量,讨论了该岩套的成因及其地幔源区的特征。石榴石橄榄岩型地幔源区经较小程度部分熔融形成了基性原始岩浆;其后经过橄榄石和单斜辉石为主的结晶分异作用,演化后的岩浆喷出地表形成玄武岩套。本区碱性玄武岩的地幔源区曾受近期富集作用影响,具有富集LREE等不相容元素的特征。  相似文献   

16.
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   

17.
Lavas and included xenoliths from the Comores Archipelago have been analysed for the rare earth elements (REE) La-Lu. Among basaltic lava types fractionation of REE rock/chondrite distribution patterns is more extreme with greater SiO2 undersaturation and contents of incompatible elements. Enrichment and slight fractionation of REE in the rock series basanite-phonolite is considered compatible with a model of fractional crystallisation at low pressures involving mainly olivine and clinopyroxene, and to a much lesser extent, plagioclase. Apatite is probably effective in curtailing further enrichment of REE. High level fractional crystallisation and eclogite fractionation at depth appear unlikely causes for the relative enrichment of light REE (La-Eu) in the undersaturated basalts. This effect is more probably due to mineralogical control during partial melting in the upper mantle. Lherzolite xenoliths are poor in REE, exhibiting a slight relative depletion in the light REE. These patterns are interpreted as those of possible mantle material subjected to small degrees of partial melting, although not necessarily related to those melts erupted as lava flows at the surface.  相似文献   

18.
The Gardar failed-rift Province is world-famous for its (per-)alkaline plutonic rocks. Elevated contents of F in the mantle source and F-enrichment in the parental melts have been suggested to account for the peculiarities of the Gardar rocks (e.g. their rare mineralogy, extreme enrichment of HFSE elements, Be or REE in the Ilímaussaq agpaites, and the formation of the unique Ivigtut cryolite deposit). To constrain the formation and chemical evolution of F-bearing melts and fluids, fluorides (fluorite, cryolite, villiaumite, cryolithionite), calcite and siderite from the Ilímaussaq, Motzfeldt and Ivigtut complexes were analysed for their trace element content focusing on the rare earth elements and yttrium (REE).The various generations of fluorite occurring in the granitic Ivigtut, agpaitic Ilímaussaq and miaskitic to agpaitic Motzfeldt intrusions all share a negative Eu anomaly which is attributed to (earlier) feldspar fractionation in the parental alkali basaltic melts. This interpretation is supported by the abundance of anorthositic xenoliths in many Gardar plutonic rocks.The primary magmatic fluorites from Ilímaussaq and Motzfeldt display very similar REE patterns suggesting a formation from closely related parental melts under similar conditions. Hydrothermal fluorites from these intrusions were used to constrain the multiple effects responsible for the incorporation of trace elements into fluorides: temperature dependence, fluid migration/interaction and complexation resulting in REE fractionation. Generally, the REE patterns of Gardar fluorides reflect the evolution and migration of a F/CO2-rich fluid leading to the formation of fluorite and fluorite/calcite veins. In certain units, this fluid inherited the REE patterns of altered host rocks. In addition, there is evidence of an even younger fluid of high REE abundance which resulted in highly variable REE concentrations (up to three orders of magnitude) within one sample of hydrothermal fluorite.The REE patterns of the granitic Ivigtut intrusion show flat to slightly heavy-REE-enriched patterns characterised by a strong tetrad effect. This effect is interpreted to record extensive fluid–rock interaction in highly fractionated, Si-rich systems.Interestingly, the fluorides appear to record different source REE patterns, as the spatially close Motzfeldt and Ilímaussaq intrusions show strong similarities and contrast with the Ivigtut intrusion located 100 km NE. These variations may be attributed to differences in the tectonic position of the intrusions or mantle heterogeneities.  相似文献   

19.
This study is aimed at understanding the behavior of monazite, xenotime, apatite and zircon, and the redistribution of Zr, REE, Y, Th, and U among melt, rock-forming and accessory phases in a prograde metamorphic sequence, the Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a section from the middle to lower continental crust. Metamorphism ranges from middle amphibolite to granulite facies and metapelites show evidence of intense partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th, and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989). Apatite is extracted from the system dissolved into partial melts. Xenotime is consumed in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar, and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are significantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer in Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concentration of Cu, Sr, Pb, Zr, Ba, LREE, and Th as amphibolite-grade metapelites (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5–6 to 14–15, the progressive increase of Nb/Ta, and the decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, and (except at the lowest degree of partial melting) monazite. Their REE patterns, especially the magnitude of the Eu anomaly, depend on the relative proportion of feldspars and monazite incorporated into the melt. The presence of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th ≈ 2.5–3. The U depletion and increase in Th/U characteristic of granulite facies only happens in monazite-bearing rocks. It is attributed to enhancement of the U partitioning in the melt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapelites, and mafic granulites maintain similar Th/U and abundance of U and Th as their unmetamorphosed equivalents, it seems that geochemical changes associated to granulitization have only a minor influence on heat-production in the lower crust.  相似文献   

20.
SAWYER  E. W. 《Journal of Petrology》1991,32(4):701-738
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   

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