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1.
Petrological and geochemical variations are used to investigatethe formation of granite magma from diatexite migmatites derivedfrom metasedimentary rocks of pelitic to greywacke compositionat St. Malo, France. Anatexis occurred at relatively low temperaturesand pressures (<800°C, 4–7 kbar), principally throughmuscovite dehydration melting. Biotite remained stable and servesas a tracer for the solid fraction during melt segregation.The degree of partial melting, calculated from modal mineralogyand reaction stoichiometry, was <40 vol. %. There is a continuousvariation in texture, mineralogy and chemical composition inthe diatexite migmatites. Mesocratic diatexite formed when metasedimentaryrocks melted sufficiently to undergo bulk flow or magma flow,but did not experience significant melt–residuum separation.Mesocratic diatexite that underwent melt segregation duringflow generated (1) melanocratic diatexites at the places wherethe melt fraction was removed, leaving behind a biotite andplagioclase residuum (enriched in TiO2, FeOT, MgO, CaO, Sc,Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocraticdiatexite (enriched in SiO2, K2O and Rb) where the melt fractionaccumulated. Leucocratic diatexite still contained 5–15vol. % residual biotite (mg-number 40–44) and 10–20vol. % residual plagioclase (An22). Anatectic granite magmadeveloped from the leucodiatexite, first by further melt–residuumseparation, then through fractional crystallization. Most biotitein the anatectic granite is magmatic (mg-number 18–22). KEY WORDS: anatexis; diatexite; granite magma; melt segregation; migmatite  相似文献   

2.
The melting relations of two proposed crustal source compositionsfor rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand,have been studied in a piston-cylinder apparatus at 10 kb totalpressure and a range of water activities generated by H2O-CO2vapour. Starting materials were glasses of intermediate composition(65 wt.% Si02 representing a metaluminous ‘I-type’dacite and a peraluminous ‘S-type’ greywacke. Crystallizationexperiments were carried out over the temperature range 675to 975?C, with aH2O values of approximately 1?0, 0?75, 0?5,and 0?25. Talc-pyrex furnace assemblies imposed oxygen fugacitiesclose to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartzand plagioclase through 675–700?C at high aH2O, 725–750?Cat aH2O0?5, and 800–875?C at aH2O0?25, corresponding to<60–70% melting. Concentrations of refractory mineralcomponents (Fe, Mg, Mn, P, Ti) in liquids increase throughoutthis melting interval with increasing temperature and decreasingaH2O. Biotite and hornblende are the only mafic phases presentnear the solidus in the dacite, compared with biotite, garnet,gedritic orthoamphibole, and tschermakitic clinoamphibole inthe greywacke. Near-solidus melting reactions are of the type:biotite + quartz + plagioclase = amphibole ? garnet, potentiallyreleasing H2O for dehydration melting in the greywacke, butproducing larger amounts of hornblende and releasing littleH2O in the dacite. At aH2O0?25 and temperatures 825–850?C,amphibole dehydration produces anhydrous mineral phases typicalof granulite fades assemblages (clinopyroxene, orthopyroxene,plagioclase?quartz in the dacite; garnet, orthopyroxene, plagioclase?quartzin the greywacke) coexisting with melt proportions as low as40%. Hornblendce-saturated liquids in the dacite are weaklyperaluminous (0?3–1?6 wt.% normative C—within therange of peraluminous TVZ rhyolites), whereas, at aH2O0?25 andtemperatures 925?C, metaluminous partial melt compositions (upto 1?8 wt.% normative Di) coexist with plagioclase, orthopyroxene,and clinopyroxene. At all water activities, partial melts ofthe greywacke are uniformly more peraluminous (1?5–2?6wt.% normative C), reflecting their saturation in the componentsof more aluminous mafic minerals, particularly garnet and Al-richorthopyroxene. A metaluminous source for the predominantly Di-normativeTVZ rhyolites is therefore indicated. With decreasing aH2O the stability fields of plagioclase andquartz expand, whereas that of biotite contracts. These changesare reflected in the proportions of normative salic componentsin partial melts of both the dacite and greywacke. At high aH2O,partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic);with decreasing aH2O they become notably poorer in An and richerin Or (granodioritic-granitic). These systematic variationsin salic components observed in experimental metaluminous tostrongly peraluminous melts demonstrate that a wide varietyof granitoid magmas may be produced from similar source rocksdepending upon P-T-aH2O conditions attending partial melting.Some peraluminous granitoids, notably trondhjemitic leucosomesin migmatites, and sodic granodiorites and granites emplacedat deep crustal levels, have bulk compositions similar to nearsolidus melt compositions in both the dacite and greywacke,indicating possible derivation by anatexis without the involvementof a significant restite component.  相似文献   

3.
Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H2O. This reflectsthe fact that conditions of low aH2O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH2O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt  相似文献   

4.
Rocks of semi-pelitic composition are common in high-grade terranes.The first appearance of orthopyroxene in these rocks marks thetransition from amphibolite- to granulite-facies conditions,and is commonly attributed to the process of fluid-absent partialmelting. We have conducted fluid-absent melting experimentson two natural semi-pelitic rocks (quartz, plagioclase, alkalifeldspar, biotite and garnet) with the specific objective ofdetermining the pressure–temperature conditions necessaryto produce orthopyroxene. In contrast to previous experimentalstudies, our starting materials were obtained from a transitionalamphibolite–granulite terrane. Importantly, the high TiO2(>5 wt %) and F (>1 wt %) contents of biotite in our experimentsare more representative of biotite found in rocks on the vergeof granulite-facies conditions than those used in earlier studies.Experiments were conducted in a piston-cylinder apparatus at800–1050°C and 7–15 kbar. We reversed the firstappearance of orthopyroxene in two-stage experiments at 7 and10 kbar. Fluid-absent melting of biotite began at  相似文献   

5.
We present results of dehydration melting experiments [3–15kbar, 810–950C f(O2) QFM (quartz-fayalite-magetite)and Ni-NiO] on two Fe-rich mixtures of biotite (37%), plagioclaseAn38 (27%), quartz (34%) and ilmenite (2%), which differ onlyin their biotite compositions (mg-number 23 and 0.4). Dehydrationmelting of metagreywackes of constant modal composition generatesa wide range of melt fractions, melt compositions and residualassemblages, through the combined effects of pressure, Fe/Mgratio and f(O2). Crystallization of garnet is the chief controlon melting behavior, and is limited by two reactions: (1) thebreakdown of garnet + quartz to orthopyroxene + plagioclaseat low P, and (2) the oxidation of garnet to magnetite + anorthite+ quartz (enstatite), which is sensitive to both f(O2) andP. Because of these reactions, melting of Mg-rich metagreywackesis rather insensitive to f(O2) but strongly sensitive to P;the converse is true for Fe-rich metagreywackes. Garnet crystallizationrequires that plagioclase break down incongruently, liberatingalbite. This increases the Na2O content of the melts and enhancesmelt production. Thus, melting of metagreywacke in a reducingdeep-crustal environment (with garnet stable) would producemore, and more sodic, melt than would garnet-absent meltingof the same source material in a relatively oxidizing, shallow-crustalenvironment. KEY WORDS: anatexis; metasediments; gneisses; granites; garnet *Corresponding author. Telephone: 706-542-2394; fax: 706-542-2425; e-mail: alpatino{at}uga.cc.uga.edu  相似文献   

6.
Calcic schists in the andalusite-type regional metamorphic terrainin the Panamint Mountains, California, contain the low-varianceassemblage quartz+epidote+muscovite+biotite+calcic amphibole+chlorite+plagioclase+spheneat low grade. Near the sillimanite isograd, chlorite in thisassemblage is replaced by garnet. The low variance in many calcicschists allows the determination of the nature of the reactionthat resulted in the coexistence of garnet+hornblende. A graphicalanalysis of the mineral assemblages shows that the reactioncan not be of the form biotite+epidote+chlorite+plagioclase+quartz=garnet+hornblende+muscovite+sphene+H2Obecause garnet+chlorite never coexisted during metamorphismand the chlorite-bearing and garnet-bearing phase volumes donot overlap. The compositions of the minerals show that withincreasing grade amphibole changed from actinolite to pargasitichornblende with no apparent miscibility gap, the partitioningof Fe and Mg between chlorite and hornblende changed from KD(Mg/Fe, chl&amp) < 1 to KD > 1, the partitioning betweenbiotite and hornblende changed from KD (Mg/Fe, bio/amp) <1 in chlorite-zone samples to KD > 1 in garnet + hornblende-zonesamples, and the transition to the garnet-bearing assemblageoccurred when the composition of plagioclase was between An55and An80. Both the graphical analysis and an analytical analysisof the compositions of the minerals using simplified componentsderived from the natural mineral compositions indicate thatat the garnet+hornblende isograd the composition of hornblendewas colinear with that of plagioclase and biotite, as projectedfrom quartz, epidote, muscovite, and H2O. During progressivemetamorphism, chlorite+biotite+epidote+quartz continuously brokedown to form hornblende+muscovite+sphene until the degeneracywas reached. At that point, tie lines from hornblende couldextend to garnet without allowing garnet to coexist with chlorite.Thus, the garnet+hornblende isograd was established throughcontinuous reactions within the chlorite-grade assemblage ratherthan through a discontinuous reaction. In this type of isograd,the low-grade diagnostic assemblage occurs only in Mg-rich rocks;whereas the high-grade assemblage occurs only in Fe-rich rocks.This relation accounts for the restricted occurrence of garnet+hornblendeassemblage in low-pressure terrains. In Barrovian terrains,garnet+chlorite commonly occurs, and the first appearana ofgarnet+hornblende can simply result from the continuous shiftof the garnet+chlorite tie line to Mg-rich compositions.  相似文献   

7.
The unusual association of cordierite and cummingtonite (? gedrite+ chlorite + biotite + ilmenite + plagioclase + quartz) definesa metamorphic facies within aluminous, low-Ca amphibolites fromthe Proterozoic rocks of the Gold Brick District, east of Gunnison,Colorado. More Fe-rich bulk chemistries in the same facies arecharacterized by assemblages consisting of cordierite+-gedrite+ garnet + chlorite + biotite + ilmenite + plagioclase + quartz,whereas more Mg-rich compositions are characterized by cordierite+ anthophyllite + chlorite + biotite + ilmenite ? plagioclase+ quartz. The assemblage gedrite 4- cummingtonite + chlorite+ biotite + ilmenite + plagioclase + quartz was also observed.Coexisting cordierite+ anthophyllite + cummingtonite was notobserved in any rocks, apparently because this assemblage isstable over only a very narrow range of bulk compositions. Metamorphosedpelitic rocks are more iron rich than the assemblage cordierite+ gedrite + garnet + chlorite + biotite + ilmenite + plagioclase+ quartz and consist of garnet ?cordierite ?staurolite ? chlorite? andalusite + biotite + ilmenite + plagioclase + quartz? microclineor muscovite. Mineral rim compositions from cordierite-bearing amphibolitesand metapelites determined by electron microprobe analysis showsystematic Fe/Mg partitioning and define assemblages that occupynon-overlapping regions of the compositional system SiO2-TiO2-Al2O3-MnO-FeO-MgO-CaO-Na2O-K2O-H2Oas determined by algebraic and statistical methods developedby Braun & Stout (1975) and Fisher (1989). Graphical methods(projections) produced spurious overlaps not confirmed by themore rigorous algebraic tests. The spurious overlaps were generatedbecause standard projective analysis was not able simultaneouslyto account for the important effects of the components Na2O,CaO, and MnO on the AFM topologies. The results of algebraicand statistical analysis are consistent with an equilibriumorigin at constant values of temperature and pressure. The cordierite-cummingtonite facies encompasses the relativelylow-pressure and moderate-temperature conditions associatedwith the stability field of andalusite. Garnet-biotite geothermo-metry,and garnet, aluminosilicate, silica, plagioclase (GASP) geobarometrysuggest that temperatures and pressures were nearly constantacross the study area at 550( ? 70) ?C and 3 kb, respectively,near the peak of metamorphism. Other geothermometers and geobarometers,and independent pressure and temperature estimates, are compatiblewith garnet-biotite thermometry and GASP geo-barometry. Gradientsin fO2 or H2O are not required to explain the compatibilityof these assemblages at constant T and P. Cordierite + cummingtonite-bearingrocks can apparently be derived from anthophyllite +garnet-bearingrocks by increasing temperature or decreasing pressure.  相似文献   

8.
The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedP–T estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordierite–K-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and aH2O < 0·3. The pressure estimate impliesan emplacement depth of  相似文献   

9.
S. Jung  S. Hoernes  K. Mezger 《Lithos》2000,50(4):15-287
The Oetmoed Granite–Migmatite Complex (OGMC), Central Damara Orogen, Namibia, consists mainly of 526 to 516 Ma garnet- and cordierite-bearing granite and subordinate 488 to 494 Ma hornblende- and titanite-bearing granite in the form of planar sheets and dykes. Additionally, a slightly elongated granite body occurs in the center of the complex. The garnet- and cordierite-bearing granite has major- and trace-element characteristics of S-type granite but the hornblende- and titanite-bearing granite has higher HFSE and REE contents similar to A-type granite. Whereas the garnet- and cordierite-bearing granite contains numerous restitic xenoliths, the hornblende- and titanite-bearing granite is xenolith-free. The country rocks are cordierite–sillimanite–K-feldspar–garnet-bearing metasedimentary rocks and migmatite. Cordierite- and garnet-rich xenoliths in the S-type granite do not represent primary restite, their depleted chemical composition is best explained by varying and large degrees of partial melting of incorporated country rocks. Most chemical variations among the garnet- and cordierite-bearing granite can be explained by processes linked with fractional crystallization of plagioclase, biotite and accessory phases, mostly monazite and zircon. Major and trace element data and high δ 18O values suggest that the least evolved members of the garnet- and cordierite-bearing granite were derived from metapelitic rocks at ca. 800°C as inferred from monazite and apatite dissolution thermometry. Higher CaO and Na2O but lower SiO2 contents and lower Rb/Sr ratios as well as lower δ 18O values of the hornblende- and titanite-bearing granite suggest that they are more likely generated by partial melting of non-pelitic sources (metagranitoids?) at temperatures in excess of 900°C. Decreasing TiO2, Na2O, FeOtot., MgO, CaO, total REE content but increasing Al2O3 and K2O indicate fractionation of mainly hornblende and titanite in the case of the hornblende- and titanite-bearing granite. The differing compositions of the garnet- and cordierite-bearing granite and the hornblende- and titanite-bearing granite are attributed to different source rocks (metapelite instead of metagranitoid) and different temperatures during melting as inferred from accessory phase dissolution thermometry. Furthermore, significant entrainment of country rock in some samples played a major role during petrogenesis of the garnet- and cordierite-bearing granite but was not important during the evolution of the hornblende- and titanite-bearing granite. Intrusion of such hot, felsic magmas close to the inferred peak of metamorphism has probably caused, in part, the high temperature metamorphism and anatexis of the country rocks at relatively low pressures.  相似文献   

10.
On the basis of the net transfer reactions among garnet, biotite,plagioclase and quartz (for both Mg and Fe end-member models),the garnet–biotite–plagioclase–quartz (GBPQ)geobarometer was empirically calibrated under physical conditionsof P = 1·0–11·4 kbar and T = 515–878°C,based on the input garnet–biotite temperatures and garnet–aluminosilicate–plagioclase–quartz(GASP) pressures of 224 natural aluminosilicate-bearing metapeliticsamples collated from the literature. The calibrations are internallyconsistent with the asymmetric quaternary solid solution modelof garnet, the symmetric quaternary solid solution model ofbiotite, and the Al-avoidance ternary solid solution model ofplagioclase in calibrating the garnet–biotite geothermometerand the GASP geobarometer. The resulting two GBPQ barometerformulae reproduce the input GASP pressures well within ±1·0kbar (mostly within ±0·5 kbar). For both aluminosilicate-bearingand aluminosilicate-absent metapelites, the two GBPQ barometryformulae yielded identical pressures, whether the sample wasincluded or not included in calibrating the GBPQ barometry.The random error of the GBPQ barometry may be expected as ±1·2kbar. The dP/dT slopes of these two GBPQ formulae are closeto that of the GASP barometer in P–T space. Applicationsof the GBPQ barometry of aluminosilicate-absent metapelitesto the rocks within a thermal contact aureole, or rocks withina limited geographical area without post-metamorphic structuraldiscontinuity, show no obvious pressure change. It may be concludedthat the two GBPQ barometry formulae derived in this study maybe used as practical tools for metamorphic pelites under theconditions of 515–878°C and 1·0–11·4kbar, in the composition range of Xgros >3% in garnet, Xan>17% in plagioclase, and  相似文献   

11.
Proterozoic migmatitic paragneisses exposed in the McCulloughRange, southern Nevada, consist of cordierite+almanditic garnet+biotite+sillimanite+plagioclase+K-feldspar+quartz+ilmenite+hercynite.This assemblage is indicative of a low-pressure fades seriesat hornblende-granulite grade. Textures record a single metamorphicevent involving crystallization of cordierite at the expenseof biotite and sillimanite. Thermobarometry utilizing cation exchange between garnet, biotite,cordierite, hercynite, and plagioclase yields a preferred temperaturerange of 590–750?C and a pressure range of 3–4 kb.Equilibrium among biotite, sillimanite, quartz, garnet, andK-feldspar records aH2O between 0?03 and 0?26. The low aH2Otogetherwith low fO2 (QFM) and optical properties of cordierite indicatemetamorphism under fluid-absent conditions. Preserved mineralcompositions are not consistent with equilibrium with a meltphase. Earlier limited partial melting was apparently extensiveenough to cause desiccation of the pelitic assemblage. The relatively low pressures attending high-grade metamorphismof the McCullough Range paragneisses allies this terrane withbiotite-cordierite-garnet granulites in other orogenic belts.aosure pressures and temperatures require a transient apparentthermal gradient ofat least 50?C/km during part of this Proterozoicevent in the southern Cordillera. *Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567  相似文献   

12.
A complex of crustally derived leucogranitic sills emplacedinto sillimanite-grade psammites in the upper Langtang Valleyof northern Nepal forms part of the Miocene High Himalayan graniteassociation. A series of post-tectonic, subvertical leucograniticdykes intrude the underlying migmatites, providing possiblefeeders to the main granite sills. The leucogranite is peraluminous and alkali-rich, and can besubdivided into a muscovite–biotite and a tourmaline–muscovitefacies. Phase relations suggest that the tourmaline leucogranitescrystallized from a water-undersaturated magma of minimum-meltcomposition at pressures around 3–4 kbar. Potential metasedimentaryprotoliths include a substantial anatectic migmatite complexand a lower-grade mica schist sequence. Isotopic constraintspreclude the migmatites as a source of the granitic melts, whereastrace-element modelling of LILEs (Rb, Sr, and Ba), togetherwith the Nd and Sr isotopic signatures of potential protoliths,strongly suggest that the tourmaline-bearing leucogranites havebeen generated by fluid-absent partial melting of the muscovite-richschists. However, REE and HFSE distributions cannot be reconciledwith equilibrium melting from such a source. Systematic covariationsbetween Rb, Sr, and Ba can be explained by variations in protolithmineralogy and P–T–aH2O. Tourmaline leucogranites with high Rb/Sr ratios represent low-fraction-melts(F{small tilde} 12%) efficiently extracted from their protolithsunder conditions of low water activity, whereas the heterogeneoustwo-mica granites may result from melting under somewhat higheraH2O conditions. The segregation of low-degree melts from sourcewas probably by deformation-enhanced intergranular flow andmagma fracturing, with the mechanisms of migration and emplacementcontrolled by variations in the uppercrustal stress regime duringlate–orogenic extensional collapse of the thickened crust.  相似文献   

13.
工作中重点对河北平山小觉地区阜平岩群两类角闪质岩石深熔作用的地球化学进行了研究。一类为厚层状斜长角闪岩，相邻新生浅色体常量元素组成上向TTG花岗质岩石方向转化，稀土和高场强元素含量明显降低，轻重稀土分离程度增高，tDM减小，εNd(t0)增大。另一类为与黑云变粒岩－片麻岩互层的条带状、石香肠状斜长角闪岩，相邻新生浅色体与之存在不同的地球化学关系：（1）稀土总量相对增高，轻重稀土分离程度有所降低；（2）稀土总量和轻重稀土分离程度都有明显增高。后者Nd同位素组成与斜长角闪岩也存在很大区别。这些现象可用熔融母岩、形成条件等差异得到合理的解释。  相似文献   

14.
《Lithos》2007,93(1-2):17-38
A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial 87Sr/86Sr ratios of up to 0.7134 and εNd150 values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial 87Sr/86Sr ratios between 0.7073 and 0.7089, and εNd150 values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial 87Sr/86Sr ratios of 0.7048 and 0.7061, and εNd100 values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.  相似文献   

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

16.
HOLTZ  F.; BARBEY  P. 《Journal of Petrology》1991,32(5):959-978
The Tourem granitic complex (North Portugal) consists of quartz-and alkali-feldspar-rich felsic granites, biotite- and plagioclase-richheterogeneous granites, and cordierite-biotite granites, containingnumerous enclaves of orthogneisses and metapelitic schists.Mineralogical, chemical, and experimental data suggest thatall the granites and the orthogneiss enclaves are geneticallyrelated. The felsic granites are characterized by normally zoned plagioclase,absence of cordierite, high SiO2 and K2O (72–74 wt.% and5?4–6?4 wt.%, respectively), moderate P2O5 and REE (0?22–0?24%and 85?0–95?7 ppm), and low Fe2O3* and Zr contents (1?3–1?5%and 80–90 ppm). These features are consistent with thoseof restite-free melts formed by low extents of melting. Meltingexperiments show that these felsic granites are likely to bederived by melting of a source material similar to the orthogneissenclaves under low water activities (0?5), at relatively hightemperature ( 800?C) and <30% melting. The heterogeneous and cordierite-biotite granites display highcordierite contents (up to 30%) in addition to biotite (5–25%),complexly zoned plagioclase, and high Fe2O3 (2?72–6?99%),CaO (0?56–1?95%), Zr (101–213 ppm), and Ce (39?8–98?1ppm) contents, suggesting that the melts contained significantproportions of residual biotite, cordierite, plagioclase, andaccessories. Experimental data indicate that the melts weregenerated under water-undersaturated conditions but by higherextents of melting (30–60% melting) with probably a largeramount of available water compared with the felsic granites. The major and trace element chemical trends of the granites,which do not define single arrays on two-element variation diagrams,and experimental data show that the generation of the Touremanatectic complex cannot be explained by the restite unmixingmodel but could have resulted from sequential low extents ofmelting with efficient melt segregation followed by higher extentsof melting with restite retention.  相似文献   

17.
The Kinsman Intrusive Suite occurs in six major plutons of westernNew Hampshire, covering a total area of 2240 km2. It is an Acadian-agesyntectonic gneissic S-type peraluminous granitoid, rangingin composition from quartz diorite to granite. Much of the Kinsmanis characterized by very large (up to 120 mm in maximum dimension)megacrysts of alkali feldspar, but the bulk chemistry of therocks indicates that these cannot be phenocrysts crystallizedfrom initially homogeneous melts. Locally, there is abundant(20 per cent) almandine-rich garnet, and graphite is a commonaccessory.In contrast to the unannealed orthoclase in surroundingmetapelites, the alkali feldspar of the Kinsman has, for themost part, inverted to maximum microcline. The garnets havecore temperatures in the range of 800 to 900 ?C, and are pseudomorphedby, or show reaction rims to, biotite. Plagioclase commonlyshows zoning, some of it oscillatory. These features are magmaticin nature, and argue against the conclusions of previous investigatorsthat the mineralogy and textures of the rock are due to regionalmetamorphism of a previously-crystallized two-mica granitoidwhich has undergone prograde reactions such as:muse + bio +3 qtz 2 Kfs + gar + 2H2O.The intrusives have also producedrecognizable contact-metamorphic features in the wallrocks andare probably coeval with the dominant M2 Acadian metamorphism.Majorelement analytical data for the Kinsman suite has been examinedby least squares mixing-model and extended Q-mode factor analysis.These calculations, supported by consideration of REE data,suggest that the most likely origin for the Kinsman magmas isby deep-crustal anatexis of slightly calcareous metapelites,and involves a reaction such as:bio + Al2SiO5 + qtz + feldspars gar + cord + Kfs + plag + melt.In non-calcareous pelites thisreaction produces a water-undersaturated peraluminous melt attemperatures above 700 ?C, and allows for the early crystallizationor recrystallization of K-feldspar, plagioclase, and garnetin a crystal-liquid mush or migma. Geochemically, garnet + plagioclaseare treated as restite, and a minimum-melt granite as the magmain the Q-mode and mixing-model calculations. The variabilityin chemistry of the Kinsman Intrusive Suite is best explainedon the basis of mixing of leucogranitic anatectic melts withgarnet-plagioclase restitic material and a quartz-feldspar-sillimanite-biotiterock, but only very slightly affected by crystal settling.  相似文献   

18.
在华北克拉通北缘大青山地区,广泛的深熔作用导致新太古代晚期石榴花岗岩发育.石榴花岗岩空间上与新太古代晚期大青山表壳岩(主要为石榴黑云母片麻岩)共生,渐变过渡.宏观上岩性具有不均一性,在包头哈德门沟一条实测地质剖面上可以观察到石榴混合闪长岩、石榴混合石英闪长岩和石榴混合花岗闪长岩等不同岩石类型.岩相学研究表明,石榴花岗岩...  相似文献   

19.
 Major and trace element models of recently published vapour-absent mica dehydration melting experiments are used to identify granitoids generated by muscovite and biotite dehydration melting, and to distinguish between plagioclase-limited and biotite-limited, biotite dehydration melting. In the case of granitoids from the Pan-African Damara mobile belt (Namibia), many of the leucogranites and Salem-type granitoids may be modelled by biotite dehydration melting. The low Rb/Sr granitoids (e.g. Donkerhuk Onanis, Salem Onanis, Donkerhuk Nomatsaus, Salem Goas) probably reflect feldspar-limited, biotite dehydration melting (a pelitic source) whereas the high Rb/Sr suites (e.g. Bloedkoppie leucogranite, Stinkbank leucogranite, Salem Swakopmund, Leucocratic Stink bank granite) reflect biotite-limited, biotite dehydration melting (a greywacke source). Alaskites from the Damara belt have major element compositions which are consistent with muscovite dehydration melting, and their positive Eu anomalies are linked to high K2O reflecting K-feldspar entrainment. Combined Zr and LREE (light rare earth element) solubility models indicate that insufficient time (probably less than 104 years) had elapsed between melt generation and melt extraction to ensure that the alaskite melts attained their equilibrium concentrations of Zr and the LREEs. In contrast, the leucogranites and Salem-type granites have attained their equilibrium inventories of these trace elements. Combined Fe2O3 and MgO contents in some samples from two granitoids (the Salem Goas and Donkerhuk Onanis intrusions) are higher than those readily attainable by biotite dehydration melting indicating either: (1) that they contain a contribution from melts generated by incipient garnet breakdown or; (2) that they contain small amounts of an entrained ferromagnesian phase. Received: 24 April 1995/Accepted: 11 December 1995  相似文献   

20.
Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. XMg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al2O3–SiO2–K2O–H2Oat high fo2, has been constructed and the effects of ZnO andFe2O3 on this grid have been explored Combining available experimentaland natural occurrence data, the high fo2 invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author  相似文献   

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