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1.
J. Dostal  S. Capedri 《Lithos》1979,12(1):41-49
A sequence of amphibolite to granulite facies metasedimentary and mafic metaigneous rocks from the western Italian Alps has been analysed for rare earth elements (REE). In this sequence, the metasedimentary granulites have probably been affected by a melting event while the metaigneous granulites remained unaffected. Metasedimentary granulites have a less fractionated chondrite-normalized REE pattern than equivalent amphibolite facies rocks. The granulites tend to have a higher content of heavy REE and lower abundances of light REE (LREE). The leucosomes of migmatitic granulites have lower REE content than the melanocratic bands and both these rock types have variable relative abundances of Eu. The mafic granulites have LREE enriched patterns while the amphibolites are slightly depleted in LREE. The differences between the mafic granulites and amphibolites are probably of pre-metamorphic origin.  相似文献   

2.
The Ascutney Mountain igneous complex in eastern Vermont, USA, is composed of three principal units with compositions ranging from gabbro to granite. Sr and O isotopic and major element relationships for mafic rocks, granites, and nearby gneissic and schistose country rock have been investigated in order to describe the petrogenesis of the mafic suite which ranges from gabbro to diorite. The entire complex appears to have been formed within a short interval 122.2±1.2 m.y. ago. The granites with 18O near +7.8 had an initial 87Sr/86Sr of 0.70395(±6) which is indistinguishable from the initial ratio of the most primitive gabbro. Initial 87Sr/86Sr ratios and 18O values for the mafic rocks range from 0.7039 to 0.7057 and +6.1 to +8.6, respectively. The isotopic ratios are highly correlated with major element trends and reflect considerable crustal contamination of a mantle-derived basaltic parent magma. The likely contaminant was Precambrian gneiss similar to exposed bedrock into which the basic rocks were emplaced. A new approach to modelling of assimilation during the formation of a cogenetic igneous rock suite is illustrated. Chemical and isotopic modelling indicate that the mafic rocks were produced by simultaneous assimilation and fractional crystallization. The relative amounts of fractionation and assimilation varied considerably. The mafic suite was not produced by a single batch of magma undergoing progressive contamination; rather, the various rocks probably were derived from separate batches of magma each of which followed a separate course of evolution. The late stage granite was apparently derived from basaltic magma by fractionation with little or no crustal assimilation. The early intrusive phases are much more highly contaminated than the final one. The observed relationships have important implications for the formation of comagmatic complexes and for isotopic modelling of crustal contamination.  相似文献   

3.
Rocks of the northeast portion of the Colorado mineral belt form two petrographically, chemically and geographically distinct rock suites: (1) a silica oversaturated granodiorite suite; and (2) a silica saturated, high alkali monzonite suite. Rocks of the granodiorite suite generally have Sr contents less than 1000 ppm, subparallel REE patterns and initial 87Sr/ 86Sr ratios greater than 0.707. Rocks of the monzonite suite are restricted to the northeast part of the mineral belt, where few rocks of the granodiorite suite occur, and generally have Sr contents greater than 1000 ppm, highly variable REE patterns and 87Sr/86Sr initial ratios less than 0.706.Despite forming simple, smooth trends on major element variation diagrams, trace element data for rocks of the granodiorite suite indicate that they were not derived from a single magma. These rocks were derived from magmas having similar REE patterns, but variable Rb and Sr contents, and Rb/Sr ratios. The preferred explanation for these rocks is that they were derived by partial melting of a mixed source, which yielded pyroxene granulite or pyroxenite residues.The monzonite suite is chemically and petrographically more complex than the granodiorite suite. It is subdivided here into alkalic and mafic monzonites, and quartz syenites, based on the textural relations of their ferromagnesian phases and quartz. The geochemistry of these three rock types require derivation from separate and chemically distinct magma types. The preferred explanation for the alkalic monzonites is derivation from a heterogeneous mafic source, leaving a residue dominated by garnet and clinopyroxene. Early crystallization of sphene from these magmas was responsible for the severe depletion of the REE observed in the residual magmas. The lower Sr content and higher Rb/Sr ratios of the mafic monzonites requires a plagioclase-bearing source.The Sr-isotope systematics of the majority of these rocks are interpreted to be largely primary, and not the result of crustal contamination. The positive correlation of Rb/Sr and 87Sr/86Sr ratios for the least fractionated samples indicate that the sources from which parent magmas of both the granodiorite and monzonite suites were derived are Precambrian in age.  相似文献   

4.
The ophiolite complex of Chamrousse (Belledonne Massif, Alps), consists of mafic to ultramafic cumulates and non-cumulates metamorphosed to amphibolite facies grade. The non-cumulitic rocks are similar in chemical composition to recent ocean-floor olivine tholeiites (both N-type and enriched P-type). The distribution of lithophile elements shows that the non-cumulitic rocks represent several magmas of different parentage. The character of the magmas varies according to the time of emplacement.Geological and geochemical data suggest that the Chamrousse complex was formed at a spreading oceanic ridge. The dynamic partial melting of an upper mantle diapir generated tholeiitic melt which decreased in amount and in REE contents. The first melt, enriched in light REE, was generated along the axis of the ridge while the second batch of melt, of lesser quantity and slightly depleted in light REE, was emplaced on the flank of the ridge. The third melt formed cross-cutting dikes with REE abundances typical of N-type (strongly light REE depleted) mid-ocean ridge basalts.  相似文献   

5.
Late Precambrian crustal evolution in the North Eastern Desert of Egypt occurred in a strongly extensional tectonic environment and was accompanied by abundant bimodal igneous activity. The extrusive and intrusive expressions of this magmatism, known as the Dokhan Volcanics and Pink Granites, respectively, were studied in detail from two areas. The Dokhan Volcanics and associated feeder dikes consist of a mafic suite dominated by andesites (60% SiO2) and smaller volumes of basalt and a felsic suite composed of rhyolite tuffs, ignimbrites and hypabyssal intrusions (72–78% SiO2). The rocks of the mafic suite display calc-alkaline trends on an AFM diagram but are enriched in incompatibles such as TiO2, P2O5, K2O, Rb, Sr, Ba, Zr, Y, Nb, and LREE. Rare earth element patterns are steep, with (Ce/Yb)n = 7.7 to 16.8. They contain moderate Ni (60 ppm) and Cr (95 ppm), indicating limited low-P fractionation. The melts of the mafic suite are interpreted to have formed either by 25% batch melting of eclogite or by 10% batch melting of LREE-enriched garnet lherzolite. The rocks of the felsic suite include Dokhan rhyolites and the epizonal Pink Granites. These contain 72–78% SiO2, are metaluminous and peraluminous, and have the high K2O/Na2O and FeO*/(FeO*+MgO) characteristic of post-tectonic, A-type granites. They are moderately enriched in incompatible elements, but their REE patterns overlap with those of the mafic suite, from which they can be distinguished by deep europium anomalies (Eu/Eu*=0.08–0.64) and flat HREE patterns=((Yb/Er)n=0.90–1.16). They share with the rocks of the mafic suite isotopic characteristics of depleted mantle, precluding anatexis of much older continental crust. The europium anomalies covary with Sr contents and indicate that plagioclase control was important, while the flat HREE patterns preclude residual garnet in the source. Hence the felsic melts could not have formed by anatexis of garnet-bearing mafic lower crust. Such melts could have formed by anatexis of amphibolite-facies crust, an interpretation which is not favored because the melts are not saturated with P2O5. Alternatively, the felsic melts may have formed via low-P fractional crystallization of the mafic melts, with about 2/3 removal of mostly plagioclase and amphibole along with minor apatite and zircon. This may have been accompanied in the latest stages of magmatic evolution by liquid-state fractionation such as thermo-gravitational diffusion or halide complexing.  相似文献   

6.
喜马拉雅造山带变泥质岩系及其地球化学特征   总被引：9，自引：7，他引：2  
高喜马拉雅结晶岩系和北喜马拉雅穹隆都发育高角闪岩相-麻粒岩相变泥质岩,岩石组合以富铝质片麻岩类为主,伴生钾质花岗片麻岩、大理岩及基性麻粒岩等.元素地球化学特征表明,其原岩主要为较富铝的长石质砂岩和石英岩质砂岩及少量粘土岩,形成于大陆边缘浅海相沉积环境.这些变泥质岩具有相似的微量元素和稀土元素地球化学特征,表现为大离子亲石元素相对高场强元素较富集,轻稀土较重稀土富集,稀土总量较高,具有较显著的Eu负异常.在变质矿物组合、元素地球化学特征及锆石组成等特征上,它们与青藏高原北缘的康西瓦和阿尔金孔兹岩系相似,可能是来源于冈瓦纳大陆边缘的同一套岩石.  相似文献   

7.
New mineralogical and chemical data for ophiolitic rocks from the southwesternmost Liguride Units are presented in order to constrain their ocean-floor origin and subsequent emplacement in an accretionary wedge. Their complete petrochemical evolution is particularly well preserved in the southern Apennine metabasites. Metadolerites show amphibolite and greenschist facies mineral assemblages of ocean-floor metamorphism. Metabasalts display greenschist facies ocean-floor metamorphism and spilitic alteration. Veins cutting the mafic rocks show mineral assemblage of the prehnite–pumpellyite metamorphic facies. HP/LT orogenic metamorphism, reflecting underplating of the ophiolitic suite at the base of the Liguride accretionary wedge during subduction of the western Tethys oceanic lithosphere produced a mineral assemblage typical of the lawsonite–glaucophane facies. Bulk-rock chemistry suggests that the mafic protoliths had a MORB-type affinity, and were affected by ocean-floor rodingitic and/or spilitic alteration. Hydrothermal alteration-induced LREE mobility and LREE enrichment may be correlated with the ocean-floor metamorphism.  相似文献   

8.
Associated rocks from the Seabrook Lake, carbonatite complex in Ontario show an increase in total REE (rare earth element) content and in light REE enrichment in the following order: fenite quartz monzohite 2 and H2O-rich fluids.  相似文献   

9.
The Serra da Providência batholith includes the type area of the homonymous suite, the oldest rapakivi magmatic assemblage in the SW of the Amazonian craton (1.60–1.53 Ga). In the midwest portion of this massif, besides wiborgites/pyterlites and granophyric syenogranites, a leucosyenogranite facies and porphyritic rhyolites constitute new rock varieties recently described in that area. UPb LA-MC-ICP-MS zircon ages of 1574 ± 9 Ma and 1604 ± 3 Ma, respectively, were obtained for these new varieties and confirm their link with the Serra da Providência magmatism, whereas the subvolcanic rocks are older than the main rock varieties and were formed in a precursor event. These granitic facies are metaluminous to peraluminous, alkali-calcic, A2-type, ferroan granites. Their FeOt/(FeOt + MgO) ratios vary from 0.83 to 0.98 and suggest that these rocks were crystallized from oxidized-to reduced-A-type magmas, where the leucosyenogranites and granophyric sienogranites tend to be formed under more reduced conditions. They show fractionated REE patterns with very pronounced to weak negative Eu anomalies. The presence of granophyric textures and miarolitic cavities in equigranular syenogranitic facies suggests that these rocks were formed at shallow crustal depths, lower than 3 km. Three samples of leucosyenogranite have silica contents higher than 75% and low K/Rb ratios (<150), similarly to the tin specialized granites described in the Amazonian craton. Two distinctive groups of mafic rocks were recognized in the study area: porphyritic and equigranular gabbronorites. They correspond to tholeiitic basalts, with #Mg varying from 37 to 41 in porphyritic gabbronorites and 51 to 65 in equigranular gabbronorites. The low to moderate #Mg suggests that these rocks were crystallized from more evolved basaltic magmas. The porphyritic gabbronorites are enriched in TiO2, FeOt, K2O, P2O5 and REE compared to the equigranular gabbronorites that are enriched in MgO and CaO. The porphyritic gabbronorites have significant negative Eu anomalies a feature not observed in the equigranular gabbronorites. Porphyritic gabbronorites geochemical characteristics are similar to calc-alkaline basalts, whereas equigranular gabbronorites are similar to continental basalts. Petrographic, geochemical, and geological data of the felsic facies and the presence of associated mafic rocks corroborate the bimodal and post-collisional character of this magmatism. The occurrence of porphyritic rhyolites associated with shallow level plutonic granites in the Serra da Providência batholith reinforces the similarities between the Rondonian granites and the classical Fennoscandian rapakivi granites.  相似文献   

10.
The ophiolites of New Caledonia are composed of ultramafics overlain by mafic rocks, all of which were affected by low P metamorphism. The mafic rocks studied (gabbroic cumulates, and basaltic flows and dikes) from Montagne des Sources are similar to recent mid-ocean ridge rocks. They are olivine-normative with Mg/Mg+Fe2+ ratios ranging from 0.69 in lavas to 0.90 in gabbroic cumulates and show tholeiitic fractionation trends such as a negative correlation of Ti and V with the Mg/Fe ratio. The lavas have a flat REE pattern with a slight depletion of light REE and a La/Yb ratio <2. The dikes have three different types of REE patterns. The first type is nearly parallel to that of lavas, the second one is enriched in LREE (La/Yb4) and the third type with the lowest REE contents and a distinct LREE depletion is similar to that of cumulitic pyroxene gabbro. The variations in chemical compositions of the mafic rocks can be accounted for by the dynamic partial melting process of Langmuir et al. (1977). In agreement with structural and tectonic observations, the geochemical data suggests that the ophiolites were formed during the spreading of a mid-ocean ridge with a spreading half-rate of about 1 cm/ year.  相似文献   

11.
Magmatic metasomatism and formation of the Merensky reef,Bushveld Complex   总被引：11，自引：0，他引：11  
The rare earth element (REE) contents of pyroxenes and other minerals from the Merensky reef and stratigraphically adjacent rocks of the Atok section, Bushveld Complex, have been determined with the ion microprobe. Merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the REE than their cumulus counterparts in rocks several meters below the reef. Chondrite-normalized Merensky clinopyroxene Ce contents vary from 10 to 90 for Ce and from 4 to 17 for Yb. They also possess deep, negative Eu anomalies, the Eu anomalics being deeper for crystals having high REE contents and relatively shallow for pyroxenes with low REE contents. Similar compositional characteristics are displayed by Cl-rich apatite, which is an accessory phase in the rocks. Interstitial pyroxene in cumulates above and below the reef also tends to have elevated REE contents and in general is not in equilibrium with coexisting cumulus minerals. The melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts; that from which Merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in REE than any normal tholeiitic or alkalic basalt. Despite their highly evolved nature in terms of the REE, the Merensky reef pyroxenes are not evolved in terms of major elements. The decoupling of incompatible trace and major elements is best explained by a metasomatic process. It is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile. The fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed.  相似文献   

12.
A suite of crustal xenoliths from Tertiary basaltic tuffs of the Northern Hessian Depression (NHD) volcanic field comprises abundant meta-igneous pyroxene granulites of mafic, noritic to anorthositic, IAT and tonalitic composition. Less abundant are granitic, tonalitic and leucogranitic gneisses and metasedimentary xenoliths. A total of 49 samples were analyzed for modal compositions, for major and trace elements (including Li, Rb, Sr, Ba, Cs, V, Sc, Cr, Co, Ni, Y, Zr, Nb, Ta, Hf, Th and REE) and oxygen isotopes. Two-pyroxene thermometry yields temperatures between 700 and 900° C for mafic and noritic granulites. Feldspar thermometry indicates temperatures of 660°–710° C for tonalitic granulites and 470°–520° C for granitic and tonalitic gneisses. One highly depleted sillimanite-rich metasediment contains cordierite and garnet which have equilibrated at temperatures of 780° C. The general lack of garnet in the mafic and noritic granulites and the presence of sillimanite in felsic xenoliths indicates that metamorphic pressures have not exceeded 10 kb. Major and trace element data and oxygen isotope compositions of the mafic granulites are compatible with an origin from spilitized enriched-type MORB rocks (enrichment in 18O to 11 and in Li to 34 ppm at average SiO2 contents of 44.1 wt%). These low-T spilites were transformed into amphibolites and then pyroxene granulites during subsequent high temperature metamorphic events. Low Si, Al, K, and Rb concentrations along with An contents in plagioclase ranging from near 50 to 98 mole percent suggest that amphibolite facies protoliths have generated tonalitic melts during partial melting at temperatures above 700° C. The mafic granulite xenoliths are interpreted as restites whereas the tonalitic samples probably represent the extracted partial melts derived by 20 to 30 percent degree of melting. Metasedimentary xenoliths strongly depleted in granitic component could represent restites from which granitic S-type partial melts have been extracted. Tonalitic and leucogranitic gneisses including one trondhjemite xenolith have many chemical characteristics (e.g. REE distribution) in common with tonalite-trondhjemite-granodiorite suites of the North Atlantic region but cannot be accounted for a more specific origin. Estimated elastic properties of the main types of NHD xenoliths yield P-wave velocities of 6.0–6.4 km-1 for granitic, tonalitic and trondhjemite gneisses including tonalitic granulites and 6.5–7.0 for the more mafic xenoliths. When compared with two seismic depths-Vp profiles these data are in accordance with a model where the mafic, andesitic, noritic and tonalitic granulites comprise abundant rock types at depths between 29 km (Moho) and 20 km which mainly consists of old oceanic crust including subduction related volcanic products. The more felsic xenoliths probably represent material from depths between 12 and 20 km.  相似文献   

13.
The Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase-porphyrite, gabbro–diabase, diabase, and pyroxene diorite. Primitive mantle-normalized multi-element patterns show that, the Xuhe mafic rocks are enriched in large ion lithophile elements(LITE), such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite-normalized REE patterns display LREE enrichment(LaN/YbN = 9.34–13.99) and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SIMS zircon U–Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low Mg O(basic rock: 3.11–7.21 wt%; pyroxene diorite: 0.89–1.21 wt%) and Mg#(0.20–0.49) for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low(5%–10%) and in garnetspinel transition facies. Sr-Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMII and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back-arc extension.  相似文献   

14.
 Mafic gneisses occur as lenses or thin layers in spatial association with tonalitic leucosomes in a granulite zone of the Quetico subprovince of the Superior Province, Ontario, Canada, and exhibit concentric zoning with a biotite-rich margin, orthopyroxene-rich outer zone, clinopyroxene-rich central zone, and, occasionally, patches of relict amphibolites within the clinopyroxene-rich zone. The granulites (biotite-, orthopyroxene- and clinopyroxene-rich zones) in the mafic gneisses are characterized by significant amounts of rare earth element (REE)-bearing fluorapatite (1–10 vol.%) and other REE-rich minerals (allanite, monazite and zircon). Fluorapatite shows an increase in modal abundance from the biotite- and orthopyroxene-rich zones to the clinopyroxene-rich zone, but is rare in the relict amphibolites. Textural evidence and element partitioning indicate that fluorapatite (and other REE-rich minerals) was part of the peak metamorphic assemblages. Whole-rock geochemical analyses confirm that the granulites in the mafic gneisses contain anomalously high contents of REE and high field strength elements (HFSE), whereas the relict amphibolites are geochemically typical of tholeiitic basalts. Mass-balance calculations reveal that REE and HFSE were introduced into the mafic gneisses during the prograde granulite facies metamorphism, pointing to REE mobility under granulite facies metamorphic conditions. The presence of high F contents in the REE-rich minerals and their associated minerals (e.g. biotite and hornblende) suggests that REE and HFSE may have been transported as fluoride complexes during the granulite facies metamorphism. This conclusion is supported by previously published results of hydrothermal experiments on the partitioning of REE between fluorapatite and F-rich fluids at 700°C and 2 kbar. Received: 2 May 1995 / Accepted: 28 September 1995  相似文献   

15.
Petrology of rodingite derived from eclogite in western Tianshan, China   总被引：8，自引：0，他引：8  
This study reports a new rodingite type which was derived from eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites, western Tianshan, China. Based on petrographical investigations, rodingite, partial rodingitized rock and completely rodingitized rock are characterized in this paper. These rocks show a continuous variation in their bulk compositions, mineralogy and their textural properties from eclogite to rodingite. The completely rodingitized rocks can be further divided into prehnite rodingites, hydrogrossular‐diopside rodingite and vesuvianite rodingites on the basis of the mineral assemblage and textural character. The chemical potential path of μ(SiO2)–μ(CaO/MgO) can be used to constrain the evolution of two stages of rodingitization. The first rodingitization possibly started under conditions of 410–430 °C and 7–9 kbar at upper greenschist facies, and resulted from a secondary serpentinization during exhumation of the subducted slab. A second and pervasive rodingitization took place under conditions of 250–350 °C and 2–10 kbar from greenschist to subgreenschist facies. The P–T path presented shows a retrograde evolution from eclogite to rodingite. We conclude that the process of rodingitization may also take place under subduction zone conditions in addition to its more common occurrence under ocean‐floor metamorphic conditions.  相似文献   

16.
Igneous rocks of broadly basaltic composition are widely distributedin the anorthosite-bearing Adirondack Highlands of New York.They constitute a mafic series of rocks that occurs on the marginsof the anorthosite series and up to 50 km away from the anorthosite.On an Sr reference diagram, the mafic series has an apparentinitial 87Sr/86Sr value of 0.7036, which is consistent with1100 Ma subcontinental mantle. The series was probably emplacedin the interval 1150–1100 Ma, during which time anorthositeand granitic magmas were also emplaced. The rocks were metamorphosedshortly thereafter ({small tilde} 1088 Ma) under upper-amphiboliteand granulite facies conditions. Individual bodies of maficrocks range in thickness from small enclaves and layers >>1 m thick to large lensoid masses several tens of meters thick.Despite deformation and metamorphic recrystallization, manyof the rocks retain chiHed margins, cross-cutting relationships,and relict igneous textures. Selected samples of the mafic series have been analyzed fortheir major and trace element compositions. Metamorphism didnot significantly alter the igneous geochemical relationships,and the rocks retain mantle-like values for Zr/Nb, K/Zr, K/Rb,Rb/Sr, and 87Sr/86Sr. The most primitive rocks of the seriesare silica-undersaturated gabbroic troctolites, and the moreevolved rocks are basaltic in composition. The mafic seriesas a whole has high abundances of A12O3, FeO, the light rareearth elements (LREE), and other incompatible trace elements.Even the most geochemically primitive compositions have highFeO contents. The Fe enrichment and Si depletion that are shownby chemically evolved compositions are consistent with a Fennertrend of fractionation. Low levels of normative di indicatethat high Fe is not a result of the extensive fractionationof cpx. The geochemical trends that are defined by the traceelements, including the REE, suggest the basaltic rocks maybe differentiates of a parental magma of gabbroic troctolitecomposition. The main compositional trend of the mafic seriescan be simulated by 61% crystallization of olivine and plagioclaseof a gabbroic troctolite, followed by 13% crystallization ofolivine, plagioclase, clinopyroxene, and titanomagnetite atthe final stages. The modelled ratio of olivine to plagioclasecrystallization changes from 1–8: 1 to 0–64: 1.These non-cotectic ratios may reflect a delay in the crystallizationof plagioclase relative to olivine, possibly as a result oflow nucleation rates. At later stages of differentiation, plagioclasewas more important in the crystallization of the series. Delayedcrystallization of plagioclase may also have resulted in thehigh A12O3 contents and enhancement of Eu and Sr relative toother trace elements at early stages of differentiation. The mafic series and the silica-saturated anorthosite seriestogether form an anorthosite-norite-troctolite (ANT) suite.More than one mantle composition may have been involved in generatingthe Adirondack mafic magmas. The rocks retain geochemical evidenceof a source that was depleted in basaltic components (cpx) butenriched in Fe, Ti, K, and the LREE. Previously documented evidenceof anticlockwise cooling paths (Bohlen, 1987) and of a regionalgravity high centered beneath the Adirondack region (Simmons,1964) suggests that of the continental crust by basaltic magmasmay have underplating of the continental crust by basaltic magmasmay have been an important feature of the tectonic evolutionofthe region. A model of mantle upwelling beneath thinning continentalcrust explains the geochemically hybrid nature of the maficseries magmas. It is also consistent with a tectonic settingof incipient or failed continental rifting, to which the generationof the anorthosites is commonly attributed.  相似文献   

17.
In the Central Dinaric Ophiolite Belt (CDOB) peridotites and associated metamorphic rocks of various grades tectonically overlie an olistostrome melange of middle to late Jurassic age. Peridotites and underlying slices of mafic granulites (partially transformed to gamet amphibolites) are intruded by doleritic dikes which do not occur in the melange. The melange contains blocks of subgreywackes and cherts as well as those of pillow lavas and massive diabase (spilites). CDOB peridotites are in the spinel peridotite facies, but locally spinel-plagioclase peridotites occur as well. All peridotites have lherzolitic compositions showing several significant element correlations: Al2O3, CaO, TiO2, Na2O and Cu are negatively correlated and Ni is positively correlated with MgO. Recent estimates of primitive mantle compositions lie near the low-MgO end point of each correlation trend. Al/Ti and Ca/Al ratios of CDOB lherzolites are for the most part higher than the range observed in chondrites. However, when a few samples with extreme compositions are excluded, Al/Ti and Ca/Al are positively correlated with MgO, and the samples at the low-MgO end have near-chondritic Ca/Al but slightly higher than chondritic Al/Ti ratios. Chondrite-normalized REE patterns of CDOB lherzolites show extreme depletions in LREE providing strong evidence for the absence of any metasomatic renrichment. The lack of correlation between highly incompatible elements (LREE) and moderately incompatible elements (HREE, Ti, Na, Al, Ca) together with the extremely low La/Sm ratios suggest that fractional or very small increment melt removal played a role in the genesis of these lherzolites. Four out of five lherzolites yield and apparent Sm-Nd isochron age of 136±15 Ma with an Nb of 6.0±1.1 (bulk rocks and clinopyroxene separates). One sample has an exceptionally high Nd of about 23. The mafic igneous rocks scatter around the lower end of the 136 Ma reference isochron allowing, but not proving, a genetic relationship with a mantle having a Nd isotopic composition which is similar to that of CDOB lherzolites. LIL element abundances of spilites and doleritic dike rocks suggest some hydrothermal alteration. In primitive mantle-normalized concentration diagrams none of these mafic igneous rocks shows a significant negative Nb-Ta anomaly. Chondrite-normalized REE patterns of both rock types are essentially flat. Whereas the inferred primary compositions of the spilites compare well with those of E-type MORBs, the doleritic dike rocks show elemental ratios similar to those normally found in back-arc basin tholeiites.  相似文献   

18.
The Bad Vermilion Lake anorthosite complex (2,700 m.y.) is exposed over an area of about 100 km2 near Rainy Lake, Ontario. As is typical of other Archean anorthosites, it is composed of coarse (1–30 cm across), equidimensional, euhedral to subhedral, calcic (An80) plagioclase, in a finer grained mafic matrix. The amount of mafic matrix in individual samples ranges from none to about 70% by volume. The complex has been variably metamorphosed to greenschist facies. Zoisite, chlorite, and hornblende are abundant, but primary plagioclase is preserved in many places. The anorthosite complex is associated with gabbro and with mafic to felsic metavolcanic rocks, and is cut by tonalite plutons and by mafic dikes. Some gabbros contain local concentrations of Fe-Ti oxides and/or apatite, but no chromite. The mafic groundmass of the anorthositic rocks is similar in major and trace element chemistry, including rare earth elements, to the associated basaltic metavolcanics, suggesting that the anorthositic complex may have accumulated from a subvolcanic magma chamber which fed mafic lavas to the surface during its crystallization. Mafic flows and dikes chemically similar to the mafic metavolcanics contain plagioclase megacrysts akin to those of the anorthositic rocks, and thus may represent a link between the anorthosite complex and associated mafic lavas. Elongate pretectonic tonalite intrusions were comagmatic with the felsic metavolcanics, but not with the anorthosites or metabasalts. These silicic rocks may represent low-pressure partial melts of the mafic rocks. There is no direct or indirect evidence for significant volumes of ultramafic material at the present exposure level of the complex. An estimate of the bulk composition of all rocks presumed to be comagmatic with the anorthosites, including gabbros and mafic metavolcanics, is an aluminous basalt with about 20 wt.% Al2O3. This composition has REE abundances unlike those of typical Archean high-Al basalts and probably does not represent that of a primary or evolved melt. The possibility must be considered, therefore, that a substantial fraction of material comagmatic with the anorthosites has been separated from the complex, either by magmatic or tectonic processes.  相似文献   

19.
A coherent ophiolitic complex of pyroxenite, serpentinite, metagabbro, mafic volcanics, felsic volcanics and sediments crops out in NW Maine, adjacent to the Chain Lakes massif. The complex (here informally referred to as the Boil Mountain ophiolitic complex) is about 500 m.y. old. The volcanic sequence is not typical of ophiolites in that it contains a large proportion of felsic volcanics. The mafic volcanics are divided into two geochemical groups. A stratigraphically lower group is depleted in Ti, Zr, Y, Cr and REE contents similar to basalts from supra-subduction zone ophiolites. An upper mafic group has trace element contents similar to normal mid-ocean ridge basalts. The felsic volcanics are mostly rhyolitic and similar to low-K rhyolites found in the forearc of the Marianas trench and in an island arc sequence in the Klamath Mountains, California. The flat REE patterns of the felsic volcanic rocks are similar to those found in siliceous rocks in the Oman ophiolite. The presence of thick sequences of felsic volcanics, the abundance of pyroxenite, the low Ti, Zr and REE contents of some mafic rocks, the flat REE patterns of the felsic volcanics, and the composition of clinopyroxene all suggest the complex was formed in the vicinity of a subduction zone. The complex may be correlated with ophiolitic fragments in the eastern part of the Dunnage Zone in Newfoundland, rather than the main ophiolite belt of the western Appalachians.  相似文献   

20.
The 1.1 Ga volcanogenic massive sulphide deposit at Deri in the Sirohi district of south Rajasthan occurs within a bimodal volcanic suite of tholeiites and rhyolites, with minor amounts of andesites and tourmaline-bearing chert, interlayered with arkosic sediments. The ores and the enclosing rocks have undergone superposed deformation and polymetamorphism initially under amphibolite facies conditions and later under hornblende hornfels facies conditions. Metamorphism, however, has not affected the bulk composition of the rocks to any significant degree.Three distinct semiconformable alteration facies, characterized by their conspicuous magnesian mineralogy, are recognized in the host rocks: (1) hornblende-biotite-plagioclase-quartz schist (AMV); (2) cordierite-anthophyllite-chlorite hornfels (AFV); and (3) biotite-chlorite(-sericite) schist/hornfels (BCS). The first is derived from the mafic volcanics, whereas the other two represent progressive alteration of felsic volcanic protoliths. Fe, Mg and water were added and Na was removed from all the alteration facies in varying amounts. The maximum enrichment is noted in BCS for Mg and Fe, whereas the maximum depletion is seen in this facies for Si, an element which is also depleted significantly in AFV. AMV on the other hand, shows enrichment of Si, Ca and to some extent, in Al. Alumina is also enriched considerably in BCS, probably due to clayey alteration and extreme leaching of silica. Amongst the trace elements, Rb, Ba, Nb and Y are gained in most of the facies, except in BCS, where Ba and Y show distinct depletion. The LREE, from La to Sm, were enriched about 1.5- to 3.0-fold in all the facies with a maximum in AFV where the flux took place at constant inter-REE proportions: 1.0 La, 0.79 Ce, 0.48 Nd and 0.35 Sm. Eu was depleted from both felsic facies, 7-fold in BCS to 4-fold in AFV, during alteration. The HREE (Er to Lu) remained immobile in all the altered facies.The chemical and mineralogical zonation in the alteration facies are interpreted to be due to the progressive reaction of an evolving sea-water hydrothermal fluid with the bimodal volcanic protoliths during convective circulation. Fluid-rock interaction, guided by vertical and lateral thermal gradients, produced a sericite-quartz assemblage in the felsic volcanics at the expense of feldspar during the initial stages (175 °C) which formed a sericite-chlorite zone upon rising temperature (200–250 °C) by base-fixing reactions. A further temperature increase (to ~ 300 °C) and deeper circulation in the mafic pile introduced more Fe and Mg, thereby transforming the previously formed assemblage to a nearly pure chloritic zone and the most intensely altered biotite-chlorite(-sericite) facies.  相似文献   

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