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1.
Carmela Freda Mario Gaeta Valeria Misiti Silvio Mollo Daniela Dolfi Piergiorgio Scarlato 《Lithos》2008,101(3-4):397-415
The Alban Hills ultrapotassic volcanic district is one of the main districts emplaced during Quaternary time along the Tyrrhenian margin of Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition is mostly K-foiditic. Differentiated products (MgO < 3 wt.%) are characterised by low SiO2 concentration (< 50 wt.%) and geochemical features indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the Alban Hills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performed experiments on the Alban Hills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonated conditions. Experiments were carried out at 1 atm, 0.5 GPa and 1 GPa, temperatures ranging from 1050 to 1300 °C, and H2O and CaCO3 in the starting material up to 2 and 7 wt.%, respectively. The experiments performed at 0.5 GPa are the most representative of the Alban Hills plumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases. Nevertheless, our experimental results demonstrate that the occurrence of CaCO3 in the starting material strongly affects phase relations. Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This early crystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic (Alban Hills parental composition) to phonolitic compositions. This is in contrast with micro-textural evidence showing late crystallization of magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO3-bearing experiments (i.e., simulating magma–carbonate interaction) the magnetite and phlogopite stability fields are strongly reduced. As a consequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in a differentiation trend moving towards K-foiditic compositions. These experimental results are in agreement with micro-textural features and chemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data. Magma–carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems. However, the uniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled by different factors, the dynamics of the plumbing system being one of the most important. 相似文献
2.
Groundwater surveys were performed by detailed(around 300 sites) grid-analysis of water temperature, pH, redox potential, electrical conductivity, 222Rn, alkalinity and by calculating the pCO2, throughout the Ciampino and Marino towns in the Alban Hills quiescent volcano (Central Italy). Following several episodes of dangerous CO2 exhalation from soils during the last 20 years and earlier ashistorically recorded, the work aimed at assessing the Natural Gas Hazard (NGH) including the indoor-Rn hazard. The NGH was defined as the probability of an area to become a site of poisonous peri-volcanic gas exhalations from soils to the lower atmosphere (comprising buildings). CO2 was found to be a ``carrier' for the other poisonous minor and in trace components (HsS, CH4, 222Rn, etc.). This assessment was performed by extrapolating in the aquifer CO2 and 222Rn conditions, and discriminating sectors where future CO2 flux in soils as well as indoor-Rn measurements have to be noted. A preliminary indoor-Rn survey was performed at about 200 sites. The highest values were found in the highest pCO2 and high 222Rn values in groundwater. This indicates convection and enhanced permeability in certain sectors of the main aquifer, i.e., along the bordering faults and inside the gas-trap of the Ciampino Horst., where ``continuous gas-phase micro-macro seepage mechanism' is invoked to explain the high peri-volcanic gases flux. 相似文献
3.
Angelo Peccerillo Marcella Federico Mauro Brilli 《Geochimica et cosmochimica acta》2010,74(10):2999-95
Magma-carbonate rock interaction is investigated through a geochemical and Sr-Nd-O isotope study of granular lithic clasts (ejecta) from the Alban Hills ultrapotassic volcano, Central Italy. Some samples (Group-1) basically represent intrusive equivalents of Alban Hills magmas. A few samples (Group-2) are ultramafic, have high MgO (∼30 to 40 wt%) and δ18O‰, and originated by accumulation of mafic phases that crystallised from ultrapotassic melts during assimilation of dolomitic rocks. Group-3 ejecta consist of dominant K-feldspar, and show major element compositions similar to phonolites, which, however, are absent among the Alban Hills volcanics. Finally, another group (Group-4) contains corroded K-feldspars, surrounded by a microgranular to porphyritic matrix, made of igneous minerals (K-feldspar, foids, clinopyroxene, phlogopite) plus wollastonite, garnet, and some cuspidine. Group-4 ejecta are depleted in SiO2 and enriched in CaO with respect to Group-3.The analysed ejecta have similar 143Nd/144Nd (0.51204-0.51217) as the Alban Hills lavas, whereas 87Sr/86Sr (0.70900-0.71067) is similar to lower. Whole rocks δ18O‰ ranges from +7.0 to +13.2, reaching maximum values in ultramafic samples. A positive correlation with CaO is observed in single rock groups. Large Ion Lithophile Element (LILE) abundances and REE fractionation are generally high, and extreme values of Th, U and LREE are found in some Group-3 and Group-4 rocks.Mineralogical, petrological and geochemical data reveal extensive interaction between magma and carbonate wall rocks, involving both dolostones and limestones. These processes had dramatic effects on magma compositions, especially on phonolites, which were transformed to foidites. Evidence of such a process is found in Group-4 samples, in which K-feldspar is observed to react with a matrix that represents strongly undersaturated melts formed by interaction between silicate magma and carbonates. Trace element data also testify to a very important role for F-CO2-H2O-S fluids during magma-wall rock interaction. Fluid transfer was responsible for extreme enrichments in Th, U, and LREE especially observed in Group-3 and Group-4 rocks. Implications of these processes for potassic magma evolution in Central Italy are discussed. 相似文献
4.
Summary A suite of lithics (ejecta) collected from the latest erupted pyroclastic products of the Alban Hills volcano (Central Italy)
has been studied to determine their mineralogical composition and to investigate their genesis. The ejecta commonly have granular
texture and consist of coarse-grained crystals often associated with a fine- to medium-grained matrix. The mineralogical composition
is variable and consists of both typical igneous minerals and contact metamorphic phases. Garnet, clinopyroxene K-feldspar
are almost ubiquitous, whereas leucite, wollastonite, sodalite-group minerals, phlogopite, nepheline and phillipsite are present
in most of the ejecta; minor and accessory phases include cuspidine, amphibole, pyrrhotite, magnetite, apatite, uranpyrochlore,
sphene, kalsilite, and melilite; anorthite, zircon and fluorine-bearing Ca, Zr silicate phases, larnite, and baryte are found
sporadically. Ca, REE, Th silicophosphates occur in many samples generally disseminated along interstices and fractures of
main minerals. Calcite is present as discrete crystals sometimes enclosed in other minerals, as granules in the fine-grained
matrix and as late microcrystalline veins. It shows high oxygen and low carbon isotope ratios with δ18O = + 17.96 to + 27.19, and δ13C = −4.74 to −19.57.
Clinopyroxene ranges from diopside to compositions strongly enriched with both Ca-Tschermak’s and esseneite components. Feldspars
are generally potassic even though Ba and Sr are found in significant concentrations in some samples. K-feldspars from wollastonite-bearing
ejecta are often rimmed with elongated felty crystals identified by X-ray diffraction analysis as leucite. These feldspars
show a depletion in Si, and enrichment in Al and K from core to rim. Significant compositional variations are also shown by
various other phases such as nepheline, apatite, Ca, REE, Th silicophosphate.
The occurrence of igneous and contact metamorphic minerals, as well as the chemical variations of clinopyroxenes and feldspars
in the investigated ejecta reveal complex genetic processes related to the interaction between potassic magma and wall rocks.
The Ca-rich composition of most phases points to a carbonate nature for the wall rocks. Textural evidence suggests that coarse-grained
rocks formed at the margin of the magma chamber were invaded by a late, volatile rich potassic liquid which crystallized as
a fine-grained matrix and produced disaggregation and reaction of early formed minerals. Fluid phases percolating through
the rocks generated infiltration metasomatism and deposited some uncommon phases rich in Ca, REE, Th, U, which are found along
cracks and at the margins of early crystallized minerals.
Overall, the all spectrum of the minerals found in this study are also typical of carbonatitic rocks. Their presence in the
Alban Hills ejecta demonstrates that their genesis can be related to interaction between ultrapotassic melts and carbonate
wall rocks, in addition to precipitation from carbonatitic melts.
Received February 20, 2001; revised version accepted September 23, 2001 相似文献
5.
Summary Mineral compositions in leucite-bearing and leucite-free rocks from Vico volcano are reported. FeO/MgO partitioning (Kdol/liq) between olivine and latite (0.14–0.22), and between olivine and trachyte (0.06–0.10) indicates a lack of equilibrium between
mineral and host rock. This suggests that mingling and/or mixing between magmas was a leading process during magmatic differentiation.
In addition, a phono-tephrite olivine population with high (0.84) and equilibrium (0.23–0.29) Kdol/liq values has been produced by the interaction of differently evolved magmas. Zoning in clinopyroxene and plagioclase from these
rocks recorded the same processes. In addition, resorbed quartz xenocrysts with coronas of clinopyroxene microlites indicate
that digestion of crustal rocks occurred during the residence of magma in a shallow level reservoir. Increasing Fe coupled
with decreasing Ca in diopside crystals from some phonolites, together with the petrographic and trace element data, indicate
that polybaric fractional crystallisation also may be involved in the genesis of magmas of the second period of Vico activity.
Leucite-free trachybasalts erupted in a late stage contain highly forsteritic olivine phenocrysts (forsterite 84–88 mol.%)
in-equilibrium (Kdol/liq = 0.24–0.35) with the host rock, which indicate that they did not suffer chemical modification at low pressure.
Received November 28, 2000; revised version accepted September 27, 2001 相似文献
6.
Chromite ores formation is still a debated topic and, in this study, detailed analyses of major, minor elements by EMPA and Rare Earth Elements (REE) by SIMS, were performed on silicate inclusions detected in chromite grains of chromitite pods, enveloping dunites and on clinopyroxene of lherzolitic host rock of well known Trodoos chromite ores. Results show the complexity of relationships among lithologies that reflects the subtlety of genetic events and of chromite ore occurrence.Analyses of textural and chemical features have led to a better understanding of the chromite ore genetic process, that is related to a supra-subduction geodynamic setting where partial melting processes were overlapped by metasomatic events.Metasomatism, that marks the general genetic context, is characterized by the presence of a hydrated boninitic melt that favors the precipitation of chromite ores. Chromite ores, typical of ophiolite complexes and usually enveloped by dunite are, in this case, characterized by the presence of two different types of dunites whose geochemical differences are reflected by olivine mineral chemistry and by different REE patterns of analyzed clinopyroxenes. Such geochemical marks, related to different magmatic and metasomatic events, could be a main key for location of fertile or barren dunites in terms of chromite ore occurrence in the field. 相似文献
7.
Field geometry, petrography and geochemistry of a dolomitization front (Late Jurassic, central Lebanon) 总被引:1,自引:1,他引:1
This contribution describes the field geometry, petrography and geochemistry of a well-exposed dolomitization front in Upper Jurassic carbonates, and attempts to highlight the sedimentological, structural and relative sea-level controls on multiphase dolomitization and related diagenetic events. The data presented reflect the superposition of various diagenetic phases which have resulted in a single dolostone body, whose dimensions are well defined in the field. Local microbial intraclastic dolomites of Late Tithonian age accumulated in a hypersaline lagoon during relative sea-level fall. These pre-date beige hydrothermal dolostones (51 to 55 mol% CaCO3 ; δ 18 O: −9·3 to −4·0‰ V-PDB; δ 13 C: −1·5 to +2·1‰ V-PDB; 87 Sr/86 Sr: 0·70742; matrix porosity: ≈6%; Klinkenberg permeability: ≈0·5 mD), whose dolomitizing fluid circulated along faults and invaded the nearby facies. First, the burrows were dolomitized, then the bulk rocks, resulting in the investigated 'tongue'-shaped dolomite body. Upon Late Jurassic–Early Cretaceous uplift, near-surface water percolated through – and altered – the underlying beige dolostones. This event was followed by a ferroan dolomite cement phase, which occurred during further burial. This contribution, featuring a well-defined geometric pattern of a dolomitization front with a large petrographic and geochemical data set, may also serve as a case study illustrating the complexity of superimposed diagenetic processes which have to be taken into account in modelling exercises of multiphase hydrothermal dolomitization. 相似文献
8.
Trace element data are reported in 21 lava samples from the Alban Hills, one of the most important volcanic complexes of the
Roman comagmatic region. The samples consist mostly of tephritic leucitites with minor phonolitic tephrites and tephritic
phonolites emplaced during two distinct phases of activity, separated by a caldera collapse.
The ferromagnesian element contents are variable (Ni=93-26 ppm; Co=37-20 ppm; Cr=359-5 ppm; Sc=35-6 ppm) and tend to have
higher values in the post-caldera rocks. Rb, Cs, Th, Sr, and LREE are extremely enriched in all the samples analyzed, with
the pre-caldera rocks displaying a lower content of Rb and Cs and a higher abundance of Th, light REE and La/Yb ratio. Ta
and Hf are not so high and are more enriched in the pre-caldera samples. Sr displays comparable values in the two groups of
rocks.
The trace element variation indicates that the rocks from the Alban Hills represent two distinct series of liquids formed
by crystal/liquid fractionation processes starting from two parental magmas. The genesis of the primary melts is hypothesized
as due to a low degree of partial melting of a mantle peridotite enriched in incompatible elements.
All of the studied samples have distribution patterns of incompatible elements normalized against a hypothetical primordial
mantle composition, which are similar to that displayed by the aeolian calc-alkaline and leucite-tephritic products and distinctively
different from those of typical K-rich volcanics from an intraplate rift environment. This strongly supports the hypothesis
that there is a close genetic connection between Roman magmatism and subductionrelated processes. 相似文献
9.
新疆尼勒克县布谷拉超钾质岩浆岩的~(40)Ar-~(39)Ar年龄和地球化学特征 总被引:1,自引:0,他引:1
本文在西天山尼勒克县布谷拉地区厘定出了具有构造指示意义的超钾质岩浆岩,它侵位在石炭纪大哈拉军山组地层中,出露面积约为0.02km2,在空间上该类岩石与铜的矿化异常带或矿化带紧密共生。布谷拉超钾质岩浆岩具有典型的斑状结构,斑晶以钾长石、橄榄石、辉石为主,偶见白榴石残留体或假象。造岩矿物及岩石化学组成特征显示,布谷拉超钾质岩浆岩为斜长白榴岩类超钾质岩浆岩。布谷拉超钾质岩浆岩微量元素蛛网图表现为Nb-Ta、Zr-Hf、Ti的负异常和Pb的正异常,暗示该超钾质岩浆岩形成于造山阶段。微量元素地球化学特征显示,布谷拉超钾质岩浆岩是含金云母的石榴石尖晶石二辉橄榄岩部分熔融的产物,其形成深度介于70~80km狭窄的范围内。布谷拉超钾质岩浆岩的40Ar-39Ar有效坪年龄为274.2±1.6Ma,等时线年龄为274.3±4.4Ma,表明该类岩石形成于早二叠世。布谷拉超钾质岩浆岩的地球化学特征及40Ar-39Ar定年研究结果显示,在早二叠世至少阿吾拉勒山的东段仍处于造山演化阶段。 相似文献
10.
Federico Casetta Pier Paolo Giacomoni Carmelo Ferlito Costanza Bonadiman Massimo Coltorti 《International Geology Review》2020,62(3):338-359
ABSTRACTThe spatial/temporal proximity of Mt. Etna to the Hyblean Plateau and the Aeolian slab makes the discussion on the nature of its mantle source/s extremely controversial. In this study, a detailed geochemical overview of the entire Mt. Etna evolutionary sequence and a comparison with the magmatism of the Hyblean Plateau was proposed to: (i) simulate the composition of Mt. Etna tholeiitic to alkaline primitive magmas in equilibrium with a fertile mantle source; (ii) model the nature, composition and evolution of the mantle source from the tholeiitic stage (600 ka) to present magmatism. According to our simulations, two amphibole + phlogopite-bearing spinel lherzolite sources are able to explain the wide range of Etnean primary magmas. The enrichment in LILE, 87Sr/86Sr, Rb and H2O of the magmas emitted after 1971 (but also discontinuously generated in both historic and prehistoric times) are caused by different melting proportions of amphibole and phlogopite in a modally and compositionally homogeneous mantle domain, with melting degrees analogous to those required to produce magmas erupted prior to 1971. The behaviour of the hydrous phases during melting could be ascribed to a variable H2O/CO2 activity in the mantle source, in turn related to the heat/fluxes supply from the asthenospheric upwelling beneath Mt. Etna. All these considerations, strengthened by numerical models, are then merged to review the complex Pliocene/Lower Pleistocene to present day’s geodynamic evolution of eastern Sicily. 相似文献
11.
《Journal of Geochemical Exploration》2003,77(2-3):93-108
In the Alban Hills area, strong areally diffuse and localised spot degassing processes occur (Tivoli, Cava dei Selci, Solforata, Tor Caldara). The gas comprises a large proportion of CO2, with minor CH4, H2S and Rn. These advective features are generated by fluid leakage from buried reservoirs hosted in the structural highs of the Mesozoic carbonate basement. Gas migration towards the surface is controlled by fault and fracture systems bordering the structural highs of the carbonate formations (e.g. Ciampino high). His release is triggered when the total pressure of the fluid phase exceeds the hydrostatic pressure, thus forming a free gas phase. Furthermore, both the sudden and catastrophic, and slow and continuous gas release at surface, of naturally occurring toxic species (CO2, H2S and Rn) poses a serious health risk to people living in this geologically active area.This paper presents data obtained from soil gas and gas flux surveys, as well as gas isotopes analyses, which suggest the presence of a deep origin gas flux enriched in carbon dioxide and minor species (CH4 and H2S), as well as a channelled migration of geogas mixtures having a Rn component which is not produced in situ.In regards to the health risk to local inhabitants, it was found that some anomalous areas had been zoned as parkland while others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil gas CO2 concentrations of over 70% and a CO2 flux of about 0.7 kg m−2 day−1, as well as radon values of more than 250 kBq/m3. In addition, an indoor radon survey has been conducted in selected houses in the town of Cava dei Selci to search for a possible correlation between the local geology and the radon concentration in indoor air. Preliminary results indicate high indoor values at ground floor levels (up to 1000 Bq/m3) and very high values in the cellars (up to 250.000 Bq/m3). It is recommended that land-use planners incorporate soil gas and/or gas flux measurements in the environmental assessment of areas of possible risk (i.e. volcanic or structurally active areas). 相似文献
12.
Sr isotope and Ca/Mg/Sr chemical compositions of freshwater ostracode tests separated from a sediment core represent the last 16 ka of sedimentation in Lake Constance, Central Europe. The chemical evolution of the paleowater's dissolved load of Lake Constance was estimated by correcting the ostracode data for Ca/Mg/Sr fractionation due to biogenic calcification. Since the Late Pleistocene deglaciation, the Ca/Sr molar ratios of paleowaters increased systematically from about 100 (a near marine signature) to about 200. Ca/Mg molar ratios varied in the range of 1–25. The 87Sr/86Sr ratios indicate Late Pleistocene paleowater compositions of 0.7086–0.7091, significantly more radiogenic than present day waters (0.7085). Sr isotopes and Ca/Mg/Sr chemical data together show that weathering of Mesozoic evaporites consistently dominated the dissolved Sr load (80–90%). Carbonate and silicate weathering were less important (1–10%). Trends of Sr dissolved loads were therefore not related to Mg which was mainly mobilized by carbonate weathering. Biotite weathering was an important source of radiogenic Sr in the paleowaters. The short-term release (duration about 600–800 years) of radiogenic Sr during glacier retreat started 15.2 ka ago and was due to enhanced biotite weathering at the glacier base. Long-term release of radiogenic Sr was due to biotite weathering in glacial soils and silicate rocks, and has gradually declined since the Late Pleistocene/Holocene transition. 相似文献
13.
青藏米巴勒地区中新世钠质方沸石(霞石)响岩中单斜辉石环带研究:对岩浆-构造演化的启示 总被引:2,自引:3,他引:2
在拉萨地块中部米巴勒地区产出的中新世钠质方沸石(霞石)响岩中的单斜辉石发育大量环带结构,环带结构包括正环带、反环带和韵律环带。其中正环带结构从核部到边部Mg#、MgO、Cr2O3呈逐渐降低趋势,而FeO、Na2O、Al2O3、TiO2呈逐渐升高的趋势,它反映了岩浆正常结晶的历史。反环带结构中Mg#、MgO、Cr2O3从核部到边部呈升高的趋势,FeO、Na2O、Al2O3的含量总体上呈降低趋势,本文认为响岩中单斜辉石的这种环带结构是受到了和它共存的钾质-超钾质火山岩岩浆混合或混染所致,两种岩石应是各自独立的母岩浆的结晶产物。具韵律环带结构的单斜辉石从核部到边部出现成分规律性波动,Na2O、Al2O3、TiO2、Cr2O3含量的变化在正反环带的范围内,Mg#值的变化范围较小,在正环带斑晶和基质的范围内,它是岩浆多次混合作用的结果。该地区钠质过碱性岩的出现可能指示青藏高原在这套火山岩喷发时(13~12Ma),构造体制发生了显著的改变,进入到一个典型的陆内伸展阶段。 相似文献
14.
M. Federico A. Peccerillo M. Barbieri T. W. Wu 《Contributions to Mineralogy and Petrology》1994,115(4):384-401
Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ultramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO2, alkalies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is highly magnesian (up to Fo%=96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indicate a genesis of ultramafic ejecta by thermal metamorphism of a siliceous dolomitic limestone, probably with input of chemical components from potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive equivalents of lavas or cumulates crystallized from variably evolved ultrapotassic magmas. One sample of the former group has major element composition resembling ultrapotassic rocks with kamafugitic affinity. Some cumulitic rocks have exceedingly high abundances of Th (81–84 ppm) and light rare-earth elements (LREE) (La+Ce=421–498 ppm) and extreme REE fractionation (La/Yb=288–1393), not justified by their modal mineralogy which is dominated by sanidine, leucite and nepheline. Finegrained phases are dispersed through the fractures and within the interstices of the main minerals. Semiquantitative EDS analyses show that Th and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the high concentration of incompatible trace elements in the whole rocks. The interstitial position of these phases and their association with fluorite support a secondary origin by deposition from fluorine-rich fluids separated from a highly evolved potassic liquid. The Nd isotopic ratios of the cjecta range from 0.51182 to 0.51217. 87Sr/86Sr ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster around 0.7105±3. This indicates either the occurrence of several isotopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires selective assimilation of host rocks in order to explain isotopic and geochemical characteristics of lavas and xenoliths. The new data indicate that the evolutionary processes in the potassic magmas of the Alban Hills were much more complex than envisaged by previous studies. Interaction of magmas with wall rocks may be an important process during magmatic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during the latest stages of magmatic evolution. Such a process was able to produce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurrence of xenoliths with kamafugitic composition points to the existence of this type of ultrapotassic magma at the Alban Hills. 相似文献
15.
《Lithos》2004,72(1-2):19-44
In the Iberian Massif, rocks of the K-rich plutonic series are not abundant, but towards internal parts of the belt represented by the Central Iberian Zone there are some sectors where shoshonitic plutonism occurs over broad areas. One of these areas is the anatectic Tormes Dome, encompassing two similar studied plutons (Pereruela and Vitigudino). A monzonitic association has been defined, ranging from monzogabbros to quartz monzonites or scarce monzogranites. Enrichment in LREE, P, Sr and Ba, a high water content (up to 5.5%) and a high degree of oxidation (Ni–NiO buffer) are the main features of magmas parental to these monzonitic rocks. These petrographic and geochemical features allow these granitoids to be ascribed to the shoshonitic type (“SH-type”), rather than to the I-type. The minimum emplacement pressure range is 410–230 MPa, whereas the estimated solidus temperature range is 940–765 °C; i.e., above water-saturated solidus. O, Sr, and Nd isotopes point to open-system processes. Apatite cathodoluminescence suggests that magma mixing was unlikely to have occurred for the most enriched rocks. Assimilation/fractional crystallisation (AFC) modelling was performed for both plutons, permitting assimilation/crystallization rates to be estimated between 0.16 and 0.25. Different contaminants have been inferred: a metapelite at upper crustal level for the Vitigudino Pluton and a granulitic orthogneiss for the Pereruela Pluton. A liquid line of descent, linking monzogabbroic members to quartz monzonites/monzogranites, can be reconstructed. Some loss of water and oxygen can be inferred, although water remained in the system below the solidus, giving rise to auto-metamorphism at ca. 500 °C. On the other hand, AFC processes are unlikely to have been the main factor in controlling the characteristic enrichment of the monzonitic series, since the least contaminated samples are the most LILE- and LREE-enriched for both plutons. Thus, a source-controlled chemical signature can be inferred. The experimental data indicate that a hybrid protholith (a mixture of peridotite, amphibolite and metapelite) can account for the enrichment and hydrated nature of the least evolved monzogabbroic magmas. Late-orogenic slab break-off and subsequent post-collisional extensional events appear to be a suitable scenario to provide the heat that triggered partial melting close to the crust/mantle boundary. 相似文献
16.
A. B. Kuznetsov G. V. Ovchinnikova M. A. Semikhatov I. M. Gorokhov O. K. Kaurova M. T. Krupenin I. M. Vasil’eva B. M. Gorokhovskii A. V. Maslov 《Stratigraphy and Geological Correlation》2008,16(2):120-137
The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran formations corresponding to middle horizons of the Lower Riphean Burzyan Group in the Taratash and Yamantau anticlinoria, respectively, the southern Urals. The least altered rock samples retaining the 87Sr/86Sr ratio of sedimentation basin have been selected for analysis using the original method of leaching the secondary carbonate phases and based on strict geochemical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024). The stepwise dissolution in 0.5 N HBr has been used to enrich samples in the primary carbonate phase before the Pb-Pb dating. Three (L-4 to L-6) of seven consecutive carbonate fractions obtained by the step-wise leaching are most enriched in the primary carbonate (in terms of the U-Pb systematics). In the 206Pb/204Pb-207Pb/204Pb diagram, data points of these fractions plot along an isochron determining age of 1550 ± 30 Ma (MSWD = 0.7) for the upper member of the Satka Formation. The initial 87Sr/86Sr ratio in the least altered limestones of this formation is within the range of 0.70460–0.70480. Generalization of the Sr isotopic data published for the Riphean carbonates from different continents showed that 1650–1350 Ma ago the 87Sr/86Sr ratio in the world ocean was low, slightly ranging from 0.70456 to 0.70494 and suggesting the prevalent impact of mantle flux. 相似文献
17.
The Naga Hills Ophiolite(NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite,diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx.Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features.The basalts are characterized by 44.1-45.6 wt.% of SiO_2 with 28-38 of Mg#, and the gabbros by38.7-43.7 wt.% of SiO_2, and 26-79 of Mg#. The ultramafic rocks are characterized by 37.4-52.2 wt.% of SiO_2, and 80-88 of Mg#. In multi-element diagrams(spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr. Nb. Sr and a gentle negative anomaly of P.However, the rare earth element(REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE(LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite(IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340-4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97-1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources(fertile and refractory). 相似文献
18.
《Chemie der Erde / Geochemistry》2015,75(2):237-260
The northeastward subduction of the Neo-Tethyan oceanic lithosphere beneath the Iranian block produced vast volcanic and plutonic rocks that now outcrop in central (Urumieh–Dokhtar magmatic assemblage) and north–northeastern Iran (Alborz Magmatic Belt), with peak magmatism occurring during the Eocene. The Karaj Dam basement sill (KDBS), situated in the Alborz Magmatic Belt, comprises gabbro, monzogabbro, monzodiorite, and monzonite with a shoshonitic affinity. These plutonic rocks are intruded into the Karaj Formation, which comprise pyroclastic rocks dating to the lower–upper Eocene. The geochemical and isotopic signatures of the KDBS rocks indicate that they are cogenetic and evolved through fractional crystallization. They are characterized by an enrichment in LREEs relative to HREEs, with negative Nb–Ta anomalies. Geochemical modeling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of partial melting of a phlogopite–spinel peridotite source to generate the KDBS rocks. Their low ISr = 0.70453–0.70535, ɛNd (37.2 Ma) = 1.54–1.9, and TDM ages ranging from 0.65 to 0.86 Ga are consistent with the melting of a Cadomian enriched lithospheric mantle source, metasomatized by fluids derived from the subducted slab or sediments during magma generation. These interpretations are consistent with high ratios of 206Pb/204Pb = 18.43–18.67, 207Pb/204Pb = 15.59, and 208Pb/204Pb = 38.42–38.71, indicating the involvement of subducted sediments or continental crust. The sill is considered to have been emplaced in an environment of lithospheric extension due to the slab rollback in the lower Eocene. This extension led to localized upwelling of the asthenosphere, providing the heat required for partial melting of the subduction-contaminated subcontinental lithospheric mantle beneath the Alborz magmatic belt. Then, the shoshonitic melt generates the entire spectrum of KDBS rocks through assimilation and fractional crystallization during the ascent of the magma. 相似文献
19.
郯庐断裂带晚中生代富碱侵入岩Nd、Sr、Pb同位素特征及源区性质探讨 总被引:2,自引:1,他引:1
郯庐断裂带是中国东部一条巨大的深大断裂带,它纵切华北克拉通、扬子克拉通、兴蒙-吉黑造山带和大别-苏鲁造山带四个大地构造单元,特别是在该断裂带出露许多晚中生代富碱侵入岩体,构成一条长达近2000km的富碱侵入岩带.本文是在前人研究基础上首次从大尺度对整个郯庐断裂带富碱侵入岩的Nd、Sr、Pb同位素做了比较系统深入的研究,发现四个大地构造单元的富碱侵入岩具有明显不同的Nd、Sr、Pb同位素特征,暗示四大构造单元富碱侵入岩的源区性质差别较大:兴蒙-吉黑造山带晚中生代富碱侵入岩源于和亏损地幔有成因联系的年青地壳(εNd(t)为ft.,平均值 2.25);华北克拉通、扬子克拉通和大别-苏鲁造山带三大构造单元晚中生代富碱侵入岩都主要源于EMI型富集地幔(εNd(t)均为负值).但是这三大构造单元岩石圈地幔富集程度差别较大.大别-苏鲁造山带岩石圈地幔相对富集程度最高(εNd(t)平均值为-17.06);华北克拉通次之(εNd(t)平均值为-10.03);扬子克拉通岩石圈地幔富集程度最低(εNd(t)平均值为-6.41).本文认为大别.苏鲁造山带岩石圈地幔相对最富集的原因可能是在早中生代扬子克拉通陆壳向华北克拉通深俯冲之后的折返过程中,因压力释放有利于熔/流体的形成、析出和向上交代并改造华北岩石圈地幔使之成为富集地幔. 相似文献
20.
《Chemie der Erde / Geochemistry》2015,75(3):301-315
Sr and Nd isotopes were applied to 5 soil profiles from the Muravera area, in south-eastern Sardinia.All the soils, which have developed during the Quaternary on the Lower Paleozoic metamorphic basement except for one on Eocene carbonates, are located far from major sources of pollution. Therefore, they are suitable for testing pedogenic processes and geochemical evolution to benefit for environmental studies.The Sr isotopic ratios range largely (δ87Sr = 1.7–65.9‰), even in each soil profile. In particular, the observed increase of δ87Sr with depth in the most of the metamorphic rock-based soils can be accounted for by the downward decrease of Sr contributions from organic matter and Saharan dust, both displaying lower isotopic ratios than the soil bedrocks. The carbonate rock-based soil exhibits δ87Sr higher (1.7–18.1‰) than the bedrock, indicating a significant contribution of radiogenic Sr from the siliciclastic fraction of the soil, and probably from dust input. The Nd isotopic ratios are slightly variable through the profiles (ɛNd from −7.8 to −14.5), confirming little mobility of Nd and Sm during the pedogenesis. Among the minerals present in the soils, phosphates, albite, and calcite are those important in providing low radiogenic Sr and Nd to organic matter of the soils.Lastly, this isotopic study has in particular allowed for evaluating the potential proportion of contribution of Saharan dust to south-eastern Sardinia, thus corroborating the findings of other studies related to soils from the central-western Mediterranean. 相似文献