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1.
L. N. Kogarko 《Geochemistry International》2006,44(1):3-10
Alkaline magmatism has occurred since 2.5–2.7 Ga and its abundance has continuously increased throughout the Earth’s history.
Alkaline rocks appeared on the Earth with changes in the geodynamic regime of our planet, i.e., when plume tectonics was supplemented
by plate tectonics. Global-scale development of plate tectonics at the Archean—Proterozoic boundary initiated subduction of
already significantly oxidized oceanic crust enriched in volatiles and large-scale mantle metasomatism caused the formation
of enriched reservoirs as sources of alkaline and carbonatite magmatism. Study of metasomatized mantle material showed the
occurrence of traces of primary carbonatite melts, which are strongly enriched in rare elements, according to ion-microprobe
analyses. The results obtained allowed us to propose a new two-stage genetic model for Ca-rich carbonatites including (1)
metasomatic wehrlitization and carbonatization of mantle material and (2) partial melting of wehrlitized mantle with formation
of carbonate-rich melts or three immiscible liquids (at high alkali contents), i.e., silicate, carbonatitic, and sulfide (at
high sulfur activity).
Original Russian Text L.N. Kogarko, 2006, published in Geokhimiya, 2006, No. 1, pp. 5–13. 相似文献
2.
According to their genesis, meteorites are classified into heliocentric (which originate from the asteroid belt) and planetocentric
(which are fragments of the satellites of giant planets, including the Proto-Earth). Heliocentric meteorites (chondrites and
primitive meteorites genetically related to them) used in this study as a characteristic of initial phases of the origin of
the terrestrial planets. Synthesis of information on planetocentric meteorites (achondrites and iron meteorites) provides
the basis for a model for the genesis of the satellites of giant planets and the Moon. The origin and primary layering of
the Earth was initially analogously to that of planets of the HH chondritic type, as follows from similarities between the
Earth’s primary crust and mantle and the chondrules of Fe-richest chondrites. The development of the Earth’s mantle and crust
precluded its explosive breakup during the transition from its protoplanetary to planetary evolutionary stage, whereas chondritic
planets underwent explosive breakup into asteroids. Lunar silicate rocks are poorer in Fe than achondrites, and this is explained
in the model for the genesis of the Moon by the separation of a small metallic core, which sometime (at 3–4 Ga) induced the
planet’s magnetic field. Iron from this core was involved into the generation of lunar depressions (lunar maria) filled with
Fe- and Ti-rich rocks. In contrast to the parent planets of achondrites, the Moon has a olivine mantle, and this fact predetermined
the isotopically heavier oxygen isotopic composition of lunar rocks. This effect also predetermined the specifics of the Earth’s
rocks, whose oxygen became systematically isotopically heavier from the Precambrian to Paleozoic and Mesozoic in the course
of olivinization of the peridotite mantle, a processes that formed the so-called roots of continents. 相似文献
3.
俯冲带部分熔融 总被引：3，自引：3，他引：0
俯冲带是地幔对流环的下沉翼,是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度,因此,一般认为俯冲板片并不会发生部分熔融,而是脱水导致上覆地幔楔发生部分熔融。但是,也有研究认为,在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下,俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时,洋壳物质可以发生饱和水或脱水熔融,基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构,俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比,俯冲的大陆地壳具有很低的水含量,较难发生部分熔融,但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融,形成S型花岗岩和埃达克质岩石。 相似文献
4.
The beginnings of hydrous mantle wedge melting 总被引：5，自引：3，他引：2
This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na2O, 0.03 wt% K2O) in the presence of excess H2O (14.5 wt% H2O) from 740 to 1,200°C at 3.2–6 GPa. Based on textural and chemical evidence, we find that the H2O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3–6 GPa. We identify both quenched solute from
the H2O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H2O-saturated solidus from 2 to 3.6 GPa, and chlorite peridotite melting experiments (containing ~6 wt% chlorite) show that
melting occurs at the chlorite-out boundary over this pressure range, which is within 20°C of the H2O-saturated melting curve. Chlorite can therefore provide sufficient H2O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H2O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly
more H2O is fluxed from the subducting slab near 100 km depth than can be bound in a layer of chloritized peridotite ~ 1 km thick
at the base of the mantle wedge. Therefore, the dehydration of serpentinized mantle in the subducted lithosphere supplies
free H2O to trigger melting at the H2O-saturated solidus in the lowermost mantle wedge. Alternatively, in cool subduction zones like the Northern Marianas, a layer
of chloritized peridotite up to 1.5 km thick could contain all the H2O fluxed from the slab every million years near 100 km depth, which suggests that the dominant form of melting below arcs
in cool subduction zones is chlorite dehydration melting. Slab P–T paths from recent geodynamic models also allow for melts of subducted sediment, oceanic crust, and/or sediment diapirs to
interact with hydrous mantle melts within the mantle wedge at intermediate to hot subduction zones. 相似文献
5.
San’kov V. A. Lukhnev A. V. Parfeevets A. V. Miroshnichenko A. I. Ashurkov S. V. 《Doklady Earth Sciences》2011,436(1):159-164
The complex analysis of parameters characterizing the modern deformations of the Earth’s crust and upper mantle in the territory
of the Mongolia-Siberian Area is made. Directions of principal tension axes of stress-tensors, calculated with the use of
earthquake source mechanisms have been taken as parameters of modern deformations at the level of the middle crust; directions
of axes of horizontal strains in the geodesic network by the GPS data have been taken as such parameters at the level of the
Earth’s surface. The strain parameters for the mantle depths are the data on seismic anisotropy derived from the published
sources about the results of studies on splitting of transversal waves from distant earthquakes. Seismic anisotropy is interpreted
as the ordered orientation of olivine crystals, which appears with great strains resulting from the flow of the mantle material.
It has been shown that directions of extensional strain axes (minimal compression) by geodesic and seismological data coincide
with anisotropy directions in the upper mantle in the region whose median value is 310°–320°. The observed mechanical coupling
of the crust and the upper mantle of the Mongolia-Siberian Mobile Area shows the participation of the lithospheric mantle
in the formation of neotectonical structures and enables us to distinguish the principal processes determining the Late Cenozoic
tectogenesis in this territory. One of the leading mechanisms for the neotectonical and modern deformations of the Mongolia-Siberian
Region is the large-scale NW-SE material flow in the upper mantle causing both motion of the entire northern part of the continent
and divergence of the Eurasia and the Amurian Plate. Lithospheric deformations in the western part of the region are related
to collision-induced compression, while those in the central part are caused by interaction of these large-scale tectonic
processes. 相似文献
6.
The study of interaction between mantle melts and crustal rocks is of great importance for deciphering the evolution of the
Earth’s crust and for better understanding the composition of mantle sources, in particular, the degree of their compositional
heterogeneity. This work presents the results of Rb-Sr and Sm-Nd isotopic studies of 37 samples taken from the Kivakka layered
intrusion, host rocks, and rocks at the contact. The studies were aimed at verifying the hypothesis of possible crustal contamination
of mafic melt during magma chamber crystallization. It was found that the section of the Kivakka layered massif is characterized
by initial Sr and Nd isotopic heterogeneity, with negative correlation between initial Nd isotopic ratio and its content.
The rocks of the massif have low ɛNd(T) values. 相似文献
7.
V. A. San’kov A. V. Parfeevets A. V. Lukhnev A. I. Miroshnichenko S. V. Ashurkov 《Geotectonics》2011,45(5):378-393
Comprehensive analysis of the parameters characterizing contemporary and neotectonic deformations of the Earth’s crust and
upper mantle developed in the Mongolia-Siberia area is presented. The orientation of the axes of horizontal deformation in
the geodetic network from the data of GPS geodesy is accepted as an indicator of current deformations at the Earth’s surface.
At the level of the middle crust, this is the orientation of the principal axes of the stress-tensors calculated from the
mechanisms of earthquake sources. The orientation of the axes of stress-tensors reconstructed on the basis of structural data
is accepted as an indicator of Late Cenozoic deformations in the upper crust. Data on seismic anisotropy of the upper mantle
derived from published sources on the results of splitting of shear waves from remote earthquakes serve as indicators of deformation
in the mantle. It is shown that the direction of extension (minimum compression) in the studied region coincides with the
direction of anisotropy of the upper mantle, the median value of which is 310–320° NW. Seismic anisotropy is interpreted as
the ordered orientation of olivine crystals induced by strong deformation owing to the flow of mantle matter. The observed
mechanical coupling of the crust and upper mantle of the Mongolia-Siberia mobile area shows that the lithospheric mantle participated
in the formation of neotectonic structural elements and makes it possible to ascertain the main processes determining the
Late Cenozoic tectogenesis in this territory. One of the main mechanisms driving neotectonic and contemporary deformations
in the eastern part of the Mongolia-Siberia area is the long-living and large-scale flow of the upper mantle matter from the
northwest to the southeast, which induces both the movement of the northern part of the continent as a whole and the divergence
of North Eurasia and the Amur Plate with the formation of the Baikal Rift System. In the western part of the region, deformation
of the lithosphere is related to collisional compression, while in the central part, it is due to the dynamic interaction
of these two large-scale processes. 相似文献
8.
Several spindle-shaped grains of zircon, which have a small size (<0.25 mm) and a distinct purplish pink coloration were found
in the crushed samples of kimberlites from the Aykhal, Komsomolskaya-Magnitnaya, Botuobinskaya (Siberian platform), and Nyurbinskaya
(Yakutia) pipes and olivine lamproites of the Khani massif (West Aldan). U-Pb SHRIMP II zircon dating performed at the VSEGEI
Center for Isotopic Research yielded the ages of 1870–1890 Ma for the pipes of the Western province (Aykhal and Komsomolskaya)
and 2200–2750 Ma for the pipes of the eastern province (Nyurbinskaya and Botuobinskaya), which allowed us to consider these
zircons to be xenogenic to kimberlites. Although these zircons resemble in their age and color those from the granulite xenoliths
in the Udachnaya pipe [2], no other granulite minerals are found there. Thus, major geological events in the mantle and lower
crust, which led to the formation of zircon-bearing rocks, happened at 1800–1900 Ma in the northern part of the kimberlite
province, whereas in the Eastern part of the province (Nakyn field) these events were much older (2220–2700 Ma). It is known
that the period of 1800–1900 Ma in the Earth’s history was accompanied by intense tectonic movements and widespread alkaline-carbonatite
magmatism. This magmatism was related to plume activity responsible for overheating the large portions of the mantle to the
temperatures at which some diamonds in mantle rocks would burn (northern part of the kimberlite province). In the Nakyn area,
the mantle underwent few or no geological processes at that time, and perhaps for this reason this area hosts more diamondiferous
kimberlites. The age of olivine lamproites from the Khani massif is 2672–2732 Ma. Thus, these are some of the world’s oldest
known K-alkaline rocks. 相似文献
9.
Proterozoic granitoid rocks in Zhejiang Province were formed in the Shengongian period (1.8–1.9 Ga) and the Late Jinningian
period (0.6–0.9 Ga), respectively. Petrogenetic problems are discussed based on chemical (major, trace elements and REEs)
and Nd-Sr isotopic compositions. The Shengongian granites resulted from partial melting of the Badu Group and the Late Jinningian
granites are of mantle derivation with or without contamination of crustal material. The crust in Zhejiang had undergone three
major periods of growth during 2.6–2.7 Ga, 0.8–1.1 Ga and 0.10–0.12 Ga after it was generated in Archean time. Compositional
fractionation in the process of crust evolution is not evident. The presence of Late Jinningian granites of mantle and mantle-crust-derivation
along the Jiangshan-Shaoxing Fault is indicative of crust subduction at that time.
This project was finantially supported by both the National Natural Science Foundation of China (No. 9490011) and the Zhongguancun
Test Center. 相似文献
10.
V. P. Kovach V. V. Yarmolyuk V. I. Kovalenko A. M. Kozlovskyi A. B. Kotov L. B. Terent’eva 《Petrology》2011,19(4):399-425
Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes
of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological,
geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental
crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic
crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic
island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction
zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents,
which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites
and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source
contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating
with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient
crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks
of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks
of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were
accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted
mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate
role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly
from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic
mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early
Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile
continental crust and allow us to distinguish a corresponding stage of juvenile crust formation. 相似文献
11.
Robert Tenzer Pavel Novák Peter Vajda Vladislav Gladkikh Hamayun 《Computational Geosciences》2012,16(1):193-207
We developed and applied a novel numerical scheme for a gravimetric forward modelling of the Earth’s crustal density structures
based entirely on methods for a spherical analysis and synthesis of the gravitational field. This numerical scheme utilises
expressions for the gravitational potentials and their radial derivatives generated by the homogeneous or laterally varying
mass density layers with a variable height/depth and thickness given in terms of spherical harmonics. We used these expressions
to compute globally the complete crust-corrected Earth’s gravity field and its contribution generated by the Earth’s crust.
The gravimetric forward modelling of large known mass density structures within the Earth’s crust is realised by using global
models of the Earth’s gravity field (EGM2008), topography/bathymetry (DTM2006.0), continental ice-thickness (ICE-5G), and
crustal density structures (CRUST2.0). The crust-corrected gravity field is obtained after modelling and subtracting the gravitational
contribution of the Earth’s crust from the EGM2008 gravity data. These refined gravity data mainly comprise information on
the Moho interface and mantle lithosphere. Numerical results also reveal that the gravitational contribution of the Earth’s
crust varies globally from 1,843 to 12,010 mGal. This gravitational signal is strongly correlated with the crustal thickness
with its maxima in mountainous regions (Himalayas, Tibetan Plateau and Andes) with the presence of large isostatic compensation.
The corresponding minima over the open oceans are due to the thin and heavier oceanic crust. 相似文献
12.
Simulation results of the equilibrium state of systems water-carbonaceous chondrite material, water-primary mantle material,
water-ultramafic rock material, and water-mafic rock material open with respect to carbon dioxide and methane at 25°C, 1 bar
indicate that highly alkaline reduced aqueous solutions with K/Na > 1 can be formed only if water is in equilibrium with compositions
close to those of continental crust and primitive mantle. Yu.V. Natochin’s hypothesis that the living cell can be formed only
in an aqueous environment with K/Na > 1 leads to the conclusion that terrestrial life could arise and further evolve on the
Earth during the differentiation of primary chondritic material into the Earth’s core and mantle (during the first few million
years of the planet’s lifetime) in an alkaline (pH 9–10) reduced (Eh = −400–500 mV) aqueous solution at a temperature of 50–60°C,
in equilibrium with an N2-bearing atmosphere, which also contained CH4 (partial pressure from 10−2 to 10−8 bar), CO2 (partial pressure from 10−5 to 10−8 bar), NH3, H2, H2S, CO, and other gases. 相似文献
13.
Theoretical ideas based on the results of numerical modeling of mantle convection are presented. The thermochemical model
has taken into consideration such factors as two-layer structure of the mantle, formation of light substance in the D” layer
(owing to transition of metallic components into the core), and heavy substances in subduction zones (eclogite-alteration
of the oceanic crust). Numerical experiments have shown that this system allows phenomena of global mantle overturns, which
make possible to model the general pattern of the Earth’s geologic evolution. The suggested theory establishes cause-effect
relationships in the sequence of geological events and is conformed to all the empirical data. 相似文献
14.
Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale (x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations. 相似文献
15.
E. V. Bibikova 《Petrology》2010,18(5):482-488
Analysis of isotope-geochemical data obtained for the early crustal complexes of the Earth provided constraints on the formation
time, scales of development, and geochemical features of protocrust. Most informative were isotope-geochemical and geochemical
data on the oldest zircons with ages up to 4.4 Ga, short-lived 146Sm/142Nd isotope system, and lead isotope composition of the oldest rocks of Greenland. The presence of positive 142Nd anomaly in the rocks of West Greenland and negative anomaly in the amphibolites of the oldest Nuvvuagittuq greenstone belt
of the Superior province (O’Neil et al., 2008) indicates the early differentiation of the Earth material into depleted mantle
and enriched (basaltic) crust (Caro et al., 2006; Benett et al., 2007a, b; O’Neil et al., 2008). Pb-Pb isotopic systematics
of the oldest crustal rocks from West Greenland and Labrador testifies that high μ enriched crust (238U/204Pb = 10.9) of basaltic composition already existed 3.9 Ga ago (Kamber et al., 2003). Based on isotope-geochemical and geochemical
features of the oldest zircons in the Late Archean greenstone belts of the Yilgarn block (Western Australia), the crust of
intermediate-felsic composition and water on the Earth’s surface already existed 4.4 Ga ago (Wilde et al., 2001). 相似文献
16.
The paper reports results of the analysis of the spatial distribution of modern (younger than 2 Ma) volcanism in the Earth’s
northern hemisphere and relations between this volcanism and the evolution of the North Pangaea modern supercontinent and
with the spatial distribution of hotspots of the Earth’s mantle. Products of modern volcanism occur in the Earth’s northern
hemisphere in Eurasia, North America, Greenland, in the Atlantic Ocean, Arctic, Africa, and the Pacific Ocean. As anywhere
worldwide, volcanism in the northern hemisphere of the Earth occurs as (a) volcanism of mid-oceanic ridges (MOR), (b) subduction-related
volcanism in island arcs and active continental margins (IA and ACM), (c) volcanism in continental collision (CC) zones, and
(d) within-plate (WP) volcanism, which is related to mantle hotspots, continental rifts, and intercontinental belts. These
types of volcanic areas are fairly often neighboring, and then mixed volcanic areas occur with the persistent participation
of WP volcanism. Correspondingly, modern volcanism in the Earth’s northern hemisphere is of both oceanic and continental nature.
The latter is obviously related to the evolution of the North Pangaea modern supercontinent, because it results from the Meso-Cenozoic
evolution of Wegener’s Late Paleozoic Pangaea. North Pangaea in the Cenozoic comprises Eurasia, North and South America, India,
and Africa and has, similar to other supercontinents, large sizes and a predominantly continental crust. The geodynamic setting
and modern volcanism of North Pangaea are controlled by two differently acting processes: the subduction of lithospheric slabs
from the Pacific Ocean, India, and the Arabia, a process leading to the consolidation of North Pangaea, and the spreading
of oceanic plates on the side of the Atlantic Ocean, a process that “wedges” the supercontinent, modifies its morphology (compared
to that of Wegener’s Pangaea), and results in the intervention of the Atlantic geodynamic regime into the Arctic. The long-lasting
(for >200 Ma) preservation of tectonic stability and the supercontinental status of North Pangaea are controlled by subduction
processes along its boundaries according to the predominant global compression environment. The long-lasting and stable subduction
of lithospheric slabs beneath Eurasia and North America not only facilitated active IA + ACM volcanism but also resulted in
the accumulation of cold lithospheric material in the deep mantle of the region. The latter replaced the hot mantle and forced
this material toward the margins of the supercontinent; this material then ascended in the form of mantle plumes (which served
as sources of WP basite magmas), which are diverging branches of global mantle convection, and ascending flows of subordinate
convective systems at the convergent boundaries of plates. Subduction processes (compressional environments) likely suppressed
the activity of mantle plumes, which acted in the northern polar region of the Earth (including the Siberian trap magmatism)
starting at the latest Triassic until nowadays and periodically ascended to the Earth’s surface and gave rise to WP volcanism.
Starting at the breakup time of Wegener’s Pangaea, which began with the opening of the central Atlantic and systematically
propagated toward the Arctic, marine basins were formed in the place of the Arctic Ocean. However, the development of the
oceanic crust (Eurasian basin) took place in the latter as late as the Cenozoic. Before the appearance of the Gakkel Ridge
and, perhaps, also the oceanic portion of the Amerasian basin, this young ocean is thought to have been a typical basin developing
in the central part of supercontinents. Wegener’s Pangaea broke up under the effect of mantle plumes that developed during
their systematic propagation to the north and south of the Central Atlantic toward the North Pole. These mantle plumes were
formed in relation with the development of global and local mantle convection systems, when hot deep mantle material was forced
upward by cold subducted slabs, which descended down to the core-mantle boundary. The plume (WP) magmatism of Eurasia and
North America was associated with surface collision- or subduction-related magmatism and, in the Atlantic and Arctic, also
with surface spreading-related magmatism (tholeiite basalts). 相似文献
17.
Carl Spandler Jörg Hermann Kevin Faure John A. Mavrogenes Richard J. Arculus 《Contributions to Mineralogy and Petrology》2008,155(2):181-198
The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models
of slab devolatilization. The eclogite-facies mélange belt of northern New Caledonia represents previously subducted oceanic
crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large
quantities of H2O and CO2 and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and
chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior
to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a mélange of mafic, ultramafic
and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at
sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400–650°C). Fluids released under
these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks,
and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these
hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon
into the deep mantle.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
18.
Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd,Sr and Pb isotopes.The results show that the gab-broic xenoliths are of more mafic basaltic composition .Their abundances show narrow variations in major elements.The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths.The gabbroic xenoliths are rich in Nd(0.51159-0.51249),Sr(0.70491-0.70768) and low in radiogenic Pb(16.283-17.046, 15.191-15.381 and 36.999-37.476),significantly different from basalts and lherzolites in isotopic space.The calculated Nd and Pb model ages are about 3.0-3.5 Ga.The rocks have relatively low equilibrium T(-850℃) and P(0.8-0.9 Gpa).They could be inter-preted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle.Their chemical and isotopic variations can be ascribed to different degrees of melting,segregation and long-term evolution. 相似文献
19.
Equilibria involving acmite, albite, nepheline, quartz, anda liquid phase constitute the petrologically important partof the system Na2O–Al2O3–Fe2O2–SiO2, and theunivariant and invariant relations provide useful analogiesfor a wide variety of alkaline igneous rocks. These relationsare dominated by the incongruent melting behaviour of acmite,which does not appear on the liquidus of the join acmite-nepheline-silica;instead, a broad field of hematite is present and acmite crystallizesonly from liquids containing potential sodium silicate. Consequently,the oversaturated and undersaturated eutectics, correspondingto granitic and nepheline syenitic liquids, are rich in sodiumsilicate and distinct from those found in Petrogeny's Residuasystem: the temperatures of the eutectics are 7285C and 7155C, respectively. Survival of peralkaline granite in the aluminouscontinental crust can be explained by the strongly peralkalinecomposition of the oversaturated eutectic. Magma of this typemay be the primitive granite of the non-orogenic zones. Theubiquitous alkali metasomatism around alkaline complexes canalso be interpreted in terms of residual liquids enriched inalkali silicates. Transition from undersaturated to oversaturatedliquids is possible by fractionation of hematite and a new processfor achieving the reverse transition has been found. This dependson the substitution of Fe3 for Al3 in feldspar and suggestsa more important role for syenite in any scheme of petrogenesis. Each of the two eutectics is linked to a corresponding peritecticat which hematite reacts to give acmite. The liquid at the undersaturated,quaternary reaction point is of ijolitic type, providing thefirst intimation that ijolite may represent a low-melting fractionin nature. The system Na2O–Al2O3–Fe2O3–SiO2thus constitutes the peralkaline residua system and on thisbasis a coherent picture of stable continental magmatism canbe constructed. Ijolite is seen as the low-melting fractionfrom a range of peralkaline compositions and from rocks suchas melilite basalt, while the frequently associated carbonatiteis considered to be the volatile-rich, fugitive material fromthe mantle. Such a relationship is consistent with the dualassociation of carbonatite with either ijolite or kimberliteunder different tectonic conditions. The more common syenite,nepheline syenite, and alkaline granite of the non-orogenicregions are regarded as low-melting fractions from basalticmaterials in the deep crust. Most of this activity, involvingmagmas of residual type, could thus be explained in terms ofpartial melting in the deep crust and upper mantle. A possiblemechanism for this would be arching of the rigid continentalcrust, the consequent relief of lithostatic load giving riseto melting, and the concentration of fugitive constituents,in the underlying zones. 相似文献
20.
We investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically. Our results show that, in a convergent tectonic setting, pressure can be considerably higher than lithostatic (the pressure resulting from the weight of the overburden). Increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads, the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root, respectively. The contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle, where rocks are thought to deform brittlely. In these domains, pressure gradients twice lithostatic can be achieved. The contribution of horizontal stresses is less important in the ductile domains as differential stresses are progressively relaxed; nevertheless, the effects are still noteworthy especially close to the brittle–ductile transition. Flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust. As a result of the combination of the two mechanisms, the pressure gradient varies vertically through the lithosphere, ranging from negative (inverted) gradients to gradients up to several times the lithostatic gradient. The pressure values range from one to two times the lithostatic values (1ρ gz to 2ρ gz ). 相似文献