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1.
R. Altherr U. Henes-Klaiber E. Hegner M. Satir C. Langer 《International Journal of Earth Sciences》1999,88(3):422-443
Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from
a subduction to a collisional setting. For ∼362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial
isotopic compositions (εNd=+3.4 to +3.8;87Sr/86Sr =0.7035–0.7053;δ18O=6.8–8.0‰) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial
melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the
central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional
setting at approximately 340–335 Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both
suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures (εNd=0.0 to –2.5;87Sr/86Sr=0.7044–0.7056) but different oxygen isotopic compositions (δ18O=7.3–8.7‰ in mafic vs 9.3–9.5‰ in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest
an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives
of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic
intrusives of the southern unit III have initial isotopic compositions (εNd=–1.1 to –1.8;87Sr/86Sr =0.7054–0.7062;δ18O=7.2–7.6‰) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched
mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of
garnet-rich metaluminous crustal source rocks or may represent hybrid magmas.
Received: 7 December 1998 / Accepted: 27 April 1999 相似文献
2.
Middle to Late Jurassic plutonic rocks in the central Mojave Desert represent the continuation of the Sierran arc south of
the Garlock fault. Rock types range from calc-alkaline gabbro to quartz monzonite. Chemical and isotopic data indicate that
petrologic diversity is attributable to mixing of crustal components with mantle melts. Evidence for magma mixing is scarce
in most plutons, but emplacement and injection of plutons into preexisting wallrocks (e.g. pendants of metasedimentary rocks)
suggests that assimilation may be locally important. Field and petrographic evidence and major and trace element data indicate
that the gabbros do not represent pure liquids but are, at least partly, cumulates. The cumulate nature of the gabbros coupled
with field evidence for open-system contamination means that trace element contents of gabbros cannot be used to fingerprint
the Jurassic mantle source, nor can isotopic data be unequivocally interpreted to reflect the isotopic composition of the
mantle. Correlation of Sr and Nd isotropic composition with bulk composition allows some constraints to be placed on the mantle
isotopic signature. Gabbros and mafic inclusions from localities north of Barstow, CA have the most depleted mantle-like isotopic
signatures (Sr
(
i
)≈0.705 and ɛNd
(t)=≈0 to +1). However, these rocks have likely seen some contamination as well, so the mantle source probably has an even more
depleted character. Gabbros with the lowest Sr(
i
) and highest ɛNd
(t) are also characterized by the highest 207Pb/204Pb and 206Pb/204Pb in the entire data set. This may be a feature of the mantle component in the Jurassic arc indicative of minor source contamination
with subducted sediment as has been observed in modern continental arcs. Locally exposed Precambrian basement and metasedimentary
rocks have appropriate Sr, Nd and Pb isotopic signatures for the crustal end members and are possible contaminants. Incorporation
of these components through combined anatexis and assimilation can explain the observed spread in isotopic composition. Evidence
for a depleted mantle component in these gabbros contrasts with the enriched subcontinental mantle component in Jurassic arc
plutons further to the east and suggests there may have been a major mantle lithosphere boundary between the two areas as
far back as the Late Jurassic. Crustal boundaries and isotopic provinces defined on the basis of initial isotopic composition
(Sr(
i
)=0.706 isopleth) are difficult to delineate because of the correlation of bulk composition with Sr and Nd isotopic composition
and because values may differ depending on the age of the rocks sampled within a given area. Data from plutons intruded into
rocks known or inferred to be Precambrian are, however, shifted dramatically (highest Sr(
i
) and lowest ɛNd(t)) toward Precambrian values. The least isotopically evolved rocks (lowest Sr(
i
) and highest ɛNd(t)) occur within the eugeoclinal belt of the Mojave Desert. This zone has been previously identified as a Precambrian rift zone
but more likely represents a zone where mantle magmas have been intruded into isotopically similar crustal rocks of the eugeocline
with minor input from old Precambrian crust.
Received: 12 August 1993/Accepted: 8 July 1994 相似文献
3.
Yan-Jie Tang Hong-Fu Zhang Eizo Nakamura Ji-Feng Ying 《Contributions to Mineralogy and Petrology》2011,161(6):845-861
Elemental and Li–Sr–Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing
models for Li isotopic fractionation in mantle peridotites and constrain the melt/fluid-peridotite interaction in the lithospheric
mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx
and olivine (<4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral
δ7Li vary greatly, with olivine (+0.7 to +5.4‰) being isotopically heavier than coexisting opx (−4.4 to −25.9‰) and cpx (−3.3
to −21.4‰) in most samples. The δ7Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances,
likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine δ7Li of −3.0 and −7.9‰, indicating the existence of low δ7Li domains in the mantle, which could be transient and generated by meter-scale diffusion of Li during melt/fluid-peridotite
interaction. The 143Nd/144Nd (0.5123–0.5139) and 87Sr/86Sr (0.7018–0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in 87Sr/86Sr and slightly higher in 143Nd/144Nd than coexisting opx, implying an intermineral Sr–Nd isotopic disequilibrium. This is observed more apparently in peridotites
having low 87Sr/86Sr and high 143Nd/144Nd ratios than in those with high 87Sr/86Sr and low 143Nd/144Nd, suggesting that a relatively recent interaction existed between an ancient metasomatized lithospheric mantle and asthenospheric
melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to
the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the lithospheric
mantle represented by the peridotites has been heterogeneously refertilized by multistage melt/fluid-peridotite interactions. 相似文献
4.
The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed
a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones.
In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly
enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429–0.70564) and lower 143Nd/144Nd(ɛNd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched
mantle source; the high positive ɛNd(T) and low 87Sr/86Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted
mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly
different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different
thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction
magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin
belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in
the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time. 相似文献
5.
A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks
from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy
and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite,
anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ
18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ
Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have
isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease
in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from
anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt
from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically,
the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ
18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors. 相似文献
6.
From basalt to dacite: origin and evolution of the calc-alkaline series of Salina, Aeolian Arc, Italy 总被引:1,自引:1,他引:0
The island of Salina comprises one of the most distinct calc-alkaline series of the Aeolian arc (Italy), in which calc-alkaline,
high-K calc-alkaline, shoshonitic and leucite-shoshonitic magma series are developed. Detailed petrological, geochemical and
isotopic (Sr, Nd, Pb, O) data are reported for a stratigraphically well-established sequence of lavas and pyroclastic rocks
from the Middle Pleistocene volcanic cycle (430–127 ka) of Salina, which is characterized by an early period of basaltic volcanism
(Corvo; Capo; Rivi; Fossa delle Felci, group 1) and a sequence of basaltic andesites, and andesites and dacites in the final
stages of activity (Fossa delle Felci, groups 2–8). Major and trace element compositional trends, rare earth element (REE)
abundances and mineralogy reveal the importance of crystal fractionation of plagioclase + clinopyroxene + olivine/ orthopyroxene ± titanomagnetite ± amphibole ± apatite
in generating the more evolved magma types from parental basaltic magmas, and plagioclase accumulation in producing the high
Al2O3 contents of some of the more evolved basalts. Sr isotope ratios range from 0.70410 to 0.70463 throughout the suite and show
a well-defined negative correlation with 143Nd/144Nd (0.51275–0.51279). Pb isotope compositions are distinctly radiogenic with relatively large variations in 206Pb/204Pb (19.30–19.66), fairly constant 207Pb/204Pb (15.68–15.76) and minor variations in 208Pb/204Pb ratios (39.15–39.51). Whole-rock δ18O values range from +6.4 to +8.5‰ and correlate positively with Sr isotope ratios. Overall, the isotopic variations are correlated
with the degree of differentiation of the rocks, indicating that only small degrees of crustal assimilation are overprinting
the dominant evolution by crystal–liquid fractionation (AFC-type processes). The radiogenic and oxygen isotope composition
of the Salina basalts suggests derivation from primary magmas from a depleted mantle source contaminated by slab-derived fluids
and subducted sediments with an isotopic signature of typical upper continental crust. These magmas then evolved further to
andesitic and dacitic compositions through the prevailing process of low-pressure fractional crystallization in a shallow
magma reservoir, accompanied by minor assimilation of crustal lithologies similar to those of the Calabrian lower crust.
Received: 29 November 1999 / Accepted: 16 April 2000 相似文献
7.
Xiba Granitic Pluton in the Qinling Orogenic Belt,Central China: Its Petrogenesis and Tectonic Implications 总被引:4,自引:0,他引:4
ZHANG Fan LIU Shuwen CHEN Xu LI Qiugen DAI Junzhi YANG Kai WU Fenghui CHEN Youzhang 《《地质学报》英文版》2012,86(5):1128-1142
Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U–Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite (218±1 Ma). The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with (La/Yb)N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 (mean= 0.97). Most of these samples from Xiba granitic pluton exhibit εNd(t) values of ?8.79 to ?5.38, depleted mantle Nd model ages (TDM) between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios (87Sr/86Sr)i from 0.7061 to 0.7082, indicating a possible Meso- to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher (87Sr/86Sr)i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested that the granitoids are most likely products in a post-collision tectonic setting. 相似文献
8.
Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley
– Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as
emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite
and the Raztocna diorite. The 87Sr/86Sr(0) ratios vary between 0.7075 and 0.7118, the ε Nd(0) values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and
a lower crustal source.
Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper
mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts
of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper
crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material. 相似文献
9.
Julian F. Menuge 《Contributions to Mineralogy and Petrology》1988,98(3):363-373
Sm-Nd and Rb-Sr isotopic analyses of charnockitic migmatite, augen gneiss, anorthosite-leuconorite and two acid plutons from the Rogaland and Vest-Agder districts of southwest Norway constrain their crustal residence ages, origin and evolution. The charnockitic migmatites, which are a major component of the metamorphic basement complex, represent the oldest and largest episode of accretion, in which new crust was derived 1.5–1.9 Ga ago from a mantle source of depleted Nd isotopic composition. The basement complex was intruded by a number of large anorthositic to granitic plutons during and after the Sveconorwegian orogenic period. Samples from the ca. 1050 Ma old, synorogenic Håland anorthosite-leuconorite massif exhibit substantial variation of initial
Nd of +2.1 to +4.4 at an anorthosite locality and –0.5 to +2.3 at a leuconorite locality, but display significant variation of initial 87Sr/86Sr ratio only between the localities (anorthosite mean=0.70369, leuconorite mean=0.70560). A model is proposed whereby the anorthosite and leuconorite were derived by major crustal contamination of, and fractional crystallization from, a picritic magma derived from isotopically-depleted mantle. Two younger acid intrusions, the 950 Ma old Lyngdal granodiorite and the 930 Ma old Farsund charnockite, both have initial Sr and Nd isotope ratios consistent with massive contamination of depleted-mantle-derived magma by old continental crustal material. 相似文献
10.
The Tasmanian dolerites, part of the Ferrar Province of Australia and Antarctica, have some trace‐element and isotopic compositions that suggest continental contamination of mantle‐derived magmas. The debate has centred on whether the contamination occurred during intrusion into the crust, or if the mantle source itself was contaminated. The behaviour of Sr and O isotopes suggests that the mantle source had a δ18O composition of +6‰ and an initial 87Sr/86Sr ratio of 0.709, which supports the latter contention. Recently published Re–Os data likewise dismiss upper crustal contamination: Re–Os isotopic compositions of magnetite‐rich mineral separates from seven Tasmanian dolerites yield an isochron that gives the same age, within uncertainties, as other dating techniques, namely 175 ± 5 Ma. Moreover, Re–Os data from a study of Antarctic Ferrar Province samples lie on the same isochron and the data together give an age of 177.3 ± 3.5 Ma. The initial 187Os/188Os of 0.125 ± 0.033 is the calculated mantle composition at the time. These results support previous models that attribute chemical features of the Ferrar magmas to re‐enrichment of a depleted mantle source region rather than processes involving assimilation of crust by basaltic magma. 相似文献
11.
V. S. Shatsky E. Jagoutz N. V. Sobolev O. A. Kozmenko V. S. Parkhomenko M. Troesch 《Contributions to Mineralogy and Petrology》1999,137(3):185-205
Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these
rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks.
The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the
upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of
the Zerenda series have very radiogenic lead isotopes. The measured μ values (238U/204Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give
a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago.
Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths.
The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma.
This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle.
We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes.
We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system
during very high grade metamorphic events.
Received: 14 August 1998 / Accepted: 1 June 1999 相似文献
12.
阿尔泰造山带岩石和矿石的氩同位素研究 总被引:1,自引:0,他引:1
通过对新疆阿尔泰造山过程中形成的岩石、矿石样品氩同位素组成的研究 ,认为阿尔泰造山带的形成与壳幔相互作用有关 ,造山过程中的地幔脱气现象是存在的。喀拉通克铜镍硫化物矿床的成矿物质主要来自于地幔。可可托海 3 脉伟晶岩型超大型稀有金属矿床的成矿物质及基性超基性岩的母岩浆主要也来自于地幔。而大喀拉苏花岗岩结晶分异型伟晶岩矿床与库威变质型伟晶岩矿床的成矿物质来自于地壳 ,将军山天河石花岗岩与富蕴县城附近的含铜石英脉也主要是地壳物质演化的结果 相似文献
13.
Mesoproterozoic olivine gabbronorites of the Bashkirian anticlinorium,the South Urals: Parental melts and specifics of magma evolution 总被引:1,自引:0,他引:1
A. A. Nosova L. V. Sazonova V. M. Gorozhanin O. F. Kuz’menkova E. O. Dubinina 《Petrology》2010,18(1):50-83
This paper is devoted to detailed study of picritic rocks (olivine melanogabbronorites) and comagmatic gabbrodolerites from
sills and dikes in the central part of the Bashkirian meganticlinorium. These rocks are ascribed to the Kama-Belsk magmatic
province (KBP) that was formed in the eastern East European Platform (EEP) in the Mesoproterozoic time. The study of minerals
(EMPA, SIMS), rocks, and their oxygen isotope compositions showed the contribution of crustal contamination, fractional crystallization
and cumulus processes in their formation. The geochemical indicators of crustal contamination (Nb/Nb*, (Nb/La)
n
, δ18O, and others) show strong variations, which indicates uneven crustal contribution in the parental melts during rock formation
(10–25%). The study of weakly contaminated (δ18O = 5.3‰) olivine melanogabbronorites (MgO = 22.55 wt %) from the small Ishlya-1 subvolcanic body, which contain subordinate
amount of cumulus (24%), high-magnesian olivine (Fo91.3), and high-Cr spinel (cr# 0.67), as well as HREE depleted clinopyroxenes, allowed us to retrieve the composition of parental
melt. The latter contained about 20 wt % MgO and was formed by 19–26% melting of mantle source (potential mantle temperature
T
m of 1530–1545°C).
Geochemical characteristics of KBP reflect the formation of primary melts by melting of mantle column at different depths,
mixing of the melts, and significant contamination by crustal material. The dominant role in the formation of the rocks of
the Ishlya area and Mashak Complex was played by derivatives of spinel peridotites, while the rocks of the Bakal-Satka area
were derived from garnet peridotites. 相似文献
14.
Jin-Hui Yang Fu-Yuan Wu Simon A. Wilde Lie-Wen Xie Yue-Heng Yang Xiao-Ming Liu 《Contributions to Mineralogy and Petrology》2007,153(2):177-190
In situ zircon U–Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the
Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis
of granites. The zircon U–Pb age of the enclaves (120 ± 1 Ma) is identical to that of the host monzogranite (120 ± 1 Ma),
establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [ε
Hf(t) = +4.5 to −6.2] is distinct from the host monzogranite [ε
Hf(t) = −15.1 to −25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle
component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data
can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wall-rock contamination
is indicated by inherited zircons with considerably older U–Pb ages and low initial Hf isotopic compositions. Hafnium isotopic
variations in Early Cretaceous zircons rule-out simple crystal–liquid fractionation or restite unmixing as the major genetic
link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled
with assimilation of wall rocks, is compatible with the data.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. 相似文献
15.
Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group 总被引:4,自引:0,他引:4
Alan D. Brandon Peter R. Hooper Gordon G. Goles Richard St J. Lambert 《Contributions to Mineralogy and Petrology》1993,114(4):452-464
Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307–0.70371, Nd=+7.7-+4.8, 18O=+5.6±6.1, and 206Pb/204Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high Nd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like 18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and 18O, lower Nd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.Deceased 相似文献
16.
Nd and Sr isotopic data are presented for the 2449–2441 Ma Olanga and Burakovka layered mafic complexes in the eastern Baltic
Shield. These complexes have similar tectonic position, but differ in two aspects: the age of the enclosing crust and the
post-crystallization metamorphic history. The Sm–Nd isotopic results for the Kivakka and Lukkulaisvaara intrusions, Olanga
Complex, are consistent with the model of closed-system crystallization of a single magma without significant wallrock assimilation.
The Rb–Sr systems of minerals were disturbed by late Rb addition during 1.75–1.50 Ga metamorphism. The Nd and Sr isotopic
systems in the Burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct
components. Isotopic variations in the Burakovka Complex can be explained by a 4–20 per cent contamination of a primary komatiitic
or picritic magma with a Mesoarchean crust, similar to that exposed in the region. A similar model, applied to the Olanga
Complex using a Neoarchean crustal isotopic composition, cannot reproduce the observed isotopic signature. The nearly uniform
initial ɛNd values between −1 and −2.3, observed in the Kivakka and Lukkulaisvaara intrusions of the Olanga Complex, as well as in the
other 2.50–2.44 Ga layered mafic intrusions throughout the eastern Baltic Shield, are better explained by a mantle plume model
with small amounts of crustal contamination and minor involvement of asthenospheric material. This model is also consistent
with the geological observations and the temporal distribution of the Paleoproterozoic mafic magmatism in the eastern Baltic
Shield. As an alternative, the enriched isotopic characteristics may be explained by melting of a metasomatically modified
lithospheric mantle source.
Received: 4 August 1994/Accepted: 5 April 1996 相似文献
17.
J. W. Valley J. S. Lackey A. J. Cavosie C. C. Clechenko M. J. Spicuzza M. A. S. Basei I. N. Bindeman V. P. Ferreira A. N. Sial E. M. King W. H. Peck A. K. Sinha C. S. Wei 《Contributions to Mineralogy and Petrology》2005,150(6):561-580
Analysis of δ18O in igneous zircons of known age traces the evolution of intracrustal recycling and crust-mantle interaction through time.
This record is especially sensitive because oxygen isotope ratios of igneous rocks are strongly affected by incorporation
of supracrustal materials into melts, which commonly have δ18O values higher than in primitive mantle magmas. This study summarizes data for δ18O in zircons that have been analyzed from 1,200 dated rocks ranging over 96% of the age of Earth. Uniformly primitive to mildly
evolved magmatic δ18O values are found from the first half of Earth history, but much more varied values are seen for younger magmas. The similarity
of values throughout the Archean, and comparison to the composition of the “modern” mantle indicate that δ18O of primitive mantle melts have remained constant (±0.2‰) for the past 4.4 billion years. The range and variability of δ18O in all Archean zircon samples is subdued (δ18O(Zrc)=5–7.5‰) ranging from values in high temperature equilibrium with the mantle (5.3± 0.3‰) to slightly higher, more evolved
compositions (6.5–7.5‰) including samples from: the Jack Hills (4.4–3.3 Ga), the Beartooth Mountains (4.0–2.9 Ga), Barberton
(3.5–2.7 Ga), the Superior and Slave Provinces (3.0 to 2.7 Ga), and the Lewisian (2.7 Ga). No zircons from the Archean have
been analyzed with magmatic δ18O above 7.5‰. The mildly evolved, higher Archean values (6.5–7.5‰) are interpreted to result from exchange of protoliths with
surface waters at low temperature followed by melting or contamination to create mildly elevated magmas that host the zircons.
During the Proterozoic, the range of δ18O(Zrc) and the highest values gradually increased in a secular change that documents maturation of the crust. After ∼1.5 Ga,
high δ18O zircons (8 to >10‰) became common in many Proterozoic and Phanerozoic terranes reflecting δ18O(whole rock) values from 9 to over 12‰. The appearance of high δ18O magmas on Earth reflects nonuniformitarian changes in the composition of sediments, and rate and style of recycling of surface-derived
material into magmas within the crust.
Electronic Supplementary Material Supplementary material is available for this article at 相似文献
18.
Y. Be’eri-Shlevin Y. Katzir J. Blichert-Toft I. C. Kleinhanns M. J. Whitehouse 《Contributions to Mineralogy and Petrology》2010,160(2):181-201
Multi-isotope study including whole-rock Nd–Sr, single zircon Hf, and SIMS δ18O analyses of zircons sheds light on magma sources in the northernmost Arabian–Nubian Shield (ANS) during ~820–570 Ma. Reconnaissance
initial Nd and Sr isotope data for the older rocks (~820–740 Ma) reaffirms previous estimates that early crustal evolution
in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed
by post-collisional calc-alkaline and alkaline igneous rocks of ~635–570 Ma across a NW-SE transect across basement of the
Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower εNd(T)–εHf(T) and higher
Sri and δ18O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios
are independent of the extent of magma fractionation, and zircon cores and rims yield similar δ18O values. Comparison with southern segments of the ANS shows that the source for most ~635–570 Ma rocks can be modeled as
the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins
(NW-region). Nevertheless, Nd–Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region
displaying the most enriched Nd–Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the
SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1–1.2 Ga and were followed
by crustal differentiation starting at ~0.9 Ga. 相似文献
19.
Brian R. Jicha Garret L. Hart Clark M. Johnson Wes Hildreth Brian L. Beard Steven B. Shirey John W. Valley 《Contributions to Mineralogy and Petrology》2009,157(2):189-207
Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and
its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources,
a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns
for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade
arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators
of slab-fluids such as (Sr/P)
n
and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle
source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification
is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most
Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and
isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass
of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization
in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher
than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ
18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding
involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope
compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile
arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. 相似文献
20.
Magma sources for Mesozoic anorogenic granites of the White Mountain magma series,New England,USA 总被引:21,自引:0,他引:21
The magma sources for granitic intrusions related to the Mesozoic White Mountain magma series in northern New England, USA, are addressed relying principally upon Nd isotopes. Many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts. Sm–Nd and Rb–Sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 Ma. The possibility of crustal sources is further examined with analyses of representative older crust including Paleozoic granitoids which serve as probes of the lower crust in the region. Multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations (<1 in
Nd). Overall, Mesozoic granite plutons range in
Nd [T] from +4.2 to -2.3, with most +2 to 0, and in initial 87Sr/86Sr from 0.7031 to 0.709. The isotopic variations are roughly inversely correlated but are not obviously related to geologic, geographic, or age differences. Older igneous and metamorphic crust of the region has much lower Nd isotope ratios with the most radiogenic Paleozoic granitoid at
Nd [180 Ma] of -2.8. These data suggest mid-Proterozoic separation of the crust in central northern New England. Moreover, the bulk of the Mesozoic granites cannot be explained as crustal melts but must have large mantle components. The ranges of Nd and Sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources. Crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation. The results indicate production of anorogenic granites from mantle-derived mafic magmas. 相似文献