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1.
P. B. Tomascak Eirik J. Krogstad Richard J. Walker 《Contributions to Mineralogy and Petrology》1996,125(1):45-59
Neodymium and lead isotope and elemental data are presented for the Sebago batholith (293±2 Ma), the largest exposed granite
in New England. The batholith is lithologically homogeneous, yet internally heterogeneous with respect to rare earth elements
(REE) and Nd isotopic composition. Two-mica granites in the southern/central portion of the batholith (group 1) are characterized
by REE patterns with uniform shapes [CeN/YbN (chondrite normalized) = 9.4–19 and Eu/Eu* (Eu anomaly) = 0.27–0.42] and ɛ
Nd(t) = −3.1 to −2.1. Peripheral two-mica granites (group 2), spatially associated with stromatic and schlieric migmatites,
have a wider range of total REE contents and patterns with variable shapes (CeN/YbN = 6.1–67, Eu/Eu* = 0.20–0.46) and ɛ
Nd(t) = −5.6 to −2.8. The heterogeneous REE character of the group 2 granites records the effects of magmatic differentiation
that involved monazite. Coarse-grained leucogranites and aplites have kinked REE patterns and low total REE, but have Nd isotope
systematics similar to group 2 granites with ɛ
Nd(t) = −5.5 to −4.7. Rare biotite granites have steep REE patterns (CeN/YbN = 51–61, Eu/Eu* = 0.32–0.84) and ɛ
Nd(t) = −4.6 to −3.8. The two-mica granites have a restricted range in initial Pb isotopic composition (206Pb/204Pb = 18.41–18.75; 207Pb/204Pb = 15.60–15.68; 208Pb/204Pb = 38.21–38.55), requiring and old, high U/Pb (but not Th/U) source component. The Nd isotope data are consistent with magma
derivation from two sources: Avalon-like crust (ɛ
Nd>−3), and Central Maine Belt metasedimentary rocks (ɛ
Nd<−4), without material input from the mantle. The variations in isotope systematics and REE patterns are inconsistent with
models of disequilibrium melting which involved monazite.
Received: 8 December 1995 / Accepted: 29 April 1996 相似文献
2.
Nd Isotopic Composition and Material Source of Pre—and Post—Sinian Sedimentary Rocks in Xiushui Area,Jiangxi Province 总被引:4,自引:1,他引:4
Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area,Jiangxi Province.Significant differences are noticed between them in their Nd isotopic dompositions.As for the pre-Sinian lightly metmorphozed sedimentary rocks,^143Nd/^144Nd=0.512000-0.512214,CNd(T)=-8.04-9.99,and TDM=18332426Ma are suggested for the Sinian-Cambrian sedimentary rocks .These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period.Mantle material ap-pears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust. 相似文献
3.
S. B. Lobach-Zhuchenko V. M. Savatenkov A. V. Kovalenko V. P. Chekulaev N. S. Guseva 《Geochemistry International》2010,48(4):366-380
This work considers geochemical and isotopic characteristics of the source of the Archean Panozero pluton derived from LILEand
LREE-enriched lithospheric mantle. Sr and Nd isotopic data on clinopyroxenes and augites define a source with Sri = 0.7017 and ɛNd(t) varying within a narrow range from + 0.7 to + 1.4 (averaging + 1.1), which is close to previously obtained whole-rock
isotopic data. Similar ɛNd(t) were obtained for the Archean alkaline rocks of Canada, whereas the Archean mafic rocks of the Baltic and Canadian Shields
formed from depleted mantle have ɛNd(t) ∼2. Lead isotope measurements on K-feldspars (KFsp) and monzonite showed that the source of the pluton has μ = 8.98 for the Stacey-Kramers two-stage model, at low U/Pb and
high Th/U ratios. Different lead isotope composition corresponding to μ = 10.43 was determined in KFsp from quartz monzonites. Diverse interpretations of obtained data have been proposed. It was noted that the Pb-Pb isotopic
system was disturbed by a later (∼ 1.9 Ga) thermal event. The ratios of elements of similar compatibility were used to determine
the geochemical specifics of source of the Panozero pluton. Their comparison with numerous literature data on metasomatized
mantle xenoliths and minerals in them showed that the mantle source strongly differed from primitive mantle in ratios of elements,
whose mineralmelt partitioning coefficients considerably differs from mineral-fluid partitioning, for instance, Nb/La. Mantle
source that was responsible for geochemical peculiarities of the Panozero pluton was made up of Phl, CPx, and Ap. 相似文献
4.
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
5.
R. Tribuzio M. F. Thirlwall B. Messiga 《Contributions to Mineralogy and Petrology》1999,136(1-2):48-62
The origin of the Sondalo gabbroic complex has been unravelled by means of a petrological study of the least evolved rocks,
troctolites to norites containing up to 20% of anhedral clinopyroxene and titanian pargasite. Pyroxenes and titanian pargasite
from the troctolites have higher Mg, Al and Cr, and lower Mn than those from the norites, whereas plagioclase does not show
systematic compositional variations (An ca. 65 mol%). The variation trend of anorthite content of plagioclase versus the forsterite
content of olivine differs from that of arc-related gabbroic rocks. Plagioclase, clinopyroxene, orthopyroxene and titanian
pargasite were analyzed for REE and selected trace elements by ion microprobe. Application of crystal/liquid partition coefficients
to trace element mineral compositions suggests that the parental liquids of both troctolites and norites had tholeiitic affinity
and were slightly LREE and LILE enriched relative to N-MORB. A troctolite and a norite give Sm-Nd mineral isochron ages of
300 ± 12 Ma and 280 ± 10 Ma. Plagioclase-amphibole Rb-Sr isochron ages are 266 ± 10 Ma and 269 ± 16 Ma for the same rock samples,
and they are interpreted to represent cooling ages. The Nd-Sr-O isotopic compositions indicate that a substantial crustal
contribution was involved in the petrogenesis of the norite, which has low ɛNd(290 Ma), high 87Sr/86Sr(290 Ma) and high δ18OPx (−2.6, 0.7057 and +7.9‰, respectively) compared with the troctolites. We thus conclude that the troctolite/norite association
formed by concomitant fractional crystallization and crustal assimilation. The somewhat elevated δ18OCpx (+6.4‰) and the relatively low forsterite contents in olivine suggest that the parental liquids of the troctolites had already
been evolved through an AFC type process. The ɛNd(290 Ma) and 87Sr/86Sr(290 Ma) of these rocks (ranging from +2.8 to +4.4 and from 0.7037 to 0.7040, respectively) probably do not reflect the
ɛNd and Sr isotopic compositions of their mantle source, and it is thus unclear whether the primary melts were derived from a
slightly enriched or from a depleted mantle source. The Sondalo gabbroic complex was most likely associated with the post-Variscan
gabbroic complexes of the Alpine belt. These gabbroic complexes can be ascribed to the intrusion at different crustal levels
of tholeiitic mantle-derived melts and were emplaced in the time span of 300–270 Ma.
Received: 14 September 1998 / Accepted: 4 January 1999 相似文献
6.
Shuan-Hong Zhang Yue Zhao Alfred Kröner Xiao-Ming Liu Lie-Wen Xie Fu-Kun Chen 《International Journal of Earth Sciences》2009,98(6):1441-1467
Recent zircon dating identified several late Carboniferous to early Permian hornblende gabbro–diorite–quartz diorite–granodiorite–tonalite–granite
plutons in lithological assemblages at the northern margin of the North China Block (NCB) that were previously regarded as
Archaean to Palaeoproterozoic. Our geochronological results indicate that emplacement of these plutons was a continuous process
during the late Carboniferous to early Permian, from 324 ± 6 to 274 ± 6 Ma, and lasted for at least 50 Ma. In this paper,
the early Permian components with compositions from gabbro to granite within the intrusive complex were studied. The early
Permian plutons exhibit calc-alkaline or high-K calc-alkaline, metaluminous geochemical features and highly variable SiO2 contents. They have no significant Eu anomaly in their REE patterns, and in primitive-mantle-normalized spidergrams they
display depletion in Th, U, Nb, Ta, P and Ti, and enrichment in Ba, K, Pb and Sr. The granitoid bodies within these plutons
display I-type and adakitic geochemical signatures. The early Permian rocks exhibit low whole-rock initial 87Sr/86Sr ratios from 0.70520 to 0.70615 and have negative whole-rock ε
Nd(t) values ranging from −17.4 to −9.3 and zircon ε
Hf(t) values of −23.2 to −10.5. The gabbros exhibit higher ε
Nd(t) values from −11.1 to −9.3 and ε
Hf(t) values from −16.5 to −10.5, and one granodiorite exhibits an even lower ε
Nd(t) value of −17.4 and zircon ε
Hf(t) values of −23.2 to −15.1. Geochemical, Sr–Nd and in situ zircon Hf isotopic compositions suggest that the hornblende gabbros
were derived from a metasomatized lithospheric mantle, and the diorite and quartz diorite were generated from a gabbroic magma
by fractional crystallization, coupled with differential assimilation of ancient lower crustal material. The granodiorite
was likely derived from partial melting of ancient lower crust with involvement of some mantle components. Involvement of
both lithospheric mantle and ancient lower crust in the generation of the early Permian plutons indicates strong crust–mantle
interaction in the northern NCB. Petrological associations as well as geochemical and Sr–Nd–Hf isotopic results show that
the early Permian plutons were emplaced along an Andean-type active continental margin during southward subduction of the
Palaeo-Asian oceanic plate beneath the NCB. Integration of our results with previously published data for late Carboniferous
and late Permian to middle Triassic intrusions suggests that the continental arc on the northern margin of the NCB existed
for at least 50 Ma during the late Palaeozoic, and final amalgamation of the Mongolian arc terranes with the northern NCB
likely occurred during a period from ~270 to ~250 Ma, i.e, in the late Permian to earliest Triassic. 相似文献
7.
Laicheng Miao Weiming Fan Dunyi Liu Fuqin Zhang Yuruo Shi Feng Guo 《Journal of Asian Earth Sciences》2008,32(5-6):348
The Hegenshan ophiolite in the Inner Mongolian-Daxinganling Orogenic Belt (IMDOB), northern China, consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros, mafic lavas and dikes, intruded by younger granodiorite dikes. The ultramafic rocks are highly depleted, serpentinized harzburgites with minor dunite, characterized by relative enrichment in large ion lithophile elements (LILE, e.g., Ba and Rb) and light rare earth elements (LREE). They are interpreted to be oceanic mantle that has undergone extensive melt extraction and variable degrees of metasomatism. The cumulate rocks consist mainly of gabbro and troctolite with LREE-depleted chondrite-normalized REE patterns showing significant positive Eu anomalies. They are enriched in LILE, depleted in Nb, and have high positive εNd(t) (+8 to +11), suggesting derivation from a subduction-modified N-MORB-like source. The gabbros and mafic dikes have essentially the same age (295 ± 15 and 298 ± 9 Ma, respectively). The mafic dikes have flat to LREE-depleted, chondrite-normalized REE patterns, are depleted in Nb, enriched in LILE and have N-MORB-type Nd isotopic signatures (εNd(t) = +8.1 to +10). The mafic lavas, erupted at 293 ± 1 Ma, can be divided into two groups; one composed of strongly deformed metabasalts similar in chemical and Nd–Sr isotopic compositions to the mafic dikes, and the other composed of undeformed and unmetamorphosed basalts with oceanic island basalt (OIB)-like trace element signatures and Nd isotopic compositions. The granodiorite dikes, which were intruded at 244 ± 4 Ma, have LREE-enriched, chondrite-normalized REE patterns with no Eu anomalies. Their abnormally high εNd(t) values (+6.3 to +6.8) and low ISr (0.70412 to 0.70450) suggest formation from melts derived from thickened oceanic crust during or shortly after closure of the Paleo-Asian Ocean. The structure, lithology and geochemistry of the Hegenshan ophiolite suggest that it is a Cordilleran-type body formed in a supra-subduction zone (SSZ) environment and amalgamated by collision of several fragments of Paleo-Asian lithosphere. Final emplacement and amalgamation occurred in the latest Permian or earliest Triassic. 相似文献
8.
The Bancun diabase dyke and the Bali hornblende gabbro dyke in western Fujian Province were emplaced in the Early and Late Cretaceous periods, respectively; the former is designated to calc-alkaline series and the latter to K-high-calc-alkaline rock series. Both the dykes are characterized by such geochemical characteristics as high Al and Na2O>K2O. As for the Bancun dyke, Al2O3=16.32%–17.54% and K2O/Na2O=0.65–0.77; as for the Bali dyke, Al2O3=16.89%–17.81% and K2O/Na2O=0.93–0.99. Both the Bancun and Bali mafic dykes are relatively enriched in LILE and LREE, but depleted in HSFE, displaying the geochemical characteristics of continental marginal arc, with high initial Sr isotopic ratios and low εNd (t) values. The (87Sr/86Sr)i ratios of the Bancun diabase dyke are within the range of 0.708556–0.70903 and their εNd (t) values vary between -6.8 and -6.3; those of the Bali hornblende dyke are within the range of 0.710726–0.710746 and their εNd (t) values are -4.7– -4.9, showing the characteristics of enriched mantle EM II. The isotope and trace element data showed that the mafic dykes have not experienced obvious crustal contamination, and metasomatism caused by subduction fluids is the main factor leading to LILE and LREE enrichments. The enriched mantle is the source region for the mafic dykes, and mixing of subduction fluid metasomatized enriched mantle and EM II-type mantle constituted the mantle source region of both the Bancun and Bali mafic dykes. Upwelling of the asthenosphere mantle provided sufficient heat energy for the generation of magmas. In accordance with the discrimination diagram of their tectonic settings as well as their trace element geochemical characteristics, it is considered that the dykes both at Bancun and Bali possess the characteristics of continental marginal arc, revealing the tectonic environment of formation of the mafic dykes, the continental dynamic background as an intraplate tensional belt in which the mafic dykes were emplaced. Meanwhile, it is also indicated that the tensional tectonic stress mechanism is responsible for the formation of the mafic dykes in western Fujian Province. 相似文献
9.
S. N. Rudnev A. E. Izokh V. P. Kovach R. A. Shelepaev L. B. Terent’eva 《Petrology》2009,17(5):439-475
The paper presents data on the structure, composition, and age of granitoid associations (Tokhtogeshil’skii Complex) composing
the Kharanur and Sharatologoi polychronous plutons in the northern part of the Ozernala zone in western Mongolia. The Tokhtogeshil’skii
Complex was determined to consist of a number of independent magmatic associations, which were formed at 540–450 Ma, within
three age intervals (540–520, 510–485, and 475–450 Ma), have different composition, were derived from different sources, and
were emplaced in different geodynamic environments. During the first, island-arc stage (540–520 Ma), high-Al plagiogranites
were produced, which belong to tonalite-plagiogranite (531 ± 10 Ma) and diorite (529 ±6 Ma) associations in the Kharanur pluton,
low-Al plagiogranites of the tonalite-plagiogranite association (519 ± 8 Ma) in the Sharatologoi pluton, and rocks of the
Khirgisnur peridotite-pyroxenite-gabbronorite complex (Kharachulu and Dzabkhan massifs). The rocks of the diorite and plagiogranite
associations of the Kharanur pluton have ɛNd(T) from +7.9 to +7.4, TNd(DM) = 0.65 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039. The plagiogranites of the Sharatologoi pluton (tonalite-plagiogranite association) are characterized by
ɛNd(T) from +6.5 to +6.6, TNd(DM) = 0.73–0.70 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039, which suggest predominantly juvenile subduction sources of the parental melts at a subordinate role of ancient
crustal material. During the second, accretionary stage (510–485 Ma), low-Al plagiogranites of the diorite-tonalite-plagiogranite
association of the Sharatologoi pluton (494 ± 10 Ma, M type) were formed. The Sr-Nd isotopic characteristics of these rocks
ɛNd(T) = +6.6, (87Sr/86Sr)0 = 0.7039 are analogous to those of the plagiogranitoids of the early type. This suggests that the melted sources were similar
in composition. During the third, postcollisional stage (475–450 Ma), rocks of the diorite-granodiorite-granite association
were formed (459 ± 10 Ma, type I) in the Kharanur pluton. These rocks have ɛNd(T) = +5.1, TNd(DM) = 0.74 Ga, and (87Sr/86Sr)0 = 0.7096. The parental melts were supposedly derived by means of partial melting of “the Caledonian” juvenile crust with
the addition of more ancient crustal material. 相似文献
10.
《地学前缘(英文版)》2020,11(6):2271-2286
In this study we present new mineral chemistry, whole-rock geochemical and zircon U–Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet (China). Modal analyses of these dykes gave averages of ~40%–65% plagioclase and ~35%–60% amphibole and small amounts of (igneous) clinopyroxene, epidote and opaque minerals. This mineral assemblage resembles that of typical orthoamphibolites. Nevertheless, due to the absence of foliation the investigated rocks are described as metamafic lithologies. These rocks have primitive mantle (PM)-normalized multi-element patterns with negative Nb and Ta anomalies as well as weak, negative Ti anomalies. In addition, they have initial 87Sr/86Sr ratios [(87Sr/86Sr)i] of 0.702844–0.703581, initial 143Nd/144Nd ratios [(143Nd/144Nd)i] of 0.512891–0.512959 and high εNd(t) values (+7.9 to +9.3). Uranium-Pb ages of magmatic zircons separated from the investigated metamafic dykes indicate that the parental melts of their protoliths intruded the Sangsang mantle at ~119.0–118.5 Ma.The metamorphic mineral assemblages recognized in the investigated dykes are suggestive of a retrograde metamorphic process, from (epidote-)amphibolite facies (~470–610 °C, ~1.9–4.3 kbar) and to prehnite-pumpellyite facies (≤280 °C, ≤ 3 kbar), active within a rift-produced oceanic lithosphere. Microtextural and geochemical data suggest that the protoliths of the dykes were most likely massive gabbros. Compositional data show that the parental magmas of the gabbroic protoliths were generated by melting of a depleted mantle (DM) source that had been weakly modified by fluids emanating from a subducted oceanic lithospheric slab. The age of the gabbroic protoliths is slightly younger than the existing ages for ophiolites from the central Yarlung-Zangbo Suture Zone (YZSZ) in the literature (~129–123 Ma). We, therefore, suggest that the gabbroic protoliths of the Sangsang metamafic dykes were formed in an incipient forearc setting during Neo-Tethyan subduction re-initiation (Aptian). Our tectonomagmatic model provides insights into the igneous accretion and post-solidification evolution of the oceanic lithosphere in South Tibet. 相似文献
11.
The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-arenaceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [ high Th/Sc (0.57-3.59) , La/Sc ( 1.46 - 12.4), La/Yb (5.84 - 19.0) ] and variable Th/U ratios, with ∑REE = 129-296μg/g, δEu =0.51 -0.86, and (La/Yb)N = 3.95 -12.9. The Nd isotopic model ages tDM of these rocks vary from 1597 to 2124 Ma. Their 143 Nd/144 Nd values are low [εNd (0) = - 11.4 to -- 15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and Krich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199 ± 26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model age tDM ( 1597 - 2124Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic ( 1100 - 1600 Ma). 相似文献
12.
Geochronological and geochemical study of mafic dykes from the northwest Chinese Altai: Implications for petrogenesis and tectonic evolution 总被引:4,自引:0,他引:4
Keda Cai Min Sun Chao Yuan Guochun Zhao Wenjiao Xiao Xiaoping Long Fuyuan Wu 《Gondwana Research》2010,18(4):638-652
Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean 206Pb/238U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO2 contents (45.2–52.7 wt.%) and total alkaline (K2O + Na2O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/YbN = 0.86–1.1), together with their high εNd(t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/YbN = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low εNd(t) values (+ 3.4 to + 5.4) and high initial 87Sr/86Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give εHf(t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai. 相似文献
13.
G. V. Ovchinnikova D. P. Krylov I. K. Kozakov V. P. Kovach N. A. Sergeeva 《Petrology》2009,17(6):570-578
Newly obtained data on Pb isotopic ratios in feldspars from Early Proterozoic granitoids of the Sangilen block of the Tuva-Mongolian
microcontinent and Caledonian structures surrounding it, considered together with earlier data on the O and Nd isotopic systems,
indicate that ancient (approximately 2 Ga) crustal material mixed in the sources of granites of the Tuva-Mongolian microcontinent
with younger juvenile material. Positive ɛNd values of granitoids from the Bashkimugor and Chgargalant massifs are accounted for by processes of crustal contamination
during the interaction of the melt with crustal material. Similar Nd isotopic characteristics of granitoids in the Khoromnug
pluton were caused by the melting of the Late Riphean crust. In granitoids of the massifs in the Kaakhem and Eastern Tannuola
zones of the surrounding Caledonian structures, the involvement of juvenile material from oceanic crust increases away from
the Sangilen block. Granites in the junction zone between the Tuva-Mongolian microcontinent and surrounding structures display
evidence of the presence (up to 10–20%) of an ancient crustal component, and the melting history of granitoids in the Eastern
Tannuola zone is dominated by an Early Paleozoic juvenile component in combination with material similar to the Vendian ophiolites
of the Agardag-Erzin zone. An increase in the δ18O value, the 206Pb/204Pb ratio, and the TNd(DM) values within a single complex (from older to younger granitoid phases) is explained by the systematic involvement of
crustal material in the melting processes. 相似文献
14.
U. K. Pandey D. V. L. N. Sastry B. K. Pandey Madhuparna Roy T. P. S. Rawat Rajeeva Ranjan V. K. Shrivastava 《Journal of the Geological Society of India》2012,79(1):30-40
The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140
Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the
time range of Bijawar sedimentation may be assigned as 2.1–1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (TDM), obtained, for Dargawan gabbros, is c. 2876–3145 Ma. High initial 87Sr/ 86Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ɛNdi (at 1.9 Ga) value of −1.5 to − 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source
magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying
sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high
initial 87Sr/86Sr ratio of 0.7098 and ɛNdi value of −3.5 to −3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These
dolerites have younger Sm-Nd Model ages (TDM) than Dargawan gabbros as c. 2462–2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments
to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives
rise to high initial 87Sr/86Sr and low initial 143Nd/144 ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of
Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic
setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum
time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora
Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently
classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic
age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup)
indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive
providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation
and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source
of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium
mineralisation although indirectly. 相似文献
15.
Granitoids from the central Mawson Escarpment (southern Prince Charles Mountains, East Antarctica) range in age from Archaean
to Early Ordovician. U–Pb dating of zircon from these rocks indicates that they were emplaced in three distinct pulses: at
3,519 ± 20, 2,123 ± 12 Ma and between 530 and 490 Ma. The Archaean rocks form a layer-parallel sheet of limited extent observed
in the vicinity of Harbour Bluff. This granitoid is of tonalitic-trondhjemitic composition and has a Sr-undepleted, Y-depleted
character typical of Archaean TTG suites. εNd and TDM values for these rocks are −2.1 and 3.8 Ga, respectively. Subsequent Palaeoproterozoic intrusions are of granitic composition
(senso stricto) with pronounced negative Sr anomalies. These rocks have εNd and TDM values of −4.8 and 2.87 Ga, indicating that these rocks were probably melted from an appreciably younger source than that
tapped by the Early Archaean orthogneiss. The remaining intrusions are of Early Cambrian to Ordovician age and were emplaced
coincident with the major orogenic event observed in this region. Cambro–Ordovician intrusive activity included the emplacement
of layer-parallel pre-deformational granite sheets at approximately 530 Ma, and the intrusion of cross cutting post-tectonic
granitic and pegmatitic dykes at ca. 490 Ma. These intrusive events bracket middle- to upper-amphibolite facies deformation
and metamorphism, the age of which is constrained to ca. 510 Ma—the age obtained from a syn-tectonic leucogneiss. Nd–Sr isotope
data from the more felsic Cambro–Ordovican intrusions (SiO2 > 70 wt%), represented by the post-tectonic granite and pegmatite dykes, suggest these rocks were derived from Late Archaean
or Palaeoproterozoic continental crust (TDM ∼ 3.5–2.3 Ga, εNd ∼ −21.8 to −25.9) not dissimilar to that tapped by the Early Proterozoic intrusions. In contrast, the compositionally more
intermediate rocks (SiO2 < 65 wt%), represented by the metaluminous pre-tectonic Turk orthogneiss, appear to have melted from a notably younger lithospheric
or depleted mantle source (TDM = 1.91 Ga, εNd ∼ −14.5). The Turk orthogneiss additionally shows isotopic (low 143Nd/144Nd and low 87Sr/86Sr) and geochemical (high Sr/Y) similarities to magmas generated at modern plate boundaries—the first time such a signature
has been identified for Cambrian intrusive rocks in this sector of East Antarctica. These data demonstrate that: (1) the intrusive
history of the Lambert Complex differs from that observed in the adjacent tectonic provinces exposed to the north and the
south and (2) the geochemical characteristics of the most mafic of the known Cambrian intrusions are supportive of the notion
that Cambrian orogenesis occurred at a plate boundary. This leads to the conclusion that the discrete tectonic provinces observed
in the southern Prince Charles Mountains were likely juxtaposed as a result of Early Cambrian tectonism. 相似文献
16.
17.
Jos María Viramonte Raúl Alberto Becchio Jos Germn Viramonte Marcio Martins Pimentel Roberto Donato Martino 《Journal of South American Earth Sciences》2007,24(2-4):167-183
New field, petrological, geochemical, and geochronological data (U–Pb and Sm–Nd) for Ordovician rock units in the southeastern Puna, NW Argentina, indicate two lithostratigraphic units at the eastern–northeastern border of salar Centenario: (1) a bimodal volcanosedimentary sequence affected by low- to medium-grade metamorphism, comprising metasediments associated with basic and felsic metavolcanic rocks, dated 485 ± 5 Ma, and (2) a plutonic unit composed of syenogranites to quartz-rich leucogranites with U–Pb zircon ages between 462 ± 7 and 475 ± 5 Ma. Felsic metavolcanic and plutonic rocks are peraluminous and show similar geochemical differentiation trends. They also have similar Sm–Nd isotopic compositions (TDM model ages of 1.54–1.78 Ga; εNd(T) values ranging from −3.2 to −7.5) that suggest a common origin and derivation of the original magmas from older (Meso-Paleoproterozoic?) continental crust. Mafic rocks show εNd(T) ranging from +2.3 to +2.5, indicating a depleted mantle source. The data presented here, combined with those in the literature, suggest Ordovician magmatism mainly recycles preexisting crust with minor additions of juvenile mantle-derived material. 相似文献
18.
H. Voshage J. C. Hunziker A. W. Hofmann A. Zingg 《Contributions to Mineralogy and Petrology》1987,97(1):31-42
The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with
607
Nd
=+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by phlogopite metasomatism. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial
293
Nd
value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic crustal residence ages of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago. 相似文献
19.
Two distinct crustal provinces have been identified in the southern mid-continent based on U–Pb crystallization ages. Both
contain large volumes of undeformed granite and rhyolite, with minor amounts of metamorphic rock and mafic intrusions. The
Eastern Granite-Rhyolite province is characterized by felsic rocks with crystallization ages of 1,470 ± 30 Ma and exposures
are restricted to the St. Francois Mountains in southeastern Missouri. Similarly, the Southern Granite-Rhyolite (SGR) province
is characterized by felsic units with ages of 1,370 ± 30 Ma with primary exposures in the eastern Arbuckle Mountains of southern
Oklahoma. Within the SGR province three magmatic pulses can be identified starting at 1,400, 1,370, and 1,340 Ma. Although
the crystallization ages are different, the Sm–Nd isotopic signatures are similar for the units exposed in these areas as
well as the buried basement in between. Depleted mantle model ages for rocks within the Arbuckle Mountains range from 1,530
to 1,430 Ma with ɛNd(t) values of +3.2 to +4.1 while units of the St. Francois Mountains range from 1,550 to 1,430 Ma and +4.5 to +4.7. Comparison
of Sm–Nd isotopic data also indicate similarities between the 147Sm/144Nd and 143Nd/144Nd ratios for the rock units in these areas suggesting a common source. 相似文献
20.
The paper presents results of petrochemical, geochemical, and isotope-geochemical study of the Patchemvarek and Severnyi gabbroanorthosite
massifs of the Kola Peninsula. It was shown that the rocks of these massifs differ from the gabbroanorthosite massifs of the
Neoarchean Keivy-Kolmozero Complex in the more calcic composition (70–85% An) of normative plagioclase, and low contents of
TiO2, FeO, and Fe2O3. In terms of chemical composition, the gabbroanorthosites of the studied massifs are close to the rocks of the Fisken?sset
Complex (Southwestern Greenland) and to the anorthosites of the Vermillion Lake Complex (Canada). U-Pb zircon dating established
Mesoarchean ages of 2925 ± 7 and 2935 ± 8 Ma for the gabbroan-orthosites of the Patchemvarek and Severnyi massifs, respectively.
It was shown that the gabbroanorthosites of the studied massifs have fairly low REE contents (Ce
n
= 2.2−4.2, Yb
n
= 1.6−2.6) and distinct positive Eu anomaly. Comagmatic ultrabasic differentiates have practically unfractionated REE pattern,
low total REE contents (Ce
n
= 1.2, Yb
n
= 1.1, La/Yb
n
= 1.3), and no Eu anomaly. The studied samples of the Archean gabbroanorthosites are characterized by positive εNd = +2.68 for the gabbroanorthosites of the Severnyi Massif and from + 2.77 to + 1.66 for the Patchemvarek Massif. Initial
strontium isotope ratios are 87Sr/86Sr
i
= 0.70204 ± 8 and 87Sr/86Sr
i
= 0.70258 ± 8 for the rocks of the Severnyi and Patchemvarek massifs, respectively. Our study showed that the obtained U-Pb
zircon ages for the gabbroanorthosites of the Patchemvarek and Severnyi massifs represent the oldest date for the Kola peninsula,
thus marking the oldest, Mesoarchean stage in the evolution of region. The differences in the initial 143Nd/144Nd ratios between the Neoarchean gabbroanorthosites of the Keivy-Kolmozero Complex and the Mesoarchean gabbroanorthosites
of the studied massifs suggest the existence of two mantle sources. One of them produced intrusions with an age of 2.67–2.66
Ga, while other was responsible for the formation of massifs with an age of 2.93–2.92 Ga. The composition and temperature
of “parental” melt of the gabbroanorthosites were simulated using COMAGMAT-3.5 program. According to the calculations, the
parental melt represented aluminous basalt, whose differentiation at T = 1280°C and P = 7 kbar at the crust-mantle boundary was accompanied by plagioclase floatation and formation of “crystal mesh” that produced
anorthosite complexes. The gabbroanorthosies of the Patchemvarek and Severnyi massifs were presumably derived from MORB-type
basalts of oceanic settings, while the Tsaga, Achinskii, and other anorthosite massifs of the Neoarchean age were generated
from subalkaline magma formed in within-plate anorogenic setting. Sm-Nd isotope data suggest the existence of several mantle
sources in the Kola region, which produced melts for different-age gabbroanorthosite massifs since Mesoarchean to the middle
Paleoproterozoic. The Archean-Early Proterozoic anorthosite magmatism of the Kola region records a complete cycle (∼ 800 Ma)
of the formation and consolidation of continental block. 相似文献