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1.
The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4. 相似文献
2.
《Chemie der Erde / Geochemistry》2016,76(1):77-93
The Mid to Late Miocene intraplate alkaline volcanic suites of western Bohemia are relict of the intensive voluminous volcanism accompanied by large-scale uplift and doming. The association with the uplift of the NE flank of the Cheb–Domažlice Graben (CDG) is uncertain in view of the mostly transpressional tectonics of the graben. The volcanism is most probably of the Ohře/Eger Rift off-rift settings. Two cogenetic volcanic suites have been recognised: (i) silica-saturated to oversaturated consisting of olivine basalt–trachybasalt-(basaltic) trachyandesite–trachyte–rhyolite (13.5 to 10.2 Ma) and (ii) silica-undersaturated (significantly Ne-normative) (melilite-bearing) olivine nephelinite–basanite–tephrite (18.3 to 6.25 Ma). A common mantle source is suggested by similar primitive mantle-normalised incompatible element patterns and Sr–Nd–Pb isotopic compositions for the assumed near-primary mantle-derived compositions of both suites, i.e., olivine basalt and olivine nephelinite. Apparently, they were generated by different degrees of partial melting of a common mantle source, with garnet, olivine and clinopyroxene in the residuum. Negative Rb and K anomalies indicate a residual K-phase (amphibole/phlogopite) and melting of partly metasomatised mantle lithosphere. The evolution of the basanite–olivine basalt–trachybasalt-(basaltic) trachyandesite–trachyte–rhyolite suite suggests the presence of an assimilation–fractional crystallization process (AFC). Substantial fractionation of olivine, clinopyroxene, Fe–Ti oxide, plagioclase/alkali feldspar and apatite accompanied by a significant assimilation of magma en route by crustal material is most evident in evolved member, namely, trachytes and rhyolites. The magmas were probably sourced by both sub-lithospheric and lithospheric partly metasomatised mantle. The evolution of the (melilite-bearing) olivine nephelinite–basanite–tephrite suite is less clear because of its limited extent. Parental magma of both these rock suites is inferred to have originated by low-degree melting of the mantle source initiated at ca. 18 Ma and reflects mixing of asthenosphere-derived melts with isotopically enriched lithospheric melts. The older Oligocene alkaline rocks (29–26 Ma) occur within the Cheb–Domažlice Graben (CDG) locally but are significant in the closely adjacent neighbouring western Ohře Rift. The Sr–Nd–Pb isotopic composition of primitive volcanic rocks of both suites is similar to that of the European Asthenospheric Reservoir (EAR). Initial Pb isotopic data plot partly above the northern hemisphere reference line at radiogenic 206Pb/204Pb ratios of ∼19 to 20, and indicate the presence of a Variscan crustal component in the source. 相似文献
3.
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
4.
T. Radhakrishna D. G. Pearson John Mathai 《Contributions to Mineralogy and Petrology》1995,121(4):351-363
Agali–Coimbatore dolerite dykes constitute an important Proterozoic magmatic event that affected the south Indian shield.
Rb-Sr whole rock isotope data yield an “errorchron” of 2369±400 Ma (2σ error) which is within error of the reported 2030±65 Ma
K-Ar age. The dyke magmas were evolved Fe-rich tholeiitic melts produced by fractionation of clinopyroxene, orthopyroxene
and olivine in the initial stages. Plagioclase became a fractionation phase during the latter stages of crystallization. The
dykes characteristically have high 87Sr/86Sri (0.703–0.706) and are enriched in large-ion lithophile and light rare earth elements relative to primordial mantle values
and show negative Nb anomalies. These compositional characteristics are interpreted as source mantle characteristics whereas
some crustal effects are visible in some samples with high initial 87Sr/86Sr. Peridotite with minor hydrous metasomatic phases like amphibole (and phlogopite) within the shallow lithospheric mantle
could be a potential source material for the dykes. However, at this stage we cannot convincingly differentiate whether the
source of the parent magmas is solely lithospheric or a product of asthenosphere-lithosphere mixing. The δ18O values of the dykes range from +5.2 to +7.2 per mil (vs standard mean oceanic water). Initial Nd isotope values at the time
of dyke intrusion (ɛNd at t=2.0 Ga) range from −2.3 to −4.8. Whole rocks define a correlation on an Sm-Nd isochron plot with a slope equivalent to an
age of 3.15±0.53 Ga (2σ error); Sm-Nd crustal residence ages average at 2.87 Ga. The isochron age does not appear to be the
result of systematic mixing with an older crustal component. These results together with trace element geochemistry suggest that the south Indian
mantle lithosphere developed by addition of enriched melts/fluids at about 3.0 Ga synchronously with major crustal gene- ration
in the south Indian shield.
Received 20 June 1994/Accepted: 17 May 1995 相似文献
5.
The western Anatolian volcanic province formed during Eocene to Recent times is one of the major volcanic belts in the Aegean–western
Anatolian region. We present new chemical (whole-rock major and trace elements, and Sr, Nd, Pb and O isotopes) and new Ar/Ar
age data from the Miocene volcanic rocks in the NE–SW-trending Neogene basins that formed on the northern part of the Menderes
Massif during its exhumation as a core complex. The early-middle Miocene volcanic rocks are classified as high-K calc-alkaline
(HKVR), shoshonitic (SHVR) and ultrapotassic (UKVR), with the Late Miocene basalts being transitional between the early-middle
Miocene volcanics and the Na-alkaline Quaternary Kula volcanics (QKV). The early-middle Miocene volcanic rocks are strongly
enriched in large ion lithophile elements (LILE), have high 87Sr/86Sr(i) (0.70631–0.71001), low 143Nd/144Nd(i) (0.512145–0.512488) and high Pb isotope ratios (206Pb/204Pb = 18.838–19.148; 207Pb/204Pb = 15.672–15.725; 208Pb/204Pb = 38.904–39.172). The high field strength element (HFSE) ratios of the most primitive early-middle Miocene volcanic rocks
indicate that they were derived from a mantle source with a primitive mantle (PM)-like composition. The HFSE ratios of the
late Miocene basalts and QKV, on the other hand, indicate an OIB-like mantle origin—a hypothesis that is supported by their
trace element patterns and isotopic compositions. The HFSE ratios of the early-middle Miocene volcanic rocks also indicate
that their mantle source was distinct from those of the Eocene volcanic rocks located further north, and of the other volcanic
provinces in the region. The mantle source of the SHVR and UKVR was influenced by (1) trace element and isotopic enrichment
by subduction-related metasomatic events and (2) trace element enrichment by “multi-stage melting and melt percolation” processes
in the lithospheric mantle. The contemporaneous SHVR and UKVR show little effect of upper crustal contamination. Trace element
ratios of the HKVR indicate that they were derived mainly from lower continental crustal melts which then mixed with mantle-derived
lavas (~20–40%). The HKVR then underwent differentiation from andesites to rhyolites via nearly pure fractional crystallization
processes in the upper crust, such that have undergone a two-stage petrogenetic evolution. 相似文献
6.
Mengist Teklay Erik E. Scherer Klaus Mezger Leonid Danyushevsky 《Contributions to Mineralogy and Petrology》2010,159(5):731-751
The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting
to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts
from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic
composition (87Sr/86Sr = 0.70336–0.70356, ε
Nd = +6.6 to +7.0, and ε
Hf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair
islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths
vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, ε
Nd = −8.1 to +2.5, and ε
Hf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield
and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks.
Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or
hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment
of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with
their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that
crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the
host basalts. 相似文献
7.
The Late Paleozoic volcanic and sedimentary rocks are widespread in the North Tianshan along the north margin of the Yili block. They consist of basalt, basaltic andesite, andesite, trachyandesite, dacite, rhyolite, tuff, and tuffaceous sandstone. According to zircon sensitive high-resolution ion microprobe (SHRIMP) dating, the age of the Late Paleozoic volcanic rocks in Tulasu basin in western part of North Tianshan is constrained to be Early Devonian to Early Carboniferous (417–356 Ma), rather than Early Carboniferous as accepted previously. Geochemical characteristics of the Early Devonian to Early Carboniferous volcanic rocks are similar to those of arc volcanic rocks, which suggest that these volcanic rocks could be the major constituents of a continental arc formed by the southward subduction of North Tianshan Oceanic lithosphere. Geochemical studies indicate that the magma source of the volcanic rocks might be the mantle wedge mixed with subduction fluid, which is geochemically enriched than primitive mantle but depleted than E-MORB. The calculation shows that the basalt could be formed by ∼10% partial melting of subduction fluid modified mantle wedge. Andesites with high initial 87Sr/86Sr (0.7094–0.7104) and negative εNd(t) (−4.45 to −4.79) values reveal the contribution of continental crust to its source. The calculation of assimilation–fractional crystallization (AFC) shows that the fractional crystallization process of the basaltic magma, which was accompanied with assimilation by different degree of continental crust, produced andesite (7–9%), dacite (∼12%) and rhyolite (>20%). 相似文献
8.
Peter Sprung Stephan Schuth Carsten Münker Leonore Hoke 《Contributions to Mineralogy and Petrology》2007,153(6):669-687
The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes
of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic
evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative
intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between
“HIMU-like” end member compositions (206Pb/204Pb: 20.57, 207Pb/204Pb: 15.77, 87Sr/86Sr: 0.7030, εHf: + 3.8, εNd: + 4.2), compositions tending towards MORB (206Pb/204Pb: 19.01, 207Pb/204Pb: 15.62, 87Sr/86Sr: 0.7028, εHf: + 9.9, εNd: + 7.0) and compositions reflecting the influence of subducted sediments (206Pb/204Pb: 18.99, 207Pb/204Pb: 15.67, 87Sr/86Sr: 0.7037, εHf: + 4.4, εNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data
set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic 206Pb/204Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting
column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting
depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the
sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric
origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting
depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving
upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of
fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled
by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension.
Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old
Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities
between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model
proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks
in former East Gondwana.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
9.
Mei-Fu Zhou Jun-Hong Zhao Liang Qi Wenchao Su Ruizhong Hu 《Contributions to Mineralogy and Petrology》2006,151(1):1-19
Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China.
The 258±3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260±3 Ma sills consist of undifferentiated
diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti
group. Rocks of the high-Ti group have FeO, TiO2 and P2O5 higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial 87Sr/86Sr ratios (0.706–0.707) lower and ɛNd (−1.5 to −0.6) higher than the low-Ti equivalents (0.710–0.715 and −9.6 to −4.0, respectively).
The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 –9.5%) of partial melting
of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like,
lithospheric mantle source. The mafic rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume
at ∼260 Ma. 相似文献
10.
Jiang-Feng Qin Shao-Cong Lai Chun-Rong Diwu Yin-Juan Ju Yong-Fei Li 《Contributions to Mineralogy and Petrology》2010,159(3):389-409
Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major
and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its
mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data
suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived
from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental
setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively.
The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element
and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and εNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive εHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the
Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent
fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of
adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90),
suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and εNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55),
Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite
show negative εHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive εHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains
the Neoproterozoic juvenile crust that has the positive εHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between
the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally
at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of
the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic
response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks. 相似文献
11.
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
12.
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
13.
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
14.
Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China 总被引:1,自引:1,他引:1
Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust. 相似文献
15.
Summary The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although
calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and
space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment
in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic
rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can
be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and
a silica undersaturated trend, charactrerised by leucite-bearing rocks.
Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively.
206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general εSr vs. εNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important
role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the
magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional
crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of εSr vs. εNd is still observed, with εSr from −2 to +180 and εNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic
leucite-bearing Roman Province rocks buffered at εSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We
may argue that mafic leucite-bearing Roman Province rocks point to εSr and εNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point
to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic
sediments as well, but bifurcation between the samples north and south of Rome is observed.
The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature
acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric
mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised.
The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly
due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might
have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one
might reflect the activity of mostly CO2-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks.
Received February 16, 2000; revised version accepted September 6, 2001 相似文献
16.
Julien Berger Renaud Caby Jean-Paul Liégeois Jean-Claude C. Mercier Daniel Demaiffe 《Contributions to Mineralogy and Petrology》2011,162(4):773-796
We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower
and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90–130-Ma-long evolution of magmatic inputs
and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost
mantle depths (25–30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source
slightly enriched by slab-derived sedimentary components ((La/Sm)N < 1; εNd: +5.4–6.2; 87Sr/86Sr: 0.7027–0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850–1,000°C;
10–12 kbar) and subject to local dehydration–melting reactions, forming trondhjemititic leucosomes with garnet–clinopyroxene–rutile
residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment
faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg
andesitic parental magma. Quartz and hornblende-gabbros (700–660 Ma) with composition typical of hydrous volcanic rocks from
mature arcs ((La/Sm)N: 0.9–1.8; εNd: +4.6 to +5.2; 87Sr/86Sr: 0.7028–0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that
magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred
either in two stages (700–660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T
path (550°C, 6–9 kbar; epidote–amphibolite and greenschist facies conditions) during the main Pan-African collision event
(620–580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation
are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and
Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong
to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent,
a similar story having occurred on the other side of the Sahara with the Mozambique Ocean. 相似文献
17.
Massimo Chiaradia Othmar Müntener Bernardo Beate Denis Fontignie 《Contributions to Mineralogy and Petrology》2009,158(5):563-588
In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon
continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to
east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc),
and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted
either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional
crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb)
data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic
centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the
frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related
geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile
elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively
lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe
widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or
amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center,
rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes
are small and their values are overall rather primitive (e.g., εNd = +1.5 to +6, 87Sr/86Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation.
We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers
characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as
by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian
rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts,
rather than of slab melts and slab melt–mantle interactions. The isotopic signatures of the least evolved adakite-like rocks
of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting
that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a
consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated
with subduction of the aseismic Carnegie Ridge. 相似文献
18.
Bin Chen K. Suzuki W. Tian B. M. Jahn T. Ireland 《Contributions to Mineralogy and Petrology》2009,158(5):683-702
We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China
craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric
zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance
of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease
in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different
rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (εNd(t) = −8 to −15), but are homogeneous in ISr ratios (0.7054–0.7066). The (187Os/188Os)i ratios are found to be anti-correlated with εNd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal
contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels.
The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction
zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along
a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the
NCC at ca. 280 Ma. 相似文献
19.
Stefanie S. Schmidberger Ernst Hegner 《Contributions to Mineralogy and Petrology》1999,135(4):373-385
Late Carboniferous (300–290 Ma) calc-alkaline basalts, andesites, and rhyolites typical of volcanic arc settings occur in
the intermontane Saar-Nahe basin (SW Germany) within the Variscan orogenic belt. The volcanic rock suite was emplaced under
a regime of tensional tectonics during orogenic collapse and its origin has been explained by melting of mantle and crust
in the course of limited lithospheric rifting. We report major, trace and rare-earth-element data (REE), and Nd-Pb-Sr-O isotope
ratios for a representative sample suite, which are fully consistent with an origin closely related to plate subduction. Major
and trace element data define continuous melt differentiation trends from a precursor basaltic magma involving fractional
crystallization of olivine, pyroxene, plagioclase, and magnetite typical of magma evolution in a volcanic arc. This finding
precludes an origin of the andesitic compositions by mixing of mafic and felsic melts as can be expected in anorogenic settings.
The mafic samples have high Mg numbers (Mg# = 65–73), and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating
derivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical
characteristics of the near-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment
in the large ion lithophile elements (LILE), negative Nb and Ti, and positive Pb anomalies relative to the neighboring REE,
suggesting melting of a subduction-modified mantle. Initial Nd values of −0.7 to −4.6, Pb, and 87Sr/86Sr(t) isotope ratios for mafic and felsic volcanics are similar and indicate partial melting of an isotopically heterogeneous and
enriched mantle reservoir. The enrichment in incompatible trace elements and radiogenic isotopes of a precursor depleted mantle
may be attributed to addition of an old sedimentary component. The geochemical characteristics of the Saar-Nahe volcanic rocks
are distinct from typical post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate
subduction at the margin of the Variscan orogenic belt not obvious from the regional geologic context.
Received: 3 August 1998 / Accepted: 2 January 1999 相似文献
20.
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