共查询到20条相似文献,搜索用时 15 毫秒
1.
R. D. Smith K. L. Cameron F. W. McDowell S. Niemeyer D. E. Sampson 《Contributions to Mineralogy and Petrology》1996,123(4):375-389
Isotopic and trace element data from mantle and granulite xenoliths are used to estimate the relative contributions of mantle
and crustal components to a large ignimbrite, referred to as the upper ignimbrite, that is representative of the voluminous
mid-Cenozoic rhyolites of northwestern Mexico. The study also uses data from the volcanic rocks to identify deep crustal xenoliths
that are samples of new crust created by the Tertiary magmatism. The isotopic composition of the mantle component is defined
by mantle-derived pyroxenites that are interpreted to have precipitated from mid-Cenozoic basaltic magmas. This component
has ɛNd≈+1.5, 87Sr/86Sr≈0.7043 and 206Pb/204Pb≈18.6. Within the upper ignimbrite and associated andesitic and dacitic lavas, initial 87Sr/86Sr is positively correlated with SiO2, reaching 0.7164 in the ignimbrite. Initial 206Pb/204Pb ratios also show a positive correlation with silica, whereas ɛNd values have a crude negative correlation, reaching values as low as −2. Of the four isotopically distinct crustal components
identified from studies of granulite xenoliths, only the sedimentary protolith of the paragneiss xenoliths can be responsible
for the high initial 87Sr/86Sr of the upper ignimbrite. The Nd, Sr, and Pb isotopic compositions of the upper ignimbrite can be modeled with relatively
modest assimilation (≤20%) of the sedimentary component ± Proterozoic granulite. Gabbroic composition granulite xenoliths
have distinctive Nd, Sr, and Pb isotope ratios that cluster closely within the range of compositions found in the andesitic
and dacitic lavas. These mafic granulites are cumulates, and their protoliths are interpreted to have precipitated from the
intermediate to silicic magmas at 32–31 Ma. These mafic cumulate rocks are probably representative of much of the deep crust
that formed during mid-Cenozoic magmatism in Mexico. Worldwide xenolith studies suggest that the relatively great depth (≤20
km) at which assimilation-fractional crystallization took place in the intermediate to silicic magma systems of the La Olivina
region is the rule rather than the exception. Oligocene ignimbrites of the southwestern United States (SWUS) have substantially
lower ɛNd values (e.g. <−6) than the upper ignimbrite and other rhyolites from Mexico. This difference appears to reflect a greater
crustal contribution to ignimbrites of the SWUS, perhaps due to a higher temperature of the lower crust prior to the emplacement
of the Oligocene basaltic magmas.
Received: 16 December 1994 / Accepted: 13 September 1995 相似文献
2.
Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada,California 总被引:1,自引:0,他引:1
Gerald K. Van Kooten 《Contributions to Mineralogy and Petrology》1981,76(4):378-385
This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (87Sr/86Sr=0.70597–0.70653; 206Pb/ 204Pb=18.862–19.018; 207Pb/204Pb=15.640–15.686; 208Pb/ 204Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (87Sr/86=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high 87Sr/86Sr (71141–0.71240) and low 206Pb/204Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H2O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas. 相似文献
3.
The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous
diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd
isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite
megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic
megacrysts have very radiogenic 206Pb/204Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an
old crustal component as reflected in the high 207Pb/204Pb ratios of many basalts from the EVP. This inference is supported by 206Pb/204Pb ratios of ∼17.6 to 18.3 and ɛNd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement.
The 87Sr/86Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions
(ɛNd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements
(LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their
Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace
element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts
(OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle.
Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K2O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an
origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest
that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western
Europe.
Received: 9 March 1995/Accepted: 24 July 1995 相似文献
4.
C. R. Paslick Alex N. Halliday R. A. Lange D. James J. B. Dawson 《Contributions to Mineralogy and Petrology》1996,125(4):277-292
Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show
evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all
appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb
isotopic compositions. The 206Pb/204Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria
vary from insignificant in the case of nepheline, to Δ 87Sr/86Sr of 0.0002 and ΔɛNd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that
did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit
none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source
variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas
that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently
erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data
to identify source reservoirs. However, mass balance calculations reveal that the 206Pb/204Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination.
Received: 15 February 1995 / Accepted: 4 May 1996 相似文献
5.
Ignacio S. Torres-Alvarado Surendra P. Verma Gerardo Carrasco-Núñez 《Journal of Earth System Science》2000,109(1):67-78
Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven
different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the
Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times
standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre
Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre
Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos
Plate were also compiled. In the MVB the isotopic ratios range as follows:87Sr/86Sr 0.703003-0.70841;143Nd/144Nd 0.512496-0.513098;206Pb/204Pb 18.567-19.580;207Pb/204Pb 15.466-15.647;208Pb/204Pb 38.065-38.632. The SMO shows a large variation in87Sr/86Sr ranging from ∼0.7033 to 0.71387.143Nd/144Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:206Pb/204Pb 18.060-18.860;207Pb/204Pb 15.558-15.636;208Pb/204Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261
for Nd). Pb isotopes for PacOc show the following range:206Pb/204Pb 18.049-19.910;207Pb/2047Pb 15.425-15.734;208Pb/204Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc.
Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have
been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle
component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data
in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr. 相似文献
6.
Friedrich Lucassen Wolfgang Kramer Viola Bartsch Hans-Gerhard Wilke Gerhard Franz Rolf L. Romer Peter Dulski 《Contributions to Mineralogy and Petrology》2006,152(5):571-589
The Jurassic to Early Cretaceous magmatic arc of the Andes in northern Chile was a site of major additions of juvenile magmas from the subarc mantle to the continental crust. The combined effect of extension and a near stationary position of the Jurassic to lower Cretaceous arc favoured the emplacement and preservation of juvenile magmatic rocks on a large vertical and horizontal scale. Chemical and Sr, Nd, and Pb isotopic compositions of mainly mafic to intermediate volcanic and intrusive rock units coherently indicate the generation of the magmas in a subduction regime and the dominance of a depleted subarc mantle source over contributions of the ambient Palaeozoic crust. The isotopic composition of the Jurassic (206Pb/204Pb: ∼ 18.2; 207Pb/204Pb: ∼ 15.55; 143Nd/144Nd: ∼ 0.51277; 87Sr/86Sr: ∼ 0.703–0.704) and Present (206Pb/204Pb: ∼ 18.5; 207Pb/204Pb: ∼ 15.57; 143Nd/144Nd: ∼ 0.51288; 87Sr/86Sr: ∼ 0.703–0.704) depleted subarc mantle beneath the Central and Southern Andes (18°–40°S) was likely uniform over the entire region. Small differences of isotope ratios between Jurassic and Cenozoic to Recent of subarc mantle-derived could be explained by radiogenic growth in a still uniform mantle source.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at . 相似文献
7.
J. M. Liotard J. M. Dautria D. Bosch M. Condomines H. Mehdizadeh J.-F. Ritz 《International Journal of Earth Sciences》2008,97(1):89-102
The activity of the Damavand volcano (Central Alborz, northern Iran) began 1.8 Ma ago and continued up to 7 ka BP. Although
the volcanic suite is clearly of shoshonitic affinity, only two petrographic types can be distinguished in the studied lavas:
(1) weakly differentiated absarokites (49 < %SiO2 < 51), scattered around the volcano but with a regional extension, (2) highly differentiated banakites (59 < %SiO2 < 63), which form the bulk of the 4,000 m thick volcanic pile. All lavas are alkalic (3.7 < %K2O < 5), REE and LILE-rich (e.g., 85 < La < 148 ppm; 9 < Th < 32 ppm) and show highly fractionated REE patterns (69 < La/Yb < 115)
and pronounced Nb–Ta negative anomalies. The absarokites are characterised by Sr (0.7045–0.7046) and Nd (0.51266–0.51269)
isotope compositions close to the Bulk Earth values, and distinct from those of the banakites (0.7047 < 87Sr/86Sr < 0.7049, 0.51258 < 143Nd/144Nd < 0.51262). The Pb isotope ratios are also slightly lower in the absarokites than in the banakites (18.71 < 206Pb/204Pb < 18.77, 15.62 < 207Pb/204Pb < 15.63, 38.85 < 208Pb/204Pb < 38.91, and 18.77 < 206Pb/204Pb < 18.84, 15.62 < 207Pb/204Pb < 15.64, 38.94 < 208Pb/204Pb < 39.06, respectively). Overall, there is a clear tendency towards higher Sr, Pb and lower Nd isotope ratios with increasing
degree of differentiation. This study suggests that the absarokites result from a low degree of partial melting (∼5%) of a
highly metasomatized mantle source, which inherited its characteristics from an old subduction setting. The initiation of
volcanic activity 1.8 Ma ago results from variations in the lithospheric thermal regime, probably related to lithospheric
delamination as proposed for Anatolia (Pearce et al. 1990). The banakites are mainly generated by extensive fractional crystallisation (∼70%) of the absarokitic magma, with a limited
amount (a few percents) of assimilation of an old crustal component, in the form of bulk assimilation or AFC processes, which
both can explain the Sr, Nd and Pb isotope data. 相似文献
8.
James H. Wittke Douglas Smith Joseph L. Wooden 《Contributions to Mineralogy and Petrology》1989,101(1):57-68
Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (87Sr/86Sr = 0.7052 to 0.7055, Nd= –9.2 to –10.7); alkali olivine basalts (87Sr/ 86Sr = 0.7049 to 0.7054, Nd= –2 to 0.2); basanite and hawaiites (87Sr/86Sr = 0.7049 to 0.7053, Nd= –3.5 to –7.8); and quartz tholeiites (87Sr/86Sr = 0.7047, Nd= –1.4 to –2.6). Post-faulting lavas have lower 87Sr/86Sr (<0.7045) and Nd from –3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible. 相似文献
9.
《Chemical Geology》2003,193(3-4):215-235
Plio–Pleistocene (3.4–0.125 Ma) post-plateau magmatism in the Meseta del Lago Buenos Aires (MLBA; 46.7°S) in southern Patagonia is linked with the formation of asthenospheric slab windows due to ridge collision along the Andean margin ∼6 Ma ago. MLBA post-plateau lavas are highly alkaline (43–49% SiO2; 5–8% Na2O+K2O), relatively primitive (6–10% MgO) mafic volcanics that have strong OIB-like geochemical signatures. Their relatively enriched Sr–Nd isotope ratios (87Sr/86Sr=0.7041–0.7049; 143Nd/144Nd=0.51264–0.51279), low 206Pb/204Pb (18.13–18.45), steep REE patterns (La/Yb=11–54), and low LILE/LREE and LILE/HFSE ratios (Ba/La<15, La/Ta<15, Ba/Ta<180; Sr/La=15–22; Th/La<0.13; Ce/Pb>15) are distinctive from most other Neogene Patagonian slab window lavas. These data are interpreted to indicate contamination of OIB-like asthenosphere-derived slab window magmas with an EM1-type component derived from the Patagonian continental lithospheric mantle (CLM). The EM1-type signature in Patagonian slab window lavas are geographically associated with the Deseado Massif and indicate important regional differences in lithospheric mantle chemistry beneath southern Patagonia. We propose that hot, upwelling subslab asthenosphere in slab window tectonic settings can cause significant thermo-mechanical erosion and thinning of the continental lithospheric mantle and, thus, may be an important process in slab window magma petrogenesis. 相似文献
10.
We report new Os-Pb-Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). 187Os/188Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260-0.1451; 58.9-168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6-15.1 ppt Os). Radiogenic Os is associated with high εHf and εNd, low 87Sr/86Sr and does not correlate with Mg∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic 187Os/188Os (0.4344, 10.3 ppt Os), lower εHf and εNd, higher 87Sr/86Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (εHf: +8.9 to +5.0) and Pb isotope compositions (206Pb/204Pb: 19.10-19.61; 207Pb/204Pb: 15.56-15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse 187Os/188Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3-2.0 ppt). Low-187Os/188Os tholeiites show Pb-Hf isotope ratios (206Pb/204Pb:18.81; 207Pb/204Pb: 15.61; εHf: +2.7) that are distinct from those in alkaline lavas with similar 187Os/188Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low 206Pb/204Pb and εHf in the other tholeiites probably reflects crustal contamination.The association at Vogelsberg of primitive alkaline and tholeiitic lavas with a range of MORB- to OIB-like Os-Pb-Hf-Nd-Sr isotopic characteristics requires at least two asthenospheric magma sources. This is consistent with trace element modelling which suggests that the alkaline and tholeiitic parent magmas represent mixtures of melts from garnet and spinel peridotite sources (both with amphibole), implying an origin of the magmas in the garnet peridotite-spinel peridotite transition zone, probably at the asthenosphere-lithosphere interface. We propose that uncontaminated Vogelsberg lavas originated in ‘metasomatized’ mantle, involving a 3-stage model: (1) early carbonatite metasomatism several 10-100 Ma before the melting event (2) deposition of low-degree asthenospheric melts from carbonated peridotite at the lithosphere-asthenosphere thermal boundary produces hydrous amphibole-bearing veins or patches, and (3) remobilization of this modified lithospheric mantle into other asthenospheric melts passing through the same area later. In keeping with ‘metasomatized’ mantle models for other continental basalt provinces, we envisage that stage (2) is short-lived (few Ma), thus producing a prominent lithospheric trace element signature without changing the asthenospheric isotopic signatures. Models of this type can explain the peculiar mix of lithospheric (prominent depletions of Rb and K) and asthenospheric (OIB-like high 187Os/188Os, 143Nd/144Nd and 176Hf/177Hf) signatures observed in the Vogelsberg and many other continental basalt suites. 相似文献
11.
S. G. Skolotnev A. A. Peyve M. E. Bylinskaya L. A. Golovina 《Doklady Earth Sciences》2017,472(1):20-25
The petrology, geochemistry, and isotope ratios of volcanics dredged during the 43rd cruise of R/V Academik Ioffe on the Bathymetrists Seamounts in the eastern equatorial Atlantic have been studied. These are alkaline volcanics of basic and ultramafic compositions. Spider diagrams of the trace elements of volcanic rocks demonstrate strong fractionation, indicating formation of their primary melts from an enriched mantle source at garnet depth facies. Considering the isotope ratio values of 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr and the character of their variations, the volcanic mantle source was chemically heterogeneous: for various volcanic rocks it was a mixture of the mantle components HIMU with EM–1 or EM–2. Limestones dredged together with the volcanics yielded microfossils suggesting a Middle Eocene age of their formation in a carbonate platform environment. 相似文献
12.
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
13.
Zhaochong Zhang Xuchang Xiao Jun Wang Yong Wang Timothy M. Kusky 《Journal of Asian Earth Sciences》2008,31(4-6):379-403
Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for post-collisional late Cenozoic shoshonitic volcanic rocks from the western Kunlun Mountains, NW China. They are distributed in two approximately E–W striking sub-belts, with the lavas in the southern sub-belt having been generated earlier than those in the northern sub-belt. The mineralogy of the rocks reflects crystallization from moderate temperature magmas (700–1000 °C) with high oxygen and water fugacities. They are geochemically characterized by relatively low TiO2, Al2O3 and FeO and high alkalies coupled with very high contents of incompatible element concentrations. Remarkably negative Nb, Ta and Ti anomalies are displayed on primitive mantle-normalized incompatible element patterns. In addition, they show a relatively broad range of low εNd (−1.8 to −8.7) at more restricted 87Sr/86Sr ratios (0.7081–0.7090). Pb isotopes are characterized by a range of 207Pb/204Pb (15.48–15.74) and 208Pb/204Pb (38.30–39.12) ratios at relatively invariant 206Pb/204Pb (18.60–18.83) values, except one sample with a ratio of 18.262, leading to near-vertical arrays. The lavas from the northern sub-belt have relatively high 87Sr/86Sr ratios. All lavas have extremely high La/Yb ratios, probably reflecting that the magmas were derived from a metasomatized lithospheric mantle source containing phlogopite–hornblende garnet peridotite affected by subducted sediments and hydrous fluids, rather than from a depleted asthenopheric mantle source or mantle plume source. However, the lavas from the southern sub-belt were derived from a lower degree of melting of more highly metasomatized sub-lithospheric mantle in comparison with those from the northern sub-belt. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in the western Kunlun could be a consequence of continuously conductive heating of upwelling, hot asthenospheric mantle following the delamination subsequent to thickening, which is consistent with the spatial and temporal geochemical variations in shoshonitic rocks in Tibet. 相似文献
14.
DING Chengwu DAI Pan BAGAS Leon NIE Fengjun JIANG Sihong WEI Junhao DING Chengzhen ZUO Pengfei ZHANG Ke 《《地质学报》英文版》2016,90(1):106-120
The Hashitu molybdenum deposit is located in the southern part of the Great Hinggan Range,NE China.Molybdenum mineralization is hosted by and genetically associated with monzogranite and porphyritic syenogranite.Sr-Nd-Pb isotopes of the intrusions show that the porphyritic syenogranite has initial~(87)Sr/~(86)Sr ratios of 0.70418-0.70952,ε_(Nd)(t)values of 1.3 to 2.1(t=143Ma),~(206)Pb/~(204)Pb ratios of 19.191-19.573,~(207)Pb/~(204)Pb ratios of 15.551-15.572,and~(208)Pb/~(204)Pb ratios of38.826-39.143.The monzogranite has initial~(87)Sr/~(86)Sr ratios of 0.70293-0.71305,ε_(Nd)(t)values of 1.1 to2.0(t=147 Ma),~(206)Pb/~(204)Pb ratios of 19.507-20.075,~(207)Pb/~(204)Pb ratios of 15.564-15.596,and~(208)Pb/~(204)Pb ratios of 39.012-39.599.The calculated Nd model ages(T_(DM))for monzogranite and porphyritic syenogranite range from 866 to 1121 Ma and 795 to 1020 Ma,respectively.The granitic rocks in the Hashitu area have the same isotope range as granites in the southern parts of the Great Hinggan Range.The isotope composition indicates that these granites are derived from the partial melting of a juvenile lower crust originating from a depleted mantle with minor contamination by ancient continental crust.The integrating our results with published data and the Late Mesozoic regional tectonic setting of the region suggest that the granites in the Hashitu area formed in an intra-continent extensional setting,and they are related to the thinning of the thickened lithosphere and upwelling of the asthenosphere. 相似文献
15.
Signatures of the source for the Emeishan flood basalts in the Ertan area: Pb isotope evidence 总被引:2,自引:0,他引:2
The Emeishan flood basalts can be divided into high-Ti (HT) basalt (Ti/Y>500) and low-Ti (LT) basalt (Ti/Y<500). Sr, Nd isotopic characteristics of the lavas indicate that the LT- and the HT-type magmas originated from distinct mantle sources and parental magmas. The LT-type magma was derived from a shallower lithospheric mantle, whereas the HT-type magma was derived from a deeper mantle source that may be possibly a mantle plume. However, few studies on the Emeishan flood basalts involved their Pb isotopes, especially the Ertan basalts. In this paper, the authors investigated basalt samples from the Ertan area in terms of Pb isotopes, in order to constrain the source of the Emeishan flood basalts. The ratios of 206Pb/204Pb (18.31–18.41), 207Pb/204Pb (15.55–15.56) and 208Pb/204Pb (38.81–38.94) are significantly higher than those of the depleted mantle, just lying between EM I and EM II. This indicates that the Emeishan HT basalts (in the Ertan area) are the result of mixing of EMI end-member and EMII end-member. 相似文献
16.
Temporal variation of isotope and rare earth element abundances in volcanic rocks from Guam: implications for the evolution of the Mariana Arc 总被引:2,自引:0,他引:2
Volcanic rocks exposed on Guam were erupted during the Late Middle Eocene (Facpi Fm.), Late Eocene-Oligocene (Alutom Fm.) and Miocene (Umatac Fm.). Four magma series are recognized: the boninite series (44 m.y.b.p.), the tholeiite and calc-alkaline series, which were erupted along with boninite series lavas at 32–36 m.y.b.p. and high-K lavas of the Umatac Fm. (14 m.y.b.p.). Isotope and and rare earth element (REE) characteristics of the four magma series are distinct. Boninite series lavas have U-shaped REE patterns, relatively low 143Nd/144Nd (0.51294–0.51298), and high 206Pb/204Pb (19.0–19.2). Tholeiite series lavas are LREE (light REE) depleted, and have high 143Nd/144Nd (0.51304–0.51306) and low 206Pb/204Pb (18.4–18.5). Calc-alkaline series lavas have Sr, Nd and Pb isotope ratios similar to tholeiite series lavas, but flat to U-shaped REE patterns. Umatac Fm. lavas are strongly LREE-enriched, and have higher 87Sr/ 86Sr (0.70375–0.70380) and 207Pb/204Pb relative to 206Pb/ 204Pb than Facpi and Alutom Fm. lavas.
Boninite and tholeiite series magmas, erupted in the position of the Palau-Kyushu Ridge, were probably derived from distinct mantle sources having OIB and N-MORB-like isotopic characteristics, together with fluids derived from subducted Pacific plate basalt. Calc-alkaline series lavas were most likely derived from the tholeiite series by extensive crystal fractionation, wallrock contamination and magma mixing. Lavas of the Umatac Fm., erupted in the position of the West Mariana Ridge, may include up to 2–3% subducted sediment, similar to some active Mariana arc lavas. 相似文献
17.
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
18.
V. A. Lebedev I. V. Chernyshev A. V. Chugaev Yu. V. Gol’tsman E. D. Bairova 《Geochemistry International》2010,48(1):41-67
Complex geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus volcano (Greater Caucasus)
experienced long (approximately 200 ka) discrete evolution, with protracted periods of igneous quiescence (approximately 50
ka) between large-scale eruptions. The volcanic activity of Elbrus is subdivided into three phases: MiddleNeopleistocene (225–170
ka), Late Neopleistocene (110–70 ka), and Late Neopleistocene-Holocene (less than 35 ka).
Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that
hybrid parental magmas of the volcano were formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic
upper crustal material of the Greater Caucasus. Mantle reservoir that participated in the genesis of Elbrus lavas as well
as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle “Caucasus” source. Primary
melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics:
87Sr/86Sr = 0.7041 ± 0.0001, ƒNd = +4.1 ± 0.2, 147Sm/144Nd = 0.105–0.114, 206Pb/204Pb = 18.72, 207Pb/204Pb = 15.62, and 208Pb/204Pb = 38.78. The temporal evolution of isotope characteristics for lavas of Elbrus volcano is well described by a Sr-Nd mixing
hyperbole between “Caucasus” source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus.
It was shown that, with time, the proportions of mantle material in the parental magmas of Elbrus gently increased: from ∼60%
at the Middle-Neopleistocene phase of activity to ∼80% at the Late Neopleistocene-Holocene phase, which indicates an increase
of the activity of deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution
of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene-Holocene phase, increasing
contribution of deep-seated mantle source in the genesis of Elbrus lavas with time as deduced from isotope-geochemical data,
as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions
may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity were to continue. 相似文献
19.
Late Cenozoic intraplate basaltic rocks in northeastern China have been interpreted as being derived from a mantle source composed of DMM and EM1 components. To constrain the origin of the enriched mantle component, we have now determined the geochemical compositions of basaltic rocks from the active Baekdusan volcano on the border of China and North Korea. The samples show LREE-enriched patterns, with positive Eu and negative Ce anomalies. On a trace element distribution diagram, they show typical oceanic island basalt (OIB)-like LILE enrichments without significant Nb or Ta depletions. However, compared with OIB, they show enrichments in Ba, Rb, K, Pb, Sr, and P. The Nb/U ratios are generally within the range of OIB, but the Ce/Pb ratios are lower than those of OIB. Olivine phenocrysts are characterized by low Ca and high Ni contents. The radiogenic isotopic characteristics (87Sr/86Sr = 0.70449 to 0.70554; εNd = −2.0 to +1.8; εHf = −1.7 to +6.1; 206Pb/204Pb = 17.26 to 18.12) suggest derivation from an EM1-like source together with an Indian MORB-like depleted mantle. The Mg isotopic compositions (δ26Mg = −0.39 ± 0.17‰) are generally lower than the average upper mantle, indicating carbonates in the source. The 87Sr/86Sr ratios decrease with decreasing δ26Mg values whereas the 143Nd/144Nd and (Nb/La)N ratios increase. These observations suggest the mantle source of the Baekdusan basalts contained at least two components that resided in the mantle transition zone (MTZ): (1) recycled subducted ancient (∼2.2–1.6 Ga) terrigenous silicate sediments, possessing EM1-like Sr–Nd–Pb–Hf isotopic signatures and relatively high values of δ26Mg; and (2) carbonated eclogites with relatively MORB-like radiogenic isotopic compositions and low values of δ26Mg. These components might have acted as metasomatizing agents in refertilizing the asthenosphere, eventually influencing the composition of the MTZ-derived plume that produced the Baekdusan volcanism. 相似文献
20.
Secular evolution of the lithosphere beneath the eastern North China Craton: evidence from Mesozoic basalts and high-Mg andesites 总被引:6,自引:0,他引:6
Hong-Fu Zhang Min Sun Mei-Fu Zhou Jian-Ping Zheng 《Geochimica et cosmochimica acta》2003,67(22):4373-4387
Geochemical and isotopic data from Mesozoic lavas from the Jianguo, Niutoushan, Wulahada, and Guancaishan volcanic fields on the northern margin of the North China Craton provide evidence for secular lithospheric evolution of the region. Jianguo lavas are alkaline basalts with LILE- and LREE-enrichment ((La/Yb)N=12.2-13.2) and MORB-like Sr-Nd-Pb isotopic ratios ((87Sr/86Sr)i<0.704; εNd=3.9-4.8; (206Pb/204Pb)i≈18). Niutoushan basalts are similar but show evidence of olivine fractionation. Wulahada lavas are high-Mg andesites (Mg#∼67) with EM1 Sr-Nd-Pb isotopic signatures. Geochemical data suggest that the basalts originated from MORB-type asthenosphere whereas the high-Mg andesites were derived an EM1 mantle source, i.e., a refractory lithospheric mantle modified by a previously subducted slab. The result, combined with the available data of the Mesozoic basalts from the southern portion of the NCC (Zhang et al., 2002), manifests a vast secular evolution of the lithospheric mantle beneath the eastern NCC from the Paleozoic refractory continental lithosphere to this Mesozoic modified lithosphere. Compared with the cratonic margin, the lithospheric mantle beneath the center of the craton was less extensively modified, implying the secular evolution was related to the subduction processes surrounding the NCC. Therefore, we suggest that the interaction of the slab-derived silicic melt with the old refractory lithospheric mantle converted the Paleozoic cratonic lithospheric mantle into the late Mesozoic fertile mantle, which was also different from the Cenozoic counterpart. A geodynamic model is proposed to illustrate such a secular lithosphere evolution. 相似文献