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1.
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   

2.
High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

3.
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   

4.
Chronological, geochemical and Sr–Nd–Pb isotopic analyses have been carried out on the Mesozoic plutons in western Shandong with the aim of characterizing crustal–mantle evolution during the tectono-thermal reactivation of the craton. Detailed SHRIMP zircon U–Pb dating reveals two main periods of Mesozoic activity with contrasting compositions. The older magmatic pulse is manifested by monzonites and monzodiorites from Tongshi for which zircon rims yield a concordant age of 177±4 Ma and the cores have a discordant age of ca. 2.5 Ga. Low MgO and Cr, high Na2O contents and especially their isotopic compositions (87Sr/86Sr < 0.7042, 206Pb/204Pb < 16.8 and Nd ~ –12) are consistent with derivation from late Archean–Paleoproterozoic lower crust. Relatively high HREE contents in these Jurassic plutons indicate a garnet-free source (<32 km), in contrast to the garnet-bearing source (>40 km) of the late Mesozoic high Sr and low Y granitoids from the same region. Distinctively different depths of crustal melting suggest dynamic thickening of the crust by magmatic underplating during the Jurassic and Cretaceous. The younger dioritic plutons from Laiwu and Yinan were emplaced at 132–126 Ma and show relatively high MgO and Cr contents and large isotopic variability. They were likely derived from enriched lithospheric mantle source and were subjected to crustal contamination during magma evolution. Early Cretaceous mantle melting is coeval with the widespread late Yanshanian granitic magmatism in North China. Early Cretaceous time may correspond to a critical period when a temperature increase due to lithospheric thinning allowed the intersection of the local geotherm and the wet peridotite solidus. While some mantle-derived magmas were erupted, most were trapped at variable crustal depths, triggering large-scale concomitant melting of the crust. Lithospheric thinning must have continued until the late Cretaceous because of the change in the source of mafic magmas from lithospheric to asthenospheric at that time. It is proposed that removal of the lithospheric keel beneath the North China craton may have been initiated as early as the Jurassic, but with the most intense period in the Cretaceous between 130–75 Ma. Such a relatively long timescale (~100 Ma) emphasizes the role of thermomechanical erosion by convective mantle in lithospheric thinning beneath this region.  相似文献   

5.
《Chemical Geology》2002,182(2-4):139-178
The Kohistan–Ladakh Terrane in the NW Himalaya is a remnant of a Cretaceous arc sequence obducted onto the Indian margin. This paper presents a geochemical study (major and trace elements and Sr, Nd, Pb isotopes) of the Mid-Cretaceous lavas of the Ladakh side of the arc sequence, which were erupted in response to northward subduction of Neo-Tethys oceanic crust.Lavas from the western Ladakh in Pakistan can be divided into three groups which, from north to south, are: (1) the Northern Group of back-arc tholeiites [0.5<(La/Yb)N<1.4; 0.3<(Nb/La)N<1.4; 4<εNd<8; 38.66<208Pb/204Pb<38.80], (2) the Southern Group of arc tholeiites [1.8<(La/Yb)N<3.9; 0.1<(Nb/La)N<0.6; 5<εNd<6; 38.40<208Pb/204Pb<38.66], and (3) the Katzarah Formation of tholeiitic Nb-rich lavas [3.4<(La/Yb)N<9.8; 1.4<(Nb/La)N<2.1; 3<εNd<5], including radiogenic Pb lavas [39.31<208Pb/204Pb<39.51] and less radiogenic lavas [38.31<208Pb/204Pb<38.55]. Magmas from the eastern Ladakh in India show a simple series of more evolved arc volcanics from basalts to rhyolites [basalts and basaltic andesites: 2.5<(La/Yb)N<5.7; 0.4<(Nb/La)N<0.5; 1.8<εNd<5.5; 38.70<208Pb/204Pb<38.80]. Isotope and trace element data of western Ladakh lavas are compatible with high-degree melting (14–21%) of a fertile MORB-mantle source. An adakitic lava [(La/Yb)N=55.8; (Nb/La)N=0.3; εNd=1.7; 208Pb/204Pb=39.00] and a Mg-poor Nb-rich basalt [(La/Yb)N=4.6; (Nb/La)N=1.3; εNd=−2; 208Pb/204Pb=39.07] are spatially associated with the tholeiitic arc lavas. Isotope compositions of all the lavas, and in particular the radiogenic Nb-rich and adakitic lavas suggest three-component mixing between depleted mantle similar to the Indian MORB mantle, and enriched components similar to the volcanogenic or pelagic sediments. The geochemical diversity of magma types is attributed to contribution of melts from the subducted crust and associated sediments, and their subsequent interaction with the mantle. Such melt–mantle interactions can also be inferred from relicts of sub-arc mantle found in Indian Ladakh. These results lead to a geodynamic reconstruction of the Kohistan–Ladakh arc as a single entity in the Mid-Cretaceous, emplaced south of the Asian margin. Slab melting imply subduction of young oceanic crust, as already proposed for the Oman ophiolite farther west. The fast northward drift of the Indian Plate could have triggered wide-scale inversion of the divergent tectonic regime responsible for the opening of the Neo-Tethys Ocean. Our results suggest breaking of the young oceanic crust initiated at the ridge rather than at passive plate boundaries.  相似文献   

6.
U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).  相似文献   

7.
The off-rift central volcano of Öræfajökull has very distinctive EM1-like isotopic compositions compared with other Icelandic lavas. New Pb–Nd–Sr isotopic data from Öræfajökull show strong correlations interpreted as a result of mixing. End-members are a depleted mantle source incorporating 0.5 % subduction-processed sediment and a mantle source with an isotopic signature similar to lavas of the Reykjanes Peninsula. Sr–Nd–Pb isotopic correlations of Icelandic Eastern Rift Zone (ERZ) lavas are almost completely distinct from those of the Reykjanes Peninsula and the Western Rift Zone (WRZ) and require a high-207Pb/204Pb, low-143Nd/144Nd end-member that resembles Öræfajökull compositions, which is very distinct from the enriched end-members suggested for the Reykjanes Peninsula and the WRZ. Given the similar depth and degree of melting at rift zones, variation in the observed enriched end-members between rift zones must indicate spatial variations in enriched mantle sources within the shallow mantle under Iceland rather than purely mixing of melts from a bi-lithological mantle. This is consistent with observations that the ERZ lavas erupted closest to Öræfajökull exhibit the most Öræfajökull-like isotopic compositions, implying that a homogenised Öræfajökull source with positive ?207Pb is focused under the Öræfajökull centre and its associated flank zone. This then mixes laterally with the dominant negative-?207Pb ERZ mantle source. Like Reykjanes Peninsula and WRZ lavas, the ERZ mantle source has strongly negative Δ207Pb and low K/Nb (<170), and these provide evidence for a recycled oceanic crust contribution. The range in 206Pb/204Pb in mantle sources with negative Δ207Pb was probably generated by heterogeneity in 206Pb/204Pb and μ in the recycled oceanic crust, which is the dominant source of incompatible elements in Icelandic lavas.  相似文献   

8.
High precision Sr-Nd isotope ratios together with Pb isotope ratios corrected for mass fractionation using a double spike are reported for an extensive suite of late Quaternary to Recent lavas of Iceland, the Kolbeinsey and Reykjanes Ridges, and a small number of basalts from further south on the Mid-Atlantic Ridge. Compared with global MORB, the Icelandic region is distinguished by having low 207Pb/204Pb for any given 206Pb/204Pb, expressed by negative Δ207Pb (−0.8 to −3.5) in all but four Icelandic samples. Most samples also have elevated 208Pb/204Pb (strongly positive Δ208Pb), which combined with their negative Δ207Pb is very unusual in MORB worldwide. The negative Δ207Pb is interpreted as a consequence of evolution in high-μ mantle sources for the last few hundred Ma. The region of negative Δ207Pb appears to correspond with the region of elevated 3He/4He, suggesting that both lithophile and volatile elements in melts from the whole region between 56 and 70°N are dominantly sourced in a plume that has incorporated recycled Palaeozoic ocean crust and unradiogenic He, probably from the deep mantle. At least four mantle components are recognized on Iceland, two with an enriched character, one depleted and one that shows some isotopic affinities to EM1 but is only sampled by highly incompatible-element-depleted lavas in this study. Within restricted areas of Iceland, these components contribute to local intermediate enriched and depleted components that display near binary mixing systematics. The major depleted Icelandic component is clearly distinct in Pb isotopes from worldwide MORB, but resembles the depleted mantle source supplying the bulk of the melt to the Kolbeinsey and southern Reykjanes Ridges. However, an additional depleted mantle source is tapped by the northern Reykjanes Ridge, which with very negative Δ207Pb and less positive Δ208Pb is distinct from all Icelandic compositions. These components must mostly mix at mantle depths because a uniform mixture of three Icelandic components is advected southward along the Reykjanes Ridge.Despite strong covariation with isotope ratios, incompatible trace element ratios of Icelandic magmas cannot be representative of old mantle sources. The observed parent-daughter ratios in depleted and enriched Icelandic lavas would yield homogeneous Sr, Nd, Hf and 206Pb isotope signatures ∼170 Ma ago if present in their sources. The heterogeneity in 207Pb/204Pb is not however significantly reduced at 170 Ma, and the negative present day Δ207Pb cannot be supported by the low μ observed in depleted lavas from Iceland or the adjacent ridges. Since μ is higher in melts than in their sources, it follows that all the depleted sources must be residues from <170 Ma partial melting events. These are thought to have strongly affected most incompatible trace element ratios.  相似文献   

9.
We report a new whole-rock dataset of major and trace element abundances and 87Sr/86Sr–143Nd/144Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNdi (??5.2 to ??9.4) and radiogenic εSri (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.  相似文献   

10.
Tchabal Gangdaba (TG) volcanic massif, which is a part of the continental sector of the Cameroon Volcanic Line (CVL), is dated between 34.4 and 25.1 Ma. It displays mafic lavas (picrobasalt and basanite, 41–43 wt % SiO2) and felsic lavas (rhyolite, 68–73 wt % SiO2). The lack of intermediate rocks evidences a pronounced Daly gap between 43 and 68 wt % SiO2, which corresponds to an important time span of 3.4 Ma. It is interpreted as due to extensive fractional crystallization under peculiar thermodynamical conditions. Felsic lavas yield strong negative anomalies in Ba, Sr and Eu (0.1?206Pb/204Pb?207Pb/204Pb?208Pb/204Pb?相似文献   

11.
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   

12.
《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   

13.
The mid‐Cretaceous Spences Bridge Group (SBG) comprises a series of basaltic to rhyolitic lavas and related volcaniclastic rocks (Pimainus Formation) overlain by a succession of mainly amygdaloidal andesites (Spius Formation) related to the closure of the Methow–Tyaughton basin and accretion of the Insular terrane in the North American Cordillera. Geochemical variation in the SBG is related primarily to metasomatic processes in the mantle wedge. Pimainus lavas include low‐ to high‐K, tholeiitic and calc‐alkaline types, and have isotopic compositions (εNd(100Ma) = + 5.2 to + 7.0, εSr(100Ma) = − 10 to − 20, 206Pb/204Pb = 18.82 to 18.91, 207Pb/204Pb = 15.55 to 15.60, 208Pb/204Pb = 38.24 to 38.43) between the ranges for primitive arcs and accreted terrane crust. Crustal sources are identified only for some low–medium K dacite and rhyolite compositions. The occurrence of intermediate compositions with high MgO contents (up to 6 wt%) and the presence of adakitic trace element features in medium–high K felsic lavas attests to metasomatism of the mantle wedge by slab melts during Pimainus volcanism. Spius lavas have comparable K2O and Pb isotopic compositions to the Pimainus, even higher MgO (up to 9.2 wt%), and display a mild intraplate character in having up to 0.6 wt% P2O5, 15 ppm Nb, and 240 ppm Zr. Spius Nd−Sr isotopic compositions (εNd(100Ma) = + 5.3 to + 6.9, εSr(100Ma) = − 14 to − 25) define an array extending from Pimainus to alkaline seamount compositions. The low εSr values, elevated high field strength element contents, and moderate silica contents suggest Spius volcanism was related to the introduction of small melt fractions from the asthenosphere into the mantle wedge which had previously generated Pimainus melts. The range of compositional types in the Pimainus Formation constrains tectonic scenarios to include an elevated slab thermal regime, likely from approach of an ocean ridge system toward the continental margin. Spius volcanism may have been generated by asthenospheric upwelling triggered by slab window development or slab‐hinge roll‐back on closure of the Methow–Tyaughton basin. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

14.
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   

15.
The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial 87Sr/86Sr ratios of 0.7044?0.7047, ?Nd(T) values of –1.5 to?+0.6, and ?Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial 206Pb/204Pb ratios up to 18.41 and 207Pb/204Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.  相似文献   

16.
We have carried out a Pb double-spike and Lu-Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U-Th-Pb and Lu-Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf (εHf = +39.6 to +2586) that reflect ∼15-25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf-Sm/Nd ratios and Nd-Hf isotopes (εNd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic (206Pb/204Pb > 19), and typically more so than the host intraplate volcanic rocks. 238U/204Pb ratios range from 17 to 68, and most samples have distinctively low 232Th/238U (<1) and 232Th/204Pb (3-22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic (εHf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. 238U/204Pb (53-111) and 232Th/204Pb ratios (157-355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle 238U/204Pb (northern and southern FMC), 232Th/238U (northern FMC) and 232Th/204Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (?80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (?70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.  相似文献   

17.
《Gondwana Research》2014,25(2):859-872
Mesozoic lamprophyres are widely present in gold province in the Jiaodong Peninsula. In this study, we analyzed major and trace elements and Sr–Nd–Pb isotopic compositions of lamprophyres from the Linglong and Penglai Au-ore districts in the Jiaodong Peninsula, in an attempt to better understand Mesozoic lithospheric evolution beneath the eastern North China Craton. These lamprophyre dikes are calc-alkaline in nature, and are characterized by low concentrations of SiO2, TiO2 and total Fe2O3, high concentrations of MgO, Mg# and compatible element, enriched in LREE and LILE but variably depleted in HFSE. They display initial 87Sr/86Sr ratios of 0.709134–0.710314, εNd(t) values of − 13.2 to − 18.3, 206Pb/204Pb of 17.364–17.645, 207Pb/204Pb of 15.513–15.571 and 208Pb/204Pb of 37.995–38.374. Interpretation of elemental and isotopic data suggests that the Linglong and Penglai lamprophyres were derived from partial melting of a phlogopite- and/or amphibole-bearing lherzolite in the spinel–garnet transition zone. The parental magma might have experienced fractionation of olivine and clinopyroxene, and minor crustal materials were incorporated during ascent of these mafic magmas. Before ~ 120 Ma of emplacement of these calc-alkaline lamprophyres, the ancient lithospheric mantle was variably metasomatized by hydrous fluids rather than melts from subducted/foundered continental crust. It is proposed that continuous modification by slab-derived hydrous fluids from the Paleo-Pacific plate converted the old cratonic lithospheric mantle to Mesozoic enriched lithospheric mantle. Geodynamic force for generation of these lamprophyres may be related to large scale lithospheric thinning coupled with upwelling of the asthenosphere beneath the North China Craton. Continental arc-rifting related to the Paleo-Pacific plate subduction is favored as a geodynamic force for the cratonic lithosphere detachment.  相似文献   

18.
The volcanic rocks of the Xiong'er Group are situated in the southern margin of the North China Craton(NCC).Research on the Xiong er Group is important to understand the tectonic evolution of the NCC and the Columbia supercontinent during the Paleoproterozoic.In this study,to constrain the age of the Xiong'er volcanic rocks and identify its tectonic environment,we report zircon LA-ICP-MS data with Hf isotope,whole-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopes of the volcanic rocks of the Xiong'er Group.The Xiong'er volcanic rocks mainly consist of basaltic andesite,andesite.dacite and rhyolite,with minor basalt.Our new sets of data combined with those from previous studies indicate that Xiong'er volcanism should have lasted from 1827 Ma to 1746 Ma as the major phase of the volcanism.These volcanics have extremely low MgO.Cr and Ni contents,are enriched in LREEs and LILEs but depleted in HFSEs(Nb,Ta,and Ti),similar to arc-related volcanic rocks.They are characterized by negative zircon ε_(Hf)_(t) values of-17.4 to 8.8,whole-rock initial ~(87)Sr/~(86)Sr values of 0.7023 to 0.7177 andε_(Nd)(t) values of-10.9 to 6.4.and Pb isotopes(~(206)Pb/~(204)Pb =14.366-16.431,~(207)Pb/~(204)Pb =15.106-15.371,~(208)Pb/~(204)Pb= 32.455-37.422).The available elemental and Sr-Nd-Pb-Hf isotope data suggest that the Xiong'er volcanic rocks were sourced from a mantle contaminated by continental crust.The volcanic rocks of the Xiong'er Group might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by oceanic subduction in the Archean.Thus,we suggest that the subduction-modified lithospheric mantle occurred in an extensional setting during the breakup of the Columbia supercontinent in the Late Paleoproterozoic,rather than in an arc setting.  相似文献   

19.
The genesis of mineralized systems across the Mountain Freegold area, in the Dawson Range Cu–Au?±?Mo Belt of the Tintina Au province was constrained using Pb and stable isotope compositions and Ar–Ar and Re–Os geochronology. Pb isotope compositions of sulfides span a wide compositional range (206Pb/204Pb, 18.669–19.861; 208Pb/204Pb, 38.400–39.238) that overlaps the compositions of the spatially associated igneous rocks, thus indicating a magmatic origin for Pb and probably the other metals. Sulfur isotopic compositions of sulfide minerals are broadly similar and their δ34S (Vienna-Canyon Diablo Troilite (V-CDT)) values range from ?1.4 to 3.6 ‰ consistent with the magmatic range, with the exception of stibnite from a Au–Sb–quartz vein, which has δ34S values between ?8.1 and ?3.1 ‰. The δ34S values of sulfates coexisting with sulfide are between 11.2 and 14.2 ‰; whereas, those from the weathering zone range from 3.7 to 4.3 ‰, indicating supergene sulfates derived from oxidation of hypogene sulfides. The δ13C (Vienna Peedee Belemnite (VPDB)) values of carbonate range from ?4.9 to 1.1 ‰ and are higher than magmatic values. The δ18O (V-SMOW) values of magmatic quartz phenocrysts and magmatic least-altered rocks vary between 6.2 and 10.1 ‰ and between 5.0 and 10.1 ‰, respectively, whereas altered magmatic rocks and hydrothermal minerals (quartz and magnetite) are relatively 18O-depleted (4.2 to 7.9 ‰ and ?6.3 to 1.5 ‰, respectively). Hydrogen isotope compositions of both least-altered and altered igneous rock samples are D-depleted (from ?133 to ?161 ‰ Vienna-Standard Mean Ocean Water (V-SMOW)), consistent with differential magma degassing and/or post-crystallization exchange between the rocks and meteoric ground water. Zircon from a chlorite-altered dike has a U–Pb crystallization age of 108.7?±?0.4 Ma; whereas, the same sample yielded a whole-rock Ar–Ar plateau age of 76.25?±?0.53 Ma. Likewise, molybdenite Re–Os model ages range from 75.8 to 78.2 Ma, indicating the mineralizing events are genetically related to Late Cretaceous volcano-plutonic intrusions in the area. The molybdenite Re–Os ages difference between the nearby Nucleus (75.9?±?0.3 to 76.2?±?0.3 Ma) and Revenue (77.9?±?0.3 to 78.2?±?0.3 Ma) mineral occurrences suggests an episodic mineralized system with two pulses of hydrothermal fluids separated by at least 2 Ma. This, in combination with geological features suggest the Nucleus deposit represents the apical and younger portion of the Revenue–Nucleus magmatic-hydrothermal system and may suggest an evolution from the porphyry to the epithermal environments.  相似文献   

20.
The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and modified in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatism could provide a unique window into deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon U-Pb dating yielded an average age of 174 ± 14 Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t) = 0.70646–0.70925, εNd(t) = −2.1 to −4.9, 206Pb/204Pb(t) = 17.14–18.12, 207Pb/204Pb(t) = 15.28–15.61, 208Pb/204Pb(t) = 37.82–38.67, and zircon εHf(t) = −17 to −21) are enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts from the Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithospheric mantle of the eastern NCC were reduced in viscosity and intensity, and finally promoted partial melting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.  相似文献   

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