Oxygen and lead isotope characteristics of granitic rocks from the Nansha block (South China Sea): Implications for their petrogenesis and tectonic affinity

The pre‐Cenozoic history of the South China Sea (SCS) region is the object of continued debate. To trace the evolution of the SCS, a better understanding of the petrogenesis and tectonic affiliation of the granitic rocks that comprise the microblocks within the region is necessary. In this study, we analyzed the whole‐rock oxygen and lead isotope ratios of granitic samples dredged from two locations in the Nansha microblock, one of the microblocks in the SCS. Oxygen isotope data combined with previously published Sr isotope data show that group I rocks (δ¹⁸O = 6.00–7.20‰; average = 6.64‰) originated from a mantle source contaminated by material and/or fluid input from a Mesozoic subduction zone in the southeastern side of the microblock. Group II rocks (δ¹⁸O = 6.86‰–9.13‰; average = 7.75‰) also came from the same source, but they were additionally affected by crustal contamination. The Nansha microblock has high radiogenic lead ratios (²⁰⁶Pb/²⁰⁴Pb_i = 18.602–18.756, ²⁰⁷Pb/²⁰⁴Pb_i = 15.660–15.713, ²⁰⁸Pb/²⁰⁴Pb_i = 38.693–38.893), which indicate that the Nansha microblock is tectonically affiliated with the Nanling–Hainan or South China block. This notion is consistent with the results of a previous Nd isotope study. As a whole, results of our study suggest that some of the other microblocks dispersed in the SCS are also possible fragments of South China block, and thus further studies are needed to better constrain the pre‐Cenozoic evolutionary history of the SCS region.     

Geochemistry of a new enriched mantle type locality in the northern hemisphere: Implications for the origin of the EM-I source

Late Cretaceous (66.2 ± 0.5 Ma amphibole and 66.7 ± 0.2 Ma phlogopite ⁴⁰Ar/³⁹Ar ages) nephelinitic volcanic rocks from Godzilla Seamount in the eastern North Atlantic (34°N latitude) have trace element and Sr–Nd–Pb–Hf-isotope compositions similar to the Enriched Mantle I (EM-I) endmember, except for their low ²⁰⁷Pb/²⁰⁴Pb relative to ²⁰⁶Pb/²⁰⁴Pb ratios (²⁰⁶Pb/²⁰⁴Pb_in = 17.7, ²⁰⁷Pb/²⁰⁴Pb_in = 15.34) plotting below the Northern Hemisphere Reference Line on the uranogenic Pb isotope diagram. O isotope data on amphibole separates are mantle-like (δ¹⁸O = 5.6–5.8‰). Age and location of the isolated Godzilla Seamount, however, preclude it from being derived from the Madeira or Canary hotspots, making a lower-mantle origin unlikely. Therefore we propose derivation from a shallow (lithospheric/asthenospheric) melting anomaly. As observed in mid-ocean-ridge and ocean-island basalts, there is a systematic decrease of ²⁰⁷Pb/²⁰⁴Pb ratios (and Δ7/4) in the individual EM-I endmember type localities towards northern latitudes with Godzilla lying on the extension of this trend. This trend is mirrored in ultra-potassic volcanic rocks such as lamproites and kimberlites, which reflect the composition of enriched subcontinental lithospheric mantle. Therefore, a global pattern in ²⁰⁷Pb/²⁰⁴Pb ratios and Δ7/4 is suggested. The geochemical composition of EM-I endmember type localities, including Godzilla lavas, and the enriched (DUPAL) anomaly in the southern hemisphere could reflect derivation from ancient, metasomatized subcontinental lithospheric mantle. We propose a two-stage model to explain the trace element and isotopic composition of the EM-I mantle endmember localities worldwide: 1) during the early history of the Earth, subcontinental lithosphere was metasomatized by melts from subducted slabs along convergent margins generating high μ (²³⁸U/²⁰⁴Pb) sources, and 2) as the Earth cooled, hydrous fluids replaced hydrous melts as the main slab component metasomatizing the subcontinental lithospheric mantle (generating EM-I sources with lower μ). In accordance with this model, the global variations in ²⁰⁷Pb/²⁰⁴Pb ratios and Δ7/4 could reflect geographic differences in μ and/or the age at which the transition from stages 1 to 2 took place in the Archaean lithosphere. The model would require a re-definition of the EM-I endmember to low ²⁰⁶Pb/²⁰⁴Pb, high ²⁰⁸Pb/²⁰⁴Pb (positive Δ8/4) but variable ²⁰⁷Pb/²⁰⁴Pb (positive and negative Δ7/4).     

Paleomagnetism, geochemistry and Ar/Ar dating of the North-eastern Paraná Magmatic Province: tectonic implications

Seventy sites of sills, flows and dikes from Northeastern Paraná Magmatic Province (PMP), were submitted to paleomagnetic, chemical and radiometric analyses. The rocks are high in TiO₂ content, and similar in composition to the rocks from the northern region of PMP. The sills intrude mainly Paleozoic sediments, and can be subdivided into two domains; the northern being characterized by sills showing reversed polarities, and the southern essentially by sills of normal polarities. ⁴⁰Ar/³⁹Ar dating of three distinct sills gave plateau ages (129.9 ± 0.1, 130.3 ± 0.1 and 131.9 ± 0.4 Ma) that are similar to surface-outcropping flows of the Northern Paraná Basin, and the Ponta Grossa dikes. The new paleomagnetic data combined with existing data from the northern PMP allowed the calculation of a paleomagnetic pole at 71.4° E and 83.0° S (N = 92; α₉₅=2.4°; k = 39). This pole is in good agreement with poles for central and southern PMP, which are slightly older than the northern PMP, as well as for the contemporaneous Central Alkaline Province (Paraguay) on the western side of PMP. In contrast, the coeval pole for the Ponta Grossa dikes (eastern border of PMP), however, is slightly displaced from that group of poles, suggesting that dikes in that area may have undergone some tectonic tilting.     

Pb‐isotope compositions of the Tasik Chini volcanic‐hosted massive sulfide deposit,Central Belt of Peninsular Malaysia: Implication for source region and tectonic setting

Lead isotope data of sulfides and host volcanic rocks from the Bukit Botol and Bukit Ketaya deposits, the two representative deposits of the Tasik Chini volcanic‐hosted massive sulfide (VHMS) deposit, Central Belt of Peninsular Malaysia, are reported. Lead isotope compositions of the associated sulfide minerals and volcanic rocks from the Bukit Botol deposit exhibit homogeneous and less radiogenic values (²⁰⁶Pb/²⁰⁴Pb showing a range of composition from 18.14 to 18.20, ²⁰⁷Pb/²⁰⁴Pb between 15.52 and 15.59 and ²⁰⁸Pb/²⁰⁴Pb from 37.96 to 38.35). Similarly, the Pb isotopic compositions of the host volcanic rocks from the Bukit Ketaya deposit yielded a narrow range to those of the sulfide samples (²⁰⁶Pb/²⁰⁴Pb from 18.04 to 18.20, ²⁰⁷Pb/²⁰⁴Pb between 15.43 and 15.57 and ²⁰⁸Pb/²⁰⁴Pb of 37.96 to 38.30). The uniform Pb‐isotope compositions of the sulfides in the ore horizon and the host volcanic rocks from both deposits suggest a derivation from a similar source reservoir and mineralization processes. In the framework of the tectonic model for the Central Belt of Peninsular Malaysia, both deposits display a range of lead isotopic compositions originated from mixing of bulk crust/juvenile arc and minor mantle sources, which are typical for VHMS deposits in an island arc–back arc setting.     

Sr-Nd-Pb isotopes of the Early Paleozoic maficultramafic dykes and basalts from South Qinling belt and their implications for mantle composition

Late Early Paleozoic mafic-ultramafic dykes and volcanic rocks from the South Qinling belt are char- acterized by εNd( t ) = 3.28― 5.02, (87Sr/86Sr)i= 0.70341― 0.70555, (206Pb/204Pb)i = 17.256― 18.993, (207Pb/204Pb)i= 15.505―15.642, (208Pb/204Pb)i=37.125―38.968, ?8/4=21.18―774.43, ?7/4=8.11―18.82. These charac- teristics suggest that they derived from a Middle Neoproterozoic mantle with isotopic compositions of mixed HIMU, EMII and minor EMI components. We interpret that these rocks were melting products of depleted mantle modified by subducted ancient oceanic crust and continental margin sediments along the northern margin of Yangtze block during Early Neoproterozoic.     

Possible origin of K-rich volcanic rocks from Virunga,East Africa,by metasomatism of continental crustal material: Pb,Nd and Sr isotopic evidence

The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system.⁸⁷Sr/⁸⁶Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative ε_Ndvalues ofε_Nd = ?10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (ε_Nd = +2.5;⁸⁷Sr/⁸⁶Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average²⁰⁶Pb/²⁰⁴Pb≈ 19.4,²⁰⁷Pb/²⁰⁴Pb= 15.79–15.84,²⁰⁸Pb/²⁰⁴Pb≈ 41.2. The basic potassic rocks have similar²⁰⁶Pb/²⁰⁴Pb values but range in²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb from the Sabinyo values to less radiogenic compositions.Excellent correlations of⁸⁷Sr/⁸⁶Sr with Rb/Sr, 1/Sr and²⁰⁷Pb/²⁰⁶Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of⁸⁷Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ～500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +ε_Nd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a ?ε_Nd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this ?ε_Nd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.     

Tadhak alkaline ring-complex (Mali): existence of UPb isochrons and “Dupal” signature 270 Ma ago

The Tadhak alkaline ring-complex of Permian age provides two whole rock UPb isochrons giving concordant ages in agreement within relative errors with the RbSr isochron age:²³⁵U²⁰⁷Pb isochron: 271 ± 32Ma(MSWD= 0.3);²³⁸U²⁰⁶Pb isochron: 254 ± 18Ma(MSWD= 7.8), both on 8 whole-rock samples. The existence of these isochrons indicates that in favorable conditions U (and Pb) can be immobile. This can be due either to the lack of hard oxidizing conditions and/or to the location of U, in very low concentrations, in weathering-resistant minerals. The initial ratios (²⁰⁶Pb/²⁰⁴Pb = 18.714 ± 70and²⁰⁷Pb/²⁰⁴Pb = 15.589 ± 16), corrected for their Permian age, lie in the range observed for oceanic island basalts or continental alkali basalts and indicate an origin in a similar mantle, without any significant crustal contamination. This was also suggested by the initial⁸⁷Sr/⁸⁶Sr ratio of 0.70457 ± 4. Moreover, these Sr and Pb isotopic characteristics belong to the field of the so-called “Dupal” anomaly and indicate that it existed already 270 Ma ago. This study shows the potential interest of isotopic investigations of within-plate alkaline ring-complexes to characterize subcontinental mantle compositions, particularly in the past.     

Isotope characteristics of submarine lavas from the Philippine Sea: implications for the origin of arc and basin magmas of the Philippine tectonic plate

Igneous rocks from the Philippine tectonic plate recovered on Deep Sea Drilling Project Legs 31, 58 and 59 have been analyzed for Sr, Nd and Pb isotope ratios. Samples include rocks from the West Philippine Basin, Daito Basin and Benham Rise (40–60 m.y.), the Palau-Kyushu Ridge (29–44 m.y.) and the Parece Vela and Shikoku basins (17–30 m.y.). Samples from the West Philippine, Parece Vela and Shikoku basins are MORB (mid-ocean ridge basalt)-like with ⁸⁷Sr/⁸⁶Sr= 0.7026−0.7032, ¹⁴³Nd/¹⁴⁴Nd= 0.51300−0.51315, and ²⁰⁶Pb/²⁰⁴Pb= 17.8−18.1. Samples from the Daito Basin and Benham Rise are OIB (oceanic island basalt)-like with ⁸⁷Sr/⁸⁶Sr= 0.7038−0.7040, ¹⁴³Nd/¹⁴⁴Nd= 0.51285−0.51291 and ²⁰⁶Pb/²⁰⁴Pb= 18.8−19.2. All of these rocks have elevated ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb compared to the Northern Hemisphere Regression Line (NHRL) and have δ²⁰⁷Pb values of 0 to +6 and δ²⁰⁸Pb values of +32 to +65. Lavas from the Palau-Kyushu Ridge, a remnant island arc, have ⁸⁷Sr/⁸⁶Sr= 7032−0.7035, ¹⁴³Nd/¹⁴⁴Nd= 0.51308−0.51310 and ²⁰⁶Pb/²⁰⁴Pb= 18.4−18.5. Unlike the basin magmas erupted before and after them, these lavas plot along the NHRL and have Pb-isotope ratios similar to modern Pacific plate MORB's. This characteristic is shared by other Palau-Kyushu Arc volcanic rocks that have been sampled from submerged and subaerial portions of the Mariana fore-arc.At least four geochemically distinct magma sources are required for these Philippine plate magmas. The basin magmas tap Source 1, a MORB-mantle source that was contaminated by EMI (enriched mantle component 1 [31]) and Source 2, an OIB-like mantle source with some characteristics of EMII (enriched mantle component 2 [31]). The arc lavas are derived from Source 3, a MORB-source or residue mantle including Sr and Pb from the subducted oceanic crust, and Source 4, MORB-source or residue mantle including a component with characteristics of HIMU (mantle component with high U/Pb [31]). These same sources can account for many of the isotopic characteristics of recent Philippine plate arc and basin lavas. The enriched components in these sources which are associated with the DUPAL anomaly were probably introduced into the asthenosphere from the deep mantle when the Philippine plate was located in the Southern Hemisphere 60 m.y.b.p.     

Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22–35°N

New trace element and Hf, Nd, and Pb isotope data are reported for 22 basalts collected between 22°N and 35°N on the Mid-Atlantic Ridge. (La / Sm)_N ratios identify the presence of enriched (E)-MORB in the northernmost part of this area and normal (N)-MORB elsewhere. A negative correlation is observed when ¹⁴³Nd / ¹⁴⁴Nd is plotted against ²⁰⁶Pb / ²⁰⁴Pb, ²⁰⁷Pb / ²⁰⁴Pb, and ²⁰⁸Pb / ²⁰⁴Pb, whereas ¹⁷⁶Hf / ¹⁷⁷Hf appears not to correlate with any of the other isotopic ratios. The E-MORB samples are characterized by high ²⁰⁶Pb / ²⁰⁴Pb, ²⁰⁷Pb / ²⁰⁴Pb, ²⁰⁸Pb / ²⁰⁴Pb, and low ¹⁴³Nd / ¹⁴⁴Nd. Principal Component Analysis (PCA) of Pb isotopes alone identifies three, and only three, significant geochemical end-members (‘components’). Including Nd and Hf isotopic data in the PCA produces spurious components, partly because of curved mixing relationships, and partly because of fractionation during melting. Our preferred interpretation of why ¹⁷⁶Hf / ¹⁷⁷Hf is decoupled from the other isotopic ratios is, as inferred from recent experimental data, that the Hf isotopic compositions of the melt and the residue fail to equilibrate during melting. A strong correlation between (Sr / Nd)_N and (Eu / Eu*)_N indicates that plagioclase is a residual phase of N-MORB, but not of E-MORB melting. The three end-members identified in this study are the depleted mantle, a common-type component, and an enriched plume-type end-member. The common, or ‘C’-type, end-member is characteristic of E-MORB and may itself be a mixture containing recycled oceanic crust (the MORB suite, terrigenous sediments, and/or oceanic plateaus). The plume-type end-member is likely to represent the lower mantle and may involve some primordial material. It is shown that mantle isochrons in general and the Pb–Pb isochron in particular do not characterize a specific geodynamic process acting to create mantle heterogeneities.     

60025: relict of primitive lunar crust?

New isotopic analyses are presented for 3 plagioclase-rich fractions and one mafic fraction from ferroan anorthosite 60025. The observed²⁰⁶Pb/²⁰⁴Pb ratios vary between 52.5 and 60.5, all much higher than the ratio for terrestrial contamination. In a²⁰⁷Pb/²⁰⁶Pb²⁰⁴Pb/²⁰⁶Pb correlation diagram, the plagioclase data define a model PbPb age of 4.520 ± 0.007 AE using meteoritic primordial lead for the non-radiogenic component. In the concordia diagram the plagioclase data yield intersections at 4.503 ± 0.007 and 0.28 AE. The meaning of the lower intercept is obscure. The earlier 60025 analysis of Tera and Wasserburg [1], with an observed²⁰⁶Pb/²⁰⁴Pb of 23.0, agrees closely with the new plagioclase data in the isotope correlation and concordia diagrams. Since the apparent age does not correlate with the²⁰⁶Pb/²⁰⁴Pb ratios and U contents of the samples, it does not appear to be controlled by terrestrial lead contamination. The time-averaged μ values for the plagioclase leads are exceptionally low, 16–55, and agree within factors of 2 with the observed μ values in the samples. These are much lower than the values observed for mafic rocks or their sources, showing that the anorthosite lead has never been associated for a substantial length of time with any high μ source. In this way the 60025 data differ substantially from UPb data for two other lunar anorthosites, 15415 and 60015. The results suggest that the averaged model Pb ages of 4.51 ± 0.01 AE closely approximate the crystallization age for the plagioclase fraction of the anorthosite, and that it dates back to an early phase in lunar history. One sample from the mafic fraction of 60025 yields a younger model Pb age of 4.42 AE. The age may have been lowered by post-crystallization disturbances or perhaps this fraction is not coeval with the plagioclase fraction.     

Origin of the Gangdise (Transhimalaya) Permian arc in southern Tibet: Stratigraphic and volcanic geochemical constraints

The well‐studied Mesozoic and Cenozoic volcanic rocks of the Gangdise Terrane, southern Tibet, are widely interpreted to have resulted from subduction of the Neotethys; however, Late Paleozoic volcanic rocks and their tectonic setting remain poorly studied. Based on new geological data, we carried out stratigraphical and geochemical analyses of Permian volcano‐stratigraphic sequences within an east–west‐trending, fault‐bounded zone of uplift in the central Gangdise Terrane. Sedimentary rocks in this area consist of platform carbonates and terrigenous clastic rocks that represent widespread shallow‐marine sedimentary basins developed around northern Gondwana. A regression or tectonic uplift event is recorded in Permian sedimentary rocks that show the local development of fluvial environments. The sedimentary succession contains evidence of two volcanic stages: a period of basaltic extrusions and younger explosive felsic magmatism. The first volcanic stage is Early and Middle Permian in age. Tholeiitic basaltic lavas are exposed around Maizhokunggar (Tangjia) and Lhunzhub in central Gangdise. The Lower Permian basalts are relatively enriched in MgO (4.58–12.19%), whereas the Middle Permian basalts are characterized by high Al₂O₃ contents (11.75–21.22%). Rocks of both ages are enriched in large‐ion lithophile elements (LILE) and light rare earth elements (LREE), and show pronounced negative Nb and Ta anomalies. Total REE contents and light (LREE)/heavy (HREE) ratios increased from the Early to Middle Permian. Observed variations in initial Sr, Nd, and Pb isotopes (⁸⁷Sr/⁸⁶Sr_i = 0.7013–0.7066, ²⁰⁷Pb/²⁰⁴Pb_i = 15.53–15.63, and ²⁰⁸Pb/²⁰⁴Pb_i = 38.04–38.64 for a given ²⁰⁶Pb/²⁰⁴Pb_i; εNd = +0.69 to ?11.55) can be explained by crustal interaction with mantle sources, as is characteristic of metasomatism by slab‐derived fluids or assimilation and fractional crystallization (AFC) processes during magmatic evolution. The observed geochemical signatures, coupled with stratigraphic constraints, support the hypothesis that an initial arc formed during the Permian due to southward subduction of the Paleotethys, predating the well‐known Mesozoic arc preserved in the Gangdise Terrane.     

The application of lead isotope ratios in the Antarctic macroalga Iridaea cordata as a contaminant monitoring tool

Lead (Pb) isotope ratios were measured in the marine macroalga Iridaea cordata collected from four locations in the Windmill Islands, East Antarctica. Based on the masses of thalli collected, samples analysed in this study were likely to be a mixture of one and two year old thalli. For a sample of thalli of various ages (<12 months to 2 years old) from the same site there was no apparent variation in Pb concentration or Pb isotope ratio with thallus mass/age, indicating that contaminant sources had been constant over the lifetime of the thalli sampled. I.cordata samples close to the Thala Valley waste disposal site (Brown Bay Inner) near the Australian Station, Casey, displayed isotopic signatures (²⁰⁸Pb/²⁰⁴Pb 35.99; ²⁰⁶Pb/²⁰⁷Pb 1.066; n = 3; average values shown) trending towards that possessed by major Australian Pb sources (Broken Hill and Mt Isa, ²⁰⁸Pb/²⁰⁴Pb 35.60; ²⁰⁶Pb/²⁰⁷Pb 1.041) suggesting that these samples had been exposed to anthropogenic Pb originating from the Thala Valley waste disposal site. Material collected hundreds of metres from the tip location at Brown Bay Outer had isotopic values (²⁰⁸Pb/²⁰⁴Pb 36.32; ²⁰⁶Pb/²⁰⁷Pb 1.088; n = 10) intermediate between Brown Bay Inner and sites further from the contaminant source at Sparkes Bay and Wilkes (²⁰⁸Pb/²⁰⁴Pb 36.46; ²⁰⁶Pb/²⁰⁷Pb 1.094; n = 4) showing that contaminant transport was predominantly restricted to Brown Bay Inner. This study demonstrates that the isotope ratios of Pb in marine macroalgae can provide valuable information as to the origin and extent of heavy metal flux in a marine environment.     

Source of lead in Central American and Caribbean mineralization,II. Lead isotope provinces

In an earlier study of Mesozoic and Cenozoic mineralization in Central America and the Caribbean region, we found that lead isotopic compositions of deposits in northern Central America, which is underlain by a pre-Mesozoic craton, ranged to higher²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb compositions than did deposits from elsewhere in the region, where the basement is Mesozoic oceanic material. Using 16 analyses for 12 new deposits, as well as new analyses for 11 of the samples studied previously, we have found that lead isotopic compositions correlate closely with crustal type but show little or no correlation with depth to the M-discontinuity. The deposits are divisible into three main groups including (in order of increasing²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratio): (1) deposits in southern Central America and all deposits in the Greater Antilles except Cuba; (2) all deposits in northern Central America; and (3) the Cuban deposits. Southern Central American and Caribbean lead is higher in²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb than most mid-ocean ridge basalts but could have been derived directly or indirectly from undepleted mantle. Northern Central America can be divided into the Maya block, which belongs to the Americas plate, and the Chortis block, which belongs to the Caribbean plate. Maya block deposits fall along a linear array whereas those of the Chortis block (except the Monte Cristo deposit) form a cluster. These results suggest that the Maya block is underlain by crust or mantle with a large range of U/Pb and Th/U ratios, whereas the Chortis block basement is more homogeneous. Two-stage model calculations indicate an age of about 2280±310 m.y. for the Maya block basement, although no such rocks are known in the region. Comparison of the Chortis block data to our recently published lead isotopic analyses of Mexican deposits shows considerable similarities suggesting that the Chortis block could have been derived from Mexico.     

The origin of Samoa: new evidence from Sr, Nd, and Pb isotopes

We report Sr, Nd and Pb isotope ratios and parent and daughter element concentrations in 34 volcanic rocks from Samoa. The highly undersaturated post-erosional volcanics, which have erupted in Recent to Historic time along a 250-km-long fissure, have isotopic compositions that define fields distinct from those of the tholeiitic to alkalic lavas of the older Samoan shield volcanoes. Most shield lavas have²⁰⁶Pb/²⁰⁴Pb of 18.9–19.4,⁸⁷Sr/⁸⁶Sr of 0.7045–0.7055 and⁸⁷Sr/⁸⁶Sr (to 0.7075). In general, isotopic compositions of the shield lavas are similar to those of the Marquesas and Society Islands. Post-erosional samples have lower²⁰⁶Pb/²⁰⁴Pb and¹⁴³Nd/¹⁴⁴Nd and higher⁸⁷Sr/⁸⁶Sr than most shield lavas. The most striking feature of the post-erosional data is a negative correlation between²⁰⁷Pb/²⁰⁴Pb and²⁰⁶Pb/²⁰⁴Pb. This suggests that post-erosional lavas are derived from mixtures of the shield source and a high-²⁰⁷Pb/²⁰⁴Pb,⁸⁷Sr/⁸⁶Sr, low-²⁰⁶Pb/²⁰⁴Pb and¹⁴³Nd/¹⁴⁴Nd post-erosional source which may contain recycled ancient sediment. This enriched mantle domain may also underlie the Ontong-Java and Manihiki Plateaus to the north and west. Although both the Samoan shield and post-erosional lavas show chemical characteristics often associated with mantle plumes, only the shield volcanism can plausibly be related to a plume. The post-erosional eruptions appear to be the result of flexure and rifting due to plate bending at the northern termination of the Tonga Trench.     

Al–Mg and Pb–Pb systematics of Allende CAIs: Canonical solar initial Al/Al ratio reinstated

The precise knowledge of the initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] is crucial if we are to use the very first solid objects formed in our Solar System, calcium–aluminum-rich inclusions (CAIs) as the “time zero” age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of ²⁷Al/²⁴Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined ²⁶Al–²⁶Mg fossil isochron with an (²⁶Al/²⁷Al)₀ of (5.23 ± 0.13) × 10^− 5. Internal mineral isochrons obtained for three of these CAIs (A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with (²⁶Al/²⁷Al)₀ = (4.96 ± 0.25) × 10^− 5, anchored to our precisely determined absolute ²⁰⁷Pb–²⁰⁶Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the “canonical” (²⁶Al/²⁷Al)₀ of 5 × 10^− 5 for the early Solar System. The uncertainty in (²⁶Al/²⁷Al)₀ corresponds to a maximum time span of ± 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the ²⁶Al–²⁶Mg and U–Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the “supra-canonical” ²⁶Al/²⁷Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supra-canonical ²⁶Al/²⁷Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS.The radiogenic ²⁰⁸Pb/²⁰⁶Pb ratio obtained as a by-product from the Pb–Pb age dating is used to estimate time-integrated ²³²Th/²³⁸U ratio (κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.     

Geochemical and Sr–Nd–Pb isotopic investigation of Triassic granitoids and basement rocks in the northern Gyeongsang Basin, Korea: Implications for the young basement in the East Asian continental margin

Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (⁸⁷Sr/⁸⁶Sr_initial = 0.7041 ~ 0.7050, ?_Nd(t) = 2.3 ~ 4.0, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr_initial = 0.7047 ~ 0.7065, ?_Nd(t) = 3.9 ~ 2.8, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.     

Pb-isotopic compositions of volcanic rocks in the West and East Philippine island arcs: presence of the Dupal isotopic anomaly

The Philippine islands are situated between two oppositely dipping zones of seismicity. With the exception of a few areas, such as in the west central Philippines where the North Palawan continental terrane (NPCT) has collided with the archipelago, these seismic zones are well defined to depths of 200 km. Active volcanic chains overlay segments in each of these zones, suggesting that subduction is presently taking place both east and west of the islands. Lavas we have studied are thus divided between what has been termed the West Philippine arc and the East Philippine arc.West Philippine arc volcanic rocks which were extruded before the Philippine archipelago collided with the NPCT, or which are younger than the collision but crop out hundreds of kilometers from the collision zone, and all but one of the rocks from the East Philippine arc fall in the MORB field on²⁰⁷Pb/²⁰⁴Pb versus²⁰⁶Pb/²⁰⁴Pb covariation diagrams. This is surprising considering the frequency with which arc materials have²⁰⁷Pb/²⁰⁴Pb ratios higher than those of MORB, the highBa/REE and Sr/REE ratios in the lavas and the possibility of sediment subduction given the small accretionary prisms. All of these rocks have high²⁰⁸Pb/²⁰⁴Pb ratios with respect to Pacific and Atlantic Ocean MORB, but are similar to Indian Ocean MORB and IOB. Thus the Philippines consist of island arcs with the peculiar Dupal isotopic anomaly documented between 0° and 60°S in the southern hemisphere and particularly in the Indian Ocean region. This demonstrates that the Dupal isotopic anomaly is not restricted to the southern hemisphere, or to MORB and OIB.Post-collision rocks cropping out near the NPCT, in the West Philippine arc, have elevated²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios that could be attributed to assimilation of the newly introduced continental crust (NPCT) by mantle-derived magmas or to the addition of a sedimentary component to mantle-derived magmas.     

Development of continental lithospheric mantle as reflected in the chemistry of the Mesozoic Appalachian Tholeiites, U.S.A.

Geochemical analyses of dikes, sills, and volcanic rocks of the Mesozoic Appalachian Tholeiite (MAT) Province of the easternmost United States provide evidence that continental tholeiites are derived from continental lithospheric mantle sources that are genetically and geochronologically related to the overlying continental crust. Nineteen olivine tholeiites and sixteen quartz tholeiites from the length of this province, associated in space and time with the last opening of the Atlantic, display significant isotopic heterogeneity: initial ε_Nd = +3.8 to −5.7; initial ⁸⁷Sr/⁸⁶Sr= 0.7044−0.7072; ²⁰⁶Pb/²⁰⁴Pb= 17.49−19.14; ²⁰⁷Pb/²⁰⁴Pb= 15.55−15.65; ²⁰⁸Pb/²⁰⁴Pb= 37.24−39.11. In PbPb space, the MAT define a linear array displaced above the field for MORB and thus resemble oceanic basalts with DUPAL Pb isotopic traits. A regression of this array yields a secondary PbPb isochron age of ≈ 1000 Ma (μ₁ = 8.26), similar to Sm/Nd isochrons from the southern half of the province and to the radiometric age of the Grenville crust underlying easternmost North America. The MAT exhibit significant trace element ratio heterogeneity (e.g., Sm/Nd= 0.226−0.327) and have trace element traits similar to convergent margin magmas [e.g., depletions of Nb and Ti relative to the rare earth elements on normalized trace element incompatibility diagrams, Ba/Nb ratios (19–75) that are significantly greater than those of MORB, and low TiO₂ (0.39–0.69%)].Geochemical and geological considerations very strongly suggest that the MAT were not significantly contaminated during ascent through the continental crust. Further, isotope and trace element variations are not consistent with the involvement of contemporaneous MORB or OIB components. Rather, the materials that control the MAT incompatible element chemistry were derived from subcontinental lithospheric mantle. Thus: (1) the MAT/arc magma trace element similarities; (2) the PbPb and Sm/Nd isochron ages; and (3) the need for a method of introducing an ancient (> 2−3 Ga) Pb component into subcontinental mantle that cannot be much older than 1 Ga leads to a model whereby the MAT were generated by the melting of sediment-contaminated arc mantle that was incorporated into the continental lithosphere during arc activity preceding the Grenville Orogeny (≈ 1000 Ma).     

Petrogenesis of an 800 m lava sequence in eastern Uruguay: insights into magma chamber processes beneath the Paraná flood basalt province

Major and trace element along with representative Sr, Nd and Pb isotope data are presented for drill core samples which intersect an 800 m lava pile in eastern Uruguay. The lavas form part of the Paraná flood basalt province, are low-Ti in composition but distinct from the low-Ti Gramado magma type, and have been termed the Treinte Y Trés magma type. The lava pile overlies a large positive gravity anomaly inferred to reflect an east–west trending, mid-crustal mafic intrusive body with a calculated volume of 35,000 km³. Smooth up-section compositional variations in the basalts are interpreted to record magma evolution within this mid-crustal magma chamber. ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb increase throughout the sequence yet Mg remains relatively constant in the lower 200 m of the sequence, suggesting a role for magma chamber recharge. Above this the lavas show a regular, up-section decrease in Mg coupled with increasing ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb and this is interpreted to reflect crystal fractionation combined with crustal contamination. The data provide further evidence that contamination of flood basalt magmas in crustal magma chambers is a common phenomenon and calculations suggest that the amount of crustal addition may be as high as 60–70%. Nevertheless, the effects of this crustal contamination do not appear able to account for the discrepancy between key incompatible trace element ratios and isotope ratios of the lavas and those of any putative mantle plume. In fact, La/Ta decreases with decreasing Mg and increasing ⁸⁷Sr/⁸⁶Sr indicating that the effects of crustal contamination were actually to reduce La/Ta and implying that the parental magmas had very high La/Ta (90). These constraints are clearly inconsistent with an asthenospheric origin for the parental magmas and so, consistent with mass balance calculations, it is inferred that they were derived from the lithospheric mantle.