A Pb isotope investigation of the Lovozero Agpaitic Nepheline Syenite,Kola Peninsula,Russia

For the first time Pb isotope composition was established in Lovozero rocks and raremetal ores, which is important for identifying their sources. The world’s largest layered intrusion of agpaitic nepheline syenite-the Lovozero alkaline massif—is located near the center of the Kola Peninsula in Russia. This superlarge complex plutonic body hosts the economically important loparite and eudiallyte deposits [1]. These deposits contain immense resources of REE, Nb, Ta, Zr, and constitute a world class mineral district. The Lovozero massif belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Previous bulk rock studies have shown that the initial Sr and Nd isotope ratios of Lovozero rocks plot in the depleted mantle quadrant of Sr-Nd diagrams [2]. More recently, Hf isotope data obtained by Kogarko et al. (3) confirm that the Lovozero and Khibina massifs with ?_Hf between 6 and 8 are derived predominantly from a depleted mantle source. It was shown that Sr, Nd, and Hf abundances are significantly elevated in the Kola alkaline rocks, and thus their isotopic compositions are relatively insensitive to minor contamination by the overlying crustal rocks. By contrast, Pb in the KACP rocks is a much more sensitive indicator of a crustal component. In this paper we investigate the lead isotopic signature of all resentative types of Lovozero rocks (Table 1) in order to further characterize their mantle sources. The Lovozero massif consists of four intrusive phases. Rocks of phase I (mostly nepheline syenites) comprise about 5% of the total volume, phase II (urtites, foyaite, lujavrites) forms the main portion of the massif comprising 77% in volume, and phase III (eudialyte lujavrites) contributes about 18%. Country rocks are represented by Devonian effusive rocks and Archean gneisses.     

Compilation of radiogenic isotope data in Mexico and their petrogenetic implications

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.     

The Pb-Sr-Nd isotope geochemistry of some recent circum-Mediterranean granites

Pb, Sr and Nd isotopic compositions have been analyzed in recent granites from Northern Africa, Northern Italy and Greece. Lead isotope compositions of K-feldspars are rather homogeneous, and cluster close to the modern lead of Stacey and Kramers (1975) but with slightly higher²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios. The Cyclades samples, however, have higher²⁰⁶Pb/²⁰⁴Pb ratios. Addition of mantle-derived lead was probably very limited, which supports a quasi-closed system evolution of this element in the continental crust. The Sr, Nd data fall in the enriched part of the¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr diagram and define a smooth hyperbolic mixing curve. Over a wide area, straddling different orogens, most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle-like component. No correlation is observed between either Pb and Sr or Nd isotopic ratios, or any isotopic ratio and major element contents. Quantitative modelling suggests that two cases fit the Sr and Nd characteristics of these granites: they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes. In the first case, the mantle-derived component may be represented by differentiated Island Arc-type magmas, and the granites result from mixing these magmas with anatectic melts. In the second case, mantle-derived igneous rocks, such as obducted ophiolites, are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations.CRPG Contribution n 630.     

Behavior of isotopic systematics during deformation and metamorphism: a Hf,Nd and Sr isotopic study of mylonitized granite

A combined compositional and Hf, Nd and Sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the Harquahala Mountains, western Arizona, to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks. The 1.4 Ga Harquahala Granite was deformed during Mesozoic thrusting along the Harquahala thrust. Granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite. In most of these mylonites, the protolith is not megascopically recognizable, and can be discerned only by the progressive transition to undeformed granite. Isotopic analyses of Hf, Nd and Sr from the shear zone document the immobility of the Hf and Nd isotopic systems relative to that of the Sr isotopic system during deformation. The Rb–Sr isotopic data show considerable scatter on an isochron plot, exhibiting both gains and losses of Rb and Sr from the whole-rock systems. In contrast, the Sm–Nd and Lu–Hf isotopic systematics are mostly well behaved on isochron diagrams, plotting mostly in tight clusters or along 1.4 Ga isochrons. These results show that while the Sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance, both Hf and Nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism.     

The genesis of felsic magmatism during the closure of the Northeastern Paleo-Tethys Ocean: Evidence from the Heri batholith in West Qinling,China

To better understand the origin of voluminous silicic rocks in a convergent continental margin, we conducted an integrated study in which we have obtained geochronological, mineralogical, and isotopic (including whole-rock Sr–Nd–Pb, in-situ zircon Hf) data of the Heri batholith in West Qinling on the NE Tibetan Plateau. The batholith is composed of metaluminous to weakly peraluminous granodiorites (235–233 Ma) and porphyritic granodiorites (230–223 Ma) with an I-type affinity. Both lithologies share similar major, trace elemental and SrNd isotopic compositions. Detailed elemental data demonstrate that these granodioritic rocks underwent fractional crystallization of hornblende and apatite, with plagioclase (i.e. sieve-textured plagioclase cores) accumulation to some extent. Except for porphyritic granodiorites, the Pb isotopes for other analyzed samples are characterized by high radiogenicity and uniformity ((²⁰⁶Pb/²⁰⁴Pb)_t: 17.263–18.472, (²⁰⁶Pb/²⁰⁴Pb)_t: 15.571–15.591, and (²⁰⁶Pb/²⁰⁴Pb)_t: 38.032–38.304), together with limited variations in initial Sr ((⁸⁷Sr/⁸⁶Sr)_t: 0.707251–0.708103) and Nd (ε_Nd(t) = −7.1 to −6.3) isotopes with two-stage model ages (T_DM2) of 1.58–1.52 Ga. These factors collectively point to a derivation from the Mesoproterozoic basement rocks at the lower crustal level, or a comprehensive mixing of different-age components that generated an average crustal residence age. The SrNd isotopic compositions of the porphyritic granodiorites are strikingly similar to those of granodiorites. Compared with the experimental melt compositions of amphibolites, the Heri granitoids are probably derived from an amphibolitic source under fluid-absent conditions due to the incongruent breakdown of amphibole and biotite. Based on the temporal–spatial distribution of granitic intrusions in West Qinling and the regional tectonic evolution, our interpretation is that the Heri batholith was formed during the initial collision between the North China Craton (NCC) and the South China Craton (SCC), which was accompanied by the closure of the Paleotethyan Ocean. Considering both previously published data and our new data, we propose that the Heri granitoids were mainly generated by the partial melting of lower crustal amphibolites, with minor mantle-derived melts.     

Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania

 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 Δɛ_Nd 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     

Isotopic systematics of He,Ar, S,Cu, Ni,Re, Os,Pb, U,Sm, Nd,Rb, Sr,Lu, and Hf in the rocks and ores of the Norilsk deposits

This paper reports the first results of a study of 11 isotope systems (³He/⁴He, ⁴⁰Ar/³⁶Ar, ³⁴S/³²S, ⁶⁵Cu/⁶³Cu, ⁶²Ni/⁶⁰Ni, ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ^206–208Pb/²⁰⁴Pb, Hf–Nd, U–Pb, and Re–Os) in the rocks and ores of the Cu–Ni–PGE deposits of the Norilsk ore district. Almost all the results were obtained at the Center of Isotopic Research of the Karpinskii All-Russia Research Institute of Geology. The use of a number of independent genetic isotopic signatures and comprehensive isotopic knowledge provided a methodic basis for the interpretation of approximately 5000 isotopic analyses of various elements. The presence of materials from two sources, crust and mantle, was detected in the composition of the rocks and ores. The contribution of the crustal source is especially significant in the paleofluids (gas–liquid microinclusions) of the ore-forming medium. Crustal solutions were probably a transport medium during ore formation. Air argon is dominant in the ores, which indicates a connection between the paleofluids and the atmosphere. This suggests intense groundwater circulation during the crystallization of ore minerals. The age of the rocks and ores of the Norilsk deposits was determined. The stage of orebody formation is restricted to a narrow age interval of 250 ± 10 Ma. An isotopic criterion was proposed for the ore-bearing potential of mafic intrusions in the Norilsk–Taimyr region. It includes several interrelated isotopic ratios of various elements: He, Ar, S, and others.     

中国东南沿海中-新生代玄武岩微量元素和Nd-Sr-Pb同位素研究

本文对中国东南沿海不含幔源包体的中生代玄武岩和含幔源包体的新生代玄武岩进行了微量元素和Nd-Sr-Pb同位素对比研究。中生代玄武岩呈Ta、Nb和Hf负异常,低Ce/Pb、Nb/U比值和高La/Nb比值,与岛弧火山岩和陆壳岩石的微量元素特征相类似,说明在岩浆生成和上升过程中,幔源组分受到了陆壳组分的混染。新生代玄武岩呈Ta、Nb正异常和Pb负异常,高Ce/Pb、Nb/U比值和低La/Nb比值,与海岛玄武岩(OIB)相类似,Nd-Sr同位素成分与夏威夷玄武岩类似,因而它们未受明显的陆壳混染。143Nd/144Nd与206Pb/204Pb之间的负相关关系和87Sr/86Sr与206Pb/204Pb之间的正相关关系说明本区新生代玄武岩起源于中等亏损程度的软流圈地幔,并与EMII富集地幔组分发生了混合。     

Intraplate volcanism in New Zealand: the role of fossil plume material and variable lithospheric properties

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.     

Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field,California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (⁸⁷Sr/⁸⁶Sr=0.7036,²⁰⁶Pb/²⁰⁴Pb=19.05,²⁰⁷Pb/²⁰⁴Pb=15.62,²⁰⁸Pb/²⁰⁴Pb= 38.63). The (initial) isotopic composition of typical rhyolite (⁸⁷Sr/⁸⁶Sr=0.7053,²⁰⁶Pb/²⁰⁴Pb=19.29,²⁰⁷Pb/²⁰⁴Pb= 15.68,²⁰⁸Pb/²⁰⁴Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.     

Pb-Sr-Nd isotopic compositions and trace element geochemistry of megacrysts and melilitites from the Tertiary Urach volcanic field: source composition of small volume melts under SW Germany

 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 ɛ_Nd 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 (ɛ_Nd +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     

H,O, Sr,Nd, and Pb isotope geochemistry of the Latir volcanic field and cogenetic intrusions,New Mexico,and relations between evolution of a continental magmatic center and modifications of the lithosphere

Over 200 H, O, Sr, Nd, and Pb isotope analyses, in addition to geologic and petrologic constraints, document the magmatic evolution of the 28.5–19 Ma Latir volcanic field and associated intrusive rocks, which includes multiple stages of crustal assimilation, magma mixing, protracted crystallization, and open- and closed-system evolution in the upper crust. In contrast to data from younger volcanic centers in northern New Mexico, relatively low and restricted primary ¹⁸O values (+6.4 to +7.4) rule out assimilation of supracrustal rocks enriched in ¹⁸O. Initial ⁸⁷Sr/⁸⁶Sr ratios (0.705 to 0.708), ¹⁸O values (-2 to-7), and ²⁰⁶Pb/²⁰⁴Pb ratios (17.5 to 18.4) of metaluminous precaldera volcanic rocks and postcaldera plutonic rocks suggest that most Latir rocks were generated by fractional crystallization of substantial volumes of mantle-derived basaltic magma that had near-chondritic Nd isotope ratios, accompanied by assimilation of crustal material in two main stages: 1) assimilation of non-radiogenic lower crust, followed by 2) assimilation of middle and upper crust by inter-mediate-composition magmas that had been contaminated during the first stage. Magmatic evolution in the upper crust peaked with eruption of the peralkaline Amalia Tuff (26 Ma), which evolved from metaluminous parental magmas. A third stage of late, roofward assimilation of Proterozoic rocks in the Amalia Tuff magma is indicated by trends in initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios from 0.7057 to 0.7098 and 19.5 to 18.8, respectively, toward the top of the pre-eruptive magma chamber. Highly evolved postcaldera plutons are generally fine grained and are zoned in initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios, varying from 0.705 to 0.709 and 17.8 to 18.6, respectively. In contrast, the coarser-grained Cabresto Lake (25 Ma) and Rio Hondo (21 Ma) plutons have relatively homogeneous initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios of approximately 0.7053 and 17.94 and 17.55, respectively. ¹⁸O values for all the postcaldera plutons overlap those of the precaldera rocks and Amalia Tuff, except for those for two late-stage rhyolite dikes associated with the Rio Hondo pluton that have ¹⁸O values of-8.6 and-9.5; these dikes are the only Latir rocks which may be largely crustal melts.Chemical and isotopic data from the Latir field suggest that large fluxes of mantle-derived basaltic magma are necessary for developing and sustaining large-volume volcanic centers. Development of a detailed model suggests that 6–15 km of new crust may have been added beneath the volcanic center; such an addition may result in significant changes in the chemical and Sr and Nd isotopic compositions of the crust, although Pb isotope ratios will remain relatively unchanged. If accompanied by assimilation, crystallization of pooled basaltic magma near the MOHO may produce substantial cumulates beneath the MOHO that generate large changes in the isotopic composition of the upper mantle. The Latir field may be similar to other large-volume, long-lived intracratonal volcanic fields that fundamentally owe their origins to extensive injection of basaltic magma into the lower parts of their magmatic systems. Such fields may overlie areas of significant crustal growth and hybridization.     

K-Ar ages and Pb,Sr isotopic characteristics of Cenozoic volcanic rocks in Shandong Province,China

28 samples of Cenozoic volcanic rocks collected from Shandong Province have been dated by K-Ar method. They are mainly Neogene with an age range of 4–19 m. y. The basalts from Linqu and Yishui in west Shandong Province are Miocene and those from Penglai and Qixia in east Shandong Province are Miocene and Pliocene in age. The basalts from Wudi in north Shandong Province are Middle-Early Pleistocene in age. In each area the duration of volcanic eruption was estimated at about 2–3 m. y. Pb and Sr isotopic compositions and U, Th, Pb, Rb, Sr, and major elements in most of the samples were determined. The isotopic compositions are:²⁰⁶Pb/²⁰⁴Pb—16.92-18.48,²⁰⁷Pb/²⁰⁴Pb—15.30-15.59,²⁰⁸Pb/²⁰⁴Pb—37.83-38.54, and (⁸⁷Sr/⁸⁶Sr)_i—0.70327-0.70632. There are some positive or negative linear correlations between²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb, Pb isotopes and Pb content, Pb isotopes and Sr isotopes, and Sr isotopes and other elements. The basaltic rocks from east and west Shandong Province have somewhat differences in isotopic composition and element content. The basalts probably are products of multi-stage evolution of the mantle. They have preserved the primary features of the source, although they were influenced, to some extent, by the contamination of crustal materials.     

Sr,Nd and Pb Isotopic Systematics of the Cenozoic Basalts of the Korean Peninsula and Their Implications for the Permo-Triassic Continental Collision Boundary

Sr, Nd and Pb isotopic compositions of the Cenozoic basalts were analyzed from Baengnyeongdo Island, Jeongok, Ganseong, and Jejudo Island of Korea. They reveal relatively enriched Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.70330∼0.70555, ¹⁴³Nd/¹⁴⁴Nd = 0.51298∼0.51256) compared with MORB.²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of all the analyzed Korean basalts lie above the Northern Hemisphere Reference Line (NHRL) defined by Hart (1984). Pb isotopic compositions of basalts from Jejudo Islands (²⁰⁶Pb/²⁰⁴Pb = 18.61∼19.12, ²⁰⁷Pb/²⁰⁴Pb = 15.54∼15.69, ²⁰⁸Pb/²⁰⁴Pb = 38.98∼39.72) are significantly more radiogenic than the rest (²⁰⁶Pb/²⁰⁴Pb = 17.72∼18.03, ²⁰⁷Pb/²⁰⁴Pb = 15.44∼15.58, ²⁰⁸Pb/²⁰⁴Pb = 37.77∼38.64). The Cenozoic Korean basalts thus can be divided into two groups based on their Sr, Nd and Pb isotopic compositions. The north group reveals mixing between DMM and EM1 while the south group displays DMM-EM2 mixing. Such a distribution is the same as Chinese Cenozoic basalts and it can be interpreted that the subcontinental lithospheric mantle under Korea represents simple lateral continuation of the South and North China Blocks. We suggest that Korean continental collision zone cross the Korean Peninsula through the region between the north and south basalt groups of Korea.     

Petrogenesis of plio-quaternary intra-plate continental alkaline lavas from the İskenderun Gulf (Southern Turkey): Evidence for metasomatized lithospheric mantle

The Quaternary alkaline volcanic field of Southern Turkey is characterized by intra-continental plate-type magmatic products, exposed to the north of the ?skenderun Gulf along a NE-SW trending East Anatolian Fault, to the west of its intersection with the N–S trending Dead Sea Fault zone. The ?skenderun Gulf alkaline rocks are mostly silica-undersaturated with normative nepheline and olivine and are mostly classified as basanites and alkaline basalts with their low-silica contents ranging between 43 and 48?wt.% SiO₂. They display Ocean Island Basalt (OIB)–type trace element patterns characterized by enrichments in large-ion-lithophile elements (LILE) and light rare earth element (LREE), and have (La/Yb)_N?=?8.8–17.7 and (Hf/Sm)_N?=?0.9–1.6 similar to those of basaltic rocks found in intraplate suites. The basanitic rocks have limited variations Sr-Nd isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.70307–0.70324, ¹⁴³Nd/¹⁴⁴Nd?=?0.512918–0.521947), whereas the alkali basalts display more evolved Sr-Nd isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.70346-0.70365, ¹⁴³Nd/¹⁴⁴Nd?=?0.512887–0.521896). The ?skenderun Gulf alkaline rocks also display limited Pb isotopic variations with ²⁰⁶Pb/²⁰⁴Pb?=?18.75–19.09 ²⁰⁷Pb/²⁰⁴Pb?=?15.61–15.66 and²⁰⁸Pb/²⁰⁴Pb?=?38.65–39.02, indicating that they originated from an enriched lithospheric mantle source. Calculated fractionation vectors indicate that clinopyroxene and olivine are the main fractionating mineral phases. Similarly, based on Sr-Nd isotopic ratios, the assimilation and fractional crystallization (AFC) modeling shows that the alkali basalts were affected by AFC processes (r?=?0.2) and were slightly contaminated by the upper crustal material.The high TiO₂ contents, enrichments in Ba and Nb, and depletions in Rb can likely be explained by the existence of amphibole in the mantle source, which might, in turn, indicate that the source mantle has been affected by metasomatic processes. The modeling based on relative abundances of trace elements suggests involvement of amphibole-bearing peridotite as the source material. ?skenderun Gulf alkaline rocks can thus be interpreted as the products of variable extent of mixing between melts from both amphibole-bearing peridotite and dry peridotite.     

松树沟元古宙蛇绿岩Nd、Sr、Pb同位素地球化学特征

松树沟蛇绿岩中镁铁质岩的Sm-Nd全岩等时年龄为1030±46Ma,∈_Nd(t)=+5.7,模式年龄在1422-1271Ma之间,矿物的内部等时年龄为983±140Ma。全岩等时年龄为其形成上限,矿物内部等时年龄为其变质年龄,说明该蛇绿岩形成于中元古代中、晚期。镁铁质岩的∈_Nd(t)在+4.2-+6.9之间,是DMM与EMI两个地幔端元的混合产物;²⁰⁶Pb/²⁰⁴Pb在18.06-18.66之间,²⁰⁷Pb/²⁰⁴Pb和²⁰⁸Pb/²⁰⁴Pb值较高,分别在15.55-15.60和37.59-38.38之间,²⁰⁷Pb/²⁰⁴Pb和²⁰⁸Pb/²⁰⁴Pb对²⁰⁶Pb/²⁰⁴Pb的关系表明,岩浆源于具DUPAL异常的源区,(²⁰⁷Pb/²⁰⁴Pb)_i相对(²⁰⁸Pb/²⁰⁴Pb)_i更偏离NHRL可能是变质流体作用的反映。∈_Sr(t)值较高且变化大,可与蛇绿岩类比,⁸⁷Sr/⁸⁶Sr比值变化大,可能与海水蚀变作用有关。据此推测,松树沟蛇绿岩代表洋壳残片。     

Transition from calc-alkaline to potassium-rich magmatism in subduction environments: geochemical and Sr, Nd, Pb isotopic constraints from the island of Vulcano (Aeolian arc)

The problem of mantle metasomatism vs. crustal contamination in the genesis of arc magmas with different potassium contents has been investigated using new trace element and Sr–Nd–Pb isotopic data on the island of Vulcano, Aeolian arc. The analysed rocks range in age from 120 ka to the present day, and cover a compositional range from basalt to rhyolite of the high-K calc-alkaline (HKCA) to shoshonitic (SHO) and potassic (KS) series. Older Vulcano products (>30 ka) consist of HKCA–SHO rocks with SiO₂=48–56%. They show lower contents of K₂O, Rb and of several other incompatible trace element abundances and ratios than younger rocks with comparable degree of evolution. ⁸⁷Sr/⁸⁶Sr ranges from 0.70417 to 0.70504 and increases with decreasing MgO and compatible element contents. ²⁰⁶Pb/²⁰⁴Pb ratios display significant variations (19.31 to 19.76) and are positively correlated with MgO, ¹⁴³Nd/¹⁴⁴Nd (0.512532–0.512768), ²⁰⁷Pb/²⁰⁴Pb (15.66–15.71) and ²⁰⁸Pb/²⁰⁴Pb (39.21–39.49). Overall, geochemical and isotopic data suggest that the evolution of the older series was dominated by assimilation–fractional crystallisation (AFC) with an important role for continuous mixing with mafic liquids. Magmas erupted within the last 30 ka consist mostly of SHO and KS intermediate and acid rocks, with minor mafic products. Except for a few acid rocks, they display moderate isotopic variations (e.g. ⁸⁷Sr/⁸⁶Sr=0.70457–0.70484; ²⁰⁶Pb/²⁰⁴Pb=19.28–19.55, but ²⁰⁷Pb/²⁰⁴Pb=15.66–15.82), which suggest an evolution by fractional crystallisation, or in some cases by mixing, with little interaction with crustal material. The higher Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr=0.70494–0.70587) of a few, low-volume, intermediate to acid rocks support differentiation by AFC at shallow depths for some magma batches. New radiogenic isotope data on the Aeolian islands of Alicudi and Stromboli, as well as new data for lamproites from central Italy, are also reported in order to discuss along-arc compositional variations and to evaluate the role of mantle metasomatism. Geochemical and petrological data demonstrate that the younger K-rich mafic magmas from Vulcano cannot be related to the older HKCA and SHO ones by intra-crustal evolutionary processes and point to a derivation from different mantle sources. The data from Alicudi and Stromboli suggest that, even though interaction between magma and wall rocks of the Calabrian basement during shallow level magma evolution was an important process locally, a similar interpretation can be extended to the entire Aeolian arc. Received: 27 September 1999 / Accepted: 24 May 2000     

Mineralization age and sources of ore‐forming material of the Nanmushu Zn‐Pb deposit in the Micangshan Tectonic Belt at the northern margin of the Yangtze Craton,China: Constraints from Rb‐Sr dating and Sr‐Pb isotopes

The Nanmushu Zn‐Pb deposit, hosted by the Neoproterozoic Dengying Formation dolostone, is located in the eastern part of the Micangshan tectonic belt at the northern margin of the Yangtze Craton, China. This study involves a systematic field investigation, detailed mineralogical study, and Rb‐Sr and Pb isotopic analyses of the deposit. The results of Rb‐Sr isotopic dating of coexisting sphalerite and galena yield an isochron age of 486.7 ± 3.1 Ma, indicating the deposit was formed during the Late Cambrian to Early Ordovician. This mineralization age is interpreted to be related to the timing of destruction of the paleo‐oil reservoir in the Micangshan tectonic belt. All initial ⁸⁷Sr/⁸⁶Sr ratios of sphalerite and galena (0.70955–0.71212) fall into the range of the Mesoproterozoic Huodiya Group basement rocks (0.70877–0.71997) and Dengying Formation sandstone (0.70927–0.71282), which are significantly higher than those of Cambrian Guojiaba Formation limestone (0.70750–0.70980), Cambrian Guojiaba Formation carbonaceous slate (0.70766–0.71012), and Neoproterozoic Dengying Formation dolostone (0.70835–0.70876). Such Sr isotope signatures suggest that the ore strontium was mainly derived from a mixed source, and both of the Huodiya Group basement rocks and Dengying Formation sandstone were involved in ore formation. Both sphalerite and galena are characterized by an upper‐crustal source of lead (²⁰⁶Pb/²⁰⁴Pb = 17.849–18.022, ²⁰⁷Pb/²⁰⁴Pb = 15.604–15.809, and ²⁰⁸Pb/²⁰⁴Pb = 37.735–38.402), and their Pb isotopes are higher than, but partly overlap with, those of the Huodiya Group basement rocks, but differ from those of the Guojiaba and Dengying Formations. This suggests that the lead also originated from a mixed source, and the Huodiya Group basement rocks played a significant role. The Sr and Pb isotopic results suggest that the Huodiya Group basement rocks were one of the most important sources of metallogenic material. The geological and geochemical characteristics show that the Nanmushu Zn‐Pb deposit is similar to typical Mississippi Valley type, and the fluid mixing may be a reasonable metallogenic mechanism for Nanmushu Zn‐Pb deposit.     

A Pb isotope investigation of the Guli massif, Maymecha-Kotuy alkaline-ultramafic complex, Siberian flood basalt province, Polar Siberia

Summary Covering a vast area of the northern Siberian platform are the Siberian flood basalts (SFB), which make up one of the world’s largest magmatic provinces. Along the northeastern margin of the SFB province lies the Maymecha-Kotuy alkaline-ultramafic complex, consisting of a large volume of alkaline lavas, numerous dykes, and the Guli massif together with numerous other, smaller alkaline plutons. The genetic link between the SFB and the Maymecha-Kotuy complex continues to be a subject of active debate. Although the rocks in both units have essentially the same age close to the Permian-Triassic boundary, questions remain as to the relative order of emplacement and the contributing source materials of each lithology. This study builds upon earlier petrologic, geochemical, and isotopic work to further an understanding of the relationship between SFB and alkaline rocks. A whole-rock U-Pb age of 250 ± 9 Ma was determined for the Guli massif, which lies within the range of ages previously reported for the SFB. The Pb isotopic composition of the Guli rocks plot mainly in the lower portion of the OIB field, and dunite and carbonatite extend downward into the MORB field suggesting for them a more depleted source than the one that produced the SFB. The combined Pb, Sr, and Nd isotopic systematics of the SFB and the Guli alkaline rocks enable the identification of several discrete source components. The first component dominates many of the Guli rocks and is characterized by low ⁸⁷Sr/⁸⁶Sr (0.7031 to 0.7038), high ε_Nd (+5.35 to +3.97), and relatively unradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb = 17.88–18.31; ²⁰⁷Pb/²⁰⁴Pb = 15.38–15.46; ²⁰⁸Pb/²⁰⁴Pb = 37.33–37.70), which we associate with the depleted (MORB source) mantle. The second component representing most of the SFB demonstrates a notable chemical and isotopic uniformity with ⁸⁷Sr/⁸⁶Sr values of 0.7046 to 0.7052, ε_Nd values of 0 to +2.5, and an average Pb isotopic composition of ²⁰⁶Pb/²⁰⁴Pb = 18.3, ²⁰⁷Pb/²⁰⁴Pb = 15.5, and ²⁰⁸Pb/²⁰⁴Pb = 38.0. This component, making up the majority of SFB, is speculated to be a relatively primitive lower mantle plume with a near-chondritic signature. Contamination by upper and lower continental crustal material, designated as components 3 and 4, is postulated to explain the isotopic characteristics of some of the higher SiO₂ Guli rocks and SFB. Finally, metasomatic processes associated with the invasion of the Siberian super-plume add a fifth component responsible for the extreme enrichment in rare-earth and related elements found in some Guli rocks and SFB.     

Nd,Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (_Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, ⁸⁷Sr/⁸⁶Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (²⁰⁶Pb/²⁰⁴Pb=18.35–18.84, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.97 and ²⁰⁸Pb/²⁰⁴Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.