Chemical and isotopic compositions of Absaroka granitoids,Southwestern Montana

Contents of major and most trace elements of granitoids in three intrusions associated with the Cretaceous Independence volcanic complex, Montana, correlate well with SiO₂. Major-element contents in granitoids in each intrusion are accurately modeled as mixtures of minimum melts and phenocryst assemblages (presumably restite). Restite assemblages are hypersthene+augite+plagioclase, hornblende+plagioclase, and biotite+plagioclase+quartz. Residues of melting are granulite or amphibolite. Melts in two of the bodies were LREE-enriched but unfractionated in MREE and HREE. REE patterns are consistent with residues dominated by pyroxene or amphibole and feldspar. Initial ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb of granitoids define a line interpreted as a secondary isochron established during crustal homogenization 3.3 Ga ago. The relatively low of source rocks (8.25) suggests that they did not spend long in U-rich environments. Source regions had variable trace element patterns; Th/Pb and U/Pb were correlated, Rb/Sr and Sm/Nd moderately well correlated, but Rb/Sr and U/Pb were decoupled. This is consistent with poor correlation of Rb, Sr and Ba with SiO₂ in some granitoids and may suggest that minor phases that concentrate these elements were inhomogeneously distributed in source regions. The source probably consisted of LREE-rich, Rb-poor metamorphic rocks. Archean amphibolites, exposed in the Beartooth Mountains, are similar to the postulated source materials. They contain plagioclase, hornblende, minor quartz, biotite, and muscovite, and have low Rb/Sr and high LREE/HREE. Certain trace-element characteristics of the granitoids indicate that the deep crust in this part of Montana may be dominated by metamorphosed mafic-intermediate lavas that formed on the sea-floor. Metapelites, intercalated with amphibolites at the surface, were rare in granitoid source regions. This buried supracrustal pile was isotopically homogenized 3.3 Ga ago. Although some material melted 2.7 Ga ago to form granites that dominate the exposed basement, enough remained fertile that heating by mantle-derived magmas 85–90 Ma ago produced the granitic rocks at Independence.     

Nd-Sr-Pb systematics and age of the Kings River ophiolite,California: implications for depleted mantle evolution

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with _Nd (483)= +10.7±0.5 and Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron. The 483 Ma data provide the first evidence of lower Paleozoic oceanic crust in the Sierran ophiolite belt. New U-Pb analyses of zircons from a plagiogranite lying on the 483 Ma Sm-Nd isochron yield upper and lower intercepts with the concordia of 430 _–60 ⁺²⁰⁰ and 183±15 Ma. Published zircon ages have underestimated the primary age of the ophiolite by 200–300 m.y. due to the effects of polymetamorphism. The 483 Ma samples have initial ⁸⁷Sr/⁸⁶Sr=0.7023–0.7030, ²⁰⁶Pb/²⁰⁴Pb=17.14–17.82, ²⁰⁷Pb/²⁰⁴Pb=15.37–15.52, ²⁰⁸Pb/²⁰⁴Pb=36.80–37.38. The 285 Ma samples have similar initial ⁸⁷Sr/⁸⁶Sr, but more radiogenic Pb. The range in Sr and Pb compositions is probably due to introduction of radiogenic Sr and Pb during multiple post-emplacement metamorphic events. The high _Nd, low ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 Ma portion of the ophiolite. Igneous activity at 285 Ma is thought to have occurred in an arc or back-arc setting, or perhaps along a leaky transform. The initial _Nd (483)=+10.7 is indistinguishable from that of the similar age Trinity Peridotite (Jacobsen et al. 1984). This value is the highest yet reported for the Mesozoic or Paleozoic depleted mantle and requires either a mantle source that was depleted 850 m.y. earlier than average or a source more highly depleted than average. Alternatively, if such values were more typical of the early Paleozoic mantle than is currently thought, then there has been little evolution of the depleted mantle over the last 500 m.y. This requires that the modern mantle has been refluxed by material with low _Nd, such as continental crust.Division Contribution # 4302 (530)     

Crustal outgassing and LILE enrichment in major lithosphere structures,Archean Abitibi greenstone belt: evidence on the source reservoir from strontium and carbon isotope tracers

Major structural discontinuities in the Abitibi greenstone belt acted as conduits for outgassing of the Archean crust, as reflected in fixation of a select group of lithophile elements including Si, C, K, Rb, Ba, Li, Cs, B and Pb, in metasomatized faults. For two of the largest structures, the Destor-Porcupine (DP) and Kirkland Lake — Cadillac (KC) fault zones 6×10¹⁵ g Si, 3×10¹⁵ g CO₂ and 10¹⁵ g K were introduced into the faults during expulsion of an estimated 6×10¹⁸ g aqueous fluids. Strontium isotope ratios of tourmaline, piemontite, actinolite and scheelite mineral separates, characterized by Rb/Sr0.02, are concordant with respect to ⁸⁷Sr/⁸⁶Sr initial ratios over local sectors of the faults. The Sr isotope data record geographic variations which, from east to west on the KC fault is 0.7031–0.7041 (Val d'Or), 0.7008–0.7022 (Bourlemaque), 0.7017–0.7019 (Bousquet), 0.7029–0.7031 (Noranda), and 0.7013 to 0.7015 (Kirkland Lake). At Timmins, on the PD fault, ⁸⁷Sr/⁸⁶Sr initial ratios cluster at 0.7010 to 0.7020. Metasomatised fault zones are systematically more radiogenic than contiguous host lithologies, and imply a source reservoir (0.7010 to 0.7041) generally more radiogenic than the upper mantle at 2690 Ma (0.700±0.001), or contemporaneous volcanic rocks of mafic to ultramafic composition (0.700 to 0.7012). Whereas certain minerals are concordant and retentive, Rb-Sr isochrons based on suites of rocks at progressive intensities of metasomatism, have been systematically reset over an elpased time of 200 Ma after termination of outgassing, due to disturbance accompanying incremental displacements on structures.Carbon isotope compositions of ferroan dolomites in faults are tightly clustered along local fault sectors, but also display a marked provinciality: from east to west ¹³C=–6.0 to –8.5 (Malartic), –8.0 to –9.0 (Cadillac), –2.0 to –4.5 (Kirkland Lake), and –0.5 to –3.5 (Timmins). The observed provinciality of both ¹³C values and ⁸⁷Sr/⁸⁶Sr initial ratios is interpreted to reflect compositional heterogeneities in a radiogenic sialic crust and the green-stone belt supracrustal sequence, both of which supplied volatiles, magmas and lithophile elements to the fault structures during late stage transpressive tectonics.     

Origin and regional significance of late Precambrian and early Palaeozoic granitoids in the Pan-African belt of Somalia

Granitoids within the Precambrian basement of north-eastern and southern Somalia are subdivided on the basis of geology, geochronology and petrology into three different assemblages. The post-kinematic assemblage in north-eastern Somalia ( 630 Ma) comprises granodiorites and granites which belong to a medium-K calc-alkaline suite. Average initial Sr, Nd and Pb isotopic ratios [Sr_i = 0.7048, _Nd = –1.8,²⁰⁶Pb/²⁰⁴Pb(i) = 17.704 and²⁰⁷Pb/²⁰⁴Pb(i) = 15.611] indicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contaminant. Two different assemblages are found in southern Somalia. The older assemblage is composed of crustal anatectic, synkinematic, parautochthonous granites ( 600 Ma) related to amphibolite facies retrogression of an intensively reworked pre-Pan-African crust [Sr_i = 0.7100, _Nd = –8.4,²⁰⁶Pb/²⁰⁴Pb(i) = 15.403 and²⁰⁷Pb/²⁰⁴Pb(i) = 15.259]. These monzo- and syenogranites are moderately potassic and peraluminous. The younger assemblage ( 470 Ma) consists of post-kinematic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage [Sr_i = 0.7114, _Nd = –13.1,²⁰⁷Pb/²⁰⁴Pb(i) = 16.913 and²⁰⁷Pb/²⁰⁴Pb(i) = 15.512] indicate a significant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic granitoids in Somalia thus express contrasting regimes, characterized by strong juvenile input in the north, close to the Arabian-Nubian Shield, whereas intense crustal reworking with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area during the Pan-African, but a zone of high crustal mobility.     

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.     

Origins of Nd–Sr–Pb isotopic variations in single scheelite grains from Archaean gold deposits,Western Australia

Accessory gangue scheelite (CaWO₄) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique.Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial ⁸⁷Sr/⁸⁶Sr and ε_Nd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have ⁸⁷Sr/⁸⁶Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone.The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that ⁸⁷Sr/⁸⁶Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite.A positive correlation between Na and REE suggests that REE³⁺ are accommodated by the coupled substitution REE³⁺+Na⁺=2 Ca²⁺ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (<m) to medium (<km) scale processes involving mixing between “fresh” batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration.The very high-ε_Nd values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny (<20 μm) grains of secondary hydroxyl-bastnäsite were found within micro-fractures of one scheelite grain containing an extremely high-ε_Nd signature. The hydroxyl-bastnäsite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high-ε_Ndi values observed in scheelite from Western Australia.     

Sr isotopes in peridotite xenoliths and their basaltic host rocks from the northern Hessian Depression (NW Germany)

⁸⁷Sr/⁸⁶Sr ratios of alkali olivine basalts, nepheline basanites and olivine nephelinites of Miocene age from the northern Hessian Depression vary between 0.7032 and 0.7036. Tholeiitic rocks from this area, which are possibly affected by crustal contamination, have more radiogenic Sr (0.7035 to 0.7042). Peridotite xenoliths with coarse protogranular (10 samples) and with porphyroclastic textures (2 samples) contain K- and Na-rich glasses which are products of reaction of metasomatic fluids with depleted peridotite. The Sr abundance in xenoliths is related to the amount of glass (and phlogopite).Sr ranges from 11 ppm to 147 ppm and ⁸⁷Sr/⁸⁶Sr ratios from 0.7033 to 0.7039. The isotopic ratios are neither correlated with Sr concentrations nor with Rb/Sr ratios. ⁸⁷Sr/ ⁸⁶Sr ratios of etched clinopyroxenes range from 0.7028 to 0.7040. In some xenoliths, clinopyroxenes differ from the whole rock samples significantly in their isotopic composition.If almost all of the pre-metasomatic Sr was located in the clinopyroxenes, the metasomatically introduced Sr ranges from 35 to 80% of the whole rock Sr. The calculated isotopic composition ranges from 0.7033 to 0.7040 for the majority of the xenoliths. For two pyroxenes which are not in isotopic equilibrium with the whole rock, the age of the metasomatic event could be estimated on the base of diffusion of Sr in clinopyroxene. Even assuming a diffusion coefficient as low as 10^–15 cm²s^–1 the time between the metasomatic alteration and the eruption of the basaltic host magma must be shorter than 1 Ma.The ⁸⁷Sr/⁸⁶Sr ratios of the basalts are interpreted as products of mixtures of a depleted component ( 0.7028) and metasomatic fluids (0.7035–0.7053) in their source peridotite.     

Western Fiordland orthogneiss: Early Cretaceous arc magmatism and granulite facies metamorphism,New Zealand

U-Pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks, here termed the Western Fiordland Orthogneiss (WFO), in Fiordland, southwest New Zealand, indicate synkinematic magmatic emplacement between 120 and 130 Ma ago. These rocks were previously interpreted as possibly being of Precambrian age. Initial Pb and Sr ratios are consistent with arc/subduction related magmagenesis with little or no involvement of ancient continental crust. Subsequent high pressure (>12 kb) metamorphism of the WFO may reflect a major collision event involving crustal thickening by overthrusting of a >15 km thick sequence. Metamorphism ceased 116 Ma ago based on²⁰⁶Pb/²³⁸U ages of zircon from a retrogressed granulite. U-Pb isotopic analysis of apatite, along with previously published Rb/Sr mineral ages, indicate that final uplift and cooling to <300–400° C was largely completed by 90 Ma. The average uplift rate during this period is inferred to have been in excess of 1 mm/yr.Unmetamorphosed gabbronorites of the Darran Complex in eastern Fiordland, inferred by some investigators to be the granulite protolith, yield concordant U/Pb zircon ages of 137±1 Ma. U-Pb ages of apatite, and previously published K/Ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years. The high-grade WFO and unmetamorphosed Darran Complex are now separated by a profound structural break. However, the ages and similarities in initial Pb and Sr isotopic ratios suggest that both suites are products of the same Early Cretaceous cycle of subduction-related magmatism. The timing of Early Cretaceous magmatism and metamorphism, collision and resultant crustal thickening, and subsequent great uplift and erosion in Fiordland has important implications for terrane accretion and the evolution of relative plate motions along the New Zealand segment of the Gondwana margin.     

Shear zone-related syenites in the Damara belt (Namibia): the role of crustal contamination and source composition

Geochemical and Nd-Sr-Pb-O isotope data for a suite of syn-collisional (ca. 520 Ma) syenites associated with a major shear zone in the Proterozoic Damara orogen (Namibia) constrain their sources and petrogenesis. Major rock types from within and outside the shear zone range from highly potassic nepheline syenites to quartz syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr and P on a primitive mantle-normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites from within the shear zone back to a hypothetical parental melt with 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (⁸⁷Sr/⁸⁶Sr: 0.705, Nd: –2, ¹⁸O: 6, ²⁰⁶Pb/²⁰⁴Pb: 19.40, ²⁰⁷Pb/²⁰⁴Pb: 15.82). More evolved quartz syenites show increasing ⁸⁷Sr/⁸⁶Sr ratios, increasing ¹⁸O values but less radiogenic Nd values and Pb isotopes with decreasing MgO, indicating assimilation of ca. 10% Archaean to Proterozoic local lower crust with unradiogenic Nd, high ⁸⁷Sr/⁸⁶Sr and low U/Pb. For samples from outside the shear zone a hypothetical parental melt with 10 wt% MgO has distinctly more radiogenic Sr but less radiogenic Nd isotopic composition (⁸⁷Sr/⁸⁶Sr: 0.712, Nd: –13), with strongly unradiogenic Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 17.40, ²⁰⁷Pb/²⁰⁴Pb: 15.50), suggesting another strongly enriched lithospheric mantle source for these rocks. Differentiated syenites from outside the shear zone show decreasing ⁸⁷Sr/⁸⁶Sr, increasing ¹⁸O values, more radiogenic Nd values and Pb isotope ratios with decreasing MgO indicating interaction with a lithospheric component with low Rb/Sr but high Sm/Nd and U/Pb.     

Magmatic and tectonic history of Jurassic ophiolites and associated granitoids from the South Apuseni Mountains (Romania)

The Jurassic ophiolites in the South Apuseni Mountains represent remnants of the Neotethys Ocean and belong to the East Vardar ophiolites that contain ophiolite fragments as well as granitoids and volcanics with island-arc affinity. New U–Pb zircon ages, and Sr and Nd isotope ratios give insights into their tectono-magmatic history. The ophiolite lithologies show tholeiitic MOR-type affinities, but are occasionally slightly enriched in Th and U, and depleted in Nb, which indicates that they probably formed in a marginal or back-arc basin. These ophiolites are associated with calc-alkaline granitoids and volcanics, which show trace element signatures characteristic for subduction-enrichment (high LILE, low HFSE). Low ⁸⁷Sr/⁸⁶Sr ratios (0.703836–0.704550) and high ¹⁴³Nd/¹⁴⁴Nd ratios (0.512599–0.512616) of the calc-alkaline series overlap with the ratios measured in the ophiolitic rocks (0.703863–0.704303 and 0.512496–0.512673), and hence show no contamination with continental crust. This excludes a collisional to post-collisional origin of the granitoids and is consistent with the previously proposed intra-oceanic island arc setting. The new U–Pb ages of the ophiolite lithologies (158.9–155.9 Ma, Oxfordian to Early Kimmeridgian) and granitoids (158.6–152.9 Ma, latest Oxfordian to Late Kimmeridgian) indicate that the two distinct magmatic series evolved within a narrow time range. It is proposed that the ophiolites and island arc granitoids formed above a long-lived NE-dipping subduction zone. A sudden flip in subduction polarity led to collision between island arc and continental margin, immediately followed by obduction of the ophiolites and granitoids on top of the continental margin of the Dacia Mega-Unit. Since the granitoids lack crustal input, they must have intruded the Apuseni ophiolites before both magmatic sequences were obducted onto the continental margin. The age of the youngest granitoid (~153 Ma, Late Kimmeridgian) yields an estimate for the maximum age of emplacement of the South Apuseni ophiolites and associated granitoids onto the Dacia Mega-Unit.     

The Ordovician igneous rocks with high Sr/Y at the Tongshan porphyry copper deposit,satellite of the Duobaoshan deposit,and their metallogenic role

A strong link between high Sr/Y arc magmas and porphyry Cu–Mo–Au deposits has been recognized in recent years. The Tongshan and Duobaoshan deposits are representative large Cu–Mo–Au deposits in NE China. We report LA–ICP–MS zircon U–Pb crystallization age of 471.5 ± 1.3 Ma for Tongshan ore-related granitoid. Re–Os isotopic analyses of the two chalcopyrite samples from Tongshan deposit show a model age range of 470.2–477.1 Ma. The Duobaoshan and Tongshan ore-related granitoids show higher Sr/Y and La/Yb ratios. The δ³⁴S values of sulphides from the Duobaoshan and Tongshan deposits vary from −2.3‰ to 0.0‰, belonging to a magmatic-hydrothermal system. The Pb isotopic ratios of the sulphides from the Duobaoshan and Tongshan deposit range from 17.201 to 18.453 for ²⁰⁶Pb/²⁰⁴Pb, 15.445 to 15.551 for ²⁰⁷Pb/²⁰⁴Pb, and 36.974 to 37.999 for ²⁰⁸Pb/²⁰⁴Pb, indicating the addition of lower crustal material. The Duobaoshan and Tongshan granitoids were formed in a subduction-related continental arc setting, produced by partial melting of juvenile hydrous basalts underplating the deep continental crust during the Ordovician.     

Chemical and isotopic (Pb,Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km²) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.Editorial responsibility: T.L. Grove     

Constraints from geochemistry and Sr–Nd isotopes for the origin of albitite deposits from Central Sardinia (Italy)

Major- and trace-element contents and Sr–Nd isotope ratios were determined in albitite, albitized and unaltered late-Variscan granitoid samples from the world-class Na-feldspar deposits of central Sardinia, Italy. The albite deposit of high economic grade has geological, textural, and chemical features typical of metasomatic alteration affecting the host granitoids. Albitization, locally accompanied by chloritization and epidotization, was characterized by strong leaching of Mg, Fe, K, and geochemically similar trace elements, and by a significant increase of Na. Ca, and P were moderately leached in the most metasomatized rocks. Other major (Si, Ti, Ca) and trace elements (U, Th, Y, and Zr), along with light (LREE) and middle (MREE) rare-earth elements, behaved essentially immobile at the deposit scale. The Nd-isotope ratios (0.512098 to 0.512248) do not provide information on the emplacement age of the unaltered late-Variscan granitoids. On the other hand, their Sr-isotope ratios fit an errorchron of 274±29 Ma (1σ error), in fair agreement with all published ages of Sardinian Variscan granitoids. The very low Rb content of albitized rocks precludes application of the Rb–Sr radiometric system to determine the age of albitization. The Sm–Nd system is not applicable either, because the ¹⁴³Nd/¹⁴⁴Nd ratios of albitized rocks and unaltered granitoids overlap. The overlap confirms that Sm and Nd were substantially immobile during albitization. On the other hand, the measured ⁸⁷Sr/⁸⁶Sr ratios of the albitized rocks are appreciably lower than those of the unaltered host granitoids, whereas, their initial Sr-isotope ratios are higher. This seems to suggest that a) albitization was induced by non-magmatic fluids rich in radiogenic Sr, and b) albitization occurred shortly after the granitoid emplacement. This conclusion is supported by Nd isotopes, because unaltered granitoids and albitites fit the same reference isochron at 274 Ma. The fluids acquired radiogenic Sr by circulation through the Lower Paleozoic metasedimentary basement. Specifically, it is estimated that Sr supplied by the non-carbonatic basement represents about 22 wt% of total Sr in albitite.     

Constraints of molybdenite Re–Os and scheelite Sm–Nd ages on mineralization time of the Kukaazi Pb–Zn–Cu–W deposit,Western Kunlun,NW China

The Kukaazi Pb–Zn–Cu–W polymetallic deposit, located in the Western Kunlun orogenic belt, is a newly discovered skarn-type deposit. Ore bodies mainly occur in the forms of lenses and veins along beddings of the Mesoproterozoic metamorphic rocks. Three ore blocks, KI, KII, and KIII, have been outlined in different parts of the Kukaazi deposit in terms of mineral assemblages. The KI ore block is mainly composed of chalcopyrite, scheelite, pyrrhotite, sphalerite, galena and minor pyrite, arsenopyrite, and molybdenite, whereas the other two ore blocks are made up of galena, sphalerite, magnetite and minor arsenopyrite and pyrite. In this study, we obtained a molybdenite isochron Re–Os age of 450.5 ± 6.4 Ma (2σ, MSWD = 0.057) and a scheelite Sm–Nd isochron age of 426 ± 59 Ma (2σ, MSWD = 0.49) for the KI ore block. They are broadly comparable to the ages of granitoid in the region. Scheelite grains from the KI ore block contain high abundances of rare earth elements (REE, 42.0–95.7 ppm) and are enriched in light REE compared to heavy REE, with negative Eu anomalies (δEu = 0.13–0.55). They display similar REE patterns and Sm/Nd ratios to those of the coeval granitoids in the region. Moreover, they also have similar Sr and Nd isotopes [⁸⁷Sr/⁸⁶Sr = 0.7107–0.7118; ε_Nd(t) = ?4.1 to ?4.0] to those of such granitoids, implying that the tungsten-bearing fluids in the Kukaazi deposit probably originate from the granitic magmas. Our results first defined that the Early Paleozoic granitoids could lead to economic Mo–W–(Cu) mineralization at some favorable districts in the Western Kunlun orogenic belt and could be prospecting exploration targets.     

S,Pb, and Sr isotope geochemistry and genesis of Pb–Zn mineralization in the Huangshaping polymetallic ore deposit of southern Hunan Province,China

The Huangshaping Pb–Zn–W–Mo polymetallic deposit, located in southern Hunan Province, China, is one of the largest deposits in the region and is unique for its metals combination of Pb–Zn–W–Mo and the occurrence of significant reserves of all these metals. The deposit contains disseminated scheelite and molybdenite within a skarn zone located between Jurassic granitoids and Carboniferous sedimentary carbonate, and sulfide ores located within distal carbonate-hosted stratiform orebodies. The metals and fluids that formed the W–Mo mineralization were derived from granitoids, as indicated by their close spatial and temporal relationships. However, the source of the Pb–Zn mineralization in this deposit remains controversial.Here, we present new sulfur, lead, and strontium isotope data of sulfide minerals (pyrrhotite, sphalerite, galena, and pyrite) from the Pb–Zn mineralization within the deposit, and these data are compared with those of granitoids and sedimentary carbonate in the Huangshaping deposit, thereby providing insights into the genesis of the Pb–Zn mineralization. These data indicate that the sulfide ores from deep levels in the Huangshaping deposit have lower and more consistent δ³⁴S values (− 96 m level: + 4.4‰ to + 6.6‰, n = 13) than sulfides within the shallow part of the deposit (20 m level: + 8.3‰ to + 16.3‰, n = 19). The δ³⁴S values of deep sulfides are compositionally similar to those of magmatic sulfur within southern Hunan Province, whereas the shallower sulfides most likely contain reduced sulfur derived from evaporite sediments. The sulfide ores in the Huangshaping deposit have initial ⁸⁷Sr/⁸⁶Sr ratios (0.707662–0.709846) that lie between the values of granitoids (0.709654–0.718271) and sedimentary carbonate (0.707484–0.708034) in the Huangshaping deposit, but the ratios decreased with time, indicating that the ore-forming fluids were a combination of magmatic and formation-derived fluids, with the influence of the latter increasing over time. The lead isotopic compositions of sulfide ores do not correlate with sulfide type and define a linear trend in a ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagram that is distinct from the composition of the disseminated pyrite within sedimentary carbonates and granitoids in the Huangshaping deposit, but is similar to the lead isotopic composition of sulfides within coeval skarn Pb–Zn deposits in southern Hunan Province. In addition, the sulfide ores have old signatures with relative high ²⁰⁷Pb/²⁰⁶Pb ratios, suggesting that the underlying Paleoproterozoic basement within southern Hunan Province may be the source of metals within the Huangshaping deposit.The isotope geochemistry of sulfide ores in the Huangshaping deposit shows a remarkable mixed source of sulfur and ore-forming fluids, and the metals were derived from the basement. These features are not found in representative skarn-type Pb–Zn mineralization located elsewhere. The ore-forming elements (S, Pb, and Zn) from the granitoids made an insignificant contribution to sulfide precipitation in this deposit. However, the emplacement of granitoids did provide large amounts of heat and fluids to the hydrothermal system in this area and extracted metals from the basement rocks, indicating that the Jurassic magmatism associated with the Huangshaping deposit was crucial to the Pb–Zn mineralization.     

Pb and Nd isotope constraints on the origin of high Mg and tholeiitic amphibolites,Kolar Schist Belt,South India

The late Archean, north-south trending Kolar Schist Belt in south India, 4 km wide by 80 km long, is thought to be a suture between two gneiss terranes (Krogstad et al. 1989). Within this volcanics-dominated belt are recognized both tholeiitic and high Mg (komatiitic and picritic) amphibolites, which make up some 70% and 5% respectively of the exposed outcrops. A massive tholeiitic amphibolite separates the belt into western and eastern parts. A Pb-Pb whole-rock age of 2732±155 Ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock. Samples of this rock have _Nd values at 2700 Ma that range between +3.8 and +6.8, ₁ (initial ²³⁸U/²⁰⁴Pb) of 7.5 and K ₁ (initial ²³²Th/²³⁸U) of about 4. Two different types of high-Mg amphibolites are recognized from the western part of the belt: a picritic or P-type, and a komatiitic or K-type. The P-type have highly variable Ce/Al ratios all greater than chondritic, Nd/Yb ratios greater than chondritic, _Nd at 2700 Ma of +1.5 to +8, and Pb isotope compositions variable in ²⁰⁷Pb/²⁰⁴Pb with ₁ of about 8.0 and k ₁ of about 4. The trace-element data suggest that the light-REE enrichment is a character of the mantle source and is not due to residual garnet. The K-type amphibolites have near chondritic Ce/Al and Nd/Yb ratios, _Nd at 2700 Ma of +1.5 to +8, and ₁ of about 8 and k ₁ of about 4. Although the P-type is light-REE enriched compared to the K-type, both types have similar Ce/Nd ratios as well as initial Pb and Nd isotopes. If the 2696±136 Ma age for the Sm-Nd isochron which includes both types of high-Mg amphibolite has any significance it dates the time of light-REE enrichment of the mantle source for the P-type komatiitic amphibolites. The high-Mg amphibolites in the eastern part of the belt are light-REE enriched, have Pb isotopic compositions that are variable in ²⁰⁷Pb/²⁰⁴Pb with a ₁ about 8.5 and _Nd at 2700 Ma of +1.8 to +4.5. Hydrothermal fluids associated with metamorphism and shearing prior to about 2400 Ma ago were responsible for the introduction of gold-quartz-carbonate veins into the Kolar Schist Belt. The Pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous Pb from adjacent older granitoid gneisses into the amphibolites, which could be responsible for the variability in the ²⁰⁷Pb/²⁰⁴Pb ratios of the samples. This extraneous Pb probably is not responsible for the distinct Pb isotope character of each type of amphibolite.     

Diagenesis of Carbonate and Siderite Deposits of the Lower Riphean Bakal Formation,the Southern Urals: Sr Isotopic Characteristics and Pb-Pb Age

The Rb-Sr and U-Pb systematics were studied for carbonate rocks of the Lower Riphean Bakal Formation of the southern Urals and related siderite ores of the Bakal iron deposit. The least-altered limestones taken at a significant distance from the Bakal ore field satisfy the strict geochemical criteria of retentivity: Mn/Sr < 0.2, Fe/Sr < 0.5, and ⁸⁷Sr/⁸⁶Sr (difference between the measured ⁸⁷Sr/⁸⁶Sr values in secondary and primary carbonate phases) < 0.001. The least-altered carbonate phases were extracted by the stepwise dissolution in 0.5 N HBr. The Pb-Pb date of limestones (1430 ± 30 Ma) defines the age of early diagenesis of carbonate sediments of the Bakal Formation. The ⁸⁷Sr/⁸⁶Sr ratio in the sedimentary environment of the Bakal carbonates (0.70457–0.70481) yields isotopic signature for the Early Riphean seawater. The Pb-Pb age of metasomatic siderites (1010 ± 100 Ma), which formed at the end of the main ore formation stage and did not undergo late epigenesis, corresponds to the final phases of the Grenville tectonogenesis. Siderites of the main ore formation stage are confined to central parts of the thickest carbonate units and have high ratios of ⁸⁷Sr/⁸⁶Sr (0.73482–0.73876) and ²⁰⁸Pb/²⁰⁴Pb (41.4–42.9). Iron-bearing solutions formed during the diagenesis of mainly Lower Riphean clayey rocks and migrated along low-density zones and faults. The solutions discharged at the interformational unconformity between the Bakal and Zigalga formations. At the contact with shales, carbonate rocks and siderites experienced the later epigenetic dolomitization (partial desideritization) caused by the circulation of solutions enriched in radiogenic ⁸⁷Sr and low-radiogenic ²⁰⁶Pb. This dolomitization occurred simultaneously with the Cadomian tectonothermal activation of the region.__________Translated from Litologiya i Poleznye Iskopaemye, No. 3, 2005, pp. 227–249.Original Russian Text Copyright © 2005 by Kuznetsov, Krupenin, Ovchinnikova, Gorokhov, Maslov, Kaurova, Ellmies.     

Age and sources of gold mineralization in the Marmato mining district, NW Colombia: A Miocene–Pliocene epizonal gold deposit

Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes, Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex; metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amagá Formation; as well as pyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9 to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au–Ag ores in the form of distensional veins, stockwork, and quartz veinlets within brecciated zones. Ore minerals are pyrite, sphalerite and galena with subordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum.Sericitized plagioclase from a porphyry dacite yielded a K–Ar age of 5.6 ± 0.6 Ma, interpreted as the age of ore deposition. This is in close agreement with the age of reactivation of the Cauca–Romeral Fault System (5.6 ± 0.4 Ma), which bounds the Calima Terrain. A porphyry andesite–dacite (6.7 ± 0.1 Ma), hosting the Au–Ag veins, shows a measured ⁸⁷Sr/⁸⁶Sr between 0.70440 and 0.70460, ε_Nd between + 2.2 and + 3.2 and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745, respectively. The ⁸⁷Sr/⁸⁶Sr and ε_Nd values of rocks from the Arquia Group range from 0.70431 to 0.73511 and − 12.91 to + 10.0, respectively, whereas the corresponding Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb) range from 18.948 to 19.652; 15.564 to 15.702; and 38.640 to 38.885, respectively. ⁸⁷Sr/⁸⁶Sr and ε_Nd values obtained on sulfides from the gold quartz veins, which occur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from − 1.11 to + 2.40. In the deepest veins, ε_Nd values lie between + 1.25 and + 3.28 and the ⁸⁷Sr/⁸⁶Sr of calcite and pyrite fall between 0.70444 and 0.70930. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively. Carbonates have an average ⁸⁷Sr/⁸⁶Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body, while the Sr, Pb and Nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the Arquia Complex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence, the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivation along a crustal-scale fault zone related to thermal processes in an accretionary oceanic–continental plate orogen.     

Crust-mantle interaction during the tectono-thermal reactivation of the North China Craton: constraints from SHRIMP zircon U–Pb chronology and geochemistry of Mesozoic plutons from western Shandong

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.     

Petrogenesis of the Mesozoic intrusive complexes from the southern Taihang Orogen,North China Craton: elemental and Sr–Nd–Pb isotopic constraints

A geochemical and isotopic study was carried out for three Mesozoic intrusive suites (the Xishu, Wuan and Hongshan suites) from the North China Craton (NCC) to understand their genesis and geodynamic implications. The Xishu and Wuan suites are gabbroic to monzonitic in composition. They share many common geochemical features like high Mg# and minor to positive Eu anomalies in REE patterns. Initial Nd–Sr isotopic compositions for Xishu suite are _Nd(135 Ma)=–12.3 to –16.9 and mostly I_Sr = 0.7056–0.7071; whereas those for Wuan suite are slightly different. Pb isotopic ratios for Xishu suite are (²⁰⁶Pb/²⁰⁴Pb)_i = 16.92–17.3, (²⁰⁷Pb/²⁰⁴Pb)_i=15.32–15.42, (²⁰⁸Pb/²⁰⁴Pb)_i=37.16–37.63, which are slightly higher than for Wuan suite. The Xishu–Wuan complexes are considered to originate from partial melting of an EM1-type mantle source, followed by significant contamination of lower crustal components. The Hongshan suite (mainly syenite and granite) shows distinctly higher _Nd(135 Ma) values (–8 to –11) and slightly higher Pb isotopic ratios than the Xishu–Wuan suites. It was formed via fractionation of a separate parental magma that also originated from the EM1-type mantle source, with incorporation of a small amount of lower crustal components. Partial melting of the mantle sources took place in a back-arc extensional regime that is related to the subduction of the paleo-Pacific slab beneath the NCC.