Evolution of Palaeoproterozoic mafic intrusions located within the thermal aureole of the Sudbury Igneous Complex, Canada: Isotopic, geochronological and geochemical evidence

Impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth. Age-dating of impact events is critical in correlating cause and effects for these catastrophic processes. The Falconbridge and Drury Township (Twp) intrusions were emplaced at the contact between Neoarchaean basement and Palaeoproterozoic volcanosedimentary rocks, and also lie at the southeast and southwest edges of the Sudbury Igneous Complex (SIC), within its thermal contact aureole. The Falconbridge Twp intrusion is dated at 2441 ± 3 Ma by U-Pb zircon, with evidence of Archaean inheritance from its host granitoids. Granitoids from the southernmost Abitibi Subprovince are dated here between 2670 ± 11 Ma for an undeformed Algoman granite, and 2696 ± 18 Ma for a foliated granitoid, consistent with existing data from the Abitibi Greenstone Belt and from the Wawa Subprovince. Major and trace element geochemical evidence, common-Pb isotopic compositions, and εNd2440 values between 0 and −1 are all consistent with a Palaeoproterozoic origin for the Falconbridge Twp intrusion, and support inclusion in the East Bull Lake-type suite of leucogabbroic plutons and sills.In contrast, the Drury Twp intrusion gives a U-Pb zircon age of 1859 ± 13 Ma, coincident with the date of SIC-emplacement. While the major and trace element compositions are comparable to the Falconbridge data, the Drury displays significant heterogeneity in εNd2440, with values ranging from +3.7 to −0.1, and contains more radiogenic Pb isotopic compositions. Field, geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the SIC itself, and indicates that the Drury too is a Palaeoproterozoic intrusion. This requires that apparently unshocked, undeformed magmatic-looking zircon has been grown or reset in a postmagmatic setting. This has significant implications for the identification of mantle-derived magmas and crustal remelts associated with large impact craters. A resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen-complexing is proposed, inducing rapid, postshock lead loss and subsequent annealing.     

Provenance of the Neoproterozoic Maricá Formation (Sul-rio-grandense Shield, Southern Brazil): Petrographic and Sm–Nd isotopic constraints

This paper reports the integrated application of petrographic and Sm–Nd isotopic analyses for studying the provenance of the Neoproterozoic Maricá Formation, southern Brazil. This unit encompasses sedimentary rocks of fluvial and marine affiliations. In the lower fluvial succession, sandstones plot in the “craton interior” and “transitional continental” fields of the QFL diagram. Chemical weathering probably caused the decrease of the ¹⁴⁷Sm/¹⁴⁴Nd ratios to 0.0826 and 0.0960, consequently lowering originally > 2.0 Ga T_DM ages to 1.76 and 1.81 Ga. ¹⁴³Nd/¹⁴⁴Nd ratios are also low (0.511521 to 0.511633), corresponding to negative ε_Nd present-day values (− 21.8 and − 19.6). In the intermediate marine succession, sandstones plot in the “dissected arc” field, reflecting the input of andesitic clasts. Siltstones and shales reveal low ¹⁴³Nd/¹⁴⁴Nd ratios (0.511429 to 0.511710), ε_Nd values of − 18.1 and − 23.6, and T_DM ages of 2.16 and 2.37 Ga. Sandstones of the upper fluvial succession have “dissected arc” and “recycled orogen” provenance. ¹⁴³Nd/¹⁴⁴Nd isotopic ratios are also relatively low, from 0.511487 to 0.511560, corresponding to ε_Nd values of − 22.4 and − 21.0 and T_DM of 2.07 Ga. A uniform granite–gneissic basement block of Paleoproterozoic age, with subordinate volcanic rocks, is suggested as the main sediment source of the Maricá Formation.     

Sm–Nd isotopic investigation of Neoproterozoic and Cretaceous igneous rocks from southern Brazil: A study of magmatic processes

Nd-evolutionary paths for diversified igneous suites from southern Brazil are here re-evaluated using published results. We interpret the ε_Nd paths considering the secondary fractionation of ¹⁴⁷Sm/¹⁴⁴Nd due to major petrogenetic processes. The inclusion of Nd isotopes and geochemical data for Precambrian and Mesozoic basic rocks allow improving the discussion on the subcontinental lithosphere beneath southern Brazil. Late Neoproterozoic rocks, mostly granitoids, are exposed in two regions of the southern Brazilian shield, an eastern collisional belt and a western foreland. The latter included two geotectonic domains amalgamated at this time, the São Gabriel Arc (900–700 Ma), and the Taquarembó cratonic block. Magma genesis mainly involved mixture of crustal and incompatible-element-enriched mantle components, both with a long residence time. Continental segments are the Neoarchaean–Paleoproterozoic lower crust (ca. 2.55 Ga) in the western foreland, and Paleoproterozoic–Neoproterozoic recycled crust (2.1–0.8 Ga) in the collisional belt. Granitoids with a single crustal derivation are limited in the southern Brazilian Shield. Mixing processes are well-registered in the western foreland, where the re-enriched old mantle was probably mixed with a 900–700 Ma-old subducted lithosphere and a 2.55 Ga-old lower crust. The contribution of the latter increased from the early 605–580 Ma to the later 575–550 Ma Neoproterozoic events, which may be due either to crustal thickening or to delamination of the lithosphere. Magma sources were diversified in the 660–630 Ma collisional belt. Initially, they involved the mixing between two components with similar Nd isotopic ratios, a 2.1–0.8 Ga-old recycled crust and a subduction-processed old mantle. Regional heating and abundant production of granitic melts, with diversified contribution of enriched mantle components, mark the end of the collisional period, at 630–580 Ma. We can also attribute this to the delamination of the lithosphere, so that the same geodynamic process may explain the magmatism in the whole shield at the end of the Dom Feliciano Orogeny. Mesozoic rocks include flood basalts from the Cretaceous Paraná Province and sub-coeval alkalic suites. Multiple processes of metasomatism affected the lithospheric mantle, resulting in some complexity but they mainly register two enriched-mantle components, both generated during Neoarchaean–Paleoproterozoic events. One end-member has a more pronounced subduction signature. The other one probably resulted from the re-enrichment of the first component at the end of the Camboriú collisional orogeny (2.0 Ga).     

Sources of Phanerozoic granitoids in the transect Bayanhongor–Ulaan Baatar, Mongolia: geochemical and Nd isotopic evidence, and implications for Phanerozoic crustal growth

The Central Asian Orogenic Belt (CAOB) is renowned for massive generation of juvenile crust in the Phanerozoic. Mongolia is the heartland of the CAOB and it has been subject to numerous investigations, particularly in metallogenesis and tectonic evolution. We present new petrographic, geochemical and Sr–Nd isotopic analyses on Phanerozoic granitoids emplaced in west-central Mongolia. The data are used to delineate their source characteristics and to discuss implications for the Phanerozoic crustal growth in Central Asia. Our samples come from a transect from Bayanhongor to Ulaan Baatar, including three tectonic units: the Baydrag cratonic block (late Archean to middle Proterozoic), the Eo-Cambrian Bayanhongor ophiolite complex and the Hangay–Hentey Basin of controversial origin. The intrusive granitoids have ages ranging from ca. 540 to 120 Ma. The majority of the samples are slightly peraluminous and can be classified as granite (s.s.), including monzogranite, syenogranite and alkali feldspar granite. Most of the rocks have initial ⁸⁷Sr/⁸⁶Sr ratios between 0.705 and 0.707. Late Paleozoic to Mesozoic granitoids (≤250 Ma) are characterized by near-zero _Nd(T) values (0 to −2), whereas older granitoids show lower _Nd(T) values (−1.5 to −7). The data confirm the earlier observation of Kovalenko et al. [Geochemistry International 34 (1996) 628] who showed that granitoids emplaced outside of the Pre-Riphean basement rocks are characterized by juvenile positive _Nd(T) values, whereas those within the Pre-Riphean domain and the Baydrag cratonic block, as for the present case, show a significant effect of ‘contamination’ by Precambrian basement rocks. Nevertheless, mass balance calculation suggests that the granitoids were derived from sources composed of at least 80% juvenile mantle-derived component. Despite our small set of new data, the present study reinforces the general scenario of massive juvenile crust production in the CAOB with limited influence of old microcontinents in the genesis of Phanerozoic granitoids.     

Coeval potassic and sodic calc-alkaline series in the post-collisional Hercynian Tanncherfi intrusive complex, northeastern Morocco: geochemical, isotopic and geochronological evidence

The post-collisional late Hercynian Tanncherfi intrusive complex (TIC) is part of a widespread intrusive episode in the Moroccan Meseta. The complex contains a wide range of rock types, from monzogabbros to monzogranites. Two distinct magmatic series are recognized: (1) a potassic (shoshonitic) series consisting of monzogabbros, quartz monzonites and monzogranites; and (2) a sodic (granodioritic) series represented by quartz monzodiorites and granodiorites. All the Tanncherfi plutonic rocks display similar spider-diagram profiles, with LILE and LREE enrichment and Nb, Ta, Ti depletion, which are typical of subduction-related magmas. Combined major, trace element compositions and Sr, Nd isotopic results indicate that the two series have been derived from a LILE- and LREE-enriched continental lithospheric mantle source, under different partial melting and/or depth conditions. Intrusion of the Tanncherfi rocks was not temporally related to subduction and the enrichment of their source is likely to be linked to preceding subduction events. The two series evolved by fractional crystallization, of clinopyroxene, plagioclase, hornblende, biotite, K-feldspar and accessories (Fe–Ti oxide minerals, titanite, apatite and zircon) for the potassic series while the sodic series combined fractional crystallization with assimilation of felsic magmas with lower Sr isotopic ratio than the more mafic term of the series, the quartz monzodiorite. The intrusion of the potassic magmas (344±6 Ma) marks a major change in the tectonic regime of eastern Meseta. These magmas intruded during post-thickening uplift and extension, both probably favored by convective thinning of the lithosphere. This model provides a reasonable mechanism for the genesis of other Hercynian intrusive complexes in Morocco.     

Sm–Nd isotopic compositions as a proxy for magmatic processes during the Neoproterozoic of the southern Brazilian shield

Combined analyses of Nd isotopes from a wide range of Neoarchaean–Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the São Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Rocks related to the formation of the São Gabriel arc (900–700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800–550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the São Gabriel arc and Taquarembó block. In the collisional belt, the old crust was intensely reworked during the São Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (São Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplate tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3–2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605–550 Ma) and younger suites from the eastern collisional belt (600–580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800–600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605–580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575–550 Ma).     

Evaluation of the effects of alteration and leaching on Sm–Nd and Lu–Hf systematics in submarine mafic rocks

Nd and Hf isotope systematics of oceanic basaltic rocks are often assumed to be largely immune to the effects of hydrothermal alteration. We have tested this assumption by comparing Nd and Hf isotope data for acid-leached Cretaceous oceanic basalts from Gorgona and DSDP Leg 15 with unleached data on the same rocks. Hf isotope values and Lu/Hf ratios are relatively unaffected by leaching, but ¹⁴³Nd/¹⁴⁴Nd values of leached samples are significantly higher than those of unleached fractions of the same sample in most cases. Furthermore, the Sm/Nd ratios of the majority of leached samples are 10–40% greater than those of unleached samples. X-ray diffraction studies indicate that selective removal of secondary minerals, such as smectite, during the acid leaching process is responsible for the fractionation of Sm/Nd ratios. These results have implications for interpretation of the Hf–Nd isotope systematics of ancient submarine rocks (older than ~ 50 Ma), as (1) the age-corrected ¹⁴³Nd/¹⁴⁴Nd ratio may not be representative of the primary magmatic signature and (2) the uncertainty of the age-corrected ε_Nd value may exceed the assumed analytical precision.     

Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico)

The El Arco porphyry copper deposit is located in central Baja California and is a resource containing >600 Mt of ore with ∼0.6% copper. It was emplaced within a relatively primitive Jurassic island arc and it was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith. The porphyritic stock intrusion and ore formation at El Arco has recently been dated at ∼165 Ma [Valencia et al. GEOS 24:189 (2004)]. This age is much older than Aptian-Albian K–Ar ages previously reported from El Arco [Barthelmy, Geology of El Arco-Calmallí area, Baja California, México. MSc Thesis, San Diego State University, CA (1975); Baja California Geology. San Diego State University, CA, pp 127–138 (1979)]. The copper mineralization at El Arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas. Mafic dikes that intruded the deposit are not mineralized, but they are affected by post-ore low-grade metamorphism. The dikes are compositionally the most primitive rocks, while host rock andesites and the porphyry stock display typical volcanic arc characteristics. The Pb isotope data from sulfides, feldspars, quartz, and whole-rock samples indicate that: (1) the copper-bearing porphyry stock and the surrounding andesites evolved from a similar source with an average μ-value of 9.43; (2) no external Pb was added during mineralization; (3) some Pb isotope compositions were slightly disturbed by a later metamorphic event. Strontium and Nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust. Our data favor a model for the formation of the El Arco deposit linked to a Triassic to Jurassic intra-oceanic arc system, cropping out at the western margin of central Baja California in the Cedros-Vizcaíno region. The intra-oceanic arc together with the El Arco deposit was accreted to the active continental margin of North America and metamorphosed during the Early Cretaceous. This model is in disagreement to earlier models that favor the El Arco deposit formation being linked to the Cretaceous continental margin.     

Early Neoproterozoic evolution of Southeast Pakistan: evidence from geochemistry,geochronology, and isotopic composition of the Nagarparkar Igneous Complex

ABSTRACT

Rocks of the early Neoproterozoic age of the world have remained in discussion for their unique identity and evolutionary history. The rocks are also present in Sindh province of Pakistan and have been in debate for a couple of years. Yet, these igneous rocks have been studied very poorly regarding U-Pb and Lu-Hf age dating. The early Neoproterozoic rocks located in Nagarparkar town of Sindh have been considered as shoulder of Malani Igneous Suite (MIS) discovered in Southwest of India. The Nagarparkar Igneous Complex (NPIC) rocks are low-grade metamorphosed, mafic and silicic rocks. These rocks are accompanied by felsic and mafic dikes. Two types of granite from NPIC have been identified as peraluminous I-type biotite granites (Bt-granites), of medium-K calc-alkaline rocks series and A-type potash granites (Kfs-granites) of high-K calc-alkaline rocks series. Geochemical study shows that these Kfs-granites are relatively high in K and Na contents and low MgO and CaO. The Bt-granites have positive Rb, Ba, and Sr with negative Eu anomalies rich with HFSEs Zr, Hf, and slightly depleted HREEs, whereas Kfs-granites have positive Rb with negative Ba, Sr, and Eu anomalies and have positive anomalies of Zr and Hf with HREEs. In addition, these rocks possess crustal material, which leads to the enrichment of some incompatible trace elements and depletion of Sr and Ba in Kfs-granites and relatively high Sr and Ba in Bt-granites, indicating a juvenile lower continental crust affinity. Zircon LA-ICP-MS U-Pb dating of these granites yielded weighted mean ²⁰⁶Pb/²³⁸U ages ranging from 812.3 ± 14.1 Ma (N = 18; MSWD = 3.7); and 810 ± 7.4 Ma (N = 16; MSDW = 0.36) for the Bt-granites, and 755.3 ± 7.1 Ma (N = 21; MSDW = 2.0); NP-GG-01 and 736.3 ± 4.3 Ma (N = 24; MSWD = 1.05) for Kfs-granites, respectively. The Bt-granites and Kfs-granites have positive zircon ε_Hf^(t) values, which specify that they are derived from a juvenile upper and lower continental crust. Based on the geochemical and geochronological data, we suggest that the Bt-granites were formed through lower continental crust earlier to the rifting time, whereas the Kfs-granites were formed via upper continental crust, during crustal thinning caused by Rodinia rifting. These zircon U-Pb ages 812 to 736 Ma, petrographic, and geochemical characteristics match with those of the adjacent Siwana, Jalore, Mount Abu, and Sirohi granites of MIS. Thus, we can suggest that NPIC granites and adjacent MIS can possibly be assumed as a missing link of the supercontinent Rodinia remnants.     

Rb–Sr and Sm–Nd isotopic compositions of the Rooiberg Group, South Africa: early Bushveld-related volcanism

The Rooiberg Group is a 6-km-thick sequence of mostly volcanic rocks, which represent the first phase of magmatic activity associated with the Bushveld Complex. These strata include, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations. Units of the lower Dullstroom Formation range from basalts to andesites and comprise two compositional suites: high Ti and low Ti. Compositional data indicate that melts represented by the overlying, more siliceous volcanic rocks, which include dacites and rhyolites, were derived from low Ti melts by fractional crystallization and assimilation of crustal material (AFC processes).

Rb–Sr isotopic data (28 samples) for units of the Dullstroom and Damwal Formations loosely constrain a crystallization age of 2071+94/−65 Ma (these errors and those below: 95% confidence limits), which agrees with previously reported age data. These data suggest an initial value for ⁸⁷Sr/⁸⁶Sr of 0.70655+0.00087/−0.00051 for the Rooiberg Group. In contrast, Rb–Sr isotopic compositions of six samples of the Kwaggasnek Formation indicate post-crystallization alteration, which was probably associated with the Lebowa Granite Suite of the Bushveld Complex. Sm–Nd isotopic data (29 samples) for volcanic units of the Rooiberg Group provide a poorly constrained age of 1837+360/−320 Ma with an initial value for ¹⁴³Nd/¹⁴⁴Nd of 0.50976+0.00026/−0.00035. These Rb–Sr and Sm–Nd isotopic compositions are similar to those indicated for melts that crystallized to form the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Extruded and intruded melts probably were derived from the same or similar sources and may have resided in the same magma chambers before emplacement.     

Charnockite genesis across the Archaean–Proterozoic terrane boundary in the South Indian Granulite Terrain: Constraints from major–trace element geochemistry and Sr–Nd isotopic systematics

Major, trace element compositions and Sr–Nd isotopic characteristics of charnockitic gneisses from the Southern Granulite Terrain (SGT), South India are presented. The study region encompasses the central segment of the Cauvery Shear Zone system (CSZ) and regions within the Madurai Block (MB) immediately south of it (designated here as the CSZ/MB and MB domains). Differences in the compositions and source characteristics between charnockitic rocks of the CSZ vis-à-vis those of the CSZ/MB and MB regions are highlighted. Foremost, the charnockites and enderbites of the CSZ show highly fractionated REE patterns with positive Eu-anomalies, depleted HREE, Y and near chondritic εNd₀ and initial-⁸⁷Sr/⁸⁶Sr at ca. 2.5 Ga, consistent with hydrous partial melting of amphibolitic crust with residual garnet and hornblende for the parental melts. By contrast, modeled at ca. 1.8 Ga and 0.8 Ga, the CSZ/MB and MB charnockitic rocks, which show a wider range of Ti and P, relatively lower degree of HREE depletion, commonly negative Eu-anomalies and undepleted Y, present clear evidence for involvement of Archaean crustal components in sources of their magmatic protoliths. There is also evidence for significant intracrustal melting processes within a thickened crust at elevated temperatures between 800 and 1000 °C. Implications to the controversial Archaean–Neoproterozoic terrane boundary problem of the SGT are discussed.     

Santa Bárbara Formation (Early Paleozoic, Caçapava do Sul, southern Brazil): Petrographic and Sm–Nd isotopic provenance parameters

Integrated petrographic and Sm–Nd isotopic data were applied in order to constrain the provenance of the Early Paleozoic Santa Bárbara Formation, Sul-rio-grandense Shield, southern Brazil. This unit comprises continental sandstones, conglomerates and siltstones deposited under semi-arid climate in a rift or pull-apart basin. Samples were collected within a stratigraphic framework composed of three sequences, in which the two basal ones present northeastwards paleoflow, and the third one marks the inversion of basin filling. Samples from sequence I show, in the south, a strong influence of intermediate volcanic (Hilário Formation) sources, and a significant increase in quartz and metamorphic fragments upsection. In the northern deposits, there is a possible influence of juvenile units (Cambaí/Vacacaí), and a more significant input of Paleoproterozoic-sourced sedimentary rocks (e.g. Maricá Formation) upsection. Samples collected from the topmost deposits of sequence II present a clear increase in the amount of volcanic fragments (mostly acidic), reflecting denudation of the “Caçapava high”. Data obtained in sequence III (Pedra do Segredo) show a progressive decrease in quartz content and a significant increase in feldspathic, plutonic fragments. A more evolved phase of denudation of the “Caçapava high”, exposing leucogranitoids of the Caçapava do Sul complex, is proposed for this interval.     

An Eburnean base metal source for sediment-hosted zinc-lead deposits in Neoproterozoic units of Namibia: Lead isotopic and geochemical evidence

The lead isotopic composition of galena from the Neoproterozoic sediment-hosted Zn-Pb sulphide deposit at Rosh Pinah and the oxidised Zn deposit at Skorpion in the Pan-African Gariep Belt, southwestern Namibia, as well as that of galena from minor occurrences in the wider Rosh Pinah ore province was investigated and is compared with that of other major sediment-hosted base metal ore deposits hosted by Neoproterozoic strata in southwestern Africa and Brazil. The isotope data were supplemented by a geochemical provenance study of the argillitic host rock to the Rosh Pinah deposit and its stratigraphic equivalents. The pre-orogenic Rosh Pinah deposit and the inferred progenitor for the secondary Skorpion deposits have very similar Pb isotopic compositions. In contrast, syn-orogenic deposits (e.g. Tsumeb-type) show a larger proportion of radiogenic Pb from the country rocks that were infiltrated by orogenic ore fluids.In all examples studied, the ore Pb has elevated ²⁰⁷Pb/²⁰⁴Pb ratios compared to Pb that evolved according to average crustal Pb growth models. The isotopic composition of the ore Pb is in agreement with that of the oldest crustal component known from the pre-Gariep basement, i.e. an Eburnean (c. 2.0 Ga) volcanic arc, best preserved in the Richtersveld Terrane, that represents the largest volume of post-Archaean juvenile crust in southern Africa. Erosion of that arc provided the main sediment source for the metasedimentary siliciclastic host to the Rosh Pinah deposit. Derivation of the Pb, and by analogy Zn, from this Eburnean volcanic arc is therefore inferred for the formation of Rosh Pinah-type syn-rift, early diagenetic replacement mineralisation.The calculated ₂- and ₂-values for the Rosh Pinah ore province (around 10.2 and 42, respectively) are higher than predicted by conventional Pb crustal growth models and are similar to those found in the wider region of southern Africa (Otavi Mountain Land) and eastern Brazil. The obtained data highlight not only that calculated Pb model ages may have no geochronological significance but might also point to a common crustal evolution of this part of SW-Gondwana.Editorial handling: M. Chiaradia     

Modification of the Sm–Nd isotopic system in garnet induced by retrogressive fluids

Garnet, as a major constitutive mineral of eclogite, is important for Sm–Nd dating of eclogite due to its high Sm/Nd ratio and its stability during retrogression. However, a comprehensive study of the petrography, mineral chemistry, garnet water content, and Sm–Nd isotopic composition of eclogites from the Bixiling massif, Central Dabie Zone (CDZ), reveals significant modification of the Sm–Nd isotopic system in garnet as a result of retrogression. This problem constitutes a challenge for Sm–Nd dating of the Bixiling eclogites, with the Sm–Nd isochron ages of 218 ± 4 to 210 ± 9 Ma reported in the literature being younger than 226 ± 3 Ma, which is the generally accepted peak metamorphic age of the CDZ. Petrographic analysis reveals heterogeneity in colour within single fractured garnet grains. There are light‐pink garnet (Grt‐P) and red garnet (Grt‐R) types that possess distinct chemical compositions. Compared to Grt‐P, Grt‐R has higher Fe and andradrite contents but lower Al and grossular contents. Grt‐P also has lower water contents (15–35 ppm) than Grt‐R (34–65 ppm), which, together with the spatial association between Grt‐R and fractures, suggests that the colour change is related to fluid alteration. Grt‐P is an ultra‐high‐pressure (UHP) mineral, and Grt‐R is the product of the interaction between Grt‐P and a fluid during retrogression. Moreover, Grt‐R features lower Sm and Nd contents but higher Sm/Nd ratios than Grt‐P. The Sm–Nd isochrons defined by UHP minerals (Grt‐P+Omp+Rt or Grt‐P+Cpx+WR) from three eclogite samples yield consistent ages of 226.0 ± 3.8 Ma, 225.0 ± 3.9 Ma and 226.2 ± 6.9 Ma, which are identical to the peak metamorphic age of 226 ± 3 Ma for the CDZ. The retrogressed garnet (i.e., Grt‐R), omphacite and rutile, together define a pseudoisochron with younger ages of 218.9 ± 5.9 to 202.8 ± 4.8 Ma, which are geologically meaningless. The increase in the Sm/Nd ratio with constant or lower ¹⁴³Nd/¹⁴⁴Nd ratios during the transformation of Grt‐P to Grt‐R was probably the cause of these younger ages.     

Pt-Os and Re-Os systematics of the Sudbury Igneous Complex, Ontario

We measured by negative thermal ionization mass spectrometry (NTIMS) Re, Os and ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os in 26 samples of 18 Ni-Cu sulfide ores from the Falconbridge, McCreedy West, and Strathcona mines at Sudbury, Ontario. At McCreedy West and Falconbridge, the isochron Re-Os ages are 1835 ± 70 Ma and 1827 ± 340 Ma, and the initial ¹⁸⁷Os/¹⁸⁸Os ratios 0.514 ± 0.019 and 0.550 ± 0.024, respectively. The ages agree with the canonical value of 1850 ± 1 Ma for the Sudbury Igneous Complex (SIC). For Hangingwall and Deep Zone ores at Strathcona, the age of 1780 ± 7 Ma may reflect resetting by dyke activity. The high initial ¹⁸⁷Os/¹⁸⁸Os of 0.934 ± 0.005 in these ores is distinct from those at McCreedy West and Falconbridge. Strathcona Deep Copper Zone ores have highly radiogenic Os giving a mean model age of 1883 ± 54 Ma that is similar to ages at McCreedy West and Falconbridge, but distinct from other Strathcona sulfides. Initial ¹⁸⁶Os/¹⁸⁸Os in two Strathcona ores with low ¹⁹⁰Pt/¹⁸⁸Os average 0.119 826 ± 0.000 009 (n = 3) and 0.119 827 ± 0.000 004 (n = 3), respectively, with a grand mean of 0.119 827 ± 0.000 003. This ratio may be slightly lower than the chondritic value at that time. Similar ores at Falconbridge and McCreedy West show more scatter, averaging 0.119 855 ± 0.000 008 (n = 6) and 0.119 867 ± 0.000 020 (n = 3), respectively. These values are substantially suprachondritic. The Re-Os isotope systematics of Sudbury ores are clearly of crustal origin and may be derived from a binary mixture of Superior Province and Huronian metasedimentary rocks, with Strathcona, Falconbridge, and McCreedy West ores containing, respectively, 55%, 16%, and 12% of Os from Superior sediments. The suprachondritic ¹⁸⁶Os/¹⁸⁸Os at McCreedy West and Falconbridge may be due to admixture of Archean or Paleozoic mafic rocks with ¹⁹⁰Pt/¹⁸⁸Os ≈ 0.1. No trace of the asteroid that produced the Sudbury Structure has been reported. At the Whistle mine S-poor olivine melanorite inclusions with high Ni and Os and low ¹⁸⁷Os/¹⁸⁸Os may contain the signature of a magmatically fractionated asteroidal core contributing 1 to 2.5 % metal. The S-poor melanorite Ni and Os data are equally well explained by admixture of ≈40% mantle peridotite, however.     

Crust-mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies

Sm-Nd isotopic compositions were determined for the peralkaline Ilímaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial _Nd(≈ 0), whereas a few samples trend towards _Nd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilímaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust.     

Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

 Single zircon U–Pb dating combined with ²⁰⁷Pb/²⁰⁶Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial e_Nd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K₂O/Na₂O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc. Received: 29 March 1999 / Accepted: 25 August 1999     

Crustal contamination of mafic magmas: evidence from a petrological, geochemical and Sr–Nd–Os–O isotopic study of the Proterozoic Isortoq dike swarm, South Greenland

The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a variety of alkaline rocks ranging from gabbroic to syenitic in composition. Major magmatic mineral phases are olivine, clinopyroxene, Fe–Ti oxides, amphibole, plagioclase and alkali feldspar. Quartz occurs in some samples as a late magmatic phase. Liquidus temperatures of olivine-bearing samples range between 1120 and 1145 °C and solidus temperatures are 850–930 °C. Calculated silica activities are highly variable between 0.53 and unity. Oxygen fugacities vary from −3 to +1 log units relative to the fayalite–magnetite–quartz buffer.

The rocks have MgO contents <6 wt.% with Mg# between 53 and 17. Primitive mantle-normalized trace element patterns show a relative enrichment of LIL elements with Ba peaks and Nb troughs. Clinopyroxenes show a general enrichment in REE relative to chondritic values with variable slightly positive to prominent negative Eu anomalies. Two of the dikes were dated with Sm–Nd three-point isochrons at 1190±44 and 1187±87 Ma, respectively. Initial ⁸⁷Sr/⁸⁶Sr ratios of mafic mineral separates range from 0.70289 to 0.70432 and initial _Nd values vary from +0.3 to −10.7. Whole-rock initial ¹⁸⁷Os/¹⁸⁸Os ratios are highly variable including very radiogenic values of up to 7.967. δ¹⁸O_v-smow values of separated clinopyroxene and amphibole range from +5.2‰ to +6.2‰ and fall within the range of typical mantle-derived rocks, although mixing with a lower crustal component is permitted by the data. Using energy-constrained assimilation-fractional crystallization (EC-AFC) modeling equations, the Sr–Nd isotope data of the more radiogenic samples can successfully be modeled by addition of up to 10% lower crustal granulite-facies Archean gneisses as contaminants. The Os isotopic data also suggest the involvement of old radiogenic crust. In accordance with seismic data, we conclude that a wedge of Archean crust extends from West Greenland further to the south below the present erosion level.     

A geochemical study of the Sweet Home Mine,Colorado Mineral Belt,USA: hydrothermal fluid evolution above a hypothesized granite cupola

Deposition of quartz–molybdenite–pyrite–topaz–muscovite–fluorite and subsequent hübnerite and sulfide–fluorite–rhodochrosite mineralization at the Sweet Home Mine occurred coeval with the final stage of magmatic activity and ore formation at the nearby world-class Climax molybdenum deposit about 26 to 25 m.y. ago. The mineralization occurred at depths of about 3,000 m and is related to at least two major fluid systems: (1) one dominated by magmatic fluids, and (2) another dominated by meteoric water. The sulfur isotopic composition of pyrite, strontium isotopes and REY distribution in fluorite suggest that the early-stage quartz–molybdenite–pyrite–topaz–muscovite–fluorite mineral assemblage was deposited from magmatic fluids under a fluctuating pressure regime at temperatures of about 400°C as indicated by CO₂-bearing, moderately saline (7.5–12.5 wt.% NaCl equiv.) fluid inclusions. LA-ICPMS analyses of fluid inclusions in quartz demonstrate that fluids from the Sweet Home Mine are enriched in incompatible elements but have considerably lower metal contents than those reported from porphyry–Cu–Au–Mo or Climax-type deposits. The ore-forming fluid exsolved from a highly differentiated magma possibly related to the deep-seated Alma Batholith or distal porphyry stock(s). Sulfide mineralization, marking the periphery of Climax-type porphyry systems, with fluorite and rhodochrosite as gangue minerals was deposited under a hydrostatic pressure regime from low-salinity ± CO₂-bearing fluids with low metal content at temperatures below 400°C. The sulfide mineralization is characterized by mostly negative δ³⁴S values for sphalerite, galena, chalcopyrite, and tetrahedrite, highly variable δ¹⁸O values for rhodochrosite, and low REE contents in fluorite. The Pb isotopic composition of galena as well as the highly variable ⁸⁷Sr/⁸⁶Sr ratios of fluorite, rhodochrosite, and apatite indicates that at least part of the Pb and Sr originated from a much more radiogenic source than Climax-type granites. It is suggested that the sulfide mineralization at the Sweet Home Mine formed from magmatic fluids that mixed with variable amounts of externally derived fluids. The migration of the latter fluids, that were major components during late-stage mineralization at the Sweet Home Mine, was probably driven by a buried magmatic intrusion.