Rb-Sr Whole-rock Age Research Articles

Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A1-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications

In the northern part of the Marrakech High Atlas (MHA), along the southern Variscan segment of the Western Meseta, a Variscan granitic intrusion crops out, intruding metasediments and meta-volcanosedimentary rocks of Early Cambrian to Ordovician age. A new whole-rock Rb-Sr isochron age of 268 ± 9 Ma for the granite, combined with a previously published whole-rock Rb-Sr radiometric dating (271 ± 3 Ma), reveals a post-kinematic (tectonic) character with regard to the main Variscan deformational event, belonging within the tectonic context of the Moroccan Variscan orogenic belt. Geochemically, the Azegour intrusion is metaluminous to peraluminous and exhibits a calc-alkaline affinity with a ferruginous composition. The massif shows an extremely differentiated character (SiO2 = 77.53–78.14 per cent), K2O and high total alkali contents, FeOt/(FeOt + MgO) and Ga/Al ratios, which have typical characteristics of an A-type granite. In addition, the granite contains high concentrations of LREE (LaN/SmN= 7.9–13.67) relative to HREE (LaN/YbN= 4.81–11.61) and a well-defined Eu negative anomaly (Eu/Eu* = 0.44–0.75). The granitic samples exhibit a strong enrichment of the most incompatible elements (RbN/YbN = 69.84–159.98) and a strong depletion of Ba, Sr, Eu, Nb, P and Ti. These characteristics are similar to those of A1-type granites. The absence of mineralogy typical of an S-type granite, combined with its weakly peraluminous character [A/CNK (molar Al2O3/CaO+Na2O+K2O) = 1,013–1,045], suggest that there is little or no significant involvement of supracrustal sources in the petrogenesis of the intrusion studied. Despite the strongly differentiated character of Azegour granitic rocks samples, their multi-element patterns shows many similarities to those of I-type granitoids, which has led to postulate that the parental liquids of A1-type were derived from partial melting of mafic magmas. The representative samples studied show less depleted εNd(t = 270 Ma)values of –0.94 to –4.85 and lower positive to slightly negative εSr(t = 270 Ma) values of –1.45 to 9.32. The isotopic data suggest that the Azegour granite was emplaced 270 myr ago, apparently generated by partial melting of a mafic/intermediate magma source in the lower crust as a result of the underplating of the asthenosphere mantle-derived Oceanic Island Basalt-like magmas. Alternatively, their isotopic signatures also can be attributed to the interaction and/or hybridisation of basaltic liquids derived from the mantle with these lower crust materials. The generated parental magma probably occurred at deep structural levels and involved fractional crystallisation processes by the separation of a mineralogical association composed of plagioclase + potassium feldspar ± biotite ± amphibole ± sphene ± apatite. The whole-rock Rb-Sr age of 268 ± 9 Ma, whole-rock geochemistry and Sr-Nd isotopic compositions of εNd(t = 270 Ma) and εSr(t = 270 Ma), combined with fieldwork data, suggest that the Azegour granite was emplaced.

In situ U-Pb geochronology and geochemistry of a 1.13 Ga mafic dyke suite at Bunger Hills, East Antarctica: The end of the Albany-Fraser Orogeny

Antarctica contains continental fragments of Australian, Indian and African affinities, and is one of the key elements in the reconstruction of Nuna, Rodinia and Gondwana. The Bunger Hills region in East Antarctica is widely interpreted as a remnant of the Mesoproterozoic Albany–Fraser Orogen, which formed during collision between the West Australian and Mawson cratons and is linked with the assembly of Rodinia. Previous studies have suggested that several generations of mafic dyke suites are present at Bunger Hills but an understanding of their origin and tectonic context is limited by the lack of precise age constraints. New in situ SHRIMP U-Pb zircon and baddeleyite dates of, respectively, 1134 ± 9 Ma and 1131 ± 16 Ma confirm an earlier Rb-Sr whole-rock age estimate of ca. 1140 Ma for emplacement of a major mafic dyke suite in the area. Existing and new geochemical data suggest that the source of the dyke involved an EMORB-like source reservoir that was contaminated by a lower crust-like component. The new age constraint indicates that the dykes post-date the last known phase of plutonism at Bunger Hills by ca. 20 million years and were emplaced at the end of Stage 2 of the Albany-Fraser Orogeny. In current models, post-orogenic uplift and progressive tectonic thinning of the lithosphere were associated with melting and reworking of lower and middle crust that produced abundant plutonic rocks at Bunger Hills. A major episode of mafic dyke emplacement following uplift, cooling, and plutonic activity with increasing mantle input, suggests that the dykes mark the end of a prolonged interval of thermal weakening of the lithosphere that may have been associated with continued mafic underplating during orogenic collapse. If the undated olivine gabbro dykes with similar trend, geochemistry and petrology at Windmill Islands are coeval with the ca. 1134 Ma dyke at Bunger Hills, this would suggest the presence of a major dyke swarm at least 400 km in extent. In such case, the dykes could have been emplaced laterally from a much more distant mantle source, possibly a plume, and interacted with the locally heterogeneous and variably metasomatised lithosphere.

複数の熱年代学的手法に基づいた江若花崗岩敦賀岩体の冷却・削剥史

In this study, multi-system thermochronology, i.e., fission-track (FT), K-Ar and U-Pb methods are used to identify the cooling and denudation history of the Tsuruga body of Kojaku granite, southwest Japan. Apatite FT age of 51.8 ± 6.5 Ma, zircon FT age of 70.4 ± 2.0 Ma, biotite K-Ar ages of 66.7–62.0 Ma, and zircon U-Pb age of 68.5 ± 0.7 Ma were obtained for granitic samples, whereas plagioclase K-Ar ages of 19.1–18.8 Ma and whole-rock K-Ar age of 19.0 ± 2.9 Ma were inferred for the basaltic dyke intruding into the granite. The zircon FT lengths are not significantly shorter than their initial lengths, implying rapid cooling at the zircon FT partial annealing zone (PAZ). On the other hand, the apatite FT length distribution shows a typical pattern for granitic pluton without reheating, indicating a slow cooling history at the apatite FT PAZ. Based on the results of these thermochronometric analyses, inverse thermal calculations using the FT data, and simple thermal conduction modeling of the granitic body, the cooling and denudation histories of the Tsuruga body are reconstructed: (1) the Tsuruga body intruded at ca. 68 Ma, late Cretaceous, at a depth of several kilometers, (2) rapidly cooled to below the zircon FT PAZ by heat conduction within a few million years or less, and (3) slowly cooled due to peneplanation during the past 50–60 million years. On the other hand, the whole-rock Rb-Sr age previously reported for the Kojaku body is younger than when the cooling curve of the Tsuruga body obtained by this study intersects with the closure temperature of the whole-rock Rb-Sr system. This may imply a time lag between the formation ages of these bodies, but more thermochronometric studies are required to draw a definitive conclusion. The K-Ar ages of the basaltic dyke are interpreted as its formation age, indicating that dyke intrusion was associated with the Green Tuff movement.

Rb-Sr Isotopic Dating and Metamorphism of a Suite of Migmatite and Granites from Sandmata Granulite Complex, Rajasthan: An Evidence of Mesoproterozoic Thermal Event

Abstract: Sandmata Granulite complex (SGC) of Rajasthan of Western India is a multiply deformed, metamorphosed and migmatised granulite units representing a Mesoproterozoic collisional orogen. It consists of migmatised metapelites, calcsilicate granulites, pyroxene granulites and charnockitic intrusives at 1.7 Ga. The granulites mostly occur as dismembered, isolated enclaves in garnet bearing migmatites and quartzofeldspathic gneiss. Intrusion of small scale post-tectonic granite in granulite documents the last thermal activity in the area. At the eastern part of SGC, greenschist to lower amphibolite facies rocks of Manglawar Complex (MC) are exposed. The P-T conditions of garnet formation during metamorphism in migmatite, leptynite and quartzo-feldspathic gneiss are 780-850°C and 7–8 kbar. Retrograded biotite forming reactions occur at much lower temperature of 500 to 600°C and pressure of 3.5-4.5 kbar. The intrusive granite is weakly deformed, calc-alkaline to sub-alkaline, peraluminous in nature, having trace element signature of volcanic-arc to syn-collisional granite. The chondrite-normalised REE patterns indicate a garnet-free source. The whole-rock Rb-Sr age of 1715 ± 24 Ma deduced for the granite mark its emplacement age. Intrusive nature of granite suggests that the age of granulite metamorphism and migmatization should be >1715 Ma, which is much closer to the whole-rock Rb-Sr age of migmatite and quartzofeldspathic gneiss. The whole-rock Rb-Sr analysis of migmatite gives an age of 1735±150 Ma with an initial 87Sr/86Sr ratio of 0.704 which is interpreted as an age when the temperature of migmatization passed below the blocking temperature of Rb and Sr for biotite and plagioclase; an upper age limit of migmatization. SGC represents a major 1.9-1.7 Ga Palaeoproterozoic, tectonothermal event. The granulite facies metamorphism is succeeded by 1.7 Ga migmatisation, intrusion of calc-alkaline granite and emplacement of charnockite protolith. The whole granulite complex is subsequently cooled and later exhumed, and became juxtaposed with MC during a late major orogeny.

Lu–Hf dating, petrography, and tectonic implications of the youngest Alpine eclogites (Tauern Window, Austria)

Isotopic dating of metamorphic minerals places fundamental constraints on the rates and mechanisms of burial and exhumation in collisional orogens. The Eclogite Zone in the Tauern Window has been the focus of many studies on subduction-related high-pressure metamorphism. However, the age and duration of the high-pressure stage remains the subject of ongoing debate. 32Ma Sr–Rb ages interpreted to date eclogite-facies metamorphism (Glodny et al., 2005) appear too young in traditional tectonic reconstructions of plate collision in the Alps. These ages have either been interpreted to indicate extremely rapid exhumation from more than 60kilometre depth to mid-crustal levels within 1Mayears or to date retrogression subsequent to high-pressure metamorphism.We present element distribution maps and lutetium–hafnium (Lu–Hf) garnet ages of three samples from the Eclogite Zone. All samples display almost unaltered eclogite-facies assemblages and garnets preserve growth zoning. Lu–Hf ages are thus considered as formation ages recording metamorphism towards peak-pressure conditions. In the sample with the smallest grain size, garnet shows regular bell-shaped element distributions with respect to manganese and the iron–magnesium ratio. A six-point isochron of this sample yields 32.8±0.5Ma (MSWD=1.06), interpreted as the age of Alpine eclogite-facies metamorphism. In one of the other two, coarser-grained samples' garnet chemistry is identical. The third sample, however, shows complex zoning in large garnet crystals. Cores with a very low iron-magnesium ratio are surrounded by a second garnet generation which is very similar to the Alpine generation in the other two samples. The two coarser-grained samples yield scattered ages between 26.9±9.8Ma and 62.7±1.8Ma for individual garnet-whole-rock pairs as the analysed garnet fractions display very different 176Hf/177Hf vs. 176Lu/177Hf ratios. This scatter reflects varying degrees of mixing between Alpine and pre-Alpine garnet fractions as represented by the cores of the third sample. The results confirm the Rb–Sr-whole rock ages of Glodny et al. (2005). Despite the problems this result causes for conventional tectonic reconstructions, the eclogites from the Eclogite Zone in the Tauern Window have to be considered as Lower Oligocene in age and are thus the youngest eclogites of the Alps identified so far.

La–Ce and Sm–Nd isotopic systematics of early Proterozoic leucogranite with tetrad REE pattern

We report La–Ce and Sm–Nd isotopic data for leucogranites having tetrad rare earth element (REE) patterns at Imweon area in the northeastern Yeongnam Massif, South Korea. The leucogranites are strongly peraluminous (A/CNK = 1.18–1.79) S-type granite generated by the partial melting of metasediments, which consist of quartz, K-feldspar, plagioclase, muscovite, biotite, garnet and sillimanite. The leucogranites have three types of chondrite-normalized REE patterns: one showing a tetrad REE pattern of M-type with a large negative Eu anomaly, another with slightly V-shaped REE pattern with a positive Eu anomaly, and third with an REE pattern of crustal type with an LREE-enriched and HREE-depleted pattern. U–Pb zircon ages and, Sm–Nd and Rb–Sr whole-rock ages from the leucogranites are 1853 ± 15 Ma − 1859 ± 14 Ma (2 σ), 1833 ± 220 Ma (2 σ) with ε Nd(1.83 Ga) = −6.6 ± 5.5 (2 σ) and 1740 ± 71 Ma (2 σ) with initial 87Sr/ 86Sr = 0.755 ± 0.050 (2 σ), respectively. These values suggest that the leucogranites were emplaced from magma formed by partial melting of the crustal material. In the ε Ce– ε Nd diagram, the present Ce and Nd isotopic compositions of the leucogranites with a tetrad REE pattern are plotted in the positive ε Nd and ε Ce field whereas those of the leucogranites without tetrad REE pattern are in the negative ε Nd and positive ε Ce field. The initial Ce isotope values of the leucogranites with a tetrad REE pattern are plotted in a range of −2.31 < ε Ce(1.85 Ga) < + 3.3. The initial Ce isotopic values of the leucogranites without a tetrad REE pattern are extremely high, + 3.3 < ε Ce(1.85 Ga) < + 21.8. The initial ε Nd(1.85 Ga) values of the leucogranites are similar in the range of −8.63 < ε Nd(1.85 Ga) < −4.19 regardless of the existence of a tetrad REE pattern, suggesting that Ce isotopes of the source materials should be more heterogeneous than Nd isotopes. Our La–Ce, Sm–Nd, and Rb–Sr isotopic data indicate that the tetrad REE pattern was formed by a specific geochemical reaction during granitic magma emplacement rather than by metamorphism, alteration, and weathering.

Petrogenesis of three Cretaceous granites in the Okcheon Metamorphic Belt, South Korea: Geochemical and Nd–Sr–Pb isotopic constraints

The Sokrisan, Weolaksan and Muamsa granites in the Okcheon Metamorphic Belt (OMB), Korea are among the representative Cretaceous granites in the Korean Peninsula. Major and trace elements, and Sr–Nd isotopic data for the three granites are reported for evaluating the source characteristics and petrogenesis. The Sokrisan and Weolaksan granites are complexes of batholith type, most of which consists of biotite granite, whereas the eastern part of the body consists of hornblende–biotite granite. The Muamsa granite is a biotite granite of stock type. The three granites all have miarolitic texture suggesting that they belong to shallow intrusive type. The Muamsa granite has a Rb–Sr whole-rock age of 88.2 ± 1.7 Ma (2 σ) with an initial Sr isotopic ratio of 0.7139 ± 0.0041 (2 σ), whereas the Weolaksan granite has 89.6 ± 2.2 Ma (2 σ) with an initial Sr isotopic ratio of 0.7132 ± 0.0014 (2 σ). Recalculated Rb–Sr whole-rock age of the Sokrisan granite is 94.8 ± 3.6 Ma (2 σ) with an initial Sr isotopic ratio of 0.7084 ± 0.0029 (2 σ). The three granites have high SiO 2, FeO t/MgO, K 2O + Na 2O, and LREE contents and Rb. They are mostly depleted in Ba, Sr, P 2O 5 and TiO 2 with strongly negative Eu anomalies. The geochemical characteristics of the granites suggest mixed type of A-type and highly fractionated granite. The ε Nd( T) of the granites ranges from − 13.3 to − 15.8. The Rb–Sr whole-rock age, consistent negative values of ε Nd( T), trace element variations and consistent REE patterns from the three granites indicate that they might have been derived from a chemically similar crustal source. In tectonic discrimination diagrams, the granites fall in the range of syn-collisional to within-plate granites. The chondrite-normalized REE patterns and spider diagrams normalized by primitive mantle are similar to those of anorogenic A-type granites which were produced during the Late Yanshanian orogeny in SE China. However, negative initial ε Nd( T) values reveal that source material of the three Cretaceous granites in the OMB should be different from those of SE China.

Initial 87Sr/86Sr ratio heterogeneity in Kamihara Tonalite, Ryoke belt, southwest Japan: Evidence from strontium isotopic analysis of apatite

Strontium isotopic analysis was carried out on magmatic apatite in the Kamihara Tonalite obtained from the Cretaceous Ryoke belt, southwestern Japan. 87Sr/86Sr isotopic compositions of euhedral apatite indicated that the initial 87Sr/86Sr ratio (SrI) in the tonalite varies from 0.70695 to 0.70790. This provides direct evidence of SrI heterogeneity in the Kamihara Tonalite. The SrI heterogeneity is inherited to Rb-Sr whole-rock age determination, and makes the age discrepancy between the Rb-Sr age and CHIME, K-Ar ages. By combining the results of the isotopic analysis, knowledge on the whole-rock chemistry, and field observations, it can be inferred that the SrI heterogeneity in the Kamihara Tonalite was caused by the assimilation of high SrI country metamorphic rocks of the Ryoke belt.

Age of granitic activity associated with copper?molybdenum mineralization at Malanjkhand, Central India

The Malanjkhand copper–molybdenum deposit in the Bhandara Craton, Central India, is hosted by a granite complex which consists of regionally dominant “grey granitoid” and “pink granitoid” confined to the mineralized zone. New SHRIMP RG data on zircons from both granite types are inferred to have crystallized during the same magmatic pulse at ca 2.48 Ga. The discrepancy between zircon age and earlier obtained Rb–Sr whole-rock age is attributed to modification of the Rb–Sr system by hydrothermal overprint. Similarity in petrographic features and chemical affinity in combination with identical age strongly indicate that the pink granite is the hydrothermally altered variety (microclinization and silicification) of the grey granite. The spatially associated, main Cu–Mo mineralization event at Malankhand appears to be broadly contemporaneous with and genetically related to the emplacement of the host granitoids at about 2.48 Ga.

Rb-Sr and40Ar/39Ar age constraints on the Variscan metamorphism recorded by Ordovician acidic metavolcanic rocks in the Eastern Southalpine basement

New Rb-Sr and40Ar/39Ar ages are presented for some acidic metavolcanic rocks coming from two different outcrops of the Eastern Southalpine basement (Val Sarentino and Levico): these ages refer to the timing of the two stages of the Variscan metamorphism, which affected the Southalpine basement. Whole-rock Rb-Sr ages of 345±9 and 353±56 Ma are consistent with Rb-Sr ages obtained for similar lithologies in the neighbouring basement outcrop of Val di Funes; together, these ages define a synchronous metamorphic stage during the Tournaisian (first Variscan stage).40Ar/39Ar ages on coarsegrained, post-kinematic phengites give values of 325–326 Ma for Val Sarentino and of 329–330 Ma for Levico samples. They probably represent cooling ages of the retrograde path of the second Variscan stage, during which the thermal climax of the whole Variscan metamorphic event occurred. The Levico ages are slightly older than Val Sarentino, possibly because of the closeness of the peak temperature and the assumed closure temperature range of K-Ar system in white mica. Taking into account the Levico and Val Sarentino cooling ages, the thermal peak in the Eastern Southalpine domain should be older than 331 Ma. Therefore, the two Variscan metamorphic stages in the Eastern Southalpine basement are probably closer in time than previously believed, being separated by less than 20 Ma.

Geochemical and isotopic characteristics of the Cretaceous Orikabe Plutonic Complex, Kitakami Mountains, Japan: magmatic evolution in a zoned pluton and significance of a subduction-related mafic parental magma

The Orikabe Plutonic Complex, northeast Japan, is a zoned pluton and one of the Cretaceous intrusions in the Circum-Pacific area. In the Main body, K-rich calc-alkaline rocks composed of marginal gabbro and a large amount of monzodiorite–quartz monzonite–monzogranite are intruded successively by innermost calc-alkaline rocks of granodiorite. The gabbro and monzodiorite–monzogranite have a continuous chemical variation, while the granodiorite has lower concentrations of K, Rb, Y, Zr, Nb and F at the same SiO2 content. The gabbro and monzodiorite–quartz monzonite have a Rb-Sr whole-rock age of 119±12 Ma with an initial 87Sr/86Sr ratio of 0.70392±0.00007. The initial 87Sr/86Sr ratio of the innermost granodiorite is estimated to be about 0.7042. The δ18O values of fresh rocks range from +6.7 to +8.3‰, indicating a positive correlation with SiO2 contents. The K-rich calc-alkaline rocks were derived through fractional crystallization from a mafic parental magma with a slightly high δ18O value, implying a major contribution of a sub-arc mantle at a continental margin. Trace element modeling indicates that the source could have been a fertile lherzolite enriched in LILE and depleted in HFSE. The innermost granodiorite was the differentiation product of a distinct parental magma, suggesting the involvement of a small amount of crustal component in the source and partial melting under a more hydrous condition.

The Proterozoic Hustad igneous complex: a low strain enclave with a key to the history of the Western Gneiss Region of Norway

The Hustad Igneous Complex (HIC) is a concentrically zoned, high K calc-alkaline intrusive complex, with a core of quartz–monzonite and granite surrounded successively by monzodiorite and pyroxenite, preserved in a 10 km2 lens within the strongly ‘caledonised’ Western Gneiss Region (WGR) of Norway. Upper intercept UPb zircon ages of 1654±1 and 1653±2 Ma, obtained from the granite and the monzodiorite, respectively, overlap with a RbSr whole rock age of 1656±49 Ma (I.R.: 0.70193±0.0009, MSWD:6.6) obtained from the felsic members of the complex and a SmNd whole rock age of 1555±180 Ma (I.R.: 0.51067±13, MSWD:3.3). The UPb age of 1654±1 Ma is interpreted to date the time of intrusion. A 50 m thick dolerite dyke that can be followed for almost 4 km across the HIC yields a UPb zircon and baddeleyite age of 1251±3 Ma. The dyke also contains a secondary polycrystalline zircon generation, which together with baddeleyite defines a lower intercept age of 384±9 Ma. The Caledonian events were also marked by the emplacement of a swarm of pegmatites, which crosscut all the other units and provide a UPb zircon age of 401±1 Ma. A RbSr whole-rock age for the pegmatites is older at 452±26 Ma (I.R.: 0.7105, MSWD:4.3) indicating some initial isotopic heterogenity and formation of the pegmatites from material with a long crustal residence time. A concordant UPb monazite age of 390±2 Ma from the pegmatite and the above 384±9 Ma lower intercept age date metamorphism caused by percolating fluids. Post pegmatite deformation transformed the complex along shear zones into gneisses indiscernible from the surrounding gneisses. The HIC is characterised by high mg-numbers (0.39–0.80), high Ni and Cr values, and depletion of Nb and Ta relative to LIL elements. Phenocrysts of ortho- and clinopyroxene have high concentrations of Cr, supporting the Cr-rich nature of the complex. The suite is LREE-enriched, has a fractionated and U- shaped REE pattern (Cen 90–143, Ybn 12–40), eNd values ranging from +0.0 to +2.7, and the felsic members display weak positive Eu anomalies while the pyroxenite has a negative Eu anomaly. The complex bears a geochemical resemblance to the late-tectonic, high-BaSr granite complexes of West Scotland, and particularly to their volcanic counterparts, the Mg-rich Old Red Sandstone andesites. Overall the data suggest generation of the suite in a subduction environment from material derived by melting of a recently enriched, but previously long-term LREE-depleted mantle. The HIC is one of the few well-preserved remnants of the igneous protoliths of the 1686–1640 Ma gneisses that make up the WGR. In fact, the pyroxenite is compositionally identical to orthopyroxene eclogite found as lenses in the surrounding gneisses and forms a likely protolith to this enigmatic rock. The metamorphism had little influence on the RbSr and SmNd systems of the felsic rocks, but it appears to have affected more strongly those in the mafic rocks, probably as a result of the more intense mineralogical transformations and metasomatic loss of Rb and LREE during high-grade metamorphism.

Nature and longevity of hydrothermal fluid flow and mineralisation in granites of the Bushveld Complex, South Africa

The Lebowa Granite Suite of the Bushveld Complex is a large, 2054 Ma old, A-type batholith, characterised by numerous relatively small magmato-hydrothermal, polymetallic ore deposits. The mineralisation is represented by a three-stage paragenetic sequence: early magmatic Sn-W-Mo-F ores (600°C &gt; T &gt; 400°C), followed by a Cu-Pb-Zn-As-Ag-Au paragenesis (400°C &gt; T &gt; 200°C) and then late-stage Fe-F-U mineralisation (&lt; 200°C). The first stage of mineralisation (typified by the endogranitic Zaaiplaats tin deposit) is related to incompatible trace element concentration during crystal fractionation and subsequent fluid saturation of the magma. Evolution of the late magmatic fluids as they were channelled along fractures, as well as mingling with externally derived connate or meteoric fluids, resulted in the deposition of the second stage of mineralisation (typified by the fracture-related, endogranitic Spoedwel and Albert deposits and the exogranitic, sediment-hosted Rooiberg mine) which is dominated by polymetallic sulphide ores. As the externally derived fluid component became progressively more dominant, oxidation of the polymetallic sulphide assemblage and precipitation of hematite, pitchblende and fluorite occurred generally along the same fracture systems that hosted the earlier sulphide paragenesis.Small hydrothermal zircons trapped along quartz growth zones from the Spoedwel deposit yield a U-Pb concordia age of 1957 ± 15 Ma. Whole-rock Rb-Sr age determinations from the Lebowa Granite Suite fall in the range 1790 ± 114 Ma to 1604 ± 70 Ma and are interpreted to reflect alkali element mobility and isotopic resetting during exhumation of the Bushveld granite. In contrast to thermal modelling which indicates that hydrothermal activity should have ceased within 4 my of emplacement, isotopic evidence suggests that mineralisation was long-lived, but episodic, and that fluid flow events were linked to major periods of Palaeo- and Mesoproterozoic orogenic activity along the margins of the Kaapvaal Craton. During these orogenic episodes, fluid flow was enhanced by tectonically induced fluid over-pressuring and/or exhumation of the Bushveld Complex.

Geochemistry and Tectonomagmatic Affinities of the Mozambique Belt Intrusive Rocks in Matuu-Masinga Area, Central Kenya

The Matuu-Masinga study area, located about 70 km north-east of Nairobi, is predominantly underlain by rocks of the Neoproterozoic Mozambique Belt (MB) of Kenya. The rocks vary from medium to high grade gneisses and granulites, that are intruded by granites, anorthosites, diorites and gabbros. Important high-grade tectonothermal events in the belt took place between about 845 and 715 Ma B.P. The subsequent cooling and uplift of the basement has been traced by K-Ar dates on biotites, which range between 438 and 528 Ma. The belt has been inferred to mark the sites for several superimposed Proterozoic subduction zones and collisional sutures (Muhongo, 1998, and references therein). The general structural trend in the survey area varies from NNW-SSE to NW-SE direction, with westerly dips and localized concentric trends around granitoid intrusions. Competent mafic lensoidal layers that have undergone ductile deformation and associated with rotated boudins and displaced micro-faults define a sinistral sense of shear. The several shear zones, cleavage patterns, boudins and thinning suggest severe strains. The mafic and granitic rocks are interpreted to be mainly metaluminous to slightly peraluminous. Geochemical data and field studies suggest that the granites are of calc-alkaline origin. The discriminant diagrams suggest a dominantly island arc-tectonic setting with subordinate within plate environment. Rb-Sr whole-rock age of 558 ± 16 Ma is given on the granite from Mavoloni hills, Matuu area. From the low initial 87Sr/ 86Sr ratio of 0.70398, the age is interpreted to indicate the time of emplacement for granite magma from a deep crustal or upper mantle material. The gabbroic and granitic rocks in Matuu-Masinga area, with high Ba (av. 1331 ppm) and Zr (av. 370 ppm) contents, are interpreted to have contained hornblende and biotite mineral assemblage phases that had a relatively high distribution coefficients for Ba and Z. The relatively high concentration of Cu (188–5810 ppm, av. 1960 ppm) and Zn (88–264 ppm, av. 155 ppm) in the mafic rocks of the study area invokes further exploration of their ore minerals.

Timing of Gothian structural evolution in SE Norway: A Rb-Sr whole-rock age study

The polydeformed basement area in SE Norway is situated within the southwest border of the Baltic Shield. The investigated area covers the northern part of the Ostfold segment (the Ostfold Complex) and adjoining parts of the Median segment (the Romerike Complex). New Rb-Sr whole-rock age determinations are integrated with the structural evolution. The oldest rock unit is the Ostfold-St. Le-Marstrand formation. During successive stages the supracrustal unit was intruded by major granitic bodies now forming the biotite gneiss (oldest), the augen gneiss and metatonalite/granite complexes (youngest). The emplacement of the granitic units was separated by orogenic deformation events, F0-F3, recognized by the development of large-scale regional folding accompanied by amphibolite facies metamorphism. Rb-Sr whole-rock age determinations indicate a maximum age for the biotite gneiss (pre-F0) at 1.71 Ga; this age also marks the maximum age of the Gothian orogeny. Intrusion of the Nordstrand-Sormarka metatonalite/gr...

Petrogenese kontrastierender Granitplutone in Westb�hmen (Tschechien)

Late-Variscan granitoid plutons in western Bohemia (Bor, Waidhaus-Rozvadov) have distinct petrographic, geochemical and isotopic features that suggest different magmatic evolutions. The Bor pluton comprises a suite of metaluminous tonalites and quartz diorites (Bor I), weakly peraluminous (monzo-)granites and granodiorites (Bor II) and medium-aluminous, late vein-forming leucomonzogranites (Bor III). The Waidhaus-Rozvadov pluton is strongly peraluminous, comprising a cordierite-biotite granitoid (CBG), the Rozvadov granite (ROG), the Barnau granite (BAG) and the subordinate, highly evolved Kreuzstein (Křižový kamen) granite (KG). Geochemical parameters and initial87Sr/86Sr ratios straddle the boundary between I- and S-type granites in the Bor pluton and are characteristic of purely S-type granites in the Waidhaus-Rozvadov pluton. The Bor II granitoids have been dated by the Rb-Sr whole-rock method at 341±17 Ma (ISr = 0.70724±0.00060). K-Ar biotite and muscovite ages of all units of the Bor pluton are mainly in the range 321-315 Ma. The K-Ar mineral ages are in good agreement with recently published U-Pb zircon data of these rocks. The different units of the Waidhaus-Rozvadov pluton have yielded less well-constrained Rb-Sr whole-rock ages, ranging from 313 to 300 Ma. However, the intrusion sequence is constrained by K-Ar muscovite ages (312-302 Ma), which define a systematic decrease towards the chemically more evolved granite types. Taken as a whole, it seems likely that the new radiometric ages characterize two temporally distinct periods of late-Variscan granitoid intrusion. The regional significance of these periods is emphasized by contemporaneous ages previously found in the adjacent northeastern Bavarian granitoids. The initial Sr and Nd isotope systematics indicate that the Bor and the WaidhausRozvadov plutons were derived from different source rocks. The Bor granitoids reflect the influence of less evolved crustal material which may have been similar to paragneisses of the Tepla-Barrandian region, including the Zone of ErbendorfVohenstraus (ZEV). The Waidhaus-Rozvadov granitoids probably resulted from anatexis of rocks resembling surrounding Moldanubian paragneisses or metapelites. In addition, the two plutons exhibit poorly defined, opposite trends of eNd(T) variation which are ascribed to assimilation processes.

Rb-Sr, Sm-Nd and K-Ar systematics of metamorphosed pillowed basalts and associated Besshi-type deposits in the Sanbagawa Belt, Japan

Samples from metamorphosed pillowed basalts and related Besshi-type deposits occurring in the Sanbagawa belt of the Shikoku Island, southwest Japan, have been analyzed for 87Sr/86Sr, 143Nd/144Nd and 40 K/40Ar. This is to investigate the tectonic settings in which the original submarine volcanism and associated Besshi-type mineralization occurred, as well as the age of metamorphism. Eight whole-rock samples of the pillow lavas metamorphosed in pumpellyite-actinolite facies conditions yield a Rb-Sr isochron age of 107 ± 15 Ma with an initial ratio of 0.70401 ± 0.00006, while they do not define a Sm-Nd isochron. We interpret the results as the metamorphic age, an interpretation consistent with the previously reported Rb-Sr whole-rock age for the Sanbagawa pelitic schists. The overall ranges of the initial epsilon values at T = 107 Ma are: ɛNd (T ) = +7.8 to +4.3; ɛSr(T ) = +2.2 to −7.0, suggesting that the most likely source for the pillowed basalts is depleted oceanic mantle, a conclusion supported by the previous Pb isotope studies. The K-Ar ages determined for twelve mineral separates from the Besshi-type deposits range from about 60 to 112 Ma, with a mean age of about 80 Ma, in agreement with the previous K-Ar and Ar-Ar data for the Sanbagawa pelitic and basic schists. The youngest age, 60 Ma, was obtained for sericite from the Hinooku deposit metamorphosed in pumpellyite- actinolite facies conditions, while the oldest one for hornblende from the spotted amphibolite in the immediate vicinity of the Shiragayama deposit metamorphosed in albite-biotite grade. The oldest age, 112 Ma, is interpreted to date the peak metamorphism, consistent with the Rb-Sr data, though a possibility of excess Ar cannot always be ruled out. In view of the closure temperatures of muscovite (350 °C) in the biotite zone, it is suggested that our K-Ar age data (<about 80 Ma) represent the age of the retrograde metamorphism or subsequent uplift. Datable microfossils found in the Sanbagawa belt of Shikoku suggest that the submarine basaltic volcanism and related Besshi-type mineralization occurred in an oceanic basin away from the trench region in Late Triassic (conodont) to Late Jurassic (radiolarian) times.

The Famatinian magmatic arc in the central Sierras Pampeanas: an Early to Mid-Ordovician continental arc on the Gondwana margin

Abstract A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Córdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 ± 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470–450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87 Sr/ 86 Sr (0.7075–0.7105) and low ɛNd t (−4.6 to −6.3), inherited from lower crust. Sm-Nd model ages of 1600–1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.

Fluid Flow and Diffusion in the Waterville Limestone, South--Central Maine: Constraints from Strontium, Oxygen and Carbon Isotope Profiles

Oxygen, carbon and strontium isotopic profiles across the margin of the Waterville limestone member are used to investigate advective and diffusive transport during metamorphism of the Waterville Formation in south–central Maine, USA. Rb–Sr isotopic systematics were homogenized on the ~10 cm hand-specimen scale at ages that are within error of the 366±6 Ma Rb–Sr whole-rock age of the syn-metamorphic Hallowell pluton. This is consistent with a plutonic heat source for this low-pressure andalusite- and sillimanite-grade Acadian metamorphic terrane. Advective displacements of all three isotope profiles at the garnet-grade Blue Rock Quarry indicate fluid flow to the east into the limestone, and the oxygen-isotope profile implies a time-integrated fluid flux of 3.2±1.5 m 3 /m 2 (2σ error). This cross layer flux is insufficient to cause the observed reaction progress of the muscovite+ankerite–Quartz to biotite+anorthite (in plagioclase)+calcite reaction in the ~100 m thick Waterville limestone member and much of the fluid flow responsible may have been layer parallel. The isotope profiles indicate advective–diffusive homogenization over distances of 1.5 m (δ 13 C) to 6 m (δ 18 O) and such homogenization distances are difficult to reconcile with observations of order of magnitude variations in reaction progression the centimetre scale or less. It is possible that infiltration occurred during events short lived compared with diffusion, that the reactions started at different temperatures dependent on bulk composition or that diffusion of water from layers with less reactants to layers with more reactants was important in driving the biotite-producing reaction. However, variations of fluid composition inferred from the mineral assemblages are apparently inconsistent with diffusion driving reactions progress, and models of precursor assemblages do not indicate significant compositional control of the temperature of the first appearance of biotite in the rocks. Irrespective of the details of flow and diffusive exchange on the centimetre scale, the average reaction progress in the Waterville limestone member requires significant layer-parallel fluid fluxes.

U-Pb dating of Mesozoic igneous rocks from Hong Kong

Twenty-two new zircon and monazite U-Pb ages have been obtained for Mesozoic plutonic and volcanic rocks of Hong Kong. These have revealed the existence of four distinct periods of Mid-Jurassic to Lower Cretaceous volcano-plutonism: 164.6 ±0.2 to 159.3 ±0.3 Ma, 146.6 ±0.2 to 146.2 ±0.2 Ma, 143.7 ±0.3 to 142.5 ±0.3 Ma, and 140.9 ±0.2 to 140.4 ±0.2 Ma. A concordant monazite age of 236.3 ± 0.8 Ma has also been obtained from one intrusive body representing the first known occurrence of a Mid-Triassic granitoid in the Territory. Among these rocks, discordant U-Pb data projected to the concordia curve have given a broad range of inheritance ages from early Phanerozoic to late Archaean. A xenocrystic zircon from one sample has yielded a concordant age of 1872 ±3 Ma. The new U-Pb ages have refined the results of recent Rb-Sr whole-rock age dating of the granitoid rocks and have considerably improved correlation between intrusive and extrusive units. The occurrence of inherited components in many of the analysed rocks indicates that they were derived from, or erupted through, older crust containing a wide variety of age components, either as primary igneous or detrital material.