Whole-rock Systems Research Articles

The unregenerate São Rafael pluton, Borborema Province, Northeastern Brazil

The late Neoproterozoic São Rafael pluton, located in the Seridó Belt of the Borborema geologic province, northeastern Brazil, is an unfoliated, metaluminous, epidote-bearing, high-K calc-alkalic granitoid. Three facies are present in its northern lobe: (i) a dominant massive granite to quartz monzonite porphyritic facies containing microcline phenocrysts, (ii) subordinate fine-grained facies, similar to the porphyritic facies except lacking amphibole, that crop out as inselbergs (“Serras”), and (iii) rare pillows of microgranular mafic enclaves (MMEs) that are present within the porphyritic facies.Chemical analyses include 19 bulk-rock data, and 33 microprobe data of epidote and amphibole. Beginning with bulk chemistry and postulated mineral compositions, an iterative procedure of solving matrices characterizes a rock sample. Derived information includes: (i) identifying what minerals are present, (ii) their relative abundances, and (iii) their chemical compositions, even for minerals (such as biotite or amphibole) with complexly variable compositions. Calculated mineral compositions are compatible with microprobe data. The matrix solutions further indicate alternative mineral assemblages, and the compositions of “phantom” minerals that could have existed in the magma source, although absent in the pluton. They establish that olivine, but not clinopyroxene, was a possible phantom mineral and that MMEs are of unrelated magmatic origin.According to matrix analysis, amphibole in an outer zone of the porphyritic facies is solely of neoformed origin, whereas amphibole in an inner zone of the porphyritic facies is of mixed origin—partly neoformed and partly residual (i.e., xenocrystic) “patchy” amphibole derived from MMEs. A qualified geothermometer provides Tsolidus = 730 ± 6 °C for the outer subfacies, and a qualified geobarometer provides P = 0.78 ± 0.06 kbar. Amphibole in MMEs crystallized at depth, at P~3.3 kbar. Corroded epidote attests to the pressure-sensitive instability of that mineral during rapid ascent decompression.Cathodoluminescence reveals complexly structured zircons with magmatic overgrowths and inherited cores. 207Pb/206Pb ages of inherited components cluster at ~2.0 Ga and ~2.4 Ga, these ages being prominent in the local basement. Oscillatory-zoned magmatic components yield a concordia upper intercept age of 580 ± 9 Ma, interpreted as the age of igneous crystallization. Values of δ18O in zircon are only slightly higher than δ18O in mantle-like zircon. At 0.58 Ga, εHf in magmatic zircon was notably unradiogenic (−22 to −26), indicating a magma source in old continental crust as adduced by ages of inherited zircon. A U-Pb titanite date is around 0.55 Ga.For the Rb-Sr and Sm-Nd whole-rock systems, isotopic inheritance from the magma protolith is so extreme and variable that isochrons are actually “scatterchrons” that provide no useable age information. Rb-Sr cooling ages of biotite are 498 ± 1 Ma. Most Sm-Nd depleted mantle ages lie in the range of 2.6 ± 0.13 Ga. São Rafael magma was “unregenerate,” not isotopically homogenized when emplaced.

The long (3.7–2.1 Ga) and multistage evolution of the Bug Granulite–Gneiss Complex, Ukrainian Shield, based on the SIMS U–Pb ages and geochemistry of zircons from a single sample

Abstract Multidisciplinary studies of zircons, rock-forming minerals and the whole-rock composition of granulite samples from the Bug Granulite–Gneiss Complex, Ukraine (including ion microprobe REE analysis, secondary ion mass spectrometry (SIMS) U–Pb and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) Lu–Hf analysis of zircons from a single sample) have revealed three major stages in the geological evolution of the complex. (i) At 3.66 Ga, a mafic intrusion contaminated with felsic rocks formed, as evidenced by 3.74 Ga zircon xenocrysts with inclusions of plagioclase, K-feldspar and quartz. (ii) At 3.59–3.55 Ga, high-temperature and high- to moderate-pressure granulite-facies metamorphism accompanied by migmatization and deformation resulted in the formation of mafic granulites. (iii) At 2.1–2.0 Ga, metamorphic overprinting occurred, and metatrachybasaltic dykes intruded at approximately 2.0 Ga. The metamorphic mineral assemblages recorded in the dykes formed at temperatures similar to those of the 3.59–3.55 Ga metamorphism but at pressures 2–3 kbar lower. This metamorphism disturbed the Sm–Nd whole-rock system, altered the Hf isotope system of the older zircons and resulted in Pb loss in small zircon grains. This complex event history recorded in zircons from a single rock corresponds to major stages of the geological evolution of both the Dniester–Bug Province and the entire Ukrainian Shield.

Nanoscale variations in 187 Os isotopic composition and HSE systematics in a Bultfontein peridotite

Abstract Understanding the mineralogical controls on radiogenic chronometers is a fundamental aspect of all geochronological tools. As with other common dating tools, it has become increasingly clear that the Re–Os system can be impacted by multiple mineral formation events. The accessory and micrometric nature of the Re–Os-bearing minerals has made assessing this influence complex. This is especially evident in cratonic peridotites, where long residence times and multiple metasomatic events have created a complex melting and re-enrichment history. Here we investigate a harzburgitic peridotite from the Bultfontein kimberlite (South Africa) which contains sub-micron Pt–Fe-alloy inclusions within base metal sulphides (BMS). Through the combination of the focused ion beam lift-out technique and low blank mass spectrometry we were able to remove and analyse the Pt–Fe-alloy inclusions for their Re–Os composition and highly siderophile element (HSE) systematics. Six repeats of the whole-rock yield 187 Os/ 188 Os compositions of 0.10893–0.10965, which correspond to Re depletion model ages (T RD ) of 2.69–2.79 Ga. The Os, Ir and Pt concentrations are slightly variable across the different digestions, whilst Pd and Re remain constant. The resulting HSE pattern is typical of cratonic peridotites displaying depleted Pt and Pd. The Pt–Fe-alloys have PUM-like 187 Os/ 188 Os compositions of 0.1294 ± 24 (2-s.d.) and 0.1342 ± 38 , and exhibit a saw-tooth HSE pattern with enriched Re and Pt. In contrast, their BMS hosts have unradiogenic 187 Os/ 188 Os of 0.1084 ± 6 and 0.1066 ± 3 , with T RD ages of 2.86 and 3.09 Ga, similar to the whole-rock systematics. The metasomatic origin of the BMS is supported by (i) the highly depleted nature of the mantle peridotite and (ii) their Ni-rich sulphide assemblage. Occurrence of Pt–Fe-alloys as inclusions within BMS grains demonstrates the genetic link between the BMS and Pt–Fe-alloys and argues for formation during a single but continuous event of silicate melt percolation. While the high solubility of HSE within sulphide mattes rules out early formation of the alloys from a S-undersaturated silicate melt and subsequent scavenging in a sulphide matte, the alignment of the Pt–Fe-alloy inclusions attests that they are exsolutions formed during the sub-solidus re-equilibration of the high temperature sulphide phases. The significant difference in 187 Os/ 188 Os composition between the included Pt–Fe-alloys and their BMS host can only be accounted for by different Re/Os. This suggests that the formation of Pt–Fe-alloy inclusions within a BMS can result in the fractionation of Re from Os. A survey experiment examining the partitioning of Re and Os confirmed this observation, with the Re/Os of the Pt–Fe-alloy inclusion up to ten times higher than the co-existing BMS. This fractionation implies that, when Re is present in the sulphide melt, the T RD ages of BMS containing alloy inclusions do not date the loss of Re due to partial melting, but rather its fractionation into the Pt–Fe-alloys. As such, BMS ages should be used with caution when dating ancient partial melting events.

The orthogneisses of the Orlica–Śniez2nik complex (West Sudetes, Poland): geochemical characteristics, the importance of pre-Variscan migmatization and constraints on the cooling history

Geochemical analyses in combination with Rb–Sr (whole-rock, phengite, biotite) and U–Pb zircon ages provide important constraints on the magmatic, metamorphic and structural evolution of the Śniez2nik and Gierałtów gneisses from the Orlica–Śniez2nik complex (West Sudetes). These two gneisses have been considered to represent distinct petrogenetic units; however, their major and trace element compositions as well as their Sr–Nd isotope characteristics show no systematic differences that are indicative for different protoliths. This striking similarity leads to the conclusion that the petrographic variability is caused by modifications superimposed during deformation and migmatization. Most εNd 500 values are in the range between −3.3 and −5.7 and suggest derivation of the protoliths from pre-existing continental crust. Two-stage T DM model ages mostly fall in the range between 1.4 and 1.6 Ga and closely correspond to other orthogneiss occurrences in the Bohemian Massif. The Rb–Sr whole-rock system is disturbed on a regional scale, to variable degrees, resulting in dates ( c . 450 Ma and c . 395 Ma) considerably younger than the time of magmatic crystallization ( c . 500 Ma). Secondary ionization mass spectrometry (SIMS) U–Pb analyses provide two groups of 206 Pb/ 238 U ages (364–341 Ma and 527–472 Ma), which largely correlate with previously established ages for protolith formation and Variscan high-temperature metamorphism. No geochronological evidence for pre-Variscan (‘Caledonian’) events was found. For phengite and biotite, the Rb–Sr system yields ages of c . 340–320 Ma, which provide further constraints on the regional cooling history.

SHRIMP U–Pb, 207Pb/ 206Pb zircon dating, and Nd isotopic signature of the Umburanas greenstone belt, northern São Francisco craton, Brazil

The Gavião block in the northern portion of the São Francisco craton, northeast Brazil, is one of the oldest Archean fragments of the South American platform. It underwent polycyclic evolution from very old juvenile components dated between 3400 and 3000 Ma. The studied parts of the Umburanas greenstone belt (UGB) formed over ‘stable’ continental crust. SHRIMP U/Pb isotopic analyses of detrital zircons from conglomeratic quartzites of the UGB's lower unit yield dates between 3335 and 3040 Ma. These dates indicate provenance from crustal materials of different ages and a maximum age of deposition as young as 3040±24 Ma (single zircon analysis) but certainly after 3147±16 Ma (three analyses). 207Pb/ 206Pb zircon evaporation analyses from a meta-andesite of the intermediate unit of the UGB yield 2744±15 Ma, which is interpreted as the time of magmatic crystallization. The Sm/Nd whole-rock systematics of three metakomatiites from the base of the lower unit indicate that they are isotopically disturbed. Additional Sm/Nd analyses of one metabasalt (lower unit) and two meta-andesites (intermediate unit) were compared with those of the country rocks. Their similarity suggests that crustal contamination processes play an important role in the formation of volcanic rocks. As a whole, the isotopic picture is in agreement with an ensialic tectonic setting of the studied parts of the UGB.

Dating fault-generated pseudotachylytes: comparison of 40Ar/39Ar stepwise-heating, laser-ablation and Rb–Sr microsampling analyses

Three different geochronological techniques (stepwise-heating, laser-ablation 40Ar/39Ar, Rb–Sr microsampling) have been evaluated for dating fault-generated pseudotachylytes sampled along the Periadriatic Fault System (PAF) of the Alps. Because pseudotachylytes are whole-rock systems composed of melt, clast and alteration phases, chemical control from both Ar isotopes (Cl/K, Ca/K ratios) and EMPA analyses is crucial for their discrimination. When applied to stepwise-heating 40Ar/39Ar analyses, this approach yields accurate melt-related ages, even for complex age spectra. The spatial resolution of laser-ablation 40Ar/39Ar analyses is capable of contrasting melt, clast and alteration phases in situ, provided the clasts are not too fine grained, the latter of which results in integrated mixed ages without geological information. Elevated Cl/K and Ca/K ratios were found to be an invaluable indicator for the presence of clast admixture or inherited 40Ar. Due to incomplete isotopic resetting during frictional melting, Rb–Sr microsampling dating did not furnish geologically meaningful ages. On the basis of isotopic disequilibria among pseudotachylyte matrix phases, and independent Rb–Sr microsampling dating of cogenetic (ultra)mylonites, the concordant 40Ar/39Ar pseudotachylyte ages are interpreted as formation ages. The investigated pseudotachylytes altogether reveal a Cretaceous to Miocene history for the entire PAF, consistent with independent geological evidence. Individual faults, however, consistently reveal narrower intervals of enhanced activity lasting a few million years. Electronic supplementary material to this paper can be obtained by using the Springer LINK server at http://dx.doi.org/10.1008/s00410-002-0381-6

Potassium sources and illitization in Texas Gulf Coast shale diagenesis

ABSTRACT The aim of this paper is to identify the controls on the rates of illitization in Texas Gulf Coast shales under diagenetic conditions. Two different mineral transformations are considered: the transformation of smectite to (nearly) illite and the precipitation of discrete illite, i.e., the formation of pure illite. Analyses of potassium feldspar and kaolinite abundance, in addition to K2O, Al2O3, and TiO2 contents, in shales from three wells are used to supplement published data in order to identify the mineral reactions responsible for the smectite-to-illite transformation in shales involving illite/smectite (I/S) mineral series. These data indicate that the first stage of the smectite-to-illite reaction in shales (R = 0 structures, ca. 0- 50% illite content) is controlled by the presence of feldspar and hence also by the dissolution kinetics of feldspar. The empirically derived activation energy for the first I/S transformation in shales is similar to that for K-feldspar dissolution. Kaolinite is not involved in this transformation in the shales investigated. The second stage of the reaction sequence (R = 1 structures, ca. 50- 100% illite content) occurs after potassium feldspar has largely or completely disappeared from the shale system. This indicates that the feldspar no longer controls the second stage of the smectite-to-illite transformation, and suggests another, external, source of potassium for the reaction in the shales. This assumption is supported by the relative increase of potassium content of the shales, at least in two of the three studied wells. Kaolinite does not seem to be involved either in this second stage of the I/S reaction sequence. The formation of discrete illite crystallites in shales is independent of the I/S transformation reactions and takes place only after a very significant period of time has elapsed. The slow rate of this reaction reflects the low activity of potassium in solutions that are in equilibrium with shale, in comparison with sandstones, where a rapid precipitation of pure illite can in some cases be observed. It is suggested that the different reactions in the smectite-to-illite transformation necessitate different sources of potassium either from within or outside an argillaceous whole-rock system. These different sources of potassium give rise to different rates for the I/S conversion reaction and the formation of illite.

Conflicting mineral and whole-rock isochron ages from the Late-Archaean Lewisian complex of northwestern Scotland: Implications for geochronology in polymetamorphic high-grade terrains

We report major and trace element (including rare-earth element, REE) geochemistry and whole-rock Sm Nd geochronology on two suites of mafic/ultramafic inclusions in an agmatite complex within a trondhjemitic host at Gruinard Bay, in the Late Archaean Lewisian Complex of northwest Scotland. A suite of medium-grained equigranular amphibolites (main amphibolite suite, MAS) has broadly chondritic high field strength element (HFSE) ratios (Ti/Zr ca. 115, Ti/Y ca. 318, Y/Cr ca. 0.39), flat heavy REE patterns at five to fifteen times chondrites, usually with mild light REE enrichment (La N < 50) and small Eu anomalies. This is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite-trondhjemite-granodiorite (TTG) suite magmas, as argued previously by Rollinson and Fowler (1987). A Sm Nd regression line (MSWD = 3.4) obtained from the MAS yields an age of 2.943 ± 0.091 Ga with an initial ε Nd value of +2.69 ± 0.62. A suite of coarser grained hornblendites and metagabbros (hornblendite-metagabbro suite, HMS) displays significant geochemical differences from the MAS, exemplified by significantly light REE-enriched patterns in which the hornblendites have the highest total REE with La N/Nd N< 1 and large negative Eu anomalies, and the most felsic metagabbros have low total REE and large positive Eu anomalies. Samples from the HMS yield a Sm Nd isochron (MSWD = 0.52) age of 2.846 ± 0.073 Ga with an initial ε Nd value of +1.10 ± 0.72. The HMS geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local TTG magmas. Combining the new data with previously published data from the host trondhjemites at Gruinard Bay (Whitehouse, 1989a) yields a Sm Nd isochron (MSWD = 1.09) age of 2.795 ± 0.028 Ga with an initial ε Nd value of +0.66 ± 0.38. The relatively low initial ε Nd values for the HMS and combined HMS-trondhjemite isochrons compared to contemporaneous depleted mantle (ε Nd(2.8) ca. + 2.1 to + 3.3) is consistent with derivation of a parental magma from light-REE-enriched components of a mafic (basaltic) precursor which was itself derived from a depleted mantle source approximately 100–200 Ma previously. The MAS suite at Gruinard Bay may be representative of such a mafic precursor. Our new geochronological data differ significantly from the ca. 3.3 Ga mineral Sm Nd and Pb Pb regression ages presented recently for two members of the HMS by Burton et al. (1994) which, if correct, require radical revision of the early geological history of the Lewisian Complex. Comparison of the relatively old mineral ages with our new whole-rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole-rock systematics. However, we argue that extensive retrogression, evident in all lithologies at Gruinard Bay, makes them less than ideal samples for a mineral isotopic study and it is likely that the ca. 3.3 Ga ages may be spurious, the product of disturbance of isotope systematics during the Laxfordian. On the basis of the geochronology presented here, evolution of the Lewisian closely follows generally accepted models for late-Archaean crust formation.

Rb-Sr and Sm-Nd Ages from the Highland Complex of Sri Lanka.

Metaigneous rocks from Gampola and Nuwara Eliya belong to the Highland Complex of Sri Lanka. Rb-Sr and Sm-Nd age data and initial Sr and Nd isotopic ratios document several geological events. (1) Separation of crustal material began in the late Archean. (2) Partial melting of crust took place on a large scale at ca. 2400Ma, leaving metaigneous rocks as restite after extraction of felsic magmas. (3) High-grade metamorphism related to the Pan-African orogeny occurred at 520Ma. (4) Partial melting of crustal material proceeded on a local scale at 490Ma-480Ma, producing slight disturbance to the Rb-Sr whole-rock system.

SmNd geochronology of Sveconorwegian granulite facies mineral assemblages in the Bamble Shear Belt, South Norway

Abstract The age of the granulite facies metamorphism in the Bamble Belt has long been a matter of debate. Samples from four localities in the coastal section of the belt have, therefore, been used to obtain SmNd mineral ages. All gave dates within the range 1070–1100 Ma. Earlier RbSr data of Field et al. had placed the maximum metamorphism of the region at 1540 Ma. These authors deduced that any subsequent metamorphism in the Sveconorwegian (∼1100 Ma) was low grade since the RbSr system had not been completely reset at this time. New UPb data, published elsewhere, indicate two periods of deformation and metamorphism; a pre-Sveconorwegian metamorphism at 1437±8 Ma and a Sveconorwegian period at 1100 Ma. Our SmNd data indicate that the Sveconorwegian period lasted for at least 50 Ma, and included granulite facies metamorphism ending at 1095 Ma. We further deduce that RbSr whole-rock systems were not necessarily totally reset during the granulite facies metamorphic event. Our conclusions represent a return to older interpretations concerning the significance of the Sveconorwegian event.

High precision U-Pb ages for granulite metamorphism and deformation in the Archean Kapuskasing structural zone, Ontario: implications for structure and development of the lower crust

Precise U-Pb isotopic ages for two subhorizontal granulite gneisses in the Kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m.y. after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust-mantle delamination. Volcanism and batholith formation had ceased by 2700 and 2680 Ma, respectively, while granulite formation occurred between 2660 and 2640 Ma and ductile deformation continued until 2585 Ma. Emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite, while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite. Widespread tectonic underplating is implied since all Superior Province granulites have similar young ages. Anomalous Ar ages that post-date accretion by about 150 m.y., and scrambled Pb-Pb and Rb-Sr whole-rock systems, also suggest that late low-temperature alteration accompanied this event. Temporal relationships and the presence of post-volcanic (2670 Ma) conglomerate cobbles at granulite grade support a model of ductile underplating of a post-accretion, thermally softened back arc-fore arc complex, with components welded onto an overlying volcanic-plutonic complex at successively deeper levels. Multistage deformation in active fault zones and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 Ma. Anomalously abundant gold production from Archean rocks may indicate that shallow underplating, granulitization and devolatization was a widespread phenomenon at this time. Post-accretion ductile underplating must be considered as a primary cause of seismic reflectors in Archean terrains.

A new approach for the determination of the age of partial or complete homogenization of Pb isotopes — Example: anchimetamorphic, detrital sediments of the Central Iberian Zone, Spain

Pb isotope systematics are discussed for lowermost Ordovician to Devonian pelites of the Almaden area (Central Iberian Zone of Spain). The rocks suffered a Carboniferous anchimetamorphic overprint which variably reset the Rb/Sr whole-rock systems at ∼ 335 Ma. The weak metamorphic event partially homogenized the Pb isotopes of pelites between ∼ 330 and ∼ 320 Ma. This time range of partial homogenization was deduced by stepwise age correction ( t 1 < t 2 < … < t sedimentation) of Pb isotopes for in situ radiogenic lead production within each whole-rock sample. For each correction age ( t x) a mean Pb isotopic ratio of all samples was calculated and the respective 1σ error was used as a measure for the scatter of Pb isotopes in the individual samples at t x. The age corresponding to the smallest scatter ( t 1 σ(min) ) of the Pb isotopes then indicates the age of partial homogenization (Carboniferous), in line with the geological constraints. The same age of partial Pb isotopic homogenization can also be obtained if the correlation coefficients of 206 Pb 204 Pb vs. 207 Pb 204 Pb are determined for the individual data sets that were stepwise corrected for in situ radiogenic Pb. The worst correlation, monitored by the lowest value of the correlation coefficient, is then indicative for the time at which the Pb isotopic homogenization reached its maximum degree. Applying the above described procedures, partial or complete homogenization of Pb isotopes during any magmatic, metamorphic or metasomatic process may readily be detected in any suite of whole-rock and/or mineral systems. Thus, we strongly recommend determinations of U and Pb concentrations on a routine basis, even if there exists the possibility of secondary U and/or Pb mobility.

Late Archaean intrusive complexes in the Olekma granite-greenstone terrain (eastern Siberia): geochemical and isotopic study

Two granitoid intrusions, the Amnunnakta and Oldongso massifs in the Olekma granite-greenstone terrain of the Aldan shield have been studied using Sm-Nd, Rb-Sr and Pb-Pb whole rock (WR) and U-Pb zircon methods. The isotopic composition of Pb in plagioclase, as well as whole-rock oxygen isotope compositions and REE patterns were also measured. The following ages were obtained for the Amnunnakta massif: U-Pb zircon 2984±22 Ma (2σ), Pb-Pb WR 2991±57 Ma, Sm-Nd WR 3094±430 Ma and for the Oldongso massif: U-Pb zircon 2999±51 Ma, Pb-Pb WR 2855±170 Ma, Sm-Nd WR 2923±144 Ma. The similar ages suggest penecontemporaneous formation of the two intrusive series. The observed resetting of the Rb-Sr WR systems (2767±116 Ma for Amnunnakta massif and 2597±82 Ma for the Oldongso massif) and opening of the plagioclase Pb isotopic systems are associated with superimposed metamorphic processes of the greenschist and epidote-amphibolite facies. Source regions of the massifs are characterized by long-term depletion in incompatible elements: the initial ϵ Nd(T) value is +2.4±0.4 for both massifs. The μ 1 values are mantle-like at 7.68 for the Amnunnakta and 7.71 for the Oldongso massifs and the initial Sr composition is low (Sr i=0.7015±5) for the Oldongso massif. Basic rocks of the Amnunnakta massif are characterized by low δ 18O values (2.7–4.7) and elevated Sr i=0.7052±7 which may be caused by both a primary low δ 18O, high rm 87Sr 86Sr magma or later interaction with a hydrothermal system. There is no evidence for recycled sedimentary material in the granitoids of either massif. One of the rocks, a fine-grained garnet-bearing trondhjemite, previously assigned to the Amnunnakta massif, yielded a T Nd model age of 3700 Ma and reflects the presence of older sialic material. Moderately fractionated REE patterns with well pronounced negative Eu anomalies in low-alumina trondhjemites of the Amnunnakta massif suggest their formation by a process of differentiation of high-alumina basaltic melts with fractionation of plagioclase. High-alumina tonalite-trondhjemites of the Oldongso massif are characterized by fractionated and depleted HREE patterns suggesting the residual presence of garnet and hornblende during partial melting of a basic source.

Age and nature of protoliths in the Caledonian blueschist-eclogite complex of western Spitsbergen: a combined approach using UPb, SmNd and REE whole-rock systems

High-pressure metamorphic rocks (≈20 kb) (eclogites, eclogitic metagabbros and glaucophane-schists) from western Spitsbergen were investigated for major and trace element (including REE) geochemistry, as well as Nd isotopes and UPb zircon characteristics. The distinct nature of protoliths of the various high-pressure rocks has been preserved to some extent. Eclogites and eclogitic metagabbros represent Lower Proterozoic (2.1 Ga) alkali basalts and transitional MORB, respectively. Glaucophane-schists probably represent Upper Proterozoic spilitized FeTi gabbros or volcaniclastic mixtures, or both. The dating results indicate that the Caledonian high-pressure metamorphism occurred at ca. 475 Ma, whereas a Lower Proterozoic magmatic activity — derived from a depleted mantle source — is also recognized. This event is the oldest magmatism detected so far in the Svalbard Caledonides. The age of the protolith of the glaucophane-schists is not strictly constrained; however, it formed between 1.0 Ga and 475 Ma.

Distribution of trace elements during breakdown of mantle garnet: an example from Zabargad

Ion probe investigations on mineral phases forming the Al-Di pyroxenites from the Zabargad peridotite body indicate that porphyroclastic pyroxenes in composite mafic layers record an unusual HREE, Zr, Sc enrichment not registered by pyroxenes in spinel websterites. Orthopyroxene in the opx+sp clusters forming the inner, cpx-free zone of layered pyroxenites shows strongly fractionated REE patterns (HREEN/LREEN>1000; Yb>100xch) and very high Zr, Sc and Y abundances (up to 30,672 and 60ppm, respectively). In the outer, cpx-rich zone porphyroclastic clinopyroxene is strongly HREE enriched (HREEN/LREEN∼29; Yb∼ 269xch) and displays very high Sc and Zr abundances (up to 819 and 164 ppm, respectively). It is suggested that the unusual trace element abundances are inherited from a precursor garnet. Composite pyroxenite layers are interpreted as former garnet clinopyroxenites characterized by gnt/cpx modal zoning. The sp+opx(cpx-free) assemblage in the inner part is a product of the break-down reaction of garnet upon decompression, with Ca of the original garnet completely entering the enstatite solid solution. The temperature at which the breakdown reaction occurred is estimated to be higher than 1000°C (P in the range 20–30 kbar). In the outer part, decompression caused the garnet to form a sp+opx assemblage; however, the grossularite component participated in the formation of new clinopyroxene which reacted with the clinopyroxene present in the original mode before the decompression reaction, thus forming a cpx2+sp+opx assemblage. As a result of garnet breakdown, pyroxenes have peculiar HFSE anomalies. Progressive upwelling during the Red Sea rifting produced incomplete reaction under pl-facies conditions. The geochemical signatures of precursor garnet in pyroxenes were partially crased during the recrystallization from granular spincl-bearing to granoblastic plagioclase-bearing assemblages, being preserved only in a few porphyroclast relies. The finding of pyroxenes with trace element characteristics of precursor garnet has important geodynamic and geochemical implications. Al-Di pyroxenite layers had a long history within the mantle, before the continental lithosphere rifting and thinning took place in the region. It is suggested that Al-Di pyroxenites were formed by deep-seated tholeiitic magmatism unrelated to the Red Sea evolution, thus representing the earliest event in the Zabargad upper mantle. Garnet breakdown significantly preceded the metasomatism induced by hydrous fluids (crystallization of Ti-rich pargasite) and the later intrusion of hydrous (Cr-Di) pyroxenite dykes. During the stages of mantle evolution, the HFSE anomalies in pyroxenes varied significantly. We note that the study of HFSE anomalies in mineral phases reveals complex geochemical histories which are not recorded by the whole-rock system.

The three-dimensional UPb method: Generalized models and implications for UPb two-stage systematics

Four forms of the three-dimensional UPb method established so far are physically generalized within the framework of a UPb two-stage evolution. Such a generalization provides an explicit insight into the principle of conventional UPb discordia dating. Its implications for both UPb geochronology and Pb isotopic geology are illustrated for whole-rock systems. When applying the three-dimensional data processing method to the experimentally determined results on the U and Pb concentrations and Pb isotopic composition of a given suite of cogenetic samples, one cannot only obtain a unique solution to timing of two episodic events (i.e. ages t 1 and t 2) directly, but also shed some light on the feature of the source region of initial Pb incorporated at time t 1 and on the nature of U/Pb differentiation at the two episodic events. Common Pb for correction in the discordia dating is clarified to be responsible for the initial Pb at t 1 rather than t 2. Such a clarification would have an impact on the conventional UPb geochronology that makes assumptions about the isotopic composition of common Pb in calculating the discordias. The three-dimensional UPb method has advantages over the two-dimensional ones (UPb discordias or PbPb isochron) for treating the results of UPb isotopic measurements on a suite of cogenetic samples, because it takes into account three variables instead of two variables in data processing and thus enables straightforward calculation of ages responsible for two episodic events without a priori assumptions. This is a particular advantage for treating the UPb isotopic data of whole-rock samples and therefore for discovering possible low-temperature U/Pb differentiation responsible for terrane uplift.

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

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

Pb/Pb, SmNd and RbSr geochronology in the Archean Craton of Zimbabwe

Pb isotope data for greenstone belt volcanic units, plutons and gneisses of the Archean Craton of Zimbabwe are presented together with SmNd isotopic analyses and model Nd mantle derivation ( t DM) ages for selected samples. Pb/Pb whole-rock isochrons yield well-determined dates for the Cardiff Hill rhyolite of the Shamva-Harare greenstone belt (2659 −39 +38 Ma), the Somabula tonalite (2752 −52 +50 Ma), the Gwenoro Dam migmatitic gneisses (2705 ∂3 +60 Ma), and for various suites from the Chingezi tonalite (from 2874±32 to 2686 −94 +88 Ma). Comparison of geochronological results from this study with those of earlier work (mainly RbSr whole-rock dating) shows some significant discordances, and their possible causes are discussed with regard to time-integrated Th/U ratios and the geological settings of the relevant rock units. The Chingezi and Sesombi plutons show good agreement between RbSr and Pb/Pb whole-rock isochron dates and also display a limited range of Th/U ratios, appropriate to a purely igneous differentiation history. Early to mid-Archean gneisses show large ranges of Th/U ratios, probably the results of U disturbances during metamorphism. In these rocks Pb/Pb dates may be older or younger than the corresponding RbSr dates, but t DM model ages generally agree with the older of the isochron results. The behaviour of the RbSr and UPb whole-rock systems during metamorphism may depend critically on the nature of the fluid phase evolved. CO 2-rich fluids appear to be implicated in U enrichment of early Archean gneisses at the Shabanie Mine. It is argued that CO 2-rich fluids may cause disturbance of the UPb system without resetting the RbSr system, while a H 2O-rich fluid phase could have the reverse effect.

Rb-Sr and K-Ar geochronology of turbidites and metavolcanics at Red Rocks, Wellington, New Zealand

Abstract Rb-Sr and K-Ar dating of various lithologies at Red Rocks, Wellington coast, New Zealand, indicate a complex thermotectonic history. Turbidites west of Red Rocks yield a Rb-Sr whole-rock age of 216 Ma, interpreted to be the time of closure of the whole-rock system during burial and low-grade metamorphism of the sediments about 15–20 Ma after deposition. Turbidites east of Red Rocks yield internally concordant Rb-Sr ages of 188–190 Ma, which are similarly interpreted, and suggest a younger sequence, with a fault between the two turbidite units. The ages are consistent with a general eastwards-younging of the Torlesse accretionary prism in the Wellington region. Red/green argillites from Red Rocks itself define an imperfect Rb-Sr isochron of about 159 Ma. K-Ar ages for these rocks range from 147 to 163 Ma and reflect argon loss during a second localised thermal event, probably related to thrusting of the eastern over the western turbidite sequence during imbrication of the Torlesse accretionary prism.

Archaean chronology in the eastern part of the Baltic Shield: a synthesis

Twenty-seven gneisses and granodiorites of the Archaean basement of eastern Finland have been chosen for Rb-Sr geochronological studies. The results were used together with other radiometric measurements to discuss the time-space relationships between the various rock units. This review can be summarized as follows: from 3.0 to 2.8 Ga a first generation of grey orthogneisses (Kivijärvi type) was emplaced. Subsequently to an important tectono-metamorphic event a second generation of grey orthogneisses (Naavala type) intruded the Kivijärvi gneisses (2.69-2.65 Ga). The two generations of grey gneisses form the basement on which were deposited the mafic and ultramafic magmas of the lower volcanic sequence of the Kuhmo-Suomussalmi greenstone belt at 2.65 Ga. Both greenstone and gneiss basement were intruded by calc-alkaline intermediate to felsic magmas. They were emplaced as volcanites in the upper part of the greenstone belt (2.5 Ga) or as granodioritic augen gneisses into the basement (2.50–2.55 Ga). The end of the Archaean magmatic activity is marked in this part of the Baltic Shield by the intrusion of pink leucogranites (2.41 Ga). These age data are consistent with the field relative chronology and show that: (1) the Finnish Archaean terrains developed over a period of at least 0.45 Ga, followed by a 0.40-Ga period of geologic quiescence; (2) there is a gneissic basement to the Kuhmo-Suomussalmi greenstone belt: the Kivijärvi grey gneisses were emplaced at least 0.25 Ga prior to the greenstones; (3) the zircons extracted from recycled magmatic rocks give c. 2.7 Ga ages and are interpreted as retaining a memory of their ancient protolith and consequently do not indicate the emplacement age of their present host rock. This interpretation is subject to great controversy and discussion among geologists; (4) the Rb-Sr whole-rock system is not modified by the Svecokarelian metamorphism, but the strongly heterogeneous rocks as well as the minerals were affected and recorded the thermal influence of that orogeny.