KAr isochron study of the Tudor Gabbro,Grenville Province,Ontario

Ten whole-rock samples from the Tudor Gabbro, Grenville Province, Ontario, Canada have been dated by the KAr method. The ages calculated by the conventional method range from 900 m.y. to 2040 m.y. On an isochron plot, three samples from a sampling site near the northern border of the gabbro lie along a 670-m.y. isochron with an initial⁴⁰Ar/³⁶Ar ratio of about 17,300 whereas all other samples lie along another 670-m.y. isochron with an initial ratio of about 5000. Although it is not certain yet as to what geological event the isochron age represents, the results clearly demonstrate that the effect of initial argon can be significant even on old samples such as these.     

Age of Grenville Belt magnetisation: Rb-Sr and palaeomagnetic evidence from Swedish dolerites

Rb-Sr mineral ages and palaeomagnetic data are presented for nine dolerite dykes and sills in central and southern Sweden. The intrusions are representative of widespread dolerites, mainly dykes, which cut pre-Sveconorwegian (Svecofennian) crust and trend parallel to the Sveconorwegian Front Zone (or Schistosity Zone) over a band up to 150 km wide along most of its 700-km length. The Rb-Sr ages suggest emplacement through the approximate interval 1000?900 m.y. Cooling ages on country rocks show little evidence of significant Sveconorwegian (e.g. 1000 m.y.) heating effects, and the new isotopic and palaeomagnetic results are taken to represent the time of dolerite intrusion.The dolerites were emplaced in response to net tensional stresses in the Svecofennian crust: it is proposed that these were caused by marginal upwarping related to post-orogenic uplift of the adjacent Sveconorwegian region.The palaeomagnetic poles for the dolerites are similar to poles from the Sveconorwegian Province and the Front Zone, indicating that these also have a ca. 1000?900-m.y. age. The similar cooling histories of the Sveconorwegian Province and the Grenville Province of Canada allow this age to be assigned also to the Grenville poles. If so, it is not necessary to postulate plate motions during Grenville-Sveconorwegian magnetisation, and the agreement between dolerite poles and Sveconorwegian poles suggests that the orogenic belt cooled essentially in place against its bounding cratons. A possible continental fit shows Scandinavia and North America contiguous in late Grenville time, though relative motions before 1000 m.y. are not ruled out.     

U–Pb systematics in heterogeneous zircon populations from the Precambrian basement of the Maryland Piedmont

The aim of the study was — besides the dating of metamorphic events — to evaluate the effects of multi-stage crystal growth, episodic and continuous Pb loss, and U gain on the discordant age patterns found for zircon populations of the polymetamorphic Baltimore Gneiss, the Precambrian basement in the Maryland Piedmont. Eight gneiss and migmatite samples were collected at two localities in the Phoenix and Towson dome, respectively. Their zircon populations were separated into twenty-three fractions of different size and optical appearance. A low-contamination method (T.E. Krogh, 1973) was used for the U-Pb analyses.Microscopy and electron-microprobe studies revealed internal heterogeneities of the zircon crystals: at least half of the grains of each population reflect more than one stage of crystal growth, with the last stage consisting of U-poor overgrowths (U: below 400 ppm, mostly below 200 ppm). Evidence for episodic U gain and overgrown material other than zircon has not been found. On a concordia diagram the “ages” obtained by upward extrapolations (1080 and 1180 m.y.) and downward extrapolations (421 and 455 m.y.) of the best-fit lines to the data points are in fair agreement with the geochronologic data found by other investigators and the probable times of metamorphic periods of Precambrian (Grenville) and early Paleozoic (Taconic) orogenies.Models of Pb loss by continuous diffusion cannot adequately explain the discordant age patterns: these are essentially the result of superposition of episodic Pb loss and zircon overgrowth during the Taconic (and Acadian?) metamorphisms. The zircon overgrowth appears to be present in all fractions, but its influence on the U-Pb systematics is generally not perceptible because it is overridden by the effect of episodic Pb loss. For the fractions showing the most discordant ages, the contribution of Pb loss to the discordancy was found to be at least 85 %.From the microscopic picture and the isotopic data, it appears that the bulk of the zircon substance crystallized during one or several high-grade metamorphisms accompanied by migmatization and granitization of the rocks in the course of the Grenville orogeny. Under consideration of zircon ages of Baltimore Gneiss rocks of Pennsylvania, the results point to a complex Grenville metamorphic history in the Maryland and Pennsylvania Piedmont, that lasted from at least 1200 m.y. until about 980 m.y. The granulite-facies metamorphism in the West Chester Prong, Pennsylvania, may be 50–200 m.y. younger than the metamorphic events in the gneiss domes of the Baltimore area. Although it seems that real differences exist with respect to the Precambrian ages of major zircon-forming events between the Phoenix and the Towson dome, the apparent difference of about 100 m.y. should be interpreted with caution, because it is impossible, so far, to evaluate quantitatively the influence of possibly much older inherited zircon components.     

The Palung granite (Himalaya); high-resolution UPb systematics in zircon and monazite

UPb analyses of fractions of zircon and monazite (3–8 grains each) and of single zircon grains resolve a lower Ordovician age of 470 ±4m.y. for the Palung granite which occurs in the High Himalayan nappes south of Kathmandu. Its thrusting during the Alpine orogeny under lower greenschist facies conditions did not affect the UPb systems in zircon and monazite. The granite crystallized from a magma which was mainly generated by anatexis of Precambrian continental crust. The magma was heterogeneous with respect to primary ages and/or metamorphic histories of the magma source rocks. This indicates either a derivation from (meta-) sediments or an intense mixing of different crustally derived magmas. The genesis of the Palung granite is possibly related to an orogeny which affected the Indian shield in lower Palaeozoic times. The detected inherited radiogenic lead in the Palung zircons occurs in perfectly homogeneous, transparent crystals; i.e. this radiogenic (“excess”) lead is not related to the presence of old, microscopically visible, overgrown zircon cores. The minimum ages of the inherited lead components range from about 800 to 1700 m.y.     

RbSr total rock isotope studies on Precambrian charnockitic gneisses from South Norway: evidence for isochron resetting during a low-grade metamorphic-deformational event

The quartzo-feldspathic charnockitic orthogneisses within the Bamble sector of the so-called Sveconorwegian (1.2–0.9 b.y.) zone are highly fractionated in K and Rb such that they comprise two chemically contrasting zones — one highly K, Rb-deficient and the other with values of the same order as upper crustal lithologies.Eight series of samples, each collected from single outcrops, have yielded Rb-Sr total rock apparent ages in two distinct groups, at ～1540 and ～1060 m.y. Outcrops in both the K-deficient and normal-K suites have produced examples of each age. The older age relates to the high-grade charnockite event, and the younger to a superimposed low-grade event which occurred at the same time as the intrusion of undeformed granite sheets and pegmatite dikes; one of the granites has yielded an isochron age of 1063 ± 20 m.y. The low-grade event involved only the partial alteration of orthopyroxenes to chlorite and/or serpentine, coupled with some corrosion of biotite; the alterations were initiated along narrow, irregularly spaced, cracks and it was their development which facilitated open system behaviour of the total rock isotopic systems at some localities. The degree of rehomogenisation is a function of the intensity of the secondary alterations.Confirmation of resetting at ～1060 m.y. is given by four mineral + host rock isochrons all yielding ages within error of the age for the intrusive granite; two of these are from outcrops where the rocks retain the older ～1540-m.y. age.The secondary total rock isotopic homogenisation cannot be explained adequately by Rb mobility or by simple mixing with a fluid having its own initial⁸⁷Sr/⁸⁶Sr composition. The primary mineralogy may have determined whether individual localities and/or samples suffered net increases or net decreases in⁸⁷Sr/⁸⁶Sr.An important implication of the results is that in this, or any similar geological situation, there would be a very real possibility of drawing erroneous conclusions from regionally-collected samples, particularly if the full significance of the later, relatively minor P-T event remained undetected and/or the scale of isotopic (re-)homogenisation, were unknown. It is only because of the methods adopted that it can be stated that there is no isotopic evidence for a high grade Sveconorwegian (Grenvillian) event in this part of southern Norway.     

SmNd isotopic systematics of Enderby Land granulites and evidence for the redistribution of Sm and Nd during metamorphism

Measurements of¹⁴³Nd/¹⁴⁴Nd and¹⁴⁷Sm/¹⁴⁴Nd are reported for whole rocks and mineral separates from granulites of the Napier Complex at Fyfe Hills. Charnockites, leuconorites and gabbros yield a whole rock SmNd isochron age of3060 ± 160m.y. and an initial¹⁴³Nd/¹⁴⁴Nd ratio of0.50776 ± 10 (?_Nd(3060m.y.) = ?2.0 ± 1.8). The negative ?_Nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of Sm and Nd and partial re-equilibration of¹⁴³Nd/¹⁴⁴Nd ratios. This probably occurred during the upper granulite facies metamorphism which has also been dated at～ 3100m.y. by RbSr and UPb zircon studies [1]. Coexisting clinopyroxene, apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 ± 300 m.y. This array indicates that redistribution of Sm and Nd and re-equilibration of¹⁴³Nd/¹⁴⁴Nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at～ 2450m.y.Due to the resetting of the SmNd system on both whole rock and mineral scales, the primary crystallization age of the igneous protolith is not well constrained by the present data, although it is clearly3100m.y. If it is assumed that the complex was derived initially from a depleted mantle reservoir(?_Nd(T) ? 2), evolution of the negative ?_Nd value of ?2.0 with the observed Sm/Nd ratios requires a prehistory of～ 380m.y. This implies a primary age of～ 3480m.y. However, substantially older primary ages can be inferred if the source reservoirs had?_Nd(T) > 2 and/or substantial reductions in the Sm/Nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m.y. Such an inferred reduction in the Sm/Nd ratio may have been the result of preferential loss of Sm relative to Nd, or introduction of a low Sm/Nd fluid with?_Nd ≥ 0 during granulite facies metamorphism.     

Palaeomagnetism and the Grenville orogeny: New Rb-Sr ages from dolerites in Canada and Greenland

Rb-Sr mineral data are presented for the Sudbury and Mackenzie dolerites of Canada and for dolerites and lamprophyres from southwest Greenland. Five mineral isochrons or errorchrons, based chiefly upon feldspar separates, agree with twelve biotites from dolerites in defining best-fit ages between 1260 and 1190 m.y. Much older K-Ar and Rb-Sr mineral ages from country rocks demonstrate that the dolerite results are not due to regional heating, and basaltic magmatism in both regions between 1260 and 1190 m.y. is indicated (λ(⁸⁷Rb) = 1.42 × 10⁻¹¹ yr⁻¹).

This activity can be correlated with extensive dolerite sills in Sweden and Finland, the subject of parallel studies, which yield six mineral isochrons or errorchrons, ten Rb-Sr biotite ages from dolerites, and determinations by other methods falling between 1290 and 1155 m.y., although the Rb-Sr biotite ages are all grouped in the 1250-1210 m.y. range.

Palaeomagnetic poles from the dolerites define tight clusters for each of Canada/Greenland and Fennoscandia. Superimposition of the two groups of poles leads to a non-unique reconstruction in which Fennoscandia may have adjoined Canada/Greenland at 1260-1190 m.y., but in an orientation quite different from that established for 1000-850 m.y. The necessary net rotation of Fennoscandia through 90° may have been achieved by splitting, separation and final reunion of the continents into a different relative disposition between 1190 and 1000 m.y. This favours the involvement of plate tectonic processes in the Grenville-Sveconorwegian orogeny. The 1260-1190 m.y. dolerites may represent the beginning of a continental separation which preceded the Grenville orogeny, although other causes for the magmatism cannot be ruled out.     

The Shabogamo Intrusive Suite,Labrador: Sr and Nd isotopic evidence for contaminated mafic magmas in the Proterozoic

The Shabogamo Intrusive Suite comprises numerous bodies of variably metamorphosed gabbro which intrude Archean and Proterozoic sequences at the junction of the Superior, Churchill, and Grenville structural provinces in western Labrador. Combined Sm-Nd and Rb-Sr systematics in two bodies, ranging from unmetamorphosed to lightly metamorphosed, document a crystallization age of about 1375 m.y., and suggest that both bodies crystallized from magmas with similar Nd and Sr isotopic compositions. This age is in accordance with the existence of a regional magmatic event in the Churchill Province at approximately 1400 m.y.Rb-Sr systematics in two bodies of amphibolite-grade gabbro suggest a regional metamorphic event at about 950 m.y., corresponding to the waning stages of Grenville activity. Sm-Nd systematics in these high-grade bodies are affected to a much lesser degree than Rb-Sr.Initial ratios for¹⁴³Nd/¹⁴⁴Nd and⁸⁷Sr/⁸⁶Sr are lower and higher, respectively, than bulk earth values at 1375 m.y. Both these displacements are in the direction of older crustal material at 1375 m.y., and a model is proposed to produce the Shabogamo magma by mixing a mantle-derived magma with a partial melt of crustal rocks (approximately 4: 1 by volume). Young volcanic rocks with anomalous Nd and Sr isotopic ratios, which have previously been taken as evidence for “enriched” mantle, may be interpreted similarly.     

40Ar/39Ar incremental-release ages of biotite from a progressively remetamorphosed Archean basement terrane in southwestern Labrador

Gneisses within an Archean basement terrane adjacent to the southwestern portion of the Labrador Trough were variably retrograded during a regional metamorphism of Grenville age (ca. 1000 Ma). Biotites from non-retrograded segments of the gneiss terrane record⁴⁰Ar/³⁹Ar plateau and isochron ages which date times of cooling following an episode of the Kenoran orogeny (2376–2391 Ma). A suite of gneiss samples displaying varying degrees of retrograde alteration was collected across the Grenville metamorphic gradient. Biotites in these samples show no petrographic evidence of retrograde alteration, however they do record internally discordant⁴⁰Ar/³⁹Ar age spectra. Although the extent of internal discordance is variable, the overall character of the release patterns is similar with younger apparent ages recorded in intermediate-temperature gas fractions. The total-gas dates range from 2257±27 Ma (northwest) to 1751±23 Ma (southeast), suggesting that variable quantities of radiogenic argon were lost from the Archean biotites during Grenville metamorphism. The “saddle-shaped” nature of the discordant spectra indicates that argon loss was not accomplished through single-stage, volume diffusion processes.Biotites in portions of the gneiss terrane which were completely recrystallized during Grenville metamorphism are petrographically and texturally distinct. A representative of this phase records a⁴⁰Ar/³⁹Ar plateau age of 2674±28 Ma. This date is markedly inconsistent with regional constraints on the timing of Grenville metamorphism, and indicates the presence of extraneous argon components. Both the extraneous and radiogenic argon components must have been liberated in constant proportions during experimental heating because the argon isotopic data yield a well-defined⁴⁰Ar/³⁶Ar vs.³⁹Ar/³⁶Ar isochron corresponding to an age (2658±23 Ma) similar to that defined by the plateau portion of the spectrum.The⁴⁰Ar/³⁹Ar biotite dates suggest that the effects of Grenville metamorphism extent 15–20 km northward into the Superior Province. The limit of this overprint is approximately coincident with the northernmost development of Grenville age thrust faults in the Archean terrane. Therefore, it is proposed that the northern margin of the Grenville Province in southwestern Labrador should be located along the northernmost Grenville thrust fault because this represents both a structural and a thermal discontinuity.     

40Ar/39Ar age and thermal history of the Kirin chondrite

The Kirin meteorite, a large (2800kg) H5 chondrite, fell in Kirin Province, China in 1976. A sample from each of the two largest fragments (K-1, K-2) yield⁴⁰Ar/³⁹Ar total fusion ages of 3.63 ± 0.02b.y. and 2.78 ± 0.02b.y. respectively.⁴⁰Ar/³⁹Ar age spectra show typical diffusional argon loss profiles. Maximum apparent ages of 4.36 b.y. (K-1) and ～4.0 b.y. (K-2) are interpreted as possible minimum estimates for the age of crystallization of the parent body.The⁴⁰Ar/³⁹Ar ages found for gas released at low temperature are about 2.2 b.y. for K-1 and about 0.5 b.y. for K-2, suggesting that this meteorite may have suffered two discrete collisional events that caused degassing of radiogenic argon. Modelling of possible thermal events in the parent body indicates that samples K-1 and K-2 were at a depth of less than 3 m from the base of an impact melt of a thickness less than 7 m and separated by no more than ～2 m from one another at the time of the heating event about 0.5 b.y. ago. Further, the duration of heating was probably less than a few years.Calculations from³⁸Ar data yield exposure ages for samples K-1 and K-2 of about 5 m.y., similar to that found for many other H chondrites.     

3600-m.y. RbSr ages from very early Archaean gneisses from Saglek Bay,Labrador

Field studies in the vicinity of Saglek Bay, Labrador, demonstrated that it was possible to subdivide the Archaean gneiss complex into distinct lithologic units and erect a geologic chronology similar to that recognized in Godthaabsfjord, West Greenland. The Uivak gneisses are the oldest quartzo-feldspathic suite in the area and are distinguished from a younger gneissic suite in the field, the undifferentiated gneisses, by the presence of porphyritic basic dykes (Saglek dykes) within the Uivak gneisses. The Uivak gneisses range in composition from tonalites to granodiorites, with the two chemically distinct suites recognized: a grey granodioritic suite and an iron-rich plutonic igneous suite which locally intrudes or grades into a grey gneiss which strongly resembles the grey Uivak gneiss. Rb-Sr isotopic studies indicate an age of 3622 ± 72 m.y. (2σ) and initial Sr isotopic composition of 0.7014 ± 0.0008 (2σ) for the Uivak gneiss suite, i.e. grey gneiss plus iron-rich suite (λ_Rb = 1.39 × 10^?11 yr^?1). The grey Uivak gneiss suite, treated independently, defines a Rb-Sr isochron with an age of 3610 ± 144 m.y. (2σ) and initial Sr isotopic composition of 0.7015 ± 0.0014 (2σ) which is indistinguishable from the age and initial ratio of the total Uivak gneiss suite, grey gneisses plus iron-rich suite. The undifferentiated gneisses define a Rb-Sr isochron with an age of 3121 ± 160 m.y. (2σ), and initial Sr isotopic composition of 0.7064 ± 0.0012 (2σ). The isotopic data support field observations suggesting the undifferentiated gneisses were derived by local remobilization of the grey Uivak gneisses. The Uivak gneisses resemble the Amitsoq gneisses of Godthaabsfjord both chemically and isotopically. The interpretation of the initial Sr isotopic composition of the Uivak gneisses is interpreted as the time of regional homogenization rather than the initial ratio of the plutonic igneous parents of the Uivak gneisses as suggested for the Amitsoq gneisses. Although the undifferentiated gneisses are contemporaneous with the Nuk gneisses of West Greenland, they do not form a well-defined calc-alkaline suite and may not be associated with major crustal thickening in the Labrador Archaean.     

The effect of initial230Th disequilibrium on young UPb ages: the Makalu case,Himalaya

Comparative UPb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic²⁰⁶Pb which originate from an excess or deficit of²³⁰Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by UTh fractionation between the crystallising mineral and the magma. Therefore, the UPb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess²⁰⁶Pb due to excess²³⁰Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of²⁰⁶Pb due to deficiency of²³⁰Th. The extent of this correction depends on the degree of ThU fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of²⁰⁶Pb were, with reference to the amount of radiogenic²⁰⁶Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the ThU partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic²⁰⁷Pb originating from the decay of initial excess²³¹Pa, also enriched during crystal growth.The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for ThU fractionation in continental crust.The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected²³⁸U²⁰⁶Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.     

RbSr radiometric age of late Precambrian fossil-bearing and associated rocks from Sinai

Six samples of metamorphic rocks from three essentially coeval stratigraphic units, two of which contain Precambrian microfossils, have been analyzed by the Rb-Sr whole-rock radiometric method. Least-squares regression of the data yields an isochron date of 934 ± 80m.y. with initial ⁸⁷Sr/⁸⁶Sr= 0.7007 ± 0.0011. This date may reflect the approximate age of formation of these rocks or, alternately, a time of major metamorphism. Regardless, the date is significant in that it (a) is in agreement with the 900-m.y. date for rocks containing similar types of fossils from Bitter Springs, Australia, and (b) coupled with structural evidence, supports the time equivalence of these rocks with 1000-m.y. old rocks (“Kibaran cycle”) of Saudi Arabia.     

KAr data for the age and evolution of Gettysburg Bank,North Atlantic Ocean

Gettysburg Bank forms the western end of the Gorringe Seamount which is situated in the North Atlantic 110 km west of the tip of the Iberian Peninsula, on the eastern end of the Azores/Gibraltar fracture zone.Gabbros dredged from the Gettysburg Bank record a complex history of events. K-Ar ages of separated mineral phases fall into three concordant groups (plus some discordant ages). The oldest ages are from three brown kaersutitic hornblendes and their mean age of135 ± 3Ma is taken to be that of their formation. Six plagioclase feldspars yielded concordant ages of105 ± 3Ma which is possibly a consequence of a thermal event occurring at that time. Ages from three deformed plagioclases are concordant with a mean of82 ± 3Ma which is believed to relate to a phase of shearing, perhaps occurring during transform motion at the plate boundary.     

Interpretation of a discordant KAr age pattern (Capo Vaticano,Calabria)

KAr age determination on whole rocks, biotites, quartz-feldspar separates and pegmatitic muscovites from a small quartz dioritic stock give a complex discordant age pattern. KAr dates from whole rocks and mineral separates define a single 116 my isochron with positive intercept, whereas muscovites from pegmatites fit a 180 my isochron with a probable negative intercept.Both ages are younger than the probable crystallization age of the stock (around 300 my), indicating a complex post-crystallization history. The fit of different mineral phases and whole rocks to a single isochron with positive intercept suggests that a thermal event caused rehomogenization of Ar among different mineral phases.     

SmNd constraints on early lunar differentiation and the evolution of KREEP

The Sm-Nd systematics of lunar KREEP basalt 15386 reflects two chronologically distinct events in the development of the incompatible element-rich materials of the moon. The measured Sm-Nd mineral isochron of 15386 indicates an age of 3.85 ± 0.08 AE which is consistent with the reported Rb-Sr and³⁹Ar-⁴⁰Ar ages of many other KREEP-rich samples. This age is interpreted as the time at which 15386 crystallized from a liquid on or near the lunar surface. The frequent occurrence of this age for KREEP-dominated samples, as well as the restricted location of KREEP near major lunar near-side impact basins, suggests that the eruption of these incompatible element-rich liquids was related to deep impact events during the postulated final bombardment phase of the surface of the moon. However, the lower than chrondritic initial¹⁴³Nd/¹⁴⁴Nd of 15386 and the essentially identical Sm-Nd evolution of other KREEP-rich samples require that the light REE enrichment which characterizes KREEP was established considerably before 3.85 AE. Within the limits imposed by model assumptions in the various radiometric systems, it is concluded that the extremely narrow spread of Sm-Nd model ages for these samples around 4.36 AE, and the compatibility of this age with that indicated by the U-Pb and Rb-Sr systems, indicate that the source of later KREEP volcanism was produced in the closing stages of an early global scale lunar differentiation episode.     

Geochronology and Nd isotopic data of Grenville-age rocks in the Colombian Andes: new constraints for Late Proterozoic-Early Paleozoic paleocontinental reconstructions of the Americas

New UPb zircon crystallization ages and ⁴⁰Ar/³⁹Ar cooling ages from the Colombian Andes confirm the existence of rocks metamorphosed during the Orinoquian Orogenic Event (ca. 1.0 Ga) of northern South America. ε_Nd (t = 1.1 Ga) for these rocks range from −3.9 to +0.91, which is interpreted as a mixture of Late Archean-Early Proterozoic crust with juvenile material produced during the 1.1 Ga orogenic event. The Colombian Grenville age rocks are part of a much longer metamorphic pericratonal belt, sporadically exposed along the Andes, in western-central Peru, southern Bolivia and northern Argentina. In addition, Nd model (T_DM) ages for the Colombian rocks range from 1.9 to 1.45 Ga, similar to those obtained in the Grenville Province of the eastern U.S. and in the Mexican basement, placing constraints on Late Proterozoic-Early Paleozoic paleocontinental reconstructions.     

Geochemistry of charnockites from Sa˜o Paulo State,Brazil

REE and Ba, Th, U, Au, Hf, Sb, Sc and Cs were determined by neutron activation techniques on samples belonging to an acidic high-K charnockitic formation outcropping along the coast of Sa?o Paulo State, Brazil. This formation was dated by the Rb-Sr method and gave whole-rock isochron ages of 546–558 m.y. and initial⁸⁷Sr/⁸⁶Sr ratios of 0.7098–0.7117. A mineral isochron gave an age of 479 m.y. Isotopic and geochemical data support the hypothesis that these rocks derive from the intrusion of a granitic magma produced by crustal anatexis. The source rocks were probably differentiated from the mantle 300–700 m.y. before the solidification of the charnockite. Th/U, K/Cs and Rb/Cs ratios and Au concentrations indicate that the source rock probably was of high metamorphic grade.     

3450-m.y.-old volcanics in the Archaean layered greenstone succession of the Pilbara Block,Western Australia

Zircon U-Pb systems from a single block sample of columnar dacite from the early Archaean Warrawoona Group from the layered “greenstone” succession of the Pilbara Craton, Western Australia, indicate an age of3452 ± 16 (2σ) m.y. This is interpreted as the age of the dacite, and is the oldest age so far determined from the Archaean of Australia. This confirms the antiquity of the Warrawoona Group and suggests a close age relationship between the Warrawoona Group in the Pilbara and the Onverwacht Group in South Africa.The Rb-Sr whole rock systems of the dacite have been disturbed since emplacement and indicate apparent ages that have no direct geological significance. This is in accord with previous published accounts of resetting of Rb-Sr whole rock ages determined on acid metavolcanics.     

Argon in Apollo 15 green glass spherules (15426):40Ar39Ar age and trapped argon

We report the results of thermal-release argon analyses of neutron-irradiated green glass spherules separated from lunar sample 15426. The gas-retention age, as determined by the⁴⁰Ar³⁹Ar method, is (3.38 ± 0.06) X 10⁹yr. This age is similar to those of local mare basalts and distinct from the ages of Appenine Front samples recovered from the same region as 15426. Trapped argon is present in near-surface regions of the spherules, and can be resolved into at least two components requiring separate origins, a shallow (0.1 μ) component with⁴⁰Ar/³⁹Ar > 30, and a deeper (2 μ) component with ⁴⁰Ar/³⁶Ar= 2.9. The ratio of trapped⁴⁰Ar to³⁶Ar is higher than found in any lunar soil and suggests that the trapped gas was implanted early in the spherules' history. The cosmic-ray exposure age is 300 my.