Rb-Sr Isochron Age Research Articles

Very low-grade metamorphic Sr isotopic resettings of magmatic rocks and minerals: evidence for a late Givetian strike-slip division of the Brabant Massif, Belgium

Rb-Sr isochron ages recorded by the Ashgillian ignimbrites from Deerlijk and Harelbeke and the Caradocian quartz diorites from Quenast and Bierghes significantly post-date their emplacement. At Quenast, two mineral isochrons (biotite-WR-plagioclase: 372 ± 13 Ma; K-feldspar-WR-epidote: 375 ± 14 Ma) reveal that the Brabantian branch of the Caledonian belts was affected by a post-Siegenian, pre-Tournaisian event. This overprint turns out to be contemporaneous with the development of the Oudenaarde–Bierghes fault zone which is dated to 373 ± 11 Ma and 379 ± 13 Ma on foliated rocks from Bierghes, Deerlijk and Harelbeke. This c . 375 Ma cluster of events is interpreted as representing late Givetian stresses developed along the Oudenaarde–Bierghes fault zone for which the balance of geological data favours strike-slip movements. Since this compressive phase of deformation is posterior to the Givetian opening of the Borain trough, the Brabant Massif appears to have been affected by successive crustal extension and shortening events in the Middle Devonian. Petrological evidence demonstrates the very low-grade character of the metamorphism in the metasediments on both sides of the Oudenaarde–Bierghes fault zone. From this, we deduce that Sr isotopic homogenization of whole-rock ignimbrite and the loss of radiogenic Sr from biotite and K-feldspar could be operative even at temperatures below 250°C.

Geochronology, geochemistry and tectonics of the NE Bayuda Desert, N Sudan: Implications for the western margin of the late Proterozoic fold belt of NE Africa

Five suites of rocks collected from the Precambrian basement in the NE Bayuda Desert of central northern Sudan give late Proterozoic whole-rock Rb Sr isochron ages. The Abu Harik Complex, thought by some previous workers to be an older basement, gives an age of 898 ± 51 Ma. Upper amphibolite-facies metasediments give a metamorphic age of 761 ± 22 Ma. The supposedly younger greenschist-facies El Koro Volcanic Series were erupted 800 ± 83 Ma ago. These are chemically similar to the volcanics which unconformably overlie the Sol Hamed ophiolite in the Red Sea Hills of NE Sudan and to some modern island are volcanics. The metasediments were intruded 678 ± 43 Ma ago by the Diefallab Granite, which is itself deformed. The younger, weakly-deformed Amaki Series, with a basal conglomerate containing basement clasts overlain by purple grits, is probably equivalent to the molasse-type Hammamat Group of the Eastern Desert of Egypt which was deposited between 616 and 596 Ma ago. Finally, the post-tectonic Shallal Granite, with within-plate geochemistry, was intruded 549 ± 12 Ma ago. Geochemical data suggest that the Abu Harik Complex, the El Koro Volcanic Series and the Diefallab Granite are arc-related magmatic rocks. They were intruded into, or thrust onto, shallow-water, shelf sediments during subduction and then collision, between c. 900 and 550 Ma. The data presented here give no support to previous views that the high-grade metasediments were metamorphosed prior to the late Proterozoic events, that they unconformably overlie a still older, perhaps Archaean, basement or that they are unconformably overlain by younger late Proterozoic low-grade volcanics. The Precambrian rocks along the E side of the Bayuda Desert must now all be assigned to the late Proterozoic and the boundary between this late Proterozoic fold belt and an older craton, known to crop out at Jebel Uweinat, must lie farther to the W.

The Anvil plutonic suite, Faro, Yukon Territory

The Anvil plutonic suite consists of three phases: a peraluminous muscovite–biotite granite (Mount Mye phase) and two metaluminous to peraluminous hornblende–biotite granodiorite and minor granite intrusions (Orchay and Marjorie phases). The suite is massive or foliated, equigranular or seriate, and contains alkali-feldspar megacrysts. The Marjorie phase is characteristically porphyritic.Geochemical trends are irregular for the suite and for individual phases. High-K2O, low-CaO, and low-MgO compositions typify the silicic, calc-alkaline suite. Hornblende-bearing phases contain less SiO2, K2O, and Rb and more cafemic oxides, TiO2, Sr, Ba, and Y than the Mount Mye phase and are compositionally similar to coeval South Fork volcanics.Isochrons from some of the Orchay phase whole-rock samples (t = 99 ± 2.5 Ma; 87Sr/86Sri = 0.7161 ± 0.0001) and from whole rocks and minerals of the Mount Mye phase (t = 100 ± 2 Ma; 87Sr/86Sri = 0.7405 ± 0.0001) indicate they are coeval but not comagmatic, accounting for the lithologic, petrographic, and geochemical distinctions. Similar K–Ar isotopic ages (81–102 Ma) suggest rapid cooling and therefore high-level emplacement. Together, the isotopic ages provide a minimum (youngest) age for the main deformation of the surrounding metasediments and a maximum (oldest) age for movement along the Tintina Fault.A petrographically and geochemically distinct sample from the Orchay phase yielded a Rb–Sr isochron age of 61 ± 1.5 Ma and an initial 87Sr/86Sr ratio of 0.7090 ± 0.0001, implying intrusive activity in the Paleocene.Field relations, lithology, petrography, geochronometry, and geochemistry suggest that the Orchay and Marjorie phases are plutonic equivalents of the South Fork volcanics. Similarities in plutonic style characterize the extensive mid-Cretaceous igneous event in southeast Yukon.

Rb [sbnd]Sr dating of the Kohistan arc-batholith in the Trans-Himalaya of north Pakistan, and tectonic implications

The Kohistan arc-batholith in northern Pakistan is situated between the Indus Suture and the Northern Suture. It belongs to the Trans-Himalayan belt which continues eastwards as the Ladakh arc-batholith in northwest India and the Kangdese batholith in southern Tibet. In Kohistan the island arc consists (upward sequence) of the Chilas stratiform complex of norites, noritic gabbros and chromite-layered dunites which formed in the sub-arc magma chamber, plutons of tonalite and diorite, one of which has a Rb Sr isochron age of 102 ± 12 Ma, the Chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites, and the Yasin Group sediments which formed in overlying intra-arc basins and which contain Albian-Aptian faunas. All these rocks were deformed by major fold structures which are correlated with the formation of the Northern Suture. NE-SW trending basic Jutal-Nomal dykes (north of Gilgit) cross-cut all the above structures and have a 39Ar/ 40Ar hornblende age of 75 Ma. Thus the Northern Suture must have formed in the period 102 ± 12 to 75 Ma. With the island arc attached to the Eurasian plate, further northward subduction of the Tethyan plate gave rise to an Andean-type batholith, two plutons of which have Rb Sr isochron ages of 54 ± 4 Ma and 40 ± 6 Ma. The Dir-Utror Group of calc-alkaline lavas (and sediments with Eocene fossils) are remnants of the volcanic cover of the Andean-type batholith. We suggest that late Cretaceous blueschists formed during subduction under the active continental margin, and that continental collision and formation of the Indus Suture was in the Eocene. The batholith was intruded by layered aplite-pegmatite sheets at 34 ± 14 Ma and 29 ± 8 Ma (Rb Sr ages) in post-collisional times. The 87Sr/ 86Sr initial ratios of all the dated rocks range between 0.7039 and 0.7052.

The timing of ore formation in Southeast Missouri; Rb-Sr glauconite dating at the Magmont Mine, Viburnum Trend

Seven Bonneterre and Davis Formation glauconite samples from the Magmont mine area, Viburnum Trend, southeast Missouri, yield a 359 + or - 22-m.y. Rb-Sr isochron with an initial 87 Sr/ 86 Sr ratio of 0.7234 + or - 0.0273 (2 sigma). Gangue calcite from the ore zone has an 87 Sr/ 86 Sr ratio of approximately 0.7112. The isochron age for the Magmont glauconite samples is about 30 percent younger than the age of their Cambrian host rocks.The Rb/Sr ratios for Magmont glauconite samples are typical of many early Paleozoic glauconites but distinctly lower than ratios for glauconites from unmineralized localities in basinal Bonneterre facies, some distance from the Vibrunum Trend.Because the Rb-Sr data from Magmont glauconite samples define an isochron, it appears that the glauconites behaved as a cogenetic suite which was isotopically homogenized 359 m.y. ago. It is very unlikely that episodic or incomplete loss or exchange of radiogenic strontium, or the addition of rubidium, would result in the formation of an isochron. Addition of common strontium (mixing) will lower and produce a greater spread in Rb/Sr ratios but will not significantly change the calculated Rb-Sr isochron age for the glauconites. Because the total thickness of stratigraphic cover in the southeast Missouri region has never exceeded a kilometer, it is unlikely that the anomalously young glauconite isochron age resulted from heating associated with deep burial. Mississippi Valley-type ore fluids offer a hot, chemically reactive medium which could promote isotopic homogenization in glauconites. Therefore, the 359-m.y. age probably reflects a real geologic event, most likely the time of ore formation in southeast Missouri.A review of other attempts at dating early Paleozoic glauconites from the midcontinental United States indicates that all Rb-Sr and K-Ar glauconite ages are 10 to greater than 30 percent younger than their known stratigraphic ages. These glauconites are from Paleozoic formations on the stable craton, and again, resetting due to burial metamorphism is unlikely. Because there was no immediately obvious explanation for the discordant young ages, early Paleozoic glauconites were simply considered to be unsuitable geochronometers. It is suggested that the anomalously young radiometric ages for early Paleozoic glauconites record disturbances in the Rb-Sr and K-Ar isotopic systems, the result of Mississippi Valley-type fluid migration in the midcontinent region.

The geologic evolution of the ancient granite—greenstone terrane of central-southern Bahia, Brazil

The granite—greenstone terrain of central-southern Bahia belongs to the São Francisco craton and is a key area for the understanding of the Archaean and lower Proterozoic of South America. In addition, it is in an area of great metallogenic potential.The granitic—migmatitic—gneissic basement complex is intimately associated with low to medium grade metasedimentary and metavolcanic sequences, such as the Brumado, Umburanas and Contendas-Mirante. In all of these sequences the metasedimentary rocks predominate, although acid metavolcanics constitute a significant fraction of the Contendas sequence and ultrabasic rocks occur in several isolated units.Early Archaean granitoid rocks (Rb-Sr isochron ages older than 3 Ga) occur in some regions such as Mata Verde-Boa Vista, Lagoa do Morro and Sete Voltas. Extensive granitic intrusive activity, with associated medium grade metamorphism and metasomatism, occurred in most of the area ∼2.7 Ga ago, and the resulting terrain is taken to be the basement, at least for the Contendas-Mirante sequence. The relationships to the Brumado and Umburanas metamorphic complexes are not clear, and these sequences may be of Archaean age.The Contendas-Mirante sequence was affected by strong lower Proterozoic deformation (∼ 2 Ga ago) and contemporaneous granitic intrusions (like the Gameleira granite). Large, domal structures, interpreted as mantled gneiss domes, such as those associated with the Boa Vista and Sete Voltas granitoid rocks, also formed at this time. A late Proterozoic to Early Palaeozoic thermal event affected the southwestern part of the region at ∼ 0.5–0.7 Ga ago as revealed by mica K-Ar ages.

RbSr glauconite ages, Sabinian, Claibornian and Jacksonian units, southeastern Atlantic Coastal Plain, U.S.A

Hand-picked, disordered to ordered authigenic glauconite from the Nanjemoy Formation (Virginia), the Santee Limestone (restricted) and Cross Formation (South Carolina), and the Castle Hayne Limestone (North Carolina) yield RbSr isochron ages ( λ 87Rb = 1.42 × 10 −11 yr −1) for Eocene units in the southeastern Atlantic Coastal Plain. The units, zonal assignment of their contained nannofossil floras and radiometric age, are: 3 Unit Nannofossil zone Age (m.y.) Castle Hayne Limestone NP 19–20 34.9 ± 1.1 Cross Formation NP 18, NP 19, NP 20 34.1 ± 1.5 Santee Limestone (upper part) NP 16–17 36.7 ± 0.6 Nanjemoy Formation NP 13 46.7 ± 3.0 These ages, and their nannofossil assemblages, provide means of correlating southeastern Atlantic Coastal Plain units with Gulf Coastal Plain and European stratoypes. Correlation to Gulf Coast stratotypes, based on nannofossil zones, indicates that the Nanjemoy Formation (in part) is equivalent to the Hatchetigbee Formation; the upper part of the Santee Limestone (restricted) is equivalent to the upper Lisbon Formation, the Gosport Sand, and the Moodys Branch Formation (upper Claibornian-lower Jacksonian); and the Cross Formation and the Castle Hayne Limestone are equivalent to the Yazoo Group (Jacksonian). Previously reported KAr glauconite ages for the Gulf Coast units are different from the RbSr ages determined in this study. As the RbSr age determinations show internal consistency when compared to their contained nannofossil zones, whereas the KAr ages do not, it is suggested that the previously reported KAr ages for the Gulf Coast Claibornian and Jacksonian are in error. Correlation of the southeastern Atlantic Coastal Plain units to European stratoypes based on nannofossil zones indicates that the Nanjemoy Formation (in part) is equivalent to the type Lutetian; the upper part of the Santee Limestone (restricted) is equivalent to the Upper Bracklesham and Barton Beds; and the Cross Formation and Castle Hayne Limestone are equivalent to the Lower and Middle Headon Beds. Previously determined KAr and RbSr radiometric ages for the type Lutetian, the Bracklesham and Barton Beds are identical to the ages determined in this study. As a result, it is suggested that Rb-Sr glauconite isochron ages can be used for regional and inter-regional correlation and that the Eocene/Oligocene boundary is younger than 34 m.y.

Geochemistry, petrology and RbSr dating of trondhjemite and granophyre associated with Jabal Tays Ophiolite, Idsas area, Saudi Arabia

Abstract Jabal Tays trondhjemite and granophyre are leucocratic acidic rocks that intrude the melange zone of an ophiolitic complex. They consist of two small stocks 200 m in diameter and some dikes and vein-like bodies. Petrographically, they are medium—fine grained hypidiomorphic granular rocks consisting mainly of plagioclase (An10–60) and quartz which commonly shows myrmekitic texture. The mafic minerals are primarily biotite and muscovite, with occasional sericite, chlorite and epidote pseudomorphs after hornblende. The bulk chemical composition and trace elements are similar to trondhjemites from other ophiolite complexes. They also show calc-alkaline affinity. The K/Rb, Rb/Sr, and initial 87Sr/86Sr ( 0.7030 ± 0001 ) ratios, as well as the depletion of the (LIL) elements suggest an origin by partial melting of basic parent material in an island-arc tectonic setting over a subduction zone. The granophyres probably evolved from the trondhjemites either by crystal fractionation or crustal contamination. The whole-rock RbSr isochron age of 620 ± 40 Ma places an upper limit on the geologic and tectonic evolution of the Jabal Tays ophiolitic complex.

Research on the isotopic age of the Sinian–Cambrian boundary at the Meishucun section in Jinning County, Yunnan Province, China

AbstractIn this paper the author describes the results of the Rb-Sr isotopic dating of black shale at the Sinian (Precambrian)–Cambrian boundary at the Meishucun section, Jinning County, Yunnan Province, China. The Sinian and Cambrian strata in eastern Yunnan are well developed, the Sinian–Cambrian boundary section in the vicinity of Meishucun Village in Jinning County being representative. These well developed strata provide excellent conditions for research on the radiometric age of the Precambrian–Cambrian boundary. An Rb-Sr isochron age of 579.7 ±8.2 Ma has been obtained on black shale from the Badaowan Member of the Early Cambrian Qiongzhusi Formation, which occurs at 20 m above the Sinian–Cambrian boundary. So an inferred age of 610±10 Ma is given for the Sinian (Precambrian)–Cambrian boundary.

Geochemistry of meta-anorthosites from Holénarasipur, Karnataka, South India

Geochemical and RbSr isotope studies indicate that the meta-anorthosites of Holénarasipur, occurring as minor differentiates in ultramafic-mafic complex are igneous intrusives with cumulus character, emplaced around 3095 m.y. ago. The fine-grained nature is secondary; relict cumulus features are preserved in less deformed bodies. In major element chemistry, they compare well with other Archean anorthosites. Abundance levels of Ti, Zr, Y and P indicate the evolution through crystal fractionation of a parental magma; cumulus olivine and pyroxenes dominated chemistry for ultramafites, cumulus plagioclase and possibly clinopyroxene controlled chemistry for anorthosite-gabbros and cumulus magnetite in magnetite-gabbros. Magnetite is not an early cumulate. REE geochemistry is dominated by plagioclase with low abundance levels, slightly LREE enriched and variable positive Eu anomaly. Sr and K Rb values vary with An content in plagioclase. Isotopic studies show low initial 87 Sr 86 Sr (=0.7016) indicating that RbSr isochron age represents the time of intrusion rather than the time of metamorphism.

The Rb-Sr isochron age of the Kennack Gneiss and its bearing on the age of the Lizard Complex, Cornwall

An acid vein from a borehole in the Kennack Gneiss has given a whole-rock Rb-Sr isochron age of 369 ± 12 Ma. This is interpreted as a metamorphic age recording the time of emplacement of the Lizard Complex. This emplacement in late Devonian times was part of an early phase of the Hercynian Orogeny which also affected the Gramscatho 'Series' of S Cornwall.

Rb-Sr isochron from Gboko and Ikyuen rhyolites and its implications for the age and evolution of the Benue Trough, Nigeria

Summary. Rhyolites overlying the basement granitic gneisses and granites adjacent to the selimentfilled Benue Trough were studied. Rb-Sr isochron age of 113 ± 3 Ma and initial 87Sr/86Sr rato of 0.7067 were obtained from two extrusive bodies which form Gboko and Ikyuen hills. It is shown that their eruption has some relation to crustal doming at the onset of the Benue rifting. The published age of the oldest exposed sedimentary infill of the trough is Albian. As a result of the investigation of these volcanic rocks from the basement margin, however, an earlier and absolute age for the initiation of the rift is implied and a late Barremian to early Aptian age is proposed for the formation of this continental rift.

Rb–Sr whole-rock age of the Klondike Schist, Yukon Territory

Seven samples of metarhyolite from a measured section of the Klondike Schist, in the Yukon Crystalline Terrane, 5 km southwest of Dawson City, give a whole-rock Rb–Sr isochron age of 202 ± 11 Ma with an initial 87Sr/86Sr ratio of 0.7140 ± 0.0001. The high initial ratio suggests that this is a metamorphic age, recording the time of shearing of the metarhyolites. An estimated minimum age of formation of the protolith of approximately 260 Ma was obtained by rotating the isochron to an assumed primary initial 87Sr/86Sr ratio of 0.707.The siliceous cataclasites of the Klondike Schist may have formed as a forearc wedge of arkose with intercalated volcanics. The unit was intensely sheared in a subduction zone, prior to obduction onto the ancient North American continental margin. The results of this isotopic study suggest an age of formation of 260 Ma and an age of cataclasis of 202 Ma. The metarhyolites remained closed to Rb–Sr isotopic equilibration during obduction, though the thermal event is recorded in K–Ar isotopic data reported by other authors. The results of this study are consistent with theories of formation of this assemblage.

Palaeomagnetism of the Proterozoic Zig-Zag Dal Basalt and the Midsommers� Dolerites, eastern North Greenland

Palaeomagnetic investigations are reported from 24 sites in the Proterozoic Zig-Zag Dal Basalt Formation and 12 sites in the Midsominersϕ Dolerites of eastern North Greenland. The Zig-Zag Dal Basalt is a typical tholeiitic flood basalt sequence, and dolerite intrusions in the underlying sandstones are thought to be genetically related to the basalts. After a detailed AF demagnetization programme 19 sites in the basalts and 10 sites in the dolerites reveal one stable component of magnetization, probably of TRM and/or CRM origin residing in small single domain titano-magnetite grains. The degree of anisotropy has not affected the direction of the remanent magnetization. The maximum axis of the anisotropy ellipsoid is parallel to the flow direction of the magma, whereas the minimum axis is perpendicular to the flow plane. Only one polarity of the geomagnetic field was found. The mean palaeomagnetic pole positions for the two rock types are not significantly different (basalt: 12.2°S, 62.8°E with A95=3.8°; dolerites: 6.9°S, 62.0°E with A95= 5.1°). After correction for Phanerozoic drift of Greenland the two mean poles compare closely to a relevant North American APW-curve for 1250–1350 Ma, in good agreement with Rb-Sr isochron ages of 1250 Ma obtained for the intrusives. The palaeogeographical position of Greenland was near equator with the major geographical axis orientated E-W.

Rb–Sr and U–Pb ages of volcanism and granite emplacement in the Michipicoten belt—Wawa, Ontario

The Michipicoten greenstone belt at Wawa, Ontario is typical of Archean volcanic belts in the Superior Province. The supracrustal rocks are divisible into lower, middle, and upper metavolcanic sequences, which are separated by iron formation and clastic metasedimentary rocks. These are intruded by granitic stocks and embayed by granitic batholiths.This study reports whole rock Rb–Sr and zircon U–Pb ages for the lower and upper metavolcanics, for the granitic rocks that are physically within the greenstone belt (internal granites), and for the granitic rocks that embay the greenstone belt (external granites). The apparent Rb–Sr ages for the lower metavolcanics are 2530 ± 90, 2285 ± 70, and 2680 ± 490 Ma. The U–Pb ages are 2749 ± 2 and 2744 ± 10 Ma. The internal granites give an Rb–Sr age of 2560 ± 270 Ma and a U–Pb age of 2737 ± 6 Ma. The external granite at Hawk Lake indicates an Rb–Sr age of 2550 ± 175 Ma and a U–Pb age of 2747 ± 7 Ma. It is possible that this unit contains elements older than 2812 Ma as it contains xenocrystic zircons. The upper volcanics give a U–Pb age of 2696 ± 2 Ma, which indicates that the belt evolved over a time period in excess of 53 Ma. The Rb–Sr ages are significantly younger than the U–Pb zircon ages and have very large uncertainties in age; hence it is unlikely that they have any stratigraphic significance. They probably reflect the Kenoran orogeny at about 2560 Ma. The 2285 ± 70 Ma Rb–Sr isochron age has an initial ratio of 0.7275 ± 0.0052, which is interpreted as a rotational isochron defining a younger post-Kenoran event in the area. The zircon ages appear to be correct chronostratigraphically. Furthermore, it appears that the granitic rocks are coeval and may also be cogenetic with the lower acid metavolcanic rocks.

Isotopic geochronology of mesozoic acid volcanic rocks in Zhejiang Province, China

Volcano-sedimentary series of the Upper Jurassic to the Lower Cretaceous are extensively developed in Zhejiang Province. But ages and stratigraphic correlation concerning these rocks have long been a controversial problem. Systematic sampling was made of volcanic rocks of the Laocun, Huangjian, Shouchang and Moshishan Formations in western Zhejiang considered thus far as the Late Jurassic. Isotopic age determinations show that U-Th-Pb zircon ages are approximately concordant with Rb-Sr isochron ages, whereas K-Ar biotite ages and K-Ar isochron ages are all slightly lower. It can therefore be established that the ages of volcanic rocks mentioned above range from 134±6 to 122±2 m.y., corresponding to the “transitional period” from Jurassic to Cretaceous. It can also be concluded that the rocks have not undergone apparent epigenetic metamorphism. The initial87Sr/86Sr ratio is about 0.7089–0.7121, on the basis of which it may be postulated that the volcanic magma seems to have originated from the upper mantle with contamination by sialic materials subsequent to differentiation.For age determinations of such acid volcanic rocks Rb-Sr isochron method is considered more suitable in view of its following advantages: the high reliability of results; wide applicability to different samples; smaller sample requirement and the possibility for further studies involving petrogenesis by use of initial87Sr/86Sr ratio.

Correlation of proterozoic sediments of western and Central Africa and South America based upon radiochronological and palaeontological data

Abstract Nearly 70 new RbSr isochron ages and many KAr conventional ages have been determined between 1975 and 1980 on Proterozoic sedimentary or metasedimentary sequences in western and Central Africa and South America. Some stratigraphic results have been established: 1. (1) five formations have been dated of the Lower Proterozoic; 2. (2) a long sedimentation gap occurs, mainly in western African and in some regions of Central Africa and South America between nearly 1600 and 1100 Ma; 3. (3) the upper Riphean assemblages of stromatolites have been dated and compared to those of the Eurasian craton; 4. (4) two main glacial events have been dated, the first one placed at ca. 950 Ma, the second one during the Vendian, at ca. 650-620 Ma; 5. (5) it can be stated that, when applied to Precambrian sequences, all stratigraphic methods must be used together.

Rb–Sr isochron ages, magmatic 87Sr/86Sr initial ratios, and oxygen isotope geochemistry of the Proterozoic lava flows and intrusions of the East Arm of Great Slave Lake, Northwest Territories, Canada

Two Rb–Sr isochrons from the oldest (Wilson Island Group) and one of the youngest (Pearson Formation) Proterozoic volcanic units in the East Arm of Great Slave Lake give dates and initial ratios of 1810 ± 19 Ma, 0.7051 ± 0.0008, and 1809 ± 30 Ma, 0.7089 ± 0.0004, respectively. These dates restrict the Great Slave Supergroup entirely to the Aphebian. The Pearson Formation date is interpreted as magmatic, but it is considered to be rapidly followed by a significant metamorphic and tectonic event within the area. Both the above suites and the volcanic formations of intermediate age have undergone metamorphism up to and including epidote–amphibolite facies. A study of remnant clinopyroxene grains from these formations has indicated general increases in δ18O of whole rocks during epidote–actinolite facies metamorphism of 0.4–2.0‰. The tholeiitic volcanic rocks and intrusions all indicated δ18O values typical of modern continental tholeiites (5.9–7.0‰). The highest δ18O came from those suites that were suspected of having been contaminated by crustal material. The estimated initial 87Sr/86Sr ratios of the magmas, both from clinopyroxene separates and isochrons, indicate a mantle origin for early tholeiitic mid-Aphebian diabase (0.7016–0.7017), Union Island Group diabase (0.7021–0.7030), and Sosan Group alkali volcanics (0.7017). The later Jackson gabbro (0.7050–0.7054) and especially the Pearson Formation tholeiitic basalt (0.7089) both show the effects of significant crustal contamination. The evidence for the Wilson Island Group is less decisive but appears to indicate a mantle origin.

Age and correlation of the late Proterozoic Grand Canyon disturbance, northern Arizona

Age and correlation of the late Proterozoic Grand Canyon disturbance, northern Arizona

Isotopic ages of Jurassic siliceous shale and Triassic bedded chert in Unuma, Central Japan.

Isotopic age determinations were carried out for Jurassic siliceous shale (assigned to Bajocian by radiolarian fossils) and for, Triassic bedded chert (assigned to Anisian by radiolarians and conodonts) in Unuma, Gifu Prefecture, Central Japan. Whole-rock samples of the Jurassic shale give a Rb-Sr isochron age of 179.8 ± 5.7 Ma with an initial 87Sr/86Sr ratio of 0.71011 ± 0.00036, whereas two of these Jurassic samples have K-Ar whole-rock ages of 151 and 152 Ma. For the Triassic chert, the Rb-Sr whole-rock isochron shows 211.9 ± 4.7 Ma with an initial 87Sr/86Sr ratio of 0.71325 ± 0.00020, and a K-Ar whole-rock age for one of these Triassic samples is 207 ± 7 Ma. Two Rb-Sr whole-rock isochron ages indicate the time when the respective sedimentary rocks became chemically closed with regard to the Rb-Sr system. The initial 87Sr/86Sr ratios in both sets of rocks are higher than the contemporaneous marine 87Sr/86Sr ratio, suggesting the existence of an older landmass. The K-Ar ages of the Jurassic siliceous shale presumably represent the time of a later geologic event.