Rb-Sr Whole-rock Age Research Articles

U-Pb zircon and Rb-Sr whole-rock and mineral ages of Proterozoic intrusives on mapsheet Lannavaara, north-eastern Sweden

Abstract Radiometric data for igneous rocks, intruding early Proterozoic supracrustals, from two separate areas in north-eastern Sweden are presented. U-Pb analyses on zircons from plutonic rocks of intermediate composition around Lake Paittasjarvi yield an age of 1880 ± 28 Ma. A whole-rock isochron age of 1915± 145 Ma and a plagioclase-hornblendemica age of 1550± 40 Ma are calculated for the same rocks with the Rb-Sr method. Correlated granitoids from the Juoloyanjarvet area, which in part have suffered replacement of plagioclase by scapolite and are intruded by gabbros and younger granites, give a whole-rock Rb-Sr reference line of 1705 Ma and mineral ages of 1535 and 1495 ± 30 Ma. The whole-rock age probably lacks direct geological significance but the mineral ages obtained from both of the above areas may reflect influences from younger plutonism on the dated rocks. One of the intruding gabbros has a Rb-Sr mineral age of 1617 ± 107 Ma. This figure is considered a minimum age of the rock. The impact of...

Geochronological data on the Amsinassene-Tefedest Block (Central Hoggar, Algerian Sahara) and evidence for its polycyclic evolution

The history of the Amsinassène-Téfedest Block, in the central Hoggar domain, is characterized by a polycyclic evolution. The unreliable 1970 Ma age obtained on the Arechchoum gneisses, known to be Suggarian-Eburnean, might represent the effect of the Eburnean orogeny. A Rb Sr whole-rock age on the leucocratic fractions of gneisses point to a kibaran migmatization. This age confirms the existence of a Kibaran orogeny shown in the Aleksod area. The Pharusian-Pan African cycle, in this domain, is characterized by the emplacement of huge granitic bodies during two distinct magmatic phases: one of Late Proterozoic age (670 Ma) or Imezzarian phase, the other of Cambrian age as shown by the 546 and 515 Ma Rb Sr whole-rock ages obtained on late kinematic granitic plutons, Iskelian phase. The younger mineral ages (470430 Ma) indicate late vertical uplift of the central Hoggar.

Rb-Sr whole-rock and K-Ar mineral ages of granitic rocks in Japan.

Rb-Sr whole-rock ages have been determined on 10 small-sized granitic masses in Japan having simple thermal history. The Rb-Sr ages are 4∼9m.y. older than the corresponding K-Ar biotite ages, except for two Neogene granites which show concordant ages. The Rb-Sr whole-rock ages for these masses probably represent the time of emplacement, whereas the K-Ar biotite ages indicate the time of uplift and cooling, thus the slight discordance in the two age systems may be explained as indicating the cooling period of the granitic masses. The comparison between the published Rb-Sr whole-rock and K-Ar mineral ages for Japanese granitic rocks reveals that the rocks are grouped into three on the basis of the concordance-discordance mode: nearly concordant, discordant with the age difference of about 50m.y., and grossly discordant. Discordant ages for granitic rocks generally have rather large errors in Rb-Sr whole-rock ages, and careful evaluation is needed for these ages.

Discussion on the Rb–Sr whole-rock age determination of the Jersey Andesite Formation, Jersey, C.I.

Drs A. C. Bishop & A. E. Mourant write: Duff ( Journal 135 , 153–6) interpreted a Rb–Sr whole-rock isochron date of 533 ± 16 …

Age data on Proterozoic granitoids from the Norvijaur area of northern Sweden

Abstract Age measurements on Svecokarelian rocks of the Norvijaur area demonstrate (a) that the microcline granite, intersecting a metasupracrustal belt, has a whole-rock Rb-Sr age of 1915 ± 20 Ma with an intercept of 0.7025 ± 0.0007 and (b) that the gneissic granodiorite, lately interpreted as a basement to the above metasupracrustals, has a U–Pb zircon age of 2010 ± 40 Ma and a Rb-Sr whole-rock age of 1925 ± 120 Ma with an intercept of 0.7033 ± 0.0006.

Rb-Sr whole-rock age determination of the Jersey Andesite Formation, Jersey, C.I

A whole-rock Rb-Sr isochron age of 533±16Ma determined for the Jersey Andesite Formation is interpreted as the age of crystallization, and suggests that Upper Brioverian sedimentation continued into the Cambrian. It provides a maximum age limit of Middle Cambrian for the late Cadomian molasse-type Rozel Conglomerate, and supports the existence of a late orogenic (post 533±16Ma) phase of Cadomian deformation, which was followed by molasse sedimentation. There are, therefore, at least two separate phases of Cadomian deformation in the northern part of the Armoricain Massif, the previously recognized earlier event being late Precambrian in age.

Petrochemical and geochronological studies of plutonic rocks in the southern Appalachians: III. Leucocratic adamellites of the Charlotte belt near Salisbury, North Carolina

The Salisbury, Yadkin, Southmont, Gold Hill, and Kannapolis plutons and related smaller bodies in the Charlotte belt of central North Carolina are leucocratic albite adamellite. The Rb-Sr whole-rock ages for these plutons are 413 to 386 m.y., and the Salisbury-group granites generally have low initial Sr 87 /Sr 86 ratios near 0.703. A mica date of 368 m.y. is an upper bracket for metamorphism. The Gold Hill fault truncates the Gold Hill pluton; major movement probably occurred between about 400 and 368 m.y. ago. Gold mineralization apparently is younger than major deformation. The gold deposits are strongly localized in shear zones, although some gold-bearing veins cut the Gold Hill and Salisbury plutons. Acadian deformation in this part of the southern Appalachians is mainly restricted to shear zones trending about 25° east of north and was accompanied or followed by lower–greenschist-facies metamorphism that overprinted an earlier greenschist- to amphibolite-facies metamorphism.

A Paleozoic age for some charnockitic-anorthositic rocks

The Arden pluton, consisting of anorthosite, charnockite, and related rocks, is located within the granulite-facies Wilmington Complex of northern Delaware and southeastern Pennsylvania. Samples from the Arden body have a Rb-Sr whole-rock age of 502 ± 20 m.y. and an initial 87 Sr/ 86 Sr ratio of 0.7065. The data indicate primary crystallization about 500 m.y. ago and metamorphism roughly 440 m.y. ago, in early Paleozoic time. Thus, the formation of massif-type anorthosites and related rocks is apparently not restricted to a limited part of Proterozoic time and, therefore, does not require conditions unique to Precambrian time.

Rb-Sr whole-rock age of the Ilivertalik Granite and other rocks from the Fiskenæsset area

Fifteen samples of the Ilivertalik granite define an isochron with an apparent age of 2770 ± 23 m. y. and (B7Sr/B6Sr) 0 = 0.7021 ± 0.0002. Samples of both older and younger rocks plot very near to the 2770 m. y. isochron and probably are of comparable age.

The RbSr age of the Hästefjorden granite and its bearing on the Precambrian evolution of southwestern Sweden

On the basis of a seven-point isochron, the RbSr whole-rock age of the Hästefjorden granite in southwestern Sweden is 1240 ± 30 Ma using a decay constant of 1.39·10 −11 yr −1. The initial 87Sr/ 86Sr ratio is 0.712. This dating sets a lower limit for the beginning of the Dalslandian (Dal sequence) sedimentation and supports suggestions of a chronological diversity of the so-called Gothian Åmål—Kroppefjäll granite complex in western Sweden. It also invalidates correlation of the Åmål—Kroppefjäll complex as a whole with the Småland plutonics of southeastern Sweden. This proves that rocks previously considered as being formed during the “Gothian Cycle” are very different in age.

Rb-Sr whole-rock age of the contact aureole of the Stillwater Igneous Complex, Montana

Fourteen whole-rock samples from three traverses through the contact aureole of the Stillwater Complex were analyzed for Rb, Sr and 87Sr/ 86Sr. Twelve of these samples yielded an age of 2750 ± 45m.y.; ( 87Sr/ 86Sr) 0 = 0.705 ± 0.003 (2σ) . In addition one whole rock and a biotite separate derived from it gave an age of 2544 m.y.; ( 87Sr/ 86Sr) 0 = 0.714 . These data support a minimum age of 2750 m.y. for the intrusion of the complex.

Strontium isotopic evidence relating to the evolution of the lower Precambrian granitic crust in Swaziland

Research Article| September 01, 1976 Strontium isotopic evidence relating to the evolution of the lower Precambrian granitic crust in Swaziland R. D. Davies; R. D. Davies 1Bernard Price Institute of Geophysical Research, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2001, South Africa Search for other works by this author on: GSW Google Scholar H. L. Allsopp H. L. Allsopp 1Bernard Price Institute of Geophysical Research, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2001, South Africa Search for other works by this author on: GSW Google Scholar Author and Article Information R. D. Davies 1Bernard Price Institute of Geophysical Research, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2001, South Africa H. L. Allsopp 1Bernard Price Institute of Geophysical Research, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2001, South Africa Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1976) 4 (9): 553–556. https://doi.org/10.1130/0091-7613(1976)4<553:SIERTT>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. D. Davies, H. L. Allsopp; Strontium isotopic evidence relating to the evolution of the lower Precambrian granitic crust in Swaziland. Geology 1976;; 4 (9): 553–556. doi: https://doi.org/10.1130/0091-7613(1976)4<553:SIERTT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Isotopic data for Swaziland Precambrian gneissic rocks, with Rb-Sr whole-rock ages in the range from 3,400 to 3,100 m.y., lie on a steep initial 87Sr/86Sr ratio versus time development line that intersects the basalt development line (or region) at about 3500 m.y. B.P. The preferred interpretation is that, locally, extensive sialic crust did not begin forming much before 3,500 m.y. ago and that the various gneissic rocks defining this steep development line attained their present state by repeated metamorphism and Sr isotopic homogenization within this early sialic crust. Constraints are noted for alternative models that involve much older separation of sialic material. At ∼3000 m.y. B.P., large-scale granite formation took place, and there is evidence that this event effectively stabilized the crust. Younger granites, intruding the earlier gneissic rocks, have initial 87Sr/86Sr ratios lying mainly outside the basalt field. The isotopic data may be interpreted in terms of a two-stage model as a first approximation to the mode of evolution of the crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An early precambrian age for migmatitic gneisses from Vikan i Bø, Vestera˚len, North Norway

Migmatitic gneisses of overall intermediate composition occur at Vikan i Bø, North Norway. A Pb-Pb whole-rock isochron on these Vikan Gneisses yields an age of 3,460 ± 70m.y. A Rb-Sr whole-rock isochron on the same rock samples yields an age of 2,300 ± 150m.y., and an initial 87Sr/ 86Sr ratio of 0.7126 ± 0.0011. The geochemical implications of the discordance of the Pb-Pb and Rb-Sr whole-rock ages are discussed, and a timetable of the geological events in the early history of Lofoten-Vestera˚len is proposed.

Age relationships between greenstone belts and “granites” in the Rhodesian Archaean craton

Whole-rock Rb-Sr age measurements on rocks from the Rhodesian Archaean craton of Southern Africa demonstrate that (a) the Mashaba area gneisses, which are typical of the Rhodesian Basement Complex, are approximately 3600 m.y. old, (b) volcanic rocks from the main greenstone belts, assigned to the Bulawayan Group, were extruded approximately 2600–2700 m.y. ago, (c) a cross-cutting pluton, the Sesombi tonalite, was emplaced 2690 ± 70 m.y. ago, (d) the Gwenoro Dam migmatites, which field evidence suggests could be older than the main greenstone belts, were emplaced 2780 ± 30 m.y. ago. The initial 87Sr/ 86Sr ratios of most of the rock units of groups b, c and d are in close agreement at about 0.701, suggesting that the later granitic (sensu lato) and andesitic rocks so far analysed were not produced by remelting of, or contamination with, ancient gneissic basement.

Rb-Sr study of granite and gneiss from Seoul, South Korea

Granite from Seoul, Korea, has Rb-Sr whole-rock ages of 161 ± 1 m.y. and an initial Sr 87 /Sr 86 ratio of 0.7116 ± 0.0003. Thermal effects associated with the intrusion of granite lowered the Rb-Sr ages of middle Precambrian gneiss.

Isotopic Composition of strontium and geologic history of the basement rocks of Wright Valley, Southern Victoria Land, Antarctica

Abstract The Sr87/Sr86 ratios of whole-rock samples from the major units of the crystalline basement complex and from the tholeiitic sills of Jurassic age (Ferrar Dolerite) in Wright Valley range from 0.710 to 0.728. The isotopic composition of strontium released from these rocks by chemical weathering is compatible with an average Sr87/ Sr86 ratio of 0.7149 in the brines of Lake Vanda, Wright Valley, that was reported earlier. Some strontium having Sr87/Sr86 ratios less than that of Lake Vanda probably originates as weathering products of the Ferrar Dolerite sills and of the marbles of the Asgard Formation as well as by import of marine strontium by meteoric precipitation. Whole-rock Rb-Sr age determinations indicate that the Wright Intrusives (Olympus granite-gneiss and Dais granite) approached isotopic homogenisation of strontium about 500 α 43 m.y. ago. A suite of rocks from the Victoria Intrusives (Vida granite and Vanda porphyry) was dated at about 480 α 44 m.y. The average initial Sr87/Sr86 ratios ...

Radiometric Age Determinations from the Florida Mountains, New Mexico

Most of the igneous rocks of the Florida Mountains, New Mexico, can be divided into three groups: (1) old (∼1.35 b.y.?) granitic rocks in the northernmost part of the area; (2) granite to (quartz) syenite of uncertain age that occurs south of the major high-angle reverse fault that roughly divides the Florida Mountains into northern and southern parts (Corbitt, 1971; Corbitt and Woodward, 1973); and (3) enigmatic syenitic rocks of uncertain age that occur north of the major high-angle reverse fault. Corbitt (1971) has proposed a Mesozoic age of emplacement for the group 3 rocks, although Griswold (1961) has placed them in the Precambrian; minor mineralization is more-or-less confined to occurrences within or near the syenitic rocks. This report summarizes preliminary geochronologic data for the major occurrences of syenitic or quartz syenitic to granitic rocks from the Florida Mountains and will attempt to interpret their history. K-Ar and Rb-Sr mineral and Rb-Sr whole-rock age data indicate a complex history; geochemical and normative data are included for assistance in interpretation.

Evaluation of Rb–Sr Dating Methods Applied to the Matachewan, Abitibi, Mackenzie, and Sudbury Dike Swarms in Canada

Re-evaluation of the age of Canadian dike swarms using Rb–Sr whole-rock data give the following results:[Formula: see text]Detailed studies of the Matachewan dike swarm show that in most cases Rb–Sr mineral isochron age values from individual samples are concordant with the Rb–Sr whole-rock age values for the dike swarm as a whole and are clearly greater than K–Ar age values on the same material. The fact that the mineral isochron ages and the whole-rock ages by the Rb–Sr method are similar suggests that there has been no significant thermal overprinting of these particular dikes since their emplacement and solidification or contamination by radiogenic 87Sr from the host environment. Furthermore, the general scatter and lack of significant grouping of the lower K–Ar values similarly suggests that there was no single period of thermal resetting of the K–Ar ratios. It is observed that the K–Ar age values are greater in samples containing mica, and are more scattered and lower in samples in which potassium is contained in late-stage or principal phases other than mica. Therefore, it seems likely that significant loss of argon may result at near or normal surface temperatures and that the rate of diffusion is dependent on the mineralogy of the sample. It is concluded that reasonably good age determinations can be made on diabase dike rocks of ancient age by the Rb–Sr whole-rock isochron method, if the mineral isochrons are concordant.

Rb-Sr Whole-Rock Age of the Siloam Granite, Georgia: A Permian Intrusive in the Southern Appalachians

The Siloam granite crops out in the southern half of Greene County, Georgia, within the Outer Piedmont. The granite is similar to the coarsely porphyritic postmetamorphic granites of the Charlotte Belt of North Carolina and South Carolina. The Rb-Sr whole-rock date of the Siloam granite is 269 ± 3 m.y., making this intrusive the youngest of the southern Appalachian region. Its low initial 87Sr/86Sr ratio of 0.7052 ± 0.0001 suggests an upper mantle affinity with no appreciable crustal contamination during emplacement.

Rb-Sr Chronology and Stratigraphic Relations of Silurian Rocks, Mt. Desert Island, Maine

A Rb-Sr whole-rock age of 429 ± 13 m.y. for the Hall Quarry and Cadillac Mountain Granites, Mt. Desert Island, Maine, establishes that plutonism in this area occurred during the Middle Silurian. The age reported is also a minimum age for the Bar Harbor Series of clastic sedimentary rocks, which are intruded by the Cadillac Mountain Granite. Rocks exposed on Flint Island, Pleasant Bay area, Maine, which contain a Middle Silurian brachiopod fauna, are correlated with the Bar Harbor Series. The minimum age of 429 ± 13 m.y. established for the Bar Harbor Series is in good agreement with other published Rb-Sr ages determined from Middle Silurian rocks elsewhere in coastal Maine.