Rb-Sr Whole-rock Age Research Articles

RbSr-isotopengeochemie hydrothermaler quarze des bayerischen pfahles und eines flusspat-schwerspatganges von nabburg-wölsendorf/bundesrepublik deutschland

The RbSr whole-rock age of the Bayerischer Pfahl, a 150 km long, hydrothermally quartz-filled system of en échelon tension gashes and feather joints, is given as 247 ± 21 Ma with an initial Sr isotopic ratio of 0.7133 ± 0.0009. The mean Sr isotopic ratio for minerals from the Nabburg—Wölsendorf mineral district — at the northern end of the Pfahl zone — is 0.7135 ± 0.0007. The concordance in Sr isotopic ratios (at the time of the Lower Triassic) points to a genetic relationship for the mineral-precipitating solutions in the two areas and which must have been homogeneous over a distance of at least 90 km with respect to Sr isotopes. The Sr isotope composition of the quartz- and fluorite/barite mineralisation is not in contradiction to the model of “tectonic pumping” for the Pfahl. The age of the mineralisation of the Pfahl quartz of 247 ± 21 Ma is thought to be connected to the development of continental rifting systems related to the beginning of the opening of the North Atlantic. A by-product of this work is that decrepitation/leaching experiments cannot be used to study the chemical compositions of fluid inclusions.

Further radiometric dating of some young granites and a Småland dike porphyry in southeastern Sweden

Abstract Radiometric dating of some young granites of Jungfrun-Gotemar type, southeastern Sweden, gives a Rb-Sr whole-rock age of 1350 ± 11 Ma which is in accordance with earlier obtained values. A U-Pb concordia age from zircons is rejected since the zircons carry cores and consequently yield a mixed age. However, a discordia line with an upper intercept of 1400 Ma, the time of intrusion for the Jungfrun and Gotemar granites, may be drawn assuming that the zircons were completely reset at the time of intrusion. A Rb-Sr mineral isochron for a post-Svecokarelian Smaland dike porphyry yields an age of 1419 ± 14 Ma. This age was reset, probably at the time of intrusion of the younger granites.

Middle Ordovician conodonts from the Buchans Group, central Newfoundland, and their significance for regional stratigraphy of the Central Volcanic Belt: Reply

Conodonts have been recovered from carbonate clasts within breccia units in the Buchans Group of central Newfoundland. The assemblage is characterized by specimens of Cordylodus horridus Barnes and Poplawski and Histiodella holodentata Ethington and Clark. These and other species recovered suggest a Whiterockian (latest Arenig–early Llanvirn) age for the clasts. This is somewhat older than the whole-rock Rb–Sr age of 447 ± 18 Ma on volcanics of the Buchans Group. The carbonate clasts are considered to be derived from the area of deposition of the Buchans Group rather than from some exotic source. This interpretation is based on knowledge of local tectonics, inferred paleogeogaphy, and geographic distribution of the conodont assemblage. All other clasts in the breccia units are considered of local origin. If the conodonts are of local Buchans Group origin then they indicate a Whiterockian age for the unit. In view of this, current regional models for the Central Volcanic Belt may require substantial revi...

Middle Proterozoic uplift events in the Dunbar dome of northeastern Wisconsin, USA

Isotopic ages of granitic and metamorphic rocks exposed in the Dunbar structural dome of northeastern Wisconsin identify a protracted series of tectonic and “hydrothermal” events that culminated in major regional uplift during Middle Proterozoic (Keweenawan; ca 1,100 Ma) continental rifting and volcanism. The major rock-forming events and the structural development of the dome occurred during the interval 1,862+/−4 Ma to 1,836+/−6 Ma. Whole-rock Rb-Sr ages are partly reset in response to a widely recognized but cryptic event in Wisconsin and Michigan at about 1,630 Ma. The scale and systematic character of the whole-rock resetting strongly suggests the presence of a fluid phase derived in situ from water dissolved in the silicates or externally from a subthrust plate of low-grade metamorphic rocks. The regional nature of the 1,630-Ma disturbance possibly indicates that it is related to a major tectonic event such as an active plate margin far to the south. Rb-Sr biotite ages for the Dunbar dome (this study), the southern complex of the Marquette district (Van Schmus and Woolsey 1975) and the Felch trough area (Aldrich and others 1965) provide a remarkably coherent pattern that reflects multiple episodes of differential uplift. Younger events superimposed on a regional 1,630-Ma imprint are recorded at 1,330 Ma and 1,140 Ma. The 1,330 Ma disturbance could reflect stabilization following intrusion of the Wolf River batholith at 1,485 Ma. The 1,140-Ma uplift event occurred during Keweenawan rifting and volcanism as a result of stresses imposed on a mosaic of fault-bounded blocks with possible subcrustal influence. The remarkably small variance in the 1,140-Ma biotite age peak argues for rapid uplift and cooling, and hence rapid erosion. Detritus from the uplift probably was being shed into nearby tectonic basins most of which did not survive subsequent uplift and erosion.

Middle Ordovician conodonts from the Buchans Group, central Newfoundland, and their significance for regional stratigraphy of the Central Volcanic Belt

Conodonts have been recovered from carbonate clasts within breccia units in the Buchans Group of central Newfoundland. The assemblage is characterized by specimens of "Cordylodus" horridus Barnes and Poplawski and Histiodella holodentata Ethington and Clark. These and other species recovered suggest a Whiterockian (latest Arenig–early Llanvirn) age for the clasts. This is somewhat older than the whole-rock Rb–Sr age of 447 ± 18 Ma on volcanics of the Buchans Group. The carbonate clasts are considered to be derived from the area of deposition of the Buchans Group rather than from some exotic source. This interpretation is based on knowledge of local tectonics, inferred paleogeogaphy, and geographic distribution of the conodont assemblage. All other clasts in the breccia units are considered of local origin. If the conodonts are of local Buchans Group origin then they indicate a Whiterockian age for the unit. In view of this, current regional models for the Central Volcanic Belt may require substantial revision. A Whiterockian age for the Buchans Group is consistent with the observation that all significant volcanogenic massive sulphides of the Canadian Appalachians are of Caradoc or older Ordovician age. It is suggested that the Buchans Group is thrust southeastwards over the Victoria Lake Group, which is a probable distal facies equivalent.

RbSr isotopic equilibrium during Sveconorwegian (= Grenville) deformation and metamorphism of the Orust dykes, S.W. Sweden

The Orust dykes and their regional correlatives are an important time marker in the polymetamorphic Östfold-Marstrand belt of S.E. Norway and S.W. Sweden. They were deformed and metamorphosed in the amphibolite facies during the Sveconorwegian (= Grenville) orogeny. Three narrow strongly deformed dykes yield RbSr whole-rock ages (1106 ± 52 m.y., 1062 ± 90 m.y., 1000 ± 153 m.y.). The weighted mean age (1087 m.y.) is interpreted as the age of the first post-dyke deformation and the accompanying amphibolite facies metamorphism. High initial 87Sr/ 86Sr ratios (0.7074 ± 9, 0.710 ± 2, 0.713 ± 3, respectively) and other geochemical evidence point to considerable exchange of mobile elements between the dykes and the country rocks. The emplacement age is not known precisely. However, the dykes may be correlated with widespread mafic magnatism which accompanied crustal instability in the N. Atlantic region early in the Sveconorwegian (= Grenville) orogeny about 1200 m.y. ago.

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.

南部阿武くま山地花こう岩類の角せん石Ｋ‐Ａｒ年代

K-Ar age determinations were carried out on 19 homblendes and 4 biotites from 9 granitic masses and two metamorphic rocks in the southern Abukuma Plateau, Northeast Japan. Hornblende ages for granitic rocks range from 96.4 to 119 Ma and are apparently not correlated with area or mass. Biotites show a limited age range of 96.1-97.8 Ma. The variation in hornblende age is attributed to the difference in cooling rate of granitic rocks. Hornblende ages for the Gosaisho and Tamadare metamorphic rocks are 115 and 103 Ma, respectively, and nearly equal to those for the adjacent granites. No old ages were found on hornblendes from the Ishikawa, Miyamoto and Samegawa masses, which constitute the older type granites and gave Rb-Sr whole-rock ages of about 400 Ma. This may impose a severe constraint on the thermal history of the older type granites.

The Rb-Sr age of the Sundsta granite in the Western Pregothian tectonic mega-unit, south-western Sweden

Abstract The Sundsta granite is a reddish acidic granite, situated in the Western Pregothian mega-unit in Varmland, south-western Sweden. Its Rb-Sr whole-rock age is 1566 ±39 Ma with an initial ratio of 0.705 ±0.003. The region is dominated by grey, migmatized granitoids presumably belonging to the Amal-I group which has ages of 1650–1700 Ma. The marked difference in degree of migmatization between these two rock-units may indicate that the intrusion of the Sundsta granite post-dates the main migmatization phase of the Amal-I granitoid in this region.

RbSr dating of low-grade metamorphics in the U.S.S.R.

Geologically well-dated low-grade metasedimentary and metavolcanic rocks from five localities have been studied using the whole-rock RbSr age method. All age values obtained are younger than those suggested by stratigraphy. When whole-rock points fit either an isochron or an errorchron with low value of the mean square of weighted deviates, the slope of this line gives the age of metamorphism.

Two high precision U–Pb zircon ages for the Sudbury Nickel Irruptive

Two independent U–Pb zircon ages were measured using norite samples from the North and South Ranges of the Sudbury Nickel Irruptive. Ages of [Formula: see text] and [Formula: see text] Ma were obtained with a probability of fit within experimental error of 51 and 57%, respectively.Previous determinations by the Rb–Sr whole-rock method gave ages between about 1700 and 2000 Ma, with errors of the order of ± 100 Ma. The difference in precision is inherent in the two methods: zircons from the norite are highly radiogenic and almost concordant whereas Rb–Sr whole-rock ages are determined by very small increments in radiogenic 87Sr.The two zircon ages have completely different discordia so that all simple models for lead loss by continuous diffusion can be excluded.

The effects of penetrative Sveconorwegian deformations on RbSr isotope systems in the Römskog-Aurskog-Höland Area, SE Norway

Abstract The area investigated consists of five granitoid complexes which range over more than 900 Ma (?1850-925 Ma RbSr whole rock) in age as well as an old series of layered para- and orthogneiss. The relative ages of the old gneiss series and the oldest granitoid complex, which is a series of augen gneisses (?1850-1710 Ma), are uncertain. The layered para- and orthogneisses were however definitely gneissified before the intrusion of a 1631 Ma old grey orthogneiss. Deformational activity took place several times during the geological evolution. The major structures developed mainly after the intrusion of both a 1277 ± 20 Ma old red granitic orthogneiss and the mafic dykes that transsect it. The post-1277 Ma structures resulted from the following: a foliation-forming event with migmatization; a fold phase giving E–W-directed folds; a fold phase giving N–S-directed folds which affect the southern part of the area only; and finally, a large-scale overthrusting towards the east along the western limit of the area. The upper age limit of the folding activity is established by a 925 Ma old undeformed granite. The post-1277 Ma structures are regarded as constituting the infrastructure of the Sveconorwegian orogeny in the area. Possible correlation with the suprastructure of this orogeny in the Dalsland area further south is discussed. Although the dated rocks have suffered from high-grade metamorphism, we argue that the RbSr whole-rock ages mark the ages of intrusion (crystallization) of the plutonic complexes.

New Rb-Sr age determinations on the Archaean basement of Eastern Sierra Leone

Abstract The Archean basement of Sierra Leone is a typical example of granite-greenstone terrains found in ancient continental nucleii. Reconnaissance field mapping showed that the area can be subdivided into old gneiss, which predates the greenstone belts, and young granite which is later than the greenstone belts. New Rb-Sr whole-rock age determinations on two suites of old tonalitic gneiss yield ages of 2786 ± 49 Ma and 2770 ± 137 Ma, which either reflect the time of formation of the original tonalites or their metamorphism. Three new Rb-Sr whole-rock age determination on young granites yield ages of 2786 ± 143 Ma, 2780 ± 79 Ma and 2770 ± 50 Ma, which are interpreted as the time of emplacement. The widespread occurrence of similar young granites, throughout the Archaean of West Africa, suggests that these results date a major event in the evolution of this segment of the crust. A published Pb-Pb age of the old gneiss and the new ages of the young granite bracket the age of the greenstone belts to 3000-2770 Ma. However, if the Rb-Sr ages of the old gneiss reported in this paper reflect the time of their formation, the age of the greenstone belts is tightly bracketed to ca. 2770 Ma. There is no isotopic evidence for rocks substantially older than 3000 Ma in the West African Archaean.

Recambrian evolution of the Beartooth Mountains, Montana-Wyoming, U.S.A.

The Beartooth Mountains ofMontana and Wyoming contam more than 5,000 km2 of exposed Archean rocks. The southeastern portion of the range contains the only well documented segments of early Archean crust. The oldest rocks identified are a supracrustal assemblage contaning basaltic, ultramafic and felsic metavolcanics, metagraywackes, quartzites, meta-ironstones, and metapelites. These rocks yield a Rb-Sr whole-rock age of 3,400 Ma which is interpreted as the time of a granulite fades metamorphic event (800°C and 6 kb). This granulite facies event was followed at about 3,000 Ma age by the development of extensive volcanic and sedimentary packages with some minor silicic plutonism. The last major episode of crustal development began with an amphibolite fades rnetamorphic deformational event (about 2,850 Ma) and culminated with the intrusion of large volumes of granitoids (about 2,800 Ma). These granitoids were subsequently intruded by mafic dikes at least four separate times between 2,800 and 700 Ma age. Many of these events, including mafic dike emplacement, are correlative with geologic events reported from other ranges in the Wyoming Province; however, the Beartooth region appears to contain the longest and most complete record.

The northward extent of the Archaean basement of Greenland — a review of RbSr whole-rock ages

An outline is given of recent Rb-Sr whole-rock ages from both the Archaean gneiss block of southern Greenland and the areas north of the Archaean block. It is probable that most of Greenland, perhaps with the Ketilidian mobile belt of South Greenland as the only exception, is underlain by Archaean rocks. However, north of the Archaean block sensu stricto, later metamorphism has generally upset the RbSr isotope systems to such an extent that precise age determination by the RbSr whole-rock method is no longer possible.

Rb-Sr ages of granitic gneisses of the Inner Piedmont belt of northwestern North Carolina and southwestern South Carolina

Granitic gneisses from the Inner Piedmont belt of northwest North Carolina have Rb-Sr whole-rock ages of 427 ± 9 m.y., 460 ± 9 m.y. and possibly 534 m.y. Initial 87 Sr/ 86 Sr ratios for these rocks are approximately 0.704. Rb-Sr data for granitic gneisses from the Inner Piedmont belt of southwestern South Carolina and adjacent Georgia suggest a whole-rock age of approximately 423 m.y. and an initial 87 Sr/ 86 Sr ratio of 0.7069. These ages are tentatively interpreted as times of igneous crystallization. Together with previously published results, these data suggest that 460 to 420 m.y. ago was a time of significant igneous activity in the Inner Piedmont of North and South Carolina; igneous activity also occurred 545 to 525 and 350 to 335 m.y. ago.

Rb-Sr and K-Ar age determinations of the Proterozoic Olden granite, central Caledonides, Jämtland, Sweden

Abstract The Olden granite is a highly differentiated Proterozoic granite occurring in a basement window within the Caledonian orogenic belt. The granite is extensively affected by the Caledonian orogeny. A minimum age of 1500 Ma and an initial 87Sr/86Sr ratio of 0.7134 ± 0.0012 have been established by a Rb-Sr whole-rock age determination. Rb-Sr mineral determinations and K-Ar determinations reflect a considerable Caledonian influence.

RbSr isotope systematics at amphibolite facies conditions, Uppsala Region, Eastern Sweden

On the basis of U-Pb, Rb-Sr and K-Ar isotope analyses of Proterozoic rocks and minerals, a chronology has been established for the tectonic, intrusive and metamorphic evolution of the Svecokarelian orogeny 1750–1950 Ma ago in the Uppsala Region, Eastern Sweden. It is suggested that when synkinematic granitoids intruded the orogenic belt, at a stage of general subsidence and at medium metamorphic conditions (600°C and 3.5–4 kbar), the U-Pb isotope system in zircons closed earlier than the Rb-Sr whole-rock system. The zircon age (1886 Ma) reflects the intrusion and crystallization of the rock melt and the Rb-Sr whole-rock age (1830 Ma) the time when the temperature had decreased below the threshold for 87Sr migration. The Rb-Sr whole rock age (1898 Ma) determined for metaandesites and metadacites reflects a recrystallization related to the intrusion of the granitoids. On the contrary, the more silicic metarhyodacitic volcanic rocks have a Rb-Sr whole rock age (1830 Ma) reflecting the cessation of the synkinematic metamorphism. The difference in the way the Rb-Sr isotope system responds in subsilicic or silicic metavolcanics is probably dependent on the amount of radiogenic 87Sr and on the fixation of 87Sr in Ca-rich minerals. Subsequent, late-kinematic, low amphibolite facies metamorphism has not altered the Rb-Sr ages of the granitoids and the recrystallized metavolcanics.

Rubidium–strontium ages from the Oxford Lake – Knee Lake greenstone belt, northern Manitoba

Rb–Sr whole-rock ages have been determined for rocks from the Oxford Lake – Knee Lake – Gods Lake greenstone belt, in the Superior Province of northeastern Manitoba.The age of the Magill Lake Pluton is 2455 ± 35 Ma (λ87Rb = 1.42 × 10−11 yr−1), with an initial 87Sr/86Sr ratio of 0.7078 ± 0.0043. This granitic stock intrudes the Oxford Lake Group, so it is post-tectonic and probably related to the second, weaker stage of metamorphism.The age of the Bayly Lake Pluton is 2424 ± 74 Ma, with an initial 87Sr/86Sr ratio of 0.7029 ± 0.0001. This granodioritic batholith complex does not intrude the Oxford Lake Group. It is syn-tectonic and metamorphosed.The age of volcanic rocks of the Hayes River Group, from Goose Lake (30 km south of Gods Lake Narrows), is 2680 ± 125 Ma, with an initial 87Sr/86Sr ratio of 0.7014 ± 0.0009.The age for the Magill Lake and Bayly Lake Plutons can be interpreted as the minimum ages of granitic intrusion in the area.The age for the Hayes River Group volcanic rocks is consistent with Rb–Sr ages of volcanic rocks from other Archean greenstone belts within the northwestern Superior Province.

Tectonic significance of the young mineral dates and the rates of cooling and uplift in the Himalaya

More than two-thirds of the published K-Ar, Rb-Sr and fission-track mineral dates from the Himalaya lie in the 5–75 m.y. range as a result of metamorphic overprint, uplift and cooling during the Late Cretaceous—Tertiary Himalayan orogeny. In contrast, the few but almost invariably old, Rb-Sr whole-rock ages reveal pre-Tertiary magmaticmetamorphic events. The pattern of distribution of these young dates vis-á-vis geological evidence reveals three phases, of the Himalayan orogeny, viz.: (1) folding and metamorphism (50–75 m.y.); (2) uplift (25–40 m.y.); and (3) major uplift, thrusting, formation of nappe structures, mylonitization and regional retrogression. The maximum concentration of dates in the 10–25 m.y. period marks this paroxysmal phase of the Himalayan orogeny. The Rb-Sr dates of co-existing muscovites and biotites have been used to compute the rates of cooling and uplift. Thus, slow cooling at the rate of about 4°C m.y. −1 from 50 to 25 m.y. and rapid cooling at the rate of 19°-21°C m.y. − from 25 m.y. to present have been inferred. The high rate of cooling over the past 25 m.y. is the result of major uplift at the rate of 0.7–0.8 mm yr −1, which is in conformity with the current rate of uplift obtained from geodetic survey.