Rb-Sr Whole-rock Research Articles

Age and provenance of granitoid clasts in Moeatoa Conglomerate, Kawhia Syncline, New Zealand

Abstract A Rb-Sr whole-rock isochron from a suite of granitoid clasts in a Late Triassic conglomerate of the Kawhia Syncline, New Zealand, yields an age of 226 ± 6 (2σ) Ma. This age falls within the range of sedimentary ages for the conglomerate (Late Carnian — Early Norian), an implies rapid uplift and erosion of the clast protolith, and rapid transport and deposition of the derived material. Petrographic, chemical and isotopic characteristics of the suite are consistent with a volcanic arc tectonic setting; a possible protolith could be a northwards continuation of the Permo-Triassic Brook Street magmatic arc.

Granite-greenstone terranes in the Pilbara Block, Australia, as coeval volcano-plutonic complexes; Evidence from U-Pb zircon dating of the Mount Edgar Batholith

The Mount Edgar Batholith in the Pilbara Block, Western Australia, consists, in part, of a gneiss complex that resembles other Archean high-grade gneiss terranes. The complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession, the Warrawoona Group. The contact between the greenstone belt and the gneiss complex is tectonic. Field evidence indicates that the banded gneisses are the oldest part of the batholith; zircon U-Pb ages for three samples are 3429 ± 13Ma, ⩾ 3443 ± 10Ma and3448 ± 8Ma, all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession ( Pidgeon, 1978 ) [1]. Two granitoids, representing pre- and post-tectonic phases respectively, give zircon ages of 3304 ± 10Ma and3314 ± 13Ma, thereby constraining the major structural events to be contemporary with granite magmatism, consistent with a diapiric model for the tectonic evolution of the batholith ( Collins, 1989 ) [2]. These ages are similar to the zircon age for terminal volcanism in the Warrawoona Group ( Pidgeon, 1984 ) [3], but are ∼ 100Ma older than the precisely-defined Rb-Sr whole-rock ages obtained throughout the batholith ( Collins and Gray, 1989 ) [4]. Batholith-wide resetting or late closure of Rb-Sr isotopic systems is implied, possibly associated with late fluid circulation. The temporal coincidence between plutonism and volcanism in the Mount Edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the ∼ 3300Ma granites and the ∼ 3440Ma gneisses in the batholith. The isotopic evidence, combined with the structural data, indicates that the granites cannot be derived from the surrounding greenstone belt. Further, there is no compelling evidence for a sialic basement to the greenstone succession.

Geochemical evidence for a revised Precambrian sequence in the northwest Adirondacks, New York

Middle Proterozoic metasedimentary and metavolcanic rocks in the northwest Adirondacks are assigned into four lithostratigraphic units: basal Hyde School Gneiss that herein is given formational status, lower marble, Popple Hill Gneiss Formation, and upper marble. A variety of stratigraphic columns have been proposed for this complexly deformed terrane, the most recent being a column by Wiener and others (1984). Their basal formation, the Alexandria Bay Gneiss, is defined from rocks that include the Rockport granite gneiss near Rockport, Ontario, and Alexandria Bay, New York, as well as leucogneisses in 14 structural domes throughout the northwest Adirondacks. We believe that these rocks have a diverse origin, one being intrusive, the other volcanic ash. Their uppermost formation, the Pleasant Lake Gneiss, includes granitoid gneisses near Rossie village and Pleasant Lake that they suggest are illitic arkoses, but we give field, geochemical, and geochronological evidence for an intrusive origin. The gneisses near Rossie village have a whole-rock Rb-Sr age of 1,160±42 m.y. (IR = 0.7036 ± 0.0004), and similar gneisses near Antwerp village are 1,197 ± 53 m.y. (IR = 0.7034 ± 0.0005). These ages are younger than the zircon U-Pb ages of 1,280-1,250 m.y. generally obtained for metasedimentary and metavolcanic rocks in the Central Metasedimentary Belt of Ontario. We urge abandonment of the five formations and two Groups proposed by Wiener and others (1984) on the basis of an intrusive origin for the gneisses near Rockport and Rossie villages and on stratigraphic correlations of recent studies. The stratigraphy of four formations proposed herein with a generalized geologic map of the northwest Adirondacks may lead to correlation with carbonate sequences in the Central Metasedimentary Belt and with rocks of the Adirondack highlands to the east.

U-Th-Pb and Rb-Sr geochronology of middle Proterozoic granite and augen gneiss, Salmon River Mountains, east-central Idaho

Coarsely porphyritic granite and augen gneiss in east-central Idaho have been dated by the U-Th-Pb zircon method. When plotted on concordia diagrams, the results give nonlinear data arrays that indicate the presence of at least one component of inherited radiogenic lead, as well as postcrystallization, probably recent, episodic lead-loss.Nevertheless, a systematic relationship between zircon size and convergence of the 207 Pb/ 206 Pb ages for the +100- and -100+150-mesh fractions from several samples are interpreted to provide a valid age of about 1,370 m.y. for these plutons. A single sample which is free of inheritance for all size fractions confirms this conclusion. Rb-Sr whole-rock samples are highly enriched in radiogenic strontium but do not define an isochron, probably because the system did not remain closed following crystallization of the plutons. Intrusive contacts of the granitic rocks indicate that deformation and regional metamorphism of the host Yellowjacket Formation occurred prior to 1370 Ma, rather than prior to 1500 Ma as previously suggested by others. This refinement of the geochronology (1) removes the necessity that the Yellowjacket Formation and overlying Proterozoic metasedimentary rocks of central and east-central Idaho be pre-Belt in age; (2) permits the traditional age correlation of the east-central Idaho metasedimentary sequence (Yellowjacket through Lawson Creek Formations) with the Belt Supergroup; and (3) suggests that a re-evaluation is in order for paleogeographic models of central Idaho, especially the proposed topographic high known as the Salmon River Arch.

Rb-Sr dating of Rahu Suite granitoids from the Paparoa Range North Westland, New Zealand

Abstract Mapping in the central Paparoa Range, West Coast of the South Island, New Zealand, has led to the recognition of three petrologically distinct granitoids. These are Steele Granodiorite, Blackwater Granite, and Buckland Granite, which were intruded in that order into the Charleston Metamorphic Group and Greenland Group in mid-Cretaceous times. Mineralogy, whole-rock chemistry, and Sr isotopic composition suggest that all three granitoids belong to the intermediate I/S-type Rahu Suite. Rb-Sr whole-rock geochronology provides an intrusive age of 116 ± 6 Ma for Buckland Granite, but is unable to establish the intrusive age of the other two granitoids. Significant1y younger muscovite ages (103–112 Ma) and biotite ages (92–99 Ma) indicate immediate postintrusive uplift of all three granitoids in the order of 0.5-1.0 mm/a. Fieldevidence suggests that Buckland Granite was intruded in several batches. The wide range of initial 87Sr/86Sr (0.7058–0.7094) indicates at least two isotopically different source components. Isotopic variation in some Blackwater Granite samples is attributed to low-temperature alteration associated with lamprophyre dike intrusion whereas similar variation in some Steele Granodiorite samples is due to contamination by metasedimentary gneiss.

Geochronology and cooling history of Mesozoic granitic rocks in the Inje-Hongcheon district, South Korea.

K-Ar and Sr isotopic age determinations were made for Mesozoic granitic rocks intruding into Precambrian gneisses and schists in the Inje-Hongcheon district, South Korea. They include hornblende-biotite granodiorite (K-Ar biotite ages of 154 to 165 Ma), porphyritic biotite granite (149 to 157 Ma) and two-mica granite (149 to 155 Ma). The hornblende-biotite granodiorite gives K-Ar hornblende ages of 168 to 180 Ma and a Rb-Sr whole-rock isochron age of 212.0±26.6 Ma with an initial 87Sr/86Sr ratio of 0.71198±0.00046. These granitic rocks have been derived from crustal melt during late Triassic to early Jurassic time. The cooling rate of the hornblende-biotite granodiorite was slow in early stage and became greater as it approached shallow depths.

Rb–Sr geochronology of the Archaean Maggo gneiss from the Hopedale block, Nain Province, Labrador

The Maggo gneiss, a typical grey gneiss from the Hopedale block of coastal Labrador (southern Nain Province), preserves structures of two major Archaean tectonothermal events, the Hopedalian and the younger Fiordian. Rb–Sr whole-rock analyses of four suites representing the Hopedalian event (3140 ± 95 Ma (1σ), 3125 ± 43 Ma, 3028 ± 199 Ma, and 3025 ± 163 Ma) fall within the early middle Archaean and represent a period of cratonization of the Hopedale block.Two suites with a younger Fiordian structural fabric (2899 ± 116 Ma and 2804 ± 100 Ma) fall within the late middle Archaean and are interpreted as dating a period of reworking that locally reset the Rb–Sr isotopic systematics of the Maggo gneiss. Fiordian ages were also determined from four suites (2927 ± 50 Ma, 2884 ± 86 Ma, 2854 ± 83 Ma, and 2764 ± 89 Ma) that do not display the Fiordian structural fabric, indicating that chemical and mechanical components of the Fiordian event did not occur simultaneously. Age determinations of the Fiordian do not date specific geologic events but seemingly reflect varying degrees of protracted isotopic homogenization during chemical and mineralogical reequilibration.One suite of analyses yields an age of 3305 ± 75 Ma and is interpreted as dating a pre-Hopedalian event. This older age supports field evidence as to the existence of old crust in the Hopedale block, typical also of rocks of the North Atlantic Craton.

Late Precambrian alkaline plutons in southwest India: Geochronologic and rare-earth element constraints on Pan-African magmatism

The Precambrian granulite facies terrain of southwestern India is intruded by a suite of alkali granite and syenite plutons. RbSr whole-rock isotope data for the Angadimogar syenite (AM) and the Peralimala alkali granite (PM), belonging to this suite, define isochron ages of 638 ± 28 and 750 ± 40 Ma, respectively, with initial 87 Sr 86 Sr ratios of 0.7032 ± 0.0008 and 0.7031 ± 0.0008, respectively. These age data, together with data from previous studies, demonstrate long-lived magmatic activity in the time span from the late Proterozoic to the early Palaeozoic, broadly contemporaneous with Pan-African events in other fragments of the Gondwana supercontinent. REE patterns are reported for four plutons of this Pan-African alkali granite-syenite suite: Chengannoor (CR), Vellingiri (VL) and the two dated intrusions (AM and PM). CR and AM are characterised by high total REE, strongly LREE-enriched patterns with no Eu anomaly, associated with low Sr, Rb, U and Th. K 2O, K 2O Na 2O , K 2O MgO and the agpaitic index are lower for these plutons as compared to the other two. The PM and VL intrusions have lower total REE and less strongly fractionated REE patterns, associated with high K 2O, K 2O Na 2O and K 2O MgO ratios, high Sr and Rb levels, but low U and Th. The geochemical patterns in these rocks compare them well with A-type granites and their tectonic relations assign affinities to magmatism of within-plate type. The alkaline magmatism manifests an extensional phase associated with the pre-rift tectonics of the Indian continent within the Gondwana assemblage. A petrogenetic model is development for these plutons, involving decompression-induced melting of deep crustal source materials characterised by low initial 87 Sr 86 Sr and high K Rb ratios.

U–Pb, Rb–Sr, and K–Ar isotopic constraints for ductile deformation and related metamorphism in the Teslin suture zone, Yukon–Tanana terrane, south-central Yukon

The Teslin suture zone (TSZ) comprises a portion of the Yukon–Tanana terrane (YT), in the Yukon, formed by steeply dipping layering and L–S tectonite foliation. The TSZ forms the fundamental tectonic boundary between rocks deposited along the ancient margin of North America and allochthonous terranes to the west. TSZ tectonites evolved during initial penetrative dip-slip deformation (Dds) and later dextral strike-slip shear (Dss) along steep, 1–3 km wide shear zones. Several workers have speculated that the TSZ and related YT heterogeneous ductile deformation and associated metamorphism are Devonian to Mississippian in age and related to the intrusion of a similar-age orthogneiss throughout the Yukon and Alaska. However, recent structural and metamorphic studies of the TSZ provide evidence contradicting this view. New isotopic evidence, presented herein, indicates that TSZ dynamothermal metamorphism was cooled by Early Jurassic time, that it cannot be related to Devonian–Mississippian and Permian granitic intrusion, and that it predates Cretaceous plutonism.U–Pb zircon dating of peraluminous orthogneiss constrains primary peraluminous granite crystallization at 355 ± 25 Ma. Three Rb–Sr whole rock + muscovite and three K–Ar muscovite cooling dates of rocks containing Dds and Dss fabrics place a younger age limit of 182–213 Ma (latest Triassic to Early Jurassic) on Dds/Dss deformation. In addition, three Rb–Sr whole-rock + muscovite isochrons and one K–Ar date on biotite indicate peraluminous orthogneisses in the eastern portion of the study area were affected by a mid-Cretaceous thermal event. These data, togemer with structural and metamorphic relationships reported elsewhere, are summarized in pressure–temperature–time–displacement diagrams illustrating the evolution of TSZ and adjacent rocks from Devonian to Late Cretaceous time. Tectonites within the TSZ can be differentiated from peraluminous orthogneiss east of the d'Abbadie fault on the basis of their respective cooling histories.

U-Pb zircon ages and Rb-Sr whole-rock isotope studies of early Proterozoic volcanic and plutonic rocks near Tampere, southern Finland

Abstract The Tampere Schist Belt, southern Finland, is a well-preserved volcanic-sedimentary belt in the early Proterozoic Svecofennian terrain. The metasedimentary rocks of the belt are dominated by turbiditic greywackes, siltstones and mudstones. The metavolcanics range from basalt to rhyolite in composition, intermediate rocks being the most common. Near Tampere, the belt comprises an east-west trending major syncline. The U-Pb zircon ages of the ∼1.9 Ga old metavolcanics cover an age interval of ∼15 Ma . The oldest age, 1904 ± 4 Ma , is from the “Intermediate Unit” at Orivesi, which is dominated by calc-alkaline. high-K dacites and andesites. This unit is the lowest in the northern limb of the syncline at Orivesi. At Ylojarvi, a sample from the Lower Volcanic Unit has an age of 1898 ± 4 Ma , while a sample from the Upper Volcanic Unit has the youngest crystallisation age of 1889 ± 5 Ma . These ages agree with the structure and stratigraphy of the belt. The implications of the 1891 ± 16 and 1889 ± 19 Ma ages of two feldspar porphyries remain obscure; here the error limits are wide and it is not known with certainty whether the rocks are dykes, sills or volcanics. An age of 1901 ± 28 Ma was obtained for the Varmala granitoid which intrudes the schists. Although the error is large, it is possible that some of the zircon has been inherited from older rocks. The Rb-Sr whole-rock ages of the volcanics agree, within error limits, with the U-Pb zircon ages. The Rb-Sr age of the Hameenkyro batholith is, however, about 100 Ma lower than the equivalent U-Pb age. The I Sr values of the metavolcanics and the 1.882 Ga old Hameenkyro batholith range between 0.7020 and 0.7033. These low figures suggest that their protoliths separated from upper mantle sources within a short time interval. The Sm-Nd and Lu-Hf isotopic data available in the literature support the conclusion that the period around 1.9 Ga represents an important episode of continental growth, which is particularly evident in the Svecofennian-Ketilidian-Trans-Hudsonian terrains.

Short Paper: A note on Rb-Sr whole-rock ages from cleaved mudrocks in the Welsh Basin

Two cleaved, epizone-grade Palaeozoic mudrock suites from Snowdonia yield a mean Rb-Sr whole-rock age of 409 ± 11 Ma. This is consistent with resetting of the Rb-Sr whole-rock isotope systems of the mudrocks during cleavage development in end-Silurian/Early Devonian (Acadian) deformation. In contrast, cleaved Palaeozoic anchizone mudrocks from north and central Wales give a Silurian mean age of 430 ± 9 Ma. This dates either a pre-tectonic isotopic event preserved through Acadian anchizone metamorphism, or mid-Silurian deformation and metamorphism at anchizone grade unaffected by the Acadian event. The precise timing of the main Caledonian deformation in Wales has never been established firmly. Powell & Phillips (1985) suggest that it occurred in north and south Wales in latest Silurian time, whereas Ashgill and end-Llandovery tectonic events were dominant in central Wales (see also George 1963 ). However, Woodcock (1984 , 1987) and Soper et al. 1987 have argued in favour of a single tectonic event of early to middle Devonian age. This chronological study was undertaken to address this controversy. The only previous attempt to define precisely the timing of cleavage development in Wales was the K-Ar dating of cleavage-parallel chlorite from a volcanic greywacke from north Wales ( Fitch et al. 1969 ). When recalculated using modern decay constants, these data give an age of 399 ± 10 Ma. Bath (1974) demonstrated the feasibility of obtaining Rb-Sr ages from cleaved mudrocks of the Welsh Caledonides, and this work produced ‘Caledonian’ ages of 409 ± 20Ma and 393 ± 22Ma from north Wales and ‘late

Resetting of the Rb–Sr whole-rock isotope system of an Ordovician microgranite during Devonian low-grade metamorphism

AbstractThe Bwlch y Cywion microgranite intrudes an Ordovician sedimentary sequence in northeast Snowdonia, North Wales. Its age is recorded by hornfelses within its metamorphic aureole which give an Ordovician age of 454 ± 20 Ma. The whole-rock Rb–Sr isotope systems of the intrusion, however, and of an associated ash-flow tuff and a rhyolite of the Llewelyn Volcanic Group (Caradoc), were reset during low-grade metamorphism and give Devonian ages of 392±11 Ma, 392±5 Ma, and 405 ± 6 Ma, respectively.

Isotopic U–Pb ages of zircon and monazite from the Leinster Granite, southeast Ireland

AbstractA more precise age for the synkinematic emplacement of the large S-type Leinster batholith (southeast Ireland) has been obtained by U–Pb dating of zircon and monazite separates from the two main granite varieties of the Northern Pluton. A concordant age of 405±2 Ma derived from monazite is considered to reflect both the age of intrusion and the main regional deformation event (D1) in southeast Ireland. The monazite age is in good agreement with an earlier, less precise whole-rock Rb–Sr isochron determination. The zircon data indicate recent Pb loss effects and the existence of a small component of inherited radiogenic Pb derived from a crustal precursor during the generation of the batholith.

Post-Acadian metasomatic origin for copper-bearing vein deposits of the Virgilina District, North Carolina and Virginia

The Virgilina mining district, located in the Carolina slate belt of North Carolina and Virginia, has 1- to 3-m-wide vein systems that contain bornite, chalcocite, and hematite, plus minor argentite and gold in quartz, calcite, and sericite gangue. Chalcopyrite is rare and pyrite is absent in major orebodies. Ore-bearing veins are restricted to greenstone tuffs, breccias, and flows of the upper Precambrian Virgilina formation. Major and trace element studies of country rocks show that ore genesis is probably related to metasomatic alteration of greenstone into epidosite and alkali-enriched schistose greenstone. This metasomatic alteration produced extensive loss of copper in the greenstone. Field and petrographic evidence indicates that vein emplacement and country-rock alteration clearly postdate regional metamorphic foliation and prograde metamorphic mineral assemblages. Rb-Sr whole-rock dating of quartz + sericite + epidote vein material from the High Hill mine indicates a 340 + or - 6-Ma age for vein formation. The calculated initial 87 Sr/ 86 Sr ratio of the vein material (0.7042 + or - 0.0004) is compatible with Cu-bearing ore fluids that equilibrated with surrounding greenstone country rock. The 340-Ma age is younger than the suggested Acadian age ( approximately 380 Ma) for regional metamorphism in the northern Carolina slate belt. Rb-Sr mineral ages for metamorphic rocks in the Virgilina region indicate that rocks remained at elevated temperatures (250 degrees -300 degrees C) until the late Paleozoic ( approximately 290 Ma). Vein formation in the Virgilina district probably resulted from post-Acadian fracturing of hot, deep-seated greenstone units, followed by the generation and migration of metasomatizing fluids that obtained copper during epidotization and alkali-enrichment of greenstones. Migration of these fluids to shalloxver structural levels during retrograde metamorphism resulted in precipitation of copper-bearing vein deposits.

Geochemical constraints on the genesis of the Tellnes ilmenite deposit, Southwest Norway

The ilmenite norite of the Tellnes Fe-Ti deposit in the Aana-Sira massif-type anorthosite is connected to a main dike which varies in composition from monzonorite to quartz mangerite. Fine-grained satellite dikes also occur in the vicinity of the main dike.A number of rock samples have been analyzed for 36 major and trace elements. An Rb-Sr whole-rock isochron (12 samples) gives an age of 930 Ma. Initial Sr isotope compositions of the ilmenite noritc, the main dike, and the satellite dikes are 0.7050, 0.7069, and 0.7100, respectively, thus precluding a comagmatic origin without invoking contamination.The evolution of the main dike can be accounted for by a closed-system fractional crystallization of a monzonoritic magma (that is, without progressive contamination) which gave rise to a series of liquids prior to intrusion. Mass balance calculation permits a two-stage process in which noritic cumulates were subtracted. Trace element behavior is also consistent with such a model.The satellite dikes represent undifferentiated monzonorites, with trace elements and isotopic ratios implying a source material quite different from that of the main dike monzonorite.The orebody is made up of ilmenite norite and small amounts of more plagioclase-rich rocks, which plot on linear trends in variation diagrams. It is interpreted as a noritic cumulate which was enriched in dense, oxide-rich magma before being emplaced as a crystal mush. The parental magma was noritic in composition, unsaturated in apatite, and probably derived from the crystallization of the enclosing massif anorthosite. It is not precluded either that it could have given rise to the main dike monzonoritic liquid after being contaminated by lower crustal material. Nevertheless, as an alternative, it is not possible to reject two distinct parental magmas for the orebody and the main dike. They would have evolved in two separated magma chambers, before the tapping of these chambers and simultaneous injection of their contents in the dike.The main dike illustrates the process by which a quartz mangerite can derive from a monzonoritic magma, and gives support to a comagmatic origin of charnockitic and monzonoritic rocks.

Geochemistry of the Shushui Complex, South Shanxi Province, China

The Shushui Complex can be divided into three rock units based on field investigation, petrography and geochemistry:(1) felsic gneisses, (2) supracrustal rocks consisting of amphibolite, marble and quartzite, and (3) late granites. Of the complex, felsic gneisses are dominant and formed in the Late Archaean, which were intruded by a basic dyke with a whole-rock Rb-Sr isochron age of 2264±219 Ma. The data on rare-earth elements as well as on major and trace elements presented for most of the rock types making up the complex suggest that (1) basic gneisses were produced by partial melting of mantle peridotite, followed by fractional crystallization, and (2) felsic gneisses produced by varying degree of melting of a mafic source. The most suitable tectonic setting to account for the generation of both types would be similar to the underplate setting.

The age and significance of Carboniferous plutons of the western Cobequid Highlands, Nova Scotia

A series of granite plutons with minor gabbro–diorite phases outcrops immediately north of the Cobequid Fault in the western Cobequid Highlands, Nova Scotia. The plutons were cut by numerous dykes, and parts of some plutons and some dykes subsequently underwent ductile deformation. Four Rb–Sr whole-rock–K-feldspar isochron dates ranging from 338 to 356 Ma have been obtained from these plutons and the dykes that cut them. The ages of these plutons are indistinguishable from an age of 341 Ma obtained for the nearby Fountain Lake Group volcanic rocks, within estimated uncertainties. Measured Carboniferous granite ages are uncommon in the Appalachian Orogen of eastern Canada. The geochemical character of these plutons indicates that they result from crustal extension. This extension and the compression observed in the granites with ductile deformation are related to transcurrent motion between the Avalon and Meguma terranes along the Minas fault zone.Preliminary Rb–Sr whole-rock data from poorly exposed plutons in the northern part of the Cobequid Highlands suggest a Middle to Late Devonian or older age. These plutons may represent an earlier phase of crustal extension.

High-resolution 40Ar/39Ar chronology of multiple intrusion igneous complexes

The Mount Brome complex of the Monteregian province of southern Quebec, Canada, consits of several major intrusions ranging compositionally from gabbro to syenite. The relative ages of these intrusives have been investigated with high-resolution 40Ar/39Ar analyses, including a specially designed irradiation configuration to cancel the effects of fluence gradients. Small yet distinct apparent age differences are observed. While a number of analytical and geological factors could be proposed to explain the small variations, evaluation of these suggests the age differences reflect those in emplacement times. The gabbro and nepheline diorite were emplaced within a short span 123.1 Ma ago. Generally more evolved lithologies (biotite monzodiorite, pulaskite, nordmarkite) appear to have been emplaced within a restricted interval 1.4±0.3 Ma later. Whole-rock Rb-Sr systematics do not give acceptable isochrons because of significant scatter interpreted to reflect initial 87Sr/86Sr heterogeneities resulting from crustal contamination. Considering the variations in initial ratio, the Rb-Sr data are consistent with the 40Ar/39Ar age.

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

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

Rb-Sr resetting ages and chemical characterization of turbidites in an accretionary wedge: Torlesse Complex, Otaki Gorge, New Zealand

Whole-rock geochemical and Rb-Sr isotopic analyses have been undertaken on turbidites from three discrete localities, each about 2 km apart across strike, within the Torlesse Complex, a Permian-Cretaceous accretionary wedge in New Zealand. Overall, the three suites show similar petrographic and geochemical compositions to turbidites from elsewhere in the Torlesse Complex, but between the suites, there are detectable differences for most major and trace elements, particularly Sr, MgO, and Na 2 O. The individual suites define three parallel Rb-Sr whole-rock isochrons with identical dates of about 205 Ma. The presence of Monotis bivalves provides a depositional age of Norian (219-225 Ma). Hence, the isochrons suggest that the Rb-Sr clock was reset some 15-20 m.y. after deposition, during low-grade metamorphism associated with accretion and deformation of the sediment pile. There is a wide variation in the initial 87 Sr/ 86 Sr ratios from 0.70683 to 0.70955, which is best explained by changes in the composition of the detritus being provided from the source area, as detected by the chemical differences. Sea water is not considered to have played a significant role in contributing to the initial ratios. The failure of some samples collected away from the main sample localities to plot on their respective isochrons suggests that domains of Sr isotopic equilibration are small, in the order of a few cubic meters. As a result of this study, we have extended the Rb-Sr whole-rock dating method of sediments, now widely applied to shales and cherts, to the more complex turbidite sequences from accretionary wedges and have shown that meaningful dates can be obtained. This should provide an invaluable method of dating these sparsely fossiliferous terranes.