Rb Sr Total-rock Research Articles

UPb and SmN edvidence for Eburnian and Pan-African high-grade metamorphism in cratonic rocks of southern Cameroon

UPb analyses of zircons and SmNd mineral and whole-rock analyses of samples from southern Cameroon document three distinct rock-forming or high-grade metamorphic events: ∼2900 Ma, ∼2050 Ma, and ∼600 Ma. Zircons from the Ebolowa charnockite in southern Cameroon yield an age of 2896±7 Ma, consistent with published RbSr total rock data for the region. SmNd crustal formation ages for these rocks are ∼3000 Ma, indicating that the granitoids and Ntem Series gneisses in the north end of the Congo Craton formed ∼2900 Ma ago and do not contain major contributions from older Archean crust. A second metamorphic event of Eburnian age is identified from re-equilibration of Sm and Nd in gneisses and metadolerites of the Ntem Complex at ∼2050 Ma. Zircons from the younger Nyong Series gneisses are a complex mixture of cores and overgrowths. The overgrowths in most cases formed during a high-grade metamorphic event between 2100 and 2000 Ma and clearly postdate the Ebolowa charnockite. The ages of the cores are not precisely known, but we believe most represent detrital zircons from 2900 Ma protoliths. SmNd crustal formation ages for samples of the Nyong Series gneisses are generally 2500 to 2900 Ma, indicating that their precursors were apparently derived mainly from the Archean craton with variable amounts of younger component. SmNd garnet-total rock isochrons for metamorphosed mafic rocks in the Nyong Series yield ages of ∼2050 Ma, and zircons from one of them yield an upper intercept age of 2037±10 Ma, further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 Ma. We interpret the Nyong Series metasedimentary gneisses as post-2900 Ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 Ma Eburnian orogeny; we cannot define precise limits on the ages of sedimentation. Preliminary data on metamorphosed intrusive rocks associated with the Yaoundé Series yield UPb concordant ages around 620 Ma, consistent with SmNd garnet—whole-rock ages from the same series. This dates the Pan African tectono-metamorphic activity in the southern end mobile belt. The Pan African lower intercepts of zircons and SmNd garnet-whole-rock ages from thNyong Series are the consequences of this event, which is further represented by a ∼590 Ma zircon age from a late syenitic pluton. Results from Cameroon are consistent with recent results from the western part of the Congo Craton farther south and from the northeastern part of the São Francisco Craton in Brazil. One interpretation of these results is that the Congo and São Francisco cratons were joined during the Ebumian-Trans-Amazonian orogeny ≈2100 Ma and are bordered to the north by a late Mesoproterozoic rift basin that was closed during the Pan-African-Brasiliano orogeny ∼600 Ma.

The evolution of a calc-alkaline basic to silicic magma system: Geochemical and Rb-Sr, Sm-Nd, and [formula omitted] isotopic evidence from the Late Hercynian Atesina-Cima d'Asta volcano-plutonic complex, northern Italy

Geochemical and Sr-Nd-O isotopic data presented for basaltic andesitic to rhyolitic and for quartz noritic to monzogranitic rock suites from the Late Hercynian calc-alkaline Atesina volcanic complex (AVC) and the Cima d'Asta pluton (CAP), Southern Alps (northern Italy), provide information on both the primary magmatic processes and the effects of (mainly Triassic) hydrothermal overprint. Fluid infiltration led to mobilization of major and trace elements (K 2O, Na 2O, CaO, Rb, Sr, and Ba), opensystem behavior in total-rock Rb-Sr, and shift in δ 18 O to elevated values (total rock up to 16.6%. and volcanic matrix up to 17.8%.). Oxygen isotopic disequilibrium between quartz-feldspar pairs suggests water-rock interaction at medium/low temperatures. The δ 18 O values of quartz, the REE characterized by regular LREE enrichment/HREE depletion, and the Sm-Nd isotopic signatures, however, remained virtually unaffected by secondary processes. The initial ϵ Nd values (at 270 Ma) of the AVC and CAP magmatites are restricted to overlapping ranges of −3.6 to −6.5 and of −2.7 to −6.5, respectively, indicating significant crustal contribution; these values and associated T DM model ages of 1.1–1.6 Ga agree well with those of typical South Alpine lower crustal magmatites. The AVC and CAP rocks do not follow the “normal” trend of increasingly crustal Nd isotopic signatures with progressive degree of magma evolution expected for a single-stage AFC-type process, but instead display an inversion of this relationship. Geochemical and isotopic constraints favor a model of a large-scale MASH-type melting and mixing zone at or near the base of the continental crust. Distinct elemental enrichment/depletion and REE crossover patterns displayed by high-silica as compared to less silicic AVC rhyolites suggest subsequent magma evolution within a shallow-level compositionally zoned chamber.

UPb and RbSr geochronology and geological evolution of the Harts Range ruby mine area of the Arunta Inlier, central Australia

Supracrustal and meta-igneous rock units from the ruby mine area of the Harts Range, eastern Arunta Inlier, central Australia, have been dated by the zircon UPb and RbSr total-rock (TR) and mica methods. A well-defined zircon discordia for a weakly deformed specimen of the Bruna granite gneiss yields an age of emplacement of 1748 −4 +5 Ma, thereby constraining the minimum age of the Irindina supracrustal assemblage. Metapelitic gneiss within the supracrustals and a meta-igneous ultramafic boudin from the associated Harts Range meta-igneous complex yield highly discordant zircon data, revealing a strong early Palaeozoic overprint. RbSr TR data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed, also apparently during the lower Palaeozoic. However, RbSr model age calculations and the zircon UPb data suggest a maximum age of about 2000 Ma for the supracrustal and meta-igneous rocks, and argue for new Proterozoic crust formation. Zircon UPb data from a deformed pegmatite, emplaced in the meta-igneous complex, yield an emplacement age of 520 −4 +5 Ma, further pointing to Lower Palaeozoic magmatism and deformation. Correlations of U content and calculated 206 Pb 238 U age for the ultramafic boudin zircons suggest that new growth of low-U zircons occurred during retrogression associated with this event. The Sr-isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of Palaeozoic Sr. Our data suggest lower Palaeozoic (possibly Delamerian) tectonothermal activity to be more important in the evolution of the Harts Range area than previously recognised. On the other hand, RbSr mica ages for deformed and undeformed pegmatites, and TR isochrons for the latter, show that pervasive tectonothernal activity had ceased by about 315 Ma and that regional cooling occurred between about 345 and 325 Ma. Local shear-zone biotite resetting may have persisted to about 300 Ma, consistent with the previously recognised Alice Springs Orogeny. Possible dilational Pb loss in the Bruna zircons occurred at about 103 Ma.

Late Archaean granites of the Napier Complex, Enderby Land, Antarctica: A comparison of Rb-Sr, Sm-Nd and U-Pb isotopic systematics in a complex terrain

Abstract Late Archaean granites have been identified within the northern part of the Napier Complex of Enderby Land, Antarctica, although intrusives of this age are not common elsewhere in the complex. The oldest intrusive (3070 ± 34 Ma), synorogenic granite at Proclamation Island, was probably derived from melting of felsic crustal rocks of igneous origin. Two younger granites were emplaced at about 2840 Ma and 2481 ± 3 Ma, and the latter has geochemical similarities with other late and post-orogenic intrusives elsewhere in the East Antarctic Shield. Because of their fractionated character, it is difficult to discern whether the granites were derived by melting of sedimentary or igneous protoliths, but a granulite-facies source is probable. TNdCHUR model ages of the three analysed granites suggest that the Napier Complex is the product of at least two temporally discrete episodes of continental crust formation. The granite precursors probably formed at about 3100 Ma, whereas most of the exposed Napier Complex is much older (back to almost 4000 Ma). Two of the granites appear to record a significant hiatus (in one case of about 600 Ma) between the formation of this crustal precursor and final emplacement. The 3100 Ma old episode of crustal formation was roughly synchronous with a widespread, intense, and high-grade tectonothermal event (D1-M1), which produced metamorphic fabrics and mineral assemblages that are widely preserved within the Napier Complex. It is therefore likely that this new crust formation, deformation and metamorphism are all attributable to a single tectonic episode, possibly related to emplacement of magma into the lower crust. The two younger granites were emplaced at about the time of two subsequent and less intense tectonothermal events (D2-M2 and D3-M3). Concordant Rb-Sr total-rock and U-Pb zircon ages, which are interpreted as emplacement ages, have been obtained for two of the intrusions. However, the third exhibits the unusual relationship of a Rb-Sr isochron age older than the U-Pb zircon age. In this case the Rb-Sr age is believed to date magmatic emplacement. The complex interpretation required for the zircon data is forewarned by a non-perfect alignment of analytical points, in contrast to the perfect analytical alignments produced by the isotopically coherent granites. Two of the three granites yield similar and relatively precise lower intercepts with the U-Pb concordia but these do not appear to have direct geological significance. The new isotopic data are combined with earlier results to derive an integrated tectonothermal evolution for the Napier Complex.

Subdivision and correlation of late Precambrian sequences in Australia

The diversity of the Australian Precambrian record is used to empirically assess the application of all available techniques of correlation of late Precambrian rocks — lithofacies, isotopic ages, palaeontology, and palaeomagnetism — within the framework of latest lithostratigraphic/geochronologic correlation charts. Lithostratigraphy is the main tool for intrabasinal correlation and mapping, but isotopic ages will continue for some time yet to provide the main criteria for interregional correlation. Lithostratigraphic correlation is demonstrated over hundreds of kilometres, but is generally constrained by tectonic controls to individual major crustal blocks. Climatic markers, such as tillites, may be correlated interregionally over thousands of kilometres. The most exciting recent development in platform geochronology is the application of U—Pb zircon dating to fine-grained tuffs. Illite may provide reliable minimum estimates, but can be shown to be up to 150 Ma too young. Glauconite and Rb—Sr total-rock shale ages have provided useful broad age estimates of sequences for which better methods are unavailable, but only when based on cautious assessment and multiple sampling; their ages may be either minimum or maximum. Stromatolites continue to demonstrate their potential for broad correlation when based on rigorous taxonomic study, but the framework of time ranges of known taxa must be expanded for stromatolites to achieve their full potential, especially for rocks older than 1000 Ma. Acritarchs in black shales provide diverse and complex assemblages, with excellent correlation potential, but once again the Australian taxonomic framework is still inadequate. Black cherts contain an evolutionarily conservative biota, with only limited correlation potential. Recent palaeomagnetic studies, based on both APWP and magnetostratigraphy, demonstrate excellent potential for detailed correlations, down to at least 1700 Ma old. However, the present chronologically-calibrated data base is sparse and must be supplemented by many more detailed studies through thick continuous rock sequences, spanning substantial intervals of time. The value of multidisciplinary correlation is elegantly demonstrated by very detailed and unambiguous correlation of combined palaeomagnetic, isotopic age, and stromatolite data across the 1700 Ma McArthur Basin. Previous regional lithostratigraphic correlations have been conclusively and significantly revised, with major implications to regional palaeogeography.

U-Pb zircon geochronology of late Archaean metamorphic rocks in the Taihangshan—Wutaishan area, North China

A geochronological study has been made in the Taihangshan—Wutaishan area, Shanxi Province, on a sequence of Precambrian supracrustal rocks and stratigraphically related intrusives. These constitute part of the North China basement block.The oldest rocks of the Fuping Group consist predominantly of mafic granulites, paragneisses, quartzites and amphibolites. This Group was always considered Archaean, and early K-Ar measurements yielded some minimum ages as old as 2300 Ma and others as young as 1500–1900 Ma. The unconformably overlying Wutai Group is considered as the base of the Proterozoic succession and consists of amphibolite-grade paragneisses, carbonates and amphibolites in its lower part, and greenschist-grade clastic sediments and volcanics in its upper part. It has given apparent K-Ar mineral ages between 1500 and 2000 Ma.Present investigations have employed the U-Pb zircon approach on selected parts of the sequences to provide direct depositional ages or age constraints for the Fuping and Wutai Groups and to find the age(s) of subsequent metamorphic processes. A minimum age for the Fuping Group and maximum age for the Wutai Group has been derived from zircon age measurements on the Lanzhishan Granite (2560 ± 9 Ma) which intrudes the Fuping Group and is unconformably overlain by the Wutai Group. This late Archaean result can be interpreted as the age of an igneous crystallization event and therefore a maximum for the depositional age of the Wutai Group.The above result is well substantiated by zircon dating of the Wutai Group itself. Zircons from a low-grade metamorphosed keratophyric lava in the upper part of the Group give a volcanic crystallization age of 2520 ± 17 Ma. This provides close constraints for the age of deposition of the Wutai Group at between 2560 and 2520 Ma ago. If the Wutai Group is indeed basal Proterozoic, the Archaean—Proterozoic boundary in this part of China is also dated at between 2560 and 2520 Ma. Further evidence as to the antiquity of the Wutai Group comes from a U-Pb zircon age of 2520 ± 30 Ma for the Ekou Granite which intrudes the Wutai Group sequence.The age of the underlying late Archaean rocks in the Tainhangshan—Wutaishan area, the Fuping Group, could not be directly determined because of the paucity of volcanic markers and the higher grade of metamorphism. Its minimum age is 2560 Ma (see above), but only an approximate maximum age can be gleaned from the zircon systematics in granulite and amphibolite-grade paragneiss samples. These data indicate a degree of Pb-inheritance which, combined with metamorphic disturbance and more recent Pb loss, inhibit any further interpretation other than that they indicate an approximate maximum age of around 2800 Ma.Several of the paragneisses and one amphibolitic schist contain zircon systems which are interpreted as having completely responded to a well-defined amphibolite facies metamorphic event at ∼2500 Ma ago. A younger but less well-defined metamorphism caused complete Pb loss from zircon and recrystallization in the amphibolite and granulite facies terrains at between 2000 and 1800 Ma ago. Rocks which show evidence of this younger metamorphic event were not recognized from the field or structural data base, but from the distribution of the few such samples analyzed it seems possible that this younger terrain may now be interpretable as a cross-cutting Proterozoic mobile belt. This Proterozoic metamorphic province is also reflected by K-Ar mica ages between 1600 and 2000 Ma and some preliminary Rb-Sr total-rock ages at ∼1700 Ma on nearby paragneisses whose U-Pb zircon age is 2500 Ma.

Possible two-stage 87Sr evolution in the Stockdale Rhyolite

The Rb-Sr total-rock data for the Stockdale Rhyolite, of significance for the Palaeozoic time scale, are more scattered about a single-stage isochron than expected from experimental error. Two-stage 87 Sr evolution for several of the samples is explored to explain this, as an alternative to variation in the initial 87 Sr/ 86 Sr which is customarily used in single-stage dating models. The deletion of certain samples having very high Rb/Sr removes most of the excess scatter and leads to an estimate of 430 ± 7 m.y. for the age of extrusion. There is a younger alignment of Rb-Sr data within each sampling site at 412 ± 7 m.y. We suggest that the Stockdale Rhyolite is at least 430 m.y. old, that its original range in Rb/Sr was smaller than now observed, and that it experienced a net loss in Sr during later hydrothermal alteration at ca. 412 m.y.

Isotopic dating and structural relationships of granitoids and greenstones in the East Pilbara, Western Australia

A structural RbSr isotopic study has been made on two small areas in the eastern Pilbara block, Western Australia. The sites were chosen because they showed good interrelationships of granitoid and greenstone units. At Warrery Gap, on the western side of the Corunna granitic dome, the acid volcanics of the Duffer Formation (at the top of the lower Talga-Talga Subgroup of the Warrawoona Group) show some updating, but a good 3506 ± 62 Ma isochron, with initial ratio (IR) of 0.7006 ± 0.0011, was recognized, consistent with the 3452 ± 16 Ma zircon measurement of Pidgeon (1978b). Both a penetrative D 1 deformation, and the doming of D 2 preceded intrusion of late tectonic granitoids, dated at 3270 ± 22 Ma (IR = 0.7015 ± 0.0003), into both the Duffer Formation and the overlying pillow lavas of the Salgash Subgroup. Thus, the Salgash Subgroup is much older than suggested by Glikson (1979) and the stratigraphic succession cannot contain a hiatus between the apparently conformable Talga-Talga and Salgash Subgroups of the magnitude he proposed. The granitic domes clearly owe their form to the D 2 deformation rather than to batholitic intrusion, but near horizontal structural lineations suggest that they were not formed by diapiric movements. A granodiorite and pink feldspar granite from just within the Corunna granitic dome are slightly deformed: pooled isochrons indicate an age of 3232 ± 27 Ma but different IRs of 0.7032 and 0.7009, respectively. At Tambourah, in the eastern Shaw granitic dome, local D 2 also deforms an intrusive microadamellite of age 3087 ± 34 Ma and IR = 0.7103 ± 0.0057. There is therefore a real spread in ages of D 2 granitoids and D 2 deformation between about 3300 and 3100 Ma. Layered megacrystic gneiss, at Tambourah, also intruded by the microadamellite, contains a nebulous foliation argued to be local S 1 . Layered and homogeneous megacrystic gneiss produce updated RbSr total rock isochrons of 2995 ± 95 and 2779 ± 38 Ma, respectively. The primary age of these D 1 gneisses is clearly greater than that of the D 2 granitoids and is probably indicated by Pidgeon's (1978c) zircon age of 3417 ± 40 Ma from the Shaw granitic dome. If so, Hickman's (1975) “Migmatite Suite” contains both D 1 gneisses and D 2 granitoids separated in age at Tambourah by ca. 300 Ma, although neither appears to be older than the lower part of the preserved layered sequence. The protocrust on which that sequence was deposited has yet to be directly identified.

Lu-Hf total-rock age for the Amîtsoq gneisses, West Greenland

Lu-Hf total-rock data for the Amîtsoq gneisses of West Greenland yield an age of 3.55±0.22Gy(2σ), based on the decay constant λ 176Lu=1.96×10 −11y −1, and an initial 176Hf/ 177Hf ratio of 0.280482±33. The result is in good agreement with Rb-Sr total-rock and U-Pb zircon ages. In spite of severe metamorphism of the area at 2.9 Gy, zircons from two of the samples have remained on the total-rock line, and define points close to the initial Hf ratio. The initial 176Hf/ 177Hf lies close to a chondritic Hf isotopic evolution curve from 4.55 Gy to present. This is consistent with the igneous precursors to the Amîtsoq gneisses having been derived from the mantle at or shortly before 3.6 Gy. Anomalous relationships between Hf concentration and the 176Lu/ 177Hf ratio may suggest that trace element abundances in the Amîtsoq gneisses are partly controlled by processes related to metamorphism.

Age relationships in the proterozoic high-grade gneiss regions of southern Norway

Abstract The Proterozoic high-grade gneiss regions of southern Norway (Bamble, Kongsberg, Rogaland and Ostfold) historically belong to the so-called Sveconorwegian (1.2—0.9 Ga) zone. This has long been correlated with the Grenville province of North America, and features prominently in Proterozoic reconstructions across the North Atlantic. Several recent radiometric studies have provided compelling evidence for an earlier high-grade gneiss-forming event at 1.6—1.5 Ga ago in parts of southern Norway. It has also been shown that at least a section of the Bamble sector suffered only a relatively low-grade thermal—deformational event during the Sveconorwegian cycle. In this paper new RbSr age determinations for metasediments and an anatectic granite from the same area confirm a pre-Grenvillian age for the high P—T event and preclude the possibility of later high-grade reworking. Previously published age determinations for all sectors are compared with the available radiometric data from the adjoining gneissic terrane of southwest Sweden, and reviewed in the light of the new data from Bamble and recent evidence that relatively low-grade events are capable of disturbing, and possibly resetting, both RbSr total-rock and UThPb zircon isotope systems. The conclusions are that the main high-grade gneiss-forming event may have been pre-Grenvillian in all sectors, and that there may have been no regional high-grade reworking during the period 1.2—0.9 Ga ago.

Secondary geologically meaningless RbSr isochrons, low 87Sr/ 86Sr initial ratios and crustal residence times of high-grade gneisses

Dark green quartzo-feldspathic charnockitic ortho-gneisses from Arendal, southern Norway, have locally been decoloured in the vicinities of intrusive, undeformed granite sheets and pegmatite dikes. The result is a series of pink rocks which are macroscopically indistinguishable from primary augen granite gneisses. The main mineralogical changes associated with the secondary decolcuration occurred under relatively low-grade conditions, and mainly involved chlorite, serpentine ± actinolitic amphibole ± biotite growth. The alterations were initiated along narrow, irregularly spaced cracks. The primary ( ∼ 1540 m.y. ) total rock RbSr isotopic systems which relate to the charnockite mineralogy were disturbed, and sometimes reset, during the secondary event which occurred at ∼ 1060 m.y . Locality by locality, the decoloured samples yield linear arrays corresponding to 1457 ± 200 m.y., 1303 ± 50 m.y. and 1125 ± 185 m.y. The combined data ( n = 19) give an apparent age of 1364 ± 43 m.y. with an 87Sr/ 86Sr initial ratio of 0.7011 ± 0.0018. This apparent age is unrelated to any geological event and there is no overlap at the 2δ level between the 87Sr/ 86Sr initial ratio and that defined by the primary event ( 0.70345 ± 0.00014). Any estimate of crustal residence time based on these decoloured samples would be in error. The weight of evidence is against significant involvement of Rb mobility in the isotopic resetting process which accompanied decolouration. A model involving modification of the 87Sr/ 86Sr ratios is preferred.

The 2.5 Ga Duxbury massif, Quebec : a remobilized piece of pre-3.0 Ga sialic basement(?)

The Duxbury massif, Eastmain District, Quebec, is a complex tonalitic intrusion that is internally divisible into a homogeneous border zone of granodiorite (Rb–Sr total-rock isochron age = 2500 ± 85 Ma; initial 87Sr/86Sr = 0.7023 ± 0.0003) and a heterogeneous core zone of tonalite (Rb–Sr total-rock errorchron age = 3060 ± 180 Ma; initial 87Sr/86Sr = 0.7014 ± 0.0003). The geologic, petrographic, and isotopic data are consistent with a model of remobilization of 3.1 Ga sialic basement during the Kenoran orogeny at 2.5 Ga. Evidence suggests that the source material was of tonalitic composition, that it was never more deeply buried than 15–20 km, and that it was remobilized in a plastic to brittle subsolidus state. Slab-sample studies of the tonalite indicate the 3.1 Ga age is a metamorphic one, and that the source material had a pre-history at that time. Sr-isotope constraints imply source region formation as juvenile material from the mantle at between 3.5 and 3.6 Ga. The Duxbury massif, hence, records a history of crustal development spanning at least 600 Ma and possibly more than 1 Ga. These results may be applicable on a larger scale to many of the regional tonalite massives found in the Superior Province of Canada and elsewhere.

A Rb-Sr age and isotope study of the Ikertôq, Nordre Strømfjord, and Evighedsfjord shear belts, West Greenland - outline and preliminary results

In the 1977 and 1978 field seasons, samples were collected from both the Ikertôq and Nordre Strømfjord shear belts and adjacent unsheared terrains. The samples represent the main rock-forming and shear zone-forming events in these two areas. In addition, samples for preliminary study were collected by S. B. Jensen and J. Allaart from sheared and unsheared gneisses in the Evighedsfjord region. Rb-Sr total rock analyses will be carried out on all samples. Suitably banded samples of both sheared and unsheared gneisses will be cut along compositionallayering to examine the Rb-Sr system on a small whole rock scale. The main Sr-bearing mineral phases of samples throughout the study area will also be analyzed. The purpose of this study is threefold. First, it will establish the geochronological frarnework of the main rock-forming and shear zone-forming events in this region. Second, it will help to clarify the thermo-tectonic history of the study area (especially in its first stages), mainly through the use of Rb-Sr mineral analyses. Two mineral age traverses will be produced, one north-south traverse along the coast between Agto and Evighedsfjord, and another east-west traverse from the coast 70 km inland along Nordre Strømfjord. The third aim of the study is to investigate the response of the Rb-Sr isotope system to intense deformation in the shear belts. This region lends itseif to such an investigation for two main reasons: (a) the age(s) of deformation ean be independently established using synto late-tectonic intrusive rocks; (b) the time period between the origin of the gneisses and the formation of the shear belts is long, from several hundred to 1000 m.y., and thus sufficient for significant isotopic heterogeneity to develop prior to deformation.

A geologically meaningless Rb–Sr total rock isochron

A major objective in total rock Rb–Sr isotope studies of relatively homogeneous igneous or high-grade metamorphic gneisses is to obtain as wide a range of 87Rb/86Sr ratios as possible, so that the resultant isochron may be well defined1. This is often achieved by collecting samples at some distance from each other. Recent studies2–6 have shown that primary Rb–Sr isotopic systems can be profoundly disturbed, and even completely reset, in response even to low-grade secondary deformations and/or mineral alterations, and in three cases2–4 isochron diagrams based on regionally collected samples have been shown to yield linear arrays corresponding to apparent ages intermediate between the true ages of the primary and secondary events. The evidence we present here shows that it is also possible for well defined intermediate isochron apparent ages to be produced where this sampling technique is used on rock suites which show evidence of only minor secondary alteration effects. An isochron so produced is geologically meaningless, that is, the apparent age is unrelated to any geological event.

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

The quartzo-feldspathic charnockitic orthogneisses within the Bamble sector of the so-called Sveconorwegian (1.2–0.9 b.y.) zone are highly fractionated in K and Rb such that they comprise two chemically contrasting zones — one highly K, Rb-deficient and the other with values of the same order as upper crustal lithologies. Eight series of samples, each collected from single outcrops, have yielded Rb-Sr total rock apparent ages in two distinct groups, at ∼1540 and ∼1060 m.y. Outcrops in both the K-deficient and normal-K suites have produced examples of each age. The older age relates to the high-grade charnockite event, and the younger to a superimposed low-grade event which occurred at the same time as the intrusion of undeformed granite sheets and pegmatite dikes; one of the granites has yielded an isochron age of 1063 ± 20 m.y. The low-grade event involved only the partial alteration of orthopyroxenes to chlorite and/or serpentine, coupled with some corrosion of biotite; the alterations were initiated along narrow, irregularly spaced, cracks and it was their development which facilitated open system behaviour of the total rock isotopic systems at some localities. The degree of rehomogenisation is a function of the intensity of the secondary alterations. Confirmation of resetting at ∼1060 m.y. is given by four mineral + host rock isochrons all yielding ages within error of the age for the intrusive granite; two of these are from outcrops where the rocks retain the older ∼1540-m.y. age. The secondary total rock isotopic homogenisation cannot be explained adequately by Rb mobility or by simple mixing with a fluid having its own initial 87Sr/ 86Sr composition. The primary mineralogy may have determined whether individual localities and/or samples suffered net increases or net decreases in 87Sr/ 86Sr. An important implication of the results is that in this, or any similar geological situation, there would be a very real possibility of drawing erroneous conclusions from regionally-collected samples, particularly if the full significance of the later, relatively minor P-T event remained undetected and/or the scale of isotopic (re-)homogenisation, were unknown. It is only because of the methods adopted that it can be stated that there is no isotopic evidence for a high grade Sveconorwegian (Grenvillian) event in this part of southern Norway.

The geochronology of Iqna Granite (wadi Kid pluton), Southern Sinai

The wadi Kid pluton of Iqna Granite, Southern Sinai, which was intruded during the last Precambrian magmatic phase, yields a Rb-Sr total rock isocrhon age of 580±23 m.y., and an initial 87Sr/ 86Sr ratio of 0.7028±0.0028. The magma of the Iqna Granite was derived from a low Rb/Sr source shortly before its crystallization. Partial resetting of biotite ages is detected by both Rb-Sr and K-Ar methods. Mineral isochrons yield higher initial values (0.7045–0.7065) as a result of Sr isotopic redistribution within a closed total rock system.

Geochemistry of Archaean Granites of the Indian Shield: A Review

Abstract Archaean granites and granite gneisses are known to occur in (a) the Iron Ore Province of Singhbhum-Orissa region in eastern India, (b) the Granite-Greenstone Belt of Karnataka and Andhra Pradesh, (c) the Granulite Belts of Tamil Nadu and Kerala and (d) the Bundelkhand-Rajasthan Precambrian province. In the Iron Ore Province of Singhbhum region, the Older Metamorphic tonalite-gneiss (OMTG-c. 3200 Ma) has strong geochemical resemblance with the 'Ancient tonalite-gneiss' of Barberton. South Africa. The low initial 87Sr/86Sr ratios (0.7018-703) for the total-rock Rb-Sr isochrons indicate derivation of the tonalitic melt from the mantle perhaps in a two-stage process. The magmatic units of the composite batholith of Singhbhum Granite (2900-3000 Ma) show distinctive trends of fractional crystallisation for the three successive but closely related phases. Multivariate analysis of variance of the trace-element data indicates statistically significant differences among the phases while Rsmode Factor analysis suggests operation of significantly different processes of trace-element distribution in the three phases. The geochemical relationship as well as the high initial 87Sr/86Sr ratio of 0.711 ± .009 suggest that the three magmatic phases were derived by partial melting successively in the same source region of the crust. The Bahalda metasomatic unit of Singhbhum Granite shows certain abnormal trends of trace-element variations in the sequence: hornblende schist?hornblende diorite?shornblende (-epidote) granodiorite. These have been explained by assuming that during progressive granitisation, (a) ionic charge and electronegativity have been relatively ineffective and (b) certain trace-elements are preferentially incorporated in some preexisting minerals. The Peninsular Gneiss and associated Archaean granitic bodies in the Granite Greenstone Belt of South India show wide compositional variations ranging from migmatitic and tonalitic gneiss through granodiorite to K-rich adamellite. These rocks show clusters of Rb-Sr isochrons at 3200-3300Ma, 2900-3000Ma and around 2600Ma. Enrichment of nickel and chromium in parts of these gneisses has been reported. Although a metasomatic origin of the Peninsular Gneiss has been suggested, the low initial 87Sr/86Sr ratios (0.701-0.704) do not support this view and instead indicate direct or multi-stage derivation of the granitic melt from the mantle. Occurrence of several abnormal U-Th enriched granite bodies within Peninsular Gneiss is of special interest. The Bundelkhand Granite is a composite body of several granitic and gneissic units and appears to be dominantly adamellitic - monzonitic in composition (2555 ± 55 Ma), but has numerous xenoliths and rafts of migmatite and gneiss, mostly of granodiorite and tonalite composition. It appears that in the Indian shield, the oldest gneisses and granites (> 3000 Ma) are dominantly tonalites while adamellitic granites become more and more abundant in the younger assemblages.

Geochronology of discrete structural-metamorphic events in a multiply deformed precambrian terrain

Rb—Sr total-rock and mineral studies are combined with detailed structural-metamorphic work to investigate polymetamorphism within the Georgetown Inlier of north Queensland, Australia. A sequence of five major, temporally discrete, regional structural events is established. The earliest events, which were marked by tight folding and prograde metamorphism, occurred at 1570 ± 20 and 1469 + 20 m.y. Subsequent deformations were less intense, and generally associated with retrogressive metamorphism. The third is documented by Rb—Sr total-rock isochrons at 967 ± 28 m.y., and the fourth by mineral and total-rock studies at about 400 m.y. The last deformation is inferred to have occurred at about 300 m.y., a time of extensive igneous activity in the east of the area. The isotopic data indicate that Sr isotopic homogenisation is approached on the scale of a cubic metre during amphibolite-facies regional metamorphism and deformation. Consequently, the Rb—Sr isochron age of a metamorphic unit is highly dependent on its deformation-metamorphic history, and in particular its last significant metamorphism; a structural-metamorphic event which forms a prominent schistosity appears to be an important requisite. The overall success of the method demonstrates its potential in unravelling the geological history of other polymetamorphic terrains.

Total rock Rb [sbnd]Sr and U [sbnd]Th [sbnd]Pb isotopic study of Precambrian metavolcanic rocks in the lower Orange River region, southern Africa

Rubidium, Sr, U, Th and Pb concentrations and Sr and Pb isotope data are presented for a suite of Precambrian metavolcanic rocks cropping out in the lower Orange River region of southern Africa. Good agreement between total rock Rb-Sr, Th-Pb and Pb/Pb ages obtained indicates an extrusion age of 1996 ± 15Ma. The total rock U-Pb system was disturbed in recent times and ages obtained are unreliable. Parental magmas to the volcanic rocks and associated plutonic intrusives were probably derived from the upper mantle, which indicates that a major crust producing event may have taken place in the lower Orange River region during the mid-Proterozoic (2.0–1.7 Ga). Reworking of part of this older crust during a 1.2-1.0-Ga event to produce gneisses within the neighbouring Namaqua Province is supported by Sr isotopes. Initial Sr and Pb in the lavas was probably derived from a mantle source that experienced a previous history of slightly high Rb/Sr and U/Pb, relative to postulated bulk earth evolution. Enrichment in Rb/Sr for the subcratonic mantle under southern Africa was previously known, and the results of this study indicate a similar pattern for U/Pb.

Rubidium-strontium geochronology of a transect of the Chilean Andes between latitudes 45° and 46° S

Plutons of quartz diorite to adamellite composition give Rb-Sr total-rock and biotite calculated ages of 12 to 176 m.y. along a transect of the Chilean Andes between 45° and 46° S. Miocene biotite dates of quartz diorite from the Puerto Aisen area are interpreted as minimum ages related to probable local structural deformation (faulting). Along a north-south traverse of the Argentine Andes between 40° and 44° S, plutons of similar composition to the Chilean Andes give K Ar and Rb Sr total-rock and mineral dates which range from late Paleozoic to Cretaceous. No systematic unidirectional migration of the radiometric ages is observed.