Isotopic dating of pre-Alpidic rocks from the island of Ios (Cyclades,Greece)

The lower tectonic unit of Ios provides evidence of an at least four stage metamorphic and intrusive history which well might be generalized for large parts of the internal Pelagonian.Metamorphic country rocks of unknown age were intruded about 500 Ma ago, as concluded from a Rb-Sr whole rock (WR) isochron on relic tonalites to granodiorites which largely escaped the polyphase postmagmatic overprints.A Hercynian amphibolite facies metamorphism, during which the igneous rocks were partly recrystallized to orthogneisses, is dated by a lower intercept age of 300–305 Ma of U-Pb determinations on zircons and by three almost concordant Rb-Sr muscovite-WR ages of 295 to 288 Ma.K-Ar analyses on these muscovites and on biotites, and Rb-Sr tie lines WR-biotite and WR with other relic magmatic minerals yielded various apparent ages between 260 and 60 Ma. They are interpreted as mixed ages between a Hercynian cooling age and the two stage Alpidic overprints.White micas formed during the Eocene high P/T and/or Oligocene/Miocene Barrovian-type overprints yielded K-Ar dates ranging from 82 to 26 Ma, as well as a single Rb-Sr date of 13 Ma. These Alpidic dates resemble the more detailed age patterns of other Cycladic islands. But they are not sufficient for an independent dating of the Tertiary evolution on Ios island.     

Isotopic dating of the emplacement of the ultramafic masses in the Serrania de Ronda,Southern Spain

A Rb-Sr analysis of suites of samples from a small intrusion of cordierite-bearing alkali granite into the peridotite of the Sierra Bermeja (Serrania de Ronda) yields an age of 22± 4 Ma ( = 1.42×10^–11 a^–1): Late Oligocene/Early Miocene. It is believed that the intrusion was derived from contact-anatectic melts produced along the hot ultramafic mass during and/or directly following its tangential, tectonic dislocation from a mantle diapir. Its age can thus be taken as dating the termination of the hot emplacement of the ultramafic masses. K-Ar dates of biotites and Rb-Sr dates of biotite/whole-rock pairs in contact-metamorphic wall rocks along the ultramafics mostly lie between 19.5 and 18.5 Ma. This probably indicates that about 19 Ma ago the contact-zones of the ultramafic masses had cooled down to the blocking temperature of biotite to Rb-Sr and K-Ar.     

Time calibration of a PT-path from the Western Tauern Window,Eastern Alps: the problem of closure temperatures

New Hornblende K-Ar and ³⁹Ar-⁴⁰Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess ⁴⁰Ar, the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500–540° C (Landshuter Hütte).The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)>hornblende K-Ar (18 Ma)>phengite K-Ar (15 Ma)>biotite Rb-Sr, K-Ar (13.3 Ma)>apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (T _c) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the T _c sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and : (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6 mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (T _c=520° C) and biotite (T _c=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.     

Paleozoic migmatites affected by high-grade tertiary metamorphism in the central Alps (Valle Bodengo,Italy)

The Lepontine Gneiss Complex of southern Switzerland and northern Italy is characterized by high-grade metamorphism and intensive deformation of Alpine age with migmatites prevalent in the area with the highest metamorphic grade. Petrological and structural observations are generally inconclusive but indicate in some places an Alpine age for the migmatite formation. To determine the time of migmatite formation a geochronologic study was undertaken in one of the best exposed areas, the Valle Bodengo, Italy. Rb-Sr whole-rock errorchrons of intrusive migmatite phases and of two rather homogeneous granitoid gneiss bodies yield apparent ages between 280 and 350 m.y. They suggest a Hercynian or older igneous history for these rocks. The U-Pb ages of the euhedral zircons are highly discordant, but they do point to the presence of zircon components more than 450 m.y. old. The concordia-intercept ages are incompatible with the Rb-Sr data and the low initial ⁸⁷Sr/⁸⁶Sr ratios of about 0.706. These low initial ratios suggest that either the bulk of the granitoid material is not much older than Hercynian, or older crustal material was isotopically homogenized on a regional scale with rocks that had low Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios (e.g. the lower crust or upper mantle) during a Hercynian metamorphism. Rb-Sr small-scale whole-rock isochrons and tie lines of adjacent, lithologically different rock phases give Alpine ages, the best isochron yielding 22 m.y. This coincides with concordant U-Pb ages of monazites of 23 to 24 m.y. Rb-Sr mineral isoohrons (muscovite, biotite, feldspars, apatite) give ages of 18–21 m.y. Our interpretation is that this age pattern resulted due to rapid cooling after the climax of the last phase of the Alpine metamorphism and we conclude that high-grade metamorphic conditions existed during the upper Oligocene or early Miocene. Other investigators have suggested that the Alpine metamorphism had a climax 35–40 m.y. ago and that the younger mineral ages are a result of simple continuous cooling due to uplift. Based on this study and other recent geochronological studies in the Lepotine Gneiss Complex we suggest that there had to be a thermal maximum at about 20–25 m.y. The example of Valle Bodengo demonstrates that the areal coincidence of the zone of highest-grade metamorphism with the occurrence of migmatites does not necessarily mean that metamorphism and migmatite formation were coeval and related to each other.     

Rb-Sr dating of the impact melt from East Clearwater,Quebec

The Rb-Sr composition of eight melt rock and three basement samples from the East Clearwater impact structure, Quebec, and two basement samples from the West Clearwater structure has been determined. The whole rock ⁸⁷Sr/⁸⁶Sr ratios of the melt samples, 0.7167–0.7253, are within the range of the basement samples, 0.7054–0.7322, and provide further evidence that the melt rocks represent shock-melted basement. A mineral isochron obtained from a relatively coarse grained melt rock gives an age of 287±26 Ma for the crystallization age of the melt. This is equivalent to K-Ar whole-rock ages of 285±30 Ma and 300±30 Ma and a Rb-Sr age of 266±15 Ma obtained on melt rocks from West Clearwater and confirms the previously generally held assumption that the East and West Clearwater structures resulted from the simultaneous impact of two bodies at 285–300 Ma ago.Contribution from the Earth Physics Branch No. 909     

Geochronometry of the Middle Paleozoic volcanics of the Omolon Massif in Northeast Asia: A comparison of the K-Ar, Rb-Sr, and U-Pb data and their geological interpretation

The results of the K-Ar, Rb-Sr, and U-Pb (SHRIMP zircon method) dating of the Middle Paleozoic volcanogenic rocks of the Omolon Massif are summarized. It was concluded that they are principally consistent with each other, as well as with the geological data. The formation of the Kedon Group, which makes up the main volume of the Middle Paleozoic volcanics, began at the Early-Middle Devonian boundary about 400 Ma ago (U-Pb dates of 400.5 ± 4.4 and 387 ± 6.4; Rb-Sr isochron age of 402 ± 6 Ma). The isotopic age of the upper boundary of the Kedon Group remains unclear due to disagreements concerning its stratigraphic assignment. The histogram based on the 111 K-Ar dates of the volcanic rocks from the Kedon Group gives a polymodal distribution, which indicates that the K-Ar isotopic system was disturbed by thermal events, which occurred 310–290 (terminal Carboniferous—beginning of the Permian) and 240–220 (Middle-beginning of the Late Triassic) Ma ago. Both thermal events were associated with mantle (ultrabasic-basic) magmatism, which spanned a significantly wider territory than the distribution area of the Kedon Group     

Equilibrium-disequilibrium relations in the Monte Rosa Granite,Western Alps: Petrological,Rb-Sr and stable isotope data

Nine samples from the Monte Rosa Granite have been investigated by microscopic, X-ray, wet chemical, electron microprobe, stable isotope and Rb-Sr and K-Ar methods. Two mineral assemblages have been distinguished by optical methods and dated as Permian and mid-Tertiary by means of Rb-Sr age determinations. The Permian assemblage comprises quartz, orthoclase, oligoclase, biotite, and muscovite whereas the Alpine assemblage comprises quartz, microcline, albite+epidote or oligoclase, biotite, and phengite. Disequilibrium between the Permian and Alpine mineral assemblages is documented by the following facts: (i) Two texturally distinguishable generations of white K-mica are 2 M muscovite (Si=3.1–3.2) and 2 M or 3 T phengite (Si=3.3–3.4). Five muscovites show Permian Rb-Sr ages and oxygen isotope fractionations indicating temperatures between 520 and 560 ° C; however, K-Ar ages are mixed or rejuvenated. Phengite always shows mid-Tertiary Rb-Sr ages, (ii) Two biotite generations can be recognized, although textural evidence is often ambiguous. Three out of four texturally old biotites show mid-Tertiary Rb-Sr cooling ages while the oxygen isotopic fractionations point to Permian, mixed or Alpine temperatures, (iii) Comparison of radiogenic and stable isotope relations indicates that the radiogenic isotopes in the interlayer positions of the micas were mobilized during Alpine time without recrystallization, that is, without breaking Al-O or Si-O bonds. High Ti contents in young muscovites and biotites also indicate that the octahedral (and tetrahedral) sites remained undisturbed during rejuvenation. (iv) Isotopic reversals in the order of O¹⁸ enrichment between K-feldspar and albite exist. Arguments for equilibrium during Permian time are meagre because of Alpine overprinting effects. Texturally old muscovites show high temperatures and Permian Rb-Sr ages in concordancy with Rb-Sr whole rock ages. For the tectonically least affected samples, excellent concordance between quartz-muscovite and quartz-biotite Permian temperatures implies oxygen isotope equilibrium in Permian time which was undisturbed during Alpine metamorphism. Arguments for equilibrium during the mid-Tertiary metamorphism are as follows: (i) Mid-Tertiary Rb-Sr mineral isochrons of up to six minerals exist, (ii) Oxygen isotope temperatures of coexisting Alpine phengites and biotites are concordant.The major factor for the adjustment of the Permian assemblages to Alpine conditions was the degree of Alpine tectonic overprinting rather than the maximum temperatures reached during the mid-Tertiary Alpine metamorphism. The lack of exchange with externally introduced fluid phases in the samples least affected by tectonism indicates that the Monte Rosa Granite stewed in its own juices. This seems to be the major cause for the persistence of Permian ages and corresponding temperatures.     

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.     

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.     

Rb-Sr, Sm-Nd and K-Ar systematics of metamorphosed pillowed basalts and associated Besshi-type deposits in the Sanbagawa Belt, Japan

Samples from metamorphosed pillowed basalts and related Besshi-type deposits occurring in the Sanbagawa belt of the Shikoku Island, southwest Japan, have been analyzed for ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and ⁴⁰ K/⁴⁰Ar. This is to investigate the tectonic settings in which the original submarine volcanism and associated Besshi-type mineralization occurred, as well as the age of metamorphism. Eight whole-rock samples of the pillow lavas metamorphosed in pumpellyite-actinolite facies conditions yield a Rb-Sr isochron age of 107 ± 15 Ma with an initial ratio of 0.70401 ± 0.00006, while they do not define a Sm-Nd isochron. We interpret the results as the metamorphic age, an interpretation consistent with the previously reported Rb-Sr whole-rock age for the Sanbagawa pelitic schists. The overall ranges of the initial epsilon values at T = 107 Ma are: ɛNd (T ) = +7.8 to +4.3; ɛSr(T ) = +2.2 to −7.0, suggesting that the most likely source for the pillowed basalts is depleted oceanic mantle, a conclusion supported by the previous Pb isotope studies. The K-Ar ages determined for twelve mineral separates from the Besshi-type deposits range from about 60 to 112 Ma, with a mean age of about 80 Ma, in agreement with the previous K-Ar and Ar-Ar data for the Sanbagawa pelitic and basic schists. The youngest age, 60 Ma, was obtained for sericite from the Hinooku deposit metamorphosed in pumpellyite- actinolite facies conditions, while the oldest one for hornblende from the spotted amphibolite in the immediate vicinity of the Shiragayama deposit metamorphosed in albite-biotite grade. The oldest age, 112 Ma, is interpreted to date the peak metamorphism, consistent with the Rb-Sr data, though a possibility of excess Ar cannot always be ruled out. In view of the closure temperatures of muscovite (350 °C) in the biotite zone, it is suggested that our K-Ar age data (<about 80 Ma) represent the age of the retrograde metamorphism or subsequent uplift. Datable microfossils found in the Sanbagawa belt of Shikoku suggest that the submarine basaltic volcanism and related Besshi-type mineralization occurred in an oceanic basin away from the trench region in Late Triassic (conodont) to Late Jurassic (radiolarian) times. Received: 5 March 1997 / Accepted: 14 May 1998     

K-Ar ages of the Brandberg and Okenyenya igneous complexes,north-western Namibia

K-Ar mineral ages from intrusive units of the Brandberg and Okenyenya igneous complexes, north-western Namibia, confirm the Early Cretaceous age of the subvolcanic centres. The two centres are contemporaneous, although the range of ages from Brandberg, 135.2 ± 1.5 to 125.4 ± 1.3 Ma, suggests a rather longer period of intrusion than is represented by the rocks of Okenyenya, 133.3 ± 1.4 to 129.2 ± 0.7 Ma. The mean K-Ar age of the Okenyenya complex is essentially equivalent to previously determined Rb-Sr ages for the Messum and Paresis complexes on the same igneous lineament, but is a little greater than that suggested recently from Rb-Sr dating of this complex (129.1–123.4Ma). K-Ar chronology for the Brandberg complex is in conflict with the order of emplacement of granite units previously inferred from field evidence. In particular, the Amis peralkaline layered intrusion yields the oldest age from the complex, 135.2 ± 1.4 Ma. The concordancy of age measurements of amphibole and biotite, having very different potassium contents, from single rock samples is compelling evidence that neither inherited radiogenic argon, nor argon loss, presents a significant problem in the dated rocks. If the K - Ar age of 135 Ma for the Amis intrusion is correct, it constrains the minimum age for the onset of Etendeka flood volcanism, associated with continental break-up, as Etendeka lavas exhibit contact metamorphism and metasomatism around the rim of the Brandberg complex.     

Differential behaviour of the Rb-Sr and K-Ar systems of spilitic flows and interbedded metasediments: the spilite group of Erquy (Brittany,France). Paleomagnetic implications

The Erquy series (Côtes du Nord, France) consists, in its upper part, of spilitic pillow lavas with some interbedded volcano-sedimentary horizons.The Rb-Sr system of the pillows allowed the construction of a whole-rock isochron at 482±10 M.a. with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7055±0.0002. These rocks and the associated keratophyres give, on the other hand, K-Ar ages of 285±16 M.a. interpreted as the consequence of late-hercynian tectonism.A volcano-sedimentary horizon interbedded with such pillow flows has been studied from petrographic, mineralogical, geochemical and isotopic (Rb-Sr and K-Ar) points of view. The sequence keeps a sedimentary memory. Its clay fractions <2 m and corresponding whole-rocks fit an isochron which is identical to that of the volcanic rocks: 494±11 M.a. with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7052±0.0005. The clay fractions give K-Ar data at about 450 M.a., but those which contain important amounts of volcanic glass, at the top of the horizon, have K-Ar values as low as 400 M.a., and those which contain almost no glass have a K-Ar age close to the Rb-Sr age at 480 M.a.This study emphasizes the possibility of a complete reset of the K-Ar system of spilitic rocks by a tectonic event without notice-able temperature increase. This result may have important implications on combined paleomagnetic and K-Ar studies: it seems that a least for spilites and keratophyres, the Curie point and Ar blocking temperature can be very different.     

Rb-Sr and K-Ar geochronologic and isotopic studies,Llano Uplift,central Texas

Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K₂O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses.Two foliated plutons in the southeast are 1,167±12m.y. (2) old, with distinctly different initial ⁸⁷Sr/⁸⁶Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial ⁸⁷Sr/ ⁸⁶Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial ⁸⁷Sr/⁸⁶Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial ⁸⁷Sr/⁸⁶Sr = 0.7061±0.0003. All of the intrusive rocks have initial ⁸⁷Sr/⁸⁶Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.     

Evolution of the Neoproterozoic Delgo suture zone and crustal growth in Northern Sudan: geochemical and radiogenic isotope constraints

In the Delgo basement area of northern Sudan, low to medium grade metamorphosed volcanic, sedimentary and plutonic rocks are surrounded by high grade gneisses. A NNE-SSW trending suture zone can be defined by the lithological, chemical and structural characteristics of several distinct units. The early Proterozoic gneiss terrain is overlain by metasedimentary units, the metamorphism of which has been dated by the Sm-Nd whole rock-mineral technique (702 ± 27 Ma in the west, 592 ± 16 Ma in the east). In the central part, the Abu Sari volcanic rocks show geochemical signatures of formation at an arc, with a protracted tholeiitic, calc-alkaline and shoshonitic evolution. The overlying El Hamri ophiolite contains chemical features of a back-arc tectonic environments. The ophiolite was dated by the Sm-Nd whole rock method on metagabbros at 752 ± 48 Ma. The further extension of this oceanic basin into the Jebel Rahib in the south-west was dated at 707 ± 54 Ma (Sm-Nd whole rock and minerals).Widespread suite of syn-tectonic granitoid intrusives displays subduction-related characteristics. They where emplaced between 650 to 760 Ma (Pb-zircon evaporation method). Their Nd and Sr isotopic compositions indicate a changing pattern of island arc to active continental margin character along an east-west transect and suggest a west to north-west dipping subduction zone. All units were juxtaposed at the minimum age of 600 Ma and rearranged during an extensional event, which was dated by the Rb-Sr thin slab technique (546 ± 19 Ma) on a migmatite. The Delgo suture provides evidence of a complex terrane pattern in north-east Africa and crustal growth during the Pan-African event by the addition of oceanic material to pre-existing continental crust.     

Radiometric constraints on hydrothermal circulation in cooling granite plutons

Improved precision of radiometric dating of ore deposits can provide information about the thermal history of hydrothermal circulation in cooling plutons. In Jales a Hercynian porphyritic two-mica granite and pre-Ordovician mica schists are cut and intensely altered by the Campo gold-quartz vein. The unaltered granite must be younger than 320 ± 6 Ma, and gives mica Rb-Sr ages of 308.5 ± 2.4 (1) Ma (muscovite) and 294.5 ± 1.1 Ma (biotite). Alteration muscovites from the granite give a weighted mean Rb-Sr age of 308.1 ± 1.5 Ma, and a mean ³⁹Ar-⁴⁰Ar age of 300.7 ± 2.8 Ma. Alteration muscovites from the mica schists give similar ³⁹Ar-⁴⁰Ar ages, averaging 303.0 ± 2.8 Ma. The results suggest that circulation of the Campo mineralising fluids took place no more than 2–4 Ma after the granite cooled through the muscovite Rb-Sr closure temperature, about 500 °C, and that subsequent cooling to biotite closure at about 300 °C took place at less than 14°C/Ma. The mean cooling rate following emplacement was 15 to 25 °C/Ma. The most detailed comparable published data, for the Cornubian ore field, imply much faster cooling rates.     

The evolution of illite to muscovite: mineralogical and isotopic data from the Glarus Alps,Switzerland

Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mössbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, ⁴⁰Ar/³⁹Ar and stable isotope methods.With increasing metamorphic grade based on illite crystallinity data (XRD and IR) the following continuous changes are observed: (i) the 1Md2M₁ polymorph transformation is completed in the higher grade anchizone; (ii) K₂O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction <2 m decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age.These results suggest that the prograde evolution from 1 Md illite to 2M₁ muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of ⁴⁰Ar and ⁸⁷Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the ⁴⁰Ar/³⁹Ar age spectra.Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant ⁴⁰Ar/³⁹Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian.     

Geochronology of the Spessart Crystalline Complex,Mid-German Crystalline Rise

Summary New Rb-Sr and K-Ar datings help to clarify the geologic history of the Spessart Crystalline Complex, Mid-German Crystalline Rise. The oldest dates, refined by new measurements, are recorded by whole-rock Rb-Sr analyses of the orthogneisses of the Rotgneiss Complex. These confirm a late Ordovician to Silurian age which is interpreted as the time of intrusion of the granitic precursors.Hornblendes, muscovites. and biotites from different lithostratigraphic units and rock types of the Spessart Crystalline Complex yielded K-Ar dates mainly in the range 324 to 318 Ma, an interval which conforms to the analytical precision. Two hornblendes and one muscovite show slightly older dates up to 328 Ma. On the other hand, there is a tail of younger hornblende dates towards 311 Ma, and two hornblendes gave dates as low as 293 and 274 Ma for no immediately obvious reason.The concordant dates around 324 Ma on the three different minerals may be interpreted as marking the time of a rapid uplift and cooling at about the boundary between Early and Late Carboniferous, presumably soon after culmination of the Variscan deformation and amphibolite facies metamorphism.

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Geochronologie des Spessart-Kristallins, mitteldeutsche Kristallinschwelle

Zusammenfassung Neue Rb-Sr- und K-Ar-Datierungen liefern einen Beitrag zum Verständnis der geologischen Geschichte des Spessart-Kristallins. Die älteste radiometrische Datierung, die bislang im Spessart-Kristallin zur Verfügung steht, leitet sich aus Rb-Sr-Gesamtgesteinsanalysen von Orthogneisen des Rotgneis-Komplexes ab. Die bereits von früheren Bearbeitern gefundenen spät-ordovizischen bis silurischen Daten wurden durch neuere Messungen bestätigt. Sie werden als Intrusionsalter des granitischen Ausgangsmaterials der Rotgneise interpretiert.Hornblenden, Muscovite und Biotite aus unterschiedlichen lithostratigraphischen Einheiten und Gesteinstypen des Spessartkristallins erbrachten K-Ar-Daten vorwiegend zwischen 324 und 318 Ma, d. h. einen Streubereich, der etwa der analytischen Genauig keit entspricht. Zwei Hornblenden und ein Muscovit ergaben etwas ältere Daten bis 328 Ma. Auf der anderen Seite beobachtet man eine Reihe von jüngereren Hornblendedaten bis 311 Ma, und zwei Hornblenden von nur 293 und 274 Ma, die sich nicht ohne weiteres erklären lassen.Die konkordanten Alterswerte um 320 Ma, die für die drei Mineralarten gewonnen wurden, können als die Zeit einer raschen Hebung und Abkühlung etwa an der Grenze Unter-/Oberkarbon interpretiert werden, die vermutlich bald nach dem Höhepunkt der variscischen Deformation und amphibolit-faziellen Metamorphose erfolgte.

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Dedicated to Borwin Grauert on the occasion of his 60th birthday.

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With 6 Figures     

A Rb-Sr geochronologic study of the pegmatites of the Middletown area,Connecticut

The pegmatites of Eastern Connecticut have a mineralogy consistent with a magmatic origin yet occur in a non-igneous environment. Various theories of genesis have been investigated by the Rb-Sr geochronologic method.Rb-Sr measurements on early stage pegmatite minerals indicate an age of 258±1 m.y. with initial Sr⁸⁷/Sr⁸⁶=0.734±0.0096. Previously reported K-Ar and U, Th-Pb ages for pegmatite minerals are 249±8 m.y. and 260±3 m.y. respectively. Rb-Sr whole rock data for the host rocks vary between 285±10 m.y. and 472±15 m.y. in age and between 0.705±002 and 0.7167±0.0016 in initial Sr⁸⁷/Sr⁸⁶. A direct genetic relationship between the pegmatites and their host rocks is thus precluded. In addition, whole rock samples of the Brimfield schist taken at variable distances from the Strickland Quarry pegmatite have remained essentially closed systems with respect to Rb and Sr and thus an in situ origin for this pegmatite is unlikely. Mixing of pegmatite and country rock systems has occurred only locally, and isotopic studies of these mixed rocks yield a date of 231±4 m.y. with initial Sr⁸⁷/Sr⁸⁶=0.7188±0.004, an age not inconsistent with previously reported K-Ar and Rb-Sr mineral dates on host rock minerals (approximately 220 to 240 m.y.).Late stage cleavelandites are anomalously enriched in radiogenic Sr-87, the source of which was most probably other zones within the crystallizing pegmatite. This is indicated by analyses of pegmatite whole rocks which show both enrichment and depletion of radiogenic Sr-87 in local systems. The conclusion is drawn that there was widespread movement of radiogenic Sr-87 within each pegmatite system, but that pegmatite-host rock reactions were minimal.     

Ar-Ar age of mylonites along the Merens Fault, central Pyrenees

Seven mylonitic samples and two coarse muscovites from the central Pyrenees have been dated by the ⁴⁰Ar-³⁹Ar method. Whole rock specimens of mylonite were cut out of thin-section chips allowing complete characterisation of mineralogy and texture. Several specimens showed rising staircase patterns in the range 50–90 Ma, with much higher ages in the highest temperature steps. This is believed to reflect mixing of argon released from micas with excess argon contained in plagioclase and released mainly at high temperatures. One biotite-quartz mylonite gave a plateau age of 93 ± 2 Ma. Other inferred mica ages are about 60–73 Ma for biotite and 50–60 Ma for muscovite; it is probable that biotite contains excess argon and that 50 Ma approximates to the cooling age in the mylonites. One coarse muscovite collected immediately below the major Mérens shear zone gave a Hercynian plateau age, while another collected within the Mérens zone gave a partially reset Hercynian age.Taken together, the data indicate that the shear zones were active in Alpine times < 100 Ma and probably about 50 Ma ago. They are believed to have formed during the early stages of Eocene compression in the Pyrenees. Deformation and resultant uplift probably terminated an important thermal event in this part of the Pyrenean basement, which may have begun at the time of the mid-Cretaceous North Pyrenean metamorphism (90–100 Ma).     

Isotopic evidence of middle proterozoic magmatism from bombay high field: Implications to crustal evolution of western offshore of India

Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446 ± 67 Ma with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7062 ± 0.0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of different grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385 ± 21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458 ± 43 Ma and 1465 ± 43 Ma, respectively. Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-Ar ages of 1452 ± 42 Ma and 1425 ± 40 Ma with an overall mean age of 1438 ± 19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450 Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400-1450 Ma on the western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.