A U-Pb zircon,and Rb-Sr and U-Th-Pb whole-rock study of a polymetamorphic terrane in the central Alps,Switzerland

U-Pb analyses of zircons from the southern paragneiss zone of the Gotthard massif in the central Alps indicate these rocks were derived from one or more source areas ≧ 1400 m.y. old and were strongly affected by both the Caledonian and Hercynian orogenies. Rb-Sr whole-rock analyses also appear to reflect the Hercynian event while Rb-Sr analyses of a metamorphic inclusion and a boudin indicate that these small-scale samples were affected by the Alpine orogeny. U-Pb whole rock data appear to reflect only the Hercynian event; these data, when corrected for primordial lead, furthermore yield an upper concordia intercept of 4500 m.y., possibly resulting from a U-Pb fractionation very early in the history of the earth. A more refined three-stage U-Pb evolution model yields an age of about 4480 m.y. rather than 4500 m.y. for this hypothesized early terrestrial differentiation. Geologically these data emphasize that: 1) the southern paragneiss zone of the Gotthard massif contains metasediments which have experienced the Caledonian orogeny and are at least 400 m.y. old; 2) the Hercynian episode in this region was pervasive indeed; and 3) the Alpine orogeny affected the Rb-Sr and U-Pb whole rock systems to a far lesser degree than the preceding orogenic episodes and apparently did not affect the U-Pb zircon systems investigated at all.     

Late Archaean—early proterozoic source ages of zircons in rocks from the Paleozoic orogen of western Galicia,NW Spain

U-Pb data are reported for nine suites of zircons and three monazites from the Paleozoic orogen in western Galicia (NW Spain): one paragneiss and six orthogneisses from the early Paleozoic basement, and two Carboniferous (ca. 310 Ma old) intrusions of two-mica granite. New whole-rock Rb-Sr analyses, along with earlier data, indicate an age of ca. 470-440 Ma (Ordovician) for the emplacement of the granitic precursors of the orthogneisses. Monazite from the paragneiss also yields an U-Pb age of ca. 470 Ma. For all nine investigated suites of zircons the U-Pb systematics signal the presence of a minor proportion of Precambrian zircon. The zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3.0 Ga and 2.0 Ga, and lower intercepts between ca. 480 and 460 Ma; the former are interpreted as approaching the age of the old zircon component, and the latter as reflecting either the time of crystallization of new zircons from the magma (orthogneisses), or the time of radiogenic lead loss from the old zircons (paragneiss). The suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470-440 Ma ago, in most cases leading to ‘false’ discordias without geochronological significance. Similarly, the zircons of the two investigated two-mica granites do not produce meaningful discordias because of post-Paleozoic disturbance. The monazite U-Pb systems of the latter granites indicate (sub)recent lead loss.From the high initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios an involvement of Precambrian continental crust material is evident in the generation of the early Paleozoic suite of granites, while the zircon U-Pb data give evidence of the presence of about 3.0-2.0 Ga old (late Archaean—early Proterozoic) components in the source material. Zircons from the oldest sedimentary rocks in the area, now present as catazonal paragneisses and a likely source for the granites, likewise reveal a provenance age of 3.0-2.0 Ga. The late Archaean—early Proterozoic source rocks in western Galicia probably formed part of an Archaean continental crust underlying much of western Europe, but mostly modified beyond recognition by younger events.     

Source contributions to Devonian granite magmatism near the Laurentian border, New Hampshire and Western Maine, USA

Radiogenic isotope data (initial Nd, Pb) and elemental concentrations for the Mooselookmeguntic igneous complex, a suite of mainly granitic intrusions in New Hampshire and western Maine, are used to evaluate petrogenesis and crustal variations across a mid-Paleozoic suture zone. The complex comprises an areally subordinate monzodiorite suite [377±2 Ma; ε_Nd (at 370 Ma)=−2.7 to −0.7; initial ²⁰⁷Pb/²⁰⁴Pb=15.56–15.58] and an areally dominant granite [370±2 Ma; ε_Nd (at 370 Ma)=−7.0 to −0.6; initial ²⁰⁷Pb/²⁰⁴Pb=15.55–15.63]. The granite contains meter-scale enclaves of monzodiorite, petrographically similar to but older than that of the rest of the complex [389±2 Ma; ε_Nd (at 370 Ma)=−2.6 to +0.3; initial ²⁰⁷Pb/²⁰⁴Pb 15.58, with one exception]. Other granite complexes in western Maine and New Hampshire are 30 Ma older than the Mooselookmeguntic igneous complex granite, but possess similar isotopic signatures.

Derivation of the monzodioritic rocks of the Mooselookmeguntic igneous complex most likely occurred by melting of Bronson Hill belt crust of mafic to intermediate composition. The Mooselookmeguntic igneous complex granites show limited correlation of isotopic variations with elemental concentrations, precluding any significant presence of mafic source components. Given overlap of initial Nd and Pb isotopic compositions with data for Central Maine belt metasedimentary rocks, the isotopic heterogeneity of the granites may have been produced by melting of rocks in this crustal package or through a mixture of metasedimentary rocks with magmas derived from Bronson Hill belt crust.

New data from other granites in western Maine include Pb isotope data for the Phillips pluton, which permit a previous interpretation that leucogranites were derived from melting heterogeneous metasedimentary rocks of the Central Maine belt, but suggest that granodiorites were extracted from sources more similar to Bronson Hill belt crust. Data for the Redington pluton are best satisfied by generation from sources in either the Bronson Hill belt or Laurentian basement. Based on these data, we infer that Bronson Hill belt crust was more extensive beneath the Central Maine belt than previously recognized and that mafic melts from the mantle were not important to genesis of Devonian granite magma.     

New and recalculated radiometric data supporting a carboniferous age for the emplacement of the Bathurst batholith,New South Wales

The Bathurst batholith is a complex of massive granitic intrusions cutting across deformed early and middle Palaeozoic rocks of the Lachlan Fold Belt of New South Wales. An adamellite from Dunkeld, near the western edge of the batholith, has yielded K‐Ar ages of 304 ± 4 m.y. (total‐rock) and 301 ± 6 m.y. (biotite).

Recalculated radiometric ages on rocks from the eastern end (Hartley) and northern edge (Yetholme), together with the new data from the western end (Dunkeld) of the Bathurst batholith yield a mean age of emplacement of 310 m.y. (8 values, standard deviation = 6.8 m.y.). This age is supported by Re‐Os data from molybdenite at Yetholme. As yet these data do not allow establishment of temporal relationships between separate intrusive phases of the Bathurst batholith, although the Durandal Adamellite at Yetholme appears to be the oldest phase yet dated.     

Petrology and Age of the Pre-Otavi Basement Granite at Franzfontein, Northern South-West Africa

Chemical, spectrographic, and isotopic analyses are presentedfor the Franzfontein alkali granite and constituent minerals.This rock has the chemical character of granties produced byliquid crystal equilibrium. Dated at 1, 700?70m.y. by the constituentzircon, its crystallization formed part of a major period (theHuabian episode) of batholithic granite emplacement in northernSouth-West Africa. The occurrence of these crystalline rocksin the core of the Huab anticline defines the maximum possibleage of the overlying Otavi Facies sediments, precluding theircorrelation with the Transvaal System of South Africa. The imprint of the Damara metamorphism (Damaran episode) isreflected in the Sr/Rb age (560?30 m.y.) obtained for the biotite:the inversion of biotite to stilpnomelane and chlorite probablyrepresents the mineralogical effects of that metamorphis. Isotopicdata indicate that changes in the relative concentrations ofRb and Sr differed significantly in plagioclase and microcline;such data from feldspars in metamorphic rocks should, therefore,be interpreted with caution.     

Arsenic variability and groundwater age in three water supply wells in southeast New Hampshire

Three wells in New Hampshire were sampled bimonthly over three years to evaluate the temporal variability of arsenic concentrations and groundwater age.All samples had measurable concentrations of arsenic throughout the entire sampling period and concentrations in individual wells had a mean variation of more than 7 μg/L.The time series data from this sampling effort showed that arsenic concentrations ranged from a median of 4 μg/L in a glacial aquifer well(SGW-65)to medians of 19μg/L and37 μg/L in wells(SGW-93 and KFW-87)screened in the bedrock aquifer,respectively.These high arsenic concentrations were associated with the consistently high pH(median≥8)and low dissolved oxygen(median0.1 mg/L)in the bedrock aquifer wells,which is typical of fractured crystalline bedrock aquifers in New Hampshire.Groundwater from the glacial aquifer often has high dissolved oxygen,but in this case was consistently low.The pH also is generally acidic in the glacial aquifer but in this case was slightly alkaline(median = 7.5).Also,sorption sites may be more abundant in glacial aquifer deposits than in fractured bedrock which may contribute to lower arsenic concentrations.Mean groundwater ages were less than 50 years old in all three wells and correlated with conservative tracer concentrations,such as chloride;however,mean age was not directly correlated with arsenic concentrations.Arsenic concentrations at KFW-87 did correlate with water levels,in addition,there was a seasonal pattern,which suggests that either the timing of or multiple sampling efforts may be important to define the full range of arsenic concentrations in domestic bedrock wells.Since geochemically reduced conditions and alkaline pHs are common to both bedrock and glacial aquifer wells in this study,groundwater age correlates less strongly with arsenic concentrations than geochemical conditions.There also is evidence of direct hydraulic connection between the glacial and bedrock aquifers,which can influence arsenic concentrations.Correlations between arsenic concentrations and the age of the old fraction of water in SGW-65 and the age of the young fraction of water in SGW-93 suggest that water in the two aquifers may be mixing or at least some of the deeper,older water captured by the glacial aquifer well may be from a similar source as the shallow young groundwater from the bedrock aquifer.The contrast in arsenic concentrations in the two aquifers may be because of increased adsorption capacity of glacio-fluvial sediments,which can limit contaminants more than fractured rock.In addition,this study illustrates that long residence times are not necessary to achieve more geochemically evolved conditions such as high pH and reduced conditions as is typically found with older water in other regions.     

U-Pb isotopic relationships in zircon suites from a para- and ortho-gneiss from the Ceneri Zone,Southern Switzerland

U-Pb isotopic analyses have been determined on four size splits from an orthogneiss zircon population and on seven size and magnetic splits from a paragneiss zircon population from the Ceneri Zone, Southern Switzerland. The orthogneiss zircon results suggest that a severe isotopic disturbance occurred in this region between 400–450 m. y. ago which, besides initiating new zircon growth, almost completely reconstituted older zircon. The U-Pb systems of the orthogneiss zircons are nearly concordant, except the coarsest size split where some older lead is present.The paragneiss zircon U-Pb systems have Precambrian apparent ages and appear to be an old detrital zircon suite which has been severely isotopically disturbed by the same Caledonian event which affected the orthogneiss zircons. Zircon U-Pb data-points of paragneiss size-and-magnetic fractions are linearly correlated on a concordia plot. The age significance of this is not fully understood, however these results indicate that Precambrian crustal rocks, at least 1,500 m. y. old, contributed detritus to the original sediments of the Alpine gneisses.The gneissic rocks in Southern Switzerland have been postulated as a possible source or root zone for petrologically and chemically similar gneisses in the Silvretta Nappe to the north. The close similarity of zircon U-Pb systems of orth- and para-gneisses between the two regions supports this hypothesis.     

The age of the Cuddapah and Kurnool systems,southern India

In southern India the older Precambrian is overlain unconformably in the Cuddapah Basin by the Cuddapah and Kurnool Systems. The former is tilted and unmetamorphosed in the west but eastwards becomes strongly folded and metamorphosed. It contains lavas and sills, particularly in the lower two groups, is intruded by dolerites and at Chelima by diatremes of kimberlitic affinities related to those intruding the older gneisses west of the Cuddapah Basin in the Wajrakarur area. The Kurnool System lacks any igneous rocks; its basal conglomerate is diamondi‐ferous.

Rb‐Sr dating of lava samples from the lowest group of the Cuddapah System shows that the age of the base of the system may be as great as 1,700 m.y. Together with data for a granite which intrudes probable Cuddapah rocks near the disturbed eastern margin of the basin the data imply that the base is unlikely to be younger than 1,555 m.y. Metamorphism affected some lavas at about 1,360 m.y. The diatremes have two ages of intrusion, about 1,225 m.y. and 1,140 m.y., the latter being the age of the Majhgawan pipe near Panna in northern India. Pre‐Kurnool dolerites have an age of 980 ±110 m.y.

The lavas and dolerites show a range of initial ⁸⁷Sr/⁸⁶Rb ratios from about 0.704 to 0.708 and possibly 0.712.

The age data suggest that no simple correlation can be made with other Precambrian sequences in northern peninsular India. Deposition of the Cuddapah System appears to have started well before the start of the deposition of the Vindhyan System, while the Kurnool System is coeval with only part of the Upper Vindhyan. The data also suggest that present interpretations of the structural development of the Cuddapah Basin may need some revision.     

Trace elements and the behaviour of the U-Pb system in inherited and newly formed zircons

The distribution of U, Y, P, and Ca was studied with an electron microprobe in detrital zircons from paragneisses of amphibolite to granulite facies as well as in zircons formed in their present-day host rocks, such as a granite, anatectic and aplitic gneisses and a migmatite from the Southern Alps. Detrital paragneiss zircons have low trace element contents, whereas zircons formed in their present-day host rocks have elevated trace element contents. According to the isotopic U-Pb age pattern one may distinguish two types of zircon populations formed in their present-day host rocks: one which lost lead without any known exogene cause, and one the U-Pb system of which was only opened in response to a geological event. Different modes of incorporation of the trace elements in the zircon crystals are thought to be responsible for this different behaviour and are discussed.     

Uranium-lead isotope systematics and apparent ages of zircons and other minerals in precambrian granitic rocks,Granite Mountains,Wyoming

Zircon suites from the two main types of granite in the Granite Mountains, Wyoming, yielded concordia-intercept ages of 2,640±20 m.y. for a red, foliated granite (granite of Long Creek Mountain) and 2,595±40 m.y. for the much larger mass of the granite of Lankin Dome. These ages are statistically distinct (40±20 m.y. difference) and are consistent with observed chemical and textural differences. The lower intercepts of the zircon chords of 50±40 and 100+ 75 m.y. for the granite of Long Creek Mountain and granite of Lankin Dome, respectively, are not consistent with reasonable continuous diffusion lead-loss curves but do correspond well with the known (Laramide) time of uplift of the rocks. Epidote, zircon, and apatite from silicified and epidotized zones in the granites all record at least one postcrystallization disturbance in addition to the Laramide event and do not define a unique age of silicification and epidotization. The lower limit of 2,500 m.y. provided by the least disturbed epidote, however, suggests that these rocks were probably formed by deuteric processes shortly after emplacement of the granite of the Lankin Dome. The earlier of the two disturbances that affected the minerals of the silicified-epidotized rock can be bracketed between 1,350 and 2,240 m.y. ago and is probably the same event that lowered mineral K-Ar and ages in the region.Zircon suites from both types of granite show well-defined linear correlations among U content, common-Pb content, and degree of discordance. One of the zircon suites has an extremely high common-Pb content (up to 180 ppm) and exhibits a component of radiogenic-Pb loss that is apparently unrelated to radiation damage.     

Application of Sr and O isotope relations to the petrogenesis of the alkaline rocks of the Red Hill complex,New Hampshire,USA

The Red Hill ring complex in central New Hampshire is composed of apparently cogenetic syenites, nepheline-sodalite syenite, and granite. The ages and petrogenetic relations among five of the six recognized units have been investigated by rubidiumstrontium and oxygen isotope analysis of whole rocks and separated minerals. Whole-rock samples from three syenite units are consistent with a single Rb-Sr isochron which gives an age of 198±3 m.y. and an initial (⁸⁷Sr/⁸⁶Sr)_o ratio of 0.70330±0.00016 (±2 sigma; =1.42× 10^–11y^–1). However, Sr isotope data for two other units, nepheline syenite and granite, are not consistent with this isochron but rather indicate higher initial ratios which range from 0.7033 to about 0.707. Whole-rock O isotope analyses give ¹⁸O values which range from+6.2 to+9.3 Sr and O isotope analyses on mineral separates indicate that observed whole-rock variations in (⁸⁷Sr/⁸⁶Sr)_o are primary and are not due to any secondary process. The fact that the isotope systematics correlate with rock type, suggests that crustal interaction is likely to have played a significant role in the development of this over-and undersaturated association. Such process(es), while still not fully delineated, could be of fundamental importance to the genesis of associations of critically undersaturated and oversaturated intrusives. The data support the idea that interaction between magmas and crustal materials strongly influenced the compositional relations of similar complexes elsewhere including those of the White Mountain magma series.     

Timing of the Acadian Orogeny in Northern New Hampshire

New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples: (1) an early syntectonic diorite that records the onset of the Acadian; (2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Acadian. Zircon from the syntectonic Wamsutta Diorite gives a 207Pb/206Pb age of circa 408 Ma, the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migrating Acadian orogenic front and records the beginning of the Acadian in this part of the Central Maine Terrane. We propose a possible Acadian tectonic model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives 207Pb/206U ages, suggesting the peak of Acadian metamorphism and intrusion of two-mica granites occurred at circa 402-405 Ma, the main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terrane of New Hampshire and western Maine range from circa 406-384 Ma, with younger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range had ended by circa 355 Ma, the 207Pb/235U age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synkinematic metamorphism, seen in central and southern New Hampshire, Massachusetts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, the orogenesis was typical of the style in parts of Maine dominated by postkinematic metamorphism.     

Dating of granulites involved in the hercynian fold-belt of Europe: An example taken from the granulite-facies orthogneisses at La Picherais,Southern Armorican Massif,France

Situated within the crystalline metamorphic complex of Champtoceaux NE of Nantes, the orthogneiss of La Picherais (near St Mars-du-Désert, Loire Atlantique, France) show relicts of a granulite facies paragenesis. Comparison with other granulitic rocks in the Hercynian fold-belt suggest possible ages ranging from Lower Proterozoic to Phanerozoic. The Rb-Sr whole rock method yields an errorchron of 570±110 m.y. for the Picherais orthogneiss, whereas the U-Pb zircon method indicates an upper intersection on Concordia at 1,880±120 m.y. and a lower intersection at 423±10 m.y. Several interpretations are possible for these data: the granite emplacement age was (1) 1,900 m.y. ago. (2) more likely Upper Proterozoic — Lower Palaeozoic. The zircons concordant at 1,900 m.y. were either present in the granitic magma at its time of origin or were introduced into the magma during emplacement. These zircons could be derived from sedimentary horizons such as found in the Lower Ordovician sandstones of the Armorican massif whose zircon age data are presented here.     

Different age response of zircon and monazite during the tectono-metamorphic evolution of a high grade paragneiss from the Ruhla Crystalline Complex,central Germany

Results of TIMS, SIMS and SEM analyses show that zircon and monazite in a high-grade paragneiss of the Ruhla Crystalline Complex, central Germany, were formed and/or altered during different stages of a tectono-metamorphic history between Early Devonian and Permian times. Detrital zircon cores of >460 Ma place an older limit on the age of anatexis, and show that the paragneiss sequence contains rocks at least as young as early Cambrian. Metamorphic zircon growth commenced at ~365 Ma, peaking at ~360–355 Ma at the same time that granite dykes were emplaced. In contrast, monazite in the paragneiss preserves little record of the metamorphic peak. Most monazite grains grew or were recrystallised in the Lower Carboniferous at ~339 Ma, contemporaneous with the emplacement of voluminous diorite and granite bodies. These intrusions and related tectonics caused some of the high-U zircon overgrowths to undergo moderate to severe Pb loss. A second Pb loss event, between 300 and 280 Ma, can be related to Late Carboniferous/Early Permian large-scale block faulting.Editorial responsibility: J. Hoefs     

Present status of the geochronology of the Precambrian rocks of Rajasthan

Age determinations, mostly by Rb---Sr analyses, of the Precambrian rocks of Rajasthan by us and by others are summarized and discussed. Broad periods of acid magmatism at (1) 3000−2900 m.y., (2) 2600−2500 m.y., (3) 2000−1900 m.y., (4) 1700−1500 m.y. and (5) 850−750 m.y. were identified. The oldest rocks in the area are the yet undated banded gneisses (BGC) east of Udaipur, intruded by the Untala granite dated at 2950 m.y. and hence of mid-Archean age. The basal status hitherto attributed to the Berach granite dated at 2600 m.y. is no longer tenable. The radiometric control on the beginning and duration of the overlying Aravalli Supergroup is not yet satisfactory, though a lower limit at 2000 m.y. is indicated. Heron's original Delhi rocks have recorded two magmatic events widely separate in space and time. While the earliest granitic activity at 1600 m.y. is recorded only in the Alwar basin in the northeast, the younger activity between 850-750 m.y. is widespread, as shown by the nearly concordant ages of “Erinpura-type” granites along the Aravalli mountain Range and the Malani rhyolites in the western plains of the Aravalli Range.     

A Rb-Sr whole rock isochron date on an igneous rock-body from the Stavanger area,South Norway

The Ormakam-Moldhesten granite, from the Stavanger area, South Norway, has been dated by the Rb-Sr whole rock method. The isochron ages obtained (1180 m.y., 1243 ±160m.y. and 1534±125 m.y.) show that the granite complex is of Precambrian age. The 1543 m. year age is thought to refer to a period of early granulite facies metamorphism, the 1180 m.y. isochron age is taken as the crystallisation age of a later intrusion of biotite granite. This is within the limits of error of the 1160 m.y. metamorphic event shown earlier to have affected the paragneisses in the area. The results demonstrate clearly the allochthonous position of the gneisses and granitic intrusives overlying the fossiliferous Cambrian beds in the Stavanger area. The tectonostratigraphic succession in this area is thus consistent with the observation of Precambrian nappes to the north (Hardangervidda-Ryfylke area).     

K-Ar Ages of Granitic Magmatism and Related Pegmatite Formation at the Umanotani-Shiroyama Mine, Shimane Prefecture, SW Japan and Their Bearings on Cooling History

Abstract. The Umanotani-Shiroyama pegmatite deposits, the largest producer of K-feldspar and quartz in Japan, are of typical granitic pegmatite. Ilmenite-series biotite granite and granite porphyry, hosting the ore deposits, and biotites separated from these rocks yielded K-Ar ages ranging from 89.0 to 81.4 Ma and 95.2 to 93.7 Ma, respectively. Muscovite and K-feldspar separated from the ore zone yielded K-Ar ages with the range of 96.2 to 93.1 Ma and 87.3 to 80.7 Ma, respectively. Muscovites from quartz-muscovite veins in the ore zone and in the granite porphyry yielded K-Ar ages of 90.4 and 76.3 Ma, respectively. K-feldspar is much younger in age than coexisting muscovite. It is noted that the K-Ar ages of biotite separates and the whole-rock ages are identical to those of muscovite and K-feldspar in the ore zone, respectively. These time relations, as well as field occurrence, indicate that the formation of the pegmatite deposits at the Umanotani-Shiroyama mine is closely related in space and time to a series of granitic magmatism of ilmenite-series nature. Using closure temperatures of the K-Ar system for biotite and K-feldspar (microcline), cooling rate of the pegmatite deposits is estimated to be about 82C/m.y. at the beginning, but slowed down to about 15C/m.y. in the later period.     

Deutung diskordanter Zirkonalter der Silvrettadecke und des Gotthardmassivs (Schweizer Alpen)

U-Pb isotopic analyses are reported for zircons of nine igneous rocks and high grade metamorphic paragneisses from the Silvretta nappe and the Gotthard massif, Switzerland.Following Silver and Deutsch (1961) the zircon populations isolated from six of these rocks were devided into series of size fractions with varying uranium content. An extensive Rb-Sr investigation has been made on some of these rock samples and their minerals (Arnold and Jäger, 1965: Gotthard massif, Grauert, 1966: Silvretta nappe). Rb-Sr measurements on biotites and mineral isochrons of individual gneiss samples yield ages which indicate the end of the Alpine and the Hercynian (=Variscan) metamorphism. Other measurements give mixed ages between these events. The U-Pb investigation of zircons, however, yield criteria for an even older event, 400 to 500 m.y. ago.The isotopic relationship indicates a strong episodic disturbance of the U-Pb systems in the paragneiss zircons with high losses of radiogenic lead. In a Concordia diagram the best fit lines to the data points of the paragneiss zircons from the Gotthard massif as well as from the Silvretta nappe trend both toward the data points of the orthogneiss zircons and intersect the Concordia at approximately 440 m.y. Rb-Sr analyses of the granitic orthogneisses from the Silvretta nappe (Flüela granite gneisses) gave a well defined isochron of 428 m.y. (=1.47×10^–11y^–1). This value agrees rather well with the age given by the intersection of the best fit lines to the paragneiss zircons with the Concordia.Considering the geological and petrographical observations we interpret this disturbance of the U-Pb systems in the paragneiss zircons as the result of an influence of regional high grade metamorphism and anatectic melting together with the formation of igneous rocks during the Caledonian petrogenesis.In the Silvretta the uranium rich zircons from the Flüela granite gneisses reflect a disturbance and lead loss probably during the Hercynian metamorphism. In the Gotthardmassif the zircons from a quartz diorite and an ultrabasic inclusion in this diorite, however, show no or only a slight influence of the later Hercynian and Alpine metamorphisms. The apparent ages of the zircons in the ultrabasic inclusion are nearly concordant (Pb²⁰⁶/U²³⁸: 448 m.y., Pb²⁰⁷/ U²³⁵: 452 m.y., Pb²⁰⁷/Pb²⁰⁶: 472 m.y.). There is in all orthogneiss zircons some evidence of older inhereted U-Pb components besides new zircon growth.If we assume an episodic model for the lead loss and a disturbance 450 m.y. ago the minimum primary age of the detrital zircon components in the paragneisses would be 1500 m.y.

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Der eine von uns (B.G.) möchte den Herren Prof. Dr. M. Grünenfelder und Prof. Dr. P. Signer vom Labor für Geochronologie des Instituts für Kristallographie und Petrographie der Eidgenössischen Technischen Hochschule Zürich seinen Dank für ihr Interesse und die großzügige Unterstützung bei der Durchführung dieser Arbeit aussprechen. Herr Prof. Dr. M. Grünenfelder und Herr Dr. R. Pidgeon haben ihn in die Arbeitsweise der U-Pb-Isotopenanalyse eingeführt. Herr Dr. V. Köppel sowie alle oben genannten Herren haben durch ihre wertvollen Ratschläge und kritischen Diskussionen diese Arbeit wesentlich gefördert. Ihnen und auch allen anderen Angehörigen des Labors für Geochronologie, die zum Gelingen der Arbeit beigetragen haben, sei herzlichst gedankt.

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Herr Dr. L. Rybach hat in dankenswerter Weise von vier Zirkonfraktionen die Urankonzentration durch Gamma-Spektrometrie bestimmt.

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Wir beide möchten Frau Prof. Dr. E. Jäger und Herrn Prof. Dr. E. Niggli vom Mineralogisch-Petrographischen Institut der Universität Bern für ihr wohlwollendes Interesse und die kritische Durchsicht des Manuskriptes danken.     

Monazite geochronology in central New England: evidence for a fundamental terrane boundary

Monazite crystallization ages have been measured in situ using SIMS and EMP analysis of samples from the Bronson Hill anticlinorium in central New England. In west‐central New Hampshire, each major tectonic unit (nappe) displays a distinctive P–T path and metamorphic history that requires significant post‐metamorphic faulting to place them in their current juxtaposition, and monazite ages were determined to constrain the timing of metamorphism and nappe assembly. Monazite ages from the low‐pressure, high‐temperature Fall Mountain nappe range from c. 455 to 355 Ma, and Y zoning indicates that these ages comprise three to four distinct age domains, similar to that found in the overlying Chesham Pond nappe. The underlying Skitchewaug nappe contains monazite ages that range from c. 417 to 307 Ma. ⁴⁰Ar/³⁹Ar ages indicate rapid cooling of the Chesham Pond and Fall Mountain nappes after 350 Ma, which is believed to represent the time of emplacement of the high‐level Chesham Pond and Fall Mountain nappes onto rocks of the underlying Skitchewaug nappe. Garnet zone rocks from western New Hampshire contain monazite that display a range of ages (c. 430–340 Ma). Both the metamorphic style and monazite ages suggest that the low‐grade belt in western New Hampshire is continuous with the Vermont sequence to the west. Rocks of the Big Staurolite nappe in western New Hampshire contain monazite that crystallized between c. 370 and 290 Ma and the same unit along strike in northern New Hampshire and central Connecticut records ages of c. 257–300 Ma. Conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks. These monazite ages confirm that the metamorphism observed in the Big Staurolite nappe occurred significantly later than that in the units structurally above and below. These data support the hypothesis that the Big Staurolite nappe represents a major tectonic boundary, along which rocks of the New Hampshire metamorphic series were juxtaposed against rocks of the Vermont series during the Alleghanian.     

40Ar-39Ar ages of biotites from Corsica and arguments for Permian age of alkaline granitic intrusion

The Western part of Corsica is underlain by Variscan calcalkaline granites which intrude the pre-Variscan basement. These granites and their basement are also intruded by Permian hyperalkaline granites. Overprinted biotites of the basement and the calcalkaline granites show only one plateau age at about 250 m.y., except for a sample of calcalkaline granite from the Cargèse region which has a high temperature plateau age of 330 m.y. related to the emplacement and a low temperature plateau age of 230 m.y. Plateau ages around 250 m.y. due to the intrusion of hyperalkaline granites whose biotites give plateau ages between 250 and 280 m.y. reflect this important thermal event.