Article

Interpretation of isotopic ages in orogenic belts

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Abstract

K-Ar and Rb-Sr methods of age-determination are the most widely applied; geological interpretation of ages so obtained is in some cases facilitated by U-Pb ages on zircons. On a broad continental scale isotopic ages have been used to delineate the time-span of evolution of structural provinces; the spread of ages so obtained is as great as 800 m.y. Within such a spread, interpretation of isotopic ages is discussed with reference to the orogenic cycle; orogenic belts may be multicyclic and involve rocks formed in a previous cycle, and there may also be repetition of phases within a cycle. The interpretation of ages obtained on rocks developed in the syn-, late- and post-orogenic phases of the orogenic cycle is considered. The question of what is being dated with respect to rocks formed in syn- and late-orogenic plutonism is considered in terms of multi-phase events and of cooling of the orogenic belt to temperatures at which radiogenic daughter products become frozen in; post-tectonic igneous activity may provide valuable datum points in the orogenic cycle. It is emphasized that full geological information is essential to any interpretation of detail within the spread of ages obtained from an orogenic cycle.

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... One of the fundamental tenets of the overprint hypothesis is that high-grade metamorphic temperatures (above 500° C.) would not be maintained for any great length of time and that cooling after the climax of metamorphism would have occurred suddenly, probably within the space of a few m.y. (Brown & Miller 1970). The writers agree with this, but believe that the temperatures to which the metamorphic complex so cooled, controlled by depth and geothermal gradients, (text- fig. 3) would have been quite sufficient to prevent the accumulation of radiogenic argon, in micas at least. ...
Article
Synopsis Three main stages are recognized in the evolution of the Scottish Caledonian orogen. Firstly, the development of the depositional framework from late Pre-Cambrian to early Ordovician times is outlined, The thick Moine and Dalradian sediments, accumulating on a continental rise, are shown to be equivalent to the shelf sequence of the foreland, and likely correlations are suggested. Secondly, the events comprising the Caledonian deformation and metamorphism of these sediments are reviewed and related to the development of a Benioff zone and coupled oceanic trench along the southern margin of the orogen through the Ballantrae complex. Comparison of stratigraphic and isotopic evidence for the age of these events leads to the conclusion that all major deformation and metamorphism occurred during a relatively short climactic episode 480–510 m.y. ago, within the Arenigian stage of the Ordovician. The third factor in the evolution of the orogen is post-climactic uplift and erosion continuing throughout Upper Ordovician, Silurian and Lower Devonian times. Contours of K–Ar mica ages are presented and related to the geological evidence for this prolonged period of isostatic recovery and thermal adjustment. The style and timing of granitic plutonism, which is closely associated with this third stage, may be indicative of crustal behaviour during uplift rather than continued metamorphism at depth.
Article
The strengths and weaknesses of the two Ar isotopic methods (K–Ar and 40Ar/39Ar) were evaluated on the basis of respective recent applications mainly on low-temperature K-bearing illite-type clay minerals. This review includes a presentation of basic, analytical and technical aspects for both methods, as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization. Whenever possible, the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates. The comparative review examines stratigraphic dating of glauconites, indirect dating of low-temperature ore deposits, dating of burial-related illitization, and dating of polyphased tectono-thermal activity, more specifically of fault gouges. Some pending questions such as the necessary encapsulation due to 39Ar recoil and its restoration into step-heating patterns are also raised, together with the new potential of Ar-dating of nanometric illite crystals.
Article
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.
Article
In diesem Artikel wird der wissenschaftstheoretische Stand der Geochronologie mit Hintergrund in den Arbeiten von Kuhn und besonders Popper diskutiert. Die Geochronologie wird untersucht mit Hinblick auf generelle methodologische Voraussetzungen, Terminologie, normalwissenschaftliche Aktivittsform und theoretische Dogmatik. Als Konklusion ergibt sich, da die fundamentalen Theoriekonstruktionen der Geochronologie nicht testbar sind, sondern als ein metaphysisches Forschungsprogramm im Sinne Poppers charakterisiert werden mssen.
Article
The significant discordance of the radiometric (Rb-Sr, Pb-U, K-Ar and fission track) ages from various orogenic cycles of the Dharwar, Satpura, Aravalli and Himalayan orogenic belts in India, coupled with their corresponding blocking temperatures for various radiometric clocks in whole rocks and minerals, has been used to evaluate the cooling and the uplift histories of the respective orogenic belts. The blocking temperatures used in the present study of various Rb-Sr (isotopic homogenization at 600°C, muscovite at 500°C and biotite at 300°C), Pb-U (monazite at 530°C), K-Ar (muscovite at 350°C and biotite at 300°C) and fission-track clock (zircon at 350°C, sphene at 300°C, garnet at 280°C, muscovite at 130°C, hornblende at 120°C and apatite at 100°C for the cooling rate l°C/Ma) have been found suitable to explain the differences in mineral ages by different radiometric techniques. The nature of the cooling curves drawn using the temperature versus age data for various orogenic cycles in India has also been discussed. The cooling and the uplift patterns determined for various orogenic cycles of India, suggest comparatively slow cooling (5.0–0.2°C/Ma) and uplift (180–2 m/Ma) for the Peninsular regions and rapid cooling (25.0–1.0° C/Ma) and fast uplift (800–30 m/Ma) during the Himalayan Orogenic Cycle (Upper Cretaceous—Tertiary) in the Extra-Peninsular region.
Article
Extract The Data for Orogenic Studies project has aimed to demonstrate that profuse and complex geological information from the Mesozoic-Cenozoic orogens can be collected in a systematic and organized fashion. The particular method used—a questionnaire designed to elicit factual information—has resulted in a collection of objective data, in which gaps in knowledge are also revealed. The accumulation of these data into a convenient and uniform format will, it is hoped, enable them to fully yield their part in the story of the evolution of the lithosphere in Mesozoic-Cenozoic times. In particular, it should facilitate the progression of geological ideas from the evidence to the hypotheses rather than (as is common) the reverse. The aim of this summary is to follow this method and indicate the major groups of data, particularly the historical information, which have been collected and briefly to evaluate their significance. The major part of the information in the volume is focussed on the rocks that build the orogenic belts. The major type of information yielded by the rocks is historical: the history of the accumulation, intrusion, vulcanism, plutonism, deformation, metamorphism, uplift, etc., which produced the present orogenic belts. Information of a geometrical nature (geometry of tectonic structures and of the orogenic crust, palaeogeography, palinspastic reconstructions, etc.) is, because of its greater complexity, not as definitely or as widely known. The volume has aimed at a world-wide coverage but is incomplete in two respects. Articles are missing for several areas, principally the U.S.S.R., the western United States, southern South
Article
Perhaps no dating method has the wide range of applicability as does the potassium argon dating method from either consideration of the ranges of ages which can be dated or the availability of suitable material to date. Minerals as young as tens of thousands of years to minerals billions of years old have been successfully dated. Many minerals retain for times of the order of billions of years the daughter, Ar40, and many minerals contain as a component K40 the parent element, potassium being a common element in the earth's crust. As a result, most rock contains at least one mineral which can be successfully dated by the potassium argon method. Even though this method has been applied for over fifteen years, there is as yet no work which summarizes the experimental techniques and the results available. The sixtieth birthday ofW. GENTNER, one of the pioneers in this field of research, is a suitable time to present such a summary.
Chapter
The most recent orogenic event in South-West Africa-the Damaran Episode-is reflected by consistent radiometric ages in the general range 450-550 (570) million years (hereinafter m.y.), yielded by regional metamorphic and granite-pegmatite minerals from orogenically deformed Upper Proterozoic rocks in two structurally continuous zones: (1) a northeast-southwest trending segment from the coast to the Kalahari Desert; and (2) a roughly north-south trending segment along the coast. The preponderance of ages within this range from correlative sequences of deformed and metamorphosed rocks in central and southern Africa, and in crystalline rocks associated with deformed correlatives in western Africa, demonstrates the regional extent of this episode. In addition, similar age patterns have been obtained from zones of gneisses, such as the Mozambique and Zambesi Belts of eastern and central Africa, in which Upper Proterozoic rocks have not been widely recognized. The tectono-thermal kinship of the zones of deformed Upper Proterozoic rocks and of the Mozambique and Zambesi Belts is indicated by the similarity of their age patterns and by their apparent geographical and structural continuity. Together these regions define a sinuous system of mountain chains-the Damarides-which, in part at least, " emerged" in Cambro-Ordovician times from an Upper Proterozoic-Lower Paleozoic mobile zone. In southern Africa this mobile zone separated and partly encircled two stable areas, termed the Congo and Kalahari Cratons, which have been stable since the Kibaran Episode ca. 1100 m.y. ago. The evidence of known orogenic intrusives and late-or syn-orogenic pegmatites and migmatites, suggests that the main pulse of the Damaran Episode took place 500 to 550(570) m.y. ago. In addition to the Damaran Episode, there is evidence that the Upper Proterozoic-Lower Paleozoic mobile zone was affected by an older episode, the Katangan Episode. This identification is based largely on U-Th-Pb ages from Katanga Province and on a limited number of similar Rb-Sr, U-Th-Pb, and K-Ar ages from western Africa and parts of the Mozambique Belt. The significance and extent of the effects of the Katangan Episode are still uncertain; its relationship to the Damaran Episode may, however, be similar to that of the Acadian and Taconian orogenies in the northern Appalachians. The evidence favors the retention of the term "Katangan Episode" for tectono-thermal events reflected by ages in the range 580-680 m.y.
Article
Reporting a radiometric date of a hypogene mineral or plutonic rock as a date of orogeny is unjustified. Many orogenies proceed without associated plutonism and many plutonic events take place without orogeny. Stratigraphic terms should not be used for tectonic or plutonic events unless the events are concurrent with the deposition of the strata. The term 'Grenville' has become meaningless because it has been used as a stratigraphic, provincial, tectonic and orogenic term spanning a great length of time and obscuring the tectonic and plutonic events of the Precambrian. 'Grenville' should be used as a stratigraphic term only and new names coined for restricted plutonic and tectonic events which involve rocks of the Grenville series.
Chapter
Although the radioactivity of potassium was discovered sixty years ago it is only during the last decade that the method of age determination based on the decay of potassium to argon has been widely used. The primary purpose of K-Ar dating has been to determine an age of geological significance.
Article
The K-Ar age pattern in the Scottish Caledonides is interpreted in terms of a cooling history. Eighty-four new K-Ar ages are reported and these are systematically related to be retentivities of the materials dated and to the structural position of the rocks sampled. Dalradian slates near the Highland Border with a flow-cleavage associated with the initial (F 1) deformation yield whole-rock ages averaging 505 ± 6 m.y. This is a minimum age for F 1 in the Scottish Dalradian. Post-500 m.y. Dalradian mica ages reflect cooling times following the main (post-F 2 pre-F 3) metamorphism, but a lower limit of 471 ±18 m.y. for this metamorphism is obtained from whole-rock analyses of high-level Dalradian slates, among the first rocks to cool after the metamorphic peak. The F 3 deformation is dated from peripheral slates with F 3 strain-slip cleavage of the Middle Dalradian and Ballapel successions, which yield whole-rock ages averaging 448 ±13 m.y. and 443 ±14 m.y. respectively. Uplift associated wim F 3 accelerated cooling of the main mass of Dalradian and Moine schists and gneisses and these yield mica averages of 451 ±17 m.y. (muscovite) and 440±13 m.y. (biotite). Intrusion of Newer Granites occurred between 425 and 390 m.y. ago, and localised high temperatures associated with this activity are evidenced in the age pattern obtained from the metamorphic rocks.
Chapter
The diffusion of rare gases in solids was first investigated by Rutherford (1901) and Strutt (1909). More intensive work was performed in the 1930’s by Hahn and his coworkers — Cook (1939), Flügge and Ziemens (1939).
Article
Data on the diffusion of Ar in sanidine, microcline, phlogopite, glauconite, and leucite are presented. We conclude from these data that: 1) Dynamic lattice changes markedly influence Ar loss at the time of the changes. 2) If Ar diffusion takes place under conditions of no lattice changes, all Ar will be lost at a single activation energy. 3) Glauconite and phlogopite have essentially identical diffusion characteristics. 4) The parameters of the diffusion equations for both feldspar and mica are only explainable on an hypothesis of extreme ordering of the diffusing Ar atoms. 5) Glauconites (and illites), due to their fine grain-size, are susceptible to high Ar loss at a temperature of 100 degrees C. if that temperature is maintained for a few million years. Thus, knowledge of the burial history of a sample is absolutely essential for each glauconite dated. 6) Feldspar diffusion appears to be a complex of lattice and grain-boundary diffusion, with the lattice unit having dimensions of 10-30 microns. 7) Heating a mica of 1 mm. thickness to 300 degrees C. for a few million years should remove all preexisting Ar, thus rendering impossible the dating of events prior to the last heating.
Article
"The cooling by conduction of an intrusive sheet in the neighborhood of a contact is discussed, taking into account the effect of heat of solidification. General formulae are given, and the important case in which the magma and country rock have the same thermal conductivity and density and the magma is intruded at its liquidus temperature is discussed in detail. Numerical values for the contact temperature are given for this case, calculated for various values of the range of solidification and latent heat of the magma. These values are more than 100 degrees C higher than those calculated by previous workers who have not adequately allowed for the effect of heat of solidification."
Article
More than 100 independent isotopic ages have been determined for minerals from an area in northern Michigan about 35 miles square. Granites, pegmatites, and metamorphosed sedimentary and volcanic rocks have yielded Rb-Sr ages for feldspar, muscovite, and biotite, K-Ar ages for hornblende, muscovite, biotite, and feldspar, and U-Pb and Th-Pb ages for zircon. It was anticipated that we would learn from the measurements both the intrusive and metamorphic history of the area and would be able to place limits on the age of the Precambrian sediments in this area. The conclusions may be summarized as follows: (1) Granites and pegmatites with approximate ages of 2,700 and 1,900 m.y. have been found. (2) The major mineral-forming metamorphic event occurred between 1,800 and 2,000 m.y. ago. (3) The Precambrian sedimentary rocks called Animikie are older than 1, 900 m.y. and younger than 2,700 m.y. (4) Biotite Rb-Sr and K-Ar ages and muscovite K-Ar ages were strongly modified by the equivalent of a rise in temperature approximately 1,350 m.y. ago, although no mineral-forming event in this interval has been observed. (5) The K-Ar system was further affected by a thermal rise at about 1,100 m.y. This later event is probably recorded geologically by a few diabase dikes.The resistance to post-formation thermal events shown by the various minerals and decay systems tested may be classified as follows (lowest to highest): feldspar K-Ar, biotite K-Ar, muscovite K-Ar, biotite Rb-Sr, zircon U 238-Pb 206, zircon Th-Pb 208, zircon U 235-Pb 207, muscovite Rb-Sr, feldspar Rb-Sr. The few hornblende K-Ar measurements indicate that the resistance of this system is about comparable to that of muscovite and feldspar Rb-Sr. The Pb 207-Pb 206 age derived from the two U-Pb ages is somewhat more resistant to change than the feldspar Rb-Sr ages.
Article
Rb-Sr, K-Ar, U-Pb, and Th-Pb ages of rocks and minerals from the Appalachian Piedmont in the vicinity of Baltimore, Maryland, and Washington, D.C. are reported. The oldest unit in this region is the Precambrian (about 1100 m. y. old) Baltimore gneiss which is overlain by the metasedimentary Glenarm series. Large post-Glenarm pegmatite swarms surrounding the Baltimore gneiss domes give Rb-Sr ages of 425±20 m. y. (λRb87 = 1.39 × 10−11 yr−1). Sometimes the age of these pegmatites has been lowered by subsequent events; in one case the pegmatite was re-equilibrated 345 m. y. ago. Small pegmatites intrusive into massive quartz monzonite (Guilford) and foliated granodiorite (Ellicott City) give younger age values (about 345 m. y.). Whole-rock Rb-Sr and zircon studies of massive granitic rocks appear to give ages similar to those of the older pegmatites but are insufficiently accurate for definitive conclusions. Most biotite (Rb-Sr and K-Ar) and hornblende (K-Ar) ages from the Paleozoic rocks which are more than 345 m. y. old, as well as from some of the Precambrian rocks, are about 300 m. y., indicating that the final closure of these minerals considerably post-dated emplacement of the rocks.
Article
The discordant U-Pb ages of sixteen minerals having Pb207/Pb206 ages of 2400 to 2700 m.y. closely fit a 2800 to 600 m.y. chord when placed on a pb207/U235-Pb206/U238 diagram. These minerals are from the continents of North America, Africa, Europe, Australia, and Asia. If the data are interpreted as evidence of an episode of loss of lead 600 m.y. ago from minerals that were crystallized 2800 m.y. ago, it is strange that the same time of loss is indicated for all four continents. Moreover, most of the samples are from shield areas where no evidence has been found for metamorphic events 600 m.y. ago. As an alternative explanation, lead may be considered to diffuse continuously from crystals at a rate governed by a diffusion coefficient (D), the effective radius (a), and the concentration gradient. Calculations of the present-day Pb206/U238 and Pb207/U235ratios as a function of the parameter D/a2 yield ratios that lie on a 2800 to 600 m.y. chord on a Pb206/U238-Pb207/U235 diagram for losses of up to two-thirds of the lead produced in the sample. This method is capable of explaining the regularity of the age patterns from one continent to another and does away with the need for episodic losses of lead simultaneously in all the areas 600 m.y. ago. Other examples have been found of suites of minerals with discordant ages which fit this hypothesis; they include minerals with apparent ages of 1900, 1700, and 1100 m.y. The behavior of uranium-lead ages in several zircons from metamorphic rocks is indicative of an activation energy for diffusion of lead of less than 10 kcal/mol over a temperature range of 50° to 400°–600°C. This inference, when considered with available data for the diffusion of argon from micas, is capable of explaining why uranium-lead ages measured from zircon may be either greater or less than the potassium-argon ages of coexisting mica.
Article
Precambrian rocks of the Front Range, Colorado, have been studied for mineralogic and geochronologic changes in a zone adjacent to a Tertiary (54 m.y.) intrusive stock. The country rocks are high-grade gneisses schists, amphibolites, and pegmatites. The only mineralogic change attributable to the contact heating takes place at a distance of one-tenth an intrusive width, where microcline changes to orthoclase. This change occurs by conversion of a highly triclinic microcline directly to orthoclase, over a distance of less than 100 feet. Study of the sodium content of the orthoclases reveals no change with distance, despite the strong thermal gradient. The Rb-Sr and K-Ar ages of biotites undergo a complete transition from unaffected Precambrian ages beyond one intrusive width from the contact (10,000 feet) to strongly affected ages near the contact. The K-Ar ages of hornblende are relatively unaffected beyond the first 100 feet. The K-Ar ages of potassium feldspar show variable response, being less affected than the biotite K-Ar age for the first 1,000 feet. The Rb-Sr ages of the potassium feldspar are apparently unaffected beyond about 50 feet. Quantitative interpretation of the biotite K-Ar and Rb-Sr ages in terms of heat flow and kinetic models shows that the data can be fitted to a wide variety of models. This treatment indicates essentially identical activation energies for the argon and strontium in the biotite, with the diffusion coefficients of the strontium being a factor of 5 smaller than that of the argon. Reasonable heat-flow models suggest a maximum value for the activation energy of about 30 kcal/mole.
Article
The red and white series A divisions of the I-Ieemskirk granite, western Tasmania, have a high internal scatter of total-rock data points about their respective isochrons. Consideration of the mineral phases of these rocks shows that the scatter is due predominantly to opensystem behavior of some of the samples, plus a small range in 'true' initial Sr7/Sr6 in at least one granite type. The open-system behavior of the mineral phases has a consistent pattern of loss of radiogenic Sr from K-feldspar and gain by plagioclase. Biotite either retains radiogenic Sr or leaks a fractional amount (up to 4%), which has led to a 14-m.y. range in indicated age. The upper limit of the range (367 _ 3 m.y.) is significantly older than the total-rock isochron estimates (red granite 346 _ m.y.,whiteseriesAgranite3524m.y.,whiteseriesBgranite3576m.y.),whichsuggestthatthelatterhavebeenbiasedtowardyoungvaluesbytheinclusionofopentotalrocksystems.TheKfeldsparagediscordancyrelatestothedegreeofsecondaryalteration(andtriclinicity),whichwasapparentlycausedbydynamicstressesassociatedwithfaulting m.y., white series A granite 352 _ 4 m.y., white series B granite 357 _ 6 m.y.), which suggest that the latter have been biased toward young values by the inclusion of open total-rock systems. The K-feldspar age discordancy relates to the degree of secondary alteration (and triclinicity), which was apparently caused by dynamic stresses associated with faulting 0 m.y. after granite formation. The youngest chlorite age of 296 m.y. supports the existence of this event, which has led by means of the isotopic redistribution response in the mineral phases, to open-system total rocks.
Article
The results of over 160 potassium–argon age determinations on Moine and Dalradian rocks are summarized. Histograms reveal a strong Moine peak at about 425 million years and a spread down to 570 million years; in the Dalradian the peak is at 435 million years (possibly influenced by geographical sampling bias) and the spread to 490 million years. Interpretation of the age pattern is discussed on the basis of (a), a rapid cooling hypothesis in which the Moine and Dalradian areas are considered to have been affected by an orogenic-metamorphic event about 425 million years ago and (b), a slow cooling hypothesis in which the spread of ages younger than about 460 million years in the combined Moine and Dalradian pattern is considered to reflect late-orogenic cooling. The merits of both hypotheses are considered in relation to the known polyphase orogenic-metamorphic history of the Caledonides. An assessment is made of the geological and radiometric evidence bearing upon the putative Pre-Cambrian metamorphisms of the Moine and Dalradian areas.
Article
Thirty-eight whole-rock samples of Connemara granites give the following Rb-Sr whole-rock ages: Galway granite, 384 ± 1 m.y.; Omey granite, 388 ± 17m.y.; Roundstone granite, 395±80m.y.; Inisk granite, 404±8m.y.; Oughterard granite, 510 ± 35m.y.; potashfeldspar gneisses, 725± 175m.y. As the field evidence shows that the Oughterard granite was intruded into the Dalradian metasediments post-F 4 in the Connemara fold-sequence, and possibly post-F 5, this dates the fold phases F 1 to F 4 and the metamorphisms M 5 to M 1 as pre-510 ±35m.y. The older isotopic age of the potash-feldspar gneisses agrees with their field-deduced age of post-F 2, pre-F 3. Muscovite, potash feldspar, and whole-rock Rb-Sr analyses show that the Oughterard granite was metamorphosed about 444 ± 7 m.y. ago, which is close to the K-Ar age obtained from three hornblendes separated from potash-feldspar gneiss. These results indicate that the post-500 m.y. dates obtained from the Connemara Dalradian rocks, and therefore presumably the Scottish Dalradian also, are the dates of late metamorphisms. While the possibility of Pre-Cambrian metamorphism and folding in the Dalradian is apparent, the results are equally in agreement with a Cambrian age for the main metamorphisms M 1 to M 5 and foldings F 1 to F 4, especially in view of the present uncertainty about the age of the Cambrian period. The base of the Arenig would seem to be younger than 510 ± 35m.y.
Article
Rubidium–strontium age-determinations are presented for minerals and whole rocks from the Lewisian, Moinian and Dalradian metamorphic complexes of Scotland and from the Connemara schists of western Ireland. Age data from the Lewisian complex confirm that it was affected by two major periods of metamorphism. Pegmatites associated with the Scourian part of the Lewisian complex are shown to be at least 2460 m.y. old, whereas the Laxfordian metamorphism occured about 1600 m.y. ago. The effect of the Laxfordian metamorphism on the Scourian pegmatites is to produce a scatter of ages in which coexisting potassium feldspars and biotites show the pattern potassium-feldspar age > biotite age. Six biotites, a microcline and a muscovite from the Moine Series have ages in the range 435 to 405 m.y., showing that a widespread Caledonian (sensu stricto) metamorphism affected the Moine Series 420 ± 15 m.y. ago. Two pegmatites from the Knoydart–Morar area yielded muscovites with ages of 740 m.y. and 665 m.y.; asurvey of the geochemical possibilities and consideration of the geological setting of the pegmatites suggest that the Moine sediments in this area are older than 740 m.y. and may have undergone an early metamorphism before this date. Specimens from the Dalradian Series of Perthshire suggest a major metamorphism at 475 ± 15 m.y. ago, interpreted as Lower or Middle Ordovician in age. Two whole-rock and three mineral analyses from the pre-metamorphic Ben Vuroeh granite-gneiss suggest that the intrusion was formed 600 ± 100 m.y. ago and that a partial reconstitution occurred 415 ± 10 m.y. ago. The Ben Vuroch granite complex as a whole appears to have behaved as a closed system with respect to rubidium and strontium during later metamorphism. Three specimens of muscovite and biotite from the Connemara schists of western Ireland have a mean age of 475 m.y.; this finding tends to support the generally supposed contemporaneity of the Dalradian and Connemara metamorphisms. Biotite from the Galway granite has an age of 365 ± 10 m.y., which suggests that this granite may be contemporaneous with other dated Caledonian granites of the British Isles. Four potassium–argon ages support the conclusions on the age of the Laxfordian and Caledonian–Moinian metamorphisms.
Article
For most of Phanerozoic time errors in radio- metric ages are usually approximately 3 per cent. except for Rb-Sr results from Mesozoic and Tertiary minerals, which must have high Rb/Sr ratios to give such low errors. Rates of uplift appear to be of the order of 10 cm/100 years for sustained movement; in consequence, uplift will normally take place within the error of an age except for post-Jurassic examples. The age obtained for a plutonic complex marks the time of closure of radiogenic systems: the available data indicate that this effectively takes place at about 200°C for the most retentive minerals, but this figure is subject to considerable uncer- tainty. Closure of systems at low temperatures poses several problems, which are discussed. It is concluded that there is much uncertainty about the stratigraphical meaning of ages obtained from minerals from large plutonic complexes.
Article
The number of isotopically dated zircons from the basement complex of the Karelian belt has been doubled to include six samples. The ages are all discordant and are compatible with loss of lead by a continuous diffusion mechanism. Two possible mechanisms are that diffusion took place at a constant value of the parameter,D/a2D/a^{2}, or under conditions such that D/a2D/a^{2} increased linearly in proportion to the amount of radiation damage. The lead age data are also compatible with an episodic loss of lead about 500 m.y. ago, but there is no evidence that episodic loss should have occurred at that time. The regional metamorphism which affected the rocks 1,700-1,900 m.y. ago, as recorded by the mica age values, did not seriously alter the uranium-lead ratios in the zircons of the pre-Karelian basement complex; however, zircon from the gneiss domes to the west may have experienced some lead loss as a result of the regional metamorphism. Four detrital zircon samples from Svecofennian schists near Tampere have d...
Article
The solution to the equation for the diffusion of lead and uranium from a sphere, including the effect of a pulse of metamorphism, is applied to the problem of discordant UPb ages. It is found that a pulse of metamorphism can greatly disturb the straight-line pattern found by Tilton, and the implications of this with regard to the thermal history of zircons are discussed. The effect of pure U diffusion is found to generate a straight-line pattern having a different slope from that for pure Pb diffusion. A very small amount of U diffusion combined with Pb diffusion can significantly aJter Tilton's straight- line pattern. This effect is relatively insensitive to the magnitude of the U diffusion. The effects of combined U and Pb diffusion with a pulse of metamorphism are found to be too complex to exhibit regularities. (auth)
Article
An attempt has been made to evaluate the significance of various determinations of equilibria between the polymorphs of Al2SiO5. A phase diagram has been constructed and free energies estimated from the data of Newton and Weill. If uncertainties involved are taken into account the results of many workers are compatible.
Article
A field and petrologic study of the Lone Grove granitic pluton and surrounding rocks from the Llano Uplift, Texas, suggests that this area has been involved in a single orogenic cycle with no later general metamorphism. Samples of granite, aplite, pegmatite, rhyolite, and metamorphic rocks were investigated in order to determine the precision in Rb-Sr and K-Ar ages between various minerals and different localities. Refined chemical and mass spectro-metric methods are capable of yielding ages on most highly radiogenic minerals to an analytical precision of ± 1 2-Jan per cent or better. Most of the ages from these rocks show a spread commensurate with the experimental error.The average Rb-Sr age on microclines, muscovites, and biotites is 1020 million years and the average K-Ar age on muscovites, biotites, and hornblendes is 1045 million years (Rb 87, λ β = 1·47× 10 -11 yr -1; K 40, λ β = 4.72 × 10 -10 yr -1, and λ ε = 0-585 × 10 -10 yr -1). A total rock Rb-Sr age on one of the granites gives no indication of being older than those of the constituent minerals.The only rock to show a real age difference is a rhyolite porphyry, which gives an average Rb-Sr microcline age of 920 million years. A metasedimentary gneiss having a total rock Rb-Sr age of 1110 million years may contain some radiogenic strontium from an earlier history. K-Ar determinations on several microcline and plagioclases give ages which are 5-20 per cent low relative to the other minerals, presumably due to argon diffusion from the feldspar. Anomalously low Rb-Sr ages occur on several fresh biotites from pegmatites and granite. Evidence is presented for strontium or rubidium migration in these rocks although the exact nature of the process is not known. Also somewhat low K-Ar ages are obtained on the pegmatitic biotites.A study of the effects of weathering on the geochronologic systems is made on two obviously altered granites. The only mineral to suffer any decrease in apparent age from such surface alteration is biotite, by the Rb-Sr method.The Sr 87/Sr 88 ratio of the original strontium incorporated into the minerals of the granite is determined on several minerals having low Rb/Sr ratios and is found to be 0.0843±0-002 (normalized to Sr 86/Sr 88 = 0.1194). A discussion of the distribution of rubidium, potassium, and normal strontium throughout the pluton is given and partitioning factors for the rubidium to potassium concentrations between different mineral species are calculated.
Article
Published isotopic ages, together with new determinations on sixteen intrusions, show that the majority of the Newer Granites were emplaced 400 ± 10 million years ago. Potassium-argon ages reveal a peak in the Moine rocks at about 420 to 430 million years and a somewhat earlier peak in the Dalradian. Although the majority of the Newer Granites were therefore emplaced in country rocks which had already become closed systems to radiogenic argon, several important exceptions to this rule have been recognized and present results indicate that some intrusions yield potassium-argon ages concordant with those of their country rocks. It is suggested that variations in intrusive style of the granitic masses may be related to their age and hence to the temperature of their country rocks and depth of cover.
Article
Rapid uplift along the Alpine Fault zone in New Zealand exposed ancient ; mica schists which are estimated to have been at depths up to 9000 ft prior to ; the fault displacement, on the assumption that the smooth surface represented by ; the present, flat topped, peak elevations is truly representative of the mature ; Pliocene surface prior to uplift and glaciation. K-Ar age measurements on the ; micas show losses of radiogenic argon that vary with depth of burial up to almost ; complete loss from biotite at a depth of 9000 ft. The mean value of the ; diffusion parameter (D/a²) calculated is 6 x 10⁻¹⁶ sec⁻¹ at an ; estimated temperature of 110 deg C. This value is much higher than expected but ; is compatible with effectively complete retention of argon at surface ; temperatures, and with measured diffusion losses at high temperatures, if in the ; relationship D/a²= (D/sub o//a²) exp (-E/RT) D/sub o//a²= 1 sec/; sup -1/ and E = 27 Kcals/ mole. A single Rb-Sr analysis on one of the low argon ; biotite samples showed anm almost equal loss of radiogenic Sr⁸⁷. (auth);
Article
The Gotthard massif in central Switzerland, part of the autochthonous basement of the Alps, has been metamorphosed during the Alpine orogeny. Structural studies of the metasedimentary hornblende rocks along the southern border reveal the existence of several mineral generations which were formed at different times and under different conditions. The sequence of formation can be deduced from their mutual intergrowth and orientation relative to the fabric of the rock. Steiger [1962] concluded that the Alpine orogeny in this region consisted of two distinct tectonic phases separated by a thermal phase. The main tectonic phase was the dragging along of the Gotthard massif by the overlying northward-moving nappes. Most of the pre-Alpine mineral assemblage was crushed and new minerals were formed which show preferential N-S orientation (direction of the nappe movements). The subsequent thermal metamorphism was linked with anatectic processes in the Lepontinic region south of the Gotthard massif. At this time porphyroblastic minerals of random orientation were formed along the southern margin of the massif. A weaker tectonic phase consisting of an E-W contraction caused N-S oriented small folds and wrinklings in mica-rich schists, and produced cross biotites. Hornblende was formed in pre-Alpine times as well as during both the main tectonic and the thermal phase of the Alpine orogeny. K-Ar ages were determined for 17 hornblendes. N-S oriented hornblendes show exclusively ages of 46 m.y.; random hornblendes yield ages of 23 to 30 m.y. Partially oriented hornblendes give apparent ages of 23 to 112 m.y. The hornblende ages are definitely higher than Rb-Sr ages of bioties (16 m.y.) from this region [Jäger, 1962]. It is concluded that the N-S oriented hornblendes give a minimum age Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W contriction of proximately date the period (23 to 30 m.y.) of the anatexis in the Lepontinic region. The Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W constriction of the massif and the Lepontinic region. Partially oriented hornblendes cannot be related to any particular event. They may in part represent relics of pre-Alpine origin which suffered differential loss of argon during the Alpine metamorphism.
Article
Volcanic and minor intrusive igneous rocks often have more closely defined geological ages than rocks of the plutonic association. Isotopic ages from these rocks are thus important in geochronology. Isotope loss and other factors often result in low ages being obtained, rather than the true age of primary crystallization of an igneous rock. In favourable circumstances, however, the date of a subsequent cleavage, metamorphism, or mineralizing event can be determined. Detailed geological and petrological analysis are essential if the isotopic data are to be correctly interpreted. A general review of published work on bentonites, tuffs, lavas, and related sub-volcanic intrusions reveals the potentialities and limitations of age studies on this category of rocks. Young volcanic and the unfolded eruptive rocks of post-tectonic environments and of the stable zones of ancient shields provide the best material for initial age studies. The significance of isotopic age studies on basic igneous rocks in the dating of orogenic and mineralizing events is discussed. Determinations of the age of certain Caledonian and Variscan events in Britain are quoted to illustrate the methods used.
Article
The theory, analytical method, and experimental errors of potassium–argon radiometric age-determination are briefly reviewed. The choice of suitable material for analysis and the application of the method to basic igneous and other rocks using whole-rock samples are described. ‘Discrepancy’ in potassium-argon age-determination is defined; geological factors that control the accuracy of potassium-argon ages, and must be taken into consideration in their interpretation, are discussed. It is asserted that, in favourable circumstances, potassium-argon age studies can provide a wide variety of alternative information of geological significance as well as, or instead of, indicating the initial age of consolidation of a rock: in particular, that it is possible to define the dates of subsequent cleavage, metasomatism, or metamorphism from a consideration of the discrepantly low ages obtained from older rocks.
Article
The ratio rλ of radiogenic daughter to residual parent (with decay constant λ) was investigated for a continuous diffusion model for a diffusion coefficient that is an arbitrary function of time. A solution for simultaneous diffusion of both parent and daughter is given in closed form. A theory is also developed that describes the trajectories rλ, rλ′, for paired decay schemes (with decay constants λ and λ′) in the neighborhood of concordia. A wide class of systems is shown to be linear in the rλ, rλ′, diagram when ∫0τD(η) dη/a2≪1. A radiation damage model is proposed foi the diffusion coefficient that relates D(τ) to the integrated irradiation damage and to the uranium (and thorium) contents. The functional relationship between rλ and the uranium content is found for three important cases. The model D(τ) = D1τ yields a rλ, rλ′, curve that is very similar to that obtained by Tilton for D = D0. The slope is somewhat greater and the linear region more extended. The theoretical curves are found to be in reasonable agreement with the existing experimental data, and they fit the existing data at least as well as the constant D model.
Article
The triple point in the Al2SiO5 polymorphic system has been determined experimentally; it lies at 6.50.5 kb and 59510 C. This result differs strongly from the data of other experimental investigations on this system; the reasons for this are discussed. The new data given here for the phase boundaries kyanite/sillimanite and andalusite/sillimanite as well as the resulting position of the triple point agree with petrologic considerations based upon both experimental work in related systems and petrographic observations.
Article
The effects of thermal metamorphism on mineral ages have been investigated near the contacts of two basic intrusives. Rb-Sr and K-Ar ages of biotite, muscovite and potassium feldspar and K-Ar ages of hornblende from the Snowbank granitic stock near the contact with the Duluth Gabbro in Minnesota and Rb-Sr and K-Ar ages of biotite and K-Ar ages of hornblende from amphibolite near the contact with a quartz dolerite dike in the Beartooth Mountains in Wyoming are the basis of the kinetic studies. The Snowbank stock is 2·60 billion (109) yr old and was transgressed by the Duluth Gabbro 1·05 billion yr ago. The gneisses and amphibolites in the Beartooth Mountains were subjected to metamorphism 2·6 billion yr ago and were intruded by the dolerite dike, 50 m thick, approximately 800 million yr ago.
Article
Potassium-argon and rubidium-strontium age measurements have been made on mica separated from many rock types of the Manhattan Prong and New Jersey-New York Highlands (Reading Prong). Most of the rocks from the Manhattan Prong were dated within experimental error at 360 m.y. (million years). Since this age is identical with those obtained by the U-Pb method by other workers on intrusive pegmatites, it probably represents the last major metamorphic event in the area. The isotopic age of mica in the Highlands at least 10 miles from the zone where complete re-crystallization took place 360 m.y. ago is about 840 m.y. Apparent ages of mica in a transition zone range between these two. The ages in the transition zone are determined by the extent of re-crystallization which in turn is strongly dependent on the local structure. If the 1150-m.y. zircon age obtained by Tilton and others (1960) in the Highlands is the primary age of the basement in this region, major metamorphic events took place at 1150, 840, and 360 m.y. Relict ages as old as 480 m.y. in the Manhattan Prong suggest a fourth event if the rocks of the Prong were deposited in the Cambrian or later. This study illustrates both the difficulties and the effectiveness of isotopic geochronometry in areas of complex metamorphic history.
Article
AN outline of the research programme undertaken from University College, London, to elucidate the history of Andean Uplift in northern Chile has recently been presented in Nature1. Locally, in the San Bartolo area, the post-Cretaceous geological history has been fairly clearly established from work carried out in 1961 (ref. 2). The chronology established, however, was entirely relative, in the absence of fossil evidence in this suite of continental sediments and volcanic rocks. Thus the San Bartolo ignimbrite formation, which is part of the Formation Liparitica of Bruggen3, was regarded by him as Miocene or younger. Other workers, however, have suggested chronologies which imply a late Pliocene or Pleistocene age4.
Article
The paper attempts to summarise recent work on time relationships of deformation and crystallisation in the Moine and Dalradian rocks of the Caledonian orogenic belt in Scotland. Three or four deformation episodes, each accompanied by some kind of metamorphism, have been detected.' Crystallisations of metamorphic minerals can be shown to have accompanied and, more especially, followed a particular episode of deformation and so the observed metamorphic fabrics result from a complex interplay of deformations and crystal lisations. Rocks may contain disequilibrium assemblages, with the metastable survival of early crystallised minerals. In the Dalradians, and probably the Moines also, the earliest known movements, which caused great thickening of the sedimentary pile, were accompanied by only low-grade, dynamic, metamorphism. The main episodes of crystallisation occurred before, during and after the second deformation. The three episodes are part of a continuous, perhaps progressive, metamorphic process, which possibly embraced some local oscillations in P/T conditions. The third, final, episode of crystallisation - an important and extended static period between the second and third fold movements - often marks the climax of the crystallisation and grain growth resulting in the build-up of schistose or gneissose fabrics. 'During this episode in the Dalradians there were regional crystallisations of kyanite-sillimanite ("Barrovian type") and andalusite-sillimanite ("Buchan type") sequences. The occurrence of these sequences at different structural levels is thought to have been controlled by orogenic P/T gradients. An account is included on current work on problems of dating the migmatites in the, Scottish Highlands. A separation in time between migmatitic and regional metamorphism has been proposed at various times by different workers. Recently it has been suggested that the Dalradian migmatites and associated sillimanite have been superimposed upon the pattern of andalusite /kyanite metamorphism. The available evidence bearing on this problem is discussed. The migmatites appear to show a different, perhaps more prolonged history of 'crystallisation than do the non-migmatitic rocks. T/P oscillations may have played an important part in the evolution of the migmatite fabric and the climax of crystallisation in the migmatite zones may not have have been attained at the same time as elsewhere. Although the history of crystallisation in relation to deformation in the Moines follows a broadly similar pattern to that in the Dalradians it has yet to be established that the regional metamorphism Moine and Dalradian tracts are coeval. Indeed, recent geochronological work, to which brief reference is made, suggests they may not be coeval.
Applied geochronology
  • E I Hamilton
  • Hamilton E. I.
The geochronology of equatorial Africa
  • L Cahen
  • N J Snelling
  • Cahen L.