The behavior of isotopic systems (Sm-Nd, Rb-Sr, K-Ar, and U-Pb) in minerals from Archean plagiogneisses in the outer-contact zone of an alkaline-ultram afic intrusion points to an age discordance that resulted from the active action of the alkaline CO2-bearing fluid. The K-Ar system of biotite is the least susceptible to the chemical influence of the fluid with the age of this mineral becoming systematically older away from the contact according to variations in the thermal field of the intrusion. At the same time, closer to the contact, the potassic feldspar loses not only 40Ar but K also, whose concentration increases by one order of magnitude (from 0.3 to 2.7%) in the plagioclase. The latter mineral also becomes richer in volatile components (Ar-40 and CO2) in the fenitization zone. The influence of the fluid on zircon is manifested in Pb and U migration and results in a newly formed metasomatic population of this mineral. The maximum U-Pb discordance arises inside zircon grains, with preservation of their thin zones of Archean age. The data obtained on the Sm-Nd and Rb-Sr systems definitely indicate that Sr and Nd were transported from the intrusion into the host gneisses and that the transport kinetics of these elements in the pore (interstitial) space was different. The character of changes in the isotopic composition of rocks during metasomatism differs from simple mixing and corresponds to the ion exchange mechanism in the rock-fluid system, when the transport of elements in fluid along an array of fractures is associated with the diffusion of these elements in the interstitial space of the rock. Thereby Sr quickly achieves equilibrium with the host rocks during the formation of metasomatic minerals in them, while Nd can be transported for long distances (in the form of soluble carbonate complex) via fractures and veinlets and is accommodated in the minerals of the vein assemblage (apatite, sphene, and pyrochlore).

This paper considers applications and limitations of the new branch in isotope geochronology that is concerned with the estimation of the temperature conditions of superimposed postcrystallization processes from the analysis of partial loss of radiogenic isotopes. The dependence of the loss on time and temperature allows one to reconstruct the metamorphic history of rocks and minerals. The closure temperature of isotopic systems, T-c (Dodson, 1973), is currently used as the main parameter connecting T-t evolution with the measured apparent age, although this is a secondary parameter. Activation energy (E) and the frequency factor C-0 (or diffusion coefficient DO) are more appropriate primary parameters. The kinetic parameters E and Do are used to define the "diffusion loss constant lambda(d)", which controls the character of temperature-time evolution. A combination of kinetic parameters for high-temperature (U-Pb for zircon and monazite) and low-temperature (K-Ar for feldspars) systems embraces practically the whole temperature interval including the peak of metamorphism, retrograde stage, and cooling from similar to1000 to similar to200degreesC and lower, if fission-track and U-He methods are involved. Examples are presented for the use of thermochronology to reconstruct the T-t history of various regions from Alpine to Archean age. The wide application of thermochronology is strongly hindered by the dependence of kinetic parameters including closure temperature on the fluid regime of metamorphism. There is a need for further E, D, and T-c measurements in various isotopic systems and minerals within a range of experimental P-T-x conditions and in various metamorphic environments (natural) from "dry" to fluid-bearing. At early stages of the application of thermochronological methods, one should choose rocks corresponding to dry metamorphic conditions and the isotopic systems that are least dependent on fluid conditions.

The Sm-Nd, Rb-Sr, and K-Ar isotopic systems were examined in Archean rocks and minerals of the Belomorian Belt, Kola Peninsula, subjected to regional metamorphism of the epidote-amphibolite facies. The Svecofennian metamorphism did not change Archean Sm-Nd characteristics of the biotite plagiogneisses and amphibolites. According to the determined T-DM = 2.88-2.94 Ma and epsilonNd(t) = + 2.5, the protolith of the gneisses has an old age of 2.9 Ma and originated from the depleted mantle not affected by any contamination by crustal material. Although the Rb-Sr system of the rocks was partly opened in Svecofennian times, their initial strontium ratio (similar to0.700) calculated from the zircon age also points to the depleted mantle as the protolith of the gneisses. In terms of the U-Pb zircon method, the Archean metamorphism has a young age of 2.73 Ma. The subsequent Proterozoic metamorphism of the epidote-amphibolite facies was responsible for the discordant ages studied by the Sm-Nd, Rb-Sr, and K-Ar methods in rock-forming and accessory minerals and ranging over the interval from 1.97 to 1.55 Ma. Most of these dates are related to sequential closing of the isotopic systems with cooling of the rocks during the retrograde stage of the Proterozoic metamorphism. When studied by the Sm-Nd or Rb-Sr method, similar minerals from gneisses and schists; have discordant age values, which shows that fluid plays an important role in the equilibration of the isotopic systems. According to Sm-Nd and Rb-Sr isotopic data on apatite (a fluid-sensitive mineral), the fluid circulation is dated at 1.77 and 1.67 Ma, respectively. The average cooling rate of Belomorian rocks was about similar to2degreesC/Ma, which is calculated from the difference between the Sm-Nd age of minerals in garnet amphibolite and the Rb-Sr age of biotite, and from the temperatures at which these isotopic systems were closed. However, the thermal-temporal pattern of Belomorian rocks shows two stages of the retrograde metamorphism: (1) slow cooling with a rate much lower than 2degreesC/Ma, which was related to the long existence of deep-seated rocks, and (2) subsequent rapid cooling with a rate over 10degreesC/Ma, which is explained by the rapid collision-related uplift of the region. This interpretation of isotopic data is consistent with the occurrence of this area within the Belomorian Belt near its junction with the Imandra-Varzuga structure.

In this contribution, we propose using the concordia diagram, which was proposed by Wetherill, to evaluate the discordant values of K-Ar and Rb-Sr mineral ages. The considerable difference between the decay constants of K-40 and Rb-87 allows us to apply Wetherill's model to these systems.

Products of hydrothermal conversion of glauconite (biotite and potassium feldspar) contain radiogenic argon inherited from a primary mineral during topotaxic solid-state transformation without the stage of complete dissolution. The degree of inheritance differs for glauconites of different chemical composition. The kinetics of release of Ar-40, H2O, and H-2 from primary glauconite and products of its recrystallization points to the existence of individual transformation stages (primary mineral-dehydroxylation-atomic reorganization) and to the factors governing the inheritance. Low temperature of conversion and contemporaneous operation of dehydroxylation and transformation decrease a diffusive loss of radiogenic argon from glauconite and determine a greater degree of its inheritance. The distortion degree of the anion framework of the mineral structure depends on the rate of Fe3+ to Fe2+ reduction and govern the inheritance. If this reaction is realized before a glauconite transformation, the migration of newly formed Fe2+ ion into the trans-position is accompanied by bond breaking in the anion framework, which results in a greater loss of radiogenic argon and a decrease of inheritance degree.

U-Pb dating of zircons from the reference granitoid complexes showed that principal thrusts in the northern flank of the junction zone between the Olekma granite-greenstone and Aldan granulite-gneiss terranes of the Aldan Shield were formed 1962 +/- 5-2398 +/- 4 Ma ago. The oldest structural and metamorphic transformations affected the nonstratified infracrustal complex 3005 +/- 4 Ma ago. These data substantiate the reliable correlation of structural units with the geochronological scales previously elaborated for the northern flank and adjacent areas of the Olekma granite-greenstone and Aldan granulite-gneiss terranes.

Excess radiogenic argon was discovered in the products of glauconite hydrothermal alteration's at P-H2O = 1-2 kbar and T = 400-600 degrees C. The newly formed minerals are biotite and potassic feldspar. An isotopic analysis of The reaction products proves that the genetically excess argon was not captured from the fluid but inherited by the biotite from the original mineral. This proves the possibility that silicate matter can be inherited during solid-state transformations that proceed according to topotaxic mechanisms without complete dissolution in fluid. The prospects of the use of radiogenic argon are considered from the standpoint of its employment as a tracer in studying the mechanisms of aluminosilicate transformations.

Excess Ar-40 of diverse geneses (entrapped and inherited) was detected in minerals (feldspars and glauconite) after hydrothermal treatment for three days at T = 200-600 degrees C and P = 1 kbar. The orthoclase contains significant amounts of air-derived argon (10(-4) cm(3)/g) at a low partial pressure, which is constrained by the solubility of argon in water. By contrast with argon in plagioclase, this element in orthoclase occurs in an adsorbed mode and has high energy in the crystal structure (in anion vacancies, which easily form in the presence of water). Excess radiogenic argon was detected in the products of complete glauconite decomposition: biotite and feldspar. As indicated by isotope data, excess argon of this type was not entrapped but inherited from the precursor mineral.

The Rb-Sr isotopic measurements of bulk samples and monomineral fractions of contact zone gneisses of alkaline-ultrabasic Ozyornaya Varka intrusion were performed. The average value of geologic age were estimated as 2760 +/- 120 m.y. and 1.5-1.6 m.y. from the bulk rock and mineral isochrone correspondingly. The first value is corresponding to the time primary metamorphism of amfibolite grade. The Rb-Sr mineral age of gneisses shows the absence minerals. It can be associated with the higher sensibility of Rb-Sr system in relation to the fluid in comparison with the K-Ar system.

Based on natural zircon specimens of different crystallization degree the experiments were carried out in respect of possibilities and constraints of recent techniques of separation of those specimen fragments which are least altered by secondary processes as only samples suitable for a mostly concordant age values. The considered techniques are as following: aeral grinding treatment; separation of heaviest mineral fraction; differential dissolution in hydrogen fluiride acid. Least discordant zircon specimens from crystalline samples and semicrystalline grains with sufficient fraction of the crystalline phase were separated by use of differential dissolution. The comparison with other independent treatment techniques proves the absence of predominant uranium and lead removal is such procedure. This techniques could not be applied to completely metamict zircons subjected to leaching of lead.

Two granitoid intrusions, the Amnunnakta and Oldongso massifs in the Olekma granite-greenstone terrain of the Aldan shield have been studied using Sm-Nd, Rb-Sr and Pb-Pb whole rock (WR) and U-Pb zircon methods. The isotopic composition of Pb in plagioclase, as well as whole-rock oxygen isotope compositions and REE patterns were also measured.The following ages were obtained for the Amnunnakta massif: U-Pb zircon 2984 +/- 22 Ma (2sigma), Pb-Pb WR 2991 +/- 57 Ma, Sm-Nd WR 3094 +/- 430 Ma and for the Oldongso massif: U-Pb zircon 2999 +/- 51 Ma, Pb-Pb WR 2855 +/- 170 Ma, Sm-Nd WR 2923 +/- 144 Ma. The similar ages suggest penecontemporaneous formation of the two intrusive series. The observed resetting of the Rb-Sr WR systems (2767 +/- 116 Ma for Amnunnakta massif and 2597 +/- 82 Ma for the Oldongso massif ) and opening of the plagioclase Pb isotopic systems are associated with superimposed metamorphic processes of the greenschist and epidote-amphibolite facies.Source regions of the massifs are characterized by long-term depletion in incompatible elements: the initial epsilon(Nd)(T) value is +2.4 +/- 0.4 for both massifs. The mu1 values are mantle-like at 7.68 for the Amnunnakta and 7.71 for the Oldongso massifs and the initial Sr composition is low (Sr(i) = 0.7015 +/- 5) for the Oldongso massif. Basic rocks of the Amnunnakta massif are characterized by low deltaO-18 values (2.7-4.7) and elevated Sr(i) = 0.7052 +/- 7 which may be caused by both a primary low deltaO-18, high Sr-87/Sr-86 magma or later interaction with a hydrothermal system. There is no evidence for recycled sedimentary material in the granitoids of either massif. One of the rocks, a fine-grained garnet-bearing trondhjemite, previously assigned to the Amnunnakta massif, yielded a T(Nd) Model age of 3700 Ma and reflects the presence of older sialic material. Moderately fractionated REE patterns with well pronounced negative Eu anomalies in low-alumina trondhjemites of the Amnunnakta massif suggest their formation by a process of differentiation of high-alumina basaltic melts with fractionation of plagioclase. High-alumina tonalite-trondhjemites of the Oldongso massif are characterized by fractionated and depleted HREE patterns suggesting the residual presence of garnet and hornblende during partial melting of a basic source.

Mechanisms of Pb diffusion in zircons are considered. The experimental data on Pb extraction kinetics from zircon specimens under dynamic. isothermal, and stepwise annealing are given. The role of grain and heating rate under dynamic annealing are studied in respect to the Pb extraction kinetics from zircon. The problems of activation energy computations for Pb extraction process and diffusion coefficient of Pb in zircon are discussed. The comparative description of the Pb extraction has been proposed involving different mechanisms of Pb migration in zircon.