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The Kokchetav Complex is a tectonic mega‐melange consisting of seven pre‐Ordovician units (units I‐VII) of contrasting lithologies and P–T conditions of metamorphism, overlain and/or intruded by four post‐recrystallization entities. Most of the constituent rock types display affinities with continental crust; paraschists and paragneisses, which carry biogenically produced carbon, clearly were laid down near the surface of the Earth. Microdiamond (and rare coesite) inclusions are contained in strong, refractory garnet, zircon, clinopyroxene, and kyanite, some of the constituent neoblastic phases of this metasedimentary unit. Systematic mineral parageneses and textural relationships support the hypothesis that the metamorphic assemblages represent a close approach to chemical equilibrium at the time of formation. Metamorphism of diamond‐bearing paragneisses and schists transpired at 535 ± 5 Ma; physical conditions included minimum pressures of 40 kbar and temperatures exceeding 900 °C. Other associated units contain mineralogic evidence of somewhat lower to considerably lower pressures and temperatures: observed magnesite + diopside pairs, coesite, grossular‐pyropic garnet, potassic clinopyroxene, Si‐rich phengite, barroisite‐crossite(?), aluminous titanite and/or Al‐rutile, and the assemblage talc + kyanite + garnet all testify to relatively elevated pressures of formation. The metamorphosed lithotectonic units represent individual, discrete stages in what initially may have been a continuous P‐T series, but intense post‐metamorphic dislocation has resulted in the preservation of a chaotically mixed sequence rather than an unbroken gradation in preserved conditions of metamorphism. Only units I‐III, and probably VIb may represent portions of a dismembered subduction zone lithologie assemblage. The uplift to mid‐crustal levels and cooling of the mega‐melange took place by about 515–517 Ma, at which time the complex was stabilized as a part of the Kazakhstan microcontinental collage. An hypothesized Late Vendian‐Early Cambrian subduction of the Kazakhstan‐North Tianshan(?) microcontinental salient to depths exceeding 125 km, followed by decoupling from the descending oceanic crust‐capped lithospheric plate is held responsible for the ultrahigh‐pressure metamorphism of the Kokchetav Complex. Inasmuch as vestiges of a calc‐alkaline volcanic/plutonic arc of approximately Early Cambrian age are preserved as only scattered relics in the general region, the plate‐tectonic setting may have involved an intra‐oceanic, Marianas‐type, incipient arc which was subsequently removed through transform faulting or erosion.

A fter the accident at the Chernobyl nuclear reactor in 1986, the concentration of radioactive caesium ( 134 Cs and 137 Cs) in fish was expected to decline rapidly. The estimated ecological half-life (the time needed to reduce the average caesium concentration by 50%) was 0.3 to 4.6 years 1,2 . Since 1986, we have measured radiocaesium in brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus), both of which are widely eaten in Scandinavia, in a lake contaminated by Chernobyl fallout 3,4 . We have measured radiocaesium in nearly 4,000 fish, taking samples 2-4 times every year from spring to autumn. We find that the decline in radiocaesium was initially rapid for 3-4 years and was then much slower. About 10% of the initial peak radioactivity declines with an ecological half-life of as long as 8-22 years.The concentration of 137 Cs, the longlived radioactive caesium isotope with a physical half-life of 30.2 years, peaked in 1986. The radioactivity was three times higher in brown trout than in Arctic charr (geometric means: 10,468 and 3,097 Bq kg ǁ1 ). The decline in 137 Cs from its maximum in 1986 to 1998 is modelled by singleand two-component decay functions: Q t ǃQe ǁkt and Q t ǃQ 1 e ǁk 1 t +Q 2 e ǁk 2 t , where Q is the caesium concentration, k is the decay rate and t is time in years after the peak. The ecological half-lives are ln2/k, and are an indication of how long it will take the fish to rid themselves of radioacti-vity. The proportional contribution of the maximum radioactivity with slow decay rates was estimated as Q 2 /(Q 1 +Q 2 ).The decline in 137 Cs was rapid during the first three (brown trout) and four (Arctic charr) years, and was then slower. Based on the initial rapid decline, ecological half-lives were estimated using a single-component decay function at 1.0 and 1.5 years for brown trout and Arctic charr, respectively, as in other post-Chernobyl studies 1,2 , but this underestimates the time that 137 Cs persists in the fish. A two-component decay function