ALBERT

Notes: Abstract The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M1-3) and two stages of deformation (S1 and S2) were observed in thin sections: M1) garnet (Alm55-60, Prp22-28, Grs16-20) + omphacite (Jd46-56) + phengite (Si ≅ 3.5) + rutile; M2) garnet + glaucophane ± lawsonite + white mica; and M3) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.

Notes: Abstract 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.

Notes: An electron microscopic description is presented of the mitochondria-rich‘chloride‘cells of the gill epithelium in the cichlid fish Tilapia aurea kept in freshwater or adapted to seawater and injected with the pesticide p,p′DDT. The‘chloride‘cells may have a light or dark (electron dense) appearance. Dark‘chloride‘cells are more common in seawateradapted than in freshwater T. aurea. Characteristic features of‘chloride‘cells include an abundance of mitochondria and an elaborate tubular reticulum, which communicates directly with the basal and lateral membranes of the cells. The apical portion of the‘chloride‘cells is often larger in seawater-adapted than in freshwater fish. Differences in‘chloride‘cell ultrastructure, which can possibly be associated with a response to different salinities, arc described and discussed. No significant ultrastructural differences are detectable in response to injections of the pesticide.