ALBERT

Notes: In the Transangarian region of the Yenisey Ridge in eastern Siberia (Russia), Fe- and Al-rich metapelitic schists of the Korda plate show field and petrological evidence of two superimposed metamorphic events. An early middle Proterozoic event with age of c.1100 Ma produced low-pressure, andalusite-bearing assemblages at c. 3.5–4 kbar and 540–560 °C. During a subsequent late Proterozoic event at c. 850 Ma, a medium-pressure, regional metamorphic overprint produced kyanite-bearing mineral assemblages that replaced minerals formed in the low-pressure event. Based on the results of geothermobarometry and P–T path calculations it can be shown that pressure increased from 4.5 to 6.7 kbar at a relatively constant temperature of 540–600 °C towards a major suture zone called the Panimba thrust. In order to produce such nearly isothermal loading of 1–7 °C km −1, we propose a model for the tectono-metamorphic evolution of the study area based on crustal thickening caused by south-westward thrusting of the 5–7 km-thick upper-plate metacarbonates over lower-plate metapelites with velocity of c. 350 m Myr−1. A small temperature increase (up to 20 ± 15 °C) of the upper part of the overlapped plate is explained by specific behaviour of steady-state geotherms calculated using lower radioactive heat production of metacarbonates as compared with metapelites. The suggested thermal-mechanical model corresponds well with P–T paths inferred from obtained thermobarometric data and correlates satisfactorily with P–T trajectories predicted by other two-dimensional thermal models for different crustal thickening and exhumation histories.

Notes: Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al2O3 on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8 kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4 kbar. The probable P–T conditions of reaction were in the range 650–790 °C, 8–10 kbar. Assuming a timescale of reaction between 106 and 108 years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10−23 to 5×10−20 m2 s−1. These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion.

Notes: Abstract Isochemical contact metamorphism was observed in 3 localities based on 184 chemical analyses of rock samples and the significance tested using simple statistical techniques. The intrusions included gabbro, granosyenite, and granitic type rocks being intruded into respective schists, clays and shales, and schists and gneisses. Conductive heat transfer appeared to be the most important heat transfer mechanism in isochemical contact metamorphism. During isochemical metamorphism both H2O and CO2 were rather mobile and impoverished in the metamorphic rocks.

Notes: Abstract The article describes the thermal metamorphism of siliceous carbonate rocks near the dolerite intrusive body in Eastern Siberia. The mineral associations at the immediate contact with dolerite are the following: wollastonite+rankinite, rankinite+spurrite (+melilite?), spurrite+melilite+merwinite+calcite and merwinite+monticellite+melilite+calcite. The melilite in these associations is usually unzoned; its composition being essentially gehlenitic. During the regressive stage of contact metamorphism new akermanite-rich melilite and calcite were formed by replacement of merwinite and earlier gehlenitic melilite through participation of CO2. The newly forming melilite grains have sharp compositional zoning. The origin of zoning was connected with the fall of temperature and decrease of the mole fraction of CO2 in the fluid equilibrated with the minerals.

Notes: Abstract At least sis or perhaps seven types of contact metamorphism may be distinguished in nature (see Table 1). They differ from each other by a set of metamorphic facies in the exocontact aureoles, also by thermodynamic conditions of metamorphism. The manifestation of some types of metamorphism depends chiefly on magma temperature and composition, on the initial temperature of the country rocks (prior to contact metamorphism) and on the depth of the intrusive formation. The movement of magma through the intrusive channel (chamber) and the kinetic peculiarities of metamorphism exert additional influence on metamorphic conditions. Since the temperature elevation with depth corresponds in a general way to the increase in the lithostatic pressure, the maximum temperature levels attained at the direct intrusive contact must differ for various pressure levels (or depths). The two series of contact metamorphic rooks may be distinguished by their pressure: 1. common hornfelsic rock low-pressure series (types of metamorphism 1–4, Table 1), and 2. thermal transformed gneissous rock series metamorphosed under moderately high pressure (types 5–6, Table 1). The initial temperature of the country rock is low in the first series. It may apparently be discounted in certain cases. The usual non-abyssal contact low-pressure metamorphism (the proper “ contact metamorphism”) depends essentially on magma temperature and composition, while in the second series, the initial temperature of the country rocks may be rather high. This and the effect of the intrusive heat and high lithostatic pressure results in rock transformation under conditions of the regional metamorphism of the facies (under moderately high pressure). Since superposition of the local temperature field on a regional high-temperature field in the neighbourhood of intrusives (in deep conditions) results in an increase in the thickness of the contact aureole, Ingersoll's criterion (the ratio of the thickness of the aureole to the thickness of the intrusive body) may be helpful for distinguishing abyssal from non-abyssal contact metamorphism. The values of this criterion cannot exceed 0.2–0.3 for the non-abyssal contact metamorphism.

Notes: Abstract The results of revised studies in analytical simulation of processes leading to development of isochemical contact metamorphic zonation by means of conductive heat transfer are reported. The peculiarities in temperature distribution in contact aureoles of intrusives are discussed for cases of convective heat transfer.