The geological, geochemical, and isotopic-geochronological data obtained for Sumian moderate-basic metavolcanites of Shombozero and Lekhta structures of the Panayarvi-Vygozero belt shows that the Tunguda Formation is confined to the Paleoproterozoic structural and material complex. This formation is represented by the complex of weakly differentiated andesitobasalts and andesites of calc-alkaline series with higher contents of MgO and moderate contents of Al2O3 and rare earth elements. The rocks of the Tunguda Formation are different from the Late Archean basic rocks of the Hiziyarvi Formation represented mainly by tholeiitic basalts with low REE contents and undifferentiated spectrum of REE distribution. The age of volcanites of the Tunguda Formation was determined to be 2439 ± 21 Ma. The xenogenic zircons from metaandesites of the Tunguda Formation have Neoarchean age according to the 207Pb/206Pb ratio (from 2536 ± 4 to 2825 ± 7 Ma). The Neoarchean zircons, a negative value of ?Nd (?3.8), and indicative geochemical parameters are evidence that the crustal component took part in formation of the protolith of the studied rocks. 相似文献
Lithofacies in the mid‐Permian Nowra Sandstone indicate a middle/upper shoreface to foreshore environment of deposition under the influence of storm‐generated waves and north‐northeasterly directed longshore currents. Palaeogeographic reconstruction for the Nowra Sandstone portrays a sand‐dominated high energy shelf and offshore shoal forming a sequence thickening seaward away from the western shore of the Sydney Basin. The shoal‐crest at the outer edge of the shelf trends north‐northeast. It is characterized by fine‐ to medium‐grained sandstone with upper flow regime structures and a high proportion of conglomerate, whereas coarser sandstone with lower energy bedforms occurs along the seaward side of the shoal. In the deeper water to the east, the lower Nowra Sandstone becomes rapidly thinner as it passes seaward, via bioturbated storm redeposited sandstone beds, into the shelf deposits of the Wandrawandian Siltstone. This sequence accumulated during a regressive event and the base of the formation becomes progressively younger eastward. The sand may have been supplied by rivers along the western coast but the major source was south of the study area. The lower Nowra Sandstone is separated from the upper part of the formation by an extensive ravinement surface overlain by the Purnoo Conglomerate Member. In contrast to the lower unit, the upper Nowra Sandstone forms a westward thickening wedge that represents a backstepping nearshore sand facies that accumulated during a transgression. The upper Nowra Sandstone passes vertically and laterally eastward into the Berry Siltstone. Thus both boundaries of the Nowra Sandstone are diachronous, first younging eastward and then westward as a response to a regressive‐transgressive episode. 相似文献
We present petrological, isotopic, and geochronological data on gabbro-picrite plutons from the Altai collisional system of Hercynides (eastern Kazakhstan). The geological, geochemical, and geochronological data suggest that these rocks are indicators of the activity of the Tarim plume. The gabbro and picrites formed in two stages (~293 and 280 Ma) in an acid-to-basic succession, explained by a model for the interaction of thermochemical plumes with the lithosphere. Early igneous activity, which gave rise to subalkalic gabbro plutons, reflects the first interaction between the ascending plume and the lithosphere, with low-melting sublithospheric protoliths. Further interaction was characterized by the sublithospheric spread of the plume head with intense heating of the lithospheric base and possible intrusion of deep melts, which resulted in the formation of Cu–Ni–PGE gabbro-picrite plutons in the Altai collisional system and Northwest China. 相似文献
The aim of this paper is to estimate syntectonic P-T conditions within albite- and garnet-bearing orthogneisses. These rocks are generally characterized by the assemblage quartz + albite + biotite + phengite + CaFe-garnet + epidote + titanite. Garnet contains up to 55 mole per cent of grossular. K-feldspar is a relict magmatic phase.
P-T conditions are estimated using several independent methods. First, it is shown that exchange reactions based on the Fe---Mg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks, due to the high grossular content of garnet. Second, maximum and minimum pressures are constrained, respectively, by the occurrence of albite instead of jadeite + quartz and by the assemblage phengite + biotite + quartz. Third, phase equilibria in albite- and garnet-bearing metagranites are modelled in the system K2O---CaO---FeO---Al2O3---SiO2---H2O. Equilibrium curves are calculated for the observed phase compositions. Uncertainties in P-T estimates mainly result from the choice of appropriate non-ideal solution models for the garnet.
An application is developed for granites from the Gran Paradiso nappe (Western Alps). These granites show an heterogeneous deformation of Alpine age expressed by mylonitic shear zones cutting across weakly deformed domains. Estimated P-T conditions for the synkinematic assemblages are 10–16 kbar at 550±50°C. 相似文献
The problem of heterogeneity of the mantle lithosphere of the southwestern portion of the Siberian Platform has been considered, and the diamond content in potential mother lodes within this area has been estimated based on original geochemical data on the rare-element composition of pyropes from diamondiferous lamproites of the Ingashin field within the Prisayan region and ancient dispersion haloes of minerals accompanying diamonds in the area between the Angara and Uda rivers. Pyropes from lamproites are characterized by low concentrations of Zr (0.18–9.05 ppm), Hf (0.03–0.37 ppm), and rare earth elements (Sm 0.04–0.49, Eu 0.02–0.16, and Dy 0.05–0.96 ppm). Pyropes from the Lower Carboniferous Baeron Formation within the Tangui-Chuksha area are significantly different from pyropes of the Ingashin lamproites in high contents of Zr (30.36–139.23 ppm) and Hf (0.4–2.22 ppm). These pyropes are characterized by elevated concentrations of rare earth elements (Sm 1.34–3.68, Eu 0.53–1.17, and Dy 1.0–2.05 ppm). The distribution patterns of rare incompatible elements in pyropes of the Lower Carboniferous Mura massif within the Mura area manifest even stronger differences with pyropes of the Ingashin lamproites and in many respects with pyropes from Lower Carboniferous sediments of the Baeron Formation within the Tangui-Chuksha area. The results obtained indicate that there is no large-scale regional spreading of pyropes from Mid-Riphean lamproite bodies in the course of washout of these bodies and that the mantle lithosphere in the southwestern portion of the Siberian Platform is laterally heterogeneous in mineralogical-geochemical terms. The chemical composition and the peculiar distribution pattern of rare elements in pyropes from lamproites of the Prisayan region indicate a depleted, primarily lherzolite composition of the upper mantle that was transformed through low-temperature potassium metasomatosis. In terms of the chemical and rare-element compositions, pyropes from Lower Carboniferous sediments of the Tangui-Chuksha and Mura areas belong to a wider range of mantle rocks: depleted peridotites, metasomatic peridotites under low (900–1000°C) and high (>1000°C) temperature conditions, and megacrysts. This suggests that the composition of the lithospheric mantle in this area of the southern portion of the Siberian Platform is characterized by a considerably differentiated stratification of mantle rocks, some of which were credibly formed in the diamond stability field. 相似文献