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One model for the large ion lithophile element (LILE) depletion of crustal rocks during granulite facies metamorphism is that extraction of melt attends emplacement of mafic magma under or near the base of the continental lower crust (magmatic accretion) (Harley, 1989). In this model, mantle magmatism, granulite facies metamorphism, and crustal anatexis are coupled processes that result in chemical differentiation of continental crust. Within the Ivrea zone (southern Alps, northern Italy), mantle-derived magma intruded metasedimentary and metaigneous rocks while the section was in the lower crust (Rivalenti et al., 1975;Voshage et al., 1990). Emplacement of mafic magmas within the supracrustal section has been traditionally interpreted as having caused or having accompanied the thermal maximum during regional granulite facies metamorphism (e.g., Schmid and Wood, 1976;Rivalenti et al., 1980;Sills, 1984). The exposure of mafic rocks thought to have caused regional metamorphism in the overlying granulite terrain has led some to consider the Ivrea zone a particularly important example of the purported relationship between regional metamorphism and magmatic accretion (e.g., Voshage et al., 1990).In this study we provide evidence supporting an alternative model, in which the emplacement of the Mafic Complex occurred after the imposition of the regional pattern of metamorphic isograds (Zingg et al., 1990). Therefore, the heat supplied by the emplacement of the exposed part of the Mafic Complex is unlikely to have caused regional granulite facies metamorphism. The metamorphism and anatexis of weakly depleted, amphibolite to granulite facies crustal rocks associated with emplacement of the Mafic Complex occurred only within an ~2-km-wide zone overlying the upper parts of the intrusion. This narrow contact aureole demonstrates that extensive regional metamorphism and anatexis may not inexorably accompany emplacement of large volumes of mafic magma against fertile crustal rocks. Studies postulating that regional effects necessarily accompany magmatic accretion (e.g., Campbell and Turner, 1987;Huppert and Sparks, 1988) may underestimate the amount of basalt required to achieve the degree of melt depletion inferred for regional granulite terrains such as the Ivrea zone. GEOLOGIC FRAMEWORKMost regional studies interpret the Ivrea zone as a cross section through attenuated continental lower crust (Burke and Fountain, 1990). There are three major lithologic divisions in the Ivrea zone ( Fig. 1): (1) supracrustal rocks of the Kinzigite Formation; (2) mantle peridotite; and (3) the Mafic Complex. The lowest grade rocks crop out along the southeastern margin of the Ivrea zone and contain upper amphibolite facies assemblages (Zingg, 1980). Granulite facies rocks are exposed in Val Strona, indicating that metamorphic grade increases toward the northwest, in accordance with pressuretemperature (P-T) estimates derived using geothermobarometry (Schmid and Wood, 1976;Sills, 1984;Henk et al., 1997).The amphibolite facies of the Kinzigite ...

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The West Siberia basin is the largest petroleum province in Russia, with 80% of the country's gas resources in the Cenomanian Pokur Formation. Significant undiscovered gas resources have been assessed as on trend with the giant gas fields. However, the origin of the large amounts of dry, isotopically light gas is still an enigma, albeit extensively addressed in the literature. This study aims at quantifying the gas contribution from all relevant thermal sources. The West Siberia Basin is the world's largest intracratonic basin, comprising up to 12 km of Mesozoic and Cenozoic clastic rocks. The Basement is composed of Palaeozoic accretionary crust. Northward-trending Permian-Triassic rifts were filled by fluvial-deltaic sediments from the south and east, punctuated by marine transgressions from the north. Cenozoic basin inversion formed traps for petroleum. A regional high-resolution 3D basin simulation was used to model the thermal evolution of the northern West Siberia basin. Geostatistical modelling was applied to assess source rock richness and quality. Basal heat flow was modelled by calibration to bottom-hole temperature and vitrinite measurements. Hydrocarbon generation kinetic parameters were derived from measurements performed on West Siberia rock samples. Thermal gas charge expelled from the hydrocarbon kitchen drainage areas of key fields were compared with the gas volumes accumulated in these fields. The study found that Cretaceous terrestrial sources can generate sufficient early thermal gas to charge accumulations in the South Kara Sea area, and additional Jurassic sources can charge the remaining accumulations of the study area if favourable conditions apply. Biogenic gas is likely to have contributed to the gas accumulations. Mixing of thermal and biogenic gas could explain the observed isotopic composition. Sensitivity analyses show that the timing of structuring and uplift is the most critical factor of the assessment. Variations in glaciation, heat flow and source kinetics show less effect on the hydrocarbon accumulation.

ABSTRACT:Dehydration melting of crustal rocks may commonly occur in response to the intrusion of mafic magma in the mid- or lower crust. However, the relative importance of melt buoyancy, shear or dyking in melt generation and extraction under geologically relevant conditions is not well understood. A numerical model of the partial melting of a metapelite is presented and the model results are compared with the Ivrea-Verbano Zone in northern Italy. The numerical model uses the mixture theory approach to modelling simultaneous convection and phase change and includes special ramping and switching functions to accommodate the rheology of crystal-melt mixtures in accordance with the results of deformation experiments. The model explicitly includes both porous media flow and thermally and compositionally driven bulk convection of a restitecharged melt mass. A range of melt viscosity and critical melt fraction models is considered. General agreement was found between predicted positions of isopleths and those from the Ivrea-Verbano Zone. Maximum melt velocities in the region of porous flow are found to be 1 × 10−7 and 1 × 10−1m per year in the region of viscous flow. The results indicate that melt buoyancy alone may not be a sufficient agent for melt extraction and that extensive, vigorous convection of partially molten rocks above mafic bodies is unlikely, in accord with direct geological examples.

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