Analysis of physical properties of rocks in the eastern zone of the north of the Urals

We summarized data on petrophysical characteristics of rocks of the main petrographic types from the eastern structural zone in the north of the Urals. Physical properties of rocks were studied taking into account the variability in density and magnetic susceptibility of rocks differing in material composition, formation conditions, the type and degree of metamorphism. Stable interrelations between density and magnetic susceptibility allow to distinguish four petrophysical groups of geological formations and to determine the causes of changes in the physical properties of rocks in the eastern zone of the northern part of the Urals.The first petrophysical group of rocks comprises ultrabasic rocks of Rayiz-Voikar ophiolite association with relatively high density (2.90-3.10 g/cm3) and average magnetization (455-680×10-6 CGS). Complicated and unstable correlations between density and magnetic susceptibility that should be considered together with the serpentinisation process were found in these rocks. As the extent of serpentinisation rises dunites and harzburgites strongly lose their density properties to 2.76 и 2.72 g/cm3 but display high magnetic attributes up to 6329×10-6 CGS.The second petrophysical group of rocks ( pyroxenites-gabbro ) is represented by rocks of dunite-wehrlite-clinopyroxenite and gabbro associations. Basites of Kershor plutonic association demonstrate high values of density and magnetic susceptibility. Wehrlites and clinopyroxenites have σ 2.87-3.05 g/cm3 and æ varies within a broad range from 800 to 9000×10-6 CGS. Density limits of hornblendites 3.10-3.23 g/cm3 and magnetic susceptibility ranges from 600 to 1000×10-6 CGS; the same values in gabbro 2.79-3.10 g/cm3 and 1500-4000×10-6 CGS respectively (Table). Rocks of the second petrophysical group demonstrate direct proportional dependencies between density and magnetic susceptibility.A classic dependence between density and basicity (silicon dioxide content) of rocks is observed. The only exclusion are strongly serpentinitized basites.The third petrophysical group of rocks ( diorite-plagiogranite ) consists of intrusive rocks of average and moderately high silica content represented by diorites, quartz diorites and granodiarites of Sob and Kongor associations. Relatively high values of density (σmean 2.80 g/cm3) and magnetic susceptibility (σmean 900×10-6 CGS) observed in diorites and quartz diorites become lower in rocks with high content of silica. Thus plagiogranites and porphyritic granites are characterized by lower density and magnetic properties (σmean 2.66 g/cm3 and σmean 400×10-6 CGS).The fourth petrophysical group of rocks ( volcanic-sedimentary ) comprises da-cites, andesite, basaltic andesites and their tuffs with interlayers of tuff sandstones and reef limestones of Malouralskaya Suit, and also by conglomerates, grit-stones and tuff sandstones of Varchatinskaya Suit. Differentiation of physical properties is more sharply expressed in volcanics. Their density (from 2.83 to2.66 g/cm3) and magnetic susceptibility (from 3100×10-6 to 400×10-6) regularlydecrease together with lowering of silica content. Density of volcanic-sedimentary rocks (tuffs) in the most cases is lower than density of volcanic rocks of the same composition that makes it possible to separate them using geo-physical data. Magnetic properties are rather constant not depending on the rock’s composition.This petrophysical study is an important part of comprehensive research program of deep structure of Timan-North Urals region. Our results are used for development of new models and clarification of the existing models of structure and evolution of lithosphere and also provide independent additional data on composition of rocks, the extent to that they were changed and the amount of low-temperature metamorphism.