Murzinka massive at the middle urals as an example of the interformational granite pluton: magmatic sources, geochemical zonality, peculiarities of formation

Murzinka massif is a sheet-like interformational body steeply deeping to the East with length about 6 km. Proterozoic metamorphic rocks of the predominantly granulite facies ( P = 5-6 kbar, T = 750-800°C) occur at the base of massif, and volcanic-sedimentary Silurian-Devonian rocks metamorphosed in the epidote-amphibolite facies - in the roof of it. Analyzes of rocks are made in the Institute of Geology and Geochemistry. A.N. Zavaritsky (Ekaterinburg, Russia) by standard methods. Petrogen elements were determined on the X-ray fluorescence spectrometers CPM-18, CPM-25, VRA-30 and the rare elements - on the ICP-MS mass spectrometer ELAN-9000 company Perkin Elmer. In the eastern direction the rocks lying in the base of the massif change their composition from predominantly basic to granitic. The gneisses of granitoid composition underwent a high degree of melting, and theirs anatectic melt formed the western part of Murzinka massif. The granites form three complexes: yuzhakovsk - vein of biotite orthoclase antiperthite granites, varying in K2O content, in the metamorphic rocks of the base of the massif, the vatikha - biotite orthoclase antiperthite granites in western part of the murzinka massif, and the murzinka s.s. - two-mica predominantly microcline granites in the eastern part of the massif. Vatikha and murzinka granites have the same isotopic age (about 255 Ma). A clear geochemical zonation is revealed in the massif: from the west to the east (from the base to the roof), the contents of Rb, Li, Nb, Ta grow in the granites of the vatikha and murzinka complexes. In the same direction, the ratios K/Rb, Zr/Hf, Nb/Ta decrease, as well as the content of Ba and Sr. Accordingly, the compositions of such rock-forming minerals as plagioclase and biotite also change. The isotope characteristics of the granites of the vatikha (Sri = 0.70868-0.70923 and εNd255 from -8.9 to -11.9) and murzinka (Sri = 0.70419-0.70549, εNd255 from -2.6 to +2.3) complexes suggest that the substratum of the former was the Proterozoic granite-gneisses, and of the second - the rocks of the newly formed crust, possibly similar to the Silurian-Devonian volcanogenic-sedimentary rocks, which contact with the murzinka granites at the west

Model of mantle-crust interaction and magma generation in the suprasubduction orogen (Paleozoic of the Urals)

A model of magma formation in the crust of the Urals mobile belt, which is the best example of epioceanic suprasubduction orogen, has been developed. Magma formation occurs both in the relic oceanic and in the newly formed orogen crust. In the first case small bodies of practically non-potassium plagiogranites are formed, in the second one can see large gabbro-tonalite-granodiorite-granite (GTGG) and essentially granite massifs. The main conclusion is that the formation of a new earth crust of the Urals mobile belt and magma generation in it was initiated by the replacement of low-water mantle magmatism with water-rich one. The latter accompanies crustal magma generation at all its stages. In the areas of intense water magmatism, centers of long-term (up to 100 million years or more) endogenous activity (CLEA) are formed, the products of which are GTGG and granite massifs. Two main stages are distinguished in the evolution of the CLEA: 1) spontaneous partial melting (automigmatization) of products of water-rich basic magmatism - hornblende gabbros and diorites, and formation of tonalite, granodiorite and plagiogranite melts; 2) partial melting of the tonalite and granodiorite which produces the melts of adamellitic and granite composition

РИФЕЙСКИЙ МАГМАТИЗМ, ПРЕДШЕСТВУЮЩИЙ РАСКРЫТИЮ УРАЛЬСКОГО ПАЛЕООКЕАНА: ГЕОХИМИЯ, ИЗОТОПИЯ, ВОЗРАСТ, ГЕОДИНАМИЧЕСКИЕ СЛЕДСТВИЯ

The rocks from different stages of the geodynamic evolution have been preserved in the Urals. In its geologic history, the least studied is the transition period between continental rifting and the beginning of oceanic spreading. This article presents the geochemical data on the Sr-, Nd-isotopes, zircon U-Pb (SHRIMP) ages for the MesoNeoproterozoic igneous rocks and associated ores from the Bashkir meganticlinorium (BMA) on the Urals western slope. A Large Igneous Province (LIP) formed there as a result of mantle plume activity during the Middle Riphean (1380–1350 Ma). Later on (1200–1100 Ma), short-term rifting took place, as evidenced by the Nazyam graben, which was followed by the complete break-up of the continental crust. For magmatic rocks in the age range of 1750–1200 Ma, the evolition of chemical composition OIB-type → E-MORB →N-MORB is observed. The εNd(t) values for the igneous rocks and the associated BMA ores vary from negative (–6) to positive ones (+5), and thus give evidence of the lithosphere mantle depletion with time. These facts and the Sr-isotope ratios for the magmatic rocks from the subsequent evolution stages confirm that the oceanic basin to the east of the East European platform started to open at the end of the Middle Riphean. For the Vendian-Cambrian, some traces of orogenes (Timanian stage) are observed. The development of the Uralian Paleozoic ocean started in the Ordovican and continued up to the Late CarboniferousPermian.Урал – одна из немногих структур, в которой сохранились породы всех стадий геодинамической эволюции. Наименее изученным в его геологической истории является период, переходный от континентального рифтинга к океаническому спредингу. В статье представлены новые данные по геохимии, изотопии Sr и Nd, U-Pb (SHRIMP) возрасту цирконов магматических пород и связанных с ними руд Башкирского мегантиклинория (западный склон Южного Урала), имеющих мезонеопротерозойский возраст. В среднем рифее (1380–1350 млн лет) здесь была сформирована крупная изверженная провинция (LIP) как возможный результат активности мантийного плюма. Затем (около 1100 млн лет) имел место полный разрыв континентальной коры, и краткое время существовала рифтовая структура (Назямский грабен). Для магматических пород с возрастом 1750–1100 млн лет фиксируется геохимическая эволюция составов: OIB →E-MORB→ N-MORB. При этом εNd изменяется от отрицательных (–6) до положительных значений (+5), указывая на обеднение литосферной мантии со временем. Эти факты, наряду с поведением изотопов Sr для пород всех последующих стадий эволюции Урала, указывают на то, что океаническое пространство к востоку от Восточно-Европейской платформы открылось в конце среднего рифея. В венде – кембрии присутствуют признаки орогенных событий (Тиманский этап). С ордовика началось развитие Уральского палеозойского океана, существовавшего до верхнего карбона – ранней перми

Biostratigraphy versus isotope geochronology: Testing the Urals island arc model

Formation of the Urals volcanic-hosted massive sulphide (VHMS) deposits is considered to be related with the intra-oceanic stage of island arc(s) development in the Upper Ordovician–Middle Devonian based on the biostratigraphic record of ore-hosting sedimentary rocks. However, the direct Re-Os dating of four known VHMS systems in the Urals gives significantly younger Re-Os isochron ages ranging from 355 ± 15 Ma up to 366 ± 2 Ma. To address this discrepancy, we performed SHRIMP U-Pb dating on zircons extracted from rhyodacites (Eifelian biostratigraphic age of 393–388 Ma) from the footwall of the Alexandrinka VHMS deposit which has a Re-Os isochron age of sulphides of 355 ± 15 Ma. New 206Pb/238U mean age of 374 ± 3 Ma (MSWD = 1.4 and probability = 0.11) is considered to be the crystallisation age of the host volcanic rock. This age is ca. 15 Ma younger than the Eifelian (393–388 Ma) biostratigraphic age and overlaps the Frasnian–Famennian boundary (372 ± 2 Ma), characterised by the final stages of Magnitogorsk Arc – East European continent collision. Such an inconsistency with geochronological age may be due to a reburial of conodonts during resedimentation as a result of erosion of older rocks in younger sedimentary sequences

Composition and mineralogy of PGE-rich chromitites in the Nurali Lherzolite-Gabbro complex, southern urals, Russia

We have investigated two subeconomic bodies of chromitite in the Nurali Iherzolite-gabbro complex, in the southern Urals, Russia, with regard to the composition of the chromian spinel and the distribution and mineralogy of the platinum-group elements (PGE). The bodies of chromitite, referred to as CHR-I and CHR-II, occur as small concordant lenses located at two stratigraphic levels within layered wehrlite and clinopyroxenite, overlying the Iherzolitie mantle tectonite. The chromian spinel is Al-rich, showing an increase of Cr/(Cr + Al), Fe2+/(Fe2+ + Mg) and TiO2 from CHR-I to CHR-II. The total PGE contents vary from 1.26 to 11.61 ppm, and show increase in (Pt + Pd)/(Os + Ir + Ru) from 0.1 to 52.2 as a result of the appearance of magmatic sulfides in the upper chromitite. The PGM assemblage shows a drastic change from laurite-erlichmanite-dominated to enriched in Pt Pd sulfides and alloys. Laurite is the first PGM to crystallize, and its composition typically reflects the Ru/Os ratio of the primitive mantle, indicating that the parent melt of the chromitite did not undergo fractionation during ascent. The Nurali chromitites are rather unusual as they have characteristics in common with chromitites associated with ophiolitic cumulates, layered intrusions, Alaskan-type complexes, and the subcontinental orogenic mantle