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

Xenoliths in the alkali basalts of Makhtesh Ramon of Desert Negev (Israel) as indicators of mantle metasomatosis and magma generation

Xenoliths in the Early Cretaceous alkali basalts of Makhtesh Ramon basin (southern Israel) are represented by essentially olivine rocks: dunite (usually including clinopyroxen) - 5% of the total amount, lherzolite - 21%, wehrlite - 28%, clinopyroxenite - 34%, gabbro - 12 %. According #Mg = Mg/(Mg + Fe), xenolith rocks form several discrete groups corresponding to the following values #Mg: >0.85 (dunite, lherzolite, some wehrlites), 0.85-0.75 (wehrlite, olivine clinopyroxenite), 0.75-0.65 (olivine clinopyroxenite, clinopyroxenite), 0.60-0.45 (gabbro). Primary mantle rocks are represented by lherzolite, other xenoliths are the products of metasomatism, which preceded and accompanied magma generation. The main minerals of ultramafic xenoliths- olivine slightly enriched by CaO, clinopyroxene with varying content of TiO2 (1-4%), Al2O3 (2-12%), Na2O (0.5-2%) and #Mg = 0.92-0.59, spinelids: chromite (Cr2O3 = 20-38%), Al spinel and titanomagnetite (TiO2 = 10-21%, Cr2O3 = 0.3-8%, Al2O3 = = 1.5-13%, MgO = 2-7%). Rich inTiO2, Al2O3 and Na2O clinopyroxene together with plagioclase, anorthoclase, kaersutite, rhenite, ilmenite, “orthopyroxene” and “feldspar” glass веlong to late ultramafic paragenesic associated with the process of partial melting. Orthopyroxene in ultramafic rocks is unstable and usually is replaced by minerals of the late paragenesis. Gabbroic xenoliths consist of low-titanium and low-aluminiferous clinopyroxene (#Mg = 0.66-0.56), orthopyroxene (#Mg ≈ 0.5), plagioklase An45-55, often with rims of anorthoclase, titanomagnetite of the same composition as in the ultramafic rocks, ilmenite. Xenoliths have the signs of partial melting and metasomatic transformation preсеding to melting. As a result the orthopyroxene from lherzolite is replaced by clinopyroxene. This leads to the width development of wehrlites and olivine clinopyroxenites. During metasomatosis the content of Mg, Cr and Ni falls while Ti, Fe, Al, Ca grows, as well as the content of large ion lithophile an high-strength elements providing increase of fertility of basalt magmatic source. Composition of the produced melt is close to basanite. The glass, cementing crystallization products, preserved in xenoliths, has a composition close to the orthopyroxene-feldspar mixtures. The mineral phases in such glass presented by clinopyroxene, kaersutite, rhenite, plagioclase, anorthoclase, nepheline, titanomagnetite and ilmenite

THE RIPHEAN MAGMATISM PRECEDING THE OPENING OF URALIAN PALEOOCEAN: GEOCHEMISTRY, ISOTOPES, AGE, AND GEODYNAMIC IMPLICATIONS

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

New data on the nature of substrate Late Paleozoic granites of the Southern Urals

Isotopic Sm-Nd and Rb-Sr systems of gneisses forming dome structures and enclosing Late Paleozoic granites in Kochkar anticlinorium of the Southern Urals were studied. Rb-Sr isochron age of gneisses - 354.9 ± 4.5 Ma - coincides with the age of the granite migmatites of Varlamovsk migmatite-pluton and age of granitic rocks widespread in the Urals gabbro-tonalite-granodiorite-granite (GTGG) massifs which are the products of hornblende gabbro water anatexis. Isotopic parameters (87Sr/86Sr)i, (εSr)i, (143Nd/144Nd)i as far as geochemical features of GTGG granitoids and gneisses were proved to be similar. Closeness of the age, isotopic characteristics and composition of the tonalite-granite-gneisses forming dome structures enclosing Late Paleozoic granite massifs, on the one hand, and the rocks of GTGG massifs, on the other hand, suggests that the gneisses are metamorphosed by Devonian granitoids. The high water content of the eutectic composition magma of Late Paleozoic granites ( PH2O = 0.7-0.8 Ptot at a value of Ptot = 4-5 kbar) prevents its vertical movement. The maximum possible ascent of granitic magma to the values of PH2O = Ptot under which it should crystallize corresponds to pressure differential not more than 0.5-1.0 kbar or in absolute numbers - 1-3 km. These values do not exceed the thickness of orthogneiss domes and do not contradict the assumption that the granite rocks occur among the possible substrate. These data testify that the main substrate Late Paleozoic (Permian) granites in the eastern part of the Southern Urals can be the rocks of Late Devonian GTGG complexes metamorphosed to amphibolite facies. This conclusion explains the isotopic parameters of granites, in particular, the low content of radiogenic strontium and increased value of143Nd/144Nd, corresponding to a positive value εNd. The rocks of Mesoproterozoic basement also participated in the granite formation, but played a minor role

The Migmatization and Granite Formation Processes within the Western Exocontact of the Aduyskiy Massif. Middle Urals

The mechanisms and sources of magma generation for the large Permian Aduyskiy granite massif, well known for rare metal, semi-precious stone and molybdenum deposits, are considered. Valuable information was obtained from study of the Kamenskiy and Krutikhinskiy massifs located at the base of Aduyskiy granite body. The migmatites of these two massifs as well as the accompanying partial melting have approximately the same zircon and Rb-Sr age. These processes range about 309-298 Ma in the Kamenskiy and 308-283 Ma in Krutikhinskiy massifs with the main and most intense stage within 296-298 Ma. That time, the main volume of the granite melt for Kamenskiy, Krutikhinskiy, and Aduyskiy massifs was formed. The migmatites of magma generation zone have banded texture in Kamenskiy massif, but, in Krutikhinskiy massif, anatectic melt formed the numerous veins and small intrusive bodies. These magmatic differences were formed mainly due to protolith composition. Камenskiy migmtites are characterized by Devonian – Carboniferous quartz diorites and granodiorites. Predominantly granitic rocks are characteristic for Krutikhinskiy massif. Basic dykes accompanied the migmatization in both massifs