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

ДЕВОН-КАМЕННОУГОЛЬНЫЙ МАГМАТИЗМ И ОРУДЕНЕНИЕ ЮЖНО-УРАЛЬСКОЙ АККРЕЦИОННО-КОЛЛИЗИОННОЙ СИСТЕМЫ

The oceanic stage in the history of the South Urals completed in the Ordovician – Early Silurian. The Ordovician through Devonian events in the region included the formation of an island arc in the East Ural zone from the Middle Ordovician to Silurian; westward motion of the subduction zone in the Late Silurian – Early Devonian and the origin of a trench along the Main Ural Fault and the Uraltau Uplift; volcanic eruptions and intrusions in the Magnitogorsk island arc system in the Devonian. The Middle-Late Paleozoic geodynamic evolution of uralides and altaides consisted in successive alternation of subduction and collisional settings at the continent-ocean transition. The greatest portion of volcanism in the major Magnitogorsk zone was associated with subduction and correlated in age and patterns of massive sulfide mineralization (VMS) with Early – Middle Devonian ore-forming events in Rudny Altai. Within-plate volcanism at the onset of volcanic cycles records the Early (D1e2) and Middle (D2ef2) Devonian slab break off. The volcanic cycles produced, respectively, the Buribay and Upper Tanalyk complexes with VMS mineralization in the Late Emsian; the Karamalytash complex and its age equivalents in the Late Eifelian – Early Givetian, as well as the lower Ulutau Formation in the Givetian. Slab break off in the Late Devonian – Early Carboniferous obstructed the Magnitogorsk island arc and supported asthenospheric diapirism. A new subduction zone dipping westward and the Aleksandrovka island arc formed in the Late Devonian – Early Carboniferous. The Early Carboniferous collision and another event of obstructed subduction led to a transform margin setting corresponding to postcollisional relative sliding of plates that produced another slab tear. Postcollisional magmatism appears as alkaline gabbro-granitic intrusives with related rich Ti-magnetite mineralization (C1). Transform faulting persisted in the Middle Carboniferous through Permian, when the continent of Eurasia completed its consolidation. The respective metallogenic events included formation of Cu-Ni picritic dolerites (C2–3), as well as large-scale gold and Mo-W deposits in granites (P1–2).Завершение океанической стадии на Южном Урале произошло в ордовик – раннесилурийское время. В среднем ордовике в Восточно-Уральской зоне начала формироваться среднеордовикско-силурийская островная дуга. В позднем силуре – раннем девоне произошел перескок зоны субдукции на запад, формирование глубоководного желоба в зоне Главного Уральского разлома – Уралтауского антиклинория и началось образование вулкано-интрузивных формаций Магнитогорской островодужной системы (D1–D3). В среднепозднепалеозойской геодинамической эволюции уралид и алтаид произошло последовательное чередование субдукционных и трансформно-коллизионных обстановок в зоне перехода континент – океан. На Южном Урале с субдукционной обстановкой связан главный объем вулканических ассоциаций Магнитогорской мегазоны. В раннем (D1e2) и среднем (D2ef2) девоне произошли разрывы слэба, фиксирующиеся проявлениями внутриплитного вулканизма, приуроченного к начальным этапам раннедевонского позднеэмсского и позднеэйфельско-раннеживетского колчеданоносных вулканических циклов. В позднем девоне – раннем карбоне произошла блокировка Магнитогорской островной дуги с разрывом слэба, и, как следствие, – главный этап астеносферного диапиризма. На рубеже позднего девона – раннего карбона сформировалась новая зона субдукции западного падения и возникла Александровская редуцированная островная дуга. Раннекаменноугольная коллизия и повторная блокировка субдукционной зоны привели к трансформной обстановке, отвечавшей постколлизионному скольжению литосферных плит, и вновь – к появлению астеносферного окна («slab-tear»). В этой обстановке были сформированы габбро-гранитные интрузивы повышенной щелочности и связанные с ними Ti-Mgt месторождения мирового класса (С1). Трансформная геодинамическая обстановка оставалась ведущей на протяжении среднего карбона – перми, когда произошла окончательная консолидация Евразийского континента

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

DEVONIAN-CARBONIFEROUS MAGMATISM AND METALLOGENY IN THE SOUTH URAL ACCRETIONARY-COLLISIONAL SYSTEM

The oceanic stage in the history of the South Urals completed in the Ordovician – Early Silurian. The Ordovician through Devonian events in the region included the formation of an island arc in the East Ural zone from the Middle Ordovician to Silurian; westward motion of the subduction zone in the Late Silurian – Early Devonian and the origin of a trench along the Main Ural Fault and the Uraltau Uplift; volcanic eruptions and intrusions in the Magnitogorsk island arc system in the Devonian. The Middle-Late Paleozoic geodynamic evolution of uralides and altaides consisted in successive alternation of subduction and collisional settings at the continent-ocean transition. The greatest portion of volcanism in the major Magnitogorsk zone was associated with subduction and correlated in age and patterns of massive sulfide mineralization (VMS) with Early – Middle Devonian ore-forming events in Rudny Altai. Within-plate volcanism at the onset of volcanic cycles records the Early (D1e2) and Middle (D2ef2) Devonian slab break off. The volcanic cycles produced, respectively, the Buribay and Upper Tanalyk complexes with VMS mineralization in the Late Emsian; the Karamalytash complex and its age equivalents in the Late Eifelian – Early Givetian, as well as the lower Ulutau Formation in the Givetian. Slab break off in the Late Devonian – Early Carboniferous obstructed the Magnitogorsk island arc and supported asthenospheric diapirism. A new subduction zone dipping westward and the Aleksandrovka island arc formed in the Late Devonian – Early Carboniferous. The Early Carboniferous collision and another event of obstructed subduction led to a transform margin setting corresponding to postcollisional relative sliding of plates that produced another slab tear. Postcollisional magmatism appears as alkaline gabbro-granitic intrusives with related rich Ti-magnetite mineralization (C1). Transform faulting persisted in the Middle Carboniferous through Permian, when the continent of Eurasia completed its consolidation. The respective metallogenic events included formation of Cu-Ni picritic dolerites (C2–3), as well as large-scale gold and Mo-W deposits in granites (P1–2)