We present the results of geological, petro-geochemical and mineralogical studies of synplutonic intrusive formations in the Chelyabinsk granitoid massif, South Urals. Numerous synplutonic intrusions in the study area are in early phases, composed of quartz diorites and granodiorites of the Late Devonian-Early Carboniferous. Such intrusions are represented by a bimodal series of rocks from gabbro-diorite to plagioleic granite. Both the mafic and salic members of the series form separate independent dykes and, jointly, compose the dyke bodies of complex structures. With respect to the relationships with host rocks, two types of the studied dykes are distinguished: (1) 'classical' synplutonic dykes with monolithic bodies that are split along strike by the enclosing granodiorite into separate fragments; and (2) 'post-granite' dykes that clearly break through the host quartz diorites and granodiorites that are older that the dykes, but show similar isotope ages: the U-Pb-Shrimp ages of zircon in the samples taken from the dyke and the host quartz diorite are 362±4 и 358±5 Ma, respectively. The first group includes the dyke of melanocratic diorite, the second-granitoid dykes and dykes of gabbro-diorites and diorites. The intrusion of acid rocks preceded the basites and was completed after their formation. As a result of the nearly simultaneous intrusion of both, the dykes of complex structures were formed. The material compositions of mafic rocks in these two groups are significantly different. The 'post-granite' dioritoids are moderately alkaline. Melanodiorite in the synplutonic dyke belongs to normal alkaline rocks. It has a very high content of MgO (12.5 mass %) and is sharply enriched with chromium (~700 ppm vs. 100-350 ppm in the 'post-granite' dykes). It is thus closer to sanukitoids. The acid 'post-granite' dykes vary in composition from plagoleic granite and adamellite to tonalite. They are normal-alkaline. Their chemical compositions often do not correspond to cotectic ones. The dioritoids have nearly zero values of Nd (from +1 to-2), and the values of (87 Sr/ 86 Sr)I vary from 0.70485 to 0.70571. The granitoids are typically characterized by negative values of Nd (from-2 to-5) and, generally, more radiogenic strontium ((87 Sr/ 86 Sr)i=0.70517-0.70567). The established isotopic compositions of Nd and Sr in the synplutonic dykes of the Chelyabinsk Massif give evidence of different sources for the coexisting salic and mafic melts, but do not fit a model of simple mixing of the two components.

Объект исследований. Высокомагнезиальные породы, ассоциированные с гранитоидными массивами Урала, представлены габбродиоритами и их меланократовыми разностями (горнблендитами), диоритами, кварцевыми диоритами, сложенными порфирокристами амфибола, часто совместно с клинопироксеном и флогопитом в базисе из кислого плагиоклаза с интерстициальными кварцем и калиевым полевым шпатом. Уникальной особенностью пород, помимо высокой магнезиальности, равной 0.5-0.8 ед., является экстремально высокое содержание хрома, достигающее 1200 г/т. Методы. Исследование состава высокомагнезиальных пород выполнено на массспектрометре с индуктивно связанной плазмой ELAN 9000, электронно-зондовом микроанализаторе Cамеса SX-100 и энергодисперсионной приставке INCA Energy 450 X-Max 80. Предел обнаружения Cr 2 O 3 на микроанализаторе составляет 0.05, на ЭДС приставке-0.2 мас. %. Результаты. Установлено различие в поведении хрома в двух главных минеральных ассоциациях, связанных с магматическими и постмагматическими процессами. Средняя концентрация оксида хрома в минералах ранней ассоциации по разным образцам составляет, мас. %: 0.10-0.50-в клинопироксене, 0.29-0.68-в амфиболе, 0.08-0.36-во флогопите при их вариациях от 0.0 до 1.6. В состав поздней ассоциации входят минералы, представляющие собой продукты постмагматического преобразования пироксенов, глиноземистого амфибола в низкоглиноземистую магнезиальную роговую обманку, актинолит, титанит, эпидот, мусковит. Преобразование хромшпинелида на этой стадии сопровождалось обменными процессами с силикатами, в результате которых последние были обогащены хромом. Средняя концентрация оксида хрома в минералах ассоциации составляет, мас. %: 0.24-0.80-в амфиболе, 1.38-3.08-в эпидоте, 1.03-в титаните, 3.5-в мусковите. Заключение. Предполагается, что кристаллизация ранней ассоциации железомагнезиальных силикатов проходила из водных высокомагнезиальных расплавов. Последующее постмагматическое изменение таких силикатов привело к развитию фаз с близким, а иногда и более высоким содержанием хрома, что можно объяснить их взаимодействием с хромитом в условиях низкой окисленности флюида, недостаточной для образования магнетита.

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).

The article is focused on the morphology, trace element composition, U‐Pb and Lu‐Hf systems in zircon in high‐Mg diorite of the Chelyabinsk granitoid massif. Our analytical studies of the U‐Pb and Lu‐Hf isotope systems and the trace element composition were performed using mass spectrometry (MS) with inductively coupled plasma (ICP) and laser ablation (LA) of samples. It is established that the zircon formed at the last stages of crystallization of the basic melt under subsolidus conditions at low (600–700 °C) temperatures, which distinguishes it from the zircon of most other high‐Mg rocks of the intermediate composition. The internal structure of the zircon and the concentration of trace elements are locally altered under the influence of a fluid, which led to a partial disruption of the U‐Pb and Lu‐Hf isotopic systems. For the least altered areas in the zircon crystals, the age of crystallization of the parent high‐Mg melt is 362±2 Ma, which coincides with the age estimated from the geological data. Considering the isotope composition of Hf in the zircon and the trace element concentrations, there are grounds to relate the formation of high‐Mg diorite in the Chelyabinsk granitoid massif with a mixed mantle‐crustal source.

—The Akhunovo–Petropavlovsk area of the late Paleozoic granite magmatism is located in the northeast of the Magnitogorsk megazone (MMZ) in the South Urals. It is a series of successively intruded rocks (Petropavlovsk, Akhunovo, Karagai, and Uiskii Bor intrusions) differing not only in composition, the depth of formation, and ore content but also in the relationship with magmatic and fluid sources and in magma generation mechanisms. This area differs significantly in the number and composition of intrusive complexes from the igneous rocks and ore associations in the central and western parts of the MMZ. The granite magmatism pulses alternated with the collisional shearing/spreading and rifting stages. The Petropavlovsk mesoabyssal granite intrusion (347.0 ± 8.6 Ma) formed at the early stage of the area evolution. Its rocks are similar in composition to a suprasubductional series (melting products of a mantle source enriched not only in water fluid but also in Cl). Later (310–306 Ma), at the collision–compression stage, crustal intrusion of the Akhunovo–Karagai granodiorite–granite complex took place. The intruded rocks are similar to the Middle Urals continental-margin gabbro-tonalite–grano-diorite–granite plutons (320–290 Ma) bearing large gold–sulfide–quartz deposits (Berezovskoe etc.). At the final stage of the area evolution, during the transition from continental-margin regime to hard collision between the East European and Kazakhstan continents (late Carboniferous) and the intense shearing/spreading deformations, the Uiskii Bor granosyenite–granite intrusion (304.0 ± 4.8 Ma) rich in K and HFSE formed. Granite intrusions of this type have been revealed in the MMZ for the first time. Thus, the granitoid complexes of the Akhunovo–Petropavlovsk area formed under changes in geodynamic settings and are characterized by different compositions, depths of occurrence, and genesis. This permits us to consider the area a typical continental-margin center of the long-term mantle–crust interaction, where magma generation proceeded at different mantle and crust levels, with the participation of both suprasubductional and enriched plume-related rift sources.