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Murzinka and Dzhabyk are continental-type batholiths of the middle and southern East Uralian domain. They comprise mainly undeformed peraluminous K-rich granites whose elemental composition is similar to some late-Variscan granites of western Europe, but with much more primitive Sr and Nd isotope ratios. Murzinka (2545 Ma) is composed of silica-rich granites forming two different rock series with a 87 Sr/ 86 Sr init of 0.709 and 0.704, respectively. Both series have enormous variations in eNd 255 (±11.9 to ±0.1 and ±8.9 to +4.1) that reveal derivation from heterogeneous sources. Dzhabyk (2914 Ma) also comprises two coeval magmas which yielded voluminous granites and quartz-monzonites, respectively, with smaller differences in 87 Sr/ 86 Sr init and eNd 290 (~0.7043, +0.8 to +1.6 and~0.7049, 0.0 to +0.8). Despite their isotope compositions both batholiths lack evidence of genetic involvement of a mantle-derived parental magma. Moreover, we suggest that Dzhabyk granitoids were generated by polybaric partial melting of Paleozoic island-arc material, whereas Murzinka granitoids derived from an extremely heterogeneous source consisting mainly of Paleozoic and Proterozoic metagreywackes. This implies a relative fast reworking of juvenile arc crust and burial of the protoliths during the orogenic evolution of the Urals. Since there is neither evidence of significant extension, nor a direct link with subduction, we suggest that the main cause for late-orogenic anatexis was elevated heat production and fertility in the protolith, perhaps combined with some additional heat from unexposed mafic intrusions.

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.

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.

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