Литосфера (Aug 2017)
The nature of zircon in the Volkovsky massif gabbro (Middle Urals): the age problem and geochronological consequences
Abstract
The article presents new isotope-geochemical data on zircon from gabbronorites of the Volkovsky massif in the Middle Urals. The massif is a complex combination of different rock associations - from ultramafic rocks to syenites and quartz diorites, whose geological and geochronological ratios are not well-defined yet. The most widespread in the massif are gabbroids, which form three blocks - the Central, Western and Southern ones. This massif is associated with: 1) industrial deposit of copper-iron-vanadium ores and 2) gold-palladium mineralization of low sulfide type. Gabbronorites, which form peripheral parts of the gabbro blocks and in some cases form dykes among olivine gabbro, have a dual relationship with mineralization. They are host for copper-iron-vanadium type and postmineral concerning the gold-palladium mineralization localized in ultramafic rocks and olivine gabbro of the Southern block. The comprehensive study of zircon (its morphology, internal structure, U-Pb age, and silicate inclusions) has shown that in gabbronorites of the Volkovsky massif, zircon aged 427.5 ± 5.3 Ma and 428 ± 7 Ma, extracted from two different samples, contains the same set of polymineral inclusions, alien to the hosting rocks. The composition of inclusions (plagioclase ( An21), biotite (f = 0.49-0.56), quartz and apatite) corresponds to gneiss or plagiogranite. However, the zircon, according to its morphology, internal structure and age characteristics is similar to that from metamorphic rocks of the dynamothermal halo of gabbro-ultramafic bodies. This suggests that the zircon in gabbronorites of the Volkovsky massif could be borrowed from the surrounding metamorphic rocks, and therefore, the gabbronorite intrusion occurred when high-temperature metamorphic rocks of the dynamothermal halo had already existed and might have undergone anatexis and retrogressive metamorphism of amphibolite facies. The zircon age (≈427 Ma) limits the lower time limit of the formation of gabbronorite intersecting olivine gabbro of the Volkovsky massif, and may be close to the time of the formation of copper-iron-vanadium and gold-palladium mineralization type controlled by the reaction processes between gabbronorites and olivine gabbro.
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