E. V. Medvedeva

Zirconology of ultrabasic rocks of the Buldym massif (Il’meno-Vishnevogorskii Complex, Southern Urals)

235 The Buldym massif (BM) of ultrabasic rocks occurs among metamorphic rocks of the Precambrian Vishnevogorskaya Suite (Fig. 1) in the northern part of the Il’meno–Vishnevogorskii Complex (IVC). The wide popularity of the massive is associated with the presence of pyrochlore carbonatite and phlogopite– vermiculite mineralization located in the sublatitudi nal zones of tectonic dislocations. Prefold, most likely Archean–Proterozoic ultrabasic rocks include regen erated olivine, enstatite–olivine rocks, and secondary lizardite serpentinite. Although the structural–mate rial peculiarities and geological setting of the massif are well known [1, 2], geochronological data on the ultrabasic rocks are almost absent. The solution of age problems is favored by SHRIMP dating [3] of zircons from ultrabasic rocks (Tables 1, 2) collected in differ ent parts of the massif (Fig. 1). The chemical compo sition of the studied ultthat reported previously [1].

Zirconology of lherzolites in the Nurali Massif

The age trend (SHRIMP U/Pb) of the evolution of zircon is obtained for the first time in lherzolites of the Nurali Massif. Zircons are subdivided into groups by the crystallomorphological and geochemical features. These specific features in zircon development are confirmed by the age dates. Precambrian dates (no younger than 1190 Ma) correspond to mantle sources of the lherzolite block. The Early Silurian (445–448 and 439–440 Ma) wass the time of lherzolite magmatism of 10–15 Ma in duration. The Middle Devonian (382.9 ± 8.7 Ma) corresponded to postmagmatic processes related to the effect of gabbro–diorite intrusions crowning in the Nurali Massif.

Zircons in Gabbro of the Nurali Massif (Southern Ural)

The formation time of 410.5 ± 1.1 Ma (zircon U/Pb SHRIMP) and the duration of rock crystallization (2–2.5 Ma) were determined for gabbro from the Nurali massif. The gabbro zircons showed a complicated polyphase structure. A new polygenous type of zonality including traces of primary growth and of the processes of crushing, dissolution, and substitution was discovered in the zircons. The formations of gabbro and ultramafites (lherzolites) of the massif are separated by a time break of 30–35 Ma determined by the genetic discontinuance.

Zirconology of rutile eclogites of the Maksutov Complex (Southern Urals)

The age data (U–Pb, SHRIMP II) of zircons from rutile eclogites of the Maksutov Complex (MC) (village Shubino, Southern Urals) were subdivided into three age groups. The Neoproterozoic zircons (561 ± 10 Ma) recorded the formation stage of eclogites, the protolith of which was diabase and gabbro-diabase bodies. The Neoarchean ancient zircons (2884 ± 36 Ma) belong to the mantle substratum, which was repeatedly transformed (2303 ± 12, 2008 ± 18, 1626 ± 59 Ma). Zircons of early Ordovician–early Silurian age (433–477 ± 6 Ma; 340 ± 40 Ma) recorded superimposed processes, corresponding to high-pressure metamorphism.

Zirconology of serpentinites from Nyashevo massif (Southern Urals)

Zircons in serpentinites from Nyashevo massif of the Ilmenogorskii complex were dated for the first time by means of the SHRIMP technique. The maximum date of 1892 ± 23 Ma for the zircons accounts for the minimum age of their mantle substrate probably constituting the restite residue. The date is comparable to those for metamorphic rocks of the Selyankino group, as well as of fenite–sand amphibolites of the Ilmenogorskii complex. The Upper Ordovician age limit of 443 ± 12 Ma is adequate for formation of the massif and conforms to the age of the Buldym massif and miaskites. The Early Permian dates of zircons (275.8 ± 2.1 Ma) represent late shear processes in the Ilmenogorskii complex.

Zirconology of ultrabasic rocks of the Karabash massif (Southern Urals)

Dating of zircon (SHRIMP) from dunite and harzburgite of the Karabash massif was carried out for the first time. Relics of ancient crystals (1940 ± 30 Ma in harzburgite, 1860 ± 16 Ma in dunite) provide evidence for the Paleoproterozoic age of the protolith. The morphological peculiarities of zircon crystals allow us to assume differentiation of the magmatic source 1720 m. y. ago. The major variety of zircons indicates stages of metamorphic evolution in the Neoproterozoic (530–560 Ma) and Early–Late Ordovician (440–480 Ma).

Zirconology of carbonate rocks (marbles-carbonatites) of the Il’meno-Vishnevogorskii Complex (Southern Urals)

504 Separate outcrops of grayish–white coarse–fine granular carbonate rocks with massive structure and granoblastic texture are observed among Precambrian metamorphic rocks in the central part of the Il’meny Mountains. They contain the signs of recrystallization and metasomatic alterations and based on the combi nation of geological and material parameters are determined as marbles of sedimentary–metamorpho genic origin [1]. Among carbonate rocks of the Il’meno–Vishnevogorskii Complex (IVC), calci phyres and carbonatites are known as well. The nature and relationships of carbonate rocks of the complex are debatable, which results in the coexistence of vari ous hypotheses and models of their formation. In par ticular, based on the morphological and mineralogical signs, some carbonate rocks previously related to mar bles changed their status to carbonatites with all the related consequences [2]. In addition to such conclu sions, there are ideas [3] that carbonatites of the IVC are controlled by melting of alternating marbles and serpentinites in the zone of influence of the Ufimskii block of the Russian Platform. The possibility of mar ble replacement by carbonatite material, i.e., the existence of apolimestone (apomarble) carbonatites is not confirmed. The obtained data for zircons from marbles and carbonatites of the IVC allow us to discuss the mentioned problems.