EVOLUTION OF GABBRO-GRANITOID MAGMATISM AND ASSOCIATING

Fe-Ti MINERALIZATION OF RIFT STRUCTURES OF THE DIFFERENT DEPTHS

(THE MIDDLE RIPHEAN, SOUTH URALS)

Kholodnov V.V.

Institute of Geology and Geochemistry of UrB RAS, Ekaterinburg, Russia, holodnov@igg.uran.ru

In rift structures of the mobile belts of the Earth granitoids of different specification have different compositions and origin. Together with the products of mantle magmas as the contrast differentiated gabbro-granitic series the products of a selective melting of earth crust are wide-spreaded.

During the Riphean period a riftogene history of the Urals consisted of the distruction stages of the Eastern European platform. At the beginning of the Middle Riphean the Kuvashsko-Mashakskaya rift system consisting of the linear grabens’ series was formed. Together with the products of contrast basalt-rhyolitic volcanism and dikes’ forming in the Northern Kuvashskii graben along the large Zyuratkulskii fault the the gabbroid intrusions (Matkalskii, Kopanskii, Medvedevskii and Kusinskii massifs) and granitoids (Ryabinovskii and Gubenskii massifs) consisting the Kusinsko-Kopanskii complex which are comagmatic earlier volcanites have been formed as a long (nearly 70 km) submeridional chain.

A time formation of gabbro intrusions of the Kusinsko-Kopanskii complex is 1385-1400 MA (Kholodnov et al., 2006; Krasnobaev et al., 2006) according to data of isotopic methods (Sm-Nd, U-Pb, Rb-Sr). A similar U-Pb age zircons of granites of the Ryabinovskii massif have confirming the conclusions about genetic relationship of the granites of this massif with gabbroids. According to U-Pb method of isotopic dating of zicons the history of the Gubenskii massif formation is appeared to be more complicated (Krasnobaev et al., 2006). At the final stage (1300 MA) the processes of metamorphism and hydrous anatexis played an essential role. These data also show to geterogeneity of substratum of this massif which consists of not only products of the Middle Riphean volcanism but also of more ancient rocks (Archean granulite, terrigene formations and Early Riphean volcanites). A source of Archean zircons could be metamorphic rocks and granitoids of the Taratashskii complex situated to the North from the Gubenskii massif on which with erosion and angular unconformity volcano-sedimentary series of the Early Riphean occur. At the same time the main petrochemical and geochemical peculiarities of the Gubenskii granites correspond to the Ryabinovskii ones. The rocks of these massifs are characterized by high ferruginousness and enriched by Ti. According to Rb and Sr relation they correspond to the products of tholeiitic magma. In them a content of high charge rare elements Nb, Ta, Zr, Hf, Y, Yb is higher. Such geochemical specification reflects a geodynamic regime different from subduction and collision environments under which a mass Paleozoic granite formation has take place. At the different diagrams offered to geodynamic interpretation of geochemical data granitoids of these massifs get into the field of the intraplate formations.

The characteristic feature of gabbro and granitoid massifs of the Kusinsko-Kopanskii complex is their forming under the sharply changing conditions of depth facies of the Kuvashskii graben’s succession: from small deep-seated hypabyssal no the South (Kopanskii and Matkalskii gabbro massifs, Ryabinovskii granite massif) to abyssal on the North (Kusinskii gabbro and Gubenskii granite massifs) and fluid regimes (water and halogens). This is reflected in the peculiarities of mineral composition and forming conditions of intrusive rocks (gabbroids and granitoids) and Fe-Ti-mineralization: from high-Ti magnetites (up to 15% TiO₂) in magnetite-ilmenite ores in the South to low-Ti magnetites (1-6% TiO₂) in magnetite-ilmenite ores – in the North (Fershtater et al., 2001; Kholodnov et al., 2006). Correspondingly the southern Ryabinovskii massif stretched in submeridional direction of more than 50 km under its thickness of 1 km formed from hypabyssal (0,5-1 kbar) conditions in the South to mezoabyssal (3-4 kbar) conditions in the North in Medvedevskii massif’s region. To the Gubenskii massif is shown a duration of depth facies’ evolution from nearly mezoabyssal at the latitude of the Medvedevskii massif to abyssal – at the latitude of the Kusinskii massif where pressure during its formation was increasing to 8-10 kbar. In this direction an evolution of volatile regime – from low hydrous at the South to essentially hydrous – at the North with a different activity of halogens (F and Cl).

Granites of the Ryabinovskii massif situated at the latitude of the Medvedevskii massif have some petrographic differences from granites of its southern part. Here the rocks become more coarse-grained, microcline, Ca-garnet and phengite are appeared, stilpnomelane and albite characterizing the granites of the southern part are disappeared. Amphibole is presented by hastingsite which is different from ferroedenite of the southern part by a high content of Al. To fine-grained hypabyssal granites of the southern part an interstitial micropegmatite containing of 40 vol% of quartz, porphyric structures, rimed feldspars, fluorite, high-Ti magnetite are characteristic. Enriched content of Cl (about 0,3%) in apatite and silicates (amphibole and biotite) with even distribution among them shows to crystallization under the conditions of low content of water in melt.

The main petrographic features of granites of the more abyssal Gubenskii pluton are their fine-middle-grained gneissous texture, granulose structure. Biotite-amphibole, biotite and double-mica kinds of rocks are distinguished. Their mineral composition is plagioclase (albite-oligoclase, oligoclase), microcline, amphibole, biotite, muscovite (phengite), garnet. Accessories are: zircon, orthite with epidote rims, sphene, fluorite, low-Ti magnetite. Amphibole (hastingsite) is characterized by high content of Al₂O₃. Biotite has a less constant ferruginousness (55-84%). Garnet is related to grossular-spessartine-almandine type. Apatite and silicates practically haven’t Cl. Apatite is enriched by It PGE which is characteristic to amphibolitic facies and hydrous granitic anatexites. Enriched content of Al₂O₃ in sphene is shown.

Financial support of RFFR, grant 07-05-96006-r-Urals-a