TECTONIC POSITION OF NIZHNEULOR GRANITIC MASSIF IN WESTERN SANGILEN

STRUCTURES (SOUTH-EAST TUVA)

Vladimirov V.G., Karmysheva I.V.

Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia, iri@uiggm.nsc.ru

Sangilen orogenic structures was formed in early Caledonian period as a result of slanting collision Tannuola island arc system with Tuva-Mongolian microcontinent (Gibsher et al, 2000; Khain et al., 2002). This process is characterized with changing geodynamic contraction conditions (570-490 ma), extension conditions (490-430 ma) (Vladimirov et al, 2005). The extension period begins at 480-460 ma, when lime-alkaline and (or) subalkaline granitic-leucogranitic crust melts intrusion in local areas of shear zones took place. For example, Nizhneulor granitic massif (474+3, 476+6 ma, Rb/Sr; 480+7 ma, biotite, Ar/Ar) (Vladimirov et al., 2005). This object is considered by this thesis.

Nizhneulor granitic massif is situated in Erzin river mean course, where right inflow of Erzin river Ulor flows into it. This stock is oriented from sourth-west to north-easth and has an oval shape in plane (4х9 km). The magmatic rock presented by one stage plagiogranite intrusion. The chemical composition variations aren’t large. The average content of rockforming elements is (mass percentage): SiO₂ – 72,40, TiO₂ – 0,45, Al₂O₃ – 14,0, FeO – 2,30, MnO – 0,05, MgO – 0,46, CaO – 1,67, Na₂O – 3,23, K₂O – 3,84, P₂O₅ – 0,15 (Kolobov, 1981).

The country rocks of granites are chinchilig series pelite and psammite, which were metamorphized in greenschist – epidote-amphibolite facieses. Late time they were exposed to contact metamorphism associated with granitic intrusion. In massif country rocks were mapped garnet, andalusite-staurolite, sillimanite zonez and hornfels zone (Lepezin, 1978; Vladimirov, Lepezin, 1996).

The data was obtained earlier that Nizhneulor massif had thermal and mechanical influence on country rocks (Kolobov, 1981; Vladimirov, Lepezin, 1996). At present time with the purpose of reconstruction condition massif formation (deformation kinematics, synkinematic mineral formation) in its metamorphic country rocks additional sections with selection structure-oriented samples were made.

The field date in the aggregate with the sample analysis and oriented thin sections gave the results.

Away from the contact shear dislocations intensity lowers, in garnet-biotite-staurolite schistes show up the indication of its initial-sedimentary nature. The shear component disappear taper, and total extension traces with boudinage its most coarse horizontes (sand and aleurosand layers) remained. As is shown in early works (Vladimirov, Lepezin, 1996), in the direction of contact the association with neogenic garnet, sillimanite, fibrolite and potassium feldspar show up, thus evident indication of the high-temperature hornfelses are observed. It is necessary recorded, that most early staurolite and andalusite are dissolved and replaced by sillimanite and fibrolite, and are suffered shear deformations.

Thus are guided by always date, was propose a intrusion and formation Nizhneulor granitic massif structure model.

The creation of shear zone where massif is located was accompanied by layers extension and fragmentation with local pull-apart formation. It was penetrating fault controlled, what were provided for melt dislocation.

In upper and low-temperature crust happened granitic consolidation and country rocks hornfels. Because regional tectonic movements continued, at contact metamorphism process neogenic minerals (garnet, sillimanite, fibrolite) aquired synkinematics characteristics. This are allowed to melt intrusion processes and Nizhneulor granitic massif formation with the regional tectonic movements, and are correlated its with 480-460 ma.

The Nizhneulor granitic massif is example inside shear magmatism in the region, and its characteristics are indicator transformed-shear extension conditions. “Inside shear magmatism” we are named the magma melts intrusion and massif formation processes within the bounds of shear zones.

This work was supported by SB RAS (IP № 6.5), RAS SB RAS (IP № 88), Russian Fund Fundamental Investigation (projects №№ 06-05-64767, 07-05-00853, 07-05-00980).

References

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