PLAGIOGRANITE MAGMATISM IN RUDNY ALTAI

Kuibida M.L.*, Kruk N.N.*, Paderin I.P.**

*Institute of geology and mineralogy SB RAS, Novosibirsk, Russia, maxkub@uiggm.nsc.ru

**A.P. Karpinsky Russian Geological Research Institute, Saint-Petersburg, Russia

The nature and age of Rudny Altai plagiogranites are discussed during many last years. At present the two plagiogranite complexes are distinguished (Shokalsky et. al., 2000; Vladimirov et. al., 2001): conventional Early Devonian Aleisky joining mesoabyssal intrusions and Late Devonian Zmeinogorsky consisting of hypabyssal mainly leucogranite massifs.

Author’s petrologic and geochronologic study of plagiogranites in Rudny Altai put in last years allows specifying former correlation scheme and getting information about sources of plagiogranite magmas.

Zmeinogorsky plagiogranites have low alkalinity and correspond to calc-alkaline island arc I-granites (Whalen et. al., 1987) or granitoids of “mixed mantle-crust” generation (Barbarin, 1990). They characterized by low Al2O3 (11.21-15.23 wt.%), middle and high SiO₂ (68.78-76.39 wt.%) and Yb (3,01-6.17 ppm), reduced Sr and Eu. It is evidence about belonging these rocks to low-aluminous plagiogranites (Arth, 1983) that forming as a result of dehydration melting of metabasites under P ≤ 10-12 (Beard & Lofgren, 1991; Rapp et al, 1995). Based on data of model estimates the plagiogranite substratum had mixed nature (oceanic basalts + terrigene sediments).

Geochronological study (U-Pb determination on zircon grains, SHRIMP-II in A.P.Karpinsky Russian Geological Research Institute) confirm the Late Devonian age for Zmeinogorsky complex; the age of melanocratic plagiogranites is 377.7±6.1 Ma, the age of leucocratic plagiogranites is 371.5±2.2 Ma.

Geochemical study of Aleisky mesoabyssal plagiogranites reveals their heterogeneity. The rock in massifs in central part of Rudny Altai (where the plagiogranites are intruded by Middle Devonian volcanites) are geochemical similar to Zmeinogorsky plagiogranites. At the same time the geochemical features of plagiogranites in Northern part of Aleisky anticlinorium are different. These rocks have normal alkalinity and on LILE- and HFSE-concentration they close to M-type granitoids (Whalen at. al., 1987). High Al2O3 (15.5 – 17.5 wt.% with SiО₂ = 67.5 - 70 wt.%) and depletion in HREE and Y evidence to belonging these rocks to high-aluminous plagiogranites (Arth, 1978) that forming as a result of dehydration melting of metabasites under P ≥ 10-12 (Rapp et. al., 1995). Based on data of model estimates this type plagiogranite substratum corresponds to OIT-similar metabasalts.

Geochronological studies (U-Pb determination of zircon in bulk sample in GEOKHI RAS, analyst E.V. Bibikova; U-Pb determination in single grains in A.P.Karpinsky Russian Geological Research Institute, analyst E.N.Lepekhina; Ar-Ar determination on amphiboles and biotites in IGM SB RAS, analyst A.V.Travin) give converging in analytical error data – 322-318 Ma (Kuibida et. al., 2007).

Comparison of obtained data with existent scenarios of the region geological evolution in Middle – Late Paleozoic (Rotarash at. al., 1982; Berzin et. al., 1994; Vladimirov et. al., 2003 etc.) evidence that “Early Aleisky” (Early Devonian) and Zmeinogorsky low-aluminous plagiogranites formed in active continental margin environment at that both metabasites of Rudny Altai oceanic basement and upper crust’s sedimentary rocks partake in granite forming processes. At the same time the forming of high-aluminous “Late Aleisky” plagiogranites correlate with peak of Siberia – Kazakhstan collision and was caused by melting of purely metabasaltic lower parts of thickened lithosphere.

Investigations was carried out with Financial Support of Presidium SB RAS (Integration project No 7.10.2) and Russian Fund of Basic Researches (projects 07-05-00853, 08-05-00974).

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