Geochemical ASSOCIATIONs BETWEEN formation OF BATHOLITHS and

RARE-METAL-GRANITE intruSIVE-DYKE belts in PHANEROZOIC

gRANITOID MAGMATISM areas

Antipin V.S., Perepelov A.B.

Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia, antipin@igc.irk.ru

The most abundant granitoid magmatism occurrences in different stages took place when large batholith, including mesoabyssal and abyssal deep facies, generated. At present we can state that batholiths are composed of granitoids of different geochemical types; they can generate in various geodynamic settings: collision, intraplate and active continental margins. For the Central Asia, in particular for Baikal Area, Transbaikalia and Mongolia, the batholith-formation problem is extremely important, as the largest batholiths of different age originated during the Phanerozoic: Angaro-Vitim, Daurian-Khentei, Khangai, etc. One of characteristic features of these large plutons is their location in "cores" of vast magmatic areas. On margins of these areas there are abundant magmatic rocks of various alkalinity, silica content and ore potential. Intrusive-dyke belts, located in the batholith surrounding, and showing evident features of mantle-crustal interaction, deep fluid-magmatic differentiation are of particular interest. They are frequently related to rare-metal mineralization, which occurs in the same tectonic structures, as dyke belts.

The Cornwall province (England) is a classical example of geologic and geochemical relations of deep-seated granitoid plutons (Kornubian batholith) and subvolcanic dyke belts being of the same age. As regard to isotope-geochemical characteristics rocks from the dyke belt which contain such high-potassium rocks as elvans, are similar to subalkaline granites of the Kornubian batholith. The Late Paleozoic magmatism occurred here could be produced from melting of the Proterozoic protolith in the lower part of the continental crust. The significance of the juvenile fluid in the evolution of granitoid-elvan magmatism results in the character of the associated mineralization (Sn, W, Cu, Mo) which is usually hosted in fluid-intrusive breccias, tourmalinization zones and frequently occupies one and the same system of fractures, together with the dyke belt. Some investigators point out genetic relation of elvan magmatism of the dyke belt and the main stage of the ore mineralization.

In the Central Asia the palingenic granite-formation combined with batholith origin was the most abundant in the Late Paleozoic and Mesozoic (Yarmolyuk, Kovalenko, 2003). The most vivid example is the largest Angaro-Vitim batholith in the Baikal Area. It is composed of mainly palingenic calc-alkaline granitoids. In addition to intrusive rocks of the heightened alkalinity magmatic rocks of the Urugudei-Utulik intrusive-dyke belt, comprising small intrusions and subvolcanic dykes of rare-metal Li-F granites, ongonites and other rocks with fluorite-topaz-cryolite mineralization, are abundant in the batholith surrounding. Intrusions of rare-metal granite (Kharagul, Bitu-Dzhida, etc.) and ultrarare-metal subvolcanic rocks (Utulik) are similar by age with granites of the Angaro-Vitim batholith. Thus, there is a general evolution trend from calc-alkaline granitoids to plumasite rare-metal Li-F granites.

The Late Paleozoic age of batholith granitoid and Urugudei-Utulik series of magmatic rocks as well as similar isotope-geochemical characteristics (⁸⁷Sr/⁸⁶Sr, εNd and the modeled age exceeding 1200 Ma) indicate similar intraplate geodynamic settings of magmatic rock evolution in the region. The obtained petrologic-geochemical data for calc-alkaline granitoids of the Angaro-Vitim batholith, as well as for silicate rocks of the intrusive-dyke belt, located on its margins, characterize them as palingenic crustal formations. However, during the fluid-magmatic evolution rare-metal granites and ongonites were extremely enriched by fluorine and a number of lithophile elements. This is an indication that these rocks belong to Li-F rocks (Antipin, 2005). The relations of calc-alkaline granitoid magmatism and plumasitic and rare-metal granites show the difference of the associated ore mineralization from Cornwall province. Intrusive-subvolcanic rocks of Khamar-Daban province in Baikal area produce Sn and Ta mineralization (tantalite-columbite, cassiterite), while the late quartz- cassiterite- wolframite veins and mineralized breccias contain ore minerals Sn, W and Be. The given province is mainly characterized by the fluorine mineralization at later stage of intrusive-dyke magmatism, when generated quartz-feld spar -topaz- cryolite veins with fine impregnation of cassiterite and wolframite.

The Daurian-Khentei batholith generated at the Early Mesozoic stage. Its formation coincides with closing of the Paleoasian basin and associated collision. The rocks of the batholith include calc-alkaline series which is predominated by granodiorites and granites enriched by dark-colored minerals. The surrounding of the Daurian-Khentei batholith contains abundant intrusive-dyke belts comprising hypabyssal-subvolcanic rocks of different geochemical types, including rare-metal (Kovalenko et al, 1999). So, multi-phase intrusions of palingenic calc-alkaline and Li-F granites as well as comagmatic belt of granite-porphyries, felsites and single dykes of ongonites with topaz and fluorite were mapped in the Abdar-Khoshutula belt. The given belt in addition to typical palingenic crustal granites hosts alkaline syenites, being of the mantle nature.

Comparison of geochemical features of batholiths and associated intrusive-dyke belts stimulates interpreting of granite-formation and disclosing of the significance of mantle and crustal sources for the origin of intrusive-dyke belts in the batholith surrounding.

Studies were supported by RFBR, grants №№ 08-05-90201-mong_а, 08-05-00403_а.

References

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Kovalenko V. I, Kostitsyn J.A., Yarmoljuk V.V., et al.. Sources of magmas and isotope (Sr, Nd) evolution of rare-metal Li-F гранитов. Petrology. 1999. v.7. №4. P.401-429.

Yarmoljuk V.V., Kovalenko V. I. Geodynamic conditions of formation of batholith in the central-Asian folded belt. Geology and Geophysics.2003. v.44. №12. P.1305-1320.