GEOCHEMICAL FEATURES, SOURCES OF SUBSTANCE AND AGE TO BITU-DZHIDA

MULTIPHASE INTRUSION OF Li-F GRANITES (KHAMAR-DABAN)

Perepelov A.B., Tatarnikov S.A., Dril S.I., Antipin V.S., Vladimirova T.A., Sandimirova G.P.

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

The purpose of the given work is to show results of the newest geological and isotope-geochemical studies of the Bitu-Dzhida multiphase intrusion of Li-F granites, located in southern spurs of Khamar-Daban Ridge (51⁰03′20″N, 102⁰11′20″E^).

The Bitu-Dzhida intrusion is found in the Upper Proterozoic metamorphic sequence (crystalline schist of the Bitu-Dzhida suite). The massif was discovered in 1933 (Naletov, 1941) and later studied via geologic survey and prospecting for Li, Rb, Ta, Nb (1954-1960) as well as via systematic scientific researches (Koval, 1975; Kosals, 1976). Using K-Ar of dating the age of the massif was determined as the Permian-Triassic - 262-218 Ma (Kosals, 1976). By the results of recent investigations (Perepelov et al, 2007) we can distinguish three main phases of intruding granite magma. The 1^st phase includes small outcrops of Pl-Kfs-Qtz-Bt middle-grained and porphyry-like granites; 2^nd phase comprises Qtz-Kfs-Pl-Bt leucocratic granites. And the predominant are amazonite-albite-zinnwaldite (Amz-Ab) rare-metal granites of the final 3^rd phase. Pegmatite-formation and greisening took place at the terminating stage of the intrusive massif formation. Exocontact zones show cataclasm, crushing and recrystallization of crystalline schist.

Rocks of all three intrusive phases of the massif belong to group of plumasite granites of Li-F geochemical type. Concentration of rare elements, namely Li, F, Sn, Ga, Rb, Nb, Ta, Pb, Th and U increase in rocks successive to intrusive phases. Compositions of rocks of the early phase differ from granites of the 2^nd and 3^rd phases in lower silica content and the lowest total alkalinity. Granites of the 1^st phase have the most differentiated REE distribution spectra (La_N/Yb_N 10.6-16.3 ) while the rocks of the 2^nd intrusive phase show decrease in REE fractionation (La_N/Yb_N 3.6-6.6) resulting from an essential enrichment by heavy elements of the spectrum and possess more considerable deficiency EuN (Eu* 0.18-0.29 as compared to 0.49-0.65 for rocks of the 1^st phase). Amz-Ab leucogranites of the final 3^rd phase are characterized by a sharp LREE depletion (La/Yb <1) and a deep Eu-minimum (Eu*0.05). Distribution pattern of rare elements including REE, correspond to models of crystallization differentiation of initial magma when granites of the 2^nd intrusive phase generated. The model of forming Amz-Ab rare-metal granites of the 3^rd phase is associated with fluid-magmatic liquation processes.

According to new data, the time of intruding the 1^st initial phase of granitoids of the Bitu-Dzhida intrusion is regarded as the Late Carbonaceous (С₂) that is 311 ± 10 Ma (Fig. 1). These data allow considering the studied intrusive complex as belonging to the age group of the Urugudeev (321 ± 5 Ma) and Kharagul (318 ± 3 Ma) massifs of Li-F granites of Khamar-Daban ridge (Kostitsyn, 2002).

The obtained isotope characteristics for granitoids of the 1^st and 2^nd intrusive phases of the Bitu-Dzhida massif, namely ⁸⁷Sr/⁸⁶Sr_(t) (0.705312-0.706187), ¹⁴³Nd/¹⁴⁴Nd_(t) (0.512088-0512290), ²⁰⁶Pb/²⁰⁴Pb_(t) (17.761-17.961), ²⁰⁷Pb/²⁰⁴Pb_(t) (15.454-15.491), ²⁰⁸Pb/²⁰⁴Pb_(t) (37.426-37.587) agree with the model of origin of initial Li-F granitoid melts at lower horizons of the continental crust (Fig. 2). The substance of the low crust most likely had the composition of relatively enriched LILE Bt-containing granulites the partial melting of which resulted from uplifting of the astenosphere mantle diaper (plume). Moreover, the composition and isotope -geochemical features of assumed magma-forming substratum agree with characteristics of the ancient Precambrian continental crust with an average modeled age T_DM2=1260 Ma and maximum - 1600 Ma. These conclusions are in agreement with results of previous studies of Li-F granites of the Central Asia (Kovalenko et al., 1999).

Studies were supported by the Russian Foundation for Basic Research, grants №№ 08-05-90201-Мong_а, 08-05-00403_а.

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