Melt Inclusions in Quartz from Topaz- and Fluorite-bearing Rare-Metal Granites and Ongonites of the Baikal Region

Victor Antipin (antipin@igc.irk.ru), Vsevolod Prokofyev, Helen Savina & Mikhail Mitichkin

P.O. Box 4019, Irkutsk, 664033, Institute of Geochemistry SB RAS, Russia

Rare-metal granites and ongonites form a complex of Paleozoic and Mesozoic hypabyssal and subvolcanic intrusions within pre-Cambrian rocks of the Khamar-Daban block (Antipin et al., 1999).

The quartz of rare-metal granites and ongonites contains small inclusions of the silicate melt. The inclusions involving the crystals of silicate phases, gas bubble (3-8 vol.%) and interstitial aqueous solution have been detected and investigated. Microthermometric studies showed that the melting of silicate phases in inclusions of fluorite-bearing granites starts at 640-650^oC, while in inclusions of topaz-bearing granites the melting begins at 600-640^oC. The melted inclusions are homogenized at higher temperatures in fluorite-bearing granites (790-870^oC) as opposed to topaz-bearing varieties (680-830^oC). The silicate phases in ongonite inclusions are melted within the range 560-650^oC, while the complete homogenization of inclusions occurs at 680-840^oC range, which is similar to that obtained for topaz-bearing granites. The water pressure, calculated using the method by V.B.Naumov (1979) for temperatures of the melting point, reaches high values both for fluorite-bearing (2.5-2.8 kbars) and topaz-bearing granites (2.3-3.1 kbars). In ongonites the PH₂O (1.1-3.5 kbars) variations completely overlap the indicated intervals for granites. The water concentration in the melt reaches the highest values in topaz-bearing granites (1.2-8.2%) and ongonites (3.6-7.7%) as compared to fluorite-bearing granites.

Thus, the study of melt inclusions in quartz indicated that the early fluorite-bearing granites crystallized from higher-temperature magmatic melt in comparison with late topaz-bearing varieties of Li-F granites of the Baikal region. It is an indication of higher content of volatile components (H₂O and F) in topaz-bearing granites and ongonites. The increased fluid pressure, amounting to 3.5 kbars was found for intrusive and subvolcanic topaz-bearing rocks of the Baikal region.

The research was supported through grants of RFBR 99-05-64765

Antipin VS, Savina EA, Mitichkin MA and Perelyaev VI, Petrology, 7 (2), 147-159, (1999).

Antipin V, Savina E, Seltmann Rand Gerel O, Mineral Deposits: Processes and Processing, Balkema, Rotterdam, 301-304, (1999).

Naumov VB, Geochemistry, 7, 997-1007, (1979).

Petrochemical and Geochemical Features of Alkaline-ultramafic and Alkaline-mafic Rocks as a Reflection of Composition and Properties of Mantle on Territory of the Baltic Shield

Anton Antonov (avlan@aa2074.spb.edu), Elena Lepekhina (selena@aa2074.spb.edu), Larisa Nikitina (nikita@ad.iggp.ras.spb.ru), Lev Levsky & Eugeny Bogomolov

Institute of Precambrian Geology and Geochronology, Russian Academy of Science, Makarova emb. 2, St-Petersburg, 199034, Russia

Alkaline-ultramafic rocks as the most deep formations leaving on a surface, represent unique objects bearing the information on the structure and condition of upper mantle, besides the deposits of diamonds connected to them. We investigated three unsimultaneous Precambrian complexes of alkaline-ultramafic rocks on a Baltic Shield. The concentric dikes of peridotite - gabbro-norites of the Kontokki complex concern to the most ancient (more than 1.8 Ga; by Sm-Nd, Rb-Sr methods) metamorphosed up to amphibolites. The received age, probably, reflects the age of metamorphism with attendant metasomatic transformations. On the diagram TAS the points of rocks compositions located in the area dividing alkaline and subalkaline rocks. The REE contents are twice as large as that in chondrites and has poorly differentiated character (La/Yb = 1-5); ISr is characterized by mantle meanings (about 0.701). Petro- and geochemical characteristics of rocks and the minerals compositions are different from once for typical diamond bearing rocks. This complex represents a new untraditional and the most ancient type of diamond bearing rocks on the Baltic Shield. The next stage is represented by Por`ya Bay dike complex of the transitional type lamproites (1720 Ma; by Sm-Nd and Rb-Sr methods; Nikitina et al., 1999). The rocks are enriched by rare alkalis, REE, especially LREE (La/Yb = 70-290). Middle-Proterozoic stage (1270 Ma; by Sm-Nd and Rb-Sr; Nikitina et al., 1999) is submitted by olivine lamproites of Kostomuksha dike complex. The rocks are enriched by REE (La/Yb = 150-690). The geochemical investigation shows following: 1. The presence of Ti-Ta-Nb anomaly in transitional type lamproites spectra of REE distribution and the absence of one in olivine bearing lamproites; 2. For transitional type lamproites is established: enrichment by Ba and Sr, abnormal high relations Ba/Nb, Rb/Nb, Ba/Th, Ba/La, abnormal low Nb/U. These facts indicates on either heterogeneity of mantle in space and/or in time, or distinction in oxidized conditions of magmas generation. The rocks from both regions demonstrate unusual for Precambrian lamproites isotopic characteristics of Sr and Nd: (epsilon)Nd=-(9-9.3) (enriched mantle) and (epsilon)Sr down to -20 (depleted mantle). Enriched component in the lamproites could originate due to mixing of depleted (MORB-type) material with enriched EM1-type mantle source. Researches of geochemistry of REE and geochemistry of isotopes in systems Rb-Sr, Sm-Nd for all complexes allows to assume existence temporary and, probably, spatial heterogeneity in upper mantle, and geochemical evolution of one under a Baltic Shield in low - middle Proterozoy.

Nikitina LP, Levskiy LK, Beliatsky BV, Juravlev BA, Lepekhina EN, Antonov AV, Petrology, 3, 40-61, (1999).