2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

U-Pb and Lu-Hf isotopic systems in zircons from pegmatitoids and granite stocks connected with the

Salmi Rapakivi Batholith, Ladoga area, Russia

 Sundbland K*., Lokhov D.K.**, Sergeev S.A.**, Kapitonov I.N**., Lokhov K.I.**

^*Turku university, Turku, Finland, ^**SPb state university,St-Petersburg, Russia,

krisun@utu.fi

The Salmi Batholith is an A-type rapakivi granite complex that intruded along the Archaean-Proterozoic boundary at 1540 Ma (Neymark, e.a., 1994, Amelin, e.a., 1997) in the Ladoga region of northwestern Russia. Several small (<1km) granite stocks and stratiform pegmatitoids occur in the Proterozoic sup racrustal sequence, immediately southwest of the Salmi Batholith.  Extensive evidence of hydrothermal products also exists in this area, partly expressed in a 20 km long zone of sulphide-bearing skarn ores and partly by fluorite veins in the granite stocks.

Although the age of the stratiform pegmatitoids and granite stocks are assumed to be similar as the main Salmi Batholith, U-Pb data fail to confirm this. We have investigated stratiform pegmatitoids at Hopunvaara (KS911) and Torppusuo (KS920) as well as a granite stock, immediately under a stockscheider pegmatite at Lupikko (KS912, KS913).

Investigation of the U-Pb isotopic system in zircons was done by SIMS SHRIMP-II, Lu-Hf isotopic system by laser ablation LAICPMS ThermoFinnigan Neptune/DUV-193 in the same areas of SIMS analysis, Sm-Nd system in rocks by conventional chemistry procedures with measurements by TIMS ThermoFinnigan Tritone at the Center of isotopic research, VSEGEI, St.-Petersburg.

In Hopunvaara rock it were found rare zircons with age estimation grouped into 250-300, 950-1100 and 2756 Ma, and abundant zircons which demonstrate concordant cluster with age 1706±13 Ma.

In Torpussuo rock was zircons demonstrate concordant cluster with age estimation 1482 ±14 Ma, two grains of

1025 ±16 Ma and one 1200Ma.

   Granitic rocks from Lupikko: KS 912 – concordant clusters in the ranges 255, 575-600, 1620-1640 Ma and discordant ones with age estimation in range 980-1200 Ma. Taken near by (<10 meters) KS913 zircons demonstrate  two concordant clusters 1710 ±16 and 1448 ±16 Ma.

These age estimations dramatically do not fit existing geochronological data on Salmi batolith. Trivial sample contamination we exclude, because some zircon extraction procedures were repeated by different facilities and with care.

In the aim to determine zircon genesis in these rocks we tried to apply isotopic Hf-Nd systematics by Lu-Hf in zircons and Sm-Nd in the rocks (Lokhov, e.a., 2009).

Fig.1. Isotopic Hf-Nd systematics. Gray lines are marking the TA correlation field (parameters for magmatic zircons).

The main result from this systematics is that all studied rocks do not contain magmatic zircons. Occurrence of some figurative points inside correlation field for magmatic rocks “terrestrial array” (TA) can be casual due to some overlapping of characteristic fields for magmatic, captured/inherited and metamorphic or metasomatic zircons.

Other interesting result is that most part of the zircons contain some excess radiogenic hafnium (figurative points above the TA field), which reflects in calculated values of εHf(T) even more than 20, and consequently calculated hafnium model ages less than U-Pb ones, and even in the future time. Such effect could originate under high fluid conditions of rock formation and alteration due to extracting of hafnium from recrystallizing minerals of host rocks with high Lu/Hf ratios (Goltsin, e.a., 2010, Valley, e.a., 2010). To transport HSFE as Zr and Hf it is needed some water-halogene ( Cl, F) fluids ( e.g. Korzhinskaya, Ivanov, 1988, and others). Taking into account presence of fluorite in studied rocks and abundant fluid during their formation, there were conditions for transport of HSFE, and presence of excess radiogenic hafnium in zircons confirms it.

From this point, zircons with U-Pb age less, that formation age of the Salmi batolith (1.54 Ga) are clearly metasomatic, and the older ones are metasomatically altered. There is an important question about robust behavior of U-Pb system in zircons under such conditions. From one hand, presence of abundant fluorine in the rock forming fluid could dramatically change the U/Pb ratios in the zircons due to high mobility of uranium under such conditions, hence age estimations by U-Pb isotopic system does not fit realistic geological processes, the age estimations are virtual. From the other side young Phanerozoic events of rocks alteration were confirmed in the near by Onega depression both by investigation of U-Pb isotopic systems in zircons and bulk rocks, clear discordias (by zircons) with the lower intercept correspond to some new concordant young zircons, which have distinct excess radiogenic hafnium (Goltsin, e.a., 2010).   

Summarizing the data and speculations we can argue, that despite we did not succeed in determination of age of the rock formation, it were found some specific zircons with measured U-Pb ages younger, that time formation of the Salmi batolith, and even Phanerozoic, and them contain essential amounts of excess radiogenic hafnium, marking high-fluid formation of the rocks, and probably pointing to metallogenic significance of the process.

This work in part was supported by SPbU grants 3.36.81.2011 и 3.31.86.2011.

Literature

Amelin Yu.,V., Larin A.M., Tucker R.D. Chronology of multiphase emplacement of the Salmi rapakivi granite-anorthosite complex, Baltic Shield: implications for magmatic evolution. Contib. Mineral. Petrol. 1997. V. 127. P. 353-368.

Goltsin N.A., Lokhov K.I., Kapitonov I.N., e.a. Polystage alteration of high carbon rocks from Ludikovi of the Onega depression. // Regional Geology and Metallogeny, 2010, №41, с.66-79. (in Russian)

Korzhinskaya V.S., Ivanov I.P. Experimental study of incongruent solubility of zircon in the system ZrO₂-SiO₂-H₂O-HCl at T=400-600^oC and P=1 kbar // Doklady Acad. Nauk USSR, 1988, v.299, N4, pp.970-973.

Lokhov K.I., Saltykova T.E., Kapitonov I.N., e.a. Corract interpretation of isotopic U-Pb zircon age by isotopic geochemistry of hafnium and neodymium (on example of some magmatic complexes of the basement of the East – European platform) // Regional Geology and Metallogeny 2009. № 38. С. 43-53. (in Russian).

Neymark L.A., Amelin Yu.V., Larin A.M. Pb-Nd-Sr isotopic and geochemical constraints on the origin of the 1.54-1.56 Ga Salmi rapakivi granite-anorthosite batholith (Karelia, Russia) // Mineral. Petrol. 1994. V. 50. P. 173-193.

Valley P.M., Fischer C.M., Hanchar J.M., e.a. Hafnium isotopes in zircon: A tracer of fluid-rock interaction during magnetite-apatite (“Kiruna – type”) mineralization // Chemical Geology, 2010, 275, p. 208-220.