2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Multistage alteration of the Sarioli andesit-basalts from the Veimarssk sequence of Nothern Karelia (Baltic shield)

Lvov P.A..*, Myskova Т.А.**, Lokhov K.I.*

^*SPb state university,St-Petersburg, Russia, **IPGG RAS, St-Petersburg, Russia

Kirill_Lokhov@vsegei.ru

Orogenic rocks of the Sarioli are constant member of the sequence of the Lekhta structure in Nothern Karelia. There are absent robust geochronological data for them, because of absence unmetamorphised rocks. In this work the rocks were specilally sampled with aim to get less altered ones from parts of sequence, stratigraphical position of which  has no apprehensions. (Ivanov, e.a., 2010).

Veimarsk sequence consists of a number of andesitic and adesito-basatic volcanics with som tuffs and brecchias. Chemically them also correspond to andesites, andesito-basalts. Except of greenshist metamorphism them suffered some event of forming blasts of biotite, hornblende and siderite which led to some change in primary (magmatic) chemical compositions. In altered rocks are elevated concentrations of TiO₂, Fe₂O_3., K₂O, Y, Zr, Rb, Cs, Nb, Hf, sum of the REE and reduced concentrations of Na₂O.

Isotopic age determinations (U-Pb, SHRIMP-II) was carried out for two samples of andesites (4244/1 и 4226). Three age clastes fro zircons were obtained and it need to be interpreted correctly. Therefore except of comparison of isotopic U-Pb age by zircon and neodymium model age T_NdDM, it was used isotopic Hf-Nd Systematics offered in (Lokhov, e.a., 2009). This method allows determinate zircons genesis (magmatic, metamorphic, captured xenogenic, etc.), and correctly works if isotopic Sm-Nd system in the whole rock samples was not altered or distorted. In This case it is not so, because were calculated unreal ancient neodymium model ages (more than 3 b.y), and using some four samples methaandesites, the Sm-Nd isochrone was obtained with the parameters: Т=1995±820 Ma, (¹⁴³Nd/¹⁴⁴Nd)i=0.50962, MSWD=0,33. For correct of the Sm-Nd isotopic system distortion during svecofenian metamorphic event, for calculation of the εNd(Т) parameter for rocks was accepted mean value of isotopic ratio in the studied rocks ¹⁴³Nd/¹⁴⁴Nd =0,51125.

Sample 4224/1 – (T_NdDM=3101 Ma) contains two zircon generations. Archean zircon with measured U-Pb age from 2672±9 to 2751±13 Ma (по ²⁰⁷Pb/²⁰⁶Pb) was found in the cores of some crystals with relict magmatic zoning and has rims of second generation. Probably these fragments are inherited or captured from Archean host rocks. REE distribution patterns are close to “hydrothermal” with clear elevated LREE and reduced Ce anomalies. Xenogenic genesis of these zircons confirms by isotopic Hf-Nd correlation plot- to the right from the Terrestrial array (TA) (fig. 1, open circle).

Second generation zircon with  U-Pb age 2412±17 Ma forms concordant cluster on the Ahrens-Wetherill plot. This generation represented by coarse zoned crystals and dark (by CL) rims. As a part of first generation zircons these second ones sometimes are a bit discordant. The age estimation obtained agree with stratigraphic position of the rocks. REE distribution patterns have typical ‘magmatic’ feature; depletion in LREE with clear Eu and Ce anomalies. Magmatic origin of these zircons is in concordance with isotopic Hf-Nd systematics. All measured areas in zircons provide calculation of the εHf(Т) values which in combination with the εNd(Т) for rocks give figurative points inside the TA.

Sample 4226 – porphyry metaandesite. In this sample two generation of zircons were found.

More abundant are zircon grains which are discordant with upper intercept about 1907±15 Ma. Low Th and U concentrations and bright CL images can suggest effects of recrystallization. These zircons form clearly distinct generation and correspond to svecofenian metamorphism. According to Hf-Nd isotopic systematics these zircons also are metamorphic, because theis figurative points fall to the left an up from the TA correlation field (fig. 1, oprn triangles). Age estimation (²⁰⁷Pb/²⁰⁶Pb)  2481±30 и 2632±10 Ma, obtained by two cores is not correlating with stratigraphic position of the rock, and Hf-Nd isotopic systematics also give evidence of their captured genesis.

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

Summary

In studied rocks several generations of zircons were found. Genetic interpretation by isotopic Hf-Nd systematics have shown excellent correlation with geological data, despite isotopic Sm-Nd system of rocks was distorted during svecofenian metamorphic event.

The zircon areas with isotopic U-Pb age 2412±17 Ma cofirmed to be magmatic and give robust estimation of the formation time of Veimarsk voclanics. Discordant zircons 1970±15 Ma (upper intercept) are connected with svecofenian metamorphism. Their discordance indicate to the younger events and sone phanerozoic zircons in these rocks really wre found. Archean zircons with U-Pb age from 2481±30 to 2751±13 Ma are captured from Archean rocks on the lower lewels of the crust.

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.

This work in part was supported by Rosnedra project:«Geological mapping of the Shombozer-Lekhta 1:200000 scale, Q-36-XXVII, Q-36-XXVIII, State geological map of Russia (GGUP «SF Mineral») , and SPbU grants 3.36.81.2011 and  3.31.86.2011.

References:

Ivanov N.M., Korsakova М.А., Dudareve G.A., e.a. State geological map of RF scale 1:200000, Karelia series, sheets Q-36-XXVII, XXVIII. Explanation paper, 2010. (in Russian).

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).