Geochronology and isotopic signature of syenite-carbonatite intrusions within crystalline basement of NE Poland
Wiszniewska J. *, Krzemińska E. *, Demaiffe D. **, Stein H. ***, Williams I.****
* Polish Geological Institute, Warsaw, Rakowiecka 4 (*email@example.com),
** Laboratoire Géochimie Isotopique, Univ. Libre de Bruxelles (U. L. B.), 1050 Bruxelles, Belgique.
** AIRIE Program, Colorado State Univ., Fort Collins CO 80512-1482, USA
****Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia
The Proterozoic basement in northeastern Poland is covered by thick Phanerozoic sedimentary pile. Geophysical prospecting and drilling operations have lead to the discovery of 3 alkaline-magmatic complexes intruding the basement: the small (~5km2) Tajno alkaline-carbonatite complex, and the much bigger (100 – 400 km2) the Elk alkaline-syenite massif and the Pisz gabbro-syenite intrusion. The complexes occur in a narrow E-W band along the southern rim of the Mesoproterozoic AMCG suite Mazury Complex, NE Poland, streaching from Lithuania along the Mazowsze Domain into the Mlawa alkaline Massif.
Undeformed carbonatites and associated alkaline rocks (mainly nepheline syenites or their volcanic equivalents) are generally emplaced in rift zones or lithospheric domes within continental cratonic blocks worldwide. Polish alkaline complexes represent subplatform intrusions related to Paleozoic rifting.
Preliminary K-Ar geochronological results estimate a Late Paleozoic age for the Pisz and Tajno complexes (Depciuch et al 1975). The Rb-Sr whole rock isochron age of 355 +/4 Ma has been obtained for the Elk Massif by Blusztajn (1994) on 9 syenites (both foid-bearing and quartz-bearing varieties). The MSWD value is quite high (5.79) possibly pointing to the hydrothermal impact of the Rb-Sr isotopic system, which is quite commonly observed in alkaline-peralkaline rocks. The mineral K-Ar and Rb-Sr ages (as well as the fission track ages) represent minimum ages related to the cooling of the intrusions. The Rb-Sr whole rock isochron age on the Elk massif, even if it is not perfect (high MSWD), is probably close to the emplacement age.
Fig.1 Zircon grains images from Pisz gabbro massif (a,b) and from Elk syenite itrusion; (a, c) in transmitted light and (b, d) in CL.
Zircons from Pisz gabbro and from Elk syenite have been recently analysed by U-Pb SHRIMP method in RSES ANU in Canberra but zircon from the Tajno albitite (trachite) by U-Pb TIMS in the “Centre de Recherches Pétrographiques et Géochimiques, CNRS (Nancy, France). Sulphides from the Tajno carbonatite were analysed by Re-Os (NTIMS) method at the Department of Geosciences, Colorado State University (Fort Collins, USA).
Fig. 2. Concordia diagrams of U-Pb zircon SHRIMP (a, b,) and TIMS (c) results for Pisz Ełk and Tajno massifs.
The new U-Pb results are very consistent: 348±8 Ma for Elk, 346±5 Ma for Pisz (SHRIMP) and 348±15Ma (TIMS) for Tajno. These ages point to an Early Carboniferous emplacement for platform mafic-alkaline magmatic activity in NE Poland. Re-Os data for a sulfide sample (pyrrhotite) from the Tajno carbonatite yield an age of 348±2 Ma, calculated using an initial Os ratio of 0.2. The sample is LLHR (low level-highly radiogenic with187Re/188Os = 2466±5); thus, selection of the initial Os has minimal effect on the age calculation.
Initial (at 348 Ma) Sr (0.7033-0.7042) and Nd (εNd: + 1.4 to +3.6) isotopic ratios point to a possible involvement of two contrasting mantle components, DMM and EM1, in proportions of 95:5 (Wiszniewska et al., 2010).
Our data suggest that these 3 intrusions constitute a new occurrence though slightly younger a widespread alkaline and carbonatite magmatism related to rifting of the East European craton, as those on Kola Peninsula (Demaiffe et al., 2005)
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Wiszniewska J., Krzemińska E., Krzemiński L., Demaiffe D., Stein H., Williams I.S. – A new Early Carboniferous Alkaline Province in the crystalline basement of NE Poland // Abstract volume of Goldschmidt Conference 12-19.06.2010., Knoxville, USA