Advances in geochronology in the Suwałki anorthosite massif and subsequent granite veins, northeastern Poland

Abstract

The Mesoproterozoic Suwałki anorthosite massif (SAM) in NE Poland, intruded into a late Palaeoproterozoic domain in southeastern Fennoscandia, comprises anorthosite, Fe-Ti-V oxide ore deposits, gabbroic rocks and rapakivi-like granitoids. In the present study, a new set of age determinations on zircon grains from AMCG components (anorthosites and rapakivi granites) from different parts of the SAM massif, from granite veins cutting the AMCG rock suite, and from micro-coronas formed on Fe-Ti oxides, is presented. The new age results obtained using a SHRIMPIIe/MC change the previous interpretation (Re-Os NTIMS method on sulfides) that the SAM anorthosites were emplaced at ca 1.55 Ga. The new zircon SHRIMP ages indicate that anorthosites were intruded in several magmatic pulses culminating at ca. 1515 Ma and 1507 Ma. This is confirmed by single spot data and concordia ages of 1513 ± 6 Ma, 1509 ± 3 Ma and 1507 ± 3 Ma obtained for anorthosites. Similar single-spot peaks at 1515 Ma, 1509 Ma and 1496 Ma characterize rapakivi-type granites which also yielded concordia ages of 1513 ± 6 Ma, 1507 ± 6 Ma and 1499 ± 8 Ma. The SHRIMP data from the SAM anorthosites indicate an age of emplacement significantly younger than the ca 1.55 Ga age determined previously, and coeval to ages obtained for rapakivi granite. Both rock types contain entrained antecrysts, reflecting a two-step crystallization in a deep-seated magma chamber and continuous crystallization during the rising of the melt and incremental emplacement of the SAM. Abundant granitic aplites and pegmatites in sharp contact with the host AMCG rocks define a time frame for the entire SAM solidification. The crystallization ages of individual veins ranging between 1489 ± 6 Ma and 1475 ± 5 Ma are clearly older than cooling ages previously obtained by Ar-Ar and K-Ar methods. Microgranites with numerous inherited zircon cores were generated by re-melting of the local sources, including late Paleoproterozoic ca 1.83 Ga crust and SAM related rocks, within a shallow felsic magma chamber. Finally, zircon coronas on Fe-Ti oxides ilmenite grains record subsolidus ilmenite exsolution processes (from ilmenite) occurring simultaneously with the crystallization of the aplite and pegmatite veins.