U–Pb and Ar–Ar geochronology of anorogenic granite magmatism of the Mazury complex, NE Poland

Abstract

The Mazury complex of northeastern Poland forms a 200 km long, E–W trending anorthosite–mangerite–charnockite–granite belt derived from bimodal, multiphase anorogenic magmatism and is similar to the classic anorogenic complexes of the Fennoscandian Shield (e.g. Wiborg, Salmi and Ragunda). The complex is dominated by felsic and intermediate rocks ranging from quartz monzonite through granodiorite and monzodiorite to diorite and is associated with anorthosite–norite plutons (Suwalki, Sejny and Ketrzyn). The ore deposits in Suwalki have 1559±37 and 1556±94 Ma Re–Os ages with a high initial 187Os/ 188Os reflecting crustal signature (Morgan et al. Miner. Deposita 35 (2000) 391). The Mazury complex crosscuts granulite and amphibolite facies metamorphic units (Pomeranian, Ciechanów and Mazovian), which are correlated with the Svecofennian rocks of Fennoscandia. U–Pb geochronology of single zircons and titanite fractions indicates that the felsic igneous activity of the Mazury complex occurred during a ∼25 m.y. period in at least three distinct pulses: (1) Krasnopol monzodiorite has a zircon 207Pb/ 206Pb age of 1525±4 Ma and titanite fractions dated at 1526±11 Ma (mean 207Pb/ 206Pb age), zircons from Bartoszyce quartz monzonite are concordant at 1522±2 Ma; (2) Boksze diorite has two concordant zircons with an age of 1512±1.1 Ma; (3) the youngest episode is represented by a 1502±2 Ma quartz monzonite from Goldap and a 1499±4 Ma quartz monzonite from Bartoszyce (Claesson et al., MAEGS meeting, St. Petersburg, 1995, p. 21). Zircon with a 207Pb/ 206Pb age of 1567±8 Ma from Boksze diorite points to inheritance from the older intrusions. Difference between U–Pb titanite and Ar–Ar biotite ages is ∼100 m.y. and suggests relatively slow cooling rates (∼2 °C/m.y.). The U–Pb ages indicate that the Mazury complex is roughly coeval with the Ragunda (Sweden) and Salmi (Russia) rapakivi complexes and show that the Fennoscandian rapakivi occurrences delineate a marked younging trend from north to south from the classic Wiborg batholith through the Riga batholith to Mazury.