An extension of the Svecofennian orogenic province into NE Poland: Evidence from geochemistry and detrital zircon from Paleoproterozoic paragneisses

The Precambrian crystalline rocks of northern Poland are known only from deep drill cores. Drilling at Jastrzebna and Monki, in the Mazowsze Domain, NE Poland, has intersected a monotonous sequence of grey paragneisses superficially similar to the paragneisses of the Svecofennian Domain exposed in Sweden and Finland. Petrographically, geochemically and isotopically the rocks from the two regions are in fact almost the same. The Jastrzebna paragneiss is an immature metasediment characterised by depleted HREE and Y, indicative of derivation from a source dominated by TTG-type rocks. The Monki metasediment, in contrast, is more mature and chemically uniform, with a REE pattern very similar to average Post-Archean Australian Shale (PAAS). Both metasediments have La–Sc–Th compositions characteristic of active continental margin or continental arc settings. The detrital zircon populations in the two Polish metasediments have the same age distributions; a dominant Paleoproterozoic population at 2.1–1.9 Ga, a subordinate late Archean population at 2.9–2.7 Ga, and rare early Archean grains up to 3.4 Ga. Both samples have very few zircons in the age range 2.6–2.1 Ga. This age distribution is extremely similar to the age distributions of detrital zircon from Svecofennian metasediments exposed in Sweden and Finland. It is highly likely that the Polish metasediments are derived from the same source region as the ‘classical’ Svecofennian, and were deposited in the same basin or basin system. The ages of the detrital zircons do not match the ages of any known igneous rocks currently exposed in the Baltic Shield. However, they do match the ages of rocks within the Osnitsk–Mikashevichi Igneous Belt on the western margin of Sarmatia, plus Archean rocks of the adjacent Ukrainian Shield. It is likely that detritus eroded from this region at ca. 1.93–1.86 Ga, during a period of rapid uplift, was deposited in a continental marginal basin system adjacent to the Archean craton. The Polish metasediments were metamorphosed up to lower amphibolite facies at ca. 1.83 Ga.