Late Paleoproterozoic deposition and Mesoproterozoic metamorphism of detrital material in the southernmost Baltic Sea region (Gdańsk IG1 borehole): monazite versus zircon and chemical versus isotopic age record

ABSTRACT

The SHRIMP ion microprobe zircon and monazite U–Th–Pb isotope geochronology of a biotite-sillimanite paragneiss derived from the Gdańsk IG1 borehole that penetrated the Precambrian basement in the southernmost Baltic Sea area, northern Poland, has constrained the ages of detrital zircon cores and metamorphic overgrowths. In addition, monazites have been dated by electron beam chemical technique either as a grain separate or in thin sections. The metasediments show a specific provenance pattern with a dominant input from the late Paleoproterozoic sources. The ages range from 2749 to 1643 Ma with peaks at 1779, 1815, 1840 and 1870 Ma. The maximum depositional age, defined by the youngest detrital zircons, was determined at 1643 Ma. Zircon rims and monazite isotope analyses record a metamorphic event at 1485, and 1498 Ma, respectively, whereas Th–U–Pb chemical dating on monazite revealed an age of 1494 Ma. These ages correspond with the time of the AMCG magmatism in the basement of northern Poland. The metamorphic monazite predates the age of the zircon rims by over 10 m.y. This discrepancy is likely caused by the different behavior of the two accessories since the zircon metamorphic crystallization requires fluid/melt-related support. Thus, zircon is not expected to record the time of peak metamorphism. In consequence, monazite tends to be a more sensitive metamorphic geochronometer. Moreover, the EPMA method provides greater spatial resolution. Additional older fragments, with ages of 1539, 1596, 1600 and 1620 Ma, were identified within monazites reflecting earlier stages of metamorphic evolution.

KEYWORDS:

• Metasediments
• zircon
• monazite
• SHRIMP
• EPMA
• Paleoproterozoic deposition
• Mesoproterozoic metamorphism

Acknowledgements

Thanks are addressed to Grzegorz Zieliński, Paweł Derkowski and Leszek Giro for monazite age analyses and BSE imaging on the CAMECA SX-100 EPMA and to Zbigniew J. Czupyt for the SHRIMP IIe/MC calibration and support. We wish to thank Evgenia Salin and the anonymous reviewer for their valuable comments on the manuscript. The authors report there are no competing interests to declare.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Polish Geological Institute-National Research Institute (Panstwowy Instytut Geologiczny-Panstwowy Instytut Badawczy) [grant number 65.2916.2020.00.0 to D.G.].