Detrital tourmaline as an indicator of source rock lithology: an example from the Ropianka and Menilite formations (Skole Nappe, Polish Flysch Carpathians)

Dorota Salata

doi:10.7306/gq.1133

Abstract

Tourmaline populations studied from the Campanian–Maastrichtian part of the Ropianka (Upper Cretaceous–Paleocene) and Menilite (Oligocene) formations of the Polish Carpathians, represent a mixture of first-cycle and polycyclic grains. The tourmalines of the deposits studied display very strong resemblance in terms of optical features and chemical composition. They belong mostly to the schorl-dravite series with a minor contribution of tourmalines of foititic or Mg-foititic composition. Euhedral tourmalines originated from metasedimentary rocks, while the rounded grains crystallised in Li-poor granitic rocks or in pegmatites, Al-poor and Al-rich metasedimentary rocks. Most of the tourmalines studied crystallised during a single igneous or metamorphic event. However, tourmalines forming in evolving chemical conditions as well as polymetamorphic grains (having a metamorphic detrital core and metamorphic overgrowths) are also present. The chemical composition of the metamorphic tourmalines studied indicates their formation in medium-grade metamorphic conditions. This is supported by the crystallisation temperature of the garnet-biotite inclusion present in one of rounded metamorphic tourmalines from the Ropianka Formation. The euhedral grains derive from metasediments, directly from a massif located close to the Skole Basin. The scarcity of euhedral grains in the tourmaline populations studied suggests that their source rocks were poor in these minerals. The direct sources of rounded tourmalines (most probably polycyclic grains), may have been Paleozoic and Mesozoic sedimentary rocks of the Skole Basin foreland or crystalline rocks of remote source areas. The initial igneous and metamorphic host rocks of the tourmalines may have been crystalline domains of the Bohemian Massif and/or the crystalline basement of Brunovistulicum.

Highlights

Tourmaline is a mineral often found as a constituent of granites and associated pegmatites and regionally metamorphosed sedimentary rocks
Tourmaline is a valuable mineral in source rock lithology determination, especially for heavy mineral assemblages of ancient or reworked sediments, usually depleted in highly diagnostic minerals, that are unstable during burial diagenesis (e.g., Morton and Hallsworth, 1999, 2007)
The zircon–tourmaline–rutile index value (ZTR; Hubert, 1962) in both formations varies across a broad range comprising 27–95% and 29–55% in the Ropianka and Menilite formations respectively

Summary

Introduction

Tourmaline is a mineral often found as a constituent of granites and associated pegmatites and regionally metamorphosed sedimentary rocks. It may be a product of diagenetic processes (e.g., Henry and Guidotti, 1985; Henry and Dutrow, 1996; Dutrow and Henry, 2011 and references therein; Hinsberg et al, 2011a, b). Tourmaline is a valuable mineral in source rock lithology determination, especially for heavy mineral assemblages of ancient or reworked sediments, usually depleted in highly diagnostic minerals, that are unstable during burial diagenesis (e.g., Morton and Hallsworth, 1999, 2007)

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Discussion

Conclusion