Detritic tourmalines with complex zonation in the Cretaceous exotic flysches of the Western Carpathians: Where did they come from?

Abstract

The period of the Cretaceous marked a time of convergence in the Tethyan realm, characterized by both collisions and local ophiolitic obduction. Sedimentary records from this period are full of detritic materials of unknown provenance, so-called exotics. Among these exotic rocks and minerals was tourmaline with complex zonation which began to appear in considerable amounts. Research conducted in Albian to Senonian exotics of the Western Carpathians has shown that there are two kinds of complex-zoned grains: 1) grains with a fabric consisting of several tourmaline phases, which are chaotically arranged as laths and 2) grains displaying a fine intergrowth of tourmaline and quartz. The first type of tourmaline shows a continuous trend of Al and its substitution by Fe3+ at the Y and Z sites, thus shifting from a schorl-dravitic composition to a bosiite one. The second type consists of a dominant presence of schorl-dravitic tourmaline, however, with a scarce occurrence of Al-enriched oxy-dravitic, foititic and magnesio-foititic compositions.Tourmaline with complex zonation occurs in metamorphics, which include HP and UHP facies, evaporitic complexes, and hydrothermal deposits. They also appear locally in granites and pegmatites. In most of the rocks, saline and boron-rich fluids played a substantial role (in the high-pressure metamorphics there was an influx of fluids during their exhumation). Because the examined exotics are rich in ophiolitic material, including HP to UHP rock pebbles it is substantiated to assume that the also represented the main source of the complex-zoned tourmaline. They may have also combined with Permian-Lower Triassic evaporites where the dismembered ophiolites are often embedded. Hydrothermal veins and granitoids did not represent a regionally significant and widespread source at that time.