First results of systematic provenance analysis of the heavy mineral assemblages from the Albian to Cenomanian exotic flysch deposits of the Klape Unit, Tatricum, Fatricum and some adjacent units

摘要

Albian was a time of large paleogeographic and tectonic changes all over the Tethys ocean, involving spreading of new branches of Central Atlantic – Ligurian – Piemont – Penninic – Vahic ocean on one side and closure of older, mainly Triassic branches, collisions and accretion of crustal blocks on the other. Closure of Triassic branches, which culminated in the Late Jurassic continued by accretion and nappe stacking in Cretaceous. In early stages, the accretion was accompanied by large-scale obduction of an oceanic crust, which left traces in form of massive input of exotic, mostly ophiolitic detritic material to the adjacent basins. In the Western Carpathians, the first massive input of exotics occurred in Albian and lasted to Cenomanian and locally even to Turonian. The main units formed by exotic-bearing sediments are Klape Unit of the Pieniny Klippen Belt and Poruba Formation in the Tatric and Fatric units of the Central Western Carpathians. They were previously systematically analyzed for the pebble material, analysis of psammitic fraction was done at selected localities and by limited provenance-discrimination methods. This paper brings results of the first systematic heavy-mineral analysis of the oldest exotics-bearing units in the Western Carpathians. The preliminary results brought new data which are important to estimate composition of the source area and for comparison with other exotics-bearing units in the Alpine-Carpathian-Dinaridic realm. Samples from 28 localities were analyzed for heavy minerals: 10 from the Klape Unit and 16 from the Poruba Formation (12 of the Tatric units, 4 of the Fatric units). Heavy mineral analysis of the samples showed that most units are dominated by chrome-spinels, zircon, tourmaline, apatite and rutile in various ratios. Garnet appears in small amounts but there are considerable excursions in a few samples. Titanite, kyanite, monazite and epidote occur only rarely; sillimanite and staurolite are very rare. There are also local excursions of blue amphiboles, pyroxenes, garnet and kyanite in some samples. The analyzed spinel grains predominantly match harzburgite field, with some overlap to the fields of podiform chromitites and cumulates of in the Mg/(Mg + Fe2+) vs. Cr/(Cr + Al) diagram. The TiO2 vs. Al2O3 diagram indicates the predominant origin of spinels in the supra-subduction zone peridotites for most of the analyses, whereas the other, aluminium-depleted and higher-titanium grains best match the arc volcanic field. The analyzed blue amphiboles from the Predmier locality belong to glaucophane to ferroglaucophane and were most likely derived from HP/UHP metamorphosed basaltic rocks in a subduction zone. Pyroxenes from the Balcová a Vyvrat localities are mostly represented by orthopyroxenes (enstatite) and less by clinopyroxenes (augite, diopside). Their common euhedral shape and fresh appearance indicate that they were probably not derived from the same ophiolitic source as the Cr-spinels and blue amphiboles, but rather from some adjacent and nearly coeval volcanics which might be of calc-alkaline provenance. Detrital tourmaline grains in the analyzed samples were mostly unzoned but some grains were zoned and some even with a complex intergrowing pattern with a mosaic appearance. According to classification diagrams, most of the grains were derived from various sorts of metasediments. Almost all tourmaline grains from Havransky vrch Hill locality were plotted to the field of Li-poor granitoid rocks, which is unusual in the Western Carpathians. The tourmalines with mosaic appearance are also uncommon and have not been described from the Western Carpathians so far. Tourmalines of similar appearance occur in some eclogites and eclogite-related rocks. The heavy mineral spectra point to large input of minerals of dominantly ophiolitic provenance, such as Cr-spinels, blue amphiboles, and eventually mosaic tourmaline. Zircon, rest of the tourmaline and rutile were likely derived from older sediments. Garnet, staurolite, kyanite, sillimanite occurring in relatively small amounts were mostly derived from metamorphic rocks of various degrees of metamorphism. No significant differences between the heavy minerals were observed among the individual units and they most likely shared the same source.