Nature and provenance of the Bucegi Conglomerate pebbles. A petrographic approach

Abstract

This study deals with the provenance and identification of the pebbles belonging to the Bucegi Conglomerates (Albian, Carpathian Bend area). More than 150 thin sections were examined, the samples including magmatic, metamorphic and sedimentary pebbles sampled from 12 occurrences located in the Bucegi Mountains. The classification schemes recommended by IUGS were used for the petrographic identification of the magmatic, metamorphic, and sedimentary clasts. The frequency of the major petrotypes is the following: 70% sedimentary lithoclasts; 10% plutonic lithoclasts; 2% volcanic lithoclasts and 18% metamorphic lithoclasts. The study defines the petrogenetic, mineralogical, structural and textural markers, which represent the basis for areal correlation and comparative analysis with the rocks of the presumed source-areas. The identified petrographic spectrum is diverse and includes
volcanic rocks, such as trachytes and rhyolites, basalts, eclogites and serpentinites as exotic elements. The petrographic markers used for the comparative analysis are: feldspar symplectites, biotite chloritization and the methamictic structures generated by the radioactivity of the zircon inclusions, the helicitic structures from albit and garnets, and the quartz and mica mechanical istorsions. The conclusions point to the existence of source-areas showing the petrographic spectrum of the present-day Leaota and Perşani Mountains. Eroded or subsided zones made of metamorphites, granitoids with secondary albite on potasium feldspar, red and metamictic biotite (MG1, MG2), calc-alkaline volcanic rocks and ultrabasites also acted as provenance areas for the investigated Bucegi Conglomerates.