Cambrian Granitoids and Their Response to Multiple Metamorphism, Northern Veporic Unit, Western Carpathians

Abstract

On example of metagranites in the northern Veporic unit of the Western Carpathians, we show how the preserved magmatic assemblage reacted to multiple metamorphic events (pre-Variscan, Variscan, Alpine), by reactions among rock-forming minerals, changes in composition and chemical age of monazite. The preserved magmatic minerals (plagioclase, K-feldspar, high-Ti biotite, garnet I) are used to estimate solidification pressure at ca. 9 kbar. Low Th, U monazite I cores preserved within dominant monazite II yielded Cambrian 525 ± 25 Ma age. The first metamorphic event of Ordovician age (470 ± 3 Ma) is identified by abundant monazite II commonly associated with ilmenite, both enclosed in large garnet II. The YAG thermometry (Grt II/Mnz II) gives 560 °C at ca. 6 kbar. Profound negative Eu anomalies of Mnz II are interpreted due to reducing conditions evidenced by the presence of graphite. The dominant metamorphic assemblage (Grt III, kyanite, rutile, low-Ti biotite, phengite) resulted from the second metamorphism, at P-T conditions of 600-650 °C and 8-9 kbar recorded by composition of garnet. Monazite II only rarely reacted to this metamorphic event by resetting its rim (Mnz III) thus recording the Carboniferous (368 ± 13 Ma) age. The resetting of Mnz III, which occurs only along the rims, indicates the lack of external hydration. Alpine monazite (Mnz IV) is only found in a sheared orthogneiss recording a low-grade metamorphic event of Cretaceous age (96 ± 23 Ma), due to extremely low-Ca composition of this rock, which shifted monazite stability to lower temperature.

Keywords: metagranite, multiple metamorphism, monazite dating, fluid-deficient conditions, Western Carpathians