Zoned and atoll garnet from the Slavonian Mountains (Croatia) and their significance for the evolution of a complex polymetamorphic terrane

The area of the Slavonian Mountains in Croatia is considered as a natural laboratory for the study of metamorphic processes on polymetamorphic rocks formed during the pre-Variscan, Variscan and Alpine orogenies. Since most lithologies contain garnet, understanding of its growth is an important to unravel the polymetamorphic evolution. Garnet up to 200 µm in size with atoll texture were found in quartz- and mica-rich rocks that have experienced peak metamorphism at intermediate temperatures and pressures in the amphibolite facies. These atoll garnet-bearing rocks occur in the oldest metamorphic complex of the Slavonian Mountains at several localities and are restricted to highly foliated mica-schists and/or paragneisses composed of quartz (~20 vol. %), plagioclase (~40-50 vol. %), biotite (20-30 vol. %), garnet (2-3 vol. %) and opaque minerals.

The observed atoll structures are practically restricted to larger garnet comprising a core, an intermediate cloudy zone composed of garnet and the minerals of the rock matrix (quartz, biotite, plagioclase) and a nearly inclusion-free rim. Smaller garnet with uniform texture and composition, which corresponds to that of the rim of larger garnet, also occurs. The core of the atoll garnet shows higher Ca contents than the rim (12-16 vs. 4-5 mol.% of grossular component), lower Fe contents (68-69 vs. 75-76 mol.% of almandine component) whereas Mg and Mn contents are similar. Furthermore, the three domains of the atoll garnet show almost regular outlines or crystallographic forms. Pseudosection modelling was used to reconstruct a preliminary and simplified P-T path, which is clockwise with maximum pressure conditions at ~1.0-1.2 GPa and temperatures of ~650 °C. However, it is likely that these conditions were followed by a significant pressure decrease accompanied by resorbtion of garnet. Shape of garnet core and cloudy zone, but also mineral phase relations, suggest a second stage of garnet growth, forming the rim, during another pre-Variscan stage of amphibolite-facies metamorphism at P-T conditions of 0.5 GPa and 530 °C. This interpretation resulted from in-situ monazite dating, yielding two mean ages at 522±6 and 473±11 Ma (2σ).

Atoll-shaped garnets are uncommon and have been recognized in contrasting metamorphic environments worldwide. Several models have been proposed to explain their formation, including preferential dissolution of garnet cores by fluid infiltration, polymetamorphism, coalescence of subgrains, and kinetic control associated with rapid growth. Here, the formation of the atoll garnet is interpreted by the following processes: external fluid infiltration into the fine-grained rocks and element exchange between the core of primary small garnet grains and matrix, dissolution of the garnet core and later replacement by a new garnet during a subsequent metamorphic event.