Geochemistry and microtextural characteristics of garnet from the Variscan medium- to high-grade metamorphic complex of Mt. Papuk (Croatia)

The area of Mt. Papuk in Croatia, which is protected as a UNESCO Global Geopark due to its exceptional geological diversity, is known for polycyclic metamorphic rocks. The study of metamorphic and igneous rocks demonstrate the occurrence of pre-Variscan, Variscan and Alpine orogenic events. Within the metamorphic lithologies (but also igneous ones, e.g., S-type granite), garnet is a key mineral to decipher details of the geological evolution even if it represents a rock volume of up to 2-3% only. The Variscan orogeny has been recognized here as the most imprinting one producing garnet-bearing medium- to high-grade rocks accompanied by partial melting. This is the only case in the Mt. Papuk area where garnet in metamorphic complex coexisted with melt, i.e. reached high temperatures.

The rock samples (mica schist, gneiss, migmatite) show a schistose fabric and a well-preserved medium to coarse-grained granolepidoblastic texture. Some of the rocks reached high-grade conditions discernible by partial melting. Schistosity is defined by the preferential orientation of elongated feldspar grains, micaceous (biotite and minor muscovite) domains and quartz bands. Feldspar (~40-50 vol%; oligoclase up to 28 mol% An) is the predominant phase, followed by biotite (20-30 vol%), quartz (20 vol%), white mica, reddish garnet and sillimanite (fibrolite). Zircon, apatite, monazite, ilmenite, rutile and titanite are accessory minerals. Monazite grains with high Ce₂O₃ content (approx. 28 wt%) were dated with the electron microprobe and yielded an age peak at 384.5±9.0 Ma (1σ).

Garnet occurs in almost all quartz- and mica-rich lithologies of the upper amphibolite facies and exhibits two size populations: 1) small, up to 100 μm large and chemically homogeneous (Alm=64, Prp=9, Grs=2 and Sps=25 mol%) garnets that are generally devoid of inclusions except some containing quartz, and 2) 1-2 mm large garnets with a slightly different chemical composition (Alm=66, Prp=12, Grs=2 and Sps=20 mol%) in inner zones containing numerous apatite, rutile, biotite, plagioclase and quartz inclusions. The thin rim of the large garnets corresponds chemically to the composition of the small garnet population.

Using Ti-in-biotite (average 645°C) and garnet-biotite thermometry (590-650°C at 500 MPa) and pseudosection modelling, a clockwise P-T path was reconstructed with maximum pressure conditions within the rutile P-T stability field where the garnet core grew at 800-900 MPa and a temperature of 590-600°C. These conditions were followed by a significant pressure drop, accompanied by melting, partial resorption of garnet and the formation of a thin garnet rim, to 450 MPa (~15 km depth) at temperatures of 680°C.

The geothermobarometric data, additionally constrained by microtextural evidences and mineral stability fields, suggest a convergence-related model reflecting burial, heating and finally rapid (tectonic?) exhumation of the moderately thickened crust. This model suggests a complex tectonometamorphic evolution of the crystalline terrain filling a gap in the Variscan belt between Central and Eastern Europe.