Contrasting behaviour of accessory phases during partial melting of orthogneiss – a case study from Mt. Papuk, Croatia

Partial melting during the tectono-metamorphic evolution of collisional orogens results in the formation of migmatites or granitoids and partial to full resorption of the precursor’s accessory phases, such as zircon and monazite. While in many cases both minerals newly crystallize from partial melt, they can also show contrasting degrees of preservation/dissolution. This study presents an example of nearly full zircon preservation but completely new monazite growth during partial melting of a mildly peraluminous orthogneiss from Mt. Papuk, Croatia. We combine U-Pb zircon and Th-U-total Pb monazite dating, zircon trace element and whole rock geochemical data with phase equilibria modelling and use solubility models of accessory phases to constrain the P-T conditions of partial melting. Zircon textures support igneous crystallization while trace elements are characteristic for an S-type granite composition. Zircon yields a Cambrian crystallization age of the protolith at 498.9 ± 6.9 Ma. Conversely, monazite records only Carboniferous anatexis of the orthogneiss at ca. 350 Ma, while zircon mostly remains unaffected. The results of phase equilibria modelling that account for nearly full zircon preservation (< 10% dissolution) and complete monazite dissolution and regrowth indicate P-T conditions of 5-8.5 kbar and 760–800 °C. Our results suggest that a low degree of partial melting (< 10 vol%) in an orthogneiss with an S-type granite signature may occur without water influx at < 800 °C. This will preserve protolith ages in zircon but erase them in original monazite. The contrasting behaviour of these accessory chronometers can be used as a tool to bracket the temperature conditions of anatexis.