Rare earth element (REE) partitioning in amphibole-bearing medium grade metamorphic rocks from the Alvand Complex (Sanandaj-Sirjan Zone, NW Iran)

Abstract

In this work we explore the distribution of rare earth elements (REE) in hornblende-bearing metamorphic rocks from the Jurassic Alvand plutonic complex (Sanandaj-Sirjan Zone, NW Iran) focusing on the understanding the effect of rock-forming silicates in controlling compositional variations during metamorphism. The studied rocks contain two distinct amphibole-dominated paragenesis: a first one including hornblende + epidote + plagioclase (“amphibolite”) and a second on made up of hornblende + garnet + epidote + plagioclase (“garnet amphibolite”). The bulk high Al2O3 (average 17.7 wt%), CaO (average 11.9 wt%) and low Fe2O3* (average 5.9 wt%), MgO (average 2.3 wt%), and TiO2 (average 0.8 wt%) contents together with the Zr/Ti (average 280), Na2O/ Al2O3 (average 0.05) and Na2O+K2O (average 1.7 wt%) values indicate these rocks are para-amphibolite formed by metamorphism of a marl (calcareous shale) protolith. With respect to the rock-forming phases, the epidotes have the highest ΣREE contents (ΣREE=86-210 ppm). The garnets (ΣREE=27-87 ppm) and hornblendes (ΣREE=10-22 ppm) have moderate values, whereas the plagioclase shows the lowest REE (2-4.7 ppm) amounts. Inverse and forward modelling thermobarometry was applied to unravel the pressure-temperature history of the studied samples and therefore to reveal the impact of each phase on the REE partitioning during the recorded metamorphic evolution, considering that REE are mostly immobile during crust processes and therefore their bulk budget is that of the protolith.