Article Rare earth element geochemistry of altered pyroclastic rocks in the Hashtjin area of north-west Iran Tohid Nouri 1 , Parvin Najafzadeh Tehrani 2 *, Rahim Masoumi 2 and George E. Christidis 3 1 Department of Civil Engineering, University of Mohaghegh Ardabili, Ardabil, Iran, 2 Department of Earth Sciences, Faculty of Natural Sciences, University of Tabriz, Iran and 3 Technical University of Crete, School of Mineral Resources Engineering, 73100 Chania, Greece Abstract This study evaluates the rare earth element (REE) geochemistry in altered trachyandesitic ignimbrites, tuff and lava flows in the Hashtjin area by assessing chondrite-normalized REE patterns and Y/Ho geochemical ratios. Modifications in the REE patterns took place along altered fault zones that were affected by hypogene and supergene alterations. The precursor volcanic and pyroclastic rocks contain phenocrysts of plagioclase accompanied by augite, zircon, apatite and pyrite. Based on X-ray diffraction analysis, the main mineral assemblages of the altered units consist of kaolinite as the main clay mineral, SiO 2 polymorphs (quartz and cristobalite) and anatase as a minor constituent. The chondrite-normalized REE patterns of argillic samples reveal fractionation of light REEs(LREEs) compared to heavy REEs(HREEs), together with a marked Eu anomaly and a weak W-type tetrad effect related to the weak non-charge radius control (CHARAC) behaviour of REEs and slightly higher Y/Ho and Zr/Hf ratios. The relationship between the Y/Ho and Zr/Hf ratios and recognizable T 3 and T 4 effects (tetrad effect) suggests that an increasing degree of water–rock interaction occurred during hypogene alteration processes by acidic hydrothermal fluids that were overprinted by supergene alteration. Water–rock interaction and adsorption by Mn-oxides and clay minerals are considered to have played important roles in determining the close to non-CHARAC behaviourof REEs during the argillic alteration of the pyroclastic rocks in the Hashtjin area. Keywords: argillic alteration, Hashtjin, ignimbrite, kaolinization, lanthanide tetrad effect, pyroclastic rocks, REEs (Received 11 August 2019; revised 29 June 2020; Accepted Manuscript online: 15 July 2020; Associate Editor: Lawrence Warr) Rare earth elements (REEs) provide useful petrological information for determining the nature of geological events such as the origin of rocks and their contamination–differentiation processes (Hanson, 1989; Bau & Knittel, 1993). A large amount of REE data has been obtained, representing the differentiation between various minerals and melts (Rollinson, 1993), as well as changes occurring in hydrothermal systems at temperatures <350°C (Hermann et al., 1974; Michard & Albarede, 1986; Wood, 1990; Jochum & Verma, 1996), under metamorphic conditions (Konrad-Schmolke et al., 2011) and during weathering (Pérez-López et al., 2010). REEs are also useful for assessing geochemical processes during the evolution of sediments in various environments (Chen & Zhao, 1997; Censi et al., 2007; Karadağ et al., 2009; Hannigan et al., 2010; Mongelli et al., 2016; Abedini et al., 2018). The distribution and differentiation of REEs have also been well studied in seawaters, marine sediments and weather- ing environments (Abedini & Calagari, 2013; Mongelli et al., 2014; Buccione et al., 2016; Liu et al., 2016; Long et al., 2017; Torr et al., 2017; Chen et al., 2018). In these cases, the distribution patterns of REEs are generally controlled by weath- ering of terrestrial material, hydrothermal activities, scavenging, oxygen fugacity, proximity to source lithologies, deposition due to biogenic conditions, diagenesis (Murphy & Dymond, 1984; Liu et al., 1988; Murray et al., 1991), changes in fluid pH and precipitation reactions due to the presence of organic and inorganic ligands and complexes (Buccione et al., 2016; Liu et al., 2016; Long et al., 2017; Torr et al., 2017; Chen et al., 2018). The main aim of this study was to evaluate the behaviour of REEs (including Y) during water–rock interaction in altered rocks and their volcanic precursors in the Hashtjin area, north- west Iran, and to investigate the origin of tetrad effects and non- chondritic Y/Ho ratios in these samples. The study area includes the western part of the Alborz Tertiary magmatic belt and Pliocene volcanic activity, which was associated with tectonic activity and the formation of numerous faults (Eftekhar Nejad, 1980). The main lithological units of the area consist of volcanic rocks and Quaternary alluvial sediments that have undergone sili- cic and argillic hydrothermal alterations. These alterations are associated with vein, veinlet and disseminated Cu-, Pb-, Zn-sulfide deposits and magnetite, goethite and hematite miner- alizations (Hajalilou, 1999). They include extensive argillic alter- ation and the accumulation of SiO 2 deposits in the form of cryptocrystalline quartz in the Tarom–Hashtjin zone. Previous studies have highlighted the role of both the hydrothermal pro- cesses and supergene alteration formation of these sequences in the circulation of acid sulfate solutions (Masoumi, 2010; Abedini et al., 2011; Nouri & Mohammady Oskouei, 2016; Nouri & Masoumi, 2020). *E-mail: parvin_tehranii@yahoo.com © The Mineralogical Society of Great Britain and Ireland, 2020 Cite this article: Nouri T, Tehrani PN, Masoumi R, Christidis GE (2020). Rare earth element geochemistry of altered pyroclastic rocks in the Hashtjin area of north-west Iran. Clay Minerals 55, 150–165. https://doi.org/10.1180/clm.2020.21 Clay Minerals (2020), 55, 150–165 doi:10.1180/clm.2020.21