Geochemistry of hydrothermal illitizations in Eocene Kösedağ magmatic rocks, Zara-Suşehri area, NE Sivas, East-Central Anatolia: Origin and age of alteration

Abstract

The study area located at the periphery of the collision zone between the Eurasian plate (i.e. Pontides) and Tauride-Anatolide platform, NE of Sivas in the east-central Turkey, which is part of the Tethyan Metallogenic Belt. Mixed-layer illite-smectite (I[sbnd]S) and illite minerals are derived within the hydrothermal alteration zones with a few km2 surface areas (up to 30 km2) in Eocene volcanic and plutonic rocks. The representative I[sbnd]S and illite samples taken from plutonic- and volcanic-hosted alteration zones are investigated by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), major and trace elements and O[sbnd]H isotope geochemistry and K[sbnd]Ar dating methods. Different types of hydrothermal alterations, such as propylitic and phyllic alteration in the plutonic rocks and argillic alteration in the volcanic rocks were developed as a result of intrusion of Kösedağ Pluton (syenite) into Karataş Volcanics (basaltic trachy-andesite and trachyte) with relations of hot-hot contact. The main phyllosilicate/clay minerals are characterized by kaolinite and I[sbnd]S in volcanic-hosted argillic alteration zones, whereas I[sbnd]S and illite in plutonic-hosted phyllic zones. The ordering types (Reichweite) of I[sbnd]S and illites are represented by R1 I[sbnd]S (I = 65–80 % in I[sbnd]S) + R3 I[sbnd]S (I = 90 % in I[sbnd]S) in the volcanic-hosted rocks, and R3 I[sbnd]S (I = 90 % in I[sbnd]S) and illite (S = 3–5 %). Dioctahedral (d060 ≤ 1.500 Å) R3 I[sbnd]S and illites have 1 Md + 1 M and 1 Md + 1 M + 2 M1 polytypes, respectively. The major and trace elements such as TiO2, Fe2O3, MgO, Na2O, P2O5, Sc, V, Cu, Ge, Sr, Hf, Zr and Y increase in the volcanic-hosted I[sbnd]S, whereas SiO2, Al2O3, K2O, Pb, W, Mo, As, Sb, Rb and U in the plutonic-hosted I[sbnd]S and illites. The chondrite-normalized distributions of I[sbnd]S and illites present a great similarity to those of host rocks. The chondrite-normalized rare earth element (REE) concentrations are more enriched in the volcanic-hosted I[sbnd]S in comparison with the plutonic-hosted I[sbnd]S and illites having distinctive Eu negative anomaly, which indicate deriving from volcanic matrix and K-feldspar, respectively. Oxygen and hydrogen isotope data of illitic clays indicate that the hydrothermal fluids are originated from magmatic water. According to stable isotopes and fluid inclusion data, I[sbnd]S and illites were formed at the temperature conditions ∼150 °C in volcanic-hosted argillic zone, whereas ∼250 °C in plutonic-hosted phyllic zones. K[sbnd]Ar dating of alunite, I[sbnd]S and illite minerals indicate that the hydrothermal alteration was started at 40.45 ± 1.28 Ma, almost 2 Ma after the Q-syenite intrusion, within the plutonic body as phyllic alteration stage, and continued up to 35.27 ± 2.81 Ma, with a duration of ∼5 Ma, and finalized before the exhumation of the Kösedağ Pluton (28–30 Ma). The geochemical characteristics of I[sbnd]S and illites were controlled by host-rock, condition, origin, and ages of alterations and they can be used as an important tool for magmatic-hydrothermal systems. © 2024 Elsevier GmbH