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NATROLITITE, AN UNUSUAL ROCK – OCCURRENCE AND PETROGRAPHIC AND GEOCHEMICAL CHARACTERISTICS (EASTERN TURKEY)

¹ Department of Geological Engineering, Nigde University, 51245 Nigde, Turkey
² Department of Geology and Geophysics, University of Missouri-Rolla, Rolla, MO 65401, USA
³ Department of Geological Engineering, Karadeniz Technical University, 61200 Trabzon, Turkey
⁴ Department of Physics, Nigde University, 51245 Nigde, Turkey

^* E-mail address of corresponding author: eciftci{at}nigde.edu.tr

Very unusual rocks consisting of natrolite (>95 vol.%) ± pargasite (<5 vol.%) and rare albite (<1 vol. %) have been discovered in the Kop mountain range, eastern Turkey. We propose to call these rocks ‘natrolitite’ and ‘pargasite natrolitite’. They were produced by Na Si metasomatism of dikes and stocks of diorite through replacement of the intermediate primary igneous plagioclase to produce natrolite. The metasomatic alteration produced concentric elliptical zones characterized by distinct mineral assemblages centered on intrusions of diorite. The Central Zone 1 consists of variably albitized diorite with preserved magmatic textures (albite ± andesine ± pargasite ± quartz). Transition Zone 2 comprises natrolite-bearing diorite (natrolite ± albite ± andesine ± pargasite ± calcite ± quartz). Marginal Zone 3 is a rock made up almost entirely of natrolite (natrolite ± pargasite ± albite ± calcite ± chlorite). Outer Zone 4 occurs along the boundary between the natrolitite and the surrounding serpentinite and consists of listvenite, a rock which comprises magnesite, quartz, calcite, mica, talc, and hematite, indicating a role for CO₂ in the metasomatic reactions, consistent with the presence of calcite in the alteration zones. Zone 5 consists essentially of brecciated serpentinite with numerous hydrothermal quartz veins and calcite veins. Whole-rock compositions document an increase in Na₂O, Al₂O₃, and H₂O from the core (central zone) to the margin while CaO, MgO, and SiO₂ decrease. Plagioclase abundance and composition also varies outwards from the central core rocks where it occurs as a primary magmatic phase (~95 vol.% An_41–38) to the alteration zones (<5 vol.% An_32–37) due to partial to complete replacement of plagioclase by natrolite with or without rare albite. The natrolites exhibit little variation in Si/Al ratios, ranging between 1.45 and 1.61, and are similar in composition to those reported in the literature. Accompanying pargasitic amphibole also becomes progressively more sodic in composition from the core rocks to the marginal zone rocks. Our analysis indicates that albitization preceded the formation of natrolite and that the formation of natrolite, instead of other more typical alteration minerals (e.g. analcime and paragonite), reflects Na metasomatism at lower chemical potentials for Al₂O₃ and SiO₂. Potential sources of Na could be hypersaline brines or leaching of country rocks, such as trondhjemites. The fluids were driven in hydrothermal convection cells set up by the intrusion of the diorites.

Key Words: Eastern Turkey • Kop Mountains • Listvenite • Metasomatism • Natrolite • Natrolitite • Pargasite • Zeolites