Geochemical, isotopic and geochronological characterization of listvenite from the Upper Unit on Tinos, Cyclades, Greece

Abstract

We describe a largely unknown listvenite deposit from Tinos, Cyclades, Greece and combine field observations with petrographic, bulk-rock geochemical, isotope (Sr, O, C), and Rb–Sr geochronological data. The volumetrically small listvenite occurrences are associated with metabasic phyllites, talc schists, meta-gabbros, ophicalcites and serpentinites of the Upper Unit. Geochemical characteristics (high Mg#, Cr, Ni), as well as preserved relic Cr-spinel and the typical mesh-texture of serpentinized Mg-silicates, document derivation from ultramafic precursors. Judging from field and textural observations it is very likely that carbonation affected serpentinite and not unaltered meta-peridotite. The direct contact or transition zones to ultramafic rocks are not preserved, but serpentinites that escaped carbonation are closely associated. The listvenites occur near a low-angle normal fault that probably focused fluid infiltration and distribution. The carbonation is associated with the influx of CO2-rich, K-bearing fluids that led to the formation of ferroan magnesite, quartz and Cr-bearing white mica (fuchsite), but otherwise the transformation of serpentinized peridotite into listvenite had been a largely isochemical process. The studied rocks do not contain elevated concentrations of precious metals (Au, Pt, Pd). Field relationships suggest that the listvenite-bearing occurrences most likely represent the same tectonostratigraphic level as Upper Unit rocks that had been thermally overprinted in the contact aureole of Miocene granitoids at ca. 15Ma. Accordingly the intrusion depth provides a minimum pressure constraint for the somewhat older carbonation. Pressure estimates for thermally overprinted rocks and the granitoids suggest an intrusion depth of ca. 7–10km that corresponds to a pressure of ca. 2–3kbar. Chlorite thermometry applied to the Tinos listvenites mostly indicates temperatures of ca. 250°C during carbonation. Internal Rb–Sr mineral isochrons (different grain-size fractions of fuchsite and magnesite) yielded apparent ages of ca. 16Ma and ca. 19Ma, respectively, which are interpreted to date carbonation and associated fuchsite formation. The new ages indicate that listvenite formation is considerably younger than the presumed Late Cretaceous or Jurassic protolith age of the ultramafic precursors and also post-dates tectonic juxtaposition of the Upper Unit onto the Lower Unit at ca. 21Ma. Although not the dominant process, a contribution of contact metamorphic decarbonation cannot completely be ruled out. The Sr isotope characteristics of magnesite and whole rocks correspond very well to the seawater curve for the formation age indicated by Rb–Sr dating. Carbonate carbon and oxygen isotopes measured for the listvenites suggest that magnesite formed following the deep circulation of fluids and their interaction with other carbonate rocks (possibly the marble units present on Tinos) including a possible contribution from magmatic CO2. The similarity in δ13C and δ18O between listvenite and some of the ophicalcite occurrences could indicate a common origin from the same circulating fluids, but remains elusive at present. Combined, P–T constraints and Sr isotope data imply infiltration of seawater-dominated fluids to a depth of several kilometers. This conclusion is supported by oxygen and carbon isotope data.