Partial high-pressure aragonitization of micritic limestones in an accretionary complex

ABSTRACT Pelagic micritic limestones within an upper Cretaceous accretionary complex in the Tavşanlı Zone, NW Turkey, preserve textures indicating incomplete prograde transformation of micritic calcite to aragonite, representing the only known example of this type. Aragonitization starts at the central parts of the micritic limestone beds and advances towards the lower and upper parts of the layers at the expense of micrite. Micrite is very fine grained (<0.003 mm) and contains radiolaria, foraminifera and thin shell fragments. Aragonite forms large crystals, up to 3 cm across, with straight grain boundaries and c-axis mostly subparallel to the carbonate beds. Relict micritic portions are devoid of any aragonite grain. Stylolites characterized by the accumulation of clay minerals, Fe-Mn-hydroxides and quartz are concentrated in the upper and lower parts of the beds. Stylolite formation precedes aragonitization. Conditions of aragonitization are estimated as 200 ± 50 C and 0.45–0.65 GPa, based on metamorphic mineral assemblages observed in associated basalts. Several features such as (i) constant composition of micritic calcite (98–99 mol. % CaCO3) throughout individual beds, (ii) enormous grain size difference between micritic calcite and aragonite (up to 1200 times), and (iii) absence of any aragonite grains within the relict micritic portions suggest that kinetic rather than thermodynamic factors controlled selective aragonite formation in the central portions of carbonate layers. Key words: aragonite; calcite-aragonite transition; high-pressure metamorphism; incipient blueschist-facies; Tavşanlı Zone; Turkey