Temporal evolution, petrography and composition of dolostones in the Upper Devonian Plavinas Regional Stage, southern Estonia and northern Latvia/Ulem-Devoni Plavinase lademe dolokivide petrograafia ja koosseisu evolutsioon Eesti louna- ning Lati pohjaosas

The Upper Devonian Plavinas Regional Stage in southern Estonia and northern Latvia is represented by dolostones containing interlayers of dolomitic marlstones and limestones. Petrographic, cathodoluminescence, electron microprobe and isotope techniques were used to investigate diagenetic evolution of dolostones. The rock succession has been affected by multiple diagenetic events. Based on petrographic and geochemical data, six dolomite textures were identified. The crystal size (5-1200 µm) and morphology of dolomites are variable. Commonly, dolomites are close to the stoichiometric composition, with low iron and manganese content. Their stable isotope composition (δ[sup.13]C, δ[sup.18]O) differs greatly from that of dolomite precipitated from Frasnian seawater. Dolomitization is more pronounced in the lower part of the studied sequence, in the Snetnaya Gora Formation and Lower Pskov unit where carbonates are completely dolomitized, whereas upwards in the section the dolomite content decreases. Voids and pores of the Lower Pskov unit are mainly open, but partly or completely occluded in the Upper Pskov unit. Void- filling dolomite has slightly and void-filling calcite notably depleted stable isotope signatures. Major dedolomitization and calcite-filling processes took place during the final uplift and emergence of the northern part of the Baltic basin, connected with the migration of karst-related meteoric waters into previously dolomitized horizons within carbonate rocks. Key words: carbonate rocks, diagenesis, dolomite types, stable isotopes, Devonian, Baltic basin. Ulem-Devoni Plavinase lademe labiloige Balti basseini pohjaosas on esindatud domeriidi ja lubjakivi vahekihte sisaldavate dolokividega. Dolokivide uuringud polarisatsiooni-, katoodluminestsentsi- ja eletronmikroskoobis ning kivimite susiniku ja hapniku isotoopide suhted viitavad kivimite keerukale diageneetilisele evolutsioonile. Petrograafiliste ja geokeemiliste andmete alusel eraldati kuus erinevat dolomiidi tuupi. Valdavalt on tegemist stohhiomeetrilise koosseisu ja vaikese raua- ning mangaanisisaldusega dolomiitidega. Isotoopkoostise alusel erinevad dolomiidid selgelt Frasne mereveest settinud dolomiitidest. Dolomitisatsioon on tugevam uuritava labiloike alumises osas. Snetnaja Gora ja Pskovi kihistu karbonaadid on taielikult dolomitiseerunud, labiloike ulemises osas on see protsess olnud mittetaielik. Pskovi kihistu alumises osas on dolokivi poorid valdavalt avatud, labiloike ulemises osa