Stratigraphy of the Kuloğlu–yaka Area and Petrographic Properties of Boyabat Metamorphites and Volcanic-Pyroclastic Rocks of Yemişliçay Formation, the Boyabat-Sinop

Abstract

The study area is located between Kuloglu and Yaka Villages in the north of Boyabat District of Sinop Province. Paleozoic-Triassic aged Boyabat metamorphites are observed at the bottom of the study area. These metamorphites are composed of phyllite, sericite-chlorite schist and metasandstones which contain intense greenschist, glaucophanitic greenschist, metagabbro and metadiabase interlevels at the bottom levels. This unit is overlain unconformably by the Lower Jurassic aged Akgöl formation, which consists of alternation of sandstone-milestone-shale. Higher up, Middle-Upper Jurassic aged Bürnük formation which consist of conglomerate is observed unconformably. The Bürnük formation is overlain by the lateral-vertical transition by Middle-Upper Jurassic aged the Akkaya formation which contain limestones. The Lower Cretaceous aged Çağlayan formation, which is composed of marls and shales with sandstone-sandy limestone interlevels, unconformably overlies the Akkaya formation. Higher up, Upper Cretaceous aged Kapanboğazı formation which consist of cherty limestones and micritic limestones is observed unconformably. This formation is conformably overlain by the Upper Cretaceous aged Yemişliçay formation which consists of basaltic-andesitic lavas, tuffs and agglomerates which contain sandstone-marl interlevels. At the top, the Upper Cretaceous aged Cankurtaran formation, which consist of alternation of sandstone-marl-shale-mudstone-limestone and contain tuff-agglomerate interlevels at the bottom levels, is conformably observed. The following mineral assemblages have been observed in metapelitic, metapsammitic and metabasic rocks belonging to Boyabat metamorphites. Phyllites and sericite-chlorite schists contain mineral assemblage which are composed of sericite + chlorite (ripidolite-picnochlorite) + quartz  calcite  plagioclase (albite)  muscovite  epidote  zoisite / clinozoisite  microcline  biotite (green)  sphene. The mineral assemblage which are composed sericite + quartz + muscovite  epidote  plagioclase (albite)  microcline  chlorite  calcite  biotite (green)  apatite are observed in metasandstones. The mineral assemblage observed in greenschists are tremolite + actinolite + chlorite (ripidolite-picnochlorite) + plagioclase (albite) + epidote + zoisite / clinozoisite  quartz  calcite  microcline  relict clinoproxene (diopside / augite) + sphene. Glaucophanitic greenschists observed as interlevels and sometimes blocks in metapelitic schists show mineral assemblage which are composed of chlorite + glaucophane + epidote + plagioclase (albite) + phengite  actinolite + zoisite / clinozoisite  stilpnomelane  calcite  quartz + sphene. Metagabbro and metadiabases consist of mineral assemblages which are composed of actinolite + plagioclase (albite-oligoclase) + chlorite (penninite-clinochlore)  relict clinoproxene (diopside / augite)  zoisite / clinozoisite + quartz  epidote  sphene. Boyabat metamorphites initially underwent to metamorphism in quartz-albite-muscovite-biotite subfacies of greenschist facies at 425-450 oC temperature and 2-3 kbar pressure conditions. Later, these metamorphites underwent the second metamorphism in the glaucophanitic greenschist (epidote-blueschist) facies at 400-500 oC temperatures and 6-12 kbar pressure conditions with increased pressure in the environment. The effects of these metamorphism are observed in the metabasites within the Boyabat metamorphites. Basalts and basaltic andesites belonging to the Yemişliçay formation show aphanitic porphyritic texture. Basalts contain common plagioclase and olivine phenocrysts and plagioclase microliths. The phenocrystalline phase of basaltic andesites consists of plagioclase, clinoproxen (diopside/augite) and very few olivine. Plagioclase microlites and volcanic glass are observed in matrix of these rocks. The tuffs observed in Yemişliçay formation include plagioclase, clinoproxene (diopside augite), amphibole (hornblende), volcanic glass and litic fragments. These rocks are andesitic tuff “according to composition and grain size” and lapilli stone “according to grain size” and lapilli tuff and lapilli stone “according to% distribution of grains” and vitric tuff “according to the volcanic glass-crystal-litic fragments compound.