Biotite from Contact-metamorphosed Rocks of the Dizi Series (the Greater Caucasus)

Author: Irakli Javakhishvili
Keywords: Dizi series, biotite, contact metamorphism
Annotation:

The Caucasus represents a component of the Alpine-Himalayan collision belt. The Dizi series is situated within the one of the five terranes – Greater Caucasian terrane and crops out in the core of the Svaneti anticlinorium in the Inguri and Tskhenistskali canions. It was formed in the conditions of the continental slope and its foot on the southern passive margin of the small ocean basin. The length of the Dizi series is about 70 km, with maximum width of 8 km, and the thickness – 2000-2200 m. The rocks of the Dizi series were faunally dated from the Devonian to the Triassic inclusive. It is composed of terrigenous phyllitic schists, sandstones, quartzite aleurolites and lenses and interlayers of marbleized limestones. Rocks of the Dizi series during the early Cimmerian orogeny underwent regional metamorphism of the chlorite-sericite subfacies of the greenschist facies. In metapelites following typical mineral associations are established: chlorite, phengite, augite, quartz, tourmaline, and in basic rocks - actinolite, fibrolite, prehnite, calcite and chlorite. Into the Dizi series polyphase intrusions of gabbro, diorite, quartz diorite, syenite-diorite, syenite and granitoid compositions are intruded. The K-Ar age of the intrusive rocks varies between 176-165 Ma that points their formation during the Bathonian orogeny. Despite the well-studied stratigraphy and tectonics of the Dizi series, the issues of the very complicated processes of its regional and contact metamorphisms are insufficiently studied. The aim of the authors was a detailed study of the Dizi series rocks contact metamorphism. The appropriate methodology was applied: preparation of key sections, collection of material and its microscopic study, X-ray analyses of elements, microprobe and structural analysis of minerals. The Dizi series rocks formed under the influence of the Bathonian magmatites on metapelites and carbonate-enriched rocks of the series. They are represented by hornfels of different mineral content, marbles, skarns. The contact metamorphism aureole reaches 350 meters. Biotite, which is developed only in contact-metamorphosed rocks and is a rather informative index mineral, has aroused particular interest. In metapelites, biotite is formed as a result of the reaction between phengite, chlorite and leucoxene, but in basite rocks it replaces actinolite or actinolitic hornblende. To study the compositional regularities of biotites, biotites from both - metapelites and metabasites were studied. In total, biotite from the basic rocks is characterized by an increased amount of titanium in contrast to biotite developed in metapelites. In their turn, biotites from metapelites are distinguished by an increased amount of aluminum. In biotites, an increased amount of titanium and aluminum is observed as they approximate the contact, while their magnesia content decreases. Metapelite biotites are characterized by an increased amount of alumina in aluminum octahedrals, in contrast to biotite of the basic rocks. In biotites of metapelites, the amount of tetrahedric aluminum is 28–34%, octahedral - 15–26%, and in basic rocks tetrahedral aluminum is 28–33%, and octahedral 7–21%. As a result of the study of minerals, including biotite, from the contact-metamorphosed rocks of the Dizi series, a number of conclusions were drawn. In particular, three exocontact zones with corresponding mineral assemblages were identified. It was established that contact metamorphism in the aureole of the Dizi series intrusions is going on at a significantly higher temperature and lower pressure than the regional metamorphism preceding the contact metamorphism.