Mineral composition of serpentinites of the tectonic melange of the Dakhovsky crystalline protrusion (Greater Caucasus)

Abstract

In the serpentinites of the tectonic melange of the northern flank of the Dakhovsky ledge, mineral associations of the lysardite-chrysotile and antigorite stages of metasomatic serpentinization are distinguished. Lizarditis is identified by synchronous thermal analysis and electron probe microanalysis. Its wide development is indicated by endothermic effects in the temperature range from ~ 640 up to 660 °С and a characteristic micro looped texture with a sectoral structure of cells. Earlier generations of serpentines represent a lyzardite replacing olivine; in this case, the a ^ p-lysardite transition gives the sectors a “cloud” structure with the central part enriched with iron and nickel and the development of fine crystalline secondary magnetite at the cell periphery. Lysarite-chrysotile varieties (which are the most prevailing diversities among serpentinites of melange)are composed of lysardite cells (with substitution a ^ p lysartite + magenetite) and suture cords and streak of chrysotile. Antigorite differences are confined to areas of intense deployment; antigorite is involved in the formation of a large-looped structure, and also forms aggregates with lysarditis and chrysotile. In antigorite serpentines development areas along the serpentines’ plate boundaries the chlorite-tremolite rocks development is observed. Chlorites (clinochlor), chrome spinels, serpentinized pyroxenes, amphiboles (tremolite), phlogopite, apatite, Y-xenotime, Nd-La-Ce monazite, pentlandite, pyrrhotite are associated with serpentines; in intergrowths with secondary magnetite, millerite and zygenite are noted. The absence of brucite indicates metasomatic serpentinization under conditions of high activity of silica and the removal of a significant part of magnesium. Primary chromic spinels (chrompicotites) bear pronounced signs of secondary changes (formation of zoning with a gradual change in composition from a chromium-enriched core to an iron and aluminum-rich rim, the development of streaks and rims of secondary chrome magnetite), indicating recrystallization under relatively low temperature conditions and exposure to hydrothermal saturated silica solutions. Serpentinite melange began to form at the Late Hercynian stage of the territory’s development after the granodiorite phase in crystalline rocks of the Macklin complex was established in synchronously with the introduction of small intrusions of Malkin granites in the zones of developing brittle deformations (subsequent deformations led to their inclusion in the serpentinite mass in the form of tectonic blocks). Serpentinization occurred under the influence of an alkaline low-temperature, enriched silica and aluminum, fluid-hydrothermal system, coupled with the final stage of development of the focus of Late Paleozoic collisional granitoid magmatism.