К вопросу образования двойников ставролита в метаморфических породах (на примере кристаллических сланцев Кейв, Кольский п-ов)

Abstract

Исследован механизм зарождения и формирования двойников ставролита в кристаллических плагиоклазставролитовых сланцах. Показано, что образование двойников в рассматриваемых метаморфических породах происходило в результате механического воздействия на зародыши ставролита растягивающих сил пластического "течения" сланцев в процессе их формирования. Внутреннее строение двойников отражает метаморфизм сланцев и помогает восстановить динамику развития процесса.

Досліджено механізм зародження і формування двійників ставроліту в кристалічних плагіоклаз-ставролітових сланцях. Показано, що утворення двійників у цих метаморфічних породах відбувалося внаслідок механічної дії розтягувальних сил пластичної "течії" сланців на зародки ставроліту в процесі їх формування. Внутрішня будова двійників відображає метаморфізм сланців і допомагає відновити динаміку розвитку процесу.

The genesis of staurolite twins in crystalline plagioclasestaurolite schists of the Keivy schists (the Kola Peninsula) has been studied. It is shown that the twins in the metamorphic rocks under question have formed as a result of staurolite nuclei, being mechanically affected by stretching forces of an elastic "flow" of schists during their formation. The twins structure reflects metamorphism of the schists and helps restore the process dynamics. A peculiar feature of staurolite in the studied rocks is its zoning, remarkably distinct in every crystal. A special paper is dedicated to the study of the zoning. Major elements of zonal crystals are a nucleus and two I-genus zones. A thin II-genus zoning is developed in I-genus zones. The structure of staurolite crystals allows defining three stages of its formation. The first stage is characterized by the formation of nuclei, i. e. the embryos of future staurolite crystals on the initial stage of the crystalline schists formation as a result of metamorphic reaction. The second stage of the staurolite crystallization comprises the nucleus stretching and the second zone forming. The way the crystal grew in this period depended on the crystal space orientation. The motive forces for the crystals growth at the second stage were deformations of the comprising environment given to the "flow" and deformation of the mineral as a "participant perceiving external stresses". It is assumed that the nucleus stretching deformation in the direction of the schists "flow" occurred under the effect of forces of the elastic schist flow. During the stretching deformation the nucleus elongated with no continuity break or divided into two parts, both of these parts being remote from the long crystal axis. In the first case the twinning took place in a number of crystals. It is supposed that the stretching of crystals with no continuity break resulted from the nucleus material deformation according to the cell "migration of dislocations" and translation gliding. The latter is accompanied by healing of defects. The first signs of the twinning are observed at the boundaries of a staurolite crystal nucleus. A wedgelike incision with a twin embryo is observed on the nucleus of twin crystals. The embryo was initially oriented according to either {231} or {031}, which resulted in the formation of an oblique or cruciform twin. The crystal-2 happened to originate not on two sides of the crystal-1, but on one side. The opposite half of the crystal-2 was formed later. However, the crystal preserved its space orientation on a plane, which provided the suggestion that a "through" twinning plane existed in the crystal-1 during the crystal-2 origination. Interestingly, the twinning joint is always close to the heart of the intergrown crystals. Thus, crystals in staurolite twins originated at different times and it is necessary to distinguish crystal-1 and crystal-2. Depending on the orientation according to {231} or {031}, crystal-2 grew and crossed the nucleus of the host crystal obliquely of perpendicularly. The third stage of the staurolite crystallization is characterized by regular tectonic conditions and a growth of crystals in all directions. Once the nucleus stopped stretching and the second zone crystallized, the formation of twins finished. All crystals, including twin ones, got overgrown by planes of the third zone, i. e. obtained the current shape.