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Russian Geology and Geophysics

2006 year, number 1

SOME ASPECTS OF METAMORPHIC EVOLUTION OF ULTRAHIGH-PRESSURE CALC-SILICATE ROCKS OF THE KOKCHETAV MASSIF

V.S. Shatsky, A.L. Ragozin, and N.V. Sobolev
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: UHP metamorphism, diamond, calc-silicate rocks, K-pyroxene, northern Kazakhstan
Pages: 105-119

Abstract

Banded samples of calc-silicate rocks with contrasting diamond contents from the Kumdy-Kol' metamorphic diamond deposit, North Kazakhstan, were examined. The specimen Kar-4 abounds in diamonds. At the same time, the specimen Kar-98-6 contains few diamond crystals, and no diamonds have been found in Kar-200. The data obtained show that the composition of minerals and zoning character may considerably vary even within one sample. In particular, in the specimen Kar-200 the content of grossular component in garnets varies from 81 to 57%. Pyroxenes from different layers are distinguished by contents of potassium impurity. For example, the pyroxene from layer A contains up to 0.6% K2O and phengite lamellae, whereas the pyroxenes from other layers contains no more than 0.2% potassium. Study of calc-silicate rocks under the scanning electron microscope has shown that dolomite is substituted by an aggregate of tremolite, talc, and calcite. Garnets from the sample Kar-4 contain rounded inclusions of dolomite, with globules in their central part composed of magnesite, amorphous hydrous silica, dolomite, and calcite. These globules are interpreted as a carbonatite melt. As a result of our studies, the following stages were distinguished in the metamorphic evolution of calc-silicate rocks. The protoliths of calc-silicate rocks are clayey calcareous dolomites. At the prograde stage of metamorphism, these rocks underwent partial melting initiated by a hydrous fluid that appeared on zoisite decomposition. At higher pressures and temperatures, the decomposition of phengite led to the formation of a high-K fluid. Diamonds are crystallized just at this stage. At the retrograde stage, in the stability field of zoisite, garnet was substituted by pyroxene-zoisite-calcite symplectite. At the final stage of exhumation, the carbonate rocks interacted with a Si-enriched hydrous fluid.