COMPOSITION AND PARAGENESIS OF GARNETS FROM ULTRAHIGH-PRESSURE CALC-SILICATE METAMORPHIC ROCKS OF THE KOKCHETAV MASSIF ( northern Kazakhstan )
N.V. Sobolev, H.-P. Schertl*, and R.D. Neuser*
Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Institute of Geology, Mineralogy and Geophysics, Ruhr University Bochum, D-44780 Bochum, Germany
Keywords: Ultrahigh-pressure metamorphism, calc-silicate rocks, diamond, Mg-Ca-garnet
Pages: 519-529
Abstract
Study of a series of samples of homogeneous and inhomogeneous (banded) calc-silicate rocks with contrasting diamond contents from the Kumdy-Kol' deposit of metamorphic diamonds (northern Kazakhstan) has shown an extremely wide diversity of compositions of pyrope-grossular garnets in association with dolomite, Mg-calcite, magnesite, diopside with up to 1.5 wt.% K2O, forsterite, Ti-clinohumite, and phlogopite. Perovskite as an inclusion in forsterite has been discovered in these rocks for the first time. In some samples of complex composition interbeds with and without diamond alternate. Garnet forms a series of compositions with wide variations of Mg# [100 · Mg/(Mg + Fe)] from 70 to 95 and Ca# [100 · Ca/(Ca + Mg + Fe + Mn)] from 42 to 85, which has first been found for these rocks. Wide variations in garnet zoning have been revealed, with a decrease in Ca# towards the rims. Relict Mg-rich garnets which were firstly recognized under natural conditions persistently occur as light irregular spots, with uniquely low amounts of FeO, within 1-2 wt.% (Mg# is 92-95 and Ca# is 60-66). These relics make up 5 to 70 vol.% of the respective grain surfaces in thin section and are revealed only by cathodoluminescence (CL) approximately in only 10-20% of garnet grains for each sample. Thus, these unusual relics are typical of the diverse samples under study, and their composition is relatively homogeneous. The outer zones of most of these garnets completely correspond to typical well-studied compositions with Mg# of 70-80 and Ca# of 40-55. Supposedly, these relics are the earliest garnets formed under the conditions corresponding to the maximum of ultrahigh-pressure metamorphism. These garnets are often surrounded by reactionary rims irregularly developed around the grains made up of clinopyroxene and spinel symplectite. Occasionally, sapphirine and corundum are found in these rims. The symplectite paragenesis was formed as a result of retrograde metamorphism, in the early stages of exhumation, and is direct evidence of the granulite facies conditions, probably, at 1.8 GPa and about 900
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