RARE-EARTH ELEMENTS AS AN INDICATOR OF THE ORIGIN OF APATITE IN PRECAMBRIAN CONGLOMERATES OF THE ALDAN SHIELD

Abstract

Apatite in apatite-bearing conglomerates previously described from Precambrian metamorphic complexes of the Aldan shield occurs as an accessory mineral in the cement and pebbles. In this article, we have estimated the role of possible sources of clastic apatite from its external mineralogic characteristics and from the distribution in it of rare-earth elements (REE), as determined with the ion microprobe. Having studied the external characteristics of apatite, we concluded that most of its fragments in conglomerate (approximately 80 percent) consist of the variety from apatite-carbonate rock. We determined the REE content of grains with the external characteristics of those in all three apatite-bearing rock groups, the potential sources of clastic apatite, as well as from the most representative apatite samples of each of these groups. Thus, we may state that apatite in the conglomerates studied was derived from apatite-carbonate rocks of the region. It entered them as a result of intraformational erosion and limited transportation. The accumulation of apatite-carbonate rocks and conglomerates was preceded by the breakdown of all apatite-bearing rocks at a considerable distance from them. While being transformed, apatite lost the original characteristics acquired upon its crystallization.