Geochemistry of Zircon from Pegmatite-bearing Leucogranites of the Laghman Complex, Nuristan Province, Afghanistan

Abstract

For the first time, a study was carried out on the geochemistry of zircon (SIMS method, 2 samples, 20 points) from leucogranites of the Laghman complex, located in the pegmatite-bearing (LCT-type) province of Nuristan (Afghanistan). Zircon is characterized by a high Hf content (up to 34 500 ppm), which tends to increase towards the edges of the grains. An increase in the content of Li, Ca, and U is correlated with an increase in Hf. The content of Ti varies slightly and corresponds to the crystallization temperature of zircon about 700 ∘C. The pattern of REE distribution is unusual. In most grains there is no positive Ce-anomaly, which is not typical for igneous zircon. For a number of grains, the marginal zones are distinguished by a reduced content of REE and a flat distribution pattern of light REE. The reason for this distribution of REE and the absence of Ce-anomaly is probably the joint crystallization of zircon with titanite, which concentrates REE and affects the fractionation of light REE. A comparison of the studied zircon with zircon from other leucogranites, including those considered as parental for LCT pegmatites, showed a high degree of fractionation of the melt of leucogranites of the Laghman complex, manifested in a high content of a number of trace elements (Hf, U and others). Therefore, the previously stated assumption about the genetic connection of leucogranites of the Laghman complex and large pegmatite lithium deposits, widely represented in the Nuristan province, in northeastern Afghanistan, is confirmed by new data on zircon geochemistry.