Two Trends of Composition Variation of Zircons and Their Significance in Origin Discrimination

Abstract

Zircons can crystallize in a wide range of physical and chemical conditions. At the same time, they have high stability and durability. Therefore zircons can grow and survive in a variety of geological processes. In addition, the diffusivity of chemical compositions in their crystals is very low. Consequently,we can trace back the evolution history of the planetary materials containing zircon by zircon U-Th-Pb geochronology and geochemistry studies. However, this depends on our ability to decipher its genesis,namely magmatic or metamorphic origins. In this paper, we have found that there are obvious differences between magmatic and metamorphic zircons in their chemical composition zonations. The magmatic zircons exhibit composition zonation of increasing HfO2, and (UO2 + ThO2) content and decreasing ZrO2/HfO2 ratio and ZrO2 content from inner to outer parts within each growth zone or from core to rim of a crysta1. The metamorphic zircons exhibit compositional variation trend opposite to that of magmatic (igneous) zircons,tending to decrease in HfO2, (UO2+ ThO2)and increase in ZrO2/HfO2 ratio and ZrO2 from core to rim of a crystal. These chemical composition variation trends are thought to be controlled by the crystal chemical features of ions themselves and the evolution trends of magmatism and metamorphism respectively, and can be used to identify the genesis of zircons. Their morphological features are also discussed.