Zircon textural and elemental evidence for the thermal state of the Carboniferous Oytag pluton in the West Kunlun Orogenic Belt, northwest China

Abstract

This study presents new cathodoluminescence images and trace element data for zircon from the Carboniferous Oytag pluton in the West Kunlun Orogenic Belt, northwest China. We identify oscillatory and sector-zoned zircon grains that reflect equilibrium and disequilibrium crystal growth within the same trondhjemite pluton. The oscillatory zircon grains display negative correlations between Hf (10,638–13,648 ppm) and Ti (1.95–4.18 ppm) contents, highlighting the dominating role of fractionation in evolving magmas. In contrast, the sector-zoned zircon grains exhibit highly variable Ti contents (0.72–6.07 ppm) uncorrelated with Hf (8951–10,085 ppm), likely reflecting rapid cooling. At a fast growth rate with high undercooling, compatible elements (e.g., Hf) are not efficiently incorporated into the crystal lattice, while incompatible elements (e.g., Nb) are not promptly removed from the crystallizing front. Hf/Nb ratios in the light prismatic sectors are higher than those in the dark pyramidal sectors but overlap with those in the equilibrium oscillatory zircon. It is suggested that the light prismatic sectors are closer to the equilibrium with melts than dark pyramidal sectors. We attribute the formation of sector-zoned and oscillatory zircon to fast and slow cooling rates at the margin and interior of the Oytag pluton, respectively. We further infer that early, small magmatic systems may solidify at faster cooling rates than their postdating counterparts during the development of the Oytag back-arc basin.