Rare twinned zircons from the granitoid–gneisses of the Bundelkhand Craton, India: A morphological insight

摘要

Abstract This communication reports the uncommon twinned zircon grains found in the Archean granitoid-gneisses of the Bundelkhand Craton, India. The twinned zircons of the Bundelkhand Craton exhibit quite different morphological characteristics compared to the twinned zircon previously reported worldwide. The newly twinned zircons exhibit the disparity in the internal growth zone pattern, the orientation of the adjacent crystals, symmetry, and also in their mode of occurrences. The Bundelkhand twinned zircons show neither symmetrical growth from the straight composition plane like typical geniculate zircon twin (also known as elbow/contact twin) nor parallel orientation of the two twinned crystals formed during synneusis. The present twinned zircons also have different internal growth zone patterns, composition plane morphology, and mode of occurrence compared to the asymmetric contact twinned zircons reported earlier. The absence of a distinct line of fracture in Bundelkhand twinned zircons imply that these zircons are not necked zircons. Based on the morphology, the twinned zircons of the Bundelkhand Craton can be classified into two types: Type I – the smaller grain of the twinned zircon penetrates (with curved composition plane) the larger grain and shows non-parallel growth. The larger grain of the twinned zircon has discontinuous internal growth zones across the part penetrated by the smaller grain. Type II – the smaller grain of the twinned zircon shows the cormel-like non-parallel growth from the larger grain. The larger member is characterized by continuous internal growth zones while small grain exhibits discontinuous growth zone from the non-penetrative composition plane. The possible modes of occurrence of the twinned zircons are also discussed.Research Highlights Rare twinned zircon grains are reported from the Bundelkhand Craton, India.The twinned zircon grains are morphologically distinct from the twinned zircon reported earlier.The twinned zircons can be classified into two types based on their internal and external morphologies.Possible modes of origin of the twinned zircons are also discussed.