Occurrence and origin of the zoned microgranular enclaves (MEs) within the Cretaceous granite in Taejongdae, SE Korea

Abstract

Microgranular Enclave (ME) is a common feature in granitic intrusive rocks. However, the zoned ME that developed in the outcrop of Cretaceous granite in the Taejongdae Geopark, Busan, represents an interesting phenomenon in formation of zoned MEs, providing useful petrogenetic information on the formation of enclaves. In this study, we define the zoned ME as an enclave composed of several contrasting orbicular rock zones. Due to the importance of the enclaves in the evolution of igneous rocks, this study aims to understand the formation mechanism of the zoned MEs, including the reasons for their diversity in the study area. The investigated MEs in the study area, have trachi-andesitic to dioritic composition, mainly composed of amphibole, plagioclase and quartz with minor orthoclase and opaque minerals. To achieve the above objectives, the relationship between the MEs and the host granite and the difference between zones were investigated based on petrological and geochemical studies. The MEs can be classified into two main types: Simple-type, composed of a single rock type, has an angular shape and sharp or sometimes crenulated contact with the host granite, and Zoned-type, composed of several zones, has a circular to elliptical shape and shows two different cooling processes. The mineral composition of the simple MEs shows some similarity with the zones of the zoned MEs. These results imply that the zoned MEs are formed by two different phases of mixing and mingling in two different magma chambers, firstly in a deep dioritic magma chamber (~7.1–7.7 km) and then in a shallower depth granitic magma chamber (~1.7–2.4 km). In the other hand, the simple MEs mainly represent xenoliths, but also possibly resulted from disruption of early formed zoned MEs before they solidified. This study is, therefore, could be useful in the understanding of the possible mechanisms for the formation of MEs and the magmatic evolution in this area.