Syenite from Ulleung Island: As a window for pre-eruptive shallow alkaline magma dynamics

Abstract

Ulleung Island is an alkaline volcano that consists of Pleistocene basalts (Stage I), effusive trachytes (Stages II–III), and Holocene explosive phonolitic volcanics (Stage IV). Syenites are found as volcanic ejecta in Stage III and IV, hence, acting as a window for pre-eruptive shallow alkaline magma dynamics. We integrate geochemical data from Ulleung Island to track the magma batch interconnection underneath this island and discuss the petrological effect of magma mixing on the resultant eruption style through shallow preconditioning. The magma mixing characteristics of syenite with melanocratic enclaves are reported in this study from a textural and geochemical context. The textural assemblage from syenite is likely to indicate a couple of magmatic conditions including quenching of hotter magma, overgrowth of orthoclase-rich feldspar, and disequilibrium of kaersutite. The magma mixing conditions were traced through the predicting models from clinopyroxene, amphibole, biotite and alkali-feldspar chemistry, and constrained by ~690–740 °C at ~77–180 MPa. In addition, the Zr/Ce versus Zr/Y plot indicates that the syenites can be subdivided into the Group I as felsic cumulate from Stage IV, and the Group II as mixing product of Stage IV and II. Consequently, the magma mixing of the Stage IV and II produced the Stage III. The syenite ejecta from Ulleung Island indicate the petrological significance of the shallow alkaline magma dynamics: the explosive eruption of Stage IV is induced by the volatile (especially, magmatic water) transportation from Stage II and subsequent saturation in Stage IV. Furthermore, a discussion on the manifested eruption style through magma mixing from comparative perspective implies that the explosive eruption of a volcano requires the transportation and saturation of volatile components and a short stagnant period, avoiding stabilization of magma chamber prior to its eruption.