Magma mingling in the Tungho area, Coastal Range of eastern Taiwan Yu-Ming Lai a , Sheng-Rong Song a, ⁎, Yoshiyuki Iizuka b a Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC b Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC ABSTRACT ARTICLE INFO Article history: Received 25 October 2007 Accepted 26 May 2008 Available online xxxx Keywords: magma mingling Luzon Arc peperite Coastal Range of eastern Taiwan breccia Complex rocks, consisting of different lithologic breccias and sediments in the Tungho area of the southern Coastal Range, eastern Taiwan, were formed by magmas and magma–sediment mingling. Based on field occurrences, petrography, and mineral and rock compositions, three components including mafic magma, felsic magma, and sediments can be identified. The black breccias and white breccias were consolidated from mafic and felsic magma, respectively. Isotopic composition shows these two magmas may be from the same source. Compared to the white breccias, the black breccias show clast-supported structures, higher An values in plagioclase, higher contents of MgO, CaO, and Fe 2 O 3 and lower SiO 2 , greater enrichment in the light rare earth elements (LREE), and depletion in the heavy rare earth elements (HREE). The white breccias show matrix-supported blocks and mingling with tuffaceous sediments to form peperite. Physical and chemical evidence shows that the characteristics of these two components (mafic and felsic magmas) are still apparent in the mingled zone. According to their petrography, mafic and felsic magmas did not have much time for mingling. White intrusive structures and black flow structures show that mingling occurred before they solidified. Finally, the occurrence of mingling between magmas and sediments suggests that the mingling has taken place at the surface and not in the magma chamber. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Magma mixing and mingling are important processes in the eruption and evolution of magmatism. When two or more magmas encounter each other, mechanical processes and chemical diffusion result in variously mixed or mingled final products (Donoghue et al., 1995; Cole et al., 2001). Magma mixing and mingling are two different kinds of processes in hybrid magmas (Vernon, 1984; Neves and Vauchez, 1995). The term ‘mixing’ will be used in this study whether the final product is homogeneous or not (Wilcox, 1999). Nevertheless, when we use the term ‘mingling (commingling)’ to explain that homogeneity had not been attained in a final rock specimen, the term ‘mixing’ is applied to homogeneous magma (Lee, 2002). Simply speaking, in this paper, if the characteristics of participating magmas is still apparent, there is magma mingling, if not, there is magma mixing. Studying magma mixing and mingling is not only helpful in explaining volcanic-rock variety but it also has significant geological meaning. Mixing and mingling of magmas occurs in different geological processes, and each exhibits different behavior and signals. When different magmas (mafic and felsic, hotter and colder etc.) come into contact, explosions or variations in chemical compositions may ensue. Investigating magma mixing and mingling gives us an under- standing of magma's evolution, volcanic eruptions, and even allows reconstruction of the paleoenvironment. Peperite is a special kind of volcaniclastic rock formed by the mingling of juvenile magma with unconsolidated sediments (Jerram and Stollhofen, 2002). It is common in arc-related and other volcano– sedimentary sequences, usually occurs at the bottom and frontal part of the lava flows, and is also distributed around shallow intrusions (Fisher, 1960; Skilling et al., 2002). Different structures of peperite reflect the interaction processes of magma and unconsolidated sediments, controlled by the characteristics of the paleoenvironment. In this study, we describe a volcanic outcrop recording a magma mingling event in the Tungho area of the Coastal Range of eastern Taiwan. Combining field observations, petrographic textures, mineral, and rock compositions, we characterized and quantified the mingling processes. It is the first time that magma mingling has been observed in this area; and it characterizes mingling as having happened during the eruption. 2. Geological setting Taiwan is a member of Ryuku–Taiwan–Philippine island arc chain rimming the western border of the Pacific Ocean. It is situated on a convergence and compressive boundary between two major plates, the continental Eurasian plate on the west and the oceanic Philippine Sea plate on the east. The Neogene Coastal Range volcanic rocks are situated in eastern Taiwan, as a result of the South China Sea subducting the Philippine Sea plate and exposing these volcanic rocks Journal of Volcanology and Geothermal Research xxx (2008) xxx-xxx ⁎ Corresponding author. Department of Geosciences, National Taiwan University, Roosevelt Rd, Sec 4, No 1, Taipei 106, Taiwan, ROC. Fax: +886 2 2362 5125. E-mail address: srsong@ntu.edu.tw (S.-R. Song). VOLGEO-04033; No of Pages 16 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.05.020 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores ARTICLE IN PRESS Please cite this article as: Lai, Y.-M., et al., Magma mingling in the Tungho area, Coastal Range of easternTaiwan, J. Volcanol. Geotherm. Res. (2008), doi:10.1016/j.jvolgeores.2008.05.020