CONSTITUCCIONES GENERALES DE LA OFS: Artículo

4.1. Introduction

Geochemical models produced for subduction zones rely significantly on information from primitive compositions (e.g. Woodhead et al., 1993, 2012). These melts provide valuable information about their source, without which, contributions from the arc crust or the subducting plate cannot be accurately measured (McCulloch and Gamble, 1991; Plank and Langmuir, 1998; Pearce et al., 1995; Turner and Hawkesworth, 1997; Straub and Zellmer, 2012). However, source compositions are rarely reflected in arc magmas, particularly in continental arcs where the crust is thick. In these settings differentiation of primary source compositions are commonly accompanied by open-system processes such as magma mixing, magma-cumulate mixing or crustal contamination (Nicholls and Whitford, 1976; Conrad et al., 1983; Thirlwall and Graham, 1984; Tepley et al., 2000; Davidson et al., 2005; Handley et al., 2008). These processes will act to alter trace element compositions and isotope ratios from their parent magmas.

Differentiation controlled by closed- system processes such as fractional crystallisation can be a useful indicator of source compositions. Removal of a particular sequence of minerals in a magma chamber will alter concentrations of major and trace elements with differentiation, but the magmas will retain their incompatible trace element and isotope ratios. Therefore, in volcanoes where magmas differentiate through fractional crystallisation without magma mixing or crustal contamination, the rocks will retain trace-element and isotope ratios inherited from source compositions (Handley et al., 2007).

In the Sunda Arc, and its eastward continuation into the Banda Arc, the arc magmas commonly show evidence for significant open-system behavoir throughout (Wirakusumah, 1993; Vroon et al., 1993; Reubi et al., 2002; Turner et al., 2003; Handley, 2006; Debaille et al., 2006). Even beneath volcanoes where whole-rock element and isotope ratios show no obvious signs of crustal contamination or magma mixing, phenocrysts often provide evidence of mineral-magma disequilibrium (Chadwick et al., 2007; Deegan et al., 2010; Troll et al., in press). In addition, cumulates collected from mid- to lower sections of the crust can be collected by rising magmas, which can act to alter their composition (e.g. chapters 2 and 3; and; Beard, 1986; Beard et al., 2005; Davidson et al., 2007; Larocque and Canil, 2010; Tiepolo et al., 2011). Source compositions beneath the Sunda Arc are still poorly known, and a number of different mantle fertilities are often invoked; including, depleted mid-ocean ridge

basalt (N-MORB), enriched mid-ocean ridge basalt (I-MORB), and ocean island basalt (OIB) compositions (Wheller et al., 1987).

Agung volcano is situated on Bali, one of a group of small islands in the western part of Lesser Sunda which include Lombok and Sumbawa. Unlike most other sections of the arc, active volcanoes from these islands contain basalt-andesite-dacite associations with near- mantle-like values for radiogenic isotopes 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb,

208_Pb/204_{Pb and} 3_He/4_{He, and strong evidence for closed-system, polybaric differentiation in}

the arc crust (Foden and Varne, 1980; Foden, 1983; Hilton and Craig, 1989; Rubin et al., 1989; Gasparon et al., 1994; Gasparon and Varne, 1998; Reubi and Nicholls, 2004, 2005). They should, therefore, provide the best insights into mantle source compositions beneath the Sunda Arc. A significant amount of data exists for the Quaternary volcanoes Batur, Rindjani, Tambora, and Sangean Api (e.g. Foden and Varne, 1980; Foden 1983, 1986; Varne and Foden, 1986; Wheller and Varne, 1986; Hilton and Craig, 1989; Rubin et al., 1989; Gasparon et al., 1994; Turner and Foden, 2001; Turner et al., 2003; Elburg et al., 2004; Reubi and Nicholls, 2004, 2005); however, relatively little has been published from Agung. This work presents a combined major and trace element and Sr-Nd-Pb-Hf study of Agung volcano; which, in combination with previously published data, will help to address the uncertainty regarding mantle compositions beneath the Sunda arc. In turn, it will provide a reference by which contributions from the arc and the subducting plate can be more readily quantified.

4.2. Geological Background and Chapter Aims

The Lesser Sunda Islands are situated in central and eastern sections of the Sunda Arc between Java and the Banda arc. These small volcanic islands are locally subdivided into Bali, West Nusa Tenggara (Lombok and Sumbawa) and East Nusa Tenggara (Flores and Wetar). The focus of this chapter involves volcanoes from Bali, and Agung in particular; but also include volcanoes from the West Nusa Tenggara which will be collectively referred to as the western Lesser Sunda Islands, or WLSI (see figure 4.1). The reason for a regional study here is because these volcanoes are situated north of a subduction trench free of collision. Therefore, magmas influenced by subduction-related processes avoid the additional complexities associated with seamount-arc and continent-arc collisions; such as the Roo Rise in central and east Java, and the NW Australian continental shelf in East Nusa Tenggara and

the Banda arc (Hall and Wilson, 2000; Audley-Charles, 2004; Elburg et al., 2005; Kopp et al., 2006; Kopp, 2011).

4.2.1. Volcanism in the WLSI

The volcanoes and their locations on Bali, Lombok and Sumbawa are shown in figure 4.1b. Previous studies show that the volcanic rocks have basalt-andesite-dacite associations which range from medium-K to high-K calc-alkaline at Batur and Rindjani, through to high-K and shoshonitic at Tambora and Sangean Api (Foden and Varne, 1980; Foden 1983, 1986; Varne and Foden, 1986; Wheller and Varne, 1986; Turner et al., 2003; Reubi and Nicholls, 2004, 2005). Sangenes and Soromundi are extinct volcanoes which have erupted some highly silica-undersaturated, leucite-bearing lithologies, similar to those discovered at Muriah, Ringgit-Besar and Batu Tara (Foden and Varne, 1980; Stolz et al., 1988; Edwards et al., 1991; van Bergen et al., 1992; Edwards et al., 1994). This is unusual because, all of the other highly alkaline volcanoes are all situated behind the Quaternary volcanic arc at positions > 200 km above the Benioff Zone, and so provides a significant argument against generalised arc-trench spatial models (Arculus and Johnson, 1978; Foden and Varne, 1980). These rocks are discussed briefly later in this chapter. On Bali, the oldest volcanic rocks are Late Pliocene in age. These rocks, represented by the extinct volcanoes of Bratan, Batukau and Seraja volcanoes, are located west of the currently active centres of Agung and Batur. The volcanism had re-commenced by ~ 500 ka BP at Batur volcanic field (Wheller and Varne, 1986; Reubi and Nicholls, 2004).

4.2.2. The Indian Ocean Plate

The ocean crust being subducted beneath the WLSI is some of the oldest in the Indian Ocean. This crust increases in age from Early Cretaceous at ~ 110°E to Late Jurassic at 120°E (Hamilton, 1979; Kopp, 2011). The closest ocean drilling site to the trench is DSDP site 261 on the Argo Abyssal Plain; a deep basin situated between the Java-Sunda trench and Australia (e.g. Plank and Langmuir, 1998). Here the basement is 160 Ma and covered by Cretaceous claystones and Miocene- to Quaternary nannofossil oozes and radiolarian clays (Gasparon and Varne, 1998; Plank and Langmuir, 1998; Kopp, 2011). Sediment thickness is on average higher in the Argo Abyssal Plain (~ 500 m) than along the Java trench (200 m –