TRATAMIENTOS ESPECIALES

Fig 1.2. (a) Tectonic reconstruction of the main continental blocks in SE Asia, after Hall & Sevastjanova (2012).

Grey fragments are those thought to derive from Gondwana, while brown blocks are thought to originate from NW Australia. The pre-Cretaceous Gondwanan terranes are modified after Metcalfe (1996, 2011a, 2011b) and Barber et al. (2005). The striped terranes represent suture zones. See text for details. (b) Shows the proposed growth of Sundaland since the Mesozoic after Hamilton (1979), Hall (2008), Hall & Wilson (2000), Hall et al. (2008) and modified by Hall & Sevastjanova (2012). Arrows indicate the direction of plate movement relative to other plates. Red trianges highlight Sunda Arc volcanoes. Other Arcs are shown with white triangles.

1985; Leterrier et al., 1990). However these rocks tend to be associated with back-arc positions and not be influenced by subduction. There are currently no rocks from the Sunda arc (outside of the region in collision with the NW Australian margin) which show evidence for the presence of ancient continental material. This suggests that either the magmas are not sampling the continental material or that is is not present beneath the arc.

1.3. Geological Background

Detailed tectonic reconstructions of Indonesia are given by Hamilton (1979); Hutchinson (1982); Hall (1996, 2002, 2012). An overview of the geology is given here and regional tectonic settings are discussed during each Chatper.

1.3.1. The Sunda Arc

The Sunda arc is one of four volcanic arcs which are focused around the plate boundaries between the Eurasian, the Indian Australian and the Pacific plates (fig 1.2b). The Sunda arc includes ~ 80% of the Indonesia‟s active volcanoes and includes the Andaman Islands, Sumatra, Java and the Lesser Sunda Islands (Hall and Smyth, 2008) where Subduction has been continuous at the Java trench since the mid Eocene (~ 45 Ma) due to the northward propagation of the Indo-Australian plate beneath the Eurasian plate at a rate of ~ 6/7 cm/yr-1 (Tregoning et al., 1994; Hall, 2002). Beneath Sumatra, subduction is oblique and poorly constrained belief zones can only be traced to depths of ~ 200 km (Curray, 1989). In contrast, there is almost normal subduction beneath Java and the seismic zone extends to depths in excess of 600 km where the north-dipping slab steepens ( to ~ 60°) below depths of 100 km (Whitford, 1979; Curray, 1989). The eastern part of the Sunda arc has been in collision with the Australian margin since ~ 3 Ma.

1.3.2. Continental SE Asia

SE Asia is a heterogeneous mosaic of continental blocks joined by suture zones (Hall, 2008; Hall, 2009). Western Indonesia consists of Gondwanan fragments brought together by the

closure of numerous Tethyan Oceans. These fragments now represent the Sundaland continental core of SE Asia and have been in there present position since the Triassic (Hall, 2012). This region includes Sumatra and west Java from the Sunda arc which are situated on the Sundaland margin and includes widespread Cretaceous post-collisional granites which were emplaced following the collision of continental blocks (Hamilton, 1979; Gasparon and Varne, 1995). This is the only part of the Sunda arc where continental-like magmatism in evident (outside of the Australia – east Indonesia collision).

The exposed basement east of Sundaland is limited to few narrow distributions of Cretaceous arc, ophiolite and melange rocks in Jiwo Hills and Luk Ulo, Central Java and also in SE Borneo (Hamilton, 1979; Parkinson et al., 1998; Wakita, 2000). This is suggested to be a continuous NE-SW ophiolite belt which runs through west Java and into the Meratus Mountains of SE Borneo which marks one of potentially many suture zones in SE Asia (Hamilton, 1979; Hall, 2012). It is now thought that much of Borneo, west Sulawesi and Java are located above Australian continental blocks which were rifted from the NW Australian margin during the Jurassic. This crust is also suspected to be located beneath most of Lesser Sunda (Hall and Sevastjanova, 2012).

1.3.3. Magmatism

Magmatism in the Sunda Arc extends back to the Mesozoic in Sumatra while the oldest rocks in the Lesser Sunda Islands are of Miocene age (McCourt et al., 1996). Pleistocene-to-recent volcanism in the Sunda Arc has been the focus of many studies (e.g. Whitford, 1975; Whitford & Nicholls, 1976; Whitford et al., 1979; Whitford & Jezek, 1982; Nicholls & Whitford, 1976; Whitford & White, 1981; Nicholls et al., 1980; Wheller et al., 1987; Hilton & Craig, 1989; Hilton et al., 1991; Edwards et al., 1990; Gasparon et al., 1994; Alves et al., 1999; Turner & Foden, 2001; Gertisser and Keller, 2003 Handley et al., 2009) and contains the widest range of magma compositions known from any other arc or orogenic setting (Wheller et al., 1987). Lavas range from tholeiitic through to highly alkaline leucititic. The most abundant rocks are calc-alkaline basaltic-andesites and andesites, with an average silica content of 55 wt. %, characteristic of many other island arcs (Whitford 1975; Nicholls & Whitford 1976). However, uniformly low abundances of Mg, Ni and Cr indicate that very few, if any, lavas represent primary mantle melts, and that most have undergone varying

degrees of differentiation (Nicholls & Whitford, 1976; Nicholls et al., 1980; Wheller et al., 1987; Handley et al., 2009).

Volcanism in Sumatra is different from elsewhere in the arc, and contains the highest number of felsic intrusive rocks with radiogenic 87Sr/86Sr and un-radiogenic 143Nd/144Nd. These are suspected to have been contaminated in Sumatran crust (Gasparon and Varne, 1995, 1998). There is no evidence for the same type of crustal material in Java, and felsic rocks are far less common. One suggestion is that the relatively basic magma compositions simply reflect a less mature basement, and that any contaminant has an isotopic signature similar to those of the ascending magmas, making assimilation difficult to detect (Handley, 2006; Handley et al., 2008, 2009). As a result, many studies have implicated the importance of a subducted component and source heterogeneity to explain chemical and isotopic anomalies in Java (Whitford & Jezek, 1982; Edwards et al., 1991; Turner & Foden, 2001; Alves, 1999; Handley et al., 2008, 2009; Sendjaja et al., 2009).

1.4. Thesis aims and objectives

The main aims of this thesis are:

1. To produce a detailed mineralogical and geochemical study of three Quaternary volcanoes across the Sunda Arc and identify the processes which occur in the crust during differentiation, the composition of the mantle source, and components added to the mantle source from the subducting slab.

2. Combine the results from these volcanoes with other published sources of data to build a profile of changes along and across the arc and examine how magmatism is linked to the tectonic setting of a particular region.

3. To identify the character of unseen source and basement rocks, particularly in relation to modern tectonic reconstructions. This will help to evaluate the use of geochemical proxies to understand tectonic process not evident at the surface.