Fig. 2.1. (a) Shows a subdivision of structural provinces (grey dotted lines) east of the Muriah-Progo lineament (dashed black line). Also shown is the location of igneous
intrusive rocks containing Cambrian to Archaean zircons (white stars) in the Southern Mountains, Quaternary volcanoes (white triangles) in the modern arc and the Pliocene and Pleistocene extinct volcanoes (black triangles) located in the Rembang Zone. The division between the Rembang and Kendeng zones is marked by the Barbaris thrust. After Smyth (2005). (b) Shows gravity data for east Java, after Smyth et al. (2008). (c) A schematic representation of the Roo Rise collision at the Java trench, redrawn after Kopp et al. (2006). (d) Seismic profiles along Java, after Widiyantoro et al. (2011). Note the gap in the slab at 113°E.
hiatus, Kelut marks a return to more frequent volcanism (Wheller et al., 1987) and the spacing between volcanoes along the rest of east Java is ~ 25 m apart. Setijadji (2010) shows that the diminished section of volcanic activity is associated with a prominent area of low heat flow terminated by the Barbaris Thrust between the Kendeng and Rembang zones. The character of the magmatism from this section of the arc will be discussed in the following sections.
The Southern Mountains
The Southern Mountains represent the southernmost province in east Java and an older Eocene to Miocene volcanic arc. A detailed account of the Southern Mountains stratigraphy is reported in Smyth (2005). This volcanic arc is currently situated ~ 50 km south of the modern arc but has been thrust north onto the volcaniclastic sequences in the Kendeng basin (Hall et al., 2007; Clements et al., 2009). Upper Oligocene to lower Miocene volcaniclastic deposits suggest that the termination of volcanism was associated with explosive eruptions and magmas of an intermediate and acidic composition. The products of which are know found extensively in the Kendeng basin. This contrasts with the modern arc where very few evolved compositions have been discovered.
The nature of the basement beneath the Southern Mountains has recently been reviewed following a number of studies by Smyth et al (2005, 2007, 2008) who discovered that many of the zircons from igneous rocks yielded Cambrian to Archaean ages. Similar age ranges have also be recorded in the Milano metamorphic basement complex in western Sulawesi (e.g. van Leeuwen et al., 2007) which is associated with widespread acidic magmatism containing isotopic characteristics typical of an old crustal source (Bergman et al., 1996; Polve et al., 1997; Polve et al., 2001). Hall et al. (2009) and Hall and Sevastjanova (2012) now interpret the deep crust in east Java and west Sulawesi to be part of the same continental block (the Argo block), which rifted from the Australian continental margin during the Jurassic. Such a model is supported by deep basement structures in Indonesia and NW Australia (Granath et al., 2011; Hall, 2011) and zircon populations from east Java, west Sulawesi and NW Australia (Smyth et al., 2007; van Leeuwen et al., 2007; Southgate et al., 2011). However, to date, there has been little evidence for the existence of such continental material outside of the Southern Mountains in east Java.
2.2.3. Seamount Collision at the Java Trench
Since the Asian Tsunami in 2004, much work has been done to increase understanding of the processes which occur along the Sunda and Java trenches. Papers which document the seismic structure, subduction erosion and material transfer beneath the Java margin provide a clearer picture of the geodynamic constraints under which magmatism may have occurred. In a geophysical study along the Java trench Kopp et al. (2006) state that “ subduction processes off central Java are dominated by the collision of the Roo Rise with the forearc between 109°E and 115°E”. Fig. 2.1c shows a schematic representation of the Roo Rise seamount colliding with the trench offshore central and eastern sections of Java. Bathymetric data shows that that the Roo Rise is ~ 70 km into the trench at its most northern point which suggests that it subduction of the plateau started at between 1.1 and 1.3 Ma (Shulgin et al., 2011). This has caused significant uplift and deformation to the forearc between the west Java and Lombok basins and a deflection of the trench on average of 40 km from its normal curvature (Kopp et al., 2006). Kelut is one of a few volcanoes clearly within this zone of deformation and therefore provides a good candidate for examining the effects of such significant changes along the trench. Offshore central and east Java the trench is reported to be devoid of any sediment other than localised ponds (Kopp, 2011; Masson et al., 1990).
2.2.4. The subducting plate: Seismic Profiles
A tear, or gap, in the slab beneath east Java has been imaged by high resolution P-wave tomography and been linked to the previous subduction of a buoyant plateau (similar to the present day Roo Rise) at ~ 8 Ma (Hall et al., 2009; Widiyantoro et al., 2011). The gap, now in the mid-upper mantle, is thought to have resulted from trench migration southwards after collision and slab break-off. Resumed subduction, behind the accreted plateau, would presumably have pulled the gap further into the mantle until its present-day position at between ~ 250 km and 550 km depth. An analogous example of slab-tear is decribed beneath the Izu-Bonin-Mariana arc, where changes in slab morphology and seismicity are related to collisions of oceanic pleateaus at the trench (Fryer & Smoot, 1985; Widiyantoro et al., 2011). Fig. 2.1d shows three sesimic profiles across Java after Widiyantoro et al. (2011). It shows that Kelut volcano is situated on a section of the arc beneath which there is a significant gap in the subducting plate related to collisional processes at the trench.
2.3. Kelut: Volcanic history, Sample Selection, and Petrography
Historically, Kelut is one of the most active and dangerous volcanoes in Java because of its close proximity to low lying cities such as Kediri and Blitar. There have been more than 30 recorded eruptions in the historical record. Since 1500 AD, a reported 15,000 lives have been lost, about 5000 of them during an explosive pyroclastic eruption in February 1990 (Sudradjat, 1991). During an eruption in 2007 a new lava dome emerged, replacing the crater lake during a series of violent eruptions (Hidayati et al., 2009).
During a field season in May 2009, more than 20 samples were collected from the volcano flanks, the crater and the new lava dome (fig. 2.2a and b). The material collected is presumed to be from the most recent eruption; however, no dating has been done to verify this. Detailed reports of the stratigraphic units at Kelut are documented in (Wirakusumah, 1993). No other studies have been published regarding petrogenesis, geochemistry or petrology. The rocks detailed in this study include 19 fresh lavas (LOI < 0.54) and three crustal cumulates.
2.3.1. Lavas
Lava compositions and mineral components are summarised in table 2A and fig. 2.5. The groups identified to best represent Kelut are based on their major and trace element geochemistry and isotope ratios and in this study are subdivided into a high-Zr (HZR) and low-Zr (LZR) group. These are discussed in section 4.
The two groups are largely indistinguishable on the basis of their petrography and contain a basic mineralogy of plagioclase, clinopyroxene, orthopyroxene and titanomagnetite. The basaltic andesites are porphyritic with a predominantly glassy (vitrophyre) groundmass. Most of these lavas contain more than 50% phenocrysts with occasional cumulophyric clots and aggregates of minerals which appear to have been scavenged from plutonic cumulates. Olivine is absent in all but one sample, where it constitutes < 1% of the sample.
Plagioclase is the most common phenocryst and measured compositions from a LZR group basaltic andesite range from An90 (anorthite) to An70 (bytownite). Like many other lavas in Java, crystals shown signs of textural disequilibrium such as resorbed or rounded cores and rims, oscillatory zoning, reverse zoning and sieve-textured cores (e.g. Handley, 2006).