Fig. 4.13. (a-f) Source melting models after (Pearce and Parkinson, 1993). Front-arc volcanoes are represented by circles, rear-arc volcanoes by squares, and back-arc by
triangles. Model shows that the nature of the sub-arc manlte contains similar concentrations at similar positions from west Java to East Sunda. See text for discussion. VHI = very highly incompatible elements (during mantle melting), HI = highly incompatible, MI = Moderately incompatible, SI = slightly incompatible, C = compatible. Degree of enrichment or depletion is largely shown by behavior of VHI. Interpretations in grey boxes after (Pearce and Parkinson, 1993). Volcano data for a-c in Table 4C. Other data includes: (d) Kelut and Southern Mountains (this study), Willis (Hartono, 1996); (e) Tangkuban Perahu (Sendjaja et al., 2009), Cereme (Edwards, 1990); (f) Ringgit-Besar, Muriah (Edwards, 1990).
Table 4C. Major element and compatible trace-element data for the samples used in model 4.13 (a-c). BZD = Benioff Zone depth (after Hutchinson, 1982).
Lesser Sunda Rocks used in Modelling
Author Elburg et al., (2005) Stolz et al (1990) This study Foden (1983) Wheller and Foden (1986) Turner et al., (2003) Stolz et al ., (1988, 1990) Varne (1986)
Island Wetar Flores Bali Lombok Bali Sumbaw a Sumbaw a Alor Alor Alor Alor
Volcano Intrusive Iya Agung Rindjani Batur Tambora Sangeang Api Batu Tara Batu Tara Batu Tara Batu Tara
Lat. 7.88 8.88 8.26 8.42 8.24 8.25 8.18 7.79 7.79 7.79 7.79 Long. 126.40 121.63 115.58 116.47 115.38 118.00 119.06 123.58 123.58 123.58 123.58 BZD (km) 120 140 160 164 165 182 248 248 248 248 248 (w t.%) SiO2 50.55 52.19 50.31 47.95 48.83 49.21 47.20 47.76 47.68 48.14 49.74 TiO2 1.05 0.82 1.13 0.83 1.02 0.98 0.83 0.90 0.90 0.93 0.78 Al2O3 17.14 17.69 17.17 13.78 17.62 17.07 13.16 13.48 13.58 13.76 15.68 Fe2O3 10.68 9.71 12.62 10.33 12.12 9.94 10.33 8.65 8.51 8.59 8.12 MnO 0.15 0.15 0.21 0.17 0.22 0.18 0.19 0.17 0.17 0.17 0.17 MgO 7.01 6.12 5.77 10.61 5.78 6.88 9.45 8.38 8.28 7.68 6.40 Mg# 57.36 56.36 48.35 67.79 49.43 58.65 65.21 66.50 66.60 64.69 61.76 CaO 9.28 9.75 8.67 13.14 10.33 10.65 13.91 12.24 12.08 12.05 10.28 Na2O 3.24 2.73 2.84 1.78 2.79 3.25 2.64 1.48 2.79 1.87 2.24 K2O 0.84 0.46 0.83 1.20 0.60 1.78 1.79 4.69 3.42 4.51 4.97 P2O5 0.06 0.12 0.19 0.21 0.14 0.35 0.39 0.87 0.84 0.85 0.78 Total 100.12 99.38 99.76 100.00 99.40 100.29 100.02 99.27 99.71 99.55 99.79 (ppm) Ni 20.00 60.00 23.39 125.00 25.00 43.00 49.00 81.00 79.00 68.00 51.00 Cr 24.00 161.00 14.35 307.00 35.00 232.00 287.00 272.00 244.00 194.00 V 434.00 250.00 360.04 383.00 307.00 294.00 276.00 302.00 307.00 278.00 Sc 42.10 31.00 30.53 36.00 33.00 33.90 49.00 36.00 37.00 42.00 32.00
modelling can be equally replicated using data from Syracuse and Geoffrey (2006), using different limits.
Variations in magma source compositions may be linked to a particular volcano‟s position relative to the subducting plate (i.e. the Benioff Zone); however, they may also be linked to their position on the upper plate itself. For instance, if the back-arc volcanoes are controlled by rifting, opposed to subduction fluxes, their depth to the subducting plate may be irrelivent. For these volcanoes it may be the upper plate rather than the lower plate which controls their magmatism through localised decompression. Further towards the trench, subduction is likely to become increasingly influential. Therefore, a volcano‟s relative position on the upper plate is just as important as its position from the subducting plate for a model such as this where the tectonic controls on mantle composition are poorly known.
The first three models include volcanoes from the Lesser Sunda region (Table 4C). The WLSI do not contain volcanoes in front-arc or back-arc positions so the model has been expanded to the entire Lesser Sunda region. The front-arc volcanoes used here (figure 4.13a) are represented by an intrusive rock from Wetar (MgO =7 wt.% , Ni = 20 ppm and Cr = 24 ppm, after Elburg et al., 2005) and a tholeiitic basalt from Iya, Flores (MgO = 6 wt.%, Ni = 60 ppm and Cr = 161 ppm, after Stolz et al., 1990). The back-arc volcano (figure 4.10c) is represented by alkaline basalts from Batu Tara (MgO = 6.4 – 8.3 wt.%, Ni = 51 – 81 ppm, Cr = 194 – 287 ppm, after Stolz et al. 1990, and van Bergen et al., 1992). Figure 4.13 (b) shows the volcanoes from the western Lesser Sunda Islands, while these are restricted to rear-arc positions, they span a range of estimated depths and compositions from medium-K rocks at Agung to shoshonites at Sangean Api (e.g. Turner et al., 2003).
The second set of three models (figure 4.13d-f) include equivalent volcanic rocks from Java based on their composition and position relative to the top of the Benioff Zone and on the arc. The source compositions of these rocks have been discussed in Chapter 2. Rather than include all of the volcanoes which qualify for the model, examples from particular regions have been used here (e.g. east Java and west Java). The modelling shows that despite the changing environment along the arc, the elements thought to represent source compositions prior to subduction display systematic variations from the front of the arc to the back. This suggests that there is a generic relationship between the pre-subduction source composition of a particular volcano and its relative position on the arc, or relative to the Benioff Zone. Furthermore, this type of mantle variation can be modelled in the most primitive rocks from
west Java to the most eastern section of Lesser Sunda, where the arc is colliding with Australia.
The concept of spatial variations in Sunda Arc magmas is not a new one. During the 70‟s and 80‟s Whitford and Nicholls published a series of papers (e.g. Whitford, 1975; Nicholls and Whitford, 1976; Whitford and Nicholls, 1976; Whitford et al., 1979; Nicholls et al., 1980; Whitford et al., 1981; Whitford and Jezek, 1982) discussing what they termed as „normal island-arc associations‟ and „high-K alkaline associations‟, and their positions from the trench. This early work identified that tholeiitic magmas are found closest to the trench at depths of 100-150 km, calc-alkaline and high-K calc-alkaline magmas are located at depths of 150-250 km, and high-K alkaline magmas are situated most distal from the trench at depths > 300 km. Their work established a number of important conclusions:
1. A number of incompatible trace elements appear to correlate with position on the arc. 2. Low Mg#, Ni and Cr concentrations in magmas indicate that few represent primary,
mantle-derived compositions.
3. Spatial variations in the lavas are best explained by melting of a chemically zoned mantle source with smaller degrees of partial melting at greater depths.
However, this type of idealised model has been challenged on the basis that a number of volcanoes on the arc do not comply to such a model (Arculus and Johnson, 1978; Foden and Varne, 1980). In the Sunda arc this argument includes the extinct volcanoes of Sangenes and Soromundi on Sumbawa which contain highly nepheline-normative alkaline magmas similar to the back-arc volcanoes advocated in model 4.13 (Foden and Varne, 1980). These volcanoes occupy a position on the arc similar to Agung, Rindjani and Batur and so cannot be explained by this type of hypothesis. The nature of these volcanoes are discussed later in this chapter, and in Chapter 5. The remainder of this section will focus on the majority of volcanoes which do identify these type of spatial variations.
The idea of melting chemically zoned mantle sources beneath island arcs is now well recognised, particulary in some oceanic arcs where there is less influence from the arc crust (Woodhead et al., 1993; Elliot et al., 1997; Hochstaedter et al., 2000; Stracke and Bourdon, 2009). A number of fractional and dynamic melting models have been devised in order to try and replicate the loss of a melt fraction from a residue beneath volcanic arcs and spreading ridges (e.g. Wood, 1979; Johnson and Dick ,1992; Phipps Morgan and Morgan, 1999; Pearce
2005). As previously mentioned, this idea suggests that a partial melt can be extracted from a source and leave behind a depleted residue. The ultimate expression of such a process is shown in ultrapotassic melts. Highly alkaline magmas such as lamproites and kimberlites are often associated with highly depleted harzburgite residues, where over long periods of time small degree melts in the lithopshere have formed enriched veins surrounded by a depleted residue (Foley et al., 1987; Foley, 1992; Pilet et al., 2008; Prelevic et al., 2012). It is suggested that at small degrees of partial melting in such environments, the vien will be preferentially melted and concentrate high quantities of incompatible elements into the magma. As melting continues, the refractory material will start to melt creating hybrid magmas as an expression of the enriched vien being progressively diluted by the surrounding refractory wallrock (e.g. Foley, 1992; Pearce, 2005). Therefore in theory, the more contrast there is between enriched and depleted mantle end-members the more compositional variation that can be produced during melting of such material.
The most widely used expression of this type of enrichment or depletion in a mafic magma is its potassium contents relative to a particular value of SiO2. At island arcs and spreading
centres the magmas associated with the lowest potassium concentrations (i.e. tholeiitic compositions) are usually found closest to the trench or at the ridge, where lower pressures produce higher degrees of melting (Whitford et al., 1979; Fitton et al., 2003). Moving progressively away from the trench, or the ridge, magma compositions can become progressively more alkaline in response to higher pressures and lower degrees of partial melting. Over time this can result in the heterogeneous mantle compositions discussed above. However, this process is incompatible with the idea of metasomatism in which potassium, being highly mobile in aqueous fluids should readily re-enrich magmas at the front of the arc. Indeed, most of the early spatial variations discovered across volcanic arcs were of incompatible trace elements such as K, Rb, Cs and Ba (e.g. Kuno, 1959; Jakes and White, 1969, 1970, 1972; Jakes and White, 1969; Whitford and Nicholls, 1976), all of which are thought to be significantly transported during slab dehyration (Tatsumi et al., 1986; Tatsumi and Kogiso, 1997). Many of these elements behave in a similar manner to the more fluid immobile elements such as HFSE and HREE in Sunda magmas across the arc.
Figure 4.14 (a) shows a plot of K2O against Zr/Nb for a range of samples from Java and the
WLSI. As for the previous model, all the samples shown here are from volcanoes where crustal contamination is minimal and MgO concentrations exceed 5 wt.%. The exception to