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Handley Etal 2014 Pb Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 139 (2014) 205–226 www.elsevier.com/locate/gca Insights from Pb and O isotopes into along-arc variations in subduction inputs and crustal assimilation for volcanic rocks in Java, Sunda arc, Indonesia Heather K. Handley a,⇑, Janne Blichert-Toft b, Ralf Gertisser c, Colin G. Macpherson d, Simon P. Turner a, Akhmad Zaennudin e, Mirzam Abdurrachman f a Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia b Laboratoire de Ge´ologie de Lyon, Ecole Normale Supe´rieure de Lyon, Universite´ Claude Bernard Lyon 1, CNRS UMR 5276, 46 Alle´e d’Italie, 69007 Lyon, France c School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom d Department of Earth Sciences, Durham University, Durham DH1 3LE, United Kingdom e Center for Volcanology and Geological Hazard Mitigation (CVGHM), Geological Agency, Bandung 40122, Indonesia f Department of Geological Engineering, Bandung Institute of Technology, Bandung 40132, Indonesia Received 29 January 2014; accepted in revised form 18 April 2014; available online 9 May 2014 Abstract New Pb isotope data are presented for Gede Volcanic Complex, Salak and Galunggung volcanoes in West Java, Merbabu and Merapi volcanoes in Central Java and Ijen Volcanic Complex in East Java of the Sunda arc, Indonesia. New O isotope data for Merbabu and new geochemical and radiogenic isotope data (Sr–Nd–Hf–Pb) for three West Javanese, upper crustal, Tertiary sedimentary rocks are also presented. The data are combined with published geochemical and isotopic data to con- strain the relative importance of crustal assimilation and subducted input of crustal material in petrogenesis in Java. Also discussed are the significance of limestone assimilation in controlling the geochemical and isotopic characteristics of erupted Javanese rocks and the geochemical impact upon central and eastern Javanese arc rocks due to the subduction of Roo Rise 18 between 105 and 109°E. The negative correlation between Pb isotopes and SiO2, combined with mantle-like d O values in Gede Volcanic Complex rocks, West Java, are most likely explained by assimilation of more isotopically-primitive arc rocks and/or ophiolitic crust known to outcrop in West Java. The negative Pb isotope-SiO2 trend cannot be explained by assimi- lation of the known compositions of the upper crustal rocks. A peak in d18O whole-rock and mineral values in Central Java- nese volcanic rocks (Merbabu and Merapi) combined with along-arc trends in Sr isotope ratios suggest that a different or additional crustal assimilant exerts control on the isotopic composition of Central Javanese volcanic rocks. This assimilant (likely carbonate material) is characterised by high d18O and high Sr isotope ratio but is not particularly elevated in its Pb isotopic ratio. Once the effects of crustal assimilation are accounted for, strong East to West Java regional variations in Ba concentration, Ba/Hf ratio and Pb isotopic composition are evident. These differences are attributed to heterogeneity in the subducted source input component along the island: a more radiogenic Pb isotopic, lower Ba/Hf component (detri- tal-rich subducted sediment) in West Java and a less radiogenic Pb isotopic, high Ba/Hf component (attributed to a greater AOC/sediment fluid component and/or dominance of pelagic, clay-rich subducted sediment) in East and possible Central ⇑ Corresponding author. Tel.: +61 2 9850 4403; fax: +61 2 9850 8943. E-mail address: [email protected] (H.K. Handley). http://dx.doi.org/10.1016/j.gca.2014.04.025 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved. 206 H.K. Handley et al. / Geochimica et Cosmochimica Acta 139 (2014) 205–226 Java. The subduction of the Roo Rise, an area of oceanic basement relief, is thought to contribute significantly to the spatial geochemical source input variations exhibited by Javanese volcanoes. Ó 2014 Elsevier Ltd. All rights reserved. 1. INTRODUCTION group-rich, low-Sr isotope, mantle-derived component. Edwards et al. (1993) used B/Be and radiogenic isotopes Understanding the genesis of volcanic rocks in subduc- of seven volcanic centres along the Sunda and Banda arc tion zone settings is complicated by the multitude of differ- (including Guntur and Cereme from the volcanic front in entiation processes such as fractional crystallisation, Java) to suggest that the composition of the subduction magma mixing and crustal assimilation and geochemical input is homogenous along the Sunda arc. In contrast, variations in source components, for example, the mantle Handley et al. (2011) argued for along-arc heterogeneity wedge, subducted oceanic crust and accompanying sedi- in this component in Java based primarily on Nd–Hf iso- ments or melts thereof, that exert control on volcanic rock tope compositions of the volcanic rocks. A recent chemical geochemistry (e.g., Davidson et al., 1987; Hildreth and and isotope (He–C–N) study of active fumarole and hydro- Moorbath, 1988; Hawkesworth et al., 1991; McCulloch thermal gas and water by Halldo´rsson et al. (2013) pro- and Gamble, 1991; Pearce and Peate, 1995). Furthermore, posed that the major volatile budget of the western Sunda within-arc variations in crustal architecture and the nature arc (Sumatra, Java and Bali) is dominantly sourced from (age and composition) of the subducting slab and its asso- subduction input of crustal material rather than magmatic ciated sediments augment the difficulty of disentangling dif- interaction with thick/old crustal basement. ferentiation effects on primary magmas from magma source Considering the nature of the debate, it is surprising that characteristics. Yet, in order to obtain an accurate under- only relatively few Pb isotope data are available for Java- standing of element transfer in subduction zones, it is of nese volcanic rocks. The significant contrast in Pb concen- primary importance to identify shallow-level geochemical tration and isotopic composition of the mantle relative to and isotopic modifications of original source compositions marine and continental sediments (e.g., Sun, 1980; Price in subduction zone petrogenesis. et al., 1986; Ben Othman et al., 1989; Rehka¨mper and The Java sector of the Sunda arc in Indonesia is the sub- Hofmann, 1997; Gasparon and Varne, 1998; Chauvel and ject of a long-standing debate on the relative importance of Blichert-Toft, 2001) dictates that Pb isotopic ratios of man- subduction input of crustal material versus magmatic inter- tle-derived magma are highly sensitive to the addition of action with the surrounding arc crust in controlling the geo- subduction-related and/or assimilated arc crustal material chemisty of erupted volcanic rocks (e.g., Whitford, 1975, during volcanic rock petrogenesis (e.g., James, 1982; 1982; Wheller et al., 1987; Gasparon and Varne, 1998; Whitford, 1982; Miller et al., 1994; Carpentier et al., Turner and Foden, 2001; Gertisser and Keller, 2003a; 2008). Previously published Pb isotope data of Javanese Chadwick et al., 2007; Handley et al., 2007, 2011; volcanoes are largely confined to studies analysing only Halldo´rsson et al., 2013). Early studies attributed a one or two samples from each volcano, with the data amal- decrease in Sr isotope composition in lavas from West Java gamated in broad arc overviews (Whitford, 1982; Turner to Bali to diminishing crustal assimilation as a result of and Foden, 2001; Woodhead et al., 2001). Prior to this eastward arc-crustal thinning (Whitford, 1975; Hamilton, study, only Guntur and Merapi volcanoes have five or more 1979). Gasparon and Varne (1998) also suggested that crus- samples with published Pb isotope analyses (Edwards, tal assimilation accounts for Sr, Pb and Nd isotope system- 1990; Gertisser and Keller, 2003a). Yet, to understand the atics along the Sunda arc, from Sumatra to Lombok, contribution of crustal assimilation and the nature of relative to mid-ocean ridge basalt (MORB). In addition, assimilants, it is essential to have tight constraints on differ- more recent detailed geochemical and isotopic studies on entiation characteristics at individual volcanoes. individual Javanese volcanoes implicate an important role The oxygen isotope compositions of subduction zone for crustal assimilation processes in the geochemical and volcanic rocks can also yield key information on crustal isotopic evolution of magma and its phenocrysts (e.g., assimilation processes during magmatic differentiation Chadwick et al., 2007; Handley et al., 2008a; and therefore, help to discriminate between subduction- Abdurrachman, 2012; Abdurrachman and Yamamoto, related and assimilated arc crustal inputs. This is due to 2012; Troll et al., 2013). However, addition of a subducted the significant contrast between the low and relatively con- crustal component to the Java sector of the Sunda arc man- stant d18O values in the upper mantle (+5.5 ± 0.2& (Eiler, tle wedge is advocated by others to exert important control 2001) to +5.7 ± 0.2& (Harmon and Hoefs, 1995)in on the crustal signature observed within the volcanic rocks MORB relative to Standard Mean Ocean Water (SMOW)) (e.g., Whitford, 1982; Wheller et al., 1987; Edwards, 1990; and generally high d18O values of upper crustal materials Turner and Foden, 2001; Gertisser and Keller, 2003a; (typically >10&; e.g., Ito and Stern, 1986; Davidson and Handley et al., 2007, 2010, 2011). Wheller et al. (1987) sug- Harmon, 1989) that have undergone low temperature inter- gested that at least
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