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Geochemistry and petrology of volcanic rocks from the Sunda arc of Indonesia
... All soils developed on volcanic ash that is attributed to the Early to Mid Holocene (Van Bemmelen, 1949;Hutchison, 1989), with possible rejuvenation by historical eruptions of the 18th and 19th centuries. Based on the total elemental composition of parent ashes across Java (Whitford, 1975), volcanic ash ranges from basaltic in East Java (calc-alkaline basaltic ash) to more silica-rich types in Central Java (basalticandesitic ash) and West Java (andesitic ash). The ashes of the volcanoes of East Java contain more Na (7 to 8% Na 2 O) but less K (0.5 to 1.5% K 2 O) and Si (48 to 53% SiO 2 ), compared to the ash of the volcanoes in Central and especially West Java (~3% Na 2 O; 2 to 3% K 2 O; 55 to 60% SiO 2 ) (Whitford, 1975). ...
... Based on the total elemental composition of parent ashes across Java (Whitford, 1975), volcanic ash ranges from basaltic in East Java (calc-alkaline basaltic ash) to more silica-rich types in Central Java (basalticandesitic ash) and West Java (andesitic ash). The ashes of the volcanoes of East Java contain more Na (7 to 8% Na 2 O) but less K (0.5 to 1.5% K 2 O) and Si (48 to 53% SiO 2 ), compared to the ash of the volcanoes in Central and especially West Java (~3% Na 2 O; 2 to 3% K 2 O; 55 to 60% SiO 2 ) (Whitford, 1975). Java is characterized by distinct wet and dry seasons. ...
... The ionic strength was then lowered Table 1 Location of sampling sites, climatic data, source, age and nature of the volcanic parent ash materials, and classification of the selected pedons. (Whitford, 1975;Utami, 1998). ...
We studied the relationship between the mineralogical composition and surface charge properties of representative Quaternary volcanic ash soils, classified as Andosols, along an East-West sequence on Java Island in Indonesia. The soil charge characteristics were determined using ‘charge fingerprinting’ procedures. Most of the studied soils have a limited ability to retain cations under their forest or grass vegetation. The surface charge properties of these soils are mainly related to allophane and organic matter contents and, to a lesser degree, to ferrihydrite content and layer silicate characteristics. In East Java, the soils are Andosols with low allophane content, dominated either by halloysitic clays or by a mixture of 2:1 and 1:1 layer silicate clays; these soils are characterized by a low to moderate permanent charge. In West Java, the soils are rich in allophane, with subordinate kaolinitic clays, gibbsitic material or a mixture of 1:1 and 2:1 layer silicate clays; in contrast to the allophane-poor soils of East Java, these soils have extreme variable charge characteristics, creating a net ‘positive’ variable surface charge at soil pH. Differences in mineralogical composition of the clay fraction are attributed mainly to more pronounced seasonality in East Java, with variations in parent ash composition, becoming more acidic from east to west, as subordinate factor. The more severely leaching environment in West Java results in a higher degree of desilication, which led to a higher point of zero net charge (PZNC) and pHo in the allophane-rich soils. This study demonstrates how a climatic gradient can affect regional variations in charge properties, through the impact of climate on the mineralogical composition of the clay fraction. Regional patterns of this type must be understood to correctly assess of variations in soil fertility status and to make correct soil management choices for sustainable crop production.
... One of the important magmatism activity event in The Indonesia is the occurrence of movement and contact subduction of tectonic plates that form a magmatic arc from the northwestern island of Weh in Sumatra to Banda Api in the eastern part of the Banda Islands (Whitford, 1975). This Magmatic arc is known by the name Sunda -Banda Arc. ...
... The evolution of Indonesian island arc has been dominated since late Paleozoic (Katili, 1975). It was marked by several linear volcano-magmatic complexes associated with sedimentary rock (Whitford, 1975). The "first cycle volcanism" is correspond to the volcanic activity at the south and south-west of Sunda arc during the Paleogene period which culminating in lower Miocene and resulting the Old Andesite Formation (van Bemmelen., 1949). ...
... Petrographic characteristic of some lavas which represent the first cycle volcanism are very similar with many of Quaternary lavas. They have porphyritic textures with abundant assemblages of plagioclase either as phenocryst phase and/or groundmass (Whitford, 1975). Both clinopyroxene and othopyroxene can be found associated with Fe-Ti-Oxide, amphibole, and olivine either as phenocryst or microphenocryst (Whitford, 1975). ...
Andesitic lavas in Kalibawang area are part of Old Andesite Formation (OAF). These lavas represent the first cycle of volcanism in Sunda arc, which marked the earliest development of Sunda arc.
The samples were collected from three locations in the east of Kulon Progo mountains. Detailed petrographic observation is used to characterize the samples.
The result of petrographic observation shows that two of the three collected samples have similar phenocryst assemblages, composed of clinopyroxene and andesine and texture. Cryptocrystalline plagioclase is present as the main component in the ground mass, showing trachytic texture. Hornblende appears only as oxidized xenocryst. The third sample is collected from a possible dike remnant, exposed in the south of the two andesitic lavas, and composed of equigranular plagioclase laths and remnant of olivine replaced by iddingsite.
The petrographic analysis suggests that the two pyroxene-andesite lavas might have resulted from the same magmatic system. However, it has not been fully understood whether or not they are coming from the same source. The third sample, the diabasic dyke, is not cut through the andesitic lavas. It may represent the sub-volcanic part either of the same system which erupted prior to the two andesitic lavas or the whole different system which presumably is older or younger (?) than the andesitic lava group.
... The 87 Sr/ 86 Sr ratios of the Merapi volcanics display significant variations between 0Á70501 and 0Á70583 (Table 1). Previously published Sr isotope ratios for Merapi indicate a similar range from 0Á70498 to 0Á70594 (Whitford, 1975aWhitford, , 1975b Whitford et al., 1981; Del Marmol, 1989; McDermott & Hawkesworth, 1991; Turner & Foden, 2001). 143 Nd/ 144 Nd ratios range from 0Á51267 to 0Á51277 (Table 1) and show a broad negative correlation with 87 Sr/ 86 Sr ratio (Fig. 9)Table 1). ...
... In a general evaluation of crustal contamination vs subducted sediment input in western Sunda arc volcanics, Gasparon & Varne (1998) strongly argued for contamination of the primary magmas by the arc crust as an important feature of Sunda arc magmatism. Therefore, before characterizing the source components of the Merapi volcanics, it is essential to evaluate the likelihood of crustal contamination Ito et al., 1987; Rehk amper & Hofmann, 1997); OIB, Indian Ocean (Dosso et al., 1979Dosso et al., , 1988; Dupr e & All egre, 1983; Barling & Goldstein, 1990; Yang et al., 1998); Java (Whitford, 1975a; Edwards et al., 1991 Edwards et al., , 1993 Turner & Foden, 2001); marine sediments, Indian Ocean (Ben Othman et al., 1989; Gasparon & Varne, 1998); published data, Merapi (Whitford, 1975a; Del Marmol, 1989; McDermott & Hawkesworth, 1991; Turner & Foden, 2001). JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003 of mantle-derived magmas during ascent through the arc crust and possible effects on magma compositions at Merapi. ...
... In a general evaluation of crustal contamination vs subducted sediment input in western Sunda arc volcanics, Gasparon & Varne (1998) strongly argued for contamination of the primary magmas by the arc crust as an important feature of Sunda arc magmatism. Therefore, before characterizing the source components of the Merapi volcanics, it is essential to evaluate the likelihood of crustal contamination Ito et al., 1987; Rehk amper & Hofmann, 1997); OIB, Indian Ocean (Dosso et al., 1979Dosso et al., , 1988; Dupr e & All egre, 1983; Barling & Goldstein, 1990; Yang et al., 1998); Java (Whitford, 1975a; Edwards et al., 1991 Edwards et al., , 1993 Turner & Foden, 2001); marine sediments, Indian Ocean (Ben Othman et al., 1989; Gasparon & Varne, 1998); published data, Merapi (Whitford, 1975a; Del Marmol, 1989; McDermott & Hawkesworth, 1991; Turner & Foden, 2001). JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003 of mantle-derived magmas during ascent through the arc crust and possible effects on magma compositions at Merapi. ...
Merapi volcano (Central Java), located within the Quaternary volcanic front of the Sunda arc, is one of the most active volcanoes of the Indonesian archipelago. During the Holocene, Merapi erupted basalts and basaltic andesites of medium-K affinity during its earlier stages of activity and high-K compositions over the past ∼ 1900 years. Merapi lavas and pyroclastic rocks are characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and higher 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios and lower 143Nd/144Nd ratios compared with Indian Ocean mid-ocean ridge basalt (MORB). Merapi high-K series rocks are enriched in LILE and LREE and slightly depleted in heavy REE (HREE) and HFSE compared with rocks from the medium-K series. The increase in K2O is accompanied by a marked increase in 87Sr/86Sr and a general decrease in 143Nd/144Nd, but not by systematic variations in δ18O values. The low δ18O nature of the Merapi magmas, and the lack of any major shift in isotopic compositions along the evolutionary trend of the two individual series, precludes extensive crustal assimilation during magma ascent and differentiation, emphasizing the importance of subducted continental material in the genesis of Merapi magmas. The compositional contrast between medium-K and high-K series volcanics largely reflects variable contributions from subducted sediment to the mantle wedge, which was similar to a MORB-source mantle before any subduction-related modification. The temporal change in the K2O content of the magmas reflects compositional variation in the mantle wedge rather than intracrustal processes occurring within the shallow volcanic system.
... For example, the two dacites from Wai Sano have very similar major-and trace-element compositions, yet have markedly different Sr-and Nd-isotope ratios, resulting in a significant displacement parallel to the 'mantle array' in Fig. 4. The Batu Tara samples also exhibit considerable variation in their St-and Ndisotope ratios. However, it is interesting to note that the two Batu Tara samples with almost identical Sr and Nd isotopic ratios ( (Figs. 5 and 6) the Flores rocks form linear arrays (with some scatter) which overlap the fields for West Java (Whitford 1975a), North Banda (J.D. Morris, in preparation), other east Sunda arc volcanics (Varne and Foden 1986; W.M. White and B. Dupre, unpublished data; authors' unpublished data), and some Indian Ocean OIB (Dosso et al. 1979;Hart 1984Hart , 1988. The Flores rocks are all characterised by relatively high 2~176 and 2~176 and therefore plot well above the NHRL (northern hemisphere reference line) defined by northern hemisphere MORB and OIB in Figs. 5 and 6, indicating an anomalous DUPAL-type Pb-isotope signature (Hart 1984). ...
... All the Flores and Batu Tara volcanics have this distinctive island-arc geochemical signature (Wheller et al. 1987;Stolz et al. 1988; Fig. 8a). However, a suite of leucite basanites from Muriah volcano, Java (Whitford 1975a;Whitford et al. 1981;Ferrara et al. 1981;Nicholls and Whitford 1983;Calanchi et al. 1983;Edwards et al. in preparation) has isotopic and trace-element characteristics which indicate the presence of a subarc mantle source that is capable of generating K-rich basic melts with many of the geochemical features of nonorogenic K-rich rocks. For example, their Zr/Nb, Sr/Nd and Nb/Pb values are very similar to the average values for OIB (Table 4), and significantly different from MORB and IAB (BVSP 1981;Sun and McDonough 1989). ...
... This is unlikely to be achieved by hydrous fluids which have low solubilities for HFSE and moderate to high solubilities for LILE (Schneider and Eggler 1986;Tatsumi et al. 1986), although the potential effects on Nb-and Tatransport of adding F or C1 to a hydrous fluid are unknown. This implied decoupling of Ti from Nb and Ta is also apparent in the leucite basanites from Muriah, Java (Fig. 8 b), which are depleted in Ti relative to N-MORB but which only exhibit a slight relative depletion in Nb (and Ta) compared with OIB (Whitford 1975a;Nicholls and Whitford 1983;Edwards et al. in preparation). ...
Major, trace-element, and Sr-, Nd-and Pbisotope data are presented for volcanics from 12 active or recently active volcanoes from the islands of Flores, Adonara, Lembata and Batu Tara in the eastern Sunda are. The volcanics vary in composition from low-K tholeiite, through medium-and high-K calcalkaline types to the K-rich leucite basanites of Batu Tara. From the tholeiites to the leucite basanites there are marked increases in the concentrations of LILE (K, Rb, Ba, Sr), LREE and La/Yb, and all the volcanics have high Ba/ Nb, La/Nb and Ba/La compared with mid-ocean ridge and intraplate eruptives. K/Cs values are generally lower than OIB values, and overlap those of other arc volcanics and northeast Indian Ocean sediments. The volcanics exhibit a broad range of 87Sr/86Sr (0.70468–0.70706), 143Nd/144Nd (0.512946–0.512447), and a moderate range in 206Pb/204Pb (18.825–19.143), 207Pb/ 204Pb (15.643–15.760) and 208Pb/204Pb (38.97–39.51). Trace-element and isotopic data suggest that the mantle beneath the eastern Sunda arc is a complex heterogeneous mixture of 3 or 4 major source components: MORB-source or depleted MORB-source, OIB-source and subducted Indian Ocean sediment. The low-K tholeiites were probably formed by relatively large degrees of melting of depleted MORB-source mantle, modified by subduction-related fluids, whereas the trace-element and isotopic characteristics of the K-rich volcanics suggest that they were derived from an OIB source which and been modified by a subduction-related melt component. The source components of the medium-to high-K calcalkaline rocks are more difficult to determine, and probably include mixtures of MORB-source or OIB-source, and melt/fluid derived from subducted oceanic sediment. Minor-and trace-element modelling calculations indicate substantial difficulties in producing the relatively low Ti-contents of arc volcanics by melting OIB-source mantle. Where OIB mantle is considered to be an important component of arc magmas it is suggested that the HFSE are buffered to relatively low concentration by a residual Ti-rich accessory phase.
... Java samples (< 53% SiO2) are for volcanoes from Java through central Flores (Whitford, 1975;Varne & Foden, 1986;Stolz et al., 1990); East Sunda samples (<54% SiO2) are for volcanoes east of central Flores (Hoogewerff et al., 1997;Stolz et al., 1990). Open symbols in Fig. 5 are samples from Rindjani, Merbabu, Krakatau and Werung not included in the regression. ...
... Average sediment Th/La is GLOSS(Plank & Langmuir, 1998), with M.C.-derived uncertainty on ratio from standard deviation of Th and La. Arc Th/La references given in table and keyed to below: 1: Electronic Appendix B; 2: Turner et al. (1997); 3: Ewart et al. (1998); 4: Peate et al. (1997); 5: Dupuy et al. (1982); 6:Gorton (1977); 7:Elliott et al. (1997); 8:Whitford (1975); 9:Varne & Foden (1986); 10:Stolz et al. (1990); 11: Hoogewerff et al. (1997); 12: Vroon (1992); 13:Gust et al. (1995); 14:Bailey et al. (1987); 15:Ryan (1995); 16:Hochstaedter et al. (1996); 17:Gravatt (1997); 18:Thirlwall & Graham (1984); 19:White & Dupre (1986); 20: Dupuy et al. (1985); 21:Pearce et al. (1995). ...
RECEIVED MARCH 17, 2003; ACCEPTED DECEMBER 1, 2004 Arc magmas and the continental crust share many chemical features, but a major question remains as to whether these features are created by subduction or are recycled from subducting sediment. This ques-tion is explored here using Th/La, which is low in oceanic basalts (<0Á2), elevated in the continents (>0Á25) and varies in arc basalts and marine sediments (0Á09–0Á34). Volcanic arcs form linear mixing arrays between mantle and sediment in plots of Th/La vs Sm/La. The mantle end-member for different arcs varies between highly depleted and enriched compositions. The sedimentary end-member is typically the same as local trench sediment. Thus, arc magmas inherit their Th/La from subducting sediment and high Th/La is not newly created during subduction (or by intraplate, adakite or Archaean magmatism). Instead, there is a large fractiona-tion in Th/La within the continental crust, caused by the preferen-tial partitioning of La over Th in mafic and accessory minerals. These observations suggest a mechanism of 'fractionation & foun-dering', whereby continents differentiate into a granitic upper crust and restite-cumulate lower crust, which periodically founders into the mantle. The bulk continental crust can reach its current elevated Th/La if arc crust differentiates and loses 25–60% of its mafic residues to foundering.
... Andosols are widespread in the Indonesian archipelago (Dudal and Soepraptohardjo, 1979). They develop from a rather wide variety of parent ashes originating from recent Pleisto-Holocene eruptions (Whitford, 1975;Utami, 1998;Van Ranst et al., 2002). The Dieng volcanic complex, located in central Java, consists of late Quaternary to recent volcanic stratocones, parasitic vents, and explosion craters (Muffler, 1970). ...
... The Dieng volcanic complex, located in central Java about 75 km northwest of Yogyakarta, is a member of the K-calc-alkali volcanoes (Whitford, 1975) and consists of separate extrusions ranging from basalt, erupted early in the sequence, through successively younger eruptions of olivine andesite, augitehypersthene andesite, hornblende andesite, and biotite andesite (Miller et al., 1983). Volcanic events of the last few thousand years consisted of phreatic eruptions, associated small hot mudflows, emission of suffocating gases, and hydrothermal activity. ...
We studied the mineralogy of a perudic Andosol developed on the Dieng Tephra Sequence in central Java, Indonesia. The objective was to confirm the presence and determine the origin and stability of 2:1 and interlayered 2:1 phyllosilicates in well-drained Andosols. This was and still is a debated topic in the literature. Total elemental and selective dissolution, as well as microscopic and X-ray diffraction analyses, were performed on the soil samples collected from this site. These analyses confirmed that andic properties were present in the soil samples. The allophane content determined by selective dissolution was 3–4% in the A horizons, and increased to 12–18% in the deeper subsoil horizons. In addition, the clay fraction contained dioctahedral smectite, hydroxy-Al-interlayered 2:1 minerals (HIS), Al-chlorite, kaolinite, pyrophyllite, mica, cristobalite and some gibbsite. The silt and sand fractions were rich in plagioclase and pyroxene. The 2:1 minerals (smectite and pyrophyllite), as well as chlorite and kaolinite were of hydrothermal origin and were incorporated in the tephra during volcanic eruption. Besides desilication during dissolution of unstable minerals, Al interlayering of 2:1 layer silicates was most likely the most prominent pedogenic process. Although hydroxy-Al polymeric interlayers would normally stabilize the 2:1 clay phases, the strong weakening, and even disappearance of the characteristic XRD peaks, indicated instability of these minerals in the upper A horizons due to the perudic and intensive leaching conditions.
... Slamet Volcano, in Central Java, Indonesia, is an unusual calc-alkaline volcano as it contains a high proportion of basalts, which occasionally contain olivines as magnesian as Fo 92 (Whitford, 1975;Vukadinovic, 1989). Despite an apparently simple tectonic setting, Slamet basalts show a wide range of phenocryst compositions (Vukadinovic, 1989), which suggests that they are the products of diverse and variable magmatic processes. ...
... They are calc-alkaline to high-K calcalkaline in nature (classi¢cation of Peccerillo and Taylor, 1976), but di¡er from 'typical' calc-alkaline lavas in containing relatively high concentrations of TiO 2 (1.05^1.8 wt%) (Whitford, 1975;Vukadinovic, 1989;Vukadinovic and Nicholls, 1989). The OS rocks are more felsic, have higher Zr/Nb and 87 Sr/ 86 Sr ratios, lower rare earth element (REE), especially light REE (LREE), abundances, and lower Nb/U and Zr/K ratios than NS rocks (Vukadinovic, 1989;Vukadinovic and Nicholls, 1989). ...
Slamet Volcano, in Central Java, Indonesia, is an active calc-alkaline stratovolcano composed largely of basalts and basaltic andesites. The phenocryst mineralogy of the most magnesian basalts (MgO s 7 wt%) has been studied in detail to investigate the nature of early magmatic processes in a large arc volcano. On the basis of stratigraphy, mineralogy, petrography and geochemistry, the studied basalts are subdivided in two groups; Old Slamet (OS) and New Slamet (NS). Olivine in the OS basalts is within the range Fo 92 ^ 65 and shows a homogenous composition distribution, with a significant proportion of crystal cores near equilibrium with whole-rock compositions. However, distinct high-and low-Ni sub-populations may be distinguished at any given Fo content (e.g. 0.25^0.10% and 0.32^ 0.27 wt% NiO for the low-and high-NiO sub-populations respectively at Fo 85). Chromian spinel inclusions within the high-NiO olivines have higher Cr# (75^80) and Fe 2þ /Fe 3þ ratio (2.6^3.2) and lower TiO 2 (0.23^0.44 wt%) contents than those within low-NiO olivines (Cr# 58^77, Fe 2þ /Fe 3þ 1.3^2.5 and TiO 2 0.73^0.91 wt%). Plagioclase and pyroxene phenocrysts display dominantly oscillatory zoning, with cores close to equilibrium with whole-rock compositions. Olivine in the NS basalts is within the range Fo 90À61 and shows a discontinuous distribution of composition including two principal peaks: the first is VFo 78À90 , close to expected equilibrium compositions; the second is VFo 62À70 , clearly too Fe-rich to be in equilibrium with whole-rock compositions and composed mainly of reversely zoned crystals. In terms of NiO content, a single (low-Ni) sub-population is observed. Chromian spinel inclusions with high Cr# and Fe 2þ /Fe 3þ ratio and low TiO 2 are not observed within the olivines of these basalts. Plagioclase and pyroxene phenocrysts are typically reversely zoned and display ubiquitous disequilibrium textures. The cores of these crystals are not in equilibrium with host basalt compositions. The mineralogy of these basalts indicates that the OS basalts resulted from mixing between two parental magmas produced from contrasted sources, probably a relatively depleted and H 2 O-poor harzburgite and a less depleted and more hydrated harzburgite to lherzolite. Subsequent evolution occurred in a magmatic system in which variations in volatile contents and/or temperature played a more important role than magma mixing. The NS basalts were produced from the less depleted source only, but magmas interacted extensively with remnants of earlier crystal mush/magma batches. Open-system processes operated early in the life of all these magmas, and influenced their geochemistry. Magma mixing was a 0377-0273 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 -0 2 7 3 (0 2) 0 0 3 5 7 -8 ubiquitous process and together with fractional crystallisation controlled the evolution of the basaltic magmas of Slamet Volcano. ß 2002 Elsevier Science B.V. All rights reserved.
... The studies of Gunung-Batur reported about values of 87 Sr / 86 Sr for basalts = 0.70399, for dacite = 0.70407 [3] . The values of 87 Sr / 86 Sr and 144 Nd / 143 Nd for lava were equal to 0.70404 and 0.512907 in 1974 respectively. ...
Sedimentary rocks can form both directly during eruptions and the destruction of effusive rocks. They can inherit the geochemical features of parent rocks. Therefore, the study of trace elements of the effusives is significant for the interpretation of the erosion source. This work provides information about trace element geochemistry of basalts from Vesuvius and Batur volcanoes, which are located at different ends of the Mediterranean-Alpine-Himalayan folded belt and represented by alkaline (subalkaline) type of magmas. The comparison of trace element concentration obtained by atomic emission spectral analysis and inductively coupled plasma mass spectrometry showed strong differences for a number of elements. Magma of Vesuvius, originating from a deep source, assimilated in its path the core material, which led to the eruption of lavas of the type EM I with high concentrations of Ba, Be, Cs, La, Rb, Mo, U, W, Zr. Batur relates to subduction arc volcanoes with the characteristic type of lavas HIMU, which are depleted of listed above chemical elements and enriched Bi. The basalts of these volcanoes have similar anomalies of the geochemical composition for most trace elements, suggesting that they have one close source that can be presented by subduction-modified oceanic lithosphere involved in generation of both HIMU and EM types.
... The characteristic of magmatism itself almost has no distinguished feature towards Quaternary magmatism. Its volcanic products either lava or pyroclastic have similar magma affinity that is calc-alkaline (Whitford, 1975). Generally, the metals bearing mineral deposit found in western Java is associated with the pre-Quaternary volcanic center. ...
The study area is located in Neglasari, Cibeber, Lebak, Banten. Western Java host a big potential in mineral deposits especially epithermal deposits and one of them was found in this studied area. This study is aimed to understand the geological framework, hydrothermal alteration and the mineralization characteristics of the prospect. Surface geological mapping is used to know the geological condition including lithologies, structural geology and hydrothermal alteration delineation. Five representative ore vein samples were analysed using FA-AAS method to determine the grade of precious and base metal contents. Stratigraphically, the lithologies found in this area can be divided into 3 units including sandstone, tuff breccia and andesitic lava units while the structural geology are strike slip fault, normal fault and some joints. Four alteration zones developed including silification, clay-silica, clay and chlorite-epidote zones. The mineralization mostly found as hydrothermal veins which can be divided into two types that is precious metal bearing vein or usually called as low sulfidation epithermal type and base metal bearing vein or usually called as intermediate sulfidation epithermal type.
Key words: epithermal, hydrothermal alteration, sulfidation.
... There may also be a significant strontium input via ash from other volcanos in the region. Javanese volcanic rock varies from about 0.7055 to 0.7040 in a northwest to southeast pattern (Whitford 1975b), so the Javanese volcanoes closest to Bali have ratios very close to Batur's own ratio. Significant long-distance deposition of atmospheric dust seems unlikely in relatively isolated Bali. ...
Two thousand years ago, maritime trade flourished in Southeast
Asia and archaeological excavations have revealed that Island Southeast
Asia played an important role within developing trading networks.
The sites of Sembiran and Pacung on the north coast of Bali,
Indonesia, have produced a wide range of artifacts that demonstrate
links to mainland and island Asia. Here, we examine faunal remains
from these sites to assess the role that livestock played in north Bali
diet and trade at that time. In addition to abundant pig (Sus cf. scrofa)
remains, the sites yielded the earliest securely dated goat (Capra hircus)
remains known from Southeast Asia. Moreover carbon, nitrogen,
oxygen, and strontium stable isotope analyses of bone collagen and
tooth apatite indicate that some of these goats were from a markedly
different environment than the pig, human, and dog remains from
the sites. It is likely that these goats were imported from a different
region—possibly South Asia—where they fed on C4 plants such as millet.
This provides evidence that livestock were included in regional
exchange networks, and prompts the question as to why goat remains
are absent from Mainland Southeast Asia archaeological sites despite
their presence in South Asia, East Asia, and Island Southeast Asia.
... The high Cr/K2θ contents throughout the Cenozoic section Fig. 13), and andesites from Java and Lesser Sunda (Merbabu volcano, Java, and Batur volcano, Bali). Data for andesites from Whitford (1975) and Wheller and Varne (1986). Open triangles = anomalously high Cr sediments in the 200-to 400-m interval at Site 765 (Fig. 4B). ...
Drilling at Site 765 in the Argo Abyssal Plain sampled sediments and oceanic crust adjacent to the Australian margin. Some day, this site will be consumed in the Java Trench. An intensive analytical program was conducted to establish this site as a geochemical reference section forcrustal recycling calculations. About 150 sediment samples from Site 765 were analyzed for major and trace elements. Downhole trends in the sediment analyses agree well with trends in sediment mineralogy, as well as in Al and K logs. The primary signal in the geochemical variability is dilution of a detrital component by both biogenic silica and calcium carbonate. Although significant variations in the nonbiogenic component occur through time, its overall character is similar to nearby Canning Basin shales, which are typical of average post-Archean Australian shales (PAAS). The bulk composition of the hole is calculated using core descriptions to weight the analyses appropriately. However, a remarkably accurate estimate of the bulk composition of the hole can be made simply from PAAS and the average calcium carbonate and aluminum contents of the hole. Most elements can be estimated within 30% in this way. This means that estimating the bulk composition of other sections dominated by detrital and biogenic components may require little analytical effort: calcium carbonate contents, average Al contents, and average shale values can be taken from core descriptions, geochemical logs, and the literature, respectively. Some of the geochemical systematics developed at Site 765 can be extrapolated along the entire Sunda Trench. However, results are general, and Site 765 should serve as a useful reference for estimating the compositions of other continental margin sections approaching trenches around the world (e.g., outboard of the Lesser Antilles, Aegean, and Eolian arcs).
... Tan (1965) reported that the parent materials from which these Andisols were developed, change from basic to acid types when moving from east to west along the volcanic rim of the Indian Ocean. As suggested by Hutchison (1989), a similar evolution was found along the Sunda Island arc (from Java to Flores), in which 80% of the volcanoes in Indonesia are concentrated, or from east to west on Java Island (Whitford, 1975). Van assessed the influence of changes in parent ash material on a number of physico-chemical properties of Andisols along an E –W sequence in Java. ...
Six highland Andisols along an E–W sequence in Java Island, Indonesia were sampled with the objective to assess the influence of the change of parent ash materials on surface reactivity of Andisols, with emphasis on short-range order mineral constituents and active Al and Fe compounds on fluoride and phosphate sorption. Because of the variety in parent ash materials and under the typical environmental conditions prevailing in East, Central and West Java, respectively, the soil's mineralogical and chemical properties differ significantly. Allophane is formed abundantly with concomitant development of active Al and Fe. The allophane content is found to increase from East to West Java, presumably due in part to the change in parent ash materials. Consequently, active Al and Fe in the soils increase in that order, which in turn raise fluoride reactivity, P-sorption maximum and P-bonding energy, implying an increase in P-fixation and/or P-retention. Estimated soil P-sorption maxima differ significantly between the three regions. There is no difference in P-bonding energy between the soils of the East and Central Java, but the values for these two regions are lower than those of the West Java. It is concluded that in terms of P-fertilizer strategy, the soils of the three regions can be divided into three separate groups: the East, Central and West Java soils display different phosphorus requirements, being lower in the east as compared to the west of the island.
... (2) The 2°Tpb/2°apb and 2°spb/2°npb ratios, however, are not as exotically high as they are in Javanese volcanic rocks of the Indonesian arc systems [42]. They are just ordinary arc-like values, similar to those of most samples from the Halmahera arc to the south [32] were collision is imminent, but has not yet culminated. ...
The Philippine islands are situated between two oppositely dipping zones of seismicity. With the exception of a few areas, such as in the west central Philippines where the North Palawan continental terrane (NPCT) has collided with the archipelago, these seismic zones are well defined to depths of 200 km. Active volcanic chains overlay segments in each of these zones, suggesting that subduction is presently taking place both east and west of the islands. Lavas we have studied are thus divided between what has been termed the West Philippine arc and the East Philippine arc.West Philippine arc volcanic rocks which were extruded before the Philippine archipelago collided with the NPCT, or which are younger than the collision but crop out hundreds of kilometers from the collision zone, and all but one of the rocks from the East Philippine arc fall in the MORB field on207Pb/204Pb versus206Pb/204Pb covariation diagrams. This is surprising considering the frequency with which arc materials have207Pb/204Pb ratios higher than those of MORB, the highBa/REE and Sr/REE ratios in the lavas and the possibility of sediment subduction given the small accretionary prisms. All of these rocks have high208Pb/204Pb ratios with respect to Pacific and Atlantic Ocean MORB, but are similar to Indian Ocean MORB and IOB. Thus the Philippines consist of island arcs with the peculiar Dupal isotopic anomaly documented between 0° and 60°S in the southern hemisphere and particularly in the Indian Ocean region. This demonstrates that the Dupal isotopic anomaly is not restricted to the southern hemisphere, or to MORB and OIB.Post-collision rocks cropping out near the NPCT, in the West Philippine arc, have elevated208Pb/204Pb and 207Pb/204Pb ratios that could be attributed to assimilation of the newly introduced continental crust (NPCT) by mantle-derived magmas or to the addition of a sedimentary component to mantle-derived magmas.
Plain Language Summary
The Eastern Anatolian High Plateau (EAHP) that formed after the collision of Arabian and Eurasian continents hosts a huge volcanic system. The nature of the source region from which these volcanics originated and the geological dynamics that triggered this widespread volcanic activity are still under debate. We suggest that volcanism occurred when the lithospheric mantle beneath the EAHP separated physically from the overlying crust and sank into the deep asthenosphere. In this study, we explore the geochemical evidence for this model by focusing on the Late Miocene Tunceli Volcanics, one of the early stage members of post‐collisional volcanics in the EAHP. Our data suggest that the Tunceli Volcanics are the products of mixed asthenospheric and lithospheric mantle melts. The lithosphere contains a dense pyroxenite component that forms when silica‐rich melt invades the lower lithosphere during the subduction process. Calculations have shown that these dense materials melted at higher depths than the base of the lithosphere beneath the region. Thus, we propose that the dense domains in the lower lithosphere resulted in gravitational instabilities and eventually led to the foundering of the eastern Anatolian lithosphere in the Late Miocene.
This chapter provides a synthesis of the geological history, chronology and magmatic evolution of Merapi. Stratigraphic field and geochronological data are used to divide Merapi into three main evolutionary stages and associated volcanic edifices (Proto-, Old and New Merapi) and eight broad volcano-stratigraphic units to characterise the eruptive activity and structural evolution of the volcano through time. Complementary petrological, geochemical and isotopic data are used to characterise the eruptive products of Merapi and shed light on the geochemical evolution and petrogenetic processes. The data indicate that the eruptive products of Merapi are mainly basaltic andesite of both medium-K and high-K type and support a two-stage petrogenetic model, where primary magmas are derived from a heterogenous, Indian Ocean MORB-like mantle source metasomatised by slab-derived components. Subsequently, these magmas are modified during transfer through the crust by complex magmatic differentiation processes, including contamination by carbonate rocks of the local upper crust. The available data indicate that, since ~ 1900 14C y BP, the lavas and pyroclastic rocks of Merapi are essentially of the high-K type and that regular geochemical variations with systematic shifts in whole rock SiO2 content occurred since at least the Late Holocene, although erupted magma compositions have remained broadly uniform since the mid-twentieth century.
Modern potassic igneous rocks occur in a wide range of tectonic environments, from continental to oceanic and within-plate settings, some of which are not apparently associated with subduction. It is therefore important, whether for improving exploration models for ancient mineral deposits or reconstructing the evolution of ancient terranes, to be able to distinguish the tectonic settings in which ancient potassic igneous rocks were generated. This Chapter seeks to provide such a distinction based on the geochemical fingerprints of potassic igneous rocks from five principal tectonic settings.
Autoclastic breccia of Peniron Formation is one of the volcanic products which represents the magmatism during Pliocene in Java. Most of the Pliocene volcanism products is covered by Quaternary products, so the magmatism characteristic during this epoch has not been studied properly and the geochemical publication are very limited. This study present major and trace elements data which are analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to understand the petrogenetic model and magmatism characteristic. The result shows that the samples range between basalt to basaltic andesite. The magma series dominated by calc alkaline and related to island arc. Based on the spider diagram some trace elements such as Ta, Nb and Ti show negative anomaly, and the other elements such as Sr and K show positive anomaly. The geochemical analysis suggested that the magmatic system in Java island being evolved. The magma source is the partial melting of mantle wedge in dry condition, become enrich in Rb, Ba and K then produced basaltic rocks. After it, primary magma is evolved because of fractional crystallization process and produced andesite basaltic rocks. The autoclastic breccia of Peniron Formation may represent the second stage volcanism product.
Autoclastic breccia of Peniron Formation is one of volcanic product which represent a volcanism activity during Pliocene. This autoclastic breccia exhumes widely in East Kebumen, Central Java and its surrounding area, however no published mineralogical and geochemical characteristic of the Peniron autoclastic breccia. The characteristics of the rocks are important to understand the evolution of magmatic in Java Island in terms of their association with Old Andesite Formation. This study presents petrographic observation and results of Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) analysis to characterize the autoclastic breccia samples of this study from Peniron Formation. Characteristic of lavas of Old Andesite Formation from another location which represents the Neogene volcanism are also provided in this study indicated by previous studies. The result of petrographic observation shows that all of breccia samples from Peniron Formation have a similar mineral assemblages and textures. The phenocryst mainly composed by andesine and clinopyroxene, while orthopyroxene appears as a minor component. These phenocryst phase show an oscillatory zoning textures. Microcrystalline plagioclase is present as the main component in the ground mass, showing intergranular texture. One of breccia sample has abundant hornblende assemblages. Some samples composed chlorite that replaces the pyroxene phenocrysts. Meanwhile, the lava of the Old Andesite Formation reported by previous studies show more abundant of feldspar. Results of the geochemical analysis suggest that the magma source of autoclastic breccia of Peniron Formation more mafic than the lava of Old Andesite Formation. The petrographic and geochemical analysis suggests that the magmatic system in Java Island unchanged but being evolved and differentiated in Miocene to Pliocene period. The Old Andesite Formation represents the first cycle volcanism product followed by the autoclastic breccia of Peniron Formation that may represent the second cycle volcanism product. Magmatic evolution of the formations presumably was caused by partial melting of mantle wedge and fractional crystallization process.
Modern potassic igneous rocks occur in a wide range of tectonic settings, from continental to oceanic and within-plate, some of which are not apparently associated with subduction (Joplin 1968; Morrison 1980; Müller et al. 1992b). It is therefore important, whether improving exploration models for ancient mineral deposits, or reconstructing ancient terranes, to be able to distinguish the tectonic settings in which ancient potassic igneous rocks were generated. The following chapter seeks to provide such a distinction.
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 constrain 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 between 105 and 109°E. The negative correlation between Pb isotopes and SiO2, combined with mantle-like δ18O 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 assimilation of the known compositions of the upper crustal rocks. A peak in δ18O whole-rock and mineral values in Central Javanese 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 δ18O 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 (detrital-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 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.
The Halmahera island arc, NE Indonesia, is the E flank of the Molucca Sea collision zone which is the site of an active arc-arc collision. One unique aspect of the arc is the vast thickness of marine sediments outboard of the trench, a result of approx 1000-1500km of closure in the Molucca Sea basin. The Halmahera arc is underlain by a 45oE dipping Benioff zone, which is present to depths of 230km. The volcanoes form a single front which lies approx 100km above the top of the slab. The arc can be separated into three regions, on the basis of tectonic setting and chemistry. Most volcanoes are part of the normal calc-alkaline oceanic arc segment. -from Authors
Leucitites from the Italian volcanic province show a marked enrichment of the large ion lithophile elements (LILE) over the high field strength elements (HFSE), indicative of a subduction-related origin. Despite Sr- and Nd-isotope ratios similar to those in lamproites, trace-element variations do not suggest the latter as a source for the K-enrichment. Instead, potassium and the other LILE and rare-earth elements (REE) are derived from subducted sediments in a multicomponent mixing process that involves both dehydration and partial melting of the subducted material. The slightly elevated delta18O and high 87Sr/86Sr values of the primitive Italian lavas are shown to be consistent with a three-component mixing model, which can also explain other unusual geochemical features of the Italian lavas. Stabilization of phlogopite in hybridized mantle above the subduction zone is regarded as an essential aspect of the petrogenesis of the orogenic leucitites.
This paper presents a compilation of partition coefficients for a suite of elements for temperatures of 1200-1300°C, oxygen fugacities of QFM ± 1 and sub-alkaline compositions. These coefficients yield good-fit mantle depletion trends for abyssal, orogenic and trench-wall peridotites. The details, but not the principles, can be varied by changing the shape of the melting column, the porosity of the mantle during melting, the potential temperature of the mantle, and the temperature and depth of initiation of melting. Bivariate plots of elements of contrasting compatibilities can be contoured according to the degree of depletion or enrichment of the mantle and the degree of melting, with selected plots also emphasizing the role of garnet (Ti-Yb) and oxygen fugacity (V-Yb). -from Authors
Gunung Slamet, Central Java, is a large stratovolcano within the Sunda magmatic arc of Indonesia. The volcanic edifice includes products of two large overlapping Quaternary stratocones. Basaltic andesites and andesites, with rare basalts, dominate the western region of the complex, known as Slamet Tua (old); and basalts and basaltic andesites compose the eastern cone, called Slamet Muda (young).On the basis of stratigraphy, trace-element content, ZrNb, ZrK and 87Sr86Sr ratios, Slamet lavas can be broadly categorized as relating to high abundance magma (HAM) and low abundance magma (LAM) types. The Tua and Lebaksiu sequences generally comprise the LAM group, and are older, more salic and have higher 87Sr86Sr ratios than those of HAM. LAM andesites contain some amphibole, but HAM andesites do not.Models involving randomized magma replenishment, tapping and fractionation (RTF) were developed to explain the geochemical features of both LAM and HAM rock groups. The salic lavas of the LAM suite can be generated if fractionation was dominant relative to replenishment and tapping in LAM magma chambers. Conversely, magma chambers with a high proportion of replenishment and tapping relative to fractionation can produce dominantly mafic lavas, such as those of the HAM suite.Concave-upward heavy-rare-earth element (HREE) patterns for LAM andesites are probably due to significant amphibole fractionation; HAM andesites display flat HREE patterns and do not require amphibole fractionation from parental basalts. The high TiO2 contents of HAM basalts and basaltic andesites (relative to those of “average” arc rocks) are due to either suppressed crystallization—or minor accumulation—of Ti-magnetite, in conjunction with RTF processes.
Quaternary lavas of the normal island-arc basalt—andesite—dacite association in the islands of Java and Bali range from those belonging to tholeiitic series over Benioff-zone depths of ∼ 150 km to high-K calc-alkaline series over Benioff-zone depths of 250 km. More abundant and diverse calc-alkaline lavas are found over intermediate Benioff-zone depths. On average, basaltic lavas become slightly more alkaline (largely due to increased K contents) with increasing depth to the Benioff zone. Levels of incompatible minor and trace elements (K, Rb, Cs, Ba, Nb, U, Th, light REE) show a corresponding increase of almost an order of magnitude.
We present Sr, Nd, and Pb isotope results and SiO2, Rb, Sr, Sm, Nd, U,
Th, and Pb data for six active volcanoes and one extinct volcanic island
distributed over the whole length of the Banda Arc. Rock types range
from low-K tholeiitic in the NE to high-K calc-alkaline in the SW. The
volcanoes in the NE have 'normal' arc signatures, whereas those in the
SW have extreme values. Serua, situated in the central part, is the most
anomalous volcano with regard to its Sr and Nd isotopic composition but
not with regard to Pb isotopes. The inactive island of Romang in the SW
overlaps the Serua trends. The volcanoes display variable within-suite
ranges in Sr-87/Sr-86 and Nd-143/Nd-144. Large ranges (e.g., at Nila)
are consistent with assimilation (10-20%) of carbonate-bearing sediments
from the arc crust. Despite the evidence for assimilation, it cannot
explain all of the Sr-Nd isotopic trends found, and Banda Arc magmas
must have already obtained a 'continental' signature at depth before
they reached the arc crust. Within-suit trends of Pb isotopes are
virtually absent. We found an extreme range in the volcanics along the
arc which coincide with a similar trend in sediments in front of the arc
and consider this as strong evidence for the contribution of subducted
continent-derived material to magma sources.
The radioisotope 10 Be has been used as a tracer to evaluate subduction and recycling of sediments in island arcs. As a cosmogenic isotope strongly enriched in oceanic sediments, it is especially suitable for monitoring sediment subduction. We report here 10 Be results for 106 arc volcanic rocks and 33 basalts from mid-ocean ridges, oceanic islands, continental rifts, and continental flood basalt provinces. The 33 basalts from non-arc environments all contain < 1 × 10 6 atom/gram (g -1 ) and form a reference or "control group" against which the arc data may be compared. Lavas from the Sunda, Mariana and Halmahera arcs, each with 12 samples, are indistinguishable from the control group. Nine samples from Japan range from 0.1 × 10 6 to 13.5 × 10 6 g -1 of 10 Be. Lavas from the Aleutians and Central America (excluding Costa Rica) consistently have 10 Be contents greater than those of the control group, extending to 15.3 × 10 6 g -1 for the Aleutians and 24 × 10 6 g -1 for Central America. Both the Japan and Central American arcs show a geographic variation in 10 Be abundances; the Aleutian arc does not. Conceptually, four models may be used to explain the incorporation of 10 Be in island arcs. Model 1 is the limiting case calculation presented in et al . (1982). Physically, it corresponds to an assumption that all 10 Be in the uppermost sediment layers is carried to depth, the 10 Be is mechanically decoupled from the sediment pile and is added to the arc magma source region. This model results in the highest calculated 10 Be contents in arc lavas. In the more physically reasonable Model 2, the 10 Be-rich upper sediments are assumed to mix with the deeper, 10 Be-poor, sediments and the sediments are subsequently incorporated into the source region. Model 3 assumes 10 Be to be incorporated from sedimentary layers encountered during magma ascent. In Model 4 only sediments contained within grabens in the downgoing slab may be subducted. As yet insufficient data exist to permit conclusive evaluation of these models, but correlations between 10 Be contents in arc volcanic rocks and recent sedimentation rates and sediment thickness, measured outboard of the trench, suggest that Model 2 may be the most important mechanism for 10 Be incorporation in many island arcs.
This study deals with fifteen Neogene and Quaternary K-rich volcanic series from the back-are area of Central Java and from South Sulawesi. The rock associations have been subdivided into three types: (1) silica-saturated or -oversaturated potassic series (SK) either potassic calc-alkaline (Lasem, Java) or shoshonitic (Patiayam, Java; Parepare, Sopeng 2 and part of Lompobatang, South Sulawesi); (2) weakly silica-saturated (ne < 10%) alkaline potassic series (AK: Muria 1, Genuk in Java; Baturape Formation, Cindako Formation, Camba 2a Formation and part of Lompobatang stratovolcano, South Sulawesi); and finally (3) strongly silica-undersaturated (ne + lc > 10%) ultrapotassic series, usually leucite-bearing (UK: Muria 2, Bawean in Java; Camba 2b Formation, Sopeng 1 Formation in Sulawesi). Comparative geochemistry of the SK, AK and UK associations show that the above distinction is consistent with the behaviour of incompatible elements (increase of K, Rb, Sr, Ba and of La/Yb ratios from SK to UK series). All these rock series show, moreover, “orogenic” chemical characteristics compatible with a subduction-related environment. However, in South Sulawesi, their emplacement clearly post-dates the latest known subduction event; in Central Java they do not fit with any model of increasing K2O with depth of the Benioff plane, and the location of the UK series is independent from the latter (the Quaternary UK Series on Bawean is located away from the 600 km isobath). Thus we strongly prefer a genetic model for the studied K-rich volcanic series by melting of mantle sources enriched in incompatible elements during previous subduction events, and possibly involving a contribution of subcontinental mantle (Central Java).
In the Sunda arc, only the Bali—Lombok—Sumbawa sector is apparently flanked both north and south by oceanic crust. South of Lombok Island the oceanic crust is probably of Early Cretaceous or Late Jurassic age, whereas the oldest rocks known from Lombok and Sumbawa islands are the Lower Miocene to Pliocene sediments and volcanics of the basement beneath the Quaternary—Recent volcanic centres.Three large active volcanoes form the northern parts of Lombok and Sumbawa. The volcanic rocks of Rindjani on Lombok belong to a basalt—andesite—dacite association, rich in plagioclase and hy- and Q-normative. East of Lombok, the volcanic rocks of Tambora and Sangeang Api on Sumbawa belong to a potassic ne—trachybasalt—trachy-andesite association. All three volcanoes occur only 150–190 km above the active north-dipping Benioff zone.Extinct Quaternary centres occur south of the active volcanoes on Sumbawa. Two of these centres, Soromundi and Sangenges, erupted markedly ne- and lc-normative leucitites together with andesites, dacites and trachybasalts.The volcanic composition—space—time relations in the Lombok—Sumbawa sector of the Sunda arc are not in accordance with the generalized island-arc schemata. Conventionally, potassic ne-mnormative island-arc associations are supposed to occur over the deep part of the Benioff zones, far from the trenches of mature island arcs. The SiO2|K2O relations of the Rindjani association are reasonably appropriate for a volcano overlying intermediate Benioff-zone depths, but both the Tambora and the Sangeang Api associations are far more potassic than would be predicted by generalized schemata, and also occur in a relatively young arc sector that apparently has developed only since Miocene time.Basalts, trachybasalts and leucitites from the Lombok—Sumbawa sector have been compared: at similar MgO contents and Mg/(Mg+Fe), the progression from hy- and Q-normative to ne- and lc-normative magmas is not marked by significant enrichment in TiO2, Na2O, Zr, Nb and P, but is accompanied by a substantial increase in K2O, Rb, Sr and LREE, by increasing and by decreasing K/Rb. ratios from Rindjani (0.70386–0.70402) and Tambora (0.70385–0.70389) are very similar and among the lowest for the Sunda arc, but from Sangeang Api (0.70460–0.70500) are significantly higher and more variable in spite of the similar tectonic setting and petrological affinities. ratios of leucities tend to be higher (0.70488–0.70529).The petrogenesis of the volcanic associations of Lombok and Sumbawa cannot be readily explained. Although even the leucitites display the poverty in TiO2 that generally characterizes volcanics from simple island-arc tectonic settings, there is very obvious uncoupling within the “incompatible elements”: enrichment in the LIL group (K, Rb, Sr but not Na) is not accompanied by similar behaviour in the group of small highly-charged ions (Ti, Zr, Nb, P). It has proved impossible to model this behaviour without invoking inhomogeneities in the source regions, both in mineralogy and in chemical composition. Similar uncoupling within the incompatible elements has also been reported from basalt groups from the Mid-Atlantic Ridge, may also occur in the Birunga province, and might not arise from processes unique to the island-arc environment.We suggest that a LIL-rich component is being progressively added to the source regions. This component could be incorporated by the crystallization of additional phases such as phlogopite or paragasite. If this component occurs deep within the mantle, it might gain passage to shallower regions either by percolating up the downgoing slab to yield the familiar arc magma zonation, or up substantial cross-arc fractures.
Potassic volcanic rocks (PVRs), such as shoshonites, are an important constituent of young volcanic suites in five main tectonic settings:Continental arcs (e.g. Andes);Post-collisional arcs (e.g. Alps);Oceanic arcs, subdivisible into initial (e.g. Marianas) and late (e.g. Sunda Arc);and Within-Plate (e.g. inland N. American Cordillera). Since some PVRs are associated with world-class gold and base metal deposits, and can be important in tectonic reconstruction of ancient terranes, new diagrams based on immobile elements (Al, Ti, P, Y, Zr, Nb, La, Ce, Hf) are presented for discriminating the tectonic setting of PVRs from their geochemistry; spidergrams and previously published discrimination diagrams based on `normal' basalts proved unsuitable. Diagrams were derived, in part via multigroup linear discriminant analysis, using a database of fresh, primitive, Cenozoic PVRs (497 analyses), itself carefully screened from a newly erected database of 2,222 PVRs of all ages and compositions. To maximise discrimination, the diagrams are hierarchical: the first applies to all five settings and removes the most distinctive group (Within-Plate), while the remainder target more subtle distinctions. Overall, concentrations of most LILE, LREE and HFSE, and ratios such as Zr/Al and Ti/Al, tend to be highest in Within-Plate PVRs, intermediate in Continental and Postcollisional Are PVRs, and lowest in Oceanic Arc PVRs. In detail, Postcollisional are distinguished from Continental Arc PVRs by higher Ce/P, and lower Zr/Ce, Ti/Nb ratios of the latter; late from initial Oceanic arc PVRs by higher Hf, La and P of the former. These geochemical differences have a firm theoretical foundation in petrogenetic processes during magma emplacement and/or in source heterogeneities related to subduction or mantle metasomatism.
Two separate series of hydrous experiments involving (1) imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite, and (2) crystallizing similar natural rock compositions progressively until apatite appears, demonstrate a close dependence between apatite saturation and silica content of the magma, and determine P2O5 levels at a given silica value and temperature at which that composition may be expected to crystallize apatite. The effect of pressure on apatite solubility is not great, and is most significant for silicic compositions.
P2O5 vs SiO2 relationships of the low-K island arc suite, calcalkaline suite and high-K calc-alkaline suite, appear regular and characteristic for each suite, and when linked with the experimental work on apatite solubility, indicate the following: (1) the low-K and calc-alkaline series have low P2O5 contents (∼0.1–0.2 wt.%) and relatively flat P2O5-SiO2 patterns; they do not show evidence of reaching apatite saturation until rhyodacite-rhyolite compositions are obtained for the low-K suite, and andesite-dacite compositions for the calc-alkaline suite; (2) the high-K calc-alkaline series has higher P2O5 contents (∼0.4–0.6 wt.%) in mafic compositions, and achieves apatite saturation over a wide compositional range for the series; (3) the calc-alkaline and high-K calc-alkaline series are probably lower temperature, and more hydrous than the low-K series; (4) anomalous P2O5-SiO2 distributions may indicate non-equilibrium crystallization of apatite, magma-mixing and crystal accumulation processes active in generation of the ‘orogenic’ volcanic series.
Since Mesozoic time, Java and Bali have formed part of an evolving system of island arcs comprising the Sunda arc of Indonesia. The present tectonic setting is relatively simple with subduction occurring at the Java Trench to the south. A north-dipping Benioff seismic zone delineates an underthrust lithospheric slab to depths of approximately 600 km beneath the Java Sea. Quaternary lavas of the ‘normal island arc association’ range from tholeiites to high-K calc-alkaline lavas over Benioff zone depths from 120–250 km, respectively. More abundant calc-alkaline lavas lie between these extremes. High-K alkaline lavas are found over Benioff zone depths in excess of 300 km.
Both within and between these groups of rocks there are consistent spatial variations in the observed geochemistry. For approximately 200 rocks, incompatible elements such as K, Rb, Cs, Sr, Ba, light REE, U and Th show an increase in abundance of almost an order of magnitude with increasing depth to the seismic zone. Abundances of compatible elements show little consistent variation and trace elements such as Ni, Co, Cr, and Sc are characteristically depleted except in some of the alkaline lavas. Major element abundances in rocks of the normal island arc association show little variation, except for K and P, which both increase in abundance across the arc and Al, which shows a relative decrease.
The major and trace element data are inconsistent with the derivation of the analyzed rocks by partial melting of the crustal component of the subducted lithosphere. On the other hand, low Ni abundances (∼20 ppm) in the basalts suggest that most of the lavas are fractionated and few if any represent primary mantle-derived melts. The spatial variations in the geochemistry of erupted lavas across Java and Bali are best explained by a combination of two processes: melting of a geochemically zoned mantle source and smaller degrees of partial melting of that material at progressively greater depths. Primary tholeiitic magmas could be formed by 20–25% melting at depths of 30–40 km, primary high-K calc-alkaline magmas by 5–15% melting at 40–60 km depth, and primary alkaline magmas by 5% melting at depths of 80–90 km. The geochemical zoning in the mantle, which is also manifested by increasing 87Sr/86Sr ratios in lavas across the arc, is interpreted to result from the addition of a small melt fraction derived from the crustal component of the subducted lithosphere.
Mineralogical, major and trace element, and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the Batu Tara volcano, eastern Sunda arc. In general, the eruptives are markedly porphyritic with phenocrysts of clinopyroxene, olivine, leucite plagioclasebiotite set in similar groundmass assemblages. These K-rich alkaline volcanics have high concentrations of large-ion-lithophile (LIL), light rare earth (LRE) and most incompatible trace elements, and are characterized by high 87Sr/86Sr (0.70571–0.70706) and low 143Nd/ 144Nd (0.512609–0.512450) compared with less alkaline volcanics from the Sunda arc. They also display the relative depletion of Ti and Nb in chondrite-normalized plots which is a feature of subalkaline volcanics from the eastern Sunda arc and arc volcanics in general. Chemical and mineralogical data for the Batu Tara K-rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions. The compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively Fe-rich (100 Mg/(Mg + Fe2+)=48–51) and is inferred to have been derived from a more primitive magma by low-pressure crystal fractionation involving olivine, clinopyroxene and spinel. Mg-rich (mg 90) and Cr-rich (up to 1.7 wt. % Cr2O3) zones in complex oscillatory-zoned clinopyroxene phenocrysts probably also crystallized from such a magma. The marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively evolved with more primitive magmas, together with their phenocrysts, along interfaces between discrete convecting magma bodies.
Olivine leucitites from the Gaussberg volcano, Antarctica are primitive members of the lamproite group of ultrapotassic rocks. They are glass rich, have an Mg number of around 70, and carry spinel lherzolite xenoliths. Liquidus phase fields and compositions were studied experimentally at atmospheric pressure with controlled oxygen fugacities. Chrome-spinels occur as inclusions in olivines in the natural rock, but it was necessary to add Cr2O3 to the experiments to stabilize spinels at the liquidus, indicating that some fractionation of spinel has almost certainly occurred.
Experimental results show thatfO2 conditions of crystallization can be characterized by ferric value (100 Fe3+/(Fe3++Fe2+)) of spinel, ferric iron content of leucite, and Mg-number (100 Mg/(Mg+Fe2+)) of olivine. The results demonstrate that the liquidus phases of the Gaussberg rocks crystallized atfO2 slightly below that of the NNO buffer. Application of the results to other lamproites indicates that they began to crystallize at oxygen fugacities varying from well above NNO (Leucite Hills) to around MW (West Kimberley and the Spanish fortunites). The Gaussberg olivine leucitite contains leucite cores poor in ferric iron with rims richer in ferric iron, indicating oxidation during emplacement. The ferric value of spinel is very sensitive to changes in oxygen fugacity and recognizing that some lamproitic magmas are known to contain diamonds, it may prove to be useful as a ‘diamond survival indicator’. The preservation of diamonds in lamproitic rocks will depend critically onfO2: diamonds are not likely to be preserved in rocks which reach the surface as liquids atfO2 near NNO or above.
Quaternary basalts from the Java-Bali sector of the Sunda arc show the following characteristics: As the vertical distance increases between the seismic Benioff zone and the respective eruptive centre, high-field-strength elements (HFSE's) increase in abundance and Zr/Nb and Hf/Nb decrease in value. If subduction processes do not affect the abundance and ratios of HFSE's, both of these trends are consistent with a progressively enriched mantle wedge with depth, independent of any metasomatism by the northward subducting Indian Ocean plate. Furthermore, the volcanic rocks display negative trends between Nb/U-Zr/Nb, Nb/U-87Sr/86Sr, and K/Nb-K and positive ones between Zr/Nb-87Sr/86Sr and Ce/Pb-Nb/U. Also, Nb-depletion (Nb/Nb*), as measured on mantle normalized incompatible-trace-element diagrams, decreases in size with increasing distance to the Benioff zone. These trends are in accord with a model in which the slab-derived metasomatic component-which is low in HFSE abundance-makes a greater chemical imprint on shallower, depleted mantle than on deeper, enrichedmantle. The nature of the chemical layering within the mantle magma source beneath Java implies that this source may be part of the lithospheric mantle.
Sr, Nd and Pb isotope data are presented for a transitional basalt-peralkaline rhyolite suite, and spatially associated calc-alkaline rhyolites from the D'Entrecasteaux Islands, eastern Papua New Guinea. Both suites have a typical convergent margin geochemical signature (i.e. high Zr/Nb, La/Nb, and low Ta/Yb compared with OIB). The transitional basalt-peralkaline rhyolite suite and calc-alkaline rhyolites have a restricted range of206Pb/204Pb (18.522–18.661),207Pb/204Pb (15.543–15.631), and208Pb/204Pb (38.31–38.63) values which overlap the fields of volcanics from Tonga, Fiji, and Pacific MORB and sediments. The transitional basalt-peralkaline rhyolite suite also displays a restricted range of143Nd/144Nd values (0.513053–0.512984), but a much broader range of87Sr/86Sr (0.703989–0.70585) values. The latter reflects uncertainties in the ages of samples with very high87Rb/86Sr values and the effects of Sr exchange with seawater. The calc-alkaline rhyolites have consistently lower143Nd/144Nd values (0.512923—0.512867), and a more restricted range of87Sr/86Sr values (0.703864–0.704028) compared with the transitional basalt-peralkaline rhyolite suite.
The isotopic and trace element data are consistent with the interpretation that the calc-alkaline rhyolites were produced by partial melting of a young arc protocrust, whereas associated calc-alkaline basic and intermediate magmas were derived from a depleted mantle source which previously had been modified by subduction along the Trobriand Trough. The transitional basalt-peralkaline rhyolite suite was produced by extensive magmatic differentiation of a parental transitional basalt magma in a relatively shallow magma chamber. The parent magma was produced by partial melting of either a depleted MORB-source mantle or a less-depleted OIB-type source which previously had been modified by subduction processes. The hy- and ol-normative transitional basalt magmas were probably generated in response to lithospheric thinning at somewhat higher pressures than qz-normative calc-alkaline magmas.
The close spatial and temporal relationship between the transitional basalt-peralkaline rhyolite suite and the tale-alkaline volcanics reflects the complex and dynamic tectonic setting of eastern Papua and the D'Entrecasteaux Islands. In particular, the change from calc-alkaline to alkaline magmatism appears to have occurred following a change from compressional to extensional tectonics resulting from the westward propogation of the Woodlark spreading ridge into eastern Papua.
Rocks from the Cordon Syenite Complex, located in the Philippine island arc, correspond to miaskitic (metaluminous) potassic lavas from the Roman Region and the Indonesian arc in terms of bulk rock chemistry. However, the mineral chemistry indicates a complex history of multiple equilibrations with liquids of different nature. T-site (Si + Al) occupancy in some clinopyroxenes and Ca contents of alkali feldspar phenocrysts indicate that these phases crystallized under the temperature-pressure subvolcanic regime from host liquids with progressively decreasing (Na + K)/Al ratio. In contrast to current views on the genesis of potassic rocks, it is suggested that differentiated leucite tephrite suites may be derived from peralkaline liquids by alkali loss. Magmas with affinities to lamproites may occur in the Philippine island arc, but a direct genetic relationship between subduction and alkaline/peralkaline magmatism remains questionable.Die Magmatite des im philippinischen Inselbogen gelegenen Syenitkomplexes von Cordon entsprechen in ihrem Gesamtgesteinschemismus den miaskitischen, K-reichen Vul kaniten der Romanischen Provinz und des indonesischen Inselbogens. Die chemische Zusammensetzung einzelner Mineralphasen deutet auf eine komplexere Genese unter Beteiligung von Magmen unterschiedlicher Alkalinitt hin. Die Variation in der Besetzung der T-Position (Si + Al) in Klinopyroxenen und der Ca-Gehalte in Alkalifeldspten wird dahingehend interpretiert, da diese Phasen unter subvulkanischen Bedingungen aus Schmelzen mit abnehmenden (Na + K)/Al-Verhltnissen kristallisierten. Im Gegensatz zu gngigen Auffassungen wird daher angenommen, da differenzierte Leucit-Tephrite aus mild-peralkalischen Magman hervorgehen knnen. Obwohl die K-reichen Magmatite des Syenitkomplexes von Cordon in einem Inselbogen gebildet wurden, ist ein direkter Zusammenhang mit Anreicherungsvorgngen im Bereich der Subduktionszone fraglich.
Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low ratios averaging 0–7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an ratio of 0.7139.The relationship between and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in from West Java to Bali and there is some evidence for increasing with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand).A model of 87Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high ratio in the Lake Toba rhyolite implies a crustal origin.
Transition from collisional to extensional tectonics in eastern Papua New Guinea (PNG) is reflected in cessation of arc-type volcanism and eruption of rifting-related transitional basalts. Pb, Sr and Nd isotopic compositions for 28 samples representing these volcanic suites and subordinate high-K trachytes of unknown origin reflect melting and mixing of three distinct magma sources. The arc volcanics have high La/Nb, low Ce/Pb and overabundances of LILE relative to the LREE consistent with melting of subduction-modified sources. and ratios are moderately heterogeneous (ϵNd = + 2.8 to + 6.3) and plot in the upper OIB field. Pb isotopic ratios are similar to modern Pacific sediment. Melting of a depleted MORB-type mantle containing up to 1.5% of Pacific sediment can account for the isotopic trends. The modeled amount of sediment is higher than that generally observed in Pacific arcs but lower than that suggested for Indonesia. Rift basalts have and (ϵNd = + 5.3 to + 7.8) intermediate between Pacific MORB and associated arc volcanics. and ratios form a mixing trend and define Pacific MORB and sediment as end-members. High La/Nb and low Ce/Pb are further evidence for an important influence of subduction-modified sources at mantle and/or crustal levels. and ratios in the high-K trachytes plot near those of the most enriched arc volcanics but low and high indicate an old source that had evolved with low and high Th/U. These characteristics are similar to those in inferred lower crust from northeast Australia and EM 1 mantle sources; however, high La/Nb and low Ce/Pb ratios suggest additional source modification. Moderately radiogenic ratios preclude a significant contribution from crustal material much older than 2 Ga. We suggest that the trachytes inherited their isotopic signatures either from lower crust that was recycled into the upper mantle possibly during Mesozoic rifting of the Australian craton, or from a yet not recognized ancient continental block within the Late Cretaceous basement of PNG.
The partitioning of La, Sm, Dy, Ho and Yb between garnet, calcic clinopyroxene, calcic amphibole and andesitic and basaltic liquids has been studied experimentally. Glasses containing one or more REE in concentrations of 500–2000 ppm were crystallized at pressures of 10–35 kbar, and temperatures of 900–1520°C. Water was added to stabilize amphibole and to allow study of partition coefficients over wide temperature ranges. Major element and REE contents of crystal rims and adjacent glass were determined by EPMA, with limits of detection for individual REE of 100–180 ppm. Measured partition coefficients, DREECryst-liq, are independent of REE concentration over the concentration ranges used.D-values show an inverse dependence on temperature, best illustrated for garnet. At a given temperature, they are almost always higher for equilibria involving andesitic liquid. Garnet shows by far the greatest range of D-values, with e.g. DLa < 0.05 and DYb ~ 44 for andesitic liquid at 940°C. DYb falls to ~ 12 at 1420°C. DSmGa-liq also correlates negatively with temperature and positively with the grossular content of garnet. Patterns of DreeCryst-Liq for calcic clinopyroxenes and amphiboles are sub-parallel, with D-values for amphibole generally higher. Both individual D-values and patterns for the crystalline phases studied are comparable with those determined for phenocryst-matrix pairs in natural dacites, andesites and basalts.D-values and patterns are interpreted in terms of the entry of REE3+ cations into mineral structures and liquids of contrasted major element compositions. The significance of the partition coefficients for models of the genesis of andesitic and Hy-normative basaltic magmas is assessed. Most magmas of these types in island arcs are unlikely to be produced by melting of garnet-bearing sources such as eclogite or garnet lherzolite.
Cold Bay and Amak Island, two Quaternary volcanic centers in the eastern Aleutians, are orthogonal relative to the trench and separated by ~50 km. Sr, Nd and Pb isotopic compositions of the calc-alkaline andesite magmas show no sign of contamination from continental crust (average , , , ). These samples plot within the mantle arrays for Sr-Nd and for Pb and are similar to arcs such as the Marianas and New Britain (Sr-Nd) and Marianas and Tonga (Pb). Incompatible element ratios for the Aleutian andesites (K/Rb ~ 332, K/Cs ~ 10,600, K/Sr ~ 22.4, K/Ba ~ 18.3, Ba/La ~ 60) are within the range reported for arc basalts, despite the difference in degree of fractionation.Average K content, K/Rb, K/Ba and K/Sr are approximately the same for basalts from arcs and from oceanic islands (OIB); K/Cs is a factor of 4 lower and Ba/La almost 3 times higher in arcs. Abundance ratio correlations indicate that arcs are enriched in Cs and depleted in La relative to OIB, with other incompatible element abundances very similar. Histograms of Sr and Nd isotopic compositions for MORB, OIB, and intraoceanic arcs show remarkably similar peaks and distribution patterns for intraoceanic arcs and OIB.A “plum pudding” model for the upper mantle best accommodates a) geochemical coherence of OIB and IAV, b) the existence of mantle plumes at some oceanic islands, and c) the presence of a MORB-type source at back arc spreading centers. In this model, OIB plums are imbedded in a MORB matrix; small degrees of melting generate OIB-type magmas while larger degrees of melting dilute the OIB magma with MORB matrix melts.OIB plums are merely less robust lower mantle plumes (i.e., blobs) which are distributed throughout the upper mantle by convection. The existence of at least two types of OIB, as indicated by Sr, Nd, and Pb isotopes, suggests that nuggets of recycled oceanic lithosphère may coexist with lower-mantle plums and that both may be tapped in arcs and intraplate environments.
The high field strength elements (HFSE) play a critical role in the interpretation of chemical variation within subduction-related magmas by providing an assumed mantle-dominated ‘baseline’ from which enrichments in many other slab-derived elements may be gauged. Of the HFSE, hafnium is unique in combining the characteristics of HFSE chemistry with a powerful isotopic tracer and should, in theory, allow the delineation of mantle domains and help constrain the timing of melt depletion processes. A detailed Hf isotope study of oceanic arc lavas and paired arc/back-arc settings has been conducted. Here we show, contrary to expectations, that the Hf isotopic compositions of arc lavas are always displaced significantly from their co-existing back-arc spreading centres which can be considered to sample the local mantle. This is true not only of those arcs in which direct sediment melting or AFC-like processes within the crust are implicated, but also in low-K tholeiitic arcs where hydrous fluids are believed to be the dominant medium of slab-to-mantle transport. This observation calls into question the concept of ‘conservative’ or ‘immobile’ elements and suggests that some transfer of material from the subducting slab into the sub-arc mantle wedge probably occurs for almost all elements. These conclusions have significant implications for models of arc geochemistry.
To assess the role of sediment subduction and recycling in island arc magma genesis and mantle evolution, we have determined Sr, Nd, and Pb isotope ratios and the concentrations of K, Rb, Cs, Ba, Sr, U, Th, Pb and rare earth elements in 36 modern marine sediments, including Mn nodules, biogenic oozes, and pelagic and hemipelagic clays from the Pacific, Atlantic and Indian Oceans. From these data we draw the following conclusions. Sr and Nd isotope ratios and the Sr/Nd concentration ratios in sediments are such that mixing between subducted sediment on the one hand and depleted mantle or subducted oceanic crust on the other can produce mixing arrays which may pass either through or outside of the oceanic basalt SrNd isotope “mantle array”. Thus whether isotope compositions of island arc volcanics (IAV) plot inside or outside of the mantle array is not a good indication of whether or not their sources contain a subducted sediment component. The presence of subducted sediment in the sources of IAV should lead to Cs/Rb and Pb/Ce ratios which are higher than those in oceanic basalts, and Ba/Rb ratios which may be either higher or lower than oceanic basalts. Simple mixing calculations suggest that as little as a percent or so sediment in island arc magma sources can account for the observed Cs/Rb, Pb/Ce, and Ba/Rb ratios in IAV. However, it does not appear that high Ba/La ratios and negative Ce anomalies in IAV are inherited from sediment in IAV magma sources. It is more likely these features reflect fractionation of alkalis and alkaline earths from rare earths during slab dehydration and metasomatism. Pb isotope ratios in sediments from the Warton Basin south of the Sunda Arc are collinear in 208Pb/204Pb-207Pb/204Pb/206Pb/204Pb space with volcanics from West Sunda, but not with volcanics from the East Sunda. This collinearity is consistent with the hypothesis that sediments similar to these are being subducted to the magma genesis zone of the West Sunda Arc. Sediment recycling to the deep mantle appears capable of explaining much of the Sr and Nd isotopic variation in oceanic basalt magma sources. However, because of the low 238U/204Pb ratios in sediments, anciently recycled sediment should have lower 206Pb/204Pb ratios than most oceanic basalts, though this effect will be in part balanced by high 238U/204Pb in hydrothermally altered oceanic crust. Unless 238U/204Pb ratios in ancient sediments were different than in modern ones, it would appear that deep sediment recycling cannot account for the Pb isotopic compositions of most oceanic basalts. In addition, deep sediment recycling should lead to higher Pb/Ce and Cs/Rb ratios and more variable Ba/Rb ratios in oceanic basalts than is observed. High Pb/Ce ratios have recently been observed in young seamounts from the Society Island chain, suggesting their source contains a recycled component. However, on the whole, the limited variation of Pb/Ce, Ba/Rb, and Ca/Rb in oceanic basalts suggests recycling of sediment of the deep mantle is limited.
Volcanic rocks of the Sunda and Banda arcs range from tholeiitic through calcalkaline and shoshonitic to leucititic, the widest compositional span of mafic magmatism known from an active arc setting.Mafic rocks in our data set, which includes 315 new analyses of volcanic rocks from twelve Quaternary volcanoes, including Batu Tara in the previously geochemically unknown Flores-Lembata arc sector, are generally similar to those from other island arcs: most contain <1.3 wt. % TiO2 and 16–22 wt. % Al2O3, and have characteristically high K/Nb and La/Nb values. Abundances of P, Ba, Rb, Sr, La, Ce, Nd, Zr and Nb increase sympathetically with increasing K2O contents of mafic rocks but those of Na, Ti, Y and Sc vary little throughout the geochemical continuum from low-K tholeiitic to high-K leucititic rocks.Excluding Sumatra and Wetar, which possess mainly dacitic and rhyolitic volcanics, the Sunda-Banda arc is divisible into four geochemical arc sectors with boundaries that correlate with major changes in regional tectonic setting and geological history. From west to east, the West Java, Bali and Flores arc sectors each comprise volcanoes which become progressively more K-rich eastwards, culminating in the leucitite volcanoes Muriah, Soromundi and Sangenges, and Batu Tara, respectively. In the most easterly Banda sector, the volcanics vary from high- to low-K eastwards around the arc.Correlations between geochemistry and 87Sr/86Sr values show separate trends for each of the four arc sectors, believed to be the result of involvement of at least three geochemically and isotopically distinct components in the source regions of the arc magmatism.A dominant source component with a low K content and a low 87Sr/86Sr value, and common to all sectors, is probably peridotitic mantle. A second component, with low K content but high 87Sr/86Sr value, appears to be crustal material. This component is most apparent in the Banda sector, in keeping with that sector's tectonic setting close to Precambrian Australian continental crust, but it is also present to lesser extents in the West Java and Flores sectors.However, the most marked geochemical and isotopic variations shown by the arc volcanics are primarily due to the involvement of a third component, which is rich in K-group elements but has relatively low 87Sr/86Sr values. This component appears to be mantle-derived and is least overprinted by crustal material in the Bali sector volcanics where the Pb, Be, U-Th and O isotope characteristics of the rocks support the suggestion that their genesis has not involved incorporation of recently subducted, continent-derived sialic material.The high, regionally persistent, Th/U value (about 4.3) of the Sunda subarc mantle, obtained from U-Th isotopic data, suggests a close association could exist between the K-rich component and the southern hemisphere ‘DUPAL’ mantle isotopic anomaly.
The northern Mariana islands in the western Pacific form a prime example of an intra-oceanic island arc system, free from the effects of continental crustal contamination. Radiogenic (Sr, Nd, Pb, Hf) and stable (O, H, S) isotope data are presented for a suite of lavas from these islands, together with rare-earth and other trace-element compositional data for the same samples. This geochemical synthesis provides an opportunity to address a number of outstanding questions concerning oceanic arc petrogenesis, in particular the nature of the mantle source and processes occurring within this region.
The extinct Pleistocene volcano Muriah, situated behind the main Pleistocene—Recent Sunda magmatic arc in north-central Java, has erupted at least two contrasted groups of lavas. One group forms a well-defined compositional series (Anhydrous Series) from leucite basanite to tephritic phonolite, with olivine and tschermakitic clinopyroxene the main phenocrysts. The other group, the “Hydrous Series”, includes compositionally variable tephrites and high-K andesites with common plagioclase, biotite and amphibole. Lavas of the Anhydrous Series are much richer in LIL trace elements than the most potassic lavas of neighbouring active volcanoes, but relative HFS element enrichment is less pronounced. REE patterns have almost constant slopes from La (250–600 times chondrites) to Yb (5–10 times chondrites), while those of lavas of active centres are less light-enriched, and show flattening in the heavy REE. Anhydrous Series initial 87Sr/86Sr ratios (0.7043–0.7046) are lower than those of active centres (0.7047–0.7053). Hydrous Series lavas are intermediate in all these geochemical characteristics.The most mafic A-series leucite basanite, with , 140 ppm Ni and 620 ppm Cr was probably derived from the primary magma for the series by fractionation of only ∼ 5 wt.% olivine. Its REE pattern suggests derivation from a garnet-bearing source. Experiments on this basanite, with up to 10% olivine and 20% orthopyroxene added, and in the presence of H2O and H2O/CO2 mixtures, have shown that for all but very high magma water contents, the olivine and garnet liquidus fields are widely separated by fields of phlogopite and clinopyroxene. There is no liquidus field of orthopyroxene. Hence, if magma production involved an equilibrium melting process alone, the most probable sources are of garnet-bearing phlogopite clinopyroxenite type. Alternatively, this magma may represent the end-product of interaction between a low-K basanite magma from a garnet lherzolite source in the asthenosphere and a phlogopite-bearing lherzolite zone in the lower lithosphere. Its production was probably related to crustal doming and extension superimposed on the dominant subduction regime. Hydrous Series magmas may have resulted from mixing between Anhydrous Series magmas and high-K calc-alkaline basaltic to andesitic magmas more directly related to subduction processes.
The Java Island, Indonesia with abundant amount of pyroclastic deposits is located in the very active and dynamic Pacific
Ring of Fires. Studying the geochemical weathering indices of these pyroclastic deposits is important to get a clear picture
about weathering profiles on deposits resulting from the eruption of Mt. Merapi. Immediately after the first phase of the
eruption (March to June 2006), moist and leached pyroclastic deposits were collected. These pyroclastic deposits were found
to be composed of volcanic glass, plagioclase feldspar in various proportions, orthopyroxene, clinopyroxene, olivine, amphibole
and titanomagnetite. The total elemental composition of the bulk samples (including trace elements and heavy metals) was determined
by wet chemical methods and X-ray fluorescence (XRF) analyses. Weathering of the pyroclastic deposits was studied using various
weathering indices. The Ruxton ratio, weathering index of Parker, Vought resudual index and chemical index of weathering of
moist pyroclastic deposits were lower than those of the leached samples, but the alteration indices (chemical and plagioclase)
were slightly higher in the moist compared to the leached pyroclastic deposits.
Systematic variations in O, Sr, Nd, and Pb isotopic ratios and in Sr, Nd, Pb, and Rb trace element abundances in central Andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization. The results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the Arequipa and Barroso volcanic groups of southern Peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of Charcani gneiss (or an equivalent rock) accompanied by dominantly plagioclase fractional crystallization. A consequence of this result is that the RbSr pseudoisochrons reported for the Arequipa and Barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation, as are all of the Pb isotopic variations measured in the same lavas, already interpreted by Tilton and Barreiro [14] to be the result of crustal contamination. Isotopic and trace element compositions of the primary magmas for the Arequipa and Barroso volcanic series are obtained by correcting for assimilation and fractional crystallization. The starting magmas are characterized over the entire range of permissible primary compositions by high87Sr/86Sr ratios, low143Nd/144Nd ratios, and high Sr concentrations relative to “normal” island arc andesites. These results appear to require prior contamination of the magmas, but not involving plagioclase fractionation. The uncertainty in Pb and O isotope ratios computed for possible primary magmatic compositions is sufficiently large, however, that it is not possible to preclude a subcrustal origin for at least some regional variation in Sr and Nd isotopic ratios.
REE data, with major element and other trace element data are reported for a suite of Archean sedimentary rocks (2800 million years old) from Kalgoorlie, Western Australia. The REE patterns fall into two groups with ϵLREE/ϵHREE ratios of 6 and 15, respectively. The first group have either no Eu anomaly relative to chondrites, or a positive Eu anomaly, in contrast to the pronounced Eu depletion (Eu/Eu ~ 0.67) shown by younger (Post-Archean) sedimentary rocks.The problem of positive Eu enrichment relative to chondritic patterns, is examined by analysing a suite of Devonian greywackes, derived from calc-alkaline volcanic rocks. Some of these samples also show positive Eu anomalies, attributable to local accumulation of feldspar. This explanation is preferred to models involving an early anorthositic crust. The group of samples showing heavy REE depletion patterns (complementary to those observed in garnet) appear to be derived from adjacent Na-rich granites which display identical REE patterns. Locally abundant K-rich granites do not appear to have made any contribution to the Archean sedimentary rocks.The majority of the sedimentary rocks have REE patterns indistinguishable from those of recent island arc calc-alkaline rocks, and so could constitute evidence that the Archean crust was principally formed by processes analogous to present day island-arc type volcanism. However, similar REE patterns may be produced by an appropriate mixture of the common bimodal tholeiitic-felsic igneous suite commonly observed in Archean terrains. The REE data presented here do not distinguish between these two models.
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