Geochemical Characteristics of Sunda Volcanic Arc in Sumatra and Andaman

DOI: 10.17014/ijog.6.1.1-16 Geochemical characteristics of Sunda volcanic belt are recognized from each characteristic of Weh Island, Tabuan Island in Semangko Bay, South Sumatra, and Andaman Islands. Trace and rare earth elements (REE) are produced by fumaroles in a marine environment of submarine volcano of Weh Island characterized by barium (Ba) as an indicator of sea water influence in the mineralization process, while sulphide minerals do not occur in this area. REE pattern compared to Mid Oceanic Ridge Basalt (MORB) shows a characteristic of subduction tectonics and is distributed in shallow coastal water of high energy. Based on comparison of REE contents in all samples, it reveals that volcanism process causes REE enrichments either in the past or in recent. Geochemical characteristics of Tabuan Island in Semangko Bay reveal the occurrence of hydrothermal mineralization followed by pervasive occurrences of sulphide minerals in vein-type disseminations enriched in Au, Ag, Zn, Pb, Cu, As, Sb, Ba, and Mn. Geochemical characteristics of Andaman Islands reveal imprint of substantial subduction component in the form of sediment fluid and melt and fluid-induced subduction component derived from altered oceanic crust.


Background
Sunda volcanic arc is located in the western coastal zone of Sumatra, to the north between Weh and Andaman Islands (Figure 1).The intense earthquake area is a transition tectonic zone between Sumatran active transform fault in the south and Andaman back -arc spreading in the north (Curray et al., 1979;Curray, 2005).
The Sunda volcanic arc of Sumatra is characterized by a chain of volcanoes formed above the subducting plate of Indo -Australian oceanic Plate below the Eurasia continental Plate (Bowin et al., 1980).This volcanic arc is parallel to an oceanic trench located in the west of Sumatra and marked an active convergent boundary.The volcanic arc was formed due to the oceanic plate saturated with water and volatiles are drastically lowering the melting point of mantle as it is further subducted.Greater pressures with increasing depth cause water to squeeze out of the plate and introduce it to the mantle and melts to form magma.The magma ascends to the surface to form an arc of volcanoes parallel to the subduction zone.The Sunda Arc is a classic example of a volcanic island arc, in which all elements of geodynamic features can be identified.In Sumatra, the volcanic arc forms the topographic summit, especially in the western coastal zone.The Sunda Arc is also the home for some of the world most dangerous and explosive volcanoes.One example was the Toba super -eruption in North Sumatra which the resulting caldera that has become Lake Toba today.
Volcanic activity periods of Sunda belt of Sumatra dated back to Jurassic to Early Cretaceous period (203 -130 Ma) based on dating of K/Ar mineral ages from the Barisan Mountains of southern Sumatra (McCourt et al., 1996).Much older period of volcanic activity was also recognized in the Permian (287 -256 Ma) from exposed plutons in western Sumatra.The periods continued to Mid -Late Cretaceous (117 -80 Ma), Early Eocene (60 -50 Ma), and Miocene -Pliocene (20 -5 Ma).On the other hand, a periodic volcanic activity in northern Sumatra had been started in the Late Eocene (Cameron et al., 1980).Three volcanic activities are recognized from three Tertiary volcanic rock supergroups which are marked by an unconformity in the Late Oligocene and a tectonic event in the Middle Miocene.The later volcanic event occurred in Pleistocene and in conjunction with sea -floor spreading commencement in the Andaman Sea, the rise of Barisan Mountains, and the growth of Sumatran Fault System.
Chemical characteristics of volcanoes around the arc systems could be correlated directly to their tectonic environment (Hutchinson, 1981).One factor that influences geochemical charac-

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Geochemical Characteristics of Sunda Volcanic Arc in Sumatra and Andaman (H.Kurnio) teristics of the arc is the depth to the underlying Benioff Zone and whether continental or oceanic crust that constructs the volcanoes.The depth to the underlying Benioff Zone beneath the volcanoes is located between 165 km and 190 km.The volcanic rocks vary in composition from calc -alkaline to highly potassic.Based on the very low Ni concentrations and low Mg/ Mg+Fe composition of andesite, Foden and Varne (1981) suggested that this volcanic rock was derived from mantle and have probably been modified by fractional crystallization processes.
They further proposed that the calc-alkaline suite probably originated by partial melting of the peridotite mantle -wedge overlying the active Benioff Zone.

Samples and Methodology
Data used are geochemical data consisting of major, trace, and rare earth elements of volcanic rocks obtained from the Semangko Bay, Weh Island, and Andaman Islands.The first two locations are the places where the author was involved in the surface rock samplings in the field as well as in its laboratory analyses, while Andaman data are secondary derived from Ray et al. (2012).Semangko Bay data were geochemically analyzed in BGR, Germany, during the programme of visiting researcher in 2004 sponsored by DAAD, and Weh Island samples were analyzed in PT Intertek Jakarta.The method used for analyses were ICPMS (Inductively Couple Plasma Mass Spectrometry) which could simultaneously determine major, trace, and rare elements of volcanic rocks.Eighteen samples were used for this study.
Methods of the study on geochemical characteristics of Sunda volcanic belt in Sumatra and Andaman are the application of diagrams such as AFM plot and multi -element spider diagrams to identify its magma origin and tectonic setting.Geochemical data used are from Kurnio et al. (2005Kurnio et al. ( , 2008)), Ray et al. (2012), and Kurnio et al. (2016), while rocks analyzed are lava and pyroclastic of basaltic -andesitic composition.

Weh Island, Aceh
Weh Island is the administrative area of Sabang City, Aceh Province, located at the westernmost of the Indonesian Territory.A zero kilometre monument was built in this island to remind its westernmost position.From Banda Aceh, the capital city of Aceh Province, it takes about one hour to reach this island using a speed ferry.
Selected seafloor samples closed to active fumaroles in Weh Island submarine volcano (Figure 2) collected either by grab sampling from the boat or directly by divers exposed plenty of trace and rare earth elements (Kurnio et al., 2016).On the other hand, examinations of main sulphide elements using statistical method, Pearson correlation coefficient (r) (Rollinson, 1995), between Fe, Zn, Ni, and S showed negative values.Pearson correlation method is a measure of strength of the association between two variables.Variables used here are element contents of trace and rare earth elements as previously mentioned.Negative statistical results interpret that sulphide minerals do not occur in Weh Island.Mineralization in Weh Island is also influenced by sea water, as shown by r high values (0.543213 -0.8638) between Pb, Zn, Ni, and Ba (Kurnio et al., 2016).Ba is an indicator of mineralization process in a marine environment (Durbar et al., 2014).Contents of trace element are shown in Table 1.
Trace element contents from seafloor rock samples (Table 2) show positive and negative correlations with the sea depth as seen in Figure 3.The negative correlations of Fe, Cr, Ti, and La are possibly due to less marine energy influence on its deposition, and they could only be deposited at high energy environment of shallow marine that is close to the coastal zone.On the other hand, positive correlations are shown in contents of Cu, Ba, Li, and La.According to Dehairs (1980), Danielsson (1980), andTill et al. (2017)

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of Sc -Th -La (Cingolani et al., 2003).The plotting diagram of Weh Island seafloor sediments falls into continental -and oceanic -island -arcs (Figure 5).This distribution is possibly correct as the studied area was regionally formed by the interaction between oceanic and continental plates of Indo -Australia and Eurasia.But the interpretation should thoughtfully be reached, because certain tectonic settings do not automatically generate rocks with unique geochemical signs (McLennan et al., 1990;Bahlburg, 1998).
The contents of light rare earth elements (LREE) of Weh Island are higher than the standard of Mid Oceanic Ridge Basalt (MORB) (Figure 4), while heavy rare earth elements (HREE) are lower than the MORB.These Weh Island geochemical characteristics resemble the subduction pattern as revealed from the figure.All analyses for REE provenance mentioned were done on andesitic lava and andesitic fragments which are close to seafloor fumaroles.
Seafloor sediment of the studied area was determined by its provenance using triangle diagram Sc -Th -La (Cingolani et al., 2003).Plotting of the three elements from seafloor sediments belong to continental and oceanic -island -arcs (Figure 5).The evidence is shown by a progressive decrease in total Fe as Fe 2 O 3 + MgO, TiO 2 , Al 2 O 3 /SiO 2 , and a decrease in K 2 O/Na 2 O and Al 2 O 3 /(CaO + Na 2 O) (in sediments as the tectonic setting changes from oceanic -island -arc to continental -island -arc (Tables 3 and 4) (Bhatia, 1981).Megascopically, plotted sediment on continental -island -arc is characterized by gravel -size andesite rock, while sediment plotted on oceanic -island -arc is sand rich in mafic minerals of magnetite.
Sediments on seafloor of Weh coastal water are determined by its provenance using diagram  explains that volcanism process causes REE enrichment either in the past (unaltered lava) and in recently (sediments closed to seafloor fumaroles).
In a volcanism process, REE mobility is significantly influenced by hydrothermal processes; but during hydrothermal alteration they are immobile.REE in the geothermal region are strongly influenced by the hydrothermal mobility formed by sulfuric acid fluids that may be found at the top of a volcano or around caldera rings.This is a place of groundwater mixing with magmatic gas rising to the surface (Lintjewas and Setiawan, 2018).On the other hand, REE primary deposits are associated with igneous rocks relatively rich   (Bhatia and Crook, 1986).5).Imprints of REE deposition were observed surrounding the active seafloor crater rim (Kurnio et al., 2016).

IDENT
Seafloor sediment samples of Sc -Th -La diagram above which falls into continentalisland -arc, megascopically are characterized by the existence of andesitic -cobble -size rock fragments.On the other hand, oceanicisland -arc samples are described enriched in  mafic minerals of magnetite, magnesium, and titanium.
Ternary diagram of Al 2 O 3 -MgO -FeO shows geochemical characteristics of Weh Island, North Sumatra, Narcondam and Barren -Andaman and Semangko Bay -South Sumatra (Figure 9).They mostly fall into a spreading centre island, except Barren volcano which is included into an orogenic tectonic regime.The tectonic regime of spreading in the centre of the island is in accordance with Sunda volcanic belt that changes from subducting related tectonism in Sumatra to back -arc spreading in Andaman Sea in the north (Ray et al., 2012; Figure 10).Orogenic tectonic of Barren Island could possibly be related to Himalaya orogenic belt in Indian Micro -continent in the north.Semangko, Southern Sumatra Geochemical characteristics of Tabuan Island in Semangko Bay, Southern Sumatra, were first studied by Kurnio et al. (2008) in Federal Institute for Geoscience and Natural Resources (BGR), Germany.The geochemical characteristics reveal the occurrence of hydrothermal mineralization which was suggested from seismic identification of small intrusive bodies which form elongated northwest -southeast ridges passing through the island.The mineralization was followed by pervasive occurrences of sulphide minerals in vein -type disseminations.Enrichments in Au, Ag, Zn, Pb, Cu, As, Sb, Ba, and Mn follow the mineralization.Great potential for epithermal type Au -Ag and metal deposits occurs in Semangko Bay area, as revealed by the association of subaerial island -arc volcanism and subvolcanic intrusive bodies, the regional extensional and strike -slip structural regime, and the occurrence of epithermal -style alteration and mineralization in the same volcanic sequence along the coastal zone (Kurnio et al., 2008).

Andaman
Geochemical characteristics of Andaman Islands reveal imprint of substantial subduction component in the form of sediment fluid (Figure 11) and melt and fluid -induced subduction component derived from altered oceanic crust (Ray et al., 2012).The characteristics were derived from the trace element ratios of arc lavas which show high Ba/La, Ba/Nb, Th/Nd, and relatively high Ba/Th ratios.REE contents of Andaman and Weh volcanic rocks are listed in Table 6.Spider diagram (Figure 12) is used to examine the changing of tectonic regime from subduction in Sumatra to back -arc spreading in Andaman.The figure clearly shows that andesitic volcanic rocks of Andaman (diamond symbol) and Weh (square symbol) belong to subduction tectonic regime as demonstrated by its diagram pattern.On the other hand, Andaman basalt (triangle symbol in Figure 12) displays REE spider diagram resemble Mid Oceanic Ridge Basalt (MORB); an evidence that tectonic regime of Andaman is back -arc spreading as mentioned earlier.

Discussion
The role of H 2 O in the dehydration and hydration reactions of down going lithosphere and the overlying mantle wedge is very important in the production of magmas at convergent plate boundaries such as Sunda subduction system where geochemical characteristics of its volcanic belt is being studied.The subduction causes liberation of H 2 O from the slab (Tatsumi, 1989).
The slab -derived H 2 O reacts with the forearc mantle wedge in the crystallization process of hydrous minerals further dragged downward to higher PT regions and is released to shallower magma source in the mantle wedge.Those hydrous minerals decompose beneath the fore -arc region (Tatsumi, 1989).Geochemical characteristics of arc magmas which include subduction components large ion lithophile elements (LILE) are governed by the migration of H 2 O through the above processes.
The LILE (Figure 13) as its abbreviation stands -Large Ion Lithophile Elements -are  indeed larger than other cations.They are lithophile in the sense that they are incompatible and usually end up with enrichment in the crust (also lithosphere).Rare Earth Elements have been considered as LILE.The LILE are fluid -mobile and hydrothermal alteration that may change their contents in the studied rock.Thus, the LILE could be used for the study of alteration processes.If fresh rocks and anomalies in the LIL systematics are found, hydrothermal processes can be learnt which occurr in the mantle that would not otherwise be able to see.The following Table 7 demonstrates some LILE occur in andesitic volcanic rocks of Andaman, Weh, and Semangko.Along the Sunda volcanic belt, there is a change of tectonic regime of volcanoes from subduction related and strike -slip -fault from Sumatra to back -arc spreading in Andaman Sea in the north.Andaman also shows an orogenic tectonic regime that could possibly be related to the Himalaya orogenic belt in Asia mainland.
The area between North Sumatra and Andaman Islands shows no volcanism activities, instead it is replaced by active seismicity that would be an interesting subject to be studied further.

Conclusions
The characteristics of Sunda volcanic belt in Sumatra are revealed through each volcano discussed.In submarine volcano of Weh Island, trace and rare earth elements (REE) are produced by seafloor fumaroles, while sulphide minerals do not occur; and mineralization in marine environment is shown by the existence of barium (Ba).
REE are more distributed in shallow coastal water of high marine energy than at a deeper sea.The pattern of REE compared to Mid Oceanic Ridge Basalt (MORB), higher in light REE but low lower in heavy REE, shows the characteristic of subduction tectonic.The comparison of rare earth element contents from seafloor sediments, altered andesitic lava, tuff, unaltered lava and sediments closed to seafloor fumaroles demonstrates that the volcanism process causes REE enrichment either in the past or recent.

Figure 1 .
Figure 1.Sunda volcanic arc Sumatra to Andaman (source: google earth) and locations of geochemical data used.No volcanic activities between Weh and Andaman Islands.

Figure 2 .
Figure 2. Sample location map of Weh Island used in this study.

Figure 3 .
Figure3.Correlation between trace elements and sea depths which shows some increased (Cu, Ba, Li, and La) and decreased (Fe, Cr, Ti, and Li) contents in deeper seas.Blue symbols in Table2represent sea floor samples.

Figure 5 .
Figure5.Seafloor sediment provenance of Weh Island is distributed among the continental and oceanic-island-arc(Bhatia and Crook, 1986).

Figure 10 .
Figure10.Major geological and tectonic features of the Indian Ocean and southeastern Asia(Ray et al., 2012).

Figure 11 .Figure 12 .
Figure11.Indication of induced sediment fluid from Indian Ocean is shown in the dark grey area(Ray et al., 2012).AOC: Altered Ocean Crust, N-MORB: Normal Mid Oceanic Ridge Basalt.

Figure 13 .
Figure13.Large ion lithophile elements are characterized by large ionic radius and small cationic charge.

Table 1 .
Trace Elements from Seafloor Samples

Table 2 .
Selected Samples of Seafloor Trace Element Contents

Table 3 .
Major Elements of Some Seafloor Sediments Used inBhatia and Crook Diagram (Figure 5)

Table 4 .
Progressive Decrease of Fe 2 O 3 + MgO, TiO 2 , Al 2 O 3 /SiO 2 , and Increase of K 2 O/Na 2 O and Al 2 O 3 /(CaO + Na 2 O) from Oceanic-Island-Arc to Continental-Island-Arc in Light -REE (LREE).The LREE consist of Sc, La, Ce, Pr, Nd, Pm, Sm, and Eu; also known as the cerium group.While heavy rare earth elements (HREE) comprise Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and La, which belong to the yttrium group.Examination and comparison of LREE and HREE of samples of Unaltered Lava (UL) and sediments are close to seafloor fumaroles (SSF) reveal more abundances of LREE in the latter sediments than the previous rocks, while HREE show almost the same contents from both (Table