Alam M., A

Capaccioni, B

Chandrasekharam, D

Manetti, Piero

Santo, Alba Patrizia

Vaselli, Orlando

English

Florence Research

Petrology of the prehistoric lavas and dyke ofthe Barren Island, Andaman Sea, Indian OceanM A Alam∗1, D Chandrasekharam1, O Vaselli2, B Capaccioni3, P Manetti4 andP B Santo31Department of Earth Sciences, Indian Institute of Technology, Bombay, Mumbai 400 076, India2Department of Earth Sciences, University of Florence, Florence 50121, Italy3Institute of Volcanology and Geochemistry, University of Urbino, Urbino 61029, Italy4CNR-Institute of Geosciences and Earth Resources, Pisa 56124, Italy∗e-mail: ayaz@iitb.ac.inAlthough Barren Island (Andaman Sea, Indian Ocean) witnessed several volcanic eruptions duringhistoric times, the eruptions that led to the formation of this volcanic island occurred mainly duringprehistoric times. It is still active and currently in the fumarolic stage. Its volcanic evolution appearsto be characterized by a constructive phase with the piling up of lava flows and scoria depositsand Strombolian activities, followed by a sudden collapse of the main cone. Deposits of a possiblecaldera-forming eruption were not recognized earlier. After a period of peri-calderic hydromagmaticactivity, whose deposits presently mantle inner and outer caldera walls, a new phase of intra-calderic Vulcanian activities took place. A prominent dyke in the SE inner side of the calderawall was recognized. Petrographically the lava flows and dyke are similar but they differ in theirchemical composition (viz., SiO2, MgO, Ni, Cr) significantly. Similarity in major, minor and traceelement composition (viz., K/La, K/Nb, K/Rb, K/Ti ratios) of these rocks together with Chondritenormalized trace element (Rb, Ba, Sr, P, Zr, Ti and Nb) and REE (La, Ce, Nd and Y) patterns of theBarren Island prehistoric lava flows and dyke and low-K lavas of Sunda Arc indicates that BarrenIsland must have evolved from a source similar to that of Sunda Arc lavas during the QuaternaryPeriod.1. IntroductionBarren Island (Andaman Sea, Indian Ocean) haswitnessed several volcanic eruptions during his-toric times: 1994–95, 1991, 1852?, 1803–04, 1795,1789, 1787 (Raina 1987; Haldar 1989; Haldar et al1992ab, 1995; Ravi Shanker et al 2001; Haldar andLuhr 2003). The present morphology of the islandis firstly the result of a huge collapse event affect-ing the central part of a pre-existing volcanic cone.This event probably occurred during the prehis-toric time, viz., late Pleistocene? (Ravi Shankeret al 2001). This prehistoric eruption is representedby the caldera wall, which forms a ridge all aroundthe island. All the later eruptions were confined tothe central part of this caldera depression. Bandy-opadhyay et al (1973) reported that the lava pileof the caldera wall was traversed by dykes, but nodetails about their location, disposition, petrogra-phy and chemistry were provided.A dyke was found by the authors during theirfield investigations in February 2003 (SmithsonianInstitution 2003). The petrography and chemistryof this dyke and the prehistoric lavas, which it hasintruded, are the subjects of this paper.2. Location and geologyBarren Island (12◦16′40′′ N, 93◦31′00′′ E), a smallvolcanic island in the Indian Ocean and a partKeywords. Barren Island; lava flows; dyke; petrology; Sunda Arc.Proc. Indian Acad. Sci. (Earth Planet. Sci.), 113, No. 4, December 2004, pp. 715–721© Printed in India. 715716 M A Alam et alof the Andaman and Nicobar chain of islands, isthe only active volcano in the Indian subcontinentregion. It is located 135 km northeast of Port Blairin east Andaman Sea. It belongs to the NeogeneInner Volcanic Arc (Haldar et al 1992a) extendingfrom the extinct volcanoes like Mt. Popa and Mt.Wuntho of Myanmar in the north to the active vol-canoes of Sumatra and Java in the south (figure 1).The island itself is a result of eruptions during thelate Pleistocene time (Ravi Shanker et al 2001).The elevation of the prehistoric cone was estimatedto be about 1100m, with its base having a diameterof about 12 km, taking into consideration the outerslope of the caldera wall (∼ 32◦) and the presentsubaerial diameter (3.5 km) of the island (Bandy-opadhyay et al 1973; Raina 1987).The northern and north-western parts ofthe caldera have been mantled by a ∼ 50mthick monotonous sequence of breccias and tuffrepresenting syn/post-caldera phreatic and hydro-magmatic activities. While the eruptive activitiesduring and immediately following the caldera for-mation were mostly hydromagmatic and from therim of the caldera itself, the historic and recenteruptions occurred from its centre, producing a250m high polygenetic tuff cone. The calderadepression could have formed either by a giant,non-eruptive lateral landslide of the original coneor by vertical collapse of a surficial magma cham-ber as a result of a paroxysmal eruption of whichwe have no depositional evidence.On the caldera wall the pre-caldera activity isrepresented by six lava flows separated by scoriabeds and minor tuff and cinder deposits. The lavaflows vary in thickness from 2 to 3m, while theother volcanic materials vary in thickness from 1to 4m. These flows occur towards the northernpart of the main caldera, which is devoid of vege-tation. Towards the south-eastern part of the innercaldera a 5m wide, NNE-SSW trending dyke isobserved. It is exposed in the flows of the calderawall at an elevation of about 140m towards thesouthern flank of a talus cone and is located atN12◦16′41.0′′, E93◦51′27.3′′. The outer portion ofthe dyke is weathered and lighter in colour, whereasthe inner portion is fresh and darker in colour. Thedyke is fine to medium grained and contains buffcoloured olivine, green pyroxene and a large num-ber of plagioclase phenocrysts. The dyke is thusbasaltic.3. Petrography and mineral chemistryThe prehistoric lava flows along the calderawall are dark grey to greyish white in colourand at places deep brown. They are largelyphyric with megacrysts and microphenocrysts ofFigure 1. Location of Barren Island on the Neogene InnerVolcanic Arc (modified after Rodolfo 1969).plagioclase, olivine (greenish but dark red orbrown when altered) and green pyroxene in finegrained grey to dark grey groundmass. Micro-scopically they show megacrysts and microphe-nocrysts of highly zoned plagioclase (An69–93%,Ab7–30%, Or0–1%) olivine (Fo66–83%) orthopyrox-ene (En70–99%, Fs0–26%, Wo1–4%) clinopyroxene(En44–46%, Fs9–19%, Wo37–47%). Mineral compo-sitions were determined using Electron ProbeMicroanalysis (EPMA) at University of Florence,Italy.Prehistoric lavas and dyke of the Barren Island 717Table 1. Major and trace element data for Barren Island rocks.Sample BAR3 BAR4 BAR5 BAR13 BAR17 BAR18 BAR19wt%SiO2 48.82 49.63 53.48 50.46 51.12 50.81 52.19Al2O3 16.68 16.68 17.67 18.58 21.29 18.25 20.58FeOt 7.88 7.97 7.07 7.63 7.02 7.39 6.86MgO 10.88 11.04 6.65 7.73 3.89 7.82 4.65CaO 10.71 10.87 9.82 11.31 11.28 10.63 10.32Na2O 2.33 2.41 3.21 2.44 2.95 2.71 3.28K2O 0.27 0.31 0.46 0.26 0.39 0.31 0.51TiO2 0.74 0.75 0.68 0.70 0.81 0.65 0.83MnO 0.15 0.15 0.15 0.15 0.15 0.16 0.14P2O5 0.08 0.08 0.11 0.07 0.10 0.08 0.14LOI 0.01 0.48 0.01 0.45 0.40 0.42 0.32Total 98.55 100.37 99.31 99.78 99.4 99.23 99.82ppmLa 5.0 4.2 6.1 4.8 5.1 4.4 7.9Ce 11.5 11.6 13.4 7.4 12.7 11.9 18.2Nd 7.1 9.8 9.1 6.3 8.7 7.7 10Nb 0.7 0.6 0.8 0.4 0.6 0.5 2.1Zr 51 50 72 49 62 55 77Y 20 20 24 21 23 20 24Sr 203 204 199 177 216 187 250Rb 5.6 5.3 8.8 5.9 10 5.4 11.2Zn 69 65 71 68 70 60 63Cu 70 55 59 73 82 37 58Ni 218 216 76 100 22 95 42Cr 496 508 216 241 36 249 81V 257 263 216 253 286 201 220Ba 71 66 98 61 70 77 107Sc 34 36 33 36 29 29 28FeOt: Total iron as FeO.LOI: Loss in weight on ignition.Olivine grains are traversed by iddingsite, whileplagioclase megacrysts show shattering effect atplaces. These megacrysts and microphenocrystsare embedded in a groundmass of dark colouredglass with microlites of plagioclase and globules ofclinopyroxene and Fe-Ti oxides. Groundmass tex-ture of the lava flows differs from flow to flowand varies from holocrystalline to aphanitic andshows intersertal to fluidal texture. Plagioclasemegacrysts are characterised by multiple twinningand contain abundant small inclusions of glass andgroundmass material zonally arranged or clusteredin the central part of the crystal with the periph-eral part of the grain corroded and surrounded byglass. Olivine megacrysts are mostly corroded andbroken. Melt inclusions grains are often seen in theolivine grains.The prehistoric dyke has a similar mineralogicalcomposition to the flows of the caldera wall, butdiffers in its grain size, the flows having a smallergrain size. Mineralogical composition of the twois similar. The dyke also has a greater amount ofolivine.4. Whole rock chemistryWhole-rock analyses were carried out by using X-ray fluorescence spectrometry (Philips PW 1480spectrometer equipped with an Rh-anode X-ray718 M A Alam et alFigure 2. Total Alkali vs. Silica (TAS) diagram (Le Bas et al 1986).tube) at the Department of Earth Sciences, Uni-versity of Edinburgh, UK. The instrument wascalibrated using international reference samples(Govindaraju 1994). Major elements were deter-mined on fused glass discs (Norrish and Hutton1969) with corrections applied for inter-elementmass absorption effects. Trace elements were deter-mined on pressed powder discs and count-rateswere corrected for line-overlap and mass absorp-tion effects. Description of the technique, analyti-cal conditions and precision estimates are given inFitton et al (1998).The prehistoric lava flows (samples BAR5,BAR13, BAR17, BAR18 and BAR19) and dyke(samples BAR3 and BAR4) have similar major ele-ment composition (table 1), but differ in their MgOcontent significantly. MgO content of the dyke is∼ 11wt%, whereas it is < wt7% in the flow. Thehigh content of MgO in the dyke is not duplicatedin any of the flows. They also differ in their SiO2,total iron as FeO, CaO contents as well, but to alesser extent. Weight loss on ignition (LOI) valuesfor the samples provides a rough idea of the subaer-ial alteration suffered by the rocks. Very low LOIvalues for both flows and dyke rocks (< 0.5wt%)show that these rocks are not much altered andhence the possibility of redistribution of the mobileelements is very small.The composition of the dyke (samples BAR3and BAR4) is basalt and the lava flows are basalt(samples BAR13, BAR17 and BAR18) and basalticandesite (samples BAR5 and BAR19) as shown inthe Total Alkali vs. Silica (TAS) diagram (Le Baset al 1986) (figure 2). However from the AFM dia-gram (figure 3) for both the dyke (samples BAR3and BAR4) and flows (samples BAR5, BAR13,BAR17, BAR18, and BAR19) tholeiitic trend isevident. Trace element compositions (table 1) ofthe flows (BAR5, BAR13, BAR17, BAR18 andBAR19) and the dyke (BAR3 and BAR4) are sim-Figure 3. Barren Island samples on the AFM diagram(after Irvine and Baragar 1971)ilar (viz., concentration of La, Ce, Nd, Nb, Y, Sr,Rb, Ba, Zn, Cu and Sc). In the dyke, concentra-tions of Ni and Cr are more than twice those in theflows.5. Discussion and conclusionsThe high MgO content of the dyke is not duplicatedin any of the flows of the caldera wall. High MgOtogether with Ni and Cr contents of the dyke maysuggest its deeper mantle origin a different degreeof evolution at a higher crustal level or an olivinecumulate.The most significant feature of these rocks, bothdyke and flows, is their similarity in major, minorand trace element chemistry and K/La, K/Nb,K/Rb, K/Ti ratios (figure 4) to the QuaternaryPrehistoric lavas and dyke of the Barren Island 719Figure 4. (Continued)720 M A Alam et alFigure 4. Discriminatory diagrams (K/La, K/Nb, K/Rb, K/Ti) for the rocks of Barren Island (dotted fields are those oflow-K Sunda Arc lavas, Wheller et al 1987).Figure 5. Spider diagram of the trace elements normalised to C1-Chondrite (Sun and McDonough 1989) for the prehistoriclava flows (BAR5, BAR13, BAR17, BAR18 and BAR19) and dyke (BAR3 and BAR4) of Barren Island and low K lavaflows of Sunda Arc (SLK1, SLK2, SLK3 and SLK4; Wheller et al 1987).low-K volcanic rocks of Sunda Arc, Indonesia. C1-Chondrite (Sun and McDonough 1989) normalisedtrace element and REE pattern (figure 5) of theBarren Island flows and Sunda Arc low-K lavas(Wheller et al 1987) are closely similar. Such astriking similarity between prehistoric flows anddyke of Barren Island and low K lavas of SundaArc indicates that Barren Island lavas may haveevolved from a source similar to that of the SundaArc lavas during the Quaternary period and is stillactive. Barren Island volcano is currently in thefumarolic stage (Smithsonian Institution 2003).Prehistoric lavas and dyke of the Barren Island 721AcknowledgementThe authors thank the Indian Coast Guard forextending support during the fieldwork. M A Alamthanks the Head, Department of Earth Sciencesand the Director, Indian Institute of TechnologyBombay, India for providing facilities to carryout this work. Authors thank Dr Robert Hall,Dr Angelo Peccerillo, Dr Francoise Chalot-Pratand Dr James F Luhr for critical reviews of themanuscript.ReferencesBandyopadhyay S, Subramanyam M R and Sharma P N1973 The geology and mineral resources of Andaman andNicobar Islands; Rec. Geol. Surv. India 105(2) 25–68Fitton J G, Saunders A D, Larsen L M, Hardason B S,Norry M J 1998 Volcanic rocks from the Southeast Green-land margin at 63◦N: composition, petrogenesis and man-tle sources. In: Proceedings of the ODP: Scientific Results(eds) A D Saunders, H C Larsen, S H Wise, College Sta-tion, TX (Ocean Drilling Program) 152 331–350Govindaraju K 1994 Compilation of working values andsample description for 383 geostandards; GeostandardsNewsletter 18 1–158Haldar D 1989 Petrology and chemistry of recent volcanicsof Barren and Narcondam islands – the only two recentvolcanoes in India; Rec. Geol. Surv. India 122(3) 48–49Haldar D and Luhr J F 2003 The Barren Island volcanismduring 1991 and 1994–95: eruptive style and lava petrol-ogy; Mem. Geol. Soc. India 52 313–338Haldar D, Chakraborty S C and Chakraborty P P 1995The 1995-eruption of the Barren Island volcano in theAndaman Sea; Rec. Geol. Surv. India 129(3) 59–61Haldar D, Lasker T, Bandyopadhyay P C, Sarkar N K andBiswas J K 1992a The volcanic eruption of the BarrenIsland volcano, Andaman Sea; J. Geol. Soc. India 39(5)411–419Haldar D, Lasker T, Bandyopadhyay P C, Sarkar N K andBiswas J K 1992b A note on the recent eruption of theBarren Island Volcano; Ind. Minerals 46(1) 77–88Irvine T N and Baragar W R A 1971 A guide to the chem-ical classification of the common volcanic rocks; Can.J. Earth. Sci. 8 523–548Le Bas M J, Le Maitre R W, Streckeisen A and Zanettin B1986 A chemical classification of volcanic rocks based onthe total alkali-silica diagram; J. Petrol. 27 745–750Norrish K and Hutton J T 1969 An accurate X-ray spec-trographic method for the analysis of a wide range ofgeological samples; Geochim. Cosmochim. Acta 33 431–453Raina V K 1987 A note on sulphur occurrence in the volca-noes of the Bay of Bengal, Ind. Minerals, 41(3) 79–86Ravi Shanker, Haldar D, Absar A and Chakraborty S C 2001Pictorial monograph of Barren Island volcano: the loneactive volcano in the Indian Subcontinent; Geol. Surv.India Sp. Pub. 67 88 pRodolfo K S 1969 Bathymetry and marine geology of theAndaman Basin and tectonic implications for SoutheastAsia; Geol. Soc. Am. Bull. 80 1203–1230Smithsonian Institution 2003 Barren Island: Fumarolicactivity noted during fieldwork in February 2003; Bull.Glob.Volc. Net. 23 12–14Sun S S and McDonough W F 1989 Chemical and isotopicsystematics of oceanic basalts: implications for mantlecomposition and processes; In: Magmatism in the OceanBasins (eds) A D Saunders and M J Norry Geol. Soc.London Sp. Pub. 42 313–345Wheller G E, Varne R, Foden J D and Abbott M J 1987 Geo-chemistry of Quaternary volcanism in the Sunda-BandaArc, Indonesia and three component genesis of island arcbasaltic magmas; J. Volcanol. Geotherm. Res. 32 137–160

Petrology of the Prehistoric Lavas and Dyke of Barren Island, Andaman Sea, Indian Ocean.

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Petrology of the Prehistoric Lavas and Dyke of Barren Island, Andaman Sea, Indian Ocean.

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