Compositional Variation in Magma through Early Neogene in the ...

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Abstract: We report unusual occurrence of glass shards with diverse morphologies and compositions in the volcanic ash associated with the early Neogene ...
JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.84, August 2014, pp.181-186

Compositional Variation in Magma through Early Neogene in the Northeast Indian Ocean: A Testimony from Glass Shards J. P. SHRIVASTAVA1* and V. SHARMA2

1

Department of Geology, University of Delhi, Delhi – 110 007 2 25, Aarohi Residency, Bopal, Ahmedabad - 380 058 *Email: [email protected]

Abstract: We report unusual occurrence of glass shards with diverse morphologies and compositions in the volcanic ash associated with the early Neogene marine stratigraphic succession (early Miocene to early middle Miocene) of Andaman-Nicobar Islands, Northeast Indian Ocean. These small, ash-size (200 to 800 µm) broken pieces of glass shards when viewed under Scanning Electron Microscope (SEM), represent distinctive - platy, sickle, bicuspate, concentric, angular, horn shape and slivers with broken angular bubble wall - morphologies. Glass shards are colourless. But, a few are grey or reddish-brown, indicate high Fe content. Chilled, juvenile, angular and blocky shards show fragments of highly viscous, silicic magma. Spindle and ribbon-shape shards form from a low viscosity basalt and rhyolite. Electron Probe Micro Analyzer (EPMA) was used to measure low concentration variations of major oxides within individual amorphous silicate solid glass shards whose disordered atomic structure is that of a liquid derived from a silicate melt. Major elemental chemistry of early Miocene glass shards from Colebrook island show low silica, alkalis, high FeO(T) MgO and CaO, whereas, early middle Miocene glass shards from Inglis island show high silica, alkalis, low FeO(T), MgO and CaO contents. These data-sets when plotted on ternary Total Alkali-Silica and Na2O+K2O-MgO-FeO (T) diagrams show that their data plots lie within the basaltic-andesite, tephri-phonolite, rhyolite and trachyte fields. These glass shards which were present in the provenance, formed by explosive eruption of lavas, ranging in composition from basalt to rhyolite with andesite/ basalt-andesite being the most common magma types erupted sub-areally, implying island arc type of tectono-magmatic setting for the formation of these lavas. However, more evolutionary variant rhyolite was most likely formed by crystal fractionation. Keywords: Glass shards, EPMA, Early Neogene, Miocene, Andaman-Nicobar Islands INTRODUCTION

Miocene marine sedimentary strata containing glass shards are found distributed widely in the Andaman-Nicobar islands in the Northeast Indian Ocean (Fig. 1a). The islands form two main arcs, an outer arc comprising major islands exposing Paleogene and older rocks and the inner volcanic arc lying east of the outer arc having two sub-aerial volcanoes. The forearc basin lying between the outer and the inner volcanic arcs is represented by islands extending from north to south and consisting mainly of marine Neogene rocks. The best developments of Neogene strata are observed on the Ritchie’s archipelago and the adjoining islands (Fig.1b), where an almost complete sequence of Neogene sediments is exposed. The volcanic arc in Andaman-Nicobar consists of two volcanoes, viz., Barren and Narcondam islands and several seamounts, like Alcock Rise and Sewell rise. Located 135 km east-northeast of Port Blair, Barren island volcano has been active in the recent past, and the

most recent eruptions took place in 1994-95. Narcondam, lying about 140 Km. north-northeast of Barren island, is possibly an extinct volcano. The lava of Barren island is represented by basalt-andesite suite, while that of Narcondam is andesite-dacite suite (Dasgupta and Mukhopadhyay, 1997) and both the volcanoes are of Pleistocene-subrecent age. In the neighbourhood of Andaman-Nicobar, volcanic activity is known in the region of Myanmar and Sumatra during the Oligocene and Miocene (Rodolfo, 1969). According to Itoh and Yaguchi (1994), volcanic activity in Myanmar reached its climax in the Miocene-Pleistocene, evidences of which they found in the form of lava flows on the surface as well as in the sub-surface sections. While the volcanic activity ceased in Myanmar in the Pleistocene, it is still continuing in Indonesia. Occasional pyroclastics in the pre-Neogene sediments are reported from the Paleogene sediments of AndamanNicobar islands (Acharyya, 1998; Bandopadhyay and

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J. P. SHRIVASTAVA AND V. SHARMA

Fig. 1 Location of Andaman-Nicobar in the Indian Ocean (a), the islands from which the samples have been collected (b) and the stratigraphic position of samples (marked with an asterisk) used for chemical analyses (c).

Ghosh, 1998). In the Neogene, volcanic ash-containing beds of Miocene age are widely distributed on these islands. The ash bearing sediments are mainly found in the early Miocene sequences, but a few records are from the early middle Miocene (Sharma and Srinivasan, 2007). Presence of volcanic ash in these marine sediments indicates magmatic activity in the region during the early Miocene and early middle Miocene which was in the form of explosive

volcanism in the region adjoining Andaman-Nicobar about 21-15 Ma (Srinivasan, 1980). Winds transported the volcanic ash particles to the site of deposition from the area of volcanic activity in the form of ash showers. As the area has been experiencing drastic rhyolitic and andesitic volcanism from the late Oligocene time, the possible source of volcanic ash in this region (during early Miocene-early middle Miocene) could be the volcanoes of the Indonesian region JOUR.GEOL.SOC.INDIA, VOL.84, AUGUST 2014

COMPOSITIONAL VARIATION IN MAGMA THROUGH EARLY NEOGENE IN THE NORTHEAST INDIAN OCEAN

183

Table 1. Biostratigraphic position and age (Sharma and Srinivasan, 2007) of the samples used in the study. Samples

Planktic Foraminiferal Zones

Epoch and Age range

ING 38 (Southwest coast section, Inglis Island) ING 02 (Southwest coast section, Inglis Island) CB 18 (Piu Bay section, Colebrook Island) CB 1 (Piu Bay section, Colebrook Island)

Praeorbulina glomerosa Globigerinatelle insueta Catapsydrax stainforthi Catapsydrax dissimilis

Early Middle Miocene (16.1 – 15.1 Ma) Early Miocene (17.3 – 16.1 Ma) Early Miocene (18.8 – 17.3 Ma) Early Miocene (21.5 – 18.8 Ma)

in the southeast. It, however, is of interest to note that the Burma central volcanic arc was also active during the same period and is characterized by the eruptions of rhyolite, calcalkaline, andesite and olivine basalt (Maung Thein, 1973). Volcanic phenomenon is commonly associated with islandarc, but, in most of the young arcs, plutonic equivalent of volcanic rocks are not exposed for direct study. Occurrences of early Miocene andesites are reported in some exploratory wells in the Irrawaddy delta basin (Yaguchi, 1981; cited from Itoh and Yaguchi, 1994; Chit Saing, 1991). No Miocene lava flows are, however, reported as yet in AndamanNicobar. Shards are ash-size broken pieces of glass representing amorphous silicate solid whose disordered atomic structure is that of a liquid formed from a silicate melt (Best and Christiansen, 2001). In this paper, we present Electron Probe Micro Analysis (EPMA) to understand chemical compositions of the magma which gave rise to glass shards in the early Neogene stratigraphic sequence of Andamn Nicobar. MATERIAL AND METHODS

For chemical analyses, glass shards were picked from two palaeontologically dated stratigraphic sections viz., Piu Bay section and southwest coast section, which contain a variety of microfossils useful for precise dating. The Piu Bay section located on the Colebrook island (Fig.1b and c)

consists mainly of nanno-foram chalk with glass shards and a bed of conglomerate near the base and limestones in the upper part of the section. The section at certain levels contains thin bands comprising almost wholly of ash. The southwest coast section at Inglis island (Fig.1b and c) is composed of alternate bands of nanno-foram chalk containing glass shards and siltstones. Study of planktic foraminifera of these sections place them in the interval spanning from Catapsydrax dissimilis Zone to Praeorbulina glomerosa zone (Early Miocene to early middle Miocene), between about 21 Ma to 15 Ma (Sharma and Srinivasan, 2007). The southwest coast section of Inglis island is younger than the Piu Bay section (Fig.1c). Glass shards in many samples show alteration to other minerals or their vesicles filled with altered minerals or sediments. Four samples (CB1, CB18, ING02 and ING38) containing clean, uncontaminated shards were chosen for the present study and were analyzed using EPMA. For EPMA study, shards were mounted on brass studs and coated with carbon. Back Scattered Electron (BSE) imaging was performed to study glass shards and selected slides were analyzed by EPMA (CAMECA, SX100 Model at Indian Bureau of Mines, Nagpur) in point and window modes using acceleration voltage of 15kV and a beam diameter of 1µm with a live counting time of 20 seconds. Qualitative elemental scanning was performed to understand distribution of various elements present in the glass shards. International standards for Si &

Table 2 . Major oxide (in wt %) data for glass shards of Piu Bay and Southwest Coast sections of the Colebrook and Inglis islands, respectively Samples / Glass shards

CB1

CB18

ING02

ING38

1

2

3

4

5

7

8

9

10

11

49.54 1.20 15.43 10.49 0.22 11.03 9.55 2.23 0.42 0.05 100.16

49.44 1.00 15.13 10.50 0.50 11.28 9.15 2.01 1.00 0.50 100.51

49.08 1.00 15.21 11.32 0.00 10.48 10.01 2.80 0.24 0.00 100.15

48.99 1.00 14.00 10.28 0.51 11.73 10.04 2.91 0.13 0.50 100.14

49.02 0.12 15.25 11.58 0.50 11.31 10.06 2.00 0.00 0.55 100.39

70.95 0.70 14.00 2.09 0.00 1.18 2.33 4.23 5.24 0.00 100.09

70.58 1.55 12.85 2.53 0.50 1.43 2.06 3.70 4.49 0.50 100.19

70.90 1.53 13.70 1.34 0.50 1.26 2.00 4.11 4.34 0.50 100.18

66.66 0.20 17.84 2.48 0.00 1.10 2.25 3.00 7.83 0.00 100.19

72.74 0.50 13.56 1.42 0.00 0.30 0.19 4.86 7.31 0.20 100.19

Oxides (wt %) SiO2 TiO2 Al2O3 FeO(t) MnO MgO CaO Na2O K2O P2O5 Total

CB1, CB18, ING02 and ING38 refer to samples from Colebrook and Inglis Island sections, whereas 1 to 11 represents analyses of glass shards JOUR.GEOL.SOC.INDIA, VOL.84, AUGUST 2014

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J. P. SHRIVASTAVA AND V. SHARMA

Na (albite on TAP crystal), Al (corundum on TAP crystal), Mg (periclase on TAP crystal), Ca (andradite on TAP crystal), K (orthoclase on PET crystal), Ti (MnTiO3 on LIF crystal), Ni (NiO on LIF crystal), Cr (Cr2O3 on LIF crystal) and Fe (Haematite on LIF crystal) were used. Glass shard specimens were analyzed along with the standards (obtained from BRGM, France), where precision obtained for the analyses is better than 2% for SiO2, TiO2, Al2O3, MgO, FeO and CaO; 5% for MnO, Na2O; and 8% for K2O P2O5, Cr2O3 and NiO. For calibration, Cu Kα X-ray emission lines were used for all the elements analyzed and ZAF data-reduction programme was applied to obtain oxide concentrations (Table 2). Details pertaining to analytical procedures, precision and accuracy of measurements are available elsewhere (Gill, 1997; Pandey, et al., 2008).

RESULTS AND DISCUSSION

BSE images (Fig. 2) of the shards when viewed under the SEM revealed presence of (a) vesicular (b) non-vesicular and (c) miscellaneous glass shards, most of them falling within the size range from 200 to 800 µm, the former two types of shards show a high electron beam reflectance in the BSE images, hence appear bright. The glass shards are of various shapes - platy (Fig 2a), sickle (Fig. 2b), bicuspate, crescentic (Fig. 2c) and horn (Fig. 2d) shapes are common. Many of these show alteration to zeolite. Several shards are angular (Fig. 2c, e, f and g) having sharp edges. A few shards show concentric layers (Fig. 2c). Fine ‘Y’ shaped, arc-like slivers (Figs. 2e) represent broken bubble walls (Fig. 2f). These glass shards show fragmentation of a highly vesiculated viscous silicic melt (Fig. 2g). Spindle or ribbon-

a

b

c

d

e

f

g

h

i

j

Fig.2. SEM-BSE images of vesicular and non-vesicular glass shards of (a) platy, (b) sickle, (c) bicuspate, concentric angular with sharp edges and (d) horn shape. Slivers (e) represent broken angular bubble wall (f) of a shard. Spindle shape shards (g) show fragmentation of a highly vesiculated viscous silicic melt. (h) Glass shard showing hallow gas channels in the form of capillary tubes. (i) Bubble within a cuspate shards and (j) Pumiceous, tiny, glass shards with vesicles commonly seen in these two sections. JOUR.GEOL.SOC.INDIA, VOL.84, AUGUST 2014

COMPOSITIONAL VARIATION IN MAGMA THROUGH EARLY NEOGENE IN THE NORTHEAST INDIAN OCEAN

shaped (Fig. 2h) glass shards observed in the sections are similar to those associated with low viscosity basaltic and per-alkaline rhyolite volcanics. Smooth teardrops and aerodynamically streamlined shapes as basaltic Pele’s tears and hair are seen in smaller-sized shards. Hollow gas channels in the form of capillary tubes (Fig.2i) are also noticed. Most of the shards are colourless in thin sections, but, a few show greyish-black or reddish-brown and are indicative of high Fe content. The dark brown glass shards show secondary hydration and devitrification. These shards are susceptible to replacement by more stable minerals. Cracked breadcrust observed in the glass shards is formed by expansion of gases trapped beneath the solidified exterior (Fig. 2g). Some of the vesicles, which are present in the shards, are oval while a few are stretched in the form of capillary tubes (Fig.2h). Bubbles within a cuspate shard (Fig.2i) are also noticed. Pumiceous, tiny curved shards with vesicles (Fig. 2k) are also found in this area and considered to have been formed by explosion due to the expanding gases in the magma during eruption (Sarna-Wojeicki, 2000). Chilled juvenile angular shards (bounded by conchoidal fractures) and blocky glass shards formed by fragmentation of highly viscous, silicic melt are also present. Size and shape of the shards largely depend on viscosity, composition of the melt and the manner in which explosion has taken place (Heiken and Wohletz, 1985). Major oxide data for glass shards when plotted on Total Alkali versus Silica (Fig. 3) and ternary Na2O+K2O - FeO(T)-

Fig. 3 Total alkali-silica diagram (Le Maitre, 1989) for glass shards from Early Miocene to Early Middle Miocene succession of the Colebrook and Inglis Islands, showing majority of data plots clustered near tephri-phonolite, trachyte, rhyolite and andacite composition. Symbols: = CB1; = CB18; + = ING 02; = ING38; Published data on glass shards by Andrei et al. (2005) shown by . Internal standard of homogeneous natural glass composition = . JOUR.GEOL.SOC.INDIA, VOL.84, AUGUST 2014

185

FeOT

Tholeiitic

Rh

lite yo

Da

e ci t

e esit And

Ba

sa lt

Calc-alkaline Na2O+K 2O

MgO

Fig. 4 Na2O + K2O –Mgo- FeO(T) data plots for glass shards from Early Miocene to Early Middle Miocene succession of the Colebrook and Inglis Islands, showing majority of data plots clustered near rhyolite field. Symbols: = CB1; = CB18; + = ING 02; = ING38; published data on glass shards by Andrei et al. (2005) shown by . Internal standard of homogeneous natural glass composition = .

MgO diagrams (Fig. 4) for the Colebrook Island samples (CB1 and CB18) show low alkali contents, and their data plots lie well within the compositional limits of basalt and basaltic-andesite fields. Glass shards of sample ING2 (late early Miocene) is rich in alkali and poor in silica content and is of tephri-phonolitic composition. The sample ING38 (middle Miocene) contains high silica and its data plots lie disposed within the limits of the rhyolite and rhyolitetrachyte fields (Fig. 3). Data plots for glass shards of these samples (ING2 and ING38) confine to corners of the Na2O+K2O and MgO corners in the ternary Na2O+K2OFeo(t)-MgO variation plot (Fig.4). All the data plots for samples ING2 and ING38 indicate soda and potash rich compositions akin to rhyolite. Majority of these plots (Fig. 4) lie close to the plots of glass shards sampled from the tephra layers of Blind Spring valley and related upper Pliocene and Pleistocene tephra layers, California, and Utah by Andrei et al. (2005). The data for the two sites on glass shard of ING2 also represent compositional closeness with a homogeneous natural glass which Andrei et al. (2005) have used as an internal standard. These observations when interpreted in terms of stratigraphy of the Andaman and Nicobar region reveal that the glass shards associated with the early Miocene succession of Colebrook island are derivatives of the basic volcanic activity. The glass shards in the later part of early Miocene and early midddle Miocene signify volcanic activity of felsic nature. Major oxide data for these glass shards show their basaltic-andesite, tephri-

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phonolite, rhyolite and trachyte compositions. These glass shards which were present in the provenance, formed by explosive eruption of lavas, ranging in composition from basalt to rhyolite with andesite/basalt-andesite being the most common magma types erupted sub-areally. Basalt and basalt andesite dominate the island arc rock spectrum, thus, implying island arc type of tectono-magmatic setting for the formation of these lava types. However, more

evolutionary variant rhyolite was most likely formed by crystal fractionation. Acknowledgements- For EPMA of samples, authors acknowledge University of Delhi (Project Grant No. Dean (R)/R&D/2007/453). SEM and EPMA studies were undertaken at the University of Delhi and at Indian Bureau of Mines, Nagpur, respectively.

References ACHARYYA , S.K. (1998) Facies, petrology and depositional environment of the Tertiary sedimentary rocks around Port Blair, South Andaman. Discussion on the paper by Bandopadhyay and Ghosh, 1998. Jour. Geol. Soc. India, v.52, pp. 610-612. ANDREI, M.S.W., MARITH, C.R., MALCOLM, S.P., ROBER,T J.F., DOUNG, B., CHARLES, E.M., JANET, L.S., ELMIRA, W., JAMES, R.B., BENNIE, T. and James, P.W. (2005) Tephra layers of Blind Spring Valley and related Upper Pliocene tephra layers, California, Nevada and Utah: Ages, correlation and magnetostratigraphy. USGS Prof. Paper 1701, pp.7-13. BANDOPADHYAY, P.C. and GHOSH, M. (1998) Facies, petrology and depositional environment of the Tertiary sedimentary rocks around Port Blair, South Andaman. Jour. Geol. Soc. India, v.52, pp.53-66. BEST, M.G. and CHRISTIANSEN, E.H. (2001) Igneous Petrology. Blackwell Science, Massachusetts, 444p. CHIT SAING (1991) Neogene events in Myanmar. In: P. Ouchanum, and B. Ratansthien (Eds.), Chiang Mai University, Thailand, pp.207-235. DASGUPTA, S. and MUKHOPADHYAY, M. (1997) Aseismicity of the Andaman subduction zone and recent volcanism. Jour. Geol. Soc. India, v.49, pp.513–521. GILL, ROBIN (1997) Modern Analytical Geochemistry. Addison Wesley Longman Limited. 317p. HEIKEN, G. and WOHLETZ, K. (1985) Volcanic Ash. Berkely, Univ.

Cal. Press, 246p. ITOH, Y. and YAGUCHI, Y. (1994) A synthesis of geological evolution of Myanmar. Proc. Intl. Workshop on Neogene Evolution of Pacific Ocean Gateways, Bandar Lampung-Indonesia, pp.66100. MAUNG, T. (1973) A preliminary synthesis of the geological evolution of Burma with refrence to the tectonic development of southeast Asia. Bull. Geol. Soc. Malaysia, v.6, pp.87-116. P ANDEY, S.K., SHRIVASTAVA, J.P. and ROONWAL , G.S. (2008) Occurrence of ferroan trevolite within olivine megacrysts of MORB from Southeastern East Pacific Rise. Curr. Sci., v.95, No.10, pp.1468-1473. RODOLFO, K.S. (1969) Bathymetry and marine geology of the Andaman Basin and tectonic implications for southeast Asia. Bull. Geol. Soc. America., v.80(2), pp.1203-1230 SARNA-WOJCICKI, A.M. (2000) Photo glossary of volcano terms, volcanic ash. United States Geological Survey, Menlo Park, California, pp.1-4. (URL http:// volcanoes.usgs.gov/ Products/ Pglossary/ash more.html). SHARMA, V. and SRINIVASAN, M.S. (2007) Geology of AndamanNicobar: The Neogene, Capital Publishing Company, New Delhi, India, pp.163. SRINIVASAN M.S. (1980) Early Neogene volcanism in Southeast Asia: Evidence of ash beds for Andaman-Nicobar. In: Kobayashi et al. (Eds.), Geology and Paleontology of Southeast Asia Symposium, pp.227-234.

(Received: 31 March 2013; Revised form accepted: 25 October 2013)

JOUR.GEOL.SOC.INDIA, VOL.84, AUGUST 2014