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JOURNAL GEOLOGICAL SOCIETY OF INDIA
Vol.76, August 2010, pp.155-163
Petrography and Geochemistry of St. Mary Islands, near
Malpe, Dakshina Kannada District, Karnataka
S. K. BHUSHAN 1 , K. N. RAO 2 and K. T. VIDYADHARAN 3
1 MSPL Limited, Baldota Enclave, Abheraj Baldota Road, Hospet – 583 203
2 F-2, Sree Essen Residency, 186/3 RT Vijay Nagar Colony, Hyderabad
3 310/Block-B, Maharaja Residency, Mangalore
Email: shibbanb@yahoo.co.in
Abstract: The present paper documents the petrography, mineral chemistry and petrochemistry of the acid volcanics of
St.Mary group of islands. These are essentially characterized by pyroxene bearing rhyolites with average 73.49 wt.%
SiO 2 . All the rhyolites from these islands are acmite normative, hence peralkaline with phenocrysts of zoned plagioclase
and sometimes hypersthene/augite. There is depletion of LREE with moderate negative Eu anomaly. The enhancement
of HFSE elements indicates their compatibility with continental crust. These rhyolites have high equilibrium temperature
(915°C) at relatively lower oxygen fugacity (10 -16 ). The acidic lava flows are anorogenic, within plate rhyolites, generated
during northern migration of Indian plate over Marion hotspot.
Keywords: Acid volcanics, Petrography, Geochemistry, St. Mary Islands, Karnataka.
INTRODUCTION
The western Indian continental margin is passive rift
related, with its southern half comprised of Archaean
and Proterozoic crystalline rocks and the northern
borders covered by the ~65 Ma Deccan flood basalt
province. The shallow inner shelf areas between Kaup
and Hangarkatta in Karnataka are dotted with a number of
small islands and islets. The prominent among them are the
St. Mary islands (SMI) a group of four islands: Coconut
island, Northern island, Darya Bahadurgarh island and
South island, off the fishing hamlet of Malpe on the
west coast (Fig.1).The islands consist entirely of flatlying,
undeformed high silica rhyolites. The geochronological
studies (Pande et al. 2001; Torsvik et al. 2000)
link the SMI to Madagascar flood basalt province (88.1±
1.2 Ma) and hence, could represent volcanic activity
associated with the break-up of Greater India (India and
Seychelles) and Madagascar.
Based on petrographic studies, the acid volcanics
identified in SMI are variously described as rhyodacite,
dacite rhyolite and granophyre (Naganna, 1964, 1966;
Balasubrahmanyam, 1975; Valsangkar et al. 1981; Subbarao
et al. 1993). The present study indicates that these are
pyroxene rhyolites with spectacular development of
columnar joints (Fig.2) as noticed in the Coconut
island.
0016-7622/2010-76-2-155/$ 1.00 © GEOL. SOC. INDIA
GEOLOGY
The acid volcanic rocks exposed in SMI are mostly fine
grained, and on the surface appear in different shades of
light pinkish red/brown, while on the fresh broken surface,
they vary in colour from light to dark grey. Randomly
distributed dirty white to greyish microphenocrysts of
feldspar and ferromagnesian minerals can be noticed by
hand lens, in an essentially aphanetic groundmass. Brown
to black, irregular to oval shaped xenoliths of varied
sizes ranging from millimeter to centimeter scale are
often seen irregularly distributed and are more abundant in
ST. MARY ISLES
(Arabian sea)
Fig.1. Location map of St. Mary islands.

156 S. K. BHUSHAN AND OTHERS
Fig.2. Columnar joints in rhyolites of Coconut island.
the rhyolites of Northern island and Darya Bahadurgarh
island.
PETROGRAPHY AND MINERAL CHEMISTRY
In thin section, almost all the rocks exposed in SMI
exhibit microporphyritic texture with phenocrysts of
plagioclase, augite and hypersthene set in fine to very
fine groundmass consisting of plagioclase, opaques,
minor chloritised biotite and cryptocrystalline, often turbid
looking felsic material containing quartz, K-feldspar and
plagioclase.
All the phenocrysts, in particular pyroxenes, frequently
exhibit minor alteration to secondary minerals along grain
boundaries and cleavage planes. Augite and hypersthene,
the common ferromagnesian microphenocrysts are euhedral
to subhedral in nature, the later (reported for the first time),
exhibiting pink pleochroism and basal section of hypersthene
show symmetrical extinction with respect to the two cleavage
directions (Fig.3).
Plagioclases commonly exhibit twinning on Albite law
but some grains exhibit twinning and combined Albite-
Fig.3. Photomicrograph of rhyolite of Northern islands exhibiting
glomeroporphyritic texture. The phenocrysts are
hypersthene (SW and northern part), augite (middle and
south) and plagioclase (east and west of centre) (Xed.
Nicols).
Carlsbad and Albite-Pericline laws. Plagioclase phenocrysts
often exhibit conspicuous concentric zoning.
Petrographic studies of the samples collected from all
the islands indicate that the groundmass is made up of fine
to very fine quartz, plagioclase and also K-feldspar and
appear pale brownish in colour. Under higher magnification,
fine grained graphic intergrowth between quartz and
feldspars can be seen in most of the specimens. EPMA data
of plagioclases, pyroxenes and ground mass of rhyolites from
Coconut island, North island and Darya Bahadurgarh island
are presented in Table 3 and 4.
The mineral chemistry (Table 4) indicates that the
plagioclase phenocrysts are essentially of andesine
composition.
The groundmass (Na 2 O 8.08%) and micro phenocrysts
(Na 2 O 7.72%) are more sodic than the core portion of
phenocryst plagioclase as is to be expected in normal course
of crystal fractionation.
In the diopsite-hedenbergite-enstatite-ferrosilitequadrilateral
(Fig.4), the ortho and clinopyroxenes
(Table 4) plot distinctly in the enstatite and augite fields
respectively. Their temperature of crystallization is around
1000°C as shown in Fig.5.
GEOCHEMISTRY
The major element analysis (Table 1) has been carried
out by XRF using fusion beads at PPOD laboratory, GSI,
AMSE, Bangalore, whereas trace and REE data (Table 2),
has been generated by ICPMS at Chemical Lab, GSI,
Hyderabad. The precision and accuracy of the analysis is
within the standard permissible limits as detailed by Reimold
Wo
Diopside Hedenbergite
Augite
Pigeonite
Enstatite Ferrosillite
En Fs
Fig.4. Plots of ortho and clinopyroxenes from rhyolites in the
pyroxene quadrilateral diagram.
JOUR.GEOL.SOC.INDIA, VOL.76,AUG.2010

PETROGRAPHY AND GEOCHEMISTRY OF ST. MARY ISLANDS, NEAR MALPE, KARNATAKA 157
JOUR.GEOL.SOC.INDIA, VOL.76,AUG.2010
Table 1. Chemical composition (major oxides Wt.%) and CIPW norms of rhyolites
Oxide STM-1 STM-2 STM-3 STM-4 STM-5 STM-6 STM-7 STM-8 Mean (8) SD
SiO 2 73.5 73.56 73.11 73.72 73.74 75.73 73.98 70.57 73.49 1.33
TiO 2 0.81 0.81 0.81 0.78 0.79 0.34 0.76 1.21 0.79 0.22
Al 2 O 3 10.74 10.72 10.74 10.7 10.78 10.54 10.71 10.35 10.66 0.13
Fe 2 O 3 3.21 3.37 3.38 3.05 3.05 1.94 2.88 4.46 3.17 0.65
MnO 0.13 0.14 0.16 0.12 0.13 0.04 0.11 0.11 0.12 0.03
MgO 1.06 0.85 0.98 0.92 0.97 0.81 0.87 1.55 1 0.22
CaO 1.43 1.58 1.38 1.51 1.5 0.63 1.37 1.81 1.4 0.32
Na 2 O 5.84 5.91 6.08 6.04 6.05 5.56 5.98 6.11 5.95 0.17
K 2 O 2.43 2.43 2.44 2.45 2.43 3.66 2.54 2.12 2.56 0.43
P 2 O 5 0.16 0.17 0.17 0.15 0.15 0.09 0.14 0.25 0.16 0.04
S 0.11 0.12 0.16 0.18 0.06 0.1 0.17 0.34 0.16 0.08
BaO 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0
LOI 0.54 0.29 0.55 0.34 0.32 0.52 0.46 1.09 0.51 0.24
Total 99.99 99.98 99.99 99.99 100 99.99 99.98 99.99 99.99 0.01
CIPW NORM (wt%)
q 29.11 29.21 28.56 29.46 29.14 33.15 29.92 25.69 29.28 1.9
or 14.43 14.43 14.49 14.55 14.43 21.7 15.08 12.58 15.21 2.55
ab 41.61 41.51 41.56 41.29 41.82 33.73 40.84 41.37 40.47 2.56
di 4.74 5.34 4.54 4.79 5.04 1.11 3.97 5.78 4.41 1.35
hy 0.59 0.01 0.69 0.07 0.13 1.5 0.33 1.54 0.61 0.57
ac 6.68 6.68 6.68 6.6 6.63 5.32 6.54 7.84 6.62 0.63
il 1.54 1.54 1.54 1.29 1.5 0.04 1.02 2.3 1.35 0.6
ap 0.37 0.39 0.39 0.35 0.35 0.21 0.32 0.58 0.37 0.1
tn 0.25 0.78 0.55 0.53 0.22
pr 0.21 0.22 0.3 0.34 0.11 0.19 0.32 0.64 0.29 0.15
NaS 0.05 0.21 0.54 0.43 1.69 0.54 0.33 0.54 0.5
Sample locations: STM-1 to STM-4 from Coconut Islands; STM-5, 6 from Northern Island and STM-7, 8 from
Darya Bahadur Garh Island. (6) = number of samples, SD = Standard Deviation LOI = loss of ignition
Table 2. REE and trace element data (in ppm) of rhyolites
STM-1 STM-2 STM-3 STM-4 STM-5 STM-6 STM-7 STM-8 Mean (8) SD
La 70.19 20.29 71.23 69.75 68.98 40.08 66.9 64.16 59 17.45
Ce 154.6 156.52 158.96 154.64 152.99 82.26 148.79 140.69 144 23.8
Pr 21.24 21.26 21.43 20.9 20.39 9.23 20.28 18.23 19 3.86
Nd 90.59 89.62 90.19 89.32 87.02 33.3 85.58 76.84 80 18.25
Eu 6.24 5.99 5.99 5.9 5.81 1.21 5.74 4.6 5 1.57
Sm 20.17 20.17 20.4 20.55 19.95 6.63 19.71 17.41 18 4.44
Tb 2.91 2.86 2.89 2.85 2.8 0.9 2.81 2.43 3 0.64
Gd 18.74 18.36 18.71 18.23 17.95 5.72 17.72 15.55 16 4.14
Dy 15.21 14.77 15.28 15.3 14.82 4.84 14.79 13.03 14 3.35
Ho 2.94 2.89 2.98 2.94 2.84 0.98 2.9 2.5 3 0.64
Er 7.25 7.01 7.14 7.05 6.92 2.48 7.05 5.99 6 1.51
Tm 1.11 1.09 1.11 1.1 1.08 0.4 1.09 0.94 1 0.23
Yb 6.21 6.26 6.35 6.36 6.18 2.38 6.33 5.43 6 1.28
Lu 0.91 0.89 0.91 0.92 0.87 0.36 0.92 0.77 1 0.18
Be 3.8 4.16 4.5 4.24 4.11 2.76 4.26 3.33 4 0.54
Co 33.16 58.75 49.67 56.79 88.96 31.92 52.72 34.57 51 17.65
Ga 26 25.92 26.6 26.19 26.35 18.31 26.46 23.23 25 2.68
Ge 2.09 1.99 2.05 2.08 2.06 1.1 2.12 1.72 2 0.33
Y 69.97 66.92 69.27 68.67 67.91 25.15 68.97 58.43 62 14.31
Zr 1292 1239 1252 1243 1207 258 1242 564 1037 370.1
Nb 48.72 53.57 50.62 49.59 49.24 18.05 47.36 37.99 44 10.82
Cs 0.53 0.62 0.57 0.71 0.69 0.88 0.95 0.72 1 0.14
Hf 19.65 19.49 19.29 19.49 18.75 6.56 19.54 15.04 17 4.28
Ta 3.49 5.36 3.86 3.83 3.63 1.6 3.9 2.9 4 0.99
Bi 0.15 0.12 0.14 0.14 0.13 0.12 0.15 0.12 0 0.01
Th 8.17 8.05 8.26 8.48 8.18 13.66 8.44 8.5 9 1.78
U 1.6 1.64 1.77 1.72 1.67 2.27 1.77 1.64 2 0.2
LREE 342.86 293.68 347.8 340.51 335.19 166.08 327.29 304.52
HREE 75.45 74.3 75.77 75.3 73.41 24.69 73.32 64.05
LR/HR 4.544 3.952 4.59 4.522 4.565 6.726 4.463 4.754
La/Lu 77.13 22.79 78.27 75.81 79.28 111.33 72.71 83.32
Eu/Eu* 0.98 1.04 1.06 1.07 1.12 1.65 1.05 1.16

158 S. K. BHUSHAN AND OTHERS
Table 3. Chemical composition of plagioclases (phenocryst and groundmass) of rhyolites
Sample No. STM-1 STM-2 STM-3 STM-4 STM-7
Description Phenocrysts Small Phenocrysts Small Ground mass Phenocrysts Phenocrysts Small Ground mass Ground mass
phenocryst phenocrysts plagioclases phenocrysts plagioclases plagioclases
Oxides mean SD Single mean SD mean SD Mean SD mean SD mean SD mean SD mean SD mean SD
(7) point (7) (4) (6) (9) (15) (6) (6) (5)
SiO 2 58.72 0.7 59.67 58.23 0.4 58.5 1.9 59.04 3 58.49 1.1 59.45 1.1 60.01 1.6 60.71 0.9 56.22 0.5
TiO 2 0.06 0 0.05 0.07 0 0.08 0 0.1 0 0.05 0 0.05 0 0.05 0 0.05 0 0.09 0
Al 2 O 3 25.53 0.9 24.1 24.78 0.9 24.62 1.3 24.18 1.4 24.81 0.6 24.53 0.5 24.29 0.3 23.22 0.7 25.99 0.5
Cr 2 O 3 0.02 0 0.05 0.01 0 0.01 0 0.01 0 0.01 0 0.01 0 0 0 0.01 0 0.01 0
FeO T 0.61 0.1 0.84 0.62 0.1 0.3 0.4 0.68 0.1 0.62 0.1 0.25 0.3 0.1 0.2 0.29 0.3 0.89 0.1
MnO 0.01 0 0 0.02 0 0.02 0 0.01 0 0.03 0 0 0 0.01 0 0.01 0 0.02 0
MgO 0.03 0 0.02 0.05 0 0.03 0 0.03 0 0.03 0 0.04 0 0.03 0 0.02 0 0.04 0
CaO 7.8 0.7 6.41 7.68 0.4 7.23 1.5 6.72 1.8 7.4 0.8 6.87 0.7 6.58 0.5 5.39 0.8 8.73 0.5
Na 2 O 7.3 0.5 7.79 7.47 0.3 7.52 0.9 7.68 1 7.56 0.5 7.7 0.3 7.21 1.4 8.32 0.3 6.61 0.3
K 2 O 0.5 0.1 0.71 0.47 0.1 0.52 0.2 0.55 0.2 0.46 0.1 0.56 0.1 0.68 0.2 0.91 0.5 0.42 0
NiO 0 0 0 0.01 0 0 0 0.01 0 0.01 0 0 0 0.01 0 0 0 0 0
Total 100.59 0.6 99.64 99.39 0.8 98.81 0.5 98.97 1 99.47 0.4 99.46 1 98.97 0.8 98.94 0.4 99.01 1.1
Cations 8(O) 8(O) 8(O) 8(O) 8(O) 8(O) 8(O) 8(O) 8(O) 8(O)
Si 2.6215 0 2.6828 2.6329 0 2.653 0.1 2.67 0.1 2.64 0 2.6736 0 2.7015 0.1 2.7386 0 2.5607 0
Ti 0.0022 0 0.0017 0.0023 0 0.003 0 0.0033 0 0.002 0 0.0016 0 0.0017 0 0.0018 0 0.0032 0
Al 1.3435 0 1.2771 1.3204 0 1.316 0.1 1.2914 0.1 1.32 0 1.3005 0 1.2893 0 1.2343 0 1.3951 0
Cr 0.0006 0 0.0018 0.0005 0 2E-04 0 0.0004 0 3E-04 0 0.0003 0 0 0 0.0003 0 0.0003 0
Fe +3
0.0205 0 0.0284 0.021 0 0.01 0 0.0187 0 0.021 0 0.0083 0 0.0032 0 0.0098 0 0.0305 0
Fe +2
0 0 0 0 0 0 0 0.005 0 0 0 0 0 0 0 0 0 0 0
Mn 0.0004 0 0 0.0008 0 8E-04 0 0.0005 0 0.001 0 0.0001 0 0.0005 0 0.0004 0 0.0008 0
Mg 0.0021 0 0.0013 0.0031 0 0.002 0 0.002 0 0.002 0 0.0024 0 0.0019 0 0.0015 0 0.0024 0
Ca 0.3732 0 0.3088 0.372 0 0.351 0.1 0.3273 0.1 0.358 0 0.3313 0 0.3177 0 0.2603 0 0.4261 0
Na 0.632 0 0.6791 0.6548 0 0.661 0.1 0.6735 0.1 0.661 0 0.6719 0 0.6304 0.1 0.7277 0 0.5835 0
K 0.0283 0 0.0407 0.0269 0 0.03 0 0.0317 0 0.027 0 0.0322 0 0.0389 0 0.0526 0 0.0245 0
Ni 0 0 0 0.0002 0 0 0 0.0003 0 3E-04 0 0.0001 0 0.0004 0 0.0001 0 0.0002 0
Table 4. Chemical composition of pyroxenes of rhyolites
Sample No. STM-1 STM-2 STM-3 STM-4
Augite Augite Augite Augite Hypersthene
Oxides Mean SD Mean SD Mean SD Mean SD Mean SD
wt%. (4) (19) (8) (3) (7)
SiO 2 51.28 0.13 51.01 0.4 51.36 0.41 50.67 0.54 52.06 0.54
TiO 2 0.58 0.12 0.59 0.07 0.58 0.04 0.57 0.16 0.29 0.03
Al 2 O 3 1.23 0.2 1.25 0.22 1.26 0.14 1.38 0.24 0.53 0.04
Cr 2 O 3 0.01 0.01 0.01 0.01 0.02 0.02 0.03 0.03 0 0.01
FeO T 10.12 0.23 10.38 0.46 10.44 0.37 10.33 0.4 19.78 0.45
MnO 1.3 0.14 1.24 0.21 1.18 0.2 1.33 0.08 2.66 0.08
MgO 14.72 0.22 14.75 0.36 14.9 0.28 15.16 0.12 22.46 0.48
CaO 19.33 0.22 19.49 0.47 19.73 0.41 20.06 0.16 1.59 0.1
Na 2 O 0.46 0.03 0.49 0.12 0.43 0.05 0.46 0.03 0.05 0.01
K 2 O 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0 0.01
NiO 0.01 0.01 0 0.01 0 0 0 0 0 0
Total 99.04 0.3 99.23 0.68 99.91 0.38 100 1.03 99.44 1
Cations 6(O) 6(O) 6(O) 6(O) 6(O)
Si 1.93 0 1.9159 0.01 1.9158 0.01 1.8851 0.01 196 0.01
Ti 0.016 0 0.0166 0 0.0164 0 0.016 0 0.0083 0
Al 0.055 0.01 0.0553 0.01 0.0556 0.01 0.0606 0.01 0.0232 0
Cr 2E-04 0 0.0003 0 0.0007 0 0.0008 0 0.0001 0
Fe +3 0.087 0.02 0.1154 0.02 0.1112 0.02 0.1703 0.03 0.072 0.03
Fe +2 0.232 0.03 0.2106 0.03 0.2145 0.03 0.1511 0.03 0.5465 0.03
Mn 0.042 0 0.0396 0.01 0.0373 0.01 0.0418 0 0.0844 0
Mg 0.826 0.01 0.8259 0.02 0.8283 0.01 0.8409 0.01 1.2516 0.02
Ca 0.779 0.01 0.7842 0.02 0.7885 0.01 0.7998 0.01 0.0636 0
Na 0.034 0 0.0356 0.01 0.0314 0 0.0333 0 0.0039 0
K 4E-04 0 0.0005 0 0.0005 0 0.0005 0 0.0002 0
Ni 2E-04 0 0.0001 0 0 0 0 0 0 0
JOUR.GEOL.SOC.INDIA, VOL.76,AUG.2010

1200C
PETROGRAPHY AND GEOCHEMISTRY OF ST. MARY ISLANDS, NEAR MALPE, KARNATAKA 159
Wo
1000c
En Fs
Fig.5. Enstatite-ferrosilite-hedenbergite-diopside showing position
of pyroxenes.
et al. (1994). The XRF and ICPMS analyses of the samples
of all the four islands clearly indicate them to be rhyolites
with SiO 2 between 70.57 to 75.73 wt%. Peccerillo and Taylor
(1976) defined rhyolites having >70% SiO 2 , whereas Ewart
(1979) classified rhyolite with >69% SiO 2 . Bhushan (2000)
also considered rhyolites of Malani Igneous Suite with >69%
SiO 2. Therefore based on silica abundances, all the felsic
volcanics of four SMI can be termed as rhyolites with
average SiO 2 of 73.49% (Table 1). Except the rhyolites from
Northern island (STM-6) and Darya Bahadurgarh (STM-
8), all the remaining six samples have nearly uniform
chemical composition. With the modest increase in silica
(STM-6), there is decrease in TiO 2 , Fe 2 O 3 , MnO, MgO, CaO
and even Na 2 O with visible enhancement of K 2 O. STM-8,
which has lowest SiO 2 when compared to other samples,
reflects increase of TiO 2 , Fe 2 O 3 , MgO, CaO and also Na 2 O.
STM-6 has the highest and STM-8, the lowest SiO 2
percentage. STM-6 has also least LREE, Be, Co, Ga, Ge, Y,
Zr, Hf, Th and U concentration. STM-8 has highest
abundance of TiO 2 , Fe 2 O 3 , MgO, CaO, Na 2 O, P 2 O 5 and
lowest K 2 O percentage, when compared to STM-6.
The CIPW normative values have been calculated by
using Igpet software (Table 1). All samples are quartz
normative reflecting their silica oversaturated nature but as
they are deficient in Al 2 O 3 , there is no anorthite in the
normative classification. All rhyolites contain normative
diopside, hypersthene and are conspicuous by being acmite
normative due to abundance of Na 2 O. Since normative
anorthite is absent, a chemical classification has been
adopted to define these acid volcanics. The criteria for the
alkalinity of rhyolites has been A/CNK ratio, Agpaitic index
AI, (Fersmann, 1929; mol. Na 2 O + K 2 O / Al 2 O 3 ) and
presence of normative acmite. All the samples have 1 and as mentioned previously are acmite
normative therefore can be defined as peralkaline rhyolites.
Since the samples contain >69% SiO 2 , all of them plot in
the rhyolite field (LeBas et al. 1986, Fig.6).
JOUR.GEOL.SOC.INDIA, VOL.76,AUG.2010
Fig.6. Plot of total alkali vs silica (after Le Bas et al. 1986).
The chondrite normalized REE (Table 2) pattern (after
Nakamura, 1974) is shown in Fig.7 There is gradual
depletion of LREE with less pronounced negative Eu
anomaly, suggesting moderate fractionation of feldspars due
to paucity of time in the magma chamber or abrupt eruption
( Storey, 1981). The Eu/Eu* values range from 0.98 to 1.65,
indicating the presence of cumulus feldspar in the melt. This
ratio has also positive relation with LREE/HREE and La/
Lu ratios. The Eu/Eu* ratio however exhibit moderate
negative correlation with REE. The higher abundance of
LREE is due to excessive presence of zirconium and titanium
in the melt which accommodate these elements (Taylor
et al. 1981). The gradual depletion of HREE also speaks
presence of ortho- and clinopyroxene in the system which
eventually partition these elements. The LREE/HREE ratios
are between 3.9 to 6.7, mostly around four times.
Figure 8 exhibits primordial mantle normalized trace
element variation spider diagram (normalization data from
Pearce et al. 1984) for these rhyolites. The relative
enrichment of Ta, K, Zr and Gd in indicative of their
compatibility with the crustal component of the melt. A
Fig.7. Chondrite normalised REE diagram (after Nakamura, 1974)

160 S. K. BHUSHAN AND OTHERS
Table 5. Chemical analyses of ulvospinels of rhyolites
Oxide STM-4 STM-4 STM-4 STM-2 STM-2 STM-2 STM-3 STM-3 STM-5 STM-7 STM-7 STM-8 Mean SD
(wt%) (12)
SiO2 0.24 0.26 0.1 0.09 0.05 0 0.22 0.14 0.17 0.08 0.13 0.04 0.13 0.1
TiO2 36.63 36.41 36.4 36.47 36.2 38.21 38.55 37.98 36.3 38.74 37.76 40.51 37.51 1.3
Al O 2 3 0.01 0.01 0.24 0.03 0.08 0.09 0.06 0.03 0.01 0.01 0.02 0.02 0.05 0.1
Cr O 2 3 0.02 0 0 0.01 0 0.03 0 0 0 0 0.02 0.03 0.01 0
FeO T
57.45 56.62 58.53 62.14 61.39 57.04 56.84 58.24 59.97 59.73 58.27 55.92 58.51 1.9
MnO 2.27 3.48 1.58 2.39 1.57 2.81 1.21 3.15 2.51 1.28 1.3 1.89 2.12 0.7
MgO 0.21 0.21 0.45 0.13 0.12 0.13 0.1 0.12 0.03 0.12 0.11 0.1 0.15 0.1
CaO 0.02 0.04 0 0 0 0 0.16 0.02 0 0 0 0 0.02 0
Na O 2 0 0 0.07 0 0 0 0 0 0.06 0.04 0 0 0.01 0
K O 2 0.02 0 0 0.01 0.01 0 0 0.01 0.02 0.01 0.01 0.01 0.01 0
NiO 0 0 0 0 0.01 0 0 0.01 0.02 0 0 0.03 0.01 0
Total 96.87 97.03 97.37 101.27 99.43 98.31 97.14 99.7 99.09 100.01 97.62 98.55 98.53 1.3
Cations 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O)
Si 0.0091 0.0098 0.004 0.0033 0.0019 0 0.008 0.0051 0.0063 0.0029 0.0049 0.0015 0.0047
Ti 1.0402 1.0337 1.029 1.0038 1.0125 1.0648 1.08 1.0477 1.017 1.0623 1.0604 1.1116 1.0469
Al 0.0004 0.0004 0.011 0.0013 0.0035 0.0039 0.003 0.0013 0.0004 0.0004 0.0009 0.0009 0.0022
Cr 0.0006 0 0 0.0003 0 0.0009 0 0 0 0 0.0006 0.0009 0.0003
Fe +3 0 0 0 0 0 0 0 0 0 0 0 0 0
Fe +2
1.8142 1.7876 1.84 1.902 1.9094 1.7677 1.77 1.7866 1.8685 1.8213 1.8198 1.7064 1.8162
Mn 0.0726 0.1113 0.05 0.0741 0.0495 0.0882 0.038 0.0979 0.0792 0.0395 0.0411 0.0584 0.0667
Mg 0.0118 0.0118 0.025 0.0071 0.0067 0.0072 0.005 0.0066 0.0017 0.0065 0.0061 0.0054 0.0085
Ca 0.0008 0.0016 0 0 0 0 0.006 0.0008 0 0 0 0 0.0008
Na 0 0 0.005 0 0 0 0 0 0.0043 0.0028 0 0 0.001
K 0.001 0 0 0.0005 0.0005 0 0 0.0005 0.001 0.0005 0.0005 0.0005 0.0004
Ni 0 0 0 0 0.0003 0 0 0.0003 0.0006 0 0 0.0009 0.0002
Table 6. Chemical composition of titanomagnetites of rhyolites
Serial STM-4 STM-4 STM-2 STM-3 STM-3 STM-8 STM-8 STM-5 STM-7 Mean SD
No. (9)
SiO 2 0.06 0.03 0.12 0.06 0.04 0.13 0.14 0.2 0.11 0.1 0.1
TiO 2 9.76 8.01 7.9 12.56 7.99 8.73 9.25 7.95 8.03 8.91 1.4
Al 2 O 3 1.09 1.21 1.32 1.32 1.79 1.07 0.97 0.93 0.39 1.12 0.4
Cr 2 O 3 0 0 0 0.01 0 0.01 0.02 0 0.04 0.01 0
FeO T 80.49 82.29 81.58 76.08 78.67 80.85 80.88 80.86 83.79 80.61 2.1
MnO 1.85 1.53 0.75 1.66 1.41 1.41 1.31 1.03 1 1.33 0.3
MgO 0.15 0.22 0.19 0.6 0.54 0.19 0.15 0.12 0.04 0.24 0.2
CaO 0 0 0 0 0 0 0 0 0 0 0
Na 2 O 0.02 0 0.08 0 0.05 0.03 0.05 0.06 0.06 0.04 0
K 2 O 0.02 0 0 0 0 0 0.01 0.02 0 0.01 0
NiO 0 0 0 0 0 0 0 0 0 0 0
Total 93.44 93.29 91.94 92.29 90.49 92.42 92.78 91.17 93.46 92.36 1
Cations 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O) 4(O)
Si 0.0023 0.0012 0.0047 0.0023 0.0016 0.0051 0.0054 0.0078 0.0042 0.0038
Ti 0.2828 0.2319 0.2317 0.368 0.237 0.2554 0.2698 0.2357 0.2331 0.2606
Al 0.0495 0.0549 0.0607 0.0606 0.0832 0.0491 0.0444 0.0432 0.0177 0.0515
Cr 0 0 0 0.0003 0 0.0003 0.0006 0 0.0012 0.0003
Fe +3 1.3826 1.4789 1.4725 1.1983 1.4433 1.4319 1.4087 1.4754 1.5109 1.4225
Fe +2 1.2112 1.1706 1.1885 1.2808 1.1521 1.1985 1.2151 1.1909 1.1934 1.2001
Mn 0.0604 0.0499 0.0248 0.0548 0.0471 0.0465 0.043 0.0344 0.0327 0.0437
Mg 0.0086 0.0126 0.011 0.0348 0.0318 0.011 0.0087 0.0071 0.0023 0.0142
Ca 0 0 0 0 0 0 0 0 0 0
Na 0.0015 0 0.0061 0 0.0038 0.0023 0.0038 0.0046 0.0045 0.003
K 0.001 0 0 0 0 0 0.0005 0.001 0 0.0003
Ni 0 0 0 0 0 0 0 0 0 0
JOUR.GEOL.SOC.INDIA, VOL.76,AUG.2010

Rock/MORB
PETROGRAPHY AND GEOCHEMISTRY OF ST. MARY ISLANDS, NEAR MALPE, KARNATAKA 161
Fig.8. Primordial mantle normalized trace element spider diagram
(after Pearce et al. 1983).
pronounced negative Ti anomaly is characteristic of its
compatible nature with titanomagnetite and ulvospinel. The
higher concentration of Y, Zr, Nb, Hf and Th lithophile
elements is observed in all the rhyolite samples, whereas
enrichment of HFSE is treated as diagnostic feature of
alkaline magma (Salvi and Jones, 1990). There is
exceptionally high concentration of Zr (Table 1) in the
melt, which suggest alkaline nature of the source material.
Bowden (1974) considered all rocks exceeding 500 ppm of
Zr as oversaturated alkaline rocks. The mineral chemistry
of ulvospinel and titanomagnetite is presented in Table 5
and 6 respectively. There is greater abundance of FeO(T)
and less TiO 2 than average titaniferous magnetite,
whereas ulvospinel has higher concentration of TiO 2 and
Fe 2 O (T) and moderate increase of MnO. Using Buddington
and Lindsley’s (1964) titanomagnetites-ulvospinel
geothermometre, indicates crystallization at 915°C
temperature with fO 2 (oxygen fugacity) of 10 –16 . This
corresponds to crystallization under QFM (Quartz + Fayalite
+Magnetite) and WM (Wustite + Magnetite) buffer
conditions. Ewart (1981) indicated similar equilibrium
temperature of 885-980°C and oxygen fugacity between
QFM and WM buffers from hypersthene rhyolites of SE
Queensland. In Ta vs Yb (Fig.9) the SMI rhyolites plot in
the ‘within plate granite’ field of Pearce et al. (1984),
indicating possible anorogenic magmatism.
DISCUSSION
Plate tectonic reconstructions propose a close link
between Madagascar and Greater India from Late
Precambrian to Cretaceous times and the separation is
believed to have begun during or immediately after a period
of late Cretaceous basic and felsic magmatism that is well
known from Madagascar (83.6-91.6 Ma; Storey et al. 1995,
Torsvik et al. 1998). During this late Cretaceous break up,
JOUR.GEOL.SOC.INDIA, VOL.76,AUG.2010
Fig.9. Ta-Yb diagram (after Pearce et al. 1984). Syn-COLG -
Collision granite; ORG - Ocean ridge granite; VAG -
Volcanic arc granite; WPG - Within plate granite.
the western margin of India presumably rifted off the eastern
margin of Madagascar. However, equivalent Cretaceous
magmatism is not well documented from western India.
Possible exceptions include mafic dykes in the mainland
south-west India (Radhakrishna and Vaidyanadhan, 1994;
Radhakrishna et al. 1999) and the acid volcanic rocks of the
SMI (Valsangkar et al. 1981). Spectacular columnar joints
in rhyolites are noticed in the Coconut island (Fig.2). The
vertical orientation of these columnar joints testifies the
absence of post magmatic structural tilting. The acid
volcanics of SMI were considered by Naganna (1966) to
represent an early phase of Deccan Flood Basalt Province
of 65 ma, but whole rock K-Ar ages ranging from 80.3±1.7
Ma to 97.6±2.3 Ma (Valsankgar et al. 1981) suggest that
this volcanism can be better correlated with an older event
related to India-Madagascar break-up. The geochronology
and palaeomagnetism of SMI were reinvestigated by
Torsvik, et al. (2000) and Pande et al. (2001). A U-Pb zircon
age of 91.2±0.2 Ma from western India (St .Mary’s islands)
confidently links India with the late Cretaceous magmatic
Province in Madagascar (84-92 Ma) and the U-Pb age is
within analytical error of the U-Pb age of the Analalava
gabbro pluton (91.6±0.3 Ma) in northeastern Madagascar
(Torsvik et al. 1998). St.Mary volcanism is linked to the
initial break-up of India and Madagascar, and the volcanism
probably resulted from rift- related extensional processes
initially induced by the Marion hotspot underlying southern
Madagascar during the late Cretaceous period (Torsvik et
al. 2000).

162 S. K. BHUSHAN AND OTHERS
The chain of St. Mary’s four islands continues up to
6 km in NW-SE direction and extend further south for
another 7 km, about 2.5 km west of Udiyavarh, where two
tiny islands outcropping 12m above sea level have been
recorded by Nambiar et al. (2006a, b). This reflects the
northern movement of plate. These felsic volcanics erupted
around 88 Ma and are attributed to the ‘hotspot’ activity
(Torsvik et al. 1998). According to Crecraft et al. (1981),
the high heat flow zone (hotspot) is provided by intrusion
of mantle derived basalt into the crust, which results in partial
melting and generation of felsic magma. The accumulated
silicic magma ascends into the upper crust, thus enhancing
the extensional stress field, resulting in further rifting.
The main burst of flood basalt has been recorded in
Madagascar and St. Mary islands perhaps represent the
tail through long chain below sea, related to Marion ‘hotspot’
(Lat. 47°S, Long. 38°E). Lack of any marine volcanic
landforms and characteristic pentagonal or hexagonal
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