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Himalayan Geology, Vol. 31
(1), 2010, pp.7-18, Printed in India
Mineralogy and Geochemistry of Ultramafic rocks of Northern Manipur
Ophiolitic Complex, North East India
A. KRISHNAKANTA SINGH1*,N. IBOTOMBI SINGH2, L. DEBALA DEVI2, TH. RANJIT2
1Wadia Institute of Himalayan Geology, Dehradun - 248001, India
2Dept. of Geology, D.M. College of Science, Imphal - 795001, India
Abstract: The northern Manipur Ophiolitic Complex in the Indo-Myanmar
Orogenic Belt of NE India presents a well-preserved sequence of
mafic-ultramafics, intrusives, volcanics, chromitites and marine sediments.
The ultramafics are characterized by variable textures viz. protogranular,
meshwork, honey-comb and ribbon. Granules of magnetite and chromian spinel
are found arranged in linear fashion along the border and fractured networks
of olivine and serpentine in the ultramafic rocks. Chemically, the
ultramafic rocks are characterized by high MgO (~ 32 wt. %), Ni (~ 1960 ppm),
Cr (~ 2005 ppm), Co (~ 130 ppm), and low TiO2 (~ 0.12 wt.%), CaO (~ 6.56
wt.%), K2O (~ 0.07 wt.%) and Na2O (~ 0.44 wt.%). Chromian spinels in these
ultramafic rocks display variation in composition between of Cr2O3
(9.93-20.60) and Al2O3 (46.69-55.10), and corroborate a reciprocal
relationship. The relationship between Cr # [100Cr/(Cr+Al)] and Mg #
[100Mg/(Mg+Fe)] of chrome-spinels in these ultramafic rocks shows affinity
with the typical abyssal/alpine peridotites. Mineralogical and geochemical
characteristics of these ultramafics suggest that these rocks have derived
probably from tholeiitic melt at low degree of partial melting in a mid
oceanic ridge setting.
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