Paper Number: 547
Jamir T, Neogi, S., Das, S. and Pal,T.
1Geological Survey of India, North Eastern Region, Shillong, Meghalaya, 793003.toshilijamir@gmail.com
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Metamorphic events of the
Gneissic complex of Meghalaya (MGC), located at the north-eastern fringe
of peninsular India are significant to build up the metamorphic
evolution of the Indian craton during Mesoproterozoic time[1]. Despite
its preservation of the different stages of metamorphism little is known
about the granulite-amphibolite terrain of this part of Indian craton.
Detailed study including mapping of an area of 300 sq km in the West of
Khasi Hills, Meghalaya shows enclaves of high-grade metasediments
(cordierite gneiss, sillimanite schist), basic granulite, charnockite
within granite gneiss of amphibolite grade. The cordierite gneiss
preserves five stages of metamorphism with two prograde (M1
& M2) and three retrograded paths (M3,
M4 & M5). In the first stage garnet grew at
the expense of biotite, plagioclase and quartz (M1). In the
second stage cordierite and spinel appeared at the expense of garnet and
sillimanite (M2). In the same PT field another type of garnet grew from
cordierite and spinel (M2) at >7500C and >5
kbar (Fig. 1). In the P-T field M3 stage is characterised by
isobaric cooling in which second type of cordierite produced from spinel
and quartz. Again an early spinel–cordierite assemblage converted to
garnet and sillimanite in the same PT regime. The remnant garnet and
sillimanite broke down to form cordierite and spinel at
750–7200C, 4.2–4.5 kbar. M4 is metamorphism marked
by decompression in which garnet, sillimanite, quartz reacted to form
cordierite at 715–6700C and 2.5 kbar. Biotite, sillimanite,
quartz, plagioclase stabilised as a final retrograde product during the
M5 stage of metamorphism through three reactions, viz. a)
garnet, K-feldspar reacted to produce plagioclase, biotite and quartz
(Fig. 2), b) garnet, K-feldspar and H2O reacted to form
biotite, sillimanite, quartz and c) cordierite, K-feldspar,
H2O formed biotite, sillimanite and quartz at
>5000C and >2 kbar. By contrast, the metamorphic
history of the granite gneiss is different from the cordierite gneiss.
The textural features show that growth of garnet and K-feldspar from
biotite, plagioclase and quartz occurred along a prograde path and
breakdown of garnet to biotite occurred along a retrograde path, which
corresponds to the M4 and M5 stages respectively
recorded by the cordierite gneiss.
The PT estimates and textural features show that the reactions of M3 stage are linked with isobaric cooling and M4 stage is isothermal decompression. The study therefore is a significant step in building up the metamorphic evolution of MGC and forms the basis of future endeavour for correlation with the Chottanagpur Gneissic Complex of the eastern part of peninsular India and with the global context with the Grenvillian orogeny.
References:
[1] Lal,R.K (1978)Contrib. Mineral. Petrol. 67:169-187