Paper Number: 545
Tectonothermal evolution of Meghalaya Gneissic Complex, NE India: a mimic of Chottanagpur Gneissic Complex during Grenvillian orogeny
Neogi, S., Zamir, T., Das, S., and Pal, T.
Geological Survey of India, North Eastern Region, Shillong, 793003, India susobhanneogi@gmail.com
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Figure 1: Geological map of the SW part of AMGC showing regional fold pattern and disposition of different lithounit.
Figure 2: Photo micrograph showing formation of cordierite after garnet during isothermal decompression.
Tectonothermal evolution of
Meghalaya Gneissic Complex (MGC) lying in north east part of the
peninsular India has significant correlation with Chottanagpur Gneissic
Complex (CGC) during Grenvillian Orogeny. The gneissic complex exposed
in the West Khasi Hills, south western part of Meghalaya records the
tectono-metamorphic history during middle Proterozoic time. Large scale
mapping (1:25000 scale) of 330km2 area show that high grade
granulite occur as enclaves within granite gneiss. This detached
lensoidal bodies of high grade metasediments along with the granite
gneiss record four stages of deformation (D1, D2,
D3, & D4). Rock types exposed in the area
include cordierite gneiss, biotite gneiss and granite gneiss, which were
intruded by granites of different time and dimension. Early gneissic
foliation (S1) preserved in metasediments and granite gneiss
are folded into very tight to isoclinal, reclined to recumbent folds
(F1) and records the earliest folding event of the area with
a pervasive axial planar cleavage (S2). Pre D1
high grade assemblage (>7500C & >4.5Kbar) in
metasediments has passed through two stages of retrogradation viz. a)
spinel-cordierite breaks down to garnet-sillimanite and b) garnet is
rimmed by cordierite due to isothermal decompression during rapid
upliftment from a depth of 4.5 to 2.5 Kb. Granite gneiss follows one
prograde path during D1 and D2 where garnet formed
at the expense of biotite, plagioclase and quartz, and granite
emplacement took place syntectonically into the core of F1
antiform. However the pre D2 granulite facies assemblage of
metasediments as reported by Chaterjee et al., 2007 [1] is pre
D1 in our record, while syn D1 deformation and
metamorphism in granite gneiss and metasediment is correlatable with
amphibolite facies condition. D3 event is responsible for
folding of the entire litho package coaxially to the early deformation
in a low to moderate easterly plunging regional antiform-synform with
ENE-WSW axial plane. During that time biotite-sillimanite stabilized at
the expense of cordierite and K-feldspar at 680-5800C and
2.5-2Kbar in cordierite gneiss. D4 results in broad warping
in map scale and cross folding with NNW-SSE axial planar cleavage where
biotite, plagioclase and quartz stabilized at 6000C below
2Kbar in both granite gneiss and metasediments. Tectonothermal evolution
of MGC recording four stages of deformation is correlatable with the
deformation pattern of CGC during Grenvillian time followed by Rodinia
amalgamation and axial planner trend of the regional fold in MGC is
exactly matching with the regional trend of CGC in further west.
References:
[1] Chatterjee N et al. (2007) Precambrian Research 152:1-26