Nagoya University
NagoyaUniversity

The birthof Gondwana: Timing and thermal spikes
chemicaldating of monazites
M. Santoshhacultyof'Science,
Kbchi Ciniversity,(LE]-mail:.ldpanfromconstrained',vaFrtoshk)cc.kocfei-u,ac,

The khondalitebelt of Kerala in southern India represent ene of the vast
supracrustal beltsin the East Gondwana fragmentwhich
wassubjected to extreme
crustalmetamorphism
at ultrahigh-temperature
conditions during lateNeoproterozoic,
coinciding
with the finalassembly of the Gondwana supercontinent.
Petrologicevidence includingthe
directassociation of spinel+quartz in various texturesin these rocks indicates
metamorphic
temperatures over leOOOC at pressuresover 7 1<barfbrthe peak assernblage (Fig,la).U-PbTh chemical datingof monazite
in texturalassociation
with spinel +quartz includedwithinshows
an age ofgamet yieldsan age of 599± 12 Ma. Monazite in the matrix assemblage
524± 1l Ma. Monazite cores includedwithin mesoperthiteshowsages of 597 ± 12 Ma to602± 13 Ma, while theirrims have ages of 548± 10 Ma to 567 ± 10 Ma. The older age fromhomogeneous monazite occuning with garnetis identicalto the core ages of monazites
occurring
withinmesoperthites± 12 Ma
to 602± 13 Ma), Monazite occurring as(597inclusions
withininthe matrix shows core ages of 599 7 Ma and 509± 9
quartzand biotite±Ma respectively.

The oldest ages from monazite
cores included
inquartzare comparable
withthose obtained from monazite
cores in association with spinel + quartzand mesoperthite.
Insomecases, the cores of monazites
includedwithin garnetshow Paleproterozoic
ages, which
are mantled
by Neeproterozoic-Cambrianrims (Fig.
Ib).
The datashow thatthe age peak at ca. 600 Ma iscommon
in all cases, marking
the timing of the early ultrahigh-temperature
granulitefaciesevent in this terrane.The 575
Ma age isalso commonly
represented
inmost cases, which comes from the rims immediately
surrounding
the cores. This study therefbre interprets
the peak ultrahigh-temperature
metamorphiceventin the Trivandrum Block to be between ca. 580-600 Ma. Homogenous
monazitein association with the second generation of spinel + quartz in the matrix showing
an age of 524 ± 11 Ma indicatesa re-heating
event that possibly
attained high temperatures ofup
to 8200C as we show
in a petrogenetic pseudosection of the rock. Monazites inmesoperthites
as well as rims of monazites
includedwithin garnet also show another peak at
ca. 550 Ma. Therefore,
the reheating event is considered
to be between 52e and 550 Ma.
From the ages ofmonazites
in various texturalassociations, itispossible to trace the thermalIc)duringthe finalassemblyand post-assembly stages of the Gondwana
perturbations(Fig.
supercontlnent.

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Fig. 1. (a)Thinfacies
photomicrographof khondalite (granulitemetapelite)from southern Kerala, India showing spinel + quartzassemblage fbrmed
during ultrahigh-temperature
metamorphism.
(b)Monazite grainincludedwithin
Paleoproterozoic cores mantled
by lategarnet from the khondaliteshowing
Neoproterozoic rims. Ages in million years.(c)Probabilityplotsof monazite
agedata from the khondalitesshowing
variousage peaks that can be correlated
withdistinct
thermal spikes duringthe syn- and post-assembly
stages ofGondwana,
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