Multistage reaction textures in xenolithic high-MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration-driven metasomatism
K. Rickers et al., Multistage reaction textures in xenolithic high-MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration-driven metasomatism, J METAMORPH, 19(5), 2001, pp. 561-580
High-MgAl rocks occur as xenoliths (up to 2 in in diameter) in mafic granul
ites at a newly discovered locality near Anakapalle. Following an early pha
se of deformation, ultrahigh-temperature (UHT) metamorphism and near-isothe
rmal decompression, the rocks were intruded in a lit-par-lit manner by fels
ic melts (charnockite), which caused local-scale metasomatism. A subsequent
deformation produced isoclinal folds and the distinct gneissic foliation o
f the charnockite still at granulite facies conditions. The sequence of mul
tiphase reaction textures in the high-MgAl xenoliths reflects the changes o
f physicochemical conditions during the polyphase evolution of the terrane,
UHT metamorphism (stage 1, >1000 degreesC, c. 10 kbar) is documented by re
lics of extremely coarse grained domains with the assemblage orthopyroxene
(opx)(1) + garnet (grt)(1) + sapphirine (spr)(1) + spinel (spl)(1) + rutile
(rt). A subsequent phase of near-isothermal decompression in the order of
1-2 kbar (stage 2) resulted in extensive replacement of grt 1 and opx I meg
acrysts by lamellar (opx(2) + spr(2)) symplectites. The intrusion of felsic
melt (stage 3) led to the development of a narrow metasomatic black wall r
eaction zone (bt + sil + plg(3) + opx(2,3) + rt) at the immediate contact o
f the xenoliths and in melt infiltration zones to the partial replacement o
f (opx(2) + spr(2)) symplectites by biotite and sillimanite and/or plg(3),
mainly at the expense of orthopyroxene, with concomitant coarsening of the
intergrowth texture. The subsequent deformation (stage 4) further modified
the symplectite textures through polygonization, recrystallization and grai
n-size coarsening. The deformation was followed by a period of cooling and
decompression (stage 5, c. 800 degreesC, 4-7 kbar) as indicated by local gr
owth of late garnet (grt(5),) at the expense of (opx + spr + plg) domains a
t static conditions, Recently published isotope data suggest that the multi
stage evolution of the high-MgAl granulites at Anakapalle followed a discon
tinuous P-T trajectory that may be related to heating of the crust through
magmatic accretion culminating in deep-crustal UHT metamorphism at 1.4 Ga (
stage 1), fast uplift of the UHT granulites into mid-crustal levels as a co
nsequence of extensional tectonics (stage 2), emplacement of felsic magmas
in the Grenvillian (at c. I Ga, stage 3) resulting in reheating of the crus
t to high-T conditions followed by a phase of compressional tectonics (stag
e 4) and a period of cooling to the stable geotherm (stage 5) still in the
Grenvillian.