1 - Cochin University of Science and Technology

and hornblende gneisses containing concordant enclaves of
amphibolite and associated granites, in the Gap proper.
Transition from the meta - sedimentary granulites to the
migmatitic gneisses is marked by a sequence of hornblende
biotite gneisses.
Khondalites are meta sedimentary gneisses consisting of
two major units namely, garnet - biotite gneisses and garnet
sillimanite gneisses. The latter is exposed in the
northern flank of the Pal ghat Gap with an E - W strike of
foliation in the west and NW - SE trend in the east, showing
sharp contacts with the calc granulites. Contact between the
E trending calc granulites and khondalite has been
established near Pullimanpatti as well as to its west and and
north east. Khondalite often passes into a garnet - gneiss,
with the disappearance of sil liminite and increase in the
amount of feldspars. Eve n t ua 1 I y , wit h the appearance of
cordierite as an essential constituent, it becomes cordierite
garnet - si 11 imanite gneiss, as observed in the Walayar
limestone quarry region.
The characteristic assemblage of these rocks is garnet +
quartz + plagioclase + K-feldspars + biotite + sillimanite +
graphite, with accessory phases including zircon, pyrrhotite,
apatite, sphene and ilmenite. Development of cordierite is
considered to be a result of progressive mineral reaction
(see chapter I I I ) . Sillimanite defines the compositional