In the South Indian basement, several crustal-scale amphibolite facies shear zones occur between high-grade

metamorphic units with a different geological history: the EW-trending Moyar Shear Zone (MSZ) is a zone of

predominantly dip-slip transport separating the Archaean Dharwar Craton in the north from the late Archaean

Nilgiri Block in the south. The NE-SW-trending, dextral-transpressive Bhavani Shear Zone (BSZ) constitutes

the southern boundary of the Nilgiri Block in its western part and bounds the southern Dharwar Craton further

east. South of the BSZ, the high-grade metasediments and metaintrusives of the Maddukarai region are separated

from the 0.6 Ga-metamorphic Madurai Block by the EW-trending dextral Palghat Shear Zone (PSZ). MSZ, BSZ

and PSZ are regarded as parts of the prominent Cauvery shear system. The N-S-trending sinistral Kollegal Shear

Zone (KSZ), which transects the Dharwar Craton, is cut off by the Cauvery shear system.

These shear zones play an important role in reconstructing the position of India within the East Gondwana

terrane assembly. A combined Sm-Nd, Rb-Sr and U-Pb isotopic study was carried out on granulite remnants,

amphibolite facies (mylonitic) gneisses and pre-, syn- und postmetamorphic intrusives in order to examine the

tectonometamorphic evolution of the MSZ, BSZ, PSZ and KSZ.

Whole rock data

The majority of relic and retrogressed granulites from the MSZ (TDM 2.3–3.1 Ga) and BSZ (TDM 2.6-2.9 Ga)

show late Archaean average crustal residence ages. With respect to their LIL-, Nb-depleted tonalitic nature and

Nd-model ages, these rocks resemble the neighbouring Nilgiri granulites. Mylonitic gneisses and granulite

remnants from the BSZ yielded a Sm-Nd-whole rock-age of 2520 ± 150 Ma (εNd(t) +0.6; MSWD = 2.2), which

is interpreted as protolith age. I-Type granites and tonalites, which intruded the MSZ ~620 Ma ago (87Sr/86Sri

0.7039), gave unusually young Nd model ages of 1.8-1.9 Ma suggesting derivation from a Mid- to

Neoproterozoic upper mantle source, presumably with minor contribution of an older crust. An allochthonous

quarzite (TDM 3.3 Ga) of the BSZ is regarded as counterpart of the Archaean Sargur group, which is exposed in

schist belts of the Dharwar Craton. As suggested by geochemical features (LIL-, Nd-rich granitoids) and

Neoarchaean-Palaeoproterozoic Nd-model ages (TDM 2.3-3.0 Ga), the PSZ-gneisses show affinity to the adjacent

Madurai Block. Sheared orthogneisses from the KSZ show Mesoarchaean average crustal residence ages (TDM

3.2-3.3 Ga) typical for Dharwar Craton gneisses.

Mineral dating on granulites

Mineral age data of relic granulites from the MSZ, BSZ and PSZ provide evidence for the metamorphic

precursor history of the shear zone rocks: gnt-plag-px-granulites from low-strain domains yielded Sm-Ndgarnet-

whole rock ages of 2355 ± 22 Ma (εNd(t) -1.4) for the MSZ and 2329 ± 19 Ma (εNd(t) -2.0) for the BSZ,

both recording late-stage Palaeoproterozoic granulitisation of the rocks and corresponding with garnet data from

the Nilgiri Block. Correlated low εNd-initial values reflect the short time span between crustal genesis and garnet

crystallisation. Further Sm-Nd mineral data from BSZ-granulites are between 1275 ± 10 Ma and 1106 ± 48 Ma

(garnet/plagioclase-whole rock-pairs; εNd(t) –5.8 to –25.4), indicating a Mesoproterozoic metamorphic imprint.

A charnockite from the southern BSZ, which is interpreted as a separate lithological unit, yielded a reproducable

Sm-Nd-garnet-whole rock age of 1705 ± 11 Ma (εNd(t) –12.4), presumably recording late Palaeoproterozoic

metamorphism.

Mineral dating on gneisses and younger intrusives

Amphibolite facies rocks with younger fabrics yielded Neoproterozoic to early Palaeozoic mineral age data for

the MSZ, BSZ and PSZ:

Sm-Nd mineral ages from gneiss-mylonites imply a first stage of early Pan-African shearing in the MSZ ~745

Ma ago (garnet/plagioclase-hornblende-pairs: 743 ± 13 Ma, 747 ± 75 Ma) and in the BSZ ~730 Ma ago (garnetwhole

rock-pair: 726