Tektonometamorphe Entwicklung von Scherzonen im präkambrischen Basement Südindiens

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 ± 9 Ma). This
tectonic stage immediately followed a period of anorogenic
alkalimagmatism in the eastern continuation of the BSZ suggesting
that it may be attributed to an overall extensive regime. A second
stage of late Pan-African shearing in the MSZ at ~620 Ma is
constrained by statistically equivalent concordant U-Pb zircon ages
that are interpreted to record crystallisation of syndeformative
intrusives with Itype characteristics (granite: 616 ± 19 Ma,
tonalite: 633 ± 23 Ma). Coeval to slightly younger metamorphic
garnet growth in adjacent MSZ-gneisses and -mylonites is reflected
by Sm-Nd-garnet-whole rock ages between 624 ± 9 Ma and 591 ± 5 Ma.
Subsequent postdeformative cooling in the MSZ is constrained by
Rb-Sr micawhole rock ages (muscovite: 594 ± 23 Ma; biotite: 603 ±
12 Ma to 547 ± 7 Ma). The 620 Ma-shearing event in the MSZ predates
late Pan-African tectonometamorphism in the BSZ, which, according
to garnet crystallisation, occurred ~550 Ma ago (Deters-Umlauf,
1997). Amphibolite facies shear deformation in the PSZ is even
younger, as suggested by a Sm-Nd-garnet-whole rock age of 521 ± 8
Ma. A lower limit for the age of ductile shearing in the BSZ is
provided by a Sm-Nd-garnet-whole rock-age of 513 ± 5 Ma reflecting
postdeformative emplacement of a pegmatitic dyke in mylonitic host
gneisses. According to Rb- Sr-mica age dating, postdeformative
cooling of the sheared BSZ-gneisses (biotite-whole rock-pairs: 508
to 491 ± 12 Ma) and the undeformed dyke (muscovite-K'feldspar: 504
± 13 Ma, biotite-K'feldspar: 488 ± 12 Ma) as well as cooling of the
PSZ-gneisses (biotite-whole rock-pairs: 486, 487 and 488 ± 12 Ma)
may have been slightly diachronous. In summary, the new
geochronological data provide evidence for non-synchronousity of
late Pan-African tectonometamorphism in the MSZ, BSZ and PSZ. At
the time of structurally-controlled amphibolite facies
metamorphism, all reworked gneisses had negative εNd(t) values
(–24.7 to -9.4) reflecting their earlier crustal evolution.
Sinistral shearing along the KSZ is not related to Pan-African
processes, but has to be attributed to Palaeoproterozoic
metamorphism in the Dharwar Craton: a lower limit for ductile
tectonics is provided by a Sm- Nd-garnet-whole rock age of 2388 ±
16 Ma (εNd(t) –7.3). Strikingly younger Rb-Sr biotite-whole rock
ages of 2137 ± 52 Ma and 2091 ± 51 Ma may be explained by local
reheating of the crust that exceeded the closure temperature of
biotite. The new results support the idea of a terrane boundary
running along the southern BSZ. This terrane boundary separates the
Archaean Dharwar/Nilgiri crustal province with 2.5 Ga metamorphism
from the Proterozoic mobile belt of the Madurai province with a 0.6
Ga high-grade imprint. According to geochronological data, South
India and Madagascar probably were subjected to different
plate-tectonic regimes in the early Neoproterozoic. Late
Neoproterozoic syndeformative emplacement of mantle-derived
granitoids with crustal contamination both in the MSZ (~620 Ma) and
in Central-Madagascar (~ 630 Ma) may point to a neighbouring
position of the two East Gondwana continents at that time.