Sm-Nd and Rb-Sr ages and P-T history of the Archean Sittampundi and Bhavani layered meta-anorthosite complexes in Cauvery shear zone,South India: evidence for Neoproterozoic reworking of Archean crust

 Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone (CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They and their host supracrustal-gneiss rocks record at least two stages of tectonometamorphic history. The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D₁-M₁) while the second stage is associated with dominantly E–W dextral transcurrent shearing and metamorphic recrystallisation (D₂-M_CSZ). Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages: Sittampundi – 2935±60 Ma, ɛ_Nd + 1.85±0.16 and Bhavani – 2899±28 Ma, ɛ_Nd + 2.18±0.14 (2σ errors). Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M₁ metamorphism occurred soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet granulites from the Sittampundi complex characterise the M_CSZ as a high-P event with metamorphic peak conditions of c. 11.8 kbar and 830°C (minimum). The M_CSZ is associated with significant isothermal decompression of the order of 4.5–3.5 kbar followed by static high-temperature rehydration and retrogression around 600°C. The timing of M_CSZ is inferred to be Neoproterozoic at c. 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for a garnet granulite from the Sittampundi complex and its agreement with the 800–600 Ma published age data on post-kinematic plutonic rocks within the CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in a model Proterozoic Gondwana supercontinent. Received: 5 December 1995 / Accepted: 3 May 1996     

High Precision Isotope Dilution Determination of Potassium by Thermal Ionisation Mass Spectrometry

Two independent articles published more than a decade ago pointed to the potential of the somewhat neglected K‐Ca system for very precise K‐Ca dating and thermochronology, but have not been followed up since. In the first of these articles, highly precise determination of K and hence the K/Ca ratio in lunar granites showed the feasibility of K‐Ca dates of micas with an analytical precision known so far in only U‐Pb and Ar/Ar dating. In the second article, it was shown that responses of the K‐Ca and Rb‐Sr isotopic systems in micas subjected to a slow post‐metamorphic cooling could provide new insights into thermochronology. Crucial to the follow‐up of these two promising leads is the development of a simple and user‐friendly method for routine high precision K determination in real samples, such as the one described in this report.     

Cosmogenic and radiogenic noble gases in the Dhajala chondrite

Whole rock and chondrules of the Dhajala chondrite were analyzed for Ne, Ar, Kr and Xe by total melting as well as by stepwise heating techniques. The cosmic ray exposure ages for the whole rock and the chondrules are6.2 ± 0.8 and6.3 ± 1.0m.y. as determined by the²¹Ne method and4.8 ± 1.5 and4.2 ± 2.0m.y. by the³⁸Ar method, respectively. The K-Ar age of the whole rock is4.2 ± 0.4b.y. The elemental composition of the trapped gas in this chondrite is of “planetary” type. The radiogenic¹²⁹Xe contents in the whole rock and chondrules are similar and this component is very retentively sited in the chondrules.     

Interannual Variation of Eddy Kinetic Energy from TOPEX Altimeter Observations

Monthly mesoscale eddy kinetic energy (EKE) per unit mass has been computed for four years, 1993-1996, from TOPEX altimeter data in the Indian Ocean. It ranges from 50 cm²/s² to 2,700 cm²/s² (about 4,000 cm²/s² near the Somali region in a few months). In the Arabian Sea and the Bay of Bengal, regions of high energies associated with various current systems under the influence of monsoonal winds have been delineated. Monthly variation of EKE near the Somali region has been studied. In this region the maximum EKE per unit mass has been observed during August every year, with variations in magnitude from year to year. The mesoscale eddy kinetic energy computed from TOPEX altimeter-derived SSH during 1993-1996 is highest near the Somali region during the SW monsoon, due to formation of mesoscale eddies and also because of upwelling. In the Bay of Bengal, high eddy kinetic energy is seen toward the western side during nonmonsoonal months due to the western boundary current. In the South Indian Ocean, it is high at a few places in some of the months. A large part of the Indian Ocean exhibits low eddy kinetic energy (less than 300 cm²/s²) year-round.