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SmNd isotopes and ree geochemistry of madras granulites, india: an introductory statement

Centre Armoricain d'Etude Structurale des Socles, Institut de Géologie, Campus de Beaulieu, 35042 Rennes France; S.K. Sen; Indian Institute of Technology, Department of Geology and Geophysics, Kharagpur 721302 India
Precambrian Research DOI:10.1016/0301-9268(87)90082-9

ABSTRACT Major and trace elements including REE as well as SmNd isotope analyses of eight granulite samples from the Madras area are presented. The data provide evidence for the origin of some of the granulites, but in other cases demonstrate metasomatic changes in major and trace element contents, including LREE. The data furthermore demonstrate that the analysed granulites are not cogenetic. However, when plotted in a SmNd isochron diagram, the samples yield a linear array which corresponds to an age T = 2555 ± 140 Ma and ϵNd(T) = +1 ± 1.4. This age of 2555 ± 140 Ma could represent the time of granulite facies metamorphism immediately preceded by the formation of the protolith. The ϵNd(T) = 1 value may be indicative of a near chondritic mantle source, or alternatively of mixing between a depleted mantle and a crustal component.

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    Article: Crustal structure and tectonics of the northern part of the Southern Granulite Terrane, India
    V. Vijaya Rao, Kalachand Sain, P.R. Reddy, Walter D. Mooney
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    ABSTRACT: Deep seismic reflection studies investigating the exposed Archean lower continental crust of the Southern Granulite Terrane, India, yield important constraints on the nature and evolution of the deep crust, including the formation and exhumation of granulites. Seismic reflection images along the Kuppam–Bhavani profile reveal a band of reflections that dip southward from 10.5 to 15.0 s two-way-time (TWT), across a distance of 50 km. The bottom of these reflections beneath the Dharwar craton is interpreted as the Moho. Further south, another reflection band dipping northward is observed. These bands of reflectivity constitute a divergent reflection fabric that converges at the Moho boundary observed at the Mettur shear zone. Reflection fabrics that intersect at a steep angle are interpreted as a collisional signature due to the convergence of crustal blocks, which we infer resulted in crustal thickening and the formation of granulites. Anomalous gravity and magnetic signatures are also observed across the Mettur shear zone. The gravity model derived from the Bouguer gravity data corroborates seismic results. The tectonic regime and seismic reflection profiles are combined in a 3-D representation that illustrates our evidence for paleo-subduction at a collision zone. The structural dissimilarities and geophysical anomalies suggest that the Mettur shear zone is a suture between the Dharwar craton in the north and another crustal block in the south. This study contributes significantly to our understanding of the operation of Archean plate tectonics, here inferred to involve collision and subduction. Furthermore, it provides an important link between the Gondwanaland and global granulite evolution occurring throughout the late Archean.
    Earth and Planetary Science Letters.
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Keywords

age T
 
analysed granulites
 
cases
 
chondritic mantle source
 
crustal component
 
depleted mantle
 
granulite facies metamorphism
 
granulite samples
 
granulites
 
linear array
 
Madras area
 
metasomatic changes
 
protolith
 
samples yield
 
SmNd isochron diagram
 
SmNd isotope analyses
 
trace element contents
 
trace elements