• MANOJ A LIMAYE

Articles written in Journal of Earth System Science

• Rootless calc-silicate folds in granite: An implication towards syn- to post-plutonic emplacement

Deformation of the Champaner Group of rocks that form a part of Southern Aravalli Mountain Belt, western India, occurred during the Grenville orogeny (ca. 1400–935 Ma). Two phases of deformation are recorded: D1, persistent throughout the group and characterised by westerly plunging tight isoclinal foldsand D2, a localized phase of deformation associated with shortening of the earlier folds from the eastern margin. Both the phases of deformation are in association with the syn-tectonically emplaced Godhra granite. The present work records rootless calc-silicate folds in granite belonging to the older formation,located at the eastern fringe of the Champaner Group. Field evidences suggest superimposition of Type 2 interference pattern trending NE–SW over rootless Type 0 of varying trends from NW–SE to N–S. The superposed pattern obtained from the field study differs in terms of structural trends with the neighbouring Precambrian stratigraphic units. These stratigraphic units include the Champaner Group to which the study area belongs, the Kadana Formation of the Lunavada Group and Pre-Chamapaner Gneissic Complex. Rootless character of folds found within the study area imply syn-post plutonic emplacement of Godhra granite.

• Geochemistry of calc-silicate rocks around Lunavada region, NE Gujarat: Implications for their protolith, provenance and tectonic setting

In this work, the calc-silicate rocks affiliated to the ‘Kadana Formation’, a youngest formation of the Lunavada Group have been investigated. These rocks are found to be embedded within associated rock types, viz., quartzites and metapelites in the form of isolated lensoidal bodies. Contact metamorphic textures and the typical mineral assemblage, viz., Act +Di + Cal + Qtz + Ttn $\pm$ Mc $\pm$ Pl $\pm$ Bt $\pm$ Ep $\pm$ Scp $\pm$ Chl with minor proportion of apatite, zircon and opaques can be observed in these calc-silicates. Major oxides, trace and rare earth elements were analysed to investigate the protolith type, provenance and tectonic setting of these rocks. Protolith must be calcareous sandstone with varied proportion of clay and deposited in shallow water environment as revealed by CaO, Al$_{2}$O$_{3}$ and FeO+MgO and Al–Zr–Ti ternary diagram, respectively. Low to moderate weathering of source rocks has been indicated by A–CN–K ternary diagram. Th/Sc vs. Zr/Sc and Th/Co vs. La/Sc plots confirm the continental source with felsic nature for these rocks and the calc-silicate samples fall within the active continental margin region of Sc–Th–Zr/10 diagram which also justify the kind of provenance for primary sediments.

$\bf{Highlights}$

$\bullet$The calc-silicate rocks show major oxides, trace and rare earth element compositions which are very similar to those of their post-archean counterparts. Similarly, in the REEs vs. sample/REE chondrite normalized pattern, a moderate negative Eu anomaly is observed; these observations suggest that the chemical composition of the continental crust of the study area was similar to that of the post-archean continental crust.

$\bullet$Data plotted in Al$_{2}$O$_{3}$, CaO and FeO + MgO diagram of Barton et al. (1991) indicated that the protolith of calc-silicate rocks of the study area is calcareous sandstone with small and varied amounts of clay within it, as most of the samples fall within or very close to the greywacke zone.

$\bullet$CIA studies are pointing towards the low to moderate weathering of the source under the cold and arid conditions.

$\bullet$The Th–Sc characteristics (Th/Sc > 1) support a predominantly continental source for these rocks; similarly, values of Zr/Sc indicate a moderate amount of sediment recycling.

$\bullet$Higher abundances of incompatible elements imply predominantly felsic rocks in the source, which is also supported by LREE-enriched patterns. As per the Th/Co vs. La/Sc plot, mostly of granitic (felsic) composition is inferred for primary sediments for these calc-silicate rocks.

$\bullet$The study concludes that the primary sediments for the calc-silicates of the Lunavada Group were deposited in an active continental margin setting.

• # Journal of Earth System Science

Volume 129, 2020
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