Chapter 14 c. 1450 Ma regional felsic volcanism at the fringe of the East Indian Craton: constraints from geochronology and geochemistry of tuff beds from detached sedimentary basins

40Ar–39Ar incremental heating studies on mineral separates from three sets of rocks in the Singbhum craton in eastern India have helped unravel the thermochronometric history of the terrane and explain an earlier discrepancy between Sm–Nd (~3800 Ma) and Rb–Sr (~3100 Ma) whole-rock ages for the Older Metamorphic Tonalitic Gneiss (OMTG). High precision plateau ages for hornblende separates from the Older Metamorphic Group of rocks (OMG) suggest that this unit is older than 3300 Ma and that enclaves of both the OMTG and OMG within the batholithic complex cooled to ~500 °C at 3300 ± 15 Ma following engulfment in magma, forming the Singbhum Granite (SG). Results from a biotite separate from the OMTG imply that slow cooling continued to a temperature of ~300 °C at ~3160 Ma. Study of a feldspar separate from the Singbhum Granite, suggests final cooling (uplift?) through the ~150 °C isotherm ~660 Ma ago.We suggest that an earlier Rb–Sr whole-rock age of 3130 ± 85 Ma on the OMTG did not yield the crystallisation age, but rather the time of cooling to strontium retention temperature for the biotite in these rocks. We also demonstrate that for hornblende and mica from Archean rocks, 40Ar–39Ar incremental heating experiments can yield very high precision plateau ages (e.g., ± 2 to ± 5 Ma at 3300 Ma). In practice, however, we suggest this technique can separate events in the early Archean spaced ~30 Ma apart.

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Geophysical studies of the major sedimentary basins of the indian craton, their deep structural features and evolution

Tectonophysics ◽

10.1016/0040-1951(76)90018-4 ◽

1976 ◽

Vol 36 (1-3) ◽

pp. 225-245 ◽

Cited By ~ 28

Author(s):

L.N. Kailasam

Keyword(s):

Sedimentary Basins ◽

Structural Features ◽

Indian Craton

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First Report of Florencite from the Singhbhum Shear Zone of the East Indian Craton

International Journal of Mineralogy ◽

10.1155/2014/978793 ◽

2014 ◽

Vol 2014 ◽

pp. 1-8 ◽

Cited By ~ 2

Author(s):

Maitrayee Chakraborty ◽

Sayan Biswas ◽

Nandini Sengupta ◽

Pulak Sengupta

Keyword(s):

Shear Zone ◽

Small Proportion ◽

Indian Shield ◽

First Report ◽

Precambrian Rocks ◽

East Indian ◽

Singhbhum Shear Zone ◽

Geological Features ◽

The World ◽

Indian Craton

Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo- to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7–11.8 mol%) and goyazite (<2 mol%) components. Florencite of this study is dominated by Ce (~49 mol%) with significant La (~30 mol%) and Nd (~21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencite-kyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks suggest that florencite was formed during Palaeoproterozoic metamorphism that culminated at the P-T range of 490±40∘C and 6.3±1 kbar. Integrating all the geological features it is postulated that florencite was formed due to metasomatism of some aluminous protolith by infiltration of acidic fluids charged with PO4-3 and LREE.

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