Abstract

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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