Constraining timing of early Archean magmatism using stepwise Pb-Pb leaching (PbSL) dating from the Bastar Craton, central India

Stepwise Pb-Pb leaching (PbSL) is a silicate digestion technique based on sequential acid treatment of a mineral, resulting in the selective recovery of radiogenic and common Pb from the crystal lattice. This technique has proven effective for directly dating rock-forming silicates associated with metamorphic reactions that define a PT path. Two Paleoarchean granitoids from the Bastar craton were selected for this study; one is a ~3.5 Ga orthopyroxene-bearing basement tonalite sample near Nagaras, CH13 ^[1] and the other is the ~3.6 Ga ‘true granite’ sample near Dalli-Rajhara ^[2], resampled as C30. Orthopyroxene (Opx) and microcline (Mc) grain separates were leached in multiple steps using HBr, HNO_3, and HF acids ^[3]. Lead was separated and purified using 100 µL and 10µL AG1-X8 anion exchange resin using HNO₃ – HBr chemistry, and the purified Pb fraction was dissolved in 0.2% HNO₃ solution. Lead isotope ratios were measured on a Thermo-Fisher Scientific Neptune Plus MC-ICPMS at the Indian Institute of Technology (IIT) Kharagpur, India, using a novel approach of combining Thallium-doping with sample-standard bracketing. Two multi-grain Opx fractions from sample CH13 yielded a combined Pb-Pb isochron age of 3594±95 Ma (2σ error, MSWD = 8.3, n = 11), which is slightly older than the zircon U-Pb crystallization age of 3453±21 Ma ^[1]. Therefore, sample CH13 is a Paleoarchean charno-enderbite formed at lower crustal depths. In contrast, Mc separated from sample C30 yielded a Pb-Pb isochron age of 3189±3 Ma (2σ error, MSWD = 1.3, n = 5), which confirms that despite an older zircon U-Pb crystallization age, this sample is not a ~3.6 Ga ‘true granite’ but is a product of a later ~3 Ga partial melting event, related to the Mesoarchean Sukma orogeny ^[1].

[1] Nandi, A., Mukherjee, S., Sorcar, N., and Vadlamani, R., 2023, Relict Mesoarchean (2.99–2.94 Ga) metamorphism overprinted by late Neoarchean tectonothermal event(s) from the Sukma Group supracrustal rocks, Bastar craton, India: Evidence from new Lu-Hf and Sm-Nd garnet isochron and Th-U-total Pb monazite ages: Precambrian Research, v. 390, p. 107056.

[2] Rajesh, H. M., Mukhopadhyay, J., Beukes, N. J., Gutzmer, J., Belyanin, G. A., and Armstrong, R. A., 2009, Evidence for an early Archaean granite from Bastar craton, India: Journal of Geological Society, London, v. 166, p. 193 – 196.

[3] Frei, R., and Kamber, B. S., 1995. Single mineral Pb-Pb dating. Earth and Planetary Science Letters, 129(1-4), 261 – 268.