Phosphate minerals as a recorder of P-T-fluid regimes of metamorphic belts: example from the Palaeoproterozoic Singhbhum Shear Zone of the East Indian shield

Abstract

The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe of the Palaeoproterozoic North Singhbhum Fold Belt (NSFB) of the East Indian Shield. Repeated folding, ductile shearing, and accompanying hydrothermal activities in the SSZ during the late Palaeoproterozoic (ca. 1.66–1.60 Ga) orogeny resulted in a highly tectonized ensemble of rocks including a suite of peraluminous kyanite-rich quartzite (KQR). Near Kanyaluka village, the KQR shows millimetre- to decimetre-thick alternation of kyanite- and quartz-rich bands. The banded rock is intensely sheared and is cross-cut by weakly deformed to undeformed kyanite-quartz veins. In many places, kyanite-rich bands show sea-green coloured pods rich in lazulite. Textural studies reveal that deformed kyanite and quartz grains are sequentially replaced by augelite and lazulite (X_Mg > 0.97) at the terminal phase of shearing in the SSZ. Modelling of observed textures and mineral compositions with the C-Space program shows the following augelite- and lazulite-forming reactions:

1. 1.829Kyanite + 0.998P + 1.5H₂O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO₂ + 0.002Fe + 0.0002Na

2. 1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO₂ + 0.001 Ca

Stoichiometry of the balanced chemical reactions suggests that a significant amount of P, Mg, and H₂O were added to, and Al and SiO₂ were subtracted from, the host kyanite-rich rock to produce augelite and lazulite. Experimental studies in the system Al₂O₃-SiO₂-FeO-MgO-P₂O₅-H₂O and the results of quantitative geothermobarometry suggest that lazulite and augelite were formed in a narrow temperature (440 ± 40°C) and pressure (~6.3 ± 1 kbar) range. Ductile shearing along the SSZ channelized the P- and Mg-rich fluids that metasomatized the kyanite-rich bands and veins to produce lazulite. The inferred P-T conditions can be explained by burial of the studied rock under an ~25 km-thick thrust sheet of NSFB during the Palaeoproterozoic orogenesis.

Keywords:

• lazulite
• augelite
• kyanite-quartzite
• P-Mg metasomatism
• thrusting
• Singhbhum Shear Zone

Acknowledgements

We are thankful to Mr Priyadarshi Chowdhury and Dr A. Gupta for their assistance during the fieldwork. Constructive comments from two anonymous reviewers and Prof. Stern, the editor-in-chief, IGR, helped a lot to improve clarity and presentation of the manuscript. Detailed comments of the editors on the revised manuscript (after first revision) helped us add a tectonic context of phosphate mineralization. We thank the editors for giving us the opportunity. We thank Prof. V.K. Singh for inviting us to contribute to this special issue.

Additional information

Funding

Maitrayee Chakraborty and Sayan Biswas acknowledge the research grants sanctioned by CSIR, New Delhi, and UGC, New Delhi, respectively. Nandini Sengupta acknowledges the financial assistance from project WOS-A, DST (Department of Science and Technology, New Delhi). Pulak Sengupta acknowledges the financial assistance from the CAS, Department of Geological Sciences and UPE-II, Jadavpur University.