On the metasomatism of carbonated peridotite mantle of the Sevattur Carbonatite Complex, southern India: insights from phlogopite and apatite compositions

In the present study, we discuss the major (electron probe micro-analyser) and trace element ( in situ laser ablation quantitative inductively coupled plasma mass spectrometry) concentrations of apatite and phlogopite occurring in the Sevattur calcite carbonatite to understand their significance in petrogenesis. Textural evidence suggests that apatite is an oval- or elliptical-shaped, early magmatic phase that occurs in association with the late magmatic–metasomatic phlogopite. Generally, the euhedral apatite grains are formed in the first stage of metasomatism in dolomite carbonatite, followed by interaction with residual carbonate melt (calcite carbonatite). As a result, euhedral apatite gets resorbed grain boundaries with the development of oval and elliptical grain boundaries during transport, along with the formation of phlogopite at the late stage owing to mantle metasomatism. The trace element signatures of apatite, viz. Sr, Y, Mn, (Ce/Yb)cn, Eu/Eu*, F, and total rare earth elements (REEs), indicate its carbonatite lineage. Moreover, apatite from the present study shows high (Eu/Eu*)cn 0.91–1.01 and (La/Yb)cn 50–87 ratios, ≤1000 ppm Mn, 5528–9313 ppm Sr and δEu 0.85–0.94 and δCe 0.99–1.09 values, which are diagnostic features of the early-formed apatite in carbonatites worldwide. Its fluorine content and concentration of heavy REEs (∑REE 0.7–1.5 wt%) indicate their crystallization in the upper mantle at >600°C temperature in greater depth within the carbonatite magma. The low Al (<12%, Al 2 O 3 ) and high BaO ( c. 0.94 wt%), Cr, Ni, Co, Mn, Nb, Sr, Sc, Zr and FeO t /MgO against FeO t /Al 2 O 3 ratios suggest the mantle metasomatic nature of phlogopite in the calcite carbonatite. Their limited variation in the Mg number and Fe# Al 2 O 3 , TiO 2 , K 2 O, MgO and FeO contents indicate that the phlogopite in the carbonatite magma was not affected during primary magmatic crystallization as well as the post-magmatic stage. The presence of a high concentration of incompatible elements in the apatite implies that it is derived from the enriched carbonatite melt, whereas their low concentration in the phlogopite is due to their formation from the residual melt. Similarly, the presence of significant fluorine content in apatite indicates its vital role during the genesis of the Sevattur carbonatite complex. The high F concentration indicates decreasing (H 2 O) water activity and a lowering of maximum melting and liquidus temperatures. It also implies that the parental magma was derived as a partial melt of the apatite–phlogopite-bearing (metasomatized) mantle source.