Petrogenesis and U–Pb zircon dating of the Chaitma Alkaline Complex from the southern margin of the Central Indian Tectonic Zone: geodynamic implications

Abstract In this study, we constrain the petrogenesis and U–Pb zircon age of a newly discovered alkaline complex, christened the Chaitma Alkaline Complex at the southern margin of the Central Indian Tectonic Zone in central India. The Chaitma Alkaline Complex comprises syenites and gabbros, emplaced coevally, and show features consistent with magma mixing. Geochemically, syenites are potassic–ultrapotassic (K2O/Na2O: 0.79–3.42), and contain high Ba (c. 800–2700 ppm) and Sr (c. 1400–3200 ppm). They show enrichment of the light rare earth elements (LREEs) relative to the heavy rare earth elements (HREEs) (La/Yb: 32–103) and do not display any Eu anomaly. Based on their geochemical signatures, such as low MgO (<0.87 wt%), Ni (8–16 ppm) and Cr (7–44 ppm) contents and prominent Zr–Hf negative anomaly, the syenites are inferred to have been derived by partial melting of a carbonated/metasomatized thickened lower crustal source. The coeval gabbros are undersaturated in silica (41–44 wt%), with relatively high total alkalis (Na2O + K2O: 3.7–5.1 wt%), Fe2O3 (17–19 wt%), P2O5 (3.1–4.9 wt%), Sr (1600–3400 ppm) and Ba (300–3500 ppm) contents. These have low MgO (<4.8 wt%), Ni (13–30 ppm) and Cr (18–84 ppm). Their chemistry is interpreted to be the result of interaction with the syenitic magma. These geochemical characters along with the high LREE/HREE ratio, negative trough in Nb–Ta, Zr–Hf, Ti, Sr and Rb, and positive spike of Pb in a multielement diagram, and enrichment of LILEs over HFSEs indicate their derivation from a metasomatized subduction-modified garnet–peridotite mantle source. Our study indicates that syenites and gabbros of the Chaitma Alkaline Complex were formed from genetically unrelated parental magmas derived from distinct sources. U–Pb dating of zircon yielded a magmatic emplacement age of 1626 ± 15 Ma for the syenites. The Chaitma Alkaline Complex was presumably formed during a short period of crustal extension in the midst of a protracted period of continent–continent collision and granulite-grade metamorphism (c. 1.71–1.58 Ga) at the southern margin of the Central Indian Tectonic Zone.