Abstract This study reports on two lamprophyre dykes from the Rapuru area along the margin of the Eastern Dharwar Craton (EDC) and the Nellore Schist Belt (NSB). The Rapuru lamprophyre (RL) dykes are situated along the southern extension of the Prakassam Alkaline Province (PAP). The RL dykes are deformed, yet still preserve a porphyritic–panidiomorphic texture, with mica phenocrysts, and amphibole and feldspars in the groundmass. Geochemically, the RL dykes have a low Mg# (0.28–0.37), and Ni (30–60 ppm) and Cr (119–228 ppm) concentrations that indicate their evolved nature, such as for other reported lamprophyres from the PAP and EDC. This is further supported by Sr–Nd isotopic ratios that show an affinity towards a mid-ocean ridge basalt (MORB)–ocean island basalt (OIB)-like signature and a juvenile magmatic nature. The RL seems to have been affected by two major influences, namely, the primary source region, which is geochemically juvenile similar to the compositional field of enriched-MORB, and the continental lithosphere. Such magmas are known to have formed in a back-arc-basin environment. The initial 87 Sr/ 86 Sr ratio ( c. 0.7012–0.7045) and initial ε Nd ratios (3.13–7.93) are in line with back-arc basin basalts recorded in other parts of the world. The field observations and bulk-rock Sr–Nd radiogenic isotope values in the present study support the Paleoproterozoic nature of the RL. This concurrence of juvenile radiogenic isotopes and fluid-related trace element compositions apparently suggest dehydration of a subducted-slab-triggered metasomatism of the overlying mantle wedge in a subduction-related geodynamic setting. Such intrusive lamprophyre rocks of older ages are limited in India as well as other parts of the world. The 2.1 and 1.8 Ga rocks are widely considered to represent the initial accretion and final break-up of an erstwhile Columbia supercontinent assembly. We argue that the RL were formed in the Paleoproterozoic during the waxing stages of the Columbia supercontinent assembly in a back-arc basin environment, most probably due to the low degree of partial melting of the asthenosphere–lithospheric interaction caused by the introduction of an influx of subduction components into the arc–back-arc basin system.

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 (K 2 O/Na 2 O: 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 (Na 2 O + K 2 O: 3.7–5.1 wt%), Fe 2 O 3 (17–19 wt%), P 2 O 5 (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.