The Khajraha, oldest carbonatite from India: Implications on carbonated-eclogite source, multi-level emplacement and its petrogenetic link with orthomagmatic fluids

Abstract

Age and petrogenesis of a Neoarchean calcio‑carbonatite dyke from the Khajraha, Bundelkhand craton (BC, India) is presented here. Zircon UPb age data show emplacement age of 2567 ± 8.0 Ma marking the discovery of the oldest Indian carbonatites. Dyke consists of coeval emplaced three litho variants, recording multi-level emplacement and concomitant exsolution of carbo-hydrothermal fluids in their micro-textures and geochemical composition. The first emplaced grey sovite (c1) grade to silico‑carbonatite (SiO2: 24–26 wt%) consist of abundant autolith-segregations set in groundmass of fine-size spinifex-habit calcite. Autolith-segregations are hydrothermally altered early-formed cumulate and their alteration products formed while deep-seated storage. The later emplaced magma batches, pink alvikite (c2) and white sovite (c3), are monomineralic of calcite and have pure calcio‑carbonatite composition represented to be residual melt formed after fractionation of silico‑carbonatite solid-residue (c1) as evident from Harker variations. During fractionation at depth, hydrous fluids exsolved are re-equilibrated with primary magmatic biotite and amphibole, forming chlorite with byproducts of titanite and K-feldspar at ~300 °C. This autometasomatism also witnessed precipitation of primary hydrothermal phases like spherulite-chlorite, epidote, titanite, K-feldspar, apatite and REE-phases. While emplacing to subvolcanic level they variably admixed, disaggregated and dispersed in calcio‑carbonatite magma defining micro- to meso-scale flow fabrics in c1. At subvolcanic level, undercooling led the calcio‑carbonatite to crystallize spinifex-habit calcite by wrapping autolith-segregations. Continuously increasing Y/Ho and Th/U ratios, negative Eu-anomaly and positive Y-anomaly in REE pattern of c1, c2 and c3 record that fractionation of carbonic fluids (Mn-rich) accompanied with this kinematic crystallization. In addition to undercooling, rate of retention and venting out of exsolved fluids in the dyke-conduit has regulated nucleation and growth rate of spinifex-habit calcite, particularly in c3, they grown to pegmatoidal size in hot CO2-rich fluid ambience as indicated the gradual lowering of δ18OSMOW values from 7.28 to 3.41 ‰ keeping δ13C values almost constant. Dyke contains low Ba (27–298 ppm), Sr (146–739 ppm) and total REE (7–696 ppm), representing to be a carbonatite of ‘trace-element depleted end-member’. The elevated δ13CPDB-1 values (−1.93 to −2.99 ‰) coupled with δ18OSMOW values (5.82–7.53 ‰, in c1 and c2) within the range of Primary Igneous Carbonatite (PIC), low Nb/Y (<1) and Nb/Ta (1.4–16.1) ratios and high Zr/Hf (40–48) and Zr/Nb (12–116) ratios similar to recycled slab component and zircons εHf(t) value ~ −4.0 with TDMC of ~3.0 Ga suggest that calcio‑carbonatite magma sourced from a Mesoarchean depleted mantle containing carbonated-eclogite.