Clay mineralogy of Ir-bearing Anjar intertrappeans, Kutch, Gujarat, India:Inferences on palaeoenvironment

Clay mineral studies on the WT boundary succession at Anjar, Kutch show sme ctite, sepiolite and palygorskite as its major clay mineral assemblage, The re is a distinct variation in the clay minerology across the section with s epiolite and palygorskite dominating the lower and upper lithounits, respec tively, in association with smectite. The KTB layer, marked by the Ir(1271 pg/g) and Os (1414 pg/g) anomaly contain sepiolite and smectite in subequal proportions. The clay stratigraphy of the succession reflects progressive increase in aridity and basicity in the depositional environment during the deposition of Anjar KTB sediments. It appears that the sepiolite-palygorsk ite-smectite complexes in the sediments were developed in a lacustrine or p eri-marine environment of high basicity under arid conditions induced and i nfluenced by Deccan volcanism. In this environment, the Si, Mg and Al ions necessary for the formation of sepiolite and palygorskite could be supplied by the hydrothermal fluids associated with volcanism by the dissociation o f silicates already available in the depositional environment or simultaneo usly by both the processes. Clay mineral associations in the Lr-bearing Anj ar intertrappean sediments across its lithounits are more compatible with t he depositional environment influenced by the volcanism, than an asteroidal impact. In the upper part of the succession, the prdominance of palygorski te and smectite suggests that the latter contributed the necessary Al, Si a nd Mg ions to result the Former. The REE signatures indicate that sepiolite -smectite is most likely the carrier phase of Ir in Anjar intertrappeans.