Origin of K-rich green clays within Late Cretaceous Deccan basalts: A local K-depository

This study focuses on compositional diversity and origin of green clays within the Late Cretaceous volcaniclastic horizons (green boles) in Deccan volcanics based on petrographical, mineralogical and geochemical investigation on outcrop samples. Green boles, showing either sharp or gradational contacts with the underlying basalt, were comprised of three different green clays: type 1 dark green void-filling clay, type 2 light green clay formed by the alteration of volcanic lithic and vitric grains, and type 3 light green clay pellet formed by the alteration of fine-grained vitric materials. FTIR and XRD data indicated 10 Å (celadonite type) and 15 Å (smectite) as dominant clay constituents. The type 1 clay showed higher oct. Fe total (∼1.03 a.p.f.u.), Mg (∼0.62 a.p.f.u.) and lower oct. Al (∼0.34 a.p.f.u.) compared to types 2 and 3. Type 2 green clay showed two subtypes. Interlayer cations in type 2A and 2B averaged 0.86 and 0.72 a.p.f.u., respectively. The high K ₂O and interlayer cation of type 1 and 2A clays confirmed celadonite. Type 2B and 3 green clays, variably mixed with smectite and quartz, belonged to interlayer cation deficient mica category. The association of type 1 celadonite with Fe, Mg-smectite and Ca-zeolite indicated its formation from pore fluid with changing chemistry. The alteration of volcanic lithic and vitric fragments under weakly reducing conditions with steady supply of relevant cations formed type 2 and 3 green clays. The study of green clays in bole beds is crucial for understanding the wide compositional diversity of potassic clay associated with continental flood basalts.