• Christian Koeberl

      Articles written in Journal of Earth System Science

    • Mineral chemistry of lava flows from Linga area of the Eastern Deccan Volcanic Province, India

      Sohini Ganguly Jyotisankar Ray Christian Koeberl Theodoros Ntaflos Mousumi Banerjee

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      Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province (EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG) sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector. A threetier classification scheme has been adopted for the characterization and classification of individual lava flows. Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ). The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively. The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing. The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range ∼Fo22 to Fo66 revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes. Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite component. Pyroxene phenocrysts are characterized by a prominent Tienrichment. Phenocrystal plagioclase grains are calcic (An52.7$–$An72.9), whereas groundmass plagioclase are relatively sodic (An39.2$–$An61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals a temperature span of 850° to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range from 1040°$–$1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption.

    • Insights into petrogenetic processes from a part of the Eastern Deccan Volcanic Province, India, using cluster analyses of mineralo-chemical data


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      Multivariate statistical analysis involving hierarchical clusters was carried out for basaltic samples (and associated units) from Khandwa (21°49'N, 76°21'E). ‘Highly significant’ or ‘significant’ linear correlation coefficient values (r) corresponding to different minerals (namely, olivine, clinopyroxene and plagioclase), denote several oxides (as for example, MgO, FeO, SiO$_2$, Al$_2$O$_3$, Na$_2$O, CaO and TiO$_2$) which were used for construction of dendrograms. Critical analysis of hierarchical patterns revealed that at the outset of magmatic crystallization, heterogeneous (${\sim}$greater symmetry) clusters are present. For the crystallization of the lava flows, the ‘bulk level of crystallization’ (in respect of clinopyroxene and plagioclase) varies from ${\sim}$30 to ${\sim}$60%, whereas their ‘ultimate crystallization’ appears to be quite high (${\sim}$80 to ${\sim}$97%). The bulk crystallization of the lava flows shows a broad control of ambient temperature. The dyke system (feeder dyke and chilled dyke) also shows bulk crystallization pattern similar to that of lava flows. Cluster analyses for basement gabbroic rock suggest that there is a wide compositional spectrum for the accumulate portion, whereas the intercumulus portion is marked by relatively restricted compositions. In general, the present CA (cluster analysis) clearly indicates progressive amalgamation of clusters (and their concomitant fall of symmetry) with advancing differentiation.


      $\bullet$ Multivariate statistical analysis from a portion of eastern Deccan volcanic province deciphers distinctive nature of crystallization.

      $\bullet$ Cluster patterns indicate distinct ‘bulk level of crystallization’ and ‘ultimate crystallization’ for the lavas.

      $\bullet$ Rapid quenching of dyke rocks only allows them to crystallize up to the bulk level.

      $\bullet$ Ambient temperature controls bulk level of crystallization for the lavas.

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