Articles written in Journal of Earth System Science

    • Mineral chemistry of tourmaline from Mashak Pahar, South Purulia Shear Zone (SPSZ), eastern Indian Shield

      Santanu Acharjee Jyotisankar Ray Payel Dey Debapriya Bhattacharyya Mousumi Banerjee Basab Chattopadhyay Shyamal Sengupta A K Bhatt D Chowdhury A K Dwivedi Sanjoy Mahato Arka Ranjan Jana P B Maithani P V Ramesh Babu

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      The area of investigation at and around Mashak Pahar, Bankura district, West Bengal, India comprises a number of rock types namely: granite gneiss, migmatized quartz tourmaline gneiss, quartz pebbleconglomerate, ferruginous quartzite, quartz tourmaline veins (as veins) and graphite schists. Interestingly, the study area lies in the region extending South Purulia Shear Zone (∼Tamar–Porapahar Shear Zone) which marks the boundary between two contrasting tectonic blocks of eastern India, namely, the Chhotanagpur Gneissic Terrane (CGC) to the north and Singhbhum Group of rocks to the south. The rocks of the study area are poly-phasedly deformed by three phases of folding, namely, F1, F2 and F3. All the tourmalines are classified to be of ‘Alkali Group’. Chemistry of tourmalines from migmatized quartz tourmaline gneiss and those from quartz tourmaline veins are in conformity with their relation to (earthquake induced) shear system evolution in this terrain. In general, the compositional evolutionof tourmaline during prograde metamorphism (∼400°–730°C) has been supported by both petrographic and chemical evidences. Assessment of mineral–chemical data of constituent tourmaline grains clearly suggests compositional variations across zonal boundaries within tourmaline that was controlled by changing metamorphic milieu in this terrane. Field and petrographic evidences clearly indicate activation of earlier and later shears in this region accompanied by infiltration of boron and formation of zoned tourmaline crystals.

    • 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.

    • Olivine decadence of continental flood basalts in the light of recent experiments: A case study from Khandwa, Eastern Deccan Volcanic Province


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      Continental flood basalts of the world are represented either by picrobasalt/picrite layers (with abundant growth of olivine) or by (almost) olivine-depleted tholeiite basalts. We have taken a case study from the olivine-depleted basaltic rocks from Khandwa (21°49'N: 76°21'E) Eastern Deccan Volcanic Province. Our research shows that recent cooling experiments (available in the literature) involving a dry basalt and construction of some selected binary major element variation diagrams (entailing lever rules) can explain (and quantify) the insignificant level (${\sim}$2–4%) of olivine crystallization (${\sim}$olivine decadence) in tholeiite basalts. Such decadent olivine retains a skeletal crystallographically-oriented geometry. We contemplate that our simplistic model of olivine decadence can be extended to other flood basalt provinces of the world as well.


      $\bullet$ First-time cogent model of olivine decadence in Deccan basalts

      $\bullet$ Integration of newly obtained experimental data and binary variation diagrams

      $\bullet$ Petrographic support validates the model

      $\bullet$ Model can be extended to other continental Flood basalts

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