• Talat Ahmad

      Articles written in Journal of Earth System Science

    • Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex

      Ashima Saikia Bibhuti Gogoi Mansoor Ahmad Talat Ahmad

      More Details Abstract Fulltext PDF

      The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, crosscutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.

    • Geochemistry and petrogenesis of Proterozoic granitic rocks from northern margin of the Chotanagpur Gneissic Complex (CGC)

      Bhupendra S Yadav Nishchal Wanjari Talat Ahmad Rajesh Chaturvedi

      More Details Abstract Fulltext PDF

      This study presents the geochemical characteristics of granitic rocks located on the northern margin of Chotanagpur Gneissic Complex (CGC), exposed in parts of Gaya district, Bihar and discusses thepossible petrogenetic process and source characteristics. These granites are associated with BarabarAnorthosite Complex and Neo-proterozoic Munger–Rajgir group of rocks. The granitic litho-units identifiedin the field are grey, pink and porphyritic granites. On the basis of geochemical and petrographiccharacteristics, the grey and pink granites were grouped together as GPG while the porphyritic graniteswere named as PG. Both GPG and PG are enriched in SiO_2, K_2O, Na_2O, REE (except Eu), Rb,Ba, HFSE (Nb, Y, Zr), depleted in MgO, CaO, Sr and are characterised by high Fe^* values, Ga/Alratios and high Zr saturation temperatures (GPG_{avg} ∼ 861^◦C and PG_{avg} ∼ 835^◦C). The REE patternsfor GPG are moderately fractionated with an average (La/Yb)_N ∼ 4.55 and Eu/Eu^* ∼0.58, than PGwhich are strongly fractionated with an average (La/Yb)_N ∼ 31.86 and Eu/Eu^* ∼0.75. These featuresindicate that the granites have an A-type character. On the basis of geochemical data, we conclude that the granites are probably derived from a predominant crustal source with variable mantle involvementin a post-collisional setting.

    • Episodic crustal growth in the Bundelkhand craton of central India shield: Constraints from petrogenesis of the tonalite–trondhjemite–granodiorite gneisses and K-rich granites of Bundelkhand tectonic zone

      Hiredya Chauhan Ashima Saikia Talat Ahmad

      More Details Abstract Fulltext PDF

      Tonalite–trondhjemite–granodiorite gneisses (TTG) and K-rich granites are extensively exposed in the Mesoarchean to Paleoproterozoic Bundelkhand craton of central India. The TTGs rocks are coarsegrained with biotite, plagioclase feldspar, K-feldspar and amphibole as major constituent phases. The major minerals constituting the K-rich granites are K-feldspar, plagioclase feldspar and biotite. They are also medium to coarse grained. Mineral chemical studies show that the amphiboles of TTG are calcic amphibole hastingsite, plagioclase feldspars are mostly of oligoclase composition, K-feldspars are near pure end members and biotites are solid solutions between annite and siderophyllite components. The K-rich granites have biotites of siderophyllite–annite composition similar to those of TTGs, plagioclase feldspars are oligoclase in composition, potassic feldspars have XK ranging from 0.97 to 0.99 and are devoid of any amphibole. The tonalite–trondhjemite–granodiorite gneiss samples have high SiO₂ (64.17– 74.52 wt%), Na₂O (3.11–5.90 wt%), low Mg# (30–47) and HREE contents, with moderate (La/Yb)CN values (14.7–33.50) and Sr/Y ratios (4.85–98.7). These geochemical characteristics suggest formation of the TTG by partial melting of the hydrous basaltic crust at pressures and depths where garnet and amphibole were stable phases in the Paleo-Mesoarchean. The K-rich granite samples show high SiO₂ (64.72–76.73 wt%), K₂O (4.31–5.42), low Na₂O (2.75–3.31 wt%), Mg# (24–40) and HREE contents, with moderate to high (La/Yb)CN values (9.26–29.75) and Sr/Y ratios (1.52–24). They differ from their TTG in having elevated concentrations of incompatible elements like K, Zr, Th, and REE. These geochemical features indicate formation of the K-granites by anhydrous partial melting of the Paleo-Mesoarchean TTG or mafic crustal materials in an extensional regime. Combined with previous studies it is interpreted that two stages of continental accretion (at 3.59–3.33 and 3.2–3.0 Ga) and reworking (at 2.5–1.9 Ga) occurred in the Bundel khand craton from Archaean to Paleoproterozoic.

    • Geochemistry and petrogenesis of Biabanak-Bafq mafic magmatism: Implication for the evolution of central Iranian terrane

      Monireh Poshtkoohi Talat Ahmad Ashwini Kumar Choudhary

      More Details Abstract Fulltext PDF

      Precambrian magmatism in the Biabanak-Bafq district represents an extensive sequence of mafic magmatic rocks. Major, trace and rare earth elements reveal that the low-Ti basement mafic rocks are magnesium tholeiite and low-Ti cover a mafic rock belongs to Fe-tholeiite, whereas, the high-Ti alkaline mafic rocks, as well as dolerites, show much more Fe–Ti enrichment. Primitive mantle normalized trace element patterns show a relative enrichment of LREE and LILE and depletion of HFSE, but have an equally distinct continental signature reflected by marked negative Nb, Sr, P, and Ti anomalies. The composition of the intrusive rocks is consistent with fractional crystallization of olivine +/- clinopyroxene +/- plagioclase, whereas variations in the Sr and Nd isotope compositions suggest heterogeneous sources and crustal contamination. Low-Ti group samples contain a crustal signature in the form of high La/Yb, Zr/Nb, and negative εNd values. In contrast, high-Ti mafic magmatic rocks display an increase in La/Yb with a decrease in Proterozoic alkaline rocks recognized across the central Iran. The presence of diverse mafic magmatic rocks probably reflects heterogeneous nature of sub-continental lithospheric mantle (SCLM) source. The mafic magmatism largely represents magmatic arc or rift tectonic setting. It is suggested that the SCLM sources were enriched by subduction processes and asthenospheric upwelling.

    • Geochemistry and petrogenesis of acidic volcanics from Betul–Chhindwara Belt, Central Indian Tectonic Zone (CITZ), central India


      More Details Abstract Fulltext PDF

      Betul–Chhindwara belt is part of Central Indian Tectonic Zone (CITZ) that includes Proterozoic basalt, rhyolite, quartzite, mafic–ultramafic rocks, volcano sediments and banded iron formation (BIF). Studied rhyolites and leuco-micro granites are deformed due to shearing and includes quartz, K-feldspar (microcline), muscovite, biotite and epidote. In some samples, feldspar has been sericitized due to interaction with hydrothermal fluids. The major element geochemistry of volcanic rocks clearly indicates acidic nature and falls in the rhyolite field. Rhyolites show difference in the enrichment of REEs and major element composition which help us divide them into two groups and also indicate heterogenous source. The rhyolites show very strong negative Eu anomaly, which indicates fractionation of feldspar. Positive anomalies of U–Th–Zr for the rhyolites indicate crustal involvement. The $\varepsilon\rm{Ndt (t=1500)}$ for the Group I rhyolites vary from –1.42 to –0.19 and for the Group II rhyolites vary from –5.81 to +0.14 and DM model ages for Group I rhyolites vary from 2284 to 2464 Ma and for Group II vary from 2174 to 2863 Ma. It is suggested that contemporary mafic magma of the Betul–Chhindwara belt while ascending from mantle sources interacted with the continental crust at different levels, supplying heat and fluids which reduced the melting points of the crustal source rocks, producing felsic melt of varying compositions. Tectonic discriminant diagrams and geochemical data indicate subduction zone tectonic environment for the genesis of the Betul–Chhindwara acidic volcanism. The acidic volcanics of Betul–Chindwara, Sakoli and the Bijli rhyolites from the adjoining areas display similarity in terms of the total alkali vs. silica diagram and many of the major and trace elements, including rare earth element characteristics. Compared to Betul Rhyolite, Sakoli Rhyolites are derived from less enriched source with less involvement of crust and/or the latter represents high degree of partial melting of similar source. They are considered contemporaneous to Betul Rhyolite based on geochronological data. Contrastingly, Bijli Rhyolite show highly fractionated patterns with high LREE enrichment indicating considerable crustal involvement which is very obvious for within plate magmatism, assigned for the Bijli rhyolites.

  • Journal of Earth System Science | News

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

© 2017-2019 Indian Academy of Sciences, Bengaluru.