• Kamal Kant Sharma

      Articles written in Journal of Earth System Science

    • An Ediacaran–Cambrian thermal imprint in Rajasthan, western India: Evidence from 40Ar-39Ar geochronology of the Sindreth volcanics

      Archisman Sen Kanchan Pande Hetu C Sheth Kamal Kant Sharma Shraboni Sarkar A M Dayal Harish Mistry

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      The Sindreth Group exposed near Sirohi in southern Rajasthan, western India, is a volcanosedimentary sequence. Zircons from Sindreth rhyolite lavas and tuffs have yielded U–Pb crystallization ages of ∼768–761 Ma, suggesting that the Sindreth Group is a part of the Malani magmatic event. Earlier 40Ar-39Ar studies of other Malani volcanic and plutonic rocks yielded disturbed argon release spectra, ascribed to a ∼550 Ma thermal event possibly related to the Pan-African orogeny. To test and confirm this possibility, we dated two whole-rock and three feldspar separate samples of the Sindreth volcanics by the 40Ar-39Ar method. All samples yield disturbed argon release spectra suggesting radiogenic argon loss and with plateau segments at 550 Ma or 490 Ma. We interpret these as events of argon loss at 550–490 Ma related to an Ediacaran–Cambrian thermal event, possibly related to the Malagasy orogeny. The combined older and new 40Ar-39Ar results are significant in showing that whereas Ediacaran–Cambrian magmatic and metamorphic events are well known from many parts of India, they left thermal imprints in much of Trans-Aravalli Rajasthan as well. The overall evidence is consistent with a model of multiphase assembly of Gondwanaland from separate continental landmasses.

    • Geology of granitoids of Pindwara–Abu Road Belt from Mesoproterozoic Delhi Supergroup: Tectonic implications

      RIYA DUTTA HARSH BHU RITESH PUROHIT KAMAL KANT SHARMA

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      Granitoids from Pindwara–Abu Road Belt (PARB) are studied to characterize their tectonostratigraphic status in relation to the associated metasediments. The PARB lies along the southern swathes of the Mesoproterozoic Delhi Supergroup (DSG) in the Aravalli Delhi Mobile Belt (ADMB) of the northwestern Indian Shield. The outcrop scale granitoids of the study area are categorized into massive and gneissic variants. The former variety is being prominently exposed as leucocratic variant intrusive into the melanocratic gneisses as well as associated metasediments. Massive intrusive granitoids have been dated previously representing three major regional thermal events of 1000, 850, and 750 Ma. These multiple tectono-thermal events have led to diminished preservation of pristine gneissic character in the granitoids outcropping as dismembered bodies in the PARB. Consequently, the field relationship between the granitoids and associated metasediments is extremely obliterated. The present study, with the help of regional and detailed mapping on different scales and petrography, has attempted to establish basement–cover relationship between the gneissic granitoids and the associated metasediments. Quartzite outcrops are delineated as marker horizons characterizing the contact lithounit between the two. The cover rocks have sheared contact with the gneissic basement, which has a limited patchy outcrop pattern as ‘Remnants’. These ‘Remnant’ outcrops, conceivably behaved as primitive relicts, perhaps acted as a cradle for the proximal metasediments. Earlier studies, based on heavy carbon isotope character, have given an age span of ${\sim}$1200–1300 Ma for the associated calcareous metasediments of the PARB. The gneissic granitoid, basement to these metasediments, is hence considered to be pre-1300 Ma, older than the massive granitoids (1000–750 Ma). The span of events reveals that the southern terrane of the DSG of rocks, especially the PARB has a younger geological history as compared to the northern terrane of the Delhi Supergroup which has records of 1700–1400 Ma. The events recorded from the PARB of the DSG are younger in age and indicate Meso-Neoproterozoic transition (${\sim}$1300–750 Ma). Globally, these are correlatable with the Grenvillian orogeny followed by Rodinia Supercontinent, amalgamation, and splitting tectonism in the northwestern Indian Shield.

      $\bf{Highlights}$

      $\bullet$ Granitoids from Pindwara-Abu Road Belt (PARB) are studied to characterize their tectonostratigraphic status in relation to the associated metasediments and are attempted to establish the basement-cover relationship between the granitoids and the associated country rocks.

      $\bullet$ Granitoids of the study area are categorized into massive and gneissic variants.Massive granitoid emplacement pertains to the threemajor Neoproterozoic thermal events that led to diminished preservation of pristine gneissic character.

      $\bullet$ Gneissic Granitoids preserve pristine characters, conceivably behaved as primitive relicts that perhaps acted as a cradle for the proximal metasediments. These gneissic granitoids are henceforth considered to be pre-1300 Ma, older than the massive granitoids (1000 Ma to 750 Ma).

      $\bullet$ Quartzite outcrops are delineated as marker horizons, characterizing the contact lithounit between the gneissic granitoids and metasediments. These granitoids and associate quartzite display regional scale enêchelon pattern of distribution reflecting impact of transpressional shearing.

      $\bullet$ The events recorded from the PARB of the Delhi Supergroup are younger in age and indicate Meso-Neoproterozoic transition (${\sim}$1300–750 Ma) that are globally correlatable with the Grenvillian orogeny followed by Rodinia Supercontinent amalgamation and splitting in the north-western Indian Shield.

    • Geochronology and geochemistry of Daba gabbro, Sirohi region: Closure of Rodinia amalgamation processes in the northwestern Indian Shield

      KAMAL KANT SHARMA C V DHARMA RAO SUNG WON KIM RITESH PUROHIT RIYA DUTTA

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      The study incorporates geochemistry and geochronology of a gabbro, located near Daba, from the Neoproterozoic Sirohi Group along the northwestern Canks of the Indian Shield. The Daba gabbro is massive, cumulate, undeformed and forms a linear body bounded by shear zones at margins. The Neoproterozoic Sirohi Group marks the end of the compressional tectonic regime and the emplacement of gabbro marks amalgamation processes of the Rodinia Supercontinent in the northwestern Indian Shield. The gabbro is characterized by enriched TiO2, P2O5, LILE, LREE, and HREE depletion; and high La/SmN, Gd/Yb$_N$, Zr/Nb ratios with no positive Eu anomalies and no Nb, Ta and Ti negative anomalies suggesting an Oceanic Floor Basalt (OFB) aDnity. The Ti–V plot of the Daba gabbro a firms OFB affinity, whereas this parameter for proximal mafics from the Delhi Supergroup shows Oceanic Island Basalts (OIB) aDnity. Primitive mantle normalized multi-element pattern and chondrite-normalized pattern for trace element and REE show OFB affinity. Depletion of Nb–Ta, P and Ti with a flat distribution of LILE and HFSE reflects limited degrees of crustal contamination (exception Zr positive anomaly). SHRIMP geochronology of magmatic zircons from the two gabbro samples has a well-defined ${}^{206}$Pb/${}^{238}$U Concordia at 825 ± 4 and 828 ± 4 Ma. This data corresponds to the gabbroic emplacement was almost synkinematic with felsic magmatism, coeval with the Sirohi orogeny (${\sim}$835 Ma), denoting the end of compression tectonic regime. This marks the closure of the Rodinia amalgamation process in the northwestern Indian shield.

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