• V S HEGDE

Articles written in Journal of Earth System Science

• Zircon SHRIMP U–Pb geochronology, geochemical and Nd isotope systematics of Neoarchean granitoids, Gadag Greenstone Belt, Dharwar Craton, southern India: Petrogenesis and tectonic significance

Coupling of the geological processes in the arc-magmatic and back-arc provinces of accretionary orogens in Neoarchean plate tectonic setting is a subject of current research all over the world. The Dharwar Craton of southern India is an example of such an accretionary orogen, with an arc-magmatic province in the east and a back-arc province in the west, referred as the Eastern Dharwar Craton and the Western Dharwar Craton, respectively. The boundary between the two provinces is considered to be marked by a 400-km long shear zone along the eastern margin of the Gadag–Chitradurga–Karighatta greenstone belt which is called as the Chitradurga Boundary Shear Zone. Potassic, metaluminous, I type, calc-alkalic to alkali-calcic, arc-magmatic granitoids are widespread in the EDC. But they are also found to occur along the western margin of the Chitradurga–Gadag greenstone belt. SHRIMP U–Pb zircon ages of the granitoids in the western back-arc province in the Gadag region occurring near Srimant Gudda, Mulgund and Chabbi have been determined as 2565  $\pm$   24 to 2591  $\pm$   64 Ma old. Within errors, the ages of these granitoids are the same as the Lakundi and Turchihal granitoids occurring to the east of the Gadag Greenstone Belt in the arc-magmatic province. Nd isotope systematics of the granitoids suggest that they were formed from magmatic melts that were produced by remelting of 3200–3500 Ma old heterogenous continental crust. Rare inherited zircons support this antiquity of the protoliths. Occurrence of granitoids of similar age and origin, in the western back-arc province and eastern arc-magmatic province in the Gadag area was attributed to thrust duplex structure in the Gadag region. However, elsewhere, along the western margin of the Chitradurga greenstone belt near Harpanhalli, Hosdurga–Nagamangala–Pandavapura sector, or away from it in the Arsikere–Banavara, where repetition by thrusting is not obvious, late potash arc-magmatic type granitoids of similar age as the Gadag arc-magmatic granitoids are observed. The arc-magmatic type granitoids appear to have overstepped the boundary shear into the back-arc province at several places. Arc-magmatic and back-arc boundary may be diffuse rather than sharp, as also suggested by some earlier workers.

$\bf{Highlights}$

$\bullet$ The ages and petrogenesis of the granitoids around GGB of both the sides of EDC (i.e., eastern magmatic-arc province) and WDC (i.e., western back-arc provinces) are very much similar.

$\bullet$ The geochemistry and isotope systematics (i.e., Nd TDM2 ages and εNdT values at 2.5–2.6 Ga) of the granitoids exposed one both the sides are also similar.

$\bullet$ This shows not only a similar antiquity of both the provinces, but also gives evidence for a possible diffusive nature of boundary between the EDC and the WDC around GGB.

• Sediment dispersal pattern along an engineered micro-tidal tropical estuarine beach

To evaluate the annual sediment dispersal pattern, a year-long beach profile monitoring and granulometric analysis were adopted along the Kundapura estuarine beaches of coastal Karnataka. The river mouth is bounded by breakwaters and the coast is armoured by seawalls and groins. Beaches adjacent to the breakwater showed progradation as a result of the shadow zone created due to the sheltering effect from incident waves. Evidence also comes from the replacement of coarser mode sediments by finer. Those away from the shadow zone, however, experienced extensive erosion. They are subjected to along-shore drift and minimal replacement from the same. It was also observed that despite being tropical estuarine beaches, sediments did not show any affinity towards a Cuvial-derived character. These points towards transport of fine sediments further offshore and retention of coarser fraction in the surf zone that is to be carried shoreward. Breakwaters in turn facilitate this by channelising the river discharge farther in the sea. Evidence towards the same is also provided by the well-sorted to moderately well-sorted nature of the sediments. Breakwaters were thus observed to highly influence the sediment dispersion and distribution along an estuarine coast. This study will provide knowledge on the role of coastal structures in sediment dispersion while framing future coastal protection strategies for the conservation of the coast.

$\bf{Highlights}$

$\bullet$ Progradation of beaches adjacent to the breakwaters due to sheltering from direct waves whereas erosion of beaches away from the influence of these structures is observed.

$\bullet$ Extensive engineered coast has resulted in the narrowing of the natural beach width making it reflective with a high-energy environment thus accelerating further erosion. Due to this, the beach has lost its recreational value.

$\bullet$ The along-shore drift directions derived considering beach morphology, beach volume change and beach sedimentology, along with the mode of transport and environmental affinity of sediments provide a comprehensive understanding of sediment dispersal pattern along the coast.

• Geochemical constraints of bulk rocks and in-situ zircon from the granitoids of the Hungund Schist Belt: An insight for hydrothermal activity

Orogenic, gold hosting greenstone belts in Eastern Dharwar Craton (EDC), India, are surrounded by granitic bodies and affected by hydrothermal activities. In this paper, field, petrographic and geochemical study of granites coupled with LA-ICP-MS analyses of the trace element abundances of zircons from the granites of the Hungund Schist Belt, the northwest continuation of Kolar and Ramgiri schist belts in EDC is discussed to report the effects of hydrothermal alteration. The geochemical characteristics such as Nb/ Ta ratio (${\sim}$5), low Zr/Hf ratio, depleted ${\Sigma}$REE from the granites close to the shear zone provide clue towards magmatic–hydrothermal interactions in the source rock, while granites away from the shear zone show feeble alteration signature. Zircons from the altered granite contain higher concentration of trace elements, LREE enrichment and high ${\Sigma}$REE with weak positive Ce anomalies and strong negative Eu anomalies indicating hydrothermal alteration. While the zircons from unaltered granites have low to moderate trace element concentration and REE patterns similar to unaltered magmatic zircons. Contrarily, in the ternary plot, the zircons from unaltered granites show minor affinity towards metasomatised hydrothermal field instead of clustering under purely magmatic Beld. Hence it is considered that these zircons from unaltered granite underwent some minor alteration effect, while the zircons from altered granite are affected completely due to hydrothermal activity. Therefore, it is concluded that movement of hydrothermal solution has taken place along the shear zone in the study area, which has affected more on granites close to the shear zone and minor effects are seen in granites away from the shear zone. Hydrothermal alterations often result in concentration of specific economic minerals and hence, can be used as an indicator for mineral exploration.

$\bf{Highlights}$

$\bullet$ Petrography and geochemistry of granites and trace element analysis of zircons from these granites of Hungund schist belt are presented. The highlights are:

$\bullet$ Sericitization, chloritization and epidotization are principle signatures observed in hydrothermally altered granites.

$\bullet$ Nb/Ta ratio (${\sim}$5), low Zr/Hf ratio, depleted ∑REE from the granites close to the shear zone suggests magmatic-hydrothermal interactions in the source rock, while the granites away from the shear zone do not reflect any significant alteration signature. Zircon geochemistry from the altered granite reveals that it contains higher concentration of trace elements, shows LREE enrichment and high ∑REE with weak positive Ce anomalies and strong negative Eu anomalies indicating hydrothermal alteration, whereas the zircons from unaltered granites have low to moderate trace element concentration and also REE patterns similar to unaltered magmatic zircons.

$\bullet$ The zircons from unaltered granites are not purely magmatic zircons and have undergone some minor alteration effect which is also evident from the LREE overabundances.

$\bullet$ Therefore, we conclude that movement of hydrothermal solution has taken place in the study area along the shear zone, showing prominent effects on the granites close to the shear zone and minor effects to those away from the shear zone. Such hydrothermal alteration signatures can be used as an indicator for mineral exploration.

• Journal of Earth System Science

Volume 132, 2023
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Continuous Article Publishing mode

• Editorial Note on Continuous Article Publication

Posted on July 25, 2019