Orals: Mon, 28 Apr | Room G2
The Cryogenian Period, which began around 720 million years ago, was marked by prolonged low-latitude glaciations known as ‘Snowball Earth’. The prevailing hypothesis is that these global cooling events were driven by enhanced weathering of continental fragments in the tropics during the breakup of the Rodinia supercontinent. To test this idea, we applied a Bayesian network analysis (cf. Gernon et al., 2021) to examine the statistical relationship between seawater chemistry (⁸⁷Sr/⁸⁶Sr) and the fraction of continental land in the tropics, as inferred from recently available plate tectonic reconstructions. Our results reveal a weak overall correlation between these variables, even when accounting for multi-million-year time lags and the effects of auto-correlation in the time series. This finding suggests that the Earth's weathering response to global tectonic reorganisation is more complex than previously assumed. We conclude that while enhanced chemical weathering may have driven Snowball Earth, it likely arose from processes other than the first-order distribution of continents in the tropics, although a secondary influence cannot be excluded. Finally, we explore alternative plate tectonic mechanisms that yield unexpectedly long time lags between continental breakup and changes in ocean chemistry and climate, which may help reconcile disparate observations from the geologic record.
Reference
Gernon, T.M., Hincks, T.K., Merdith, A., Rohling, E.J., Palmer, M.R., Foster, G.L., Bataille, C.P. and Muller, D. Global chemical weathering dominated by continental arcs since the mid-Palaeozoic. Nature Geoscience 14, 690–696, doi: 10.1038/s41561-021-00806-0 (2021).
How to cite: Gernon, T. and Hincks, T.: Testing enhanced surface weathering hypotheses for Snowball Earth, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7188, https://doi.org/10.5194/egusphere-egu25-7188, 2025.
There were at least two major glaciation events (i.e., “Snowball Earth” events) in the Neoproterozoic Era when ice sheets dominated Earth’s surface. Radioisotopic dating indicates the first, Sturtian glaciation lasted ~56 Myr, and the second, Marinoan glaciation lasted ~4 Myr.
Why do the two glaciation events – which are only separated by a ~22 Myr interglacial gap – have a ~14x difference in duration? To first order, the glacial termination, and thus duration, is determined by (1) changes in the albedo of Earth’s surface and/or (2) changes in greenhouse warming from the atmosphere, likely driven by enhanced CO2 and the geologic carbon cycle.
Here, we simulated the evolution of atmospheric CO2 via the geologic carbon cycle during the Sturtian and Marinoan glaciation events to determine what conditions could explain the difference in their duration. While the 4 Myr Marinoan glaciation was reproduced in >30% of model runs with a variety of model parameter values, we find that only 0.05% of model runs reproduced the 56 Myr Sturtian glaciation. The Sturtian model runs require very low levels of CO2 outgassing from volcanos and extremely efficient seafloor weathering (which consumes CO2) to keep atmospheric CO2 levels low enough to sustain glacial conditions for 56 Myr. To reproduce the Marinoan glaciation, the opposite is required: a 1.6x increase in CO2 outgassing and a 10x decrease in seafloor weathering.
What could cause such drastic changes in CO2 outgassing and seafloor weathering in successive glaciations separated by only 22 Myr? Possible explanations relate to the Franklin Large Igneous Province (LIP), the depth of mid-ocean ridges, and high-temperature anhydrite production in the seafloor; however, none of these explanations are directly indicated by geologic evidence.
Therefore, the differing durations of the two glaciation events – and particularly the long duration of the 56 Myr Sturtian – indicate that an important aspect of the Neoproterozoic carbon cycle is not being captured. We suggest several possibilities and look forward to open discussions that may illuminate the solution.
How to cite: Thomas, T., Macdonald, F., and Catling, D.: Long duration of the ~56 Myr Sturtian Snowball Earth event suggests missing link in geologic carbon cycle., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14406, https://doi.org/10.5194/egusphere-egu25-14406, 2025.
The two Cryogenian ‘snowball Earth’ glaciations, the Sturtian (~717-658 Ma) and Marinoan (~654-635 Ma), represent extreme climate states when ice reached equatorial latitudes and persisted for millions of years. Varve-like laminites deposited before and after the Sturtian glaciation reflect high-frequency climate cycles linked to solar, ocean and atmospheric dynamics. However, to date, no evidence of such cycles has been documented during the snowball Earth interval. Here, we analyze a ~5.5 m thick bed of laminites within the Port Askaig Formation, Scotland—a Sturtian glaciogenic succession—to reconstruct short-term climate variability on snowball Earth. Petrographic analysis indicates the laminites represent annual varves, reflecting freeze-thaw cycles and seasonal sediment contributions to a glacio-lacustrine environment. Spectral analysis of laminar set thickness reveals statistically significant periodicities of similar length to the present-day Quasi-Biennial Oscillation, Schwabe cycle, and Gleissberg cycle. This finding supports linkages between solar forcing, dynamic ocean circulation and regional climatic variability, which modulated glaciogenic sedimentation during the Sturtian. The preservation of multiannual to multidecadal cycles within the laminites provides important new insight into the persistence of solar-ocean-atmospheric interactions during the Cryogenian.
How to cite: Griffin, C., Gernon, T., Rugen, E., Spencer, A., Warrington, G., and Hincks, T.: Interannual to multidecadal climate oscillations in the Cryogenian, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2138, https://doi.org/10.5194/egusphere-egu25-2138, 2025.
The late Ediacaran Nama Group of southern Namibia and northwestern South Africa hosts a mixed carbonate-siliciclastic Proterozoic succession and is key for resolving the timing of early metazoan evolution, leading to a suite of geochronology studies of these rocks. Important outcrops of the upper Nama Group are found in the Swartpunt area, where the depositional sequence is preserved as a series of thrust plates that formed during compression associated with the Gariep orogeny. Here, numerous silicified volcanic tuff interbeds are present, but different interpretations regarding the fidelity of associated tuff bed ages result in very different regional stratigraphic correlations. We use geological mapping, integrated with lithostratigraphy, carbonate carbon isotope (δ13Ccarb) chemostratigraphy and high-precision radioisotope geochronology from outcrop and recently acquired drill core from the ICDP project GRIND-ECT (Geological Research through Integrated Neoproterozoic Drilling – Ediacaran-Cambrian Transition) in an attempt to address this issue. A compilation of new and published zircon U-Pb ages from the Swartpunt area shows systematic age repetition within the upper Nama Group, that either reflects pervasive zircon reworking or points to the presence of a cryptic décollement. We investigate the evidence for and against both scenarios, and consider their implications for stratigraphic and δ13Ccarb correlations between the Swartpunt area and coeval autochthonous exposures along the Orange River border with South Africa.
The first scenario implies that some published ash bed ages may be >1 Myr older than their depositional age, increasing the uncertainty of the chronostratigraphic correlation between these two areas by up to 0.22% of the age compared with an analytical uncertainty as low as ±0.02% from the youngest coherent zircon populations. If this scenario is preferred, then a cautious approach would be to consider all ash bed zircon U-Pb ages to reflect maximum depositional ages, thereby highlighting an insidious complication for calibrating rates of paleoenvironmental change and biotic innovation at the dawn of the Cambrian explosion. Given that these issues are revealed in an area that benefits from numerous silicified ash beds and extensive exposure, the inability to confidently discount either scenario highlights a level of compounding uncertainty in stratigraphic correlation that should be carefully considered when constructing global chronostratigraphic frameworks in any interval of the geologic record.
How to cite: Bowyer, F., Messori, F., Wood, R., Linnemann, U., Rojo-Perez, E., Zieger-Hofmann, M., Zieger, J., Ndeunyema, J., Shipanga, M., Mataboge, B., Condon, D., Rose, C., Uahengo, C.-I., Gaynor, S., Muller, I., Geyer, G., Vennemann, T., and Ovtcharova, M.: Geochemistry and high-precision zircon U-Pb geochronology of the Nama Group reveal foundational uncertainties in terminal Ediacaran chronostratigraphy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18792, https://doi.org/10.5194/egusphere-egu25-18792, 2025.
The late Neoproterozoic–early Cambrian signify a time bracket when the Indian craton got separated from the supercontinent Rodinia and was in a process of becoming an integral part of supercontinent Gondwana. The late Neoproterozoic–early Cambrian Bilara Group (BG) of the Marwar Supergroup provides a scope for study of depositional processes and spatio-temporal evolution of a distally-steepened carbonate ramp that developed on westward dipping greater Indian shelf; arguably formed in Indo-Arabian geological province during the time period. Deposited within an intracratonic rift/sag set up fringing the northern margin of the Aravalli craton, the BG succession, is subdivided under three Formations viz. Dhanapa, Gotan and Pondlu, in order of superposition.
Six different facies are identified within BL succession with their depositional environment spanning from supra-peritidal to intertidal to shallow and deep subtidal. The cabbage-headed stromatolites and crinkly laminites with tepee structure represent the shallowest supratidal-peritial setting whereas the LLH-type stromatolite in alternation with algal laminite and plane-laminated carbonates represent products of intertidal to shallow subtidal set-up. The limestone-shale heterolithics having signature of storm action is interpreted as deposit of subtidal shelf above storm wave base. The occurrence of calaclastite, intraclastic conglomerate with carbonate mass flows origin are indicative of steep slope at the distal part of the ramp. A distally-steepened ramp geometry is visualized for the Bilara carbonate platform. Additionally, metres-thick soft sediment deformation (SSD) structure layers including disharmonic folds, low-angle thrusts, distorted laminae, fluidisation pipes, slump and load structures, homogeneities, diapirs, etc. at different stratigraphic levels through the BG succession, traceable over hundreds of metres in outcrop, bear indication of basin-scale instability in course of Bilara carbonate platform development.
From delineation of facies succession and documentation of facies stacking pattern, two cycles of deposition inferred from the Bilara lithopackage; DC1 and DC2. While the DC-1 is transgressive and represented by superimposition of facies types of increasing bathymetry, the second cycle DC-II is progradational, shallowing upward and represented by progressively shallow water facies types.
Stable isotope (C ) study on Bilara carbonate with systematic sampling from different facies associations reveal four major intervals of negative isotope excursion (EN1, EN2, EN3 and EN4) of medium to long duration and three positive excursions (EP1, EP2, EP3) of short duration. The EN1- records highest negative value; where δ13Ccarb value as low as -10.4 ‰ but for significantly of short duration. The other three negative excursions (i.e. EN2, EN3 and EN4) δ13C values are quite similar and ranges in between –8 to -6.5‰; in terms of stratigraphic interval (i.e time duration) the EN3 records the longest.
In most of the cases , high negative δ13C values are in close association and succeeded upward by SSD horizons. The presence of SSD structures, presence of Bitumen in pore spaces and large scale negative carbon isotope suggest, destabilization of methane clatharate as possible cause of carbon isotope excursion.
How to cite: Sharma, R. and Chakraborty, P. P.: Depositional architecture of a Neopreoterozoic distally-steepened carbonate ramp from the Bilara Group, Marwar Supergroup, Rajasthan, India and a few clues on Supercontinent and ocean- atmosphere interaction , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12442, https://doi.org/10.5194/egusphere-egu25-12442, 2025.
The present study focuses on the complex fluid-induced processes involved in the evolution of two suites of high-grade rocks from the northern part of the Eastern Ghats Belt, India. We make a comparative study on the role of fluids in the modification of lower crustal rocks; the felsic gneiss/granite mylonite in the Mahanadi Shear Zone (MSZ) and investigate the origin of several micrometres to meters thick syenite veins hosted in and at the contacts of mafic granulite and charnockite away from the MSZ. The syenite (K-feldspar-hyalophane-clinopyroxene-titanite-fluorapatite-allanite-epidote-calcite/REEcarbonates-actinolite-quartz with or without ilmenite-thorite-zircon) is coarse-grained and bears mineralogical features distinct from either side of the contact. We document features like orthopyroxene changing to clinopyroxene, anorthitic rims on plagioclase, and myrmekite patches at the syenite and mafic granulite interface. K-feldspar and hyalophane occur in coarse-recrystallized pockets with the latter often occurring along grain boundaries of the former. Fluorapatite grains occur as euhedral megacrysts and are marginally replaced by patchy as well as large crystals of allanite (typically zoned and consisting of thorite inclusions), epidote, and actinolite grains. The entire assemblage is infiltrated by calcite veins and patches. We interpret this as a metasomatic transformation of the original charnockite rock (orthopyroxene- K-feldspar- quartz ± ilmenite and titanite) that was driven by late-stage magmatic fluid, charged with CO2-F-H2O species. This late fluid could have mobilised Ca from the mafic granulite and formed the syenite veins. An alternative mechanism by syenite magmatism looks like a distant possibility as found in the north-western margin of the belt. The felsic gneiss from the MSZ also hosts evidence of a channelised fluid flow associated with shearing. Textures of K-feldspar micro-veins and patches in and around quartz and plagioclase matrix are likely to be caused by fluid action. Monazites in this rock preserve extensive fluid alteration signatures, indicated by compositional zoning, sub-domains and resetting of ages, suspected to have been caused by the process of coupled dissolution-reprecipitation. The unaltered monazite shows distinct age signatures in the range ca. 1000-900 Ma which presumably implies the timing of a major tectonothermal event that joined the Angul domain in the north and the Phulbani domain in the south. The fluid-mediated monazite domains, on the other hand, show a spectrum of ages in the range ca. 890-810 Ma. Our combined mineralogical-textural and geochronological study thus identifies a channelised fluid event during pervasive shearing associated with the amalgamation of two crustal domains of the Eastern Ghats Belt.
How to cite: Mondal, N., Bose, S., Ganguly, P., and Ghosh, G.: Fluid-induced changes in suites of high-grade rocks along the Mahanadi Shear Zone in northern Eastern Ghats Belt, India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14862, https://doi.org/10.5194/egusphere-egu25-14862, 2025.
Understanding the significance of the Indian Shield in the reconstruction of tectonic plate amalgamation in geological history requires an analysis of the evolution of the Chotanagpur Granite Gneiss Complex (CGGC), which is regarded as a component of the Rodinia supercontinent. The major lithology of the CGGC is felsic orthogneiss, which contains enclaves of mafic granulites, metapellites, and calc-silicates. The present study focuses on the mafic enclaves hosted within the felsic orthogneiss located in the Bero-Saltora area of the eastern part of CGGC. The rocks comprise of orthopyroxene (Opx), clinopyroxene (Cpx1) and plagioclase (Pl1) representing the high-grade granulite facies assemblage and variable amount of amphibole. The amphibole is found to replace the earlier minerals suggested by their occurrence as relict within this phase. The rock shows occurrence of a second generation clinopyroxene (Cpx2) and plagioclase (Pl2) developed as symplectite at a high angle to the amphibole's grain boundary. This feature suggests that the rock witnessed a metamorphic dehydration reaction post to the hydration event that formed amphibole in the anhydrous rock. Rarely occurring and locally developed, the symplectite texture indicates that the majority of the rock did not record the conditions under which this assemblage formed. The reaction sequences after stabilization of granulite facies assemblage (Opx+Cpx1+Pl1) can be drawn as Opx + Cpx1 + Pl1 → Amp, followed by Amp → Cpx2 +Pl2. Previous research from this region has modeled the granulite facies event and the ensuing hydration; however, despite the fact that similar symplectite assemblages have been documented, to our knowledge no study has studied the detailed petrological significance of the texture from the studied area. In this study, we will focus on the petrological significance and their connection to the geological evolutionary history of CGGC using detailed petrography and thermodynamic modeling.
How to cite: Banerjee, S., Adak, V., and Dutta, U.: Imprint of metamorphic dehydration reactions subsequent to amphibolitization of mafic granulite from Eastern part of Chotanagpur Granite Gneiss Complex (CGGC), India., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16901, https://doi.org/10.5194/egusphere-egu25-16901, 2025.
Two coal mines namely Kurja and Jhiriya were selected for investigation using multiproxy approach which included organic petrology, geochemistry and stable carbon and nitrogen isotope to reveal coal characteristics, paleo-depositional conditions and precursors material responsible for the paleomire. The results from the analysis reveal that the degraded humic material for the coal deposits was supplied from the surrounding terrestrial vegetation typical as vitrinite dominates the maceral groups followed by inertinite and liptinites. The degraded organic detritus in the samples suggest regular inundation of the mire aiding in degradation of the organic matter. The mineral matter observed through coal microscopy are dominantly argillaceous which is supported by mineral phases supported by x-ray diffraction and x-ray fluorescence studies. It is more likely that the Hasdeo basin coals were possibly deposited in the lacustrine environment with intermittent influx of siliceous detrital matter through fluvial channels. The geological setting and tectonics probably aided syn-rift sedimentation during the Permian. The mires shifted from rheotropic to mesotrophic regime due to the fluctuations in the water table during the evolution of the mire. The various indices such as the CIA, ICV, PIA and CIW are suggestive of moderate to intense weathering condition prevailing in the basin, the sediments were chiefly sourced dominantly from felsic with lesser input from mafic sources. The samples are rich in volatile matter (25.34 - 42.44 wt% on daf basis) in Kurja and (30.2 - 40.12 wt% on daf basis) in Jhiriya revealing low in rank which is also corroborated by the maturity parameter, vitrinite reflectance (random) having mean 0.40% in Kurja and 0.43% in Jhiriya. High oxygen to carbon and low hydrogen to carbon ratio suggest oxidation of the organic detritus in the mire. The elemental ratio in conjunction with stable carbon and nitrogen isotope are indicative of sedimentary organic matter sourced from C3 terrestrial plants. The carbon isotopic excursion in the basin based on delta δ13C aligns well with the global data in the isotopic shift in the coal and carbonaceous material revealing the paleo atmospheric carbon during the Permian periods. The study establishes the coeval nature of evolution of the isolated Gondwana sequences in the Indian sub-continent and in the various parts of the world.
How to cite: Naik, A. S. and Kumar, G.: Evolution of Permian coals of Hasdeo Basin, India: Insights from Organic Petrology, Geochemistry, and Stable Isotope analysis., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20370, https://doi.org/10.5194/egusphere-egu25-20370, 2025.
Posters on site: Mon, 28 Apr, 16:15–18:00 | Hall X2
Zircon geochemistry provides critical information on the melt from which they form. Specifically, Eu and Ce anomalies in zircons can be used to infer the evolution of average crustal thickness over time. However, they are typically influenced by multiple factors, such as the depth of magmagenetic processes, the nature of the parental magma, magma hydration, oxidation state, and the crystallization of minerals like plagioclase, apatite, and garnet. As a result, translating these data into paleo-depth is challenging and can introduce significant biases into interpretations of crustal evolution.
We tested these proxies on detrital zircons from the well-known Pan-African Anti-Atlas orogen. Current geodynamic models suggest an initial igneous phase (>800 Ma) dominated by rifting mafic magmatism, and the formation of oceanic basins and passive margins along the northern boundary of the West African Craton. Between 760 and 650 Ma, magmatic arcs developed, characterized by juvenile mantle-derived magmas. This period is followed by the closure of oceanic domains around 630 Ma and the subsequent development of syn-orogenic flysch basins. Abundant post-collisional to Cadomian felsic magmatism ignited around 610 Ma and lasted until 550 Ma.
A dataset of 827 Neoproterozoic zircons was statistically analyzed using bootstrap approach to produce chemical timeseries for both Eu and Ce anomalies. The results are the following: (i) pre-760 Ma (12% of zircons data): shows a slightly increasing trend in Eu (negative) and Ce (positive) anomalies. (ii) 760 - 710 Ma: zircon's age-frequency diagram suggests a first magmatic inflation around 750 Ma, Eu anomaly trends decrease while Ce anomaly remains constant. (iii) 710 - 630 Ma: Ce anomaly is still constant, but Eu anomaly shows a gradual decrease. (iv) 630 - 600 Ma: both proxies drop sharply and synchronously, coinciding with a negative shift from a compilation of whole-rock Nd signatures. This marks the implication of the West African Craton crust in the source of post-collision magmas. (v) 600 - 550 Ma: both proxies rise significantly and remain closely correlated.
Our analysis reveals that the paroxysm of the magmatic flare-up occurs at the transition from oceanic subduction to continental collision (at ~630 Ma) in the Anti-Atlas orogenic belt. If used as a proxy for crustal thickness, the Eu/Eu* ratio in zircons would be expected to increase around 630 Ma, as most geological markers indicate crustal thickening related to continental collision. However, it instead shows a sharp decline strongly correlated with Ce anomaly and coinciding with a major shift in magma sources—from mantle-dominated to crust-dominated. Conversely, intervals associated with variations in crustal thickness (from 760 to 700 Ma for example) exhibit a clear decorrelation between the Eu and Ce anomalies time series. This shows that magmatic changes associated with geodynamic transitions (e.g., from rifting to subduction to collision) have a significant impact on zircon trace element composition which inhibits other variations related to petrogenetic processes or crustal architecture.
How to cite: Triantafyllou, A., Calassou, E., Bisch, A., El Kabouri, J., Bosch, D., Berger, J., Bruguier, O., Ganne, J., Mahéo, G., Christophoul, F., and Ducea, M. N.: Tracking the transition from subduction to continental collision using Ce and Eu anomaly in detrital zircons, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12980, https://doi.org/10.5194/egusphere-egu25-12980, 2025.
The late Neoproterozoic Era saw deep glaciation and possible rises in atmospheric and marine oxygen levels. It has been suggested that these environmental changes could be the consequence of supercontinent breakup and amplified continental weathering rates, which could have drawn down CO2 and liberated nutrients. But this idea has not been tested using recent paleogeographic reconstructions and paleoclimate modelling. Here we present a suite of HadCM3L climate model simulations covering the late Neoproterozoic Era, specifically the descent into the Sturtian glaciation (800 – 715 Ma), based upon a new full-plate model and palaeogeographic framework. We outline the modelling strategy which includes representation of continental-scale icesheets and investigate the sensitivity of the climate to changing palaeogeography. To assess the implications for the carbon cycle, the resulting suite of climatologies are incorporated into the SCION climate-chemical model to produce a self-consistent reconstruction of biogeochemistry (including chemical weathering and atmospheric O2 and CO2) and climate.
How to cite: Hunter, S., Mills, B., Merdith, A., and Haywood, A.: Climate Modelling of the late Neoproterozoic Era., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8991, https://doi.org/10.5194/egusphere-egu25-8991, 2025.
The Arabian-Nubian Shield (ANS) is a juvenile crust formed over more than 300 my of Neoproterozoic crustal evolution. In the northern ANS two major igneous cycles were most significant in manufacturing the continent: the older comprises Tonian intra-oceanic island arcs whereas the second saw widespread, Early Ediacaran late to post-tectonic granitoids and volcanics. A swarm of schistose dikes of basic to intermediate composition occurs in the Neoproterozoic basement of the Eilat area, in the northern ANS. These dikes were metamorphosed in epidote-amphibolite facies, are vertically-oriented, striking WNW-ENE, with marked vertical schistosity parallel to their walls. They are abundant in a 740 Ma Eilat granite gneiss and crosscut the regional foliation which dips moderately to the south. They are commonly thought to mark a break in the prolonged Pan-African orogenic history but their age is not well defined.
We located a unique field occurrence of a schistose dike crosscut by a granite pegmatite vein which in turn was deformed and folded parallel to the vertical schistosity. The marked foliation in the hinge zone of the folded granite vein formed by crystalline plasticity at elevated temperatures during metamorphism. This key outcrop provides the opportunity to tie high-resolution field observations to accurate, multi-system U-Pb geochronology and to evaluate the relations between dike intrusion, metamorphism, and the invasion of late to post-orogenic granitoids.
Zircon U-Pb geochronology from the schistose dike yielded 645±4 Ma, considered to mark the age of crystallization of the igneous protolith. Zircon from the deformed granite vein yielded an age of 617±17Ma, indicating the vein pertains to the abundant late- to post-orogenic granitoids that invaded the juvenile crust in the aftermath of Pan-African orogeny. Titanite from the schistose dike yielded a lower intercept U-Pb age of 626±4 Ma. With a closure temperature for Pb of ~550-650◦C, titanite records the age of its crystallization during metamorphism. Apatite yielded a lower intercept U-Pb age of 611±12Ma. With an effective closure temperature for Pb of 450-550°C, apatite serves as an important medium-temperature thermochronometer. Similarly, an apatite U-Pb age of 593±12Ma was determined for the adjacent, garnet-grade Eilat schist. We interpret apatite U-Pb age as representing the timing of cooling of the entire crustal edifice in the Eilat area.
Our study demonstrates that the Pan-African tectonometamorphic history in the Eilat area was punctuated by the intrusion of basic dikes at ~645 Ma. They penetrated an already accreted and metamorphosed island-arc sequence and were subsequently deformed and metamorphosed with their country rocks in the Early Ediacaran. The Early Ediacaran deformation and metamorphism partly overlapped the intrusion of late-orogenic granitoids (see also Elisha et al, 2017) and was immediately followed by rapid exhumation. Our previous work (Katz et al 2004) showed that the protoliths of schistose dikes from the nearby Roded area were high-Mg andesites, resembling boninites which are currently restricted to active subduction zones. We propose, as a working hypothesis, that the schistose dikes signify southward subduction of the proto-Tethys below the Gondwana margin in the late stages of Pan-African orogeny.
How to cite: Avigad, D., Vardi, C., Glazer, A., Millonig, L., Gerdes, A., Albert, R., and Geller Lutzky, Y.: The Early Ediacaran orogenic history of the northern Arabian-Nubian Shield in a nutshell (Eilat area, Israel) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9990, https://doi.org/10.5194/egusphere-egu25-9990, 2025.
The South Delhi Fold Belt (SDFB) is a Proterozoic fold belt trending NE-SW in northwest India. Its western boundary is defined by the crustal-scale Phulad Shear Zone (PSZ). To the west of the SDFB lies the Marwar Craton, and the timing of its amalgamation with the rest of India has been a subject of long debate. Scattered occurrences of ~1 Ga granites near the PSZ within the SDFB are temporally associated with the assembly of the Rodinia supercontinent. Our detailed field investigations reveal distinct pre-shearing deformation patterns in the SDFB and the Marwar Craton rocks. Geochemical and geochronological analyses of rocks from the SDFB and Marwar Craton indicate an extensional regime around ~1 Ga, with the collision and suturing of the Marwar Craton and SDFB occurring as late as 820 Ma. Our findings suggest that northwest India lacks geological evidence supporting the assembly of Rodinia, a critical insight for reconstructing Rodinia's paleogeography and clarifying India's role within the supercontinent.
How to cite: Chatterjee, S., Roy, A., Sarkar, A. K., and Manna, A.: Neoproterozoic Tectonics of Northwest India: Insights from Field Evidence, Geochemistry, and Geochronology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19819, https://doi.org/10.5194/egusphere-egu25-19819, 2025.
The Ediacaran-Cambrian transition represents a pivotal geological time, denoting the decline of the Ediacaran biota and the emergence of most modern animal phyla in an interval marked by perturbations to the carbon cycle (as evidence by carbonate carbon isotopes, δ13Ccarb), biotic turnover, dynamic paleoredox regimes, and magnetic field instability. Thick and laterally extensive exposure of marine sedimentary rock along the Anti-Atlas (AA) belt of Morocco constitute an especially important succession for global Ediacaran-Cambrian (E-C) chronostratigraphy. Here, numerous attempts have sought to pinpoint the exact stratigraphic level of the E-C boundary. The AA belt comprises ca. 3 km of continuous carbonate rocks, providing one of the most complete successions for the establishment of a global δ13Ccarb chemostratigraphic reference curve.
A growing number of publications in recent years have enhanced the stratigraphic, paleontological and geochronological record of the AA belt. However, despite extensive efforts, the precise position of the E-C boundary in the Anti-Atlas remains ambiguous. The δ13Ccarb data from this region have been used to inform rates of change in global palaeomarine redox conditions, biotic innovation and turnover, but significant inconsistencies remain in global correlation.
Here, we conduct a comprehensive examination of the available chemostratigraphic, paleontological, and geochronological data associated with the late Ediacaran-Cambrian Ouarzazate Group and Adoudou Formation within the AA belt. The objective is to refine our understanding of the regional expression of the E-C boundary and offer clarity on the inconsistencies observed among biostratigraphic, chemostratigraphic and geochronological datasets. This review highlights that the stratigraphic level currently assumed to represent the E-C boundary in the AA belt relies primarily on δ13Ccarb data and, in particular, a prominent negative δ13Ccarb excursion. However, the precise level of the E-C boundary in this region lacks corroborating evidence from other independent markers such as geochronological data or, crucially, the presence of the boundary-defining ichnospecies Treptichnus pedum.
Through the integration of newly available data and interrogation of global chemostratigraphic, biostratigraphic, and geochronological information, our findings suggest that the E-C boundary within the Western Anti-Atlas may be positioned as low as within the upper unit of the Tabia Member. However, this interpretation relies heavily on the presumed Fortunian age of the ichnotaxa Monomorphichnus, because no co-occurring specimens of T. pedum are yet known. Moreover, a revised litho- and chemostratigraphic correlation that employs a compilation of published geochronological markers indicates that the Tabia and Tifnout members in the Central and Eastern Anti-Atlas do not correlate with the same named members in the Western Anti-Atlas. Both the Tabia and Tifnout members of the Central-Eastern Anti-Atlas may instead correlate with the middle part of Tifnout Member in the Western Anti-Atlas. This implies a late Ediacaran to early Cambrian ca. 10 m.y stratigraphic gap in the Central-Eastern Anti-Atlas and hence the E-C boundary in the Central-Eastern Anti-Atlas is likely situated within the unconformity between the Ouarzazate and Taroudant Groups.
Keywords: Anti-Atlas, Ediacaran-Cambrian boundary, Lower Cambrian ichnozone, Ouarzazate Group Adoudou Formation
How to cite: El kabouri, J., Errami, E., Bowyer, F. T., Beker-Kerber, B., Belkacim, S., and Triantafyllou, A.: Ediacaran-Cambrian Boundary in the Anti-Atlas belt (Morocco): A review of biostratigraphy, chemostratigraphy and geochronology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3309, https://doi.org/10.5194/egusphere-egu25-3309, 2025.
The resurgence of Banded Iron Formations (BIFs) during the Neoproterozoic, following a billion-year hiatus, reflects significant geodynamic and climatic transition. Newly discovered Neoproterozoic BIFs in the central Anti-Atlas region of Morocco provide key insights into these processes. The studied BIFs units are exposed within the Bou Azzer-El Graara inlier (Central Anti-Atlas), an oceanic paleo-suture zone between the Paleoproterozoic West African Craton and remnants of a Neoproterozoic magmatic arc. This inlier comprises 750 to 680 Ma magmatic arcs and ophiolitic remnants, both intruded by ~650 Ma dioritic plutons and overlain by Ediacaran metasedimentary sequences. The studied BIFs are hosted in meta-volcano-sedimentary units, intercalated between magmatic arc and ophiolitic complexes, and locally intruded by igneous bodies. Neither the BIFs nor their host volcano-sedimentary schists are associated with glacio-derived sediments.
Petrological, geochemical, and geochronological analyses were conducted to reconstruct the paleo-depositional environment and identify the mechanisms of BIF formation. In situ U-Pb dating on hematite yielded a crystallization age of 641 ± 41 Ma. Hematite dating could be interpreted as an early diagenetic age probably close to BIF deposition.
The whole-rock major and trace element composition of the Bou Azzer BIFs exhibits a high correlation among terrigenous proxies (e.g., Al, Zr, Hf) and silica content, with trends strongly aligning with the felsic host rocks. This suggests that the BIFs’ whole-rock geochemical signature, specifically the siliceous layers, is predominantly controlled by detrital inputs. Multi-element geochemistry, (e.g. mean La/YbSN ratio of 0.36, low TiO₂ content of 0.24 wt%, Y/Ho ratio of 26, Nb-Ta depletion) combined with Nd-Sr isotopic data from the host rocks (εNdᵗ +4.0 to +4.5), indicates a juvenile arc source, consistent with presence of igneous minerals, such as feldspar, epidote, and amphibole, in both the host rocks and BIF samples.
Hematite in BIFs show two habitus: large euhedral grains surrounded by platy hematite. Petrographic evidence suggests that euhedral hematite precipitated at a more precocious stage, while platy hematite is distinctly aligned with the foliation of the host sediments. In situ LA-ICP-MS analyses of hematite from the two habitus reveal distinct geochemical signatures from each other and from the whole-rock compositions. Overall, hematite exhibits significantly lower ΣREE and superchondritic Y/Ho ratios up to 42, with a median value of ~28. Large euhedral hematite displays a pronounced negative Ce anomaly, indicative of precipitation from oxygenated seawater and distant from hydrothermal sources, as shown by low positive Eu anomaly (~1.06). The chemical composition of platy hematite shows no Eu or Ce anomalies, suggesting anoxic conditions during diagenetic crystallization.
The Bou Azzer BIFs are Cryogenian and were deposited in an arc-bounded basin, with no evidence of glaciogenic influence. This paleo-depositional context emphasizes the role of limited arc-related basins during the Neoproterozoic, which facilitated the development of unique suboxic conditions.
How to cite: Calassou, E., Triantafyllou, A., Bisch, A., Debret, B., Bosch, D., Bruguier, O., El Kabouri, J., Fang, L., Margirier, A., Fellah, C., Berger, J., Gardien, V., and Maheo, G.: Geochronological and Petrochemical Study of Non-Glaciogenic Neoproterozoic Banded Iron Formations (Anti-Atlas, Morocco): Insights into Their Formation in a Suboxic Arc-Related Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4074, https://doi.org/10.5194/egusphere-egu25-4074, 2025.
The Neoproterozoic tectonics at the northeast–southwest trending western margin of the Aravalli-Delhi Mobile Belt (ADMB) along the South Delhi Fold belt (SDFB) is highly debated due to the spread of geochronological data from different parts of the belt. The dataset of the magmatic and metamorphic ages of the granitic rocks ranges from Stenian to Tonian Period. However, there is a lack of clarity on whether the orogenesis of the belt (SDFB) along Phulad-Ranakpur Lineament (PRL) is associated with the Grenvillian orogeny or it is a much younger Pan-African orogeny. Therefore, the tectono-stratigraphy of the region to elucidate the overall formation of the Greater Indian Landmass (GIL) is difficult to understand. To solve this problem, a comprehensive study through systematic geological mapping, structural analysis, metamorphic and geochronological study has been conducted in and around Kumbhalgarh-Sayra-Ranakpur area, Rajasthan, India. The study demarcates three phases of deformation (D1, D2 and D3) and their subsequent prograde/retrograde metamorphic events in the calcareous rocks. The first generation isoclinal, reclined (F1) folds are the result of D1 event which are synchronous with prograde amphibolite facies metamorphism (~5 kbar, 650 oC). This is followed by (D2) formation of outcrop to map-scale upright (F2) folds. D3 is marked by ‘partitioned transpression’ along subvertical shear zones (Steep zones) within the folded sequences of SDFB. Oblique slip dextral-reverse movement (D3a) along the Kumbhalgarh Steep Zone (KSZ) formed an outcrop-scale positive flower like structure. At the western limit of the SDFB, along the PRL, the Ranakpur Shear Zone (RSZ) shows rotated and steepened hinges of the F2 folds (D3b). U-Pb zircon dating is done for the zircon grains derived from the intrusive granites associated with different phases of deformation. The oldest granitic intrusion (strongly deformed pink granite from RSZ: pre-D2) occurred at ca. 990 Ma which indicates a Grenville-age orogeny for the SDFB rocks. The leucocratic granites from the KSZ (post-D2, but pre-D3) suggests ca. 850 Ma age. However, the third deformation (D3 = D3a and D3b), a progressive interlinked transpression, is identified as a ca. 822-819 Ma event from the granite ages. Undeformed leucocratic granite from RSZ shows the youngest age of ca. 819 Ma as a late-tectonic event and that marks the final suturing event between the ADMB and MC along the Phulad-Ranakpur paleo-suture zone to form the GIL. The geochemical signatures of the different varieties of granites from different parts of the SDFB also supports a collisional setting for the granite magmatism. The spacial distribution of early (ca. 1000-900 Ma) and late (ca. 800-700 Ma) Tonian magmatic and metamorphic ages over the SDFB demarcate that there is a younging trend from Beawar and Sendra area in the north to Mt. Abu and Ambaji area in the south. Hence, it can be concluded that the collision of the two blocks (ADMB and MC) started in the northern part and eventually the growth of the GIL took place through different stages of oblique collision in a pulsating manner through the Early Neoproterozoic.
How to cite: Hatui, K., Chattopadhyay, A., and Das, K.: Evidence of oblique collision between the Aravalli Delhi Mobile Belt and Marwar Craton along the Phulad – Ranakpur paleo-suture zone: Implication for the partition transpression style deformation during the Grenville-age orogeny, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15268, https://doi.org/10.5194/egusphere-egu25-15268, 2025.
The Phulad Shear Zone, a NE–SW trending ductile transpressional shear zone with a southeasterly dip, developed between ca. 820–810 Ma and marks the tectonic boundary between the Marwar Crustal Block and the South Delhi Fold Belt to the east. The evolution of the Marwar Crustal Block, particularly before its accretion to Greater India, is poorly understood but involves three phases of ductile deformation: D1, D2, and D3. The D1 deformation is restricted to enclave gneisses, while the Megacrystic granite was emplaced syn-tectonically during D2 deformation, forming NNW–SSE magmatic foliation oblique to the PSZ. D3 deformation coincides with the PSZ and includes the emplacement of the porphyritic Phulad granite (~820 Ma) along and across the shear zone. Field evidence indicates that the Phulad granite crystallized during the regional deformation associated with Phulad Shear Zone. Magmatic foliation in this Phulad granite is characterized by parallel alignment of feldspar phenocrysts and microgranitoid enclaves, transitioning to solid-state foliation due to ongoing deformation. Structural analyses reveal that releasing bends of N–S orientation within the Phulad Shear Zone provided the space for the granite’s emplacement under a transpressional regime. Geochronological data further constrain the tectonic history. U-Pb zircon ages in the Marwar Crustal Block document magmatic events at ~890 Ma and ~860 Ma, with monazite ages peaking at ~820 Ma, marking significant tectono-thermal activity. EPMA U-Pb-Th monazite and U-Pb LA-ICP-MS zircon ages from the Phulad granite confirm its magmatic age at ~819 Ma, supporting its role as a stitching pluton during the accretion of the Marwar Crustal Block with the Indian landmass Integrating structural, geochronological, and field data suggests that the accretion of the Marwar Crustal Block postdated ~860 Ma and culminated during ~820–810 Ma along the Phulad Shear Zone. This event marked the assembly of the Greater India landmass, with the Phulad Shear Zone acting as a significant suture zone. These findings highlight the distinct geological evolution of the Marwar Crustal Block and its role in the tectonic assembly of northwest India within the broader framework of Rodinia’s fragmentation and reassembly.
How to cite: Sarkar, A. K., Roy, A., Chatterjee, S. M., and Manna, A.: Early to Mid‐Neoproterozoic Tectonics of Northwestern India and it’s implications for Rodinia reconstruction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19357, https://doi.org/10.5194/egusphere-egu25-19357, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 2
EGU25-14725 | Posters virtual | VPS28
Circum-Indian craton-margin orogenic reactivation during ca. 800-700 Ma: Tectonometamorphic characterizationTue, 29 Apr, 14:00–15:45 (CEST) | vP2.13
The Precambrian history of the Indian continent centers around several Archean cratonic nuclei, e.g. the Singhbhum Craton, the Bundelkhand Craton and the Aravalli Craton in the north; the Bastar Craton in the central-east, and the Dharwad Craton in the south. Apart from a group of less disturbed and unmetamorphosed Meso- to Neoproterozoic platformal sedimentary packages resting over deformed and metamorphosed Archean to Paleoproterozoic basement, several Neoproterozoic orogenic belts occur at the margins of these Archean cratonic blocks. These craton-margin orogenic belts are the areas of intense deformation and multiple phases of deep- to intermediate depth, and hence constitute the sites of major records of crustal-scale material recycling through plate movements. They occur on the east, south and west of the Archean cratonic clusters (conjugate north and south Indian cratonic blocks). Though major deep-crustal deformation and metamorphism in these craton-margin orogenic belts can be tracked mostly up to the earliest Neoproterozoic, exhumation-related reactivation seems to be more common in these belt around ca. 800–750 Ma.
In this study, we shall highlight the east and west Indian marginal belts. We shall present the new data showing conditions of metamorphic pulses and their age from the rocks of the Mercara Shear Zone, marking the south-western boundary of the Archean (>3000 Ma) Dharwar craton. The results indicate at least four events; (1) ~2900 Ma; basin formation with supply from craton, (2) 2900–2700 Ma; age of prograde metamorphism, (3) 2700–2500 Ma, age of charnockite magmatism during Dharwar Orogeny with metamorphic peak, and (4) final reactivation at 830–730 Ma marking exhumation of deep crust during retrograde metamorphism along the crustal scale shear zone (stretching lineation and S-C fabric formation as last deformation event. We shall also review our group’s recent published data on the pressure-temperature-deformation-fluid-age histories during the orogenic reactivation of the western boundary, and the Chilka Domain of the northern Eastern Ghats Belt. Together we shall try to collate data showing the idea of a near-synchronous orogenic reactivation surrounding Indian cratonic cluster during middle to late Tonian Period with various preceding age gaps.
How to cite: Das, K., Bose, S., Ganguly, P., and Chatterjee, A.: Circum-Indian craton-margin orogenic reactivation during ca. 800-700 Ma: Tectonometamorphic characterization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14725, https://doi.org/10.5194/egusphere-egu25-14725, 2025.
EGU25-1504 | ECS | Posters virtual | VPS28
Neoproterozoic Tectonics of the Kaliguman Shear Zone: Implications for the Delhi-Aravalli Fold Belt Contact, NW IndiaTue, 29 Apr, 14:00–15:45 (CEST) | vP2.14
The Kaliguman Shear Zone (KSZ) in northwestern India marks the boundary between the South Delhi Fold Belt to the west and the Aravalli Fold Belt to the east. Structural analysis reveals a narrow, high-strain zone characterized by the development of mica schist along this boundary. The principal structural orientation trends in the NE-SW direction. Strain analysis indicates that the rocks in this zone formed under transpressional deformation conditions.
The metamorphic history of the KSZ is well-preserved in the mica schists, which predominantly contain garnet and staurolite. Petrological and textural studies have helped establish the relative crystallization sequence of mineral phases during the metamorphic events. Examination of garnet porphyroblasts reveals a complex deformation pattern, reflecting pre-, syn-, and post-tectonic events associated with fabric formation. Geothermobarometric analysis indicates that the mica schists underwent amphibolite facies metamorphism. Phase equilibria analysis, supported by PT pseudosections, shows peak metamorphic conditions at approximately 590±10 °C and 4.7 kbar. Garnet isopleth plots suggest increasing pressure and temperature during metamorphism, which is consistent with the inferred PT path. Variations in the modal abundance of index minerals further corroborate this evolutionary trajectory. These findings support a model of crustal thickening for the KSZ. The textural control monazite age data from the mica schists confirms that the shear zone was formed during the early Neoproterozoic. The study provides valuable insights into the tectonic evolution of the contact between the Delhi and Aravalli Fold Belts, highlighting the role of shear zones in accommodating deformation and facilitating metamorphic processes during Neoproterozoic orogenic events.
Keywords: Kaliguman Shear Zone, Aravalli, Delhi Fold Belt, Neoproterozoic, NW India
How to cite: Mondal, S., Roy, A., and Chatterjee, S.: Neoproterozoic Tectonics of the Kaliguman Shear Zone: Implications for the Delhi-Aravalli Fold Belt Contact, NW India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1504, https://doi.org/10.5194/egusphere-egu25-1504, 2025.
EGU25-5552 | ECS | Posters virtual | VPS28
Neoproterozoic magmatism in NW India and its implication for crustal evolutionTue, 29 Apr, 14:00–15:45 (CEST) | vP2.15
In northwest India, the South Delhi Fold Belt (SDFB) is a NE-SW trending region of the Neoproterozoic age, consisting of poly-deformed and poly-metamorphosed rocks. To the west lies the Marwar Craton, and the boundary between them is defined by a crustal-scale shear zone, dated to 810 Ma, known as the Phulad Shear Zone (PSZ). The syn-tectonic Phulad Granite, which runs along the PSZ, played a key role in stitching together the Marwar Craton and the SDFB during the 810 Ma tectonic event. Approximately 30 km to the east of the PSZ, a quartz monzonite pluton, emplaced within the calc-silicates of the SDFB, is observed. This study focuses on the meso- and micro-structures, as well as the geochemistry of the quartz monzonite, to better understand its emplacement conditions and the tectonic processes at that time.
In the field, the quartz monzonite exhibits a saccharoidal texture with a crude foliation, defined by the alignment of feldspar grains. The foliation in the monzonite has a mean orientation of 14°/67° E. The quartz monzonite is primarily composed of k-feldspar and plagioclase feldspar, with minor amounts of quartz, amphibole, and titanite. Microstructural analysis reveals features indicative of sub-magmatic, high-temperature deformation, suggesting that the rock underwent solid-state deformation. These microstructural characteristics of the quartz monzonite suggest a syn-magmatic deformation event. The foliation in the monzonite is broadly parallel to the mylonitic foliation in PSZ, further supporting the idea of a syn-tectonic emplacement. The geochemical study of the quartz monzonite displays a syn-collisional granite-type geochemical signature with a distinctly negative REE pattern. The REE pattern features suggest that garnet played a significant role in the petrogenesis. By integrating micro and meso-structural analyses with geochemical data, we infer that the emplacement of the quartz monzonite coincided with the development of the PSZ and the intrusion of the Phulad Granite. Despite the temporal overlap, the quartz monzonite and the Phulad Granite display significant geochemical differences, denoting distinct petrogenetic processes. Based on the integration of all available data, we propose that the quartz monzonite was emplaced during the 810 Ma collisional event, resulting from the partial melting of garnet-bearing mafic crust. While both quartz monzonite and Phulad Granite likely share a common source, the depth of melting was different. The greater depth of melting in the eastern portion suggests an eastward subduction of the Marwar Craton during this tectonic event.
How to cite: Ghosh, D. D. and Chatterjee, S. M.: Neoproterozoic magmatism in NW India and its implication for crustal evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5552, https://doi.org/10.5194/egusphere-egu25-5552, 2025.
EGU25-11645 | Posters virtual | VPS28
Behavior of monazite during incipient dehydration melting of charnockite at the northern Eastern Ghats Belt, India: Insights on the mobility of REE at amphibolite-granulite facies transitionTue, 29 Apr, 14:00–15:45 (CEST) | vP2.16
Monazite has the potential to place temporal constraints on the crustal melting in high-grade metamorphic rocks like granulites and migmatites. Melt loss in granulite-grade metamorphic rocks plays a key role in progressively depleting LREE in the residue and enhancing the dissolution of monazite during heating to the metamorphic peak. Newly formed monazite are therefore more abundant in leucosomes than the residue. Higher degree of partial melting and subsequent melt loss, therefore, poses a major hindrance to constraining the mobility of these elements in the micro-domain scale, particularly during the early stage of melting at amphibolite to granulite facies transition. To overcome such an issue and to understand the behavior of this mineral during the onset of granulite facies metamorphism, metamorphic rocks that have reached the P-T conditions culminating at the aforesaid transition should be targeted. Considering this, the present study has been carried out on charnockite from the northern Eastern Ghats Belt, India which underwent such transition (M2) following crystallization during an earlier granulite facies metamorphic event (M1). The rock is composed of plagioclase (Pl), K-feldspar (Kfs), quartz, orthopyroxene, biotite, and garnet with apatite, allanite, and monazite as accessory phases. The rock has well-developed gneissic foliation, demarcated by alternate biotite +garnet-rich and quartzofeldspathic layers. While both the feldspars show grain boundary migration recrystallization, quartz grains are deformed by sub-grain rotation recrystallization. Garnet is porphyroblastic and post-kinematic as it overgrows the matrix biotite. The former phase is closely associated with cuspate Kfs and quartz grains which developed as a result of incipient dehydration melting of moderately fluorine rich biotite during the aforesaid transition. Monazite grains are coarse (up to 200 µm across), mostly elliptical and either partially or completely replaced by the reaction rim of apatite+ thorite with an external corona of allanite in the biotite+garnet-rich layers. In case of partial replacement, the oscillatory-zoned relict monazite core is preserved. Th-rich patches are present in such cores. Interestingly, the coronitic assemblage overgrows the matrix biotite is always associated with porphyroblastic garnet. On the contrary, corona-free monazite grains are abundant in quartzofeldspathic layers. Spot dates from the oscillatory-zoned relict monazite core yield a weighted mean age of 960±6 Ma. Th-rich patches, showing prominent huttonite substitution, yield a weighted mean age of 938±7 Ma. Integrating monazite textural and age data, we interpret that the ca. 960 Ma represents the crystallization age of the charnockite magma which coincides with the M1 metamorphic event of the Eastern Ghats Province (EGP). The ca. 938 Ma, additionally, corresponds to the age of the M2 event when biotite dehydration melting occurred and porphyroblastic garnet was formed. Based on the textural evidence and mineral phase chemical data, we propose that the replacement of primary monazite occurred via coupled dissolution precipitation process in the presence of incipient melt originated during biotite dehydration melting. Such melt was fluorine rich and helped to mobilize REEs by forming REE-fluoride complexes and was incorporated in allanite corona. Monazite grains in quartzofeldspathic layers must have escaped the melting reaction and the melt-induced element mobility.
How to cite: Ganguly, P., Banerjee, A., and Das, K.: Behavior of monazite during incipient dehydration melting of charnockite at the northern Eastern Ghats Belt, India: Insights on the mobility of REE at amphibolite-granulite facies transition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11645, https://doi.org/10.5194/egusphere-egu25-11645, 2025.
EGU25-19572 | ECS | Posters virtual | VPS28
Geochemistry of ~1Ga granite and associated mafic rocks from the South Delhi Fold Belt, NW India, and its tectonic significanceTue, 29 Apr, 14:00–15:45 (CEST) | vP2.17
The South Delhi Fold Belt (SDFB) within the northwestern Indian Shield is a Proterozoic NE-SW trending fold belt. The western boundary of the SDFB is defined by the Phulad Shear Zone, formed during a transpression regime around 820-810Ma. Granite rocks of ~1Ga have been documented from the western part of the fold belt and are linked with the formation of the Rodinia Supercontinent. These granites are closely associated with gabbroic rocks. The present study focuses on the geochemistry of these granites and the mafic rocks, as well as their field structure and petrography.
The granites and the mafic rocks are confined to a narrow linear belt along the western part of the fold belt. Detailed field studiesreveal that the foliations in the granites, mafic and mylonites within PSZ share a common stress regime and are broadly synchronous. Geochemically these granites are ferroan, calc-alkalic, metaluminous to weakly peraluminous and their classification in granite discrimination diagrams confirms A-type and within plate granite. The mafic rocks exhibit a compositional range fromtholeiitic to calc-alkaline, with atrace element ratio resembling enriched mid-oceanic ridge basalt (E-MORB) type magma. The tectonic discrimination diagram suggestsrift-relatedmagmatism. Geochemical analysis of these bimodal magmatic compositions in the SDFB, encompassing both mafic rocks and A-type granites are typically associated with areas experiencing extensional tectonics, particularly rift-related magmatism. Integrating field structures, petrography and geochemistry of these granite and the mafic rocks suggests that the ~1Ga granite and the associated mafic rocks formed in an extensional regime and are not directly linked to the collisional assembly of the Rodinia Supercontinent.
How to cite: Manna, A., Chatterjee, S. M., Roy, A., and Sarkar, A. K.: Geochemistry of ~1Ga granite and associated mafic rocks from the South Delhi Fold Belt, NW India, and its tectonic significance, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19572, https://doi.org/10.5194/egusphere-egu25-19572, 2025.
