The Jonnagriri greenstone belt (JGB) of eastern Dharwar craton is one of the unique and economically important greenstone belts of southern India. This greenstone sequence consists of metabasalts, meta-felsic volcanics and schistose meta-tuffs surrounded by granitoid intrusives. These intrusives are the only known granitoid in India that hosts orogenic type gold deposit in the Dona sector of JGB. In our present work, we are focusing on the granitoids emplaced surrounding the JGB that are broadly classified into two main groups; (i) The Pagadarayi granodiorite (PGD) and (ii) The Chennampalli granites (CGR). The present study is to examine the geochemical evolution, decipher the tectonic settings and determine the timing of emplacements of the granitoids using whole rock geochemistry and U-Pb zircon dating. This will also help us to understand the crustal architecture of the part of the eastern Dharwar Carton in the southern Indian peninsula. The PGD compositionally ranges from tonalite to granodiorite suits and the adjacent CGR ranges from granite to syenogranite suits of rocks. The petrogenetic evolution of these granitoid clearly showed a transition from calc-alkaline to a more fractionated potassic rich granite. The major oxide and trace element behavior showed that they are metaluminous to peraluminous in nature where the PGD are geochemically linked to the TTGs while the potassic rich CGR phase majorly shares its genetic link with hybrid and two-mica granites. The source of PGD suits of rocks is inferred to be an outcome of partial melting of TTG crust and the CGR suits are ferromagnesian deficit phase with an increase in K₂O/Na₂O due to crustal contamination, which indicates a result of crustally reworked magmatic phase. Therefore, an enrichment of LREE and LILE relative to HREE and HFSE with depletion of Nb-Ta-Ti notably is observed, which represents a convergent setting signature for these granitoids. The PGD suit of rocks dominantly showed I-type characteristics affirmed by their ASI index, and with the typical SiO₂, CaO and FeO contents, indicating that they are part of volcanic arc to island arc granitoid settings. The CGR showed distinctive characteristics of A₂ type granites due to its Y/Nb value ranging from 1.2 to 1.83 depicting a shallow depth subduction regime which are formed within the crust, indicating a within plate post-colliotional settings. Zircon U-Pb dating by LA-ICP-MS reveals that the first emplacement of PGD recorded a concordant emplacement age of 2651±14 Ma and the CGR yielded concordant ages of 2557±10 Ma and 2532±22 Ma respectively. This implies that the Jonnagiri granitoid represents an orogenic type tectonic settings where diversification in the mode of granitoid intrusion showed a change in volcanic-arc magmatism to within plate magmatism, from the older phase of tonalite-granodiorite emplacement to the successive younger potassic rich syenogranite emplacement. 

How to cite: Das, S. and Chinnasamy, S. S.: Granitoids of the Jonnagiri greenstone belt of eastern Dharwar craton: Constraints from geochemistry and U-Pb zircon geochronology., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6082, https://doi.org/10.5194/egusphere-egu24-6082, 2024.

The Southern Granulite Terrane (SGT) is an integral part of the Neoproterozoic Gondwana supercontinent. The Nilgiri block situated in the northern part of the SGT is characterized by Neoarchean magmatism. The major rock types found in the western part of the Nilgiri block include charnockite, hornblende biotite gneiss (HBG), pyroxenite, gabbro, amphibolite, mafic granulites, and granite. This study focuses on the geochemical signatures of major lithologies in the western Nilgiri block to understand the geological evolution within the broader Southern Granulite Terrane. Oxide data show charnockite samples with SiO₂ from 51 to 67 wt% and HBG with SiO₂ from 63 to 71 wt%. Normative Ab-An-Or classification diagrams indicate tonalitic and granodioritic signatures for charnockite and HBG, respectively. Both exhibit high Al₂O₃ (>15 wt%) and low K₂O (<3 wt%). Charnockites and HBG display a magnesian and calcic-to-calc-alkalic composition, while mafic granulite shows a low-K tholeiite and calc-alkali composition. Zr vs. Ti diagrams reveal a volcanic arc signature for charnockite, HBG, and mafic granulites. In the study area, Peralimala Granite (PG), Ambalavayal Granite (AG), and Kalpetta Granite (KG) are present as plutons. KG and PG are magnesian, while AG is ferroan. KG and PG have an alkali-calcic composition, which contrasts with the calc-alkalic nature of AG. Comparative analysis with charnockites from the eastern Nilgiri block shows variations in SiO₂ and Al₂O₃ content, with the western region exhibiting lower SiO₂ and higher Al₂O₃. HBG in the western part shows a slightly elevated Na₂O+K₂O value. The charnockite consistently shows depletion in Y, Cs, Ta, Th, and U, coupled with Zr, Ba, Pb, and La enrichment. Similarly, HBG displays depletion in Y, Ta, Th, and U and enrichment in Rb, Sr, Zr, and Ba. Both rock types consistently exhibit enrichment in LREE and depletion in HREE. Mafic granulites show depletion in Cs, Th, Ta, and U, along with Nb, Ba, and Pb enrichment, displaying a flat REE pattern without significant fractionation. KG and PG exhibit enriched LREE and depleted HREE, whereas AG displays a strong negative Eu anomaly and a flat REE pattern. KG and PG are enriched in Ba, K, and La, with depletion in Ta, Nb, Ti, and Y. Conversely, AG is enriched in Ba, K, and Sr, with minimal Rb content and depletion in Th, U, Ta, Nb, and Ti, suggesting AG partial melts formed from the deep crustal level or upper mantle region. Charnockite and HBG show an affinity towards Archean TTG, suggesting their formation through the partial melting of Archean-hydrated, low K-metabasic source rocks, resulting in garnet amphibolite restite. U-Pb zircon data reveals an age of 2555±15 Ma for charnockite and 2512±12 Ma for HBG, indicating Neoarchean magmatism. Discordant ages from HBG align on a Pb-loss trend toward the lower intercept age of ~1100 Ma, suggesting a Late-Mesoproterozoic thermal event. U-Pb zircon data of Kalpetta Granite gives an age of 544±5 Ma, implying a Pan-African thermal event. These findings highlight distinct magmatic and thermal events in the western Nilgiri block, including Neoarchean activity and notable Late-Mesoproterozoic and Neoproterozoic thermal events.

How to cite: Shamil, V. T. M., -Kumar, C. I., and -Kumar, M. S.: Insights into the Neoarchean Magmatic Evolution of the Western Nilgiri Block, Southern India., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10081, https://doi.org/10.5194/egusphere-egu24-10081, 2024.

Combined with geophysical, geochronological and structural data, isotope geochemical proxies (including Pb, Nd, Hf and Sr) are commonly used to correlate basement terranes and hence to test and refine supercontinent configurations. In the case of Pb, many such studies rely on whole-rock isotopic data, which being time-dependent, can be difficult to interpret.

By contrast, feldspars, by virtue of their low U/Pb and Th/Pb ratios, have time invariant Pb isotope compositions. They are also modally abundant rock-forming minerals and are amenable to in situ analysis and textural discrimination. This study evaluates the utility of Pb isotopes in feldspar as a mechanism to identify terrane boundaries and evaluate large-scale continental reconstruction models.

We are systematically collating feldspar Pb isotope data from Archaean and Paleoproterozoic basement rocks in the North Atlantic region (Laurentia-Baltica and the intervening continental plateaux) from literature sources as well as new measurements from outcrop and offshore wellbores from Scotland, Norway, Greenland and Russia. 

A particular focus is on the Lewisian Complex of Scotland because these rocks are argued to link with terranes in both Greenland and Baltica. Our Pb isotopic data identify four Pb isotopic domains in Scotland.  The NAC, Nagssugtoqidian and Rinkian terranes are recognised onshore and offshore west of Shetland and correlate strongly with Greenland.  Moreover the boundaries between them are consistent with large-scale sutures proposed on other grounds although they do not support the traditional nine-terrane model for the Lewisian Complex [1]. Feldspar Pb isotopic data from the Kola Peninsula and northern Fennoscandia are consistent with the suggestion that the sub-Moine inliers (previously considered to have Archaean protoliths, equivalent to the Lewisian Complex) have an affinity with Baltica [2].

References:

[1] Kinny et al. (2005) Journal of the Geological Society, London 162, 175-186.

[2] Strachan et al. (2020) Geology 48, 1094-1098.

How to cite: Daly, J. S., Flowerdew, M., and Badenszki, E.: Mapping ancient geochemical terranes using feldspar Pb isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11081, https://doi.org/10.5194/egusphere-egu24-11081, 2024.

The Late Archean Gadag greenstone belt (GGB) in the northern continuity of extensive Chitradurga greenstone belt in the western Dharwar craton has extensive meta volcano-sedimentary cover apart from older TTGs (Tonalite-trondhjemite-granodiorite), pink granite and other intrusives. The GGB encompasses a very unique and classical orogenic type gold deposit known as Gadag gold field (GGF) where auriferous lodes are hosted by metabasalt, greywacke and BIFs in western, central and eastern part of GGF respectively. In this study, we focus on the deformation history and P-T conditions of metamorphism of greenstone rocks which could have influenced in the generation of auriferous fluid in GGB. Two events of deformation were suggested in GGB so far where the first event (D1) is represented by NW-SE trending pervasive fabrics (S1) and schistocity, whereas the second event (D2) marking the regional disposition of NNW-SSE tectonic trend. This is further corroborated by microscopic evidences of Q-M microlithons development and garnet porphyroblast growth that is post tectonic to D1 deformation, syn-tectonic to early D2 and stage 2 porphyroblast development of non-rotating porphyroblast model in garnetiferous amphibolite in the north-eastern part of GGB. However, the matrix continued to get deformed and reached stage 6 in garnetiferous schist with dominant sinistral sense of shear in the southern part of GGB. The estimated peak metamorphic temperature of garnetiferous amphibolite using hornblende-plagioclase thermobarometry is 578 ± 21°C and the pressures are 7.1 ± 3kb and 7.4 ± 2.3kb. Garnet-biotite thermometric estimation for the same garnetiferous amphibolite from north eastern part of GGB yielded 525 ± 25°C indicating a lower amphibolite facies of metamorphism. The auriferous quartz veins were associated with both the D1 and D2 phases of deformation, and the genesis of this auriferous fluid may be related to lower amphibolite facies of metamorphism of these host rocks as evidenced from the fluid inclusion study.

Keywords: craton, P-T conditions, thermobarometry

How to cite: Saikia, M. J. and Chinnasamy, S. S.: Deformation history and P-T condition of metamorphism of Gadag greenstone belt of western Dharwar craton, southern India: Implications on the genesis of orogenic gold mineralisation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10873, https://doi.org/10.5194/egusphere-egu24-10873, 2024.

The Son Valley Crystallines (SVC) of the Precambrian Chhotanagpur granite-gneiss complex (CGGC) of the Sonbhadra district Uttar Pradesh contain gneisses and migmatites which are highly deformed and metamorphosed. The study area encompasses different parts of SVC which are Arangapani, Kudri, Anjangira, Jarha, Nawatola, and Naktu. Here we found gneiss and migmatite except Nawatola and Naktu where we found felsic breccia. The foliation in migmatite is E-W but at places it changes to ENE-WSW. The leucosome part of the migmatite occasionally shows pinching and swelling and ptygmatic folding structures. Petrography of the leucosome of this gneiss and migmatite shows the dominance of microcline, orthoclase, quartz, and rare plagioclase with a minor amount of biotite and amphibole whereas the melanosome is dominated by biotite, rarely with amphibole and minor quartz and, feldspars. These rocks contain a higher amount of REE-bearing phases viz. zircon, garnet, apatite, titanite, monazite, and xenotime. The REE-bearing phases particularly zircon, monazite, and xenotime are found as inclusions mostly within biotite and occasionally in feldspars whereas apatite and titanite were found as inclusions exclusively within feldspars. Few zircon and monazite are found in intergranular space between different mineral grains. REE-bearing phases were analyzed in EPMA for total rare earth oxide (TREO) quantification and it was observed that TREO is 0.84 - 1.91 wt% (avg. 1.37 wt%) for apatite and titanite, and for monazite, it is 44.54 - 68.51 wt% (avg. 58.98 wt%). The chemical ages of monazite from Kudri migmatite is 1166 - 748 Ma (avg. 936 Ma; n=10) and Jarha gneiss is 1339 - 837 Ma (avg. 1088 Ma; n=13) these ages coincide with Grenville orogeny (c.1250 - 980 Ma). Hence, REE mineralization in the study area may be the result of the Grenville orogenic event. Most monazite grains are anhedral to elongate in shape and they are found as inclusion in biotite and feldspar however, some of the monazite grains are found in the intergranular space. These two contrasting modes of occurrences of monazite suggest an earlier dominantly of syngenetic (orogenic) mineralization and later with an epigenetic overprint. The geochemical analysis of these rocks shows high SiO₂ and low MgO, FeO, and CaO contents indicating a felsic source which is also supported by the TAS diagram in which most of the samples fall in the granite region. The A/NK vs A/CNK plot shows the rocks are peraluminous in nature. The tectonic discrimination diagram of Y vs Nb, Ta+Nb vs Rb, and Yb vs Ta suggests that most of the rock samples fall within the plate field with few in the volcanic arc and few in the syn-collisional setting which indicates the complexity of the terrain.

How to cite: Verma, A. and Chinnasamy, S. S.: The implication of Grenville orogeny on REE mineralization in migmatites and gneisses of Son Valley Crystallines Sonbhadra, Uttar Pradesh, North India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15661, https://doi.org/10.5194/egusphere-egu24-15661, 2024.

The Eoarchean Isua supracrustal belt (ISB) in West Greenland comprises one of the oldest rock records, yet its geological evolution and geodynamic setting are still debated. Major order questions involve the timing and number of the tectono-metamorphic events leading to the formation of mineral assemblages and structures observed today. Interpreted cross-cutting relationships and crystallization ages of the Ameralik (mafic) dykes have been used to suggest an Eoarchean metamorphic age for the most pervasive event (e.g., Nutman et al., 2004, JGS 161, 421-430). Recent studies, in contrast, argue that this event is Neoarchean in age, as constrained via garnet (-plagioclase-hornblende) dating (e.g., Eskesen et al. 2023, Geology 51, 1017-1021; Ramírez-Salazar et al., unpublished data). In this study, we present data of eight garnet grains from two metapelitic samples from the eastern limb of the ISB to acquire further information about the metamorphic evolution. Both samples share a similar mineral assemblage, featuring a matrix predominantly composed of porphyroblastic garnet with ±chlorite, biotite, white mica, quartz, and ±plagioclase. The garnets exhibit a diversity of unevenly distributed inclusions dominated by quartz, plagioclase, white mica, chlorite, and ilmenite. Additionally, apatite, ±monazite, ±allanite, ±rutile, and ±zircon, as well as some iron-arsenide and -sulphide minerals, are abundant as inclusions. Textural evidence in combination with major and trace element zoning reveals three distinct garnet core to rim domains referred to as garnet I to garnet III. The porphyroblasts’ inclusion-rich cores (garnet I) are characterized by a bell-shaped spessartine component and heavy rare earth element zoning pattern, along with a relatively flat grossular and pyrope pattern. The garnet annuli (garnet II) are grossular-rich, with a constant or slightly increasing pyrope and spessartine components already at their minimum. Garnet II typically exhibits less inclusions. A drop in grossular together with an increase in pyrope components, and the virtual absence of inclusions mark the third garnet domain (garnet III). Our examination of the chemical, mineralogical, and textural characteristics across the different garnet domains reveals the growth of garnet through distinct mineral reactions under variable metamorphic conditions. These domains may be linked to either distinct metamorphic growth stages within a single event or disparate metamorphic events in the tectono-thermal history of the ISB. To unravel the chronological sequence of garnet growth, we present texturally resolved dating of individual garnet domains using the Sm-Nd isochron method.

How to cite: Wolf, J., Sorger, D., Ramírez-Salazar, A., Hauzenberger, C., Webb, A. A. G., Willbold, M., and Müller, T.: Unravelling the metamorphic history of the Isua supracrustal belt: A garnet-based study in West Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18808, https://doi.org/10.5194/egusphere-egu24-18808, 2024.

In the Southern Brasília Orogen (south-eastern Brazil), a nappe system that represents the roots of a magmatic arc records HT-UHT metamorphic conditions in lower to mid-crustal rocks. It is divided into two segments by a major shear zone, of which the northern nappe hosts the most extreme metamorphism and has been targeted for most petrochronological studies. These rocks carry insights into the stages of orogeny, as well as the first direct evidence of the paleo-active margin basement, and time-constraint (1) a metamorphism related to the magmatic arc consolidation on the active margin at 670-640 Ma and (2) an enduring UHT event related to collision and decompression at 630-590 Ma. The southern nappe (Socorro Nappe) hosts felsic and mafic granulites, amphibolites and migmatites with intricate occurrences and complex pressure-temperature-time histories that may preserve distinct age populations in the inner nappe and its outward boundaries (Embu Terrane and São Roque Domain). The mafic lower to mid-crustal rocks of the Socorro Nappe lack detailed comprehensive studies of their P-T-t evolution. We present new preliminary LA-ICPMS U-Pb and Lu-Hf systematics in zircon retrieved from metamafic rocks such as granulites, amphibolites and orthogneisses, and partial results on conventional thermobarometry and thermodynamic modelling. We investigate the significance of a wide timespan from ca. 750 to 570 Ma where granulites tend to preserve older ages in contrast to amphibolites. However, Hf signatures are complex and also show a wide range of ε values from 0 to weakly radiogenic, and strongly radiogenic that are not straightforwardly related to a clear time evolution. Herein we discuss preliminary insights into the zircon petrochronology and P-T-t evolution of metamafic high-grade rocks as a tool to unravel the tectonic evolution of the southernmost segment of the Southern Brasília Orogen in relation to the adjacent São Roque Domain and Embu Terrane, in the context of the Western Gondwana amalgamation. 

How to cite: Balis, M., Schulz, B., and Campos Neto, M. D. C.: Zircon U-Pb and Hf petrochronology of mafic high-grade rocks of a Neoproterozoic nappe system in West Gondwana (Socorro Nappe, SE Brazil), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19548, https://doi.org/10.5194/egusphere-egu24-19548, 2024.

The Eoarchean Isua supracrustal belt (ISB) in southern West Greenland exposes one of the oldest rock records on Earth. Its tectono-metamorphic evolution remains debated, particularly with respect to its timing and metamorphic peak conditions recorded by the various mafic to ultramafic lithologies. A first-order question is the presence or absence of metamorphic gradients within the belt. To that end, phase equilibria modelling combined with multiple geothermobarometric data by our group suggested a homogeneous distribution of peak metamorphic conditions (550-600 °C; 0.8-1.0 GPa). However, recent studies of a specific dunite lens from the ISB suggested different metamorphic conditions, varying from ultra-high pressure (UHP) to prograde low-pressure deserpentinization.

In this study, the metamorphism affecting ultramafic rocks from the specified lens B in the northwestern limb of the ISB is investigated along a transect from the rim towards the center of the lens. A total of four samples are examined for their petrography, mineral assemblages, and reaction textures, using EPMA, SEM, EBSD, ICP-MS data together with thermodynamic modelling.

Results reveal a strong foliation in the rim of the lens, with antigorite (X_Mg=0.91) + magnesite + magnetite ± ilmenite forming the stable assemblage. Deformation decreases towards the center of the lens, where antigorite (X_Mg=0.98) + fosterite (X_Mg=0.97) + magnesite + magnetite ± Ti-chondrodite/Ti-clinohumite form the observed assemblage.

The presence of accessory Ti-phases has been used as an indicator for retrograde decompression after UHP metamorphism through the breakdown reaction of Ti-chondrodite to form Ti-clinohumite. We present textural evidence of Ti-clinohumite being replaced by Ti-chondrodite, pointing to a different reaction along a cooling path at lower pressures. The presence of carbonate instead of brucite together with the absence of talc highlights the role of CO₂ – which must be assessed to accurately describe phase relations for the metamorphic evolution of ultramafic rocks. Thermodynamic modelling indicates that antigorite stability is strongly CO₂-dependent, limiting the stability of antigorite + magnesite to pressures < 1 GPa at X_CO2 > 0.005. EBSD analysis reveals clear evidence of olivine experiencing internal deformation, and thus preceding the growth of antigorite in the sample. A second observation further supports this finding: Magnetite, magnesite and Ti-humite phases are spatially associated with olivine breakdown reaction textures.

In summary, this study suggests that UHP conditions or deserpentinization are not required to explain the metamorphic records of these mafic lenses, thus removing a strong argument for the ISB to exhibit a metamorphic gradient and UHP rocks. Instead, we interpret the lens to exhibit a fluid-mediated reaction front that can be readily achieved at the homogeneous, amphibolite-facies conditions reported for the rest of the belt.

How to cite: Mueller, T., Wolf, J., Sorger, D., Piazolo, S., Hauzenberger, C., Zuo, J., and Webb, A.: Amphibolite facies conditions recorded in CO2-bearing fluid-mediated reaction fronts preserved in an ultramafic lens from the Isua supracrustal belt, southern West Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19290, https://doi.org/10.5194/egusphere-egu24-19290, 2024.

Cratonic margin basins are crucial in tracking the provenance characteristics. This study aims to characterize one of the Proterozoic basins in Southeastern India known as Pranhita Godavari (PG) basin. Nestled between the famous Dharwar and Bastar cratons, the PG basin is about 450 Km long NW-SE trending rift-related basin with a huge thickness of siliciclastics. The basin consists of three cycles of deposition. In this study, we characterize the geochemistry, U-Pb geochronology and Lu-Hf systematics of the detritus as well as the adjacent basement Gneiss in order to define the sediment source.

                        K₂O/Na₂O v SiO₂ and the Discriminant function diagrams (Verma & Armstrong-Altrin, 2013) indicate a passive margin setting for the sediments. Together with Eu/Eu*, the high La/Yb_N ratios (8.23-37.9) and LREE enriched and flat HREE patterns indicate a felsic source. The other geochemical indicators like TiO₂ versus Zr and Ni, Al₂O₃/TiO₂, Th/Co, La/Sc ratios along with high Th/U also indicate felsic continental sources.   The different weathering indices like CIA.PIA, and  ICV indicate moderate to slightly intense weathering of the source terrains producing mostly mature sediments.

The metamorphic zircons (average Th/U=0.88) from the basement Gneiss yield a Concordia age of 2490±6 Ma and a weighted mean age of 2490±6 Ma. Three magmatic zircon cores yield a weight mean average of 2677±36 Ma. Previous studies from the Bastar and Dharwar cratons also report the presence of 2.5 Ga old zircons.

The sediment was mainly sourced from the Paleoarchean to Mesoproterozoic sources with detrital zircon ages spread from 3571-1622 Ma. The Probability Density Plots show major peaks at 1842, 2498, 2637 Ma and minor peaks at 1637,3340, 3567 Ma. The Youngest detrital zircon method brackets the maximum depositional age of the Mulug Formation 1622±33 Ma.

The zircons from the basement gneiss of Peninsular Gneissic complex show negative Hf isotope values (εHf= -15.27 and 0.17). Similarly, the detrital zircons show negative values (εHf =-12.58-0.70) indicating their derivation from evolved sources.

The age peaks and the εHf composition from the Mulug formation perfectly match with the basement gneiss ages. This together with published literature indicates that the Mulug formation was deposited at ~1.6 Ga and was sourced from the eastern Dharwar and Bastar cratons

How to cite: Ganaie, A. M.: Detrital Zircon U-Pb Geochronology and Lu-Hf isotope systematics from the Pranhita Godavari basin: Implications for the Provenance and Mesoproterozoic tectonics  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3845, https://doi.org/10.5194/egusphere-egu24-3845, 2024.

The northern margin of the North China Craton is an important tectonic zone, bordered by the globe’s largest Phanerozoic accretionary orogen, the Central Asian Orogenic Belt. The Paleoproterozoic strata on the northern margin of the North China Craton (NCC) comprise a relatively intact low-grade (meta-)sedimentary sequence, but whether there are breaks in the sequence and the tectonic setting in which the sequence was deposited are uncertain. In this work, Lu–Hf isotopic, and zircon U–Pb geochronological analyses were performed on (meta-)sedimentary rocks from the Paleoproterozoic strata, to investigate the potential provenance, depositional age, and depositional environment, and discuss the likely tectonic setting of sedimentation on the northern margin of the NCC during the Paleoproterozoic. The distribution of detrital zircon ages shows that the lower meta-feldspathic sandstone unit of the Paleoproterozoic sequence was deposited after ca. 2.2 Ga, whereas the maximum depositional age of (meta-)quartz siltstone and quartzites were around 1.89 Ga and at from ca. 1.86 to 1.81 Ga, respectively. Integration of our U–Pb and Hf isotope data with data from the other regions of the NCC, indicate that the Paleoproterozoic strata have experienced a depositional environment between ca. 2.2–1.81 Ga in an extensional basin system. We further proposed that the northern margin of the NCC evolved from an extensional tectonic setting, possibly related to slab rollback. Subsequently, the Paleoproterozoic sequence was affected by a ~1.8 Ga metamorphic event to form these (meta-)sedimentary rocks in the region.

How to cite: Chen, N. H.-C.: Zircon U-Pb ages and Lu-Hf isotopes of Paleoproterozoic metasedimentary rocks in the North China Craton: Implications for its crust and evolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1331, https://doi.org/10.5194/egusphere-egu24-1331, 2024.

The Earth's crust, an incredibly thin layer of rock encompassing the planet's outermost solid shell, plays a crucial role in sustaining human existence by housing vital natural resources. Moreover, actively engaged in a multitude of geological processes such as plate tectonics, volcanic activities, and erosion, it significantly molds landscapes and influences natural phenomena, thereby serving as a cornerstone in maintaining Earth's habitable conditions. However, substantial debate persists regarding the mechanisms governing the growth of continental crust, and the broader implications of its role within Earth's tectonic framework remain elusive. In this study, we sought to predict the historical evolution of crustal thickness during the Archean era using machine learning based on global geochemical data of igneous rocks. The model predicted that crustal thickness decreased progressively from approximately ~35km during the Paleoarchean period to a nadir of roughly ~33km in the Early Mesoarchean era. Subsequently, there was a trend of thickening observed from the Early Mesoarchean to the Neoarchean period. Our findings reveal a dynamic history of Archean crustal thickness characterized by an initial phase of thinning followed by subsequent thickening from the Paleoarchean to the Neoarchean. These fluctuations align cohesively with the transition from a pre-plate tectonic to a regime of sustained plate tectonics. The observed phenomena were attributed to the initiation of ultrathick primary crust formation, accompanied by a tectonic regime primarily influenced by Rayleigh–Taylor instabilities. The instabilities arising from an excessive buildup of primary crust likely played a pivotal role in causing the subsequent thinning of the continental crust, marking the transition phase towards a predominantly modern-style subduction.

How to cite: Zhang, Y., Liu, H., and Sun, W.: Machine Learning-Based Quantification of Archean Crustal Thickness (Moho Depth), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7316, https://doi.org/10.5194/egusphere-egu24-7316, 2024.

Research on sampling and analysis of fluids and rocks within both continental and oceanic crusts has revealed a significant portion of Earth's prokaryotic biomass residing in the rock-hosted biosphere, extending to depths of several kilometers. This deep subsurface life within the lithosphere may host a substantial amount of Earth's biomass. However, more constrained estimates of this biomass are still lacking. In this study, we first determine the habitable volume of the lithospheric biosphere and then estimate its potential biomass. Temperature, a critical factor influencing the limits of life, is considered the primary focus in this context. We employ a machine learning approach to estimate the habitable volume of the lithosphere above the 122°C isotherm, which is considered the upper limit for prokaryotic life.

Our methodology involves selecting a range of geophysical and geological features that are thought to impact geothermal gradients. These include Moho depth, lithosphere-asthenosphere boundary (LAB) depth, topography, susceptibility, tectonic units, gravity mean curvature, vertical magnetic field, and distances to ridges, trenches, transform faults, young rifts, and volcanoes, as well as P-wave and S-wave velocities in the crust. We used a Gradient Boosted Regression Tree algorithm to develop two models correlating geothermal gradients in continental and oceanic settings with these geophysical and geological features. The models were applied to a binned grid with a 0.5° x 0.5° resolution, enabling the estimation of the depth to the 122°C isotherm for each grid based on the machine learning model's predictions of geothermal gradients.

Our results reveal substantial habitable volumes within the lithosphere. The habitable volume in the continental crust may account for about 7% of the total crustal volume, while in the oceanic crust, it could be around 5%. Given the cell density range of 10² to 10⁶ cells per gram in oceanic rocks and 10² to 10⁴ cells per gram in continental rocks, along with an estimated carbon mass of 2x10^-14 grams per cell, the lithospheric biomass is estimated to be between 0.008 and 33.2 Gt Carbon. For context, the biomass found in plants and marine microbes is estimated to be around 450 Gt Carbon and 2.9 Gt Carbon, respectively.

How to cite: Zhao, W. and Zhangzhou, J.: Estimating Biomass in the Lithosphere: A Machine Learning Approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13794, https://doi.org/10.5194/egusphere-egu24-13794, 2024.

Vp/Vs (or the Poisson's ratio) provides critical information to constrain the bulk crustal composition, stress state, and tectonic evolution. The crustal Vp/Vs beneath the seismic station can be effectively determined by the receiver function H-κ stacking technique. However, complex crustal structures often cause large uncertainty in Vp/Vs measurements. Additionally, Vp/Vs observations are sparse in many regions of the world due to the uneven distribution of seismic stations. While interpolation methods have been widely applied to obtain a continuous distribution of Vp/Vs, these results are not accurate enough and may be strongly biased by the interpolation artifacts. Therefore, reliable mapping of the Vp/Vs variation in the crust remains a major challenge in seismological study.

We present a machine learning approach to estimate the Vp/Vs using multiple geophysical datasets. This approach assumes that the Vp/Vs is related to the physical and chemical properties of the crust. Specifically, we implement a Gradient Boosted Regression Tree algorithm (XGBoost) to develop an optimal prediction model for North America. To train the model, we use Vp/Vs as the target values and a compilation of geophysical observations as the predictor variables. These measurements are composed of two types of data: (1) continuous data, such as crustal velocities and gravity anomalies; (2) categorical data (tectonic type). We use 80% of the measurements to train the model, and the remaining 20% to validate the model. We first examine the reliability of this method by predicting Vp/Vs for the United States, where extensive geophysical data are available. Overall, the model achieves a high R² value of 0.88 in all measured results versus prediction results, indicating robust prediction results at most locations. In the second, a more challenging test, we predict Vp/Vs for Canada where measurements are sparse and uneven, and the amount of data is only 14% of that in the United States. The results show an R² value of 0.87 between the measured and predicted values. Feature importance analysis indicates that crustal shear wave velocity and tectonic type contribute most significantly to reducing the loss function. The prediction results show that bulk Vp/Vs varies between 1.70 and 1.90 across Canada, with a mean value of 1.82.  The cratonic regions generally exhibit high (>1.80) Vp/Vs with a relatively small (<0.05) variation, whereas Proterozoic orogens are characterized by a large Vp/Vs variation from 1.75 and 1.90. The lowest Vp/Vs of 1.66 is observed in the Phanerozoic orogenic belts of Cordillera, contrasting sharply with the basement rocks (~1.90) beneath the Alberta foreland basin. Overall, our study highlights the capability of the machine learning in discovering complex relationships between multi-dimensional geophysical data sets and resolving crustal Vp/Vs in continents globally.

How to cite: Gao, X., Chen, Y., Zhang, Z., and Zhao, W.: Predicting Vp/Vs in North America with a Machine Learning Approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9828, https://doi.org/10.5194/egusphere-egu24-9828, 2024.

Delineating subsurface interfaces is a crucial step in site selection and characterization for various subsurface applications, such as the geologic carbon sequestration, the radioactive waste disposal and hydrocarbon exploration and production. 3D seismic surveys are widely used for identifying subsurface interfaces and geologic features. Due to the large volumes and the complexity of seismic data, manual interpretation of subsurface interfaces is extremely time-consuming, and the interpretation results can be greatly affected by the subjectivity of a particular interpreter. With the latest advances in deep neural networks (DNNs), automatic seismic interpretation methods based on DNNs emerged. Most of the DNN-based seismic interpretation methods are supervised learning methods, which require large amount of labeled data for network training. We have developed an unsupervised learning method with deep fully convolutional networks (FCNs) for rapid subsurface interface identification based on self-learning algorithms, which does not require manual data labeling and specific training datasets. The characteristics of subsurface interfaces are represented as numerical constraints added to the specially designed loss function for constructing the FCN model. Physical constraints are further applied in postprocessing the outputs of the FCN model to improve the resolution of the identified subsurface interfaces. Application of the unsupervised learning method on a real seismic dataset collected at a potential CO₂ storage site demonstrates that the proposed method yields rapid and accurate identification of subsurface interfaces with relatively strong acoustic impedance contrast in seismic images. The proposed approach may assist in automatic subsurface interface identification in real time and facilitate building geological models for subsurface applications.

How to cite: Wang, Z., Qi, S., Liu, Y., Zheng, B., Sun, P., and Han, Y.: Unsupervised Deep Learning for Rapid Subsurface Interface Identification using Geophysical Measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19107, https://doi.org/10.5194/egusphere-egu24-19107, 2024.

The Angul-Tikarpara domain of the northern Eastern Ghats Province (EGP) of Eastern India is characterized by high-grade rocks like felsic gneiss, khondalite (garnet-sillimanite-K-feldspar-quartz bearing granulite), charnockite, mafic granulite and aluminous granulite (spinel-magnetite-hematite-garnet-cordierite-sillimanite-K-feldspar-quartz-biotite-plagioclase). Two distinct mineralogical varieties of mafic granulite are present as enclaves within felsic gneiss and coarse-grained charnockite. Type-A mafic granulite is composed of orthopyroxene, clinopyroxene, hornblende, plagioclase, magnetite, ilmenite, biotite, and/or garnet with a subordinate amount of quartz. Type-B mafic granulite, on the other hand, contains titanite along with clinopyroxene, plagioclase, hornblende, garnet, calcite, ilmenite, magnetite, and biotite. The present study is focused on the latter variety where the peak (M_A1) assemblage is represented by clinopyroxene, plagioclase, titanite, ilmenite, magnetite, calcite, and hornblende. Porphyroblastic clinopyroxene in the aforesaid assemblage is characterized by well-developed Al-zoning, with Al₂O₃-content being maximum at the core (5.3–3.8 wt.%) and minimum at the rim (3.6–0.22 wt.%). It appears that the high-aluminous core had stabilized at the peak stage, while the low-aluminous rim formed during the retrogressive stage characterized by decompression (M_A1R). From the zoning pattern, the peak M_A1 and M_A1R conditions were estimated to be 8.6‒6.8 kbar, ~850˚C, and 4.2‒3 kbar, 660 ˚C, respectively. During the decompression, subidioblastic titanite broke down to low-Al clinopyroxene (2.5-0.5 wt.% Al₂O₃) + ilmenite intergrowth surrounding the former phase. The decompression was associated with cooling during which coronal garnet grew surrounding porphyroblastic clinopyroxene, plagioclase, and calcite. Zn-bearing spinel was exsolved from magnetite during this stage. Phase diagram modeling of Type-B mafic granulite shows that M_A1 metamorphism occurred along a clockwise P-T trajectory. Further development of grossular-rich coronal garnet around hornblende suggests that the rock underwent a second prograde metamorphic event (M_A2). In the chondrite-normalized rare earth element (REE) diagram, Type-B mafic granulite shows a more fractionated pattern than Type-A mafic granulite.

Textural, thermobarometric, and phase equilibria data indicate that the metamorphic evolution of Type-B mafic granulite is comparable to the recently published P-T evolution of Type-A mafic granulite, aluminous granulite, khondalite, and fine-grained charnockite of the area where the M_A1 metamorphism involved decompression and associated cooling from ∼850^◦C, 7–8 kbar to ∼760^◦C, 4–5.8 kbar at ca. 1200 Ma and followed by subsequent prograde M_A2 metamorphism at ca. 990 Ma. This study provides additional evidence that the geological evolution of the Angul-Tikarpara domain is different than the rest of the EGP lying at the southern part of the Mahanadi Shear Zone (MSZ). The latter segment of the EGP is characterized by ultra-high temperature metamorphism (UHT) at ca. 1030–990 Ma which is absent in the present study area. Such contrasting ages and P-T evolution of high-grade rocks on either side of the MSZ confirm the fact that the shear zone represents a terrane boundary which was formed by juxtaposition of the Angul-Tikarpara domain with the EGP only after ca. 960 Ma.

How to cite: Banerjee, A., Ganguly, P., Bose, S., Sorcar, N., Mukherjee, S., and Das, K.: Reaction textures in titanite-bearing mafic granulite: Constraints on the metamorphic evolution of Angul-Tikarpara domain of the Eastern Ghats Province, India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2374, https://doi.org/10.5194/egusphere-egu24-2374, 2024.

The Belomorian eclogite province (BEP) of the eastern Fennoscandian Shield (Russia) is one of the oldest eclogite metamorphic complexes on Earth, providing an opportunity to study the most ancient subduction processes. The BEP gained prominence due to the numerous publications on petrology and geochemistry of Archean-Paleoproterozoic HP metamorphic rocks from Gridino, Salma, and Kuru-Vaara areas. Here, we present results from a detailed petrological study of a representative retrograde eclogite sample from a new HP locality in the Kem-Ludy Islands, where eclogites form lenticular bodies and have conformable contacts with country gneisses.

The studied retrograde eclogite sample has a massive structure with a fine- to medium-grained granoblastic texture. At least three metamorphic stages were identified in the rock based on the mineral assemblages. The pre-eclogite stage is indicated by epidote inclusions in the cores of garnet porphyroblasts. Garnet, matrix omphacite, and quartz represent peak metamorphism stage. Plagioclase-clinopyroxene symplectites after the omphacite and amphibole-plagioclase kelyphytes around the garnet were formed at the retrograde stage, while peak metamorphism minerals were then replaced to different extents by secondary amphibole during fluid-rock interaction.

The application of garnet-clinopyroxene geothermometer and clinopyroxene-plagioclase-quartz geobarometer assessed peak metamorphic temperature of 625-670°C and minimum pressure of 1.2-1.3 GPa, respectively. The retrograde stage corresponds to conditions of 600-725°C and 0.6-0.7 GPa as determined by garnet-amphibole, amphibole-clinopyroxene, amphibole-plagioclase geothermometers, and clinopyroxene-plagioclase-quartz and amphibole-plagioclase geobarometers.

The established P-T metamorphic evolution of the retrograde eclogite from Kem-Ludy area differs from most of the eclogites of Gridino and Salma areas by lower pressure at the peak of metamorphism and a lack of decompressional heating up to the UHT granulite facies conditions. Therefore, Kem-Ludy area likely pertains to another tectonic slice than Gridino and Salma areas, where the Neoarchean and Paleoproterozoic ages of HP metamorphism are under debate. Further investigation is needed to clarify the tectonic position of Kem-Ludy area.

Financial support of Russian Science Foundation project 23-77-00066.

How to cite: Grigorieva, V., Perchuk, A., Kozlovsky, V., and Sandalov, F.: Metamorphic evolution of eclogite of Kem-Ludy Islands, Early Precambrian Belomorian eclogite province, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10822, https://doi.org/10.5194/egusphere-egu24-10822, 2024.