Cratonic Domains Research Articles

Source‐To‐Sink Sedimentary Budget of the African Equatorial Atlantic Rifted Margin

AbstractDespite their very low relief and erosion rates, non‐orogenic (i.e., cratonic) continental domains account for over 60% of the Earth's exposed lands. Therefore, they contribute significantly to the clastic sediments and solutes exported to the ocean and should be accounted for in global studies. Nonetheless, they have been much less studied than orogenic domains. In this study, we establish the source‐to‐sink sedimentary budget of the sub‐saharan West African cratonic domain and its Equatorial Atlantic rifted margin using published low‐temperature thermochronological data to estimate onshore denudation and regional geological cross‐sections to estimate offshore accumulation. We show that during and immediately following rifting (130‐94 Ma), the build‐up and subsequent erosion of rift‐related relief resulted in a transient, 100–200 km wide strip along the margin recording high denudation rates (>50 m/Myr), while the inland domain underwent steady and very low denudation (<10 m/Myr). Afterward, the whole onshore domain underwent very low and steady denudation. Thus, the changes in post‐rift accumulation rates documented along the rifted margin were caused by changes in the climate and/or drainage network. During the Late Cretaceous, we document a regional rise in accumulation rates caused by the enlargement of drainage areas feeding the basins by a hinterlandward migration of the continental divide. During the Paleogene, we document a general drop in accumulation rates in all the basins of the African Atlantic margins caused by the global greenhouse climate, which enhanced the development of lateritic weathering mantles, storing clastic sediments on the continent and favoring solute exports to the ocean.

Insights into the petrogenetic evolution of the Khallari layered intrusion and coeval granites of the Paleoproterozoic Dongargarh Supergroup, Bastar Craton, India

Several layered igneous complexes around the globe host ultramafic-mafic lithologies at the base to more evolved lithologies at the roof sections. In this study, we report U-Pb zircon ages and geochemistry of Paleoproterozoic granites and Khallari layered intrusion from the Dongargarh Supergroup of Bastar craton, central India. The Khallari layered intrusion hosts pyroxenites at the base to more evolved lithologies including gabbro, layered gabbro and anorthositic gabbros towards the roof section of the magma chamber. The estimated weighted mean U-Pb zircon ages of granites (2443 ± 13 Ma) and layered intrusion (2494 ± 19 Ma and 2469.2 ± 5.2 Ma) are in conjunction with the major Siderian crust building peak identified at the craton. The granitic plutons of the Dongargarh Supergroup are chemically similar to post-collisional/anorogenic granitic rocks, in which the rapakivi granites exhibit a fractional crystallization relation with the Khallari layered intrusion. The Dongargarh granites and enclaves were derived by a combination of fractional crystallization and mixing of partial melts from the pre-existing crust of the Bastar craton. Petrogenetic modeling indicates Khallari layered intrusion is formed by the fractional crystallization of lithospheric mantle melt followed by crystal accumulation. The parental melt also experienced localized crustal contamination (up to 10%) during its evolution. The layered intrusion and the granites formed in a post-collisional rift setting where magmatism postdates the proposed subduction and collisional orogenesis at 2.5 Ga involves cratonic domains of Eastern and Western Bastar cratons.

Sediment routing systems to the Atlantic rifted margin of the Guiana Shield

Abstract Sediment routing systems of cratonic domains have not been studied extensively because their relief and erosion rates are very low, although their vast dimensions allowed them to contribute to a significant proportion of the sediments exported to the global ocean. To gain further insights into the behavior of cratonic sediment routing systems at geological time scales, we investigated the Guiana Shield and its Atlantic rifted margin (i.e., the Guiana-Suriname and Foz do Amazonas Basins, northern South America) over the Meso-Cenozoic with an emphasis on paleoenvironment and accumulation histories of the offshore sediments. We show that the basins of the Guiana Shield rifted margin record (1) periods of very low siliciclastic supply concomitant with the development of carbonate platforms, alternating with (2) phases of higher siliciclastic supply associated with sand-dominated clastic deposits and turbidites. Low siliciclastic supplies reflect either very limited rift-related relief growth and erosion such as during the Central Atlantic rifting in the Late Jurassic or intense lateritic weathering of the cratonic source area during Paleogene–Miocene climate optima. Higher siliciclastic supplies correspond either to (1) periods of rapid rift-related relief growth and erosion such as during the Equatorial Atlantic rifting (Early Cretaceous), (2) periods of drainage reorganization over a steadily eroding cratonic domain (Late Cretaceous), or (3) periods of tapping of sediments stored in the Andean retro-foreland basins via the presentday Orinoco and Amazon Rivers (Plio-Pleistocene).

Nature and extent of shear deformation along the eastern and southern terrane boundaries of the Motloutse Complex, eastern Botswana

The Motloutse Complex at the southwestern margin of the Zimbabwe Craton is an Archean medium-grade allochthonous terrane in eastern Botswana. The high-grade gneisses of the Limpopo Complex lie east of the Motloutse Complex and those of the Mahalapye Complex to the south. A number of shear zones are postulated to delineate the aerial extent of the Motloutse Complex, and separate it from the cratonic domain and high-grade terranes. Except the northern margin, these terrane boundaries are poorly understood. This study aims to characterize the nature and extent of shear deformation along the eastern (Dikalate Shear Zone) and southern (Sunnyside Shear Zone) boundaries of the Motloutse Complex. Representative areas from the eastern (Masikate Hills), and southern (Radisele-Mogome) regions were selected for geologic mapping. In view of the poor exposures outside these areas, aeromagnetic data is used to delineate the regional extent of the shear zones. Shear deformation is prominent along the N–S elongate Masikate Hills made up of tonalite gneiss, amphibolite, anorthosite and ultramafic rocks. A dominant right lateral sense of movement, with minor left lateral movement is inferred for the shear deformation. The extent of the N–S trending Dikalate Shear Zone, and its branches, is delineated in the aeromagnetic imagery.In comparison, the Radisele-Mogome area, exposes two set of rocks – tonalite gneiss, amphibolite, ultramafic rocks (part of the Motloutse Complex) to the NE, and granodiorite, diorite gneiss, leucogranite (part of the Mahalapye Complex) to the SW. This is well exemplified in the aeromagnetic image, with change in magnetic signature between rocks in the NE and SW parts. Shear deformation variably overprints rocks along the contact zone. Mylonitic deformation is prominent in the SW part. As along the eastern terrane boundary, a dominant right lateral sense of shear movement is inferred. The extent of the NW-SE trending Sunnyside Shear Zone and its branch is delineated in the aeromagnetic imagery. The results highlight an example of using a combination of geological and geophysical methods to delineate the nature and extent of shear zones that constitute terrane boundaries.

Episodic mafic dike swarms (2.0 and 1.7 Ga) in the Quadrilátero Ferrífero Province, Brazil: Implications for the refinement of the São Francisco Craton magmatic barcode and the North China link

Late Archean to Paleoproterozoic dike swarms crosscut the basement and supracrustal sequences of the São Francisco Craton. Their spatial distribution and orientation patterns, as paleomagnetic signature, provide key evidence to the positioning of the cratonic area in ancient paleocontinental reconstructions through the refinement of the “magmatic barcode”. Zircon U–Pb data from dikes in the cratonic domain of the Quadrilátero Ferrífero Mineral Province, in the southern of São Francisco Craton, allowed the recognition of two magmatic stages: a Rhyacian-Orosirian pulse, belonging to the Paraopeba swarm, and a Statherian pulse that belongs to the Pará de Minas II swarm. The Ediacaran Brasiliano Orogeny later affected dikes from both swarms.The Paraopeba swarm consists of pulses related to the Rhyacian-Orosirian orogenic cycle, with dikes providing U–Pb zircon ages of 2019 + 23–11, 1944 ± 29 and 1918 ± 12 Ma. The zircon εHf values from these dikes are predominantly negative, suggesting reworking of preexisting crust, whereas the TDM ages vary from 4000 to 2500 Ma, which can be interpreted as a long period of crustal residence. The Statherian Pará de Minas II swarm, with a U–Pb zircon age of 1717 ± 9 Ma, is chronocorrelated to the lower stratigraphic sequence of the Espinhaço rift basin. Zircon grains from this dike have predominantly positive εHf values and TDM model ages between 3050 and 1950 Ma. Unlike the Paraopeba swarm, the Pará de Minas magmatism is marked by a striking input of juvenile mantle material with minor crustal contamination. The Ediacaran Brasiliano tectono-thermal event caused isotopic disturbance in both Paraopeba and Pará de Minas II swarms, resulting in lower intercepts at 595 ± 65 and 584 ± 27 Ma, respectively.Our data pinpoint dike emplacement related to tectonic events described in the southern of São Francisco. In the final stages of Rhyacian orogenic cycle, at ca. 2019–1918 Ma, data suggests an important crustal contamination. Thereafter, a crustal rifting event at ca. 1717 Ma involved dike swarm emplacement with magmatic source from a mantle plume. The Rhyacian-Orosirian ages obtained here are the first zircon U–Pb geochronological data presented for the Paraopeba dike swarm and allow for a refinement of the São Francisco Craton barcode, reinforcing previous suggestions of proximity with the North China Craton crosscut by the similarly aged Hengshan and Xiwangshan/Xuwujia dike swarms, whether or not those two cratonic pieces were part of the Columbia supercontinent.

Evidence of secular variation in Archean crust formation in the Eastern Indian Shield

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0–2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT.

The Brazilian Midwest lateritic domains: Regolith features and dynamic

Intracontinental deformations cause the uplift of erosive surfaces and are consequence of lithosphere destabilization processes in cratonic domains. The Atlantic Ocean opening influenced processes 2000 km away from the coast with reflections in the regolith. The Brazilian Midwest regolith is characterised by a stepwise landscape of highlands, mountains, plateaus, inselbergs, canyons, and lowlands that form four morphostratigraphic surfaces. Surface 1 is sustained by quartzite mountains. Surfaces 2 and 3 are mantled by ferruginous lateritic duricrusts. The duricrusts in surface 2 are quartz-hematitic, hardened, columnar, reddish-pink to yellowish-red, have protonodular, fragmental, and pisolitic textures, and reach up to 3 m thick, whereas the duricrusts in surface 3 are more friable, quartz-goethitic, columnar, yellowish-red to orangish-yellow, have a vermiform texture with bioturbated micro-aggregated clay-ferruginous matrix, reaching up to 2 m thick. Surface 4 includes the lower alluvial plains. The purpose of this study was to understand the formation of these regolith surfaces, their relationships, the impact of the lateritic dynamic, and the continental influences as a landscape-shaping agent. The area was studied using SRTM images, swath profiles, and the Roughness Concentration Index, as well as fieldwork control, textures, mineralogy, and geochemistry analysis. Surface 1 marks the regional erosional event, while surfaces 2 and 3 mark two stable well-drained ferruginization weathering episodes and a seasonal drier to humid climate change. Surface 4 sculpts the lowlands. These regolith sequential landforms surfaces were produced in a complex landscape process modelled by an extensive erosional regime as a result of tectonic movements during the Atlantic Ocean evolution evolution and tropical weathering climate since the Neogene, as indicated by the lateritic duricrusts features (textures, mineral, and chemical composition). This landscape is younger than expected for Archean to Paleozoic terrains subjected to tropical weathering, dating back at least to the Paleogene, the age of the majority of older lateritic duricrusts in Brazil and around the world. These younger morphostratigraphic surfaces in cratonic domains, as well as the geological processes involved in tectonic and climate influence, had significant implications for the Brazilian South American Platform regolith.

Regional gravity field distribution over cratonic domains of the Indian shield: Implications for lithospheric evolution and destruction

• Detailed analysis of regional gravity field over Indian cratons. • Extremely low Bouguer gravity of −120 mGal associated with Dharwar Craton. • Gravity field is largely positive over the north Indian cratonic segments. • Cratons with positive gravity have thinner lithosphere and higher heat flow. The nature of crust and lithospheric mantle evolution of the Archean shields and their subsequent destruction through intraplate tectonic processes are important in understanding continental dynamics and resources. The Indian shield, which has remained dynamic throughout the past for more than three billion years of geological history, is unique in terms of its lithospheric architecture compared to other Archean terranes on the globe. It consists of five major cratons, the Dharwar, Singhbhum, Bundelkhand, Bastar and Aravalli, which are separated by active rift valleys, mega lineaments and sutures zones. Here, we present a detailed analysis of gravity field over these areas, and their possible relationship with lithospheric thickness variations. Our results show a large variation in residual gravity field among the different cratons. A highly negative anomaly of around -70 mGal occurs over the western part of the Dharwar Craton, beneath which the lithosphere is about 160–200 km thick. In comparison, the anomalies are conspicuously positive, reaching around (i) +10 mGal over Central Dharwar Craton, Eastern Ghats Belt and the eastern part of Singhbhum Craton, (ii) +35 mGal over the Nellore Schist and Aravalli-Delhi Fold Belts, and (iii) +60 to +70 mGal over eastern part of SGT. In these regions, the lithosphere is markedly thinner between 70 and 135 km. Such areas have been associated with episodic magmatism, moderately high heat flow and elevated Moho temperatures, indicating a large-scale upwarping of isotherms leading to lithospheric destruction to the tune of 100–150 km, caused by mechanical, thermal and chemical erosion. Our study confirms that many of the Archean cratons on the globe, like the Indian shield, have undergone substantial erosion of their roots through subsequent geotectonic processes.

Chrome-diopside Xenocrysts Entrained in a Neoproterozoic Lamprophyre Dyke from the Mysuru Area: Their Origin and Implications for Lithospheric Thickness Beneath the Western Dharwar Craton, Southern India

Abstract In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2 – 1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895 - 1026 °C (± 30 °C) and 32 – 38 kbar respectively and correspond to depth range of 106 – 127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ~115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.

The Devonian of the Paraná Basin, Brazil: Sequence stratigraphy, paleogeography, and SW Gondwana interregional correlations

The Devonian Period is characterized by an overall mean sea-level higher than is observed today. This pattern is recognized in several basins worldwide and permitted, for example, the flooding of the Rheic ocean through SW Gondwana's cratonic domain. Nonetheless, this flooding was not purely eustatic. The Ordovician-Devonian geodynamic Famatinian episode also played its role. This tectonic-eustatic interplay favored shallow-marine deposition recorded in several SW Gondwana basins, such as the Paraná Basin in Brazil. Even so, the evolution of the Paraná Basin through the Devonian and the factors responsible for the accommodation space remain speculative, which leads to bias regarding the correlation of transgressive-regressive events in SW Gondwana. This work aims, by integrating outcrop and subsurface data, to provide a basin-scale stratigraphic framework and a paleogeographical reconstruction of the Ponta Grossa (Early Devonian) and São Domingos formations (Middle to Late Devonian) of the Paraná Basin. It also correlates the studied interval with adjacent basins to identify the impact of tectonic and eustatic processes on the evolution of SW Gondwana tectono-sedimentary cycles. We identified two 2nd order Transgressive-Regressive sequences that correspond to the lithostratigraphic occurrence of Ponta Grossa and São Domingos formations, respectively, and whose maximum transgressive and regressive surfaces and high-hierarchy unconformities are traceable through adjacent basins. The Ponta Grossa-São Domingos boundary also delineates an important tectonic event associated with far-field stresses probably caused by Precordilleran orogeny related phenomena. The Ponta Grossa and São Domingos formations share several similarities with coeval strata in SW Gondwana basins, indicating that they can be extrapolated and correlated to contemporaneous processes of interregional scale. The first, namely Precordilleran I (Early Devonian) was dominated by tectonics and encompasses the Ponta Grossa and Las Pavas composite sequence set. The second, Precordilleran II (Middle to Late Devonian), was mostly eustatic and encompasses São Domingos and Aguarave composite sequence sets. Both intervals are part of the Famatinian geodynamic cycle.

Devonian to Permian post-orogenic denudation of the Brasília Belt of West Gondwana: insights from apatite fission track thermochronology

The Brasília Belt in southern Brazil is a Neoproterozoic orogenic belt that represents the northern border of the Paraná Basin, a long-lived early Paleozoic intracratonic basin. The belt is surrounded by cratonic domains and were modestly affected and reactivated by the opening of the South Atlantic Ocean during the Cretaceous. Here we present new apatite fission track data from crystalline rocks of the Brasília Belt. The apatite fission track central ages range from 386 to 243 Ma and the mean track lengths range between 13.4 and 14.5 μm. Thermal history modeling reveals Devonian to Permian cooling, which we interpret as the main final exhumation of the Brasília Belt, implying that indeed the influence of posterior Mesozoic and Cenozoic geodynamic history on the belt was limited. The Paraná Basin and surrounding elevated terranes therefore provide a time window to study the tectonic history of Gondwana and the effect of far-field intraplate stresses on the interior of West Gondwana. Rapid basement exhumation of the Brasília Belt is coeval with extension and tectonic subsidence in the Paraná Basin as a result of continent-scale tectonic forces occurring over entire West Gondwana.

Evidence for ca. 2046 Ma high-grade metamorphism in Paleoproterozoic metasedimentary rocks of the northern Borborema Province, NE Brazil: constraints from U-Pb (LA-ICP-MS) zircon ages

This study presents LA-ICP-MS U-Pb detrital zircon ages on a migmatitic paragneiss of the Jaguaretama Complex, northern Borborema Province, NE-Brazil. This metasedimentary rock comprises a stromatic migmatite with alternating layers of metapelite, metapsammite, and garnet-tourmaline-bearing leucosome. The paleosome displays a well-developed schistosity, with fine to medium-grained textures, and locally garnet porphyroblasts. Backscattered electron images show that many of the zircon grains display metamorphic overgrowths (recrystallization) and cores with igneous oscillatory zoning. The detrital zircon grains show 207Pb/206Pb ages clustering from 2.51 to 2.12 Ga (with 90% conc.), evidencing that the igneous sources were generated mostly during the Transamazonian/Eburnean orogeny. The age of 2137±9 Ma of the youngest and most concordant detrital zircon (core) is interpreted as the maximum depositional age of the protolith. The youngest and most concordant (conc. 99%) 207Pb/206Pb apparent age obtained in a metamorphic zircon overgrowth was 2046±6 Ma, which is interpreted as the age of high-grade metamorphism. This age of metamorphism is similar to other records of Paleoproterozoic high-grade metamorphic events in the Borborema Province and other cratonic domains (e.g. Sao Francisco and West Africa cratons). This age also represents the minimum depositional age of the protolith. Therefore, the depositional age of the studied supracrustal rock may be bracketed between ca. 2.14 and 2.05 Ga.

Fluid inclusion and stable isotope (O, H, C) constraints on the genesis of the Pedra Branca gold deposit, Troia Massif, Borborema Province, NE Brazil: An example of hypozonal orogenic gold mineralization

At the Archean-Paleoproterozoic Troia Massif, NE Brazil, two major Paleoproterozoic greenstone belts are documented (Algodões and Serra das Pipocas). They share similar lithostratigraphic characteristics with other 2.2–2.1 Ga greenstone belts of the surrounding cratonic domains (e.g. Guiana shield and West Africa craton). Gold mineralization in the Serra das Pipocas greenstone belt is associated with a regional NE-trending shear zone. In the mineralized area (the Pedra Branca gold deposit) the main gold mineralization is found in association with quartz veins, calc–silicate alteration (e.g. diopside, K-feldspar, amphibole, titanite, biotite, pyrite, albite, magnetite ± carbonates) and albitization. Pyrrhotite is commonly found as inclusions in pyrite, and late magnetite replaces pyrite, suggesting progressive oxidation of the ore–forming fluid. Free-milling gold commonly precipitates at the late stages of alteration, in close association with magnetite and tellurides. The main tellurides found are hessite, altaite, sylvanite, calaverite and tellurobismuthite. Two fluid inclusion assemblages were identified from mineralized quartz veins. Assemblage 1 is characterized by pseudo-secondary trails that show the coexistence of CO2–rich and low salinity (0–8 wt% NaCl equiv.) CO2–H2O–NaCl and H2O–NaCl inclusions, suggesting formation during phase separation (fluid immiscibility). Assemblage 2 is represented by late secondary low–temperature (Th < 200 °C) H2O–NaCl inclusions, probably unrelated to gold mineralization. The δ18O, δD and δ13C values of hydrothermal minerals (quartz, calcite, biotite, hornblende and magnetite) and fluid inclusions are compatible with a magmatic–hydrothermal ore-forming fluid that underwent fluid–rock interaction with the greenstone sequence. The calculated fluid δ18O values range from +8.3 to +11.0‰ (n = 59), fluid δD from −98 to –32‰ (n = 24) and δ13C values of calcite from −6.35 to −9.40‰ (n = 3). Oxygen isotope thermometry for quartz–magnetite pairs yields temperatures from 467 to 526 °C (n = 7, average 503 °C). This probably represents the temperature of gold deposition, because gold is generally associated with magnetite in the ore mineral assemblage. The estimated PT conditions of the gold mineralization range from 467 °C and 1.93 kbar (7 km) to 526 °C and 3.48 kbar (13 km), repectively, and they were obtained from the intersection of isochores with the temperature range derived from the quartz–magnetite isotope geothermometer. This PT range is akin to hypozonal orogenic gold deposits. The association of gold with magnetite and tellurides implies an ore–forming fluid derived from oxidized magmas, similar to those interpreted as ‘orogenic oxidized intrusion-related gold deposits’ in other Precambrian greenstone belts (e.g. Abitibi and Eastern Goldfields).

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.

Depositional history and provenance of cratonic “Purana” basins in southern India: A multipronged geochronology approach to the Proterozoic Kaladgi and Bhima basins

Peninsular India is a collage of Archaean cratonic domains separated by Proterozoic mobile belts. A number of cratonic basins, known as “Purana basins” in the Indian literature, formed in different parts of the Indian Peninsula during extensional tectonic events, from Paleoproterozoic through Neoproterozoic times. In this contribution, we present a diversity of new geochronological data for different units within the Kaladgi and the Bhima basins, which overlie the western and eastern Dharwar cratons, respectively. The new geochronology data are discussed in terms of depositional history and provenance of these poorly understood Proterozoic intracratonic basins. For the Kaladgi Group, a U–Pb baddeleyite age of 1,861 ± 4 Ma obtained for a dolerite dyke intruding the Yendigere Formation is used to constrain the minimum age of deposition of the lower Kaladgi Group. This result demonstrates that this part of the succession is comparable in age to the Papaghni Group of the Cuddapah Basin, heralding onset of Purana sedimentation at ~1,900 Ma. The detrital zircon populations from the clastic rocks of the Kaladgi and Bhima basins show unique and distinct age patterns indicating different source of sediments for these two basins. Palaeocurrent analysis indicates a change in provenance from south or southeast to west or northwest between the Kaladgi and Bhima clastic sedimentation. New U–Th–Pb and Rb–Sr radiometric dates of limestones and glauconite‐bearing sandstones of the Bhima Group (Bhima Basin) and the Badami Group (Kaladgi Basin) indicate deposition at around 800–900 Ma, suggesting contemporaneity for the two successions. Thus, the unconformity between the Kaladgi Group and the overlying Badami Group represents a time gap of up to 1,000 Myr. These new results demonstrate the complex multistage burial and unroofing history of the Archaean Dharwar Craton throughout the Proterozoic, with important implications for exploration of metal deposits and diamonds in Peninsular India.

Building up the first continents: Mesoarchean to Paleoproterozoic crustal evolution in West Troms, Norway, inferred from granitoid petrology, geochemistry and zircon U-Pb/Lu-Hf isotopes

Combining geological, petrographic, geochemical and zircon U-Pb and Lu-Hf isotopic data on granitoids provides powerful insights into the architecture, growth and geodynamic setting of Precambrian continents. Using such a dataset, we explore ca. 1.3 Ga of continent construction in the West Troms Basement Complex (WTBC), northern Norway, between the Mesoarchean and Paleoproterozoic. The new data show that the WTBC consists of two Archean lithotectonic segments: (1) a northeastern (NE) segment made up by juvenile (εHf(t) = +2 to +3) TTGs, formed between 2.92 and 2.83 Ga by increasingly deeper melting (Sr/Y from <50 to >90) of a young mafic crust; (2) a southwestern (SW) segment dominated by 2.71–2.67 Ga, sanukitoid-like (qz-)diorite-granodiorite-granite, derived from mantle-TTGs interactions and unrelated to the NE segment, as shown by positive εHf(t) (+1 to +2). The intervening domain comprises 2.74–2.69 Ga sanukitoids and granites with minor 2.96 Ga remnants, and may correspond to either reworked NE segment (εHf(t) = −1) or a third segment. The Archean crust was intruded at 1.87–1.86 and 1.80–1.75 Ga by qz-monzonite, monzodiorite and monzo-/syenogranite plutons belonging to the Transscandinavian Igneous Belt (TIB). These show negative εHf(t) (−7 to −13) pointing to melting of Archean mafic-intermediate lower crust and enriched lithospheric mantle.These observations are explained by a geodynamic model involving: (i) stabilization of the NE segment as a 3.0–2.8 Ga proto-cratonic nucleus, by formation and repeated melting of a progressively thickening mafic plateau; and (ii) drifting of this proto-craton towards the SW at 2.75–2.65 Ga, leading to “passive” subduction at its leading edge and formation of the SW segment as an arc-like complex. The subsequent Svecofennian accretionary orogeny (1.97–1.81 Ga) led to over-thickening and delamination of the lower crust and lithospheric mantle, forming the 1.87–1.75 Ga granitoids. A similar transition from intraplate to horizontal tectonics has been recorded in other cratonic domains, yet at different times ranging from Paleoarchean to Paleoproterozoic. This suggests that the onset of plate tectonics is not a globally synchronous event and depends on the regional lithospheric evolution.

Geochemistry and U–Pb–Hf zircon data for plutonic rocks of the Troia Massif, Borborema Province, NE Brazil: Evidence for reworking of Archean and juvenile Paleoproterozoic crust during Rhyacian accretionary and collisional tectonics

Rhyacian (c. 2.30–2.05 Ga) subduction-accretion and collisional processes are thought to represent an important event of crustal growth and reworking, often reported in the so-called Tranzamazonian/Eburnean granite-greenstone terranes of N/NE Brazil and West African craton. In this work, we present whole-rock geochemistry and U–Pb–Hf zircon ages for Rhyacian plutonic rocks that intrude Paleoproterozoic granite-greenstone terranes and Archean basement rocks of the Troia Massif, Borborema Province, NE Brazil. U–Pb SHRIMP zircon age of 2185 ± 4 Ma was obtained for a metatonalite sample (Mirador metatonalites). The metatonalites show geochemical affinity similar to those of adakites-like rocks, with all “classic” (slab-derived) adakitic signature (e.g. high Sr/Y, La/Yb and low HREE), but more akin to those evolved from partial melting of the metasomatized mantle wedge, followed by fractional crystallization. Zircon Hf crustal model ages of the metatonalite sample range between 2800 and 2660 Ma, evidencing that Archean crustal components contributed to its magma genesis, probably via subduction of continental-derived sediments to the mantle wedge. For potassic plutons of the Troia Massif (Bananeira suite) we obtained U–Pb SHRIMP zircon age of 2079 ± 4 Ma for a deformed quartz-monzonite with geochemical affinities similar to those derived from low-pressure partial melting of a K-rich mafic protolith. For less-deformed equigranular granites, we obtained a U–Pb LA-ICPMS zircon age of 2068 ± 5 Ma. They are high-K calc-alkaline and slightly peraluminous granites that probably derived from low-pressure partial melting of tonalitic crustal sources (metatonalites) and/or metagraywackes. Zircon Hf crustal model ages of a granite sample range from 2713 to 2535 Ma, evidencing Archean crustal contribution to magma genesis. However, two analyzed c. 2307 Ma inherited zircon grains show εHf(t) values of c. +4.87, indicating that crustal reworking of less-radiogenic Paleoproterozoic sources also participated in its genesis. Regional correlations of our results with the Paleoproterozoic record of the Troia Massif, Borborema Province and surrounding cratonic domains, suggest that the studied plutons are related to the c. 2.2–2.0 Ga Transamazonian/Eburnean orogenic cycle. The c. 2185 Ma metatonalites are associated to pre-collisional setting, while the c. 2079–2068 Ma quartz monzonites and granites evolved during collisional setting.

Magnetotelluric imaging across the tectonic structures in the eastern segment of the Central Indian Tectonic Zone: Preserved imprints of polyphase tectonics and evidence for suture status of the Tan Shear

The Central Indian Tectonic Zone (CITZ), a major tectonic feature extending across the Indian sub-continent, was formed by the accretion of the cratonic domains in the north and south blocks of peninsular India during the Proterozoic. It is characterized with prominent shear/fault zones and granulite belts that carry the evolutionary history of the central Indian region. This study presents the two-dimensional resistivity structure of the hitherto least studied eastern segment of the mega lineament in the Indian subcontinent. Magnetotelluric (MT) data along the Rajnandgaon-Bareli profile, which extends from the southern margin (Central Indian Shear-CIS) to the northern limit (Narmada-Son Lineament-NSL) of the CITZ, were used to investigate the resistivity character of the crust and shallow upper mantle. The study imaged conductive to moderately conductive structures, which show correlation to the various shear/faults and granulitic belts mapped in the region, interspersed in a resistive background. An upper crustal isolated conductor was observed below the Bhandra – Balaghat Granulite belt, which is exposed along the CIS. A vertical moderately conductive structure noticed in the middle to upper mantle depths under the Ramakona-Katangi Granulite (RKG) belt and this is interpreted as fault/shear zone that acted as the channel/pathway for the emplacement of parental magma of the RKG rocks. A more interesting feature in the MT model is a north dipping conductive horizon extending from the upper crust to the Moho. This feature shows further extension into the upper mantle as a subvertical moderately conductive zone and can be traced onto the surface mark of the Tan Shear, which suggests suture status to this prominent shear zone within the CITZ. Resistivity section also shows mid-lower crustal conductive/moderately conductive horizons under the Deccan trap and NSL, which are produced due to the large-scale magma intrusions into the crust during the Cretaceous-Tertiary Deccan volcanism through the pre-existing weak zones in the CITZ. The conductive to moderately conductive features of the MT model are indicative of the complex tectonic history of the region that range from accretion processes started in the Paleoproterozoic, followed by multiple metamorphic and deformation cycles in different geological times, to the Late Cretaceous volcanism.

New insights into the crust and lithospheric mantle structure of Africa from elevation, geoid, and thermal analysis

AbstractWe present new crust and lithosphere thickness maps of the African mainland based on integrated modeling of elevation and geoid data and thermal analysis. The approach assumes local isostasy, thermal steady state, and linear density increase with depth in the crust and temperature‐dependent density in the lithospheric mantle. Results are constrained by a new comprehensive compilation of seismic Moho depth data consisting of 551 data points and by published tomography models relative to LAB depth. The crustal thickness map shows a N‐S bimodal distribution with higher thickness values in the cratonic domains of southern Africa (38–44 km) relative to those beneath northern Africa (33–39 km). The most striking result is the crustal thinning (28–30 km thickness) imaged along the Mesozoic West and Central African Rift Systems. Our crustal model shows noticeable differences compared to previous models. After excluding the Afar plume region, where the modeling assumptions are not fulfilled, our model better fits the available seismic data (76.3% fitting; root mean square error = 4.3 km). The LAB depth map shows large spatial variability (90 to 230 km), with deeper LAB related to cratonic domains and shallower LAB related to Mesozoic and Cenozoic rifting domains, in agreement with tomography models. Though crustal and lithosphere thickness maps show similar regional patterns, major differences are found in the Atlas Mountains, the West African Rift System, and the intracratonic basins. The effects of lateral variations in crustal density as well as the nonisostatic contribution to elevation in the Afar plume region, which we estimate to be ~1.8 km, are also discussed.

Unraveling crustal growth and reworking processes in complex zircons from orogenic lower-crust: The Proterozoic Putumayo Orogen of Amazonia

High-grade basement massifs exposed in the northern Andes and the buried basement of the adjacent Putumayo foreland basin contain a record of Amazonia's involvement in the supercontinent Rodinia. Metasedimentary granulites and orthogneisses, strongly deformed during at least one metamorphic episode dated at ca. 0.99Ga, provide critical information on the pre-collisional history of the Mesoproterozoic continental margin. Here, new U–Pb, Lu–Hf, Sm–Nd and O isotopic data from outcrop samples of the Garzón and Las Minas Cordilleran basement massifs as well as fragments of drill-core recovered from the Putumayo basin basement are reported. We explore the application of a dual-ICP-MS approach to obtain concurrent U–Pb and Lu–Yb–Hf information on complexly zoned zircon from orogenic lower-crust, and demonstrate its use to retrieve reliable pre-metamorphic information despite possible complexities introduced by mixed-domain ablation and isotopic disturbance of the U–Pb system by thermally induced recrystallization. In combination with δ18O compositions from the same zircon growth domains, and bulk-rock Nd isotope information, we reconstruct segments of the tectonic and crustal evolution of a long-lived accretionary orogen that developed along the (modern) NW margin of Amazonia during most of the Mesoproterozoic. Inherited zircons in metaigneous samples from the Cordilleran massifs, with protolith crystallization ages in the range from ca. 1.47 to 1.15Ga, have Hf–O compositions that indicate significant crustal reworking in their source region, but denote a trend of increasing 176Hf/177Hf with decreasing age that can be attributed to rejuvenation by progressive addition of radiogenic components during this time interval. Detrital zircons within this same age range found in metasedimentary granulites of the Garzón massif also follow this trend, further supporting previous inferences that their protoliths were deposited in arc-proximal basins with little to no coarse-grained detritus delivered from an older cratonic domain. A shift in orogenic deformation style starting at ∼1.15–1.10Ga, inferred to be associated with the accretion of fringing-arc terranes against the continental margin, triggered an early amphibolite-grade metamorphic episode; this was accompanied by pervasive partial melting and migmatite development in fertile metasedimentary units and is interpreted to be responsible for enhanced crustal reworking evidenced from the shallowing of 176Hf/177Hf vs. age trends in detrital and metamorphic zircons from the Garzón and Las Minas massifs. Convergent tectonism along the Putumayo margin came to an end during the final incorporation of Amazonia to the core of the Rodinia supercontinent, possibly during collision against the Sveconorwegian segment of Baltica at 0.99Ga. Although the position and role of Amazonia within Rodinia remains controversial, the new Nd and Hf isotope data provide additional evidence to link the evolution of this orogenic segment with the basement of Oaxaquia, as well as continue to draw fundamental differences with the timing and nature of the tectonic processes associated with the development of the Sunsás-Aguapeí Orogen of SW Amazonia.