Dyke Swarm Research Articles

Regional control structures of kimberlite emplacement in the southern São Francisco Craton basement: A multidisciplinary approach including a representative example from Junco diatreme (Divinópolis, MG, Brazil)

In the southernmost São Francisco Craton, the well-exposed Archean to Paleoproterozoic basement shows kimberlite bodies that represent the last southeastern clusters associated with the Azimuth 125° structure. The absence of Mesozoic to Cenozoic overlying cover rocks in the region allows in-depth aeromagnetic geophysical interpretation of the structural controls responsible for the emplacement of kimberlites. The main controlling structures are depicted by the crosscut NW-SE dyke swarms of Paraopeba, Pará de Minas, and Transminas with the E-W lineaments, characterized by quartz veins and hydrothermalites in the area. The Junco kimberlite intrusion occurs in the region, and reproduces some regional controls at a local scale: the ascent mechanism along a mafic dyke (Pará de Minas swarm) and the placement in the intersection of the mafic dyke with E-W structures. Geological mapping confirmed the remote interpretation and defined the intrusion as oval-shaped, with the magnetic model indicating a bulky diatreme body up to 500 m depth. The goal of this research is to show the importance of the regional controls, their relation to the Junco intrusion, and combine aerogeophysical analysis, geological mapping, mineral chemistry, and lithosphere information to unravel potentially linking processes operating at variable scales. In this context, regolith material overlying the intrusion was sampled, and the indicator minerals pyrope garnet, clinopyroxene, ilmenite, and chromite were selected and analyzed for mineral compositions using electron probe microanalysis. Pyrope garnet and chromite do not show a relation to diamond's potential fields. Ilmenite analyzes indicate relatively high level of oxidation, a characteristic that differentiates the Junco diatreme from kimberlites occurring in the Alto Paranaíba Igneous Province within the southern Brasília Fold Belt. Single-clinopyroxene thermobarometry calculations provided pressure-temperature array values ranging from 745 °C to 832 °C and 31.3 kbar to 38.9 kbar, respectively. A geotherm model was fitted resulting in an estimated surface heat flow of 49.0 mW/m2 and a lithospheric thickness of 202 km. The average equilibration depth of the clinopyroxene at 99.6 km points to pressure-temperature conditions consistent with the stability field of graphite.

Multiple Sulfur Isotope Geochemistry of the Precambrian Mafic Dykes and Komatiites in the Dharwar Craton, Southern India: Evidence for Crustal Recycling and Enrichment in the Subcontinental Lithospheric Mantle

Abstract Komatiites, greenstone volcanic rocks, and mafic dyke swarms are constituents of early earth magmatic activity, crucial for understanding the chemical evolution of the Archean mantle. The composition of the subcontinental lithospheric mantle (SCLM) is systematically modified throughout the Earth’s history by the addition of geochemically diverse oceanic and continental crustal materials through subduction and can be sampled through intraplate mafic/ultramafic volcanic activities. Here, we present a first report on the multiple sulfur isotope characteristics of the mafic dyke swarms and komatiites from the Dharwar craton in southern India and discuss the geochemical modifications of SCLM through crustal recycling. δ34SV-CDT values of the samples are all negative ranging from -0.15 to -2.91‰. Δ33S values for all the samples are close to 0 with the lowest value of -0.060‰ and highest of 0.146‰. Δ36S values are mostly negative with very few exceptions, ranging from -1.184 to 1.111‰. Near zero values of Δ33S and negative values for δ34S indicate an early formed mantle reservoir with a possible mixture of sulfur from subducting oceanic sediments. Together with trace element geochemistry, we suggest a depleted MORB source mantle (DMM) modified by oceanic crustal components and a depleted mantle (DM) modified by recycled continental crustal sediments as the two end members of the mantle source that produced mafic dyke swarms in the Late Archean to Proterozoic Dharwar craton.

Mafic to felsic dyke swarms in coastal Guangdong, China: geochemical and geochronological constraints on the latest Mesozoic geodynamics on the southern margin of the South China Block

AbstractThe coastal region of Guangdong Province, China, is characterized by well-developed dyke swarms. The dykes with widths of decimetres to metres and lengths of tens of metres occur along straight and planar fractures cutting granites or volcanic rocks with Jurassic to Early Cretaceous ages. They show steep attitudes with strikes varying from NNW to NNE and from NE to SE, consistent with a stress regime transition from E–W to N–S extension. Major-element analysis on representative dyke samples reveals a composition range from basaltic to andesitic with a few dacitic outliers. Trace elements of most samples show notable Nb–Ta negative anomalies and Pb positive anomalies on the primitive mantle normalized spidergrams, characteristic of arc-related rocks. Among these, several dacite samples show notable fractionated medium rare earth elements and heavy rare earth elements and high Sr/Y (47–74) and La/Yb (15–21) ratios indicative of adakitic affinity. A few samples lack anomalies in Nb and Ta and have primitive trace-element ratios (e.g. Nb/La and Sr/Nd) or initial ϵNd values resembling ocean island basalt-like rocks. Rare earth element distribution patterns of all samples are right inclined and generally absent of Eu anomalies, which together with other trace-element indexes indicate an origin from heterogeneous mantle sources with depths below the stability field of plagioclase. Radiogenic isotopes, especially those of Nd and Sr (or Nd–Pb), define an array extended from the depleted to enriched mantle (EMII) provenance. LA-ICP-MS dating of zircon obtains ages between 110 and 70 Ma indicating emplacement of these dykes in latest Early to Late Cretaceous time, a time just before initiation of the South China Sea basin. In the context of regional geodynamics, it is proposed that these dykes were formed in a tectonic setting transiting from a Pacific-related back-arc to a passive continental margin pertaining to the development of the South China Sea basin.

Petrogenesis and geodynamic implications of the Late Cretaceous granitoids in Puerto Libertad, Sonora, México: Insights into geochemical signatures of adakites, adakitic affinity and calc-alkaline rocks in NW Mexico

Paleozoic and Mesozoic stratigraphic sequences in the Puerto Libertad area, NW Sonora, Mexico, are intruded by Laramide granitic plutons with dominantly alkaline affinity. We conducted a petrogenetic study of these bodies across a north-south 35 km long transect at oriented NW-SE which includes the Sierra Picú (SP), El Destierro (ED), Sierra Tordilla (ST) and Sierra Los Mochos (SLM). The intrusive bodies display abundant Mafic Microgranular Enclaves (MME) and are intruded by aplitic and pegmatitic dike swarms. U/Pb zircon dates of the intrusive rocks yield crystallization ages between 71.9 and 74.9 Ma. Granodioritic and mozogranitic rocks dominate the SP and ED, while syenogranitic rocks dominate the ST and SLM regions. In addition, these rocks show notable effects, by normal and listric faults which includes cataclastic rocks. Geothermobarometry data from amphiboles and plagioclase of the granodiorites suggest that the temperatures and pressures reached 711–740 °C and 1.67–2.43 kb. respectively, and were emplaced at depths between ∼5 and 6 km. Geochemically, this magmatism shows a relatively high silica content (56–75 wt%), values of Sr/Y = 29–50, La/Yb = 26–39, Sr up to 600 ppm and Ba up to 1400 ppm and depleted in highfield strength elements. Collectively these geochemical traits accentuate the adakite signature of these rocks. This work raises the current problems related to the petrogenetic and tectonic aspects for the generation of calc-alkaline and adakitic magmas while proposing ternary discrimination diagrams to aid in the discrimination from classic calk-alkaline, adakites and adakitic affinity magmatism.

TitaniQ temperatures and textural analysis as a record of the deformation history in a major continental shear zone system, Borborema Province, Brazil

We combine quartz textural analysis with Titanium-in-quartz (TitaniQ) geothermometry to investigate the late stages of deformation in mylonitic rocks from the Sucuru dike swarm in the Borborema Province. The wide range in TitaniQ temperatures (>340 to 740 °C) closely related to grain sizes are attributed to inefficient Ti resetting during bulging (BLG) and subgrain rotation (SGR) recrystallization. The highest temperature values (>500–740 °C) occur in the core of quartz ribbons and porphyroclasts. They are interpreted as the record of early-stage plastic deformation, likely during the intrusion of magmatic bodies, and are progressively overprinted during a lower-temperature stage (>340–500 °C) associated with intense dynamic recrystallization and production of a large amount of fine-grained matrix. The dominantly weak crystallographic preferred orientations (CPOs) can be attributed to a large variety in crystallographic orientations of parent grains, which control to a large extent the CPO evolution of daughter grains, and to activation of multiple slip systems in the <a > direction (basal, rhomb and prism) due to synkinematic temperature decrease. The CPOs are consistent with strain localized by operation of dislocation creep-accommodated grain boundary sliding (DisGBS) during the low-temperature stage. DisGBS seems to be less important in coarse pure quartz domains, where moderate to strong CPOs indicate the dominance of dislocation creep. Several evidence of intracrystalline deformation in quartz ribbons and porphyroclasts point to dominant activation of dislocation creep at the early high-temperature stage of deformation, with activation of prism <c>. The dike orientations, geometry and evidence of solid-state deformation in their margins, fit well with progressive E-W dextral shearing, where they intrude and crystallize in a NW-SE direction and rotate clockwise during shearing. This implies that the dextral shear regime that dominated the Borborema province from ca. 600 Ma was still in effect into the Cambrian some 60–70 million years later.

The Carbonatite-Related Morro do Padre Niobium Deposit, Catalão II Complex, Central Brazil

Abstract The Morro do Padre deposit contains a valuable niobium resource estimated at 14.5 Mt at 1.52 wt % Nb2O5 (at a cut-off grade of 0.5% Nb2O5) hosted in carbonatite-related rocks and their regolith in the southern part of the Catalão II Complex, in central Brazil. Morro do Padre shares numerous geologic features with some of the biggest niobium producers in the world (the Boa Vista mine, also in Catalão II, the Mina II in Catalão I, and the CBMM mine in the Araxá Complex) and can help advance our understanding of the ore formation processes involved. The Morro do Padre hypogene zone is characterized by E-W–trending dike swarms of tetraferriphlogopite phoscorites (magnetite-apatite-carbonate-tetraferriphlogopite-pyrochlore rocks) and carbonatites intrusive within Precambrian rocks. The magmatic origin of these Nb-rich rocks is supported by country rock xenoliths within dikes and ponding into a stratified sill with repetitive cumulus layers. At least two tetraferriphlogopite phoscorite phases (apatite-rich or pegmatoidal P1 and the magnetite-rich P2) and two carbonatite phases (C1 calcite carbonatite and C2 dolomite carbonatite) are present. The bulk of hypogene mineralization is primarily controlled by the emplacement of P2 dikes and secondarily by C1 and C2 dikes where pyrochlore is accessory. Whole-rock and pyrochlore chemistry and textural and spatial relationships suggest that the genesis of P2 (and that of the niobium deposit) is due to the emplacement of a parental dolomite carbonatite magma that crystallized medium- to coarse-grained magnetite, apatite, tetraferriphlogopite, and pyrochlore on dike walls upon cooling, in an elaborate magmatic type of “cumulate dike build-up.” Weathering generated the regolith zone, where the dissolution of barren phases compounded the Nb concentration even further. Morro do Padre showcases the role of carbonatite-phoscorite magmatism in producing Fe-P-Nb–rich rocks and economic niobium mineralization.

Petrogenesis of low-Ti dolerite sills from Paraná-Etendeka LIP in Cerro do Coronel region, southernmost Brazil

ABSTRACT In Large Igneous Provinces (LIPs), sills are a key component of the plumbing system responsible for magma transport through the crust where magma fractionation takes place. In this work, we investigate four dolerite sills related to the Paraná-Etendeka LIP from Cerro do Coronel region, southern Brazil. They are basaltic andesites composed of plagioclase, augite+pigenonite, Ti-magnetite, and rare olivine, divided into a high- and a low-MgO group. The former is more depleted in SiO2, alkalis, and trace elements, with lower FeOt/MgO ratios, higher Eu/Eu*(N), less radiogenic 87Sr/86Sr and more radiogenic 143Nd/144Nd. The composition of Cerro do Coronel dolerites resemble those of evolved rocks from Morungava Intrusions, the silica saturated tholeiite dykes in southern Brazil, and the Cuaró Dyke Swarm dolerites from Uruguay. They evolved by fractional crystallization from primitive Gramado-type basalts under FMQ-2 fO2 conditions at low pressures (<3 kbar) and underwent assimilation of variable upper and lower crust, representing shallow-depth equilibration of magmas sourced from deeper crustal levels. The studied sills share some geochemical features with the Vale do Sol Fm. lavas, although we cannot assume they have the same age, and are part of a major plumbing system related to the low-Ti magmas of the Paraná-Etendeka LIP.

A petrological and structural assessment of mafic dike swarms in the Patagonian fold-thrust belt during seismic ridge collision

Between Viedma and San Martín lakes, in an area of 1200 km2 that partly covers the Southern Patagonian cordillera and the Miocene-Pliocene extra-Andean basaltic plateaus, mafic dike swarms crop out which have not been studied previously. As a result of mapping on satellite images and in the field more than 1500 dikes were identified. The dikes crop out in a zone between rear-arc calc-alkaline intrusions to the west and tholeiitic-alkaline plateau basalts to the east, within the internal and external fold-thrust belt. In order to understand the genetic relationship between the dikes, other igneous units in the region and the structures of the Andean orogeny, a petrological study (including petrography and geochemistry) and a structural analysis of the dike orientations were carried out. The dikes were emplaced close to the surface (<1 km). Three predominant compositions were recognized: basalts, which predominate, followed by tephrites and andesites, the latter restricted to a single sector. We propose preliminarily the following correlations: (i) the dikes of tephritic composition with the Río Carbón Essexite (Eocene), (ii) the andesites with the Chaltén Adakite (Miocene) and (iii) the basaltic dikes with the Pliocene basaltic plateaus. All these units are related in the literature to different stages of ridge subduction and slab window development. The dikes present variable, but not random, preferential orientations, with no relation with composition. Our analysis suggests that dike emplacement was controlled by previous anisotropies in a heterogeneous and very shallow crust. The anisotropy in the internal fold-thrust belt was achieved mainly during Eocene and/or Late Cretaceous contraction, whereas that in the external fold-thrust belt was developed during a less significant Miocene compressive event, thus resulting in a more homogeneous dike orientation. The vertical disposition of the dikes indicates horizontal stretching, as previously proposed for the last stages of Andean growth.

New Insights into Feeder Dike Swarms in Scoria Cones and Their Structural Control: A Case Study in the Michoacán-Guanajuato Volcanic Field

HomeJoinGSA StoreDonateContact Sign in Toggle search Keyword search Toggle navigation Keyword search Abstract View Volume 33 Issue 2 (February 2023) GSA Today Article, p. 4-10 | Full Text | PDF New Insights into Feeder Dike Swarms in Scoria Cones and Their Structural Control: A Case Study in the Michoacán-Guanajuato Volcanic Field Martha Gabriela Gómez-Vasconcelos* CONACYT–Instituto de Investigaciones en Ciencias de la Tierra, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán, México, gabriela.gomez@umich.mx Denis-Ramón Avellán CONACYT–Instituto de Geofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán, México, denisavellan@gmail.com José Luis Macías Instituto de Geofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán, México, jlmv63@gmail.com Guillermo Cisneros-Máximo Instituto de Geofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán, México, gcisneros@igeofisica.unam.mx Juan Manuel Sánchez-Núñez Instituto Politécnico Nacional–CIIEMAD, Gustavo A. Madero, Ciudad de México, México, jsancheznu@ipn.mx Daniel P. Miggins 40Ar/39Ar Geochronology Laboratory, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA, daniel.miggins@oregonstate.edu Abstract Understanding the feeder systems in scoria cones is essential because they serve as the conduits that feed the most common eruptions worldwide. Feeder dikes and their emplacement are presumably controlled by the tectonic stress field. However, the mechanism of dike propagation and structural control in monogenetic scoria cones remains poorly understood, as well as the conditions that allow dike swarms in scoria cones and in low magma-flux monogenetic volcanic fields. This is the first direct study of a magma feeder system in the Michoacán-Guanajuato Volcanic Field in central México. Quarrying in the Cerrito Colorado scoria cone displays six orthogonal feeder dikes—four of them are N-S oriented, parallel to the least compressive stress, intruding preexisting faults, and two are E-W oriented, perpendicular to the least compressive stress, forming their own fracture at the time of the eruption. Single feeder dikes are common in monogenetic volcanoes, but dike networks (swarms) can develop locally in the vicinity of scoria cones and other vent structures. We suggest that bifurcation of feeder dikes can result from temporary blockages of the conduit and during changes in the magma ascent rate and magma pressure. Feeder dikes at the surface can appear as tabular dikes, cylindrical conduits, or as a combination of both geometries. We suggest that tabular dikes splay-off tangentially, and cylindrical conduits bifurcate radially and axially to the main vent. Our study attests to the complexity and structural control that even small scoria cones can present. *Corresponding author Manuscript received 17 Mar. 2022. Revised manuscript received 30 June 2022. Manuscript accepted 18 July 2022. Posted 10 Aug. 2022. © The Geological Society of America, 2022. CC-BY-NC. https://doi.org/10.1130/GSATG539A.1 Cover Image Search Google Scholar for Martha Gabriela Gómez-Vasconcelos Denis-Ramón Avellán José Luis Macías Juan Manuel Sánchez-Núñez Daniel P. Miggins Search GSA Today About Membership Publications Events Education & Careers Science Policy News Advertising Place a Job Ad FAQs Jobs at GSA Center for Professional Excellence Code of Ethics Events Code of Conduct Privacy Policy Join GSA Contact Us www.geosociety.org © 2023 The Geological Society of America, Inc.

Secular evolution of the subcontinental lithospheric mantle beneath Indian cratons: Insights from geochemistry and geochronology of the Precambrian mafic dykes

The Indian Shield is comprised of the Dharwar, Bastar, Singhbhum, Bundelkhand, and Aravalli Cratons, intruded by distinct mafic dyke swarms of different generations (ca. 2.8–0.8 Ga). Most of these dykes are tholeiitic basalt to basaltic-andesite, including boninite. Some other subordinate dyke rocks are of komatiitic, picritic, and andesitic compositions. The vast areal extent of these dykes indicates that they are remnants of Precambrian Large Igneous Provinces (LIPs). The present study reviews the existing geochronological and geochemical data of various Precambrian mafic dyke swarms intruding in all Indian cratons, to track temporal changes in composition of the underlying subcontinental lithospheric mantle (SCLM). Most of these dykes have crust-like abundances of incompatible trace-element. Even the primitive dykes (Mg# = 82–64) exhibit crust-like incompatible element patterns. However, some also have depleted mantle-like abundances of incompatible elements. Most of the dykes also have high concentrations of compatible trace elements (e.g., Cr and Ni). The age-corrected radiogenic Nd isotope compositions (ɛ Nd(i) ) of these dykes vary between the upper continental crust and depleted mantle ɛ Nd growth arrays. The elemental composition and ɛ Nd(i) of these dykes suggest their derivation from a heterogeneous SCLM comprising enriched (metasomatized) and depleted mantle components. The negative as well as positive ɛ Nd(i) values exhibited by the ca. 2.8 Ga (oldest) dykes suggest the presence of enriched mantle materials distributed within a depleted-SCLM already by ca. 2.8 Ga. The enrichment must have occurred before ca. 2.8 Ga due to subduction-released fluids. Later, the SCLM beneath Indian cratons evolved with a composition consisting of an enriched/metasomatized mantle component and a depleted mantle component. • Geochemistry of 2.8–0.8 Ga mafic dyke swarms of Indian cratons have been reviewed. • They originated from heterogeneous SCLM containing enriched & depleted components. • Subduction-driven metasomatism of global SCLM occurred during the Archean.

Targeting REE deposits associated with carbonatite and alkaline complexes in northeast India

Rare earth elements (REEs) are considered critical metals globally. About 62% of the global resources of REEs occur associated with carbonatites and alkaline complexes. However, the entire production of REEs in India currently comes from monazite-bearing beach sands, although a variety of REE enriched source rocks, particularly carbonatites and alkaline complexes occur in different parts of the country. There is, therefore, a significant potential in the county for new REE deposit discoveries associated with carbonatites and alkaline complexes. This paper describes a generalised carbonatite-alkaline complex related REE mineral systems model and applies a knowledge-driven model to demarcate REE exploration targets in the Karbi-Meghalaya plateau, NE India. The main components of the mineral systems are (1) pockets of metasomatised subcontinental lithospheric mantle (SCLM) which form fertile source regions for REE-bearing fluids; (2) extensional geodynamics; (3) permeable lithosphere architecture for tapping REE-enriched fluids from SCLM and focusing them to near-surface levels, and (4) a post-emplacement tectonic regime that preserves the deposits. Spatial proxies representing each of these components are mapped and integrated using fuzzy inference system (FIS) to identify prospective targets. Systemic and stochastic uncertainties associated with the model were quantified to aid target selection and further work.Main recommendations of this exercise are the following: (1) project-scale ground exploration for the Sung valley and Jasra complexes; (2) further regional-scale data collection for the Mikir Hills in the areas surrounding Samchampi and Barpung Complexes and around the swarms of dykes in the Garo Hills around Swangkre; (3) follow-up exploration in the areas north of Silchar and south of Nongstoin and (4) detailed geochemical sampling and surface or air-borne radiometric surveys for the Mawpyut Ultramafic Complex.The REE-mineral-systems model and the workflow demonstrated in this article could be used for targeting REE deposits in geologically similar terrains worldwide.

Dike swarms of the west-central Arabian Shield: a key to its tectono-magmatic evolution

Dike swarms of the west-central Arabian Shield: a key to its tectono-magmatic evolution

The origin of Patagonia: insights from Permian to Middle Triassic magmatism of the North Patagonian Massif

AbstractWe conducted a U–Pb–Hf isotope study on zircon crystals from ignimbrites of the Changhsingian to Olenekian (253–248 Ma) Los Menucos Basin in the North Patagonian Massif (NPM), Argentina, in order to evaluate the age and petrogenesis of the magmas. Additionally, a compilation of whole-rock geochemistry and U–Pb–Hf in zircon isotope data for the Permian to Middle Triassic rocks of the NPM, for comparison with our data, was made to assess whether Patagonia would have been an exotic terrane accreted to SW Gondwana during the late Palaeozoic. We interpret the available U–Pb–Hf data to suggest that northern Patagonia experienced eastward arc expansion from the early Permian, about 273 Ma ago. This ∼820 km arc expansion event involved crustal shortening and magmatism with high-silica adakitic affinity, resulting in Hf-isotopic pull-down. At 253 Ma, slab steepening became associated with the coeval emplacement of ignimbrites of the Los Menucos Basin, which involved post-orogenic to intraplate magmatism. During the Middle Triassic, a slab break-off triggered uplift and basaltic underplating, promoting the emplacement of dike swarms with C-type adakitic signature at 246–244 Ma. The Hf isotope data for SW Gondwana for the same period indicate distinct trends that are explained here by differential slab roll-back since the Guadalupian, in a slab-tearing setting. Therefore, Permian to Middle Triassic magmatism is interpreted as having been associated with an eastward-directed proto-Pacific subduction system, which ultimately supports an autochthonous origin for Patagonia.

Geodynamic framework of the magmatic activity of Cretaceous Léré basin (Southwestern Chad, Central Africa)

Numerous 5 to 20 m-wide dolerite dykes crosscut vertically to sub-vertically basement granitoids around and within the Cretaceous Léré basin, SW Chad (Central Africa). They extend along strike on several hundred meters length. Léré dolerites exhibit various textures, from classical doleritic texture to sub-ophitic via ophitic and intersertal textures. They are composed of euhedral and subhedral crystals of andesine and labradorite, anorthoclase, augite, olivine, ilmenite and Ti-magnetite. Calcite and amphibole are present. Estimated crystallization temperatures are between 1050 and 1105 ± 50 °C. Occurrence of dyke swarms in Léré basin is controlled by Pan-African strike-slip faults created during an extensional tectonic phase during Paleozoic to Early Mesozoic. These lavas, analyzed with ICP-MS and ICP-AES, are basalt, trachybasalt and basaltic trachyandesite with some continental tholeiite affinity. Dolerites of Léré basin have evolved by limited fractional crystallization differentiation, accompanying by minor assimilation process. Three groups of rocks are identified (according to Y/Nb ratios), linked to different mantle sources which have intermediate compositions between E-MORB and OIB.

New CA-ID-TIMS U–Pb zircon ages for the Altenberg–Teplice Volcanic Complex (ATVC) document discrete and coeval pulses of Variscan magmatic activity in the Eastern Erzgebirge (Eastern Variscan Belt)

The Altenberg–Teplice Volcanic Complex (ATVC) is a large ~ NNW–SSE trending volcano-plutonic system in the southern part of the Eastern Erzgebirge (northern Bohemian Massif, south-eastern Germany and northern Czech Republic). This study presents high precision U–Pb CA-ID-TIMS zircon ages for the pre-caldera volcano-sedimentary Schönfeld–Altenberg Complex and various rocks of the caldera stage: the Teplice rhyolite, the microgranite ring dyke, and the Sayda-Berggießhübel dyke swarm. These data revealed a prolonged time gap of ca. 7–8 Myr between the pre-caldera stage (Schönfeld–Altenberg Complex) and the climactic caldera stage. The volcanic rocks of the Schönfeld–Altenberg Complex represent the earliest volcanic activity in the Erzgebirge and central Europe at ca. 322 Ma. The subsequent Teplice rhyolite was formed during a relatively short time interval of only 1–2 Myr (314–313 Ma). During the same time interval (314–313 Ma), the microgranite ring dyke intruded at the rim of the caldera structure. In addition, one dyke of the Sayda-Berggiesshübel dyke swarm was dated at ca. 314 Ma, while another yielded a younger age (ca. 311 Ma). These data confirm the close genetic and temporal relationship of the Teplice rhyolite, the microgranite ring dyke, and (at least part of) the Sayda-Berggießhübel dyke swarm. Remarkably, the caldera formation in the south of the Eastern Erzgebirge (caldera stage of ATVC: 314–313 Ma) and that in the north (Tharandt Forest caldera: 314–312 Ma) occurred during the same time. These data document a large ~ 60 km NNW–SSE trending magmatic system in the whole Eastern Erzgebirge. For the first time, Hf-O-isotope zircon data was acquired on the ring dyke from the ATVC rocks to better characterize its possible sources. The homogeneous Hf-O-isotope zircon data from the microgranite ring dyke require preceding homogenization of basement rocks. Some small-scale melts that were produced during Variscan amphibolite-facies metamorphism show similar Hf-O-isotope characteristics and can therefore be considered as the most probable source for the microgranite ring dyke melt. In addition, a second source with low oxygen isotope ratios (e.g. basic rocks) probably contributed to the melt and possibly triggered the climactic eruption of the Teplice rhyolite as well as the crystal-rich intrusion of the ring dyke.

IN-SIMS zircon U-Pb geochronology from the Snowbird tectonic zone Large igneous Province (STZ LIP), western Churchill Province, Canada

IN-SIMS (in-situ secondary ionization mass spectrometry) micro-zircon geochronology resolves the age of the Chipman and Kazan mafic dike swarms exposed along the central segment of the Snowbird Tectonic Zone (STZ), the geophysically defined feature separating the Rae and Hearne subprovinces of the western Churchill Province. Long-considered syn-kinematic with respect to Paleoproterozoic transpressional deformation, the primitive composition of the STZ Large Igneous Province (LIP) and granulite grade metamorphic overprint has made establishing crystallization ages difficult. In-situ U-Pb isotopic analysis of micro-zircon yielded scattered and discordant results, except for four zircon crystals, which yielded a weighted mean of 2113 Ma +/- 22 (n = 8 analyses; MSWD: 0.46). This age is similar to other mafic complexes in the western Churchill Province including the Griffin Gabbro sills interpreted to have formed during opening of the Manikewan Ocean. Nd isotopic analyses of Kazan dikes paired with geochemical data suggest that the Chipman and Kazan dikes were asthenospherically-derived but contaminated by a metasomatized lithospheric root. At ca. 2.1 Ga, the STZ was the site of intracratonic extension within a coherent Rae-Hearne crustal block (Snowbird LIP Phase I). Existing data suggests that subsequent tectonism led to focused dextral transpression along the trace of the dike swarm and attests to a long-lived interplay of preexisting weaknesses and deformation.

Anatomy of an intracratonic Son-Mahanadi Gondwana rift basin in Peninsular India: An integrated gravity, magnetic and remote sensing approach

The NW-SE trending Son-Mahanadi basin is one of the largest intra-cratonic Gondwana rift basins of Peninsular India covering an area of approximately 48,000 sq. km. Here we employed a combined analysis of gravity and magnetic (GM) and remote sensing (RS) techniques to delineate the subsurface structures to tracing the tectonic evolution of the Son-Mahanadi basin. Landsat remote sensing data, ground gravity, and EMAG2 magnetic data were used to understand the surface and subsurface structures. The Bouguer gravity anomaly map of the region shows short wavelength gravity lows and highs due to depressions and upwarps in the basement. Interestingly, short wavelength gravity anomalies are superposed on a long wavelength regional gravity low centered over the basin. The regional gravity low bears an inverse correlation with the regional topography suggesting that the excess topographic load is compensated at depth and the required buoyancy might be due to thinning of the lithosphere as a consequence of Deccan volcanism.The ENE-WSW structural grain inferred from remote sensing data coincides with the trend of dyke swarms and the WNW-ESE trend possibly corresponds to the rift boundary faults in the south. The lineament trends show preferred spatial distribution in three sub-basins (Son, Hasdo-Arand, and Mahanadi), each separated by basement ridges. Integration of remote sensing, geological, and potential field data suggests reactivation of ENE-WSW trending structural lineaments which are associated with the dyke emplacement during Deccan volcanism. Thus, the Son-Mahanadi Gondwana basin witnessed a post-rift tectonic event associated with the Deccan volcanism.

The generation and evolution of the Archean continental crust: The granitoid story in southeastern Brazil

The Archean Eon was a time of geodynamic changes. Direct evidence of these transitions come from igneous/metaigneous rocks, which dominate cratonic segments worldwide. New data for granitoids from an Archean basement inlier related to the Southern São Francisco Craton (SSFC), are integrated with geochronological, isotopic and geochemical data on Archean granitoids from the SSFC. The rocks are divided into three main geochemical groups with different ages: (1) TTG (3.02–2.77 Ga); (2) medium- to high-K granitoids (2.85–2.72 Ga); and (3) A-type granites (2.7–2.6 Ga). The juvenile to chondritic (Hf-Nd isotopes) TTG were divided into two sub-groups, TTG 1 (low-HREE) and 2 (high-HREE), derived from partial melting of metamafic rocks similar to those from adjacent greenstone belts. The compositional diversity within the TTG is attributed to different pressures during partial melting, supported by a positive correlation of Dy/Yb and Sr/Zr, and batch melting calculations. The proposed TTG sources are geochemically similar to basaltic rocks from modern island-arcs, indicating the presence of subduction processes concomitant with TTG emplacement. From ∼2.85 Ga to 2.70 Ga, the dominant rocks were K-rich granitoids. These are modeled as crustal melts of TTG, during regional metamorphism indicative of crustal thickening. Their compositional diversity is linked to: (i) differences in source composition; (ii) distinct melt fractions during partial melting; and (iii) different residual mineralogies reflecting varying P–T conditions. Post-collisional (∼2.7–2.6 Ga) A-type granites reflect rifting in that they were closely followed by extension-related dyke swarms, and they are interpreted as differentiation or partial melting products of magmas derived from subduction-modified mantle. The sequence of granitoid emplacement indicates subduction-related magmatism was followed by crustal thickening, regional metamorphism and crustal melting, and post-collisional extension, similar to that seen in younger Wilson Cycles. It is compelling evidence that plate tectonics was active in this segment of Brazil from ∼3 Ga.

Geological history and supercontinent cycles of the Arctic

AbstractThe geological history of the Arctic is constrained within the framework of the assembly and breakup of three supercontinents. The first of these was preceded by the crystallization of the oldest dated rocks on Earth and consolidation of the Arctic region’s Archean cratons between 2.82 and 2.54 Ga. Following the emplacement of regional mafic dike swarms between 2.51 and 2.03 Ga, the cratons were amalgamated into the Nuna (Columbia) supercontinent between 2.0 and 1.6 Ga, and the distribution of low-thermalgradient eclogite (indicative of continental subduction) and ophiolite (indicative of obduction of oceanic crust onto a continental margin) suggests that diagnostic plate-tectonic processes were well in place by the early Paleoproterozoic. Basin formation, flood basalts, and dike swarms are features of the partial(?) breakup of Nuna (Columbia) by 1.5–1.27 Ga. The extent to which specific dike swarms led to continental breakup and a rift-to-drift transition remains unclear. Assembly of the second supercontinent (Rodinia, 1.4–0.9 Ga) is recorded by a network of Grenvillian and Sveconorwegian collisional orogenic belts. Prominent features of Rodinia breakup (780–615 Ma) in the Arctic are extensive dike swarms and regional-scale glacial-periglacial deposits associated with the Sturtian (717–661 Ma) and Marinoan (ca. 645 ± 6 to ca. 635 Ma) snowball Earth glaciations. Assembly of the third supercontinent, Pangea, between 600 Ma and ca. 250 Ma, was accomplished through stitching of four orogens in the Arctic (Timan-Varanger, Caledonian, Ellesmerian, and Urals-Taymyr). Pangea breakup (rifting since 250 Ma and oceanic spreading since the Cretaceous) led to the emplacement of Cretaceous and Paleogene flood basalts, new oceanic crust in the Labrador Sea, North Atlantic Ocean, and Arctic Ocean, and orogens characterized by relatively small but far-traveled accreted terranes with provenance in Laurentia, Baltica, and Siberia. Paleogeographic similarities and geological correlations among Laurentia, Baltica, Siberia, and the North China craton suggest that Rodinia formed following incomplete breakup of Nuna (Columbia) and/or by introversion, whereas unique paleogeographic traits for Pangea within the Arctic region point to supercontinent formation by extroversion.

Stress states during the emplacement of the eastern Rio Ceará-Mirim dike swarm, Borborema Province, northeastern Brazil

Stress states during the emplacement of the eastern Rio Ceará-Mirim dike swarm, Borborema Province, northeastern Brazil