Mafic Dykes Research Articles

Aspect ratio analysis of distinct Paleoproterozoic mafic dyke swarms and related fracture systems in the Eastern Dharwar Craton, India: Implications for emplacement mechanism and depth of origin

Understanding emplacement mechanism and depth of origin of continental mafic dyke swarms is pivotal in resolving outstanding questions regarding magma flow dynamics, plumbing system architecture, and amount of crustal dilation related to large igneous provinces. Paleoproterozoic mafic dyke swarms are often deemed as suitable candidates to establish the crustal evolution of cratonic regions. Here, we present the geometric and statistical analyses of five distinct Paleoproterozoic mafic dyke swarms and associated fracture systems in the Eastern Dharwar Craton (EDC) to constrain their magmatic overpressures and magma chamber depths. The geometric analysis is further validated with field evidence and measurements. It is suggested that mafic dykes of different swarms are formed primarily due to vertical magma injection from deep-seated magma reservoirs, followed by lateral magma flow at shallower depths. This study suggests a direct derivation of magma from deep-seated magma reservoirs, extending from lower crustal depths (∼17 km) to the crust-mantle boundary (∼36 km). It is also suggested that the emplacement of mafic dykes at ca. 2.37–2.36 Ga, ca. 2.26–2.25 Ga, ca. 2.22 Ga, and ca. 2.21 Ga can be traced back to the development of fracture system formed during and subsequent to the Neoarchean accretion event of Eastern and Western Dharwar Cratons (< 2.52 Ga). Therefore, the emplacement systems of the studied mafic dyke swarms have been largely controlled by the regional stress field and large-scale structural architecture of the Dharwar Craton.

Application of Satellite Remote Sensing, UAV-Geological Mapping, and Machine Learning Methods in the Exploration of Podiform Chromite Deposits

The irregular and sporadic occurrence of chromite pods in podiform chromite deposits (PCD), especially in mountainous terranes with rough topography, necessitates finding innovative methods for reconnaissance and prospecting. This research combines several remote sensing methods to discriminate the highly serpentinized peridotites hosting chromite pods from the other barren ultramafic and mafic cumulates. The case study is the area of the Sabzevar Ophiolite (NE Iran), which hosts several known chromite and other mineral deposits. The integration of satellite images [e.g., Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite sensor, Landsat series, and Sentinel-2] coupled with change detection, band rationing, and target detection algorithms [including the Spectral Angle Mapper (SAM)] were used to distinguish potential lithological units hosting chromites. Results have been verified by an initial on-field checking and compared with the high-resolution (GSD ~6 cm) orthomosaic images obtained by the processing of photographs taken from an Unmanned Aerial Vehicle (UAV) at a promising area of 35 km2. The combination of visual interpretation and supervised classification by machine learning methods [Support Vector Machine (SVM)] yielded the production of a geological map, in which the lithological units and structures are outlined, including the crust-mantle transition zone units, mafic cumulates, crosscutting dykes, and mantle sequences. The validation of the results was performed through a second phase, made up of field mapping, sampling, chemical analysis, and microscopic studies, leading to the discovery of new chromite occurrences and mineralized zones. All ultramafic units were classified into four groups based on the degree of serpentinization, represented by the intensity of their average spectral reflectance. Based on their presumed protolith, the highly serpentinized ultramafics and serpentinites were classified into two main categories (dunite or harzburgite). The serpentinite with probable dunitic protolith, discriminated for a peculiar Fe-rich Ni-bearing lateritic crust, is more productive for chromite prospecting. This is particularly true at the contact with mafic dykes, akin to some worldwide chromite deposits. The results of our work highlight the potential of multi-scale satellite and UAV-based remote sensing to find footprints of some chromite mineral deposits.

Estimation of magma overpressure from partially exposed dykes - A new approach

We present a new method to estimate full length and maximum thickness of dykes from their partial exposures having at least one tip. The method is particularly applicable to dykes which originate as dominantly opening mode fractures. The idealized canonical model for Mode I fracture shows it to open up in the shape of an ellipse. We solve the equation of this ellipse using every possible combination of two points lying on the dyke margin measured from the observed tip to acquire the results. The method has been validated with observations made on felsic dykes found within intrusive porphyritic granitoids of Chotanagpur Granite Gneiss Complex (CGGC), Purulia, India which are completely exposed (showing tips at both ends) or have an exposed centre with at least one tip. The method has also been tested against published data collected from incomplete dyke outcrops found in the caldera walls of Miyake-jima volcano, Japan. Acquired results are found to be in good agreement with the ones estimated through a previously published method. The present method has been successfully employed to calculate the aspect ratios (maximum thickness/total length) of partially exposed mafic dykes emplaced within younger granite of Chitradurga Schist Belt, Western Dharwar Craton, India. We comment on the magma overpressure ranges and depth of origin of these dykes and on the fracture toughness of the host granite.

Baddeleyite dating of a 2.34 Ga mafic dyke in the Western Shandong Province, North China Craton, and its tectonic implications

Baddeleyite dating of a 2.34 Ga mafic dyke in the Western Shandong Province, North China Craton, and its tectonic implications

Petrogenesis of Mesozoic Magmatic Suites in the Jiaodong Peninsula: Implications for Crust-Mantle Interactions and Decratonization

Abstract The North China Craton (NCC) has thick lithosphere in the Paleozoic (&amp;gt;200 km) but appears to be decratonized in the Mesozoic. However, the actual processes operated in the lower crust-deep mantle are still unclear. The Mesozoic granitic rocks can provide important clues to the NCC decratonization mechanism. Here, we conducted trace element modeling to check whether partial melting of the Archean lower crust can generate these Mesozoic magmatic suites. Meanwhile, zircon Hf isotope analysis was conducted to reveal crust-mantle interaction processes and further give constraints on the decratonization of the NCC. Zircon Hf isotope data of the Linglong, Guojialing, and Aishan suites, the mafic microgranular enclaves (MMEs) in the Guojialing suite, and mafic dykes display minor differences: the Linglong (160–150 Ma), Guojialing (~130 Ma), and Aishan (118–116 Ma) suites have zircon εHft=–25.4 to –14.5, –15.3 to –10.4, and –23.1 to –11.9, respectively. The Cretaceous mafic dyke (126 Ma) has a highly negative εHft value (–22.8 to –17.7). Meanwhile, the MMEs (in the Guojialing granodiorite, DCW-2A, 129 Ma) have zircon εHft=–13.0 to –8.9. Temperature-pressure conditions calculated using amphibole compositions for both the Guojialing granodiorite and its MMEs are basically identical, implying possible magma mixing. Our modeling results show that certain trace elements (e.g., Tb, Yb, and Y) have to be retained in the source to match the composition of the Linglong suite, which requires substantial garnet residues (high-pressure melting) in the Jurassic. The Early Cretaceous garnet-dominated lower crust is Yb-/Y-enriched but depleted in elements like Sr and La. Therefore, it could not form geochemical features like high Sr/Y and La/Yb ratios akin to the Guojialing suite. Integrating the modeling results and zircon Hf isotope data, we propose that the crust in the eastern NCC had thickened and partially melted by dehydration to produce an eclogitic residue containing a large amount of garnet (&amp;gt;50% by weight) during the Jurassic (Linglong granite), whereas upwelling of hot and hydrous mafic magma from the asthenospheric mantle induced fluxed melting of both the lower crust and lithospheric mantle in the Early Cretaceous, during which the lithospheric mantle and part of the lower crust in the Jiaodong were removed by the convective mantle. About 10 Mys later while the Aishan suite formed, the crust was not thick anymore, and melting occurred under moderate pressure which does not necessarily require abundant garnet as the residue phase.

Polyphase deformation of the Youjiang fold-and-thrust belt during the Mesozoic: Implications for the tectonic transition of the South China block

The South China block (SCB) experienced the tectonic transition from the Paleo-Tethys to the Paleo-Pacific tectonic domains during the Mesozoic, but the transition process is hotly debated. The Youjiang fold-and-thrust belt (YFTB), in the interior of the Youjiang Basin in the southwestern SCB, is located in a junction of these two tectonic domains and thus witnessed their tectonic evolution. It also separates the northern thin-skinned structures from the southern thick-skinned structures in the basin. Therefore, the YFTB is an intriguing window into the Mesozoic evolution in the southwestern SCB and the Mesozoic tectonic transition of the SCB. In this study, we conduct structural analysis at the middle of the YFTB and discuss the Mesozoic tectonic transition of the SCB. Four phases of deformation are identified in the YFTB during the Mesozoic. The first phase of deformation (D1) is characterized by a series of conjugate joints, NW–SE trending thrust faults and folds resulting from NE–SW shortening that was related to the collision between the SCB and Indochina block. The second phase of deformation (D2) is manifested by the pre-existing NW–SE striking thrust faults transformed to normal faults and the Late Triassic mafic magmatism. Our structural observations, combined with previous geochronological data for mafic dykes, suggest the study area experienced post-orogenic extension in the Late Triassic. The third phase of deformation (D3), accompanied with the westward subduction of the Paleo-Pacific oceanic plate, is represented by a series of conjugate joints and NE–SW-striking faults formed by NW–SE compression. As a result of the continuous subduction of the Paleo-Pacific plate and its subsequent slab rollback, the fourth phase of deformation (D4) is featured with normal faults, magmatic-hydrothermal activities, and regional mineralization, which are associated with the nearly E–W extension setting. Our study results indicate that, in the Early Mesozoic, the Youjiang Basin was dominated by the Paleo-Tethys domain and then transitioned to the Paleo-Pacific domain. Together with the magmatic lull between the Triassic and the Early-Middle Jurassic identified in the SCB, we propose that the tectonic transition process manifested in the YFTB more likely initiated in the Early Jurassic.

GEOCHEMISTRY AND AGE OF MAFIC DYKES OF THE Be-DEPOSIT SNEZNOE: LATE PALEOZOIC MANTLE PLUME MAGMATISM EVIDENCE IN EASTERN SAYAN

Within the East Sayan rift zone of the Late Paleozoic Barguzin salic large igneous province (SLIP), along with alkaline-peralkaline granitoids with ore mineralization, mafic subvolcanic bodies (gabbroids, mafic dykes) are common. One of the typomorphic mafic – peralkaline granitoid association is the Snezhnoye beryllium deposit. Using the U–Pb LA–ICP–MS method, mafic dykes of the Snezhnoye beryllium deposit were dated. The age of zircons from the diabase (dolerite) dyke was 301 ± 6 Ma, and from the microgabbro one, 297 ± 2 Ma. The obtained dates are close to the age of fluorite-beryl mineralization (306 Ma) and peralkaline granites with Ta-Nb mineralization attributed to the Ognite complex (311–295 Ma). The geochemical features of mafic dykes suggest the generation of mafic magmas from an asthenospheric mantle source, which, taking into account the intracontinental setting, testifies in favor of the plume-mantle nature of mafic – alkaline-peralkaline granitoid magmatism of the East Sayan rift (rare-metal) zone.

Remote sensing applied to geological, structural, and mass movements characterization in the connection between Curral Homocline and Moeda Syncline, Quadrilátero Ferrífero Region, Brazil

Mass movements naturally occurring along slopes, especially after heavy rains, cause heavy losses of materials and life and affect highways, pipelines, and bridges, among other structures. Among the most common mass movements, landslides and subsequent mass flows especially stand out. Remote sensing and geoprocessing techniques are useful tools for geological and geotechnical analysis, here applied to characterization of mass movements from the analysis of the influence of slopes, lineaments or fractures, and geological-geotechnical at the junction of the Curral homocline and the Moeda syncline, Quadrilátero Ferrífero, Southeast Brazil. The area consists of Archean granitic-gneissic basement and the greenstone belt sequence of the Rio das Velhas Supergroup with Proterozoic metasedimentary rocks of the Minas Supergroup covering these two older successions. Based on satellite images, a preliminary map of lineaments was created, and fractures were grouped according to their direction before being rasterized. Each lineament group was associated with a previously known geological and/or structural feature. Geological units mapped in the area were numerically classified according to their mass movement susceptibility and related geotechnical units. Using a Shuttle Radar Topography Mission (SRTM) image, a terrain slope raster was generated. The objective was to obtain a raster indicating low and high probabilities of mass movements. Four sets of lineaments were identified: the first set associated with the northeast strike stratification of the rocks of the Moeda Formation; the second set related to eastward-trending warp axis lineations; the third set related to the southeast trend of stretch lineaments or mafic dykes directions; and the fourth set evidently associated with the watershed pattern of the Curral Range crest, perpendicular to the strike of the rocks in the area. Simulations with different combinations of parameters produced eight scenarios were generated were constructed to classify the area as to its susceptibility to rupture through changes in slope, lithotype, and lineament density; the first four (1 to 4) were classified by using the slope percentage raster without interval reclassification, so its values in the cartographic algebra were not grouped. The last for (5 to 8), consider the slope percentage classified. This methodology successfully combines mathematical models for predicting mass movements and determining areas in the urban landscape most susceptible to these phenomena. As such, it comprises a useful tool for government planning of preventative actions for areas of high risk.

Early Cretaceous mafic dykes from the Chhota Nagpur Gneissic Terrane, eastern India: Evidence of multiple magma pulses for the main stage of the Greater Kerguelen mantle plume

Early Cretaceous NNW- to WNW-trending dolerite dykes of the eastern Indian Shield, collectively termed the Raniganj-Koderma swarm, are studied for their emplacement ages and petrogenetic history, and assessed for a possible linkage with the Greater Kerguelen plume. 40Ar/39Ar dates of four dolerite dykes from different locations in the Chhota Nagpur Gneissic Terrane, indicate three pulses of emplacement ca. 118–116 Ma, ca. 112–111 Ma, and ca. 109 Ma. Geochemistry of 19 samples analysed herein (and an additional 12 samples from literature) shows sub-alkaline high-Mg tholeiitic basaltic andesite compositions. All the dyke sets belonging to the Raniganj-Koderma swarm with sub-sets WNW-trending ca. 118 Ma, NNW-trending ca. 116 Ma Salma, and a N-S trending ca. 109 Ma dyke showing similar chemistry. However, NW-trending dykes emplaced ca. 112–111 Ma have slightly different geochemical characteristics. Key trace element geochemistry data, particularly Nb/Y, Zr/Y, Nb/Yb, Ti/Yb, Th/Nb and Th/Yb ratios, indicate derivation from mantle melts generated from interaction of a plume with a spreading ridge (producing OIB – E-MORB melts) with a minor role for interaction with an enriched lithospheric mantle that was metasomatized during an earlier (pre-Mesozoic time) subduction event. Spatiotemporal distribution of the studied dolerite dykes connects them with the second plume head phase of the Greater Kerguelen mantle plume.

Early Paleozoic lithospheric extension law, dynamic mechanism, origin of mafic dykes in Ziyang, South Qinling Block, China

南秦岭地块紫阳地区广泛出露早古生代基性岩墙群，其研究具有重要的地球动力学意义。虽然如此，已有相关研究目前主要集中于志留纪基性岩墙群方面，而对早古生代（如，寒武纪、奥陶纪）基性岩墙群的研究仍相对薄弱。从而制约了对南秦岭早古生代岩石圈伸展过程相关问题（如，时空分布规律、地幔性质、动力学机制和相关成矿作用）的总体把握。鉴于尚存的科学问题，本研究选取康家坪、梨树梁、大竹坪、清明寨、苟家山、庙梁上、曾家山和毛坝村的基性岩墙群为研究对象，开展了矿物学、岩石学、锆石U-Pb年代学、地球化学和Sr-Nd-Hf同位素方面的研究，对其精细时代格局、成因及成岩动力学背景进行了系统探讨。研究结果显示，紫阳早古生代基性岩墙以辉绿岩和辉绿玢岩为主，形成时代为奥陶纪（478.8~486Ma），属碱性系列岩石（K₂O+Na₂O变化范围为4.10%~5.14%）。另外，基性岩墙群具有富集LREE、Rb、Ba、Sr、Nb、Ta、Zr、Hf和Eu （Eu/Eu^*=1.13~1.35），亏损HREE、U、Pb和Ti的典型地球化学特征。（⁸⁷Sr/⁸⁶Sr）_i=0.7044~0.7050、<i>ε</i>_Nd（<i>t</i>）=3.1~3.6、<i>ε</i>_Hf（<i>t</i>）=4.4~12.1，¹⁷⁶Hf/¹⁷⁷Hf=0.282634~0.282846，暗示基性岩墙为地幔柱作用过程亏损岩石圈或软流圈地幔部分熔融作用的产物。另外，在辉绿岩成岩过程经历了橄榄石和单斜辉石的分离结晶，但地壳混染的影响不明显。

Formation of Tarim Large Igneous Province and Strengthened Lithosphere Revealed Through Machine Learning

AbstractEarth history is punctuated by voluminous magmatism and the formation of large igneous provinces (LIPs). Although anomalous mantle temperatures are known to be involved in many LIPs formation, the potentially critical role of fluids remains elusive. Here we apply machine learning methods (e.g., random forest, deep neural network, and support vector machines) to train models based on global datasets of basalts associated with different settings. The trained models predict that the basalts of Tarim LIP in northwestern China show a spatial decrease in their island arc affinity from northeast to southwest, which can be correlated to fluids released from earlier southward oceanic subduction. Temporally, the fluid activity declined from 290 Ma basalts to 270 Ma mafic dykes, suggesting that the fusible components in the mantle source were waning over time and ultimately a strengthened lithosphere was generated. Our study provides new insights into the crucial role of fluids in the generation of LIPs, particularly those related to ancient subducted slabs.

Geochemistry, Petrogenesis and Tectonic Setting of the Mafic Dykes from the Amgaon and Khairagarh Regions, Bastar Craton, Central Indian Shield: Constraints on the Precambrian Crustal Evolution

The presence of dominantly dykes, dyke swarms and sills along with the basement rocks indicate that the study area has undergone deep erosion exposing the basement rocks and these magmatic bodies (dykes, dyke swarms and sills) actually represent the plumbing system for the mantle derived melts, which contributed significantly to the Precambrian crustal evolution processes in the Central Indian shield

Early Cambrian forearc ophiolite-hosted VMS-type Cu deposit in the North Qaidam belt, northern Tibetan Plateau

The North Qaidam belt is an important polymetallic metallogenic belt in the northwestern region of China. However, the tectonic setting and age of related VMS deposits remain debated. Here we performed an integrated analysis of field relationship, geochemistry, and geochronology for hosting rocks of the Lüliangshan VMS-type Cu deposit and surrounding mafic-ultramafic rocks. These rocks, including serpentinite, pyroxenite, chromitite, mafic dykes with associated meta-plagiogranite, lava, chert, and limestone, constitute a relatively complete ophiolite complex, indicating that the Lüliangshan Cu deposit can also be introduced as an ophiolite-hosted VMS deposit. Geochemical data show that the meta-dolerite and ore-hosting lava exhibit geochemical features similar to tholeiitic forearc basalt and are probably generated by partial melting of a depleted mantle source metasomatized by hydrous fluids. Some lavas have boninitic compositions and are formed by partial melting of residual mantle after extraction of forearc basalt. Some ore-hosting lavas also have geochemical affinities to island arc tholeiites as a result of more SSZ components involved in their magma source. The chert samples have remarkably high Fe2O3T contents and are classified as iron-rich one of hydrothermal origin, which is deposited in a ridge-proximal environment. These rocks, together with chromitites with subducted-related geochemical features, collectively indicate that the ophiolite-hosted VMS-type Cu deposit was formed in the forearc setting. Meta-gabbros intruding the ore-hosting lavas yield zircon U-Pb ages mainly ranging from 527 Ma to 518 Ma. The new ages of forearc ophiolite and the oldest age of island-arc rocks (514 Ma) suggest that the Lüliangshan Cu deposit formed in the early Cambrian during early-stage subduction of Proto-Tethys Ocean.

Geochemistry, magma flow characteristics and petrogenesis of Paleoproterozoic NW–NNW trending mafic dykes from central Bastar craton, India

Geochemistry, magma flow characteristics and petrogenesis of Paleoproterozoic NW–NNW trending mafic dykes from central Bastar craton, India

Possible source of Au in the Jiaodong area from lower crustal sulfide cumulates: Evidence from oxygen states and chalcophile elements contents of Mesozoic magmatic suites

The genesis of giant Jiaodong gold province remains perplexing although many ore-forming models have been proposed in the past decades. The key issues are what caused the burst of gold around 120 Ma and possible sources of gold. Here we report gold and platinum group elements (PGE) data and magmatic oxidation states of the pre-mineralization Mesozoic plutonic rocks and some mafic dykes to discuss their potential effects on gold mineralization. Gold concentrations of the Late Jurassic Linglong granite are 0.015–0.278 ppb. The Early Cretaceous plutonic rocks (129–127 Ma) have the same levels of Au concentrations (Au = 0.031–0.047 ppb). Both of the Jurassic and Cretaceous magmatic rocks are depleted in Au compared to normal granitic rocks worldwide (∼1 ppb). Palladium and platinum concentrations in the Linglong granite are 0.160–0.385 ppb and 0.046–0.116 ppb, respectively. Meanwhile, Pd and Pt contents of Cretaceous magmatic rocks are 0.069–0.363 ppb and 0.063–0.465 ppb, respectively.Oxygen fugacities of the Late Jurassic Linglong granite calculated by biotite oxybarometer are FMQ + 0.5 to FMQ + 1.2 (where FMQ refers to fayalite-magnetite-quartz buffer). Oxidation states of the Early Cretaceous magmatic rocks estimated via amphibole oxybarometer are from FMQ + 0.3 to FMQ + 1.1 for Guojialing granodiorite. Zircon trace element data yield overall higher fO2 values than those estimated with biotite and amphibole compositions. Oxygen fugacities are from FMQ – 0.6 to FMQ + 1.9 (sample CJ-3) and FMQ – 0.7 to FMQ + 1.1 (sample LL-1) for the two Linglong granite samples. Zircons from the Early Cretaceous Guojialing pluton yield oxidation states between FMQ – 0.8 and FMQ + 1.9 (DCW-1), which are about one log unit higher than those of the Beijie pluton (BJ-1: FMQ – 1.0 to FMQ + 0.7). However, oxidation states from nearly syn-mineralization mafic dyke reach FMQ + 2.0 to + 3.0 when estimated via zircon oxybarometer. Oxidation states given by whole rock Fe3+/Fe2+ ratios also support the results calculated by mineral compositions.There is no fractionation crystallization of redox-sensitive minerals, like magnetite or garnet, as indicated by geochemical data, which implies that the oxidation states of magmas may reflect their source regions. However, the depleted nature of Au and PGE must imply that these elements are missing. We propose that sulfide saturation at the deep crust can be invoked to explain the extremely low gold and PGE contents of these magmatic rocks. The sulfide cumulates act as sinks for sulfide-loving elements (e.g., Au), which are pre-enriched source for later large-scale gold mineralization in Jiaodong area. The underplated water-rich mafic magma from enriched mantle source ponded at the lower crust, continuously crystallized anhydrous minerals (e.g., olivine and clinopyroxene) and reached volatile saturation at the very moment. This volatile, probably carrying gold, resorbed the sulfide cumulate and escaped to shallow levels to form the giant Jiaodong gold deposits.

Petrology, geothermobarometry and geochemistry of granulite facies wall rocks and hosting gneiss of gemstone deposits from the Mogok area (Myanmar)

The Mogok Metamorphic Belt (MMB) of Myanmar formed during the Paleogene collision between the West Burma block and the Shan-Thai block. The MMB is mainly composed of medium to high-grade metamorphic marble, calc-silicate rocks, gneiss, quartzite, peridotite and igneous rocks such as granite, syenite and gabbro. The Mogok area in the central part of the MMB is well-known for magnificent quality ruby, spinel, sapphire, and peridot. To unravel the metamorphic PT-conditions prevailing during the formation of spinel and ruby from primary marble deposits in the Mogok area, three different types of high-grade quartz-garnet gneiss from the neighbourhood of gemstone mines were investigated by electron microprobe. Geothermobarometry reveals granulite facies PT-conditions of 756–792 °C at 7.4–7.6 kbar, which is reproduced by Theriak-Domino modelling within the error of both methods at water activities of 0.34–0.4. Shoshonitic and high-K calc-alkaline mafic dykes occur within marble forming conspicuous garnet-nepheline and clinopyroxene-clinoamphibole gneiss. Petrologic and geochemical investigations of these metadykes verify their granulite facies metamorphism and classify them as subduction-related magmatic rocks, which intruded the marble sequences. These investigations as well as previous studies show that spinel and ruby in marble of the Mogok area may have formed not only by metasomatism around alkaline intrusions, but also by granulite facies regional metamorphism.

Petrochemistry and Sr-Nd isotopes of post-collisional Neoproterozoic (ca. 950 Ma) amphibolite dykes of continental flood basalt affinity from the Simdega area: Implications for the geodynamic evolution of the Chhotanagpur Gneissic Complex, Eastern India

Metamorphosed mafic dykes are significant stratigraphic markers which also represent a shift from extensional to compressional geodynamic processes. We present petrology, bulk-rock geochemistry, and Sr-Nd isotope data on nineteen E-W to ENE-WSW trending mafic dykes metamorphosed under amphibolite facies in the Simdega area in the south western domain of the Chhotanagpur Gneissic Complex (CGC). The CGC is widely regarded as an eastern extension of the Central Indian Tectonic Zone (CITZ) which separates the Northern Indian and the Southern Indian blocks. Calcic amphiboles and plagioclases are the dominant minerals whereas clinopyroxene (diopside), biotite, chlorite, quartz, apatite, titanite, ilmenite and magnetite are present in minor amounts. Relict igneous textures are also present in some of the samples. Conventional thermobarometry and pseudosection modelling gave consistent peak temperatures (619 ± 13 °C) and pressures (5.8 ± 0.1 kbar) corresponding to the amphibolite facies. Major oxide geochemistry of the studied samples show differentiation trends and display arc-tholeiitic, high Fe-tholeiitic, high Mg-tholeiitic and calc-alkaline characters. Based on the LREE and HREE fractionation (La N /Yb N , La N /Sm N and Gd N /Yb N ) the studied samples can be classified into two broad groups: Group-I and Group II. In terms of the 87 Sr/ 86 Sr initial and 143 Nd/ 144 Nd initial contents, both the Group I and II samples are confined to an array of continental flood basalts and EM-1 and EM-2 reservoirs. The T DM (depleted mantle) model age of ca. 1.9 Ga suggest the source enrichment of the dykes during the Paleoproterozoic. Trace element ratios of both groups of the amphibolite dykes reveal that the (i) crustal contamination to be minimum, (ii) subduction-related fluid enrichment influenced their source regions, and (iii) mixed arc-, OIB-, and oceanic plateau- type geochemical signatures to be their characteristic features. Petrogenetic modelling involving REE suggest variable degrees of partial melting ranging from 5 to 10% for most of the samples of G-I and 1–3% for G-II dykes from a mixed source comprising predominantly of spinel lherzolite and minor amounts of garnet lherzolite. A three-stage model is proposed for the evolution of the CGC amphibolite dykes which involves: (i) sub-continental lithopheric mantle (SCLM) source enrichment due to the fluids derived from the subduction zone prior to ca. 1.9 Ga, (ii) emplacement of the mafic dykes in a post-collisional intra-continental extensional tectonic setting due interaction between the mantle plume and SCLM during the Tonian period at ca. 0.95 Ga and (iii) subsequent amphibolite facies thermal event at 0.92–0.88 Ga, associated with the amalgamation of the Rodinia supercontinent, that metamorphosed the mafic dykes as well as the much of the south-western part of CGC. Our findings support models proposing a northward subduction of the southern Indian block beneath the northern Indian block along the CITZ as well as bring out the role of a mantle plume in the generation of the Precambrian mafic dykes in the CGC. • Emplacement in a post-collisional intra-continental extensional tectonic setting • Trace elements show arc- as well as plume-derived components. • Nd isotope model ages highlight ancient (1.9 Ga) subduction event in their genesis. • Amphibolite grade metamorphism linked to Rodinia amalgamation

Ancient crust-derived syenitic and A-type granitic intrusions in the Emeishan large igneous province, SW China

Within the inner zone of the Emeishan large igneous province (ELIP), SW China, there are several small syenitic and A-type granitic intrusions not spatially related to giant Fe–Ti–V-deposit-bearing mafic–ultramafic layered intrusions. The representative Heime syenitic and Huili granitic intrusions as well as their spatially associated mafic dykes were developed at ca. 260 Ma, contemporaneous with the ELIP. The Heime metaluminous syenites are K-rich and have moderate SiO2 (60.1–61.4 wt%) and high total-alkali (Na2O + K2O = 9.68–10.7 wt%) contents with high K2O/Na2O ratios (1.18 to 1.74). The Huili metaluminous granites are K-rich A-type granites with moderate–high SiO2 (62.7–68.9 wt%), high total-alkali (7.96–8.62 wt%) and Zr (324–853 ppm) contents, and high 10,000 × Ga/Al ratios (2.68–3.22). Both of them are enriched in high-field-strength elements and light rare-earth elements, with significantly negative Nb–Ta–Sr–P–Ti anomalies. They also have highly evolved whole-rock SrNd isotopic compositions (Heime syenites, (87Sr/86Sr)i = 0.7092–0.7105, εNd(t) = −7.77 to −6.09; Huili granites, (87Sr/86Sr)i = 0.7116–0.7146, εNd(t) = −6.34 to −5.32). Zircon δ18O values of the Heime syenites (5.73‰–6.40‰) are slightly higher than those of the mantle zircons, and lower than those of the Huili granites (5.98‰–7.52‰). Zircon Hf 8isotopic compositions of the Huili granites (εHf(t) = −3.30 to +0.73) are higher than those of the Heime syenites (εHf(t) = −7.08 to −2.97), possibly indicating a more contribution of mantle component. In addition, the spatially associated mafic dykes display considerably higher εNd(t) values (Heime mafic dykes, −3.95 to −3.01; Heime mafic dykes, −0.58 to −0.51) than those of the Heime syenites and Huili granites. The parental magmas of both the Heime syenites and the Huili A-type granites show a close affinity to the K-rich, metaluminous syenitic magmas. Based on the Sr–Nd–O–Hf isotopic compositions of the Heime syenites, Huili granites, and spatially associated mafic dykes, we suggest that the K-rich syenitic magmas parental to the Heime syenites were produced mainly by remelting of Paleo–Mesoproterozoic mafic meta-igneous rocks, whereas those parental to the Huili A-type granites were generated by the remelting of Paleo–Mesoproterozoic basement rocks and mixing with small amounts of coeval mantle-derived basaltic melts.

Paleoproterozoic Plate Tectonics Recorded in the Northern Margin Orogen, North China Craton

AbstractThe occurrence of plate tectonic processes on Earth during the Paleoproterozoic is supported by ca. 2.2–1.8 Ga subduction‐collision orogens associated with the assembly of the Columbia‐Nuna supercontinent. Subsequent supercontinent breakup is evidence by global ca. 1.8–1.6 Ga large igneous provinces. The North China craton is notable for containing Paleoproterozoic orogens along its margins, herein named the Northern Margin orogen, yet the nature and timing of orogenic and extensional processes of these orogens and their role in the supercontinent cycle remain unclear. In this contribution, we present new field observations, U‐Pb zircon and baddeleyite geochronology dates, and major/trace‐element and isotope geochemical analyses from the northern margin of the North China craton that detail its Paleoproterozoic tectonic and magmatic history. Specifically, we record the occurrence of ca. 2.2–2.0 Ga magmatic arc rocks, ca. 1.9–1.88 Ga tectonic mélange and mylonitic shear zones, and folded lower Paleoproterozoic strata. These rocks were affected by ca. 1.9–1.8 Ga granulite‐facies metamorphism and ca. 1.87–1.78 Ga post‐collisional, extension‐related magmatism along the cratonal northern margin. We interpret that the generation and emplacement of these rocks, and the coupled metamorphic and magmatic processes, were related to oceanic subduction and subsequent continent‐continent collision during the Paleoproterozoic. The occurrence of ca. 1.77–1.73 Ga mafic dykes and ca. 1.75 Ga mylonitic shear zones along the northern margin of the North China craton may have been related to a regional mantle plume event. Our results are consistent with modern style plate tectonics, including oceanic subduction‐related plate convergence and continent‐continent collision, operating in the Paleoproterozoic.

Constraints on the lithospheric structure and tectonics of Archean Dharwar Craton, southern India from geophysical and geological data: Evidence for modified lithosphere

• Mantle plume activities eroded the lithospheric keel of the Archean Dharwar Craton. • Small-scale convection destabilized and promoted erosion of lithospheric mantle. • SCLM is metasomatized as indicated by low S-wave velocities (Vs = 4.1-4.4 km/s). • Effective distance of mantle plume activity is ∼1500 km from the source region. Cratons are the stable portions of the continents for at least the past two billion years. Their longevity is generally attributed to their thick, cold, rheologically strong and chemically depleted subcrustal mantle lithosphere. The lithosphere achieved both buoyancy and strength by relatively low thermal regime and compositional differences due to dehydration, presence of melt-depleted peridotites and low iron content. Low temperature of the mantle lithosphere leads to high viscosity, which prevents deformation and thus preserves the cratonic root. However, significant modifications/erosion of the lithospheric root is observed for some cratons from different parts of the globe. The present paper discusses some aspects of lithospheric modification of the Archean Dharwar Craton, southern India. Southern India is a mosaic of several Archean cratonic blocks, the Western and Eastern Dharwar cratons, and Southern Granulite Terrain. It has experienced several tectonic and magmatic events, including mafic dykes of various periods since Paleoproterozoic. The Dharwar Craton was accreted with the Karwar and Coorg blocks on the west and Eastern Ghats and Nallamalai fold belts on the east during the Proterozoic. The Cuddapah, Bhima and Kaladgi basins were formed during the Proterozoic period. A 1.1 Ga kimberlite magmatism and 550 Ma East-African orogen are other important geodynamic events of the region. The western and eastern continental margins, together with the Deccan Large Igneous Province, one of the world's largest igneous provinces, were formed due to mantle plume activities during 130-65 Ma. 2500 km long Himalayan Mountain chain was formed during ∼55 Ma. Active and passive seismic, magnetotelluric, gravity, xenolith and elevation data complimented with geological data were used to understand the lithospheric structure. Subduction, collision and mantle plume activities, which are main processes for crustal evolution are also found responsible for lithospheric destruction in the region. The detached subducted slabs from various geodynamic activities of the region might have led to rheological weakening of the subcontinental lithospheric mantle and thereby craton modification/erosion in the southern India. Mantle plume activities are responsible for epeirogenic uplift and lithospheric thinning in the southern India.