Crustal Accretion Research Articles

Tectonic juxtaposition of plume and subduction derived magmatic sequences in the Bababudan greenstone terrane, western Dharwar Craton, India: Constraining crustal accretion processes in a Neoarchean subduction-collision orogeny

Archean greenstone belts in the Western Dharwar Craton preserve important clues on Early Earth geodynamics. Here we investigate the ultramafic-mafic metavolcanic rock sequences from the Santaveri Formation of Bababudan greenstone terrane to address the tectonic evolution, mantle dynamics and crustal growth processes of western Dharwar Craton. The ultramafic volcanic rocks are geochemically classified as komatiite-picrite-basanites, whereas the mafic volcanic rocks correspond to magnesian andesite -Niobium-enriched basaltic andesite association. The pristine mineralogy for these ultramafic volcanic rocks is overprinted by tremolite-actinolite-biotite assemblage marking greenschist facies of metamorphism. The komatiites include Al-undepleted, Al-depleted and Ti-enriched types and share overlapping geochemical characteristics with the picrites. The basanites are distinguished as alkaline ultramafic rocks with total alkali content >3 wt% and MgO > 18 wt%. The coherent association of komatiite-picrite-basanite suggests pervasive plume-lithosphere interaction and complies with polybaric melting of a chemically heterogeneous mantle plume through a subduction-modified sub-continental lithospheric mantle at a rifted cratonic margin. Our results substantiate the role of an Archean mantle refertilized by subduction-driven recycling of Mesoarchean supracrustals concurrent with the onset of subduction event at ≥3.2 Ga. The mafic rocks were derived through melting of a hot oceanic slab and mantle hybridization. The xenocrystic zircon grains from komatiite suggest lower crustal inheritance and reworking of basement gneisses by ascending ultramafic melts. The 3177 Ma magmatic age obtained from these zircon grains indicate a Mesoarchean magmatic event in the genesis of the basement gneisses. The emplacement of komatiite-picrite-basanite association and magnesian andesite -Niobium-enriched basaltic andesite lithologies envisages a post 3.1 Ga Neoarchean crustal growth over Mesoarchean protocontinent of WDC. This crust generation episode can be translated in terms of plume-driven bottom-up and subduction-driven top-down tectono-magmatic processes at continental rift and ocean-continent convergence set-up respectively. Geochemical characteristics and petrogenetic aspects suggest that the studied volcanic supracrustals of Bababudan greenstone terrane encapsulate a Neoarchean subduction-collision orogenic system comprising plume-derived komatiite-picrite-basanite and subduction-derived magnesian andesite -niobium-enriched basaltic andesite magmatic sequences tectonically juxtaposed during Neoarchaen (~2.7 Ga) subduction-accretion-collision event.

The Heterogeneous Tethyan Oceanic Lithosphere of the Alpine Ophiolites

The Alpine–Apennine ophiolites are lithospheric remnants of the Jurassic Alpine Tethys Ocean. They predominantly consist of exhumed mantle peridotites with lesser gabbroic and basaltic crust and are locally associated with continental crustal material, indicating formation in an environment transitional from an ultra-slow-spreading seafloor to a hyperextended passive margin. These ophiolites represent a unique window into mantle dynamics and crustal accretion in an ultra-slow-spreading extensional environment. Old, pre-Alpine, lithosphere is locally preserved within the mantle sequences: these have been largely modified by reaction with migrating asthenospheric melts. These reactions were active in both the mantle and the crust and have played a key role in creating the heterogeneous oceanic lithosphere in this branch of the Mesozoic Western Tethys.

Age and geochemistry of Cambaí Complex, São Gabriel Terrane, Brazil: Arc-related TTG-like rocks

The Cambaí Complex TTG-like rock assemblage representing a Neoproterozoic juvenile magmatic arc is part of the São Gabriel Terrane that crops out in the SW domain of the Dom Feliciano Belt in S-Brazil. Geochemistry of dioritic, tonalitic, trondhjemitic, and granodioritic rocks of this complex indicate their low-medium-K calc-alkaline geochemical affinity being metaluminous and with fractional crystallization as the main magmatic process of formation. Whole-rock geochemistry enrichment in LILE as Sr and Ba in relations to HFSE as Y and Zr, with flat patterns in an REE spider diagram, with enrichment to chondrite ((La/Yb) N = 1.3 to 17.2) and low Eu anomaly (Eu/Eu* = 1.2), with 87Sr/86Sr(i) from 0.704260 to 0.706430, 143Nd/144Nd(t) from 0.512070 to 0.512080, Nd-TDM model age from 820 to 852 Ma, εNd(t) from 3.9 to 4.2 and 207Pb/204Pb and 206Pb/204Pb ranged from 15.5040 to 15.5776 and 17.5313 to 18.5696, suggest an immature magmatic arc-related rock setting. Geochronology U–Pb LA-ICP-MS in zircons from diorites shown Tonian crystallization ages (745 ± 4 Ma, 748 ± 5 Ma, 752 ± 4 Ma). Any model proposed to explain the history of the evolution of the West Gondwana Supercontinent needs to consider the accretion of juvenile crust from primitive mantle source with minor crustal contamination in this segment of the orogenic collage during the Neoproterozoic.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

The crust and mantle both in ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are critical to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogues for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it with the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dykes and sills cutting both mantle and lower crust. The gabbro units are thin ( c . 350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable with the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analogue of ultraslow-spreading ocean lithosphere. Supplementary material: Single-spot analysis and average compositions for mineral major element compositions (wt.%) of the Jiding gabbro section, Xigaze ophiolite are available at https://doi.org/10.17632/dtzpmwkscp.1

Triassic granitoids in the East Kunlun Orogenic Belt, Northwestern China: magmatic source and implications for geodynamic evolution

ABSTRACT Granitoids, an important part of the continental crust, are the direct and indelible evidence of the accretion and reworking process of the continental crust in different tectonic settings. Therefore, these early geodynamic mechanisms can be roughly reconstructed by analysing the geochemical characteristics of granitoids. In this study, the Permian–Triassic Granitoids widely distributed in the East Kunlun Orogenic Belt (EKOB) are studied to determine their chronology and petrogenesis to restrict their source regions and establish the significance of their geodynamic and crustal growth. Zircon inductively coupled plasma mass spectrometry (ICP–MS) dating results show that the crystallization ages of these granitoids are between 252–240 Ma, during the early and middle Triassic. These granitoids have high SiO2 and medium Na2O + K2O contents, plotting into the granite region, and calc-alkaline to high-K calc-alkaline series on the SiO2–Na2O + K2O and SiO2–K2O diagrams, respectively. All the samples have aluminium saturation index (A/CNK) values from 1.00 to 1.25 showing peraluminous characteristics. In terms of trace and rare earth elements, the samples are depleted in high field strength elements (HFSEs) such as Nb, Ta, P, and Ti and are enriched in large ion lithophile elements (LILEs) such as Rb, K, Ba, and U. Moreover, all granitoids show right-inclined rare earth element (REE) distribution patterns with weak to moderate Eu anomalies (δEu = 0.50–0.93). In addition, the samples have low P2O5 contents, 10,000*Ga/Al ratios (1.65–2.39), and Zr + Nb + Ce + Y (212–262 ppm) displaying affinities to I-type granite. Combined with their εHf (t) values of −2.31 to 4.44, with 1.0–1.4 Ga two stage model ages (TDM2), they are considered to be the products of the reworking of the Mesoproterozoic lower crust; therefore, they are not related to the continental net growth in the EKOB. On the tectonic discrimination diagrams of granitoids, all the samples fall into the area of volcanic arc granite (VAG) and continental margin arc; this suggests that the Paleo-Tethys Ocean slab was still in the subduction stage during 252–240 Ma, and the continental collision had not yet started. Based on the plenty of magmatic rock dating results published previously, it appears there are two magmatic peak periods (260–240 Ma and 230–210 Ma) and one magmatic intermittence period (240–230 Ma) during the late Permian to Triassic of the EKOB, corresponding to three geodynamic evolution stages of subduction, post-collision extension, and syn-collision, respectively.

Formation of Igneous Layering in the Lower Oceanic Crust From the Samail Ophiolite, Sultanate of Oman

AbstractAs the largest and best exposed example of paleo fast‐spreading oceanic crust on land, the Samail ophiolite in the Sultanate of Oman represents an ideal natural laboratory for investigating deep crustal processes at fast‐spreading mid‐ocean ridges. We studied two layered gabbro sequences from different stratigraphic depths: one from the middle of the plutonic crust showing decimeter‐scale modal layering (i.e., varying phase proportions) with olivine abundances gradually decreasing from layer bases to tops (Wadi Somerah, Sumail block) and one located near the crust‐mantle boundary showing millimeter‐scale olivine‐rich layers (Wadi Wariyah, Wadi Tayin block). Our multimethod approach of field, petrographic, geochemical, and microstructural observations focuses on documenting layered textures that are widely observed within the lower oceanic crust as well as understanding their formation mechanisms within the context of small scale crustal accretion processes beneath fast‐spreading mid‐ocean ridges. Results from the mid‐crustal sequence indicate moderate cooling rates (Ca‐in‐olivine: log[dT/dt; °C yr−1] = −2.21 ± 0.7) and correlated variations in mineral compositions and microstructures. We infer that decimeter‐scale layers in Wadi Somerah were deposited by density currents of crystal‐laden magma within a sill environment that potentially experienced occasional magma replenishment. The millimeter layering in Wadi Wariyah is best explained by Ostwald ripening emphasizing initial heterogeneities possibly being provoked by cyclical nucleation of olivine through the competing effects of element diffusion and rapid cooling. Fast cooling is recorded for the crustal base (Ca‐in‐olivine: log[dT/dt; °C yr−1] = −1.19 ± 0.5, Mg‐in‐plagioclase: log[dT/dt; °C yr−1] = −1.35°C ± 0.6) demonstrating that heat locally can be lost very efficiently from the lowermost crust.

Lithospheric density structure of the southern Central Andes constrained by 3D data-integrative gravity modelling

The southern Central Andes (SCA) (between 27° S and 40° S) is bordered to the west by the convergent margin between the continental South American Plate and the oceanic Nazca Plate. The subduction angle along this margin is variable, as is the deformation of the upper plate. Between 33° S and 35° S, the subduction angle of the Nazca plate increases from sub-horizontal (< 5°) in the north to relatively steep (~ 30°) in the south. The SCA contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated Cenozoic deformation within the Andean orogen. The distribution of the deformation within the SCA has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities. However, the possible influence that the thickness and composition of the continental crust have had on both short-term and long-term deformation of the SCA is yet to be thoroughly investigated. For our investigations, we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the Andean orogeny and, in particular, investigated the short- and long-term nature of the present-day deformation processes. We established a 3D model of lithosphere beneath the orogen and its foreland (29° S–39° S) that is consistent with currently available geological and geophysical data, including the gravity data. The modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains: the crystalline crust in the orogen (the magmatic arc and the main orogenic wedge) is thicker (~ 55 km) and less dense (~ 2900 kg/m3) than in the forearc (~ 35 km, ~ 2975 kg/m3) and foreland (~ 30 km, ~ 3000 kg/m3). Crustal thickening in the orogen probably occurred as a result of stacking of low-density domains, while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion. No clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early Paleozoic. Areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal-density contrast. This suggests that the short-term deformation within the interior of the Andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers. A thicker, denser, and potentially stronger lithosphere beneath the northern part of the SCA foreland is interpreted to have favoured a strong coupling between the Nazca and South American plates, facilitating the development of a sub-horizontal slab.

Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: the Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)

AbstractThe architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2 gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-metre IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-metre shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe–Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe–Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.

Insights on the Indian Ocean Tectonics and Geophysics Supported by GMT.

This paper presented analyzed and summarized data on geological and geophysical settings about the tectonics and geological structure of the seafloor of the Indian Ocean by thematic visualization of the topographic, geophysical and geological data. The seafloor topography of the Indian Ocean is very complex which includes underwater hills, isolated mountains, underwater canyons, abyssal and accumulative plains, trenches. Complex geological settings explain seismic activity, repetitive earthquakes, and tsunami. Understanding and prognosis of the disastrous and catastrophic geological events is strongly based on correct data analysis, modelling and visualization. An important feature of this paper is mapping multi-source high-resolution data by GMT. Data include raster grids in NetCDF and GRD formats: ETOPO1, geologic and marine free-air gravity data, EGM96, age, spreading rates, and spreading asymmetry of the ocean crust by NOAA, total sediment thickness. Data were visualized by GMT modules to compare and analyze geophysical and geological settings of the Indian Ocean. Visualization reveled correlations between high bathymetric variations of the oceanic seafloor, distribution of main geological seafloor fabric: Southwest, Southeast, Mid and Carlsberg ridges. Tectonic maps were plotted to perform comparative analysis of several variables: crust age, spreading half rates (mm/yr), asymmetries in crustal accretion on conjugate ridge flanks (%), variations in the geopotential and gravimetric models. Being the warmest of the world’s ocean, Indian Ocean has specific climatic conditions (repetitive monsoons, tsunamis, cyclones and storms), complex geologic seafloor structure with triple junction and unique geographic settings. Presented paper contribut ed to the regional studies of the Indian Ocean.

Lithogeochemical and Nd isotopic constraints on felsic magmatism in the Makkovik Orogen, Labrador, Canada: Implications for assembly of the supercontinent Nuna

The formation of the supercontinent Nuna was a significant global crustal accretion event during the interval 1900 to 1800 Ma. Insight into its amalgamation is preserved in felsic volcanic of the Aillik Group and associated porphyritic intrusive rocks from the Makkovik Province. Lithogeochemical data indicate compositions typical of ferroan (A-type) felsic magmas. Trace-element ratios, such as high La/Yb(pm), are indicative of partial melting at high temperatures (~850–900 °C). Most samples have εNd(t) between +0.1 and −5.2, with T(DM) from 2773 to 2160 Ma, suggesting derivation through anatexis of predominantly Neoarchean to Paleoproterozoic sialic crust. One sample, however, has εNd(t =1860 Ma) = +3.0 and T(DM) of 1927 Ma, comparable with two other published values, indicative of a large component of juvenile asthenospheric material in its origin. This suggests that the felsic rocks may, alternatively, have been derived from a highly fractionated mantle melt with variable Neoarchean to Paleoproterozoic crustal contamination. It is likely that a mixture of both end-member models, anataxis and fractional crystallization, were active in the generation of these felsic magmas.The Aillik Group is genetically more closely related to the Cape Harrison arc than previously thought. It is, herein, proposed that there are likely only two domains (Kaipokok and Adlavik) preserved in the Makkovik Province. The Aillik Group was deposited in a backarc setting during the initial stage of the Cape Harrison arc, and was subsequently thrust over the Kaipokok arc. This thrusting resulted in the formation of crustal-scale shear zones that localized fluids and increased the metallogenic tenor of the region, consistent with the mode of formation of other accretionary belts of Nuna.

Li-isotope exchange during low-temperature alteration of the upper oceanic crust at DSDP Sites 417 and 418

The record of the Li-isotopic composition of ancient-seawater has the potential to provide important insight into the Earth system. However, we still have an incomplete understanding of the controls on the magnitude and isotopic compositions of the fluxes of Li into and out of the ocean. An important sink for Li from seawater is through low-temperature hydrothermal alteration of the upper oceanic crust. Here we present a detailed study of three adjacent drill cores in 120 Myr old Atlantic crust (Holes 417A, 417D and 418A) that had very different sedimentation histories. Samples from Hole 417A, drilled on a topographic high and exposed to seawater for >20 Myr before sediment started to accumulate, have much higher Li contents (up to ∼70 ppm) than samples from the other sites where sediment began accumulating immediately after crustal accretion (up to ∼20 ppm). Despite this difference in Li content, the δ7Li of the altered basalts in all cores increases with depth over the upper ∼40 m from around −1‰ at the lava-sediment interface to 6 ± 2‰ at ∼40 m depth, remaining roughly constant with depth after this. If the uppermost lavas equilibrated with seawater, with a bulk fractionation factor of ∼0.980 ± 0.003, then the isotopic composition of seawater 120 Myr ago was ∼19 ± 3‰, within the range of other estimates for Cretaceous seawater. Fluid-rock reaction models, with the hydrological regime varying as a function of depth and sediment accumulation history, can explain the observed rock compositions. However, modelling shows that complexities related to the time-varying hydrological regime, along with the possibility of time-varying δ7Li of seawater, mean that the Li-isotopic composition of altered oceanic lavas cannot be uniquely interpreted. The bulk Li content and isotopic composition of the well-sampled 120 Myr crust in Holes 417A, 417D and 418A suggest that Cretaceous upper oceanic crust was a sink for ∼4×109 mol yr−1 of Li. Surprisingly, the bulk Li-isotopic composition of the lavas sampled at these sites (∼6‰) is not much higher than that of fresh MORB, and within uncertainty of that of high-temperature hydrothermal vent fluids; i.e., high- and low-temperature alteration processes approximately balanced one another for Li-isotopes in the late Cretaceous. This could be fortuitous; alternatively, if the ocean was at steady state the main source of Li to the ocean at this time may have had a δ7Li similar to that of high-temperature hydrothermal fluids.

Regionally variable timing and duration of celadonite formation in the Troodos lavas (Cyprus) from Rb-Sr age distributions

Celadonite (K[Mg2+,Fe2+][Fe3+,Al3+](Si4O10)(OH2)) is a common void-filling mineral in ocean floor and ophiolite lavas that forms during low-temperature, off-axis, hydrothermal alteration. Its occurrence provides important information about the sink of K from seawater into the upper oceanic crust. New age determinations for celadonitic samples from a ~20 km section of the lavas in the Troodos ophiolite, along with published ages, show that most celadonite formed within the first ~20 Myrs after crust accretion. However, significant regional differences in the duration of celadonite formation are observed. In comparison to a region of flat paleo-seafloor, in a paleo-seafloor topographic low celadonite started forming later and continued forming for ~20 Myrs longer. The paleo-seafloor low is associated with abundant hydrothermal sediments which are interpreted to have played a key role in controlling the chemical conditions required for celadonite precipitation. It is hypothesised that celadonite formed for longer largely because these hydrothermal sediments acted as a source for labile Fe for a longer time than in the region where the Fe required for celadonite formation was mainly leached from silicate phases. In the region of flat paleo-seafloor, sediment did not begin accumulating for ~20 Myrs after crustal accretion allowing prolonged oxidative alteration of the upper crust. This led to Fe released by silicate phase breakdown being incorporated in stable Fe3+ containing phases, such as Fe-oxyhydroxides, rather than being transported as aqueous Fe2+ to sites where celadonite could form. If this model is correct, the uptake of K from seawater into the oceanic crust may be highly spatially heterogeneous and intimately linked to the subsurface redox conditions and the distribution of hydrothermal sediments.

Crustal Accretion in a Slow Spreading Back‐Arc Basin: Insights From the Mado Megamullion Oceanic Core Complex in the Shikoku Basin

AbstractOceanic core complexes (OCCs) represent tectonic windows into the oceanic lower crust and mantle; they are key structures in understanding the tectono‐magmatic processes shaping the oceanic lithosphere. We present a petrological and geochemical study of gabbros collected at the Mado Megamullion, a recently discovered OCC located in the extinct Shikoku back‐arc basin. Bathymetry of the Mado Megamullion reveals spreading‐parallel corrugations extending 25 km from the breakaway to the termination. Samples from several locations include peridotites, gabbros, dolerite, and rare pillow basalts. Gabbros range from granular to varitextured olivine gabbros and oxide gabbros. The emplacement of these gabbroic rocks within the oceanic lithosphere was followed by a multiphase tectono‐metamorphic evolution including (i) dynamic recrystallization within shear zones, developed under granulite‐ to upper‐amphibolite‐facies conditions, and (ii) intrusion of highly evolved melts forming felsic segregations. This tectono‐metamorphic evolution recalls that of the lower crust from other OCCs worldwide, demonstrating that this OCC exposes deep‐seated intrusions progressively exhumed by detachment faulting. Nonetheless, the Mado Megamullion lower crustal gabbros show an unusual crystal line of descent, different from what is reported from mid‐ocean ridge lower crustal rocks. We infer that the water‐bearing character of the primary melts in this back‐arc basin triggered the early precipitation of clinopyroxene, soon followed by amphibole and Fe‐Ti oxides. Such modifications in phase saturation are likely to be directly related to the back‐arc setting of the Mado Megamullion. If so, the phase assemblages of oceanic gabbros may be a diagnostic for the tectonic setting of lower crustal rocks in ophiolites.

Delineation of the early Neoarchean (2.75 to 2.70 Ga) crustal growth and reworking processes in the southeast base of Taihang Mountain, North China Craton

Numerous studies suggest that various secular geologic and geochemical transitions occurred between ~3.2 Ga and 2.5 Ga. During this age-window, the ~2.70 Ga tectono-thermal event of the Neoarchean is by far the most influential, and is unusual in terms of coeval mafic-felsic magmatic rocks which were interpreted to reflect widespread crustal accretion. Here we report the early Neoarchean TTG and potassic granite association preserved in the Yunmengshan area, Taihang Mountain, North China Craton (NCC). TTG and potassic granites emplaced at 2712 ± 65 to 2644 ± 25 Ma, followed by ~2.57 to 2.50 Ga metamorphism and partial anatexis. TTG has geochemical features corresponding to high-Al medium-pressure (MP) TTG. Their high CaO, low to moderate Al2O3/(FeOT + MgO), and positive εHf (t) values (+3.8 to +7.2) with TDM2 of 2.96 to 2.75 Ga, indicate that they formed by partial melting of juvenile low-K mafic rocks, representative of crustal accretion. Besides, TTG can be subdivided into two groups based on Eu anomalies. Those with Eu/Eu* < 1.0 underwent advanced amphibole and plagioclase fractionation. The possible reason for their weakly negative Eu anomalies is that plagioclase has negated the effects of amphibole fractionation which is theoretically accompanied by plagioclase removal. Those with positive Eu anomalies accord with “slab-melt” identification criterion (Sr > 300 ppm plus elevated Sr/Y > 40, (La/Yb)N > 12 and Eu/Eu* > 1.0). The high Eu/Eu*, Sr/Y and low Yb probably represent the contribution of amphibole fractionation and plagioclase accumulation during the magma evolution. The evidence favors an arc-related setting for the TTG. The early Neoarchean potassic granites are monzogranite to syenogranite, and formed shortly after TTG. They belong to high-K calc-alkaline to shoshonitic I-type granite, and display depleted and concave-upward REE patterns between middle and heavy REEs and higher Zr/Sm ratios (69.6 on average) than TTG (45.4). The calculated εHf (t) values are mainly positive (+1.4 to +7.0) with TDM2 from 3.03 Ga to 2.72 Ga, together with low Al2O3/(FeOT + MgO), low to moderate CaO and high K2O/Na2O ratios, indicating a high-K mafic crustal source with metapelite involvement. Meanwhile, we reviewed geochemical data of the early Neoarchean TTGs published in the NCC and other cratons abroad. The results show that samples from Trans-North China Orogen in NCC have lower Y contents, higher Sr/Y and (La/Yb)N ratios, and more depleted HREE than those from Eastern Block, which could be interpreted to reflect an increase in the depth of melting. Alternatively, according to previous studies and comparison globally, elevated Sr/Y values could also reflect increased consumption of plagioclase in melting reactions due to higher temperatures, or magma fractionation. Therefore, it mostly lacks a clear distinction between the “arc” and “non-arc” settings based on geochemistry. However, many researchers put forward evidence that subduction was well attested in the late Archean (3.0 to 2.5 Ga), but it might be unstable.

SEAFLOOR MAPPING OF THE ATLANTIC OCEAN BY GMT: VISUALIZING MID ATLANTIC RIDGE SPREADING, SEDIMENT DISTRIBUTION AND TECTONIC DEVELOPMENT

The study presents the insights of the tectonic development and geological settings of the Atlantic Ocean supported by cartographic visualization in Generic Mapping Tools (GMT). The aim is to study geologic situation and trends in the tectonic development of the Mid-Atlantic Ridge and Atlantic Ocean seafloor. The objective is to find out impact of various factors (such as volcanic, tectonic, hydrothermal and sedimentary processes) that sculpt seafloor geomorphology, and correlation between early history of crust formation, geological processes and present submarine landforms. Other assignments in this work refer to mutual comparison of raster grids on sedimentation, topography, geology, seafloor fabric and highlighting similarities among the landforms and sediment thickness. Asymmetry in crustal accretion is explained by the tectonic history of the lithosphere formation. Correlation between plate subduction and development of the submarine landforms is explained by the Earth's crust extension resulting in formation of cracks, elongations, faults, rifts. Ocean seafloor geomorphology is shaped by a variety of factors that impact its form at different scales. These drivers (tectonic evolution, oceanic currents, hydrology, sedimentation) have effects on geomorphic landforms of the seafloor in context of historic geological development and during Quaternary. Technical part of this work was performed by GMT scripting toolset with all maps plotted in American polyconic projection. The results are received by overlay, cartographic analysis and synthesis of the multi-source geodata through mapping and interpreting grids (ETOPO1, EGM96, GlobSed, crustal age). This work contributes to expand the knowledge on geological and tectonic development of the Atlantic Ocean seabed in order to complete the view of its submarine geomorphology

Large-scale asymmetry in thickness of crustal accretion at the Southeast Indian Ridge due to deep mantle anomalies

Abstract Hot mantle plumes and ancient cold slabs have been observed beneath modern mid-ocean ridges, but their specific and detailed effects on mid-ocean ridge crustal accretion are poorly understood. The oceanic lithosphere beneath the Southeast Indian Ocean displays unique morphological, geophysical, and geochemical characteristics, which may reflect the influence of both mantle anomalies and upwelling plumes on seafloor spreading. In this study, we combined gravity-derived oceanic crustal thickness with plate tectonic reconstructions to investigate patterns of asymmetry in thickness of crust accreted at the Southeast Indian Ridge over the last 50 m.y. Our results reveal several distinct features: (1) small-scale, short-lived asymmetries in the thickness of crustal accretion of up to 0.75 km are alternatively distributed on the southern and northern flanks of the 90°–120°E Southeast Indian Ridge segment. These can be explained by variations in mantle depletion or mantle temperature. (2) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of &amp;gt;2.5 km are observed around the Kerguelen Plateau and Balleny Islands, which we attribute to excess crust from the off-axis Kerguelen and Balleny mantle plumes. (3) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of 0.75–2.5 km are observed on the northern flank of the westernmost (70°–80°E) Southeast Indian Ridge and the southern flank of the eastern (120°–140°E) Southeast Indian Ridge segment, respectively. We attribute these to asymmetry in mantle temperature of up to 20–53 °C. We suggest these asymmetric temperatures across the Southeast Indian Ridge are associated with the foundered lithospheric fragments of the Indian Craton triggered by the African Large Low-Shear-Velocity Province during the breakup of Gondwanaland and an intraplate subducted slab of the Paleo-Tethys Ocean, respectively. The remnant craton fragments and subducted oceanic slab may have moved north in concert with the northward-migrating Southeast Indian Ridge beginning at 50 m.y. ago.

Young and Old Granulites: A Volatile Connection

Granulite facies metamorphism of the lower crust has decreased in scale since the Late Archean, but many of its definitive features have persisted: (1) Punctuated, sometimes relatively short-lived, episodes of high-grade metamorphism. These are recorded, in favorably simple cases, by discrete growth rims on zircons. (2) A consistent age gap of a few to several tens of millions of years between juvenile magmatism (crustal accretion) and high-temperature metamorphism. The secondary thermal pulse is an event distinct from primary crustal accretion. (3) Involvement of mineralizing pore fluids of lowered H2O activity, that is, with high CO2 and saline concentrations. Very high oxidation states of some granulites implicate sulfur as an important fluid component. (4) Transcurrent faulting as a conspicuous feature of synmetamorphic deformation. This gives rise to characteristic transposed foliation and lineation. (5) Emplacement of coeval postorogenic K-rich granites at midcrust levels. These features can be rationalized by concepts of modern plate tectonics. High-angle plate collision is succeeded by orogen-parallel transport. This change of plate motion necessarily detaches the underthrust portion of the lithosphere, liberating asthenospheric melts and/or fluids in a postorogenic resurgence. A generation of volatile-rich mafic magmas invades the continental margin; high CO2 and halogen contents cause outgassing and freezing of the magmas at depth. Liberated volatiles effect granulite facies metamorphism by leaching H2O and lithophile elements, importantly K, and transporting these components and heat upward. Extensive melting of the lower crust is inhibited by the low H2O activity of saline-carbonic pore fluids at high pressure. Melting of orthogneiss and supracrustal rocks occurs at midcrust levels by increase of H2O activity as pressure on alkali chloride solutions falls below 0.6–0.5 GPa. The foregoing hypothesis is an alternative to the classical view that granite results from fluid-absent partial melting of, and extraction from, the lower crust, leaving granulites.

Correlations between oceanic crustal thickness, melt volume, and spreading rate from global gravity observation

Oceanic crustal accretion at mid-ocean ridges is a function of spreading rate, mantle temperature, and composition, which are intertwined in affecting melt production and oceanic crustal thickness. Sparse and irregular seismic and geochemical observations on a global scale showed no correlation between crustal thickness and spreading rate along slow to fast-spreading centers. Here, we compile a global oceanic crustal thickness model from gravity data to revisit this issue at a high resolution. Gravity-observed melt volume shows a positive correlation with spreading rate globally, implying that spreading rate is the dominant factor in melt production. However, oceanic crustal thickness is negatively correlated with spreading rate from slow to fast-spreading centers, and this trend is further consolidated by the increase in Curie point depth (a geothermal proxy) and ridge depth with increasing spreading rate. Thus, a decreasing near-ridge temperature probably contributes to crustal thinning from slow to fast-spreading centers. Inferred low melt volume anomalies beneath fast-spreading centers are consistent with a 20 °C temperature drop in the near-ridge mantle, likely caused by efficient hydrothermal cooling.

Archean and Paleoproterozoic crustal evolution and evidence for cryptic Paleoarchean-Hadean sources of the NW São Francisco Craton, Brazil: Lithochemistry, geochronology, and isotope systematics of the Cristalândia do Piauí Block

The Cristalândia do Piauí Block, located in the northwestern margin of the São Francisco Craton, represents the basement of the Rio Preto Fold Belt. It is composed of Archean orthogneisses of ca. 3.2 Ga reworked at 2.81 and 2.68 Ga with juvenile to moderately juvenile εHf values between −1.51 and −8.07, and high-K syenogranites dated at 2.65 Ga with crustal εHf values between −10.37 and −19.54, both with model ages (TDMc) varying from 3.57 to 4.33 Ga, indicating cryptic Paleo- to Eoarchean and even Hadean sources. Metamafic-ultramafic rocks, iron formations, metacherts, and graphite schists occur in association with the Archean orthogneiss. The whole set is intruded by Paleoproterozoic (ca. 2.2 Ga) metagranitoids with compositions varying from granodioritic with sanukitoid-type signatures to monzogranitic, and alkali-feldspar granitic with crustal signatures. They are related to the Rhyacian-Orosirian orogeny, responsible for the complex deformation patterns printed in the Archean basement. Orosirian metasedimentary rocks are represented by garnet-biotite paragneiss with maximum depositional age of ca. 1.95 Ga. Intrusive mafic dikes in the complex show ages of ca. 2.07 Ga and isotopic features of mantle-derived magmas. Considering the presented data, the Cristalândia do Piauí Block represents a metacratonic domain corresponding to part of the Guanambi-Correntina Paleoplate, wich had been involved in crustal accretion and reworking from the Archean to the Paleoproterozoic. Many of the elements of the evolutionary stages wich are present in the São Francisco-Congo Paleocontinent can be recognized, suggesting an evolution of this crustal segment amounts to the Eoarchean era and disclosing the existence of cryptic Paleoarchean or even Hadean nuclei, reworked in at least three metamorphic events during the Rhyacian-Orosirian orogeny.

Strain localization during burial and exhumation of the continental upper crust: A case study from the Northern Sporades (Pelagonian thrust sheet, Greece)

Abstract Extension is a key process controlling the post-orogenic exhumation of metamorphic rocks in subduction and collision zones. Previous studies have largely focused on the mechanics of localized core-complex style post-orogenic extension and the large-scale effects of various internal (e.g. rheology) and external (e.g. plate motions) parameters on the mode of extension. However, many regions on earth underwent rock exhumation during post-orogenic extension, which is characterized by distributed rather than localized deformation. We explore conditions of distributed deformation as illustrated by the Pelagonian unit in the Aegean subduction system. In particular, we explore the influence of structural inheritance related to the pre-extension shortening and mechanical stratigraphy on the localization of extension on the scale of the upper crust through detailed structural analysis on the islands of Skiathos and Skopelos. Additionally, the time frame of deformation has been established by 40Ar/39Ar dating of key shear zones. Shortening on the islands predominantly took place by ductile top-SW thrusting under low-grade metamorphic conditions, localized in weak calcite marble layers within the Upper Cretaceous and Upper Triassic carbonates at ~55 Ma. We show that the presence of shallow decoupling levels in the upper crust resulted in the formation of thin (several 100 m thick) thrust sheets that are defined for the first time on Skiathos. The Early Paleogene accretion of the Pelagonian upper crust to the upper plate (Eurasia/Rhodopia) was followed by the extensional inversion of the nappe stack. Extension was accommodated by opposite-sense, generally top-NE, ductile to brittle shearing, which localized at inherited heterogeneities such as reverse-sense shear zones and stratigraphic contacts at around 35 Ma, as suggested by our 40Ar/39Ar age spectra. The dense network of such northerly-dipping, inherited weakness zones resulted in a highly distributed pattern of extensional deformation dominated by layer-parallel shearing. We argue that the distribution of crustal heterogeneities substantially influences the style of post-orogenic extension.