Magmatic Research Articles

Alkaline Devonian magmatism of the Menorca Island: Tracking the mantle isotopic sources in the realm of the western Paleo-Tethys Ocean

Several locations with alkaline magmatism are recognised in Silurian-Devonian basins along the southern variscan autochthon units (e.g. Central Iberian Zone) of the northern Gondwana margin. The origin of the Devonian basins and their magmatism has not been studied in the context of the passive margin of Gondwana. The basement of Menorca, Balearic Islands, consists of a deep Devonian-Carboniferous basin with mafic igneous rocks, the Tramuntana Gabbros. In this study, we trace the geodynamic setting and isotopic sources of the Tramuntana Gabbros through elemental geochemistry, isotopic geochemistry (SrNd) and UPb geochronology in zircons. These gabbros are the product of an intraplate alkaline magmatism with immobile trace element and REE contents similar to those of Ocean Island Basalts. Average 87Sr/86Sr(370) of 0.708456 and εNd(370) of +4.0 indicate a source similar to a Type-2 enriched mantle with average TDM of 665 Ma, suggesting a relatively old metasomatized mantle. Concordant UPb ages of c. 597 Ma (Ediacaran, radiometric age) from a single population of 31 zircons separated from the Tramuntana Gabbros (Devonian, biostratigraphic age) reinforce the presence of older units in the corresponding lithospheric mantle. The Tramuntana Gabbros and the Devonian-Carboniferous sequences of Menorca limit the westward extension of the Paleo-Tethys Ocean, whose development never reached these westernmost regions. Assuming a common sublithospheric mantle source for the peri-Gondwanic Devonian alkaline rocks and considering the previous Cadomian (Neoproterozoic) subduction to be the most favourable origin of the separated zircons, the bulk rock TDM and zircon UPb data obtained from the Tramuntana Gabbros track the mixing, recycling, and mantle accretion in this peri-Gondwanic section from Precambrian to Devonian times.

Fabricating nepheline syenite-chitosan composite for heavy metals uptake: Mechanism insight via statistical physics modeling

Given the substantial costs associated with conventional wastewater treatment methods, researchers are increasingly exploring mineral compounds for their cost-effectiveness, widespread availability, and efficiency. In this study, nepheline syenite, an alkaline igneous rock, was selected among these compounds. The research investigates the single and binary adsorption of heavy metals (Ni(II) and Cd(II)) from synthetic and electroplating industrial effluents using a nepheline syenite-chitosan composite (NS-CS). The physical and chemical properties of the adsorbents were characterized using XRD, XRF, FT-IR, BET, and SEM analyses. The study examined the effects of solution acidity, contact time, initial concentration, and adsorbent dosage to determine the optimal conditions for the uptake of Ni(II) and Cd(II). The isothermal data were best described by the Langmuir isotherm model, with NS-CS showing adsorption capacities of up to 40.48 mg/g for Ni(II) and 41.79 mg/g for Cd(II). Additionally, the competitive adsorption of nickel and cadmium in the binary system was more accurately described by the modified Freundlich equation. Kinetic data were best fitted by the pseudo-second-order model. Thermodynamic analysis indicated that the adsorption of both ions was endothermic and nonspontaneous. Statistical physics modeling was employed to elucidate the adsorption mechanism of heavy metals on the NS-CS composite. The single-layer model with one energy level provided the most precise fit to the experimental data. Using this model, key factors were calculated, revealing that the adsorption energies for both Ni(II) and Cd(II) ions were below 20 kJ/mol, suggesting that the adsorption process is likely physical in nature.

Formation of hydrothermal ferromanganese oxides in the Daini-Nishi-Yamato Seamount, Sea of Japan: Do they really contain critical-metal particles?

In submarine environments, ferromanganese (Fe–Mn) oxides are formed via various processes. Although hydrothermal Fe–Mn oxides are generally valueless as mineral resources because of their low critical metal content, they reflect temporal changes in hydrothermal activity. Metal particles, including native metals, have been observed in hydrothermal Fe–Mn oxides and associated igneous rocks that are extensively distributed in the Sea of Japan. This finding can have significant implications for metal transport in submarine hydrothermal systems. However, the existence of these metal particles requires verification because their thermodynamic coexistence with Mn oxides is challenging and has only been reported by one research team. Here, we show the growth structure and geochemistry of hydrothermal Fe–Mn oxides and volcanic rocks from the Daini-Nishi-Yamato Seamount in the Sea of Japan. The chemical and Nd–Sr isotopic compositions of the volcanic rock indicated that the Daini-Nishi-Yamato Seamount formed through magmatism after back-arc spreading in the Yamato Basin at 10–17 Ma. The hydrothermal Fe–Mn oxides consisted of Ti-rich layers that formed earlier and Ti-poor layers that formed later. The Ti-rich layers precipitated from Ti-rich hydrothermal fluids because of water–rock interactions at higher temperatures during the early stages of hydrothermal activity. During the late stages of hydrothermal activity, Ti-poor layers formed, likely because of a decrease in the temperature of the water–rock interactions and/or the depletion of Ti in the host rocks. However, unlike in previous studies, metal particles such as Ag and rare earth elements were not detected in our samples, which were impregnated with resin to prevent contamination during the polishing process. These results strongly suggest that the metal particles reported in the previous studies were contaminants.

Comparative mineralogy, geochemistry and petrology of the Beloziminsky Massif and its aillikite intrusions

The Beloziminsky Massif (BZM) is an alkaline ultramafic carbonatite complex that includes carbonatites, ijolites, meltegites, and syenites (abbreviated as the CIMS suite) as well as aillikite intrusions that range in age from 645–621 Ma. Aillikite intrusions also occur in the Yuzhnaya Pipe (YuP), located about 16 km eastward of the BZM. Over 5400 analyses in total were conducted to compare mineralogy and geochemistry of different rock types in this study; of these, 24 CIMS samples (>1100 analyses) and about 16 aillikites (>2300 analyses) were collected from within the BZM; the rest are aillikite mineral samples from pipes and dykes outside the massif (>2000 analyses). The results suggest significant differences in sources for rock-forming minerals, less so for the accessories. The pyroxenes in aillikite correspond either to mantle Cr-diopside xenocrysts or megacrystic augites. Low-Na Ti-augites and diopsides as well as aegirines are prevalent in the CIMS intrusive suite. Amphiboles show a considerably long compositional trend, from hornblendes to richterites. Dolomitic carbonatites include admixtures of Na, K, and Ba while calcium carbonatites often contain Sr. The carbonate-rich aillikitics are enriched either in Mg or Ca. The CIMS rocks, particularly the Ca-Mg carbonatites, often include siderites. Thermobarometry for the YuP samples, collected from outside the BZM and containing Cr-diopsides, Cr-phlogopites and Cr-spinels, suggest a formation pressure of 2–4 GPa and a temperature of 800–1250°C; augite xenocrysts with elevated HFSE, U, Th, and Al-augites trace a 90 mW/m2 geotherm.The huge thermal impact of the plume that triggered the break-up of Rodinia also created a series of ultramafic–alkaline–carbonatite massifs. Initially, the aillikites in the mantle were likely produced by the plume-induced melting of carbonated metasomatites containing ilmenite, perovskites, apatites, amphiboles and phlogopites which, in turn, were created by subduction-related melts. Any additional enrichment in the ore components might have occurred subsequentlty in the lower crust, due to liquation. The aillikites inside the BZM contain low-temperature clinopyroxenes tracing a steep advective geotherm (0.4–1.5 GPa); they also contain clots, related to intermediate depth magma chambers, together with CIMS pyroxenes and amphiboles. This suggests that the liquation of aillikites was accompanied by density separation and assimilation and fractional crystallization (AFC) fractionation with the participation of crustal material. Trace elements (especially REEs) in silicate minerals, carbonates, apatites, and accessories (perovskites, pyrochlores, monazites, columbites, zircons, ancylites, etc.) show a general rise in REE levels and La/Ybn ratios from aillikites to ijolites, and later to Fe- carbonatites. The presence of zircons, monazites, columbite-tantalites, and other Zr-Hf and Ta-Nb minerals like perovskites and tantalites in the BZM aillikites occurred due to the mixing of the silicate melts with carbonate-rich magmas at deeper levels in the crust and later, in the massif. In the aillikites, xenocrysts. Further, apatites and perovskites show high REE levels. The carbonate-silicate magmas likely passed through a system of polybaric magmatic chambers and liquated carbonatites. Consequently, the later-formed aillikites captured and mixed all varieties of xenocrysts.

High mantle helium flux unveils active mantle melting beneath the cathaysia block, south China

Helium (He) in hydrological systems is used to constrain the deep structure and improve interpretations of geophysical techniques beneath the Cathaysia Block (CB), of which the Cenozoic tectonic and geodynamic processes are controversial. The air-corrected 3He/4He ratio of both geothermal and non-geothermal waters in the CB range from 0.10 to 6.41Ra (Ra is the atmospheric 3He/4He ratio), displaying an increasing trend from inland to coastal area, aligning with thinning crust and younger magmatic activities. Mantle-derived He fluxes in the CB vary from 0.11 to 33.41 × 1010 atoms m−2s−1, surpassing those in stable continental areas by up to three orders of magnitude. Due to the absence of active volcanic surface manifestations and identifiable crustal magma chambers, the mantle-derived volatiles are possibly transported from the mantle through the faults with advective flow rates ranging from 1.26 to 154 mm y−1. Two distinct modes of mantle He releases lead to differences in mantle He fluxes between the interior and coastal CB. Seismic activity enhances permeability in the interior CB, leading to the leakage of mantle He. In the coastal CB, high mantle He fluxes with characteristics of volcanic degassing imply degassing from the partial mantle melting. The presence of high 3He/4He ratios (up to 6.41 Ra) and regional thermal anomaly provide evidence for an ongoing process of crustal underplating by mantle melting. Combined with the underplate layer revealed by geophysical results, this implies the continuous compensation mechanism involving mantle influx to counter extension-induced crustal thinning since the Mesozoic–Cenozoic in the coastal CB.

Forearc magmatism in the Pannonian basin recorded by metasomatised skarnoid xenoliths in Pliocene basalt (Novohrad-Gemer Volcanic Field, Slovakia)

Anorthite-diopside-spinel-forsterite xenoliths occur in basaltic lava flow in northern part of the Pannonian basin in the Novohrad-Gemer Volcanic Field (NGVF). Several lines of geochemical evidence suggest that the typical skarn assemblage originated by multiple carbonatitic and alkaline silicate metasomatism of pristine basalt to picrobasalt protolith rather than by the modification of crustal carbonates around shallow magma chambers. Interaction with mantle-derived magnesite or dolomite carbonatite melt triggered the crystallization of neoformed forsterite at the expense of primary clinopyroxene. The alkaline silicate metasomatism resulted in the replacement of primary diopside and augite by neoformed aegirine and aegirine-augite accompanied by amphiboles (richterite, katophorite, arfvedsonite). Weakly metasomatised Group 1 xenoliths retained their fore-arc magmatic geochemical signatures: flat REE patterns, low concentrations of high-field strength elements (Ti, P, Zr) and Ni compared to MORB, low ratios of highly-to-less incompatible elements (Ti/V, Zr/Y) relative to MORB, low Na2O, low total REE, and supra-subduction affinity of primary clinopyroxenes, being thus analogues to fore-arc basalts (FAB). In contrast, strongly metasomatised Group 2 xenoliths exhibit LREE-enriched patterns sub-parallel with those of host trachybasalts. Some Group 1 xenoliths also exhibit large positive Eu anomalies and strongly depleted signatures diagnostic of low-pressure plagioclase-pyroxene cumulates. One xenolith with complementary negative Eu, Sr and positive Rb anomalies has been identified as high-Mg basaltic andesite melt residuum. Sr and Nd isotopes plot along the OIB-MORB array. The Group 1 converges at initial 87Sr/86Sr and 143Nd/144Nd ratios of 0.7038–0.7041 and 0.5127–0.5128, respectively, thus recording a 140–190 Ma old mantle source. Thermobarometry data on metasomatically unmodified pyroxenes have been arranged along decompression cooling trend, propagating from ∼1230 °C and ∼ 1 GPa and ending at ∼1160 °C around atmospheric pressure. The maximum pressure indicates the melt generation zone ∼35 km deep, reflecting thin lithospheric mantle. The FABs identified for the first time in the AlCaPa megaunit corroborate the affinity of the NGVF lithospheric segment to Dinarides.

Formation of Large to Giant Porphyry Mo Deposits: Constraints From Whole-Rock and In Situ Mineral Geochemistry of Causative Magmatic Rocks in the Tongbai-Hong’an-Dabie Orogens, Central China

Formation of Large to Giant Porphyry Mo Deposits: Constraints From Whole-Rock and In Situ Mineral Geochemistry of Causative Magmatic Rocks in the Tongbai-Hong’an-Dabie Orogens, Central China

Relationship between electrical conductivity, metallogeny, and lithospheric structure in south China

The South China Block (SCB) is located at the junction of the Pacific, Eurasian, and Indo-Australian plates. Their interaction led to large-scale multi-stage mineralization in the SCB during the Mesozoic. Several regional ore-concentration areas, such as the Middle-Lower Yangtze Metallogenic Belt (MLYMB), the Qinzhou-Hangzhou Metallogenic Belt (QHMB), the Wuyishan Metallogenic Belt (WYMB), and the Nanling Metallogenic Belt (NLMB) were formed during this process. However, the dominant mineral types of these metallogenic belts are different. To study the deep mechanisms of the different metallogenic types developed in the same tectonic background at almost the same period, the magnetotelluric sounding (MT) data from 691 sites located mainly within the eastern SCB are employed to obtain a regional lithospheric 3-D resistivity model.It could be concluded from the resistivity model that deep low-resistivity anomalies and mantle upwelling channels mainly controlled almost all the Mesozoic deposits. Large-scale low-resistivity bodies extend from the crust to the upper mantle beneath the MLYMB and the QHMB, interpreted as channels of mantle-derived magma and melts/fluids containing metallogenic elements upwelling. However, the MLYMB and the QHMB have different upper crustal ore-conducting and ore-controlling structures, thus forming porphyry copper and copper-bearing tungsten deposits. Small-scale low-resistivity anomalies are invading the high-resistivity crust in the NLMB. And remelting the upper crust, mixing, and intrusion of crust-source magma, enriching tungsten-tin elements near the NLMB. Larger-scale low-resistivity anomalies exist deep in the WYMB, remelting the lower crust and resulting in the formation of porphyry copper in it. Significantly, the upper-mantle low-resistivity anomalies beneath the eastern SCB show a spatial distribution that is gradually shallowing from south to north, probably indicating that the asthenospheric materials are upwelling from south to north, corresponding with the changing progressively of magma and metallogenic activities. We propose that the lithospheric delamination and asthenospheric upwelling caused by the far-field effects of the Paleo-Pacific Plate subduction are the sources of solid magmatic activities and related metallogeny.

Two Mechanisms Generating Intraplate Volcanism in Southeast Asia: Insights From Mantle Transition Zone Discontinuities

AbstractDetermining the mechanisms responsible for intraplate volcanism ‐ such as slab devolatilization melting versus active mantle plumes ‐ remains a challenge. The greater South China Sea (SCS) region has experienced extensive intraplate Cenozoic volcanism across areas including Hainan, Southeast Indochina, northern Borneo, the northern SCS, and the post‐spreading SCS basin. The prevalence of volcanism distributed widely across this region prompts fundamental questions about the key geodynamic processes driving such diverse magmatic activities. In this study, we elucidate the mantle transition zone (MTZ) discontinuities in this region using SS precursors, which helps to overcome the sparse seismic coverage due to its predominantly oceanic setting. We collected over 16,000 high‐quality seismograms that sample the upper mantle and MTZ beneath this region from global earthquakes and stations. After correcting for the effects of shallow crustal variations and upper mantle heterogeneity on traveltimes of SS phases and their precursors, we unveil lateral variations in the MTZ boundaries (d410 and d660) and intricate features of the mid‐MTZ reflectors (S520S). Significant MTZ thinning and normal S520S waveforms beneath Hainan provide compelling evidence for mantle upwelling through the MTZ. Conversely, the evident splitting of S520S beneath the northern SCS, Southeast Indochina, and northern Borneo, all characterized by stagnant subducted slabs, indicates that the volcanism in these regions likely originated from a mechanism distinct from the active upwelling beneath Hainan. Dehydration melting attributed to devolatilizing stagnant slabs in the MTZ is a potential cause for Cenozoic volcanism in these regions.

Cognate Mafic Enclaves Formed by Cumulated Mush Convection in a Granitic Magma Chamber: Evidence from Mineral Chemistry of the Triassic Zhashui Pluton, Qinling Orogen

AbstractMafic enclaves in granites are generally considered to represent coeval mafic melts that derived from metasomatized mantle, which can provide valuable information about crust–mantle interaction. Exploring the genetic links between the mafic enclaves and their host monzogranite from the Triassic Zhashui Pluton, Qinling orogenic belt. The enclaves (220 ± 4.6 Ma) and the monzogranite (220 ± 2.8 Ma) display identical zircon U‐Pb ages, and they also share similar trace element and zircon Lu‐Hf isotopes, indicating a cognate source. The monzogranite displays zircon εHf(t) values of –0.99 to +1.98, while the mafic enclaves show similar values of –0.45 to +3.26; however, differences in mineral chemistry reveal different crystallization conditions. The amphibole from the mafic enclaves has higher temperature and pressure (757°C; 2.65 kbar) compared to those of the host monzogranite (733°C; 1.96 kbar), suggesting that mafic minerals in the enclaves crystallized at an early stage. Moreover, apatite in the mafic enclaves displays slightly higher volatile contents (0.72 wt%) than those of the monzogranite (0.66 wt%), indicating a volatile‐rich condition. These results suggest that the mafic enclaves represent early hydrous mafic cumulates in the granitic chamber, and subsequent magma convection would have led to the formation of the mafic enclaves.

U-Pb dating of Barra Velha carbonates reveals the influence of Santos Basin sedimentary and tectonothermal history on pre-salt carbonate ages

The depositional age and diagenetic history of the Upper Barra Velha carbonates are poorly understood. To clarify the geochronology of these events and the behavior of the U-Pb isotopic system in carbonates, we conducted an integrated petrographic, QEMSCAN imaging, and in situ U-Pb dating study of calcites and dolomites from the Barra Velha Formation, in the central Santos Basin (SE Brazil). Our results span three geochronological intervals, which are ∼125 Ma, ∼116-114 Ma, and ∼105-100 Ma. The U-Pb isotopic system of calcite allochems was disturbed, yielding ages of ∼115 Ma and ∼105-100 Ma. These ages coincide with magmatic and hydrothermal events in the pre-salt. Dolomites showed that geochronological disturbance of carbonate components may not be pervasive but rather related to fluid-rock interactions and properties of rocks and their constituents such as mineralogy, porosity and permeability. This is evidenced by the U-Pb system of non-porous, non-permeable, and less reactive dolomitic aggregates remaining undisturbed. Eodiagenetic microcrystalline and fine dolomite rhombs provided evidence of Barremian ages, challenging the belief that the Upper Barra Velha deposits are Aptian. Other eodiagenetic dolomites occur as ∼117 Ma eodiagenetic lamellar dolomites, as pseudomorphic dolomites that underwent U-Pb alteration at ∼100 Ma, and as ∼112 Ma pervasive rhombohedral dolomites associated with Mg-rich fluids percolation in the Upper Barra Velha carbonates. Blocky dolomite and saddle dolomite cements have ages of ∼100 Ma, with saddle dolomite constraining the latest silicification event as > 100 Ma. This research elucidates significant geological events, processes, and factors (including magmatic activity, hydrothermal alteration, fluid-rock interactions, and petrographic and mineralogical controls) that influenced the ages of the Upper Barra Velha carbonates. Moreover, it aids in reconstructing the depositional and diagenetic history of the Barra Velha Formation and enhances the understanding of the Santos Basin tectonothermal evolution.

Evidence for, and significance of, the Neoproterozoic Xuefeng Orogeny, South China

The recognition of unconformities is important for determining the long-term evolution of orogenic belts. We document an unconformity between Tonian and Cryogenian strata of the Nanhua Basin of South China. This stratigraphic break has previously been termed the Xuefeng Movement and is developed within the Yangtze Block and western Cathaysia Block. Deformation characteristics and detrital zircon U-Pb geochronology from rock units underlying and overlying the unconformity (i.e., the Niuguping and Gucheng formations, respectively) from the Anhua region of the northern Hunan Province in the southeastern Yangtze Block are documented. The Niuguping Formation displays a set of top-to-northwest imbricate thrust faults and related fault-propagation folds, NEE-plunging subparallel crenulation lineations, a foliation, and shows widespread greenschist facies metamorphism. In contrast, the overlying Gucheng Formation exhibits no obvious stratal deformation (apart from a mild inclination of bedding) and is unmetamorphosed. Detrital zircon age spectra of the samples from the Niuguping Formation are characterized by unimodal age peaks of ca. 750–730 Ma, while those of the samples from the Gucheng Formation yield a multimodal age distribution, with a few syn-sedimentary detrital zircons (<700 Ma), and much older zircons with age peaks of ca. 800 Ma, 2.1–1.9 Ga, and 2.6–2.3 Ga. Combining this data with previous geochronology indicates deposition of the Niuguping and Gucheng formations in the late Tonian (725–720 Ma) and Cryogenian (690–660 Ma), respectively. The time gap across the unconformity is some 30 Ma (720–690 Ma). The unconformity and associated structural and metamorphic break extend across the Yangtze Block and western Cathaysia Block, defining a major regional orogenic event. The significantly different detrital zircon spectra of strata across the unconformity suggest a dramatic change in the source due to the Xuefeng Orogeny in the Yangtze Block. Zircon U-Pb-Hf isotopic evidence indicates that detritus of the Niuguping Formation below the unconformity was derived from syn-depositional magmatic activity in the northern Yangtze Block, while a significant increase of Archean to Paleoproterozoic ages for the Gucheng Formation, suggests that the Xuefeng Orogeny resulted in crustal thickening and denudation in the northern Yangtze Block. These changes in provenance, along with changing deformational and metamorphic patterns, indicate evolution of the Nanhua Basin from a compressional to an extensional back-arc basin, which we infer is related to tectonic switching from an advancing to a retreating subduction system in the northern Yangtze Block at the time of the unconformity.

Neoproterozoic tectonic shift along the western margin of the Yangtze block, South China craton: Implications for its paleogeographic position during the Rodinia−Gondwana transition

Neoproterozoic igneous rocks in the Panxi region from the western margin of the Yangtze block, South China craton, record major changes in tectonic evolution and crustal thickness, and provide important constraints on the paleogeographic position and climate changes of the block in Rodinia and Gondwana reconstructions. Our new dating results show that the Pengguan complex from the Panxi region comprises ca. 743 Ma adakitic granodiorites and ca. 741 Ma A-type granites. The former is plagioclase-rich and has relatively low SiO2 content (65.22−67.36 wt%), high Mg# values (44.40−52.33), Sr/Y (23.22−61.94) and (La/Yb)N ratios (normalized to chondritic values; 13.41−24.00), with positive whole-rock εNd(t) values (+1.17 to +5.15), and high zircon εHf(t) (+5.53 to +8.83) and low zircon δ18O (4.21‰ to 5.51‰) values, indicating derivation from the partial melting of thickened juvenile mafic lower crust. By contrast, the granites are rich in K-feldspar and have high SiO2 contents (78.71−84.62 wt%), 10,000 × Ga/Al ratios (&amp;gt;2.6), and Fe/Mg ratios, with negative εNd(t) values (−1.47 to −0.02), high zircon εHf(t) (+2.94 to +9.77), and high saturation temperatures (828−920 °C). These features indicate fractional crystallization and partial melting of continental crust in a low-pressure, high-temperature, extensional environment. Integrated with the zircon Eu anomalies (Eu/Eu*) data and Hf-O isotopic data on pre-740 Ma magmatism in the Panxi region, we find that a rapid increase of crustal thickening (820−740 Ma) was synchronous with a gradual decrease of zircon δ18O and an increase of zircon εHf(t) values. These variations indicate an increase in the degree of hybridization between mantle sources and subducting slab-derived materials, thus generating a great number of low-δ18O melts, and continuous crustal thickening through the underplating of juvenile magmas. In addition, thinning of the crust since ca. 740 Ma, along with the gradual increase in zircon δ18O and εHf(t), suggests a shift from regional compression to extension. This documented shift, together with regional data from along the western margin of the Yangtze block, indicates a change from the well-developed and long-established Panxi convergent plate margin arc system (at least ca. 870−750 Ma) to passive-margin development and Cryogenian (720−635 Ma) glaciation. The long-lived subduction zone requires a peripheral location of South China that was linked to northern India during the Rodinia breakup. The subsequent and contemporaneous passive-margin development of the Yangtze block and India, along with the similar tectonic affinities of the Cathaysia block with India, indicate that South China lay outboard of the northern Gondwana margin during the assembly of this supercontinent.

Provenance of cretaceous sediments in the West Kunlun piedmont belt and implications for tectonic evolutionary events

The West Kunlun orogenic belt located in the northwestern margin of the Qinghai-Tibet Plateau is an important record of the formation and northward extension of the plateau, but the current research mainly focuses on the tectonic activities of the Cenozoic era, and there is still considerable controversy regarding the formation and evolutionary history of pre-Cenozoic orogenic belts. This study focuses on Cretaceous sandstone samples from the Kedong region in the piedmont belt of the West Kunlun orogenic belt. U-Pb geochronological analysis was performed on 200 detrital zircon grains from the core samples. Combined with stratigraphic data and previous research, the main provenance direction was investigated to constrain the tectonic evolutionary history of the orogenic belt’s peripheral regions. The results show that the detrital zircons are aged from 290 to 208 Ma, 520–310 Ma, 810–580 Ma, 1,400–880 Ma and 2,548–1,730 Ma, reflecting the complexity of provenance in this area. Based on a comprehensive analysis of the characteristics of igneous rocks, zircon age composition and stratigraphic conditions in potential source areas, it is concluded that the primary source regions include the East Kunlun orogenic belt and the North and South Kunlun terranes, with a low likelihood of contributions from within the Tarim Basin. The evolution of the West Kunlun orogenic belt can generally be divided into two opening and two closing phases. The detrital zircon ages predominantly exhibit two peak values at 259 Ma and 459 Ma, respectively representing the ages of transition from oceanic crust subduction to continent-continent collision for the Paleo-Tethys Ocean and the Proto-Tethys Ocean. Additionally, there is a temporal gap between the evolution of the Proto-Tethys Ocean and the Paleo-Tethys Ocean. The Triassic period marks a transitional phase in tectonic evolution, shifting into an intracontinental evolutionary stage. This study provides new geochronological evidence for the early developmental history of the West Kunlun orogenic belt.

Lower crustal copper enrichment and mobilization indicated by Late Permian amphibole gabbros and granodiorites in the eastern Kunlun Orogen, northern Tibet

Lower crustal fractional crystallization accompanied by sulfide saturation in thick continental arcs has been viewed as an essential step in forming porphyry Cu systems. These processes are supposed to lock Cu in the lower crust and, therefore, can be unfavorable to the generation of ore-forming magmas. In this study, we present in situ mineral major-trace elements, pyrrhotite S isotopes, and zircon UPb ages, as well as whole-rock major-trace elements and SrNd isotopes for amphibole gabbros and contemporary granodiorites in the eastern Kunlun Orogen to elucidate Cu variations during the transcrustal magmatic evolution of these rocks. The amphibole gabbros can be divided into two types based on field relationships with the granodiorites. These rocks were derived from an oxidized metasomatic mantle in a steep subduction setting at ∼250 Ma and experienced a transcrustal magmatic evolution. Orthopyroxene relicts, resorbed clinopyroxene, calcic plagioclase, high-Al2O3 amphibole, and magnetite in the amphibole gabbros of the first type are lower crustal cumulate minerals. Pyrrhotite with minor amounts of chalcopyrite, interstitial to cumulate amphibole or as inclusions in magnetite from the amphibole gabbros of the first type, is lower crustal Ni-rich monosulfide solid solution (mss) crystallized from immiscible sulfide melts with residual Cu-rich sulfide liquids. These lines of evidence together with field, mineralogical, and geochemical data indicate that the Cu-rich parental magmas experienced lower crustal MASH (melting, assimilation, storage, and homogenization) processes and became Cu-depleted. The evolved magmas after lower crustal evolution became moderately oxidized and Cu-normal according to amphibole, biotite, zircon, and whole-rock compositions. The moderately oxidized magmas scavenged and mobilized Cu from Cu-rich sulfide liquids to the upper crustal magma chambers where degassing dispersed the Cu and thus was unfavorable to porphyry Cu mineralization.

Early-stage intracontinental rifting in the Neoproterozoic Centralian Superbasin: Systematic U–Pb and Lu–Hf isotopes in detrital and inherited zircons from the Yeneena Basin, northwest Australia

Detrital zircons from different lithostratigraphic units of Supersequence 1 across the Neoproterozoic northwest Officer and Yeneena basins were investigated using LA-ICPMS U−Pb and Lu − Hf isotope analyses to understand the source of detritus and potential links to regional tectono-magmatic events, as well as the crustal evolution of the sedimentary sources terranes. All data show that Proterozoic detritus dominate the age spectra of samples from the Yeneena Basin and northwest Officer Basin, with signatures varying in the heights of peaks rather than the presence or absence of age groups in different formations. The Archean and Proterozoic ages overlap with known magmatic events in the Pilbara Craton, Fortescue Basin, Rudall Province, Capricorn Orogen, Musgrave Province, Madura Province, and the Coompana Province suggesting these terranes are possible sources of the detritus. The detrital zircon population of the samples have main age peaks in the range 1300 to 1000 Ma, suggesting magmatic rocks of the Rudall and Musgrave provinces are the main sources of detrital zircons, which can be differentiated by Hf signature. The distal Musgrave Province persistently dominates the provenance of detritus in the northwestern Officer and Yeneena basins, while detritus from the proximal Rudall Province contributes to a lesser extent. This suggests that the distal Musgrave Province remained emergent during the evolution of the northwestern Officer Basin and the Yeneena Basin, whereas the proximal Rudall Province potentially became buried by the accumulating sedimentary rocks and contributed less detrital zircons. The 833.6 ± 1.4 Ma maximum depositional age obtained in this study for the sedimentary rocks of the northwestern Officer and Yeneena basins point to the approximate time of the start of basins development. The extensional event in this age period is related to emplacement and extrusion of the ca. 830 Ma Willouran Large Igneous Province (LIP). The Duke gabbro sills observed in the Yeneena Basin are part of this LIP, which extends from the Adelaide Rift Complex, through the Musgrave Province, central–northwest Officer Basin and into the Yeneena Basin. The sediment source terranes suggest a southeast–northwest oriented depositional system between central and Western Australia, possibly because of intracontinental rifting related to widespread mafic–ultramafic magmatic events that could include the earlier Ngaanyatjarra Rift formed by the 1090–1040 Ma Warakurna LIP as well as the ca. 830 Ma Willouran LIP, synchronous with the breakup of Rodinia in the mid-Neoproterozoic.

Felsic xenoliths in Triassic quartz-monzodiorite dykes from the central Jebilet (Morocco): Detailed petrography and geochemistry, with a critical assessment of the so-called “granulitic” samples

Felsic xenoliths occur locally in the Early Triassic quartz-monzodiorite dykes that crosscut the Sarhlef basin in Central Jebilet, Morocco. Irrespective of the secondary features linked to entrapment, three types of xenoliths are distinguished on the basis of textural evidence: 1) isotropic rocks of broad granitic appearance; 2) foliated samples whose planar fabric undoubtedly corresponds to a ghost foliation, 3) a specific xenolith with a blastomylonitic structure.Geochemically, samples with Na > K and moderately negative εNd values (-1.5 to - 3.8), reflecting a hybrid origin, depart from samples with K > Na and strongly negative εNd values (-9.1 to - 11.7), pointing to a stronger or even pure crustal derivation. The blastomylonitic sample differs drastically from the rest by its plagiogranitic composition and elevated Nb content, associated with a highly radiogenic Nd isotope signature (εNd240 = + 6.3). Altogether, these features and available radiometric data suggest a connection of the xenoliths with the Variscan igneous bodies that crop out in the central Jebilet. The potassic xenoliths with unradiogenic Nd are broadly similar to the granodiorite plutons. The xenoliths with Na > K and mildly negative εNd and the blastomylonitic plagiogranite are reminiscent of the bimodal association of microgranites and gabbros of the Sarhlef basin.From a structural point of view, samples with a gneissic fabric may reflect the possible occurrence of a metamorphic, migmatitic complex at somewhat deeper crustal level. However, based on detailed mineralogical and geochemical data, the samples previously known as “mid-crustal granulites” turn out to be highly differentiated, aplo-pegmatitic rocks, interpreted as early, incipient partial melts. The presence of high-grade metamorphic rocks at depth is only documented by one small, HT garnet-biotite xenolith, while peraluminous lenses (fibrolite + spinel ± corundum) dispersed in the quartz-monzodiorite dykes suggest that a metapelitic component was involved in the melting process generating these lavas. In summary, this set of felsic xenoliths does not provide much information on the crustal composition under the Jebilet, apart from fairly superficial levels.

Age and Petrogenesis of Paleoproterozoic Diorites from Dikes of the Baikal Uplift of the Siberian Craton

Abstract ––We carried out a detailed geological, geochronological, geochemical, and isotope study of diorites from a dike located in the central part of the Baikal uplift of the Siberian craton. The geochemical and isotope data obtained for diorites of the studied intrusion were compared with coeval mafic and intermediate igneous rocks of the southern part of the Siberian craton. The U–Pb (ID-TIMS) baddeleyite age of 1862 ± 7 Ma has been estimated for diorite from a dike located in the area of the Onguren Village. The obtained data are the first reliable age determination for the Paleoproterozoic mafic and intermediate igneous rocks of the Baikal uplift, which are part of the South Siberian postcollisional magmatic belt. The dike is of NE strike. The dike rocks correspond in chemical composition to diorites and are highly differentiated varieties (mg# = 36.5–37.4). There are no significant variations in the composition of diorites in the marginal and central parts of the dike. The rocks are characterized by low contents of TiO2, P2O5, and Nb, high contents of Th, Zr, Ba, and LREE, and low negative values of εNd(T) (–5.9 to –6.2). We assume that the diorites formed from the enriched subcontinental lithospheric mantle. The obtained geochemical and isotope data show similar negative values of εNd(T) from –4.3 to –11.6 for most of Paleoproterozoic mafic and intermediate igneous rocks of the South Siberian postcollisional magmatic belt within the Aldan Shield, the Baikal uplift, and the Irkut block of the Sharyzhalgai uplift. These rocks correspond in geochemical features to rocks resulted from the melting of subduction-modified lithospheric mantle sources in the postcollisional extension setting at the final Paleoproterozoic stage of formation of the Siberian craton.

Mapping paleoelevations along active continental margins with igneous geochemistry: A case study from South America

Mountains and mountain ranges, often situated at convergent plate margins, play a pivotal role in many fields of the Earth, climate, and biological sciences. Reconstructing past episodes of mountain building from the geological rock record is one of the main challenges for unravelling the ancient physical geography of Earth’s surface. Established methods for quantifying past elevations traditionally relied on sedimentary rocks, but in recent years, alternative approaches have emerged on the basis that geochemical signatures of magmatic rocks formed in convergent settings correlate with crustal thickness or elevation. Consequently, methods based on igneous samples have the potential to allow quantitative mapping of past topographic change for large spans of space and time where existing maps are largely based on a qualitative approach. Here, we investigate the application of paleoelevation estimates derived from geochemistry using the western margin of South America as a case study. We investigate their consistency with independent indicators of past elevations such as stratigraphy, stable isotopes, fossils etc. for Cenozoic samples along the Andean margin. For older times, we compare the estimated paleoelevations with more general aspects of the geological record, as well as paleoelevations from global paleogeography models widely used in climate modelling studies, to evaluate the extent to which these models are consistent with the igneous geochemical proxies. We find that estimates based on multiple geochemical proxies, and which account for variations in elevation as a function of MgO, are preferable to those based on individual proxies. The multi-mohometer estimates yield estimates of elevation that better match both present-day topography and Cenozoic paleoelevations, and retain more samples during the data filtering stage. In deeper time, we show that igneous geochemistry quantifies changes in elevation related to documented phases of crustal thickening and thinning, and is thus likely to allow improvements to existing maps of paleotopography.

Plagioclase Megacrysts in Mesoproterozoic Amphibolites from the New Jersey Highlands, USA: Indicators of Mixed-Source Magma and Fractionation Interruption in Anorthosite-Dominated Terrains

Rare amphibolite in the New Jersey Highlands containing plagioclase megacrysts ≤13 cm long forms bodies 0.5 to 2 m thick that preserve a penetrative metamorphic fabric and have sharp, conformable contacts against Mesoproterozoic country rocks. The megacrystic amphibolites were emplaced as thin dikes along extensional faults between 1160 and 1130 Ma. Amphibolites contain weakly zoned, subhedral andesine megacrysts (An29–44) in a matrix of plagioclase (An18–38), magnesio-hastingsite, biotite, diopside, Fe-Ti oxides, and apatite. The whole-rock major oxide composition of the megacrystic amphibolite matrix has high TiO2 (2.07 wt.% ± 2.0%), Al2O3 (17.03 wt.% ± 0.87%), and Fe2O3t (12.84 wt.% ± 3.2%) that represents an Al-Fe-rich mafic magma type that is characteristic of anorthosite associations globally. The whole-rock, rare earth element (REE) composition of the megacrystic amphibolite matrix is characterized by enrichments in light rare earth elements (LREEs) (La/YbN = 1.73–10.69) relative to middle (MREEs) and heavy (HREEs) rare earth elements (Gd/YbN = 1.30–1.85), and most samples have small positive Eu anomalies (Eu/Eu* = 0.95–1.25). The megacrystic amphibolite matrix is also enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) (e.g., Ba/Nb = 24–22). Megacrystic amphibolites formed through partial melting of subduction-modified lithospheric mantle that fractionated olivine and plagioclase, producing a high-Al-Fe mafic magma. Plagioclase megacrysts formed through extraction of a plagioclase-rich crystal-liquid mush from anorthosite that mixed with mafic magma and collected in the upper part of the mid-crustal magma (depth of ~20 km based on Al-in-hornblende geobarometry) chamber through flotation. Periodic tapping of this mixed source by extensional fractures led to emplacement of the amphibolites as dikes and may have interrupted the extensive fractionation and plagioclase separation necessary to form voluminous anorthosite intrusions.