Magmatic Arc Research Articles

Petrogenetic evolution of the Jalapa del Marqués pluton: Miocene arc magmatism in southern Mexico and its tectonic implications

Arc-related plutonic rocks in southern Mexico occur along the Pacific coast in the Sierra Madre del Sur (SMS) igneous province. These rocks record the migration of the Caribbean–North America–Farallon/Cocos junction towards the east between the Late Cretaceous and Miocene. However, little is known about the petrogenesis of igneous rocks that were emplaced during the latest expressions of magmatism in the SMS (i.e., early–middle Miocene). Based on fieldwork, petrography, whole-rock and mineral geochemistry, thermobarometric calculations, and zircon UPb dating, we document the petrogenetic evolution of the Jalapa del Marqués pluton and adjoining intrusions, which are the youngest so far dated in the SMS and are located ∼150 km inland from the present-day trench. This subduction-related magmatic complex consists of granites, granodiorites, diorites, dacite porphyries, and intermediate-to-felsic dikes, whose geochemical features suggest early evolutionary processes driven by fractionation of amphibole ± clinopyroxene (moderate–high pressures) or plagioclase (low pressures). Pulses of evolved magmas were episodically injected into upper-crustal reservoirs (7–14 km) over ∼9 Myr, starting at 22.14 ± 0.65 Ma (2σ)—earlier than previously thought (∼15.7 Ma)—and producing subsequent pulses at 19.45 ± 0.36 Ma, from 16.36 ± 0.37 to 15.8 ± 0.34 Ma, and from 13.9 ± 0.5 to 13.3 ± 0.2 Ma. These ages correlate with the onset and early stages of volcanic centers in the Trans-Mexican Volcanic Belt (TMVB), some of which were located up to 500 km landward from the present-day trench. The large difference in the distance-to-trench between coeval magmatism in the Jalapa del Marqués pluton and the TMVB, indicates a variation in the subduction angle between two segments of the Farallon/Cocos Plate that were likely separated by a slab-tear. The eastward migration of this slab-tear (along with the triple junction) terminated the magmatic activity in the SMS by switching the locus of magmatism to the TMVB. This probably occurred shortly after the studied magmatic complex was assembled.

Slab melting boosts the mantle wedge contribution to Li-rich magmas

The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration. This subduction-related metasomatism was poorly effective on enhancing the Li inventory of the sub-arc lithospheric mantle, underpinning the inefficiency of slab-derived fluids on mobilizing Li through the mantle wedge. However, major and trace element compositions of mantle minerals in other xenoliths also record transient thermal and chemical anomalies associated with the percolation of slab window-related magmas, which exhibit an “adakite”-type geochemical fingerprint inherited by slab-derived melts produced during ridge subduction and slab window opening event. As these melts percolated through the shallow (7.2–16.8 kbar) and hot (952–1054 °C) lithospheric mantle wedge, they promoted the crystallization of metasomatic clinopyroxene having exceptionally high Li abundances (6–15 ppm). Numerical modeling shows that low degrees (< 10%) of partial melting of this Li-rich and fertile sub-arc lithospheric mantle generates primitive melts having two-fold Li enrichment (~13 ppm) compared with average subduction-zone basalts. Prolonged fractional crystallization of these melts produces extremely Li-enriched silicic rocks, which may stoke the Li inventory of mineralizing fluids in the shallow crust.

Constraints on the subduction and closure of the Hegenshan Ocean: Magmatic and sedimentary records from central Inner Mongolia, China

Petrological, geochronological, and geochemical data from the volcano-sedimentary sequences, granitoids, and ophiolite relics of central Inner Mongolia, China, were used to reconstruct the subduction and final closure of the Hegenshan Ocean. Geochronological dating and compilation reveal four phases (ca. 360−355 Ma, 348−320 Ma, 320−310 Ma, and 310−275 Ma) of magmatism in the Uliastai continental margin. The ca. 356 Ma I-type Halatumiao granodiorite and Amanwusu ophiolite relics are subduction-related, and the Halatumiao granodiorite provides solid evidence of the northward subduction of the Hegenshan Ocean beneath the Uliastai continental margin at ca. 360−355 Ma. The ca. 348−320 Ma and 320−310 Ma volcanic rocks and granitoids constitute two linear magmatic belts roughly parallel to the Erenhot-Hegenshan ophiolite belt, which record two phases of continental arc magmatism in the Uliastai continental margin. Overall, the ca. 360−310 Ma arc magmatism shows landward migration and then oceanward migration in the Uliastai continental margin, which indicates advancing subduction and subsequent slab steepening of the Hegenshan Ocean. By contrast, the ca. 310−275 Ma magmatic rocks are dominated by I- and A-type felsic volcanic rocks, granites, and dikes, which are post-accretionary, extension-related, and pervasive in the Uliastai continental margin and Erenhot-Hegenshan ophiolite belt. A provenance shift was identified between the Benbatu and Amushan formations of the Amanwusu area of the Erenhot-Hegenshan ophiolite belt. The early detritus was derived from the early Paleozoic rocks in the Sonid Zuoqi arc belt, whereas the late detritus originated from the Early Carboniferous ophiolite relics in the Erenhot-Hegenshan ophiolite belt. The provenance shift and emplacement of pervasive extension-related magmatic rocks imply a Late Carboniferous closure of the Hegenshan Ocean. The Late Carboniferous oceanic closure event in the north of the southeast Central Asian Orogenic Belt is also evidenced by the transition of Hf isotopic composition of zircons dated between ca. 360−310 Ma and 310−275 Ma.

Evolution of Devonian and Carboniferous pre-Andean arc magmatism in the Frontal Cordillera (27°-35°S), Argentina: Insights from U–Pb zircon and isotopic studies

We propose a conceptual model for the magmatic system that gave rise to the Devonian and Carboniferous subduction-related magmatism in the Frontal Cordillera of Argentina, (representing the pre-Andean margin of SW Gondwana), integrating previous geochronological studies with new geochronological data from granitoids and subvolcanic dikes, which include reported petrological, geochemical, and isotopic data. This conceptual magmatic system postulates an extended magmatic activity, similar to that reported in a previous study for the Devonian foreland magmatism located in the present-day Sierras Pampeanas of Argentina. The geochronological data play a central role in the model, and lead us to postulate the presence of a deep mush reservoir, where a prolonged magmatic activity, permitted the extended crystallization of Devonian (ca. 400 ± 3 and 414 ± 3 Ma) and Carboniferous antecrysts (334 ± 2 Ma, 341 ± 2, and 348 ± 2, where the age of 348 Ma maybe considered as derived from the source, i.e. xenocrysts). Migration of the parental magma from the mush reservoir zone occurred near the time of emplacement and culminated in the formation of an ephemeral magma chamber located in shallow levels, where Devonian and Carboniferous zircon autocrysts crystallized (382 ± 5 Ma and 325 ± 2 Ma, respectively). Age spectra reported within individual sample, favors the idea of a massive migration of magma when conditions were favorable (e.g., thermally matured crust). Notably, the Carboniferous geochronological data is strongly consistent with previous geochronological data recently reported for the granitic rocks of the Tabaquito batholith. Although this model supports a protracted magmatic system for the Devonian and Carboniferous subduction-related magmatism, previous and new isotopic data reveal a significant difference in the petrogenetic processes. While the Carboniferous arc magmatism shows significant juvenile material contribution, this is not the case for the Devonian magmatic arc, where isotopic data suggest an older continental lithosphere as the dominant source. These different sources are attributed to two contrasting geodynamic settings, with advancing and retreating oceanic slabs, respectively.

Geology and Ore Mineralization Characteristics of the Selodong Prospect, Lombok Island, Indonesia

Abstract The Selodong prospect is located in the southwest sector of Lombok Island which tectonically lies within the W-E trending Sunda-Banda magmatic arc, which hosts various gold-base metal mineralizations including porphyry and epithermal deposits. Lombok Island, particularly southwestern portion of the island is known to hosts gold, silver and copper prospects, including Selodong prospect. The Selodong prospect is divided into several sub-prospect areas which include the Blongas, Montong Botek, Kekalik and Belikat. However, this study only focuses on the Blongas sub-prospect. The study is aimed to discusses the geology, characteristics of alteration and mineralization of the sub-prospect. To achieve the objective, some methods such as mapping and numerous laboratory works were conducted. Petrography to identify alteration minerals, ore microscopy to identify type of ore minerals, and fluid inclusion microthermometry to characterize ore fluid were performed. The geological conditions of the study area are composed of andesite lava, andesite breccia, diorite intrusion and alluvial deposit. Typical porphyry alteration type i.e., potassic is present. Argillic, silicic, advanced argillic, propylitic and phyllic alterations are also identified. The distribution of potassic and some argillic alteration is controlled by lithology, while silicic alteration, advanced argillic, propylitic and some argillic alterations are controlled by geological structures with an NW-SE and NE-SW trending patterns. Propylitic alteration is characterized by chlorite + epidote + calcite ± clay. Argillic alteration is marked by the presence of clay minerals such as clay + quartz. Advanced argillic alteration is typified by quartz + alunite + pyrolusite ± paragonite. Phyllic alteration is typified by sericite + quartz ± clay. Potassic alteration is marked by secondary biotite + quartz ± clay. Silicic alteration is marked by quartz + pyrolusite + epidote. Ore mineralization in the study area is associated with massive and stockwork vein textures. Ore and metalliferous minerals are found as magnetite, chalcopyrite, pyrite, hematite, goethite, jarosite, sphalerite, pyrrhotite, chalcocite and native gold. Generally, homogenization temperatures in the research area range from 543 - &gt;600 °C, with salinity of 26.3 - 73.95 wt.% NaCl eq associated with potassic alteration, whereas homogenization temperatures range from 251 to 349 °C, with salinity of 5.46 - 8.38 wt.% NaCl eq. associated with silicic alteration. Based on those characteristics, the prospect in the study area indicates a porphyry deposit type, which is transition to high sulfidation epithermal.

Mineralogical and ore-fluid characteristics of the Onto Porphyry-HSE transition in Hu’u district, West Nusa Tenggara, Indonesia

Abstract Onto prospect is located in the eastern part of Sumbawa Island in West Nusa Tenggara Province, Indonesia. The island is situated within the Sunda-Banda magmatic arc, which extends from west to east, consists of volcanic rocks from the early Miocene to the Holocene. Onto is one of the highly potential prospects and holds significant economic value among ore deposits in Indonesia. The aim of this research is to explain the geological setting and determine the genesis of mineralization based on petrographic, ore microscopic, and fluid inclusion analyses. Eight lithological units have been identified, namely early andesites, polymictic diatreme breccia, upper sedimentary package, caping andesite, quartz-phyric andesite sill, early porphyry, early intra-mineral porphyry, and late intra-mineral porphyry. Nine samples were analyzed for petrography to identify the alteration types consisting of illite-smectite, quartz-dickite, residual vuggy quartz, silicic, quartz-alunite, quartz-pyrophyllite±diaspore, potassic, phyllic-chloritic, and propylitic alterations. Additionally, pyrite, chalcopyrite, and covellite are predominantly present in the study area. Meanwhile, native gold is found as an inclusion in covellite and pyrite. From micro thermometric analysis of fluid inclusions in quartz veins associated with multiphase intrusive rocks, it is shown that the homogenization temperature of early porphyry ranges on average from 279°C to 377°C, early inter-mineral porphyry ranges from 263°C to 361°C, and late intra-mineral porphyry ranges from 275°C to 336°C. The salinity level of the measurements ranges from 5.71 to 10.27% by weight NaCl equivalent, with an average of approximately 8.15% by weight NaCl equivalent. The mineralization formation during the early or prograde phase is associated with the EDM vein system with a mineralization set of biotite-magnetite, digenite, chalcocite, and coevellite, type A veins with quartz-sulfides + anhydrite, M veins type associated with quartz-magnetite-chalcopyrite-bornite mineralization, AB veins type associated with quartz-sulfides + anhydrite textures, and B veins type with sulfide centerline characteristics. The mineralization system during the early stage is generally associated with potassic alteration (K-feldspar-magnetite-secondary biotite). Whereas the retrograde phase, the vein system is associated C veins type with sulfide mineralization groups consisting of massive chalcopyrite + quartz with phyllic alteration (quartz-muscovite-chlorite ± dickite), D veins type with quartz-pyrite+chalcopyrite associated with advanced argillic alteration (quartz-alunite-pyrophyllite ± diasporite), E veins representing the epithermal stage associated with minerals such as quartz-carbonates, galena, sphalerite, chalcopyrite, tetrahedrite-tennatite in the alteration stage of kaolinite-illite-smektite-sericite.

Mineralogy and Hydrothermal Fluids Characteristics of the Mekarbakti Low Sulfidation Epithermal Gold Prospect at Garut Regency, West Java, Indonesia

Abstract Mekarbakti prospect is situated in the western sector of Java Island which tectonically lies within the W-E trending Sunda-Banda magmatic arc. This arc hosts numerous prospects for ore mineralization with various types of mineral deposits, one of which is epithermal that found in the study area. The Mekarbakti prospect is located close to the Arinem prospect, about 8 km to the east, which is confirmed as a low sulfidation epithermal deposit type. Arinem is proven to be economical to mine, thus it becomes interesting to conduct study on the Mekarbakti prospect, to elucidate its mineralization model. This paper is dealing with the characteristics of hydrothermal alteration using petrographic and X-Ray Diffraction (XRD) to identify the alteration minerals, using ore microscopy analysis to identify the type of ore minerals, and fluid inclusion to reveal the hydrothermal fluid characteristics. The geological condition of the study area is composed of andesite lava, andesite breccia, tuff, lapilli, and andesite intrusion. The distribution of silicification alteration is controlled by geological structures with an NW-SE pattern, while argillic and propylitic alterations tend to have a gradient trend towards silicification alteration. Propylitic alteration is characterized by chlorite+illite+smectite±epidote. Argilic alteration is marked by the presence of clay minerals such as kaolinite+illite+smectite+quartz. Silicification alteration typified by quartz+kaolinite+diaspore+illite. Ore mineralization in the study area is associated with massive, brecciated, comb, cockade and, stockwork vein textures. Ore and metalliferous minerals are found as pyrite, hematite, goethite, chalcopyrite, sphalerite, galena, pyrrhotite, covellite, chalcocite, acanthite, native gold, and native silver. Generally, the mean homogenization temperature in the research area ranged from 248.05 – 266.12 °C, with salinity ranged from 1.01 – 1.68 wt.% NaCl eq. The epithermal deposit in the study area is classified as deep low sulfidation epithermal deposit specifically in transition between base metal and precious metal interval.

Machine-learning oxybarometer developed using zircon trace-element chemistry and its applications

Abstract Magmatic oxygen fugacity (fO2) is a fundamental property to understanding the long-term evolution of the Earth’s atmosphere and the formation of magmatic-hydrothermal mineral deposits. Classically, the magmatic fO2 is estimated using mineral chemistry, such as Fe-Ti oxides, zircon, and hornblende. These methods, however, are only valid within certain limits and/or require a significant amount of a priori knowledge. In this contribution, a new oxybarometer, constructed by data-driven machine learning algorithms using trace elements in zircon and their corresponding independent fO2 constraints, is provided. Seven different algorithms are initially trained and then validated on a data set that was never utilized in the training processes. Results suggest that the oxybarometer constructed by the extremely randomized trees model has the best performance, with the largest R2 value (0.91 ± 0.01), smallest RMSE (0.45 ± 0.03), and low propagated analytical error (~0.10 log units). Feature importance analysis demonstrates that U, Ti, Th, Ce, and Eu in zircon are the key trace elements that preserve fO2 information. This newly developed oxybarometer has been applied in diverse systems, including arc magmas and mid-ocean ridge basalts, fertile and barren porphyry systems, and global S-type detrital zircon, which provide fO2 constraints that are consistent with other independent methods, suggesting that it has wide applicability. To improve accessibility, the oxybarometer was developed into a software application aimed at enabling more consistent and reliable fO2 determinations in magmatic systems, promoting further research.

Fluid-mediated Cu and Zn isotope fractionation in subduction zones and implications for arc volcanism: Constraints from high pressure veins within eclogites in the Dabie Orogen

High-pressure (HP) veins in eclogites are the products of fluid-rock interaction and provide insight into the composition and evolution of fluids in subduction zones. We here present the Cu and Zn isotope data for different types of HP veins and their host eclogites from Ganghe and Hualiangting in the Dabie Orogen to reveal the behavior of Cu and Zn isotopes during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the Omp-Ep vein formation. The early Omp-Ep veins have variably lower δ65Cu (−1.32 to −1.12‰ at Ganghe, −0.70 to −0.66‰ at Hualiangting) but higher δ66Zn (0.36 to 0.38‰ at Ganghe, 0.40 to 0.41‰ at Hualiangting) relative to the host eclogites (δ65Cu: −0.71 vs. 1.84‰, δ66Zn: 0.22 vs. 0.33‰), indicating CuZn isotope fractionation during slab dehydration in subduction zones with the lighter Cu and heavier Zn isotopes preferentially entering the vein-forming fluids from the eclogites. Systematic increase of δ65Cu from the Omp-Ep (−0.70 to −0.66‰) through Ep-Qtz (0.01‰) to Ky-Ep-Qtz veins (0.46 to 0.95‰) can be attributed to isotope fractionation induced by redox changes during the evolution of metamorphic fluids, as revealed by the negative correlations of δ65Cu with redox-sensitive ratios of Fe3+/ΣFe and (Eu/Eu*)N in those veins. Additionally, the higher δ66Zn of the Omp-Ep veins relative to the Ky-Ep-Qtz veins can be explained by equilibrium isotope fractionation between crystallized minerals and evolved metamorphic fluids. Our results thus demonstrate that CuZn isotope fractionation occurred during the evolution of slab-derived metamorphic fluids in subduction zones. Binary mixing calculations show that fluids derived from dehydration of mafic rocks in subducted slabs, represented by the multistage HP veins in the present study, cannot account for the heavier Cu and lighter Zn isotope compositions in most arc magmas than in mid-ocean ridge basalts. This offset can be resolved by addition of forearc serpentinite-derived fluids enriched in heavy Cu and light Zn isotopes into the mantle source of arc magmas.

Evolving Subduction Zone Thermal Structure Drives Extensive Forearc Mantle Wedge Hydration

AbstractHydration of the subduction zone forearc mantle wedge influences the downdip distribution of seismicity, the availability of fluids for arc magmatism, and Earth's long term water cycle. Reconstructions of present‐day subduction zone thermal structures using time‐invariant geodynamic models indicate relatively minor hydration, in contrast to many geophysical and geologic observations. We pair a dynamic, time‐evolving thermal model of subduction with phase equilibria modeling to investigate how variations in slab and forearc temperatures from subduction infancy through to maturity contribute to mantle wedge hydration. We find that thermal state during the intermediate period of subduction, as the slab freely descends through the upper mantle, promotes extensive forearc wedge hydration. In contrast, during early subduction the forearc is too hot to stabilize hydrous minerals in the mantle wedge, while during mature subduction, slab dehydration dominantly occurs beyond forearc depths. In our models, maximum wedge hydration during the intermediate phase is 60%–70% and falls to 20%–40% as quasi‐steady state conditions are approached during maturity. Comparison to global forearc H2O capacities reveals that consideration of thermal evolution leads to an order of magnitude increase in estimates for current extents of wedge hydration and provides better agreement with geophysical observations. This suggests that hydration of the forearc mantle wedge represents a potential vast reservoir of H2O, on the order of 3.4–5.9 × 1021 g globally. These results provide novel insights into the subduction zone water cycle, new constraints on the mantle wedge as a fluid reservoir and are useful to better understand geologic processes at plate margins.

Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

AbstractRecycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S &gt; 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H2O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at log fO2 from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite at fO2 conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ34S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke 34S‐rich subducted lithologies.

Early fluid exsolution suppressed sulfide saturation in Triassic arc volcanic rocks from the Gangdese belt: Implications for Cu depletion in arc magmas and mineralization potential

The formation of porphyry–epithermal CuAu deposits depends critically on Cu and Au fertility of arc magmas. Arc magmas commonly exhibit decreasing Cu and Au contents during magmatic differentiation, which is believed to result from sulfide saturation. However, the Cu and Au contents of the arc magmas would also be impacted by fluid exsolution, particularly when magma differentiation occurs in thin arcs. Here we report the variations of chalcophile element contents, including Cu and platinum-group elements (PGEs), with magmatic differentiation in the Qushui–Changguo (QS–CG) arc volcanic rocks from the Gangdese magmatic belt, Tibet. Zircon ages for QS–CG arc volcanic rocks range from 235 to 232 Ma and represent the earliest magmatism correlated to the subduction of the Neo-Tethys Ocean. Magmatic zircons yield an oxygen fugacity of FMQ +1.0 ± 0.6, which is within the range of typical arc magmas worldwide. The crystallization pressures (2.5 kbar) and magmatic H2O contents (5 wt%) estimated from amphibole and clinopyroxene compositions suggest the arc magmas were hydrous and differentiated at low pressures. The primitive rocks have Cu/Pd ratios within the mantle range and relatively high PGE contents, indicating that most chalcophile metals were transferred to the melt during partial melting to form the chalcophile fertile arc magma. The Cu contents decreased from >100 ppm in the primitive rocks to <30 ppm in the evolved rocks (e.g., ∼2 wt% MgO), which resembles the trend observed in global arc magmas. However, the Pt contents exhibit a slight reduction as the MgO decreases, which is in accordance with the crystallization of Pt-rich alloys. No decrease in Pd contents is observed in the samples, and Pd contents increase to ∼10 ppb in the most evolved samples. The absence of a dramatic decrease in PGE contents reveals that the magma did not reach sulfide saturation during differentiation, even in the highly evolved magma after magnetite crystallization. In contrast, the decoupling of Cu and PGE contents in the QS–CG arc magmas and decreasing Cu/Pd ratios are indicative of early aqueous volatile exsolution during magmatic differentiation. We suggest that early volatile exsolution in the arc magmas suppressed sulfide saturation during magmatic differentiation. As a result, early fluid exsolution may also be a key factor in reducing Cu contents in arc magmas in thin arcs, especially for magmas with high volatile contents that undergo differentiation at low pressures. Cu and Au would have been enriched in exsolved fluid prior to sulfide saturation. The Triassic metallogenic potential of the Gangdese belt should be re-evaluated based on recent discoveries. Our results indicate the Triassic magmatism could also be fertile, with the ability to form subduction-related porphyry CuAu deposits in the Gangdese metallogenetic belt.

Problemy ophiolitów Zagros i ciał bazaltowych, przykłady z Regionu Kurdystanu, północny Irak

In the Iraqi Zagros, there are ten ophiolites and basaltic bodies, the famous ones are Penjween, Mawat, Bulfat and Peshashan Ophiolite complexes in addition to basaltic bodies such as Kata Rash, Avroman, Gercus, Chalki, and Hamrin basaltic bodies. The present study describes more than 12 significant problems concerning the previous assigning of the bodies as igneous rocks. These problems are observable in the field, laboratory, and in most previous works of literature that oppose the magmatic origin of these bodies. Our study explicated all aspects of each problem and clarified how the problems contradict magmatic crystallization and aid the sedimentary origin of these claimed igneous bodies. Finally, the interpretations of all the problems were collected as conjugate pieces of evidence for appraisal of the new origin of all igneous bodies in the Iraqi and Iranian Zagros belt. The outcomes consider the ophiolitic and basaltic rocks metamorphosed volcaniclastic sandstones (greywackes or mafic sandstone). These sandstones belong to fresh or metamorphosed greywackes of stratigraphic units of the Paleocene-Eocene Walash Formation (as distal facies) and Kata Rash Conglomerate (as proximal facies) which were previously considered volcanic rocks. These sediments are sourced originally from Urumeiah-Dokhtur Magmatic Arc (ADMA) and deposited inside Neo-Tethys, present Sanandaji-Sirjan Zone (SSZ), during the Jurassic-Early Cretaceous. Later, the sediments were metamorphosed and uplifted during the Paleocene and deposited inside the Iraqi Zagros belt by turbidity currents inside the Zagros Foreland basin. These ideas are shown in detail by tectonic and paleogeographic models.

Origin and multi-episodic melt/fluid interactions revealed by the subducted serpentinites in the North Qilian Belt, NW China: Implications for the compositional heterogeneity of the fossil oceanic mantle

In nature, serpentinites act as reservoirs of water and fluid-mobile elements and play an essential role in arc magmatism, fluid metasomatism, and elemental circulation in subduction zones. This paper presents new mineral and whole-rock geochemical analyses of the Qingshuigou serpentinite massif and discusses its petrogenesis, tectonic setting, and mantle metasomatism. The chemical composition (e.g., REE, Y, Mg, and Al) and petrological observations (e.g., pseudomorph and hourglass textures) of serpentines demonstrate that the serpentinites’ protoliths belong to the peridotites. Their degrees of partial melting were estimated to be 15–20 % using Yb, Cr, Ti, and HREEs, indicating that these peridotite protoliths were residual in origin after high degrees of mantle melting. They were relics of the subducted lithospheric mantle in the subduction zone, based on their geochemical similarities with subducted serpentinites. Notably, the Qingshuigou serpentinites show several geochemical fingerprints of melt-rock interactions, such as the U-shaped REE patterns, elevated Zr/Ti, Zr/Hf, and Nb/Ta ratios; this led us to propose that the peridotite protoliths of the serpentinites were not merely melting remnants but were significantly modified by melt metasomatism. Also, they experienced two stages of fluid/rock interactions in the subduction zone: early-stage serpentinization at shallow depths and lizardite/chrysotile to antigorite transition at an elevated depth. Combined with the published data of ophiolitic peridotites from the North Qilian Belt, the compositional heterogeneity of these peridotites and associated serpentinites might be associated with the nature of melt/fluid metasomatism under different geodynamic backgrounds.

The spatial–temporal evolution of Neotethyan subduction in central to Southeast Iran: Constraints from geochemistry, zircon U–Pb–Hf, and Sr–Nd–Pb isotopes

Igneous rocks in central Iran preserve proof of several episodes of Late Mesozoic–Cenozoic Neotethyan subduction, and record temporal and spatial variations from subduction to collisional geodynamic regimes that remain a subject of debate. We revisit magmatic rocks from the central Urumieh–Dokhtar magmatic arc (UDMA; 32° 30′ N to 36° 00′ N) and report new major and trace element analyses, whole-rock Sr–Nd–Pb-isotopic data and zircon U–Pb–Hf ages on plutonic rocks from the Kajan region. The data reveal magmatic pulses between 32 and 22 Ma that are represented by igneous rocks with geochemical and isotopic features resulting from melting of a metasomatized, enriched mantle. Modeling indicates the samples represent a mixture of 97% batch melt (from a source with 98.5% mantle melt +1.5% terrigenous sediment) with 3% upper continental crust. The oldest yielded age (i.e., 32 Ma) rules out a lull in magmatism in the central UDMA between 37 and 26 Ma and speaks against distinct flare-up magmatic episodes at ∼54–37 Ma and ∼ 20–5 Ma in the frontal arc. We hypothesize that magmatism records down-going slab tearing into two pieces beneath the central to SE UDMA, which then changes the geometry and buoyancy of subducting Neotethyan lithosphere. The down-going slab remained involved in central UDMA (i.e., rollback) during Early Miocene whilst arc retreat beneath the SE UDMA proceeded with detachment of subducted oceanic lithosphere (i.e., break-off). We hypothesize a diachronous collision in the UDMA, first in the southeastern segments, and subsequently in the central UDMA. These findings bear important implications for the geodynamic evolution of the Zagros orogen and run counter to a model suggesting that the collision initiated in the northwest to central and propagated progressively to the southeast along the Zagros suture zone.

What controls structural variations along the Zagros Collision Zone? Insights from geophysical observations and thermo-mechanical modelling

The Zagros Collision Zone is a complex tectonic region formed as a consequence of the collision between Arabia and Eurasia after the subduction of the Neo-Tethys ocean. The NW-SE striking Zagros orogen consists of the following parallel tectonic units (from SW to NE): Zagros Fold and Thrust Belt (ZFTB), Sanandaj–Sirjan Metamorphic Zone (SSZ), and Urumieh–Dokhtar Magmatic Arc (UDMA). In this study, we perform a combined analysis of recent geophysical data, revealing pronounced differences in the crustal and lithospheric structure along the Zagros Mountains. The northwestern sector shows a fairly uniform crustal thickening across the broad symmetric orogen from the ZFTB to the UDMA. In contrast, in the central Zagros, the transition from a relatively narrow zone of high elevations and high-frequency relief in the ZFTB to a smoother surface topography of the SSZ and UDMA occurs with an abrupt increase in Moho depth below the SSZ. The last observation has recently been interpreted as a result of “relamination” process, where the felsic upper crust of the Arabian plate underthrust the mafic crust of the Iranian plate. We present geodynamic numerical models of crustal relamination during continental collision and compute static gravity field of the resulting structures. We show that oblique closure of the Neo-Tethys affects lateral variations in the style and extent of crustal relamination, which control the observed along-strike changes in crustal configuration and orogen altitude. In particular, a narrow and higher orogen (as in the central Zagros) develops in the experiments with a young and wide oceanic plate, whereas an old and narrow subducting plate tends to form a broad and lower topography (as in the northwestern Zagros). This is geometrically consistent with the progressive closure of the Neo-Tethys from NW to SE during the oblique continental collision between Arabia and Eurasia.

Implications of Late Triassic–Middle Jurassic detrital zircons from the southeastern margin of the South China Block for the Paleo-Pacific Plate subduction

There existed an early Mesozoic tectonic regime transformation in the South China Block (SCB). Although it was believed that this transformation was closely related to the subduction of Paleo-Pacific Plate, the process and initial time of the subduction of the Paleo-Pacific Plate have been controversial for a long time. Based on the published Upper Triassic and Middle Jurassic succession detrital zircons geochronology from the southeastern margin of the SCB, the newly obtained Late Triassic to Middle Jurassic detrital zircons Hf isotope data, and the available magmatic age data and Hf isotope data, this study discussed the subduction of the Paleo-Pacific Plate. The Hf isotope composition of zircon grains selected from the Early Mesozoic strata of the SCB, spanning from the Late Triassic to the Middle Jurassic, exhibits a consistent temporal trend with that of the Triassic–Middle Jurassic magmatic rocks. Both display a transition from negative to positive εHf(t) values, indicative of a gradually increasing contribution of mantle-derived materials from the Triassic to the Middle Jurassic. Zircon trace elements indicate that a magmatic arc appeared outside the southeastern margin of the SCB at 200–190 Ma and continued to develop into the Middle Jurassic, which may have been generated by the Paleo-Pacific Plate subduction. This study proposed that the Paleo-Pacific Plate subduction was confined to the southeastern coast of the SCB in the Late Triassic, and the subduction of the flat slab was halted by obstruction at the end of the Late Triassic. In the Early Jurassic, the Paleo-Pacific Plate began to subduction again, and arc-related magmatic rocks were formed along the coast of SCB. At the same time, due to the remote effect of the subduction of the Paleo-Pacific Plate, the weak tectonic belt existing in the Nanling area was reactivated, resulting in the Nanling area in the intraplate extensional setting. Subsequently, the continuous subduction of the Paleo-Pacific Plate led to the thickening of the crust along the SCB in the Middle Jurassic, and the Nanling area was still under in the extensional setting.

Petrography, U–Pb geochronology and geochemistry of Varcheh intrusions: Insight into younging trend of mid‐Cretaceous subduction in the northern Sanandaj–Sirjan Zone, western Iran

The Neotethyan Sanandaj–Sirjan Zone of western Iran has recorded major magmatic activities due to its continental arc tectonic setting during the Mesozoic. The Varcheh mafic intrusions were less‐studied plutons in the northern Sanandaj–Sirjan Zone (SSZ). Field evidence, petrography, geochemistry and U–Pb geochronological data were used to determine petrographic composition, geochemical nature, crystallization age and also to suggest a conceptual tectonomagmatic model for their emplacement. Small plutonic bodies are dominantly composed of monzogabbro that have intruded into the Cretaceous sedimentary rocks. Based on U–Pb zircon datings, these rocks have crystallized at 125–118 Ma in late Early Cretaceous (Barremian–Aptian), and are older than the supposed ages reported on geological maps. Varcheh rocks are not just typical calc‐alkaline rocks and some show alkaline affinity. Negative anomalies in Nb–Ta–Ti and enrichments in some large‐ion lithophile elements on spider diagrams are consistent with a subduction‐zone setting. Potential deep source for magma generation is partial melting of subcontinental lithospheric mantle wedge above a subducting slab of oceanic lithosphere. The spaces for the Varcheh mafic intrusions are accommodated by dominant dextral strike‐slip movement in a continental arc experiencing extension during late Early Cretaceous subduction. According to the zircon U–Pb geochronology results in this paper and previous U–Pb ages in the northern part of the SSZ, the mid‐Cretaceous magmatism reveals a significant NW‐ward younging trend and migration of the magmatic arc from the Barremian–Aptian in south‐east to the Albian–Cenomanian in the north‐west.

The electrical structure of the lithosphere in the southeastern margin of the Amazonian craton: Contribution to West Gondwana tectonics

A high electrical conductivity belt near the southeastern margin of the Amazonian craton, known as Paraguay-Araguaia Conductivity Anomaly (PACA), has been discovered and mapped over a distance of more than 1,000 km by a large array of magnetometer measurements. To precisely determine the location and structure of the conductive anomaly, we conducted a 200-km-long magnetotelluric (MT) profile with typical site spacing of 8 km over the northern part of PACA and crossing the Goiás magmatic arc and Araguaia belt in Central Brazil. Three-dimensional resistivity models derived from this data indicate that the PACA is not a single conductor but comprises at least two contiguous bodies at the upper to middle crust. The enhanced conductivity of the northern part of PACA is likely due to sulfide minerals deposited in an inter-arc or back-arc basin that were deformed and subsequently emplaced tectonically beneath the western Goiás magmatic arc. The PACA is truncated to the west by a prominent sub-vertical resistor, attributed to the Transbrasiliano lineament (TBL), a continental-scale structural feature interpreted here as a mylonitic shear zone developed late in the Neoproterozoic to Cambrian tectonic history involving the collision between Amazonian craton and proto-West Gondwana. We cannot confirm the prevailing view of an east-dip subduction of oceanic lithosphere from the Amazonian plate under the Goiás arc terranes. Instead, our results probably represent the record of an oblique continental collision. Regardless of its origin, the PACA may serve as a proxy for the paleo border of the Amazonian plate. Therefore, additional geophysical mapping, especially in areas covered by thick Phanerozoic sediments, is essential to constrain the tectonic evolution of West Gondwana.

The bending of a supra-subduction zone produced crustal thickening and arc migration of the Mongolian Orocline

Orogenic curvatures have been widely recognized along global convergent plate boundaries. Understanding the impact of such curvatures on the tectonic evolution of orogens and their three-dimensional architecture has been challenging. Here we address this issue by studying magmatism around the tightly curved Mongolian Orocline in Central Asia. Our results show that during the Permian–Triassic, arc magmatism around the inner hinge of the orocline became younger towards the core of the orocline. During the same period, the crust was thickened, as indicated by Lanthanum-Ytterbium ratio proxy. These findings, together with the observation that the present-day hinge zone of the Mongolian Orocline is wider, indicate that this zone was subjected to significant crustal-scale contraction. This contractional deformation accounts for the relatively thicker crust around the inner hinge of the Mongolian Orocline, and offers a novel perspective on the formation of elevated topography around the hinge of curved plate boundaries.