Arc Crust Research Articles

Extremely rapid up-and-down motions of island arc crust during arc-continent collision

Mountain building and the rock cycle often involve large vertical crustal motions, but their rates and timescales in unmetamorphosed rocks remain poorly understood. We utilize high-resolution magneto-biostratigraphy and backstripping analysis of marine deposits in an active arc-continent suture zone of eastern Taiwan to document short cycles of vertical crustal oscillations. A basal unconformity formed on Miocene volcanic arc crust in an uplifting forebulge starting ~6 Ma, followed by rapid foredeep subsidence at 2.3–3.2 mm yr−1 (~3.4–0.5 Ma) in response to oceanward-migrating flexural wave. Since ~0.8–0.5 Ma, arc crust has undergone extremely rapid (~9.0–14.4 mm yr−1) uplift to form the modern Coastal Range during transpressional strain. The northern sector may have recently entered another phase of subsidence related to a subduction polarity reversal. These transient vertical crustal motions are under-detected by thermochronologic methods, but are likely characteristic of continental growth by arc accretion over geologic timescales.

Arc Crust Formation of Lesser Antilles Revealed by Crustal Xenoliths from Petit St. Vincent

Abstract The Lesser Antilles volcanic arc is known for its magmatic diversity and unusually abundant plutonic xenoliths. Xenoliths from Petit St. Vincent (Grenadines’ archipelago) are particularly interesting because of their textural and petrogenetic range. Here we combine petrographic observations, Electron Backscatter Diffraction (EBSD) analysis, major and trace element chemistry of xenoliths and lavas and geochemical and thermal modelling to explore the construction of arc crust beneath Petit St. Vincent. Petit St. Vincent xenoliths are dominated by calcic plagioclase, clinopyroxene and amphibole, and can be divided into two main categories, igneous and meta-igneous. Igneous xenoliths typically have cumulate textures; meta-igneous xenoliths range texturally from those that preserve vestiges of primary magmatic fabrics to intensely deformed varieties characterised by grain-size reduction and foliation development. Meta-igneous xenoliths also contain the most calcic plagioclase (An98–100). The presence of both meta-igneous and igneous xenoliths provides evidence for reworking of older arc crust and antecedent igneous intrusions. The latter have a protolith composition similar to high-MgO, low-Sr picrites and high-Ca, high-Sr ankaramites from the neighbouring islands of Petite Martinique and Grenada. The meta-igneous xenoliths derive from older, mafic arc crust present at the onset of subduction. Trace element chemistry and EBSD analyses of meta-igneous xenoliths are consistent with a complex history of re-melting and deformation mediated by chlorine-bearing H2O rich fluids (including melts). Thermal modelling supports crustal reworking through repeated magma intrusions and indicates that the observed thermal structure and thickness of crust beneath Petit St. Vincent could have developed on a timescale of approximately 4 million years at rates compatible with the regional arc magma flux. Based on evidence from thermodynamic models and exhumed ancient arc crust sections, Collins et al. (Nature Geoscience, 13, 331–338, 2020) have proposed that water-fluxed melting may be an important aspect of deep arc crust sections world-wide. Textures and mineralogy of xenoliths from Petit St. Vincent, including their characteristic high-An plagioclase, testify to such a process beneath an active, intra-oceanic arc.

Magma hybridization and crystallization in coexisting gabbroic and granitic bodies in the mid-crust, Akechi district, central Japan

Petrological and geochemical features of gabbros and fine-grained mafic rocks (mafic microgranular enclaves; MMEs) in the Inagawa Granite of the Ryoke Plutonic Complex were investigated to assess the interactions between coexisting mafic and silicic magmas, and the petrogenetic relationships between the MMEs and surrounding gabbros. The MMEs exhibit mingling textures that imply the coexistence of mafic and silicic magmas that did not undergo complete mixing, but the geochemical compositions of the MMEs require substantial hybridization and homogenization. The gabbroic rocks exhibit disequilibrium textures and mineral compositions, such as quartz–hornblende ocellar textures and patchy plagioclase crystals with bimodal anorthite contents. These textures and compositions record an abrupt decrease in crystallization temperature and mechanical mixing between crystallizing gabbroic mush and silicic (granitic) melt. Geochemical variations of the gabbroic rocks can be explained by hybridization and fractional crystallization (HFC) processes between crystallizing gabbroic mush and granitic melt. Extrapolation of the mixing trend to a basaltic composition suggests that the primitive mafic end-member was a low-K basaltic magma. Given that HFC yields magnesian andesite by the addition of a small amount of silicic melt to a primitive mafic end-member, the compositional modification of mafic magmas by magma mixing might be an essential process in the formation of andesitic magma in arc crust.

Tectonic burial of sedimentary rocks drives the building of juvenile crust of magmatic arc

AbstractContinental arcs grow primarily by addition of mantle-derived magmas, thus forming juvenile crust, although geophysical evidence, alongside field investigation of exhumed terranes, show that supracrustal rocks are common components of the lower portions of continental arcs. The mechanisms by which metasedimentary rocks are transported to the deep arc crusts and their contributions to the juvenile arc crusts are ambiguous. Here, we conduct a systematic petrological, geochronological, and geochemical study of pelitic migmatites within Late Cretaceous meta-gabbros from the lower crust of the eastern Gangdese arc, southern Tibet. Our results show that the pelitic migmatites were derived from the Late Carboniferous sedimentary rocks of the upper arc crust, have significantly enriched Sr-Nd-Hf-O isotopic compositions, and underwent Late Cretaceous (95–80 Ma) high-pressure granulite-facies metamorphism and partial melting at ~850 °C and 15 kbar to generate voluminous granitic melts. This indicates that the eastern Gangdese arc underwent significant crustal thickening and chemical differentiation during the final stages of subduction of the Neo-Tethys. We suggest that the metasedimentary rocks were transported into the lower crust of the Gangdese arc by underthrusting of arc crust and accretion of mantle-derived magma, and the deeply buried supracrustal rocks altered the petrological constitution and chemical compositions of juvenile lower arc crust, and assimilated the mantle- and juvenile crust-derived melts. We propose that tectonic burial of sedimentary rocks is a key mechanism driving the building and reworking of juvenile crust of magmatic arcs throughout most of geological time.

Detrital orthopyroxene as a tracer of geodynamic setting:: A Raman and SEM-EDS provenance study

Raman spectroscopy is a powerful and efficient tool that is being used more and more frequently in high-resolution provenance analysis. This study is dedicated to Ca-poor orthorhombic pyroxene, which for its low durability during diagenesis can be considered as a diagnostic first-cycle phase. We focused on 15 samples of modern sands representing a full range of rock units exposed at different structural levels of continental, arc, or oceanic lithosphere and generated in diverse geodynamic settings. By coupling optical observations, SEM-EDS mineral-chemistry and Raman spectroscopy analyses on the same grains we show how orthopyroxene from mantle, deep crustal, and volcanic rocks ranging in composition from andesite to dacite and felsic differentiates can be robustly distinguished. The Mg# [Mg/(Mg + Fe)] content of orthopyroxene grains can be accurately estimated by recording the characteristic position of their six main characteristic peaks (ν1 to ν6, vibrational modes) even during routine point-counting under the Raman spectroscope. In particular, the position of the strong and narrow ν3 peak (~655 cm−1) also allows estimation of the Ca content when compared to the position of other peaks (especially ν1, ~330 cm−1). High-Mg orthopyroxene is exclusively derived from mantle rocks, whereas metagabbros of the lower continental crust and gabbroic rocks of arc crust are distinguished by their lower Ca. The lowest Mg# was recorded in detrital orthopyroxene derived from the silicic Amiata Volcano in central Italy, whereas trachytes in rift-related settings did not show unique characteristics apart from their slightly higher Ca content.

Early Eocene high-Sr/Y magmas from the Urumieh-Dokhtar paleo-arc, Iran: Implications for the origin of high-flux events in magmatic arcs

High-flux magmatic events, or flare-ups, are seen in many continental and transitional arcs but the underlying cause is not fully understood. Throughout Iran, voluminous calc-alkaline rocks emplaced between ~53 Ma and ~37 Ma are manifestations of a high-flux event. This high-flux event has been attributed to extension in the upper plate of one or more Neo-Tethyan subduction zones. Here, we present new findings in the Urumieh-Dokhtar paleo-arc, Iran where emplacement of Early Eocene granitic intrusions (ca. 55–52 Ma) immediately preceded the high-flux event. Geochemically, the studied rocks display high-K calc-alkaline compositions, continental crust-like trace element patterns, variable depletion of Y and heavy rare-earth elements, and SrNd isotopes that are close to or slightly more enriched than the Bulk Silicate Earth. The SrNd isotopic compositions indicate that the intrusions derived from upper plate materials. Relatively high Sr/Y and La/Yb of the studied rocks indicate that the residue of melt extraction, the crystalline assemblage that was left behind during magma differentiation, or both, to have existed in the deep arc crust. Following a ~ 20 Myr magmatic lull, the Early Eocene crustal thickening was most likely related to underthrusting of rear-arc crust towards the main arc, a process that not only promoted rapid root growth but also triggered the Eocene high-flux event. In the Iranian case example in particular, magmas formed during the high-flux event do not display elevated Sr/Y, implying that dense, locally thickened root might have foundered from the base of the arc lithosphere to the convecting mantle soon after their formation.

Late Cretaceous Metamorphism and Anatexis of the Gangdese Magmatic Arc, South Tibet: Implications for Thickening and Differentiation of Juvenile Crust

AbstractMagmatic arcs are the primary sites of growth of post-Archean continental crust; however, the mechanisms and processes for transforming primary arc crust into mature continental crust are subject to disagreement. We conducted a detailed petrologic and geochronological study on mafic and felsic migmatites from the eastern Gangdese magmatic arc, which is typical of continental arcs worldwide. The studied mafic migmatites contain amphibole, garnet, plagioclase, epidote, white mica, quartz, rutile and ilmenite in melanosomes, and plagioclase, garnet, epidote, amphibole, white mica, and quartz in leucosomes. The leucosomes occur as diffuse patches, concordant bands, or concordant and discordant networks and veins in the melanosomes. The migmatites have protolith ages between ~157 and ~86–87 Ma, and metamorphic ages of ~83–87 Ma and underwent high-pressure granulite-facies metamorphism at peak P–T conditions of ~850–880°C and 15–17 kbar. Heating, burial, and associated partial melting preceded near-isobaric cooling with residual melt crystallization. Significant melt (>16 wt.%) generated during heating and loading had a granitic composition. Compositional comparison to low-grade meta-gabbros implies that any extracted melt had adakitic affinities (high Sr/Y and highly fractionated REE patterns). The eastern Gangdese magmatic arc experienced crustal thickening during Late Cretaceous late-stage evolution of the arc due to magma loading and tectonic shortening and thrusting of the arc crust. Crustal thickening and chemical differentiation of the Gangdese arc occurred during late subduction of the Neo-Tethys, prior to the India–Asia collision. Metamorphism nearly completely erased all prior igneous mineralogy and mineral chemistry, and consequent partial melting represents a potential source for Late Cretaceous granitoids of the upper arc crust. Although prior studies demonstrate the significance of fractional crystallization, deep-seated metamorphic processes largely drove chemical differentiation to produce mature continental crust in the Gangdese arc during the late Cretaceous.

Eoarchean subduction-like magmatism recorded in 3750\xa0Ma mafic\u2013ultramafic rocks of the Ukaliq supracrustal belt (Qu\xe9bec)

Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.

Late Paleozoic Southward Migration of the Dananhu Arc in the Eastern Tianshan (NW China)

AbstractThe process of continental growth and closure of the Paleo‐Asian Ocean are controversial. To address this problem, we report new geochemical and geochronological data of granitic rocks along a N‐S corridor of the Dananhu arc. These rocks yield 401–270 Ma U‐Pb zircon ages. The arc‐related granites are enriched in the light rare earth elements (LREE), Rb, Ba, Th, Sr and Pb, but depleted in Nb and Ta, and have high zircons values of εHf(t) (+7.7−+19.6) and εNd(t) (+5.4−+7.2); they were possibly generated by partial melting of juvenile intermediate or mafic arc crust. A 356 Ma high‐K calc‐alkaline granite has high Ga/Al (3.28–3.37), low Mg# (14–16), and high zircon εHf(t) (+8.9−+12.0); it can be classified as A2‐type‐granite, suggesting it was a product of substantial fractional crystallization of a mantle‐derived arc‐related magma in an intra‐arc extensional setting. The youngest granite (254 Ma) is an adakite‐like granite which is characterized by high Sr/Y (26–38), depletion in heavy rare earth elements (HREE), and high εNd(t) (+7.0) and εHf (t) (+10.4 to +15.4) values, most likely it was derived from a subducted oceanic slab. Therefore, all the granitic rocks formed in subduction‐related settings. The granitic rocks are generally young southwards from the Kezier area (ca. 401–335 Ma) to the Tuwu area (334–315 Ma), and to the Kanguer area (ca. 280–254 Ma), indicating that the arc grew southwards, and the Paleo‐Asian Ocean closed possibly after 254 Ma. Therefore, our data provide a robust constraint on the prolonged growth of the Altaids.

Geodetic and Geological Deformation of the Island Arc in Northeast Japan Revealed by the 2011 Tohoku Earthquake

Northeast Japan is a typical island arc related to the Pacific plate subduction. The 2011 Mw9.0 Tohoku-oki earthquake provided a unique opportunity to analyze crustal deformation with different boundary conditions, similar to a gigantic rock deformation experiment. We review findings obtained through various observations and data analyses in Northeast Japan, focusing on the crustal deformation in different timescales. The occurrence of the M9 earthquake solved the ongoing paradox that the geodetic strain rate is an order of magnitude larger than the geologic estimate, showing that the centennial geodetic observation had mainly captured the elastic strain accumulation. Along the localized contraction zone along the Japan Sea coast, a comparison of postseismic and interseismic deformation patterns revealed a significant contribution of inelastic deformation, which plays an essential role in long-term deformation. Along the Pacific coast, rapid interseismic subsidence and unexpected coseismic subsidence were followed by a rapid postseismic uplift, indicating that viscous relaxation in the mantle is of essential importance. These findings advance our understanding of plate interactions and the tectonic evolution of the island arc. ▪ The 2011 Tohoku-oki earthquake provided the most complete crustal deformation data set ever for interseismic, coseismic, and postseismic periods. ▪ The discrepancy between the geologic and geodetic deformation rates in Northeast Japan is attributed to an elastic strain due to interplate locking. ▪ A significant contribution of inelastic deformation in the island arc crust is identified through a comparison of interseismic and postseismic deformations.

The formation of high-Sr/Y plutons in cordilleran-arc crust by crystal accumulation and melt loss

Bulk-rock data are commonly used in geochemical studies as a proxy for melt compositions in order to understand the evolution of crustal melts. However, processes of crystal accumulation and melt migration out of deep-crustal, crystal-rich mush zones to shallower storage regions raise questions about how faithfully bulk-rock compositions in plutons approximate melt compositions. This problem is particularly acute in the lower crust of arcs, where melt reservoirs are subject to periodic melt extraction that leaves behind a cumulate residue. Here, we examine bulk-rock data from the perspective of high-Sr/Y plutonic rocks in the lower crust of a well-exposed Early Cretaceous cordilleran-arc system in Fiordland, New Zealand. We test the validity of using high-Sr/Y bulk-rock compositions as proxies for melts by comparing bulk-rock compositions to melts modeled from >100 major- and trace-element analyses of 23 magmatic clinopyroxene grains from the same samples. The sampling locations of the igneous clinopyroxenes and encompassing bulk rocks are distributed across ~550 km2 of exhumed lower crust and are representative of Mesozoic lower-crustal arc rocks in the Median batholith. We confirm that bulk-rock data have characteristics of high-Sr/Y plutons (Sr/Y >50, Na2O >3.5 wt%, Sr >1000 ppm, and Y <20 ppm), features that have been previously interpreted to indicate the presence of garnet as a residual or fractionating phase. In contrast to bulk rocks, igneous clinopyroxenes have low Sr (<100 ppm), high Y (25–100 ppm), and low molar Mg# [100 × Mg/(Mg + Fe)] values (60–70), which are consistent with derivation from fractionated, low-Sr/Y melts. Chondrite-normalized rare-earth-element patterns and Sm/Yb values in clinopyroxenes also show little to no evidence for involvement of garnet in the source or in differentiation processes. Fe-Mg partitioning relationships indicate that clinopyroxenes are not in equilibrium with their encompassing bulk rocks but could have been in equilibrium with melt compositions determined from chemometry of coexisting igneous hornblendes. Moho-depth calculations based on bulk-rock Sr/Y values also yield Moho depths (average = 69 km) that are inconsistent with Moho depths based on bulk-rock Ce/Y, contact aureole studies, Al-in-horn- blende crystallization pressures, and our modeled clinopyroxene crystallization pressures. These data indicate that most Mesozoic high-Sr/Y bulk rocks in the lower crust of Fiordland are cumulates formed by plagioclase + amphibole + clinopyroxene accumulation and interstitial melt loss from crystal-rich mush zones. Our data do not support widespread fractionation of igneous garnet nor partial melting of a garnet-bearing source in the petrogenesis of these melts. We speculate that melt extraction and the production of voluminous cumulates in the lower crust were aided by unusually high heat flow and high magma addition rates associated with an Early Cretaceous arc flareup. We conclude that bulk-rock compositions are poor proxies for melt compositions in the lower crust of the Median batholith, and geochemical modeling of these high-Sr/Y bulk rocks would overemphasize the role of garnet in their petrogenesis.

What drives Fe depletion in calc-alkaline magma differentiation: Insights from Fe isotopes

Abstract The continental crust is strongly depleted in iron relative to mid-oceanic ridge basalt, broadly identical to the calc-alkaline magmas, suggesting that calc-alkaline differentiation is key to continent formation. However, it remains contentious as to what drives Fe depletion during magmatic differentiation in the crust. The two competing hypotheses for calc-alkaline differentiation—magnetite versus garnet (± amphibole) fractionation—predict contrasting Fe isotopic fractionation pathways in evolved melts because magnetite preferentially depletes ferric, isotopically heavy Fe whereas garnet (± amphibole) does the opposite. We report whole-rock Fe isotope data for two suites of igneous rocks from the central Andes, which represent magmas traversing normal and thickened arc crust, respectively. The magmas traversing thickened crust show a strong Fe depletion trend and consistently high δ56Fe values (0.14‰ ± 0.02‰, 1 standard deviation [SD]), while those traversing normal crust are less depleted in Fe and show variable δ56Fe values (0.10‰ ± 0.05‰, 1SD). The two Andean suites are both isotopically heavier than Mariana arc (Pacific Ocean) magmas that differentiate along tholeiitic (Fe-enriching) paths. These results confirm that garnet (± amphibole) fractionation/retention is the primary driver of Fe depletion in calc-alkaline magmas, and highlight a role for crustal thickening in generating calc-alkaline magmas.

A new geological map of the Lau Basin (southwestern Pacific Ocean) reveals crustal growth processes in arc-backarc systems

A 1:1,000,000-scale lithostratigraphic assemblage map of the Lau Basin (southwestern Pacific Ocean) has been created using remote predictive mapping (RPM) techniques developed by geological surveys on land. Formation-level geological units were identified in training sets at scales of 1:100,000–1:200,000 in different parts of the basin and then extrapolated to the areas where geological data are sparse. The final compilation is presented together with a quantitative analysis of assemblage-level crustal growth based on area-age relationships of the assigned units. The data sets used to develop mapping criteria and an internally consistent legend for the compilation included high-resolution ship-based multibeam, satellite- and ship-based gravity, magnetics, seafloor imaging, and sampling data. The correlation of units was informed by published geochronological information and kinematic models of basin opening. The map covers >1,000,000 km2 of the Lau-Tonga arc-backarc system, subdivided into nine assemblage types: forearc crust (9% by area), crust of the active volcanic arc (7%), backarc rifts and spreading centers (20%), transitional arc-backarc crust (13%), relict arc crust (38%), relict backarc crust (8%), and undivided arc-backarc assemblages (<5%), plus oceanic assemblages, intraplate volcanoes, and carbonate platforms. Major differences in the proportions of assemblage types compared to other intraoceanic subduction systems (e.g., Mariana backarc, North Fiji Basin) underscore the complex geological makeup of the Lau Basin. Backarc crust formed and is forming simultaneously at 12 different locations in the basin in response to widely distributed extension, and this is considered to be a dominant pattern of crustal accretion in large arc-backarc systems. Accelerated basin opening and a microplate breakout north of the Peggy Ridge has been accommodated by seven different spreading centers. The result is an intricate mosaic of small intact assemblages in the north of the basin, compared to fewer and larger assemblages in the south. Although the oldest rocks are Eocene (~40 m.y. old basement of the Lau and Tonga Ridges), half of the backarc crust in the map area formed within the last 3 m.y. and therefore represents some of the fastest growing crust on Earth, associated with prolific magmatic and hydro-thermal activity. These observations provide important clues to the geological evolution and makeup of ancient backarc basins and to processes of crustal growth that ultimately lead to the emergence of continents.

Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits.

Machine learning for geochemical exploration: classifying metallogenic fertility in arc magmas and insights into porphyry copper deposit formation

A current mineral exploration focus is the development of tools to identify magmatic districts predisposed to host porphyry copper deposits. In this paper, we train and test four, common, supervised machine learning algorithms: logistic regression, support vector machines, artificial neural networks (ANN) and Random Forest to classify metallogenic ‘fertility’ in arc magmas based on whole-rock geochemistry. We outline pre-processing steps that can be used to mitigate against the undesirable characteristics of geochemical data (high multicollinearity, sparsity, missing values, class imbalance and compositional data effects) and therefore produce more meaningful results. We evaluate the classification accuracy of each supervised machine learning technique using a tenfold cross-validation technique and by testing the models on deposits unseen during the training process. This yields 81–83% accuracy for all classifiers, and receiver operating characteristic (ROC) curves have mean area under curve (AUC) scores of 87–89% indicating the probability of ranking a ‘fertile’ rock higher than an ‘unfertile’ rock. By contrast, bivariate classification schemes show much lower performance, demonstrating the value of classifying geochemical data in high dimension space. Principal component analysis suggests that porphyry-fertile magmas fractionate deep in the arc crust, and that calc-alkaline magmas associated with Cu-rich porphyries evolve deeper in the crust than more alkaline magmas linked with Au-rich porphyries. Feature analysis of the machine learning classifiers suggests that the most important parameters associated with fertile magmas are low Mn, high Al, high Sr, high K and listric REE patterns. These signatures further highlight the association of porphyry Cu deposits with hydrous arc magmas that undergo amphibole fractionation in the deep arc crust.

Middle–Late Triassic southward-younging granitoids: Tectonic transition from subduction to collision in the Eastern Tianshan–Beishan Orogen, NW China

Abstract To constrain the closure mechanism and time of the Paleo-Asian Ocean, we report new geochronological and geochemical data for Triassic granites along a NW–SE corridor from Eastern Tianshan to Beishan, NW China. Seven granites have U-Pb ages that young southwards from 245 Ma to 234 Ma in the Kanguer accretionary complex, to 237 Ma to 234 Ma in the eastern Central Tianshan block, to 229 Ma to 223 Ma in the Liuyuan accretionary complex. Granites in the Kanguer accretionary complex formed by fractional crystallization and are peraluminous, high-K, calc-alkaline, and crust-derived. They have very low MgO (Mg# = 6–9), Cr, and Ni contents, and their high εNd(t) (+3.40) and εHf(t) (+4.49 to +11.91) isotopes indicate that the Dananhu arc crust was juvenile. The Huaniushan pluton in the Liuyuan accretionary complex displays the geochemical signatures of both A1- and A2-type granites (Y/Nb = 0.32–3.39). All other granites in the Central Tianshan block and Liuyuan accretionary complex are aluminous A2-types with high K2O + Na2O, Al, rare earth elements (REE), Zr + Nb + Y, Ga, Fe/Mg, and Y/Nb and remarkable depletions of Eu, Ba, Nb, Ta, Sr, P, and Ti. They have a broad range of MgO (Mg# = 9–59), Cr, and Ni contents, Isr (0.70741–0.70945) values, negative εNd (t) (−2.98 to −1.14), and low to moderate εHf(t) (−1.22 to +7.78), which suggests a mixture of mantle and crustal components. These 245–223 Ma granitoids show marked Nb-Ta depletions that point to a subduction origin. Notable enrichments in Nd-Hf isotopes of Late Triassic granites are likely an indication of collision. Integration with previous data enables us to conclude that the delamination of an oceanic slab and mantle upwelling induced partial melting of thickened arc crust during a tectonic transition from a multiple supra-subduction margin to a collisional setting in the Late Triassic.

A Crustal Control on the Fe Isotope Systematics of Volcanic Arcs Revealed in Plutonic Xenoliths From the Lesser Antilles

Lavas produced at subduction zones represent the integration of both source heterogeneity and an array of crustal processes, such as: differentiation; mixing; homogenisation; assimilation. Therefore, unravelling the relative contribution of the sub-arc mantle source versus these crustal processes is difficult when using the amalgamated end products in isolation. In contrast, plutonic xenoliths provide a complementary record of the deeper roots of the magmatic plumbing system and provide a unique record of the true chemical diversity of arc crust. Here, we present the δ56Fe record from well characterised plutonic xenoliths from two distinct volcanic centres in the Lesser Antilles volcanic arc–the islands of Martinique and Statia. The primary objective of this study is to test if the Fe isotope systematics of arc lavas are controlled by sub-arc mantle inputs or during subsequent differentiation processes during a magma’s journey through volcanic arc crust. The Fe isotopic record, coupled to petrology, trace element chemistry and radiogenic isotopes of plutonic xenoliths from the two islands reveal a hidden crustal reservoir of heavy Fe that previously hasn’t been considered. Iron isotopes are decoupled from radiogenic isotopes, suggesting that crustal and/or sediment assimilation does not control the Fe systematics of arc magmas. In contrast to arc lavas, the cumulates from both islands record MORB-like δ56Fe values. In Statia, δ56Fe decreases with major and trace element indicators of differentiation (SiO2, Na2O + K2O, Eu/Eu*, Dy/Yb), consistent with fractionating mineral assemblages along a line of liquid descent. In Martinique, δ56Fe shows no clear relationship with most indicators of differentiation (apart from Dy/Yb), suggesting that the δ56Fe signature of the plutonic xenoliths has been overprinted by later stage processes, such as percolating reactive melts. Together, these data suggest that magmatic processes within the sub-arc crust overprint any source variation of the sub-arc mantle and that a light Fe source is not a requirement to produce the light Fe isotopic compositions recorded in volcanic arc lavas. Therefore, whenever possible, the complimentary plutonic record should be considered in isotopic studies to understand the relative control of the mantle source versus magmatic processes in the crust.

Intrusive Complexes of the Late Neoproterozoic Island Arc Structure of the Lake Zone (Mongolia): Isotope Systematics and Sources of Melts

Abstract –We present data on the geochemical and Sr–Nd isotope compositions of rocks and on the Lu–Hf isotope composition of magmatic and xenogenic zircons from granitoids and gabbroids of the late Neoproterozoic island arc structure of the Lake Zone. Plagiogranitoids, gabbroids, and quartz diorites (559–542 Ma) formed at the late Neoproterozoic subduction stage of magmatism, and two-feldspathic granites (~483 Ma) mark Cambrian–Ordovician accretion–collision processes. We have established that the volcanic rocks of the late Neoproterozoic island arc and/or its oceanic base, which formed from the depleted mantle, were the mafic source of plagiogranitoids. This is proved by the overlapping positive εNd values of plagiogranitoids and the host volcanic rocks and by the commensurate εHf values of magmatic zircons from the plagiogranitoids and depleted mantle. The lower εNd values of gabbro and quartz diorites from the Tavan Hayrhan and Shuthuyn plutons, the lower εHf values of zircons from these rocks, and the high (87Sr/86Sr)0 ratios and K2O, Rb, and Th contents point to the generation of these rocks from a less depleted mantle source, namely, mantle wedge peridotites. The isotope composition of the latter changed at the previous subduction stage under the impact of fluids and with the contribution of subducted sediments. The least radiogenic Hf isotope composition of magmatic and xenogenic zircons from Ordovician accretion–collisional two-feldspathic granites of the Ih Zamiin pluton suggests their formation through the melting of the late Neoproterozoic–Cambrian island arc crust with the contribution of more differentiated crustal sources enriched in Th, Nb, and LREE and characterized by low εNd values. The age of xenogenic zircons (≤716 Ma) in the studied granitoids and gabbroids and their similarity in Hf isotope composition to magmatic zircons from the same rocks confirm the formation of the late Neoproterozoic island arc of the Lake Zone in an intraoceanic setting far from ancient continental sources similar to the Dzavhan microcontinent.

Crustal Forensics at Pūtauaki (Mt. Edgecumbe), New Zealand reveal the influence of deep crustal arc processes on magma evolution in the Taupo Volcanic Zone

Crustal Forensics at Pūtauaki (Mt. Edgecumbe), New Zealand reveal the influence of deep crustal arc processes on magma evolution in the Taupo Volcanic Zone

Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)

Abstract The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz-in-garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near-peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near-peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step-over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.