Volcanic Arc Magmas Research Articles

Global volcanic arc magma composition correlates with thickness of both arc crust (Moho depth) and arc lithosphere (LAB depth): A revealing message on arc basement histories and arc magmatism

Past fifty years have seen mounting publications on the genesis of volcanic arc magmas. While details remain debated, it is generally agreed that arc magmas result from slab-dehydration induced mantle wedge melting followed by crustal level differentiation of varying extent and sophistication. Two recent arc magma studies deserve particular attention because they attempt to discuss globally unifying controls on arc magma composition. Both Harvard study (Turner and Langmuir, 2015a,b) and Rice study (Farner and Lee, 2017) show correlations of arc magma composition with crustal thickness and both ascribe the crustal thickness as the principal control on their observed magma compositional variations, yet the physical role of the crustal thickness in their interpretations is markedly different because of (1) the ambiguous use of “crust” and (2) their different magma compositional ranges chosen in discussion. The Harvard study only uses basaltic samples corrected to MgO = 6.0 wt.% to discuss mantle processes and interprets the arc crustal thickness as restricting the mantle wedge melting, i.e., the extent of melting decreases with increasing crustal thickness. The Rice study uses samples of all compositions (basaltic to rhyolitic), whose extent of differentiation increases with increasing crustal thickness, interpreted as Moho-crossing mantle wedge melts travelling greater vertical distance with greater degree of cooling and erupting more evolved compositions above thicker crust than melts erupted above thinner crust without need of invoking mantle wedge processes. We commend these efforts and approve their different approaches but emphasize that the unifying understanding of global arc magmatism requires clearly defined Moho (the base of the crust) and LAB (the lithosphere-asthenosphere boundary) and their intrinsic controls on mantle wedge melting (Harvard Study model) and crustal level magma differentiation (Rice Study model) beneath global arcs. In this study, we use chemical compositions of 36,945 global arc volcanic samples provided by the Rice study together with the literature data on seismic Moho and LAB depths of these sample locations to establish (1) the correlation of crustal thickness interval averaged magma composition with crustal thickness and (2) the correlation of lithosphere thickness interval averaged magma composition with lithosphere thickness. These correlations reaffirm our understanding that the lithospheric mantle must exist beneath volcanic arc crust with a globally averaged LAB/Moho depth ratio of 3.26 ± 0.63, i.e., the arc crust is on average about 31.8% ± 6.1% of the lithosphere thickness. This knowledge forms a solid constraint essential for models of global arc magmatism.

Zircon U–Pb geochronology and Sm–Nd and Rb–Sr isotope systematics of Neoproterozoic granitoïds from Bou Azzer (Anti-Atlas - Morocco): The obduction trigger of the central Anti-Atlas terrane

This study presents new U–Pb zircon ages for the Neoproterozoic granitoids of the Bou Azzer inlier at the central part of the Anti-Atlas chain (Morocco), in the northwestern edge of the West African Craton (WAC). Analyzes of zircon grains yield the following ages: 658 ± 3 Ma for Ait Ahmane pluton, 647 ± 5 Ma for Bou-Izbane intrusion, 656 ± 3 Ma for Bou Azzer quartz-diorites, 583 ± 12 Ma in the Oustrate intrusion and 644 ± 4 Ma for the Ait Ahmane leucogranite. These ages confirm a major magmatic event between 640 and 660 Ma contemporaneous with the obduction of oceanic crust marking the formation of the ophiolites of Bou Azzer and Sirwa. The emplacement of granitoids would have led to a decrease in the density of the oceanic crust and triggered the process of obduction. The granitoids display the geochemical features of volcanic arc magmas and have positive initial ϵNd (+4.67 to +6.57) and low initial 87Sr/86Sr (0.7002–0.7055). The quartz-diorite group displays young Depleted Mantle Model (TDM) ages that range from 904 to 1065 Ma. The leucogranites have younger TDM of 869–884 Ma. These new data attest to their juvenile nature and the absence of any crustal contribution. The NNW-SSE extended inlier band of Bou Azzer - Sirwa constitutes an exotic terrane of Neoproterozoic age and of a juvenile nature wedged between the SW and NE domains, respectively, formed by or surmounting a Meso-Paleoproterozoic or even Archean basement. The Bleida granodiorite, dated at 559 ± 4, displays older U–Pb ages from inherited zircon cores at 1234, 1748 and 2085 Ma. The Bleida granodiorite was emplaced during a later magmatic episode that belongs to the Ouarzazate Group, generated during the post-collisional metacratonic evolution of the northern edge of the West African craton.

Editorial: Fluid-mobile element tracers of subduction processes—the record in volcanic arc magmas and exposed subduction complexes

Editorial: Fluid-mobile element tracers of subduction processes—the record in volcanic arc magmas and exposed subduction complexes

The petrogenesis and tectonic setting of the New Hampshire Plutonic Suite: Towards a more comprehensive model for the magmatism of the Acadian Orogeny

The Wamsutta and Nineteenmile Brook Diorites, two small plutons located in the White Mountains of New Hampshire, have bearing on the tectonic setting of magmatism between 410 and 407 Ma in this portion of the northern Appalachians. The unmetamorphosed, undated Nineteenmile Brook pluton has arc basalt affinities, produced in the same arc as the mafic magmas that were mingled with the ∼410 Ma Meredeth Porphyric Granite of central New Hampshire. These volcanic arc magmas from a westerly dipping subduction zone contributed both heat and mass to the petrogenesis of the New Hampshire Plutonic Suite (NHPS), producing the high temperature melts of the Kinsman Granodiorite of the NHPS. These peraluminous NHPS magmas were emplaced during collision of Avalonia with Laurentia, forming the bases of Acadian thrust sheets. The ∼408 Ma Wamsutta Diorite has appinite-like textures and chemically is a low SiO₂ adakite, with Sr/Y ratios of ∼ 400 and (La/Yb)_N between 80 and 130. These magmas were generated after flat slab, subduction erosion mixed basaltic rocks into the mantle wedge and partially melted the mafic rocks in the garnet stability field. The melts interacted with the surrounding peridotite to attain the low SiO₂ adakite characteristics. At this same time, the ∼407 Ma Exeter Diorite and other arc plutons were emplaced in the Merrimack belt of southeastern New Hampshire. By 400 Ma, continued westerly dipping subduction provided mafic magma underplating to partially melt lower crustal amphibolites, generating the Spaulding Tonalite. Subsequently, lower crustal delamination and asthenospheric upwelling provided the heat source that produced a younger, post-tectonic suite of magmas between 390 and 370 Ma that, while having arc signatures because of the heritage of their crustal source rocks, are not arc magmas because subduction is thought to have ceased by this time. These plutons include the mafic rocks of the Northeast Kingdom of Vermont and the Mooselookmeguntic Igneous Complex of NH and ME. This same heat source may have contributed to melting lower to midcrustal metasediments to produce the widespread peraluminous Concord Granite of Vermont, New Hampshire, and western Maine.

Adakites, High-Nb Basalts and Copper–Gold Deposits in Magmatic Arcs and Collisional Orogens: An Overview

Adakites are Y- and Yb-depleted, SiO2- and Sr-enriched rocks with elevated Sr/Y and La/Yb ratios originally thought to represent partial melts of subducted metabasalt, based on their association with the subduction of young (<25 Ma) and hot oceanic crust. Later, adakites were found in arc segments associated with oblique, slow and flat subduction, arc–transform intersections, collision zones and post-collisional extensional environments. New models of adakite petrogenesis include the melting of thickened and delaminated mafic lower crust, basalt underplating of the continental crust and high-pressure fractionation (amphibole ± garnet) of mantle-derived, hydrous mafic melts. In some cases, adakites are associated with Nb-enriched (10 ppm < Nb < 20 ppm) and high-Nb (Nb > 20 ppm) arc basalts in ancient and modern subduction zones (HNBs). Two types of HNBs are recognized on the basis of their geochemistry. Type I HNBs (Kamchatka, Honduras) share N-MORB-like isotopic and OIB-like trace element characteristics and most probably originate from adakite-contaminated mantle sources. Type II HNBs (Sulu arc, Jamaica) display high-field strength element enrichments in respect to island-arc basalts coupled with enriched, OIB-like isotopic signatures, suggesting derivation from asthenospheric mantle sources in arcs. Adakites and, to a lesser extent, HNBs are associated with Cu–Au porphyry and epithermal deposits in Cenozoic magmatic arcs (Kamchatka, Phlippines, Indonesia, Andean margin) and Paleozoic-Mesozoic (Central Asian and Tethyan) collisional orogens. This association is believed to be not just temporal and structural but also genetic due to the hydrous (common presence of amphibole and biotite), highly oxidized (>ΔFMQ > +2) and S-rich (anhydrite in modern Pinatubo and El Chichon adakite eruptions) nature of adakite magmas. Cretaceous adakites from the Stanovoy Suture Zone in Far East Russia contain Cu–Ag–Au and Cu–Zn–Mo–Ag alloys, native Au and Pt, cupriferous Ag in association witn barite and Ag-chloride. Stanovoy adakites also have systematically higher Au contents in comparison with volcanic arc magmas, suggesting that ore-forming hydrothermal fluids responsible for Cu–Au(Mo–Ag) porphyry and epithermal mineralization in upper crustal environments could have been exsolved from metal-saturated, H2O–S–Cl-rich adakite magmas. The interaction between depleted mantle peridotites and metal-rich adakites appears to be capable of producing (under a certain set of conditions) fertile sources for HNB melts connected with some epithermal Au (Porgera) and porphyry Cu–Au–Mo (Tibet, Iran) mineralized systems in modern and ancient subduction zones.

Old/New Subduction Zone Paradigms as Seen From the Cascades

This paper briefly reviews the magmatic manifestations of volcanic arcs, and aspects of how such magmas form. Common concepts about ‘how subduction zones work’ are reassessed – with particular emphasis on the atypical Cascades volcanic arc. This arc is associated with perhaps one of the warmest modern subduction zones, with accelerated slab dehydration and low flux of slab-derived fluids compared to most others. With minimized leverage of fluids, other aspects of arc magmatism become more evident. We see multiple distinct basaltic magma types, all represented by relatively primitive high-MgO end members that allow assessment of their conditions of formation. Dominant types include two distinct intraplate varieties: low-K tholeiite with similarities to MORB and another variety with affinities to many ocean island basalts. In addition, there is a spectrum of more typical calcalkalic basalts that range to fairly alkali-rich compositions (absarokites) as well as high-Mg basaltic andesites. These magma types are variably present throughout much of the arc and all are present coevally in a transect paralleling the Columbia River. Their complex spatial distributions may in part reflect along-strike differences in tectonic controls and/or dynamics of mantle convective flow. The Cascades arc is broader than most other arcs, with significant basaltic volcanism locally in frontal and back arc regions. Prominence of basaltic magmatism has emerged since the late Miocene, and may be related to changes in the regional tectonic regime over time. These mafic magmas provide energy to drive melting in the crust, and perhaps in the lithospheric mantle, that feeds the more prominent stratovolcanoes distributed along the arc axis. Thermobarometry of the most primitive basalts implies unusual relations between depth and composition, as well as conflicting views regarding melt generation. Some traditional interpretations concerning [1] the conditions of magma formation, [2] the role of subducting oceanic plates (± sediments) in forming volcanic arc magmas, and [3] the nature of magmatic source domains in general appear to be inconsistent with current observations to some degree. These topics are reviewed and their implications assessed regarding physical and tectonic aspects of subduction zones.

Tectonic Settings and Petrogenesis of Pan-African S-Type Granitoids in Anka Schist Belt, NW, Nigeria

Research into geotectonic settings of the Anka schist belt was regionally carried out by (Holt, 1982) stating that there is no evidence of significant partial melting or metasomatism. This research uses chemical data of some Pan-African granitoids within the Anka schist belt to explain the tectonic setting and petrogenesis of the S-type granitoids intruding the metasediments within the Anka schist belt. Twenty-five (25) representative samples of granitoids was analyzed for major oxides and trace elements composition. Results obtained from the chemical data was used to plot various classification diagrams. The granitoids in the study area plots within the orogenic class and the use of trace element discrimination diagrams as a means of fingerprinting the tectonic environments revealed that the granitoids ranges from syn-collision to late orogenic field. The granitoids which plotted in the syn-collision field is a characteristic of chemical affinity of syn-collision peraluminous granites with highly fractionated volcanic-arc magmas. The petrogenesis of the S-type granitoids in the study area are greywacke sourced which is largely metagreywacke and metapelitic sourced during partial melting. Keywords: Granitoids, Pan-African, S-type, Metasediments, Partial melting DOI : 10.7176/JNSR/10-2-08 Publication date: January 31 st 2020

Rhenium enrichment in the northwest Pacific arc

Rhenium (Re) is one of the least abundant elements in the silicate Earth. However, independent rhenium sulfide was reported in the Usu and Kudryavy volcanos, namely rheniite (ReS2, 74.5 wt% of Re). This raises questions about the sources of the Kurile–Kamchatka volcanic island arc magmas and geochemical cycling of Re. The oxidation-reduction cycle on the Earth’s surface is the major process that concentrates Re in reduced sediments. The northwest part of the Pacific plate mainly consists of Jurassic to Cretaceous oceanic crust, with abundant organic-rich sediments. During plate subduction, Re is remobilized early due to the devolatilization of organic-rich sediments and then the decomposition of sulfide (e.g., pyrite), and is subsequently added to the mantle wedge at shallow depths, which is usually not sampled by arc magmas unless there is a major change in the subduction regime. Ridge subduction in the northwest Pacific was initially characterized by low angle subduction. When slab rollback started, rhenium-enriched components were involved in arc magmas, and further concentrated in volcanic vents because of the volatility of Re, forming rheniite.

Uranium isotope fractionation during slab dehydration beneath the Izu arc

Fluids released from subducted slabs impart characteristic geochemical signatures on volcanic arc magmas and residual slabs transported into the deeper mantle. Yet, the sources and transport mechanisms of trace elements released from the slab are speculative. We investigate fluids released from subducted slabs from the perspective of 238U/235U and radiogenic Pb isotope ratios in lavas from the Izu volcanic arc in the Pacific ocean. Izu arc lavas are fluid-dominated end-member type magmas that allow a close characterisation of slab fluids. The Izu arc lavas have low 238U/235U ratios compared to the bulk Earth and mid-ocean ridge basalt (MORB). The low 238U/235U (δ238U=−0.46 to −0.33‰, where δ238U=238U/235Usample/238U/235UCRM145 − 1) is associated with slab-derived fluids low in Th/U that are added to the magma sources. The radiogenic Pb isotope ratios of the lavas form an array between ‘Indian’ type MORB and subducting sediments that is inconsistent with fluids derived from the altered mafic oceanic crust (AMOC). We infer that ‘fluid-mobile’ elements, including U and Pb are mobilised from largely unaltered, deeper sections of the mafic crust by migrating fluids that are derived from the dehydration of underlying serpentinites. Uranium is only fluid-mobile as UVI and needs to be oxidised from predominant UIV in unaltered magmatic rocks in order to be mobilised by fluids. Uranium isotope fractionation of ∼0.2‰ in δ238U during this process is required to generate the low 238U/235U in the fluids. We propose that channelised fluid flow through the metamorphosed sheeted dyke and gabbroic sections of the mafic crust locally oxidises and mobilises U. We suggest that U isotope fractionation occurs within the fluid channels and is related to equilibrium isotope fractionation during the oxidation of U and the incorporation of UIV into secondary phases such as epidote, apatite and zircon that grow within the channels. These phases are predicted to carry isotopically heavy U into the deeper mantle beyond subduction zones. The δ238U is thus tracing the dehydration process of subducting slabs. Similar observations have been made for other, ‘stable isotope’ systems in different arcs and subduction-related metamorphic rocks, thus highlighting their potential for studying processes occurring within the slabs during subduction. This information is essential for understanding and the partitioning of elements between subducted slabs and the mantle wedge and constraining the role of subduction zones in global geochemical cycles.

Heavy oxygen recycled into the lithospheric mantle

Magmas in volcanic arcs have geochemical and isotopic signatures that can be related to mantle metasomatism due to fluids and melts released by the down-going oceanic crust and overlying sediments, which modify the chemistry and mineralogy of the mantle wedge. However, the effectiveness of subduction-related metasomatic processes is difficult to evaluate because the composition of arc magmas is often overprinted by interactions with crustal lithologies occurring during magma ascent and emplacement. Here, we show unequivocal evidence for recycling of continental crust components into the mantle. Veined peridotite xenoliths sampled from Tallante monogenetic volcanoes in the Betic Cordillera (southern Spain) provide insights for mantle domains that reacted with Si-rich melts derived by partial melting of subducted crustal material. Felsic veins crosscutting peridotite and the surrounding orthopyroxene-rich metasomatic aureoles show the highest 18O/16O ratios measured to date in upper mantle assemblages worldwide. The anomalously high oxygen isotope compositions, coupled with very high 87Sr/86Sr values, imply the continental crust origin of the injected melts. Isotopic anomalies are progressively attenuated in peridotite away from the veins, showing 18O isotope variations well correlated with the amount of newly formed orthopyroxene. Diffusion may also affect the isotope ratios of mantle rocks undergoing crustal metasomatism due to the relaxation of 18O isotope anomalies to normal mantle values through time. Overall, the data define an O isotope “benchmark” allowing discrimination between mantle sources that attained re-equilibration after metasomatism (>5 Myr) and those affected by more recent subduction-derived enrichment processes.

Origin of negative cerium anomalies in subduction-related volcanic samples: Constraints from Ce and Nd isotopes

Negative Cerium (Ce) anomalies are observed in chondrite-normalized Rare Earth Element patterns from various volcanic arc suites. These anomalies are well defined in volcanic rocks from the Mariana arc and have been interpreted as the result of addition of subducted sediments to the arc magma sources. This study combines 143Nd/144Nd and 138Ce/142Ce isotope measurements in Mariana volcanic rocks that have Ce anomalies ranging from 0.97 to 0.90. The dataset includes sediments sampled immediately before subduction at the Mariana Trench (Sites 801 and 802 of ODP Leg 129) and primitive basalts from the Southern Mariana Trough (back-arc basin). Binary mixing models between the local depleted mantle and an enriched end-member using both types of sediment (biosiliceous and volcaniclastic) found in the sedimentary column in front of the arc are calculated. Marianas arc lavas have Ce and Nd isotopic compositions that require <2.5% of a sediment component derived from the volcaniclastics. With this proportion of sediment, most of the Ce/Ce* range measured in lavas is reproduced. Thus, this study confirms that the origin of the Ce anomalies in the Mariana arc magmas can be principally attributed to recycling of trench sediments through active subduction. The participation of a component derived from biosiliceous sediments does not explain the Ce-Nd isotope composition of the lavas because the involved proportion is too high (up to 8%) in comparison to results obtained from other geochemical proxys. Using this end-member, the modeled Ce anomalies are also too high (0.91–0.84) in comparison to those measured in lavas. Various processes and conditions are able to generate Ce anomalies: oxygen fugacity, residual mineral phases, partial melting, fractional crystallization and tropical weathering. Their influence in the case of Mariana volcanic arc magmas seems to be very limited but partial melting effect may explain the lowest measured Ce/Ce* values. Magmatic processes cannot be definitely ruled out in producing Ce anomalies in other arc system environments. Additional experimental data, however, are needed for a better understanding of the behavior of cerium relative to its neighboring elements. Also, this study highlights the importance of using local depleted mantle and sediments to model the isotopic compositions of arc lavas.

Magnesium Isotopes as a Tracer of Crustal Materials in Volcanic Arc Magmas in the Northern Cascade Arc

Fifteen North Cascade Arc basalts and andesites were analyzed for Mg isotopes to investigate the extent and manner of crustal contributions to this magmatic system. The δ26Mg of these samples vary from within the range of ocean island basalts (the lightest being -0.33 ± 0.07‰) to heavier compositions (as heavy as -0.15 ± 0.06‰). The observed range in chemical and isotopic composition is similar to that of other volcanic arcs that have been assessed to date in the circum-pacific subduction zones and in the Caribbean. The heavy Mg isotope compositions are best explained by assimilation and fractional crystallization within the deep continental crust with a possible minor contribution from the addition of subducting slab-derived fluids to the primitive magma. The bulk mixing of sediment into the primitive magma or mantle source and the partial melting of garnet-rich peridotite are unlikely to have produced the observed range of Mg isotope compositions. The results show that Mg isotopes may be a useful tracer of crustal input into a magma, supplementing traditional methods such as radiogenic isotopic and trace element data, particularly in cases in which a high fraction of crustal material has been added.

Whole rock geochemistry, Zircon U–Pb and Hf isotope systematics of the Çangaldağ Pluton: Evidences for Middle Jurassic Continental Arc Magmatism in the Central Pontides, Turkey

The Central Pontides (Turkey) includes several igneous complexes, which are predominantly Permo-Carboniferous and Middle Jurassic in age. The Çangaldağ Pluton is one of the largest igneous bodies located in the northern Central Pontides, whose age and tectono-magmatic evaluation is important to constrain the geological evolution of the Northern Neotethys. The pluton tectonically overlies the Çangaldağ Metamorphic Complex, which represents an arc–back-arc complex of Middle Jurassic age. The Çangaldağ Pluton consists mainly of non-metamorphic gabbroic diorite, dacite porphyry, and lesser amount of granitic rocks. Geochemically, the various rock types are akin to volcanic arc magmas displaying Th/Nb and light rare earth element/heavy rare earth element enrichments. In-situ U–Pb age results of zircons from dacite porphyry and granite samples yield ages of 161±5Ma and 170±2Ma, respectively. Corresponding in-situ average 176Hf/177Hf initial ratios are 0.28287±0.00004 and 0.28213±0.00002 for the dacite porphyry and granite samples, respectively. These results are consistent with derivation from a subduction-modified mantle source. Based on Hf isotope compositions, TDM model ages vary between 624 and 1512Ma and suggest that the arc magmatism associated with the Çangaldağ Pluton may have involved partial melting of Neoproterozoic/Mesoproterozoic crustal rocks, a common feature in Gondwana-derived terranes. The geochemical, Hf isotope, and geochronological data reported here confirm the presence of a continental arc system during the Middle Jurassic in the Central Pontides, and suggests that the Çangaldağ Pluton formed during northward subduction of the Intra-Pontide branch of the Northern Neotethys.

The role of crystallization‐driven exsolution on the sulfur mass balance in volcanic arc magmas

AbstractThe release of large amounts of sulfur to the stratosphere during explosive eruptions affects the radiative balance in the atmosphere and consequentially impacts climate for up to several years after the event. Quantitative estimations of the processes that control the mass balance of sulfur between melt, crystals, and vapor bubbles is needed to better understand the potential sulfur yield of individual eruption events and the conditions that favor large sulfur outputs to the atmosphere. The processes that control sulfur partitioning in magmas are (1) exsolution of volatiles (dominantly H2O) during decompression (first boiling) and during isobaric crystallization (second boiling), (2) the crystallization and breakdown of sulfide or sulfate phases in the magma, and (3) the transport of sulfur‐rich vapor (gas influx) from deeper unerupted regions of the magma reservoir. Vapor exsolution and the formation/breakdown of sulfur‐rich phases can all be considered as closed‐system processes where mass balance arguments are generally easier to constrain, whereas the contribution of sulfur by vapor transport (open system process) is more difficult to quantify. The ubiquitous “excess sulfur” problem, which refers to the much higher sulfur mass released during eruptions than what can be accounted for by amount of sulfur originally dissolved in erupted melt, as estimated from melt inclusion sulfur concentrations (the “petrologic estimate”), reflects the challenges in closing the sulfur mass balance between crystals, melt, and vapor before and during a volcanic eruption. In this work, we try to quantify the relative importance of closed‐ and open‐system processes for silicic arc volcanoes using kinetic models of sulfur partitioning during exsolution. Our calculations show that crystallization‐induced exsolution (second boiling) can generate a significant fraction of the excess sulfur observed in crystal‐rich arc magmas. This result does not negate the important role of vapor migration in sulfur mass balance but rather points out that second boiling (in situ exsolution) can provide the necessary yield to drive the excess sulfur to the levels observed for crystal‐rich systems. In contrast, in crystal‐poor systems, magma recharge that releases sulfur‐rich bubbles is necessary and most likely the primary contributor to sulfur mass balance. Finally, we apply our model to account for the effect of sulfur partitioning during second boiling and its impact on sulfur released during the Cerro Galan supereruption in Argentina (2.08 Ma) and show the potential importance of second boiling in releasing a large amount of sulfur to the atmosphere during the eruption of large crystal‐rich ignimbrites.

Geochronology and geochemistry of the major host rock of the Dong’an gold deposit, Lesser Khingan Range: Implications for petrogenesis and metallogenic setting during the Early–Middle Jurassic in northeast China

The Dong’an gold deposit is a large-sized epithermal gold deposit recently discovered in the Lesser Khingan Range, NE China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of a medium–coarse grained alkali-feldspar granite, the major host rock of the Dong’an gold deposit. The LA–ICP–MS zircon U–Pb dating of the medium–coarse grained alkali-feldspar granite yields an early Jurassic age of 176.3±1.1Ma (MSWD=0.62). The whole-rock geochemical data indicate that the samples are felsic, ferroan, alkali-calcic and peraluminous with relatively high alkali (K2O+Na2O) content. They are enriched in LREEs and LILEs (e.g., Rb, Ba, K), but are depleted in HFSEs (e.g., Nb, Ta, P, Ti), especially in P and Ti, showing characteristics of volcanic arc magmas and similarities with the Early–Middle Jurassic granitic rocks in Xing’an Mongolian orogenic belt. Meanwhile, the negative Eu, Nb, Ta, Ti, and P anomalies are consistent with fractional crystallization of plagioclase, Ti-bearing phases (rutile, ilmenite, titanite, etc.) and apatite during magma evolution. The samples have low Nb/Ta ratios (8.65–14.91) and low Mg# values (18–36), which are indicative of crustal derived magmas and no interaction between source magmas and the mantle. In-situ Hf isotopic analyses of the zircons from the medium–coarse grained alkali-feldspar granite yield εHf(t) values of +3.38–+5.68 and two-stage model ages (TDM2) of 772–900 Ma, indicating the magmas formed this intrusion were generated by partial melting of Neoproterozoic basaltic materials in the young lower crust, and the magma source could be derived from a depleted mantle. The medium–coarse grained alkali-feldspar granite most likely formed in the late stage of Toarcian subduction of the Pacific plate, which can be identified on the tectonic setting discrimination diagrams, and the formation of this intrusion was associated with underplating of mantle-derived magmas, which provided heat for crustal partial melting. Similar to the medium–coarse grained alkali-feldspar granite, large amounts of granitic rocks and a series of nonferrous metal hydrothermal deposits (Mo, Cu, Au) formed in northeast China as results of magmatic activities triggered by subduction of the Pacific plate during the Early–Middle Jurassic.

U–Pb dating, geochemistry, and Sr–Nd isotopic composition of a granodiorite porphyry from the Jiadanggen Cu–(Mo) deposit in the Eastern Kunlun metallogenic belt, Qinghai Province, China

The Jiadanggen porphyry Cu–(Mo) deposit is newly discovered and located in the Eastern Kunlun metallogenic belt of Qinghai Province, China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of the mineralization causative granodiorite porphyry. The new data indicate that the granodiorite porphyry is characterized by high SiO2 (68.21–70.41wt.%) and Al2O3, relatively high K2O, low Na2O, and low MgO and CaO concentrations, and is high-K calc-alkaline and peraluminous. The granodiorite porphyry has low Mg# (38–46) values that are indicative of no interaction between the magmas and the mantle. The samples that we have examined have low Nb/Ta (9.17–10.3) and Rb/Sr (0.28–0.39) ratios, which are indicative of crustal-derived magmas. Source region discrimination diagrams indicate that the magmas that formed the granodiorite porphyry were derived from melting of a mixed amphibolite source in the lower crust. The samples have ISr values of 0.70954–0.70979, εNd(t) values of −8.3 to −7.9, and t2DM ages ranging from 1644 to 1677Ma. These indicate that the magmas that formed this intrusion were generated by melting of Mesoproterozoic lower crustal material. Higher K(Rb) contents of the samples indicate that the magma source is high potassium basaltic material in the lower crust, which could be derived from an enriched mantle source. LA-ICP-MS zircon U–Pb dating of the granodiorite porphyry yields a late Indosinian age (concordia age of 227±1Ma; MSWD=0.31), which is close to the molybdenite Re–Os isochron age (227.2±1.9Ma), indicating further the close relationship between the granodiorite porphyry and the Cu–(Mo) mineralization. These samples are LREE and LILE (e.g., Rb, K, Ba, and Sr) enriched, and HFSE (e.g., Nb, Ta, P, and Ti) depleted, especially in P and Ti, similar to the characteristics of volcanic arc magmas. This intrusion most likely formed during the later stage of Indosinian deep subduction of oceanic slab. This was associated with underplating of mantle-derived magmas, which provided heat for crustal melting. Similar to the Jiadanggen granodiorite porphyry, Indosinian hypabyssal intermediate-felsic intrusive rocks, formed under subduction tectonism or a transitional regime from subduction to syn-collision, make up the most important targets for porphyry Cu(Mo) deposits in the Eastern Kunlun metallogenic belt.

Late Oligocene post‐obduction granitoids of New Caledonia: A case for reactivated subduction and slab break‐off

Abstract In southern New Caledonia, Late Oligocene granodiorite and adamellite are intruded into an ultramafic allochthon emplaced in the Late Eocene period. Previous studies of these granitoids proposed an origin associated with the melting of the underlying continental crust, but our new data show that these high‐K to medium‐K calc‐alkaline granitoids display the geochemical and isotopic features of volcanic arc magmas uncontaminated by crust‐derived melts. These magmas were probably generated in a post‐Eocene and pre‐Miocene subduction, the geophysical traces of which have been detected along the western coast of New Caledonia. Sr, Nd and Pb isotopic ratios indicate derivation from an almost isotopically homogeneous mantle wedge, but in contrast, some variation in trace element ratios uncorrelated to differentiation is indicative of source heterogeneity. Prominent heavy rare earth element (HREE) depletion of some of the younger granitoids may be the result of an equilibrium achieved with garnet‐bearing subcrustal material (granulite) found as xenoliths, while a relative Nb, Ta and Hf enrichment, irrespective of crystal fractionation, may be related to either a modest contamination by previously underplated mafic material, heterogeneous hydration of the mantle wedge, or mixing with uplifted Nb‐rich mantle. Post‐obduction slab break‐off can be proposed to have played a role in sublithospheric mantle mixing and the subsequent heterogeneity. The Late Oligocene subduction described here may be tentatively extended southward into northern New Zealand allochthons.

Geochemical and isotope data from the Acatlán Volcanic Field, western Trans-Mexican Volcanic Belt: Origin and evolution

The Quaternary Acatlán Volcanic Field (AVF) is located at the western edge of the Trans-Mexican Volcanic Belt (TMVB). This region is related to the subduction of the Pacific Cocos and Rivera plates beneath the North American plate since the late Miocene. AVF rocks are products of Pleistocene volcanic activity and include lava flows, domes, erupted basaltic andesite, trachyandesite, trachydacite, and rhyolite of calc–alkaline affinity. Most rocks show depletion in high field-strength elements and enrichment in large ion lithophile elements and light rare earth elements as is typical for magmas in subduction-related volcanic arcs. 87Sr/ 86Sr values range from 0.70361 to 0.70412, while ɛNd values vary from +2.3 to +5.2. Sr–Nd isotopic data plot along the mantle array. On the other hand, lead isotope compositions ( 206Pb/ 204Pb=18.62–18.75, 207Pb/ 204Pb=15.57–15.64, and 208Pb/ 204Pb=38.37–38.67) give evidence for combined influences of the upper mantle, fluxes derived from subducted sediments, and the upper continental crust involved in magma genesis at AVF. Additionally δ 18O whole rock analyses range from +6.35‰ in black pumice to +10.9‰ in white pumice of the Acatlán Ignimbrite. A fairly good correlation is displayed between Sr as well as O isotopes and SiO 2 emphasizing the effects of crustal contamination. Compositional and isotopic data suggest that the different AVF series derived from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source.

Progressive left ventricular remodeling in patients with hypertrophic cardiomyopathy and severe left ventricular hypertrophy

The Central Pontides (Turkey) includes several igneous complexes, which are predominantly Permo-Carboniferous and Middle Jurassic in age. The Çangaldağ Pluton is one of the largest igneous bodies located in the northern Central Pontides, whose age and tectono-magmatic evaluation is important to constrain the geological evolution of the Northern Neotethys. The pluton tectonically overlies the Çangaldağ Metamorphic Complex, which represents an arc–back-arc complex of Middle Jurassic age. The Çangaldağ Pluton consists mainly of non-metamorphic gabbroic diorite, dacite porphyry, and lesser amount of granitic rocks. Geochemically, the various rock types are akin to volcanic arc magmas displaying Th/Nb and light rare earth element/heavy rare earth element enrichments. In-situ U–Pb age results of zircons from dacite porphyry and granite samples yield ages of 161 ± 5 Ma and 170 ± 2 Ma, respectively. Corresponding in-situ average 176Hf/177Hf initial ratios are 0.28287 ± 0.00004 and 0.28213 ± 0.00002 for the dacite porphyry and granite samples, respectively. These results are consistent with derivation from a subduction-modified mantle source. Based on Hf isotope compositions, TDM model ages vary between 624 and 1512 Ma and suggest that the arc magmatism associated with the Çangaldağ Pluton may have involved partial melting of Neoproterozoic/Mesoproterozoic crustal rocks, a common feature in Gondwana-derived terranes. The geochemical, Hf isotope, and geochronological data reported here confirm the presence of a continental arc system during the Middle Jurassic in the Central Pontides, and suggests that the Çangaldağ Pluton formed during northward subduction of the Intra-Pontide branch of the Northern Neotethys.

Petrology, geochemistry and tectonics of the Sabzevar ophiolite, North Central Iran

The Sabzevar ophiolite is a highly dismembered ophiolite complex located along the northern boundary of the central Iranian microcontinent (CIM), and is one of the internal Iranian group of ophiolites and colored melanges. The igneous rocks of this complex consist of peridotites (harzburgite, dunite and lherzolite), serpentinite, minor pyroxenite, gabbros, and a volcanic sequence that exhibits a wide range of composition from basalts and basaltic andesites to rhyodacite–dacites, rhyolites and basanites. Sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-marine rocks. These include pelagic fossiliferous carbonates that are mixed with the pillow basalt and basaltic andesite as interlayers or exotic blocks ranging in size from 10 to 100 m. Also present are extensive units of radiolarian chert, which are interbedded within the basalts and basaltic andesites. A combination of petrographic observations and analyses of incompatible trace elements and rare earth elements indicates the presence of at least four different types of extrusive rocks in the Sabzevar ophiolite. The geochemical data clearly identifies some of the extrusive rocks to have formed from three distinct types of basaltic melts; (i) the group-1 basaltic rocks, which formed from an initial melt with N-MORB-like (LREE depleted) chemical signatures and are petrogenetically related to the gabbros, (ii) group-2 basaltic rocks which have E-MORB chemical signatures, and (iii) group-3 basaltic rocks with LREE-enriched signatures and incompatible trace element patterns that suggest an island arc affinity. The data also suggest that the gabbros and the group-1 basaltic rocks are petrogenetically related and produced by fractionation of plagioclase and to a lesser extent clinopyroxene and hornblende. Similar to other studied Iranian ophiolites (e.g. Khoy, Band-e-Zeyarat/Dar Anar, Neyriz, Kermanshah and Baft) the results from the Sabzevar ophiolite demonstrate the presence of relics of mid-ocean ridge basalt (MORB)-like signatures in the extrusive units of Neo-Tethyan ophiolites and suggests that the fossil oceanic lithosphere preserved in these ophiolites developed at an oceanic spreading center. In addition, the Sabzevar ophiolite contains other volcanic rocks that have non-MORB-like geochemical signatures (e.g. within-plate, island arc). The differences between these geochemical signatures could be a result of a differing upper mantle composition, or different degrees of partial melting of the same upper mantle. The presence of sizable Nb anomalies in the extended rare earth element (REE) patterns for andesites and the group-3 basalts is characteristic of volcanic arc magmas, or possibly are products of the ascending magma that was modified by crustal contamination. The presences of the basanites and the silica-undersaturated rocks could be attributed to extrusive activity following ophiolite emplacement. Plate reconstructions suggest that rocks of the Sabzevar ophiolite were part of the Tethyan oceanic crust that was formed during rifting of a narrow but deep seaway that separated the CIM from the Eurasian plate. The ophiolite was emplaced during northeast dipping subduction (i.e. closure) of this segment of the Tethyan seaway (Sabzevar Ocean).