Supra-subduction Mantle Wedge Research Articles

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.

Позднемеловые-раннепалеогеновые вулканические породы Восточной Камчатки (особенности состава и изотопно-геохимическая гетерогенность)

New and previously published isotope-geochemical material on the Upper Cretaceous-Lower Paleogene volcanic complexes of the Ozernovsky-Valaginsky segment of the Achaivayam-Valaginsky paleoarc are presented. It is found that composition of rocks composing separate volcanic fields within the Achayvayam-Valagin paleoarc (terrane) is different. On Karaginsky Island, volcanics of the moderate-potassium calc-alkaline and shoshonite series are widespread. On Ozernaya Peninsula, low-potassium basalts of the tholeiitic series are predominantly developed. The rocks of the Kumroch Ridge include low-potassium basalt-basaltic andesite, medium-potassium calc-alkaline and high-potassium shoshonitic. Volcanic rocks of the Tumrok Ridge are represented by low-potassium picrites and high-potassium trachybasalt-trachyandesites. In the Valaginsky Ridge, low-potassium tholeiitic, moderate-potassium calc-alkaline and shoshonite-latite series, as well as picrites and lamproitoids are distinguished. Isotopic (Sr, Nd and Pb) and geochemical composition of the rocks indicate their formation from a depleted mantle source within an ensimatic volcanic arc. It is shown that in the absence of significant differences in the material composition of rocks, regional differences are manifested. It is suggested that large-ion lithophile elements enrichment of volcanics may relate to fluids introduced into the melts during the melting of the supra-subduction mantle wedge. Issues of geodynamics of the region are considered.

Magmatic Native Gold: Composition, Texture, Genesis, and Evolution in the Earth’s Crust

Abstract —Here we report results of microforms’ studies of native gold and its alloys in igneous rocks, modified to varying degrees by secondary processes. We discuss the composition and occurrence of both the deep-seated magmatic gold-bearing alloys and the products of their transformation under conditions of the upper Earth’s crust. Gold-bearing Kamchatka adakites and ankaramites, Ildeus massif mafic-ultramafic intrusions and adakites from the Stanovoy fold system as well as dacites from the Bolivian Andes were formed during melting of either the suprasubduction mantle wedge or the subducted oceanic crust. In depleted peridotites from the Avachinsky Volcano in Kamchatka as well as suprasubduction ophiolites from Polar Urals, Eastern Sayan and the Western Mediterranean Betic–Rifean belt, the gold-bearing mantle was hybridized by subduction-related melts and high-temperature fluids. Volcanic rocks associated with the Lesser Khingan Fe–Mn deposits and Zolotaya Gora Au deposit in Southern Urals as well as Taragai ultramafic rocks in the South Khingan Range display subduction-related geochemical characteristics. Gold-bearing trachytes in the Virginian Appalachians (USA) represent felsic differentiates of mafic intraplate magmas. We propose that one of the principal forms of gold transport into the upper crustal environments is represented by Cu–Ag–Au alloys, which precipitated from mantle-derived silicate melt enriched in chalcophile and siderophile elements. Such Cu–Ag–Au alloy-rich magmatic rocks can either constitute primary sources of precious metals in the mantle-crust system or serve as geochemical precursors to the formation of native gold assemblages in epithermal and mesothermal ore deposits. Presence of magmatic gold particles in subduction-related igneous rocks and mantle restites hybridized by subduction-derived melts and high-temperature fluids suggest the existence of gold-rich horizons in the Earth’s mantle at depths comparable to typical depths of generation of primary convergent zone and some within-plate magmas.

The Formation of Magnesite Ores by Reactivation of Dunite Channels as a Key to Their Spatial Association to Chromite Ores in Ophiolites: An Example from Northern Evia, Greece

Ophiolite magnesite deposits are among the main sources of magnesite, a raw material critical for the EU. The present work focuses on magnesite occurrences at Kymasi (Evia Island, Greece), in close spatial association with chromitite within the same peridotite massif, and on the relationship between ultramafic rocks and late magnesite veins. Chromitite lenses are hosted within dunite, in contact with a partially serpentinized peridotite cut by magnesite veins. Close to the veins, the peridotite shows evidence of carbonation (forming dolomitized peridotite) and brecciation (forming a serpentinite–magnesite hydraulic breccia, in contact with the magnesite veins). Spinel mineral chemistry proved to be crucial for understanding the relationships between different lithologies. Spinels within partially serpentinized peridotite (Cr# 0.55–0.62) are similar to spinels within dolomitized peridotite (Cr# 0.58–0.66). Spinels within serpentinite–magnesite hydraulic breccia (Cr# 0.83–0.86) are comparable to spinels within dunite and chromitite (Cr# 0.79–0.84). This suggests that older weak zones, such as dunite channels, were reactivated as fluid pathways for the precipitation of magnesite. Magnesite stable isotope composition, moreover, points towards a meteoric origin of the oxygen, and to an organic source of carbon. The acquired data suggest the following evolution of Kymasi ultramafic rocks: (i) percolation of Cr-bearing melts in a supra-subduction mantle wedge within dunite channels; (ii) obduction of the ophiolitic sequence and peridotite serpentinization; (iii) uplift and erosion of mantle rocks to a shallow crustal level; (iv) percolation of carbon-rich meteoric waters rich at shallow depth, reactivating the dunite channels as preferential weak zones; and (v) precipitation of magnesite in veins and partial brecciation and carbonation of the peridotite host rock.

Carbonatite-melilitite-phosphate immiscible melts from the aragonite stability field entrained from the mantle by a Pliocene basalt

A plagioclase megacryst containing composite crystal-glass inclusions was ejected in a Pliocene basaltic diatreme in the Carpathian back-arc basin (Pannonian Basin). The megacryst grew from phonolitic melt, relics of which are preserved in the crystal-glass inclusions. Most of the pristine melt has undergone substantial compositional resetting by interaction with several batches of a low-viscosity carbonated, P-rich melilitite melt, which infiltrated and largely replaced the original inclusion content. The melilitite melt also caused partial resorption of the host megacryst and crystallisation of new calcic plagioclase forming stringers and palisades. A P-rich calcic carbonatite melt exsolved from the melilitite and later crystallised to aragonite at ~ 800 °C and 1.9 GPa. The phosphate melt fraction exsolved from the carbonatite and solidified as CO32−-rich A-B type apatite. At a very late evolutionary stage, K- and Si-rich fluids caused potassic and silicic alteration of the solidified melilitite glass along cracks and interfaces between calcic carbonate globules and glass at temperatures below 680 °C. The oxygen isotope composition of the plagioclase megacryst (6.2 ‰ V-SMOW; Vienna Standard Mean Ocean Water) and the 87Sr/86Sr isotope ratio of carbonates in the inclusions (0.7034) are consistent with a mantle-derived melt. 87Sr/86Sr isotope ratios (0.7047–0.7051) in interstitial carbonates from associated syenite and carbonatite xenoliths indicate a metasomatised mantle source contaminated with radiogenic crustal material or altered marine carbonate. The O-isotope ratios in the carbonates, 22.7 ± 0.6 ‰ V-SMOW in calcite and 23.6 ± 0.7 ‰ V-SMOW in aragonite, are also consistent with a sedimentary precursor. Contrasting δ13C values in the calcite, -12.7 ± 0.5 ‰ V-PDB (Vienna PeeDee Belemnite), and the aragonite (-4.6 ± 0.5 ‰ V-PDB) indicate low-temperature modification of calcite assisted by delta13C-depleted CO2 and preservation of primary magmatic delta13C values in aragonite. The microstructural and geochemical evidence points towards heterogeneous silicate-carbonate melt fractions generated during the metasomatism and partial melting of a supra-subduction mantle wedge.

Isotopes track Tethyan seamount subduction beneath the Troodos spreading centre, Cyprus

Supra-subduction zone volcanoes record the chemical transfer of sedimentary and igneous material from the subducting plate to the overlying mantle and lithosphere. This transfer is thought to preferentially involve fluid-mobile elements, which swamp the signature of similar trace elements in the supra-subduction mantle. Using high-resolution isotope measurements of fluid-mobile Pb, we have traced the evolution and inputs beneath the supra-subduction spreading ridge that is now the Troodos Ophiolite, Cyprus. It is apparent that variable but significant amounts of Pb are actually derived from the mantle wedge, and substantial amounts of the fluid-immobile elements like Nb are introduced from the slab. Our study identifies a chemical transformation of the spreading crust, which reflects a spatial change in the Neo-Tethyan slab. The west was influenced by the subduction of pelagic/carbonate sediments overlying MORB, whereas the east and south also include a high 206Pb/204Pb component, similar to high-μ mantle plumes distributed around northern Africa. This chemo-tectonic switch is interpreted to represent the location of supra-subduction spreading relative to the path of a seamount chain colliding with and subducting beneath the Troodos trench. The Hecataeus Rise, located to the southeast of Cyprus, is a candidate to be a foundered relic of this seamount chain, locked in the subduction zone. We also demonstrate that the supra-subduction mantle wedge was of an Atlantic/Pacific-type composition, but the subducting Neo-Tethyan oceanic crust had an Indian Ocean signature. As such, this destructive plate boundary can be defined as the Cretaceous interface between the Atlantic and Indian mantle domains.

Tectonomagmatic reconstruction of the Upper Mesozoic-Cenozoic Neotethyan arcs in the Lut block, East Iran: a review and synthesis

The Lut Block in east Iran with a widespread record of arc-type Mesozoic-Cenozoic magmatic rocks is bordered by the Birjand (Sistan) ophiolitic assemblage to the east and the Sabzevar ophiolitic assemblage to the north. The nature and tectonomagmatic setting of these sutures and associated arcs are controversial. The geochemical attributes of the Upper Cretaceous-Paleogene intrusive and extrusive rocks in the central (CLB) and the northern Lut block (NLB) and in the Sabzevar ophiolite zone (SOZ), Late Cretaceous calc-alkaline and adakitic rocks to the east of the Sistan suture (CAS), and the Bazman intrusives to the west are consistent with a subduction-related volcanic arc setting. The Najmabad granitoids in the NLB compositionally overlap with the Eocene Sabzevar calcalkaline and adakitic lava flows (ESA), implying a common magmatic source and a similar trend of evolution. The island arc basalt-like geochemical patterns and the initial Sr and Nd isotope compositions for the CLB intrusive bodies suggest that the parent magmas formed by partial melting in a suprasubduction mantle wedge. The initial 87Sr/861Sr and εNd(t) values for the CAS, the Bazman intrusives, the ESA, and the Najmabad granitoids are similar and overlap those of the Sabzevar ophiolitic rocks. Here, we review the previously proposed one-sided subduction models for the Lut block and the Sabzevar suture zone, and discuss in favour of a two-sided asymmetric subduction as an alternative geodynamic model.

ГЕОХИМИЯ НЕОГЕНОВЫХ ВУЛКАНИТОВ СЕВЕРНОЙ ЧАСТИ ЦЕНТРАЛЬНО-КАМЧАТСКОГО ВУЛКАНИЧЕСКОГО ПОЯСА

New geological and geochemical data on the age and composition of Neogene rocks in the northern part of the Central Kamchatka volcanic belt are presented. K-Ar dating of volcanic rocks determined the age of all studied volcanic rocks to be Late Miocene. The geochemical characteristics of the rocks show that the northern part of the Central Kamchatka volcanic belt is characterized by a complex geological history, in which magma generation processes are governed by the type of mantle matter. The “earliest” among the Late Miocene rocks that make up the Umuvayam and Tolyatovayam complexes (6.7–9.0 Ma) are represented by typical island-arc rocks. Whereas, “later” (5.4–7.7 Ma) volcanic rocks of the Alney and Veemgetver complexes are characterized by an association of rocks with a high proportion of enrichment from the OIB-type source. The magma source corresponded in composition to spinel lherzolite. The enrichment of volcanic rocks in large-ion lithophile elements is explained by the role of fluids introduced into the melts during the melting of the supra-subduction mantle wedge, which probably also underwent modification as a result of supra-subduction metasomatism in the geological past.

Secular variations of magma source compositions in the North Patagonian batholith from the Jurassic to Tertiary: Was mélange melting involved?

Abstract This study of Sr-Nd initial isotopic ratios of plutons from the North Patagonian batholith (Argentina and Chile) revealed that a secular evolution spanning 180 m.y., from the Jurassic to Neogene, can be established in terms of magma sources, which in turn are correlated with changes in the tectonic regime. The provenance and composition of end-member components in the source of magmas are represented by the Sr-Nd initial isotopic ratios (87Sr/86Sr and 143Nd/144Nd) of the plutonic rocks. Our results support the interpretation that source composition was determined by incorporation of varied crustal materials and trench sediments via subduction erosion and sediment subduction into a subduction channel mélange. Subsequent melting of subducted mélanges at mantle depths and eventual reaction with the ultramafic mantle are proposed as the main causes of batholith magma generation, which was favored during periods of fast convergence and high obliquity between the involved plates. We propose that a parental diorite (= andesite) precursor arrived at the lower arc crust, where it underwent fractionation to yield the silicic melts (granodiorites and granites) that formed the batholiths. The diorite precursor could have been in turn fractionated from a more mafic melt of basaltic andesite composition, which was formed within the mantle by complete reaction of the bulk mélanges and the peridotite. Our proposal follows model predictions on the formation of mélange diapirs that carry fertile subducted materials into hot regions of the suprasubduction mantle wedge, where mafic parental magmas of batholiths originate. This model not only accounts for the secular geochemical variations of Andean batholiths, but it also avoids a fundamental paradox of the classical basalt model: the absence of ultramafic cumulates in the lower arc crust and in the continental crust in general.

The Ore and Petroleum Regions of the South Okhotsk Province and Deep Geodynamics

The South Okhotsk Sea province, which includes the Sakhalin, Kunashir, Iturup, Urup islands and surrounding sea areas, comprises numerous occurrences of rare, noble metals and other mineralization, as well as petroleum fields, gas hydrate accumulations, and, locally, active emission of water–hydrocarbon gases. The occurrences and deposits of solid, liquid, and gaseous mineral resources are controlled by hidden deep-seated transform-type fault zones: Nosappu (Tuscarora), Iturup, and Urup. These long-lived extended (over 1000 km) zones are distinguished on the NW Pacific megaplate margin, near the SE flank of the Kuril–Kamchatka trough. Using seismotomographic methods we have established their extension to the west from the seismic focal zone in the oceanic slab subsided into mantle transition zone. In the areas of pull-apart extension, the faults provided active formation of drainage channels for seawater penetration in the lithosphere with subsequent serpentinization of its ultramafic rocks, and for decompressional generation of ascending mantle-derived abiogenic fluid flows. The latter penetrated from the subslab asthenosphere into the suprasubduction mantle wedge and sublithospheric mantle and caused metasomatism. The subsequent migration of the flows led to the generation of the primary magma reservoirs in the lower parts of the continental lithosphere, and intermediate and peripheral chambers in the Earth’s crust. The injection of melts from the chambers in the consolidated crust led to the formation of abyssal and hypabyssal intrusive massifs, arch-dome uplifts, and magmatogenic-ore systems predominantly among the rocks of the pre-Pliocene basement. The concentration of oil and gas accumulations from the mantle-derived abiogenic hydrocarbons bearing mercury, gold, rhenium, and PGE in reservoirs beneath fluid-impermeable beds among Cenozoic sedimentary basins also was related to deep sub- and supraslab fluid flows.

Petrology, petrogenesis, and geochronology review of the Cenozoic adakitic rocks of northeast Iran: Implications for evolution of the northern branch of Neo‐Tethys

Cenozoic adakitic rocks of the northern part of the Central Iran Structural Zone (CISZ) are among the notable geological features of the terrains in northeast Iran, so a comprehensive comparison of several of these adakitic sequences is presented. This lithogeochemical analysis is constrained to examining adakitic magmatism of the three magmatic belts within the CISZ, which from southeast to northeast and from oldest to youngest are as follows: (a) south of Shahrood‐Damghan, (b) north‐northwest of Sabzevar‐Neyshabour, and (c) south of Qouchan and west of Esfarayen. Radiogenic isotope analysis using Rb–Sr and Sm–Nd methods show that the adakitic rocks associated with Qouchan‐Esfarayen magmatism have 0.512581 to 0.51288 initial 143Nd/144Nd and 0.703903 to 0.705627 initial 87Sr/86Sr, with εNd −0.86 to 4.98. Adakitic rocks in south to southeast Shahrood have 0.512775 to 0.512893 initial 143Nd/144Nd and 0.703746 to 0.705314 initial 87Sr/88Sr, with εNd 3.69 to 6.0, and adakites emplaced into the Sabzevar ophiolite have 0.512846 to 0.512911 initial 143Nd/144Nd and 0.70379 to 0.705019 initial 87Sr/86Sr contents with εNd of 5.26 to 6.54. Isotopic initial ratios of Nd and Sr support an origin involving partial melting of the subducting oceanic lithosphere of the northern branch of Neo‐Tethys and the associated suprasubduction mantle wedge in producing these adakitic rocks.

Permian post-collisional basic magmatism from Corsica to the Southeastern Alps

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals.The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab.Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.

Lithospheric mantle, asthenosphere, slab and crustal contribution to petrogenesis of Eocene to Miocene volcanic rocks from the west Alborz Magmatic Assemblage, SE Ahar, Iran

AbstractSignificant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ∼42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ∼19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts.

Density Structure of the Lithosphere of the Sikhote-Alin Orogenic Belt

For the first time, a detailed density 3D model of the lithosphere of the Sikhote-Alin orogenic belt to a depth of 130 km has been created. Two extended submeridional linear zones of low density are distinguished in the upper mantle and characterized. Large tin and gold deposits in the northern Sikhote-Alin are spatially confined to these zones. The present-day density structure of the lithosphere in the region, the formation of which began at the turn from the Late Jurassic to the Early Cretaceous, reflects its density differentiation resulting from the two main geodynamic processes: (1) a subduction regime replaced by a transform regime of the Bureya-Sikhote-Alin active continental margin, the stage (s) of which was (were) characterized by the emergence of mantle low-density zones, associated with the ascent of fluids and melts in the suprasubduction mantle wedge and the formation of intermediate magma chambers; and (2) intraplate rifting, spatially localized in zones of mantle thickening.

Long-lived, Eocene-Miocene stationary magmatism in NW Iran along a transform plate boundary

The Eocene-Miocene Mianeh-Hashtroud igneous district in NW Iran is part of the Turkish–Caucasus–Iranian collision zone, a key region to decipher the assembly and differentiation of Gondwana-derived terranes along the Alpine-Himalayan convergence zone. Major inherited tectonic structures control in space and time the Mesozoic-Cenozoic transition from oceanic subduction to continental collision in the region. The geology of the study area is dominated by a polyphase, long-lived magmatic activity, spanning from ~45 to ~6 Ma. The igneous products are subalkaline to alkaline, with intermediate to acid compositions and a high-K calcalkaline to shoshonitic affinity. Evidence of crustal contamination is attested by inherited zircons in the oldest (Eocene-Oligocene) samples, with ages spanning from Neo-Archean to Paleocene. The Sr-Nd isotopic compositions of the Eocene-Oligocene samples plot close to the Bulk Silicate Earth estimate, whereas the Miocene samples document stronger crustal contamination. The lack of correlation between Nd-Sr isotopes and SiO2 supports a scenario of magma differentiation of different magma batches rather than crustal contamination. Major oxide and Sr-Nd isotopic variation lead us to suggest that magmatism is the consequence of re-melting of earlier underplated (Mesozoic-Tertiary) magmatic products, controlled by amphibole-dominated fractionation processes. Regional scale correlations show long-lived Cenozoic magmatism in NW Iran and Caucasus region, where the main porphyry and epithermal deposits occur. We propose that the Cenozoic collisional magmatism and the associated mineralisation at the junction between NW-Iran and Caucasus was controlled by the activity of a major, lithosphere-scale inherited boundary, transverse to the convergence zone. In such a geodynamic setting, the along-strike segmentation of the lithosphere slab generated asthenospheric melts, their upwelling into the metasomatised supra-subduction mantle wedge and the potential activation of different mantle and crustal sources, with consequent mineral endowment in the region.

Three‐Dimensional Modeling of Mount Etna Volcano: Volume Assessment, Trend of Eruption Rates, and Geodynamic Significance

Abstract3‐D modeling of Mount Etna, the largest and most active volcano in Europe, has for the first time enabled acquiring new information on the volumes of products emitted during the volcanic phases that have formed Mount Etna and particularly during the last 60 ka, an issue previously not fully addressed. Volumes emitted over time allow determining the trend of eruption rates during the volcano's lifetime, also highlighting a drastic increase of emitted products in the last 15 ka. The comparison of Mount Etna's eruption rates with those of other volcanic systems in different geodynamic frameworks worldwide revealed that since 60 ka ago, eruption rates have reached a value near to that of oceanic‐arc volcanic systems, although Mount Etna is considered a continental rift strato‐volcano. This finding agrees well with previous studies on a possible transition of Mount Etna's magmatic source from plume‐related to island‐arc related. As suggested by tomographic studies, trench‐parallel breakoff of the Ionian slab has occurred north of Mount Etna. Slab gateway formation right between the Aeolian magmatic province and the Mount Etna area probably induced a previously softened and fluid‐enriched suprasubduction mantle wedge to flow toward the volcano with consequent magmatic source mixing.

U-Pb zircon geochronology, geochemistry, and petrogenesis of the Hamech intrusions in the Kuh-e-Shah volcano-plutonic complex, Eastern Iran

The Hamech area is located in the west of Kuh-e-Shah volcano-plutonic complex, close to the boundary between the Lut Block and Sistan Suture Zone (SSZ), and is composed of monzonite to diorite porphyries and rare gabbro that intruded into older volcanic rocks. U-Pb zircon dating indicates an age of 36.9 ± 1 Ma for gabbro (Late Eocene, Priabonian) and 38.6 ± 0.5 Ma for dacite (Late Eocene, Bartonian). Geochemically, the studied intrusions are dominantly I-type, high-K calc-alkaline, and metaluminous features. Primitive mantle-normalized trace-element spider diagrams of these rocks present enrichment in ion lithophile elements such as Cs, Rb, Ba, K, and Sr and depletion in high field strength elements such as Nb, Ti, Zr, and heavy rare earth elements. Besides, chondritenormalized rare earth elements plots of the rocks show enrichment in light rare earth elements (6.85 < La N /Yb N < 9.72) and a lack or weak negative Eu anomaly (Eu/Eu* = 0.81-1.02). The initial 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios for the rocks from 0.704541 to 0.704880 and from 0.512633 to 0.512691, respectively, when recalculated to an age of 39 Ma. The eNd (i) values vary from +0.87 to +1.99, which fits into a suprasubduction mantle wedge source for the parental melts. All data suggest that the Hamech intrusions developed in a low maturity continental margin arc setting related to the convergence of the Afghan and Lut Blocks and subduction of Sistan oceanic crust during the Eocene.

Extensive crustal melting during craton destruction: Evidence from the Mesozoic magmatic suite of Junan, eastern North China Craton

The cratonic destruction associated with the Pacific plate subduction beneath the eastern North China Craton (NCC) shows a close relationship with the widespread magmatism during the Late Mesozoic. Here we investigate a suite of intrusive and extrusive magmatic rocks from the Junan region of the eastern NCC in order to evaluate the role of extensive crustal melting related to decratonization. We present petrological, geochemical, zircon U-Pb geochronological and Lu-Hf isotopic data to evaluate the petrogenesis, timing and tectonic significance of the Early Cretaceous magmatism. Zircon grains in the basalt from the extrusive suite of Junan show multiple populations with Neoproterozoic and Early Paleozoic xenocrystic grains ranging in age from 764Ma to 495Ma as well as Jurassic grains with an age range of 189–165Ma. The dominant population of magmatic zircon grains in the syenite defines three major age peaks of 772Ma, 132Ma and 126Ma. Zircons in the granitoids including alkali syenite, monzonite and granodiorite yield a tightly restricted age range of 124–130Ma representing their emplacement ages. The Neoproterozoic (841–547Ma) zircon grains from the basalt and the syenite possess εHf(t) values of −22.9 to −8.4 and from −18.8 to −17.3, respectively. The Early Paleozoic (523–494Ma) zircons from the basalt and the syenite also show markedly negative εHf(t) values of −22.7 to −18.0. The dominant population of Early Cretaceous (134–121Ma) zircon grains presented in all the samples also displays negative εHf(t) values range from −31.7 to −21.1, with TDM of 1653–2017Ma and TDMC in the range of 2193–3187Ma. Accordingly, the Lu-Hf data suggest that the parent magma was sourced through melting of Mesoarchean to Paleoproterozoic basement rocks. Geochemical data on the Junan magmatic suite display features similar to those associated with the arc magmatic rocks involving subduction-related components, with interaction of fluids and melts in the suprasubduction mantle wedge. From the data presented, we propose that the Late Mesozoic intrusive and extrusive suites of Junan represent extensive lower and middle crustal melting, possibly triggered by mantle upwelling during the back-arc spreading associated with the Pacific plate subduction beneath the NCC. Intense asthenospheric upwelling resulted in lithospheric thinning and partial delamination during the Early Cretaceous, marking one of the peak decratonization stages of the NCC.

Migration of fluids and melts in subduction zones and general aspects of thermophysical modeling in geology

Modeling of fluid-magmatic systems in a suprasubduction mantle wedge is considered for the case of Kamchatka with reference to data on peridotites from other known subduction and oceanic rock complexes. This modeling has to take account of magma storage in several intermediate reservoirs at different depths, up to six such reservoirs, as in the case of Avacha Volcano. Comparison of available data on melt inclusions in spinels indicates crystallization of the Avacha peridotites in magmatic systems progressively decreasing in temperature (>1200 °C → 1100 °C → 900 °C) and pressure (from 13.8 to 4.5 kbar) in intermediate reservoirs at depths of 30-40 and 15-20 km. The Avacha harzburgites do not belong to primary oceanic mantle as they lack both signatures of high-temperature plastic flow and effects of mantle melts known for sheared mantle peridotites from ophiolite suites. The vP/vS ratio estimated from jointly analyzed P- and S-wave velocities (vP and vs, respectively), an important indicator for seismic tomographic reconstructions of subduction zones, allows discriminating between regions saturated mainly with liquid (melts) and gas phases beneath volcanoes. Only specially tested tomographic data can provide reliable reference for modeling of mantle wedge processes.

Geochemistry and petrogenesis of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan marginal seas

The results of ICP-MS trace-element (LILE, HFSE, REE) study of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and geochronological K-Ar dating of the Eocene volcanic rocks are presented. Specifics of volcanism developed on submarine rises of these seas was characterized for the first time, and magma sources and geodynamic settings of the volcanic complexes predating the formation of the deep-water basins were determined. It is established that the Late Mesozoic magmas were formed in a subduction setting from spinel peridotites of suprasubduction mantle wedge, which was metasomatically reworked by aqueous fluids that were released by dehydration of sedimentary layer of subducting oceanic plate. This follows from the elevated concentrations of H2O, alkalis, potassium, LILE and LREE, and lowered HFSE (including Ta-Nb minimum) and HREE contents, at lowered Sm/Yb, Nb/Ta, Nb/Y and elevated La/Nb, Ba/La, and Zr/Y ratios. Eocene adakite-like volcanic rocks were identified for the first time in the Sea of Okhotsk. They vary from andesitic to more felsic compositions with elevated MgO (>4%) and elevated La/Yb (>14) and Sr/Y (50–60) ratios. Identification of adakite-like volcanic rocks serves as evidence in support of the transform continental-margin (or plate sliding) setting, which is characterized by breaking apart of subduction slab and formation of slab “windows” acting as pathways for the transfer of asthenospheric mantle into continental lithosphere. New geochemical data on the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and analysis of literature data were used to distinguish two geodynamic settings within these seas: subduction and transform margin. Similar settings operated at that time in the adjacent continental- margin volcanic belts (Akinin and Miller, 2011; Martynov and Khanchuk, 2013; et al.).