Melting Of Mantle Wedge Peridotite Research Articles

High-magnesium igneous associations record final-stage geodynamic process of the southeastern segment of the Central Asian Orogenic Belt

Much controversy exists regarding the evolution and termination of accretionary orogens where typical continent-continent collision features are lacking. The Central Asian Orogenic Belt provides a key opportunity to explore the genetic relationships between the formation of high-Mg igneous associations and the evolution of accretionary orogens. In this study, two-stage Triassic magmatic events were identified in the southeastern segment of the Central Asian Orogenic Belt (including ca. 247 Ma andesites and ca. 222 Ma diorites), through precise secondary ion mass spectrometry (SIMS) and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon dating. The Olenekian andesites show typical sanukitoid affinities with high TiO2, MgO, Ni, and Cr contents, and enrichment in large ion lithophile elements (LILEs; K, Ba, and Sr). They yield depleted Hf isotopic compositions and high δ18O values, which along with Hf-O isotope modeling results indicate that they originated from the partial melting of mantle wedge peridotites with input of 20% terrigenous sediments. The Norian diorites have moderate SiO2, relatively high MgO, Cr, and Ni contents, Mg# values of 63−65, high Sr, low Y and Yb contents, relevant high La/Yb ratios, and negligible Eu anomalies, which makes them comparable to high-Mg andesites with adakitic affinity. Their depleted isotopic compositions are similar to coeval A-type granites that resulted from lithospheric delamination after the final collision. In addition, with high K2O/Na2O ratios and incompatible element ratios similar to those of the continental crust (e.g., Nb/U, Ce/Pb, and Nd/Sm), they conform to an origin in which partial melting of delaminated mafic lower crust subsequently interacted with mantle materials. These two-stage abnormal Triassic high-Mg rocks archive a hot slab window triggered by the break-off of the Paleo-Asian oceanic slab and lower crustal delamination related to collapse of the southeastern segment of the Central Asian Orogenic Belt. These findings provide beneficial information for the reconstruction of the East Asian blocks in Pangea.

Eocene dike swarm and felsic stock in Central Iran: Roles of metasomatized mantle wedge and Neo-Tethyan slab

The Eocene felsic stock and dike swarm with intermediate composition from Central Iran (southwest of Jandaq) are studied to understand geodynamic evolution of the Central-East Iranian Microcontinent (CEIM). Field relationships show that the dikes, composed of trachyandesite and basaltic trachyandesite, cross-cut the felsic stock. Whole rock geochemical data reveal the calc-alkaline nature of stock and high-K calc-alkaline to shoshonitic nature of dikes. The felsic stock and intermediate dikes show LREE and LILE enrichment and HFSE (Ta, Nb and Ti) depletion, suggesting that they probably formed in a subduction zone and are similar to the volcanic arc rocks. The rock samples of felsic stock have geochemical characteristics of the high silica adakites (HSA) produced by melting of a subducted oceanic crust. On the other hand, the dikes have characteristics of low silica adakites (LSA) produced by partial melting of a lithospheric mantle peridotite previously metasomatized by slab-derived melts. The Eocene felsic stock and dikes probably formed by subduction of the CEIM-confining oceanic crusts beneath the CEIM. The field and geochemical studies indicate that the magma originated from melting of basic section (amphibolite) of the subducted slab (HSA) will arrive to the Earth surface before the magma formed by melting of mantle wedge peridotites (LSA).

An Intra-Oceanic Subduction System Influenced by Ridge Subduction in the Diyanmiao Subduction Accretionary Complex of the Xar Moron Area, Eastern Margin of the Central Asian Orogenic Belt

This study focuses on the geology, geochemistry, Sr-Nd isotopes and their tectonic settings of three types of basalts in Diyanmiao ophiolite in the Xar Moron area located on the eastern margin of the Central Asian Orogenic Belt. Type I basalts are oceanic tholeiites with a depleted light rare earth element (LREE) pattern, which are similar to the typical N-mid-oceanic ridge basalt (MORB) and suggests that they were formed at a mid-oceanic ridge. The initial 87Sr/86Sr ratios of Type I basalts range from 0.703 966 to 0.705 276 and the eNd(t) values are from 16.49 to 17.15, indicating that they were derived from a depleted mantle source. Type II basalts belong to the medium-potassium calc-akaline series and have the geochemical characteristics of Nb-enriched basalt (NEB) with high Nb content (14.5 ppm) and strong enrichment in LREEs, implying that they were created by the partial melting of mantle wedge peridotite that previously metasomatized by slab melts. Type III basalts are high-Al basalt (HAB) with high-Al contents (Al2O3=16.75 wt.%–18.00 wt.%), distinct Nb depletion and high Th/Yb ratios. Thus they were likely generated in a normal island-arc setting. Therefore, the association of MORB, NEB, and HAB in the study area may be due to the subduction of a mid-oceanic ridge, and the Diyanmiao ophiolite is proposed to be formed in the forearc setting of a mid-oceanic ridge subduction system.

Simultaneous growth and reworking of the Lhasa basement: A case study from Early Cretaceous magmatism in the north-central Tibet

Magmatic rocks preserve a lot of information about the formation and evolution of continental crust, and they play an important role in understanding the growth and reworking of ancient continent. Cretaceous magmatic rocks occurring in the Lhasa terrane are a window to investigate magmatic processes and continental evolution of the Tibetan Plateau. In this study, we report zircon UPb ages and Hf isotopic data, as well as whole-rock major and trace elemental compositions for Early Cretaceous magmatic rocks, including andesite and dacite from the Zenong Group, and associated diorite and granite in the Xinji area, north-central Tibetan Plateau. Zircon from andesite and dacite yielded weighted-mean 206Pb/238U ages of 120–108 Ma and variable zircon εHf(t) values between −12.3 and +2.2. Geochemical compositions suggested that they both displayed high-K calc-alkaline features. Andesite was probably formed by mixing between crust- and mantle-derived melts, whereas dacite was most likely generated by partial melting of ancient crust with input of mantle-derived material. Dioritic and granitic intrusions were intruded into the Precambrian strata and Zenong Group, respectively. Zircon from diorite gave a weighted-mean 206Pb/238U age of ca. 116 Ma and zircon εHf(t) values of −0.5 to +7.3. Diorite displayed medium-K calc-alkaline series signatures and was most likely formed by partial melting of mantle wedge peridotite accompanied by crustal assimilation. Granite yielded zircon weighted-mean 206Pb/238U ages of 107–104 Ma, and positive zircon εHf(t) values of +1.5 to +6.7, indicating a major source of juvenile crust. Granitic samples show irregular REE distribution with variable Ce anomalies, which were probably inherited from subducted pelagic sediments. The Early Cretaceous magmatic rocks in the Xinji area records distinct formation processes involved partial melting of mantle, re-melting of crust, and mixing between mantle and crust. Such processes, however, had a complicated source from mantle, ancient and juvenile continental crust, and recycled pelagic sediments, suggesting simultaneous crustal growth and reworking of the Lhasa basement during the Early Cretaceous.

Zircon U-Pb-Hf isotopes and geochemistry of Jurassic igneous rocks from the southern Zhangguangcai Range, NE China: constraints on magmatism, petrogenesis and tectonic implications

ABSTRACT In this study, we present new zircon U-Pb and Hf isotope data and geochemical analyses for Jurassic igneous rocks from the southern Zhangguangcai Range, NE China. In the context of our new U/Pb data together with other published data (n = 165), we recognize three major magmatic pulses in the eastern Songnen Block during the Jurassic: earliest Jurassic (200–195 Ma), late Early Jurassic (183–175 Ma) and early Late Jurassic (ca. 163 Ma). According to the petrological classification scheme, rocks are divided into two categories of andesite and I-type granitoids. Generally, Early Jurassic monzogranites have more evolved compositions with SiO2, Al2O3, and K2O+Na2O concentrations of 67–74 wt.%, 13.28–17.04 wt.% and 7.25–9.31 wt.%, respectively than Late Jurassic granodiorite. Early Jurassic andesite has SiO2, Al2O3, Na2O+K2O contents of 54.64–56 wt.%, 17.27–17.44 wt.% and 3.47–4.06 wt.%, respectively with low Mg# values of 0.18–0.26. In addition, all samples show enrichment in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depletion in high strength field elements (HSFEs, such as Nb, Ta, P and Ti) and low Sr/Y and (La/Yb)N ratios, all of which are characteristic of arc-type magmatic signatures. In view of the Hf isotope signatures, these rocks show high initial 176Hf/177Hf ratios (0.282711–0.283069) corresponding to positive εHf(t) values (+0.39 to +13.66) and Neoproterozoic two-stage model ages (1154–550 Ma). In summary, the geochemical results suggest that the primary magmas of Jurassic granitoids were derived from variable degrees of partial melting of juvenile lower crust, whereas Early Jurassic andesite was derived from the partial melting of mantle wedge peridotite metasomatized by hydrous melts from subducting sediments and/or oceanic crust. The Jurassic arc-type magmatism in the eastern Songnen Block indicates that the Mudanjiang oceanic plate was still subducting in the Jurassic.

The syncollisional granitoid magmatism and crust growth during the West Qinling Orogeny, China: Insights from the Jiaochangba pluton

The West Qinling Orogenic Belt (WQOB) is a major portion of the Qinling‐Dabie‐Sulu Orogen and holds essential information for understanding the protracted evolution of the north‐eastern branch of the Paleo‐Tethys in East Asia. In this study, we report our petrological, geochemical, and geochronological study on the five Triassic granitoid plutons of West Qinling with emphasis on the poorly studied Jiaochangba pluton with zircon U–Pb ages of 217.5 ± 1.6 Ma and 215.2 ± 1.2 Ma. The new data and the existing data on the other four plutons support the view that the West Qinling granitoids represent a magmatic response to the continental collision of the Yangtze Block (YB) with the North China Craton (NCC) in the Triassic. Like the other four plutons, the Jiaochangba pluton shows strong light rare earth element (REE) enrichment and weak heavy REE depletion ([La/Sm]N ≈ 7.14 ± 1.89; [Sm/Yb]N ≈ 4.63 ± 1.85) with varying negative Eu anomalies (Eu/Eu* ≈ 0.65 ± 0.20). In the N‐MORB normalized diagram, all the samples show relative enrichment in Rb, Pb, U, and K with negative Nb, Ta, P, and Ti anomalies, resembling those of the model continental crust. The Jiaochangba pluton has relatively lower (87Sr/86Sr)i (0.7062 to 0.7081), higher εNd(t) (−6.91 to −2.09), and εHf(t) (−5.57 to −0.14) than mature continental crust, which are consistent with their source being dominated by lower crust with significant mantle contributions. Mantle‐derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. Evolution of such parental magmas in open system crustal magma chambers with continued evolution/replenishment and crustal contamination and assimilation give rise to the observed petrological and geochemical characteristics of these granitoids.

Petrogenesis of Early Cretaceous intermediate-felsic dikes in the Jiaodong Peninsula, south-eastern North China Craton: Constraints from geochronology, geochemistry and Sr-Nd-Pb-Hf isotopes

Early Cretaceous dike swarms are widely developed in the Jiaodong Peninsula, NE China. In this study, we newly investigated the spatial-temporal distribution, petrography, geochronology and whole-rock geochemistry of the intermediate-felsic dikes from the Jiaobei terrane and the Sulu orogenic belt in the Jiadong Peninsula. The zircon U-Pb dating has constrained the timing of the emplacement of intermediate-felsic dikes to 128–108 Ma. The quartz diorite dikes in Jiaobei show adakitic geochemical features, including high SiO2 (66.3–67.5 wt%) contents and high Sr/Y (76–149) and La/Yb (41–91) ratios. The combination of a series of isotopic data, including initial 87Sr/86Sr ratios (0.7098–0.7104) and negative εNd(t) (−20.1 to −14.7) and zircon εHf(t) values (−19.9 to −9.5), indicates that these quartz diorite dikes were likely derived from partial melting of thickened ancient lower crust with involvement of underplated mafic magmas. Additionally, the diorite dikes in Jiaobei and those in Sulu show similar chemical compositions, as both yield the high-Mg andesite (or andesitic rocks) (HMAs) characteristics with a high Mg# value (60–72), high MgO, Cr, and Ni contents, and low Na2O (average = 3.25 wt%) contents. They also exhibit crustal geochemical signatures, such as depletion in Nb, Ta, and Ti, but enrichment in Th and U; high initial 87Sr/86Sr ratios (0.7063–0.7094), and low εNd(t) (−16.7 to −9.6) and εHf(t) values (−29.4 to −9.8). The entire geochemical evidences imply that they derived from the partial melting of mantle wedge peridotite metasomatized by hydrous fluids from the subduction of the oceanic slab with marine sediments. In combination with the Early Cretaceous intrusions and mafic dikes at Jiaodong, the intermediate-felsic dikes represent a magmatic response to lithospheric thinning resulted from the prolonged thermo-mechanical-chemical erosion processes caused by slab rollback of the Paleo-Pacific plate.

Petrogenesis and tectonic implications of Early Jurassic volcanic rocks of the Raohe accretionary complex, NE China

The Raohe accretionary complex, located at the border between the Russian Far East and Northeastern China, is a significant part of the western Pacific Oceanic tectonic regime. Due to lack of precise age and geochemical constraints, the tectonic setting and petrogenesis of the magmatic rocks in this area remain undefined, resulting in debate about crustal growth mechanisms and subduction-related accretionary processes in Northeastern China. Here, we report whole-rock major and trace element and Sr-Nd isotope data, together with zircon U-Pb ages and in situ zircon Hf isotope data for calc-alkaline andesites, dacites, rhyolites, rhyolitic crystal tuffs, Nb-enriched andesites and basaltic andesites, and high-Mg andesites of the Raohe accretionary complex in NE China. Samples were collected from Late Triassic to Early Jurassic strata. However, geochronological results in this study indicated that the studied magmatism occurred in the Early Jurassic (187–174Ma). The calc-alkaline volcanic rocks possess geochemical characteristics typical of arc magmas that form at active continental margins, such as moderate enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletions in high field strength elements (HFSEs). They have positive εHf(t) values of +3.4 to +10.6 and relatively high (87Sr/86Sr)i values of 0.7047–0.7102. While the Nb-enriched andesites and basaltic andesites have higher TiO2, Hf, Nb, and Zr contents and higher Nb/Ta (24.0–87.6), Nb/U (11.9–75.9), (Nb/Th)PM (0.67–2.70), and (Nb/La)PM (1.95–5.00) ratios than typical arc basalts. They have negative εNd(t) values (−5.5 to −6.0) and relatively variable (87Sr/86Sr)i values of 0.7047–0.7114, suggesting an origin via the partial melting of mantle wedge peridotite that had been metasomatized by slab-derived melt. The high-Mg volcanic rocks, characterized by high MgO and Mg#, TiO2, Al2O3, Cr, Ni, (La/Yb)N and (La/Sm)N, but low Ba/Th ratios, are geochemically similar to sanukitoids in southeastern Japan. The occurrence of calc-alkaline, Nb-enriched, and high-Mg volcanic rocks at modern arcs where oceanic slabs are subducting has been well documented. Therefore, we propose that a similar mechanism may explain the generation of the Early Jurassic volcanic rocks of the Raohe accretionary complex in Northeastern China.

Zircon U–Pb geochronology, Sr–Nd–Hf isotopic composition and geological significance of the Late Triassic Baijiazhuang and Lvjing granitic plutons in West Qinling Orogen

The Qinling Orogen was a consequence of continental collision of the South China Craton with the North China Craton in the Triassic and caused widespread granitoid magmatism. However, the petrogenesis of these granitoids remains controversial. In this paper, we choose the Baijiazhuang (BJZ) and Lvjing (LJ) plutons in the West Qinling Orogen for a combined study of the zircon U–Pb geochronology, whole-rock major and trace element compositions and Sr–Nd–Hf isotopic characteristics. We obtained zircon crystallization ages of ~216Ma and ~212Ma for the BJZ and the LJ plutons, respectively. The granitoid samples from both plutons have high K2O metaluminous to peraluminous compositions. They are enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field-strength elements (HFSEs) with significant negative Eu anomalies. The BJZ samples have initial Sr isotopic ratios of 0.7032 to 0.7078, εNd(t) of −10.99 to −8.54 and εHf (t) of −10.22 to −6.41. The LJ granitoids have initial Sr isotopic ratios of 0.7070 to 0.7080, εNd(t) of −5.37 to −4.58 and εHf(t) of −3.64 to −1.78. The enriched isotopic characteristics of the two plutons are consistent with their source being dominated by ancient continental crust. However, two BJZ samples show depleted Sr isotope compositions, which may infer possible involvement of mantle materials. Mantle-derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. That is, the two granitic plutons are magmatic responses to the closure of the Mianlue ocean basin and the continental collision between the Yangtze and South Qinling crustal terranes.

U–Pb zircon geochronology, Sr–Nd geochemistry, petrogenesis and tectonic setting of Mahoor granitoid rocks (Lut Block, Eastern Iran)

The Mahoor Cu–Zn-bearing porphyritic granitoid rocks belong to the Lut Block volcanic–plutonic belt (central Eastern Iran). These granitoid rocks occur mainly as dykes and stocks that intrude into Eocene volcanics and pyroclastic rocks. Petrographically, all the studied intrusives display porphyritic textures with mm-sized phenocrysts, most commonly of plagioclase and hornblende, embedded in a fine-grained groundmass with variable amounts of plagioclase, hornblende, clinopyroxene, quartz and opaque minerals. Hydrothermal alteration affected these granitoid rocks, as revealed by the common occurrence of sericite, chlorite, titanite, epidote and calcite. Chemical classification criteria show that the intrusives may be named as gabbrodiorites, diorites, monzodiorites and tonalites. Major elements geochemistry reveals that all the studied lithologies are typically metaluminous (A/CNK⩽0.9). Magnetic susceptibility (1485×10−5 SI) together with mineralogical and geochemical features shows that they belong to magnetite granitoid series (I-type). Trace element patterns normalized to chondrite and primitive mantle are very similar to each other and show enrichments in LREE relative to HREE and in LILE relative to HFSE, as well as negative anomalies of Ta, Nb and Ti. Eu/Eu∗ ratios vary from 0.88 (in the most mafic composition) to 0.65, showing that plagioclase played a role in magma differentiation. LA-MC-ICP-MS U–Pb zircon data from a diorite, yielded similar concordia ages of ca. 31.88±0.2Ma (Error: 2σ), which corresponds to the Oligocene period. These granitoid rocks have (87Sr/86Sr)i values vary between 0.7055 and 0.7063. In terms of isotopic compositions, while εNdi is between −0.6 and −2.5, suggesting that magmas underwent contamination through being exposed to the continental crust. The whole set of geochemical data agree with the emplacement of the studied intrusions in a magmatic belt above a subduction zone. Primitive magmas should have formed by melting of mantle wedge peridotite, and during magma ascent to crustal levels, both magma differentiations took place by crystal fractionation and crustal contamination. Sulfide mineralizations (pyrite, chalcopyrite and sphalerite) related to these granitoid rocks is common and occurs as both disseminated and hydrothermal veins, indicating a high mineralization potential for this area.

Early Silurian (~440Ma) adakitic, andesitic and Nb-enriched basaltic lavas in the southern Altay Range, Northern Xinjiang (western China): Slab melting and implications for crustal growth in the Central Asian Orogenic Belt

As an important part of the Central Asian Orogenic Belt (CAOB), the Altay Range contains large-scale Paleozoic magmatic rocks. However, owing to the lack of precise age constraints, the tectonic setting and petrogenesis of the magmatic rocks in this area have been controversial, which has led to the debate on Phanerozoic crustal growth mechanisms and accretionary orogenic processes in the CAOB. Herein, we report geochronological and geochemical data of the Suoerkuduke adakitic, andesitic and Nb-enriched basaltic (NEB) lavas in the southern margin of the southern Altay Range. LA-ICP-MS zircon U–Pb analyses for five adakitic, andesitic and NEB samples indicate that they were coevally generated in the Early Silurian (~ 440 Ma). The adakites and basaltic andesites are geochemically characterized by high Na2O/K2O, Sr/Y, Al2O3, Sr, εNd(t) and zircon εHf(t) values and relatively low (87Sr/86Sr)i ratios. The NEBs are sodium-rich and have higher TiO2, P2O5, Zr, Nb, and Nb/U values than those of typical arc basalts. They also have positive εNd(t) values and positive and variable zircon εHf(t) values. We suggest that the Suoerkuduke adakites were derived by a partial melting of the subducted oceanic crust with minor overlying sediments, and the continuous compositional variations between adakites and basaltic andesites confirm that the interaction between slab melts and mantle peridotite played an important role in the formation of basaltic andesites. The associated NEBs were possibly generated by a partial melting of mantle wedge peridotites metasomatized by slab-derived adakitic melts and minor fluids. In combination with the occurrence of voluminous Silurian–Devonian granitoids, coeval ophiolite mélanges, and a series of intra-arc basins, a slab window model triggered by slab tearing is proposed to account for the formation of the Suoerkuduke adakite–basaltic andesite–NEB suites. The upwelling of the asthenospheric mantle through the slab window probably caused significant extension in the overlying lithosphere; extensive melting of the subducted oceanic crust, mantle and juvenile lower crust; and intense interaction between slab-derived melts or fluids and the mantle wedge. Slab tearing and subsequent asthenospheric upwelling possibly played an important role in the crustal growth of the CAOB during the Phanerozoic.

Northward subduction of Bangong–Nujiang Tethys: Insight from Late Jurassic intrusive rocks from Bangong Tso in western Tibet

New zircon LA-ICPMS U–Pb age data, whole-rock major and trace elements, and zircon Hf isotopic data of intrusive rocks from Larelaxin and Caima plutons in the southern margin of the Western Qiangtang subterrane provide important evidence of northward subduction of the Bangong–Nujiang Tethyan Ocean lithosphere. Host granitoids including quartz diorites, granodiorites, and syenogranites and associated mafic varieties including gabbroic enclaves, dioritic dikes and enclaves, and quartz dioritic enclaves are investigated in this study. Five host granitoid samples are dated at 163–160 Ma, and one dioritic dike and three dioritic enclave samples are dated at 162–158 Ma, which indicates that these rocks were contemporaneous. The quartz diorites and granodiorites are normal calc-alkaline I-type granitoids. The syenogranites are characterized by high SiO2 (74–77 wt.%) and differentiation index (DI = 92–97) and marked depletion in Ba, Nb, Ta, Sr, P, Ti, and Eu, thus indicating that they are highly fractionated I-type granitoids. The host granitoids exhibit uniform zircon εHf(t) values (− 1.4 to + 1.9). Considering the current data and their negative whole-rock εNd(t) values (− 4.7 to − 3.5) reported in recent studies, these rocks can be interpreted to have resulted from the partial melting of ancient mafic lower crust with varying contributions from mantle-derived or juvenile crust-derived components. One gabbroic enclave sample exhibits high Al2O3 (18.3 wt.%), low MgO (3.3 wt.%), high TiO2 (1.4 wt.%), and high Nb (24 ppm) and is geochemically similar to high-alumina basalt (HAB) and high-Nb basalt (HNB). This indicates that the gabbroic enclave originated from the partial melting of mantle wedge peridotite that was metasomatized by slab melting and subsequently experienced significant fractional crystallization of olivine and clinopyroxene. The dioritic dike and enclave samples yield εHf(t) values (− 1.6 to + 3.1) similar to those of the host granitoids. They most likely originated from magma mixing between mantle-derived and crust-derived melts at or close to the Moho. The intrusive rocks with geochemical diversity of the Larelaxin and Caima plutons from Bangong Tso in western Tibet can be interpreted to have resulted from melting, assimilation, storage, and homogenization (a MASH process) above subduction zones, thus indicating that the Bangong–Nujiang Tethyan Ocean lithosphere was subducted northward beneath the Western Qiangtang subterrane during the Late Jurassic.

The tectonomagmatic evolution of structure-lithologic complexes in the Tannu-Ola zone, Tuva, in the Late Vendian–Early Cambrian ( from geochemical, Nd isotope, and geochronological data)

In the Tannu-Ola zone, there are island-arc (Late Vendian) and subduction-accretionary (first half of the Early Cambrian) complexes whose mantle sources were generated from melts produced in suprasubduction conditions as a result of the melting of mantle wedge peridotites. The island arc, whose fragment is present in the Tannu-Ola zone, formed tentatively at 570 Ma and ceased as an individual structure at the Vendian–Cambrian boundary, though the subduction continued till the middle Early Cambrian. From the Vendian–Cambrian boundary to middle Early Cambrian, the accretion of island-arc, back-arc, oceanic, and metamorphic structure-lithologic complexes took place at this segment of the Central Asian Fold Belt. From middle to late Early Cambrian, the Tannu-Ola zone was an active continental margin, where basalts with island-arc characteristics, quartz microdiorites of adakite composition, basalts, dolerites, and quartz microdiorites of intraplate type, and diorite-tonalite-plagiogranite association of island-arc type formed. To explain the spatial and temporal conjugation of different magmatism manifestations on the active margin, we applied the slab–window model.

Arc Basalt Simulator version 2, a simulation for slab dehydration and fluid‐fluxed mantle melting for arc basalts: Modeling scheme and application

Convergent margin magmas typically have geochemical signatures that include elevated concentrations of large‐ion lithophile elements; depleted heavy rare earth elements and high field strength elements; and variously radiogenic Sr, Pb, and Nd isotopic compositions. These have been attributed to the melting of depleted mantle peridotite by the fluxing of fluids or melts derived from subducting oceanic crust. High Mg # basalts and high Mg # andesites are inferred to make up the bulk of subduction‐related primary magmas and may be generated by fluid or melt fluxing of mantle peridotite. The difference in contributions from the subducted slab found among various arcs appears to be mostly controlled by thermal structure. Cold slabs yield fluids, and hot slabs yield melts. Recent experimental studies and thermodynamic models better constrain the phase petrology of the slab components during prograde metamorphism and melting, mantle wedge melting, and mantle slab melt reaction. Experimental results also constrain the behavior of many elements in these processes. In addition, geodynamic models allow increasingly realistic, quantitative modeling of the temperature and pressure in the subducted slab and mantle wedge. These developments together enable generation of forward models to explain arc magma geochemistry. The Arc Basalt Simulator (ABS) version 2 (ABS2) uses an Excel® spreadsheet‐based calculator to predict the partitioning of incompatible element and Sr‐Nd‐Pb isotopic composition in a slab‐derived fluid and in arc basalt magma generated by an open system fluid‐fluxed melting of mantle wedge peridotite. The ABS2 model is intended to simulate high Mg # basalt geochemistry in relatively cold subduction zones. The modeling scheme of ABS2 is presented and is applied to primitive arc magmas.

Magnesian andesite and dacite lavas from Mt. Shasta, northern California: products of fractional crystallization of H2O-rich mantle melts

Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.

Geochemical modeling of dehydration and partial melting of subducting lithosphere: Toward a comprehensive understanding of high‐Mg andesite formation in the Setouchi volcanic belt, SW Japan

Possible mechanisms for the production of mantle‐derived, high‐Mg andesite magmas, including (1) partial melting of mantle wedge peridotite by addition of aqueous fluids from the subducting lithosphere and (2) partial melting of the subducting sediments and altered oceanic crust, and subsequent melt‐mantle interaction, were examined by geochemical formulation of dehydration, partial melting and melt‐solid reactions. The modeling results demonstrate that both mechanisms can reasonably explain the incompatible trace element characteristics of high‐Mg andesites in the Setouchi volcanic belt, SW Japan. However, simple hydrous melting of mantle wedge peridotite cannot account for the Sr‐Nd‐Pb‐Hf isotopic compositions of such andesites. By contrast, the latter mechanism, which is consistent with thermal structures beneath the Setouchi volcanic belt, can well reproduce the isotopic signature of those high‐Mg andesites.

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge, Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ɛNd (+8.0–+8.4) and low 87Sr/86Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (ɛNd=+7.5−+8.4; 87Sr/86Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ɛNd (+4.6–+4.9) and (87Sr/86Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.