Subduction Component Research Articles

Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics

AbstractBoninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si boninites (Si8 < 57) and high-Si boninites (Si8 > 57). Satisfying one but not both of the boninite criteria are rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main boninite sources. Nonetheless, arc-basin boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic boninites, they provide evidence for some form of subduction during the Archean.

Radiogenic isotopes document the start of subduction in the Western Pacific

Subduction initiation is one of the least understood aspects of plate tectonics. In an effort to obtain the first in situ magmatic record of subduction initiation, the International Ocean Discovery Program Expedition 352 drilled at four sites in the inner trench wall of the Bonin Trench to recover 1.22 km of oceanic upper crust accreted within a few m.y. of subduction initiation. The two sites nearer to the trench (U1440 and U1441) yielded axial and off-axis fore-arc basalts (FAB), while those c. 15 km further from the trench (U1439 and U1442) yielded axial low-silica boninites and high-Mg andesites overlain by off-axis high-silica boninites. This study uses Hf–Nd–Sr–Pb isotope analyses from c. 50 stratigraphically representative core samples to trace the evolution of the mantle source during the brief period of FAB-through-boninite magmatism immediately following subduction initiation. Results show that: 1) the FAB have high εHf relative to εNd and were derived from variably depleted mantle of ‘Indian’ provenance with no detectable subduction input; 2) the axial boninites follow mixing trends between a residual FAB mantle source and a subduction component derived from shallow (amphibolite facies) melting of oceanic crust of ‘Pacific’ provenance; and 3) the off-axis boninites define mixing trends between a hybrid mantle wedge (residual mantle + slab melt) and an additional subduction component with lower εNd and higher 207Pb/204Pb that requires a significant contribution from pelagic sediment. This incoming of pelagic sediments may signify a change from an accretionary to non-accretionary margin as subduction evolves. The results thus indicate a rapidly evolving system in terms of geodynamics, magma genesis and crustal accretion immediately following subduction initiation.

Geology and geochemistry of the Middle Miocene Yağcıköy volcanic complex, western Turkey: Wide-rift alkaline volcanism associated with incipient stages of slab tearing

Alkaline and ultrapotassic volcanic rocks in the Afyon volcanic complex are regarded as the first products with asthenospheric signature after post-orogenic magmatism in western Turkey. The Yağcıköy volcanic complex (YVC), a key locality in the western off-axis of the Afyon volcanic complex, comprises three units consisting of domes, dykes, lava flows and associated volcaniclastic rocks intercalated with Middle Miocene fluvial/lacustrine deposits. Emplacement of the domes and dykes appears to have been controlled by NE-trending faults in a rift environment. The YVC is alkaline and shoshonitic in character, and all the volcanic units in the complex have similar geochemical patterns (enriched in LILE and LREE and depleted in Nb, Ta and Ti). The 87Sr/86Sr and 143Nd/144Nd ratios range from 0.704989 to 0.706662 and 0.512492 to 0.512595, respectively. The geochemical and isotopic data suggest that fractionation of parental basaltic trachyandesite magmas generated a magma chamber with chemically normal zonation. The geochemical data and modeling results suggest that the YVC originated from a spinel peridotite-bearing lithospheric mantle source that had been metasomatized by subduction components. The rifting environment that hosts the volcanic products is thought to have resulted from slab tear that caused the rise of fertile asthenospheric material to relatively shallow depths. The upwelling of hot asthenospheric melts through a tear subsequently melted the spinel peridotite-bearing lithospheric mantle source.

Late Cryogenian magmatic activity in the North Lhasa terrane, Tibet: Implication of slab break-off process

The North Lhasa terrane in Tibet is generally interpreted to be paleotectonically unrelated to the East African Orogen (EAO) and is instead thought to have derived from northeastern India or northwestern Australia. In this study, we present petrogenetic and geochronological results pertaining to the analysis of gabbros (ca. 652 Ma), diorites (ca. 658 and 646 Ma), and tonalites (ca. 652 Ma) from the North Lhasa terrane. The gabbros are calc-alkaline and exhibit arc-like geochemical features. Low positive zircon εHf(t) values (+1.0 to +3.8), high zircon δ18O (6.25‰ to 7.94‰), and low negative whole-rock εNd(t) values (−3.5 to −1.4) indicate that the gabbros were derived from the lithospheric mantle, with geochemical modification by a subduction component. The diorite suite is characterized by a wide range of whole-rock chemistries (e.g., SiO2 = 51.33–61.98 wt%) and Hf–O–Sr isotopic compositions (εHf(t) = −10.8 to −0.1; δ18O = 5.17‰ to 7.11‰; ISr = 0.706 to 0.710), and negative whole-rock εNd(t) values (−7.0 to −4.7). These diorites are geochemically similar to OIB and are interpreted to be products of the partial melting of a relatively deep mantle source (>85 km) prior to extensive modification by continental crustal material. The tonalites are adakitic and have moderate Mg# values (47–54), low compatible element abundances, positive zircon εHf(t) values (+3.4 to +6.2), high ISr values (0.714 to 0.715), and small negative whole-rock εNd(t) values (−1.6 to −0.4). These tonalites most likely formed by the melting of thickened Mesoproterozoic continental crust. The generation of these ca. 650 Ma magmatic rocks was related to slab break-off in a collision zone. By integrating the findings of previous studies with the data of the present study, we suggest that the North Lhasa terrane was most likely located in the northern segment of the EAO in paleotectonic reconstructions of the Gondwana supercontinent.

The role of subduction recycling on the selenium isotope signature of the mantle: Constraints from Mariana arc lavas

Investigating the isotope systematics and behavior of selenium in subduction zones provides valuable insights into mechanisms that contribute to chalcophile and moderately volatile element distribution between terrestrial reservoirs. In this study, we present high-precision Se isotope and Se–Te elemental data on subduction zone lavas from the Mariana arc system. Our results indicate that Se–Te concentrations are unaffected by submarine degassing but are affected by magmatic differentiation. In contrast, Se isotopes of submarine lavas are unaffected by both magmatic differentiation and degassing and thus may preserve their magmatic source signature. Compared to the average Se isotope composition previously obtained for mantle-derived samples (δ82/76Se = 0.23 ± 0.12‰, 2 s.d., n = 5), suitable to represent the Mariana pre-subduction wedge, Mariana lavas show a larger overall range (δ82/76Se from 0.03 to −0.33‰, n = 21) with a clear tendency towards lighter Se isotope compositions. The variable Se isotope signatures within the Mariana suite can further be attributed to different slab-derived fluid and melt-like subduction components. This provides evidence for a significant role of subduction recycling of hydrothermal sulfide-containing altered oceanic crust and pelagic sediments with possible implications for the Se isotope evolution of the crust-mantle system throughout geological time.

Middle Jurassic subduction-related ophiolite fragment in Triassic accretionary complex (Mamu Dağı ophiolite, Northern Turkey)

ABSTRACTThe Mamu Dağı ophiolite, ca. 13 km long and 5 km across (Tokat, Sakarya Zone), consists of peridotites, pyroxenites, gabbros, and basalts, which are crosscut by dolerite dykes. These rocks show variable degrees of serpentinization and alteration. Gabbroic rocks consisting of plagioclase + clinopyroxene ± orthopyroxene ± olivine ± amphibole ± sphene ± opaque minerals have commonly the ophitic and the cumulate textures. Similar mineral paragenesis is observed in the basalts and the dolerites, which are commonly characterized by the sub-ophitic and the microlitic porphyric textures.Primitive mantle-normalized rare earth and trace element diagrams of gabbros and basalts display subduction-related geochemical characteristics such as high Th concentrations, negative Nb, Zr, and Ti anomalies. Some of the gabbros are interpreted to be the cumulate rocks. They have mostly positive europium anomaly (Eu/Eu* 1.77–0.83) and relatively low SiO2 and incompatible element (e.g. Zr, Ti) contents. The initial 87Sr/86Sr and 143Nd/144Nd values of gabbro/dolerite and basalt samples vary between 0.7036 and 0.7049, between 0.51259 and 0.51278, respectively. The isotope data and the whole rock geochemistry suggest that the Mamu Dağı ophiolite was derived from a mantle source that was affected by the subduction component rather than MORB or depleted mantle source.Hornblendes from a gabbro sample of the Mamu Dağı ophiolite yielded 40Ar/39Ar plateau age of 159 ± 1 Ma. This age data is similar to those of many ophiolites located along the İzmir-Ankara-Erzincan suture zone but is different from the ages reported for the Tokat Massif.

Geochemical Characteristics of Sunda Volcanic Arc in Sumatra and Andaman

DOI: 10.17014/ijog.6.1.1-16Geochemical characteristics of Sunda volcanic belt are recognized from each characteristic of Weh Island, Tabuan Island in Semangko Bay, South Sumatra, and Andaman Islands. Trace and rare earth elements (REE) are produced by fumaroles in a marine environment of submarine volcano of Weh Island characterized by barium (Ba) as an indicator of sea water influence in the mineralization process, while sulphide minerals do not occur in this area. REE pattern compared to Mid Oceanic Ridge Basalt (MORB) shows a characteristic of subduction tectonics and is distributed in shallow coastal water of high energy. Based on comparison of REE contents in all samples, it reveals that volcanism process causes REE enrichments either in the past or in recent. Geochemical characteristics of Tabuan Island in Semangko Bay reveal the occurrence of hydrothermal mineralization followed by pervasive occurrences of sulphide minerals in vein-type disseminations enriched in Au, Ag, Zn, Pb, Cu, As, Sb, Ba, and Mn. Geochemical characteristics of Andaman Islands reveal imprint of substantial subduction component in the form of sediment fluid and melt and fluid-induced subduction component derived from altered oceanic crust.

Temporal changes in the subduction of the Paleo-Pacific plate beneath Eurasia during the late Mesozoic: Geochronological and geochemical evidence from Cretaceous volcanic rocks in eastern NE China

Subduction of the Paleo-Pacific Plate played an important role in the evolution of the eastern margin of the Eurasian continent. However, the details of the processes of subduction during the late Mesozoic remain poorly constrained. Here we examine the geochronology and geochemistry of Cretaceous volcanic rocks from eastern NE China, with the aim of understanding the variation of subduction process. Cretaceous volcanism in the region mainly occurs in the Raohe–Mishan-Suifenhe–Wangqing and Jiamusi–Jixi areas in eastern Jilin and Heilongjiang provinces, as well as in the Lesser Xing'an Range where volcanism extends from the continental margin farther inland. The Cretaceous volcanic rocks in the Raohe–Mishan–Suifenhe–Wangqing (RMSW) area are composed of basalts, basaltic andesites, andesites, dacites, and rhyolites with late Early Cretaceous (122–106 Ma) ages and dacites that yield early Late Cretaceous (96–93 Ma) ages. A mixed source of compositionally heterogeneous mantle and crustal rocks is assumed based on the compositional range of volcanic rocks. Adakitic andesites (114–110 Ma) were derived from a mantle source modified by slab-derived melt that infiltrated the mantle wedge. Adakitic dacites (122–106 and 96–93 Ma) were produced by partial melting of thickened lower crust that formed in response to earlier subduction of the Paleo-Pacific Plate beneath Eurasia. The rhyolites and dacites that lack adakitic signatures were derived from partial melting of a depleted lower-crustal block that was accreted during the Neoproterozoic. In particular, the late Early Cretaceous (106 Ma) basalt was derived from a depleted lithospheric mantle that was metasomatized by slab-derived fluids and subsequently by asthenospheric melts at a relatively shallow depths, implying rollrback of the subducted Paleo-Pacific Plate. The Cretaceous volcanism in the Jiamusi–Jixi area formed during the late Early Cretaceous (124–110 Ma) and shows a bimodal distribution of compositions with a prominent ‘Daly Gap’. The mafic members were generated by re-melting of depleted lithospheric mantle that was modified by subduction-related fluids and underwent fractional crystallization. The coeval felsic end-members, with or without A-type characteristics, were generated by partial melting of juvenile crustal material, consistent with an extensional environment related to subduction of the Paleo-Pacific Plate. Cretaceous volcanism in the Lesser Xing'an Range shows a bimodal compositional distribution of volcanic rocks with a ‘Daly Gap’ and a period without volcanism at 108–97 Ma. Andesites with and without adakitic signatures were derived from partial melting of mantle that was hybridized by slab-derived melts and metasomatized by a fluid-rich subduction component. In contrast, the coeval A-type rhyolites were generated by partial melting of juvenile crust, implying the migration of the extensional environment. The formation of, and spatio-temporal relationships between, Cretaceous volcanic rocks in NE China, combined with the regional geological data, suggest a transition from low-angle subduction to slab rollback and subsequent changes in the direction of motion of the Paleo-Pacific Plate during the late Early Cretaceous to the Late Cretaceous.

The Thickness of the Mantle Lithosphere and Collision-Related Volcanism in the Lesser Caucasus

The Lesser Caucasus mountains sit on a transition within the Arabia–Eurasia collision zone between very thin lithosphere (<100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from NW to SE in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb–Ta anomalies. Magmatic rocks from the SE, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic ⁸⁷Sr/⁸⁶Sr, and less radiogenic ¹⁴³Nd/¹⁴⁴Nd. Across the region, there is no correlation between SiO₂ content and Sr–Nd isotope ratios, revealing a lack of crustal contamination. Instead, ‘spiky’ mid-ocean ridge basalt normalized trace element patterns are the result of derivation from a subduction-modified mantle source, which probably inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent–continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the SE. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the SE must occur at greater depths in the mantle. Temperatures of melting below 1200°C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in NW Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran. The mid-lithosphere magma source has a distinct composition compared with the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation.

Geochemistry of diverse lava types from the Lau Basin (South West Pacific): Implications for complex back‐arc mantle dynamics

Lavas from back‐arc basins are generally diverse, and their geochemical compositions indicate that they received variable contributions from subduction components during petrogenesis. Obtaining comprehensive geochemistry characteristics for the entire back‐arc basin are helpful in gaining a better understanding of the complexities of back‐arc evolution. In this study, we systematically reviewed the major and trace element compositions of Lau Basin lavas. This data set covered all of the major tectonic units of the basin, for example, the Eastern Lau spreading center (ELSC), the Valu Fa ridge (VFR), the Central Lau spreading center (CLSC), the Northwestern Lau spreading center (NWLSC), the Northeastern Lau spreading center (NELSC), the Mangatolu Triple Junction (MTJ), and the Fonualei Spreading Center (FSC). The lava types from these locations are diverse, including tholeiitic basalts, alkali basalts, basaltic andesites, andesites, dacites, and rhyolites. In addition, the NELSC and FSC contain boninites. In this study, the geochemical compositions of these diverse rock types are used to investigate the spreading dynamics of the Lau Basin back‐arc. We conclude that (a) the subduction components mainly include hydrous fluids released from the subducted slab, and hydrous melt directly from the melting subducted sediment, and the influence of the subducted components on the mantle wedge and the petrogenesis of the central and southern Lau Basin lavas decreases with increasing distance from the Tonga Trench; (b) during the propagation and spreading of the Samoan mantle into the Lau Basin, lavas from the NELSC, NWLSC, MTJ, and even from the northern part of the FSC may receive variable degrees of influence from the Samoan plume; and (c) the migration of Indian MORB‐like mantle southwards into the Pacific mantle is consistent with the opening of the Lau Basin. With the gradual migration of Indian mantle into the Lau Basin, a series of spreading centers were formed, for example, in order, the NWLSC, CLSC, ELSC, and the propagating VFR.

The zircon Hf isotope archive of rapidly changing mantle sources in the south Patagonian retro-arc

Rapid changes in geochemical and isotopic signatures of arc-related magmatic products can be used to trace magmatic processes in subduction zones across many scales, from the regional response of magmatism to large-scale geodynamic changes in the subduction system, to the emplacement of single intrusions. In this contribution, we use the south Patagonian subduction system as a natural laboratory to investigate magmatic processes in continental arcs. We use diverse intrusions and dikes from the retro-arc region at 49−51°S to investigate these processes both at the subduction zone scale, and within the exceptionally well-exposed Torres del Paine sheeted intrusion. We present Hf isotope data for zircon from 30 to 12 Ma magmatic units that were emplaced ∼50 km inboard of the main subduction-related batholith. These samples record an ∼18 m.y. period during which the region experienced profound geodynamic changes, resulting in transient migration of arc magmatism into the retro-arc, which then vanished to be replaced by more alkaline retro-arc magmatism. Integrating published whole rock geochemistry, we show that the Hf isotope signatures of these magmatic units directly record their mantle sources, with negligible assimilation of continental crust into magmas during transport through and storage in the crust. This allows us to trace the appearance and disappearance of the subduction component in the retro-arc mantle. Our data show that migration of calc-alkaline magmatism into the retro-arc produced magmas with a more enriched Hf isotope composition that was remarkably consistent over ∼200 km and >4 m.y. (eHf(i) of −1 to +2.5). These signatures record addition of subducted continental crust to the mantle wedge during a period of subduction erosion that was associated with arc migration, and show that only a few m.y. of fluxing by a subduction component is sufficient to leave a distinct Hf isotope imprint on a mantle wedge previously unmodified by subduction. The Torres del Paine laccolith was built up by discrete pulses of magmatism in <200 k.y. Our data show isotopic differences between magmatic batches, and an abrupt shift to more juvenile Hf isotope compositions during the buildup of the youngest part of this magmatic complex, recording the rapid input of new mantle-derived melts during its formation. This rapid rejuvenation occurred within 20 ± 10 k.y., demonstrating that different batches of magmatism within a single intrusive complex can tap geochemically distinct mantle reservoirs on very short timescales of <200 k.y.

A re-interpretation of the petrogenesis of Paricutin volcano: Distinguishing crustal contamination from mantle heterogeneity

Paricutin volcano is the youngest and most studied monogenetic cinder cone in the Michoacán-Guanajuato volcanic field (Mexico), with an excellent historical record of its nine years (February 1943 to March 1952) of eruptive activity. The eruption produced lavas and tephras that range in composition from basalt to andesite. We have conducted new major and trace element and isotopic studies (whole rock Sr-Nd-Pb-Os) of the Paricutin lavas and tephras spanning the entire duration of the eruption, together with xenoliths found in early erupted lavas and bombs, and crustal samples representative of the Paricutin basement. This work contributes to our understanding of the potential roles of mantle source heterogeneity, subduction-related metasomatism, and crustal assimilation in the petrogenesis of arc magmas; moreover this study underscores the complexity of magma generation and evolution in monogenetic volcanoes. Although Paricutin has been traditionally considered as the classical example of magma evolution by fractional crystallization and crustal contamination, our multi-isotopic study has revealed that Paricutin compositional variations are inconsistent with significant crustal assimilation. Alternatively, we suggest that Paricutin's geochemical evolution can be explained by a combination of variable degrees of fractional crystallization of magmas produced by melting heterogeneous mantle beneath the TMVB that has been metasomatized by subduction components including sediment- and oceanic crust-derived fluids.

Petrology, geochemistry and geodynamic setting of Eocene-Oligocene alkaline intrusions from the Alborz-Azerbaijan magmatic belt, NW Iran

The Kaleybar, Razgah and Bozqush (KRB) intrusions were studied to better understand subduction-related Eocene-Oligocene alkaline magmatism in NW Iran. The bulk of intrusions mainly consist of Si-undersaturated rocks including foid-bearing monzonite and syenite (nepheline syenite, pseudoleucite syenite) with some foid-bearing diorite and gabbro. In addition, they are spatially associated with Si-saturated rocks ranging in composition from monzo-diorite to syeno-granite. The main mafic rock-forming minerals of the studied rocks are olivine (Fo44Fa56), clinopyroxene (diopside to augite), biotite (Mg-biotite through Fe-biotite), amphibole (ferro-pargasite and magnesio-hastingsite with Mg#<0.55), and garnet (Ti-andradites). Based on whole rock geochemistry, the foid-syenites and associated rocks show mildly alkaline (shoshonitic) affinity. The content of SiO2, K2O + Na2O, and K2O/Na2O ratio ranges from 47.8 to 60.7 wt.%, 5.31 to 16.33 wt.%, and 0.6 to 3.2, respectively. The intrusions are commonly metaluminous, with an aluminum-saturation index (ASI) ranging from 0.66 to 1.01. Almost all the rocks display similar arc-related geochemical features characterized by the enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) together with the depletion in high field strength elements (HFSE). The chondrite-normalized REE patterns show no to marked negative Eu anomaly (Eu/Eu* = 0.55 to 1.12), (La/Yb)N = 8.16 to 31, (La/Sm)N = 2.80 to 10.59, and (Tb/Yb)N = 0.84 to 2.40. The evaluation of the REE patterns for the KRB magmas and the comparison of the trace element ratios with experimental studies indicate a chemically enriched lithospheric mantle source composed of garnet-spinel-lherzolite that have underwent a low degree of partial melting <5% to generate the KRB intrusions. Based on the present data, we infer that the mantle source was contaminated by a subduction component and the melting of the mantle lithosphere occurred by local extension in an overall convergent regime in NW Iran. The extension regime during the Eocene is proposed to be the result of the Neo-Tethys slab roll-back and the Sevan-Akera-Qaradagh (SAQ) slab break-off.

Neogene–Quaternary Magmatism of the Çaldıran Plain and its Vicinity (Eastern Turkey): an Example of Post-Collisional Transition from Subduction to Intraplate Type

This paper is aimed at studying the chronological evolution of the Neogene–Quaternary volcanic activity within the Caldiran plain and its mountainous framing (Eastern Turkey). It is shown that the last pulse of continental-margin magmatism related to the subduction and closure of Neotethys oceanic basin occurred in the Middle Miocene (13.5–12.5 Ma). The post-collision volcanism proceeding simultaneously with large-scale regional tectonic rearrangement and initiation of the long-term Caldiran fault began in the Late Miocene (7–6 Ma), and reached maximum activity in the Middle Pliocene (4.7–3.6 Ma). The Quaternary period in the region evolution was marked by the abundant within-plate magmatic activity restricted to the regional SW–NE trending zone, and the formation of Eastern Turkey’s largest Tendurek shield volcano (Late Pleistocene–Holocene). Petrological-geochemical data indicate that magmas during the overall evolution of young volcanism of the Caldiran plain was generated from a single mantle reservoir, whose composition gently one-way evolved with time. Calculations show that melting occurred in the upper part of the asthenosphere (immediately near the boundary with thinned lithospheric mantle), which was metasomatized by pre-existing long-continued subduction. The chemical variations of mantle source with time (from the Middle Miocene to Quaternary) were mainly determined by a decrease of subduction component and the presence of aqueous phases, with a general trend from E-MORB to OIB-type for generated magmas. The composition of Late Quaternary basic lavas of Tendurek Volcano in terms of most petrological-geochemical characteristics corresponds to within-plate alkaline basalts. The main trend of geochemical evolution of mantle source is correlated with a systematic change of the predominant serial affinity of igneous rocks from calcalkaline through moderately alkaline to Na-alkaline varieties. Discrete character of young magmatism within the Caldiran plain, and its subsequent evolution (sulrasubduction → post-collision → within-plate) were mainly determined by periodical large-scale changes in geotectonic setting within the Eurasian–Arabian collision zone: (1) cessation of subduction, (2) break-up and deepening of oceanic slab with its subsequent break off, (3) inferred emergence of incipient rift setting under conditions of intense submeridional compression.

Zinc isotopic systematics of Kamchatka-Aleutian arc magmas controlled by mantle melting

Geochemical characteristics of arc magmas reflect incorporation of subducted materials to their mantle wedge sources in subduction zones. Subduction component addition has been proposed to modify the Zn isotopic budget of arc magmas. However, the lack of a systematic study on Zn isotopic compositions of arc magmas hampers a better understanding of Zn isotope behavior in subduction zones. To address this issue, we have determined Zn isotopic compositions of 37 well-characterized arc rocks from the Kamchatka and Central-Eastern Aleutian arcs. These rocks record contributions of fluids and melts derived from altered oceanic crust (AOC) without overprints of sediment melts and thus allow focus on the potential effects of AOC-derived fluids and melts on the Zn isotopic budget of arc magmas. For comparison, nine basalts from the Gakkel, Mid-Atlantic and Southeast Indian Ridges, and the Lau Basin and nine adakites from Central America were also analyzed. Rocks from the Kamchatka-Aleutian arcs have δ66Zn from 0.16 to 0.31‰ that are mostly similar to those of mid-ocean ridge basalts (MORBs), back-arc basin basalts (BABBs), and adakites (δ66Zn = 0.23–0.33‰), but a significant number of arc samples also display δ66Zn higher than that of the depleted MORB-type mantle (DMM), indicating Zn isotope fractionation during magmatic processes and/or modifications of the mantle wedge Zn isotopic budget by incorporation of AOC-derived fluids and melts.The lack of correlations of δ66Zn with geochemical indicators of magma differentiation (e.g., MgO, SiO2, and Zn/FeT) indicate that fractionation of olivine, pyroxene, and magnetite has a limited effect on the Zn isotopic compositions of arc magmas. Even though the mantle sources of arc rocks investigated here are strongly affected by AOC-derived fluids and melts that have higher δ66Zn compared to the DMM, we observe no systematic variations of δ66Zn with indicators of subduction components (e.g., Ba/La, Ba/Th, Sr/Y, Hf/Lu and 87Sr/86Sr). This suggests that insignificant transport of Zn from the subducting Pacific slab to the Kamchatka and Central-Eastern Aleutian mantle wedge. Our model calculations suggest that the observed offset of δ66Zn between the mantle and arc magmas can be attributed to isotope fractionation during partial melting with no need for contributions from subduction components.

Precambrian origin of the North Lhasa terrane, Tibetan Plateau: Constraint from early Cryogenian back-arc magmatism

The origin of ancient continental blocks in the Tibetan plateau and their paleogeographic locations in the Rodinia supercontinent remain enigmatic. Here, we report the early Cryogenian metamorphic magmatic rocks (including amphibolites and granitic gneisses) from the North Lhasa terrane of the central Tibetan Plateau. The amphibolites (ca. 822 Ma) are tholeiitic and exhibit both MORB- (e.g., flat patterns of rare-earth and high field strength elements) and arc-like (e.g., elevated Th/Yb tatios) geochemical affinities. In combination with high positive zircon εHf(t) (+6.9 to +12.4) and whole-rock εNd(t) (+4.4 to +10.4) and low zircon δ18O (4.87 to 5.81‰) values, their geochemical data indicate a depleted mantle source affected by subduction components. The granitic gneisses (ca. 810 and 806 Ma) are A2-type granitoids and have relatively lower zircon εHf(t) (+4.7 to +6.9) and whole-rock εNd(t) (+3.5) and higher zircon δ18O (5.44–8.08‰) values. Their protoliths were probably generated by partial melting of Mesoproterozoic crustal rocks. The amphibolites and granitic gneisses are geochemically distinct from the Cryogenian rift-related magmatic rocks in the interior of the Rodinia supercontinent but similar to the coeval back-arc magmatic rocks at the northwestern edge of the Rodinia supercontinent, thus providing new constraints on the early Cryogenian paleogeographic location of the North Lhasa terrane.

Sequence and petrogenesis of the Jurassic volcanic rocks (Yeba Formation) in the Gangdese arc, southern Tibet: Implications for the Neo-Tethyan subduction

The Yeba Formation volcanic rocks in the Gangdese arc recorded important information regarding the early history of the Neo-Tethyan subduction. To explore their magmatic evolution and tectonic significance, we performed a systematic petrological, geochronological and geochemical study on these volcanic rocks. Our data indicated that the Yeba Formation documents a transition from andesite-dominated volcanism (which started before 182 Ma and continued until ~176 Ma) to bimodal volcanism (~174–168 Ma) in the earliest Middle Jurassic. The early-stage andesite-dominated volcanics are characterized by various features of major and trace elements and are interpreted as the products of interactions between mantle-derived arc magmas and lower crustal melts. Their positive εNd(t) and εHf(t) values suggest a significant contribution of asthenosphere-like mantle. The late-stage bimodal volcanism is dominated by felsic rocks with subordinate basalts. Geochemical signatures of the basalts indicate a composite magma source that included a “subduction component”, an asthenosphere-like upper mantle domain and an ancient subcontinental lithospheric mantle component. The felsic rocks of the late stage were produced mainly by the melting of juvenile crust, with some ancient crustal materials also involved. We suggest that the occurrence and preservation of the Yeba Formation volcanic rocks were tied to a tectonic switch from contraction to extension in the Gangdese arc, which probably resulted from slab rollback of the subducting Neo-Tethyan oceanic slab during the Jurassic.

Origin of Primitive Tholeiitic and Calc‐Alkaline Basalts at Newberry Volcano, Oregon

AbstractMajor and trace element and radiogenic isotopic characteristics of primitive mafic Pleistocene and Holocene lavas from Newberry Volcano, Oregon, define two groups. The first consists of dry tholeiitic high‐alumina olivine basalts that are slightly enriched in highly incompatible elements. The second group consists of calc‐alkaline basalts that contained 2–4 wt % H2O prior to eruption and shows strong enrichment in the light rare earth elements, Ba, and Sr, and deficits in Nb, Ta, Hf, and Zr. The tholeiitic basalts reflect 6–11% anhydrous adiabatic decompression melting of spinel peridotite. The calc‐alkaline basalts derived from compositionally distinct sources with strong LIL enrichment and relative depletion in HFSE, but with Sr, Nd, Hf, and Pb isotopic composition only slightly distinct from the sources of the tholeiitic magmas. Radiogenic Os correlates with LREE enrichment in the calc‐alkaline magmas, which indicates that their source materials include a contribution from a mafic component that was melted in the garnet stability field. The calc‐alkaline magmas were derived by melting of peridotite metasomatized by a fluid/melt that originated by melting of a mixture of the sediment plus MORB basalt/mantle in the underlying subducting oceanic plate. While the trace element characteristics of the calc‐alkaline magmas were determined by the subduction component, their isotopic characteristics were modified during transit through the mantle by interaction with the highly magmatically processed mantle wedge beneath Newberry Volcano that, without the slab component, serves as the source of the tholeiitic magmas.

Subduction contribution to the magma source of the Okinawa Trough—Evidence from boron isotopes

Boron (B) has been an excellent geochemical tracer for the investigation of magmatism in arcs and back‐arc basins, due to its high fluid mobility and distinct isotope compositions in the mantle wedge and subducting slabs. In this study, B isotope compositions of the volcanic rocks collected from the northern, middle, and southern Okinawa Trough were determined to discuss the sources of B in the magma and assess the contribution of subduction components to the magma source. The results show that B in the volcanic rocks from the Okinawa Trough are mainly from the underlying mantle wedge but influenced by the fluids derived from the subducted slab. Compared with volcanic rocks from the middle Okinawa Trough, samples from the southern Okinawa Trough are generated from the source with larger proportion of subduction components. For the volcanic rocks from the northern Okinawa Trough, the influence of the subduction components is the most significant and the proportion of slab‐derived B in the magma reaches up to 30%. It is estimated that the subduction components are composed of altered oceanic crust/serpentinite fluid and sediment fluid with the proportion of 94:6. The B‐rich fluid, loaded by cold and fast dipping slab, released into overlying mantle wedge resulting in partial melting of mantle is the genetic mechanism of B‐rich volcanic rocks in the Okinawa Trough.

Immature intra-oceanic arc-type volcanism on the Izanagi Plate revealed by the geochemistry of the Daimaruyama greenstones in the Hiroo Complex, southern Hidaka Belt, central Hokkaido, Japan

The Izanagi Plate is assumed to have underlain the western Panthalassa Ocean to the east of Eurasia, and to have been subducting under the Eurasian continent. Although the Izanagi Plate has been lost to subduction, the subduction complexes of the circum-Panthalassa continental margins provide evidence that subduction-related volcanism occurred within the Panthalassa Ocean, and not just along its margins. The Daimaruyama mass is a kilometer-sized allochthonous greenstone body in the Hiroo Complex in the southeastern part of the Nakanogawa Group in the southern Hidaka Belt, northern Japan. The Hiroo Complex is a subduction complex that formed within the Paleo-Kuril arc–trench system at 57–48 Ma. The Daimaruyama greenstones consist mainly of coarse volcaniclastic rocks with lesser amount of lava. Red bedded chert, red shale, and micritic limestone are also observed as blocks associated with the greenstones. The presence of Early Cretaceous (Aptian–Albian) radiolaria in red bedded cherts within the greenstones indicates that the Daimaruyama greenstones formed after this time. An integrated major and trace element geochemical dataset for whole-rocks and clinopyroxenes of the greenstones indicates a calc-alkaline magmatic trend with low TiO2 contents and increases in SiO2 and decreases in FeO* with increasing differentiation. Negative anomalies of Nb, Ta, and Ti in normal mid-ocean-ridge basalt type normalized patterns are interpreted as “arc-signatures”. Using “rhyolite-MELTS”, we conducted a numerical simulation of magmatic differentiation under conditions of 1.5 kbar and H2O = 3 wt% to reproduce the liquid line of descent of the Daimaruyama greenstones. Back-calculations of the equilibrium melt compositions from the trace element chemistry of the clinopyroxenes generally agree with the whole-rock rare earth element compositions of the Daimaruyama greenstones, therefore providing support for the conditions used for the rhyolite-MELTS calculations as well as the actual results. Clinopyroxene trace element compositions indicate substantial enrichment of Ba in the magma, reflecting the participation of shallow subduction components such as aqueous fluids. Geochemical investigations reveal that the Daimaruyama greenstones were probably submarine volcanic rocks that formed as a result of the subduction of the Izanagi Plate within the Thalassa Ocean (the Thalassa Ocean was the eastern realm of Panthalassa, and represents the proto-Pacific Ocean) after the Early Cretaceous (Aptian–Albian; 125–101 Ma), and they were eventually accreted onto the Paleo-Kuril arc–trench system at 57–48 Ma to form an allochthonous block as part of the mélange facies of the Hiroo Complex on the landward slope of the trench.