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Eocene igneous rocks along the Gangdese belt in southern Tibetan Plateau are important for understanding the India–Eurasia collision and Tibetan Plateau uplift. These magmatic rocks are widely considered to be related to roll-back or break-off of the Neotethyan slab during northward subduction. However, Eocene rhyolites in the northwestern Gangdese belt (SW Qiangtang margin) do not fit either the rollback or breakoff models. This paper investigates these Early Eocene lavas and compares them with contemporaneous igneous rocks along the Gangdese belt via detailed field observations, petrology, zircon geochronology, zircon Lu–Hf isotopes and whole-rock geochemistry. These felsic lavas form large outcrops in western Tibet and have two zircon U–Pb ages of 54.71 ± 0.14 Ma and 54.74 ± 0.27 Ma. They have high SiO2 and alkali contents, FeO*/MgO, and Ga/Al ratios as well as strongly negative Eu anomalies and slightly positive zircon εHf(t) values (+2.5 to +4.4). Both Ti-in-zircon and Zr saturation thermometers confirm high magmatic temperatures (~900°C). They are ferroan, A-type rhyolites. Furthermore, the distinctive low εHf(t) values imply that these rhyolites are likely partial melting products of the southwestern Qiangtang Terrane. The lithosphere in this region undergone long-term shortening and thickening since 80–95 Ma. Pre-thickened lithosphere may be a prerequisite for the Early Eocene delamination, and the simultaneous inducement is a far-field tectonic effect from the India–Eurasia collision. We suggest that: (1) India collided with Eurasia before 55 Ma and triggered delamination to form the Shiquanhe A-type rhyolites on the southwestern Qiangtang margin thereafter; and (2) tectonic stresses related to India–Eurasia collision during the Eocene were transmitted efficiently over large distances. [ABSTRACT FROM AUTHOR]

Broadly similar Early to Middle Jurassic stratigraphic sequences including bimodal igneous rocks of the Sanandaj–Sirjan Zone of Iran and the Sakarya Zone of Turkey suggest that these formed in a common tectonic setting in an extensional basin that evolved from a terrestrial magmatic rift to a marine shelf and passive continental margin. Whole‐rock chemistry and Sr–Nd isotope signatures indicate derivation of mafic melts from partial melting of the subcontinental lithosphere. Decompression associated with extension led to 5%–30% partial melting of spinel–garnet lherzolite with minor involvement of continental crust, producing tholeiitic to transitional basaltic magma. Extensional basins inverted during the Mid‐Late Jurassic. These relationships suggest the Early to Middle Jurassic formation of a volcanic rifted margin on the SW Eurasian margin, similar to that of offshore Norway. [ABSTRACT FROM AUTHOR]

The compositional similarity between high‐Mg andesite‐dacite from accretionary orogens and bulk continental crust (CC) provides an opportunity to unravel the CC formation paradox. Compositional data from a global compilation of Quaternary magmatic arcs indicate the presence of low‐Mg series (LMS) and high‐Mg series (HMS). The LMS show trends of crystal fractionation and can be subdivided into high Ba/Th and high La/Sm groups, which likely originate from fluid‐ and sediment melt‐modified mantle wedge, respectively. In contrast, the HMS have variably mixed compositions (e.g., high Mg#, Ba/Th, and La/Sm) and can be explained by partial melting of mélange diapirs rising into the mantle wedge, which are mixtures of subducted sediment, eroded arc crust or CC, buoyant oceanic crust, and peridotite. We, therefore, propose a two‐step process for creating CC involving extraction of LMS from the mantle followed by re‐melting of recycled LMS in the mantle to generate HMS and thus CC. Plain Language Summary: Continental crust (CC) is extracted from the mantle primarily by subduction‐related magmatism in accretionary orogens. However, igneous rocks sourced from the mantle have low SiO2 (basalt), whereas average CC has high SiO2 (andesite) and high MgO. To resolve this paradox, we analyzed Quaternary subduction‐related igneous rocks worldwide and find that they can be divided into low‐Mg series (LMS) and high‐Mg series (HMS). We compare existing compositional data for LMS and HMS and find that LMS are dominated by basalt whereas HMS are dominated by andesite and similar to the average CC. Petrogenetic analyses reveal that LMS likely originate from the mantle and represent primitive CC, whereas HMS likely originate from recycled crustal materials in the mantle and represent mature CC. We propose a two‐step process for creating CC, involving extraction of LMS from the mantle followed by re‐melting of recycled LMS to generate HMS and thus CC. Our results provide a new solution to the CC formation paradox. Key Points: Low‐Mg series (LMS) arc igneous rocks originate from fluid‐/sediment melt‐modified mantle wedge, followed by crystal fractionationHigh‐Mg series (HMS) arc igneous rocks likely originate from recycled crustal materials as mélange diapirs rising into the mantle wedgeContinental crust is formed by extracting LMS from the mantle followed by re‐melting of recycled LMS in the mantle wedge to generate HMS [ABSTRACT FROM AUTHOR]

Garnets show wide ranges of chemical compositions and are key minerals for reconstructing the thermodynamic evolution of metamorphic terranes. The properties of garnets – including Mohs hardness ~7, lack of cleavage, vitreous luster and multiple colors – also makes them useful as semi-gemstones. Garnets are widespread accessory minerals in igneous and metamorphic rocks of western and southern Iran and are especially abundant in skarns and regional metamorphic rocks. 45 garnet deposits of western and southern Iran show six associations: (1) skarn (35 locations); (2) peraluminous granitoid and rhyolite (4 locations); (3) alkaline granite (1 location); (4) metamorphic rocks (3 locations); and (5) ophiolites (2 locations). Distinct garnet compositions are found in each association: mostly grossular-andradite in skarn, Ti-andradite in alkaline granite, almandine in peraluminous granite and volcanic rocks, almandine-grossular in metamorphic rocks and andradite- uvarovite associated with ophiolites. Western and southern Iran garnets are mostly related to skarns around Cenozoic granitoid intrusions. Ti-rich garnet formed at the expense of clinopyroxenites in alkaline igneous complexes due to alkaline metasomatism. These associations are useful for understanding garnet semi-gemstone deposits in western and southern Iran. [ABSTRACT FROM AUTHOR]

We studied nine samples of igneous rocks from the inner wall of the Mariana Trench above the Challenger Deep from 4150 to 6100 m depth recovered by manned submersible and ROV. Samples from two regions that bracket the Moho were studied: (i) 7 samples from a N‐S transect a few km to the west of the Shinkai Seep Field; and (ii) 2 samples from the Shinkai Seep Field. Transect samples include olivine‐2 pyroxene hornblendites, amphibole basalts, basaltic andesite, and hornblende andesite. We analyzed three transect samples for 40Ar/39Ar ages; two yielded good plateau ages of 46.5 ±0.5 Ma (hornblendite) and 46.60 ±0.15 Ma (hornblende andesite). These results combined with previously published results, indicate that this crust formed during an intense 46–47 Ma magmatic episode that occurred 5–6 my after subduction initiation. Hornblendites and hornblende basalts formed from primitive magmas, as shown by high MgO (11–21 wt%), Ni (222–885 ppm) and Cr (412–1145 ppm) contents. Electron microprobe analyses indicate that hornblende is Na‐rich (up to 3.0 wt% Na2O) and that many samples have an atypically large range in plagioclase composition (i.e. individual samples have An < 10 to An 90 plagioclase). Two subgroups can be identified: a mostly deeper depleted suite and a mostly shallower enriched suite. These results indicate that (i) the crust–mantle boundary in this region is transitional, occurring over a ~ 1.5 km interval, with interlayered peridotite and hornblendites between 5800 and 4300 m; and (b) extension to form the Challenger Deep forearc segment occurred by combined stretching of old crust and injection of young basaltic magmas. In contrast to the mostly fresh nature of transect samples, the two samples from the Shinaki Seep Field are intensely altered peridotite and basalt. [ABSTRACT FROM AUTHOR]

Jurassic igneous bodies of the Sanandaj–Sirjan zone (SaSZ) in SW Iran are generally considered as a magmatic arc but critical evaluation of modern geochronology, geochemistry and radiogenic isotopes challenges this conclusion. There is no evidence for sustained igneous activity along the ~1,200 km long SaSZ, as expected for a convergent plate margin; instead activity was brief at most sites and propagated NW at ~20 mm/a. Jurassic igneous rocks define a bimodal suite of gabbro‐diorite and granite. Chemical and isotopic compositions of mafic rocks indicate subcontinental lithospheric mantle sources that mostly lacked subduction‐related modifications. The arc‐like features of S‐type granites reflect massive involvement of Cadomian crust and younger sediments to generate felsic melts in response to mafic intrusions. We conclude that Jurassic SaSZ igneous activity occurred in a continental rift, not an arc. SaSZ igneous rocks do not indicate that subduction along the SW margin of Eurasia began in Jurassic time. [ABSTRACT FROM AUTHOR]

China covers approximately 10 million km 2 and its crust has a complicated evolution of amalgamation, igneous activity, and sedimentation. Many studies address various aspects of China's crust, but few provide a simple geological and geophysical overview that is accessible to students and non-specialists; Filling this void is the objective of this review. China is characterized by thick (40–75 km) crust in the west due to Cenozoic collision with India and thin (30–40 km thick) crust in the east due to E–W Mesozoic-Cenozoic backarc extension. In contrast, overall crustal fabric trends E–W, defined by ophiolite belts and ultra-high pressure metamorphic rocks. This crustal fabric indicates that China has grown like a sandwich, with crust progressively added through Phanerozoic time by closing various E–W oriented Tethys oceans and seaways. In map view, China consists of five E–W trending tiers. Tier 1 is defined by the Central Asian Orogenic Belt (CAOB) along the northern margin of China, which consists of the Xing'an-Mongolia orogenic belt in the NE and the Tianshan Orogen in the NW. The CAOB formed during ~1000 Ma to ~250 Ma and is an accretionary orogen of mostly Paleozoic age that formed through closure of the Paleo-Asian Ocean and collision between the Siberian Craton and Archean-Paleoproterozoic crust to the south, which constitutes Tier 2. The CAOB has a strong aeromagnetic signature. Sediments from the Amur River show detrital U-Pb zircon age peaks at 2.8–2.3 Ga, 1.8 Ga, 450–250 Ma, and 200–100 Ma, which is expected for erosion of the Xing'an-Mongolia belt. Tier 1 igneous rocks are mainly Paleozoic except in the NE (Xing'an-Mongolia orogenic belt) and reflect subduction of the Paleo-Asian Ocean and associated accretion events, whereas Paleozoic CAOB crust in the east is overprinted by Jurassic and Cretaceous igneous rocks related to subduction of ancient Pacific basin oceanic lithosphere. Tier 2 includes the North China Craton (NCC) to the east and Tarim Craton to the west. The NCC contains the oldest rocks in China and is dominated by Archean and Paleoproterozoic ages. The extent of Archean rocks in the NCC may have been overestimated, as suggested by detrital zircons from the Yellow River, which flows across the craton, showing age peaks at 2.5–2.2 Ga, ~1.9 Ga, 500–400 Ma, and 300–200 Ma. The Tarim Craton is dominated by Palaeoproterozoic- Mesoproterozoic metamorphic strata along with a significant proportion of Neoproterozoic (~0.8 Ga) rocks. U-Pb ages for detrital zircons from Tarim River sediments reflect this basement geology, with strong peaks of Early and Late Paleozoic age, less abundant Neoproterozoic ages, and scattered ages back to the Archean. The NCC also was affected by abundant Mesozoic igneous activity with voluminous Early Cretaceous rocks that are associated with lithospheric thinning and decratonization. Tier 3 – also known as the Central China Orogen - is composed of the Sulu-Dabie-Qinling-Kunlun Orogen and records closing of an arm of Prototethys during the Ordovician to Silurian and Paleotethys during the Triassic. Tier 3 contains one of Earth's three giant ultra-high pressure (UHP) terranes with well-documented peak metamorphism of 650–850 °C and 4 GPa, indicating that some of these rocks were deeply subducted and then exhumed from depths of over 120 km in Triassic time. Tier 3 magmatism occurred in two episodes, early-middle Paleozoic and Triassic. Tier 4 contains blocks rifted from Gondwana, which include the Songpan-Ganzi, Qiangtang, and Lhasa terranes of Tibet in the west and the South China Block in the east. These terranes are marked by broad magnetic anomalies with a NE-SW trend along the Pacific margin, and a broad N–S trending anomaly between Tibet and South China. The South China Block is made up of Proterozoic and minor Archean crust of the Yangtze and Cathaysia blocks, which collided at 1.0–0.8 Ga to form the Jiangnan Orogen and the South China Block. Age spectra for detrital zircons from the Yangtze and Pearl Rivers shows major peaks at ~1.8 Ga, 900–800 Ma, ~400 Ma, and 300–150 Ma, which is consistent with the age of South China Block crust. Early-Middle Paleozoic igneous rocks are also found in South China. Mesozoic igneous rocks are widespread in both South China and Tibet and are related to subduction of the Paleo-Pacific and Tethyan oceanic plates, respectively. The accretion of Tibetan terranes to southern Eurasia occurred in the Mesozoic before collision with India at ~55 Ma. Tier 5 is represented by the island of Taiwan on the SE margin of China and marks where China crust continues to grow. Taiwan lies on a complex convergent boundary between the South China Block to the NW, the Philippine Sea Plate to the SE, and the Sunda Plate to the SW. [ABSTRACT FROM AUTHOR]

The Cadomian orogen of NW Iran includes a series of metamorphic rocks with zircon U-Pb ages between ca 562 and 505 Ma (Ediacaran to middle Cambrian). The Ediacaran-Cambrian basement is intruded by a series of Late Eocene-Late Oligocene I-type granitic rocks. U-Pb geochronology, integrated with geochemical and isotopic data for the basement rocks in NW Iran, provides further evidence of a Cadomian (562-505 Ma) arc-related magmatic event lasting ∼ 60 Myr. Cadomian magmatism in Iran was a part of a ∼ 100 Myr long episode of subduction-related arc magmatism at the northern margin of Gondwana. Zircon Hf-isotope compositions show that during Cadomian magmatic arc activity, juvenile arc magmas interacted with reworked Archean crust to generate the Ediacaran-Cambrian igneous rocks. Our results document both inheritance of old zircons and the presence of zircons with juvenile signatures in NW Iran, suggesting that the geotectonic setting for the Cadomian rocks was an Ediacaran continental magmatic arc and probably a neighboring back-arc basin. The occurrence of Ediacaran ophiolitic slices in NW Iran may provide evidence of back-arc basin opening at that time. Cenozoic plutonism in NW Iran is part of an Eocene-Oligocene magmatic 'flare-up' along the Urumieh-Dokhtar Magmatic Belt in central Iran, which lasted for ca 30 Myr. The melts responsible for the formation of these rocks had an essentially juvenile signature with minor contamination by Archean to Cadomian middle-lower continental crust. Continuous convergence between Arabia and Iran was accompanied by the transition of SW Eurasia from a compressional to an extensional convergent plate margin in Eocene-Oligocene times, leading to orogenic collapse, core-complex formation, exhumation of Cadomian crust and a major increase in arc magmatism. [ABSTRACT FROM AUTHOR]

An Ediacaran mafic intrusion of south Jordan is a distinctive appinitic igneous rock with a possibly unique texture, characterized by spherical clots up to 40 mm in diameter composed of amphibole cores from which plagioclase euhedra radiate; we call it the Rosetta Gabbro. It is exposed as a small (ca. 750 m 2 ) outcrop in the Neoproterozoic basement of south Jordan. A second outcrop of otherwise similar gabbro is located about 400 m to the north of the Rosetta Gabbro, but it lacks the distinctive texture. The Rosetta Gabbro could represent a magma pipe. It intrudes the Aqaba Complex (~ 600 Ma) granitoids and metasediments of the Janub Metamorphic Complex (633–617 Ma). The gabbro is an Ol- to QZ tholeiite with the following chemical characteristics: SiO 2 = 46.2–47.8 wt.%; Al 2 O 3 = 16.4–17.7 wt.%, TiO 2 = 1.70–2.82 wt.%, Na 2 O = 1.27–2.83 wt.%. K 2 O = 0.82–1.63 wt.%; Mg# 58–63; Σ REE = 70–117 ppm; La/Yb ~ 6 to 8; and Eu/Eu* = 1.05–1.2. The investigated gabbro has the geochemical features of a continental flood tholeiitic basalt emplaced in a within-plate tectonic setting. Two varieties of amphiboles are found: 1) large, 3–5 mm, brown ferri-titanian-tschermakite (K 0.09 Na 0.28 )(Na 0.20 Ca 1.80 )(Mn 0.04 Fe 3 + 1.1 Mg 2.34 Fe 2 + 0.90 Ti 0.29 Al 0.22 )(Al 1.85 Si 6.15 )O 22 (OH) 1.95 of the calcic amphibole group which is riddled with opaques; and 2) acicular yellowish-light green ferrian-magnesiohornblende (K 0.04 Na 0.153 )(Ca 1.755 Na 0.245 ) (Fe 3 + 0.66 Mn 0.01 Fe 2 + 1.01 Mg 3.03 Ti 0.06 Al 0.22 )(Al 1.03 Si 6.97 )O 22 (OH) 1.95 . Scattered flakes of phlogopite also occur. Tabular radiating plagioclase (An 64–79 ) are complexly twinned, with broad lamellae that show no zoning. Laser-ablation ICP-MS analyses of amphibole and plagioclase reveal considerable variation in trace element abundance, in spite of more subtle major element variations except for TiO 2 in amphibole. The REE in the amphibole shows an order of magnitude variation with a concave-downward pattern and a positive Eu anomaly Eu/Eu* = 0.6–2, though far less pronounced compared to the Eu/Eu* = 5–45 of plagioclase. The 3D dandelion-like texture of the rosettas is broadly similar to “Chrysanthemum Stone”, which is a diagenetic growth in sedimentary rock, but we can find no description of similar textures in igneous rocks. The formation of the rosettas is thought to reflect loss of magmatic water resulting in supersaturation of plagioclase, which grew rapidly around amphibole and may have floated in the magma. This implies magmatic evolution in shallow (10 to 12 km deep) crust where temperatures were nevertheless in the range of ca. 750 to 900 °C. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Atud diamictite in Wadi Kareim and Wadi Mobarak in the Eastern Desert of Egypt and the Nuwaybah formation in NW Saudi Arabia consist of poorly sorted, polymictic breccia, with clasts up to 1 m of granitoid, quartz porphyry, quartzite, basalt, greywacke, marble, arkose, and microconglomerate in fine-grained matrix. Stratigraphic relations indicate that the diamictite was deposited in a marine environment. Integrated field investigation, petrographic study and U–Pb SHRIMP zircon ages demonstrate that the Atud and Nuwaybah are correlative. The distribution of zircon ages indicate that ~750 Ma ages are dominant with a significant component of older materials, characterized by minor Mesoproterozoic and more abundant Paleoproterozoic and Neoarchean ages. Some matrix and metasedimentary clast zircons yield ages that are a few 10s of Ma younger than the age of the youngest clast (754 ± 15 Ma), suggesting Atud/Nuwaybah diamictite deposition ~750 Ma or slightly later, broadly consistent with being deposited during the Sturtian glaciation (740–660 Ma). The Paleoproterozoic and Neoarchean clasts have no source within the ensimatic Arabian–Nubian Shield. The distribution of the pre-Neoproterozoic ages are similar to the distribution of the pre-Neoproterozoic ages in Yemen and Saharan Metacraton, suggesting that these clasts have been transported hundreds of kilometers, maybe by ice-rafting. The Atud diamictite may represent important evidence for Cryogenian “Snowball Earth” in the Arabian–Nubian Shield. [ABSTRACT FROM AUTHOR]

New geochemical, isotopic, and geochronological data and interpretations are presented for late Neoproterozoic intrusive carbonates and related rocks of southern Sinai, Egypt (northernmost Arabian–Nubian Shield). The Tarr carbonates are coarsely crystalline and related to explosive emplacement of hypabyssal and volcanic albitite at 605 ± 13 Ma. The carbonates associated with the albitites are divisible into two types: primary dolomitite and secondary breunneritite (Fe-rich magnesite). The dolomitite was clearly intrusive but differs from classic igneous carbonatites, containing much lower abundances of incompatible elements, such as REE, U, Th, Rb, Nb, Y, P, Sr, Zr, Ba, and total alkalies. The breunneritite is a secondary replacement of dolomitite, probably marking the roots of a vigorous hydrothermal system. Albitites show pristine abundances of major and trace elements and were not subjected to a major metamorphic overprint. They are relatively more fractionated, alkaline and related to within-plate A-type magmas, were emplaced in an extensional or non-compressive tectonic regime in the cupola of high-level A-type granite. Tarr albitites may represent residual magma remaining after near-total crystallization of an A-type granite pluton at depth, forcibly emplaced into the roof above the cooling pluton. The intrusive dolomitite exsolved from highly differentiated albitite melt, in the apical regions of a still-buried alkaline “A-type” granite pluton that was rich in CO2; these volatiles migrated upwards and towards the cooler margins of the magma body. Late NNE-SSW extension allowed a shallow-level cupola to form, into which albitite melts and carbonate fluids migrated, culminating in explosive emplacement of albitite breccia and intrusive carbonate. Isotopic compositions of Tarr dolomitite and albitite indicate these are consanguineous and ultimately of mantle origin. Magmatic volatiles fenitized the wall rock, while submarine hydrothermal activity transformed some of the dolomitite into breunneritite. Recognition of Tarr-type should encourage similar hypabyssal complex intrusions to be sought for in association with A-type granitic plutons elsewhere. [ABSTRACT FROM AUTHOR]

West Rota Volcano (WRV) is a recently discovered extinct submarine volcano in the southern Mariana Arc. It is large (25 km diameter base), shallow (up to 300 m below sealevel), and contains a large caldera (6 × 10 km, with up to 1 km relief). The WRV lies near the northern termination of a major NNE-trending normal fault. This and a second, parallel fault just west of the volcano separate uplifted, thick frontal arc crust to the east from subsiding, thin back-arc basin crust to the west. The WRV is distinct from other Mariana Arc volcanoes: (i) it consists of a lower, predominantly andesite section overlain by a bimodal rhyolite-basalt layered sequence; (ii) andesitic rocks are locally intensely altered and mineralized; (iii) it has a large caldera; and (iv) WRV is built on a major fault. Submarine felsic calderas are common in the Izu and Kermadec Arcs but are otherwise unknown from the Marianas and other primitive, intraoceanic arcs. 40Ar–39Ar dating indicates that andesitic volcanism comprising the lower volcanic section occurred 0.33–0.55 my ago, whereas eruption of the upper rhyolites and basalts occurred 37–51 thousand years ago. Four sequences of rhyolite pyroclastics each are 20–75 m thick, unwelded and show reverse grading, indicating submarine eruption. The youngest unit consists of 1–2 m diameter spheroids of rhyolite pumice, interpreted as magmatic balloons, formed by relatively quiet effusion and inflation of rhyolite into the overlying seawater. Geochemical studies indicate that felsic magmas were generated by anatexis of amphibolite-facies meta-andesites, perhaps in the middle arc crust. The presence of a large felsic volcano and caldera in the southern Marianas might indicate interaction of large normal faults with a mid-crustal magma body at depth, providing a way for viscous felsic melts to reach the surface. [ABSTRACT FROM AUTHOR]

Abstract: There is an increasing evidence for the involvement of pre-Neoproterozoic zircons in the Arabian–Nubian Shield, a Neoproterozoic crustal tract that is generally regarded to be juvenile. The source and significance of these xenocrystic zircons are not clear. In an effort to better understand this problem, older and younger granitoids from the Egyptian basement complex were analyzed for chemical composition, SHRIMP U–Pb zircon ages, and Sm–Nd isotopic compositions. Geochemically, the older granitoids are metaluminous and exhibit characteristics of I-type granites and most likely formed in a convergent margin (arc) tectonic environment. On the other hand, the younger granites are peraluminous and exhibit the characteristics of A-type granites; these are post-collisional granites. The U–Pb SHRIMP dating of zircons revealed the ages of magmatic crystallization as well as the presence of slightly older, presumably inherited zircon grains. The age determined for the older granodiorite is 652.5±2.6Ma, whereas the younger granitoids are 595–605Ma. Xenocrystic zircons are found in most of the younger granitoid samples; the xenocrystic grains are all Neoproterozoic, but fall into three age ranges that correspond to the ages of other Eastern Desert igneous rocks, viz. ∼710–690, ∼675–650 and ∼635–610Ma. The analyzed granitoids have (+3.8 to +6.5) and crystallization ages, which confirm previous indications that the Arabian–Nubian Shield is juvenile Neoproterozoic crust. These results nevertheless indicate that older Neoproterozoic crust contributed to the formation of especially the younger granite magmas. [Copyright &y& Elsevier]

Southwestern Japan during Paleogene time was affected by subduction of the Kula and Pacific plates beneath the eastern margin of Eurasia, followed by a brief episode of transform faulting to allow the Shikoku Basin to open at 27-15 Ma. Subsequent opening of the Japan Sea back-arc basin in middle Miocene time broke Japan away from Eurasia, and SW Japan rotated clockwise ∼45° as it drifted south (17-15 Ma). Southward drift required subduction of the Philippine Sea plate beneath SW Japan, leading to the formation of a magmatic arc. Subduction of the hot Shikoku Basin lithosphere and spreading ridge resulted in distinctive volcanism in the SW Japan forearc between 17 and 12 Ma. This volcanism included midoceanic-ridge basalt (MORB)-like intrusions and oceanic-island basalt (OIB)-type alkali basalts, felsic plutons in the accretionary prism, and high-magnesium andesites along the Setouchi forearc basin. Mafic magmas in the outermost forearc originated from the subducted Shikoku Basin spreading ridge, whereas Setouchi high-magnesium andesites (HMA) may have resulted from the interaction of melts of the subducted Philippine Sea plate with the overlying mantle. Felsic magmas in the forearc resulted from melting of Shimanto Belt sediments caused by intrusion of HMA magmas. Persistent rear-arc volcanism was caused by upwelling astheno sphere associated with opening of the Sea of Japan. Rear-arc volcanism began ca. 25 Ma as rift-fill low-alkali tholeiite volcanism and was replaced gradually after 12 Ma by alkali basalt volcanism. Upwelling-related alkali volcanism continues up to the present, whereas forearc volcanism ceased at 12 Ma. The volcanic arc narrowed with time as the Philippine Sea slab descended and slowly cooled. Adakitic dacites erupted after 1.7 Ma above the 100-km-depth contour of the subducted Philippine Sea plate, suggesting that melting resulted from interaction of the slab with upwelling asthenosphere. Interactions between upwelled rear-arc asthenosphere and subduction of the hot Philippine Sea slab appear to have been the main controls of magmatism for the Neogene SW Japan arc. [ABSTRACT FROM AUTHOR]

Earth is the only known planet with subduction zones and plate tectonics, and this fact demonstrates that special conditions are required for this mode of planetary heat loss. Sinking of cold, dense lithosphere in subduction zones is the principal plate-driving force, so plate tectonics could not have begun until Earth cooled sufficiently to allow lithosphere to collapse into the underlying asthenosphere. Direct geologic evidence for when the modern episode of subduction tectonics began focuses on the first appearance of ophiolitic graveyards, blueschist facies metamorphic rocks, and ultrahigh-pressure metamorphic terranes. Ophiolites manifest two modes of lithospheric motion expected from subduction tectonics: seafloor spreading and obduction. High-pressure, low-temperature metamorphic blueschists and ultrahigh-pressure terranes indicate subduction and exhumation of oceanic and continental crust, respectively. These lines of evidence indicate that the modern style of subduction tectonics began in Neoproterozoic time. This revolution in the functioning of the solid Earth may have driven wild fluctuations in Earth's climate, described under the "snowball Earth" hypothesis. These conclusions may be controversial, but suggest fruitful avenues for research in geodynamics and paleoclimate. [ABSTRACT FROM AUTHOR]

The tectonic setting in which Jurassic igneous rocks of the Sanandaj-Sirjan Zone (SaSZ) of Iran formed is controversial. SaSZ igneous rocks are mainly intrusive granodiorite to gabbroic bodies, which intrude Early to Middle Jurassic metamorphic basement; Jurassic volcanic rocks are rare. Here, we report the age and petrology of volcanic rocks from the Panjeh basaltic-andesitic rocks complex in the northern SaSZ, southwest of Ghorveh city. The Panjeh magmatic complex consists of pillowed and massive basalts, andesites and microdioritic dykes and is associated with intrusive gabbros; the overall sequence and relations with surrounding sediments indicate that this is an unusually well preserved submarine volcanic complex. Igneous rocks belong to a metaluminous sub-alkaline, medium-K to high-K calc-alkaline mafic suite characterized by moderate Al 2 O 3 (13.7–17.6 wt%) and variable Fe 2 O 3 (6.0–12.6 wt%) and MgO (0.9–11.1 wt%) contents. Zircon U-Pb ages (145–149 Ma) define a Late Jurassic (Tithonian) age for magma crystallization and emplacement. Whole rock compositions are enriched in Th, U and light rare earth elements (LREEs) and are slightly depleted in Nb, Ta and Ti. The initial ratios of 87 Sr/ 86 Sr (0.7039–0.7076) and εNd(t) values (−1.8 to +4.3) lie along the mantle array in the field of ocean island basalts and subcontinental metasomatized mantle. Immobile trace element (Ti, V, Zr, Y, Nb, Yb, Th and Co) behavior suggests that the mantle source was enriched by fluids released from a subducting slab (i.e. deep-crustal recycling) with some contribution from continental crust for andesitic rocks. Based the chemical composition of Panjeh mafic and intermediate rocks in combination with data for other gabbroic to dioritic bodies in the Ghorveh area we offer two interpretations for these (and other Jurassic igneous rocks of the SaSZ) as reflecting melts from a) subduction-modified OIB-type source above a Neo-Tethys subduction zone or b) plume or rift tectonics involving upwelling metasomatized mantle (mostly reflecting the ~550 Ma Cadomian crust-forming event). [ABSTRACT FROM AUTHOR]

It is important to understand the Jurassic tectonic setting of SW Eurasia, and the igneous rocks of the Sanandaj-Sirjan zone in SW Iran provide critical evidence. Bimodal volcanic and hypabyssal rocks exposed around Songhor in the central part of the Sanandaj-Sirjan zone have zircon U-Pb crystallization ages of 156 Ma for felsic and 153 to 146 Ma for mafic rocks. Felsic volcanic and subvolcanic rocks with affinity to A-type granite have high SiO 2 (62.6 to 73.9 wt%) and total alkalis (K 2 O + Na 2 O = 5.1–9.8 wt%), low MgO (0.1–3.1 wt%) and slightly positive εNd(t) (+0.6 to +3.8) and moderate 87Sr/86Sr(i) ratios (0.7041 to 0.7061). The mafic rocks (basalt and micro gabbro/dolerite) with lower contents of SiO 2 (47.0 to 54.6 wt%), high MgO (4.1 to 10.7 wt%) with variable Na 2 O (2.0–5.9 wt%) and K 2 O (0.1–2.6 wt%) can be divided into two groups based on TiO 2 contents: 1) low-Ti and 2) high-Ti. The low-Ti group has 0.14 to 0.62 wt% TiO 2 with εNd(t) = +3.2 to +5.8 and 87Sr/86Sr(i) = 0.7032–0.7059. The high-Ti basalts have 1.4–1.6 wt% TiO 2 with εNd(t) = +1.2 to +4.2 and 87Sr/86Sr(i) =0.7039 to 0.7057. The low-Ti suite is less fractionated, with Mg# between 55 and 74 compared to 43 to 63 for the high-Ti suite. Felsic rocks may have formed by remelting of underplated mafic rocks which were derived from subcontinental lithospheric mantle along with minor assimilation of continental crust in the early stage of extension. Thinning of continental lithosphere continued with increasing partial melting to produce high-Ti mafic melts early and low-Ti mafic melts in the late stage. Similar Sr-Nd isotope ratios for the mafic and felsic rocks with an 8–10 Ma interval between these two groups hint at gradual thinning of the continental lithosphere. This is consistent with formation of transitional crust of a passive continental margin between Neo-Tethys and Iran continental crust in Jurassic time. [Display omitted] • Bimodal magmatism occurred from 156 to 144 Ma in west Iran (Songhor). • Felsic rocks were generated from the mantle with minor involvement of older crust. • Mafic rocks are divided into low-Ti (tholeiitic) and high-Ti (alkaline) groups. • Magmatic activity suggest that SW Iran was a Jurassic volcanic rifted margin. [ABSTRACT FROM AUTHOR]

Across-arc geochemical variations between igneous rocks from the magmatic front (MF) to rear-arc (RA) are commonly observed for convergent margins, but the processes that are responsible for these variations are unclear. To address these questions, we studied a well-exposed Cenozoic continental arc, the Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran and its rear-arc counterpart to the north. North Iran RA magmatism is bimodal, characterized by mafic to felsic igneous rocks of calc-alkaline to shoshonitic affinity. The new zircon U -Pb ages show that plutonic rocks are ~ 42–38 Ma whereas volcanic rocks are older at ~52 Ma. 87Sr/86Sr (t) and εNd (t) values of UDMB RA igneous rocks range from 0.70483 and 0.70783 and –2.5 and + 3.6, respectively. Zircons from plutonic rocks show εHf(t) values ranging from +1.8 to +9.8, higher than volcanic rocks with εHf (t) between +2.6 and –9.2. Isotopic data agrees with inferences from incompatible trace element ratios (e.g. , Th/Yb, Zr/Nb and La/Yb) that mafic plutonic rocks originated from an enriched mantle source, whereas felsic volcanic rocks show more interaction with continental crust. High magmatic fluxes did not occur at the same time in the MF and RA; UDMB MF flared-up at ~ 54–37 Ma and 20–5 Ma, overlapping RA magmatic flare-ups at 45–40 Ma and 30–25 Ma. The available major and trace element data from the UDMB show that most magmatic rocks experienced strong fractional crystallization and assimilation-fractional crystallization processes, making it difficult to identify across-arc geochemical variations. Moreover, the bulk rock εNd(t) and zircon εHf(t) values are highest at the MF, but the across-arc isotopic variability seems to be mainly controlled by crustal influence. High-flux magmatism in the UDMB was related to protracted heating of the crust as a result of subduction of Neotethys oceanic lithosphere. This weakened the continental lithosphere, leading to strong extension in Paleogene time accompanied by decompression melting and magmatic flare-ups. • Iran was the site of a Cenozoic continental arc with bimodal rear-arc magmatism. • Cenozoic rear-arc magmatic flare-up occurred in two pulses at 45–40 and 30–25 Ma. • Subduction, extension and crustal heating led to Cenozoic Iran high-flux magmatism. [ABSTRACT FROM AUTHOR]

Understanding how new subduction zones form is essential for complete articulation of plate tectonic theory. Formation of new subduction zones by collapse of oceanic transform faults or fracture zones is suggested on the basis of empirical evidence. This process has heretofore been investigated with two-dimensional (2-D) numerical models, which thus ignore its intrinsic three-dimensional (3-D) geometry, lateral propagation, and dynamics. Here, we investigate a 3-D thermomechanical model, in which old and thick oceanic lithosphere (plate) is separated by a transform fault from a thinner and younger oceanic plate containing a transform-orthogonal spreading ridge. The results suggest that the older plate starts to sink spontaneously at the ridge-transform fault junction, and then subduction initiation laterally propagates along the transform away from the ridge. Two key factors control the 3-D subduction initiation (SI) dynamics in nature: (1) the age of the sinking plate, which controls its negative buoyancy; and (2) the thermal structure of the overriding plate, which reflects its spreading history. Our numerical models not only shed new light on the SI dynamics of Cenozoic subduction zones (e.g., the Izu-Bonin-Mariana zone in the Pacific Ocean), but also have implications for fossil convergent plate margins (e.g., the Bitlis-Zagros suture zone, west of the Strait of Hormuz). In the latter case, systematic variations in ages of supra-subduction zone ophiolites may reflect diachronous SI and its lateral propagation. [ABSTRACT FROM AUTHOR]

Intra-plate basalts of ~ 35–0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature ( T p ), while the Shandong basalts have the lowest T p . We infer that a deep high- T p plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low- T p plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at ~ 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation. [ABSTRACT FROM AUTHOR]

Continental-arc igneous rock compositions change in response to the transition from subduction to collision and these changes can reveal how the crust, lithosphere and magma sources evolved. Neotethys-related Late Cretaceous to Pleistocene subduction- and collision-related magmatic rocks from the ~350 km long southeast Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran provide an excellent natural laboratory to better understand these changes. These igneous rocks are well-exposed and moderately eroded to reveal a nearly complete record since subduction initiation at ~95 Ma. We analyzed new samples for major and trace elements (83 samples), Sr Nd isotopic compositions (47 samples), and U Pb zircon ages (26 samples) and compiled geochemical and geochronological data on the southeast segment of the UDMB. The geochronological data reveal two magmatic pulses at ~80–70 Ma and ~50–0 Ma. Important changes in magmatic compositions reflect initial collision with Arabia at ~32 Ma, changing from normal calc-alkaline to increasingly adakitic immediately after collision began. Five stages can be identified: 1) normal continental-arc magmatism during the Late Cretaceous; 2) arc quiescence in Paleocene and Early Eocene time; 3) Middle-Late Eocene extensional arc magmatism related to slab rollback; 4) early collision and crustal thickening during the Early Oligocene; and 5) slab breakoff, asthenospheric upwelling, and associated adakitic magmatism from Middle Miocene onward. Temporal changes in UDMB magmas reflect the response of the overriding plate to changes in the geometry of the subducting Neotethyan lithosphere and to collision between Arabia and Iran. Crustal thickening and arc narrowing during Miocene to Pleistocene post-collisional magmatism caused adakitic magmatism and associated Cu mineralization. Zircon O Hf and apatite O isotopes as well as bulk-rock Nd isotopes of Cu-bearing adakitic rocks are similar to other barren rocks, but nearly all fertile rocks have higher Hf/Y, Eu/Eu⁎ (n) in zircon and higher Sr/Y, V/Y, Eu/Eu⁎ (n) in apatite than barren rocks. • Magmatism in southeast Iran occurred during neotethyan subduction and collision. • Magmatism changed from pre-collision calc-alkaline to Syn -collisional adakitic. • Syn-collisional magmatism at ca 32 Ma was characterized by perturbation of isotopic systems. [ABSTRACT FROM AUTHOR]

Abstract: A remarkable record of early arc volcanism in the Izu–Bonin–Mariana (IBM) arc is exposed in and around the Bonin Islands, an uplifted segment of the IBM forearc. New 40Ar/39Ar dating results imply that the boninitic volcanism on Chichijima Island occurred in a brief period during Eocene time, between 46–48 Ma. A slightly younger volcanic succession is identified along the Bonin Ridge, including 44.74±0.23 Ma high-Mg andesite from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, 44.0±0.3 Ma tholeiitic to calcalkaline andesite from Hahajima Island, and 3 samples of andesite collected by the submersible SHINKAI 6500 from the Bonin Ridge Escarpment (BRE) that range in age from 41.84±0.14 to 43.88±0.21 Ma. Four SHINKAI 6500 dives (YK 04–05) on the BRE mapped an elongated constructional volcanic ridge atop the escarpment; we observed steeply west-dipping volcaniclastic debris flows shed from the summit of this ridge into the Ogasawara Trough to the west. These dives recovered fresh andesitic clasts from debris flows along the northern segment of the ridge, and high-Mg andesite lava blocks and Nummulitic limestone of middle Eocene age from the escarpment northwest of Chichijima. Our results also confirm previous inferences that melting of depleted mantle at shallow levels beneath the length of the arc with the aid of hydrous fluids from newly subducted slab to produce boninitic volcanism occurred nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. BRE–Mikazukiyama Formation–Hahajima andesites represent a transitional stage from forearc spreading (represented by ODP site 786-Chichijima boninites) and the stable, mature arc that developed in the Oligocene. These OPX-bearing high-Mg or tholeiitic to calcalkaline andesites were erupted along the BRE, as the arc magmatic axis localized and retreated from the trench. [Copyright &y& Elsevier]