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Trace elements in magmatic zircon exhibit characteristic trends during magma compositional evolution. Zircon geochemistry presents the advantage over whole rock geochemistry of showing progressive 'snapshots' of accompanying melt chemistry during magma crystallization, and in situ methods such as LA-ICPMS and SIMS can provide multiple analyses in zoned zircons for higher resolution of magmatic processes. However, one limitation is that zircon is typically a late-crystallizing phase, leaving much of the magmatic history unrecorded. Using the CAMECA ims1290 ion microprobe with Hyperion-II ion source, we explore whether micro-zircon (ca. 5–40 µm) captured by relatively early-crystallizing magmatic accessory minerals can meaningfully enhance the interpretations of zircon trace element records of magmatic evolution in two granites from Southern California: the La Posta 2-mica granite and the granite of Butler Peak (Big Bear Lake Intrusive Suite). We find that contamination of the inclusion measurement by major structural constituents of the host phase is a problem for most measurements, but that species which are not structural components of the host phases follow typical magmatic zircon trace element evolution trends. Compared to free grains from a mineral separate, zircon inclusions in ilmenite and apatite from the La Posta 2-mica granite are on average lower in Hf, recording plausible extensions of these magmatic evolution trends to earlier periods in magma evolution. Free zircons from the granite of Butler Peak's mineral separate form a more complex story, recording both mixing between Cretaceous magmas derived from different depths (based on their U/Yb and Eu/Eu* behavior) and inheritance of older Mesozoic to Proterozoic zircon. Most zircon inclusions in Butler Peak magnetite and ilmenite are chemically like the apparently shallow magma-derived Cretaceous zircons in the free matrix, while a small proportion of the included zircon resemble more closely the apparently deeper magma or the inherited grains. [ABSTRACT FROM AUTHOR]

Oceanic gabbros are the most abundant rocks close to Earth's surface. Here we present new data from a consistent profile through the paleocrust of the Samail Ophiolite (Oman), which is thought to provide the best analog for modern fast‐spreading oceanic crust. Incompatible trace elements of co‐existing plagioclase and clinopyroxene fractionate from the mineral core to rim and up section from layered to foliated to varitextured gabbros. Layered gabbro parental melts correspond to mid‐ocean‐ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady‐state boundary conditions. Further upsection, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open‐system fractional crystallization within a heterogeneous magma plumbing structure, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). REE‐in‐plagioclase‐clinopyroxene thermometry compared to and supported by anorthite‐in‐plagioclase thermometry reveals a relationship of TREEAn ${T}_{\text{REE}}^{\text{An}}$ [°C] = 6.1 ± 0.2 × An + 706 ± 19. Crystallization temperatures of the layered gabbros cover a narrow range of 1216 ± 14°C. Considerable temperature variability of 1077–1231°C is observed further upsection, featuring a thermal minimum within the foliated gabbros. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels. Our findings suggest hybrid accretion of fast‐spread crust. Plain Language Summary: Oceanic crust, the crystalline basement beneath the ocean‐floor sediments is difficult to be sampled by shipboard drilling operations, but can be obducted onto continents by tectonic movement of lithospheric crustal fragments to form ophiolites. These rock formations serve as an analog to in‐situ crust. They can be samples along a profile from extrusive basaltic rocks to gabbros to the upper mantle giving access to the earth's interior. The geochemical and thermal evolution along a profile of paleocrust is crucial for gaining knowledge of the crust's accretional dynamics and formation processes. Our data suggest that the lower part the layered gabbros, are formed by a fundamentally different process than the upper foliated, and varitextured gabbros. While geochemical signatures systematically evolve upsection and the temperature structure is homogeneous for the lower part, the upper part is characterized by much higher dynamics and lower crystallization temperatures. Therefore, the crust forms in a hybrid process, where the lower part is dominated by near steady‐state in situ crystallization in equilibrium with the parental melt. The upper part is formed in a more heterogeneous environment, dominated by subsiding crystal mushes, which mix with more primitive melts under disequilibrium conditions, yielding enhanced chemical variability. Key Points: Geochemical profiles reveal continuous evolution of incompatible elements for the layered, and diverse fractionation for the upper gabbrosREE and anorthite thermometry shows a homogenous temperature structure for the lower, and a variable temperature record for the upper crustBoth suggests a hybrid model, with lower crust replenished in situ accretion and upper crust melt mixing and variable equilibrium dynamics [ABSTRACT FROM AUTHOR]

The Shiquanhe-Num Co ophiolitic mélange belt (SNOB) is one of two such belts along the Bangong-Nujiang suture zone (BNSZ) separating the Lhasa and Qiantang terranes of central Tibet. This study reports new geochronological, petrological and geochemical data for high-Mg dioritic dikes intruding the Shiquanhe ophiolitic peridotite. Based on LA-ICP-MS zircon U–Pb dating, dioritic dikes yield a Late Jurassic age of 162.8 ± 1.3 Ma. These are high-Mg diorites (53.0%–56.8% SiO2, 7.07%–8.82% MgO) with moderate TiO2 (0.95%–1.01%), high K2O+Na2O (6.02%–8.01%), and high contents of Cr (150–182 ppm), Ni (91–145 ppm) and Th (2.8–5.9 ppm). They are very similar to sanukites of the Setouchi volcanic rock belt (SW Japan). Shiquanhe diorites have high Sr/Y ratios (up to 41.3), distinct positive Pb anomalies, negative Nb anomalies and continental crust-like trace element patterns without negative Eu anomalies (average Eu/Eu* = 0.96). These geochemical characteristics indicate that Shiquanhe diorities were derived by partial melting of mantle peridotite as a result of interaction with hydrous melts of subducted sediments. Similar high-Mg magmas are common in the early evolution of a new subduction zone. Integrated with field observations and results from other studies of Late Jurassic high-Mg andesites and diorites along the SNOB, we argue that the Shiquanhe high-Mg diorites formed at an early stage of northward subduction of the Bangong-Nujiang Tethys ocean (BNO) beneath the Qiangtang terrane. Therefore, these results provide critical constraints on the Jurassic tectonic evolution of the Shiquanhe-Num Co ophiolitic mélange belt and are significant for understanding the evolution of the BNO. [ABSTRACT FROM AUTHOR]

Abstract Detection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long‐term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short‐lived 146Sm‐142Nd and 182Hf‐182W systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between 142Nd/144Nd and 182W/184W ratios among Réunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of 182Hf. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long‐lived 143Nd‐176Hf isotopic systematics mirror classical models invoking mixing of recycled trace‐element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources. If the core provides a detectible contribution to the tungsten element budget of the silicate Earth, this represents a critical component to planetary‐scale tungsten mass balance. A basic model is explored that reconciles the W abundance and isotopic composition of the bulk silicate Earth resulting from both late accretion and core‐mantle interaction. The veracity of core‐mantle interaction as proposed here would have many implications for long‐term thermochemical cycling.

Detection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long‐term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short‐lived 146Sm‐142Nd and 182Hf‐182W systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between 142Nd/144Nd and 182W/184W ratios among Réunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of 182Hf. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long‐lived 143Nd‐176Hf isotopic systematics mirror classical models invoking mixing of recycled trace‐element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources. If the core provides a detectible contribution to the tungsten element budget of the silicate Earth, this represents a critical component to planetary‐scale tungsten mass balance. A basic model is explored that reconciles the W abundance and isotopic composition of the bulk silicate Earth resulting from both late accretion and core‐mantle interaction. The veracity of core‐mantle interaction as proposed here would have many implications for long‐term thermochemical cycling. Plain Language Summary: Radioactive elements with relatively short half‐lives can be used as tools to study the geological processes that took place in the earliest part of Earth's history. Two of these short‐lived radioactive tools, the samarium‐neodymium and hafnium‐tungsten systems, are correlated in the Réunion hotspot source and it is suggested that this results from influences from Earth's metallic core and the preservation of four‐billion‐year old crust in the deep Earth. The idea that a geochemical fingerprint of Earth's core may make it to the surface has important consequences for broader understanding of Earth's thermal and chemical evolution and possibly changes previous assumptions about the role of late addition of meteorites in establishing Earth's modern tungsten isotopic composition. Key Points: There is a negative trend between the μ142Nd‐μ182W compositions of Réunion ocean island basalts (OIB) and primitive Deccan continental flood basaltsRéunion hotspot Nd‐W isotopic compositions may reflect early magma ocean crystallization and core‐mantle interactionStripping of W during subduction can explain the difference in μ142Nd‐μ182W compositions between Réunion OIB and Samoan or Hawaiian OIB [ABSTRACT FROM AUTHOR]

The fundamental origins of metamorphic rocks as sedimentary or igneous are integral to the proper interpretation of a terrane's tectonic and geodynamic evolution. In some cases, the protolith class cannot be determined from field relationships, texture, and/or compositional layering. In this study, we utilize machine learning to predict a metamorphic protolith from its major element chemistry so that accurate interpretation of the geology may proceed when the origin is uncertain or to improve confidence in field predictions. We survey the efficacy of several machine learning techniques to predict the protolith class (igneous or sedimentary) for whole rock geochemical analyses using 9 major oxides. The data are drawn from a global geochemical database with > 533 000 geochemical analyses. In addition to metamorphic samples, igneous and sedimentary analyses are used to supplement the dataset based on their similar chemical distributions to their metamorphic counterparts. We train the classifiers on most of the data, retaining ∼ 10% for post-training validation. We find that the RUSBoost algorithm performs best overall, achieving a true-positive rate of > 95% and > 85% for igneous- and sedimentary-derived samples, respectively. Even the traditionally-difficult-to-differentiate metasedimentary and metaigneous rocks of granitic–granodioritic composition were consistently identified with a > 75% success rate (92% for granite; 85% for granodiorite; 88% for wacke; 76% for arkose). The least correctly identified rock types were iron-rich shale (58%) and quartzolitic rocks (6%). These trained classifiers are able to classify metamorphic protoliths better than common discrimination methods, allowing for the appropriate interpretation of the chemical, physical, and tectonic contextual history of a rock. The preferred classifier is available as a MATLAB function that can be applied to a spreadsheet of geochemical analyses, returning a predicted class and estimated confidence score. We anticipate this classifier's use as a cheap tool to aid geoscientists in accurate protolith prediction and to increase the size of global geochemical datasets where protolith information is ambiguous or not retained. • Classical chemical-based methods for determining protoliths are non-unique. • PCA filtering and log-ratio transforms do not significantly improve classification. • Non-ensemble machine learning methods poorly classify sedimentary samples. • RUSBoosted identifies 95% igneous and 85% sedimentary global samples correctly. • Results vary by rock type, least accurate for shale and wacke, best for mafic igneous. [ABSTRACT FROM AUTHOR]

Highlights • 2045.8 ± 1.0 Ma and 2042.7 ± 3.0 Ma U–Pb ages for the lower Union Island Group (UIG) • UIG is the oldest Paleoproterozoic strata in the circum-Slave region. • Stratigraphy of the East Arm basin is revised. • UIG basalt geochemistry indicates a continental rift origin for the East Arm basin. • Implications for a connected Rae–Slave craton prior to ca. 2.05 Ga. Abstract The Slave craton underwent widespread extension during the Paleoproterozoic. The East Arm basin of Great Slave Lake formed during this period and preserves a protracted record of sedimentation and volcanism in the southeastern Slave craton. Within the East Arm basin package, the Union Island Group represents voluminous mafic volcanism with subordinate sedimentary strata. We report the first high precision U–Pb zircon and baddeleyite dates for the Union Island Group; 2045.8 ± 1.0 Ma for a volcaniclastic unit from the lower basalt formation and 2042.7 ± 3.0 Ma for a diabase intrusion within volcaniclastic horizons in the lower basalt formation. We fundamentally revise the lower basin stratigraphy and demonstrate that the Union Island Group is the oldest identified supracrustal package in the East Arm basin, ∼120 Myr older than previously thought. This revised stratigraphy is supported by detrital zircon provenance age distribution in two Union Island Group sedimentary samples, which is dominated by 2.76–2.56 Ga ages, reflecting prominent input from the local Archean Slave basement. The Union Island Group contains two geochemically distinct mafic volcanic packages. The lower basalt package is predominantly alkaline, characterized by an OIB-like chemical signature with high and variable incompatible element contents (108–438 ppm Zr, 13–62 ppm Nb). In contrast, the upper basalt package is tholeiitic, contains much lower and uniform incompatible element contents (83–101 ppm Zr, 2–4 ppm Nb), and has an E-MORB-like chemical signature. Both packages display overlapping, depleted time-integrated εNd(i) values (+1.1 to +3.2). Petrological modeling suggests the lower package originated as OIB-like melts from the asthenosphere that interacted with a depleted reservoir; the upper package was produced by larger degree of partial melting of a similar mantle source, in addition to possible lithospheric input. The petrogenesis established in this study is consistent with a passive continental rift origin for the Union Island Group magmatism and is further supported by geochemical similarities with Proterozoic and Phanerozoic rift successions. We therefore propose that the 2046 Ma Union Island Group represents an incipient rift basin sequence formed during Paleoproterozoic extension that marked the initial breakup of the southern margin of the Slave craton from a pre-Laurentia supercontinent. [ABSTRACT FROM AUTHOR]

One of the most impassioned topics in large igneous province (LIP) research is how prolonged the duration of these large-scale magmatic events are, as LIP magmatism has considerable impact on models of associated reconstructions, of climate variability or tectonic events. High-precision geochronology is pivotal to LIP basalt emplacement rate, and thus to unravel the role these enormous magmatic events have throughout Earths geological and environmental history. Four high-precision 40Ar/39Ar plagioclase plateau ages for the Tasmanian dolerites (Ferrar) indicate ∼1.6 ± 0.4 Ma of resolvable, continuous magmatic activity; 184.27 ± 0.24 to 182.69 ± 0.54 Ma (2 σ). The 40Ar/39Ar results provide evidence of distinctly older intrusions and a more prolonged duration than the observed 182.4-182.9 Ma age range and duration indicated by the main zircon record. Moreover, the precision of our 40Ar/39Ar results coupled with secondary electron microscopy analyses provide evidence of plagioclase crystal inheritance from slightly older magmatism entrained into younger magmatic pulses by exploiting pre-existing conduits. Numerical diffusion models, calculated for a theoretical age spectrum resulting from two slightly different plagioclase ages, provide an excellent match for measured data. Coupling geochemical data to the new age data indicates a silica and incompatible element evolution of the Ferrar magmatic system through time. The older generation of intrusions (ca. Zr: 92 ppm, SiO 2 : 53.67 wt.%) are seemingly less enriched in incompatible elements and silica than the youngest generation (ca. Zr: 147 ppm, SiO 2 : 56.5 wt.%). Here, we suggest that the magma chambers differentiated to more incompatible/silica-rich compositions saturating zircon only at evolved magmatic stages. This implies that plagioclase dates the full duration of magmatic Ferrar LIP activity of ca. 1.6 Myrs whilst zircon ages might be naturally biased and restricted to post-Zr saturation stage. The extended duration of Ferrar magmatism indicates that it is coeval with the Pliensbachian-Toarcian boundary. Therefore, we speculate that Ferrar (±Karoo) magmatism triggered the Pliensbachian-Toarcian extinction event and contributed to the Toarcian oceanic anoxic event, from which the environment did not begin to recover until only after the waning and cessation of Ferrar magmatic activity at ∼182 Ma, with zircon crystals recording the final flux of magma. • High-precision 40Ar/39Ar plagioclase geochronology indicate ∼1.6 Ma duration. • Potential plagioclase crystal inheritance revealed within 40Ar/39Ar age spectrum. • Extended Ferrar duration indicates coeval magmatism with the Pl/To extinction event. [ABSTRACT FROM AUTHOR]

The older eastern Galápagos are different in almost every way from the historically active western Galápagos volcanoes. Geochemical, geologic, and geophysical data support the hypothesis that the differences are not evolutionary, but rather the eastern volcanoes grew in a different tectonic environment than the younger volcanoes. The western Galápagos volcanoes have steep upper slopes and are topped by large calderas, whereas none of the older islands has a caldera, an observation that is supported by recent gravity measurements. Most of the western volcanoes erupt evolved basalts with an exceedingly small range of Mg#, Lan/Smn, and Smn/Ybn. This is attributed to homogenization in a crustal-scale magmatic mush column, which is maintained in a thermochemical steady state, owing to high magma supply directly over the Galápagos mantle plume. In contrast, the eastern volcanoes erupt relatively primitive magmas, with a large range in Mg#, Lan/Smn, and Smn/Ybn. These differences are attributed to isolated, ephemeral magmatic plumbing systems supplied by smaller magmatic fluxes throughout their histories. Consequently, each batch of magma follows an independent course of evolution, owing to the low volume of supersolidus material beneath these volcanoes. The magmatic flux to Galápagos volcanoes negatively correlates to the distance to the Galápagos Spreading Center (GSC). When the ridge was close to the plume, most of the plume-derived magma was directed to the ridge. Currently, the active volcanoes are much farther from the GSC, thus most of the plume-derived magma erupts on the Nazca Plate and can be focused beneath the large young shields. We define an intermediate sub-province comprising Rabida, Santiago, and Pinzon volcanoes, which were most active about 1 Ma. They have all erupted dacites, rhyolites, and trachytes, similar to the dying stage of the western volcanoes, indicating that there was a relatively large volume of mush beneath them. The paradigm established by the evolution of Hawaiian volcanoes as they are carried away from the hotspot does not apply to most archipelagos.

Tectonic evolution models for the Cretaceous Russia Sikhote‐Alin and eastern NE China continental margin and interior remain controversial. To understand the magmatic evolution over time and assess regional geodynamic processes, we sampled a diverse array of igneous rocks and employed zircon U‐Pb dating, hornblende and plagioclase 40Ar‐39Ar dating, whole‐rock major and trace element analysis, and 87Sr/86Sr and 143Nd/144Nd isotopic analysis. The west Sikhote‐Alin Pikeshan Formation volcanics and associated granites occurred at a peak of ~118 Ma and are hosted by the Triassic‐Jurassic accretionary complex. Their whole rock geochemistry shows that SiO2 increased in a linear trend, Eu/Eu* values decreased from 0.91 to 0.38, and εNd(t) values decreased from +0.6 to −2.9, indicating magma mixing of a juvenile mantle wedge source and continental crust, consistent with a continental arc. The arc thickened over time with a felsic dike hosted in the Pikeshan granites showing depletion in heavy rare earth elements. The termination of the arc front is documented by the ~107‐Ma intermediate lamprophyre and felsic dikes with εNd(t) values of +4.5 to +1.1, indicating an increased mantle contribution over time. Lithospheric extension of the Jiamusi Block to the west occurred at ~100 Ma, characterized by bimodal volcanism and composite dike emplacement, suggestive of asthenosphere upwelling. Based on the spatial and temporal distribution of these igneous rocks, the continental arc and intraplate magmatism migrated eastward contemporaneously. We favor a model invoking rollback of the subducting Paleo‐Pacific slab affecting a long‐lived continental arc. Key Points: Continental arc and back‐arc magmatism in eastern NE China migrated eastward over time in Cretaceous, triggered by Paleo‐Pacific rollbackIntermediate‐felsic calc‐alkaline volcanics and plutons and their alkaline lamprophyre dikes mark the arc front and its terminationTransition from Early Cretaceous intraplate extension to Late Cretaceous compression was triggered by subduction of an increasingly younger slab [ABSTRACT FROM AUTHOR]

The Salave Gold deposit is located at the northern end of the Oscos Gold Belt, which forms part of the Asturian-Leonese Zone of the Variscan Massif. The Gold Belt and the Salave deposit are genetically related to the emplacement of post-kinematic granitoids, which were largely controlled by Variscan fracture systems. In the Salave area, three main igneous complexes, Porcia, Salave and Represas, outcrop in an approximately E-W direction. Gold deposits are only found in the intense hydrothermal alterations affecting the western part of the Salave outcrop, just where the Mondoñedo thrust reaches the Cantabrian Sea. The geometry of the mineralized body is approximately tabular and is roughly parallel to the Mondoñedo thrust. A network of fractures controlled the gold ore. It consisted of sub-horizontal and sub-vertical fractures, the latter acting as feeders. Along the fracture network an intense albitization replaced the granodiorite and partially filled the open fractures. Farther from the fracture system the granodiorite rock is affected by sericitic-propylitic alteration halo. The most external alteration halo is the chloritic alteration. The transition from one alteration to another is gradual and irregular. The albitized areas and some areas slightly beyond them are then intensely sericitized (advanced sericitic alteration), sulfurized and carbonatized along the network of fractures, and becomes the richest areas in gold. The sulfurized areas are rich in arsenopyrite, pyrite and stibnite, always over 10% and reaching as much as 40% in volume. All the altered rocks mentioned have associated late carbonates as veins or micro-veins, disseminated or filling open spaces. The isocon diagrams (using Zr and Nb as immobile elements) and representative analytical data show that in chloritic alteration there is a small gain of mass, in propylitic alteration the gain is negligible, in albitic there is no gain or loss, in less intense advanced sericitic there is a small loss of mass and in the intense advanced sericitic the loss of mass is significant. As far as the metallic elements, sulfur, arsenic and antimony, are concerned, significant amounts of S, Au, As, Sb and Bi were consistently added to the original rock, mainly during advanced sericitic alterations. Ag is added during albitization to advanced sericitic alteration. Pb, Cu, W, Mo and U are more erratic but appear to have been added from the beginning, together with some Bi, S and As, and to have remained quite constant during all the alterations as if they had been introduced previously to gold mineralization. Four mineral associations have been described at Salave, Cu-Fe-Zn-S, Mo-Bi-S-(W), S-As-Fe-(Au) and S-Sb-Au-(Pb-Bi-Ag-Cu-Zn). The two gold related ones, are post granodiorite consolidation. Visible gold (or electrum) is scarce. Invisible gold at Salave was detected using microprobe analyses. It accounts for most of the Salave gold. The invisible gold in arsenopyrite is in solid solution, while in pyrite a small amount appears in solid solution, but most is probably in the form of microinclusions. Invisible gold is higher in arsenopyrite than in pyrite. The Salave gold deposit is a fracture-controlled deposit whose characteristics, such as stable isotope data, fluid inclusions and geochemistry, indicate a strong igneous influence, albeit partially influenced by meteoric waters and host-rocks. Thus, the deposit should be considered as a reduced intrusion-related gold deposit. [ABSTRACT FROM AUTHOR]

The "boring billion" (1.8 to 0.8 billion years ago) is an enigmatic period of Earth's evolution viewed by some as a time of tectonic quiescence reflecting secular changes in our planet's convection regime, either from sluggish global plate tectonics regime or from a wholly different regime. The confusion about this time-span stems from diverging interpretations of an ambiguous geological record, which points to a Mesoproterozoic world dominated by volcanic arcs, but also includes the formation of the massive Grenville Orogen. Here we present a compilation of multidisciplinary geological datasets, that are incompatible with the current tectonic paradigm, whereby the Grenville Orogen developed as an Andean-like arc at the margin of the Laurentia continent (now North America) and was reworked during a collision with the Amazonia continent (now South America). We argue, instead, that the geological record and proxies, with emphasis on those from the main exposure of the Grenville Orogen in the Grenville Province of eastern Canada, are consistent with the assembly of a distinct continent, which we coin Shawiniga (1.2-1.15 Ga), through the amalgamation of mature oceanic arcs/back-arcs offshore of Laurentia followed by a Shawiniga-Laurentia collision (∼1.03 Ga). This new model reveals that the transition from the supercontinent Columbia to Rodinia involved long-lived passive margins and subduction zones, as well as continental assembly through arc collisions, resolving the longstanding contradiction between tectonic proxies and the geological record and confirming the Mesoproterozoic was actually an exciting Era. • Geological data from the Grenville Province are incompatible with Andean tectonics. • The Allochthonous Belt consists of arcs amalgamated far away from Laurentia. • Laurentia traveled thousands of km southward to collide with these arcs at ca. 1030 Ma. • Tectonic proxies are better reconciled with this model than the current Andean paradigm. • The Mesoproterozoic was tectonically active. [ABSTRACT FROM AUTHOR]

The Scandinavian Caledonides are a late Silurian–early Devonian orogen that is an amalgam of thrust sheets derived from the Baltican margin, Iapetan arcs, and the Laurentian margin. Here we discuss intrusive rocks in two Iapetan arc complexes of Ordovician–early Silurian age that are now part of the Upper Allochthon in the central-western part of Norway. Samples from the older of the two arcs were collected near Lensvik and Rissa, on both sides of Trondheimsfjord. These include mafic to felsic, mostly tholeiitic metamorphosed intrusive rocks, with one previously reported age of 482 Ma. These plutons intrude volcanic rocks of the Støren Group, a suite of 495 to 482 Ma oceanic arc rocks that include ophiolites. We assign these plutonic rocks to the Støren Group, based on their similar ages and strong geochemical similarities with Støren Group plutonic and volcanic rocks in the inner Trondheimsfjord region. Part of the Støren Group is unconformably overlain by earliest Middle Ordovician sedimentary sequences that contain warm-water Laurentian faunal assemblages. Plutonic rocks of the younger arc have previously reported ages of 477 to 440 Ma, are calc-alkaline, span a composition range from ultramafic to granitic, and include a large fraction of adakites. Rocks in the Upper Allochthon with similar ages and lithologic and geochemical characteristics also occur farther north, across the Grong-Olden culmination, and in southwestern Norway, indicating an extensive arc complex. Throughout that extent, the plutonic rocks are spatially associated with older ophiolite-bearing oceanic arc assemblages, thought to be equivalent to the Støren Group. The Helgeland Nappe of the Uppermost Allochthon contains an extensive suite of igneous rocks that have almost identical characteristics to the younger Upper Allochthon arc we describe. They are also spatially associated with 500 to 480 MA ophiolites, and in addition cut metamorphosed sedimentary rocks that contain evidence for development near or on the Laurentian margin. This evidence suggests that the calc-alkaline rocks of the Upper and Uppermost Allochthons may belong to the same arc complex. The fact that similar-age calc-alkaline arc rocks occur in the Taconian arc system of western New England, the Notré Dame arc of Newfoundland, and in the Midland Valley Terrane and related units in Scotland and Ireland, suggest extensive calc-alkaline Iapetan arc complexes that developed before and during late Ordovician closing of an arm of Iapetus. [ABSTRACT FROM AUTHOR]

During the Early Carboniferous, significant intracontinental magmatism was developed in the retro-arc region of the SW margin of Gondwana between ca. 27° and 31°S. This magmatism consisted of metaluminous to weakly peraluminous A-type granites, strongly peraluminous A-type granites (A- to S- hybrid granitoids) and, to a lesser extent, (sub-)volcanic felsic (rhyolites and trachytes) and mafic (alkaline basalts and lamprophyres) rocks. The Vinquis Intrusive Complex (VIC) registers these two compositional varieties of A-type granites: the Vinquis Unit (VU) represents the strongly peraluminous granites and the Andaluca Unit (AU) the weakly peraluminous granites. The adjacent Zapata Intrusive Complex (ZIC) also contains metaluminous to weakly peraluminous A-type granites. The AU is a semi-elliptical plutonic body of approximately 60 km2 located in the southwestern part of the VIC, intruding the Vinquis Unit. The AU comprises three dominant rock types: i) quartz monzonite, ii) syenogranite and iii) alkali feldspar granite that can be classified as F-rich ferroan and weakly peraluminous A-type granitoids (calculated F content 1950–5700 ppm, [FeOt/(FeOt + MgO] = 0.86–0.95, ASI = 1.03–1.10, [Zr + Nb + Ce +Y] = 364–570 ppm, Ga = 20–23 ppm, SiO 2 = 68.5–75.1%). This unit was emplaced in early Carboniferous time with U-Pb zircon crystallization ages of 346 ± 3 Ma and 342 ± 3 Ma. The AU has 87Sr/86Sr t , εNd t and εHf t values ranging from 0.7092 to 0.7140, −2.6 to −3.6 and −16.5 to −3.3, respectively. These isotopic compositions suggest that the parental magma of AU involved variable mixtures of mantle-derived components (magmas/fluids) and peraluminous continental crust. The extremely felsic granites of the AU (SiO 2 > ∼72%) have a strongly fractionated composition (Rb > 336 and Sr < 70 ppm and Eu/Eu* < 0.22) that suggests extensive fractional crystallization. The high contents of fluorine and HFSE could be explained as derived from a parental mafic magma rich in F, HFSE, and alkalis plus contributions from F-bearing minerals assimilated from the country rock. Magmatic differentiation of these reduced mafic magmas could also explain the ferroan character of the AU granitoids. We envisage three stages in the origin of these rocks: (1) fluids and magmas from the asthenospheric mantle metasomatize and melt the subcontinental lithospheric mantle (SCLM), the alkali-rich mafic melts produced are emplaced at the base of the crust, transfer heat to the crust and melt it, generating strongly peraluminous A-type magmas; (2) continued introduction of alkali-rich mafic magmas into the deep crust produces extensive assimilation of the preheated crust in equilibrium with mafic cumulates (generated by reaction and/or fractionation); (3) the hot magmas so produced migrate into the middle-upper crust as weakly peraluminous ferroan A-type granitoids. • The Andaluca plutonic unit of the Vinquis Intrusive Complex crystallized between 346 ± 3 Ma and 342 ± 3 Ma. • Andaluca Unit shows a F-rich weakly peraluminous ferroan A 2 -type affinity. • The high contents of fluorine and HFSE in Andaluca Unit can be explained from an F-rich parental alkaline mafic magma. • The Andaluca Unit granitoids can be generated through AFC processes from the metasomatized SCLM-derived magmas. [ABSTRACT FROM AUTHOR]

The Seve Nappe Complex of the Scandinavian Caledonides is predominately metamorphosed rift-related igneous and sedimentary rocks formed during Rodinia breakup in the Neoproterozoic. The Kebne Dyke Complex of the Kebnekaise Massif, arctic Sweden, is one such unit within the Seve Nappe Complex and is mostly composed of dolerite dykes metamorphosed and thrust onto Baltica during the Caledonian orogeny. Structurally adjacent to the dyke complex are geochemically similar amphibolites that have a common origin as the metadolerite dykes. Collectively, these rocks have transitional mid-ocean ridge basalt geochemistry similar to correlative Seve Nappe Complex rocks to the north (Indre Troms dykes) and south (Sarek Dyke Swarm). U–Pb single-crystal chemical abrasion–thermal ionization mass spectrometry of zircon from a metagabbro and a metagranitoid, showing co-mingling magmatic textures with the metadolerite, produced 11 concordant analyses with206Pb/238U ages ranging from 608 to 596 Ma, which includes the magmatic age of the dyke complex. These results support the idea that the Kebne Dyke Complex, Sarek Dyke Swarm and Indre Troms dykes constitute the tholeiitic continent–ocean transition subdivision within the Seve Nappe Complex. However, the Kebne Dyke Complex differs from the Sarek Dyke Swarm and Indre Troms dykes as it is the least enriched among these, nearly lacks rift-related metasedimentary rocks, and may be younger than the Sarek Dyke Swarm by up to 14 Myr. [ABSTRACT FROM PUBLISHER]

Extensive magmatism occurred in southeast China during Late Jurassic time, forming large-scale granitic and volcanic rocks associated with non-ferrous, rare earth and rare, radioactive metal deposits. The Shuikoushan Pb–Zn–Au orefield is a typical example located in Hunan Province. This study reports LA-ICP-MS zircon U–Pb ages, whole-rock chemistry, and Sr–Nd–Pb isotopic compositions, andin situHf isotopic geochemistry of zircons from the Laomengshan rhyodacite in the Shuikoushan Pb–Zn–Au orefield. Zircon U–Pb dating yields a weighted average age of 156.7 ± 1.6 Ma for the intrusion of the rhyodacite. The rhyodacite samples are mainly shoshonitic series, having metaluminous to weakly peraluminous A/CNK values ranging from 0.96 to 1.09, with moderately high magnesium content (Mg# = 42.4–47.5). Samples display high (87Sr/86Sr)ivalues (0.71165–0.71176), lowεNd(T) values (−10.7 to −10.3), old Nd model ages (TDM = 1.73–1.86 Ga), and relatively homogeneous Pb isotopic compositions [(206Pb/204Pb)i = 18.365–18.412, (207Pb/204Pb)i = 15.663–15.680, and (208Pb/204Pb)i = 38.625–38.666]. The zircons exhibit enrichedεHf(T) values (−16.22 to −9.86) and old two-stage Hf model ages (TDM2 = 1.82–2.22 Ga). All the above data indicate that the Laomengshan rhyodacite originated from melting Palaeoproterozoic basement, perhaps contaminated by subordinate mantle melts. Intense extension and thinning of the continental lithosphere during Late Jurassic time resulted in melting of upwelling asthenosphere, and mafic mantle melts interacted with and melted Palaeoproterozoic lower crust, thus forming the Laomengshan rhyodacite. [ABSTRACT FROM PUBLISHER]

Abstract: The Parashi granitoid of northeasternmost Colombia intrudes the Upper Cretaceous to Lower Paleocene accretionary complex formed by the collision of the Caribbean arc and the continental margin of South America. This granitoid presently separated of the continental margin includes a major quartzdiorite body with andesite to dacite dikes and mafic enclaves. Zircon U–Pb LA-MC-ICP-MS and K–Ar geochronology on the quartzdiorite and the dikes suggest that crystallization extended from ca. 47 to 51 Ma. Major and trace elements are characterized by a medium-K, immature continental arc signature and high Al2O3, Na2O and Ba–Sr contents. Initial 87Sr/86Sr isotopic values range between 0.7050 and 0.7054, with 143Nd/144Nd = 0.51235–0.51253, εNd and εHf values from −0.81 to −4.40 and −4.4 and −5.2. Major and trace element ratios and isotopic modeling suggest that sedimentary and/or quartzofeldspathic crustal sources were mixed with a mafic melt input. The petrotectonic and geological constraints derived from this granitoid suggest that Parashi plutonism records an immature, oblique subduction-zone setting in which the presence of a high-temperature mantle realm and strong plate coupling associated to upper crust subduction caused the partial fusion of a previously tectonically underplated mafic crust and associated metasediments exposed in the continental margin. The limited temporal expression of this magmatism and the transition to a regional magmatic hiatus are related to a subsequent change to strongly and slow oblique tectonics in the Caribbean–South America plate interactions and the underflow of a relatively thick slab of Caribbean oceanic crust. [Copyright &y& Elsevier]

Abstract: The Linares deposit is an intrusion-hosted gold deposit located in the NW of the Iberian Variscan Belt. The igneous rocks hosting the ore are the Linares adamellite and the Arganzúa leucogranite. They are high-K, calc-alkaline and slightly peraluminous post-collisional igneous rocks, a mixture of I- and S-types, the Arganzúa leucogranite representing a late-stage fractionated phase. The intrusions have a reduced to intermediate oxidation state which spans the boundary between the ilmenite and magnetite series. The gold mineralization occurs disseminated or along a network of microfractures and veins of variable size, commonly in sheeted vein array. The alteration observed includes both potassic, with K-feldspar and secondary biotite developed mainly in the Arganzúa leucogranite, and sericitic and propylitic alterations. A sericite–chlorite–carbonate alteration, related to hairline fractures, is broadly distributed but rarely pervasive. Wolfram-bearing mineralization is followed by sulphide precipitation, mainly pyrrhotite, chalcopyrite, pyrite, arsenopyrite, molybdenite and galena, with minor löllingite and sphalerite. Gold is commonly associated with Bi–Te-bearing minerals with Bi/Te(S+Se)≥1, consistent with a pyrrhotite-buffered environment. This is reflected in the strong correlation observed between Au and Bi. The fluids involved in the sulphide precipitation are aqueous-carbonic (CO2 and ±CH4), with low salinity (0.5–6.3wt.% NaCl eq.) and variable amounts of other volatiles (N2 and H2S). The fluid inclusion study registered an adiabatic drop from a lithostatic pressure of 1.8–2.6kbar to a hydrostatic pressure of 0.3–0.9kbar, at temperatures of 300–400°C. The fault-valve phenomena could explain the pressure drop producing a volatile release from an early CO2-rich fluid. A later trapped fluid with a wide-ranging CO2/CH4 ratio suggests an isothermal mixture with a CH4(±N2)-rich fluid external to the granitoid (metamorphic). The effervescence and the fluid-mixing could be the possible mechanisms of ore precipitation in this system. The Linares deposit presents a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. The existence of intrusion-related gold systems would have potentially important implications for exploration in the NW of the Iberian Variscides. [Copyright &y& Elsevier]

This paper reports the first petrological and geochemical evidence for the Meso-Neoproterozoic metamorphic and metaintrusive rocks of the Shaw Mountain massif (Prince Charles Mountains, East Antarctica). It was shown that the orthogneisses (plagiogneisses) and metabasites of the massif were formed as constituents of a volcanoplutonic complex, which included a variety of igneous rocks of normal and subalkaline groups, from ultrabasic to silicic and was assigned to the volcanic tholeiite basalt-andesite-rhyolite, plutonic peridotite-gabbro, and late (?) calc-alkaline gabbro-diorite-plagiogranite associations. The distribution of major and compatible trace elements indicates the fractionation of the primary melts that produced the volcanoplutonic association of the Shaw massif. With respect to the distribution of REE and trace elements and some trace element ratios, the metabasic rocks of the Shaw massif correspond mainly to enriched and normal basalts of mid-ocean ridges, continental rifts, and ocean islands, which suggests a contribution from a plume mantle source. It was found that the region of the Shaw massif is a high-grade metamorphosed margin of the Fisher volcanoplutonic complex, a Mesoproterozoic structure of single geodynamic nature. This is supported by the spatial proximity of the Shaw and Fisher regions, the similar behaviors of most major elements and distribution patterns of trace elements and REE in comparable magmatic associations, and the similar ages of some plutonic associations. Based on the petrological and geochemical data, an alternative geotectonic model was proposed for this region. According to this model, the Fisher complex was formed in a setting of continental rifting coupled with the processes of mantle diapirism, which was subsequently changed by the compression stage. During rifting, the structure could experience significant opening accompanied by ultrabasic-basic tholeiitic magmatism with a significant contribution of mantle material. A subsequent inversion resulted in that the rift structure underwent considerable horizontal compression accompanied by calc-alkaline magmatism and the formation of narrow intracratonic fold zones. The cyclic character of rifting processes and superposition of young rift systems on older ones was also established in the Phanerozoic geotectonic history of the region of the Prince Charles Mountains. [ABSTRACT FROM AUTHOR]

Abstract: A magmatic arc was active along the Antarctic Peninsula''s western margin during the Mesozoic and most of the Cenozoic. We report new constraints on the geodynamic history of King George Island (South Shetland Islands, northern Antarctic Peninsula), based on a multidisciplinary approach combining structural, petrological, geochemical, geochronological and paleomagnetic data from magmatic dike swarms and their host rocks. The deformation of the dikes is expressed as jointing and faulting. Joint systems present in the host rocks occur also within the dikes, indicating that the stress field outlasted dike intrusion. No folding axis could be determined for the investigated areas, but the orientation of the folding axis evident on nearby Livingston Island shows that the corresponding tectonic stress field there was almost identical to the one on King George Island. This suggests a uniform tectonic pattern throughout the area during the time of dike intrusion. Dikes from King George Island yielded Thanetian to Lutetian 40Ar/39Ar ages. A pronounced peak of intrusive activity is evident at 47–45Ma and coincides well with a dramatic change in subduction zone parameters that caused a change from a compressional to an extensional tectonic regime, as also reflected by the uplift of the Smith Island metamorphic rocks at the same time. The dikes comprise a subalkaline series of predominantly calc-alkaline affinity. Tholeiites occur also, apparently related to the initial stages of intrusive activity in the respective areas and thus a time of strongly increasing velocity of the subducting plate. Crustal contamination is not evident from the dikes on King George Island and the main intrusive phase during the Lutetian is characterized by increasing contribution of subducted sediments to arc magma genesis. Enrichment of the depleted mantle source by fluids derived from the subducted slab as reflected by elevated Ba/Th ratios is not uniform in the studied areas, indicating heterogeneities in the mantle wedge that are also confirmed by other HFSE systematics. Paleomagnetic studies undertaken in order to check for Cenozoic tectonic rotations indicate high between-site dispersion probably associated with strong secular variation during Paleocene/Eocene times. However, an interpretation in combination with previously published data suggests that no post-formational vertical-axis rotations and latitudinal displacement occurred. [Copyright &y& Elsevier]

Abstract: Volcanism along Mid-Ocean Ridges is known to exhibit significant isotopic and elemental variations, traditionally regarded as reflecting global scale variations in mantle depletion history and refertilization processes. The ∼400km long Mid-Atlantic Ridge (MAR) between the Ascension and Bode Verde Fracture Zones (7°30′S to 11°30′S) has been sampled at high spatial resolution (c. 10km along-axis scale) and large elemental and isotopic variations have been identified covering almost the entire compositional spectrum of MAR basalts. In this paper we employ a multi-isotope (Sr–Nd–Hf–Pb) and trace element approach in order to explore the geodynamic implications of along-ridge compositional variations and their relationship to off-axis volcanism on nearby Ascension Island, commonly regarded as plume-related. The studied portion of the MAR consists of four major segments. Isotopic data from the deeply incised northern and southern segments A1 and A4 define a trend involving a high-εHf depleted mantle endmember. This isotopic signature is inferred to result from an ancient melting event in the garnet stability field, causing high time-integrated Lu/Hf and elevated 176Hf/177Hf. In contrast, a low-εHf depleted mantle endmember is indicated by the compositional trend for samples from the topographically elevated central portion (segments A2 and A3). Both trends converge at a common enriched endmember with least diluted compositions represented by samples from the subaerial eruptive stage of nearby Ascension Island. Because linear co-variations between isotope compositions and the respective parent–daughter ratios cannot be explained as representing “mantle isochrons” we infer that our data reflect arrays related to physical mixing of depleted and enriched mantle domains. This implies that distinctive mantle domains at kilometer scale may survive convection processes over time spans in the order of 1–2Ga. This finding corroborates the results of recent Os-isotope studies of abyssal peridotite which returned similar conclusions. The systematics of along-axis isotopic variations show that mantle upwelling is highly variable. A systematic decrease of the enriched isotopic signature from the central segment A3 toward the northern termination of the segment A2 suggests a northward flow of enriched mantle material. In marked contrast, the compositional variations along the marginal segments A1 and A4 are random indicating heterogeneous mixing of mantle domains at a spatial resolution of <50km to 10km (along-axis scale). With regard to Ascension Island we show that the submarine volcanic stage, sampled by a 3216m long drill hole, was fed by a distinctive enriched mantle source currently inaccessible to partial melting. We interpret this observation within the framework of recent local-scale geophysical investigations and infer an on-axis origin of the bulk of the Ascension volcanic edifice at around 5 to 6Ma, synchronous with the surrounding oceanic crust. Off-axis partial melting of the current common enriched mantle endmember accounts for rejuvenation of volcanism on Ascension generating the volumetrically subordinate subaerial portion of the island. [Copyright &y& Elsevier]

Abstract: The southernmost Izu–Bonin arc and northernmost Mariana arc are characterized by K-rich and shoshonitic lavas, referred to as the alkalic volcano province (AVP). These compositions are unusual for intra-oceanic arcs and the interpretation of the AVP is controversial. Rifting to form the Mariana Trough back-arc basin occurs just south of the AVP although back-arc seafloor spreading has not begun. Here we report the results of dredge sampling of the West Mariana Ridge (WMR) in the region of rift propagation; this recovered exclusively medium K to shoshonitic basalts that show clear arc-like geochemical signatures. Ar–Ar ages of WMR shoshonitics systematically young northward. Age of c. 6Ma was obtained at 21.5°N, c. 3Ma at 23–23.5°N, and zero-age shoshonites occur on Io-to Island (formerly Iwo Jima) at 24.8°N. Shoshonitic magmatism migrated northward at 4.3cm/year, in advance of northward-propagating Mariana Trough rifting. This implies that AVP shoshonitic magmatism manifests processes and sources that are uniquely associated with earliest back-arc basin rifting. High-precision Pb isotopic analyses reveal that WMR lavas form a single trend between 2 components, one with lower 206Pb/204Pb and high Δ7/4 (arc-like), and another with high 206Pb/204Pb as well as low Δ 7/4 and 8/4 (HIMU-like). These components could correspond respectively to subducted pelagic sediment and subducted seamounts and volcaniclastics with HIMU isotopic signature. These slab-derived components alone, however, cannot fully explain chemical characteristics of WMR shoshonitic lavas. These lavas require a component with high Δ7/4 and high Ce/Pb, which is not likely to be either pelagic sediment or seamount volcanics. This component is only expressed when rifting begins, suggesting that it resides in enriched lithosphere or uppermost asthenosphere, which is easily melted due to decompression caused by rifting, when the lithosphere is first ruptured. This component might be linked to slow Vs anomalies in the mantle wedge beneath the AVP. [Copyright &y& Elsevier]

Abstract: Amongst island arcs, Izu–Bonin is remarkable as it has widespread, voluminous and long-lived volcanism behind the volcanic front. In the central part of the arc this volcanism is represented by a series of seamount chains which extend nearly 300km into the back-arc from the volcanic front. These back-arc seamount chains were active between 17 and 3Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active rifting just behind the Quaternary volcanic front. In this paper we present new age, chemical and isotopic data from the hitherto unexplored seamounts which formed furthest from the active volcanic front. Some of the samples come from volcanoes at the western limit of the back-arc seamount chains. Others are collected from seamounts of various sizes which lie on the Shikoku Basin crust (East Shikoku Basin seamounts). The westernmost magmatism we have sampled is manifested as a series of volcanic edifices that trace the extinct spreading centre of the Shikoku Basin known as the Kinan Seamount Chain (KSC). Chemically, enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that the back-arc seamount chains and the East Shikoku Basin seamounts have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism and distinct from the MORB-like lavas of the Shikoku back-arc basin. 40Ar/39Ar ages range from 15.7 to 9.6Ma for the East Shikoku Basin seamounts, indicating this arc magmatism started immediately after the Shikoku Basin stopped spreading. Although the KSC volcanoes are found to be contemporaneous with the seamount chains and East Shikoku Basin seamounts, their chemical characteristics are very different. Unlike the calc-alkaline seamount chains, the KSC lavas range from medium-K to shoshonitic alkaline basalt. Their trace element characteristics indicate the absence of a subduction influence and their radiogenic isotope systematics reflect a mantle source combining a Philippine Sea MORB composition and an enriched mantle component (EM-1). One of the most remarkable features of the KSC is that their geochemistry has a distinct temporal variation. Element ratios such as Nb/Zr and concentrations of incompatible elements such as K2O increase with decreasing age and reach a maximum at ca. 7Ma when the KSC ceased activity. Based on the chemical and temporal information from all the data across the back-arc region, we have identified two contrasting yet contemporaneous magmatic provinces. These share a tectonic platform, but have separate magmatic roots; one stemming from subduction flux and the other from post-spreading asthenospheric melting. [Copyright &y& Elsevier]

Abstract: Peridotite xenoliths from the late-Cretaceous Alto Paranaíba and Goiás mafic–alkalic magmatic provinces of central and southeast Brazil reveal the existence of compositionally and temporally distinct lithospheric mantle beneath these areas. Garnet and spinel–lherzolites and spinel–harzburgites from the Alto Paranaíba province are generally depleted in Ca, Al and Re, which indicates that they are residues of melt extraction. Old Re-depletion model ages (average 2.4 Ga) for these peridotites show that this area is underlain by the early-Proterozoic to late-Archean melt-depleted lithospheric mantle of the São Francisco Craton. In contrast, spinel–lherzolites from the Goiás alkalic province, located 500–600 km northwest of the Alto Paranaiba province, have major- and trace-element compositions similar to modern fertile mantle. Most Goiás peridotites have fertile mantle 187Os/188Os (0.1261–0.1292), but two samples with lower 187Os/188Os define Re-depletion model ages of ∼1.2 Ga. The compositional distinction between the lithospheric mantle samples is mirrored in the kamafugitic rocks of these two provinces, which suggests that the composition of these magmas was strongly influenced, or determined entirely, by the lithospheric mantle. Most of the kamafugitic rocks have 187Os/188Os higher than observed for peridotite, which suggests that the source of these magmas is a mixture of lithospheric peridotite of varying age, veined and/or interlayered with various olivine-poor components. The relatively limited range in Sr, Nd, and Hf isotopic composition of the mafic–alkalic magmas indicates that the olivine-poor component was added regionally, overprinting the incompatible-element characteristics of the lithospheric mantle beneath these provinces. The age of metasomatism in the lithospheric mantle of this area is poorly constrained, but the isotopic characteristics of the mafic–alkalic magmatism suggest that this event may have occurred during the mid- to late-Proterozoic assembly of the Brasília mobile belt. [Copyright &y& Elsevier]

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]

The surface of the Cenozoic Kerguelen Archipelago, constructed on the Kerguelen Plateau in the southern Indian Ocean, is dominantly flood basalt. With the objective of understanding the Cenozoic history of the Kerguelen mantle plume, the age and geochemical characteristics of this flood basalt province are being determined by studying stratigraphic sections of basalt flows at several locations. Sections from the NW, north–central, east and SE parts of the archipelago have been studied. Here we report results for a 597 m succession of lavas from Mont Tourmente from the Plateau Central, a region of the archipelago that has not been studied in detail. Mont Tourmente lavas range from ∼26 Ma dominantly transitional basalts in the lower 80% of the section to ∼25·3 Ma dominantly alkalic basalts in the upper part of the section. The timing of this change from transitional to alkalic volcanism within the Mont Tourmente section is consistent with that defined by the older ∼28–29 Ma transitional basalts in the north and the ∼25 Ma alkalic lavas erupted in the east. This change in basalt composition may be related to migration of the archipelago away from the plume or to increasing lithosphere thickness over the ∼5 Myr of flood basalt volcanism. The alkalic and transitional Tourmente lavas are nearly homogeneous in isotopic ratios of Sr, Nd and Pb. They have lower 87Sr/86Sr and higher 143Nd/144Nd compared with lavas from the eastern sections, which have been proposed to be characteristic of the Kerguelen plume. Consequently, the Mont Tourmente isotopic data may reflect heterogeneity within the plume or a constant proportion of a depleted component mixed with the high 87Sr/86Sr and low 143Nd/144Nd plume. In contrast to many of the Cretaceous Kerguelen Plateau lavas, there is no evidence in trace element abundances or Sr and Nd isotopic ratios that the Cenozoic Kerguelen Archipelago lavas were influenced by continental lithosphere. [ABSTRACT FROM PUBLISHER]

Submarine lavas erupted onto the Hawaiian arch 200–400 km north of Oahu show that the areal extent of Hawaiian volcanism is much larger than previously recognized. The North Arch volcanic field comprises 25 000 km2 of ∼0·5–1·15 Ma, volatile-rich, olivine-phyric alkalic lavas (alkalic basalt to nephelinite). These lavas are similar in composition to rejuvenated-stage lavas such as the Koloa Volcanics (Kauai) and Honolulu Volcanics (Oahu). North Arch lavas that encompass the compositional extremes have similar Sr, Nd and Pb isotopic ratios. Olivine accumulation and fractionation was the major post-melting process that affected the compositions of North Arch lavas. After correction for these processes, the inferred primary magma compositions show that they were derived by variable, factor of four, and relatively low extents of melting of garnet peridotite. Garnet and olivine were important residual phases during partial melting; in contrast to the Honolulu Volcanics, there is little evidence for residual hydrous phases, sulfides or Fe–Ti oxides. The mantle source for the North Arch lavas had Sr, Nd and Pb isotopic ratios intermediate between those of Pacific Ocean lithosphere and the inferred range for Hawaiian plume components. These data are consistent with a mixed lithosphere–plume source. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis is that during the middle Cretaceous, South Pacific lithosphere was contaminated by plumes that formed large oceanic plateaux (e.g. Ontong Java). This mixed source was subsequently partially melted as it passed near the Hawaiian plume. [ABSTRACT FROM PUBLISHER]

Magmatic zircon tends to exhibit characteristic trends in trace element contents in response to magma cooling and fractionation, such that zircon may provide a window into melt evolution not accessible by whole rock geochemistry. The Peninsular Ranges Batholith of southern California and Baja California consists of older rocks of oceanic arc affinity in the west. Younger rocks to the east been interpreted via whole rock chemistry to reflect (Transition Zone) deeper origins and (Eastern Zone) deeper origins with significant crustal assimilation compared to the Western Zone. The youngest magmas in the Eastern Zone form large, typically zoned plutons and are high in Sr/Y. We present zircon trace element and oxygen isotope measurements in selected Western and Transition Zone samples and in many units from the high-Sr/Y La Posta and San Jacinto plutons. Most Western and Transition Zone populations exhibit typical shallow fractionation-related trace element trends, while one Eastern Transition Zone sample and most samples from the high-Sr/Y Eastern Zone lack Eu/Eu* evidence for plagioclase fractionation. Oxygen isotopes, zircon P contents, and Ce-based redox estimates suggest that the most evolved unit (2-mica granodiorite) of the La Posta pluton is significantly contaminated by sediments, with other high-Sr/Y units displaying zircon oxygen isotope and redox estimates falling between the 2-mica granodiorite and the relatively uncontaminated Transition Zone in these parameters. However, most of these zircons lack unambiguous markers for supracrustal contamination that could be interpreted even in a detrital context. The contrasting behavior of zircon U/Yb and Eu/Eu* during shallow fractionation and their similar response to melting depth variations can be used to distinguish the effects of shallow fractional crystallization, depth of melting, and introduction of exotic materials by either crustal assimilation or magma mixing. On the basis of U/Yb vs Eu/Eu* relationships, we interpret the behavior of Western Zone and most Transition Zone zircons as reflecting mainly shallow fractionation. One Eastern Transition Zone tonalite appears to result from mixing of magma from different depths. We interpret the high-Sr/Y Eastern Zone magmas as resulting from mixing of magmas that have assimilated varying amounts of metasediment. Moderate levels of sediment assimilation may complicate magma depth interpretations which are based on zircon (Th, U)/Yb plots or detrital zircon Eu/Eu* – without yielding clear zircon geochemical evidence for supracrustal assimilants. Magma mixing is less problematic for regional-scale detrital zircon Eu/Eu* studies and may be easier to detect based on the relationships among (Th, U)/Yb and Eu/Eu*. [ABSTRACT FROM AUTHOR]

A 200-m section of Koolau basalt was sampled in the 1.6-km Trans-Koolau (T–K) tunnel. The section includes 126 aa and pahoehoe lava flows, five dikes and ten thin ash units. This volcanic section and the physical characteristics of the lava flows indicate derivation from the nearby northwest rift zone of the Koolau shield. The top of the section is inferred to be 500–600 m below the pre-erosional surface of the Koolau shield. Therefore, compared with previously studied Koolau lavas, this section provides a deeper, presumably older, sampling of the shield. Shield lavas from Koolau Volcano define a geochemical end-member for Hawaiian shields. Most of the tunnel lavas have the distinctive major and trace element abundance features (e.g. relatively high SiO2 content and Zr/Nb abundance ratio) that characterize Koolau lavas. In addition, relative to the recent shield lavas erupted at Kilauea and Mauna Loa volcanoes, most Koolau lavas have lower abundances of Sc, Y and Yb at a given MgO content; this result is consistent with a more important role for residual garnet during the partial melting processes that created Koolau shield lavas. Koolau lavas with the strongest residual garnet signature have relatively high 87Sr/86Sr, 187Os/188Os, 18O/16O, and low 143Nd/144Nd. These isotopic characteristics have been previously interpreted to reflect a source component of recycled oceanic crust that was recrystallized to garnet pyroxenite. This component also has high La/Nb and relatively low 206Pb/204Pb, geochemical characteristics which are attributed to ancient pelagic sediment in the recycled crust. Although most Koolau lavas define a geochemical endmember for Hawaiian shield lavas, there is considerable intrashield geochemical variability that is inferred to reflect source characteristics. The oldest T–K tunnel lava flow is an example. It has the lowest 87Sr/86Sr, Zr/Nb and La/Nb, and the highest 143Nd/144Nd ratio found in Koolau lavas. In most respects it is similar to lavas from Kilauea Volcano. Therefore, the geochemical characteristics of the Koolau shield, which define an end member for Hawaiian shields, reflect an important role for recycled oceanic crust, but the proportion of this crust in the source varied during growth of the Koolau shield. [ABSTRACT FROM AUTHOR]