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Comprehensive U-Pb detrital zircon geochronology of Ediacaran sandstones from Oman indicates that they originated from Neoproterozoic basement with a peak magmatic age of ca. 850-780 Ma (Tonian), with lesser Paleoproterozoic and Ediacaran sources and renewed magmatic input starting at ca. 550 Ma. Comparison with detrital zircons from the Arabian-Nubian and NW Indian Shields supports an eastern Gondwana affinity for the Ediacaran succession of Oman. Tonian basement sediment sources are present in Oman. Sources for older Paleoproterozoic zircons (ca. 2500 and 1800 Ma) are not found in Oman but are known from the cratonic Indian Shield. The signal of the main magmatic events of the juvenile Arabian-Nubian Shield, peaking at ca. 640-620 Ma, is rare or absent in the Ediacaran rock succession of Oman. However, deformed Ediacaran clastic units with an Arabian-Nubian Shield affinity occur in western Oman. Influx of latest Ediacaran-early Cambrian zircons (550-525 Ma) is interpreted as due to final Cambrian Angudan/Malagasy orogeny-related magmatism. These results, together with new ages for volcano-sedimentary terranes in the subsurface of SW Oman and seismic observations, support the interpretation that the Ediacaran succession of Oman was deposited along the western, passive margin of the Greater Indian Shield, on the eastern (east and west in this paper refer to present-day coordinates) side of the Mozambique Ocean. Oman then collided with the Arabian-Nubian Shield in the early Cambrian, when the Angudan/Malagasy orogeny led to final consolidation of Gondwana. Our data support the inference that the youngest and final suture between the Indian and Arabian Shields lies in the subsurface of Oman along the early Cambrian western deformation front. [ABSTRACT FROM AUTHOR]

A big mantle wedge (BMW) is defined as the broad region of upper mantle above a stagnant slab in the mantle transition zone (MTZ). It is a common and significant structure within Earth's interior at modern convergent plate margins as revealed by seismic data yet rarely identified in fossil convergent systems. We propose the existence of a BMW beneath the Western Mongolia Collage during the early to middle Paleozoic based on a comprehensive chronology of geological events that characterized the accretionary orogen in this region. The trench-arc system initially developed above a NE-dipping subduction zone, with subduction-related arc magmatism clustered at ca. 530-490 Ma and accumulations of flysch-like sequences from the Cambrian to early Silurian constituting the accretionary wedge of the Altai Zone. The westward migration of the arc was likely driven by slab rollback and trench retreat, leading to gradual formation of a BMW as the slab stagnated at the MTZ. The BMW influenced the tectonic evolution of the entire Western Mongolia Collage, inducing Ordovician--Silurian intraplate magmatism in regions inboard of the migrating magmatic arc and the potential opening of the Mongol-Okhotsk Ocean. Westward movement of the trench-arc continued until the Devonian, resulting in back-arc basin formation in the Chinese Altai and intraplate magmatism in the Hovd and Lake Zones of the Western Mongolia Collage, forming a trench--arc--back-arc and intraplate tectonic system. Mantle flow within the BMW is inferred to have impacted magmatism, basin migration, and the stress and thermal state of the overriding plate. [ABSTRACT FROM AUTHOR]

The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25-0.24 +0.67 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ∼2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ∼2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift. [ABSTRACT FROM AUTHOR]

Mafic microgranular enclaves (MMEs) are a ubiquitous feature of post-collisional magmatism, receiving much attention among earth scientists over the last decades. While recent advances point to the large-scale involvement of the lithospheric mantle in granite petrogenesis, MMEs have received less attention in such discussion. Because MMEs are commonly acknowledged to represent the mafic end member with a mantle affinity that is related to early-stage batholith petrogenesis, they constitute a good proxy for the mantle role in the process. Using MME data from Los Pedroches batholith in southwestern Iberia, we conduct a geochemical comparative study between MMEs and the mafic-intermediate (sanukitoid) suite of post-collisional batholiths. An accurate overlap between the two groups is revealed, implying a potential genetic link between MMEs and the sanukitoid suite. Together with evidence from experimental cotectic liquids, the link between the high-Mg signature of postcollisional magmas and the predominance of amphibole in the studied MME samples is used to account for the composition of post-collisional magmatism. Implications for post-collisional batholith petrogenesis is then discussed in a qualitative manner, suggesting a heterogeneous yet common two-stage origin for all post-collisional magmatism in which the relationship between MMEs, sanukitoid, and the host felsic magmas is a differentiation process, thus representing a major input of juvenile magma into the crust. [ABSTRACT FROM AUTHOR]

We report the discovery of one of the largest ancient (>3.0 Ga) crustal terranes on Earth. Granitoids with crystallization ages >3.0 Ga and/or Sm-Nd depleted mantle model ages ≥3.2 Ga define a -1000 x 100 km belt on the western margin of the Rae craton, Canada, referred to herein as the Perry River terrane (PRT). Zircon U-Pb-Hf-O isotope and whole-rock geochemical data from granitoids show that the PRT is a predominantly juvenile 3.3--3.2 Ga terrane that was partially reworked by more evolved ca. 3.1 Ga magmatism. These findings call for a reassessment of the timing and extent of ancient continental growth on Earth. A global compilation of zircon Hf isotope data from 3.6 to 3.0 Ga igneous rocks reveals clusters of relatively juvenile (initial εHf -2 to +3) rocks at ca. 3.31 and ca. 3.23 Ga, which include samples from the PRT and 13 other terranes worldwide. Other global zircon data sets also document age peaks between 3.3 and 3.2 Ga, and a cluster of broadly chondritic initial εHf values around 3.2 Ga. The 3.3--3.2 Ga period may therefore have been a time of enhanced net continental growth on Earth, and the PRT is one of the largest terranes preserved from that time. Furthermore, zircon Hf isotope data from 3.3--3.1 Ga PRT granitoids and 3.5--3.0 Ga igneous rocks worldwide yield little evidence for parent magmas that interacted with or derived from pre--3.6 Ga continental material. Contrary to some continental-growth models, this latter observation suggests that the volume of continental crust established by 3.6 Ga was relatively small. [ABSTRACT FROM AUTHOR]

New high-precision trace-element analyses of magmatic olivines point to a pyroxenitedominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (<∼5%) melts of pyroxenite and peridotite. [ABSTRACT FROM AUTHOR]

The White Mountain magma series is the largest Mesozoic felsic igneous province on the eastern North American margin. Previous geochronology suggests that magmatism occurred over 50 m.y., with ages for the oldest units apparently coeval with the ca. 201 Ma Central Atlantic Magmatic Province, the flood basalt province associated with the end-Triassic mass extinction and the opening of the Atlantic Ocean. We use zircon U-Pb geochronology to show that emplacement of White Mountain magma series plutons was already underway at 207.5 Ma. The largest volcanic-plutonic complex, the White Mountain batholith, was emplaced episodically from ca. 198.5 Ma to ca. 180 Ma and is -25 m.y. older than published ages suggest, and all samples we dated from the Moat Volcanics are between ca. 185 Ma and 180 Ma. The Moat Volcanics and the White Mountain batholith are broadly comagmatic, which constrains the age of a key Jurassic paleomagnetic pole. Our data indicate that a regional mantle thermal anomaly in eastern North America developed at least 5 m.y. prior to the main stage of Central Atlantic Magmatic Province flood basalt volcanism and suggest a geodynamic link between the White Mountain magma series and the Central Atlantic Magmatic Province. [ABSTRACT FROM AUTHOR]

Orogenic belts have been thought to form through plate convergence, involving subduction of oceanic lithosphere at continental margins (accretionary orogens), which may ultimately lead to ocean closure and continent-continent collision (collisional orogens). Intraplate orogens away from plate margins have been known, but the mechanisms controlling their evolution are poorly understood. The South China craton, including the Yangtze and Cathaysia blocks, underwent a Paleozoic orogenesis that formed a >500-km-wide orogenic belt with widespread granitoids that are unconformably overlain by Devonian cover sequences. The pre-Devonian basement rocks were subjected to strong deformation and greenschist-to amphibolite-facies metamorphism at 460-400 Ma. Paleozoic magmatism was characterized by voluminous crustally derived Silurian granitoids associated with incorporation of ancient crustal materials at 450-440 Ma and addition of juvenile mantle-derived melts at 420-410 Ma. Based on the absence of arc-like magmatism and the existence of ophiolites in the West Cathaysia terrane, geochemical evidence that oceanic crust existed beneath the East Cathaysia terrane, and geophysical evidence of contrasting lithospheres on both sides and two discrete slabs beneath their fault boundary, we propose that this Paleozoic orogenic belt was formed by collision between the two terranes that was driven by far-field forces during the assembly of Gondwana, and the East Cathaysia terrane represents oceanic lithosphere that was overthrusted by the continental crustal materials of the West Cathaysia terrane. Numerical modeling shows that this type of collision can explain the dynamics of the Paleozoic orogenesis in the South China craton and may be a mechanism for some orogens in which subduction-related igneous and metamorphic rocks are lacking. [ABSTRACT FROM AUTHOR]

Difficulties in dating Mississippi Valley-type (MVT) mineral deposits and the often closely associated dolomitization have led to controversy regarding their origin. We report the first radiometric ages for the Gorno mining district in northern Italy, an example of the Alpine subclass of MVT deposits. U-Pb ages of hydrothermal carbonates pre- and postdating the ore-forming event show that base-metal mineralization occurred shortly after the deposition of the Carnian host rocks. This implies that the Gorno ore deposits formed at shallow burial depth prior to the Early Jurassic western Tethys rifting phase. Contemporaneous Triassic magmatism and extensional tectonics likely contributed to the high geothermal heat fluxes required to drive the mineralizing system. Our study reinforces the need for reliable geochronological data for metallogenic models and warns against a general application of classic North American MVT models to similar deposits worldwide. [ABSTRACT FROM AUTHOR]

Oceanic core complexes (OCCs) and detachment faults play a key role in crustal accretion at slow and ultraslow spreading centers. We investigated the effect of different magma supply at three OCCs of the Southwest Indian Ridge using high-resolution deep-sea bathy- metric and magnetic data. The average equivalent thickness of extrusive basalt deduced from the magnetic anomalies, a proxy for magma supply, decreases from west to east, from the Yuhuang (49.25°E) to Longqi (49.65°E) to Junhui (51.75°E) OCCs. Conversely, serpentinite outcrops become more abundant, the domal OCC morphology flattens as the footwall rotation (measured by the magnetization vector inclination) increases, and hydrothermal evidence becomes sparse. Combined with results from the amagmatic easternmost Southwest Indian Ridge, our study shows that the magma supply controls the character and evolution of the OCCs and detachment faults on the Southwest Indian Ridge. [ABSTRACT FROM AUTHOR]

The timing of the emergence of subaerial landmasses is equivocally constrained as post-Archean and continues to be a much-debated issue. In this study, we document exceptionally 18O-depleted (δ18O < 4.7‰) Mesoarchean to early Neoarchean magmatism in India that shows a similarity with the coeval low-δ18O magmas reported from Australia, South America, and northern China. Such global-scale low-δ18O magmatism would require high-temperature meteoric water-rock interaction in the uppermost crust synchronous with magma generation, necessitating the emergence of a substantial volume of the continental crust. The timing of this low-δ18O magmatism coincides with the development of extensive, subaerial large igneous provinces, a downward shift in δ18O and Δ17O values in pelitic rocks, the rise of normalized 87Sr/86Sr in seawater, and an intermittent upsurge in the proportion of atmospheric oxygen. We propose that the emergence of substantial volumes of continental crust initiated at ca. 3.2 Ga and peaked at 2.8-2.6 Ga, facilitating the generation of globally distributed low-δ18O magmas, and this event can be linked to the first appearance of atmospheric oxygen. [ABSTRACT FROM AUTHOR]

The 1.87-1.84 Ga Black Hills dike swarm of the Kalahari craton (South Africa) is coeval with several regional magmatic provinces used here to resolve the craton's position during Columbia assembly. We report a new 1850 ± 4 Ma (U-Pb isotope dilution-thermal ionization mass spectrometry [ID-TIMS] on baddeleyite) crystallization age for one dike and new paleomagnetic data for 34 dikes of which 8 have precise U-Pb ages. Results are constrained by positive baked-contact and reversal tests, which combined with existing data produce a 1.87-1.84 Ga mean pole from 63 individual dikes. By integrating paleomagnetic and geochronological data sets, we calculate poles for three magmatic episodes and produce a magnetostratigraphic record. At 1.88 Ga, the Kalahari craton is reconstructed next to the Superior craton so that their ca. 2.0 Ga poles align. As such, magmatism forms part of a radiating pattern with the coeval ca. 1.88 Ga Circum-Superior large igneous province. [ABSTRACT FROM AUTHOR]

The tectonic history of the Philippine Sea plate is an essential piece in understanding the tectonic evolution of Southeast Asia, but it is still unclear and controversial. We present the first geochemical data obtained from lavas from the Gagua Ridge (GR) within the Philippine Sea. The GR lavas exhibit geochemical signatures typical of subduction-related arc magmatism. Plagioclase Ar-Ar ages of ca. 124-123 Ma and subduction-related geochemical signatures support the formation of GR lavas in the vicinity of an arc during the Early Cretaceous induced by subduction of the oceanic plate along East Asia. The ages of trapped zircon xenocrysts within the GR lavas cluster at 250 Ma, 0.75 Ga, and 2.45 Ga and match well the ages of zircons recovered from the Cathaysian block, southern China. Our results imply that the GR basement is partially composed of continental material that rifted away from the Eurasian margin during opening and spreading of the Huatung Basin. The depleted mantle wedge-derived magmas evolved and picked up the continental zircons during ascent. The youngest zircon ages and the GR lava Ar-Ar ages (ca. 124-123 Ma) presented in this study newly constrain an Early Cretaceous age for the Huatung Basin. Our study provides further evidence that the Huatung Basin is a remnant of a Mesozoic-aged ocean basin that dispersed from southern China during the Cretaceous. Transport of continental slivers by growth and closure of marginal seas along the East Asia margin may have been more prevalent than previously recognized. [ABSTRACT FROM AUTHOR]

Oxygen isotope ratios of garnet provide well-established means to investigate crustal fluid histories. Traditionally, d18O values from skarn garnets have been used to track the hydrothermal evolution of an individual skarn body through time. We, however, use garnet from 14 skarns from the Jurassic (ca. 175 to ca. 148 Ma) Cordilleran margin arc (southwestern United States) to provide regional tectonic context to arc magmatism and hydrothermal activity. We document arc-wide garnet d18O variability of ~19‰ (-8.9‰ to +10.3‰, n = 159), providing a record of contrasting meteoric fluid ingress between northern (Sierra Nevada) and southern (Mojave Desert) arc segments. Strongly negative garnet d18O values (=-3‰) are limited to the Mojave Desert arc segment and can only form in the presence of meteoric fluid, requiring shallow formation in subaerial crust. When combined with U-Pb garnet ages, the d18O data provide a minimum radiometric age of local subaerial arc emergence and temporal constraint on the migration of the Jurassic paleoshoreline in the Mojave Desert section of the arc. [ABSTRACT FROM AUTHOR]

We integrated new and existing bedrock and detrital zircon dates from the Zealandia Cordillera to explore the tempo of Phanerozoic arc magmatism along the paleo-Pacific margin of southeast Gondwana. We found that episodic magmatism was dominated by two high-magma-addition-rate (MAR) events spaced ~250 m.y. apart in the Devonian (370-368 Ma) and the Early Cretaceous (129-105 Ma). The intervening interval between high-MAR events was characterized by prolonged, low-MAR activity in a geographically stable location for more than 100 m.y. We found that the two high-MAR events in Zealandia have distinct chemistries (S-type for the Devonian and I-type for the Cretaceous) and are unlikely to have been related by a repeating, cyclical process. Like other well-studied arc systems worldwide, the Zealandia Cordillera high-MAR events were associated with upper-plate deformation; however, the magmatic events were triggered by enhanced asthenospheric mantle melting in two distinct arc-tectonic settings--a retreating slab and an advancing slab, respectively. Our results demonstrate that dynamic changes in the subducting slab were primary controls in triggering mantle flare-up events in the Phanerozoic Zealandia Cordillera. [ABSTRACT FROM AUTHOR]

We present new coupled detrital zircon trace-element and U-Pb age data from Valanginian-Santonian strata of the Nacimiento forearc basin (California, USA) to enhance provenance discrimination and investigate the evolution of the late Mesozoic California margin. Our data document at least five different Jurassic-earliest Cretaceous zircon populations with variable U/Yb ratios, and zircon that displays systematically increasing U/Yb from 130 to 80 Ma. Based on the presence of a distinctive population of geochemically primitive, 168-157 Ma low-U/Yb zircon that is found in Albian-Lower Cenomanian strata but not in older Valanginian strata, we infer a period of uplift and Albian-early Cenomanian erosion of forearc basement (the Coast Range ophiolite) that was coincident with increasing Cordilleran arc magmatic flux. [ABSTRACT FROM AUTHOR]

Along the present-day circum-Pacific subduction girdle, seamount subduction is known to have significant effects on subduction dynamics including on seismicity and arc magmatism. Because seamount subduction should have occurred throughout much of Earth history, its effects on orogenesis in the overriding plate should be identifiable in ancient orogens. In this study, we investigate the Paleozoic South Tianshan orogen of Central Asia, for which abundant evidence of seamount subduction exists, further bolstered by the continuation of a long-lived plume-induced large igneous province on the subsequently accreted Tarim craton. We find that semi-continuous seamount subduction from ca. 400 to 330 Ma temporarily shut down arc magmatism, and once the seamounts were completely subducted, then arc magmatism resumed and eclogites were quickly exhumed. If such an orogenic fingerprint of seamount subduction can be identified in ancient orogens, a much more complete picture of plumesubduction interaction and its influence on both crustal and mantle processes can be developed. [ABSTRACT FROM AUTHOR]

At slow-spreading ridges, plate separation is commonly partly accommodated by slip on long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. However, the mechanics of this process, the subsurface structure, and the interaction of these faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismographs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although both detachments exhibited high levels of seismicity, they are separated by an ~8-km-wide aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic activity, possibly indicating magmatism, crosscuts the 13°30°N domed detachment surface, confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS network spatially overlapped with a similar survey done in 2014, significant changes in the patterns of seismicity between these surveys are observed. These changes suggest that oceanic detachments undergo previously unobserved cycles of stress accumulation and release as plate spreading is accommodated. [ABSTRACT FROM AUTHOR]

The Duluth Complex (Minnesota, USA) is one of the largest mafic intrusive complexes on Earth. It was emplaced as the Midcontinent Rift developed in Laurentia's interior during an interval of magmatism and extension from ca. 1109 to 1084 Ma. This duration of magmatic activity is more protracted than is typical for large igneous provinces interpreted to have formed from decompression melting of upwelling mantle plumes. While the overall duration was protracted, there were intervals of more voluminous magmatism. New 206Pb/238U zircon dates for the anorthositic and layered series of the Duluth Complex constrain these units to have been emplaced ca. 1096 Ma in <1 m.y. (duration of 500 ± 260 k.y.). Comparison of paleomagnetic data from these units with Laurentia's apparent polar wander path supports this interpretation. This rapid emplacement bears similarities to the geologically short duration of well-dated large igneous provinces. These data support hypotheses that call upon the co-location of lithospheric extension and anomalously hot upwelling mantle. This rapid magmatic pulse occurred >10 m.y. after initial magmatism following >20° of latitudinal plate motion. A likely scenario is one in which upwelling mantle encountered the base of Laurentian lithosphere and flowed via "upside-down drainage" to locally thinned lithosphere of the Midcontinent Rift. [ABSTRACT FROM AUTHOR]

The dearth of cumulate magmatic roots in accretionary orogens is a cornerstone of models that postulate redistribution of mass and energy within the crust for the genesis of intermediate to silicic magmatism. Likewise, the origin of the evolved Acadian (Devonian) plutonism in the New England Appalachians (northeastern USA) has long been explained by closed-system crustal melting due to the absence of associated coeval deep mafic counterparts. Here, we report the discovery of Acadian hydrous ultramafic cumulate rocks that formed by deepseated (~1.1 GPa) fractional crystallization processes from a mantle-derived parental melt (Connecticut, southern New England, USA). These rocks are the first of their kind identified in the Appalachian orogen, and one of only a handful of preserved deep subarc hydrous cumulates worldwide. We propose a genetic link between the studied rocks and the evolved coeval plutonism in central-southern New England, where the former represent the missing deep cumulate roots of the same magmatic arc. Our findings support the hypothesis that differentiation of mantle-derived hydrous magmas by fractional crystallization and assimilation processes in the deep crust is a fundamental process in the production of intermediate to silicic magmatism and the geochemical evolution of the continental crust. [ABSTRACT FROM AUTHOR]

We report on a ca. 239 Ma mafic dike swarm intruded in the Southern Qiangtang terrane, central Tibet, that was generated on the passive continental margin of a subducting lower plate. The dikes are tholeiitic basalts and exhibit light rare earth element enrichment, modest negative anomalies in Nb and Ta, and enriched isotopic signatures. The dikes are coeval with a back-arc basin formed in the upper plate as a result of the rollback of the Paleo-Tethys oceanic slab. Thus, after ocean-ridge subduction, enhanced slab-pull forces related to slab rollback on one side of the ocean induced extension and magmatism in the passive margin on the opposite side. We argue that enhanced slab-pull forces are a previously unrecognized mechanism for the generation of lower-plate passive-margin magmatism. [ABSTRACT FROM AUTHOR]

The Valanginian Stage is marked by a period of global positive δ13C carbon cycle perturbation and biotic crises, which are collectively referred to as the Valanginian event (VE). Many attempts have been made to link the Paraná-Etendeka large igneous province volcanism with the VE. However, currently there is no conclusive proof to support this hypothesis, since the timing and duration of the volcanic activity are not known with sufficient precision. In this study, we significantly revise the time scales of magmatism and environmental impact of the Paraná magmatic province (PMP) in Brazil with new high-precision zircon U-Pb ages from the low-Ti Palmas and high-Ti Chapecó sequences. Our data demonstrate that signifi- cant volumes of low-Ti silicic rocks from the PMP erupted rapidly at ca. 133.6 Ma within 0.12 ± 0.11 k.y. The age of the high-Ti Chapecó sequence from central PMP is constrained at ca. 132.9 Ma and thus extends the duration of magmatic activity by ~700 k.y. Our new ages are systematically younger than previous ages and postdate the major positive carbon isotope excursion, indicating that PMP silicic magmatism did not trigger the VE but could have contributed to extending its duration. Within the framework of the stratigraphic column of the PMP, the earliest low-Ti basalts could have been responsible for the VE if they are at least 0.5 m.y. older than the low-Ti silicic rocks dated herein. [ABSTRACT FROM AUTHOR]

Death Valley (California, USA) hosts iconic Cryogenian snowball Earth deposits, but the lack of direct geochronological constraints has permitted a variety of correlations and age models. Here, we report two precise zircon U-Pb isotope dilution-thermal ionization mass spectrometry dates for the Kingston Peak Formation: a volcanic eruptive age of 705.44 ± 0.28 Ma from the synglacial Limekiln Spring Member, and a maximum depositional age of 651.69 ± 0.64 Ma from the nonglacial Thorndike submember, which is below the Wildrose diamictite. These dates confirm that the Limekiln Spring and Surprise Members were deposited during the Sturtian glaciation, while the Wildrose submember is a Marinoan glacial deposit, and the overlying Sentinel Peak Member of the Noonday Formation is a Marinoan cap carbonate. Additionally, the age from the Thorndike submember supersedes existing radioisotopic ages from the Datangpo Formation in South China as the youngest constraint on the onset of the Marinoan glaciation, demonstrating that the Cryogenian nonglacial interlude lasted for at least 9 m.y. and the Marinoan glaciation was <17 m.y. long. Cryogenian glaciation in western Laurentia occurred against the backdrop of -85 m.y. of episodic rift-related subsidence and magmatism within laterally discontinuous, fault-bound basins. [ABSTRACT FROM AUTHOR]

Deposition of trona, nahcolite, and other Na-carbonate evaporite minerals in lakes is commonly closely associated with active volcanism, suggesting that the excess alkalinity required for their formation may arise from fluid-rock interactions involving hydrothermal waters that contain magmatic CO2. Paradoxically, the world's largest Na-carbonate occurrence, contained within the Eocene Green River Formation in Wyoming, USA, was not associated with nearby active magmatism. Magmatism was active ~200 km southeast in the Colorado Mineral Belt, however, suggesting that a river draining this area could have supplied excess alkalinity to Eocene lakes. Sedimentologic studies in southwestern Wyoming, along the course of the hypothesized Aspen paleoriver, document fluvial and deltaic sandstone with generally northwest-directed paleocurrent indicators. Sandstone framework grain compositions and detrital zircon ages are consistent with derivation from the Colorado Mineral Belt and its host rocks. These results provide the first confirmation of a fluvial connection to downstream Eocene lakes, and indicate that lake deposits may offer a unique perspective on upstream magmatic and hydrothermal histories. [ABSTRACT FROM AUTHOR]

Uplift of the Tibetan Plateau is thought to be one of the most important orogenic and climate forcing events of the Cenozoic Era, associated with geodynamic changes related to India-Asia collision and subsequent continental lithosphere subduction. However, the fate and scale of the subducted continental lithosphere segments remain highly controversial. Using a comprehensive compilation of the spatiotemporal distribution of postcollisional magmatic rocks across Tibet, together with new geochemical and Sr-Nd-Pb isotopic data and modeling simulations, we propose a holistic, two-stage evolutionary model to explain the link between genesis of the magmas and continental subduction. The magmatism prior to 25 Ma resulted from continuous upwelling of a carbonate-rich upper-mantle plume induced by northward underthrusting of Indian oceanic and continental lithosphere with its cover of Tethyan platform carbonate sediments, whereas magmatism after 25 Ma was related to opposing northdirected and south-directed continental subduction. Our model indicates a transformation in the distribution and nature of the magmatism in Tibet at ca. 25 Ma, which reflects a significant change in the Himalayan-Tibetan orogen and associated mantle dynamic processes in the early Miocene. Understanding this transformation could have important implications for the utility of the Himalayan-Tibetan system as a modern analogue for ancient orogens. [ABSTRACT FROM AUTHOR]

The evolution of the mantle source beneath a terrane is recorded by the Sm-Nd isotopic systematics of mafic rocks over extended periods of time. The evolution of the mantle beneath the Variscan massifs of continental Europe is tracked in order to examine the coupling between the crust and underlying continental lithospheric mantle during the late Paleozoic Variscan orogeny. Late Neoproterozoic and early Paleozoic (Cambrian-Devonian) extension-related mafic rocks have εNd(t) values similar to depleted mantle and so were probably derived from juvenile mantle sources. Thus, weak deformation and low-grade metamorphism associated with the ca. 540 Ma Cadomian orogeny did not change the mantle source beneath the Variscan massifs. In contrast, late Carboniferous-Permian mafic magmatism, which erupted in the aftermath of the collision between Gondwana and Laurussia, is characterized by negative εNd(t) values and significantly older depleted mantle model ages (TDM = 1800-800 Ma), suggesting derivation from a lithospheric mantle contaminated by late Paleozoic subduction. This characteristic is shared by most Variscan massifs, and it is consistent with the vast tracts of allochthonous peri-Gondwanan terranes exposed within the orogen, which suggest regional crust-mantle decoupling during Variscan collision. The regionally extensive Cenozoic European Volcanic Province has higher εNd(t) values (+5 to +7) and significantly lower TDM ages (ca. 500-100 Ma), suggesting that the continental lithospheric mantle was refertilized. [ABSTRACT FROM AUTHOR]

Magmatic arcs typically exhibit non-steady-state evolution with episodic flare-ups and lulls, yet the main drivers remain contentious. Situated in the southwest margin of Southeast Asia, Sumatra records a long-lived magmatic arc that is still poorly constrained in age and tempo. Detrital zircon data from Sumatra delineate major arc magmatic pulses at ca. 212, 102-85, 52, and 22-11 Ma. The mid-Cretaceous to early Eocene zircons mostly yield high positive εHf(t) values, indicating magma derivation from juvenile sources and matching well with those of the Gangdese batholiths in the southern Lhasa terrane. These similarities substantiate an extended (~6000 km) Neo-Tethyan arc system from southern Tibet to Sumatra that exhibits concurrent magmatic lulls (ca. 150-105 and 85-65 Ma) and flare-ups (ca. 105-85 and 65-40 Ma). The Late Cretaceous magmatic lull coincided with a period of strong regional deformation and increasingly fast northward drift of India, likely attributable to Neo-Tethyan flat slab subduction. Periodic pulses of Neo-Tethyan arc magmatism most likely correlated with repeated steepening and shallowing of slab dip, rather than India-Eurasia convergence rates. [ABSTRACT FROM AUTHOR]

Kimberlite magmas are volatile-rich, potassic, and ultramafic, and they are host to most of the world's diamond deposits. A continental-scale kimberlite magmatic belt (the central Cretaceous kimberlite corridor [CCKC]) is found in the interior of the North American continent. Parallel to and coeval with the CCKC, the Cretaceous Omineca magmatic belt (OMB) is located in the Cordilleran orogen. Cordilleran magmatism, including the OMB, is commonly explained through long-lived east-dipping subduction beneath the western margin of the continent. However, this does not explain the temporal and spatial relationships between the OMB and CCKC. We suggest that west-dipping subduction of North American lithosphere beneath the eastern side of the Cordillera explains both. In this model, subduction resulted in arc magmatism of the OMB. The contemporaneous CCKC was formed by extensional stress acting on the continent as it flexed upon entry into the trench. Using a semi-infinite elastic beam model, we show that flexure of a subducting continental plate (elastic thickness = 120 km) produces tensile stresses in the deep continental lithosphere, coincident with the location of the CCKC. [ABSTRACT FROM AUTHOR]

The magmatic processes responsible for accretion of the lower oceanic crust remain one of the least-constrained components of the global seafloor spreading system. Samples of gabbroic rocks recovered by scientific ocean drilling are too limited to allow effective assessment of spatial variations in magmatic flow within in situ lower crust. Extensive exposures of gabbros in ophiolites, on the other hand, provide opportunities to study accretion processes in three dimensions across wide areas and at a resolution that allows variations in magmatic fabrics through the crust to be quantified. Here, we show that magnetic anisotropy provides a reliable proxy for lower-crustal magmatic fabrics in the world's largest ophiolite in Oman. Important differences in magnetic fabrics are detected that reflect variations in magmatic processes on a range of scales. Fabrics in layered gabbros are aligned with modal layering and display a consistency in the orientation of maximum principal axes of anisotropy between localities at a regional scale. These fabrics are compatible with subhorizontal preferred alignment of crystals orthogonal to the inferred orientation of the Oman spreading axis, resulting from magmatic flow or deformation of melt-rich crystal mushes during spreading. In contrast, magnetic anisotropy in foliated gabbros at the top of the lower crust reveals for the first time distinctly different linear and anastomosing fabric styles between localities sampled at the same pseudostratigraphic level. These differences reflect spatial variations in the style and trajectory of flow in the crystal mush beneath the axial melt lens during upward melt migration at the spreading axis. [ABSTRACT FROM AUTHOR]

We report on pipe-like bodies and dikes of carbonate rocks related to sodic alkaline intrusions and amphibole mantle peridotites in the Ivrea zone (European Southern Alps). The carbonate rocks have bulk trace-element concentrations typical of low-rare earth element carbonatites interpreted as cumulates of carbonatite melts. Faintly zoned zircons from these carbonate rocks contain calcite inclusions and have trace-element compositions akin to those of carbonatite zircons. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon dating yields concordant ages of 187 ± 2.4 and 192 ± 2.5 Ma, coeval with sodic alkaline magmatism in the Ivrea zone. Cross-cutting relations, ages, as well as bulk and zircon geochemistry indicate that the carbonate rocks are carbonatites, the first ones reported from the Alps. Carbonatites and alkaline intrusions are comagmatic and were emplaced in the nascent passive margin of Adria during the Early Jurassic breakup of Pangea. Extension caused partial melting of amphibole-rich mantle domains, yielding sodic alkaline magmas whose fractionation led to carbonatite-silicate melt immiscibility. Similar occurrences in other rifts suggest that small-scale, sodic and CO2-rich alkaline magmatism is a typical result of extension and decompressiondriven reactivation of amphibole-bearing lithospheric mantle during passive continental breakup and the evolution of magma-poor rifts. [ABSTRACT FROM AUTHOR]

The concave-inboard (concave toward the overriding plate) geometry of most convergent margins is considered a natural consequence of the depression of the edge of a thin spherical cap, whereas concave-outboard margin segments commonly form around indenters on the subducting plate. At the Cascadia subduction zone, the apex of a >500-km-long concave-outboard bend in the trench presently shows no obvious subduction of an indenter, but does coincide with the axis of an outboard-facing concavity in upper-plate rocks arched around the Olympic Peninsula in northwestern Washington State, USA. Here we synthesize paleomagnetic and structural data together with new analyses of Global Navigation Satellite System data to show that the upper plate at Cascadia has been folded from the Miocene to the present into an orocline with an axial trace that bisects the Olympic Peninsula. The processes that accommodate bending, which we suggest include (1) folding by flexural slip on the orocline limbs and (2) shortening, uplift, and escape within the core of the fold at the Olympic Mountains, have the combined result of relative motion of the forearc towards the arc at the core of the orocline, and sustained opposing rotations of the upper plate on the orocline limbs. We propose that oroclinal bending is promoted and maintained by along-strike variations in plate-boundary tractions resulting from the geometry of the plate interface at depth and suggest that these processes can contribute to the development of concave-outboard margins without the need for a subducting indenter. [ABSTRACT FROM AUTHOR]

A-type magmatism is commonly generated in extensional settings. Cenozoic postcollisional extensional structures and magmatism widely occur in the well-known Himalayan-Tibetan orogen (HTO). So far, however, no Cenozoic A-type magmatism has been identified in the orogen. Here, we report on newly identified 24-23 Ma felsic lavas (trachytes + rhyolites) in the Konglong area of the Lhasa block, southern Tibet. They have diagnostic minerals (sodalites) and geochemical characteristics (high total-alkali, FeO/MgO, Zr, and Ga/Al, low CaO, and strongly negative Eu, Sr, and Ba anomalies), demonstrating their close affinity with A-type granitoids. Generation of the A-type lavas required high-temperature melting (up to 944 °C) of crustal source rocks in the pressure range of 0.5-1.2 GPa. Our study not only identifies the first postcollisional A-type magmatism in the HTO, but it also indicates an extension event contemporaneous with the onset of the Main Central thrust and South Tibetan detachment system, and development of the Kailas basin and ultrapotassic magmatism of the southern HTO. These events straddled the Himalaya and Lhasa blocks and can be reconciled with foundering of the subducted Indian plate. Thus, we suggest a tectonic model of Indian plate flat subduction and foundering beneath the western Lhasa block for the postcollisional evolution of the southern HTO. [ABSTRACT FROM AUTHOR]

A Jurassic extensional detachment associated with carbonated serpentinites and basalts is preserved at Falotta (Platta nappe, southeastern Switzerland). Structural data indicate that fluid circulation occurred during late increments of extension along the detachment plane at the contact between serpentinites and basalts. The homogeneity of the isotopic signatures (δ18O ~16‰ and δ13C centered on 0‰-1‰) can be best explained by a single, sudden seawater-derived carbonation event at temperatures of ~100 °C. Carbonation was focused in the high-permeability zone along the detachment. Our model yields new insights for carbonation processes related to mantle exhumation. [ABSTRACT FROM AUTHOR]

Porphyry copper deposits (PCDs) represent the most important type of Cu reservoir on Earth, but the mechanism of Cu enrichment in PCDs is debated due to the lack of direct constraints. This issue may be resolved by the study of copper isotopes (δ65Cu), which are strongly fractionated during formation and/or precipitation of sulfides. Here we report high-precision Cu-isotope data on a large set of porphyries, mafic magmatic enclaves (MMEs), and sulfide ores from PCDs in southern Tibet. For comparison, barren intrusions from southern Tibet were also analyzed. The fertile porphyries and MMEs from PCDs have high Cu contents and elevated δ65Cu values compared with the barren intrusions and global average felsic rocks. These features are inconsistent with the known supergene processes after sulfide formation (e.g., leaching and weathering). Because sulfides formed from secondary Cu-rich fluids are enriched in heavy Cu isotopes, the elevated Cu contents and Cu-isotope ratios indicate that the Cu source for PCDs was a refertilized lithosphere enriched in sulfides. This suggests that initial Cu enrichment in magma sources could be a key step in the formation of giant PCDs in continental collision zones. [ABSTRACT FROM AUTHOR]

Mantle lithosphere heterogeneities are well documented, are ubiquitous, and have often been thought to control lithosphere-scale deformation. Here, we explore the influence of deep scarring in crustal deformation in three dimensions by considering the Ouachita orogeny in the southeastern United States, an example of a continental collision where mantle structure is present but not previously linked to the regional crustal tectonics. We present state-of-the-art continental compressional models in the presence of inherited three-dimensional lithospheric structure. Our models find that the surface expression of the Ouachita orogeny is localized by, and projected from, the controlling mantle scarring, in keeping with geological and geophysical observations. We are able to produce a large-scale arcuate orogeny with associated basin development appropriate to the Ouachita orogeny, alongside smaller-scale crustal faulting related to the region. This study offers a new and alternative hypothesis to the tectonic history of the Ouachita orogeny, with previous research having focused exclusively on crustal structures. The findings have broad implications, demonstrating the important potential role of the mantle lithosphere in controlling crustal dynamics and highlighting the requirement to consider deeper structure and processes when interpreting tectonic evolution of lithospheric-scale deformation. [ABSTRACT FROM AUTHOR]

Models of orogens identify subduction of oceanic crust as the key mechanism leading to continental collision. Such models, based inter alia on thermobarometric and geochronological evidence preserved in high-pressure metamorphic rocks and subduction-related magmatism, have been used to explain the convergence of Europe and Adria in the Cretaceous-Cenozoic and the subsequent Alpine orogen. Here, we review the metamorphic, igneous, and sedimentary record of the past 300 Ma of the Alpine orogen to show that there is no evidence of igneous activity during subduction initiation and prograde high-pressure metamorphism, leading to an ~50 Ma hiatus in magmatism, or "arc gap." The closure of rift basins forming the Piemont-Liguria ocean did not follow a classical Wadati-Benioff-type subduction. Instead, subduction initiation at passive margins allowed for the accretion of the hydrated portion of the subducting plate within an orogenic wedge as subduction of dry subcontinental lithosphere inhibited magmatism during subduction initiation and ocean closure. [ABSTRACT FROM AUTHOR]

We present a global compilation of major element, as well as Re-Os isotope, data on mantle xenoliths from continental lithosphere to constrain the secular evolution of mantle depletion since the early Archean. Whereas a temporal dichotomy in the degree of mantle depletion has long been recognized in previous regional studies of mantle xenoliths, this global compilation reveals, for the first time, a smooth secular trend in mantle depletion, which is in remarkable agreement with what is expected from the secular cooling of the ambient mantle as inferred from the petrology of non-arc basalts. Depleted mantle now composing continental lithosphere is likely to have been originally formed beneath mid-ocean ridges or similar spreading environments, and a greater degree of depletion in the past can be seen as a corollary of the secular cooling of the mantle. [ABSTRACT FROM AUTHOR]

La Gloria Pluton (LGP) in central Chile is a shallow, north-northwest- elongated granitoid body of 18 km length and 4-6 km width, belonging to a regional north-south trend of Miocene plutons, from the San Francisco Batholith and porphyries in the north, to Mesón Alto, and San Gabriel in the south. New U-Pb zircon ages of the LGP indicate that crystallization occurred mostly within an interval between 11.3-10.2 Ma, with a pattern of decreasing ages along the pluton axis from south to north. The progression of zircon ages can be explained either by gradual northwestward migration of the feeder zone that supplied magma to the shallow pluton or, more likely, by shallow lateral magma propagation southeastward from a fixed feeder zone located beneath the northern margin of the pluton. The age progression, together with existing data of subhorizontal mineral and magnetic lineations in the LGP parallel to the pluton axis, indicate lateral propagation of magma during reservoir construction along the hinge of an anticline of the volcanic host sequences. In addition to controlling the position of possible volcanic output, such horizontal migration of silicic magmas in the upper crust significantly increases the surface footprint over which fluids are exsolved and outgas, strongly decreasing the potential for magmatic-hydrothermal ore formation above laterally emplaced laccoliths in the shallow crust. [ABSTRACT FROM AUTHOR]

Accurate and precise temperatures of metamorphism and magmatism can be obtained from O-isotope compositions of minerals using a system of linear equations that defines fractionation and mass balance of O isotopes among all minerals in a rock. This approach--linearequation thermometry (LinT)--does not require that cooling rates or absolute O-diffusion parameters be known. If O diffusivities are known, closure temperatures for each phase can be calculated and inverted to determine a temperature-dependent cooling rate that is accurate to a factor of three. The highest and most precise temperatures will be preserved in rocks containing quartz and garnet, because O diffuses slowly in garnet and garnet-quartz O-isotope fractionation is relatively large. The technique will be most useful in rocks with low-variance mineral assemblages in which precise pseudosection analysis is inhibited by wide spacing of reactions in pressure-temperature space. Application of LinT to ultrahigh-temperature gneisses from Madagascar yields temperatures of 927 ± 31 °C, consistent with independent thermometry. [ABSTRACT FROM AUTHOR]

An understanding of the long-term thermal evolution or thermal history of magma storage is key to understanding the behavior of crustal magmatic systems. The ability to measure both age and crystallization temperature in zircon crystals is increasingly being used to infer the thermal history of magmas in young volcanic rocks. However, the temperature dependence of zircon crystallization and the effects of analytical uncertainties also affect the ability of zircons to uniquely record thermal variations during magma storage. We have developed a probabilistic forward modeling approach to explore zircon age--crystallization temperature systematics produced by user-generated thermal histories. Results suggest that zircon data sets acquired by secondary ion mass spectrometry currently have limited ability to uniquely resolve magmatic thermal history. Uncertainties in measured ages are typically longer than both the time taken for zircon growth at the analyzed scale (~1-20 µm), and the expected duration of thermal rejuvenation events. This strongly limits the extent to which short thermal pulses can be distinguished. Episodic and continuous temperature histories produce remarkably similar zircon systematics, particularly when sample sizes are small. Natural zircon samples that show a range of apparent temperatures through time, and that have been interpreted to reflect extended magma storage at relatively elevated temperatures, can also be produced by near-solidus magma storage punctuated by a limited number of short thermal rejuvenation events. [ABSTRACT FROM AUTHOR]

Hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes, including the Namibian Damara belt. Here, subduction-related deformation occurred at temperatures σ550 °C, and vein geometry is consistent with plate interface shear, low effective normal stresses, and mixed-mode deformation. Quartz vein δ18O values relative to Standard Mean Ocean Water (SMOW) range from 9.4‰ to 17.9‰ (n = 30), consistent with precipitation from metamorphic fluids. A dominant subset of quartz veins away from long-lived high-strain zones and basaltic slivers have δ18O values in a smaller range of 14.9‰ ± 1‰, requiring precipitation from a fluid with δ18O of 12‰ ± 1‰ at 470-550 °C. This uniform fluid isotope value is consistent with progressive local breakdown of chlorite allowing extensive hydrofracture at temperatures typical of the plastic regime. In active subduction zones, brittle deformation within the plastic regime is inferred from observations of tectonic tremor, a noise-like seismic signal including overlapping low- and very low-frequency earthquakes, which occurs below the seismogenic zone. Both tremor and hydrothermal veins correlate with zones of inferred high fluid pressure, could represent a mixture of shear and dilatant failure, and may therefore be controlled by episodic hydrofracturing within a dominantly plastic and aseismic regime. [ABSTRACT FROM AUTHOR]

Tin (Sn) and tungsten (W) mineralization are commonly associated with each other in relation to highly evolved granites, but economical ore grades are restricted to rare global occurrences and mineralization styles are highly variable, indicating different mechanisms for ore formation. The Sn-W Zinnwald deposit in the Erzgebirge (Germany and the Czech Republic) in the roof zone of a Variscan Li-F granite hosts two contrasting styles of mineralization: (1) cassiterite (Sn) in greisen bodies, and (2) cassiterite and wolframite (W) in predominantly subhorizontal quartz-rich veins. The relative timing and causes for ore formation remain elusive. Studies of fluid inclusion assemblages in wolframite, cassiterite, and quartz samples from greisen and veins by conventional and infrared microthermometry and by laser ablation-inductively coupled plasma-mass spectrometry analyses have revealed compelling evidence that all elements required for the formation of the Zinnwald Sn-W deposit were contained in a single parental magmatic-hydrothermal fluid that underwent two main processes: (1) fluid-rock interaction during Sn-greisen formation, and (2) depressurization and vapor loss leading to ore precipitation in quartz-Sn-W veins. The results also show that fluid inclusion assemblages in ore minerals can document fluid processes that are absent in the fluid inclusion record of gangue minerals. The study further highlights the role of phase separation in the formation of W-rich Sn deposits and indicates that W deposits in distal parts of evolved granites may be restricted to fluids derived from deeper-seated plutons. [ABSTRACT FROM AUTHOR]

The high values of La/Yb (>15-20) obtained from late Eocene to Pliocene volcanic rocks of central Tibet indicate that source regions were in a thickened (>35 km) crust. The higher Neogene La/Yb values compared to lower Paleogene La/Yb values required continuous crustal thickening, as La/Yb values are proportional to crustal thickness. Although the Paleogene high La/Yb values can be explained by coeval thrusting, this mechanism is unable explain the Neogene higher La/Yb values because central Tibet is covered by undeformed flat-lying early Miocene strata. Here we correlate the temporal variation of Cenozoic volcanic geochemistry with the regional geology of central Tibet, which was part of a larger Eocene-Oligocene thrust belt and the related foreland basin. The Paleogene foreland basin was partitioned into the southern (Hoh Xil) and northern (Qaidam) subbasins by the Kunlun fault ca. 25-20 Ma. The two subbasins were at similar elevations at the time of their formation, but the southern basin, at ~5 km, is >2 km higher than the northern basin at ~2.8 km. Because the southern basin and the Paleogene thrust belt have been associated with Neogene volcanism and high La/Yb values, we suggest that the elevation difference between the two basins was induced by >15 km of magmatic inflation sourced from mantle melting. The magnitude of magmatic inflation is comparable to that of tectonic thickening in central Tibet, highlighting the important and often neglected role of syncollisional mantle melting and its induced magmatic inflation in constructing the thickened Tibetan crust. [ABSTRACT FROM AUTHOR]

Laser ablation-inductively coupled plasma-quadrupole mass spectrometry analysis on hydrothermal cassiterite was applied to solve the long-standing debate on the age of the central European Erzgebirge-Krušné hory tin province of eastern Germany and the western Czech Republic. Cassiterite samples from the Sadisdorf, Ehrenfriedersdorf, Altenberg, Cinovec-Zinnwald, and Krupka tin deposits have U-Pb ages that overlap within the 2σ uncertainty range, and define a relatively narrow time window of 326 ± 3 to 320 ± 3 Ma. These dates also define the minimum age of the associated highly evolved alkali feldspar granite intrusions, which so far could not be reliably dated by conventional U-Pb, Sm-Nd, Rb-Sr, or 40Ar-39Ar techniques due to high-U metamict zircons and resetting by later thermal events. The time span of ca. 326-320 Ma characterizes the age range of regional rare-metal granite magmatism and associated hydrothermal tin mineralization in the Erzgebirge as a result of exten-sional collapse of the Variscan orogen. [ABSTRACT FROM AUTHOR]

The origin of intraplate carbonatitic to alkaline volcanism in Africa is controversial. A tectonic control, i.e., decompression melting associated with far-field stress, is suggested by correlation with lithospheric sutures, repeated magmatic cycles in the same areas over several million years, synchronicity across the plate, and lack of clear age progression patterns. Conversely, a dominant role for mantle convection is supported by the coincidence of Cenozoic volcanism with regions of lithospheric uplift, positive free-air gravity anomalies, and slow seismic velocities. To improve constraints on the genesis of African volcanism, here we report the first radiometric and isotopic results for the Catanda complex, which hosts the only extrusive carbonatites in Angola. Apatite (U-Th-Sm)/He and phlogopite 40Ar/39Ar ages of Catanda aillikite lavas indicate eruption at ca. 500-800 ka, more than 100 m.y. after emplacement of abundant kimberlites and carbonatites in this region. The lavas share similar high-μ (HIMU)-like Sr-Nd-Pb-Hf isotope compositions with other young mantle-derived volcanics from Africa (e.g., Northern Kenya Rift; Cameroon Line). The position of the Catanda complex in the Lucapa corridor, a long-lived extensional structure, suggests a possible tectonic control for the volcanism. The complex is also located on the Bié Dome, a broad region of fast Pleistocene uplift attributed to mantle upwelling. Seismic tomography models indicate convection of deep hot material beneath regions of active volcanism in Africa, including a large area encompassing Angola and northern Namibia. This is strong evidence that intraplate late Cenozoic volcanism, including the Catanda complex, resulted from the interplay between mantle convection and preexisting lithospheric heterogeneities. [ABSTRACT FROM AUTHOR]

Hydrothermal venting, an important cooling mechanism of the Earth, supports a diverse array of seafloor and sub-seafloor ecosystems that are sustained by large thermal and chemical fluxes. Vents have been found along even the slowest and coldest spreading centers, calling into question the driving heat source for these vents. The ultraslow-spreading Mid-Cayman Spreading Center in the Caribbean Sea, which hosts the axial-flank Von Damm Vent Field (VDVF), provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates. Using active-source seismic data from the 2015 CaySeis (Cayman Seismic) experiment, we determined the seismic velocities in the large massif beneath the VDVF. We propose that this massif was produced by a pulse of on-axis magmatism at ca.2 Ma, which was then followed by exhumation, cooling, and fracturing. A low seismic velocity anomaly 5 km below the VDVF is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills, both of which could drive ongoing deep hydrothermal fluid circulation. We conclude that the transient magmatism and variable crustal thickness at ultraslow-spreading centers create conditions for long-lived hydrothermal venting that may be widespread, and other VDVF-like vents may be common in these areas. [ABSTRACT FROM AUTHOR]

Earth history has been punctuated by episodes of short-lived (<10 m.y.), high-volume (>106 km³) magmatism. The origin of these events and their manifestations as large igneous provinces (LIPs) with associated continental flood basalts do not fit in the current plate-tectonic paradigm. Upper-mantle processes have been invoked for some LIPs, whereas the origin of others appears to be related to plumes rising from the deep mantle. The Paraná-Etendeka LIP has remained enigmatic and highly contested in terms of plume versus upper-mantle models. Here, we provide evidence for a plume origin based on new isotopic (He, O, Sr, Nd, Pb) and trace-element data from olivine-rich dikes from Namibia. The composition of the dikes can be explained by mixing at shallow depths between a plume source with high ³He/4He (>26 RA) and ambient asthenospheric mantle, before ascent through the thinning lithosphere. [ABSTRACT FROM AUTHOR]

The use of volatile-bearing (H2O-Cl-F-C-S) minerals as proxies for constraining the volatile content of lunar magmas remains controversial. In this study, we apply published F-Cl-S signature trends in apatite from lunar mare basalts, new experimentally determined apatitemelt partition coefficients for S, and thermodynamic modeling to demonstrate unequivocally that fractional crystallization of lunar silicate melt could not have produced the observed S signatures of lunar apatite. Instead, metasomatic (cryptic) alteration by a S-Cl-bearing, F-poor volatile phase plausibly explains the volatile element zoning in lunar apatite, consistent with some lunar magmas being more volatile rich than previously suggested. [ABSTRACT FROM AUTHOR]

The tectonic origin of the Pennsylvanian-Permian Ancestral Rocky Mountain system is debated. Most models invoke stress from collision along the Ouachita-Marathon belt as the primary driver for Ancestral Rocky Mountain (ARM) (western USA) tectonism, but no existing model has convincingly explained the timing, northeastsouthwest contractional kinematics, and amagmatic nature of this system. We propose that stress generated by transpressional deformation along the southwestern (Sonora) margin of Laurentia combined with that from collision along the Ouachita-Marathon and Nevada margins drove uplift and basin development of the ARM. This model explains ARM timing, uplift kinematics, and lack of magmatism, and is supported by a new generation of late Paleozoic global plate reconstructions. [ABSTRACT FROM AUTHOR]