A multiple-hotspot origin has been proposed for the Cook-Austral volcanic lineament in the southwest (SW) Pacific. One candidate, the Rarotonga hotspot, is associated with volcanism on only two islands: Rarotonga (1.157 +/- 0.003 to 1.697 +/- 0.055 Ma) and Aitutaki (youngest stage: 1.382 +/- 0.058 to 1.941 +/- 0.035 Ma), located similar to 250 km apart in a north-south (N-S) direction. Their overlapping eruption ages and lack of clear age progression preclude a robust evaluation of a hotspot origin. This study identifies and investigates a new volcanic feature, Tama Seamount-located similar to 80 km east-southeast (ESE) of Rarotonga Island along a Pacific plate flowline-potentially associated with the Rarotonga hotspot. New ages from two lavas (Ar-40/Ar-39 groundmass ages: 0.664 +/- 0.027 and 0.816 +/- 0.020 Ma) and from a nepheline syenite xenolith-including zircon U-Pb dating (weighted mean age: 0.74 +/- 0.02 Ma) and a biotite Ar-40/Ar-39 age (0.730 +/- 0.007 Ma)-constrain Tama's age. These ages align with a Rarotonga hotspot age progression based on Pacific plate motion, and support a Rarotonga hotspot origin for Tama. We also report on the first lava samples dredged from the submarine flanks of Rarotonga Island, which exhibit geochemical fingerprints and an age (1.493 +/- 0.007 Ma) consistent with those of previous Rarotonga samples. Furthermore, radiogenic isotopes of Tama samples in this study overlap with existing Rarotonga Island data, supporting a genetic relationship between Tama Seamount and Rarotonga Island. Together, these new results suggest Tama Seamount represents the youngest known expression of the Rarotonga hotspot.

Metamorphic soles beneath ophiolites are thought to record subduction initiation. However, there is ambiguity about the tectonic and thermal mechanisms operative during subduction initiation, arising partly from uncertainty in the duration of sole metamorphism. Here we use chemical mapping and diffusion speedometry of garnet crystals from the metamorphic sole of the Samail Ophiolite (Oman and United Arab Emirates) to show that high-temperature (≥750 °C) metamorphism was rapid, lasting ≤1 Myr (potentially ≤100 kyr) at peak temperature conditions. The short durations are supported by zircon U–Pb ages and new garnet–whole-rock–zircon Lu–Hf data from the same rocks, contrasting with previous inferences for ≥8 Myr metamorphic durations. These observations are nominally consistent with the spontaneous sinking of a dense lower plate. However, the rapid metamorphic timescales cannot be accounted for solely by conductive thermal equilibration with juxtaposed oceanic mantle. One potential explanation is dissipative heating driven by relative motion across the nascent plate interface. This interpretation accounts for the timescales, the spatial pattern of metamorphism and the global similarities in sole pressure–temperature conditions independent of other geodynamic variables. Metamorphic soles beneath ophiolites record rapid subduction initiation, with high-temperature metamorphism that may be driven by relative motion across the plate interface, according to diffusion speedometry of garnets combined with isotopic data.

The Late Cretaceous Semail ophiolite, Oman was emplaced onto the previously passive continental margin of Arabia. Recent geochronology from the ophiolite, its metamorphic sole, and associated high-pressure (HP) metamorphic rocks have resulted in a proliferation of new tectonic models explaining the sequence of events associated with ophiolite obduction and later continental subduction. These models have argued for (a) subduction initiation similar to 8 Myr prior to Semail ophiolite formation, (b) two distinct HP metamorphic events, including HP subduction metamorphism before and after ophiolite emplacement, and (c) the presence of two separate, synchronous subduction zones operating during ophiolite formation and emplacement. We review published data and demonstrate that these new models are unsupported by robust geochronological and geological constraints; rather, the existing data are best explained by the initiation of a single NE-dipping subduction zone by similar to 96.7 Ma, over which the ophiolite crystallized and along which the continental margin eventually subducted beneath it. The overlap between U-Pb zircon age dates on the Semail ophiolite crustal sequence (96.1-95.2 Ma), and U-Pb zircon (96.7-95.2 Ma) and internally consistent Lu-Hf garnet (97.9-93.0 Ma) dates from the metamorphic sole indicate synchronous Cenomanian ophiolite crystallization and sole metamorphism, consistent with ophiolite formation in a supra-subduction zone environment. By similar to 79 Ma, the Oman continental margin reached the subduction zone, and Early Permian shelf carbonates and basement slices were dragged down to depths similar to 90-100 km and metamorphosed to eclogite facies (>= 20 kbar), which is recorded by Sm-Nd, U-Pb, and Rb-Sr ages from multiple phases (garnet, zircon, rutile, phengite).

High-precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well-preserved and well-characterized ophiolite, we performed coupled zircon laser-ablation inductively coupled mass spectrometry trace element analyses and high-precision isotope dilution-thermal ionization mass spectrometry U-Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman-United Arab Emirates). Zircon grains from amphibolite- to granulite-facies (0.8-1.3 GPa, similar to 700-900 degrees C), garnet- and clinopyroxene-bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet-clinopyroxene amphibolite samples, Ti-in-zircon temperatures increase, and U-Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti-in-zircon temperature with decreasing HREE slope, and the combined U-Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic P-T-t path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 +/- 0.094 and 95.161 +/- 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at similar to 104 Ma. Garnet-free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are similar to 0.5-1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon epsilon(Hf) (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7-95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1-95.2 Ma). The similar to 600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (similar to 8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first similar to 1.5 Myr of subduction. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0-95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite.

During the Cryogenian (720 to 635 Ma ago) Snowball Earth glaciations, ice extended to sea level near the equator. The cause of this catastrophic failure of Earth’s thermostat has been unclear, but previous geochronology has suggested a rough coincidence of glacial onset with one of the largest magmatic episodes in the geological record, the Franklin large igneous province. U-Pb geochronology on zircon and baddeleyite from sills associated with the paleo-equatorial Franklin large igneous province in Arctic Canada record rapid emplacement between 719.86 ± 0.21 and 718.61 ± 0.30 Ma ago, 0.9 to 1.6 Ma before the onset of widespread glaciation. Geologic observations and (U-Th)/He dates on Franklin sills are compatible with major post–Franklin exhumation, possibly due to development of mafic volcanic highlands on windward equatorial Laurentia and increased global weatherability. After a transient magmatic CO2 flux, long-term carbon sequestration associated with increased weatherability could have nudged Earth over the threshold for runaway ice-albedo feedback.

The Semail Thrust in the Oman-UAE mountains is mapped along the base of the Semail Ophiolite, a 10-15 km thick sequence of Cenomanian oceanic crust and upper mantle emplaced from NE to SW onto the previously passive, Mid-Permian to Cenomanian continental margin of Oman. The juxtaposition of the Semail ophiolite with a range of different rock types sourced from different depths suggest a complex tectonic history for this major fault and shear zone. Here we summarize previous work and present an overview of the tectonic history of the fault. The Semail Thrust is mapped along the base of the ophiolite as a single line on the geological map, yet it covers a variety of structural features spanning depths of 40-45 km to the surface, and a time scale from ~96 Ma (or earlier) to Eocene time. The structural evolution of the Semail Thrust includes (a) the roof fault or ductile shear zone of an exhumed oceanic subduction zone (granulites, amphibolites and greenschists of the metamorphic sole), (b) a deep mantle ductile shear zone (Banded Ultramafic Unit), (c) a brittle fault above a foreland directed fold-thrust belt, (d) an out-of-sequence brittle fault exhuming a higher ophiolite thrust sheet above deeper level lower crust granulites (e.g. Bani Hamid, UAE), (e) a late out-of-sequence thrust truncating underlying structural units (e.g. Hawasina Window), (f) a passive roof fault beneath exhuming HP rocks (e.g. 'Semail Thrust' below the Muscat peridotite, above the Ruwi me ' lange and high-pressure rocks of northern Saih Hatat), and (g) a reactivated normal fault bounding rising footwall culminations, notably of the Jebel Akhdar, Jebel Nakhl, and Saih Hatat anticlines. Different stages in the evolution of the Semail Thrust can be mapped out and interpreted from different regions along the Oman Mountains.

The Bahia massif exposes the lower crustal section of the Oman ophiolite located close to the thrust front of the Semail nappe. It is affected by intense faulting previously attributed to tectonic events that dismembered a classical ophiolitic sequence during or after the obduction. Here we show that most of this complexity is primary, inherited from syn-accretion tectonics. The crustal section is exposed in a 15 by 8 km tectonic enclave surrounded by mantle peridotite. Its northern boundary corresponds to a major, steeply dipping normal fault striking WNWESE, at low angle to the paleo-ridge axis. Movement along this fault was accommodated by intense plastic deformation of the crustal cumulates and adjacent mantle peridotites at temperature conditions >= 900 degrees C. The thickness of the deformed zone reaches several hundred meters. The flattening of the cumulate layering away from the fault is correlated to a decrease in the deformation intensity. Undeformed olivine-gabbro dykes crosscut this "tectonic Moho" indicating that the tilting occurred before the end of the igneous activity. To the southwest, the crustal enclave is bounded by a NW-SE trending transtentional shear zone that was active in the amphibolite to greenschist facies and was intensely injected by syn- to post-kinematic gabbronorite and tonalite/ trondhjemite dykes and plugs. The age of one felsic sample (95.214 +/- 0.032 Ma, high-precision U-Pb zircon dating) is within error of the age of intrusive felsic intrusions into the mantle and lowermost axial crust from the length of the Oman ophiolite, which slightly post-dates the mean crystallization age of the Semail crust (V1 magmatism; 96.1-95.6 Ma). Other contacts are low temperature features including cataclastic faults, serpentine-carbonate breccias and flat-lying decollements. Parent melts of the Bahia crustal cumulates were more siliceous and hydrous, i.e. more andesitic, than typical mid-ocean ridge basalt (MORB) as deduced from the frequent occurrence of early crystallizing orthopyroxene (opx) and late crystallizing amphibole. Some facies such as cumulate harzburgite and opx-troctolite have not been documented elsewhere in the Oman ophiolite and may be specific to the tectonic context in which the frontal massifs accreted. The chemical composition of the lower crustal cumulates can be accounted for by the hybridization in various proportions between MORB and a primitive andesite from a depleted source whose origin can be looked for in melts from a nascent subduction zone or from high temperature hydrothermal processes. The structure of the Bahia lower crustal section is reminiscent of the plutonic growth faults documented along present-day slow-spreading centres in both mid-ocean ridge and back arc settings. The distinctive characteristics of the Moho and lower crustal section in the Bahia massif are tentatively related to their position at the leading edge of the ophiolite, i.e. closer to the Arabian continental margin at the time of accretion than the massifs from the internal part of the ophiolite that have a more continuous and less deformed lower crust. It indicates that the style of crustal accretion may have changed during the opening of the oceanic basin from which the Oman ophiolite issued.

Studies of the high‐pressure (HP) As Sifah eclogites in the NE Saih Hatat Window, Oman, have used different combinations of radiometric dating results (Ar/Ar, Sm‐Nd vs. U‐Pb, Rb‐Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction—including its relationship to the Samail Ophiolite. To determine the timing of continental subduction, we coupled petrochronological analyses of major (garnet) and accessory phases (zircon and rutile) from the highest grade metamorphic rocks (As Sifah eclogites) in the Saih Hatat. Early Permian (283.8 ± 0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions and metamorphic assemblages consistently suggest a single metamorphic event with garnet growth from ∼81 to 77 Ma—though with slight, consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous interpretations for continental HP metamorphism in a single NE‐dipping subduction zone beneath the obducted Samail Ophiolite; there is no robust evidence for an ∼110 Ma event or for SW‐dipping subduction beneath the Arabian plate. Combined with other constraints, our data suggest that the As Sifah unit was subducted at rates similar to other small continental HP bodies, followed by two stages of cooling and exhumation—likely associated with the dragging to mantle depths by a mafic root, an initial phase of rapid exhumation from mantle depths, and a lengthy residence (≤40 My) in the lower to middle crust.

The end-Triassic mass extinction is one of the big fiveextinction events in Phanerozoic Earth history. It is linked with the emplacement of the Central Atlantic Magmatic Province and a host of interconnected environmental and climatic responses that caused profound deterioration of terrestrial and marine biospheres. Current understanding, however, is hampered by (i) a geographically limited set of localities and data; (ii) incomplete stratigraphic records caused by low relative sea-level in European sections during the Late Triassic and earliest Jurassic; and (iii) major discrepancies in the estimated duration of the latest Triassic Rhaetian that limit spatiotemporal evaluation of climatic and biotic responses locally and globally. Here, we investigate the Late Triassic-Early Jurassic time interval from a stratigraphically well-preserved sedimentary succession deposited in tropical oceanic Panthalassa. We present diverse new data from the lower McCarthy Formation exposed at Grotto Creek (Wrangell Mountains, southern Alaska), including ammonoid, bivalve, hydrozoan, and conodont biostratigraphy; organic carbon isotope (d13Corg) stratigraphy; and CA-ID TIMS zircon U-Pb dates. These data are consistent with a Norian-Rhaetian Boundary (NRB) of similar to 209 Ma, providing new evidence to support a long duration of the Rhaetian. They also constrain the Triassic-Jurassic boundary (TJB) to a similar to 6 m interval in the section. Our TJB delta C-13(org) record from Grotto Creek, in conjunction with previous data, demonstrates consistent features that not only appear correlative on a global scale but also shows local heterogeneities compared to some Tethyan records. Notably, smaller excursions within a large negative carbon isotope excursion [NCIE] known from Tethyan localities are absent in Panthalassan records. This new comparative isotopic record becomes useful for (i) distinguishing regional overprinting of the global signal; (ii) raising questions about the ubiquity of smaller-scale NCIEs across the TJB; and (iii) highlighting the largely unresolved regional vs. global scale of some presumed carbon cycle perturbations. These paleontological and geochemical data establish the Grotto Creek section as an important Upper Triassic to Lower Jurassic succession due to its paleogeographic position and complete marine record. Our record represents the best documentation of the NRB and TJB intervals from Wrangellia, and likely the entire North American Cordillera. (C) 2021 Elsevier B.V. All rights reserved.

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO 2 ‐rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO 2 . Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid‐rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H 2 O and CO 2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front.

Understanding the tectonic setting in which ophiolites form is necessary to determine how they can be used to study ocean spreading and subduction zone processes. Here, we present high‐precision U‐Pb zircon dates and Sm‐Nd isotopic data from two late magmatic series in the Samail ophiolite in Oman and the United Arab Emirates (UAE), which constrain its tectonic development. Volcanic rocks in the ophiolite record a progression from MORB‐like V1 lavas to subduction‐related V2 lavas. Plutonic rocks related to V2 magmatism yielded 206 Pb/ 238 U dates of 95.557 ± 0.063–95.289 ± 0.067 Ma. A second late magmatic series consists of felsic dikes and sills that intermittently intrude the upper mantle and are attributed to melting of a subducting slab. These dikes in Oman range from 95.201 ± 0.032 to 94.95 ± 0.10 Ma, with ε Nd (t) = −5.05 to −1.63. A single dike, attributed to V2 magmatism, has a higher ε Nd (t) = 7.35 and a date of 95.478 ± 0.032 Ma. Similar intrusions in the UAE are younger, ranging from 94.119 ± 0.057 to 90.998 ± 0.052 Ma. Our new and existing data indicate the following timeline of ophiolite formation during subduction initiation: (1) Initial sole metamorphism at ≥96.2 Ma; (2) Formation of the crust through primarily decompression‐related V1 magmatism from 96.1 to 95.6 Ma; (3) V2 magmatism related to H 2 O‐fluxed mantle melting from 95.6 to 95.2 Ma; and (4) Intrusion of slab‐derived felsic dikes from 95.2 to 95.0 Ma. The temporal progression of magmatism is similar to the timescales of subduction initiation predicted by geodynamic models and observed in the Izu‐Bonin‐Mariana forearc.