Zircon Age Constraints Research Articles

Heavy Mineral and Zircon Age Constraints on Provenance of Cenozoic Sandstones in the Gulf of Mexico Subsurface

Combined heavy mineral analysis and detrital zircon geochronology have enabled us to track detritus supplied by the ancestral river systems draining the North American continent into the deep subsurface of the Gulf of Mexico, in both the coastal plain and the offshore deep water areas. During deposition of the Paleocene–Eocene Wilcox Group, sandstones in the western part of the area are interpreted as the products of the Rosita system derived via paleo-Rio Grande material, with a large component of sediment shed from the Western Cordillera. By contrast, samples from wells further east have high proportions of zircons derived from the Yavapai-Mazatzal Province and are attributed to the Rockdale system with sediment fed predominantly by the paleo-Colorado or paleo-Colorado-Brazos. There is evidence that sediment from the Rosita system occasionally extended into the central Gulf of Mexico, and, likewise, data indicate that the Rockdale system sporadically supplied sediment to the western part of the basin. During the Late Eocene of the central Gulf of Mexico (Yegua Formation) there was a distinct shift in provenance. The earlier Yegua sandstones have a large Grenville zircon component and are most likely to have had a paleo-Mississippi origin, whereas the later Yegua sandstones are dominated by zircons of Western Cordilleran origin, similar to Wilcox sandstones fed by the Rosita system via the paleo-Rio Grande. The switch from paleo-Mississippi to paleo-Rio Grande sourcing implies there was a major reorganisation of drainage patterns during the Late Eocene. Miocene sandstones in the deepwater Gulf of Mexico were principally sourced from the paleo-Mississippi, although the paleo-Red River is inferred to have contributed to the more westerly-located wells.

Stratigraphic and U-Pb zircon age constraints on the timing of the Yanliao Biota in northern China

The Yanliao Biota from northern China is one of the most famous Mesozoic terrestrial lagerstätten in the world, with well-preserved fossil records in the Jurassic volcanic-sedimentary Ningcheng and Jianchang basins. However, the temporal evolution of the Yanliao Biota remains controversial, mainly due to the confusing stratigraphic framework and the unclear age of fossiliferous deposits in the Ningcheng basin. To address this issue, we carried out detailed field investigations and secondary ion mass spectrometry (SIMS) U-Pb age dating of zircons on fossiliferous and related strata in the southern part of the Ningcheng basin. The Jurassic infill of this area is composed of, in ascending order, fossil-bearing tuffaceous clastic rocks (unit 1), andesite and pyroclastic rocks (unit 2), fossil-bearing clastic rocks interlayered with minor volcanic rocks (unit 3), and andesite and pyroclastic rocks (unit 4). Integration of stratigraphic and geochronological data indicates that the Yanliao Biota in the Ningcheng basin appeared no later than ca. 163 Ma and lasted until ca. 156 Ma. A comparison with other analogous basins in northern China (e.g., the Luanping, Jianchang, and Beipiao basins) leads to the conclusion that the Yanliao Biota evolved in two phases and lasted for more than 11 m.y.: the first phase from ca. 167 Ma to ca. 163 Ma in the middle Bathonian to middle Callovian, followed by the second phase between ca. 161 Ma and ca. 156 Ma in the lower and middle Oxfordian.

Detrital zircon age constraints on low-grade sedimentary successions of the eastern Circum-Rhodope Belt, Bulgaria

Detrital zircon age constraints on low-grade sedimentary successions of the eastern Circum-Rhodope Belt, Bulgaria

Geochemical and detrital zircon age constraints on Meso- to Neoproterozoic sedimentary basins in the southern Yangtze Block: Implications on Proterozoic geodynamics of South China and Rodinia configuration

The late Mesoproterozoic to early Neoproterozoic sedimentary sequences in the southwestern Yangtze Block, represented by the Julin, Huili, and Kunyang groups, provide vital evidence for deciphering the tectonic evolution of the Yangtze Block that have implications on the reconstruction of the Rodinia supercontinent. Zircon geochronological data reveal that the Julin and Huili groups were deposited at 1167–982 Ma and 1176–958 Ma, respectively. The Haizishao Formation, the conventionally thought the upper most of the Julin Group, was deposited after 982 Ma. The stratigraphicphic position of the Haizishao Formation basal conglomerate and diversity in its detritus source, and tectonic setting, suggest that the Haizishao Formation may not be a part of the Julin Group but represents an independent sedimentary event. Geochemical tectonic discrimination characteristics based on major elements, Th-Sc-Zr/10, Ti/Zr-La/Sc, La/Th-Hf and Sc/Th-La/Sc plots, in combination with detrital zircon age distribution pattern and bimodal magmatism, point toward a transition from continental rift to passive continental margin setting followed by an active continental margin setting for Julin and Huili groups. However, the Huili Group is significantly different from both the Julin and Kunyang groups in terms of lithological composition, depositional environment, detrital zircon age distribution, and tectonic setting, implying that the Julin and Huili basins may have been located in different microcontinents during the late Mesoproterozoic - early Neoproterozoic times.Detrital zircon U-Pb ages and Lu-Hf isotopic characteristics of Julin and Huili groups indicate detritus sourced from the continental interior and exotic terranes, such as the Yudongzi, Douling, and Kongling complexes and magmatic rocks at the southwestern margin of Yangtze Block, the Gawler Craton in Australia, the Transantarctic Mountains in East Antarctica, and the Ongole domain in the Eastern Dharwar Craton of India. Combining provenance analyses and comparison of detrital zircon data, the Yangtze Block was paleogeographically placed in the external position to the Rodinia supercontinent, close to north-central India, Australia, and East Antarctica during the late Mesoproterozoic to early Neoproterozoic.

U-Pb zircon age constraints on the vertebrate assemblages and palaeomagnetic record of the Guadalupian Abrahamskraal Formation, Karoo Basin, South Africa

The rich fossil record of the Permo-Triassic Beaufort Group of the main Karoo Basin serves as a reference sequence for tetrapod biochronology and provides a unique window into the evolution of terrestrial ecosystems between the Guadalupian and Middle Triassic. Absolute temporal constraints on the Beaufort Group have improved in the last decade with the application of U-Pb zircon geochronology but precise ages have not yet been reported for its basal Abrahamskraal Formation. Consequently, the age of the Eodicynodon Assemblage Zone and the lower boundary of the Tapinocephalus Assemblage Zone have remained less clear than younger Permian biozones. An improvement in temporal constraints also has implications for the correlation of palaeomagnetic logs reported from the Abrahamskraal Formation. In this paper we present new CA-ID-TIMS U-Pb geochronology from tuff beds that provide the first high-resolution chronostratigraphy for the upper two thirds of Abrahamskraal Formation. These ages constrain the base of the Tapinocephalus Assemblage Zone to be older than 264.4 Ma, and closely constrain the boundary between its two subzones to ca. 262 Ma. Combined with published ages from the overlying Teekloof Formation, our data show the Tapinocephalus Assemblage Zone is latest Wordian to late Capitanian in age. These complement previously published CA-ID-TIMS U-Pb ages for younger Beaufort rocks and suggest that the normal polarity interval observed in the middle Abrahamskraal Formation represents a normal (probably third) chron within the mixed-polarity Illawarra Superchron.

Detrital zircon age constraints for Late Permian to Late Triassic clastic sedimentation in the northern-western Sakar-Strandzha Zone, SE Bulgaria

Detrital zircon age constraints for Late Permian to Late Triassic clastic sedimentation in the northern-western Sakar-Strandzha Zone, SE Bulgaria

Detrital Zircon Age Constraints on the Evolution of Paleo‐Tethys in NE Iran: Implications for Subduction and Collision Tectonics

AbstractThe timings of the onset of oceanic spreading, subduction and collision are crucial in plate reconstructions, but not always straightforward to resolve. The Paleo‐Tethys evolution dominated the Paleozoic‐Early Mesozoic tectonics of West Asia, but the timeline of events is still poorly constrained. We present new detrital zircon ages from the Binalud Mountains of NE Iran, in order to determine the timing of tectonic events in the region, and the wider implications for regional tectonics, paleogeography and climate change. Consistent age patterns in Paleozoic clastic rocks show a dominant provenance from the Neoproterozoic basement of northern Gondwana. We interpret this as deposition on a long‐lasting passive continental margin after the initial spreading of the Paleo‐Tethys Ocean. Central Iran‐Eurasia collision caused coarse clastic deposition of the Mashhad Phyllite in a peripheral foreland basin on the Paleozoic sequence of the northern Central Iran Block. The Mashhad Phyllite yields age clusters at 450–250 Ma and 1,900–1,800 Ma, with a clear provenance from a long‐lived continental arc at the active Eurasian margin since the latest Ordovician. Analysis of the age spectra allows us to constrain the maximum depositional age of the Mashhad Phyllite as no later than 228 Ma. This age provides a tight constraint on the timing of initial collision between Central Iran and Eurasia, which also coincides with the Carnian Pluvial Event (CPE). Our new timing evidence of collision reinforces the previous speculation on the Paleo‐Tethys closure as a causal mechanism for the CPE.

A high-precision U–Pb zircon age constraints the timing of the faunistic and palynofloristic events of the Carnian Ischigualasto Formation, San Juan, Argentina

The Late Triassic represents records many of the most critical biotic crises in life-history (from Carnian to the end-Triassic extinction, ETE). Some hypotheses propose that these life changes were linked with shifts in the environmental setting during the initial stages of the Pangea breakup. These crises were likely not sudden events but a series of step-changes that reached their peak at the end of the Triassic. The Carnian-Norian Ischigualasto Formation is a world-renown terrestrial sequence that records both the Triassic Dinosaur Diversification Event (DDE) and at least one crucial terrestrial environmental and biotic turnover. A notable increase in humidity and volcanism was the setting of critical faunal replacement represented by the passage of the Hyperodapedon-Exaeretodon-Herrerasaurus to Exaeretodon tetrapod biozones. That environmental modification, together with the paleofaunistic turnover, has been accurately constrained by a new high-precision CA-TIMS U/Pb zircon age of 228.91 ± 0.14 Ma obtained from a tuff layer located in the exact stratigraphic position of the major biotic-paleoenvironmental disturbance in the middle part of the Valle de la Luna Member, at ~310 m from the base of the Ischigualasto Formation. Palynological assemblages containing the first records in westernmost Gondwana of European Tethyan and Onslow palynological species are also recognized close to the dated tuff level. Thus, this age is the first isotopic record for the Onslow Microflora diagnostic species in Gondwana. Moreover, the CA-TIMS date promotes a discussion about the accurate chronostratigraphic position for the beginning (after the Carnian Pluvial Event) of a series of environmental-biological disturbances that culminated at the end of the Triassic.

Detrital zircon age constraints on depositional history and provenance of the Murihiku Supergroup, Murihiku Terrane, North Island, New Zealand

In the Murihiku Terrane of New Zealand, U-Pb detrital zircon ages in Murihiku Supergroup sandstones of Late Triassic, Jurassic and possibly earliest Cretaceous age have a marked youngest age component that is close to, and sometimes coincident with, established biostratigraphic ages, thus reflecting contemporary volcanism. However, youngest Huriwai Group samples yield 137–142 Ma zircon age components (earliest Early Cretaceous) in conflict with palynofloras that suggest only a latest Jurassic age. This is resolved if the age of the Jurassic/Cretaceous boundary is lowered to ca. 140 Ma. Older, reworked zircons are mainly Early Jurassic, Late Triassic and Late Permian reflecting an enduring exhumed magmatic arc source nearby. This might be in the adjacent Median Batholith but as a Murihiku sediment source its Jurassic, Triassic and Permian elements are not well-matched in terms of extent, age and bulk compositions. A connection between the Murihiku (proximal forearc) and Waipapa Composite (distal accretionary wedge) terranes is probable, with a common magmatic arc, speculatively situated in the New England Orogen, eastern Australia.

Geological record of Paleoarchean oceanic flake tectonics preserved in the c. 3.3 Ga Kromberg volcanic type-section, Barberton greenstone belt, South Africa

The geology and existing geodynamic models for the c. 3.3 Kromberg mafic–ultramafic sequence of rocks in the Barberton greenstone belt (South Africa) have been the subject of intense debate as, in addition to their regional significance, they have bearing also upon possible Paleoarchean tectonic processes on the early Earth. Models for the origin of the Kromberg sequence of rocks range from formation in an autochthonous, plume-type setting to that of an allochthonous, ophiolite thrust sheet emplaced in a modern-style plate-tectonic regime. This study reports new high-resolution field mapping results, drill core observations and secondary ion microprobe U-Pb detrital zircon age constraints across the Kromberg type-section on both the eastern and western sides of the Komati River. The new observations indicate that the Kromberg type-section represents a ca. 1.65 km thick dismembered and variably silicified mélange of oceanic supracrustal rocks with minor metadunites, which are structurally bound by chlorite-carbonate-quartz shear zones. Field observations and microstructural analysis allow for the rheological behaviour of the Paleoarchean oceanic lithosphere to be explored. It is proposed herein that the observed structural and volcanic geology is compatible with a Paleoarchean oceanic flake tectonic process, involving detachment along a brittle-ductile transition zone and horizontal accretion of thin, rigid, silicified slivers of upper Archean oceanic crust. These accreted allochtonous oceanic fragments would have delaminated from thick, weaker, lower oceanic crust that was recycled into the hot Paleoarchean mantle, and thus removed from the geological record. The model invoked here suggests that, although the well-preserved mafic–ultramafic Kromberg volcanic sequence may share some significant similarities with modern Phanerozoic ophiolites in the field, it most likely did not form by modern-style plate-tectonic processes. The new field observations herein support highly elevated oceanic geothermal gradients at the time due to a hotter, more vigorously convecting, Paleoarchean mantle and crustal geodynamic processes unique to the Paleoarchean era of Earth history in the absence of a global-scale, fully developed plate-tectonic regime.

U-Pb zircon age constraints on the earliest eruptions of the Deccan Large Igneous Province, Malwa Plateau, India

Climate instability driven by emission of volatiles during emplacement of large igneous provinces (LIPs) is frequently invoked as a potential cause of mass extinctions. However, documenting this process in the geologic record requires a holistic understanding of eruption rates, the location of eruptive centers, and potential sources of climate-changing volatiles. We present new chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology from Malwa Plateau basalts on the northern margin of the Deccan LIP, India. These basalts have been previously interpreted as either an extension of the province's main volcanic stratigraphy or as an independent eruptive center active up to millions of years prior to the main eruptive phase. Our data instead demonstrate that the lower Malwa Plateau basalts are temporally correlative with the first pulse of Deccan volcanism and provide new constraints on its initiation and duration. Paleomagnetic data further indicate that upper Malwa Plateau basalts may be age-equivalent to the second, third, and fourth pulses of Deccan volcanism. The relative thicknesses of age-equivalent packages of basalt are consistent with eruption of the Deccan LIP from a southward-migrating eruptive center. The first eruptive pulse is coeval with a ∼200 kyr Late Maastrichtian warming event preserved globally in contemporaneous stratigraphic sections. We propose that the first pulse of Deccan magmatism was more voluminous in the north, where it erupted through organic-rich sedimentary rocks of the Narmada-Tapti rift basin. Thermal metamorphism of these sediments could have been a source of sufficient CO2 to drive the Late Maastrichtian warming event, which if true would reconcile the apparent dampened warming signals associated with later Deccan eruptive pulses.

High-precision U-Pb zircon age constraints on the Guadalupian in West Texas, USA

The Guadalupian Epoch was characterized by major changes in paleogeography, paleoclimate, and biodiversity. Yet, the paucity of precise and accurate radioisotopic dates from the Guadalupian stages in their type area, Guadalupe Mountains National Park in West Texas has rendered their calibration inadequate. In this study, we report high-precision U-Pb zircon geochronology by the CA-ID-TIMS method from three ash beds (2σ internal errors only) in the Rader Member of the Bell Canyon Formation at the Back Ridge Section (262.127 ± 0.097 Ma, MSWD = 0.89, n = 3), the lower Pinery Member of the Bell Canyon Formation at the Frijole Section (264.23 ± 0.13 Ma, MSWD = 0.89, n = 8) and the basal South Wells Member of the Cherry Canyon Formation at the Monolith Canyon Section (266.525 ± 0.078 Ma, MSWD = 0.62, n = 5). The Bayesian interpolation statistics method is used to establish an age-stratigraphy model that estimates the base of the Capitanian to be 264.28 ± 0.16 Ma, serving as the best age estimate for the Capitanian Stage at present. In addition, we review the existing geochronology from the Guadalupian Series in West Texas and seek to propose more precise temporal estimates of Guadalupian geological and biological events. These data constrain the high-frequency sequences of the Cherry Canyon and Bell Canyon formations in the Guadalupe Mountains National Park area. Accordingly, the base of the Wordian is estimated at 266.9 ± 0.4 Ma and the Illawarra geomagnetic polarity reversal in West Texas at 267.4 ± 0.4 Ma to 266.5 ± 0.3 Ma. The global end-Guadalupian extinction began in the conodont zone of Jinogondolella altudaensis above the base of the Reef Trail Member of the Bell Canyon Formation and might continue to the Clarkina postbitteri postbitteri Zone in the earliest Wuchiapingian. The conodonts display a rapid evolutionary rate during this interval. This constrains the biotic crisis from ca. 260 Ma to 259 Ma based on our conodont age estimation. The emplacement of the Emeishan Large Igneous Province (ELIP) in South China has been constrained to ca. 260 Ma to 257.4 Ma based on zircon U-Pb geochronology by the CA-ID-TIMS method, overlapping with the end-Guadalupian extinction, which provides support for the temporal relationship between them. Additionally, the ELIP persisted into the early Wuchiapingian and may have hampered ecosystem restoration during the post-extinction interval.

Detrital zircon age and Sr isotopic constraints for a Late Palaeozoic carbonate platform in the lower Rhodope thrust system, Pirin, SW Bulgaria

AbstractWe focused on the Pirin–Pangeon–Thasos carbonate sequence of the Rhodope thrust system, combining Sr isotopes from marble with U–Pb dating of detrital zircons from interlayered schists with outcrop near the villages of Ilindentsi and Petrovo in Bulgaria. The youngest zircon age at Ilindentsi is 266 Ma, i.e. Middle Permian, while the youngest zircon at Petrovo yielded an age of 290 Ma, i.e. Early Permian. Strontium isotopes range from 0.707420 to 0.707653, and are consistent with a Middle Permian maximum depositional age. Middle Permian sedimentation of this carbonate platform most likely occurred along the Eurasian margin rather than the Gondwana margin.

Coupled stratigraphic and U-Pb zircon age constraints on the late Paleozoic icehouse-to-greenhouse turnover in south-central Gondwana

Abstract The demise of the Late Paleozoic Ice Age has been hypothesized as diachronous, occurring first in western South America and progressing eastward across Africa and culminating in Australia over an ∼60 m.y. period, suggesting tectonic forcing mechanisms that operate on time scales of 106 yr or longer. We test this diachronous deglaciation hypothesis for southwestern and south-central Gondwana with new single crystal U-Pb zircon chemical abrasion thermal ionizing mass spectrometry (CA-TIMS) ages from volcaniclastic deposits in the Paraná (Brazil) and Karoo (South Africa) Basins that span the terminal deglaciation through the early postglacial period. Intrabasinal stratigraphic correlations permitted by the new high-resolution radioisotope ages indicate that deglaciation across the S to SE Paraná Basin was synchronous, with glaciation constrained to the Carboniferous. Cross-basin correlation reveals two additional glacial-deglacial cycles in the Karoo Basin after the terminal deglaciation in the Paraná Basin. South African glaciations were penecontemporaneous (within U-Pb age uncertainties) with third-order sequence boundaries (i.e., inferred base-level falls) in the Paraná Basin. Synchroneity between early Permian glacial-deglacial events in southwestern to south-central Gondwana and pCO2 fluctuations suggest a primary CO2 control on ice thresholds. The occurrence of renewed glaciation in the Karoo Basin, after terminal deglaciation in the Paraná Basin, reflects the secondary influences of regional paleogeography, topography, and moisture sources.

Tectonics of the Paleoproterozoic Jiao-Liao-Ji orogenic belt in the Liaodong peninsula, North China Craton: A review

A global collisional orogenic event that formed the Columbia supercontinent occurred during 2100–1800 Ma. In the eastern North China Craton (NCC), the Paleoproterozoic Jiao-Liao-Ji orogenic belt (JLJOB), composed of metamorphosed volcanic-sedimentary rocks and intrusions, may have preserved records of the collisional event. Tectonics of the JLJOB thus becomes the key to the understanding of the tectonic evolution of the NCC, and may provide important information on assembly of the Columbia supercontinent. Two classical models, i.e. the rift closure model and arc-continent collision model, have been widely cited for the tectonic evolution of the JLJOB. The former is based on occurrence of A-type granites and bimodal volcanics within the orogenic belt, and comparable Archean blocks to the south and north of it, while the latter is supported by occurrence of calc-alkaline volcanics, I-type granites, paired metamorphic belts and detrital zircon age constraints. Integration of literature results and our data, we suggest that the JLJOB was formed by closure of a back-arc basin at an active continental margin. Tectonic evolution of the JLJOB is subdivided into four stages, i.e., pre-orogenic stage at 2200–2060 Ma, transitional stage at 2060–1940 Ma, orogenic stage at 1940–1860 Ma and post-orogenic stage after 1860 Ma.

Detrital zircon age and provenance constraints on late Paleozoic ice-sheet growth and dynamics in Western and Central Australia

U–Pb dating and Hf-isotope provenance analysis of detrital zircons from the glaciogenic lower Permian Grant Group of the Canning Basin indicate sources principally from basement terranes in central Australia, with subordinate components from terranes to the south and north. Integrating these data with field outcrop and subsurface evidence for ice sheets, including glacial valleys and striated pavements along the southern and northern margins of the basin, suggests that continental ice sheets extended over several Precambrian upland areas of western and central Australia during the late Paleozoic ice age (LPIA). The youngest zircons constrain the maximum age for contemporaneous ice sheet development to the late Carboniferous (Kasimovian), whereas palynology provides a minimum age of early Permian (Asselian–Sakmarian). Considering the palynological age of the Grant Group within the context of regional and global climate proxies, the main phase of continental ice sheet growth was possibly in the Ghzelian–Asselian. The presence of ice sheets older than Kasimovian in western and central Australia remains difficult to prove given a regional gap in deposition possibly covering the mid-Bashkirian to early Ghzelian within the main depocentres and even larger along basin margins, and the poor evidence for older Carboniferous glacial facies. There is also no evidence for extensive glacial facies younger than mid-Sakmarian in this region as opposed to eastern Australia where the youngest regional glacial phase was Guadalupian.

Mantle derived crystal-poor rhyolitic ignimbrites: Eruptive mechanism from geochemical and geochronological data of the Piedra Parada caldera, Southern Argentina

Trace elements, isotopic modeling and U-Th-Pb SHRIMP zircon age constraints are used to reconstruct the eruption history and magmatic processes of the Piedra Parada Caldera. In the early Eocene, the crystal-poor Barda Colorada ignimbrite (BCI), having >>15% micro-porphyritic crystals with respect to magmatic components, erupted a volume estimated in more than 300 km3. The Piedra Parada caldera is located in the Patagonian Andes foreland, at the southern end of the calderas field of the Pilcaniyeu Volcanic Belt (PVB). This belt is related to an extensional tectonic setting as a result of the collision of the Farallon-Aluk ridge with South America, which enabled the development of a transform ocean/continental plate margin followed by the detachment of the Aluk plate and the opening of a slab window. The BCI extra-caldera Plateau is a >100 m thick deposit, having a lower unit with high silica (SiO2 > 76 wt.%), potassium poor rhyolitic composition (trondhjemitic like magma), and an upper unit with normal to high potassium rhyolitic composition (granitic like magma). A trace elements modeling of the BCI units shows that the BCI lower and upper units did not evolve from fractionation or immiscibility in the shallow magma reservoir. The BCI also have a primitive isotopic signature (initial 87Sr/86Sr = 0.7031–0.7049 and εNd = +3.4 to +3.65). Thus, tectonic, compositional and isotopic constraints suggest the fast ascent of high silica magmas to a shallow reservoir, and point to an upper mantle origin for these rhyolitic magmas in a transitional (Orogenic-Anorogenic) tectono-magmatic setting. U-Th-Pb SHRIMP zircon crystallization ages of the Syn-caldera stage BCI units (56–51.5 Ma) show a protracted life of 5 Ma for this caldera reservoir. The age of 52.9 ± 0.3 Ma is considered the best fit for the possible maximum age for the caldera collapse. The Late-caldera magmatism has trachyandesitic and rhyolitic compositions. The trace element modeling suggests that these rhyolites evolve from the trachyandesites and do not evolve from the BCI residual magma. The trachyandesites have U-Th-Pb SHRIMP zircon crystallization ages of 52 ± 1 Ma, suggesting that the caldera eruption was triggered by the arrival of the trachyandesitic magma.

Tholeiitic to calc-alkaline metavolcanic transition in the Archean Nigerlikasik Supracrustal Belt, SW Greenland

We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and metamorphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929±5Ma, which is slightly older than a protolith intrusion age of 2902±4Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO2 and Al2O3, coupled with a decrease in MgO, total iron (FeOT) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeOT, low Al2O3/TiO2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeOT, high Al2O3/TiO2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb∗) and LREE-enrichment (La/Sm)N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende+garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.

U–Pb zircon age constraints for the Ordovician Fishguard Volcanic Group and further evidence for the provenance of the Stonehenge bluestones

New U–Pb zircon ages from rhyolite samples of the Fishguard Volcanic Group, SW Wales, confirm a Middle Ordovician (Darriwilian) age for the group. One of the samples is from Craig Rhos-y-felin, which has recently been identified on petrological and geochemical grounds as the source of much of the debitage (struck flakes) at Stonehenge. Analysis of a Stonehenge rhyolite fragment yields an age comparable with that of the Craig Rhos-y-felin sample. Another Stonehenge fragment, thought to come from orthostat (standing stone) 48 and on petrographical grounds to be derived from the Fishguard Volcanic Group (but not Craig Rhos-y-felin), yields an age also consistent with a Fishguard Volcanic Group source. Supplementary material: Details of analytical methods and a table of data are available at https://doi.org/10.6084/m9.figshare.c.3518175 .

Biostratigraphic and detrital zircon age constraints on the basement of the Himalayan Foreland Basin: Implications for a Proterozoic link to the Lesser Himalaya and cratonic India

AbstractThe Himalayan Foreland Basin in the Ganga Valley is key to assessing the pre‐collision relationship between cratonic India and the Himalaya – the world's largest mountain chain. The subsurface Ganga Supergroup, representing the sedimentary basement of the Ganga Valley, has been interpreted as a northern extension of the Proterozoic Vindhyan Supergroup in cratonic India. This interpretation is contentious because the depositional age of the Ganga Supergroup is not resolved: whereas the lower Ganga Supergroup is widely regarded as Proterozoic, the upper Ganga Supergroup has been variously inferred to include Neoproterozoic, lower Palaeozoic, or Cretaceous strata. Here, we integrate biostratigraphic and detrital zircon data from drill cores to show that the entire Ganga Supergroup is likely Proterozoic and can be correlated with Proterozoic successions on the northern Indian craton and in the Lesser Himalaya. This helps redefine the first‐order stratigraphic architecture and indicates broad depositional continuity along the northern Indian margin during the Proterozoic.