Fred Jourdan

Pre-Miocene birth of the Yangtze River

The development of fluvial systems in East Asia is closely linked to the evolving topography following India–Eurasia collision. Despite this, the age of the Yangtze River system has been strongly debated, with estimates ranging from 40 to 45 Ma, to a more recent initiation around 2 Ma. Here, we present 40Ar/39Ar ages from basalts interbedded with fluvial sediments from the lower reaches of the Yangtze together with detrital zircon U–Pb ages from sand grains within these sediments. We show that a river containing sediments indistinguishable from the modern river was established before ∼23 Ma. We argue that the connection through the Three Gorges must postdate 36.5 Ma because of evaporite and lacustrine sedimentation in the Jianghan Basin before that time. We propose that the present Yangtze River system formed in response to regional extension throughout eastern China, synchronous with the start of strike–slip tectonism and surface uplift in eastern Tibet and fed by strengthened rains caused by the newly intensified summer monsoon.

The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende ( 40Ar/ 39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ∼56Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ∼0.5GPa at ∼800-950C. The thermal gradient inferred from the metamorphic assemblage (∼60°Ckm -1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ∼53Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ∼56Ma.

Late Oligocene–early Miocene birth of the Taklimakan Desert

Significance The formation of the Taklimakan Desert marked a major geological event in central Asia during the Cenozoic, with far-reaching impacts. Deposition of both eolian sand dunes in the basin center and the genetically equivalent loessite along the basin margins provide evidence for the birth of the Taklimakan Desert. This paper resolves a long-standing debate concerning the age of the Taklimakan Desert, specifically whether it dates to ∼3.4–7 Ma, currently the dominant view. Our result shows that the desert came into existence during late Oligocene–early Miocene, between ∼26.7 Ma and 22.6 Ma, as a result of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan. As the world’s second largest sand sea and one of the most important dust sources to the global aerosol system, the formation of the Taklimakan Desert marks a major environmental event in central Asia during the Cenozoic. Determining when and how the desert formed holds the key to better understanding the tectonic–climatic linkage in this critical region. However, the age of the Taklimakan remains controversial, with the dominant view being from ∼3.4 Ma to ∼7 Ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert. In this study, we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy. We constrained the initial desertification to be late Oligocene to early Miocene, between ∼26.7 Ma and 22.6 Ma. We suggest that the Taklimakan Desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan, which had reached a climatically sensitive threshold at this time.

40Ar/39Ar age of the Lonar crater and consequence for the geochronology of planetary impacts

Asteroid impacts play an important role in the evolution of planetary surfaces. In the inner solar system, the large majority of impacts occur on bodies (e.g., asteroids, the Moon, Mars) covered by primitive igneous rocks. However, most of the impacts recorded on Earth occur on different rock types and are poor proxies for planetary impacts. The Lonar crater is a 1.88-km-diameter, Quaternary age crater (Fig. 1) located on the ca. 66 Ma Deccan basaltic traps in Maharashtra (India), and is one of the very few craters on Earth emplaced directly on basaltic lava flows. We carried out 12 40 Ar/ 39 Ar step-heating experiments on 4 melt rock samples in order to (1) obtain a precise age for the Lonar crater; (2) study the response of isotopic chronometers during impacts on mafic target rocks; and (3) better understand the dating of extraterrestrial impact craters. We obtained 10 plateau and 9 inverse isochron ages on various aliquots. Combination of selected data into a global inverse isochron yielded an age of 570 ± 47 ka (MSWD = 1.1; P = 0.24). In comparison, previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages (ca. 12–62 ka). The measured 40 Ar/ 36 Ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40 Ar* degassing capacity of basaltic impact melt rocks. The 40 Ar/ 36 Ar ratio of 296.5 ± 1.7 is indistinguishable from the atmospheric composition and suggests that inherited 40 Ar* is absent from the melt rock. This result substantiates diffusion models that predict a near-complete degassing of low-viscosity melt (e.g., basalts) during impact, and demonstrates for the first time that inherited 40 Ar* is less problematic for 40 Ar/ 39 Ar dating of impact events in basaltic igneous rocks compared to Si-rich rocks. These results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and, as far as dating is concerned, should be the preferred targets of sample recovery by future missions.

Petrology and geochemistry of calc-alkaline volcanic and subvolcanic rocks, Dalli porphyry copper–gold deposit, Markazi Province, Iran

Early Miocene igneous rocks associated with the Dalli porphyry ore body are exposed within the Urumieh-Dokhtar Magmatic Arc (UDMA). The Dalli porphyry Cu–Au deposit is hosted by subduction-related subvolcanic plutons with chemical composition from diorite to granodiorite, which intruded andesitic and dacitic volcanic rocks and a variety of sedimentary sequences. 40Ar/39Ar age data indicate a minimum emplacement age of ∼21 million years for a potasically altered porphyritic diorite that hosts the porphyry system. The deposit has a proven reserve of 8 million tonnes of rock containing 0.75 g/t Au and 0.5% Cu. Chondrite-normalized rare earth element (REE) patterns for the subvolcanic rocks are characterized by light REE enrichments [(La/Sm) n  = 2.57–6.40] and flat to gently upward-sloping profiles from middle to heavy REEs [(Dy/Yb) n  = 0.99–2.78; (Gd/Yb) n  = 1.37–3.54], with no significant Eu anomalies. These characteristics are generated by the fractionation of amphibole and the suppression of plagioclase crystallization from hydrous calc-alkaline magmas. In normalized multi-element diagrams, all analysed rocks are characterized by enrichments in large ion lithophile elements and depletions in high field strength elements, and display typical features of subduction-related calc-alkaline magmas. We used igneous mineral compositions to constrain the conditions of crystallization and emplacement. Biotite compositions plot above the nickel–nickel oxide (NNO) buffer and close to oxygen fugacity values defined by the hematite–magnetite (HM) buffer, indicating oxidizing conditions during crystallization. Assuming a minimum crystallization temperature of 775°C, the oxygen (fO2) and water (fH2O) fugacities are estimated to be 10−10.3 bars (∼ΔNNO+4) and ≤748 bars, respectively, during the crystallization of biotite phenocrysts. The temperature and pressure conditions, estimated from temperature–corrected Al-in-hornblende barometry and amphibole-plagioclase thermometry, suggest that the hornblende phenocrysts in Dalli rocks crystallized at around 780 ± 20°C and 3.8 ± 0.4 kbar. An alternative method using the calcic amphibole thermobarometer indicates that the Dalli magmas were, on average, characterized by an H2O content of 4.3 wt.%, a relatively high oxygen fugacity of 10−11.0 bars (ΔNNO+1.3), and a hornblende phenocryst crystallization temperature of 880 ± 68°C and pressure of 2.6 ± 1.7 kbar.

Multiple migmatite events and cooling from granulite facies metamorphism within the Famatina arc margin of northwest Argentina

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure-temperature path in kyanite-K-feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ∼12 kbar and 868°C at 461 ±1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One-dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20–30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ∼436 Ma. The second migmatite event was regionally localized and occurred at ∼700°C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike-slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane.

Modelling effect of sericitization of plagioclase on the 40K/40Ar and 40Ar/39Ar chronometers: implication for dating basaltic rocks and mineral deposits

Abstract The 40Ar/39Ar technique is the most commonly used technique to date basaltic rocks. For basaltic rocks older than about 30 Ma, the dating of plagioclase separates is preferred over groundmass as the latter is susceptible to containing cryptic alteration due to fluid circulations, difficult if not impossible to remove during sample preparation. Alteration under such metamorphic conditions progressively forms K-rich sericite after plagioclase. Owing to its transparency, plagioclase allows a distinction to be made optically between partially–completely altered grains and fresh grains. However, practice shows that grains that contain less than about 1% of sericite are hard to identify under the stereomicroscope. Owing to the high K2O content (c. 10 wt%) of sericite, such compromised grains can have dramatic effects on the age determination of plagioclase. Here, we investigate and quantify the effect of sericite on the 40Ar/39Ar age determination of plagioclase using a numerical model with multiple variable parameters. We show that the most influential parameter is the time difference between the crystallization of plagioclase and the sericitization event. We also show that for some continental flood basalts, even 0.1 wt% of sericite can bias the apparent age of a plagioclase separate by several hundred thousand years. The presence of sericite can be identified using a combination of Ca/K ratios, age spectra, and 39Ar and 37Ar degassing curves obtained during a conventional 40Ar/39Ar step-heating procedure. When the age of the fresh plagioclase and its Ca/K ratio are known, the percentage of sericitization and the age of the alteration event can be estimated. Ultimately, above approximately 65% of sericitization, the apparent age measured on the altered plagioclase is within ±1% of the age of the alteration event, with implications for accurately dating low-temperature metamorphism and mineral deposit formations. Supplementary material: Further details of calculation are available at www.geolsoc.org.uk/SUP18609.

Evidence of multi-phase Cretaceous to Quaternary alkaline magmatism on Tore-Madeira Rise and neighbouring seamounts from 40Ar/39Ar ages

Abstract: The Tore–Madeira Rise is a seamount chain located 300 km off the Portugal and Morocco coasts attributed to hotspot activity. U–Pb ages of lavas from the northern and central Tore–Madeira Rise range between 103 and 80.5 Ma whereas 40Ar/39Ar ages from the central and southern Tore–Madeira Rise yield ages ranging from 94.5 to 0.5 Ma. We performed new 40Ar/39Ar measurements to better understand the geodynamic history of the Tore–Madeira Rise. Plagioclase ages from the Bikini Bottom and Torillon seamounts suggest ages of >90 Ma and ≥60 Ma, respectively. Amphiboles from the Seine seamount yield an age of 24.0 ± 0.8 Ma. Biotites from lavas of the Ashton seamount give ages of 97.4 ± 1.1 Ma and 97.8 ± 1.1 Ma. The geochronological database available on the Tore–Madeira Rise has been filtered on statistical criteria to eliminate unreliable ages. The resulting database reveals three pulses of alkaline magmatism on the Tore–Madeira Rise at 103–80.5 Ma, at c. 68 Ma and between 30 Ma and the present. The magmatism was continuous from 103 Ma until c. 68 Ma and from c. 30 Ma until the present on the Tore–Madeira Rise, the surrounding seamounts and the Portugal coast. We suggest that the space–time distribution of this magmatism results from the interaction between a wide thermal anomaly emitting magmatic pulses and the complex motion of the Iberian plate. Supplementary material: A detailed Ar measurements dataset is available at http://www.geolsoc.org.uk/SUP18359.

Neoproterozoic to early Paleozoic extensional and compressional history of East Laurentian margin sequences: The Moine Supergroup, Scottish Caledonides

Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laurentia and are related to rifting associated with the breakout of Laurentia from the supercontinent Rodinia. Detrital zircons from the Moine Supergroup, NW Scotland, yield Archean to early Neoproterozoic U-Pb ages, consistent with derivation from the Grenville-Sveconorwegian orogen and environs and accumulation post–1000 Ma. U-Pb zircon ages for felsic and associated mafi c intrusions confi rm a widespread pulse of extension-related magmatism at around 870 Ma. Pegmatites yielding U-Pb zircon ages between 830 Ma and 745 Ma constrain a series of defor ma tion and metamorphic pulses related to Knoydartian orogenesis of the host Moine rocks. Additional U-Pb zircon and monazite data, and 40 Ar/ 39 Ar ages for pegmatites and host gneisses indicate high-grade metamorphic events at ca. 458–446 Ma and ca. 426 Ma during the Caledonian orogenic cycle. The presence of early Neoproterozoic silici clastic sedimentation and deformation in the Moine and equivalent successions around the North Atlantic and their absence along strike in eastern North America refl ect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with the inferred development of convergent platemargin activity along the periphery of the supercontinent. In contrast in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (ca. 760 Ma) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession.

High-precision dating of the Kalkarindji large igneous province, Australia, and synchrony with the Early-Middle Cambrian (Stage 4-5) extinction

The voluminous Kalkarindji flood basalts erupted in Australia during the Cambrian and covered >2 x 10(6) km(2). New U-Pb and Ar-40/Ar-39 age data from intrusive rocks and lava flows yielded statistically indistinguishable ages at ca. 511 Ma, suggesting a relatively brief emplacement for this province. A zircon age of 510.7 +/- 0.6 Ma shows that this province is temporally indistinguishable at the few-hundred-thousand-year level from the Early-Middle Cambrian (Stage 4-5) boundary age of 510 +/- 1 Ma, which marks the first severe extinction of the Phanerozoic and an extended marine anoxia period. Sulfur concentration measurements ranging from < 50 to 1900 mu g/g, and fractal analysis of extensive explosive volcanic breccias, suggest that blasts and phreatomagmatic explosions have contributed to injection of large amounts of sulfur into the stratosphere. In addition, magma intrusions in oil, gas, and sulfate deposits may have generated significant emission of CH4 and SO2 which, along with volcanic gases, would have combined to cause an oscillation of the climate and led to the Cambrian extinction. (Less)