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The Triassic was a crucial period in the tectonic evolution of the South China Block. Research on tectonic deformation during this period provides information on intracontinental orogenic mechanisms in South China. In this study, alongside thermochronological analyses, we examine the macroscopic and microscopic structural features of the Rongxian ductile shear zone, located south of the Darongshan granite in the southeastern part of Guangxi Province, on the southern margin of South China. Sinistral shear is indicated by the characteristics of rotated σ‐type feldspar porphyroclasts, stretching lineations defined by elongated quartz grains and the orientations of quartz c‐axes. LA‐ICP‐MS U‐Pb dating of zircons from two samples of granitic mylonite and one of granite yielded ages of ca. 256 Ma. Furthermore, two samples of granitic mylonite yield muscovite 40Ar/39Ar plateau ages of 249–246 Ma. These results indicate that the Rongxian ductile shear zone resulted from Early Triassic deformation of the late Permian Darongshan granite. This deformation was likely related to the closure of the eastern Paleo‐Tethys Ocean and the subsequent collision of the South China and Indochina blocks, during the early stage of the Indosinian orogeny. [ABSTRACT FROM AUTHOR]

The Tatun Volcano Group (TVG) is located at the northern end of Taiwan, ~ 15 km from the center of Taipei, a metropolitan area of over seven million inhabitants. A volcanic eruption by the TVG of any type or scale could cause catastrophic human and economic impacts. This paper summarizes previous geochemical, geophysical, and geochronological reports and highlights why the widely accepted age model does not comport with the latest observations. This study also reports novel 40Ar/39Ar ages for two andesite samples and one basalt sample from the TVG. A sample collected from Chihsingshan yields a robust 40Ar/39Ar age of 0.081 ± 0.005 Ma. This provides the first direct evidence of TVG volcanic activity after 0.1 Ma. Two samples yield 0.28 ± 0.02 Ma for Tatunshan and 0.159 ± 0.017 Ma for Honglushan. The younger ages refute previously proposed age models for the TVG. Along with new drone photos and LiDAR images, the age data help resolve eruptive history and advance understanding of volcanic hazards and hazard mitigation in Taiwan and surrounding areas. [ABSTRACT FROM AUTHOR]

Abstract The Tatun Volcano Group (TVG) is located at the northern end of Taiwan, ~ 15 km from the center of Taipei, a metropolitan area of over seven million inhabitants. A volcanic eruption by the TVG of any type or scale could cause catastrophic human and economic impacts. This paper summarizes previous geochemical, geophysical, and geochronological reports and highlights why the widely accepted age model does not comport with the latest observations. This study also reports novel 40Ar/39Ar ages for two andesite samples and one basalt sample from the TVG. A sample collected from Chihsingshan yields a robust 40Ar/39Ar age of 0.081 ± 0.005 Ma. This provides the first direct evidence of TVG volcanic activity after 0.1 Ma. Two samples yield 0.28 ± 0.02 Ma for Tatunshan and 0.159 ± 0.017 Ma for Honglushan. The younger ages refute previously proposed age models for the TVG. Along with new drone photos and LiDAR images, the age data help resolve eruptive history and advance understanding of volcanic hazards and hazard mitigation in Taiwan and surrounding areas.

The Guodian Fe deposit is representative of the newly discovered Qihe–Yucheng high-grade Fe skarn ore cluster, Luxi Block, eastern North China Craton (NCC). The age of the Pandian Fe deposit remains elusive, which hinders the understanding of its metallogenic tectonic background. Phlogopites are recognized in syn-ore stages, and they are closely associated with magnetite in the Guodian skarn Fe deposit. Here, we carried out 40Ar/39Ar dating of phlogopite, which can place a tight constraint on the timing of Guodian iron mineralization and shed light on the geodynamic framework under which the Guodian Fe deposit formed. Ore-related phlogopite 40Ar/39Ar dating yielded 40Ar/39Ar plateau ages of 131.6 ± 1.7 Ma at 890–1400 °C, with the corresponding isochron age being 131.1 ± 2.6 Ma. These two ages are consistent within the error, indicating that they can represent the formation age of the Guodian iron deposit. The mineralization age overlaps the zircon U-Pb age of 124.4 Ma for ore-related Pandian pluton. This age consistency confirms that the iron skarn mineralization is temporally and likely genetically related to Pandian diorite. The present results, coupled with existing isotopic age data, indicate the Guodian skarn Fe deposit formed contemporaneously with large-scale skarn iron mineralization over the Luxi Block in the Late Mesozoic. The available data demonstrated that the eastern NCC was "destructed" in the Late Mesozoic, as marked by voluminous igneous rocks, faulted-basin formation, high crustal heat flow, and widespread metamorphic core complexes in the eastern part of the NCC. It is thus suggested that the Guodian Fe skarn deposits, together with other deposits of similar ages in the Luxi Block and even in the eastern NCC, were products of this craton destruction. Lithospheric extension and extensive magmatism related to the craton destruction may have provided sufficient heat energy, fluid, chlorine, and Fe for the formation of the Fe deposit. [ABSTRACT FROM AUTHOR]

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The Lower Palaeolithic site of Cimitero di Atella is located in the Basilicata region (Southern Italy), about 10 km south of the extinct Monte Vulture volcano. The site was discovered in the early 1990s and was continuously excavated for nearly twenty years under the supervision of Professor E. Borzatti von Löwenstern (University of Florence). This open-air site contained a 5-m-thick fluvio-lacustrine sequence characterized by the occurrence of two main archaeological units with lithic industries and faunal remains. Based on the composition of the lithic assemblages, and in particular the presence of handaxes in the Lower unit, Borzatti von Löwenstern (et al., 1997) attributed the site to the Early Acheulean. Cimitero di Atella was interpreted as the result of various lake shore occupations linked to the exploitation of large mammals (Palaeoloxodon antiquus and Bison sp.) and the opportunistic use of raw materials to produce simple small and large lithic tools (Borzatti von Löwenstern et al., 1997). The purpose of this paper is to present the last five years of research at the site. Geochronological, palaeontological and palynological investigations have been conducted to shed light on the environmental and chronological context of the site and the associated human occupations levels. This essential step enables us to evaluate the nature and potential of archaeological data in order to place the results in a broader framework. The accurate study of the context and of the archaeological material of Cimitero di Atella enable us to rethink the crucial question of the emergence of the bifacial phenomenon in Europe. [ABSTRACT FROM AUTHOR]

The Guodian Fe deposit is representative of the newly discovered Qihe–Yucheng high-grade Fe skarn ore cluster, Luxi Block, eastern North China Craton (NCC). The age of the Pandian Fe deposit remains elusive, which hinders the understanding of its metallogenic tectonic background. Phlogopites are recognized in syn-ore stages, and they are closely associated with magnetite in the Guodian skarn Fe deposit. Here, we carried out 40Ar/39Ar dating of phlogopite, which can place a tight constraint on the timing of Guodian iron mineralization and shed light on the geodynamic framework under which the Guodian Fe deposit formed. Ore-related phlogopite 40Ar/39Ar dating yielded 40Ar/39Ar plateau ages of 131.6 ± 1.7 Ma at 890–1400 °C, with the corresponding isochron age being 131.1 ± 2.6 Ma. These two ages are consistent within the error, indicating that they can represent the formation age of the Guodian iron deposit. The mineralization age overlaps the zircon U-Pb age of 124.4 Ma for ore-related Pandian pluton. This age consistency confirms that the iron skarn mineralization is temporally and likely genetically related to Pandian diorite. The present results, coupled with existing isotopic age data, indicate the Guodian skarn Fe deposit formed contemporaneously with large-scale skarn iron mineralization over the Luxi Block in the Late Mesozoic. The available data demonstrated that the eastern NCC was “destructed” in the Late Mesozoic, as marked by voluminous igneous rocks, faulted-basin formation, high crustal heat flow, and widespread metamorphic core complexes in the eastern part of the NCC. It is thus suggested that the Guodian Fe skarn deposits, together with other deposits of similar ages in the Luxi Block and even in the eastern NCC, were products of this craton destruction. Lithospheric extension and extensive magmatism related to the craton destruction may have provided sufficient heat energy, fluid, chlorine, and Fe for the formation of the Fe deposit.

During Cretaceous–Eocene time, North America was involved in a succession of shortening events that produced major orogenic belts. Previous work permitted the characterization of the kinematics and dynamics of some of these orogenic events along various transects. However, the timing of shortening is not well constrained everywhere. This has led to misinterpretation of the tectonic history of some sectors of the North American Pacific margin and to an incomplete understanding of the subduction dynamics. Mexico is one of those sectors in which the shortening history is not completely defined. We present structural and 40Ar/39Ar isotopic data from major shortening structures in southern Mexico, documenting two episodes of shortening: a first one at ~ 118–112 Ma, not considered in previous works, and a second one at ~ 104–90 Ma, which was previously interpreted as the record of shortening initiation. Integrated with previous data, our 40Ar/39Ar results indicate that, in Mexico, the shortening history was developed in two main stages: 1) a Late Aptian–Early Albian stage of localized shortening, during which the oceanic Arperos Basin was closed and the Guerrero terrane arc accreted to nuclear Mexico forming a ~ 80 km-wide suture belt, and 2) a latest Albian–Eocene stage, in which shortening was propagated from the North American trench to the plate interior, producing the development of a critically tapered wedge. Whether these two stages represent two superposed orogenic cycles or two steps within the same orogenic cycle remains to be established. In any case, the Late Aptian–Early Albian stage of deformation, which was not fully recognized in most previous works, is a key for our understanding of the causes that triggered shortening in southern North America after ~ 100 m.y. of extension associated with Pangea break-up [ABSTRACT FROM AUTHOR]

Various interference reactions producing unwanted Ar isotopes from K, Ca, Cl and Ar require correction to satisfy the 40Ar/39Ar age equation. Using GEANT4, we design and build a model Cadmium Lined In Core Irradiation Tube (CLICIT) irradiation facility, as used in the Oregon State TRIGA Reactor (OSTR). We illustrate the complexity of the irradiation of geologic samples within this framework and determine an overlooked production channel of 36Ar. The production of 36Ar is fed from the 39K(n, α)36Cl nuclear channel, 36Cl subsequently decays to 36Ar (39K(n, α , β) 36Ar). Simulations in this work using a 235U fission neutron energy spectrum and modelled CLICIT facility, determine a production ratio for this reaction (36Cl/39Ar) K = 0.40 ± 0.01 (1 σ); greater than an order of magnitude larger than any other K interference. The magnitude of the resulting age bias for an unknown sample will be a function of the integrated neutron flux, the length of irradiation (fluence), the time elapsed since irradiation, and the age relationship between the unknown and neutron fluence monitor. We show using the raw data of (Niespolo et al., 2017) that the age of Alder Creek sanidine can be modified to be ca. 0.1% older (1 σ), at the 2σ level of current analytical precision for the Alder Creek age for this study. The 39K(n, α , β)36Ar inference should be incorporated into routine data analysis and may be especially important in the intercalibration of the 40Ar/39Ar system with other chronometers (e.g., 206Pb/238U). [ABSTRACT FROM AUTHOR]

There is a consensus that volcanism along the East African Rift System (EARS) is related to plume activities. However, because of our limited knowledge of the local lithospheric mantle, the dynamics of the plume are poorly constrained by magma chemistry. The Turkana Basin is one of the best places to study plume‐related volcanism because the lithospheric mantle there is unusually thin. New Ar‐Ar geochronology and geochemical data on lavas from western Turkana show that Eocene volcanics have relatively low 206Pb/204Pb (<19.1) and high εNd (>3.78). Their relatively high Ba/Rb (35–78) ratios suggest contributions from the shallow lithospheric mantle. Oligo‐Miocene Turkana volcanics have HIMU‐ and EMI‐ type enriched mantle signatures with overall lower Ba/Rb ratios, which is consistent with partial melting of plume material. Pliocene and younger Turkana volcanics have low Ba/Rb and Sr‐Nd‐Pb isotope ratios that resemble those of Ethiopian volcanics with elevated 3He/4He ratios. This temporal variation can be reconciled with a layered plume model where an outer layer of ancient recycled oceanic crust and sediment overlies more primitive lower mantle material. Beneath Ethiopia, the outer layer of the plume is either missing or punctured by the delamination of the thicker overlying lithospheric mantle at ca. 30 Ma, an event that would have facilitated the rapid upwelling of the inner portion of the plume and triggered the Ethiopian flood volcanism. The outer layer of the plume may be thicker in the southern EARS, which could explain the occurrence of young HIMU‐ and EMI‐type volcanics with primordial noble gas signatures. Plain Language Summary: We studied lavas from the Turkana Basin, Kenya, to understand the nature of the mantle beneath the East African Rift System (EARS). We found that only the oldest Turkana lavas (>35 Ma) show contributions from the shallow lithospheric mantle while younger lavas show more contributions from a mantle plume. Oligocene‐Miocene lavas (32 Ma −12 Ma) from Turkana show signatures of ancient oceanic crust and sediments recycled deep into the mantle while Pliocene and younger Turkana volcanics (<5 Ma) show signatures of the lower mantle. We proposed a layered plume model to explain these observations: the initial arrival of the mantle plume under Turkana first caused partial melting of the enriched components in the shallow lithospheric mantle in the Eocene; further upwelling caused the outer layer of the plume to melt and form lavas with HIMU‐ and EMI‐type of geochemical signatures; the inner part of the plume that consists of lower mantle material was finally able to melt and generate Afar‐type magmatism in the Turkana Basin in the Pliocene. Variability in the thicknesses of the outer layer of the plume could explain the geographic differences in volcanism across the EARS. Key Points: HIMU and EMI Turkana volcanism occurred mostly in the Miocene while Pliocene Turkana volcanics resemble high 3He/4He Afar volcanicsTemporal variation of Turkana volcanism indicates a layered plume with an outer layer of ancient recycled oceanic crust and sedimentsLithospheric mantle delamination may have punctured the outer layer of the plume and triggered Ethiopian flood volcanism [ABSTRACT FROM AUTHOR]

We integrate 10 new with five published 40Ar/39Ar age determinations, both on primary volcanic deposits and on detrital sanidine, which provide precise geochronologic control on the Marine Isotope Stage (MIS) 5.5 and MIS 5.3 sea‐level indicators that occur at three coastal caves in a tectonically stable region of the central Tyrrhenian Sea of Italy. The age of a Strombus‐bearing bioclastic conglomerate, associated with a tidal notch occurring at 9.5 m a.s.l. at Cape Circeo, is constrained to between 121.5 ± 5.8 and 116.2 ± 1.2 ka. Moreover, backbeach deposits intercalated in the sedimentary filling of Guattari and Capre coastal caves are directly correlated with a tidal notch at ∼2.5 m associated with another bioclastic conglomerate at Cape Circeo and dated to 110.4 ± 1.4–104.9 ± 0.9 ka. The latter deposit is also correlated with the adjacent marine terrace, occurring at 3–5 m on the coast between Capes Circeo and Anzio, for which a maximum age of 100.7 ± 6.6 ka was previously reported. These data provide evidence for a maximum sea level around 9.5 m above the present sea level and a duration of MIS 5.5 highstand until 116 ka, in agreement with estimates from other regions in the world. In contrast, they suggest a maximum sea level during MIS 5.3 highstand that is similar to the present level, and only ∼7 m lower than the MIS 5.5 highstand, challenging the reconstructions of the MIS 5 ice‐sheet volumes and derived global sea levels that are based on benthic oxygen isotope records. Key Points: We present 10 40Ar/39Ar ages on the Marine Isotope Stage (MIS) 5.5 and MIS 5.3 sea‐level indicators on the Tyrrhenian Sea coastMIS 5.5 paleo‐sea level at ∼9.5 m a.s.l. is dated 121.5 ± 5.8 through 116.2 ± 1.2 kaWe find a MIS 5.3 sea level ca. 20 m higher than the δ18O‐derived values [ABSTRACT FROM AUTHOR]

The accuracy and traceability of geochronometers are of vital importance to questions asked by many Earth scientists. The widely applied 40Ar/39Ar geochronometer relies on the co‐irradiation of samples with neutron fluence monitors (reference materials) of known ages; the ages and uncertainties of these monitors are critical to our ability to apply this chronometer. Previously, first principles, astronomical and optimisation calibrations have been made. The first principles method for determining the age of monitor minerals is the K‐Ar method, which involves measurement of their 40K and 40Ar* abundances. The AQuA (Absolute Quantities of Argon) pipette system, which emits calibrated quantities of 40Ar* via the ideal gas law, was used to calibrate the sensitivity of the system across a range of source pressures and estimate 40Ar* abundances in neutron fluence monitors. These 40Ar abundances were combined with existing 40K abundance data for these monitors. Ages for HD‐B1 and MD2 (GA1550) biotite fluence monitors were calculated and combined with intercalibration data for HD‐B1 and Fish Canyon sanidine (FCs) to determine ages for FCs. Current results do not have the targeted accuracy when compared with previous calibrations; however, we show how the extensive methodology development presented here can be used towards making reliable future measurements. [ABSTRACT FROM AUTHOR]

The extrusion tectonic model for the southeastern margin of the Himalayan orogeny links the crustal shear activity along the Red River Shear Zone (RRSZ) to the opening of the South China Sea (SCS). The Day Nui Con Voi (DNCV) metamorphic massif in northern Vietnam strikes NW-SE, is bounded by the RRSZ to the south and continues along the strike where it meets the SCS. The DNCV is thus a critical area to document thermotectonic history in order to advance our understanding of the tectonic evolution of Indochina extrusion and its relationship to the opening of the SCS. Our new 40Ar/39Ar data combined with microstructural and petrological analyses constrained the timing of the left-lateral shearing of the RRSZ and revealed the thermal evolution of the DNCV metamorphic massif. Three ductile deformation events were observed. D1 formed NNW-SSE striking upright folds under granulite to upper amphibolite facies conditions. D2 was a horizontal to sub-horizontal folding event that occurred at amphibolite facies conditions. D3 was a doming event that formed NW-SE striking up-right folds bounded by left-lateral shearing mylonite belts along the two limbs. The S/C fabrics were defined by muscovite fish, quartz + albite + K-feldspar aggregates, and muscovite folia. The D3 doming event exhumed the DNCV metamorphic massif from amphibolite facies conditions to the lower greenschist facies conditions. The 40Ar/39Ar ages obtained from amphibole (∼26 Ma), phlogopite (∼25 Ma), muscovites (∼24-23 Ma), biotite (∼25-23 Ma), and K-feldspars (∼25-22 Ma) from different structural domains of the DNCV metamorphic massif indicated a rapid exhumation ∼26–22 Ma. We interpreted this as the time period for the D3 event, with the onset of left-lateral shearing occurring around 24 Ma based on ages obtained from syn-kinematic muscovites. This age was much younger than the initiation of sea-floor spreading of the SCS (since 32 Ma) but coincided with the age for the ridge jump event in the SCS. Based on these new data, we proposed that extrusion tectonism cannot be the cause for the initial opening of the SCS. Rather, the extrusion of the Indochina block was temporally correlative with the southward ridge jump event of the already opened SCS.

Porphyry and epithermal deposits are important sources of base and precious metals. Most actively mined deposits have been exhumed such that ore bodies are relatively close to the surface and are therefore locatable and economic to extract. Identifying and characterizing concealed deposits, particularly more deeply buried porphyry deposits, represents a far greater challenge for mineral exploration, and will become progressively more important as near-surface resources are gradually exhausted over time. We report high-precision 40Ar/39Ar dates for coarsely crystalline alunite that precipitated from magmatic steam in open fractures in Oligocene dacitic volcanic rocks, and a SHRIMP 206Pb/238U zircon date for one of several rhyolite dikes present at Alunite Ridge and Deer Trail Mountain, Utah. Both the magmatic-steam alunite and rhyolite dikes are related to concealed intrusions. The rhyolite dike yielded an age of 30.72 ± 0.36 Ma, which is older than a commonly cited 27.1 Ma age estimate for the Three Creeks Tuff Member of the Bullion Canyon Volcanics that is cut by the dike. 40Ar/39Ar data for samples of magmatic-steam alunite and sericite from six mines and prospects provide evidence for at least two periods of episodic hydrothermal activity at ca. 15.7–15.1 Ma and ca. 14.7–13.8 Ma, with the older and younger pulses of activity recorded at the more eastern and western sites, respectively. These two periods of hydrothermal activity are consistent with previous interpretations that Alunite Ridge and Deer Trail Mountain are underlain by two concealed porphyry stocks. 40Ar/39Ar analyses of individual bands in a sample of massive, centimeter-scale banded vein alunite yield indistinguishable ages with a weighted mean of 13.98 ± 0.12 Ma, consistent with a short-lived (≲250 ka) magmatic event with episodic vapor discharge recurring on short timescales (≲36 ka). 40Ar/39Ar geochronology of magmatic-steam alunite is a valuable tool to constrain the timing and duration of magmatic hydrothermal activity associated with unexposed intrusions and potentially porphyry deposits, and therefore may be useful in exploration. [ABSTRACT FROM AUTHOR]

[Display omitted] • The plateau age of 40Ar/39Ar were indicate a duration of mineralizing hydrothermal activity of approximately 170 Ma. • The ore-forming fluid was a medium–high and medium-salinity H 2 O– NaCl- CO 2 -CH 4 system. • The later fluid immiscibility may be one of the factors for the unloading and precipitation of mine. • The deposit has undergone at least two stages fluid interaction. The Bailongshan Pegmatite deposit, located in the West Kunlun Orogenic Belt, Northwest China, is a newly discovered, super-large Li–Rb (Be–Ta–Nb) rare-metal deposit. Since complex magmatic-hydrothermal processes are responsible for the mineralization of such rare-element pegmatites, it is desirable to study the evolution and sources of ore-forming fluids to analyze the genesis of ore deposits. In this study, the 40Ar/39Ar plateau ages of muscovite and biotite were determined to be 171.36 ± 1.87 and 172.39 ± 1.66 Ma, respectively, indicating that the duration of hydrothermal mineralization was approximately 170 Ma. Based on the zonal nature of the mineral assemblage, the Bailongshan area was divided into four zones and stages (I–IV), namely the albite–quartz–lithium tourmaline (AQT, stage I), albite–quartz-bearing mica (AQM, stage II), albite–quartz–spodumene (AQS, stage Ⅲ), and spodumene–quartz (SQ, stage IV) zones. Among these, AQS and SQ were the main ore-bearing areas. In terms of the fluid inclusions found in quartz and spodumene, the different types include a gas-rich phase (V-type), a liquid-rich phase (L-type), a daughter mineral-bearing three-phase (S-type), and a carbon dioxide-bearing phase (C-type). In stage I, the homogeneous temperatures of the V- and S-type fluid inclusions varied from 365 to 415 °C, while their corresponding salinities were 8.5–12.9 and 44.8–47.2 wt% NaCl equiv., respectively. In stage II, the homogeneous temperature and salinity of the L-type inclusions were 315–365 °C and 9.9–13.3 wt% NaCl equiv., respectively, while in stages Ⅲ and IV, the homogeneous temperatures of the L- and S-type fluid inclusions were between 235 and 335 °C, while their salinities were 7.2–12.3 and 32.1–37.0 wt% NaCl equiv., respectively. Furthermore, for the C-type inclusions, the homogeneous temperature and salinity were 235–320 °C and 4.9–10.6 wt% NaCl equiv., respectively. The laser Raman results showed that the fluid in the metallogenic stage was an H 2 O–NaCl–CO 2 –CH 4 system. Based on the homogeneous temperature and salinity results, the fluid capture pressure from stage III to stage IV was calculated to be 280–150 MPa, and the depth of the capture was >6 km. Moreover, the H–O isotope results suggested that the early ore-forming fluids are mainly magmatic hydrothermal fluids, whereas the later (stage IV) mineralizing fluids may be mixed with a small amount of meteoric water. The subsequent immiscibility of the fluid may be one of the factors responsible for the discharge and precipitation of minerals. [ABSTRACT FROM AUTHOR]

The diamond-bearing Seve Nappe Complex (SNC) in the Scandinavian Caledonides records subduction of continental margin rocks to (ultra)high-pressure conditions at mantle depths, and exhumation thereafter from beneath the hinterland to the Earth's surface in the foreland. Structural data of the Upper, Middle, and Lower SNC in central Jämtland, Sweden demonstrate a triclinic bulk deformation during the exhumation of the still ductile SNC from crustal levels where migmatites formed. 40Ar/39Ar data from the Upper SNC constrain the timing of cooling through 450‒300 °C to be ~ 418 to 416 Ma. In combination with a review of the published pressure–temperature–time data and regional geology of the central Scandinavian Caledonides, four stages of exhumation of the central Jämtland SNC are summarized: (1) buoyancy-driven exhumation during ~ 455 to 433 Ma from ultrahigh-pressure depths to granulite-facies depths triggered by tectonic under-pressure; (2) tectonic exhumation in ~ 433 to 418 Ma at lower- to mid-crustal levels resulted from accretion of the Lower Köli Nappes onto Baltica; (3) eduction of the Western Gneiss Region lithosphere and piggyback transport of the SNC in ~ 418 to 375 Ma from hinterland to foreland, coupled with extensional faulting at mid- to upper-crustal levels; and (4) gravitational collapse- and erosion-driven exhumation following the end of the Scandian Orogeny at ~ 375 Ma. This multi-stage exhumation transported the SNC for > 100 km vertically from mantle depths to the Earth's surface and > 350 km horizontally from the Caledonian hinterland to the foreland. This contribution provides a typical example of the complex exhumation of deeply subducted continental rocks in orogenic belts. [ABSTRACT FROM AUTHOR]

Discussions continue about whether Middle Eocene magmatism in the Eastern Pontides is associated with collision or subduction. This paper presents new whole-rock geochemistry, Sr-Nd isotopic and 40Ar-39Ar age data for Middle Eocene volcanic rocks from the Bayburt area of the Eastern Pontides (NE Turkey) to investigate their sources and evolutionary history. The new 40Ar–39Ar ages reveal that these volcanic rocks erupted between 44.6 ± 0.1 Ma and 43.5 ± 0.1 Ma, within the Lutetian (Middle Eocene). The studied volcanic rocks are composed of basalt, andesite, basaltic andesite and minor dacite lava and pyroclastic rocks. These rocks consist of plagioclase, amphibole, pyroxene, olivine, biotite, sanidine and minor quartz phenocrysts with Fe-Ti oxides. They have microlithic, hyalo-microlithic, porphyritic and rarely glomeroporphyritic textures. The volcanic rocks have low to high-K calc-alkaline affinities. They display enrichment in large-ion lithophile elements and depletion in high-field strength elements with high Th/Yb ratios, which indicate that the magmas forming the volcanic rocks were derived from lithospheric mantle sources enriched by mostly slab-derived fluids in the spinel stability field. 87Sr/86Sr(i) values vary between 0.70485 and 0.70551 and 143Nd/144Nd(i) values vary between 0.51255 and 0.51267. These data correspond to the mantle array on the isotope ratio diagram. The main solidification processes consist of fractional crystallization with minor assimilation. In light of the data obtained in this study together with data from previous studies, petrogenetic character of the Middle Eocene magmas from the southern parts of the Eastern Pontides may be explained by melting of an enriched lithospheric mantle source initially metasomatized by subduction fluids in a post-collisional extensional-related tectonic setting. [ABSTRACT FROM AUTHOR]

As a result 40Ar/39Ar geochronological studies, the age of gold mineralization has been determined for the Elga deposit that is located in the eastern part of the Mongol–Okhotsk fold belt. The age of ore metasomatites and ore-hosting hydrothermally altered carbonaceous sericite–feldspar–quartz schists is estimated at 139–137 Ma. The formation of ore mineralization of the Elga deposit is not likely related to magmatic processes, because the age of igneous complexes within the studied region is either younger or significantly older than the mineralization. An almost identical age of 139 Ma is identified for sericite from schists of the Talyma Formation beyond the ore zone. This indicates that the final stage of metamorphism and regional deformations, on one hand, and the formation of ore metasomatites, on the other hand, has similar age. In our opinion, the key role in mobilization and redistribution of ore matter and the formation of the Elga deposit belongs to postcollisional dislocation processes, which were accompanied by hydrothermal alteration. [ABSTRACT FROM AUTHOR]

In 40Ar/39Ar geochronology, excess Ar is 40Ar that does not derive from the in situ radioactive decay of 40K, or from the measurable input of atmospheric Ar, and results in increased daughter-parent ratios that correspond to anomalously old apparent dates without geological age significance. Excess 40Ar is commonly identified by a saddle-shaped 40Ar/39Ar degassing spectrum. However, biotite from the east Albany-Fraser Orogen of Western Australia contains excess 40Ar, but yields well-defined 40Ar/39Ar plateau dates, reproducible upon replicate analysis of biotite from the same sample. This "cryptic excess 40Ar" is inferred where plateau dates (1) are older than existing time constraints on cooling, such as U/Pb zircon ages for amphibolite to granulite facies metamorphism, and (2) vary between multiple samples from the same outcrop that have experienced the same thermal history. Rb/Sr geochronology is used to test the geologic significance of 40Ar/39Ar plateau dates, as the closure temperatures of both chronometers are similar, and the two chronometers are expected to yield similar cooling dates in an undisturbed system. Six Rb/Sr biotite-whole rock isochron ages from three outcrops yield a weighted mean age of 1133 ± 3 Ma (MSWD = 1.13, P = 0.34), and are interpreted to record post-orogenic cooling. 40Ar/39Ar plateau dates from the same samples are 31–394 Ma older, and in six of nine samples cannot be explained by simple monotonic cooling, by low-temperature biotite recrystallisation, or by alteration; instead, biotite contains cryptic excess 40Ar. Diffusion modelling of 40Ar/39Ar step-heating plateaus suggests that Ar, both excess and radiogenic, is homogeneously distributed within the crystal lattice, and was incorporated during biotite crystallisation. Biotite likely crystallised in a rock with a high partial pressure of Ar, possibly due to a high rate of radiogenic 40Ar generation in a K-rich lithology, together with a poorly-connected intergranular fluid network that inhibited Ar loss from the rock volume. This is supported by a homogeneous distribution of Ar at the cm scale, with reproducible biotite 40Ar/39Ar dates from each sample. Patchy fluid circulation during metamorphism led to the pervasive but heterogeneous distribution of excess 40Ar across outcrops. Due to the presence of the 40Ar/39Ar plateau, cryptic excess 40Ar is difficult to identify a priori , without comparison to additional geochronological constraints. Where cryptic excess 40Ar on a regional scale has been identified, such as in the eastern Biranup Zone of the Albany-Fraser Orogen, Rb/Sr geochronology may provide a more robust alternative to constrain the cooling path of rocks from high temperatures. [ABSTRACT FROM AUTHOR]

40Ar/39Ar geochronology is one of the most important techniques for constraining the timing of basaltic events due to the paucity of suitable minerals in basalts for other geochronological techniques such as U–Pb (e.g., zircon, baddeleyite). Among a variety of materials from basaltic rocks that have been used for 40Ar/39Ar dating, plagioclase is the most important due to its common presence in basalts as a primary crystallizing phase, and its transparency so that fresh grains can be selected during sample preparation. However, plagioclase 40Ar/39Ar geochronology has often been compromised by alteration (e.g., sericitization by hydrothermal events), which, in practice, is difficult to identify using a petrographic microscope when the amount of alteration is low (e.g., <1%). We used laboratory step-heating experiments and theoretical simulations to characterize the 40Ar/39Ar age and Ca/K spectra of altered plagioclase so that 40Ar/39Ar dating results on altered samples can be identified and better interpreted. The step-heating experiments and theoretical simulations yielded consistent results, and show that with the presence of even a tiny amount of sericite (~0.01% for K-poor samples and ~0.1% for K-rich samples), the plagioclase samples yielded alteration plateau ages that are 3%–4% younger than the crystallization age. The difference between the alteration age of sericitized plagioclase and its crystallization age is primarily controlled by the time lapse between the crystallization and sericitization events, but also by the Ca/K ratios of the plagioclase. For plagioclase samples that experienced the same alteration event, the higher the Ca/K ratio is, the more sensitive the 40Ar/39Ar age is to alteration. We propose that the alteration signatures of plagioclase can be effectively identified through inspecting the 40Ar/39Ar age spectra, the Ca/K spectra, and the degassing curves. We also investigated the effect of sericitization of plagioclase microliths in basaltic groundmass and modelled the 40Ar/39Ar age and Ca/K spectra of altered groundmass samples. We validate our approach by revisiting published 40Ar/39Ar dating results for large igneous provinces, and showed that these dates should have been interpreted as alteration ages (minimum eruption ages) rather than crystallization ages. Finally, we demonstrate that with high degrees of alteration (~50% for K-poor and >70% for K-rich plagioclase samples), the age of hydrothermal alteration can be successfully dated. [ABSTRACT FROM AUTHOR]

Abstract We present new high-resolution 40Ar/39Ar ages for alkaline layered gabbros and cross-cutting mafic-ultramafic and felsic dykes from the São Sebastião Island, in the northern sector of the Serra do Mar Alkaline Province, São Paulo, Brazil. Duplicate analysis of single kaersutite crystals yielded plateau ages of 88 ± 1 and 87.9 ± 0.8 Ma for a gabbro and 87 ± 1 and 86.0 ± 0.8 Ma for a picrite dyke. Two biotite aliquots from a trachyte yielded ages of 86.0 ± 0.5 and 86.2 ± 0.5 Ma, while an alkali feldspar concentrate yielded ages of 86.0 ± 0.5 Ma. The similar ages obtained for minerals with contrasting closure temperatures suggest that the results are crystallization ages and that the rocks were emplaced at shallow crustal levels. Our results, along with available high-resolution data, point to a relatively narrow time span (ca. 88-85 ± 0.6 Ma) for the entire alkaline magmatism on the island. Other alkaline occurrences both nearby and in the continent’s interior reveal similar ages, reinforcing the hypothesis that mantle decompression and upwelling of heterogeneous mantle sources led to crustal extension and fast ascension of a variety of alkaline magma types in this segment of South America.

We present new high-resolution 40Ar/39Ar ages for alkaline layered gabbros and cross-cutting mafic-ultramafic and felsic dykes from the São Sebastião Island, in the northern sector of the Serra do Mar Alkaline Province, São Paulo, Brazil. Duplicate analysis of single kaersutite crystals yielded plateau ages of 88 ± 1 and 87.9 ± 0.8 Ma for a gabbro and 87 ± 1 and 86.0 ± 0.8 Ma for a picrite dyke. Two biotite aliquots from a trachyte yielded ages of 86.0 ± 0.5 and 86.2 ± 0.5 Ma, while an alkali feldspar concentrate yielded ages of 86.0 ± 0.5 Ma. The similar ages obtained for minerals with contrasting closure temperatures suggest that the results are crystallization ages and that the rocks were emplaced at shallow crustal levels. Our results, along with available high-resolution data, point to a relatively narrow time span (ca. 88-85 ± 0.6 Ma) for the entire alkaline magmatism on the island. Other alkaline occurrences both nearby and in the continent's interior reveal similar ages, reinforcing the hypothesis that mantle decompression and upwelling of heterogeneous mantle sources led to crustal extension and fast ascension of a variety of alkaline magma types in this segment of South America. [ABSTRACT FROM AUTHOR]

The Ross orogenic belt in Antarctica is one of several Neoproterozoic-early Palaeozoic orogens that crisscrossed Gondwana and are associated with Gondwana's assembly. We present new age data from the Queen Maud Mountains, Ross orogen, from areas that hitherto have lacked precise ages from the local plutonic rocks. The zircon U-Pb igneous crystallization ages (n = 7) and a hornblende 40Ar/39Ar cooling age (n = 1) constrain plutonism to primarily lie within the Cambrian to Ordovician. Cumulative zircon U-Pb crystallization age data yield polymodal age distributions (516 Ma, 506–502 Ma, and 488 Ma age peaks) that are similar to other areas of the Queen Maud-Horlick Mountains, consistent with regional magmatic flare-ups along the Pacific-Gondwana margin during these times. The ages of deformed plutons constrain deformation to the Cambrian (Series 2) to Ordovician (Lower), with some regions indicating a transition to post-tectonic magmatism and cooling at ~509-470 Ma. Collectively, the data indicate that the Queen Maud-Horlick Mountains share a similar petrotectonic history with other regions of the Pacific-Gondwana margin, providing new evidence that this tectonostratigraphic province is part of and not exotic to the larger igneous-sedimentary successions developed in the peri-Gondwana realm under a broadly convergent margin setting. [ABSTRACT FROM AUTHOR]

Porphyry and epithermal deposits are important sources of base and precious metals. Most actively mined deposits have been exhumed such that ore bodies are relatively close to the surface and are therefore locatable and economic to extract. Identifying and characterizing concealed deposits, particularly more deeply buried porphyry deposits, represents a far greater challenge for mineral exploration, and will become progressively more important as near-surface resources are gradually exhausted over time. We report high-precision 40Ar/39Ar dates for coarsely crystalline alunite that precipitated from magmatic steam in open fractures in Oligocene dacitic volcanic rocks, and a SHRIMP 206Pb/238U zircon date for one of several rhyolite dikes present at Alunite Ridge and Deer Trail Mountain, Utah. Both the magmatic-steam alunite and rhyolite dikes are related to concealed intrusions. The rhyolite dike yielded an age of 30.72 ± 0.36 Ma, which is older than a commonly cited 27.1 Ma age estimate for the Three Creeks Tuff Member of the Bullion Canyon Volcanics that is cut by the dike. 40Ar/39Ar data for samples of magmatic-steam alunite and sericite from six mines and prospects provide evidence for at least two periods of episodic hydrothermal activity at ca. 15.7–15.1 Ma and ca. 14.7–13.8 Ma, with the older and younger pulses of activity recorded at the more eastern and western sites, respectively. These two periods of hydrothermal activity are consistent with previous interpretations that Alunite Ridge and Deer Trail Mountain are underlain by two concealed porphyry stocks. 40Ar/39Ar analyses of individual bands in a sample of massive, centimeter-scale banded vein alunite yield indistinguishable ages with a weighted mean of 13.98 ± 0.12 Ma, consistent with a short-lived (≲250 ka) magmatic event with episodic vapor discharge recurring on short timescales (≲36 ka). 40Ar/39Ar geochronology of magmatic-steam alunite is a valuable tool to constrain the timing and duration of magmatic hydrothermal activity associated with unexposed intrusions and potentially porphyry deposits, and therefore may be useful in exploration.

Situated in the western Erzgebirge metallogenetic province (Vogtland, Germany), the Eichigt prospect is associated with several quartz-Mn-Fe-oxyhydroxide veins that are exposed at surface. Bulk-rock geochemical assays of vein material yield high concentrations of Li (0.6–4.1 kg/t), Co (0.6–14.7 kg/t), and Ni (0.2–2.8 kg/t), as well as significant quantities of Mn, Cu, and light rare earth elements, a very unusual metal tenor closely resembling the mixture of raw materials needed for Li-ion battery production. This study reports on the results of a first detailed investigation of this rather unique polymetallic mineralization style, including detailed petrographic and mineralogical studies complemented by bulk rock geochemistry, electron microprobe analyses, and laser ablation inductively coupled mass spectrometry. The mineralized material comprises an oxide assemblage of goethite hematite, hollandite, and lithiophorite that together cement angular fragments of vein quartz. Lithiophorite is the predominant host of Li (3.6–11.1 kg/t), Co (2.5–54.5 kg/t), and Ni (0.2–8.9 kg/t); Cu is contained in similar amounts in hollandite and lithiophorite whereas light rare earth elements (LREE) are mainly hosted in microcrystalline rhabdophane and florencite, which are finely intergrown with the Mn-Fe-oxyhydroxides. 40Ar/39Ar ages (~ 40–34 Ma) of coronadite group minerals coincide with tectonic activity related to the Cenozoic Eger Graben rifting. A low-temperature hydrothermal overprint of pre-existing base metal sulfide-quartz mineralization on fault structures that were reactivated during continental rifting is proposed as the most likely origin of the polymetallic oxyhydroxide mineralization at Eichigt. However, tectonically enhanced deep-reaching fracture-controlled supergene weathering cannot be completely ruled out as the origin of the mineralization. [ABSTRACT FROM AUTHOR]

The Campi Flegrei (CF) caldera, in southern Italy, is the source of some of the most powerful Late Pleistocene eruptions of the European sub-continent (e.g., Campanian Ignimbrite, Neapolitan Yellow Tuff eruptions). Although the CF caldera has been continuously and intensively investigated for decades, relatively little is known regarding its earliest volcanic activity. In this work, integrating existing and new tephrostratigraphic data, we provide a comprehensive and updated framework for the CF volcanic activity which has occurred at ∼160 ka and between ∼110 ka and ∼90 ka. The new tephrostratigraphic, geochemical (EMPA + LA-ICP-MS), chronological (40Ar/39Ar dating) and grain-size distribution data relate to CF tephra deposits preserved in mid-proximal (Campanian Plain), distal (Tyrrhenian Sea) and ultra-distal (Lower Danube area) sedimentary archives. Our results allowed us to recognize the presence of at least 13 CF eruptions covering the investigated time frame, with 12 eruptions occurring between 110 and 90 ka. Our high-resolution stratigraphic and chronological investigation also allowed us to recognize that the Triflisco/C-22 tephra, previously considered as a single marker layer, can be actually separated into three different events, sourced from within the CF area in the short time interval of ∼93-90 ka, suggesting a more complex and intense volcanic history than previously thought. Moreover, a Bayesian age-depth model, constrained by previous and new high precision 40Ar/39Ar ages, has led to a reliable estimate of the ages of those undated CF eruptions. Overall, the updated framework on the stratigraphy, chronology, dispersion, and geochemistry of the CF tephra of ∼160 ka and between 110 ka and 90 ka consolidates the notion that the Middle-Late Pleistocene activity in theCF area represents a significant stage of its volcanic evolution, characterised by intense and frequent explosive eruptions. • Widespread Middle-Late Pleistocene Mediterranean tephra layers were investigated. • New geochemical and chronological constraints for the volcanic activity at CF. • Unknown, older than ∼90 ka large magnitude events recognized and correlated to CF. • New volcanic history reconstruction could be pivotal for future hazard assessment. [ABSTRACT FROM AUTHOR]

Montagna Grande and Monte Gibele represent the northwestern and southeastern portions of a shield volcano (hereafter "MGV") located entirely within the Cinque Denti caldera structure on Pantelleria. The Cinque Denti caldera formed during the 45.7 ± 1.0 ka eruption of the pantelleritic Green Tuff. Subsequent isostatic compensation was achieved by the eruption within the caldera of ∼3 km3 of trachyte lavas that comprise the MGV and are thought to represent renewed tapping of the Green Tuff magma reservoir. The time interval between the eruption of the Green Tuff and the MGV trachytes has not been well constrained, with previous K Ar ages from alkali feldspar yielding ages between 44 ± 8 and 28 ± 16 ka. In this study we report the results of new single-crystal anorthoclase incremental heating and total fusion 40Ar/39Ar ages collected from three trachyte lava flows collected at different sites and at different stratigraphic heights that provided ages of 26.2 ± 2.0, 22.5 ± 0.8 ka, and 22.3 ± 2.8 ka (uncertainties at 2σ). The results indicate that there was a repose period of up to ∼20,000 years between the eruption of the caldera-forming ignimbrite and the caldera-filling trachytes. The timing of our oldest dated sample is coincident with renewed basaltic volcanism in the northwest part of the island at ∼28 ka and suggests that the eruption of the trachyte was promoted by mafic recharge. We briefly review the petrogenetic processes operative in the reservoir during the repose period. Modelling of the thermal and compositional evolution of the fractionating system indicates an eruption rate of 7.5 × 10−4 km3/yr over the ∼4 ka eruption period. [ABSTRACT FROM AUTHOR]

Here, we report on the Quaternary Volsci Volcanic Field (VVF, central Italy). In light of new 40Ar/39Ar geochronological data and compositional characterization of juvenile eruptive products, we refine the history of VVF activity, and outline the implications on the pre-eruptive magma system and the continental subduction processes involved. Different from the nearby volcanic districts of the Roman and Campanian Provinces, the VVF was characterized by small-volume (0.01–0.1 km3) eruptions from a network of monogenetic centers (mostly tuff rings and scoria cones, with subordinate lava occurrences), clustered along high-angle faults of lithospheric depth. Leucite-bearing, high-K (HKS) magmas (for which we report for the first time the phlogopite phenocryst compositions) mostly fed the early phase of activity (∼761–539 ka), then primitive, plagioclase-bearing (KS) magmas appeared during the climactic phase (∼424–349 ka), partially overlapping with HKS ones, and then prevailed during the late phase of activity (∼300–231 ka). The fast ascent of primitive magma batches is typical of a tectonically controlled volcanic field, where the very low magma flux is a passive byproduct of regional tectonic strain. We suggest that the dominant compressive stress field acting at depth was accompanied by an extensional regime in the upper crust, associated with the gravity spreading of the Apennine chain, allowing the fast ascent of magma from the mantle source with limited stationing in shallow reservoirs. [ABSTRACT FROM AUTHOR]

The north–north‐east‐trending (NNE) shear zones, which are defined by the Jiekou shear zone and Huangmao shear zone, are located in the eastern Jiangnan Orogen. This article provides kinematics, geochronological data, and dynamics on the Jiekou and Huangmao shear zones and serves as a reference for understanding the tectonic evolution in the South China Block during the Early Palaeozoic orogeny. All shearing signatures show that the Huangmao shear zone is dextral, and the Jiekou shear zone is sinistral. Both shear zones have certain genetic connections. The rocks from the Jiekou and Huangmao shear zones are metamorphosed and deformed into protomylonite and mylonite. Results of the deformation fabrics of feldspar and quartz and quartz c‐axis fabric analysis indicate that the ductile deformation temperatures of the Jiekou shear zone sinistral shearing are ~400–500°C. Dextral shearing with medium–high temperature (450–550°C) is mainly recorded in the Huangmao shear zone. In combination with the previous geochronology, NNE‐trending sinistral and dextral shearing in the eastern Jiangnan Orogen occurred at 465–430 Ma, coeval with the Early Palaeozoic orogeny. Therefore, the geodynamics of the NNE‐trending shear zones is related to a major strain transformation from NW to WNW, which is remotely affected by the intracontinental convergence of Yangtze and Cathaysia blocks during the Early Palaeozoic orogeny. [ABSTRACT FROM AUTHOR]

Jarosite [KFe 3 (SO 4) 2 (OH) 6 ] occurs both as a hydrothermal mineral and as the product of weathering and chemical sedimentation. It has been used in 40Ar/39Ar geochronology to date water–rock interaction and weathering processes on the surfaces of Earth and Mars, but the lack of information about Ar diffusivity parameters relevant to specific types of jarosites makes the interpretation of geochronological results tentative. We have filled this gap by investigating Ar diffusion parameters in representative supergene and hypogene jarosites. Detailed diffusion studies were carried out on a hypogene jarosite sample from Gilbert, Nevada, and two supergene jarosite samples from Baiyin, China. The diffusion studies were accompanied by in-situ heating investigations in a transmission electron microscope to directly determine the thermal stability and the phase transformations that jarosite undergoes under progressive heating under ultra-high vacuum. The TEM results suggest that jarosite is stable under vacuum up to ∼400 °C, when it undergoes phase transition to yavapaiite [KFe(SO 4) 2 ] and hematite (Fe 2 O 3). Incremental-heating experiments reveal average diffusion parameters of E a = 138.6 ± 4.2 kJ/mol and ln(D o /a2) = 9.9 ± 0.9 ln(s−1) for hypogene jarosite; E a = 110.3 ± 3.2 kJ/mol and ln(D o /a2) = 5.7 ± 0.7 ln(s−1) for one supergene jarosites; and E a = 141.2 ± 7.9 kJ/mol and ln(D o /a2) = 11.3 ± 1.7 ln(s−1) for the other supergene jarosite sample from the same weathering profile. Jarosite closure temperatures depend strongly on sieve size. For samples between 500–200 µm (grain size usually used for samples in Ar geochronology), at a cooling rate 100 °C·Ma−1, the closure temperatures are 143 ± 18 °C for the hypogene jarosite, and 105 ± 8 and 113 ± 14 °C, respectively, for the two supergene jarosites. Forward modelling of incremental-heating results predicts that coarse-grained hypogene jarosite is retentive of Ar below 50 °C for 100 Ma and below 25 °C for 4 Ga. Densely packed supergene jarosite grains larger than 200 µm are suitable for 40Ar/39Ar geochronology at the timescales suitable for investigating water–rock interaction at the surface of Earth and Mars. Fine-grained, porous jarosites require detailed diffusion analyses prior to geochronology due to possible high Ar losses over Ma timescales. The absence of jarosites older than ∼40 Ma on Earth suggests that jarosite may require continuous exposure to acid oxidizing conditions, and that it does not survive burial and exhumation. Therefore, the occurrence of jarosite on Earth and Mars may identify segments of the planets' surfaces continuously exposed to acid-oxidizing conditions since jarosite precipitation. [ABSTRACT FROM AUTHOR]

The Alastuo gold deposit, located in the Narati region of the Chinese West Tianshan, is a newly discovered gold deposit within the world-class Tianshan gold belt. The gold mineralization is hosted by an Early Carboniferous granodiorite intrusion and controlled structurally by subsidiary faults of the transcrustal North Narati Fault. The deposit consists of auriferous alteration assemblages and minor vein ores. Field relations and petrographic data suggest four stages of mineralization with gold precipitated coevally with quartz, chlorite, epidote, pyrite, and galena during the second and third stages that are characterized by tectonic transformation from ductile shearing to brittle deformation. The temperature range of gold precipitation is estimated to be 280–330 °C using chlorite geothermometry. Ten auriferous pyrite samples yielded a well-defined Re-Os isochron age of 325 ± 3 Ma and two sericite samples yielded plateau-like 40Ar/39Ar ages of 321.7 ± 3.0 Ma and 321.2 ± 2.8 Ma. These consistent ages suggest that the gold mineralization in the Narati region was emplaced in a transitional regime from subduction to continental collision between the Tarim block and Middle Tianshan terrane (325–310 Ma), rather than during the post-collisional stage as in most part of the Tianshan gold belt. Gold-bearing pyrite has an initial Osi value of 0.7 ± 0.1 and lead isotopes ratios of 17.897 to 18.723 for 206Pb/204Pb, 15.474 to 15.643 for 207Pb/204Pb, and 37.522 to 38.299 for 208Pb/204Pb, while samples of pyrite, galena, and sphalerite have 34SV-CDT of +1.9 to +7.8‰. The isotope data suggest that the sulfur and metals are most likely derived from mixed mantle and crustal sources including metamorphic devolatilization of subducted oceanic slab and overlying sedimentary rocks. The mineralization styles, structural controls, metal and sulfur sources, and the timing of gold mineralization with respect to the orogenic event all suggest that the Alastuo gold deposit represents an orogenic type deposit. This newly recognized Carboniferous orogenic gold mineralization event calls for a re-evaluation of the gold metallogeny in the Middle Tianshan. [ABSTRACT FROM AUTHOR]

During the Early Cretaceous, the North China Craton (NCC) underwent dramatic lithospheric thinning and destruction, resulting in widespread granitic magmatism and lithospheric extension. The geochronology of basic–felsic dikes provides a unique perspective on a crustal‐extension event that was related to the NCC destruction. Previous studies usually used zircon U–Pb dating to constrain the time of dike emplacement. This method is limited in terms of dating basic dikes because the authigenic zircons are rare or absent. Additionally, recording later reheating events during multiple phases of extension that occurred in the NCC after the Mesozoic is impossible. Here, we present 40Ar/39Ar geochronological data for basic–felsic dikes in the Sulu Orogen, Shandong Peninsula, East China. Our results indicate that these dikes were emplaced between ca. 127 and 95 Ma, with only two felsic samples recording ages <100 Ma, suggesting that crust‐derived magmatism decreased after this time. Several of these dikes that were adjacent to faults also recorded a later Cretaceous reheating event, which may have been related to fault movement and magmatic activity in the deeper crust. Combined with previous research results, Cretaceous dike emplacement was contemporaneous with crustal extension and widespread magmatic activity, slightly later than the peak time of the NCC destruction, suggesting an intrinsic connection between the dikes' emplacement and NCC destruction. The crustal magma and tectonic activity are direct evidence of the craton destruction, so these age data constrain the timing of destruction in the southeastern portion of the NCC to approximately 95 Ma. [ABSTRACT FROM AUTHOR]

Vorticity estimates based on porphyroclasts analysis are limited by the extrapolation to three dimensions of two‐dimensional data. We describe a 3D approach based on the use of X‐ray micro‐computed tomography that better reflects the real 3D geometry of the porphyroclasts population. This new approach for kinematic vorticity analysis in the Munsiari Thrust mylonites, the lower boundary of the Main Central Thrust zone (MCTz) in Indian Himalaya, indicates a large pure shear component during non‐coaxial shearing. 40Ar/39Ar ages of micas along the mylonitic foliation of the Munsiari and Vaikrita thrusts (the upper boundary of the MCTz) constrain thrust activity to 5–4 and 8–9 Ma, respectively. Available kinematic vorticity analyses of the Vaikrita mylonites suggest the dominance of a simple shear component. Combining these data, we suggest that the southward and structurally downward shift of deformation along the MCTz was accompanied by a progressive increase in the pure shear component in a general shear flow. [ABSTRACT FROM AUTHOR]

40Ar/39Ar radio-isotopic dating of volcanic tuffs intercalated in sediments can provide high accuracy age control on the deposition of sedimentary rocks. State-of-the-art mass spectrometers such as the ARGUS VI+ are able to acquire highly precise ages for relatively small single grains (~90-250 µm for Miocene samples). Single grain measurement can provide insight into the sometimes complex age distributions within volcanic tuffs. The results show that 40Ar/39Ar ages based on multiple grain fusions will not necessarily reflect eruption ages, which can lead to (slight) overestimation of the depositional age. The paper compares multiple and single grain data from different Miocene tuffs in the Central Paratethys, which plays an important role in the establishment of a geological time frame for this area. The examples come from three middle Miocene tuff horizons that span from the Badenian transgression to the Badenian--Sarmatian Extinction Event. The new ages obtained from the Quellgraben section in the Styrian Basin (14.31± 0.27 Ma and 14.03 ± 0.04 Ma) are much younger than the previous dating and together with the new data from the Bernhardsthal-4 well, Vienna Basin (15.12 ± 0.19 Ma) indicate, that the Badenian (Langhian) marine flooding did not reach this area before 15.2 Ma. The new weighted mean age of 12.56 ± 0.10 Ma from the Kamenica nad Hronom section in the Danube Basin dates the transition from marine to terrestrial setting, which is possibly connected with a sea level lowstand at the beginning of the Sarmatian. [ABSTRACT FROM AUTHOR]

The age of the Gow Lake impact structure (Saskatchewan, Canada) is poorly constrained, with previous estimates ranging from 100 to 250 Ma. Using a combination of step-heating and UV laser in situ 40Ar/39Ar analyses we have sought to understand the 40Ar/39Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for 40Ar/39Ar geochronology due to its small size (∼5 km diameter), the silicic composition of the target rock, and the large difference in age between the impact event and the target rock (∼1.2 Ga). These factors can serve to inhibit argon mobility in impact melts, leading to retention of 'extraneous' 40Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous 40Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ 40Ar/39Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of 40Ar inherited from silica-rich melts, data still had to be corrected for extraneous 40Ar by using isotope correlation plots to define the initial trapped 40Ar/36Ar components. Our inverse isochron age of 196.8 ± 9.6/9.9 Ma (2σ, analytical/external precision) demonstrates that the Gow Lake event occurred within uncertainty of the Triassic-Jurassic boundary, but there is no evidence that it was part of an impact cluster. [ABSTRACT FROM AUTHOR]

This study presents 40Ar/39Ar geochronology on the mafic dykes emplaced in the Damodar valley Gondwana sedimentary basins of the Chhotanagpur Gneissic Complex (CGC) to authenticate prolonged mafic magmatic activities during Maastrichtian period. A couple of earlier and one new 40Ar/39Ar plateau ages, which range in age from ca. 70.5 to 65.5 Ma, suggest prolonged (~5 myr) magmatic activities in the CGC. These syn- and pre-Deccan LIP magmatic intrusive activities in the CGC are supposedly related to the Réunion mantle plume. The reported age of 70.5 ± 0.9 Ma of a NE-trending mafic dyke emplaced within the Raniganj basin could probably be the earliest record of the Réunion mantle plume activity in the Indian shield. A number of other early magmatic rocks, related to the Réunion mantle plume induced Deccan LIP event, are also recorded elsewhere in the Indian shield and supportive of prolonged magmatic activities. Finally, this study also provides a better constraint on the initiation and lateral extent of the Réunion mantle plume induced Deccan LIP. [ABSTRACT FROM AUTHOR]

The Jiawula Ag–Pb–Zn deposit is located in the northern part of the Great Xing'an Range metallogenic belt within the eastern segment of the Central Asian Orogenic Belt. Here, we report results from muscovite 40Ar/39Ar geochronology and fluid inclusion study and formulate a vertical projection map of the ore grade in this deposit. The muscovite from the Jiawula deposit yields a plateau age of 133.27 ± 0.66 Ma and a 40Ar/39Ar isochron age of 131.88 ± 0.83 Ma. The muscovite 40Ar/39Ar data indicate a discrete second hydrothermal event postdating the mineralization, which we correlate with post‐collisional extension after the subduction direction of the Palaeo‐Pacific Plate changed. Low‐salinity aqueous fluid inclusions in quartz from the Jiawula deposit represent meteoric water or groundwater. Based on the fluid inclusion study, the fluids were trapped during cooling and decompression, which may have resulted in metal precipitation. We envisage that the copper precipitated from a high‐temperature fluid in the southern domain whereas lead, zinc, and silver precipitated at a lower temperature in the north. The spatial distribution of the ore‐forming elements, therefore, reflects the ore fluid migration‐cooling path from the south to north. [ABSTRACT FROM AUTHOR]

The Leucite Hills Volcanic Field, southwest Wyoming comprises two dozen volcanic features including necks, flows, dikes, and plugs. It has been the focus of many petrologic studies as its volcanic and shallow intrusive rocks are one of the only surficial manifestations of ultrapotassic lamproite. We build on paleomagnetic findings of Sheriff and Shive (1980) by providing further paleomagnetic data from the Boars Tusk dike and Black Rock flows. We also characterize the magnetic mineral assemblage of these lamproites. Principal component analysis of alternating field (AF) and thermal demagnetization data indicate that the dike and breccias of Boars Tusk record a reversed magnetic polarity and the Black Rock lava records a normal polarity, both consistent with previous findings. This recording is typically carried by minerals with coercivities > 15 mT and susceptibility measurements indicate magnetite, maghemite, and titanomagnetite as likely magnetic carriers. AF and thermal demagnetization experiments evince secondary magnetizations held by lower coercivity grains, likely caused by lightning strikes. 40Ar/39Ar incremental heating experiments from Boars Tusk and Black Rock give plateau ages of ∼ 2500 ka and ∼ 900 ka, respectively. Recent advances in the chronology of geomagnetic field reversals and excursions during the Quaternary permit integration of the Boars Tusk dike into the lower Matuyama chron, whereas the Black Rock lavas most probably record the Kamikatsura excursion. Notably, Black Rock records high inclinations that suggest the short-lived excursion achieved a full geomagnetic reversal, something not observed at other localities recording the Kamikatsura excursion. The Leucite Hills offer further opportunities to refine the Quaternary geomagnetic instability time scale (GITS), and to improve understanding of the eruptive and geomorphic evolution of this unusual volcanism. • New 40Ar/39Ar data constrains eruptive history of Leucite Hills volcanic complex. • New paleomagnetic and electron microprobe data reported for Leucite Hills. • First paleomagnetic record of Kamikatsura geomagnetic excursion in North America. [ABSTRACT FROM AUTHOR]

The composite Albian–Eocene orogenic wedge of the northern part of the Inner Western Carpathians (IWC) comprises the European Variscan basement with the Upper Carboniferous–Triassic cover and the Jurassic to Upper Cretaceous sedimentary successions of a large oceanic–continental Atlantic (Alpine) Tethys basin system. This paper presents an updated evolutionary model for principal structural units of the orogenic wedge (i.e., Fatricum, Tatricum and Infratatricum) based on new and published white mica 40Ar/39Ar geochronology and P–T estimates by Perple_X modeling and geothermobarometry. The north-directed Cretaceous collision led to closure of the Jurassic–Early Cretaceous basins, and incorporation of their sedimentary infill and a thinned basement into the Albian–Cenomanian/Turonian accretionary wedge. During this compressional D1 stage, the subautochthonous Fatric structural units, including the present-day higher Infratatric nappes, achieved the metamorphic conditions of ca. 250–400 °C and 400–700 MPa. The collapse of the Albian–Cenomanian/Turonian wedge and contemporary southward Penninic oceanic subduction enhanced the extensional exhumation of the low-grade metamorphosed structural complexes (D2 stage) and the opening of a fore-arc basin. This basin hemipelagic Coniacian–Campanian Couches-Rouges type marls (C.R.) spread from the northern Tatric edge, throughout the Infratatric Belice Basin, up to the peri-Pieniny Klippen Belt Kysuca Basin, thus tracing the south-Penninic subduction. The ceasing subduction switched to the compressional regime recorded in the trench-like Belice “flysch” trough formation and the lower anchi-metamorphism of the C.R. at ca. 75–65 Ma (D3 stage). The Belice trough closure was followed by the thrusting of the exhumed low-grade metamorphosed higher Infratatric complexes and the anchi-metamorphosed C.R. over the frontal unmetamorphosed to lowest anchi-metamorphosed Upper Campanian–Maastrichtian “flysch” sediments at ca. 65–50 Ma (D4 stage). Phengite from the Infratatric marble sample SRB-1 and meta-marl sample HC-12 produced apparent 40Ar/39Ar step ages clustered around 90 Ma. A mixture interpretation of this age is consistent with the presence of an older metamorphic Ph1 related to the burial (D1) within the Albian–Cenomanian/Turonian accretionary wedge. On the contrary, a younger Ph2 is closely related to the late- to post-Campanian (D3) thrust fault formation over the C.R. Celadonite-enriched muscovite from the subautochthonous Fatric Zobor Nappe meta-quartzite sample ZI-3 yielded a mini-plateau age of 62.21 ± 0.31 Ma which coincides with the closing of the Infratatric foreland Belice “flysch” trough, the accretion of the Infratatricum to the Tatricum, and the formation of the rear subautochthonous Fatricum bivergent structure in the Eocene orogenic wedge.

The Tiantangshan tin‐polymetallic deposit, located in the Nanling Range of South China, is a medium‐sized polymetallic deposit found in the region in recent years. The deposit is hosted within volcanic rocks, and the orebodies occur in the cupola and outer contact zone of the concealed quartz porphyry, as well as the altered fracture zone of the volcanic rocks close to the intrusive contact. In light of field evidence and petrographic observations, the mineralization can be divided into four stages: greisenization stage (stage I), quartz–cassiterite–wolframite stage (stage II), quartz–fluorite–cassiterite–sulfides stage (stage III), and post‐ore stage (stage IV). Three types of fluid inclusions are present in the hydrothermal topaz, quartz, and fluorite, including H2O‐rich (W‐type, WL‐ and WV‐ subtypes), CO2‐bearing (C‐type), and solid‐bearing (S‐type). Four stages of fluid evolution are observed by detailed fluid inclusion studies: (a) Stage I fluids are trapped under two‐phase immiscible condition, as evidenced by the coexistence of primary aqueous (W‐type) and aqueous‐carbonic (C‐type) fluid inclusions preserved in topaz and quartz; the fluid inclusions display homogenization temperatures of 378–448°C and salinities of 6.0–17.5 wt.% NaCl equiv. (b) Similarly, fluid inclusions in stage II quartz also record immiscible condition, as identified by the coexistence of W‐type and C‐type fluid inclusions with lower homogenization temperatures (308–400°C) and salinities (1.8–14.5 wt.% NaCl equiv.). (c) Stage III fluids are characterized by the coexistence of widespread WL‐subtype, minor S‐type, and rare WV‐subtype fluid inclusions, with similar homogenization temperatures (230–369°C) but contrasting salinities (0.5–39.5 wt.% NaCl equiv.), which indicates an episode of fluid boiling occurred in this stage. (d) Stage IV marks the end of the hydrothermal system characterized by the lower temperatures (175–298°C) and salinities (0.2–3.9 wt.% NaCl equiv.) W‐type fluid inclusions trapped. Microthermometry and H–O isotopes indicate that the early ore‐forming fluids (in stage I) exsolved from the granitic magma and underwent progressive mixing with meteoric water during subsequent ore‐forming process (in stages III and IV). The water–CO2 fluid immiscibility is the main mechanism of cassiterite and wolframite precipitation, while fluid boiling and mixing are probably thought to be the dominant mechanisms for the deposition of sulfide at Tiantangshan. 40Ar/39Ar dating of hydrothermal biotite intergrown with cassiterite shows that tin‐polymetallic mineralization occurred at ~133 Ma which is coeval with the hidden intrusions. Taken together, these lines of evidence confirm that the Tiantangshan deposit is a magmatic hydrothermal greisen–quartz–vein type tin‐polymetallic deposit that formed in the lithospheric extension and thinning setting associated with the postsubduction Paleo‐Pacific Plate tectonic regime that influenced South China. [ABSTRACT FROM AUTHOR]

The Cook‐Austral islands do not exhibit a simple linear age progression from a single point of active volcanism, in contrast to other Pacific island chains. Explaining this anomaly, however, has been difficult due to the low reliability of the existing K/Ar ages for these islands. Here we present 56 new incremental heating 40Ar/39Ar age determinations for eight of the Cook‐Austral islands. We show that these 40Ar/39Ar ages are on average 10–40% different and generally older than the K/Ar ages for the same samples, are more reproducible within single lava flows, exhibit less scatter among data from a single island, and are more reliable than published K/Ar age determinations. Our new data result in two clearly defined and matching age‐progressive trends, with origins at the Macdonald and Arago seamounts. This supports the hypothesis that the Macdonald and Rurutu tracks formed by two contemporaneous hotspots aligned in the direction of plate motion. Compared to other volcanic chains on the Pacific plate, the Cook‐Austral hotspot tracks record angular rotational plate velocities (0.96 ± 0.05 to 1.09 ± 0.04°/Ma) similar to Hawaii (1.15°/Ma) but faster than Samoa (0.63°/Ma). This may imply that hot spot location relative to tectonic boundaries may have an effect on the age progressions recorded by volcanic chains. Specifically, the Cook‐Austral and Hawaii hot spots have over the last 30 Myr been located truly intraplate and thus far away from any tectonic boundary, while Samoa has been proximal to the active Tonga‐Kermedec subduction zone. Key Points: Macdonald and Arago hotspot trails are similar to Hawaii but different from SamoaLocal recorded plate velocities at seamount trails are likely modulated at plate tectonic boundariesPrimary and secondary hotspots together remain viable candidates for recording plate and plume motions [ABSTRACT FROM AUTHOR]

The rhyolite of Bodie Hills (California) is characterized compositionally and the geochronology of selected phases is studied. Sanidine (BHs) from the rhyolite is well suited as a 40Ar/39Ar reference material with high K/Ca and radiogenic yield. Intercalibration with GA1550 biotite from the Dromedary igneous complex (New South Wales, Australia) yields an age of 9.7946 ± 0.0031 Ma for BHs relative to an age of 98.79 ± 0.54 Ma for GA1550 and a calibration factor (R BHs/GA1550) of 0.096719 ± 0.000032. BHs is also intercalibrated with sanidines of the Taylor Creek Rhyolite (TCs; R TCs/BHs of 2.90874 ± 0.00067), Fish Canyon Tuff (FCs; R FCs/BHs of 2.88339 ± 0.00088), and rhyolite of Alder Creek (ACs; R ACs/BHs of 0.12028 ± 0.00024). These calibration factors yield ages of 28.344 ± 0.011 Ma, 28.099 ± 0.013 Ma, and 1.1809 ± 0.0024 Ma for TCs, FCs, and ACs, respectively, relative to GA1550. Full propagation of errors increases these uncertainties and that of BHs to ±0.9% of their ages. Calibration of BHs using the astronomically tuned age of FCs determined by Kuiper et al. (2008) yields an age of 9.8295 ± 0.0036 Ma. Stepwise heating of BHs reveals the same small, progressive increase in age across the age spectrum reported for FCs, ACs, and other potassium feldspars (e.g., Foland and Xu, 1990; Phillips et al., 2017). This increase is consistent with mass fractionation of argon during step heating and favors use of single-step fusion ages of all sanidines for monitor and calibration purposes. Zircons from the rhyolite of Bodie Hills are strongly zoned in U and Th, and U/Pb geochronologic analyses suggest multiple generations of zircon growth. The youngest ages indicate a crystallization age of 9.97 ± 0.08 Ma (2σ); arguably similar to the 40Ar/39Ar results when residence time is considered, but previous episodes of zircon growth began at least 400 ka prior to eruption. Precise (U - Th)/He thermochronology of these zircons is difficult because crystal-to-crystal variations in the magnitude and complexity of their U + Th zoning complicates accurate alpha ejection calculations. Replicate conventional (single-crystal) (U - Th)/He ages are more widely dispersed than predicted by analytical uncertainties, but laser ablation (U - Th)/He ages are more reproducible, with an inverse variance-weighted mean of 9.71 ± 0.58 Ma. • Characterization of the Bodie Hills sanidine standard with 40Ar/39Ar, U/Pb, and (UTh)/He geochronology and geochemistry. • Intercalibration of a high K/Ca, highly radiogenic 9.8-Ma sanidine standard with widely used 40Ar/39Ar mineral standards. • Further documentation of mass fractionation of Ar isotopes during stepwise heating argues against its use for standards. • SHRIMP U - Pb analyses of Bodie Hills zircons show at least 400 ka of pre-emplacement history similar to Fish Canyon Tuff. [ABSTRACT FROM AUTHOR]

Argon release mechanisms and diffusivity were quantified for coarsely crystalline hypogene alunite [KAl 3 (SO 4) 2 (OH) 6 ] from Marysvale, Utah, and microcrystalline supergene alunite aggregates from Coober Pedy, South Australia. Prior to diffusivity studies, all alunite samples, sorted in the 500–200, 200–100, 100–50, 50–10, and <10 µm sieve-size ranges, were vacuum encapsulated to quantify 39Ar recoil losses. Incremental-heating of single capsules inserted into Ta-crucibles and heated by a projector-lamp in a specially designed diffusion cell permits quantifying 39Ar released at precisely measured temperature steps (±1–3 °C). Incremental-heating 40Ar/39Ar analyses of the various sieve-size ranges yield reproducible ages that are indistinguishable from single grain (1–2 mm) laser-heating 40Ar/39Ar dating of the same samples. The results, cast in Arrhenius plots, yield activation energies (E a) ranging from 248.0 ± 18.5 to 281.7 ± 15.2 kJ/mol and ln(D o /a2) from 26.2 ± 2.0 to 27.8 ± 3.2 ln(s−1) for hypogene alunite; supergene alunite yield E a ranging between 233.3 ± 5.4 and 293.8 ± 13.7 kJ/mol and ln(D o /a2) between 27.3 ± 1.0 and 36.8 ± 2.6 ln(s−1). These diffusion parameters correspond to closure temperatures of 264 ± 22 °C and 246 ± 19 °C for hypogene and supergene alunite, respectively, assuming a cooling rate 100 °C·Ma−1. In-situ TEM experiments on aliquots of alunite crystals from the same samples indicate that alunite single crystals undergo transformation to nanocrystalline aggregates at 430–460 °C, showing that alunite releases Ar by volume diffusion below ∼430 °C, retains a significant amount of Ar during phase transformation, and proceeds to release Ar by volume or multipath diffusion from a modified polycrystalline structure at T > 460 °C. Isothermal holding time and AGESME modelling using our calculated diffusion parameters indicate that alunite should preserve Ar quantitatively for long periods (4.0 Ga) at Earth and Mars surface conditions, and both hypogene and supergene alunite should preserve original formation ages, independently of precipitation mechanism. [ABSTRACT FROM AUTHOR]

Geomorphological and sedimentary records near the confluences of the Tana River and major tributaries draining the eastern slopes of Mt. Kenya and the Nyambeni Range, indicate impacts of Late Quaternary volcanic activity in their fluvial records. The main reconstructed event was triggered by a 366.9 ka basalt flow (40Ar/39Ar dated) which flowed along Kazita River from the Nyambeni Range blocking both Kazita River and Tana River near Kibuka Grand Falls, causing a temporary lake. Consequently, Tana River and Kazita River started to build volcanoclastic Gilbert type deltas. The preserved pro-delta sediments rich in trachytic pumice fragments display a mineralogical and age match with known Ithanguni trachytic tuffs, indicating delta build up right after a contemporary Ithanguni eruption. This trachytic eruption caused the deposition of lahars and fluvial volcaniclastic sediments in all river records draining the Eastern side of Mt. Kenya. The multiple lahars seem to be triggered by eruptions under glacial conditions (basalt age indicates MIS 10). The lava dammed lake was only short lived (estimated to have lasted only a few years to decades) and breached before a complete lake infill could occur. The current Kibuka Grand Falls can be viewed as the delayed incisional response of this lava dam breach, indicating that after >366.9 ka, Tana River is still responding and adjusting to this short-lived disruptive phase. The current Kazita River has re-incised adjacent to a MIS 4 basalt flow down into the crystalline Basement System rocks. The MIS 10 pre-volcanic sedimentary record indicates that more sediments were in the fluvial system during glacial conditions than during the interglacial conditions. An implication of our reconstruction is that the Late Quaternary fluvial record of Tana River is of only limited use to reconstruct uplift rates because reconstructed Quaternary incision rates are reflecting both volcanic disruptions as climate change trends of aridification and decreasing glaciation extents. Unlabelled Image • Excellent examples of fluvial volcanic interactions in a tropical continental rift setting. • New insights into the geochronological fluvial – volcanic framework of Tana river and Mt. Kenya. • In a dominant incising system with limited fluvial preservation, volcanic activity and glaciations are driving large fluvial aggradation phases. • Quaternary climate change of aridification is inferred by decreasing sediment supply and decreasing fluvial aggradation. [ABSTRACT FROM AUTHOR]

Hypogene and supergene alunite and jarosite grains in the 500–200, 200–100, 100–50, 50–10, and <10 µm sieve-size ranges were neutron-irradiated in vacuum-sealed Si-glass capsules to permit direct quantification of 39Ar displaced by recoil during irradiation. Coarsely crystalline hypogene alunite and jarosite samples break into single crystals or equant fragments of single crystals when sieve size is larger than 10 µm. Recoil losses from these samples is less than 0.5%, and only 0.1% for alunite and 0.19% for jarosite in the sieve-size fractions > 200 µm, the typical size used in single crystal 40Ar/39Ar geochronology. The < 10 µm sieve-size fraction is composed of crystal shards with large surface:volume ratios; these sample powders may undergo up to 2.44% (alunite) and 2.24% (jarosite) 39Ar recoil losses. Supergene alunite samples are composed of aggregates of 1–5 µm pseudo-cubic crystallites, and 39Ar recoil losses are ∼ 1.0 ± 0.2%, independently of the sieve-size fraction dated. Supergene jarosite, composed of aggregates of 1–30 µm hexagonal plates, undergoes < 2% 39Ar recoil loss in sieve-size samples > 10 µm. Single crystals and shards of jarosite crystals in sieve-size fractions smaller than 10 µm show the largest recoil losses at 4.22%. The large 39Ar recoil losses in sieve-size samples < 10 µm is irrelevant for geochronology since routine 40Ar/39Ar analysis does not target grains in that size range. SRIM and TRIM modeling shows that recoil range (mean displacement distance) is 0.16 µm in amorphous targets of both alunite [KAl 3 (SO 4) 2 (OH) 6 ] (ρ = 2.74 g/cm3) and jarosite [KFe 3 (SO 4) 2 (OH) 6 ] (ρ = 3.09 g/cm3) compositions, resulting in 39Ar-depleted rims of 0.114 µm in alunite and 0.116 µm in jarosite. Modeling results show that most of the 39Ar ions ejected by recoil from individual crystallites in supergene samples gets re-implanted in nearby crystallites, resulting in relatively low 39Ar losses. The experimental and modeling results confirm that 39Ar recoil loss is not an impediment in dating of hypogene and supergene alunites and jarosites to precision and accuracy comparable to that obtained for silicates commonly used in 40Ar/39Ar geochronology. [ABSTRACT FROM AUTHOR]

La Poruña (21°53′S; 68°30′W) is a 140 m high scoria cone composed of pyroclastic material and an extensive basaltic-andesite to andesite lava flow that is up to 8 km in length. Automated mineralogical analysis describes a suite of porphyritic mafic samples, comprising olivine- and pyroxene-bearing rocks. Well-defined major element compositional trends, as well as trace and rare earth element characteristics (e.g. Sr/Y < 47; Sm/Yb < 4), likely reflect magmatic differentiation at middle-upper crustal levels. Additionally, magma mixing and assimilation and fractional crystallization processes act on these La Poruña magmas within the thickened continental crust, which is typical in Andean volcanic systems. A remarkable compositional feature is the unusual reversed isotopic behaviour of increasing silica with decreasing 87Sr/86Sr compositions. In a process of crustal assimilation during turbulent magma ascent (ATA), the least differentiated rocks are the most contaminated ones since the turbulent hottest magmas effectively assimilate the crustal material. We relate the inverse Sr isotope trend to latter magmatic evolution involving ATA at shallow crustal levels prior to eruption, therefore differing from the broadly accepted Central Andean magmatic model. The older volcanics (>96 ka) from San Pedro volcano exhibit similar isotopic characteristics, therefore evidence of similar magmatic processes. This new dataset clearly defines magma compositional changes during the La Poruña eruption (ca. 100 ka), revealing an increase in crustal contamination at shallow crustal levels for the younger San Pedro lavas (<96 ka), likely controlled by increasing amounts of deep-sourced basaltic input over time. Unlabelled Image • La Poruña is a mafic scoria cone coeval with eruptions of San Pedro volcano. • 87Sr/86Sr decreases as SiO 2 increases for La Poruña scoria cone. • La Poruña magmas evolved by selective assimilation during turbulent ascent (ATA) process. • San Pedro eruptives are related to similar ATA and late stages of fractional crystallization processes. [ABSTRACT FROM AUTHOR]