Age of the Rockland tephra, western USA

The age of the Rockland tephra, which includes an ash-flow tuff south and west of Lassen Peak in northern California and a widespread ash-fall deposit that produced a distinct stratigraphic marker in western North America, is constrained to 565,000 to 610,000 yr by ⁴⁰Ar/³⁹Ar and U-Pb dating. ⁴⁰Ar/³⁹Ar ages on plagioclase from pumice in the Rockland have a weighted mean age of 609,000 ± 7000 yr. Isotopic ages of spots on individual zircon crystals, analyzed by the SHRIMP-RG ion microprobe, range from ∼500,000 to ∼800,000 yr; a subpopulation representing crystal rims yielded a weighted-mean age of 573,000 ± 19,000 yr. Overall stratigraphic constraints on the age are provided by two volcanic units, including the underlying tephra of the Lava Creek Tuff erupted within Yellowstone National Park that has an age of 639,000 ± 2000 yr. The basaltic andesite of Hootman Ranch stratigraphically overlies the Rockland in the Lassen Peak area and has ⁴⁰Ar/³⁹Ar ages of 565,000 ± 29,000 and 565,000 ± 12,000 yr for plagioclase and groundmass, respectively. Identification of Rockland tephra in ODP core 1018 offshore of central California is an important stratigraphic age that also constrains the eruption age to between 580,000 and 600,000 yr.     

Detailed <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of geologic events associated with the Mantos Blancos copper deposit,northern Chile

The ⁴⁰Ar/³⁹Ar geochronological method was applied to date magmatic and hydrothermal alteration events in the Mantos Blancos mining district in the Coastal Cordillera of northern Chile, allowing the distinction of two separate mineralization events. The Late Jurassic Mantos Blancos orebody, hosted in Jurassic volcanic rocks, is a magmatic-hydrothermal breccia-style Cu deposit. Two superimposed mineralization events have been recently proposed. The first event is accompanied by a phyllic hydrothermal alteration affecting a rhyolitic dome. The second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit. Because of the superposition of several magmatic and hydrothermal events, the obtained ⁴⁰Ar/³⁹Ar age data are complex; however, with a careful interpretation of the age spectra, it is possible to detect complex histories of successive emplacement, alteration, mineralization, and thermal resetting. The extrusion of Jurassic basic to intermediate volcanic rocks of the La Negra Formation is dated at 156.3 ± 1.4 Ma (2σ) using plagioclase from an andesitic lava flow. The first mineralization event and associated phyllic alteration affecting the rhyolitic dome occurred around 155–156 Ma. A younger bimodal intrusive event, supposed to be equivalent to the bimodal stock and sill system inside the deposit, is probably responsible for the second mineralization event dated at ca. 142 Ma. Other low-temperature alteration events have been dated on sericitized plagioclase at ca. 145–146, 125, and 101 Ma. This is the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in the metallogenic belt of the Coastal Cordillera of northern Chile. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar,and U-Pb geochronology

Mineralization and alteration events at ten Miocene porphyry Cu and porphyry-related epithermal mineral deposits in southern, central, and northern Ecuador were dated by means of molybdenite Re-Os, biotite and alunite ⁴⁰Ar/³⁹Ar, and titanite U-Pb geochronology. Most of these hydrothermal events show a spatio-temporal correlation with porphyry intrusion emplacement as constrained by zircon U-Pb ages. The total age range for these events spans the 23.5–6.1 Ma period, without displaying systematic along- or across-arc age distribution trends. While epithermal deposits tend to be spatially associated with volcanic rocks of a similar age, porphyry Cu deposits in Ecuador are frequently spatially associated with deeper-seated basement units and batholith-scale precursor intrusive systems assembled over ≥5 m.y. time periods. In most cases, formation of the porphyry Cu deposits is related to the youngest magmatic (-hydrothermal) event in a given area, postdating batholith construction at a regional scale. The majority of Miocene deposits occurs in southern Ecuador where areally extensive, post-mineralization (late Miocene to recent) volcanic sequences with the potential to conceal mineralization at depth are lacking. Only few Miocene deposits occur in northern-central Ecuador, where they mainly crop out in the Western Cordillera, west of the productive present-day volcanic arc. The surface distribution of post-mineralization arc volcanism reflects along-arc variations in subducting slab geometry. Porphyry Cu and epithermal deposits in Ecuador define a Miocene metallogenic belt broadly continuous with its coeval counterpart in northern-central Peru. Although both belt segments were formed in an overall similar tectonomagmatic and metallogenic setting, their respective metal endowments differ significantly.     

滇西腾冲更新世粗面安山岩Ar-Ar年代学、地球化学特征及其构造意义

腾冲芒棒地区广泛发育新近纪以来的火山岩,本文首次对该区粗面安山岩进行的高精度40Ar/39Ar同位素定年,其坪年龄分别为2.1±0.1Ma和2.0±0.1Ma,代表其喷溢时代为早更新世。岩石地球化学特征研究显示,粗面安山岩属高钾钙碱性系列,富集大离子亲石元素、亏损高场强元素,具有弧火山岩与大陆板内火山岩的双重属性,推测其形成于后造山环境。综合研究表明,腾冲地块更新世火山岩的形成与印度-欧亚板块俯冲-碰撞造山隆升后的伸展垮塌、富集地幔部分熔融作用密切相关,而印支陆块向东南逃逸以及腾冲弧形走滑断裂带(龙川江断裂带、大盈江断裂带)的活化促使了该期火山岩的喷发与侵位。     

40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field,Saudi Arabia: Implications for duration and migration of volcanic activity

New ⁴⁰Ar/³⁹Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma–Recent. The new ⁴⁰Ar/³⁹Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the ⁴⁰Ar/³⁹Ar plateau ages show that volcanism in this area began in the Neogene (∼10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.     

Age and Chemistry of Miocene Volcanic. Rocks from the Kiraz Basin of the Küçük Menderes Graben: Its Significance for the Extensional Tectonics of Southwestern Anatolia,Turkey'

Neogene volcanic rocks and granitoid plutons are among the most important geological components of western Turkey. Although they are voluminous north of the Gediz Graben, they are very scarce to the south, where volcanic rocks occur as isolated small exposures in a small number of localities. The Kiraz Basin of the Küçük Menderes Graben is a key locality, in which Tertiary volcanic rocks crop out at three locations. These rocks have been chemically analysed and dated (³⁹Ar-⁴⁰Ar whole rock and biotite analyses) in order to understand their tectonic setting of emplacement and its relation to the wider structure of western Anatolia. Whole rock and biotite ³⁹Ar-⁴⁰Ar ages vary between 13.9 ± 0.2 Ma and 14.6 ± 0.2 Ma.

The Kiraz volcanic rocks are calc-alkaline, with a compositional range from basaltic andesite to dacite. They are strongly enriched in the light ion lithophile elements (LILE) and have chemistries typical of lavas erupted in subduction-related settings. Their close association with rift-bounding faults suggests eruptions via conduits flanking grabens in an extensional environment. The difference in chemical composition and age between the Kiraz volcanic rocks and the slightly older calc-alkaline volcanic rocks north of the Gediz Graben is attributed to their relatively younger ages and greater proximity to the Aegean Arc. Their calc-alkaline chemistry reflects magma generation influenced by the slab descending beneath this arc and eruption/emplacement in an extensional setting.     

^40Ar／^39Ar Plateau Age Spectra of Feldspars and Their Geological Implications

The paragenic minerals plagioclase,perthite,biotite,hornblende and pyroxene in acid-granite alkali-granite,monzonite and volcanic rocks collected from seven areas different in thermal history have been determined.On the basis of 14 plateau age spectra and isochron ages of paragenic miner-als in conjunction with the observed mineral textures ,the suitability of plateau age spectra of plagioclase and perthite and their thermo-chronological significance are discussed in this paper.The results indicate that undisturbed feldspar gives satisfactory^40Ar-^39Ar plateau ages in consis-tence with those of paragenic minerals.This means that feldspars from the undisturbed area are suita-ble for ^40Ar-^39Ar dating .On the other hand,the age spectra of feldspars as well as of biotite,pyroxene and hornblende affected by tectonic or thermodynamic events appear unsmooth at varying temperatures,thus complicating their interpretation.Feldspars may give an emplacement age of a rock and /or that of the latest thermodynamic event,depending on the intensity of the event and the retentivity of Ar in the mineral.     

40Ar/39Ar Geochronology of Volcanic and Intrusive Rocks in the Papandayan Metallic Prospect Area,West Java,Indonesia

This study presents new ⁴⁰Ar/³⁹Ar ages on the volcanic and intrusive rocks from the Papandayan metallic district in West Java, Indonesia. The vein system in the Arinem area, one of the prospective areas in the district, has been considered as an epithermal gold–silver–base metal deposit, however, no published age results are available for the host volcanic rocks in the district. The ages of these rocks are critical in terms of their association with mineralization and are important to understand the evolution of volcanism in the region, which has implications for mineral exploration in the district. ⁴⁰Ar/³⁹Ar plateau ages of two typical basalt and one andesite samples of the Jampang Formation volcanic rocks yielded ages of 11.65 ± 0.52 Ma, 18.15 ± 0.46 Ma and 7.69 ± 0.05 Ma, respectively. ⁴⁰Ar/³⁹Ar ages of three intrusive rock samples from Gunung Halang diorite, Gunung Lingga diorite, and Gunung Buligir fine‐grained quartz diorite yielded ages of 12.98 ± 0.20 Ma, 10.81 ± 0.15 Ma, and 7.37 ± 0.05 Ma, respectively. The age of the youngest fine‐grained diorite (Gunung Wayang dike) is 3.95 ± 0.03 Ma. An ⁴⁰Ar/³⁹Ar age obtained from adularia in the Arinem mineralized vein (18.30 ± 0.20 Ma) is older than the age of altered basalt sample of this study (11.65 ± 0.52 Ma) and the K–Ar illite ages of the Arinem vein (9.4 ± 0.3 Ma and 8.8 ± 0.3 Ma) which resulted from a previous study. The age results suggest that the Papandayan district may have experienced multiple hydrothermal and mineralization events. This study, therefore, provides crucial age data to support future mineral exploration in the district.     

Temporal and chemical connections between plutons and ignimbrites from the Mount Princeton magmatic center

The Mount Princeton magmatic center, located in central Colorado, consists of the epizonal Mount Princeton batholith, the nested Mount Aetna caldera, and volumetrically minor leucogranites. New CA-TIMS U/Pb zircon ages indicate the majority of the Mount Princeton batholith was emplaced during a period of regional ignimbrite quiescence. The structurally highest unit of quartz monzonite yields a ²⁰⁶Pb/²³⁸U age of 35.80 ± 0.10 Ma, and the youngest dated unit of the quartz monzonite is a porphyritic unit that yields a ²⁰⁶Pb/²³⁸U age of 35.37 ± 0.10 Ma. Using the exposed, dated volume of the quartz monzonite and new geochronology yields an estimated pluton filling rate of ~0.002 km³/a. This rate is comparable to the accumulation rates published for other plutons, and at least an order of magnitude slower than fluxes necessary to support accumulation of large eruptible magma volumes. Geochronology for the two large ignimbrites spatially associated with the batholith indicates a temporal disconnect between the vast majority of pluton building and explosive eruption of magma. The Wall Mountain Tuff erupted from a source in the same geographic area as the Mount Princeton batholith at 37.3 Ma (Ar/Ar sanidine), but no structural evidence of a caldera or temporally associated plutonic rocks is known. The Badger Creek Tuff erupted at 34.3 Ma (Ar/Ar sanidine) during the formation of the Mount Aetna caldera in the southern portion of the batholith. Our ²⁰⁶Pb/²³⁸U age for the Badger Creek Tuff is 34.47 ± 0.05. The only analyzed plutonic rocks of similar age to the Badger Creek Tuff are an extra-caldera dike with a ²⁰⁶Pb/²³⁸U age of 34.57 ± 0.08 Ma, a ring dike with a ²⁰⁶Pb/²³⁸U age of 34.48 ± 0.09 Ma, and a portion of the Mount Aetna pluton with a ²⁰⁶Pb/²³⁸U age of 34.60 ± 0.13 Ma. The small volume intrusions related to the eruption of the Badger Creek Tuff are chemically similar to the ignimbrite and show no signature of crystal–liquid separation in the shallow crust.     

40Ar/39Ar and K–Ar geochronology of magmatic and hydrothermal events in a classic low-suphidation epithermal bonanza deposit: El Peñon, northern Chile

The epithermal El Peñon gold–silver deposit consists of quartz–adularia veins emplaced within a late Upper Paleocene rhyolitic dome complex, located in the Paleocene–Lower Eocene Au–Ag belt of northern Chile. Detailed K–Ar and ⁴⁰Ar/³⁹Ar geochronology on volcano–plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events. The Paleocene to Lower Eocene magmatism in the El Peñon area is confined to a rhomb-shaped basin, which was controlled by N–S trending normal faults and both NE- and NW-trending transtensional fault systems. The earliest products of the basin-filling sequences comprise of Middle to Upper Paleocene (～59–55 Ma) welded rhyolitic ignimbrites and andesitic to dacitic lavas, with occasional dacitic dome complexes. Later, rhyolitic and dacitic dome complexes (～55–52 Ma) represent the waning stages of volcanism during the latest Upper Paleocene and the earliest Eocene. Lower Eocene porphyry intrusives (～48–43 Ma) mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime. ⁴⁰Ar/³⁹Ar geochronology of hydrothermal adularia from the El Peñon deposit yields ages between 51.0±0.6 and 53.1±0.5 Ma. These results suggest that mineralization occurred slightly after the emplacement of the El Peñon rhyolitic dome at 54.5±0.6 Ma (⁴⁰Ar/³⁹Ar age) and was closely tied to later dacitic–rhyodacitic bodies of 52 to 53 Ma (K–Ar ages), probably as short-lived pulses related to single volcanic events.     

New radiometric dating of the dykes from the Hurd Peninsula, Livingston Island, South Shetland Islands

Fifteen new K–Ar ages in the range of 79–31 Ma are partially confirmed by three ⁴⁰Ar/³⁹Ar plateaus and isochron data of 64.9±0.4, 55.5±0.1 and 52.8±0.6 Ma. The new geochronological data reveal a much more detailed picture of the subduction imprint in the Hurd Peninsula. Using cutting relationships, the dyke emplacement history is divided into four episodes. The Late Cretaceous–Paleocene dykes in the range of 80–60 Ma are related to the main magmatism in Livingston Island and most likely reflect the final stages of subduction of the proto-Pacific oceanic crust. The Early Eocene dykes (56–52 Ma) fill the gap in volcanic activity 70–50 Ma ago. They are the only magmatic event manifested at this time in the region. The 45–42 Ma dykes may be related to the intrusion of the Barnard Point tonalite. Three samples of Oligocene age appear to represent the last igneous activities on the Hurd Peninsula prior to the opening of the Bransfield Strait.     

应用40Ar—39Ar定年技术研究某些火山岩及陨石样品的受热历史

常规的K-Ar法是基于⁴⁰K通过K-层电子捕获衰变成⁴⁰Ar^*这一机理,应用衰变定律而定年的。它具有测定对象广、测定年龄范围大等优点,是同位素地质定年的主要方法之一。但是,由于⁴⁰Ar^*是气体,当岩石、矿物形成以后受到搅动时(如岩浆的侵入、构造活动、宇宙物质的冲击等),⁴⁰Ar^*容易丢失,使年龄值偏低。1962年由Sigurgeirsson提出的,后经Merrihue、Turner等人逐步完善的⁴⁰Ar-³⁹Ar快中子活化定年技术,很好地克服了K-Ar法的局限性。⁴⁰Ar-³⁹Ar定年分为两种:一是全熔融法(total fusion),样品被快中子照射后一次加热熔融,然后计算年龄,此值与常规K-Ar法结果相当;另一种是阶段加热法(step-heating),被照射的样品从低温到高温被逐步加热,分别计算各温度阶段的⁴⁰Ar/³⁹Ar视年龄,并进而得到一条年龄谱和一个坪年龄(plateau age)。后一种方法对研究地质体是否受过热的挠动、岩石矿物的早期结晶年代、后期热挠动次数、热挠动年代、岩石矿物对氢的保存性、过剩氩的存在状态等具有独特作用,它开辟了同位素地质年代学的一个新领域。本文将着重介绍应用⁴⁰Ar-³⁹Ar阶段加热技术研究地球物质及陨石受热历史的某些成果。     

The age and thermal history of Cerro Rico de Potosi, Bolivia

 Cerro Rico de Potosi, Bolivia, is the world’s largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8±0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a ⁴⁰Ar/³⁹Ar date of 13.76±0.10 Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5±1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and widespread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of ³⁹Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system. Received: 14 October 1995/Accepted: 29 January 1996     

Time relationships between volcanism–plutonism–alteration–mineralization in Cu-stratabound ore deposits from the Michilla mining district,northern Chile: a <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronological approach

The Michilla mining district comprises one of the most important stratabound and breccia-style copper deposits of the Coastal Cordillera of northern Chile, hosted by the Middle Jurassic volcanic rocks of the La Negra Formation. ⁴⁰Ar/³⁹Ar analyses carried out on igneous and alteration minerals from volcanic and plutonic rocks in the district allow a chronological sequence of several magmatic and alteration events of the district to be established. The first event was the extrusion of a thick lava series of the La Negra Formation, dated at 159.9 ± 1.0 Ma (2σ) from the upper part of the series. A contemporaneous intrusion is dated at 159.6 ± 1.1 Ma, and later intrusive events are dated at 145.5 ± 2.8 and 137.4 ± 1.1 Ma, respectively. Analyzed alteration minerals such as adularia, sericite, and actinolite apparently give valid ⁴⁰Ar/³⁹Ar plateau and miniplateau ages. They indicate the occurrence of several alteration events at ca. 160–163, 154–157, 143–148, and 135–137 Ma. The first alteration event, being partly contemporaneous with volcanic and plutonic rocks, was probably produced in a high thermal gradient environment. The later events may be related either to a regional low-grade hydrothermal alteration/metamorphism process or to plutonic intrusions. The Cu mineralization of the Michilla district is robustly bracketed between 163.6 ± 1.9 and 137.4 ± 1.1 Ma, corresponding to dating of actinolite coexisting with early-stage chalcocite and a postmineralization barren dyke, respectively. More precisely, the association of small intrusives (a dated stock from the Michilla district) with Cu mineralization in the region strongly suggests that the main Michilla ore deposit is related to a magmatic/hydrothermal event that occurred between 157.4 ± 3.6 and 163.5 ± 1.9 Ma, contemporaneous or shortly after the extrusion of the volcanic sequence. This age is in agreement with the Re–Os age of 159 ± 16 Ma obtained from the mineralization itself (Tristá-Aguilera et al., Miner Depos, 41:99–105,2006).     

Central Mediterranean tephrochronology between 313 and 366 ka: New insights from the Fucino palaeolake sediment succession

Thirty-two tephra layers were identified in the time-interval 313–366 ka (Marine Isotope Stages 9–10) of the Quaternary lacustrine succession of the Fucino Basin, central Italy. Twenty-seven of these tephra layers yielded suitable geochemical material to explore their volcanic origins. Investigations also included the acquisition of geochemical data of some relevant, chronologically compatible proximal units from Italian volcanoes. The record contains tephra from some well-known eruptions and eruptive sequences of Roman and Roccamonfina volcanoes, such as the Magliano Romano Plinian Fall, the Orvieto–Bagnoregio Ignimbrite, the Lower White Trachytic Tuff and the Brown Leucitic Tuff. In addition, the record documents eruptions currently undescribed in proximal (i.e. near-vent) sections, suggesting a more complex history of the major eruptions of the Colli Albani, Sabatini, Vulsini and Roccamonfina volcanoes between 313 and 366 ka. Six of the investigated tephra layers were directly dated by single-crystal-fusion ⁴⁰Ar/³⁹Ar dating, providing the basis for a Bayesian age–depth model and a reassessment of the chronologies for both already known and dated eruptive units and for so far undated eruptions. The results provide a significant contribution for improving knowledge on the peri-Tyrrhenian explosive activity as well as for extending the Mediterranean tephrostratigraphical framework, which was previously based on limited proximal and distal archives for that time interval.     

Ar-Ar Geochronology in a gneiss dome within the Zagros Orogénic Belt

Exhumation of the Tutak mantled gneiss dome without significant cooling has taken place in a doubly plunging anticline within the Sanandaj-Sirjan HP-LT metamorphic belt in the Zagros Thrust System of Iran. Reconstruction of structural evolution of the Tutak gneiss dome at the contact between Arabian and Iranian plates by ⁴⁰Ar/³⁹Ar geochronology exhibits history of the closure of Neo-Tethyan Ocean. There are two granites of different ages in the core of dome; the oldest corresponds to the central Iranian continental crust and was deformed at about 180 Ma. The younger granite was emplaced in the NE–SW transpression system. The timing of strain-related fabrics and exhumation history of the region illustrates the closure of Neo-Tethys and beginning of continent-continent collision at about 77 Ma, as constrained by a well defined plateau ⁴⁰Ar/³⁹Ar age obtained on biotite. Then, the biotite age corresponds to the second stage of emplacement of the Bendenow granite-gneiss which illustrating repeated orogenic events. Continuing deformation without interruption that by now has been created at about 77 Ma, was largely restricted to the transpression and high proportion of simple shear components relative to the pure shear components along the NE–SW.     

The Geysers - Cobb Mountain Magma System, California (Part 1): U-Pb zircon ages of volcanic rocks, conditions of zircon crystallization and magma residence times

Combined U-Pb zircon and ⁴⁰Ar/³⁹Ar sanidine data from volcanic rocks within or adjacent to the Geysers geothermal reservoir constrain the timing of episodic eruption events and the pre-eruptive magma history. Zircon U-Pb concordia intercept model ages (corrected for initial ²³⁰Th disequilibrium) decrease as predicted from stratigraphic and regional geological relationships (1σ analytical error): 2.47 ± 0.04 Ma (rhyolite of Pine Mountain), 1.38 ± 0.01 Ma (rhyolite of Alder Creek), 1.33 ± 0.04 Ma (rhyodacite of Cobb Mountain), 1.27 ± 0.03 Ma (dacite of Cobb Valley), and 0.94 ± 0.01 Ma (dacite of Tyler Valley). A significant (∼0.2-0.3 Ma) difference between these ages and sanidine ⁴⁰Ar/³⁹Ar ages measured for the same samples demonstrates that zircon crystallized well before eruption. Zircons U-Pb ages from the underlying main-phase Geysers Plutonic Complex (GPC) are indistinguishable from those of the Cobb Mountain volcanics. While this is in line with compositional evidence that the GPC fed the Cobb Mountain eruptions, the volcanic units conspicuously lack older (∼1.8 Ma) zircons from the shallowest part of the GPC. Discontinuous zircon age populations and compositional relationships in the volcanic and plutonic samples are incompatible with zircon residing in a single long-lived upper crustal magma chamber. Instead we favor a model in which zircons were recycled by remelting of just-solidified rocks during episodic injection of more mafic magmas. This is consistent with thermochronologic evidence that the GPC cooled below 350° C at the time the Cobb Mountain volcanics were erupted.     

The stratoid granites of central Madagascar: paleomagnetism and further age constraints on neoproterozoic deformation

A paleomagnetic and ⁴⁰Ar/³⁹Ar study of a 630-Ma alkaline granite suite in Madagascar, the so-called ‘stratoid’ granites, reveals a complex history of remagnetization during the formation of the Antananarivo Zone de Virgation at ∼560 Ma (D₂) and the Angavo shear zone at ∼550 Ma (D₃). ⁴⁰Ar/³⁹Ar dating of hornblende, biotite and potassium feldspar from rocks affected by D₂/D₃ show initial cooling rates of 8 °C/Ma during the 550–520 Ma interval followed by slower cooling of 2.5 °C/Ma. The thermal effects of the D₂ and D₃ events appear to be restricted to regions surrounding the shear zones as evidenced by a ⁴⁰Ar/³⁹Ar biotite age of 611.9±1.7 Ma north of the virgation zone. The paleomagnetic data from the stratoid granites are complex and some sites, particularly in areas to the north of the virgation zone, may have been rotated about non-vertical axes following their emplacement and cooling. Because of these possible rotations, our best estimate for the paleomagnetic pole for Madagascar is derived from sites within the virgation zone. This pole falls at 6.7°S, 352.6°E (a₉₅=14.2°). A post-metamorphic cooling history for the virgation zone indicates a magnetization age of 521.4±11.9 Ma. Our work in central Madagascar, coupled with previous studies, suggests that emplacement of the 630 Ma stratoid granites followed a collisional (?) tectonic event beginning around 650 Ma, recently recognized in southern Madagascar and in Tanzania. Subsequently, the stratoid granites in the Antananarivo virgation zone were reheated (∼750–800 °C) at pressures between 3.5 and 3.6 kbars resulting in a pervasive remagnetization. We suggest that the younger shear events are genetically related to collisional tectonics elsewhere during the final stages of Gondwana assembly and are a consequence of the Kuunga Orogeny further south.     

(极)年轻火山岩激光熔蚀40Ar/39Ar定年

对中国大量年轻或/和极年轻火山岩的定年实践研究表明,(极)年轻火山岩的激光熔蚀40Ar/39 Ar定年具有不同于第四纪以前喷发火山岩定年的显著特点.激光熔蚀40Ar/39Ar定年技术因为本底低、样品用量小以及与现代惰性气体同位素质谱设备在灵敏度、高精度方面的相一致,在年轻火山岩的定年中得到深入运用.借助激光在年轻或/和极年轻火山岩的40 Ar/39 Ar定年中,实践证明,样品形成时限越年轻(特别是相当于第四纪时期的样品),Nier值与样品中初始氩比值的偏离会引起K-Ar和40Ar/39 Ar表观年龄的偏差越大.对于小于0.2Ma的样品,Nier值与样品中初始氩比值的偏离对K-Ar和40Ar/39Ar表观年龄的偏差影响呈指数增长;当样品年龄相对较老(老于第四纪)时,Nier值和初始氩比值的偏离对K-Ar和40Ar/39 Ar表观年龄的影响较小.以40Ar/ArAr定年为出发点,定量给出界定年轻与极年轻火山岩的年龄:2～0.2Ma的火山岩界定为年轻火山岩,0.2Ma以来的火山岩称为极年轻火山岩.实验结果还证实,测定(极)年轻火山岩基质年龄时要尽量剔除非同源分馏的斑晶,以便去除斑晶可能带来的过剩氩影响;年轻火山岩样品的测年,应根据岩石结构和粒度特征选取合适的粒度,通常情况下,推荐0.2mm颗粒直径(60～80目)为理想粒径;年轻火山岩样品在快中子辐照后冷却放置时间不宜过长,否则造成37 Ar测不准,影响数据结果,带来较大偏差;激光40Ar/39Ar精细定年对标准样品的均一性有很高的要求,通过标定常用的国内外监测标样发现,标样SB-778-Bi,Bem4M,BT-1均一性很好,适合用作激光熔蚀40Ar/39Ar定年监测;测试数据的处理中,火山岩喷发后冷却结晶中同时形成的斑晶和基质的等时线处理能够帮助获得客观真实和精细的年龄结果.在此基础上,北京大学惰性气体同位素实验室建成了专用于(极)年轻火山岩精细定年的激光熔蚀40Ar/39Ar定年实验流程.