Ar Dating Research Articles

A Middle Miocene basalt dike in the Tomikusa area in southern Nagano Prefecture, central Japan

©The Geological Society of Japan 2013 732 Abstract: We discovered a NNW–SSE-trending, 85-cmwide basalt dike that was intruded into Early Miocene Tomikusa Group sediments in southern Nagano Prefecture, central Japan. K–Ar dating of plagioclase within this dike yielded an age of ca. 13.6 Ma (Middle Miocene), indicative of the timing of emplacement/ cooling of the dike. Intrusion of the dike was coeval with late-stage volcanic activity that formed the Shitara igneous rocks a few tens of kilometers to the southwest of the study area. 報 告

Timing of Thermal Overprints in the Silvermines Granite and Associated Diabase Intrusions, St. Francois Mountains, Missouri

Igneous rocks exposed in the St. Francois Mountains record the geologic history of volcanic activity and plutonic intrusions contributing to the growth and stabilization of the continental crust during the Precambrian. These igneous rocks also contain evidence of thermal overprinting both in the isotope chemistry and the crystalline structures. This study presents new isotopic, mineral and petrologic data to support the timing of dike emplacement and thermal overprinting in the host rock. The study area for this research is along the East Fork of the St. Francis River at the Silver Mines Recreation Area where diabase dikes intrude the surrounding granite. Samples of the Silvermines granite and diabase were collected for petrologic study and isotopic analyses. Thin section analyses revealed minerals and textures that suggest magmatic segregation as well as thermal variation in the diabase intrusions. The surrounding granite exhibits thermal alteration at the contact with the diabase. X-ray diffraction patterns support the thin section analyses and provide clarification on opaque minerals. An x-ray diffraction pattern for the granite also indicates orthoclase, supporting existing data sets for structural resetting of K-feldspar in both the volcanic and plutonic rocks in the area. Mineral separates were prepared for granite samples at the contact with the diabase and at a distance of 10 m from the contact. Gas ages were determined using Ar-Ar methodologies for the diabase groundmass as well as K-feldspar, biotite, and amphibole in the host granite. According to the Ar-Ar data collected, the total gas age for the diabase is 1167.1  7.9 Ma. Ar-Ar data from the granite is more complex indicating Precambrian timing with total gas ages of 904, 1066, 1119, and 1258 Ma. Based on this information, it is evident that a significant thermal event altered the Silvermines granite as early as 1258 Ma to as late as 904 Ma. This thermal event may be associated with the intrusion of the Silver Mines dike swarm and its magmatic source.

Detecting the thermal aureole of a magmatic intrusion in immature to mature sediments: a case study in the East Greenland Basin (73°N)

The Cretaceous and Triassic argillaceous rocks from the passive margin of Greenland have been investigated in order to detect the thermal aureole of magmatic intrusions, ranging from metric dyke to kilometric syenite pluton. Rock-Eval data (Tmax generally <468 °C), vitrinite reflectance data (R0 < 0.9 per cent) and illite cristallinity data (ICI > 0.3), all indicate a maximum of 5 km burial for the argillaceous rocks whatever the distance to an intrusion. The K–Ar dating of the clays <2 μm fraction suggests that illites are mostly detrital, except near magmatic intrusions where younger ages are recorded. To get more information about the extent of the thermal aureole, rock magnetism data were determined. At distance away from the thermal aureole of the syenite intrusion, Triassic argillaceous rocks reveal a standard magnetic assemblage compatible with their burial (R0 ∼ 0.4 per cent). It is constituted essentially by neoformed stoichiometric magnetite (Fe3O4). In contrast, within the thermal aureole of the magmatic intrusions, the Cretaceous argillaceous rocks contain micron-sized pyrrhotite (Fe7S8), firmly identified through the recognition of Besnus transition at 35 K. The thermal demagnetization of natural remanence carried by this pyrrhotite shows a diagnostic ‘square shouldered’ pattern, indicating a narrow grain size distribution of pyrrhotite. The extension of this diagnostic pyrrhotite maps a ∼10-km-thick aureole around the syenitic pluton. Away from this aureole, the magnetic assemblage is diagnostic of those found in argillaceous rocks where organic matter is mature.

40Ar/39Ar geochronology of Volcan Tepetiltic, western Mexico: Implications for the origin of zoned rhyodacite-rhyolite liquid erupted explosively from an andesite stratovolcano after a prolonged hiatus

40 Ar/ 39 Ar geochronology is used to determine the eruptive history of Volcan Tepetiltic, a predominantly andesitic stratovolcano that underwent caldera collapse during explosive eruption of zoned rhyodacite–rhyolite. The main edifice was largely constructed between 560 and 450 ka, but it was not complete until ca. 416 ka, during which time ∼42 km 3 of phenocryst–rich (25–40 vol%) lavas ranging from 57 to 69 wt% SiO 2 were erupted. After a hiatus of ∼180 k.y., there was a climactic Plinian eruption of ∼4–8 km 3 of zoned magma (68–75 wt% SiO 2 ). Afterward, a small rhyodacite dome (69 wt% SiO 2 ; 190 ± 22 ka) and an andesite dome were emplaced on the caldera floor. There has been no subsequent volcanic activity. The crystal–poor (0–5 vol%) Plinian pumice could not be dated directly, owing to the hydration of glass and the absence of a K–rich mineral phase. Instead, the age of the climactic eruption was bracketed to be ca. 236 ± 52 ka from 40 Ar/ 39 Ar dates (±2δ) on peripherally erupted basaltic andesite flows, which are found underneath (241 ± 47 ka) and overlying (220 ± 36 ka) the associated fallout deposits. A volume of ∼9 km 3 of basaltic andesite was erupted from three peripheral vents surrounding Volcan Tepetiltic, most of it from a shield volcano that was active before, during, and after the Plinian event. Thermal models indicate that the magma chamber beneath Volcan Tepetiltic would have cooled below its solidus within 36 k.y. in the absence of new injections of magma. Given the long hiatus (∼180 k.y.) between the cone–building episode that built the andesite stratovolcano and the explosive eruption of rhyodacite–rhyolite, it is proposed that the influx of voluminous basaltic andesite into the upper crust drove partial melting of the subsolidus magma chamber beneath Volcan Tepetiltic, which explains the synchronicity of the basaltic andesite volcanism with the Plinian eruption of zoned, crystal–poor rhyolitic melt.

Episodic magmatism at 105 Ma in the Kinki district, SW Japan: Petrogenesis of Nb-rich lamprophyres and adakites, and geodynamic implications

Cretaceous episodic magmatism produced Nb-rich lamprophyres and adakitic granitoids in the Kinki district of SW Japan. K–Ar dating of minerals from the lamprophyres, adakites, and hornblende peridotite xenoliths yielded ages of 109–99Ma, indicating a short-lived episodic magmatism. The lamprophyres generally display primitive high-Mg basaltic to basaltic andesite compositions with high Mg# and high Cr and Ni contents that preclude substantial differentiation. Some high-Nb basalt (HNB) and Nb-enriched basalt (NEB) compositions also occur. The lamprophyres have high large-ion lithophile element (LILE) and high field-strength element (HFSE) contents and variable (La/Yb)n ratios, and can be divided into high-(La/Yb)n (12.5–22.1) and low-(La/Yb)n (3.6–6.1) groups. The former contains nepheline-normative rocks with positive initial εNd(T) values, whereas the latter contains hypersthene-normative subalkaline rocks with negative initial εNd(T) values. The adakitic granitoids have relatively high TiO2, Nb, and Ta contents compared to more typical high-silica adakites elsewhere, indicating that they were produced by high temperatures (ca. 920 to 970°C) during slab melting.Early Cretaceous slab rollback and the accompanying asthenospheric upwelling at 105Ma could form a transitory thermal anomaly, and hence induce melting of the subducted slab to form adakitic granitoids, and produce metasomatized wedge mantle to form the lamprophyres. The high-(La/Yb)n lamprophyres originated from small degrees of partial melting of an enriched metasomatized mantle wedge within the garnet stability field at depths of ≥70km, whereas the low-(La/Yb)n lamprophyres originated from a different mantle source by a comparatively larger degree of partial melting in a relatively shallow part of the mantle wedge. The magmatic diversity of the Kyoto lamprophyres thus derives primarily from a heterogeneous mantle source that has been variably affected by the results of subduction.

Geology, geochemistry, and geochronology of Fe-oxide Cu (± Au) mineralization associated with Şamlı pluton, western Turkey

The Şamlı (Balıkesir) Fe-oxide Cu (±Au) deposit, one of several iron (+Cu±Au) deposits in western Turkey, is hosted by porphyritic rocks of the multi-phase Şamlı pluton and metapelitic–metadiabasic rocks of Karakaya Complex. Two successive mineralization events are recognized in the area as; i) early magnetite and sulfide and ii) late hematite–goethite-native copper (±Au). Alteration associated with the mineralization in Şamlı is characterized by four distinct mineralogical assemblages. They are, in chronological order of formation, (1) plagioclase–early pyroxene (±scapolite), (2) garnet–late pyroxene, (3) chlorite–epidote, and (4) chalcedony–calcite alteration. Geochemical, isotopic (Sr, Nd, O, S) and geochronological (Ar–Ar) data from alteration and magmatic rocks suggest a temporal and genetic link between the multiphase Şamlı pluton and the hydrothermal system that controls the Fe-oxide-Cu (±Au) mineralization. 40Ar/39Ar geochronology on hornblende and biotite separates of the Şamlı pluton yielded an age range between 23.20±0.50 and 22.42±0.11Ma, overlapping with 40Ar/39Ar age of 22.34±0.59Ma from alteration.The close spatial and temporal associations of Şamlı mineralization with porphyritic intrusions, pervasive Ca-rich alteration (calcic plagioclase, andraditic garnet, diopsidic pyroxene, scapolite, and epidote) are considered as common features akin to calcic assemblages in typical IOCG deposits. Besides abundant low-Ti (≤0.5%) magnetite/hematite, high Cu–moderate Au (up to 8.82ppm) association, structural control and lithologic controls of mineralization, low S-sulfide content (chalcopyrite>pyrite) in the deposit; and the derivation of causative magma from subduction-modified subcontinental lithospheric mantle under a transpressional to transtensional regime, are collectively considered as the features in favor of IOCG-type mineralization for the Şamlı deposit.

The role of heterogenetic mantle in the genesis of adakites northeast of Sanandaj, northwestern Iran

In the northern part of the Sanandaj-Sirjan Zone (SSZ), northwestern Iran, there are a number of young (10–11Ma) dismembered volcanic complexes that extend parallel to the Zagros Suture Zone in a northwest–southeast direction. The major lithologies associated with these complexes include trachyte, andesite, dacite and rhyolite. These rocks are characterized by high SiO2 (>60wt.%) and Al2O3 (>15wt.%), low MgO (<2wt.%) and high Sr/Y (>30) and La/Yb (>25) ratios. Initial 87Sr/86Sr ratios based on an age of 11Ma (Ar–Ar dating) are relatively high (0.7070–0.7079), and εNdt values are relatively low (between −4 and −3). Values of fSm are negative and fluctuate between −0.5 and 0; values of fRb vary between +3.5 and +8. Analyses of chemical compositions confirm that these rocks belong to the adakite group. The Sr–Nd isotope ratios indicate that these rocks are distinct from adakites derived from partial melting of oceanic slabs or lower continental crust and suggest that the parent magma originated from the metasomatic mantle wedge above the oceanic slab in an active continental margin. Furthermore, the eruption of high-Nb basalts (HNBs) in this area supports the role of metasomatic mantle in the genesis of these adakites. Adakites in the northern SSZ are likely produced by (1) crystallization at high pressure of minerals such as garnet or amphibole from normal basaltic magma (e.g., high-Mg basalt) above the subduction zone, or (2) partial melting of metasomatic mantle at high pressure in the garnet stability field.

40 Ar/ 39 Ar hornblende provenance clues about Heinrich event 3 (H3)

Abstract Iceberg discharges into the North Atlantic are important sources of fresh water, and the sediments they deposit can provide constraints on which sectors of different ice sheets were contributing icebergs. 40 Ar/ 39 Ar ages of sand-sized hornblende grains provide useful constraints on IRD (ice-rafted detritus) source areas. Heinrich events are intervals of anomalously high percentages of IRD in marine sediment cores of the North Atlantic IRD belt. In contrast to the others, Heinrich event 3 (H3) records a significantly lower flux of IRD. This study compares 40 Ar/ 39 Ar hornblende age distributions from the interval around and including H3 in giant gravity core EW9303-GGC31 from Orphan Knoll, in the southern part of the Labrador Sea, with piston core V28-82 in the eastern part of the North Atlantic IRD belt. Collectively, these results confirm that H3 represents a Hudson Strait IRD event, but that it was smaller than during H1, H2, H4 and H5, and therefore comprises only a small fraction of the detritus at the eastern North Atlantic location of V28-82. These results support a previously published interpretation of across-strait ice flow during H3 at Hudson Strait. Supplementary material: Appendix 1 is 40 Ar/ 39 Ar data from core EW9303-GGC31; Appendix 2 is grain counts across H3 from core V28-82; Appendix 3 is 40 Ar/ 39 Ar data from core V28-82; these are available at http://www.geolsoc.org.uk/SUP18631 .

Post-Taconic tilting and Acadian structural overprint of the classic Barrovian metamorphic gradient in Dutchess County, New York

Field mapping and ⁴⁰Ar/³⁹Ar age spectrum dating of white mica across the classic metamorphic gradient in Dutchess County, New York indicate that post-peak metamorphic and structural events complicate the interpretation of this sequence as an intact Taconic metamorphic field gradient. Oriented samples were collected and fabrics were measured along two ∼7 km transects. These transects extend from west of the biotite zone (west of Clove Mountain) to the sillimanite zone (Swamp River Valley) in eastern Dutchess County. Field and petrographic analysis reveals three pervasive foliations (S1, S2 and S3). S1 is folded in the lower grade rocks of the sequence, but is increasingly overprinted at garnet and higher zones by a penetrative foliation, S2. S2 dips steeply to the SE and is axial planar to folds in S1. S2 is overgrown by porphyroblasts of biotite, biotite + chloritoid, biotite + garnet ± staurolite ± kyanite ± sillimanite, at respective metamorphic grades, that appear to approximate peak temperature conditions. S1, S2, and peak temperature conditions are constrained as late Taconic based on a depositional age of ∼460 Ma (Potter, 1972) for the Walloomsac Formation and a staurolite age of ∼454 Ma (Lanzirotti and Hanson, 1997) from kyanite-sillimanite grade rocks. S3 is a greenschist facies foliation composed of muscovite ± biotite ± chlorite that truncates or overprints S2 in garnet through sillimanite grade rocks in the eastern part of the study area. S3 dips gently to the S-SE and becomes more penetrative to the east, such that in the garnet zone, S2 is overprinted by a weakly developed spaced S3 cleavage, while in many samples in the staurolite and higher metamorphic zones S2 micas are significantly overprinted by S3 micas. S3 is observed wrapping around and truncating peak temperature porphyroblasts. Three samples of lepidoblastic white mica (with varying proportions of S1, S2, and S3) from the biotite through staurolite zone all produce ⁴⁰Ar/³⁹Ar age spectra suggesting closure ages of ∼385 Ma. These white mica ages are interpreted to represent the time of cooling through white mica closure (∼350 °C). The ⁴⁰Ar/³⁹Ar ages in conjunction with field mapping and petrographic analysis suggest that S3 developed in the Devonian Acadian orogeny. Furthermore, the ⁴⁰Ar/³⁹Ar data constrains the timing of tilting and uplift of the Ordovician Barrovian sequence from the Middle Silurian to Early Devonian. This study suggests mid-crustal wedging during the Salinic orogeny as a possible mechanism for tilting the rocks of this region and exposing the Taconic-aged Barrovian field gradient at the surface today. We conclude the low-grade Acadian metamorphic overprint of these rocks involved deformation that produced a new (third) cleavage that was coincident with regional cooling from peak Taconic metamorphic conditions and thus required strain but no reheating during the Acadian.

The first 40Ar–39Ar date from Oxfordian ammonite-calibrated volcanic layers (bentonites) as a tie-point for the Late Jurassic

AbstractEight volcanic ash layers, linked to large explosive events caused by subduction-related volcanism from the Vardar Ocean back-arc, interbedded with marine limestones and cherts, have been identified in the Rosso Ammonitico Veronese Formation (northeastern Italy). The thickest ash layer, attributed to the Gregoryceras transversarium ammonite Biozone (Oxfordian Stage), yields a precise and reliable 40Ar–39Ar date of 156.1 ± 0.89 Ma, which is in better agreement with GTS2004 boundaries than with the current GTS2012. This first biostratigraphically well-constrained Oxfordian date is proposed as a new radiometric tie-point to improve the Geologic Time Scale for the Late Jurassic, where ammonite-calibrated radiometric dates are particularly scarce.

Hydrothermal and unexpected diagenetic alteration in Permian shales of the Lodève epigenetic U-deposit of southern France, traced by K–Ar illite and K-feldspar dating

Mineralogical and chemical analyses combined with K–Ar dating of illite and K-feldspars were carried out on bulk samples and different fractions from Permian shales around the Lodève U-deposit to identify diagenetic and hydrothermal alteration episodes and their timing. Authigenic 2M1 and 1M illite with K–Ar ages between 270 and 245Ma are the predominant mineral types in the shales. They are of early diagenetic origin relative to deposition and formed apparently directly from pyroclastic materials in a lagoonal to deltaic environment under semi-arid conditions. Albite and a first generation of K-feldspar developed by a replacement of analcime, the K-feldspar growing only in the former lagoonal environment that favored also early diagenetic dolomitization at the expense of calcite.The hydrothermally altered shales next to the U-deposit contain authigenic quartz, Fe-chlorite, neoformed illite, and a second-generation K-feldspar with K–Ar ages beginning at about 220Ma (Triassic), although Jurassic and Cretaceous ages were also obtained. This younger age spectrum (Jurassic/Cretaceous) is close to previous U–Pb isotope data from the U-deposit, probably reflecting several episodes of U-mobilization. Therefore, the hydrothermal activity extended episodically from Triassic to Cretaceous time within the U-deposit. The hydrothermal temperatures attained locally >220°C as the K–Ar system of illite from early Permian diagenesis is partially reset.Triassic hot fluids ascended along faults into the northern area of the Lodève basin during an initial rifting of the nearby western Tethys Ocean. Further fluid pulses were recorded at Lodève during the opening of the North Atlantic and separation of Europe from Africa (Jurassic/Cretaceous), which also caused the peak hydrothermal activities in Central and Western Europe. As the Lodève area is located between two independently developing oceans it could have been supplied with hot fluids for a long period of time.

The Strawberry Volcanics: generation of ‘orogenic’ andesites from tholeiite within an intra-continental volcanic suite centred on the Columbia River flood basalt province, USA

Abstract The widely distributed, mid-Miocene lavas of the Strawberry Volcanics of NE Oregon are compositionally diverse, ranging from basalt to rhyolite. They are composed mainly of calc-alkaline and mildly tholeiitic basaltic andesite and andesite. Ar–Ar dating and stratigraphic relationships indicate that the volcanic field was active from &gt;16 Ma to c. 12 Ma ago and thus is coeval for the first 1–2 Ma with strongly tholeiitic flood basalts of the Columbia River Province that encircle the Strawberry Volcanics. Tholeiitic and calc-alkaline compositions develop subtle but noticeable differences towards higher silica contents. At silica contents of &lt;55 wt% SiO 2 , calc-alkaline and tholeiitic lavas are essentially indistinguishable. Trace element constraints among Strawberry Volcanics and crustal rocks indicate that open-system processes, such as assimilation or magma mixing, are responsible for evolution along a calc-alkaline trend leading to ‘orogenic’ andesites from tholeiite. Exclusively tholeiitic basalts carrying evidence for a metasomatized mantle source erupted during the mid-Miocene of eastern Oregon. Consequently, tholeiite imparted the ‘subduction signals’ and crustal processing generated the calc-alkaline character to end up with compositions of typical ‘orogenic’ andesites at the Strawberry Volcanics. No primitive calc-alkaline basalt from the mantle is needed as parental magma here and possibly at other similar intra-continental calc-alkaline suites.

Sierra Crest graben-vent system: A Walker Lane pull apart within the ancestral Cascades arc

We show here that transtensional rifting along the eastern boundary of the Sierra Nevada microplate (Walker Lane rift) began by ca. 12 Ma in the central Sierra Nevada (USA), within the ancestral Cascades arc, triggering voluminous high-K intermediate volcanism (Stanislaus Group). Flood andesite (i.e., unusually large-volume effusive eruptions of intermediate composition) lavas erupted from fault-controlled fissures within a series of grabens that we refer to as the Sierra Crest graben-vent system. This graben-vent system includes the following. 1. The north-northwest–south-southeast Sierra Crest graben proper consists of a single 28-km-long, 8–10-km-wide full graben that is along the modern Sierra Nevada crest between Sonora Pass and Ebbetts Pass (largely in the Carson-Iceberg Wilderness). This contains fissure vents for the high-K intermediate lavas. 2. A series of north-northwest-south-southeast half-grabens on the western margin of the full graben, which progressively disrupted an ancient Nevadaplano paleochannel that contains the type section of Stanislaus Group (Red Peak–Bald Peak area). These Miocene half-grabens are as much as 15 km west of the modern Sierra Nevada crest, and vented high-K lavas from point sources. 3. Series of northeast-southwest grabens define a major transfer zone along the northeast side of the Sierra Crest graben. These extend as much as ∼30 km from the modern range crest down the modern Sierra Nevada range front, in a zone ∼30 km wide, and vented high-K lavas and tuffs of the Stanislaus Group from point sources. Range-front north-south and northeast-southwest faults to the south of that, along the southeast side of the Sierra Crest graben, did not vent volcanic rocks (although they ponded them); those will be described elsewhere. We present evidence for a dextral component of slip on the north-northwest–south-southeast normal faults, and a sinistral component of slip on the northeast-southwest normal faults. The onset of transtension immediately preceded the high-K volcanism (within the analytical error of 40 Ar/ 39 Ar dates), and triggered the deposition of a debris avalanche deposit with a preserved volume of ∼50 km 3 . The grabens are mainly filled with high-K lava flows, ponded to thicknesses of as much as 400 m; this effusive volcanism culminated in the development of the Little Walker caldera over a relatively small part of the field. Trachydacite outflow ignimbrites from the caldera also became ponded in the larger graben-vent complex, where they interfingered with high-K lavas vented there, and escaped the graben-vent complex on its west margin to flow westward down two paleochannels to the western foothills. The Sierra Crest graben-vent system is spectacularly well exposed at the perfect structural level for viewing the controls of synvolcanic faults on the siting and styles of feeders, vents, and graben fills under a transtensional strain regime in an arc volcanic field.

Localisation of deformation in the thermal contrast at a granite batholith margin, New Zealand

The Wainui Shear Zone is a ∼ 1.2 km wide steeply dipping ductile shear zone that is intimately associated with the western margin of a very extensive granite batholith in central New Zealand. Shear sense indicators and lineation orientations within the central high-strain portion of shear zone consistently show a reverse dip-slip east-side-up sense of motion. Shallower foliation in the margins of the shear zone is interpreted to pre-date the central high-strain zone. However, U–Pb and Ar–Ar data reveal that all ductile deformation occurred between ∼114 and 109 Ma (and probably between 114 and 111 Ma) requiring: (1) that deformation kinematics within the shear zone changed over a short time period, and (2) the shear fabrics formed immediately after emplacement of the voluminous granite batholith. The temporal and spatial link between the deformation and plutonism is hypothesised to be a result of thermal weakening of the cooler Paleozoic crust by Cretaceous granite intrusion, which promoted localisation of deformation onto the intrusive contact. Deformation was initially distributed over a wide zone but became focused into a central mylonite zone. Continental-scale shortening is recorded as a network of ductile shear zones that formed on thermal contrasts within the upper, middle and lower crust along the New Zealand Gondwana margin.

The origin of the Canary Island Seamount Province - New ages of old seamounts

The Canary Island Seamount Province forms a scattered hotspot track on the Atlantic ocean floor ~1300 km long and ~350 km wide, perpendicular to lithospheric fractures, and parallel to the NW African continental margin. New 40Ar/39Ar datings show that seamount ages vary from 133 Ma to 0.2 Ma in the central archipelago, and from 142 Ma to 91 Ma in the southwest. Combining 40Ar/39Ar ages with plate tectonic reconstructions, I find that the temporal and spatial distribution of seamounts is irreconcilable with a deep fixed mantle plume origin, or derivation from passive mantle upwelling beneath a mid-ocean ridge. I conclude that shallow mantle upwelling beneath the Atlantic Ocean basin off the NW African continental lithosphere flanks produced recurrent melting anomalies and seamounts from the Late Jurassic to Recent, nominating the Canary Island Seamount Province as oldest hotspot track in the Atlantic Ocean, and most long-lived preserved on earth.

Persistent long-term ( c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

Abstract To address Miocene–present episodic v. persistent exhumation, we utilize a simple graphical procedure that vertically stacks spatially diverse K-feldspar 40 Ar/ 39 Ar multi-domain diffusion (MDD) models from the length of the approximately 100 km-long high-peak region of the Eastern Alaska Range. We supply additional constraints with 40 Ar/ 39 Ar mica dating because the higher closure-temperature-window places limits on the initiation of rapid Eastern Alaska Range exhumation. We also provide a broad 40 Ar/ 39 Ar K-feldspar minimum closure age data set to add more detail on spatial patterns in the regional exhumation history for the Eastern Alaska Range. We find that rapid and persistent exhumation has occurred in the Eastern Alaska Range since about 24 Ma at a long-term rate of approximately 0.9 km/Ma, but that this rapid exhumation is spatially variable through time. Onset of rapid Eastern Alaska Range exhumation is coincident with the initiation of rapid exhumation in SW Alaska, the Western Alaska Range and the Chugach–Saint Elias Range at around 25 Ma, implying a region-wide deformational response to a change in tectonic forcing. The initiation of highly coupled flat-slab subduction of the Yakutat microplate is probably responsible for this prolonged period of rapid exhumation in southern Alaska. Supplementary material: Sample locations from the Eastern Alaska Range, and 40 Ar/ 39 Ar data tables and age spectrum figures are available at www.geolsoc.org.uk/SUP18603 .

An Embedded Feature Extraction Algorithm with MMC for Face Recognition

Feature extraction is the most crucial part of face recognition system, which has important role in the field of pattern recognition. There are a lot of classic algorithms about feature extraction at present, such as the method based on linear and nonlinear. An effective classification and dimension reduction method, local embedded graph attribute selection algorithm with maximal region is to generate the inherent graph and penalty graph, to overcome the low efficiency problem of local linear embedding (LLE) method and maximum margin criterion (MMC) method. With inherent picture, the structured nonlinear can be found on the high-dimensioned space by using local geometry of the restructured linear, which leads to the same instances gathering together as more as possible. Meanwhile, different class instances are as far as possible from each other in penalty picture. Since LLE is an unsupervised method, not enhance visual clustering classification ability, so compact figure within the class to consider the sample class information, can sample the same category as compact. In this method, the smallest size instance issue was tackled by the employment of MMC and the neighborhood relationship can be better described by an adequate improvement of the adjacency matrix. The effectiveness of algorithm proposed by this paper is present by a large amount of experiences in the standard face databases of Yale, AR and ORL facial data.

PROBING THE SHALLOW CONVECTION ZONE: RISING MOTION OF SUBSURFACE MAGNETIC FIELDS IN THE SOLAR ACTIVE REGION

In this Letter we present a seismological detection of a rising motion of magnetic flux in the shallow convection zone of the Sun, and show estimates of the emerging speed and its decelerating nature. In order to evaluate the speed of subsurface flux that creates an active region, we apply six Fourier filters to the Doppler data of NOAA AR 10488, observed with SOHO/MDI, to detect the reduction of acoustic power at six different depths from -15 to -2 Mm. All the filtered acoustic powers show reductions, up to 2 hours before the magnetic flux first appears at the visible surface. The start times of these reductions show a rising trend with a gradual deceleration. The obtained velocity is first several km s^-1 in a depth range of 15--10 Mm, then ~1.5 km s^-1 at 10-5 Mm, finally ~0.5 km s^-1 at 5-2 Mm. If we assume that the power reduction is actually caused by the magnetic field, the velocity of order of 1 km s^-1 is well in accordance with previous observations and numerical studies. Moreover, the gradual deceleration strongly supports the theoretical model that the emerging flux slows down in the uppermost convection zone before it expands into the atmosphere to build an active region.

Jadeitite formed during subduction: In situ zircon geochronology constraints from two different tectonic events within the Guatemala Suture Zone

Jadeitite is a rare rock type associated with high-pressure–low-temperature blocks within serpentinite matrix mélanges. Models of formation involve precipitation from subduction-zone aqueous fluids veining the overlying mantle wedge (P-type), or metasomatism of igneous and/or sedimentary protoliths previously emplaced into the mélange (R-type). Age determinations of mélange lithologies provide constraints on the timing of “peak metamorphism” and subsequent exhumation. The timing of jadeitite formation, particularly in the rich source of the Guatemala Suture Zone (GSZ), is a controversial subject needing further attention.Over 80 in situ zircon crystals from three jadeitites and two mica–albite rocks from the North Motagua Mélange and one phengite jadeitite from the South Motagua Mélange of the GSZ were studied for age and trace-element determination. Most of these zircons are characterized by low Th/U ratios, depleted chondrite-normalized REE patterns relative to zircons from oceanic gabbros, and contain fluid and mineral inclusions that reflect the primary mineralogy (i.e., jadeite) and context (i.e., crystallization from an aqueous fluid) of the host rock, and thus formed during jadeitite crystallization. The SHRIMP-RG and LAM-ICP-MS U–Pb dates from zircon indicate that jadeitites and mica–albite rocks from the GSZ were formed through vein precipitation at ~98−80 and ~154–158Ma, respectively. These data show (a) older ages that indicate jadeitite crystallization occurred ~10–30Ma before the preserved subduction-zone peak metamorphism (e.g., exhumed eclogite), and (b) a second group of ages slightly younger than, or similar to, exhumation ages given by Ar–Ar dates from micas. Similar relationships occur at other jadeitite occurrences, such as the Syum-Keu ultramafic complex in the Polar Urals (Russia) and the serpentinite mélanges of the Río San Juan complex (Dominican Republic). The data argue for formation of jadeitite within the mantle wedge during active subduction. Thus, jadeitite provides a record of fluid introduction into the mantle wedge during subduction rather than during exhumation.

U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone, Kyrgyzstan, and Ar–Ar age of deformations along the fault

A 2000km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U–Pb zircon SHRIMP ages 728±11Ma and 778±11Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and their calculated tHfc ages varied from 2.42 to 2.71Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical 206Pb/238U ages of 279±5Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and calculated tHfc ages from 2.42 to 2.71Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400°C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227Ma to 199Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic.