Hornblende Ages Research Articles

Mechanisms and timing of exhumation of collision‐related metamorphic rocks, southern Brooks Range, Alaska: Insights from 40Ar/39Ar thermochronology

New 40Ar/39Ar thermochronologic data document the spatial distribution of cooling ages and rates across a metamorphic culmination in the south central Brooks Range, providing important constraints on the timing and processes responsible for exhumation of deep‐seated metamorphic rocks. The data indicate widespread episodic exhumation‐related cooling, with short‐lived events in the mid‐Albian to early Cenomanian and Paleocene to Eocene. Hornblende ages indicate that cooling from the metamorphic peak began at 105–103 Ma across the epidote‐amphibolite facies core of the culmination. Mica ages, however, increase southward from ∼90 Ma to ∼100 Ma, indicating that average cooling rates increased southward. Thermal modeling suggests that Albian cooling is the result of a short‐lived exhumation event that ended by the early Cenomanian, and that exhumation rates were between ∼1 and 8 mm/yr, with higher rates toward the south. The southward increase in cooling/exhumation rates is attributed to a southward increase in the component of tectonic exhumation by normal faulting on the south flank of the range. Rapid erosion, which is recorded by thick molasse deposits in flanking basins that are coeval with rapid cooling, may have been aided by surface uplift in the footwall of these faults. The northward younging of mica ages may not be explained by a simple core complex model involving metamorphic rocks being progressively dragged out from beneath an extensional fault. The overall spatial distribution of metamorphic zones and cooling ages/rates is best explained by a two‐step process: (1) a brief episode of extensional faulting producing a regional northward tilt with superimposed doming during the Albian‐Cenomanian and (2) relative uplift of the core during renewed Tertiary contraction, which enhanced the domal geometry. Diffusion‐domain modeling of K‐feldspar data indicates a period of little or no cooling that spanned much of the Late Cretaceous followed by rapid increase in cooling rates in the Paleocene to Eocene. Increased Tertiary cooling rates are attributed to renewed contraction leading to surface uplift and erosion.

Emplacement depths and radiometric ages of Paleozoic plutons of the Neukirchen–Kdyně massif: differential uplift and exhumation of Cadomian basement due to Carboniferous orogenic collapse (Bohemian Massif)

The igneous complex of Neukirchen–Kdyně is located in the southwestern part of the Teplá–Barrandian unit (TBU) in the Bohemian Massif. The TBU forms the most extensive surface exposure of Cadomian basement in central Europe. Cambrian plutons show significant changes in composition, emplacement depth, isotopic cooling ages, and tectonometamorphic overprint from NE to SW. In the NE, the Všepadly granodiorite and the Smržovice diorite intruded at shallow crustal levels (<ca. 7 km depth) as was indicated by geobarometric data. K–Ar age data yield 547±7 and 549±7 for hornblende and 495±6 Ma for biotite of the Smržovice diorite, suggesting that this pluton has remained at shallow crustal levels ( T<ca. 350 °C) since its Cambrian emplacement. A similar history is indicated for the Všepadly granodiorite and the Stod granite. In the SW, intermediate to mafic plutons of the Neukirchen–Kdyně massif (Všeruby and Neukirchen gabbro, Hoher–Bogen metagabbro), which yield Cambrian ages, either intruded or were metamorphosed at considerably deeper structural levels (>20 km). The Teufelsberg (Čertův kámen) diorite, on the other hand, forms an unusual intrusion dated at 359±2 Ma (concordant U–Pb zircon age). K–Ar dating of biotite of the Teufelsberg diorite yields 342±4 Ma. These ages, together with published cooling ages of hornblende and mica in adjacent plutons, are compatible with widespread medium to high-grade metamorphism and strong deformation fabrics, suggesting a strong Variscan impact under elevated temperatures at deeper structural levels. The plutons of the Neukirchen area are cut by the steeply NE dipping Hoher–Bogen shear zone (HBSZ), which forms the boundary with the adjacent Moldanubian unit. The HBSZ is characterized by top-to-the-NE normal movements, which were particularly active during the Lower Carboniferous. A geodynamic model is presented that explains the lateral gradients in Cambrian pluton composition and emplacement depth by differential uplift and exhumation, the latter being probably related to long-lasting movements along the HBSZ as a consequence of Lower Carboniferous orogenic collapse.

Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains

The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet+ diopside amphibolites, enstatite+cordierite+sillimanite+spinel±sapphirine quartzites, alkaline mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain garnet+diopside+scapolite+plagioclase±wollastonite. P–T conditions of granulite facies metamorphism are in the range 800–860°C and 10.5±1.1 kbar to 14.7±2.8 kbar. Garnet+clinopyroxene+hornblende+plagioclase amphibolites from the metamorphic sole record peak P–T conditions of 840±70°C and 11.6±1.6 kbar (THERMOCALC average P–T mode) and 840–870°C and 13.9–11.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 57–46 km beneath oceanic crust, much deeper than can be accounted for by the thickness of the ophiolite. 40Ar39Ar hornblende ages from the amphibolites range from 95–93 Ma, synchronous with formation of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic sequence and deposition of Cenomanian–Turonian radiolaria in metalliferous sediments between the Geotimes (V1) and Lasail (V2) lavas. Protoliths of the metamorphic sole were Triassic–Jurassic and early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites which were at or around 1100–1500°C. All data from the sub-ophiolite metamorphic sole in Oman and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting and that obduction occurred along a NE-dipping high-temperature subduction zone during Late Cretaceous times.

Neoproterozoic tectonothermal evolution of the Central Eastern Desert, Egypt: a slow velocity tectonic process of core complex exhumation

Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by 40Ar/ 39Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These 40Ar/ 39Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at ∼650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until ∼580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography.

Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

We present new 40Ar/39Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the Yukon–Tanana Upland, Alaska. Integration of our data with published 40Ar/39Ar, kinematic, and metamorphic pressure (P) and temperature (T) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215–212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ~188–186 Ma 40Ar/39Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high-P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40Ar/39Ar cooling ages, most yield Early Cretaceous 40Ar/39Ar ages: hornblende ~135–115 Ma, and muscovite and biotite ~110–108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

北上山地南部，千厩‐気仙沼地域の火成岩類のＫ‐Ａｒ年代

K-Ar ages of six hornblendes and two biotites were determined for the igneous rocks in the area around Senmaya and Kesennuma, southern Kitakami Mountains. The Niitsuki Formation, mainly composed of andesitic-basaltic volcanic rocks, is exposed in the eastern part. The Orikabe Plutonic Complex, characterized by quartz monzodiorite and granodiorite, intrudes into the Niitsuki Formation, and is intruded by porphyrite dike swarms in the northern part. The Senmaya Tonalitic Pluton, mainly composed of homogeneous tonalite, is widely exposed in the western part. Hornblende ages from andesitic fragments of the Niitsuki Formation, a granodiorite of the Orikabe Plutonic Complex, and a hornblende porphyrite dike are 121, 120 and 107 Ma, respectively. Hornblende and biotite ages for the Senmaya Tonalitic Pluton range from 105 to 108 Ma and from 109 to 112 Ma, respectively. The hornblende ages for the Niitsuki Formation and the Orikabe Plutonic Complex imply their close relationship, and they are typical K-Ar ages for Cretaceous volcanic and plutonic rocks in the Kitakami Mountains. On the other hand, the hornblende and biotite ages for the Senmaya Tonalitic Pluton and porphyrite dikes are the youngest among the plutonic rocks in the Kitakami Mountains, indicating they are the products of the later igneous activity. The obtained K-Ar ages in this area imply a shift of plutonic activity to the western continental side after the main plutonism in the Kitakami Mountains in the Early Cretaceous.

Fluvial Terrace Development and Crustal Movement along the Souzu and Matsuda Rivers, Southwestern Shikoku, Japan

The purposes of this paper are to discuss the development of fluvial terraces along the Souzu and Matsuda Rivers in southwestern Shikoku using widespread marker tephras and to estimate the crustal movement in the region on the basis of deformed terrace profiles. First the author identified several tephras from the terrace and tales deposits, including the Zentsubo, Kubo, and Ogawa tephras to establish terrace chronology. The fission-track age of the Zentsubo tephra with the zeta calibration is 0.58±0.11 Ma. The Kousenji II tephra located below the Zentsubo tephra shows a fission-track age of 0.66±0.15 Ma. In terms of the similarities of mineral composition, age, and refractive indices of volcanic glass and hornblende, the Zentsubo tephra can be correlated with the Yufugawa pyroclastic flow deposits that erupted from the Yufuin basin at about 0.6 Ma. The fluvial terrace surfaces along the Souzu and Matsuda Rivers are classified into four levels: H1, H2, M, and L in descending order. The Zentsubo and Kousenji II tephras are intercalated in the upper part of the H1 terrace deposits, indicating that the age of the H1 terrace surface is approximately 0.6 Ma. The occurrence of the Ogawa tephra in L terrace deposits or in tales deposits on L terrace deposits suggests that the L terrace surface was formed around or after 70-80 ka. The H1 terrace is a broad fill-top terrace formed by the paleo-Minamiuwa River, taking a different course from the present courses of the Souzu and Matsuda Rivers, whereas the lower terraces occur along the present river systems. The paleo-Minamiuwa River disappeared after the H1 terrace was formed by a large-scale river capture due to the beginning of upwarping in the middle reaches. The thickness of the H1 terrace deposits indicates that the tectonic setting in this area was stable or subsiding until 0.6 Ma. Subsequently, the upwarping of the Shikoku Mountains began at around 0.6 Ma and has continued up to the present, causing the deformation of the longitudinal profile of the H1 terrace surface at Uwaodo and Kamiari along the Souzu and Matsuda Rivers. This upwarping triggered a large-scale river capture by the Matsuda and Akagi Rivers at around 0.6 Ma.

Geological and chronological evidence of Indo-Chinese strike- slip movement in the Altyn Tagh fault zone

A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these rocks are the product of the syntectonic anatexis in the process of the left-lateral strike-slip shear, and are the result of the ductile transpression. There are two types of zircon sorted from the mylonite formed with synshear anatexis. Among them, one is the anatectic long columnar zircon and another is the residual metamorphic sub-rounded columnar zircon. Two groups of age for single zircon measured by ion microprobe (SHRIMP) are obtained: one is 461–547 Ma for the sub-rounded columnar residual metamorphic zircon, and the other is 239–244 Ma for the long-columnar anatectic zircon. This type of zircon is directionally spread in rock, and the long axis direction of its crystal is identical to that of stretching lineation, representing the direction of tectonic stress in the process of the strike-slip. 40Ar-39Ar age of the directional growth hornblende in the same mylonite sample measured is 223–226 Ma. The above geological and chronological data prove that the syntectonic anatexis occurred during Indo-Chinese Epoch, showing that the strong strike-slip movement occurred in the Altyn Tagh fault zone at least as early as Indo-Chinese Epoch.

Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large‐scale transport

Samples for geochronologic, geobarometric, and paleomagnetic analyses were collected across the northern portion of the Ecstall pluton southeast of Prince Rupert, British Columbia. Al‐in‐hornblende geobarometry indicates pressures from 740 ± 10 to 840 ± 30 MPa corresponding to crystallization depths of ∼25 to ∼30 km. U/Pb analyses of zircons from western, central, and eastern localities within the pluton yield crystallization ages of 91.5 ± 1.0 Ma, 90.8 ± 1.0 Ma, and 90.5 ± 1.0 Ma, respectively. Rock magnetic experiments, reflected light microscopy, and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low‐Ti titanohematite. Unblocking temperatures of the characteristic remanent magnetization (ChRM) are dominantly in the 560°C to 630°C range, with age of magnetization approximated by the40Ar/39Ar hornblende ages of 84.2 ± 0.10 Ma on the western margin and 76.4 ± 0.6 Ma in the center of the pluton. Site‐mean ChRM directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at ∼340° azimuth. ChRM directions from the central portion of the pluton are concordant with the expected Cretaceous magnetic field direction, while ChRM directions from the western margin are discordant by &gt;70°. Folding of the Ecstall pluton, either during Late Cretaceous west directed thrust transport above the convex upward Prince Rupert Shear Zone or during younger deformation of the pluton and underlying shear zone, can account for the paleomagnetic data and is consistent with the geochronologic, geobarometric, and structural geologic observations.

Origin of the Wadi Haimur–Abu Swayel gneiss belt, south Eastern Desert, Egypt: petrological and geochronological constraints

The Wadi Haimur–Abu Swayel gneiss belt is composed of biotite-gneiss, biotite–hornblende-gneiss and garnet-gneiss. Amphibole and muscovite compositions indicate that these gneisses belong to low- to intermediate-pressure metamorphic facies. Temperature values calculated using the hornblende–plagioclase geothermometer are in the range of 495–550 °C. The retrograde part of the metamorphic evolution for these rocks is documented by the presence of epidote and chloritization of biotite. The chlorite geothermometer indicates that chloritization occurred around 260–300 °C. The sedimentary origin of the gneisses is indicated by the high values of normative quartz and corundum. It is suggested that the sedimentary protoliths of the Wadi Haimur–Abu Swayel gneisses evolved in a back-arc basin. Compression of this basin led to intense deformation, metamorphism and granite magmatism. K/Ar ages have been determined on hornblende and biotite. The hornblende ages ≈600 Ma and the ≈585 Ma of coexisting biotite from the gneisses are interpreted as cooling ages following the peak of metamorphism. These ages are within the range of granitic emplacement in the study region (570–675 Ma), which indicates that the peak of metamorphism could have been contemporaneous with the granitic magmatism in the Wadi Haimur–Abu Swayel gneisses.

Early History of the Carthage‐Colton Shear Zone, Grenville Province, Northwest Adirondacks, New York (U.S.A.)

Abstract The Adirondack Mountains expose two distinct tectonic elements of the Proterozoic Grenville Province of northeastern North America: the Adirondack Lowlands and Highlands. The Lowlands are located along the eastern edge of the Metasedimentary Belt, and the Highlands form the western portion of the Granulite Terrane. The two are separated by the Carthage‐Colton Shear Zone (CCSZ). U/Pb titanite and 40Ar‐39Ar hornblende ages in the Lowlands are ca. 100 m.yr. older than in the Highlands, across the CCSZ. While both the Lowlands and the Highlands record a history of metamorphism during the Elzevirian Orogeny (ca. 1150 Ma), only the Highlands record evidence of a major phase of Grenvillian thermotectonic activity (Ottawan Orogeny) at ca. 1090–1030 Ma. Proposed tectonic models require that the Lowlands were either laterally separated from the Highlands or were at structurally higher levels during this metamorphism. New U/Pb and 40Ar‐39Ar ages from titanite and hornblende‐bearing mylonites in the Dana Hil...

Constraints on the timing of granite emplacement, deformation and metamorphism in the Shamva area, Zimbabwe

In order to constrain the temporal relationship between granite (sensu lato) emplacement and metamorphism, isotope work was carried out on the minerals zircon and apatite (U–Pb), garnet (Pb–Pb) and hornblende (Ar–Ar) from wall rock samples in the Shamva area in Zimbabwe. The area, encompassing parts of the Chinamora and Murehwa batholiths and a wedge-shaped greenstone belt segment in between, is commonly quoted in the literature as an example illustrating pluton emplacement processes and deformational models for the Archean. New U–Pb dating of apatite from a boudinaged pegmatite within mafic schists in the batholith–greenstone contact zone has yielded an age of 2619 +28/–24 Ma. This age is interpreted as the best estimation of the intrusion age of this unit, depending on the assumed closure temperature, and provides an upper age limit for the syntectonic emplacement of the now gneissic granites. Pb–Pb dating of late kinematic garnets in cordierite-bearing rocks within the greenstone belt wall rocks gives an age of 2623±8 Ma. Together, this timing of relatively late, syntectonic plutonism and metamorphic mineral growth at ca. 2.62 Ga compares well with existing zircon crystallization ages for felsic volcanics (2645±4 Ma, 2643±8 Ma) and post-tectonic porphyritic monzogranites (2601±14 Ma). Ar–Ar hornblende ages for mafic schists from different areas within the greenstone belt wall rocks range between 2621 and 2498 Ma and have been interpreted to indicate mixing between metamorphic ages and cooling ages. The data support a geological model whereby volcanism and sedimentation are associated with an early phase of regional deformation at ca. 2.64 Ga, which may have started earlier and lasted longer, and evolves into the voluminous emplacement of granites (now gneissic granites) in the batholiths at approximately 2.62 Ga. Emplacement of post-tectonic tabular monzogranites takes place at ca. 2.60 Ga.

Early to Middle Carboniferous hornblende 40 Ar/ 39 Ar ages of amphibolites and gabbros from the Bergsträsser Odenwald

Pre- to Early Variscan metamorphic rocks and intrusive calc-alkaline rocks are constituting the Odenwald accretionary complex. The predominant regional metamorphic formation of the amphibolite is postdated by the intrusion of the Frankenstein gabbro complex in the NW Odenwald at about 362 Ma. Thus indications of older metamorphic events might be preserved in the amphibolites. However, for the central and southern Odenwald zircon data revealed a thermal peak of regional metamorphism at about 336 Ma. Nine amphibole separates from two gabbroic intrusives and from seven amphibolites were dated by the K-Ar method, mainly using the 40Ar/39Ar technique. The two gabbro amphibole separates yielded ages of 335 and 329 Ma; i.e. they are about 30 Ma younger than the Frankenstein gabbro. The total argon ages of the amphibolite hornblendes are at about 359 Ma in the north and about 330 to 320 Ma in the south (early and late stages of the Early Carboniferous). The ages of the three metamorphic amphiboles from the north are in the age range of the Frankenstein gabbro intrusion, however, the four from the south slightly postdate the regional metamorphic climax. Two of the older amphibole separates of the north and two of the younger ones from the south yielded plateau ages slightly older than the total argon ages (about 358 Ma and 329 Ma, respectively). The southernmost amphibolite with a total argon age of 320 Ma shows an irregular spectrum with a maximum age step for 40% of its argon corresponding to an age of 344 Ma. It possibly has partly preserved an older argon component during the younger phase of regional metamorphism. Together with published data the results show that hornblende of amphibolites from the Odenwald offer the chance to unravel the metamorphic veil of the Mid-Carboniferous regional metamorphism.

Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40 Ar/ 39 Ar and Rb–Sr chronology

40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally &gt;600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.

Precise K–Ar, 40Ar/ 39Ar, Rb–Sr and U/Pb mineral ages from the 27.5 Ma Fish Canyon Tuff reference standard

The accuracy of ages measured using the 40Ar/ 39Ar technique is affected by uncertainties in the age of radiation fluence-monitor minerals. At present, there is lack of agreement about the ages of certain minerals used as fluence monitors. The accuracy of the age of a standard may be improved if the age can be measured using different decay schemes. This has been done by measuring ages on minerals from the Oligocene Fish Canyon Tuff (FCT) using the K–Ar, 40Ar/ 39Ar, Rb–Sr and U/Pb methods. K–Ar and 40Ar/ 39Ar total fusion ages of sanidine, biotite and hornblende yielded a mean age of 27.57±0.36 Ma. The weighted mean 40Ar/ 39Ar plateau age of sanidine and biotite is 27.57±0.18 Ma. A biotite–feldspar Rb–Sr isochron yielded an age of 27.44±0.16 Ma. The U–Pb data for zircon are complex because of the presence of Precambrian zircons and inheritance of radiogenic Pb. Zircons with 207Pb/ 235U<0.4 yielded a discordia line with a lower concordia intercept of 27.52±0.09 Ma. Evaluation of the combined data suggests that the best age for FCT is 27.51 Ma.

40 Ar/39/Ar dating and cooling history of the Pangeon granitoids, Rhodope Massif (Eastern Macedonia, Greece)

The Pangeon granitoids are distinguished into two pétrographie types with sharp contacts: (a) heterogranular, medium- to coarse-grained, hornblende+biotite- bearing porphyritic tonalités and granodiorites (PTG), and, (b) equigranular, medium-grained, biotite±muscovite-bearing granodiorites and granites (MGG). Dark-coloured, medium-grained monzodioritic enclaves occur in PTG rocks. Hornblende 40Ar/39Ar spectra from the PTG rocks yielded cooling ages of 21.7±0.5 Ma to 18.8±0.6 Ma. With the exception one sample, the corresponding hornblende ages from enclaves coincide well with the above ages. The age of 21.7±0.5 Ma is considered as the lower limit for the PTG rocks emplacement. Muscoviteplateau ages of c. 15.7±0.5 Ma and total gas biotite ages of 15.2±0.4 Ma to 13.8±0.5 Ma from the studied rocks, constrain the cooling history of the Pangeon granitoids (with some local variations) in the range 430 - 300Ί C.

山形県南陽市周辺のマイロナイト帯（梨郷マイロナイト帯）の発見と棚倉構造線の北方延長問題

We found a mylonite zone up to 1300 m in width in a granodioritic pluton in Nanyo City, Yamagata Prefecture, and named it the Ringou Mylonite Zone. Geological and petrographical features of the mylonite zone, situated between the Tanagura Tectonic Line in the south and the Oisawa Tectonic Zone in the north, are described. The mylonite zone shows sinistral shear sense, which is in harmony with that of the Tanagura Tectonic Line in the southern Abukuma Mountains. A K-Ar hornblende age of granodiorite (114 Ma) distributed about 5 km east of the mylonite zone is similar to the isotopic ages obtained from Abukuma Granites. On the other hand, granodiorites on the western side of the mylonite zone are petrographically identical to some of the granitic rocks in the Asahi Mountains. From these facts, we infer that the Ringou Mylonite Zone corresponds to the northern extension of the Tanagura Tectonic Line.

One-dimensional thermal modelling of Acadian metamorphism in southern Vermont, USA

One-dimensional thermal (1DT) modelling of an Acadian (Devonian) tectonothermal regime in southern Vermont, USA, used measured metamorphic pressures and temperatures and estimated metamorphic cooling ages based on published thermobarometric and geochronological studies to constrain thermal and tectonic input parameters. The area modelled lies within the Vermont Sequence of the Acadian orogen and includes: (i) a western domain containing garnet-grade pre-Silurian metasedimentary and metavolcanic rocks from the eastern flank of an Acadian composite dome structure (Rayponda–Sadawga Dome); and (ii) an eastern domain containing similar, but staurolite- or kyanite-grade, rocks from the western flank of a second dome structure (Athens Dome), approximately 10 km farther east. Using reasonable input parameters based on regional geological, petrological and geochronological constraints, the thermal modelling produced plausible P–T paths, and temperature–time (T –t) and pressure–time (P–t) curves. Information extracted from P–T –t modelling includes values of maximum temperature and pressure on the P–T paths, pressure at maximum temperature, predicted Ar closure ages for hornblende, muscovite and K-feldspar, and integrated exhumation and cooling rates for segments of the cooling history. The results from thermal modelling are consistent with independently obtained pressure, temperature and Ar cooling age data on regional metamorphism in southern Vermont. Modelling results provide some important bounding limits on the physical conditions during regional metamorphism, and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as much as 2.5 kbar lower than the actual maximum pressure experienced by rocks along various particle paths. In addition, differences in peak metamorphic grade (garnet-grade versus staurolite-grade or kyanite-grade) and peak temperature for rocks initially loaded to similar crustal depths, differences in calculated exhumation rates, and differences in 40Ar/39Ar closure ages are likely to have been consequences of variations in the duration of isobaric heating (or ‘crustal residence periods’) and tectonic unroofing rates. Modelling results are consistent with a regional structural model that suggests west to east younging of specific Acadian deformational events, and therefore diachroneity of attainment of peak metamorphic conditions and subsequent 40Ar/39Ar closure during cooling. Modelling is consistent with the proposition that regional variations in timing and peak conditions of metamorphism are the result of the variable depths to which rocks were loaded by an eastward-thickening thrust-nappe pile rooted to the east (New Hampshire Sequence), as well as by diachronous structural processes within the lower plate rocks of the Vermont Sequence.

Determining the extent and nature of Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake Superior region, north–central USA

Twenty-one hornblende and mica 40Ar/ 39Ar dates from central and northwest Wisconsin, USA, provide important information on the timing, spatial extent, and intensity of Mazatzal-age metamorphism and deformation which overprinted the Paleoproterozoic (1870–1820 Ma) Penokean orogenic belt in the southern Lake Superior region. 1760–1750 Ma mica plateau ages from bedrock beneath undeformed 1750–1630 Ma quartzites are interpreted as the time of lower temperature (∼300–350°C) cooling and crustal stabilization after the Penokean orogeny. Six mica ages from bedrock underlying deformed Paleoproterozoic quartzites cluster around 1600 Ma (1576–1614 Ma). The complete absence of 1760–1750 Ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300–350°C during Mazatzal-related deformation and metamorphism. The ∼1600 Ma mica ages are interpreted to date the cooling phase of this metamorphism. Two anomalously young biotite dates are interpreted to indicate partial resetting associated with Mesoproterozoic rifting at 1100 Ma. Ten hornblende 40Ar/ 39Ar dates obtained in this study address the higher-temperature overprinting effects in the southern Lake Superior region. One latest Archean age of 2503±18 Ma and two ages of 1853 and 1830 Ma are interpreted as remnant evidence of Archean and Penokean age amphibolite metamorphic events, respectively. The majority of hornblende ages are younger than the Penokean orogeny, scattering between 1796 and 1638 Ma. Microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages. This suggests that shearing and recrystallization did not play an important role in the retention or loss of argon. The 1638 Ma hornblende age is concordant with the Mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid-related resetting associated with that event. Six other post-Penokean ages scatter over a 70 million year interval (1796–1723 Ma) and probably reflect variable retention of radiogenic argon. They are interpreted to indicate variable degrees of partial intermediate-temperature (350–500°C) resetting of hornblende argon systematics at ∼1650–1630 Ma. Collectively, these data suggest that the effects of the Mazatzal orogeny in the southern Lake Superior region involved 350–500°C metamorphism and penetrative deformation.

Dynamic versus anorogenic setting for Mesoproterozoic plutonism in the Wet Mountains, Colorado: Does the interpretation depend on level of exposure?

Research Article| January 01, 2000 Dynamic versus anorogenic setting for Mesoproterozoic plutonism in the Wet Mountains, Colorado : Does the interpretation depend on level of exposure? Christine S. Siddoway; Christine S. Siddoway 1Department of Geology, Colorado College, Colorado Springs, Colorado 80903, U.S.A. Search for other works by this author on: GSW Google Scholar Rima M. Givot; Rima M. Givot 1Department of Geology, Colorado College, Colorado Springs, Colorado 80903, U.S.A. Search for other works by this author on: GSW Google Scholar Christopher D. Bodle; Christopher D. Bodle 2Department of Geology, College of Wooster, Wooster, Ohio 44691, U.S.A. Search for other works by this author on: GSW Google Scholar Matt T. Heizler Matt T. Heizler 3New Mexico Geochronological Research Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, U.S.A. Search for other works by this author on: GSW Google Scholar Rocky Mountain Geology (2000) 35 (1): 91–111. https://doi.org/10.2113/35.1.91 Article history received: 10 Oct 1998 rev-recd: 05 Jun 1999 accepted: 08 Jul 1999 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Christine S. Siddoway, Rima M. Givot, Christopher D. Bodle, Matt T. Heizler; Dynamic versus anorogenic setting for Mesoproterozoic plutonism in the Wet Mountains, Colorado : Does the interpretation depend on level of exposure?. Rocky Mountain Geology 2000;; 35 (1): 91–111. doi: https://doi.org/10.2113/35.1.91 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyRocky Mountain Geology Search Advanced Search Abstract New field investigations in the Wet Mountains of Colorado reveal informative structural-plutonic relationships surrounding Mesoproterozoic intrusions. Gneisses and schists of the Wet Mountains host syntectonic ∼1.7-Ga and ∼1.4-Ga plutons plus two to three generations of sills and stocks. Comparison of two study areas reveals a variation in metamorphic grade, crustal position, and structural rigidity of gneisses hosting the 1.4-Ga intrusions, with implications for the interpretation of dynamic versus anorogenic intrusive settings. An episode of post-1.4-Ga mineral growth was recorded in the Wet Mountains by overprinting mineral textures and 40Ar/39Ar hornblende ages of 1369 ± 4 to 1342 ± 6 Ma.Mineral textures and rock fabrics provide evidence of three significant Proterozoic deformational events in the Wet Mountains. Predominant northwest- to west-striking foliation is a second-phase fabric, S2, developed during regional plutonism at 1.66–1.7 Ga. S0, relict sedimentary layering, and S1, an earlier penetrative foliation, are preserved within cordierite.Fabric development and metamorphism during 1.4-Ga magmatism varied across the range. Middle amphibolite-grade gneisses of the Arkansas River Canyon in the north give way to stromatic migmatites in the central Wet Mountains. S2 was completely transposed in two discrete shear zones. The Five Points Gulch shear zone strikes approximately north-south and records sinistral-oblique displacement along a sillimanite mineral lineation. The Newlin Creek shear zone strikes northwest, with top-southwest transport.Fabric within 1.4-Ga intrusions varies from locally developed foliation on discordant margins (northern Wet Mountains) to strong concordant foliation in extensive sills (central Wet Mountains). Later sills are less well-foliated and slightly discordant, indicating syntectonic granitic emplacement. Blocks of host gneiss were assimilated along some sill margins, attesting to a similarity in temperature between intruded material and country rock.The variation in degree of metamorphic recrystallization, degree of transposition, and style of intrusion from north to south in the Wet Mountains is attributed to southward increase in temperature and structural depth. This study suggests that pluton emplacement depth influenced structural development and thus bears on the interpretation of dynamic versus anorogenic context for 1.4-Ga magmatism. You do not currently have access to this article.