Age of intrusion and rapid cooling of the Frankenstein gabbro (Odenwald,SW-Germany) evidenced by40Ar/39Ar and single-zircon207Pb/206Pb measurements

The Frankenstein gabbro complex in the northern Odenwald/FR Germany is one of the Hercynian plutonic bodies of the Saxothuringian zone in the Variscan belt.⁸⁷Sr/⁸⁶Sr isotopic investigations on pyroxene,⁴⁰Ar/³⁹Ar age determinations on hornblendes, plagioclases, biotites and pyroxenes and²⁰⁷Pb/²⁰⁶Pb dating on single zircon crystals have been carried out. The⁸⁷Sr/⁸⁶Sr initial ratio of the gabbro (0.70380) indicates uncontaminated derivation of the magma from the mantle. The results on hornblendes, plagioclases and zircons establish the isotopic age of intrusion and the mode of cooling. Pyroxene and biotite data indicate disturbed K-Ar systems of these minerals. The average ages of the hornblendes (363±7 Ma), of the plagioclases (359±3 Ma) and of the zircons (362 ±9 Ma) agree within the 1-sigma levels and constrain the early history of the plutonic intrusion at the turn from Devonian to Carboniferous times. These minerals must have been cooled below their closure temperatures within a time-interval shorter than the error margins of 10 Ma. The ages therefore are, unlike those of the neighbouring Bergsträßer Odenwald, not long-time cooling ages due to largescale and slow geological uplift of the crustal segment. Instead, they will closely estimate the time of intrusion of the Frankenstein pluton into a shallow crustal level. The hiatus of about 20 Ma compared to the average ages of the crystalline rocks of the Bergsträßer Odenwald in the south characterizes the mafic Frankenstein pluton as a separated unit of the Odenwald crystalline, and underlines the importance of the Carboniferous tectonic fault zone in-between. The present data set clearly demonstrates the importance of the applied combination of methods and techniques to constrain the history of intrusion and cooling of mafic plutonic rocks.     

Variable Variscan thermal rejuvenation in the St Malo region, Cadomian Orogen, France: evidence from 40Ar/39Ar mineral ages

Abstract The St Malo region in north-west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. ⁴⁰Ar/³⁹Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting. Eleven muscovite concentrates and one whole-rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record ⁴⁰Ar/³⁹Ar plateau ages of c. 527 and 521 Ma. An internally discordant ⁴⁰Ar/³⁹Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain-Deux Evailles-Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north-western sector of the St Malo region exhibit internally discordant ⁴⁰Ar/³⁹Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south-eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole-rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan-Bobital Granite Complex, yields a ⁴⁰Ar/³⁹Ar plateau age of c. 307 Ma. The ⁴⁰Ar/³⁹Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.     

U/Pb- und K/Ar-Datierungen des Uranvorkommens Höhenstein/Oberpfalz

K/Ar dating of micas from rocks of Fichtelgebirge and the northern Oberpfalz (NE-Bavaria) covers the rather wide range from 330 to 300 Ma. In combination with Rb/Sr wholerock isochron ages of post-tektonic granites four Variscian events can be traced:

1. Regional metamorphism ～ 330 Ma,
2. Intrusion of the older post-deformative granite G1 about 320 Ma (Falkenberger Granit) followed by rapid cooling.
3. About 300 Ma ago the older mineral ages were more or less reset by the intrusion of younger granites (G2 to G4, Flossenbürger Granit) again followed by a rapid cooling.
4. Local influences later than 290 Ma are recorded by some white micas in fault gauges at least up to the middle Jurassic (< 150 Ma).

The different U-black ore types yielded strongly discordant U/Pb dates. Applying the three dimensional U/Pb-discordia plane method the following results have been obtained: the older pitchblende generation with an upper concordia intersection of 336±17 Ma can be correlated with the regional metamorphism as well as with the intrusion of the older post-deformative granites. The younger generation with 298±4 Ma is of equal age as the intrusion of the younger granites. The brannerite mineralisation which according to thin section evaluation must be still younger could be only roughly dated to 288±78 Ma. The low precision is due to low uranium concentration and high common lead contents of the samples. The lower concordia intersection ages are 0 to 7 Ma. They were obtained more precisely by dating of secondary U-minerals: 10 torbernite samples with extremely high²³⁸U/²⁰⁶Pb-ratios of 30 000 to 50 000 yielded a positive slope of the discordia which points to an age of (137±12)×10³a. The²³⁰Th/²³⁸U-activity ratio of 0.68 determined byα-spectrometry indicates an age of (130.4±5.0)×10³ a. Pyrite and chalcopyrite samples which had U-concentration of some 100 ppm and which according to thin section evaluation are younger than the brannerite showed a positive slope of the²⁰⁷Pb/²⁰⁶Pb vs.²³⁸U/²⁰⁶Pb discordia, too. They could not be dated because the²⁰⁷Pb/²⁰⁶Pb-ratios corrected for common lead contribution are lower than 0.046 (radiogenic lead of zero age). This may be explained by an addition of²⁰⁶Pb due to²²²Rn migration from the pitchblende in only a few cm distance.     

Constraints on Variscan granite emplacement in north-east Bavaria,Germany: further clues from a petrogenetic study of the Mitterteich granite

Petrochemical and Rb-Sr, K-Ar and Sm-Nd isotopic data presented for the Mitterteich granite provide information on whole rock and mineral compositional characteristics, intrusion and cooling history, and protolith nature and put further constraints on the Variscan magmatic evolution in north-east Bavaria.The compositional characteristics classify the Mitterteich granite as a peraluminous (monzo-)granite (SiO₂ 67.3–73.5 wt.% ). Values for K₂O/Na₂O (> 1.2 and Al₂O₃/(CaO + N₂O + K₂O) (>1.1) are in the range of S-type granites. The rare earth elements show fractionated chondrite-normalized patterns (La _N /Yb _N =24–19) with negative Eu anomalies (Eu _N /Eu _N ^*=0.35–0.19). The micas have restricted ranges of major element composition, but reveal notable variations in trace element concentrations. Different biotite fractions of single specimens show a trend to lower concentrations of compatible elements in the finer fraction which can be explained as a result of asynchronous growth during the fractionation process. The PT conditions of crystallization of the magma based on muscovite and biotite is 600–640°C at 3 kbar. Regression of the whole rock samples gives an isochron corresponding to a ⁸⁷Rb-⁸⁷Sr age of 310 ± 7 Ma, initial ⁸⁷Sr/⁸⁶Sr of 0.7104±0.0010 (2 errors) and MSWD =0.03. Muscovite and biotite yield concordant K-Ar ages between 310 and 308 Ma, indicating a fast cooling rate of the granite intrusion. _{Nd T310}values average –4.2±1.0. Nd model ages of 1.4 Ga suggest a source region of mid-Proterozoic age.The Rb-Sr isochron age and initial Sr ratio of the Mitterteich granite are indistinguishable from those of the adjacent Falkenberg granite, establishing a genetic link. However, the K-Ar mica ages suggest that the Mitterteich granite must have undergone a faster uplift or cooling history than Falkenberg. Confronted with the geochronological record of granite emplacement in north-east Bavaria, the new results substantiate the view of three key periods of magmatic activity around 330–325, 315–305 and 290 Ma.     

Late-Variscan magmatism revisited: new implications from Pb-evaporation zircon ages on the emplacement of redwitzites and granites in NE Bavaria

Pb-evaporation zircon analyses coupled with a detailed cathodoluminescence (CL) study on the complete series of granitoids from the northern Oberpfalz, NE Bavaria, provide new evidence for the commencement and timing of late-Variscan magmatism. All granitoids analysed in this study were dated before by Rb-Sr and/or K-Ar methods. Investigated samples comprise medium-grained, I-type dioritic rocks (redwitzites), I/S-type granites (Leuchtenberg, Marktredwitz (G1), Zainhammer) and S-type granites (Falkenberg, Liebenstein, Mitterteich, Friedenfels, Steinwald, Flossenbürg, Bärnau). The zircon evaporation technique reveals three groups of ²⁰⁷Pb/²⁰⁶Pb ages which are interpreted to represent magmatic crystallisation: (1) ages of 324-321 Ma are found in all analysed redwitzites and in almost all I/S-type granites; (2) the granites of Falkenberg and Liebenstein yield ages of ~315 Ma; (3) ages between 312 and 310 Ma are recorded in the Mitterteich, Friedenfels, Steinwald and Flossenbürg granites. Titanite crystals from different redwitzite bodies yield conventional U-Pb ages of 325-322 Ma, identical to the Pb-evaporation zircon data of these rocks. The S-type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages, indicating that some of the cores must have lost their radiogenic Pb. From the geochronological data, we infer that metamorphic conditions of the Variscan crust produced different granite types at different times. The data support a model involving an early period of mantle-related magmatism which postdates the final convergence stage of the Variscan orogen. This magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis, which is probably related to post-collisional extension of the thickened Variscan crust.     

U-Pb and 39Ar/40Ar dating and Sm-Nd and Pb-Pb isotopic study of the Kalguty molybdenum-tungsten ore-magmatic system, Southern Altai

The Kalguty ore-magmatic system comprises two intrusive complexes: the Kalguty granite-leucogranite complex and Eastern Kalguty complex of dikes and small intrusions. U-Pb dating of individual zircon grains from granites of the main intrusive phase demonstrated that the crystallization age of small grains of magmatic habits and outer rims of large grains is almost concordant and is 216 ± 3 Ma. Ar-Ar isotope study shows that the K-Ar system of biotites from granites of the main phase within the Kalguty ore field was disturbed (radiogenic Ar was partially lost) and gave an age of 202 ± 1 Ma. The Ar-Ar dating of muscovites from intraore and postore dikes of the Eastern Kalguty complex devoid of signatures of postmagmatic recrystallization and superimposed greisenization gave similar ages of 205–201 Ma. This date is considered as the emplacement age of the Eastern Kalguty dikes and associated complex W-Mo-Bi-Be ore mineralization. Sm-Nd and Pb-Pb isotopic study of granites, ongonites, and elvans of the Kalguty ore-magmatic system and host rocks shows that these systems were closed. For example, recalculation of Nd isotopic ratios for corresponding ages of crystallization of magmatic systems (216 and 205 Ma) shows that ?_Nd(T) values decrease from ?1.9 to ?3.5 ... ?5.08 with transition from granite-leucogranite to subvolcanic granite porphyry, ongonite, and elvan dikes with corresponding increase of model ages of protoliths from 1.0 to 1.25 Ga. Lead isotopic ratios for leaching residues of whole-rock samples of all rock varieties (²⁰⁶Pb/²⁰⁴Pb = 18.305–18.831; ²⁰⁷Pb/²⁰⁴Pb = 15.527–15.571) are plotted well below the line of average crustal lead evolution according to the Stacey-Kramers model.     

Genetic types of tantalum ore deposits and their economic importance

Metasediments and meta-igneous rocks of the Willyama Supergroup in the Paleoproterozoic Olary Block of South Australia were deposited at ～1700 Ma. Intrusion by I-type granitoids at 1630 Ma was followed by the Olarian Orogeny, comprising two events of deformation and high-grade metamorphism at 1590 ± 20 Ma. Regional S-type granites and rare-metal pegmatites also formed during the Olarian Orogeny. The K-Ar isotopic system in primary pegmatitic muscovite closed at ～1505 ± 7 Ma, and the third event (regressive) of deformation and metamorphism together with minor granite emplacement, associated with the Olarian Orogeny, occurred at 1500 ± 20 Ma. A widespread thermal event occurred at 1100 to 1200 Ma and resulted from the Musgravian Orogeny. This was followed by crustal extension, tholeiitic dolerite dike intrusion, and rifting at 700 to 800 Ma. Cooling after the Delamerian Orogeny is recorded at ～466 to 475 Ma in the muscovite data. The ⁴⁰Ar/³⁹Ar data from many mica samples are complex because of multiple phases of thermal resetting and regression. This partial resetting of the K-Ar system is characterized by multiple age components and mixtures between them.     

Electron-microprobe dating of monazites from Western Carpathian basement granitoids: plutonic evidence for an important Permian rifting event subsequent to Variscan crustal anatexis

Accessory monazites from 35 granitoid samples from the Western Carpathian basement have been analysed with the electron microprobe in an attempt to broadly constrain their formation ages, on the basis of their Th, U and Pb contents. The sample set includes representative granite types from the Tatric, Veporic and Gemeric tectonic units. In most cases Lower Carboniferous (Variscan) ages have been obtained. However, a much younger mid-Permian age has been recorded for the specialised S-type granites of the Gemeric Unit, and several small A- and S-type granite bodies in the Veporic Unit and the southern Tatric Unit. This distinct Permian plutonic activity in the southern part of the Western Carpathians is an important, although previously little considered geological feature. It appears to be not related to the Variscan orogeny and is interpreted here to reflect the onset of the Alpine orogenic cycle, with magma generation in response to continental rifting. The voluminous Carboniferous granitoid bodies in the Tatric and Veporic units comprise S- and I-type variants which document crustal anatexis accompanying the collapse of a compressional Variscan orogen sector. The Variscan magmas were most likely produced through the remelting of a subducted Precambrian volcanic arc-type crust which included both igneous and sedimentary reworked volcanic-arc material. Although the 2C errors of the applied dating method are quite large and typically ᆞ-20 Ma for single samples, it would appear from the data that the Variscan S-type granitoids (333-367 Ma) are systematically older than the Variscan I-type granitoids (308-345 Ma). This feature is interpreted in terms of a prograde temperature evolution in the deeper parts of the post-collisional Variscan crust. In accordance with recently published zircon ages, this study shows that the Western Carpathian basement must be viewed as a distinct "eastern" tectonomagmatic province in the Variscan collision zone, where the post-collisional crustal melting processes occurred ~20 Ma earlier than in the central sector (South Bohemian Batholith, Hohe Tauern Batholith).     

Zircon dating,Sr and Nd isotopes,and element geochemistry of the Khalifan pluton,NW Iran: Evidence for Variscan magmatism in a supposedly Cimmerian superterrane

The Khalifan pluton of the Sanandaj-Sirjan Zone is composed of A-type peraluminous leucogranites with Nd model ages around 1.2 Ga. It intrudes an unfossiliferous sedimentary sequence previously considered as Cretaceous by analogy with neighboring areas. However, zircon U–Pb SHRIMP and Pb–Pb sequential evaporation dating have revealed that Khalifan granites are Carboniferous, with a precise age of 315 ± 2 Ma. This is the first reliable Variscan age obtained so far in this area, but there are geological indicators that other small plutonic bodies might also be Variscan. The existence of Carboniferous granite rocks in the Sanandaj-Sirjan Zone casts doubts on whether it was a part of the Cimmerian superterrane detached from east Gondwanide terranes because these characteristically lack Variscan magmatism. The age, chemical features and Nd isotopes of Khalifan granites are similar to those of the southernmost Urals and Tien Shan, so that we tentatively suggest that this area bears a Middle Asian instead of Cimmerian linkage.     

40Ar/39Ar Dating of Xuebaoding Granite in the Songpan-Garzê Orogenic Belt, Southwest China, and its Geological Significance

<正>Thus far,our understanding of the emplacement of Xuebaoding granite and the occurrence and evolution of the Songpan-Garze Orogenic Belt has been complicated by differing age spectra results.Therefore,in this study,the ~(40)Ar/~(39)Ar and sensitive high resolution ion micro-probe(SHRIMP) U-Pb dating methods were both used and the results compared,particularly with respect to dating data for Pankou and Pukouling granites from Xuebaoding,to establish ages that are close to the real emplacements.The results of SHRIMP U-Pb dating for zircon showed a high amount of U,but a very low value for Th/U.The high U amount,coupled with characteristics of inclusions in zircons,indicates that Xuebaoding granites are not suitable for U-Pb dating.Therefore,muscovite in the same granite samples was selected for ~(40)Ar/~(39)Ar dating.The ~(40)Ar/~(39)Ar age spectrum obtained on bulk muscovite from Pukouling granite in the Xuebaoding,gave a plateau age of 200.1±1.2 Ma and an inverse isochron age of 200.6±1.2 Ma.The ~(40)Ar/~(39)Ar age spectrum obtained on bulk muscovite from Pankou granite in the Xuebaoding gave another plateau age of 193.4±1.1 Ma and an inverse isochron age of 193.7±1.1 Ma. The ~(40)Ar/~(36)Ar intercept of 277.0±23.4(2σ) was very close to the air ratio,indicating that no apparent excess argon contamination was present.These age dating spectra indicate that both granites were emplaced at 200.6±1.3 Ma and 193.7±1.1 Ma,respectively.Through comparison of both dating methods and their results,we can conclude that it is feasible that the muscovite in the granite bearing high U could be used for ~(40)Ar/~(39)Ar dating without extra Ar.Based on this evidence,as well as the geological characteristics of the Xuebaoding W-Sn-Be deposit and petrology of granites,it can be concluded that the material origin of the Xuebaoding W-Sn-Be deposit might partially originate from the Xuebaoding granite group emplacement at about 200 Ma.Moreover,compared with other granites and deposits distributed in various positions in the Songpan-Garze Orogenic Belt,the Xuebaoding emplacement ages further show that the main rare metal deposits and granites in peripheral regions occurred earlier than those in the inner Songpan-Garze.Therefore,~(40)Ar/~(39)Ar dating of Xuebaoding granite will lay a solid foundation for studying the occurrence and evolution of granite and rare earth element deposits in the Songpan-Garze Orogenic Belt.     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

K-Ar ages of the Brandberg and Okenyenya igneous complexes,north-western Namibia

K-Ar mineral ages from intrusive units of the Brandberg and Okenyenya igneous complexes, north-western Namibia, confirm the Early Cretaceous age of the subvolcanic centres. The two centres are contemporaneous, although the range of ages from Brandberg, 135.2 ± 1.5 to 125.4 ± 1.3 Ma, suggests a rather longer period of intrusion than is represented by the rocks of Okenyenya, 133.3 ± 1.4 to 129.2 ± 0.7 Ma. The mean K-Ar age of the Okenyenya complex is essentially equivalent to previously determined Rb-Sr ages for the Messum and Paresis complexes on the same igneous lineament, but is a little greater than that suggested recently from Rb-Sr dating of this complex (129.1–123.4Ma). K-Ar chronology for the Brandberg complex is in conflict with the order of emplacement of granite units previously inferred from field evidence. In particular, the Amis peralkaline layered intrusion yields the oldest age from the complex, 135.2 ± 1.4 Ma. The concordancy of age measurements of amphibole and biotite, having very different potassium contents, from single rock samples is compelling evidence that neither inherited radiogenic argon, nor argon loss, presents a significant problem in the dated rocks. If the K - Ar age of 135 Ma for the Amis intrusion is correct, it constrains the minimum age for the onset of Etendeka flood volcanism, associated with continental break-up, as Etendeka lavas exhibit contact metamorphism and metasomatism around the rim of the Brandberg complex.     

Anticorrelated Rb−Sr and K−Ar age discordances,Leuchtenberg granite,NE Bavaria,Germany

The Leuchtenberg granite (Oberpfalz, NE Bavaria) displays a continuous differentiation trend ranging from mildy peraluminous, coarse-grained, porphyritic biotite granites (BG) to strongly peraluminous, medium- to fine-grained, garnet-bearing muscovite granites (GMG). The Rb–Sr and K–Ar age determinations of whole-rock and mineral samples from the granite and associated intermediate rocks (redwitzites) have revealed two divergent age gradients: Rb–Sr wholerock dates decrease and initial ⁸⁷Sr/⁸⁶Sr ratios increase for successively more evolved subsets of the granite. All BG samples (⁸⁷Rb/⁸⁶Sr=2–16) yield a date of 326±2 Ma with a low initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70778±0.00013 (1), while all GMG samples (⁸⁷Rb/⁸⁶Sr=70 to 1000) yield a younger date of 317±2 Ma with an enhanced initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7146±0.0039. The K–Ar measurements on biotites and muscovites give closely concordant dates for the GMG (326–323 Ma) and the southern lobe of the BG (324–320 Ma). The northern lobe of the BG, including the redwitzites, shows a well-defined trend of decreasing K–Ar dates from 320 Ma to 300 Ma towards the northwest. Critical consideration of both isotope systems leads to the conclusion that the Rb–Sr system of the GMG was disturbed by a later hydrothermal event. The ca. 326 Ma whole-rock Rb–Sr date for the BG is not in conflict with any of the K–Ar mineral dates and is taken as approaching the crystallization age of the Leuchtenberg granite. The K–Ar age progression within the northern lobe of the BG indicates that this part either cooled down over a protracted period of some 20 Ma or experienced reheating at ca. 300 Ma. The study highlights the potential of combined Rb–Sr and K–Ar dating in deciphering detailed chronology on the scale of a single igneous intrusion.     

Evolution of the Mesozoic Granites in the Xiong＇ershan-Waifangshan Region, Western Henan Province, China, and Its Tectonic Implications

Based on the new data of isotopic ages and geochemical analyses, three types of Mesozoic granites have been identified for the Xiong'ershan-Waifangshan region in western Henan Province: high-Ba-Sr I-type granite emplaced in the early stage (～160 Ma), I-type granite in the middle stage (～130 Ma) and anorogenic A-type granite in the late stage (～115 Ma).Geochemical characteristics of the high-Ba-Sr I-type granite suggest that it may have been generated from the thickened lower crust by partial melting with primary residues of amphibole and garnet. Gradual increase of negative Eu anomaly and Sr content variations reflect progressive shallowing of the source regions of these granites from the early to late stage. New 40Ar/39Ar plateau ages of the early-stage Wuzhangshan granite (156.0±1.1 Ma, amphibole) and middle-stage Heyu granite (131.8±0.7 Ma, biotite) are indistinguishable from their SHRIMP U-Pb ages previous published, indicating a rapid uplift and erosion in this region. The representative anorogenic A-type granite, Taishanmiao pluton, was emplaced at ～115 Ma. The evolution of the granites in this region reveals a tectonic regime change from post-collisional to anorogenic between ～160 Ma and ～115 Ma. The genesis of the early- and middle-stage I-type granites could be linked to delamination of subducted lithosphere of the Qinling orogenic belt, while the late-stage A-type granites represent the onset of extension and the end of orogenic process. In fact, along the Qinling -Dabie-Sulu belt, the Mesozoic granitoids in western Henan, Dabieshan and Jiaodong regions are comparable on the basis of these temporal evolutionary stages and their initial 87Sr/86Sr ratios,which may suggest a similar geodynamic process related to the collision between the North China and Yangtze cratons.     

Variscan vs Cadomian tectonothermal activity in northwestern sectors of the Teplá-Barrandian zone, Czech Republic: constraints from 40Ar/39Ar ages

Five muscovite concentrates from high-grade, pelitic metasedimentary basement rocks exposed in northwestern sectors of the Teplá-Barrandian zone (Czech Republic) record ⁴⁰Ar/³⁹Ar mineral plateau ages which range between ca. 376 and 362?Ma. Hornblende concentrates from metagabbro (Mariánské Lánzě complex) and fine-grained basement amphibolite display plateaux which define ³⁶Ar/⁴⁰Ar vs ³⁹Ar/⁴⁰Ar isotope-correlation ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower-grade metasedimentary rocks in southeastern portions of the Teplá-Barrandian zone is characterized by an internally discordant ⁴⁰Ar/³⁹Ar whole-rock age spectrum which suggests partial Variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed diorite of the Kdyn? complex records a well-defined ⁴⁰Ar/³⁹Ar age plateau which corresponds to an isotope-correlation age of ca. 516?Ma. This is interpreted to date post-magmatic cooling following emplacement.     

Comparative geochronology in the reversely zoned plutons of the Bottle Lake Complex,Maine: U-Pb on zircons and Rb-Sr on whole rocks

The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379±5 m.y. and 381±4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende ⁴⁰Ar/³⁹Ar determinations from Passadumkeag River which indicate an age of 378±4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496±14 m.y., with an initial Sr isotopic ratio of 0.70262.Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399±8 m.y. for Whitney Cove, 388±6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396±32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the ²⁰⁷Pb/²⁰⁶Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381±3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data.Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement.     

Petrographic, geochemical and geochronological investigation on granitic pebbles from Permotriassic metasediments of the Tisia terrain (eastern Papuk, Croatia)

Granitic pebbles occurring in the Permotriassic metasedimentary sequence of eastern Papuk, Slavonian Mountains, Croatia, were recognized to represent a coherent group of felsic, muscovite-albite metagranites. Fabrics, modal compositions and geochemical data imply that the rocks are derivatives of S-type granites formed through a combination of igneous and subsequent metasomatic processes. A Variscan formation age is demonstrated by K-Ar dating on coarse muscovite (range of 329?C317?Ma) as well as by electron microprobe based Th-U-Pb monazite dating (338?±?15?Ma). Additionally to the Variscan metasomatic processes of albitization and greisenisation, which led to an almost complete replacement of K-feldspar and biotite by albite and coarse muscovite, pebbles were affected by a younger phase of alteration resulting in the formation of a fine-grained sericitic matrix. The fine sericite yields K-Ar ages of 91?C83?Ma. A substantial reheating of the rocks during the Cretaceous is also indicated by the growth of new monazite dated at 106?±?10?Ma. Yttrium-contents of the Cretaceous monazite from the granite pebbles (0.3?C0.9?wt% Y₂O₃) are compatible with metamorphic temperatures of ~350?C400°C. These data confirm recent concepts according to which large parts of the Slavonian Mountains received a pervasive Cretaceous low-T regional metamorphic overprint. Furthermore, the pebbles provide useful information on the nature of the eroded Variscan crust of the Tisia Terrain, which has obviously contained considerable amounts of evolved high-level S-type granites modified through albitization and greisenization.     

U–Pb geochronology of the Southern Black Forest Batholith (Central Variscan Belt): timing of exhumation and granite emplacement

Some granites, granitoid dykes and volcanic rocks of the Southern Black Forest were dated by U–Pb techniques using zircon and monazite. An effusive rhyolite, which is interbedded in upper Visean sedimentary sequences of the Badenweiler-Lenzkirch zone, was dated at 340 ±2?Ma. This weakly metamorphic zone of supracrustal rocks borders high-grade gneiss terrains in the north and the south, which are intruded by a series of granitoid intrusions: the strongly sheared Schlächtenhaus granite is dated by monazite at 334±2?Ma and the hypothesis of a Devonian emplacement is therefore discarded. The emplacement of all other granites, crosscutting dykes and of an ignimbrite were all within analytical uncertainty: St. Blasien granite 333±2?Ma; Bärhalde granite 332±3?Ma; Albtal granite 334±3?Ma; and a porphyry dyke at Präg 332+2/-4?Ma. Deformation and thrusting of the basement units near the Badenweiler-Lenzkirch zone occurred after the emplacement of the Schlächtenhaus granite, but before the intrusion of the other granitoids, and may therefore be constrained to the time period unresolved between 334±2 and 333±2?Ma. The ignimbritic rhyolite of Scharfenstein was deposited in a caldera 333±3?Ma ago. This age coincides within error limits with published U–Pb monazite and Rb–Sr small slab ages of mimatitic gneisses, Ar–Ar hornblende ages of metabasites and Sm–Nd mineral isochron ages of eclogitic rocks in the underlying basement. This suggests that exhumation and cooling of this basement unit must have been active at rates of approximately 20?km and a few 100°C per million years. The silicic melts are interpreted to be of hybrid crust/mantle origin and their formation was most likely linked to these exhumation tectonics. A phase of mantle upwelling and heat advection into the crust is proposed to be the reason for this short-episodic magmatic pulse.     

Isotopic dating of the emplacement of the ultramafic masses in the Serrania de Ronda,Southern Spain

A Rb-Sr analysis of suites of samples from a small intrusion of cordierite-bearing alkali granite into the peridotite of the Sierra Bermeja (Serrania de Ronda) yields an age of 22± 4 Ma ( = 1.42×10^–11 a^–1): Late Oligocene/Early Miocene. It is believed that the intrusion was derived from contact-anatectic melts produced along the hot ultramafic mass during and/or directly following its tangential, tectonic dislocation from a mantle diapir. Its age can thus be taken as dating the termination of the hot emplacement of the ultramafic masses. K-Ar dates of biotites and Rb-Sr dates of biotite/whole-rock pairs in contact-metamorphic wall rocks along the ultramafics mostly lie between 19.5 and 18.5 Ma. This probably indicates that about 19 Ma ago the contact-zones of the ultramafic masses had cooled down to the blocking temperature of biotite to Rb-Sr and K-Ar.