Rb−Sr and Ar−Ar systematics of Malani volcanic rocks of southwest Rajasthan: Evidence for a younger post-crystallization thermal event

A new Rb−Sr age of 779±10 Ma has been obtained for a suite of andesite-daciterhyolite from the Malani Igneous Province of southwestern Rajasthan, dated earlier at 745±10 Ma by Crawford and Compston (1970). The associated basalts may be slightly younger than the felsic volcanics and have a mantle source. The felsic volcanics on the other hand were most probably derived by fractional crystallization of a crustal magma (Srivastavaet al 1989a, b).⁴⁰Ar−³⁹Ar systematics of three samples viz., a basalt, a dacite and a rhyolite show disturbed age spectra indicating a thermal event around 500–550 Ma ago. This secondary thermal event is quite wide-spread and possibly related to the Pan-African thermo-tectonic episode observed in the Himalayas and south India.     

Eruption of the Continental Flood Basalts at -259 Ma in the Emeishan Large Igneous Province, SW China： Evidence from Laser Microprobe ^40Ar/^39Ar Dating

A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province (LIP), SW China was investigated for laser microprobe 40Ar/39Ar dating. There are two 40Ar/39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger 40Ar/39Ar ages for some basalts with low-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The 40Ar/39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration (about 3 Ma) of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.     

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

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

40Ar-39Ar age of carbonatite-alkaline magmatism in Sung valley, Meghalaya, India

⁴⁰Ar-³⁹Ar analyses of one alkali pyroxenite whole rock and two phlogopite separates of calcite carbonatites from the Sung Valley carbonatite-alkaline complex, which is believed to be a part of the Rajmahal-Bengal-Sylhet (RBS) flood basalt province, yielded indistinguishable plateau ages of 108.8 ± 2.0Ma, 106.4 ± 1.3Ma and 107.5 ± 1.4Ma, respectively. The weighted mean of these ages, 107.2 ± 0.8 Ma, is the time of emplacement of this complex. This implies that Sung Valley complex and probably other such complexes in the Assam-Meghalaya Plateau postdate the main flood basalt event (i.e., the eruption of tholeiites) in the RBS province by ∼10Ma.     

A new 40Ar/39Ar eruption age for the Mount Widderin volcano,Newer Volcanic Province,Australia, with implications for eruption frequency in the region

The Mount Widderin shield volcano is located near Skipton, western Victoria, in the Western Plains subprovince of the monogenetic Pliocene–Holocene Newer Volcanic Province (NVP). Radiometric ages for lavas in the Hamilton–Skipton–Derrinallum area are few, owing to limited suitable outcrop for K–Ar or ⁴⁰Ar/³⁹Ar geochronology studies. Existing age constraints for flows in this area have been inferred from Regolith Landform Units (RLUs), complemented by a small number of K–Ar studies on ≥1 Ma flows. Although the RLU approach provides a valuable overview of relative eruption ages across the NVP, it is of limited use in eruption frequency studies. Additional radio-isotopic ages are required to refine age ranges for individual RLUs, and to validate previous assignment of individual flows to specific RLUs. We report a new, high-precision ⁴⁰Ar/³⁹Ar age of 389 ± 8 ka (2σ) for a Mount Widderin basalt sample. Based on this age and geomorphic observations, we propose that both the Widderin and Elephant lava flows be reassigned from the Eccles RLU to the Rouse RLU. We use the 389 ± 8 ka (2σ) age for Widderin, along with published K–Ar ages, to anchor a stratigraphic sequence of 15 individual flows in the Hamilton–Skipton–Derrinallum area, demonstrating that intermittent volcanism has occurred in this area from ≥3 Ma to ≤0.389 Ma. Within the limits of available data for the NVP, this time span of volcanic activity is second only to that of the Melbourne area. We consider the significance of the Widderin eruption age, in conjunction with published age constraints for maars and scoria cones of the Western Plains subprovince, building on previous studies that have focused solely on lava flow ages. The inclusion of the additional data weakens the argument for a decrease in volcanic activity after ca 0.9 Ma as implied by published ages for lava flows only. Additional detailed combined geochronology–geomorphology studies of lavas, scoria cones and maars in strategically selected small areas are advocated to better understand eruption frequency across the NVP.     

哈萨克斯坦北东天山浅成低温热液型金矿床成矿时代及构造背景

哈萨克斯坦北东天山地区是中亚造山带重要的浅成低温热液型金矿床产出地区,但其成矿年代学研究非常薄弱。为确定成矿时代,作者运用高精度激光40Ar/39Ar定年法对阿尔哈尔雷金矿床和乌仁科布拉克金矿床的赋矿围岩进行了年龄测定。获得阿尔哈尔雷金矿床安山岩样品20个点40Ar/39Ar等时线年龄为304±7Ma(MSWD=6),乌仁科布拉克金矿床安山玄武岩样品21个点40Ar/39Ar等时线年龄为280±6Ma(MSWD=2.4),表明哈萨克斯坦北东天山地区浅成低温热液型金矿床主要形成于晚石炭世末期-早二叠世。初步的岩石学、地球化学研究表明,哈萨克斯坦北东天山地区晚石炭世末期-二叠纪火山岩主要为流纹岩、粗安岩、玄武粗安岩、玄武安山岩、粗面玄武岩和玄武岩,具双峰式特征,主体属于高钾钙碱性和橄榄玄粗岩系列。哈萨克斯坦北东天山地区晚石炭世末期浅成低温热液型金矿床形成于碰撞晚期向裂谷的转换阶段,而二叠纪浅成低温热液型金矿床产出于陆内裂谷环境。     

Hunter–Bowen deformation in South Percy Island,northeastern Australia

South Percy Island is located approximately 50 km off the central Queensland coast and comprises a disrupted ophiolite mass alongside a diverse array of metamorphosed felsic and mafic rocks that record several episodes of magmatism, volcanism and deformation from the Permian to Early Cretaceous. This paper aims to constrain the age, affinity and deformation history of these units, as well as to establish the tectonic significance of the terrane. The trace-element compositions of mafic and felsic meta-igneous rocks record a change from MORB-like prior to ca 277 Ma to subduction-related by ca 258 Ma. Overprinting relationships between intrusive phases and deformation features reveal a relative chronology for the tectonothermal evolution of the area, while U–Pb and ⁴⁰Ar/³⁹Ar geochronology provides absolute age constraints. Deformation is localised around a NNE-striking tectonic contact that separates serpentinised ultramafic rocks from metamorphosed pillow lavas. Early formed ductile fabrics associated with the main episode of deformation (D₁) preserve bulk flattening strains at greenschist-facies conditions. Emplacement and post-kinematic cooling ages of a pre-D₁ quartz-monzonite dyke constrain the age of D₁/M₁ deformation and metamorphism to the period between ca 258 and ca 248 Ma. Minor brittle deformation (D₂) occurred at ca 230 Ma, based on U–Pb dating of a syn-D₂ diorite dyke (ca 231 ± 10 Ma) and several ca 230 Ma ⁴⁰Ar/³⁹Ar cooling ages. The deformation, metamorphism, and supra-subduction zone magmatism preserved on South Percy Island is correlated with the nearby Marlborough Terrane and more broadly with the second pulse of the Hunter–Bowen Orogeny, which affected much of the central and northern parts of eastern Australia in the late Permian and Early Triassic. Our results support previous suggestions that the second pulse of the Hunter–Bowen Orogeny involved coeval thrust systems in both the inboard and outboard parts of the orogen.     

40Ar/39Ar evidence for the timing of Paleoproterozoic gold mineralisation at the Sandpiper Deposit,Tanami region,northern Australia

At the Sandpiper gold deposit in the Tanami region of northern Australia sericite is intimately intergrown with arsenopyrite in gold-bearing quartz veins and breccias, suggesting sericite crystallisation synchronous with gold-bearing fluid flow. This ore-stage sericite yields a ⁴⁰Ar/³⁹Ar plateau age of 1785 ± 32 Ma (2σ including both analytical and systematic uncertainties). Recalculation using revised and more precise values for the ⁴⁰K decay constants and the age of the Fish Canyon Sanidine standard shifts the age to 1794 ±12 Ma (2σ including all known uncertainties). Given the possibility of post-mineralisation isotopic resetting this age can be conservatively interpreted as a minimum constraint on the timing of gold deposition although, given local geological relationships and estimates for the argon retentivity of white mica, we consider complete isotopic resetting to be unlikely. The preferred interpretation is, therefore, that the sericite ⁴⁰Ar/³⁹Ar age indicates the timing of gold mineralisation. Thesericite age accords with a limited dataset of ²⁰⁷Pb/²⁰⁶Pb xenotime ages of ca 1800 Ma from other gold deposits in the Tanami region, interpreted as mineralisation ages. The agreement between independently derived ages from several gold deposits lends support for a widespread gold-mineralising event at ca 1800 Ma in the Tanami region.     

Collision Event during 177-135 Ma on the Eastern Marginof the Qinghai-Tibet Plateau: Evidence from ~(40)Ar/ ~(39)Ar Dating for Basaltson the Western Margin of the Yangtze Platform

Geochronology of continental flood basalts sampled from the Emei large igneous province (LIP) on the western margin of the Yangtze platform was investigated by the laser microprobe 40Ar/39Ar dating technique. These basalts yield a fairly wide range of 40Ar/39Ar ages, varying from 259 to 135 Ma. One basalt sample, at least altered, recorded the oldest 40Ar/39Ar age of about 259 Ma, corresponding to a peak eruption age of the Emei LIP continental flood basalts. Most of the samples yield much younger ages from 135 to 177 Ma, which are consistent with the K-Ar ages for the same samples (122.8-172.1 Ma). The dating data suggest that these Permian basalts had been widely affected by the regional tectonothermal event at 177-135 Ma. The event was probably caused by the convergence and collision among the Laurasia, Yangtze and Qiangtang-Qamdo continental blocks on the eastern margin of the Qinghai-Tibet plateau after the late Triassic. The age of the event reflects the timing of the peak collisional orogeny.     

Tectono-thermal evolution of the India-Asia collision zone based on<Superscript>40</Superscript>Ar-<Superscript>39</Superscript>Ar thermochronology in Ladakh,India

New⁴⁰Ar-³⁹Ar thermochronological results from the Ladakh region in the India-Asia collision zone provide a tectono-thermal evolutionary scenario. The characteristic granodiorite of the Ladakh batholith near Leh yielded a plateau age of 46.3 ± 0.6 Ma (2σ). Biotite from the same rock yielded a plateau age of 44.6 ± 0.3 Ma (2σ). The youngest phase of the Ladakh batholith, the leucogranite near Himya, yielded a cooling pattern with a plateau-like age of ∼ 36 Ma. The plateau age of muscovite from the same rock is 29.8 ±0.2 Ma (2σ). These ages indicate post-collision tectono-thermal activity, which may have been responsible for partial melting within the Ladakh batholith. Two basalt samples from Sumdo Nala have also recorded the post-collision tectono-thermal event, which lasted at least for 8 MY in the suture zone since the collision, whereas in the western part of the Indus Suture, pillow lava of Chiktan showed no effect of this event and yielded an age of emplacement of 128.2 ±2.6 Ma (2σ). The available data indicate that post-collision deformation led to the crustal thickening causing an increase in temperature, which may have caused partial melting at the base of the thickened crust. The high thermal regime propagated away from the suture with time.     

A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic–ultramafic intrusions in the Emeishan Large Igneous Province, SW China

Based on published data, we reappraise the classification of high-Ti and low-Ti basalt from the Emeishan large igneous province (ELIP) and the correlations between basalts and mafic–ultramafic intrusions. Because of the lack of clear spatial and temporal variations of different types of basalts, we suggest that the basalts in the ELIP cannot be classified into high-Ti and low-Ti groups, by TiO₂ contents and/or Ti/Y ratios. The distinctive characteristics of these high-Ti and low-Ti lavas probably result largely from the different fractionating assemblages. Whether or not fractional crystallization of the Fe–Ti oxides occurred probably is the key factor that controls the Ti abundances and Ti/Y ratios in the residual melts, e.g., lavas, although the nature of the mantle sources, variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas. Therefore, neither Ti abundance nor Ti/Y ratios in basalts can reflect the nature of their mantle source. Moreover, the different types of mafic–ultramafic intrusions in the ELIP cannot simply be attributed to be genetically related special types of basalts, either high-Ti or low-Ti basalts. It is likely that they are merely the cumulus phases, i.e. chamber or conduit of the basaltic lavas. Hence, caution should be exercised in the use of high-Ti or low-Ti basalts as prospecting vectors for ore deposits in the region. Potential implications are proposed that both the Fe–V–Ti oxide and Cu–Ni–(PGE) sulfide mineralization in the ELIP intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes.     

大兴安岭地区德尔布干断裂带北段构造年代学研究

德尔布干断裂带是大兴安岭隆起西侧NE向的重要断裂带,处在海拉尔-拉布达林-根河盆地西缘,是著名德尔布干成矿区东南边界断裂带.为了确定德尔布干断裂带运动性质、活动时间,深入探讨该断裂带与中生代海拉尔-拉布达林-根河盆地及大兴安岭盆山格局、认识德尔布干断裂带多金属矿床成因等问题,本文应用锆石SHRIMP和云母40Ar/39Ar定年技术,分别对断裂带内的细粒黑云母花岗岩侵入体、韧性变形的花岗闪长质片麻岩、白云母石英片岩,进行了同位素年代学研究.其中花岗闪长质片麻岩岩浆型锆石SHRIMP谐和年龄300.6±9.3Ma,为花岗闪长质片麻岩海西期的侵位年龄;而花岗闪长质片麻岩中黑云母40Ar/39Ar坪年龄是130.9±1.4Ma,白云母石英片岩的白云母40Ar/39Ar坪年龄是115.6±1.6Ma,代表早白垩世伸展构造变形年龄;细粒黑云母花岗岩侵入体岩浆型锆石SHRIMP谐和年龄130.1±1.4Ma,为同伸展构造变形侵位的岩浆事件.上述地质年代说明德尔布干断裂带是早白垩世(110～130Ma)该区最年轻的重大伸展构造变形产物.控制NE向大兴安岭隆起和中生代海拉尔-拉布达林-根河等火山沉积盆地的发育格局、以及中生代以来的地壳演化与成矿类型.     

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

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

Age and composition of dykes emplaced before and during the opening of the Tasman Sea—source implications

Abstract

Dykes are common in the wave-cut platforms along the coast from Newcastle to Sydney. According to some authors, they may be related to the opening of the Tasman Sea that commenced ca 84?Ma ago. However, there are few detailed radiogenic dating and geochemical studies to evaluate this. We attempt to resolve this by K–Ar dating of plagioclase in and geochemical studies of, basaltic dykes intruding Permo-Triassic sequences on the wave-cut platforms and Carboniferous and Permo-Triassic sequences inland. The plagioclase separated from the dykes give K–Ar ages ranging from 266 to 53?Ma with the majority older than 84?Ma indicating that most dykes were emplaced before the Tasman Seafloor formation. The dykes are generally mildly alkaline, high-Ti basalts; fewer are tholeiitic and calc-alkaline, low-Ti basalts. Strongly light rare earth element (LREE)-enriched patterns typify the former and flat, LREE-depleted or slightly to moderately enriched LREE patterns, the latter. High-Ti basalts have ocean-island-basalt-like and low-Ti basalts, calc-alkaline or mid-ocean ridge basalt (MORB)-like patterns. Most high-Ti and some low-Ti basalts show plume-like characteristics, others N-type MORB and arc-like characteristics. Dykes intruding the Carboniferous sequences show a distinct contamination signature that could be crustal or due to subduction-related metasomatism of the subcontinental lithospheric mantle. The sources of the basaltic magmas vary substantially and in places changes with time. All alkali basalts are derived from enriched asthenospheric sources at varying depths (90–147?km) and most tholeiitic, low-Ti basalts have been extracted from asthenospheric and depleted asthenospheric–lithospheric sources indicating substantial compositional heterogeneity of the mantle. Further, Nd model ages varying from Neoproterozoic (940–580?Ma) to Paleozoic (460–370?Ma) suggest variation in the age of mantle sources for the basalts.     

40Ar‐39Ar and K‐Ar age constraints on the Early Proterozoic Tennant Creek Block,northern Australia,and the age of its gold deposits∗

⁴⁰Ar‐³⁹Ar age spectra on minerals from granitic, metamorphic and hydrothermal rocks confirm that the Early Proterozoic Tennant Creek Block was affected by two thermal events during its evolution. Although extensive alteration of biotite and feldspar within the granites precludes the direct determination of their cooling history, ⁴⁰Ar‐³⁹Ar analyses for hydrothermal muscovite from several nearby gold‐copper deposits indicate that regional cooling to below ～ 300°C was not prolonged. Flat, uniform muscovite age spectra were obtained from gold deposits east of the Tennant Creek town site and indicate a minimum age of 1825–1830 Ma for their formation. These ages are within error of those for the felsic volcanism of the Flynn Subgroup, and a genetic relationship between the two may exist. Samples from gold deposits elsewhere in the area indicate disturbance of the K‐Ar isotope system. The second thermal event to affect the region occurred at around 1700 Ma, and is confirmed by the ⁴⁰Ar‐³⁹Ar muscovite ages for the ‘Warrego’ granite (1677 ± 4 Ma) and for the metamorphism of the Wundirgi Formation (1696 ± 4 Ma).     

U-Pb and Ar/Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, ⁴⁰Ar/³⁹Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The ⁴⁰Ar/³⁹Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite ⁴⁰Ar/³⁹Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.     

新疆西准噶尔卡拉岗组火山岩40Ar-39Ar年龄

对新疆西准噶尔盆地萨吾尔地区卡拉岗组3个火山岩样品(英安岩、玄武岩和流纹岩)进行了⁴⁰Ar-³⁹Ar精确测年,获得英安岩坪年龄为283±2Ma(2σ),玄武岩加权平均年龄为281.2±3.0Ma(2σ),流纹岩坪年龄为280±2Ma(2σ),因此确定该组火山岩形成于距今283～280Ma,为早二叠世亚丁斯克期火山活动的产物。     

40Ar/39Ar geochronology constraints on the Middle Tertiary basement extensional exhumation,and its relation to ore-forming and magmatic processes in the Eastern Rhodope (Bulgaria)

The interaction of distinct geologic processes involved during late orogenic extensional exhumation history of the metamorphic units in the Eastern Rhodope is refined by new and reviewing ⁴⁰Ar/³⁹Ar geochronological and structural data. Minerals with different closure temperatures from metamorphic rocks investigated in this study are combined with those from magmatic and ore-forming hydrothermal rocks in two late stage metamorphic domes – the Kesebir-Kardamos and the Biala reka-Kehros domes. The 38-37 Ma muscovite and biotite cooling ages below 350°-300°C characterize basement metamorphic rocks that typified core of the Kesebir-Kardamos dome, constraining their exhumation at shallow crustal levels in the footwall of detachment. These ages are interpreted as reflecting last stage of ductile activity on shear zone below detachment, which continued to operate under low-temperature conditions within the semi-ductile to brittle field. They are close to and overlap with existing cooling ages in southern Bulgaria and northern Greece, indicating supportively that the basement rocks regionally cooled between 42-36 Ma below temperatures 350°-300°C. The spatial distribution of ages shows a southward gradual increase up structural section, suggesting an asymmetrical mode of extension, cooling and exhumation from south to the north at latitude of the Kesebir-Kardamos dome. The slightly younger 36.5-35 Ma crystallization ages of adularia in altered rocks from the ore deposits in the immediate hanging-wall of detachments are attributed to brittle deformation on high-angle normal faults, which further contributed to upper crustal extension, and thus constraining the time when alteration took place and deformation continued at brittle crustal levels. Silicic dykes yielded ages between 32-33 Ma, typically coinciding with the main phase of Palaeogene magmatic activity, which started in Eastern Rhodope region in Late Eocene (Priabonian) times. The ⁴⁰Ar/³⁹Ar plateau ages from the above distinct rock types span time interval lasting approximately ca. 6 Ma. Consequently, our geochronologic results consistently indicate that extensional tectonics and related exhumation and doming, epithermal mineralizations and volcanic activity are closely spaced in time. These new ⁴⁰Ar/³⁹Ar age results further contribute to temporal constraints on the timing of tectonic, relative to ore-forming and magmatic events, suggesting in addition that all above mentioned processes interfered during the late orogenic extensional collapse in the Eastern Rhodope region.     

Two distinct Miocene age ranges of basaltic rocks from the Rhön and Heldburg areas (Germany) based on Ar/Ar step heating data

New ⁴⁰Ar/³⁹Ar age data are determined for Cenozoic basaltic rocks from the Thuringian Rhön and Heldburg Gangschar (area also referred to as Grabfeld), integral parts of the Central European Volcanic Province. Applying the incremental heating technique on groundmass and plagioclase separates provided data which considerably specify our knowledge on the eruption ages in these volcanic fields and narrow down the duration of volcanic activity compared to earlier studies. All data but one outlier range between 20 and 14 Ma, being thus similar to those of the neighbouring Vogelsberg volcanic complex. The spectrum of ages is clearly divided into two distinct subsets: the Rhön ages are between 20 and 18 Ma, those of the Heldburg Gangschar are between 16 and 14 Ma. Thus, the present data clearly indicate a striking regional and temporal division of the Thuringian Miocene volcanism. The composition of the volcanic rocks in the two volcanic fields is remarkably diverse, ranging from tholeiitic basalts over alkali basalts and basanites to nephelinites. However, radiometric ages do not correlate with geochemical or petrological characteristics of the volcanics within each volcanic field, indicating that the different magma types erupted broadly contemporaneously.The outlier in age (29 Ma) is from a volcanic dyke of the NE Rhön area close to the NW end of the Thuringian Forest. However, more data are required to approve the significance of this age value, in particular since the rock showed isotopic age disturbance.     

Development of high precision Rb-Sr phlogopite and biotite geochronology; an alternative to Ar/Ar tri-octahedral mica dating

Potassium-Ar and Rb-Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K-Ar and, in particular, ⁴⁰Ar/³⁹Ar dating of micas is still widely applied, Rb-Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb-Sr ages than the K-Ar or ⁴⁰Ar/³⁹Ar system. Moreover, a reduction of uncertainties typically reported for Rb-Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb-Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% (n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield ⁴⁰Ar/³⁹Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb-Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb-Sr and ⁴⁰Ar/³⁹Ar ages demonstrates the robustness of the Rb-Sr system in tri-octahedral micas and cautions against the sole use of ⁴⁰Ar/³⁹Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb-Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb-Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma.Combining the mica Rb-Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb-Sr system in 1-2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.